Defective lysosomal egress of free sialic acid (N-acetylneuraminic acid) in fibroblasts of patients with infantile free sialic acid storage disease

Exploration of the Sialic Acid World

Sialic acids are cytoprotectors, mainly localized on the surface of cell membranes with multiple and outstanding cell biological functions. The history of their structural analysis, occurrence, and functions is fascinating and described in this review. Reports from different researchers on apparently similar substances from a variety of biological materials led to the identification of a 9-carbon monosaccharide, which in 1957 was designated "sialic acid." The most frequently occurring member of the sialic acid family is N-acetylneuraminic acid, followed by N-glycolylneuraminic acid and O-acetylated derivatives, and up to now over about 80 neuraminic acid derivatives have been described. They appeared first in the animal kingdom, ranging from echinoderms up to higher animals, in many microorganisms, and are also expressed in insects, but are absent in higher plants. Sialic acids are masks and ligands and play as such dual roles in biology. Their involvement in immunology and tumor biology, as well as in hereditary diseases, cannot be underestimated. N-Glycolylneuraminic acid is very special, as this sugar cannot be expressed by humans, but is a xenoantigen with pathogenetic potential. Sialidases (neuraminidases), which liberate sialic acids from cellular compounds, had been known from very early on from studies with influenza viruses. Sialyltransferases, which are responsible for the sialylation of glycans and elongation of polysialic acids, are studied because of their significance in development and, for instance, in cancer. As more information about the functions in health and disease is acquired, the use of sialic acids in the treatment of diseases is also envisaged.

Lysosomal storage diseases

Lysosomes are cytoplasmic organelles that contain a variety of different hydrolases. A genetic deficiency in the enzymatic activity of one of these hydrolases will lead to the accumulation of the material meant for lysosomal degradation. Examples include glycogen in the case of Pompe disease, glycosaminoglycans in the case of the mucopolysaccharidoses, glycoproteins in the cases of the oligosaccharidoses, and sphingolipids in the cases of Niemann-Pick disease types A and B, Gaucher disease, Tay-Sachs disease, Krabbe disease, and metachromatic leukodystrophy. Sometimes, the lysosomal storage can be caused not by the enzymatic deficiency of one of the hydrolases, but by the deficiency of an activator protein, as occurs in the AB variant of GM2 gangliosidosis. Still other times, the accumulated lysosomal material results from failed egress of a small molecule as a consequence of a deficient transporter, as in cystinosis or Salla disease. In the last couple of decades, enzyme replacement therapy has become available for a number of lysosomal storage diseases. Examples include imiglucerase, taliglucerase and velaglucerase for Gaucher disease, laronidase for Hurler disease, idursulfase for Hunter disease, elosulfase for Morquio disease, galsulfase for Maroteaux-Lamy disease, alglucosidase alfa for Pompe disease, and agalsidase alfa and beta for Fabry disease. In addition, substrate reduction therapy has been approved for certain disorders, such as eliglustat for Gaucher disease. The advent of treatment options for some of these disorders has led to newborn screening pilot studies, and ultimately to the addition of Pompe disease and Hurler disease to the Recommended Uniform Screening Panel (RUSP) in 2015 and 2016, respectively.

Sialic acid deposition impairs the utility of AAV9, but not peptide-modified AAVs for brain gene therapy in a mouse model of lysosomal storage disease

Recombinant vector systems have been recently identified that when delivered systemically can transduce neurons, glia, and endothelia in the central nervous system (CNS), providing an opportunity to develop therapies for diseases affecting the brain without performing direct intracranial injections. Vector systems based on adeno-associated virus (AAV) include AAV serotype 9 (AAV9) and AAVs that have been re-engineered at the capsid level for CNS tropism. Here, we performed a head-to-head comparison of AAV9 and a capsid modified AAV for their abilities to rescue CNS and peripheral disease in an animal model of lysosomal storage disease (LSD), the mucopolysacharidoses (MPS) VII mouse. While the peptide-modified AAV reversed cognitive deficits, improved storage burden in the brain, and substantially prolonged survival, we were surprised to find that AAV9 provided no CNS benefit. Additional experiments demonstrated that sialic acid, a known inhibitor of AAV9, is elevated in the CNS of MPS VII mice. These studies highlight how disease manifestations can dramatically impact the known tropism of recombinant vectors, and raise awareness to assuming similar transduction profiles between normal and disease models.

