Chem. Senses 27: 577-580,
2002
© Oxford University Press 2002
SYMPOSIUM: Mechanisms of Differentiation and Migration of Olfactory Progenitors |
The Progenitor Cells of the Embryonic Telencephalon and the Neonatal Anterior Subventricular Zone Differentially Regulate their Cell Cycle
Department of Cell Biology, Emory University School of Medicine, Atlanta, GA 30322, USA
Correspondence to be sent to: Marla B. Luskin, Department of Cell Biology, Emory University School of Medicine, Whitehead Biomedical Research Building, 615 Michael St., Atlanta, GA 30322, USA. e-mail: luskin{at}cellbio.emory.edu
Abstract
For the last 10 years our laboratory has been studying the proliferation, migration and differentiation of neuronal progenitor cells located in the anterior part of the postnatal forebrain subventricular zone (SVZa). SVZa-derived cells possess a number of proliferative characteristics that distinguish them from the other progenitor cells in the central nervous system. This review summarizes our recent findings, in which we compared the pattern of cell cycle inhibitory proteins expressed by the neonatal SVZa to that of telencephalic ventricular zone cells.
Introduction
Most neurons of the mammalian forebrain arise prenatally from progenitor cells within the ventricular zone (VZ) and subventricular zone (SVZ) that line the lateral ventricles. The immature neurons generated at the ventricular surface migrate through the overlying intermediate zone (IZ) to reach their destinations in the cortical plate (CP). Once the immature neurons of the VZ cease proliferation and begin to differentiate and migrate, they remain forever postmitotic.
Not all neural progenitor cells of the central nervous system (CNS) obey
the order of `proliferation arrest prior to migration and differentiation'.
The neuronal progenitor cells, which concurrently undergo division while they
migrate, even though they express a neuronal phenotype, are located within a
distinct region of the anterior part of the postnatal forebrain subventricular
zone [the SVZa (Luskin, 1993
;
Menezes et al.,
1995)]. SVZa-derived cells migrate along a highly restricted
pathway, the rostral migratory stream (RMS), to reach the subependymal zone in
the middle of the olfactory bulb. Subsequently, SVZa-derived progenitor cells
migrate radially to their final destinations in the granule cell and
glomerular layers, where they become postmitotic interneurons. Thus, unlike
the progenitor cells of the embryonic telencephalic VZ, SVZa-derived cells
initiate differentiation without becoming postmitotic.
The decision to proliferate or become postmitotic is made in the
G1 phase of the cell cycle, where intrinsic and extrinsic signals
come together to act on different groups of proteins that inhibit or
facilitate cell cycle progression (Sherr,
1994). Transition from G1 to S phase is negatively
regulated by two families of cyclin-dependent kinase inhibitors (CKIs): the
CIP/KIP family (including p21CIP1, p27KIP1 and
p57KIP2) and the INK4 family (including p15INK4b,
p16INK4a, p18INK4c and p19INK4d)
(Elledge and Harper, 1994).
Among these CKIs, p18INK4c, p19INK4d and
p27KIP1 are expressed in the CNS during development
(Zindy et al., 1997;
van Lookeren Campagne and Gill,
1998) and may play pivotal roles governing neurogenesis.
As indicated above, SVZa-derived cells can undergo cell division despite the initiation of differentiation and migration, unlike the progenitor cells of the embryonic telencephalic VZ. This unique property of the SVZa cells prompted us to examine whether they regulate their cell cycle in a different manner from other CNS progenitors. To determine whether the differential regulation of the G1—S progression can account for the unusual proliferative behavior of SVZa progenitor cells, we have compared the pattern of p19INK4d expression by the cells of the neonatal SVZa to that of telencephalic VZ cells.
p19INK4d is expressed by postmitotic immature neurons of the telencephalic VZ
VZ progenitors of the developing cerebral cortex withdraw from the cell
cycle at the ventricular surface, and the newly generated postmitotic neurons
initiate differentiation and migrate to their final destinations in the CP
(Bayer et al., 1991).
Our staining of the developing cerebral cortex with an antibody to
p19INK4d revealed that the newly generated neurons at the
ventricular surface express p19INK4d, while the progenitor cells
that are in the S phase at the basal border of the VZ are devoid of
p19INK4d immunoreactivity
(Coskun and Luskin, 2001). This
suggests that p19INK4d plays a role in regulating the timely
withdrawal of VZ progenitors from the cell cycle.
