Abstract: Spinal cord injury has devastating consequences because adult central nervous system (CNS) neurons do not regenerate their axons after injury. Two key reasons for axon regeneration failure are extrinsic inhibitory factors and a low intrinsic capacity for axon regrowth. Research has therefore focused on overcoming extrinsic growth inhibition, and enhancing intrinsic regeneration capacity. Both of these issues will need to be addressed to enable optimal repair of the injured spinal cord.
To re-establish motor function after spinal cord injury, descending corticospinal axons need to regenerate over long distances and past the site of injury before making meaningful connections (Tedeschi and Bradke, 2017). Current approaches to overcome inhibitory molecules stimulate sprouting and plasticity leading to some recovery of function, but do not enable long-range axon regrowth. Approaches to enhance the neurons intrinsic capacity for regeneration also stimulate short-range growth leading to limited functional recovery, however there are currently no interventions that stimulate the regeneration of descending motor axons over long distances through the adult spinal cord. Long-range regeneration is possible through the spinal cord, as has been recently demonstrated for sensory neurons regenerating their axons from the periphery towards the brain (Cheah et al., 2016). This was made possible by providing the dorsal root ganglia (DRG) with an activated integrin, which allows axon growth over the extracellular matrix (ECM) molecule tenascin-C (which is upregulated in the spinal cord after injury). Integrins are cell surface receptors for ECM molecules that mediate axon growth during CNS development and adult peripheral nervous system (PNS) regeneration after injury. Integrin α9β1 is one of the receptors for tenascin-C and had been shown to promote axon growth and regeneration. Expression of α9 integrin together with its activator kindlin-1 endows sensory axons with the ability to ignore inactivation by injury-induced molecules leading to vigorous effects on regeneration and functional recovery (Cheah et al., 2016). This method works for ascending sensory axons because PNS neurons efficiently transport integrins into their axons, allowing them to drive regeneration from the axon surface. The approach could be used to drive long-range regeneration of descending motor axons in the corticospinal tract (CST), however integrins are not transported into these axons. AAV mediated delivery of α9 integrin into CST neurons allows transport of integrins into dendrites but not into axons (Andrews et al., 2016). Endogenous integrins are similarly not transported into adult CNS axons but instead confined to dendrites. Examining the mechanisms controlling axonal integrin transport could identify ways of directing integrins into CNS axons. This would mean that the integrin method which drives long-range sensory regeneration could be applied to CST motor neurons. It might also help us to understand whether the CNS blockade of integrin axon transport contributes to regenerative failure.
Spinal cord injury can lead to damage of the corticospinal tract and thereby result in paralysis. My PhD project aims to promote regeneration of corticospinal tract in vivo because this is the key event to restore motor function.
The research strategy is based on an integrin engineering protocol that has produced substantial regeneration of sensory axons in the spinal cord. This was achieved by expressing integrins and their activator kindlin in injured sensory neurons. However, that approach will not work for corticospinal neurons because integrins are selectively blocked at the axon initial segment. We therefore plan to overcome the transport block though demolition of the axon initial segment and co-transduce the neurons with alpha9 integrin and kindlin. The hypothesis is that this combinatorial intervention could lead to successful regeneration of the corticospinal tract after spinal cord injury.
The research is conducted at the University of Cambridge (United Kingdom) and the Netherlands Institute for Neuroscience.