l-Tryptophan-Induced Vasodilation Is Enhanced in Preeclampsia: Studies on Its Uptake and Metabolism in the Human Placenta

Periconceptional Non-medical Maternal Determinants Influence the Tryptophan Metabolism: The Rotterdam Periconceptional Cohort (Predict Study)

Background

The vital role of the maternal tryptophan (TRP) metabolism in maternal health and pregnancy is well established. However, non-medical maternal determinants influencing the TRP metabolism have been poorly investigated. We hypothesise that periconceptional maternal non-medical determinants alter the TRP metabolism, affecting both kynurenine (KP) and serotonin pathway (SP) metabolite concentrations. Therefore, we investigated the influence of non-medical maternal determinants on the TRP metabolism during the periconception period.

Methods

About 1916 pregnancies were included from the Rotterdam Periconceptional Cohort between November 2010 and December 2020. Data on periconceptional non-medical maternal determinants were collected through questionnaires. Serum samples were collected at 8.5 (SD = 1.6) weeks of gestation and TRP, kynurenine (KYN), 5-hydroxytryptophan (5-HTP), 5-HT (5-hydroxytryptamine) and 5-hydroxyindole acetic acid (5-HIAA) were determined using validated liquid chromatography (tandem) mass spectrometry. Mixed models were used to determine associations between periconceptional non-medical maternal determinants and these metabolites.

Results

In total 11 periconceptional non-medical maternal determinants were identified. Protein intake was positively associated with TRP (β = .12, 95% CI = 0.07-0.17), while age, energy intake and body mass index (BMI) (β = -.24, 95% CI = -0.37 to -0.10) were negatively associated with TRP. Age, BMI and total homocysteine were associated with higher KYN, whereas non-western geographical origin was associated with lower KYN (β = -.09, 95% CI = -0.16 to -0.03). Protein intake and total homocysteine (β = .07, 95% CI = 0.03-0.11) had a positive association with 5-HTP, while a negative association was found for energy intake. A non-western geographical origin and drug use were associated with higher 5-HT, and BMI with lower 5-HT (β = -6.32, 95% CI = -10.26 to -2.38). Age was positively associated with 5-HIAA (β = .92, 95% CI = 0.29-1.56), and BMI negatively.

Conclusions

Periconceptional non-medical maternal determinants, including age, geographical origin, drug use, energy and protein intake, BMI and total homocysteine, influence KP and SP metabolite concentrations.

Statins Prevent the Deleterious Consequences of Placental Chemerin Upregulation in Preeclampsia

BACKGROUND

Chemerin, an inflammatory adipokine, is upregulated in preeclampsia, and its placental overexpression results in preeclampsia-like symptoms in mice. Statins may lower chemerin.

METHODS

Chemerin was determined in a prospective cohort study in women suspected of preeclampsia and evaluated as a predictor versus the sFlt-1 (soluble fms-like tyrosine kinase-1)/PlGF (placental growth factor) ratio. Chemerin release was studied in perfused placentas and placental explants with or without the statins pravastatin and fluvastatin. We also addressed statin placental passage and the effects of chemerin in chorionic plate arteries.

RESULTS

Serum chemerin was elevated in women with preeclampsia, and its addition to a predictive model yielded significant effects on top of the sFlt-1/PlGF ratio to predict preeclampsia and its fetal complications. Perfused placentas and explants of preeclamptic women released more chemerin and sFlt-1 and less PlGF than those of healthy pregnant women. Statins reversed this. Both statins entered the fetal compartment, and the fetal/maternal concentration ratio of pravastatin was twice that of fluvastatin. Chemerin constricted plate arteries, and this was blocked by a chemerin receptor antagonist and pravastatin. Chemerin did not potentiate endothelin-1 in chorionic plate arteries. In explants, statins upregulated low-density lipoprotein receptor expression, which relies on the same transcription factor as chemerin, and NO release.

CONCLUSIONS

Chemerin is a biomarker for preeclampsia, and statins both prevent its placental upregulation and effects, in an NO and low-density lipoprotein receptor-dependent manner. Combined with their capacity to improve the sFlt-1/PlGF ratio, this offers an attractive mechanism by which statins may prevent or treat preeclampsia.

