Host functions used by hepatitis B virus to complete its life cycle: Implications for developing host-targeting agents to treat chronic hepatitis B

Abstract

Similar to other mammalian viruses, the life cycle of hepatitis B virus (HBV) is heavily dependent upon and regulated by cellular (host) functions. These cellular functions can be generally placed in to two categories: (a) intrinsic host restriction factors and innate defenses, which must be evaded or repressed by the virus; and (b) gene products that provide functions necessary for the virus to complete its life cycle. Some of these functions may apply to all viruses, but some may be specific to HBV. In certain cases, the virus may depend upon the host function much more than does the host itself. Knowing which host functions regulate the different steps of a virus' life cycle, can lead to new antiviral targets and help in developing novel treatment strategies, in addition to improving a fundamental understanding of viral pathogenesis. Therefore, in this review we will discuss known host factors which influence key steps of HBV life cycle, and further elucidate therapeutic interventions targeting host-HBV interactions.

Keywords: Antiviral agents; Direct acting antiviral agents; Hepatitis; Hepatitis B virus (HBV); Host targeting agents; Liver; Virus-host interaction.

Fig. 1.. Binding to and entry in to hepatocytes.

The hepatitis B virion is shown in (A) making an initial “docking” on to heparin sulfate proteoglycans (HSPG), followed by (B) “rolling” to Sodium Taurocholate Polypeptide (NTCP) receptors, which are believed to be the higher affinity receptor for the virus. Cellular functions mediating these steps are indicated in orange, research phase compounds that interfere with these steps are shown in pinkish red, with compounds that are clinical phase, or approved, in light blue. This scheme was illustrated by using the Biology Bundle of Motifolio Drawing Toolkits (www.motifolio.com) (the same as following figures). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 2.. Transport of the virus nucleocapsid to the nucleus.

Following entry in to the cell, the virus is de-enveloped and the nucleocapsid is transported to the nucleus. Cellular functions mediating these steps are indicated in orange, research phase compounds that interfere with these steps are shown in pinkish red, with compounds that are clinical phase, or approved, in light blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3.. Deproteination of the viral DNA, and synthesis of viral cccDNA.

The viral polymerase protein within the capsid is covalently attached to the 5′ end of “minus” strand of DNA, and must be removed for the successful production of covalently closed circular DNA (cccDNA), which is the template for all viral transcripts in the normal infection (shown as the 3 interlocked circles). The host functions mediating this process are not definitively known, but TDP2 may be involved. Host polymerase kappa (polK), FEN1, and DNA ligase (LIG) 1 and 3 are needed to complete the formation of cccDNA. Topoisomerases and various DNA repair functions may be involved. Cellular functions mediating these steps are indicated in orange, research phase compounds that interfere with these steps are shown in pinkish red, with compounds that are clinical phase, or approved, in light blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4.. Transcription of cccDNA and transport of viral RNA out of the nucleus.

HBV cccDNA is associated with, and presumably regulated by numerous host transcriptional factors, and unmodified and modified histones. Host SMC5/6 complex is shown, in this illustration, “attacking” (and transcriptionally repressing) HBV cccDNA, with HBx binding host protein DBB1 which, causes the degradation of SMC5/6, and thus promotes HBV cccDNA transcription. Host pol II transcribes HBV cccDNA as well as HBV DNA integrated in to the host chromosomes, and these transcripts are processed and transported out of the nucleus using numerous host functions, including polyadenylation, although the precise pathways of processing are not well established. Cellular functions mediating these steps are indicated in orange, research phase compounds that interfere with these steps are shown in pinkish red, with compounds that are clinical phase, or approved, in light blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 5.. HBV RNA translation, capsid formation, encapsidation, reverse transcription and recycling of the nucleocapsid to the nucleus.

Five main species of HBV mRNA are translated in the cytoplasm in to the viral polypeptides precore (precursor of HBeAg, from the 3.5 kb precore mRNA), core and pol (from the 3.5 kb pgRNA), envelope polypeptides L (from the 2.4 kb mRNA), envelope polypeptide, M and S (from the 2.1 kb mRNA) and X from the 0.7 kb mRNA. Core rapidly dimerizes, is modified by phosphorylations, and cooperates with pol and pgRNA to form the nucleocapsid. Host factors modulate translation, phosphorylation, and even folding through chaperons of the viral functions. Encapsidated viral pgRNA is reverse transcribed, and nucleocapsids are either enveloped and secreted (see Fig. 6), or not enveloped and recycled to the nucleus, where they may then begin the cycle of cccDNA production, and transcription of new viral gene products. Cellular functions mediating these steps are indicated in orange, research phase compounds that interfere with these steps are shown in pinkish red, with compounds that are clinical phase, or approved, in light blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 6.. Morphogenesis, envelopment and secretion of HBV.

Nucleocapsids associated with newly synthesized, envelope polypeptides, S, M, and L, which would have been co- and post-translationally modified by N-glycosylations and glycan processing (L, M, S) and O-glycosylation (M), and myristolyation (L). M, and possibly L, are “folded” by Calnexin, and BiP. L, M and S are all heavily disulfide bonded, are use Protein Disulphide Isomerase (PDI) is their maturation. Numerous cell functions mediate vesicle transport. Subviral particles of oligomerized S bud in to the lumen of the ER-Golgi and are secreted out of the cell via constitutive secretory mechanisms. Envelopment of nucleocapsids, to form virions, probably occurs at the ER and then a Multi Vesicular Body (MVP). MVP derived exosomes containing enveloped virions are thought to fuse with plasma membranes and result in virion release in to the blood. Note that this occurs in a “polarized” hepatocyte, in which secretion of virions is through the basolateral side of the cell. Cellular functions mediating these steps are indicated in orange, research phase compounds that interfere with these steps are shown in pinkish red, with compounds that are clinical phase, or approved, in light blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)