Decreased ganglioside neuraminidase activity in fibroblasts from mucopolysaccharidosis patients. Inhibition of the activity in vitro by sulfated glycosaminoglycans and other compounds

Pathogenic Roles of Heparan Sulfate and Its Use as a Biomarker in Mucopolysaccharidoses

Heparan sulfate (HS) is an essential glycosaminoglycan (GAG) as a component of proteoglycans, which are present on the cell surface and in the extracellular matrix. HS-containing proteoglycans not only function as structural constituents of the basal lamina but also play versatile roles in various physiological processes, including cell signaling and organ development. Thus, inherited mutations of genes associated with the biosynthesis or degradation of HS can cause various diseases, particularly those involving the bones and central nervous system (CNS). Mucopolysaccharidoses (MPSs) are a group of lysosomal storage disorders involving GAG accumulation throughout the body caused by a deficiency of GAG-degrading enzymes. GAGs are stored differently in different types of MPSs. Particularly, HS deposition is observed in patients with MPS types I, II, III, and VII, all which involve progressive neuropathy with multiple CNS system symptoms. While therapies are available for certain symptoms in some types of MPSs, significant unmet medical needs remain, such as neurocognitive impairment. This review presents recent knowledge on the pathophysiological roles of HS focusing on the pathogenesis of MPSs. We also discuss the possible use and significance of HS as a biomarker for disease severity and therapeutic response in MPSs.

iPS-derived neural stem cells for disease modeling and evaluation of therapeutics for mucopolysaccharidosis type II

Mucopolysaccharidosis type II (MPS II), also known as Hunter syndrome, is a rare, lysosomal disorder caused by mutations in a gene encoding iduronate-2-sulfatase (IDS). IDS deficiency results in an accumulation of glycosaminoglycans (GAGs) and secondary accumulations of other lipids in lysosomes. Symptoms of MPS II include a variety of soft and hard tissue problems, developmental delay, and deterioration of multiple organs. Enzyme replacement therapy is an approved treatment for MPS II, but fails to improve neuronal symptoms. Cell-based neuronal models of MPS II disease are needed for compound screening and drug development for the treatment of the neuronal symptoms in MPS II. In this study, three induced pluripotent stem cell (iPSC) lines were generated from three MPS II patient-derived dermal fibroblast cell lines that were differentiated into neural stem cells and neurons. The disease phenotypes were measured using immunofluorescence staining and Nile red dye staining. In addition, the therapeutic effects of recombinant human IDS enzyme, delta-tocopherol (DT), and hydroxypropyl-beta-cyclodextrin (HPBCD) were determined in the MPS II disease cells. Finally, the neural stem cells from two of the MPS II iPSC lines exhibited typical disease features including a deficiency of IDS activity, abnormal glycosaminoglycan storage, and secondary lipid accumulation. Enzyme replacement therapy partially rescued the disease phenotypes in these cells. DT showed a significant effect in reducing the secondary accumulation of lipids in the MPS II neural stem cells. In contrast, HPBCD displayed limited or no effect in these cells. Our data indicate that these MPS II cells can be used as a cell-based disease model to study disease pathogenesis, evaluate drug efficacy, and screen compounds for drug development.

Mucopolysaccharidosis Type I: A Review of the Natural History and Molecular Pathology

Mucopolysaccharidosis type I (MPS I) is a rare autosomal recessive inherited disease, caused by deficiency of the enzyme α-L-iduronidase, resulting in accumulation of the glycosaminoglycans (GAGs) dermatan and heparan sulfate in organs and tissues. If untreated, patients with the severe phenotype die within the first decade of life. Early diagnosis is crucial to prevent the development of fatal disease manifestations, prominently cardiac and respiratory disease, as well as cognitive impairment. However, the initial symptoms are nonspecific and impede early diagnosis. This review discusses common phenotypic manifestations in the order in which they develop. Similarities and differences in the three animal models for MPS I are highlighted. Earliest symptoms, which present during the first 6 months of life, include hernias, coarse facial features, recurrent rhinitis and/or upper airway obstructions in the absence of infection, and thoracolumbar kyphosis. During the next 6 months, loss of hearing, corneal clouding, and further musculoskeletal dysplasias develop. Finally, late manifestations including lower airway obstructions and cognitive decline emerge. Cardiac symptoms are common in MPS I and can develop in infancy. The underlying pathogenesis is in the intra- and extracellular accumulation of partially degraded GAGs and infiltration of cells with enlarged lysosomes causing tissue expansion and bone deformities. These interfere with the proper arrangement of collagen fibrils, disrupt nerve fibers, and cause devastating secondary pathophysiological cascades including inflammation, oxidative stress, and other disruptions to intracellular and extracellular homeostasis. A greater understanding of the natural history of MPS I will allow early diagnosis and timely management of the disease facilitating better treatment outcomes.

