The long term goal of this research is to understand the basic developmental events which control the morphogenesis of the endocardial cushions during heart development. The endocardial cushions are the progenitors of portions of the valves and membranous septa in the adult heart. Since the majority of congenital heart defects appear to involve lesions of the valves and septa of the heart, it is thought that maldevelopment of endocardial cushion tissue may be responsible for many of the defects. The mesenchymal cells which make up the major portion of the endocardial cushions originate from the endothelium in restricted regions of the developing heart by an epithelial-mesenchymal transformation. These mesenchymal cells invade and proliferate in the cardiac jelly (i.e. extracellular matrix) underlying the endothelium and eventually help form the valves and septa. The mechanisms which mediate the formation, migration and proliferation of the mesenchymal cells involves a complex sequence of developmental events which are just beginning to be described in molecular terms. Several members of the TGFbeta family and fibronectin have been shown to be directly involved in the regulation of these events, and it is highly probable that additional growth factors and matrix components are also critically involved. Preliminary data suggests that several members of the FGF and FGF receptor families are present in developing hearts during the early stages of cushion tissue morphogenesis. In addition, we have identified what appears to be a novel, int-2/FGF-3 related protein which is localized in the cardiac jelly at high levels prior to the invasion of mesenchymal cells but then becomes undetectable after the cells have populated the cushions. The objective of this proposal is to determine the distribution and functions of aFGF, bFGF, int2/FGF-3, FGF receptors 1 and 2, and the int-2 related protein during the development of the endocardial cushions. Immunohistochemical and in situ hybridization studies will be used to determine the distribution and source of the proteins. The function of the proteins will be addressed using antibodies and antisense oligodeoxynucleotides in two complementary assays. A mouse whole embryo culture system in which the intact heart develops normally will be used be used to investigate the function of these proteins during heart development. An in vitro collagen gel bioassay which closely mimics the events seen in vivo will be utilized to investigate which specific step of the epithelial-mesenchymal transformation, migration or proliferation processes, if any, are mediated by these proteins. Biochemical and molecular approaches will be used to further characterize the int-2 related protein. These studies should yield information which will help to define the underlying mechanisms which mediate the normal, critical events which occur during the formation of the anlage of the valves and septa of the heart. Such information may provide the basis for a better understanding of the causes of congenital heart defects.
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