Endocardial cells form the coronary arteries by angiogenesis through myocardial-endocardial VEGF signaling.

The origins and developmental mechanisms of coronary arteries are incompletely understood. We show here by fate mapping, clonal analysis, and immunohistochemistry that endocardial cells generate the endothelium of coronary arteries. Dye tracking, live imaging, and tissue transplantation also revealed that ventricular endocardial cells are not terminally differentiated; instead, they are angiogenic and form coronary endothelial networks. Myocardial Vegf-a or endocardial Vegfr-2 deletion inhibited coronary angiogenesis and arterial formation by ventricular endocardial cells. In contrast, lineage and knockout studies showed that endocardial cells make a small contribution to the coronary veins, the formation of which is independent of myocardial-to-endocardial Vegf signaling. Thus, contrary to the current view of a common source for the coronary vessels, our findings indicate that the coronary arteries and veins have distinct origins and are formed by different mechanisms. This information may help develop better cell therapies for coronary artery disease.

Methylprednisolone for Heart Surgery in Infants - A Randomized, Controlled Trial.

BACKGROUND: Although perioperative prophylactic glucocorticoids have been used for decades, whether they improve outcomes in infants after heart surgery with cardiopulmonary bypass is unknown. METHODS: We conducted a multicenter, prospective, randomized, placebo-controlled, registry-based trial involving infants (<1 year of age) undergoing heart surgery with cardiopulmonary bypass at 24 sites participating in the Society of Thoracic Surgeons Congenital Heart Surgery Database. Registry data were used in the evaluation of outcomes. The infants were randomly assigned to receive prophylactic methylprednisolone (30 mg per kilogram of body weight) or placebo, which was administered into the cardiopulmonary-bypass pump-priming fluid. The primary end point was a ranked composite of death, heart transplantation, or any of 13 major complications. Patients without any of these events were assigned a ranked outcome based on postoperative length of stay. In the primary analysis, the ranked outcomes were compared between the trial groups with the use of odds ratios adjusted for prespecified risk factors. Secondary analyses included an unadjusted odds ratio, a win ratio, and safety outcomes. RESULTS: A total of 1263 infants underwent randomization, of whom 1200 received either methylprednisolone (599 infants) or placebo (601 infants). The likelihood of a worse outcome did not differ significantly between the methylprednisolone group and the placebo group (adjusted odds ratio, 0.86; 95% confidence interval [CI], 0.71 to 1.05; P = 0.14). Secondary analyses (unadjusted for risk factors) showed an odds ratio for a worse outcome of 0.82 (95% CI, 0.67 to 1.00) and a win ratio of 1.15 (95% CI, 1.00 to 1.32) in the methylprednisolone group as compared with the placebo group, findings suggestive of a benefit with methylprednisolone; however, patients in the methylprednisolone group were more likely than those in the placebo group to receive postoperative insulin for hyperglycemia (19.0% vs. 6.7%, P<0.001). CONCLUSIONS: Among infants undergoing surgery with cardiopulmonary bypass, prophylactic use of methylprednisolone did not significantly reduce the likelihood of a worse outcome in an adjusted analysis and was associated with postoperative development of hyperglycemia warranting insulin in a higher percentage of infants than placebo. (Funded by the National Center for Advancing Translational Sciences and others; STRESS ClinicalTrials.gov number, NCT03229538.).

Myocardial differentiation is dependent upon endocardial signaling during early cardiogenesis in vitro.

The endocardium interacts with the myocardium to promote proliferation and morphogenesis during the later stages of heart development. However, the role of the endocardium in early cardiac ontogeny remains under-explored. Given the shared origin, subsequent juxtaposition, and essential cell-cell interactions of endocardial and myocardial cells throughout heart development, we hypothesized that paracrine signaling from the endocardium to the myocardium is crucial for initiating early differentiation of myocardial cells. To test this, we generated an in vitro, endocardial-specific ablation model using the diphtheria toxin receptor under the regulatory elements of the Nfatc1 genomic locus (NFATc1-DTR). Early treatment of NFATc1-DTR mouse embryoid bodies with diphtheria toxin efficiently ablated endocardial cells, which significantly attenuated the percentage of beating EBs in culture and expression of early and late myocardial differentiation markers. The addition of Bmp2 during endocardial ablation partially rescued myocyte differentiation, maturation and function. Therefore, we conclude that early stages of myocardial differentiation rely on endocardial paracrine signaling mediated in part by Bmp2. Our findings provide novel insight into early endocardial-myocardial interactions that can be explored to promote early myocardial development and growth.

