The viral puzzle of why humans are susceptible to hepatitis B virus, but monkeys are not

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 stabilizing 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."

In a world where the majority of HBV infections are concentrated in low- and middle-income countries, the high costs of treatment pose not only a healthcare crisis but also an economic burden that ripples through societies. This groundbreaking study sheds light on how natural evolution has equipped certain species with defenses against this debilitating disease, marking a pivotal advancement in our understanding of viral interactions. By unraveling 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.