Similar molecular determinants on Rem mediate two distinct modes of inhibition of CaV1.2 channels

Inhibitory effects on L- and N-type calcium channels by a novel CaVβ1 variant identified in a patient with autism spectrum disorder

Voltage-gated calcium channel (VGCC) subunits have been genetically associated with autism spectrum disorders (ASD). The properties of the pore-forming VGCC subunit are modulated by auxiliary β-subunits, which exist in four isoforms (Ca_Vβ_1-4). Our previous findings suggested that activation of L-type VGCCs is a common feature of Ca_Vβ₂ subunit mutations found in ASD patients. In the current study, we functionally characterized a novel Ca_Vβ_1b variant (p.R296C) identified in an ASD patient. We used whole-cell and single-channel patch clamp to study the effect of Ca_Vβ_{1b_R296C} on the function of L- and N-type VGCCs. Furthermore, we used co-immunoprecipitation followed by Western blot to evaluate the interaction of the Ca_Vβ_1b-subunits with the RGK-protein Gem. Our data obtained at both, whole-cell and single-channel levels, show that compared to a wild-type Ca_Vβ_1b, the Ca_Vβ_{1b_R296C} variant inhibits L- and N-type VGCCs. Interaction with and modulation by the RGK-protein Gem seems to be intact. Our findings indicate functional effects of the Ca_Vβ_{1b_R296C} variant differing from that attributed to Ca_Vβ₂ variants found in ASD patients. Further studies have to detail the effects on different VGCC subtypes and on VGCC expression.

Into the spotlight: RGK proteins in skeletal muscle

The RGK (Rad, Rem, Rem2 and Gem/Kir) family of small GTPases are potent endogenous inhibitors of voltage-gated Ca²⁺ channels (VGCCs). While the impact of RGK proteins on cardiac physiology has been investigated extensively, much less is known regarding their influence on skeletal muscle biology. Thus, the purpose of this article is to establish a basis for future investigation into the role of RGK proteins in regulating the skeletal muscle excitation-contraction (EC) coupling complex via modulation of the L-type Ca_V1.1 VGCC. The pathological consequences of elevated muscle RGK protein expression in Type II Diabetes, Amyotrophic Lateral Sclerosis (ALS), Duchenne's Muscular Dystrophy and traumatic nerve injury are also discussed.

The life cycle of voltage-gated Ca2+ channels in neurons: an update on the trafficking of neuronal calcium channels

Neuronal voltage-gated Ca2+ (CaV) channels play a critical role in cellular excitability, synaptic transmission, excitation-transcription coupling and activation of intracellular signaling pathways. CaV channels are multiprotein complexes and their functional expression in the plasma membrane involves finely tuned mechanisms, including forward trafficking from the endoplasmic reticulum (ER) to the plasma membrane, endocytosis and recycling. Whether genetic or acquired, alterations and defects in the trafficking of neuronal CaV channels can have severe physiological consequences. In this review, we address the current evidence concerning the regulatory mechanisms which underlie precise control of neuronal CaV channel trafficking and we discuss their potential as therapeutic targets.

Regulation of cardiovascular calcium channel activity by post-translational modifications or interacting proteins

Voltage-gated calcium channels are the major pathway for Ca²⁺ influx to initiate the contraction of smooth and cardiac muscles. Alterations of calcium channel function have been implicated in multiple cardiovascular diseases, such as hypertension, atrial fibrillation, and long QT syndrome. Post-translational modifications do expand cardiovascular calcium channel structure and function to affect processes such as channel trafficking or polyubiquitination by two E3 ubiquitin ligases, Ret finger protein 2 (Rfp2) or murine double minute 2 protein (Mdm2). Additionally, biophysical property such as Ca²⁺-dependent inactivation (CDI) could be altered through binding of calmodulin, or channel activity could be modulated via S-nitrosylation by nitric oxide and phosphorylation by protein kinases or by interacting protein partners, such as galectin-1 and Rem. Understanding how cardiovascular calcium channel function is post-translationally remodeled under distinctive disease conditions will provide better information about calcium channel-related disease mechanisms and improve the development of more selective therapeutic agents for cardiovascular diseases.

