In 2016, the Zika virus (ZIKV) became the forefront of public attention with its unexpected spread into the Americas and the Caribbean from its endemic regions of Africa and Asia. Of significant concern was the link between ZIKV and the neurological birth defect microcephaly (Li et al. 2016).
To address this emerging crisis, the research community increased its efforts to understand the biology of the virus as well as explore effective diagnostic tools and vaccines. This was supported by the mobilization of research funds to scientists studying other family member viruses such as dengue, yellow fever and West Nile virus. Since January 2016, there have been over 1680 ZIKV associated research articles listed in the scientific database PubMed. This blog post highlights major research findings on ZIKV, and provides a perspective on how scientists addressed this public health issue.
One of the first steps to designing a vaccine or treatment for ZIKV was to identify appropriate experimental models with which to study the disease. Dowall et al. (2016) studied the effect of ZIKV infection in type-I interferon receptor deficient (A129) mice. These mice were challenged with the virus in the lower leg via subcutaneous infection to mimic a mosquito bite. They showed that A129 mice were highly susceptible to ZIKV, demonstrating detectable viral RNA in the blood, spleen, liver and ovaries with no clinical symptoms (note that only 20% of ZIKV infections are clinically detectable). This study therefore identified A129 mice as a suitable small animal model for testing potential vaccines and antivirals.
Another relevant ZIKV mouse model was identified by Lazear et al. (2016). These researchers showed that Ifnar1-/- mice that lack components of the antiviral response can sustain high ZIKV burden in the brain, spinal cord and testes, which makes this model also suitable for studying sexual transmission of ZIKV.
The most recently proposed Zika mouse model was developed by the United States Food and Drug Administration (FDA) (Manangeeswaran et al. 2016). Using neonatal C57BL/6 mice, the research team demonstrated successful ZIKV infection, which led to clinical signs of neurological disease that subsided after two weeks. These mice also showed viral antigen predominantly in the cerebellum at the peak of disease as well as infiltration of key immune cells and molecules in the CNS. This mouse model is ideal for studying how ZIKV therapeutics affect the association of the virus with neurological diseases and congenital defects, including microcephaly.
One approach to treating ZIKV infection has been to identify proteins that could be targeted to inhibit virus replication. These proteins are essentially the Achilles heel of the virus as ZIKV uses certain proteins to enter the host cell and divide. Savidis et al. (2016) performed genetic screening analyses of the human genome to identify such proteins. Using this approach, they identified the AXL protein as a possible drug or vaccine target for treating ZIKV infection. AXL is critical during the early stage of infection, as the virus needs this protein to enter the cell.
Another therapeutic target was identified by Grant et al. (2016). They discovered that altering or removing the ZIKV protein NS5 allows immune defense against the virus. NS5 prevents ZIKV infected cells from inducing an interferon mediated immune response. Therefore, it was proposed that a vaccine could be developed using an attenuated form of ZIKV with altered NS5 function. Interestingly, NS5 is also a vaccine target for West Nile Virus infection, Yellow Fever and Encephalitis.
The ZIKV outbreak of 2016 highlighted the need not only for ZIKV therapies, but also for low-cost diagnostics that can be conveniently distributed in pandemic regions. The serious implications of ZIKV emphasize the importance of timely diagnosis for patient health and limiting rapid viral proliferation. To address this, Pardee et al. (2016) developed a cell-free, freeze-dried paper-based platform that hosts synthetic gene networks and can be used outside the lab without biosafety concerns. ZIKV can be detected using this platform in a small sample of saliva, urine or blood.
This diagnostic tool uses toehold switch RNA sensors and isothermal RNA amplification to detect clinically relevant concentrations of ZIKV, while demonstrating specificity against closely related Dengue virus sequences. Therefore, it addresses the limitations of traditional tests which often demonstrate cross-reactivity to related flaviviruses, leading to false positive results.
The biomolecular components of the tool remain stable at room temperature, which allows for easy storage and distribution in global settings, particularly where a lab may not be accessible. This new technology was developed using the engineering principles of synthetic biology and could be applied to other future health crises. This demonstrates the unique contributions that scientists with expertise outside of infectious diseases can make in addressing major public health concerns.
To determine whether FDA approved drugs could also be effective at inhibiting ZIKV, Burrows et al. (2016) tested 774 such drugs in a human hepatoma cell line. The team identified 20 compounds that could target ZIKV. Selected compounds were validated in human neural stem cells and primary amnion cells. The drugs identified as having anti-ZIKV activity include those known to inhibit other flaviviruses such as bortezomib and mycophenolic acid, as well as others with no known anti-viral activity such as daptomycin. Importantly, several of these drugs reduced ZIKV infection across multiple cell lines.
These drugs are not yet being evaluated for treatment of ZIKV infection. However, a number of ZIKV vaccines are currently being evaluated in clinical trials and animal models. These are primarily based on variations of other therapeutics designed against similar viruses such as dengue. History has demonstrated that vaccination is perhaps the most successful method of eliminating a disease. Therefore significant resources from both government and the pharmaceutical industry have been mobilized towards the search for an effective vaccine (Myshko 2017).
Although, the World Health Organization declared the Zika public health emergency over, research studies could help to prevent another outbreak. Accordingly, the research community is still working to address the many unanswered questions regarding ZIKV biology in order to develop effective preventative and therapeutic strategies against ZIKV.
Burrows NJ et al. (2016). A screen of FDA-approved drugs for inhibitors of Zika virus infection. Cell Host Microbe 20, 259-270.
Dowall SD et al. (2016). A susceptible mouse model for Zika virus infection. PLoS Negl Trop Dis 10, e0004658.
Grant A et al. (2016). Zika virus targets human STAT2 to inhibit type I interferon signaling. Cell Host Microbe 19, 882-890.
Manangeeswaran M et al. (2016). Zika (PRVABC59) infection is associated with T cell infiltration and neurodegeneration in CNS of immunocompetent neonatal C57Bl/6 mice. PLoS Pathog 12, e1006004.
Myshko D (2017). Update on Zika research. Pharma Voice. Accessed February 1, 2017.
Lazear HM et al. (2016). A mouse model of zika virus pathogenesis. Cell Host Microbe 19, 1-11.
Li H et al. (2016). Zika virus infects neural progenitors in the adult mouse brain and alters proliferation. Cell Stem Cell 19, 593-598.
Pardee K et al. (2016). Rapid, low-cost detection of Zika virus using programmable biomolecular components. Cell 165, 1-12.
Savidis G et al. (2016). Identification of Zika virus and dengue virus dependency factors using functional genomics. Cell Rep doi: http://dx.doi.org/10.1016/j.celrep.2016.06.028 [Epub ahead of print].