L6 Ion Channels PDF

26 Ion charmels ProfIan McFadiean Physiology & Pharmacology of the Central Nervous System (5BBM0218) Ion channels Ian McFadzean Pharmacology & Therapeutics [email protected] Learning outcomes Having studied this lecture material, students should be able to; ◼ Describe what an ion channel is an...

26 Ion charmels ProfIan McFadiean Physiology & Pharmacology of the Central Nervous System (5BBM0218) Ion channels Ian McFadzean Pharmacology & Therapeutics [email protected] Learning outcomes Having studied this lecture material, students should be able to; ◼ Describe what an ion channel is and what it does. List the structural requirements for an ion channel  Draw a simple diagram of the protein structure of voltage-dependent potassium channels, voltage-dependent sodium channels and nicotinic AChRs  Explain the role ion channels play in determining the membrane potential, why opening or closing channels changes the membrane potential and why membrane depolarisation is an excitatory event  ◼ List the main families of ion channels and describe how they are classified Provide examples of classification by physiology  Provide examples of classification by pharmacology  Provide examples of classification by structure  ◼ Describe the properties of different families of ion channels voltage-gated ion channels  ligand-gated ion channels  The basic properties of ion channels Changes in membrane hey potential: outsideonde ↑Part Ion channels are transmembrane proteins – or more commonly complexes of transmembrane proteins – that form water-filled pores through which ions in aqueous solution can flow down their electrochemical gradient. Ion channels demonstrate Cpassive selective permeability and are gated, allowing them to control copenclose the flow of different ion species across the cell membrane. Ionic currents flowing across the cell membrane through ion channels lead to changes in the cell membrane potential, which in turn brings about changes in cell activity. process in response to stimuli Structural requirements for any ion channel ◼ ◼ ◼ ◼ A pore A selectivity filter A sensor of some description One or more gates Hille's model of a voltage-gated ion channel (from B Hille, Ionic channels of excitable membranes 2nd ed). Ion channels are formed by transmembrane proteins – e.g. the nicotinic ACh receptor (NAChR) sensor: Does bind? Crceptor Ach operated in channel N C Ag pore -save on extracellular Pentameric x5 structure 1 M2- lines intracellular aqueous pane Nicotinic AChR GABAA receptor 5HT3 receptor Recepter gated ion charney Ion channels are formed by transmembrane proteins – e.g. the voltage-dependent potassium channel x4 same structure ↓ Y SUBUMT!!! The archetypal voltage gated potassium channel 2 proteins is a single polypeptide that has a similar predicted came togetherstructure to a single domain of the voltage-gated ↓ sodium channel (next slide). various The protein contains 6 transmembrane spanning combos of small proteins -helices and a 'P' loop. The 4th transmembrane ↓ -helix is a voltage sensor. greater heterogen of t city - channels comparedIn the membrane FOUR protein subunits come to Wat channels. together to form a tetramer with the voltage sensors aligned and the 'P' loops forming the pore (hence P(ore) loops). Ion channels are formed by transmembrane proteins – e.g. the voltage-dependent sodium channel Y of proop x1 ⑤ Voltage ploops causes I upstroke action potential dama ↳8 transmembrane senser ploop-forms Drawthis!! pare The archetypal voltage gated sodium channel is a single polypeptide of about 2000 amino acids. It contains 4 homologous domains (I – IV) that each contain 6 transmembrane spanning -helices and a 'P' loop. The 4th transmembrane -helix in each domain is a voltage sensor. In the membrane the protein is folded so that each repeat domain acts as a subunit with the voltage sensors aligned and the 'P' loops forming the pore. Ions flow through ions channels down their electrochemical gradient (Passive) ◼ A key difference between ion channels and transporter proteins (e.g. sodium-potassium ATPase) is that ionic flow through ion channels is entirely passive. In other words ions only flow down their electrochemical flux gradient high This happens at remarkably high transport rates; for example a voltage-dependent calcium channel passes flow around 1 million ions per second it is open High rates Perhaps more remarkable is that it achieves this whilst being able to “select” for calcium ions over other ions. stops For example it will allow approximately 1200 calcium ions to pass into the cell before it “mistakenly” lets through a sodium ion This is despite the fact that there are around ten times more sodium ions present outside the cell than calcium ions ionic ◼ ◼ ◼ I & sodium ions-highly selective 7 a Not simple filter What determines whether a given ion will flow through a given ion channel? Three things to consider 1. Can the ion pass through the channel’s selectivity filter – that part of the protein that determines which types of ion are allowed to pass through 2. Are the gates