Increased extracellular Ca²⁺ concentrations ([Ca²⁺]_ex) trigger activation of the NLRP3 inflammasome in monocytes through calcium-sensing receptor (CaSR). To prevent extraosseous calcification in vivo, the serum protein fetuin-A stabilizes calcium and phosphate into 70-100 nm-sized colloidal calciprotein particles (CPPs). Here we show that monocytes engulf CPPs via macropinocytosis, and this process is strictly dependent on CaSR signaling triggered by increases in [Ca²⁺]_ex. Enhanced macropinocytosis of CPPs results in increased lysosomal activity, NLRP3 inflammasome activation, and IL-1β release. Monocytes in the context of rheumatoid arthritis (RA) exhibit increased CPP uptake and IL-1β release in response to CaSR signaling. CaSR expression in these monocytes and local [Ca²⁺] in afflicted joints are increased, probably contributing to this enhanced response. We propose that CaSR-mediated NLRP3 inflammasome activation contributes to inflammatory arthritis and systemic inflammation not only in RA, but possibly also in other inflammatory conditions. Inhibition of CaSR-mediated CPP uptake might be a therapeutic approach to treating RA.

The interplay of microbiota and the human host is physiologically crucial in health and diseases. The beneficial effects of lactic acid bacteria (LAB), permanently colonizing the human intestine or transiently obtained from food, have been extensively reported. However, the molecular understanding of how LAB modulate human physiology is still limited. G protein-coupled receptors for hydroxycarboxylic acids (HCAR) are regulators of immune functions and energy homeostasis under changing metabolic and dietary conditions. Most mammals have two HCAR (HCA₁, HCA₂) but humans and other hominids contain a third member (HCA₃) in their genomes. A plausible hypothesis why HCA₃ function was advantageous in hominid evolution was lacking. Here, we used a combination of evolutionary, analytical and functional methods to unravel the role of HCA₃ in vitro and in vivo. The functional studies included different pharmacological assays, analyses of human monocytes and pharmacokinetic measurements in human. We report the discovery of the interaction of D-phenyllactic acid (D-PLA) and the human host through highly potent activation of HCA₃. D-PLA is an anti-bacterial metabolite found in high concentrations in LAB-fermented food such as Sauerkraut. We demonstrate that D-PLA from such alimentary sources is well absorbed from the human gut leading to high plasma and urine levels and triggers pertussis toxin-sensitive migration of primary human monocytes in an HCA₃-dependent manner. We provide evolutionary, analytical and functional evidence supporting the hypothesis that HCA₃ was consolidated in hominids as a new signaling system for LAB-derived metabolites. Although it has been known for 15 years that HCA₃ is present in humans and other hominids but absent in all other mammals, no study so far aimed to understand why HCA₃ was functionally preserved during evolution. Here, we take advantage of evolutionary analyses which we combine with functional assays of hominid HCA₃ orthologs. In search for a reasonable scenario explaining the accumulated amino acid changes in HCA₃ of hominids we discovered D-phenyllactic acid (D-PLA), a metabolite produced by lactic acid bacteria (LAB), as the so far most potent agonist specifically activating HCA₃. Further, oral ingestion of Sauerkraut, known to contain high levels of D-PLA, caused subsequent plasma concentrations sufficient to activate HCA₃. Our data interpreted in an evolutionary context suggests that the availability of a new food repertoire under changed ecological conditions triggered the fixation of HCA₃ which took over new functions in hominids. These findings are particularly important because they unveiled HCA₃, which is not only expressed in various immune cells but also adipocytes, lung and skin, as a player that transfers signals of LAB-derived metabolites into a physiological response in humans. This opens up new directions towards the understanding of the versatile beneficial effects of LAB and their metabolites for humans.

