Abstract:
Methods for treating cardiovascular disease, and in particular heart failure, are provided comprising administering a therapeutically effective amount of a modulator of SERCA2a post-translation modification such as SUMOylation or acetylation. Also provided are methods of treating cardiovascular disease by inhibiting SERCA2a degradation. Further provided are methods of diagnosing a propensity to develop heart failure comprising determining if a SERCA2a mutant is present or determining the level of expression of SUMO1 in cardiomyocytes. The disclosure also provides methods of screening for therapeutics that modulate the post-translational modification of SERCA2a, such as by modulating post-translational SUMOylation and/or acetylation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of provisional U.S. Patent Application No. 61/507,526 filed Jul. 13, 2011, which is incorporated herein by reference in its entirety. 
     
    
     FIELD 
       [0002]    The disclosure relates generally to the prevention and treatment of cardiac disease and, more specifically, to the modulation of SERCA2a to prevent and/or treat cardiac disease or ameliorate a symptom thereof. 
       BACKGROUND 
       [0003]    SERCA2a is a critical ATPase responsible for Ca 2+  re-uptake by cardiac muscle cells during excitation-contraction coupling. The down-regulation of SERCA2a is one of the primary abnormalities found in failing hearts. Consistent with this observation, restoration of SERCA2a by gene transfer has proven to be effective in normalizing cardiac function in humans as well as model animals. 
         [0004]    Heart failure (HF) represents complex patho-physiological conditions that are often final consequences of various cardiovascular disorders including atherosclerosis, cardiomyopathy, and hypertension. The incidence of HF continues to grow worldwide. HF is characterized by contractile dysfunction that is in large part due to abnormalities in sarcoplasmic reticulum (SR) Ca 2+  cycling (Gwathmey et al., 1987; Gwathmey and Hajjar, 1990). In normal human cardiomyocytes, the activity of SERCA2a contributes to the removal of more than 70% of cytosolic Ca 2+  into the SR during diastole. SERCA2a, therefore, affects muscle contraction kinetics by determining the SR Ca 2+  content in the subsequent beat (MacLennan and Kranias, 2003). Impaired SR Ca 2+  uptake due to a decreased expression level and a reduced activity of SERCA2a has been reported in failing human hearts (Meyer et al., 1995; Minamisawa et al., 1999; Zarain-Herzberg et al., 1996). It is known that restoration of SERCA2a levels by gene transfer improves systolic and diastolic dysfunction in rodent (del Monte et al., 2001) and porcine models of HF (Byrne et al., 2008; Kawase et al., 2008). 
         [0005]    Post-translational modification (PTM) is an important way to modulate the function of diverse cellular proteins by affecting their enzymatic activity, localization, stability, or turnover rates in response to environmental stimuli. It was previously shown that SERCA2a activity could be modulated by PTM such as glutathiolation (Adachi et al., 2004; Dremina et al., 2007; Lancel et al., 2009) and nitration (Knyushko et al., 2005). It is also known that the isoelectric point of SERCA2a becomes both more acidic and basic in the failing heart compared to the normal heart. In addition, restoration of SERCA2a levels by gene transfer also partially restored this shifted isoelectric point of SERCA2a in the failing heart (Figure S 1 ). These data indicated that there existed multiple PTMs of SERCA2a, which are associated with the development of HF. 
         [0006]    Small ubiquitin-related modifier (SUMO), which shares 18% sequence homology with ubiquitin, can be conjugated to lysine residues of target proteins. This PTM is referred to as SUMOylation. In humans, three SUMO isoforms (SUMO1-3) appear to modify both common and distinct substrates (Welchman et al., 2005). Specifically, SUMO1 has been shown to play important roles in modulating diverse cellular processes including transcriptional regulation, nuclear transport, DNA repair, cell cycle, plasma membrane depolarization, and signal transduction both in normal and pathogenic conditions (Sarge and Park-Sarge, 2009). SUMO-mediated regulation of cardiac transcriptional factors such as GATA4 (Wang et al., 2004) and Nkx2.5 (Wang et al., 2008) is associated with differentiation of cardiomyocytes and development of cardiac structures. SUMO-mediated modification also regulates cardiac ion channel activity including voltage-gated potassium channels (Benson et al., 2007). In addition, SUMOylation of ERK5 has recently been linked to diabetes-related heart conditions (Shishido et al., 2008). 
         [0007]    Over the past few years, a host of studies has shown that SUMOylation can regulate the activities of a variety of proteins both in normal and human pathogenesis, including neurodegenerative diseases, cancer, and familial dilated cardiomyopathy (Kim and Baek, 2006; Steffan et al., 2004; Zhang and Sarge, 2008). Recently, the critical role of SUMOylation of ERK5 in diabetic heart has been reported (Woo and Abe, 2010). 
         [0008]    SUMOylation can affect biochemical properties of target proteins such as enzymatic activities and stabilities. The underlying molecular mechanism is largely unknown, but three possibilities have been suggested. SUMO attachment may alter interaction between the target and its binding partners (DNA or protein) by masking of existing binding sites or addition of interfaces that are present in SUMO. Alternatively, SUMOylation may induce a conformational change of the target proteins, which can either increase or decrease the enzymatic activities. Finally, SUMOylation can block other PTMs at lysine residues such as ubiquitination and acetylation, which lead to alterations in the functional properties of target proteins. 
         [0009]    It has been shown that SERCA2a is a target of oxidative PTMs. Accumulated nitration of SERCA2a has been observed in skeletal muscle undergoing electrical stimulation, in hypercholesteremic aorta, and in ischemic human heart. The nitration at tyrosines 294 and 295 was correlated with the reduced Ca 2+−  ATPase activity of SERCA2a. The position of these tyrosines within a functionally key membrane region of SERCA2a and close to a negatively charged side chain would seem to ensure both efficient nitration and a mechanism for decreased rates of calcium transport. In addition, oxidation of a redox-sensitive cysteine residue of SERCA2a (cysteine 674) was detected in diabetic pigs (Ying et al., 2008). This oxidative modification may be related to the accelerated SERCA degradation in ischemic heart (French et al., 2006) and in H9c2 cells exposed to hydrogen peroxide Mara et al., 2005). 
