(Circulation. 2001;103:2402.)
© 2001 American Heart Association, Inc.
Basic Science Reports |
From the Divisions of Molecular Cardiovascular Biology (X.W., H.O., J.G., J.R.) and Pediatric Cardiology (S.W., T.K.), Childrens Hospital Research Foundation, Cincinnati, Ohio; Departments of Medicine (G.W.D.) and Molecular and Cellular Physiology (M.N., J.N.L.), University of Cincinnati Medical Center, Cincinnati, Ohio; and South Dakota Health Research FoundationCardiovascular Research Institute (A.M.G.), Sioux Falls, South Dakota.
Correspondence to Jeffrey Robbins, Division of Molecular Cardiovascular Biology, Childrens Hospital Research Foundation, 3333 Burnet Ave, Cincinnati, OH 45229. E-mail robbj0{at}chmcc.org
| Abstract |
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Methods and ResultsMultiple transgenic mouse lines that expressed either murine wild-type desmin or a 7amino acid deletion (R173 through E179) desmin (D7-des) mutation linked to DRM were made. The distribution of desmin protein was unchanged, and no overt phenotype was detected in the wild-type desmin transgenic mice. In contrast, the D7-des mouse heart showed aberrant intrasarcoplasmic and electron-dense granular filamentous aggregates that were desmin-positive and characteristic of human DRM. The desmin filament network was significantly disrupted, and myofibril alignment was visibly compromised. Although systolic function at the whole-organ level was substantially conserved in the young adult animals, the ability of the heart to respond to ß-agonist stimulation, as measured in the intact animal, was significantly blunted.
ConclusionsUpregulation of desmin protein at moderate levels is not detrimental. However, the D7-des mutation is dominant negative, and expression of the mutant protein leads to the appearance of aggregates that are characteristic of and diagnostic for human desmin-related cardiomyopathy.
Key Words: heart diseases cardiomyopathy molecular biology physiology pathology
| Introduction |
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The desminopathies are a heterogeneous group of human myopathies characterized by abnormal intrasarcoplasmic desmin accumulation.11 Clinically, DRM can present as a generalized myopathy, although often only cardiac muscle or cardiac and skeletal muscle is affected. To date, a putative 7amino acid deletion (R173 through E179; D7-des) and missense mutations (A337P, A360P/N393I, L345P, N342D, R406W) of desmin have been linked to DRM.7 8 12 Several in vitro studies using desmin filament assembly and/or cell culture manipulations suggest that the mutations are causative.8 9 However, in vivo causative data outside genetic linkage studies do not exist, nor is it clear what pathogenic processes are involved in the development of the different cardiomyopathies.
Both skeletal and cardiac myopathies have been described in desmin-null mice.13 14 However, the relationship of the desmin-null mutation to DRM is not clear because the total desmin complement, which consists of both normal and mutated protein, is significantly increased in a patients skeletal and cardiac muscles. Thus, the disease does not appear to be caused by the absence of protein. To unambiguously establish that the R173 through E179 deletion mutation is sufficient to cause DRM, we created an animal model of DRM using cardiac-specific transgenic (TG) expression of D7-des protein. TG expression of murine wild-type desmin (WT-des) mRNA and protein did not result in a discernible phenotype. In contrast, TG expression of D7-des protein led to aberrant intrasarcoplasmic desmin aggregation, perturbed myofibril alignment, and defective myocyte mechanical function, recapitulating aspects of human DRM.
