Insulin secretory granules (SGs) mediate the regulated secretion of insulin, which is essential for glucose homeostasis. The basic machinery responsible for this regulated exocytosis consists of specific proteins present both at the plasma membrane and on insulin SGs. The protein composition of insulin SGs thus dictates their release properties, yet the mechanisms controlling insulin SG formation, which determine this molecular composition, remain poorly understood. VPS41, a component of the endolysosomal tethering homotypic fusion and vacuole protein sorting (HOPS) complex, was recently identified as a cytosolic factor involved in the formation of neuroendocrine and neuronal granules. We now find that VPS41 is required for insulin SG biogenesis and regulated insulin secretion. Loss of VPS41 in pancreatic β-cells leads to a reduction in insulin SG number, changes in their transmembrane protein composition, and defects in granule-regulated exocytosis. Exploring a human point mutation, identified in patients with neurological but no endocrine defects, we show that the effect on SG formation is independent of HOPS complex formation. Finally, we report that mice with a deletion of VPS41 specifically in β-cells develop diabetes due to severe depletion of insulin SG content and a defect in insulin secretion. In sum, our data demonstrate that VPS41 contributes to glucose homeostasis and metabolism.

Spinocerebellar ataxia type 3 (SCA3, also known as Machado Joseph disease) is a fatal neurodegenerative disease caused by the expansion of the trinucleotide repeat region within the ATXN3/MJD gene. Mutation of ATXN3 causes formation of ataxin‐3 protein aggregates, neurodegeneration, and motor deficits. Here we investigated the therapeutic potential and mechanistic activity of sodium butyrate (SB), the sodium salt of butyric acid, a metabolite naturally produced by gut microbiota, on cultured SH‐SY5Y cells and transgenic zebrafish expressing human ataxin‐3 containing 84 glutamine (Q) residues to model SCA3. SCA3 SH‐SY5Y cells were found to contain high molecular weight ataxin‐3 species and detergent‐insoluble protein aggregates. Treatment with SB increased the activity of the autophagy protein quality control pathway in the SCA3 cells, decreased the presence of ataxin‐3 aggregates and presence of high molecular weight ataxin‐3 in an autophagy‐dependent manner. Treatment with SB was also beneficial in vivo, improving swimming performance, increasing activity of the autophagy pathway, and decreasing the presence of insoluble ataxin‐3 protein species in the transgenic SCA3 zebrafish. Co‐treating the SCA3 zebrafish with SB and chloroquine, an autophagy inhibitor, prevented the beneficial effects of SB on zebrafish swimming, indicating that the improved swimming performance was autophagy‐dependent. To understand the mechanism by which SB induces autophagy we performed proteomic analysis of protein lysates from the SB‐treated and untreated SCA3 SH‐SY5Y cells. We found that SB treatment had increased activity of Protein Kinase A and AMPK signaling, with immunoblot analysis confirming that SB treatment had increased levels of AMPK protein and its substrates. Together our findings indicate that treatment with SB can increase activity of the autophagy pathway process and that this has beneficial effects in vitro and in vivo. While our results suggested that this activity may involve activity of a PKA/AMPK‐dependent process, this requires further confirmation. We propose that treatment with sodium butyrate warrants further investigation as a potential treatment for neurodegenerative diseases underpinned by mechanisms relating to protein aggregation including SCA3.

Type 2 diabetes (T2D) is a pressing global health challenge, primarily driven by modern dietary and lifestyle patterns. Central to T2D progression is the dysfunction of insulin-secreting pancreatic β-cells, which critically disrupts glucose homeostasis. The progression to T2D relies on the β-cells’ inability to compensate for increasing insulin resistance. Initially, β-cells enhance the insulin output, but chronic nutrient overload, ER stress and inflammation ultimately compromise their function and survival. This review examines the molecular and cellular drivers of β-cell failure, focusing on endoplasmic reticulum stress, mitochondrial dysfunction and inflammatory pathways amid chronic metabolic stress. We also explore the loss of β-cell identity and altered interactions within the islet microenvironment. Understanding these mechanisms is essential for developing strategies to prevent β-cell dysfunction and slow T2D progression, ultimately supporting better metabolic health outcomes.

In this study, we show that acute VPS41 deletion results in rapid degradation of insulin, whereas chronic VPS41 deletion results in downregulation of β-cell identity. In acute VPS41 depletion in vitro, the loss of insulin is associated with increased degradative pathway activity, increased Adapter Protein 3 complex colocalization with lysosomes, increased nuclear localization of transcription factor E3, and downregulation of PDX1 and INS mRNA expression. Inhibition of lysosomal degradation rescues the rapidly depleted insulin content.