Aims/hypothesis The pathogenesis of diabetes and the success of islet transplantation depend on the control of pancreatic beta cell fate.

No abstract

Pancreatic ␤-cell death is a critical event in type 1 diabetes, type 2 diabetes, and clinical islet transplantation. We have previously shown that prolonged block of ryanodine receptor (RyR)-gated release from intracellular Ca 2؉ stores activates calpain-10-dependent apoptosis in ␤-cells. In the present study, we further characterized intracellular Ca 2؉ channel expression and function in human islets and the MIN6 ␤-cell line. All three RyR isoforms were identified in human islets and MIN6 cells, and these endoplasmic reticulum channels were observed in close proximity to mitochondria. Blocking RyR channels, but not sarco/endoplasmic reticulum ATPase (SERCA) pumps, reduced the ATP/ADP ratio. Blocking Ca 2؉ flux through RyR or inositol trisphosphate receptor channels, but not SERCA pumps, increased the expression of hypoxia-inducible factor (HIF-1␤). Moreover, inhibition of RyR or inositol trisphosphate receptor channels, but not SERCA pumps, increased the expression of presenilin-1. Both HIF-1␤ and presenilin-1 expression were also induced by low glucose. Overexpression of presenilin-1 increased HIF-1␤, suggesting that HIF is downstream of presenilin. Our results provide the first evidence of a presenilin-HIF signaling network in ␤-cells. We demonstrate that this pathway is controlled by Ca 2؉ flux through intracellular channels, likely via changes in mitochondrial metabolism and ATP. These findings provide a mechanistic understanding of the signaling pathways activated when intracellular Ca 2؉ homeostasis and metabolic activity are suppressed in diabetes and islet transplantation.

The Swi/Snf chromatin remodeling complex has been previously demonstrated to be required for transcriptional activation and repression of a subset of genes in Saccharomyces cerevisiae. In this work we demonstrate that Swi/Snf is also required for repression of RNA polymerase II-dependent transcription in the ribosomal DNA (rDNA) locus (rDNA silencing). This repression appears to be independent of both Sir2 and Set1, two factors known to be required for rDNA silencing. In contrast to many other rDNA silencing mutants that have elevated levels of rDNA recombination, snf2⌬ mutants have a significantly decreased level of rDNA recombination. Additional studies have demonstrated that Swi/Snf is also required for silencing of genes near telomeres while having no detectable effect on silencing of HML or HMR.