Neuronal-specific impairment of heparan sulfate degradation in Drosophila reveals pathogenic mechanisms for Mucopolysaccharidosis type IIIA

Structure of a truncated human GlcNAc-1-phosphotransferase variant reveals the basis for its hyperactivity

Mutations that cause loss of function of GlcNAc-1-phosphotransferase (PTase) lead to the lysosomal storage disorder mucolipidosis II. PTase is the key enzyme of the mannose 6-phosphate (M6P) targeting system that is responsible for tagging lysosomal hydrolases with the M6P moiety for their delivery to the lysosome. We had previously generated a truncated hyperactive form of PTase termed S1S3 which was shown to notably increase the phosphorylation level of secreted lysosomal enzymes and enhance their uptake by cells. Here, we report the 3.4 Å cryo-EM structure of soluble S1S3 lacking both transmembrane domains and cytosolic tails. The structure reveals a high degree of conservation of the catalytic core to full-length PTase. In this dimeric structure, the EF-hand of one protomer is observed interacting with the conserved region four of the other. In addition, we present a high-quality EM 3D map of the UDP-GlcNAc bound form of the full-length soluble protein showing the key molecular interactions between the nucleotide sugar donor and side chain amino acids of the protein. Finally, although the domain organization of S1S3 is very similar to that of the Drosophila melanogaster (fruit fly) PTase homolog, we establish that the latter does not act on lysosomal hydrolases.

Drosophila melanogaster models of MPS IIIC (Hgsnat-deficiency) highlight the role of glia in disease presentation

Sanfilippo syndrome (Mucopolysaccharidosis type III or MPS III) is a recessively inherited neurodegenerative lysosomal storage disorder. Mutations in genes encoding enzymes in the heparan sulphate degradation pathway lead to the accumulation of partially degraded heparan sulphate, resulting ultimately in the development of neurological deficits. Mutations in the gene encoding the membrane protein heparan-α-glucosaminide N-acetyltransferase (HGSNAT; EC2.3.1.78) cause MPS IIIC (OMIM#252930), typified by impaired cognition, sleep-wake cycle changes, hyperactivity and early death, often before adulthood. The precise disease mechanism that causes symptom emergence remains unknown, posing a significant challenge in the development of effective therapeutics. As HGSNAT is conserved in Drosophila melanogaster, we now describe the creation and characterisation of the first Drosophila models of MPS IIIC. Flies with either an endogenous insertion mutation or RNAi-mediated knockdown of hgsnat were confirmed to have a reduced level of HGSNAT transcripts and age-dependent accumulation of heparan sulphate leading to engorgement of the endo/lysosomal compartment. This resulted in abnormalities at the pre-synapse, defective climbing and reduced overall activity. Altered circadian rhythms (shift in peak morning activity) were seen in hgsnat neuronal knockdown lines. Further, when hgsnat was knocked down in specific glial subsets (wrapping, cortical, astrocytes or subperineural glia), impaired climbing or reduced activity was noted, implying that hgsnat function in these specific glial subtypes contributes significantly to this behaviour and targeting treatments to these cell groups may be necessary to ameliorate or prevent symptom onset. These novel models of MPS IIIC provide critical research tools for delineating the key cellular pathways causal in the onset of neurodegeneration in this presently untreatable disorder.

Models to study basic and applied aspects of lysosomal storage disorders

The lack of available treatments and fatal outcome in most lysosomal storage disorders (LSDs) have spurred research on pathological mechanisms and novel therapies in recent years. In this effort, experimental methodology in cellular and animal models have been developed, with aims to address major challenges in many LSDs such as patient-to-patient variability and brain condition. These techniques and models have advanced knowledge not only of LSDs but also for other lysosomal disorders and have provided fundamental insights into the biological roles of lysosomes. They can also serve to assess the efficacy of classical therapies and modern drug delivery systems. Here, we summarize the techniques and models used in LSD research, which include both established and recently developed in vitro methods, with general utility or specifically addressing lysosomal features. We also review animal models of LSDs together with cutting-edge technology that may reduce the need for animals in the study of these devastating diseases.

Lysosomal positioning diseases: beyond substrate storage

Lysosomal storage diseases (LSDs) comprise a group of inherited monogenic disorders characterized by lysosomal dysfunctions due to undegraded substrate accumulation. They are caused by a deficiency in specific lysosomal hydrolases involved in cellular catabolism, or non-enzymatic proteins essential for normal lysosomal functions. In LSDs, the lack of degradation of the accumulated substrate and its lysosomal storage impairs lysosome functions resulting in the perturbation of cellular homeostasis and, in turn, the damage of multiple organ systems. A substantial number of studies on the pathogenesis of LSDs has highlighted how the accumulation of lysosomal substrates is only the first event of a cascade of processes including the accumulation of secondary metabolites and the impairment of cellular trafficking, cell signalling, autophagic flux, mitochondria functionality and calcium homeostasis, that significantly contribute to the onset and progression of these diseases. Emerging studies on lysosomal biology have described the fundamental roles of these organelles in a variety of physiological functions and pathological conditions beyond their canonical activity in cellular waste clearance. Here, we discuss recent advances in the knowledge of cellular and molecular mechanisms linking lysosomal positioning and trafficking to LSDs.