         [0010]    Accordingly, a need continues to exist in the art for therapeutics and methods of treating cardiovascular disease such as heart failure in a manner that is safe and effective for humans and other animals. 
       SUMMARY 
       [0011]    The subject matter disclosed herein satisfies at least one of the aforementioned needs in the art for therapeutics and methods of treating cardiovascular disease. In particular, the experiments disclosed herein establish that SERCA2a is SUMOylated. The SUMO1 level and SUMOylation of SERCA2a was greatly reduced in failing hearts. SUMO1 overexpression restored impaired cardiac function in failing hearts partly through enhancing enzymatic activity and stability of SERCA2a, whereas SUMO1 down-regulation resulted in cardiac dysfunction. The data provide novel insight on the regulation of SERCA2a function by PTM and provide the basis for the design of novel therapeutic strategies for HF. 
         [0012]    Various aspects of the disclosed subject matter are described in the following enumerated paragraphs. 
         [0013]    1. A method of treating cardiac dysfunction in a subject comprising administering a therapeutically effective amount of a modulator of SERCA2a post-translational modification to the subject. 
         [0014]    2. The method according to paragraph 1 wherein the cardiac dysfunction is selected from the group consisting of heart failure, pressure overload-induced cardiac dysfunction, and cardiac dysfunction induced by inhibited calcium decay. 
         [0015]    3. The method according to paragraph 2 wherein the heart failure comprises contractile dysfunction. 
         [0016]    4. The method according to paragraph 2 wherein the heart failure is TAC-induced heart failure. 
         [0017]    5. The method according to paragraph 1 wherein the subject is a human. 
         [0018]    6. The method according to paragraph 1 wherein the modulator modulates SERCA2a post-translational SUMOylation. 
         [0019]    7. The method according to paragraph 6 wherein the modulator is a vector comprising an expressible coding region encoding a protein selected from the group consisting of SERCA2a and SUMO1, and wherein the coding region is operably linked to at least one expression control element. 
         [0020]    8. The method according to paragraph 7 wherein the vector is a recombinant adeno-associated virus. 
         [0021]    9. The method according to paragraph 8 wherein the recombinant adeno-associated virus is rAAV1. 
         [0022]    10. The method according to paragraph 1 wherein the modulator modulates SERCA2a post-translational acetylation. 
         [0023]    11. The method according to paragraph 10 wherein the modulator is Sirt1 deacetylase. 
         [0024]    12. A method of treating a cardiovascular disorder in a subject by inhibiting SERCA2a degradation comprising administering a therapeutically effective amount of a SUMO1 agent. 
         [0025]    13. The method according to paragraph 12 wherein the SUMO1 agent is a vector comprising an expressible coding region encoding a protein selected from the group consisting of SERCA2a and SUMO1, and wherein the coding region is operably linked to at least one expression control element. 
         [0026]    14. The method according to paragraph 13 wherein the vector is recombinant adeno-associated virus. 
         [0027]    15. The method according to paragraph 14 wherein the recombinant adeno-associated virus is rAAV1. 
         [0028]    16. A method of diagnosing a propensity to develop heart failure comprising determining the amino acid corresponding to a position selected from the group consisting of any of positions 479-482 and/or position 584-587 of human SERCA2a (SEQ ID NO:2). 
         [0029]    17. A method of diagnosing a propensity to develop heart failure comprising determining the polynucleotide sequence encoding an amino acid corresponding to any of amino acids 479-482 or 584-587 of human SERCA2a (SEQ ID NO:1). 
         [0030]    18. A method of diagnosing a propensity to develop heart failure comprising determining the level of expression of SUMO1 in a cardiomyocyte of a subject and comparing that level to the level of expression of SUMO1 in a cardiomyocyte of a healthy control, wherein reduced expression of SUMO1 relative to the control is indicative of a propensity to develop cardiac failure. 
         [0031]    19. A method of screening for a therapeutic to treat heart failure comprising contacting SUMO1 and SERCA2a in the presence and absence of a candidate therapeutic and identifying the candidate therapeutic as a therapeutic if the level of SERCA2a SUMOylation is greater in the presence compared to the absence of the candidate therapeutic. 
         [0032]    Other features and advantages of the present disclosure will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating preferred embodiments, are provided for illustration only, because various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0033]      FIG. 1 . SERCA2a complex. (A) Two-dimensional SDS-PAGE gels of SERCA2a complex. Silver-stained SDS-PAGE gel shows a 12 kDa spot (arrow) immunoprecipitated with SERCA2a. Enrichment of SERCA2a was confirmed by Western blotting obtained from of the immunoprecipitated SERCA2a complex with rabbit anti-SERCA2a antibody (anti-S2a). Rabbit IgG (anti-IgG) complex used as a negative control. (B) The representative peptide fingerprint for SUMO1 identified by tandem mass spectrometric analysis. Protein sequence of SUMO1 with matched peptide shown in bold. (C) in vivo SERCA2a SUMOylation in HEK293 cells expressing flag-tagged SUMO1. Precipitated flag-tagged SUMO1 conjugates were analyzed by anti-SERCA2a. (D) Interaction between SERCA2a and Ubc9. HEK293 cells were transfected with expressing vectors for myc-tagged Ubc9 and SERCA2a or pcDNA vector (negative control). Cell lysates were subjected to immunoprecipitation (IP) with anti-myc and the resulting precipitates were subjected to Western blotting with anti-SERCA2a. 
           [0034]      FIG. 2 . Endogenous SUMO1 protein level is decreased in both experimental model and human heart failure. (A) SERCA2a SUMOylation in human cardiac tissues. Representative blots showing protein expressions with subsequent quantification (n=5 per each group). S-SERCA2a: SUMOylated SERCA2a. (B) Representative Western blotting of SUMO1 and SERCA2a in TAC-induced mouse model of heart failure with subsequent quantification. TAC-induced failing hearts (TAC, n=5); sham-operated control hearts (Sham, n=8). (C) Western blotting of one representative experiment with subsequent quantification. The protein amounts of SUMO1 and SERCA2a in the heart of eight pigs were analyzed. MR-induced failing porcine hearts (MR, n=3); Sham-operated control hearts (Sham, n=5). Protein band intensities were assessed by densitometry using the ImageJ system. The internal standard GAPDH was used to normalize for equal protein loading. All data represent means±SD. * p&lt;0.05 vs. respective control using Student t-test. TAC: trans-aortic constriction, MR: mitral valve regurgitation, NF: non-failing, or normal, heart. 
