Cell Signaling and Signal Transduction Vocabulary

Introduction to Cell Signaling and Basic Elements

• Communication between Cells: Cell signaling and signal transduction represent the processes by which cells communicate and interpret messages to coordinate biological functions.

• The Basic Elements of a Cell Signaling System:     * Extracellular Messenger Molecules: These are the primary signals transmitted between cells.     * Autocrine Signaling: The signaling cell possesses receptors on its own surface that respond to the messenger it produces, effectively communicating with itself.     * Paracrine Signaling: Messenger molecules travel relatively short distances through the extracellular space to reach neighboring cells.     * Endocrine Signaling: Messenger molecules (hormones) enter the bloodstream to reach distant target cells throughout the body.

• Receptors and Ligands:     * Target Cells: These cells possess receptors (on the surface or internally) that receive the extracellular message.     * Ligand: A specific molecule that binds to a receptor.     * Recruitment: Signaling often involves cytoplasmic protein recruitment to the activated receptor.

• Signal Transduction Pathways:     * Protein Cascades: Signaling pathways consist of a series of proteins that alter the conformation of the next protein in the sequence.     * Signal Transduction: The overall process of converting an extracellular signal into an intracellular response.     * Phosphorylation and Dephosphorylation: The primary mechanism for altering protein conformation and activity.         * Kinases: Enzymes that add phosphate groups to proteins.         * Phosphatases: Enzymes that remove phosphate groups.     * Functional Changes via Phosphorylation:         * Activating or inactivating an enzyme.         * Increasing or decreasing protein-protein interactions.         * Changing the subcellular location of a protein.         * Triggering protein degradation.     * Target Proteins: These are the ultimate recipients of the message that change cell activity.

Chemical Nature of Extracellular Messengers and Receptors

• Classes of Extracellular Messengers:     * Small Molecules: Includes amino acids and their derivatives. Examples: acetylcholine, epinephrine, dopamine, glutamate, and $\gamma$-aminobutyric acid (GABA).     * Gases: Signaling molecules like Nitric Oxide ($NO$) and Carbon Monoxide ($CO$).     * Steroids: Derived from cholesterol. Examples include:         * Estradiol (an estrogen).         * Progesterone.         * Testosterone.         * Cortisol (a glucocorticoid).     * Eicosanoids: Molecules derived from arachidonic acid.     * Peptides and Proteins: For example, Insulin or Epidermal Growth Factor (EGF).     * Other Related Molecules: Thyroid hormone, Vitamin $D_3$, Retinoic acid, and Indole-3-acetic acid (an auxin plant hormone).

• Receptor Types:     * G-protein coupled receptors (GPCRs).     * Receptor protein-tyrosine kinases (RTKs).     * Ligand-gated channels.     * Steroid hormone receptors.

• Glucocorticoid Action Example:     * Glucocorticoids cross the plasma membrane and bind to an inactive receptor bound to $Hsp90$ in the cytosol.     * Binding releases $Hsp90$, and the receptors dimerize.     * The receptor dimer enters the nucleus, binds DNA, recruits coactivators and Histone Acetyltransferase (HAT), leading to the acetylation of nucleosomes and initiation of transcription.

Signal Transduction by G Protein-Coupled Receptors (GPCRs)

• GPCR Characteristics:     * Largest superfamily of proteins encoded by animal genomes (thousands of members).     * Contain seven $\alpha$-helical transmembrane domains.     * Interact with heterotrimeric G proteins.

• Natural Ligands for GPCRs:     * Hormones (animal and plant).     * Neurotransmitters.     * Opium derivatives.     * Chemoattractants (odorants, tastants, and photons).

• Physiological Examples of GPCR-Mediated Processes:     * Epinephrine: Binds $\beta$-Adrenergic receptor $\rightarrow$ activates Adenylyl cyclase $\rightarrow$ Glycogen breakdown.     * Serotonin: Binds Serotonin receptor $\rightarrow$ activates Adenylyl cyclase $\rightarrow$ Behavioral sensitization/learning in Aplysia.     * Light: Binds Rhodopsin $\rightarrow$ activates $cGMP$ phosphodiesterase $\rightarrow$ Visual excitation.     * IgE-antigen complexes: Binds Mast cell IgE receptor $\rightarrow$ activates Phospholipase C $\rightarrow$ Secretion.     * f-Met peptide: Binds Chemotactic receptor $\rightarrow$ activates Phospholipase C $\rightarrow$ Chemotactic response.     * Acetylcholine: Binds Muscarinic receptor $\rightarrow$ opens Potassium channel $\rightarrow$ Slowing of pacemaker activity.

