Vascular Biology |
From the Department of Pharmacology and Toxicology and Cardiovascular Research Institute Maastricht (P.M.H.S., H.v.E., G.E.F., J.G.R.D.M.), Universiteit Maastricht, Maastricht, the Netherlands; Institut National de la Santé et de la Recherche Médicale (INSERM) U141 (D.H., C.M.B., B.I.L.), IFR Circulation Lariboisière, Université Paris VII, Paris, France; and Unité de Biologie du Developpement (E.C.-G., F.L.-V.), Institute Pasteur, Paris, France.
Correspondence to Dr J.G.R. De Mey, Department of Pharmacology and Toxicology, Universiteit Maastricht, PO Box 616, 6200 MD Maastricht, Netherlands. E-mail j.demey{at}farmaco.unimaas.nl
| Abstract |
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Key Words: vimentin arterial remodeling blood flow mice carotid arteries
| Introduction |
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The intermediate filament protein vimentin is a component of the cytoskeleton of vascular endothelial cells.21 Vimentin-deficient mice (V-/-)22 exhibit a blunted, acute, flow-induced arterial vasodilatation23 24 and an altered balance between endothelin-1 and nitric oxide (NO), 2 endothelium-derived vasoactive factors that may also be involved in arterial remodeling.13 25 26
In the present study, we evaluated whether vimentin plays a pivotal role in the capacity of arterial blood vessels to adjust their lumen diameter and wall mass in response to altered BF. Therefore, we applied unilateral carotid artery ligations27 in wild-type mice (V+/+) and V-/-.23 24 Our findings provide direct support for an important role of vimentin in diameter and wall mass changes during flow-induced arterial remodeling.
| Methods |
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Unilateral Carotid Artery Ligation
At 3 to 4 months of age, V+/+ (n=19) and V-/- (n=19) were
anesthetized with ketamine and xylazine (100 and 10
mg/kg SC). Both common carotid arteries were exposed through a midline
incision in the neck. In half of both groups of animals (experimental
animals), the left common carotid artery (LCA) was ligated with 5.0
surgical suture just proximal to the carotid bifurcation as described
by Kumar and Lindner.27 This intervention resulted in
cessation of BF in the unbranched occluded vessel. One of the V+/+
(sham) and 4 of the V-/- (2 sham and 2 experimental) died during or
after the surgery. The animals were allowed to recover for 4 weeks
after sham surgery or unilateral carotid artery ligation. At this
stage, they were anesthetized again with ketamine and
xylazine.
BF Measurements
During initial surgery and 4 weeks later, body temperature was
maintained at 37.5°C by a thermostatically controlled heating
platform. BF in the LCA and right carotid artery (RCA) was recorded
for V+/+ and V-/- that had undergone either sham surgery or
unilateral carotid artery ligation. We used a transit-time ultrasonic
flow probe (0.5-mm V series, Transonic Systems) that was mounted on a
micromanipulator and positioned halfway between the aortic arch and the
carotid artery bifurcation. The pulsatile BF signal was recorded
(Figure 1
), and mean BF was obtained by
averaging during a 5-minute period for the LCA and RCA of each animal.
Complete ligation of the LCA resulted in BF recordings that
fell below the detection limit of the flowmeter, indicating that there
is no net forward BF in these occluded arteries (Table
and
Figure 1
).
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Pressure-Diameter Curves
After the flow measurements, which were performed at 4 weeks
after the initial surgery, the LCA and the RCA were isolated. The
proximal end of each vessel was mounted on a steel cannula (diameter,
0.3 mm), after which the distal end was ligated with 7-0 surgical
suture, and the arterial segments were incubated in an
organ chamber at 37°C in a calcium-free
physiological salt solution (KRB) containing
10 µmol/L sodium nitroprusside to inactivate the
arterial smooth muscle. The cannulated ("blind sac")
arterial segments were connected to a feedback-controlled
pressure source (Living Systems Instrumentation) and were mounted on
the stage of an Invertoscope (Nikon TMS) equipped with a video camera
(Stemmer) and a digital device (LSI) for the recording of
arterial outer diameter. Diameter analysis was
limited to the outer diameter because (especially at low distending
pressure) mice carotid arteries were not sufficiently translucent to
monitor lumen diameter. Intra-arterial pressure was
increased in 10 mm Hg steps from 20 to 150 mm Hg. Outer
diameter was recorded at 2 minutes after each pressure increment.
After the highest pressure was reached, pressure was returned to
100 mm Hg, and 30 minutes later, the arterial
preparations were exposed to phosphate-buffered (pH 7.4) formaldehyde
(4%).
