The 43-kDa neuronal growth-associated protein (GAP-43) is present in plasma membranes of rat astrocytes.

Abstract

One of the neuronal growth-associated proteins, GAP-43 (molecular mass, -43 kDa; pI 4.3), is abundant in growth-cone membranes and corresponds to a major protein kinase C substrate, the 46-kDa phosphoproteiw (pp46), of a growth-cone-enriched subcellular fraction. This protein has the following additional designations (depending on context): B-50 (phospholipid metabolism), F1 (synaptic plasticity), and p57 (calmodulin binding). We show that a protein with the same molecular mass and isoelectric point as GAP-43, which interacts with anti-GAP-43 antibodies on immunoblots, is present in the plasma membranes of cultured neonatal rat cortical astrocytes. Double-immunofluorescence labeling of cells with a serum against glial fibrillary acidic protein and anti-GAP-43 antibody was observed. Furthermore, astrocytic protein was phosphorylated in vitro by protein kinase C and comigrated in two-dimensional PAGE with GAP-43. The data indicate that GAP-43, heretofore believed to be neuronspecific, is present in at least one class of glial cells.Astrocytes constitute about one-half of all glial cells in the central nervous system. Although their precise functions are unknown, data indicate that they may be involved in a symbiotic relationship with neurons and thereby play important roles in neuronal development and function. This suggests that a significant aspect of astrocytic activity involves its plasma membrane. However, astrocytic plasma membranes remain to a large extent biochemically uncharacterized and, as such, their capacity for interacting with their environment is unknown. In this report, the proteins of the astrocytic plasma membrane are characterized by twodimensional PAGE, immunoblotting, and comparison with membranes from other cell types.We have described two brain-specific plasma membrane proteins that are very abundant in rat cortex (1). The smaller polypeptide was identified as growth-associated protein 43 (GAP-43) because of its relative molecular mass (=43 kDa), pI value (pI 4.3), in vitro phosphorylation by protein kinase C (PKC), and reactivity with anti-GAP-43 antibody. A polypeptide with these characteristics has been identified by several groups and designated F1, B-50, and a 46-kDa phosphoprotein (pp46) (2). Since this protein is developmentally regulated and its synthesis increases in regenerating axons, it was named growth-associated protein 43 (3). We show here that a protein with a molecular mass and a pI value similar to those of neuronal GAP-43 is present in the plasma membranes of cultured rat astrocytes. Two anti-GAP-43 antibodies interacted with this protein on immunoblots and mediated fluorescent labeling of cultured astrocytes. It is currently believed that GAP-43 is exclusively a neuronal protein. The presence of GAP-43 in astrocytic membranes may have important implications for the role of this protein in nervous system development.

Neuropathy-Induced Spinal GAP-43 Expression Is Not a Main Player in the Onset of Mechanical Pain Hypersensitivity

“…As an alternative to neuronal expression, GAP-43 expression may be regulated in glial cells. Schwann cells have been reported to upregulate GAP-43 following peripheral neuropathy (Plantinga et al, 1993a(Plantinga et al, ,1993b, and astroglial GAP-43 expression has been found in vitro (Vitkovic et al, 1988). Importantly, we did not obtain any evidence for neuropathy-induced GAP-43 regulation in astroglial cells within the superficial dorsal horn.…”

Cell cultures of neuroblasts from rat olfactory epithelium that show odorant responses.

“…3B) prepared from extracts of SNIF-11 and -12. A protein with these properties, presumably GAP-43, may also appear in certain glial cell cultures (24) and in the C6 rat glioma (Fig. 3B); however, immunoblots of SNIF-11 and -12 do not show the glial marker, glial fibrillary acidic protein (data not shown).…”

GAP‐43 as a plasticity protein in neuronal form and repair

“…GAP-43 is neuron specific in expression (Karns et al, 1987;Basi et al, 1987;Biffo et al, 1990), except for some expression in cultured glial precursor cells (Vitkovic, Steisslinger, Aloyo, and Mersel, 1988;Deloulme et al, 1990). GAP-43 protein and RNA levels increase after injury to peripheral nerves (Skene and Willard, 198 la;Benowitz and Lewis, 1983;Hoffman, 1989).…”