Leucine-Rich Repeat Kinases.

Activating mutations in leucine-rich repeat kinase 2 (LRRK2) represent the most common cause of monogenic Parkinson's disease. LRRK2 is a large multidomain protein kinase that phosphorylates a specific subset of the ∼65 human Rab GTPases, which are master regulators of the secretory and endocytic pathways. After phosphorylation by LRRK2, Rabs lose the capacity to bind cognate effector proteins and guanine nucleotide exchange factors. Moreover, the phosphorylated Rabs cannot interact with their cognate prenyl-binding retrieval proteins (also known as guanine nucleotide dissociation inhibitors) and, thus, they become trapped on membrane surfaces. Instead, they gain the capacity to bind phospho-Rab-specific effector proteins, such as RILPL1, with resulting pathological consequences. Rab proteins also act upstream of LRRK2 by controlling its activation and recruitment onto membranes. LRRK2 signaling is counteracted by the phosphoprotein phosphatase PPM1H, which selectively dephosphorylates phospho-Rab proteins. We present here our current understanding of the structure, biochemical properties, and cell biology of LRRK2 and its related paralog LRRK1 and discuss how this information guides the generation of LRRK2 inhibitors for the potential benefit of patients.

Mechanisms of Pediatric Inflammatory Bowel Disease.

Inflammatory bowel disease (IBD), including Crohn disease and ulcerative colitis, is characterized by chronic intestinal inflammation due to a complex interaction of genetic determinants, disruption of mucosal barriers, aberrant inflammatory signals, loss of tolerance, and environmental triggers. Importantly, the incidence of pediatric IBD is rising, particularly in children younger than 10 years. In this review, we discuss the clinical presentation of these patients and highlight environmental exposures that may affect disease risk, particularly among people with a background genetic risk. With regard to both children and adults, we review advancements in understanding the intestinal epithelium, the mucosal immune system, and the resident microbiota, describing how dysfunction at any level can lead to diseases like IBD. We conclude with future directions for applying advances in IBD genetics to better understand pathogenesis and develop therapeutics targeting key pathogenic nodes.

Targeted suppression of human IBD-associated gut microbiota commensals by phage consortia for treatment of intestinal inflammation.

Human gut commensals are increasingly suggested to impact non-communicable diseases, such as inflammatory bowel diseases (IBD), yet their targeted suppression remains a daunting unmet challenge. In four geographically distinct IBD cohorts (n = 537), we identify a clade of Klebsiella pneumoniae (Kp) strains, featuring a unique antibiotics resistance and mobilome signature, to be strongly associated with disease exacerbation and severity. Transfer of clinical IBD-associated Kp strains into colitis-prone, germ-free, and colonized mice enhances intestinal inflammation. Stepwise generation of a lytic five-phage combination, targeting sensitive and resistant IBD-associated Kp clade members through distinct mechanisms, enables effective Kp suppression in colitis-prone mice, driving an attenuated inflammation and disease severity. Proof-of-concept assessment of Kp-targeting phages in an artificial human gut and in healthy volunteers demonstrates gastric acid-dependent phage resilience, safety, and viability in the lower gut. Collectively, we demonstrate the feasibility of orally administered combination phage therapy in avoiding resistance, while effectively inhibiting non-communicable disease-contributing pathobionts.

Epigenetic reader SP140 loss of function drives Crohn's disease due to uncontrolled macrophage topoisomerases.

How mis-regulated chromatin directly impacts human immune disorders is poorly understood. Speckled Protein 140 (SP140) is an immune-restricted PHD and bromodomain-containing epigenetic "reader," and SP140 loss-of-function mutations associate with Crohn's disease (CD), multiple sclerosis (MS), and chronic lymphocytic leukemia (CLL). However, the relevance of these mutations and mechanisms underlying SP140-driven pathogenicity remains unexplored. Using a global proteomic strategy, we identified SP140 as a repressor of topoisomerases (TOPs) that maintains heterochromatin and macrophage fate. In humans and mice, SP140 loss resulted in unleashed TOP activity, de-repression of developmentally silenced genes, and ultimately defective microbe-inducible macrophage transcriptional programs and bacterial killing that drive intestinal pathology. Pharmacological inhibition of TOP1/2 rescued these defects. Furthermore, exacerbated colitis was restored with TOP1/2 inhibitors in Sp140-/- mice, but not wild-type mice, in vivo. Collectively, we identify SP140 as a TOP repressor and reveal repurposing of TOP inhibition to reverse immune diseases driven by SP140 loss.

