(Hypertension. 2000;36:449.)
© 2000 American Heart Association, Inc.
Scientific Contributions |
ko Gajovi
From the International Center for Genetic Engineering and Biotechnology, Trieste, Italy (M.L.M., F.P., S.G., F.E.B., A.F.M.); Department of Physiology, School of Medicine, University of California at Los Angeles (O.U.S., M.C.J., L.H., K.P.R.); Veterans Affairs GLA Healthcare System, Los Angeles, California (O.U.S., L.H.); and Department of Histology and Embryology, School of Medicine, University of Zagreb, Croatia (S.G.).
Correspondence to A.F. Muro, ICGEB, Padriciano, 99, 34012 Trieste, Italy. E-mail muro{at}icgeb.trieste.it
| Abstract |
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0.0001), diastolic
blood pressure (-/-: 83.54±1.07; +/+: 74.87±2.23;
P
0.005), and pulse blood pressure (-/-: 43.32±1.10;
+/+: 33.19±1.96; P
0.001) in ß-adducindeficient
mice. Western blot analysis showed that as a result of the
introduced genetic modification, ß-adducin was not present in
heart protein extracts from -/- mice. Consequently, this deficiency
produced a sharp decrease of
-adducin and a lesser reduction in
-adducin levels. However, we found neither cardiac remodeling nor
modification of the heart function in these animals. This is the first
report showing direct evidence that hypertension is triggered by a
mutation in the adducin gene family.
Key Words: hypertension, genetic adducin mice, knockout echocardiography cytoskeleton telemetry electrocardiography
| Introduction |
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-, ß-, and
-adducin.1 2 The
and ß subunits, but not the
subunit, are found in membrane cytoskeletons of human erythrocytes at
the actin-spectrin junctions as a mixture of heterodimers and
heterotetramers. Combinations of
/ß and
/
oligomers are
found in the actin cytoskeleton at cell-cell contact sites in other
cells.2 3
Adducin, an unexpected cytoskeletal player as blood pressure (BP)
modulator, has been the subject of several association studies that
attempted to link adducin variants to hypertension. Since the
publication of the first positive association of adducin
polymorphism with human hypertension,4 contradictory
results have appeared in the literature. In fact, the human
-adducin
gene polymorphism (G460W) has been found to be significantly
associated with hypertension and salt sensitivity in certain patient
groups,4 5 6 7 8 although this observation was not confirmed by
other groups.9 10 11 12 13 14
The attention to adducin in hypertension was originally drawn by a rat
model (Milan hypertensive strain [MHS]), after our characterization
of the
- and ß-adducinspecific variants associated with high BP
levels.15 These mutated adducins lead to a higher level of
filamentous actin, enhanced actin bundling in cell-free systems, and
increased Na-K pump activity when transfected into kidney epithelial
cells.16 However, in both humans and rats, there has been
no proof for direct involvement of any of the adducins in the
modulation of BP. Furthermore, adducin polymorphism cosegregation
analysis and kidney cross-transplantation experiments in the
MHS rat model showed that, in a manner similar to that observed in
humans,4 5 7 adducin accounted for only a portion of the
hypertension.15 Therefore, the simultaneous
action of other genes and other organs may be involved in the primary
cause of hypertension in both human and rats.15 17
The knockout technology allows elimination of all differences in genetic background except for the desired mutation. Taking advantage of this possibility, we have created a ß-adducindeficient mouse strain by targeted disruption of the ß-adducin gene.18 The shape and osmotic fragility of red blood cells (RBCs) of homozygous mutant animals were altered, and consequently the mice suffered from a mild anemia with compensated hemolysis, similar to human spherocytic hereditary elliptocytosis.18
Our knockout model also gave us the opportunity to study the direct association between ß-adducin and hypertension in a well-defined genetic background. Therefore, we studied the effect of ß-adducin deficiency on arterial BP, and we found that ß-adducinmutant animals developed hypertension. We also studied cardiac remodeling and heart function through postmortem analysis and in vivo dynamic assessment of left ventricular dimensions and electrocardiography to discover any possible pathology associated with hypertension or a possible effect of the ß-adducin deficiency on excitable cell membrane ion channels.
| Methods |
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BP Determinations
Arterial BP was measured in 5 -/- and 6 +/+
nonanesthetized, freely moving animals with the use of
telemetry implants (PA-C20 BP device, Data Sciences
International) for a period of 6 hours. BP determinations
started 30 minutes after discontinuation of halothane
anesthesia. The first hour was considered from 30 minutes
after discontinuation of anesthesia until 59 minutes later,
the second hour was from 60 minutes after discontinuation of
anesthesia until 119 minutes later, and so on every 60
minutes. During this time, abdominal aortic BP waveforms were
continuously sampled, and the values are means of 5-second
periods every 3 minutes (a total of twenty 5-second periods per hour,
except that in the first hour only ten 5-second periods were
considered). The device calculated a mean of all individual pulses
(
40 pulses) during that time. Therefore, every hourly mean includes
800 pulses.
