Iron is an essential micronutrient, and, in the case of bacteria, its availability is commonly a growth-limiting factor. However, correct functioning of cells requires that the labile pool of chelatable “free” iron be tightly regulated. Correct metalation of proteins requiring iron as a cofactor demands that such a readily accessible source of iron exist, but overaccumulation results in an oxidative burden that, if unchecked, would lead to cell death. The toxicity of iron stems from its potential to catalyze formation of reactive oxygen species that, in addition to causing damage to biological molecules, can also lead to the formation of reactive nitrogen species. To avoid iron-mediated oxidative stress, bacteria utilize iron-dependent global regulators to sense the iron status of the cell and regulate the expression of proteins involved in the acquisition, storage, and efflux of iron accordingly. Here, we survey the current understanding of the structure and mechanism of the important members of each of these classes of protein. Diversity in the details of iron homeostasis mechanisms reflect the differing nutritional stresses resulting from the wide variety of ecological niches that bacteria inhabit. However, in this review, we seek to highlight the similarities of iron homeostasis between different bacteria, while acknowledging important variations. In this way, we hope to illustrate how bacteria have evolved common approaches to overcome the dual problems of the insolubility and potential toxicity of iron.

Chorismate mutase (CM), an essential enzyme at the branch-point of the shikimate pathway, is required for the biosynthesis of phenylalanine and tyrosine in bacteria, archaea, plants, and fungi. MtCM, the CM from Mycobacterium tuberculosis, has less than 1% of the catalytic efficiency of a typical natural CM and requires complex formation with 3-deoxy-d-arabino-heptulosonate 7-phosphate synthase for high activity. To explore the full potential of MtCM for catalyzing its native reaction, we applied diverse iterative cycles of mutagenesis and selection, thereby raising kcat/Km 270-fold to 5 × 105 m−1s−1, which is even higher than for the complex. Moreover, the evolutionarily optimized autonomous MtCM, which had 11 of its 90 amino acids exchanged, was stabilized compared with its progenitor, as indicated by a 9 °C increase in melting temperature. The 1.5 Å crystal structure of the top-evolved MtCM variant reveals the molecular underpinnings of this activity boost. Some acquired residues (e.g. Pro52 and Asp55) are conserved in naturally efficient CMs, but most of them lie beyond the active site. Our evolutionary trajectories reached a plateau at the level of the best natural enzymes, suggesting that we have exhausted the potential of MtCM. Taken together, these findings show that the scaffold of MtCM, which naturally evolved for mediocrity to enable inter-enzyme allosteric regulation of the shikimate pathway, is inherently capable of high activity.

Identification of antibody-binding epitopes is crucial to understand immunological mechanisms. It is of particular interest for allergenic proteins with high cross-reactivity as observed in the lipid transfer protein (LTP) syndrome, which is characterized by severe allergic reactions. Art v 3, a pollen LTP from mugwort, is frequently involved in this cross-reactivity, but no antibody-binding epitopes have been determined so far. To reveal human IgE-binding regions of Art v 3, we produced three murine high-affinity mAbs, which showed 70–90% coverage of the allergenic epitopes from mugwort pollen–allergic patients. As reliable methods to determine structural epitopes with tightly interacting intact antibodies under native conditions are lacking, we developed a straightforward NMR approach termed hydrogen/deuterium exchange memory (HDXMEM). It relies on the slow exchange between the invisible antigen-mAb complex and the free 15N-labeled antigen whose 1H-15N correlations are detected. Due to a memory effect, changes of NH protection during antibody binding are measured. Differences in H/D exchange rates and analyses of mAb reactivity to homologous LTPs revealed three structural epitopes: two partially cross-reactive regions around α-helices 2 and 4 as well as a novel Art v 3–specific epitope at the C terminus. Protein variants with exchanged epitope residues confirmed the antibody-binding sites and revealed strongly reduced IgE reactivity. Using the novel HDXMEM for NMR epitope mapping allowed identification of the first structural epitopes of an allergenic pollen LTP. This knowledge enables improved cross-reactivity prediction for patients suffering from LTP allergy and facilitates design of therapeutics.

Arabinogalactan proteins (AGPs) are plant proteoglycans with functions in growth and development. However, these functions are largely unexplored, mainly because of the complexity of the sugar moieties. These carbohydrate sequences are generally analyzed with the aid of glycoside hydrolases. The exo-β-1,3-galactanase is a glycoside hydrolase from the basidiomycete Phanerochaete chrysosporium (Pc1,3Gal43A), which specifically cleaves AGPs. However, its structure is not known in relation to its mechanism bypassing side chains. In this study, we solved the apo and liganded structures of Pc1,3Gal43A, which reveal a glycoside hydrolase family 43 subfamily 24 (GH43_sub24) catalytic domain together with a carbohydrate-binding module family 35 (CBM35) binding domain. GH43_sub24 is known to lack the catalytic base Asp conserved among other GH43 subfamilies. Our structure in combination with kinetic analyses reveals that the tautomerized imidic acid group of Gln263 serves as the catalytic base residue instead. Pc1,3Gal43A has three subsites that continue from the bottom of the catalytic pocket to the solvent. Subsite −1 contains a space that can accommodate the C-6 methylol of Gal, enabling the enzyme to bypass the β-1,6–linked galactan side chains of AGPs. Furthermore, the galactan-binding domain in CBM35 has a different ligand interaction mechanism from other sugar-binding CBM35s, including those that bind galactomannan. Specifically, we noted a Gly → Trp substitution, which affects pyranose stacking, and an Asp → Asn substitution in the binding pocket, which recognizes β-linked rather than α-linked Gal residues. These findings should facilitate further structural analysis of AGPs and may also be helpful in engineering designer enzymes for efficient biomass utilization.

Replication protein A (RPA) is a eukaryotic ssDNA-binding protein and contains three subunits: RPA70, RPA32, and RPA14. Phosphorylation of the N-terminal region of the RPA32 subunit plays an essential role in DNA metabolism in processes such as replication and damage response. Phosphorylated RPA32 (pRPA32) binds to RPA70 and possibly regulates the transient RPA70-Bloom syndrome helicase (BLM) interaction to inhibit DNA resection. However, the structural details and determinants of the phosphorylated RPA32–RPA70 interaction are still unknown. In this study, we provide molecular details of the interaction between RPA70 and a mimic of phosphorylated RPA32 (pmRPA32) using fluorescence polarization and NMR analysis. We show that the N-terminal domain of RPA70 (RPA70N) specifically participates in pmRPA32 binding, whereas the unphosphorylated RPA32 does not bind to RPA70N. Our NMR data revealed that RPA70N binds pmRPA32 using a basic cleft region. We also show that at least 6 negatively charged residues of pmRPA32 are required for RPA70N binding. By introducing alanine mutations into hydrophobic positions of pmRPA32, we found potential points of contact between RPA70N and the N-terminal half of pmRPA32. We used this information to guide docking simulations that suggest the orientation of pmRPA32 in complex with RPA70N. Our study demonstrates detailed features of the domain-domain interaction between RPA70 and RPA32 upon phosphorylation. This result provides insight into how phosphorylation tunes transient bindings between RPA and its partners in DNA resection.

