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1.  GSK3β Phosphorylates Newly Identified Site in the Pro-Ala Rich Region of Cardiac Myosin Binding Protein C and Alters Cross-Bridge Cycling Kinetics in Human 
Circulation research  2012;112(4):633-639.
Rationale
Cardiac myosin binding protein C (cMyBP-C) regulates cross-bridge cycling kinetics and thereby fine-tunes the rate of cardiac muscle contraction and relaxation. Its effects on cardiac kinetics are modified by phosphorylation. Three phosphorylation sites (Ser275, Ser284, Ser304) have been identified in vivo, all located in the cardiac-specific M-domain of cMyBP-C. However recent work has shown that up to four phosphate groups are present in human cMyBP-C.
Objective
To identify and characterize additional phosphorylation sites in human cMyBP-C.
Methods and results
Cardiac MyBP-C was semi-purified from human heart tissue. Tandem mass-spectrometry analysis identified a novel phosphorylation site on serine 133 in the proline-alanine (Pro-Ala) rich linker sequence between the C0 and C1 domains of cMyBP-C. Unlike the known sites, Ser133 was not a target of protein kinase A. In silico kinase prediction revealed glycogen synthase kinase 3β (GSK3β) as the most likely kinase to phosphorylate Ser133. In vitro incubation of the C0C2 fragment of cMyBP-C with GSK3β showed phosphorylation on Ser133. In addition, GSK3β phosphorylated Ser304, although the degree of phosphorylation was less compared to PKA-induced phosphorylation at Ser304. GSK3β treatment of single membrane-permeabilized human cardiomyocytes significantly enhanced the maximal rate of tension redevelopment.
Conclusion
GSK3β phosphorylates cMyBP-C on a novel site, which is positioned in the Pro-Ala rich region and increases kinetics of force development, suggesting a non-canonical role for GSK3β at the sarcomere level. Phosphorylation of Ser133 in the linker domain of cMyBP-C may be a novel mechanism to regulate sarcomere kinetics.
doi:10.1161/CIRCRESAHA.112.275602
PMCID: PMC3595322  PMID: 23277198
Cardiac myosin binding protein-C; phosphorylation; GSK3β; heart failure; contractile proteins; hypertrophy/remodeling; myocardial contractility
2.  Localisation of AMPK γ subunits in cardiac and skeletal muscles 
The trimeric protein AMP-activated protein kinase (AMPK) is an important sensor of energetic status and cellular stress, and mutations in genes encoding two of the regulatory γ subunits cause inherited disorders of either cardiac or skeletal muscle. AMPKγ2 mutations cause hypertrophic cardiomyopathy with glycogen deposition and conduction abnormalities; mutations in AMPKγ3 result in increased skeletal muscle glycogen. In order to gain further insight into the roles of the different γ subunits in muscle and into possible disease mechanisms, we localised the γ2 and γ3 subunits, along with the more abundant γ1 subunit, by immunofluorescence in cardiomyocytes and skeletal muscle fibres. The predominant cardiac γ2 variant, γ2-3B, gave a striated pattern in cardiomyocytes, aligning with the Z-disk but with punctate staining similar to T-tubule (L-type Ca2+ channel) and sarcoplasmic reticulum (SERCA2) markers. In skeletal muscle fibres AMPKγ3 localises to the I band, presenting a uniform staining that flanks the Z-disk, also coinciding with the position of Ca2+ influx in these muscles. The localisation of γ2-3B- and γ3-containing AMPK suggests that these trimers may have similar functions in the different muscles. AMPK containing γ2-3B was detected in oxidative skeletal muscles which had low expression of γ3, confirming that these two regulatory subunits may be co-ordinately regulated in response to metabolic requirements. Compartmentalisation of AMPK complexes is most likely dependent on the regulatory γ subunit and this differential localisation may direct substrate selection and specify particular functional roles.