Vascular reactivity is altered in the placentas of fetuses with congenital diaphragmatic hernia

INTRODUCTION

Infants with congenital diaphragmatic hernia (CDH) often develop pulmonary hypertension but frequently fail to respond to vasodilator therapy, for instance because of an altered pulmonary vasoreactivity. Investigating such alterations in vivo is impossible. We hypothesised that these alterations are also present in fetoplacental vessels, since both vasculatures are exposed to the same circulating factors (e.g. endothelin-1) and respond similarly to certain stimuli (e.g. hypoxia). As proof-of-concept, we compared fetoplacental vasoreactivity between healthy and CDH-affected placentas.

METHODS

Fetoplacental vascular function of healthy and antenatally diagnosed left-sided CDH fetuses was assessed by wire myography. Placental expression of enzymes and receptors involved in the altered vasoreactive pathways was measured using quantitative PCR.

RESULTS

CDH arteries (n = 6) constricted more strongly to thromboxane A2 agonist U46619 (p < 0.001) and dilated less to bradykinin (p = 0.01) and nitric oxide (NO)-donor sodium nitroprusside (p = 0.04) than healthy arteries (n = 8). Vasodilation to prostacyclin analogue iloprost and adenylate cyclase stimulator forskolin, and vasoconstriction to endothelin-1 were not different between both groups. Angiotensin II did not induce vasoconstriction. Phosphodiesterase inhibitors sildenafil and milrinone did not affect responses to sodium nitroprusside, forskolin, or U46619. The mRNA expression of guanylate cyclase 1 soluble subunit alpha 1 (p = 0.003) and protein kinase cyclic guanine monophosphate (cGMP)-dependent 1 (p = 0.02) were reduced in CDH versus healthy placentas.

DISCUSSION

The identified changes in the thromboxane and NO-cGMP pathways in the fetoplacental vasculature correspond with currently described alterations in the pulmonary vasculature in CDH. Therefore, fetoplacental arteries may provide an opportunity to predict pulmonary therapeutic responses in infants with CDH.

Two distinct molecular faces of preeclampsia revealed by single-cell transcriptomics

INTRODUCTION

Preeclampsia is a multisystemic, pregnancy-specific disorder united by new-onset hypertension but with considerable variation in clinical manifestation, onset, and severity. For symptoms to regress, delivery of the placenta is required. For symptoms to regress, delivery of the placenta is required, making the placenta central to preeclampsia pathophysiology. To dissect which placental functions were impacted in two forms of preeclampsia, we studied molecular changes across the cell types of the placenta.

METHODS

We performed a transcriptomic survey of single-cells and single-nuclei on cases of early- and late-onset preeclampsia with gestation-matched controls.

FINDINGS

Our data revealed massive dysregulation of gene expression in all cell classes that was almost exclusive to early preeclampsia. For example, an important known receptor/ligand imbalance hallmarking angiogenic disfunction, sFLT1/placental growth factor (PGF), was reflected in striking, cell-autonomous dysregulation of FLT1 and PGF transcription in the syncytium in early preeclampsia only. Stromal cells and vasculature echoed an inflamed, stressed, anti-angiogenic environment. Finally, the placental immune niche set the tone for inflammation in early but not late preeclampsia. Here, fetal-origin Hofbauer and maternal-origin TREM2 macrophages were revealed as surprising main actors, while local cells of the adaptive immune system were largely unaffected. Late preeclampsia showed minimal cellular impact on the placenta.

CONCLUSIONS

Our survey provides systematic molecular evidence for two distinct diseases. We resolved systematic molecular dysregulation to individual cell types with strong implications for definition, early detection, diagnosis, and treatment.

FUNDING

Funded by the Preeclampsia Foundation through the Peter Joseph Pappas Research Grant.

Maternal Tryptophan Catabolites and Insulin Resistance Parameters in Preeclampsia

Preeclampsia (PE) is a pregnancy-related disorder characterized by high blood pressure and proteinuria in the third trimester. The disease is associated with many metabolic and biochemical changes. There is a need for new biomarkers for diagnosis and follow-up. The present study examined the diagnostic ability of tryptophan catabolites (TRYCATs) and insulin resistance (IR) parameters in women with PE. This case-control study recruited sixty women with preeclampsia and 60 healthy pregnant women as a control group. Serum levels of TRYCATs (tryptophan, kynurenic acid, kynurenine, and 3-hydroxykynurenine) and IR parameters (insulin and glucose) were measured by ELISA and spectrophotometric methods. The results showed that PE women have a significantly lower tryptophan level than healthy pregnant women. However, there was a significant increase in kynurenic acid, kynurenic acid/kynurenine, kynurenine/tryptophan, and 3-hydroxykynurenine levels. PE women also have a state of IR. The correlation study indicated various correlations of IR and TRYCATs with clinical data and between each other, reflecting the role of these parameters in the pathophysiology of PE. The ROC study showed that the presence of IR state, reduced tryptophan, and increased 3-HK predicted PE disease in a suspected woman with moderate sensitivities and specificities. In conclusion, the pathophysiology of PE involves a state of IR and an alteration of the TRYCAT system. These changes should be taken into consideration when PE is diagnosed or treated.

