Modulation of OPC migration: improving remyelination potential in multiple sclerosis

Abstract

In the brain, axons are wrapped by myelin sheaths which ensure fast saltatory conduction of impulses and provide metabolic support. In multiple sclerosis (MS), the myelin sheaths are lost which leaves the axon denuded. This not only results in slower conduction of action potentials, but if prolonged, can also lead to axon death due to the loss of metabolic support. This neurodegeneration is the main cause of permanent disability in multiple sclerosis patients.

The axon death and disability which stem from it could be prevented by restoring the myelin wrap before axon damage has occurred. This remyelination process is carried out by oligodendrocyte precursor cells which are present throughout life. To remyelinate, OPCs migrate to the area of damage and differentiate into myelinating oligodendrocytes which ensheathe axons with new myelin. In multiple sclerosis, this process occurs but is insufficient to overcome the damage.

Therefore, central to the therapeutic efforts in multiple sclerosis is the aim to improve endogenous remyelination. Enhancing recruitment of oligodendrocyte precursor cells (OPCs) to the areas of damage is a clinically unexplored target. To investigate the therapeutic potential of OPC recruitment modulators, I have looked at 2 different targets involved in migration NDST1/HS and Sema3A/NP1.

The first target, heparan sulfate (HS) is a proteoglycan which is important to OPC migration, investigated by Pascale Durbec’s group in France. In a demyelinating mouse model, its key synthesising enzyme, NDST1, is upregulated by oligodendroglia in a belt around the lesion to aid OPC recruitment. Loss of NDST1 in oligodendrocytes was found to impair remyelination and reduce OPC migration in mice. In collaboration with them, I investigated the relevance of this molecule in post-mortem MS human tissue. I found that in human as well as mouse, NDST1 was primarily expressed by oligodendroglia.

The protein level and the proportion of oligodendroglia expressing NDST1 was increased in MS compared to control indicating NDST1 upregulation as a disease response in human. We also found that low numbers of NDST1+ oligodendroglia correlate with bigger sizes of lesions and chronic lesion types that fail to repair, highlighting its importance in repair.

Moreover, high numbers of NDST1+ cells in a patient correlated with increased remyelination potential. This indicates that in human, intra-patient variation in NDST1 level may explain differences in potential for endogenous repair.

Secondly, I looked at Sema3A, a chemorepulsive molecule which is upregulated in demyelinated injury rodent models aswell as multiple sclerosis lesions, particularly in OPC-depopulated chronic active lesions. Research has consistently found that the level of Sema3A negatively correlates to remyelination because Sema3A hinders OPC migration.

This has highlighted Sema3A as a potential target to improve OPC recruitment in MS however the size and shape of the molecule make it hard to design therapeutics against it.

Therefore, I looked at its druggable receptor, Neuropilin 1 (NP1), to see whether inhibition of NP1 had the same positive effect on OPC recruitment and remyelination as lowering the level of Sema3A. NP1 is a tyrosine kinase receptor for both Sema3A and vascular endothelial growth factor (VEGF) and is found in many cell types.

To check if NP1 inhibition is beneficial, I assessed remyelination in a mouse where the Sema3A binding site of NP1 has been mutated to prevent Sema3A binding and exerting its effect. This is a proxy for a (currently unavailable) ideal NP1 inhibitor of the Sema3A site only.

Contrary to my expectations, OPC recruitment and remyelination in the mutant mice were not improved. However, the NP1 mutation resulted in an altered immune response. To exclude the possibility that no improvement in the OPC recruitment and remyelination of those mice was seen because it was negated by the altered immune response, I explored a cell specific mutant mouse in which NP1 was deleted in oligodendroglia only. In this mutant as well, I did not see improvement of OPC recruitment and remyelination. I therefore propose that Neuropilin 1 is not imperative for Sema3As action in remyelination and is not suitable as a therapeutic target in multiple sclerosis.

Loss of the whole NP1, but not loss of the Sema3A site also resulted in biggermyelinated and unmyelinated axons as well as a different myelin thickness post remyelination. This showed that VEGF and the VEGF site on NP1 in oligodendroglia have a previously unknown but important role in determining axon size and myelin thickness which should be further investigated.

To further elucidate those results in a simple system, I looked at how Sema3A, NP1-Sema3A inhibitors, VEGF and NP1-VEGF inhibitor affect OPC behaviour. I confirmed Sema3As chemorepulsive effect but also showed that at different concentrations it can improve proliferation and survival of OPCs. Inhibiting the Sema3A site and the VEGF site of NP1 by specific blocking antibodies also affects OPC proliferation and maturation. This suggested that NP1s ligands are involved in more than just OPC migration.

In summary, this work supports the relevance of the mouse findings that NDST1 is upregulated in demyelination and important for repair for human illustrating that it might be a suitable therapeutic target to investigate. However, despite the importance of Sema3A in MS models, its only reported receptor, NP1, is not essential for Sema3As action. Therefore, it is an unsuitable therapeutic target. The fact that NP1 is an inappropriate drug target for MS is further demonstrated by the involvement of its ligands in multiple OPC behaviours both in positive and negative aspects.