| Methods |
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-myosin heavy chain
promoter.15 The 7 amino
acids deleted in the mutant linked to DRM, as well as the immediate
flanking sequences, are essentially conserved
(Figure 1a
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Total ventricular RNA was isolated separately from 3 hearts of each line, and the integrity of the TG RNA was confirmed by Northern analysis and sequencing. RNA levels were quantified with dot blots hybridized to transcript-specific oligonucleotides. Total desmin protein levels were determined with SDS-PAGE followed by Western blots with a desmin monoclonal antibody (Sigma). Protein extracts were prepared from cytoskeletal (detergent insoluble) and soluble fractions of myocardial homogenates. Approximately 20 mg apical ventricular myocardium was homogenized on ice in PBS, pH 7.4, with 1% Triton X-100, EDTA (5 mmol/L), phenylmethylsulfonyl fluoride (1 mmol/L), and 1 tablet/10 mL protease inhibitor cocktail (Roche). The homogenates were centrifuged at 14 000g at 4°C, and the supernatant (soluble fraction) was separated from the pellet. The pellet was resuspended in SDS-PAGE loading buffer, boiled, and centrifuged, and the resultant supernatant (containing the desmin) was kept as the insoluble fraction. The ECF Western blotting protocol (Amersham Life Sciences) was used for quantification of desmin protein, and an enhanced chemiluminescent protocol (F. Hoffmann-La Roche Ltd) was used for film exposure.
At least 3 tissue samples from each area of the heart were
picked randomly for extensive ultrastructural
analysis.16
Immunoelectron microscopy (EM) was used to detect the localization of
desmin protein at ultrastructural level. The hearts were
perfused with 1% glutaraldehyde/3%
paraformaldehyde in cardioplegic buffer (5% dextrose,
30 mmol/L KCl in PBS), further fixed in the same fixative in
cacodylate buffer, incubated in 0.1 mol/L glycine/PBS, dehydrated in
series of N,
N-dimethyl formamide, and
embedded in LRWhite resin. Ultrathin sections were picked up on nickel
grids, dried, and etched with a saturated solution of sodium
m-periodate and 0.1N HCl. The immunolabeling procedure consisted of a
PBS rinse; a 1.5-hour incubation in 1% BSA, 0.1% cold water fish skin
gelatin, and 1% Tween 20 in PBS; overnight incubation in rabbit
anti-desmin antibody (Biomeda) diluted 1:25 with 1% BSA/PBS; a short
rinse in PBS; a 1.5-hour incubation in the blocking solution; and then
a 2-hour incubation with goat anti-rabbit IgG tagged with colloidal
gold (10-nm particle size, diluted 1:20 with 1% BSA/PBS;
Aurion/Electron Microscopy Sciences). After rinsing in PBS, postfixing
in 2% glutaraldehyde, and rinsing in water, sections
were counterstained with uranium/lead and viewed in a Zeiss Omega 912
at 100 kV. Indirect immunofluorescence staining was
carried out on both fixed isolated myocytes and myocardial
cryosections.4 Hearts were
excised, fixed by coronary perfusion with 4%
paraformaldehyde, saturated with 30% sucrose solution,
and embedded in Tissue-Tek OCT (Sakura Finetek USA). Cryosections or
mounted isolated myocytes were air dried, incubated with 0.1 mol/L
glycine in PBS (pH 7.2) for 30 minutes, treated with 1% Triton X-100
for 60 minutes, and blocked with 0.5% BSA/10% goat serum in PBS. The
specimens were then incubated with mouse monoclonal antibodies to
-actinin (Sigma) and/or rabbit polyclonal antibodies to desmin
(Biomeda) overnight at 4°C and subsequently with TRITC-conjugated
anti-mouse IgG and FITC-conjugated anti-rabbit IgG antibodies (Sigma)
for 1 hour for confocal microscopy.
In addition to gravimetric analyses of different compartments of the heart, the size and area of isolated cardiomyocytes,17 transcripts characteristic of the hypertrophic response,18 and echocardiography16 of 8- to 10-week-old mixed-sex animals were used to determine the extent of hypertrophy.
| Results |
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Lines 520 (WT-des) and 641 (D7-des) were chosen for detailed
characterization because analysis showed that both lines had
3-fold the amount of desmin present relative to NTG littermates
(Figure 1b
and 1c
). Diagnosis of DRM is based largely on
morphological criteria; the hallmark of DRM is the presence of aberrant
desmin aggregates in the myocytes of the affected muscles. These
aggregates display a unique histology and ultrastructure that are
diagnostic for
DRM.11 19 To
determine whether the D7-des mutation was causative for DRM, light
microscopy, immunofluorescence confocal microscopy,
transmission EM, and immuno-EM were used to characterize the TG hearts.
