Evolutionary History of Ion Channels and Neurotransmitters Neuro Journal Club, Peter HANTZ, Arendt Laboratory

Voltage, and voltage/intracellular ligand-gated ion channles K+ VG: fastly and slowly activated, inward rectifier, leak-channels V/ILG: Ca-activated, ATP-activated, cyclic-nucleotide gated Ca+ VG: High voltage activated, Low voltage activated V/ILG: Ca-activated, IP3 activated Na+, Cl-, H+: mostly VG Types of Ion Channels Stretch-activated ion channels Large-conductance MSc Low-conductance MSc Light-gated ion channels

Neurotransmitter systems Ionotropic receptors: (ligand-gated ion channels) Cys-loop receptors "fast activating" Anionic(inhibitory) Gly, GABA Cationic(excitatory) 5HT, Ach (nicotinic) Glutamate Gated Channels Glutamate, agonists: AMPA, NMDA, kainate G-protein coupled (metabotropic) receptors ACh, Glu (AMPA, KAIN, NMDA), GABA, 5-HT, DA, NE,... Shortcut pathway, Second Messenger Cascade Other neurotransmitter receptors peptides, NO, CO Types of Ion Channels/Receptors

Precursors of the VG-like ion channels small synthetic peptides: -fold into a-helixes -voltage: inserted into membranes -spontaneously build ion 5-7 mer channels 1-TM ion channels: Influenza M2 tetramer 2-TM ion channels: simplest K+ channels: KcsA, Kir (inwardly rectifier) tetramer Present in all three domains of life No voltage-sensitivity

Gene fusion/duplication: subunits containing six transmembrane crossings S1-S4: voltage gating S4: + charged voltage sensor S5, S6: conserved selectivity filter KvAP Voltage-sensitive Potassium Channels

6TM-type channels Voltage and cyclic nucleotide-gated channels: one-domain, 6TM homotetramers Usual structure of Na and Ca channels: α1 subunits = four LINKED DOMAINS Each: six transmembrane elements

Possible origins of the 6TM channels Two rounds of gene duplications? -Similarities: domains I and III; domains II and IV -Two-domain channels were identified (TPC) They evolved from one-domain 6TM multimers? Ca or Na-channels are more ancestral? 4-domain Ca-channel: already in yeast 4-domain Na-channel: only in multicellular animals not detected in protozoa, in plants But: There is an ancestral bacterial 6TM homotetrameric Na-channel Note: two types of "inactivation" mechanisms(following activation) "ball and chain" or "inactivation loop" Cl channels: conservative, structurally distinct (10-12 domains)

Evolution of the LG-like ion channels Structure (Cys-loop "fast activating" channels): Mostly: pentamers of 1-domain 4-TM proteins N-term extracellular domain: ligand binding site Made of several unrelated proteins? Homologs in bacteria Ancient role: nutrient seeking? osmotic regulation? Now: intercellular communication

Evolution of the LG-like ion channels Ancient: ACh, 5HT and GABA Gly derived from GABA despite Gly is "more simple" Root:?

Structure of the GPCR Single, 7TM polypeptide 2 extracellular loops: transmitter binding site 2 intracellular loops: activate G-proteins Two major groups: PLC-activating (IP3) cAMP decrease Ev. connection between ionotropic (LGICh) and metabotropic (GPCR) receptors ?

7-TM architecture in procaryotes: bacteriorhodopsins, no GPC...linkage to eucaryotic GPCR ? (disputed) GPCR: Present in plants, fungi and animals: common ancestor, 1.2 Gy ago Evolution of the GPCR