Lecture 5: Cell Signaling Overview

Introduction to Cell Signaling

  • Course: Biochemistry 384, Module 7, Topic 5

  • Topic: Overview of Cell Signaling Pathways

  • Importance: Cell signaling is a crucial aspect of biochemistry studied for decades.

  • Structure of Signaling Pathways:

    • Receptor Proteins: The key proteins that initiate the signaling pathways.

    • First Messengers: Include hormones, ligands, etc.

    • Second Messengers: Molecules that transmit signals within the cell following the activation by first messengers.

Overview of Cell Signaling Pathways

  • Cell signaling consists of a series of sequential steps leading to various cellular responses.

  • The initiation of signaling is generally due to a first messenger binding to a receptor protein, inducing a conformational change.

    • Example first messengers: Insulin, glucagon, epinephrine (adrenaline).

  • Signal Transduction: The term used when the signal passes through the membrane after a conformational change occurs in the receptor.

  • Downstream Effects: The receptor activation can lead to actions like enzyme activation, formation of binding partners, and generation of second messengers.

Structure of Cell Signaling Pathways

  • First Messengers:

    • Can be hormones that cause effects by binding to receptors.

  • Second Messengers: Examples include cyclic AMP (cAMP), cyclic GMP (cGMP), calcium ions (Ca²⁺).

  • Amplification: Signaling pathways amplify the initial signal, leading to multiple downstream effects.

Genetic Composition of Cell Signaling

  • Percentage of genes devoted to signaling varies across organisms:

    • Humans: About 9% of 22,000 genes are involved in cell signaling.

    • Comparison to lower organisms:

    • Roundworm (C. elegans): Lower percentage of genes for signaling.

    • Yeast: Approximately 3% of genes involved.

  • Conclusion: Increased complexity in organisms correlates with a higher number of signaling genes.

Historical Context: Discovery of Insulin

  • Insulin, a key first messenger, was discovered by scientists Banting and Best.

  • Discovery: They used dogs to purify insulin and received a Nobel Prize for their work.

  • Significance in medicine: Critical for understanding glucose regulation; advances in treatment of diabetes through insulin pumps and sensors.

Types of Signaling Mechanisms

  • Endocrine Signaling: Hormones are produced in glands and travel through the bloodstream to distant target cells.

  • Paracrine Signaling: Local signaling where hormones affect nearby cells.

  • Autocrine Signaling: Where a cell produces a hormone that binds back to its own receptors, forming a feedback loop.

Examples of First Messengers

  • Hormonal:

    • Beta estradiol (estrogen) - hormone for female reproduction.

  • Neurotransmitter:

    • Acetylcholine - involved in neural signal transmission.

  • Gas:

    • Nitric oxide (NO) - a non-hormonal first messenger.

  • Metabolites:

    • Glutamate (signaling amino acid), ATP (involved in cyclic AMP formation).

Example of a Signaling Pathway: Nitric Oxide in Kissing Bug

  • Kissing bug uses nitric oxide to assist in blood feeding with the protein nitroforein.

  • Mechanism:

    • Nitric Oxide binds to guanylate cyclase, producing cyclic GMP (cGMP) as a second messenger.

    • Result: Cause vasodilation in the host, allowing easier feeding.

    • Additional effect: Binding of nitric oxide also dampens the inflammatory response by interacting with histamine.

Understanding Second Messengers

  • Cyclic AMP (cAMP):

    • Derived from ATP; synthesized by adenylate cyclase.

    • Activates Protein Kinase A (PKA), which initiates downstream signaling.

    • Cyclic AMP Phosphodiesterase: Enzyme that degrades cAMP to AMP, thus providing an off switch for signaling.

  • Cyclic GMP (cGMP):

    • Similar to cAMP, produced by guanylate cyclase.

    • Cyclic GMP Phosphodiesterase: Degrades cGMP to dampen the signal.

Example: Viagra and Vasodilation

  • Viagra (Sildenafil) inhibits cGMP phosphodiesterase, leading to amplified effects of cGMP and prolonged vasodilation.

Other Second Messengers

  • Diacylglycerol (DAG) and Inositol trisphosphate (IP3) are produced from PIP2 when cleaved by phospholipase C.

  • Operate alongside calcium ions (Ca²⁺), which serve as second messengers that activate enzymes including Protein Kinase C (PKC) and calmodulin for downstream signaling.

Signal Amplification Importance

  • Amplification occurs when each step in the signaling pathway initiates additional signaling events, leading to rapid and specific cellular responses.

Focus on Receptor Classes

  • The next series of lectures will cover five key receptor classes:

    • Gated Ion Channels: Example - nicotinic acetylcholine receptor.

    • G Protein-Coupled Receptors (GPCRs): Target of many drugs.

    • Receptor Tyrosine Kinases (RTKs): Insulin and epidermal growth factor receptors.

    • TNF Receptors: Involved in apoptosis.

    • Nuclear Receptors: Include steroid hormone receptors.

  • Each receptor type initiates a different downstream signaling response but fundamentally starts with a receptor interaction.

Conclusion

  • Expect further and more detailed discussions on these receptor classes in upcoming lectures.