At the light microscope level, no differences presented between
the non-TG (NTG) littermates and the WT-des mice. Cell size,
organization, and appearance were unremarkable. Confocal
analysis confirmed that the WT-des staining pattern
(Figure 2d
through 2f) was essentially identical to the NTG
controls
(Figure 2a
through 2c).
-Actinin and desmin staining
colocalized and were restricted to the Z band
(Figure 2c
and 2f
). In contrast,
immunostaining of cryosections of the D7-des TG hearts
showed abnormal desmin arrangements, confirming that the abnormal
aggregates were desmin positive
(Figure 2h
and 2i
). The aberrant desmin aggregates clearly
disrupted the continuity and overall organization of the desmin network
throughout the cell from the sarcolemma to the nuclear envelope
(Figure 2l
). Line 83, which had lower levels of mutant
protein expression, also displayed some aggregates, but they were much
less abundant (data not shown).
|
The aggregates appeared as electron-dense granular
filamentous structures proximal to the nuclei and in the
intermyofibrillar space
(Figure 3
). Some appeared to be associated with the nuclear
envelope; others, with the Z band
(Figure 3c
and 3d
). Numerous fragmented filaments with
diameters characteristic of the intermediate filaments were found in
the immediate surrounding area of the aggregates
(Figure 3d
). The aggregates and filaments were desmin
positive as demonstrated by immuno-gold labeling
(Figures 3e
and 3f
). Consistent with the hypothesis
that desmin helps to maintain the structural integrity of the
cardiomyocyte, both immunostaining of
-actinin and ultrastructural analyses showed that alignment
of adjacent myofibrils was perturbed at the Z line in the D7-des TG
hearts
(Figures 2
and 3
). The high degree of uniformity that is
normally present at the boundary was absent
(Figure 3c
).
|
Desmin Mutation Causes Cardiac
Hypertrophy
At the molecular level, activation of fetal
genetic programs is common during the early stages of
hypertrophy, with upregulation of atrial
natriuretic factor and ß-myosin and downregulation of
-myosin.18 The
transcripts for phospholamban and the sarcoplasm reticulum
Ca2+ pump, which can often serve as markers
for changes in calcium handling, were also determined. The relative
amounts of these transcripts, endogenous desmin, and total
desmin
(Figure 4a
) indicate that expression of D7-des protein but
not overexpression of WT-des protein triggers a hypertrophic response.
The response, as manifested by increases in the ratios of ventricle to
body weight
(Figure 4b
), is most pronounced between 4 to 8 weeks of age
but is progressively ameliorated in the adult. This apparent
compensation is consistent with the lack of morbidity/mortality
observed in the animals during the first 17 months of life.
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To determine whether the cardiomyocytes
themselves had hypertrophied, we analyzed the cell volume, profile
area, length, sarcomere length, the transverse sectional area, and its
minor and major diameters
(Table
I).17 In
both ventricles, neither the sarcomere length nor the myocyte length
was significantly increased
(Table
I); rather the increases observed in the D7-des
cardiomyocyte size were due to increases in the transverse
sectional area. In the left ventricle (LV), this was caused by growth
in the minor diameter and in both the minor and major diameters in the
right ventricle myocytes, indicating that a concentric
hypertrophy at the cellular level occurs in both
ventricles. Consistent with these data,
echocardiography showed a significant increase in
posterior wall thickness in the absence of significant changes in
either diastolic and systolic LV chamber dimensions
in the D7-des TG hearts
(Table
II). No abnormalities were observed in the cohort
that overexpressed WT-des.
