In contrast to adult vertebrates, which have limited capacities for neurogenesis, adult planarians undergo constitutive cellular turnover during homeostasis and are even able to regenerate a whole brain after decapitation. This enormous plasticity derives from pluripotent stem cells residing in the planarian body in large numbers. It is still obscure how these stem cells are programmed for differentiation into specific cell lineages and how lineage identity is maintained. Here we identify a Pitx transcription factor of crucial importance for planarian regeneration. In addition to patterning defects that are co-dependent on the LIM homeobox transcription factor gene islet1, which is expressed with pitx at anterior and posterior regeneration poles, RNAi against pitx results in islet1-independent specific loss of serotonergic (SN) neurons during regeneration. Besides its expression in terminally differentiated SN neurons we found pitx in stem cell progeny committed to the SN fate. Also, intact pitx RNAi animals gradually lose SN markers, a phenotype that depends neither on increased apoptosis nor on stem cell-based turnover or transdifferentiation into other neurons. We propose that pitx is a terminal selector gene for SN neurons in planarians that controls not only their maturation but also their identity by regulating the expression of the Serotonin production and transport machinery. Finally, we made use of this function of pitx and compared the transcriptomes of regenerating planarians with and without functional SN neurons, identifying at least three new neuronal targets of Pitx.KEY WORDS: Planaria, Schmidtea mediterranea, Regeneration, Serotonergic neuron, Pitx, Islet1, Stem cell differentiation
INTRODUCTIONTranscription factors are key regulators of differentiation processes and the misexpression of a small number of transcription factors can change the fate of a cell completely (reviewed by Sancho-Martinez et al., 2012). The family of paired class homeobox/pituitary homeobox (Pitx) transcription factors has been implicated in a number of differentiation decisions in mammals. For instance, Pitx proteins can act as terminal selectors, which are transcription factors that regulate both the terminal differentiation of a late progenitor cell and the establishment and maintenance of its cell type-specific functions (reviewed by Flames and Hobert, 2011;Hobert, 2011;
RESEARCH ARTICLE STEM CELLS AND REGENERATIONMax Planck Research Group Stem Cells and Regeneration, Max Planck Institute for Molecular Biomedicine, Von-Esmarch-Strasse 54, 48149 Münster, Germany.*Author for correspondence (kerstin.bartscherer@mpi-muenster.mpg.de) This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution and reproduction in any medium provided that the original work is properly attributed.Received 20 February 2013; Accepted 22 August 2013 Hobert et al., 2010). In the mouse, Pitx2 and Pitx3 ar...