Abstract
Bidirectional signaling between Eph receptor tyrosine kinases and their cell-surface protein signals, the ephrins, comprises one mechanism for guiding motor axons to their proper targets. During projection of motor axons from the lateral motor column (LMC) motor neurons of the spinal cord to the hindlimb muscles in chick embryos, ephrin-A5 has been shown to be expressed in the LMC motor axons until they reach the base of the limb bud and initiate sorting into their presumptive dorsal and ventral nerve trunks, at which point expression is extinguished. We tested the hypothesis that this dynamic pattern of ephrin-A5 expression in LMC motor axons is important for the growth and guidance of the axons to, and into, the hindlimb by knocking down endogenous ephrin-A5 expression in the motor neurons and their axons. No perturbation of LMC motor axon projections was observed in response to this treatment, suggesting that ephrin-A5 is not needed for LMC motor axon growth or guidance.
Highlights
Proper muscle function requires that muscles be innervated by appropriate motor neurons and their axons from the central nervous system
For any ephrin-A5 knockdown to have an effect on lateral motor column (LMC) motor axons, it would have to be achieved while ephrin-A5 is still expressed in the LMC motor neurons, prior to or at stage 23
Our results indicate that perturbation of normal ephrin-A5 expression in LMC motor neurons and their axons with specific shRNAs against ephrin-A5 had no effect on the growth or migration of the motor axons
Summary
Proper muscle function requires that muscles be innervated by appropriate motor neurons and their axons from the central nervous system. Medial motor neurons in the LMC are thought to extend their motor axons into the ephrin-A5 expressing ventral mesoderm, because the motor axons are thought to express no EphAs and aren’t repelled (Figure 1) Complicating this scheme is the observation that, during their growth and initial migration out of the spinal cord, all axons of the LMC express ephrin-A5 (Figure 1) [5]. Ephrin-A5 could affect growth or migration either via its receptor function, signaling back to its expressing cell, or through its ligand function, by binding to EphA4 on the surface of axons in the same tract, or both We have tested this hypothesis by using an shRNA approach to knock down normal ephrin-A5 expression in LMC axons exiting the spinal cord. Our results show that this loss-of-function approach resulted in no motor axonal growth or migration defects
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