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Objective Elevations in pancreatic α-cell intracellular Ca2+ ([Ca2+]i) lead to glucagon (GCG) secretion. Although glucose inhibits GCG secretion, how lactate and pyruvate control α-cell Ca2+ handling is unknown. Lactate enters cells through monocarboxylate transporters (MCTs) and is also produced during glycolysis by lactate dehydrogenase A (LDHA), an enzyme expressed in α-cells. As lactate activates ATP-sensitive K+ (KATP) channels in cardiomyocytes, lactate may also modulate α-cell KATP. Therefore, this study investigated how lactate signaling controls α-cell Ca2+ handling and GCG secretion. Methods Mouse and human islets were used in combination with confocal microscopy, electrophysiology, GCG immunoassays, and fluorescent thallium flux assays to assess α-cell Ca2+ handling, Vm, KATP currents, and GCG secretion. Results Lactate-inhibited mouse (75 ± 25%) and human (47 ± 9%) α-cell [Ca2+]i fluctuations only under low-glucose conditions (1 mM) but had no effect on β- or δ-cells [Ca2+]i. Glyburide inhibition of KATP channels restored α-cell [Ca2+]i fluctuations in the presence of lactate. Lactate transport into α-cells via MCTs hyperpolarized mouse (14 ± 1 mV) and human (12 ± 1 mV) α-cell Vm and activated KATP channels. Interestingly, pyruvate showed a similar KATP activation profile and α-cell [Ca2+]i inhibition as lactate. Lactate-induced inhibition of α-cell [Ca2+]i influx resulted in reduced GCG secretion in mouse (62 ± 6%) and human (43 ± 13%) islets. Conclusions These data demonstrate for the first time that lactate entry into α-cells through MCTs results in KATP activation, Vm hyperpolarization, reduced [Ca2+]i, and inhibition of GCG secretion. Thus, taken together, these data indicate that lactate either within α-cells and/or elevated in serum could serve as important modulators of α-cell function., Graphical abstract Image 1, Highlights • Lactate reduces islet α-cell Ca2+ entry under low glucose conditions. • Lactate does not alter β- or δ-cell Ca2+ handling under low glucose conditions. • Lactate enters islet α-cells through monocarboxylate transporters. • Lactate hyperpolarizes islet α-cell membrane potential by activating KATP channels. • Lactate reduces mouse and human islet glucagon secretion.

Key points Tetraspanin (TSPAN) proteins regulate many biological processes, including intracellular calcium (Ca2+ ) handling. TSPAN-7 is enriched in pancreatic islet cells; however, the function of islet TSPAN-7 has not been identified. Here, we characterize how β-cell TSPAN-7 regulates Ca2+ handling and hormone secretion. We find that TSPAN-7 reduces β-cell glucose-stimulated Ca2+ entry, slows Ca2+ oscillation frequency and decreases glucose-stimulated insulin secretion. TSPAN-7 controls β-cell function through a direct interaction with L-type voltage-dependent Ca2+ channels (CaV 1.2 and CaV 1.3), which reduces channel Ca2+ conductance. TSPAN-7 slows activation of CaV 1.2 and accelerates recovery from voltage-dependent inactivation; TSPAN-7 also slows CaV 1.3 inactivation kinetics. These findings strongly implicate TSPAN-7 as a key regulator in determining the set-point of glucose-stimulated Ca2+ influx and insulin secretion. Abstract Glucose-stimulated insulin secretion (GSIS) is regulated by calcium (Ca2+ ) entry into pancreatic β-cells through voltage-dependent Ca2+ (CaV ) channels. Tetraspanin (TSPAN) transmembrane proteins control Ca2+ handling, and thus they may also modulate GSIS. TSPAN-7 is the most abundant islet TSPAN and immunostaining of mouse and human pancreatic slices shows that TSPAN-7 is highly expressed in β- and α-cells; however, the function of islet TSPAN-7 has not been determined. Here, we show that TSPAN-7 knockdown (KD) increases glucose-stimulated Ca2+ influx into mouse and human β-cells. Additionally, mouse β-cell Ca2+ oscillation frequency was accelerated by TSPAN-7 KD. Because TSPAN-7 KD also enhanced Ca2+ entry when membrane potential was clamped with depolarization, the effect of TSPAN-7 on CaV channel activity was examined. TSPAN-7 KD enhanced L-type CaV currents in mouse and human β-cells. Conversely, heterologous expression of TSPAN-7 with CaV 1.2 and CaV 1.3 L-type CaV channels decreased CaV currents and reduced Ca2+ influx through both channels. This was presumably the result of a direct interaction of TSPAN-7 and L-type CaV channels because TSPAN-7 coimmunoprecipitated with both CaV 1.2 and CaV 1.3 from primary human β-cells and from a heterologous expression system. Finally, TSPAN-7 KD in human β-cells increased basal (5.6 mM glucose) and stimulated (45 mM KCl + 14 mM glucose) insulin secretion. These findings strongly suggest that TSPAN-7 modulation of β-cell L-type CaV channels is a key determinant of β-cell glucose-stimulated Ca2+ entry and thus the set-point of GSIS.