Anatomical changes and pathophysiology of the brain in mucopolysaccharidosis disorders

Mucopolysaccharidosis (MPS) disorders are caused by deficiencies in lysosomal enzymes, leading to impaired glycosaminoglycan (GAG) degradation. The resulting GAG accumulation in cells and connective tissues ultimately results in widespread tissue and organ dysfunction. The seven MPS types currently described are heterogeneous and progressive disorders, with somatic and neurological manifestations depending on the type of accumulating GAG. Heparan sulfate (HS) is one of the GAGs stored in patients with MPS I, II, and VII and the main GAG stored in patients with MPS III. These disorders are associated with significant central nervous system (CNS) abnormalities that can manifest as impaired cognition, hyperactive and/or aggressive behavior, epilepsy, hydrocephalus, and sleeping problems. This review discusses the anatomical and pathophysiological CNS changes accompanying HS accumulation as well as the mechanisms believed to cause CNS abnormalities in MPS patients. The content of this review is based on presentations and discussions on these topics during a meeting on the brain in MPS attended by an international group of MPS experts.

The lysosomal storage disease continuum with ageing-related neurodegenerative disease

Lysosomal storage diseases and diseases of ageing share many features both at the physiological level and with respect to the mechanisms that underlie disease pathogenesis. Although the exact pathophysiology is not exactly the same, it is astounding how many similar pathways are altered in all of these diseases. The aim of this review is to provide a summary of the shared disease mechanisms, outlining the similarities and differences and how genetics, insight into rare diseases and functional research has changed our perspective on the causes underlying common diseases of ageing. The lysosome should no longer be considered as just the stomach of the cell or as a suicide bag, it has an emerging role in cellular signalling, nutrient sensing and recycling. The lysosome is of fundamental importance in the pathophysiology of diseases of ageing and by comparing against the LSDs we not only identify common pathways but also therapeutic targets so that ultimately more effective treatments can be developed for all neurodegenerative diseases.

Mucopolysaccharidosis type I: current knowledge on its pathophysiological mechanisms

Mucopolysaccharidosis type I is one of the most frequent lysosomal storage diseases. It has a high morbidity and mortality, causing in many cases severe neurological and somatic damage in the first years of life. Although the clinical phenotypes have been described for decades, and the enzymatic deficiency and many of the mutations that cause this disease are well known, the underlying pathophysiological mechanisms that lead to its development are not completely understood. In this review we describe and discuss the different pathogenic mechanisms currently proposed for this disease regarding its neurological damage. Deficiency in the lysosomal degradation of heparan sulfate and dermatan sulfate, as well as its primary accumulation, may disrupt a variety of physiological and biochemical processes: the intracellular and extracellular homeostasis of these macromolecules, the pathways related to gangliosides metabolism, mechanisms related to the activation of inflammation, receptor-mediated signaling, oxidative stress and permeability of the lysosomal membrane, as well as alterations in intracellular ionic homeostasis and the endosomal pathway. Many of the pathogenic mechanisms proposed for mucopolysaccharidosis type I are also present in other lysosomal storage diseases with neurological implications. Results from the use of methods that allow the analysis of multiple genes and proteins, in both patients and animal models, will shed light on the role of each of these mechanisms and their combination in the development of different phenotypes due to the same deficiency.

