Aortic elastin and collagen content and synthesis in two strains of rats with different susceptibilities to rupture of the internal elastic lamina

Idiopathic Aneurysms of the Ascending Aorta in the Mouse and Rat

Aneurysms of the ascending aorta, unrelated to xenobiotic administration, are described in 5 rats and 2 mice in nonclinical safety studies conducted at Charles River Laboratories (CRL) sites over the past 10 years. The most prominent microscopic finding was focal dilation with disruption of the wall of the ascending aorta with chronic adventitial inflammation or fibroplasia. The pathogenesis of this finding is unknown. There were no associated macroscopic findings, clinical abnormalities, or vascular lesions elsewhere. The results of a search of historical control data from toxicology studies of 1 day to 72 weeks' duration performed at CRL for aortic findings from 5900 mice and 23,662 rats are also reported. Aortic lesions are uncommon in mice and rats used in nonclinical safety studies, but toxicologic pathologists should be aware that aneurysms of the ascending aorta with fibroplasia and inflammation in the aortic wall and adventitia may occur spontaneously or iatrogenically, as they have the potential to impact interpretation in toxicology studies.

Potassium channel openers increase aortic elastic fiber formation and reverse the genetically determined elastin deficit in the BN rat

Hypertension is a cardiovascular disorder that appears in more than half of the patients with Williams-Beuren syndrome, hemizygous for the elastin gene among 26 to 28 other genes. It was shown that the antihypertensive drug minoxidil, an ATP-dependent potassium channel opener, enhances elastic fiber formation; however, no wide clinical application was developed because of its adverse side effects. The Brown Norway rat was used here as an arterial elastin-deficient model. We tested 3 different potassium channel openers, minoxidil, diazoxide, and pinacidil, and 1 potassium channel blocker, glibenclamide, on cultured smooth muscle cells from Brown Norway rat aorta. All tested potassium channel openers increased mRNAs encoding proteins and enzymes involved in elastic fiber formation, whereas glibenclamide had the opposite effect. The higher steady-state level of tropoelastin mRNA in minoxidil-treated cells was attributable to an increase in both transcription and mRNA stability. Treatment of Brown Norway rats for 10 weeks with minoxidil or diazoxide increased elastic fiber content and decreased cell number in the aortic media, without changing collagen content. The minoxidil-induced cardiac hypertrophy was reduced when animals simultaneously received irbesartan, an angiotensin II-receptor antagonist. This side effect of minoxidil was not observed in diazoxide-treated animals. In conclusion, diazoxide, causing less undesirable side effects than minoxidil, or coadministration of minoxidil and irbesartan, increases elastic fiber content, decreases cell number in the aorta and, thus, could be suitable for treating vascular pathologies characterized by diminished arterial elastin content and simultaneous hypertension.

Changes in aortic stiffness related to elastic fiber network anomalies in the Brown Norway rat during maturation and aging

Adult Brown Norway (BN) rats exhibit numerous internal elastic lamina (IEL) ruptures in the abdominal aorta (AA) and a lower aortic elastin-to-collagen ratio (E/C) compared with other strains. We studied here AA mechanical properties in BN compared with control strains. AA stiffness (assessed by plotting elastic modulus/wall-stress curves obtained under anesthesia), thoracic aorta elastin and collagen contents, and IEL ruptures in AA were measured in male BN and LOU rats aged 6, 10, and 15 wk. The Long Evans (LE) control strain was compared with BN at more advanced ages (15, 28, and 64 wk). At all ages, aortic E/C was lower in BN than in control strains. At 6 wk, AA stiffness was greater in BN than in LOU. In both strains, AA stiffness decreased between 6 and 10 wk, more so in BN than in LOU, and then increased, reaching similar values at 15 wk. BN AA stiffness was not different from that of LE at 15 and 28 wk, but was significantly lower at 64 wk. The increased stiffness in young BN rat AA may be due to the decreased E/C. IEL rupture onset in the BN around 7–8 wk, which decreases stiffness, as suggested by its pharmacological modulation, abolished such differences by 15 wk. Thereafter, age-related AA stiffness increased less in BN than in LE, likely due to the numerous IEL ruptures. We conclude that, in the BN rat, the lower E/C and the presence of IEL ruptures have opposing effects on arterial stiffness.

