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Signal Transduction Part I: Understanding Communication between Cells and Tissues

Fundamental Concepts and Modes of Signaling

  • Intercellular signaling vs intracellular signaling: communication between cells/tissues vs signaling events inside a cell that translate external cues into responses.

  • Signaling is essential for coordinated tissue responses in health and disease (e.g., regulation of blood glucose levels).

  • Signaling occurs through organized networks where multiple receptors and pathways interact to produce integrated outcomes.

Receptor Superfamilies (Five Major Classes)

  • Receptor superfamily 1: Cytokine Receptors

  • Receptor superfamily 2: Ion-Linked Receptors

  • Receptor superfamily 3: Ennzyme-Linked

  • Receptor superfamily 4: G Protein-Coupled Receptors

  • These 4 families constitute the major classes of cell-surface in Part I; Part II will cover Ion Channel Receptors and G Protein-Coupled Receptors in more depth.

Intercellular Signaling and Regulation in the Intact Organism: Blood Glucose Example

  • An integrated example showing how signaling networks regulate glucose uptake, glucose synthesis, and storage in health and disease.

  • Signals involved include insulin, IGF-1, TNF-α, and reproductive/cytokine signals, which modulate pathways such as PI3K-AKT-mTOR and RAS-ERK to control metabolism and growth.

  • Key downstream nodes include:

    • PI3K, PDK1/2, AKT isoforms (AKT1/2/3), AS160

    • mTOR, GSK3, FOXO1, p90RSK

    • Ras/ERK cascade (ERK1/2, p90RSK) influencing protein synthesis and cell growth

  • The network demonstrates how signals from multiple receptors converge on shared intracellular pathways to regulate metabolic outcomes.

Signaling Networks: Interactions of Multiple Receptors and Pathways

  • Signaling networks underlie most physiological or pathological responses, not isolated pathways.

  • Nodes in networks (e.g., Node1, Node2, Node3) represent integrated signaling hubs such as PI3K, AKT/mTOR, ERK/MAPK, and their regulators (PTEN, SHC, IRS proteins, JAKs, STATs).

  • Example pathways shown in the figure include:

    • Receptor tyrosine kinases (RTKs) such as IGF1R and IR (insulin receptor)

    • TNF receptor signaling

    • Cytokine receptors and JAK-STAT signaling

    • Lipid-activated second messengers and downstream kinases (PDK1/2, PKC, AKT, mTOR)

  • The end result includes regulation of glucose uptake, protein synthesis, and cell growth/differentiation through coordinated network activity.

Major Concepts of Intercellular Signaling

  • Space and distance: target cell proximity, dilution of secreted signals, and how many cells are influenced by a signal.

  • Time and duration: time for signal to reach target cells and how long signaling persists (milliseconds to hours).

  • Modes of signaling:

    • Endocrine: long-range endocrine signals traveling through blood.

    • Paracrine: signaling to nearby cells.

    • Autocrine: signaling to the same cell that released the signal.

  • Juxtacrine (contact-dependent) signaling requires direct cell-cell contact for signal transmission.

Endocrine vs Paracrine vs Autocrine Signaling

  • Paracrine and autocrine signaling play crucial roles during embryonic development and immune responses after birth.

  • Endocrine signaling coordinates systemic physiology via circulating hormones.

  • Juxtacrine signaling provides a localized, contact-dependent mechanism frequently used in development and immune interactions.

Axon Synaptic Signaling: A Highly Specialized Paracrine Signaling in the CNS and PNS

  • Electrical stimulus triggers the release of acetylcholine (ACh) from a nerve terminal.

  • ACh binds to acetylcholine receptors on the muscle cell, activating Na^+ entry and local membrane depolarization.

  • Acetylcholinesterase degrades acetylcholine, terminating the signal.

  • The example illustrates fast, transient synaptic signaling with rapid onset and termination.

Juxtacrine Signaling in Development and Immunity

  • Contact-dependent signaling requires direct cell contact to transmit signals via membrane-bound ligands or receptors.

  • Critical for tissue patterning, cell differentiation, and immune cell communication.

Intracellular Signaling: Major Concepts

  • Extracellular signaling molecule binds to a receptor protein on or in the target cell.

  • Intracellular signaling proteins relay the signal to target proteins to elicit responses.

