Ion Channels John Koester jdk3 References: 4/15/2017 Ion Channels John Koester jdk3 References: •Kandel, Schwartz and Jessell (2000): Principles of Neural Science, 4th edition, chapter 5 •Hille, B. (2001) Ion Channels of Excitable Membranes, 3rd edition

Outline Why ion channels? Channel structure 4/15/2017 Outline Why ion channels? Channel structure Ion channels have three basic functional properties Conduct Select Gate Evolutionary relationships between ion channels Various factors contribute to ion channel diversity

4/15/2017 Ions Cannot Diffuse Across the Hydrophobic Barrier of the Lipid Bilayer

4/15/2017 Ion Channels Provide a Polar Environment for Diffusion of Ions Across the Membrane

Specialized Functions of Ion Channels 4/15/2017 Specialized Functions of Ion Channels Mediate the generation, conduction and transmission of electrical signals in the nervous system Control the release of neurotransmitters and hormones Initiate muscle contraction Transfer small molecules between cells (gap junctions) Mediate fluid transport in secretory cells Control motility of growing and migrating cells Provide selective permeability properties important for various intracellular organelles

Outline Why ion channels? Channel structure 4/15/2017 Outline Why ion channels? Channel structure Ion channels have three basic functional properties Conduct Select Gate Evolutionary relationships between ion channels Various factors contribute to ion channel diversity

Channels are Made Up of Subunits 4/15/2017 Channels are Made Up of Subunits

Outline Why ion channels? Channel structure 4/15/2017 Outline Why ion channels? Channel structure Ion channels have three basic functional properties Conduct Select Gate Evolutionary relationships between ion channels Various factors contribute to ion channel diversity

•Ion Channels Act As Catalysts 4/15/2017 Conduction •Ion Channels Conduct Up to 108 Ions/sec •Ion Channels Act As Catalysts •Speed up fluxes •Do not impart energy •Driving force is provided by electrochemical potential Several orders of mag faster than faster carrier or transporter

4/15/2017 Unlike Channels, Ion Pumps Do Not Provide a Continuous Pathway Through the Membrane Na+ K+

Outline Why ion channels? Channel structure 4/15/2017 Outline Why ion channels? Channel structure Ion channels have three basic functional properties Conduct Select Gate Evolutionary relationships between ion channels Various factors contribute to ion channel diversity

Ion Channels are Selectively Permeable 4/15/2017 Ion Channels are Selectively Permeable Cation Permeable Na+ K+ Ca++ Na+, Ca++, K+ Anion Permeable Cl -

Structure of K+ Channel Has Multiple Functional Adaptations 4/15/2017 Structure of K+ Channel Has Multiple Functional Adaptations Selectivity Filter Alpha helices are polarized, because H-bonds that hold it together act as dipoles

Outline Why ion channels? Channel structure 4/15/2017 Outline Why ion channels? Channel structure Ion channels have three basic functional properties Conduct Select Gate Evolutionary relationships between ion channels Various factors contribute to ion channel diversity

4/15/2017 Single Channel Openings are All-or-None in Amplitude, With Stochastically Distributed Open and Closed Times Closed Open 2 pA 20 msec

There are Two Major Types of Gating Actions 4/15/2017 There are Two Major Types of Gating Actions

Gating Can Involve Conformational Changes Along the Channel Walls 4/15/2017 Gating Can Involve Conformational Changes Along the Channel Walls

Gating Can Involve Plugging the Channel 4/15/2017 Gating Can Involve Plugging the Channel

4/15/2017 Gating Can Result from Plugging by Cytoplasmic or Extracellular Gating Particles

There are Five Types of Gating Controls 4/15/2017 There are Five Types of Gating Controls

4/15/2017 1) Ligand Binding Extracellular Cytoplasmic

4/15/2017 2) Phosphorylation

4) Mechanical Force-Gated 4/15/2017 3) Voltage-gated Change Membrane Potential 4) Mechanical Force-Gated Stretch

5) Temperature-gated Current Temperature (º C.) Cold-Sensitive 4/15/2017 5) Temperature-gated Current Cold-Sensitive Heat-Sensitive 20 25 30 35 40 45 50 Temperature (º C.)