The dimeric ectonucleotidase CD73 catalyzes the hydrolysis of AMP at the cell surface to form adenosine, a potent suppressor of the immune response. Blocking CD73 activity in the tumor microenvironment can have a beneficial effect on tumor eradication and is a promising approach for cancer therapy. Biparatopic antibodies binding different regions of CD73 may be a means to antagonize its enzymatic activity. A panel of biparatopic antibodies representing the pairwise combination of 11 parental monoclonal antibodies against CD73 was generated by Fab-arm exchange. Nine variants vastly exceeded the potency of their parental antibodies with ≥90% inhibition of activity and subnanomolar EC50 values. Pairing the Fabs of parents with nonoverlapping epitopes was both sufficient and necessary whereas monovalent antibodies were poor inhibitors. Some parental antibodies yielded potent biparatopics with multiple partners, one of which (TB19) producing the most potent. The structure of the TB19 Fab with CD73 reveals that it blocks alignment of the N- and C-terminal CD73 domains necessary for catalysis. A separate structure of CD73 with a Fab (TB38) which complements TB19 in a particularly potent biparatopic shows its binding to a nonoverlapping site on the CD73 N-terminal domain. Structural modeling demonstrates a TB19/TB38 biparatopic antibody would be unable to bind the CD73 dimer in a bivalent manner, implicating crosslinking of separate CD73 dimers in its mechanism of action. This ability of a biparatopic antibody to both crosslink CD73 dimers and fix them in an inactive conformation thus represents a highly effective mechanism for the inhibition of CD73 activity.

Sirtuin 6, SIRT6, is critical for both glucose and lipid homeostasis and is involved in maintaining genomic stability under conditions of oxidative DNA damage such as those observed in age-related diseases. There is an intense search for modulators of SIRT6 activity, however, not many specific activators have been reported. Long acyl-chain fatty acids have been shown to increase the weak in vitro deacetylase activity of SIRT6 but this effect is modest at best. Herein we report that electrophilic nitro-fatty acids (nitro-oleic acid and nitro-conjugated linoleic acid) potently activate SIRT6. Binding of the nitro-fatty acid to the hydrophobic crevice of the SIRT6 active site exerted a moderate activation (2-fold at 20 μm), similar to that previously reported for non-nitrated fatty acids. However, covalent Michael adduct formation with Cys-18, a residue present at the N terminus of SIRT6 but absent from other isoforms, induced a conformational change that resulted in a much stronger activation (40-fold at 20 μm). Molecular modeling of the resulting Michael adduct suggested stabilization of the co-substrate and acyl-binding loops as a possible additional mechanism of SIRT6 activation by the nitro-fatty acid. Importantly, treatment of cells with nitro-oleic acid promoted H3K9 deacetylation, whereas oleic acid had no effect. Altogether, our results show that nitrated fatty acids can be considered a valuable tool for specific SIRT6 activation, and that SIRT6 should be considered as a molecular target for in vivo actions of these anti-inflammatory nitro-lipids.

Aminopeptidase N (APN, CD13) is a transmembrane ectopeptidase involved in many crucial cellular functions. Besides its role as a peptidase, APN also mediates signal transduction and is involved in the activation of matrix metalloproteinases (MMPs). MMPs function in tissue remodeling within the extracellular space and are therefore involved in many human diseases, such as fibrosis, rheumatoid arthritis, tumor angiogenesis, and metastasis, as well as viral infections. However, the exact mechanism that leads to APN-driven MMP activation is unclear. It was previously shown that extracellular 14-3-3 adapter proteins bind to APN and thereby induce the transcription of MMPs. As a first step, we sought to identify potential 14-3-3–binding sites in the APN sequence. We constructed a set of phosphorylated peptides derived from APN to probe for interactions. We identified and characterized a canonical 14-3-3–binding site (site 1) within the flexible, structurally unresolved N-terminal APN region using direct binding fluorescence polarization assays and thermodynamic analysis. In addition, we identified a secondary, noncanonical binding site (site 2), which enhances the binding affinity in combination with site 1 by many orders of magnitude. Finally, we solved crystal structures of 14-3-3σ bound to mono- and bis-phosphorylated APN-derived peptides, which revealed atomic details of the binding mode of mono- and bivalent 14-3-3 interactions. Therefore, our findings shed some light on the first steps of APN-mediated MMP activation and open the field for further investigation of this important signaling pathway.

Members of the B-cell lymphoma (BCL-2) protein family regulate mitochondrial outer membrane permeabilization (MOMP), a phenomenon in which mitochondria become porous and release death-propagating complexes during the early stages of apoptosis. Pro-apoptotic BCL-2 proteins oligomerize at the mitochondrial outer membrane during MOMP, inducing pore formation. Of current interest are endogenous factors that can inhibit pro-apoptotic BCL-2 mitochondrial outer membrane translocation and oligomerization. A mitochondrial-derived peptide, Humanin (HN), was reported being expressed from an alternate ORF in the mitochondrial genome and inhibiting apoptosis through interactions with the pro-apoptotic BCL-2 proteins. Specifically, it is known to complex with BAX and BID. We recently reported the fibrillation of HN and BAX into β-sheets. Here, we detail the fibrillation between HN and BID. These fibers were characterized using several spectroscopic techniques, protease fragmentation with mass analysis, and EM. Enhanced fibrillation rates were detected with rising temperatures or pH values and the presence of a detergent. BID fibers are similar to those produced using BAX; however, the structures differ in final conformations of the BCL-2 proteins. BID fibers display both types of secondary structure in the fiber, whereas BAX was converted entirely to β-sheets. The data show that two distinct segments of BID are incorporated into the fiber structure, whereas other portions of BID remain solvent-exposed and retain helical structure. Similar analyses show that anti-apoptotic BCL-xL does not form fibers with humanin. These results support a general mechanism of sequestration of pro-apoptotic BCL-2 proteins into fibers by HN to inhibit MOMP.

