Altered GM1 catabolism affects NMDAR-mediated Ca2+ signaling at ER-PM junctions and increases synaptic spine formation in a GM1-gangliosidosis model

Endoplasmic reticulum-plasma membrane (ER-PM) junctions mediate Ca2+ flux across neuronal membranes. The properties of these membrane contact sites are defined by their lipid content, but little attention has been given to glycosphingolipids (GSLs). Here, we show that GM1-ganglioside, an abundant GSL in neuronal membranes, is integral to ER-PM junctions; it interacts with synaptic proteins/receptors and regulates Ca2+ signaling. In a model of the neurodegenerative lysosomal storage disease, GM1-gangliosidosis, pathogenic accumulation of GM1 at ER-PM junctions due to β-galactosidase deficiency drastically alters neuronal Ca2+ homeostasis. Mechanistically, we show that GM1 interacts with the phosphorylated N-methyl D-aspartate receptor (NMDAR) Ca2+ channel, thereby increasing Ca2+ flux, activating extracellular signal-regulated kinase (ERK) signaling, and increasing the number of synaptic spines without increasing synaptic connectivity. Thus, GM1 clustering at ER-PM junctions alters synaptic plasticity and worsens the generalized neuronal cell death characteristic of GM1-gangliosidosis.

AAV-mediated gene therapy for Sialidosis

Sialidosis is a glycoprotein storage disease caused by deficiency of the lysosomal sialidase NEU1, which leads to pathogenic accumulation of sialylated glycoproteins and oligosaccharides in tissues and body fluids. The disease belongs to the group of orphan disorders with no therapy currently available. Here, we have tested the therapeutic potential of AAV-mediated gene therapy for the treatment of sialidosis in a mouse model of the disease. One-month-old Neu1 -/- mice were co-injected with two scAAV2/8 vectors, expressing NEU1 and its chaperone PPCA, and sacrificed at 3 months post-injection. Treated mice were phenotypically indistinguishable from their WT controls. Histopathologically, they showed diminished or absent vacuolization in cells of visceral organs, including the kidney, as well as the choroid plexus and other areas of the brain. This was accompanied by restoration of NEU1 activity in most tissues, reversal of sialyl-oligosacchariduria, and normalization of lysosomal exocytosis in the CSF and serum of treated mice. AAV injection prevented the occurrence of generalized fibrosis, which is a prominent contributor of disease pathogenesis in Neu1 -/- mice and likely in patients. Overall, this therapeutic strategy holds promise for the treatment of sialidosis and may be applicable to adult forms of human idiopathic fibrosis with low NEU1 expression.

Glycosphingolipids within membrane contact sites influence their function as signaling hubs in neurodegenerative diseases

Intracellular organelles carry out many of their functions by engaging in extensive interorganellar communication through specialized membrane contact sites (MCSs) formed where two organelles tether to each other or to the plasma membrane (PM) without fusing. In recent years, these ubiquitous membrane structures have emerged as central signaling hubs that control a multitude of cellular pathways, ranging from lipid metabolism/transport to the exchange of metabolites and ions (i.e., Ca2+ ), and general organellar biogenesis. The functional crosstalk between juxtaposed membranes at MCSs relies on a defined composite of proteins and lipids that populate these microdomains in a dynamic fashion. This is particularly important in the nervous system, where alterations in the composition of MCSs have been shown to affect their functions and have been implicated in the pathogenesis of neurodegenerative diseases. In this review, we focus on the MCSs that are formed by the tethering of the endoplasmic reticulum (ER) to the mitochondria, the ER to the endo-lysosomes and the mitochondria to the lysosomes. We highlight how glycosphingolipids that are aberrantly processed/degraded and accumulate ectopically in intracellular membranes and the PM change the topology of MCSs, disrupting signaling pathways that lead to neuronal demise and neurodegeneration. In particular, we focus on neurodegenerative lysosomal storage diseases linked to altered glycosphingolipid catabolism.

Altered GM1 catabolism affects NMDAR-mediated Ca 2+ signaling at ER-PM junctions and increases synaptic spine formation

Endoplasmic reticulum-plasma membrane (ER-PM) junctions mediate Ca 2+ flux across neuronal membranes. The properties of these membrane contact sites are defined by their lipid content, but little attention has been given to glycosphingolipids (GSLs). Here, we show that GM1-ganglioside, an abundant GSL in neuronal membranes, is integral to ER-PM junctions; it interacts with synaptic proteins/receptors and regulates Ca 2+ signaling. In a model of the neurodegenerative lysosomal storage disease, GM1-gangliosidosis, pathogenic accumulation of GM1 at ER-PM junctions due to β-galactosidase deficiency drastically alters neuronal Ca 2+ homeostasis. Mechanistically, we show that GM1 interacts with the phosphorylated NMDAR Ca 2+ channel, thereby increasing Ca 2+ flux, activating ERK signaling, and increasing the number of synaptic spines without increasing synaptic connectivity. Thus, GM1 clustering at ER-PM junctions alters synaptic plasticity and exacerbates the generalized neuronal cell death characteristic of GM1-gangliosidosis.

Distinct functions of dimeric and monomeric scaffold protein Alix in regulating F-actin assembly and loading of exosomal cargo

Alix is a ubiquitously expressed scaffold protein that participates in numerous cellular processes related to the remodeling/repair of membranes and the actin cytoskeleton. Alix exists in monomeric and dimeric/multimeric configurations, but how dimer formation occurs and what role the dimer has in Alix-mediated processes are still largely elusive. Here, we reveal a mechanism for Alix homodimerization mediated by disulfide bonds under physiological conditions, and demonstrate that the Alix dimer is enriched in exosomes and F-actin cytoskeleton subcellular fractions. Proteomic analysis of exosomes derived from Alix^-/- primary cells underlined the indispensable role of Alix in loading syntenin into exosomes, thereby regulating the cellular levels of this protein. Using a set of deletion mutants, we define the function of Alix Bro1 domain, which is solely required for its exosomal localization, and that of the V domain, which is needed for recruiting syntenin into exosomes. We reveal an essential role for Cys⁸¹⁴ within the disordered proline rich domain (PRD) for Alix dimerization. By mutating this residue, we show that Alix remains exclusively monomeric and, in this configuration, is effective in loading syntenin into exosomes. In contrast, loss of dimerization affects the ability of Alix to associate with F-actin, thereby compromising Alix-mediated cytoskeleton remodeling. We propose that dimeric and monomeric forms of Alix selectively execute two of the protein's main functions: exosomal cargo loading and cytoskeleton remodeling.

AAV-mediated gene therapy for galactosialidosis: A long-term safety and efficacy study

AAV-mediated gene therapy holds promise for the treatment of lysosomal storage diseases (LSDs), some of which are already in clinical trials. Yet, ultra-rare subtypes of LSDs, such as some glycoproteinoses, have lagged. Here, we report on a long-term safety and efficacy preclinical study conducted in the murine model of galactosialidosis, a glycoproteinosis caused by a deficiency of protective protein/cathepsin A (PPCA). One-month-old Ctsa -/- mice were injected intravenously with a high dose of a self-complementary AAV2/8 vector expressing human CTSA in the liver. Treated mice, examined up to 12 months post injection, appeared grossly indistinguishable from their wild-type littermates. Sustained expression of scAAV2/8-CTSA in the liver resulted in the release of the therapeutic precursor protein in circulation and its widespread uptake by cells in visceral organs and the brain. Increased cathepsin A activity resolved lysosomal vacuolation throughout the affected organs and sialyl-oligosacchariduria. No signs of hyperplasia or inflammation were detected in the liver up to a year of age. Clinical chemistry panels, blood cell counts, and T cell immune responses were normal in all treated animals. These results warrant a close consideration of this gene therapy approach for the treatment of galactosialidosis, an orphan disease with no cure in sight.

Galactosialidosis: preclinical enzyme replacement therapy in a mouse model of the disease, a proof of concept

Galactosialidosis is a rare lysosomal storage disease caused by a congenital defect of protective protein/cathepsin A (PPCA) and secondary deficiency of neuraminidase-1 and β-galactosidase. PPCA is a lysosomal serine carboxypeptidase that functions as a chaperone for neuraminidase-1 and β-galactosidase within a lysosomal multi-protein complex. Combined deficiency of the three enzymes leads to accumulation of sialylated glycoproteins and oligosaccharides in tissues and body fluids and manifests in a systemic disease pathology with severity mostly correlating with the type of mutation(s) and age of onset of the symptoms. Here, we describe a proof-of-concept, preclinical study toward the development of enzyme replacement therapy for galactosialidosis, using a recombinant human PPCA. We show that the recombinant enzyme, taken up by patient-derived fibroblasts, restored cathepsin A, neuraminidase-1, and β-galactosidase activities. Long-term, bi-weekly injection of the recombinant enzyme in a cohort of mice with null mutation at the PPCA (CTSA) locus (PPCA -/- ), a faithful model of the disease, demonstrated a dose-dependent, systemic internalization of the enzyme by cells of various organs, including the brain. This resulted in restoration/normalization of the three enzyme activities, resolution of histopathology, and reduction of sialyloligosacchariduria. These positive results underscore the benefits of a PPCA-mediated enzyme replacement therapy for the treatment of galactosialidosis.

Lysosomes and Cancer Progression: A Malignant Liaison

During primary tumorigenesis isolated cancer cells may undergo genetic or epigenetic changes that render them responsive to additional intrinsic or extrinsic cues, so that they enter a transitional state and eventually acquire an aggressive, metastatic phenotype. Among these changes is the alteration of the cell metabolic/catabolic machinery that creates the most permissive conditions for invasion, dissemination, and survival. The lysosomal system has emerged as a crucial player in this malignant transformation, making this system a potential therapeutic target in cancer. By virtue of their ubiquitous distribution in mammalian cells, their multifaced activities that control catabolic and anabolic processes, and their interplay with other organelles and the plasma membrane (PM), lysosomes function as platforms for inter- and intracellular communication. This is due to their capacity to adapt and sense nutrient availability, to spatially segregate specific functions depending on their position, to fuse with other compartments and with the PM, and to engage in membrane contact sites (MCS) with other organelles. Here we review the latest advances in our understanding of the role of the lysosomal system in cancer progression. We focus on how changes in lysosomal nutrient sensing, as well as lysosomal positioning, exocytosis, and fusion perturb the communication between tumor cells themselves and between tumor cells and their microenvironment. Finally, we describe the potential impact of MCS between lysosomes and other organelles in propelling cancer growth and spread.

Isolation and Characterization of Exosomes from Skeletal Muscle Fibroblasts

Exosomes are small extracellular vesicles released by virtually all cells and secreted in all biological fluids. Many methods have been developed for the isolation of these vesicles, including ultracentrifugation, ultrafiltration, and size exclusion chromatography. However, not all are suitable for large scale exosome purification and characterization. Outlined here is a protocol for establishing cultures of primary fibroblasts isolated from adult mouse skeletal muscles, followed by purification and characterization of exosomes from the culture media of these cells. The method is based on the use of sequential centrifugation steps followed by sucrose density gradients. Purity of the exosomal preparations is then validated by western blot analyses using a battery of canonical markers (i.e., Alix, CD9, and CD81). The protocol describes how to isolate and concentrate bioactive exosomes for electron microscopy, mass spectrometry, and uptake experiments for functional studies. It can easily be scaled up or down and adapted for exosome isolation from different cell types, tissues, and biological fluids.

Conventional and Unconventional Therapeutic Strategies for Sialidosis Type I

Congenital deficiency of the lysosomal sialidase neuraminidase 1 (NEU1) causes the lysosomal storage disease, sialidosis, characterized by impaired processing/degradation of sialo-glycoproteins and sialo-oligosaccharides, and accumulation of sialylated metabolites in tissues and body fluids. Sialidosis is considered an ultra-rare clinical condition and falls into the category of the so-called orphan diseases, for which no therapy is currently available. In this study we aimed to identify potential therapeutic modalities, targeting primarily patients affected by type I sialidosis, the attenuated form of the disease. We tested the beneficial effects of a recombinant protective protein/cathepsin A (PPCA), the natural chaperone of NEU1, as well as pharmacological and dietary compounds on the residual activity of mutant NEU1 in a cohort of patients’ primary fibroblasts. We observed a small, but consistent increase in NEU1 activity, following administration of all therapeutic agents in most of the fibroblasts tested. Interestingly, dietary supplementation of betaine, a natural amino acid derivative, in mouse models with residual NEU1 activity mimicking type I sialidosis, increased the levels of mutant NEU1 and resolved the oligosacchariduria. Overall these findings suggest that carefully balanced, unconventional dietary compounds in combination with conventional therapeutic approaches may prove to be beneficial for the treatment of sialidosis type I.

MYC competes with MiT/TFE in regulating lysosomal biogenesis and autophagy through an epigenetic rheostat

Coordinated regulation of the lysosomal and autophagic systems ensures basal catabolism and normal cell physiology, and failure of either system causes disease. Here we describe an epigenetic rheostat orchestrated by c-MYC and histone deacetylases that inhibits lysosomal and autophagic biogenesis by concomitantly repressing the expression of the transcription factors MiT/TFE and FOXH1, and that of lysosomal and autophagy genes. Inhibition of histone deacetylases abates c-MYC binding to the promoters of lysosomal and autophagy genes, granting promoter occupancy to the MiT/TFE members, TFEB and TFE3, and/or the autophagy regulator FOXH1. In pluripotent stem cells and cancer, suppression of lysosomal and autophagic function is directly downstream of c-MYC overexpression and may represent a hallmark of malignant transformation. We propose that, by determining the fate of these catabolic systems, this hierarchical switch regulates the adaptive response of cells to pathological and physiological cues that could be exploited therapeutically.

Genes related to lysosomal and autophagic systems are transcriptionally regulated by the Mit/TFE family of transcription factors. Here the authors show that MYC, in association with HDACs, suppresses the expression of lysosomal and autophagy genes by competing with the Mit/TFE transcription factors for occupancy of their target gene promoters.

Excessive exosome release is the pathogenic pathway linking a lysosomal deficiency to generalized fibrosis

Lysosomal exocytosis is a ubiquitous process negatively regulated by neuraminidase 1 (NEU1), a sialidase mutated in the glycoprotein storage disease sialidosis. In Neu1-/- mice, excessive lysosomal exocytosis is at the basis of disease pathogenesis. Yet, the tissue-specific molecular consequences of this deregulated pathway are still unfolding. We now report that in muscle connective tissue, Neu1-/- fibroblasts have features of myofibroblasts and are proliferative, migratory, and exocytose large amounts of exosomes. These nanocarriers loaded with activated transforming growth factor-β and wingless-related integration site (WNT)/β-catenin signaling molecules propagate fibrotic signals to other cells, maintaining the tissue in a prolonged transitional status. Myofibroblast-derived exosomes fed to normal fibroblasts convert them into myofibroblasts, changing the recipient cells' proliferative and migratory properties. These findings reveal an unexpected exosome-mediated signaling pathway downstream of NEU1 deficiency that propagates a fibrotic disease and could be implicated in idiopathic forms of fibrosis in humans.

Regulated lysosomal exocytosis mediates cancer progression

Understanding how tumor cells transition to an invasive and drug-resistant phenotype is central to cancer biology, but the mechanisms underlying this transition remain unclear. We show that sarcomas gain these malignant traits by inducing lysosomal exocytosis, a ubiquitous physiological process. During lysosomal exocytosis, the movement of exocytic lysosomes along the cytoskeleton and their docking at the plasma membrane involve LAMP1, a sialylated membrane glycoprotein and target of the sialidase NEU1. Cleavage of LAMP1 sialic acids by NEU1 limits the extent of lysosomal exocytosis. We found that by down-regulation of NEU1 and accumulation of oversialylated LAMP1, tumor cells exacerbate lysosomal exocytosis of soluble hydrolases and exosomes. This facilitates matrix invasion and propagation of invasive signals, and purging of lysosomotropic chemotherapeutics. In Arf (-⁄-) mice, Neu1 haploinsufficiency fostered the development of invasive, pleomorphic sarcomas, expressing epithelial and mesenchymal markers, and lysosomal exocytosis effectors, LAMP1 and Myosin-11. These features are analogous to those of metastatic, pleomorphic human sarcomas, where low NEU1 levels correlate with high expression of lysosomal exocytosis markers. In a therapeutic proof of principle, we demonstrate that inhibiting lysosomal exocytosis reversed invasiveness and chemoresistance in aggressive sarcoma cells. Thus, we reveal that this unconventional, lysosome-regulated pathway plays a primary role in tumor progression and chemoresistance.

Neuraminidase-1 mediates skeletal muscle regeneration

Neuraminidase-1 (NEU1) is the sialidase responsible for the catabolism of sialoglycoconjugates in lysosomes. Congenital NEU1 deficiency causes sialidosis, a severe lysosomal storage disease associated with a broad spectrum of clinical manifestations, which also include skeletal deformities, skeletal muscle hypotonia and weakness. Neu1(-/-) mice, a model of sialidosis, develop an atypical form of muscle degeneration caused by progressive expansion of the connective tissue that infiltrates the muscle bed, leading to fiber degeneration and atrophy. Here we investigated the role of Neu1 in the myogenic process that ensues during muscle regeneration after cardiotoxin-induced injury of limb muscles. A comparative analysis of cardiotoxin-treated muscles from Neu1(-/-) mice and Neu1(+/+) mice showed increased inflammatory and proliferative responses in the absence of Neu1 during the early stages of muscle regeneration. This was accompanied by significant and sequential upregulation of Pax7, MyoD, and myogenin mRNAs. The levels of both MyoD and myogenin proteins decreased during the late stages of regeneration, which most likely reflected an increased rate of degradation of the myogenic factors in the Neu1(-/-) muscle. We also observed a delay in muscle cell differentiation, which was characterized by prolonged expression of embryonic myosin heavy chain, as well as reduced myofiber cross-sectional area. At the end of the regenerative process, collagen type III deposition was increased compared to wild-type muscles and internal controls, indicating the initiation of fibrosis. Overall, these results point to a role of Neu1 throughout muscle regeneration.

Broad and direct interaction between TLR and Siglec families of pattern recognition receptors and its regulation by Neu1

Both pathogen- and tissue damage-associated molecular patterns induce inflammation through toll-like receptors (TLRs), while sialic acid-binding immunoglobulin superfamily lectin receptors (Siglecs) provide negative regulation. Here we report extensive and direct interactions between these pattern recognition receptors. The promiscuous TLR binders were human SIGLEC-5/9 and mouse Siglec-3/E/F. Mouse Siglec-G did not show appreciable binding to any TLRs tested. Correspondingly, Siglece deletion enhanced dendritic cell responses to all microbial TLR ligands tested, while Siglecg deletion did not affect the responses to these ligands. TLR4 activation triggers Neu1 translocation to cell surface to disrupt TLR4:Siglec-E interaction. Conversely, sialidase inhibitor Neu5Gc2en prevented TLR4 ligand-induced disruption of TLR4:Siglec E/F interactions. Absence of Neu1 in hematopoietic cells or systematic treatment with sialidase inhibitor Neu5Gc2en protected mice against endotoxemia. Our data raised an intriguing possibility of a broad repression of TLR function by Siglecs and a sialidase-mediated de-repression that allows positive feedback of TLR activation during infection.

DOI:http://dx.doi.org/10.7554/eLife.04066.001

Many living things have an immune system that is able to detect invading bacteria, viruses and other pathogens and trigger a response targeted against the threat before it causes lasting damage. Cells employ a number of different receptors that can detect these pathogens or the molecules that they produce.

In animals, toll-like receptors (or TLRs) are a type of protein that recognizes patterns or structures that are found in many different types of pathogen, known as pathogen-associated molecular patterns (or PAMPs). Injured cells release proteins that are also recognized by toll-like receptors and are called danger associated molecular patterns (or DAMPs). An immune response is triggered when PAMPs and DAMPs are recognized, but the response must be properly controlled. If it goes awry, it can result in an over-activation of the immune cells that can lead to life-threatening conditions, one of which is called sepsis.

Siglecs are proteins that bind to a sugar molecule, which is found attached to many other proteins, and are known to inhibit the immune response. However, it remained unclear how Siglecs do this and if they can interact directly with toll-like receptors. Chen et al. now show that most (although not all) Siglecs bind to TLRs, and that deleting the gene for a Siglec protein that can bind to multiple TLRs boosted the response of the immune cells to a range of microbial PAMPs. Deleting the gene for another Siglec that did not bind to any TLRs had no effect on the immune response.

Chen et al. suggest that the Siglec proteins that interact with toll-like receptors act a bit like a brake that slows down the activation of the receptors. However, when an immune cell detects a foreign molecule through a TLR, an enzyme called Neu1 is relocated from the inside of the cell to the cell's surface, where it removes the sugar molecules from the TLRs. This disrupts the interaction between the TLRs and the Siglecs, thus activating the receptors and triggering an immune response against the invading pathogen or damaged cells. This represents a newly discovered mechanism that can regulate the signaling of TLRs.

Chen et al. also show that a chemical compound that stops the function of the Neu1 enzyme prevents the toll-like receptors—and hence the immune cells—from becoming overly activated. Mice treated with this compound are protected against sepsis triggered by the presence of a bacterial PAMP. These results suggest that the Neu1 enzyme may be a promising new target for treating sepsis; further work will now be required to assess the potential side effects caused by inhibiting this enzyme.

DOI:http://dx.doi.org/10.7554/eLife.04066.002

Chaperone-mediated gene therapy with recombinant AAV-PPCA in a new mouse model of type I sialidosis

The lysosomal storage disease sialidosis is caused by a primary deficiency of the sialidase N-acetyl-α-neuraminidase-1 (NEU1). Patients with type I sialidosis develop an attenuated, non-neuropathic form of the disease also named cherry red spot myoclonus syndrome, with symptoms arising during juvenile/ adult age. NEU1 requires binding to its chaperone, protective protein/cathepsin A (PPCA), for lysosomal compartmentalization, stability and catalytic activation. We have generated a new mouse model of type I sialidosis that ubiquitously expresses a NEU1 variant carrying a V54M amino acid substitution identified in an adult patient with type I sialidosis. Mutant mice developed signs of lysosomal disease after 1year of age, predominantly in the kidney, albeit low residual NEU1 activity was detected in most organs and cell types. We demonstrate that the activity of the mutant enzyme could be effectively increased in all systemic tissues by chaperone-mediated gene therapy with a liver-tropic recombinant AAV2/8 vector expressing PPCA. This resulted in clear amelioration of the disease phenotype. These results suggest that at least some of the NEU1 mutations associated with type I sialidosis may respond to PPCA-chaperone-mediated gene therapy.

LPS-induced cytokine production in human dendritic cells is regulated by sialidase activity

Removal of sialic acid from glycoconjugates on the surface of monocytes enhances their response to bacterial LPS. We tested the hypothesis that endogenous sialidase activity creates a permissive state for LPS-induced cytokine production in human monocyte-derived DCs. Of the four genetically distinct sialidases (Neu1-4), Neu1, Neu3, and Neu4 are expressed in human monocytes, but only Neu1 and Neu3 are up-regulated as cells differentiate into DCs. Neu1 and Neu3 are present on the surface of monocytes and DCs and are also present intracellularly. DCs contain a greater amount of sialic acid than monocytes, but the amount of sialic acid/mg total protein declines during differentiation to DCs. This relative hyposialylation of cells does not occur in mature DCs grown in the presence of zanamivir, a pharmacologic inhibitor of Neu3 but not Neu1, or DANA, an inhibitor of Neu1 and Neu3. Inhibition of sialidase activity during differentiation to DCs causes no detectable change in cell viability or expression of DC surface markers. Differentiation of monocytes into DCs in the presence of zanamivir results in reduced LPS- induced expression of IL-6, IL-12p40, and TNF-α by mature DCs, demonstrating a role for Neu3 in cytokine production. A role for Neu3 is supported by inhibition of cytokine production by DANA in DCs from Neu1⁻/⁻ and WT mice. We conclude that sialidase-mediated change in sialic acid content of specific cell surface glycoconjugates in DCs regulates LPS-induced cytokine production, thereby contributing to development of adaptive immune responses.