Maximal SERCA Activity Research Articles

GSK3-inhibition improves maximal SERCA activity in a murine model of Arrhythmogenic cardiomyopathy

•Arrhythmogenic cardiomyopathy is characterized by cardiac arrhythmias, myocardial remodelling, and exercise-induced premature death.•Recent work in a homozygous Desmoglein-2 mutant (Dsg2mut/mut) mouse model of ACM has identified pathogenic roles of increased glycogen synthase kinase-3 (GSK3) activity, calcium (Ca2+) overload, and mitochondrial dysfunction, which ultimately contributes to apoptosis and myocardial remodelling.•GSK3 negatively regulates the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) pump, a key regulator of Ca2+ uptake in the heart.•Here, our results demonstrate that the benefits of GSK3 inhibition, previously observed in Dsg2mut/mut mice, could be partly attributed to improvements in cardiac SERCA function.

Sarco(endo)plasmic reticulum Ca2+-ATPase function is impaired in skeletal and cardiac muscles from young DBA/2J mdx mice

SummaryThe DBA/2J (D2) mdx mouse is a more severe model of Duchenne muscular dystrophy when compared to the traditional C57BL/10 (C57) mdx mouse. Here, we questioned whether sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) function would differ in muscles from young D2 and C57 mdx mice. Both D2 and C57 mdx mice exhibited signs of impaired Ca2+ uptake in the gastrocnemius, diaphragm, and left ventricle; however, the level of impairment was more severe in D2 mdx mice. Reductions in maximal SERCA activity were also more prominent in the D2 mdx gastrocnemius and diaphragm when compared to those from C57 mdx mice; however, there were no differences detected in the left ventricle. Across all muscles, D2 mdx mice had the highest levels of oxidative stress as indicated by protein nitrosylation and/or nitration. In conclusion, our study shows that SERCA function is more impaired in young D2 mdx mice compared with age-matched C57 mdx mice.

The Role of GSK3β in SERCA Dysfunction and the Development of Arrhythmogenic Cardiomyopathy

The sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) pump catalyzes the active transport of Ca2+ into the sarcoplasmic reticulum and is responsible for over 70% of Ca2+ turnover after muscle contraction. Phospholamban (PLN) is a well-known SERCA regulator, and impaired SERCA function due to reductions in SERCA and/or enhanced inhibitory PLN action has been linked to the development of various cardiomyopathies. Arrhythmogenic cardiomyopathy (ACM) is an inherited, non-ischemic heart disease presenting with ventricular arrhythmias, hypo/dyskinesia, and fibrotic remodeling. Over 60% of ACM cases are caused by pathogenic variants in genes encoding the cardiac desmosome, where Desmoglein-2 (DSG2) is the second most prevalent desmosomal gene variant in the ACM patient population. Using Dsg2 mutant (Dsg2mut/mut) mice, previous studies showed that the enzyme glycogen synthase kinase-3β (GSK3β), plays a major role in ACM pathogenesis. Inhibiting GSK3β in these mice improved cardiac contractility and ACM pathological phenotypes. We have shown that GSK3β inhibition in cardiomyocytes can also improve SERCA function in wild-type (WT) mice; however, the role of GSK3β activity on SERCA function in Dsg2mut/mut mice is currently unknown. Here, we sought to examine the effects of GSK3β inhibition on SERCA function using a GSK3β-specific inhibitor, SB216763 (SB2), in Dsg2mut/mut mice. At 3 weeks of age, both WT and Dsg2mut/mut mice were treated with SB2 (2.5 mg/kg/day) or equivalent volume/kg/day of vehicle (DMSO) for 13 weeks via intraperitoneal injection. After treatment, mice were sacrificed and hearts were collected and homogenized. Our results show that cardiomyocytes obtained from vehicle treated Dsg2mut/mut mice had significantly lower SERCA2a expression (-52%, p<0.01) compared to vehicle treated WT mice, leading to reduced SERCA2a:PLN (-65%, p=0.04). Maximal SERCA activity was also significantly lower in vehicle treated Dsg2mut/mut mice (-65%, p=0.001). However, in myocardium from SB2 treated Dsg2mut/mut mice, maximal SERCA activity, SERCA2a expression, and SERCA2a:PLN ratio were all restored to near WT levels. We also examined the effects of heterozygous and homozygous GSK3β S9A mutation in the Dsg2mut/mut mice, which prevents phosphorylation and maintains GSK3β in its constitutively active form. When comparing WT with Dsg2mut/mutheterozygous and homozygous GSK3β S9A mutants, we saw a significant but non-progressive decrease in maximal SERCA activity (-60%, p=0.02 and -58%, p=0.03 respectively). In conclusion, these results show that SERCA function is negatively impacted in Dsg2mut/mut cardiomyocytes. Conversely, GSK3β inhibition with SB2 treatment improved SERCA content and activity to healthy WT levels. These results further illuminate the role of GSK3β in ACM pathogenesis and support the use of GSK3β inhibitors in its treatment, as our new promising results indicate this therapeutic strategy can also improve Ca2+ handling.

