The physiological role of Homer2a and its novel short isoform, Homer2e, in NMDA receptor-mediated apoptosis in cerebellar granule cells

Teiichi Furuichi,Yo Shinoda,Yuko Muto,Yumi Sato,Yoko Shiraishi-Yamaguchi,Kanehiro Hayashi,Tetsushi Sadakata

doi:10.1186/s13041-021-00804-8

Abstract

Homer is a postsynaptic scaffold protein, which has long and short isoforms. The long form of Homer consists of an N-terminal target-binding domain and a C-terminal multimerization domain, linking multiple proteins within a complex. The short form of Homer only has the N-terminal domain and likely acts as a dominant negative regulator. Homer2a, one of the long form isoforms of the Homer family, expresses with a transient peak in the early postnatal stage of mouse cerebellar granule cells (CGCs); however, the functions of Homer2a in CGCs are not fully understood yet. In this study, we investigated the physiological roles of Homer2a in CGCs using recombinant adenovirus vectors. Overexpression of the Homer2a N-terminal domain construct, which was made structurally reminiscent with Homer1a, altered NMDAR1 localization, decreased NMDA currents, and promoted the survival of CGCs. These results suggest that the Homer2a N-terminal domain acts as a dominant negative protein to attenuate NMDAR-mediated excitotoxicity. Moreover, we identified a novel short form N-terminal domain-containing Homer2, named Homer2e, which was induced by apoptotic stimulation such as ischemic brain injury. Our study suggests that the long and short forms of Homer2 are involved in apoptosis of CGCs.

Highlights

Homer proteins are scaffold proteins that predominantly exist in the postsynaptic density (PSD) of neurons and tether a variety of target proteins within the PSDs, including the group I metabotropic glutamate receptor 1α/5, inositol 1,4,5-trisphosphate receptor (IP3R), and Shank, a scaffold protein for the N-methyl-Daspartate receptor (NMDAR) complex [1,2,3,4]
Overexpression of exogenous Homer2a in cerebellar granule cells (CGCs) To clarify the function of Homer2a in CGCs, four recombinant AdV vectors were constructed (Fig. 1A): AdV carrying enhanced green fluorescent protein (EGFP) only, and three AdV constructs consisting of EGFP fused to full length (H2a-F), C-terminal or N-terminal Homer2a, which structurally resemble a short form of Homer, like Homer1a
Our study showed that overexpression of the Homer2a N-terminal domain decreased NMDA receptor 1 (NR1) localization and the NMDA currents, and had an antiapoptotic function in CGCs

Summary

Introduction

Homer proteins are scaffold proteins that predominantly exist in the postsynaptic density (PSD) of neurons and tether a variety of target proteins within the PSDs, including the group I metabotropic glutamate receptor 1α/5 (mGluR1α/5), inositol 1,4,5-trisphosphate receptor (IP3R), and Shank, a scaffold protein for the N-methyl-Daspartate receptor (NMDAR) complex [1,2,3,4]. There are three distinct genes in the Homer family, Homer1/vesl-1, Homer2/Cupidin/vesl-2, and Homer, and they are classified into two major forms, long and short [5, 6]. Long Homers, such as Homer1b/c/d, 2a/b, and 3a/b, consist of two main domains, the N-terminal enabled/vasodilator-stimulated phosphoprotein homology 1 (EVH-1) domain for the target binding and the C-terminal coiledcoil domain for the self-multimerization [7]. The short Homer, such as Homer1a, which has been reported as an immediate-early gene induced by electroconvulsive seizure in an activity-dependent manner [8], only has the N-terminal target-binding domain. Homer1a is thought to act as a natural dominant negative regulator by competing with long Homer proteins that form Homer-target protein multimers via the C-terminal. Homer1a is known to promote cell protection from apoptosis [11,12,13]

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Results

Conclusion