Structure based analysis of KATP channel with a DEND syndrome mutation in murine skeletal muscle

Functional Regulation of KATP Channels and Mutant Insight Into Clinical Therapeutic Strategies in Cardiovascular Diseases

ATP-sensitive potassium channels (K_ATP channels) play pivotal roles in excitable cells and link cellular metabolism with membrane excitability. The action potential converts electricity into dynamics by ion channel-mediated ion exchange to generate systole, involved in every heartbeat. Activation of the K_ATP channel repolarizes the membrane potential and decreases early afterdepolarization (EAD)-mediated arrhythmias. K_ATP channels in cardiomyocytes have less function under physiological conditions but they open during severe and prolonged anoxia due to a reduced ATP/ADP ratio, lessening cellular excitability and thus preventing action potential generation and cell contraction. Small active molecules activate and enhance the opening of the K_ATP channel, which induces the repolarization of the membrane and decreases the occurrence of malignant arrhythmia. Accumulated evidence indicates that mutation of K_ATP channels deteriorates the regulatory roles in mutation-related diseases. However, patients with mutations in K_ATP channels still have no efficient treatment. Hence, in this study, we describe the role of K_ATP channels and subunits in angiocardiopathy, summarize the mutations of the K_ATP channels and the functional regulation of small active molecules in K_ATP channels, elucidate the potential mechanisms of mutant K_ATP channels and provide insight into clinical therapeutic strategies.

Compound heterozygous variants of the NARS2 gene in siblings with developmental delay, epilepsy, and neonatal diabetes syndrome

Neonatal diabetes mellitus (NDM) with developmental delay and epilepsy is classified as developmental delay, epilepsy, and neonatal diabetes (DEND) syndrome. The majority of DEND syndrome are due to severely damaging variants of K-ATP channels, and few mitochondria-related genes have been reported. We report here two Japanese siblings who were clinically diagnosed with DEND syndrome in whom NARS2 compound heterozygous variants were detected. Patient 1 was a 3-year-old girl and presented with diabetes ketoacidosis at 3 months old. Patient 2 was a 1-year-old boy who presented with severe hyperglycemia and started insulin therapy at 3 days old. After the first episodes, they both presented with severe developmental delay, hearing loss and treatment-resistant epilepsy accompanied by progressive brain atrophy. Whole-exome sequencing revealed compound heterozygous NARS2 p.R159C and p.L217V variants, and the GATA4 p.P407Q variant in both patients. They were treated by mitochondrial supportive therapy of vitamin B1, L-carnitine, and coenzyme Q10. Patient 2 was withdrawn from insulin therapy at 6 months old. This is the first report of NDM in which variants of the NARS2 gene coding mitochondrial protein were detected. Genetic analysis including mitochondrial genes should be considered in patients with neonatal onset diabetes associated with neurogenic symptoms.

Consequences of SUR2[A478V] Mutation in Skeletal Muscle of Murine Model of Cantu Syndrome

(1) Background: Cantu syndrome (CS) arises from gain-of-function (GOF) mutations in the ABCC9 and KCNJ8 genes, which encode ATP-sensitive K⁺ (KATP) channel subunits SUR2 and Kir6.1, respectively. Most CS patients have mutations in SUR2, the major component of skeletal muscle KATP, but the consequences of SUR2 GOF in skeletal muscle are unknown. (2) Methods: We performed in vivo and ex vivo characterization of skeletal muscle in heterozygous SUR2[A478V] (SUR2^wt/AV) and homozygous SUR2[A478V] (SUR2^AV/AV) CS mice. (3) Results: In SUR2^wt/AV and SUR2^AV/AV mice, forelimb strength and diaphragm amplitude movement were reduced; muscle echodensity was enhanced. KATP channel currents recorded in Flexor digitorum brevis fibers showed reduced MgATP-sensitivity in SUR2^wt/AV, dramatically so in SUR2^AV/AV mice; IC₅₀ for MgATP inhibition of KATP currents were 1.9 ± 0.5 × 10⁻⁵ M in SUR2^wt/AV and 8.6 ± 0.4 × 10⁻⁶ M in WT mice and was not measurable in SUR2^AV/AV. A slight rightward shift of sensitivity to inhibition by glibenclamide was detected in SUR2^AV/AV mice. Histopathological and qPCR analysis revealed atrophy of soleus and tibialis anterior muscles and up-regulation of atrogin-1 and MuRF1 mRNA in CS mice. (4) Conclusions: SUR2[A478V] “knock-in” mutation in mice impairs KATP channel modulation by MgATP, markedly so in SUR2^AV/AV, with atrophy and non-inflammatory edema in different skeletal muscle phenotypes.