Swi-independent 3a and 3b (Sin3a and Sin3b) are paralogous transcriptional coregulators that direct cellular differentiation, survival, and function. Here, we report that mouse Sin3a and Sin3b are coproduced in most pancreatic cells during embryogenesis but become much more enriched in endocrine cells in adults, implying continued essential roles in mature endocrine cell function. Mice with loss of Sin3a in endocrine progenitors were normal during early postnatal stages but gradually developed diabetes before weaning. These physiological defects were preceded by the compromised survival, insulin-vesicle packaging, insulin secretion, and nutrient-induced Ca(2+) influx of Sin3a-deficient β-cells. RNA sequencing coupled with candidate chromatin immunoprecipitation assays revealed several genes that could be directly regulated by Sin3a in β-cells, which modulate Ca(2+)/ion transport, cell survival, vesicle/membrane trafficking, glucose metabolism, and stress responses. Finally, mice with loss of both Sin3a and Sin3b in multipotent embryonic pancreatic progenitors had significantly reduced islet cell mass at birth, caused by decreased endocrine progenitor production and increased β-cell death. These findings highlight the stage-specific requirements for the presumed “general” coregulators Sin3a and Sin3b in islet β-cells, with Sin3a being dispensable for differentiation but required for postnatal function and survival.

During Glucose-Stimulated Insulin Secretion (GSIS), β-cells exhibit oscillations in cytosolic Ca2+ ([Ca2+]c) and endoplasmic reticulum (ER) Ca2+ ([Ca2+]ER), which are regulated by the two-pore domain K+ channel, TALK-1. TALK-1 is the most abundant β-cell K+ channel and it controls Ca2+ homeostasis by modulating K+ permeability at both the plasma and ER membrane. However, how TALK-1 modulates ER stress and β-cell failure during the pathogenesis of diabetes has not been determined. To understand how TALK-1 controls β-cell function under diabetic conditions, we assessed how a mutation in TALK-1 (ND) which causes neonatal diabetes, impacts TALK-1 channel activity and [Ca2+]ER handling. While plasmallemal TALK-1 activity was inhibited with the ND mutation, [Ca2+]ER storage was significantly reduced by the ND mutation compared to control TALK-1. This suggests that TALK-1 ND enhances TALK-1 activity on the ER membrane leading to reduced [Ca2+]ER storage. Reducing [Ca2+]ER enhances ER stress, which would be expected to cause β-cell dysfunction. Indeed, TALK-1 ND significantly reduces β-cell insulin secretion compared to control. To further investigate the role of TALK-1 we developed a β-cell-specific TALK-1 KO mouse model (β-TALK1-KO). On a high fat diet, β-TALK1-KO mice show improved glucose tolerance compared to controls. Mitochondrial calcium ([Ca2+]mito) was also examined because it is modulated by [Ca2+]ER; we found that β-TALK1-KO mice have increased [Ca2+]mito. As reactive oxygen species (ROS) production is a byproduct of mitochondrial respiration, we investigated if TALK-1 affects ROS production. TALK1 KO islets were found to have reduced ROS production compared to wild type islets. Together, these data indicate that increased TALK-1 channel activity reduces [Ca2+]ER, [Ca2+]mito, and increases ROS; suggesting that TALK-1 plays an important role in modulating the β-cell responses to stress associated with diabetic conditions. Disclosure S.M. Graff: None. P. Dadi: None. C.E. Ibsen: None. M. Dickerson: None. K.L. Jordan: None. D. Jacobson: None.