Regeneration and maintenance of the planarian nervous system

Abstract

Planarians can regenerate all tissues, including the central nervous system and the eyes. This process depends on a population of cells in the adult, the neoblasts, that includes pluripotent stem cells. Whether the neoblast population also includes progenitors specialized for forming specific lineages has not been demonstrated. This thesis describes the identification of progenitors that contribute to eyes during regeneration. Expression and functional analyses identified the genes eyes absent, six- 1/2 and ovo as critical for the formation of all cells of the eye. otxA and soxB were specifically required for photoreceptor regeneration, and sp6-9 and dlx were required for regeneration of the optic pigment cup. Expression analysis of these transcription factors in situ revealed that eye progenitors were distributed in mesenchymal trails extending posteriorly from the regenerating eye. These progenitors originate in the neoblasts, and promixity to the eye primordium correlates with increased differentiation. The spatial and genetic identification of a progenitor population in planarians elucidates migratory and morphogenetic mechanisms underlying organ regeneration in these animals. RNA sequencing of eye tissue also identified hundreds of genes with highly enriched expression in the eye, including numerous orthologs of eye pathology-related gene as well as likely components of key visual processes such as phototransduction and optic pigment cell function. The planarian brain is composed of dozens of cell types with regionalized distribution. The function of the planarian hedgehog gene in the patterning of CNS regions was investigated. hedgehog was expressed in the medial planarian brain, flanked by nkx2 and nkx6, then pax6b, and finally dlx-1 and msx at the most distal positions. This organization is similar to the expression domains of orthologous transcription factors in the vertebrate neural tube. However, in contrast to vertebrates, the expression of nkx2, nkx6, and pax6b in planarians was not affected by loss of hedgehog expression. RNA sequencing analysis identified a strong effect of Hedgehog signaling genes on a medially positioned cell with glia-like features. Therefore, Hedgehog signaling affects formation of at least one cell type in the planarian brain, but does not broadly regulate transcription factor expression domains and cell type identity.