Unmasking the Molecular Mechanisms of Hepatitis B Virus Entry

An infection with the hepatitis B virus (HBV), which is spread by blood or bodily fluids, is one of the main causes of chronic liver diseases. The World Health Organization reports that there are 1.2 million new HBV infections worldwide each year. These infections, which are caused by HBV, only affect a few species, such as chimpanzees and humans.

Old-world monkeys are immune to HBV infections, even though they share a close evolutionary relationship with these animals.

Research led by Dr. Kaho Shionoya from the Tokyo University of Science, Dr. Jae-Hyun Park, Dr. Toru Ekimoto, Dr. Mitsunori Ikeguchi, and Dr. Sam-Yong Park from Yokohama City University, along with Dr. Norimichi Nomura from Kyoto University, worked together under the direction of Visiting Professor Koichi Watashi from the Tokyo University of Science, to discover the reason why monkeys are inherently resistant to HBV infection in a new study published in the journal Nature Communications.

Researchers used cryo-electron microscopy to determine the structure of the sodium taurocholate co-transporting polypeptide (NTCP), a membrane receptor present in macaque liver cells. HBV uses the preS1 region of the surface protein to attach itself to human NTCP.

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.”

Koichi Watashi, Visiting Professor, Tokyo University of Science

Professor Watashi and his colleagues compared the structures of hNTCP and mNTCP to identify variations in amino acid residues essential for HBV binding and entry into liver cells. This helped them comprehend this “interspecies barrier” against viral transmission.

Despite having 14 different amino acids, hNTCP and mNTCP have 96% amino acid homology. The large arginine side chain at position 158 in mNTCP, which inhibits deep preS1 insertion into the NTCP bile acid pocket, is a crucial distinction among these variations.

A smaller amino acid, such as glycine, which is present in hNTCP, is required for the virus to enter liver cells successfully.

It is interesting to note that arginine replaced glycine in mNTCP at a location that was far from the bile acid binding site.

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.”

Koichi Watashi, Visiting Professor, Tokyo University of Science

An amino acid at position 86 is also essential for maintaining NTCP's bound state with HBV's preS1 domain, according to additional laboratory tests and simulations. Macaques have asparagine, which helps them resist HBV, while non-susceptible species lack lysine at this location, which has a long side chain.

Additionally, the researchers observed that bile acids and HBV's preS1 competed for binding to NTCP, with the bile acid's long tail-chain structure preventing preS1 from binding.

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.”

Koichi Watashi, Visiting Professor, Tokyo University of Science

Since low- and middle-income nations account for the majority of HBV infections worldwide, the high expense of treatment presents both a medical emergency and a financial strain that affects entire communities.

This ground-breaking study represents a significant breakthrough in the understanding of viral interactions by illuminating how natural evolution has given some species defenses against this crippling illness. Researchers have created new therapeutic opportunities by figuring out the structure of mNTCP and identifying the amino acids that allow the virus to enter liver cells.

Moreover, the consequences go beyond HBV, providing important information about other viruses, such as SARS-CoV-2, and their capacity to transcend species boundaries. In addition to improving the knowledge of viral dynamics, this research is an essential resource in the continuous effort to anticipate and stop pandemics in the future.

These findings, which offer a route toward more fair access to therapies and a more robust defense against new viral threats, are crucial for the future of global health.