"receptor-ligand interactions - cell signaling, adhesion, motility, cell migration"

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"Receptor-ligand interactions - cell signaling, adhesion, motility, cell migration". Patricia Zuk, PhD Research Director Regenerative Bioengineering and Repair (REBAR) Lab Department of Surgery David Geffen School of Medicine at UCLA. No cell lives in isolation. - PowerPoint PPT Presentation


  • "Receptor-ligand interactions - cell signaling, adhesion, motility, cell migration" Patricia Zuk, PhDResearch DirectorRegenerative Bioengineering and Repair (REBAR) LabDepartment of SurgeryDavid Geffen School of Medicine at UCLA

  • No cell lives in isolationsurvival depends on an elaborate intercellular communication network that coordinates growth, differentiation and metabolismcells adjacent to one another frequently communicate through cell-cell contactother forms of communication cover larger distances = extracellular signaling molecules

  • Extracellular Signallingsignaling molecules are released by signaling cellsthe signal is called the ligandthe ligand binds to its specific receptor on a target cellthis ligand-receptor interaction induces a conformational or shape-change in the receptorproduces a specific response - called the cellular responsecan include a vast array of compoundse.g. small amino acid derivatives, small peptides, proteins

  • Cell-to-cell communication by extracellular signaling usually involves six steps(1) synthesis of the signaling molecule by the signaling cell(2) release of the signaling molecule by the signaling cell(3) transport of the signal to the target cell(4) detection of the signal by a specific receptor protein receptor-ligand specificity(5) a change in cellular metabolism, function, or development = cellular response triggered by the receptor-ligand complex specific to the ligand-receptor complex(6) removal of the signal, which usually terminates the cellular response degredation of ligand

  • Signaling molecules operate over various distances in animals-extracellular signaling can occur over:1. large distances or endocrine signaling signaling molecules are called hormones- act on target cells distant from their site of synthesis-usually carried through the bloodstream2. short distances or paracrine signaling affects target cells within proximityto the cell that synthesized the molecule-usually mediated by neurotransmitters and some growthfactors

  • Signaling molecules operate over various distances in animals-extracellular signaling can occur over:3. no distance or autocrine signaling the signal feeds-back and affects itself-action of many growth factors-these compounds generally act on themselves toregulate proliferation-seen frequently in tumor cells

    -many compounds can act through two or even three types of cell signalinge.g. growth factors (e.g. EGF) paracrine and autocrine and endocrine-epinephrine endocrine and paracrine

  • Circulating & Local HormonesCirculating hormonesact on distant targetstravel in bloodendocrine hormonesLocal hormonesparacrine hormones & autocrine hormones

  • Hormonestwo typeslipid soluble water soluble

  • Lipid-Soluble Hormones-lipid-soluble hormones can easily enter a cell by diffusing through the plasma membrane-PROBLEM: how do they travel in thewater-based blood??-SOLUTION: they are carried by carrier-proteins-these hormones then enter their target cell where they result in a specific cellular effect or response

  • Water-soluble Hormones-water soluble hormones can easily travel within the blood-PROBLEM: how do they enter a cell and result in a cellular response??-SOLUTION: binding to specific cell-surface receptors-this binding activates the receptor and results in a series of cellular events calledthe second messenger system

  • Lipid-soluble HormonesSteroidslipids derived from cholesterol in SERdifferent functional groups attached to core of structure provide uniquenessinteract with specific intracellular receptors (within the cell) to turn specific genes on or offeffective for hours or daysThyroid hormonestyrosine ring plus attached iodines are lipid-solubleactivate enzymes involved in the catabolism of fats and glucosehelp set our basal metabolic rateRetinoidsvitamin A derivativeshave dramatic effects on proliferation and differentiation plus cellular death (i.e. apoptosis)

  • Water-soluble HormonesAmino acid derivatives, small peptides and protein hormonesmodified amino acids or amino acids put togetherserotonin, melatonin, histamine, epinephrinelarger peptide hormonesinsulin and glucagon

    Eicosanoidsderived from arachidonic acid (fatty acid)prostaglandins or leukotrienesprostaglandins despite being lipidphilic bind to cell surface receptors

  • Action of Lipid-Soluble HormonesHormone diffuses through phospholipid bilayer & into cell Binds to receptor turning on/off specific genesNew mRNA is formed & directs synthesis of new proteinsNew protein alters cells activity

  • Action of Water-Soluble HormonesCan not diffuse through plasma membraneHormone receptors are integral membrane proteins act as first messengerReceptor protein activates G-protein in membrane G-protein activates adenylate cyclase to convert ATP to cAMP in the cytosol

    Cyclic AMP is the 2nd messengerActivates kinases in the cytosol to speed up/slow down physiological responsesPhosphodiesterase inactivates cAMP quicklyCell response is turned off unless new hormone molecules arrive

