Developmental NMDA receptor signaling regulates cerebellar unipolar brush cell number and dampens excitability

Abstract

Unipolar brush cells (UBCs) are excitatory interneurons that have a characteristic dendritic brush that amplifies and extends incoming signals in the cerebellum. UBCs transform synaptic input through their ionotropic and metabotropic glutamate receptors. Differential regulation of receptor subunits is a critical developmental process, but how the expression of glutamatergic receptors changes in UBCs as they develop is unclear. NMDA-type glutamate receptors (NMDARs) are particularly important for development and plasticity. We examined the expression of NMDAR subunits during development and tested whether signaling through these receptors is necessary for the development of the elaborate dendritic structure and unusual synaptic function of UBCs. Whole-cell patch clamp recordings from UBCs in acute brain slices revealed tonic and synaptic NMDAR-mediated currents in early postnatal UBCs that decrease during development. RNAscope in situ hybridization revealed differential developmental regulation of GluN2C/D subunits. Cell-type specific constitutive NMDAR knockout had no apparent effect on dendritic brush development, but increased UBC number in adulthood, suggesting a role in programmed cell death. Both pharmacological blockade or genetic deletion of NMDARs produced a paradoxical increase in excitability, which was calcium dependent and was occluded by inhibition of calcium activated potassium channels. Thus, NMDA receptors are dispensable for migration and dendritic development but may be involved in cell death pathways. Their functional roles include synaptic signaling as well as providing a tonic calcium flux that dampens excitability in developing UBCs and may influence transformations of vestibular signals essential for smooth movements and balance. SIGNIFICANCE STATEMENT Unipolar brush cells (UBCs) amplify and transform incoming sensorimotor signals to support cerebellar function. Their disruption could lead to disorders such as ataxia and nystagmus. Factors contributing to UBC development are unclear. NMDA receptors play a pivotal role in maturation of other cerebellar neuron types. Here we show that NMDA receptors are expressed by developing UBCs and that they regulate the number of UBCs that survive through development, but do not contribute to their dendritic brush morphology. These receptors generate a tonic current due to their GluN2C/D subunit expression that paradoxically dampens excitability through the activation of calcium activated potassium channels. Thus, the expression of NMDA receptors in these neurons is a mechanism to regulate their excitability and their number.