Nelson Ong'Ondo Gekara

Nucleotide-binding oligomerization domain 2 (NOD2) signaling is a critical arm of host defense and immune homeostasis but requires strict regulation to prevent self-injury. Central to this regulation is the polyubiquitination of the adaptor protein RIP2, yet the precise ubiquitin linkages and their dynamic control remain incompletely understood. Here, we identify K27-linked polyubiquitination of RIP2 by the E3 ligase XIAP as a novel inhibitory signal that attenuates RIP2 complex assembly and downstream NF-κB activation, thereby fine-tuning NOD2-induced inflammatory signaling. We show that XIAP-mediated K63-linked ubiquitination promotes NOD2-RIP2 activation, whereas K27-linked ubiquitination impairs the recruitment of TAK1 and NEMO to the NOD2-RIP2 complex, dampening downstream signaling and cytokine production. We further demonstrate that this regulation is counterbalanced by the deubiquitinase MYSM1, which selectively removes K27-, K63-. and M1-polyubiquitin chains via its SWIRM-MPN domains to restore balanced signaling. Together, these findings define a dynamic XIAP-RIP2-MYSM1 axis in which opposing ubiquitin linkages fine-tune NOD2 responses, establishing a molecular rheostat for immune signaling with implications for inflammatory disease pathogenesis and opportunities for targeted therapeutic intervention.

We investigated how the gut microbiota modulate anti-viral immunity in mouse models of infection. We showed that gut bacteria are vital for the presence of low level of type-I interferons, key mediators in anti-viral immunity. The gut bacteria deliver DNA in vesicles to host cells, priming anti-viral immunity.

Radiation is an essential preparative procedure for bone marrow (BM) transplantation and cancer treatment. The therapeutic efficacy of radiation and associated toxicity varies from patient to patient, making it difficult to prescribe an optimal patient‐specific irradiation dose. The molecular determinants of radiation response remain unclear. AIM2‐like receptors (ALRs) are key players in innate immunity and determine the course of infections, inflammatory diseases, senescence, and cancer. Here it is reported that mice lacking ALRs are resistant to irradiation‐induced BM injury. It is shown that nuclear ALRs are inhibitors of DNA repair, thereby accelerate genome destabilization, micronuclei generation, and cell death, and that this novel function is uncoupled from their role in innate immunity. Mechanistically, ALRs bind to and interfere with chromatin decompaction vital for DNA repair. The finding uncovers ALRs as targets for new interventions against genotoxic tissue injury and as possible biomarkers for predicting the outcome of radio/chemotherapy.

The gut epithelium serves to maximize the surface for nutrient and fluid uptake, but at the same time must provide a tight barrier to pathogens and remove damaged intestinal epithelial cells (IECs) without jeopardizing barrier integrity. How the epithelium coordinates these tasks remains a question of significant interest. We used imaging and an optical flow analysis pipeline to study the dynamicity of untransformed murine and human intestinal epithelia, cultured atop flexible hydrogel supports. Infection with the pathogen Salmonella Typhimurium (S.Tm) within minutes elicited focal contractions with inward movements of up to ∼1,000 IECs. Genetics approaches and chimeric epithelial monolayers revealed contractions to be triggered by the NAIP/NLRC4 inflammasome, which sensed type-III secretion system and flagellar ligands upon bacterial invasion, converting the local tissue into a contraction epicenter. Execution of the response required swift sublytic Gasdermin D pore formation, ion fluxes, and the propagation of a myosin contraction pulse across the tissue. Importantly, focal contractions preceded, and could be uncoupled from, the death and expulsion of infected IECs. In both two-dimensional monolayers and three-dimensional enteroids, multiple infection-elicited contractions coalesced to produce shrinkage of the epithelium as a whole. Monolayers deficient for Caspase-1(-11) or Gasdermin D failed to elicit focal contractions but were still capable of infected IEC death and expulsion. Strikingly, these monolayers lost their integrity to a markedly higher extent than wild-type counterparts. We propose that prompt NAIP/NLRC4/Caspase-1/Gasdermin D/myosin–dependent contractions allow the epithelium to densify its cell packing in infected regions, thereby preventing tissue disintegration due to the subsequent IEC death and expulsion process. Significance Contractile movements in the mammalian intestine typically rely on dedicated muscle cells. Here, we however show that untransformed intestinal epithelial cell layers initiate immediate focal contractions, affecting hundreds to thousands of epithelial cells, in response to bacterial infection. This epithelial contraction response occurs in the absence of other mucosal cell types. Instead, the epithelium itself senses the pathogen intrusion through a pattern recognition receptor complex—NAIP/NLRC4—and initiates actomyosin contractions that propagate across the epithelial layer. Within minutes, this response densifies the cell packing at infection sites and may prevent tissue disintegration during the subsequent stage of epithelial cell death and expulsion. Our results highlight a previously unappreciated dynamic behavior of intestinal epithelia.

DNA repair via homologous recombination (HR) is indispensable for genome integrity and cell survival but if unrestrained can result in undesired chromosomal rearrangements. The regulatory mechanisms of HR are not fully understood. Cyclic GMP‐AMP synthase (cGAS) is best known as a cytosolic innate immune sensor critical for the outcome of infections, inflammatory diseases, and cancer. Here, we report that cGAS is primarily a chromatin‐bound protein that inhibits DNA repair by HR, thereby accelerating genome destabilization, micronucleus generation, and cell death under conditions of genomic stress. This function is independent of the canonical STING‐dependent innate immune activation and is physiologically relevant for irradiation‐induced depletion of bone marrow cells in mice. Mechanistically, we demonstrate that inhibition of HR repair by cGAS is linked to its ability to self‐oligomerize, causing compaction of bound template dsDNA into a higher‐ordered state less amenable to strand invasion by RAD51‐coated ssDNA filaments. This previously unknown role of cGAS has implications for understanding its involvement in genome instability‐associated disorders including cancer.

Hydrogen peroxide (H₂O₂) has a major function in host-microbial interactions. Although most studies have focused on the endogenous H₂O₂produced by immune cells to kill microbes, bacteria can also produce H₂O₂. How microbial H₂O₂influences the dynamics of host-microbial interactions is unclear. Here we show that H₂O₂released byStreptococcus pneumoniaeinhibits inflammasomes, key components of the innate immune system, contributing to the pathogen colonization of the host. We also show that the oral commensal H₂O₂-producing bacteriaStreptococcus oraliscan block inflammasome activation. This study uncovers an unexpected role of H₂O₂in immune suppression and demonstrates how, through this mechanism, bacteria might restrain the immune system to co-exist with the host.