In any developmental system cell proliferation and cell death need to be tightly regulated to ensure proper growth, morphogenesis and patterning. During regeneration these cellular processes must be also coordinated in order to achieve a well-proportioned animal de novo upon regeneration completion. A recent paper from the laboratory of Emili Saló and Teresa Adell (http://www.ncbi.nlm.nih.gov/pubmed/24922054) reports on the key function of the JNK pathway in regulating these events in regenerating and degrowing planarians. JNK is a stress-activated protein kinase belonging to the MAPK family that, in other systems, has been implicated in the regulation of cell cycle, wound healing, neurodegenerative disorders and cancer.
Planarian JNK is expressed in the central nervous system as well as in the neoblasts (planarian adult pluripotent stem cells). Upon silencing of this gene by RNAi, regeneration was severely inhibited as the treated animals regenerated very small blastemas with aberrant differentiation of the new structures within them. JNK RNAi did not affect the early expression of the polarity determinants notum and wnt1, at those stages in which polarity is re-established. However, the latter expression of these genes was significantly attenuated indicating that JNK is somehow required for the maintenance of the expression of such polarity genes. Whereas in other systems such as Drosophila the JNK pathway is required for wound closure, this was not affected after JNK silencing in planarians. However, JNK RNAi resulted in the failure to activate the expression of several wound-induced genes.
Upon amputation neoblast proliferation dynamics displays a bimodal response. There is a first proliferative peak at 6 h after amputation that is systemic throughout all the regenerating pieces, and that it has been associated to wounding. A second mitotic peak is seen at 48 h of regeneration concentrated around the wound region; this second peak is associated to tissue loss and the regenerative response that leads to blastema formation. JNK RNAi does not affect the number of neoblasts or the proportion of actively cycling cells. However, the authors observed that the first mitotic peak was elevated and the second peak occurred about 10 h earlier than in controls. CldU labeling combined with piwi1 (a neoblast-specific marker) and phosphohistone H3 (a marker of the entering to the M phase of the cell cycle) suggests that JNK RNAi induces the shortening of the G2 phase of the cell cycle and, therefore, neoblast enter faster into mitosis. This shortening of the G2 did not affect the capacity of those neoblasts to give rise to normal numbers of post-mitotic progeny.
In addition to neoblast proliferation and blastema formation, the pre-existing tissues must go through a remodeling process so the regenerated animal achieves proper body proportions. This remodeling is largely dependent on cell death. After amputation apoptotic cell death, in planarians, follows also a bimodal response with a first apoptotic peak at 4 h post amputation concentrated at the wound region, and a second apoptotic peak at 3 days of regeneration systemically found throughout the entire regenerating fragment. This second apoptotic peak has been associated to the remodeling of the pre-existing tissues. JNK RNAi inhibited these two apoptotic peaks. Remarkably, the inhibition of apoptosis was accompanied by an increased proliferation in those pre-existing tissues that need to go through remodeling. As a consequence, these treated animals were incapable of readjusting the position of pre-existing organs such as the pharynx to restore proper body proportions. Overall, these results indicate that JNK is necessary to trigger a proper apoptotic response.
During planarian regeneration the second mitotic and apoptotic peaks are related to tissue loss whereas the first peaks are related to a general systemic response to wounding. Thus, after a wound that does not imply tissue loss only the first mitotic and apoptotic peaks are observed. Interestingly, JNK RNAi did not affect neither the wound-associated proliferative and apoptotic responses or the normal expression of wound-induced genes. In contrast, the proliferative and apoptotic response after small injuries that imply loss of small amounts of tissues depended on the function of JNK, as it happens upon amputation of large regions. Therefore, JNK is required for regeneration in those contexts in which tissue has been lost.
In addition to their amazing regenerative capabilities, planarians are very plastic animals as they constantly grow and degrow depending on food availability. Planarian growth and degrowth depend upon the balance of cell proliferation and cell death. Again these two cellular processes must be tightly regulated as animals keep proper body proportions at any time. In starved animals that consequently will degrow, the silencing of JNK inhibits apoptosis without affecting the proliferation rates. This inhibition of apoptosis is accompanied by the impairment of proper body re-scaling during degrowth. Remarkably, JNK RNAi did not affect the apoptotic response in growing animals and they underwent through proper body re-scaling during their growth. Therefore, the authors conclude that JNK is required for the apoptotic-driven remodeling that takes place in degrowing animals to maintain proper body proportions.
In summary, JNK is required to trigger a proper regenerative response after any wound that results in tissue loss. There, JNK is required to induce apoptosis, regulate the onset of mitosis in neoblasts and trigger the expression of wound-induced genes. Moreover, in intact starved animals that are degrowing JNK is necessary to induce the apoptotic driven tissue remodeling and rescaling of proper body proportions.