Translocation of Viable Gut Microbiota to Mesenteric Adipose Drives Formation of Creeping Fat in Humans.

A mysterious feature of Crohn's disease (CD) is the extra-intestinal manifestation of "creeping fat" (CrF), defined as expansion of mesenteric adipose tissue around the inflamed and fibrotic intestine. In the current study, we explore whether microbial translocation in CD serves as a central cue for CrF development. We discovered a subset of mucosal-associated gut bacteria that consistently translocated and remained viable in CrF in CD ileal surgical resections, and identified Clostridium innocuum as a signature of this consortium with strain variation between mucosal and adipose isolates, suggesting preference for lipid-rich environments. Single-cell RNA sequencing characterized CrF as both pro-fibrotic and pro-adipogenic with a rich milieu of activated immune cells responding to microbial stimuli, which we confirm in gnotobiotic mice colonized with C. innocuum. Ex vivo validation of expression patterns suggests C. innocuum stimulates tissue remodeling via M2 macrophages, leading to an adipose tissue barrier that serves to prevent systemic dissemination of bacteria.

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.

Somatic Evolution in Non-neoplastic IBD-Affected Colon.

Inflammatory bowel disease (IBD) is a chronic inflammatory disease associated with increased risk of gastrointestinal cancers. We whole-genome sequenced 446 colonic crypts from 46 IBD patients and compared these to 412 crypts from 41 non-IBD controls from our previous publication on the mutation landscape of the normal colon. The average mutation rate of affected colonic epithelial cells is 2.4-fold that of healthy colon, and this increase is mostly driven by acceleration of mutational processes ubiquitously observed in normal colon. In contrast to the normal colon, where clonal expansions outside the confines of the crypt are rare, we observed widespread millimeter-scale clonal expansions. We discovered non-synonymous mutations in ARID1A, FBXW7, PIGR, ZC3H12A, and genes in the interleukin 17 and Toll-like receptor pathways, under positive selection in IBD. These results suggest distinct selection mechanisms in the colitis-affected colon and that somatic mutations potentially play a causal role in IBD pathogenesis.

The Lymphatic Vasculature in the 21st Century: Novel Functional Roles in Homeostasis and Disease.

Mammals have two specialized vascular circulatory systems: the blood vasculature and the lymphatic vasculature. The lymphatic vasculature is a unidirectional conduit that returns filtered interstitial arterial fluid and tissue metabolites to the blood circulation. It also plays major roles in immune cell trafficking and lipid absorption. As we discuss in this review, the molecular characterization of lymphatic vascular development and our understanding of this vasculature's role in pathophysiological conditions has greatly improved in recent years, changing conventional views about the roles of the lymphatic vasculature in health and disease. Morphological or functional defects in the lymphatic vasculature have now been uncovered in several pathological conditions. We propose that subtle asymptomatic alterations in lymphatic vascular function could underlie the variability seen in the body's response to a wide range of human diseases.

Diet Diurnally Regulates Small Intestinal Microbiome-Epithelial-Immune Homeostasis and Enteritis.

Throughout a 24-h period, the small intestine (SI) is exposed to diurnally varying food- and microbiome-derived antigenic burdens but maintains a strict immune homeostasis, which when perturbed in genetically susceptible individuals, may lead to Crohn disease. Herein, we demonstrate that dietary content and rhythmicity regulate the diurnally shifting SI epithelial cell (SIEC) transcriptional landscape through modulation of the SI microbiome. We exemplify this concept with SIEC major histocompatibility complex (MHC) class II, which is diurnally modulated by distinct mucosal-adherent SI commensals, while supporting downstream diurnal activity of intra-epithelial IL-10+ lymphocytes regulating the SI barrier function. Disruption of this diurnally regulated diet-microbiome-MHC class II-IL-10-epithelial barrier axis by circadian clock disarrangement, alterations in feeding time or content, or epithelial-specific MHC class II depletion leads to an extensive microbial product influx, driving Crohn-like enteritis. Collectively, we highlight nutritional features that modulate SI microbiome, immunity, and barrier function and identify dietary, epithelial, and immune checkpoints along this axis to be potentially exploitable in future Crohn disease interventions.

