Molecular Characterization of the Regulatory Subunits of Voltage -Dependent Calcium Channels From Rat Heart
Chu, Po-Ju
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https://hdl.handle.net/2142/87217
Description
Title
Molecular Characterization of the Regulatory Subunits of Voltage -Dependent Calcium Channels From Rat Heart
Author(s)
Chu, Po-Ju
Issue Date
2002
Doctoral Committee Chair(s)
Best, Philip M.
Department of Study
Molecular and Integrative Physiology
Discipline
Molecular and Integrative Physiology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Cell
Language
eng
Abstract
Voltage-dependent Ca2+ channels are multimeric proteins consisting of pore-forming alpha1 and regulatory alpha 2delta, beta, and gamma subunits. The alpha1 subunit is essential for current generation, voltage sensing and drug interaction, whereas the regulatory subunits modulate channel properties including current amplitude, kinetics and voltage-dependence. To date, 10 alpha1, 4 alpha2delta, 4 beta, and 8 gamma genes have been identified. Although the a1 subunits have been relatively well characterized, our understanding of the regulatory subunits is still primitive. By inspecting the genes encoding the regulatory subunits systematically, I have identified expression of 4 beta, 3 alpha2delta and 3 gamma subunits from rat heart. Using phylogenetic analysis, I have studied the mammalian gamma subunit gene family and proposed the following relationship ((((gamma2, gamma 3), (gamma4, gamma8)), (gamma5, gamma 7)), (gamma1, gamma6)) indicating that these gamma subunits evolved from a common ancestral gamma subunit. Interestingly, the gamma 6 subunit, which is highly expressed in cardiac muscle, most closely resembles gamma1 and shares with it the lack of a PDZ-binding motif that is characteristic of most other gamma subunits. Additionally, the expression of gamma1 mRNA and the long isoform of gamma 6 mRNA is most robust in muscle. These results indicate that gamma 1 and gamma6 may share common physiological functions. Among the three alpha2delta subunits, only the alpha 2delta-3 subunit mRNA level is upregulated upon IGF-1 stimulation in atrial myocytes, where IGF-1 also enhances LVA Ca2+ current density. Thus, the expression of cardiac LVA current stimulated by IGF-1 is probably regulated by the expression of alpha2delta-3. Multiple beta subunits and their splice variants are expressed in rat heart. My studies demonstrate that the expression of cardiac beta subunits varies during postnatal development. I have shown that beta4 mRNA expression is higher in atria of young animals than that of adult animals, which parallels the postnatal change of LVA current density in atrial myocytes. These results suggest that the beta4 subunit may be involved in atrial LVA current expression. In summary, this work has demonstrated the complexity and expression variation of cardiac Ca2+ channel regulatory subunits, and provides a detailed molecular foundation to study their modulatory effects on Ca2+ channels.
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