RNA Profiling of the Human and Mouse Spinal Cord Stem Cell Niches Reveals an Embryonic-like Regionalization with MSX1+ Roof-Plate-Derived Cells

摘要

class="head no_bottom_margin" id="sec1title">IntroductionThe adult central nervous system maintains neural stem cells in specific areas called niches (). The main stem cell pools are in the subventricular zone (SVZ) and in the hippocampus. A third stem cell niche is found in the adult spinal cord around the central canal in anamniotes, rodents, and humans (, , ). This niche originates from the embryonic neuroepithelium and forms the spinal cord ependymal zone (EZ) organized as a pseudo-epithelium. As in the brain, this stem cell niche is highly organized and contains stem and non-stem cells. Four different cell types, namely ependymocytes, cerebrospinal-fluid-contacting neurons (CSF-N), vessels, and long radial cells have been described in mice. In particular, the EZ presents a dorsal-ventral regionalization with long radial glial fibrillary acidic protein (GFAP)⁺ cells in the dorsal part (). In contrast to brain, spinal cord ependymal cells slowly proliferate to self-renew (, ). In cultures, a fraction of these cells can also generate passageable neurospheres (i.e. clonal expansion of neural precursor cells) which can generate astrocytes, oligodendrocytes, and neurons after differentiation (). The identity of these neurosphere-forming cells in the EZ is still not completely clear as both GFAP⁺ and GFAP⁻ ependymal cells can behave as neural stem cells in vitro (, , , ). Recent single cell analysis has identified neurogenesis in the adult spinal cord (); however, whether these new neurons are derived from the EZ is not yet established. It has been known since 1962 () that the EZ can readily activate and produce new cells upon injury (). Depending on the lesion type and severity, EZ-derived cells can significantly contribute to the glial scar formation (href="#bib38" rid="bib38" class=" bibr popnode">Ren et al., 2017, href="#bib43" rid="bib43" class=" bibr popnode">Stenudd et al., 2015).In comparison with the brain niches, less is known about the adult spinal cord EZ. Reminiscent of the mouse niche, in human, ependymal cells around the central canal display immature features such as expression of NES (nestin), VIM (vimentin), and SOX2 (href="#bib7" rid="bib7" class=" bibr popnode">Becker et al., 2018). However, with aging the central lumen can disappear and the EZ is disorganized (href="#bib16" rid="bib16" class=" bibr popnode">Garcia-Ovejero et al., 2015). Multipotent neurospheres with a limited proliferation ability have been derived from the human spinal cord (href="#bib12" rid="bib12" class=" bibr popnode">Dromard et al., 2008) and using alternative culture conditions, href="#bib32" rid="bib32" class=" bibr popnode">Mothe et al. (2011) were able to maintain a sustained proliferation of multipotent human-derived neural stem cells.A detailed transcriptomic profiling of the human and mouse EZ is currently lacking. This would help us to understand the specificity and diversity of these cells as well as identify gene expressions and molecular pathways conserved between primates and rodents. It would also provide important insights into why, in contrast to anamniotes, mammalian ependymal cells cannot regenerate neurons after spinal cord injury (href="#bib7" rid="bib7" class=" bibr popnode">Becker et al., 2018).Here we provide a cellular and molecular resource for the mouse and human EZ based on RNA profiling, immunostaining, and fluorescent transgenic mice. This uncovered the conserved expression of 1,200 genes specifically expressed in the EZ, including 120 transcription factors (TFs). Unexpectedly, the EZ maintains an embryonic-like dorsal-ventral pattern of expression of spinal cord developmental TFs. New subpopulations of cells expressing specific genes were identified in the dorsal and ventral part of the EZ. In mice, dorsal ependymal cells were found to be derived from the embryonic spinal cord roof plate.