Cupidin/Homer2 regulates postsynaptic molecular organization and dendritic spine morphology

Teiichi Furuichi

Abstract

Homer is a postsynaptic adaptor protein that binds various target proteins including the class I metabotropic glutamate receptors (mGluR1_/5), IP3 receptors, Shank (a protein that binds to the GKAP/PSD-95/NMDAR complex and to an actin-binding protein Cortactin), and Drebrin (a dendritic actin-binding protein) via the N-terminal EVH1 domain. In addition, Homer proteins self-assemble via the C-terminal coiled-coil domain and Leu zipper motifs. Thus, Homer is thought to generate physical and functional links among these target proteins in postsynapses. Three distinct genes of the Homer family (Homer1/Vesl-1/PSD-Zip45, Homer2/Vesl-2/Cupidin, and Homer3) and their alternative-splicing isoforms have been identified to date. The long Homer forms (Homer1b/c, Cupidin/Homer2a/b, and Homer3a/b) contain both N- and C-terminal domains. On the other hand, the activity-dependently expressed short Homer1a/Vesl-1S lacks the C-terminal self-assembly domain, and is suggested to act as a natural dominant negative regulator that intervenes with a physical link between multiple targets generated by self-assembled long Homer forms.

We analyzed the expression and function of Cupidin/Homer in synapse development. In the developing cerebellum, Cupidin/Homer2 is clustered in postsynapses of granule cells connecting mossy fiber terminals. The declustering of Cupidin was induced by Glu treatment within a minute via the NMDA receptor Ðmediated Ca2+ influx followed by MAPK/ERK cascades. Exogenous Cupidin overexpressed in granule cells increased NMDA currents, whereas overexpression of the N-terminal domain that resembles the short Homer1a had the opposite effect. These results suggest the involvement of Cupidin in synaptic transmission and its negative regulation by Homer1a in cerebellar granule cells. In hippocampal neurons, the clustering of Cupidin coincided with that of the NMDA receptor complex proteins NR2B and PSD-95 in the dendritic clustering and synaptic targeting. Overexpression of Cupidin increased the number of mushroom-like mature-type spines and enhanced the synaptic clustering of the NR2B, PSD-95, and N-cadherin. In contrast, overexpression of the N-terminal domain increased the number of long protrusions and decreased the synaptic clustering of PSD-95 and Drebrin.

In conclusion, these results indicate that Cupidin/Homer has two modes in regulating synaptic protein localizations and functions: 1) activity-dependent reversible declustering of long Homer proteins and 2) short-Homer (Homer1a)-mediated disruption of target links generated via self-assembled long Homers. The former regulatory mode appears to be fast response to synaptic activities, whereas the latter mode via the short Homer is slower response induced by strong stimulations such as sustained Glu treatment, electric convulsion, LTP, cocaine administration, etc. In addition, the long Homer is likely involved in the organization and links of postsynaptic proteins throughout dendritic and synaptic development, whereas the short Homer regulates the morphology of dendritic spines.

Shiraishi, Y., Mizutani, A., Yuasa, S., Mikoshiba, K., and Furuichi, T. (2004). Differential expression of Homer family proteins in the developing mouse brain. J. Comp. Neurol. 473(4):582-599.

Datwani, A.*, Iwasato, T.*+, Itohara, S., and Erzurumlu, R.S.+, Lesion-induced thalamocortical axonal plasticity in the S1 cortex is independent of NMDA receptor function in excitatory cortical neurons. J. Neurosci. 22, 9171-9175 (2002). *The first two authors contributed equally to this work. +The corresponding authors

Shiraishi, Y., Mizutani, A., Mikoshiba, K., and Furuichi, T. (2003). Coincidence in dendritic clustering and synaptic targeting of homer proteins and NMDA receptor complex proteins NR2B and PSD95 during development of cultured hippocampal neurons. Mol. Cell. Neurosci. 22(2):188-201.

Shiraishi, Y., Mizutani, A., Haruhiko, B., Fujisawa, K., Narumiya, S., Mikoshiba, K., and Furuichi, T. (1999). Cupidin, an isoform of Homer/Vesl, interacts with the actin cytoskeleton and activated Rho family small GTPases and is expressed in developing mouse cerebellar granule cells. J. Neurosci. 19:8389-8400.

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