Quantitative genetic basis of arterial phenotypes in the Brown Norway rat

The Brown Norway (BN) rat presents several genetically determined arterial phenotypes of interest, i.e., ruptures of the internal elastic lamina (RIEL) in the abdominal aorta (AA), iliac (IAs), and renal arteries, aortic elastin deficit and higher frequency of persistent ductus arteriosus (PDA) than other strains. We investigated the genetic basis of these phenotypes. We established a backcross between BN and the LOU reference strain and performed a genome-wide scan on 104 males and 105 females with 193 microsatellite markers followed by linkage analysis. RIEL in AA and IAs showed highly significant linkage to a locus on chromosome 5 and suggestive linkage to a locus on chromosome 10, which is syntenic to one linked to a syndrome of thoracic aortic aneurysms with PDA in humans. In contrast, renal artery RIEL mapped to a chromosome 3 locus and thoracic aortic elastic content to two loci on chromosome 2. PDA was significantly linked to two different quantitative trait loci (QTL) on chromosomes 8 and 9. This is the first study in rats to identify genetic loci for PDA. We identified 21 candidate genes by functional relevance or integration of our mapping data with global expression analysis. Sequencing these genes identified 47 single nucleotide polymorphisms, but no functionally relevant amino acid changes. By expression analysis, myosin heavy chain 10, nonmuscle, in the chromosome 10 QTL, emerged as a candidate for RIEL in AA and IAs. Furthermore, production of a congenic line for the chromosome 5 QTL proved implication of this locus in RIEL formation.

High-Flow-Induced Arterial Remodeling in Rats with Different Susceptibilities to Cerebral Aneurysms

BACKGROUND

The higher incidence of cerebral aneurysms (CAs) induced by enhanced arterial blood flow in Long Evans (LE) compared to Brown Norway (BN) rats suggests that intrinsic differences in high-flow arterial remodeling may be involved in determining CA susceptibility. Some aspects of this remodeling were compared in LE and BN rats after creation of an abdominal aortocaval fistula (ACF).

METHODS AND RESULTS

At 4 days with ACF, aortic luminal cross-sectional area (LCSA) determined by morphometry was increased by 20% in LE but not in BN rats. mRNA levels, determined by RT-PCR, were higher in LE than in BN rats for collagen alpha1(I), collagen alpha1(III), MMP2 and its inhibitor TIMP1 at 19 days with ACF. Nitric oxide synthase (NOS) mRNA levels were higher in LE rats at 4 days for the inducible (NOS2) isoform and at 4 and 19 days for the neuronal (NOS1) isoform. Aortic LCSA and NOS1 mRNA levels were tightly correlated and NOS inhibition prevented ACF-induced aortic remodeling in the LE rat. MMP2 and MMP7 activity, evaluated by zymography at 4 days with ACF, did not greatly differ between BN and LE.

CONCLUSIONS

These data suggest that a higher intrinsic ability for high-flow-induced arterial enlargement associated with NOS gene overexpression may be a possible genetic determinant in CA susceptibility.

Semicarbazide-sensitive amine oxidase in annulo-aortic ectasia disease: relation to elastic lamellae-associated proteins

Lysyl oxidases (Lox), which are members of the amine oxidase family, are involved in the maturation of elastic lamellae and collagen fibers. Modifications of amine oxidases in idiopathic annulo-aortic ectasia disease (IAAED) have never been investigated. Our aim was to examine the expression of several proteins that might interfere with elastic fiber organization in control (n=10) and IAAED (n=18) aortic tissues obtained at surgery. Expression of amine oxidases and semicarbazide-sensitive amine oxidase (SSAO), and cellular phenotypic markers were examined by immunohistopathology and confocal microscopy. The expression of these proteins was assessed in relation to clinical and histomorphological features of the arterial wall. In control aorta, SSAO staining was expressed along elastic lamellae, whereas in aneurysmal areas of IAAED, SSAO was markedly decreased, in association with severe disorganization of elastic lamellae. Smooth muscle myosin heavy chain was also decreased in IAAED compared with controls, indicating smooth muscle cell dedifferentiation. Multiple regression analysis showed that elastic lamellar thickness (ELT) was correlated positively with the SSAO:elastin ratio and negatively with the Lox:elastin ratio, and that the clinical features of IAAED (aneurysm, thoracic aorta diameter, and aortic insufficiency) were positively correlated with ELT but not with SSAO. The relationship between SSAO expression and ELT suggests that this amine oxidase may be involved in elastic fiber organization. However, in advanced IAAED, the deficit in SSAO expression could be secondary to the decrease and fragmentation of elastic fibers and/or to vascular smooth muscle cell dedifferentiation.

