Role of Endogenous Neural Stem Cells in Spinal Cord Injury and Repair

Clinical Review & Education

Clinical Implications of Basic Neuroscience Research Role of Endogenous Neural Stem Cells in Spinal Cord Injury and Repair

Moa Stenudd; Hanna Sabelström, PhD; Jonas Frisén, MD, PhD

Spinal cord injury is followed by glial scar formation, which has positive and negative effects on recovery from the lesion. More than half of the astrocytes in the glial scar are generated by ependymal cells, the neural stem cells in the spinal cord. We recently demonstrated that the Author Affiliations: Department of neural stem cell–derived scar component has several beneficial functions, including Cell and Molecular Biology, Karolinska restricting tissue damage and neural loss after spinal cord injury. This finding identifies Institute, Stockholm, Sweden. endogenous neural stem cells as a potential therapeutic target for treatment of spinal cord Corresponding Author: Jonas Frisén, injury. MD, PhD, Department of Cell and Molecular Biology, Karolinska Institute, Berzelius väg 25, Stockholm JAMA Neurol. 2015;72(2):235-237. doi:10.1001/jamaneurol.2014.2927 17177, Sweden ([email protected]). Published online December 22, 2014. Section Editor: Hassan M. Fathallah-Shaykh, MD, PhD.

pinal cord injury causes severance of axons and death of Neural stem cells self-renew and are multipotent, which means neurons, which lead to permanent functional impairments. that they can make copies of themselves and generate different ma- S There is no curative treatment available, but spinal cord ture cell types.1 Neural stem cells are present in all main subdivi- injury is often mentioned as one of the most attractive indications sions of the adult mammalian central nervous system, including the for the development of stem cell transplantation therapies. The dis- spinal cord, which is a nonneurogenic region.2-4 Transplantation of covery of endogenous neural stem cells in the adult spinal cord has neural stem cells derived from the adult spinal cord can improve re- raised the hope for future noninvasive therapy for spinal cord covery from spinal cord injury in rodents.3 The improved recovery injury. It is crucial to understand more about endogenous neural suggests that endogenous spinal cord neural stem cells have ben- stem cells’ properties to use them for spinal cord injury treatment eficial features that could be used in the development of therapies paradigms. for spinal cord injury.

Figure 1. Generation and Distribution of Cells After Spinal Cord Injury

A B

A, Type A pericytes (orange) around the vessel wall divide, leave the vessel wall, and generate stromal cells after spinal cord injury. Resident astrocytes (red) divide and become reactive after spinal cord injury. Ependymal neural stem cells (green) divide, leave the central canal region, and generate both astrocytes and oligodendrocytes after spinal cord injury. B, Type A pericyte progeny generates stromal cells forming the fibrotic scar in the core of the lesion (orange). Resident astrocytes contribute to the outer portion of the glial scar (red). Ependymal neural stem cell progeny generates both remyelinating oligodendrocytes in the parenchyma and astrocytes in the inner portion of the glial scar (green).

