Role of the NS1 Protein in Flavivirus Pathogenesis of Placental Function and Fetal Brain Development
Introduction. Flaviviruses are a class of RNA viruses that consist of Zika, Dengue, West Nile, Yellow Fever, and Japanese encephalitis viruses.1 Vertical transmission of Zika virus from mother to fetus can result in serious congenital defects such as microcephaly, a condition termed congenital Zika syndrome.1,2 The transmission of Zika virus to the developing fetus could be explained by the mechanisms initiated by the NS1 protein, a glycoprotein secreted from the flavivirus viral particles that plays a role in viral replication, immune evasion, and vascular leakage.3 The NS1 protein is conserved across flaviviruses and maintains a 20-40% identity and 60-80% similarity.3 The NS1 protein displays disease tropism and results in dysfunction of the endothelium in the placenta and brain microvascular endothelial cell layer. 3 Methods. Transendothelial electrical resistance assay (TEER) was used to measure permeability of the placental layer, specifically of different trophoblast cell lineages. 4 TEER was also used to quantify the permeability effects of the NS1 protein on the brain microvascular endothelial cell layer.5 Results. Zika NS1 induced placental hyperpermeability was the result of increased amount of hyaluronidase expressed at the placental barrier which resulted in shedding of glycosaminoglycans, which are critical components in maintaining structural integrity of the placental layer. Additionally, placental tight junction and adherens junction proteins, ZO-1 and E-cadherin, were altered following exposure to NS1, which contributes to the hyperpermeability and viral dissemination through the placenta.4 On the brain microvascular endothelial cell layer, the Zika NS1 protein upregulated NADPH dependent enzymes, NOX2 and NOX4, which resulted in an increase in reactive oxidative species (ROS.) This increase in ROS activated a redox specific kinase that phosphorylates VE-cadherin and B-catenin, which lead to disruption of Claudin 5. The disruption of junctional proteins results in brain microvascular hyperpermeability. A cytoplasmic tyrosine phosphatase, SHP2, is responsible for dephosphorylating VE-cadherin and B-catenin, and thus plays an important role in the regulation of brain endothelial integrity. However, following NS1 introduction, SHP2 production was downregulated.5 Conclusions. Vertical transmission of Zika virus through the placental barrier and developing fetal brain barrier can result in congenital brain deformities.1 Thus, understanding the role of the Zika NS1 protein as it relates to viral dissemination and endothelial pathogenesis is important. Vaccine development in mice models targeting the NS1 protein has shown promise in decreasing the amount of virus that disseminates through the placenta and brain and could potentially be applied clinically to reduce the prevalence of congenital Zika syndrome.6
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