Liver tissue organization and function
The liver is a key organ performing tasks vital to life, such as metabolism of nutrients and xenobiotics, as well as detoxification. It is remarkably dynamic, being able to constantly renew its cells and regenerate 2/3 of its mass after partial hepatectomy. Liver function depends on its unique tissue architecture. Hepatocytes exhibit a complex apico-basal polarity: their apical membranes form a three-dimensional (3D) narrow belt between adjacent cells, which collectively give rise to the bile canaliculi (BC) network, an essential component for bile secretion and overall liver function. The different cell-cell contacts in the tissue allow the hepatocytes to develop multiple apical and basolateral surfaces per cell giving rise to a complex 3D tissue organization. It is, thus, very important to understand the specific structural organization of hepatocytes and their interactions with the sinusoidal endothelial cells but also stellate and Kupffer cells in order to understand liver function and homeostasis. This project aims at identifying and characterizing the key factors that are responsible for the maintenance and loss of hepatocyte polarity as well as the 3D organization of the BC and sinusoidal network in liver tissue. We are addressing three questions:
- How is hepatocyte polarity regulated?
- How do cells with complex polarity assemble into tissue?
- How is tissue architecture maintained when there is cell turnover due to tissue regeneration?
Reconstruction of liver tissue structure in 3D
The aim is to develop a quantitative understanding of liver structure using image analysis and multi-scale modelling. We use a pipeline of state-of-the-art imaging techniques and novel image analysis algorithms to reconstruct the 3D-structure of liver tissue and its dynamics in vivo. By applying high- and super-resolution light microscopy, electron microscopy (EM) and intra-vital imaging we aim at developing an unprecedented quantitative understanding of liver tissue organization at multi-scale, ranging from the sub-cellular to the lobule levels. The quantitative parameters extracted will be used to generate a tissue structure model. Predictions from the model will then be tested by introducing genetic Exploiting endocytosis for siRNA delivery and chemical perturbations. Silencing of genes in primary hepatocytes or in mouse liver is possible using state-of-the-art RNA interference (RNAi) (Zeigerer et al., 2012). Using this approach, it was possible to describe phenotypes at multiple scales, from the molecular to the cellular level up to the organ and organism physiology. Validation studies will be conducted on the developing, adult and regenerating liver after partial hepatectomy.
Functional genomics in mouse liver
The efficient delivery of siRNAs and gene silencing in hepatocytes provides the unique possibility to conduct a systems biology analysis in mouse liver, as mammalian organ model system. An example of the enormous potential of this approach is the multi-scale analysis of the liver upon depletion of Rab5 by siRNAs delivered via lipid nanoparticles (LNPs) in collaboration with Victor Koteliansky (Zeigerer et al., Nature 2012).
It is becoming widely accepted that modern biological research can no longer be limited to studying single molecular components in isolation but has to encompass multiple levels of complexity, hierarchically ordered from the molecular to sub-cellular, to cells, tissues, organs and entire organisms. We aim to integrate functional information within each level and bridge between the scales, to understand how perturbations at the molecular level are manifested at all levels of complexity.
Regulation of liver size during development and regeneration
Organ size and shape are accurately controlled during development and regeneration. In Drosophila and mammals, studies over the past years have uncovered a critical role of the Hippo signal transduction pathway in the regulation of organ size: For example, Hippo de-regulation leads to liver overgrowth. So far, we have obtained a good understanding of the molecular components of the Hippo signalling pathway, their effects on proliferation and apoptosis, their role in the self-renewal and expansion of stem cells and tissue-specific progenitor cells, and their function in tissue regeneration. However, we still lack information on how cells “sense” at the molecular level the completion of organ development or repair in order to shut down cell proliferation and prevent organ overgrowth. We are developing a theoretical and experimental framework to measure, describe and model how signaling pathways acting in liver development and homeostasis such as Hippo, HGF and Wnt are regulated to ensure the correct morphogenetic process during liver development and repair.