Hepatoblast iterative apicobasal polarization is regulated by extracellular matrix remodeling

Abstract

Hepatocytes have a unique multiaxial polarity with several apical and basal surfaces. The prevailing model for the emergence of this multipolarity and the coordination of lumen formation between adjacent hepatocytes is based on asymmetric cell division. Here, investigating polarity generation in liver cell progenitors, the hepatoblasts, during liver development in vivo and in vitro, we found that this model cannot explain the observed dynamics of apical lumen formation in the embryonic liver. Instead, we identified a new mechanism of multi-axial polarization: We found that polarization can be initiated in a cell-autonomous manner by re-positioning apical recycling endosomes (AREs) to the cell cortex via fibronectin sensing through Integrin αV. Using live cell imaging we showed that this process repeats, leading to multiaxial polarity independently of cell division. We found that the establishment of oriented trafficking leads to the secretion of the metalloprotease MMP13, allowing neighboring hepatoblasts to synchronize their polarization by sensing extracellular matrix (ECM) distribution and enabling lumen opening. Finally, active remodeling of ECM in the proximity of nascent apical surfaces closes a positive feedback loop of polarization, whereas disruption of this loop by either blocking MMP13 or downregulating Integrin αV prevents the formation of the bile canaliculi network. Integration of this feedback loop into a simple mathematical model reproduces the observed dynamics of bile canaliculi network formation during liver development quantitatively. Our combined findings thus suggest a new mechanism of polarization coupling to self-organization at the tissue scale.

Shaping epithelial lumina under pressure

“…This symmetry breaking could indeed result from pre-existing mechanical asymmetries of the biological system, or asymmetries caused by biological signalling during lumenogenesis. In the early, spherical stage of a forming bile canaliculus [ 35 ], the junction belt constitutes an example of such a mechanical asymmetry [ 21 ], but it remains unknown whether and how this asymmetry is coupled to the sphere-to-tube transition of the bile canaliculus. Moreover, as noted above, during the later extension of tubular bile canaliculi, anisotropic mechanics help to guide the bile canaliculi via anisotropic tension [ 18 ] and support them via apical bulkheads [ 21 , 29 ].…”

“…Hepatocytes from mice and rats also form pressurised lumina in culture [ 21 , 32 , 33 ]. In vivo , these structures extend and connect to form the bile canaliculi network [ 34 , 35 ], which transports bile from the hepatocytes that produce it to the bile ducts which eventually drain the bile into the intestine. As in MDCK cysts, the apical surfaces of hepatocytes face the lumen [ 36 ].…”

“…The mechanics of the lumen are, therefore, dominated by the mechanics of the apical surfaces of these two cells. When first formed (Figure 1 D), bile canaliculi lumina are roughly spherical [ 29 , 35 ], in contrast with their eventual tubular structure. The transition from spherical to tubular shapes occurs during their early development (Figure 1 D).…”