           [0035]      FIG. 3 . SUMO1 is conjugated with lysines 480 and 585 of SERCA2a and is required for SERCA2a function. (A) N-domain of SERCA2a. Protein sequence alignment of SERCA2a of human, pig, rat and mouse from positions 360-603 of SEQ ID NOS:2, 4, 6 and 8, respectively. Putative SUMO consensus motifs (ΨKXE, where Ψ is a hydrophobic amino acid) contained the Lysine 480 and Lysine 585 of SERCA2a. SUMO modification sites of SERCA2a are marked by a black outline. Hydrolase domains are outline in a dotted line. (B) in vivo SUMOylation of SERCA2a. HEK293 cells were co-transfected with plasmids expressing flag-tagged SUMO1 and myc-tagged Ubc9 and WT or mutants SERCA2a. SUMOylated forms of SERCA2a were detected by Western blotting using anti-SERCA2a antibody. (C) Dose response of SUMO1 expression in SUMOylation of SERCA2a. HEK293 cells were co-transfected with myc-tagged Ubc9 and WT or K480R/K585R SERCA2a mutant. Indicated amount of flag-tagged SUMO1 or equal amount of the corresponding empty vector were added. Western blotting shows dose-effects of SUMO1 on SERCA2a SUMOylation. (D) Ca 2+ -dependent ATPase activity of WT and K480R/K585R SERCA2a mutant in the presence and absence of additional SUMO1. The data shown were acquired from three independent experiments with each carried out in duplicate. (E) ATP binding capacity of WT and K480R/K585R SERCA2a. Transfected HEK293 cell lysates with indicated plasmids were affinity-precipitated with ATP-sepharose and subsequently subjected to Western blotting with anti-SERCA2a antibody. (F) Western blotting was also performed with indicated antibodies at different time points, i.e., a temporal series of HEK293 cell lysates were affinity-purified with ATP-sepharose and subjected to Western blot analysis with an anti-SERCA2a antibody. (G) Effects of SUMO1 overexpression on the protein stability of WT and K480R/K585R SERCA2a mutant in HEK293 cells. Quantification data are represented as the relative ratio of day 0 (n=3). All data represent means±SD. * p&lt;0.05; ** p&lt;0.001 vs. respective control using Student t-test. 
           [0036]      FIG. 4 . SUMO1 overexpression enhances cardiomyocytes contractility and calcium transients in both normal and failing cardiomyocytes. Average parameters of cardiomyocytes contraction were determined (Sham contractility, n=6 mice/143 cells; HF contractility, n=8 mice/181 cells). Average parameters of Ca 2+  transient properties were determined (Sham calcium transient, n=4 mice/65 cells; HF calcium transient, n=6 mice/131 cells). All data represent means±SD. * p&lt;0.05; ** p&lt;0.001 vs. respective control using Student t-test. 
           [0037]      FIG. 5 . SUMO1 overexpression restores TAC-induced cardiac dysfunctions. (A) Generation of conditional SUMO1 transgenic mice. Cardiac SUMO1 express under the control of the α-MHC promoter. (B) Western blotting represents cardiac SUMO1 overexpression in transgenic mice. WT: wild type littermates, TG: SUMO1 transgenic mice. (C) Representative gross hearts (top left), hematoxylin and eosin staining (bottom left) and M-mode imaging (right) shows morphology of the heart, left ventricular function and dimensions in WT and SUMO1 transgenic mice at 3 months post-TAC operations time point. (D) Echocardiographic measurements of internal diameters in end-diastole (LVIDd), end-systole (LVIDs), fractional shortening (FS) and ejection fraction (EF). Sham: WT, n=12; TG, n=10; TAC: WT, n=14; TG, n=12. (E) Kaplan-Meier survival curves in response to TAC operations. WT represents a dotted line (n=15) and SUMO1 transgenic mice represents closed line (n=14). (F) Representative Western blotting shows alternations of cardiac protein expressions in SUMO1 transgenic mice (left). Band intensities were quantified by densitometry measurement and normalized with GAPDH control. Data are represented as the relative ratio (n=4 per each group, right). (G) Ca 2+ -dependent ATPase activity of SERCA2a in preparations from sham-operated WT (◯), sham-operated SUMO1 transgenic mice (), preparations from TAC-operated WT (∇), and TAC-operated SUMO1 transgenic mice (▴) hearts (n=3 per each group). All data represent means±SD. * p&lt;0.05; ** p&lt;0.001 vs. respective control using Student t-test. NS: non-significant. 
           [0038]      FIG. 6 . Reduction of SUMO1 level accelerated cardiac dysfunction. (A) SUMO1 shRNA construct design. The rAAV expresses a SUMO1 shRNA sequence under the control of U6 promoter. Scramble expression cassette was cloned into the same viral vectors. (B) Representative Western blotting of cardiac tissue extracts from mice delivered with rAAV9 expressing scrambled control shRNA (rAAV9/SC) or shRNA against SUMO1 (rAAV9/shSUMO1) at 3 weeks after injection (left). Total extracts were probed with indicated antibodies Inhibition of cardiac SUMO1 expression was measured (n=5 per each group, right). (C) Representative gross (top left), section of the hearts (top right) and M-mode imaging (bottom) shows heart morphology, cardiac function and dimensions in mice injected rAAVs at 6 weeks after injection. (D) Assessments of the internal diameters in end-diastole (LVIDd), end-systole (LVIDs), fractional shortening (FS), and ejection fraction (EF) of rAAV9/shSC and rAAV9/shSUMO1. Echocardiographic parameters were determined (n=14 per each group). (E) Survival of animals with rAAV9-mediated cardiac knockdown of SUMO1. Kaplan-Meier method was used to analyze lifespan of animals obtained by different dose of rAAV9/shSUMO1 (n=14 per each group) or rAAV9/shSC injection (n=24). (F) Representative Western blotting shows expression levels of cardiac proteins (left panel). 6 weeks after tail-vein injection with rAAV9/shSC (n=4) or rAAV9/shSUMO1 (n=7) were analyzed by Western blotting with indicated antibodies. Data are represented as the relative ratio. (G) Ca 2+ -dependence of SERCA2a&#39;s ATPase activity was demonstrated in preparations from scramble (•) injected and shRNA against SUMO1 (•) injected hearts (n=3 per each group). For each concentration, the upper data point is rAAV9/SC; the lower data point is rAAV9/shSUMO1. All data represent means±SD. * p&lt;0.05; ** p&lt;0.001 vs. respective control using Student t-test. NS: non-significant. 