Restoration of Sarcoplasmic Reticulum Ca2+ ATPase (SERCA) Activity Prevents Age-Related Muscle Atrophy and Weakness in Mice.

Sarcopenia has a significant negative impact on healthspan in the elderly and effective pharmacologic interventions remain elusive. We have previously demonstrated that sarcopenia is associated with reduced activity of the sarcoplasmic reticulum Ca2+ ATPase (SERCA) pump. We asked whether restoring SERCA activity using pharmacologic activation in aging mice could mitigate the sarcopenia phenotype. We treated 16-month male C57BL/6J mice with vehicle or CDN1163, an allosteric SERCA activator, for 10 months. At 26 months, maximal SERCA activity was reduced 41% in gastrocnemius muscle in vehicle-treated mice but maintained in old CDN1163 treated mice. Reductions in gastrocnemius mass (9%) and in vitro specific force generation in extensor digitorum longus muscle (11%) in 26 versus 16-month-old wild-type mice were also reversed by CDN1163. CDN1163 administered by intra-peritoneal injection also prevented the increase in mitochondrial ROS production in gastrocnemius muscles of aged mice. Transcriptomic analysis revealed that these effects are at least in part mediated by enhanced cellular energetics by activation of PGC1-α, UCP1, HSF1, and APMK and increased regenerative capacity by suppression of MEF2C and p38 MAPK signaling. Together, these exciting findings are the first to support that pharmacological targeting of SERCA can be an effective therapy to counter age-related muscle dysfunction.

Phospholamban and sarcolipin prevent thermal inactivation of sarco(endo)plasmic reticulum Ca2+-ATPases

Na+-K+-ATPase from mice lacking the γ subunit exhibits decreased thermal stability. Phospholamban (PLN) and sarcolipin (SLN) are small homologous proteins that regulate sarco(endo)plasmic reticulum Ca2+-ATPases (SERCAs) with properties similar to the γ subunit, through physical interactions with SERCAs. Here, we tested the hypothesis that PLN and SLN may protect against thermal inactivation of SERCAs. HEK-293 cells were co-transfected with different combinations of cDNAs encoding SERCA2a, PLN, a PLN mutant (N34A) that cannot bind to SERCA2a, and SLN. One-half of the cells were heat stressed at 40°C for 1 h (HS), and one-half were maintained at 37°C (CTL) before harvesting the cells and isolating microsomes. Compared with CTL, maximal SERCA activity was reduced by 25-35% following HS in cells that expressed either SERCA2a alone or SERCA2a and mutant PLN (N34A) whereas no change in maximal SERCA2a activity was observed in cells that co-expressed SERCA2a and either PLN or SLN following HS. Increases in SERCA2a carbonyl group content and nitrotyrosine levels that were detected following HS in cells that expressed SERCA2a alone were prevented in cells co-expressing SERCA2a with PLN or SLN, whereas co-expression of SERCA2a with mutant PLN (N34A) only prevented carbonyl group formation. In other experiments using knock-out mice, we found that thermal inactivation of SERCA was increased in cardiac left ventricle samples from Pln-null mice and in diaphragm samples from Sln-null mice, compared with WT littermates. Our results show that both PLN and SLN form a protective interaction with SERCA pumps during HS, preventing nitrosylation and oxidation of SERCA and thus preserving its maximal activity.

