Cell Signaling: Mechanisms, Pathways, and Monitoring in Cell Biology

Module 7: Cell Signaling

Overview

Cell signaling is a fundamental process by which cells communicate with their environment and with each other, enabling them to respond to external and internal cues. This module covers the major themes, mechanisms, and experimental approaches in cell signaling, focusing on receptor-ligand interactions, G proteins, signaling pathways, and methods for monitoring signaling events.

Module 7A: Signaling Themes

Cell Communication and Signaling

• Cell signaling is the process by which cells collect information from their environment and respond with appropriate actions at the correct time.

• Signals can induce diverse cellular responses such as growth, differentiation, or apoptosis.

• Extracellular signals can have different effects in different cell types, depending on gene expression at a given moment.

Key Terms in Cell Signaling

• Signal transduction: The process by which cells respond to ligands (signaling molecules) via receptors, which may be intracellular or extracellular.

• Signal transduction pathways: Chains of molecules inside the cell that relay signals from receptors to effectors.

Signaling Cascade

• A cascade is a relay event where the signal is passed from one molecule to another, often involving phosphorylation or dephosphorylation.

• Steps in a signaling cascade:

  1. Signal release in response to conditions.

  2. Production of signaling molecules.

  3. Signal travels to target proteins.

  4. Receptors bind signaling molecules and transduce the signal.

  5. Signal molecules are released, and the process resets.

Types of Signaling Modes

• Signaling modes differ in distance (how far the signal travels), speed (how quickly it travels), and sensitivity (how responsive the receptor is).

Types of Molecules Involved

• Proteins act as molecular switches (e.g., phosphorylation/dephosphorylation).

• Second messengers such as Ca2+ and cyclic AMP (cAMP) relay signals inside the cell.

Slow and Rapid Responses

• Some signaling systems generate both rapid and slow responses, allowing for immediate and long-term cellular changes.

• Rapid responses often involve changes in protein function; slow responses typically involve changes in gene expression.

Signaling Ranges

• Endocrine signaling: Long-distance, via hormones in the bloodstream.

• Neuronal signaling: Long-distance, via neurotransmitters.

• Paracrine signaling: Short-medium distance, affecting nearby cells.

• Autocrine signaling: Short-medium distance, affecting the same cell that released the signal.

• Juxtacrine signaling: Direct contact between signaling and target cells.

Big Picture Themes

• Cascade: Sequential relay of signals.

• Integration: Multiple receptors feed into a single downstream response.

• Feedback: Later events influence earlier steps (positive or negative).

• Amplification: Messenger molecules increase the strength and spread of the signal.

Module 7B: Receptor-Ligand Interactions and G Proteins

Three Classes of Surface Receptors

• Ion-channel-coupled receptors: Open or close ion channels upon activation (e.g., GABA receptor).

• G-protein-coupled receptors (GPCRs): Activate G proteins, involved in sensory perception and many signaling pathways.

• Enzyme-coupled receptors: Activate kinases, often leading to phosphorylation cascades.

GABA Receptor

• A ligand-gated ion channel that allows ion flow upon ligand binding, affecting neuronal signaling.

G Protein Signaling: GPCR and Its Ligand

• Ligand binding induces a conformational change in GPCR, enabling G protein interaction.

• GPCR acts as a GEF (guanine nucleotide exchange factor), exchanging GDP for GTP on the G protein's α subunit.

• GTP binding causes dissociation of the α subunit from the βγ complex, both of which can regulate downstream effectors.

• Signal termination occurs when the α subunit hydrolyzes GTP to GDP and reassociates with βγ.

G Protein Subunits and Types

• G proteins are composed of α, β, and γ subunits, each with multiple isoforms.

• Three broad types of Gα subunits:

  • Gi: Inhibitory, reduces cAMP.

  • Gs: Stimulatory, increases cAMP.

  • Gq: Activates phospholipase C.

Downstream of Gq Activation

• Activated GPCR stimulates phospholipase Cβ (PLCβ) via Gq protein.

• PLCβ hydrolyzes PIP2 into IP3 and DAG (second messengers).

• IP3 releases Ca2+ from the ER; DAG activates protein kinase C (PKC).

Downstream of Gs Activation

• GPCR activates adenylyl cyclase via Gs, increasing cAMP.

• cAMP activates protein kinase A (PKA), which phosphorylates CREB, stimulating gene transcription.

Examples of G Protein Signaling

• β-adrenergic receptors: Couple to Gs, increase cAMP (heart rate, contraction).

• α1-adrenergic receptors: Couple to Gq, activate PLC, increase Ca2+ (vasoconstriction).

• α2-adrenergic receptors: Couple to Gi, inhibit adenylyl cyclase, reduce cAMP.

Module 7B: Enzyme-Coupled Receptors

Types of Enzyme-Coupled Receptors

• Some have intrinsic phosphorylation capacity (e.g., receptor tyrosine kinases).

• Others are closely associated with kinases but lack intrinsic activity.

Tyrosine Kinases

• Largest class of kinases associated with receptors; phosphorylate tyrosine residues.