Designer genetically encoded voltage-dependent calcium channel inhibitors inspired by RGK GTPases

High‐voltage‐activated calcium (Ca_V1/Ca_V2) channels translate action potentials into Ca²⁺ influx in excitable cells to control essential biological processes that include; muscle contraction, synaptic transmission, hormone secretion and activity‐dependent regulation of gene expression. Modulation of Ca_V1/Ca_V2 channel activity is a powerful mechanism to regulate physiology, and there are a host of intracellular signalling molecules that tune different aspects of Ca_V channel trafficking and gating for this purpose. Beyond normal physiological regulation, the diverse Ca_V channel modulatory mechanisms may potentially be co‐opted or interfered with for therapeutic benefits. Ca_V1/Ca_V2 channels are potently inhibited by a four‐member sub‐family of Ras‐like GTPases known as RGK (Rad, Rem, Rem2, Gem/Kir) proteins. Understanding the mechanisms by which RGK proteins inhibit Ca_V1/Ca_V2 channels has led to the development of novel genetically encoded Ca_V channel blockers with unique properties; including, chemo‐ and optogenetic control of channel activity, and blocking channels either on the basis of their subcellular localization or by targeting an auxiliary subunit. These genetically encoded Ca_V channel inhibitors have outstanding utility as enabling research tools and potential therapeutics.

Engineering selectivity into RGK GTPase inhibition of voltage-dependent calcium channels

Genetically encoded inhibitors for voltage-dependent Ca²⁺ (Ca_V) channels (GECCIs) are useful research tools and potential therapeutics. Rad/Rem/Rem2/Gem (RGK) proteins are Ras-like G proteins that potently inhibit high voltage-activated (HVA) Ca²⁺ (Ca_V1/Ca_V2 family) channels, but their nonselectivity limits their potential applications. We hypothesized that nonselectivity of RGK inhibition derives from their binding to auxiliary Ca_Vβ-subunits. To investigate latent Ca_Vβ-independent components of inhibition, we coexpressed each RGK individually with Ca_V1 (Ca_V1.2/Ca_V1.3) or Ca_V2 (Ca_V2.1/Ca_V2.2) channels reconstituted in HEK293 cells with either wild-type (WT) β_2a or a mutant version (β_2a,TM) that does not bind RGKs. All four RGKs strongly inhibited Ca_V1/Ca_V2 channels reconstituted with WT β_2a By contrast, when channels were reconstituted with β_2a,TM, Rem inhibited only Ca_V1.2, Rad selectively inhibited Ca_V1.2 and Ca_V2.2, while Gem and Rem2 were ineffective. We generated mutant RGKs (Rem[R200A/L227A] and Rad[R208A/L235A]) unable to bind WT Ca_Vβ, as confirmed by fluorescence resonance energy transfer. Rem[R200A/L227A] selectively blocked reconstituted Ca_V1.2 while Rad[R208A/L235A] inhibited Ca_V1.2/Ca_V2.2 but not Ca_V1.3/Ca_V2.1. Rem[R200A/L227A] and Rad[R208A/L235A] both suppressed endogenous Ca_V1.2 channels in ventricular cardiomyocytes and selectively blocked 25 and 62%, respectively, of HVA currents in somatosensory neurons of the dorsal root ganglion, corresponding to their distinctive selectivity for Ca_V1.2 and Ca_V1.2/Ca_V2.2 channels. Thus, we have exploited latent β-binding-independent Rem and Rad inhibition of specific Ca_V1/Ca_V2 channels to develop selective GECCIs with properties unmatched by current small-molecule Ca_V channel blockers.

Molecular mechanisms and physiological relevance of RGK proteins in the heart

The primary route of Ca²⁺ entry into cardiac myocytes is via 1,4-dihydropyridine-sensitive, voltage-gated L-type Ca²⁺ channels. Ca²⁺ influx through these channels influences duration of action potential and engages excitation-contraction (EC) coupling in both the atria and the myocardium. Members of the RGK (Rad, Rem, Rem2 and Gem/Kir) family of small GTP-binding proteins are potent, endogenously expressed inhibitors of cardiac L-type channels. Although much work has focused on the molecular mechanisms by which RGK proteins inhibit the Ca_V 1.2 and Ca_V 1.3 L-type channel isoforms that expressed in the heart, their impact on greater cardiac function is only beginning to come into focus. In this review, we summarize recent findings regarding the influence of RGK proteins on normal cardiac physiology and the pathological consequences of aberrant RGK activity.

Ca2+ leak-What is it? Why should we care? Can it be managed?

For arrhythmia triggers that are secondary to dysfunctional intracellular Ca²⁺ cycling, there are few, if any, agents that specifically target the Ca²⁺ handling machinery. However, several candidates have been proposed in the literature. Here we review the idea that these agents or their derivatives will prove invaluable in clinical applications in the future.