Outer diameter was expressed as a function of distending pressure. To determine the maximal outer diameter, pressure-diameter (P-D) curves were analyzed by a 4-parametric logistic sigmoidal curve fitting [y=ymin+(ymax-ymin)/(1+10(log E50-x) · Hill slope)] of individual curves (Graphpad Prism 2.01). The correlation coefficient of each curve fit exceeded 0.99 in each case.
Morphometry
After overnight fixation in formaldehyde at 100 mm Hg, the
arteries were removed from the arteriograph system and stored in 70%
ethanol for, at most, a week. To concentrate
histological analysis on the central part of
the vessels, fixed arteries were cut in half, and both parts were
embedded side by side in paraffin. Cross sections (4 µm) were
stained with Lawsons solution (Boom BV), which highlights the elastic
laminae. Medial cross-sectional area (CSAm), lumen diameter, and medial
thickness were determined as previously described29 with
the use of video images generated by a Zeiss Axioscope, a standard CCD
camera (Sony), and commercial software (JAVA 1.21, Jandell
Scientific).
Data Analysis
Results are shown as mean±SEM. Statistical significance of
differences between experimental and sham-operated animals and between
V+/+ and V-/- were evaluated by a 2-way ANOVA, followed by a post hoc
comparison with a Newman-Keuls significant difference test. A value of
P<0.05 was considered significant.
| Results |
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Carotid Arteries of V-/- Compared With V+/+
In both groups of sham-operated mice, the passive P-D curves could
be superimposed (Figure 2
), and the CSAms
were identical for the RCA and LCA (Figure 3
).
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The outer diameters of LCA and RCA tended to be smaller in V-/- than
in V+/+. This was statistically significant for the maximal diameters
calculated by curve fitting of the P-D curves (Figures 2
and 4
). CSAm was significantly smaller
for sham-operated V-/- than V+/+ (Figure 3
). Combined, the LCA
and RCA diameters of V-/- tended to be smaller and the medial mass
was significantly smaller than corresponding values in V+/+.
Surprisingly, the smaller medial mass did not seem to alter mechanical
properties, such as circumferential compliance (reflected by the
tangent to the P-D curve) in the arteries of V-/- (Figure 2
),
or their distensibility (not shown).
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Effects of Altered BF in V+/+
In V+/+, LCA ligation resulted in a narrower, stiffer, and thicker
vessel (Figure 5
). CSAm was significantly
increased (Figure 3
), whereas the steepness of the P-D curve was
markedly reduced (Figure 5
), as was the maximal outer diameter
(Figure 4
).
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Despite the doubling of BF, the RCA of V+/+ displayed no significant
alteration of wall mechanics and diameter (Figure 5
). If
anything, medial mass tended to be reduced in these hyperperfused
vessels (Figure 3
).
Effects of Altered BF in V-/-
The diameter reduction and stiffening of the ligated LCA was
considerably less in V-/- than in V+/+ (Figure 5
). The maximal
outer diameter was reduced by 77 µm in V-/-; this was
approximately the double (162 µm) of that in V+/+ (Figure 4
). Medial hypertrophy, on the other hand, was more
pronounced in the occluded LCA of V-/-
(10x103 µm2) than
of V+/+ (5x103 µm2,
Figure 3
).
In contrast to V+/+, the hyperperfused RCA of V-/- exhibited a
statistically significant increase in maximal diameter (Figures 4
and 5
) and in CSAm (Figure 3
).
| Discussion |
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Kumar and Lindner27 introduced unilateral carotid artery ligation in mice for the study of neointimal formation after BF cessation. Application of this approach in knockout mice revealed a key role of P-selectin30 and of endothelial NO synthase26 in neointimal formation and overall arterial structural changes, respectively, in response to a drastic fall in arterial BF. We reasoned that this experimental approach could also be used to address the role of a cytoskeletal protein, such as vimentin, and that not only arterial structural responses in the absence of BF but also responses to chronically increased BF could be evaluated. We strengthened this experimental approach by combining measurements of BF, mechanics, and structure at the level of the mouse carotid artery.
