5, part 2 Cell Signaling CAMPBELL BIOLOGY IN FOCUS Urry • Cain • Wasserman • Minorsky • Reece 5, part 2 Cell Signaling Kathleen Fitzpatrick and Nicole Tunbridge, Simon Fraser University Modified by James R. Jabbur, Houston Community College © 2016 Pearson Education, Inc

Overview: The Cellular Internet Cell-to-cell communication is essential for multicellular organisms (sensible enough…) Biologists have discovered some universal mechanisms of cellular regulation The combined effects of multiple signals determine cellular response

Figure 11.1 How do the effects of Viagra (multicolored) result from its inhibition of a signaling-pathway enzyme (purple)? Viagra

External signals are converted to responses within the cell A signal transduction pathway is a series of steps by which a signal on a cell’s surface is converted into a specific cellular response Pathway similarities suggest that ancestral signaling molecules evolved in prokaryotes and were modified later in eukaryotes

Detecting population density Prokaryotic example: Detecting population density 1 Individual rod- shaped cells 2 Aggregation in process 0.5 mm 3 Spore-forming structure (fruiting body) Figure 11.3 Communication among bacteria Fruiting bodies

Eukaryotic example: Mating in yeast  factor Receptor 1 Exchange of mating factors a  a factor Yeast cell, mating type a Yeast cell, mating type  Mating a  2 Figure 11.2 Communication between mating yeast cells a/ 3 New a/ cell

Concept 5.6: The plasma membrane plays a key role in most cell signaling Cells in a multicellular organism communicate by chemical messengers in the following way: - Animal and plant cells have cell junctions that directly connect the cytoplasm of adjacent cells (Gap Junctions or Plasmodesmata) - Animal cells may communicate by direct contact, also termed cell to cell recognition - In local signaling, animal cells communicate using locally secreted regulators or neurotransmitters - Over long-distances, plants and animals signal using chemicals called hormones

Gap junctions between animal cells Plasmodesmata between plant cells Plasma membranes Gap junctions between animal cells Plasmodesmata between plant cells (a) Direct Connection via Cell junctions Figure 11.4 Communication by direct contact between cells (b) Cell to cell recognition

Long-distance signaling Local signaling Long-distance signaling Target cell Electrical signal along nerve cell triggers release of neurotransmitter Endocrine cell Blood vessel Neurotransmitter diffuses across synapse Secreting cell Secretory vesicle Hormone travels in bloodstream to target cells Local regulator diffuses through extracellular fluid Target cell is stimulated Target cell Figure 11.5 Local and long-distance cell communication in animals (a) Paracrine signaling (b) Synaptic signaling (c) Hormonal signaling

The Three Stages of Cell Signaling: A Preview EXTRACELLULAR FLUID CYTOPLASM Plasma membrane 1 Reception 2 Transduction 3 Response Receptor Activation of cellular response Relay molecules in a signal transduction pathway Signaling molecule Animation: Overview of Cell Signaling

Reception: A signal molecule binds to a receptor protein, causing it to change shape The binding between a signal molecule (ligand) and its receptor is highly specific A shape change in a receptor is often the initial transduction of the signal Most signal receptors are plasma membrane proteins. Some signal receptors are located within the cell Receptor location depends on the size and polarity of the ligand

Receptors in the Plasma Membrane Most large and water-soluble (hydrophilic) ligands bind to specific sites outside of the cell on receptor proteins embedded in the plasma membrane There are three main types of membrane receptors: G protein-coupled receptors Receptor tyrosine kinases Ion channel receptors

G protein-coupled receptors A G protein-coupled receptor is a plasma membrane receptor that works with the help of a G protein The G protein acts as an on/off switch: - If GDP is bound to the G protein, the G protein is inactive (termed Gi) - If GTP is bound to the G protein, the G protein is active (termed Gs)

Segment that interacts with G proteins Ligand binding site Figure 11.7 Membrane receptors—G protein-coupled receptors, part 1 Segment that interacts with G proteins G protein-coupled receptor

Plasma membrane EXTRACELLULAR FLUID G protein-coupled receptor Inactive enzyme Activated receptor Signaling molecule Gi Gs GDP G protein (inactive) Enzyme GDP GTP CYTOPLASM 1 2 Activated enzyme Gs Gi Figure 11.7 Membrane receptors—G protein-coupled receptors, part 2 GTP GDP P i Cellular response 3 4

Receptor Tyrosine Kinases (not discussed in text) Receptor tyrosine kinases are membrane receptors that attach phosphates to tyrosines A receptor tyrosine kinase can trigger multiple signal transduction pathways at once

Fully activated receptor Trans-phosphorylation Ligand EXTRACELLULAR FLUID Signaling molecule  Helix Tyr Tyr Tyrosines Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Receptor tyrosine kinase proteins Dimer CYTOPLASM 1 2 Activated relay proteins Figure 11.7 Membrane receptors—receptor tyrosine kinases Cellular response 1 Tyr Tyr P Tyr Tyr P P P Tyr Tyr Tyr Tyr P Tyr Tyr P Tyr Tyr P P Tyr Tyr P Tyr Tyr P Tyr P Cellular response 2 6 ATP 6 ADP P Tyr Activated tyrosine kinase regions Fully activated receptor tyrosine kinase Inactive relay proteins Trans-phosphorylation 3 4

