Friday, December 26, 2008

Cell Communication and Signal Transduction Pathways

The cells, unicellular or multicellular, respond to changes in the environment. In multicellular organisms, cells communicate with the neighboring cell in order to coordinate the functions of the system and body. Cell-to-cell communication is essential for normal functioning of multicellular organisms, and this is because they carry special abilities to maintain a well-defined communication network. In recent past, "cell signaling" has become a fascinating field. It explains a lot of critical issues regarding embryological development, hormone action, and the development of diseases such as cancer.

Cell Communication

Cell-to-cell communication is essential for multicellular organisms for coordinating various metabolic activities including activation of immune systems and gene expression. Interestingly, the same fundamental cell communication strategies are evident in many different types of cells. The cell-to-cell communication usually occurs through certain chemicals called as signal molecules. Signal molecules are small organic molecules, which can interact with certain specific proteins known as receptors. The signal molecules can be an amino-acid-like tyrosine and its derivatives or small peptides such as insulin or steroid hormones and growth regulators such as cytokines. Certain environmental factors also can impart some signals, which can be received by some receptor proteins. Signals that originate from environmental factors include temperature, electromagnetic waves of different spectrum such as visible light, osmolarity, ions such as iron, etc. Recipient cells, which receive these signals through receptors, are located on the surface of the cell membranes. Then the cells can respond to these signals by accepting them or by transmitting them to the next target molecule or cells.

Communicating cells may be close together or far apart: There are local regulators that influence cells in the more immediate vicinity. Growth factors are examples of local regulators that stimulate nearby cells to grow and multiply.

Paracrine signaling is another local signaling in animals. When a nerve cell produces a chemical signal, called a neurotransmitter, that diffuses to a single target cell that is very close to the first cell. This is known as synaptic signaling. Local signaling in plants is not well understood, although we do know that they must use different mechanisms since they have cell walls. In both animals and plants, hormones are used for cell signaling at greater distances. In endocrine (hormone) signaling, the specialized cells release hormone molecules into the circulatory system that then carry the hormones to the target cells in other parts of the body.

Cells may also communicate by direct contact. Cell junctions provide cytoplasmic continuity between cells and signaling substances in one cell can therefore diffuse into the cytoplasm of the adjacent cell.

Signal Transduction

There are three stages in the process of cell signaling or communication:

1. Reception-a protein at the cell surface detects chemical signals.

2. Transduction-a change in protein stimulates other changes including signal-transduction pathways.

3. Response-almost any cellular activity.

Once the target cell receives the signal molecule it converts the signal to a form that can bring about a specific cellular response. This often occurs in a series of steps called a signal transduction pathway. Signal molecules bind to receptor protein, in cell membranes, and generally cause a conformational change in the proteins. This change in conformation is transmitted to the cytoplasmic domain or part of the receptor molecule. The transformed molecule interacts with the information-relaying molecules in the cytoplasm. These molecules are small molecules present in the cytoplasm known as secondary messengers. Calcium ions and Cyclic AMP (cAMP) are examples. This further starts a series of chain reactions, which ultimately reaches the target gene and causes its expression or repression. A single cell may have several types of receptors each binding to a specific signal molecule. A cell can receive a number of different types of signal molecules simultaneously. Once the signals are relayed into the cells, they are selectively routed through various signal pathways to the target, which may be a gene or a protein. Usually the cellular response for a signal molecule may be a change in gene expression, change in ion permeability, or a change in the enzyme activity or protein three-dimensional structure, which ultimately affects the metabolism of the cell or organism.

Some types of signal molecules pass through the cell membrane and directly activate the gene or proteins, without the involvement of any secondary messengers. Lipid-soluble molecules such as steroids (steroid hormones) or small molecules (such as nitric oxide) are examples. Molecules such as interferon and interleukin can also do the same type of direct activation of a gene. Through cell-to-cell signaling and signal transduction, the information that the cells acquire from the environment and from other neighboring cells is effectively monitored and responded into the appropriate manner.

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