Regulation and function of developmentally controlled polyploidization

The development of a multicellular organism requires intricate regulation of cellular behaviour, especially on the cell division cycle. While we have a lot of knowledge on how cells make the decision to divide, insight on how cell cycle alterations are regulated in development and disease is lacking. Two main cell cycle variations, endoreplication and endomitosis, give rise to polyploid cells, containing more than two chromosome sets divided over one or two nuclei. Polyploidization is a process that occurs frequently in plants and animals, with endoreplication occurring often in plants and insects, while endomitosis is more prevalent in mammalian tissues. Although a general molecular mechanism that regulates endoreplication has been established, we do not yet understand what controls endomitosis. The scope of this thesis comprises the regulation and function of developmentally controlled polyploidization cycles in the C. elegans intestine. With this, we aim to broaden our understanding of the molecular mechanisms that control endomitosis and the functional significance of polyploidization in development, providing insights that aid the investigation of polyploidization in other model organisms and tissues as well. In chapter 2, we describe observed differences between canonical mitosis and endomitosis in the C. elegans intestine. We find that many cytokinesis and mitosis genes are downregulated on a transcriptional level during endomitosis. By inducing additional cell cycles in the intestinal lineage, we also establish that the switch from canonical cell cycle regulation to endomitosis is made during embryogenesis. Finally, we identify the transcriptional regulator SIN-3 as an important player in the regulation of endomitosis. In chapter 3, we investigate the transcriptional changes that occur during embryonic differentiation of the intestine. We use fluorescence activated cell sorting and single cell RNA sequencing to characterize expression profiles of intestinal cells throughout embryonic development and identify transcriptional regulators with temporal expression patterns as potential players in the regulation of endomitosis. In chapter 4, we describe our investigation of the functional difference between endomitosis and endoreplication. Using an auxin-inducible degradation system, we specifically block binucleation of the C. elegans intestine and find that this affects adult hermaphrodite tissue-specific gene expression needed to support progeny growth. Altogether, we show that binucleation of polyploid cells facilitates rapid upregulation of gene expression needed during development and upon heat stress induction. In chapter 5, we provide our perspective on the function of polyploid cells in development and disease. Based on the observation that polyploid cell types often function to support other, proliferating cells, we suggest that polyploidization is advantageous for this supporting role, possibly enhancing the capacity to provide nutrients and signals and form a barrier, thereby creating a (micro-)environment for dividing cells. Finally, the findings presented in this thesis are summarized and discussed in chapter 6, along with suggestions for possible future directions