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Cell-Cell Interactions 7 Cell-Cell Interactions Concept Outline 7.1 Cells signal one another with chemicals. Receptor Proteins and Signaling between Cells. Receptor proteins embedded in the plasma membrane change shape when they bind specific signal molecules, triggering a chain of events within the cell. Types of Cell Signaling. Cell signaling can occur between adjacent cells, although chemical signals called hormones act over long distances. 7.2 Proteins in the cell and on its surface receive signals from other cells. Intracellular Receptors. Some receptors are located within the cell cytoplasm. These receptors respond to lipid- soluble signals, such as steroid hormones. Cell Surface Receptors. Many cell-to-cell signals are water-soluble and cannot penetrate membranes. Instead, the signals are received by transmembrane proteins protruding out from the cell surface. 7.3 Follow the journey of information into the cell. FIGURE 7.1 Persimmon cells in close contact with one another. These Initiating the Intracellular Signal. Cell surface receptors plant cells and all cells, no matter what their function, interact often use “second messengers” to transmit a signal to the with their environment, including the cells around them. cytoplasm. Amplifying the Signal: Protein Kinase Cascades. Surface receptors and second messengers amplify signals as id you know that each of the 100 trillion cells of your they travel into the cell, often toward the cell nucleus. Dbody shares one key feature with the cells of tigers, bumblebees, and persimmons (figure 7.1)—a feature that 7.4 Cell surface proteins mediate cell-cell interactions. most bacteria and protists lack? Your cells touch and com- The Expression of Cell Identity. Cells possess on their municate with one another. Sending and receiving a variety surfaces a variety of tissue-specific identity markers that of chemical signals, they coordinate their behavior so that identify both the tissue and the individual. your body functions as an integrated whole, rather than as a Intercellular Adhesion. Cells attach themselves to one massive collection of individual cells acting independently. another with protein links. Some of the links are very strong, The ability of cells to communicate with one another is the others more transient. hallmark of multicellular organisms. In this chapter we will Tight Junctions. Adjacent cells form a sheet when connected by tight junctions, and molecules are encouraged look in detail at how the cells of multicellular organisms in- to flow through the cells, not between them. teract with one another, first exploring how they signal one Anchoring Junctions. The cytoskeleton of a cell is another with chemicals and then examining the ways in connected by an anchoring junction to the cytoskeleton of which their cell surfaces interact to organize tissues and another cell or to the extracellular matrix. body structures. Communicating Junctions. Many adjacent cells have direct passages that link their cytoplasms, permitting the passage of ions and small molecules. 123 7.1 Cells signal one another with chemicals. Receptor Proteins and Signal Signaling between Cells molecules Communication between cells is com- mon in nature. Cell signaling occurs in all multicellular organisms, provid- Extracellular ing an indispensable mechanism for surface cells to influence one another. The cells of multicellular organisms use a variety of molecules as signals, includ- ing not only peptides, but also large proteins, individual amino acids, nu- cleotides, steroids and other lipids. Even dissolved gases are used as Cytoplasm signals. Nitric oxide (NO) plays a role in mediating male erections (Viagra FIGURE 7.2 functions by stimulating NO release). Cell surface receptors recognize only specific molecules. Signal molecules will bind Some of these molecules are at- only to those cells displaying receptor proteins with a shape into which they can fit snugly. tached to the surface of the signaling cell; others are secreted through the plasma membrane or released by exocytosis. Monoclonal antibodies. The first method uses mono- Cell Surface Receptors clonal antibodies. An antibody is an immune system pro- tein that, like a receptor, binds specifically to another Any given cell of a multicellular organism is exposed to a molecule. Each individual immune system cell can make constant stream of signals. At any time, hundreds of differ- only one particular type of antibody, which can bind to ent chemical signals may be in the environment surround- only one specific target molecule. Thus, a cell-line de- ing the cell. However, each cell responds only to certain rived from a single immune system cell (a clone) makes signals and ignores the rest (figure 7.2), like a person fol- one specific antibody (a monoclonal antibody). Mono- lowing the conversation of one or two individuals in a clonal antibodies that bind to particular receptor pro- noisy, crowded room. How does a cell “choose” which sig- teins can be used to isolate those proteins from the nals to respond to? Located on or within the cell are re- thousands of others in the cell. ceptor