The Inflammation in the Cytopathology of Patients With Mucopolysaccharidoses- Immunomodulatory Drugs as an Approach to Therapy

Mucopolysaccharidoses (MPS) are a group of lysosomal storage diseases (LSDs), characterized by the accumulation of glycosaminoglycans (GAGs). GAG storage-induced inflammatory processes are a driver of cytopathology in MPS and pharmacological immunomodulation can bring improvements in brain, cartilage and bone pathology in rodent models. This manuscript reviews current knowledge with regard to inflammation in MPS patients and provides hypotheses for the therapeutic use of immunomodulators in MPS. Thus, we aim to set the foundation for a rational repurposing of the discussed molecules to minimize the clinical unmet needs still remaining despite enzyme replacement therapy (ERT) and hematopoietic stem cell transplantation (HSCT).

Drosophila D-idua Reduction Mimics Mucopolysaccharidosis Type I Disease-Related Phenotypes

Deficit of the IDUA (α-L-iduronidase) enzyme causes the lysosomal storage disorder mucopolysaccharidosis type I (MPS I), a rare pediatric neurometabolic disease, due to pathological variants in the IDUA gene and is characterized by the accumulation of the undegraded mucopolysaccharides heparan sulfate and dermatan sulfate into lysosomes, with secondary cellular consequences that are still mostly unclarified. Here, we report a new fruit fly RNAi-mediated knockdown model of a IDUA homolog (D-idua) displaying a phenotype mimicking some typical molecular features of Lysosomal Storage Disorders (LSD). In this study, we showed that D-idua is a vital gene in Drosophila and that ubiquitous reduction of its expression leads to lethality during the pupal stage, when the precise degradation/synthesis of macromolecules, together with a functional autophagic pathway, are indispensable for the correct development to the adult stage. Tissue-specific analysis of the D-idua model showed an increase in the number and size of lysosomes in the brain and muscle. Moreover, the incorrect acidification of lysosomes led to dysfunctional lysosome-autophagosome fusion and the consequent block of autophagy flux. A concomitant metabolic drift of glycolysis and lipogenesis pathways was observed. After starvation, D-idua larvae showed a quite complete rescue of both autophagy/lysosome phenotypes and metabolic alterations. Metabolism and autophagy are strictly interconnected vital processes that contribute to maintain homeostatic control of energy balance, and little is known about this regulation in LSDs. Our results provide new starting points for future investigations on the disease’s pathogenic mechanisms and possible pharmacological manipulations.

An Engineered sgsh Mutant Zebrafish Recapitulates Molecular and Behavioural Pathobiology of Sanfilippo Syndrome A/MPS IIIA

Mucopolysaccharidosis IIIA (MPS IIIA, Sanfilippo syndrome type A), a paediatric neurological lysosomal storage disease, is caused by impaired function of the enzyme N-sulfoglucosamine sulfohydrolase (SGSH) resulting in impaired catabolism of heparan sulfate glycosaminoglycan (HS GAG) and its accumulation in tissues. MPS IIIA represents a significant proportion of childhood dementias. This condition generally leads to patient death in the teenage years, yet no effective therapy exists for MPS IIIA and a complete understanding of the mechanisms of MPS IIIA pathogenesis is lacking. Here, we employ targeted CRISPR/Cas9 mutagenesis to generate a model of MPS IIIA in the zebrafish, a model organism with strong genetic tractability and amenity for high-throughput screening. The sgsh^Δex5-6 zebrafish mutant exhibits a complete absence of Sgsh enzymatic activity, leading to progressive accumulation of HS degradation products with age. sgsh^Δex5-6 zebrafish faithfully recapitulate diverse CNS-specific features of MPS IIIA, including neuronal lysosomal overabundance, complex behavioural phenotypes, and profound, lifelong neuroinflammation. We further demonstrate that neuroinflammation in sgsh^Δex5-6 zebrafish is largely dependent on interleukin-1β and can be attenuated via the pharmacological inhibition of Caspase-1, which partially rescues behavioural abnormalities in sgsh^Δex5-6 mutant larvae in a context-dependent manner. We expect the sgsh^Δex5-6 zebrafish mutant to be a valuable resource in gaining a better understanding of MPS IIIA pathobiology towards the development of timely and effective therapeutic interventions.

