An ependymal cell quest : identification and functional role of spinal cord neural stem cells

Few injuries have as profound and long-lasting consequences as spinal cord injury. The primary areas of impaired function typically include sensation, mobility, bladder, bowel and sexual function. The economic, social and personal effects can also be devastating. Today there is no cure available but the discovery of neural stem cells in the adult spinal cord has raised the hope for a treatment. It has, however, proven difficult to identify these stem cells and several different cell types including ependymal cells, astrocytes and oligodendrocyte progenitors have been proposed to function as neural stem or progenitor cells in the adult spinal cord. We assessed the generation of new cells from these cell populations under physiological conditions and after spinal cord injury. In Paper I, we identify ependymal cells as latent neural stem cells. They have in vitro stem cell potential and are multipotent in vivo after injury, giving rise to scarforming astrocytes and remyelinating oligodendrocytes. We also show that oligodendrocyte progenitors and astrocytes are lineage restricted progenitors in the adult spinal cord. We offer an integrated view of how several different endogenous cell populations generate new cells under physiological and pathological conditions. Scar formation has traditionally been seen as an impediment to functional recovery after spinal cord injury. However, in Paper II, we show that scarring by ependymal cell-derived astrocytes is required to reinforce the tissue to prevent expansion of the lesion and further damage. We also identify ependymal cell progeny as a major source of neurotrophic support, and find substantial neuronal loss in the absence of this component of the scar tissue. In order to modify the endogenous stem cell response following an injury it is important to understand if all spinal cord ependymal cells have stem cell properties, or if this feature is limited to a subpopulation. In Paper III, we show that ependymal cells are functionally heterogeneous with proliferating progenitors and quiescent stem cells. The latent neural stem cell population in the adult spinal cord is made up of a small subpopulation of ependymal cells that are activated, proliferate and give rise to a large number of cells both in vitro and in response to injury. This thesis is an ependymal cell quest that provides new insights to the identity and function of a latent neural stem cell population residing in the center of the spinal cord. POPULAR SCIENCE ABSTRACT (SWEDISH) I slutet av forra milleniet visade forskare for forsta gangen att det bildas nya nervceller i var vuxna hjarna och att dessa nervceller kommer fran omogna stamceller. Sedan dess har jakten pa hjarnans stamceller varit i full gang och forhoppningar har hojts om att kunna anvanda dem for att ersatta forlorade celler i en rad olika sjukdomar och skador i nervsystemet. Vid en ryggmargsskada kapas nervfibrerna som leder signaler mellan hjarnan och resten av kroppen, vilket bland annat kan leda till forlamning. Efter en ryggmargsskada aktiveras manga olika sorters celler i ryggmargen och dar skadan skett bildas ett arr. I det forsta delarbetet anvander vi laboratoriemoss for att identifiera vilka av cellerna i ryggmargen som ar stamceller och hur de tillsammans med andra celltyper hjalps at att bilda arrvavnad. Ryggmargens stamceller ar sa kallade ependymceller som tacker ytan kring de vatskefyllda halrummen som finns i hjarnan och ryggmargen. I detta delarbete visar vi aven att stamcellerna ar viktiga for lakningsprocessen, detta da de bildar stodjeceller som isolerar och skyddar overlevande nervbanor. Fram tills nu har forskare varit mycket oense kring huruvida arret som bildas efter en skada ar bra eller daligt for lakningen. En av anledningarna till att det varit sa omtvistat ar att arret bestar av flera olika celltyper och komponenter dar stamcellerna bara utgor en viss del. I det andra delarbetet anvander vi oss av ett genetisk trick for att stanga av stamcellerna i moss sa att de inte kan aktiveras efter en skada. Vi visar att nar arrbildningen fran stamceller blockerats vaxer sig skadan djupare med tiden och fler nervfibrer gar av. Detta tyder pa att stamcellerna hindrar skadan fran att vaxa och bli storre. Vi visar aven att stamceller producerar livsviktiga overlevnadsfaktorer till nervceller i narheten av skadan utan vilka dessa nervceller dor. I det tredje delarbetet studerar vi ependymceller i mer detalj och visar att de bestar av flera olika grupper av celler varav endast en liten del ar stamceller. Huvudsyftet men den har avhandlingen ar att identifiera ryggmargens stamceller och visa att de ar viktiga under lakningsprocessen efter en ryggmargsskada genom att bilda bade arrvavnad som framjar lakning och stodjeceller som isolerar nervtradar. I ryggmargen finns det celler som har olika stamcellsegenskaper och om vi kan lara oss mer om hur olika sorters stamceller reagerar pa skada sa kan vi kanske styra dem till att bli battre pa att laka skador, potentiellt genom att bilda fler stodjeceller. LIST OF PUBLICATIONS I. Barnabe-Heider F, Goritz C, Sabelstrom H, Takebayashi H, Pfrieger FW, Meletis K, Frisen J. Origin of new glial cells in the intact and injured adult spinal cord. Cell Stem Cell, 2010, 7, 470-482. II. Sabelstrom H, Stenudd M, Reu P, Dias DO, Elfineh M, Zdunek S, Damberg P, Goritz C, Frisen J. Resident neural stem cells restrict tissue damage and neuronal loss after spinal cord injury in mice. Science, 2013, 342, 637-40. III. Sabelstrom H, Stenudd M, Stahl F, Goritz C, Basak O, Clevers H, Brismar H, Barnabe-Heider F, Frisen J. Spinal cord ependymal cells are functionally heterogeneous with proliferating progenitor cells and quiescent stem cells. Manuscript. Publications not included in this thesis: Barnabe-Heider F, Meletis K, Eriksson M, Bergmann O, Sabelstrom H, Harvey M. , Mikkers H, Frisen J. Genetic manipulation of adult mouse neurogenic niches by in vivo electroporation. Nature Methods. 2008, Feb;5(2):189-96. Sabelstrom H, Stenudd M, Frisen J. Neural stem cells in the adult spinal cord. Experimental Neurology. 2013, doi: 10.1016/j.expneurol. 2013.01. 026