The Synaptic Targeting of mGluR1 by Its Carboxyl-Terminal Domain Is Crucial for Cerebellar Function

Abstract

The metabotropic glutamate receptor subtype 1 (mGluR1, Grm1) in cerebellar Purkinje cells (PCs) is essential for motor coordination and motor learning. At the synaptic level, mGluR1 has a critical role in long-term synaptic depression (LTD) at parallel fiber (PF)-PC synapses, and in developmental elimination of climbing fiber (CF)-PC synapses. mGluR1a, a predominant splice variant in PCs, has a long carboxyl (C)-terminal domain that interacts with Homer scaffolding proteins. Cerebellar roles of the C-terminal domain at both synaptic and behavior levels remain poorly understood. To address this question, we introduced a short variant, mGluR1b, which lacks this domain into PCs of mGluR1-knock-out (KO) mice (mGluR1b-rescue mice). In mGluR1b-rescue mice, mGluR1b showed dispersed perisynaptic distribution in PC spines. Importantly, mGluR1b-rescue mice exhibited impairments in inositol 1,4,5-trisphosphate receptor (IP 3 R)-mediated Ca 2ϩ release, CF synapse elimination, LTD induction, and delay eyeblink conditioning: they showed normal transient receptor potential canonical (TRPC) currents and normal motor coordination. In contrast, PC-specific rescue of mGluR1a restored all cerebellar defects of mGluR1-KO mice. We conclude that the long C-terminal domain of mGluR1a is required for the proper perisynaptic targeting of mGluR1, IP 3 R-mediated Ca 2ϩ release, CF synapse elimination, LTD, and motor learning, but not for TRPC currents and motor coordination.

Numb deficiency in cerebellar Purkinje cells impairs synaptic expression of metabotropic glutamate receptor and motor coordination

“…These results support the notion that endocannabinoid signaling, P/Q-type Ca 2+ channels, and CF synapse elimination in PCs do not contribute to impaired PF-LTD in Numb-cKO mice. Previous studies have reported that CF elimination is impaired in mGlu1 null-mutant mice (30,53) but can be partially restored by transgenic expression of mGlu1 (27,30). Our data extend these findings and suggest that partial preservation of mGlu1 can be sufficient for CF elimination.…”

“…Likewise, LTD of PF excitatory postsynaptic potentials (EPSPs) induced by a conjunction of double PF shocks and PC depolarization (200 ms, 1 nA) repeated at 1 Hz for 5 min in current-clamp mode was also blocked in NumbcKO mice (Fig. S3) (26,27). The ratio of paired-pulse facilitation (PPF) measured at an interval of 80 ms was not changed after LTD induction in control and Numb-cKO mice (Fig.…”

The neural circuitry and molecular mechanisms underlying delay and trace eyeblink conditioning in mice

“…Some findings indicate that CR acquisition has been disturbed in DEC in mGluR1 mutant mice, Purkinje cell degenerative (pcd) mice, GluR␦2-deficient mice, Purkinje cell-specific protein kinase C inhibitor over expressing mice, granule-cell specific reversible neurotransmission blocked (RNB) mice, Ngsk Prnp (0/0) mice and Purkinje neuron Scn8a KO mice, suggesting the importance of the cerebellar cortex in this paradigm [23,[28][29][30][31][32][33][34]. Besides, the latest research indicated that both parallel fiber (PF)-Purkinje cell (PC) synapses and the metabotropic glutamate receptor subtype 1 (mGluR1, Grm1) in cerebellar Purkinje cells (PCs) are essential for DEC in mice [35,36]. Interestingly, results in C57BL/6 mice aged 4, 8, 12, 18, and 24 months suggest that individual variation in Purkinje neuron number is related to DEC in young organisms [37].…”

Chronic imaging of movement-related Purkinje cell calcium activity in awake behaving mice

“…Thus substantial effort has been expended on understanding the link between this plasticity and learned aspects of motor-driven behavior. For example, LTD in PCs can affect conditioned eyeblink responses (Koekkoek et al 2003;Ohtani et al 2014; but see Welsh et al 2005), spatial learning (Burguiere et al 2005;Rochefort et al 2011), adaptation of the horizontal optokinetic reflex , and smooth pursuit eye movements (Medina and Lisberger 2008). However, climbing fiber-driven LTD alone has failed to fully account for behavioral learning in many cases including adaptation of the vestibuloocular reflex or conditioned eyeblink responses (Ke et al 2009;Kimpo et al 2014;Ly et al 2013;Schonewille et al 2010Schonewille et al , 2011Tanaka et al 2013;Wetmore et al 2014).…”