  • Target outcomes include:

    • Metabolic changes

    • Gene regulatory changes

    • Cytoskeletal rearrangements and altered cell shape/movement

  • Diagrammatic pathway: Extracellular signal molecule → Receptor protein → Intracellular signaling proteins → Target proteins (metabolic enzymes, transcription factors, cytoskeletal components).

Signaling by Secreted Hydrophobic Hormones and Vitamins; Intracellular Receptors

  • Hydrophobic ligands (steroids, some vitamins) cross the plasma membrane and bind intracellular receptors.

  • Receptors translocate to the nucleus and regulate gene expression directly (DNA binding as transcription factors).

  • Ligand-bound receptors often form homodimers or heterodimers to regulate target genes.

Cell Surface Receptor Superfamilies: Overview

  • Secreted signaling molecules that are too hydrophilic or too large to cross the plasma membrane engage cell surface receptors.

  • Two Major Classes of Receptors for Secreted Signaling Molecules:

    • Intracellular (nuclear) receptors for hydrophobic ligands (already covered).

    • Cell surface receptors, which include multiple superfamilies.

  • Cell surface receptor superfamilies discussed include:

    • Ion channel receptors

    • G protein-coupled receptors (GPCRs)

    • Enzyme-linked (catalytic) receptors

    • Cytokine receptors

  • The Ion Channel Receptors and GPCRs are the focus of Part II of the course.

Targeted Receptors and their Ligands

  • Receptors for most neurotransmitters are GPCRs or ion channels.

  • Receptor-subtype diversity explains tissue-specific responses and drug side effects.

  • Three or more receptor subtypes can be expressed in different tissues, illustrating why drugs targeting a single neurotransmitter system can have varied effects across tissues.

Receptor-Ligand Binding and Biological Response: Key Terms

  • Ligand: any molecule that binds to a receptor.

  • Agonist: a ligand that activates receptor function and transduces signal.

  • Antagonist: a ligand that prevents receptor activation and signal transduction.

  • Receptor + Ligand → Receptor-Ligand Complex → Physiological Response (signal transduction steps follow).

Receptor Occupancy and Pharmacodynamics: Kd vs EC50

  • In many physiological receptor-ligand pairs, maximal response occurs when only a fraction of receptors are occupied.

  • EC50 < Kd reflects the amplification inherent in intracellular signaling cascades.

  • This means a lower concentration of ligand is required to achieve half-maximal response than would be required to occupy half of the receptors at equilibrium.

Potency versus Efficacy: Examples

  • Agonist B is less potent (higher EC50) than agonist A but has equivalent efficacy (same maximal response).

  • Agonist C is equally potent (same EC50) as A but has lower efficacy (lower maximal response).

  • These concepts are critical for understanding drug actions and therapeutic profiles.

Receptor Subtypes and Pharmacological Implications

  • Receptors exist as multiple subtypes across tissues, contributing to tissue-specific responses and potential side effects of drugs targeting a single receptor type.

  • Example: three different acetylcholine receptor subtypes expressed by different genes in various tissues.

Second Messengers and Intracellular Signaling

  • Second messengers rapidly propagate signals from receptors to intracellular targets.

  • Major second messengers and related molecules:

    • 3',5'-Cyclic Adenosine Monophosphate (cAMP): 3',5'- ext{Cyclic~AMP}

    • 3',5'-Cyclic Guanosine Monophosphate (cGMP): 3',5'- ext{Cyclic~GMP}

    • Diacylglycerol (DAG)

    • Inositol 1,4,5-trisphosphate (IP3)

    • Ca^{2+} ions (calcium)

  • Lipid-derived second messengers also contribute to signaling cascades.

Phosphorylation and GTP-Binding Proteins as Molecular Switches

  • Protein phosphorylation (serine/threonine and tyrosine kinases) modulates enzyme activity, protein interactions, and transcription factor activity.

  • GTP-binding proteins (G proteins) act as molecular switches, toggling between active (GTP-bound) and inactive (GDP-bound) states to propagate signals.

  • Major kinase classes include:

    • Serine/Threonine-specific kinases

    • Tyrosine kinases

  • These switches are central to translating extracellular cues into intracellular responses.

Protein Kinase Cascades

  • Cascades amplify signals: receptor activation triggers a sequence of kinases that propagate and diversify the response.

  • Common kinase cascade components: membrane receptors, adenylate/guanlylate cyclases, PKA, PKG, PKC, Ca^{2+}-dependent kinases, and MAP kinases.

