Second messengers are small intracellular signaling molecules that relay and amplify signals from cell surface receptors such as G protein-coupled receptors to intracellular targets. They are produced or released in response to receptor activation and diffuse to activate downstream effectors.
Cyclic AMP
Cyclic AMP is synthesized from ATP by adenylyl cyclase, an integral membrane protein activated by G-alpha-s and inhibited by G-alpha-i. cAMP levels rise within seconds of receptor activation and are rapidly returned to baseline by phosphodiesterases that hydrolyze cAMP to AMP. The principal effector of cAMP is protein kinase A, which phosphorylates serine and threonine residues on target proteins.
PKA phosphorylates a wide range of substrates, including metabolic enzymes such as phosphorylase kinase, hormone-sensitive lipase, and pyruvate kinase. In the nucleus, PKA phosphorylates the transcription factor CREB at serine 133, which recruits the coactivator CBP and activates transcription of genes containing cAMP response elements. Exchange proteins activated by cAMP are additional cAMP effectors that activate the small GTPase Rap1.
Cyclic GMP
Cyclic GMP is synthesized from GTP by guanylyl cyclase, which exists in membrane-bound and soluble forms. Membrane-bound GC is activated by natriuretic peptides such as atrial natriuretic peptide. Soluble GC is activated by nitric oxide, which binds to its heme group. cGMP is hydrolyzed by specific phosphodiesterases.
cGMP activates protein kinase G, which phosphorylates targets involved in smooth muscle relaxation, platelet inhibition, and phototransduction. In the visual system, cGMP directly binds and opens ion channels in rod photoreceptors. The cGMP signaling pathway is targeted by drugs including sildenafil, which inhibits cGMP-specific PDE5, prolonging NO-mediated vasodilation.
Calcium
Calcium is one of the most versatile second messengers, regulating processes from muscle contraction to gene expression. The cytoplasmic calcium concentration is maintained at around 100 nanomolar at rest, more than 10,000-fold lower than the extracellular concentration. This steep gradient allows rapid calcium influx through opened channels.
Calcium signals are generated by two mechanisms. Influx from the extracellular space occurs through voltage-gated calcium channels, ligand-gated channels, and store-operated channels. Release from intracellular stores, primarily the endoplasmic reticulum, occurs through IP3 receptors and ryanodine receptors. The calcium signal is terminated by calcium ATPases that pump calcium back into the ER or out of the cell.
Calcium effects are mediated by calcium-binding proteins. Calmodulin is the most ubiquitous, binding four calcium ions and undergoing a conformational change that enables it to activate target proteins including CaM kinases, calcineurin, and myosin light chain kinase. Troponin C mediates calcium-triggered muscle contraction in striated muscle.
Inositol Trisphosphate and Diacylglycerol
IP3 and DAG are produced simultaneously by phospholipase C-mediated cleavage of phosphatidylinositol 4,5-bisphosphate. IP3 is water-soluble and diffuses to the ER, where it binds IP3 receptors and triggers calcium release. The calcium signal can propagate as waves across the cell and show complex spatiotemporal patterns including oscillations.
DAG remains in the plasma membrane, where it helps recruit and activate protein kinase C. PKC is also dependent on calcium and phosphatidylserine. Different PKC isoforms have distinct regulatory properties and tissue distributions. PKC phosphorylates diverse targets regulating cell growth, differentiation, apoptosis, and secretion.
Phosphoinositides
Phosphatidylinositol can be phosphorylated at the 3, 4, and 5 positions of the inositol ring to generate seven different phosphoinositide species. These phospholipids are concentrated in specific membrane domains and serve as docking sites for proteins with phosphoinositide-binding domains. PIP2 is the most abundant phosphoinositide in the plasma membrane. PIP3, generated by PI3K, recruits AKT and other PH domain-containing proteins. Phosphoinositide phosphatases such as PTEN and SHIP negatively regulate PI3K signaling.
Nitric Oxide
Nitric oxide is a unique gaseous second messenger synthesized from arginine by nitric oxide synthase. It diffuses freely across membranes and cannot be stored. NO activates soluble guanylyl cyclase by binding to its heme iron, increasing cGMP production. NO signaling is terminated by diffusion away from targets and by reaction with hemoglobin. NO mediates vasodilation, neurotransmission, and immune defense. Nitroglycerin used for angina is converted to NO, while sildenafil prolongs NO-cGMP signaling.
