Ch 6 Cell Signals and Responses
Cell Signals and Responses
Introduction to Cell Signaling
Firefly Light Organ Example:
Female firefly observes specific light flash patterns from a male.
She responds with her own flash, triggered by a nerve impulse from her brain to her light organ.
Question: How do light organ cells convert a nerve impulse into a flash of light? (Answer provided at the end of the text).
Key Concepts (Chapter Overview):
6.1: Cells Detect a Variety of Signals
6.2: Signal Molecule Receptors Can Be Classified into Several Groups
6.3: Signal Transduction Allows a Cell to Respond Appropriately to a Signal
6.4: Signal Transduction Is Highly Regulated
6.1 Cells Detect a Variety of Signals
Importance of Environmental Detection:
Critical for all organisms to survive and adapt.
Led to the evolution of cell-to-cell signaling, where one cell produces a signal for detection by another.
Organisms Detect Environmental Variables:
The environment varies spatially and temporally.
All organisms (prokaryotes, multicellular eukaryotes) detect and respond to light, temperature, sound, touch, magnetic fields, gravitational fields, pH, and chemicals.
Examples:
Plants grow toward light (phototropism).
Bacteria alter gene transcription rates in response to temperature.
Barnacle larvae attach to solid substrates.
Pigeons navigate using Earth's magnetic field.
Carrion beetles fly toward the smell of dead animals.
Environmental variables are detected by individual cells, leading to changes and appropriate responses.
This ability maintains stable intracellular conditions.
Mechanism of Environmental Detection:
Cells use various sensory receptors (chemo-, photo-, thermo-, mechanoreceptors).
Detection fundamentally involves changes in the tertiary (three-dimensional) structure of a sensory receptor.
These structural changes initiate a cascade of events leading to a cellular (and organismal) response.
Examples of Receptor Changes:
Plant Pigments: Beyond chlorophyll, plants use other protein-bound pigment molecules to detect light (e.g., for flowering, seed germination). Absorption of a photon by the pigment causes a conformational change in the attached protein, leading to a cellular response.
Rhodopsin in Eyes: Retinal (derived from vitamin A) is covalently attached to opsin, forming rhodopsin. Photon absorption by retinal changes the shape of opsin.
Cell-to-Cell Signals vs. Environmental Signals:
Cell signals differ in three key ways:
They are produced by a cell (the signaling cell).
They convey specific information.
They have an intended recipient (the receiver or target cell).
The goal of an intercellular signal is to elicit a response in the target cell.
Three Steps of Cellular Response to a Signal (FIGURE 6.1):
Binding of the chemical signal to its receptor:
A chemical signal (or ligand) is a molecule that binds to a receptor protein.
Ligands include proteins, lipids, and other molecules.
Binding is highly specific; receptors bind only to particular ligands.
This allows signals to be targeted to specific cell groups (those with the receptor).
Cells without the receptor for a ligand will not detect the signal.
Binding changes the receptor's three-dimensional structure (conformation), altering its enzymatic activity or binding properties.
A cascade of events inside the cell (signal transduction):
Often, receptor binding initiates a series of intracellular events called signal transduction.
This commonly involves a cascade (series) of events, often with enzymes exhibiting allosteric regulation (conformational change due to binding at a site other than the active site).
Phosphorylation (binding of phosphate groups) frequently causes allosteric changes in signal transduction enzymes.
Signal Amplification: The involvement of enzymes is crucial because a single activated enzyme can catalyze many reactions, leading to amplification. A single chemical signal molecule can activate tens, hundreds, or thousands of downstream molecules, enabling robust responses from extremely low signal concentrations.
The cell's response:
The signal transduction pathway culminates in the activation of proteins that generate the cellular response.
Short-term responses: Enzyme activation (increasing reaction rate), ion channel opening (changing membrane voltage).
Long-term responses: Modification of regulatory proteins, leading to altered gene expression.
Evolutionary Significance:
Cell signaling was essential for the evolution of multicellularity and cellular differentiation.
Complex multicellular organisms have a significantly higher percentage of genes encoding cell-to-cell signaling molecules compared to single-celled organisms.
Reception of Signals in Multicellular Organisms:
Cells are exposed to many signals but respond only to a subset.
Chemical signals reach target cells via:
Local diffusion.
Direct contact.
Bulk flow (e.g., circulatory system in animals, vascular system in plants).
Signals are usually present in very low concentrations (e.g., 10^{-10} ext{ M}).