MED12 mutation activates the tryptophan/kynurenine/AHR pathway to promote growth of uterine leiomyomas

Uterine leiomyomas cause heavy menstrual bleeding, anemia, and pregnancy loss in millions of women worldwide. Driver mutations in the transcriptional mediator complex subunit 12 (MED12) gene in uterine myometrial cells initiate 70% of leiomyomas that grow in a progesterone-dependent manner. We showed a distinct chromatin occupancy landscape of MED12 in mutant MED12 (mut-MED12) versus WT-MED12 leiomyomas. Integration of cistromic and transcriptomics data identified tryptophan 2,3-dioxygenase (TDO2) as the top mut-MED12 target gene that was significantly upregulated in mut-MED12 leiomyomas when compared with adjacent myometrium and WT-MED12 leiomyomas. TDO2 catalyzes the conversion of tryptophan to kynurenine, an aryl hydrocarbon receptor (AHR) ligand that we confirmed to be significantly elevated in mut-MED12 leiomyomas. Treatment of primary mut-MED12 leiomyoma cells with tryptophan or kynurenine stimulated AHR nuclear translocation, increased proliferation, inhibited apoptosis, and induced AHR-target gene expression, whereas blocking the TDO2/kynurenine/AHR pathway by siRNA or pharmacological treatment abolished these effects. Progesterone receptors regulated the expression of AHR and its target genes. In vivo, TDO2 expression positively correlated with the expression of genes crucial for leiomyoma growth. In summary, activation of the TDO2/kynurenine/AHR pathway selectively in mut-MED12 leiomyomas promoted tumor growth and may inform the future development of targeted treatments and precision medicine.

Discovery of novel metabolic signatures for early identification of women at risk of developing gestational hypertension

INTRODUCTION

Gestational hypertension (GH) is defined as the presence of systolic blood pressure (BP) ≥ 140 mm Hg and/or diastolic BP ≥ 90 mm Hg, measured at least 4 h apart after 20 weeks of gestation. Early identification of women at high-risk of developing GH could contribute significantly towards improved maternal and fetal outcomes.

OBJECTIVES

To determine early metabolic biomarkers in women with GH as compared with normotensive women.

METHODS

Serum samples were collected from subjects during three stages of their pregnancy: 8-12 weeks, 18-20 weeks and after 28 weeks (< 36 weeks) of gestation and studied using nuclear magnetic resonance (NMR) metabolomics approach. Multivariate and univariate analyses were performed to determine the significantly altered metabolites in GH women.

RESULTS

A total of 10 metabolites, including isoleucine, glutamine, lysine, proline, histidine, phenylalanine, alanine, carnitine, N-acetyl glycoprotein and lactic acid were observed to be significantly downregulated during all pregnancy stages in women with GH as compared with controls. Furthermore, expression of 5 metabolites in the first trimester i.e., phenylalanine [area under the curve (AUC) = 0.745], histidine [AUC = 0.729], proline [AUC = 0.722], lactic acid [AUC = 0.722], and carnitine [AUC = 0.714] exhibited highest potential in discriminating GH from normotensive women.

CONCLUSION

The present study is the first of its kind to identify significantly altered metabolites that have the potential to discriminate between women at risk of developing GH and normotensive women across three trimesters of pregnancy. This opens up the possibility of exploring these metabolites as potential early predictive markers of GH.

Tryptophan degradation enzymes expression in the placenta and the Kynurenine/Tryptophan ratio in maternal plasma after elective cesarean section

Indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase (TDO) metabolize tryptophan in the kynurenine pathway. We evaluated these enzymes' mRNA expression in maternal and fetal sides of the placenta of uncomplicated, unlabored full-term pregnancies after elective cesarean section and compared it with that of placentas obtained from vaginal delivery. Tryptophan and kynurenine plasmatic levels after cesarean section were measured, to investigate their possible correlation with IDO1 and TDO mRNA (TDO2) expression. The results suggested that IDO1 and TDO2 expression was higher in the maternal side of the placenta and that labor significantly affects TDO2 expression and the plasma Kynurenine/Tryptophan ratio.