Secondary storage of dermatan sulfate in Sanfilippo disease

Mucopolysaccharidoses are a group of genetically inherited disorders that result from the defective activity of lysosomal enzymes involved in glycosaminoglycan catabolism, causing their intralysosomal accumulation. Sanfilippo disease describes a subset of mucopolysaccharidoses resulting from defects in heparan sulfate catabolism. Sanfilippo disorders cause severe neuropathology in affected children. The reason for such extensive central nervous system dysfunction is unresolved, but it may be associated with the secondary accumulation of metabolites such as gangliosides. In this article, we describe the accumulation of dermatan sulfate as a novel secondary metabolite in Sanfilippo. Based on chondroitinase ABC digestion, chondroitin/dermatan sulfate levels in fibroblasts from Sanfilippo patients were elevated 2-5-fold above wild-type dermal fibroblasts. Lysosomal turnover of chondroitin/dermatan sulfate in these cell lines was significantly impaired but could be normalized by reducing heparan sulfate storage using enzyme replacement therapy. Examination of chondroitin/dermatan sulfate catabolic enzymes showed that heparan sulfate and heparin can inhibit iduronate 2-sulfatase. Analysis of the chondroitin/dermatan sulfate fraction by chondroitinase ACII digestion showed dermatan sulfate storage, consistent with inhibition of iduronate 2-sulfatase. The discovery of a novel storage metabolite in Sanfilippo patients may have important implications for diagnosis and understanding disease pathology.

Why are behaviors of children suffering from various neuronopathic types of mucopolysaccharidoses different?

Mucopolysaccharidoses (MPS) are inherited metabolic disorders from the group of lysosomal storage diseases (LSD). They arise from mutations causing dysfunction of one of enzymes involved in degradation of glycosaminoglycans (GAGs) in lysosomes. Impaired degradation of these compounds results in their accumulation in cells and dysfunction of most tissues and organs of patients. If heparan sulfate (HS) is the sole or one of stored GAGs, brain functions are also affected. However, despite the fact that products of incomplete degradation of the same chemical, HS, are accumulated in brains of patients suffering from Hurler disease (MPS type I), Hunter disease (MPS type II), Sanfilippo disease (MPS type III) and Sly disease (MPS type VII), and obvious deterioration of brain functions occur in these patients, their behavior is considerably different between various types of MPS. Here we asked the question about biochemical reasons of these differences. We performed theoretical analysis of products of incomplete HS degradation that accumulate in tissues of patients diagnosed for these diseases. A correlation between chemical structures of incompletely degraded HS and behaviors of patients suffering from particular MPS types was found. We propose a hypothesis that particular chemical moieties occurring at the ends of incompletely degraded HS molecules may determine characteristic behavioral disturbances, perhaps due to chemical reactions interfering with functions of neurons in the brain. A possible experimental testing of this hypothesis is also proposed. If the hypothesis is true, it might shed some new light on biochemical mechanisms of behavioral problems occurring not only in MPS but also in some other diseases.

Secondary lipid accumulation in lysosomal disease

Lysosomal diseases are inherited metabolic disorders caused by defects in a wide spectrum of lysosomal and a few non-lysosomal proteins. In most cases a single type of primary storage material is identified, which has been used to name and classify the disorders: hence the terms sphingolipidoses, gangliosidoses, mucopolysaccharidoses, glycoproteinoses, and so forth. In addition to this primary storage, however, a host of secondary storage products can also be identified, more often than not having no direct link to the primary protein defect. Lipids - glycosphingolipids and phospholipids, as well as cholesterol - are the most ubiquitous and best studied of these secondary storage materials. While in the past typically considered nonspecific and nonconsequential features of these diseases, newer studies suggest direct links between secondary storage and disease pathogenesis and support the view that understanding all aspects of this sequestration process will provide important insights into the cell biology and treatment of lysosomal disease.

Abnormal neuronal metabolism and storage in mucopolysaccharidosis type VI (Maroteaux-Lamy) disease