  • Outcomes of cascades include:

    • Regulation of metabolic enzymes

    • Regulation of gene expression

    • Regulation of protein synthesis and cytoskeletal dynamics

  • Example arrangement: Receptor → Ras/RAF → MEK → ERK (MAPK) → transcription factors → gene expression changes.

Termination of Signal Transduction by Cell Surface Receptors

  • Multiple strategies ensure signals are transient and properly controlled:

    • Decrease extracellular ligand concentration via diffusion, metabolism, or reuptake.

    • Down-regulation: internalization and degradation of functional cell surface receptors.

    • Desensitization: covalent modification or other inactivation of receptor signaling.

  • Common process: receptor sequestration into endosomes, lysosomal degradation, and inhibition by regulatory proteins.

  • These processes prevent perpetual signaling and restore cellular sensitivity.

Enzyme-Linked Receptors (Catalytic Receptors)

  • Agonist ligands are large polypeptides (hormones or growth factors).

  • Receptors often possess intrinsic enzyme activity; common activities include protein tyrosine kinases, protein tyrosine phosphatases, serine/threonine kinases, and guanylate cyclase activity.

  • Examples of catalytic domains include:

    • Guanylyl cyclases (receptor for ANP)

    • Receptor serine/threonine kinases (e.g., TGF-β and BMP receptor families)

    • Receptor tyrosine kinases (RTKs, e.g., EGFR family)

    • Receptor tyrosine kinase-associated receptors with associated tyrosine kinases (e.g., JAKs in cytokine signaling)

Four Types of Enzyme Activities in an Enzyme-Linked Receptor

  • A. Guanylyl cyclases (extracellular space ↔ cytosol) generating cGMP as a second messenger

  • B. Receptor serine/threonine kinases (e.g., TGF-β, BMP pathways)

  • C. Receptor tyrosine kinases (RTKs) that autophosphorylate and recruit downstream signaling proteins

  • D. Tyrosine-kinase-associated receptors with cytosolic tyrosine kinases (e.g., JAK-STAT pathways)

  • Additional activities include protein tyrosine phosphatases and other enzymatic modules attached to receptors

Growth Factor/Hormone Receptors with Intrinsic Tyrosine Kinase Activity

  • Dysregulation of insulin receptor signaling and related RTKs is a major contributor to insulin resistance and type 2 diabetes.

  • RTKs initiate signaling via ligand-induced dimerization and autophosphorylation of tyrosine residues in the cytoplasmic kinase domain.

  • Autophosphorylated tyrosines serve as docking sites for signaling proteins containing SH2 or PTB domains, propagating cascades (e.g., PI3K-AKT, RAS-MAPK).

Activation Steps in Receptor Tyrosine Kinase (RTK) Signaling

  • A ligand-induced dimer forms, bringing kinase domains into proximity.

  • Tyr kinase activity is activated and receptor tyrosines are phosphorylated (autophosphorylation).

  • Phosphotyrosines recruit signaling proteins; intracellular signaling is relayed to effectors.

  • Phosphorylation cascades lead to downstream responses such as gene expression changes and metabolic adjustments.

MAP Kinases and RTK Signaling

  • MAP kinases couple RTKs to gene expression changes required for growth, division, and metabolic regulation.

  • Activation of the MAPK cascade is a central link between surface receptor activation and transcriptional responses.

Oncogenic RTK Activation

  • Overexpression or mutations in growth factor-activated RTKs can drive oncogenesis (transforming oncogenes).

  • Example: HER2 (Human Epidermal Growth Factor Receptor 2) amplification/overexpression in breast cancer.

TGF-β and BMP Receptor Signaling

  • TGF-β receptor has intrinsic serine/threonine kinase activity; directly phosphorylates SMAD transcription factors, regulating gene expression.

  • BMP receptors employ similar Smad-dependent signaling pathways, affecting development and tissue homeostasis.

Cytokine Receptors: Structure and Signaling

  • Cytokine receptors are generally multimeric and bind large polypeptide ligands.

  • They lack intrinsic enzymatic activity but recruit non-receptor tyrosine kinases (e.g., JAKs).

  • This receptor class activates downstream STAT transcription factors in response to ligand binding.

Key Cytokines and Their Roles

  • Some cytokines regulate immune and inflammatory responses; many utilize JAK-STAT signaling pathways.