Simultaneous quantification of tryptophan metabolites by liquid chromatography tandem mass spectrometry during early human pregnancy

OBJECTIVES

In this study we describe the development and validation of a liquid chromatography mass spectrometry method (LC-MS/MS) to quantify five tryptophan (TRP) metabolites within the kynurenine- and serotonin pathway and apply the method to serum samples of women in the first trimester of pregnancy. A secondary aim was to investigate the correlation between body mass index (BMI) and the five analytes.

METHODS

A LC-MS/MS was developed for the analysis of TRP, kynurenine (KYN), 5-hydroxytryptophan (5-HTP), hydroxytryptamine (5-HT), and 5-hydroxyindole acetic acid (5-HIAA). Serum samples (n=374) were analyzed of pregnant women (median gestational age: 8 ± 2 weeks) participating in a subcohort of the Rotterdam Periconceptional Cohort (Predict study).

RESULTS

The LC-MS/MS method provided satisfactory separation of the five analytes (7 min run). For all analytes R² was >0.995. Within- and between-run accuracies were 72-97% and 79-104%, and the precisions were all <15% except for the between-run precisions of the low QC-samples of 5-HTP and 5-HT (both 16%). Analyte concentrations were determined in serum samples of pregnant women (median (IQR)); TRP (µmol/L): 57.5 (13.4), KYN (µmol/L): 1.4 (0.4), 5-HTP (nmol/L): 4.1 (1.2), 5-HT (nmol/L): 615 (323.1), and 5-HIAA (nmol/L): 39.9 (17.0). BMI was negatively correlated with TRP, 5-HTP, and 5-HIAA (TRP: r=-0.18, p<0.001; 5-HTP: r=-0.13, p=0.02; natural log of 5-HIAA: r=-0.11, p=0.04), and positively with KYN (r=0.11, p=0.04).

CONCLUSIONS

The LC-MS/MS method is able to accurately quantify kynurenine- and serotonin pathway metabolites in pregnant women, providing an opportunity to investigate the role of the TRP metabolism in the (patho)physiology of pregnancy.

The Role of the Kynurenine Pathway in the (Patho) physiology of Maternal Pregnancy and Fetal Outcomes: A Systematic Review

INTRODUCTION

Tryptophan is the precursor of kynurenine pathway (KP) metabolites which regulate immune tolerance, energy metabolism, and vascular tone. Since these processes are important during pregnancy, changes in KP metabolite concentrations may play a role in the pathophysiology of pregnancy complications. We hypothesize that KP metabolites can serve as novel biomarkers and preventive therapeutic targets. This review aimed to provide more insight into associations between KP metabolite concentrations in maternal and fetal blood, and in the placenta, and adverse maternal pregnancy and fetal outcomes.

METHODS

A systematic search was performed on 18 February 2022 comprising all KP metabolites, and keywords related to maternal pregnancy and fetal outcomes. English-written human studies measuring KP metabolite(s) in maternal or fetal blood or in the placenta in relation to pregnancy complications, were included. Methodological quality was assessed using the ErasmusAGE quality score (QS) (range: 0-10). A meta-analysis of the mean maternal tryptophan and kynurenine concentrations in uncomplicated pregnancies was conducted.

RESULTS

Of the 6262 unique records, 37 were included (median QS = 5). Tryptophan was investigated in most studies, followed by kynurenine, predominantly in maternal blood (n = 28/37), and in the second and third trimester of pregnancy (n = 29/37). Compared to uncomplicated pregnancies, decreased tryptophan in maternal blood was associated with an increased prevalence of depression, gestational diabetes mellitus, fetal growth restriction, spontaneous abortion, and preterm birth. Elevated tryptophan was only observed in women with pregnancy-induced hypertension compared to normotensive pregnant women. In women with preeclampsia, only kynurenic acid was altered; elevated in the first trimester of pregnancy, and positively associated with proteinuria in the third trimester of pregnancy.

CONCLUSIONS

KP metabolite concentrations were altered in a variety of maternal pregnancy and fetal complications. This review implies that physiological pregnancy requires a tight balance of KP metabolites, and that disturbances in either direction are associated with adverse maternal pregnancy and fetal outcomes.

