Microglia‐derived plasminogen enhances neurite outgrowth from explant cultures of rat brain

Recently, we reported the production and secretion of plasminogen (Pg) in cultured rat brain microglia [Nakajima et al., (1992) Fedn. Eur. Biochem. Socs Lett.308, 179–182]. To investigate the physiological significance of Pg, we determined the effect of Pg on neurite outgrowth of cultured neocortical explants of an embryonic rat brain in serum‐free chemically defined medium. Pg markedly enhanced the neurite outgrowth. Although plasmin, which is derived from Pg by activation by urokinase (UK), had a similar effect in this explant culture system, UK itself did not show any effect. Furthermore, we studied the characteristics of Pg binding to cultured neocortical neurons dissociated from an embryonic 16‐day‐old rat brain by using 125I‐Pg. Specific binding of Pg to neocortical neurons was detected and Scatchard plot analysis revealed high‐ and low‐affinity binding sites on the neurons. The estimated dissociation constants of high‐ and low‐affinity binding sites were approximately 16.1 and 124.2 nM, respectively. These results suggest that microglia‐derived Pg plays certain roles in the regulation of neurite extension through binding to the surface of neocortical neurons.