Cell Communication

In the human body there are many parts that work together in order for everything to function properly, but even these parts need to have a way to know what to do. This is where cell communication comes into play. Tiny cells in the body contain astounding networks that allow for this communication. Scientists are discovering why these messages from cell to cell are so efficient, and this could lead to new therapies for diseases.

In the past 15 years, scientists have discovered more of the code the cells use for their internal communications. Signal transmission begins when a messenger docks temporarily with a specific receptor on a recipient cell. This receptor is physically connected to the cytoplasm, and because of this is able to relay a message. The receptor is usually a protein that includes three domains: an external docking region for the messenger, a component that spans the cells outer membrane, and a tail that extends into the cytoplasm. When the messenger binds to the external site, it generates a change in the shape of the tail. This eases the tails interactions with the information-relaying molecules in the cytoplasm, which lead to more cellular signaling.

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Through this knowledge, the question of how messages reached their destinations without being diverted along the way was aroused. Efforts began to identify the first cytoplasmic proteins that are contacted by activated (messenger-bound) receptors in a large and important family: the receptor tyrosine kinases. The receptors transmit the commands of hormones, which control replication, specialization, or metabolism of cells.

The binding of hormones to receptor tyrosine kinases at the cell surface causes the individual receptor molecules to cluster into pairs and to attach to phosphates to the tyrosines on each others cytoplasmic tails. The altered receptors interact directly with proteins that contain a module they call an SH2 domain, which refers to a sequence of about 100 amino acids, that adopts a defined structure within a protein.

Knowledge of the time held that the messages were transmitted within cells primarily through enzymatic reactions, where one molecule alters a second without tightly binding to it and without being distorted. The receptors did not necessarily alter the chemistry of the SH2-containing proteins. Many merely stimulated the SH2 domains to latch onto the phosphate-decorated tyrosines.
The scientists were also left to figure how the nonenzymatic modules contribute to swift and specific communication in these cells. They discovered that when a protein that bears a linker also includes an enzymatic module, attachment of the linker region to another protein could position the enzymatic where it most needs to be. In the case of certain SH2-containing proteins, the linker module may originally be folded around the enzymatic domain in a way that blocks the enzymes activity. When the SH2 domain unfurls to engage an activated receptor, the move releases the enzyme to work on its target.

Even when a full protein is formed from nothing but protein-binding modules, it can function as an indispensable adapter. One module plugs into a developing signaling complex, and the other modules allow more proteins to join the network. The adapters enable cells to make use of enzymes that otherwise might not fit into a particular signaling circuit.

Nonenzymatic modules can support communication in other ways. Some molecules contain a protein-binding module and a DNA-binding module that meshes with a specific sequence of DNA nucleotides. When a protein attaches, through its linker module, to an activated receptor kinase, the interaction spurs the bound protein to detach, move to the nucleus and bind to a particular gene, which induces the synthesis of a protein. In this illustration, the only enzyme in the signaling chain is the receptor itself.

Another area of progress developed in the study of the cytoplasm, as the work demonstrated that cytoplasm is not really amorphous as it was once thought to be. It is packed densely with organelles and proteins. These findings proved that high fidelity signaling within cells depends on the interlocking of selected proteins through dedicated linker modules and adapter proteins. These complexes secure that enzymes and their targets are brought together swiftly and in the proper sequence as soon as a receptor at the surface of the cell is activated.

Studies of receptor tyrosine kinases and of SH2 domains have also helped clarify how cells guarantee that only the precise proteins combine to form any chosen signaling pathway. Diverse hormones and receptors produce different effects on the cells. SH2 domains have been found to be present in over 100 separate proteins. Every SH2 domain includes a region that fits securely over a phosphate-bearing tyrosine, as well as a second region. These regions differ from one SH2 domain to another, and the region recognizes a particular sequence of three or so amino acids next to the phosphate-bearing tyrosine, or phosphotyrosine. For this reason, all SH2 domains can bind to phosphorylated tyrosine, but they do differ in their inclination for the adjacent amino acids in a receptor. The amino acids around the tyrosine thereby serve as a code to specify which version of the SH2 domain can attach itself to a given phosphotyrosine-bearing receptor. Each SH2 domain is attached to a different enzymatic domain, and this code dictates which pathway with be activated later from the current receptors.

The signaling networks headed by receptor tyrosine kinases seem to rely on adapter proteins. Communication circuits in nerve cells of the brain show, through analyses, that some proteins in neuronal pathways have a large number of linker domains. These proteins are often called scaffolding molecules, as they permanently hold groups of signaling proteins together in one place. These scaffolds prove that certain signaling networks are hardwired into cells, which can enhance the speed and accuracy of information transfer.

Scaffolding begins as signals pass from one neuron to another at synapses, or contact points. The first neuron releases a chemical messenger into a crevice between the cells. Receptors on the second cell grab the chemical messenger, or neurotransmitter, and then cause ion channels in the membrane to open. This arrival of ions triggers enzymes that are needed to distribute an electrical impulse. Once produced, the impulse sends for more neurotransmitter to be released. This impulse is produced only when several components of this communication system have jumped into action almost simultaneously.

Kinases and phosphatases control most activities in cells. If one kinase activates a protein, a phosphatase will be charged with inactivating that protein, or vice versa. Human cells will manufacture hundreds of different kinases and phosphatases. Scaffolding proteins seem to be a common strategy for preventing the wrong kinases and phosphatases from acting on a target; they create the proper reactions by holding selected kinases and phosphatases near the precise proteins they are supposed to maintain.

The research we have today has enabled us to live better lives, and improve our understanding and knowledge of the human body. The advent of a modular signaling system would be useful to cells. A cell can generate many molecules and combination of molecules and can builds an array of interconnected pathways without having to invent a huge inventory of building blocks, simply by mixing and matching existing modules. The human body is an amazing masterpiece of science and art, and we have only just begun to understand it. Soon we will realize that our body has created ways of doing things that we can only dream of, or dissect.