Antiviral tamarindo?

Sugar-coated viruses

Glycans are complex biomolecules characterized by extensive branching of carbohydrates, and a variety of glycosylation patterns have been identified on viral surface proteins. Glycosylation is carried out by the host cell during viral biosynthesis and is critical for virus propagation and survival.

Being at the cell surface, these viral glycoproteins are often the sole component of the virus that interacts with the external environment. Thus, the majority of the interactions between viral pathogens with their hosts are influenced by the pattern of glycans and glycan-binding receptors that each express.

Because multiple enveloped viruses have surface glycoproteins that mediate attachment and fusion with their target cell membrane, blocking viral surface glycoprotein epitopes has been suggested as a powerful antiviral strategy. As a result, much effort has been directed towards developing antiviral agents that block viral surface glycoprotein epitopes.

Sugar-binding lectins as powerful antiviral agents

Naturally occurring molecules often outperform rationally-designed antimicrobial agents. It seems nature somehow knows better.

Lectins are carbohydrate-binding proteins that are highly specific in their recognition of sugar moieties. This specificity makes lectins incredibly useful, and they are involved in numerous recognition phenomena. This includes mediating the attachment and binding viruses to their intended targets.

Lectins are ubiquitous in nature and are found in many foods. Some lectins are beneficial, while others may be powerful toxins that need to be removed by cooking or fermentation, such as ricin from castor beans!

Given the importance of the glycans decorating the viral envelope, the sugar-binding lectins found in nature are recognized as a potential source of selective antivirals. Indeed, lectins have been shown to be able to target specific glycans present on the surface of the viral glycoprotein envelope, and several such lectins have been developed as antiviral therapeutics. Griffithsin (GRFT), an algae-derived lectin, binds terminal mannoses on the surface of HIV and is one of the most potent viral entry inhibitors in vitro. A plant lectin isolated from the fruit of bananas, named BanLec, was shown to have antiviral activity against HIV, hepatitis C virus, and influenza virus, all of which have high-mannose-type N-glycans on their surfaces.

Tamarind power: From ancient tradition to modern laboratory analyses

Ayurveda is a system of medicine with historical roots in the Indian subcontinent, but that might include traditions that have existed from the time of the Indus Valley Civilization or even earlier. Today, globalized and modernized practices derived from Ayurveda traditions form a type of alternative medicine.

Tamarind is a leguminous tree indigenous to tropical Africa that produces pod-like fruit filled with an edible pulp and is an important part of ayurvedic herbal medicine. Indeed, tamarind seeds have been mooted to have numerous health benefits, including as an antiviral. Do you see where this might be leading?

Given the long traditional use of tamarind and its apparent efficacy in treating various ailments and conditions, a team of Indian biologists set out to investigate which molecules might account for tamarind seeds’ beneficial effects.

Using a chromatography approach, the researchers isolated and purified a chitinase (chi)-like lectin (TCLL) from tamarind seeds. As one of the most abundant molecules found in the seeds, this lectin had been previously analyzed by X-ray diffraction and biochemical and biophysical approaches and shown to have an N-acetylglucosamine (NAG) binding property.

Given that the crystal structures of the E2-E1 proteins of Sindbis virus (SINV) and the E3-E2-E1 complex of chikungunya (CHIKV) have been shown to possess N-glycan sites, the authors hypothesized that the TCLL lectin found in tamarind seeds might be able to bind to the N-glycan rich surface of alphaviruses, thus blocking the virus’ entry into the cells.

TCLL interacts with the N-glycans of SINV and CHIKV envelope

First, using the maximal non-toxic dose of TCLL, the researchers tested the interaction between the lectin and the NAG sugar moieties found on SINV and CHIKV. Using an enzyme-linked immunosorbent assay the group were able to demonstrate TCLL binding to these viruses, and that this occurred through direct interaction with the viral glycans.

TCLL has a SINV and CHIKV antiviral effect

The antiviral activity of TCLL was then assessed using an assay where the amount of luciferase signal correlates with the virus infection. By this approach, it was shown that the CHKV and SINV treated with the TCLL protein had a reduced ability (by 64% and 55% respectively) to infect the host cells.

When this experiment was repeated by first pre-treating the TCLL with NAG, the reductions in CHIKV and SINV infectivity was just 14% and 18% respectively. Here it seems likely that the TCLL binding sites become saturated by the NAG pretreatment, thereby preventing TCLL interaction with the glycosylation sites on the envelopes of these two alphaviruses.

The researchers performed a further experiment to eliminate the possibility that the TCLL was binding to the cell surface receptors required for the virus entry – rather than to the glycosylation sites on the viral envelopes – but detected no TCLL-host cell interaction.

Overall, these data suggest that the TCLL molecule behaves much like an antibody that binds to the virus surface, thereby blocking the virus from binding to and infecting the host cells.

Reduction of SINV and CHIKV infectivity occur in a dose-dependent manner

As additional lines of evidence supporting the antiviral potential of TCLL, the authors performed dose dependency experiments, and report that a significant reduction in SINV and CHIKV infectivity could be achieved with 100 micromolar of TCLL incubated with the virus for just 30 min. The antiviral effect of the protein was also assessed by measuring the SINV and CHIKV RNA levels inside the infected cells. RNA levels tested in TCLL-treated cells were found to be significantly reduced relative to controls (by 37% and 45% respectively).

Based on these in vitro findings, TCLL has potential to someday contribute to fighting off a range of alphaviruses. However, much work (including in vivo analyses) remains to be done before the TCLL lectin can be confirmed as a bone fide antiviral agent.

The sugar code

Relative to the genetic code, the scientific community remains largely in the dark about the role of the sugar code. However, some in the field have proposed that the so-called sugar code could be as important – if not more important – as DNA. It is becoming increasingly clear that sugars, and the lectins that attach to them, have key roles in cell functioning, including immunity. Tamarind may have been used as a herbal remedy for thousands of years, but Kaur et al.’s work raises some very timely questions about the host-virus interactions.