Types of Cell-to-Cell Signaling Systems (FIGURE 6.2):
Juxtacrine Signaling:
Requires direct contact between signaling and receiving cells.
Involves signals diffusing intercellularly through gap junctions (animals) or plasmodesmata (plants).
Alternatively, involves interaction between signal and receptor molecules bound to the surfaces of the two cells.
The extracellular matrix may also play a role.
Paracrine Signaling:
Signals diffuse to nearby target cells.
Synaptic signaling (neurotransmitter diffusion to adjacent nerve cells) is an example.
Autocrine Signaling:
A cell releases a signal for which it also has a receptor, affecting itself.
Crucial in tumor cells, which produce growth factors to stimulate their own uncontrolled division.
Endocrine Signaling:
Signals act over long distances.
Hormones are chemical signals transported by bulk flow (e.g., circulatory system in animals, vascular system in plants).
In animals, hormones are produced by specialized cells in specific tissues.
Plant hormones can be produced in multiple locations.
Receptor Specificity:
Only cells with the necessary receptors can respond to a signal.
While all cells contain the genes for all receptor types, these genes are only expressed in certain cells (e.g., photoreceptor genes expressed in eye cells, not liver cells).
6.2 Signal Molecule Receptors Can Be Classified into Several Groups
Diversity of Receptors:
Multicellular eukaryotes possess hundreds of distinct signal molecule receptor proteins.
Each receptor binds a specific signal molecule.
Related receptors often belong to gene families, sharing similar structure and function.
Both cell signals and environmental signals are detected by related receptor proteins, causing similar changes.
Receptor Location Based on Ligand Chemistry (FIGURE 6.3):
Intracellular Receptors:
Located in the cytosol or nucleus.
Bind ligands that are small or nonpolar, allowing them to diffuse directly across the cell membrane (e.g., lipid bilayer).
Example: Animal steroid hormones (estrogen, cortisol) (FIGURE 6.4).
Cortisol diffuses into the cytosol, binds to its receptor.
Binding causes the receptor to change shape and release a chaperone protein (which normally prevents nuclear entry).
The hormone-bound receptor then enters the nucleus and affects gene expression.
Membrane Receptors:
Embedded in the cell membrane.
Bind ligands that are large or polar, and thus cannot diffuse through the cell membrane (e.g., protein hormones).
Example: Insulin (a 51-amino acid protein hormone) is too large and hydrophilic to cross the membrane.
These are transmembrane proteins with:
An extracellular ligand-binding region.
A hydrophobic region spanning the membrane.
A cytosolic region responsible for initiating signal transduction.
The ligand binds to the extracellular region and acts as an allosteric regulator, exposing the active site of the cytosolic region.
The ligand itself doesn't directly contribute to the cellular response beyond initiating it.
Ligand Binding Properties:
Ligands bind to receptors noncovalently and reversibly.
Binding is generally favored, even at low ligand concentrations.
Specificity arises from the receptor's protein sequence and glycosylation (carbohydrate residues attached, especially in the extracellular region), which also affects protein folding and receptor response.
Regulation of Cell Signaling (Inhibitors):
Cells can regulate signaling by producing inhibitors that prevent normal ligand binding.
These inhibitors bind to the ligand-binding site but do not cause activation (similar to competitive enzyme inhibition).
Example: Caffeine and Adenosine (FIGURE 6.6):
Adenosine binds to its receptor in the brain, leading to drowsiness.
Caffeine has a similar enough structure to adenosine to bind to the adenosine receptor but does not activate it.
Caffeine acts as a competitive inhibitor, preventing adenosine binding and thus reducing drowsiness and increasing alertness.
Major Classes of Membrane Receptors (Categorized by Action):
Ligand-Gated Ion Channels (FIGURE 6.7):
Protein channels in cell membranes for ions ( ext{Na}^+, ext{K}^+, ext{Ca}^{2+}, ext{Cl}^−).
Their function (opening/closing) depends on ligand binding.
Example: Acetylcholine Receptor (AChR):
A ligand-gated sodium channel in skeletal muscle cell membranes.
Acetylcholine (ACh), a neurotransmitter, binds to two of the five AChR subunits.
This causes the channel to change shape and open, allowing ext{Na}^+ (more concentrated outside) to diffuse into the cell.
ext{Na}^+ influx initiates muscle contraction.
G Protein-Coupled Receptors (GPCRs) (FIGURE 6.8):
Activate GTP-binding proteins (G proteins).
Structure: Seven hydrophobic membrane-spanning regions.