• Tyrosine kinases can be receptors or directly associated with receptors.

Serine/Threonine Kinases

• Phosphorylate serine or threonine residues; structurally similar due to reactive hydroxyl groups.

• Examples: PKA, PKC, TGFβ signaling.

Receptor Tyrosine Kinases (RTKs): Classical Model

• RTKs are inactive as monomers; ligand binding induces dimerization.

• Each receptor phosphorylates the other, activating enzymatic activity.

• Phosphorylation creates docking sites for other proteins, propagating the signal.

Serine/Threonine Kinase Example: TGF Beta Signaling

• Smad-dependent pathway regulates development, cancer, and cell death.

Module 7C: Signaling Pathways

EGFR Pathway

• EGF receptor (EGFR) is inactive as a monomer; EGF binding induces dimerization and autophosphorylation.

• Phosphorylated EGFR recruits SH2/SH3 domain proteins, activating three main routes: PLC, PI3K, and Ras.

Table: Ras Superfamily of Monomeric GTPases

Family	Some Family Members	Functions
Ras	H-Ras, K-Ras, N-Ras	Relay signals from RTKs
Rho	RhoA, Rac, Cdc42	Activate mTOR, cytoskeletal regulation
Rab	Rab1, Rab5, Rab7	Regulate vesicle traffic
Ran	Ran	Regulate nuclear import/export
Arf	Arf1-6	Regulate vesicle formation

EGFR Pathway: Ras Activation

• EGFR binds EGF, leading to dimerization and autophosphorylation.

• Adaptor proteins (Grb2, SOS) promote Ras GTP loading.

• GTP-bound Ras activates downstream effectors.

Downstream of Ras Activation

• MAP kinase cascade: Raf → MEK → Erk.

• Erk phosphorylates proteins, affecting gene expression and cell behavior.

EGFR Pathway: PI3K Activation

• PI3K phosphorylates PIP2 to generate PIP3, which recruits PH domain proteins (e.g., Akt).

• Akt regulates cell survival, growth, and metabolism.

EGFR Pathway: Summary

• EGFR activation leads to three main branches: Ras/MAPK, PI3K/Akt, and PLC/PKC/CaMK.

Insulin Signaling Pathway

• Insulin receptor is an RTK; associates with IRS1, which recruits SH2 domain proteins.

RTKs and GPCRs: Overlapping Signaling

• RTKs and GPCRs can activate parallel intracellular pathways, often converging on common kinases and effectors.

Module 7D: Monitoring Signaling

Key Methods for Monitoring Cell Signaling

Method	Measurement Target	Resolution	Timing
Western Blotting	Protein levels, post-translational modifications	Population	Endpoint (fixed cells)
Flow Cytometry	Proteins, modifications	Single-cell	Endpoint
Mass Spectrometry	Proteins, phosphorylation	Population/single-cell	Endpoint
FRET	Protein-protein interactions	Single-cell/high	Real-time
Reporter Gene Assays	Transcriptional activity	Population/single-cell	Real-time/time-lapse
RNA sequencing (RNA-seq)	RNA expression	Single-cell/population	Endpoint

Western Blotting

• Used to analyze protein levels and modifications (e.g., phosphorylation).

• Steps: Electrophoresis, transfer, blocking, antibody incubation, detection.

• Detection by chemiluminescence or fluorescence; provides size and relative concentration of proteins.

SDS PAGE

• Separates proteins based on size; used before Western blotting.

• Can separate antibody light and heavy chains.

RNAseq

• High-throughput sequencing to measure gene expression levels.

• Allows analysis of transcriptional output in response to signaling.

Module 7E: Turning Off Signals: Phosphorylation and Proteolysis

Proteolysis and Ubiquitin-Proteasome System

• Ubiquitin is covalently attached to target proteins via E1, E2, and E3 enzymes.

• Ubiquitinated proteins are targeted to the proteasome for degradation into peptide fragments.

Turning Off Signals: Desensitization Mechanisms

• Negative feedback

• Delayed feed-forward

• Receptor inactivation

• Receptor sequestration (endosome)

• Receptor destruction (lysosome)

Example: Turning Off TGF Beta Signaling

• SARA recruits phosphatase to reverse receptor phosphorylation.

• SnoN/Ski complex prevents Smad activation.

• Smad2/3 ubiquitination leads to proteasomal degradation.

Example: Turning Off GPCR Signaling

• GPCR kinase phosphorylates GPCR; arrestin binds, leading to desensitization and internalization.

Additional info:

• Some figures and tables were inferred and described based on standard cell biology knowledge.

• Key terms such as ligand, receptor, second messenger, and phosphorylation are foundational for understanding cell signaling.

• Equations:

  • cAMP production: $\text{ATP} \xrightarrow{\text{adenylyl cyclase}} \text{cAMP} + \text{PPi}$

  • Phosphorylation: $\text{Protein} + \text{ATP} \xrightarrow{\text{Kinase}} \text{Protein-P} + \text{ADP}$

  • GTPase cycle: $\text{GTP} \xrightarrow{\text{hydrolysis}} \text{GDP} + \text{P}_i$