Premature axon-oligodendrocyte interaction contributes to stalling of experimental axon regeneration after injury to the white matter

Ephraim F.Trakhtenberg

Studies from nearly 3 decades ago suggested that,in the central nervous system (CNS),myelination of axons by oligodendrocytes not only helps improve axonal conductivity but also stabilizes circuitry(Colello and Schwab,1994).Over the years,myelin sheaths produced by oligodendrocytes have been found to contain multiple molecules that are inhibitory to axonal growth (e.g.,MAG,NogoA,OMgp,Semaphorins) (Yiu and He,2006;Silver et al.,2014).After white matter injury in the adult CNS,myelin debris from damaged axons and dead oligodendrocytes accumulates in the forming glial scar and exposes these myelin-associated axon growth-inhibitory molecules to the injured axonal stumps,thereby contributing to the inhibition of axonal regrowth.During development,CNS axons reach their postsynaptic targets and stop growing before oligodendrocytes appear and myelinate them (Foran and Peterson,1992;Dangata et al.,1996).Therefore,myelin-associated axon growthinhibitory molecules interacting with already grown axons during myelination were thought to block axons from promiscuous sprouting and miswiring,thereby stabilizing neural circuitry in the CNS (Colello and Schwab,1994).

Later,in culture experiments showed that oligodendrocytes can secrete factors,which help co-cultured neurons to survive and extend neurites better (Meyer-Franke et al.,1995;Wilkins et al.,2003).CNS axons experimentally stimulated to regenerate after injury become myelinated even while they are still growing (Marin et al.,2016),and studies have suggested that therapeutically promoting myelination of axons during their experimentally-stimulated regrowth (before they reached their post-synaptic targets) may enhance the axonal regeneration process (Wang et al.,2020).However,we raised the question,as to whether premature interaction of the injured regenerating CNS axons with oligodendrocyte lineage cells at the initiation of their myelination,while the axons are still growing,could contribute to stalling of the axonal regeneration process (Xing et al.,2023).In all experimental approaches for stimulating CNS axon regeneration after injury,the vast majority of the regenerating axons stall growth prior to reaching their original respective postsynaptic targets (see studies we recently cited elsewhere;Xing et al.,2023).We hypothesized that because during developmental axon growth oligodendrocytes are not present to myelinate the axons until after they have reached postsynaptic targets and stopped growing,premature interaction of the oligodendrocyte lineage cells with the axons during their growth after injury may contribute to stalling of axonal regrowth.We thought that not only myelin debris from damaged axons and dead oligodendrocytes but also live oligodendrocytes (particularly newly-born which are primed-to-myelinate) interacting with the regenerating axons could stall axon regrowth in thein vivoenvironment.

We tested our hypothesis and found that optic nerve injury kills most oligodendrocytes in the injury site by the next day,and their myelin debris fills the injury site (persisting for at least 3 days),however,by 2 weeks myelin debris is cleared from the injury site and it is repopulated by newlyborn oligodendrocytes (which incorporate into the forming glial scar).Then,we found that postinjury born oligodendrocyte lineage cells are susceptible to the demyelination cuprizone-based diet,which reduced their presence in the glial scar and enhanced Pten knockdown-stimulated axon regeneration (Xing et al.,2023).We also showed using scRNA-seq that,post-injury newlyborn oligodendrocytes express the axon growthinhibitory myelin-associated molecules,which are presented on the membrane surface and could interact with the growing axons (Xing et al.,2023).Thus,as the myelin-associated inhibitors on the debris of dead oligodendrocytes are cleared from the injury site,post-injury-born oligodendrocyte lineage cells emerge within the glial scar and present on their surface similar axon growthinhibitory molecules as the myelin debris did,thereby positioning them to inhibit regrowth of the axons attempting to regenerate.Together,these two mechanisms continuously preserve this aspect of the inhibitory environment.Some of the surviving mature oligodendrocytes also could reacquire myelinating capacity (Duncan et al.,2018) and provide myelin-associated inhibitors beyond the injury site as well.

Our study suggests that the interaction of regenerating axons with oligodendrocytes should be suspended until the axons regenerate the full length to reach their postsynaptic targets because live oligodendrocytes contribute to the inhibition of axon regenerationin vivowhereas axons have the capacity to grow full-length without oligodendrocytes (as they do during normal developmentin vivo).Therefore,our findings imply that,in certain types of white matter lesions,experimental promotion of myelination may need to be postponed until after the regenerating axons reached their postsynaptic targets,whereas factors secreted by oligodendrocytes that could promote axon growth may complement other axon regeneration-stimulating treatments.The improvement of axonal conductivity by myelination is relevant after regenerating axons have synapsed on postsynaptic targets and stopped growing,at which point myelination may need to be enhanced experimentally.