Exploiting the Potential of Drosophila Models in Lysosomal Storage Disorders: Pathological Mechanisms and Drug Discovery

Lysosomal storage disorders (LSDs) represent a complex and heterogeneous group of rare genetic diseases due to mutations in genes coding for lysosomal enzymes, membrane proteins or transporters. This leads to the accumulation of undegraded materials within lysosomes and a broad range of severe clinical features, often including the impairment of central nervous system (CNS). When available, enzyme replacement therapy slows the disease progression although it is not curative; also, most recombinant enzymes cannot cross the blood-brain barrier, leaving the CNS untreated. The inefficient degradative capability of the lysosomes has a negative impact on the flux through the endolysosomal and autophagic pathways; therefore, dysregulation of these pathways is increasingly emerging as a relevant disease mechanism in LSDs. In the last twenty years, different LSD Drosophila models have been generated, mainly for diseases presenting with neurological involvement. The fruit fly provides a large selection of tools to investigate lysosomes, autophagy and endocytic pathways in vivo, as well as to analyse neuronal and glial cells. The possibility to use Drosophila in drug repurposing and discovery makes it an attractive model for LSDs lacking effective therapies. Here, ee describe the major cellular pathways implicated in LSDs pathogenesis, the approaches available for their study and the Drosophila models developed for these diseases. Finally, we highlight a possible use of LSDs Drosophila models for drug screening studies.

Modeling Mucopolysaccharidosis Type II in the Fruit Fly by Using the RNA Interference Approach

Mucopolysaccharidosis type II (MPS II) is a lysosomal storage disorder that occurs due to the deficit of the lysosomal enzyme iduronate 2-sulfatase (IDS) that leads to the storage of the glycosaminoglycan heparan- and dermatan-sulfate in all organs and tissues. It is characterized by important clinical features and the severe form presents with a heavy neurological involvement. However, almost nothing is known about the neuropathogenesis of MPS II. To address this issue, we developed a ubiquitous, neuronal, and glial-specific knockdown model in Drosophila melanogaster by using the RNA interference (RNAi) approach. Knockdown of the Ids/CG12014 gene resulted in a significant reduction of the Ids gene expression and enzymatic activity. However, glycosaminoglycan storage, survival, molecular markers (Atg8a, Lamp1, Rab11), and locomotion behavior were not affected. Even strongly reduced, IDS-activity was enough to prevent a pathological phenotype in a MPS II RNAi fruit fly. Thus, a Drosophila MPS II model requires complete abolishment of the enzymatic activity.

An Insulin-Sensitive Circular RNA that Regulates Lifespan in Drosophila

Circular RNAs (circRNAs) are abundant and accumulate with age in neurons of diverse species. However, only few circRNAs have been functionally characterized, and their role during aging has not been addressed. Here, we use transcriptome profiling during aging and find that accumulation of circRNAs is slowed down in long-lived insulin mutant flies. Next, we characterize the in vivo function of a circRNA generated by the sulfateless gene (circSfl), which is consistently upregulated, particularly in the brain and muscle, of diverse long-lived insulin mutants. Strikingly, lifespan extension of insulin mutants is dependent on circSfl, and overexpression of circSfl alone is sufficient to extend the lifespan. Moreover, circSfl is translated into a protein that shares the N terminus and potentially some functions with the full-length Sfl protein encoded by the host gene. Our study demonstrates that insulin signaling affects global circRNA accumulation and reveals an important role of circSfl during aging in vivo.

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Accumulation of circRNAs with age is slowed down in long-lived insulin mutant flies

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A circRNA encoded by the sulfateless gene is induced in long-lived insulin mutants

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Overexpression of circSfl extends the lifespan of fruit flies

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CircSfl is translated, and the resulting peptide is sufficient to extend lifespan

Pathogenesis of Mucopolysaccharidoses, an Update

The recent advancements in the knowledge of lysosomal biology and function have translated into an improved understanding of the pathophysiology of mucopolysaccharidoses (MPSs). The concept that MPS manifestations are direct consequences of lysosomal engorgement with undegraded glycosaminoglycans (GAGs) has been challenged by new information on the multiple biological roles of GAGs and by a new vision of the lysosome as a signaling hub involved in many critical cellular functions. MPS pathophysiology is now seen as the result of a complex cascade of secondary events that lead to dysfunction of several cellular processes and pathways, such as abnormal composition of membranes and its impact on vesicle fusion and trafficking; secondary storage of substrates; impairment of autophagy; impaired mitochondrial function and oxidative stress; dysregulation of signaling pathways. The characterization of this cascade of secondary cellular events is critical to better understand the pathophysiology of MPS clinical manifestations. In addition, some of these pathways may represent novel therapeutic targets and allow for the development of new therapies for these disorders.

