Signal Transduction Mechanisms II: Messengers and Receptors

Overview of Intercellular Communication and Signal Transduction

• Regulatory Chemical Messengers: Intercellular communication is primarily achieved through regulatory chemical messengers. The process involves cells producing signals by releasing a chemical signal or by displaying molecules on their surfaces. Multicellular organisms utilize these signals to control the activities of specialized cells.

• Signal Transduction Definition: Signal transduction is the ability of a cell to respond to ligand-receptor binding by altering its behavior or gene expression. This process often involves a cascade of biochemical changes in the receiving cell.

Classification of Signaling Molecules by Distance

• Endocrine Signals: These are produced at a site far from the target tissues. They reach their targets by traveling through the circulatory system (bloodstream). Hormones are a primary example of endocrine signals.

• Paracrine Signals: These are diffusible signals that act over a short range on nearby target cells. They are often referred to as local mediators.

• Juxtacrine Signals: These signaling events require physical contact between the sending cell and the receiving (target) cell.

• Autocrine Signals: These molecules act on the same cell that produced and released them.

Receptor-Ligand Interactions and Quantitative Dynamics

• Ligands: A messenger that binds to a target receptor is called a ligand. Ligands can bind to receptors located on the surface of the cell or inside the cytosol.

• Receptor Occupancy and Saturation: The binding of a receptor and ligand is analogous to the binding of an enzyme and its substrate. A receptor is considered "occupied" when bound to its ligand.     * As ligand concentration increases, the system reaches saturation, meaning most receptors are occupied.     * Receptor Affinity: This describes the relationship between the ligand concentration in solution and the number of receptors occupied.     * Dissociation Constant ($K_d$): This often represents the hormone concentration at which $50\%$ of receptors are bound.

• Specificity of Binding: Messengers bind via highly specific noncovalent bonds. Specificity is determined by:     * The binding site (or binding pocket) on the receptor fits the messenger very closely.     * The specific positioning of amino acid side chains that form chemical bonds with the messenger.

• Synthetic Ligands:     * Agonists: Synthetic drugs that bind to and activate the receptor they are bound to.     * Antagonists: Molecules that bind to receptors without triggering a change, effectively preventing the naturally occurring messenger from activating the receptor.

Signal Amplification and Information Flow

• The Signaling Cascade: Information flows from the primary messenger (ligand) to receptor-ligand binding, then through signal transduction (often via second messengers) in the cytosol, leading to cellular responses and/or changes in gene expression in the nucleus.

• Signal Amplification: Very small quantities of ligand are sufficient to elicit a massive response due to multiplication effects at each step of the cascade. At each step, a signaling intermediate stimulates the production of many molecules for the next step.

• Example: Epinephrine Signaling Pathway:     * 1 molecule of Epinephrine binds to a G protein-coupled receptor.     * $10^2$ molecules of G protein are activated.     * $10^2$ molecules of Adenylyl cyclase are activated.     * $10^4$ molecules of Cyclic AMP ($\text{cAMP}$) are produced from ATP.     * $10^4$ molecules of Protein Kinase A ($\text{PKA}$) are activated.     * $10^5$ molecules of Phosphorylase kinase are activated.     * $10^6$ molecules of Glycogen phosphorylase are activated.     * $10^8$ molecules of Glucose-1-phosphate are released from glycogen.

G Protein-Coupled Receptors (GPCRs)

• Structure: GPCRs are named because ligand binding induces a conformational change that activates a G protein. They typically feature seven-transmembrane helices.

• G Protein Definition: A G protein is a guanine-nucleotide binding protein. They function as molecular switches.     * "On" State: Bound to guanosine triphosphate ($\text{GTP}$).     * "Off" State: Bound to guanosine diphosphate ($\text{GDP}$).

• Heterotrimeric G Proteins: These consist of three subunits: $G_\alpha$, $G_\beta$, and $G_\gamma$.

