| Cell-Cell Communication and Signal Transduction |
Although development begins with a "master plan," initiated by coordinate genes and carried on through a series of genetic cascades, cells also communicate with one another to coordinate development. In addition, cell-cell communication is essential throughout the life of the organism. Indeed, many cancers are due in part to failures of normal cell-cell communication. (See the Cell Biology and Cancer unit.)
There are some similarities between the way cells communicate and the way individual organisms communicate: in both cases there are signalers and receivers. Cell-cell communication, like many forms of communication between organisms, involves the transfer of information by using molecules between signalers and receivers. The signaling cell sends out molecules called ligands; these can be proteins or small molecules such as vitamin D. Ligands attach to proteins embedded in the membrane of the receiver cell;these proteins are sometimes called receptor proteins.
Once the receptor protein receives the message (the ligand), the nucleus still needs to receive the information because that's where transcription occurs. How does that happen? Most often, the binding of the ligand causes the receptor protein to change its conformation. This conformational change sets up a series of changes, and sometimes cascades, which eventually lead to changes in transcriptional activity of genes.
One example of a signaling pathway involves the "hedgehog" gene in Drosophila. This gene was so named because larvae with the mutant phenotype are covered with hair and look somewhat like a hedgehog. The protein encoded by the hedgehog gene is a ligand and interacts with several receptors. Among other functions, it triggers the early steps in development of postsynaptic neurons. It also is involved in the differentiation of the photoreceptor cells of the eye.