Currently available antiviral drugs for the treatment of coronavirus disease 2019 (COVID-19) are neither broadly accepted nor highly effective. Therefore, there is a pressing need to find biomolecules and synthetic compounds exhibiting antiviral properties for effective COVID-19 treatment.
Study: Fighting SARS-CoV-2 with green seaweed Ulva sp. extract: extraction protocol predetermines crude ulvan extract anti-SARS-CoV-2 inhibition properties in in vitro Vero-E6 cells assay. Image Credit: Chokniti Khongchum / Shutterstock.com
Macroalgae, or seaweed, which are marine multicellular organisms encompassing several thousand species, are previously known to exhibit antiviral properties. Several studies have suggested that seaweed sulfated polysaccharides (SSPS), located in the cell wall of seaweeds, play a vital role in their defense against pathogens and other opportunistic organisms.
The effectiveness of inhibitory effects of SSPS on viral replication under in vitro conditions has also been known for more than 60 years. Subsequently, they are effective against several enveloped ribonucleic acid (RNA) viruses, including the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
The antiviral properties exhibited by SSPS, a polysaccharide, against SARS-CoV-2 partially correlate with the presence of sulfonate groups and partly with its structural diversity and high charge density of its sulfated polymer. All these structural and functional aspects help SSPS efficiently bind to the proteins located on the surface of viruses like SARS-COV-2.
About the study
The authors of a recently published study on the Peerj* preprint server investigated the antiviral activities of SSPS extracted from a cultivated green seaweed Ulva sp. to demonstrate the anti-SARS-CoV-2 activity of ulvans. The extraction was carried out with the help of an in vitro cytopathic effect (CPE) reduction assay using Vero E6 cells. These cells express the angiotensin-converting enzyme 2 (ACE2) receptor, to which the SARS-CoV-2 virus typically binds.
The researchers used two distinct extraction protocols including hydrochloric acid (HCl) and ammonium oxalate (AOx) extraction protocols. The extraction protocols changed the ulvan extracts qualitatively and quantitatively.
Subsequently, the extracted ulvans had different purity, molecular weight (MW) distribution, sulfate content, and bioactivity. The team analyzed the composition of extracts using infrared spectroscopy, carbon, hydrogen, nitrogen, and sulfur (CHNS) elemental analysis, as well as size exclusion (SE) and anion exchange (AE) chromatography.
Differential antiviral activity by SSPS
The researchers used several advanced analytical methodologies such as Fourier-transform infrared (FT-IR) spectroscopy analyses, SE chromatography, and AE chromatography to analyze crude Ulva sp. extracts. They found that the AOx and HCl extraction protocols of Ulva sp. resulted in mixtures of compounds having a different distribution of MW, overall molecular charges, and nitrogen-containing contaminating molecules. Both the extracts exhibited slightly different antiviral activity, thereby indicating variability in the biological activity of SSPS compounds obtained through two different extraction protocols.
The AOx extracts showed positive in vitro antiviral activity, as demonstrated by their ability to protect VERO E6 cells against the cytopathic effect of the SARS-CoV-2 as a result of a higher charge density on SSPS biopolymers, higher average MWs compared to HCl-based extracts and the flexibility of the polysaccharide backbone.
Further, these results indicated that only a specific fraction of this AOx-based extract is active against SARS-CoV-2. To this end, this fraction exhibited an 11.3-fold difference in its maximum inhibition activity against SARS-CoV-2 as compared to the HCl-based extracts. Furthermore, these AOx-based extracts, compared to the HCl-based extracts, exhibited a higher level of alkaloids, peptides, proteins, and nucleic acids fragments.
These nitrogen-containing contaminating compounds could be responsible for the difference in the antiviral activity of the two extracts. Interestingly, lectins, co-extracted with the SSPS from the Ulva sp., could also be used to exhibit antiviral activities, including against SARS-CoV-2.
Future challenges and conclusion
This current study is just a starting point, as it would be challenging to develop thorough bioassay-guided fractionation approaches for the isolation and precise identification of the active components in AOx-based SSPS extracts in the future. Also, future in vivo studies will have to address the challenge of delivering the active fractions of Ulva sp. crude extract.
Despite global vaccination campaigns, it is crucial to find new antiviral candidates against SARS-CoV-2 infection for two reasons. One, SARS-CoV-2 is continuously producing new mutant strains that exhibit resistance to both vaccine-based immunity and known antivirals. Second, the vast antiviral activity of certain SSPS presents an opportunity to develop broadly active and highly potent antivirals with high specific activity against certain viruses, including SARS-CoV-2.
The current study provides experimental evidence that Ulva sp. could be a promising antiviral as a source of SSPS, as its crude extract could direct the development of natural product-based therapeutic agents against SARS-CoV-2.
peerj publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
- Shefer, S., Robin, A., Chemodanov, A., et al. (2021). Fighting SARS-CoV-2 with green seaweed Ulva sp. extract: extraction protocol predetermines crude ulvan extract anti-SARS-CoV-2 inhibition properties in in vitro Vero-E6 cells assay. Peer Journal 9(e12398). doi:10.7717/peerj.12398. https://peerj.com/articles/12398/.