doi:10.1007/s10974-013-9359-4
PMCID: PMC3853370  PMID: 24037260
AMPK; γ Subunits; Sub-cellular localisation; Cardiomyopathy
3.  Embryonic expression of AMPK γ subunits and the identification of a novel γ2 transcript variant in adult heart 
AMP-activated protein kinase (AMPK), the key sensor and regulator of cellular energy status, is a heterotrimeric enzyme with multiple isoforms for each subunit (α1/α 2; β1/β2; γ1/γ2/γ3). Mutations in PRKAG2, which encodes the γ2 regulatory subunit, cause a cardiomyopathy characterized by hypertrophy and conduction abnormalities. The two reported PRKAG2 transcript variants, γ2-short and γ2-long (encoding 328 and 569 amino acids respectively), are both widely expressed in adult tissues. We show that both γ2 variants are also expressed during cardiogenesis in mouse embryos; expression of the γ3 isoform was also detected unexpectedly at this stage. As neither γ2 transcript is cardiac specific nor differentially expressed during embryogenesis, it is paradoxical that the disease is largely restricted to the heart. However, a recently annotated γ2 transcript, termed γ2-3B as transcription starts at an alternative exon 3b, has been identified; it is spliced in-frame to exon 4 thus generating a protein of 443 residues in mouse with the first 32 residues being unique. It is increasingly expressed in the developing mouse heart and quantitative PCR analysis established that γ2-3B is the major PRKAG2 transcript (~ 60%) in human heart. Antibody against the novel N-terminal sequence showed that γ2-3B is predominantly expressed in the heart where it is the most abundant γ2 protein. The abundance of γ2-3B and its tissue specificity indicate that γ2-3B may have non-redundant role in the heart and hence mediate the predominantly cardiac phenotype caused by PRKAG2 mutations.
Highlights
► We have identified a novel PRKAG2 transcript of intermediate length (γ2-3B). ► γ2-3B is the most abundant cardiac AMPK γ2 at both mRNA and protein levels. ► Functional changes in AMPK containing γ2-3B may mediate PRKAG2 cardiomyopathy. ► γ2 and γ3 are the early embryonic AMPK γ subuits.
doi:10.1016/j.yjmcc.2012.05.017
PMCID: PMC3477313  PMID: 22683324
AMP-activated protein kinase; PRKAG2 transcripts; Cardiomyopathy
4.  Fumarate Is Cardioprotective via Activation of the Nrf2 Antioxidant Pathway 
Cell Metabolism  2012;15(3):361-371.
Summary
The citric acid cycle (CAC) metabolite fumarate has been proposed to be cardioprotective; however, its mechanisms of action remain to be determined. To augment cardiac fumarate levels and to assess fumarate's cardioprotective properties, we generated fumarate hydratase (Fh1) cardiac knockout (KO) mice. These fumarate-replete hearts were robustly protected from ischemia-reperfusion injury (I/R). To compensate for the loss of Fh1 activity, KO hearts maintain ATP levels in part by channeling amino acids into the CAC. In addition, by stabilizing the transcriptional regulator Nrf2, Fh1 KO hearts upregulate protective antioxidant response element genes. Supporting the importance of the latter mechanism, clinically relevant doses of dimethylfumarate upregulated Nrf2 and its target genes, hence protecting control hearts, but failed to similarly protect Nrf2-KO hearts in an in vivo model of myocardial infarction. We propose that clinically established fumarate derivatives activate the Nrf2 pathway and are readily testable cytoprotective agents.
Highlights
► Cardiac fumarase deletion (cFH1-KO) results in mice with elevated cardiac fumarate ► cFH1-KO is compensated for by amino acid influx into the citric acid cycle ► Nrf2 and its target genes are activated in the hearts of cFH1-KO mice ► Fumarate-related Nrf2 activation is cytoprotective and may be of therapeutic use
doi:10.1016/j.cmet.2012.01.017
PMCID: PMC3314920  PMID: 22405071
5.  Familial Dilated Cardiomyopathy caused by an Alpha-Tropomyosin Mutation: The Distinctive Natural History of Sarcomeric DCM 
Objectives
To further define the role of sarcomere mutations in DCM and associated clinical phenotypes.
Background
Mutations in several contractile proteins contribute to DCM, but definitive evidence for the roles of most sarcomere genes remains limited by the lack of robust genetic support.
Methods
Direct sequencing of 6 sarcomere genes was performed on 334 probands with DCM. A novel D230N missense mutation in the gene encoding α-tropomyosin (TPM1) was identified. Functional assessment was performed using an in vitro reconstituted sarcomere complex to evaluate ATPase regulation and Ca2+ affinity as correlates of contractility.
Results
TPM1 D230N segregated with DCM in two large unrelated families. This mutation altered an evolutionarily conserved residue and was absent in >1000 control chromosomes. In vitro studies demonstrated major inhibitory effects on sarcomere function with reduced Ca2+-sensitivity, maximum activation, and Ca2+ affinity compared to wildtype TPM1. Clinical manifestations ranged from decompensated heart failure or sudden death in those presenting early in life, to asymptomatic left ventricular dysfunction in those diagnosed during adulthood. Notably, several affected infants had remarkable improvement.
