A PCTAIRE family kinase regulates eye and brain size in freshwater planarians

During embryonic development and regeneration, the growth of any organ must be tightly regulated in order to achieve their optimal final size and become fully functional. Freshwater planarians, with their remarkable plasticity and ability to regenerate any part of their body based upon the presence of adult pluripotent stem cells, provide an ideal model to study how the final organ size is regulated during this process. Also, the fact that planarians are constantly growing and degrowing depending on culture conditions, allows us to study how the size of the different organs is determined under homeostatic cell renewal. Here, we investigate the role of Smed-pctk-1, a cyclin dependent kinase that belongs to the PCTAIRE subfamily of CDKs, which remains largely understudied. Functional analyses show that Smed-pctk-1 silencing disrupts the normal size of the cephalic ganglia and results in an overgrowth of the eyes both in homeostatic and regenerating planarians. The increase in eye size correlates to an increase in the number of both progenitor and differentiated eye cell types. Phototaxis behavioral assays reveal that Smed-pctk-1 RNAi planarians exhibit a precocious sensitivity to light. Overall, our findings identify Smed-pctk-1 as a key regulator of eye and neural size in planarians, highlighting its contribution to the mechanisms that control organ growth during both regeneration and homeostasis.