During zebrafish fin regeneration different genes and signalling pathways are locally upregulated in the wound area and play important roles to trigger a successful regenerative response. However, less is known about the existence and function that putative factors provided by tissues away from the wound could have during these regeneration events. A recent paper from the laboratory of Atsushi Kawakami reports on a diffusible signal derived from hematopoietic cells that would support cell survival and proliferation during zebrafish fin regeneration (http://www.ncbi.nlm.nih.gov/pubmed/25533245).
The authors studied a mutant called cloche (clo) previously identified from a phenotype that lacks most hematopoietic and endothelial cells. In wild-types (fish larvae), the fin fold was normally regenerated (in size and shape) by 3 days after amputation. In contrast, clo mutants displayed aberrant regeneration with the formation of tissues with a blister-like appearance. The authors thought that this might represent dying cells so they checked with TUNEL assay to find out a large number of dying cells in this region, compared to wild-types. Uncut clo mutants did not display higher levels of cell death so it seemed that this increase in cell death in clo mutants was regeneration-dependent. This was further confirmed because in a wound healing assay there was no increase in cell death in these mutants. This increase in cell death was accompanied by a decrease in cell proliferation as assayed by BrdU labelling. Again, here, the cell proliferation levels in the head and trunk (away from the wound) were normal compared to wild-types.
Immediately after amputation (0,5 hours), necrotic cell death was similarly induced in the injured region of wild-types and clo mutants. This necrotic cell death disappeared by 6 hours in wild-types and clo mutants, but then at 12 hours after amputation clo mutants displayed a high number of TUNEL positive cells. This cell death was mostly suppressed when overexpressing Bcl2 (a negative regulator of apoptosis) in these mutants, suggesting that cell death was most probably caused by a Bcl2-regulated apoptosis pathway.
In clo mutants this increased apoptosis coincided with the stage in which regenerative cell proliferation begins. The authors checked then the expression of junb and junbl early markers induced in the wound epithelium and the blastema, respectively. These two genes appeared to be normally induced in the clo mutants, suggesting that the initial regenerative response might be normal in them. Next, they carried out some transplantation experiments between clo mutants and wild-types to investigate the cell autonomy of Clo protein function during regeneration, concluding that Clo function was non-cell autonomous for the survival of the regenerative cells. These results prompted them to wonder whether the lack of hematopoietic cells in clo mutants was responsible for these reduced cell survival during regeneration. Thus, they analysed tal1, another hematopoietic mutant, and found out similar phenotypes: impaired fin fold regeneration with increased apoptosis and reduced cell proliferation. As the hematopoietic lineage contains several cell types they wanted to identify the cell types required for the survival of the regenerative cells in these mutants. In red blood and endothelial cell mutants they did not observed apoptosis of the regenerative cells. However, when they knocked down the myeloid lineage through the silencing of pu.1, a transcription factor required for myeloid cell differentiation, they obtained similar results to those observed in clo and tal1 mutants, suggesting that myeloid cells would have an important function on the survival of the regenerative cells.
Finally, they wanted to investigate how this putative signal was mediated from the hematopoietic tissues to the amputation site. They developed an in vitro assay for a tail explant cultured for 12 hours post-amputation. Wild-type and clo and tal1 mutant explants normally induced the expression of junb and junbl. Whereas in wild-types no apoptosis was observed, explants from clo and tal1 mutants displayed an induced cell death. These results suggest that the defects observed in clo mutants could not be caused by an impaired circulation. Remarkably, when the mutant explants were treated with body extracts prepared from wild-types there was a rescue of the apoptosis. On the other hand, body extracts from the mutants did not rescue the apoptosis in the tail explants from those mutants. These results suggested that the survival of the regenerative cells might be supported by a diffusible factor in the wild-type body.
In summary, this study suggests that a diffusible, and still unknown, factor derived from the hematopoietic cells would support the survival and proliferation of primed regenerative cells during fin fold regeneration in zebrafish.