Most cells adhere to their neighbors and to the extracellular matrix, a fibrillar meshwork surrounding or underlying most cells in the body. Cell adhesion plays important roles in the normal functions of cells, contributing to cellular organization and structure, proliferation and survival, metabolism, and gene expression. During embryological development, cell adhesion is important for the correct movements of cells modeling the embryo. In the adult, appropriate cell adhesion is necessary for numerous physiological processes and can be deranged in many diseases, including thrombosis, inflammation, and cancer.

Our laboratory seeks to understand the proteins involved in cell adhesion and the ways in which these proteins control adhesion and migration of cells in both normal and pathological processes. Cell adhesion is mediated by several families of proteins, called adhesion receptors, specialized for adhesion between adjacent cells or between cells and the extracellular matrix. Adhesion receptors do much more for cells than merely sticking them down in the correct locations, although that in itself is important. They also form physical linkages between the extracellular environment and the internal structures of cells and thus control cell shape and motility.

Adhesion receptors also act as two-way transducers of signals both into and out of cells. Therefore, cells can control whether or not their adhesion receptors are functional; this is important to ensure appropriate cell adhesion. For example, when a blood vessel is damaged, blood platelets must adhere to staunch bleeding—this process is called hemostasis. They must not, however, adhere at the wrong time or place—that produces thrombosis. Similarly, leukocytes must adhere in appropriate places to fight infections; if they adhere at the wrong place or time, the result is inflammation. Alterations in cell adhesion also play important roles in the control of cell behavior during invasion and metastasis of malignant cancer cells. Thus, control of adhesion receptors is a matter of life and death. In their role as signal transducers into cells, adhesion receptors control cell proliferation, cell survival, and the expression of specific genes. Our aim is to understand these processes at the molecular level.

One approach we use to decipher the roles of the proteins involved is to generate mice with mutations in the genes encoding these proteins. In that way, we can discover which processes require specific adhesion proteins by "knocking out" the genes encoding them so that the resulting mice cannot make the proteins. We have used this approach to dissect the roles of various adhesion receptors in the recruitment of white blood cells to sites of inflammation and in the adhesion of platelets during hemostasis and thrombosis. We have also generated mouse models of human diseases affecting cell adhesion. For example, mice lacking the αIIbβ3 integrin exhibit bleeding, just as do human patients with the same defect. These mice also have defects in bone remodeling, because specialized bone cells (called osteoclasts) lack a related integrin, αvβ3, necessary for their function. Therefore, inhibition of the function of the αvβ3 integrin should ameliorate osteoporosis, which arises from overactivity of osteoclasts. Drugs blocking the function of platelet αIIbβ3 integrin are already in use to reduce thrombosis after angioplasty.

Many of our mutations produce defects in the development of new blood vessels, or angiogenesis, which involves multiple cell adhesion and migration events. Our work has shown particularly important roles for the extracellular matrix proteins, fibronectins, and their cell surface receptors. Fibronectins, which comprise a group of closely related proteins all encoded by a single gene, promote cell adhesion and cell migration and affect many other cellular processes.

Cells use cell surface integrin receptors to recognize fibronectins. There are about two dozen different integrins. They are the major receptors for extracellular matrix, and some integrins also participate in cell-cell adhesion; their extracellular portions recognize specific binding sites in proteins such as fibronectins. Their intracellular portions bind to cytoplasmic proteins, including both structural proteins of the cytoskeleton (which can be viewed as the "bones and muscles" of cells) and signaling proteins, which send messages into the cell affecting cell behavior. Thus, integrins serve as transmembrane linkers between the extracellular matrix outside and the cytoskeleton and signaling systems inside cells. Both fibronectin and several integrin receptors are essential for angiogenesis. We are analyzing the relative contributions of different splice isoforms of fibronectin and different integrins. Some integrins have been suggested as targets for antiangiogenic drugs, and it is important to determine which ones are the most crucially involved. Our recent work has led to new interpretations of the efficacy of certain candidate antiangiogenic drugs.

Another example of the potential for novel antiadhesive therapeutics comes from our research on other cell adhesion receptors, called selectins. These proteins mediate adhesion between circulating white blood cells or platelets and the walls of blood vessels. They, along with integrins, play important roles in ensuring that white blood cells circulate to their correct locations in the body and home on sites of infection or inflammation. We have made mice mutated in all possible permutations of their selectin genes. These mice are alive but have defects in their ability to recruit white blood cells. We have used these selectin-deficient mice to study the roles of selectins in various inflammatory responses.

We are now applying our understanding of cell adhesion to analyses of cancer. Numerous steps in the progression of cancer, including invasion and metastasis, involve altered adhesive properties of cells. Invasion and metastasis are what make tumors malignant; a better understanding of these events would be invaluable. As one example, human carcinomas that express ligands for selectins have a poorer prognosis than do those that lack those ligands, suggesting that the malignant cells may use selectins during their progression. We have used our selectin-deficient mice to investigate this question. We also use DNA arrays, which allow screening for large numbers of genes, to search for genes selectively expressed in metastatic cells. We have found that several genes involved in extracellular matrix assembly, organization of the cytoskeleton and regulation of cell migration are reproducibly dysregulated. We have been able to demonstrate that several of these changes are essential for tumor progression and metastasis. We are continuing to discover other alterations contributing to invasion and metastasis. Now that the complete genomic sequences of humans and mice are available, it is possible to determine exactly how many genes contribute to cell adhesion—our current estimates are 2,000–2,500. Using approaches such as DNA arrays and proteomics, we are investigating which of these genes are altered in their expression in different steps of invasion and metastasis.

Our work on adhesion molecules in intact animals is complemented by studies in cell culture and with purified proteins, allowing more detailed analysis of the specific binding interactions between fibronectins and integrins and between integrins and their cytoskeletal connections. This strategy offers great promise for the design of drugs to block adhesion in the intact organism. In that way, one can hope to combat pathological processes involving adhesion such as thrombosis, inflammation, angiogenesis, and cancer. Our mouse models help to refine these strategies by showing which molecules are most important for particular processes.