The epilepsy and intellectual disability-related gene tbc1d24 encodes a novel synaptic protein that regulates dendritic spine morphogenesis in neuron

Abstract

The majority of excitatory synapses are located on dendritic spines of the postsynaptic neuron. Spine formation and turnover is considered as an important mechanism underlying brain development as well as learning and memory. Several missense mutations of the human tbc1d24 gene have been associated with epilepsy and intellectual disability. However, the physiological role of the TBC1D24 protein remains largely unexplored. Here we report an essential role of TBC1D24 in regulating the density and morphology of dendritic spines in hippocampal neurons. We found that TBC1D24 protein was enriched in the synaptic plasma membrane fraction of adult mouse brains. Immunocytochemistry further revealed that TBC1D24 was present in close proximity to dendritic spines and the postsynaptic scaffold protein PSD-95. Notably, the expression of TBC1D24 in hippocampal neurons was bi-directionally regulated in response to elevation and blockade of neuronal activity. Using short-hairpin RNA (shRNA) to knock down its expression in mature hippocampal neurons, we demonstrated that the maintenance of dendritic spines critically depends on TBC1D24. Moreover, the small GTPase ARF6 was identified as the downstream mediator of TBC1D24 in the regulation of spine morphogenesis. These findings suggest that TBC1D24 is involved in activity-dependent spine morphogenesis in the postsynaptic neuron, and defects in spine development might contribute to the pathophysiology of intellectual disability in individuals harboring the loss-of-function gene mutations. This study was supported in part by the Research Grant Council of Hong Kong [General Research Fund (GRF) 16100814 and Early Career Scheme (ECS) 27119715].