Electrocardiography
Two platinum needle electrodes were implanted in the
subcutaneous tissue overlying the right scapula and the apex of the
heart in 5 -/- and 7 +/+ halothane-anesthetized mice.
Halothane was then discontinued; the ECG was recorded for 30
minutes with the use of ICM-01 amplifiers on a Gilson polygraph and
digitized with a Labmaster A-D board at 10 KHz. Analysis of ECG
parameters was performed with Axotape software (Axon,
Inc).
Heart Morphology and Mass
Measurements of heart mass and volume of ventricular
walls were made by 2 methods: planimetry of
histological section images and weight of dissected
ventricles. In the first (6 -/- and 5 +/+ mice), hearts were removed,
flash-frozen in methylbutane at dry ice temperature, embedded in OCT
compound (Miles Laboratories), serially cut in 20-µm slices
perpendicular to their long axis on a Microhm cryostat (Carl Zeiss),
and stained with hematoxylin-eosin, and images of slices were
digitized. Thickness and total area of the ventricular
walls were measured with IPPLUS software (Media Cybernetics, Inc).
Volume of ventricular walls was calculated from slice areas
and interslice distances.
In a separate series of animals (7 -/- and 8 +/+ mice), hearts were dissected to eliminate the great vessels and atria. The remaining left and right ventricles were opened to discard any blood contents and weighed.
Echocardiography
Mice (6 +/+ and 5 -/-) were sedated with Avertin
(2,2,2-tribromethanol, 2.5% solution, 0.016 mL/g body mass,
Aldrich Chemical Co). Two-dimensionally guided M-mode and
Doppler images were obtained with a 7.5-MHz probe and 10-mm
standoff on an ATL Apogee CX200.19 20 Animals were
positioned in the supine or left decubitus position. The images were
digitized for analysis (SigmaScan software, Systat, Inc) of
left ventricular dimensions during systole and
diastole. Ejection times were determined from flow velocity
profiles of the aorta.
Western Blot Analysis
Heart homogenates were electrophoresed on a 10%
Laemmli gel, blotted onto nitrocellulose membranes, probed with rabbit
anti
-, ß-, and
-adducin polyclonal antibodies (1:500, 1:1000,
and 1:1000 dilution, respectively), and analyzed by
densitometry, as previously described.18 The experiments
were repeated, and identical results were obtained with 2 independent
protein preparations.
Creatinine and Kidney Weight Analysis
Fresh blood from wild-type and ß-adducin -/- mice (10 mice
per group) was collected in Eppendorf tubes. Creatinine
levels were analyzed according to standard methods. For kidney
weight analysis, mice were killed (9 mice per group) and
weighed; then the kidneys were removed, and left and right kidneys were
weighed separately. Data of kidney weight correspond to the sum of both
kidneys and are expressed as percentage of total animal weight.
Statistical Methods
Data analysis was performed with the use of the software
package Stata, version 6.0 (StataCorp, 1999). The data were summarized
with the mean as a measure of central tendency and the standard error
as a measure of dispersion. The difference in BP between the wild-type
and mutant mice was assessed by a repeated-measures ANOVA according to
a split-plot factorial design. A P value of 0.05 was chosen
as the limit of statistical significance. Students t test
was used for the rest of the analyzed
parameters.
| Results |
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1x10-4;
17.47% increase), diastolic BP (-/-: 83.54±1.07; +/+:
74.87±2.23, P
0.005; 11.58% increase), and pulse BP
(-/-: 43.32±1.10; +/+: 33.19±1.96; P
0.001; 30.51%
increase) were observed in ß-adducindeficient mice with the use of
repeated-measures ANOVA (Figure 1). However, heart rate was not
significantly higher in mutant mice (Figure 1, inset).
Repeated-measures ANOVA of the BP data showed that there was a
nonsignificant effect for time and for the interaction mutation-by-time
in BP determinations (data not shown), except for a significant effect
for time in the case of pulse BP. Further data analysis showed
that the differences between the time point at hour 1 and the time
points from hours 3 to 6 account for the overall significant effect of
time for pulse BP. In fact, if the data between hours 2 to 6 are
considered, there is no effect of time for pulse BP (Figure 1),
suggesting that it might be generated by a residual effect of
anesthesia in the first hour after its discontinuation.
|
We analyzed the expression levels of the different adducin
subunits (
, ß, and
) in heart protein extracts by Western blot.
