Reduction of EphA4 receptor expression after spinal cord injury does not induce axonal regeneration or return of tcMMEP response.

Spinal cord injury (SCI) causes an increase of inhibitory factors that may restrict axonal outgrowth after trauma. During the past decade, the Eph receptors and ephrin ligands have emerged as key repulsive cues known to be involved in neurite outgrowth, synapse formation, and axonal pathfinding during development. Given the non-permissive environment for axonal regeneration after SCI, we questioned whether enhanced-expression of the EphA4 receptor with repulsive activity for axonal outgrowth is potentially responsible for the regenerative failure. To address this possibility, we have examined the expression of EphA4 after SCI in adult rats following a contusion SCI. EphA4 expression studies demonstrated a time-dependent change for EphA4 protein without alterations in beta-actin. EphA4 was downregulated initially and upregulated 7 days after injury. Blockade of EphA4 upregulation with antisense oligonucleotides did not produce an anatomical or physiological response monitored with anterograde tracing studies or transcranial magnetic motor evoked potentials (tcMMEP), respectively. These results demonstrated that upregulation of EphA4 receptors after trauma is not related to axonal regeneration or return of nerve conduction across the injury site.

Blocking EphA4 upregulation after spinal cord injury results in enhanced chronic pain.

Spinal cord injury (SCI) is characterized by a total or partial loss of motor and sensory functions due to the inability of neurons to regenerate. This lack of axonal regenerative response has been associated with the induction of inhibitory proteins for regeneration, such as the Eph receptor tyrosine kinases. One member of this family, the EphA4 receptor, coordinates appropriate corticospinal fibers projections during early development and is expressed in spinal commissural interneurons. Its mechanism of action is mediated by repulsive activity after ligand binding, but its role after trauma is unknown. We examined the temporal expression profile of this receptor after spinal cord contusion in adult rats by RT-PCR and immunohistochemistry. SCI induced a biphasic gene expression profile with an initial downregulation at 2 and 4 days post-injury (DPI) followed by a subsequent upregulation. Double labeling studies localized EphA4 immunoreactivity in neurons from the gray matter and astrocytes of the white matter. To test the role of this receptor, we reduced gene upregulation by intrathecal/subdural infusion of EphA4-antisense oligodeoxynucleotide (ODN) and subsequently assessed behavioral outcomes. No locomotor recovery was observed in the rats treated with the EphA4-antisense ODN. Interestingly, reducing EphA4 expression increased mechanical allodynia, as observed by the Von Frey test and decreased exploratory locomotor activity. These results indicate that upregulation of EphA4 receptor after trauma may prevent the development of abnormal pain syndromes and could potentially be exploited as a preventive analgesic mediator to chronic neuropathic pain.