proteins, each with a three-dimensional shape that fits the shape of a specific signal molecule. When a signal Gene analysis. The study of mutants and isolation of molecule approaches a receptor protein of the right shape, gene sequences has had a tremendous impact on the the two can bind. This binding induces a change in the re- field of receptor analysis. In chapter 19 we will present a ceptor protein’s shape, ultimately producing a response in detailed account of how this is done. These advances the cell. Hence, a given cell responds to the signal mole- make it feasible to identify and isolate the many genes cules that fit the particular set of receptor proteins it pos- that encode for various receptor proteins. sesses, and ignores those for which it lacks receptors. Remarkably, these techniques have revealed that the enormous number of receptor proteins can be grouped into The Hunt for Receptor Proteins just a handful of “families” containing many related recep- tors. Later in this chapter we will meet some of the mem- The characterization of receptor proteins has presented a bers of these receptor families. very difficult technical problem, because of their relative scarcity in the cell. Because these proteins may constitute Cells in a multicellular organism communicate with less than 0.01% of the total mass of protein in a cell, puri- others by releasing signal molecules that bind to fying them is analogous to searching for a particular grain receptor proteins on the surface of the other cells. of sand in a sand dune! However, two recent techniques Recent advances in protein isolation have yielded a have enabled cell biologists to make rapid progress in this wealth of information about the structure and function of these proteins. area. 124 Part II Biology of the Cell Types of Cell Signaling Secretory cell Cells communicate through any of four basic mechanisms, depending primarily on the distance between the signaling and responding cells (figure 7.3). In ad- Gap dition to using these four basic mecha- junction nisms, some cells actually send signals to themselves, secreting signals that Adjacent target cells bind to specific receptors on their own plasma membranes. This process, called autocrine signaling, is thought (a) Direct contact (b) Paracrine signaling to play an important role in reinforcing developmental changes. Hormone secretion into blood by endocrine gland Neurotransmitter Direct Contact As we saw in chapter 6, the surface of a Target cell eukaryotic cell is a thicket of proteins, carbohydrates, and lipids attached to Nerve cell Blood vessel and extending outward from the Distant target cells Synaptic gap plasma membrane. When cells are very close to one another, some of the mol- (c) Endocrine signaling (d) Synaptic signaling ecules on the cells’ plasma membranes may bind together in specific ways. FIGURE 7.3 Many of the important interactions Four kinds of cell signaling. Cells communicate in several ways. (a) Two cells in direct between cells in early development contact with each other may send signals across gap junctions. (b) In paracrine signaling, occur by means of direct contact be- secretions from one cell have an effect only on cells in the immediate area. (c) In endocrine signaling, hormones are released into the circulatory system, which carries them to the target tween cell surfaces (figure 7.3a). We’ll cells. (d) Chemical synaptic signaling involves transmission of signal molecules, called examine contact-dependent interac- neurotransmitters, from a neuron over a small synaptic gap to the target cell. tions more closely later in this chapter. Paracrine Signaling Synaptic Signaling Signal molecules released by cells can diffuse through the In animals, the cells of the nervous system provide rapid extracellular fluid to other cells. If those molecules are communication with distant cells. Their signal molecules, taken up by neighboring cells, destroyed by extracellular neurotransmitters, do not travel to the distant cells enzymes, or quickly removed from the extracellular fluid in through the circulatory system like hormones do. Rather, some other way, their influence is restricted to cells in the the long, fiberlike extensions of nerve cells release neuro- immediate vicinity of the releasing cell. Signals with such transmitters from their tips very close to the target cells short-lived, local effects are called paracrine signals (figure (figure 7.3d). The narrow gap between the two cells is 7.3b). Like direct contact, paracrine signaling plays an im- called a chemical synapse. While paracrine signals move portant role in early development, coordinating the activi- through the fluid between cells, neurotransmitters cross the ties of clusters of neighboring cells. synapse and persist only briefly. We will examine synaptic signaling more fully in chapter 54. Endocrine Signaling If a released signal molecule remains in the extracellular fluid, Adjacent cells can signal others by direct contact, while it may enter the organism’s circulatory system and travel nearby cells that are not touching can communicate widely throughout the body.
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