Overexpression of SIRT3FL Improves Cardiac SERCA2a Function Following High Fat‐High Sucrose Feeding in Mice

BackgroundDiabetes induces a distinct form of cardiomyopathy, termed diabetic cardiomyopathy (DCM). A key pathophysiological mechanism of DCM is reduced level and activity of sarco(endo)plasmic reticulum Ca2+ ATPase 2a (SERCA2a). Recently, it has been discovered that the function of SERCA2a is negatively regulated by the post‐translational modification, acetylation. The sirtuin (SIRT) isoform, SIRT3 full‐length (SIRT3FL), deacetylates protein substrates, though it is unknown if SIRT3FL deacetylates SERCA2a. Therefore, the objective of this study was to determine if the overexpression of SIRT3FL alters cardiac SERCA2a acetylation and function in a mouse model of diabetes, induced by high fat‐high sucrose (HFHS) feeding.Methods and ResultsAdult (12 weeks of age) male C57BL/6J Wild Type (WT) mice and adult male MCK‐mSIRT3‐M1‐Flag transgenic (SIRT3FLTG) mice on a C57BL/6J background, who overexpress the SIRT3FL isoform in skeletal and cardiac muscle, were randomized into groups fed either a standard lab diet (Control) or a HFHS diet for 4‐months. After 4‐months, HFHS‐fed mice exhibited increased body weight, left ventricle (LV) weight, LV weight relative to body weight, and liver weight, compared to control fed‐mice. LV systolic parameters were unchanged by HFHS feeding, while the A and E wave forms of HFHS‐fed mice were slowed, compared to control‐fed mice. HFHS‐fed mice exhibited LV thickening, compared to control‐fed mice. Glucose tolerance was impaired in HFHS‐fed mice, compared to control‐fed mice. Diet did not alter SIRT3 protein content. Likewise, HFHS feeding did not alter the level or function of cardiac SERCA2a. LV SERCA2a protein content was not altered by genotype. However, SIRT3FLTG mice, regardless of diet, had 21% greater maximal SERCA activity (Vmax) and an 11% greater Hill coefficient, compared to WT mice. The Ca50, which is the Ca2+concentration that elicits 50% of Vmax, was 24% lower in SIRT3FLTG‐HFHS‐fed mice, compared to WT‐HFHS‐fed mice, indicating an increase in Ca2+ sensitivity.ConclusionHFHS feeding induced a diabetic phenotype in WT and SIRT3FLTG mice but level and function of cardiac SERCA2a was unchanged. However, overexpression of the SIRT3FL isoform improved cardiac SERCA2a function, regardless of HFHS feeding.Support or Funding InformationThis work was supported by a grant‐in‐aid from the Heart and Stroke Foundation of Canada

Choline administration attenuates aspects of the dystrophic pathology in mdx mice

