Animal Models of Cardiovascular Complications of Pregnancy

Abstract

Cardiovascular complications of pregnancy have risen substantially over the past decades, and now account for the majority of pregnancy-induced maternal deaths, as well as having substantial long-term consequences on maternal cardiovascular health. The causes and pathophysiology of these complications remain poorly understood, and therapeutic options are limited. Pre-clinical models represent a crucial tool for understanding human disease. We review here advances made in preclinical models of cardiovascular complications of pregnancy, including preeclampsia and peripartum cardiomyopathy, with a focus on pathologic mechanisms elicited by the models, and on relevance to human disease.

Introduction

Pregnancy and delivery are a high-risk period in a woman’s life. As recently as 1920, 600 of every 100,000 women died in or around childbirth, usually from sepsis, hemorrhage, or hypertension¹. The development of aseptic technique, antibiotics, and blood transfusions in the 20^th century have dramatically decreased maternal mortality from these causes. Today, in the US, the main causes of maternal mortality are cardiovascular in nature, including cardiomyopathy, preeclampsia, thromboembolic disease, strokes, and others². Strikingly, maternal mortality in the US has increased three-fold in the last 40 years, likely a reflection of advancing maternal age, preexisting morbid conditions including obesity and diabetes, improved management of congenital heart disease allowing these patients to become pregnant, and environmental factors². The US is one of only two countries in which maternal mortality has increased since 2000. Equally strikingly, there are profound socioeconomic, geographic, and racial inequalities in outcomes. For example black women are three times as likely to die during or after childbirth than are white women³. In addition, there is an emerging recognition that pregnancy complications presage substantial long-term risk of cardiovascular disease. For example, the life-long risk of CVD in women who have had preeclampsia is as much as tripled, on par with the risk conferred by smoking or diabetes⁴, and the AHA now recommends that eliciting a history of preeclampsia be an integral component of routine clinic visits⁵. Pregnancy-associated maternal disease also frequently affects the fetus or the newborn, both short and long-term. Therapeutic options for treating or preventing cardiovascular complications of pregnancy remain limited, with no existing disease-specific therapies for most of them. There is thus an urgent need to better understand the causes and pathophysiology of cardiovascular complications of pregnancy.

General considerations

Before describing specific models of cardiovascular complications of pregnancy, it is useful to consider certain general concepts.

• Placental pregnancy is restricted to mammals (rudimentary placenta-like organs do exist in some fish)⁶. Models of cardiovascular complications of pregnancy must therefore be in mammals. To date, most models have used rodents, predominantly mice. Numerous parallels between rodent and human development and physiology, as well as the availability of genetic manipulations in mice, make rodents ideal model systems. Comparative systems biology of human and mouse placental studies suggest that there is close to 90% concordance between proteins expressed in placental tissue from humans and mice⁷. Most known hormonal changes of human pregnancy are recapitulated in the mouse and rat, though not all⁸. Maternal changes in physiology, such as calorie retention via expansion of fat stores, and development of relative insulin resistance, also mirror those seen in human pregnancy⁹. Many cardiovascular changes are also recapitulated, including decreased blood pressure throughout most of pregnancy, increase in plasma volume with consequent decrease in hematocrit, increase in cardiac output, and the development of cardiac hypertrophy^{10, 11}. However, it is also important to recognize the many limitations of the rodent models. Unlike human pregnancies, rodent pregnancies are always multiparous, typically yielding 3–12 pups. Although primates and rodents share a hemochorial type of placentation, placentation in rodents is significantly different than in humans⁸. For example, the invasion of the uterine wall by fetal trophoblasts is substantially shallower in mice, and trophoblast invasion in mice follows a perivascular route rather than an endovascular route noted in humans¹². In addition, gestational length is rodents is significantly shorter (21 days) in contrast to ~9 months in humans, and genome-wide expression profiling of mouse and human placentae revealed clusters of genes with distinct co-expression patterns across gestation, suggesting that the developmental timeline in mouse runs parallel to the first half of human placental development¹³. Rodent hemodynamics are also significantly different from humans: the rodent heart, for examples, beats 600–700 times per minute, an order of magnitude faster than the human heart¹¹.

• Hormones play a major role in disease. Pregnancy profoundly changes maternal homeostasis, and does so rapidly, creating numerous vulnerabilities. Parturition and the immediate post-partum period reverses many of these changes, and introduces new ones, at an even faster rate¹⁴. These rapid changes are largely elicited and controlled by hormones such as progesterone emanating from the placenta. A simple way to conceptualize these profound effects is that the placenta, a tissue mostly of fetal origin, uses a panoply of hormones to developmentally reprogram maternal physiology to suit the needs of the fetus first, and then, anticipatorily at the end of gestation, of the newborn second. It is therefore not surprising that most uncovered mechanisms of cardiovascular complications of pregnancy involve aberrant hormonal effects, as discussed in detail below.

• Pregnancy and the post-partum period represent a unique relationship between two genetically distinct organisms. During pregnancy, this interaction largely occurs within the placenta. The placenta thus must be an immune-privileged setting, where neither maternal nor fetal immune system rejects the other¹⁵. For the same reason, maternal immunity is generally suppressed during gestation, e.g., studies have suggested that pregnancy is characterized by upregulation of T-regulatory cells that suppress immunity and maintain tolerance¹⁶. This immune suppression, and the rebound immunological activation post-partum, can also contribute to disease.

Disease models of cardiovascular complications of pregnancy

Preeclampsia

Preeclampsia typically presents during the third trimester with hypertension and proteinuria and if left untreated can progress to end-organ damage in the kidney (acute kidney injury), liver (HELLP syndrome), and brain (eclampsia)¹⁷. While delivery of placenta is the definitive treatment for this disease, preeclamptic women can have both-short term and long-term cardiovascular complications¹⁸. In the short-term, severe hypertension in the post-partum period can occur and is often associated with repeat hospitalization and rarely seizures. Preeclampsia is also a significant risk factor for peripartum cardiomyopathy (see below) and for heart failure with preserved ejection fraction (HFpEF)¹⁹. In the long-term, preeclampsia is associated with 3-fold excess of cardiovascular disease (CVD)¹⁸. Many pathways related to angiogenesis, inflammation, syncytiotrophoblast stress, and autoantibodies have been proposed to play a role in the pathogenesis of preeclampsia¹⁷. Pathogenesis likely involves two distinct stages. The first, asymptomatic phase of the disease is associated with shallow invasion of the placenta into the decidua and myometrium, and decreased uteroplacental blood flow. The second stage occurs during the third trimester, when women present with clinical features of hypertension, proteinuria, and microangiopathy in various organs such as kidney, liver and brain. During this phase, there is syncytiotrophoblast stress that can be measured by high circulating levels of anti-angiogenic factors such as soluble fms-like tyrosine kinase-1 (sFLT1), soluble endoglin (sENG)²⁰, and increased urinary levels of prolactin and its anti-angiogenic 16kDa fragment²¹. Animal models are critical to study the cause/effect of the various pathways that have been proposed to play a role in preeclampsia. Tools to genetically modify animals, radiotelemetry to measure blood pressures non-invasively, and ultrasound/dopplers to evaluate uteroplacental blood flow, have enabled the continuous monitoring of healthy and preeclamptic pregnancies in rodents²². Several animal models that recapitulate the first and/or second phase of the syndrome have been characterized. However, no single model fully recapitulates all the features of humans with preeclampsia, and therefore extrapolation of the experimental findings to the clinical situation should be interpreted with caution. Nevertheless, animal models have been invaluable in dissecting the pathogenesis of preeclampsia and validating potential targets for therapeutic intervention²³. Here, we will summarize validated animal models currently used to study the pathophysiology of this disease (See Table 1).

