Second Messengers and Signal Transduction

Tim Bloom, Ph.D.

104A Hall of Science

893-1712

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Overview

• Signal transduction- a concept

• Second messengers– Characteristics– Examples– Benefits

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Signal Transduction

• Intracellular communication– Detection of extracellular event– Generation of internal change

• Bottom line- presence of external ligand causes a change inside cell

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Signaling Examples

• Ligands – Estrogen

– Acetylcholine

– Insulin

– Epinephrine

• Receptors – Transcription factor

– Sodium channel

– Receptor kinase

– G protein-coupled receptor

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Second Messengers

• Molecules whose presence is a signal

• Synthesized or released from storage

• Act as intracellular ligands

• Commonly used in G protein-coupled receptor signaling– Made or released by effector proteins– (Ligand for GPCR is 1st messenger)

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Second Messengers

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

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Second Messengers

• Cyclic nucleotides– cAMP– cGMP

• Calcium

• Lipid derivatives– IP3

– DAG– Many others

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Cyclic AMP

• Made from ATP• Adenylate cyclase

– Membrane effector

– Needs a G-protein

• Hydrolysis of cAMP to AMP terminates signal

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cAMP

• Regulates some ion channels as ligand

• Regulates a kinase – Phosphorylates many substrates– Substrate determines effect seen

• Increase glycogen breakdown

• Decrease glycogen synthesis

• Increase cardiac beat strength

• Etc.

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cAMP System

• Example of typical second messenger set up– Receptor– G protein– Effector protein– Second messenger– “2nd Messenger binding protein(s)”– Target(s) of ‘SMBPs” – Termination mechanism

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cAMP Signaling Path

RGs

E

ATP

cAMP

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cAMP Signaling Path

RGs

E

ATP

cAMP

PKA

+

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cAMP Signaling Path

RGs

E

ATP

cAMP

PKA

+

substrate Substrate-P

PDE-

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Cyclic GMP

• Made from GTP• Guanylate cyclase

– Membrane or soluble

– Acts as receptor

– No G-protein involved

• Hydrolysis of cGMP to GMP terminates signal

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cGMP

• Regulates several proteins as ligand– Ion channels– Protein kinase

• Important in smooth muscle relaxation

• Important in visual system

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Calcium as a 2nd Messenger

• Low cytoplasmic Ca++ at rest

• Channels open with voltage and/or ligands

• Channels in PM and in “calciosome”

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Calcium as a 2nd Messenger

• High cytoplasmic Ca+

+ when stimulated

• Pumps move calcium out of cytoplasm

• Pumps in PM and in “calciosome”

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Calcium as a 2nd Messenger

• Many binding proteins mediate Ca++ action

• Activated by calcium– Troponin C– Calmodulin

• Calmodulin is multifunctional

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Calmodulin

Ca++ Ca++

Ca++ Ca++

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Calmodulin

Ca++

Ca++ Ca++

Ca++

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Calmodulin Targets

• Adenylate cyclase (some versions)

• Phosphodiesterase (some versions)

• Myosin light chain kinase

• Calmodulin-dependent kinases

• Calcineurin (a phosphatase)

• And so on…

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Phospholipase C as Effector

• Inositol tri-phosphate• Hydrophilic• Agonist for internal

calcium channel

• [Ca++]i rises

• Multiple effects through Ca++-binding proteins

• Diacylglycerol• Hydrophobic • Targets PKC (a kinase)• PKC requires Ca++ and

DAG• Mimicked by phorbol

esters (tumorogenic)

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IP3/DAG Signaling Path

RG

E

PI

IP3 & DAG

PKC

substrateSubstrate-P

calciosome

Ca++

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Why 2nd Messengers?

• Complicated pathways– Difficult to understand– Hard to learn– Many parts makes easy to be disrupted

• Complexity allows flexibility– More graded cellular responses– Interaction of pathways– Signal amplification

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Signals and Effects

• Signal is required to produce cellular change– Signal generated by receptor (or effector)– Signal size related to # of active receptors– (dose response!)

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Signal Amplification

• Example from G protein-coupled receptor– Activated receptor hits several G proteins– Each effector makes/ releases many molecules– 2nd messenger receptor also amplifies,

especially if it’s a kinase

• Bottom line: one molecule of extracellular ligand generates many modified proteins inside the cell ( a large signal)

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Review

• Ligand binding alters receptor• Some receptors use signaling molecules

– Synthesized from precursors– Released from storage site– Called second messengers

• Second messengers activate other proteins– Activation acts as signal, changes things– Second messenger is removed to end signal