Microscopic analysis of the cellular events during scatter factor/hepatocyte growth factor‐induced epithelial tubulogenesis

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Abstract
Scatter factor/hepatocyte growth factor (SF/HGF), a large multifunctional polypeptide growth and motility factor, is known to play important roles during embryonic development, adult tissue growth and repair. In an established three‐dimensional type I collagen model, SF/HGF induces Madin–Darby canine kidney (MDCK) epithelial cysts to form long, branching tubules (tubulogenesis). In addition, the composition of the surrounding extracellular matrix (ECM) has been shown to modulate SF/HGF‐induced morphogenesis, where tubulogenesis was completely abrogated in Matrigel basement membrane. Many cellular events that occur during SF/HGF‐mediated remodelling, and its modulation by the ECM, remain unclear. We have investigated these mechanisms through microscopic examination of the time‐course of SF/HGF‐induced responses in MDCK cysts cultured in type I collagen or Matrigel. We found that early responses to SF/HGF were matrix‐independent. Changes included increased paracellular spacing between normally closely apposed lateral membranes, and the formation of filopodial processes, indicating a partial motile response. Cell–cell contact was maintained, with the persistence of cell junctions. Therefore, while one or a number of ECM components are preventing SF/HGF‐primed cells from undergoing an invasive and/or migratory programme, non‐permissive matrices are not preventing SF/HGF signalling to the cell. Later matrix‐dependent responses, which occurred in type I collagen but not Matrigel, included the formation of basal protrusions that comprise two or more neighbouring cells, which extend to form nascent tubules. Modified polarity of cells comprising the basal protrusions was evident, with a marker for the apical membrane being found in the same region as adherens junctions and desmosomes, typically localized at lateral membranes. We propose a model for SF/HGF‐induced tubulogenesis in which tubules form from basal protrusions of adjacent cells. This mechanism of in vitro tubule formation has many similarities to reported in vivo epithelial tubulogenesis.