Exploring Maturation of Pancreatic β-cells
The β-cells of the pancreatic Islets of Langerhans play a major role in glucose homeostasis through their production of insulin, and they change in their ability to release insulin in response to glucose during development. Fetal and neonatal β-cells secrete more insulin basally and in response to low glucose concentrations than mature β-cells, but they respond poorly to a high glucose stimulus. This led Vanderbilt Basic Sciences investigators Guoqiang Gu, David Jacobson, and Roland Stein to investigate mechanisms that regulate β-cell maturation. A Ca2+ signal is required for insulin release, so the investigators first examined the ability of immature mouse β-cells to release insulin in response to KCl-mediated membrane depolarization, which leads to an influx of Ca2+. They found that immature β-cells actually released more insulin under these conditions than mature β-cells even though the mature cells contained more insulin prior to the stimulus and exhibited a higher Ca2+ increase. These findings suggested that immature β-cells are more sensitive to Ca2+ than mature cells. RNA-seq analysis of mouse β-cells at varying ages revealed a marked increase in expression of Syt4 during the course of maturation. Syt4 encodes the protein synaptotagmin 4 (Syt4), which is known to modulate Ca2+-dependent secretion in some cell types. They used super-resolution structured illumination microscopy to show that Syt4 colocalizes with insulin vesicles in β-cells, supporting the hypothesis that Syt4 plays a role in regulating insulin secretion. To further test the hypothesis, they created a Syt4 knockout mouse model and found that immature β-cells from these mice exhibit higher release of insulin to KCl or low glucose, but no change in the response to high glucose when compared to cells from wild-type mice. In contrast, β-cells from older mice demonstrated a blunted response to high glucose, suggesting a failure to mature properly. The investigators also created a mouse model enabling induced overexpression of Syt4 in β-cells. This resulted in suppressed basal insulin secretion in immature β-cells with no effects on the response to glucose. However, by 10 days after birth, islets from these mice exhibited abnormal morphology, gene expression, and proliferation, and glucose-stimulated insulin secretion was blunted. Overexpression of Syt4 also led to a decrease in the number of insulin-containing vesicles close to the plasma membrane in immature β-cells, whereas the number of these vesicles was increased in the absence of Syt4. The results suggested that Syt4 works by suppressing Ca2+-mediated insulin secretion. In most cells, Syt4 works in concert with another synaptotagmin that promotes Ca2+-mediated secretion. Consistently, the investigators identified Syt7 as the likely protein carrying out this function in β-cells. They concluded that the absence of Syt4 in immature β-cells promotes release of insulin at low glucose and Ca2+ concentrations so that pools of readily releasable insulin are depleted. Increased Syt4 during development suppresses insulin release, enabling the cells to accumulate stores of insulin so that a strong response to high glucose is possible. Studies using human β-cells in culture indicated that Syt4 plays a similar, if not identical role in humans. The work is published in the Developmental Cell [C. Huang, et al. Dev. Cell (2018) published online Apr. 12, DOI: 10.1016/j.devcel.2018.03.013].