The olfactory receptor Olfr78 promotes differentiation of enterochromaffin cells in the mouse colon.

The gastrointestinal epithelium constitutes a chemosensory system for microbiota-derived metabolites such as short-chain fatty acids (SCFA). Here, we investigate the spatial distribution of Olfr78, one of the SCFA receptors, in the mouse intestine and study the transcriptome of colon enteroendocrine cells expressing Olfr78. The receptor is predominantly detected in the enterochromaffin and L subtypes in the proximal and distal colon, respectively. Using the Olfr78-GFP and VilCre/Olfr78flox transgenic mouse lines, we show that loss of epithelial Olfr78 results in impaired enterochromaffin cell differentiation, blocking cells in an undefined secretory lineage state. This is accompanied by a reduced defense response to bacteria in colon crypts and slight dysbiosis. Using organoid cultures, we further show that maintenance of enterochromaffin cells involves activation of the Olfr78 receptor via the SCFA ligand acetate. Taken together, our work provides evidence that Olfr78 contributes to colon homeostasis by promoting enterochromaffin cell differentiation.

The human bitter taste receptor T2R38 is broadly tuned for bacterial compounds.

T2R38 has been shown to be a specific bacterial detector implicated in innate immune defense mechanism of human upper airway. Several clinical studies have demonstrated that this receptor is associated with the development of chronic rhinosinusitis (CRS). T2R38 was previously reported to bind to homoserine lactones (HSL), quorum sensing molecules specific of Pseudomonas Aeruginosa and other gram negative species. Nevertheless, these bacteria are not the major pathogens found in CRS. Here we report on the identification of bacterial metabolites acting as new agonists of T2R38 based on a single cell calcium imaging study. Two quorum sensing molecules (Agr D1 thiolactone from Staphylococcus Aureus and CSP-1 from Streptococcus Pneumoniae) and a list of 32 bacterial metabolites from pathogens frequently implicated in CRS were tested. First, we observed that HSL failed to activate T2R38 in our experimental system, but that the dimethylsulfoxide (DMSO), used as a solvent for these lactones may, by itself, account for the agonistic effect previously described. Secondly, we showed that both Agr D1 thiolactone and CSP-1 are inactive but that at least 7 bacterial metabolites (acetone, 2-butanone, 2-pentanone, 2-methylpropanal, dimethyl disulfide, methylmercaptan, γ-butyrolactone) are able to specifically trigger this receptor. T2R38 is thus much more broadly tuned for bacterial compounds than previously thought.

Deorphanization and characterization of human olfactory receptors in heterologous cells.

Olfaction plays an indispensable role in human and animals in self and environmental recognition, as well as intra- and interspecific communication. Following the discovery of a family of olfactory receptors (ORs) by Buck and Axel in 1991, it has been established that the sense of smell begins with the molecular recognition of a chemical odorant by one or more ORs expressed in the olfactory sensory neurons. Therefore, characterization of the molecular interactions between odorant molecules and ORs is a key step in the elucidation of the general properties of the olfactory system and in the development of applications, i.e., design of new odorants, search for blockers, etc. The process putted in place at ChemCom to improve the expression of ORs at the cytoplasmic membrane of the HEK293 cell and assays enabling large-scale deorphanization, and to characterize the interaction between chemical odorants and ORs is described. The family of human ORs includes ca. 400 putatively functional ORs which are GPCRs (G protein-coupled receptors); to date over 100 human ORs have been deorphanized.

Stroma cell-derived factor-1α signaling enhances calcium transients and beating frequency in rat neonatal cardiomyocytes.

Stroma cell-derived factor-1α (SDF-1α) is a cardioprotective chemokine, acting through its G-protein coupled receptor CXCR4. In experimental acute myocardial infarction, administration of SDF-1α induces an early improvement of systolic function which is difficult to explain solely by an anti-apoptotic and angiogenic effect. We wondered whether SDF-1α signaling might have direct effects on calcium transients and beating frequency.Primary rat neonatal cardiomyocytes were culture-expanded and characterized by immunofluorescence staining. Calcium sparks were studied by fluorescence microscopy after calcium loading with the Fluo-4 acetoxymethyl ester sensor. The cardiomyocyte enriched cellular suspension expressed troponin I and CXCR4 but was vimentin negative. Addition of SDF-1α in the medium increased cytoplasmic calcium release. The calcium response was completely abolished by using a neutralizing anti-CXCR4 antibody and partially suppressed and delayed by preincubation with an inositol triphosphate receptor (IP3R) blocker, but not with a ryanodine receptor (RyR) antagonist. Calcium fluxes induced by caffeine, a RyR agonist, were decreased by an IP3R blocker. Treatment with forskolin or SDF-1α increased cardiomyocyte beating frequency and their effects were additive. In vivo, treatment with SDF-1α increased left ventricular dP/dtmax.These results suggest that in rat neonatal cardiomyocytes, the SDF-1α/CXCR4 signaling increases calcium transients in an IP3-gated fashion leading to a positive chronotropic and inotropic effect.