Loss of flow responsive Tie1 results in ImpairedAortic valve remodeling.

The mechanisms regulating endothelial cell response to hemodynamic forces required for heart valve development, especially valve remodeling, remain elusive. Tie1, an endothelial specific receptor tyrosine kinase, is up-regulated by oscillating shear stress and is required for lymphatic valve development. In this study, we demonstrate that valvular endothelial Tie1 is differentially expressed in a dynamic pattern predicted by disturbed flow during valve remodeling. Following valvular endocardial specific deletion of Tie1 in mice, we observed enlarged aortic valve leaflets, decreased valve stiffness and valvular insufficiency. Valve abnormalities were only detected in late gestation and early postnatal mutant animals and worsened with age. The mutant mice developed perturbed extracellular matrix (ECM) deposition and remodeling characterized by increased glycosaminoglycan and decreased collagen content, as well as increased valve interstitial cell expression of Sox9, a transcription factor essential for normal ECM maturation during heart valve development. This study provides the first evidence that Tie1 is involved in modulation of late valve remodeling and suggests that an important Tie1-Sox9 signaling axis exists through which disturbed flows are converted by endocardial cells to paracrine Sox9 signals to modulate normal matrix remodeling of the aortic valve.

Overcoming underpowering: Trial simulations and a global rank end point to optimize clinical trials in children with heart disease.

BACKGROUND: Randomized controlled trials (RCTs) in children with heart disease are challenging and therefore infrequently performed. We sought to improve feasibility of perioperative RCTs for this patient cohort using data from a large, multicenter clinical registry. We evaluated potential enrollment and end point frequencies for various inclusion cohorts and developed a novel global rank trial end point. We then performed trial simulations to evaluate power gains with the global rank end point and with use of planned covariate adjustment as an analytic strategy. METHODS: Data from the Society of Thoracic Surgery-Congenital Heart Surgery Database (STS-CHSD, 2011-2016) were used to support development of a consensus-based global rank end point and for trial simulations. For Monte Carlo trial simulations (n = 50,000/outcome), we varied the odds of outcomes for treatment versus placebo and evaluated power based on the proportion of trial data sets with a significant outcome (P < .05). RESULTS: The STS-CHSD study cohort included 35,967 infant index cardiopulmonary bypass operations from 103 STS-CHSD centers, including 11,411 (32%) neonatal cases and 12,243 (34%) high-complexity (Society of Thoracic Surgeons-European Association for Cardio-Thoracic Surgery mortality category ≥4) cases. In trial simulations, study power was 21% for a mortality-only end point, 47% for a morbidity and mortality composite, and 78% for the global rank end point. With covariate adjustment, power increased to 94%. Planned covariate adjustment was preferable to restricting to higher-risk cohorts despite higher event rates in these cohorts. CONCLUSIONS: Trial simulations can inform trial design. Our findings, including the newly developed global rank end point, may be informative for future perioperative trials in children with heart disease.

Rationale and design of the STeroids to REduce Systemic inflammation after infant heart Surgery (STRESS) trial.

For decades, physicians have administered corticosteroids in the perioperative period to infants undergoing heart surgery with cardiopulmonary bypass (CPB) to reduce the postoperative systemic inflammatory response to CPB. Some question this practice because steroid efficacy has not been conclusively demonstrated and because some studies indicate that steroids could have harmful effects. STRESS is a randomized, placebo-controlled, double-blind, multicenter trial designed to evaluate safety and efficacy of perioperative steroids in infants (age < 1 year) undergoing heart surgery with CPB. Participants (planned enrollment = 1,200) are randomized 1:1 to methylprednisolone (30 mg/kg) administered into the CPB pump prime versus placebo. The trial is nested within the existing infrastructure of the Society of Thoracic Surgeons Congenital Heart Surgery Database. The primary outcome is a global rank score of mortality, major morbidities, and hospital length of stay with components ranked commensurate with their clinical severity. Secondary outcomes include several measures of major postoperative morbidity, postoperative hospital length of stay, and steroid-related safety outcomes including prevalence of hyperglycemia and postoperative infectious complications. STRESS will be one of the largest trials ever conducted in children with heart disease and will answer a decades-old question related to safety and efficacy of perioperative steroids in infants undergoing heart surgery with CPB. The pragmatic "trial within a registry" design may provide a mechanism for conducting low-cost, high-efficiency trials in a heretofore-understudied patient population.

