Background: Fps/Fes and Fer are the only two members of a distinct subclass of cytoplasmic protein tyrosine kinases. Fps/Fes was previously implicated in Semaphorin 3A (Sema3A)-induced growth cone collapse signaling in neurons from the dorsal root ganglion (DRG) through interaction with and phosphorylation of the Sema3A receptor component PlexinA1, and members of the collapsin response mediator protein (CRMP) family of microtubule regulators. However, the potential role of the closely related Fer kinase has not been examined.
Results: Here we provide novel biochemical and genetic evidence that Fer plays a prominent role in microtubule regulation in DRG neurons in response to Sema3A. Although Fps/Fes and Fer were both expressed in neonatal brains and isolated DRGs, Fer was expressed at higher levels; and Fer, but not Fps/Fes kinase activity was detected in vivo. Fer also showed higher in vitro kinase activity toward tubulin, as an exogenous substrate; and this activity was higher when the kinases were isolated from perinatal relative to adult brain stages. CRMP2 was a substrate for both kinases in vitro, but both CRMP2 and PlexinA1 inhibited their autophosphorylation activities. Cultured mouse DRG neurons retracted their axons upon exposure to Sema3A, and this response was significantly diminished in Fer-deficient, but only slightly attenuated in Fps/Fes-deficient DRG neurons.
Conclusion: Fps/Fes and Fer are both capable of phosphorylating tubulin and the microtubule regulator CRMP2 in vitro; and their in vitro kinase activities were both inhibited by CRMP2 or PlexinA1, suggesting a possible regulatory interaction. Furthermore, Fer plays a more prominent role than Fps/Fes in regulating the axon retraction response to Sema3A in DRG neurons. Therefore, Fps/Fes and Fer may play important roles in developmental or regenerative axon pathfinding through signaling from Sema3A to the microtubule cytoskeleton.