Cell Communication and Signaling

Cell Communication

• Cells in multicellular organisms need to respond to external and internal changes, which often involves adapting or coordinating behavior.
• Cells need to send and receive signals.
• Cellular communication involves different forms of information that can require conversion from one form to another.
• Signal transduction is a process of converting a signal.
• Typical cellular communication describes the signalling cell producing a signal and this signal molecule being detected by a receptor protein on a target cell.

Signal Transmission Range

• Hundreds of different extracellular signal molecules exist, including proteins, peptides, amino acids, nucleotides, steroids, fatty acid derivatives, and dissolved gases.
• Four main ways to transfer signals between cells: endocrine, paracrine, neuronal, and contact-dependent.

Endocrine Signaling

• Hormones produced in endocrine glands are released into the bloodstream and travel to distant target cells to initiate a response.

Paracrine Signaling

• Cells release local mediators into the extracellular fluid, affecting nearby target cells.

Neuronal Signaling

• Signals are transmitted along axons to distant target cells, often via neurotransmitters released at synapses.

Contact-Dependent Signaling

• Cell-surface-bound signal molecules bind to receptor proteins on adjacent cells, initiating a response.

Cellular Responses

• Each cell type has a specific set of receptor proteins allowing it to respond only to certain extracellular signalling molecules.
• Multiple signalling molecules can interact to regulate cellular behavior.

Classes of Extracellular Signaling Molecules

• Two classes: cell-surface receptors and intracellular receptors.
• Cell-surface receptors are associated or embedded in the plasma membrane; intracellular receptors are located inside the cell (e.g. in the cytoplasm or the nucleus).

Cell Surface Receptors Relay Signals Via intracellular Signaling Pathways

• A signal from the cell surface receptor is relayed inside the target cell through intracellular signalling molecules.
• This process ultimately alters the activity of effector proteins.

Intracellular Signaling Proteins Act as Molecular Switches

• Molecular switches are turned on or off, allowing cells to reset these proteins.
• Two classes: protein phosphorylation (addition/removal of phosphate groups) and GTP-binding proteins (binding to GTP or GDP).

Cell Surface Receptors Fall Into Three Main Classes

• Ion-channel-coupled receptors -respond to binding of a signal molecule by opening or closing a channel thereby allowing the flow of ions across the membrane to change the membrane potential thus producing an electrical current.
• G-protein-coupled receptors (GPCRs) - activated receptor signals GTP binding protein (G protein) on the inside of the plasma membrane, either switching an enzyme or an ion channel on or off.
• Enzyme-coupled receptors - Receptor own enzyme activity or associates with enzymes.

G-Protein-Coupled Receptors (GPCRs)

• Most frequent targets for G proteins are the enzymes: adenylyl cyclase and phospholipase C.
• These enzymes create second messengers that are involved in intracellular signalling.
• Cyclic AMP (cAMP) is a crucial second messenger. It increases rapidly in response to extracellular signals.
• cAMP activates cyclic AMP dependent protein kinase (PKA).
• PKA affects downstream effects (e.g. glycogen breakdown in skeletal muscle).
• Other second messengers are produced by phospholipase C, such as inositol 1,4,5-trisphosphate (IP3), and diacylglycerol (DAG).

Receptors Are Often Targets For Chemicals

• Multiple chemical substances (e.g. barbiturates, nicotine, morphine) can either activate or block cell surface receptors.

Some Hormones Can Cross the Plasma Membrane and Bind Intracellular Receptors

• Steroid Hormones (e.g. cortisol) are hydrophobic molecules that pass through the plasma membrane and bind to intracellular receptors within the cytoplasm or nucleus, thus affecting gene expression.