Background & aimsDystrophic muscle fibres are fragile and prone to breakage, leading to impaired Ca2+ homeostasis and excessive inflammation, resulting in muscle wasting and weakness. Choline, an essential water-soluble nutrient, is involved in multiple biological processes, including modulation of inflammation and oxidative stress. We tested the hypothesis that choline supplementation would ameliorate the dystrophic pathology in mdx mice.MethodsThree-week old male mdx mice (n = 40) were fed control purified laboratory chow (CON; n = 20) or a choline-enriched diet (5 g/kg choline; CHL, n = 20) for four weeks. Rotarod performance, grip strength and running (wheel) distance were assessed during treatment. Markers of Ca2+-handling, inflammation, oxidative stress and fibrosis were measured in the diaphragm, quadriceps muscle and the liver.ResultsCholine-treated mdx mice displayed less macrophage (CD68 -33%, P < 0.05) and collagen infiltration (−34%, P < 0.05) and reduced Tgfβ3 mRNA expression (P < 0.05) in the diaphragm compared to untreated mdx mice. Choline supplementation increased maximal SERCA activity (38%, P < 0.05) and reduced markers of inflammatory (Tnfα, F4/80 and Cd206 mRNA, P < 0.05) processes compared with untreated mdx mice. In the liver there was a reduction in Acta2 mRNA (P < 0.05) with choline treatment, as well as an improvement in serum ALT levels (P < 0.01). There were no differences between the groups for the whole-body functional analyses.ConclusionsCholine supplementation attenuated the progression of the dystrophic pathology. Although choline did not alter functional performance, the reduction in fibrosis is clinically relevant for increasing the efficacy of therapies for DMD. Dystrophic muscle fibres are fragile and prone to breakage, leading to impaired Ca2+ homeostasis and excessive inflammation, resulting in muscle wasting and weakness. Choline, an essential water-soluble nutrient, is involved in multiple biological processes, including modulation of inflammation and oxidative stress. We tested the hypothesis that choline supplementation would ameliorate the dystrophic pathology in mdx mice. Three-week old male mdx mice (n = 40) were fed control purified laboratory chow (CON; n = 20) or a choline-enriched diet (5 g/kg choline; CHL, n = 20) for four weeks. Rotarod performance, grip strength and running (wheel) distance were assessed during treatment. Markers of Ca2+-handling, inflammation, oxidative stress and fibrosis were measured in the diaphragm, quadriceps muscle and the liver. Choline-treated mdx mice displayed less macrophage (CD68 -33%, P < 0.05) and collagen infiltration (−34%, P < 0.05) and reduced Tgfβ3 mRNA expression (P < 0.05) in the diaphragm compared to untreated mdx mice. Choline supplementation increased maximal SERCA activity (38%, P < 0.05) and reduced markers of inflammatory (Tnfα, F4/80 and Cd206 mRNA, P < 0.05) processes compared with untreated mdx mice. In the liver there was a reduction in Acta2 mRNA (P < 0.05) with choline treatment, as well as an improvement in serum ALT levels (P < 0.01). There were no differences between the groups for the whole-body functional analyses. Choline supplementation attenuated the progression of the dystrophic pathology. Although choline did not alter functional performance, the reduction in fibrosis is clinically relevant for increasing the efficacy of therapies for DMD.

Elevated whole muscle phosphatidylcholine: phosphatidylethanolamine ratio coincides with reduced SERCA activity in murine overloaded plantaris muscles

BackgroundAn increase in phosphatidylcholine:phosphatidylethanolamine (PC:PE) and a decrease in fatty acyl chain length, monounsaturated:polyunsaturated (MUFA:PUFA) fatty acyl ratio reduces SERCA activity in liposomes and in mouse models of obesity and muscular dystrophy. We have previously shown that maximal SERCA activity is significantly reduced in mechanically overloaded (OVL) plantaris, however, whether changes in PC:PE ratio or fatty acyl composition may contribute to the alterations in maximal SERCA activity remain unknown. Here, we tested the hypotheses that in OVL plantaris 1) PC:PE ratio would negatively correlate with maximal SERCA activity and 2) PC fatty acyl chain length (ACL) and/or MUFA:PUFA ratio would positively correlate with maximal SERCA activity.MethodsTo overload plantaris in mice, we transected the soleus and gastrocnemius tendons from one leg, while the contralateral leg underwent a sham surgery. After two weeks, plantaris muscles were extracted, homogenized and processed for SERCA activity and lipid analyses. Specifically, we performed HPTLC densitometry to examine changes in PC, PE, and the ratio of PC:PE. We also performed gas chromatography to assess any potential changes to fatty acyl composition.ResultsSERCA activity was significantly reduced in OVL plantaris compared with sham. Coinciding with this, we found a significant increase in PC but not PE in OVL plantaris. In turn, there was an increase in PC:PE but did not reach significance (p = 0.09). However, we found a significant negative correlation between PC:PE and maximal SERCA activity. Fatty acyl composition of PE remained similar between OLV and sham and PC demonstrated higher percent mole fraction of 17:1, 18:1, and ACL compared to sham. In addition, PC ACL, % MUFA, % PUFA, or MUFA:PUFA did not significantly correlate with maximal SERCA activity.ConclusionsOur results indicate that the phospholipid headgroup PC:PE negatively correlated and could potentially contribute to reductions in SERCA activity seen in functionally overloaded plantaris. In contrast, fatty acyl chain (ACL, % MUFA, % PUFA, MUFA:PUFA) did not correlate with maximal SERCA activity. Future studies will determine whether altering PC:PE with genetic and dietary interventions can influence SERCA activity and ultimately change the physiological outcome in response to muscle overloading.

