Molecular basis of inherited calcium channelopathies: role of mutations in pore-forming subunits

A survey of protein interactions and posttranslational modifications that influence the polyglutamine diseases

The presence and aggregation of misfolded proteins has deleterious effects in the nervous system. Among the various diseases caused by misfolded proteins is the family of the polyglutamine (polyQ) disorders. This family comprises nine members, all stemming from the same mutation—the abnormal elongation of a polyQ repeat in nine different proteins—which causes protein misfolding and aggregation, cellular dysfunction and disease. While it is the same type of mutation that causes them, each disease is distinct: it is influenced by regions and domains that surround the polyQ repeat; by proteins with which they interact; and by posttranslational modifications they receive. Here, we overview the role of non-polyQ regions that control the pathogenicity of the expanded polyQ repeat. We begin by introducing each polyQ disease, the genes affected, and the symptoms experienced by patients. Subsequently, we provide a survey of protein-protein interactions and posttranslational modifications that regulate polyQ toxicity. We conclude by discussing shared processes and pathways that bring some of the polyQ diseases together and may serve as common therapeutic entry points for this family of incurable disorders.

Current molecular insight to reveal the dynamics of CAG repeating units in spinocerebellar ataxia

Spinocerebellar ataxia (SCA) is a heterogeneous genetic disorder with overlapping clinical phenotypes arising from the degeneration of purkinje cells and other regions of the brain. There are approximately 36 different subtypes of SCA, but SCA 1, 2, 3, 6 and 7 are most prevalent in the Indian population. Many findings suggested that cerebellar Purkinje cells region may be a uniquely vulnerable neuronal cell type, and more susceptible to a wider variety of genetic or cellular problems than other neuron types. In this review we emphasized mainly five common subtypes of SCA (1, 2, 3, 6 and 7) their pathophysiology, therapeutics, drugs studies and the technical challenges in the field of molecular genetic diagnosis.

Review: Cav2.3 R-type Voltage-Gated Ca2+ Channels - Functional Implications in Convulsive and Non-convulsive Seizure Activity

Background:

Researchers have gained substantial insight into mechanisms of synaptic transmission, hyperexcitability, excitotoxicity and neurodegeneration within the last decades. Voltage-gated Ca²⁺ channels are of central relevance in these processes. In particular, they are key elements in the etiopathogenesis of numerous seizure types and epilepsies. Earlier studies predominantly targeted on Ca_v2.1 P/Q-type and Ca_v3.2 T-type Ca²⁺ channels relevant for absence epileptogenesis. Recent findings bring other channels entities more into focus such as the Ca_v2.3 R-type Ca²⁺ channel which exhibits an intriguing role in ictogenesis and seizure propagation. Ca_v2.3 R-type voltage gated Ca²⁺ channels (VGCC) emerged to be important factors in the pathogenesis of absence epilepsy, human juvenile myoclonic epilepsy (JME), and cellular epileptiform activity, e.g. in CA1 neurons. They also serve as potential target for various antiepileptic drugs, such as lamotrigine and topiramate.

Objective:

This review provides a summary of structure, function and pharmacology of VGCCs and their fundamental role in cellular Ca²⁺ homeostasis. We elaborate the unique modulatory properties of Ca_v2.3 R-type Ca²⁺ channels and point to recent findings in the proictogenic and proneuroapoptotic role of Ca_v2.3 R-type VGCCs in generalized convulsive tonic–clonic and complex-partial hippocampal seizures and its role in non-convulsive absence like seizure activity.

Conclusion:

Development of novel Ca_v2.3 specific modulators can be effective in the pharmacological treatment of epilepsies and other neurological disorders.

Genetic basis of ventricular arrhythmias

Sudden cardiac death caused by malignant ventricular arrhythmias is the most important cause of death in the industrialized world. Most of these lethal arrhythmias occur in the setting of ischemic heart disease. A significant number of sudden deaths, especially in young individuals, are caused by inherited ventricular arrhythmic disorders, however. Genetically induced ventricular arrhythmias can be divided in two subgroups: the primary electrical disorders or channelopathies, and the secondary arrhythmogenic cardiomyopathies. This article focuses on the genetic background of these electrical disorders and the current knowledge of genotype-phenotype interactions.

Cloning and characterization of voltage-gated calcium channel alpha1 subunits in Xenopus laevis during development

Voltage-gated calcium channels play a critical role in regulating the Ca2+ activity that mediates many aspects of neural development, including neural induction, neurotransmitter phenotype specification, and neurite outgrowth. Using Xenopus laevis embryos, we describe the spatial and temporal expression patterns during development of the 10 pore-forming alpha1 subunits that define the channels' kinetic properties. In situ hybridization indicates that CaV1.2, CaV2.1, CaV2.2, and CaV3.2 are expressed during neurula stages throughout the neural tube. These, along with CaV1.3 and CaV2.3, beginning at early tail bud stages, and CaV3.1 at late tail bud stages, are detected in complex patterns within the brain and spinal cord through swimming tadpole stages. Additional expression of various alpha1 subunits was observed in the cranial ganglia, retina, olfactory epithelium, pineal gland, and heart. The unique expression patterns for the different alpha1 subunits suggests they are under precise spatial and temporal regulation and are serving specific functions during embryonic development.