           [0039]      FIG. 7 . A working model for regulation of SERCA2a function by SUMOylation. Under basal conditions, SUMOylation enhances SERCA2a protein stability and Ca 2+  pump functions to regulate cardiac contractility however, increasing unSUMOylated SERCA2a forms followed by low SUMO1 protein pool trigger impaired SERCA2a and cardiac dysfunction under the pathophysiological condition. 
           [0040]      FIG. 8 . Aligned amino acid sequences. Aligned amino acid sequences of wild-type SERCA2a from human ( Homo sapiens , SEQ ID NO:2), pig ( Sus scrofa , SEQ ID NO:4), rat ( Rattus norvegicus , SEQ ID NO:6) and mouse ( Mus musculus , SEQ ID NO:8) are presented. Residues noted herein as being involved in SERCA2a modifications are conserved, as evidenced by the conserved lysine residues at positions 480 and 585, as well as the conserved cysteine at position 674. 
           [0041]      FIG. 9 . Aligned polynucleotide sequences. Aligned polynucleotide coding region sequences encoding wild-type SERCA2a in human, pig, rat and mouse. These polynucleotide coding region sequences are presented in the sequence listing at positions 564-3557 of SEQ ID NO:1 for human SERCA2a, positions 14-3007 of SEQ ID NO:3 for pig SERCA2a, positions 507-3500 for rat SERCA2a, and positions 541-3537 for mouse SERCA2a. 
       
    
    
     DETAILED DESCRIPTION 
       [0042]    Disclosed herein are data establishing that SERCA2a is SUMOylated at lysine residues 480 and 585 and that this SUMOylation preserves the ATPase activity and stability of SERCA2a. The significance of SUMOylation was further demonstrated by the observation that a SERCA2a variant (K480R/K585R) lacking the SUMOylated residues possessed a significantly reduced ATPase activity and stability. In isolated cardiomyocytes, adenovirus-mediated SUMO1 overexpression augmented contractility and calcium transients with an accelerated calcium decay. Transgene-mediated SUMO1 overexpression rescued pressure overload-induced cardiac dysfunction concomitantly with increased SERCA2a function. In contrast, down-regulation of SUMO1 level using shRNA accelerated pressure overload-induced deterioration of cardiac function accompanied by a decreased SERCA2a function. Taken together, the work disclosed herein shows that SUMOylation is a critical post-translational modification regulating SERCA2a function, and provides a method for treating a cardiac dysfunction or disorder, e.g., heart failure, by modifying intracellular calcium in the heart. Human clinical trials with a recombinant adeno-associated virus encoding SERCA2a (rAAV1/SERCA2a) have been initiated and the results indicate that targeting SERCA2a is a safe and effective modality for the treatment of human HF. 
         [0043]    Disclosed herein for the first time is the SUMOylation of SERCA2a at two lysine residues. Interestingly, both SERCA2a levels and SUMOylation of SERCA2a were significantly reduced in failing hearts. Compelling evidence, disclosed in the Detailed Description below, established that the reduced SUMOylation of SERCA2a is a direct result of the reduced SUMO1 level in failing hearts. This reduced SUMOylation was strictly correlated with reduced ATPase activity of SERCA2a and with reduced SERCA2a stability. Moreover, restoration of SUMO1 reversed contractile dysfunctions in failing hearts. These results are summarized in  FIG. 7 . 
         [0044]    The conserved nature of the amino acid sequence of SERCA2a is illustrated in  FIG. 8 , which provides an aligned amino acid sequences for the SERCA2a of human (SEQ ID NO:2), pig (SEQ ID NO:4), rat (SEQ ID NO:6) and mouse (SEQ ID NO:8). In  FIG. 9 , aligned polynucleotide sequences encoding these SERCA2a amino acid sequences, with human (SEQ ID NO:1), pig (SEQ ID NO:3), rat (SEQ ID NO:5) and mouse (SEQ ID NO:7) polynucleotide sequences being presented. The SUMOylation motif noted in the brief description of  FIG. 3  are found at positions 479-482 (MKKE, SEQ ID NOS:10, 12, 14, and 16 for human, pig, rat and mouse, respectively) and at positions 584-587 (IKYE, SEQ ID NOS:10, 12, 14, and 16). It is expected that single amino acid changes in either or both of these four-amino-acid motifs will modify SERCA2a in a manner that modulates its effect on cardiac function. It is further expected that most of these variations will interfere or inhibit SUMOylation of SERCA2a, leading to increased likelihood of cardiovascular disease such as HF. Accordingly, one method for diagnosing a disposition towards cardiovascular disease comprises obtaining a biological sample from a patient and determining the amino acid sequence of SERCA2a at positions 479-482 and/or positions 584-587 and diagnosing a disposition towards cardiovascular disease if the amino acid sequence varies from the wild-type sequence disclosed herein. Analogous diagnostic methods are contemplated for the encoding polynucleotide sequence(s). 
         [0045]    Amino acid sequences of the SUMO-1 protein mediating the PTM of SERCA2a that affects cardiac function are also presented for human (SEQ ID NO:10), pig (SEQ ID NO:12), rat (SEQ ID NO:14) and mouse (SEQ ID NO:16). Polynucleotide sequences encoding these amino acid sequences are set forth in SEQ ID NOs:9, 11, 13, and 15 for human, pig, rat, and mouse, respectively. 