Mucopolysaccharidosis (MPS) type VI, also known as Maroteaux-Lamy disease, is an inherited disorder of glycosaminoglycan catabolism caused by deficient activity of the lysosomal hydrolase, N-acetylgalactosamine 4-sulphatase (4S). A variety of prominent visceral and skeletal defects are characteristic, but primary neurological involvement has generally been considered absent. We report here that the feline model of MPS VI exhibits abnormal lysosomal storage in occasional neurones and glia distributed throughout the cerebral cortex. Abnormal lysosomal inclusions were pleiomorphic with some resembling zebra bodies and dense core inclusions typical of other MPS diseases or the membranous storage bodies characteristic of the gangliosidoses. Pyramidal neurones were shown to contain abnormal amounts of GM2 and GM3 gangliosides by immunocytochemical staining and unesterified cholesterol by histochemical (filipin) staining. Further, Golgi staining of pyramidal neurones revealed that some possessed ectopic axon hillock neurites and meganeurites similar to those described in Tay-Sachs and other neuronal storage diseases with ganglioside storage. Some animals evaluated in this study also received allogeneic bone marrow transplants, but no significant differences in neuronal storage were noted between treated and untreated individuals. These studies demonstrate that deficiency of 4S activity can lead to metabolic abnormalities in the neurones of central nervous system in cats, and that these changes may not be readily amenable to correction by bone marrow transplantation. Given the close pathological and biochemical similarities between feline and human MPS VI, it is conceivable that children with this disease have similar neuronal involvement.

Mucopolysaccharidosis IIIA (Sanfilippo syndrome) in a New Zealand Huntaway dog with ataxia

AIM

To investigate the nature of a progressive ataxia in a New Zealand Huntaway dog.

METHODS

The affected dog was examined clinically before being humanely killed and necropsied. Selected tissues were submitted to light and electron microscopy and to biochemical analyses.

RESULTS

The histological lesions were interpreted as indicative of one of the forms of mucopolysaccharidosis type-III (MPS-III), a lysosomal storage disease. Biochemically there was a deficiency of heparan sulphamidase. All the heparan sulphate chains had non-reducing-end glucosamine-N-sulphate residues.

CONCLUSION

The disease is MPS-IIIA (Sanfilippo syndrome). An autosomal recessive mode of inheritance can be provisionally assumed from the nature of this disease in other species.

Differential subcellular localization of cholesterol, gangliosides, and glycosaminoglycans in murine models of mucopolysaccharide storage disorders

The mucopolysaccharidoses (MPSs) are a complex family of lysosomal storage disorders characterized by failure to degrade heparan sulfate (HS) and/or other types of glycosaminoglycans (GAGs) secondary to the absence of specific lysosomal enzymes. An accompanying storage of glycosphingolipids (GSLs), most notably GM2 and GM3 gangliosides, has also been documented to occur in many types of MPS disease and is believed to be caused by secondary inhibition of GSL-degradative enzymes by intracellular GAG accumulation. We have documented the presence of secondary ganglioside accumulation in mouse models of several MPS disorders (types I, IIIA, IIIB, and VII) and report that this storage is accompanied by sequestration of free cholesterol in a manner similar to that observed in primary gangliosidoses. Using confocal microscopy, we evaluated the cellular distribution of cholesterol, GM2 and GM3 gangliosides, and HS in brains of mice with MPS IIIA disease. Unexpectedly, we found that although both gangliosides often accumulated in the same neurons, they were consistently located in separate populations of cytoplasmic vesicles. Additionally, GM3 ganglioside only partially co-localized with the primary storage material (HS), and cholesterol likewise only partially co-localized with the GM2 and GM3 gangliosides. These findings raise significant questions about the mechanism(s) responsible for secondary accumulation of storage materials in MPS disease. Furthermore, given that GSLs and cholesterol are constituents of membrane rafts believed critical in signal transduction events in neurons, their co-sequestration in individual neurons suggests the presence of defects in the composition, trafficking, and/or recycling of raft components and thus possible new mechanisms to explain neuronal dysfunction in MPS disorders.

Enzyme-replacement therapy from birth delays the development of behavior and learning problems in mucopolysaccharidosis type IIIA mice

Mucopolysaccharidosis type IIIA (MPS IIIA; Sanfilippo syndrome) is a lysosomal storage disorder characterized by severe CNS degeneration, resulting in behavioral abnormalities and loss of learned abilities. Early treatment is vital to prevent long-term clinical pathology in lysosomal storage disorders. We have used naturally occurring MPS IIIA mice to assess the effects of long-term enzyme-replacement therapy initiated either at birth or at 6 wk of age. MPS IIIA and normal control mice received weekly i.v. injections of 1 mg/kg recombinant human sulfamidase until 20 wk of age. Sulfamidase is able to enter the brain until the blood-brain barrier completely closes at 10-14 d of age. MPS IIIA mice that were treated from birth demonstrated normal weight, behavioral characteristics, and ability to learn. MPS IIIA mice that were treated from birth performed significantly better in the Morris water maze than MPS IIIA mice that were treated from 6 wk of age or left untreated. A reduction in storage vacuoles in cells of the CNS in MPS IIIA mice that were treated from birth is consistent with the improvements observed. These data suggest that enzyme that enters the brain in the first few weeks of life, before the blood-brain barrier matures, is able to delay the development of behavior and learning difficulties in MPS IIIA mice.

