Signal relay by guidance receptors at the axonal growth cone is a process essential for the assembly of a functional nervous system. We
investigated the in vivo function of Src family kinases (SFKs) as growth cone guidance signaling intermediates in the context of spinal
lateral motor column (LMC) motor axon projection toward the ventral or dorsal limb mesenchyme. Using in situ mRNA detection we
determined that Src and Fyn are expressed in LMC motor neurons of chick and mouse embryos at the time of limb trajectory selection.
Inhibition of SFK activity by C-terminal Src kinase (Csk) overexpression in chickLMCaxons using in ovo electroporation resulted inLMC
axons selecting the inappropriate dorsoventral trajectory within the limb mesenchyme, with medial LMC axon projecting into the dorsal
and ventral limb nerve with apparently random incidence. We also detected LMC axon trajectory choice errors in Src mutant mice
demonstrating a nonredundant role for Src in motor axon guidance in agreement with gain and loss of Src function in chickLMCneurons
which led to the redirection ofLMCaxons. Finally, Csk-mediated SFK inhibition attenuated the retargeting ofLMCaxons caused by EphA
or EphB over-expression, implying the participation of SFKs in Eph-mediated LMC motor axon guidance. In summary, our findings
demonstrate that SFKs are essential for motor axon guidance and suggest that they play an important role in relaying ephrin:Eph signals
that mediate the selection of motor axon trajectory in the limb.This work was supported by a grant from the Canadian Institutes of Health Research (Institute of Genetics and
Institute of Neurosciences, Mental Health, and Addiction) to A.K. (MOP-77556 and IG-74068)