         [0046]    Without wishing to be bound by theory, it is possible that the effect of SUMOylation on SERCA2a ATPase activity results from an induced conformational change in SERCA2a; alternatively, SUMOylation may lead to an additional interface for ATP binding, leading to an increase in ATPase activity. It is also possible that SUMOylation may affect other post-translational modifications (PTMs) of SERCA2a, such as the acetylation of particular residues. Previous studies indicated that a host of regulatory proteins were reciprocally and competitively regulated by SUMOylation and acetylation. For example, the transcriptional activity of tumor repressor gene HIC1 is promoted by SUMOylation and inhibited by acetylation (Van Rechem et al., 2010). Interestingly, acetylation of SERCA2a has been recently identified in a large-scale analysis of human acetylome in cancer cell lines (Choudhary et al., 2009). Also, it has been found that SERCA2a is acetylated and that this acetylation is more prominent in failing hearts and can be reversed by Sirt1 deacetylase. 
         [0047]    The reciprocal regulation of SUMOylation and ubiquitination is consistent with SUMOylation stabilizing SERCA2a. This type of SUMOylation-mediated inhibition of protein degradation has been shown for other proteins. For example, SUMOylation of Axin, a negative regulator of Wnt signaling, prevented ubiquitination and thus conferred a prolonged half-life of Axin (Kim et al., 2008). Similarly, SUMOylation of p68 and p72 RNA helicases increased the stabilities of these proteins by reducing ubiquitin-proteasome-mediated protein degradation (Mooney et al., 2010). 
         [0048]    The data disclosed herein establish that the SUMO1 level is significantly reduced in failing hearts, providing experimental basis for the position that cellular SUMO1 level should be precisely maintained and controlled for proper functions of cardiomyocytes. The finding disclosed herein that replenishment of SUMO1 reversed TAC-induced failing phenotypes indicated that reduced SUMO1 level is the direct cause of contractile dysfunction. The therapeutic effect of SUMO1 gene transfer was profound. In contrast to the reduced SUMO1 level, the protein level of the SUMOylating and de-SUMOylating enzymes, Ubc9 and SENP1, were unaltered in failing hearts and when shSUMO1 was administered. The level of Ubc9 and SENP1 was also unaltered when the SUMO1 level was restored. Therefore, the specificity and capacity of SUMOylation is unlikely to be changed in failing hearts. What matters most appears to be the reduced supply of SUMO1. In this regard, it is intriguing to note that depletion of cellular ubiquitin level is sufficient to cause neuronal dysfunction and death (Ryu et al., 2008). 
         [0049]    In the experiments disclosed hereinbelow, a novel regulatory mechanism is disclosed whereby SUMOylation affects or modulates SERCA2a activity and overall contractile properties of the cardiac muscle cells. In addition, the impressive beneficial effects of SUMO1 on cardiac contractility and survival indicates that targeting SUMO1 will have a therapeutic value in the treatment of heart failure. 
         [0050]    The following examples illustrate embodiments of the disclosure. Example 1 discloses the materials and methods used in the experiments described herein. Example 2 discloses data establishing that SUMO1 interacts with SERCA2a. Example 3 shows that SUMOylation of SERCA2a is reduced in failing hearts. Example 4 reveals that SERCA2a is SUMOylated at lysine residues K480 and K585. Example 5 shows that SUMOylation of SERCA2a increases the ATPase activity of SERCA2a. Example 6 establishes that SUMOylation enhances SERCA2a stability. Example 7 shows that SUMO1 overexpression enhances cardiomyocyte contractility and enhances Ca 2+  transients in isolated cardiomyocytes. Example 8 further shows that SUMO1 overexpression improves cardiac function in TAC-induced heart failure. Example 9 shows that small hairpin RNA mediates down-regulation of SUMO1, which accelerates cardiac dysfunction. 
       Example 1 
       [0051]    This example provides a description of the materials and methods used in the experiments disclosed herein. 
       In Vivo SUMOylation Assay 
       [0052]    To analyze SUMOylation within cells, Lipofectamine 2000 was used to transfect HEK293 cells with plasmids encoding SERCA2a wild type (WT) or SERCA2a SUMOylation site mutants, along with flag-tagged SUMO1 and myc-tagged Ubc9. Cells were lysed by sonication in ice-cold lysis buffer (50 mM Tris-Cl, pH 8.0, 150 mM NaCl, 0.1% Triton X-100, 10 mM EDTA, complete protease inhibitor [one tablet per 10 ml; Roche], and protein phosphatase inhibitor cocktail (Sigma)) containing 20 mM N-ethylmaleimide. Lysates were cleared by centrifugation at 30,000 g for 20 minutes. Cell lysates were then subjected to immune-precipitation by incubation with a flag-specific affinity matrix gel (Sigma) overnight at 4° C., after which the immunoprecipitates were washed in cold lysis buffer Immunocomplexes were resolved by SDS-PAGE, and subjected to Western blotting with SERCA2a-specific antibody, i.e., anti-SERCA2a antibody. 
         [0053]    Fresh tissue extracts were prepared in lysis buffer for in vivo SUMOylation assays. Hearts from each experimental and control group were frozen in liquid nitrogen. Frozen tissues were crushed and homogenized in lysis buffer, as described above, using the MP homogenate system (FastPrep homogenizer). The insoluble portion was removed by centrifugation at 30,000 g for 20 minutes. Extracts were incubated with anti-SUMO1 agarose resin with agitation overnight. The SUMO conjugated forms were detected by Western blotting with specific primary antibodies. 
       SERCA2a Activity Assay 
       [0054]    Crude microsome was prepared as previously described (Clarke et al., 1989). SERCA2a activity assays were performed using pyruvate/NADH-coupled reactions, as previously described (Hajjar et al., 1997). The activity of the Ca 2+ -ATPase was calculated as follows: Δabsorbance/6.22×protein×time (in nmol ATP/mg protein×min) All assays were done in triplicate. 