Advances in MALDI imaging mass spectrometry of proteins in cardiac tissue, including the heart valve.

Significant progress has been made for tissue imaging of proteins using matrix-assisted laser desorption ionization imaging mass spectrometry (MALDI IMS). These advancements now facilitate mapping of a wide range of proteins, peptides, and post-translational modifications in a wide variety of tissues; however, the use of MALDI IMS to detect proteins from cardiac tissue is limited. This review discusses the most recent advances in protein imaging and demonstrates application to cardiac tissue, including the heart valve. Protein imaging by MALDI IMS allows multiplexed histological mapping of proteins and protein components that are inaccessible by antibodies and should be considered an important tool for basic and clinical cardiovascular research. This article is part of a Special Issue entitled: MALDI Imaging, edited by Dr. Corinna Henkel and Prof. Peter Hoffmann.

Postnatal lethality and abnormal development of foregut and spleen in Ndrg4 mutant mice.

NDRG4 is a member of the NDRG family (N-myc downstream-regulated gene), which is highly expressed in brain and heart. Previous studies showed that Ndrg1-deficient mice exhibited a progressive demyelinating disorder of peripheral nerves and Ndrg4-deficient mice had spatial learning deficits and vulnerabilities to cerebral ischemia. Here, we report generation of Ndrg4 mutant alleles that exhibit several development defects different from those previously reported. Our homozygous mice showed growth retardation and postnatal lethality. Spleen and thymuses of Ndrg4(-/-) mice are considerably reduced in size from 3 weeks of age. Histological analysis revealed abnormal hyperkeratosis in the squamous foregut and abnormal loss of erythrocytes in the spleen of Ndrg4(-/-) mice. In addition, we observed an abnormal hind limb clasping phenotype upon tail suspension suggesting neurological abnormalities. Consistent to these abnormalities, Ndrg4 is expressed in smooth muscle cells of the stomach, macrophages of the spleen and neurons. Availability of the conditional allele for Ndrg4 should facilitate further detailed analyses of the potential roles of Ndrg4 in gut development, nervous system and immune system.

Notch1 Mutation Leads to Valvular Calcification Through Enhanced Myofibroblast Mechanotransduction.

OBJECTIVE: Calcific aortic valve disease (CAVD) is a significant cardiovascular disorder, and controversy exists as to whether it is primarily a dystrophic or osteogenic process in vivo. In this study, we sought to clarify the mechanism of CAVD by assessing a genetic mutation, Notch1 heterozygosity, which leads to CAVD with 100% penetrance in humans. APPROACH AND RESULTS: Murine immortalized Notch1(+/-) aortic valve interstitial cells (AVICs) were isolated and expanded in vitro. Molecular signaling of wild-type and Notch1(+/-) AVICs were compared to identify changes in pathways that have been linked to CAVD-transforming growth factor-ß1/bone morphogenetic protein, mitogen-activated protein kinase, and phosphoinositide 3-kinase/protein kinase B-and assessed for calcification potential. Additionally, AVIC mechanobiology was studied in a physiologically relevant, dynamic mechanical environment (10% cyclic strain) to investigate differences in responses between the cell types. We found that Notch1(+/-) AVICs resembled a myofibroblast-like phenotype expressing higher amounts of cadherin-11, a known mediator of dystrophic calcification, and decreased Runx2, a known osteogenic marker. We determined that cadherin-11 expression is regulated by Akt activity, and inhibition of Akt phosphorylation significantly reduced cadherin-11 expression. Moreover, in the presence of cyclic strain, Notch1(+/-) AVICs exhibited significantly upregulated phosphorylation of Akt at Ser473 and smooth muscle α-actin expression, indicative of a fully activated myofibroblast. Finally, these Notch1-mediated alterations led to enhanced dystrophic calcific nodule formation. CONCLUSIONS: This study presents novel insights in our understanding of Notch1-mediated CAVD by demonstrating that the mutation leads to AVICs that are fully activated myofibroblasts, resulting in dystrophic, but not osteogenic, calcification.

Making a heart: advances in understanding the mechanisms of cardiac development.

