SUCNR1 regulates insulin secretion and glucose elevates the succinate response in people with prediabetes.

Pancreatic ß-cell dysfunction is a key feature of type 2 diabetes, and novel regulators of insulin secretion are desirable. Here we report that the succinate receptor (SUCNR1) is expressed in ß-cells and is up-regulated in hyperglycemic states in mice and humans. We found that succinate acts as a hormone-like metabolite and stimulates insulin secretion via a SUCNR1-Gq-PKC-dependent mechanism in human ß-cells. Mice with ß-cell-specific Sucnr1 deficiency exhibit impaired glucose tolerance and insulin secretion on a high-fat diet, indicating that SUCNR1 is essential for preserving insulin secretion in diet-induced insulin resistance. Patients with impaired glucose tolerance show an enhanced nutritional-related succinate response, which correlates with the potentiation of insulin secretion during intravenous glucose administration. These data demonstrate that the succinate/SUCNR1 axis is activated by high glucose and identify a GPCR-mediated amplifying pathway for insulin secretion relevant to the hyperinsulinemia of prediabetic states.

SUCNR1 signaling in adipocytes controls energy metabolism by modulating circadian clock and leptin expression.

Adipose tissue modulates energy homeostasis by secreting leptin, but little is known about the factors governing leptin production. We show that succinate, long perceived as a mediator of immune response and lipolysis, controls leptin expression via its receptor SUCNR1. Adipocyte-specific deletion of Sucnr1 influences metabolic health according to nutritional status. Adipocyte Sucnr1 deficiency impairs leptin response to feeding, whereas oral succinate mimics nutrient-related leptin dynamics via SUCNR1. SUCNR1 activation controls leptin expression via the circadian clock in an AMPK/JNK-C/EBPα-dependent manner. Although the anti-lipolytic role of SUCNR1 prevails in obesity, its function as a regulator of leptin signaling contributes to the metabolically favorable phenotype in adipocyte-specific Sucnr1 knockout mice under standard dietary conditions. Obesity-associated hyperleptinemia in humans is linked to SUCNR1 overexpression in adipocytes, which emerges as the major predictor of adipose tissue leptin expression. Our study establishes the succinate/SUCNR1 axis as a metabolite-sensing pathway mediating nutrient-related leptin dynamics to control whole-body homeostasis.

Soluble factors influencing the neural stem cell niche in brain physiology, inflammation, and aging.

Within the adult central nervous system (CNS) of most mammals resides a resident stem cell population, known as neural stem cells (NSCs). NSCs are located within specific niches of the CNS and maintain a self-renewal and proliferative capacity to generate new neurons, astrocytes, and oligodendrocytes throughout adulthood. The NSC niches are dynamic and active environments that are within proximity to the systemic circulation and the cerebrospinal fluid (CSF). Therefore, NSCs respond not only to factors present in the local microenvironment of the niche but also to factors present in the systemic macroenvironment. The factors can be soluble forms such as cytokines and chemokines located in the circulation or directly from local cells, such as microglia and astrocytes. Additionally, recent evidence points towards physiological aging and its association with a progressive loss of function and a decline in the self-renewal and regenerative capacities of CNS NSCs, which can be further exacerbated by changes in the local and systemic milieu. This review will highlight the main intrinsic and extrinsic regulators of neural stem cell function under homeostatic and inflammatory conditions including those trafficked within extracellular membrane vesicles. Further, discussion will center around how intrinsic and extrinsic factors impact normal homeostatic functions within the adult brain and in aging.