Drosophila as a model to understand autophagy deregulation in human disorders

Autophagy has important functions in normal physiology to maintain homeostasis and protect against cellular stresses by the removal of harmful cargos such as dysfunctional organelles, protein aggregates and invading pathogens. The deregulation of autophagy is a hallmark of many diseases and therapeutic targeting of autophagy is highly topical. With the complex role of autophagy in disease it is essential to understand the genetic and molecular basis of the contribution of autophagy to pathogenesis. The model organism, Drosophila, provides a genetically amenable system to dissect out the contribution of autophagy to human disease models. Here we review the roles of autophagy in human disease and how autophagy studies in Drosophila have contributed to the understanding of pathophysiology.

Molecular Bases of Neurodegeneration and Cognitive Decline, the Major Burden of Sanfilippo Disease

The mucopolysaccharidoses (MPS) are a group of diseases caused by the lysosomal accumulation of glycosaminoglycans, due to genetic deficiencies of enzymes involved in their degradation. MPS III or Sanfilippo disease, in particular, is characterized by early-onset severe, progressive neurodegeneration but mild somatic involvement, with patients losing milestones and previously acquired skills as the disease progresses. Despite being the focus of extensive research over the past years, the links between accumulation of the primary molecule, the glycosaminoglycan heparan sulfate, and the neurodegeneration seen in patients have yet to be fully elucidated. This review summarizes the current knowledge on the molecular bases of neurological decline in Sanfilippo disease. It emerges that this deterioration results from the dysregulation of multiple cellular pathways, leading to neuroinflammation, oxidative stress, impaired autophagy and defects in cellular signaling. However, many important questions about the neuropathological mechanisms of the disease remain unanswered, highlighting the need for further research in this area.

Changes in cellular processes occurring in mucopolysaccharidoses as underestimated pathomechanisms of these diseases

Mucopolysaccharidoses (MPS) are a group of genetic disorders belonging to lysosomal storage diseases. They are caused by genetic defects leading to a lack or severe deficiency of activity of one of lysosomal hydrolases involved in degradation of glycosaminoglycans (GAGs). Partially degraded GAGs accumulate in lysosomes, which results in dysfunctions of cells, tissues, and organs. Until recently, it was assumed that GAG accumulation in cells is the major, if not the only, mechanism of pathogenesis in MPS, as GAGs may be a physical ballast for lysosomes causing inefficiency of cells due to a large amount of a stored material. However, recent reports suggest that in MPS cells there are changes in many different processes, which might be even more important for pathogenesis than lysosomal accumulation of GAGs per se. Moreover, there are many recently published results indicating that lysosomes not only are responsible for degradation of various macromolecules, but also play crucial roles in the regulation of cellular metabolism. Therefore, it appears plausible that previous failures in treatment of MPS (i.e., possibility to correct only some symptoms and slowing down of the disease rather than fully effective management of MPS) might be caused by underestimation of changes in cellular processes and concentration solely on decreasing GAG levels in cells.

Heparan sulfate proteoglycans: The sweet side of development turns sour in mucopolysaccharidoses

Heparan sulfate proteoglycans (HSPGs) are complex carbohydrate-modified proteins ubiquitously expressed on cell surfaces, extracellular matrix and basement membrane of mammalian tissues. Beside to serve as structural constituents, they regulate multiple cellular activities. A critical involvement of HSPGs in development has been established, and perturbations of HSPG-dependent pathways are associated with many human diseases. Recent evidence suggest a role of HSPGs in the pathogenesis of mucopolysaccharidoses (MPSs) where the accumulation of undigested HS results in the loss of cellular functions, tissue damage and organ dysfunctions accounting for clinical manifestations which include central nervous system (CNS) involvement, degenerative joint disease and reduced bone growth. Current therapies are not curative but only ameliorate the disease symptoms. Here, we highlight the link between HSPG functions in the development of CNS and musculoskeletal structures and the etiology of some MPS phenotypes, suggesting that HSPGs may represent potential targets for the therapy of such incurable diseases.

Mucopolysaccharidosis and Autophagy: Controversies on the Contribution of the Process to the Pathogenesis and Possible Therapeutic Applications

Mucopolysaccharidosis (MPS) consists of a group of 11 enzymatic defects which result in accumulation of undegraded glycosaminoglycans (GAG) in lysosomes. MPS is a severe metabolic disease for which only bone marrow/hematopoietic stem cell transplantation and enzyme replacement therapy are current therapeutic options. However, they are available for only a few of MPS types, and are ineffective in treatment of central nervous system. Recent studies indicated that the autophagy process can be impaired in MPS, but various contradictory conclusions have been published in this matter. Nevertheless, stimulation of autophagy has been proposed as a potential therapeutic option for MPS, and very recent results suggest that such approach might be effective in improving MPS symptoms. Still the mechanisms of autophagy changes in MPS are not clear, and efficiency of autophagy activation in clearing the storage material requires further investigation. These problems are summarized and discussed in this review.