Integrins are the major class of receptors by which cells interact with the extracellular matrix, promoting cell adhesion and cell migration. Signalling from extracellular matrix molecules to the inside of cells can occur via integrin receptors. These signalling events are thought to play roles in processes important for embryonic development such as induction of cell-cell adhesion molecules, stimulation of enzyme secretion, and tissue organization. Integrins generally contain two relatively short cytoplasmic domains. The roles of these domains in controlling the location of receptors, signalling, and regulating cell behavior is being explored using molecular biology and biochemical methods. Functions of isolated domains are being tested using chimeric receptors containing a reporter domain consisting of a subunit of the interleukin-2 receptor and various integrin cytoplasmic tails. The beta1 cytoplasmic domain of the fibronectin receptor contains sufficient information to target receptors to adhesion sites of cells, whereas the alpha5 domain of this same integrin does not. Preliminary studies indicate that beta3 tails can also provide such targeting information, and that this activity can be regulated by alternative splicing of precursor RNA. Since the chimeric beta1 receptor concentrated at sites where endogenous ligand-occupied receptors normally localize, the possibility was tested that localization of endogenous receptors is regulated by occupancy with its ligand molecule. Fibronectin fragments or synthetic peptide containing the binding site for the fibronectin receptor were able to drive redistribution of the integrin fibronectin receptor directly by occupancy; similar results were obtained with beta3 integrin receptors. Further studies are characterizing the roles of other integrin domains in localization as well as in regulation of phosphorylation and other messenger systems. These studies should help provide an in-depth understanding of how cells communicate with their extracellular environment in normal and abnormal embryonic development, where such signalling is essential for coordinating the complex rearrangements and final organization of oral, facial, and other developing tissues.
