Requirement of focal adhesion kinase in branching tubulogenesis

Epithelial branching tubulogenesis plays an essential role in the development of several tissues such as kidney, lung, salivary and mammary glands, and the study of the underlying mechanisms has been of both basic and clinical interest. Mardin-Darby canine kidney (MDCK) cells cultured in collagen gel are widely used as the simplest and most convenient model for studying tubulogenesis. Clone II 3B5, isolated from MDCK cells, can undergo morphogenesis and develop into polarized, elongated and branching tubule structures under the stimulation of hepatocyte growth factor (HGF) [1,2]. In our previous study, we successfully isolated several MDCK sub-clones, y224 and m634, that exhibited either cystic or tubular phenotypes in low concentration of collagen gel without HGF stimulation [3,4]. These cells became excellent models for studying novel regulatory mechanisms of branching tubulogenesis.It has been demonstrated that extracellular matrix proteins (ECM) are key regulators for branching morphogenesis. Fibronectin, for example, is synthesized by groups of adherent cells that assemble it into a fibrillar network [5,6]. Fibronectin is known to provide survival cues for MDCK cell morphogenesis and development of mouse submandibular salivary gland, kidney as well as lung [7-9]. Laminin, another example of ECM, is crucial in the end bud persistence and ductal elongation in the developing mammary gland [10]. Despite the importance of extracellular matrix proteins, integrins (the membrane receptors of ECM) are also of particular significance, as the potential of branching tubulogenesis in MDCK cells was inhibited by the decrease in α3 integrin expression [3]. Monoclonal antibodies against either α6 or β1 subunit were shown to reduce kidney tubulogenesis in vitro [11], while temporal and spatial changes in integrin expression promoted branching morphogenesis of the developing collecting system in vivo [12].Focal adhesion proteins are downstream of ECM and interact with integrins.