PURPOSE OF REVIEW: The study of cardiac development is critical to inform management strategies for congenital and acquired heart disease. This review serves to highlight some of the advances in this field over the past year. RECENT FINDINGS: Three main areas of study are included that have been particularly innovative and progressive. These include more precise gene targeting in animal models of disease and in moving from animal models to human disease, more precise in-vitro models including three-dimensional structuring and inclusion of hemodynamic components, and expanding the concepts of genetic regulation of heart development and disease. SUMMARY: Targeted genetics in animal models are able to make use of tissue and time-specific promotors that drive gene expression or knockout with high specificity. In-vitro models can recreate flow patterns in blood vessels and across cardiac valves. Noncoding RNAs, once thought to be of no consequence to gene transcription and translation, prove to be key regulators of genetic function in health and disease.

Erg is a crucial regulator of endocardial-mesenchymal transformation during cardiac valve morphogenesis.

During murine embryogenesis, the Ets factor Erg is highly expressed in endothelial cells of the developing vasculature and in articular chondrocytes of developing bone. We identified seven isoforms for the mouse Erg gene. Four share a common translational start site encoded by exon 3 (Ex3) and are enriched in chondrocytes. The other three have a separate translational start site encoded by Ex4 and are enriched in endothelial cells. Homozygous Erg(ΔEx3/ΔEx3) knockout mice are viable, fertile and do not display any overt phenotype. By contrast, homozygous Erg(ΔEx4/ΔEx4) knockout mice are embryonic lethal, which is associated with a marked reduction in endocardial-mesenchymal transformation (EnMT) during cardiac valve morphogenesis. We show that Erg is required for the maintenance of the core EnMT regulatory factors that include Snail1 and Snail2 by binding to their promoter and intronic regions.

Evaluating registry-based trial economics: Results from the STRESS clinical trial.

Background: Registry-based trials have the potential to reduce randomized clinical trial (RCT) costs. However, observed cost differences also may be achieved through pragmatic trial designs. A systematic comparison of trial costs across different designs has not been previously performed. Methods: We conducted a study to compare the current Steroids to Reduce Systemic inflammation after infant heart surgery (STRESS) registry-based RCT vs. two established designs: pragmatic RCT and explanatory RCT. The primary outcome was total RCT design costs. Secondary outcomes included: RCT duration and personnel hours. Costs were estimated using the Duke Clinical Research Institute's pricing model. Results: The Registry-Based RCT estimated duration was 31.9 weeks greater than the other designs (259.5 vs. 227.6 weeks). This delay was caused by the Registry-Based design's periodic data harvesting that delayed site closing and statistical reporting. Total personnel hours were greatest for the Explanatory design followed by the Pragmatic design and the Registry-Based design (52,488 vs 29,763 vs. 24,480 h, respectively). Total costs were greatest for the Explanatory design followed by the Pragmatic design and the Registry-Based design ($10,140,263 vs. $4,164,863 vs. $3,268,504, respectively). Thus, Registry-Based total costs were 32 % of the Explanatory and 78 % of the Pragmatic design. Conclusion: Total costs for the STRESS RCT with a registry-based design were less than those for a pragmatic design and much less than an explanatory design. Cost savings reflect design elements and leveraging of registry resources to improve cost efficiency, but delays to trial completion should be considered.

Spatial transcriptional profile of the chick and mouse endocardial cushions identify novel regulators of endocardial EMT in vitro.

Valvular Interstitial Cells (VICs) are a common substrate for congenital and adult heart disease yet the signaling mechanisms governing their formation during early valvulogenesis are incompletely understood. We developed an unbiased strategy to identify genes important in endocardial epithelial-to-mesenchymal transformation (EMT) using a spatial transcriptional profile. Endocardial cells overlaying the cushions of the atrioventricular canal (AVC) and outflow tract (OFT) undergo an EMT to yield VICs. RNA sequencing (RNA-seq) analysis of gene expression between AVC, OFT, and ventricles (VEN) isolated from chick and mouse embryos at comparable stages of development (chick HH18; mouse E11.0) was performed. EMT occurs in the AVC and OFT cushions, but not VEN at this time. 198 genes in the chick (n=1) and 105 genes in the mouse (n=2) were enriched 2-fold in the cushions. Gene regulatory networks (GRN) generated from cushion-enriched gene lists confirmed TGFß as a nodal point and identified NF-κB as a potential node. To reveal previously unrecognized regulators of EMT four candidate genes, Hapln1, Id1, Foxp2, and Meis2, and a candidate pathway, NF-κB, were selected. In vivo spatial expression of each gene was confirmed by in situ hybridization and a functional role for each in endocardial EMT was determined by siRNA knockdown in a collagen gel assay. Our spatial-transcriptional profiling strategy yielded gene lists which reflected the known biology of the system. Further analysis accurately identified and validated previously unrecognized novel candidate genes and the NF-κB pathway as regulators of endocardial cell EMT in vitro.

