Tenascin glycoproteins in developing neural tissues: only decoration?
Tenascin glycoproteins constitute a growing family of extracellular matrix molecules that are transiently expressed by astrocytes during the development of the central nervous system (CNS). In some areas, tenascin distribution is discrete and tenascin boundaries delineate emerging functional processing units, for example, in the somatosensory cortex. The intact adult CNS displays low levels of expression and up-regulation of tenascin after lesion or in glial tumors. In vitro studies using the purified glycoprotein, bacterially expressed tenascin fusion proteins, monoclonal antibodies, and defined cell culture models have established that tenascin glycoproteins possess cell-binding sites for neural cells, support neuronal migration and the formation of neurites. On the other hand, tenascin also embodies repulsive, inhibitory properties that could serve to conceal neuronal assemblies and to segregate emerging fiber tracts. These functional properties are encoded by distinct domains of the molecule and suggest that tenascin glycoproteins are involved in neural pattern formation and regeneration. This interpretation is discussed with reference to a recent report that the elimination of the tenascin gene does not cause obvious abnormalities of neural tissues.