of the channel open? - 3. Is the electrochemical gradient favourable? Passive) B The “steepness” of the electrochemical gradient is measured as the “electrochemical driving force” ◼ First - recall the Nernst equation (Dr Pini’s lectures) that allows us to calculate the equilibrium potential for an ion las Eion = (RT/zF) ln([ion]o/[ion]i) Remember which at be in equilibrium these! EK = -90 mV ENa = +60 mV Voltage to want ECl = -70 mV ECa = ~+130mV ions equilibrium one at If the cell membrane is suddenly made permeable to an ion (i.e. ion channels open), that ion will flow across the membrane in either direction in order to move the membrane potential towards its equilibrium potential Question; what direction do K+ ions have to flow across the membrane to take the membrane potential to EK+ from -60mV? ; or from -120 mV? K+ Vm -60 mV EK+ -90 mV K+ K+ + - - + K+ - + K+ K+ Vm -120 mV LOOK AT LECTURE CAPTURE FOR REVISION QUESTIONS The voltage that pushes ions through the channel is the electrochemical driving force ◼ ◼ The electrochemical driving force is the difference between the membrane potential (Vm or Em) and the equilibrium potential (Eeq) for that ion. So, for a potassium channel; At a membrane potential of -60 mV, the electrochemical driving force is (-60-(-90)) = +30 mV 30m gradient  At a membrane potential of -120 mV, the electrochemical driving force is (-120-(-90)) = -30 mV same magnitude Farce differentdirection  At a membrane potential of -90 mV, the electrochemical driving force is (-90-(-90)) = 0 mV -> No not change of potassium  -> ◼ ◼ in Although the magnitude of the driving force at -60 mV and -120 mV is the same (30 mV), the different sign tells us that the force will be pushing the potassium ions in the opposite direction; out of the cell at -60 mV and into the cell at -120 mV Notice that at -90 mV there will be no current (the driving force is zero) and the potassium current reverses direction. So for a potassium current EK+ is also the reversal potential (Erev) Opening (and closing) of ion channels determines the membrane potential many things ◼ ◼ ◼ ◼ can open and close At any instant in time, the membrane potential of a cell is determined by which ion channels are open in the membrane A typical nerve cell will express multiple copies of a given ion channel type (from tens to tens of thousands!); and perhaps dozens of different types of ion channel These channels open in response to environmental changes (e.g. the presence of a neurotransmitter outside the cell; changes in intracellular calcium; changes in membrane potential) The permeant ions then flow through these channels trying to drag the membrane potential towards their respective equilibrium potentials A simplified model of a nerve cell at “rest” mare Spermeable to omU Vm -90 mV EK / AtreSt makapotassiseem um permeability & potassium +60 mV ENa pK+ and pNa+ are the relative permeabilities of the membrane to potassium ions and sodium ions i.e. how many potassium and sodium ion channels are open Opening or closing ion channels will change the membrane potential Vm -90 mV EK We can hyperpolarise the cell by either; closing Na+ channels or Opening K+ channels +60 mV ENa We can depolarise the cell by either; Opening Na+ channels or Closing K+ channels Why are changes in nerve cell membrane potential important? ◼ ◼ ◼ ◼ Within the CNS, nerve cells exist in networks; communication within and between these networks determines our moods, our emotions, how we move our limbs, how we perceive pain etc etc So nerve cells must communicate with each other; they do this by a mix of electrical (action potentials traveling along axons) and chemical (neurotransmitters being released at synapses between nerve cells) means The membrane potential determines whether a nerve cell is communicating with others in the network; is it firing an action potential; is it releasing its neurotransmitter? Voltage-dependent sodium and voltage dependent calcium channels are critical here Membrane depolarisation is excitatory Vm -90 mV EK ↑voltage serve -nonan ion galed AP threshold close on +60 mV ENa The action potential threshold (approx. -55mV) is the membrane potential above which voltagedependent Na+ channels open to trigger off an all-or-nothing action potential (Dr Pini’s lectures) That action potential is propagated from the cell body, along the axon, to the nerve terminal Features of a typical nerve cell (neuron) Dendrites are branched processes (dendritic tree) that receive information from other neurons The cell body (soma) contains the nucleus and drives maintenance and metabolism. It also integrates the information received by the dendrites. The axon (nerve) terminals release neurotransmitters onto other cells to pass on information. The axon hillock is where the action potential is initiated. The axon is of variable length and conducts action potentials. The axon may be insulated by myelin or it may not. Insulated axons conduct more quickly than uninsulated axons. At the nerve terminal, voltage- dependent calcium channels are key Excitatory release) -> NT -30 mV Vm EK