CHAPTER 15Cell Signaling and Signal

Transduction: Communication Between Cells

Introduction

• Cells must respond adequately to external stimuli to survive.

• Cells respond to stimuli via cell signaling.• Some signal molecules enter cells; others bind

to cell-surface receptors.

15.1 The Basic Elements of Cell Signaling Systems (1)

• Extracellular messenger molecules transmit messages between cells.– In autocrine signaling, the cell has receptors on its

surface that respond to the messenger.– During paracrine signaling, messenger molecules

travel short distances through extracellular space.– During endocrine signaling, messenger molecules

reach their target cells through the bloodstream.

Types of intercelular signaling

The Basic Elements of Cell Signaling Systems (2)

• Receptors on or in target cells receive the message.– Some cell surface receptors generate an

intracellular second messenger through an enzyme called an effector.

– Other surface receptors recruit proteins to their intracellular domains.

Overview of signaling pathways

The Basic Elements of Cell Signaling Systems (3)

• Signaling pathways consist of a series of proteins.– Each protein in a pathway alters the conformation

of the next protein.– Protein conformation is usually altered by

phosphorylation.– Target proteins ultimately receive a message to

alter cell activity.– This overall process is called signal transduction.

A signal transduction pathway

15.2 A Survey of Extracellular Messengers and Their Receptors (1)

• Extracellular messengers include:– Small molecules such as amino acids and their

derivatives.– Gases such as NO and CO– Steroids– Eicosanoids, which are lipids derived from fatty

acids.– Various peptides and proteins

A Survey of Extracellular Messengers and Their Receptors (2)

• Receptor types include:– G-protein coupled receptors (GPCRs)– Receptor protein-tyrosine kinases (RTKs)– Ligand gated channels– Steroid hormone receptors– Specific receptors such as B-and T-cell receptors

15.3 G Protein-Coupled Receptors and Their Second Messengers (1)

• G protein-coupled receptors (GPCRs) are numerous.

• GPCRs have seven transmembrane domains and interact with G proteins.

A GPCR and a G protein

A GPCR and a G protein

Examples of GPCRs and their ligands

G Protein-Coupled Receptors and Their Second Messengers (2)

• Signal Transduction by G Protein-Coupled Receptors– Ligand binding on the extracellular domain

changes the intracellular domain.– Affinity for G proteins increases, and the receptor

binds a G protein intracellularly.– GDP is exchanged for GTP on the G protein,

activating the G protein.– One ligand-bound receptor can activate many G

proteins.

Mechanism of receptor-mediated

activation/inhibition by G proteins

G Protein-Coupled Receptors and Their Second Messengers (3)

• Termination of the Response– Desensitization – by blocking active receptors

from turning on additional G proteins.– G protein-coupled receptor kinase (GRK) activates

a GPCR via phosphorylation.– Proteins called arrestins compete with G proteins

to bind GPCRs.– Termination of the response is accelerated by

regulators of G protein signaling (RGSs).

G Protein-Coupled Receptors and Their Second Messengers (4)

• Bacterial Toxins, such as cholera toxin and pertussis virulence factors, target GPCRs and G proteins.

• Second Messengers– The Discovery of Cyclic AMP• It is a second messenger, which is released into the

cytoplasm after binding of a ligand.• Second messengers amplify the response to a single

extracellular ligand.

The localized formation of cAMP

G Protein-Coupled Receptors and Their Second Messengers (5)

• Phosphatidylinositol-Derived Second Messengers– Some phospholipids of cell membranes are

converted into second messengers by activated phospholipases.

• Phosphatidylinositol Phosphorylation– Phosphoinositides (PI) are derivatives of

phosphatodylinositol.

Phospholipid-based second messengers

G Protein-Coupled Receptors and Their Second Messengers (6)

• Phosphatidylinositol-specific phospholipase C- produces second messengers derived from phosphatidylinositol-inositol triphosphate (IP3) and diacylglycerol (DAG).

