1. Second Messengers and Signal Transduction
Tim Bloom, Ph.D.
104A Hall of Science

2. Overview Signal transduction- a concept Second messengers
Characteristics
Examples
Benefits

3. Signal Transduction Intracellular communication
Detection of extracellular event
Generation of internal change
Bottom line- presence of external ligand causes a change inside cell

4. Signaling Examples Ligands Receptors Estrogen Acetylcholine Insulin
Epinephrine
Receptors
Transcription factor
Sodium channel
Receptor kinase
G protein-coupled receptor

5. 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)
Second messengers are not exclusively found in the path of G protein-coupled receptors- other receptor signal paths can also use second messengers

6. Second Messengers General characteristics Low amounts in resting state
Regulated synthesis
Regulated destruction
Act through other proteins
Acting through other proteins can be a make-or-break part of the definition. There are many molecules whose synthesis and destruction are regulated that DO NOT act as second messengers because their presence does not prompt proteins to change their behavior, i.e. because they do not act as a signal.

7. Second Messengers Cyclic nucleotides Calcium Lipid derivatives cAMP
cGMP
Calcium
Lipid derivatives
IP3
DAG
Many others

8. Cyclic AMP Made from ATP Adenylate cyclase
Membrane effector
Needs a G-protein
Hydrolysis of cAMP to AMP terminates signal

9. 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.

10. 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
Remember: receptors other than G protein-coupled receptors may use second messenger

11. cAMP Signaling Path E Gs R
cAMP
ATP

12. cAMP Signaling Path E Gs R
cAMP +
ATP
PKA

13. cAMP Signaling Path E
PDE - Gs R
cAMP +
ATP
PKA
Substrate-P
substrate

14. Cyclic GMP Made from GTP Guanylate cyclase
Membrane or soluble
Acts as receptor
No G-protein involved
Hydrolysis of cGMP to GMP terminates signal

15. cGMP Regulates several proteins as ligand
Ion channels
Protein kinase
Important in smooth muscle relaxation
Important in visual system

16. Calcium as a 2nd Messenger
Low cytoplasmic Ca++ at rest
Channels open with voltage and/or ligands
Channels in PM and in “calciosome”

17. Calcium as a 2nd Messenger
High cytoplasmic Ca++ when stimulated
Pumps move calcium out of cytoplasm
Pumps in PM and in “calciosome”

18. Calcium as a 2nd Messenger
Many binding proteins mediate Ca++ action
Activated by calcium
Troponin C
Calmodulin
Calmodulin is multifunctional

19. Calmodulin
Ca++
Ca++
Ca++
Ca++

20. Calmodulin Ca++ Ca++ Ca++ Ca++
This change in shape allows calcium-bound calmodulin to bind (and regulate) other proteins

21. Calmodulin Targets Adenylate cyclase (some versions)
Phosphodiesterase (some versions)
Myosin light chain kinase
Calmodulin-dependent kinases
Calcineurin (a phosphatase)
And so on…

22. 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)
This enzyme (PLC) can be activated in a number of ways, one of which is by a G-protein. When on, PLC synthesizes the two second messengers described above.

23. IP3/DAG Signaling Path PKC E G Substrate-P R substrate IP3 & DAG PI
Ca++
calciosome

24. Why 2nd Messengers? Complicated pathways Complexity allows flexibility
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

25. 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!)

26. 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)
With cyclic AMP as a specific example: one epinephrine molecule binds its receptor; multiple G proteins are activated by the activated receptor (more than one per epinephrine molecule = amplification); each G protein activates one adenylate cyclase enzyme (one-to-one = no amplification); each adenylate cyclase makes many molecules cyclic AMP (amplification); cyclic AMP activates a kinase which phosphorylates many substrates (more amplification)

27. 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