Secondary messengers system

Brief description about cAMP, cGMP, IP3-DAG pathways and calcium as second messenger

1. SECOND MESSENGER SYSTEM BY: FOZIYA KHAN PHARMACOLOGY BRANCH SEM I

2. Contents Introduction Second messengers Classification cAMP Pathway cGMP Pathway Nitric oxide as 2 nd messenger IP3/ DAG Pathway Calcium as 2 nd messenger

3. INTRODUCTION Second messengers are molecules that relay signals from receptors on the cell surface to target molecules inside the cell. They greatly amplify the strength of the signal, cause some kind of change in the activity of the cell. They are a component of cell signaling pathways . Earl Wilbur Sutherland Jr., discovered second messengers, for which he won the 1971 Nobel Prize.

4. SECOND MESSENGERS Short lived intracellular signaling molecules Elevated concentration of second messenger leads to rapid alteration in the activity of one or more cellular enzymes Removal or degradation of second messenger terminate the cellular response Four classes of second messengers Cyclic nucleotides Membrane lipid derivatives Ca2+ Nitric oxide/carbon monoxide

5. TYPES OF SECOND MESSENGERS

6. cAMP pathway • cGMP pathway • IP3 / DAG pathway • Calcium as a second messenger Second Messengers

7. Second Messengers • General characteristics – Low amounts in resting state – Regulated synthesis – Regulated destruction – Act through other proteins

8. c AMP PATHWAY Ligands : Epinephrine Ach Primary Effector : Adenyl cyclase Secondary messenger : cAMP

9. CYCLIC AMP cAMP is a second messenger that is synthesized from ATP by the action of the enzyme adenylyl cyclase . Binding of the hormone to its receptor activates a G protein which, in turn, activates adenylyl cyclase. Leads to appropriate response in the cell by either (or both): using Protein Kinase A(PKA) —a cAMP-dependent protein kinase that phoshorylates target proteins; cAMP binds to a protein called CREB(cAMP response element binding protein), and the resultant complex controls transcription of genes. Ex of cAMP action -adrenaline, glucagon, LH

10. C AMP Pathway

11. Cyclic AMP activates Protein Kinase A (PKA) • In addition to signaling in the cytoplasm, the catalytic subunit of PKA can enter the nucleus of cells and phosphorylate and activate the transcription factor cAMP response element binding (CREB) protein. • Phospho-CREB protein increases the transcription of many genes • Indirect effect of cAMP mediated by PKA

12. C GMP PATHWAY Ligands : ANP & NO Primary Effector : Guanylate cyclase Secondary messenger : cGMP

13. CYCLIC GMP cGMP is synthesized from the nucleotide GTP using the enzyme guanylyl cyclase . Nitric oxide stimulates the synthesis of cGMP. Many cells contain a cGMP-stimulated protein kinase that contains both catalytic and regulatory subunits. Some of the effects of cGMP are mediated through Protein Kinase G(PKG) cGMP serves as the second messenger for nitric oxide (NO) the response of the rods of the retina to light.

14. cGMP Pathway Ligand-receptor Guanylyl cyclase PDE GTP cGMP GMP Protein kinase G Active protein kinase G Effects

15. Nitric oxide (NO) NO, a simple gas, is able to diffuse across the membrane, and alters the activity of intracellular target enzymes. It’s extremely unstable, so its effects are local. Ex. It signals the dilation of blood vessels. Mechanism : Acetylcholine is released from the terminus of nerve cell in the blood vessel wall. The endothelial cells are stimulated to produce NO (from Arginine), which causes an increased synthesis of cGMP , a second messenger responsible for blood vessel dilation.

16. PHOSPHATIDYLINOSITOL-DERIVED SECOND MESSENGERS Phosphatidylinositol (PI) is a negatively charged phospholipid and a minor component in eukaryotic cell membranes. The inositol can be phosphorylated to form Phosphatidylinositol-4-phosphate (PIP) Phosphatidylinositol-4,5-bis-phosphate (PIP 2 ) Phosphatidylinositol-3,4,5-trisphosphate (PIP 3 ) Intracellular enzyme Phospho lipase C (PLC), hydrolyzes PIP 2 which is found in the inner layer of the plasma membrane. Hydrolysis of PIP 2 yields two products: Diacylglycerol (DAG) Inositol-1,4,5-trisphosphate (IP 3 )

17. IP 3 /DAG LIGAND+RECEPTOR G protein Phospholipase C PIP2 IP 3 + DAG Endoplasmic reticulum opening of Ca channels Ca++ Protein kinase C Effects

18. IP 3 /DAG Inositol triphosphate Hydrophilic Agonist for internal calcium channel [Ca++] i rises Multiple effects through Ca++ binding protein Diacylglycerol Hydrophobic Target PKC(a kinase) PKC requires Ca++ and DAG

19. Calcium as a 2nd Messenger • Calcium ions - once they enter the cytoplasm exert allosteric regulatory effects on many enzymes and proteins. • Calcium acts as a second messenger by indirect signal transduction pathways such as via G protein-coupled receptors.

20. • Low cytoplasmic Ca++ at rest (10–100 nM). • To maintain this low concentration, Ca2+ is actively pumped from the cytosol to the extracellular space and into the endoplasmic reticulum (ER) • Certain proteins of the cytoplasm and organelles act as buffers by binding Ca2+. • Signaling occurs when the cell is stimulated to release calcium ions (Ca2+) from intracellular stores, and/or when calcium enters the cell through plasma membrane ion channels.

21. • sudden increase in the cytoplasmic Ca2+ level up to 500–1,000 nM by opening channels in the endoplasmic reticulum or the plasma membrane. • Phospholipase C pathway – IP3 & DAG • Many of Ca2+-mediated events occur when the released Ca2+ binds to and activates the regulatory protein calmodulin. • Calmodulin may activate calcium-calmodulin dependent protein kinases, or may act directly on other effector proteins. • Besides calmodulin, there are many other Ca2+-binding proteins such as troponin C that mediate the biological effects of Ca2+.

22. Calmodulin Targets • Adenylate cyclase • Phosphodiesterase • Myosin light chain kinase • Calmodulin-dependent kinases • Calcineurin (a phosphatase)

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