• DAG activates protein kinase C, which phosphorylates serine and threonine residues on target proteins.

• The phosphorylated phosphoinositides form lipid-binding domains called PH domains.

The generation of second messengers as a result of breakdown of PI

G Protein-Coupled Receptors and Their Second Messengers (7)

• One IP3 receptor is a calcium channel located at the surface of the smooth endoplasmic reticulum.

• Binding of IP3 opens the channel and allows Ca2+ ions to diffuse out.

Examples of responses mediated by Protein Kinase C

Cellular responses elicited by adding IP3

G Protein-Coupled Receptors and Their Second Messengers (8)

• Regulation of Blood Glucose Levels– Different stimuli acting on the same target cell

may induce the same response.• Glucagon and epinephrine bind to different receptors

on the same cell.• Both hormones stimulate glucoses breakdown and

inhibit its synthesis.• cAMP is activated by the G protein of both hormone

receptors

– Responses are amplified by signal cascades.

The reactions that lead to glucosestorage or mobilization

G Protein-Coupled Receptors and Their Second Messengers (9)

• Glucose Metabolism: An Example of a Reponse Induced by cAMP– cAMP is synthesized by adenylyl cyclase.– cAMP evokes a reaction cascade that leads to

glucose mobilization.– Once formed, cAMP molecules diffuse into the

cytoplasm where they bind a cAMP-dependent protein kinase (protein kinase A, PKA).

Formation of cAMP from ATP

The response by a liver cell to glucagonor epinephrine

G Protein-Coupled Receptors and Their Second Messengers (10)

• Other Aspects of cAMP Signal Transduction Pathways– Some PKA molecules phosphorylate nuclear

proteins.– Phosphorylated transcription factors regulate

gene expression.– Phosphatases halt the reaction cascade.– cAMP is produced as long as the external stimulus

is present.

The variety of processes that can be affected by changes in [cAMP]

Examples of hormone-induced responses mediated by cAMP

PKA-anchoring protein signaling

G Protein-Coupled Receptors and Their Second Messengers (11)

• The Role of GPCRs in Sensory Perception– Rhodopsin is a photosensitive protein for black-

and-white vision that is also a GPCR.– Several color receptors are GPCRs.– Odorant receptors in the nose are GPCRs.– Taste receptors for bitter and some sweet flavors

are GPCRs.

The Human Perspective: Disorders Associated with G Protein-Coupled Receptors (1)

• Several disorders are caused by defects in receptors or G proteins.

• Loss of function mutations result in nonfunctional signal pathways.– Retinitis pigmentosa, a progressive degeneration

of the retina, can be caused my mutations in rhodopsin’s ability to activate a G protein.

Transmembrane receptor responsible for causing human diseases

Human diseases linked to the G

protein pathway

The Human Perspective: Disorders Associated with G Protein-Coupled Receptors (2)

• Gain of function mutations may create a constitutively activated G protein. – Some benign thyroid tumors are caused by a

mutation in a receptor.

• Certain polymorphisms in G protein-related genes may result in an increased susceptibility to asthma or high blood pressure, as well as decreased susceptibility to HIV.

15.4 Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction (1)

• Protein-tyrosine kinases phosphorylate tyrosine residues on target proteins.

• Protein-tyrosine kinases regulate cell growth, division, differentiation, survival, and migration.

• Receptor protein-tyrosine kinases (RTKs) are cell surface receptors of the protein-tyrosine kinase family.

Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction (2)

• Receptor Dimerization– Results from ligand binding.– Protein kinase activity is activated.• Tyrosine kinase phosphorylates another subunit of the

receptor (autophosphorylation).• RTKs phosphorylate tyrosines within phosphotyrosine

motifs.