• Activation Cycle of G Proteins:     1. Resting State: Receptor is not bound to ligand; $G_\alpha$ is bound to $\text{GDP}$ and associated with $G_{\beta\gamma}$.     2. Ligand Binding: The receptor binds a G protein; $G_\alpha$ releases $\text{GDP}$ and acquires $\text{GTP}$.     3. Separation: $G_\alpha$ and $G_{\beta\gamma}$ subunits dissociate.     4. Transduction: Subunits activate or inhibit target proteins.     5. Hydrolysis: The $G_\alpha$ subunit hydrolyzes bound $\text{GTP}$ to $\text{GDP}$, becoming inactive.     6. Recombination: Subunits recombine to form the inactive G protein complex.

• Subunit Signaling Examples:     * $\beta\gamma$ Subunits: In the muscarinic acetylcholine receptor, the $\beta\gamma$ subunit causes potassium channels in the membrane to open.     * $\alpha$ Subunits: Often activate or inhibit enzymes like adenylyl cyclase.

Second Messengers: Cyclic AMP (cAMP)

• Formation: $\text{cAMP}$ is formed from cytosolic $\text{ATP}$ by the enzyme adenylyl cyclase. Adenylyl cyclase remains inactive until bound to activated $G_{s\alpha}$ (stimulated by ligand binding and $\text{GTP}$ acquisition).

• Function: The primary target of $\text{cAMP}$ is Protein Kinase A ($\text{PKA}$). $\text{cAMP}$ regulates $\text{PKA}$ by separating its regulatory and catalytic subunits.

• Action: Active $\text{PKA}$ phosphorylates various proteins on serine or threonine residues using $\text{ATP}$ as a phosphate source.

• Degradation: $\text{cAMP}$ is converted to $\text{AMP}$ by the enzyme phosphodiesterase ($\text{H}_2\text{O}$ is required).

The IP3, Diacylglycerol (DAG), and Calcium Pathway

• Mechanism: A ligand binds a receptor, activating the G protein $G_q$. $G_q$ activates Phospholipase C ($\text{PLC}_\beta$).

• Cleavage: Phospholipase C cleaves Phosphatidylinositol-4,5-bisphosphate ($\text{PIP}_2$) into two second messengers:     1. Inositol-1,4,5-trisphosphate ($\text{IP}_3$): A water-soluble molecule released into the cytosol.     2. Diacylglycerol ($\text{DAG}$): A hydrophobic molecule that remains in the plasma membrane.

• Secondary Actions:     * $\text{IP}_3$ binds to the $\text{IP}_3$ receptor channel (a ligand-gated calcium channel) on the Endoplasmic Reticulum ($\text{ER}$), releasing $Ca^{2+}$ into the cytosol.     * $\text{DAG}$ activates members of the Protein Kinase C ($\text{PKC}$) family.     * $Ca^{2+}$ and $\text{DAG}$ together activate $\text{PKC}$, which luego phosphorylates serine and threonine residues on target proteins.

Calcium as a Signal Mediator

• Concentration Maintenance: Cytosolic $Ca^{2+}$ levels are kept low through:     * Calcium ATPases: In the plasma membrane (transport $Ca^{2+}$ out of the cell) and the $ER$ (sequester $Ca^{2+}$ in the $ER$ lumen).     * Sodium-Calcium Exchangers: Reduce cytosolic concentration further.     * Mitochondria: Transport $Ca^{2+}$ into the mitochondrial matrix.

• Calcium Release: Triggered by $\text{GPCR}$ signaling (activating $\text{PLC}$) or neuronal signaling (opening channels in the plasma membrane).

• Calcium-Induced Calcium Release: The $\text{IP}_3$ receptor and ryanodine receptor channels are sensitive to calcium. A rapid increase in $Ca^{2+}$ can cause ryanodine receptor channels to open, releasing more $Ca^{2+}$ from the $ER$.

• Calmodulin: A mediator protein for calcium-activated processes. It has an "arm and hand" structure. Each calmodulin molecule binds four $Ca^{2+}$ ions (two per globular "hand"). Upon binding, it changes conformation to wrap around and activate target proteins.

