Schwann Cell-Specific TDP-43 Rescue Improves Peripheral Nerve Myelin Pathology Without Altering Motor Behaviour in a Mouse Model of ALS

Amyotrophic lateral sclerosis (ALS) is a terminal disease caused by motor neuron loss. Schwann cells, the myelinating cells of the peripheral nervous system, metabolically and structurally support neurons. ALS patients exhibit Schwann cell pathology, such as TDP-43 proteinopathy, therefore Schwann cell dysfunction may contribute to disease progression. Here, we have characterised myelinating Schwann cell pathology in a TDP-43Q331K (TDP-43) transgenic mouse model of ALS. We also crossed the floxxed TDP-43 mouse with a myelin protein zero (P0)-cre mouse to excise the transgene from Schwann cells alone (P0-cre/TDP-43) to assess rescue. Compared to wild-type (WT) littermates, 10 mo TDP-43 mice exhibited changes to myelin architecture, including loss of myelin binding proteins at the paranodes, decreased node of Ranvier length, and non-compact, degenerating myelin. In P0-cre/TDP-43 mice these myelin disruptions were rescued. However, this improved histology did not lead to a functional rescue, with both P0-cre/TDP-43 and TDP-43 mice exhibiting slowed sciatic nerve conduction and worsened motor behaviour. Further histological analyses revealed that Bungner Schwann cells, a subtype of Schwann cells triggered by neuronal injury, were activated in both TDP-43 and P0-cre/TDP-43 mice. Activation of Bungner Schwann cells can trigger damaging inflammation through the recruitment of macrophages, which can hinder motor and electrophysiological performance, potentially underpinning the lack of functional rescue in the P0-cre/TDP-43. We established that the rescue of Schwann cells indeed protects myelin in this ALS model, however understanding how Bungner Schwann cells exacerbate neuronal pathology is essential for developing effective therapeutics that can improve functional output. Significance StatementAmyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease with no cure and limited treatments. Given that the average patient life expectancy is 3-5 years following diagnosis, finding novel treatment targets is of the utmost importance. Recent research has revealed that non-neuronal cells, such as Schwann cells, contribute to the disease, however the extent of their pathology remains elusive. Investigating Schwann cell and peripheral myelin pathology in ALS may lead to the identification of previously unrecognized disease mechanisms, opening novel avenues for therapeutic development. Identifying approaches through which to target glial and neuronal pathology concurrently would enable more holistic treatment of the various aspects of ALS pathobiology to improve patient outcomes.