Steps in the activation of RTK

Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction (3)

• Phosphotyrosine-Dependent Protein-Protein Interactions– Phosphorylated tyrosines bind effector proteins

that have SH2 domains and PTB domains.– SH2 and PTB domain proteins include:• Adaptor proteins that bind other proteins.• Docking proteins that supply receptors with other

tyrosine phosphorylation sites.• Signaling enzymes (kinases) that lead to changes in cell.• Transcription factors

The interaction between SH2 domain and a peptide contain a phosphotyrosine

A diversity of signaling proteins

Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction (4)

• The Ras-MAP Kinase Pathway– Ras is a G protein embedded in the membrane by

a lipid group.– Ras is active when bound to GTP and inactive

when bound to GDP.

The structure of a G protein and theG protein cycle

Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction (5)

• Ras-MAP kinase pathway (continued)– Accessory proteins play a role:• GTPase-activating proteins (GAPs) shorten the active

time of Ras.• Guanine nucleotide-exchange factors (GEFs) stimulate

the exchange of GDP for GTP.• Guanine nucleotide-dissociation inhibitors (GDIs)

inhibit release of GDP.

Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction (6)

• Ras-MAP kinase pathway (continued)– The Ras-MAP kinase cascade is a cascade of

enzymes resulting in activation of transcription factors.

– Adapting the MAP kinase to transmit different types of information:• End result differs in different cells/situations.• Specificity of the MAP kinase response due to

differences in the types of kinases participating and differences in spatial organization of components.

The steps of a generalized MAP kinase cascade

Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction (7)

• Signaling by the Insulin Receptor– Insulin regulates blood glucose levels by

increasing cellular uptake of glucose.– The insulin receptor is a protein-tyrosine kinase• Autophosphorylated receptor associates with insulin

receptor substrate proteins (IRSs). • IRSs bind proteins with SH2 domains, which activate

downstream signal molecules.• SH2 domain-containing proteins are kinases that

phosphorylate a lipid, PI 3-kinase (PI3K).

The response of the insulin receptorto ligand binding

The role of tyrosine-phosphorylated IRS in activating a variety of signaling pathways

Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction (8)

• Glucose Transport– PKB regulates glucose uptake by GLUT4

transporters.• GLUT4 transporters reside in intracellular membrane

vesicles.• Vesicles fuse with the membrane in response to ligand

binding to the IR.

– Diabetes mellitus is caused by defects in insulin signaling and Type 2 diabetes is caused by gradual insensitivity to insulin.

Regulation of glucose uptake in muscleand fat cells by insulin

Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction (9)

• Insulin Signaling and Lifespan– Several studies demonstrate that the lifespan can

be increased by decreasing the level of insulin.– Human who live along life show high insulin

sensitivity.– Laboratory studies show that calorie restriction

leads to decreased insulin levels and increased insulin sensitivity.

Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction (10)

• Signaling Pathways in Plants– Plants lack cyclic nucleotides and RTKs.– Plants have protein kinases that phosphorylate

histidine residues.• The downstream cascade is similar to MAP kinase

cascade.• The target of the cascade is usually transcription

factors.

15.5 The Role of Calcium as an Intracellular Messenger (1)

• Cytoplasmic calcium levels are determined by events within a membrane.– Calcium levels are low in the cytosol because it is

pumped out into the extracellular space and the membrane is highly impermeable to the ion.

– Calcium channels can be transiently opened by action potential or calcium itself.

– Calcium binds to calcium-binding proteins (such as calmodulin), which affects other proteins.

Experimental demonstration of localized release of intracellular Ca2+

Calcium-induced calcium release

Calcium wave in a starfish egg

Examples of mammalian proteinsactivated by Ca2+

The Role of Calcium as an Intracellular Messenger (2)

• Recent research indicates a phenomenon called store-operated calcium entry (SOCE).

• During SOCE the depleted calcium levels trigger a response that lead to opening of calcium channels.

• The mechanism responsible for SOCE is a signaling system between the ER and plasma membrane.

