Heparan sulfate in baculovirus binding and entry of mammalian cells.

Makkonen and his colleagues published an article in the Journal of Virology entitled “6-Oand N-Sulfated Syndecan-1 Promotes Baculovirus Binding and Entry into Mammalian Cells” (1). Some parts of the data and conclusions seem misleading and not fully justified. Heparan sulfate (HS) alone can bind baculovirus to cells during entry and transduction. Syndecan-1 (SDC-1) contains a protein core and HS chains. Therefore, it is called heparan sulfate proteoglycan (HSPG). I have examined the ability of baculovirus to enter mammalian cells after treatment with heparinase III (EC 4.2.2.8), which specifically digests HS chains from the protein core. For HT1080 cells, heparinase III could significantly reduce baculovirus entry compared with untreated control cells (Fig. 1A). The reduced susceptibility of cells with the baculovirus vector after enzymatic removal of HS indicates that HS plays an important role in baculovirus entry. The protein core of syndecan-1 does not have any role in baculovirus binding. The pgsD-677 cell has a single mutation affecting both N-acetylglucosaminyltransferase and glucuronosyltransferase, which are necessary for polymerization of HS chains, and does not synthesize HS but does produce three times more chondroitin sulfate (CS) than the wild-type CHO-K1 cell does (2). A baculovirus vector was used to evaluate entry into the CHO-K1 cell and its mutant, pgsD-677. The lack of HS expression severely impaired the ability of baculovirus vector to enter into the mutant cell (Fig. 1B). Despite overproduction of CS in pgsD-677 cells, poor transduction in mutant cells further demonstrated the specificity of baculovirus for HS. The authors masked SDC-1 with various concentrations of anti-SDC-1 antibody and tested whether baculovirus binding could be inhibited. They showed that anti-SDC-1 antibody was able to decrease baculovirus binding and transduction of cells. The anti-SDC-1 antibody (sc-5632; Santa Cruz Biotechnology, CA) used in this study was raised against the core protein of SDC-1, but not against HS. Therefore, an explanation should be given for how blocking of SDC-1 (not HS) with its antibody can block the binding of baculovirus. The authors also showed that increasing concentrations of recombinant SDC-1 (ab83609; Abcam, Cambridge, MA) could decrease baculovirus entry and transduction in a dose-dependent manner. Abcam produces recombinant SDC-1 protein in Escherichia coli which does not efficiently glycosylate SDC-1. As a result, recombinant SDC-1 cannot compete with mammalian cell surface SDC-1. Therefore, an explanation of how recombinant SDC-1 can inhibit baculovirus entry into cells is needed. The authors further mentioned that baculovirus could bind 293T cells despite low-level expression of SDC-1. They argued that this could be due to significant differences in the disaccharide composition of HS chains of SDC-1. I found that expression of HS in 293T cells was low and resulted in poor transduction of foamy virus (3), but good transduction of baculovirus. In addition to HS, herpes simplex virus 1 uses herpesvirus entry mediator (HveA) and members of the nectin family receptor (HveC) for binding to cells (4, 5). It should be investigated whether baculovirus might use any other binding molecules during entry into 293T cells.