AUTHOR=Chernoff Ellen A. G. , Sato Kazuna , Salfity Hai V. N. , Sarria Deborah A. , Belecky-Adams Teri 

TITLE=Musashi and Plasticity of Xenopus and Axolotl Spinal Cord Ependymal Cells

JOURNAL=Frontiers in Cellular Neuroscience

VOLUME=12

YEAR=2018

URL=https://www.frontiersin.org/journals/cellular-neuroscience/articles/10.3389/fncel.2018.00045

DOI=10.3389/fncel.2018.00045

ISSN=1662-5102

ABSTRACT=<p>The differentiated state of spinal cord ependymal cells in regeneration-competent amphibians varies between a constitutively active state in what is essentially a developing organism, the tadpole of the frog <italic>Xenopus laevis</italic>, and a quiescent, activatable state in a slowly growing adult salamander <italic>Ambystoma mexicanum</italic>, the Axolotl. Ependymal cells are epithelial in intact spinal cord of all vertebrates. After transection, body region ependymal epithelium in both <italic>Xenopus</italic> and the Axolotl disorganizes for regenerative outgrowth (gap replacement). Injury-reactive ependymal cells serve as a stem/progenitor cell population in regeneration and reconstruct the central canal. Expression patterns of mRNA and protein for the stem/progenitor cell-maintenance Notch signaling pathway mRNA-binding protein <italic>Musashi</italic> (msi) change with life stage and regeneration competence. Msi-1 is missing (immunohistochemistry), or at very low levels (polymerase chain reaction, PCR), in both intact regeneration-competent adult Axolotl cord and intact non-regeneration-competent <italic>Xenopus</italic> tadpole (Nieuwkoop and Faber stage 62+, NF 62+). The critical correlation for successful regeneration is <italic>msi-1</italic> expression/upregulation after injury in the ependymal outgrowth and stump-region ependymal cells. <italic>msi-1</italic> and <italic>msi-2</italic> isoforms were cloned for the Axolotl as well as previously unknown isoforms of <italic>Xenopus msi-2</italic>. Intact <italic>Xenopus</italic> spinal cord ependymal cells show a loss of <italic>msi-1</italic> expression between regeneration-competent (NF 50–53) and non-regenerating stages (NF 62+) and in post-metamorphosis froglets, while <italic>msi-2</italic> displays a lower molecular weight isoform in non-regenerating cord. In the Axolotl, embryos and juveniles maintain Msi-1 expression in the intact cord. In the adult Axolotl, Msi-1 is absent, but upregulates after injury. Msi-2 levels are more variable among Axolotl life stages: rising between late tailbud embryos and juveniles and decreasing in adult cord. Cultures of regeneration-competent <italic>Xenopus</italic> tadpole cord and injury-responsive adult Axolotl cord ependymal cells showed an identical growth factor response. Epidermal growth factor (EGF) maintains mesenchymal outgrowth <italic>in vitro</italic>, the cells are proliferative and maintain <italic>msi-1</italic> expression. Non-regeneration competent <italic>Xenopus</italic> ependymal cells, NF 62+, failed to attach or grow well in EGF+ medium. Ependymal Msi-1 expression <italic>in vivo</italic> and <italic>in vitro</italic> is a strong indicator of regeneration competence in the amphibian spinal cord.</p>