Table 1:

Animal models of preeclampsia.

Animal Models	Species	Pathways	Limitations
Surgical ischemia of placenta24–28, 31, 33, 34, 43	Rats, Mice and Baboons	Hypoxia, Oxidative stress, Inflammation, Angiogenic Imbalance	Lack of severe disease Cannot study trophoblast invasion
Angiotensinogen/Renin transgenics38, 39, 41, 44	Rats, MIce	Oxidative stress, Angiotensin receptor autoantibodies, Inflammation	Less relevant to humans as preeclampsia is characterized by suppressed renin and angiotensin II
sFLT1 overexpression46–51, 53	Rats, Mice	Angiogenic imbalance, Endothelial dysfunction	Short gestation and not optimal to study early placentation events
BPH/5 model58, 61	Mice	Complement activation, Angiogenic imbalance	Lack of severe disease Short gestation
CBA/J x DBA/2	Mice	Complement activation, Angiogenic imbalance, immune dysfunction	Lack of severe disease Short gestation
COMT KO66	Mice	Hormonal and placental hypoxia	Lack of severe disease; no phenotype in rats; short gestation
STOX1 transgenic73	Mice	Inflammation, Angiogenic Imbalance	Relevance to humans unclear as loss of function mutations variable depending on populations
RGS2 KO81	Mice	Histone deacetylation, cyclic AMP pathway	Relevance to humans unclear as no evidence of placental ischemia
AVP infusion82, 83	Mice, Rats	Immune dysfunction, G protein signaling	Lack of severe disease; short gestation
L-NAME infusion85	Mice, Rats	Endothelial Dysfunction	Partial phenotype
Dahl Salt-sensitive Rats86–88	Rats	Hypoxia, Endothelial Dysfunction	Lack of severe disease; not a model for de novo preeclampsia

Acute placental ischemia models

Since the major defect in preeclampsia is placental ischemia related to poor placentation, researchers have modeled placental ischemia in rodent and primates using surgical approaches. The most common model is induction of partial ischemia in pregnant rats using surgical clips of the uterine artery, referred to as reduced uterine perfusion pressure (RUPP) model²⁴. Pregnant rats entering the RUPP procedure undergo clipping procedure at day 14 of gestation. Typically, a silver clip is placed around the aorta above the iliac bifurcation to reduce uterine perfusion pressure in the gravid rat by approximately 40%. Since compensation of blood flow to the placenta occurs in pregnant rats through an adaptive increase in ovarian blood flow, additional clips on both right and left uterine arcades at the ovarian end right before the first segmental artery helps maintain the placental ischemia²⁵ (see Figure 1). RUPP rats develop reduced GFR, third trimester hypertension, and variable degree of fetal growth restriction. Importantly, pathways implicated in humans with preeclampsia such as angiogenic imbalance, autoantibodies against angiotensin receptor, and pro-inflammatory factors have all been noted in this model^26–29. Many therapeutic compounds such as recombinant endothelial growth factors have been tested successfully in these models and provide valuable preclinical data needed prior to testing in pregnant humans^{26, 27}. However, there are some caveats of this model. Preterm delivery has not been observed with this model and therefore it is difficult to assess prolongation of pregnancy with specific therapies. Also, RUPP is a model of non-severe preeclampsia, as these animals have not been reported to develop HELLP syndrome or seizures to suggest eclampsia.

Fig 1:

Schematic of the Reduced Uterine Perfusion Pressure (RUPP) model

The RUPP model is made by clipping the ovarian arteries and abdominal aorta with silver clips during the 3rd trimester in rats or in mice. Pregnant RUPP animals develop hypertension, decreased glomerular filtration rate, variable degrees of proteinuria, and fetal growth restriction. (Illustration credit: Ben Smith)

RUPP model has also been performed in mice by several groups, and preclinical studies to evaluate the safety and efficacy of nicotinamide and tadalafil were recently published^{30, 31}. The advantage of using mice over rats is the availability of ready-to-use genetically modified mice from several repositories that can allow one to test the role of specific pathways in the pathogenesis of preeclampsia. To model ischemia, some groups have also exposed pregnant mice to relative hypoxia (9.5% O2) from gestational day 7 to 17, inducing preeclampsia-like features including elevated preeclampsia biomarker levels³².

Placental ischemia has also been modeled in non-human primates, typically with baboons³³. Since pregnancy duration is significantly longer in baboons (180 days) than rodents, and since there is a single placenta and fetus, this surgical model closely approximates the clinical condition. Again, many therapies have been tested in this model that are now being pursued in clinical trials^{34, 35}. While there are several advantages to using non-human primates for preclinical proof-of-concept studies, a disadvantage is the inability to study early placentation events leading to preeclampsia, because the induction of surgical ischemia may actually promote trophoblast invasion³⁶.

Renin-Angiotensin models of preeclampsia

Preeclampsia is characterized by enhanced angiotensin II sensitivity even prior to the onset of hypertension³⁷. Models to study enhanced angiotensin II sensitivity and signaling have largely used transgenic rodents in which the hypertension is induced by placental renin that acts on maternal angiotensinogen. When transgenic female mice expressing angiotensinogen are mated with transgenic males expressing renin, pregnant females display gestational hypertension during the 3^rd trimester due to secretion of placental renin into the maternal circulation³⁸. Generalized seizures occur in 15% of these mice and the hypertension reverses after delivery of the placenta. Subsequently other groups have reproduced this model in rats and reported that gestational hypertension was associated with the presence of agonistic antibodies against the angiotensin receptor^{39, 40} (see Figure 2). Follow-up studies suggested that these agonistic autoantibodies not only induce angiotensin II sensitivity⁴¹, but also may be sufficient to induce preeclampsia-like phenotypes when purified and injected into wild-type pregnant mice^{42, 43}. While several of the features of preeclampsia such as gestational hypertension, albuminuria and angiotensin II sensitivity are similar to humans with preeclampsia, this model has very high renin and angiotensin II which is in contrast to humans with preeclampsia, where renin and angiotensin II levels are suppressed. Nevertheless, the model has been particularly useful to dissect the early events of placentation. For example, studies suggest that enhanced angiotensin II signaling may block trophoblast invasion and that neutralization of the RAS may enhance placentation^{44, 45}.

