Cell Signaling and Communication Lesson 5.6

Cell Signaling

Introduction to Cell Signaling

Cell signaling is the process by which cells communicate with each other to coordinate their activities and respond to environmental changes. This communication is essential for maintaining homeostasis, regulating growth, and responding to external stimuli.

• Cell communication allows multicellular organisms to function as integrated units.

• Cells use signaling pathways to transmit information and elicit specific responses.

• Signaling can occur over short or long distances.

Pathway Diagram Analysis

Pathway diagrams illustrate the flow of information in signaling pathways, showing activators, inhibitors, and the sequence of molecular events.

• Identify activators and inhibitors in the pathway.

• Determine which molecules are involved in gene regulation or second messenger systems.

• Example: p53 is a protein that inhibits the cell cycle in response to DNA damage, while MDM2 inhibits p53, allowing cell cycle progression.

Topic 4.1: Cell Communication

Mechanisms of Cell Communication

Cells communicate through various mechanisms, including direct contact and the release of signaling molecules.

• Cell-to-cell contact: Physical interaction between cells via membrane-bound proteins.

• Paracrine signaling: Release of signaling molecules that diffuse to nearby target cells.

• Endocrine signaling: Signaling molecules travel long distances through the bloodstream.

• Only cells with the correct receptor protein can respond to a specific signal.

Basics of Cell Signaling

Steps in Cell Signaling

Cell signaling typically involves three main steps: reception, transduction, and response.

• Step 1: Reception

  • Ligand binds to the receptor, causing a conformational (shape) change.

  • This change transmits the signal inside the cell.

• Step 2: Transduction

  • Signal is relayed from the cell membrane to the nucleus via phosphorylation of proteins.

  • Phosphorylation is the addition of a phosphate group to a protein, often activating it.

• Step 3: Response

  • The final molecule in the pathway initiates transcription of target genes in the nucleus.

  • This leads to the production of proteins that cause the cell response.

Mechanisms of Ligand Delivery

Cell to Cell Contact vs. Paracrine Signaling

• Traditional Cell-Cell Contact:

  • Originating cell expresses a ligand on its membrane.

  • Target cell expresses receptor protein on its membrane.

• Gap Junctions:

  • Occur in animal cells; allow signals to quickly diffuse between cells.

• Plasmodesmata:

  • Plant cell-specific; pores open between cell walls for signal transfer.

• Paracrine Signaling:

  • Cell releases a signaling molecule (ligand) that moves to the target cell.

Topic 4.2: Reception and Transduction Basics

Membrane Bound vs. Intracellular Receptors

• Membrane Bound Receptors:

  • Hydrophilic ligands bind to receptors embedded in the cell membrane.

  • Ligand does not cross the membrane.

  • Signal is amplified and relayed to the nucleus via a transduction pathway.

• Intracellular Receptors:

  • Hydrophobic ligands diffuse across the membrane and bind to receptors inside the cell.

  • Signal travels directly to the nucleus to initiate cell response.

Signal Transduction Pathway

• Ligand binds to the receptor, causing a shape change and transmitting the signal.

• A second messenger (e.g., cyclic AMP) is activated and phosphorylates the next protein in the chain.

• This process continues until a transcription factor enters the nucleus and initiates the cell response.

Topic 4.3: Cell Response to Transduction

Types of Cellular Responses

• Cells use signaling to respond to their environment.

• Transduction pathways initiate transcription of target genes, resulting in protein production.

• Examples of cell responses:

  • Initiation of gene expression (protein synthesis)

  • Cell growth and entry into the cell cycle

  • Secretion of molecules

  • Initiation of cell death (apoptosis)

G Protein Coupled Receptors (GPCRs)

GPCR Signaling Example

• Ligand binds to the GPCR, causing it to phosphorylate and activate the G protein.

• Phosphorylated G protein activates the second messenger, initiating transduction and leading to the cell response.

Topic 4.4: Changes in Signal Transduction Pathways

Pathway Malfunction and Interference

• Mutations that change the shape or chemical affinity of signaling molecules, receptors, or transduction proteins can alter or stop the cell response.

• Medicines and toxins can activate or inhibit signaling pathways.

  • Example: Botulinum Toxin (Botox) breaks down SNARE protein, preventing neurotransmitter release and causing paralysis.

Topic 4.5: Feedback Mechanisms

Negative and Positive Feedback

• Negative Feedback: Slows down or stops a response, helping maintain homeostasis.

  • Example: Body temperature regulation via sweat glands.

• Positive Feedback: Amplifies or increases a response, often producing large systemic changes.

  • Example: Initiation and amplification of labor contractions during childbirth.

Feedback Mechanism Table

Type	Function	Example
Negative Feedback	Stops or slows response to maintain homeostasis	Body temperature regulation
Positive Feedback	Amplifies response to produce systemic change	Labor contractions

Negative Feedback Practice Example

• If enzyme 3 is inhibited in a metabolic pathway, substrate B will build up because it cannot be converted to intermediate C.

• If enzyme 5 is inhibited, the end product (isoleucine) will not be produced, affecting the pathway's regulation.

Additional info: Feedback mechanisms are crucial for regulating metabolic pathways and maintaining cellular and organismal stability.