Abnormal changes of neuronal Tau protein, such as phosphorylation and aggregation, are considered hallmarks of cognitive deficits in Alzheimer's disease. Abnormal phosphorylation is thought to precede aggregation and therefore to promote aggregation, but the nature and extent of phosphorylation remain ill-defined. Tau contains ∼85 potential phosphorylation sites, which can be phosphorylated by various kinases because the unfolded structure of Tau makes them accessible. However, methodological limitations (e.g. in MS of phosphopeptides, or antibodies against phosphoepitopes) led to conflicting results regarding the extent of Tau phosphorylation in cells. Here we present results from a new approach based on native MS of intact Tau expressed in eukaryotic cells (Sf9). The extent of phosphorylation is heterogeneous, up to ∼20 phosphates per molecule distributed over 51 sites. The medium phosphorylated fraction Pm showed overall occupancies of ∼8 Pi (± 5) with a bell-shaped distribution; the highly phosphorylated fraction Ph had 14 Pi (± 6). The distribution of sites was highly asymmetric (with 71% of all P-sites in the C-terminal half of Tau). All sites were on Ser or Thr residues, but none were on Tyr. Other known posttranslational modifications were near or below our detection limit (e.g. acetylation, ubiquitination). These findings suggest that normal cellular Tau shows a remarkably high extent of phosphorylation, whereas other modifications are nearly absent. This implies that abnormal phosphorylations at certain sites may not affect the extent of phosphorylation significantly and do not represent hyperphosphorylation. By implication, the pathological aggregation of Tau is not likely a consequence of high phosphorylation.

Programmed ribosomal frameshifting (PRF) is a mechanism used by arteriviruses like porcine reproductive and respiratory syndrome virus (PRRSV) to generate multiple proteins from overlapping reading frames within its RNA genome. PRRSV employs −1 PRF directed by RNA secondary and tertiary structures within its viral genome (canonical PRF), as well as a noncanonical −1 and −2 PRF that are stimulated by the interactions of PRRSV nonstructural protein 1β (nsp1β) and host protein poly(C)-binding protein (PCBP) 1 or 2 with the viral genome. Together, nsp1β and one of the PCBPs act as transactivators that bind a C-rich motif near the shift site to stimulate −1 and −2 PRF, thereby enabling the ribosome to generate two frameshift products that are implicated in viral immune evasion. How nsp1β and PCBP associate with the viral RNA genome remains unclear. Here, we describe the purification of the nsp1β:PCBP2:viral RNA complex on a scale sufficient for structural analysis using small-angle X-ray scattering and stochiometric analysis by analytical ultracentrifugation. The proteins associate with the RNA C-rich motif as a 1:1:1 complex. The monomeric form of nsp1β within the complex differs from previously reported homodimer identified by X-ray crystallography. Functional analysis of the complex via mutational analysis combined with RNA-binding assays and cell-based frameshifting reporter assays reveal a number of key residues within nsp1β and PCBP2 that are involved in complex formation and function. Our results suggest that nsp1β and PCBP2 both interact directly with viral RNA during formation of the complex to coordinate this unusual PRF mechanism.

Zinc is a critical element for insulin storage in the secretory granules of pancreatic beta cells. The islet-specific zinc transporter ZnT8 mediates granular sequestration of zinc ions. A genetic variant of human ZnT8 arising from a single nonsynonymous nucleotide change contributes to increased susceptibility to type-2 diabetes (T2D), but it remains unclear how the high risk variant (Arg-325), which is also a higher frequency (>50%) allele, is correlated with zinc transport activity. Here, we compared the activity of Arg-325 with that of a low risk ZnT8 variant (Trp-325). The Arg-325 variant was found to be more active than the Trp-325 form following induced expression in HEK293 cells. We further examined the functional consequences of changing lipid conditions to mimic the impact of lipid remodeling on ZnT8 activity during insulin granule biogenesis. Purified ZnT8 variants in proteoliposomes exhibited more than 4-fold functional tunability by the anionic phospholipids, lysophosphatidylcholine and cholesterol. Over a broad range of permissive lipid compositions, the Arg-325 variant consistently exhibited accelerated zinc transport kinetics versus the Trp-form. In agreement with the human genetic finding that rare loss-of-function mutations in ZnT8 are associated with reduced T2D risk, our results suggested that the common high risk Arg-325 variant is hyperactive, and thus may be targeted for inhibition to reduce T2D risk in the general populations.

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In addition to their well-known role in the control of cellular proliferation and cancer, cell cycle regulators are increasingly identified as important metabolic modulators. Several GWAS have identified SNPs near CDKN2A, the locus encoding for p16INK4a (p16), associated with elevated risk for cardiovascular diseases and type-2 diabetes development, two pathologies associated with impaired hepatic lipid metabolism. Although p16 was recently shown to control hepatic glucose homeostasis, it is unknown whether p16 also controls hepatic lipid metabolism. Using a combination of in vivo and in vitro approaches, we found that p16 modulates fasting-induced hepatic fatty acid oxidation (FAO) and lipid droplet accumulation. In primary hepatocytes, p16-deficiency was associated with elevated expression of genes involved in fatty acid catabolism. These transcriptional changes led to increased FAO and were associated with enhanced activation of PPARα through a mechanism requiring the catalytic AMPKα2 subunit and SIRT1, two known activators of PPARα. By contrast, p16 overexpression was associated with triglyceride accumulation and increased lipid droplet numbers in vitro, and decreased ketogenesis and hepatic mitochondrial activity in vivo. Finally, gene expression analysis of liver samples from obese patients revealed a negative correlation between CDKN2A expression and PPARA and its target genes. Our findings demonstrate that p16 represses hepatic lipid catabolism during fasting and may thus participate in the preservation of metabolic flexibility.

Cellular energy demands are met by uptake and metabolism of nutrients like glucose. The principal transcriptional regulator for adapting glycolytic flux and downstream pathways like de novo lipogenesis to glucose availability in many cell types is carbohydrate response element–binding protein (ChREBP). ChREBP is activated by glucose metabolites and post-translational modifications, inducing nuclear accumulation and regulation of target genes. Here we report that ChREBP is modified by proline hydroxylation at several residues. Proline hydroxylation targets both ectopically expressed ChREBP in cells and endogenous ChREBP in mouse liver. Functionally, we found that specific hydroxylated prolines were dispensable for protein stability but required for the adequate activation of ChREBP upon exposure to high glucose. Accordingly, ChREBP target gene expression was rescued by re-expressing WT but not ChREBP that lacks hydroxylated prolines in ChREBP-deleted hepatocytes. Thus, proline hydroxylation of ChREBP is a novel post-translational modification that may allow for therapeutic interference in metabolic diseases.

