Genetic ablation of serotonin receptor 2B improves aortic valve hemodynamics of Notch1 heterozygous mice in a high-cholesterol diet model

Effects of LP533401 on vascular and bone calcification in hyperlipidemic mice

Peripheral serotonin levels are associated with cardiovascular disease risk. We previously found that serum serotonin levels are higher in hyperlipidemic mice than wild-type mice. Evidence also suggests that serotonin regulates biomineralization, in that serotonin treatment augments TNF-a-induced matrix calcification of aortic valve interstitial cells and that a selective inhibitor of peripheral serotonin, LP533401, rescues bone loss induced by ovariectomy in mice. Thus, in the present study, we examined the effects of LP533401 on both skeletal bone mineral density (BMD) and aortic calcification in both young and older hyperlipidemic mice susceptible to calcific atherosclerosis and bone loss. By serial in vivo microCT imaging, we assessed BMD and aortic calcification of Apoe-/- mice fed an atherogenic (high cholesterol) diet alone or mixed with LP533401. Results show that in the young mice, LP533401 blunted skeletal bone loss in lumbar vertebrae but not in femurs. LP533401 also blunted the initial development of aortic calcification but not its progression. Echocardiographic analysis showed that LP533401 blunted both hyperlipidemia-induced cardiac hypertrophy and left ventricular dysfunction. In the older mice, LP533401 increased the BMD of lumbar vertebrae but not of femurs. The aortic calcification progressed in both controls and LP533401-treated mice, but, at post-treatment, LP533401-treated mice had significantly less aortic calcification than the controls. These findings suggest that LP533401 mitigates adverse effects of hyperlipidemia on skeletal and vascular tissues in site- and stage-dependent manners.

Structure activity relationships of 5-HT2B and 5-HT2C serotonin receptor antagonists: N6, C2 and 5'-Modified (N)-methanocarba-adenosine derivatives

(N)-Methanocarba adenosine derivatives were structurally modified to target 5-HT2B serotonin receptors as antagonists, predominantly containing branched N6-alkyl groups. N6-Dicycloalkyl-methyl groups, including their asymmetric variations, as well as 2-iodo, were found to generally favor 5-HT2Rs, while only N6-dicyclohexyl-methyl derivative 35 showed weak 5-HT2AR affinity (Ki 3.6 μM). The highest 5-HT2BR affinities were Ki 11-23 nM (N6-dicyclopropyl-methyl-2-iodo 11, 2-chloro-5'-deoxy-5'-methylthio 15 and N6-((R)-cyclobuty-cyclopropyl-methyl)-2-iodo 43), and Ki 73 nM at 5-HT2CR for 36. Direct comparison of adenine ribosides and their corresponding rigid (N)-methanocarba derivatives (cf. 51 and MRS8099 45) indicated a multifold affinity enhancement with the bicyclic ring system. Compounds 43, 45 and 48 were functional 5-HT2BR (KB 2-3 nM) and 5-HT2CR (KB 79-328 nM) antagonists in a Gq-mediated calcium flux assay, with 5-HT2BR functional selectivity ranging from 45- (48) to 113-fold (43). Substantial adenosine receptor (AR) affinity (Ki, A1AR < Ki, A3AR < Ki, A2AAR) was still present in this series, suggestive of dual acting compounds: 5-HT2B antagonist and A1AR agonist, potentially useful for treating chronic conditions (fibrosis; pain). Given its affinity (17 nM) and moderate 5-HT2BR binding selectivity (32-fold vs. 5-HT2CR, 4-fold vs. A1AR), 43 (MRS7925) could potentially be useful for anti-fibrotic therapy.

2B Determined: The Future of the Serotonin Receptor 2B in Drug Discovery

The cardiotoxicity associated with des-ethyl-dexfenfluramine (norDF) and related agonists of the serotonin receptor 2B (5-HT2B) has solidified the receptor's place as an "antitarget" in drug discovery. Conversely, a growing body of evidence has highlighted the utility of 5-HT2B antagonists for the treatment of pulmonary arterial hypertension (PAH), valvular heart disease (VHD), and related cardiopathies. In this Perspective, we summarize the link between the clinical failure of fenfluramine-phentermine (fen-phen) and the subsequent research on the role of 5-HT2B in disease progression, as well as the development of drug-like and receptor subtype-selective 5-HT2B antagonists. Such agents represent a promising class for the treatment of PAH and VHD, but their utility has been historically understudied due to the clinical disasters associated with 5-HT2B. Herein, it is our aim to examine the current state of 5-HT2B drug discovery, with an emphasis on the receptor's role in the central nervous system (CNS) versus the periphery.