Diaphragm assessment in mice overexpressing phospholamban in slow-twitch type I muscle fibers.

AimsPhospholamban (PLN) and sarcolipin (SLN) are small inhibitory proteins that regulate the sarco(endo)plasmic reticulum Ca2+‐ATPase (SERCA) pump. Previous work from our laboratory revealed that in the soleus and gluteus minimus muscles of mice overexpressing PLN (Pln OE), SERCA function was impaired, dynamin 2 (3–5 fold) and SLN (7–9 fold) were upregulated, and features of human centronuclear myopathy (CNM) were observed. Here, we performed structural and functional experiments to evaluate whether the diaphragm muscles of the Pln OE mouse would exhibit CNM pathology and muscle weakness.MethodsDiaphragm muscles from Pln OE and WT mice were subjected to histological/histochemical/immunofluorescent staining, Ca2+‐ATPase and Ca2+ uptake assays, Western blotting, and in vitro electrical stimulation.ResultsOur results demonstrate that PLN overexpression reduced SERCA's apparent affinity for Ca2+ but did not reduce maximal SERCA activity or rates of Ca2+ uptake. SLN was upregulated 2.5‐fold, whereas no changes in dynamin 2 expression were found. With respect to CNM, we did not observe type I fiber predominance, central nuclei, or central aggregation of oxidative activity in diaphragm, although type I fiber hypotrophy was present. Furthermore, in vitro contractility assessment of Pln OE diaphragm strips revealed no reductions in force‐generating capacity, maximal rates of relaxation or force development, but did indicate that ½ relaxation time was prolonged.ConclusionsTherefore, the effects of PLN overexpression on skeletal muscle phenotype differ between diaphragm and the postural soleus and gluteus minimus muscles. Our findings here point to differences in SLN expression and type I fiber distribution as potential contributing factors.

Differential regulation of the fiber type-specific gene expression of the sarcoplasmic reticulum calcium-ATPase isoforms induced by exercise training.

The regulatory role of adenosine monophosphate-activated protein kinase (AMPK)-α2 on sarcoplasmic reticulum calcium-ATPase (SERCA) 1a and SERCA2a in different skeletal muscle fiber types has yet to be elucidated. Sedentary (Sed) or exercise-trained (Ex) wild-type (WT) and AMPKα2-kinase dead (KD) transgenic mice, which overexpress a mutated and inactivated AMPKα2 subunit, were utilized to characterize how genotype or exercise training influenced the regulation of SERCA isoforms in gastrocnemius. As expected, both Sed and Ex KD mice had >40% lower AMPK phosphorylation and 30% lower SERCA1a protein than WT mice (P < 0.05). In contrast, SERCA2a protein was not different among KD and WT mice. Exercise increased SERCA1a and SERCA2a protein content among WT and KD mice, compared with their Sed counterparts. Maximal SERCA activity was lower in KD mice, compared with WT. Total phospholamban protein was higher in KD mice than in WT and lower in Ex compared with Sed mice. Exercise training increased phospholamban Ser(16) phosphorylation in WT mice. Laser capture microdissection and quantitative PCR indicated that SERCA1a mRNA expression among type I fibers was not altered by genotype or exercise, but SERCA2a mRNA was increased 30-fold in WT+Ex, compared with WT+Sed. In contrast, the exercise-stimulated increase for SERCA2a mRNA was blunted in KD mice. Exercise upregulated SERCA1a and SERCA2a mRNA among type II fibers, but was not altered by genotype. Collectively, these data suggest that exercise differentially influences SERCA isoform expression in type I and type II fibers. Additionally, AMPKα2 influences the regulation of SERCA2a mRNA in type I skeletal muscle fibers following exercise training.