|
Myocyte and Cardiac Dysfunction
Cardiac function in patients with DRM is often
significantly compromised.19
Considering the dramatic effects of D7-des protein expression on
cellular structure and organization, we first determined the impact of
D7-des or WT-des TG protein expression on cardiomyocyte
function. Isolated cardiomyocytes are often more sensitive
than whole-organ or whole-animal analyses in reflecting a
primary pathology at the functional level because they are unloaded and
isolated from potential compensatory mechanisms that the heart is able
to bring to bear on the compromised cells through both internal and
external mechanisms. Cardiomyocytes isolated from 8- to 10-week-old
animals were field stimulated, and the cell mechanics were
analyzed. Both the contractile and relaxation functions of the
D7-des cardiomyocytes were significantly compromised in the
D7-des TG group with respect to NTG cardiomyocytes
(Figure 5a
). The percent shortening of unloaded myocytes
isolated from D7-des mice was decreased by 38% compared with the NTG
controls (P<0.05). The first
derivatives of both shortening and relengthening were decreased by 42%
and 35%, respectively
(P<0.05,
P<0.01), in the D7-des group.
Interestingly, the percentage shortening of ventricular
myocytes isolated from age-matched WT-des mice was 27% greater than
the NTG littermate controls
(P<0.05), whereas the first
derivatives of the shortening and relengthening of these cells were
unchanged compared with the control cohorts.
|
In intact mice instrumented with a Millar pressure
transducer,16 LV
diastolic function was markedly compromised in the D7-des
TG group but not in the WT-des TG mice
(Figure 5b
and 5c
). LV minimum dP/dt was significantly higher
in the D7-des TG animals under baseline conditions compared with both
NTG and WT-des animals. LV systolic function as determined by
dP/dtmax was also affected at this stage; at
baseline, the values for the NTG, WT-des, and D7-des mice were
9071±521, 8327±238, and 7168±275, respectively. In addition, the
response to the ß-agonist dobutamine was substantially
blunted in the D7-des TG animals. These data confirm that the deficits
in ventricular performance described in the
isolated myocytes extend to the intact animal and that systemic
alterations in cardiovascular dynamics are unable to
compensate for the myocardial deficit at this stage. It is interesting
to note that baseline LV end-diastolic pressure was
significantly elevated in the D7-des TG animals compared with the other
2 groups (14.8±2.0 mm Hg in D7-des TG versus 7.6±1.0 and
4.2±1.0 mm Hg in NTG and WT-des TG). The relatively large effect
on dP/dtmin was confirmed by determining the
time constant of relaxation,
, a load-independent measure of
diastolic function, which was markedly prolonged in the
D7-des TG mice both at baseline and during dobutamine
stimulation
(Figure 5d
).
| Discussion |
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The
-helical rod domain of desmin is critical for the
polymerization of protein and subsequent filament
formation.20 In vitro
studies showed that the ability of D7-des to assemble normally is
compromised.8 In the D7-des
cardiomyocytes, although the immunostaining
pattern of desmin in the cortical region of the cell appeared to be
overtly normal, the presence of aberrant desmin aggregates interrupted
network architecture and presumably its function. Myofibril alignment
is perturbed, even in areas that do not contain the aggregates. These
data indicate that the mutant desmin protein disrupts desmin filament
formation in vivo with a resultant myofibrillar misalignment at the
Z-line level.
The structural defects caused by D7-des protein expression are accompanied by functional abnormalities. The percent shortening and the rates of shortening and relengthening of the unloaded isolated ventricular myocytes were significantly lower compared with both the NTG and WT-des TG controls. These findings demonstrate that cardiac function at the cellular level is compromised by structural abnormalities resulting from D7-des protein. Results from experiments in intact anesthetized mice at 8 months showed that the defective diastolic function persisted in vivo. The rates of ventricular contraction and relaxation were markedly decreased in D7-des TG animals under baseline conditions and during maximal ß-adrenergic stimulation. We believe that it is likely that this impairment in contractility and relaxation leads to the observed increase in LV end-diastolic pressure (and presumably volume) that may further contribute to the development of cardiomyopathy.
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| Acknowledgments |
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| Footnotes |
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Received October 16, 2000; revision received December 31, 2000; accepted January 10, 2001.
| References |
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