Glial Fibrillary Acidic Protein in the Cytoskeletal and Soluble Protein Fractions of the Developing Rat Brain

: The distribution of glial fibrillary acidic protein (GFAP) into cytoskeletal and soluble protein fractions during development of the rat brain has been studied by quantitative immunoblotting and enzyme‐linked immunosorbent assay (ELISA). These assays indicate that cytoskeletal GFAP accounts for nearly all the total GFAP in the adult rat brain, and that the developmental increase in the GFAP content of the rat brain is due to accumulation of GFAP into the cytoskeleton. A small and constant amount of the total GFAP was detected in the soluble protein fraction. This GFAP had an apparent molecular mass (Mr) similar to that of the highest Mr form of GFAP detected in the cytoskeletal fraction. In contrast to the assays for cytoskeletal GFAP, no significant increase in the GFAP concentration of the soluble protein fraction could be measured during development. Sensitive, calibrated immunoblotting of cytoskeletal and soluble protein with [125I]protein A confirmed these findings, and showed that both cytoskeletal and soluble GFAP are first detected during the same period of foetal rat brain development. A finite and reproducible amount of lower Mr forms of GFAP were observed in the cytoskeletal fraction even when prepared in the presence of stringent proteolytic inhibitors. These presumed proteolytic degradation products of GFAP increased in abundance during development, parallel to the increase in cytoskeletal GFAP content of the rat brain. However, the abundant proteolytic degradation products of GFAP found in the cytoskeletal fraction were not detected in the soluble protein fraction at any age studied. These findings, and the failure to detect a significant increase in the amount of soluble GFAP during development, strongly suggest that the very small pool of soluble GFAP is unlikely to be derived from the much larger pool of cytoskeletal GFAP. The possible role of soluble GFAP as a precursor to glial filaments is discussed.