USP14 promotes tryptophan metabolism and immune suppression by stabilizing IDO1 in colorectal cancer

Indoleamine 2,3 dioxygenase 1 (IDO1) is an attractive target for cancer immunotherapy. However, IDO1 inhibitors have shown disappointing therapeutic efficacy in clinical trials, mainly because of the activation of the aryl hydrocarbon receptor (AhR). Here, we show a post-transcriptional regulatory mechanism of IDO1 regulated by a proteasome-associated deubiquitinating enzyme, USP14, in colorectal cancer (CRC). Overexpression of USP14 promotes tryptophan metabolism and T-cell dysfunction by stabilizing the IDO1 protein. Knockdown of USP14 or pharmacological targeting of USP14 decreases IDO1 expression, reverses suppression of cytotoxic T cells, and increases responsiveness to anti-PD-1 in a MC38 syngeneic mouse model. Importantly, suppression of USP14 has no effects on AhR activation induced by the IDO1 inhibitor. These findings highlight a relevant role of USP14 in post-translational regulation of IDO1 and in the suppression of antitumor immunity, suggesting that inhibition of USP14 may represent a promising strategy for CRC immunotherapy.

Kynurenine metabolites predict survival in pulmonary arterial hypertension: A role for IL-6/IL-6Rα

Activation of the kynurenine pathway (KP) has been reported in patients with pulmonary arterial hypertension (PAH) undergoing PAH therapy. We aimed to determine KP-metabolism in treatment-naïve PAH patients, investigate its prognostic values, evaluate the effect of PAH therapy on KP-metabolites and identify cytokines responsible for altered KP-metabolism. KP-metabolite levels were determined in plasma from PAH patients (median follow-up 42 months) and in rats with monocrotaline- and Sugen/hypoxia-induced PH. Blood sampling of PAH patients was performed at the time of diagnosis, six months and one year after PAH therapy. KP activation with lower tryptophan, higher kynurenine (Kyn), 3-hydroxykynurenine (3-HK), quinolinic acid (QA), kynurenic acid (KA), and anthranilic acid was observed in treatment-naïve PAH patients compared with controls. A similar KP-metabolite profile was observed in monocrotaline, but not Sugen/hypoxia-induced PAH. Human lung primary cells (microvascular endothelial cells, pulmonary artery smooth muscle cells, and fibroblasts) were exposed to different cytokines in vitro. Following exposure to interleukin-6 (IL-6)/IL-6 receptor α (IL-6Rα) complex, all cell types exhibit a similar KP-metabolite profile as observed in PAH patients. PAH therapy partially normalized this profile in survivors after one year. Increased KP-metabolites correlated with higher pulmonary vascular resistance, shorter six-minute walking distance, and worse functional class. High levels of Kyn, 3-HK, QA, and KA measured at the latest time-point were associated with worse long-term survival. KP-metabolism was activated in treatment-naïve PAH patients, likely mediated through IL-6/IL-6Rα signaling. KP-metabolites predict response to PAH therapy and survival of PAH patients.

The Footprint of Kynurenine Pathway in Cardiovascular Diseases

Kynurenine pathway is the main route of tryptophan metabolism and produces several metabolites with various biologic properties. It has been uncovered that several cardiovascular diseases are associated with the overactivation of kynurenine pathway and kynurenine and its metabolites have diagnostic and prognostic value in cardiovascular diseases. Furthermore, it was found that several kynurenine metabolites can differently affect cardiovascular health. For instance, preclinical studies have shown that kynurenine, xanthurenic acid and cis-WOOH decrease blood pressure; kynurenine and 3-hydroxyanthranilic acid prevent atherosclerosis; kynurenic acid supplementation and kynurenine 3-monooxygenase (KMO) inhibition improve the outcome of stroke. Indoleamine 2,3-dioxygenase (IDO) overactivity and increased kynurenine levels improve cardiac and vascular transplantation outcomes, whereas exacerbating the outcome of myocardial ischemia, post-ischemic myocardial remodeling, and abdominal aorta aneurysm. IDO inhibition and KMO inhibition are also protective against viral myocarditis. In addition, dysregulation of kynurenine pathway is observed in several conditions such as senescence, depression, diabetes, chronic kidney disease (CKD), cirrhosis, and cancer closely connected to cardiovascular dysfunction. It is worth defining the exact effect of each metabolite of kynurenine pathway on cardiovascular health. This narrative review is the first review that separately discusses the involvement of kynurenine pathway in different cardiovascular diseases and dissects the underlying molecular mechanisms.

