UC Davis

CaV1.2 channels are critical players in cardiac excitation-contraction coupling, yet we do not understand how they are affected by an important therapeutic target of heart failure drugs and regulator of blood pressure, angiotensin II. Signaling through Gq-coupled AT1 receptors, angiotensin II triggers a decrease in PIP2, a phosphoinositide component of the plasma membrane and known regulator of many ion channels. PIP2 depletion suppresses CaV1.2 currents in HEK cells but the mechanism of this regulation and whether a similar phenomenon occurs in cardiomyocytes is unknown. Previous studies have shown that CaV1.2 currents are also suppressed by angiotensin II. This dissertation sought to understand the connection between these two observations: the role of AngII-mediated PIP2 depletion in the regulation CaV1.2 and the effect on cardiac excitability in physiological conditions and in disease. We used a combination of cutting-edge microscopy, electrophysiology, biochemistry, and lipid mass spectrometry techniques to test the hypothesis that PIP2 stabilizes CaV1.2 expression at the plasma membrane and angiotensin II depresses cardiac excitability by stimulating PIP2 depletion and destabilization of CaV1.2 expression in acute and chronic conditions. We found that acute AngII signaling depletes PIP2 and attenuates ICa and EC-coupling by inducing dynamin-dependent CaV1.2 internalization. Chronic AngII infusion leads to a sustained deficit in PIP2 and CaV1.2 levels, but compensatory PKA and CaMKII phosphorylation of CaV1.2 and RyR2 sustained functional output. Finally, we used a rabbit heart failure model to test how phosphoinositide levels are altered in disease. We found in this model that PIP2 levels significantly increase, but that they correspond with a decrease in circulating AngII levels. Together, the data in this dissertation demonstrate an essential role of PIP2 in the regulation of CaV1.2. It supports a model wherein acute AngII signaling fine-tunes the level of PIP2 to modulate EC-coupling, but that with chronic AngII and in heart failure that tuning mechanism can become maladaptive. These data could inform much future study on the role of phosphoinositides in ion channel regulation, and in disease progression.