Comprehensive Study Guide on Cell Signaling: Principles, GPCRs, and Enzyme-Coupled Pathways

General Principles of Cell Signaling

• Signal Transduction Definition: Signal transduction is the process whereby one type of signal is converted into another. This is essential for animal cells to communicate and change cell behaviors in response to various extracellular signals.
• Communication Ranges: Signals can act over a long or short range. Mechanisms include:     * Endocrine: Hormone molecules traveling through the bloodstream to distant cells.     * Paracrine: Local mediators affecting cells in the immediate environment.     * Autocrine: A cell responding to a signal it secretes itself (a form of paracrine signaling).     * Neuronal (Synaptic): Neurotransmitters delivered across synapses to specific target cells.     * Contact-Dependent: A transmembrane signal protein on the signaling cell binds to a receptor on the adjacent target cell.
• Types of Receptors: Extracellular signal molecules bind either to cell-surface receptors or to intracellular receptors.     * Cell-Surface Receptors: Used for large and/or hydrophilic signal molecules that cannot cross the plasma membrane.     * Intracellular Receptors: Used for small and/or hydrophobic molecules (e.g., steroid hormones, nitric oxide) that can diffuse across the plasma membrane. These receptors may be in the cytosol or the nucleus.
• Diversity of Response: The same signal molecule can induce different responses in different target cells based on the receptor and the cell's internal machinery. For example, Acetylcholine can:     * Decreases the rate of firing in heart pacemaker cells.     * Induce secretion in salivary gland cells.     * Induce contraction in skeletal muscle cells.
• Combinatorial Signaling: An animal cell typically depends on multiple extracellular signals to determine its fate:     * Survive: Interaction of signals A, B, and C.     * Grow + Divide: Interaction of signals A, B, C, D, and E.     * Differentiate: Interaction of signals A, B, C, F, and G.     * Die: In the absence of survival signals, the cell undergoes apoptosis (programmed cell death).
• Speed of Signaling:     * Fast Response: Occurs in \text{<sec to mins}. It involves altered protein function and changes in the cytoplasmic machinery.     * Slow Response: Occurs in $\text{mins to hrs}$. It involves signals moving into the nucleus to alter DNA, RNA synthesis, and protein synthesis, leading to altered cell behavior.

Catalog of Extracellular Signal Molecules (Table 16-1)

• Hormones:     * Epinephrine (Adrenaline): Originates in the adrenal gland; derivative of tyrosine; increases blood pressure, heart rate, and metabolism.     * Cortisol: Originates in the adrenal gland; steroid derivative of cholesterol; affects metabolism of proteins, carbohydrates, and lipids.     * Estradiol: Originates in the ovary; steroid derivative of cholesterol; maintains secondary female sexual characteristics.     * Insulin: Originates in $\beta$ cells of the pancreas; protein; stimulates glucose uptake, protein synthesis, and lipid synthesis.     * Testosterone: Originates in the testis; steroid derivative of cholesterol; maintains secondary male sexual characteristics.     * Thyroid Hormone (Thyroxine): Originates in the thyroid gland; derivative of tyrosine; stimulates metabolism in many cell types.
• Local Mediators:     * Epidermal Growth Factor (EGF): Originates in various cells; protein; stimulates proliferation of epidermal and other cells.     * Platelet-Derived Growth Factor (PDGF): Originates in various cells/blood platelets; protein; stimulates cell proliferation.     * Nerve Growth Factor (NGF): Originates in innervated tissues; protein; promotes survival and axonal growth of specific neurons.     * Histamine: Originates in mast cells; derivative of histidine; causes blood vessels to dilate and become leaky (causes inflammation).     * Nitric Oxide (NO): Originates in nerve cells and endothelial cells; dissolved gas; causes smooth muscle relaxation and regulates nerve-cell activity.
• Neurotransmitters:     * Acetylcholine: Originates in nerve terminals; derivative of choline; excitatory neurotransmitter at nerve-muscle synapses.     * $\gamma$-Aminobutyric Acid (GABA): Originates in nerve terminals; derivative of glutamic acid; inhibitory neurotransmitter in the Central Nervous System (CNS).
• Contact-Dependent Molecules:     * Delta: Originates in prospective neurons and developing cell types; transmembrane protein; inhibits neighboring cells from specializing in the same way as the signaling cell.

