The Cacna1h mutation in the GAERS model of absence epilepsy enhances T-type Ca2+ currents by altering calnexin-dependent trafficking of Cav3.2 channels

Juliane Proft,Gerald W Zamponi,Norbert Weiss,Terrance P Snutch,Yuriy Rzhepetskyy,Fang-Xiong Zhang,Stuart M Cain,Joanna Lazniewska

doi:10.1038/s41598-017-11591-5

Abstract

Low-voltage-activated T-type calcium channels are essential contributors to the functioning of thalamocortical neurons by supporting burst-firing mode of action potentials. Enhanced T-type calcium conductance has been reported in the Genetic Absence Epilepsy Rat from Strasbourg (GAERS) and proposed to be causally related to the overall development of absence seizure activity. Here, we show that calnexin, an endoplasmic reticulum integral membrane protein, interacts with the III-IV linker region of the Cav3.2 channel to modulate the sorting of the channel to the cell surface. We demonstrate that the GAERS missense mutation located in the Cav3.2 III-IV linker alters the Cav3.2/calnexin interaction, resulting in an increased surface expression of the channel and a concomitant elevation in calcium influx. Our study reveals a novel mechanism that controls the expression of T-type channels, and provides a molecular explanation for the enhancement of T-type calcium conductance in GAERS.

Highlights

Generalized non-motor epilepsies are often associated with an hereditary component[1]
Note that the immunoprecipitated reactive species above 250 KDa that corresponds to the Cav3.2 channel is not present in the co-immunoprecipitation performed from Cav3.2 knock out (KO) brain, demonstrating the specificity of the anti-Cav3.2 antibody used in these experiments
The Cav3.2/calnexin interaction was observed in tsA-201 cells expressing a recombinant HA-tagged human Cav3.2 channel (HA-hCav3.2) (Fig. 1C) where the channel is highly colocalized with calnexin (Fig. 1D)

Summary

Introduction

Generalized non-motor epilepsies are often associated with an hereditary component[1]. As for human generalized non-motor epilepsies, SWDs in GAERS are inherited, and notably segregate with a missense mutation in the gene Cacna1h encoding for the voltage-gated Cav3.2 T-type calcium channel[6]. A preceding period of hyperpolarization may be required to recruit T-type channels from inactivation, they are typically triggered by subthreshold membrane depolarizations to generate a Ca2+ transient which in turn gives rise to high frequency bursts of action potentials that support various forms of neuronal rhythmogenesis[8,9,10,11] These aspects of T-type channel function are of direct relevance to the functioning of the thalamocortical network, a brain circuit that is critically involved in the development and propagation of SWDs12–14. Other reported alterations in GAERS include elevated levels of thalamic Cav3.2 mRNA expression[30] and whole cell T-type currents[25], the genetic and molecular mechanisms by which upregulation of T-type channel activity might occur in GAERS and other rodent models of absence epilepsy remain unknown

Methods

Results

Conclusion