Protein Kinase-Associated Receptors

• Definition: Receptors that also function as kinases. Ligand binding stimulates their kinase activity, triggering a phosphorylation cascade.

• Receptor Tyrosine Kinases (RTKs):     * Structure: Usually a single polypeptide chain with one transmembrane segment. The extracellular side is the ligand-binding domain, and the cytosolic side is the tyrosine kinase domain.     * Examples: Insulin receptor, Insulin-like growth factor-1, Fibroblast growth factor ($\text{FGF}$), Epidermal growth factor ($\text{EGF}$), Nerve growth factor ($\text{NGF}$).

• RTK Activation:     * Dimerization/Clustering: Ligand binding (e.g., $FGF$) causes receptors to dimerize or (e.g., $EGF$) causes receptor clustering.     * Autophosphorylation: Receptors phosphorylate each other on tyrosine residues because they phosphorylate the same type of receptor as themselves.

The Ras/MAP Kinase Pathway

• Recruitment: Phosphotyrosine residues are recognized by cytosolic proteins with SH2 domains (Src Homology 2).

• Ras Activation:     * $GRB2$ (containing an SH2 domain) binds the receptor and recruits Sos.     * Sos is a guanine-nucleotide exchange factor ($\text{GEF}$) that stimulates Ras (a small monomeric G protein) to release $\text{GDP}$ and bind $\text{GTP}$.

• Phosphorylation Cascade:     1. Raf: The first protein kinase in the cascade, activated by Ras.     2. MEK: Phosphorylated by Raf on serine and threonine residues.     3. MAP Kinases (MAPKs): Phosphorylated by MEK on threonine and tyrosine residues.

• Nuclear Function: Activated MAPKs enter the nucleus to phosphorylate transcription factors (e.g., Jun and Ets family members) to regulate genes needed for cell growth and division.

• Inactivation: GAP (GTPase activating protein) facilitates the hydrolysis of $\text{GTP}$ on Ras, returning it to an inactive $\text{GDP}$-bound state.

Receptor Mutations

• Dominant Negative Mutations: Mutant receptors dimerize with normal receptors but interfere with normal function, overriding the healthy receptors.

• Constitutively Active Mutations: Mutations that cause receptors to stay "on" even without a ligand.     * Achondroplasia: Caused by a dominant mutation in $\text{FGFR-3}$.     * Cancer: Certain constitutively active $\text{EGFR}$ mutations are linked to various cancers.

Alternative Signaling and Integration

• Receptor Serine-Threonine Kinases: The Transforming Growth Factor $\beta$ ($\text{TGF}\beta$) family is the major class. Signaling initiates when the growth factor binds the receptor, eventually phosphorylating Smad proteins.

• Signal Integration and Scaffolding: Cells coordinate multiple signals using scaffolding proteins like Ste5 in yeast mating. Ste5 recruits kinases (Ste20, MEKK, MEK, MAPK) into a large complex at the plasma membrane to increase efficiency.

• Signaling Crosstalk: Occurs when components of one pathway affect components of another. It represents a network of biochemical pathways rather than linear sequences.

Specialized Signaling: Hormones and Gases

• Hormone Categories:     1. Amino Acid Derivatives: (e.g., Epinephrine).     2. Peptides: (e.g., Vasopressin).     3. Proteins: (e.g., Insulin).     4. Lipid-like Hormones: Steroids (e.g., Testosterone, Progesterone, Estrogen, Glucocorticoids).

• Steroid Hormones: Hydrophobic molecules that pass through the plasma membrane. They bind to cytosolic receptors (e.g., Glucocorticoid receptor bound to Heat shock proteins/Hsp). The hormone-receptor complex moves into the nucleus to activate or inhibit gene transcription.

• Gaseous Signaling:     * Nitric Oxide (NO): Particularly important in the nervous system.     * Ethylene Gas: Mediates fruit ripening in plants.     * $\text{O}_2$ and $\text{CO}_2$: Key long-range signals in animal respiration.