Intracellular sensing of stress and danger signals initiates inflammatory innate immune responses by triggering inflammasome assembly, caspase-1 activation and pyroptotic cell death as well as the release of interleukin 1β (IL-1β), IL-18 and danger signals. NLRP3 broadly senses infectious patterns and sterile danger signals, resulting in the tightly coordinated and regulated assembly of the NLRP3 inflammasome, but the precise mechanisms are incompletely understood. Here, we identified NLRP11 as an essential component of the NLRP3 inflammasome in human macrophages. NLRP11 interacted with NLRP3 and ASC, and deletion of NLRP11 specifically prevented NLRP3 inflammasome activation by preventing inflammasome assembly, NLRP3 and ASC polymerization, caspase-1 activation, pyroptosis and cytokine release but did not affect other inflammasomes. Restored expression of NLRP11, but not NLRP11 lacking the PYRIN domain (PYD), restored inflammasome activation. NLRP11 was also necessary for inflammasome responses driven by NLRP3 mutations that cause cryopyrin-associated periodic syndrome (CAPS). Because NLRP11 is not expressed in mice, our observations emphasize the specific complexity of inflammasome regulation in humans.

Gut microbiota has emerged as a modulator of host metabolism and energy balance. However, the precise microbial metabolites mediating thermogenic activation in obesity remain largely undefined. We investigated the effect of antibiotic treatment under a high-fat diet on metabolites and its contribution to lipolysis and thermogenesis. Antibiotic treatment in high-fat diet-fed rats reduced adiposity and enhanced adaptive thermogenesis. Metabolomics revealed elevated taurine levels in the cecum content and plasma of antibiotic-treated animals, correlating with increased expressions of UCP1 and TGR5 in brown adipose tissue. Taurine enhanced lipolysis and oxygen consumption in mouse adipose tissue and human adipocytes. Thereby, taurine modulated lipolysis dependent on TGR5 signaling in adipose tissue. Human data confirmed that taurine promotes browning of white adipocytes and that acute cold exposure leads to a marked drop in circulating taurine, suggesting its rapid recruitment into thermogenic tissues. Besides its synthesis in the liver and dietary uptake, taurine can be a microbiota-derived metabolite that activates adipose thermogenesis and lipolysis through TGR5 and possibly taurine transporter-dependent mechanisms. These findings uncover a gut–adipose axis with therapeutic potential for metabolic disease.

Pathogenic variants inTP53have been classically thought to cause Li-Fraumeni syndrome (LFS), a cancer predisposition with high risks for various childhood- and adult-onset malignancies. However, increased genetic testing has lately revealed, that pathogenic variant carriers exhibit a broader range of phenotypes and that penetrance may be dependent both on variant type and modifiers. Using next generation sequencing and short tandem repeat analysis, we identified germline pathogenic variants inTP53andRAD51Clocated incison chromosome 17 in a 43-year-old male, who has developed a rare sebaceous gland carcinoma (SGC) but so far no tumors of the LFS spectrum. This course mirrors aTrp53-Rad51c-double-mutantcismouse-model, which similarly develops SGC, while the characteristicTrp53-associated tumor spectrum occurs with significantly lower frequency. Therefore, we propose that co-occurent pathogenic variants inRAD51CandTP53may predispose to SGC, reminiscent of Muir-Torre syndrome. Further, this report supports the diversity of clinical presentations associated with germlineTP53alterations, and thus, the proposed expansion of LFS to heritableTP53-related cancer syndrome.

Inflammasomes are cytosolic multiprotein complexes facilitating the maturation and release of the inflammatory cytokines interleukin (IL)−1β and IL-18 and pyroptosis. ASC (apoptosis-associated-speck-like protein containing a CARD) is the central inflammasome adaptor. ASC polymerization is crucial for inflammasome assembly, and ASC particle release propagates inflammasome responses to bystander cells. However, control of inflammasome and ASC particle assembly to limit chronic inflammation and the emergence of autoinflammatory diseases is still incompletely understood. Here, we show that the E3 ubiquitin ligase TRIM (tripartite-motif-containing protein) 21, a common autoantigen in autoimmune diseases, is involved in inflammasome assembly. Specifically, TRIM21 binds to and ubiquitinates ASC to facilitate ASC/NLRP3 interactions, ASC polymerization and the release of ASC/TRIM21-containing particles during pyroptosis in human and mouse macrophages. Furthermore, we detect systemic ASC/TRIM21 particles and autoantibodies in human and mouse autoinflammatory disease. Thus, our findings highlight a previously unrecognized role of TRIM21 as an inflammasome component and driver of autoinflammation.