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- Worms in space |
There exists a general theme that cell-cell interactions are central to the proliferation and development of cells in an organism. A system by which one cell alters the properties of a second cell must have two components. The originating cell must produce a signal in the form of a cell-surface or secreted molecule for which the target cell has a specific receptor. The times and places in which these signaling molecules and their receptors are made will determine which cells can exchange particular types of information. Equally important, the binding of the signaling molecule to its receptor must induce an activity intrinsic to or associated with the receptor, and thereby modulate biochemical pathways within the target cell. This in turn triggers changes in gene expression, the cell cycle, cytoskeletal architecture or metabolism. Tremendous progress has been made in identifying the genes that control signal transduction in a wide range of organisms. Remarkably, the gene products that regulate these events are conserved not only between vertebrates and invertebrates, but also in simple unicellular eukaryotes. This has allowed biologists to use fruit flies, nematodes, and yeast as model genetic organisms for studying signal transduction. At the same time, these discoveries have medical implications, since human diseases such as cancer and diabetes involve errors in signal transduction, often as a result of mutations in the genes that control signal transduction. Indeed, the discovery of cancer-causing genes in certain viruses and in the DNA of tumor cells has been central to understanding how signal transduction works. The convergence of biochemical and molecular genetic data from cancer research with genetic data from simple animal models has focused attention on conserved signaling modules that regulate cell behavior. Signaling molecules and their receptors fall into at least five families based on structural similarities. Each family couples to a different biochemical pathway inside cells to transduce information. The pathways within cells are formed by chains of intercommunicating proteins. Each protein component of a pathway integrates signals from upstream proteins and passes them to downstream proteins. Individual transducing proteins work in a variety of ways. They may respond to acquired signals by amplifying them, damping them down, or processing them in complex ways before they transmit them to downstream proteins. Pathways that use such components allow a cell to integrate a variety of signals received at its surface. This description vaguely has the ring of electronic circuitry, and one day we will surely be able to depict these pathways in a form recognizable to the electrical engineer.
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