Cimetidine-associated patent ductus arteriosus is mediated via a cytochrome P450 mechanism independent of H2 receptor antagonism.

Persistent patency of the ductus arteriosus (PDA) is a common problem in preterm infants. The antacid cimetidine is a potent antagonist of the H2 histamine receptor but it also inhibits certain cytochrome P450 enzymes (CYPs), which may affect DA patency. We examined whether cimetidine contributes to PDA and is mediated by CYP inhibition rather than H2 blockade. Analysis of a clinical trial to prevent lung injury in premature infants revealed a significant association between cimetidine treatment and PDA. Cimetidine and ranitidine, both CYP inhibitors as well as H2 blockers, caused relaxation of the term and preterm mouse DA. CYP enzymes that are inhibited by cimetidine were expressed in DA subendothelial smooth muscle. The selective CYP3A inhibitor ketoconazole induced greater DA relaxation than cimetidine, whereas famotidine and other H2 antagonists with less CYP inhibitory effects caused less dilation. Histamine receptors were developmentally regulated and localized in DA smooth muscle. However, cimetidine caused DA relaxation in histamine-deficient mice, consistent with CYP inhibition, not H2 antagonism, as the mechanism for PDA. Oxygen-induced DA constriction was inhibited by both cimetidine and famotidine. These studies show that antacids and other compounds with CYP inhibitory properties pose a significant and previously unrecognized risk for PDA in critically ill newborn infants.

Cdc42 promotes host defenses against fatal infection.

The small Rho GTPase Cdc42 regulates key signaling pathways required for multiple cell functions, including maintenance of shape, polarity, proliferation, invasion, migration, differentiation, and morphogenesis. As the role of Cdc42-dependent signaling in fibroblasts in vivo is unknown, we attempted to specifically delete it in these cells by crossing the Cdc42(fl/fl) mouse with an fibroblast-specific protein 1 (FSP1)-Cre mouse, which is thought to mediate recombination exclusively in fibroblasts. Surprisingly, the FSP1-Cre;Cdc42(fl/fl) mice died at 3 weeks of age due to overwhelming suppurative upper airway infections that were associated with neutrophilia and lymphopenia. Even though major aberrations in lymphoid tissue development were present in the mice, the principal cause of death was severe migration and killing abnormalities of the neutrophil population resulting in an inability to control infection. We also show that in addition to fibroblasts, FSP1-Cre deleted Cdc42 very efficiently in all leukocytes. Thus, by using this nonspecific Cre mouse, we inadvertently demonstrated the importance of Cdc42 in host protection from lethal infections and suggest a critical role for this small GTPase in innate immunity.

Abnormal embryonic lymphatic vessel development in Tie1 hypomorphic mice.

Tie1 is an endothelial receptor tyrosine kinase that is essential for development and maintenance of the vascular system; however, the role of Tie1 in development of the lymphatic vasculature is unknown. To address this question, we first documented that Tie1 is expressed at the earliest stages of lymphangiogenesis in Prox1-positive venous lymphatic endothelial cell (LEC) progenitors. LEC Tie1 expression is maintained throughout embryonic development and persists in postnatal mice. We then generated two lines of Tie1 mutant mice: a hypomorphic allele, which has reduced expression of Tie1, and a conditional allele. Reduction of Tie1 levels resulted in abnormal lymphatic patterning and in dilated and disorganized lymphatic vessels in all tissues examined and in impaired lymphatic drainage in embryonic skin. Homozygous hypomorphic mice also exhibited abnormally dilated jugular lymphatic vessels due to increased production of Prox1-positive LECs during initial lymphangiogenesis, indicating that Tie1 is required for the early stages of normal lymphangiogenesis. During later stages of lymphatic development, we observed an increase in LEC apoptosis in the hypomorphic embryos after mid-gestation that was associated with abnormal regression of the lymphatic vasculature. Therefore, Tie1 is required for early LEC proliferation and subsequent survival of developing LECs. The severity of the phenotypes observed correlated with the expression levels of Tie1, confirming a dosage dependence for Tie1 in LEC integrity and survival. No defects were observed in the arterial or venous vasculature. These results suggest that the developing lymphatic vasculature is particularly sensitive to alterations in Tie1 expression.

Nfatc1 coordinates valve endocardial cell lineage development required for heart valve formation.