Fig 2:

Angiotensin II model of Pregnancy-Associated Hypertension (PAH)

Female human angiotensinogen (hAogen) transgenic mice mated with male human renin (hRen) transgenic mice develop severe hypertension during pregnancy as human renin made from the fetus/placenta acts on maternal human angiotensinogen to induce excess angiotensin II that occurs only during pregnancy. Endogenous mouse renin will not cleave human angiotensinogen, making this a valuable model to study PAH. The opposite crosses (female hRen x male hAogen) can serve as controls as human angiotensinogen made in the fetus does not cross to the maternal circulation. (Illustration credit: Ben Smith)

sFLT1 overexpression model

Clinical studies suggest that high levels of sFLT1 correlate with adverse maternal and fetal outcomes related to preeclampsia. Adenoviral-mediated expression of sFLT1 in pregnant mice and rats reproduces many of the features of preeclampsia including severe hypertension, proteinuria, glomerular endotheliosis and fetal growth restriction^{22, 46–49} (see Figure 3). In this model, sFLT1 is expressed ectopically from the liver and circulating concentrations achieved are higher than the concentrations noted in women with preeclampsia. To evaluate a more physiologically relevant model, transgenic over-expression of sFLT1 from the placenta was recently reported⁵⁰. In this model, the animals developed full-blown maternal disease as well as placental disease, including abnormal spiral artery remodeling and impaired blood flow to the feto-placental unit⁵¹. Other groups have also studied other synergistic anti-angiogenic factors, for example soluble endoglin (sENG). Transgenic expression of sENG during pregnancy leads to hypertension and fetal growth restriction⁵². Co-expression of sENG with sFLT1 either through adenoviral vector or as infusion of recombinant proteins has been used as a model of HELLP syndrome, a severe complication of preeclampsia^{53, 54}. The sFLT1 over-expression model has been used to test several novel therapies, such as VEGF-121 and statins^{46, 55}. Kumasawa et al, showed that pravastatin upregulated PlGF and rescued preeclampsia-like features in a placental lentiviral sFLT1 overexpression model⁵⁶. Since clinical studies have reported that elevated circulating sFLT1 and low PlGF is present is most cases of preterm preeclampsia and that angiogenic imbalance correlates with adverse maternal and fetal outcomes related to preeclampsia⁵⁷, there is strong interest in the sFLT1 model to test novel therapies, but also to understand downstream mechanisms by which sFLT1 induces vascular disease.

Fig 3:

sFLT1 overexpression model of preeclampsia

Adenoviral sFLT1, versus adenoviral Fc protein as control, delivered into pregnant rats or mice induces systemic angiogenic imbalance and preeclampsia-like features including hypertension (panel A), proteinuria (panel B) and glomerular endotheliosis (panel C). Figure adapted from Maynard et al 47.

Genetic Models:

a. BPH/5 model:

BPH/5 mice are an inbred sub-line derived from 20 generations of brother-sister matings of the inbred spontaneously hypertensive strain BPH/2⁵⁸. During the nonpregnant states, BPH/5 has mildly elevated blood pressures when compared to the C57BL/6 strain, but does not develop proteinuria. However, during pregnancy BPH/5 strain mice develop severe hypertension during 3^rd trimester that is accompanied by proteinuria and glomerular damage⁵⁸. Interestingly, these maternal phenotypes are accompanied by low birth weight and smaller litter size in these mice. However, BPH/5 mice do not develop features of severe disease such as HELLP syndrome⁵⁹. Many groups have attempted to study the mechanisms of why this strain develops preeclampsia-like states. Sones et al have suggested that anti-angiogenic sFLT1 protein was upregulated in the BPH/5 placentas and that aberrant sFLT1 expression was responsible for the complement activation and poor placentation in this model⁶⁰. Interestingly, adenoviral delivery of VEGF121 in early pregnancy prevented the spontaneous development of preeclampsia in this model thus confirming that angiogenic imbalance was central to the pathogenesis of the maternal syndrome⁶¹.

b. CBA/J x DBA/2 model:

Immunological mechanisms have been hypothesized to play a central role in early placentation defects in preeclampsia. CBA/J x DBA/2 mating combinations has been extensively used to study miscarriages and preeclampsia^{62, 63}. Initial studies reported embryos derived from mating CBA/J females with DBA/2 males showed an increased frequency of resorption and that surviving embryos showed consistent and significant intrauterine growth restriction. Administration of soluble CD200, an immune check point inhibitor has been shown to prevent abortions by augmenting T regulatory cells⁶⁴. Ahmed et al characterized the maternal phenotypes of this mating combination and reported several features like albuminuria and glomerular fibrin deposits that are similar to humans with preeclampsia⁶⁵. Although these animals at baseline did not develop hypertension, they developed exaggerated sensitivity to Angiotensin II. This animal model was explored to study several therapies such as pravastatin and pro-angiogenic factor VEGF121 with beneficial effects and without inducing toxicity.

c. COMT knockout mice:

Kanasaki et al. first reported that mice lacking Catechol-O-methyltransferase (COMT) in the placentas and in the mother developed 3^rd trimester hypertension, proteinuria and kidney damage⁶⁶. They further reported that the phenotype was mediated by a deficiency of 2-methoxyestradiol that is normally produced by COMT. Clinical studies suggest that genetic polymorphisms in the COMT pathway may be associated with the development of preeclampsia⁶⁷. However, COMT knock-out in rats was associated with no preeclampsia-like phenotype⁶⁸. Whether this discrepancy is related to differences in placentation between mice and rats has not been resolved.

d. ASB4 Knock-out mice:

Rownley-Tilson et al. reported that mice genetically deficient in ubiquitin ligase Ankyrin repeat, SOCS box-containing 4 (ASB4) develop preeclampsia-like states including 3^rd trimester hypertension and proteinuria⁶⁹. They further demonstrated that ASB4 promotes trophoblast differentiation through the degradation of inhibitor of DNA binding (ID2). Aberrant expression of ID2 has been observed in preeclamptic placentas, suggesting a role for this pathway in the human condition⁷⁰. Li et al. reported that nicotinamide rescues the preeclampsia phenotypes and improved fetal survival in mice lacking ASB4⁷¹.