Conclusions
Genetic segregation in 2 unrelated families and functional analyses conclusively establish a pathogenic role for TPM1 mutations in DCM. In vitro results demonstrate contrasting effects of DCM and HCM mutations in TPM1, suggesting that specific functional consequences shape cardiac remodeling. Along with prior reports, our data support a distinctive, age-dependent phenotype with sarcomere-associated DCM where presentation early in life is associated with severe, sometimes lethal, disease. These observations have implications for the management of familial DCM.
doi:10.1016/j.jacc.2009.11.017
PMCID: PMC3000630  PMID: 20117437
cardiomyopathy; heart failure; genetics
6.  Normal passive viscoelasticity but abnormal myofibrillar force generation in human hypertrophic cardiomyopathy 
Hypertrophic cardiomyopathy (HCM) is characterized by left ventricular hypertrophy, increased ventricular stiffness and impaired diastolic filling. We investigated to what extent myocardial functional defects can be explained by alterations in the passive and active properties of human cardiac myofibrils. Skinned ventricular myocytes were prepared from patients with obstructive HCM (two patients with MYBPC3 mutations, one with a MYH7 mutation, and three with no mutation in either gene) and from four donors. Passive stiffness, viscous properties, and titin isoform expression were similar in HCM myocytes and donor myocytes. Maximal Ca2+-activated force was much lower in HCM myocytes (14 ± 1 kN/m2) than in donor myocytes (23 ± 3 kN/m2; P < 0.01), though cross-bridge kinetics (ktr) during maximal Ca2+ activation were 10% faster in HCM myocytes. Myofibrillar Ca2+ sensitivity in HCM myocytes (pCa50 = 6.40 ± 0.05) was higher than for donor myocytes (pCa50 = 6.09 ± 0.02; P < 0.001) and was associated with reduced phosphorylation of troponin-I (ser-23/24) and MyBP-C (ser-282) in HCM myocytes. These characteristics were common to all six HCM patients and may therefore represent a secondary consequence of the known and unknown underlying genetic variants. Some HCM patients did however exhibit an altered relationship between force and cross-bridge kinetics at submaximal Ca2+ concentrations, which may reflect the primary mutation. We conclude that the passive viscoelastic properties of the myocytes are unlikely to account for the increased stiffness of the HCM ventricle. However, the low maximum Ca2+-activated force and high Ca2+ sensitivity of the myofilaments are likely to contribute substantially to any systolic and diastolic dysfunction, respectively, in hearts of HCM patients.
Research Highlights
► The passive stiffness of skinned HCM cardiac myocytes was similar to that of normal (donor) myocytes. ► Maximum Ca-activated force production was reduced by 40% in HCM vs donor myocytes. ► This loss of force could contribute to systolic dysfunction in HCM hearts. ► Myofibrillar Ca sensitivity was higher in HCM than in donor myocytes. ► The enhanced Ca sensitivity could compensate for the smaller maximum force but would tend to cause diastolic dysfunction. ► These characteristics were common to all HCM patients studied, suggesting the changes were secondary consequence of the underlying genetic variants.
doi:10.1016/j.yjmcc.2010.06.006
PMCID: PMC2954357  PMID: 20615414
Hypertrophic cardiomyopathy; Skinned cardiac myocytes; Viscoelasticity; Ca2+ sensitivity; Cross-bridge kinetics
7.  Determination of AMP-activated protein kinase phosphorylation sites in recombinant protein expressed using the pET28a vector: A cautionary tale 
AMP-activated protein kinase (AMPK) is responsible for sensing of the cell’s energetic status and it phosphorylates numerous substrates involved in anabolic and catabolic processes as well as interacting with signaling cascades. Mutations in the gene encoding the γ2 regulatory subunit have been shown to cause hypertrophic cardiomyopathy (HCM) with conduction abnormalities. As part of a study to examine the role of AMPK in the heart, we tested whether specific domains of the thick filament component cardiac myosin binding protein-C (cMyBP-C) were good in vitro AMPK substrates. The commercially available pET28a expression vector was used to generate a recombinant form of the cMyBP-C C8 domain as a fusion protein with a hexahistidine tag. In vitro phosphorylation with activated kinase showed that the purified fusion protein was a good AMPK substrate, phosphorylated at a similar rate to the control SAMS peptide and with phosphate incorporation specifically in serine residues. However, subsequent analysis of alanine replacement mutants and thrombin digestion revealed that the strong AMPK phosphorylation site was contained within the thrombin cleavage sequence encoded by the vector. As this sequence is common to many commercial pET vectors, caution is advised in the mapping of AMPK phosphorylation sites when this sequence is present.
doi:10.1016/j.pep.2009.02.016
PMCID: PMC2691924  PMID: 19269329
AMP-activated protein kinase; Recombinant protein; Phosphorylation

Results 1-7 (7)