Molecular analysis of brain development and function

Recent progress in the ®eld of vertebrate developmental neurobiology has been made not only in areas which have become classical (pattern formation, neural crest) but also in relatively new subjects such as the behavioural analysis of mutations affecting CNS development, and the properties of stem cells and how to exploit them to treat disease. Participants in the meeting (`Molecular Analysis of Brain Development and Function', 14±19 June, Tourtour, France) organized by Peter Gruss and Nicole Le Douarin and funded by the Fondation Les Treilles use approaches to developmental biology that re ̄ect this variety of subjects. Marnie Halpern (Carnegie Institution, Baltimore) discussed some of the early events of patterning of the neural tube that are already initiated during gastrulation. Previous work has shown that the notochord induces the formation of the ̄oor plate and motor neurons in the ventral neural tube via the signalling molecule sonic hedgehog (shh). However, the analysis of zebra®sh mutants that affect notochord and/or ̄oor plate development suggests that earlier events also control the formation of ̄oor plate and motor neurons. For example, shh and the related twhh gene are coexpressed in the shield, the zebra®sh equivalent of the `organizer', that contains precursors for the prechordal mesoderm, notochord and ̄oor plate. Subsequently, the notochord and ̄oor plate express only shh and twhh, respectively. However, in the nil mutant in which notochord development is blocked, shh and twhh continue to be coexpressed and there is an expansion of the ̄oor plate, consistent with a switch of notochord precursors to a ̄oor plate fate. The ̄oor plate fails to form and shh is not expressed in a mutant of the cyclops gene, that encodes a nodal-related signalling molecule expressed in the shield and the prechordal mesoderm. These defects in cyclops mutants are rescued by injection of RNA to express cyclops in prechordal mesoderm (but not when expressed in notochord), suggesting that this tissue has an important role in induction as it migrates past the presumptive spinal cord. Later in development, cyclops, antivin (a TGFb-related gene), and the pitx2 transcription factor are expressed in an asymmetric manner, on the left side of the presumptive pineal organ in the dorsal diencephalon. These genes form part of a regulatory cascade in which one-eyed pinhead (oep), required for nodal signalling, is essential. By injecting oep RNA into oep mutants, early gastrulation defects are rescued, but not the later role in left-right asymmetric gene expression. Intriguingly, this leads to a mispositioning of the normally asymmetric location of the pineal organ, indicating a role in the generation of anatomical asymmetry in the forebrain. One major theme of the meeting was the role of transcription factors in pattern formation along the anteroposterior and dorsoventral axes in the mouse midbrain and forebrain. Antonio Simeone (IIGB, Milan, and Guys Hospital, London) discussed the relative roles of Otx1 and Otx2 in the speci®cation of the anterior brain. The forebrain, midbrain and anterior hindbrain are missing in Otx2 2/2 mutants, whereas Otx1 2/2 mutants only have later defects in the telencephalon and eye, and in semicircular canals in the inner ear. Otx2 function is required at early stages in anterior visceral endoderm (AVE) that induces anterior neural tissue, and is subsequently expressed in, and required for the maintenance of, forebrain and midbrain territory. Furthermore, the interface of Otx2 and Gbx2 expression at the presumptive midbrain/hindbrain interface is required for formation of the isthmic organizer, a signalling centre expressing FGF8 that induces the midbrain and anterior hindbrain. Whereas the isthmus forms normally in Otx2 1/2 mutants, in Otx1 2/2, Otx2 1/2 mutants FGF8 expression shifts anteriorly into the posterior forebrain and fails to become restricted to its normal narrow domain. However, forebrain tissue is not transformed into midbrain, suggesting that Otx gene function is required for the competence to respond to FGF8. These results suggest that Otx1 and Otx2 have at least partly overlapping roles, and that a minimum Otx gene dosage is required for anterior brain development. This raises the question of whether the different phenotypes of Otx1 and Otx2 mutants re ̄ect divergent biochemical properties of these transcription factors, or Mechanisms of Development 98 (2000) 193±198