A model for store-operated calcium entry

Calmodulin

The Role of Calcium as an Intracellular Messenger (3)

• Regulating Calcium Concentrations in Plant Cells– Cytosolic calcium changes in response to several

stimuli, including light, pressure, gravity, and hormones.

– Calcium signaling aids in decreasing turgor pressure in guard cells.

A model of the role of Ca2+ in guard cell closure

15.6 Convergence, Divergence and Crosstalk Among Different Signaling Pathways (1)

• Signaling pathways can converge , diverge, and crosstalk as follows:– Signals form unrelated receptors can converge to

activate a common effector. – Identical signals can diverge to activate a variety

of effectors.– Signals can be passed back and forth between

pathways as a result of crosstalk.

Examples of convergence, divergence, and crosstalk among signal transduction pathways

Convergence, Divergence and Crosstalk Among Different Signaling Pathways (2)

• Convergence – GPCRs, receptor tyrosine kinases, and integrins bind to different ligands but they all can lead to a docking site for Gbr2.

Convergence of signals transmitted from a GPCR, an integrin, and receptor tyrosine kinase

Convergence, Divergence and Crosstalk Among Different Signaling Pathways (3)

• Divergence – all of the examples of signal transduction so far are evidence of divergence of how a single stimulus sends signals along a variety of different pathways.

Convergence, Divergence and Crosstalk Among Different Signaling Pathways (4)

• Crosstalk – more and more crosstalk is found between signaling pathways:– cAMP can block signals transmitted through the

MAP kinase cascade.– Ca2+ and cAMP can influence each other’s

pathways.

An example of crosstalk betweentwo major signaling pathways

15.7 The Role of NO as an Intracellular Pathway

• Nitric oxide (NO) is both an extracellular and intercellular messenger with a variety of functions.

• NO is produced by nitric oxide synthase.– NO stimulates guanylyl cyclase, making cGMP.– cGMP decreases cytosolic calcium and relaxes

smooth muscle.– NO also plays a role in male arousal.

Signal transduction by means of NO and cGMP

15.8 Apoptosis (Programmed Cell Death) (1)

• Apoptosis is an ordered process involving cell shrinkage, loss of adhesion to other cells, dissection of chromatin, and engulfment by pahgocytosis.

A comparison of normal and apoptotic cells

Apoptosis (Programmed Cell Death) (2)

• Apoptotic changes are activated by proteolytic enzymes called caspases, which target:– Protein kinases, some of which cause detachment

of cells.– Lamins, which line the nuclear envelope.– Proteins of the cytoskeleton– Caspase activated DNase (CAD)

Apoptosis (Programmed Cell Death) (3)

• The Extrinsic Pathway of Apoptosis– It is initiated by external stimuli:• Tumor necrosis factor (TNF) is detected by a TNF cell

surface receptor.• Bound TNF receptors recruit “procaspases” to the

intracellular domain of the receptor.• Procaspases convert other procaspases to caspases.• Caspases activate executioner caspases, leading to

apoptosis.

The extrinsic (receptor-mediated) pathway of apoptosis

Apoptosis (Programmed Cell Death) (4)

• The Intrinsic Pathway of Apoptosis– It is initiated by intracellular stimuli.• Proapoptotic proteins stimulate mitochondria to leak

proteins, mostly cytochrome c.• Release of apoptotic mitpchondrial proteins irreversibly

commits the cell to apoptosis.

The intrinsic (mitochondria-

mediated) pathway of apoptosis

Release of cytochrome c and nuclear fragmentation during apoptosis

Apoptosis (Programmed Cell Death) (5)

• Antiapoptotic proteins promote survival.• Cell fate depends on the balance between

pro- and anti-apoptotic signals.• Apoptotic cell death occurs without spilling

cellular contents to prevent inflammation.• Apoptotic cells are cleared by phagocytosis.

Clearance of apoptotic cells isaccompanied by phagocytosis