Formation and function of binucleated human hepatocytes

Abstract

Development of multicellular organisms, including humans, begins with one cell. Through many cell divisions and differentiations, this cell ultimately develops into an organism. All the information that is needed for the development of an organism is coded in its DNA and it is crucial that the DNA is passed on correctly to all cells. To protect the DNA during cell division, DNA is carefully packaged into X-shaped structures called chromosomes. Most human cells are diploid, which means they contain two sets of chromosomes. However, some cells accumulate more than two sets of chromosomes as part of their normal development. Cells that contain more than two sets of chromosomes are called polyploid cells. In humans, polyploid cells can be found in the liver, heart, mammary gland, and bone marrow.

Even though polyploid cells are found in many organs, we do not completely understand how and why they are formed. In this thesis, I present the results of our studies on the mechanism, regulation, and function of polyploid cells in diploid organisms, with a focus on human hepatocytes.

In Chapter 1, we present an overview of previous studies on the regulation and formation of polyploid cells during animal development. In addition to polyploid cells that are formed as part of normal development, polyploid cells can also arise due to errors in cell division or in response to organ damage. The parallels in the function of polyploidy in development and disease are reviewed in Chapter 2.

One of the ways in which polyploid cells are formed is through a process called endomitosis. In endomitosis, a cell replicates its DNA and package it into chromosomes as it would during a normal division; however, the cell does not physically divide in two, resulting in a polyploid cell that has twice the amount of DNA. How this process happens in human cells is not completely understood. In Chapter 3, we look into the mechanisms of endomitosis in human hepatocytes.

Apart from E2F proteins, not much is known about which other proteins regulate endomitosis. In Chapter 4, we use a CRISPR-based method to look for genes that promote endomitosis. This study identifies new candidate regulators of endomitosis, but a lot of work is still required to understand the role of these candidate regulators in the regulation of endomitosis.

Polyploid cells are often thought to have increased capacity for cellular processes, such as protein synthesis, but there are not many studies that have tested this hypothesis directly. In Chapter 5, we look at the effect of polyploidy towards cellular processes in human hepatocyte organoids and in the intestines of the nematode Caenorhabditis elegans, which represents an ideal model organism to study the effect of extreme polyploidy in a diploid organism.

Finally, in Chapter 6 we present how the results of our studies on polyploidy contribute to the field of developmental biology. The studies presented in this thesis represent one of the first steps on our journey to understand the mechanism and function of polyploidy.