• Introduction: The sodio-taurocholic acid cotransport polypeptide (NTCP) is a 9-transmembrane domain protein. NTCP is mainly expressed in the basement membrane side of hepatocytes (blood sinus side), which promotes the absorption of bile acid in portal vein blood into the liver, and is associated with cholestasis, liver fibrosis, and nonalcoholic fatty liver disease. In addition, it is also a functional receptor for human hepatitis B virus (HBV) and hepatitis D virus (HDV). The two marketed drugs targeted are synthetic polypeptides of HBV Pre-S1 structure, which can competitively bind to NTCP to block the entry of hepatitis virus and bile acids into liver cells. There are also oligonucleotide drugs that target NTCP, and ADAR-based RNA editing introduces NTCP nonsense mutations to relieve cholestasis.
• Targeting strategy: The exons 1-6 of mouse Ntcp gene that encode the whole molecule (ATG to STOP codon), including 3’UTR were replaced by human counterparts in B-hNTCP mice. The promoter and 5’UTR region of the mouse gene are also replaced. The human NTCP expression is driven by human NTCP promoter, while mouse Ntcp gene transcription and translation will be disrupted.
• mRNA expression analysis: Mouse Ntcp mRNA was only detectable in wild-type mice. Human NTCP mRNA was exclusively detectable in homozygous B-hNTCP mice but not in wild-type mice.

Gene targeting strategy for B-hNTCP mice. The exons 1-6 of mouse Ntcp gene that encode the whole molecule (ATG to STOP codon), including 3’UTR were replaced by human counterparts in B-hNTCP mice. The promoter and 5’UTR region of the mouse gene are also replaced. The human NTCP expression is driven by human NTCP promoter, while mouse Ntcp gene transcription and translation will be disrupted.

Strain specific analysis of NTCP mRNA expression in wild-type C57BL/6JNifdc and B-hNTCP mice by RT-PCR. Liver RNA were isolated from wild-type C57BL/6JNifdc (+/+) and homozygous B-hNTCP mice (H/H), then cDNA libraries were synthesized by reverse transcription, followed by PCR with mouse or human NTCP primers. Mouse Ntcp mRNA was only detectable in wild-type mice. Human NTCP mRNA was exclusively detectable in homozygous B-hNTCP mice but not in wild-type mice. Human sequences were confirmed via Sanger Sequencing.

The inhibitory efficiency of the NTCP-targeted small nucleic acid drug in homozygous B-hNTCP mice. B-hNTCP mice were randomly divided into 2 groups (n=5, 12 weeks old, male). The human NTCP-targeted nucleic acid drug (synthesized according to patents) and saline were administered to the mice individually. The nucleic acid drug was administered in the form of saline aqueous solution. The mice were sacrificed on day 14. (A) The schematic diagram of experimental processing. (B) The changes in NTCP mRNA expression levels in liver tissue on day 14 after administration, compared to the control group. Values are expressed as mean ± SEM.