Preclinical Molecular Signatures of Spinal Cord Functional Restoration: Optimizing the Metamorphic Axolotl (Ambystoma mexicanum) Model in Regenerative Medicine.

Regenerative medicine offers hope for patients with diseases of the central and peripheral nervous system. Urodele amphibians such as axolotl display an exceptional regenerative capacity and are considered as essential preclinical model organisms in neurology and regenerative medicine research. Earlier studies have suggested that the limb regeneration ability of this salamander notably decreases with induction of metamorphosis by thyroid hormones. Metamorphic axolotl requires further validation as a negative control in preclinical regenerative medicine research, not to mention the study of molecular substrates of its regenerative abilities. In this study, we report new observations on the effect of experimentally induced metamorphosis on spinal cord regeneration in axolotl. Surprisingly, we found that metamorphic animals were successful to functionally restore the spinal cord after an experimentally induced injury. To discern the molecular signatures of spinal cord regeneration, we performed transcriptomics analyses at 1- and 7-days postinjury (dpi) for both spinal cord injury (SCI)-induced (experimental) and laminectomy (sham) groups. We observed 119 and 989 differentially expressed genes at 1- and 7-dpi, respectively, while the corresponding mouse orthologous genes were enriched in junction-, immune system-, and extracellular matrix-related pathways. Taken together, our findings challenge the prior notions of limited regenerative ability of metamorphic axolotl which exhibited successful spinal cord regeneration in our experience. Moreover, we report on molecular signatures that can potentially explain the mechanistic substrates of the regenerative capacity of the metamorphic axolotl. To the best of our knowledge, this is the first report on molecular responses to SCI and functional restoration in metamorphic axolotls. These new findings advance our understanding of spinal cord regeneration, and may thus help optimize the future use of axolotl as a preclinical model in regenerative medicine and integrative biology fields.

Experimentally induced metamorphosis in highly regenerative axolotl (ambystoma mexicanum) under constant diet restructures microbiota.

Axolotl (Ambystoma mexicanum) is a critically endangered salamander species and a model organism for regenerative and developmental biology. Despite life-long neoteny in nature and in captive-bred colonies, metamorphosis of these animals can be experimentally induced by administering Thyroid hormones (THs). However, microbiological consequences of this experimental procedure, such as host microbiota response, remain largely unknown. Here, we systematically compared host bacterial microbiota associated with skin, stomach, gut tissues and fecal samples, between neotenic and metamorphic axolotls based on 16S rRNA gene sequences. Our results show that distinct bacterial communities inhabit individual organs of axolotl and undergo substantial restructuring through metamorphosis. Skin microbiota among others, shifted sharply, as highlighted by a major transition from Firmicutes-enriched to Proteobacteria-enriched relative abundance and precipitously decreased diversity. Fecal microbiota of neotenic and metamorphic axolotl shared relatively higher similarity, suggesting that diet continues to shape microbiota despite fundamental transformations in the host digestive organs. We also reproduced the previous finding on reduction in regenerative capacity in limbs of axolotl following metamorphosis, highlighting the need to investigate whether shifts in microbiota is causally linked to regenerative capacity of axolotl. The initial results on axolotl microbiota provide novel insights into microbiological aspects of axolotl metamorphosis and will establish a baseline for future in-depth studies.