MPP Lecture 6 LO PDF

Learning Objectives: Cell Communication Fundamentals Distinguish between intercellular and intracellular communication Intercellular vs. Intracellular Communication & Signaling Mechanisms 1. Introduction to Cellular Communication Cellular Communication: The process by which cells det...

Learning Objectives: Cell Communication Fundamentals Distinguish between intercellular and intracellular communication Intercellular vs. Intracellular Communication & Signaling Mechanisms 1. Introduction to Cellular Communication Cellular Communication: The process by which cells detect, interpret, and respond to signals from their environment, which can occur within a cell (intracellular) or between cells (intercellular). Importance: Cellular communication is crucial for maintaining homeostasis, coordinating growth, differentiation, and responding to environmental changes. 2. Distinguishing Between Intercellular and Intracellular Communication Intracellular Communication: ○ Definition: Communication within a single cell. ○ Mechanisms: Signal Transduction Pathways: Series of molecular events initiated by a signal (e.g., hormone binding) that results in a cellular response. Second Messengers: Molecules like cAMP, Ca²⁺, and IP₃ that propagate the signal inside the cell. Gene Expression Regulation: Transcription factors activated by signaling pathways to regulate gene expression. ○ Types Ligand gated ion channels GPCRs Catalytic receptors Intracellular receptors ○ Examples: cAMP Pathway: Activation of protein kinase A (PKA) by cAMP leads to the phosphorylation of target proteins. Calcium Signaling: Ca²⁺ release from intracellular stores triggers various cellular responses, including muscle contraction and neurotransmitter release.  Intercellular Communication: ○ Definition: Communication between different cells. ○ Mechanisms: Signal Molecules: Hormones, neurotransmitters, cytokines, and growth factors. Receptors: Proteins on the cell surface or within the cell that bind signal molecules and initiate a response. Junctions: Direct cell-to-cell connections, like gap junctions, allowing small molecules to pass between cells. ○ Examples: Paracrine signaling: cells release signaling molecules that affect nearby cells within the same tissue. Endocrine Signaling: Hormones released by endocrine glands travel through the bloodstream to target distant organs. Autocrine signaling: cell produces signaling molecules that act on the same cell, leading to self-regulation. Direct Signaling (Juxtacrine): direct cell-to-cell contact, where membrane-bound signaling molecules interact with receptors on adjacent cells. Synaptic Signaling: in the nervous system, where neurons release neurotransmitters across a synapse to target adjacent neurons, muscle cells, or glands. Compare and contrast endocrine, paracrine, autocrine, direct, and synaptic signaling Comparison and Contrast of Signaling Mechanisms Endocrine vs. Paracrine: ○ Distance: Endocrine signals travel long distances Paracrine signals act locally. ○ Response Time: Endocrine responses are slower due to signal transport Paracrine responses are quicker. ○ Specificity: Endocrine signaling is less specific as it affects distant cells  Paracrine signaling is more specific, affecting only neighboring cells. Paracrine/Endocrine vs. Autocrine: ○ Target: Paracrine signals affect neighboring cells endocrine signaling affects distant cells Autocrine signals affect the signaling cell itself. ○ Regulation: Autocrine signaling is a form of self-regulation, often seen in growth control and immune responses. ○ Feedback: Autocrine signaling is often involved in feedback loops to regulate cellular activity. Direct vs. Synaptic: ○ Mechanism: Direct signaling involves physical contact between cells Synaptic signaling involves neurotransmitter release across a synapse. ○ Specificity: Synaptic signaling is highly specific, targeting only the cell at the synapse Direct signaling can affect any adjacent cell with the appropriate receptor. Receptor Types and Functions Differentiate between ligand-gated ion channels and G protein-coupled receptors (GPCRs) Ligand-Gated Ion Channels: Structure: Typically composed of multiple subunits that form a pore through the plasma membrane. Function: Act as channels that open or close in response to the binding of a specific ligand (e.g., neurotransmitter), allowing ions (such as Na⁺, K⁺, Ca²⁺, or Cl⁻) to flow across the membrane. Response Time: Extremely fast, typically within milliseconds.  