e. STOX1 transgenic mice:

Storkhead box 1 (STOX1) is a transcription factor belonging to the enlarged Forkhead Box gene family that has been associated with preeclampsia, and is able to modulate trophoblast proliferation and migration. STOX1 is maternally expressed in a specific cell type of the placenta, the column extravillous trophoblasts. Overexpression of STOX1 in human choriocarcinoma cells induces transcriptional alterations that mimic those of preeclamptic placentas⁷². Wildtype females crossed with males expressing STOX1 under a constitutive promoter were reported to develop severe gestational hypertension, proteinuria, and increased plasma level of soluble antiangiogenic factors, as well as kidney and placenta histological alterations⁷³. Interestingly, aspirin use in this model was shown to reverse the hypertension phenotype, resembling the human situation⁷⁴. However, the role of STOX1 in human preeclampsia is still controversial. While mutations in STOX1 gene were first reported in Dutch families⁷⁵, follow-up studies have not reported increased prevalence of STOX1 mutations in preeclampsia in other populations^{76, 77}.

f. Corin knockout mice:

Corin, also known as atrial natriuretic peptide (ANP) converting enzyme, has been linked with trophoblast invasion and spiral artery remodeling⁷⁸. Consistent with this hypothesis, pregnant corin mice developed preeclampsia-like features including hypertension, proteinuria and impaired spiral artery remodeling. However, mice lacking ANP, the Corin target, do not express any of the phenotypes of preeclampsia including hypertension or fetal growth restriction⁷⁹. Furthermore, human studies have suggested that Corin levels may be elevated during preeclampsia⁸⁰ suggesting that deficiency of Corin is unlikely to play significant role in preeclampsia.

g. RGS2 Knockout mice:

Regulator of G protein signaling (RGS) proteins have been demonstrated to contribute to numerous cardiovascular diseases. Genetically modified mice with fetal and placental deficiency of RGS2 develop preeclampsia-like features including hypertension and proteinuria during late gestation⁸¹. Interestingly, the RGS2 knockout model of preeclampsia does not have evidence of placental hypoxia or features of anti-angiogenic factor upregulation. Reduced expression of RGS2 has been reported in some cases of human preeclampsia⁸¹. RGS2 deficient mice may thus be useful to study sub-classes of preeclampsia that are not mediated by placental hypoxia or anti-angiogenic factors.

Other models of preeclampsia:

a. Pharmacological compounds:

Timed administration of compounds during pregnancy has been reported by some groups to model features of preeclampsia. Infusion of arginine vasopressin (AVP) during pregnancy was reported to exhibit robust elevations in systolic blood pressure, but without changes in diastolic blood pressure⁸². Changes in blood pressure were associated with increase in proteinuria and glomerular endotheliosis, both hallmarks of preeclampsia. Mechanistically, AVP was shown to induce pro-inflammatory Th1 interferon gamma and Th17 associated IL17 in the placenta⁸³. Human studies have suggested that co-peptin, a surrogate biomarker of vasopressin, is elevated in humans with preeclampsia, but the differences in alterations were more apparent closer to the diagnosis of clinical preeclampsia⁸⁴. Administration of L-NAME, an inhibitor of endothelial nitric oxide synthase (eNOS), in pregnant rats also led to the development of hypertension, fetal growth restriction and variable degrees of proteinuria⁸⁵.

b. Dahl-salt sensitive rats:

Pregnant Dahl salt-sensitive rats have been used as a model of superimposed preeclampsia⁸⁶. Mean arterial blood pressures were elevated in the Dahl S rats before pregnancy⁸⁷. However, during pregnancy these rats have elevated blood pressures that are not seen in other hypertensive rats (for example, SHR). Dahl-S rats exhibited proteinuria, glomerulomegaly and elevated sFLT1, all features of superimposed preeclampsia in humans. Sildenafil, a phosphodiesterase 5 inhibitor, ameliorated the maternal syndrome of preeclampsia and fetal growth restriction suggesting impaired nitric oxide signaling may be central to signs noted in this model⁸⁶. Consistent with this hypothesis, L-citrulline therapy in drinking water was also shown to reduce gestational hypertension and improved fetal growth in this model⁸⁸. More work is needed to better characterize the early placentation events in this model.

Preeclampsia models to study CVD

Since preeclampsia is associated with long-term hypertension and CVD, some investigators have used the preeclampsia animal models to study whether preeclampsia is directly involved in the CVD risk, in contrast to the alternative hypothesis that CVD and preeclampsia share underlying risk factors. Pruthi et al, first reported that experimental preeclampsia using sFLT1 overexpression causes an enhanced vascular response to future vessel injury⁸⁹. Mice with prior exposure to preeclampsia or normal pregnancies were challenged with unilateral carotid injury. Preeclampsia-exposed mice had significantly enhanced vascular remodeling with increased vascular smooth muscle cell proliferation and vessel fibrosis compared with control pregnancy⁸⁹. This new paradigm may contribute to the substantially increased risk of CVD in woman exposed to preeclampsia. Bytauteiene et al reported that animals exposed to sFLT1 during pregnancy had no changes in blood pressure or vascular function six to eight months post-partum⁹⁰, but cardiovascular stressors such as angiotensin II or salt were not tested. In a follow-up study, the same group reported that sFLT1 exposure during pregnancy altered over 150 plasma proteins that could contribute to long term cardiovascular disease⁹¹. STOX1 transgenic mice develop, 8 months post-pregnancy, cardiac fibrin deposits and mildly abnormal cardiac function in response to dobutamine stress⁹² In summary, there is some evidence from mouse models that preeclampsia is directly involved in accentuating long-term CVD, but more work is needed in solidify this conclusion. Investigators have also used preeclampsia mouse models to study cardiovascular disease in offspring. For example, offspring of BPG/5 mice have white adipose tissue accumulation and evidence of hypertension and cardiomegaly⁹³. Here too, more work is needed to better understand how in utero exposure to preeclampsia-like states may lead cardiovascular disease in the offspring.