Co-Expression of SERCA Isoforms, Phospholamban and Sarcolipin in Human Skeletal Muscle Fibers

Sarcolipin (SLN) and phospholamban (PLN) inhibit the activity of sarco(endo)plasmic reticulum Ca2+-ATPases (SERCAs) by reducing their apparent affinity for Ca2+. A ternary complex between SLN, PLN, and SERCAs results in super-inhibition of SERCA activity. Analysis of skeletal muscle homogenate has limited our current understanding of whether SLN and PLN regulate SERCA1a, SERCA2a, or both in skeletal muscle and whether SLN and PLN are co-expressed in skeletal muscle fibers. Biopsies from human vastus lateralis were analyzed through single fiber Western blotting and immunohisto/fluorescence staining to circumvent this limitation. With a newly generated SLN antibody, we report for the first time that SLN protein is present in human skeletal muscle. Addition of the SLN antibody (50 µg) to vastus lateralis homogenates increased the apparent Ca2+ affinity of SERCA (K Ca, pCa units) (-Ab, 5.85 ± 0.02 vs. +Ab, 5.95 ± 0.02) and maximal SERCA activity (μmol/g protein/min) (-Ab, 122 ± 6.4 vs. +Ab, 159 ± 11) demonstrating a functional interaction between SLN and SERCAs in human vastus lateralis. Specifically, our results suggest that although SLN and PLN may preferentially regulate SERCA1a, and SERCA2a, respectively, physiologically they both may regulate either SERCA isoform. Furthermore, we show that SLN and PLN co-immunoprecipitate in human vastus lateralis homogenate and are simultaneously expressed in 81% of the fibers analyzed with Western blotting which implies that super-inhibition of SERCA may exist in human skeletal muscle. Finally, we demonstrate unequivocally that mouse soleus contains PLN protein suggesting that super-inhibition of SERCA may also be important physiologically in rodent skeletal muscle.

Effects of buthionine sulfoximine treatment on diaphragm contractility and SR Ca2+pump function in rats

The purpose of this study was to examine the effects of glutathione (GSH) depletion and cellular oxidation on rat diaphragm contractility and sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) function in vitro under basal conditions and following fatiguing stimulation. Buthionine sulfoximine (BSO) treatment (n = 10) for 10 days (20 mM in drinking water) reduced (P < 0.05) diaphragm GSH content (nmol/mg protein) and the ratio of GSH to glutathione disulfide (GSH/GSSG) by 91% and 71%, respectively, compared with controls (CTL) (n = 10). Western blotting showed that Hsp70 expression in diaphragm was not increased (P > 0.05) with BSO treatment. As hypothesized, basal peak twitch force (g/mm(2)) was increased (P < 0.05), and fatigability in response to repetitive stimulation (350-ms trains at 100 Hz once every 1 s for 5 min) was also increased (P < 0.05) in BSO compared with CTL. Both Ca(2+) uptake and maximal SERCA activity (mumol.g protein(-1).min(-1)) measured in diaphragm homogenates that were prepared at rest were increased (P < 0.05) with BSO treatment, an effect that could be partly explained by a twofold increase (P < 0.05) in SERCA2a expression with BSO. In response to the 5-min stimulation protocol, both Ca(2+) uptake and maximal SERCA activity were increased (P < 0.05) in CTL but not (P > 0.05) in BSO diaphragm. We conclude that 1) cellular redox state is more optimal for contractile function and fatigability is increased in rat diaphragm following BSO treatment, 2) SERCA2a expression is modulated by redox signaling, and 3) regulation of SERCA function in working diaphragm is altered following BSO treatment.