Cell Signalling 1

Chapter 15: Cell Signaling

Overview of Cell Signaling

  • Cells, tissues, and organs can sense and respond to environmental changes.

  • Communication between distant cells requires messengers that can travel between cells.

General Principles and Characteristics of Signaling

  • Extracellular Signaling: Communication by signaling molecules released from a cell.

  • Intracellular Signaling: Signals processed within the cell.

  • Effector Proteins: Proteins that result in metabolic changes, altered gene expression, or changes in cell movement (e.g., transcriptional activity).

Types of Signaling

  • G-Protein-Coupled Receptors (GPCRs): Largest receptor family, involved in many physiological responses by activating G-proteins.

  • Enzyme-Coupled Receptors: Serve as an enzyme or activate other enzymes in signaling pathways.

Forms of Intercellular Signaling

  1. Contact-Dependent: Requires direct contact between cells.

  2. Paracrine Signaling: Local signaling where cells communicate over short distances.

  3. Synaptic Signaling: Involves neurotransmitter release across synapses.

  4. Endocrine Signaling: Hormonal signals travel through the bloodstream to distant target cells.

Key Concepts in Cell Signaling

  • Response Timing: Varies depending on the nature of the signal and type of response required.

  • Sensitivity and Adaptation: Cells can adjust their sensitivity to signals based on prior exposure.

  • Signal Processing: Simple signals can lead to complex cellular responses.

Molecular Switches and Signal Amplification

  • Molecular Switches: Proteins that toggle between active and inactive states, primarily via phosphorylation (protein kinases add phosphate, phosphatases remove phosphate).

  • Signal Amplification: Small amounts of signaling molecules can lead to large cellular responses; achieved through secondary messengers like cAMP.

GPCR Signaling Pathway

  1. Activation: Ligand binding activates GPCRs, leading to G-protein activation (GDP exchanged for GTP).

  2. G-Protein Activation: The GTP-bound alpha subunit dissociates, activating downstream effectors (e.g., adenylyl cyclase).

  3. Cyclic AMP: Produced from ATP by adenylyl cyclase, amplifies the signal and activates Protein Kinase A (PKA).

Target Proteins and Cell Responses

  • PKA phosphorylates various target proteins, modulating cellular functions such as metabolism and gene expression.

  • Responses can be fast (seconds) or slow (hours) depending on the downstream effects (e.g., enzyme activity vs. gene transcription).

Nuclear Receptors

  • Nuclear Receptors: Ligand-modulated transcription regulators that control gene expression by binding to DNA upon activation.

  • Hormones like steroid hormones can cross the cell membrane and bind to these receptors, leading to changes in gene transcription.

Adaptation to Signals

  • Prolonged exposure to a stimulus can lead to receptor inactivation, adaptation, or even production of inhibitory proteins which modulate the signaling response.

Conclusion

  • The complexity of intercellular signaling systems allows multicellular organisms to function cohesively, coordinating many different cell types and responses.