Investigating coupling of metabolism with gene transcription to support the axonal regeneration programme for repair

Spinal cord injury (SCI) is a cause of permanent severe disability in 350,000 people in the US and 60,000 in the UK alone. While neurorehabilitation approaches have made important advances, no treatment is available to significantly improve the many neurological impairments spanning from sensory, motor, cardiovascular, urinary and sexual dysfunction. Following SCI, functional recovery fails due to the inability of central nervous system (CNS) axons to regenerate and re-establish lost circuits. Conversely, peripheral nervous system (PNS) axons are able to partially regenerate and reinnervate their targets such as after a compression of the sciatic nerve for example. A direct comparison of the mechanisms underpinning this opposite ability can lead to identify new targets for regeneration after SCI. Recent studies in my laboratory comparing this opposite regenerative ability led to the discovery of a metabolic pathway called Pentose Phosphate Pathway (PPP) that promotes regeneration and repair after SCI. The PPP uses sugar (glucose) to produce NADPH that is important to reduce free radicals and ribose-5-phosphate (R5P), needed for nucleoside biosynthesis. This pathway does not use energy in the form of ATP to generate its products and it is therefore ideally suited in a post-injury state where damaged neurons have an extreme need for new metabolites with a low energy demand. However, nothing is known about the role of the PPP in axonal regeneration and plasticity after injury. Initial experiments in my lab showed that while PPP production of NADPH does not affect regenerative growth, PPP-dependent R5P and downstream ribonucleoside production are needed for DRG neurite outgrowth. Importantly, my lab found that overexpressing the PPP enzyme transketolase strongly increases neurite outgrowth in cultured DRG neurons and it promotes axonal regeneration after a spinal cord injury in vivo. Altogether, this led us to hypothesise that boosting the PPP could promote axonal regeneration and repair after SCI by increasing the availability of nucleotides that are the elements needed for a transcriptional regenerative response.
This proposal aims to provide evidence for the regenerative potential of the PPP after SCI and to unravel the molecular mechanisms underpinning this ability. Lastly, it will leverage upon the PPP-dependent regenerative response to propose treatment after SCI in order to promote locomotor and sensory recovery in a clinically suitable delivery modality of nucleosides with translational potential. The specific aims will investigate: (1) the ability of transketolase overexpression to promote regeneration and synaptic plasticity after SCI; (2) the molecular mechanisms linking PPP activation with transcription for regenerative gene expression; (3) in vivo ribonucleosides delivery as a treatment to increase plasticity, regeneration and recovery post-SCI in mouse models of spinal cord injuries. This research project will shed a completely new light on the regenerative response to injury by providing an alternative view of the metabolic control of the axonal regenerative ability by investigating PPP-dependent repair mechanisms that will also offer novel translational opportunities.

Following spinal cord injury (SCI), functional recovery fails due to the inability of central nervous system axons to regenerate and re-establish lost circuits; conversely, peripheral nervous system axons can partially regenerate. A direct comparison of the mechanisms underpinning this opposite ability can allow the identification of new targets for regeneration after SCI. Recent proteomics studies in my laboratory comparing the central vs the peripheral projecting axoplasm of sciatic dorsal root ganglia (DRG) neurons revealed a significant enrichment for the Pentose Phosphate Pathway (PPP) in the regenerative peripheral axoplasm only. The PPP is a metabolic pathway branching from glycolysis to produce NADPH and ribose-5-phosphate (R5P), needed for nucleoside biosynthesis. Initial experiments in my lab showed that PPP-dependent R5P and ribonucleoside production are needed for DRG neurite outgrowth. Importantly, my lab found that overexpressing the PPP enzyme transketolase strongly increases neurite outgrowth in DRG neurons and axonal regeneration after sciatic and spinal cord injury. This led to hypothesise that boosting the PPP would promote axonal regeneration and plasticity after SCI by increasing R5P to meet the enhanced demand of nucleotides for a transcriptional regenerative response. This proposal aims to provide evidence for the regenerative potential of the PPP after SCI and to unravel the underlying molecular mechanisms. The specific aims will investigate: (1) the ability of transketolase overexpression to promote regeneration and synaptic plasticity after SCI; (2) the molecular mechanisms linking PPP activation with transcription for regenerative gene expression; (3) in vivo ribonucleosides delivery as a treatment to increase plasticity, regeneration and recovery in experimental models of SCI. This research will shed a new light on the previously unrecognised PPP-dependent metabolic control of axonal regeneration, suggesting novel translational pathways