TLR4-Endothelin Axis Controls Syncytiotrophoblast Motility and Confers Fetal Protection in Placental Malaria

Abstract

Pregnancy associated malaria is often associated with adverse pregnancy outcomes. Placental circulatory impairments are an intriguing and unsolved component of malaria pathophysiology. Here, we uncovered a TLR4-TRIF-endothelin axis that controls trophoblast motility and is linked to fetal protection during Plasmodium infection. In a cohort of 401 pregnancies from Northern Brazil we found that infection during pregnancy reduced expression of endothelin receptor B in syncytiotrophoblasts while endothelin expression was only affected during acute infection. We further show that quantitative expression of placental endothelin and endothelin receptor B proteins are differentially controlled by maternal and fetal TLR4 alleles. Using murine malaria models, we identified placental autonomous responses to malaria infection mediated by fetally encoded TLR4 that not only controlled placental endothelin gene expression but also correlated with fetal viability protection. In vitro assays showed that control of endothelin expression in fetal syncytiotrophoblasts exposed to Plasmodium -infected erythrocytes was dependent on TLR4 via the TRIF pathway but not MyD88 signaling. Time-lapse microscopy in syncytiotrophoblast primary cultures and cell invasion assays demonstrated that ablation of TLR4 or endothelin receptor blockade abrogate trophoblast collective motility and cell migration responses to infected erythrocytes. These results cohesively substantiate the hypothesis that fetal innate immune sensing, namely the TRL4-TRIF pathway exerts a fetal protective role during malaria infection by mediating syncytiotrophoblast vasoregulatory responses that counteract placental insufficiency.

Placental malaria is associated with a TLR–Endothelin-3–oxidative damage response in human placenta tissues

“…The analysis revealed that when compared with PM-negative controls, PM-positive samples expressed significantly higher levels of TLR4 and TLR7, but not TLR9 (Figure 3A-C, P = 0.002, 0.03, and 0.59, respectively). This is consistent with mouse data showing that PM upregulates TLR4-mediated expression of endothelin-1 [15]. We therefore wondered if human PM alters the expression of Endothelin genes.…”

“…Considering that TLRs are important drivers of inflammation [36], which is implicated in PM pathogenesis [37], taken together with the observed changes in placental histological features, our findings indicate for the first time, that TLR-mediated responses to PM may contribute to local placental inflammation, which may underlie the observed PM-associated low birth and placental weights, although the precise mechanisms remain unclear. Mouse data show that PM-driven TLR4 expression drives placental endothelin-1 expression [15], and for the first time, our findings show that PM is also associated with the upregulation of Endothelin-3. The Endothelin ligands 1, 2, and 3 are a family of vasoactive factors that influence a range of cellular processes, such as vascular remodeling and angiogenesis [38].…”

“…This suggests the presence of TLR-mediated innate immune responses to PM, although this has not been reported in the context of human PM. Here, considering that mouse data show that TLR4 modulates endothelin-1 expression [15], malaria is inflammatory and oxidative [16], oxidative DNA damage upregulates TLR4 [17], and TLR signaling is thought to promote DNA repair [18], we used biobank placenta samples donated by women living in a malaria-endemic region of Kenya to examine the hypothesis that human PM triggers a TLR–Endothelin–oxidative damage signaling response.…”

Genetic variants of TLR4, including the novel variant, rs5030719, and related genes are associated with susceptibility to clinical malaria in African children

“…Noteworthy amongst these genes is TLR4 , which have been identified in all analyses described above (Tables 1 and 2 , and 3 ). Supporting literature has also identified this gene as integral in the pathological response to malaria [ 13 , 15 ], and is demonstrable as an anti-malarial vaccine target [ 14 ]. TLRs, including TLR4 identified herein, are part of the innate immune system, being expressed primarily by monocytes and playing a role in recognition of malarial molecular markers, in particular GPI, ostensibly implicating variations in TLR4 with the elicited immune response [ 18 ].…”

Homeostasis Maintenance in Plasmodium-Infected Placentas: Is There a Role for Placental Autophagy During Malaria in Pregnancy?

“…Autophagy might also be triggered upon parasite sensing by TLRs (92), which induce autophagy through the MyD88-TRIF interaction with the Bcl2-Beclin1 complex (43). In fact, TLR-MyD88 signaling, which initiates inflammation by promoting the production of cytokines such as TNF-a, and IL-1b (by inducing its transcription and posttranslational maturation through the inflammasomes), is implicated in the outcomes of MiP, since genetic depletion of TLR4 and MyD88 improves pregnancy outcomes in an experimental murine model of MiP (93)(94)(95)(96). In parallel, MiP-associated hypoxia and ROS (3,27) can promote mitochondrial instability, increasing oxidative stress and cell death via apoptotic pathways.…”