Functional tuning of Vascular L-type Ca2+ channels

Abstract

Vascular smooth muscle contraction is intimately tied to membrane potential and the rise in intracellular Ca²⁺ enabled by the opening of L-type Ca²⁺ channels. While voltage is often viewed as the single critical factor gating these channels, research is starting to reveal a more intricate scenario whereby their function is markedly tuned. This emerging concept will be the focus of this three-part review, the first part articulating the mechanistic foundation of contractile development in vascular smooth muscle. Part two will extend this foundational knowledge, introducing readers to functional coupling and how neighboring L-type Ca²⁺ channels work cooperatively through signaling protein complexes, to facilitate their open probability. The final aspect of this review will discuss the impact of L-type Ca²⁺ channel trafficking, a process tied to cytoskeleton dynamics. Cumulatively, this brief manuscript provides new insight into how voltage, along with channel cooperativity and number, work in concert to tune Ca²⁺ responses and smooth muscle contraction.

Keywords: Ca2+ sensitization; L-type Ca2+ channels; channel trafficking; cooperative gating; functional coupling; vascular smooth muscle.

FIGURE 1

Mechanisms of cross-bridge cycling and MLC20 phosphorylation in vascular smooth muscle. Mechanosensor/receptor activation induces depolarization by activating transient receptor potential channels and inhibiting hyperpolarizing K+ currents. Depolarization opens L-type Ca2+ channels, with the resulting influx of Ca2+ enhancing MLCK and cross-bridge cycling through binding to CaM. Conversely, G-protein coupled receptors further enhance contraction by inhibiting MLCP (aka-Ca2+ sensitization) through two regulatory pathways. The first entails PKC activation by PLC-β and DAG, a series/threonine kinase that phosphorylates: 1) CPI-17 to inhibit PP1c; or 2) target proteins that set actin stabilization/polymerization. The second involves G12-13 activation of RhoA and Rho-kinase, the latter of which controls the phosphorylation state of: 1) MYPT1 (Threonine–696 and Threonine–853 (mouse numberings)); or 2) target proteins that set actin polymerization. Note, Ca2+ release from the sarcoplasmic reticulum, resulting from receptor-induced IP3 production, can also contribute to MLCK activation. Abbreviations: MLCK, Myosin light chain kinase; MLCP, Myosin light chain phosphatase, PLC, Phospholipase C; DAG, Diacylglycerol; CPI-17, Cytosolic phosphatase inhibitory protein of 17-kDa; IP3, Inositol trisphosphate; MYPT1, Myosin light chain phosphatase targeting subunit 1; MLC, Myosin regulatory light chain; CaM, Calmodulin; PKC, Protein Kinase C; PP1c, Type 1 protein phosphatase 1c.

FIGURE 2

Cooperative gating and trafficking of L-type Ca²⁺ channels. (A) L-type Ca²⁺ channels can work as independent channels or in a cooperative gating manner due to proximate localization and the formation of connections at the Ca_V1.2 C-terminus. Cooperatively gated channels exhibit functional coupling properties, such as synchronous opening, higher Po, and more persistent current compared to individually gated channels. AKAP5 (mouse numbering), PKC, and PKA play a critical role in the promotion of functional coupling properties. (B) Depolarization enhances the trafficking of L-type Ca²⁺ channel subunits to the plasma membrane. This key biological process entails the movement of vesicles from the Golgi apparatus to caveolae strategically positioned on the plasma membrane. Abbreviations: AKAP5, A-kinase anchoring protein -5; PKA, Protein kinase A; PKC, Protein kinase C; MLCK, Myosin light chain kinase; MLCP, Myosin light chain phosphatase.