Characteristics of the aortic elastic network and related phenotypes in seven inbred rat strains

Extracellular matrix (ECM) molecules such as elastin and collagen provide mechanical support to the vessel wall and are essential for vascular function. Evidence that genetic factors influence aortic ECM composition and organization was concluded from our previous studies showing that the inbred Brown Norway (BN) rat differs significantly from the outbred Long-Evans (LE) and the inbred LOU rat with respect to both thoracic aortic elastin content and internal elastic lamina (IEL) rupture in the abdominal aorta and iliac arteries. Here, we measured aortic elastin and collagen contents as well as factors that may modulate these parameters [insulin growth factor (IGF)-I, transforming growth factor (TGF)-beta(1), and matrix metalloproteinase (MMP)-2] in seven inbred rat strains, including BN and LOU. We also investigated whether IEL ruptures occur in strains other than BN. We showed that LOU, LE, BN, and Fischer 344 (F344) rats were significantly different for aortic elastin content and elastin-to-collagen ratio, whereas LE, Lewis, WAG, and Wistar-Furth (WF) were similar for these parameters. BN and F344 had the lowest values. BN was the only strain to present numerous IEL ruptures, whereas F344, LE, and WF presented a few and the other strains presented none. In addition, IGF-I and TGF-beta(1) levels in the plasma and aorta differed significantly between strains, suggesting genetic control of their production. Because inbred rat strains provide interesting models for quantitative trait locus analysis, our results concerning elastin, collagen, IEL ruptures, and cytokines may provide a basis for the search for candidate genes involved in the control of these phenotypes.

Arterial sympathetic innervation and cerebrovascular diseases in original rat models

The role of the arterial sympathetic innervation in cerebrovascular pathology was investigated in new experimental models using Brown Norway (BN) and Long-Evans (LE) rats. The BN rat is susceptible to intracerebral hemorrhage (ICH) within the cerebral cortex when rendered hypertensive whereas the LE rat is prone to cerebral aneurysms (CAs) in arteries of the circle of Willis with hypertension and carotid ligation. Noradrenaline (NA) content, determined by high performance liquid chromatography (HPLC), was lower both in the caudal and cerebral arteries in the BN than in the LE rat. Denervation of cerebral arteries by superior cervical ganglionectomy did not increase ICH lesion incidence in BN hypertensive rats. A possible link between the level of caudal artery NA content and the occurrence of ICH lesions and CAs was studied in rats from two distinct BNXLE crosses: back-cross (BC) rats (F1XBN) and F2 rats (F1XF1) which respectively display, with hypertension and carotid ligation, a high incidence of either ICH lesions or CAs. In BC rats, the level of caudal artery NA content was not related to ICH lesion occurrence. However, in F2 rats a low caudal artery NA content was associated with a high incidence of ruptured CAs. Thus, a low arterial sympathetic innervation may participate in mechanisms leading to rupture of CAs.

Identification of two susceptibility loci for vascular fragility in the Brown Norway rat

A trait of vascular fragility, characterized by the formation of abrupt defects within the elastic laminae of the abdominal aorta, has been identified in Brown Norway (BN) rats. These lesions are greatly exacerbated in F(1) rats from a BN x New Zealand genetically hypertensive (GH) intercross, implying that the genetic background provided by the GH rat influences lesion severity. The F(2) progeny of the BN x GH intercross were used to identify susceptibility loci for the lesions as well as exacerbating loci. Two major quantitative trait loci (QTLs) for number of internal elastic lamina lesions were identified on rat chromosomes 5 and 10, with the maximum "log of the odds ratio" (LOD) scores at D5Rat119 (LOD 5.0) and at D10Mit2 (LOD 4.5), respectively, together contributing 33.5% to the genetic variance. Further analysis revealed that the chromosome 10 locus exhibits a dominant mode of inheritance, with BN alleles being associated with increased lesion number (P < 0.0002) compared with GH homozygotes. This locus was in epistasis to a modifier locus on rat chromosome 2 at D2Mit14 (LOD score 2.12). A second major locus was identified on chromosome 5, exhibiting a semidominant mode of inheritance, again with the BN allele being significantly associated with increased lesion number (P < 0.0001). Furthermore, a locus influencing lesion severity was identified on chromosome 3 wherein GH alleles associated with increased severity. This is the first study to identify susceptibility loci for vascular elastic tissue fragility.