Molecular basis for zinc transporter 1 action as an endogenous inhibitor of L-type calcium channels

The L-type calcium channel (LTCC) has a variety of physiological roles that are critical for the proper function of many cell types and organs. Recently, a member of the zinc-regulating family of proteins, ZnT-1, was recognized as an endogenous inhibitor of the LTCC, but its mechanism of action has not been elucidated. In the present study, using two-electrode voltage clamp recordings in Xenopus oocytes, we demonstrate that ZnT-1-mediated inhibition of the LTCC critically depends on the presence of the LTCC regulatory beta-subunit. Moreover, the ZnT-1-induced inhibition of the LTCC current is also abolished by excess levels of the beta-subunit. An interaction between ZnT-1 and the beta-subunit, as demonstrated by co-immunoprecipitation and by fluorescence resonance energy transfer, is consistent with this result. Using surface biotinylation and total internal reflection fluorescence microscopy in HEK293 cells, we show a ZnT-1-dependent decrease in the surface expression of the pore-forming alpha(1)-subunit of the LTCC. Similarly, a decrease in the surface expression of the alpha(1)-subunit is observed following up-regulation of the expression of endogenous ZnT-1 in rapidly paced cultured cardiomyocytes. We conclude that ZnT-1-mediated inhibition of the LTCC is mediated through a functional interaction of ZnT-1 with the LTCC beta-subunit and that it involves a decrease in the trafficking of the LTCC alpha(1)-subunit to the surface membrane.

Calcium channels, neuromuscular synaptic transmission and neurological diseases

Voltage-dependent calcium channels are essential in neuronal signaling and synaptic transmission, and their functional alterations underlie numerous human disorders whether monogenic (e.g., ataxia, migraine, etc.) or autoimmune. We review recent work on Ca(V)2.1 or P/Q channelopathies, mostly using neuromuscular junction preparations, and focus specially on the functional hierarchy among the calcium channels recruited to mediate neurotransmitter release when Ca(V)2.1 channels are mutated or depleted. In either case, synaptic transmission is greatly compromised; evidently, none of the reported functional replacements with other calcium channels compensates fully.

[Human calcium channelopathies. Voltage-gated Ca(2+) channels in etiology, pathogenesis, and pharmacotherapy of neurologic disorders]

Voltage-gated calcium channels are key components in a variety of physiological processes. Within the last decade an increasing number of voltage-gated Ca(2+) channelopathies in both humans and animal models has been described, most of which are related to the neurologic and muscular system. In humans, mutations were found in L-type Ca(v)1.2 and Ca(v)1.4 Ca(2+) channels as well as the non-L-type Ca(v)2.1 and T-type Ca(v)3.2 channels, resulting in altered electrophysiologic properties. Based on their widespread distribution within the CNS, voltage-gated calcium channels are of particular importance in the etiology and pathogenesis of various forms of epilepsy and neuropsychiatric disorders. In this review we characterise the different human Ca(2+) channelopathies known so far, further illuminating basic pathophysiologic mechanisms and clinical aspects.

Hippocampal Seizure Resistance and Reduced Neuronal Excitotoxicity in Mice Lacking the Cav2.3 E/R-Type Voltage-Gated Calcium Channel

Voltage-gated calcium channels are key components in the etiology and pathogenesis of epilepsies. Former studies mainly focused on P/Q-type Cav2.1 and T-type Cav3.2 Ca2+ channels involved in absence epileptogenesis, but recent findings also point to an intriguing role of the Cav2.3 E/R-type Ca2+ channel in ictogenesis and seizure propagation. Based on the observation that Cav2.3 is thought to induce plateau potentials in CA1 pyramidal cells, which can trigger epileptiform activity, our recent investigation revealed reduced PTZ-seizure susceptibility and altered seizure architecture in Cav2.3−/− mice compared with controls. In the present study we tested hippocampal seizure susceptibility in Cav2.3-deficient mice using surface and deep intrahippocampal telemetric EEG recordings as well as phenotypic seizure video analysis. Administration of kainic acid (30 mg/kg ip) revealed clear alteration in behavioral seizure architecture and dramatic resistance to limbic seizures in Cav2.3−/− mice compared with controls, whereas no difference in hippocampal EEG seizure activity between both genotypes could be detected at this suprathreshold dosage. The same tendency was observed for NMDA seizure susceptibility (150 mg/kg ip) approaching the level of significance. In addition, histochemical analysis within the hippocampus revealed that excitotoxic effects after kainic acid administration are absent in Cav2.3−/− mice, whereas Cav2.3+/+ animals exhibited clear and typical signs of excitotoxic cell death. These findings clearly indicate that the Cav2.3 voltage-gated calcium channel plays a crucial role in both hippocampal ictogenesis and seizure generalization and is of central importance in neuronal degeneration after excitotoxic events.