Single-Cell Analysis of Crohn's Disease Lesions Identifies a Pathogenic Cellular Module Associated with Resistance to Anti-TNF Therapy.

Clinical benefits of cytokine blockade in ileal Crohn's disease (iCD) are limited to a subset of patients. Here, we applied single-cell technologies to iCD lesions to address whether cellular heterogeneity contributes to treatment resistance. We found that a subset of patients expressed a unique cellular module in inflamed tissues that consisted of IgG plasma cells, inflammatory mononuclear phagocytes, activated T cells, and stromal cells, which we named the GIMATS module. Analysis of ligand-receptor interaction pairs identified a distinct network connectivity that likely drives the GIMATS module. Strikingly, the GIMATS module was also present in a subset of patients in four independent iCD cohorts (n = 441), and its presence at diagnosis correlated with failure to achieve durable corticosteroid-free remission upon anti-TNF therapy. These results emphasize the limitations of current diagnostic assays and the potential for single-cell mapping tools to identify novel biomarkers of treatment response and tailored therapeutic opportunities.

The landscape of immune dysregulation in Crohn's disease revealed through single-cell transcriptomic profiling in the ileum and colon.

Crohn's disease (CD) is a chronic gastrointestinal disease that is increasing in prevalence worldwide. CD is multifactorial, involving the complex interplay of genetic, immune, and environmental factors, necessitating a system-level understanding of its etiology. To characterize cell-type-specific transcriptional heterogeneity in active CD, we profiled 720,633 cells from the terminal ileum and colon of 71 donors with varying inflammation status. Our integrated datasets revealed organ- and compartment-specific responses to acute and chronic inflammation; most immune changes were in cell composition, whereas transcriptional changes dominated among epithelial and stromal cells. These changes correlated with endoscopic inflammation, but small and large intestines exhibited distinct responses, which were particularly apparent when focusing on IBD risk genes. Finally, we mapped markers of disease-associated myofibroblast activation and identified CHMP1A, TBX3, and RNF168 as regulators of fibrotic complications. Altogether, our results provide a roadmap for understanding cell-type- and organ-specific differences in CD and potential directions for therapeutic development.

Phage-display immunoprecipitation sequencing of the antibody epitope repertoire in inflammatory bowel disease reveals distinct antibody signatures.

Inflammatory bowel diseases (IBDs), e.g., Crohn's disease (CD) and ulcerative colitis (UC), are chronic immune-mediated inflammatory diseases. A comprehensive overview of an IBD-specific antibody epitope repertoire is, however, lacking. Using high-throughput phage-display immunoprecipitation sequencing (PhIP-Seq), we identified antibodies against 344,000 antimicrobial, immune, and food antigens in 497 individuals with IBD compared with 1,326 controls. IBD was characterized by 373 differentially abundant antibody responses (202 overrepresented and 171 underrepresented), with 17% shared by both IBDs, 55% unique to CD, and 28% unique to UC. Antibody reactivities against bacterial flagellins dominated in CD and were associated with ileal involvement, fibrostenotic disease, and anti-Saccharomyces cerevisiae antibody positivity, but not with fecal microbiome composition. Antibody epitope repertoires accurately discriminated CD from controls (area under the curve [AUC] = 0.89), and similar discrimination was achieved when using only ten antibodies (AUC = 0.87). Individuals with IBD thus show a distinct antibody repertoire against selected peptides, allowing clinical stratification and discovery of immunological targets.

Multiomics analyses reveal a critical role of selenium in controlling T cell differentiation in Crohn's disease.

