Cell Biology

Edited by: Ricki Lewis S. Miyamoto, H. Teramoto, O.A. Coso, J.S. Gutkind, P.D. Burbelo, S.K. Akiyama, K.M. Yamada, "Integrin function: Molecular hierarchies of cytoskeletal and signaling molecules," Journal of Cell Biology, 131:791-805, 1995. (Cited in more than 123 publications through August 1997) Comments by Kenneth M. Yamada, Craniofacial Developmental Biology and Regeneration Branch, and J. Silvio Gutkind, Oral and Pharyngeal Cancer Branch, National Institute of Dental Research 'A NEW APPROACH': Kenneth Yamada notes that the organization and function of adhesion and signaling complexes can be divided into specific steps. The ways in which cells detect and respond to environmental signals are complex and diverse. Typically, proteins called integrins span cell membranes and serve as receptors for specific molecules that are part of the extracellular matrix (ECM). Stimulated integrins then set off cascades of enzymatic activity that carry out signal transduction, which ultimately alters gene expression. The ends of integrins that dip into the cytoplasm cause other molecules to aggregate and trigger reorganization of the underlying actin-containing cytoskeleton. "In the past few years, it has become clear that there are strong functional ties linking the extracellular matrix, the cytoskeleton, and an extraordinary range of signaling pathways," says Kenneth Yamada, chief of the Craniofacial Developmental Biology and Regeneration Branch of the National Institute of Dental Research (NIDR). A cell's response to an incoming signal begins locally. Shingo Miyamoto, a postdoctoral fellow working with Yamada who recently returned to Japan, along with J. Silvio Gutkind's group in the Oral and Pharyngeal Cancer Branch at NIDR, scrutinized the molecular choreography that occurs in a defined area of cell membrane as an incoming signal is detected. This approach enabled the researchers to group by function several of the molecules important in signal transduction and cytoskeletal responses to stimulation. They defined the groups by conditions that must be met for the molecules to aggregate. "This is the first time that most of the molecules that participate in integrin-mediated signaling have been defined, and that information has been organized in terms of sequences of events," notes Gutkind, acting chief of NIDR's Laboratory of Cellular Development and Oncology. DEFINITIVE WORK: J. Silvio Gutkind says this paper marks "the first time that most of the molecules that participating in integrin-mediated signaling have been defined." The researchers used latex beads coated with ligands or antibodies to bind integrin receptors on human foreskin fibroblasts. After 20 minutes of incubation, the cells were analyzed using immunofluorescence microscopy. The cytoplasmic molecules that bound to these probes defined distinct parts of signal transduction and cytoskeletal assembly pathways. Explains Gutkind, "The latex beads are covered with antibodies that bind the extracellular domains of integrin proteins and cause them to cluster together. If a molecule is associated with the intracellular domain of the integrin, you can detect it by coclustering-this is seen as a halo around the bead." Adds Yamada, "Miyamoto did a heroically comprehensive job of comparing the ability of more than 30 proteins to accumulate in complexes on the cytoplasmic side of the plasma membrane under a whole series of different conditions of integrin stimulation. He was also able to confirm biochemically the accumulation of such molecules." Yamada explains the rationale of "molecular hierarchies" in signal processing: "The overall concept was that integrin-induced transmembrane assembly of adhesion and signaling complexes could be understood in terms of a stepwise hierarchy of assembly. The behavior of integrins and the accumulation of the specific cytoskeletal and signal transduction molecules that comprise these complexes depend on whether the integrin receptors were clustered, occupied by ligand, or both." This paper, plus several others published at about the same time, filled in many of the gaps in understanding how a cell's adhesion to the ECM triggers signal transduction and cytoskeletal response, Yamada points out. Their work "provided a new approach to dissecting the organization and function of adhesion and signaling complexes into specific steps or hierarchies. The paper established that these steps of molecular assembly involve distinctive subsets of well-known signal transduction and cytoskeletal proteins." The study also confirmed the complexity of these molecular assemblies. "This is to be expected for a system that can play important roles in regulating growth, differentiation, apoptosis, cell migration, and the formation of tissues and extracellular matrices," Yamada concludes. Ricki Lewis, a freelance science writer based in Scotia, N.Y., is the author of several biology textbooks. She can be reached online at 76715.3517@compuserve.com.

Cell Biology

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