Cell Communication Flashcards

Cell Communication

9.1 Signaling Molecules and Cellular Receptors

Learning Objectives

• Describe four types of signaling mechanisms found in multicellular organisms.

• Compare internal receptors with cell-surface receptors.

• Recognize the relationship between a ligand’s structure and its mechanism of action.

Four Categories of Signaling

• Ligand: A molecule that brings a signal to a cell and initiates a response.

• Autocrine signaling: Signaling cells that can also bind to the ligand that is released, such that the signal and target cell can be the same or similar to each other (e.g., cell death signaling).

• Direct signaling across gap junctions: Intracellular mediators that allow small signaling molecules to move between cells.

• Paracrine signaling: Signals move by diffusion through the extracellular matrix; examples include synaptic signals and neurotransmitters.

• Endocrine signaling: Signals from distant cells, typically producing a slower response with a long-lasting effect (e.g., hormones).

Paracrine Signaling Example

• Synapse between nerve cells: The distance between the presynaptic cell and the postsynaptic cell is very small, allowing for rapid diffusion of the neurotransmitter.

• Enzymes in the synaptic cleft degrade some types of neurotransmitters to terminate the signal.

Cell-Surface Receptors

• Ion channel-linked receptors

• G-protein-linked receptors

• Enzyme-linked receptors

Cell-Surface Receptors Example: Ion Channel-Linked Receptor

• Gated ion channels form a pore through the plasma membrane that opens when the signaling molecule binds.

• The open pore then allows ions to flow into or out of the cell.

G-Protein-Linked Receptor Example

• When a signaling molecule binds to the G-protein-coupled receptor, the G protein α subunit exchanges GTP for GDP.

• The α subunit dissociates from the β and γ subunits and triggers a cellular response.

• GTP is hydrolyzed to GDP.

Enzyme-Linked Receptor Example

1. A receptor tyrosine kinase is an enzyme-linked receptor with a transmembrane region, and extracellular and intracellular domains.

2. Binding of a signaling molecule to the extracellular domain causes the receptor to form a dimer.

3. Tyrosine residues on the intracellular domain are then autophosphorylated, triggering a downstream cellular response.

4. The signal is terminated by a phosphatase that removes the phosphates from the phosphotyrosine residues.

Internal Receptors

• Hydrophobic signaling molecules typically diffuse across the plasma membrane and interact with intracellular receptors in the cytoplasm because they can easily cross the phospholipid bilayer.

• Many intracellular receptors are transcription factors that interact with DNA in the nucleus and regulate gene expression.

Signaling Molecules

• Small hydrophobic ligands

  • Steroid hormones have similar chemical structures to their precursor, cholesterol.

  • Can diffuse directly across the plasma membrane into the cell, where they interact with internal receptors.

• There are also water-soluble ligands (typically bind to cell-surface receptors) and gas ligands (e.g., nitric oxide).

9.2 Propagation of the Signal

Learning Objectives

• Explain how the binding of a ligand initiates signal transduction throughout a cell.

• Recognize the role of phosphorylation in the transmission of intracellular signals.

• Evaluate the role of second messengers in signal transmission.

Propagation of the Signal

• Signal transduction: When a ligand binds to a receptor, and the signal is transmitted through the cell membrane and into the cytoplasm, continuing the signal.

• Dimerization: Two receptors bind to each other to form a stable complex.

• Signaling pathway: A chain of events including second messengers, enzymes, and activated proteins that follow ligand binding to a receptor.

• Signal integration: Signals from two or more different cell-surface receptors merge to activate the same response in the cell.

Example of a Signaling Pathway: EGFR

• The epidermal growth factor (EGF) receptor (EGFR) is a receptor tyrosine kinase involved in the regulation of cell growth, wound healing, and tissue repair.

• When EGF binds to the EGFR, a cascade of downstream events causes the cell to grow and divide.

• If EGFR is activated at inappropriate times, uncontrolled cell growth (cancer) may occur.

Second Messengers

• First messenger: The ligand that brings a signal to a cell.

• Second messenger: A small molecule that propagates a signal within the cell.

• Example: Cyclic AMP (cAMP) serves as a second messenger to activate or inactivate proteins within the cell.

• Termination of the signal occurs when an enzyme called phosphodiesterase converts cAMP into AMP.

9.3 Response to the Signal

Learning Objectives

• Describe how signaling pathways direct protein expression, cellular metabolism, and cell growth.

• Recognize the role of apoptosis in the development and maintenance of a healthy organism.

Response to the Signal: Gene Expression

• Gene expression as a response to cell signaling: ERK is a MAP kinase that activates translation when it is phosphorylated.

• ERK phosphorylates MNK1, which in turn phosphorylates eIF-4E.

• When eIF-4E becomes phosphorylated, the mRNA unfolds, allowing protein synthesis in the nucleus to begin.

Responses to Cell Signaling

• The same signal may produce different responses in different cells.

• Increase in cellular metabolism

  • Adrenaline activates β-adrenergic receptors.

  • These increase cyclic AMP (cAMP), which activates PKA.

  • PKA phosphorylates two enzymes, which lead to a ready supply of glucose and an increase in metabolism.

• Cell growth

  • Growth factors bind to tyrosine kinases.

  • These initiate a pathway (including a G-protein called RAS) which activates the MAP kinase pathway.

  • MAP Kinase stimulates protein expression that eventually leads to cell division.

Programmed Cell Death: Apoptosis

• Cell signals are terminated by degradation of ligands or by other signals.

• Ex. Phosphatases: enzymes that remove the phosphate group attached to proteins by kinases.

9.4 Signaling in Single-Celled Organisms

Learning Objectives

• Describe how single-celled yeasts use cell signaling to communicate with one another.

• Relate the role of quorum sensing to the ability of some bacteria to form biofilms.

Signaling in Single-Celled Organisms: Yeast

• Yeast cells can communicate by releasing a signaling molecule called mating factor.

• Mating factor binds to cell-surface receptors in nearby yeast cells.

• They stop their normal growth cycles and initiate a cell signaling cascade that includes protein kinases and GTP-binding proteins that are similar to G-proteins.

Signaling in Single-Celled Organisms: Bacteria

• In bacteria, population density is often the key factor for signaling.

• Bacterial signaling is called quorum sensing because it occurs when there are a sufficient number of cells.

• It utilizes molecules called autoinducers.