The development of a chronic, low-grade inflammation originating from adipose tissue in obese subjects is widely recognized to induce insulin resistance, leading to the development of type 2 diabetes. The adipose tissue microenvironment drives specific metabolic reprogramming of adipose tissue macrophages, contributing to the induction of tissue inflammation. Uncoupling protein 2 (UCP2), a mitochondrial anion carrier, is thought to separately modulate inflammatory and metabolic processes in macrophages and is up-regulated in macrophages in the context of obesity and diabetes. Here, we investigate the role of UCP2 in macrophage activation in the context of obesity-induced adipose tissue inflammation and insulin resistance. Using a myeloid-specific knockout of UCP2 (Ucp2ΔLysM), we found that UCP2 deficiency significantly increases glycolysis and oxidative respiration, both unstimulated and after inflammatory conditions. Strikingly, fatty acid loading abolished the metabolic differences between Ucp2ΔLysM macrophages and their floxed controls. Furthermore, Ucp2ΔLysM macrophages show attenuated pro-inflammatory responses toward Toll-like receptor-2 and -4 stimulation. To test the relevance of macrophage-specific Ucp2 deletion in vivo, Ucp2ΔLysM and Ucp2fl/fl mice were rendered obese and insulin resistant through high-fat feeding. Although no differences in adipose tissue inflammation or insulin resistance was found between the two genotypes, adipose tissue macrophages isolated from diet-induced obese Ucp2ΔLysM mice showed decreased TNFα secretion after ex vivo lipopolysaccharide stimulation compared with their Ucp2fl/fl littermates. Together, these results demonstrate that although UCP2 regulates both metabolism and the inflammatory response of macrophages, its activity is not crucial in shaping macrophage activation in the adipose tissue during obesity-induced insulin resistance.

Pectins are a major dietary nutrient source for the human gut microbiota. The prominent gut microbe Bacteroides thetaiotaomicron was recently shown to encode the founding member (BT1017) of a new family of pectin methylesterases essential for the metabolism of the complex pectin rhamnogalacturonan-II (RG-II). However, biochemical and structural knowledge of this family is lacking. Here, we showed that BT1017 is critical for the metabolism of an RG-II–derived oligosaccharide ΔBT1017oligoB generated by a BT1017 deletion mutant (ΔBT1017) during growth on carbohydrate extract from apple juice. Structural analyses of ΔBT1017oligoB using a combination of enzymatic, mass spectrometric, and NMR approaches revealed that it is a bimethylated nonaoligosaccharide (GlcA-β1,4-(2-O-Me-Xyl-α1,3)-Fuc-α1,4-(GalA-β1,3)-Rha-α1,3-Api-β1,2-(Araf-α1,3)-(GalA-α1,4)-GalA) containing components of the RG-II backbone and its side chains. We showed that the catalytic module of BT1017 adopts an α/β-hydrolase fold, consisting of a central twisted 10-stranded β-sheet sandwiched by several α-helices. This constitutes a new fold for pectin methylesterases, which are predominantly right-handed β-helical proteins. Bioinformatic analyses revealed that the family is dominated by sequences from prominent genera of the human gut microbiota, including Bacteroides and Prevotella. Our re-sults not only highlight the critical role played by this family of enzymes in pectin metabolism but also provide new insights into the molecular basis of the adaptation of B. thetaiotaomicron to the human gut.

Oxygen regulates hypoxia-inducible factor (HIF) transcription factors to control cell metabolism, erythrogenesis, and angiogenesis. Whereas much has been elucidated about how oxygen regulates HIF, whether lipids affect HIF activity is un-known. Here, using cultured cells and two animal models, we demonstrate that lipoprotein-derived fatty acids are an independent regulator of HIF. Decreasing extracellular lipid supply inhibited HIF prolyl hydroxylation, leading to accumulation of the HIFα subunit of these heterodimeric transcription factors comparable with hypoxia with activation of downstream target genes. The addition of fatty acids to culture medium suppressed this signal, which required an intact mitochondrial respiratory chain. Mechanistically, fatty acids and oxygen are distinct signals integrated to control HIF activity. Finally, we observed lipid signaling to HIF and changes in target gene expression in developing zebrafish and adult mice, and this pathway operates in cancer cells from a range of tissues. This study identifies fatty acids as a physiological modulator of HIF, defining a mechanism for lipoprotein regulation that functions in parallel to oxygen.

Peptidoglycan (PG) is an essential constituent of the bacterial cell wall. During cell division, the machinery responsible for PG synthesis localizes mid-cell, at the septum, under the control of a multiprotein complex called the divisome. In Escherichia coli, septal PG synthesis and cell constriction rely on the accumulation of FtsN at the division site. Interestingly, a short sequence of FtsN (Leu75–Gln93, known as EFtsN) was shown to be essential and sufficient for its functioning in vivo, but what exactly this sequence is doing remained unknown. Here, we show that EFtsN binds specifically to the major PG synthase PBP1b and is sufficient to stimulate its biosynthetic glycosyltransferase (GTase) activity. We also report the crystal structure of PBP1b in complex with EFtsN, which demonstrates that EFtsN binds at the junction between the GTase and UB2H domains of PBP1b. Interestingly, mutations to two residues (R141A/R397A) within the EFtsN-binding pocket reduced the activation of PBP1b by FtsN but not by the lipoprotein LpoB. This mutant was unable to rescue the ΔponB-ponAts strain, which lacks PBP1b and has a thermosensitive PBP1a, at nonpermissive temperature and induced a mild cell-chaining phenotype and cell lysis. Altogether, the results show that EFtsN interacts with PBP1b and that this interaction plays a role in the activation of its GTase activity by FtsN, which may contribute to the overall septal PG synthesis and regulation during cell division.

Poly(ADP-ribose) polymerase (PARP) superfamily members covalently link either a single ADP-ribose (ADPR) or a chain of ADPR units to proteins using NAD as the source of ADPR. Although the well-known poly(ADP-ribosylating) (PARylating) PARPs primarily function in the DNA damage response, many noncanonical mono(ADP-ribosylating) (MARylating) PARPs are associated with cellular antiviral responses. We recently demonstrated robust up-regulation of several PARPs following infection with murine hepatitis virus (MHV), a model coronavirus. Here we show that SARS-CoV-2 infection strikingly up-regulates MARylating PARPs and induces the expression of genes encoding enzymes for salvage NAD synthesis from nicotinamide (NAM) and nicotinamide riboside (NR), while down-regulating other NAD biosynthetic pathways. We show that overexpression of PARP10 is sufficient to depress cellular NAD and that the activities of the transcriptionally induced enzymes PARP7, PARP10, PARP12 and PARP14 are limited by cellular NAD and can be enhanced by pharmacological activation of NAD synthesis. We further demonstrate that infection with MHV induces a severe attack on host cell NAD+ and NADP+. Finally, we show that NAMPT activation, NAM, and NR dramatically decrease the replication of an MHV that is sensitive to PARP activity. These data suggest that the antiviral activities of noncanonical PARP isozyme activities are limited by the availability of NAD and that nutritional and pharmacological interventions to enhance NAD levels may boost innate immunity to coronaviruses.

War in Ukraine: The world reaction - Brazil and Africa Audio NCapeling 9 June 2022

The ninth episode of our podcast mini-series examines perspectives from Brazil and the Africa region and the impact on their relations with Russia.

