The authors are grateful to Dr. Joshua Beckham at the University of Texas at Austin for technical support with molecular docking. This study was partly supported by funding from the Ministry of Science and Technology of Taiwan (MOST107-2320-B-037-002 to C.-J.L. and L.-T.L.; MOST106-2320-B-038-021 and MOST107-2320-B-038-034-MY3 to L.-T.L.).

NOTE: All cell culture and virus infections must be conducted in certified biosafety hoods that are appropriate for the biosafety level of the samples being handled. The two tannin-class of small molecules chebulagic acid (CHLA) and punicalagin (PUG), that were observed to efficiently block CVA16 infection9, are used as examples of candidate inhibitory agents. For basic principles in virology techniques, virus propagation, determination of virus titer, and concepts of plaque formi...

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T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hydrolyzable+tannins+(chebulagic+acid+and+punicalagin)+target+viral+glycoprotein-glycosaminoglycan+interactions+to+inhibit+herpes+simplex+virus+1+entry+and+cell-to-cell+spread.">Hydrolyzable tannins (chebulagic acid and punicalagin) target viral glycoprotein-glycosaminoglycan interactions to inhibit herpes simplex virus 1 entry and cell-to-cell spread.</a> Journal of Virology. 85, 4386-4398 (2011).</li><li>Lin, L. 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In this report, we described the protocols that are useful for the identification of antiviral candidates that target viral entry, in particular against the non-enveloped CVA16. The assays are designed in ways to dissect the early events during viral entry, which is helpful to clarify the mechanism(s) of action and potential target(s) of the test agents&#39; antiviral activity. The &#39;time-of-drug-addition assay&#39; permits to broadly determine the potential target of the test compounds, for instance the uninfected ho...

Hand, foot, and mouth disease (HFMD) is a disease most commonly caused by coxsackievirus A16 (CVA16) and enterovirus 71 (EV71) in young children. Recently across the Asia-Pacific region, there has been a significant uptick in CVA16-induced HFMD. While symptoms can be mild, severe complications can occur that affect the brain and the heart, with potential fatality1,2. At present, there are no licensed antiviral therapies or vaccinations available for CVA16, and thus there is a pressing need to develop antiviral strategies to curb future outbreaks and the associated complications.
CVA16 is a non-enveloped virus which has an icosahedral capsid assembled from pentamers that each contain 4 structural proteins namely VP1, VP2, VP3, and VP4. Encircling each five-fold axis in the pentamer is a &#39;canyon&#39; region that shows as a depression and is noted for its role in receptor binding3. At the bottom of this canyon lies a hydrophobic pocket in the VP1 region that contains a natural fatty ligand named sphingosine (SPH). Cellular receptors, such as human P selectin glycoprotein ligand 1 (PSGL-1) and scavenger receptor class B member 2 (SCARB2), have been suggested to play a role in viral binding by displacing this ligand which results in conformational changes to the capsid and the subsequent ejection of viral genome into the host cell4,5,6. Identifying possible inhibitors that block the successive events in the viral entry process could provide potential therapeutic strategies against CVA16 infection.
The steps in the virus life cycle can be dissected through experimental approaches as targets to help identify mode-specific antiviral agents. A time-of-drug-addition analysis examines the drug treatment effect at different times during the viral infection, including pre-entry (added prior to the virus infection), entry (added concurrent to the virus infection), and post-entry (added following the virus infection)7. The impact can be assessed using a standard plaque assay by quantitating the number of viral plaques formed in each of the treatment conditions. The flow cytometry-based viral binding assay determines if the drug prevents viral attachment to host cells. This is achieved by shifting the temperature from 37 &#176;C, at which the majority of human virus infections occur, to 4 &#176;C, where the virions are able to bind to the host cell surface but are unable to enter the cells7. The cell membrane-bound virus particles are then quantified through immunostaining against viral antigens and assessed by flow cytometry. The viral inactivation assay on the other hand helps to assess potential physical interactions of the drug with free virus particles, either shielding or neutralizing the virions, or causing aggregations or conformational changes that render them inactive for subsequent interactions with the host cell surface during the infection8,9. In this experiment, the viral inoculum is allowed to first incubate with the drug before being diluted to titrate out the drug prior to infecting the host cell monolayer and performing a standard plaque assay8. Finally, molecular docking is a powerful tool to predict potential drug interaction sites on the virion surface, including the viral glycoproteins from enveloped viruses and the viral capsid proteins from non-enveloped viruses, by using computational algorithms. This helps to mechanistically pinpoint targets of the drug&#39;s mode of action and provide useful information that can be further validated by downstream assays.
We recently employed the above described methods to identify antiviral compounds that efficiently blocked infection by the non-enveloped CVA169. Herein, the detailed protocols that were used are described and discussed.

