Cell Signaling Fundamentals

Flashcards covering the essential components, types, and pathways of cell signaling, including receptor types and their roles in development and disease.

Signaling Cell

The specific cell that produces and releases a signaling molecule to communicate with other cells.

Signaling Molecule

A chemical messenger, such as a peptide, lipid, or gas, released by a signaling cell to deliver information.

Receptor Protein

A protein located on or in a responding cell that specifically recognizes and binds to a signaling molecule.

Responding Cell

A cell that receives a chemical message through its receptors and changes its behavior as a result.

Adrenaline (Epinephrine)

A hormone released by the adrenal glands during stress that travels through the blood to trigger effects like a faster heartbeat.

Quorum Sensing

A process used by bacteria, like Streptococcus pneumoniae, to count neighbors and coordinate group behaviors like DNA uptake once a population density threshold is crossed.

Receptor Activation

The first step in cell signaling, where the binding of a signaling molecule causes a conformational change that turns the receptor "on."

Signal Transduction

A chain reaction or cascade of biochemical events inside a cell where one molecule activates the next, often resulting in signal amplification.

Amplification

A feature of signal transduction where each step in a cascade multiplies the effect, allowing a small amount of external signal to produce a large internal response.

Response

The cellular outcome of a signaling pathway, such as the activation of an enzyme or the turning on of gene transcription.

Termination

The process of shutting off a cellular response to prevent overreacting and to reset the system for new signals.

Endocrine Signaling

Long-distance signaling where molecules are released into the bloodstream and carried by circulation to reach distant target cells.

Paracrine Signaling

Short-distance signaling where molecules move by diffusion between nearby cells, typically traveling a maximum of $20$ cell-diameters.

Growth Factors

Molecules, often acting as paracrine signals, that stimulate target cells to grow, divide, or differentiate.

Sonic Hedgehog

A paracrine signaling molecule that helps position motor neurons in the spinal cord and determines the correct placement of digits on the hand.

Neuronal Signaling

A special case of short-range signaling where neurotransmitters are released to diffuse across a synapse to an adjacent neuron or muscle cell.

Synapse

The tiny physical gap between a neuron and its target cell across which signaling molecules diffuse.

Autocrine Signaling

A signaling method where the signaling cell and the responding cell are the same, often used to reinforce developmental decisions in embryos.

Contact-Dependent Signaling

The shortest-range signaling where a transmembrane protein on one cell binds to a receptor on a physically touching adjacent cell without any diffusion.

Ligand

A term for a signaling molecule that binds specifically to the ligand-binding site of a receptor protein.

Delta/Notch Signaling

A contact-dependent process in nervous system development where Delta-expressing cells become neurons and Notch-activated neighbors become glial cells.

Polar Signaling Molecules

Hydrophilic molecules that cannot cross the hydrophobic cell membrane and must bind to receptors on the cell surface.

Extracellular Domain

The part of a cell-surface receptor located on the outside of the cell that contains the ligand-binding site.

Transmembrane Domain

The portion of a receptor protein that spans the hydrophobic interior of the plasma membrane.

Cytoplasmic Domain

The interior portion of a cell-surface receptor that undergoes a conformational change to begin downstream signaling.

Nonpolar Signaling Molecules

Hydrophobic molecules, such as steroid hormones, that pass directly through the cell membrane to bind internal receptors in the cytosol or nucleus.

G Protein-Coupled Receptors (GPCRs)

A class of evolutionarily conserved receptors that, when bound to a ligand, couple with an intracellular G protein to initiate signaling.

Receptor Kinases

Cell-surface receptors that act as enzymes by transferring phosphate groups from ATP onto target molecules.

Phosphorylation

The chemical addition of a phosphate group to a protein, usually carried out by a kinase to activate the protein's function.

Phosphatases

Enzymes that remove phosphate groups from proteins (dephosphorylation), typically inactivating the protein.

Ligand-Gated Ion Channels

Channel proteins that open or close in response to the binding of a signaling molecule, allowing ions to flow across the membrane.

Kit Receptor

A receptor kinase that controls pigment production in vertebrates and has remained functionally conserved for over $500$ million years.

Dimerization

The process where two signaling receptors pair up after binding a ligand, such as when PDGF binds to its receptors.

Ras

A single-subunit G protein that becomes active when it swaps GDP for GTP, acting as a key target for receptor kinases.

MAP Kinase Pathway

A cascade of sequentially activating kinases that eventually enters the nucleus to phosphorylate transcription factors for cell division.

Receptor Overexpression

A signaling error where a cell has $10$ to $100$ times the normal number of receptors, leading to exaggerated responses and uncontrolled division.

Mutant Ras

A signaling error found in more than $30\%$ of cancers where Ras is locked in its active, GTP-bound state, keeping the MAP kinase pathway continuously active.