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

Abstract

(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] (SUR2wt/AV) and homozygous SUR2[A478V] (SUR2AV/AV) CS mice. (3) Results: In SUR2wt/AV and SUR2AV/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 SUR2wt/AV, dramatically so in SUR2AV/AV mice; IC50 for MgATP inhibition of KATP currents were 1.9 ± 0.5 × 10−5 M in SUR2wt/AV and 8.6 ± 0.4 × 10−6 M in WT mice and was not measurable in SUR2AV/AV. A slight rightward shift of sensitivity to inhibition by glibenclamide was detected in SUR2AV/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 SUR2AV/AV, with atrophy and non-inflammatory edema in different skeletal muscle phenotypes.

Zoledronic Acid Blocks Overactive Kir6.1/SUR2-Dependent KATP Channels in Skeletal Muscle and Osteoblasts in a Murine Model of Cantú Syndrome

“…Therefore, we propose that increased ZOL sensitivity in osteoblasts and soleus muscle likely reflects increased Kir6.1/SUR2B contributions to native KATP channels in these tissues relative to FBD and EDL muscle. Importantly, this is also consistent with our previous finding that the Kir6.1 V65M mutation, which reduces glibenclamide inhibition [ 11 ], has a greater effect on GLIB sensitivity of soleus KATP channels versus FDB KATP channels [ 9 ]. The high expression ratio of KCNJ8 -Kir6.1/ KCNJ11 -Kir6.2 relates to the potency of the zoledronic acid in the musculoskeletal tissues ( Table 2 ).…”

“…However, in Kir6.1 V65M mice, skeletal muscle was severely affected, muscles were atrophic and necrotic, and there was marked loss of muscle mass with fibrotic replacement, inflammatory cell infiltration, up-regulation of autophagy genes, and reduced muscle strength in slow twitch muscle, as well as severe impairment of glibenclamide response- [ 9 ]. These symptoms are very different from what previously observed in other neuromuscular disorders known as Hypokalemic Periodic Paralysis associated with the loss of function of the fast-twitch KATP channels in humans [ 11 , 13 ] and animals [ 14 , 15 ]. The human patients and the animals show muscle weakness with vacuolar features [ 16 ], but no necrosis or inflammation, and flaccid paralysis induced by insulin injection is associated with the lowering of extracellular K + ions concentration [ 17 , 18 ].…”

“…The reversal of cardiovascular phenotypes in SUR2 A478V mice [ 10 ] and the apparent benefit of glibenclamide, with minimal disruption of glycemic control in one human case [ 10 ] are consistent with this action. However, the inhibitory effect of glibenclamide is reduced in mutant channels with more severe GOF, particularly in Kir6.1 GOF mutants [ 11 , 12 ]. Accordingly, glibenclamide is much less effective against KATP channels in cells from CS mutant mice carrying the Kir6.1 V65M mutation [ 8 , 9 , 12 ].…”

Bisphosphonates Targeting Ion Channels and Musculoskeletal Effects

“…The bone marrow cells from Kir6.1 (V65M) and the SUR2 (A478V) CS mice following differentiation failed to mineralize in our hands suggesting of abnormal mineralization process driven by the KATP channels. Histopathological analysis of femora bone sections from these mice, however, did not reveal severe abnormalities like those observed for instance in the skeletal muscles ( Scala et al, 2021 ; Scala et al, 2020a ) of the same animals, unless some osteoblasts aggregate were observed in some sections. We have demonstrated that CS mutation in the KCNJ8 genes encoding for the Kir6.1WT/VM is responsible for severe injuries in the skeletal muscle of a murine model of CS, reporting how GOF mutation caused atrophy, necrosis, and loss of muscle with fibrotic replacement, inflammatory cell infiltration, up-regulation of autophagy genes, and reduced muscle strength in the slow twitching muscle and impairment of glibenclamide response ( Scala et al, 2020b ).…”

“…We have demonstrated that CS mutation in the KCNJ8 genes encoding for the Kir6.1WT/VM is responsible for severe injuries in the skeletal muscle of a murine model of CS, reporting how GOF mutation caused atrophy, necrosis, and loss of muscle with fibrotic replacement, inflammatory cell infiltration, up-regulation of autophagy genes, and reduced muscle strength in the slow twitching muscle and impairment of glibenclamide response ( Scala et al, 2020b ). We also reported that in SUR2WT/AV and SUR2AV/AV mice, forelimb strength was reduced associated with loss of modulation of KATP channels by MgATP with metabolic decoupling and atrophy in different muscles ( Scala et al, 2021 ). A slight rightward shift of sensitivity to inhibition by glibenclamide was detected in SUR2AV/AV mice indicative however of a conserved response to this drug.…”