Disease relevance of KCNJ2

• LQT was the primary cardiac manifestation, present in 71% of KCNJ2 mutation carriers, with ventricular arrhythmias present in 64% [1].
• KCNJ2 mutation results in Andersen syndrome with sex-specific cardiac and skeletal muscle phenotypes [2].
• Recently, strong evidences showed that Kir2.1 channels were associated with mouse atrial fibrillation (AF), therefore we hypothesized that KCNJ2 was associated with familial AF [3].
• Novel KCNJ2 mutation in familial periodic paralysis with ventricular dysrhythmia [4].
• METHODS AND RESULTS: To clarify this situation we generated known Andersen disease mutants of KCNJ2 which did not yield any measurable K(+) currents in CHO cells indicating that the Andersen mutants failed to form functional homomultimeric complexes [5].

High impact information on KCNJ2

• A missense mutation in KCNJ2 (encoding D71V) was identified in the linked family [6].
• We have mapped an Andersen's locus to chromosome 17q23 near the inward rectifying potassium channel gene KCNJ2 [6].
• Expression of two of these mutations in Xenopus oocytes revealed loss of function and a dominant negative effect in Kir2.1 current as assayed by voltage-clamp [6].
• Mutations in Kir2.1 cause the developmental and episodic electrical phenotypes of Andersen's syndrome [6].
• These findings suggest that Kir2.1 plays an important role in developmental signaling in addition to its previously recognized function in controlling cell excitability in skeletal muscle and heart [6].

Chemical compound and disease context of KCNJ2

• Long-term exposure to retinoic acid induces the expression of IRK1 channels in HERG channel-endowed neuroblastoma cells [7].
• PIP2 binding residues of Kir2.1 are common targets of mutations causing Andersen syndrome [8].

Biological context of KCNJ2

• 1. In this report, we characterized the functional consequences of three novel and seven previously described KCNJ2 mutations using a two-microelectrode voltage-clamp technique and correlated the findings with the clinical phenotype [1].
• Molecular genetics analysis identified two novel missense mutations in the KCNJ2 gene leading to amino acid changes C154F and T309I of the Kir2.1 open reading frame [9].
• A reduction in Kir2.1 prolonged the terminal phase of the cardiac action potential, and in the setting of reduced extracellular K(+), induced Na(+)/Ca(2+) exchanger-dependent delayed afterdepolarizations and spontaneous arrhythmias [1].
• Mutations in KCNJ2 may provide a new genotype (LQT7) of LQTS [10].
• The CaMK pathway, although Ca(2+) dependent, is unaffected by changes in membrane potential or block of Kir2.1 channels [11].

Anatomical context of KCNJ2

• These findings support the suggestion that, in addition to its recognized role in function of cardiac and skeletal muscle, KCNJ2 plays an important role in developmental signaling [2].
• Andersen-Tawil syndrome is a skeletal and cardiac muscle disease with developmental features caused by mutations in the inward rectifier K+ channel gene KCNJ2 [12].
• To gain insight into the mechanism of arrhythmia susceptibility, we simulated the effect of reduced Kir2.1 using a ventricular myocyte model [1].
• When expressed in Xenopus oocytes, Kir5.1 is efficiently targeted to the cell surface and forms electrically silent channels together with Kir2.1, thus negatively controlling Kir2.1 channel activity in native cells [13].
• In the brain, Kir5.1 mRNA is restricted to the evolutionary older parts of the hindbrain, midbrain and diencephalon and overlaps with Kir2.1 in the superior/inferior colliculus and the pontine region [13].

Associations of KCNJ2 with chemical compounds

• The affected members of a single family had a G514A substitution in the KCNJ2 gene that resulted in a change from aspartic acid to asparagine at position 172 (D172N) [14].
• (iii) When Kir2.1 and Kir2.2 channels were coexpressed in Xenopus oocytes the IC(50) for Ba(2+) block of the inward rectifier current differed substantially from the value expected for independent expression of homomeric channels [15].
• In human myoblasts triggered to differentiate, a hyperpolarization, resulting from K(+) channel (Kir2.1) activation, allows the generation of an intracellular Ca(2+) signal [11].
• Here we report a direct interaction between the strong inward rectifier, Kir2.1, and a recently identified splice variant of postsynaptic density-93 (PSD-93), a protein involved the subcellular targeting of ion channels and glutamate receptors at excitatory synapses [16].
• This non-blockade by LY294002 indicates that Kir2.1 acts upstream of myogenin and MEF2 [17].
• We could show that increased Kir2.1 activity requires dephosphorylation of tyrosine 242; replacing this tyrosine in Kir2.1 by a phenylalanine abolished inhibition by bpV(Phen) [18].

