Infection with Zika virus (ZIKV) in pregnancy can lead to neurological disorders, fetal abnormalities, and fetal death. Until now it’s not been clear how the virus manages to cross the placenta, which forms a strong barrier against microbes and chemicals that could harm the fetus. The results of a laboratory study carried out by researchers at Baylor College of Medicine, in collaboration with a team at Pennsylvania State University, have identified a strategy that Zika virus uses to covertly spread in placental cells, raising little alarm in the immune system. The team suggests their findings could point to new therapeutic strategies against the virus.
“The Zika virus, which is transmitted by mosquitoes, triggered an epidemic in the Americas that began in 2015 and by 2018 had reached as many as 30 million cases,” said Indira Mysorekar, PhD, E.I. Wagner Endowed, M.D., Chair Internal Medicine II, chief of basic and translational research and professor of medicine–infectious diseases at Baylor. “Understanding how Zika virus spreads through the human placenta and reaches the fetus is critical to prevent or control this devastating condition.
Zika virus is a mosquito-borne virus in the Flaviviridae family, and infection can lead to neurological disorders and fetal abnormalities such as microcephaly, and fetal death, “collectively known as congenital Zika syndrome,” the authors explained. “The propensity for horizontal and vertical transmission, and the ability to traverse blood-tissue barriers, including the blood-placental barrier, of ZIKV are unique among Flaviviridae.” The researchers noted that their own studies in mice, and work by others, have shown that ZIKV can infect fetal trophoblasts and endothelial cells of the placenta, which form the primary barrier between the maternal and fetal circulations. By infecting these cells the virus can enter the fetal circulation.
The researchers’ newly reported laboratory study has now discovered that Zika virus builds underground tunnels, a series of tiny tubes called tunneling nanotubes TNTs, in the placental trophoblasts, which facilitate the transfer of viral particles to neighboring uninfected cells. This ability is reliant on a viral protein NS1. “TNT formation is driven exclusively via ZIKV non-structural protein 1 (NS1),” they wrote.
“Zika is the only virus in its family, which includes dengue and West Nile viruses among others, whose NS1 protein triggers the formation of tunnels in multiple cell types,” Michita said. “Other viruses unrelated to Zika, such as HIV, herpes, influenza A, and SARS-CoV-2, the virus that causes COVID-19, also can induce tiny tunnels in cells they infect and use the tunnels to spread to uninfected cells. This is the first time that tunneling has been shown by Zika virus infection in placental cells.”
Interestingly, the tiny conduits provided a means to transport not only viral particles, but also RNA, proteins, and mitochondria, a cell’s main source of energy, from infected to neighboring cells. “We demonstrate that ZIKV infection or NS1 expression induces elevated mitochondria levels in trophoblasts and that mitochondria are siphoned via TNTs from healthy to ZIKV-infected cells,” the team wrote. Added co-author Long B. Tran, a graduate student in the Mysorekar lab, “We propose that transporting mitochondria through the tunnels may provide an energetic boost to virus-infected cells to promote viral replication.”
The study findings showed how TNT-mediated trafficking also allows Zika cell-to-cell transmission that is “camouflaged from host defenses.” Michita further commented, “We also show that traveling through the tiny tunnels can potentially help Zika virus avoid the activation of large-scale antiviral responses, such as interferon lambda (IFN-lambda) defenses implemented by the placenta. Mutant Zika viruses that do not make tiny tunnels induce robust antiviral IFN-lambda response that can potentially limit the spread of the virus.”
Mysorekar continued, “Altogether, we show that Zika virus uses a tunneling strategy to covertly spread the infection in the placenta while hijacking mitochondria to augment its propagation and survival. We propose that this strategy also protects the virus from the immune response. These findings offer vital insights that could be used to develop therapeutic strategies targeted against this stealth transmission mode.”
In their paper, the team concluded, “Our investigation reveals a previously unknown mechanism of intercellular transmission exploited by ZIKV, setting it apart from other orthoflaviviruses … Together our findings identify a stealth mechanism that ZIKV employs for intercellular spread among placental trophoblasts, evasion of antiviral interferon response, and the hijacking of mitochondria to augment its propagation and survival and offers a basis for novel therapeutic developments targeting these interactions to limit ZIKV dissemination.”
The team acknowledged that further research will be needed to investigate the molecular mechanism by which ZIKV NS1 induces TNTs, and whether monoclonal antibodies or NS1-based vaccines target TNT formation in ZIKV-infected cells, to potentially limit viral infection and spread.