Second Messengers

Second messengers are small, rapidly diffusing intracellular molecules and ions whose concentrations change in response to receptor activation and which relay, amplify, and distribute a signal received at the cell surface to intracellular targets. Classic examples include cyclic AMP, cyclic GMP, inositol 1,4,5-trisphosphate, diacylglycerol, and calcium ions.

Definition

A second messenger is a non-protein intracellular signalling molecule or ion that is rapidly produced or released following activation of a cell-surface receptor and that propagates the signal by binding and regulating downstream effector proteins.

Scope

The topic covers the principal second-messenger systems, the enzymes that generate and degrade them, their downstream effectors, and the way they amplify and shape signals. It is treated as a biochemical and molecular subject within signal transduction mechanisms.

Core questions

• How are second messengers generated and degraded so that signals are both rapid and reversible?
• How does a single second messenger produce many distinct downstream effects?
• How do cells confine second-messenger signals in space and time?

Key concepts

• Cyclic AMP (cAMP)
• Inositol 1,4,5-trisphosphate (IP3)
• Diacylglycerol (DAG)
• Calcium ion as a messenger
• Adenylyl cyclase and phosphodiesterase
• Signal amplification
• Effector proteins (PKA, PKC)

Mechanisms

An activated receptor stimulates an enzyme that produces a second messenger: for example, adenylyl cyclase converts ATP to cyclic AMP, while phospholipase C cleaves a membrane phospholipid to release inositol trisphosphate and diacylglycerol. These messengers bind effectors such as protein kinase A (activated by cyclic AMP) or protein kinase C (activated by diacylglycerol and calcium), while inositol trisphosphate triggers release of calcium from intracellular stores. Because one activated enzyme generates many messenger molecules, and each effector can act on many substrates, the system amplifies the original signal. Dedicated enzymes such as phosphodiesterases degrade cyclic nucleotides, ensuring the response is transient and reversible. Cyclic AMP, acting through protein kinase A, also reaches the nucleus to regulate gene transcription.

Clinical relevance

Second-messenger systems are the point of action for many physiological regulators and pharmacological agents, and altered messenger signalling features in numerous disease processes. This entry describes the mechanisms at a reference level and is not a basis for individual diagnostic or treatment decisions.

Evidence & guidelines

The understanding of second messengers rests on biochemical and molecular research and authoritative reviews and textbooks; it is foundational laboratory science rather than a clinical-guideline domain.

History

Earl Sutherland's discovery of cyclic AMP in the late 1950s introduced the concept that hormones act through an intracellular second messenger, work for which he received the Nobel Prize. The framework expanded with the recognition of the phosphoinositide pathway, in which Berridge and Irvine identified inositol trisphosphate as a second messenger that mobilises intracellular calcium, and with later studies showing how cyclic AMP links to transcriptional control.

Key figures

• Earl Sutherland
• Martin Rodbell
• Michael Berridge
• Robin Irvine
• Marc Montminy

Seminal works

• berridge-1984
• sutherland-1972
• montminy-1997

Frequently asked questions

What makes a molecule a second messenger?

It is a small intracellular molecule or ion that is produced or released in response to a receptor signal and that carries the signal onward by regulating downstream proteins, in contrast to the first messenger, which is the extracellular signal itself.

How is a second-messenger signal turned off?

Dedicated enzymes and transporters remove the messenger; for example, phosphodiesterases break down cyclic nucleotides and pumps and exchangers restore low resting calcium, so the response ends when the stimulus stops.