GENETIC-CONTROL OF BRAIN MORPHOGENESIS THROUGH OTX GENE DOSAGE REQUIREMENT

Understanding the genetic mechanisms that control patterning of the ve rtebrate brain represents a major challenge for developmental neurobio logy, Previous data suggest that Otx1 and Otx2, two murine homologs of the Drosophila orthodenticle (otd) gene, might both contribute to bra in morphogenesis. To gain insight into this possibility, the level of OTX proteins was modified by altering in vivo the Otx gene dosage, Her e we report that Otx genes may cooperate in brain morphogenesis and th at a minimal level of OTX proteins, corresponding either to one copy e ach of Otx1 and Otx2, or to only two copies of Otx2, is required for p roper regionalization and subsequent patterning of the developing brai n, Thus, as revealed by anatomical and molecular analyses, only Otx1(- /-); Otx2(+/-) embryos lacked mesencephalon, pretectal area, dorsal th alamus and showed an heavy reduction of the Ammon's horn, while the me tencephalon was dramatically enlarged occupying the mesencencephalic a rea, In 8.5 days post coitum (d.p.c.) Otx1(-/-); Otx2(+/-) embryos, th e expression patterns of mesencephalic-metencephalic (mes-met) markers such as En-1 and Wnt-1 confirmed the early presence of the area fated to give rise to mesencephalon and metencephalon while Fgf-8 transcrip ts were improperly localized in a broader domain, Thus, in Otx1(-/-); Otx2(+/-) embryos, Fgf-8 misexpression is likely to be the consequence of a reduced level of specification between mes-met primitive neuroep ithelia that triggers the following repatterning involving the transfo rmation of mesencephalon into metencephalon, the establishment of an i sthmic-like structure in the caudal diencephalon and, by 12.5 d.p.c., the telencephalic expression of Wnt-1 and En-2. Taken together these f indings support the existence of a molecular mechanism depending on a precise threshold of OTX proteins that is required to specify early re gional diversity between adjacent mes-met territories and, in turn, to allow the correct positioning of the isthmic organizer.