An important aspect of stem cell biology is to determine the genes responsible for the maintenance of the different stem cell populations as well as to understand both how they are regulated and how they exert their function. Now, a paper from the laboratories of Ian C. Scott and Bret J. Pearson reports on the identification of the Mediator subunit Med14 as an important regulator of stem cell maintenance in zebrafish and planarians (http://www.ncbi.nlm.nih.gov/pubmed/25772472).
The Mediator complex was first identified in yeasts and its core consists of three modules (head, middle and tail) with an additional kinase module present sometimes. Because in yeast Mediator is located at the promoters of nearly all protein coding genes it has been suggested that it may be part of the general transcription machinery. However, several reports in some plant and animal models do not seem to fully support this model. Interestingly, several subunits of this Mediator complex have been identified as regulators of pluripotency of mouse ESCs. However little is known about the putative role of Mediator on stem cells in vivo.
In this paper the authors characterized the function of the subunit Med14 in stem and progenitor cells and regeneration in two different models: zebrafish and freshwater planarians. First they found that the zebrafish mutant logelei (log) corresponds to mutation of med14. Pleiotropic effects that suggest a developmental arrest characterize the phenotype of this mutant. Accordingly, morpholinos of med14 recapitulate many of these phenotypes, which can be then rescued by the injection of wild-type med14. Analyses of genome-wide transcript levels and mRNA levels did not support a general affectation of transcription in these mutants. In contrast, when the authors focused on stem and progenitor cells and regeneration in the log mutants they observed specific defects. Thus, using different markers they saw that retinal, hematopoietic and gut stem cells were reduced in the log mutant and that med14 seemed to be important in heart progenitor cells. Also, when the tail fin of these animals was amputated it did not regenerate.
In order then to better characterize the putative function of med14 on stem cells and regeneration the authors studied it in planarians, an attractive model in which to study stem cells in vivo. In planarians, as in zebrafish, med14 was ubiquitously expressed including in neoblasts (planarian totipotent stem cells and the only proliferative cells in them). The silencing of med14 by RNAi in intact planarians resulted in a phenotype resembling that obtained after depletion of the neoblasts. That is, the animals curled ventrally, lost their heads and finally lysed. On the other hand, the silencing of med14 also blocked regeneration. After med14 silencing there was a gradual loss of cells expressing the neoblast marker Smedwi-1 (a piwi homologue). There was also a loss of proliferative activity detected with different markers of mitosis and S-phase such as phosphorylated histone H3, histone h2b and pcna. Using a marker of neoblast early progeny the authors found that this cell population was also gradually depleted. In contrast, med14 RNAi did not affect the expression of specific markers for differentiated cell types including neurons, gut cells, muscle, pharynx and eye. Moreover they found a significant increase in apoptosis all throughout the treated animals. These results suggested that med14 silencing affected specifically the neoblasts and their progeny and that transcription in general was not compromised.
In summary, the results presented here indicate that the Mediator complex has an important role in stem cell maintenance in two distant models such as zebrafish and planarians. Further analyses should help determining how the Mediator exerts its function, maybe through establishing the epigenetic landscape essential for pluripotency and stem cell maintenance in these animals.