NeuroD2 controls inhibitory circuit formation in the molecular layer of the cerebellum

Ingo Bormuth,Stephanie Rudolph,Jacques I Wadiche,Bettina Duverge Castillo,Markus H Schwab,Tilmann Götze,Tomoko Yonemasu,Ulli Bode,Alexander Pieper,Hendrik Mießner,Sandra Goebbels,Iva Tzvetanova,Kuo Yan,Georg L Wieser

doi:10.1038/s41598-018-37850-7

Abstract

The cerebellar cortex is involved in the control of diverse motor and non-motor functions. Its principal circuit elements are the Purkinje cells that integrate incoming excitatory and local inhibitory inputs and provide the sole output of the cerebellar cortex. However, the transcriptional control of circuit assembly in the cerebellar cortex is not well understood. Here, we show that NeuroD2, a neuronal basic helix-loop-helix (bHLH) transcription factor, promotes the postnatal survival of both granule cells and molecular layer interneurons (basket and stellate cells). However, while NeuroD2 is not essential for the integration of surviving granule cells into the excitatory circuit, it is required for the terminal differentiation of basket cells. Axons of surviving NeuroD2-deficient basket cells follow irregular trajectories and their inhibitory terminals are virtually absent from Purkinje cells in Neurod2 mutants. As a result inhibitory, but not excitatory, input to Purkinje cells is strongly reduced in the absence of NeuroD2. Together, we conclude that NeuroD2 is necessary to instruct a terminal differentiation program in basket cells that regulates targeted axon growth and inhibitory synapse formation. An imbalance of excitation and inhibition in the cerebellar cortex affecting Purkinje cell output may underlay impaired adaptive motor learning observed in Neurod2 mutants.

Highlights

The cerebellum controls motor behavior and adaptive motor learning
X-gal histochemistry combined with immunostaining for parvalbumin (PV) in Neurod2-lacZ ‘knock in’ reporter mice[24] confirmed Neurod[2] expression in molecular layer interneurons (MLIs) at P20
A corresponding analysis of Neurod1-lacZ ‘knock in’ reporter mice[29] revealed similar expression of Neurod[1] in granule cells and MLIs, which was absent from Purkinje cells (Fig. 1A)

Summary

Introduction

The cerebellum controls motor behavior and adaptive motor learning. Purkinje cells serve as the sole output neurons of the cerebellar cortex and provide inhibitory input to neurons of the deep cerebellar and vestibular nuclei[1,2]. A distinct precursor pool in the primitive cerebellar neuroepithelium, which expresses the proneural bHLH protein Ptf1a8, gives rise to all GABAergic neurons of the cerebellum. This includes Purkinje cells, the major neuronal subtype derived from the Ptf1a+ ventricular zone, in addition to Golgi cells and MLI lineages. ML preferentially innervate Purkinje cell dendrites while basket cells in the deeper ML contact the perisomatic compartment of Purkinje cells[12,13] It is not settled whether these interneurons represent two distinct cell types or one functionally continuous population[14]. Since expression of the Neurod[2] gene was observed in MLIs24,27, members of the NeuroD subfamily represent plausible candidate transcription factors to regulate excitatory and inhibitory circuit formation in the cerebellum. We hypothesize that an imbalance of excitatory and inhibitory neurotransmission in the cerebellar cortex could contribute to impaired motor learning in Neurod[2] mutants

Methods

Results

Conclusion