Cell Communication

Communication Cells must communicate with other cells and respond to their environment Basic communication processes are shared across evolutionary lines Communication pathways direct many processes Differentiation Timing of cell cycle Physiological responses Immunity Many, many more

Communication Most cells communicate via chemical messengers directed toward target cells Can be due to cell-to-cell contact or over very short distances Local regulators – travel short distances to nearby cells Paracrine signaling – growth factors Synaptic signaling – neurons release neurotransmitters into the synapse Plasmodesmata and animal cell junctions Substances pass between adjacent cells Direct contact of molecules on surfaces

Communication May involve chemical signals over long distances Long distance regulators – use hormones Endocrine signaling – travel through circulatory system to a different area of the body

Cell Signaling Three stages: Reception – target cell detects signal when it binds to protein (usually on the surface) Transduction – converts the signal to a form that can trigger a response May be a single step or a series of changes (cascade) Response – transduced signal triggers a specific cellular response

Signal-transduction pathway Process, or series of steps, by which a signal on a cell’s surface is converted into a specific response 1. Chemical messenger (ligand) binds to a receptor protein Reception is specific – due to protein structure

Signal Transduction Pathway 2. Upon recognition, receptor protein changes shape (conformation), which initiates transduction Examples: G-protein linked receptors (usually activate enzymes) Tyrosine kinase receptors (trigger multiple pathways) Ligand-gated ion channels (allow for flow of ions)

G-protein-linked receptor

Tyrosine-kinase receptors

Ligand-gated ion channel

Signal Transduction Pathway 3. In transduction, signal is converted into a response Signaling cascades relay messages to target cells, often amplifying the signals Second messengers are often essential May include protein modifications or phosphorylation

Second messengers Small, nonprotein, water-soluble molecules or ions Rapidly spread through cell by diffusion carry message from membrane to the interior of the cell Most common are cAMP and Ca 2+ Allow the cell to respond to a single membrane event with many events inside the cell—they distribute the signal. They amplify the signal by activating more than one enzyme target.

Regulating protein activity Protein kinases – enzyme that phosphorylates proteins by transferring a phosphate from ATP Act as relay molecules in signal-transduction pathways May create a “phosphorylation cascade” A single cell may have hundreds of different kinds Help to regulate cell reproduction Defect may contribute to cancer

Regulating protein activity Protein phosphatases – remove phosphate groups from proteins Reverse effect of kinases Shuts down pathway and response In the cell, activities of proteins regulated by phosphorylation depends on the balance of kinase and phosphatase molecules

Cell Response 4. Signal transduction pathways lead to regulation of cell activities (responses): Opening or closing of ion channels Change in metabolism Regulate enzymes Cell division Regulate protein synthesis gene control (transcription factors) Nerve impulses Hormone release Immune response Muscle Contraction

Response Signal transduction ends after the cell responds— enzymes convert each transducer back to its inactive precursor. The balance between the regulating enzymes and the signal enzymes determines the cell’s response. Cells can alter the balance of enzymes in two ways: Synthesis or breakdown of the enzyme Activation or inhibition of the enzymes by other molecules

Specificity Different kinds of cells have different collections of proteins The response of a particular cell to a signal depends on its collection of signal receptor proteins, relay proteins, and proteins needed to carry out the response

Changes in the pathway… If transduction is blocked or defective the results could be severe or detrimental Diseases such as diabetes, autoimmune disease, cancer Faulty development Death Medications often have an effect on signal transduction pathways Birth control, anti-depressants, metabolic control

Apoptosis Programmed cell death Elaborate system of cell signaling Triggered by signals that activate a cascade of “suicide” proteins in the cell The cell is dismantled and digested Protects neighboring cells from damage Normal for development and the operation of many body systems