No abstract

BackgroundGut microbiota composition and function are symbiotically linked with host health and altered in metabolic, inflammatory and neurodegenerative disorders. Three recognised mechanisms exist by which the microbiome influences the gut–brain axis: modification of autonomic/sensorimotor connections, immune activation, and neuroendocrine pathway regulation. We hypothesised interactions between circulating gut-derived microbial metabolites, and the blood–brain barrier (BBB) also contribute to the gut–brain axis. Propionate, produced from dietary substrates by colonic bacteria, stimulates intestinal gluconeogenesis and is associated with reduced stress behaviours, but its potential endocrine role has not been addressed.ResultsAfter demonstrating expression of the propionate receptor FFAR3 on human brain endothelium, we examined the impact of a physiologically relevant propionate concentration (1 μM) on BBB properties in vitro. Propionate inhibited pathways associated with non-specific microbial infections via a CD14-dependent mechanism, suppressed expression of LRP-1 and protected the BBB from oxidative stress via NRF2 (NFE2L2) signalling.ConclusionsTogether, these results suggest gut-derived microbial metabolites interact with the BBB, representing a fourth facet of the gut–brain axis that warrants further attention.Electronic supplementary materialThe online version of this article (10.1186/s40168-018-0439-y) contains supplementary material, which is available to authorized users.

Formyl-peptide receptor type 2 (FPR2), also called ALX (the lipoxin A4 receptor), conveys the proresolving properties of lipoxin A 4 and annexin A1 (AnxA1) and the proinflammatory signals elicited by serum amyloid protein A and cathelicidins, among others. We tested here the hypothesis that ALX might exist as homo-or heterodimer with FPR1 or FPR3 (the two other family members) and operate in a ligand-biased fashion. Coimmunoprecipitation and bioluminescence resonance energy transfer assays with transfected HEK293 cells revealed constitutive dimerization of the receptors; significantly, AnxA1, but not serum amyloid protein A, could activate ALX homodimers. A p38/MAPK-activated protein kinase/heat shock protein 27 signaling signature was unveiled after AnxA1 application, leading to generation of IL-10, as measured in vitro (in primary monocytes) and in vivo (after i.p. injection in the mouse). The latter response was absent in mice lacking the ALX ortholog. Using a similar approach, ALX/FPR1 heterodimerization evoked using the panagonist peptide Ac2-26, identified a JNK-mediated proapoptotic path that was confirmed in primary neutrophils. These findings provide a molecular mechanism that accounts for the dual nature of ALX and indicate that agonist binding and dimerization state contribute to the conformational landscape of FPRs.inflammation | leukocyte | resolution signaling G -protein-coupled receptors (GPCRs) constitute a large family of cell surface receptors that share structural characteristics and perform pivotal biological functions, transducing signals from hormones, autacoids, and chemokines. The human GPCR termed "ALX/FPR2" (formyl peptide receptor type 2 or lipoxin A 4 receptor, hereafter referred to as "ALX") is a unique GPCR, shown to convey signals induced by proteins, peptides, and lipid ligands (1). ALX belongs to a small family of receptors that is also activated by formylated peptides, short amino acid sequences with an N-terminal formyl group released by pathogenic and commensal bacteria, as well as by mitochondria upon cell damage. There are three human FPRs and they are termed FPR1, ALX, and FPR3 (2). In view of their different nature and potential engagement with a large number endogenous and exogenous ligands, elucidation of FPR functions may reveal important biological pathways.ALX is an unconventional receptor for the diversity of its agonists and because it can convey contrasting biological signals. The proresolving and anti-inflammatory properties of the protein annexin A1 (AnxA1) and the lipid lipoxin A 4 (LXA 4 ), which include neutrophil apoptosis and macrophage efferocytosis, are mediated by this receptor, as shown using pharmacological approaches (1, 3) and more recently with knockout mouse models (4). At the same time, the proinflammatory responses elicited by the cathelicidin-associated antimicrobial peptide LL-37 and serum amyloid protein A (SAA) are also mediated by ALX, which modulates leukocyte activation, recruitment to the site of inflammation, and lifespan (5-7). Moreover,...

The blood-brain barrier (BBB), a critical guardian of communication between the periphery and the brain, is frequently compromised in neurological diseases such as multiple sclerosis (MS), resulting in the inappropriate passage of molecules and leukocytes into the brain. Here we show that the glucocorticoid anti-inflammatory messenger annexin A1 (ANXA1) is expressed in brain microvascular endothelial cells, where it regulates BBB integrity. In particular, ANXA1 −/− mice exhibit significantly increased BBB permeability as a result of disrupted interendothelial cell tight junctions, essentially related to changes in the actin cytoskeleton, which stabilizes tight and adherens junctions. This situation is reminiscent of early MS pathology, a relationship confirmed by our detection of a selective loss of ANXA1 in the plasma and cerebrovascular endothelium of patients with MS. Importantly, this loss is swiftly restored by i.v. administration of human recombinant ANXA1. Analysis in vitro confirms that treatment of cerebrovascular endothelial cells with recombinant ANXA1 restores cell polarity, cytoskeleton integrity, and paracellular permeability through inhibition of the small G protein RhoA. We thus propose ANXA1 as a critical physiological regulator of BBB integrity and suggest it may have utility in the treatment of MS, correcting BBB function and hence ameliorating disease.T he presence of narrow and dense tight junctions between adjacent endothelial cells is peculiar to the cerebral vasculature, and their integrity is essential for the maintenance of correct blood-brain barrier (BBB) function as the primary regulator of cross-talk between the brain and the rest of the body (1). Increasing evidence indicates that the integrity of this structural and functional barrier is compromised in neurological conditions such as multiple sclerosis (MS), Alzheimer's, and Parkinson diseases, leading to the failure of the normal mechanisms controlling passage of substances into the brain (2) and to the sensitization and/or worsening of pathologic conditions. Pharmacological intervention to prevent or correct BBB alteration in such diseases is a difficult task, but potential therapeutic leads can be gained from the study of endogenous mediators regulating barrier integrity.Annexin A1 (ANXA1) is an important anti-inflammatory protein, principally known as a regulator of peripheral leukocyte migration and a promoter of macrophage phagocytosis (3). ANXA1 is expressed in several cell types within the brain, including ependyma and microglia, but in particular in the endothelium of the brain microvasculature (4), although its role in these cells remains obscure. We have previously shown glucocorticoids to up-regulate expression of ANXA1 in the cerebral endothelium (5), and, given that glucocorticoids enhance BBB tightness (6), we hypothesized that ANXA1 may play a role in the regulation of BBB permeability. Through combined in vitro and in vivo approaches, we have identified a dual role for ANXA1 in organizing the interendothelial cell...