L-type calcium channel targeting and local signalling in cardiac myocytes

Abstract

In the heart, Ca(2+) influx via Ca(V)1.2 L-type calcium channels (LTCCs) is a multi-functional signal that triggers muscle contraction, controls action potential duration, and regulates gene expression. The use of LTCC Ca(2+) as a multi-dimensional signalling molecule in the heart is complicated by several aspects of cardiac physiology. Cytosolic Ca(2+) continuously cycles between ~100 nM and ~1 μM with each heartbeat due to Ca(2+) linked signalling from LTCCs to ryanodine receptors. This rapid cycling raises the question as to how cardiac myocytes distinguish the Ca(2+) fluxes originating through L-type channels that are dedicated to contraction from Ca(2+) fluxes originating from other L-type channels that are used for non-contraction-related signalling. In general, disparate Ca(2+) sources in cardiac myocytes such as current through differently localized LTCCs as well as from IP3 receptors can signal selectively to Ca(2+)-dependent effectors in local microdomains that can be impervious to the cytoplasmic Ca(2+) transients that drive contraction. A particular challenge for diversified signalling via cardiac LTCCs is that they are voltage-gated and, therefore, open and presumably flood their microdomains with Ca(2+) with each action potential. Thus spatial localization of Cav1.2 channels to different types of microdomains of the ventricular cardiomyocyte membrane as well as the existence of particular macromolecular complexes in each Cav1.2 microdomain are important to effect different types of Cav1.2 signalling. In this review we examine aspects of Cav1.2 structure, targeting and signalling in two specialized membrane microdomains--transverse tubules and caveolae.

Figure 1

Schematic of the cardiac Ca_V1.2 LTCC complex. The channel complex consists of a pore-forming α_1C subunit assembled with accessory proteins (β, α₂δ, γ, and calmodulin). The α_1C subunit contains four homologous domains (I–IV), each with six transmembrane segments (S1–S6). The S1–S4 segments comprise the voltage sensor, and the S5–S6 segments from each domain collaborate to form the channel pore and selectivity filter.

Figure 2

Forward trafficking of cardiac ion channels. Post-transcription and translation, cardiac ion channels are packaged into vesicles and transported by the microtubule cytoskeleton to specific sub-domains on the plasma membrane. Specificity of final delivery of connexon 43 channels to gap junctions is due, in part, to the connexon protein, the microtubule plus-end-tracking proteins EB1 and p150(Glued) and the adherens junction complex serving as membrane anchor. Specificity of final delivery of Cav1.2 channels to T-tubules is due, in part, to Cav1.2 protein, and BIN1 serving as membrane anchor. Microtubule plus-end-tracking proteins have not yet been determined for Cav1.2 to T-tubule trafficking. It has also been identified that the actin cytoskeleton slows down forward delivery and is associated with the majority of cytoplasmic Connexin 43 containing vesicles, possibly serving as sorting and reservoirs related rest stops on the way to the plasma membrane. Presumably actin fibres function in a similar capacity for Cav1.2 delivery.

Figure 3

Schematic illustrating proposed dichotomous LTCC targeting and signalling in the heart. Dyadic Ca_V1.2 channels trigger calcium-induced calcium release that leads to contraction, whereas caveolae-localized Ca_V1.2 channels initiate NFAT signalling by activating the Ca²⁺-CaM-dependent protein phosphatase, calcineurin.