How have Brazil and Africa reacted to the war in Ukraine? With impending elections in Brazil, how is President Bolsonaro’s relationship with Putin received by the public? How will food insecurity affect African nations’ response to the war?

This episode was produced by David Dargahi and Anouk Millet of Earshot Strategies on behalf of Chatham House.

Enhancing the role of women in peacebuilding and politics in Ethiopia 29 June 2022 — 1:00PM TO 2:30PM Anonymous (not verified) 16 June 2022 Online

Panellists discuss the priorities for promoting the agency of women in politics and peacebuilding in Ethiopia and approaches for combatting gender-based discrimination and violence.

The war in northern Ethiopia and conflicts elsewhere have disproportionately affected women and girls – including through the infliction of physical and sexual violence, the heightened impacts of displacement and disruptions to education, and the co-option of women’s experiences in narratives by aggressors of conflict.

Hard-won political gains in women’s rights have been undermined and deep-rooted gender inequalities exacerbated. Despite this, women remain central actors in politics, as well as in conflict resolution and mediation efforts. However, more needs to be done to promote the security and inclusion of women in finding sustainable solutions for Ethiopia’s long-term recovery and to institutionalize reforms for gender equity and development.

At this public event, panellists will discuss the priorities for improving women’s participation and equality in public decision-making in Ethiopia and how to strengthen the implementation of legislation on women’s rights. They will also discuss what societal shifts and approaches are needed to combat gender-based discrimination and violence and to promote the agency of women in peacebuilding.

This webinar is part of a series of events and outputs on Ethiopia’s political transition.

This event will also be broadcast live on the Chatham House Africa Programme’s Facebook page.

What challenges does the new president of Somalia face? Explainer Video aboudiaf.drupal 28 June 2022

Ahmed Soliman examines the challenges the new president Hassan Sheikh Mohamud faces in his first 100 days as president.

Key issues for the new administration are a deteriorating situation with regards to drought as close to half the population are facing food insecurity due to a fourth failed rainy season.

Combined with an inflation rate above ten per cent, many Somalis are at risk of famine and starvation. Many areas of the country are affected from the pastoralist regions to those which house IDP camps around the capital city and other towns, all being exacerbated by the war in Ukraine as Somalia was importing much of its wheat imports from Ukraine and Russia.

Will Africans’ calls for better democracy be met? The World Today mhiggins.drupal 29 July 2022

Voters want the continent’s ageing leaders to step aside to usher in a new age of political engagement and robust democracy, say the experts of Afrobarometer.

Across Africa, recent years have been marked by both encouraging democratic highs and troubling anti-democratic lows. Notable advances from last year include the Gambia’s successful presidential election, a ruling-party transition in Zambia and the first democratic transfer of power in Niger. In the lead up to this, add Malawi’s retake of its flawed presidential election in 2020 and an earlier succession of oustings of long-serving autocrats in Sudan, Zimbabwe and the Gambia. 

Contrast these gains, though, with setbacks elsewhere, including increasing restrictions on opposition parties in Benin, Senegal and Tanzania; the use of violence and intimidation during elections in Côte d’Ivoire and Uganda; and military coups, with the latest in Burkina Faso this year and last year in Chad, Mali, Sudan and Guinea.

These contradictory developments join dire warnings from experts that democracy is losing ground on the continent. But what can we learn about the state of democracy on the continent from Africans themselves?

Afrobarometer, a pan-African, non-partisan research network, has been surveying people about their views on democracy, governance and quality of life for more than 20 years. After interviewing nearly 50,000 citizens across 34 countries during Afrobarometer Round 8, which spans 2019-2021, we find that despite the efforts of some leaders to undermine democratic norms, Africans remain committed to democracy and democratic institutions.

They believe that the military should stay out of politics, that political parties should freely compete for power, that elections are an imperfect but essential tool for choosing their leaders, and that it is time for the old men who cling to power to step aside.

But their political reality falls short of these expectations. The perception of widespread and worsening corruption is particularly corrosive, leaving people increasingly dissatisfied with political systems that are yet to deliver on their aspirations to live in societies that are democratically and accountably governed. And although citizens find myriad ways to voice their concerns, they feel that their governments are not listening.

Simply put, Africans want more democratic and accountable governance than they think they are getting.

Africans’ democratic aspirations

Over the past decade, democracy watchers have been alarmed by declining trends in Africa. Concerns have been exacerbated in the past two years as some governments have taken advantage of the Covid pandemic to limit freedoms, restrict fair campaigning or postpone elections. Activists fear that supposedly temporary rollbacks in hard-won governance reforms could become permanent.

But for the most part, African citizens remain committed to democracy and democratic institutions. Across 30 countries that Afrobarometer has surveyed consistently since Round 5 (2011–2013), most indicators are strong and quite steady.

For example, seven in 10 Africans say that ‘democracy is preferable to any other kind of government’. While this is down modestly from 73 per cent a decade ago, more specific indicators seem to affirm popular commitment to democracy. Large and steady majorities consistently reject authoritarian alternatives, including one-person or ‘strongman’ rule (82 per cent), one-party rule (77 per cent) and military rule (75 per cent), which is clearly rejected even in many of the countries rocked by recent military coups.

Africans also express strong support for a limit to presidential terms, a feature of democratic governance that researchers and activists argue nurtures political participation, demonstrates that change via the ballot box is possible, and reduces the risk of personality cults, authoritarianism, corruption and coups. Across 34 countries, an average of 76 per cent favour limiting their presidents to two terms, including a majority (54 per cent) who ‘strongly’ support this rule. Term limits enjoy majority support in every surveyed country. 

The public’s democratic commitment is undergirded by strong and in some cases growing support for core democratic institutions. Support for multiparty competition and parliamentary oversight of leaders remains steady, while expectations that governments should be accountable to the courts have increased significantly over the past decade.

In addition, growing numbers of people say it is more important to have a government that is accountable to the people rather than one that just ‘gets things done’, an especially strong indicator of deepening commitment to democratic norms among citizens. 

Trouble at the polling booth

Elections remain a central, though controversial, institution of democracy for Africans. They have served as the foundation for real change, as in Zambia last year. But in other cases, such as Uganda’s January 2021 poll, they have been marred by violence and human rights abuses, as well as the weaponization of Covid to justify restrictions on campaigning.

The public is also sceptical about the capacity of elections to bring about real change: fully 50 per cent say they do not think elections are effective in enabling voters ‘to remove from office leaders who do not do what the people want’.

At the same time, large majorities report positively on their country’s election environment. Asked about their most recent election, at least eight in 10 say they did not observe intimidation (87 per cent) or interference (81 per cent) by security forces and did not fear violence (80 per cent).