RATIONALE: Formation of heart valves requires early endocardial to mesenchymal transformation (EMT) to generate valve mesenchyme and subsequent endocardial cell proliferation to elongate valve leaflets. Nfatc1 (nuclear factor of activated T cells, cytoplasmic 1) is highly expressed in valve endocardial cells and is required for normal valve formation, but its role in the fate of valve endocardial cells during valve development is unknown. OBJECTIVE: Our aim was to investigate the function of Nfatc1 in cell-fate decision making by valve endocardial cells during EMT and early valve elongation. METHODS AND RESULTS: Nfatc1 transcription enhancer was used to generate a novel valve endocardial cell-specific Cre mouse line for fate-mapping analyses of valve endocardial cells. The results demonstrate that a subpopulation of valve endocardial cells marked by the Nfatc1 enhancer do not undergo EMT. Instead, these cells remain within the endocardium as a proliferative population to support valve leaflet extension. In contrast, loss of Nfatc1 function leads to enhanced EMT and decreased proliferation of valve endocardium and mesenchyme. The results of blastocyst complementation assays show that Nfatc1 inhibits EMT in a cell-autonomous manner. We further reveal by gene expression studies that Nfatc1 suppresses transcription of Snail1 and Snail2, the key transcriptional factors for initiation of EMT. CONCLUSIONS: These results show that Nfatc1 regulates the cell-fate decision making of valve endocardial cells during valve development and coordinates EMT and valve elongation by allocating endocardial cells to the 2 morphological events essential for valve development.

Enhanced sensitivity for high spatial resolution lipid analysis by negative ion mode matrix assisted laser desorption ionization imaging mass spectrometry.

We have achieved enhanced lipid imaging to a ~10 µm spatial resolution using negative ion mode matrix assisted laser desorption ionization (MALDI) imaging mass spectrometry, sublimation of 2,5-dihydroxybenzoic acid as the MALDI matrix, and a sample preparation protocol that uses aqueous washes. We report on the effect of treating tissue sections by washing with volatile buffers at different pHs prior to negative ion mode lipid imaging. The results show that washing with ammonium formate, pH 6.4, or ammonium acetate, pH 6.7, significantly increases signal intensity and number of analytes recorded from adult mouse brain tissue sections. Major lipid species measured were glycerophosphoinositols, glycerophosphates, glycerolphosphoglycerols, glycerophosphoethanolamines, glycerophospho-serines, sulfatides, and gangliosides. Ion images from adult mouse brain sections that compare washed and unwashed sections are presented and show up to 5-fold increases in ion intensity for washed tissue. The sample preparation protocol has been found to be applicable across numerous organ types and significantly expands the number of lipid species detectable by imaging mass spectrometry at high spatial resolution.

A novel technique for quantifying mouse heart valve leaflet stiffness with atomic force microscopy.

BACKGROUND AND AIM OF THE STUDY: The use of genetically altered small animal models is a powerful strategy for elucidating the mechanisms of heart valve disease. However, while the ability to manipulate genes in rodent models is well established, there remains a significant obstacle in determining the functional mechanical properties of the genetically mutated leaflets. Hence, a feasibility study was conducted using micromechanical analysis via atomic force microscopy (AFM) to determine the stiffness of mouse heart valve leaflets in the context of age and disease states. METHODS: A novel AFM imaging technique for the quantification of heart valve leaflet stiffness was performed on cryosectioned tissues. Heart valve leaflet samples were obtained from wild-type mice (2 and 17 months old) and genetically altered mice (10-month-old Notch1 heterozygous and 20-month-old ApoE homozygous). Histology was performed on adjacent sections to determine the extracellular matrix characteristics of the scanned areas. RESULTS: The 17-month-old wild-type, 10-month-old Notch1, and 20-month-old ApoE aortic valve leaflets were all significantly stiffer than leaflets from 2-month-old wild-type mice. Notch1 leaflets were significantly stiffer than all other leaflets examined, indicating that the Notch1 heterozygous mutation may alter leaflet stiffness, both earlier and to a greater degree than the homozygous ApoE mutation. However, these conclusions must be considered only preliminary due to the small sample size used in this proof-of-concept study. CONCLUSION: It is believed that this technique can provide a powerful end-point analysis for determining the mechanical properties of heart valve leaflets from genetically altered mice. Further, the technique is complementary to standard histological processing, and does not require excess tissue for mechanical testing. In this proof-of-concept study, AFM was shown to be a powerful tool for investigators of heart valve disease who develop genetically altered animals for their studies.