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(Circulation. 2000;101:2674.)
© 2000 American Heart Association, Inc.
Brief Rapid Communications |
From Klinik III für Innere Medizin, Universität zu Köln, Köln, Germany (O.Z., M.B.), and Friedrich Miescher Institute, Basel, Switzerland (P.C.).
Correspondence to Prof Dr Michael Böhm, Klinik III für Innere Medizin, Joseph-Stelzmann-Str 9, 50924 Köln, Germany. E-mail michael.boehm{at}medizin.uni-koeln.de
| Abstract |
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Methods and ResultsImmunohistochemistry and in situ
hybridization confirmed the expression of MLP protein and mRNA in human
cardiomyocytes. Western blot analysis revealed that
the MLP peptide was present in the contractile protein fraction but
not in the cytosolic or membrane fraction and that the binding of MLP
to myofibrils required functional zinc finger domains. MLP
immunoreactivity was decreased
50% (P<0.05) in the
left ventricular myocardium of patients with
chronic heart failure due to dilated or ischemic
cardiomyopathy compared with non-failing donor
hearts. MLP mRNA expression, as assessed by Northern blot experiments,
was not significantly different between failing and non-failing control
hearts, which suggests that decreased MLP synthesis or increased MLP
protein turnover, rather than a decreased number of RNA transcripts,
may play a role.
ConclusionsBecause MLP may promote myofibril assembly, the down-regulation of this adapter protein might play an essential role in myofibril derangement or impaired myofibril rearrangement in the failing human myocardium.
Key Words: heart failure cardiomyopathy genes RNA
| Introduction |
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In the heart, MLP is expressed at high levels in atrial and ventricular myocytes during fetal development and in the adult.3 Overexpression of MLP in cultured myoblasts augments differentiation, which suggests that this gene serves as a positive regulator of myogenesis. The importance of MLP for normal cardiac function was demonstrated in mice bearing a targeted disruption of the gene. MLP-deficient animals developed dilated cardiomyopathy with myocyte hypertrophy and heart failure after birth.4 To understand the role of MLP in normal cardiac physiology and pathophysiology in humans, we analyzed its expression and subcellular localization in left ventricular myocardium derived from explanted human hearts.
| Methods |
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Immunoblot Analysis
Crude homogenates were obtained by
homogenization of left ventricular
tissue in 10 volumes of ice-cold buffer (50 mmol/L Tris-HCl [pH
7.5], 5 µg/mL aprotinin, 5 µg/mL leupeptin, 0.1 mmol/L PMSF,
and 10 mmol/L EDTA). Cytosol, membranes, and myofibrils were
prepared according to standard protocols.5 6 To extract
myofibrillar proteins sensitive to EDTA, myofibrils were incubated in
50 mmol/L Tris-HCl (pH 7.5) and 10 mmol/L EDTA for 30 minutes
at 4°C and centrifuged for 15 minutes at
20 000g.
SDS-PAGE was performed using a 12.5% gel. Separated proteins were transferred to a nitrocellulose membrane and probed with polyclonal MLP antiserum. This polyclonal antibody against the C-terminal peptide of rat MLP (GGLTHQVEKKE) also recognizes human MLP. The immunoreactive bands were visualized using a chemoluminescence detection system.
Immunohistochemistry
Cryostat sections of left ventricular tissue were
fixed in formalin and stained by the indirect streptavidin method. The
horseradish peroxidase substrate aminoethylcarbazole was used.
Expression of Recombinant MLP
A cDNA fragment corresponding to the coding sequence of
human MLP was generated by polymerase chain reaction and cloned into
the pGEX-1
T vector. Glutathione-S-tranferase (GST)-MLP fusion
proteins expressed and purified from clarified bacterial lysate were
incubated with a 50% slurry of glutathione agarose beads. Loaded beads
were washed 3 times, and GST-MLP was eluted with reduced glutathione
and recombinant MLP was separated by thrombin-cleavage.
Blot Hybridization Experiments
Northern blots were prepared from 10 µg of total RNA as
described previously.5 A
[32P]-labeled 375-bp fragment of human MLP cDNA
was used as a specific probe.