Signal Transduction: Direct; the binding of the ligand to the receptor immediately changes the ion flow across the membrane, altering the cell's electrical potential and initiating a rapid cellular response. Examples: ○ Nicotinic acetylcholine receptor: Opens in response to acetylcholine, allowing Na⁺ ions to enter the cell, leading to depolarization and muscle contraction. ○ GABA-A receptor: Opens in response to GABA, allowing Cl⁻ ions to enter the cell, leading to hyperpolarization. G Protein-Coupled Receptors (GPCRs): Structure: Single polypeptide chain that spans the plasma membrane seven times (7-transmembrane domains). Function: Activate intracellular signaling pathways through the interaction with G proteins when a specific ligand (e.g., hormone, neurotransmitter) binds to the receptor, undergo conformational change. Response Time: Slower, prolonged response Signal Transduction: Indirect; the binding of the ligand activates the associated G protein, which then triggers various intracellular signaling cascades (e.g., activation of adenylate cyclase, phospholipase C, ion channels), leading to a broad range of cellular responses. Key Differences: Mechanism of Action: ○ Ligand-gated ion channels directly alter ion flow across the membrane ○ GPCRs initiate a signaling cascade through G protein activation. Response Time: ○ Ligand-gated ion channels act rapidly ○ GPCRs have a slower, more prolonged response. Signal Amplification: ○ ligand-gated ion channels have a direct and immediate effect. ○ GPCRs can amplify the signal through multiple steps of the signaling cascade Identify the five major classes of catalytic receptors and their specific ligands 1. Receptor Tyrosine Kinases (RTKs) Function: These receptors have intrinsic tyrosine kinase activity, meaning they can phosphorylate tyrosine residues on themselves (autophosphorylation) and on downstream signaling proteins. Ligands: ○ Epidermal Growth Factor (EGF) ○ Insulin ○ Platelet-Derived Growth Factor (PDGF) ○ Vascular Endothelial Growth Factor (VEGF) ○ Nerve Growth Factor (NGF) 2. Receptor Serine/Threonine Kinases Function: These receptors phosphorylate serine or threonine residues on themselves and downstream proteins. Ligands: ○ Transforming Growth Factor-β (TGF-β) ○ Bone Morphogenetic Proteins (BMPs) 3. Receptor Guanylyl Cyclases Function: These receptors catalyze the conversion of GTP to cyclic GMP (cGMP), which acts as a second messenger. Vascular smooth muscle Ligands: ○ Atrial Natriuretic Peptide (ANP) ○ Brain Natriuretic Peptide (BNP) 4. Receptor Tyrosine Phosphatases Function: These receptors remove phosphate groups from tyrosine residues on themselves or other proteins, counteracting the action of tyrosine kinases. Ligands: ○ No well-established ligands (These receptors are involved in cell adhesion and signaling processes, often interacting with other proteins rather than traditional ligands). 5. Tyrosine-Kinase-Associated Receptors  Function: These receptors do not have intrinsic kinase activity but are associated with cytoplasmic tyrosine kinases, such as Janus kinases (JAKs), that are activated upon ligand binding. Ligands: ○ Cytokines (e.g., Interleukins, Interferons) ○ Growth Hormone (GH) ○ Prolactin GPCR Signaling Mechanisms Explain G-protein activation (via GTP) and inactivation (via GDP) Describe key enzymes and second messengers in GPCR-mediated cellular responses Outline how phospholipase C activates IP3 and DAG increase intracellular calcium and activate protein kinase C G-protein-coupled receptor (GPCR) signaling is a key mechanism in cellular communication, and it involves the activation and inactivation of G-proteins, which transduce extracellular signals into intracellular responses. G-Protein Activation and Inactivation: 1. Activation (via GTP): ○ When a ligand binds to a GPCR, it causes a conformational change in the receptor. ○ This change allows the receptor to interact with a heterotrimeric G-protein (composed of α, β, and γ subunits). ○ The GPCR acts as a guanine nucleotide exchange factor (GEF), facilitating the exchange of GDP for GTP on the G-protein's α subunit. ○ Once GTP is bound, the α subunit dissociates from the βγ complex. Both the α-GTP and βγ subunits can then interact with downstream effectors to propagate the signal. 2. Inactivation (via GDP): ○ The G-protein’s intrinsic GTPase activity hydrolyzes GTP to GDP, inactivating the α subunit. ○ The α subunit then reassociates with the βγ subunits, returning the G-protein to its inactive state, ready for another activation cycle. Key Enzymes and Second Messengers in GPCR-Mediated Responses: Adenylate Cyclase: Activated by the Gαs subunit, this enzyme catalyzes the conversion of ATP to cyclic AMP (cAMP), a second messenger. ○ cAMP activates protein kinase A (PKA), which phosphorylates various target proteins to regulate cellular responses like metabolism and gene transcription. Phospholipase C (PLC): Activated by the Gαq subunit, PLC plays a crucial role in producing second messengers involved in calcium signaling. Phospholipase C Activation and Second Messenger Cascade: 1. Activation of IP3 and DAG: ○ Activated PLC hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2), a membrane phospholipid, into two important second messengers: inositol trisphosphate (IP3) and diacylglycerol (DAG). 