We must keep in mind that these encouraging averages can obscure deep problems in some countries. For example, while only 3 per cent of Namibians say votes are ‘often’ not counted fairly, between a quarter and one-third cite inaccurate counts as a frequent problem in Zimbabwe, Sudan and Gabon. 

In addition, confidence in the fairness of the media environment is drastically lower, on average just 36 per cent.

But perhaps most importantly, almost nine in 10 Africans (87 per cent) say they are free to vote as they choose, including sizeable majorities in every surveyed country. And a solid majority of 63 per cent rate their most recent election as completely or mostly free and fair. 

All of this may help to explain still-strong support for competitive elections as the best system for selecting leaders. A robust three-quarters confirm their commitment to elections, though this has fallen slightly over the past decade, probably reflecting disillusionment with electoral processes that are too often torn by violence and produce contested results. 

A growing number of people may also be recognizing that elections, especially poor-quality ones, are not enough to guarantee democracy and better governance, and that a healthy democracy must include such other features as government accountability, respect for the rule of law, responsiveness and citizen participation.

The ‘democratic disappointment’ gap

To what extent does political reality align with Africans’ democratic aspirations? Our findings suggest that it is falling well short of expectations.

While a slim majority has steadily reported that their country is a ‘full democracy’ or one ‘with minor problems’ over the past decade, satisfaction, however, has dropped to 43 per cent in that time. 

What explains this growing dissatisfaction? Other indicators of democratic supply offer some clues. While ratings of election quality have held steady, favourable public assessments of presidential accountability to parliament and to the courts have both declined.

The rising scourge of corruption

But one of the most significant driving factors may be burgeoning corruption, a trend that appears to parallel declining democratic satisfaction. On average across 34 countries, around six in 10 say both that corruption in their country increased over the past year, and that their government is doing a poor job of controlling it.

These perceptions matter. Over time, when perceptions of corruption rise or fall, levels of dissatisfaction with democracy tend to follow suit. 

In South Africa, dissatisfaction with democracy grew steadily alongside scandals involving President Jacob Zuma, and has continued to rise under his successor, Cyril Ramaphosa, whose office has been tainted by ‘Farmgate’ and a major Covid-relief scandal. The ‘Fishrot’ scandal in Namibia has had similar consequences.

Are governments listening?

African citizens are raising their voices, calling on their governments to fulfil their democratic aspirations. Since April 2017, the Carnegie Endowment for International Peace has recorded more than 70 episodes in 35 African countries of protests focused on issues ranging from demands for democracy in eSwatini to resisting police brutality, presidential third-term attempts and Covid restrictions. 

Citizen participation and government responsiveness are cornerstones of democracy. But are governments listening?

Voting is the most obvious and popular way for citizens to express themselves, and Africans take advantage of this opportunity. Two-thirds said they voted in their most recent national election. But elections occur only occasionally, and they force individuals to compress a wide array of views into very few choices. How do Africans find their voice during the long intervals between elections?

Many invest in personal efforts to act as agents of change. In fact, nearly half say they joined with others to raise an issue at least once in the past year, and a third contacted a political leader. A quarter report they acted with others to request government action. Less common but still important modes of engagement include asking for help from or lodging a complaint with government, contacting the media, and joining a demonstration.

These robust levels of citizen engagement suggest that people feel they can make a difference. Unfortunately, decision-makers aren’t always receptive or responsive to citizen voices. Less than a quarter of people think local government officials listen to them – and even fewer think their members of parliament do. 

What is more troubling is that fully two-thirds say they are at risk of retaliation or some form of negative consequences if they take action by reporting incidents of corruption. 

Lack of government responsiveness and respect for popular voices may have direct implications for both citizen engagement and citizen satisfaction. For example, we find that people are more likely to contact leaders or take other actions to solve problems if they believe that government officials respect and listen to them; that they will get a response if they raise an issue; and if they do not need to fear retaliation. 

Similarly, when we compare country averages for government responsiveness to the percentage of citizens who are satisfied with democracy, we again find positive associations. 

When governments are responsive, citizens are more likely to engage in addressing community needs and to be satisfied with their political system and optimistic about the future. Respectful and responsive governance has the potential to spur citizen action to solve critical development challenges – and may be the cure for what ails democracy.

Now is the moment to launch an African vaccine industry The World Today mhiggins.drupal 31 July 2022

The continent plans to make 60 per cent of its vaccines by 2040. After the failure of the world to help in the pandemic, it’s high time, says Ngozi Erondu.

The lack of an African vaccine industry has been a glaring concern for decades. Before the pandemic, 99 per cent of Africa’s vaccines were manufactured outside the continent. As well as endangering the lives of millions, this situation has inhibited social and economic progress on the continent.

In response, the Africa Centres for Disease Control and Prevention (Africa CDC) has undertaken an ambitious plan, outlined in the Partnerships for African Vaccine Manufacturing (PAVM) Framework for Action, to develop the nascent African vaccine manufacturing sector into an end-to-end industry by 2040. The framework aims to raise the share of African-manufactured vaccines used across the continent to 60 per cent by 2040, or the equivalent to up to 1.7 billion doses annually.

Seven of every 10 vaccines used in Africa are currently donated through Gavi, the Vaccine Alliance. Most are administered within childhood immunization programmes and are largely manufactured either in India, or by  multinational vaccine manufacturers in North America or Japan.

Vaccine donations have inhibited the development in Africa of vaccines and other countermeasures against diseases.

Though the Ebola virus was discovered in Central Africa in 1976, vaccine development was not adequately funded until it emerged in Europe in 2014. Human monkeypox resurfaced in Nigeria in 2017, yet the global Coalition for Epidemic Preparedness Innovations only targeted it for vaccine development in July this year.

The pandemic highlighted Africa’s fatal dependency on imported vaccines. Only 20 per cent of Africans are fully vaccinated against Covid-19, due to the failure of countries in the Global North to ensure the equitable distribution of vaccines via the COVAX facility to 40 per cent of the world’s most vulnerable people. 

The pandemic also confirmed that Africa could not rely on fellow states of the Global South. At the height of the Delta variant outbreak in early 2021, India halted vaccine exports to Africa, where only 1.5 per cent of the population had at that time received any vaccine doses.

After decades of discussions, there are signs that Africa could soon succeed in creating its own vaccine industry. First, the 55-member African Union is in the process of establishing the African Medicines Agency, a regional regulatory body. 
 

‘The new public health order’

Additionally, the African Export-Import Bank and African Development Bank (AfDB) have established a foundation to provide financial and strategic support for the development of the pharmaceutical industry and the consolidation of regional vaccination programmes in Africa (the foundation would potentially negotiate intellectual property rights and licensing issues but that remains to be seen).