       Generation of Conditional SUMO1 Transgenic Mouse 
       [0055]    The αMHC-flox-mouse SUMO1 transgene was subcloned into the pML2G vector, which has an EGFP cDNA between two loxP sites. The DNA construct was microinjected into fertilized eggs from B6C3 mice and transgenic integration was confirmed by PCR. 
       Statistical Analysis 
       [0056]    The analysis was performed using the Student&#39;s t test, with significant differences demarcated by a single asterisk (*), indicating p&lt;0.05, or by a double asterisk (**), indicating p&lt;0.001. Data in figures represent mean±SD. 
       Example 2 
     SUMO1 Interacts with SERCA2a 
       [0057]    As an approach to identify novel modifiers of SERCA2a, the SERCA2a-associated protein complex was isolated from porcine heart lysates by immunoprecipitation with anti-SERCA2a antibody and then analyzed by two-dimensional electrophoresis ( FIG. 1A ). MS/MS analysis of the stained protein spots revealed that SUMO1 is co-precipitated with SERCA2a. A representative MS/MS peptide fingerprint for SUMO1 is shown in  FIG. 1B . HEK293 cells were co-transfected with plasmids encoding SERCA2a and flag-tagged SUMO1 Immunoprecipitation with anti-flag antibody followed by Western blotting with anti-SERCA2a antibody showed that SERCA2a indeed binds to SUMO1 ( FIG. 1C ). A similar experiment performed with HEK293 cells co-transfected with plasmids encoding SERCA2a and MYC-tagged Ubc9, a SUMO conjugating enzyme, also confirmed that SERCA2a binds to Ubc9 ( FIG. 1D ). These data establish that SERCA2a is a target of SUMOylation. 
       Example 3 
     SUMOylation of SERCA2a is Reduced in Failing Hearts 
       [0058]    We then examined whether SERCA2a is indeed SUMOylated in hearts. Human heart lysates were immunoprecipitated with anti-SUMO1 antibody and probed with anti-SERCA2a antibody. In addition to a normal SERCA2a band (˜110 kDa), slowly migrating SERCA2a bands (150˜250 kDa) were detected ( FIG. 2A , top) which represent the SUMOylated SERCA2a. The level of SUMOylated SERCA2a was significantly reduced in failing hearts (HF) compared to normal hearts (NF). In addition, SERCA2a levels were significantly reduced in the failing hearts, consistent with previous reports, and supporting the validity of the heart sample preparations. Along with this reduced SERCA2a level, SUMO1 level was also significantly reduced in the failing hearts. In contrast, the levels of Ubc9 and SENP1, the critical SUMOylating and de-SUMOylating enzymes, respectively, were unaltered in the failing hearts ( FIG. 2A , bottom). These data indicated that the reduced SUMOylation of SERCA2a might be primarily due to the reduced level of SUMO1 but not to the reduced SUMOylating or elevated de-SUMOylating activities. 
         [0059]    We have also observed that SUMO1 level as well as SERCA2a level were significantly reduced in a murine model of HF induced by pressure-overload ( FIG. 2B ) and in a porcine model of HF induced by volume-overload ( FIG. 2C ). These results established that reduced SUMOylation of SERCA2a caused by a reduced SUMO1 level is a prevailing characteristic associated with the development of HF in diverse mammalian species. 
       Example 4 
     SERCA2a is SUMOylated at Lysines 480 and 585 
       [0060]    SUMOylation of target proteins is known to occur on lysine residues in the context of a highly conserved recognition motif, γΨKχE/D (where Ψ stands for a large hydrophobic amino acid and χ for any amino acid) (Sampson et al., 2001). Three independent SUMOylation prediction programs (http://bioinformatics.lcd-ustc.org, http://www.abgent.com.cn/doc/sumoplot, http://sumosp.biocuckoo.org/prediction.php) identified two putative SUMO conjugating sites in SERCA2a, lysines 480 (K480) and 585 (K585). These lysine residues are located in the cytosolic nucleotide-binding domain where ATP binds and are perfectly conserved in mouse, rat, pig, and human SERCA2a ( FIG. 3A ). 
         [0061]    To investigate the role of SERCA2a K480 and K585 during SUMOylation, we generated three SERCA2a variants in which K480 or K585 was replaced by arginine (K480R and K585R, respectively) or both K480 and K585 were replaced by arginine (K480R/K585R). HEK293 cells were transfected with plasmids encoding wild type (WT) and these SERCA2a variants, and then the cell lysates were immunoprecipitated with anti-SUMO1 antibody and probed with anti-SERCA2a antibody. While K480R and K585R were SUMOylated indistinguishably from WT SERCA2a, K480R/K585R was completely unSUMOylated ( FIG. 3B ). The SUMOylation of WT SERCA2a was enhanced when increasing amounts of the SUMO1 plasmid were co-transfected in a dose-dependent manner, whereas SUMOylation of K480R/K585R was completely abolished ( FIG. 4C ). Taken together, these results indicate that SERCA2a is SUMOylated at the K480 and K585 residues. 
       Example 5 
     SUMOylation Increases SERCA2a ATPase Activity 
       [0062]    Since the SUMOylated lysine residues, K480 and K585, reside in the nucleotide-binding domains of SERCA2a, SUMOylation may affect the SERCA2a ATPase activity. WT and SUMOylation-defective K480R/K585R SERCA2a were immune-precipitated from the lysates of HEK293 cells transfected with the corresponding plasmids, along with the empty or SUMO1-expressing plasmids, and ATPase activities were determined. K480R/K585R possessed a significantly decreased Vmax compared to WT SERCA2a (WT; 94.60±1.63, K480R/K585R; 37.95±5.40 nmol/min/mg) and a significantly increased EC50 value compared to WT SERCA2a (WT; 0.24±0.09, K480R/K585R; 0.76±0.17 nmol Ca 2+ /L). Co-expression of SUMO1 significantly increased Vmax (98.58±1.83 nmol/min/mg) and decreased EC50 (0.11±0.09 nmol Ca 2+ /L) in WT SERCA2a, whereas it does not affect the ATPase activity of K480R/K585R ( FIG. 3D ). 
