Folding of viral glycoproteins in the endoplasmic reticulum

Competition for calnexin binding regulates secretion and turnover of misfolded GPI-anchored proteins

In mammalian cells, misfolded glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) are cleared out of the ER to the Golgi via a constitutive and a stress-inducible pathway called RESET. From the Golgi, misfolded GPI-APs transiently access the cell surface prior to rapid internalization for lysosomal degradation. What regulates the release of misfolded GPI-APs for RESET during steady-state conditions and how this release is accelerated during ER stress is unknown. Using mutants of prion protein or CD59 as model misfolded GPI-APs, we demonstrate that inducing calnexin degradation or upregulating calnexin-binding glycoprotein expression triggers the release of misfolded GPI-APs for RESET. Conversely, blocking protein synthesis dramatically inhibits the dissociation of misfolded GPI-APs from calnexin and subsequent turnover. We demonstrate an inverse correlation between newly synthesized calnexin substrates and RESET substrates that coimmunoprecipitate with calnexin. These findings implicate competition by newly synthesized substrates for association with calnexin as a key factor in regulating the release of misfolded GPI-APs from calnexin for turnover via the RESET pathway.

Protective role of PERK-eIF2α-ATF4 pathway in chronic renal failure induced injury of rat hippocampal neurons

PURPOSE

Chronic renal failure (CRF) is associated with impairment of hippocampal neurons. This study investigated the effect of PERK-eIF2α-ATF4 pathway in CRF.

METHODS

Rat CRF model was established and rat hippocampal neurons were separated. Xanthine Oxidase method, fluorescence spectrophotometry and flow cytometry were applied to detect superoxide dismutase (SOD) content, reactive oxygen species (ROS) level and apoptosis in hippocampal neurons, respectively. The levels of phosphorylated (p)-PERK, phosphorylated (p)-eIF2α, CHOP, Bax, C-Caspase-3 and Bcl-2 in rats were measured using Western blot. Then, the neurotoxicity of serum from CRF rats was assessed in rat hippocampal neurons after treatment with rat CRF serum and transfection with or without PERK overexpression or knockdown plasmid.

RESULTS

SOD activity was reduced, while ROS level and apoptosis rate were increased in hippocampal tissues of CRF rats. PERK-eIF2α-ATF4 and apoptosis pathways were activated in CRF rats. Cells treated with serum from CRF rats showed increases in apoptosis rate and LDH and ROS levels, and decreases in cell viability and SOD activity. However, overexpressed PERK could reverse the cytotoxic effect of serum from CRF rats. PERK overexpression could enhance the activation of PERK-eIF2α-ATF4 pathway in hippocampal neurons induced by serum from CRF rats. Furthermore, PERK overexpression could alleviate the increases in CHOP, Bax, C-Caspase-3 expressions and the reduction of Bcl-2 expression in hippocampal neurons induced by serum from CRF rats.

CONCLUSION

PERK-eIF2α-ATF4 pathway induced by increased endoplasmic reticulum stress may alleviate CRF-induced hippocampal neuronal damage.

Human endogenous retrovirus HERV-K(HML-2) encodes a stable signal peptide with biological properties distinct from Rec

Background

The human endogenous retrovirus HERV-K(HML-2) family is associated with testicular germ cell tumors (GCT). Various HML-2 proviruses encode viral proteins such as Env and Rec.

Results

We describe here that HML-2 Env gives rise to a 13 kDa signal peptide (SP) that harbors a different C-terminus compared to Rec. Subsequent to guiding Env to the endoplasmatic reticulum (ER), HML-2 SP is released into the cytosol. Biochemical analysis and confocal microscopy demonstrated that similar to Rec, SP efficiently translocates to the granular component of nucleoli. Unlike Rec, SP does not shuttle between nucleus and cytoplasm. SP is less stable than Rec as it is subjected to proteasomal degradation. Moreover, SP lacks export activity towards HML-2 genomic RNA, the main function of Rec in the original viral context, and SP does not interfere with Rec's RNA export activity.

Conclusion

SP is a previously unrecognized HML-2 protein that, besides targeting and translocation of Env into the ER lumen, may exert biological functions distinct from Rec. HML-2 SP represents another functional similarity with the closely related Mouse Mammary Tumor Virus that encodes an Env-derived SP named p14. Our findings furthermore support the emerging concept of bioactive SPs as a conserved retroviral strategy to modulate their host cell environment, evidenced here by a "retroviral fossil". While the specific role of HML-2 SP remains to be elucidated in the context of human biology, we speculate that it may be involved in immune evasion of GCT cells or tumorigenesis.

Factors that influence VSV-G pseudotyping and transduction efficiency of lentiviral vectors—in vitro andin vivo implications

Pseudotyping viral vectors with vesicular stomatitis virus glycoprotein (VSV-G) enables the transduction of an extensive range of cell types from different species. We have discovered two important parameters of the VSV-G-pseudotyping phenomenon that relate directly to the transduction potential of lentiviral vectors: (1) the glycosylation status of VSV-G, and (2) the quantity of glycoprotein associated with virions. We measured production-cell and virion-associated quantities of two isoform variants of VSV-G, which differ in their glycosylation status, VSV-G1 and VSV-G2, and assessed the impact of this difference on the efficiency of mammalian cell transduction by lentiviral vectors. The glycosylation of VSV-G at N336 allowed greater maximal expression of VSV-G in HEK293T cells, thus facilitating vector pseudotyping. The transduction of primate cell lines was substantially affected (up to 50-fold) by the degree of VSV-G1 or VSV-G2 incorporation, whereas other cell lines, such as D17 (canine), were less sensitive to virion-associated VSV-G1/2 quantities. These data indicate that the minimum required concentration of virion-associated VSV-G differs substantially between cell species/types. The implications of these data with regard to VSV-G-pseudotyped vector production, titration, and use in host-cell restriction studies, are discussed.