The human central nervous system (CNS) has very limited capacity for renewing cells and is not capable of compensating for cells lost after injury or disease. Studies of organisms that can regenerate the CNS could improve our understanding of how adult stem cells can be harnessed for therapeutic purposes. The goal of this work is to establish planarians as models to gain insights into mechanisms regulating stem cell differentiation in vivo. We are specifically interested in understanding how the remarkable ability of these animals to regenerate the central nervous system after injury or amputation. We have capitalized on the molecular tools available to study planarian regeneration to identify and functionally characterize genes with critical roles in nervous system repair, patterning and function.
To advance our knowledge of how regeneration and patterning is achieved in the CNS, we are analyzing when and where neurons are replaced during regeneration. During the reporting period, we identified neural-specific transcription factor belonging to the basic Helix-Loop-Helix (bHLH) gene family. We found bHLH genes that were expressed in stem cells and neurons and were also required for normal CNS regeneration. Our laboratory has identified some of the first neural progenitor populations in planarians and has contributed to the hypothesis that planarians possess lineage-committed neural progenitor cells that might serve to generate new neurons in uninjured and regenerating animals. To identify genes that are differentially expressed during regeneration of the CNS, we completed high-throughput gene expression studies using samples obtained from tissue replaced in the early phases of regeneration of the planarian head after amputation. We have performed gene inhibition studies for 168 genes chosen from the microarray list based on expression pattern or homology and found that knockdown of 25 of these genes produced phenotypes such as loss of the stem cells, reduced or delayed regeneration, and patterning defects. Results from these experiments have revealed interesting conserved genes with potential roles in stem cell regulation. In addition, we have examined in detail the function of the transcription factor COE. COE proteins have been recently implicated in CNS diseases in adult organisms. Using RNA interference, we found that COE is essential for regeneration and to maintain nervous system architecture in planarians. Using genomic techniques, we examined gene expression changes following inhibition of COE expression and uncovered conserved genes required for CNS regeneration. We also found that COE is important for stem cell homeostasis, suggesting that studies using planarians could provide insights into how COE dysfunction leads to CNS diseases such as cancer.