Gated Channels A ligand-gated ion channel receptor acts as a gate when the receptor changes shape When a signal molecule binds as a ligand to the receptor, the gate allows specific ions through a channel in the receptor The nervous system uses voltage-gated ion channels to conduct nervous impulses

Gate closed Plasma membrane Gate open 1 Gate closed Ions Ligand EXTRACELLULAR FLUID Receptor Plasma membrane CYTOPLASM 2 Gate open Cellular response Figure 11.7 Membrane receptors—ion channel receptors 3 Gate closed

Intracellular Receptors Some receptor proteins are intracellular, found in the cytosol or nucleus of target cells Small or hydrophobic ligands can readily cross the membrane and activate receptors Examples of hydrophobic ligands are the steroid (hydrophobic) and thyroid (small) hormones of animals An activated hormone-receptor complex can act as a transcription factor, turning on specific genes

Hormone (testosterone) Plasma membrane Receptor protein DNA NUCLEUS EXTRACELLULAR FLUID Plasma membrane Receptor protein DNA Figure 11.8 Steroid hormone interacting with an intracellular receptor NUCLEUS CYTOPLASM

Hormone (testosterone) Plasma membrane Receptor protein Hormone- EXTRACELLULAR FLUID Plasma membrane Receptor protein Hormone- receptor complex DNA Figure 11.8 Steroid hormone interacting with an intracellular receptor NUCLEUS CYTOPLASM

Hormone (testosterone) Plasma membrane Receptor protein Hormone- EXTRACELLULAR FLUID Plasma membrane Receptor protein Hormone- receptor complex DNA Figure 11.8 Steroid hormone interacting with an intracellular receptor NUCLEUS CYTOPLASM

Hormone (testosterone) Plasma membrane Receptor protein Hormone- EXTRACELLULAR FLUID Plasma membrane Receptor protein Hormone- receptor complex DNA Figure 11.8 Steroid hormone interacting with an intracellular receptor mRNA NUCLEUS CYTOPLASM

Hormone (testosterone) Plasma membrane Receptor protein Hormone- EXTRACELLULAR FLUID Plasma membrane Receptor protein Hormone- receptor complex DNA Figure 11.8 Steroid hormone interacting with an intracellular receptor mRNA NUCLEUS New protein CYTOPLASM

Transduction by Cascades of Molecular Interaction Signal transduction involves multiple steps Multistep pathways amplify a signal: A few molecules can produce a larger response Multistep pathways coordinate and regulate the response The molecules that relay a signal from receptor to response are mostly proteins At each step, the signal is transduced into a different form, usually causing a change of shape Animation: Signal Transduction Pathways

Protein Phosphorylation In many pathways, the signal is transmitted by a cascade of protein phosphorylations Protein kinases transfer phosphates from ATP to a protein in a process called phosphorylation Protein phosphatases remove phosphates from a protein in a process called dephosphorylation This phosphorylation and dephosphorylation system acts as a molecular switch, turning activities on and off

Phosphorylation cascade Signaling molecule Receptor Activated relay molecule Inactive protein kinase 1 Active protein kinase 1 Inactive protein kinase 2 ATP Phosphorylation cascade ADP Active protein kinase 2 P PP P i Figure 11.9 A phosphorylation cascade Inactive protein kinase 3 ATP ADP Active protein kinase 3 P PP P i Inactive protein ATP ADP P Active protein Cellular response PP P i

Small Molecules and Ions as Second Messengers Second messengers are small, non-protein, water-soluble molecules or ions that spread throughout a cell by diffusion Second messengers participate in pathways initiated by G protein-coupled receptors and receptor tyrosine kinases Cyclic AMP and calcium ions are common second messengers

Cyclic AMP Cyclic AMP (cAMP) is one of the most widely used second messengers Adenylyl cyclase, an enzyme in the plasma membrane, converts ATP to cAMP in response to an extracellular signal Further regulation of cell metabolism is provided by G-protein systems that inhibit adenylyl cyclase Adenylyl cyclase Phosphodiesterase Pyrophosphate P P i ATP cAMP AMP

Ligand (First messenger) Adenylyl cyclase G protein GTP G protein-coupled receptor ATP Second messenger cAMP Figure 11.11 cAMP as second messenger in a G-protein-signaling pathway cAMP usually activates protein kinase A, which phosphorylates various other proteins Protein kinase A Cellular responses

Calcium Ions and Inositol Triphosphate (IP3) A signal relayed by a signal transduction pathway can also trigger an increase in cytosolic calcium Calcium ions (Ca+2) act as a second messenger in so many pathways because cells can tightly regulate its cytosolic concentration (slide) Pathways leading to the release of calcium involve inositol trisphosphate (IP3) and diacylglycerol (DAG) as additional second messengers (slide)

