Lecture Notes: Cellular Signaling and Fight-or-Flight Response

Introduction

• Lecture Topic: Fight-or-flight response as a cellular signaling paradigm.

Key Concepts in Cellular Signaling

Dynamic and Transient Nature of Cellular Signaling

• Cellular signaling is dynamic and transient.
• Characteristics of signaling include rapid response and definitive timing.

Signal, Response, and Output

• Signal: Usually a molecule, outside or inside the cell.
• Response: Biochemical processes within the cell.
• Output: Biological effect or change.

Types of Signals

• Signals in cytoplasm that diffuse across the plasma membrane.
• Importance of transmembrane receptors: Allow a wider range of signals (small polar molecules, proteins, etc.).

Cellular Switches: G-proteins and Phosphorylation

G-proteins

• Bind guanidine nucleotides (GDP, GTP).
• GDP-bound (inactive/off), GTP-bound (active/on).
• Structure change upon GTP binding.
• Types: Small (monomeric) and trimeric G-proteins.

Phosphorylation

• A post-translational modification involving serine, threonine, and tyrosine.
• Catalyzed by kinases, reversed by phosphatases.
• A significant portion of the genome is dedicated to kinases.

Fight-or-Flight Response

Overview

• Uses G-protein coupled receptors (GPCRs).
• GPCR: 7 transmembrane helices, n-terminal outside, c-terminal inside.
• Involves adrenaline/epinephrine as a signal.

Signal Transduction Pathway

• Adrenaline binds to GPCR, activates trimeric G-protein.
• G-protein's alpha subunit changes from GDP to GTP.
• GTP-bound alpha subunit activates adenylate cyclase.
• Adenylate cyclase converts ATP to cyclic AMP (cAMP).
• cAMP activates protein kinase A (PKA).

Amplification and Feedback

• Amplification: Adenylate cyclase produces many cAMP molecules.
• Feedback: cAMP breakdown by specific enzymes.

Biological Response

• Glycogen breakdown in the liver to release glucose.
• Provides energy for immediate, rapid response.

Detailed Summary of Lecture Content

This section provides a comprehensive overview of the lecture's key points: The lecture began with an apology for the absence of candy (promised for the following week) and transitioned into the topic of cellular signaling, using the fight-or-flight response as a central example. The instructor highlighted the dynamic and transient nature of cellular signaling, emphasizing the importance of rapid response and precise timing in cellular processes. The core components of cellular signaling were defined as the signal (often an extracellular molecule), the response (intracellular biochemical events), and the biological output (the resulting cellular change). Different types of signals were discussed, with a focus on transmembrane receptors, which allow for a wider array of signal molecules compared to intracellular receptors. The lecture then delved into the molecular details of two crucial cellular switches: G-proteins and phosphorylation.

G-proteins: These proteins bind guanine nucleotides, existing in either an inactive GDP-bound state or an active GTP-bound state. The transition between these states involves a conformational change in the protein's structure, triggered by the binding of GTP. Both monomeric (small) and trimeric G-proteins were mentioned, with the trimeric form consisting of alpha, beta, and gamma subunits. The alpha subunit is the key player in GTP binding and subsequent activation.

Phosphorylation: This post-translational modification involves the addition of a phosphate group to serine, threonine, or tyrosine residues on a protein. Kinases catalyze this reaction, while phosphatases reverse it. The lecture highlighted the vast number of kinases encoded in the genome, indicating their significant role in cellular processes and their importance as therapeutic targets in disease. The fight-or-flight response served as a detailed case study. It begins with the binding of epinephrine (adrenaline) to a G-protein coupled receptor (GPCR), a transmembrane receptor with seven transmembrane helices. Ligand binding causes a conformational change in the GPCR, activating a trimeric G-protein. The alpha subunit then exchanges GDP for GTP, activating it. This activated alpha subunit subsequently binds to and activates adenylate cyclase. Adenylate cyclase catalyzes the conversion of ATP to cyclic AMP (cAMP), a second messenger that amplifies the signal. cAMP then activates protein kinase A (PKA), leading to a cascade of phosphorylation events.

The overall effect is the breakdown of glycogen in the liver, releasing glucose into the bloodstream to provide energy for the body's response to a perceived threat. The process involves amplification at the cAMP production step and feedback mechanisms that regulate cAMP levels.

The lecture also explored receptor tyrosine kinases (RTKs), another class of transmembrane receptors. Unlike GPCRs, RTKs have a single transmembrane domain and an intracellular kinase domain. Ligand binding induces dimerization, leading to cross-phosphorylation and activation of the kinase domains. This often results in downstream signaling cascades that ultimately affect gene transcription and cell proliferation. The example of epidermal growth factor (EGF) was used to illustrate this pathway. The lecture concluded by highlighting the significance of these signaling pathways in human health and disease. Aberrant signaling, such as constitutive activation (where a pathway is permanently "on"), can contribute to diseases like cancer. The substantial investment in developing drugs that target GPCRs and kinases reflects their importance as therapeutic targets. The lecture emphasized the complex interplay between different signaling pathways, including integration where multiple pathways converge to produce a coordinated response. The concepts of specificity, amplification, feedback, and integration were repeatedly underscored throughout the lecture.

Drug Targets and Therapeutic Implications

• GPCRs are major drug targets (45% of cellular receptors).
• Examples of drugs targeting GPCRs for various diseases.
• Computational modeling and bioinformatics in drug discovery.

Receptor Tyrosine Kinases (RTKs)

Overview

• Single transmembrane domain with an extracellular and kinase domain.
• Ligand binding induces dimerization and cross-phosphorylation.

Signaling Pathway

• EGF binds, causing dimerization and phosphorylation.
• Activation of small G-protein (Ras) and subsequent phosphorylation cascades.
• Often leads to gene transcription and cell proliferation.

Pathways and Disease

• Aberrant signaling pathways lead to diseases such as cancer.
• Kinase signaling pathways are key therapeutic targets.

Integration in Signaling Pathways

• Pathways often share signals and integrate to produce complex responses.

Conclusion

• Constitutive activation: Key concept where pathways are permanently on.
• Economic and therapeutic importance of targeting signaling pathways due to their role in disease.

These notes summarize key concepts and details from the lecture on cellular signaling, emphasizing the fight-or-flight response, the role of G-proteins and phosphorylation, and their implications in health and disease.