Drug TargetsSlide 2Slide 3Drug Targets: ReceptorsSlide 5ReceptorsSlide 7Calcium as Second MessengerSlide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19G-Protein-coupled ReceptorsSlide 21Slide 22Slide 23Slide 24Cyclic NucleotidesSlide 26Slide 27Slide 28Regulation of ReceptorsSlide 30Receptor DesensitizationReceptor Down-RegulationNuclear ReceptorsSlide 34BIMM118Drug TargetsFour major drug targets:Exceptions: Colchicin (acts on tubulin), Cyclosporin (acts via immunophillins), etc.BIMM118Drug TargetsBIMM118Drug TargetsBIMM118Responses to the extracellular environment involve cell membrane or intracellular receptors whose engagement modulates cellular components that generate, amplify, coordinate and terminate postreceptor signaling via (cytoplasmic) second messengers. Transmembrane signaling is accomplished by only a few mechanisms:–Transmembrane ion channels: open or close upon binding of a ligand or upon membrane depolarization–G-protein-coupled receptors: Transmembrane receptor protein that stimulates a GTP-binding signal transducer protein (G-protein) which in turn generates an intracellular second messenger –Nuclear receptors: Lipid soluble ligand that crosses the cell membrane and acts on an intracellular receptor–Kinase-linked receptors: Transmembrane receptor proteins with intrinsic or associated kinase activity which is allosterically regulated by a ligand that binds to the receptor’s extracellular domainDrug Targets: ReceptorsBIMM118Drug Targets: ReceptorsBIMM118Receptors•Rapidly acting (milliseconds) transmembrane ion channels: Multi-unit complexes with central aqueous channel. Upon binding of a ligand, channel opening allows a specific ion travel down its concentration gradient ==> Transient cell membrane depolarization (post synaptic potential)•Example: Nicotinic acetylcholine receptor: Pentameric structure (two alpha chains, one each beta, gamma, and delta chain). Activation occurs by binding of two molecules ACh to the alpha subunits, triggering the opening of the channel for Na+ and K+ ions.Myasthenia gravis: Autoimmune disease caused by inactivating antibodies against NAchRBIMM118ReceptorsMany other types of transmembrane ion channels ==> Ion channels are common drug targets!•Voltage-gated channels: •Gating: controlled by membrane polarization/depolarization•Selectivity: Na+, K+ or Ca+ ions•Intracellular ligand-gated channels: •Ca+ controlled K+ channel•ATP-sensitive K+ channel•IP3-operated Ca+ channel (in the ER membrane)BIMM118Calcium as Second MessengerCa++ ==> very important in regulating cellular and physiological responses –Extracellular concentrations are 2 mM (EM, blood), and levels in cytoplasmic vesicles and the ER can reach up to 10mM.–Baseline cytosolic Ca2+ concentration is around 100 nM in resting cells.–High gradient makes this a very fast and sensitive signaling system: only slight changes in membrane permeability will result in dramatic changes in the concentration of [Ca2+]i.–Low level of [Ca2+]i is also necessary to facilitate a phosphate oriented cellular metabolism (high calcium and high phosphate concentrations are incompatible!!)==> Evolutionary challenge: Maintain calcium gradient !!!–Evolvement of proteins that bind Ca2+ with high affinity, but reject magnesium!–Two classes of Ca-binding proteins: •membrane-integrated (unlimited capacity --> transporter systems: Ca-channels, calcium pumps)•non-membranous (limited capacity --> not only buffering, but processing of signal through conformational changes that enable interaction with target proteins: Calmodulin, Troponin C ...)Conc in mMECFICFK+4.5160Na+1447Cl-1147Ca++2.20.0001BIMM118Calcium as Second MessengerSources of Ca++: •Extracellular compartment: (predominantly in nerve, cardiac and smooth muscle cells)Three types of plasma-membrane localized calcium channels:–Voltage operated calcium channels:Action potental depolarizes plasma membrane, which results in the opening of “voltage” dependent calcium channels (channels can be opened by increase in extracellular K+).Each channel protein has four homologous domains, each containing six membrane spanning -helices (the fourth one functions as the “voltage” sensor.BIMM118Calcium as Second Messenger–Ligand gated calcium channels:Calcium channels opened after ligand binding to the receptor (e.g. glutamate/NMDA receptor; ATP receptor; nicotinic ACh receptors ( muscarinic ACh receptors signal through G-Proteins--> slower), prostaglandin receptors–Store operated calcium channels:Activated by emptying of intracellular stores, exact mechanism unknownType Properties Location/Function BlockersLHigh activation threshold; slow inactivationPlasma membrane of many cells; main Ca++ source for contraction in smooth and cardiac muscleDihydropyridines; verapamil; diltiazemNLow activation threshold;slow inactivationMain Ca++ source for transmitter release by nerve terminals-Conotoxin (snail venom)TLow activation threshold;fast inactivationWidely distributed; important in cardiac pacemaker and Purkinje cellsMibefradil; (verapamil; diltiazem)Three types:BIMM118Calcium as Second MessengerIntracellular compartment: (predominantly in muscle cells)–Calcium stored in mM concentrations in endo/sarcoplasmatic reticulum bound to Calsequestrin . (Previously mitochondria were thought to play an important role as Ca++-stores, but the uptake rate is 10x lower than that of the ER/SR -> not useful)Calcium release from the ER/SR is regulated by two receptors in the ER/SR membrane:–Ryanodine receptors (RyR):•Named after sensitivity to Ryanodine: plant alkaloid, irreversible inhibitor•Very important in skeletal muscle: direct coupling of RyRs with the dihydropyridine receptors of the T-tubules (dihydropyridine receptors are closely related to the L-type Ca++ channels) (see Ca++ effects)•Activity of RyRs in non-muscle cell lacking T-tubules regulated by cyclic ADP ribose•Caffeine: reversible activator of RYRs–IP3- Receptors (IP3R):•Inositol-1,4,5-triphosphate is produced through theactivity of receptor activated phospholipases C --> diffuses through cytoplasm and binds IP3RBIMM118Calcium as Second MessengerRemoval of Ca++ :•Ca++ - pumps:Activity of these pumps is induced by increases in cytosolic calcium.–Plasma membrane Na+/Ca2+ exchanger (mainly in excitable cells, e.g. cardiac cells)three Na+- ions are exchanged for one Ca++-ionDigitalis alkaloids: Na+/
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