Stromal Cells Covering Omental Fat-Associated Lymphoid Clusters Trigger Formation of Neutrophil Aggregates to Capture Peritoneal Contaminants

The omentum is a visceral adipose tissue rich in fat-associated lymphoid clusters (FALCs) that collects peritoneal contaminants and provides a first layer of immunological defense within the abdomen. Here, we investigated the mechanisms that mediate the capture of peritoneal contaminants during peritonitis. Single-cell RNA sequencing and spatial analysis of omental stromal cells revealed that the surface of FALCs were covered by CXCL1+ mesothelial cells, which we termed FALC cover cells. Blockade of CXCL1 inhibited the recruitment and aggregation of neutrophils at FALCs during zymosan-induced peritonitis. Inhibition of protein arginine deiminase 4, an enzyme important for the release of neutrophil extracellular traps, abolished neutrophil aggregation and the capture of peritoneal contaminants by omental FALCs. Analysis of omental samples from patients with acute appendicitis confirmed neutrophil recruitment and bacterial capture at FALCs. Thus, specialized omental mesothelial cells coordinate the recruitment and aggregation of neutrophils to capture peritoneal contaminants

Adipose tissue macrophage heterogeneity and the role of Tim4⁺ macrophages in lipid homeostasis

Resident macrophages are essential for the maintenance of tissue homeostasis as they participate in clearance of apoptotic cells and tissue remodelling and repair. In recent years, there has been an increased interest in the study of adipose tissue macrophages (ATMs). In lean individuals, ATMs are important for the control of insulin sensitivity, thermogenesis, angiogenesis and adipose tissue development. In obesity, the number and phenotype of ATMs is altered, and is associated with chronic low grade systemic and local inflammation. These “pro-inflammatory” changes are postulated to contribute to the manifestation of metabolic syndrome. These findings have suggested that the pool of ATMs is heterogeneous and may change, especially during obesity. To date, the characterisation of ATMs has been limited largely to the F4/80/CD11b markers, however the hypothesis of this thesis is that ATMs have distinct phenotype and function that could influence, in different ways, tissue homeostasis. This thesis aims to characterise and phenotype ATM subsets in order to better understand their potential specific role in the tissue. During the course of this research, a novel population of Tim4+ resident ATMs were identified. An additional aim of this thesis was to elucidate their role in adipose tissue homeostasis. Partial bone marrow chimeras were used to identify macrophage origin. The main AT depots were shielded from irradiation and a donor BM was injected intravenously. After 8 weeks, the origin of macrophages was analysed using flow cytometry. Tim4, a phosphatidylserine receptor mediating phagocytosis of apoptotic cells and a marker found on resident macrophages in other tissues, was used for the first time in adipose tissue. Four subsets of ATMs were identified: F4/80highCD11c-Tim4+, F4/80highCD11c- Tim4-; F4/80lowCD11c+Tim4-; F4/80lowCD11c-Tim4-. Interestingly, this newly described F4/80highTim4+ ATM subset showed the lowest non-host chimerism compared to the other ATMs, suggesting this is a main self-replenishing resident ATM population. To study the impact of obesity on ATM turnover, partial chimeric mice were fed HFD for 8 weeks. This increased the number of macrophages in AT. However, the different subsets of ATMs were differentially affected by the diet. Indeed, only a small proportion of Tim4+ ATMs derived from the bone marrow. In contrast, replenishment of the 3 other subsets was almost fully dependent on the arrival of monocyte-derived cells from the bone marrow. TIMD4, the gene encoding for Tim4, has been highlighted in genetic studies as being linked with dyslipidaemia. This suggests that Tim4+ ATMs might play a role in lipid homeostasis. Further characterisation of Tim4 ATMs demonstrated that these Tim4+ ATMs are highly charged in neutral lipid, and also have an increased lysosomal activity (shown by lysotracker staining) compared to the other ATM subsets. Using blocking anti-Tim4 antibodies in vivo, I found that Tim4 contributed markedly to free fatty acid (FFA) release into the plasma after short-term and long term HFD feeding. In addition, in vitro and in vivo experiments demonstrated that Tim4 could be required for the uptake of neutral lipids and their integration into lysosomes for degradation, though this seems to be dependent on the nature of the lipid. Collectively, these results indicate that Tim4 plays a crucial role in the control of lipid trafficking under conditions when dietary lipid is in excess. Tim4 allows uptake of lipids by Tim4+ ATMs and subsequent release of FFA into the circulation. Finally, the presence of Tim4+ lipid laden ATMs was demonstrated in the human omentum. This finding may lead to the discovery of new targets to improve metabolic health in obese patients. This work stresses the importance of resident ATM population in body lipid homeostasis as they could be involved in coping with lipid availability in the body and influence the amount of FFA in the plasma

