2/7/16. Ion channel properties

Transcription

1 The Wellcome Trust - Biomedical Vacation Scholarships provide promising undergraduates with hands-on experience of research during the summer vacation, with the aim of encouraging them to consider a career in research. The deadline for applications is Friday 12 February WTD htm Ion channel properties 1. Activation Gating (voltage; ligands/neurotransmitters and others.) determines rate and conditions for channel opening. 2. Ion Selectivity and Conductance determines electrical drive. 3. Inactivation/ desensitization (a state different from the closed state) determines conditions and duration for channel availability after activation. 4. Refractory period (recovery from inactivation) Point 1: Diverse ion-channel properties and combinations underlie the diversity of action potential wave forms and neuronal firing properties Point 2: Primary structure of voltage-gated channels (understand how this occurs In terms of the ions and their equilibrium potentials) Tanouye, 1988 Jan and Jan, Rod McKinnon, 2003 Na + -Channel Ca ++ -Channel K + -Channel 1

2 Genetics, Mutageneses and Functional Studies Have Identified Molecular Explanations or Channel Properties Eg. Voltage clamp Eg. Patch clamp Ion-channel genes are mutated in 50+ channelopathies. - Henry Stewart Lecture (online from TCD library) Human Channelopathies Ptacek, L. J. et al. Arch Neurol 2004;61: Regulation of a K+ channel inactivation Hyperkalemic Periodic Paralysis Quarter mile horses with Impressive periodic paralysis. LGi1 is mutated in a dominant epilepsy, associated with auditory hallucinations Kaczmarek, Neuron 2006; Schulte et al., Neuron Impressive sired 2250 foals 55,000 descendants by

3 Mutations in a human voltage-gated sodium channel Na V 1.7 have no effect on general cognitive, motor functions in humans. However, depending on the exact allele, they may cause either: a) congenital analgesia (the inability to sense pain); or, b) hyperalgesia (extreme sensitivity to pain). Based on your knowledge of sodium channel function and nervous system organization discuss/ speculate on: a) how the high specificity of the mutations to pain perception may be explained; b) how different mutant alleles of the same gene can cause contrasting phenotypes. Your models here should explain how the alleles alter specific activities of the ion-channel (eg. activation, inactivation, ionic conductance) c) how you might test any one of the hypotheses you suggest in a) or b) above. Science 2013 Bark scorpion venom induces pain by activating the voltage-gated Na(+) channel Nav1.7, but has no effect on Nav1.8. Grasshopper mice Nav1.8 has amino acid variants that bind bark scorpion toxins and inhibit Na(+) currents, blocking action potential propagation and inducing analgesia. 3

4 Channelopathy phenotypes also depend. Genetic background. Shaker mutations affect a 4AP-sensitive K-channel (A model for human episodic ataxia - EA1) Genetic interactions: suppression Increased nerve firing Extracellular nerve recordings Increased muscle response Intracellular muscle recording Reduced muscle response Suppression of Sh phenotypes 4

5 Genetic interactions: synergism These double mutant effects in model organisms illustrate Extracellular nerve recordings Intracellular muscle recording 1. Polygeneic disease/ phenotypes 2. Drug targets for therapeutics Humans homozygous for CCR5-delta32 show resistance to AIDs. Human with PCSK9 mutations have low LDL and low heart disease. Neuronal excitability and synaptic transmission Recording evoked responses at the frog NMJ Presynaptic voltage-gated calcium channel Luigi Galvani (1786?) Membrane potential Action potential Synaptic transmission (fast active membrane response) From Bill Stark, Missouri 5

6 Fatt and Katz, Frog NMJ recordings showng mini ejps Synaptic vesicles mediate transmitter release Many proteins regulate synaptic-vesicle exocytosis The mechanics and regulation of synaptic vesicle exocytosis 1. Lipid bilayers and their dynamics 2. Vesicle content integrity 3. Target specificity of fusion 4. Speed of fusion 5. Calcium regulation 6. Replacement of vesicles and transmitter after they are used 1. Vesicle target recognition 2. Vesicle target docking 3. Calcium binding 4. Exocytosis 5. Various forms of regulation (including localization) 6

7 Synaptic-vesicles recycle Excitation and Inhibition The Knee-Jerk Reflex: a simple behavioral circuit Fusion Uncoating proteins Dynamin Stretch receptor Endocytosis 7