Associate with specific heterotrimeric G proteins (alpha, beta, gamma subunits) partially embedded on the cytosolic side of the membrane.
The ext{alpha} subunit binds GDP or GTP.
Mechanism:
Inactive state: No signal, GPCR not activated; ext{alpha} subunit of G protein has GDP bound.
Activation: Ligand binds to GPCR, causing a conformational change.
This change causes the G protein ext{alpha} subunit to exchange GDP for GTP.
The GTP-bound ext{alpha} subunit detaches and activates an effector protein (an enzyme).
Effector protein causes further cellular changes, leading to signal amplification and response.
Abundance: Over 800 GPCRs encoded in the human genome.
Importance: Especially significant in sensory systems.
Protein Kinase Receptors (FIGURE 6.9):
Upon ligand binding, undergo a conformational change that exposes or activates an active site in their cytosolic region.
The cytosolic region has protein kinase activity: it modifies specific target proteins by adding phosphate groups (usually from ATP) to their side chains.
Target Amino Acids: Serine, threonine, and tyrosine (due to their hydroxyl (- ext{OH}) functional groups).
Reaction: ext{ATP} + ext{protein}
ightarrow^{ ext{Protein kinase}} ext{ADP} + ext{phosphorylated protein}This covalent modification (phosphorylation) changes the activity of the target protein.
Abundance: Over 500 protein-coding genes in humans are protein kinases.
Example: Insulin Receptor:
Insulin binds to the receptor.
Causes autophosphorylation (phosphorylation of tyrosine residues on the receptor itself).
Activated receptor then binds and phosphorylates other target proteins (insulin-response substrates).
This triggers a cascade, including the insertion of glucose transport proteins into the cell membrane.
6.3 Signal Transduction Allows a Cell to Respond Appropriately to a Signal
Signal Transduction Pathway Overview:
Activation of a membrane receptor by its signal initiates a cascade of events inside the cell.
Proteins interact (sometimes through nonprotein intermediates) until responses are achieved.
Cascades allow for amplification and distribution of the initial signal.
Also facilitate the integration of different signaling pathways.
Common Cellular Responses to Signals:
1. Opening of ion channels:
Rapidly changes ion concentrations across the membrane.
Alters electric potential (voltage) across the membrane (critical for nerve and muscle cells).
2. Alteration of enzyme activities:
Changes in enzyme activity quickly alter metabolic pathway rates within the cell.
Example: Epinephrine activates specific enzymes in liver cells to mobilize glucose (discussed below).
3. Alterations in gene expression:
Transcription of some genes may be upregulated (switched on) or downregulated (switched off).
Alters the abundance of encoded proteins (often enzymes), thus changing cell function.
These changes are generally slower to manifest but longer-lasting.
Same Signal, Different Responses in Different Cell Types:
The same signal (e.g., epinephrine) can elicit different responses based on the cell type.
Example: Epinephrine in muscle cells:
Heart muscle cells: Epinephrine stimulates a pathway activating enzymes for glucose mobilization (energy) and muscle contraction.
Smooth muscle cells (digestive tract): Epinephrine stimulates a pathway inhibiting a target enzyme, causing muscle relaxation, increasing blood vessel diameter to carry more nutrients.
This difference is due to different signal transduction pathways being stimulated in different cell types, even by the same hormone.
Signal Amplification and Regulation in Cascades:
While most pathways involve multiple steps, some are direct (e.g., steroid hormones binding cytosolic receptors directly alter gene expression; ligand-gated ion channels open directly upon binding).
A signal transduction cascade often involves sequential activation/inhibition of enzymes.
Protein kinases add phosphate groups (covalent change) to activate/inhibit target proteins by altering conformation or exposing/masking active sites.
Example: Mitogen-Activated Protein Kinases (MAPKs) Cascade (FIGURE 6.10):
Mitogens are signaling molecules that stimulate cell division.
1. A mitogen binds to its membrane receptor.
2. The receptor phosphorylates itself (autophosphorylation).
3. The activated receptor initiates events that activate RAS (a small-GTPase G protein, different from heterotrimeric G proteins associated with GPCRs) by causing it to bind GTP.
4. Activated RAS binds and activates MAP3K (MAPK kinase kinase).
5. Activated MAP3K (a protein kinase) phosphorylates many molecules of MAP2K (MAPK kinase).
6. Activated MAP2K (a protein kinase) phosphorylates many molecules of MAPK (MAP kinase).
7. Activated MAPK enters the nucleus and phosphorylates target proteins, promoting the expression of genes involved in cell division.