Neurobiology and cellular pathogenesis of glycolipid storage diseases

Disorders of lysosomal metabolism often involve the accumulation of specific types of glycolipid, particularly gangliosides, because of either degradative failure or other currently unknown mechanisms. Although the precise role of gangliosides in cells remains enigmatic, the presence of specific abnormalities secondary to ganglioside accumulation in lysosomal diseases has suggested important biological functions. Chief among these is the growth of new dendrites on particular classes of mature neurons secondary to an increase in GM2 ganglioside. That GM2 has also been shown to be elevated in normal immature neurons coincident with dendritic sprouting provides a compelling argument that this ganglioside plays a role in dendritic initiation. This discovery has led to the search for other regulators of dendritic differentiation that may in some way be linked to the expression and/or function of GM2 ganglioside. Principal candidates that have emerged include tyrosine kinase receptors, small GTPases and calcium/calmodulin-dependent protein kinase II. Understanding the mechanism underlying ectopic dendritogenesis in lysosomal diseases can be expected to generate significant insight into the control of dendritic plasticity in normal brain. The detrimental aspects of ganglioside accumulation in storage diseases as well as the potential link between gangliosides and dendritogenesis also provide a strong rationale for developing pharmacological means to manipulate ganglioside expression in neurons.

Histological diagnosis of mucopolysaccharidosis IIIA in a wire-haired dachshund

A four-year-old wire-haired dachshund developed progressive neurological signs of ataxia, intention tremor and finally dysuria. Two years later, histopathology showed that neurons throughout the brain and spinal cord were distended with lipopigment which was also present in macrophages. Ultrastructurally, the pigment in the neurons occurred predominantly as electron-dense membranous whorls and stacks. There were a few vacuolated macrophages in the meninges. Hepatocytes were highly vacuolated and electron microscopy suggested that they were empty membrane-bound vesicles. The disease was diagnosed as mucopolysaccharidosis IIIA because of its similarity to other biochemically confirmed cases in the same breed and in a New Zealand huntaway dog. Additional lesions included calcium oxalate uroliths, severe secondary calcification of tissues including the brain and storage deposits in some neurons, and lesions which may have been associated with high levels of the substrate, heparan sulphate.

A mouse model for mucopolysaccharidosis type III A (Sanfilippo syndrome)

Mucopolysaccharidosis type III A (MPS III A, Sanfilippo syndrome) is a rare, autosomal recessive, lysosomal storage disease characterized by accumulation of heparan sulfate secondary to defective function of the lysosomal enzyme heparan N- sulfatase (sulfamidase). Here we describe a spontaneous mouse mutant that replicates many of the features found in MPS III A in children. Brain sections revealed neurons with distended lysosomes filled with membranous and floccular materials with some having a classical zebra body morphology. Storage materials were also present in lysosomes of cells of many other tissues, and these often stained positively with periodic-acid Schiff reagent. Affected mice usually died at 7-10 months of age exhibiting a distended bladder and hepatosplenomegaly. Heparan sulfate isolated from urine and brain had nonreducing end glucosamine- N -sulfate residues that were digested with recombinant human sulfamidase. Enzyme assays of liver and brain extracts revealed a dramatic reduction in sulfamidase activity. Other lysosomal hydrolases that degrade heparan sulfate or other glycans and glycosaminoglycans were either normal, or were somewhat increased in specific activity. The MPS III A mouse provides an excellent model for evaluating pathogenic mechanisms of disease and for testing treatment strategies, including enzyme or cell replacement and gene therapy.

