Ion channels & secondary messengers

Anu K RLecturer

Dept.of PharmacologyELIMS College of Pharmacy

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Ion channels• Passage to communicate through.• When ion channels are open, they allow specific ions to move

across the plasma membrane, down their electrochemical gradient—a concentration (chemical) difference plus an electrical difference.

• Ions move from areas of higher concentration to areas of lower concentration (the chemical part of the gradient). Also, positively charged cations move toward a negatively charged area, and negatively charged anions move toward a positively charged area (the electrical aspect of the gradient).

• As ions move, they create a flow of electrical current that can change the membrane potential.

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• Ion channels open and close due to the presence of “gates.”

• The gate is a part of the channel protein that can seal the channel pore shut or move aside to open the pore.

• The electrical signals produced by neurons and muscle fibers rely on four types of ion channels: leak channels, ligand-gated channels, mechanically gated channels, and voltage-gated channels:

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Leak channels

• The gates of which randomly alternate between open and closed positions .

• More potassium ion (K) leak channels than sodium ion (Na) leak channels, and they are more leakier too.

• Thus, the membrane’s permeability to K is much higher than its permeability to Na.

• Found in nearly all cells, including the dendrites, cell bodies, and axons of all types of neurons.

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Ligand gated channels• A ligand-gated channel opens and closes in response to

the binding of a ligand (chemical) stimulus. • Either chemical ligands—including neurotransmitters,

hormones, or particular ions—can open or close ligand-gated channels.

• The neurotransmitter acetylcholine, for example, opens cation channels that allow Na and Ca2 to diffuse inward and K to diffuse outward.

• Located in the dendrites of some sensory neurons, such as pain receptors, and in dendrites and cell bodies of interneurons and motor neurons.

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Mechanically gated channels

• A mechanically gated channel opens or closes in response to mechanical stimulation in the form of vibration (such as sound waves), touch, pressure, or tissue stretching .

• The force distorts the channel from its resting position, opening the gate.

• Examples of mechanically gated channels arethose found in auditory receptors in the ears, in receptors that monitor stretching of internal organs, and in touch receptors and pressure receptors in the skin.

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Voltage gated channels

• A voltage-gated channel opens in response to a change in membrane potential (voltage).

• Voltage-gated channels participate in the generation and conduction of action potentials in the axons of all types of neurons.

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Secondary messengers

• Molecules that relay signals from receptors on the cell surface to target molecules inside the cell i.e. Cytoplasm or nucleus.

• Causing some kind of change in the activity of the cell.

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• Short lived• 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(cAMP,cGMP)– Membrane lipid derivatives(IP3,DAG)– Ca2+

– Nitric oxide/carbon monoxide

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cAMP• 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

phosphorylates target proteins; – cAMP binds to a protein called CREB (cAMP response element binding

protein), and the resultant complex controls transcription of genes.

• Eg.of cAMP action - adrenaline, glucagon, LH

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1. The ligand binds to the receptor, altering its conformation and increasing its affinity for the G protein to which it binds.

2. The G subunit releases its GDP, which is replaced by GTP.

3. The α subunit dissociates from the G complex and binds to an effector (in this case adenylyl cyclase), activating the effector.

4. Activated adenylyl cyclase produces cAMP.

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• 5. The GTPase activity of G hydrolyzes the bound GTP, deactivating G.

• 6. G reassociates with G, reforming the trimeric G protein, and the effector ceases its activity.

• 7. The receptor has been phosphorylated by a GRK

• 8. The phosphorylated receptor has been bound by an arrestin molecule, which inhibits the ligand-bound receptor from activating additional G proteins.

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cGMP• 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.

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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 (PIP2)

– Phosphatidylinositol-3,4,5-trisphosphate (PIP3)

• Intracellular enzyme phospholipase C (PLC),hydrolyzes PIP2 which is found in the inner layer of the plasma membrane. Hydrolysis of PIP2 yields two products:– Diacylglycerol (DAG)– Inositol-1,4,5-trisphosphate (IP3)

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DIACYLGLYCEROL

• Diacylglycerol stimulates protein kinase C activity by greatly increasing the affinity of the enzyme for calcium ions.

• Protein kinase C phosphorylates specific serine and threonine residues in target proteins.

• Known target proteins include calmodulin, the glucose transporter, HMG-CoA reductase, cytochrome P450 etc.

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INOSITOL TRIPHOSPHATE, IP3

This soluble molecule diffuses through the cytosol and binds to receptors on the endoplasmic reticulum causing the release of calcium ions (Ca2+) into the cytosol.

The rise in intracellular calcium triggers the response.

Example: the calcium rise is needed for NF-AT (the "nuclear factor of activated T cells") to turn on the appropriate genes in the nucleus.

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Mode of action

• Peptide and protein hormones like vasopressin, TSH, and neurotransmitters like GABA bind to GPCRs

• This activate the intracellular enzyme phospholipase C (PLC).

• PLC in turn cleaves PIP2 to yield two products – DAG and IP3.

• Both of these products act as second messengers.

• So, the cleavage of PIP2 by PLC is the functional equivalent of the synthesis of cAMP by adenylyl cyclase.

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Formation of IP3& DAG

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Nitric oxide• Nitric oxide (NO) acts as a second messenger because it is a

free radical that can diffuse through the plasma membrane and affect nearby cells.

• It is synthesised from arginine and oxygen by the NO synthase.

• It activates soluble guanylyl cyclase, which when activated produces another second messenger, cGMP.

• It is toxic in high concentrations , but is the cause of many other functions like relaxation of blood vessels, apoptosis etc.

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Calcium ions• Many cells respond to extracellular stimuli by altering their intracellular

calcium concentration.

• Ca++ acts as a second messenger in two ways:– it binds to an effector molecule, such as an enzyme, activating it;– it binds to an intermediary cytosolic calcium binding protein such as

calmodulin.

• The binding of Ca++ causes profound conformational changes in calmodulin that increase calmodulin`s affinity for its effector molecules.

• Calmodulin, when activated, causes contraction of smooth muscles.

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Reference • Tortora and Derrickson

• Karp, Gerald. Cell and Molecular biology, 6th edition, John Wiley and Sons, Inc.

• Rastogi S.C, Cell and Molecular biology, 3rd edition (2010), New Age International (P) Limited, publishers.

• Twyman R.M, Advanced Molecular Biology (2003), Viva Books Private Limited, New Delhi.

• http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Second_messengers.html

• http://en.wikipedia.org/wiki/Second_messenger_system