Relevance to Cancer: Mutations in RAS proteins that make them permanently active are common in cancer, leading to uncontrolled cell division by constantly upregulating this mitogen signaling cascade.
Second Messengers (FIGURE 6.11):
Small, nonprotein molecules produced during one or more steps of a signaling cascade.
First messengers are the initial signal molecules (ligands).
Types:
Hydrophilic: Cyclic AMP (cAMP), inositol trisphosphate (IP ext{_3}), ext{Ca}^{2+} ions.
Hydrophobic: Diacylglycerol (DAG), phosphatidylinositol trisphosphate (PIP ext{_2}) (insoluble in water, associated with membranes).
Gaseous: Nitric oxide (NO).
Function: Regulate target enzymes by binding noncovalently (allosteric regulation).
Benefit: Allow a cell to convert a single event at the membrane into many intracellular events, effectively distributing the initial signal.
Location/Action:
Hydrophobic: Act on cell membrane-associated proteins.
Hydrophilic: Act on cytosolic proteins.
Gaseous: Rapidly diffuse through membranes, initiating responses in nearby cells.
Example: Epinephrine Signaling Cascade with Second Messenger (FIGURE 6.12):
Activation pathway for liver cells in response to epinephrine (fight-or-flight response).
1. Epinephrine binds to its G protein-coupled receptor.
2. Activates a G protein.
3. The activated G protein activates the membrane-integrated enzyme adenylyl cyclase.
4. Adenylyl cyclase catalyzes the production of the second messenger cAMP from ATP.
5. cAMP activates protein kinase A.
6. Protein kinase A phosphorylates two target enzymes with opposite effects:
Inhibits glycogen synthase: Prevents glucose molecules from forming glycogen (storage).
Activates phosphorylase kinase: Which then phosphorylates and activates glycogen phosphorylase.
Glycogen phosphorylase catalyzes the breakdown of glycogen, releasing glucose.
7. The released glucose enters the bloodstream.
Net Result: Mobilizes glucose from glycogen stores, preparing for emergency (e.g., 1 epinephrine molecule can lead to $10,000$ glucose molecules released).
Amplification: Occurs at each enzymatic step; steps involving allosteric regulators (G protein, cAMP) do not amplify.
Discovery of a Second Messenger (INVESTIGATION - FIGURE 6.13):
Experiment by Earl Sutherland and colleagues: Aimed to confirm the existence of a soluble second messenger in epinephrine's pathway.
Hypothesis: Phosphorylation of liver glycogen phosphorylase after epinephrine binding involves a soluble second messenger in the cytosol.
Method: Homogenized liver cells, separated into cytosolic and membrane components. Incubated membrane components with/without epinephrine. Removed membranes. Added the resulting membrane-free solution to a cytosolic fraction. Measured glycogen phosphorylase phosphorylation activity.
Key Findings: Phosphorylation of glycogen phosphorylase increased when the membrane-free solution from epinephrine-incubated membranes was added to the cytosol, even without the membranes being present.
Conclusion: A soluble second messenger, produced by hormone-activated membranes, is present in the solution and activates cytosolic enzymes.
Implications: Suggested that the active factor was stable to heating (not a protein) and could be mimicked by cAMP.
6.4 Signal Transduction Is Highly Regulated
Regulation and Resetting of Signaling Pathways:
Signal transduction is a temporary event and must be turned off and reset so the cell can respond to new signals.
Three Ways to Turn Off a Signaling Pathway (FIGURE 6.14):
Receptor Recycling:
Membrane receptors can be endocytosed and degraded in lysosomes.
Intracellular receptors (e.g., estrogen receptor) have a rapid turnover rate (every few hours), ending the response.
Chemical Reversion of Signal Transduction Molecules:
Active molecules are converted back to inactive precursors.
Protein phosphatases remove phosphate groups from target proteins, reversing the effects of protein kinases.
G proteins possess GTPase activity, hydrolyzing GTP to GDP, which inactivates the ext{alpha} subunit and ends its allosteric activation of enzymes.
The enzyme phosphodiesterase converts cAMP to AMP, ending the activation of cAMP-dependent enzymes.
Loss of the Signal Molecule:
Some second messengers, like gaseous nitric oxide (NO), simply diffuse out of the cell, rapidly lowering their concentration and shutting down the response.
Integration of Multiple Signaling Pathways:
Cells often detect several signals and must integrate this information for an appropriate response.
Signaling pathways are frequently interconnected; some activate responses, others switch them off.