Genetic exchange of the S2 and S3 subunits in pertussis toxin.

Bordetella pertussis, the causative agent of whooping cough, produces a complex hetero-oligomeric exotoxin, named pertussis toxin (PTX), which is responsible for several of the clinical manifestations associated with whooping cough. The toxin is composed of five dissimilar subunits, named S1 through S5 and arranged in a hexameric structure with a 1S1:1S2:1S3:2S4:1S5 stoichiometry. Although S2 and S3 share 70% amino acid identity, these two subunits were previously thought not to be able to substitute for each other in toxin assembly/secretion and the biological activities of PTX. Here, we show that toxin analogues containing two S3 subunits and lacking S2 (PTXdeltaS2), or containing two S2 subunits and lacking S3 (PTXdeltaS3), can be produced, assembled and secreted by B. pertussis strains, in which the S2-encoding cistron or the S3-coding cistrons have been inactivated by internal in-frame deletions that avoid downstream effects. In fact, PTXdeltaS3 was produced in higher amounts in the bacterial culture supernatants than natural PTX, whereas PTXdeltaS2 was produced in lower amounts than PTX. The action of the toxin analogues on the clustering of Chinese Hamster Ovary cells was also affected differentially by the S2-S3 substitution. These toxin analogues constitute thus interesting probes for the study of cellular functions, in particular immune cell functions, for which natural PTX has already shown its usefulness.

Origin, originality, functions, subversions and molecular signalling of macropinocytosis.

Macropinocytosis refers to the formation of primary large endocytic vesicles of irregular size and shape, generated by actin-driven evaginations of the plasma membrane, whereby cells avidly incorporate extracellular fluid. Macropinosomes resemble "empty" phagosomes and show no difference with the "spacious phagosomes" triggered by the enteropathogenic bacteria Salmonella and Shigella. Macropinosomes may fuse with lysosomes or regurgitate their content back to the extracellular space. In multiple cell types, macropinocytosis is a transient response to growth factors. When amoebas are cultured under axenic conditions, macropinocytosis is induced so as to fulfil nutritional requirements. In immature dendritic cells, macropinocytosis allows for extensive sampling of soluble antigens; after a few days of maturation, this activity vanishes as processed peptides are being presented. Macropinosomes are also formed at the leading edge of motile leukocytes or neurons. In all these examples, macropinocytosis appears tightly regulated. Transformation of fibroblasts by Src or Ras also results in constitutive formation of macropinosomes at "ruffling" zones, that could be related to accelerated cell motility. Like phagocytosis, macropinocytosis depends on signalling to the actin cytoskeleton. We have explored this signalling in transformed cells. v-Src and K-Ras activate PI3K and PLC, as demonstrated by in situ production of the corresponding lipid products. Pharmacological inhibitors of PI3K and PLC and stable transfection leading to a dominant-negative PI3-kinase construct in transformed fibroblasts abolish macropinocytosis, demonstrating that both enzyme activities are essential. Conversely, stable transfection leading to a dominant-positive P13K in non-transformed fibroblasts is sufficient to induce macropinocytosis. Combination of experiments allows to conclude that P13K and PLC act in sequential order. In non-polarized cells expressing a thermosensitive v-Src mutant, v-Src kinase activation accelerates fluid-phase endocytosis. In polarized MDCK cells, this stimulation occurs selectively at the apical domain and the response is selectively abrogated by pharmacological inhibitors of P13K and PLC. Thus, two paradigmatic oncogenes cause constitutive macropinocytosis. For v-Src, this response is polarized at the apical membrane. It is suggested that, in enterocytes that do not normally phagocytose, the P13K-PLC signalling pathway leading to selective induction of macropinocytosis at the luminal surface has been subverted by enteropathogenic bacteria to penetrate via "spacious phagosomes".