Neoblast specialization during regeneration of the planarian S. mediterranea

Abstract

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Biology, 2018.Cataloged from PDF version of thesis. "Due to the condition of the original material, there are unavoidable flaws in this reproduction. Table 3.1 is missing from page 167"-- Disclaimer Notice page.Includes bibliographical references (pages 175-186).Planarians are well known for their ability to regenerate an entire animal from small tissue fragments. Planarian regeneration requires a population of dividing cells called neoblasts that are distributed throughout the body. Historically, neoblasts have been considered a homogeneous population of stem cells capable of differentiating into all cell types. Most studies, however, analyze neoblasts at the population rather than the single cell level, making it difficult to determine how heterogeneous the neoblast population is. A bulk RNA sequencing approach with expression screening identified 33 new transcription factors transcribed in specific differentiated cells that were also expressed in small fractions of neoblasts during regeneration. Transcription factors of distinct differentiated tissues were expressed in different subsets of neoblasts, whereas transcription factors expressed in the same differentiated tissues were expressed in the same neoblasts. These results suggest roles for neoblast-expressed transcription factors in the specification of distinct tissues. Furthermore, the transcription factors klf, Pax3/7, and FoxA were required for the differentiation of cintillo-expressing sensory neurons, dopamine- beta-hydroxylase-expressing neurons, and the pharynx, respectively. The planarian nervous system is comprised of numerous different cell types, providing an opportunity to study how neoblasts acquire the diverse cell fates that comprise a particular tissue. We used single-cell sequencing to identify the transcriptomes of hundreds of planarian neurons and neoblasts. Using computational analysis of these data we identified the transcriptomes of several specific types of planarian neuronal cells, including cholinergic, dopaminergic, and serotonergic neurons, as well as glial cell types. In neoblasts, we identified a population of cells that expressed both markers of differentiated neurons and transcription factors expressed in various neural cell types, which we hypothesize to be neural specialized neoblasts. We found a number of unique populations of neural neoblasts that correspond with specific neural sub-types. Interestingly, however, these neural specialized neoblasts do not express a detectable unified gene regulatory network. These results are consistent with direct specification of neural sub-types in neoblasts and suggest that neoblasts do not differentiate down a highly hierarchical lineage path as has been described for many developmental lineages.by Kellie M. Kravarik.Ph. D