Characterization of simian virus 40 on its infectious entry pathway in cells using fluorescence correlation spectroscopy

Reticulon protects the integrity of the ER membrane during ER escape of large macromolecular protein complexes

Escape of large macromolecular complexes from the endoplasmic reticulum (ER), such as a viral particle or cellular aggregate, likely induces mechanical stress initiated on the luminal side of the ER membrane, which may threaten its integrity. How the ER responds to this threat remains unknown. Here we demonstrate that the cytosolic leaflet ER morphogenic protein reticulon (RTN) protects ER membrane integrity when polyomavirus SV40 escapes the ER to reach the cytosol en route to infection. SV40 coopts an intrinsic RTN function, as we also found that RTN prevents membrane damage during ER escape of a misfolded proinsulin aggregate destined for lysosomal degradation via ER-phagy. Our studies reveal that although ER membrane integrity may be threatened during ER escape of large macromolecular protein complexes, the action of RTN counters this, presumably by deploying its curvature-inducing activity to provide membrane flexibility and stability to limit mechanical stress imposed on the ER membrane.

Cre Recombinase Mediates the Removal of Bacterial Backbone to Efficiently Generate rSV40

Gene therapy has been shown to be a feasible approach to treat inherited disorders in vivo. Among the currently used viral vector systems, adeno-associated virus (AAV) vectors are the most advanced and have been applied in patients successfully. An important drawback of non-integrating AAV vectors is their loss of expression upon cell division, while repeating systemic administration lacks efficacy due to the induction of neutralizing antibodies. In addition, a significant percentage of the general population is not eligible for AAV-mediated gene therapy due to pre-existing immunity. Development of additional viral vectors may overcome this hurdle. Simian virus 40 (SV40)-derived vectors have been reported to transduce different tissues, including the liver, and prevalence of neutralizing antibodies in the general population is very low. This renders recombinant SV40 (rSV40) vector an interesting candidate for effective (re-)administration. Clinical use of SV40 vectors is in part hampered by less advanced production methods compared to AAVs. To optimize the production of rSV40 and make it better suitable for clinical practice, we developed a production system that relies on Cre recombinase-mediated removal of the bacterial plasmid backbone.

A Non-enveloped Virus Hijacks Host Disaggregation Machinery to Translocate across the Endoplasmic Reticulum Membrane

Mammalian cytosolic Hsp110 family, in concert with the Hsc70:J-protein complex, functions as a disaggregation machinery to rectify protein misfolding problems. Here we uncover a novel role of this machinery in driving membrane translocation during viral entry. The non-enveloped virus SV40 penetrates the endoplasmic reticulum (ER) membrane to reach the cytosol, a critical infection step. Combining biochemical, cell-based, and imaging approaches, we find that the Hsp110 family member Hsp105 associates with the ER membrane J-protein B14. Here Hsp105 cooperates with Hsc70 and extracts the membrane-penetrating SV40 into the cytosol, potentially by disassembling the membrane-embedded virus. Hence the energy provided by the Hsc70-dependent Hsp105 disaggregation machinery can be harnessed to catalyze a membrane translocation event.

How non-enveloped viruses penetrate a host membrane to enter cells and cause disease remains an enigmatic step. To infect cells, the non-enveloped SV40 must transport across the ER membrane to reach the cytosol. In this study, we report that a cellular Hsp105-powered disaggregation machinery pulls SV40 into the cytosol, likely by uncoating the ER membrane-penetrating virus. Because this disaggregation machinery is thought to clarify cellular aggregated proteins, we propose that the force generated by this machinery can also be hijacked by a non-enveloped virus to propel its entry into the host.

