Biology subjects

Vikram, A.

Publications and source records attributed to Vikram, A..

4 recordsLinked to original sources

bioRxiv2025

ALPHA/BETA HYDROLASE DOMAIN CONTAINING 5 SUPPORTS GLUCOSE-STIMULATED-LIPOLYSIS AND INSULIN SECRETION IN PANCREATIC BETA CELLS

PNPLA2 (aka adipose triglyceride lipase) is the rate limiting enzyme of triglyceride (TG) catabolism at the surface of lipid droplets (LDs). Lipolysis by PNPLA2 increases with glucose in pancreatic beta cells and produces metabolites that support glucose-stimulated insulin secretion (GSIS). Upregulation of lipolysis by glucose is blunted in human islets affected by type 2 diabetes. Here, we aim to determine how glucose regulates lipolysis in beta cells. We found that glucose recruits alpha/beta hydrolase domain containing 5 (ABHD5), a co-activator of PNPLA2 to LDs where lipolysis occurs. ABHD5 recruitment to LDs was reduced by the addition of H-89 and the expression of a mutant ABHD5 that is resistant to phosphorylation by a cAMP dependent kinase (PKA) indicating partial dependence of recruitment on PKA. Importantly, ABHD5 was indispensable for glucose responsiveness of lipolysis in INS-1 832/13 (INS-1) cells. Downregulation of ABHD5 increased LDs and TG content in INS-1 cells and human islets indicating that ABHD5 is highly active in beta cells. Additionally, glucose-stimulated insulin secretion was impaired in ABHD5 downregulated INS-1 and human pseudoislets, which agrees with impaired GSIS in ATGL downregulated beta cells. Thus, ABHD5 plays an important role in conferring glucose responsiveness of lipolysis and supporting insulin secretion in beta cells.

cell biology↗

bioRxiv2024

Lipid droplet protein Perilipin 2 is critical for the regulation of insulin secretion through beta cell lipophagy and glucagon expression in pancreatic islets

Knockdown (KD) of lipid droplet (LD) protein perilipin 2 (PLIN2) in beta cells impairs glucose-stimulated insulin secretion (GSIS) and mitochondrial function. Here, we addressed a pathway responsible for compromised mitochondrial integrity in PLIN2 KD beta cells. In PLIN2 KD human islets, mitochondria were fragmented in beta cells but not in alpha cells. Glucagon but not insulin level was elevated. While the formation of early LDs followed by fluorescent fatty acids (FA) analog Bodipy C12 (C12) was preserved, C12 accumulated in mitochondria over time in PLIN2 KD INS-1 cells. A lysosomal acid lipase inhibitor Lali2 prevented C12 transfer to mitochondria, mitochondrial fragmentation, and the impairment of GSIS. Direct interactions between LD-lysosome and lysosome-mitochondria were increased in PLIN2 KD INS-1 cells. Thus, FA released from LDs by microlipophagy cause mitochondrial changes and impair GSIS in PLIN2 KD beta cells. Interestingly, glucolipotoxic condition (GLT) caused C12 accumulation and mitochondrial fragmentation similar to PLIN2 KD in beta cells. Moreover, Lali2 reversed mitochondrial fragmentation and improved GSIS in human islets under GLT. In summary, PLIN2 regulates microlipophagy to prevent excess FA flux to mitochondria in beta cells. This pathway also contributes to GSIS impairment when LD pool expands under nutrient load in beta cells.

molecular biology↗

bioRxiv2024

Acute Inhibition of Adipose Triglyceride Lipase by NG497 Dysregulates Insulin and Glucagon Secretion from Human Islets

Adipose triglyceride lipase (ATGL), which catalyzes the breakdown of triglycerides in lipid droplets (LDs), plays a critical role in releasing fatty acids to support insulin secretion in pancreatic beta cells. Based on genetic downregulation of ATGL in beta cells, multiple mechanisms are proposed that acutely or chronically regulate insulin secretion. Currently, the contribution of acute versus chronic mechanisms in the regulation of insulin secretion is unclear. Also, little is known whether ATGL affects alpha cell function. Using the human-specific ATGL inhibitor, NG497, this study investigates the impact of acute inhibition of ATGL on hormone secretion from human islets. When lipolysis by ATGL was assessed via morphological differences in LDs in confocal images of beta and alpha cells, beta cells exposed to NG497 showed notable increases in LD size and number under glucose-sufficient culture. The effect of NG497 on LD accumulation in alpha cells was more prominent under fasting-simulated conditions than glucose-sufficient conditions, pointing toward a critical role for ATGL lipolysis under conditions that stimulate hormone secretion in beta and alpha cells. When exposed to NG497 acutely, human islets reduced glucose-stimulated insulin secretion mildly, particularly first-phase insulin secretion, to an extent less pronounced than the impacts of chronic ATGL downregulation. Thus, chronic mechanisms play a predominant role in reducing insulin secretion when ATGL is downregulated. Acute exposure of human islets to NG497 significantly reduced glucagon secretion at low glucose concentration, highlighting an important potential role of ATGL lipolysis in promoting hormone secretion acutely from alpha cells.

cell biology↗

bioRxiv2022

SUMOylation of the Cardiac Sodium Channel NaV1.5 Modifies Inward Current and Cardiac Excitability

BackgroundDecreased peak sodium current (INa) and increased late sodium current (INa,L), through the cardiac sodium channel NaV1.5 encoded by SCN5A, cause arrhythmias. Many NaV1.5 post-translational modifications have been reported by us and others. A recent report concluded that acute hypoxia increases INa,L by increasing a Small Ubiquitin-like MOdifier (SUMOylation) at K442-NaV1.5. ObjectiveTo determine whether and by what mechanisms SUMOylation alters INa, INa,L and cardiac electrophysiology. MethodsSUMOylation of NaV1.5 was detected by immunoprecipitation and immunoblotting. INa was measured by patch clamp with/without SUMO1 overexpression in HEK293 cells expressing wild type (WT) or K442R-NaV1.5 and in neonatal rat cardiac myocytes (NRCMs). SUMOylation effects were studied in vivo by electrocardiograms and ambulatory telemetry using Scn5a heterozygous knockout (SCN5A+/-) mice and the de-SUMOylating protein SENP2 (AAV9-SENP2) or the SUMOylation inhibitor anacardic acid. NaV1.5 trafficking was detected by immunofluorescence. ResultsNaV1.5 was SUMOylated in HEK293 cells, NRCMs and human heart tissue. HyperSUMOylation at NaV1.5-K442 increased INa in NRCMs and in HEK cells overexpressing WT but not K442R-Nav1.5. SUMOylation did not alter other channel properties including INa,L. AAV9-SENP2 or anacardic acid treatment of SCN5A+/- mice decreased INa, prolonged QRS duration, and produced heart block and ventricular arrhythmias. SUMO1 overexpression enhanced membrane localization of NaV1.5. ConclusionSUMOylation of K442-Nav1.5 increases peak INa without changing INa,L, at least in part by altering membrane abundance. Our findings do not support SUMOylation as a mechanism for changes in INa,L. Nav1.5 SUMOylation may modify arrhythmic risk in disease states and represents a potential target for pharmacological manipulation.

physiology↗