         [0063]    Further tests addressed whether SUMOylation affected the ATP-binding affinity of SERCA2a. HEK293 cells were transfected with WT or K480R/K585R SERCA2a-expressing plasmids, along with the empty or SUMO1-expressing plasmids. Cell lysates were incubated with ATP-sepharose and the resulting precipitates were probed with anti-SERCA2a antibody. The results indicated that co-expression of SUMO significantly increased the ATP-binding affinity of SERCA2a. In contrast, K480R/K585R possessed a significantly reduced ATP-binding affinity, which was not affected by the co-expression of SUMO1 ( FIG. 3E ). These data establish that SUMOylation increased the ATPase activity of SERCA2a, at least partly by enhancing ATP-binding affinity. 
       Example 6 
     SUMO1 Enhances the Stability of SERCA2a Protein 
       [0064]    The data disclosed herein establish that the SERCA2a level was reduced in failing hearts concomitantly with a reduced SUMO1 level ( FIG. 2 ). As a consequence, the possibility that SUMOylation affects the stability of SERCA2a was investigated. HEK293 cells were transfected with WT or K480R/K585R SERCA2a expressing plasmids, along with the empty or SUMO1-expressing plasmids. At forty-eight hours after transfection, the cells were treated with cycloheximide, an inhibitor of protein synthesis, to block further de novo synthesis of SERCA2a. After incubation for an additional three and five days, the SERCA2a levels were determined by Western blotting. The estimated half-life of WT SERCA2a was 4.90±0.70 days, whereas it increased to 5.90±0.20 days when SUMO1 was co-expressed. The estimated half-life of K480R/K585R was significantly reduced (2.42±0.50 and 2.35±0.80 days with and without co-expression of SUMO1, respectively) compared to WT SERCA2a ( FIG. 3F ). These data indicate that SUMOylation increased the stability of SERCA2a. 
       Example 7 
     SUMO1 Overexpression Enhances Cardiomyocyte Contractility and Ca 2+  Transients in Isolated Cardiomyocytes 
       [0065]    To examine the physiological function of SUMO1, mouse adult cardiomyocytes were isolated from normal (Sham) or TAC-induced failing hearts (HF), and then infected with either adenovirus expressing β-gal (Ad-β-gal) or SUMO1 (Ad-SUMO1). Contractile properties were determined using a dual-excitation spectrofluorometer equipped with a video-edge detection system. When infected with Ad-SUMO1, normal cardiomyocytes showed enhanced contractility with an 11% increase in cell shortening, a 17% increase in maximal rate of contraction, and a 9% increase in the maximal relaxation in comparison with the Ad-β-gal-infected cardiomyocytes. More prominent enhancement in contractility was observed when the failing cardiomyocytes were infected with Ad-SUMO1 with a 27% increase in cell shortening, a 30% increase in maximal rate of contraction, and a 27% increase in maximal relaxation. Ad-SUMO1-infected cardiomyocytes showed increased calcium amplitude and Ca 2+  decay in comparison with Ad-β-gal-infected cardiomyocytes. The overall inotropic effect of SUMO1 overexpression was comparable to the effect when SERCA2a is overexpressed ( FIG. 4 ). It is expected that SUMO1 overexpression enhanced cardiomyocyte contractility, at least partly through increasing the enzymatic activity and stability of SERCA2a. 
       Example 8 
     SUMO1 Overexpression Improves Cardiac Function in TAC-Induced Heart Failure 
       [0066]    We proceeded to define the physiological consequences of SUMO1 overexpression in vivo. For this purpose, we utilized a Cre/loxP conditional expression system in which administration of tamoxifen induced heart-specific SUMO1 overexpression in exchange of EGFP expression ( FIG. 5A ). No apparent cardiac dysfunctions were seen in this transgenic mouse (Table S 1 ). Western blotting revealed that tamoxifen-induced SUMO1 expression level in transgenic mice (TG) was approximately 5-fold higher than that of wild type littermates (WT) ( FIG. 5B ). 
         [0067]    WT and TG mice were subjected to TAC operation. HF with an approximately 40-50% decrease in fractional shortening (FS) was developed in two months. Tamoxifen was then administered for four days to induce SUMO1 overexpression. Along with the increased SUMO1 level, SUMOylation and the protein level of SERCA2a were significantly induced by the administration of tamoxifen ( Figure S2 ). One month later (three months post-TAC in total), cardiac functions were examined by histology and echocardiography. Representative heart sections and M-mode echocardiographic data are shown in  FIG. 5C . WT and TG mice were indistinguishable at baseline. At three months post-TAC, WT mice exhibited severe failing phenotypes with significant left ventricular (LV) dilation and reduced FS and ejection fraction (EF). Tamoxifen-induced SUMO1 overexpression, however, dramatically reversed these failing phenotypes with less LV dilation (LV internal diastolic dimension (LVIDd), 3.24±0.33 mm in TG vs. 4.40±0.57 mm in WT, p&lt;0.001; LV internal systolic dimension (LVIDs), 1.44±0.30 mm in TG vs. 2.93±0.54 mm in WT, p&lt;0.001) and improved FS (55.91±6.70% in TG vs. 34.30±5.56% in WT, p&lt;0.001) and EF (90.26±4.27% in TG vs. 69.57±7.28% in WT, p&lt;0.001) ( FIG. 5D ). 
         [0068]    Hemodynamic analyses also showed improved LV function in TG. The end-systolic pressure-volume relationship (ESPVR) in LV was slightly steeper in TG animals than WT, suggesting an increased cardiac contractility (Figure S 3 ). In contrast, the slope of LV end-diastolic pressure-volume relationship (EDPVR) was decreased in TG mice, indicating a decreased end-diastolic LV chamber stiffness. Parameters of LV dilation including stroke volume, end-diastolic volume, and end-systolic volume were likewise restored in TG. In addition, an increase in heart weight to body weight ratio was significantly inhibited in TG (Table S 2 ). 
