Collagen receptor control of epithelial morphogenesis and cell cycle progression.

To define the unique contributions of the alpha subunit cytoplasmic tails of the alpha(1)beta(1) and alpha(2)beta(1) integrin to epithelial differentiation and branching morphogenesis, a variant NMuMG cell line lacking alpha(1)beta(1) and alpha(2)beta(1) integrin expression was stably transfected with the full-length alpha(2) integrin subunit cDNA (X2C2), chimeric cDNA consisting of the extracellular and transmembrane domains of the alpha(2) subunit and the cytoplasmic domain of the alpha(1) subunit (X2C1), or alpha(2) cDNA truncated after the GFFKR sequence (X2C0). The X2C2 and X2C1 transfectants effectively adhered, spread, and formed focal adhesion complexes on type I collagen matrices. The X2C0 transfectants were less adherent to low concentrations of type I collagen, spread less well, and formed poorly defined focal adhesion complexes in comparison to the X2C2 and X2C1 transfectants. The X2C2 and X2C1 transfectants but not the X2C0 transfectants proliferated on collagen substrates. Only the X2C2 transfectants developed elongate branches and tubules in three-dimensional collagen gels and migrated on type I collagen. These findings suggest a unique role for the alpha(2) integrin cytoplasmic domain in postligand binding events and cooperative interactions with growth factors that mediate epithelial differentiation and branching morphogenesis. Either intact alpha(1) or alpha(2) integrin subunit cytoplasmic domain can promote cell cycle progression.

The spatial and temporal expression of the alpha 2 beta 1 integrin and its ligands, collagen I, collagen IV, and laminin, suggest important roles in mouse mammary morphogenesis.

To begin to determine the role of the alpha 2 beta 1 integrin and its ligands, collagen I, collagen IV, and laminin, in mammary epithelial differentiation in vivo, we determined the expression of these molecules by in situ hybridization and immunofluorescence in the developing mouse mammary gland. Expression of collagen I, collagen IV, and laminin mRNAs in the mammary gland during puberty corresponded to the period of greatest growth of the gland, 4-7 weeks postnatally. Collagen I expression preceded collagen IV expression, both of which preceded laminin expression, suggesting an important temporal sequence of extracellular matrix (ECM) production. When growth of the epithelium ceased in the adult virgin gland, expression of all three mRNAs became undetectable. Following the onset of pregnancy these molecules were re-expressed with the same chronology observed during puberty. Collagen I, collagen IV, and laminin were expressed by stromal cells immediately surrounding the developing ductal epithelium. Surprisingly, we found no expression of ECM components in the epithelial cells, suggesting the mammary epithelium does not synthesize its own basement membrane. The distribution of collagen I was consistent with a role in duct formation, since collagen I was strikingly abundant around larger mammary ducts, but was sparse around growing endbuds or alveoli. Conversely, there was abundant laminin near growing endbuds and around alveoli, and less around large ducts, suggesting its role is different than collagen I. The alpha 2 beta 1 integrin was present on the basal, lateral, and apical surfaces of the mammary epithelium throughout postnatal development and pregnancy. The alpha 2 beta 1 integrin expression was strongest at midpregnancy, suggesting a role for alpha 2 beta 1 integrin in the alveolar formation that occurs at this time. The alpha 2 beta 1 integrin expression decreased dramatically in the lactating gland. Our results suggest that alpha 2 beta 1 integrin interactions with its temporally and spatially regulated ligands, collagen I, collagen IV, and laminin, could play an important role in mammary morphogenesis in vivo.