Regulation and pathophysiological implications of UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE) as the key enzyme of sialic acid biosynthesis

The key enzyme for the biosynthesis ofN-acetylneuraminic acid, from which all other sialic acids are formed, is the bifunctional enzyme UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE). GNE is a highly conserved protein found throughout the animal kingdom. Its highest expression is seen in the liver and placenta. GNE is regulated by a variety of biochemical means, including tetramerization promoted by the substrate UDP-GlcNAc, phosphorylation by protein kinase C and feedback inhibition by CMP-Neu5Ac, which is defect in the human disease sialuria. GNE knock-out in mice leads to embryonic lethality, emphasizing the crucial role of this key enzyme for sialic acid biosynthesis. The metabolic capacity to synthesize sialic acid and CMP-sialic acid upon ManNAc loads is amazingly high. An additional characteristic of GNE is its interaction with proteins involved in the regulation of development, which might play a crucial role in the hereditary inclusion body myopathy. Due to the importance of increased concentrations of tumor-surface sialic acid, first attempts to find inhibitors of GNE have been successful.

A dileucine signal situated in the C-terminal tail of the lysosomal membrane protein p40 is responsible for its targeting to lysosomes

Transport of newly synthesized lysosomal membrane proteins from the TGN (trans-Golgi network) to the lysosomes is due to the presence of specific signals in their cytoplasmic domains that are recognized by cytosolic adaptors. p40, a hypothetical transporter of 372 amino acids localized in the lysosomal membrane, contains four putative lysosomal sorting motifs in its sequence: three of the YXXphi-type (Y(6)QLF, Y(106)VAL, Y(333)NGL) and one of the [D/E]XXXL[L/I]-type (EQERL(360)L(361)). To test the role of these motifs in the biosynthetic transport of p40, we replaced the most critical residues of these consensus sequences, the tyrosine residue or the leucine-leucine pair, by alanine or alanine-valine respectively. We analysed the subcellular localization of the mutated p40 proteins in transfected HeLa cells by confocal microscopy and by biochemical approaches (subcellular fractionation on self-forming Percoll density gradients and cell surface biotinylation). The results of the present study show that p40 is mistargeted to the plasma membrane when its dileucine motif is disrupted. No role of the tyrosine motifs could be put forward. Taken together, our results provide evidence that the sorting of p40 from the TGN to the lysosomes is directed by the dileucine EQERL(360)L(361) motif situated in its C-terminal tail.

Molecular physiology and pathophysiology of lysosomal membrane transporters

In contrast to lysosomal hydrolytic enzymes, the lysosomal membrane remains poorly characterized. In particular, although the genetic study of cystinosis and sialic acid storage disorders led to the identification of two lysosomal transporters for cystine and sialic acids, respectively, ten years ago, most transporters responsible for exporting lysosomal hydrolysis products to the cytosol are still unknown at the molecular level. However, two lines of investigation recently started to fill this gap in the knowledge of lysosomal biology. First, novel proteomic approaches are now able to provide a reliable inventory of lysosomal membrane proteins. On the other hand, a novel functional approach based on intracellular trafficking mechanisms allows direct transport measurement in whole cells by redirecting recombinant lysosomal transporters to the cell surface. After surveying the current state of knowledge in this field, the review focuses on the sialic acid transporter sialin and shows how recent functional data using the above whole-cell approach shed new light on the pathogenesis of sialic acid storage disorders by revealing the existence of a residual transport activity associated with Salla disease.

Embryonic expression patterns of Drosophila ACS family genes related to the human sialin gene

The anion/cation symporter (ACS) family is a large subfamily of the major facilitator superfamily (MFS) of transporters. ACS family permeases are widely distributed in nature and transport either organic or inorganic anions in response to chemiosmotic cation gradients. The only protein in the ACS family to which a human disease has been linked, is sialin, the proton-driven lysosomal carrier for sialic acid. Genetic defects in sialin cause a lysosomal storage disease in humans. Here we have identified a group of conserved Drosophila ACS family genes related to sialin and studied their expression patterns throughout embryogenesis. Drosophila sialin-related genes are expressed in a wide variety of tissues. Expression is frequently observed throughout various parts of the intestinal tract, including Malpighian tubules and salivary glands. Additionally, some genes are expressed in vitellophages (yolk nuclei), nervous system, respiratory tract and a number of other embryonic tissues. These data will aid the establishment of a fruitfly model of human lysosomal storage disorders, the most common cause of neurodegeneration in childhood.

