The baculovirus cysteine protease has a cathepsin B-like S2-subsite specificity

Origins of truncated supplementary capsid proteins in rAAV8 vectors produced with the baculovirus system

The ability to produce large quantities of recombinant Adeno-Associated Virus (rAAV) vectors is an important factor for the development of gene therapy-based medicine. The baculovirus/insect cell expression system is one of the major systems for large scale rAAV production. So far, most technological developments concerned the optimization of the AAV rep and cap genes in order to be expressed correctly in a heterologous system. However, the effect of the baculovirus infection on the production of rAAV has not been examined in detail. In this study we show that the baculoviral cathepsin (v-CATH) protease is active on several (but not all) rAAV serotypes, leading to a partial degradation of VP1/VP2 proteins. Subsequently, we identified the principal v-CATH cleavage site in the rAAV8 capsid proteins and demonstrated that the cleavage is highly specific. The proteolytic degradation of VP1/VP2 AAV capsid proteins reduces the infectivity of rAAV vectors but can be prevented by the use of a baculovirus vector with a deletion of the chiA/v-cath locus or by addition of the E64 protease inhibitor during production. Moreover, the codon optimization of AAV cap performed for several serotypes and originally aimed at the removal of potential alternative initiation codons, resulted in incorporation of additional forms of truncated VP1 into the rAAV capsids.

Increased expression and activation of cathepsin K in human osteoarthritic cartilage and synovial tissues

Few studies have analyzed Cathepsin K (CatK) expression in human osteoarthritic tissues. We investigated CatK expression and activation in human articular cartilage using clinical specimens. Human osteoarthritic cartilage was obtained during surgery of total hip arthroplasty (n = 10), and control cartilage was from that of femoral head replacement for femoral neck fracture (n = 10). CatB, CatK, CatL, CatS, and Cystatin C (CysC) expressions were evaluated immunohistochemically and by real-time PCR. Intracellular CatK protein was quantified by ELISA. Intracellular CatK activity was also investigated. Osteoarthritis (OA) chondrocytes were strongly stained with CatK, particularly in the superficial layer and more damaged areas. CatB, CatL, CatS, and CysC were weakly stained. CatK mRNA expression was significantly higher in OA group compared to that in control group (p = 0.043), whereas those of CatB, CatL, CatS, and CysC did not differ significantly. Mean CatK concentration (4.83 pmol/g protein) in OA chondrocytes was higher than that (3.91 pmol/g protein) in control chondrocytes (p = 0.001). CatK was enzymatically more activated in OA chondrocytes as compared with control chondrocytes. This study, for the first time, revealed increased CatK expression and activation in human OA cartilage, suggesting possible crucial roles for it in the pathogenesis of osteoarthritic change in articular cartilage.

Interaction of Autographa californica multiple nucleopolyhedrovirus cathepsin protease progenitor (proV-CATH) with insect baculovirus chitinase as a mechanism for proV-CATH cellular retention

The insect baculovirus chitinase (CHIA) and cathepsin protease (V-CATH) enzymes cause terminal host insect liquefaction, enhancing the dissemination of progeny virions away from the host cadavers. Regulated and delayed cellular release of these host tissue-degrading enzymes ensures that liquefaction starts only after optimal viral replication has occurred. Baculoviral CHIA remains intracellular due to its C-terminal KDEL endoplasmic reticulum (ER) retention motif. However, the mechanism for cellular retention of the inactive V-CATH progenitor (proV-CATH) has not yet been determined. Signal peptide cleavage occurs upon cotranslational ER import of the v-cath-expressed protein, and ER-resident CHIA is needed for the folding of proV-CATH. Although this implies that CHIA and proV-CATH bind each other in the ER, the putative CHIA-proV-CATH interaction has not been experimentally verified. We demonstrate that the amino-terminal 22 amino acids (aa) of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) preproV-CATH are responsible for the entry of proV-CATH into the ER. Furthermore, the CHIA-green fluorescent protein (GFP) and proV-CATH-red fluorescent protein (RFP) fusion proteins colocalize in the ER. Using monomeric RFP (mRFP)-based bimolecular fluorescence complementation (BiFC), we determined that CHIA and proV-CATH interact directly with each other in the ER during virus replication. Moreover, reciprocal Ni/His pulldowns of His-tagged proteins confirmed the CHIA-proV-CATH interaction biochemically. The reciprocal copurification of CHIA and proV-CATH suggests a specific CHIA-proV-CATH interaction and corroborates our BiFC data. Deletion of the CHIA KDEL motif allowed for premature CHIA secretion from cells, and proV-CATH was similarly prematurely secreted from cells along with ΔKDEL-CHIA. These data suggest that CHIA and proV-CATH interact directly with each other and that this interaction aids the cellular retention of proV-CATH.

