The role of cellular calcium channels in planarian biology

Abstract

Identification of signaling pathways and therein drugable targets, to manipulate stem cell behavior in vivo is a major focus of regenerative medicine. This dissertation focuses on the role of Ca2+ channels in stem cell differentiation and regeneration in a simple in vivo model, the planarian flatworm. These animals maintain a totipotent population of stem cells that give rise to all the cell types in the worm. Previously, we discovered that the isoquinoline drug praziquantel (PZQ) caused a robust (100%) and complete duplication of the entire anterior-posterior (AP) axis during flatworm regeneration to yield two-headed (bipolar) organisms. My studies mechanistically dissect these observations to show that PZQ subverted regeneration via activation of a specific neuronal voltage-gated Ca2+ channel (VGCC) isoform (Cav1A). Surprisingly, another isoform Cav1B was found to play opposing roles in axis formation to promote tail regeneration, suggesting a delicate interplay between Ca2+ signals critical for nervous system regeneration. Further dissection of the downstream pathway showed that RNAi of Cav1A blocked PZQ-evoked bipolar regeneration, Ca2+ entry and decreases in Wnt levels, the output of Hedgehog signaling. Thus, these data demonstrated that calcium signaling regulated regeneration through modulating Hedgehog signaling, a pathway that has been shown to regulate neuronal stem cell behavior, patterning and growth in diverse development processes. Taken together, these findings add new insights into the mechanisms that govern planarian regeneration. Additionally, my work on intracellular Ca2+ release channels in this system led to the identification of the planarian inositol 1, 4, 5-trisphosphate receptor (IP3R). Studies designed to elucidate the biological significance of this protein by in vivo RNAi knockdown led to the discovery that sexual planarians underwent severe defects of laying eggs in the absence of IP3R, although it failed to produce an obvious phenotype in asexual worms. Thus, these data provided genetic evidence that IP3R plays an important role in regulating reproductive physiology in planarian flatworms. In summary, the data obtained in this thesis have revealed essential roles of Ca2+ signaling in regulating planarian stem cell differentiation and reproductive physiology.