A subset of signal transduction pathways is required for hippocampal growth cone collapse induced by ephrin-A5

The Eph family tyrosine kinase receptors and their ligands, ephrins, play key roles in a wide variety of physiological and pathological processes including tissue patterning, angiogenesis, bone development, carcinogenesis, axon guidance, and neural plasticity. However, the signaling mechanisms underlying these diverse functions of Eph receptors have not been well understood. In this study, effects of Eph receptor activation on several important signal transduction pathways are examined. In addition, the roles of these pathways in ephrin-A5-induced growth cone collapse were assessed with a combination of biochemical analyses, pharmacological inhibition, and overexpression of dominant-negative and constitutively active mutants. These analyses showed that ephrin-A5 inhibits Erk activity but activates c-Jun N-terminal kinase. However, regulation of these two pathways is not required for ephrin-A5-induced growth cone collapse in hippocampal neurons. Artificial Erk activation by expression of constitutively active Mek1 and B-Raf failed to block ephrin-A5 effects on growth cones, and inhibitors of the Erk pathway also failed to inhibit collapse by ephrin-A5. Inhibition of JNK had no effects on ephrin-A5-induced growth cone collapse either. In addition, inhibitors to PKA and PI3-K showed no effects on ephrin-A5-induced growth cone collapse. However, pharmacological blockade of phosphotyrosine phosphatase activity, the Src family kinases, cGMP-dependent protein kinase, and myosin light chain kinase significantly inhibited ephrin-A5-induced growth cone collapse. These observations indicate that only a subset of signal transduction pathways is required for ephrin-A5-induced growth cone collapse.