Intracellular Signaling Pathways and Molecular Switches

• Functions of Intracellular Signaling Pathways:     * Relay: Help the signal spread through the cell.     * Amplify: Make the signal stronger (e.g., small messenger molecules like $cAMP$, $IP_3$, Diacylglycerol, or $Ca^{2+}$).     * Integrate: Combine more than one signaling pathway before relaying downstream.     * Distribute: Evoke a complex response by branching to multiple effector proteins.     * Modulate (Feedback): Adjust the response via positive or negative feedback loops.
• Molecular Switches: Proteins that toggle between an active and inactive state.     * Signaling by Protein Phosphorylation:         * Protein Kinase: Adds a phosphate group from $ATP$ to the signaling protein to turn it "ON."         * Protein Phosphatase: Removes the phosphate group to turn the protein "OFF."     * Signaling by GTP-Binding Proteins:         * Activation: Binding of $GTP$ turns the protein "ON."         * Deactivation: The protein has intrinsic $GTP$ hydrolysis activity, converting $GTP$ to $GDP$ to turn itself "OFF."     * Regulation of Monomeric GTPases:         * GEF (Guanine Nucleotide Exchange Factor): Promotes the exchange of $GDP$ for $GTP$, activating the protein.         * GAP (GTPase-activating protein): Stimulates $GTP$ hydrolysis, inactivating the protein.

Classes of Cell-Surface Receptors and Foreign Substances

• Three Main Classes:     * Ion-Channel-Coupled Receptors: Change the permeability of the plasma membrane to specific ions, altering membrane potential.     * G-Protein-Coupled Receptors (GPCRs): Activate membrane-bound, trimeric GTP-binding proteins (G proteins), which then activate an enzyme or an ion channel.     * Enzyme-Coupled Receptors: Act as enzymes themselves or associate with enzymes inside the cell when activated.
• Interactions with Foreign Substances (Table 16-2):     * Barbiturates and Benzodiazepines (Valium/Ambien): Stimulate GABA-activated ion-channel-coupled receptors; result in anxiety relief and sedation.     * Nicotine: Stimulates acetylcholine-activated ion-channels; causes constriction of blood vessels and elevated blood pressure.     * Morphine and Heroin: Stimulate G-protein-coupled opiate receptors; result in analgesia (pain relief) and euphoria.     * Curare: Blocks acetylcholine-activated ion-channels; leads to paralysis (neuromuscular transmission blockage).     * Strychnine: Blocks glycine-activated ion-channels; leads to seizures and muscle spasms.     * Capsaicin: Stimulates temperature-sensitive ion-channels (heat); induces painful burning sensations and can lead to paradoxically pain relief.     * Menthol: Stimulates temperature-sensitive ion-channels (cold); induces cool sensations or burning pain at high doses.

G-Protein-Coupled Receptors (GPCRs)

• GPCR Structure: All GPCRs possess a similar structure consisting of a single polypeptide chain that threads back and forth across the lipid bilayer seven times (7-pass transmembrane protein). It has an exoplasmic face and a cytosolic face.
• Activation Mechanism:     1. Extracellular signal binds to the GPCR.     2. The GPCR undergoes a conformational change that activates a G protein on the cytosolic side.     3. The activated receptor encourages the $\alpha$ subunit of the G protein to expel its $GDP$ and pick up $GTP$.     4. The $\alpha$ subunit often dissociates from the $\beta\gamma$ complex; both components can interact with target proteins.
• Deactivation: The $\alpha$ subunit eventually hydrolyzes its bound $GTP$ to $GDP$, causing it to reassociate with the $\beta\gamma$ complex, switching the signal off.
• Ion Channel Regulation: In heart pacemaker cells, G proteins directly couple receptor activation to the opening of $K^+$ channels, slowing the heart rate.
• Second Messenger Production:     * Adenylyl Cyclase Pathway: Activated G proteins ($G_s$) stimulate adenylyl cyclase to produce $cAMP$ from $ATP$. $cAMP$ is degraded by cyclic AMP phosphodiesterase.     * Phospholipase C Pathway: Activated G proteins ($G_q$) activate phospholipase C, which cleaves an inositol phospholipid into Inositol 1,4,5-trisphosphate ($IP_3$) and Diacylglycerol (DAG).         * $IP_3$ binds to and opens $Ca^{2+}$ channels in the Endoplasmic Reticulum (ER).         * DAG remains in the membrane and, along with $Ca^{2+}$, helps activate Protein Kinase C (PKC).
• Cyclic AMP Mediated Responses (Table 16-3):     * Heart (Epinephrine): Increase in heart rate and force of contraction.     * Skeletal Muscle (Epinephrine): Glycogen breakdown.     * Fat (Epinephrine, Glucagon): Fat breakdown.     * Adrenal Gland (ACTH): Cortisol secretion.
• Glycogen Breakdown Mechanism: Epinephrine binds to an adrenergic receptor $\rightarrow$ activates $G_s$ $\rightarrow$ increases $cAMP$ $\rightarrow$ activates Protein Kinase A (PKA) $\rightarrow$ activates glycogen phosphorylase and inactivates glycogen synthase $\rightarrow$ decreased glycogen.
• Gene Transcription: PKA can translocate to the nucleus, phosphorylate transcription regulators, and activate target genes.
• Calcium ($Ca^{2+}$) Signaling:     * A rise in cytosolic $Ca^{2+}$ (e.g., during fertilization of an egg) triggers many biological processes.     * The protein Calmodulin acts as a calcium sensor; upon binding $Ca^{2+}$, it changes shape and wraps around target proteins like CaM-kinase.

