A central issue in developmental neuroscience is the control of nerve cell differentiation by morphogenic signals. A recent clue into how neurons may transduce one class of morphogenic signals, associated with integrin function, has come from this group's molecular-level studies of Alzheimer's pathogenesis. In testing the idea that amyloid-evoked cell death may involve aberrant protein tyrosine phosphorylations, it was discovered that developing neurons express high levels of the novel nonreceptor protein tyrosine kinase known as focal adhesion kinase (FAK). In non-neuronal cells, FAK is regulated by integrin activity, the first specific kinase to show this transductional linkage; FAK moreover is part of the molecular machinery that controls actin organization. Strong precedent thus exists for FAK to play an important role in integrin-dependent morphogenic signal transduction, and possibly for other morphogenic signal pathways as well. Preliminary data amply support this possibility: FAK is expressed in brain, its tyrosine phosphorylation drastically down-regulates with development, it colocalizes with clusters of vinculin in neurites and growth cones, and it forms an endogenous complex with Fyn, another nonreceptor tyrosine kinase implicated in axon outgrowth.
Three aims are proposed: (1) Characterize changes in FAK concentration, tyrosine phosphorylation and distribution for rat brain cells developing in vivo and in culture, testing for predicted correlations with a family of actin-organizing proteins. (2) Test the impact of FAK knockdown on morphogenic behavior and structure, using assays for adhesion, neurite extension, growth cone movements, and neuritogenic cytoarchitecture. (3) Identify signal-dependent neuronal FAK complexes, comparing biochemically-isolated complexes with those identified at adhesive loci in situ by immunogold whole mount electron microscopy. Data will characterize critical aspects of neuronal FAK development, function and mechanisms of action, testing the hypothesis that FAK, functioning in tandem with a family of actin-organizing proteins, is a transduction factor that couples morphogenic signals to cytoarchitectural control of the neuronal cytoskeleton.
