HPV caught in the tetraspanin web?

Tetraspanins are master organizers of the cell membrane. Recent evidence suggests that tetraspanins themselves may become crowded by virus particles and that these crowds/aggregates co-internalize with the viral particles. Using microscopy, we studied human papillomavirus (HPV) type 16-dependent aggregates on the cell surface of tetraspanin overexpressing keratinocytes. We find that aggregates are (1) rich in at least two different tetraspanins, (2) three-dimensional architectures extending up to several micrometers into the cell, and (3) decorated intracellularly by filamentous actin. Moreover, in cells not overexpressing tetraspanins, we note that obscurin-like protein 1 (OBSL1), which is thought to be a cytoskeletal adaptor, associates with filamentous actin. We speculate that HPV contact with the cell membrane could trigger the formation of a large tetraspanin web. This web may couple the virus contact site to the intracellular endocytic actin machinery, possibly involving the cytoskeletal adaptor protein OBSL1. Functionally, such a tetraspanin web could serve as a virus entry platform, which is co-internalized with the virus particle.

The mesoscale organization of syntaxin 1A and SNAP25 is determined by SNARE-SNARE interactions.

SNARE proteins have been described as the effectors of fusion events in the secretory pathway more than two decades ago. The strong interactions between SNARE domains are clearly important in membrane fusion, but it is unclear whether they are involved in any other cellular processes. Here, we analyzed two classical SNARE proteins, syntaxin 1A and SNAP25. Although they are supposed to be engaged in tight complexes, we surprisingly find them largely segregated in the plasma membrane. Syntaxin 1A only occupies a small fraction of the plasma membrane area. Yet, we find it is able to redistribute the far more abundant SNAP25 on the mesoscale by gathering crowds of SNAP25 molecules onto syntaxin clusters in a SNARE-domain-dependent manner. Our data suggest that SNARE domain interactions are not only involved in driving membrane fusion on the nanoscale, but also play an important role in controlling the general organization of proteins on the mesoscale. Further, we propose these mechanisms preserve active syntaxin 1A-SNAP25 complexes at the plasma membrane.

Anatomy of a viral entry platform differentially functionalized by integrins α3 and α6.

During cell invasion, human papillomaviruses use large CD151 patches on the cell surface. Here, we studied whether these patches are defined architectures with features for virus binding and/or internalization. Super-resolution microscopy reveals that the patches are assemblies of closely associated nanoclusters of CD151, integrin α3 and integrin α6. Integrin α6 is required for virus attachment and integrin α3 for endocytosis. We propose that CD151 organizes viral entry platforms with different types of integrin clusters for different functionalities. Since numerous viruses use tetraspanin patches, we speculate that this building principle is a blueprint for cell-surface architectures utilized by viral particles.

ADAM17-dependent signaling is required for oncogenic human papillomavirus entry platform assembly.

Oncogenic human papillomaviruses (HPV) are small DNA viruses that infect keratinocytes. After HPV binding to cell surface receptors, a cascade of molecular interactions mediates the infectious cellular internalization of virus particles. Aside from the virus itself, important molecular players involved in virus entry include the tetraspanin CD151 and the epidermal growth factor receptor (EGFR). To date, it is unknown how these components are coordinated in space and time. Here, we studied plasma membrane dynamics of CD151 and EGFR and the HPV16 capsid during the early phase of infection. We find that the proteinase ADAM17 activates the extracellular signal-regulated kinases (ERK1/2) pathway by the shedding of growth factors which triggers the formation of an endocytic entry platform. Infectious endocytic entry platforms carrying virus particles consist of two-fold larger CD151 domains containing the EGFR. Our finding clearly dissects initial virus binding from ADAM17-dependent assembly of a HPV/CD151/EGFR entry platform.

The packing density of a supramolecular membrane protein cluster is controlled by cytoplasmic interactions.

Molecule clustering is an important mechanism underlying cellular self-organization. In the cell membrane, a variety of fundamentally different mechanisms drive membrane protein clustering into nanometre-sized assemblies. To date, it is unknown whether this clustering process can be dissected into steps differentially regulated by independent mechanisms. Using clustered syntaxin molecules as an example, we study the influence of a cytoplasmic protein domain on the clustering behaviour. Analysing protein mobility, cluster size and accessibility to myc-epitopes we show that forces acting on the transmembrane segment produce loose clusters, while cytoplasmic protein interactions mediate a tightly packed state. We conclude that the data identify a hierarchy in membrane protein clustering likely being a paradigm for many cellular self-organization processes.

Seroprevalence of arboviruses among blood donors in French Polynesia, 2011-2013.

OBJECTIVES: French Polynesia is a high epidemic/endemic area for arthropod-borne viruses (arboviruses). We recently reported the silent circulation of Ross River virus and absence of active transmission of chikungunya virus (CHIKV) among blood donors sampled before the emergence of Zika virus (ZIKV) and CHIKV in French Polynesia. In this study, the prevalence of the four serotypes of dengue virus (DENV) and the occurrence of circulation of other arboviruses were investigated in blood donors in French Polynesia. METHODS: Serum samples from 593 blood donors collected between July 2011 and October 2013 were tested by ELISA for the presence of immunoglobulin G antibodies against each of the four DENV serotypes, ZIKV, Japanese encephalitis virus (JEV), and West Nile virus (WNV). RESULTS: It was found that 80.3%, 0.8%, 1.3%, and 1.5% of blood donors were seropositive for at least one DENV serotype, ZIKV, JEV, and WNV, respectively. CONCLUSIONS: These results corroborate the expected high transmission of DENV and conversely suggest that no active circulation of ZIKV, JEV, and WNV occurred in French Polynesia before 2011. Information provided by this study may be useful for public health authorities to improve surveillance and implement strategies to prevent the transmission of arboviruses.

Silent Circulation of Ross River Virus in French Polynesia.

OBJECTIVES: Ross River is an emerging mosquito-borne disease in the Western Pacific. Ross River virus (RRV) circulation has been sporadically reported in some Pacific Island Countries and Territories but never in French Polynesia. To determine if RRV has circulated locally among the French Polynesian population, we conducted a seroprevalence study on blood donors. METHODS: Sera of 593 blood donors were collected from July 2011 to October 2013 and tested by ELISA for the presence of RRV-specific Immunoglobulin G (IgG) antibodies. RESULTS: A total of 204 (34.40%) blood donors were found seropositive for RRV. Among the 132 blood donors that were born in French Polynesia and had never travelled abroad, 56 (42.42%) had RRV-specific IgGs. DISCUSSION: Our results support the existence of autochthonous RRV transmission and suggest that this pathogen has silently circulated in French Polynesia. These findings raise the question of possible undetected circulation of RRV in other Pacific Island Countries and Territories.