N-linked glycans located in the pro-region of Bombyx mori nucleopolyhedrovirus V-CATH are essential for the proper folding of V-CATH and V-CHIA

Post-mortem host degradation by infection of Bombyx mori nucleopolyhedrovirus (BmNPV) requires the synergistic activation of two virus-encoded genes, cathepsin (v-cath) and chitinase (v-chiA). Previous studies have suggested that V-CHIA is essential for the proper folding of the nascent V-CATH polypeptide in the endoplasmic reticulum, and that the putative V-CHIA-V-CATH interaction might be mediated by N-linked glycans of V-CATH. Sequence analysis shows that BmNPV V-CATH includes three consensus N-linked glycosylation sites (asparagine 38, 65 and 158). To clarify the role of N-linked glycans of V-CATH in its biological activity, we generated three recombinant BmNPVs expressing mutant V-CATHs, and found that the two residues, asparagine 38 and 65, which are localized in the pro-region of V-CATH, are the glycosylation sites of BmNPV V-CATH. Western blot analysis also showed that removal of N-linked glycans from BmNPV V-CATH resulted in production of the insoluble forms of V-CATH and V-CHIA. These results demonstrate that N-linked glycans located in the pro-region of BmNPV V-CATH are essential for the proper folding of V-CATH and V-CHIA.

Trypsin cleavage of the baculovirus occlusion-derived virus attachment protein P74 is prerequisite in per os infection

Baculovirus occlusion-derived virions (ODVs) contain a number of infectivity factors essential for the initiation of infection in larval midgut cells. Deletion of any of these factors neutralizes infectivity by the per os route. We have observed that P74 of the group I alphabaculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is N-terminally cleaved when a soluble form of the protein was incubated with insect midgut tissues under alkaline conditions and that cleavage was prevented by soybean trypsin inhibitor (SBTI). Presently, biological assays were carried out that suggest SBTI inhibits and trypsin enhances baculovirus per os infectivity. We developed a method to rescue per os infectivity of a P74 null virus involving co-transfection of viral DNA with a plasmid that transiently expresses p74. We used this plasmid rescue method to functionally characterize P74. A series of site-directed mutants were generated at the N terminus to evaluate if trypsin cleavage sites were necessary for function. Mutagenesis of R195, R196 and R199 compromised per os infectivity and rendered P74 resistant to midgut trypsin.

Arabidopsis thaliana beta1,2-xylosyltransferase: an unusual glycosyltransferase with the potential to act at multiple stages of the plant N-glycosylation pathway

XylT (beta1,2-xylosyltransferase) is a unique Golgi-bound glycosyltransferase that is involved in the biosynthesis of glycoprotein-bound N-glycans in plants. To delineate the catalytic domain of XylT, a series of N-terminal deletion mutants was heterologously expressed in insect cells. Whereas the first 54 residues could be deleted without affecting the catalytic activity of the enzyme, removal of an additional five amino acids led to the formation of an inactive protein. Characterization of the N-glycosylation status of recombinant XylT revealed that all three potential N-glycosylation sites of the protein are occupied by N-linked oligosaccharides. However, an unglycosylated version of the enzyme displayed substantial catalytic activity, demonstrating that N-glycosylation is not essential for proper folding of XylT. In contrast with most other glycosyltransferases, XylT is enzymatically active in the absence of added metal ions. This feature is not due to any metal ion directly associated with the enzyme. The precise acceptor substrate specificity of XylT was assessed with several physiologically relevant compounds and the xylosylated reaction products were subsequently tested as substrates of other Golgi-resident glycosyltransferases. These experiments revealed that the substrate specificity of XylT permits the enzyme to act at multiple stages of the plant N-glycosylation pathway.

