Calcium Channel Subunit α2δ4 Is Regulated by Early Growth Response 1 and Facilitates Epileptogenesis

Abstract

Transient brain insults, including status epilepticus (SE), can trigger a period of epileptogenesis during which functional and structural reorganization of neuronal networks occurs resulting in the onset of focal epileptic seizures. In recent years, mechanisms that regulate the dynamic transcription of individual genes during epileptogenesis and thereby contribute to the development of a hyperexcitable neuronal network have been elucidated. Our own results have shown early growth response 1 (Egr1) to transiently increase expression of the T-type voltage-dependent Ca 2ϩ channel (VDCC) subunit Ca V 3.2, a key proepileptogenic protein. However, epileptogenesis involves complex and dynamic transcriptomic alterations; and so far, our understanding of the transcriptional control mechanism of gene regulatory networks that act in the same processes is limited. Here, we have analyzed whether Egr1 acts as a key transcriptional regulator for genes contributing to the development of hyperexcitability during epileptogenesis. We found Egr1 to drive the expression of the VDCC subunit ␣2␦4, which was augmented early and persistently after pilocarpine-induced SE. Furthermore, we show that increasing levels of ␣2␦4 in the CA1 region of the hippocampus elevate seizure susceptibility of mice by slightly decreasing local network activity. Interestingly, we also detected increased expression levels of Egr1 and ␣2␦4 in human hippocampal biopsies obtained from epilepsy surgery. In conclusion, Egr1 controls the abundance of the VDCC subunits Ca V 3.2 and ␣2␦4, which act synergistically in epileptogenesis, and thereby contributes to a seizure-induced "transcriptional Ca 2ϩ channelopathy."

Presynaptic α2δ subunits are key organizers of glutamatergic synapses

“…However, the severity of the phenotype of specific α 2 δ loss-of-function models strongly correlated with the expression level of the particular isoform in the affected cells or tissues: knockdown of α 2 δ-1 affected synapse formation in retinal ganglion cells (Eroglu et al, 2009), lack of α 2 δ-2 causes pre-and postsynaptic defects in hair cells of the inner ear (Fell et al, 2016), knockout of α 2 δ-3 alters presynaptic morphology of auditory nerves (Pirone et al, 2014) and in invertebrates lossof-function of the homologous subunit resulted in abnormal presynaptic development in motoneurons (Caylor et al, 2013;Kurshan et al, 2009), and finally, the predominant expression of α 2 δ-4 in the retina (Knoflach et al, 2013) is mirrored by retinal defects and consequences on the organization of rod and cone photoreceptor synapses (Kerov et al, 2018;Wang et al, 2017;Wycisk et al, 2006). Contrary to these specialized cell types and tissues, the mammalian brain expresses all four known α 2 δ isoforms (Cole et al, 2005;van Loo et al, 2019), whereby the isoforms α 2 δ-1, -2, and -3 are strongly and most ubiquitously expressed (Geisler et al, 2019;Schlick et al, 2010). While the increasing severity of the phenotypes between α 2 δ subunit single and double knockout mice already suggested a functional redundancy, this was ultimately revealed in the cellular triple loss-of-function model established for the present study.…”

Neuronal α2δ proteins and brain disorders

“…However, we would like to highlight two prominent mutations (Fig. 2, hα 2 δ-4), which affect α 2 δ-4 expression or structure and may, in theory, also affect the previously suggested expression of α 2 δ-4 in the brain [95,117]. A mutation found in night blindness (p.Tyr802Ter) results in a premature stop at amino acid 802.…”

“…Until recently, protein and mRNA expression levels of human and murine α 2 δ-4 seemed negligible in all previously examined CNS regions [40,88,95]. A recent study, however, reported increased levels of α 2 δ-4 mRNA in human hippocampal biopsies obtained from epileptic patients, an interesting finding in spite of the lack of information on absolute mRNA levels [117]. Considering that standard curve-based qRT-PCR studies from our group identified very low amounts of α 2 δ-4 mRNA in the mouse brain [40,95], the recent findings suggest the existence of a subpopulation of hippocampal neurons expressing α 2 δ-4.…”

“…α 2 δ-4 is almost exclusively expressed in the retina, pituitary gland, and adrenal gland [29,88]. However, very low levels are also detected in the hippocampus and are upregulated during development and status epilepticus [95,117]. A possible role in the brain, which is further outlined below, is suggested by several SNPs linked to numerous neurological disorders.…”

Transcriptional Regulation of Channelopathies in Genetic and Acquired Epilepsies

“…2(Bernard et al, 2004;Monaghan et al, 2008), K ir 2 channels(Young et al, 2009), small-conductance (SK) calcium-activated potassium channels(Oliveira et al, 2010), big potassium channels(BK-channels;Pacheco Otalora et al, 2008;Shruti et al, 2008), persistent sodium channels(Agrawal et al, 2003;Chen et al, 2011), the T-type calcium channel Ca V 3.2(Su et al, 2002;Becker et al, 2008) and the calcium channel subunit α2δ4(van Loo et al, 2019).…”