GIT1,a G-protein-coupled receptor kinase interacting protein,has been reported to be involved in neurite outgrowth.However,the neurobiological functions of the protein remain unclear.In this study,we found that GIT1 was highly expressed in the nervous system,and its expression was maintained throughout all stages of neuritogenesis in the brain.In primary cultured mouse hippocampal neurons from GIT1 knockout mice,there was a significant reduction in total neurite length per neuron,as well as in the average length of axon-like structures,which could not be prevented by nerve growth factor treatment.Overexpression of GIT1 significantly promoted axon growth and fully rescued the axon outgrowth defect in the primary hippocampal neuron cultures from GIT1 knockout mice.The GIT1 N terminal region,including the ADP ribosylation factor-GTPase activating protein domain,the ankyrin domains and the Spa2 homology domain,were sufficient to enhance axonal extension.Importantly,GIT1 bound to many tubulin proteins and microtubule-associated proteins,and it accelerated microtubule assembly in vitro.Collectively,our findings suggest that GIT1 promotes neurite outgrowth,at least partially by stimulating microtubule assembly.This study provides new insight into the cellular and molecular pathogenesis of GIT1-associated neurological diseases. GIT1, a G-protein-coupled receptor kinase interacting protein, has been reported to be involved in neurite outgrowth. However, the neurobiological functions of the protein remain unclear. In this study, we found that GIT1 was highly expressed in the nervous system, and its expression was maintained throughout all stages of neuritogenesis in the brain. In vivo cultured mouse hippocampal neurons from GIT1 knockout mice, there was a significant reduction in total neurite length per neuron, as well as in the average length of axon-like structures, which could not be prevented by nerve growth factor treatment. Overexpression of GIT1 significantly promoted axon growth and fully rescued the axon outgrowth defect in the primary hippocampal neuron cultures from GIT1 knockout mice. GIT1 N terminal region, including the ADP ribosylation factor-GTPase activating protein domain, the ankyrin domains and the Spa2 homology domain, were sufficient to enhance axonal extension. Implantantly, GIT1 bound to many tubul in proteins and microtubule-associated proteins, and it accelerated microtubule assembly in vitro. Collectively, our findings suggest that GIT1 promotes neurite outgrowth, at least partially by stimulating microtubule assembly. This study provides new insights into the cellular and molecular pathogenesis of GIT1-associated neurological diseases.