Tissue regeneration is fundamental for multi-cellular organisms to maintain their integrity, but the competence of tissue restoration is different depending on tissues, species, and ages. In spite of the recent progresses of the molecular basis of regeneration, little is known about its regulative processes. We previously identified the junb and junb-like (junbl) as transcripts induced in response to tissue injury in zebrafish. It has been demonstrated that the mammalian JunB is not phosphorylated by the Jun N-terminal kinase (JNK) due to the absence of a target site. Here, we show that the zebrafish Junb proteins retain the target site and are phosphorylated by the JNK. Significantly, we found that the phosphorylated Junb proteins (pJunbs) are necessary for adult and larval tissue regeneration, suggesting that the regulation of Junb proteins by phosphorylation is one of the molecular bases for the higher regeneration ability in zebrafish. We also show that the prolonged expression of junbs and their protein phosphorylation by the JNK after the wound healing stage are the unique and necessary features for regeneration. Thus, our data suggest that the transcriptional and post-transcriptional controls of junbs and their protein products are the important regulative steps that enable tissue regeneration in zebrafish.

Multicellular organisms maintain body integrity by constantly regenerating tissues throughout their lives; however, the overall mechanism for regulating regeneration remains an open question. Studies of limb and fin regeneration in teleost fish and urodeles have shown the involvement of a number of locally activated signals at the wounded site during regeneration. Here, we demonstrate that a diffusible signal from a distance also play an essential role for regeneration. Among a number of zebrafish mutants, we found that the zebrafish cloche (clo) and tal1 mutants, which lack most hematopoietic tissues, displayed a unique regeneration defect accompanying apoptosis in primed regenerative tissue. Our analyses of the mutants showed that the cells in the primed regenerative tissue are susceptible to apoptosis, but their survival is normally supported by the presence of hematopoietic tissues, mainly the myeloid cells. We further showed that a diffusible factor in the wild-type body fluid mediates this signal. Thus, our study revealed a novel mechanism that the hematopoietic tissues regulate tissue regeneration through a diffusible signal.