Signal Amplification Case Study: Rod Photoreceptors

• The Cascade:     1. One rhodopsin molecule absorbs 1 photon.     2. 500 G protein (transducin) molecules are activated.     3. 500 cyclic GMP phosphodiesterase molecules are activated.     4. $10^5$ cyclic GMP molecules are hydrolyzed.     5. 250 cation channels in the plasma membrane close.     6. $10^6$ to $10^7$ $Na^+$ ions per second are prevented from entering the cell for approximately 1 second.     7. Membrane potential is altered by 1 mV, relaying the signal to the brain.

Enzyme-Coupled Receptors: Receptor Tyrosine Kinases (RTKs)

• Activation: Signal molecules (often in the form of a dimer) bind to RTKs, causing the two receptor tails to come together and phosphorylate each other on specific tyrosines.
• Assembly of Signaling Complex: Phosphorylated tyrosines serve as docking sites for intracellular signaling proteins containing SH2 domains or other interaction domains.
• The Ras Pathway:     1. Activated RTKs recruit an adaptor protein and a Ras-GEF.     2. Ras-GEF activates Ras (a monomeric GTPase) by exchanging $GDP$ for $GTP$.     3. Ras initiates a MAP-kinase signaling module:         * Activated Ras $\rightarrow$ MAP kinase kinase kinase $\rightarrow$ MAP kinase kinase $\rightarrow$ MAP kinase.         * MAP kinase phosphorylates effector proteins and transcription regulators to change protein activity and gene expression.
• The PI-3-Kinase-Akt Pathway:     1. RTK activates PI 3-kinase, which phosphorylates inositol phospholipids in the membrane.     2. These lipids serve as docking sites for Akt (also called Protein Kinase B).     3. Akt promotes cell survival by phosphorylating and inactivating Bad (a pro-apoptotic protein), which releases Bcl2 (an anti-apoptotic protein).     4. Akt also stimulates cell growth by activating the serine/threonine kinase Tor (which stimulates protein synthesis and inhibits protein degradation).

Specialized Signaling: Nitric Oxide (NO)

• Mechanism in Blood Vessels:     1. Acetylcholine binds to a GPCR on endothelial cells.     2. Through the $IP_3/Ca^{2+}$ pathway, NO synthase (NOS) is activated.     3. NOS produces Nitric Oxide (NO) from arginine.     4. NO diffuses rapidly across membranes into adjacent smooth muscle cells.     5. NO binds to guanylyl cyclase, stimulating the production of cyclic GMP.     6. Cyclic GMP causes rapid relaxation of smooth muscle cells, leading to vessel dilation.
• Clinical Application (Sildenafil/Viagra):     * Sexual stimulation leads to NO release in terminal nerves and vascular endothelium.     * This raises $cAMP$ levels via guanylate cyclase.     * Sildenafil acts as a PDE5 inhibitor, preventing the degradation of cyclic GMP by Phosphodiesterase 5.     * Increased $cAMP$ maintains low $Ca^{2+}$ levels, sustaining arterial smooth muscle relaxation, increased blood inflow, and erection.