Endoplasmic reticulum stress occurs in association with the extrusion of toxic extracellular vesicles from human placentae treated with antiphospholipid antibodies

Abstract

Antiphospholipid autoantibodies (aPLs), a major maternal risk factor for preeclampsia, are taken into the syncytiotrophoblast where they bind intracellular vesicles and mitochondria. Subsequently, large quantities of extracellular vesicles (EVs) extruded from syncytiotrophoblast into the maternal circulation are altered such that they cause maternal endothelial cell activation. However, the mechanism driving this change is unknown. First trimester placental explants were treated with aPL for 18 h. The EVs were then collected by different centrifugation. The levels of HSP 70, misfolded proteins, caspase 8 activity, and Mixed Lineage Kinase domain-Like (MLKL) were measured in placental explants and EVs. In addition, the levels of TNF-α and CD95 in conditioned medium were also measured. Treating placental explants with aPL caused an increase in levels of HSP 70, misfolded proteins and MLKL in placental explants and EVs. Increased activity of caspase 8 was also seen in placental explants. Higher levels of TNF-α were seen conditioned medium from aPL-treated placental explant cultures. aPLs appear to induce endoplasmic reticulum stress in the syncytiotrophoblast in a manner that involved caspase 8 and TNF-α. To avoid accumulation of the associated misfolded proteins and MLKL, the syncytiotrophoblast exports these potentially dangerous proteins in EVs. It is likely that the dangerous proteins that are loaded into placental EVs in preeclampsia contribute to dysfunction of the maternal cells.

Extracellular Vesicles and Antiphospholipid Syndrome: State-of-the-Art and Future Challenges

“…Micro- and nanovesicles isolated from aPL-stimulated placental explants contained more cell death pathway proteins (e.g., mixed lineage kinase domain like pseudokinase (MLKL), misfolded proteins, mitochondrial DNA, and possessed increased ER stress (increased heat shock protein 70 (HSP70)) [ 89 , 93 ]. Mitochondrial DNA serves as a danger signal that can trigger the TLR signaling pathway in recipient cells, leading to endothelial cell dysfunction and consequently contributing to the increased risk of PE in women with aPL [ 89 ].…”

“…One way to remove these danger signals and avoid apoptosis is to package them in the extruded EVs. While removal of these danger signals by EVs might protect the stressed STB, the cargo is potentially toxic to the maternal cells that are the ultimate targets of these EVs [ 93 ].…”

MLKL: Functions beyond serving as the Executioner of Necroptosis

“…This release appears to inhibit cell death. For example, syncytiotrophoblasts release EVs that contain Hsp 70, misfolded proteins, and MLKL, to eliminate intracellular increased toxins 94 . However, these dangerous/toxic EVs can lead to maternal endothelial cell activation, causing preeclampsia.…”

Antiphospholipid antibodies and extracellular vesicles in pregnancy

“…Typically, the release of cytochrome C from the mitochondria into the cytosol activates and perpetuates the apoptotic cascade via formation of the apoptosome. Indeed, activation of caspase 9, a downstream effector of cytochrome C, was increased in placentae exposed to aPL 61 . However, since the syncytiotrophoblast is one single cell covering the entire surface of the placenta, the syncytiotrophoblast cannot die without jeopardizing the pregnancy.…”

“…However, since the syncytiotrophoblast is one single cell covering the entire surface of the placenta, the syncytiotrophoblast cannot die without jeopardizing the pregnancy. Therefore it is likely that activation of caspases in the syncytiotrophoblast is indicative of processes other than cell death 61 . It is possible that proteins of the cell death pathways may have alternative functions in the syncytiotrophoblast, such as being involved in EV generation 61 …”