Mechanism of uptake and incorporation of the non-human sialic acid N-glycolylneuraminic acid into human cells

N-Glycolylneuraminic acid (Neu5Gc) is a widely expressed sialic acid in mammalian cells. Although humans are genetically deficient in producing Neu5Gc, small amounts are present in human cells in vivo. A dietary origin was suggested by human volunteer studies and by observing that free Neu5Gc is metabolically incorporated into cultured human carcinoma cells by unknown mechanisms. We now show that free Neu5Gc uptake also occurs in other human and mammalian cells. Inhibitors of certain non-clathrin-mediated endocytic pathways reduce Neu5Gc accumulation. Studies with human mutant cells show that the lysosomal sialic acid transporter is required for metabolic incorporation of free Neu5Gc. Incorporation of glycosidically bound Neu5Gc from exogenous glycoconjugates (relevant to human gut epithelial exposure to dietary Neu5Gc) requires the transporter as well as the lysosomal sialidase, which presumably acts to release free Neu5Gc. Thus, exogenous Neu5Gc reaches lysosomes via pinocytic/endocytic pathways and is exported in free form into the cytosol, becoming available for activation and transfer to glycoconjugates. In contrast, N-glycolylmannosamine (ManNGc) apparently traverses the plasma membrane by passive diffusion and becomes available for conversion to Neu5Gc in the cytosol. This mechanism can also explain the metabolic incorporation of chemically synthesized unnatural sialic acids, as reported by others. Finally, to our knowledge, this is the first example of delivery to the cytosol of an extracellular small molecule that cannot cross the plasma membrane, utilizing fluid pinocytosis and a specific lysosomal transporter. The approach could, thus, potentially be generalized to any small molecule that has a specific lysosomal transporter but not a plasma membrane transporter.

Varied mechanisms underlie the free sialic acid storage disorders

Salla disease and infantile sialic acid storage disorder are autosomal recessive neurodegenerative diseases characterized by loss of a lysosomal sialic acid transport activity and the resultant accumulation of free sialic acid in lysosomes. Genetic analysis of these diseases has identified several unique mutations in a single gene encoding a protein designated sialin (Verheijen, F. W., Verbeek, E., Aula, N., Beerens, C. E., Havelaar, A. C., Joosse, M., Peltonen, L., Aula, P., Galjaard, H., van der Spek, P. J., and Mancini, G. M. (1999) Nat. Genet. 23, 462-465; Aula, N., Salomaki, P., Timonen, R., Verheijen, F., Mancini, G., Mansson, J. E., Aula, P., and Peltonen, L. (2000) Am. J. Hum. Genet. 67, 832-840). From the biochemical phenotype of the diseases and the predicted polytopic structure of the protein, it has been suggested that sialin functions as a lysosomal sialic acid transporter. Here we directly demonstrate that this activity is mediated by sialin and that the recombinant protein has functional characteristics similar to the native lysosomal sialic acid transport system. Furthermore, we describe the effect of disease-causing mutations on the protein. We find that the majority of the mutations are associated with a complete loss of activity, while the mutations associated with the milder forms of the disease lead to reduced, but residual, function. Thus, there is a direct correlation between sialin function and the disease state. In addition, we find with one mutation that the protein is retained in the endoplasmic reticulum, indicating that altered trafficking of sialin is also associated with disease. This analysis of the molecular mechanism of sialic acid storage disorders is a further step in identifying therapeutic approaches to these diseases.

Functional characterization of wild-type and mutant human sialin

The modification of cell surface lipids or proteins with sialic acid is essential for many biological processes and several diseases are caused by defective sialic acid metabolism. Sialic acids cleaved off from degraded sialoglycoconjugates are exported from lysosomes by a membrane transporter, named sialin, which is defective in two allelic inherited diseases: infantile sialic acid storage disease (ISSD) and Salla disease. To develop a functional assay of human sialin, we redirected the protein to the plasma membrane by mutating a dileucine-based internalization motif. Cells expressing the plasmalemmal construct accumulated neuraminic acid at acidic pH by a process equivalent to lysosomal efflux. The assay was used to determine how pathogenic mutations affect transport. Interestingly, while two missense mutations and one small, in-frame deletion associated with ISSD abolished transport, the mutation causing Salla disease (R39C) slowed down, but did not stop, the transport cycle, thus explaining why the latter disorder is less severe. Since neurological symptoms predominate in Salla disease, our results suggest that sialin is rate-limiting to specific sialic acid-dependent processes of the nervous system.