Second, studies show there is an emerging middle class in Africa. In a 2011 report by the AfDB, this was estimated at some 56 million households. Potentially, this means many people will be able to buy vaccines and medicines made in Africa.

About a third of African countries currently pay for their vaccine needs. According to PAVM forecasts, the value of the total African market could reach between $3 billion and $17 billion by 2040.

The recent entry into effect of the African Continental Free Trade Area should also prove conducive to African vaccine development. Through economic integration, free movement and harmonized regional standards, countries that invest in their biopharmaceutical and medical technology sectors may attract employees, regional and international businesses, and investment. Further, the pandemic has encouraged people to relocate to countries with, or planning for, universal healthcare.

Building an African pharmaceutical industry from the ground up could take much longer than two decades and cost tens of billions of dollars. Nevertheless, the moment seems ripe, and timely support has been forthcoming from influential regional actors, including Rwandan President Paul Kagame, South Africa’s Cyril Ramaphosa, and private sector business executives, including the Zimbabwean-born billionaire Strive Masiyiwa.

With a pandemic treaty embedding equity in prevention, preparedness and response some way off, and given the limitations surrounding the recent World Trade Organization compromise on the TRIPS waiver – which temporarily waives Covid-19 vaccine patent protections for poorer countries – it is doubly important for Africa to build up its own pharmaceutical industry and emergency systems. 

In 2021, John Nkengasong, then director of Africa CDC, wrote of the necessity of a post-pandemic ‘new public health order’ for Africa. Such a change may threaten the global health organizations, industries and institutes who derive payment from ‘saving Africa’ during emergencies. Additionally, through strengthening Africa CDC, other actors such as the World Health Organization may find that they have a diminished strategic role on the continent.
 
While Africa should not dismiss these valuable and long-standing partnerships, it must take the opportunity to advance its interests and to assume leadership in this important area.

Why Africa needs to be in space The World Today mhiggins.drupal 1 August 2022

From agriculture and navigation to banking and tele-education, satellite technology can have a huge impact on rapidly developing societies, says Val Munsami.

Africa’s socio-economic and environmental development is widely acknowledged as being crucial to its growth and long-term sustainability – and the prosperity of its more than one billion residents. 

Increasingly, though, attention is also turning to the contribution that the space industry can make to progress on the continent. Space-based products and services have a critical role to play in meeting national and continental priorities, as underpinned by the UN’s Sustainable Development Goals and the African Union’s Agenda 2063 – the bloc’s strategic framework for development, democracy and peace. 

With this in the mind of policymakers, the African Union’s space policy and strategy is embedded in Agenda 2063 as one of its 15 key programmes. It guides the sector’s development and the nascent African Space Agency, to become operational later this year from its headquarters in Egypt.

The continental agency is expected to leverage the benefits of space science and technology for socio-economic and environmental development. It will lead on bridging the space divide, especially for those countries that do not have a focus on, or activities in, space science and technology – and simultaneously inject some momentum into improving capabilities of existing national space programmes.

The African Union’s commitment to space has accelerated the growth of the African space industry. More than 20 national space agencies or space-related institutions have been established on the continent over the past five or so years. 

Our modern lifestyles are intimately dependent on space products and services. Meteorological and communication satellites are placed in geostationary orbits at an altitude of 36,000km above the equator. At this point above the Earth, they complete one orbit every 24 hours in the direction of the planet’s rotation, appearing, essentially, motionless – and providing a constant gaze on the same geographic location.

They provide a wealth of information that fuels the everyday services we take for granted, but that are essential for our everyday lives, from health to education to the economy.

From their vantage point, geostationary orbit satellites provide our daily weather reports, monitor climate-related cycles and offer a platform for near-instantaneous communications across the globe to relay multimedia, live sporting events and up-to-the-minute global news. 

This lightning-fast communication is also indispensable for tele-education and tele-medicine, by which professionals in urban areas can deliver educational content and health services to rural schools and clinics, respectively. Banking transactions also rely on telecommunication satellites to communicate between an automated teller machine and the data servers located at the bank. 

How satellites can detect disease

Other satellites are placed in low Earth orbits. These complete on average one polar orbit around the Earth every 100 minutes. Because the planet rotates across the plane of the orbit, such a satellite eventually covers the whole Earth, which is immensely useful for remote sensing and navigation and positioning applications. 

Remote sensing applications provide a myriad of products and services, including monitoring the state of our natural resources, observing ship traffic in our coastal economic zones and providing information for precision farming that can help a farmer decide, for example, when to irrigate and how much fertilizer to use.

They can also detect changes that might indicate encroaching water-borne diseases, aid peacekeeping missions and help ensure public safety and security. Navigation applications are vital for aviation and marine navigation, whereas positioning applications are important for safety-of-life services. 

Another way that positioning applications in developing countries are put to good use is the assignment of geolocation addresses to dwellings in informal settlements where postbox addresses do not exist. This then allows the overlaying of key vector data about populations on to geophysical base maps. This type of data is vital for town planning in terms of how many schools and clinics are needed to serve the population, and the extent of the road, water, sanitation and electrical infrastructure needed.

The rich source of information derived from satellites, overlaid with in-situ data, is vital for evidence-based decision and policymaking. Datasets accessed from historical archives can be used to observe the time evolution of environmental and statistical data. 

When policy decisions are taken, we can utilize the same satellite and in-situ platforms to monitor progress after their implementation. The utility of data to inform decision-making is being enhanced through the adoption of AI and big-data analytics, which is placing key information at our disposal in near real time. 

It is therefore not surprising to notice the increasing focus on space science and technology activities on the continent. However, to ensure the effective uptake and utilization of space products and services, certain building blocks are needed to establish robust national and regional space ecosystems. 

Africa’s route into space

These ecosystems must include four primary elements to function: the human capital required to establish and operate the space initiatives; a significant industry base to capitalize on the commercial aspects of the space sector; the requisite infrastructure needed to support the space value chain; and international cooperation to ensure knowledge transfer and diffusion – so that we don’t have to reinvent the wheel.

The applications and problem-solving innovations provided by space products and services are endless. To take advantage of this, Africa needs strong governance and institutional architectures. 

The evolution of the space ecosystem on the continent must be premised on key instruments such as a space policy – which areas to focus on and why – and a space strategy that outlines which programmes and performance indicators to pursue. 

The conceptualization of a space ecosystem is by no means a simple endeavour and there is certainly a dearth of skills and experience on the African continent to establish effective and relevant space ecosystems. 

There are many institutions leading efforts to build space capacity and skills on the continent, such as the International Space University in France, which offers programmes that provide a holistic overview of the complex global space sector, and the African Space Leadership Institute, which has been recently created to develop capacity in space policy, law and strategy. 

With the right approach, commitment and investment, Africa can rapidly change the fate of its citizens by effectively using space science and technology to support and drive its developmental agenda.

