Cell-extracellular matrix interactions in the fluidic phase direct the topology and polarity of self-organized epithelial structures

Abstract

In vivo, cells are surrounded by extracellular matrix (ECM). To build organs from single cells, it is generally believed that ECM serves as a large-scale scaffold to coordinate cell positioning and differentiation. Nevertheless, how cells utilize cell-ECM interactions to spatiotemporally coordinate their positioning and differentiation to different ECM at the whole-tissue scale is not fully understood. Here, using in vitro assay with engineered MDCK cells co-expressing H2B-mCherry (nucleus) and gp135 (Podocalyxin)-GFP (apical marker), we show that such spatiotemporal coordination for epithelial morphogenesis and polarization can be initiated and determined by cell-soluble ECM interaction in the fluidic phase. The coordination depends on the native topology of ECM components such as sheet-like basement membrane (BM, mimicked by Matrigel in experiments) and linear fiber-like type I collagen (COL). Two types of coordination are found: scaffold formed by BM (COL) facilitates a close-ended (open-ended) coordination that leads to the formation of lobular (tubular) epithelium, where polarity is preserved throughout the entire lobule/tubule. During lobular formation with BM, polarization of individual cells within the same cluster occurs almost simultaneously, whereas the apicobasal polarization in the presence of COL can start at local regions and proceed in a collective way along the axis of tubule, which might suggest existence of intercellular communications at the cell-population level. Further, in the fluidic phase, we found that cells can form apicobasal polarity throughout the entire lobule/tubule without a complete coverage of ECM at the basal side. Based on reconstructions from time-lapse confocal imaging, this is likely derived from polarization occurring at early stage and being maintained through growth of the epithelial structures. Under suspension culture with COL, the polarization was impaired with formation of multi-lumens on the tubes, implying the importance of ECM microenvironment for tubulogenesis. Our results suggest a mechanism for cells to form polarity and coordinate positioning in vivo, and a strategy for engineering epithelial structures through cell-soluble ECM interaction and self-assembly in vitro.