MTCL2 Is Essential for the Bipolar-to-Multipolar Transition in the Dendrite Extension of Cerebellar Granule Neurons

The dynamic regulation of neuronal polarity is essential for the formation of neural networks during brain development. Primary cultures of rodent neurons recapitulate several aspects of this polarity regulation, providing valuable insights into the molecular mechanisms underlying axon specification, dendrite formation, and neuronal migration. However, the process by which the preexisting bipolarity of migrating neurons is disrupted to form multipolar dendrites remains to be elucidated. In this study, we demonstrate that MTCL2, a microtubule-crosslinking protein associated with the Golgi apparatus, plays a crucial role in this type of polarity transformation exhibited by cerebellar granule neurons (CGNs) in mice of either sex. MTCL2 is highly expressed in CGNs and gradually accumulates in dendrites as the cells develop polarity. MTCL2 knockdown inhibited the bipolar-to-multipolar transition of dendrite extension observed in their differentiation in vitro as well as in vivo. During this transformation, the Golgi apparatus shifts from the base of the preexisting bipolar neurites to the lateral or apical side of the nucleus in the cell body. There, it forms a close association with the microtubule cage that wraps around the nucleus. The resulting upward extension of the Golgi apparatus is tightly coupled with the randomization of its position in the x-y plane. Knockdown and rescue experiments demonstrated MTCL2 promotes these changes in the Golgi position in a microtubule- and Golgi-binding activity-dependent manner. These results suggest that MTCL2 promotes the development of multipolar short dendrites by sequestering the Golgi apparatus from the base of the preexisting neurite into the microtubule cage.