Beijing Institute of Microbiology & Epidemiology

In the SARS-CoV-2 replication-transcription complex (RTC), the nascent template-product duplex is unwound into a template strand for recycling and a product strand that needs to be capped. Here, we determined structures of the SARS-CoV-2 RTC in the pre- and post-capping initiation (CI) states. In the pre-CI state, the RTC has a dimer-of-dimeric architecture (ddRTC). The upstream RNA duplex in one RTC is reciprocally unwound by a helicase in a head-to-head-positioned RTC in the 3'-5' direction. The helicases bind either ADP or ADP⋅P_i in their ATP-binding pockets, suggesting a mechanism for ATP-hydrolysis-driven unwinding. In the post-CI state, the binding of nsp9 to the nsp12 nidovirus RdRp-associated nucleotidyltransferase (NiRAN) disrupts the ddRTC. The N terminus of nsp9 and the triphosphorylated 5' end of the product strand co-localize in NiRAN's catalytic site, exhibiting the state prior to nsp9 RNAylation for capping. These results provide an insight into the concurrence of template recycling and RNA capping in the SARS-CoV-2 RTC.

Severe fever with thrombocytopaenia syndrome virus (SFTSV) was identified by the World Health Organization as a priority pathogen due to its high case-fatality rate in humans and rapid spread. It is maintained in nature through three transmission pathways: systemic, non-systemic and transovarial. Understanding the relative contributions of these transmission pathways is crucial for developing evidence-informed public health interventions to reduce its spillover risks to humans. Using next-generation matrices, sensitivity analyses, elasticity analyses and random forest models, we estimated the basic reproduction number R₀, relative contribution of each pathway, and identified the most sensitive model parameters across 27 scenarios. Results showed that [Formula: see text] ranged from 0.72 to 2.08 across scenarios, increasing with higher tick abundance and longer viraemia. Transovarial transmission dominated in 26 scenarios, while the importance of the other two varied, with non-systemic transmission more important under high tick abundance, short viraemia or aggregated tick distribution. [Formula: see text] dropped below 1 in all scenarios when transovarial transmission was excluded. These findings emphasize the need for interventions targeting transovarial transmission, such as reducing female adult tick survival and limiting large vertebrate host movement, and underscore the importance of laboratory studies measuring sensitive parameters including transovarial transmission efficiency, tick survival probabilities and the duration of viraemia and potential for non-systemic transmission for key animal host species with high seroprevalence rates.

Hyperactivated inflammatory responses induced by cytokine release syndrome are the primary causes of tissue damage and even death. The translation process is precisely regulated to control the production of proinflammatory cytokines. However, it is largely unknown whether targeting translation can effectively limit the hyperactivated inflammatory responses during acute hepatitis and graft-versus-host disease.

By using in vitro translation and cellular overexpression systems, we have found that the nonstructural protein gene NS2A of Zika virus functions as RNA molecules to suppress the translation of both ectopic genes and endogenous proinflammatory cytokines. Mechanistically, results from RNA pulldown and co-immunoprecipitation assays have demonstrated that NS2A RNA interacts with the translation initiation factor eIF2α to disrupt the dynamic balance of the eIF2/eIF2B complex and translation initiation, which is the rate-limiting step of translation. In the acetaminophen-induced, lipopolysaccharide/D-galactosamine-induced, viral infection-induced acute hepatitis, and graft-versus-host disease mouse models, mice with myeloid cell-specific knock-in of NS2A show decreased levels of serum proinflammatory cytokines and reduced tissue damage.

Zika virus NS2A dampens the production of proinflammatory cytokines and alleviates inflammatory injuries by interfering translation process as RNA molecules, which suggests that NS2A RNA is potentially used to treat numerous acute inflammatory diseases characterized by cytokine release syndrome.