1.
ROLE OF SECONDARY MESSENGER SYSTEMS IN RECEPTOR
SIGNAL TRANSDUCTION
Date: 13th October 2021
Presented by: PRIYANSHA SINGH
Subject: GE (511)
(M.S.- Pharmacology & Toxicology)
NIPER- Guwahati

2.
CONTENTS
1. WHAT ARE SECONDARY MESSENGERS
2. REQUIREMENTS FOR A COMPOUND TO ACT LIKE A SECOND MESSENGER
3. CLASSIFICATION OF SECOND MESSENGERS
• A) cAMP
• B) cGMP
• C) IP3/ DAG
• D) Ca2+
• E) NO
4. RECENT DISCOVERIES
5. CONCLUSIONS
6. REFERENCES

3.
EARL WILBUR SUTHERLAND Jr. WON NOBEL PRIZE IN PHYSIOLOGY IN 1971
THE DISCOVERY OF SECOND MESSENGER cAMP

4.
WHAT ARE SECONDARY MESSENGER SYSTEM
• Secondary messenger system is a part of cellular signaling process in which proteins of
different kind are activated through generation of diffusible signaling molecules. The
activated proteins then participate in a cellular response.
• Second messengers are produced catalytically in response to the extracellular signals
(primary messengers) and amplify their response, thus second messengers are a part of
signal transduction cascades.
• In the transduction process, extracellular signal is passed from one intracellular molecule
to another through the secondary messengers until the cellular behavior alters.
• G-protein linked receptors activate a class of membrane bound proteins which then
migrates in the plane of plasma membrane initiating the cascade effects. This results in
enzymatic alteration and generates host of additional signals, called as second
messengers.

5.
(i) 1st messenger
(ligand/ drug)
SIGNAL TRANSDUCTION
CASCADE SHOWING THE
ACTIVITY OF SECONDARY
MESSENGER
(vi) Cascade of cellular responses (metabolic & structural changes
Effector enzyme for
e.g.- Adenyl Cyclase,
PLC (b and g), IP3 gated
calcium channels
cAMP, cGMP, IP3/DAG, Ca2+
(Active secondary messenger)
GDP GTP
ATP, GTP PIP2
(inactive secondary
messenger)
(v) Activated (phosphorylated) kinases
Membrane
receptor
(ii) G- protein relay
(iv) Activation
of second
messenger by
effector

6.
MESSENGERS

7.
REQUIREMENT OF A COMPOUND TO ACT LIKE A
SECOND MESSENGER
• 1. During hormone- receptor binding, concentration of second
messenger should increase in order to exert the biological effect.
• 2. When the hormone is removed, the concentration of second
messenger and the biological response shown by the hormone should
decrease in concentration and activity.

8.
CLASSIFICATION OF SECOND MESSENGER

9.
CYCLIC NUCLEOTIDES
cAMP & cGMP are nucleotides. The
cyclic nucleotides cyclic adenosine
monophosphate (cAMP) and cyclic
guanosine monophosphate (cGMP)
regulate the activity of protein
kinase A (PKA) and protein kinase
G (PKG), respectively.

10.
cAMP
cAMP is a second messenger that is synthesized from ATP by the action
of the enzyme Adenylyl Cyclase.
Binding of the hormone/ ligand to its receptor activates a G protein
which, in turn activates Adenyl cyclase.
Leads to appropriate cell 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.
Examples of cAMP action- Adrenaline, LH, glucagon

11.
Ligand
AMP
Phosphodiesterase
Receptor
Adenyl Cyclase
ATP
GDP GTP
cAMP
Plasma
membrane
G- Protein
Cell specific response occurs
cAMP
pathway
cAMP Pathway
1. Ligands- epinephrine, Ach
2. Primary effector- Adenyl cyclase
3. Secondary messenger- cAMP

12.
• Cyclic AMP activates protein kinase A
• In addition to signaling in the cytoplasm, the catalytic subunit of PKA can enter the nucleus of
cell & phosphorylate & activate the transcription factor cAMP response element binding (CREB)
protein.
• Phospho- CREB protein increases transcription of many genes.

13.
Glucose mobilization: an example of a
response induced by cAMP
• Binding of hormone on receptor protein
• Activation of effector enzyme and formation of
secondary messenger cAMP.
• cAMP binds to PKA and activates it, which further
phosphorylates 2 enzymes
1. Phosphorylase kinase- phosphorylates glycogen
Phosphorylase- stimulates glycogen breakdown
2. Glycogen synthetase- inhibition- prevents
conversion of glucose to glycogen.

