Summary of Hormone Receptor Pathways and Signaling

## Introduction G-protein-mediated signal transduction is a mechanism by which cells transmit signals from their surface into the cytoplasm or organelles. In this material, we will focus on one of the main pathways mediated by the Gq protein, discussing the steps, key molecules, and functional consequences of activating this pathway. This material is intended for university-level students and assumes basic knowledge of cell structure. ## Key Concepts > **G-protein:** A heterotrimeric membrane-bound protein composed of Gα, Gβ, and Gγ subunits that transmits signals between receptors and effectors. > **Gq protein:** A type of Gα subunit that, upon activation, binds GTP and subsequently activates the enzyme phospholipase C (PLC). > **Phospholipase C (PLC):** A membrane-bound enzyme that cleaves phosphatidylinositol-4,5-bisphosphate (PIP2) into inositol-1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). > **IP3:** A diffusible signaling molecule that binds to IP3 receptors in the smooth endoplasmic reticulum or sarcoplasmic reticulum, triggering the release of Ca^{2+}. > **DAG:** A lipid second messenger that remains in the membrane and activates protein kinase C (PKC). ## Step-by-Step: Gq Pathway Activation 1. Receptor and Ligand - A hormone or factor (e.g., oxytocin) binds to receptors coupled to Gq protein. 2. Gq Activation - Upon ligand binding, the receptor undergoes a conformational change and acts as a GEF for Gα_q: Gα_q releases GDP and binds GTP. - Gα_q-GTP dissociates from Gβγ and is active. 3. Effector Activation (PLC) - Gα_q-GTP binds to phospholipase C (PLC) in the plasma membrane and activates it. 4. PIP2 Cleavage - PLC cleaves the membrane lipid PIP2 into two molecules: IP3 and DAG. - Chemically: PIP2 → IP3 + DAG 5. IP3 Signaling - IP3 diffuses into the cytoplasm and binds to IP3 receptors in the smooth ER or sarcoplasmic reticulum. - Upon binding, Ca^{2+} channels open and Ca^{2+} is released into the cytoplasm. 6. Role of Ca^{2+} and DAG - Ca^{2+} binds to calmodulin and/or directly activates Ca^{2+}-dependent kinases. - DAG, along with Ca^{2+}, activates protein kinase C (PKC). 7. Consequences of Phosphorylation - PKC and other kinases phosphorylate target proteins, altering their activity, localization, or stability. ### Signal Flow Visualization (Summary) - Receptor + ligand → Gα_q activation → Gα_q-GTP → PLC activation → PIP2 cleavage → IP3 + DAG - IP3 → Ca^{2+} release from ER; DAG + Ca^{2+} → PKC activation → target phosphorylation ## Functional Consequences in the Cell - Change in membrane permeability (e.g., opening of ion channels) - Activation of contractile mechanisms in muscle cells (via Ca^{2+}-calmodulin kinases) - Regulation of metabolism and pathways leading to the synthesis or degradation of molecules - Change in gene expression via kinases and transcription factors ## Practical Examples and Applications - Oxytocin in uterine contraction: oxytocin activates the Gq pathway in uterine muscle, increasing Ca^{2+} and promoting contraction. - Neurotransmitters and hormones: some neurotransmitters and hormones utilize the Gq-PLC/IP3-DAG pathway to rapidly regulate intracellular Ca^{2+} levels. - Pharmacology: inhibitors or modulators target receptors or downstream effects (e.g., PKC modulation) in disease treatment. ## Comparison of Gq with other Gα Subtypes | Property | Gq | Gs | Gi/o | |---|---:|---:|---:| | Main effector | PLC (→ IP3 + DAG) | adenylyl cyclase (↑ cAMP) | inhibition of adenylyl cyclase or activation of ion channels | | Main type of second messengers | Ca^{2+}, DAG | cAMP | cAMP (decreased), modulation of ion currents | | Typical functions | contraction, secretion, enzyme activation | increased metabolism, PKA activation | inhibition, hyperpolarization | ## Signal Termination Mechanisms - Hydrolytic activity of Gα_q: GTP → GDP (inactivation of Gα_q). - Ca^{2+} reuptake into the ER (SERCA pumps) and Ca^{2+} binding by proteins. - IP3 degradati