“In people with type 2 diabetes, the VPS39 gene is significantly less active in the muscle cells than it is in other people, and the stem cells with less activity of the gene do not form new muscle cells to the same degree. The gene is important when muscle cells absorb sugar from blood and build new muscle. Our study is the first ever to link this gene to type 2 diabetes,” says Charlotte Ling, professor of epigenetics at Lund University who led the study.
In type 2 diabetes, the ability to produce insulin is impaired, and patients have chronically elevated blood sugar. Muscles are generally worse at absorbing sugar from food, and muscle function and strength are impaired in patients with type 2 diabetes.
A muscle consists of a mixture of fiber types with different properties. Throughout life, muscle tissue has the ability to form new muscle fibers. There are also immature muscle stem cells that are activated in connection with, for example, injury or exercise.
Two groups were included in the study: 14 participants with type 2 diabetes and 14 healthy people in a control group. The participants in the groups were matched by age, gender and BMI (body mass index). The researchers studied epigenetic changes in the muscle stem cells in both groups, and under exactly the same conditions, they also extracted mature muscle cells and compared them.
n total, they identified 20 genes , including VPS39, whose gene expression differed between the groups in both immature muscle stem cells and mature muscle cells. The researchers also compared the epigenetic patterns of muscle cells before and after cell differentiation in both groups.
“Despite the fact that both groups’ muscle stem cells were grown under identical conditions, we saw more than twice as many epigenetic changes in the type 2 diabetes group during the differentiation from muscle stem cell to mature muscle cells. Muscle-specific genes were not regulated normally, and epigenetics did not function in the same way in cells from people with type 2 diabetes,” says Charlotte Ling.
“The study clearly showed that muscle stem cells that lack the function of the gene VPS39, which is lower in type 2 diabetes, also lack the ability to form new mature muscle cells. This is because muscle stem cells that lack VPS39 due to altered epigenetic mechanisms cannot change their metabolism in the same way as muscle stem cells from controls – the cells therefore remain immature or break down and die,” says Johanna Säll Sernevi, postdoc researcher at Lund University.
To confirm the findings, the researchers also used animal models with mice that had a reduced amount of the VPS39 gene, to mimic the disease. The mice subsequently had altered gene expression and reduced uptake of sugar from blood into the muscle tissue, just like the individuals with type 2 diabetes.
The comprehensive study is a collaboration between Swedish, Danish and German researchers, who believe that the findings open up new avenues for treating type 2 diabetes.
For example, by regulating proteins, stimulating or increasing the amount of the VPS39 gene, it would be possible to affect the muscles’ ability to regenerate and absorb sugar,” concludes Charlotte Ling.
Proper temporal release of peptide hormones plays a critical role in the body’s ability to maintain homeostasis. Pancreatic β-cells produce insulin and store it within intracellular vesicles, known as insulin secretory granules (SGs), which undergo exocytosis in response to extracellular stimuli (e.g., hyperglycemia).
The basic machinery responsible for this regulated exocytosis consists of proteins present both at the plasma membrane and on insulin SGs. Thus, the protein composition of insulin SGs partly dictates their release properties. SGs form at the trans-Golgi network (TGN), where they bud as immature granules.
These vesicles then undergo a series of maturation steps, which include luminal acidification, processing of proinsulin to insulin, insulin crystallization with zinc, and removal of immature granule markers (1–8). Surprisingly, the exact mechanisms dictating SG biogenesis and maturation, which is thought to determine their molecular composition, remain poorly understood.
VPS41 and VPS39 associate with the VPS class C core proteins to form the homotypic fusion and vacuole protein sorting (HOPS) complex, which acts as a tether for late endosome–lysosome fusion (9–12). Disruption in HOPS impairs delivery of endocytic cargo to lysosome (13) and causes defects in autophagosome clearance (14,15). Independently of its role in HOPS, VPS41 functions in the fusion of TGN-derived LAMP carriers with late endosomes (16).
Interestingly, gene expression analysis revealed a significant reduction in VPS41 mRNA levels in islets of mice susceptible to develop diabetes (17), and VPS41 variants have been associated with type 2 diabetes in humans, suggesting that they might be a factor contributing to the development of the disease (18). Furthermore, we previously found that VPS41 affects regulated secretion of neuropeptides from neuroendocrine cells and neurons (19). Altogether, this led us to investigate if VPS41 contributes to glucose homeostasis by influencing insulin SG biology.
Using clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 in rat insulinoma INS-1 cells, we now show that VPS41 is required for proper insulin storage and regulated secretion. Lack of VPS41 decreases SG number and affects their ability to undergo regulated exocytosis, which is concomitant with a defect in sorting of SG transmembrane proteins. We show that a patient mutation of VPS41, which is deficient for HOPS function (20), rescues insulin-regulated secretion in VPS41 knockout (KO) INS-1 cells, indicating VPS41 function in insulin-regulated secretion is independent of HOPS. Finally, we report that mice with a targeted deletion of VPS41 in β-cells develop diabetes due to a defect in insulin SG biogenesis and secretion. These studies, for the first time, implicate VPS41 in regulating β-cell function and control of glucose homeostasis.
reference link: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7881869/
More information: Cajsa Davegårdh et al. VPS39-deficiency observed in type 2 diabetes impairs muscle stem cell differentiation via altered autophagy and epigenetics, Nature Communications (2021). DOI: 10.1038/s41467-021-22068-5
