EXPRESSION OF 2 NEURONAL MARKERS, GROWTH-ASSOCIATED PROTEIN-43 AND NEURON-SPECIFIC ENOLASE, IN RAT GLIAL-CELLS

Recent studies have revealed that proteins such as growth-associated p rotein 43 (GAP-43) and neuron-specific enolase (NSE), believed for man y years to be expressed exclusively in neurons, are also present in gl ial cells under some circumstances. Here we present an overview of the se observations. GAP-43 is expressed both in vitro and in vivo transie ntly in immature rat oligodendroglial cells of the central nervous sys tem, in Schwann cell precursors, and in non-myelin-forming Schwann cel ls of the peripheral nervous system. GAP-43 mRNA is also present in ol igodendroglial cells and Schwann cells, indicating that GAP-43 is synt hesized in these cells. GAP-43 is also expressed in type 2 astrocytes (stellate-shaped astrocytes) and in some reactive astrocytes but not i n type 1 astrocytes (flat protoplasmic astrocytes). These results sugg est that GAP-43 plays a more general role in neural plasticity during development of the central and peripheral nervous systems. NSE enzymat ic activity and protein and mRNA have been detected in rat cultured ol igodendrocytes at levels comparable to those of cultured neurons. NSE expression increases during the differentiation of oligodendrocyte pre cursors into oligodendrocytes. In vivo, NSE protein is expressed in di fferentiating oligodendrocytes and is repressed in fully mature adult cells. The upregulation of NSE in differentiating oligodendrocytes coi ncides with the formation of large amounts of membrane structures and of protoplasmic processes. Similarly, NSE becomes detectable in glial neoplasms and reactive glial cells at the time when these cells underg o morphological changes. The expression of the glycolytic isozyme NSE in these cells, which do not normally contain it, could reflect a resp onse to higher energy demands. This expression may also be related to the neurotrophic and neuroprotective properties demonstrated for this enolase isoform. NSE activity and protein and mRNA have also been foun d in cultured rat type 1-like astrocytes but at much lower levels than in neurons and oligodendrocytes. Thus GAP-43 and NSE should be used w ith caution as neuron-specific markers in studies of normal and pathol ogical neural development.