The floor plate is a transient epithelial cell group which begins to differentiate at the midline of the neural plate and later occupies the ventral midline of the neural tube and developing central nervous system (CNS). Recent studies indicate that cells of the floor plate have a central role in the early development of the vertebrate CNS. The floor plate, together with the underlying notochord, appears to control the pattern of cell differentiation along the dorsoventral axis of the neural tube, and may also contribute to the regionalization of the neural plate along its anteroposterior axis. Later, the floor plate appears to guide the axons of a subset of central neurons by releasing a diffusible chemoattractant which promotes the outgrowth and orientation of axons in vitro and in vivo. In addition, the floor plate appears to act as an intermediate target involved in the contact-dependent guidance of axons that cross the ventral midline of the CNS. The molecular mechanism that underlie these cellular functions of the floor plate remain poorly characterized. In an attempt to analyze the molecular basis of floor plate function we have begun to identify the novel floor plate-specific transcripts. Using a new subtractive hybridization strategy we have isolated several floor plate enriched or specific cDNAs and assessed their distribution by in situ hybridization. In this application we propose to perform a series of experiments to determine the identity and function of floor plate specific genes. One of the genes that has already been cloned and characterized, FP5, encodes on novel diffusible protein with striking homologies to proteins implicated in cell adhesion, chemotaxis and haptotaxis. We will examine the function of the FP5 protein, focussing on its potential roles in the control of neural cell differentiation and axon growth and guidance. In addition, we will complete the structural and functional characterization of two other floor plate-specific genes using similar assays. Finally, we will use refined subtractive hybridization methods to identify additional floor plate specific genes that may be involved in the developmental signalling properties of the floor plate. The information that derives from these studies should provide a clearer understanding of the cellular and molecular mechanisms that control the early development of the vertebrate central nervous system.
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