<table border="0" cellpadding="0" cellspacing="0" style="width:824px;" width="824">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">4% Paraformaldehyde</td>
			<td style="width:205px;">Sigma</td>
			<td style="width:171px;">AL-158127-500G</td>
			<td style="width:232px;"></td>
		</tr>
		<tr height="41">
			<td height="41" style="height:41px;width:216px;">Alexa 488-conjugated anti-mouse IgG</td>
			<td style="width:205px;">Invitrogen</td>
			<td style="width:171px;">A11029</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Amphotericin B</td>
			<td style="width:205px;">GIBCO</td>
			<td style="width:171px;">15290-018</td>
			<td></td>
		</tr>
		<tr height="61">
			<td height="61" style="height:61px;width:216px;">Anti-VP1 antibody</td>
			<td style="width:205px;">Merck-Millipore</td>
			<td style="width:171px;">MAB979</td>
			<td style="width:232px;">Anti-Enterovirus 71 Antibody, cross-reacts with Coxsackie A16, clone 422-8D-4C-4D</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Beckman Coulter Cytometer</td>
			<td style="width:205px;">Beckman Coulter</td>
			<td style="width:171px;">FC500</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Corina</td>
			<td style="width:205px;">Molecular Networks GmbH</td>
			<td style="width:171px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Crystal violet</td>
			<td style="width:205px;">Sigma</td>
			<td style="width:171px;">C3886-100G</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">DMEM</td>
			<td style="width:205px;">GIBCO</td>
			<td style="width:171px;">11995-040</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">DMSO</td>
			<td style="width:205px;">Sigma</td>
			<td style="width:171px;">D5879</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">FBS</td>
			<td style="width:205px;">GIBCO</td>
			<td style="width:171px;">26140-079</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Formaldehyde</td>
			<td style="width:205px;">Sigma</td>
			<td style="width:171px;">F8775</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Graphpad Prism</td>
			<td style="width:205px;">GraphPad</td>
			<td style="width:171px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Heparin sodium salt</td>
			<td style="width:205px;">Sigma</td>
			<td style="width:171px;">H3393</td>
			<td></td>
		</tr>
		<tr height="41">
			<td height="41" style="height:41px;width:216px;">In vitro toxicology assay kit, XTT-based</td>
			<td style="width:205px;">Sigma</td>
			<td style="width:171px;">TOX2</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Methylcellulose</td>
			<td style="width:205px;">Sigma</td>
			<td style="width:171px;">M0512-100G</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">PBS pH 7.4</td>
			<td style="width:205px;">GIBCO</td>
			<td style="width:171px;">10010023</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Penicillin-Streptomycin</td>
			<td style="width:205px;">GIBCO</td>
			<td style="width:171px;">15070-063</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">PyMol</td>
			<td style="width:205px;">Schr&#246;dinger</td>
			<td style="width:171px;"></td>
			<td></td>
		</tr>
		<tr height="41">
			<td height="41" style="height:41px;width:216px;">UCSF Chimera</td>
			<td style="width:205px;">University of California, San Francisco</td>
			<td style="width:171px;"></td>
			<td></td>
		</tr>
	</tbody>
</table>

use of viral entry assays and molecular docking analysis for the identification of antiviral candidates against coxsackievirus a16

Antiviral assays that mechanistically examine viral entry are pertinent to discern at which step the evaluated agents are most effective, and allow for the identification of candidate viral entry inhibitors. Here, we present the experimental approaches for the identification of small molecules capable of blocking infection by the non-enveloped coxsackievirus A16 (CVA16) through targeting the virus particles or specific steps in early viral entry. Assays include the time-of-drug-addition analysis, flow cytometry-based viral binding assay, and viral inactivation assay. We also present a molecular docking protocol utilizing virus capsid proteins to predict potential residues targeted by the antiviral compounds. These assays should help in the identification of candidate antiviral agents that act on viral entry. Future directions can explore these possible inhibitors for further drug development.

The time-of-drug-addition assay is indicated in Figure 1 and shows the influence from treatment using the small molecules CHLA and PUG on CVA16 infection either pre-viral entry (pretreatment), during viral entry (co-addition), or post-viral entry (post-infection). Both small molecules only produced marginal impact against CVA16 infectivity whether in the pretreatment of the host cells prior to viral infection (Figure 1A) or in th...

