THE ROLE OF SECONDARY MESSENGER SYSTEM IN SIGNAL TRANSDUCTION

About Authors:
Mr. Salahuddin Mohammed
Associate Professor of Pharmacology, College of Health Sciences,
Department of Pharmacy, Mizan Tepi University,
Mizan Teferi, Ethiopia.
salahuddin_pharma48@yahoo.com

Introduction
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.

REFERENCE ID: PHARMATUTOR-ART-2031

In the transduction process, extracellular signal is passed from one intracellular molecule to another through the secondary messengers until al cellular behavior alters and the cytoskeleton is tweaked into new configuration.

G-protein Linked Receptor activation
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.

Figure 2: G-Protein receptor activation

· When the G-protein is activated it dissociates in to two signaling proteins.

· After the regulation of the target protein is completed, the G-protein α subunit switches off by hydrolyzing the bound GTP into GDP by GTPase. Then the reassociation of α and βγ complex takes place making the G-protein ready to couple with other receptor.

Figure 3: Activation cycle of G-proteins by G-protein-coupled receptors

Source by Sven Jähnichen

REQUIREMENTS FOR A COMPOUND TO ACT AS A SECOND MESSENGER

During hormone-receptor binding, concentration of second messenger should increase in order to exert the biological effect.
When the hormone is removed, the concentration of second messenger and the biological response shown by the hormone should decrease in concentration and activity.

type of intracellular signaling molecules

They are produced depending upon the interaction of the G-proteins with target enzymes as given below:

S.No	TARGET ENZYME	INTRACELLULAR SIGNALING MOLECULE PRODUCED
1.	Adenylyl cyclase	Cyclic AMP
2.	Phospholipase C	Inositol triphosphate
3.	Phospholipase C	Diacylglycerol
4.	Guanylyl cyclase	Cyclic GMP

Table 1: Types of Intracellular signalling molecules

I. CYCLIC AMP AS A SECONDARY MESSENGER

Cyclic-3’,5’-adenosine monophosphate is second messenger in intracellular signaling cascade. Wide variety of exogenous stimuli, such as hormones , neurotransmitters, physical and chemical signals control the intracellular level of cyclic nucleotides by regulating the enzyme systems directly or indirectly. These physiological effects are exerted through binding to a number of proteins which includes cyclic nucleotide dependant protein kinases, cyclic nucleotide gated channels and cyclic nucleotide-regulated phosphodiesterases.The phosphorylated state of various proteins and cyclic nucleotides are controlled by extracellular signals.

The regulatory mechanism of adenylyl cyclase is a typical G-protein dependant signal transduction.

Figure 4: Cyclic AMP activation

Source: universe-review.ca/F11-monocell.htm

Regulation by G-protein subunits
G-protein consists of two functional subunits, G_s (stimulatory protein) and G_i (inhibitory protein). The GTP bound G_sαassociates with adenylyl cyclase and stimulates the rate of cAMP synthesis and in contrast GTP bound G_iαinhibits the catalytic activity.

The regulation of different adenylyl cyclases types are given below:

REGULATOR	MODE OF REGULATION	ADENYLYL CYCLASE TYPES
Gsα	STIMULATION	1-6
Giα	INHIBITION	1-6
Gβγ	STIMULATION	2,4
FORSKOLIN	INHIBITION	1-6
CALMODULIN	STIMULATION	1,3,8
CALCIUM	INHIBITION	5,6
PROTEIN KINASE C	STIMULATION	2,5
PROTEIN KINASE A	INHIBITION	6

Table 2: Regulation of cyclic AMP by G Proteins

Regulation by calcium
Recent studies showed that some adenylyl cyclases are regulated by calcium/calmodulin. Type 1,3,8 enzymes are stimulated by calcium.Types 4 and 2 are not sensitive to calcium. Types 5 and 6 appear to be inhibited by low concentrations of free calcium.

