Lysosome-associated membrane protein-2 (LAMP-2) is a highly glycosylated protein and an important constituent of the lysosomal membrane. Here we show that LAMP-2 deficiency in mice increases mortality between 20 and 40 days of age. The surviving mice are fertile and have an almost normal life span. Ultrastructurally, there is extensive accumulation of autophagic vacuoles in many tissues including liver, pancreas, spleen, kidney and skeletal and heart muscle. In hepatocytes, the autophagic degradation of long-lived proteins is severely impaired. Cardiac myocytes are ultrastructurally abnormal and heart contractility is severely reduced. These findings indicate that LAMP-2 is critical for autophagy. This theory is further substantiated by the finding that human LAMP-2 deficiency causing Danon's disease is associated with the accumulation of autophagic material in striated myocytes.

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Bone marrow stromal antigen 2 (BST-2, also known as tetherin) is a recently identified interferon-inducible host restriction factor that can block the production of enveloped viruses by trapping virus particles at the cell surface. This antiviral effect is counteracted by the human immunodeficiency virus type 1 (HIV-1) accessory protein viral protein U (Vpu). Here we show that HIV-1 Vpu physically interacts with BST-2 through their mutual transmembrane domains and leads to the degradation of this host factor via a lysosomal, not proteasomal, pathway. The degradation is partially controlled by a cellular protein, ␤-transducin repeat-containing protein (␤TrCP), which is known to be required for the Vpu-induced degradation of CD4. Importantly, targeting of BST-2 by Vpu occurs at the plasma membrane followed by the active internalization of this host protein by Vpu independently of constitutive endocytosis. Thus, the primary site of action of Vpu is the plasma membrane, where Vpu targets and internalizes cell-surface BST-2 through transmembrane interactions, leading to lysosomal degradation, partially in a ␤TrCP-dependent manner. Also, we propose the following configuration of BST-2 in tethering virions to the cell surface; each of the dimerized BST-2 molecules acts as a bridge between viral and cell membranes. Viral protein U (Vpu)2 is an 81-amino acid type I integral membrane phosphoprotein expressed by human immunodeficiency virus type 1 (HIV-1) (1, 2) and several simian immunodeficiency viruses (3-6). Vpu is not incorporated into virus particles (7), indicating that it acts exclusively in virus-producer cells. Indeed, Vpu is known to play two distinct roles during the later stages of infection. First, Vpu interacts with newly synthesized CD4 molecules complexed with the gp160 envelope glycoprotein precursor in the endoplasmic reticulum (8, 9) and recruits the ␤-transducin repeat-containing protein 1 (␤TrCP-1) subunit of the Skp1-Cullin1-F-box ubiquitin ligase complex (10) as well as ␤TrCP-2 (11) through its phosphoserine residues at positions 52 and 56 in the cytoplasmic (CT) domain (12,13). This event results in proteasome-mediated degradation of CD4 (10, 14, 15) allowing gp160 to resume transport toward the cell surface for virion incorporation. Second, Vpu mediates the enhancement of virion release (16 -18) in a cell type-dependent manner (e.g. HeLa cells require Vpu, whereas COS7 cells do not (19,20)), and its absence leads to the accumulation of viral particles at the cell surface (21).In contrast to the effect of Vpu on CD4 degradation, little had been known about the mechanism by which Vpu enhances the release of virions. The finding that HeLa-COS7 heterokaryons exhibited HeLa-type properties suggested that Vpu-responsive HeLa cells might harbor endogenous a restriction factor(s) that could be counteracted by this viral protein (22), as seen in Vifresponsive cells harboring the antiretroviral factor APOBEC3G counteracted by Vif (23). Neil et al. (24) showed that Vpu-deficient viral particles accumulated ...

In LAMP-2–deficient mice autophagic vacuoles accumulate in many tissues, including liver, pancreas, muscle, and heart. Here we extend the phenotype analysis using cultured hepatocytes. In LAMP-2–deficient hepatocytes the half-life of both early and late autophagic vacuoles was prolonged as evaluated by quantitative electron microscopy. However, an endocytic tracer reached the autophagic vacuoles, indicating delivery of endo/lysosomal constituents to autophagic vacuoles. Enzyme activity measurements showed that the trafficking of some lysosomal enzymes to lysosomes was impaired. Immunoprecipitation of metabolically labeled cathepsin D indicated reduced intracellular retention and processing in the knockout cells. The steady-state level of 300-kDa mannose 6-phosphate receptor was slightly lower in LAMP-2–deficient hepatocytes, whereas that of 46-kDa mannose 6-phosphate receptor was decreased to 30% of controls due to a shorter half-life. Less receptor was found in the Golgi region and in vesicles and tubules surrounding multivesicular endosomes, suggesting impaired recycling from endosomes to the Golgi. More receptor was found in autophagic vacuoles, which may explain its shorter half-life. Our data indicate that in hepatocytes LAMP-2 deficiency either directly or indirectly leads to impaired recycling of 46-kDa mannose 6-phosphate receptors and partial mistargeting of a subset of lysosomal enzymes. Autophagic vacuoles may accumulate due to impaired capacity for lysosomal degradation

Mice double deficient in LAMP-1 and -2 were generated. The embryos died between embryonic days 14.5 and 16.5. An accumulation of autophagic vacuoles was detected in many tissues including endothelial cells and Schwann cells. Fibroblast cell lines derived from the double-deficient embryos accumulated autophagic vacuoles and the autophagy protein LC3II after amino acid starvation. Lysosomal vesicles were larger and more peripherally distributed and showed a lower specific density in Percoll gradients in double deficient when compared with control cells. Lysosomal enzyme activities, cathepsin D processing and mannose-6-phosphate receptor expression levels were not affected by the deficiency of both LAMPs. Surprisingly, LAMP-1 and -2 deficiencies did not affect long-lived protein degradation rates, including proteolysis due to chaperone-mediated autophagy. The LAMP-1/2 double-deficient cells and, to a lesser extent, LAMP-2 single-deficient cells showed an accumulation of unesterified cholesterol in endo/lysosomal, rab7, and NPC1 positive compartments as well as reduced amounts of lipid droplets. The cholesterol accumulation in LAMP-1/2 double-deficient cells could be rescued by overexpression of murine LAMP-2a, but not by LAMP-1, highlighting the more prominent role of LAMP-2. Taken together these findings indicate partially overlapping functions for LAMP-1 and -2 in lysosome biogenesis, autophagy, and cholesterol homeostasis.