Based on the p19INK4d expression pattern, the VZ can be divided into a p19INK4d(-) upper (VZu) and p19INK4d(+) lower (VZl) subdivision. Furthermore, our experiments demonstrated that p19INK4d expression is down-regulated while the cells ascend through VZu and reactivated upon entering the IZ. Migrating cortical neurons in the IZ start to express p19INK4d in a punctate manner in the apical domain of the cell at the cytoplasmic—nuclear border, while the neurons of the CP express p19INK4d in a more diffuse pattern (i.e. crescent shaped) (Figure 1C).
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The suggestion has been put forward that CKIs not only coordinate the cell
cycle exit of the progenitor cells, but also maintain the postmitotic cells in
a quiescent state (Zindy et al.,
1999). In agreement with this notion, our data revealed that
p19INK4d expression persists in the differentiating postmitotic
neurons of the cerebral cortex. This persistent expression of
p19INK4d in quiescent cells suggests that p19INK4d
prevents postmitotic cells from re-entering the proliferative cycle; the
neurons in p19INK4d-p27KIP1 double knock out mice
undergo extra rounds of cell division in regions where only quiescent neurons
reside in wild type animals (Zindy et
al., 1999). Collectively, our results revealed that in the
developing telencephalon, p19INK4d is expressed mainly by the
postmitotic immature neurons and has a characteristic perinuclear distribution
that is correlated with a cell's laminar position and state of
differentiation.
SVZa-derived cells down-regulate p19INK4d expression in the RMS and re-enter the cell cycle
In order to determine whether the pattern of p19INK4d expression
by SVZa neuronal progenitor cells can account for their unusual proliferative
behavior, we analyzed the spatiotemporal expression pattern of
p19INK4d by the cells of the rodent RMS. Our data revealed that
SVZa-derived cells exhibit an
anteriorhigh—posteriorlow gradient of
p19INK4d expression along the RMS
(Coskun and Luskin, 2001).
Detection of progressively more p19INK4d-immunoreactive cells in
the RMS as the olfactory bulb is approached indicates that few cells withdraw
from the cell cycle in the SVZa and increasingly more do so as they reach the
bulb.
To reconcile the observation that cells of the SVZa/RMS undergo division as they migrate and that SVZa-derived cells in the proximal portion of the RMS initiate p19INK4d expression (usually indicative of a postmitotic cell), we investigated the hypothesis that SVZa cells may down-regulate the expression of p19INK4d and re-enter the cell cycle to undergo another round of cell division before reaching the subependymal zone. To test this hypothesis, we administered the proliferation marker bromodeoxyuridine (BrdU) to rat pups at 3 and 9 h before their perfusion and examined the expression of p19INK4d by the BrdU(+) cells along the RMS. We observed that at 3 h following BrdU administration, very few SVZa-derived cells along the RMS co-localize BrdU and p19INK4d. However, at 9 h following BrdU administration, a significant fraction of the BrdU(+) cells were immunoreactive for both BrdU and p19INK4d. Taken together, these findings indicate that SVZa-derived cells in the RMS successively down-regulate their p19INK4d expression prior to undergoing division, which may enable them to repeatedly exit and re-enter the cell cycle. Consistent with the idea that p19INK4d expression persists in postmitotic cells to maintain them in a quiescent state, postmitotic neurons at their final destinations in the olfactory bulb were also p19INK4d-immunoreactive.
Conclusions and future directions
In conclusion, based on the p19INK4d immunoreactivity and cell cycle kinetics, SVZa-derived cells appear to successively exit and re-enter the cell cycle, despite expressing a neuronal phenotype. This is in contrast to telencephalic VZ cells, in which p19INK4d expression is initiated even before they undergo differentiation. Since the SVZa-derived cells in the RMS appear to repeatedly down-regulate p19INK4d expression, this might indicate that SVZa cells continue to undergo multiple rounds of de-differentiation and division. Therefore, the unique proliferation characteristics of the SVZa-derived cells can, to some extent, be attributed to the dynamic regulation of p19INK4d expression.
It had been suggested that different members of the CIP/KIP and INK4
families might cooperate to ensure timely cell cycle exit of proliferating
cells (Thullberg et al.,
2000). In order to determine the cooperative interactions between
the CKIs in the regulation of SVZa cell proliferation and differentiation, our
future experiments will determine which other CKIs are expressed by SVZa cells
in addition to p19INK4d. We have also initiated studies to
determine the extracellular signaling molecules that act on CKIs to regulate
the proliferation of SVZa-derived cells
(Coskun et al.,
2001).
Acknowledgments
This work was supported in part by a grant awarded to MBL from the National Institute of Deafness and Other Communicative Disorders (RO1DC03190).
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Accepted May 1, 2002
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