Towards democracy in Sudan: Reflections on the transitional period 20 September 2022 — 2:00PM TO 4:00PM Anonymous (not verified) 31 August 2022 Online

In this webinar, panellists discuss the key outcomes of the evaluation workshop and reflect on how the experience of the transition to date should inform the realization of Sudan’s democratic aspirations going forward.

The military coup on 25 October 2021 halted Sudan’s transition to democracy and prospects for sustainable peace. Since then, members of the former regime have regained political influence, with many reinstated to senior positions.

The coup has provoked a strong reaction from the country’s pro-democracy movement and youth-led resistance committees who have led continuous peaceful protests demanding civilian democratic transition, despite a brutal crackdown by state security forces.

Those supportive of the coup have blamed the Forces of Freedom and Change (FFC), a wide coalition of pro-democracy political and civilian groups, for the mistakes of the transitional government.

However, the country’s political, security and economic situation has sharply deteriorated since the military’s action, and the progress achieved by the transitional government has been reversed, leading to an accelerating economic crisis, increased food insecurity and political instability.

The FFC, which played a key role in appointing the transitional government, has acknowledged its errors. In July 2022, it held a workshop in Khartoum, which was broadcast on social media, to evaluate its performance and identify lessons learned, in discussion with civil society actors and activists.

In this webinar, leading pro-democracy movement figures and independent experts discuss the key outcomes of the evaluation workshop and reflect on how the experience of the transition to date should inform the realization of Sudan’s democratic aspirations going forward.

This event is part of a Chatham House Africa programme project on supporting Sudan’s civilian-led democratic transition.

Sudan’s gold boom: Connections to conflict and transnational impacts 7 December 2022 — 2:00PM TO 3:30PM Anonymous (not verified) 24 November 2022 Online

At this event, experts will discuss Sudan’s gold sector, its connections to conflict, and transnational impacts. 

At this webinar panellists will discuss Sudan’s gold sector, its connections to conflict, and transnational impacts.

Sudan is one of the largest gold producers on the continent, with the industry constituting Sudan’s foremost source of hard currency since the secession of South Sudan in 2011 and resulting loss of oilfields.

The gold rush that has ensued has had important implications for domestic and transnational conflict dynamics. Military actors and armed groups have sought control of gold-producing areas in the peripheries and to capitalize on the flow of labour migrants, against a wider backdrop of conflict partly stemming from contestation for control between central and local actors.

International interests are prominent, including increased Russian involvement in the sector, while gold smuggling has also interlaced with mercenary activity in neighbouring CAR, Chad and Libya.
 
At this event, panellists will discuss Sudan’s gold trade, its connections to conflict, and transnational impacts, including the international politics of Sudan’s gold extraction and role of armed groups. It will also explore the environmental and socio-economic dimensions of gold in Sudan’s border areas. 
 
This roundtable is an output of the Cross-Border Conflict: Evidence, Policy and Trends (XCEPT) research programme, funded by UK Aid from the UK government.
 

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PREAMBLE

The Society of Nuclear Medicine and Molecular Imaging (SNMMI) is an international scientific and professional organization founded in 1954 to promote the science, technology, and practical application of nuclear medicine. The European Association of Nuclear Medicine (EANM) is a professional nonprofit medical association that facilitates communication worldwide between individuals pursuing clinical and research excellence in nuclear medicine. The EANM was founded in 1985. The EANM was founded in 1985. SNMMI and EANM members are physicians, technologists, and scientists specializing in the research and practice of nuclear medicine.

The SNMMI and EANM will periodically define new guidelines for nuclear medicine practice to help advance the science of nuclear medicine and to improve the quality of service to patients throughout the world. Existing practice guidelines will be reviewed for revision or renewal, as appropriate, on their fifth anniversary or sooner, if indicated.

Each practice guideline, representing a policy statement by the SNMMI/EANM, has undergone a thorough consensus process in which it has been subjected to extensive review. The SNMMI and EANM recognize that the safe and effective use of diagnostic nuclear medicine imaging requires specific training, skills, and techniques, as described in each document. Reproduction or modification of the published practice guideline by those entities not providing these services is not authorized.

These guidelines are an educational tool designed to assist practitioners in providing appropriate care for patients. They are not inflexible rules or requirements of practice and are not intended, nor should they be used, to establish a legal standard of care. For these reasons and those set forth below, both the SNMMI and the EANM caution against the use of these guidelines in litigation in which the clinical decisions of a practitioner are called into question.

The ultimate judgment regarding the propriety of any specific procedure or course of action must be made by the physician or medical physicist in light of all the circumstances presented. Thus, there is no implication that an approach differing from the guidelines, standing alone, is below the standard of care. To the contrary, a conscientious practitioner may responsibly adopt a course of action different from that set forth in the guidelines when, in the reasonable judgment of the practitioner, such course of action is indicated by the condition of the patient, limitations of available resources, or advances in knowledge or technology subsequent to publication of the guidelines.

The practice of medicine includes both the art and the science of the prevention, diagnosis, alleviation, and treatment of disease. The variety and complexity of human conditions make it impossible to always reach the most appropriate diagnosis or to predict with certainty a particular response to treatment.

Therefore, it should be recognized that adherence to these guidelines will not ensure an accurate diagnosis or a successful outcome. All that should be expected is that the practitioner will follow a reasonable course of action based on current knowledge, available resources, and the needs of the patient to deliver effective and safe medical care. The sole purpose of these guidelines is to assist practitioners in achieving this objective.

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Researchers use dynamic PET imaging with target-selective tracer molecules to probe molecular processes. Kinetic models have been developed to describe these processes. The models are typically fitted to the measured PET data with the assumption that the brain is in a steady-state condition for the duration of the scan. The end results are quantitative parameters that characterize the molecular processes. The most common kinetic modeling endpoints are estimates of volume of distribution or the binding potential of a tracer. If the steady state is violated during the scanning period, the standard kinetic models may not apply. To address this issue, time-variant kinetic models have been developed for the characterization of dynamic PET data acquired while significant changes (e.g., short-lived neurotransmitter changes) are occurring in brain processes. These models are intended to extract a transient signal from data. This work in the PET field dates back at least to the 1990s. As interest has grown in imaging nonsteady events, development and refinement of time-variant models has accelerated. These new models, which we classify as belonging to the first, second, or third generation according to their innovation, have used the latest progress in mathematics, image processing, artificial intelligence, and statistics to improve the sensitivity and performance of the earliest practical time-variant models to detect and describe nonsteady phenomena. This review provides a detailed overview of the history of time-variant models in PET. It puts key advancements in the field into historical and scientific context. The sum total of the methods is an ongoing attempt to better understand the nature and implications of neurotransmitter fluctuations and other brief neurochemical phenomena.

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