Fig. 3

The interplay between canonical and extracellular vesicle-mediated HBV life cycle. A The schematic depicts the genome structure of HBV, encompassing four overlapping open reading frames (ORFs) that give rise to crucial components such as HBsAg (LHBs, MHBs, and SHBs), the nucleocapsid containing HBeAg (precore) and core antigens (HBcAg), polymerase (Pol) with reverse transcriptase and DNA polymerase activities, RNase activities, and the potentially hepatocarcinogenic X protein (HBx). B Canonically, in HBV-infected cells, the process begins with the binding of the virus to the NTCP receptor, followed by Rab5-medaited endocytosis and Rab7-dependent membrane fusion to facilitate virus uncoating. The relaxed circular HBV DNA (rcDNA) is then transported into the nucleus, where it undergoes conversion into covalently closed circular DNA (cccDNA). The cccDNA serves as a template for the generation of various HBV RNAs, including pregenomic RNA (pgRNA), precore/core mRNA, preS1/preS2/S mRNA, and HBx mRNA, transcribed from different promoters. These transcripts yield protein products such as precore/core, polymerase (Pol), LHBs, MHBs, SHBs, and HBx. The core protein assembles into an icosahedral capsid that encapsulates pgRNA associated with Pol. LHBs, MHBs, and SHBs are translated in the ER lumen and anchored into the ER membrane. These anchored membranes can form vesicles, fusing with endosomes or MVBs, enriching their membranes with LHBs, MHBs, and SHBs. The assembled HBV capsid can enter MVBs via ESCRT proteins like ALIX, VPS4B, and Rab33, overlapping with the ESCRT-dependent exosomal pathway. Host MVB functions are crucial for efficient HBV virion release after fusion with the plasma membrane. Exosomal HBV, a double-layer membrane structure, reportedly contains CD63, CD81, and HBsAg (LHBs, MHBs, and SHBs) on its surface. HBsAg is implicated in binding to the NTCP receptor, potentially facilitating entry via Clathrin/dynamin-mediated endocytosis. Exosomal HBV-containing endosomes may fuse with other endosomes to form MVBs. These MVBs then faces two possible fates: fusion with the lysosome to release the viral genome or re-secretion into the extracellular space as an exosome. The crosstalk between canonical and exosomal HBV replication and transmission involves neutral sphingomyelinase (nSMase), specifically membrane-associated nSMase 2, catalyzing the conversion of sphingomyelin to ceramide, leading to ceramide enrichment on the membrane composition of exosomal HBV. The question mark denotes an unknown or unclear process and molecular mechanism