Development of Pharmacophore Models for the Important Off-Target 5-HT2B Receptor

Toxicity is a major cause of attrition in the development of pharmaceuticals, and the off-target effects are a frequent contributor. The 5-HT_2B receptor agonism is known to be responsible for a variety of safety concerns including valvular heart disease which was the cause for the withdrawal of several compounds from the market. An early detection of potential binding to this receptor is thus desirable. Herein, we present the identification of key amino acid residues in the active site of 5-HT_2B by molecular dynamics simulations, the development of pharmacophore models and their performance on in-house data, and a structurally highly diverse subset of Enamine REAL labeled for 5-HT_2B activity by a machine learning model. These models may be used as filters employed on screening compound sets for the early filtration of compounds with potential 5-HT_2B off-target liabilities.

Sclerostin ablation prevents aortic valve stenosis in mice

The objective of this study was to test the hypothesis that targeting sclerostin would accelerate the progression of aortic valve stenosis. Sclerostin (mouse gene, Sost) is a secreted glycoprotein that acts as a potent regulator of bone remodeling. Antibody therapy targeting sclerostin is approved for osteoporosis but results from a stage III clinical trial showed multiple off-target cardiovascular effects. Wild-type (WT, Sost+/+) and Sost-gene knockout-expression (Null, Sost-/-) mice were generated and maintained to 12 mo of age on a high-cholesterol diet to induce aortic valve stenosis. Mice were examined by echocardiography, histology, and RNAseq. Immortalized valve interstitial cells were developed from each genotype for in vitro studies. Null mice developed a bone overgrowth phenotype, similar to patients with sclerosteosis. Surprisingly, however, WT mice developed hemodynamic signs of aortic valve stenosis, whereas Null mice were unchanged. WT mice had thicker aortic valve leaflets and higher amounts of α-smooth muscle actin, a marker myofibroblast activation and dystrophic calcification, with very little evidence of Runx2 expression, a marker of osteogenic calcification. RNAseq analysis of aortic roots indicated the HOX family of transcription factors was significantly upregulated in Null mice, and valve interstitial cells from Null animals were enriched with Hoxa1, Hoxb2, and Hoxd3 subtypes with downregulated Hoxa7. In addition, Null valve interstitial cells were shown to be less contractile than their WT counterparts. Contrary to our hypothesis, sclerostin targeting prevented hallmarks of aortic valve stenosis and indicates that targeted antibody treatments for osteoporosis may be beneficial for these patients regarding aortic stenosis.NEW & NOTEWORTHY We have found that genetic ablation of the Sost gene (protein: sclerostin) prevents aortic valve stenosis in aged, Western diet mice. This is a new role for sclerostin in the cardiovascular system. To the knowledge of the authors, this is one of the first studies directly manipulating sclerostin in a cardiovascular disease model and the first to specifically study the aortic valve. We also provide a potential new role for Hox genes in cardiovascular disease, noting pan-Hox upregulation in the aortic roots of sclerostin genetic knockouts. The role of Hox genes in postnatal cardiovascular health and disease is another burgeoning field of study to which this article contributes.

Indoleamine 2,3-Dioxygenase 1 Deletion-Mediated Kynurenine Insufficiency in Vascular Smooth Muscle Cells Exacerbates Arterial Calcification

BACKGROUND

IDO1 (indoleamine 2,3-dioxygenase 1) is the rate-limiting enzyme for tryptophan metabolism. IDO1 malfunction is involved in the pathogenesis of atherosclerosis. Vascular smooth muscle cells (VSMCs) with an osteogenic phenotype promote calcification and features of plaque instability. However, it remains unclear whether aberrant IDO1-regulated tryptophan metabolism causes VSMCs osteogenic reprogramming and calcification.

METHODS

We generated global Apoe (apolipoprotein E) and Ido1 double knockout mice, and Apoe knockout mice with specific deletion of IDO1 in VSMCs or macrophages. Arterial intimal calcification was evaluated by a Western diet-induced atherosclerotic calcification model.

RESULTS

Global deficiency of IDO1 boosted calcific lesion formation without sex bias in vivo. Conditional IDO1 loss of function in VSMCs rather than macrophages promoted calcific lesion development and the abundance of RUNX2 (runt-related transcription factor 2). In contrast, administration of kynurenine via intraperitoneal injection markedly delayed the progression of intimal calcification in parallel with decreased RUNX2 expression in both Apoe-/- and Apoe-/-Ido1-/- mice. We found that IDO1 deletion restrained RUNX2 from proteasomal degradation, which resulted in enhanced osteogenic reprogramming of VSMCs. Kynurenine administration downregulated RUNX2 in an aryl hydrocarbon receptor-dependent manner. Kynurenine acted as the endogenous ligand of aryl hydrocarbon receptor, controlled resultant interactions between cullin 4B and aryl hydrocarbon receptor to form an E3 ubiquitin ligase that bound with RUNX2, and subsequently promoted ubiquitin-mediated instability of RUNX2 in VSMCs. Serum samples from patients with coronary artery calcification had impaired IDO1 activity and decreased kynurenine catabolites compared with those without calcification.

