A Unique Heparin-Binding Domain in the Envelope Protein of the Neuropathogenic PVC-211 Murine Leukemia Virus May Contribute to Its Brain Capillary Endothelial Cell Tropism

Previous studies from our laboratory demonstrated that PVC-211 murine leukemia virus (MuLV), a neuropathogenic variant of Friend MuLV (F-MuLV), had undergone genetic changes which allowed it to efficiently infect rat brain capillary endothelial cells (BCEC) in vivo and in vitro. Two amino acid changes from F-MuLV in the putative receptor binding domain (RBD) of the envelope surface protein of PVC-211 MuLV (Glu-116 to Gly and Glu-129 to Lys) were shown to be sufficient for conferring BCEC tropism on PVC-211 MuLV. Recent examination of the unique RBD of PVC-211 MuLV revealed that the substitution of Lys for Glu at position 129 created a new heparin-binding domain that overlapped a heparin-binding domain common to ecotropic MuLVs. In this study we used heparin-Sepharose columns to demonstrate that PVC-211 MuLV, but not F-MuLV, can bind efficiently to heparin and that one or both of the amino acids in the RBD of PVC-211 MuLV that are associated with BCEC tropism are responsible. We further showed that heparin can enhance or inhibit MuLV infection and that the mode of action is dependent on heparin concentration, sulfation of heparin, and the affinity of the virus for heparin. Our results suggest that the amino acid changes that occurred in the envelope surface protein of PVC-211 MuLV may allow the virus to bind strongly to the surface of BCEC via heparin-like molecules, increasing the probability that the virus will bind to its cell surface receptor and efficiently infect these cells.

本实验室既往研究显示，PVC-211鼠白血病病毒（murine leukemia virus, MuLV）——一种Friend鼠白血病病毒（Friend MuLV, F-MuLV）的神经致病变异株——发生了遗传改变，使其可在体内外高效感染大鼠脑毛细血管内皮细胞（rat brain capillary endothelial cells, BCEC）。研究证实，PVC-211 MuLV的包膜表面蛋白（envelope surface protein）的推定受体结合结构域（putative receptor binding domain, RBD）中存在两处与F-MuLV不同的氨基酸突变（Glu-116突变为Gly、Glu-129突变为Lys），仅这两处突变即可赋予PVC-211 MuLV BCEC嗜性。近期针对PVC-211 MuLV独特RBD的分析发现，129位Glu被Lys取代后形成了一个新的肝素结合结构域（heparin-binding domain），该结构域与嗜亲性鼠白血病病毒（ecotropic MuLVs）共有的肝素结合结构域存在重叠。本研究采用肝素-琼脂糖凝胶柱（heparin-Sepharose columns）实验证实，PVC-211 MuLV（而非F-MuLV）可高效结合肝素，且该特性与PVC-211 MuLV RBD中参与BCEC嗜性的一处或两处氨基酸突变密切相关。我们进一步证实，肝素可增强或抑制MuLV感染，其作用模式取决于肝素浓度、肝素硫酸化程度以及病毒与肝素的结合亲和力。本研究结果提示，PVC-211 MuLV包膜表面蛋白发生的氨基酸突变，可能使病毒通过类肝素分子强力结合BCEC表面，从而提升病毒结合其细胞表面受体并高效感染此类细胞的概率。