Stem cells are essential for growth, development, gamete production and tissue homeostasis but what regulates their maintenance and function in vivo? On , Phillip Newmark and colleagues report that the conserved protein arginine methyltransferase PRMT5 promotes stem cell function in planarian flatworms. These organisms contain a population of adult stem cells called neoblasts that can regenerate all the worm’s tissues. Neoblasts characteristically contain chromatoid bodies, large cytoplasmic ribonucleoprotein (RNP) granules similar to structures that are present in the germline of many organisms. The researchers show that, like germline RNP granules, chromatoid bodies contain proteins bearing symmetrical dimethylarginine (sDMA) modifications, probably including the PIWI family member SMEDWI-3. PRMT5 is responsible for sDMA modification of these proteins, they report, and PRMT5 depletion results in fewer chromatoid bodies, fewer neoblasts, and defects in regeneration, growth and homeostasis. Together, these results identify new chromatoid body components that are involved in neoblast function and add to the evidence that suggests that sDMA modification of proteins stabilises RNP granules.
