Edited by Jeffrey E. Pessin Alterations in endoplasmic reticulum (ER) calcium (Ca 2؉) levels diminish insulin secretion and reduce ␤-cell survival in both major forms of diabetes. The mechanisms responsible for ER Ca 2؉ loss in ␤ cells remain incompletely understood. Moreover, a specific role for either ryanodine receptor (RyR) or inositol 1,4,5-triphosphate receptor (IP 3 R) dysfunction in the pathophysiology of diabetes remains largely untested. To this end, here we applied intracellular and ER Ca 2؉ imaging techniques in INS-1 ␤ cells and isolated islets to determine whether diabetogenic stressors alter RyR or IP 3 R function. Our results revealed that the RyR is sensitive mainly to ER stress-induced dysfunction, whereas cytokine stress specifically alters IP 3 R activity. Consistent with this observation, pharmacological inhibition of the RyR with ryanodine and inhibition of the IP 3 R with xestospongin C prevented ER Ca 2؉ loss under ER and cytokine stress conditions, respectively. However, RyR blockade distinctly prevented ␤-cell death, propagation of the unfolded protein response (UPR), and dysfunctional glucose-induced Ca 2؉ oscillations in tunicamycin-treated INS-1 ␤ cells and mouse islets and Akita islets. Monitoring at the single-cell level revealed that ER stress acutely increases the frequency of intracellular Ca 2؉ transients that depend on both ER Ca 2؉ leakage from the RyR and plasma membrane depolarization. Collectively, these findings indicate that RyR dysfunction shapes ER Ca 2؉ dynamics in ␤ cells and regulates both UPR activation and cell death, suggesting that RyR-mediated loss of ER Ca 2؉ may be an early pathogenic event in diabetes. This work was supported by National Institutes of Health Grants R01 DK093954 and UC4 DK 104166 (to C. E-M.); Department of Veterans Affairs Merit Award I01BX001733 (to C. E-M.); and Sigma Beta Sorority, Ball Brothers Foundation, and George and Frances Ball Foundation gifts (to C. E-M.).

The phosphatase Rtr1 has been implicated in dephosphorylation of the RNA Polymerase II (RNAPII) C-terminal domain (CTD) during transcription elongation and in regulation of nuclear import of RNAPII. Although it has been shown that Rtr1 interacts with RNAPII in yeast and humans, the specific mechanisms that underlie Rtr1 recruitment to RNAPII have not been elucidated. To address this, we have performed an in-depth proteomic analysis of Rtr1 interacting proteins in yeast. Our studies revealed that hyperphosphorylated RNAPII is the primary interacting partner for Rtr1. To extend these findings, we performed quantitative proteomic analyses of Rtr1 interactions in yeast strains deleted for CTK1, the gene encoding the catalytic subunit of the CTD kinase I (CTDK-I) complex. Interestingly, we found that the interaction between Rtr1 and RNAPII is decreased in ctk1Δ strains. We hypothesize that serine-2 CTD phosphorylation is required for Rtr1 recruitment to RNAPII during transcription elongation.