Calcium concentrations in the cell EXTRACELLULAR FLUID Plasma membrane Ca+2 pump ATP Mitochondrion Nucleus CYTOSOL Ca+2 pump Endoplasmic reticulum (ER) Figure 11.12 The maintenance of calcium ion concentrations in an animal cell Ca+2 pump ATP Key High [Ca+2] Low [Ca+2]

G protein-coupled receptor EXTRA-CELLULAR FLUID Ligand (first messenger) G protein DAG GTP G protein-coupled receptor (for this example) PIP2 Phospholipase C IP3 (second messenger) IP3-gated calcium channel Figure 11.13 Calcium and IP3 in signaling pathways Endoplasmic reticulum (ER) Various proteins activated Cellular responses Ca2+ Ca+2 (second messenger) CYTOSOL

Response: Cell signaling can lead to the regulation of cytoplasmic or nuclear activity The cell’s response to an extracellular signal is sometimes called the “output response” The response may occur in the cytoplasm or may involve action in the nucleus Ultimately, a signal transduction pathway leads to the regulation of one or more cellular activities Many signaling pathways regulate the synthesis of other proteins, usually by turning genes on or off in the nucleus by activating a transcription factor (Nuclear Synthesis – transcription) Other pathways regulate cytosolic enzyme activity Figure 11.14 Nuclear responses to a signal: the activation of a specific gene by a growth factor

Nuclear Synthesis CYTOPLASM NUCLEUS Growth factor (Ligand) Receptor Reception Phosphorylation cascade Transduction CYTOPLASM Inactive transcription factor Active transcription factor Figure 11.14 Nuclear responses to a signal: the activation of a specific gene by a growth factor Response P DNA Gene NUCLEUS mRNA

Cytosolic Enzyme Activity Reception Binding of epinephrine to G protein-coupled receptor (1 molecule) Transduction Inactive G protein Active G protein (102 molecules) Inactive adenylyl cyclase Active adenylyl cyclase (102 molecules) Amplified Response!!! ATP Cyclic AMP (104 molecules) Inactive protein kinase A Active protein kinase A (104 molecules) Figure 11.15 Cytoplasmic response to a signal: the stimulation of glycogen breakdown by epinephrine Inactive phosphorylase kinase Active phosphorylase kinase (105 molecules) Inactive glycogen phosphorylase Active glycogen phosphorylase (106 molecules) Response Glycogen Glucose-1-phosphate (108 molecules)

Specificity of Cell Signaling & Response Coordination Different kinds of cells have different functions and hence, different collections of proteins These different proteins allow cells to detect and respond to different signals, differently Even the same signal can have different effects in cells with different proteins and pathways Pathway branching and “cross-talk” further help the cell coordinate incoming signals (next slide) (how many times did I mention ‘different’ here)

Figure 11.17 The specificity of cell signaling molecule Receptor Relay molecules Response 1 Response 2 Response 3 Cell A. Pathway leads to a single response. Cell B. Pathway branches, leading to two responses. Figure 11.17 The specificity of cell signaling Activation or inhibition Response 4 Response 5 Cell C. Cross-talk occurs between two pathways. Cell D. Different receptor leads to a different response.

Signaling Efficiency: Scaffolding Proteins and Signaling Complexes Scaffolding proteins are large relay proteins to which other relay proteins are attached, thereby increasing signal transduction efficiency by grouping together different proteins involved in the same signal transduction pathway Signaling molecule Plasma membrane Receptor Three different protein kinases Scaffolding protein

Signal Termination Inactivation mechanisms are an essential aspect of cell signaling When signal molecules leave the receptor, the receptor reverts to its inactive state (easy, huh?)

Signal transduction pathways

Fine-Tuning the Response Multistep pathways have two valued benefits: Amplifying the signal and thus the response through a cascade effect (already discussed) Contributing to the specificity and coordination of the response (next slide)

Concept 11.5: Apoptosis (programmed cell death) integrates multiple cell-signaling pathways Apoptosis is programmed or controlled cell suicide and is important in biological processes, including cell development, differentiation, and cancer In this process, a cell is chopped up and packaged into vesicles for digestion by scavenger cells, preventing dying cell contents from leaking out, damaging neighboring cells (see next slide) Apoptosis results when specific proteins that “accelerate” apoptosis override those that “put the brakes” on apoptosis (see following slide) The role of apoptosis was first studied in Caenorhabditis elegans (C. elegans), a uniquely structured soil worm

Figure 11.19 Apoptosis of human white blood cells

(inhibits Ced-4 activity) Active Ced-9 protein (inhibits Ced-4 activity) Mitochondrion Ced-4 Ced-3 Receptor for death- signaling molecule Inactive proteins No Death signal Cell forms blebs Inactive Ced-9 protein Death- signaling molecule Figure 11.20 Molecular basis of apoptosis in C. elegans Active Ced-4 Active Ced-3 Proteases Nucleases Activation cascade Death signal

Apoptotic Pathways and Trigger Signals Caspases are the main proteases (enzymes that cut up proteins) that carry out apoptosis Apoptosis can be triggered by: An extracellular death-signaling ligand DNA damage in the nucleus Protein misfolding in the endoplasmic reticulum