Vimentin is expressed by various cell types, including vascular endothelial and smooth muscle cells.21 This expression is developmentally regulated31 32 and changes during the course of the repair of arterial and myocardial injuries.33 34 35 36 Yet, vimentin-deficient mice, V-/-, which do not show upregulation of other intermediate filament or cytoskeletal proteins, display a relatively mild phenotype.22 They develop and reproduce normally,22 and their arterial blood pressure does not differ from that of wild-type control mice, V+/+.23 24 It has been reported, however, that V-/- cannot cope with a drastic reduction of renal mass24 and display impaired arterial vasodilatation in response to elevated BF23 24 as a result of an altered balance between the vascular levels of endothelin-1 and NO.24 The evolutionary advantage of the highly conserved vimentin sequence has been proposed to lie not only within the role of the intermediate filament in cellular motility and contractility but also in its possible role in pathological conditions that require vascular adaptations.24
In various previous experimental settings, chronic flow increases and flow decreases have been reported to result in outward and inward arterial remodeling, respectively.3 4 8 10 13 17 The structural diameter changes were observed to lead to a normalization of wall shear stress and to be accompanied by a compensatory change in medial mass, which restores circumferential wall stress. Changes in arterial smooth muscle cell size and number have been suggested to participate in these compensatory or adaptive forms of arterial remodeling in response to altered BF.4 8 10
In mice, however, the BF changes resulting from unilateral carotid artery ligation did not result in compensatory adaptive remodeling. In the occluded vessel, the diameter decreased, but medial mass increased (Reference 2727 and the present study), undoubtedly entailing a decrease in circumferential wall stress. Moreover, a significant neointima develops in the distal part of these vessels.27 30 Even more surprising is the lack of structural changes in the contralateral carotid arteries. Although we previously described compensatory outward hypertrophic remodeling in rat mesenteric muscular arteries exposed to a doubling of BF,8 the diameter and medial mass of mouse carotid arteries seemed to be refractory to a comparable intervention. We cannot firmly exclude that the contralateral hyperperfused vessel regulated wall shear stress primarily by removal of vasomotor tone (flow-induced vasodilatation). Because shear stress is directly proportional to flow and inversely proportional to the third power of vessel radius,3 maintenance of wall shear stress during doubling of BF would, however, require a 26% increase in arterial diameter. This exceeds by far the maximal dilatation that can be induced in carotid arteries of rodents,37 including mice (D.H., unpublished observations, 1997). We have currently no firm explanation for the differences between mouse carotid artery structural responses to altered BF. These differences may involve age,38 39 interspecies and regional differences regarding the pulsatility of the local hemodynamic factors, and the abundance of smooth muscle cells, extracellular matrix components, and cytoskeletal features.18 40
In the present study, we focused our attention on the central part of the vessels where BF measurements were performed and where alterations in vessel mechanics, CSAm, and vessel diameter occurred in the absence of significant neointimal formation. We constructed P-D curves in vitro to ensure that differences in diameter between groups were not due to differences in transmural pressure or arterial smooth muscle tone.
Occluded carotid arteries of V-/- displayed a less marked reduction of arterial diameter and a more pronounced medial hypertrophy than occluded carotid arteries of V+/+. This finding displays similarities to previous results in endothelial cell NO synthasedeficient mice in which unilateral carotid artery ligation did not alter arterial diameter but resulted in exaggerated media hypertrophy.26 As proposed with respect to acute flow-induced vasodilatation,23 24 the presence of vimentin may be required to translate altered arterial shear stress into a chronic alteration of endothelial cell NO synthase activity.
The contralateral hyperperfused carotid artery of V-/- displayed significant increases in diameter and medial mass, whereas the arteries of V+/+, which experienced a more marked increase in BF, showed no significant structural changes. The blunted contralateral hyperemia seen in V-/- during unilateral carotid artery ligation may find its origin in impaired flowinduced vasodilatation23 24 of the vessels that interconnect the areas perfused by both carotid arteries.
Chronic structural responses of arteries to altered BF have been considered to reflect the summation of many short-term vasomotor events.41 42 Findings of the present study indicate, however, that flow-induced vasodilatation and flow-induced remodeling are not directly related. Impaired flowinduced dilatation has been observed in mesenteric23 and renal24 arteries of V-/-. In the assumption that this also applies to the carotid arteries of V-/-, their outward flow-induced remodeling suggests that remodeling does not develop as a consequence of vasomotor changes but rather in conditions in which tone cannot provide for optimal regulation.
Despite increased diameter and medial mass responses to an imposed increase in BF, carotid arteries of sham-operated V-/- tended to be narrower and were equipped with a considerably smaller media than those of V+/+ littermates. This indicates that imposed flow-induced arterial remodeling in the adult does not represent the recapitulation of processes that operate during the development of the arterial system. Changes in carotid artery BF during early development are most likely much more gradual than the ones that we imposed by arterial occlusion, and the effects of endothelium-derived factors may differ between an immature and a fully differentiated vessel wall.43
In conclusion, the present study provides the first evidence that vimentin modulates arterial structural responses to altered BF. It expands previous conclusions regarding the role of vimentin in the mechanotransduction of shear stress from acute vasomotor responses to chronic arterial remodeling, although the structural changes do not seem to be the direct consequence of altered arterial contractility.
| Acknowledgments |
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Received May 14, 1999; accepted September 24, 1999.
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