Impact of inflammation and infection on the expression of amino acid transporters in the placenta: A minireview

Amino acid transporters expressed in the placenta help to regulate the transfer of amino acids from maternal to fetal circulation. Nutritional or hormonal factors are known to potentially impact the expression of amino acid transporters in the placenta. A relatively new field of inquiry has also demonstrated that inflammation, whether associated with infection or not, also alters the expression of amino acid transporters in the placenta. Indeed, studies over the past 15 years have demonstrated that malaria, viral and bacterial models of infection, preeclampsia, and direct administration of proinflammatory cytokines can alter placental amino acid transporter expression. While such studies have largely focused on System A and System L transporters, other transporters are also affected. p38 MAPK, STAT3, mTORC1, and AMPK signaling have all been implicated in these changes, but the underlying mechanism(s) remain to be fully elucidated. Furthermore, the implications of such changes warrant further investigation. This review will summarize studies that have investigated the impact of inflammation on placental amino acid transporter expression, identify questions that remain unanswered, and propose future areas of research to advance the field. As amino acid transporters are now being considered for drug targeting and drug delivery, furthering our understanding of the regulation of these transporters during disease states will be of increasing clinical value. Significance Statement While this is a relatively new field of research, multiple studies have demonstrated that inflammation alters placental amino acid transporter expression. This review will serve to summarize, for the first time, studies in this field and identify gaps in current knowledge as research in this area moves beyond identifying changes in transporter expression to investigating the implications of such changes and the mechanisms underlying them.

The Role of Amino Acids in Endothelial Biology and Function

The vascular endothelium acts as an important component of the vascular system. It is a barrier between the blood and vessel wall. It plays an important role in regulating blood vessel tone, permeability, angiogenesis, and platelet functions. Several studies have shown that amino acids (AA) are key regulators in maintaining vascular homeostasis by modulating endothelial cell (EC) proliferation, migration, survival, and function. This review summarizes the metabolic and signaling pathways of AAs in ECs and discusses the importance of AA homeostasis in the functioning of ECs and vascular homeostasis. It also discusses the challenges in understanding the role of AA in the development of cardiovascular pathophysiology and possible directions for future research.

The Placental Innate Immune System Is Altered in Early-Onset Preeclampsia, but Not in Late-Onset Preeclampsia

Preeclampsia is a severe placenta-related pregnancy disorder that is generally divided into two subtypes named early-onset preeclampsia (onset <34 weeks of gestation), and late-onset preeclampsia (onset ≥34 weeks of gestation), with distinct pathophysiological origins. Both forms of preeclampsia have been associated with maternal systemic inflammation. However, alterations in the placental immune system have been less well characterized. Here, we studied immunological alterations in early- and late-onset preeclampsia placentas using a targeted expression profile approach. RNA was extracted from snap-frozen placenta samples (healthy n=13, early-onset preeclampsia n=13, and late-onset preeclampsia n=6). The expression of 730 immune-related genes from the Pan Cancer Immune Profiling Panel was measured, and the data were analyzed in the advanced analysis module of nSolver software (NanoString Technology). The results showed that early-onset preeclampsia placentas displayed reduced expression of complement, and toll-like receptor (TLR) associated genes, specifically TLR1 and TLR4. Mast cells and M2 macrophages were also decreased in early-onset preeclampsia compared to healthy placentas. The findings were confirmed by an immunohistochemistry approach using 20 healthy, 19 early-onset preeclampsia, and 10 late-onset preeclampsia placentas. We conclude that the placental innate immune system is altered in early-onset preeclampsia compared to uncomplicated pregnancies. The absence of these alterations in late-onset preeclampsia placentas indicates dissimilar immunological profiles. The study revealed distinct pathophysiological processes in early-onset and late-onset preeclampsia placentas and imply that a tailored treatment to each subtype is desirable.

Early Pregnancy Exposure to Ambient Air Pollution among Late-Onset Preeclamptic Cases Is Associated with Placental DNA Hypomethylation of Specific Genes and Slower Placental Maturation

Exposure to ambient air pollution during pregnancy has been associated with an increased risk of preeclampsia (PE). Some suggested mechanisms behind this association are changes in placental DNA methylation and gene expression. The objective of this study was to identify how early pregnancy exposure to ambient nitrogen oxides (NO_x) among PE cases and normotensive controls influence DNA methylation (EPIC array) and gene expression (RNA-seq). The study included placentas from 111 women (29 PE cases/82 controls) in Scania, Sweden. First-trimester NO_x exposure was assessed at the participants’ residence using a dispersion model and categorized via median split into high or low NO_x. Placental gestational epigenetic age was derived from the DNA methylation data. We identified six differentially methylated positions (DMPs, q < 0.05) comparing controls with low NO_x vs. cases with high NO_x and 14 DMPs comparing cases and controls with high NO_x. Placentas with female fetuses showed more DMPs (N = 309) than male-derived placentas (N = 1). Placentas from PE cases with high NO_x demonstrated gestational age deceleration compared to controls with low NO_x (p = 0.034). No differentially expressed genes (DEGs, q < 0.05) were found. In conclusion, early pregnancy exposure to NO_x affected placental DNA methylation in PE, resulting in placental immaturity and showing sexual dimorphism.