Summary of Preeclampsia models

In sum, while the use of animal models to study preeclampsia has increased dramatically in recent years, no one model faithfully recapitulates all the signs and symptoms of the human syndrome. Placental ischemia models in rodents have been most informative as researchers have elucidated a pathogenic role for oxidative stress, anti-angiogenic factors, and angiotensin autoantibodies in the pathogenesis of the maternal syndrome⁹⁴. However, a significant limitation of ischemia models is that features of severe disease such as HELLP syndrome, eclampsia, and abruption are not noted. Transgenic or adenoviral overexpression of sFLT1 alone or in combination with sENG develop more robust preeclampsia phenotypes, including severe hypertension, nephrotic range proteinuria, cerebral edema and HELLP like phenotypes^{50, 51, 53, 95}. Taken together with the data that, in humans, high levels of sFLT1 and sENG correlate with adverse maternal and fetal outcomes²⁰, there has been great interest in testing targeted therapies for preeclampsia in anti-angiogenic overexpression model. A significant limitation of the rodent model, however, is lack of expression of all the isoforms of sFLT1 expressed in humans⁹⁶, making it difficult to study molecular therapies such as antibodies that preferentially bind one isoform over others. It would therefore be critical to develop transgenic non-human primate models that express human sFLT1 isoforms and in whom the duration of pregnancy approximates the human situation. Furthermore, since there are no mechanical compromises to the uterine circulation in these transgenic models, the models can be used to evaluate whether therapies with maternal benefit would also benefit fetuses. Carter et al have suggested that smaller primates such as common marmoset (a New World monkey) should be used to model preeclampsia⁹⁷, however it is not known whether uterine invasion by trophoblasts occur in marmosets to the same degree as in humans.

Peripartum cardiomyopathy

Peripartum cardiomyopathy (PPCM) is characterized by systolic heart failure presenting in the last month of pregnancy or the 1st few months postpartum^98–103. It affects anywhere from 1:100 to 1:3000 births, with certain geographic hot spots like Haiti and Nigeria.^98–103 PPCM accounts for >50% of cases of maternal cardiogenic shock in the US¹⁰⁴ and is a leading cause of maternal peripartum death, with mortality rates in the US are as high as 25%.^{101, 102, 105–107} The disease affects all socioeconomic and racial sectors, but is more common and bears significantly worse prognosis in women of African descent^108–110. The precise cause of PPCM remains elusive. However, work with preclinical models over the last 20 years, as outlined below (see also Table 2), has elucidated the current model that PPCM is in large part a vascular disease, triggered by aberrant hormonal changes of late pregnancy and early post-partum.^111–115 In parallel, genetic studies in human cohorts has uncovered that rare genetic variants in various genes, including TTN, FLNC, DSP, and others, can predispose to PPCM.^{116, 117} The implications of these observations for building of pre-clinical models are also discussed below.

Table 2:

Mouse models of peripartum cardiomyopathy.

Model	Proposed mechanisms	Rescue experiments	Evidence for human relevance	Limitations
GαQ transgenic118–121	Promotion of cardiomyocyte apoptosis			Supraphysiologic overexpression. Baseline dysfunction.
STAT3 cardiac KO112, 124–126	Cardiac secretion of cathepsin D cleaves prolactin to vasculotoxic 16KD fragment	Suppression of prolactin with bromocriptine	Some clinical support for treatment with bromocriptine. STAT3 suppressed in cardiac PPCM tissue.	No genetic evidence for role of STAT3 in human PPCM
PGC-1α cardiac KO113, 133	Blunted cardiac pro-angiogenic signals create vulnerability to vasculotoxic hormones, including sFLT1	VEGF injections, treatment with bromocriptine	sFLT1 elevations in PPCM. Epidemiological association with preeclampsia, and cardiac dysfunction.	No genetic evidence for role of PGC-1α in human PPCM
TTNtv knock-in139–141	Unknown		Incontrovertible evidence that variants in TTN contribute to human PPCM	Mild, if any, pregnancy-induced cardiac dysfunction
γ-secretase inhibition150	Suppression of Notch1 activation, causing blunted angiogenesis.			Unclear mechanism, pleiotropic effects of γ-secretase inhibition
Npr1 whole-body KO151	Indirect mechanism leads to lactation-dependent activation of aldosterone in CNS	CNS deletion of aldosterone receptor		Largely driven by elevated BP. Npr1 signaling likely elevated in PPCM, rather than blunted. No genetic evidence in humans.
HSPB6/Hsp20 transgenic152–154	Cardiomyocyte apoptosis and suppressed autophagy			Unknown role of Hsp20 in human PPCM. High expression of mutant protein.
PERK transgenic155	Microvascular rarefaction			Unknown if PERK or the UPR is activated in PPCM hearts
Encephalo-myocarditis virus (EMCV) infection171	Postpartum myocarditis			Unclear role of viral infection in PPCM
Erbb4 cardiac heterozygous KO173	ERBB4 protects maternal heart from peripartum stress		Multiple microRNAs that target Erbb4 are elevated in serum of PPCM patients	Significant baseline cardiac dysfunction. Multiple microRNAs may have unpredictable targets. No genetic evidence in humans.
caAkt cardiac trangenic +/− STAT3 cardiac KO174	Enhanced oxidative stress and hypertrophy	Treatment with bromocriptine		Exhuberant hypertrophy not reflective of normal pregnancy.
Zfp36l2 cardiac KO157	Disinhibition of mTORC1	Treatment with rapamycin	Some evidence low Zfp36l2 in PPCM hearts	Baseline cardiac dysfunction. No genetic evidence in humans.

Genetic models of PPCM

Cardiac expression of Gαq

The first mouse model of PPCM was described in 1998. Numerous hypertrophic signals in the heart, including angiotensin and adrenergic signals, are coupled to heterotrimeric guanine nucleotide-binding proteins of the Gq family. Transgenic models expressing various levels of Gαq in the heart were generated and shown to promote hypertrophy and subsequent systolic heart failure, a process that was markedly accelerated by repeated pregnancies.^{118, 119} Subsequent work demonstrated that the systolic failure was caused by cardiomyocyte loss.^{120, 121} This model had numerous limitations, however. First, the transgenic expression of Gαq was supraphysiologic, raising the possibility that the observed mechanism of cardiomyopathy is not relevant to human disease (transgenic lines with less induction of Gαq expression revealed no phenotype). Second, the transgenic mice revealed significant systolic dysfunction at baseline, even in the absence of pregnancy, while such dysfunction is not seen in women with PPCM¹²², and indeed the absence of such preexisting dysfunction is included in the definition of PPCM. This model was thus important as a first demonstration that pregnancy could accelerate cardiomyopathy in mice, but the model has not been used further. How much cardiomyocyte death contributes to PPCM in more faithful models and in the human disease remains uncertain.

