LQT_PARASYMPA-SYMPA

The classic form of arrythmias related with long QT is the so called twisting of points (ON THE ECG). During arrythmias , big QRS complexes broaden and seem to twist around the isoelectic line.

 In some cases Long Qt degenarates further to Ventricular fibrilation leading cause of death. Arrhythmias are due to abnormal electrical activity in the heart. Loss of normal rhythmic contraction prevents effiecient pumping of blood. Patients die from heart attack due to inssufficient  blood supply to the CNS

Long QT syndrome arises from inherited or acquired dysfunction of ion channels. Inherited LQTS involves genetic mutations passed from generation to generation. Acquired LQTS is mediated by drugs or metabolic imbalances (Low K, Ca or Mg). Most frequent reason for LQTS is block of Ikr (carried by HERG channels) by drugs. A wide variety of drugs both cardivovascular and non-cardiovascular (e.g. Antibiotics, antipsychotics) can cause LQTS by binding to HERG and block Ikr currents causing lengthening of the ventricular action potential

Lots of potassium channels involved in the ventricular AP. Nearly all the drugs that cause long qt target HERG.WHY HERG? These drugs bind at the inner cavity near the selectivity filter. Two residues, a phenylalanine and a tyrosine are really important part of the drug binding site. Mutating those resulted in 50% channel block. HERG is the only channel having phenylalanine and tyrosine facing the inner cavity (unique) thats why HERG targets these channels. They have these cyclic rings on them and interact with drugs cause drugs have aromatic rings on them and interact with electrostatic interactions. 8 of these aromatic residues in the pore interact through hydrophobic and electrostatic interactions. Inner cavity is larger than other channels thus it can fit larger molecules compared to other potassium channels.

Genetic analysis and cloning stratagies show that families suffering from LQTS have mutations in ion channel genes and ion-channel related genes (13 genes identified until now). In nearly all cases LQT is due to mutations in ion channel subunits. Mutations at LQT1 AND LQT2 by far more frequent. Mutations take place at pore forming subunits. Point, nonsense and frameshift mutations

Single channel recordings in WT and  sodium chanels carrying a deletion mutation (prevents complete inactivation of Na current)

...wt: depol result very brief openings at the begining of the depol. On the other hand (deletion mutation) leads to Late openings of Na channels results in sustained component of inward current lengthening AP plateu

LQT3 mutations in sodium channel  in domains 3 and 4 (region important for inactivation). These mutations can either affect the inactivation gate and stop it from working properly and others affect the receptor of the inactivation gate

Mutations in Na channels increase channel activity – gain of function mutations

Mutations in K channels reduce current during AP – loss of function mutations

Transfecting HERG in HEK cells and record responses. Mutations completely abolished HERG currents and only endogenous currents are visible.

Western blots show that protein is expressed for all mutants, but there are differences in protein glycosylation

2 bands for the WT (different molecular weights) One heavier than other due to some proteins glycocylated (increases MW). Not the case for mutants while Glycosilated forms some times is absent all together.. Glycoilation is important for surfgace expression..takes place at the ER. Only fully glycosilated proteins are corectly trafficted to the cell membrane thus taking part in the action potential.

Double immunofluorescence staining of HERG and an ER located protein shows that Y611H gets retained in the ER and doesn’t move to the membrane

One copy of each gene in one chromosomes (2 copies) 1 copy might have a mutations however there are NO function channels although 1 copy is good. Why is that? Downstream of the expression.. U get wild type sus and mutated sus...in the er they are not segregated o they are gona mix...u get channels only with WT and few with tetrameric mutated...15 of the 16 subunit  possibilities will have one ore more mutated subunits...Only 1 16th of the channels will be fully functional. contain at least 1 mutations u get more than 50% reduction in current.

Some mutations affect channel gating...  Although resulting currents are bigger. WT HERG currents have slow deactivation while mutated has FAST deactivation

Net result of LQTS mutations is prolongation of the cardiac AP, which increases the incidence of early afterdepolarisations (EAD). EADs are premature, secondary depolarisations. EADs trigger additional APs at multiple loci - causing TdP and ventricular fibrillation. EADs occur because with a long AP Ca channels tend to re-open. Treatments for LQT patients are inhibitors of b-adrenergic receptors or implantable defibrillators.