Yamamoto Hayato, Tobisawa Yuki, Inubushi Toshihiro, Irie Fumitoshi, Ohyama Chikara, Yamaguchi Yu
From the Human Genetics Program, Sanford Burnham Prebys Medical Discovery Institute, La Jolla, California 92037 and.
the Department of Urology, Hirosaki University Graduate School of Medicine, Hirosaki 036-8562, Japan.
J Biol Chem. 2017 May 5;292(18):7304-7313. doi: 10.1074/jbc.M116.770149. Epub 2017 Feb 28.
Hyaluronan (HA) is an extremely large polysaccharide (glycosaminoglycan) involved in many cellular functions. HA catabolism is thought to involve the initial cleavage of extracellular high-molecular-weight (HMW) HA into intermediate-size HA by an extracellular or cell-surface hyaluronidase, internalization of intermediate-size HA, and complete degradation into monosaccharides in lysosomes. Despite considerable research, the identity of the hyaluronidase responsible for the initial HA cleavage in the extracellular space remains elusive. HYAL1 and HYAL2 have properties more consistent with lysosomal hyaluronidases, whereas CEMIP/KIAA1199, a recently identified HA-binding molecule that has HA-degrading activity, requires the participation of the clathrin-coated pit pathway of live cells for HA degradation. Here we show that transmembrane protein 2 (TMEM2), a mammalian homolog of a protein playing a role in zebrafish endocardial cushion development, is a cell-surface hyaluronidase. Live immunostaining and surface biotinylation assays confirmed that mouse TMEM2 is expressed on the cell surface in a type II transmembrane topology. TMEM2 degraded HMW-HA into ∼5-kDa fragments but did not cleave chondroitin sulfate or dermatan sulfate, indicating its specificity to HA. The hyaluronidase activity of TMEM2 was Ca-dependent; the enzyme's pH optimum is around 6-7, and unlike CEMIP/KIAA1199, TMEM2 does not require the participation of live cells for its hyaluronidase activity. Moreover, TMEM2-expressing cells could eliminate HA immobilized on a glass surface in a contact-dependent manner. Together, these data suggest that TMEM2 is the long-sought-after hyaluronidase that cleaves extracellular HMW-HA into intermediate-size fragments before internalization and degradation in the lysosome.
透明质酸（HA）是一种极大的多糖（糖胺聚糖），参与多种细胞功能。HA分解代谢被认为涉及细胞外或细胞表面的透明质酸酶将细胞外高分子量（HMW）HA初步切割为中等大小的HA，中等大小的HA内化，以及在溶酶体中完全降解为单糖。尽管进行了大量研究，但负责细胞外空间中HA初始切割的透明质酸酶的身份仍然难以确定。HYAL1和HYAL2的特性与溶酶体透明质酸酶更一致，而CEMIP/KIAA1199是最近鉴定出的具有HA降解活性的HA结合分子，其HA降解需要活细胞的网格蛋白包被小窝途径参与。在这里，我们表明跨膜蛋白2（TMEM2）是一种在斑马鱼心内膜垫发育中起作用的蛋白质的哺乳动物同源物，是一种细胞表面透明质酸酶。活细胞免疫染色和表面生物素化分析证实，小鼠TMEM2以II型跨膜拓扑结构在细胞表面表达。TMEM2将HMW-HA降解为约5 kDa的片段，但不切割硫酸软骨素或硫酸皮肤素，表明其对HA具有特异性。TMEM2的透明质酸酶活性依赖于Ca；该酶的最适pH约为6-7，与CEMIP/KIAA1199不同，TMEM2的透明质酸酶活性不需要活细胞参与。此外，表达TMEM2的细胞可以以接触依赖的方式消除固定在玻璃表面的HA。总之，这些数据表明TMEM2是长期寻找的透明质酸酶，它在溶酶体内化和降解之前将细胞外HMW-HA切割为中等大小的片段。