CONCLUSIONS

Kynurenine, an IDO1-mediated tryptophan metabolism main product, promotes RUNX2 ubiquitination and subsequently leads to its proteasomal degradation via an aryl hydrocarbon receptor-dependent nongenomic pathway. Insufficient kynurenine exerts the deleterious role of IDO1 ablation in promoting RUNX2-mediated VSMCs osteogenic reprogramming and calcification in vivo.

Evaluation of early bilateral ovariectomy in mice as a model of left heart disease

Post-menopausal women tend to have worse cardiovascular outcomes in a manner that is associated with osteoporosis severity. In this study we performed the first evaluation of the left ventricle and aortic valve phenotype of ovariectomized mice aged on Western diet to one year. Disease was monitored in vivo using echocardiography and dual x-ray absorptiometry imaging and ex vivo using quantitative histological and immunostaining analysis. Mice had decreased bone mineral density in response to ovariectomy and increased fat mass in response to Western diet. Ovariectomized mice had a significantly increased left ventricle mass compared to control animals, absent of fibrosis. There was a slight increase in aortic valve peak velocity but no change in mean pressure gradient across the valve in the ovariectomy group. There was no evidence of leaflet hypertrophy, fibrosis, calcification, or protein markers of dystrophic or osteogenic calcification. This model of ovariectomy may present a novel method of studying left ventricle hypertrophy in female populations but does not have a phenotype for study of aortic stenosis. This is particularly useful as it does not require genetic manipulation or drug treatment and more faithfully mimics the aging, high-cholesterol diet, and post-menopausal osteoporosis many female patients experience potentially resulting in a more translatable disease model.

Serotonin—A Driver of Progressive Heart Valve Disease

It is well known that some serotoninergic drugs and neuroendocrine tumors producing serotonin (5-HT) may induce valvular heart disease by stimulation of proliferation of valvular cells via interaction with a 5-HT receptor type 2B. Serotonin could play a role in the pathogenesis of progressive valvular disease for example as a complication of rheumatic fever, in patients with congenital bicuspid aortic valves or in degenerative aortic valve stenosis. The initial inflammation in acute rheumatic fever seems to affect both right and the left-side cardiac valves. Some patients develop chronic right-sided valve disease, particularly in connection with septum defects, though left-sided valves typically are predominantly affected, indicating that high flow velocity and systemic pressure close to the valves may be central in the pathogenesis. Serotonin is transported in granules in blood platelets. Changes in platelet number and concentrations of substances released from platelets in patients with valvular disease indicate that serotonin is released locally by shear stress when passing through an abnormal valve. Accordingly, any functional changes (like bicuspid aortic valves and changes secondary to degeneration) in the valves may progress due to locally released serotonin. Unfortunately, due to serotonin release by sampling and preparation of plasma, local serotonin assessment is not possible. Nevertheless, we suggest that serotonin may play a role in valvular disease in general and that patients may benefit from treatment reducing the effect of serotonin on the heart.

Comparison of Serotonin-Regulated Calcific Processes in Aortic and Mitral Valvular Interstitial Cells

Calcification is an important pathological process and a common complication of degenerative valvular heart diseases, with higher incidence in aortic versus mitral valves. Two phenotypes of valvular interstitial cells (VICs), activated VICs and osteoblastic VICs (obVICs), synergistically orchestrate this pathology. It has been demonstrated that serotonin is involved in early stages of myxomatous mitral degeneration, whereas the role of serotonin in calcific aortic valve disease is still unknown. To uncover the link between serotonin and osteogenesis in heart valves, osteogenesis of aortic and mitral VICs was induced in vitro. Actin polymerization and serotonin signaling were inhibited using cytochalasin D and serotonin inhibitors, respectively, to investigate the role of cell activation and serotonin signals in valvular cell osteogenesis. To evaluate calcification progress, calcium and collagen deposits along with the expression of protein markers, including the rate-limiting enzyme of serotonin synthesis [tryptophan hydroxylase 1 (TPH1)], were assessed. When exposed to osteogenic culture conditions and grown on soft surfaces, passage zero aortic VICs increased extracellular collagen deposits and obVIC phenotype markers. A more intense osteogenic process was observed in aortic VICs of higher passages, where cells were activated prior to osteogenic induction. For both, TPH1 expression was upregulated as osteogenesis advanced. However, these osteogenic changes were reversed upon serotonin inhibition. This discovery provides a better understanding of signaling pathways regulating VIC phenotype transformation and explains different manifestations of degenerative pathologies. In addition, the discovery of serotonin-based inhibition of valvular calcification will contribute to the development of potential novel therapies for calcific valvular diseases.