In Situ Hybridization
Cryostat sections of left ventricular
myocardium were fixed in 4% formaldehyde in PBS and
treated with proteinase K. Sense and antisense digoxigenin-labeled RNA
probes were prepared by in vitro transcription from a human MLP cDNA
insert. Hybridization was performed overnight at 42°C in
hybridization buffer (50% formamide, 10% dextran sulfate, 2.5x
Denhardts, 2.25x SSPE, 100 µg/mL salmon sperm DNA, 5 mmol/L
dithiothreitol, and 0.2 µg/mL denatured probe). After
high-stringency washes and treatment with RNase A, the sections were
stained with an alkaline phosphatase-coupled anti-digoxigenin-coupled
antibody and the chromogen 4-nitro blue tetrazolium
chloride/5-bromo-4-chloro-3-indolyl-phosphate (NBT/BCIP).
Statistics
Data are expressed as mean±SEM. Statistical significance was
estimated with 1-way ANOVA and the Bonferroni t test.
P<0.05 was considered significant.
| Results |
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MLP Is Associated With Myofibrils
To investigate the cellular localization of MLP,
immunohistochemical staining of cryostat sections of human left
ventricular myocardium derived from donor
hearts and from explanted hearts with dilated or ischemic
cardiomyopathy was performed. The specific MLP
antiserum detected a cytoplasmic antigen in cardiomyocytes
in a mainly striate pattern (Figure 2A
).
Furthermore, MLP immunoreactivity accumulated at the cell periphery.
MLP was obviously not localized to nuclei because nuclear staining was
absent, even when fixative or fixation time were changed.
Consistent with immunocytochemistry findings, in situ
hybridization using a specific MLP riboprobe revealed strong
cardiomyocyte staining (Figure 2B
).
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MLP Binding to Myofibrils Requires Functional Zinc Finger
Domains
In an effort to investigate the subcellular distribution of MLP
protein, protein fractions containing membrane proteins, cytosolic
proteins, or myofibrillar proteins were analyzed for MLP
expression. These experiments demonstrated that MLP immunoreactivity
accumulates in the fraction containing contractile proteins (Figure 2C
). If the binding of MLP requires the presence of coordinated
zinc in the LIM motif, the MLP-myofibril association should be
sensitive to EDTA. Indeed, MLP was enriched in the EDTA-soluble protein
fraction after the incubation of myofibrils with 10 mol/L divalent
metal chelator (Figure 2D
). Furthermore, the MLP present in
the myofibril extract from the left ventricular
myocardium binds to recombinant GST-MLP
immobilized with glutathione sepharose (Figure 2E
).
This binding was reduced in the presence of EDTA.
| Discussion |
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Recent studies using animal models suggest an important role for MLP, both in the developing and in the adult heart. During mouse embryogenesis, expression of MLP occurred at embryonic day 9 in the developing atrial and ventricular heart chambers and it remained at high levels through the end of gestation and into adulthood.7 Disruption of the MLP gene in mice leads to progradient alterations in the cytoarchitecture of cardiac muscle cells. Because the MLP knockout animals reproduce the clinical and morphological picture of dilated cardiomyopathy and heart failure in humans, it has been hypothesized that altered expression of MLP might play a role in the pathogenesis of human heart failure.
To address this question, we investigated MLP expression in the failing left ventricular myocardium from patients with dilated and ischemic cardiomyopathy. MLP protein concentrations were significantly decreased in failing hearts, whereas MLP mRNA concentrations were unchanged compared with non-failing donor hearts. These results indicate that decreased MLP synthesis or enhanced MLP protein turnover, rather than a decreased number of MLP transcripts, may play a critical role in human heart failure. Interestingly, these alterations are not confined to dilated cardiomyopathy. They can also be detected in hearts dilated due to coronary heart disease.
It has been suggested that MLP is important for the organization of actin-based structures and the regulation of cardiomyocyte cytoarchitecture.4 8 These studies, however, all referred to animal experiments. Our data now demonstrate that MLP is also associated with myofibrils in the human myocardium and that its binding requires the presence of Zn2+ in the LIM motif. Our results from immunohistochemistry studies are in line with the suggestion that MLP is associated with lateral myofibril anchorage sites and Z-lines of myofibrils.4 In chronic heart failure in humans, ultrastructural alterations, such as a disruption of myofibrillar organization, a rarefaction of sarcomeres, and an increase in nonmyofibrillar space, have been identified as morphological correlates of reduced myocardial function.9 The results from the present study suggest that reduced MLP content might promote the deterioration of the contractile function of the myocardium afflicted with dilated or ischemic cardiomyopathy. The discovery of this cellular alteration could provide a target for novel therapeutic interventions.
| Acknowledgments |
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Received January 19, 2000; revision received April 17, 2000; accepted April 18, 2000.
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