Ortega-Barria E, Boothroyd J C
Department of Microbiology & Immunology, Lucile Packard Children's Hospital, Stanford University School of Medicine, Stanford, California 94305, USA.
J Biol Chem. 1999 Jan 15;274(3):1267-76. doi: 10.1074/jbc.274.3.1267.
Toxoplasma gondii is one of the most widespread parasites of humans and animals. The parasite has a remarkable ability to invade a broad range of cells within its mammalian hosts by mechanisms that are poorly understood at the molecular level. This broad host cell specificity suggests that adhesion should involve the recognition of ubiquitous surface-exposed host molecules or, alternatively, the presence of various parasite attachment molecules able to recognize different host cell receptors. We have discovered a sugar-binding activity (lectin) in tachyzoites of T. gondii that plays a role in vitro in erythrocyte agglutination and infection of human fibroblasts and epithelial cells. The ability to agglutinate erythrocytes can be reversed by a variety of soluble glycoconjugates, of which heparin, fucoidan, and dextran sulfate were the most effective. Interestingly, infectivity of tachyzoites for human foreskin fibroblasts, cells that are commonly used to grow T. gondii in vitro, was increased by low concentrations of the sulfated glycoconjugates that inhibited hemagglutination activity (i.e. dextran sulfate and fucoidan) whereas high concentrations inhibited parasite infection. Furthermore, inhibition of glycosaminoglycan biosynthesis and sulfation on the host cells reduced Toxoplasma infectivity. Finally, Toxoplasma tachyzoites showed a reduced ability to infect epithelial cell mutants deficient in the biosynthesis of surface proteoglycans. The probable identity of the hemagglutinin(s) was investigated by 1) direct binding of red blood cells to filter blots of Toxoplasma proteins separated by polyacrylamide gel electrophoresis, and 2) binding of metabolically labeled parasite proteins to fixed mammalian cells. Three parasite bands were thus identified as candidate adhesins. These results suggest that attachment of T. gondii to its target cell is mediated by parasite lectins and that sulfated sugars on the surface of host cells may function as a key parasite receptor.
刚地弓形虫是人和动物中分布最广泛的寄生虫之一。该寄生虫具有非凡的能力，能够通过在分子水平上了解甚少的机制侵入其哺乳动物宿主内的多种细胞。这种广泛的宿主细胞特异性表明，黏附作用应涉及对普遍存在的表面暴露宿主分子的识别，或者是存在能够识别不同宿主细胞受体的各种寄生虫附着分子。我们在刚地弓形虫速殖子中发现了一种糖结合活性（凝集素），它在体外对红细胞凝集以及人类成纤维细胞和上皮细胞的感染中发挥作用。多种可溶性糖缀合物可逆转红细胞凝集能力，其中肝素、岩藻依聚糖和硫酸葡聚糖最为有效。有趣的是，低浓度抑制血凝活性的硫酸化糖缀合物（即硫酸葡聚糖和岩藻依聚糖）可提高速殖子对人包皮成纤维细胞（体外培养刚地弓形虫常用的细胞）的感染性，而高浓度则抑制寄生虫感染。此外，宿主细胞上糖胺聚糖生物合成和硫酸化的抑制会降低弓形虫的感染性。最后，刚地弓形虫速殖子对表面蛋白聚糖生物合成缺陷的上皮细胞突变体的感染能力降低。通过以下方法研究了血凝素的可能身份：1）红细胞与聚丙烯酰胺凝胶电泳分离的弓形虫蛋白的滤膜印迹直接结合，以及2）代谢标记的寄生虫蛋白与固定的哺乳动物细胞结合。由此确定了三条寄生虫条带为候选黏附素。这些结果表明，刚地弓形虫与其靶细胞的附着是由寄生虫凝集素介导的，宿主细胞表面的硫酸化糖可能作为关键的寄生虫受体发挥作用。
