Hepatitis B virus (HBV) infection is a leading cause of chronic liver diseases, that spreads among individuals through blood or body fluids. According to the World Health Organization, globally 1.2 million new HBV infections are reported every year – most in low- and middle-income countries. HBV infections are limited to a few species, including humans and chimpanzees. Despite their close evolutionary relationship with these animals, old-world monkeys are not susceptible to HBV infections.
In a new study published in Nature Communications, scientists led by Visiting Professor Koichi Watashi from the Tokyo University of Science to uncover why monkeys are naturally resistant to HBV infection.
Using cryo-electron microscopy, scientists solved the structure of a membrane receptor found in liver cells called the sodium taurocholate co-transporting polypeptide (NTCP) in macaques. HBV binds to human NTCP using its preS1 region in the surface protein. Prof Watashi explains, “We identified a binding mode for NTCP-preS1 where two functional sites are involved in human NTCP (hNTCP). In contrast, macaque NTCP (mNTCP) loses both binding functions due to steric hindrance and instability in the preS1 binding state.”
To understand this ‘interspecies barrier’ against viral transmission, Prof Watashi and his team compared the structures of hNTCP and mNTCP, identifying differences in amino acid residues critical for HBV binding and entry into liver cells. hNTCP and mNTCP share 96% amino acid homology, with 14 amino acids distinct between the two receptors. A key distinction among these differences is the bulky side chain of arginine at position 158 in mNTCP, which prevents deep preS1 insertion into the NTCP bile acid pocket. For successful viral entry into liver cells, a smaller amino acid like glycine, as found in hNTCP, is necessary.
Interestingly, the substitution of Glycine by Arginine in mNTCP was at a position far away from the binding site for bile acid. Prof Watashi adds, “These animals probably evolved to acquire escape mechanisms from HBV infections without altering their bile acid transport capacity. Consistently, phylogenetic analysis showed strong positive selection at position 158 of NTCP, probably due to pressure from HBV. Such molecular evolution driven to escape virus infection has been reported for other virus receptors.” Further lab experiments and simulations revealed that an amino acid at position 86 is also critical for stabilising NTCP’s bound state with HBV’s preS1 domain. Non-susceptible species lack lysine at this position, which has a large side chain; macaques instead have asparagine, which contributes to HBV resistance.
The researchers also noted that bile acids and HBV’s preS1 competed to bind to NTCP, where the long tail-chain structure of the bile acid inhibited the binding of preS1. Commenting on these findings, Prof Watashi stated, “Bile acids with long conjugated chains exhibited anti-HBV potency. Development of bile acid-based anti-HBV compounds is underway and our results will be useful for the design of such anti-HBV entry inhibitors.”
By unravelling the structure of mNTCP and pinpointing the amino acids that facilitate viral entry into liver cells, researchers have opened the door to new therapeutic avenues. Furthermore, the implications extend beyond HBV, offering critical insights into other viruses, including SARS-CoV-2, and their potential to cross species barriers. This research not only enhances our understanding of viral dynamics but also serves as a crucial tool in the ongoing quest to predict and prevent future pandemics.
The future of global health hinges on these revelations, promising a path toward more equitable access to treatments and a stronger defence against emerging viral threats.
Source: Tokyo University of Science
