A long-standing question within the field of planarian regeneration relates to the nature of the neoblasts, planarian totipotent stem cells. These cells are the only cells with proliferative capabilities in these animals and are absolutely indispensable for regeneration. Based on morphological criteria (size, shape, nucleus/cytoplasm ratio) about 20-30% of the planarian cells have been considered as neoblasts. However, it has not been clear for many years whether all these cells represent a uniform neoblast population with identical o similar proliferative abilities and potentiality or, alternatively, neoblast comprise a quite heterogeneous cell population respect to these criteria. Recent studies from several laboratories have identified several genes and transcription factors that are expressed in different populations of neoblasts and are required for the differentiation of specific cell types. Based on these data Peter Reddien proposed two models for planarian neoblasts: 1) the naïve model in which neoblasts are a rather homogeneous cell population with the same potentiality and in which fate specification occurs only in the non-dividing neoblast progeny; in contrast, 2) the specialized model predicts that neoblasts are an heterogeneous population containing many different lineage-committed dividing cells.
Based on several recent studies planarian neoblasts seem to follow the specialized model as several cell type-specific genes are co-expressed with Smedwi-1, a planarian piwi homologue, considered as a neoblast marker. Now, a recent study from the laboratory of Peter Reddien further supports the specialized neoblast model (http://www.ncbi.nlm.nih.gov/pubmed/25254346). In this work, the authors first purified by FACS neoblast that were in the S or G2/M phases of the cell cycle (dividing neoblasts) and analysed their expression profiles by RNAseq. Then, they focussed on transcription factors as these proteins regulate cell differentiation. Here, they identified a list of transcription factors that were upregulated in dividing neoblasts during regeneration. To validate these results they next carried out in situ hybridizations on purified dividing neoblasts and all of the transcription factors tested were expressed within these cells although, evidently, with different percentages of positive cells.
They extended then these analyses to other previously characterized transcription factors specific for different cell types. Again, here, they identified a percentage of dividing neoblasts in which these factors were also expressed. Next, they focussed on the planarian nervous system as a target to test the specialized neoblast model as many different neuronal subpopulations have been identified. From the analyses of the transcription factors identified in their RNAseq experiments as well as for the search of conserved factors with conserved functions on the development of the nervous system they found 26 neural transcription factors that were expressed in dividing neoblasts. One of these factors, a klf homologue was coexpressed with cintillo in mechanosensory cells located around the head periphery. Remarkably, klf RNAi lead to the absence of these sensory cells during regeneration, despite these treated animals were capable to normally differentiate many other neuronal cell types. Similarly, a pax3/7 homologue was found to be expressed in the medial region of the brain in cells some of which also expressed a dopamine β-hydroxylase (DBH) gene. Animals in which pax3/7 was silenced by RNAi regenerated a significant reduced number of DBH positive cells. Altogether, these results suggest that these transcription factors would be necessary for the differentiation of specific neural lineages from distinct progenitor neoblast subpopulations.
Importantly, the authors also found that transcription factors associated to specific cell lineages (pharynx, central nervous system, eye, protonephridia or muscle) were not co-expressed in the same dividing neoblasts. These strongly suggest that distinct neoblast populations expressed specific combinations of transcription factors associated to different differentiated cell types.
In summary, this study reports 36 transcription factors expressed in dividing neoblasts from regenerating planarians. These factors are expressed in different cell types and tissues in adult planarians, which suggests that they may specify distinct subpopulations of lineage-committed dividing neoblasts, further supporting the specialized model.