Close Encounters of the Lysosome-Peroxisome Kind

Lysosomes provide a major source for cellular cholesterol; however, most of this cholesterol is trafficked to the plasma membrane via unknown mechanisms. Chu et al. identify an unexpected role for peroxisomes in the transport of cholesterol from the lysosome to the plasma membrane via a lysosome-peroxisome membrane contact site. Lysosomes provide a major source for cellular cholesterol; however, most of this cholesterol is trafficked to the plasma membrane via unknown mechanisms. Chu et al. identify an unexpected role for peroxisomes in the transport of cholesterol from the lysosome to the plasma membrane via a lysosome-peroxisome membrane contact site. Cholesterol is an essential determinant of membrane fluidity, permeability, and organization in animal cells. While the vast majority of cellular cholesterol (about 60%–80%) is localized at the plasma membrane (PM) (Maxfield and Wüstner, 2002Maxfield F.R. Wüstner D. J. Clin. Invest. 2002; 110: 891-898Crossref PubMed Scopus (279) Google Scholar), cholesterol originates from the ER via biosynthesis and the lysosome via import of exogenous cholesterol. These observations raise a fundamental question, how is cholesterol transported from the ER or lysosome to the PM? In this issue of Cell, Chu et al., 2015Chu B.-B. Liao Y.-C. Qi W. Xie C. Du X. Wang J. Yang H. Miao H.-H. Li B.-L. Song B.-L. Cell. 2015; 161 (this issue): 291-306Abstract Full Text Full Text PDF PubMed Scopus (232) Google Scholar discover that peroxisomes play a critical role in the transport of cholesterol from the lysosome to the PM and uncover an unexpected membrane contact site between the peroxisome and lysosome (Figure 1). Exogenous cholesterol enters the cell as low density lipoproteins (LDL) via endocytosis of the LDL receptor. Upon delivery to the lysosome, LDL-derived cholesterol esters are de-esterified into free cholesterol then exported to other compartments including the PM (Maxfield and Wüstner, 2002Maxfield F.R. Wüstner D. J. Clin. Invest. 2002; 110: 891-898Crossref PubMed Scopus (279) Google Scholar). The physiological importance of cholesterol transport out of the lysosome is underscored by Niemann-Pick disease type C (NPC). NPC is a fatal, predominantly neurodegenerative disorder caused by mutations in NPC1 or NPC2, which results in cholesterol accumulation in the lysosome. NPC1 and NPC2 work together to transport free cholesterol out of the lumen to the limiting membrane of the lysosome (Du et al., 2011Du X. Kumar J. Ferguson C. Schulz T.A. Ong Y.S. Hong W. Prinz W.A. Parton R.G. Brown A.J. Yang H. J. Cell Biol. 2011; 192: 121-135Crossref PubMed Scopus (224) Google Scholar, Kwon et al., 2009Kwon H.J. Abi-Mosleh L. Wang M.L. Deisenhofer J. Goldstein J.L. Brown M.S. Infante R.E. Cell. 2009; 137: 1213-1224Abstract Full Text Full Text PDF PubMed Scopus (485) Google Scholar, Vanier, 2015Vanier M.T. J. Inherit. Metab. Dis. 2015; 38: 187-199Crossref PubMed Scopus (189) Google Scholar). The molecular mechanisms of subsequent steps, exit of cholesterol from the lysosomal membrane and delivery to the PM, were largely uncharacterized. To identify proteins required for transport of LDL-derived cholesterol, Chu et al., 2015Chu B.-B. Liao Y.-C. Qi W. Xie C. Du X. Wang J. Yang H. Miao H.-H. Li B.-L. Song B.-L. Cell. 2015; 161 (this issue): 291-306Abstract Full Text Full Text PDF PubMed Scopus (232) Google Scholar design an elegant screen that takes advantage of the antibiotic Amphotericin C, which permeablilizes the PM through association with exposed cholesterol, as well as U18666A, which enables them to stage the release of LDL-derived cholesterol from the lysosome. Using shRNA, they identify 341 candidate genes. Surprisingly, several candidates are related to peroxisomal function and biogenesis. Knockdown of these peroxisome related genes results in the accumulation of cholesterol in the lysosome lumen. Analysis of cultured wild-type cells by both fluorescence microscopy and transmission electron microscopy reveals a previously unrecognized membrane contact site between lysosomes and peroxisomes. Further evidence for the lysosome-peroxisome contact site is provided by multiple in vitro studies demonstrating an interaction between these organelles. Chu et al. find that the lysosome-peroxisome contact site is bridged, at least in part, by the integral lysosomal membrane protein, synaptotagmin 7 (syt7), through binding to PI(4,5)P2 on the peroxisomal membrane. The lysosome-peroxisome contact site is transient and cholesterol-dependent. Notably, efficient formation of the lysosome-peroxisome contact site also requires NPC1 suggesting that this contact is important for cholesterol exit from the lysosome. Three of the peroxisomal genes identified in this study have been implicated in human diseases. X-linked adrenoleukodystrophy, Infantile Refsum disease, and Zellweger syndrome are caused by mutations in ABCD1, PEX1, and PEX26 (Aubourg and Wanders, 2013Aubourg P. Wanders R. Handb. Clin. Neurol. 2013; 113: 1593-1609Crossref PubMed Scopus (47) Google Scholar). Strikingly, Chu et al., 2015Chu B.-B. Liao Y.-C. Qi W. Xie C. Du X. Wang J. Yang H. Miao H.-H. Li B.-L. Song B.