MAP5 expression in proliferating neuroblasts

MAPS, a microtubule-associated protein present in immature neurons, was fou nd to be expressed in the embryonic mouse telencephalic ventricular zone (V Z). Since the VZ contains proliferating neuroblasts, the source of most of the neurons of the cerebral cortex, this observation raised the possibility that MAPS is expressed by proliferating neuronal progenitors. MAPS-positiv e mitotic cells were observed at the ventricular surface, a finding consist ent with progenitors expressing MAPS prior to their last division. This pos sibility was investigated using dissociated, cortical cells in vitro by mea suring the expression of MAPS and the neuroepithelial marker nestin, togeth er with the incorporation of bromodeoxyuridine (BrdU), a thymidine analogue that labels the DNA of proliferating cells in the S-phase of the cell cycl e. All of the proliferating cells expressed nestin. A population of MAPS-po sitive cells was also found to incorporate BrdU; some cells expressed MAPS within 30 min of BrdU labeling. The results suggest that uncommitted neurob lasts express only nestin, with expression of MAPS occurring near the time the cell commits to become a postmitotic neuron after the next cell divisio n. Subsequently, cells expressing both MAPS and nestin leave the cell cycle and exit the VZ, lose nestin, and differentiate into neurons. Since some c ells expressed MAPS during or shortly after S-phase but before mitosis, MAP S may be the earliest marker to identify neuronal progenitors that will bec ome post-mitotic neurons following their next mitosis. (C) 1999 Published b y Elsevier Science B.V. All rights reserved.