RIM4γ deficiency causes alterations in cerebellar Purkinje cell function in vivo

Eva Maria Schönhense1, Hyun Tae Kim2, Katrin Michel3, Albert J. Becker3, Dirk Dietrich2, Laura A. Ewell4, Susanne Schoch3

1 Institut of Neuropathology and Department of Epileptology, University Clinic Bonn
2 Experimental Neurophysiology, Department of Neurosurgery, University Clinic Bonn
3 Institute of Neuropathology and Department of Epileptology, University Clinic Bonn
4 Institute of Experimental Epileptology and Cognition Research, University Clinic Bonn

RIMs (Rab3-interacting molecules) are multidomain proteins, enriched at presynaptic active zones. The large isoforms (RIM1α/β, RIM2α/β) have been shown to be important for mediating presynaptic active zone function by coupling synaptic vesicles to voltage-gated calcium channels and by regulating neurotransmitter release as well as presynaptic plasticity. The functional role of the small RIM isoforms, RIM3γ and RIM4γ, in particular in vivo has so far remained unresolved. In newly generated constitutive RIM4γ knock-out (KO) mice we observed spontaneous episodes of strong hind limb impairments with rapid uncontrolled movements accompanied by weight loss after weaning. In order to uncover if this phenotype resulted from a dysfunction in the cerebellum, we generated Pcp2-Cre(Mpin):RIM4γ KO mice, in which Cre recombination is mainly restricted to Purkinje cells. Behavioral experiments with these mice reproduced the deficits in fine motor coordination and less exploration in a novel environment found in the constitutive KO line. In vivo single unit recordings in freely moving Pcp2-Cre(Mpin):RIM4γ KO mice unveiled a reduced cerebellar firing frequency under baseline conditions. Interestingly, the firing frequency strongly changes during the motor episodes of the KO mice, induced by a single injection of caffeine. Surprisingly, Pcp2-Cre(Mpin):RIM1/2 DKO mice did not exhibit any motor problems. Taken together, our data for the first time reports that RIM4γ in Purkinje cells is required to maintain normal electrophysiological properties of the cerebellar network and that in turn, RIM4γ deficiency results in a phenotype resembling human dyskinesias. Our results also suggest that RIM4γ acts independently of the long RIM isoforms, RIM1/2.