  • Note: Other cytokine receptor types (e.g., TNF receptors) can utilize kinases beyond JAKs and transcription factors beyond STATs.

Ligand-Activated Nuclear Receptors

  • Receptors are intracellular soluble proteins; localized to the nucleus or cytosol with translocation to the nucleus upon activation.

  • Ligands are small hydrophobic molecules such as steroids, certain vitamins (A and D), some fatty acids, or thyroid hormones.

  • Receptors function as homodimers or heterodimers.

Structure of Nuclear Receptor Subunits

  • Nuclear receptors contain modular domains: transcriptional activation domains, DNA-binding domains, dimerization domains, ligand-binding domains, and localization signals.

  • Common receptor examples include Glucocorticoid Receptor (GR), Thyroid Hormone Receptor (TR), Retinoid X Receptor (RXR), Vitamin D Receptor (VDR), Estrogen Receptor (ER), Progesterone Receptor (PR), and PPAR family (PPARα/β/γ).

  • Domains are arranged with N-terminal activation domain (A/B), DNA-binding domain (C), hinge region (D), and ligand-binding domain (E).

Mechanisms of Nuclear Receptor Activation and Signaling

  • A. Ligand binding to the receptor induces a conformational change enabling transcriptional activation.

  • B. Inactive receptor is held in the cytosol by inhibitory proteins (e.g., Hsp90) and translocates to the nucleus upon activation.

  • C. Active receptor binds to DNA at hormone response elements and recruits coactivator proteins to promote transcription.

  • D. Receptors can bind DNA either before or after dimerization with RXR to regulate target gene transcription.

  • Activation leads to recruitment of RNA polymerase II and initiation of transcription.

Context-Dependent and Cell-Specific Responses

  • Tissue- or cell-specific responses arise from integration of multiple intracellular signaling pathways initiated by different signaling molecules.

  • The same ligand can elicit different outcomes depending on the cellular context and the network of pathways engaged.

Signaling Networks: Insulin Resistance and Beyond

  • Insulin resistance arises from complex interactions within signaling networks involving insulin/IGF-1 receptors, IRS proteins, PI3K-AKT, and MAPK pathways.

  • This illustrates how dysregulated network signaling can lead to metabolic disease.

Questions and Review Prompts

  • What are the major receptor superfamilies and their core signaling modalities?

  • How do endocrine, paracrine, autocrine, and juxtacrine signaling differ in range and mechanism?

  • How do RTKs transduce signals from ligand binding to gene expression changes?

  • What roles do second messengers play in transducing extracellular signals to intracellular responses?

  • How do nuclear receptors regulate gene expression, and what determines tissue-specific responses?

  • What mechanisms terminate signaling to ensure proper cellular responses?

  • How do signaling networks contribute to disease states such as insulin resistance or cancer?

Notes on Figures and References

  • Diagrammatic networks show connections among insulin/IGF signaling, cytokine receptors, PI3K/AKT/mTOR, MAPK, and downstream targets like Glucose uptake and Protein synthesis.

  • Figures referenced: GPCR/ion channel signaling, RTK activation steps, MAPK cascades, receptor internalization and downregulation.

  • Texts cited: Boron & Boulpaep Medical Physiology (2nd ed.), Molecular Biology of the Cell (4th ed., MBOC) for figures and conceptual diagrams.

Summary of Key Concepts to Remember

  • Five receptor superfamilies (intrinsic enzyme activity, cytokine receptors, intracellular/nuclear, neurotransmitter-gated ion channels, GPCRs).

  • Endocrine, paracrine, autocrine, and juxtacrine modes of signaling with distinct spatial/temporal characteristics.

  • Receptor-ligand binding concepts: ligand, agonist, antagonist; EC50 vs Kd and signaling amplification.

  • Second messengers and their roles in rapid intracellular responses (cAMP, cGMP, DAG, IP3, Ca^{2+}).

  • Kinase cascades and the concept of molecular switches (GTPases) that propagate signals.

  • Termination mechanisms: ligand removal, receptor downregulation, receptor inactivation, sequestration.

  • Distinct receptor types: RTKs, serine/threonine kinases, tyrosine-kinase-associated receptors, guanylyl cyclases, cytokine receptors, nuclear receptors.

  • Context-dependent, tissue-specific signaling results from integration of multiple pathway inputs.