Cardiac deletion of STAT3

The most extensively studied model of PPCM is the Myh6-Cre;Stat3^lox/lox mouse, in which the transcription factor Signal transducer of transcription 3 (STAT3) is selectively deleted in cardiomyocytes¹¹². This model, while revealing no apparent dysfunction at baseline, develops systolic heart failure after one or two pregnancies. STAT3 is a ubiquitously expressed transcription factor that, in cardiomyocytes, regulates expression of antioxidant enzymes and pro-angiogenic factors. STAT3 is normally activated by phosphorylation in cardiomyocytes during pregnancy, and loss of cardiac STAT3 in the Myh6-Cre;Stat3^lox/lox model causes unrestrained production of reactive oxygen species (ROS) and blunted angiogenesis¹¹². The latter is in part caused by the ROS-induced secretion of the peptidase Cathepsin D, which cleaves the circulating nursing hormone prolactin to a 16kDa fragment well-established to be vasculotoxic (often called vasoinhibin) (see Figure 4)¹¹². The 16kDa prolactin fragment interacts with circulating PAI-1 to bind to the urokinase-type plasminogen activator receptor (uPAR) on endothelial cells¹²³, triggering a cascade of cardiotoxic events, including endothelial apoptosis and vascular insufficiency, and endothelial secretion of exosomes containing microRNA-146a that, when taken up by adjacent cardiomyocytes, suppress pro-survival signaling by Erbb4 (Figure 4). Thus, the Myh6-Cre;Stat3^lox/lox model was first to illustrate a vascular/hormonal paradigm to explain PPCM, whereby aberrant signaling by a pregnancy-induced hormone (prolactin) leads to vasculotoxicity and in turn cardiomyopathy. Most tellingly, suppression of prolactin secretion with the dopamine agonist bromocriptine prevented pregnancy-induced heart failure, demonstrating the critical role of prolactin in the pathology of the Myh6-Cre;Stat3^lox/lox model. The model has been used to study the role of the pregnancy hormone relaxin-2 on PPCM progression¹²⁴, the effect of the metabolic modulator perhexiline¹²⁵, and the role of catecholamines, especially β1-adrenergic agonism¹²⁶. In the latter case, β1-adrenergic stimulation substantially worsened cardiac function, mirroring observations in a small group of women with PPCM in whom treatment with dobutamine correlated with worse outcomes¹²⁶.

Fig 4:

Vasculo-hormonal hypothesis of the pathophysiology of peripartum cardiomyopathy. Pregnancy triggers the systemic secretion of hormones from the pituitary and placenta. Some of these, including prolactin (PRL) and soluble FLT1 (sFLT1) can be toxic to cardiac microvasculature, either via proteolytic conversion to vasculotoxic 16Kd PRL or via inhibition of VEGF signaling, respectively, ultimately leading to cardiac ischemia and apoptosis. See text for further details. (Illustration credit: Ben Smith)

Numerous lines of evidence support the clinical relevance of the Myh6-Cre;Stat3^lox/lox model and the pathophysiological mechanism outlined above. Levels of STAT3 protein are decreased in left ventricular tissue from patients with PPCM, compared to non-failing donors¹¹². Circulating cathepsin D activity and levels of microRNA-146a are elevated in women with PPCM, compared to pregnancy-matched healthy women.^{112, 127} And there is substantial, though still somewhat controversial, support for the use of bromocriptine in women with PPCM. Three randomized studies have suggested a benefit of bromocriptine use^128–130, but the studies had limitations including small size, patients lost to follow-up, and the lack of a placebo arm, respectively. There thus remains some equipoise on the use of bromocriptine for PPCM, and a large (n=200) randomized placebo-controlled study has now been initiated in the US. In sum, there is substantial support for the human relevance of the Myh6-Cre;Stat3^lox/lox model of PPCM.

Cardiac deletion of PGC-1α

The next model of PPCM to garner attention was the Myh6-Cre;Ppargc1a^lox/lox mouse, in which the transcriptional coactivator PGC-1α is deleted selectively in cardiomyocytes¹¹³. Like the STAT3 model, Myh6-Cre;Ppargc1a^lox/lox have minimal cardiac dysfunction at baseline, but develop dilated cardiomyopathy after one or two pregnancies. PGC-1α is a pleitropic coactivator that coordinates multiple transcriptional programs, including the promotion of oxidative metabolism and angiogenesis, the latter in part via secretion of vascular endothelial growth factor (VEGF)^{131, 132}. Loss of cardiac PGC-1α in the Myh6-Cre;Ppargc1a^lox/lox model leads to pregnancy-induced defects in cardiac microvasculature, cardiac ischemia, and cardiomyopathy¹¹³. These effects were rescued by ectopic administration of VEGF and bromocriptine, but not either alone, supporting the likely role of prolactin in pregnancy-induced cardiomyopathy, but also indicating the presence of additional anti-angiogenic factors. A search for the latter identified sFLT1, the placental-derived suppressor of VEGF activity described above (see preeclampsia section, and also Figure 4). sFLT1 was sufficient to trigger cardiomyopathy in Myh6-Cre;Ppargc1a^lox/lox mice in the absence of pregnancy¹¹³. Activin-A, another placental derived hormone, was also recently implicated in cardiomyopathy in this model: inhibition of Activin-A receptors partially reversed pregnancy-induced cardiomyopathy in Myh6-Cre;Ppargc1a^lox/lox mice¹³³. Thus, as with the Myh6-Cre;Stat3^lox/lox model, the Myh6-Cre;Ppargc1a^lox/lox model illustrates the vascular/hormonal paradigm of PPCM, in which pregnancy-induced hormones (sFlt1, Activin-A) lead to vasculotoxicity and cardiomyopathy.

Numerous lines of evidence also support the clinical relevance of the Myh6-Cre;Ppargc1a^lox/lox model and the role of sFLT1, Activin-A, and likely other placental hormones, in PPCM. PGC-1α expression has been reported to be reduced in human cardiomyopathy, although not specifically in PPCM. Circulating levels of sFlt1 and Activin-A are elevated in PPCM patients, and correlate with heart failure severity and with adverse outcomes^{113, 133}. Preeclampsia, in which sFlt1 and Activin-A levels are markedly elevated (see above), is one of the strongest risk factors for PPCM, present in 25–50% of cases^{101, 102, 134}. In fact, even in the absence of PPCM, women with preeclampsia exhibit subclinical cardiac dysfunction that closely correlates with plasma sFlt1 levels¹³⁵. It is unknown, however, if PGC-1α is altered in cardiac tissue of human PPCM, and in fact has been reported as elevated in other forms of human cardiomyopathy¹³⁶. Nevertheless, loss of cardiac PGC-1α in mice is clearly a reliable model in which the murine heart is artificially made susceptible to pregnancy-induced vasculo-hormonal insults, allowing mechanistic studies of the latter.