Regulation by phosphorylation
Adenylyl cyclase is a direct target for phosphorylation by protein kinaseC, It alters the responsiveness to G-protein regulation.Adenylyl cyclase isoforms are differentially regulated by protein kinase A dependent phosphorylation.

The regulation of adenylyl cyclases by protein kinases is also connected to the regulatory mechanism of protein kinases including various growth factors and other activation pathways. Thus, adenylyl cyclases system is involved in many number of signal transduction systems.

Figure 5: Effects of Cyclic AMP as a secondary messenger

Some effects of protein phosphorylation by c-AMP dependent protein kinase are listed in the table below:

ENZYME	EFFECT
Phosphorylase kinase	Phosphorylation of phosphorylase and glycogen breakdown
Glycogen synthetase	Inhibition of glycogen synthesis
Hormone sensitive triglyceride lipase	Stimulation of triglyceride breakdown
Smooth chain myosin light chain kinase	Inhibition of contraction
Phospholamban	Increased calcium uptake into sarcoplasmic reticulum of heart muscle

Table 3: Effects of protein phosphorylation by cyclic AMP dependent protein kinase.

II. LIPID DERIVED SECONDARY MESSENGERS: IP₃ AND DAG
In this signaling cascade system, the extracellular signals activate phospholipase C instead of adenylyl cyclase through the G-protein receptors which results in the Inositol-phospholipid pathway which takes place as follows:

1. Activated phospholipase C cleaves PIP₂ into DAG and IP₃

Figure 6: Formation of IP₃ and DAG from PIP₂

2. IP₃being a hydrophilic sugar molecule diffuses into the cytosol and attaches to the endoplasmic reticulum and opens the calcium channels embedded on the endoplasmic reticulum to cause a sharp increase in the cytosolic concentration of free calcium.

3. DAG together with the released calcium activates protein kinase C(PKC) causing its traslocation from the cytosol to the plasma membrane.

4. PKC phosphorylates set of intracellular proteins to exert their effects.

Figure 7: Activation of IP₃as a secondary messenger

SOME FIRST MESSENGERS WHICH ACTIVATE PI BREAKDOWN

MESSENGER	TARGET CELLS	PHYSIOLOGICAL EFFECTS
Adrenaline, ATP Vasopressin	LIVER(α₁, P_2,V₁ receptors respectively)	Glycogen breakdown
Acetylcholine	Exocrine pancreas	Secretion
Acetylcholine	Smooth muscle	Contraction
PDGF	Platelets	Cell proliferation
Thrombin	Fibroblasts	Aggregation

Table 4: First messengers which activate PI breakdown.

EFFECTS OF CALCIUM RELEASE IN THE BODY
Calcium plays important role in embryonic development, acts on skeletal muscle and causes contraction. It also acts on nerve cells(secretary cells) and triggers secretion.

Indirectly calcium acts through transducer proteins where calcium binds to calmodulin and this complex activates the target proteins.

III. c-GMP SiGNALLING: NITRIC OXIDE and ATRIAL NATURETIC PEPTIDES
Several hormones like Insulin, Oxytocin elevate cGMP levels. Effects shown by cGMP are opposite to that shown by cAMP and increase in cGMP is often accompanied by decrease levels of cGMP.

Nitric Oxide and cGMP sinalling:
Ca -calmodulin complex results in activation of Nitric oxide synthase, resulting in release of nitric oxide (NO). Effect of NO on the smooth muscle is mediated by cGMP as follows:

Figure 8: Effect of NO on the smooth muscle mediated by cGMP

Atrial Naturetic Factor(ANF) and other peptides:
Atrial naturetic factor is released from granules of cardiac atrial tissue and uses cGMP as a second messenger. When cGMP content increases, it activates membrane bound guanylate cyclase and inturn cGMP dependent protein kinase, which mediates activation of ANF by changing the shapes of both regulatory and catalytic site subunits.The stimulation of it subsequently causes relaxation of smooth muscle.

Figure 9: Cyclic GMP as a secondary messenger

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