14.
Increase in
heart rate
and
contraction
force
Glycogen
breakdown
cAMP
effects as
second
messengers
Thyroid
hormone
synthesis
Sodium and
water
reabsorption
Bone
resorption

15.
DRUGS ACTING ON cAMP
ligand
+

16.
NEURODEGENERATIVE DISORDERS
 Target 2nd messengers used by
multiple neurotransmitters
 AC1 most attractive target
 AC1 activators- used for cognitive
decline
ANALGESIA
 Increase cAMP- increased
nociception
 AC1 & AC5 involved
 Selective AC1 & AC5 inhibitors-
Analgesics
 Preclinical stages
CHRONIC HEART FAILURE
 Alteration in B adrenoreceptors- AC- cAMP
pathway.
 Downregulation of B1 receptors
 Upregulation of inhibitory G proteins & G- proteins
coupled receptor kinases.
 Catecholamine refractoriness, exercise intolerance.
 Protective phenomenon- shields myocytes from
arrhythmogenic, hypertrophic & apoptotic effects
of Catecholamines
 Genetic variant of GRK5- Accelerated
desensitization – better prognosis
DRUG DEPENDENCE
 Repeated opioid exposure- up
regulation of AC activity
 3 specific isoforms- AC1, AC5 & AC8
 Selective inhibitors for opioid
dependence
NOVEL DRUG
TARGETS OF
cAMP
SPERM FUNCTION
 sAC regulates sperm motility &
capacitation.
 Increased cAMP- mediates
capacitation
 Inhibitors for male contraception
in early stages

17.
cGMP
• cGMP is synthesized from nucleotide GTP using the enzyme Guanyl 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
• cGMP serves as secondary messenger for nitric oxide (NO) & response of the rods of the retina to the light.
cGMP pathway
Ligands- ANP & NO
Primary effector- guanylate
cyclase
Secondary messenger- cGMP

18.
NITRIC OXIDE (NO)
• Nitric oxide acts as a second messenger because it is a free radical that can diffuse
through the plasma & affect nearby cells.
• It is synthesized from Arginine & oxygen by the NO synthase.
• It activates soluble guanylyl cyclase, which when activated produces another
second messenger, cGMP.
• It is toxic in higher concentrations, but is the cause of many other functions like
relaxation of blood vessels, apoptosis etc.

19.
Nitric oxide
synthase
Smooth muscle relaxation
Myosin phosphatase
Changes in gene expression
Transcription factors
Soluble
guanylate
cyclase
cGMP
Protein kinase G
VASP
Platelet inhibition
Plasma
membrane
 The Ach (neurotransmitter) acts on
endothelial cells to stimulate NO
synthesis.
 NO diffuses to neighbouring
smooth muscle cells where it
interacts with the guanylyl cyclase

20.
Therapeutic
Applications
of cGMP

21.
Phosphodiesterase (PDE)
PDE comprises family of enzymes expressed in almost all cells of the body & of prime importance in cellular
functioning. Hydrolysis of phosphodiester bond in the second messengers cAMP & cGMP  inactive forms 5AMP &
5GMP. Inactive forms lead to termination of intracellular signals and hence physiological functions.
Following is the therapeutic application of Phosphodiesterase (PDE)

22.
THERAPEUTIC APPLICATIONS OF PDE

23.
PHOSPHATIDYLINOSITOL DERIVED SECOND MESSENGERS
Phosphatidylinositol (PI) is a negatively
charged phospholipid and a minor
component in eukaryotic cell
membranes.
Inositol can be phosphorylated to form:-
• Phosphatidylinositol-4-phosphate
• Phosphatidylinositol-4,5- bisphosphate
• Phosphatidylinositol- 3,4,5- triphosphate
Intracellular enzyme phospholipase-c
(PLC), hydrolyses PIP2 which is found in
the inner layer of the plasma
membrane. Hydrolysis of PIP2 yields:
DAG and IP3

24.
Ca2+
Protein
kinase C
EFFECTS
Mechanism of IP3/
DAG pathway
Ligand + Receptor
G- Protein
Phospholipase C
PIP2 IP3 DAG
Endoplasmic
reticulum
Opening of Ca2+
channels

25.
(i) A signalling molecule binds to a
receptor leading to activation of PLC
(ii) PLC then cleaves plasma membrane
phospholipid PIP2 in IP3 & DAG
(iii) DAG functions as second
messenger in other pathways
(iv) IP3 quickly diffuses
through the cytosol &
binds to an IP3 gated
calcium channel in the
E, causing it to open

26.
Phosphatidylinositol-3,4,5-
triphosphate
oHydrophilic
oAgonistic for internal calcium
channel
oCa2+ concentration rises
oMultiple effects through Ca2+
binding protein
Diacylglycerol (DAG)
oHydrophobic
oTarget PKC (a kinase)
oPKC requires Ca2+ and DAG
IP3/DAG