Dissecting Calcific Aortic Valve Disease-The Role, Etiology, and Drivers of Valvular Fibrosis

Calcific aortic valve disease (CAVD) is a highly prevalent and progressive disorder that ultimately causes gradual narrowing of the left ventricular outflow orifice with ensuing devastating hemodynamic effects on the heart. Calcific mineral accumulation is the hallmark pathology defining this process; however, fibrotic extracellular matrix (ECM) remodeling that leads to extensive deposition of fibrous connective tissue and distortion of the valvular microarchitecture similarly has major biomechanical and functional consequences for heart valve function. Significant advances have been made to unravel the complex mechanisms that govern these active, cell-mediated processes, yet the interplay between fibrosis and calcification and the individual contribution to progressive extracellular matrix stiffening require further clarification. Specifically, we discuss (1) the valvular biomechanics and layered ECM composition, (2) patterns in the cellular contribution, temporal onset, and risk factors for valvular fibrosis, (3) imaging valvular fibrosis, (4) biomechanical implications of valvular fibrosis, and (5) molecular mechanisms promoting fibrotic tissue remodeling and the possibility of reverse remodeling. This review explores our current understanding of the cellular and molecular drivers of fibrogenesis and the pathophysiological role of fibrosis in CAVD.

Targeting 5-HT2B Receptor Signaling Prevents Border Zone Expansion and Improves Microstructural Remodeling After Myocardial Infarction

BACKGROUND

Myocardial infarction (MI) induces an intense injury response that ultimately generates a collagen-dominated scar. Although required to prevent ventricular rupture, the fibrotic process is often sustained in a manner detrimental to optimal recovery. Cardiac myofibroblasts are the cells tasked with depositing and remodeling collagen and are a prime target to limit the fibrotic process after MI. Serotonin 2B receptor (5-HT2B) signaling has been shown to be harmful in a variety of cardiopulmonary pathologies and could play an important role in mediating scar formation after MI.

METHODS

We used 2 pharmacological antagonists to explore the effect of 5-HT2B inhibition on outcomes after MI and characterized the histological and microstructural changes involved in tissue remodeling. Inducible 5-HT2B ablation driven by Tcf21MCM and PostnMCM was used to evaluate resident cardiac fibroblast- and myofibroblast-specific contributions of 5-HT2B, respectively. RNA sequencing was used to motivate subsequent in vitro analyses to explore cardiac fibroblast phenotype.

RESULTS

5-HT2B antagonism preserved cardiac structure and function by facilitating a less fibrotic scar, indicated by decreased scar thickness and decreased border zone area. 5-HT2B antagonism resulted in collagen fiber redistribution to thinner collagen fibers that were more anisotropic, enhancing left ventricular contractility, whereas fibrotic tissue stiffness was decreased, limiting the hypertrophic response of uninjured cardiomyocytes. Using a tamoxifen-inducible Cre, we ablated 5-HT2B from Tcf21-lineage resident cardiac fibroblasts and saw similar improvements to the pharmacological approach. Tamoxifen-inducible Cre-mediated ablation of 5-HT2B after onset of injury in Postn-lineage myofibroblasts also improved cardiac outcomes. RNA sequencing and subsequent in vitro analyses corroborate a decrease in fibroblast proliferation, migration, and remodeling capabilities through alterations in Dnajb4 expression and Src phosphorylation.

CONCLUSIONS

Together, our findings illustrate that 5-HT2B expression in either cardiac fibroblasts or activated myofibroblasts directly contributes to excessive scar formation, resulting in adverse remodeling and impaired cardiac function after MI.

Structure, Function, and Pharmaceutical Ligands of 5-Hydroxytryptamine 2B Receptor

Since the first characterization of the 5-hydroxytryptamine 2B receptor (5-HT2BR) in 1992, significant progress has been made in 5-HT2BR research. Herein, we summarize the biological function, structure, and small-molecule pharmaceutical ligands of the 5-HT2BR. Emerging evidence has suggested that the 5-HT2BR is implicated in the regulation of the cardiovascular system, fibrosis disorders, cancer, the gastrointestinal (GI) tract, and the nervous system. Eight crystal complex structures of the 5-HT2BR bound with different ligands provided great insights into ligand recognition, activation mechanism, and biased signaling. Numerous 5-HT2BR antagonists have been discovered and developed, and several of them have advanced to clinical trials. It is expected that the novel 5-HT2BR antagonists with high potency and selectivity will lead to the development of first-in-class drugs in various therapeutic areas.