Watch this Scientific Journal Video about Use of Viral Entry Assays and Molecular Docking Analysis for the Identification of Antiviral Candidates against Coxsackievirus A16 at JoVE.com

Use of Viral Entry Assays and Molecular Docking Analysis for the Identification of Antiviral Candidates against Coxsackievirus A16

The goal of the protocol is to illustrate the different assays relating to viral entry that can be used to identify candidate viral entry inhibitors.

Antiviral assays that mechanistically examine viral entry are pertinent to discern at which step the evaluated agents are most effective, and allow for ...

This protocol can aid in the discovery of potential antiviral small molecules through a mechanism-driven approach. The time of addition assay determines at which step of the infection the small molecule exhibits its antiviral activity. Molecular docking predicts the interaction between the small molecule and the viral proteins.To evaluate the influence of drugs on host cells prior to viral infection, seed RD cells in 12-well plates at a two times 10 to the fifth cells per well plating density. Incubate the cells at 37 degrees Celsius in a 5%carbon dioxide incubator overnight. The next morning, treat each well of the RD monolayer with a test compound of interest, in triplicate, at non-toxic concentrations in one milliliter of basal medium for one or four hours.At the end of the treatment incubation, wash the cells with one milliliter of PBS before adding 50 plaque-forming units of virus in 300 microliters of basal medium per well for one hour, with rocking every 15 minutes. At the end of the infection incubation, wash the cells with PBS and overlay the cells with one milliliter of fresh basal medium containing 0.8%methylcellulose. After 72 hours in the cell culture incubator, wash each well with two milliliters of PBS and fix the cells with 0.5 milliliters of 37%formaldehyde per well for 15 minutes.At the end of the fixation, wash the wells with PBS and stain the cells with 0.5 milliliters of 0.5%crystal violet solution per well. After two minutes, wash the wells with a gentle stream of water and allow the plate to air dry. Then place the plate on a white light box for counting and calculate the percent of coxsackievirus infected cells according to the formula.For molecular docking analysis, download 3D molecules of the test compounds from PubChem. If a molecule does not have a 3D structure uploaded, download the 2D structure or use the SMILE string sequence to transform the structure into a 3D molecule via an appropriate molecular program. Next, download a viral biological assembly unit from RCSB Protein Data Bank.Using an appropriate bio-computing program, delete the solvents from the Protein Data Bank file, replace the incomplete side chains using data from the Dunbrack 2010 rotamer library and add hydrogens and charges to the structure as previously reported. To dock the test compounds onto the prepared virus unit, upload the test compound file into University of California San Francisco Chimera as the ligand and select the whole prepared viral protein as the receptor to perform blind docking. For additional docking, confine the docking site onto the viral protein to regions of interest derived from the blind docking results by further reducing the search volume.Then upload the docking file into an appropriate molecular graphics system to analyze the binding mode positions. Select the ligand to find polar contacts from the compound to the viral protein, identifying the polar contacts with the to any atoms option. Both of the small molecules tested in this representative experiment, only produced a marginal impact against the coxsackievirus A16 infectivity, whether in the pretreatment of the host cells prior to viral infection or in the post-infection treatment.In contrast, the molecules efficiently abrogated the infection by greater than 80%in the co-addition treatment, suggesting that the two compounds are the most effective when they are concurrently present with the virus particles on the host cell surface during the infection. Flow cytometry based binding analysis confirms that the two tannins prevented coxsackievirus A16 infectivity entry by preventing viral particle binding to the host cells. Molecular docking of the tannins indicates that they are both predicted to bind in the canyon region of the coxsackievirus pentamer, just above the pocket entrance that holds the pocket factor and plays an important role for mediating coxsackievirus binding and entry into the host cell.In these surface projections, the unique residues predicted from the polar contacts of the small molecules around the pocket entrance can be observed, with asparagine-85, lysine-257, and asparagine-417 in common between the two tannins. For molecular docking, the topology of the viral protein needs to be taken into account when ranking the binding frames. Additional experiments could include testing the antiviral activity of the compound on recombinant viruses, with mutations on the amino acids discovered to validate their importance to the drug's efficacy.

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7.8K Views.  University of Texas at Austin. This protocol can aid in the discovery of potential antiviral small molecules through a mechanism-driven approach. The time of addition assay determines at which step of the infection the small molecule exhibits its antiviral activity. Molecular docking predicts the interaction between the small molecule and the viral proteins.To evaluate the influence of drugs on host cells prior to viral infection, seed RD cells in 12-well plates at a two times 10 to the fifth cells per well plating ...