脳神経科学を社会へ還流する教育研究拠点　東北大脳科学グローバルCOE

ALESSANDRA PIERANI
(INSTITUT JACQUES-MONOD, UNIVERSITE PARIS 6 ET 7)

Forebrain development and facial morphogenesis are intimately linked processes, involving the establishment of a molecular dialog between facial ectoderm, cephalic mesenchyme and the underlying neurectoderm from early steps of head development. Growing evidences indicate that the coordinated activity of signalling factors such as Fgfs, BMPs, retinoids and Shh is required to ensure the correct maturation of both craniofacial structures and neural tissues. We study the role of small subsets of cells expressing Dbx1 (a homeobox transcription factor) at early steps of head development in mice. Around E8.5 (12 somites) we identified four discrete populations of Dbx1+ progenitors located respectively (i) in the ventral diencephalon (hypothalamus), (ii) in the dorsal diencephalon/mesencephalon, (iii) in the ventral anterior neural ridge (ANR) and (iv) in the facial ectoderm overlying the optic vesicle. We have performed genetic ablation in mice using the conditional expression of diphtheria toxin (Dbx1-IRES-loxP-stop-loP-DTA) following Cre/Lox recombination. Complete ablation of Dbx1+ subpopulations using pGK:CRE animals results in a total loss of forebrain and dorsal facial tissues at E12.5. Massive apoptosis can be observed from E8.5 and occurs in the presumptive tissues of the forebrain and face. Genetic tracing experiments clearly show that the cells committing apoptosis neither express Dbx1 nor derive from Dbx1+ progenitors. Dbx1+ cells, therefore, appear to have a signalling function that is essential to ensure the survival of the surrounding tissues. In order to test the relative involvement of the four Dbx1+ subsets, we have performed a selective genetic ablation of each of them by crossing Dbx1DTA mice with various Cre-expressing animals (Nkx2.1:Cre, Wnt1:Cre, Foxg1Cre and Le:Cre). Our results suggest that each Dbx1+ population has a specific role in patterning correlating with the known morphogen function at their site of origin. Nevertheless, ablation of each Dbx1+ population individually does not lead to an absence of forebrain and face, suggesting that several Dbx1+ populations ensure the surviving activity.