The Function of the Kynurenine Pathway in the Placenta: A Novel Pharmacotherapeutic Target?

(L-)tryptophan is metabolized via the kynurenine pathway into several kynurenine metabolites with distinct functions. Dysfunction of the kynurenine pathway can lead to impairments in vascular regulation, immune regulation, and tolerance. The first and rate limiting enzyme of this pathway, indoleamine 2,3-dioxygenase (IDO), is highly expressed in the placenta and reduced in placentas from complicated pregnancies. IDO is essential during pregnancy, as IDO inhibition in pregnant mice resulted in fetal loss. However, the exact function of placental IDO, as well as its exact placental localization, remain controversial. This review identified that two isoforms of IDO; IDO1 and IDO2, are differently expressed between placental cells, suggesting spatial segregation. Furthermore, this review summarizes how the placental kynurenine pathway is altered in pregnancy complications, including recurrent miscarriage, preterm birth, preeclampsia, and fetal growth restriction. Importantly, we describe that these alterations do not affect maternally circulating metabolite concentrations, suggesting that the kynurenine pathway functions as a local signaling pathway. In the placenta, it is an important source of de novo placental NAD⁺ synthesis and regulates fetal tryptophan and kynurenine metabolite supply. Therefore, kynurenine pathway interventions might provide opportunities to treat pregnancy complications, and this review discusses how such treatment could affect placental function and pregnancy development.

Differential Distribution of Tryptophan-Metabolites in Fetal and Maternal Circulations During Normotensive and Preeclamptic Pregnancies

Preeclampsia (PE) is a hypertensive pregnancy, which is a leading cause of maternal and fetal morbidity and mortality during pregnancy. L-Tryptophan (Trp) is an essential amino acid, which can be metabolized into various biologically active metabolites. However, the levels of many circulating Trp-metabolites in human normotensive pregnancies (NT) and PE are undetermined. This study quantified the levels of Trp-metabolites in maternal and umbilical vein sera from women with NT and PE. Paired maternal and umbilical blood samples were collected from singleton pregnant patients. Twenty-five Trp-metabolites were measured in serum samples using liquid chromatography with tandem mass spectrometry. The effects of L-kynurenine (Kyn) and indole-3-lactic acid (ILA), on function of human umbilical vein endothelial cells (HUVECs), were also determined. Twenty Trp-metabolites were detected. The levels of 9 Trp-metabolites including Kyn and ILA were higher (P < 0.05) in umbilical vein than maternal serum, whereas 2 (5-hydroxy-L-tryptophan and serotonin) were lower (P < 0.05) in umbilical vein compared to maternal serum. PE significantly (P < 0.05) elevated ILA levels in maternal and umbilical vein sera. Kyn dose-dependently decreased (P < 0.05) cell viability. Kyn and ILA dose- and time-dependently (P < 0.05) increased monolayer integrity in HUVECs. These data suggest that these Trp-metabolites are important in regulating endothelial function during pregnancy, and the elevated ILA in PE may antagonize increased endothelial permeability occurring in PE.

Tryptophan metabolism and immune regulation in the human placenta

The placenta represents the maternal-fetal vascular interface. It is capable of supplying the bioenergetic needs of the developing conceptus. It is composed of different cell types that engage in highly varied functions, ranging from attachment, invasion and vascular remodeling to cell fusion, hormone production, and nutrient transport. A deep knowledge of the immunological mechanisms responsible for maintaining an active tolerance towards an allogeneic fetus and the anti-inflammatory properties of the placenta can be useful to clarify the pathogenesis of adverse events in pregnancy. While the systemic mechanisms of this immunological regulation in pregnancy have been well studied, the metabolic processes involved in the placental immune response are still poorly understood. The aim of this review is to summarize the most important information concerning the immune regulation in pregnancy, focusing on the role of tryptophan (Trp) catabolism performed by indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO) in the placenta.

