Stem cells and cell type complexity in a sea anemone

Humans as all other animals in general are composed of many different cell types, organized into tissues and organs. Thus, the complexity of animals largely depends on the complexity of cell types, their organization and interaction between each other. Yet, how the observed cell type complexity has evolved and to what extent this is related to genetic complexity is one of the open fundamental questions in biology. Furthermore, how the cell types are maintained by stem cells throughout adulthood has important biomedical implications. Our skin, intestine and hair are constantly replaced by new cells that differentiate from stem cells. Different from humans, who age when their stem cells are depleted due to an imbalance between self- renewal and differentiation, simpler organisms such as the cnidarians (sea anemones, corals, jellyfish) continuously generate new cells from stem cells without any signs of senescence. However, the possible connection between cell type complexity, its evolution, and the non- senescent properties of stem cells in these organisms remain to be elucidated. In this project, we will investigate the cell type diversity and identify the hitherto enigmatic stem cells in a virtually immortal animal, the sea anemone Nematostella vectensis. To this end, we will assess the molecular profile of cells by a novel method, which allows sequencing of active gene transcripts from single cells in a high-throughput manner. We will also investigate the role of many genes commonly associated with pluripotent stem cells and germline stem cells in other organisms. The identified candidate stem cells will be monitored by lineage tracing in transgenic animals to visualize their differentiation potential and the role of the putative stem cell genes will be tested genetically. The molecular profiling will provide an objective and comprehensive basis for assessing the relation of genetic and organismal complexity. Revealing the identity and molecular features of stem cells in this immortal animal will be key for our understanding of regenerative capacity and longevity.

In humans, most differentiated cells and tissues either originate from early embryonic processes and are very long lived, such as neurons of the central nervous system, or they have to be constantly replenished by adult stem cells, such as hair, colon epithelium or blood cells. Maintenance of cellular homeostasis therefore relies on the balanced proliferation and differentiation of the adult stem cell pool throughout the life time, a process which is often becoming hampered during disease and aging. How to maintain this balance could be studied in animals that appear to be immortal or at least very long lived such as sea anemones. Sea anemones, which are closely related to corals, belong to an ancient group of animals called the Cnidaria. They are not only known for their longevity but also for their stunning regenerative capacity. By studying these animals, we can deduce the types of cells that were already present when different animal forms diversified during early evolution. In this study, we take advantage of a state-of-the-art technology, called single cell transcriptomics, which allows us to document gene usage in different cells across developmental time, at the level of the individual cells. For each developmental time point, we can capture single cells from dissociated tissues in a micro-droplet and sequence only the active portions of the genome, the transcriptome in any individual cell. Using this strategy, we generated an atlas of cell-type development from the sea anemone Nematostella vectensis. We also generated multiple transgenic lines in which expression of a fluorophore is under the control of regulatory promoters of specific genes that are predicted to be cell-type specific. These transgenic animals confirm the specificity of gene expression that we inferred from the gene expression dataset. While in vertebrates, and many other animals, neurogenesis is largely restricted to early developmental stages, we show that in the sea anemone differentiation of neuroglandular cells is maintained throughout all life stages, and follows the same molecular trajectories from embryo to adulthood, ensuring lifelong homeostasis of neuroglandular cell lineages. Neurons, glands, and stinging cells all derive from a pool of putative stem cells, which is identifiable molecularly, but whose anatomical location was obscure. The generation of transgenic animals allows us now to localize these putative stem cells in live animals and monitor their differentiation into a variety of cell types like neurons and gland cells. The identification and future study of putative stem cells may therefore offer insights into the processes of how sea anemones are able to replenish all cell types of the body throughout development and adulthood.