Secretion of amyloid precursor protein and laminin by cultured astrocytes is influenced by culture conditions

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Abstract
Although normally quiescent, astrocytes in the adult brain respond to various types of brain injury by rapidly dividing, swelling, extending cellular processes, and expressing increased amounts of glial fibrillary acidic protein (GFAP). These phenomena are collectively referred to as “astrogliosis.” Similarly, astroglia in primary culture stop dividing when they attain confluency, yet, as seen in situ, they retain their proliferative capacity for extended periods and resume rapid division when subcultured. To examine the impact of glial division on secretion of neuritepromoting factors, conditioned medium (CM) was removed from subconfluent, newly confluent, and longterm confluent (“aged”) neonatal rat astrocyte cultures, and from aged confluent cultures that had been repassaged, “Lesioned” (scraping with a rubber policeman), or triturated 3 days before harvest. Secretion of neurite-promoting factor(s) by glial cells into these CM was then assayed by treating neuroblastoma cultures with these various CM and quantitating neurite elaboration. Extensive neurite sprouting was elicited by CM from cultures just reaching confluency and from repassaged, lesioned, or triturated cultures. CM from aged confluent cultures did not induce sprouting. These results indicate that secretion of neurite-promoting factors(s) is regulated by glial division, and suggest that gliosis in situ may contribute to neurite sprouting by similar mechanisms. Immunoblot analysis demonstrated the presence in CM of varying amounts of laminin and amyloid precursor protein (APP), including isoforms containing the Kunitz-type protease inhibitor domain. CM from subconfluent cultures contained trace amounts of these protiens, but CM from cultures just reaching confluency contained significant amounts. Although CM from aged cultures contained barely detectable levels of either protein, trituration or repassage of aged cultures dramatically increased secretion of these proteins. APP-and laminin-enriched CM fractions promoted neuritogenesis to a similar level as respective unfractionated CM; anti-APP and antilaminin antisera blocked this effect. Purified human brain APP promoted neuritogenesis when added to non-conditioned medium and aged CM. Increased secretion of APP and laminin therefore mediates at least a portion of CM-induced neuronal sprouting; these proteins may perform analogous functions during astrogliosis in situ.

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