No ß-adducin (normal or deleted form) was detected in total heart
homogenates of ß-adducin -/- mice (Figure 2A). By using an anti
-adducin
antibody, we detected a sharp decrease (70%) in the amount of this
subunit in -/- mice (Figure 2B). In addition, when we used an
anti
-adducin antibody, we observed a downregulation (50%) of
-adducin levels in these mice (Figure 2C). Although adducin
mRNAs had been previously found in hearts of some
organisms,2 21 22 the presence of
-, ß-, and
-adducin protein in this tissue has never been reported.
|
Qualitative histological analysis showed no obvious morphological differences between hearts from wild-type and mutant mice (not shown). The quantitative histological study of frozen hearts did not show any difference in dimensions of ventricular walls (Figure 3A and 3B). This result was confirmed by the measurement of left and right combined ventricular masses in a separate series of animals used for ECG recording (0.401±0.019% and 0.418±0.086% of body mass for -/- and +/+ mice, respectively). In addition, the in vivo M-mode echocardiography study did not reveal differences among the 2 strains in septum and posterior left ventricular wall thickness, end-diastolic and end-systolic diameters, and left ventricular fractional shortening (Figure 3C). The Doppler echocardiography study also showed similar values of ejection time in both strains (Figure 3C). ECG demonstrated ECG waveforms characterized by a QRS complex with a small or absent Q wave, a prominent R wave, and an S wave followed immediately by a T wave with no discernible ST interval in both mice strains. Values of QRS complex duration and of PR and QT intervals did not differ among the 2 strains (Figure 3D). These ECG data indicate that the absence of ß-adducin does not produce significant changes in the muscle cell membrane ion channels of the heart to induce changes in the ECG parameters. It is noteworthy that despite the marked changes in adducin expression observed, no alterations of heart morphological, mechanical, or electric properties have been detected in this model, suggesting a lack of involvement of these proteins in heart function.
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| Discussion |
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One of the clear consequences of the absence of ß-adducin in some
tissues (References 18 and 2218 22 and data not shown) is the concomitant
modification of
- and
-adducin levels, supporting the hypothesis
that the abnormal adducin complex assembly might be an important
determinant of BP.23 Hence, ß-adducin deficiency could
be directly responsible for BP changes, or it may be acting through the
consequent modification of
- and
-adducin levels or other as yet
undetected effects. ß-Adducindeficient mice have a lower hematocrit
than wild-type animals (43.5 versus 46.1;
P
0.005),18 and this cannot explain the
change in BP.
Although in hearts of -/- mice, ß-adducin deficiency produced
altered
- and
-adducin levels, we have found neither cardiac
remodeling nor modification of the heart function in these animals.
This phenomenon is probably related to the moderate magnitude of the
change in arterial BP present and/or the relatively
young age of the animals under study. However, we cannot exclude the
involvement of adducin on heart function under different types of load
and stress.
The involvement of the kidney in the sequence of events connecting adducin polymorphism to the development and maintenance of hypertension was shown in the MHS strain of rats17 24 and was suggested in humans.6 8 However, preliminary analysis of kidneys of our ß-adducindeficient mice showed no difference by gross histological analysis with +/+ mice, and the kidney weight of -/- mice was similar to that of +/+ animals (0.56±0.09% and 0.58±0.06% of animal weight for +/+ and -/- animals, respectively [mean±SD]; n=9 per group; P=0.226, Students t test). Moreover, serum creatinine levels showed no difference between -/- and +/+ animals (0.416±0.044 mg/dL and 0.419±0.025 mg/dL for +/+ and -/- animals, respectively [mean±SD]; n=10 per group; P=0.853, Students t test).
The interest of our work for clinical research lies primarily in the definitive proof of the direct involvement of adducin in BP modulation. Contrary to the partial contribution of adducin genes to hypertension in both different human populations and MHS rats, the mutation of the ß-adducin gene in the knockout mouse is the only one responsible for BP changes in this strain. Considering this and the very high degree of sequence homology between mouse and human adducins (Reference 1818 and A.F. Muro, unpublished data, 1999), it is possible that the molecular mechanisms of hypertension in these mice are relevant to humans. Furthermore, since in our mouse model RBC abnormalities were present, causing mild hemolytic anemia, it will be of great interest to search for adducin polymorphisms in selected groups of patients with elevated BP accompanied by erythrocyte changes similar to those found in our mouse model.
| Acknowledgments |
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| Footnotes |
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Received February 1, 2000; first decision February 28, 2000; accepted April 10, 2000.
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