Localization of Indoleamine 2,3-Dioxygenase-1 and Indoleamine 2,3-Dioxygenase-2 at the Human Maternal-Fetal Interface

Immunohistochemical localization of indoleamine 2,3-dioxygenase-1 and indoleamine 2,3-dioxygenase-2, the first and rate-limiting enzyme in tryptophan metabolism along the kynurenine pathway, has been studied in order to better understand the physiological significance of these enzymes at the maternal-fetal interface of human pregnancy with a gestational age of 7 weeks (n = 1) and term placentas (37-40 weeks of gestation, n = 5). Indoleamine 2,3-dioxygenase-1 protein immunoreactivity was found in glandular epithelium of the decidua and the endothelium of the fetal blood vessels in the villous stroma with some additional positive cells in the villous core and in the decidua. The syncytiotrophoblast stained strongly for indoleamine 2,3-dioxygenase-2. Immunoreactivity of kynurenine, the immediate downstream product of indoleamine 2,3-dioxygenase-mediated tryptophan metabolism, showed the same localization as that of indoleamine 2,3-dioxygenase-1 and indoleamine 2,3-dioxygenase-2, suggesting these are functional enzymes. Interferon-γ added to placental villous explant culture markedly stimulated expression level of both mRNA and immunoreactivity of indoleamine 2,3-dioxygenase-1. The different cellular expression and interferon-γ sensitivity of these enzymes at the maternal-fetal interface suggests distinct physiological roles for each enzyme in normal human viviparity.

Tryptophan 2,3-Dioxygenase Expression Identified in Murine Decidual Stromal Cells Is Not Essential for Feto-Maternal Tolerance

Indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase (TDO) catalyze the rate-limiting step of tryptophan catabolism along the kynurenine pathway, which has important immuno suppressive properties, particularly in tumor cells and dendritic cells. The prominent expression of IDO1 in the placenta also suggested a role in preventing immune rejection of fetal tissues, and pharmacological inhibition of IDO1 induced abortion of allogeneic fetuses in mice. However, this was later challenged by the lack of rejection of allogeneic fetuses in IDO1-KO mice, suggesting that other mechanisms may compensate for IDO1 deficiency. Here we investigated whether TDO could contribute to feto-maternal tolerance and compensate for IDO1 deficiency in IDO1-KO mice. Expression of TDO mRNA was previously detected in placental tissues. We developed a new chimeric rabbit anti-TDO antibody to confirm TDO expression at the protein level and identify the positive cell type by immunohistochemistry in murine placenta. We observed massive TDO expression in decidual stromal cells, starting at day E3.5, peaking at day E6.5 then declining rapidly while remaining detectable until gestation end. IDO1 was also induced in decidual stromal cells, but only at a later stage of gestation when TDO expression declined. To determine whether TDO contributed to feto-maternal tolerance, we mated TDO-KO and double IDO1-TDO-KO females with allogeneic males. However, we did not observe reduced fertility. These results suggest that, despite its expression in decidual stromal cells, TDO is not a dominant mechanism of feto-maternal tolerance able to compensate for the absence of IDO1. Redundant additional mechanisms of immunosuppression likely take over in these KO mice. The massive expression of TDO during decidualization might suggest a role of TDO in angiogenesis or vessel tonicity, as previously described for IDO1.

Interactions of coronaviruses with ACE2, angiotensin II, and RAS inhibitors-lessons from available evidence and insights into COVID-19

The rapid spread of a novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has led to an ongoing pandemic of coronavirus disease 2019 (COVID-19). Recently, angiotensin-converting enzyme 2 (ACE2) has been shown to be a functional receptor for SARS-CoV-2 to enter host target cells. Given that angiotensin receptor blockers (ARBs) and an ACE inhibitor (ACEI) upregulated ACE2 expression in animal studies, the concern might arise regarding whether ARBs and ACEIs would increase the morbidity and mortality of COVID-19. On the other hand, animal data suggested a potential protective effect of ARBs against COVID-19 pneumonia because an ARB prevented the aggravation of acute lung injury in mice infected with SARS-CoV, which is closely related to SARS-CoV-2. Importantly, however, there is no clinical or experimental evidence supporting that ARBs and ACEIs either augment the susceptibility to SARS-CoV-2 or aggravate the severity and outcomes of COVID-19 at present. Until further data are available, it is recommended that ARB and ACEI medications be continued for the treatment of patients with cardiovascular disease and hypertension, especially those at high risk, according to guideline-directed medical therapy based on the currently available evidence.