CHAPTER 15 Cell Signaling and Signal Transduction Communication

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 receptormediated 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 c. AMP

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 (IP 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. • c. AMP 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 c. AMP – c. AMP is synthesized by adenylyl cyclase. – c. AMP evokes a reaction cascade that leads to glucose mobilization. – Once formed, c. AMP molecules diffuse into the cytoplasm where they bind a c. AMP-dependent protein kinase (protein kinase A, PKA).

Formation of c. AMP from ATP

The response by a liver cell to glucagon or epinephrine

G Protein-Coupled Receptors and Their Second Messengers (10) • Other Aspects of c. AMP Signal Transduction Pathways – Some PKA molecules phosphorylate nuclear proteins. – Phosphorylated transcription factors regulate gene expression. – Phosphatases halt the reaction cascade. – c. AMP is produced as long as the external stimulus is present.

The variety of processes that can be affected by changes in [c. AMP]

Examples of hormone-induced responses mediated by c. AMP

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 blackand-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 SH 2 domains and PTB domains. – SH 2 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 SH 2 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 SH 2 domains, which activate downstream signal molecules. • SH 2 domain-containing proteins are kinases that phosphorylate a lipid, PI 3 -kinase (PI 3 K).

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 GLUT 4 transporters. • GLUT 4 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 Gbr 2.

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: – c. AMP can block signals transmitted through the MAP kinase cascade. – Ca 2+ and c. AMP 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 c. GMP. – c. GMP decreases cytosolic calcium and relaxes smooth muscle. – NO also plays a role in male arousal.

Signal transduction by means of NO and c. GMP

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 (receptormediated) 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 (mitochondriamediated) 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 proand 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