An overriding principle of development is that neurons become permanently postmitotic once they initiate differentiation. Work in our laboratory, however, has provided evidence for a population of progenitor cells in mammalian forebrain that express properties of differentiated neurons, even though they continue to divide. These neuronal progenitor cells are situated in the rostral migratory stream (RMS), which extends from a specialized portion of the subventricular zone surrounding the anterior tip of the lateral ventricle, referred to as the SVZa, to the middle of the olfactory bulb. As SVZa-derived cells migrate to the olfactory bulb, they undergo cell division, and they never deviate from the RMS. Once they reach their final destinations, they become terminally postmitotic interneurons. This Mini-Review concerns findings from our recent experiments designed to reveal the intrinsic and extrinsic mechanisms governing the proliferation and differentiation of the unique SVZa neuronal progenitor cells. We have investigated the role(s) of cell cycle regulatory proteins, in particular, the cell cycle inhibitor p19(INK4d), in the control of SVZa cell proliferation. Several studies have indicated that cells withdraw from the cell cycle once they express p19(INK4d). To begin to investigate whether p19(INK4d)(+) SVZa-derived cells are postmitotic, we analyzed the pattern of p19(INK4d) expression by the cells of the RMS. A pronounced gradient of p19(INK4d) expression was demonstrated; progressively more cells are p19(INK4d) immunoreactive as the olfactory bulb is approached. In addition, the capacity of p19(INK4d)(+) cells to incorporate bromodeoxyuridine was investigated. From the results of these studies, we conclude that SVZa cells in the RMS can successively down-regulate their expression of p19(INK4d) as they migrate and that they repeatedly exit and reenter the cell cycle while en route to the olfactory bulb. These studies led us to investigate whether bone morphogenetic proteins (BMPs) are involved in the regulation of SVZa cell proliferation and p19(INK4d) expression, because, elsewhere in the CNS, BMPs modulate cell proliferation and influence cell fate decisions. To determine the effects of BMP signaling on SVZa cell proliferation and differentiation, we altered the expression of the BMP receptor Ia (BMPR-Ia) using retrovirally mediated gene transfer. The cells in the SVZa encoding the wild-type BMPR-Ia exit the cell cycle and do not appear to migrate through the RMS. Conversely, both within the SVZa and along the RMS, the majority of SVZa-derived cells encoding a dominant-negative BMPR-Ia gene do not express p19(INK4d). These findings indicate that p19(INK4d) expression is suppressed when BMP signaling is inhibited. Furthermore, SVZa-derived cells with both augmented and inhibited BMP signaling retain their neuronal commitment. Collectively, these studies have revealed that SVZa cell proliferation and differentiation is under the control of several interacting intrinsic and extrinsic factors.

Copyright 2002 Wiley-Liss, Inc.