Sinh học - Chapter 11: Cell communication

Chapter 11Cell CommunicationOverview: The Cellular InternetCell-to-cell communication is essential for multicellular organisms.Biologists have discovered some universal mechanisms of cellular regulation.The combined effects of multiple signals determine cell response. For example, the dilation of blood vessels is controlled by multiple molecules.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsEvolution of Cell SignalingA signal transduction pathway is a series of steps by which a signal on a cell’s surface is converted into a specific cellular response.Signal transduction pathways convert signals on a cell’s surface into cellular responses.Pathway similarities suggest that ancestral signaling molecules evolved in prokaryotes and were modified later in eukaryotes.The concentration of signaling molecules allows bacteria to detect population density.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsLocal and Long-Distance SignalingCells in a multicellular organism communicate by chemical messengers.Animal and plant cells have cell junctions that directly connect the cytoplasm of adjacent cells.In local signaling, animal cells may communicate by direct contact, or cell-cell recognition.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsCell SignalingPlasma membranesGap junctionsbetween animal cells(a) Cell junctionsPlasmodesmatabetween plant cells(b) Cell-cell recognitionIn many other cases, animal cells communicate short distances using local regulators, messenger molecules that travel only.In long-distance signaling, plants and animals use chemicals called hormones delivered via the blood.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Local and Long-Distance Signaling Local signalingTarget cellSecretingcellSecretoryvesicleLocal regulatordiffuses throughextracellular fluid(a) Paracrine signaling(b) Synaptic signalingTarget cellis stimulatedNeurotransmitter diffuses across synapseElectrical signalalong nerve celltriggers release ofneurotransmitterLong-distance signalingEndocrine cellBloodvesselHormone travelsin bloodstreamto target cellsTargetcell(c) Hormonal signalingThe Three Stages of Cell Signaling: A PreviewEarl W. Sutherland discovered how the hormone epinephrine acts on cells.Sutherland suggested that cells receiving signals went through three processes:ReceptionTransduction (relay / cascade amplifies)ResponseCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Signal Transduction PathwayEXTRACELLULARFLUIDPlasma membraneCYTOPLASMReceptor Signalingmolecule: ligand Relay molecules in a signal transduction pathwayActivationof cellularresponseTransductionAmplification of signalResponse23Reception1Protein Receptors in the Plasma MembraneMost water-soluble signal molecules bind to specific sites on receptor proteins in the plasma membrane.There are three main types of membrane receptors:G protein-coupled receptorsReceptor tyrosine kinasesIon channel receptorsCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsSignalingmolecule(ligand)GateclosedIonsLigand-gatedion channel receptorPlasmamembraneGate openCellularresponseGate closed321Transduction: Cascades of molecular interactions relay signals from receptors to target molecules in the cellSignal transduction usually involves multiple steps.The molecules that relay a signal from receptor to response are mostly proteins.Multistep pathways can amplify a signal: A few molecules can produce a large cellular response.Multistep pathways provide more opportunities for coordination and regulation of the cellular response.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsSignaling moleculeReceptorActivated relaymoleculeInactiveprotein kinase1Activeproteinkinase1Inactiveprotein kinase2ATPADPActiveproteinkinase2PPPPInactiveprotein kinase3ATPADPActiveproteinkinase3PPPPiATPADPPActiveproteinPPPiInactiveproteinCellularresponsePhosphorylation cascadeiAmplification -- CascadeSmall Molecules and Ions as Second MessengersThe extracellular signal molecule (ligand) that binds to the receptor is a pathway’s “first messenger.”Second messengers are small, nonprotein, water-soluble molecules or ions that spread throughout a cell by diffusion.Cyclic AMP and calcium ions are common second messengers.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsCalcium Ions -- Second MessengersCalcium ions (Ca2+) act as a second messenger in many pathways.Calcium is an important second messenger because cells can regulate its concentration.A signal relayed by a signal transduction pathway may trigger an increase in calcium in the cytosol.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsEXTRACELLULARFLUIDCa+ Ions as Second MessengersATPNucleusMitochondrionCa2+ pumpPlasmamembraneCYTOSOLCa2+pumpEndoplasmicreticulum (ER)Ca2+pumpATPKeyHigh [Ca2+]Low [Ca2+] Calcium and IP3 in signaling pathwaysG proteinEXTRA-CELLULARFLUIDSignaling molecule(first messenger)G protein-coupledreceptorPhospholipase CPIP2DAGIP3(second messenger)IP3-gatedcalcium channelEndoplasmicreticulum (ER)Ca2+CYTOSOLVariousproteinsactivatedCellularresponsesCa2+(secondmessenger)GTPNuclear and Cytoplasmic ResponsesUltimately, a signal transduction pathway leads to regulation of one or more cellular activities.The response may occur in the cytoplasm or may involve action in the nucleus.Many signaling pathways regulate the synthesis of enzymes or other proteins, usually by turning genes on or off in the nucleus.The final activated molecule may function as a transcription factor.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsNuclear responses to a signal: the activation of a specific gene by a growth factorGrowth factorReceptorPhosphorylationcascadeReceptionTransductionActivetranscriptionfactorResponsePInactivetranscriptionfactorCYTOPLASMDNANUCLEUSmRNAGeneOther signaling pathways regulate the activity of enzymes.Signaling pathways can also affect the physical characteristics of a cell, for example, cell shape.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFine-Tuning of the ResponseSignal Transduction multi-step pathways have two important benefits:Amplifying the signal (and thus the response)Contributing to the specificity of the responseCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsSignal AmplificationEnzyme cascades amplify the cell’s response.At each step, the number of activated products is much greater than in the preceding step.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsThe Specificity of Cell Signaling and Coordination of the ResponseDifferent kinds of cells have different collections of proteins.These different proteins allow cells to detect and respond to different signals.Even the same signal can have different effects in cells with different proteins and pathways.Pathway branching and “cross-talk” further help the cell coordinate incoming signals.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsSpecificityof Cell SignalingSignalingmoleculeReceptorRelaymoleculesResponse 1Cell A. Pathway leadsto a single response.Response 2Response 3Cell B. Pathway branches,leading to two responses.Response 4Response 5Activationor inhibitionCell C. Cross-talk occursbetween two pathways.Cell D. Different receptorleads to a different response.Signaling Efficiency: Scaffolding Proteins and Signaling ComplexesScaffolding proteins are large relay proteins to which other relay proteins are attached.Scaffolding proteins can increase the signal transduction efficiency by grouping together different proteins involved in the same pathway.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsTermination of the SignalInactivation mechanisms are an essential aspect of cell signaling.When signal molecules leave the receptor, the receptor returns to its inactive state.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsApoptosis (programmed cell death) integrates multiple cell-signaling pathwaysApoptosis is programmed or controlled cell suicide.Apoptosis can be triggered by:An extracellular death-signaling ligand DNA damage in the nucleusProtein misfolding in the endoplasmic reticulum.For example, apoptosis in the cells that form webbing between fetal human fingers and toes is a normal part of development. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Review: Signal Transduction PathwayReceptionTransductionResponseReceptorRelay moleculesSignalingmoleculeActivationof cellularresponse123You should now be able to:Describe the nature of a ligand-receptor interaction and state how such interactions initiate a signal-transduction system.List two advantages of a multistep pathway in the transduction stage of cell signaling.Explain how an original signal molecule can produce a cellular response when it may not even enter the target cell.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings4. Define the term second messenger ; briefly describe the role of these molecules in signaling pathways.5. Explain why different types of cells may respond differently to the same signal molecule.6. Describe the role of apoptosis in normal development.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings