Transport Of Lysosomal Enzymes Research Articles

LYSET is a New Factor Critical for Lysosomal Enzyme Transport and Viral Infection

LYSET is a New Factor Critical for Lysosomal Enzyme Transport and Viral Infection

LYSET is a New Factor Critical for Lysosomal Enzyme Transport and Viral Infection

LYSET is a New Factor Critical for Lysosomal Enzyme Transport and Viral Infection

Molecular characterization of a cation-dependent mannose-6-phosphate receptor gene in Crassostrea hongkongensis and its responsiveness in Vibrio alginolyticus infection.

The cation-dependent mannose-6-phosphate receptor (CD-M6PR) is a P-type lectin that plays a crucial role in lysosomal enzyme transport, bacterial resistance, and viral entry. In this study, we cloned and analyzed the ORF of the CD-M6PR gene from Crassostrea hongkongensis and named it ChCD-M6PR. We analyzed the nucleotide and amino acid sequence of ChCD-M6PR, its tissue expression pattern and immune response to Vibrio alginolyticus. Our results showed that the ORF of ChCD-M6PR was 801 bp long and encoded a protein of 266 amino acids with a signal peptide at the N-terminus, as well as Man-6-P_recep, ATG27 and transmembrane structural domains. Phylogenetic analysis indicated that Crassostrea hongkongensis shared the highest similarity with Crassostrea gigas in the terms of CD-M6PR. The ChCD-M6PR gene was found to be expressed in various tissues, with the highest expression observed in the hepatopancreas and the lowest in the hemocytes by the fluorescence quantitative PCR. Furthermore, the expression of ChCD-M6PR gene was significantly up-regulated for a short time in response to Vibrio alginolyticus infection in the gill and hemocytes, while it was down-regulated in the gonads. The expression patterns of ChCD-M6PR also varied in the other tissues. The 96h cumulative mortality rate of Crassostrea hongkongensis infected with Vibrio alginolyticus after knockdown the ChCD-M6PR gene was significantly higher. Overall, our findings suggests that ChCD-M6PR plays a crucial role in the immune response of Crassostrea hongkongensis to Vibrio alginolyticus infection, and its tissue-specific expression patterns may be indicatitive of varied immune responses across tissues.

The human disease gene LYSET is essential for lysosomal enzyme transport and viral infection.

Lysosomes are key degradative compartments of the cell. Transport to lysosomes relies on GlcNAc-1-phosphotransferase-mediated tagging of soluble enzymes with mannose 6-phosphate (M6P). GlcNAc-1-phosphotransferase deficiency leads to the severe lysosomal storage disorder mucolipidosis II (MLII). Several viruses require lysosomal cathepsins to cleave structural proteins and thus depend on functional GlcNAc-1-phosphotransferase. We used genome-scale CRISPR screens to identify lysosomal enzyme trafficking factor (LYSET, also named TMEM251) as essential for infection by cathepsin-dependent viruses including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). LYSET deficiency resulted in global loss of M6P tagging and mislocalization of GlcNAc-1-phosphotransferase from the Golgi complex to lysosomes. Lyset knockout mice exhibited MLII-like phenotypes, and human pathogenic LYSET alleles failed to restore lysosomal sorting defects. Thus, LYSET is required for correct functioning of the M6P trafficking machinery and mutations in LYSET can explain the phenotype of the associated disorder.

Lizozomal Depo Hastalıkları: Kırıkkale Üniversitesi Deneyimi

Objective: Lysosomal storage diseases which were first described in 1880; are important group of metabolic disorders characterized by the deposition of the substrates in lysosomes due to defects of the activity or transport of lysosomal enzymes or a defect in the receptor proteins. LSDs usually show a progressive clinical course and may not be represented with any clinical signs during the neonatal period. The overall prevalence of LSDs is 1 / 7000-8000. The aim of this study was to share the clinical characteristics of our LSDs patients and the experiences of our pediatric metabolic diseases department.Material and Methods: This retrospective cohort study was conducted at Kırıkkale University Hospital with 56 patients diagnosed as lysosomal storage disease among 315 patients diagnosed with metabolic diseases. Data were collected from outpatient clinic patient files who were diagnosed between 2011- 2018. Results: A total of 315 patients diagnosed with inherited metabolic disease were followed in our clinic and 56 (17.7 %) of them were diagnosed as LSDs. The 56 patients were suffering from the following diseases: 10 patients with Mucopolysaccharidosis, 1 patient with mucolipidosis type 2 (I-cell disease), 41 patients with sphingolipidoses, two patients with cystinosis, one patient with Infantile Pompe Disease and one patient with beta-mannosidosis.The mean age of the patients with Fabry Disease and the other patients diagnosed with other LSDs were 34.7±14.2 years (minimum 8, maximum 64) and 2.67±3.4 years (minimum 0, maximum 10.5) respectively. All diagnoses were verified by specific enzyme analysis and/or by conducting genetic mutation analysis. Conclusion: The most common lysosomal storage disease among our patients were Mucopolysaccharidosis and sphingolipidosis. Treatment options, such as enzyme replacement therapy and bone marrow transplantation exist, and 24 of these patients are receiving enzyme replacement therapy.

MiR-143 Regulates Lysosomal Enzyme Transport across the Blood-Brain Barrier and Transforms CNS Treatment for Mucopolysaccharidosis Type I

During brain maturation, cation-independent mannose-6-phosphate receptor (CI-MPR), a key transporter for lysosomal hydrolases, decreases significantly on the blood-brain barrier (BBB). Such a phenomenon leads to poor brain penetration of therapeutic enzymes and subsequent failure in reversing neurological complications in patients with neuropathic lysosomal storage diseases (nLSDs), such as Hurler syndrome (severe form of mucopolysaccharidosis type I [MPS I]). In this study, we discover that upregulation of microRNA-143 (miR-143) contributes to the decline of CI-MPR on the BBB during development. Gain- and loss-of-function studies showed that miR-143 inhibits CI-MPR expression and its transport function in human endothelial cells invitro. Genetic removal of miR-143 in MPS I mice enhances CI-MPR expression and improves enzyme transport across the BBB, leading to brain metabolic correction, pathology normalization, and correction of neurological functional deficits 5months after peripheral protein delivery at clinically relevant levels that derived from erythroid/megakaryocytic cells via hematopoietic stem cell-mediated gene therapy, when otherwise no improvement was observed in MPS I mice at a parallel setting. These studies not only uncover a novel role of miR-143 as an important modulator for the developmental decline of CI-MPR on the BBB, but they also demonstrate the functional significance of depleting miR-143 for "rescuing" BBB-anchored CI-MPR on advancing CNS treatment for nLSDs.

MiR-143 regulates lysosomal enzyme transport across blood-brain barrier and improves CNS treatment for Hurler syndrome

miR-143 regulates lysosomal enzyme transport across blood-brain barrier and improves CNS treatment for Hurler syndrome

Alterations in EGF-Endocytosis, Lysosomal Enzyme Transport and Maturation of Cathepsins in Juvenile Neuronal Ceroid Lipofuscinosis Fibroblasts

Background: Juvenile neuronal ceroid lipofuscinosis (JNCL), one of the most frequent forms of the NCL storage diseases, is known to be caused by loss-of-function mutations in ceroid-lipofuscinosis neuronal protein 3 (CLN3), but its cell function has not been fully elucidated. We previously reported increased lysosomal pH in CLN3 deficient cells. In the present study, we analysed the consequences of this effect in the endo-lysosomal pathways in CLN3 cells. Methods: The present study investigated different endo-lysosomal pathways in control, CLN2, CLN3 human skin fibroblasts under high and low proteolysis conditions. Cell surface biotinylation after EGF (2 ng/mL) stimulation, EGF phosphorylation (Tyr-845), retromer and cation-independent mannose-6- phosphate receptor (CI-MPR) levels and stability, EGF degradation pathways and cathepsin L and D levels were analysed by western blots. Caveolae mediated endocytosis was analysed by flow cytometry. CIMPR subcellular localization was ascertained by immunocytochemistry, confocal microscopy and further image analysis. Results: Whereas caveolae-mediated endocytosis was not affected in CLN3 cells, clathrin-mediated epidermal growth factor (EGF) internalization was reduced, along with EGF receptor (EGFR) phosphorylation. In addition, cell surface EGFR levels and recycling to the cell membrane were increased. EGFR lysosomal degradation was impaired and our results suggest that the receptor was diverted to proteasomal degradation. We also analysed the machinery responsible for lysosomal hydrolase transport to the lysosome and found increased stability of CIMPR, a major receptor implicated in the transport of hydrolases. The subcellular distribution of the CI-MPR was also altered in CLN3 cells, since it accumulated within the Trans-Golgi network (TGN) and did not progress into the lysosomes. In addition, we found a reduced turnover of retromer subunits, a complex that retrieves the CI-MPR from endosomes to the TGN. Finally and as a possible consequence of these alterations in lysosomal enzyme transport, cathepsin L and D maturation were found suppressed in CLN3 cells. Conclusion: Altogether, these results point to increased lisosomal pH as a pivotal event causing various alterations in intracellular traffic associated to the development of JNCL disease.

CLN8 is an endoplasmic reticulum cargo receptor that regulates lysosome biogenesis.

Organelle biogenesis requires proper transport of proteins from their site of synthesis to their target subcellular compartment1–3. Lysosomal enzymes are synthesized in the endoplasmic reticulum (ER) and traffic through the Golgi complex before being transferred to the endolysosomal system4–6, but how they are transferred from the ER to the Golgi is unknown. Here we show that ER-to-Golgi transfer of lysosomal enzymes requires CLN8, an ER-associated membrane protein whose loss of function leads to the lysosomal storage disorder, Neuronal Ceroid Lipofuscinosis 8 (a type of Batten disease)7. ER-to-Golgi trafficking of CLN8 requires interaction with the COPII and COPI machineries via specific export and retrieval signals localized in the cytosolic C-terminus of CLN8. CLN8 deficiency leads to depletion of soluble enzymes in the lysosome, thus impairing lysosome biogenesis. Binding to lysosomal enzymes requires CLN8’s second luminal loop and is abolished by some disease-causing mutations within this region. Our data establish an unanticipated example of an ER receptor serving the biogenesis of an organelle and suggest that impaired transport of lysosomal enzymes underlies Batten disease caused by mutations in CLN8.

Sortilin and retromer mediate retrograde transport of Glut4 in 3T3-L1 adipocytes.

Sortilin is a multiligand sorting receptor responsible for the anterograde transport of lysosomal enzymes and substrates. Here we demonstrate that sortilin is also involved in retrograde protein traffic. In cultured 3T3-L1 adipocytes, sortilin together with retromer rescues Glut4 from degradation in lysosomes and retrieves it to the TGN, where insulin--responsive vesicles are formed. Mechanistically, the luminal Vps10p domain of sortilin interacts with the first luminal loop of Glut4, and the cytoplasmic tail of sortilin binds to retromer. Ablation of the retromer does not affect insulin signaling but decreases the stability of sortilin and Glut4 and blocks their entry into the small vesicular carriers. As a result, Glut4 cannot reach the insulin-responsive compartment, and insulin-stimulated glucose uptake in adipocytes is suppressed. We suggest that sortilin- and retromer-mediated Glut4 retrieval from endosomes may represent a step in the Glut4 pathway vulnerable to the development of insulin resistance and diabetes.

Effect of Phosphodiesterase in Regulating the Activity of Lysosomes in the HeLa Cell Line.

The transport of lysosomal enzymes into the lysosomes depends on the phosphorylation of their chains and the binding of the phosphorylated residues to mannose-6-phosphate receptors. The efficiency of separation depends more on the phosphodiesterases (PDEs) than on the activity of the phosphorylation of mannose residues and can be determined in vitro. PDEs play important roles in regulation of the activation of lysosomes. The expression of proteins was confirmed by western blotting. All PDE4 series protein expression was reduced in high concentrations of rolipram. As a result of observing the fluorescence intensity after rolipram treatment, the lysosomal enzyme was activated at low concentrations and suppressed at high concentrations. High concentrations of rolipram recovered the original function. Antimicrobial activity was not shown in either 10 or 100 µ concentrations of rolipram in treated HeLa cells in vitro. However, the higher anticancer activity at lower rolipram concentration was shown in lysosomal enzyme treated with 10 µ of rolipram. The anticancer activity was confirmed through cathepsin B and D assay. Tranfection allowed examination of the relationship between PDE4 and lysosomal activity in more detail. Protein expression was confirmed to be reduced. Fluorescence intensity showed decreased activity of lysosomes and ROS in cells transfected with the antisense sequences of PDE4 A, B, C, and D. PDE4A showed anticancer activity, whereas lysosome from cells transfected with the antisense sequences of PDE4 B, C, and D had decreased anticancer activity. These results showed the PDE4 A, B, C, and D are conjunctly related with lysosomal activity.

Prenatal Diagnosis of Mucolipidosis Type II: Comparison of Biochemical and Molecular Analyses

Purpose: Mucolipidosis type II (ML II), also known as I-cell disease is an autosomal recessive inherited disorder of lysosomal enzyme transport caused by a deficiency of the uridine diphosphate (UDP)-N-acetylglucosamine:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase (GlcNAc-phosphotransferase). Clinical manifestations are skeletal abnormalities, mental retardation, cardiac disease, and respiratory complications. A severely and rapidity progressive clinical course leads to death before 10 years of age. Methods/Results: In this study we diagnosed three cases of prenatal ML II in two different at-risk families. We compared two procedures -biochemical analysis and molecular analysis - for the prenatal diagnosis of ML II. Both methods require an invasive procedure to obtain specimens for the diagnosis. Biochemical analysis requires obtaining cell cultures from amniotic fluid for more than two weeks, and would result in a late diagnosis at 19 to 22 weeks of gestation. Molecular genetic testing by direct sequence analysis is usually possible when mutations are confirmed in the proband. Molecular analysis has an advantage in that it can be performed during the first-trimester. Conclusion: Molecular diagnosis is a preferable method when a prompt decision is necessary.

Pharmacologic manipulation of lysosomal enzyme transport across the blood–brain barrier

The adult blood–brain barrier, unlike the neonatal blood–brain barrier, does not transport lysosomal enzymes into brain, making enzyme replacement therapy ineffective in treating the central nervous system symptoms of lysosomal storage diseases. However, enzyme transport can be re-induced with alpha-adrenergics. Here, we examined agents that are known to alter the blood–brain barrier transport of large molecules or to induce lysosomal enzyme transport across the blood–brain barrier ((±)epinephrine, insulin, retinoic acid, and lipopolysaccharide) in 2-week-old and adult mice. In 2-week-old adolescent mice, all these pharmacologic agents increased brain and heart uptake of phosphorylated human β-glucuronidase. In 8-week-old adult mice, manipulations with (±)epinephrine, insulin, and retinoic acid were significantly effective on uptake by brain and heart. The increased uptake of phosphorylated human β-glucuronidase was inhibited by mannose 6-phosphate for the agents (±)epinephrine and retinoic acid and by L-NG-nitroarginine methyl ester for the agent lipopolysaccharide in neonatal and adult mice. An in situ brain perfusion study revealed that retinoic acid directly modulated the transport of phosphorylated human β-glucuronidase across the blood–brain barrier. The present study indicates that there are multiple opportunities to at least transiently induce phosphorylated human β-glucuronidase transport at the adult blood–brain barrier.

Single-chain antibody-fragment M6P-1 possesses a mannose 6-phosphate monosaccharide-specific binding pocket that distinguishes N-glycan phosphorylation in a branch-specific manner†.

The acquisition of mannose 6-phosphate (Man6P) on N-linked glycans of lysosomal enzymes is a structural requirement for their transport from the Golgi apparatus to lysosomes mediated by the mannose 6-phosphate receptors, 300 kDa cation-independent mannose 6-phosphate receptor (MPR300) and 46 kDa cation-dependent mannose 6-phosphate receptor (MPR46). Here we report that the single-chain variable domain (scFv) M6P-1 is a unique antibody fragment with specificity for Man6P monosaccharide that, through an array-screening approach against a number of phosphorylated N-glycans, is shown to bind mono- and diphosphorylated Man6 and Man7 glycans that contain terminal αMan6P(1 → 2)αMan(1 → 3)αMan. In contrast to MPR300, scFv M6P-1 does not bind phosphodiesters, monophosphorylated Man8 or mono- or diphosphorylated Man9 structures. Single crystal X-ray diffraction analysis to 2.7 Å resolution of Fv M6P-1 in complex with Man6P reveals that specificity and affinity is achieved via multiple hydrogen bonds to the mannose ring and two salt bridges to the phosphate moiety. In common with both MPRs, loss of binding was observed for scFv M6P-1 at pH values below the second pKa of Man6P (pKa = 6.1). The structures of Fv M6P-1 and the MPRs suggest that the change of the ionization state of Man6P is the main driving force for the loss of binding at acidic lysosomal pH (e.g. lysosome pH ∼ 4.6), which provides justification for the evolution of a lysosomal enzyme transport pathway based on Man6P recognition.

Lrp1/LDL Receptor Play Critical Roles in Mannose 6-Phosphate-Independent Lysosomal Enzyme Targeting.

Most lysosomal enzymes require mannose 6-phosphate (M6P) residues for efficient receptor-mediated lysosomal targeting. Although the lack of M6P residues results in missorting and hypersecretion, selected lysosomal enzymes reach normal levels in lysosomes of various cell types, suggesting the existence of M6P-independent transport routes. Here, we quantify the lysosomal proteome in M6P-deficient mouse fibroblasts (PT(ki)) using Stable Isotope Labeling by Amino acids in Cell culture (SILAC)-based comparative mass spectrometry, and find unchanged amounts of 20% of lysosomal enzymes, including cathepsins D and B (Ctsd and Ctsb). Examination of fibroblasts from a new mouse line lacking both M6P and sortilin, a candidate for M6P-independent transport of lysosomal enzymes, revealed that sortilin does not act as cargo receptor for Ctsb and Ctsd. Using fibroblast lines deficient for endocytic lipoprotein receptors, we could demonstrate that both LDL receptor and Lrp1 mediate the internalization of non-phosphorylated Ctsb and Ctsd. Furthermore, the presence of Lrp1 inhibitor increased the secretion of Ctsd from PT(ki) cells. These findings establish Lrp1 and LDL receptors in M6P-independent secretion-recapture targeting mechanism for lysosomal enzymes.

Overexpression of the IGF-II/M6P receptor in mouse fibroblast cell lines differentially alters expression profiles of genes involved in Alzheimer's disease-related pathology.

Alzheimer’s disease (AD) is the most common type of senile dementia affecting elderly people. The processing of amyloid precursor protein (APP) leading to the generation of β-amyloid (Aβ) peptide contributes to neurodegeneration and development of AD pathology. The endocytic trafficking pathway, which comprises of the endosomes and lysosomes, acts as an important site for Aβ generation, and endocytic dysfunction has been linked to increased Aβ production and loss of neurons in AD brains. Since insulin-like growth factor-II (IGF-II) receptor plays a critical role in the transport of lysosomal enzymes from the trans-Golgi network to endosomes, it is likely that the receptor may have a role in regulating Aβ metabolism in AD pathology. However, very little is known on how altered levels of the IGF-II receptor can influence the expression/function of various molecules involved in AD pathology. To address this issue, we evaluated the expression profiles of 87 selected genes related to AD pathology in mouse fibroblast MS cells that are deficient in murine IGF-II receptor and corresponding MS9II cells overexpressing ∼500 times the human IGF-II receptors. Our results reveal that an elevation in IGF-II receptor levels alters the expression profiles of a number of genes including APP as well as enzymes regulating Aβ production, degradation and clearance mechanisms. Additionally, it influences the expression of various lysosomal enzymes and protein kinases that are involved in Aβ toxicity. IGF-II receptor overexpression also alters expression of several genes involved in intracellular signalling as well as cholesterol metabolism, which play a critical role in AD pathology. The altered gene profiles observed in this study closely match with the corresponding protein levels, with a few exceptions. These results, taken together, suggest that an elevation in IGF-II receptor levels can influence the expression profiles of transcripts as well as proteins that are involved in AD pathogenesis.

Glycan Structure Determinants for Cation-Independent Mannose 6-Phosphate Receptor Binding and Cellular Uptake of a Recombinant Protein

The cation-independent mannose 6-phosphate receptor (CI-MPR) plays a critical role in intracellular transport of lysosomal enzymes as well as the uptake of recombinant proteins. To define the minimal glycan structure determinants necessary for receptor binding and cellular uptake, we synthesized a series of glycans containing mono-, di-, tri-, tetra-, and hexamannoses terminated with either one or two phosphates for conjugating to a model protein, recombinant human acid α-glucosidase. A high affinity interaction with the CI-MPR can be achieved for the enzyme conjugated to a dimannose glycan with a single phosphate. However, tightest binding to a CI-MPR affinity column was observed with a hexamannose structure containing two phosphates. Moreover, maximal cellular uptake and a 5-fold improvement in in vivo potency were achieved when the bisphosphorylated hexamannose glycan is conjugated to the protein by a β linker. Nevertheless, even a monophosphorylated dimannose glycan conjugate showed stronger binding to the receptor affinity column, higher cellular uptake, and significantly greater in vivo efficacy compared to the unconjugated protein which contains a low level of high affinity glycan structure. These results demonstrate that the phosphorylated dimannose moiety appears to be the minimal structure determinant for enhanced CI-MPR binding and that the orientation of the glycan is critical for maximum receptor interaction. In summary, we have improved the understanding of the mechanism of CI-MPR binding and developed a simple alternative for CI-MPR targeting.

Toward the successful delivery of lysosomal enzymes across the blood–brain barrier

AbstractThe development of enzyme replacement therapy (ERT) has been one of the most remarkable successes of translational studies, bridging laboratory discoveries to clinical settings. Although ERT is very effective in clearing peripheral tissue storage in many lysosomal storage diseases (LSD), negotiating the blood‐brain barrier (BBB) remains a key challenge in delivering corrective lysosomal enzymes into the brain. In recent years, brain delivery of lysosomal enzymes has been vigorously pursued for treating neurodegenerative LSD. Delivery strategies include high‐dose and long‐term treatment regimens, manipulation of receptor‐mediated transport, biochemical engineering of enzymes, use of transport vectors, and enzyme administration by alternative routes. Most studies have investigated biochemical or histological improvements in central nervous system (CNS) storage after treatment. Few studies, however, have directly focused on the mechanisms of lysosomal enzyme transport across the BBB. In previous studies, we discovered that the luminally expressed mannose 6‐phosphate/insulin‐like growth factor‐II receptor is a universal transporter of lysosomal enzymes. We and others have also successfully manipulated receptor‐mediated transport by using endogenous characteristics of the BBB. Interestingly, accumulating evidence indicates that some lysosomal enzymes employ multiple mechanisms for their transport across the BBB. This review provides a detailed discussion of the kinetics of lysosomal enzyme transport across the BBB, and the manipulation of transport characteristics based on studies of the transport of β‐glucuronidase, sulfamidase, arylsulfatase A and acid α‐glucosidase. Furthermore, recent achievements in treating abnormalities in CNS storage and their contribution to developing new therapeutic approaches are also discussed.

Enhanced activity of lysosomal proteases in activated rat hepatic stellate cells is associated with a concomitant increase in the number of the mannose‐6‐phosphate/insulin‐like growth factor II receptor

Activated hepatic stellate cells (HSCs) play a central role during hepatic tissue repair through their influence on extracellular matrix remodelling. We have determined whether the activity levels of cathepsin B and D are affected by in vitro activation of rat HSCs, and whether the enzymes were released from the cells. Furthermore, given the important role of the mannose-6-phosphate/insulin-like growth factor II receptor (M6P/IGF-IIR) in the intracellular transport of lysosomal enzymes, we have examined whether changes in the activity of these proteases were associated with parallel changes in the level of the M6P/IGF-IIR. The activity of cathepsin B and D increased ∼4 times between 2 and 8 days of HSC culture. This result was supported by analysing mRNA expression by RT-PCR. The cells released the enzymes into the culture medium, amounting to ∼10% of the cell-associated activity over 24 h. The release of enzymes was not affected by reducing medium pH from 7.4 to 6.2, indicating that the enzymes were transported to the medium independently of the M6P/IGF-II-R. The released cathepsin B was mostly in the inactive proenzyme form. HSC activation led to a particularly large increase in M6P/IGF-IIR expression. A large proportion of the receptors was located on the cell surface and was found to be very suitable for measuring endocytosis of (125) I-IGF-II. The results show that the endocytic activity increased in parallel with the increase in surface receptors and activity of lysosomal enzymes. Degradation of the ligand was reduced by inhibitors of lysosomal proteases and therefore took place in lysosomes.

HVps41 and VAMP7 function in direct TGN to late endosome transport of lysosomal membrane proteins

Targeted delivery of lysosome-associated membrane proteins is important for lysosome stability and function. Here we identify a pathway for transport of lysosome-associated membrane proteins directly from the trans-Golgi network to late endosomes, which exists in parallel to mannose 6-phosphate receptor and clathrin-dependent transport of lysosomal enzymes to early endosomes. By immunoelectron microscopy we localized endogenous LAMP-1 and -2 as well as LAMP-1-mGFP to non-coated, biosynthetic carriers at the trans-Golgi network and near late endosomes. These LAMP carriers were negative for mannose 6-phosphate receptor, adaptor-protein complex-1, secretory albumin and endocytic markers, but contained the homotypic fusion and protein sorting complex component hVps41 and the soluble N-ethylmaleimide-sensitive factor attachment protein receptors protein VAMP7. Knockdown of hVps41 or VAMP7 resulted in the accumulation of lysosome-associated membrane protein carriers, whereas knockdown of hVps39 or hVps18 did not, indicating that the effect of hVps41 is independent of CORVET/HOPS. Mannose 6-phosphate receptor carriers remained unaffected upon hVps41 or VAMP7 knockdown, implicating that hVps41 and VAMP7 are specifically involved in the fusion of trans-Golgi network-derived lysosome-associated membrane protein carriers with late endosomes.