Inhibition of acid sialidase by inorganic sulfate

Sulfated glycosaminoglycans are known to inhibit mammalian acid-active sialidase. Although the inhibition depends clearly on the presence of sulfate groups on these macromolecules, there was no information on the intrinsic inhibitory potency of inorganic sulfate. In this study, we demonstrate that inorganic sulfates inhibit acid-active Mu-Neu5Ac sialidase of U937 cells. This inhibition was found to be reversible and it appeared to be of the mixed competitive type. Sulfate-induced inhibition was also observed in other cells as well as with other substrates such as sialyl lactose and bovine mixed brain gangliosides. We conclude that the intrinsic inhibitory potency of sulfate groups may be significantly involved in the inhibition of acid-active sialidase by sulfated glycosaminoglycans. In addition, inorganic sulfate by its apparent potency to selectively inhibit acid sialidases might constitute an interesting tool for the characterisation of the minor forms of sialidases occurring in mammalian cells.

Coincident neuraminidase and aspartoacylase deficiency associated with chromosome 9Q paracentric inversion in a Saudi family

A large, consanguineous Saudi family with three members with sialidosis type 1 and five members with infantile central nervous system spongy degeneration of the brain (ICNSSD, or Canavan-Bertrand-van Bogaert disease) is described. The patients with sialidosis had normal aspartoacylase activity, while neuraminidase activity in the patients with ICNSSD was reduced. All patients had normal carboxypeptidase activity in their fibroblasts. In an additional member there was photic-induced epilepsy, but he had normal enzymes. Two of the patients and one normal brother, but not the parents, had pericentric inversion of chromosome 9q. We postulate that an unidentified gene function is responsible for varied expression of these neurodegenerative diseases in this family.

Lysosomal and plasma membrane ganglioside GM3 sialidases of cultured human fibroblasts. Differentiation by detergents and inhibitors

Cultured human fibroblasts contain two sialidases that degrade gangliosides such as GM3: a lysosomal activity that appears identical with the activity towards water-soluble substrates and that is deficient in the genetic lysosomal disorder sialidosis, and another enzyme that seems localized on the external surface of the plasma membrane. In this report we show that both enzymes can be differentiated in the presence of each other by choice of the detergent used for activation, and also by the inhibitory action of some polyanionic compounds such as sulphated glycosaminoglycans. The lysosomal ganglioside GM3 sialidase is greatly stimulated by sodium glycodeoxycholate and, to lesser degrees, by sodium glycocholate and sodium cholate. The ganglioside GM3 sialidase of the plasma membrane is not measurably active under the conditions of the lysosomal enzyme but is specifically activated by the non-ionic detergent Triton X-100. The glycodeoxycholate-stimulated, but not the Triton-activated, ganglioside GM3 sialidase activity was profoundly diminished in cell lines from patients with the lysosomal disorders sialidosis and galactosialidosis; however, both activities were normal in fibroblasts from patients with mucolipidosis IV, previously thought to be a ganglioside sialidase deficiency disorder. Both the lysosomal and the plasma membrane ganglioside GM3 sialidases were inhibited by sialic acids, suramin, dextran sulphate and sulphated glycosaminoglycans. Among the latter, heparin and heparan sulphate showed a much higher inhibitory potency towards the plasma membrane ganglioside GM3 sialidase than towards the lysosomal onw.(ABSTRACT TRUNCATED AT 250 WORDS)

Ganglioside GM3 sialidase activity in fibroblasts of normal individuals and of patients with sialidosis and mucolipidosis IV. Subcellular distribution and and some properties

Sensitive assays for the determination of the ganglioside sialidase activity of fibroblast homogenates were established using ganglioside GM3, 3H-labelled in the sphingosine moiety, as a substrate. Ganglioside GM3 sialidase activity was greatly stimulated by the presence of the non-ionic detergent Triton X-100 and was further enhanced by salts such as NaCl; the optimal pH was 4.5. The subcellular localization of this activity was determined by fractionation using free-flow electrophoresis and found to be exclusively associated with the marker for the plasma membrane, but not with that for lysosomes. This Triton-stimulated ganglioside sialidase activity was selectively inhibited by preincubating intact cells in the presence of millimolar concentrations of Cu2+, suggesting that the activity resides on the external surface of the plasma membrane. In normal fibroblasts homogenates, ganglioside GM3 sialidase was also greatly stimulated by sodium cholate. In contrast to the Triton X-100-activated reaction, however, it was not diminished by prior incubation of intact cells in the presence of Cu2+. Only after cell lysis was Cu2+ inhibitory. the cholate-stimulated ganglioside sialidase activity thus paralleled the behaviour of the lysosomal 4-methylumbelliferyl-alpha-D-N-acetylneuraminic acid (4-MU-NeuAc) sialidase. In fibroblasts from sialidosis patients, the cholate-stimulated ganglioside GM3 sialidase activity, but not that of the Triton-activated enzyme, was profoundly diminished. In fibroblasts from patients with mucolipidosis IV (ML IV), both the Triton X-100- and the cholate-stimulated ganglioside GM3 sialidase activities were in the range of normal controls. The Triton-activated enzyme was associated with the plasma membrane in the same manner as in normal cells. Our findings suggest that, in human fibroblasts, there exist two sialidases that degrade ganglioside GM3: one on the external surface of the plasma membrane, and another that is localized in lysosomes and seems identical with the activity that acts on sialyloligosaccharides and 4-MU-NeuAc. As neither activity was found to be deficient in ML IV fibroblasts, our results argue against the hypothesis of a primary involvement of a ganglioside GM3 sialidase in the pathogenesis of ML IV.

