Specification and differentiation of neural cells in Nematostella vectensis

Abstract

During embryonic development, early neurogenesis can be divided into several components, such as the origin, proliferation and movement of neural stem cells and progenitor cells, which are regulated by conserved genes and signalling pathways. These fundamental aspects of neurogenesis have been extensively studied in only a few bilaterian model organisms, leaving many questions regarding the evolution of this process open. The cnidarian and bilaterian lineages are sister groups that separated approximately 600 million years ago. Cnidarians have an informative position to study the early evolution of cellular and molecular aspects of neurogenesis and to understand common principles of neural development. Nematostella vectensis is a sea anemone, member of the phylum Cnidaria. They possess epithelial neural progenitor cells that express NvSoxB(2) and Atonal-like transcription factors. The Notch signalling pathways regulates the number of progenitor cells and achaete-scute is involved in further development. While some aspects of neural progenitor cells have been identified, little is known regarding the specification and differentiation of neural subtypes. The present thesis focuses on those aspects of neurogenesis. Through a candidate gene approach, two transcriptions factors were selected for further analysis. Expression analysis and generation of a transgenic reporter line for the single POU class 4 gene in Nematostella vectensis, revealed that this gene is expressed in diverse post-mitotic neural cell types. I analysed its function by first generating a mutant line with CRISPR/Cas9 and secondly by analysing and comparing transcriptomes derived from the mutants and from different neural cell populations. This study shows that NvPOU4 is involved in the terminal differentiation program of different neural cells, a function conserved with many bilaterians. I further discuss the relevance of POU4 genes, and terminal selectors in general, for studying the evolution of cell types in metazoans. The second candidate gene involved in neural differentiation is Insulinoma associated 1 (Insm1). Using expression analysis and a transgenic reporter line, I show that NvInsm1-expressing cells give rise to sensory and ganglion neurons as well as to gland cells. In this study, I demonstrate that those cell types originate from a population of progenitor cells expressing NvSoxB(2). I further discuss the implications of these results regarding the developmental and evolutionary origin of neural and gland cells in metazoans. The new findings and molecular tools generated in this thesis provide the foundation for a better understanding of evolutionary and developmental aspects of nervous system formation.