The somatostatin receptor in human pancreatic β-cells

The peptide hormone somatostatin (SST) is produced in the brain, the gut, and in δ-cells in pancreatic islets of Langerhans. SST secretion from δ-cells is stimulated by glucose, amino acids, and glucagon-like peptide-1. Exogenous SST strongly inhibits the secretion of the blood glucose-regulating hormones insulin and glucagon from pancreatic β-cells and α-cells, respectively. Endogenous SST secreted from δ-cells is a paracrine regulator of insulin and glucagon secretion, although the exact physiological significance of this regulation is unclear. Secreted SST binds to specific receptors (SSTRs), which are coupled to Gi/o proteins. In both β- and α-cells, activation of SSTRs suppresses hormone secretion by reducing cAMP levels, inhibiting electrical activity, decreasing Ca²⁺ influx through voltage-gated Ca²⁺ channels and directly reducing exocytosis in a Ca²⁺ and cAMP-independent manner. In rodents, β-cells express predominantly SSTR5, whereas α-cells express SSTR2. In human islets, SSTR2 is the dominant receptor in both β- and α-cells, but other isoforms also contribute to the SST effects. Evidence from rodent models suggests that SST secretion from δ-cells is dysregulated in diabetes mellitus, which may contribute to the metabolic disturbances in this disease. SST analogues are currently used for the treatment of hyperinsulinism and other endocrine disorders, including acromegaly and Cushing's syndrome.

Pancreatic endocrine cell arrangement during human ontogeny

In the human pancreas, various forms of endocrine cell arrangement are found: single endocrine cells, endocrine cell clusters, and mantel, bipolar and mosaic cell (mixed) islets. Our aim was to analyse the distribution and dynamics of insulin-, glucagon- and somatostatin-containing cells within the various forms of endocrine pancreas arrangement during human prenatal development and in adults and to suggest a mechanism of change in the endocrine cell ratio in adult islets. Pancreatic autopsies derived from human foetuses from the 10^th to the 40^th weeks of development and from adults were examined using histological, immunohistochemical and morphometric methods. During development, the human endocrine pancreas undergoes not only de novo differentiation of endocrine cells and islet formation, but morphogenetic restructuring, which is revealed as a change of the α-, β- and δ-cell ratio in the islets. In particular, increased proportion of glucagon- and somatostatin-containing cells and decreased proportion of β-cells were shown in the largest mosaic islets in adults. Our results indicate that the distribution and proportion of α-, β- and δ-cells depend on the islets size and vascularisation. Studying of the mechanism of such restructuring may contribute to the development of new approaches in the treatment of diabetes mellitus.

Dnmt1 activity is dispensable in δ-cells but is essential for α-cell homeostasis

In addition to β-cells, pancreatic islets contain α- and δ-cells, which respectively produce glucagon and somatostatin. The reprogramming of these two endocrine cell types into insulin producers, as observed after a massive β-cell ablation in mice, may help restoring a functional β-cell mass in type 1 diabetes. Yet, the spontaneous α-to-β and δ-to-β conversion processes are relatively inefficient in adult animals and the underlying epigenetic mechanisms remain unclear. Several studies indicate that the conserved chromatin modifiers DNA methyltransferase 1 (Dnmt1) and Enhancer of zeste homolog 2 (Ezh2) are important for pancreas development and restrict islet cell plasticity. Here, to investigate the role of these two enzymes in α- and δ-cell development and fate maintenance, we genetically inactivated them in each of these two cell types. We found that loss of Dnmt1 does not enhance the conversion of α- or δ-cells toward a β-like fate. In addition, while Dnmt1 was dispensable for the development of these two cell types, we noticed a gradual loss of α-, but not δ-cells in adult mice. Finally, we found that Ezh2 inactivation does not enhance α-cell plasticity, and, contrary to what is observed in β-cells, does not impair α-cell proliferation. Our results indicate that both Dnmt1 and Ezh2 play distinct roles in the different islet cell types.