ENa ECa Voltage-dependent calcium channels typically have a threshold for opening around -30 mV When they open, calcium ions enter the cell down their electrochemical gradient Calcium entry further depolarises the cell plus the resultant increase in the intracellular concentration of calcium triggers the release of neurotransmitter by exocytosis Don’t forget chloride ion channels Sinhibitary) opens GABA GABA-A via recepter Vm EK ↓ ECl ENa ECa Opening of chloride ion channels is an important inhibitory mechanism For example, the GABAA receptor is a ligand-gated chloride ion channel, opened by the inhibitory neurotransmitter gamma-amino butyric acid (GABA) So in summary! Vm EK ECl Inhibitory Opening K+ channels Opening Cl- channels Closing already open Na+ channels Closing already open Ca2+ channels ENa ECa Excitatory Opening Na+ channels Opening Ca2+ channels Closing already open K+ channels Closing already open Cl- channels How do we deal with nonselective cation channels e.g. permeable to both Na+, and K+ ions? ◼ ◼ ◼ ◼ ◼ ◼ ◼ If you open such a channel at a membrane potential of -60 mV, Na+ ions will flow into the cell through the channel and K+ ions will flow out! At some value of membrane potential the two will cancel each other out, and this will be the reversal potential (Erev) for the non-selective cation channel current. The value of Erev depends upon the relative permeability of the channel to the ions. Consider a channel that was 1000 times more permeable to potassium than to sodium. This is almost a potassium channel, so its reversal potential would be very close to EK+ Now consider a channel that was equally permeable to Na+ and K+. Its reversal potential would be half-way between EK+ and ENa+ i.e. half-way between -90 mV and +60 mV, at -15 mV Most nonselective cation channels are slightly more permeable to Na+ than K+ ions, with reversal potentials around zero mV, e,g. Nicotinic AChR or glutamate receptors So when ACh or glutamate activate their respective channels, the overall effect is membrane depolarisation to around zero mV Church leccap for questions prev to this slide! How might we classify ion channels? 1) by a description of their physiological properties ◼ Gating mechanism e.g.     ◼ work? they Voltage-gated Ligand (or receptor)-gated Intracellular calcium or ATP Temperature Ionic selectivity e.g.   ◼ I know do Na+ channel Non-selective cation channel (Na+, K+ and Ca2+) permeant Kinetics e.g.    - ->? Sensor detection Fast activating Slow inactivating Transient or sustained How might we classify ion channels? 2) on the basis of pharmacology ◼ Channel blockers e.g.    ◼ Channel openers e.g.  ◼ Tetrodotoxin (some voltage-dependent sodium channels) Ketamine (NMDA subtype of glutamate receptor gated channels) Dihydropiridines eg nifedipine (L-type voltage-gated calcium channels) Minoxidil (ATP-dependent potassium channels) Channel allosteric modulators e.g.  Benzodiazepines eg diazepam (GABAA receptor gated channel) How might we classify ion channels? 3) by structure e.g. ◼ Voltage dependent sodium channels Formed from a single large protein BUT  Nine genes encoding such proteins  Each of the nine proteins has the same general structure, but subtle differences in amino acid sequence that lead to subtle differences in function and pharmacology  NaV1.1 ………….NaV1.9  ◼ Nicotinic AChR   PENTAMERIC structure with each subunit crossing the membrane 4 times Lots of subunits identified o o o  9 -subunits ↑ 4 -subunits#  which combination of these? · Functional receptors vary in composition e.g. 111 or 33444 or 44222 or 77777 And finally- my tip for success! Whenever you come across an ion channel in physiology, ask yourself two questions;   Ach? Voltage?Ligard? What opens it? What goes through it? Na?Ca?he? CI? itexcite will or inhibit? Only by knowing the answer to those two questions can you understand the role it might play in physiology. And come exam time, bear both questions in mind when describing an ion channel. DON’T simply say “sodium channels open during the action potential” for example say “voltage-dependent sodium channels open during the action potential” as that describes the channel more completely and demonstrates a deeper understanding Learning outcomes Having studied this lecture material, students should be able to; ◼ Describe what an ion channel is and what it does. List the structural requirements for an ion channel  Draw a simple diagram of the protein structure of voltage-dependent potassium channels, voltage-dependent sodium channels and nicotinic AChRs  Explain the role ion channels play in determining the membrane potential, why opening or closing channels changes the membrane potential and why membrane depolarisation is an excitatory event  ◼ List the main families of ion channels and describe how they are classified Provide examples of classification by physiology  Provide examples of classification by pharmacology  Provide examples of classification by structure  ◼ Describe the properties of different families of ion channels voltage-gated ion channels  ligand-gated ion channels 