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ION CHANNELS, TRANSMITTERS, RECEPTORS & DISEASE



Channels & disorders
  Anions
  ATPase
  Calcium
  Cation
  Chloride
  Concepts
  Cyclic nucleotide-gated
  Gap junctions
  Long QT Syndromes
  Magnesium
  Mitochondrial solute carriers
  Na+, K+, Cl- Co-transporters
  Piezo
  Potassium
    HCN
    KCN
    K+/H+ ATPase
  Proton-gated
  Sodium
    Na+/H+ exchangers
    Non-voltage-gated
    Voltage-gated
  Toxins
  Transient receptor potential

Channel binding proteins

Transmitters/Receptors
  Acetylcholine
  ATP
  Capsaicin
  Catecholamines
  Dopamine
  Glutamine
  Glycine
  Purines

Diagrams

CHANNEL TYPES: General 9

  • Extracellular ligand-gated channels: Nicotinoid
    • 5 Homologous polypeptide subunits
    • Subunits have 4 membrane spanning regions
    • Ligands: Neurotransmitters
    • Specific receptors
      • Nicotinic AChR
      • GABAA & GABAC
      • Glycine
      • 5-HT3
      • Glutamate activated anionic channels
        • Types: NMDA; AMPA; Kainate
        • 4 Homologous subunits
  • Intracellular ligand-gated channels
    • Ligands: cAMP, cGMP, Ca++, G-proteins, Phosphorylation
  • Voltage-gated channels
    • 4 domains
      • Na+ & Ca+ channels: In single polypeptide chain
      • K+ channel: Tetramer of 4 similar subunits
    • Each domain has 6 membrane spanning regions
    • S4 sequence
      • Contains + charged amino acids (lysine and/or arginine)
      • "Senses" voltage across membrane: Regulates pore opening
    • Selective channel pore (Bacterial K+ channel model)
      • Dimensions: 12 Å long; 3 Å wide
      • Lined by main chain oxygen atoms
    • Ion selectivity: Na+, Ca++ or K+
  • Inward rectifier
    • P domain: "Selectivity filter"
    • 2 Flanking transmembrane region
    • Homo- or heterooligomers in membrane
    • Ion selectivity: K+
  • Gap junction channels
    • 6 polypeptide subunits
    • Each subunit has 4 membrane spanning regions
  • ATP gated channels: 3 Homologous polypeptide subunits


CHLORIDE CHANNELS

Classes
  Voltage gated (CLCN; CLCK)
  Intracellular (CLIC)
  Calcium activated (CLCA)
  Anion channels (SLC4)
  Na-K-Cl Cotransporters
Principles
Disorders

Chloride channels: Principles 14
Chloride channels: Disorders

SODIUM CHANNELS

Figure
Principles: Na+ channels
  Exchangers
  Non-voltage-gated
  Voltage-gated
Subunits
  SCNA; SCNB; SCNN
  NAH exchangers: SLC9; SLC other
  Cation (CNG)
Na+ channel disorders

Sodium channels: Principles Sodium channels: Disorders

CALCIUM CHANNELS

Ca++ channel disorders
Ca++ channel: Figures
Types
  Voltage-gated Ca++ channels
    Classes
      L; N; P; Q; R; T
    Principles
    CACN: A; B; G
  Other
    Ligand gated (ATP2)
    Intracellular activation (RYR; IP3)
    Ca++ sensors
    Cation (CNG-gated)
    Other (NAADP; EDG1)

Voltage-gated Ca++ entry channels: Principles Other Ca++ channels Ca++ sensors Ca++ channel disorders

POTASSIUM CHANNELS

Figure
K+ channel disorders
Principles
  Structure
  Functions
    Voltage gated
    Inwardly rectifying
    KCa
  Subunits
Types
  HCN
  KCN
    A; B; C; D;
    E; F; G; H;
    J; K; M; N;
    Q; S; T; V
  KCTD
  H+/K+-ATPase
  Plasmolipin
  SUR

Principles & Types of K+ Channels Disorders of K+ Channels

MAGNESIUM

Magnesium Hypomagnesemia
Gitelman Syndrome (Familial Hypokalaemia–Hypomagnesemia) 37
  Na-Cl cotransporter (SLC12A3) ; Chromosome 16q13; Recessive

ANION CHANNELS, EXCHANGERS & TRANSPORTERS


CATION CHANNELS


Cyclic Nucleotide-Gated


Cation Leak

Sodium leak channel, nonselective (NALCN)

PROTON-GATED ION CHANNELS: Neural




Na-K-Cl CO-TRANSPORTERS (Solute carrier family (SLC) 12)



