Transcript File
CHAPTER 15
Cell 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 (IP produces second messengers derived from phosphatidylinositol-inositol triphosphate 3 ) 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 IP 3 receptor is a calcium channel located at the surface of the smooth endoplasmic reticulum.
Binding of IP 3 opens the channel and allows Ca 2+ ions to diffuse out.
Examples of responses mediated by Protein Kinase C
Cellular responses elicited by adding IP 3
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 glucose storage 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 glucagon or 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 the G 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 receptor to 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 muscle and 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 Ca 2+
Calcium-induced calcium release
Calcium wave in a starfish egg
Examples of mammalian proteins activated by Ca 2+
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 Ca 2+ 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.
– Ca 2+ and cAMP can influence each other’s pathways.
An example of crosstalk between two 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 is accompanied by phagocytosis