Reelin, an extracellular glycoprotein has an important role in the proper migration and positioning of neurons during brain development. Lack of reelin causes not only disorganized lamination of the cerebral and cerebellar cortex but also malpositioning of mesencephalic dopaminergic (mDA) neurons. However, the accurate role of reelin in the migration and positioning of mDA neurons is not fully elucidated. In this study, reelin-deficient reeler mice exhibited a significant loss of mDA neurons in the substantia nigra pars compacta (SNc) and a severe alteration of cell distribution in the retrorubal field (RRF). This abnormality was also found in Dab1-deficinet, yotari mice. Stereological analysis revealed that total number of mDA neurons was not changed compared to wild type, suggesting that the loss of mDA neurons in reeler may not be due to the neurogenesis of mDA neurons. We also found that formation of PSA-NCAM-positive tangential nerve fibers rather than radial glial fibers was greatly reduced in the early developmental stage (E14.5) of reeler. These findings provide direct evidence that the alteration in distribution pattern of mDA neurons in the reeler mesencephalon mainly results from the defect of the lateral migration using tangential fibers as a scaffold.

Disabled 1 (Dab1), a cytoplasmic adaptor protein expressed predominantly in the CNS, transduces a Reelin-initiated signaling that controls neuronal migration and positioning during brain development. To determine the role of Dab1 in neural stem cell (NSC) differentiation, we established a culture of neurospheres derived from the embryonic forebrain of the Dab1 −/− mice, yotari. Differentiating Dab1 −/− neurospheres exhibited a higher expression of GFAP, an astrocytic marker, at the expense of neuronal markers. Under Dab1-deficient condition, the expression of NeuroD, a transcription factor for neuronal differentiation, was decreased and the JAK-STAT pathway was evidently increased during differentiation of NSC, suggesting the possible involvement of Dab1 in astrocyte differentiation via JAK-STAT pathway. Notably, expression of neural and glial markers and the level of JAK-STAT signaling molecules were not changed in differentiating NSC by Reelin treatment, indicating that differentiation of NSC is Reelin-independent. Immunohistochemical analyses showed a decrease in the number of neurons and an increase in the number of GFAP-positive cells in developing yotari brains. Our results suggest that Dab1 participates in the differentiation of NSCs into a specific cell lineage, thereby maintaining a balance between neurogenesis and gliogenesis.

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The extracellular protein, reelin, which binds to ApoER2, VLDLR controls the positioning of radially migrating neurons and synaptic plasticity. During neurodevelopment activation of Go/i, a heterotrimeric G protein, facilitates the axon growth. Here we investigated the relationship between of Reelin signaling and Go/i during neurite formation of primary hippocampal neurons. In reeler, reelin‐deficient mice, neuritogenesis was significantly decreased. Inhibition of neuritogenesis in reeler was rescued by reelin treatment. The localization of Gαi1, Gαi3 and Gαo were obvious in the terminal portion of growing neurites in wild type, but not in reeler. In addition, treatment of pertussis toxin, Go/i inhibitor, suppressed the neurite formation by reelin. In study using F11 cells transfected Gαo mutant (Q205L), induction of neurite outgrowth by reelin could not be observed. To address the underlying mechanism, we investigated the downstream signaling pathway of reelin and Go/i using specific inhibitors. Reelin treatment induced the activation of MAPK‐JNK pathway, which is well‐known as a major pathway in neuritogenesis. Taken together, we suggest that Go/i signaling may be involved in induction of neuritogenesis by reelin.