Stretch-activated non-selective cation channels (SA channels)



TRANSIENT RECEPTOR POTENTIAL (TRP) ION CHANNELS

General Features TRP families 12
Group 1
  TRPC (canonical)
  TRPV (vanilloid)
  TRPM (melastatin)
  TRPA (ankyrin)
  TRPN (NOMPC-like)
Group 2
  TRPP (polycystin)
  TRPML (mucolipin)


GAP JUNCTIONS 23

Gap junction

ION CHANNEL-BINDING PROTEINS: INTRACELLULAR

Ion channel Binding protein Mechanism & Effect
K+ channel, Voltage-gated
  Shaker type
NMDA receptor
  NR2 subunit
Chapsyns*: PSD-95 ;
SAP97 ; Chapsyn-110 ;
Sap102 ; Dlg
Binding via PDZ** domains
  1st & 2nd on PSD-95
Post-synaptic densities in CNS
NMDA receptor
  NR1 subunit
α-actinin Actin binding protein
Concentrated in dendritic spines
Glycine receptor (GlyR) Gephyrin Binds to β intracellular loop
  of GlyR & tubulin
AChR: Nicotinic Rapsyn/43K Neuromuscular junction localization
Na+ channel
  Voltage-gated
Ankyrin G Node of Ranvier localization
AMPA receptor
GluR2 subunit
Glutamate receptor
interacting protein (GRIP)
Binding via PDZ domain
Couples receptor to cytoskeletal
  & signaling molecules
Glutamate receptor
Metabotropic
Subunits: mGluR1a
  & mGluR5
Homer Binding via PDZ-like domain
Expression by synaptic activity
Cerebellar development
* Belong to Membrane Associated Guanylate Kinase (MAGUK) family
    Chapsyn = Channel associated protein of synapse
** PDZ domains: Homologous 90 amino acid sequence repeats; Bind other proteins


Acetylcholine Receptors: Disorders


Glycine Receptors


Glutamate Receptors


Dopamine Receptors


Long QT Syndromes 16


Short QT (SQT) Syndromes




Concepts in channelopathies

What are the properties of the mutations in the chloride channel gene (CLC1) that determine whether a syndrome is inherited in a dominant or recessive pattern?

The dominant or recessive
nature of a mutation depends on the ability of the mutant chloride channel monomers to polymerize with normal channel monomers. Dominant mutations complex with normal monomers producing defective channels. For some mutations one abnormal monomer is sufficient to destroy the function of a tetramer complex (e.g. Pro480Leu). For other mutations (e.g. Gly230Glu) it requires two abnomal monomers to destroy the channel function of a tetramer. In either case, only a minority of tetramers remain functional and myotonia results. Recessive mutations do not complex with normal monomers. Normal monomers are then free to complex with other normal monomers. This produces enough functional tetramers in heterozygotes (50% of the usual amount) to preserve normal membrane excitablity and myotonia does not occur.



What are the properties of the mutations in the sodium channel gene (SCN4A) that determine whether a syndrome presents with myotonia, paramyotonia, or weakness?

Many mutations produce abnormal inactivation of the sodium channel. This results in increased sodium conductance and membrane depolarization. Mild depolarization is associated with increased membrane excitability and myotonia. Strong depolarization produces membrane inexcitability and weakness. Some mutations only reduce inactivation at low temperatures producing paramyotonic disorders (myotonia or weakness worse in the cold). Mutations in the inactivation gate (amino acid 1306) produce different degrees of disease severity depending on the size and charge of the side chain of the new amino acid. Alanine, with a short side chain produces mild myotonia fluctuans. Valine, with an intermediate side chain, produces paramyotonia congenita. Glutamic acid, with a long side chain and a negative charge, results in myotonia permanens.




CHANNEL TOXINS

Marine toxins
  Ciguatoxin
  Conotoxins
  Palytoxin (Clupeotoxism)
  Tetrodotoxin
  Shell fish
    Saxitoxin: Paralytic
    Domoic acid: Encephalopathic
    Brevetoxins: Neurotoxic
    Diarrheic
Other
  Lidocaine
  Potassium channel


Marine toxins: General 24
Ciguatera toxins 7, 8, 11

Epidemiology
Toxicity
Clinical
Laboratory

Clupeotoxism

From NCI
Palytoxin

Conotoxins
Lidocaine
Saxitoxin
Tetrodotoxin
Brevetoxins

From FDA

Domoic Acid
Diarrheic shellfish poisoning (DSP)

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References
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5/3/2024