Effets du L-Lactate de magnésium alimentaire sur la fonction du myocarde et la masse musculaire chez le rongeur sain

poster commentéDans l’organisme, le L-Lactate est pris en charge par la déshydrogénase qui, en le transformant en pyruvate, produit des équivalents réduits et pourrait être impliqué dans la lutte contre le stress oxydant. L’objectif de notre travail consistait à savoir l’effet de cette molécule sur la fonction du cœur et la masse musculaire quand elle est administrée par voie alimentaire chez le rongeur sain et sédentaire. Des rats mâles ont absorbé à leur convenance une solution de L-Lactate de magnésium pendant 3 mois. L’activité mécanique du cœur a alors été mesurée par échographie. Les cœurs ont ensuite été perfusés selon le mode Langendorff dans des conditions parfaitement standard et la masse des muscles de la patte arrière a été estimée. Enfin, le stress oxydant a été déterminé dans le cœur et dans le plasma. Les animaux ont consommé de façon régulière une faible dose de L-Lactate (28 nmoles/Kg/J). Le composé n’a modifié ni le gain de poids corporel, ni les masses grasses et maigres. Il a accru la masse du tibialis antérieur (+12%). Il a réduit la longueur du ventricule gauche en diastole, mais n’a pas affecté la fraction d’éjection systolique in vivo. Dans le cœur isolé, il a accru les vitesses de contraction (+33%) et de relaxation (+21%) du myocarde, sans altérer le débit coronaire. Au niveau du plasma, il a réduit le stress oxydant des protéines sans altérer celui des lipides. En conclusion, le L-Lactate de magnésium mime les effets de l’entraînement physique régulier chez les animaux sédentaires sur le muscle squelettique et le cœur, peut être en réduisant le stress oxydant plasmatiqu

A chronic low-dose magnesium L-lactate administration has a beneficial effect on the myocardium and the skeletal muscles

International audienceThe purpose of this study was to determine whether magnesium L-lactate is responsible for having a beneficial effect on the myocardium and the skeletal muscles and how this substrate acts at the molecular level. Twenty seven young male Wistar rats were supplied with a magnesium L-lactate (L) solution, a magnesium chloride (M) solution and/or water (W) as a vehicle for 10 weeks. The treated animals absorbed the L and M solutions as they wished since they also had free access to water. After 9 weeks of treatment, in vivo cardiac function was determined ultrasonically. The animals were sacrificed at the end of the tenth week of treatment and the heart was perfused according to the Langendorff method by using a technique allowing the determination of cardiomyocyte activity (same coronary flow in the two groups). Blood was collected and skeletal muscles of the hind legs were weighed. The myocardial expressions of the sodium/proton exchange 1 (NHE1) and sodium/calcium exchange 1 (NCX1), intracellular calcium accumulation, myocardial magnesium content, as well as systemic and tissue oxidative stress, were determined. Animals of the L group absorbed systematically a low dose of L-lactate (31.5 ± 4.3 µg/100 g of body weight/day) which was approximately four times higher than that ingested in the W group through the diet supplied. Ex vivo cardiomyocyte contractility and the mass of some skeletal muscles (tibialis anterior) were increased by the L treatment. Myocardial calcium was decreased, as was evidenced by an increase in total CaMKII expression, without any change in the ratio between phosphorylated CaMKII and total CaMKII. Cardiac magnesium tended to be elevated. Our results suggest that the increased intracellular magnesium concentration was related to L-lactate-induced cytosolic acidosis and to the activation of the NHE1/NCX1 axis. Interestingly, systemic oxidative stress was reduced by the L treatment whereas the lipid profile of the animals was unaltered. Taken together, these results suggest that a chronic low-dose L-lactate intake has a beneficial health effect on some skeletal muscles and the myocardium through the activation of the NHE1/NCX1 axis, a decrease in cellular calcium and an increase in cellular magnesium. The treatment can be beneficial for the health of young rodents in relation to chronic oxidative stress-related diseases