BISC 421 1st Edition Lecture 9 Outline of Current LectureI.Synaptc Transmission Contnued II. Neurotransmitters and Their ReceptorsCurrent LectureSynaptic Transmission (continued) Vesicle docking and fusionSynaptobrevin, syntaxin, and SNAP- 25 form a complex that spans both membranes. None will bind Ca2+. Synaptotagmin is thought to be the Ca2+-sensitive component. Binds Ca2+ at concentratons known to evoke fusion. Mutatons alter vesicle fusion. Deleton is lethal in mice. How Ca2+-binding leads to exocytosisis unclear.•These proteins work as docking proteins •They also appear to bind and detect changes in calcium-‐ important because this initates the process •Calcium sensing is crucial •These three proteins will form a complex that will span the membrane (synaptobrevin, syntaxin, and SNAP 25) •The anchor is syntaxin •The other anchor is synaptobrevin •SNAP 25 seems to be interlinking these two-‐ associates with the membrane•However, none of these bind calcium-‐ this is the role of synaptotagmin•Synaptotagmin binds calciummolecular model of fusion•1. Docking-‐ these proteins dock the vesicle to the membrane but there is a bit a of a gap, not totally fused•2. Priming-‐next slideMolecular model of fusion cont•Priming-‐ the vesicle is fused to the membrane and the vesicle opens up•Think that Ca2+ entry through the voltage gated channels binds to synaptotagmin and causes a change in conformaton to fuse the vesicle to the membrane•Need the docking and priming and synaptotagmin for this to occur•Don’t know exactly how these dock•JUST REMMEBER: roles of those specified proteins•The vesicles are actually recycledClathrin-‐ mediated endocytosisDistinct proteins work to recover the vesicle from the membrane after fusion. Endocytosis. Plasma membrane protein clathrin is the most critical. Has D ³triskelion´ structure. Forms a dome-like structure which confers stability to the vesicle as it is retrieved from the membrane.Other proteins facilitate clathrin. Adaptor proteins AP-2 & AP-180 connect to the membrane. Uncoat´ clathrin from the vesicle. •Due to its shape it allows the vesicle to form spherical shape•Clathrin coatng covers the vesicles to make this shape•Facilitates the fusion of the vesicle and the budding and recycling•How? There are adapter proteins that are involved in connectng clathrin to the membraneDynamin-‐ involved in the last part of pinching off the vesicle to get it back and recycleClathrin mediated endocytosis•Due to adapter proteins, the clathrin can associate and know what part of the membrane is vesicular membrane•1. They can bind to these parts of the membrane and start to cause a budding off of the membrane•This is how recycling occurs•2. Dynamin-‐ forms ring structure to pinch off the edge of the vesicle•3. The vesicle is now back in the cytosol. Now these proteins associate with thecytosol and the vesicle goes back to its original form•This process is not just for neurons-‐ for a number of different cellsDiseases that affect the presynaptic terminal•Congenital myasthenic-‐ weakening of muscles. This can be defects in the budding process•Also in coming off of the endosome•Migraines associated with the defect in Ca2+ channels•In additon Botulinum and tetatus toxin affect SNARE proteins-‐ like botoxToxins that affect neurotransmitters•These toxins are proteases that will cut off essental proteins making it so that vesiclefusion cannot occurNeurotransmitters and their receptorsWhere Does the initial depolarization come from?•Where are the places in the postsynaptc cell where this happens? The dendrites-‐ axo-‐dendritc, axo-‐axonic, axo-‐somatc, dendro-‐dendritc•This can pretty much happen anywhere but we will primarily talk about dendrites•To get an acton potental we must initate a depolarizaton of the postsynaptc cell of sufficient value to cross threshold (controlled by Na+ channels)•If we record in the soma in the cell we can get a depolarizaton•Neurotransmitters diffuse across synaptc cleft and bind to either ion channels or Gcoupled protein receptors•These proteins have high affinity for NTs to bind and this will open up channel to allow Na+ or Ca2+ in-‐ the influx of positve charge will give us an inital depolarizaton in the membrane•G protein coupled-‐ metabotropic. Have receptors in the membrane that will initate acascade of events that will modulate cell functonLigand Gated ion channels•Ionotropic versus metabotropic•Ionotropic: very fast and the signal is INTRAmolecularly coupled•We will get not only a flow of positve ions but also negatve ions through channels(EPSP or IPSP)-‐ certain channels depolarize and certain ones hyperpolarizeG Protein coupled receptors•MUCH slower-‐ INTERmolecularly coupled-‐ use several proteins to get an effect•Cascade of events using G proteins inducing a change of events that will lead to actvaton of other proteins•More involved because of duraton in things like homeostasis of the cell or developmentNeurotransmitters can work through both receptor types•All the different types of classes of these receptors•They have a lot of similarites•Glutamate receptors-‐ for major excitatory response-‐ 3 different types of ionotropicreceptors and there are also metabotropic receptors•There are different types of receptors for the same neurotransmitter•Various forms of glutamate receptors with multple subunits•Can be pentamers-‐ continued•Not all of them have transmitter binding regions but they come together‐the structure of nACh receptory Pentameric protein that has different subunit composition depending on where it is expressed.Ń At the neuromuscular junction:x Two D-subunits: each binds one AChx Plus E, G, !"#$J subunits.Ń Neuronal ACh receptors contain 3 D- subunits and 2 E-subunits.y Are non-selective cationchannels.Ń Allow Na+ and K+ to pass.•Nicotnic-‐ACh receptor-‐ the proteins that are in the muscle•Ligand gated ion channels•This is a pentamer with different compositons depending on where it is expressed•If at the neuromuscular juncton it will have multple subunits-‐ only the two alpha subunits bind ACh•Different propertes depending on where expressed in the body•These are non selectve caton channels-‐ aka they cannot discriminate between Na+ andK+: seems strange to us•Patch clamp measuring outside outopening of ACh