The aim of our work is to use freshwater planarians (non-parasitic flatworms) as model organisms to understand how stem cells replace lost neurons after injury. Adult planarians possess a population of pluripotent stem cells, which serve to replace cells lost after wounding or during normal physiological turnover. One of the truly remarkable properties of planarians is their ability for repair and regeneration of the central nervous system (CNS), a capacity that is limited in most animal models currently studied. Thus, planarians are excellent models in which to investigate fundamental mechanisms regulating stem cells. We are capitalizing on the molecular tools available to study planarian regeneration to identify and functionally characterize genes with critical roles in CNS repair.
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 have generated antibodies that recognize planarian neurons and their projections. These antibodies are being used in combination with staining of dividing cells to analyze the transition of the stem cells to specific neuronal cell types. To identify genes that are differentially expressed during regeneration of the CNS, we have performed high-throughput gene expression studies (microarray analyses) using samples obtained from tissue replaced in the early phases of regeneration of the planarian head after amputation. Our experiments have revealed approximately 700 genes changing their level of expression in the regenerating tissues compared to control tissues obtained from uninjured animals. We have begun to examine the pattern of expression of these genes in the animal; thus far, screening of 289 genes has revealed 50 genes expressed in the CNS, 71 expressed in the stem cells (another 68 genes are potentially expressed in the stem cells and will be confirmed), and 225 are expressed in the early regenerating tissue. Based on the patterns of expression and sequence annotation from comparisons to genes of other organisms (including humans), we will analyze close to 225 genes in studies aimed at characterizing their function during regeneration. Results from these on-going experiments have already revealed interesting genes with potential roles in stem cell regulation that share high similarity with human genes.
Successful identification of genes with no known or emerging roles in stem cell regulation would help to improve our knowledge of biological mechanisms regulating proliferation and differentiation of stem cells in the CNS. This information has the potential to contribute to our ability to manipulate pluripotent stem cells to divide and acquire neuronal fates in vivo, which would be valuable for therapeutic applications.