The long and winding road: human papillomavirus entry and subcellular trafficking

Abstract

Human papillomaviruses (HPVs) infect and replicate in differentiating mucosal and cutaneous epithelium. Most HPV infections are asymptomatic or cause transient benign neoplasia. However, persistent infections by oncogenic HPV types can progress to cancer. During infectious entry into host keratinocytes, HPV particles interact with many host proteins, beginning with major capsid protein L1 binding to cellular heparan sulfate and a series of enzymatic capsid modifications that promote infectious cellular entry. After utilizing the endosomal pathway to uncoat the viral genome (vDNA), the minor capsid protein L2/vDNA complex is retrograde trafficked to the Golgi, and thereafter, to the nucleus where viral transcription initiates. Post-Golgi trafficking is dependent on mitosis, with L2-dependent tethering of vDNA to mitotic chromosomes before accumulation at nuclear substructures in G1. This review summarizes the current knowledge of the HPV entry pathway, the role of cellular proteins in this process, and notes many gaps in our understanding.

Figure 1.. Extracellular interactions of HPV on human keratinocytes.

(A–C) The context of HS and HSPG molecules on cells may ordain the fate of an interacting HPV virion and help account for asynchronous uptake. (A) HPV particles can bind directly to laminin 322 (LN332) or to shed HS and/or Sdc HSPG ectodomains attached to LN332 in the ECM; there, they may remain inert until released by enzymes (e.g., heparanase) and/or transferred to active filopodia for transport to the cell body. (B) Attachment to plasma membrane-resident Sdc HSPGs await ectodomain shedding by MMPs and ADAMs or HS processing by heparinase to release soluble HPV/HS/GF complexes for transfer to uptake receptor platforms. (C) LN332 interacts with Sdc1, CD151 tetraspannin and α6β4 integrin on the basal cell to provide cell anchorage to the ECM/basement membrane, termed the hemidesmosome. Sdc-1 and Sdc-4 capture EGFR family members and integrins via binding sites in their ectodomains and thereby, may provide rapid signaling and recruitment of uptake platforms for some incoming virions. (D) The HPV entry receptor complex comprises enlarged TEMs with CD151 and EGFR, and potentially CD63, integrins, and A2t. Currently, potential roles of Sdc, HS and GF during entry are unclear. The cytosolic obscurin-like 1 protein (OBSC1) adaptor colocalizes with HPV PsV and CD151, potentially linking the complex to the actin cytoskeleton for endocytosis. Created with BioRender.com.

Figure 2.. Endosomal, retrograde, and mitosis-dependent nuclear trafficking of HPV.

(A) Endocytosed virions, bound to the entry receptor complex recruit cytosolic p120-catenin (B) to facilitate interaction with γ-secretase. Partial uncoating of the capsid enables exposure of L2 for (C) interaction with γ-secretase resulting in initial L2 membrane insertion. Furin cleavage of L2 and the C-terminal polybasic CPP are also required for this step. Binding of partially cytosolic L2 to the trimeric retromer (D) stabilizes endosomal L2 membrane protrusion, and also promotes (E) interaction of L2-retromer with TBC1D5 and activated Rab7-GTP, resulting in GTP hydrolysis, Rab7 cycling, and disassembly of the L2-retromer complex. Further uncoating and disassembly of the L1 capsid likely frees L2/vDNA (in complex with some L1 pentamers) to partition away from degradative late endosomal and lysosomal compartments. (F) Collectively these events lead to retrograde sorting/transport of the subviral L2/vDNA/L1 complex to the Golgi. Upon the onset of mitosis (G) L2 recruits dynein motors to these post-Golgi vesicles via RanBP10, KPNA2, and DYNLT3, for minus-end transport along the spindle microtubules. Presumably a motor-switch occurs for (H) plus-end kinesin-mediated transport of vesicle-bound L2/vDNA toward the mitotic chromosomes. (I) L2-dependent tethering of vesicular L2/vDNA/L1 ensures nuclear entry and infection of daughter cells. Created with BioRender.com.