• Mechanism of GPCR Activation:     1. Ligand binds to the extracellular domain, inducing a conformational change.     2. GDP is exchanged for GTP on the $G\alpha$ subunit of the heterotrimeric G protein.     3. $G\alpha$ dissociates from the $\beta\gamma$ complex and activates an effector (e.g., adenylyl cyclase).     4. The effector produces a second messenger (e.g., $cAMP$).

• G Protein Families:     * $G_s$: Couples receptors to adenylyl cyclase stimulation.     * $G_q$: Contains $G\alpha$ subunits that activate $PLC\beta$.     * $G_i$: Inhibits adenylyl cyclase.     * $G_{12/13}$: Less well characterized.

• The G Protein Cycle and Termination:     * Activity is terminated by the hydrolysis of GTP to GDP and $P_i$ by the $G\alpha$ subunit's intrinsic GTPase activity.     * RGS proteins (Regulators of G protein Signaling) stimulate this hydrolysis.     * Inactive GDP-bound $G\alpha$ reassociates with the $\beta\gamma$ complex.     * Arrestins: Binding of arrestin to a phosphorylated GPCR competes with G protein binding, leading to desensitization.     * Recycling: GPCRs can be internalized; if returned to the surface, the cell is "resensitized."

Second Messengers: Cyclic AMP (cAMP) and Bacterial Toxins

• Cyclic AMP ($cAMP$):     * Synthesized from ATP by Adenylyl cyclase.     * Degraded into AMP by $cAMP$ phosphodiesterase.     * Enables large-scale coordinated responses from a single extracellular signal.

• Bacterial Toxins Targeting Signaling:     * Cholera Toxin: Modifies $G\alpha$ to keep adenylyl cyclase active by inhibiting GTPase activity, leading to massive intestinal water loss.     * Pertussis Toxin: Inactivates $G\alpha$ subunits, resulting in immune defense inhibition (whooping cough).

• cAMP-Induced Glucose Mobilization:     * Triggered by Epinephrine or Glucagon.     * Adenylyl cyclase produces $cAMP$, which activates Protein Kinase A (PKA).     * PKA phosphorylates Phosphorylase kinase, which then activates Glycogen phosphorylase.     * Glycogen phosphorylase breaks down glycogen into Glucose-1-phosphate.     * Inhibition: PKA also phosphorylates Glycogen synthase, inactivating it to prevent concurrent glycogen storage.

• cAMP in Gene Expression:     * Activated PKA translocates to the nucleus.     * PKA phosphorylates CREB (cAMP Response Element-Binding protein).     * Phosphorylated CREB dimerizes and binds to CRE (cAMP Response Element) on DNA to initiate transcription of genes like those for gluconeogenesis.

Phosphatidylinositol-Derived Second Messengers and Ca2+ Signaling

• Phospholipid Signaling Molecules:     * Membrane phospholipids are converted to messengers by phospholipases, kinases, and phosphatases.     * $PLC\beta$ (Phospholipase C-$\beta$): Splits $PI(4,5)P_2$ into two second messengers:         1. Diacylglycerol (DAG): Remains in the membrane; recruits and activates Protein Kinase C (PKC). Phorbol esters mimic DAG and can cause malignant cell behavior.         2. Inositol 1,4,5-trisphosphate ($IP_3$): Diffuses into the cytosol; binds to $IP_3$ receptors (tetrameric $Ca^{2+}$ channels) on the Smooth ER.