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Spinal cord injury is followed by the formation of a scar with a Spinal Cord Injury and Scar Formation fibrotic and a glial compartment. The fibrotic compartment is lo- cated at the center of the scar and consists of stromal cells that are Spinal cord injury leads to massive cell death and disruption of the derived from blood vessel–associated type A pericytes (Figure 1). blood–spinal cord barrier, followed by infiltration of immune cells. The fibrotic scar is necessary to seal the injury; in its absence, open Inflammation, free radical formation, and other cellular events at tissue defects develop.7 The glial component of the scar consists of the lesion site cause a secondary injury cascade that kills addi- astrocytes, which are derived from self-duplicating astrocytes and tional cells, including oligodendrocytes that myelinate axons of ependymal cells.8 surviving neurons.5 Demyelinated axons are vulnerable to degen- eration; without rapid remyelination, the neurons may die, result- 6 ing in worsened damage and functional impairment. Neural Stem Cells’ Role in Spinal Cord Injury Ependymal cells are ciliated cells lining the ventricular system of the Figure 2. Role of Endogenous Neural Stem Cells brain and the central canal of the spinal cord. They are responsible After Spinal Cord Injury for propulsion of cerebrospinal fluid and function as a barrier to the brain and spinal cord parenchyma.3 In the intact spinal cord, epen- A dymal cells rarely divide, but in cell culture they start dividing vig- orously and demonstrate multipotency by giving rise to astrocytes,oligodendrocytes,andneurons.8 Invivoafterspinalcordinjury, ependymal cells start dividing rapidly and generate more than half the astrocytes in the glial scar and a small amount of oligodendrocytes that myelinate axons (Figure 1).8 Oligodendrocyte progenitor cells also generate mature oligodendrocytes. Oligodendrocyte progenitor cells are the main dividing cell population in the intact adult spinal cord, and they increase their rate of division after spinal cord injury and generate large num- bers of remyelinating oligodendrocytes.8 Astrocytes divide sporadi- B cally in the intact spinal cord to maintain their population. After injury,astrocytes become reactive, divide rapidly,and form the border of the glial scar (Figure 1).8,9 Astrocytes and oligodendrocyte progenitor cells self-renew but are not multipotent, which indicates that they are not stem cells.8 However, ependymal cells display neural stem cell properties in culture and after spinal cord injury by generating new ependymal cells as well as astrocytes and oligodendrocytes. Therefore, ependymal cells represent a latent neural stem cell population in the adult spinal cord.8 The scar formed after spinal cord injury was long seen mainly as a physical barrier preventing axonal regeneration. Glial scar as- C trocytes produce inhibitory factors, such as chondroitin sulfate pro- teoglycans,thatpreventaxonsfromgrowingthroughthescar.9 How- ever, the view of the scar has become more nuanced as numerous beneficial functions of the scar have been reported. Astrocytes in the glial scar have been killed or inactivated in several studies to shed light on their functions. Ablating astrocytes in the glial scar may affect infiltration of immune cells and lead to larger lesion volume, increased neuronal death, and worsened functional outcome, sug- gesting beneficial effects of glial scar astrocytes on the outcome after spinal cord injury.However, the glial scar is generated by different cell types, which produce scar components with detrimental and/or beneficialeffectsonrecovery.7-9 Sincepreviousstudiestargetedscar-

A, Intact adult spinal cord showing latent neural stem cells in the ependymal forming astrocytes produced by self-duplicating astrocytes and neu- layer around the central canal (green) and an intact corticospinal tract (light ral stem cells, it has been difficult to draw any definite conclusions blue fibers). B, Chronic spinal cord incision injury with normal neural stem cell about the function of each separate scar component. progeny formation. Ependymal neural stem cell–derived astrocytes occupy the To study specific functions of the neural stem cell–derived glial lesion site. Many neurons (dark blue) are lost in chronic lesions; however, the corticospinal tract is left largely intact in this injury paradigm. C, Chronic spinal scar component, our laboratory generated mice with an inducible cord incision injury without ependymal neural stem cell progeny formation. knockout of all Ras genes.10 The Ras gene knockout rendered the Injuries grow deeper and consequently sever the corticospinal tract. The endogenous neural stem cells unable to proliferate, and conse- neuronal loss increases without neural stem cell progeny. quently, the neural stem cell–derived component of the glial scar

