Title: READ: Cell Communication
Course: Biology (A)

Cell Communication

To survive and grow, cells need to be able to "talk" with their cell neighbors and be able to detect change in their environment. Talking with neighbors is even more important to a cell if it is part of a multicellular organism. The billions of cells that make up your body need to be able to communicate with each other to allow your body to grow, and to keep you alive and healthy. The same is true for any organism. Cell signaling is a major area of research in biology today. Recently scientists have discovered that many different cell types, from bacteria to plants, use similar types of communication pathways, or cell-signaling mechanisms. This suggests that cell-signaling mechanisms evolved long before the first multicellular organism did.

The Language of Cells

For cells to be able to signal to each other, a few things are needed:

- a signal
- a cell receptor, which is usually on the plasma membrane, but can be found inside the cell
- a response to the signal

Cells that are communicating may be right next to each other or far apart. The type of chemical signal a cell will send differs depending on the distance the message needs to go. For example, hormones, ions, and neurotransmitters are all types of signals that are sent depending on the distance the message needs to go.

The target cell then needs to be able to recognize the signal. Chemical signals are received by the target cell on receptor proteins. As discussed earlier, most receptor proteins are found in the plasma membrane. Most receptors proteins are found on the plasma membrane, but some are also found inside the cell. These receptor proteins are very specific for only one particular signal molecule, much like a lock that recognizes only one key. Therefore, a cell has lots of receptor proteins to recognize the large number of cell signal molecules. There are three stages to sending and receiving a cell "message:" reception, transduction, and response.

Signal Receptors

Cell-surface receptors are integral proteins—they reach right through the lipid bilayer, spanning from the outside to the inside of the cell. These receptor proteins are specific for just one kind of signal molecule. The signaling molecule acts as a ligand when it binds to a receptor protein. A ligand is a small molecule that binds to a larger molecule. Signal molecule binding causes the receptor protein to change its shape. At this point the receptor protein can interact with another molecule. The ligand (signal molecule) itself does not pass through the plasma membrane.

In eukaryotic cells, most of the intracellular proteins that are activated by a ligand binding to a receptor protein are enzymes. Receptor proteins are named after the type of enzyme that they interact with inside the cell. These enzymes include G proteins and protein kinases, likewise there are G-protein-linked receptors and tyrosine kinase receptors. A kinase is a protein involved in phosphorylation. AG-protein linked receptor is a receptor that works with the help of a protein called a G-protein. A G-protein gets its name from the molecule to which it is attached, guanosine triphosphate (GTP), or guanosine diphosphate (GDP). The GTP molecule is similar to ATP.

Once G proteins or protein kinase enzymes are activated by a receptor protein, they create molecules called second messengers. A second messenger is a small molecule that starts a change inside a cell in response to the binding of a specific signal to a receptor protein. Some second messenger molecules include small molecules called cyclic nucleotides, such as cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Calcium ions (Ca^2+) also act as secondary messengers. Secondary messengers are a part of signal transduction pathways.

Signal Transduction

A signal-transduction pathway is the signaling mechanism by which a cell changes a signal on it surface into a specific response inside the cell. It most often involves an ordered sequence of chemical reactions inside the cell which is carried out by enzymes and other molecules. In many signal transduction processes, the number of proteins and other molecules participating in these events increases as the process progresses from the binding of the signal. A "signal cascade" begins. Think of a signal cascade as a chemical domino-effect inside the cell, in which one domino knocks over two dominos, which in turn knock over four dominos, and so on. The advantage of this type of signaling to the cell is that the message from one little signal molecule can be greatly amplified and have a dramatic effect.

How a G-protein linked receptor works with the help of a G-protein. In panel C, the second messenger cAMP can be seen moving away from the enzyme.

G protein-linked receptors are only found in higher eukaryotes, including yeast, plants, and animals. Your senses of sight and smell are dependent on G-protein linked receptors. The ligands that bind to these receptors include light-sensitive compounds, odors, hormones, and neurotransmitters. The ligands for G-protein linked receptors come in different sizes, from small molecules to large proteins. G protein-coupled receptors are involved in many diseases, but are also the target of around half of all modern medicinal drugs.

The process of how a G-protein linked receptor works is outlined in Figure above.

A. | A ligand such as a hormone (small, purple molecule) binds to the G-linked receptor (red molecule). Before ligand binding, the inactive G-protein (yellow molecule) has GDP bound to it.
B. | The receptor changes shape and activates the G-protein and a molecule of GTP replaces the GDP.
C. | The G-protein moves across the membrane then binds to and activates the enzyme (green molecule). This then triggers the next step in the pathway to the cell's response. After activating the enzyme, the G-protein returns to its original position. The second messenger of this signal transduction is cAMP, as shown in C.

The sensing of the external and internal environments at the cellular level relies on signal transduction. Defects in signal transduction pathways can contribute or lead to many diseases, including cancer and heart disease. This highlights the importance of signal transductions to biology and medicine.

Signal Responses

In response to a signal, a cell may change activities in the cytoplasm or in the nucleus that include the switching on or off of genes. Changes in metabolism, continued growth, movement, or death are some of the cellular responses to signals that require signal transduction.

Gene activation leads to other effects, since the protein products of many of the responding genes include enzymes and factors that increase gene expression. Gene expression factors produced as a result of a cascade can turn on even more genes. Therefore one stimulus can trigger the expression of many genes, and this in turn can lead to the activation of many complex events. In a multicellular organism these events include the increased uptake of glucose from the blood stream (stimulated by insulin), and the movement of neutrophils to sites of infection (stimulated by bacterial products). The set of genes and the order in which they are activated in response to stimuli are often called a genetic program.

CK-12 Biology I book:
- CK-12 Foundation, Barabara Akre, Jean Brainard, Niamh Gray-Wilson, Douglas Wilson