TTN knock-in mice

Recent work has demonstrated that 10–15% of women with PPCM harbor heterozygous truncating variants (i.e, nonsense, frameshift, or splice site variants) in various genes known also to predispose to non-ischemic dilated cardiomyopathy (DCM)^{116, 117}. Most prominent among these, present in ~10% of women with PPCM^{116, 117} as well as patients with DCM^{137, 138}, is TTN, encoding for the large sarcomeric protein titin. These observations have led to the generation of knock-in mice, bearing heterozygous Ttn truncating variants analogous to those seen in human patients. These models do not exhibit spontaneous cardiomyopathy, but cardiac stressors including minipump infusions with angiotensin II, treatment with doxorubicin, or acute volume overload, can elicit a mild decline in systolic function^139–141. The impact of pregnancy of these models has not been reported. Why these models are so mild, compared to the clinical disease, is unclear, but it should be noted that TTN truncating variants in humans are poorly penetrant for disease, with 95% of carriers exhibiting no cardiac dysfunction¹⁴², strongly suggesting the requirement for “second or third hits” that may not be present in the mouse models. It is also worth noting that the truncated titin protein encoded by TTN variant-bearing allele is present in human hearts^{143, 144}, but not in rat¹⁴⁵ or mouse (Z.A.) hearts, suggesting that protein quality control in the mouse may be better at clearing the mutated protein and thus averting disease. Thus, even though mice bearing Ttn truncating variants reflect genetic underpinnings of the human disease more faithfully than do the Myh6-Cre;Ppargc1a^lox/lox and Myh6-Cre;Stat3^lox/lox models, the latter recapitulate better the disease phenotype. Mouse models with heterozygous deletions of other genes known to be mutated in cases of PPCM exist (Bag3^{146, 147}, Flnc¹⁴⁸, Dsp¹⁴⁹), but the impact of pregnancy on their cardiac function has not been reported.

Other mouse models of PPCM

a. Pharmacological compounds:

Several groups have recently reported other mouse models that develop cardiac failure during or after pregnancy. Treatment of pregnant wild type mice during the last 3 days of gestation and first 3 weeks postpartum with an inhibitor of γ-secretase, which is required to activate Notch1 and normal angiogenesis, led to systolic cardiac failure, accompanied by blunted angiogenesis and lower circulating levels of cathepsin D and of sFlt1¹⁵⁰. The study did not elucidate if γ-secretase inhibition elicits this cardiac phenotype primarily by acting on endothelial cells (where Notch1 plays a strong role) versus in cardiomyocytes (where cathepsin D is regulated) versus the placenta (the primary source of sFlt1 during pregnancy). Known effects of γ-secretase inhibition beyond suppression of Notch1 may also have contributed to the phenotype. Thus this model recapitulates some mechanistic aspects of the well-established Myh6-Cre;Stat3^lox/lox model, and benefits from not needing genetic modifications, but the clinical relevance of Notch1 inhibition in human PPCM is not clear, and the pleiotropic effects of γ-secretase inhibition may render mechanistic investigations difficult.

b. Loss of natriuretic peptide signaling.

Another recent model of PPCM involved mice lacking Npr1, the receptor for the natriuretic peptides ANP and BNP that are secreted by the heart to modulate kidney function and blood pressure¹⁵¹. Npr1^−/− mice have marked elevations in blood pressure, by ~20mmHg, accompanied by significant cardiac hypertrophy. After two pregnancies, the cardiac hypertrophy is accentuated, and the mice develop systolic heart failure. Mechanistic studies with various genetic crosses implicated lactation and a neuronal circuit involving signaling by aldosterone in the central nervous system¹⁵¹. The model appears to be driven largely by the elevated blood pressure and the ensuing cardiac hypertrophy, which is not typically observed in human PPCM. The model is also accompanied by activation of STAT3 phosphorylation in cardiac tissue, in contrast to loss of STAT3 in Myh6-Cre;Stat3^lox/lox mice or in human PPCM hearts¹¹². Moreover, PPCM and other forms of heart failure are likely accompanied by elevated, rather than blunted, ANP/BNP signaling. Nevertheless, the study suggests that it may be worth testing aldosterone inhibition with readily available FDA-approved inhibitors in this and other models of PPCM, as well as in the human disease.

c. Protein folding defects.

Two recent studies have suggested that aberrations in protein folding and the cellular response to misfolding may contribute to PPCM. HSPB6/Hsp20 is a small heat-shock protein found primarily in muscle tissues, and protective in multiple models of heart failure. Female transgenic mice with cardiac-specific expression of a mutant form of HSPB6/Hsp20, identified in a small subset of patients with DCM, develop mild systolic failure after 2–4 pregnancies, accompanied by cardiomyocyte apoptosis and evidence of impaired autophagic flux^{152, 153}. Male transgenic mice spontaneously develop similar dysfunction starting at 6 months of age, indicating that the developed heart failure is not specific to pregnancy¹⁵⁴. And Myh6-Cre;Perk^lox/lox mice, in which PERK, one of the effectors of the unfolded protein response (UPR), is deleted selectively in cardiomyocytes, develop systolic heart failure after 3–4 pregnancies, accompanied by a marked decrease in VEGF expression, and microvascular rarefaction¹⁵⁵. Treatment with bromocriptine did not rescue pregnancy-induced systolic dysfunction in this model, but co-treatment with VEGF or other pro-angiogenic agent was not tested¹⁵⁵. Interestingly, treatment of hypoxic isolated cardiomyocytes with 23kD prolactin promoted PERK activation, suggesting a role for PERK activation during lactation, but in vivo evidence for this conclusion remains lacking, nor is it known if PERK is activated in human PPCM hearts. Both of these studies thus suggest that aberrations in the UPR may contribute to PPCM, but further rescue studies will be needed to test this notion. The relevance of these models to the human disease is also difficult to gauge because there is little information available on the status of the UPR or of protein folding in human PPCM.

d. mTORC hyperactivity.

During mouse pregnancy, cardiac Akt and mTORC1 activity rise, concomitant with marked physiological hypertrophy and increasing cardiac mass, and this process reverses post-partum^{10, 156}. Kouzu et al. recently demonstrated that preventing the post-partum suppression of cardiac mTORC1 activity may predispose to PPCM¹⁵⁷. Myh6-Cre;Zfp36l2^lox/lox mice lacking cardiac expression of Zfp36l2, a mRNA-destabilizing protein that suppresses mTORC1 via a Mdm2/p53/Redd1 pathway, develop profound mTORC1 hyperactivity and systolic dysfunction post-partum. The mice do spontaneously develop cardiac dysfunction in the absence of pregnancy, although pregnancy markedly accelerates the process. Importantly, systolic function is partially restored by treatment with rapamycin, a mTORC1 inhibitor. Interestingly, persistent mTORC1 activity can promote UPR response^{158, 159}, thus potentially linking to the mouse models described above. It will be of interest to evaluate mTORC1 signaling in other models of PPCM, as well as their response to rapamycin, to test the generalizability of this model. Vascularity or response to bromocriptine treatment was not tested in this model.

