Brain cells and glioma cells in culture release a protein which induces neurite outgrowth in neuroblastoma cells. This neurite‐promoting factor (NPF), which has been purified from serum‐free glioma conditioned medium, has an apparent mol. wt. of 43 000. NPF inhibits urokinase as well as plasminogen activator‐dependent caseinolysis or fibrinolysis. NPF and urokinase form an SDS‐resistant complex. The fact that this glia‐derived NPF is a potent protease inhibitor indicates that glial cells modulate the proteolytic activity associated with neuronal cells and suggests that this phenomenon is one of the biochemical events involved in the regulation of neurite growth.

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. Cultured explants from early postnatal mouse cerebellum were used to examine the influence of a 43-kDa glia-derived neurite-promoting factor (GdNPF) on the migration of [3H]thymidine-labeled granule cell neurons.GdNPF, which is a potent serine protease inhibitor, significantly reduced the extent of granule cell migration in a dose-dependent manner. This effect could be neutralized by addition of thrombin, which binds GdNPF. Other protease inhibitors such as aprotinin, hirudin, soybean trypsin inhibitor, leupeptin, 6-aminocaproic acid, and D-Phe-Pro-ArgCH2CI do not show this inhibitory effect. These results demonstrate that a glia-derived protein can regulate the migration of postmitotic neurons, an important cellular event in the development of the nervous system.