Study guide - Cellular communication

Principles of cell signalling

The principle of cell signalling is hinged on the fact that cellular communication frequently involves converting signals that carry information from one form to another. During cell communication, the signalling cell releases a particular signalling molecule that is then detected by the target cell. Most animal cells send and receive signals and as such act as both signalling and target cells. Animal cells can communicate through direct contact or by secreting local regulators such as growth factors or neurotransmitters.

There are three stages of cell signalling:

  • Reception – where the target cell detects a signalling molecule present in the exogenous environment.
  • Transduction – the conversion of the signal to a form that can bring about a specific cellular response.
  • Response – the specific cellular effect brought about by the signalling molecule.
 
Reception: a signal molecule (ligand) binds to a receptor protein (receptor), causing it to change shape. The interaction between a ligand and receptor is highly specific. A conformational change in a receptor is often the initial stage in the transduction of a signal. Receptors are found in two places;
  • Intracellular proteins are found inside the plasma membrane in the cytoplasm or nucleus. The signalling molecule must cross the plasma membrane and therefore must be hydrophobic (for instance the steroid hormone testosterone), or very small (Nitric Oxide).
  • Cell-surface proteins are embedded in the plasma membrane, and these receptors bind to water-soluble ligands.

Transduction: cascades of molecular interactions relay signals from receptors to target molecule in the cell. Signal transduction pathways often involve a phosphorylation cascade. Because the pathway is usually a multistep one, the possibility of greatly amplifying the signal exists. At each step, enzymes called protein kinases phosphorylate and thereby activate many proteins at the next level. This cascade of phosphorylation greatly enhances the signal, allowing for a large cellular response. Not all components of signal transduction pathways are proteins, some are small non-protein water-soluble ions called second messengers, for example cyclic AMP (Figure 1).

Response: cell signalling leads to the regulation of transcription or cytoplasmic activities. Many signalling pathways ultimately affect protein synthesis, usually by turning specific genes on or off within the nucleus. Often, the final activated molecule in a signalling pathway functions as a transcription factor. In the cytoplasm, signalling pathways often regulate the activity of proteins rather than their synthesis. For example, the final step in the signalling pathway may affect the activity of enzymes or cause cytoskeleton rearrangement.

 

Signalling

Figure 1: Example illustration of an intracellular signalling pathway from a cell membrane receptor through to transcriptional changes within the nucleus. The cAMP pathway is activated by the binding of a ligand to its appropriate receptor (R2) which leads to the activation of cAMP-dependent protein kinase (PKA) by adenylate cyclase (AC). This activated PKA then phosphorylates HePTP at Ser23, inhibiting its ability to bind to Erk and subsequently inhibiting the MAPK pathway. [Author: Hasban, original source]