Heritability of intracerebral hemorrhagic lesions and cerebral aneurysms in the rat

BACKGROUND AND PURPOSE

Under certain conditions, the Brown Norway (BN) rat is susceptible to intracerebral hemorrhagic vascular (ICV) lesions within the cerebral cortex, whereas the Long-Evans (LE) rat is prone to develop aneurysms in the circle of Willis. The incidence of these 2 pathological phenotypes was studied in progeny of different BNXLE crosses to determine their heritability in these new rat models. In addition, a possible link between ICV lesion occurrence and either the susceptibility to spontaneous rupture of the arterial internal elastic lamina (IEL) or basal plasma angiotensin-converting enzyme (ACE) activity was also studied in back-cross (BC) F1XBN rats, the only second-generation group with a high incidence of ICV lesions.

METHODS

To induce cerebrovascular lesions, rats were submitted to experimental hypertension associated with ligation of 1 carotid artery. After death, the brain was examined for cerebral lesions. Numbers of arterial IEL ruptures were determined microscopically with the use of en face preparations. Plasma ACE activity was determined before the induction of hypertension.

RESULTS

In general, groups that developed ICV lesions presented a low incidence of aneurysms. ICV lesion incidence was similar in F1 hybrids and BC(F1XBN) and greatly decreased in F2 and BC(F1XLE) rats compared with BN rats. No cerebral aneurysms developed in F1 rats. Aneurysmal incidence was 24% (20% ruptured) in LE, 42% (59% ruptured) in F2, and 50% (75% ruptured) in BC(F1XLE) rats. In BC(F1XBN) rats, neither the incidence of IEL rupture nor the plasma ACE activity was higher in the rats with ICV lesions. However, the mean blood pressure level was higher in these rats, and peak blood pressure was higher in rats with the most severe grades of ICV lesions.

CONCLUSIONS

These data suggest a polygenic and dominant mode of inheritance of ICV pathology. The formation of aneurysms in the circle of Willis tended to be favored, and their rupture was clearly increased by the presence of BN rat alleles within the LE rat genome. These data may provide the basis for future studies to determine, in new rat models, which genes are involved in these pathologies.

Aortic wall damage in mice unable to synthesize ascorbic acid

By inactivating the gene for L-gulono-gamma-lactone oxidase, a key enzyme in ascorbic acid synthesis, we have generated mice that, like humans, depend on dietary vitamin C. Regular chow, containing about 110 mg/kg of vitamin C, is unable to support the growth of the mutant mice, which require L-ascorbic acid supplemented in their drinking water (330 mg/liter). Upon withdrawal of supplementation, plasma and tissue ascorbic acid levels decreased to 10-15% of normal within 2 weeks, and after 5 weeks the mutants became anemic, began to lose weight, and die. Plasma total antioxidative capacities were approximately 37% normal in homozygotes after feeding the unsupplemented diet for 3-5 weeks. As plasma ascorbic acid decreased, small, but significant, increases in total cholesterol and decreases in high density lipoprotein cholesterol were observed. The most striking effects of the marginal dietary vitamin C were alterations in the wall of aorta, evidenced by the disruption of elastic laminae, smooth muscle cell proliferation, and focal endothelial desquamation of the luminal surface. Thus, marginal vitamin C deficiency affects the vascular integrity of mice unable to synthesize ascorbic acid, with potentially profound effects on the pathogenesis of vascular diseases. Breeding the vitamin C-dependent mice with mice carrying defined genetic mutations will provide numerous opportunities for systematic studies of the role of antioxidants in health and disease.

Influence of elastin gene polymorphism on the elastin content of the aorta: A study in 2 strains of rat

The elastin content in the thoracic aorta of male Brown-Norway (BN) rats is 31.4+/-1.2% (dry weight), whereas that of male LOU rats is 37.2+/-1.0%. A similar difference in the elastin content of the thoracic aorta is also observed in female animals. Furthermore, in the thoracic aorta of young, growing rats as well as in cultured aortic smooth muscle cells, the steady-state level of elastin mRNA is significantly lower in the BN than in the LOU strain. These results suggested that 1 or more genes control the elastin mRNA level and the elastin content in the aortas of BN and LOU rats. A possible relationship between a polymorphism in the elastin gene and the elastin content of the aorta was tested. For this purpose, the aortic elastin content was measured in F(1) and F(2) generations bred from LOU and BN rats and was compared with that of the F(0) (parental) generation. A polymorphic marker located in intron 25 of the elastin gene has been used to genotype the F(2) rats. The degree of genetic determination of aortic elastin content was estimated to be 73% in the F(2) cohort, but the elastin locus accounts for only 3. 9% of the total variance in aortic elastin content. Other genes are thus responsible for the major part of the observed interstrain difference by regulating the transcription of the gene, the stability of elastin mRNA, and/or posttranslational events.

