The role of ependymal cells in axolotl spinal cord regeneration

The urodele amphibians such as the axolotl are capable of regenerating injured spinal cord throughout adult life. The ependymal cells, which are the glial population that lines the central canal, react to injury by proliferating and migrating into the lesion site. We have shown that upon cord transection the ependyma undergo an epithelial-to-mesenchymal transition as demonstrated by a change in expression of the intermediate filament protein cytokeratin to expression of vimentin. The epithelial-to-mesenchymal transition can also be correlated with the appearance and disappearance of fibronectin in the lesion site. The presence of mesenchymal ependymal cells correlates with the absence of fibrous astrocytes in the lesion. We have isolated ependymal explants in culture in the state of mesenchymal outgrowth when cytokeratin and vimentin are co-expressed and fibronectin is the required substratum. We have shown, in vitro, that the ependymal cells require EGF for cellular proliferation and migration. TGF-$\beta$ and PDGF act synergistically to cause a change in cellular morphology that results in the ependyma forming multi-layered cords of cells suggesting that TGF-$\beta$ and PDGF may be important in cell reorganization characteristic of later stages of regeneration. We have also shown that cultured ependymal cells secrete 4 gelatinases; at least one of these is a Type I collagenase. The 4 gelatinases have approximate molecular weights of 62.8, 59.3, 55.8, and 51.6 kDa. Expression of the 51.6 kDa proteinase appears to be inhibited by both TGF-$\beta$ and PDGF. In addition, the 55.8 kDa proteinase requires the presence of TGF-$\beta$, PDGF, or EGF for its expression. The Type I collagenase also requires the presence of one of the above mentioned growth factors for expression. Lastly, lack of casein- or fibronectin-degrading enzymes suggests that stromelysin and Type IV collagenase are not produced by ependymal cells. Ependymal bulb homogenates produced several more gelatinases, reflecting the presence of both secreted enzymes and unsecreted proenzymes. The Type I collagenase is also present in the tissue homogenates of regenerating ependymal cells.