-L. Cell. 2015; 161 (this issue): 291-306Abstract Full Text Full Text PDF PubMed Scopus (232) Google Scholar show that cells from patients suffering from each of these diseases accumulate cholesterol in lysosomes to a similar extent as those from NPC patients. The potential contribution of defects in cholesterol trafficking to symptoms of these diseases must now be considered. The precise roles of the lysosome-peroxisome contact site in facilitating cholesterol transport out of the lysosome on its path to the PM remains to be determined. Chu et al., 2015Chu B.-B. Liao Y.-C. Qi W. Xie C. Du X. Wang J. Yang H. Miao H.-H. Li B.-L. Song B.-L. Cell. 2015; 161 (this issue): 291-306Abstract Full Text Full Text PDF PubMed Scopus (232) Google Scholar provide evidence from in vitro, as well as cell-based studies, that cholesterol is transferred from lysosomes to peroxisomes via the lysosome-peroxisome membrane contact site, raising the possibility that cholesterol may transit through the peroxisome on its way to the PM. It is also possible that the contact site facilitates transport of cholesterol out of the lysosome directly to a different organelle, such as the ER. In support of this possibility, knockdown of the oxysterol binding protein-related protein 5 (ORP5), which is localized to the ER, has been shown to result in accumulation of NPC1 dependent pools of cholesterol in the limiting membrane of the lysosome (Du et al., 2011Du X. Kumar J. Ferguson C. Schulz T.A. Ong Y.S. Hong W. Prinz W.A. Parton R.G. Brown A.J. Yang H. J. Cell Biol. 2011; 192: 121-135Crossref PubMed Scopus (224) Google Scholar). Orp5 may act in a parallel pathway, or downstream of Syt7 and ABCD1 in the transfer of cholesterol out of the lysosome, either to the ER or to the peroxisome. Different pathways of cholesterol transport out of the lysosome may function in different cell types or under different conditions and perhaps only a subset of these pathways are directed to the PM. A comprehensive assessment of the molecular components of the lysosome-peroxisome contact, including the consideration of a possible three-way lysosome-peroxisome-ER junction, is necessary. This knowledge will be critical to understanding the peroxisome-dependent mechanisms of cholesterol transport out of the lysosome and for the development of disease therapies. The lysosome-peroxisome contact site joins a growing list of inter-organelle contact sites. The known membrane contact sites currently include: ER-mitochondria, ER-PM, ER-lysosome/vacuole, ER-endosome, ER-Golgi, mitochondria-lysosome/vacuole, and mitochondria-PM (Prinz, 2014Prinz W.A. J. Cell Biol. 2014; 205: 759-769Crossref PubMed Scopus (279) Google Scholar). Note that the lysosome-peroxisome contact site is now the third example of a critical contact between the lysosome and another organelle. Among the identified contact sites, some share partial functional redundancy. For example, the ER-mitochondrial encounter structure (ERMES) and the vacuole and mitochondria patch (vCLAMP) are distinct membrane contact sites that connect the mitochondria to the ER and the yeast vacuole, respectively (Elbaz-Alon et al., 2014Elbaz-Alon Y. Rosenfeld-Gur E. Shinder V. Futerman A.H. Geiger T. Schuldiner M. Dev. Cell. 2014; 30: 95-102Abstract Full Text Full Text PDF PubMed Scopus (261) Google Scholar, Hönscher et al., 2014Hönscher C. Mari M. Auffarth K. Bohnert M. Griffith J. Geerts W. van der Laan M. Cabrera M. Reggiori F. Ungermann C. Dev. Cell. 2014; 30: 86-94Abstract Full Text Full Text PDF PubMed Scopus (232) Google Scholar). Loss of either the ERMES or the vCLAMP has minimal phenotypic consequences, whereas simultaneous loss of both is lethal. Additional inter-organelle contacts are likely to be discovered. In addition to specialized functions including calcium homeostasis and storage, intracellular signaling, organelle division, and lipid biosynthesis, membrane contact sites have repeatedly been shown to facilitate lipid transfer between membranes (Elbaz and Schuldiner, 2011Elbaz Y. Schuldiner M. Trends Biochem. Sci. 2011; 36: 616-623Abstract Full Text Full Text PDF PubMed Scopus (174) Google Scholar, Prinz, 2014Prinz W.A. J. Cell Biol. 2014; 205: 759-769Crossref PubMed Scopus (279) Google Scholar). It is tempting to speculate that inter-organelle contacts are the major routes of lipid transfer between cellular compartments and fundamental to the accurate distribution of distinct lipid species throughout the cell. Currently, we are limited in our ability to observe the movements of lipids within cells because robust assays for tracking most lipids via microscopy do not yet exist. The development of methods facilitating such observations will be invaluable to advancement of the field. This work was supported by NIH grants R01-NS064015, R01-GM050403, and R37- GM062261 (to L.S.W.). B.S.S. was supported by a postdoctoral fellowship from the Jane Coffin Childs Memorial Fund for Medical Research. Cholesterol Transport through Lysosome-Peroxisome Membrane ContactsChu et al.CellApril 09, 2015In BriefLysosome forms dynamic membrane contacts with peroxisome, and cholesterol is transported from lysosome to peroxisome. Massive cholesterol accumulates in the cells from patients with peroxisomal disorders. Full-Text PDF Open Archive