Calcium modulation of cardiac sodium channels

Abstract

Modification of voltage‐gated Na⁺ channel (Na_V) function by intracellular calcium ([Ca²⁺]_i) has been a topic of much controversy. Early studies relied on measuring Na_V function in the absence or presence of [Ca²⁺]_i, and generated seemingly disparate results. Subsequent investigations revealed the mechanism(s) of [Ca²⁺]_i driven Na_V modulation are complex and involve multiple accessory proteins. The Ca²⁺ sensing protein Calmodulin (CaM) has a central role in tuning Na_V function to [Ca²⁺]_i, but the mechanism has been obscured by other proteins (such as Fibroblast Growth Factors (FGF) or CaM dependent Kinase II (CaMKII)) as they can also modify channel function, or exert an influence in a Ca²⁺ dependent manner. Significant progress has been made in understanding the architecture of full‐length ion channels and the structural and biophysical details of Na_V‐accessory protein interactions. Interdisciplinary structure‐function studies are beginning to resolve the effect each interaction has on Na_V gating. Carefully designed structure‐guided or strategically selected disease‐associated mutations are able to impair Na_V‐accessory protein interactions without altering other properties of channel function. Recently CaM was found to engage part of Na_V1.5 that is required for channel inactivation with high affinity. Careful impairment of this interaction disrupted Na_V1.5's ability to recover from inactivation. Such results support a paradigm of CaM Facilitated Recovery from Inactivation (CFRI). Understanding how Na_V‐CaM, CaMKII, and FGF/FHF interactions affect the timing or function of CFRI in cardiomyocytes remain open questions that are discussed herein. Moreover if CFRI dysfunction or premature activation underlie certain Na_V channelopathies are important questions that will require further investigation.

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