Title: Elucidating Regulatory Mechanisms of Cardiac CaV1.2 and NaV1.5 Channels
Description: In the heart, sodium (Na+) influx via NaV1.5 channels initiates the action potential, and calcium (Ca2+) influx via CaV1.2 channels has a key role in excitation-contraction coupling and determining the plateau phase of the action potential. Mutations in the genes that encode these ion channels or in proteins that modulate them are linked to arrhythmias and cardiomyopathy, underscoring the need for characterizing mechanisms of regulation. The work presented in this thesis is subdivided into three different chapters, each with a distinct focus on ion channel modulation. The first chapter details our investigation of the functional PKA phosphorylation target for β-adrenergic regulation of CaV1.2. Physiologic β-adrenergic activation of PKA during the sympathetic “fight or flight” response increases Ca2+ influx through CaV1.2 in cardiomyocytes, leading to increased cardiac contractility. The molecular mechanisms of β-adrenergic regulation of CaV1.2 in cardiomyocytes are incompletely known, but activation of PKA is required for this process.

Recent data suggest that β-adrenergic regulation of CaV1.2 does not require any combination of PKA phosphorylation sites conserved in human, guinea pig, rabbit, rat, and mouse α1C subunits. To test if any non-conserved sites are required for regulation, we generated mice with inducible cardiac-specific expression of α1C with mutations at both conserved and non- conserved predicted PKA phosphorylation sites (35-mutant α1C). Additionally, we createdanother mouse with inducible cardiac-specific expression of β2 with mutations at predicted PKA phosphorylation sites (28-mutant β2B). In each of these mice, β-adrenergic stimulation of Ca²⁺ current was unperturbed. Finally, to test the hypothesis that redundant functional PKA phosphorylation sites exist on the α1C subunit and β2 subunit or that several sites confer incremental regulation, we crossed the 35-mutant α1C mice with the 28-mutant β2B mice to generate offspring expressing both mutant subunits. In these offspring, intact regulation was observed.

These results provide the definitive answer that phosphorylation of the α1C subunit or β2 subunit is not required for β-adrenergic regulation of CaV1.2 in the heart. In the second chapter, we study the influence of calmodulin and fibroblast growth homologous factor (FHF) FGF13 on late Na+ current. Studies in heterologous expression systems show that the Ca²⁺-binding protein calmodulin plays a key role in decreasing late Na⁺ current. The effect of loss of calmodulin binding to NaV1.5 on late Na+ current has yet to be resolved in native cardiomyocytes. We created transgenic mice with cardiac-specific expression of human NaV1.5 channels with alanine substitutions for the IQ motif (IQ/AA), disrupting calmodulin binding to the C-terminus. Surprisingly, we found that the IQ/AA mutation did not cause an increase late Na⁺ current in cardiomyocytes. These findings suggest the existence of endogenous protective mechanisms that counteract the increase in late Na+ current that occurs with loss of calmodulin binding.

We reasoned that FGF13, a known modulator of late Na+ current that is endogenously expressed in cardiomyocytes but not HEK cells, might play a protective role in limiting late Na+ current. Finally, we coexpressed the IQ/AA mutant NaV1.5 channel in HEK293 cells with FGF13 and found that FGF13 diminished the late Na⁺ currentcompared to cells without FGF13, suggesting that endogenous FHFs may serve to prevent late Na⁺ current in mouse cardiomyocytes. The third chapter of this thesis focuses on the use of proximity labeling and multiplexed quantitative proteomics to define changes in the NaV1.5 macromolecular complex in Duchenne muscular dystrophy (DMD), in which the absence of dystrophin predisposes affected individuals to arrhythmias and cardiac dysfunction.. Standard methods to characterize macromolecular complexes have relied on candidate immunoprecipitation or immunocytochemistry techniques that fall short of providing a comprehensive view of the numbers and types of interactors, as well as the potential dynamic nature of the interactions that may be perturbed by disease states.

To provide an inclusive understanding of NaV1.5 macromolecular complexes, we utilize live-cell APEX2 proximity labeling in cardiomyocytes. We identify several proximal changes that align with the electrophysiological NaV1.5 phenotype of young dystrophin-deficient mice, including a decrease in Ptpn3 and Gdp1l and an increase in proteasomal machinery. Whole-cell protein expression fold-change results were used to reveal the altered global expression profile and to place context behind NaV1.5-proximal changes. Finally, we leveraged the neighborhood- specificity of proteins at the lateral membrane, intercalated disc, and transverse tubules of cardiomyocytes to demonstrate that NaV1.5 channels can traffic to all three membrane compartments even in the absence of dystrophin. Thus, the approach of proximity labeling in cardiomyocytes from an animal model of human disease offers new insights into molecular mechanisms of NaV1.5 dysfunction in DMD and provides a template for similar investigations in other cardiac diseases.