Enhanced expression of β cell Cav3.1 channels impairs insulin release and glucose homeostasis

Voltage-gated calcium 3.1 (Ca(v)3.1) channels are absent in healthy mouse beta cells and mediate minor T-type Ca2+ currents in healthy rat and human beta cells but become evident under diabetic conditions. Whether more active Ca(v)3.1 channels affect insulin secretion and glucose homeostasis remains enigmatic. We addressed this question by enhancing de novo expression of beta cell Ca(v)3.1 channels and exploring the consequent impacts on dynamic insulin secretion and glucose homeostasis as well as underlying molecular mechanisms with a series of in vitro and in vivo approaches. We now demonstrate that a recombinant adenovirus encoding enhanced green fluorescent protein-Ca(v)3.1 subunit (Ad-EGFP-Ca(v)3.1) efficiently transduced rat and human islets as well as dispersed islet cells. The resulting Ca(v)3.1 channels conducted typical T-type Ca2+ currents, leading to an enhanced basal cytosolic-free Ca2+ concentration ([Ca2+](i)). Ad-EGFP-Ca(v)3.1-transduced islets released significantly less insulin under both the basal and first phases following glucose stimulation and could no longer normalize hyperglycemia in recipient rats rendered diabetic by streptozotocin treatment. Furthermore, Ad-EGFP-Ca(v)3.1 transduction reduced phosphorylated FoxO1 in the cytoplasm of INS-1E cells, elevated FoxO1 nuclear retention, and decreased syntaxin 1A, SNAP-25, and synaptotagmin III. These effects were prevented by inhibiting Ca(v)3.1 channels or the Ca2+ -dependent phosphatase calcineurin. Enhanced expression of beta cell Ca(v)3.1 channels therefore impairs insulin release and glucose homeostasis by means of initial excessive Ca2+ influx, subsequent activation of calcineurin, consequent dephosphorylation and nuclear retention of FoxO1, and eventual FoxO1-mediated down-regulation of beta cell exocytotic proteins. The present work thus suggests an elevated expression of Ca(v)3.1 channels plays a significant role in diabetes pathogenesis.