Improvement of GFPuv-beta 3GnT2 fusion protein production by suppressing protease in baculovirus expression system

The effects of protease inhibitors on the production of recombinant protein were investigated using a recombinant baculovirus containing GFPuv-human beta 1,3-N-acetylglucosaminyltransferase 2 (beta 3GnT2) connected to the prepromelittin signal sequence. The addition of leupeptin as a cysteine protease inhibitor at 2.5 microg/ml improved intra- and extracellular beta 3GnT activities 5- and 3-fold, respectively, compared to those without addition, which was due to a suppression of protease activity. With the leupeptin addition only four degraded molecular bands lower than 32 kDa appeared, but 9 degraded molecular bands between 29 kDa and 41 kDa existed without addition. In contrast, pepstatin A as a carboxyl protease inhibitor had no influence on the improvement of beta 3GnT production, judging from SDS-PAGE. Moreover, when 50 microM carbobenzoxy-L-leucyl-L-leucyl-L-leucinal (MG-132), known as a proteasome inhibitor, was used in combination with the leupeptin, a ladder of low molecular mass bands of fusion protein was diminished. The intracellular beta 3GnT activity increased 9-fold, to as high as that without addition of two kinds of protease, but the extracellular activity was not different from that with the addition of only leupeptin. These findings indicate that the decrease in cell viability causes the decrease in the secretion rate of intracellular fusion protein, resulting the accumulation of the full-length of fusion protein.

Characterization of recombinant mannan-binding lectin-associated serine protease (MASP)-3 suggests an activation mechanism different from that of MASP-1 and MASP-2

Mannan-binding lectin (MBL)-associated serine proteases (MASP-1, -2, and -3) are homologous modular proteases that each associate with MBL and L- and H-ficolins, which are oligomeric serum lectins involved in innate immunity. To investigate its physicochemical, interaction, and enzymatic properties, human MASP-3 was expressed in insect cells. Ultracentrifugation analysis indicated that rMASP-3 sedimented as a homodimer (s(20,w) = 6.2 +/- 0.1 S) in the presence of Ca(2+), and as a monomer (s(20,w) = 4.6 +/- 0.1 S) in EDTA. As shown by surface plasmon resonance spectroscopy, it associated with both MBL (K(D) = 2.6 nM) and L-ficolin (K(D) = 7.2 nM). The protease was produced in a single-chain, proenzyme form, but underwent slow activation upon prolonged storage at 4 degrees C, resulting from cleavage at the Arg(430)-Ile(431) activation site. Activation was prevented in the presence of protease inhibitors iodoacetamide and 1,10-phenanthroline but was not abolished upon substitution of Ala for the active site Ser(645) of MASP-3, indicating extrinsic proteolysis. In contrast, the corresponding mutations Ser(627)-->Ala in MASP-1 and Ser(618)-->Ala in MASP-2 stabilized the latter in their proenzyme form. Likewise, the MASP-1 and MASP-2 mutants were each activated by their active counterparts, but MASP-3 S645A was not. Activated MASP-3 did not react with C1 inhibitor; had no activity on complement proteins C2, C4, and C3; and only cleaved the N-carboxybenzyloxyglycine-L-arginine thiobenzyl ester substrate to a significant extent. Based on these observations, it is postulated that MASP-3 activation and control involve mechanisms that are different from those of MASP-1 and -2.

Autographa californica M nucleopolyhedrovirus ProV-CATH is activated during infected cell death

V-CATH, a cathepsin L-like cysteine protease encoded by the baculovirus Autographa californica M nucleopolyhedrovirus, has been shown to play an essential role in host liquefaction. Similar to cellular cathepsin L, V-CATH is synthesized as an inactive proenzyme and is activated by cleavage of the propeptide. Previous studies indicated that removal of the propeptide was rapid, occurring as soon as the protein could be detected by Western blot, 22 h postinfection. We found, however, that these results reflected artifactual processing of the proenzyme. When the protease inhibitor E-64 was used to prevent this aberration, we found that proV-CATH accumulated in infected cells and activation did not begin until the onset of cell death, at approximately 80 h postinfection. Western blot analysis of fractions of live and dead cells isolated by fluorescence-activated cell sorting revealed that mature V-CATH was found only in dead cells. The regulation of activation of proV-CATH, therefore, was quite different from that of cellular cathepsins. Acridine orange staining revealed that lysosome integrity was lost in dead cells, an occurrence that could lead to the activation of proV-CATH by lysosomal proteases.

