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