Forcing neural progenitor cells to cycle is insufficient to alter cell-fate decision and timing of neuronal differentiation in the spinal cord.
BACKGROUND
During the development of the nervous system, neural progenitor cells can either stay in the pool of proliferating undifferentiated cells or exit the cell cycle and differentiate. Two main factors will determine the fate of a neural progenitor cell: its position within the neuroepithelium and the time at which the cell initiates differentiation. In this paper we investigated the importance of the timing of cell cycle exit on cell-fate decision by forcing neural progenitors to cycle and studying the consequences on specification and differentiation programs.


RESULTS
As a model, we chose the spinal progenitors of motor neurons (pMNs), which switch cell-fate from motor neurons to oligodendrocytes with time. To keep pMNs in the cell cycle, we forced the expression of G1-phase regulators, the D-type cyclins. We observed that keeping neural progenitor cells cycling is not sufficient to retain them in the progenitor domain (ventricular zone); transgenic cells instead migrate to the differentiating field (mantle zone) regardless of cell cycle exit. Cycling cells located in the mantle zone do not retain markers of neural progenitor cells such as Sox2 or Olig2 but upregulate transcription factors involved in motor neuron specification, including MNR2 and Islet1/2. These cycling cells also progress through neuronal differentiation to axonal extension. We also observed mitotic cells displaying all the features of differentiating motor neurons, including axonal projection via the ventral root. However, the rapid decrease observed in the proliferation rate of the transgenic motor neuron population suggests that they undergo only a limited number of divisions. Finally, quantification of the incidence of the phenotype in young and more mature neuroepithelium has allowed us to propose that once the transcriptional program assigning neural progenitor cells to a subtype of neurons is set up, transgenic cells progress in their program of differentiation regardless of cell cycle exit.


CONCLUSIONS
Our findings indicate that maintaining neural progenitor cells in proliferation is insufficient to prevent differentiation or alter cell-fate choice. Furthermore, our results indicate that the programs of neuronal specification and differentiation are controlled independently of cell cycle exit.
Lobjois
Valérie
V
Centre de Biologie du Développement, UMR5547, Institut d'Exploration Fonctionnelle des Génomes IFR109, Université Toulouse III et Centre National de Recherche Scientifique, 31062 Toulouse, France. lobjois@cict.fr
Bel-Vialar
Sophie
S
Trousse
Françoise
F
Pituello
Fabienne
F
D000818:Animals; D001369:Axons / Q000502:physiology; D015415:Biomarkers; D002453:Cell Cycle / Q000502:physiology; D002454:Cell Differentiation / Q000502:physiology; D002552:Cerebral Ventricles / Q000166:cytology / Q000196:embryology; D002642:Chick Embryo; D002645:Chickens; D019938:Cyclin D1 / Q000235:genetics; D018507:Gene Expression Regulation, Developmental; D009046:Motor Neurons / Q000166:cytology* / Q000502:physiology / Q000648:ultrastructure; D046569:Neuroepithelial Cells / Q000166:cytology / Q000502:physiology; D009836:Oligodendroglia / Q000166:cytology* / Q000502:physiology; D013116:Spinal Cord / Q000166:cytology* / Q000196:embryology*; D013234:Stem Cells / Q000166:cytology* / Q000502:physiology; D014162:Transfection
D015415:Biomarkers; D019938:Cyclin D1