The role of retinoids in the regeneration of the axolotl spinal cord

Retinoids play an important role in tissue patterning during development as well as in epithelial formation and health. In the mammalian central nervous system, the meninges are a source of retinoids for brain tissue. Retinoid production has been described in juvenile Axolotl ependymal cells. Retinoid effects may possess a significant role in the regeneration-permissive interaction of the meninges and ependyma of the Axolotl spinal cord after penetrating injury. During spinal cord regeneration in urodele amphibians, the pattern of retinoid production changes as the meninges interact with the injury-reactive ependymal cells reconstructing the injured spinal cord. In order to determine which components of the retinoid metabolism and intracellular signaling pathway act in Urodele spinal cord regeneration, we employed antibody/horseradish peroxidase staining of both intact and regenerating Axolotl spinal cord tissues obtained from adult animals as well as cell culture techniques to determine expression of three retinoid pathway components: Cellular Retinoic Acid Binding Protein II (CRABP 2), Cellular Retinol Binding Protein I (CRBP 1), and Retinaldehyde Dehydrogenase II (RALDH 2). Current results demonstrate the following in the intact cord: 1) CRBP 1 is expressed in the pia and dura mater meningeal layers, in gray matter neurons (including their axonal processes), and the ependymal cell radial processes that produce the glia limitans, 2) CRABP 2 is expressed in the arachnoid and/or dura mater meningeal layers surrounding the spinal cord, and 3) RALDH 2 is expressed in the meninges as well as cytoplasm of grey matter neurons and some ependymal/sub-ependymal cells. In the regenerating cord, CRBP 1 is expressed in ependymal cells that are undergoing epithelial-to-mesenchymal transition (EMT), as is CRABP 2. RALDH 2 staining is very strong in the reactive meninges; in addition, expression is also upregulated in the cytoplasmic and perinuclear regions of reactive grey matter neurons, including motor neurons and in the apical region of ependymal. Preliminary studies culturing reactive meninges and ependymal cells together suggested that the meninges could drive re-epithelialization of the reactive ependymal cells. Experiments to characterize this interaction show an unusual proliferation pattern: Proliferating Cell Nuclear Antigen (PCNA) labeling is present in intact and regenerating cord ependymal cells. However, in culture, the presence of meninges results in no proliferation proximal to the explant, but extensive proliferation in leading cell outgrowth; also, the cultured meninges is positive for RALDH2. In summary, the intact adult cord shows meningeal production of RA, which is upregulated following injury; in addition, during this time, RA production is upregulated in the adult ependymal cells as well. In culture, the reactive meninges appears to modulate the behavior of reactive ependymal cells.