27.
2. DRUGS ACTING ON PHOSPHOLIPASE IP3/DAG
UBO- QIC,
GP2A,
YM254890
Lithium,
neomycin

28.
CALCIUM IONS
• Calcium ions- once they enter the cytoplasm exert allosteric regulatory affects on
many enzymes & proteins.
• Calcium acts as a second messenger by indirect signal transduction pathways
such as via G protein coupled receptors.
• Low cytoplasmic Ca2+ at rest (10- 100nM)
• 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 & organelle act as buffer by binding Ca2+.
• Signaling occurs when the cell is stimulated to release Calcium ions from the
intracellular stores , or when the calcium enters the cell through plasma
membrane ion channels

29.
• Sudden increase in the cytoplasmic Ca2+ level
upto 500- 1000nM by opening channels in the
endoplasmic reticulum/ plasma membrane.
• IP3/DAG pathway stimulates the endoplasmic
reticulum Ca2+ ion channels.
• Many of the 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 the other effector proteins.
• Besides calmodulin, there are many other Ca2+
- binding proteins like Troponin C that mediate
the biological effects of Ca2+
CALMODULIN TARGETS
1. PHOSPHODIESTERASE
2. ADENYLATE CYCLASE
3. MYOSIN LIGHT CHAIN
KINASE
4. CALMODULIN- DEPENDENT
KINASES
5. CALCINEURIN (A
PHOSPHATASE)

30.
Activated
protein
binds
calmodulin
Changed
conformation Binds to other
proteins
IP3
Sarcoplasmic/ endoplasmic reticulum
Ca2+ ions

31.
Ca2+ CHANNEL REGULATION
Increases Ca2+ concentration
intracellular- Adrenergic b1
agonists (heart & skeletal
muscle)
Decreases Ca2+
concentration intracellular-
agonists of Dopamine (D2),
GABAB, Opioid- k,
Adenosine- A1,
Somatostatin
Activated G proteins (Go, Gs & Gi) can also
open or inhibit Ca2+ ion channels without
the intervention of second messengers like
IP3 and cAMP and bring about
hyperpolarization/ depolarization/ changes
in intracellular Ca2+.
Direct channel regulation is mostly the
function of bg dimer of the dissociated G
protein.
The Gs opens Ca2+ channels in myocardium
& skeletal muscles while Gi & Go open K+
channels in heart & smooth muscle as well
as inhibit neuronal Ca2+ channels

32.
RECENT DEVELOPMENTS
• A new second messenger, c-di-AMP was discovered in Staphylococcus aureus with a role in controlling cell size and
envelope stress. This work was published in September 2011 issue of PloS Pathogen. Most of the work on c-di-AMP
has been done in Gram +ve bacteria & actinobacteria, where c-di-AMP signalling pathways affect potassium
transport, cell wall structure, and antibiotic resistance. These findings greatly expand the c-di-AMP signalling role &
reveal a central metabolic regulatory role for a cyclic dinucleotide. However, the molecular mechanisms of c-di-AMP
signalling are still poorly understood.
• Feng- Yen Li, a student of PhD in biomedical sciences at UCSF, discovered a “second messenger” role for Magnesium
in T cell signalling, by studying a family of 2 boys suffering from chronic Epstein- Barr Virus infections.
• Laber Congenital Amaurosis is a congenital blindness due to mutations in retinal Guanyl Cyclase leading to lack of
cGMP.
• Cone rod dystrophy is caused by deterioration of photoreceptors of cone & rod cells due to increased cGMP because
of mutation in Guanyl Cyclase encoding gene.

33.
CONCLUSION
• SIGNAL TRANSDUCTION PATHWAYS ALWAYS ALLOW CELLS TO RESPOND TO ENVIRONMENTAL
SIGNALS.
• IN THESE PATHWAYS, A SIGNAL IS AMPLIFIED.
• THIS SIGNAL AMPLIFICATION IS BROUGHT ABOUT BY SECOND MESSENGERS LIKE cAMP, cGMP, Ca
ions, IP3/DAG & NO.
• SECOND MESSENGERS ESSENTIALLY SERVE AS CHEMICAL RELAYS FROM THE PLASMA MEMBRANE
TO THE CYTOPLASM, THUS CARRYING OUT INTRACELLULAR SIGNAL TRANSDUCTION.

34.
REFERENCES
• https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book
%3A_Biology_(Kimball)/04%3A_Cell_Metabolism/4.14%3A_Secondary_Messeng
ers
• https://www.researchgate.net/publication/324314598_Second_Messenger_Syst
em
• https://www.sciencedirect.com/topics/neuroscience/second-messenger-system
• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4968160/
• www.pharmatutor.org