Summary of models PPCM

Developing faithful models of PPCM is imperative to better study this still poorly understood disease. The variety of models associated with PPCM supports the notion that PPCM is a syndrome with multiple origins. Most clinical and pre-clinical evidence indicates that the disease is vascular and hormonal in nature, and two mouse models, the Myh6-Cre;Stat3^lox/lox and Myh6-Cre;Ppargc1a^lox/lox models, best capture these aspects of the disease, thereby also illustrating convergence of various pathologies on common pathways. However, both models are limited by the physiological differences between mouse and human pregnancies. Various other aspects of human PPCM are also poorly reflected in these models, including the observation that most women with PPCM recover cardiac function, while the mouse models do not. Better characterization of PPCM pheno- and genotypes in humans and the subsequent development of appropriate preclinical models of PPCM would significantly advance the field.

Models of other cardiovascular complications of pregnancy

Pregnancy-associated vascular aneurysm/dissection:

Pregnancy is associated with increased risk of accelerated aneurysm progression and aortic dissection, as well as spontaneous coronary artery dissection (SCAD). These catastrophic events often are seen late in pregnancy and in the immediate post-partum period. Habashi et al recently reported that mice with severe deficiency in fibrillin-1 expression, a model of Marfan’s syndrome, showed accelerated aortic growth during pregnancy, accompanied by high incidence of dissection and markedly decreased survival, which were blocked by oxytocin antagonists or by prevention of lactation¹⁶⁰. Similarly, pregnancy was noted to exacerbate aortic dissection in a mouse model of vascular Ehlers-Danlos syndrome that carried a dominant mutation in a major collagen (COL3A1) in blood vessels. Recently Zekavat et al. used whole exome sequencing to identify rare variants in ten fibrillar collagen genes that predisposed to SCAD, and showed that mice with heterozygous deficiency in Col5a1 or both Col5a1 and Col3a1 were prone to spontaneous aortic dissection, but they did not report if pregnancy accentuated the phenotype¹⁶¹. Interestingly, similar to the Marfan mouse, oxytocin antagonisms prevented pregnancy-associated aortic dissection in the COL3A1 mutant mice¹⁶². Since oxytocin is frequently used to support the delivery process, the present model may indicate more careful use of this hormonal treatment. Thus, as in preeclampsia and PPCM, pregnancy-associated hormones appear to play a dominant role in disease progression.

Pregnancy-associated thromboembolism:

Pregnancy is characterized by a hyper-coagulable state and is associated with increased thrombosis and vascular complications such as recurrent miscarriages, venous thrombosis and fetal growth restriction. Iseramman et al. first reported that mice lacking thrombomodulin developed abortions due to activation of tissue-factor-initiated activation of a blood coagulation cascade at the maternal-fetal interface¹⁶³. Similarly, in order to model thrombosis related to anti-phospholipid (aPL) antibody syndrome, several groups have performed passive transfer of the aPL antibodies purified from patients into pregnant mice. These aPL antibody-injected mice develop miscarriages or severe fetal growth restrictions due to placental thrombosis¹⁶⁴. Other groups have suggested that mice immunized with beta-2-glycoprotein I (β2GPI) also develop aPL-like syndrome¹⁶⁵. β2GPI is a phospholipid-binding protein and it is thought that aPL antibodies are induced by binding of β2GPI to endogenous phospholipids. Many groups have tested therapeutic compounds such as complement inhibitors and statins to ameliorate vascular thrombosis and improve adverse pregnancy outcomes in these aPL models¹⁶⁶. Recently, aPL actions via apolipoprotein E receptor 2 in placental trophoblasts was also shown to induce hypertension and fetal growth restriction in pregnant mice¹⁶⁷. Interestingly, in this mouse model protein phosphatase 2A inhibition was found to afford complete protection from aPL-associated pregnancy complications.

Pregnancy-associated cardiac arrhythmias:

Cardiac arrhythmias due to pre-existing conditions may be exacerbated during pregnancy due to physiologic tachycardia¹⁶⁸. El Khouri et al. confirmed that pregnancy exacerbates supraventricular tachyarrhythmias using programmed electrical stimulation protocols in anesthetized pregnant mice^{169, 170}. Mechanistically, cell-intrinsic differences in sinoatrial node cell activity and increased L-type Ca2+ current and spontaneous Ca2+ release from sarcoplasmic reticulum could be demonstrated between nonpregnant and pregnant mice, and chronic administration of oestrogen to nonpregnant mice was sufficient to increase heart rate, in a manner dependent on the estrogen receptor α^{169, 170}. Thus the model suggests a hormonal, rather than hemodynamic, contribution to pregnancy-associated tachycardia.

Conclusion

Cardiovascular complications of pregnancy are frequent and long-lasting, but their pathophysiology remains poorly understood. Clinical studies are challenging, in part due to especially high regulatory hurdles in this patient population, and to frequent skepticism and even disinformation. Pre-clinical studies in animal models are thus critical to advance mechanistic insight. The placenta plays a dominant role in pathophysiology, and all animal models are thus in placental mammals. To date most of these models are surgical, pharmacological, or genetic in nature. Few have incorporated environmental factors such as air pollution, or high-altitude modeling, which are also thought to contribute to cardiovascular complications of pregnancy. A potential role for infection, e.g. myocarditis, has also been only minimally modeled^{171, 172}. Nevertheless, animal models to date have provided substantial insight into disease pathogenesis, and have led to numerous potential treatments currently under evaluation, such as bromocriptine for PPCM (NCT05180773, NCT02590601, NCT00998556), or extracorporeal removal of sFlt1 for preeclampsia (NCT02923206, NCT03231657).

Nonstandard Abbreviations and Acronyms

HELLP

Hemolysis, Elevated Liver Enzymes, Low Platelet syndrome

HFpEF

Heart Failure with preserved ejection fraction

sFLT1

Soluble fms-like tyrosine kinase 1

sENG

Soluble Endoglin

CVD

Cardiovascular Disease

VEGF

Vascular Endothelial Growth Factor

RUPP

Reduced Uterine Perfusion Pressure Model

GFR

Glomerular Filtration Rate

COMT

Catechol-O-Methyl Transferase

AVP

Arginine Vasopressin

STOX1

Storkhead box 1

ANP

Atrial natriuretic peptide

PPCM

Peripartum Cardiomyopathy

STAT3

Signal transducer of transcription 3

BP

Blood pressure

PGC-1α

PPARγ-coactivator-1α

MYH

Myosin heavy chain

DCM

Dilated cardiomyopathy