secondmessenger

Second messenger is an intracellular signaling molecule that relays information from cell surface receptors to internal targets, helping transduce and amplify signals initiated by extracellular stimuli such as hormones, neurotransmitters, or growth factors. The term contrasts with the primary, or first messenger, which is the extracellular ligand that binds to receptors on the cell surface. Typical second messengers are small, diffusible molecules or ions produced or released in response to receptor activation and act to regulate a broad array of proteins and pathways inside the cell.

Common examples include cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP), calcium ions (Ca2+), inositol trisphosphate (IP3) and diacylglycerol (DAG). In many pathways, G protein coupled receptors activate adenylyl cyclase to convert ATP to cAMP, which activates protein kinase A (PKA) and alters metabolism and gene expression. Phospholipase C, activated by certain receptors, cleaves PIP2 to generate IP3 and DAG. IP3 triggers release of Ca2+ from the endoplasmic reticulum, while DAG helps activate protein kinase C (PKC); Ca2+ itself acts as a versatile messenger by binding calmodulin and other targets.

Second messengers are tightly regulated by enzymes such as phosphodiesterases that degrade cAMP and cGMP, and by pumps that sequester Ca2+. Rapid synthesis and degradation allow cells to respond quickly and precisely to stimuli. Bacteria also use second messengers such as cyclic di-GMP and cyclic di-AMP to control processes like biofilm formation and virulence.