Fluid shear stress induces a shift from glycolytic to amino acid pathway in human trophoblasts

BACKGROUND

The human placenta, a tissue with a lifespan limited to the period of pregnancy, is exposed to varying shear rates by maternal blood perfusion depending on the stage of development. In this study, we aimed to investigate the effects of fluidic shear stress on the human trophoblast transcriptome and metabolism.

RESULTS

Based on a trophoblast cell line cultured in a fluidic flow system, changes caused by shear stress were analyzed and compared to static conditions. RNA sequencing and bioinformatics analysis revealed an altered transcriptome and enriched gene ontology terms associated with amino acid and mitochondrial metabolism. A decreased GLUT1 expression and reduced glucose uptake, together with downregulated expression of key glycolytic rate-limiting enzymes, hexokinase 2 and phosphofructokinase 1 was observed. Altered mitochondrial ATP levels and mass spectrometry data, suggested a shift in energy production from glycolysis towards mitochondrial oxidative phosphorylation. This shift in energy production could be supported by increased expression of glutamic-oxaloacetic transaminase variants in response to shear stress as well as under low glucose availability or after silencing of GLUT1. The shift towards amino acid metabolic pathways could be supported by significantly altered amino acid levels, like glutamic acid, cysteine and serine. Downregulation of GLUT1 and glycolytic rate-limiting enzymes, with concomitant upregulation of glutamic-oxaloacetic transaminase 2 was confirmed in first trimester placental explants cultured under fluidic flow. In contrast, high fluid shear stress decreased glutamic-oxaloacetic transaminase 2 expression in term placental explants when compared to low flow rates. Placental tissue from pregnancies with intrauterine growth restriction are exposed to high shear rates and showed also decreased glutamic-oxaloacetic transaminase 2, while GLUT1 was unchanged and glycolytic rate-limiting enzymes showed a trend to be upregulated. The results were generated by using qPCR, immunoblots, quantification of immunofluorescent pictures, padlock probe hybridization, mass spectrometry and FRET-based measurement.

CONCLUSION

Our study suggests that onset of uteroplacental blood flow is accompanied by a shift from a predominant glycolytic- to an alternative amino acid converting metabolism in the villous trophoblast. Rheological changes with excessive fluidic shear stress at the placental surface, may disrupt this alternative amino acid pathway in the syncytiotrophoblast and could contribute to intrauterine growth restriction.

Growth restricted placentas show severely reduced volume of villous components with perivascular myofibroblasts

INTRODUCTION

The restricted placental growth in IUGR is associated with a simultaneous weight and volume restriction for the placental villous tree. It is unknown whether the whole villous tree or only specific parts of it are growth restricted in IUGR. In the case of uniform growth restriction of the villous tree, IUGR placentas could be interpreted as symmetrically smaller versions of normal placentas. Otherwise, IUGR placentas would be morphologically, developmentally and, therefore, functionally different from normal placentas.

METHODS

We investigated ten normal and eleven IUGR placentas with quantitative microscopic techniques. Using immunohistochemical detection of placental myofibroblasts (γ-sm-actin) and foetoplacental endothelium (CD34), we distinguished between more centrally located villi showing the presence of myofibroblasts (contractile villi; C-villi) and more peripherally located villi showing the absence of myofibroblasts (noncontractile villi; NC-villi).

RESULTS

Compared to normal placentas, IUGR placentas showed significantly reduced mean volume of C-villi, but not of NC-villi. The volume of vessels in both, C-villi and NC-villi, was significantly reduced in IUGR. Additional stereologic estimates confirmed the known alterations in the morphology of NC-villi in IUGR.

DISCUSSION

Our results suggest that IUGR placentas are not just smaller but morphologically (and therefore functionally) different from normal placentas. We propose that the reduced volume of C-villi and vessels in C-villi reflects a developmental disturbance in the formation of C-villi, which are mostly composed of stem villi. As such, key pathological villous alterations in IUGR placentas could begin before the formation of intermediate and terminal villi, possibly already in the late first trimester of pregnancy.

Air contrast of the intervillous space enables non-disruptive Micro-CT analysis of paraffin-embedded archival placental tissue

The morphometric parameters of the villous tree are a strong indicator of deviant placentas. Methods have been established to digitally reconstruct small peripheral branches by tracing with 3D Microscopy at subcellular resolution. Micro-CT can help scale up the scanning of villous trees with resolution in the range of a few micrometers. As placental tissue samples are routinely conserved and archived by fixation and paraffin embedding, the villous structures are inaccessible to Micro-CT imaging due to poor contrast between paraffin and paraffinized tissue. We present a novel procedure for contrast enhancement by selectively replacing wax by air in the intervillous space.