FAMIN Is a Multifunctional Purine Enzyme Enabling the Purine Nucleotide Cycle.

Mutations in FAMIN cause arthritis and inflammatory bowel disease in early childhood, and a common genetic variant increases the risk for Crohn's disease and leprosy. We developed an unbiased liquid chromatography-mass spectrometry screen for enzymatic activity of this orphan protein. We report that FAMIN phosphorolytically cleaves adenosine into adenine and ribose-1-phosphate. Such activity was considered absent from eukaryotic metabolism. FAMIN and its prokaryotic orthologs additionally have adenosine deaminase, purine nucleoside phosphorylase, and S-methyl-5'-thioadenosine phosphorylase activity, hence, combine activities of the namesake enzymes of central purine metabolism. FAMIN enables in macrophages a purine nucleotide cycle (PNC) between adenosine and inosine monophosphate and adenylosuccinate, which consumes aspartate and releases fumarate in a manner involving fatty acid oxidation and ATP-citrate lyase activity. This macrophage PNC synchronizes mitochondrial activity with glycolysis by balancing electron transfer to mitochondria, thereby supporting glycolytic activity and promoting oxidative phosphorylation and mitochondrial H+ and phosphate recycling.

C13orf31 (FAMIN) is a central regulator of immunometabolic function.

Single-nucleotide variations in C13orf31 (LACC1) that encode p.C284R and p.I254V in a protein of unknown function (called 'FAMIN' here) are associated with increased risk for systemic juvenile idiopathic arthritis, leprosy and Crohn's disease. Here we set out to identify the biological mechanism affected by these coding variations. FAMIN formed a complex with fatty acid synthase (FASN) on peroxisomes and promoted flux through de novo lipogenesis to concomitantly drive high levels of fatty-acid oxidation (FAO) and glycolysis and, consequently, ATP regeneration. FAMIN-dependent FAO controlled inflammasome activation, mitochondrial and NADPH-oxidase-dependent production of reactive oxygen species (ROS), and the bactericidal activity of macrophages. As p.I254V and p.C284R resulted in diminished function and loss of function, respectively, FAMIN determined resilience to endotoxin shock. Thus, we have identified a central regulator of the metabolic function and bioenergetic state of macrophages that is under evolutionary selection and determines the risk of inflammatory and infectious disease.

Recent advances in inflammatory bowel disease: mucosal immune cells in intestinal inflammation.

The intestine and its immune system have evolved to meet the extraordinary task of maintaining tolerance to the largest, most complex and diverse microbial commensal habitat, while meticulously attacking and containing even minute numbers of occasionally incoming pathogens. While our understanding is still far from complete, recent studies have provided exciting novel insights into the complex interplay of the many distinct intestinal immune cell types as well as the discovery of entirely new cell subsets. These studies have also revealed how proper development and function of the intestinal immune system is dependent on its specific microbiota, which appears to have evolutionarily co-evolved. Here we review key immune cells that maintain intestinal homeostasis and, conversely, describe how altered function and imbalances may lead to inflammatory bowel disease (IBD). We highlight the latest developments within this field, covering the major players in IBD including intestinal epithelial cells, macrophages, dendritic cells, adaptive immune cells, and the newly discovered innate lymphoid cells, which appear of characteristic importance for immune function at mucosal surfaces. We set these mucosal immune pathways in the functional context of IBD risk genes where such insight is available. Moreover, we frame our discussion of fundamental biological pathways that have been elucidated in model systems in the context of results from clinical trials in IBD that targeted key mediators secreted by these cells, as an attempt of 'functional' appraisal of these pathways in human disease.

A purine metabolic checkpoint that prevents autoimmunity and autoinflammation.

Still's disease, the paradigm of autoinflammation-cum-autoimmunity, predisposes for a cytokine storm with excessive T lymphocyte activation upon viral infection. Loss of function of the purine nucleoside enzyme FAMIN is the sole known cause for monogenic Still's disease. Here we discovered that a FAMIN-enabled purine metabolon in dendritic cells (DCs) restrains CD4+ and CD8+ T cell priming. DCs with absent FAMIN activity prime for enhanced antigen-specific cytotoxicity, IFNγ secretion, and T cell expansion, resulting in excessive influenza A virus-specific responses. Enhanced priming is already manifest with hypomorphic FAMIN-I254V, for which ∼6% of mankind is homozygous. FAMIN controls membrane trafficking and restrains antigen presentation in an NADH/NAD+-dependent manner by balancing flux through adenine-guanine nucleotide interconversion cycles. FAMIN additionally converts hypoxanthine into inosine, which DCs release to dampen T cell activation. Compromised FAMIN consequently enhances immunosurveillance of syngeneic tumors. FAMIN is a biochemical checkpoint that protects against excessive antiviral T cell responses, autoimmunity, and autoinflammation.

Inverse regulation of plasticity-related immediate early genes by calcineurin in hippocampal neurons.

In the mammalian hippocampus, changes in the expression of immediate early genes (IEGs) is thought to contribute to long term plastic changes in neurons brought about by learning tasks and high frequency stimulation of synapses. The phosphatase calcineurin has emerged as an important negative regulator of hippocampus-dependent learning and long term potentiation. Here we investigated the possibility that the constraining action of calcineurin on hippocampal plasticity is mediated in part by regulation of gene expression through negative control of transcription factors, such as cAMP-response element (CRE)-binding protein (CREB). We assessed the effect of calcineurin inhibitors on CREB activation by neuronal activity and show that calcineurin activity is in fact required for CREB-mediated gene expression. However, inhibition of calcineurin had disparate effects on the transcriptional induction of CREB-dependent IEGs. We find that the IEG c-fos is unaffected by suppression of calcineurin activity, the plasticity-related genes Egr1/Zif268 and Egr2/Krox-20 are up-regulated, and genes encoding the orphan nuclear hormone receptors Nor1 and Nur77 are down-regulated. We further show that the up-regulation of particular IEGs is probably due to the presence of serum response elements (SREs) in their promoters, because SRE-mediated gene expression is enhanced by calcineurin blockers. Moreover, expression of the c-fos gene, which is unaffected by calcineurin inhibitors, could be down-regulated by mutating the SRE. Conversely, SRE-mediated c-fos induction in the absence of a functional CRE was enhanced by calcineurin inhibitors. Our experiments thus implicate calcineurin as a negative regulator of SRE-dependent neuronal genes.

Differential effects of Ca2+ and cAMP on transcription mediated by MEF2D and cAMP-response element-binding protein in hippocampal neurons.

In neurons, the second messengers Ca(2+) and cAMP are mediators of transcriptional responses that are important for the development and function of the nervous system. The pro-survival neuronal transcription factors cAMP-response elementbinding protein (CREB) and myocyte enhancer factor-2 (MEF2) both stimulate gene expression in response to activity-dependent increases in the concentration of intracellular Ca(2+) ions. CREB is also activated by increases in intracellular cAMP. Here we have investigated whether the MEF2 family member MEF2D, similar to CREB, is also activated by cAMP in hippocampal neurons. We have shown that, unlike CREB, MEF2D is not activated by agents that increase intracellular cAMP. Moreover, increases in cAMP inhibit Ca(2+)-activated MEF2D-mediated gene expression. We have also shown that cAMP inhibits Ca(2+)-induced nuclear export of the MEF2 co-repressor HDAC5 and prevents Ca(2+)-stimulated nuclear import of the MEF2 co-activator NFAT3/c4. Our results suggest that cAMP interferes with MEF2D-mediated gene expression at multiple levels by antagonizing the derepression of MEF2D by HDAC5 and by inhibiting recruitment of the co-activator NFAT.