Axon Degeneration and the Schwann Cell Early Injury Response: A Study in Mouse and Zebrafish
After peripheral nervous system injury, axons degenerate through a process termed Wallerian degeneration and Schwann cells transform into a repair phenotype. Axon degeneration is regulated by a signalling pathway controlled by the pro-degenerative axon death molecule sterile-alpha and toll/interleukin 1 receptor motif containing protein 1 (SARM1). Meanwhile, Schwann cells activate a distinct transcriptional response, digest myelin using myelinophagy, attract macrophages, and support the survival of damaged neurons and their growth and guidance to their target. The early Schwann cell injury response prior to and around the timing of axon degeneration has, however, not been investigated in great detail, and the identity of an axonal injury signal that induces this Schwann cell injury response remains elusive. In order to investigate axon Schwann cell interactions in vitro, I developed a novel compartmentalised dissociated dorsal root ganglion neuron and Schwann cell coculture model. I show that in this model, Schwann cells are initially axo-protective, as their presence delays degeneration, irrespective of their myelination status. In later phases after injury, they are then axo-destructive and fragment and phagocytose axons. To further investigate the role of Schwann cells after injury, I then characterised an in vivo model of peripheral nervous system injury in larval zebrafish. I describe the rate of axon degeneration after laser axotomy of the peripheral lateral line nerve in wildtype, as well as in sarm1 mutant animals. I show a characteristic delay in axon degeneration in sarm1 mutant zebrafish, while their myelination is normal. I then performed cell specific reexpression experiments with human SARM1 and show that neuronal SARM1 is sufficient to rescue axon degeneration. In a more traditional mouse model of peripheral nervous system injury, I performed a bulk RNA sequencing study of the distal tibial nerve after cut injury at the sciatic notch at early timepoints after injury in both Wildtype and Sarm1 knockout mice. Previous studies have mainly focussed on late timepoints, but I show a much earlier induction of the Schwann cell injury response, prior to myelinated axon degeneration. I then further investigated the timing of unmyelinated axon degeneration and show that these degenerate before myelinated axons do, at timepoints that correspond to the induction of the early Schwann cell injury response. I further show that Schwann cells likely do not express SARM1 and are insensitive to SARM1 activation, suggesting the delayed degeneration in Sarm1 knockout mice is solely due to the axonal absence of Sarm1. Overall, this thesis details novel methods to investigate peripheral nervous system injury and provides novel insights into early events after injury | both in Schwann cells and axons. Results provide insights into Schwann cell axon interactions after injury, and highlight key differences between myelinated and unmyelinated axons that warrant further investigation.
Axon degeneration, Peripheral nervous system injury, Schwann cell
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