Physical interactions of KCNJ2

• Antibodies directed against a hemagglutinin epitope tag on Kir2.1 coimmunoprecipitated AKAP79, indicating that the two proteins exist together in a complex within intact cells [19].
• Depending on the subcellular localization, a fraction of 20 to 60% of PSD-95 molecules interacted with Kir2.1 channels, approximating their fluorescent labels by less than 5 nm [20].
• Likewise, outward current properties of heterologously expressed Kir2.1-Kir2.3 complexes in normal and 10 mmol/L [K+]o were similar to Kir2.1 but not Kir2 [21].
• 1. These results indicate that GRIF-1 binds to Kir2.1 and facilitates trafficking of this channel to the cell surface [22].

Regulatory relationships of KCNJ2

• 1. Functionally, the presence of AKAP79 enhanced the response of Kir2.1 to elevated intracellular cAMP, suggesting a requirement for a pool of PKA anchored close to the channel [19].
• Electrophysiological properties of inward rectifier potassium current (I (K1)) and hyperpolarization-activated inward current (I (f)) and the protein expression of the Kir2.1 subfamily and the hyperpolarization-activated cation channel 2 (HCN2) and HCN4 were studied in control and hypertrophied myocytes [23].
• GRIF-1 specifically enhanced Kir2.1-dependent growth in yeast and Kir2.1-mediated (86)Rb(+) efflux in HEK293 cells [22].

Other interactions of KCNJ2

• Recently, two further gain-of-function mutations in the KCNQ1 gene encoding the alpha-subunit of the KvLQT1 (I(Ks)) channel and in the KCNJ2 gene encoding the strong inwardly rectifying channel protein Kir2.1 confirmed a genetically heterogeneous disease [24].
• 4. These studies suggest that Kir2.2 and Kir2.1 are primary determinants of endogenous K(+) conductance in HAECs under resting conditions and that Kir2.2 provides the dominant conductance in these cells [25].
• No significant EPI-MID differences were observed in the expression of the other channel proteins studied (Kir2.1, alpha1C, HERG and MiRP1) [26].
• We conclude that the Kir2.1-induced hyperpolarization triggers human myoblast differentiation via the activation of the calcineurin pathway, which, in turn, induces expression/activity of myogenin and MEF2 [11].
• Together, these findings suggest that AKAP79 associates directly with the Kir2.1 ion channel and may serve to anchor kinase enzymes in close proximity to key channel phosphorylation sites [19].

Analytical, diagnostic and therapeutic context of KCNJ2

• 3. By reverse transcription-polymerase chain reaction (RT-PCR) using suitable primers on human eosinophils mRNA, an inward rectifier channel, Kir2.1, was identified, which is known from expression studies to have very similar properties to those found in this study [27].
• Western blot analysis demonstrated greater expression of Kir2.1 protein in proliferative cells, consistent with the higher current density [28].
• Here we identify a new variant, "SQT3", which has a unique ECG phenotype characterized by asymmetrical T waves, and a defect in the gene coding for the inwardly rectifying Kir2.1 (I(K1)) channel [14].
• One of the PCR products we obtained was virtually identical to IRK1 (cloned from a mouse macrophage cell line); the other, which we named hIRK, exhibited < 70% identity to IRK1 [29].
• The presence of G-protein-gated inward rectifier potassium channel-2 immunoreactivity in substantia nigra pars compacta dopaminergic neurons was confirmed by showing its co-localization with tyrosine hydroxylase by double immunocytochemistry, and also by selectively lesioning dopaminergic neurons with the neurotoxin 6-hydroxydopamine [30].

References