Autographa californica M nucleopolyhedrovirus chiA is required for processing of V-CATH

Infection of permissive insect hosts by the baculovirus Autographa californica M nucleopolyhedrovirus results in liquefaction, a pathogenic effect that enhances the dispersal of progeny virions. Two viral gene products-a protease, V-CATH, and a chitinase, chiA-have been shown to be required for liquefaction to occur. It has been generally accepted that the primary functions of these proteins is to degrade the proteinaceous and chitinous components of the host cadaver, respectively. We have generated suggestive evidence, however, that chiA may also serve as a molecular chaperone for proV-CATH, the precursor of V-CATH. When cells were infected with virus lacking a functional chiA gene, proV-CATH failed to undergo processing in vivo and in vitro and formed insoluble aggregates in the endoplasmic reticulum of infected cells. Thus, expression of chiA may be required for the proper folding of the nascent V-CATH polypeptide in the endoplasmic reticulum. Identical results were obtained when tunicamycin was used to block N-linked glycosylation in cells infected with wildtype virus, suggesting that the putative chiA/V-CATH interaction is mediated by N-linked oligosaccharides.

Proteolytic activity in infected and noninfected insect cells: degradation of HIV-1 Pr55gag particles

In this work the proteolytic activity in the supernatant and inside insect cells in culture was evaluated for different multiplicities of infection (MOI) and times of infection (TOI). Several methods to detect proteolytic activity in insect cells were tested and that using fluorescein thiocyanite-casein as a substrate was chosen. It was observed that infection caused not only a reduction in the concentration of proteases by decreasing their synthesis but also an inhibition of the intracellular proteolytic activity by increasing the intracellular ATP level (measured by in vivo nuclear magnetic resonance, NMR). The maximum proteolytic activity in the supernatant was observed at 72 hpi except when the cells were infected in the late exponential growth phase or with very low MOI, yielding a nonsynchronous infection. The proteolytic degradation of Pr55gag particles was studied during culture and after harvest. In this particular case it was concluded that the supernatant should be stored at low temperature or quickly purified, since the degradation after 24 h is only 3% at 4 degrees C while at 27 degrees C this value rises to 23%. There is a complex relationship between MOI, TOI, proteolytic activity, and product titer and quality. Thus, the optimal conditions for each case will be a compromise between the final product titer, the desired product quality, and operational issues like process time and capacity, requiring proper integration between bioreaction and downstream processing.

Cathepsin K knockout mice develop osteopetrosis due to a deficit in matrix degradation but not demineralization

Cathepsin K is a cysteine protease expressed predominantly in osteoclasts. Activated cathepsin K cleaves key bone matrix proteins and is believed to play an important role in degrading the organic phase of bone during bone resorption. Mutations in the human cathepsin K gene have been demonstrated to be associated with a rare skeletal dysplasia, pycnodysostosis. The degree of functional activity of the mutated forms of cathepsin K in these individuals has not been elucidated, but is predicted to be low or absent. To study the role of cathepsin K in bone resorption, we have generated mice deficient in the cathepsin K gene. Histologic and radiographic analysis of the mice revealed osteopetrosis of the long bones and vertebrae, and abnormal joint morphology. X-ray microcomputerized tomography images allowed quantitation of the increase in bone volume, trabecular thickness, and trabecular number in both the primary spongiosa and the metaphysis of the proximal tibiae. Not all bones were similarly affected. Chondrocyte differentiation was normal. The mice also had abnormalities in hematopoietic compartments, particularly decreased bone marrow cellularity and splenomegaly. The heterozygous animals appeared normal. Close histologic examination of bone histology revealed fully differentiated osteoclasts apposed to small regions of demineralized bone. This strongly suggests that cathepsin K-deficient osteoclasts are capable of demineralizing the extracellular matrix but are unable to adequately remove the demineralized bone. This is entirely consistent with the proposed function of cathepsin K as a matrix-degrading proteinase in bone resorption.