Alterations in neuron morphology in mucopolysaccharidosis type I. A Golgi study

Morphological changes in neurons with inborn defects of the lysosomal hydrolase, alpha-L-iduronidase, and with concomitant storage of glycosaminoglycans, were evaluated by Golgi staining in two animal models and compared to a similar study of a child with the same disease. Cortical pyramidal neurons in feline mucopolysaccharidosis type I often displayed axon hillock enlargements (meganeurites) and/or ectopic, secondary neuritic processes sprouting from this same region of the cell. The latter structures were prominent and often appeared longer than similar neurites reported in other neuronal storage diseases. Although most meganeurites were aspiny, a few were observed which possessed spine-like processes or neurites. Other than these morphological changes in cortical pyramidal neurons, few other cell types displayed abnormalities demonstrable by Golgi impregnation. In the canine model of this disorder, abnormal Golgi-impregnated cortical neurons resembled more closely those seen in human mucopolysaccharidosis. That is, they possessed meganeurites which typically were aspiny in appearance. Ectopic neurite growth was not observed on any Golgi-impregnated neurons in the cases of canine or human mucopolysaccharidosis used in this study. The latter finding, given the advanced ages of these cases, is consistent with the view that ectopic neuritogenesis seen in neuronal storage diseases may be subject to a developmental window, albeit one open well beyond the period of early postnatal maturation.

Prenatal diagnosis and confirmation of infantile sialic acid storage disease

Amniocentesis was performed in a pregnancy at risk for infantile sialic acid storage disease. Greatly elevated levels of free sialic acid were found in cell-free amniotic fluid as well as in cultured amniotic cells from the fetus at risk. After incubation of the cultured amniocytes with fetuin labelled in its sialic acid moiety, pulse and chase experiments respectively showed accumulation and impaired release of TCA-soluble radioactive material in the amniotic cells at risk. These data thus clearly indicated that the fetus was affected. After pregnancy termination, ultrastructural studies of fetal organs and placenta showed a generalized storage picture characterized by clear membrane-bound inclusions. The diagnosis was further confirmed by the finding of greatly increased amounts of free sialic acid in fetal organs and cultured fibroblasts.

Internalization of exogenous gangliosides in cultured skin fibroblasts for the diagnosis of mucolipidosis IV

The internalization of exogenous mixed brain gangliosides in ML IV cultured skin fibroblasts indicated an impairment of ganglioside catabolism in these cells. Incubation of ML IV, normal and various other lysosomal storage disorders cell lines for five days with exogenous tritium labelled GM3, GD1a or GT1 gangliosides allowed accurate quantitation of the retained gangliosides. This in vitro approach provides a reliable method for the diagnosis of ML IV.

Specificity studies on the oligosaccharide neuraminidase of human fibroblasts

Competition and thermal inactivation experiments with different potential natural substrates indicated that in homogenates of human fibroblasts one single enzyme is acting on both (alpha 2-3) and (alpha 2-6) sialosyl linkages of oligosaccharides and glycoproteins, but not of the ganglioside GM3. N-Acetylneuraminic and 2-deoxy-2,3-dehydro-N-acetylneuraminic acids are competitive inhibitors, whereas chondroitin 4-sulphate and the drug Suramin are potent inhibitors of undefined type.