P2X Receptors and Acid-Sensing Ion Channels (ASICs) PDF

P2X Receptors and Acid-Sensing Ion Channels (ASICs) Dr Stephen Kelley Senior Lecturer in Pharmacology [email protected] 1  Learning outcomes Be able to discuss the structure and cellula...

P2X Receptors and Acid-Sensing Ion Channels (ASICs) Dr Stephen Kelley Senior Lecturer in Pharmacology [email protected] 1  Learning outcomes Be able to discuss the structure and cellular and physiological function of P2X receptors and acid-sensing ion channels. Be able to appreciate the difficulties of developing drugs that act at P2X receptors. Recommended reading: See noted sections in the slides  The Ligand-Gated Ion Channel Receptors Cys-loop receptors- nicotinic acetylcholine receptors, 5- HT3 receptors, GABA-A receptors, the strychnine sensitive glycine receptor and the zinc-activated channel (ZAC) Ionotropic glutamate receptors- NMDA receptors, AMPA and kainate receptors o P2X receptors o Acid sensing ion channels 3  Purinergic P2X receptors Trimers- that means composed three protein subunits P2X receptors are permeable to Na+, K+ and Ca2+ , exceptionally, Cl- Like the cys-loop receptors and ionotropic glutamate receptors, P2X receptors can be either homo-oligomeric or hetero-oligomeric 7 subunits identified. P2X1-7 Note that P2Y receptors are G-protein-coupled receptors For further reading, see Rang and Dale’s Pharmacology, pp 232-235 4 P2X receptors: Basic Structure The receptor is a trimer- three subunits surround a centrally-located ion channel Adapted from Illes et al., 2020 5 P2X receptor subunits: Basic Structure P2X subunit – both transmembrane domains contribute to the pore of the ion channel 6 Adapted from Samways, Li and Egan 2014 P2X receptors: Ion Flow 7 Adapted from Samways, Li and Egan 2014 P2X receptors Native receptors may be homo-oligomeric (e.g. P2X1 in smooth muscle) or hetero-oligomeric (e.g. P2X2:P2X3 in the nodose ganglion) All the receptors are gated by ATP and mediate excitatory neurotransmission in the central nervous system and smooth muscle. ATP is often released from cells by vesicular exocytosis during damage before being converted to ADP (which binds to P2Y receptors, which are GPCRs) then adenosine (which binds to adenosine receptors, also GPCRs. P2X receptors are expressed in a variety of tissues including the nervous system, smooth muscle and skeletal muscle 8 P2X receptors Agonists for P2X receptors include ATP, selective agonists include αβ-me-ATP and BzATP Antagonists include suramin (an anti-parasitic drug) at P2X1, P2X2, P2X3 and P2X5 receptors Involved in synaptic transmission, cardiac function, excitation in the gut, kidney function, vascular tone and nociception Also involved with apoptosis and inflammatory responses Allosterically modulated by changes in pH 9 P2X Receptor Tissue Distribution Physiological Function P2X1 Smooth muscle and urinary Urinary bladder contraction bladder P2X2, P2X3, P2X2/3 Nervous system, Sensory Taste perception neurons P2X4 Central nervous system Neuroinflammation P2X5 B cells and T cells Inflammation, Cl- conductance P2X6 B cells Pro-inflammatory immune response P2X7 Heart, liver, skeletal muscle, Inflammation pancreas, thymus, tonsils, monocytes, macrophages, osteoclasts 10 Adapted from Iles et al., 2020 and the IUPHAR Guide to Pharmacology  P2X receptors play a prominent role in gut motility in concert with P2Y receptors. 11 Adapted from Burnstock 2013  Acid-sensing (H+-gated) ion channels (ASICs) Trimer, conducts Na+ Three subunits identified: ASIC1, ASIC2, ASIC3 Activated by H+ Channel blocked by amiloride Expressed in CNS and PNS where they modulate neuronal sensitivity to acidosis ASICs have been detected in taste receptor cells, photoreceptors, lung epithelial cells urothelial cells, adipose cells vascular smooth muscle cells), immune cells and bone Related to epithelial Na+ channels (ENaC) which are not ligand-gated but constituently active. 12 (see table 6.2 in Medical Physiology) Phylogenetic Tree of ASIC and ENaC 13 Kellenberger & Schild 2015 Structure of ASICs Adapted from Heusser & Pless, 2021  ASIC Pharmacology Initial desensitisation followed by activation with increasing H+ concentration in ASIC1a homo-oligomers. Adapted from Heusser & Pless, 2021 ASIC subunit H+ (pEC50) Amiloride (pIC50) ASIC1a 6.2 - 6.8 5.0 ASIC2 4.1 - 5.0 4.6 ASIC3 6.2 - 6.7 4.2 - 4.8 Adapted the IUPHAR Guide to Pharmacology Summary of LGICs Receptor family Receptors General tissue Physiological function Notable pharmacology distribution Cys-loop receptors Nicotinic acetylcholine CNS, PNS, motor endplate Non-selective cation Agonists-nicotine, receptors depending upon subunit channel, permeable to Na+, varenicline. Antagonists composition K+ and Ca2+ to a lesser such as neostigmine of degree. Fast synaptic AChE boost activity at excitation, motor end-plate nAChRs potentials 5-HT3 receptors CNS (chemoreceptor zone Non-selective cation 5-HT3 receptor antagonists of hindbrain), PNS, enteric channel, permeable to Na+, such as ondansetron are nervous system. K+ and Ca2+ to a lesser anti-emetics. degree. GABA-A receptors Primarily CNS Non-selective anion Positive allosteric channel, permeable to Cl-. modulators include the Primary mechanism of following classes of drugs: inhibition in the brain. Fast benzodiazepines, synaptic inhibition. barbiturates, anaesthetics, alcohol, neurosteroids. All can evoke sedation at high enough doses. Drugs that increase synaptic GABA concentrations can act as anti-epileptics. Summary of LGICs Receptor family Receptors General tissue Physiological function Notable pharmacology distribution Ionotropic glutamate receptors NMDA CNS, particularly the limbic Non-selective cation channel, Open channel blockers include regions of the brain. permeable to Na+, K+ and very ketamine. Note that not many permeable to Ca2+. Fast clinically relevant drugs in synaptic excitation. Over-activity current use act at these = cell death. receptors. AMPA and Kainate receptors CNS, particularly the limbic Non-selective cation channel, Note that not many clinically regions of the brain. permeable to Na+, K+ and limited relevant drugs in current use act permeability to Ca2+. Fast at these receptors. synaptic excitation. Over-activity = cell death via down-stream NMDA activity. P2X receptors P2X1-7 CNS, PNS, smooth muscle and Non-selective cation channel, Difficult due to diffuse immune cells. permeable to Na+, K+ and limited expression and physiological permeability to Ca2+. Synaptic modes of action. Noted lack of transmission (presynaptic selective ligands. modulation), cardiac function, excitation in the gut, apoptosis and inflammatory responses and bladder control. ASICs ASIC1a,b, ASIC2,3 Sensory neurons. Related to ENaC, activated by Amiloride (ENaC channel changes in pH blocker) will act as an channel blocker at ASICs. NSAIDs but at very high concentrations.