The role of intracellular transport on synapse development in neuron

Abstract

The neuronal intracellular transport enables proteins and RNAs to reach the axon and dendrites from the soma. Kinesin 1 motor protein, which have three highly homologous members (KIF5A, KIF5B and KIF5C), is crucial for long-distance transport, and plays a key role in axonal transport. However, the exact functions of kinesin 1 family for dendritic transport and synapse development on dendrite are not well-defined. Furthermore, whether the three KIF5s have non-overlapping functions in synapse development remains unexplored. Through specific knockdown of individual KIF5, I found that the knock-down of KIF5B or KIF5C, but not KIF5A could reduce the density of mature spines in cultured hippocampal neurons. Moreover, overexpression of the full-length KIF5A could not rescue the decreased mushroom spines caused by KIF5B knock-down indicating that the two homologous kinesin KIF5A and KIF5B play non-overlapping roles in dendritic spine development. Whole-cell patch-clamp recording of miniature excitatory postsynaptic current (mEPSCs) confirmed the differential functions of KIF5A and KIF5B knockdown on excitatory synaptic transmission of hippocampal neurons. Amino acid sequence alignment reveals that the C-terminal tails among KIF5A, KIF5B and KIF5C are the most diverse. Pull-down experiments revealed that only the C-terminal of KIF5B and KIF5C but not that of KIF5A could bind to the RNA-binding protein FMRP. Importantly, the C-terminal of KIF5B could recover the ability of KIF5A to interact with FMRP and rescue the decreased mushroom spines caused by the KIF5B knock-down. Moreover, a new role of arginine methylation on the tail of KIB5B was revealed. Conditional KIF5B knock-out mice also showed the loss of dendritic spines and impaired excitatory synaptic transmission in the hippocampus. These findings together revealed a specific role of KIF5B in synapse development and its underlying molecular basis and identify the C-terminal tail as a crucial determinant for the specific function played by the individual KIF5. Besides the long-distance transport driven by motor proteins, the endocytosis and endosomal trafficking could regulate the local transport of materials at the cell periphery, such as the turnover of receptor proteins on the cell surface. TBC1D24, a protein implicated in epilepsy and intellectual disability, contains a highly conserved TBC domain that possesses the GAP activity for the endosomal trafficking associated proteins Rab. Knock-down of TBC1D24 decreased frequency of both excitatory and inhibitory transmission in hippocampal neurons. In contrast, the mutant F251L at the TBC domain could increase the amplitude of both excitatory and inhibitory transmission currents. These findings suggest that the TBC1D24-related endosomal trafficking could regulate the excitatory and inhibitory synapse function.