• Calcium ($Ca^{2+}$) as a Messenger:     * Normal cytosolic levels are kept extremely low by active pumps.     * $IP_3$ binding opens ER channels, releasing $Ca^{2+}$ into the cytoplasm.     * Bursts of $Ca^{2+}$ release cause oscillations; triggered by stimuli like Vasopressin in the liver.     * Calmodulin: The best-studied calcium-binding protein; has 4 binding sites. When bound to $Ca^{2+}$, it undergoes a conformational change to activate effectors like CaM kinase II or MLCK.     * Other Ca-activated proteins: Troponin C (muscle), Calpain (protease), $\alpha$-actinin.

• Cellular Responses to IP3: Contraction in smooth muscle, actin polymerization in slime molds, shape change in blood platelets, and membrane depolarization in sea urchin eggs.

Receptor Protein-Tyrosine Kinases (RTKs) and the Ras-MAPK Pathway

• Receptor Protein-Tyrosine Kinases (RTKs):     * Activated by growth factors (PDGF, EGF) and differentiation factors.     * Activation Mechanism:         1. Ligand-mediated dimerization (e.g., PDGF) or Receptor-mediated dimerization (e.g., EGF).         2. Trans-autophosphorylation: Kinase domains phosphorylate each other on tyrosine residues.         3. The activation loop moves away from the substrate-binding site, activating the kinase.

• Downstream Signaling:     * Phosphotyrosine residues serve as binding sites for proteins with SH2 (Src-homology 2) or PTB (Phosphotyrosine-binding) domains.     * Adaptor Proteins: e.g., Grb2, which contains one SH2 and two SH3 domains (SH3 binds proline-rich regions).

• The Ras-MAP Kinase Pathway:     * Ras: A small monomeric G protein (GTPase) anchored to the plasma membrane.     * Active state: Ras-GTP; Inactive state: Ras-GDP.     * Accessory Proteins:         * GEFs (Guanine nucleotide-exchange factors): Stimulate GDP-to-GTP exchange (activates Ras). Example: Sos.         * GAPs (GTPase-activating proteins): Stimulate GTP hydrolysis (inactivates Ras).         * GDIs (Guanine nucleotide-dissociation inhibitors): Inhibit GDP release.     * MAPK Cascade: Ras activates Raf (MAPKKK) $\rightarrow$ Raf activates MEK (MAPKK) $\rightarrow$ MEK activates ERK (MAPK) $\rightarrow$ ERK phosphorylates transcription factors.

• Scaffolding Proteins: Tether pathway members together to ensure specificity and enhance interaction efficiency (e.g., $AKAPs$, $KSR$, yeast $Pbs2$).

Insulin Receptor Signaling and Glucose Homeostasis

• Insulin Receptor Structure: A dimer consisting of $\alpha$ and $\beta$ chains.

• Mechanism:     1. Insulin binds $\rightarrow$ RTK autophosphorylation.     2. Activation of IRS-1 and IRS-2 (Insulin Receptor Substrates).     3. PI 3-kinase (PI3K) binds IRS via SH2 domains and converts membrane lipids into $PIP_3$.     4. $PIP_3$ recruits PKB (Protein Kinase B) and PDK1.

• Regulation of Glucose Transport:     * Recruitment of GLUT4 transporters from intracellular vesicles (tethered by TUG protein) to the plasma membrane.     * Inhibition of GSK-3 kinase leads to an increase in Glycogen synthase activity, promoting glucose storage.

Sensory Perception and NO Signaling

• GPCRs in Sensory Systems:     * Vision: Rhodopsin (black-and-white) and color receptors in cones are GPCRs.     * Smell: Nasal epithelium contains over 400 types of odorant GPCRs.     * Taste: Receptors detect five basic qualities: salty, sour, sweet, bitter, and savory (umami).

• Nitric Oxide (NO) as a Messenger:     * Produced by Nitric oxide synthase.     * Mechanism: $\text{NO}$ diffuses to smooth muscle cells and activates Guanylyl cyclase to produce $cGMP$.     * $cGMP$ leads to smooth muscle relaxation and blood vessel dilation.     * Nitroglycerine: Used to treat angina because it is metabolized into $\text{NO}$, increasing heart blood flow.     * Viagra (Sildenafil): Inhibits $cGMP$ phosphodiesterase 5 (PDE5). This prevents the breakdown of $cGMP$, maintaining blood vessel dilation for male arousal. It does not inhibit PDE3 (vital for heart muscle contraction).