phospholipase C

Physiology/Biochemistry (Cell Signaling)CardiovascularGastrointestinalEndocrineReproductiveNervousMusculoskeletal (smooth muscle)

Summary

Phospholipase C (PLC) is a membrane-bound enzyme that hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) into two second messengers: inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). It is a key effector in Gq-protein-coupled receptor signaling pathways, mediating diverse physiologic responses including smooth muscle contraction, secretion, and platelet activation.

Detail

Phospholipase C is activated downstream of Gq-coupled GPCRs (e.g., alpha-1 adrenergic, M1/M3 muscarinic, V1 vasopressin, H1 histamine, angiotensin II AT1, and TRH receptors). Upon receptor activation, the Gαq subunit activates PLC-β, which cleaves PIP2 in the plasma membrane into IP3 and DAG. IP3 diffuses into the cytoplasm and binds IP3 receptors on the smooth endoplasmic reticulum, triggering calcium release into the cytosol. This calcium can then bind calmodulin to activate downstream kinases (e.g., myosin light-chain kinase in smooth muscle) or work synergistically with DAG. DAG remains in the membrane and activates protein kinase C (PKC), which phosphorylates numerous target proteins, contributing to processes like cell proliferation, secretion, and inflammatory responses. Receptor tyrosine kinases (RTKs) can also activate PLC-γ isoforms directly via phosphorylation, independent of G proteins, important in growth factor signaling (e.g., EGF, PDGF, FGF receptors). Clinically, this pathway is important in understanding drug mechanisms: alpha-1 agonists (e.g., phenylephrine) cause vasoconstriction via PLC-mediated calcium increases; toxins like Bordetella pertussis toxin inhibit Gi (not Gq) affecting cAMP pathways, while cholera toxin affects Gs. Lithium, used in bipolar disorder, inhibits inositol monophosphatase, depleting free inositol needed to regenerate PIP2, dampening this pathway. PLC signaling is also relevant in platelet activation (via thromboxane A2 and thrombin receptors), gastric acid secretion (via M3 and gastrin receptors), and pupillary constriction/lacrimal secretion (M3 receptors). Mnemonic 'Gq activates PLC to make PIP2 into DAG and IP3' is essential board knowledge, often tested via receptor-drug pairing (e.g., 'HAV-1' receptors: Histamine H1, Alpha-1, Vasopressin V1).

Sources

First Aid for the USMLE Step 1
Lippincott Illustrated Reviews: Biochemistry
Goodman & Gilman's The Pharmacological Basis of Therapeutics
Molecular Biology of the Cell (Alberts et al.)

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