Cell Signaling “Principles”

Dr. Fridoon Jawad Ahmad

HEC Foreign ProfessorKing Edward Medical University

Visiting Professor LUMS-SSE

2nd Biggest Leap

Multicellular = Specialization = CoordinationAbility to sense & respond to external and internal

environment

2.5 billion Years

Why Signaling System 

In order to survive even simplest organisms need to sense and respond to their environment.

 It is critical that the cells of multicellular organisms communicate to coordinate their efforts (Running).

Cells in a multicellular organism are specialized and rely on each other for the support (brain sugar).

 During development there have to be checks balances on differentiation (analogy society).

Signals Can

Instruct Cells to Perform Various

functions(Manipulating

Gene expression)

Design

1) Ligand binding2) Conformational change

Cytoplasmic domain3) Mediators

4) Cell function modified

Expression of One Gene Can Alter Phenotype of Cells

Modes

Low Affinity

Receptors & Cell Machinery

Receptor combinations confer cell behavior in an environment

flooded with hundreds of ligands

Cellular machinery specifies cell response to a particular ligand

High Turnover (NO)

Ach Receptor-ACh NO synthase DeaminationNO Diffusion G-cyclase cGMP Relaxation

NO half life 5 seconds

Receptors: Intracellular (ICR)

Blood transport via carrier proteins longer life (thy days Ach ms) Carrier left outside

Inactive ICR may be DNA bound or in cytoplasm (NLS nonfunctional)

Activated receptor binds DNA induces gene transcription

Small Hydrophobic Lipid soluble molecules eg steroid & thyroid hormones, retinoids & Vit D etc

ICR Specificity

Different cells with identical ICRs regulate different genes due to other cell specific mediators

Right combination of co-activators/gene regulators required to transcribe specific genes (testosterone)

ICR Transcription

Ligand binding removes inhibitory proteins and facilitates binding of transcription activators

Cell-Surface Receptors (CSR)

CSR Response Time

Neurotransmitters produce all or noting response

Small IC Mediators

SICMs are produced/released in response to signal received

by the receptor

SICMs donot have an enzymatic activity of their

own however they modify the function of other molecules

IC Proteins1 Relay proteins simply pass the message to the next

signaling component in the chain.2 Messenger proteins carry the signal from one part of the cell to another, such as from the cytosol to the

nucleus.3 Adaptor proteins link one signaling protein to

another, without themselves conveying a signal.4 Amplifier proteins, which are usually either

enzymes or ion channels, greatly increase the signal they receive, either by producing large amounts of

small intracellular mediators or by activating large numbers of downstream

intracellular signaling proteins. When there are multiple amplification steps in a relay chain, the chain is often referred to as a signaling cascade.

5 Transducer proteins convert the signal into a different form. The enzyme that makes cyclic AMP is

an example: it both converts the signal and amplifies it, thus acting as both a transducer and an

amplifier.6 Bifurcation proteins spread the signal from one

signaling pathway to another.7 Integrator proteins receive signals from two or

more signaling pathways and integrate them before relaying a signal onward.

8 Latent gene regulatory proteins are activated at the cell surface by activated receptors and then migrate

to the nucleus to stimulate gene transcription.

1

2

3

4 & 5

6

7

Signaling in E. coli 

After ligand binding change in tertiary

structure of extra cellular part of EnvZ leads to

structural change in its cytoplasmic domain

making it a kinase (auto..).

EnvZ-P can now phosphor-ilate OmpR (responder)

outside signal in and amplified.

Signaling in E. coli

Receptor conformational change after ligand binding which activates

kinase activity.

Phosphorilation alters responder function.

Signal amplified.

Transcription factor activated.

Protein synthesis results in altered cell activity.

G Protein-Linked Receptors

Ligand binding causes a structural change permitting G protein to bind receptor.

Binding of G protein to activated receptor causes it to exchange GDP for GTP (receptor releases ligand).

G Protein-Linked Receptors

Subunit of G protein separates and activates an effector molecule (causing a functional change).

Epinephrine effects different cells differently (heart muscle contracts, intestinal vascular smooth muscle relaxes more nutrients absorbed (Adnl C inhibition).