Intracellular trafficking pathway of BK Virus in human renal proximal tubular epithelial cells

Intracellular trafficking of BK Virus (BKV) in human renal proximal tubular epithelial cells (HRPTEC) is critical for BKV nephritis. However, the major trafficking components utilized by BKV remain unknown. Coincubation of HRPTEC with BKV and microtubule disrupting agents prevented BKV infection as detected by immunofluorescence and western blot analysis with antibodies which recognize BKV large T antigen. However, inhibition of a dynein, cellular motor protein, did not interfere with BKV infection in HRPTEC. A colocalization study of BKV with the markers of the endoplasmic reticulum (ER) and the Golgi apparatus (GA), indicated that BKV reached the ER from 6 to 10 h, while bypassing the GA or passing through the GA too transiently to be detected. This study contributes to the understanding of mechanisms of intracellular trafficking used by BKV in the infection of HRPTEC.

"Alternative" endocytic mechanisms exploited by pathogens: new avenues for therapeutic delivery?

Some pathogens utilize unique routes to enter cells that may evade the intracellular barriers encountered by the typical clathrin-mediated endocytic pathway. Retrograde transport and caveolar uptake are among the better characterized pathways, as alternatives to clathrin-mediated endocytosis, that are known to facilitate entry of pathogens and potential delivery agents. Recent characterization of the trafficking mechanisms of prion proteins and certain bacteria may present new paradigms for strategizing improvements in therapeutic spread and retention of therapy. This review will provide an overview of such endocytic pathways, and discuss current and future possibilities in using these routes as a means to improve therapeutic delivery.

Fluorescence correlation spectroscopy: novel variations of an established technique

Fluorescence correlation spectroscopy (FCS) is one of the major biophysical techniques used for unraveling molecular interactions in vitro and in vivo. It allows minimally invasive study of dynamic processes in biological specimens with extremely high temporal and spatial resolution. By recording and correlating the fluorescence fluctuations of single labeled molecules through the exciting laser beam, FCS gives information on molecular mobility and photophysical and photochemical reactions. By using dual-color fluorescence cross-correlation, highly specific binding studies can be performed. These have been extended to four reaction partners accessible by multicolor applications. Alternative detection schemes shift accessible time frames to slower processes (e.g., scanning FCS) or higher concentrations (e.g., TIR-FCS). Despite its long tradition, FCS is by no means dated. Rather, it has proven to be a highly versatile technique that can easily be adapted to solve specific biological questions, and it continues to find exciting applications in biology and medicine.

Ultrafiltration membrane for electrophoretic capturing of pathogens for AFM imaging

This communication describes a simple and rapid technique for electrophoretically assisted capture of phages, viruses, and other pathogens on the surface of an ultrafiltration membrane that can be considered smooth at the nanoscale. The surface was prepared by coating commercial dialysis membrane with a micrometer-thick layer of cross-linked dextran or globular proteins. To ensure strong adherence of the coating, the surface of the dialysis membrane was activated in cold plasma. It was shown that the root-mean-square roughness of the coating was well below 1 nm when the polymer solution used for coating was allowed to slowly dry through a dialysis membrane left in direct contact with mica. Relatively small viral particles (e.g., fd phages 0.7 microm long and only 3.5 nm high in the dry state) are readily visible by AFM following electrophoretic capture from suspensions containing as few as 1 x 10(6) particles/mL onto membranes prepared as described.

Viral entry, lipid rafts and caveosomes

Lipid rafts and caveolae are detergent-insoluble plasma membrane microdomains, involved in cellular endocytic processes and signalling. Several viruses, including a human pathogen, echovirus 1, and an extensively studied simian virus 40 utilize these domains for internalization into the host cells. Interaction of viruses with receptors on the cell surface triggers specific conformational changes of the virus particle and can give rise to signalling events, which determine the mechanisms of virus entry. After internalization via cell surface lipid rafts or caveolae, virus-containing vesicles can fuse with caveosomes, pre-existing cytoplasmic organelles, or dock on other intracellular organelles. These pathways may deliver viruses further to different cellular destinations, where the viral replication cycle then takes place. The information concerning the viral entry processes is important for understanding the details of the infections, for finding new targets for antiviral therapy and for elucidating the cellular internalization pathways in general.