  • Cell-surface receptors belong to four major classesGPCRs are involved in a range of signaling pathways, including light detection, odorant detection, and detection of certain hormones and neurotransmittersMany different mammalian cell-surface receptors including GPCRs are coupled to a trimeric signal-transducing G proteinmade of an alpha, beta and gamma subunit complexLigand binding activates the receptor, which activates the G protein, which activates an effector enzyme to generate an intracellular second messengere.g. adenylyl cyclase converts ATP to cAMPdepending on regulation at the effector enzyme this pathway can be either activated or inhibitedby the type of G protein activated by the hormone-receptor complexGs proteins result in stimulation of the effector enzymeGi proteins result in inhibition of the effector enzyme

  • Four classes of cell-surface receptors-ligand binding changes the confirmation of the receptor so that specific ions flow through it-the resultant ion movement alters the electric potential across the plasmamembrane-found in high numbers on neuronal plasma membranese.g. ligand-gated channels for sodium and potassium-also found on the plasma membrane of muscle cells-binding of acetylcholine results in ion movement andeventual contraction of muscle

  • -lack intrinsic catalytic activity-binding of the ligand results in the formation of a receptor dimer (2 receptors)-this dimer than activates a class of protein called tyrosine kinases-this activation results in the phosphorylation of downstream targets by these tyrosine kinases (stick phosphate groups onto tyrosines withinthe target protein)-receptors for cytokines such as XXXX, interferons (XXXXXXX)

  • -also called receptor tyrosine kinases OR ligand-triggered protein kinases-similar to tyrosine-linked receptors - ligand binding results in formation of a dimer-BUT: they differ from tyrosine-linked receptors intrinsic catalytic activity-means that ligand binding activates it and the activated receptor acts as a kinase -recognize soluble or membrane bound peptide/protein hormones that act as growth factorse.g. NGF, PDGF, insulin-binding of the ligand stimulates the receptors tyrosine kinase activity, -results in phosphorylation of multiple amino acid residues within its target such as serine and threonine residues-this phosphorylation activates downstream targets-its targets are generally other protein kinases which phosphorylate their own downstream targets (other kinases??)

    Signal transductionCascade

  • Signal transduction cascades-the successive phosphorylation/activation of multiple kinases results in a cascade of phosphorylation/activation -this cascade is frequently called a signal-transduction cascade -this cascade eventually leads to a specific cellular responsee.g. changes in cell physiology and/or patterns of gene expression-RTK pathways are involved in regulation of cell proliferation and differentiation, promotion of cell survival, and modulation of cellular metabolism


  • Second messengersproduced by the activation of GPCRs and RTKsHormone stimulation of Gs protein-coupled receptors leads to activation of adenylyl cyclase and synthesis of the second messenger cAMPmost commonly studied second messengercAMP does not function in signal pathways initiated by RTKscAMP and other second messengers activate specific protein kinases (cAMP-dependent protein kinases or PKAs)cAMP has a wide variety of effects depending on the cell type and the downstream PKAs and other kinasesin adipocytes, increased cAMP activates a PKA that stimulates production of fatty acidsin ovarian cells another PKA will respond to cAMP by increase estrogen synthesissecond messenger systems allow for amplification of an extracellular signalone epinephine molecule can bind one GPCR this can result in the synthesis of multiple cAMP molecules which can go on to activate and amplified number of PKAsa blood level as low as 10-10M epinephrine can raise blood glucose levels by 50%

  • Second messengersother second messengers include:IP3 and DAG breakdown products of phosphotidylinositol (PI)produced upon activation of multiple hormone receptor types (GPCRs and RTKs)calcium IP3 production results in the opening of calcium-channels on the plasma membrane of the ER release of calciuma rise in calcium in pancreatic beta cells triggers the exocytosis of insulina rise in intracellular calcium also triggers contraction of muscle cellsmuch study has been done on the binding of calcium to a protein called calmodulin and the effect of this complex on gene expression

  • MAP kinase pathwaysbest characterized signal transduction pathwayactivation of RTKs by growth factors, hormones etc..result in activation of an adaptor protein called Ras GTPaseras induces a kinase signal cascade that starts with a kinase called rac and culminates in activation of a MAP kinase (MAPK)in between are a series of kinases that are part of the cascadeMAPK activation results in translocation into the nucleus and phosphorylate many different proteins, including transcription factors that regulate gene expression

  • MAPK kinase pathscdc42/rac then activates one of three MAPK paths: Activation of ras/MEK/ERK path: proliferation & differentiationp38MAPK: stress response & apoptosis (MAPKAP-2, HSP27) Stress, cytokines, hormones & mitogens signal through cdc42/racJNK: stress response & proliferation (jun)transcription factors (nucleus)MAPK kinase

  • Common signaling pathways are initiated by different receptors in a class The effects of activation of GPCRs and RTKs is more complicated than a simple step-by-step cascadeStimulation of either GPCRs or RTKs often leads to production of multiple second messengers, and both types of receptors promote or inhibit production of many of the same second messengersin addition, RTKs can promote a signal transduction cascade that eventually acts on the same target as the GPCR therefore the same cellular response may be induced by multiple signaling pathways by distinct mechanismsInteraction of different signaling pathways permits fine-tuning of cellular activities