Protection of the arterial internal elastic lamina by inhibition of the renin-angiotensin system in the rat

Spontaneous rupture of the internal elastic lamina (IEL) occurs in some arteries of the rat during growth and aging. Inbred, normotensive, Brown Norway (BN) rats are particularly susceptible to rupture of the IEL, especially in the abdominal aorta (AA). Preliminary experiments showed that different angiotensin-converting enzyme (ACE) inhibitors protect against rupture of the IEL in the BN rat to a greater extent than hydralazine, suggesting a role of the renin-angiotensin system (RAS) in this phenomenon. To explore this possibility, we have treated male BN rats from 4.5 to 14 weeks of age with either enalapril or losartan (both at 1, 3, and 10 mg x kg(-1) x d(-1)) or with the calcium antagonists mibefradil (at 3, 10, 30, and 45 mg x kg(-1) x d(-1)) and amlodipine (at 30 mg x kg(-1) x d(-1)). Systolic blood pressure (SBP) was measured weekly, and at the end of treatment we (1) recorded body and heart weights, (2) measured various parameters of the RAS in plasma, (3) quantified interruptions in the IEL on "en face" preparations of AA, and (4) quantified elastin, collagen, and cell proteins in the media of the thoracic aorta. Results showed that enalapril and losartan similarly decrease SBP and rupture of the IEL in the AA, suggesting that enalapril inhibits the latter via a decrease in the production of angiotensin II (Ang II) and not via another effect on ACE. The decrease in IEL rupture and in SBP, as well as the modifications in the parameters of the RAS, were all dose dependent. Mibefradil had little effect on the RAS and, at the highest doses, decreased SBP to an extent similar to that for enalapril at 3 mg x kg(-1) x d(-1) but did not significantly inhibit IEL rupture. Amlodipine decreased SBP, increased plasma renin concentration, and was without effect on IEL rupture. All treatments at the highest doses had a hypotrophic effect on the aortic media but differed in their effects on the heart, with enalapril and losartan decreasing and mibefradil and amlodipine increasing heart weight, suggesting that the inhibition of IEL rupture may be related to a cardiac hypotrophic effect. All these results, taken together, suggest that Ang II plays a role in the rupture of the IEL that is, in part, independent of SBP.

Genetic susceptibility to experimental cerebral aneurysm formation in the rat

BACKGROUND AND PURPOSE

The susceptibility to experimental cerebral aneurysm formation in arteries of the circle of Willis was studied in four strains of rats presenting different susceptibilities to the spontaneous rupture of the internal elastic lamina in extracerebral arteries: Brown-Norway (BN) > Wistar > Long-Evans (LE) > LOU.

METHODS

Rats (150 g body weight) of the four strains were subjected to hypertension and a change in local cerebral blood flow by ligation of one common carotid artery for about 7 months. Six-month-old BN and LE rats were subjected to carotid ligation only for 11 to 13.5 months and treated or not (from 3 to 7 months of age) with an inhibitor of connective tissue fiber maturation, beta-aminopropionitrile (BAPN).

RESULTS

Aneurysmal structures (AS) occurred mainly in the anterior cerebral/anterior communicating arterial complex and proximal part of the posterior artery. In hypertensive rats, the AS incidence was LE, 56%; Wistar, 33%; BN, 17%; and LOU, 11%. When normotensive and subjected to carotid ligation only, LE rats showed an even greater susceptibility to AS formation (86%) than BN (7%). BAPN treatment did not influence AS formation: LE (60%) versus BN (8%).

CONCLUSIONS

These results suggest that genetic factors are involved in cerebral aneurysm formation in the rat. The susceptibility of the internal elastic lamina of extracerebral arteries to spontaneous rupture does not appear to be a determinant genetic trait in the propensity to develop aneurysms in arteries of the circle of Willis. The comparison of these different rat strains may be very useful for studying factors contributing to cerebral aneurysm pathogenesis.