Future studies also need to investigate the potential inhibition of axon regeneration by the surviving mature oligodendrocytes,particularly those that reacquire myelinating capacity (Duncan et al.,2018),as well as by oligodendrocyte progenitor cells,which proliferate after injury.In order to assist future studies,we developed a Subtype Gene Browser web application (https://health.uconn.edu/neuroregeneration-lab/subtypes-gene-browser),as a resource for comparing gene expression in oligodendrocyte lineage cell subtypes from the uninjured and injured adult optic nerves.Furthermore,concurrent targeting of other extraaxonal inhibitors of axon regeneration,along with therapeutic activation of neuronal intrinsic mechanisms of axon growth (Costa et al.,2022),may enable more axons to overcome the signaling that stalls their regeneration,and ultimately restore injured circuits (Figure 1).Additionally,future studies using inducible conditional knockout mice,which can prevent the birth of newly formed oligodendrocytes after injury,will be able to address whether or not cuprizone might have also affected axon regeneration through yet unknown mechanisms and also prevent a possible transient increase in myelin debris by cuprizone,which may have reduced the full potential of disinhibition of axon regeneration by this approach.Furthermore,because the inhibitory interaction of newly formed oligodendrocyte processes/soma with the regenerating axons precedes myelination itself,it is necessary to investigate whether subsequent myelination elicits opposing inhibitory and enhancing effects on axon regeneration,as myelin sheaths that wrap the axons contain established myelin-associated axon growth-inhibitors but at the same time,through other mechanisms,may also positively affect the axons.

Figure 1｜Predominantly inhibitory effects of extra-axonal factors on axon regeneration after optic nerve injury.

Our findings highlight the importance of developing a therapeutic method that would temporarily pause the emergence of post-injury born oligodendrocytes,or in other ways prevent their interaction with regenerating axons until the axons regrow the full length to reach their respective postsynaptic targets,after which time myelination should be promoted.However,in complex white matter lesions involving acute severing of some axons while predisposing spared axons to the ensuing secondary damage,experimental promotion of myelination of the intact spared axons (which became demyelinated due to acute or secondary damage) can be protective.Myelin sheaths are expected to protect the axons from various noxious factors of secondary damage,which may otherwise sever the intact axons that were spared in the preceding acute phase of the lesion.Thus,timely protection of the demyelinated spared intact axons from further secondary damage may take precedence,depending on the lesion type and location,even if it would interfere with concurrent axon regeneration treatment.Future studies are needed to determine,for which lesions delaying experimental myelination for the benefit of enhanced experimental axon regeneration is justified.Ultimately,secondary damage attenuated pharmacologically,while oligodendrocyte linage cells-secreted growth factors delivered locally,may allow to delay experimental promotion of myelination in any type of white matter lesion.This would,in turn,allow experimental pausing of the emergence of post-injury born oligodendrocytes and delaying promotion of myelination,without concerns in any type of white matter lesion,and thereby facilitate unhindered utilization of the full potential of experimental axon regeneration.

This work was supported by grants from The University of Connecticut School of Medicine (Start-Up Fund),the National Institutes of Health (NIH)(Grant R01-EY029739),the Connecticut Institute for the Brain and Cognitive Sciences (Research Seed Grant),and the BrightFocus Foundation (Grant G2017204) (all to EFT).

Ephraim F.Trakhtenberg*

Department of Neuroscience,University of Connecticut School of Medicine,Farmington,CT,USA

*Correspondence to:Ephraim F.Trakhtenberg,PhD,trakhtenberg@uchc.edu.https://orcid.org/0000-0003-2844-4191(Ephraim F.Trakhtenberg)

Date of submission:April 9,2023

Date of decision:May 25,2023

Date of acceptance:June 8,2023

Date of web publication:July 20,2023

https://doi.org/10.4103/1673-5374.380883

How to cite this article:Trakhtenberg EF (2024)Premature axon-oligodendrocyte interaction contributes to stalling of experimental axon regeneration after injury to the white matter.Neural Regen Res 19(3):469-470.

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