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was never formed. Large cysts developed at the lesions when the proliferation of neural stem cells was blocked, while no cyst forma- Conclusions tion occurred in mice with normal neural stem cell function. This outcome implies that neural stem cell progeny functions as a scaf- Neural stem cells are a source of glial scar astrocytes with benefi- fold within the scar to restrict secondary enlargement of the lesion cialfunctions,includingpreservingtissueintegrityandsupplyingneu- and prevents the lesion from expanding after the initial insult. rotrophic support for surviving neurons. The beneficial effects of en- Indeed, spinal cord injuries in mice without the scar component dogenous neural stem cells after spinal cord injury highlight them from neural stem cells grew progressively deeper over time and as a potential therapeutic target. It is important to investigate the severed additional axonal tracts after the initial insult (Figure 2). identity, potential, and regulation of endogenous neural stem cells There was also an increased loss of neurons after spinal cord injury to successfully modulate their injury response. Interesting venues in mice without neural stem cell progeny formation (Figure 2). Neu- to explore include increasing neural stem cell progeny formation and ral stem cell progeny was found to be necessary for production of redirecting neural stem cells to produce more oligodendrocytes af- several neurotrophic factors that support neuronal survival after ter spinal cord injury. To improve recovery from spinal cord injury insults in the central nervous system. The increased loss of neurons by modulating the scar, it is essential to examine the specific role of was attributed to the loss of neurotrophic support from neural resident astrocytes in scar formation and the interactions between stem cell progeny.10 astrocyte-derived and neural stem cell–derived scar components.

ARTICLE INFORMATION Regenerative Medicine at Karolinska Institutet cord central canal zone contains proliferative cells Accepted for Publication: August 21, 2014. (StratRegen), Torsten Söderbergs Stiftelse, and Knut and closely resembles the human. J Comp Neurol. och Alice Wallenbergs Stiftelse. 2014;522(8):1800-1817. Published Online: December 22, 2014. doi:10.1001/jamaneurol.2014.2927. Role of the Funder/Sponsor: The funding sources 5. Silva NA, Sousa N, Reis RL, Salgado AJ. From had no role in the design and conduct of the study; basics to clinical: a comprehensive review on spinal Author Contributions: Dr Frisén had full access to collection, management, analysis, and cord injury. Prog Neurobiol. 2014;114:25-57. all the data in the study and takes responsibility for interpretation of the data; preparation, review, or the integrity of the data and the accuracy of the 6. Franklin RJM, Ffrench-Constant C. approval of the manuscript; and decision to submit Remyelination in the CNS: from biology to therapy. data analysis. the manuscript for publication. Study concept and design: All authors. Nat Rev Neurosci. 2008;9(11):839-855. Acquisition, analysis, or interpretation of data: REFERENCES 7. Göritz C, Dias DO, Tomilin N, Barbacid M, Stenudd, Sabelström. Shupliakov O, Frisén J. A pericyte origin of spinal Drafting of the manuscript: All authors. 1. Gage FH, Temple S. Neural stem cells: generating cord scar tissue. Science. 2011;333(6039):238-242. Critical revision of the manuscript for important and regenerating the brain. Neuron. 2013;80(3): 588-601. 8. Barnabé-Heider F, Göritz C, Sabelström H, et al. intellectual content: All authors. Origin of new glial cells in intact and injured adult Obtained funding: Frisén. 2. Weiss S, Dunne C, Hewson J, et al. Multipotent spinal cord. Cell Stem Cell. 2010;7(4):470-482. Administrative, technical, or material support: All CNS stem cells are present in the adult mammalian authors. spinal cord and ventricular neuroaxis. J Neurosci. 9. Burda JE, Sofroniew MV. Reactive gliosis and the Study supervision: Frisén. 1996;16(23):7599-7609. multicellular response to CNS damage and disease. Neuron. 2014;81(2):229-248. Conflict of Interest Disclosures: None reported. 3. Sabelström H, Stenudd M, Frisén J. Neural stem 10. Sabelström H, Stenudd M, Réu P, et al. Resident Funding/Support: Work in the authors’ laboratory cells in the adult spinal cord. Exp Neurol.2014; 260C:44-49. doi:10.1016/j.expneurol.2013.01.026. neural stem cells restrict tissue damage and neuronal was supported by grants from the Swedish Research loss after spinal cord injury in mice. Science.2013; Council, the Swedish Cancer Society, the Karolinska 4. Alfaro-Cervello C, Cebrian-Silla A, 342(6158):637-640. Institute, Tobias Stiftelsen, AFA Försäkringar, the Soriano-Navarro M, et al. The adult macaque spinal Strategic Research Programme in Stem Cells and

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