         [0069]    The recovery of cardiac dysfunction by SUMO1 was also manifested by increased survival of TG under prolonged pressure-overload ( FIG. 5E ). The survived mice were 14 out of 14 (100%) in TG, whereas they were only 7 out of 15 (47%) in WT at 100 days after administration of tamoxifen. 
         [0070]    We performed Western blotting to monitor expression levels of key regulatory proteins involved in Ca 2+  homeostasis. Notable changes under TAC were a reduction of the SERCA2a level (65% decrease vs. sham), which is consistent with the numerous previous reports, and an increase in NCX1 level (56% increase vs. sham) NCX1 is responsible for cytosolic Ca 2+  elimination during diastole. It was previously shown that a decrease in SERCA function is coupled with an increase in NCX function in failing hearts (Schillinger et al., 2003; Studer et al., 1994) and in isolated cardiomyocytes after delivery of siRNA against SERCA2a (Seth et al., 2004). These TAC-induced changes in the levels of SERCA2a and NCX1 were normalized in TG ( FIG. 5F ). 
         [0071]    TAC resulted in a significant reduction in the ATPase activity of SERCA2a in WT with a 50% decrease in Vmax (TAC; 40.39±5.08, Sham; 81.03±7.11 nmol/min/mg) and a 120% increase in EC50 (TAC; 0.31±0.021, Sham; 0.14±0.08 μnmol Ca 2+ /L). This TAC-induced reduction in the ATPase activity was significantly ameliorated in TG with a 15% decrease in Vmax (TAC; 70.32±5.54, Sham; 82.29±5.39 nmol/min/mg) and a 59% increase in EC50 (TAC; 0.27±0.12, Sham; 0.166±0.09 μnmol Ca 2+ /L), however. 
         [0072]    Taken together, these data indicate that SUMO1 overexpression restores cardiac dysfunction induced by pressure-overload. 
       Example 9 
     shRNA-Mediated Down-Regulation of SUMO1 Accelerates Cardiac Dysfunction 
       [0073]    To evaluate the effects of down-regulation of SUMO1 in hearts, we generated recombinant adeno-associated viruses serotype 9 (rAAV9) that express SUMO1-directed short hairpin RNA, or shRNA, (rAAV9/shSUMO1), or a scrambled sequence (rAAV9/SC) under the control of the U6 promoter ( FIG. 6A ). These rAAV constructs were injected into B6C3/F1 male mice through the tail vein at a dose of 5×10 10  viral genomes (vg)/mouse. At three weeks after injection, Western blotting with heart extracts revealed that SUMO1 levels were reduced by approximately 70% in the rAAV9/shSUMO1-injected hearts but not in the rAAV9/SC-injected hearts. Expression levels of SUMO2, SUMO3, Ubc9, and SENP1 were not affected by rAAV9/shSUMO1 ( FIG. 6B ). 
         [0074]    At six weeks after injection, cardiac functions were evaluated. Gross morphology of the hearts and representative M-mode images of echocardiographic analyses are shown in  FIG. 6C . Hearts from rAAV9/shSUMO1-injected mice showed prominent left ventricle (LV) dilation and functional deterioration compared with the hearts from rAAV9/SC-injected mice with an increased Left Ventricular Internal Dimension-diastole, or LVIDd, (shSUMO1; 3.85±0.17 mm, SC; 3.42±0.19 mm, p&lt;0.001), and Left Ventricular Internal Dimension-systole, or LVIDs, (shSUMO1; 2.02±0.17 mm, SC; 1.30±0.12 mm, p&lt;0.001), and decreased FS (shSUMO1; 47.63±3.07%, SC; 61.77±4.77%, p&lt;0.001) and EF (shSUMO1; 84.51±2.63%, SC; 93.85±1.23%, p&lt;0.001) ( FIG. 6D ). 
         [0075]    Hemodynamic analyses showed that injection of rAAV9/shSUMO1 resulted in a rightward shift of the LV pressure-volume loops and a decreased End-Systolic Pressure-Volume Relationship, or ESPVR, indicating negative inotropic effects of SUMO1 down-regulation. Injection of an increased dose of rAAV9/shSUMO1 resulted in a more severe cardiac dysfunction (Figures S 4 A and S 4 B). An increased heart weight to body weight ratio was also observed in rAAV9/shSUMO1-injected hearts (Table S 3 ). 
         [0076]    The rAAV9/shSUMO1-induced cardiac dysfunction was manifested by sudden deaths of the rAAV9/shSUMO1-injected mice. All the mice received 1×10 11  vg of rAAV9/shSUMO1 died within three weeks. Death rates of mice received 3×10 10  vg and 5×10 10  vg of rAAV9/shSUMO1 were slightly higher than, but not statistically significant from, that of control mice received rAAV9/SC. During the period of experiments, none of the control mice died ( FIG. 6E ). 
         [0077]    Western blotting revealed that SERCA2a protein level was decreased by approximately 40% in rAAV9/shSUMO1-injected hearts. Related to this, the sodium/calcium exchanger NCX1 protein level was slightly elevated, although the elevation was not statistically significant. PLN (phospholamban) and RyR2 (ryanodine receptor 2) protein levels, however, were not altered. As expected, SUMOylation of SERCA2a was also significantly blunted ( FIG. 6F ). SUMOylation of PLN and RyR2 were not altered ( Figure S4C ). These results establish that SUMO1 increases SERCA2a SUMOylation and SERCA2a stability. 
         [0078]    SUMO1 down-regulation by injection of rAAV9/shSUMO1 suppressed the ATPase activity of SERCA2a with a reduced Vmax (shSUMO1; 48.11±6.34, SC; 61.12±6.49 nmol/min/mg) and an increased EC50 (shSUMO1; 5.69±0.23, SC; 0.10±0.07 μmol/L) ( FIG. 6G ). The ATPase activity of SERCA2a was more severely impaired when a higher dose of rAAV9/shSUMO1 was injected ( Figure S4D ). 
         [0079]    Taken together, the data disclosed herein establish that SUMO1 is an essential regulator of SERCA2a function in the heart. 
         [0080]    Each of the references cited herein is hereby incorporated by reference in its entirety. 
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         [0121]    From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention.