Zeng Jibin, Racicott Jesse, Morales Carlos R
Department of Anatomy and Cell Biology, McGill University, Montreal, Canada.
Exp Cell Res. 2009 Nov 1;315(18):3112-24. doi: 10.1016/j.yexcr.2009.08.016. Epub 2009 Sep 2.
Lysosomes are intracellular organelles which contain enzymes and activator proteins involved in the digestion and recycling of a variety of cellular and extracellular substances. We have identified a novel sorting receptor, sortilin, which is involved in the lysosomal trafficking of the sphingolipid activator proteins, prosaposin and GM(2)AP, and the soluble hydrolases cathepsin D, cathepsin H, and acid sphingomyelinase. Sortilin belongs to a growing family of receptors with homology to the yeast Vps10 protein, which acts as a lysosomal sorting receptor for carboxypeptidase Y. In this study we examined the effects of the sortilin gene inactivation in mice. The inactivation of this gene did not yield any noticeable lysosomal pathology. To determine the existence of an alternative receptor complementing the sorting function of sortilin, we quantified the concentration of prosaposin in the lysosomes of the nonciliated epithelial cells lining the efferent ducts. These cells were chosen because they express sortilin and have a large number of lysosomes containing prosaposin. In addition, the nonciliated cells are known to endocytose luminal prosaposin that is synthesized and secreted by Sertoli cells into the seminiferous luminal fluids. Consequently, the nonciliated cells are capable of targeting both exogenous and endogenous prosaposin to the lysosomes. Using electron microscope immunogold labeling and quantitative analysis, our results demonstrate that inactivation of the sortilin gene produces a significant decrease of prosaposin in the lysosomes. When luminal prosaposin was excluded from the efferent ducts, the level of prosaposin in lysosomes was even lower in the mutant mice. Nonetheless, a significant amount of prosaposin continues to reach the lysosomal compartment. These results strongly suggest the existence of an alternative receptor that complements the function of sortilin and explains the lack of lysosomal storage disorders in the sortilin-deficient mice.
溶酶体是细胞内的细胞器，含有参与各种细胞内和细胞外物质消化与循环利用的酶和激活蛋白。我们鉴定出一种新型分选受体——sortilin，它参与鞘脂激活蛋白、prosaposin和GM(2)AP以及可溶性水解酶组织蛋白酶D、组织蛋白酶H和酸性鞘磷脂酶的溶酶体运输。Sortilin属于一个不断增加的受体家族，与酵母Vps10蛋白具有同源性，后者作为羧肽酶Y的溶酶体分选受体。在本研究中，我们检测了sortilin基因敲除对小鼠的影响。该基因的敲除未产生任何明显的溶酶体病理变化。为了确定是否存在替代受体来补充sortilin的分选功能，我们对输出小管内衬的非纤毛上皮细胞溶酶体中prosaposin的浓度进行了定量分析。选择这些细胞是因为它们表达sortilin且有大量含有prosaposin的溶酶体。此外，已知非纤毛细胞可内吞由支持细胞合成并分泌到生精管腔液中的管腔prosaposin。因此，非纤毛细胞能够将外源性和内源性prosaposin靶向运输到溶酶体。通过电子显微镜免疫金标记和定量分析，我们的结果表明sortilin基因敲除会导致溶酶体中prosaposin显著减少。当输出小管中排除管腔prosaposin时，突变小鼠溶酶体中prosaposin的水平甚至更低。尽管如此，仍有大量prosaposin继续到达溶酶体区室。这些结果强烈表明存在一种替代受体来补充sortilin的功能，并解释了sortilin缺陷小鼠中缺乏溶酶体贮积症的原因。
