G protein coupled receptors (Type II Hormone Signaling)

Cell signaling, Endocrinology, Physiology, Biochemistry

1. G Protein – Coupled Receptors PRADEEP SINGH, MANSI MODI M.Sc. Medical Biochemistry HIMSR, JAMIA HAMDARD

2. Introduction The signal represents information that is detected by specific receptors and converted to a cellular response which always involves a chemical process. Several representative signal- transduction systems, classified according to the type of receptor. A signal interacts with a receptor, the activated receptor interacts with cellular machinery, producing a second signal or a change in the activity of a cellular protein. T he metabolic activity of the target cell undergoes a change and finally the tranduction event ends.

3. Six General Types Of Signal Transducers

4. G Protein- Coupled Receptors and Second Messengers Guanosine nucleotide – binding protein (G protein) family. Three essential components define signal transduction through GPCRs: a plasma membrane receptor with seven transmembrane helical segments. The G protein, stimulated by the activated receptor, exchanges bound GDP for GTP, then dissociates from the occupied receptor and binds to the near by effector enzymes, altering its activity. The activated enzyme then generates a second messenger that affects downstream targets.

5. Medical point:- GPCRs have been implicated in many common human diseases, including allergies, depression, blindness, diabetes and various cardiovascular defects with serious healt h consequences. Close to half of all drugs on the market target one GPCR or another. For example, the β adrenergic receptor, which mediates the effects of epinephrine, is the target of the ‘beta blockers’, prescribed for such diverse conditions as hypertension, cardiac arrhythmia, anxiety and migraine headache.

6. The β - Adrenergic Receptor System Acts through t he Second Messenger cAMP Adrenergic receptors are of four general types, α ₁, α₂ , β₁ and β₂ , defined by differences in their affinities and response to a group of agonists and antagonists. Agonists are structural analogs that bind to a receptor and mimic the effects of its natural ligand. Antagonists are analogs that bind the receptor without triggering the normal effects and there by block the effects of agonists, including the biological ligand.

7. The 4 types of adrenergic receptors are found in different target tissues and mediate different responses to epinephrine. Here we focus on the β - adrenergic receptors of muscle, liver, and adipose tissue.

8. Transduction of the Epinephrine Signal: the β Adrenergic Pathway

9. Protein kinase

10. cAMP binds to a PK called PKA, a heterotetrameric molecule. It consisting of two regulatory subunits that inhibits the activity of two catalytic subunits when bound as a tetrameric complex. The active catalytic subunits phosphorylate a number of target protein on serine and threonine residues.

11. cAMP Mediated Cascade

12. The net effect of the cascade is amplification of the hormonal signal by several orders of magnitude, which accounts for the very low concentration of epinephrine(or any other hormone) required for hormone activity.

13. G - Proteins: Clinical point of view Cholera toxin, secreted by Vibrio cholerae in the intestine of an infected person, is heterometric protein. Cholera toxin ADP ribosylates G protein and its permanently activation of adenylyl cyclase of intestinal epithelial. Chronically high cAMP Chronically active PKA Phosphorylates exchanger in intestinal epithelia Ultimate result is efflux of Cl ̄ triggers and water loss into gut Severe Diarrhoea

14. The β - Adrenergic Receptor Is desensitized by Phosphorylation and by Association with Arrestin

15. The arrestin - receptor complex recruits two proteins involved in vesicle formation the AP-2 complex and clathrin , which initiate membrane invagination, leading to the formation of endosomes containing adrenergic receptor. In this state the receptors are inaccessible to epinephrine and therefore inactive. Receptors in the endocytic vesicles are eventually dephosphorylated and returned to the plasma membrane completing the circuit and resensitizing the system to epinephrine.

16. β ARK is a member of a family of G protein- coupled receptor kinases, and play roles similar to that of β ARK is desensitization and resensitization of their receptors.

17. G-Protein Subunits With Second Messenger

18. DAG, IP₃ and Ca²⁺ Have Related Roles as Second Messengers

19. Phospholipase C/Inositol Phosphate System 1950s by Hokin . PIP₂ is the substrate for a membrane bound enzyme, phospholipase C. Which splits into DAG and inositol (1,4,5) triphosphate. Both function as second messenger. IP₃ receptor- a ligand gated calcium channel present on the membrane of the endoplasmic reticulum.

20. Diacylglycerol and Protein Kinase C DAG, unlike the IP₃, is highly lipophilic and remains within the membrane. Binds to a specific site on the PKC molecule, which migrates from the cytosol to the cell membrane in the presence of DAG, thereby becoming activated. Kinases is general play a central role in signal transduction and control many different aspects of cell function.

21. Some Signals That Act through PhospholipaseC , IP₃ and Ca²⁺

22. Ca²⁺ Calcium is an important intracellular regulator of cell function like contraction of muscles, secretion of hormones and neurotransmitters, cell division and regulation of gene regulation. The intracellular calcium concentration is low (10 ̄⁷) where as extracellular calcium concentration is very high (10 ̄³), maintaining a 10,000 fold calcium gradient across the membrane.

23. There are mainly 3types of calcium transport systems:- - Voltage gated calcium channels - Sodium/calcium antiport transporter - Calcium transporting ATPase The calcium transporting ATPase transporter accumulates calcium within the lumen of ER in muscle. These Ca²⁺ ions can be released into the cytoplasm by an IP₃ gated calcium channel or by a ligand gated calcium channel (ryanodine receptor).

24. On entering a cell, calcium ions bind with the protein c almodulin . C almodulin changes its shape and initiates multiple effects inside the cell, including activation or inhibition of PK. Activation of calmodulin - dependent protein kinases causes, via phosphorylation, activation or inhibition of proteins involved in the cell’s response to the hormone.

25. Some P roteins Regulated by Ca²⁺ and Calmodulin

26. Summary

27. Conclusion GPCR will continue to be highly important in clinical medicine because of their large number, wide expression and role in physiologically important responses. Nearly 40%of the drugs approved for marketing by the FDA target GPCRs. Future discoveries will reveal new GPCR drugs, in part because it is relatively easy to screen for pharmacologic agents that access these receptors and stimulate or block receptor mediated biochemical or physiological responses.

28. Thank You