Second Messenger

Second messengers are allosteric regulators and do not have enzymatic activity

Cyclic AMP (cAMP) can bind ion channels to open them or bind

enzymes to exposing their active sites.

Enzyme Activation Via Second messenger

The cAMP-dependent protein kinases (PKA) are tetramers, consisting of two regulatory (R) subunits and

two catalytic (C) subunits. In the tetrameric form PKA is enzymatically inactive.

Binding of cAMP to the R subunits causes dissociation of the two C subunits, which then can phosphorylate

specific acceptor proteins.

cAMP-dependent protein kinase (cAPK), glycogen phosphorylase kinase (GPK), and glycogen phosphorylase (GP) — are all

regulated, directly or indirectly, by cAMP by phosphoprotein phosphatase, which removes the phosphate residues from the inactive form of glycogen synthase At high cAMP levels, cAPK

phosphorylates an inhibitor of phosphoprotein phosphatase (PP)

CRE

Gs vs Gi

PKC Activationvia Gq

Cell type Organ/systemActivators

ligands --> Gq-GPCRs Effects

smooth muscle cell (gastrointestinal tract sphincters)

digestive system•prostaglandin F2α

[4] -->•thromboxanes[4]

contraction

•smooth muscle cells in:iris dilator muscle (sensory system)•urethral sphincter (urinary system)•uterus (reproductive system)•arrector pili muscles(integumentary system)•ureter (urinary system)•urinary bladder (urinary system)[5][6]

Various •adrenergic agonists --> α1 receptor contraction

•smooth muscle cells in:iris constrictor muscle•ciliary muscle

sensory system acetylcholine --> M3 receptor contraction

smooth muscle cell (vascular) circulatory system•5-HT --> 5-HT2A receptor•adrenergic agonists --> α1 receptor

•vasoconstriction[7][8]

smooth muscle cell (seminal tract[9]) reproductive system •adrenergic agonists --> α1 receptor ejaculation

smooth muscle cell (GI tract) digestive system•5-HT --> 5-HT2A or 5-HT2B receptor[7]

•acetylcholine (ACh) --> M3 receptor•contraction[10]

smooth muscle cell (bronchi) respiratory system•5-HT --> 5-HT2A receptor•adrenergic agonists --> α1 receptor•acetylcholine --> M3[11] andM1 receptor[12]

bronchoconstriction[7]

proximal convoluted tubule cell kidney•angiotensin II --> AT1 receptor•adrenergic agonists --> α1 receptor

•stimulate NHE3 --> H+ secretion & Na+ reabsorption[13]

•stimulate basolateral Na-K ATPase --> Na+ reabsorption[13]

neurons in autonomic ganglia nervous system acetylcholine --> M1 receptor EPSP

neurons in CNS nervous system•5-HT --> 5-HT2A receptor•acetylcholine --> M1 receptor

•neuronal excitation (5-HT)[7]

•memory? (acetylcholine)[14]

platelets circulatory system 5-HT --> 5-HT2A receptor[7] aggregation[7]

ependymal cells (choroid plexus) ventricular system 5-HT --> 5-HT2C receptor[7] ↑cerebrospinal fluid secretion[7]

heart muscle circulatory system •adrenergic agonists --> α1 receptor positive ionotropic effect[5]

serous cells (salivary gland) digestive system•acetylcholine --> M1 andM3 receptors•adrenergic agonists --> α1 receptor

•↑secretion[5]

•increase salivary potassium levels.

serous cells (lacrimal gland) digestive system •acetylcholine --> M3 receptor •↑secretion[8]

adipocyte digestive system/endocrine system •adrenergic agonists --> α1 receptor •glycogenolysis andgluconeogenesis[5]

hepatocyte digestive system •adrenergic agonists --> α1 receptor •glycogenolysis andgluconeogenesis[5]

sweat gland cells integumentary system •adrenergic agonists --> α1 receptor •↑secretion[5]

parietal cells digestive system acetylcholine --> M1 receptors[12] ↑ gastric acid secretion

Receptor Tyrosine Kinases & Ras

Autophosphorylation

Activated RTKs Indirectly Bindand Activate RAS

RAS Helpers

Protein Kinase Cascade

Ras Experiment

Signal Amplification

AlternateNames

Comparison