Mechanisms of type 1 diabetic serum-induced hyperactivation of CaV1 channels in the pancreatic β cell

The pancreatic β cell relies on appropriate Ca2+ entry through voltage-gated calcium (CaV) channels to accomplish its unique function insulin secretion and to guarantee its viability. Well-regulated β cell CaV channels are critical to ensure adequate functional β cell mass, thereby maintaining adequate insulin release and glucose homeostasis in the body. When β cell CaV channels mediate insufficient or excessive Ca2+ influx due to either inherited or acquired defects, β cell becomes malfunctioning and even dies. Type 1 diabetic (T1D) serum hyperactivates β cell CaV1 channels driving Ca2+-dependent β cell apoptosis via previously unappreciated mechanisms. The present PhD work has mechanistically dissected T1D serum-induced hyperactivation of CaV1 channels in the β cell by combining patchclamp techniques, confocal microscopy, as well as molecular and cellular approaches. It reveals the following findings:

Functional CaV1.3 channels reside in 20 % of mouse islet CaV1.2-/- β cells. They characteristically show a large unitary Ba2+ conductance with long-lasting openings in plasma membrane patches of islet cells endowed with undetectable voltage-gated Na+ currents, larger cell capacitance (> 7 pF) and insulin mRNA. These observations pinpoint β cell-specific CaV1.2-/- mice as a convenient small animal model for investigation of human β cell CaV1.3 channel-related disorders such as T1D serum-induced hyperactivation of β cell CaV1.3 channels.

T1D serum hyperactivates both CaV1.2 and CaV1.3 channels by elevating their conductivity and number in the β cell plasma membrane. This finding emphasizes that both CaV1.2 and CaV1.3 channels are potential druggable targets for prevention of Ca2+ overload-induced β cell death.

Apolipoprotein CIII (ApoCIII) in T1D serum is electrophysiologically validated to be the actual factor enhancing CaV channel currents in the β cell. This validation opens up the possibility to deplete or neutralize ApoCIII in T1D serum for medical intervention of CaV channel hyperactivation-driven β cell destruction.

ApoCIII activates both PKA and Src kinase in a scavenger receptor class B type I/β1 integrin-dependent fashion to selectively hyperactivate β cell CaV1 channels without altering β cell CaV1 channel expression. ApoCIII-induced hyperactivation of β cell CaV1 channels results from the enriched density and increased activity of functional CaV1 channels in the β cell plasma membrane. This newly-identified signaling pathway shows great potential as a set of novel druggable targets for prevention of Ca2+-dependent β cell death in association with diabetes.

The key endocytic protein syndapin I/PACSIN 1 (PCS1) is richly expressed in β cells to govern endocytic activity. PCS1-mediated endocytosis acts as a homeostatic control system to fine-tune the CaV1 channel density in the β cell plasma membrane. These findings add a new layer of complexity to the mechanisms of β cell CaV1 channel regulation.

ApoCIII impairs both constitutive and regulated β cell endocytosis with no influence on PCS1 expression. Consequently, ApoCIII abrogates PCS1-dependent endocytic trafficking, thereby accumulating excessive CaV1 channels in the β cell plasma membrane. These results delineate a novel mechanism of Ca2+-dependent β cell destruction in diabetes development and reveal a promising and attractive option to counteract the critical diabetogenic process of Ca2+-dependent β cell death.

Overall, the aforementioned findings depict a mechanistic picture of how ApoCIII renders CaV1 channels highly enriched and excessively activated in the β cell plasma membrane, thereby resulting in pathologically exaggerated Ca2+ influx and Ca2+-dependent β death. These findings lay the foundation for novel treatment strategies for diabetes.

List of scientific papers

I. Guang Yang, Yue Shi, Jia Yu, Yuxin Li, Lina Yu, Andrea Welling, Franz Hofmann, Jörg Striessnig, Lisa Juntti-Berggren, Per-Olof Berggren, Shao-Nian Yang (2015) CaV1.2 and CaV1.3 channel hyperactivation in mouse islet β cells exposed to type 1 diabetic serum. Cell Mol Life Sci 72: 1197-1207.
https://doi.org/10.1007/s00018-014-1737-6

II. Yue Shi, Guang Yang, Jia Yu, Lina Yu, Ruth Westenbroek, William A. Catterall, Lisa Juntti-Berggren, Per-Olof Berggren, Shao-Nian Yang (2014) Apolipoprotein CIII hyperactivates β cell CaV1 channels through SR-BI/β1 integrin-dependent coactivation of PKA and Src. Cell Mol Life Sci 71: 1289-1303.
https://doi.org/10.1007/s00018-013-1442-x

III. Yue Shi, Guang Yang, Jia Yu, Yuxin Li, Britta Qualmann, Michael M. Kessels, Lina Yu, Per-Olof Berggren, Shao-Nian Yang (2015) Inositol hexakisphosphate primes syndapin I/PACSIN 1 activation in endocytosis. [Manuscript]

IV. Yue Shi, Guang Yang, Jia Yu, Lina Yu, Lisa Juntti-Berggren, Per-Olof Berggren, Shao-Nian Yang (2015) Apolipoprotein CIII accumulates CaV1 channels in the β cell plasma membrane via abrogation of syndapin I/PACSIN 1-dependent endocytic trafficking. [Manuscript]