Receptors, Hormones, & Cell Signaling Ch. 15

Chapter 15: Receptors, Hormones, and Cell Signaling

Overview

This chapter discusses the role of receptors in cell signaling, with a focus on G protein-coupled receptors (GPCRs) and their mechanisms of action. Key concepts include signal transduction pathways, the structure and function of GPCRs, methods of studying receptors, and the regulation of metabolic processes by signaling molecules.

15.1 Signal Transduction Pathways: From Extracellular Signal to Cellular Response

Key Points:

  • Cell Communication: Cells interact with their environment to sense stimuli (e.g., light, temperature, nutrients) to adapt and respond appropriately.
      - Example: Algae exhibit phototaxis; human retinal cells process light signals to the brain.

  • Hormones and Extracellular Signals: Cells communicate via chemical signals such as hormones and neurotransmitters, controlling various processes such as metabolism and gene expression. Hormonal signals can indicate stress (e.g., adrenaline) or nutrient availability (e.g., insulin).

  • Target Cells: These are specific cells that receive signals. The signaling molecule (ligand) binds to a receptor, typically leading to a conformational change in the receptor, activating downstream pathways.
      - The receptor is highly specific for its ligand and changes shape after binding.

  • Signal Transduction Pathways: These pathways include multiple steps, leading to various cellular responses. They may involve effector proteins like enzymes or transcription factors, ultimately influencing gene expression and cellular activity.

15.2 Studying Cell-Surface Receptors and Signal Transduction Proteins

Key Techniques:

  • Binding Assays: Used to quantify and determine the affinity of receptors for their ligands. Typically involves radioactively or fluorescently labeled ligands.

  • Western Blotting and Immunoprecipitation: Techniques to measure the activity of kinases and GTP-binding proteins by detecting protein expression and modifications (e.g., phosphorylation).

  • Affinity Chromatography: A method for purifying receptors, involving ligand-binding sites on beads that attract the receptor proteins from a mixture of membrane proteins.

  • Pull-Down Assays: Used to measure the activity of GTP-binding proteins by immobilizing their target proteins to isolate the active GTP-bound form.

15.3 Structure and Mechanism of G Protein–Coupled Receptors

Key Features of GPCRs:

  • Basic Structure: All GPCRs contain seven transmembrane alpha-helical regions with extracellular and intracellular segments. The N-terminus is extracellular, while the C-terminal tail is cytosolic.

  • Ligand Binding: GPCRs interact with various ligands including neurotransmitters and hormones, resulting in receptor activation and G protein engagement.

  • G Protein Activation: Upon ligand binding, GPCRs act as guanine nucleotide exchange factors (GEFs), facilitating the exchange of GDP for GTP on the associated G protein.

  • Diversity of Function: Approximately 800 GPCRs exist in humans, playing pivotal roles in varied physiological processes. Drugs targeting GPCRs represent about 35% of therapeutic agents.

15.4 Regulating Metabolism of Many Cells: GPCRs That Activate or Inhibit Adenylyl Cyclase

Role of Epinephrine:

  • Fight-or-Flight Response: Epinephrine binds to β-adrenergic receptors activating G protein-coupled pathways that promote energy mobilization through glycogenolysis (glycogen breakdown) and lipolysis (fat breakdown).

  • Adenylyl Cyclase Activation: GTP-bound α subunits activate adenylyl cyclase, leading to increased cyclic AMP (cAMP) levels that activate protein kinase A (PKA), which phosphorylates target proteins influencing glucose and fatty acid metabolism.

15.5 Regulating Protein Secretion and Muscle Contraction: Ions as Second Messengers

Calcium as a Second Messenger:

  • Cellular Responses to Calcium: Increases in cytosolic Ca²⁺ lead to various responses, including hormone secretion, neurotransmitter release, and muscle contraction.
      - Examples: Acetylcholine triggers secretion of digestive enzymes; thrombin activates platelet aggregation.

  • Release Mechanism: Inositol 1,4,5-trisphosphate (IP₃) facilitates Ca²⁺ release from the endoplasmic reticulum, further inducing cellular responses.

  • DAG Role: Diacylglycerol (DAG) activates protein kinase C (PKC), amplifying cellular signaling responses.

Key Concepts for Review

  1. Signal Transduction Components: Most cell signaling systems share common features such as receptor-ligand binding, activation of G proteins, and modulation of second messengers.

  2. Types of Chemical Signals: Distinctions between endocrine, paracrine, and autocrine signals based on their target cells' location and signaling processes.

  3. Receptor Dynamics: Understanding GPCR activation and the roles of intrinsic GTPases.

  4. Feedback Mechanisms: Mechanisms regulating GPCR activity include desensitization through phosphorylation and recruitment of β-arrestins, which initiate receptor internalization.

Conclusion

This chapter provides a comprehensive understanding of cellular communication mechanisms facilitated by receptors, emphasizing GPCRs and their extensive role in various physiological responses. For effective biological responses and homeostasis, cells must balance signal amplification and desensitization.