Example: Blood Glucose Regulation (Insulin, Glucagon, Epinephrine) (FIGURE 6.15 & 6.16):
Glucose is vital for energy metabolism.
Epinephrine and Glucagon:
Both bind to G protein-coupled receptors.
Activate G proteins, which activate adenylyl cyclase.
This increases cAMP levels.
Lead to increased glucose release from glycogen stores (as seen with epinephrine above).
They also decrease fatty acid synthesis, increase triglyceride breakdown, and increase gluconeogenesis (glucose from non-carbohydrate sources), all increasing glucose availability.
Insulin (FIGURE 6.15):
Binds to a protein kinase receptor.
The pathway primarily acts to reduce blood glucose levels.
Mechanism (Simplified):
Insulin binding leads to receptor phosphorylation.
Activated receptor phosphorylates an insulin response substrate (IRS).
Activated IRS activates phosphoinositide kinase.
Phosphoinositide kinase phosphorylates PIP ext{2} to form PIP ext{3}.
PIP ext{_3} binds to and activates protein kinase B (PKB).
PKB activates phosphodiesterase, which hydrolyzes cAMP to AMP.
Reduced cAMP levels then inhibit the glucagon and epinephrine pathways, preventing glycogen breakdown and glucose release.
Another effect of insulin signaling (not shown in Figure 6.15) is to cause vesicles containing glucose carrier proteins to fuse with the cell membrane in tissues like muscle, facilitating rapid glucose uptake from the blood.
Overall: Insulin counteracts the effects of glucagon and epinephrine, reducing blood glucose.
Patterns in Integrated Signaling (FIGURE 6.16 Review):
Same Targets: Different pathways can affect the same enzymes (e.g., all three pathways target glycogen synthase).
Interference: Pathways can interfere with each other (e.g., cAMP increases with epinephrine/glucagon but decreases with insulin).
Complexity: Predicting effects of multiple simultaneous signals is challenging due to varying receptor/protein concentrations and other unmentioned signals.
Misregulation of Signaling in Health Disorders:
Diabetes: Caused by problems with insulin signaling (reaching epidemic proportions).
Type I Diabetes: Immune system mistakenly destroys insulin-producing pancreatic cells.
Type II Diabetes: Accounts for > 90 ext{ percent} of cases; primarily linked to obesity. Liver cells become insensitive to even low levels of insulin produced by the pancreas.
Consequences (Both Types): Insulin pathway is inactive or reduced, allowing epinephrine and glucagon pathways to dominate, leading to abnormally high blood glucose levels.
High glucose levels damage small blood vessels (capillaries), causing them to become leaky (e.g., retinal damage leading to blindness, nerve damage in feet/hands).
Treatments: Type I treated with insulin injections. Type II treated with drugs like Metformin, which inhibits glucagon-stimulated cAMP rise, reducing glucose.
Cancer: Characterized by uncontrolled cell division due to cell cycle misregulation.
Example: Permanently active RAS protein mutations are common in cancer cells, constantly upregulating mitogen signaling and stimulating cell division even without an external signal.
Answer to Firefly Light Organ Question
Mechanism:
When a nerve signal reaches the light organ, the neurotransmitter octopamine is released from the nerve cell.
Octopamine binds to its G protein-coupled receptor.
This activates a G protein.
The active G protein initiates a signal transduction pathway that culminates in a flash of light.
Light Production:
The flash is caused by the light reaction catalyzed by the enzyme luciferase, which oxidizes its substrate, luciferin.
Reaction: ext{Luciferin} + ext{ATP} + ext{O}2 ightarrow ext{oxyluciferin} + ext{AMP} + ext{PP}i + ext{light}
Trigger for the Flash:
The reaction, and thus the flash, is thought to be triggered by a brief pulse of oxygen supplied to the light organ.
Current Hypotheses for the Oxygen Pulse (Ongoing Research):
Nitric Oxide (NO) Hypothesis 1: Production of NO temporarily shuts off mitochondrial electron transport, causing oxygen levels to rise.
Nitric Oxide (NO) Hypothesis 2: Production of NO leads to an increase in hydrogen peroxide ( ext{H}2 ext{O}2) from peroxisomes, which is then used in place of oxygen in the light reaction.
Ion Pump Hypothesis: Activation of ion pumps causes fluid to be pumped out of small tubes, allowing oxygen to rapidly diffuse deep into the light organ.
The specific details of the signal transduction pathway from octopamine to the oxygen pulse and flash are still an active area of scientific research.