Bearing in mind the central dogma of molecular biology `DNA makes RNA makes protein' we know that most RNA molecules are involved in communicating genetic information from the nucleus to the translational machinery in the cytoplasm. More recently, however, it has become evident that RNAs can do far more, in particular the so-called microRNAs, which are small RNAs that comprise only 21–23 nucleotides. MicroRNAs do not encode proteins themselves but bind to complementary target RNAs that do, and thereby interfere with ribosomal translation to make proteins. Hundreds of microRNAs have been identified in plants and animals so far, but their specific functions remain largely unknown. Now, a team of scientists from Rockefeller University led by Markus Stoffel has discovered a novel microRNA function: regulating insulin secretion by pancreatic islet beta cells.
Since a few studies in invertebrates have revealed that specific microRNAs have important regulatory functions, Stoffel and co-workers wondered whether microRNAs have comparable functions in mammals. To address this question the team studied mammalian pancreatic beta cells, which secrete the blood-sugar-regulating hormone insulin in response to elevated glucose levels. The team cloned more than 60 microRNAs from a pancreatic beta cell line. Some microRNAs were highly specific to beta cells, and the function of the most abundant beta-cell-specific microRNA, miR-375, aroused the team's interest.
To study the in vivo function of miR-375, they over-expressed and inhibited endogenous miR-375 in mammalian pancreatic cells and observed stunning effects on glucose-stimulated insulin secretion. Over-expression of miR-375 decreased insulin secretion, while inhibition of miR-375 enhanced insulin secretion, prompting the team to investigate the underlying molecular mechanism.
Stoffel and co-workers designed an elegant series of experiments to discover where miR-375 acts in the insulin signaling cascade. Using an adenoviral system to express miR-375 in pancreatic cells, they observed that miR-375 inhibits insulin secretion regardless of intracellular ATP and Ca2+ concentrations, both early signals of insulin secretion. In the end, the team determined that miR-375 affects exocytosis, the final step in insulin secretion during which insulin-containing vesicles fuse with the pancreatic beta cells' outer membrane to release their content.
But how does miR-375 affect exocytosis and what RNA does it target? To identify RNA targets of miR-375, the Rockefeller group used computer programs to search for consecutively matching base pairs between the microRNA and putative target RNAs. This analysis pinpointed myotrophin, a protein previously shown to be involved in neuronal secretion. Stoffel and co-workers discovered that knocking out myotrophin by RNA interference inhibits insulin secretion, indicating that regulation of myotrophin expression is the mechanism by which miR-375 affects exocytosis in pancreatic islet beta cells.
Stoffel and his team present an array of convincing data supporting the notion that miR-375 regulates insulin secretion by regulating myotrophin levels and influencing the response of pancreatic islet beta cells to glucose. Although the molecular details of the miR-375/myotrophin axis and possible roles of miR-375 in physiological feedback loops remain to be elucidated,miR-375 is a promising target of new strategies in the treatment of diabetes.
