The attenuation of ancestral pro-regenerative pathways may explain why humans do not efficiently regenerate damaged organs. Vertebrate lineages that exhibit robust regeneration, including the teleost zebrafish, provide insights into the maintenance of adult regenerative capacity. Using established models of spinal cord, heart, and retina regeneration, we discovered that zebrafish T-like (zT) cells rapidly homed to damaged organs. Conditional ablation of zT cells blocked organ regeneration by impairing precursor cell proliferation. In addition to modulating inflammation, infiltrating zT cells stimulated regeneration through interleukin-10-independent secretion of organ-specific regenerative factors (Ntf3: spinal cord; Nrg1: heart; Igf1: retina). Recombinant regeneration factors rescued the regeneration defects associated with zT cell depletion, whereas Foxp3a-deficient zT cells infiltrated damaged organs but failed to express regenerative factors. Our data delineate organ-specific roles for T cells in maintaining pro-regenerative capacity that could potentially be harnessed for diverse regenerative therapies.

*Zebrafish proves to be an excellent model system to study spinal cord regeneration because it can repair its disengaged axons and replace lost cells after injury, allowing the animal to make functional recovery. We have characterized injury response following crush injury, which is comparable to the mammalian mode of injury. Infiltrations of blood cells during early phases involve macrophages that are important in debris clearance and probably in suppression of inflammatory response. Unlike mammals where secondary injury mechanisms lead to apoptotic death of both neurons and glia, here we observe a beneficial role of apoptotic cell death. Injury-induced proliferation, presence of radial glia cells, and their role as progenitor all contribute to cellular replacement and successful neurogenesis after injury in adult zebrafish. Together with cell replacement phenomenon, there is creation of a permissive environment that includes the absence or clearance of myelin debris, presence of Schwann cells, and absence of inflammatory response. Developmental Dynamics 239:2962-2979,

BackgroundAmong the vertebrates, teleost and urodele amphibians are capable of regenerating their central nervous system. We have used zebrafish as a model to study spinal cord injury and regeneration. Relatively little is known about the molecular mechanisms underlying spinal cord regeneration and information based on high density oligonucleotide microarray was not available. We have used a high density microarray to profile the temporal transcriptome dynamics during the entire phenomenon.ResultsA total of 3842 genes expressed differentially with significant fold changes during spinal cord regeneration. Cluster analysis revealed event specific dynamic expression of genes related to inflammation, cell death, cell migration, cell proliferation, neurogenesis, neural patterning and axonal regrowth. Spatio-temporal analysis of stat3 expression suggested its possible function in controlling inflammation and cell proliferation. Genes involved in neurogenesis and their dorso-ventral patterning (sox2 and dbx2) are differentially expressed. Injury induced cell proliferation is controlled by many cell cycle regulators and some are commonly expressed in regenerating fin, heart and retina. Expression pattern of certain pathway genes are identified for the first time during regeneration of spinal cord. Several genes involved in PNS regeneration in mammals like stat3, socs3, atf3, mmp9 and sox11 are upregulated in zebrafish SCI thus creating PNS like environment after injury.ConclusionOur study provides a comprehensive genetic blue print of diverse cellular response(s) during regeneration of zebrafish spinal cord. The data highlights the importance of different event specific gene expression that could be better understood and manipulated further to induce successful regeneration in mammals.

Highlights d Glutamylated tubulin is enriched in cilia of foxj1-expressing cells in the zebrafish d Motile ciliated ependymal cells in the zebrafish forebrain are highly diverse d Gmnc drives the transition from mono-to multiciliated cells at juvenile stage d Lack of multiciliation does not impact brain and spine morphogenesis

Zebrafish can repair their injured brain and spinal cord after injury unlike adult mammalian central nervous system. Any injury to zebrafish spinal cord would lead to increased proliferation and neurogenesis. There are presences of proliferating progenitors from which both neuronal and glial loss can be reversed by appropriately generating new neurons and glia. We have demonstrated the presence of multiple progenitors, which are different types of proliferating populations like Sox2+ neural progenitor, A2B5+ astrocyte/ glial progenitor, NG2+ oligodendrocyte progenitor, radial glia and Schwann cell like progenitor. We analyzed the expression levels of two common markers of dedifferentiation like msx-b and vimentin during regeneration along with some of the pluripotency associated factors to explore the possible role of these two processes. Among the several key factors related to pluripotency, pou5f1 and sox2 are upregulated during regeneration and associated with activation of neural progenitor cells. Uncovering the molecular mechanism for endogenous regeneration of adult zebrafish spinal cord would give us more clues on important targets for future therapeutic approach in mammalian spinal cord repair and regeneration.