Inflammatory bowel disease (IBD) mainly includes Crohn's disease (CD) and ulcerative colitis (UC). Immune disorders play an essential role in the pathogenesis of these two IBDs, but the differences in the immune microenvironment of the colon and their underlying mechanisms remain poorly investigated. Here we examined the immunological features and metabolic microenvironment of untreated individuals with IBD by multiomics analyses. Modulation of CD-specific metabolites, particularly reduced selenium, can obviously shape type 1 T helper (Th1) cell differentiation, which is specifically enriched in CD. Selenium supplementation suppressed the symptoms and onset of CD and Th1 cell differentiation via selenoprotein W (SELW)-mediated cellular reactive oxygen species scavenging. SELW promoted purine salvage pathways and inhibited one-carbon metabolism by recruiting an E3 ubiquitin ligase, tripartite motif-containing protein 21, which controlled the stability of serine hydroxymethyltransferase 2. Our work highlights selenium as an essential regulator of T cell responses and potential therapeutic targets in CD.

Ileitis-associated tertiary lymphoid organs arise at lymphatic valves and impede mesenteric lymph flow in response to tumor necrosis factor.

Lymphangitis and the formation of tertiary lymphoid organs (TLOs) in the mesentery are features of Crohn's disease. Here, we examined the genesis of these TLOs and their impact on disease progression. Whole-mount and intravital imaging of the ileum and ileum-draining collecting lymphatic vessels (CLVs) draining to mesenteric lymph nodes from TNFΔARE mice, a model of ileitis, revealed TLO formation at valves of CLVs. TLOs obstructed cellular and molecular outflow from the gut and were sites of lymph leakage and backflow. Tumor necrosis factor (TNF) neutralization begun at early stages of TLO formation restored lymph transport. However, robustly developed, chronic TLOs resisted regression and restoration of flow after TNF neutralization. TNF stimulation of cultured lymphatic endothelial cells reprogrammed responses to oscillatory shear stress, preventing the induction of valve-associated genes. Disrupted transport of immune cells, driven by loss of valve integrity and TLO formation, may contribute to the pathology of Crohn's disease.

Development of antifibrotic therapy for stricturing Crohn's disease: lessons from randomized trials in other fibrotic diseases.

Intestinal fibrosis is considered an inevitable complication of Crohn's disease (CD) that results in symptoms of obstruction and stricture formation. Endoscopic or surgical treatment is required to treat the majority of patients. Progress in the management of stricturing CD is hampered by the lack of effective antifibrotic therapy; however, this situation is likely to change because of recent advances in other fibrotic diseases of the lung, liver, and skin. In this review, we summarize data from randomized controlled trials (RCTs) of antifibrotic therapies in these conditions. Multiple compounds have been tested for antifibrotic effects in other organs. According to their mechanisms, they were categorized into growth factor modulators, inflammation modulators, 5-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors, intracellular enzymes and kinases, renin-angiotensin system (RAS) modulators, and others. From our review of the results from the clinical trials and discussion of their implications in the gastrointestinal tract, we have identified several molecular candidates that could serve as potential therapies for intestinal fibrosis in CD.

Spatial features of skip lesions in Crohn's disease.

Skip lesions are an enigmatic spatial feature characterizing Crohn's disease (CD). They comprise inflamed and adjacent non-inflamed tissue sections with a clear demarcation. Currently, spatial features of the human gastrointestinal (GI) system lack clarity regarding the organization of microbes, mucus, tissue, and host cells during inflammation. New technologies with multiplexing abilities and innovative approaches provide ways of examining the spatial organization of inflamed and non-inflamed tissues in CD, which may open new avenues for diagnosis, prognosis, and treatment. In this review, we present evidence of the relevance of spatial context in patients with CD and the methods and ideas recently published in studies of spatiality during inflammation. With this review, we aim to provide inspiration for further research to address existing gaps.

Microbiota-Induced TNF-like Ligand 1A Drives Group 3 Innate Lymphoid Cell-Mediated Barrier Protection and Intestinal T Cell Activation during Colitis.