  • Integrating Cells into Tissues:Cell-Cell Adhesion and Communication-a key event in the evolution of multicellularity is the ability for cells to adhere to oneanother and be able to communicate with each other-evolved a series of cell-adhesion molecules or CAMs that allow interaction witheach other and with the surrounding extracellular matrix (ECM)-this results in coordinated functioning of tissues

    -HOW??-these interactions result in the activation of specific signal transduction cascades eventually resulting in the desired cellular effect-therefore the physical interaction of CAMs with the ECM can turn pathways on oroff cellular effecte.g. cellular interactions with the adhesion protein b1-integrin can result in activation of the MAPK cascade

  • -various classes of CAMs found on cells1. cadherins cell-cell adhesion-calcium dependente.g, E-cadherin, P-cadherin2. Ig superfamily of CAMs cell-cell adhesione.g. N-CAM, V-CAM-calcium-independent-some are found enriched on specific cell types N-CAM3. Integrins major cell-matrix adhesion moleculee.g. a1 integrin, b1 integrin

  • Cell-Cell adhesion: Cadherin-containing junctionsAdherens junctionDesmosome-cadherins are a family of calcium-dependentCAMs-major molecules of cell-cell adhesion (homophilic)-over 40 different are knowne.g. E or epithelial cadherin N or neural cadherin-tissues have specialized junctions made of cadherins

    adherens junction continuous band of cadherins found in epithelial cells-connects cells tightly and interactwith the actin portion of the cytoskeletondesmosomes also found in high #s inepithelial tissues-cells attach via cadherins connectto protein plaques that attach to theplasma membrane and to intermediatefilaments within the cytoskeleton

  • Cell Matrixthree components to the ECMinsoluble collagens structural framework of the ECM, provides strength and resiliencyproteoglycans cushions cellsadhesive matrix proteins bind these components to receptors on the cell surfacedifferent combinations result in unique ECM compositions which can directly affect the activity of the cellsthe interaction of ECM components with specific CAMs (i.e. ECM receptors) can turn certain signaling paths ON or OFFalso the ECM is capable of sequestering growth factors and hormones by binding them and making them unavailable to turn cellular signaling on or off

  • Cell-matrix interactions: integrinsintegrins allow connection between the extracellular matrix with the cytoskeleton of the cell also can mediate cell-cell interactions (weak)made of an alpha and a beta subunit 17 types of a chains, 8 types of b chainsthese ab complexes act as receptors to specific matrix componentse.g. a1b1 collagen and laminin a5 b1 - fibronectincells will express multiple integrin typesattachment can be regulated by up- or down-regulated expression of these integrinsintegrins can also form very specific adhesive junctionsfocal adhesions (FN to actin) complex of more than 20 proteinscan increase and decrease based on physical stresshemidesmosomes (intermediate filaments to collagens and laminins) epithelial cellsactivation of these integrins can promote cell signalinge.g. formation and activation of focal adhesions triggers a cascade initaited by a Focal Adhesion Kinase (FAK) modulates cell growth and motility

  • Cell-Cell Communication: Gap Junctions-almost all cells have specialized junctionsthat allow the free passage of materials(e.g. signaling molecules) back and forth-these junctions = gap junctions-made of a channel protein called connexon-connexons interact to form channels betweentwo cells-one important compound small enough to traverse this junction is the second messengercAMP-also allows passage of calcium ions and otherions crucial to signaling (sodium, phosphate)-these gaps are not free-swinging gates-they actually open and close inresponse to ion concentrations thereby restricting the flow of toomany ions-they also exclude materials basedon size

  • Cell-Cell and Cell-Matrix interactions can activate signal transduction cascadesJ Biomed Sci. 2006 Feb 23; Crosstalk between hepatocyte growth factor and integrin signaling pathways. Chan PC, Chen SY, Chen CH, Chen HC

    J Neurochem. 2006 Jan;96(1):148-59 Fibronectin promotes brain capillary endothelial cell survival and proliferation through alpha5beta1 and alphavbeta3 integrins via MAP kinase signalling. Wang J, Milner R.

    Immunopharmacol Immunotoxicol. 2005;27(3):371-93. Interleukin-3, -5, and granulocyte macrophage colony-stimulating factor induce adhesion and chemotaxis of human eosinophils via p38 mitogen-activated protein kinase and nuclear factor kappaB. Ip WK, Wong CK, Wang CB, Tian YP, Lam CW.

    Cell Commun Adhes. 2004 Sep-Dec;11(5-6):137-53. ERK signaling pathways regulate the osteogenic differentiation of human mesenchymal stem cells on collagen I and vitronectin. Salasznyk RM, Klees RF, Hughlock MK, Plopper GE.

    Microsc Microanal. 2005 Jun;11(3):200-8. Cross talk between cell-cell and cell-matrix adhesion signaling pathways during heart organogenesis: implications for cardiac birth defects.Linask KK, Manisastry S, Han M.