Tyrosine phosphorylation in early embryonic growth cones

A large and growing body of evidence suggests that the regulation of tyrosine phosphorylation is important in the induction of axon growth. We have examined the subcellular distribution of enzymes regulating tyrosine phosphorylation in early embryonic brain, employing a preparation of isolated growth cone particles (GCPs). Because of the early developmental age and well-characterized nature of our tissue source, our GCP preparation offers some advantages over those described previously. As was found with other GCPs, our GCPs had relatively high levels of both the growth-associated protein GAP-43 and the intracellular tyrosine kinase pp60c-arc. In addition, we found that both total tyrosine kinase activity and total tyrosine phosphatase activity were concentrated two- to threefold in the GCPs relative to a neuronal membrane fraction. Two other nonreceptor tyrosine kinases, YES and FYN, were concentrated in the GCPs to a similar degree as that seen for SRC. In addition, we examined the developmental expression in brain of the three tyrosine kinases, using both a quantitative ELISA and Western blot analysis. Our results show that FYN, like SRC, reaches a peak of expression early in development, and declines thereafter. In contrast, expression of YES peaks later, and remains high in the adult brain. Immunofluorescence staining suggests that FYN is expressed both by neurons and by glia, and possibly by neuronal precursor cells. Our results implicate multiple tyrosine kinases as well as tyrosine phosphatases in growth cone function. In addition, the concentration of FYN in early embryonic growth cones combined with its early peak of expression suggests an important role for FYN in early neuronal development.