Investigation into the role of α2β1 integrin in biliary morphogenesis and tissue homeostasis through cell-matrix interactions

Abstract

Ductular reaction (DR) is a stereotypical pattern of response to inflammation in the portal area of the liver. This pattern of injury, which occurs in diseases including primary sclerosing cholangitis and primary biliary cholangitis, is characterised by the infiltration of inflammatory cells, changes to the extracellular matrix (ECM), and the emergence of an expanded network of bile ducts which migrate through the changing ECM. This expansion and migration of bile ducts into a network of interconnected tubules is known as biliary morphogenesis.

The aim of this thesis was to scrutinise the role of integrin α2β1, a collagen-binding cell surface receptor which has been shown to be involved in abnormal patterns of bile duct growth, in biliary morphogenesis. Specifically, this thesis focuses on biliary morphogenesis in biliary injury and ductular reaction, where cholangiocyte regeneration and migration take place.

Integrin α2β1 is newly expressed by injured cholangiocytes, and particularly in mouse models of DR caused by obstructive cholangiopathy, which show similarities to human diseases. The expression of integrin α2β1 is spatially and temporally correlated with an increase in the deposition and bioavailability of structural collagens, and particularly collagen I, around newly regenerating bile ducts.

These integrin α2β1-high cholangiocytes, which are preferentially expressed at branch points and in the budding epithelium, have a specific transcriptomic signature which suggests a change in cell function: Fewer metabolic and secretory genes are expressed, whereas multiple genes typical of epithelial-to-mesenchymal transition such as Ncad and Zeb1 are upregulated; genes believed to be involved in regulating branching of epithelial structures, including RGS4 and Inhba, are also differentially expressed.

Inhibiting integrin α2β1 in vitro reduced the migration of cholangiocytes across collagen, and the specific knockout of integrin α2β1 in vivo led to both reduced regeneration and reduced migration of cholangiocytes in DR.

Mechanistically, FAK, PI3K/Akt and STAT3 pathways are found to be modulated by integrin α2β1 in cholangiocytes, and inhibiting FAK led to a similar migratory deficit both in vitro and in vivo as that seen with integrin α2 inhibition.

In summary, integrin α2β1 identifies an emerging population of migratory cholangiocytes in ductular reaction, and mediates the migration of biliary cells through FAK signalling.