Inflammatory bowel disease (IBD) results from a dysregulated interaction between the microbiota and a genetically susceptible host. Genetic studies have linked TNFSF15 polymorphisms and its protein TNF-like ligand 1A (TL1A) with IBD, but the functional role of TL1A is not known. Here, we found that adherent IBD-associated microbiota induced TL1A release from CX3CR1+ mononuclear phagocytes (MNPs). Using cell-specific genetic deletion models, we identified an essential role for CX3CR1+MNP-derived TL1A in driving group 3 innate lymphoid cell (ILC3) production of interleukin-22 and mucosal healing during acute colitis. In contrast to this protective role in acute colitis, TL1A-dependent expression of co-stimulatory molecule OX40L in MHCII+ ILC3s during colitis led to co-stimulation of antigen-specific T cells that was required for chronic T cell colitis. These results identify a role for ILC3s in activating intestinal T cells and reveal a central role for TL1A in promoting ILC3 barrier immunity during colitis.

Structural Activation of Pro-inflammatory Human Cytokine IL-23 by Cognate IL-23 Receptor Enables Recruitment of the Shared Receptor IL-12Rß1.

Interleukin-23 (IL-23), an IL-12 family cytokine, plays pivotal roles in pro-inflammatory T helper 17 cell responses linked to autoimmune and inflammatory diseases. Despite intense therapeutic targeting, structural and mechanistic insights into receptor complexes mediated by IL-23, and by IL-12 family members in general, have remained elusive. We determined a crystal structure of human IL-23 in complex with its cognate receptor, IL-23R, and revealed that IL-23R bound to IL-23 exclusively via its N-terminal immunoglobulin domain. The structural and functional hotspot of this interaction partially restructured the helical IL-23p19 subunit of IL-23 and restrained its IL-12p40 subunit to cooperatively bind the shared receptor IL-12Rß1 with high affinity. Together with structural insights from the interaction of IL-23 with the inhibitory antibody briakinumab and by leveraging additional IL-23:antibody complexes, we propose a mechanistic paradigm for IL-23 and IL-12 whereby cognate receptor binding to the helical cytokine subunits primes recruitment of the shared receptors via the IL-12p40 subunit.

The Crohn's Disease Risk Factor IRGM Limits NLRP3 Inflammasome Activation by Impeding Its Assembly and by Mediating Its Selective Autophagy.

Several large-scale genome-wide association studies genetically linked IRGM to Crohn's disease and other inflammatory disorders in which the IRGM appears to have a protective function. However, the mechanism by which IRGM accomplishes this anti-inflammatory role remains unclear. Here, we reveal that IRGM/Irgm1 is a negative regulator of the NLRP3 inflammasome activation. We show that IRGM expression, which is increased by PAMPs, DAMPs, and microbes, can suppress the pro-inflammatory responses provoked by the same stimuli. IRGM/Irgm1 negatively regulates IL-1ß maturation by suppressing the activation of the NLRP3 inflammasome. Mechanistically, we show that IRGM interacts with NLRP3 and ASC and hinders inflammasome assembly by blocking their oligomerization. Further, IRGM mediates selective autophagic degradation of NLRP3 and ASC. By suppressing inflammasome activation, IRGM/Irgm1 protects from pyroptosis and gut inflammation in a Crohn's disease experimental mouse model. This study for the first time identifies the mechanism by which IRGM is protective against inflammatory disorders.

The Xenobiotic Transporter Mdr1 Enforces T Cell Homeostasis in the Presence of Intestinal Bile Acids.

CD4+ T cells are tightly regulated by microbiota in the intestine, but whether intestinal T cells interface with host-derived metabolites is less clear. Here, we show that CD4+ T effector (Teff) cells upregulated the xenobiotic transporter, Mdr1, in the ileum to maintain homeostasis in the presence of bile acids. Whereas wild-type Teff cells upregulated Mdr1 in the ileum, those lacking Mdr1 displayed mucosal dysfunction and induced Crohn's disease-like ileitis following transfer into Rag1-/- hosts. Mdr1 mitigated oxidative stress and enforced homeostasis in Teff cells exposed to conjugated bile acids (CBAs), a class of liver-derived emulsifying agents that actively circulate through the ileal mucosa. Blocking ileal CBA reabsorption in transferred Rag1-/- mice restored Mdr1-deficient Teff cell homeostasis and attenuated ileitis. Further, a subset of ileal Crohn's disease patients displayed MDR1 loss of function. Together, these results suggest that coordinated interaction between mucosal Teff cells and CBAs in the ileum regulate intestinal immune homeostasis.