The bipolar-to-multipolar transition of dendrite extension of cerebellar granule neurons requires MTCL2 to link the Golgi apparatus to the microtubule cage

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 observed during the differentiation of cerebellar granule neurons (CGNs). MTCL2 is highly expressed in CGNs and gradually accumulates in dendrites as the cells develop polarity. MTCL2 knockdown resulted in the generation of longer and fewer dendrites by suppressing the bipolar-to-multipolar transition of dendrite extension observed in the normal polarization process. During this process, 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 that MTCL2 promotes these Golgi position changes 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 into the microtubule cage from the base of the preexisting neurite.