Signal transduction

All cells of multicellular organisms constantly exchange signals with each other during their life cycle. Through this "signal exchange", cells can influence each other's behaviour and thus adapt the entire organism to the given physiological conditions. During this transfer, the different messages received by a cell are specifically processed and passed on as an internal biochemical signal to perceptive systems such as enzymes.

A large number of enzymes and secondary messengers are often involved in this signal transduction. The totality of all signals and signalling pathways in a cell is also known as its signalling network.

The messenger substances include:

• hormones
• Neurotransmitter
• Growth factors

Although a very large number of different messenger substances are known, the biochemical processes of intracellular signal transduction are characterized by very similar or even identical transmission cascades. For example, the messenger (ligand) binds to a receptor of its target cell independent of its molecular composition. The receptor recognizes the signal molecule and enters into a highly specific, non-covalent interaction with it. The binding between ligand and receptor is characterized by a very high affinity, so that most signal molecules are effective even at very low concentrations. The interaction of ligand/receptor triggers a cascade of molecular changes in the cell. This leads to the specific response of the cell to the information received.

In the target cell, receptors can be located at different sites: in the cell membrane (transmembrane receptors) or intracellularly. For example, steroid hormones, thyroid hormones, vitamin D and retinoids are so lipophilic that they can penetrate cell membranes. They can therefore dock to intracellular receptors which are bound to activating proteins in the cytosol. Docking of the agonist to its receptor initially leads to the detachment of inactivating proteins. The agonist-receptor complex then docks to another agonist-receptor complex and enters the cell nucleus as a dimer. There, this complex binds to specific DNA sequences and promotes or inhibits the transcription of certain target genes.

Receptor-like proteins: This term covers cellular proteins that do not mediate the effects of transmitters, hormones or cytokines, but have other tasks in the cell. These include:

• enzymes (Na+, K+ or ATPases)
• transporters (e.g. the neuronal norepinephrine or serotonin transporters blocked by certain antidepressants)
• Cellular structural proteins (e.g. microtubils blocked by vinca alkaloids or taxanes)

The most common type of receptor, however, is located in the cell membrane and penetrates the membrane. The cell membrane receptors bind to their ligand via their extracellular domain and transmit the information via their intracellular domain into the interior of the cell. Such transmembrane receptors are used by messenger substances (e.g. those with peptide structure) which are unable to penetrate a lipophilic cell membrane. The cell membrane receptors can be divided into four groups (see receptors below):

• G-protein coupled receptors
• Ion channel receptors
• enzyme receptors
• Receptors with associated tyrosine kinase

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