Epilepsy protein myoclonin1 interacts with inositol 1,4,5-trisphosphate receptor and reduces calcium ions stored in endoplasmic reticulum

Abstract

Aim: Mutations in the EFHC1 gene have been identified in patients with various epilepsies, including juvenile myoclonic epilepsy (JME). Mice with Efhc1 deficiency also exhibit epileptic phenotypes. The protein myoclonin1, encoded by EFHC1, is not expressed in neurons but in cells with motile cilia, including choroid plexus and ependymal cells lining of brain ventricles. However, the molecular mechanisms by which EFHC1 mutations cause epilepsy remain unclear. Because of the involvement of inositol 1,4,5-trisphosphate receptor type 1 (IP₃R1) in epileptic phenotypes and the involvement of myoclonin1 in calcium ions (Ca²⁺) signaling, we investigated possible functional interplay between myoclonin1 and IP₃R1.

Methods: We performed immunohistochemical staining of brain tissues and co-immunoprecipitation assay of myoclonin1 and IP₃R1, and Ca²⁺ imaging analyses using human HeLa.S3, mouse embryonic fibroblasts, or glial cells derived from Efhc1 homozygous knockout (Efhc1^–/–) and wild-type (WT) littermates.

Results: Myoclonin1 was revealed to be well co-expressed with IP₃R1 at choroid plexus and ependymal cells, and these two proteins bound to each other. Endoplasmic reticulum (ER) of Efhc1-deficient mouse (Efhc1^–/–) cells showed larger amounts of Ca²⁺ than that of WT mice, and IP₃-induced Ca²⁺ release (IICR) from ER was higher in Efhc1^–/– cells than that of WT. Furthermore, myoclonin1 was revealed to interact with beta subunit of glucosidase II (PRKCSH), also known as a protein kinase C substrate 80K-H, which interacts with IP₃R1. Myoclonin1 further binds to IP₃R2 and IP₃R3.

Conclusions: These results indicate that myoclonin1 modulates ER-Ca²⁺ homeostasis through interactions with IP₃Rs and PRKCSH, and suggest that myoclonin1 dysfunctions cause impaired intracellular Ca²⁺ mobilization. Its relevance to the epileptic phenotypes of patients with EFHC1 mutations is now of interest.

Keywords

EFHC1 / myoclonin1 / juvenile myoclonic epilepsy / inositol 1 / 4 / 5-trisphosphate receptor / protein kinase C substrate 80K-H / endoplasmic reticulum-calcium ions store

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