Biology subjects

Sharma, R. B.

Publications and source records attributed to Sharma, R. B..

3 recordsLinked to original sources

bioRxiv2026

MEF2D impairs mitochondrial respiration, glucose-stimulated insulin secretion, and survival in INS-1 β-cells.

Myocyte Enhancer Factor 2D (Mef2D) is a member of the Mef2 family. As a transcription factor, Mef2D regulates the expression of genes that impinge on cellular viability, tissue development, and fuel metabolism in a tissue dependent manner. Mef2D is expressed in the beta-cell, and overexpression and knockdown have been shown to modulate glucose stimulated insulin secretion. We sought to understand the role of Mef2D on beta-cell function and survival. To determine the function of Mef2D in the beta-cell, we built overexpression and knockdown INS-1 832/13 cell lines. We determined the effect of Mef2D overexpression or knockdown on mitochondrial respiration, insulin secretion, cell survival, and gene expression. Our data demonstrates that Mef2D knockdown enhances mitochondrial respiration, insulin secretion, and cell survival. Conversely, Mef2D overexpression inhibits mitochondrial respiration, insulin secretion, and cell survival. We demonstrate that some of this effect is due to modulated expression of the mitochondrial gene mtND6. These findings demonstrate that Mef2D overexpression is detrimental to beta-cell function and that Mef2D knockdown is beneficial. These data suggest that Mef2D may be a viable target to enhance functional beta-cell mass as a treatment for Type 1 and Type 2 Diabetes.

cell biology↗

bioRxiv2022

Noncanonical CDK4 signaling rescues diabetes in a mouse model by promoting beta cell differentiation

Expanding beta cell mass is a critical goal in the fight against diabetes. CDK4, an extensively characterized cell cycle activator, is required to establish and maintain beta cell number. Beta cell failure in the IRS2-deletion mouse type 2 diabetes model is in part due to loss of CDK4 regulator Cyclin D2. We set out to determine whether replacement of endogenous CDK4 with the inhibitor-resistant mutant CDK4-R24C rescued the loss of beta cell mass in Irs2-deficient mice. Surprisingly, not only beta cell mass but also beta cell dedifferentiation status was effectively rescued, despite no improvement in insulin sensitivity. Ex vivo studies in primary islet cells revealed a novel mechanism in which CDK4 intervened downstream in the insulin signaling pathway to prevent FOXO1-mediated transcriptional repression of critical beta cell transcription factor Pdx1. FOXO1 inhibition was not related to E2F1 activity, to FOXO1 phosphorylation, or even to FOXO1 subcellular localization, but rather was related to deacetylation of FOXO1 and reduced FOXO1 abundance. Taken together, these results demonstrate a novel differentiation-promoting activity of the classical cell cycle activator CDK4 and support the concept that beta cell mass can be expanded without compromising function.

cell biology↗

bioRxiv2020

Beta-cell specific insulin resistance promotes glucose-stimulated insulin hypersecretion

Abstract Insulin receptor (Insr) protein can be found at higher levels in pancreatic {beta}-cells than in most other tissues, but the consequences of {beta}-cell insulin resistance remain enigmatic. Ins1cre allele was used to delete Insr specifically in {beta}-cells of both female and male mice. Experimental mice were compared to Ins1cre-containing littermate controls at multiple ages and on multiple diets. RNA-seq of purified recombined {beta}-cells revealed transcriptomic consequences of Insr loss, which differed between female and male mice. Action potential and calcium oscillation frequencies were increased in Insr knockout {beta}- cells from female, but not male mice, whereas only male {beta}InsrKO mice had reduced ATP-coupled oxygen consumption rate and reduced expression of genes involved in ATP synthesis. Female {beta}InsrKO and {beta}InsrHET mice exhibited elevated insulin release in perifusion experiments, during hyperglycemic clamps, and following i.p. glucose challenge. Deletion of Insr did not alter {beta}-cell area up to 9 months of age, nor did it impair hyperglycemia-induced proliferation. Based on our data, we adapted a mathematical model to include {beta}-cell insulin resistance, which predicted that {beta}-cell Insr knockout would improve glucose tolerance depending on the degree of whole-body insulin resistance. Indeed, glucose tolerance was significantly improved in female {beta}InsrKO and {beta}InsrHET mice when compared to controls at 9, 21 and 39 weeks, and also in insulin-sensitive 4-week old males. We did not observe improved glucose tolerance in older male mice or in high fat diet-fed mice, corroborating the prediction that global insulin resistance obscures the effects of {beta}-cell specific insulin resistance. The propensity for hyperinsulinemia was associated with mildly reduced fasting glucose and increased body weight. We further validated our main in vivo findings using the Ins1-CreERT transgenic line and found that male mice had improved glucose tolerance 4 weeks after tamoxifen-mediated Insr deletion. Collectively, our data show that loss of {beta}-cell Insr contributes to glucose-induced hyperinsulinemia, thereby improving glucose homeostasis in otherwise insulin sensitive dietary and age contexts.

physiology↗