Modifiers of Channel Gating 4/15/2017 Modifiers of Channel Gating

Binding of Exogenous Ligands Can Block Gating 4/15/2017 Binding of Exogenous Ligands Can Block Gating (Curare) (BTx) (ACh)

Ion Permeation Can be Prevented by Pore Blockers 4/15/2017 Ion Permeation Can be Prevented by Pore Blockers PCP Phencyclidine (commonly known as PCP or "angel dust") Glutamate-Activated Channel

Exogenous Modulators Can Modify the Action of Endogenous Regulators 4/15/2017 Exogenous Modulators Can Modify the Action of Endogenous Regulators Current Time Open Closed Open Closed

Outline Why ion channels? 4/15/2017 Outline Why ion channels? Ion channels have three basic functional properties Conduct Select Gate Evolutionary relationships between ion channels Various factors contribute to ion channel diversity

Evolution Operates More Like a Tinkerer Than an Engineer 4/15/2017 Evolution Operates More Like a Tinkerer Than an Engineer

Ion Channel Gene Superfamilies 4/15/2017 Ion Channel Gene Superfamilies I) Channels Activated by Neurotransmitter-Binding (pentameric channel structure): •Acetylcholine •GABA •Glycine •Serotonin II) Channels Activated by ATP or Purine Nucleotide- Binding (quatrameric or trimeric channel structure)

Ion Channel Gene Superfamilies 4/15/2017 Ion Channel Gene Superfamilies III) Channels With Quatrameric Structure Related to Voltage-Gated, Cation-Permeant Channels: A) Voltage-gated: •K+ permeant •Na+ permeant •Ca++ permeant •Cation non-specific-permeant B) Cyclic Nucleotide-Gated (Cation non-specific- permeant) C) TRP Family (Cation Non-specific); Gated by: • osmolarity • pH • mechanical force (hearing, etc.) • ligand binding • temperature D) Channels Activated by Glutamate-Binding •quatrameric channel structure •cation non-specific permeability

Ion Channel Gene Superfamilies 4/15/2017 Ion Channel Gene Superfamilies IV) “CLC” Family of Cl--Permeant Channels (dimeric structure): Gated by: •Voltage •Cell Swelling •pH V) Gap Junction Channels (non-specific permeability; hexameric structure)

Outline Why ion channels? Channel structure 4/15/2017 Outline Why ion channels? Channel structure Ion channels have three basic functional properties Conduct Select Gate Evolutionary relationships between ion channels Various factors contribute to ion channel diversity

Different Genes Encode Different Pore-Forming Subunits 4/15/2017 Different Genes Encode Different Pore-Forming Subunits Many more channels than genes

Different Pore-Forming Subunits Combine in Various Combinations 4/15/2017 Different Pore-Forming Subunits Combine in Various Combinations

4/15/2017 The Same Pore-Forming Subunits Can Combine with Different Accessory Subunits

Alternative Splicing of Pre-mRNA 4/15/2017 Alternative Splicing of Pre-mRNA Hundreds of splice variants in cochlea

Post-Transcriptional Editing of pre-mRNA 4/15/2017 Post-Transcriptional Editing of pre-mRNA Deamination? of adneosine = inosine, changes codon

4/15/2017 Generator Potentials, Synaptic Potentials and Action Potentials All Can Be Described by the Equivalent Circuit Model of the Membrane PNS, Fig 2-11

Equivalent Circuit Model of the Neuron 4/15/2017 Equivalent Circuit Model of the Neuron The Nerve (or Muscle) Cell can be Represented by a Collection of Batteries, Resistors and Capacitors

The Lipid Bilayer Acts Like a Capacitor 4/15/2017 The Lipid Bilayer Acts Like a Capacitor Vm = Q/C + + + + - - - - ∆Vm = ∆Q/C ∆Q must change before ∆Vm can change

4/15/2017 Change in Charge Separation Across Membrane Capacitance is Required to Change Membrane Potential - + + + - - + + - - + + - - + - + - + + - - + - - + - - + + - +

The Bulk Solution Remains Electroneutral 4/15/2017 The Bulk Solution Remains Electroneutral PNS, Fig 7-1

Each K+ Channel Acts as a Conductor (Resistance) 4/15/2017 Each K+ Channel Acts as a Conductor (Resistance) PNS, Fig 7-5

4/15/2017 Ion Channel Selectivity and Ionic Concentration Gradient Result in an Electromotive Force PNS, Fig 7-3

An Ion Channel Acts Both as a Conductor and as a Battery 4/15/2017 An Ion Channel Acts Both as a Conductor and as a Battery RT [K+]o EK = •ln zF [K+]i PNS, Fig 7-6

4/15/2017 An Ionic Battery Contributes to VM in Proportion to the Membrane Conductance for that Ion

Experimental Set-up for Injecting Current into a Neuron 4/15/2017 Experimental Set-up for Injecting Current into a Neuron PNS, Fig 7-2

Because of Membrane Capacitance, Voltage Always Lags Current Flow 4/15/2017 Because of Membrane Capacitance, Voltage Always Lags Current Flow t = Rin x Cin t PNS, Fig 8-3

Length Constant l = √rm/ra 4/15/2017 Length Constant l = √rm/ra PNS, Fig 8-5