2. IP3 and Calcium Release: ○ IP3 binds to its receptor on the endoplasmic reticulum (ER), causing the release of stored calcium into the cytoplasm. ○ The increase in intracellular calcium concentrations triggers various calcium-dependent processes, including muscle contraction and secretion. 3. DAG and Protein Kinase C (PKC) Activation: ○ DAG remains in the membrane and activates protein kinase C (PKC). ○ PKC, once activated, phosphorylates a variety of target proteins, influencing cellular responses such as cell growth, differentiation, and apoptosis. This signaling pathway involving PLC, IP3, DAG, and PKC is a critical part of many cellular processes, particularly those regulating calcium homeostasis and protein phosphorylation. Catalytic Receptor Signaling Explain guanylyl cyclase receptor function in increasing intracellular cGMP levels  Describe the structure and function of receptor tyrosine kinases (RTKs) Outline the activation of the Ras-MAP cascade through the EGFR receptor Guanylyl Cyclase Receptor Function in Increasing Intracellular cGMP: Guanylyl cyclase receptors are a type of catalytic receptor that, when activated by ligands such as natriuretic peptides or nitric oxide (NO), convert GTP to cyclic GMP (cGMP). cGMP acts as a second messenger, playing key roles in vasodilation, smooth muscle relaxation, and phototransduction in the retina. The receptor typically exists in two forms: 1. Membrane-bound guanylyl cyclase receptors: These receptors, like the atrial natriuretic peptide receptor, are integral membrane proteins with an extracellular ligand-binding domain and an intracellular guanylyl cyclase catalytic domain. 2. Soluble guanylyl cyclase (sGC): Activated by nitric oxide (NO), this enzyme is cytoplasmic and leads to increased cGMP production upon NO binding. Once activated, cGMP can activate protein kinase G (PKG), which phosphorylates target proteins to mediate cellular responses, such as relaxation of vascular smooth muscle. Structure and Function of Receptor Tyrosine Kinases (RTKs): RTKs are single-pass transmembrane proteins that act as receptors for growth factors, cytokines, and hormones. Structure: ○ Extracellular domain: Binds to specific ligands like growth factors (e.g., epidermal growth factor, EGF). ○ Transmembrane domain: Anchors the receptor in the cell membrane. ○ Intracellular tyrosine kinase domain: Contains the enzymatic activity responsible for autophosphorylation on specific tyrosine residues. Function: ○ Upon ligand binding, RTKs undergo dimerization or oligomerization, which brings the intracellular kinase domains into close proximity. ○ This leads to autophosphorylation on tyrosine residues within the intracellular domain, creating docking sites for various signaling proteins.  ○ Phosphorylated tyrosines serve as binding sites for SH2 domain-containing proteins, which propagate downstream signaling pathways involved in cell growth, differentiation, and survival. Activation of the Ras-MAP Kinase Cascade through EGFR: 1. EGFR Activation: ○ Epidermal growth factor receptor (EGFR), a type of RTK, is activated upon binding to its ligand, such as epidermal growth factor (EGF). ○ Ligand binding induces dimerization of EGFR, which leads to autophosphorylation of tyrosine residues on the receptor's intracellular domain. 2. Recruitment of Adapter Proteins: ○ The phosphorylated tyrosines on EGFR recruit Grb2, an adapter protein that contains an SH2 domain. ○ Grb2, in turn, binds to Sos (son of sevenless), a guanine nucleotide exchange factor (GEF) for Ras. 3. Activation of Ras: ○ Sos facilitates the exchange of GDP for GTP on the small GTPase Ras, activating Ras in its GTP-bound state. 4. MAP Kinase Cascade Activation: ○ Activated Ras interacts with Raf, a serine/threonine kinase, initiating the MAP kinase (mitogen-activated protein kinase) cascade. ○ Raf phosphorylates and activates MEK, which then phosphorylates ERK (extracellular signal-regulated kinase). ○ ERK translocates into the nucleus, where it phosphorylates transcription factors that regulate genes involved in cell proliferation, differentiation, and survival. This Ras-MAP kinase pathway is a critical mechanism in controlling cellular growth and development and is frequently dysregulated in cancers. Clinical Relevance Recognize the link between dysregulated tyrosine kinase signaling and diseases like cancer and Alzheimer's EGFR Signaling and Inhibition Explain the role of Gefitinib (an EGFR inhibitor) in the EGFR signaling pathway and its potential influence on transcription factors Myc, Jun, and Fos PI3K-AKT Signaling Pathway Identify the key components of the PI3K-AKT signaling pathway, including PI3K, AKT, Foxo, and mTOR 14. Recognize PTEN and mTOR inhibitors as potential therapeutic targets for cancer treatment and regulation of cell growth Nuclear Receptors Define the functions and mechanisms of nuclear receptors, focusing on their interaction with five major steroid hormones Identify androgen receptors and thyronine receptors as examples of nuclear receptors and describe their role in gene transcription regulation