Cell Communication in Multicellular and Single-Celled Organisms

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 signal and target cell can be the same or similar to each other.

Direct signaling across gap junctions

Intracellular mediators that allow small signaling molecules to move between cells.

Paracrine signaling

Signals that move by diffusion through extracellular matrix, examples include synaptic signals and neurotransmitters.

Endocrine signaling

Signals from distant cells, typically produce a slower response with a long-lasting effect, such as hormones.

Paracrine signaling example

Synapse between nerve cells where the distance between the presynaptic cell and the postsynaptic cell allows for rapid diffusion of the neurotransmitter.

Ion channel-linked receptors

Receptors that form a pore through the plasma membrane that opens when the signaling molecule binds, allowing ions to flow into or out of the cell.

G-protein-linked receptors

A type of cell-surface receptor that interacts with G-proteins to transmit signals inside the cell.

Enzyme-linked receptor

A receptor that has an enzymatic activity and is involved in signaling pathways.

Receptor tyrosine kinase

An enzyme-linked receptor with a transmembrane region, and extracellular and intracellular domains.

Dimerization

The process where binding of a signaling molecule to the extracellular domain causes the receptor to form a dimer.

Autophosphorylation

The process where tyrosine residues on the intracellular domain are phosphorylated after dimerization, triggering a downstream cellular response.

Phosphatase

An enzyme that removes phosphates from phosphotyrosine residues to terminate the signal.

Internal receptors

Receptors that interact with hydrophobic signaling molecules that typically diffuse across the plasma membrane.

Transcription factors

Many intracellular receptors that interact with DNA in the nucleus and regulate gene expression.

Hydrophobic signaling molecules

Molecules that can easily cross the phospholipid bilayer due to their hydrophobic nature.

Signal propagation

The process by which a signal is transmitted from one cell to another or within a cell.

Cell-surface receptors

Receptors located on the plasma membrane that bind signaling molecules and initiate cellular responses.

Extracellular matrix

A network of proteins and carbohydrates outside of cells that facilitates cell signaling.

Neurotransmitters

Chemical messengers that transmit signals across a synapse from one neuron to another.

Signal termination

The process of stopping a signaling pathway, often involving the degradation of signaling molecules.

Downstream cellular response

The series of events that occur inside a cell after a signaling molecule binds to its receptor.

Steroid hormones

Small hydrophobic ligands that have similar chemical structures to their precursor, cholesterol, and can diffuse directly across the plasma membrane into the cell.

Water soluble ligands

Ligands that typically bind to cell-surface receptors.

Gas ligands

Examples include nitric oxide.

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

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.

EGFR

The epidermal growth factor receptor, a receptor tyrosine kinase involved in regulation of cell growth, wound healing, and tissue repair.

First messenger

The ligand that brings a signal to a cell.

Second messenger

A small molecule that propagates a signal within the cell.

Cyclic AMP (cAMP)

A second messenger that activates or inactivates proteins within the cell, formed from ATP.

Phosphodiesterase

An enzyme that converts cAMP into AMP, terminating the signal.

Internal receptors

Receptors that interact with ligands that can diffuse across the plasma membrane.

Cell-surface receptors

Receptors that typically bind to water soluble ligands.

Phosphorylation

A process that plays a role in the transmission of intracellular signals.

Second messengers role

They are involved in signal transmission following ligand binding.

Cascade of downstream events

The series of reactions initiated by EGF binding to EGFR that causes the cell to grow and divide.

Uncontrolled cell growth

A potential outcome of inappropriate activation of EGFR, which may lead to cancer.

Learning Objectives

Goals set for understanding signaling mechanisms, receptor comparisons, and ligand structure relationships.

Multicellular organisms

Organisms composed of multiple cells that utilize various signaling mechanisms.

Example of a signaling pathway

The process involving EGFR and EGF that illustrates how signaling pathways operate.

Ligand

A molecule that binds to a receptor to initiate signal transduction throughout a cell.

Phosphorylation

The addition of a phosphate group to a protein, which plays a crucial role in the transmission of intracellular signals.

Second messengers

Molecules that relay signals received at receptors on the cell surface to target molecules inside the cell.

ERK

A MAP kinase that activates translation when it is phosphorylated.

MNK1

A protein that is phosphorylated by ERK, which in turn phosphorylates eIF-4E.

eIF-4E

A protein that, when phosphorylated, allows mRNA to unfold and initiates protein synthesis in the nucleus.

Cellular metabolism

The set of chemical reactions that occur within a cell to maintain life, which can be increased by certain signals.

Adrenaline

A hormone that activates β-adrenergic receptors, leading to an increase in cyclic AMP (cAMP) and activation of PKA.

cAMP

Cyclic adenosine monophosphate, a second messenger that activates PKA.

PKA

Protein kinase A, an enzyme that phosphorylates other proteins to promote glucose supply and increase metabolism.

Growth factors

Proteins that bind to tyrosine kinases to initiate pathways leading to cell division.

Tyrosine kinases

Enzymes that phosphorylate tyrosine residues on proteins, initiating signaling pathways for cell growth.

RAS

A G-protein that is activated in the MAP kinase pathway, stimulating protein expression for cell division.

MAP kinase pathway

A signaling pathway that regulates various cellular activities, including protein expression and cell division.

Apoptosis

Programmed cell death that is essential for the development and maintenance of a healthy organism.

Phosphatases

Enzymes that remove phosphate groups from proteins, terminating cell signals.

Mating factor

A signaling molecule released by yeast cells to communicate with nearby cells.

Quorum sensing

A process in bacteria where signaling occurs based on population density, allowing for coordinated behavior.

Autoinducers

Molecules used in quorum sensing that signal to bacteria when a sufficient number of cells are present.