Clinical, biochemical, and molecular diagnosis of a free sialic acid storage disease patient of moderate severity

The allelic autosomal recessive lysosomal storage disorders Salla disease and infantile free sialic acid storage disease (ISSD) result from mutations in SLC17A5. This gene codes for sialin, a lysosomal membrane protein that transports the charged sugar, N-acetylneuraminic acid (sialic acid), out of lysosomes. ISSD has a severe phenotype with infantile onset, while the Finnish variant, Salla disease, has a milder phenotype with later onset. Both disorders cause developmental delay, and ISSD is generally fatal in early childhood. We describe a 30-month old non-Finnish, Caucasian child with global developmental delay of postnatal onset, language, and motor skills stagnant at a 3-4 month level, hypotonia, and mild but progressive coarsening of facial features. Urinary excretion of free sialic acid was elevated 4.5 times above control. EM of a skin biopsy revealed enlarged secondary lysosomes consistent with oligosaccharide storage. Free sialic acid in fibroblasts was 3.8+/-0.9 nmol/mg protein (concurrent normal controls, 0.5+/-0.1); differential centrifugation indicated a lysosomal location. Genomic analysis revealed compound heterozygosity for two new SLC17A5 mutations. This child's clinical manifestations of a lysosomal free sialic acid storage disease are consistent with her sialin mutations and biochemical findings. The differential diagnosis of postnatal developmental delay should include free sialic acid storage disorders such as ISSD and Salla disease.

Sialic acid storage disease and related disorders

This paper gives an overview of the two sialic acid storage disorders, Salla disease and infantile sialic acid storage disease, and the related disorders cystinosis, sialuria, sialidosis, and galactosialidosis. Sialic acid storage disease and cystinosis are models for a deficient lysosomal transport of monosaccharides and amino acids, respectively. Several gene mutations leading to the production of the faulty membrane proteins sialin and cystinosin have been identified in recent years. Knowledge of the underlying pathophysiology is a prerequisite for future research projects, which will focus on the expression of the disease genes in living systems and the physical characterization of these proteins by X-ray crystallography and nuclear magnetic resonance spectroscopy.

Sialic acid storage disease of the Salla phenotype in American monozygous twin female sibs

Salla disease, one of three disease phenotypes that manifest increased urinary excretion of unconjugated sialic acid, is an autosomal recessive condition caused by a mutation in SLC17A5. This gene encodes sialin, a lysosomal membrane transporter for sialic acid. Salla disease is rare outside of individuals of Finnish ancestry. In this report we describe the disorder in non-Finnish monozygous twin siblings, the first reported American cases of Salla disease.

Biochemical and molecular analyses of infantile free sialic acid storage disease in North American children

The differential diagnosis of developmental delays and growth retardation in early childhood includes the allelic lysosomal sialic acid storage disorders, Salla disease and infantile free sialic acid storage disease (ISSD). These diseases, due to defective free sialic acid transport out of lysosomes, derive from mutations in the SLC17A5 gene coding for the protein sialin. We present two patients with clinical, biochemical, and molecular data indicative of lysosomal free sialic acid storage disorders. One patient, with a severe clinical course typical of ISSD, had 86-fold elevated levels of fibroblast free sialic acid, with 62% in the lysosomal fraction. His SLC17A5 mutations include a 148-bp deletion of exon 9, due to a G >A splice site mutation in position 1 of intron 9, and a 15-bp deletion (del 801-815) in exon 6. Another patient, with "intermediate severe" Salla disease, had 9-fold elevated levels of free sialic acid in cultured fibroblasts, of which 87% resided in the lysosomal fraction. This girl is compound heterozygous for the SLC17A5 mutation commonly found in Finnish Salla disease patients (R39C) and a 15-bp deletion found in ISSD patients (del 801-815). These observations emphasize the importance of considering free sialic acid disorders in infants with developmental delays and growth retardation, regardless of whether they are of Finnish ancestry.

Benzyl-N-acetyl-alpha-D-galactosaminide induces a storage disease-like phenotype by perturbing the endocytic pathway

The sugar analog O-benzyl-N-acetyl-alpha-d-galactosaminide (BG) is an inhibitor of glycan chain elongation and inhibits alpha2,3-sialylation in mucus-secreting HT-29 cells. Long-term exposure of these cells to BG is associated with the accumulation of apical glycoproteins in cytoplasmic vesicles. The mechanisms involved therein and the nature of the vesicles have not been elucidated. In these cells, a massive amount of BG metabolites is synthesized. Because sialic acid is mainly distributed apically in epithelial cells, it has been proposed that the BG-induced undersialylation of apical membrane glycoproteins is responsible for their intracellular accumulation due to a defect in anterograde traffic and that sialic acid may constitute an apical targeting signal. In this work, we demonstrate that the intracellular accumulation of membrane glycoproteins does not result mainly from defects in anterograde traffic. By contrast, in BG-treated cells, endocytosed membrane proteins were retained intracellularly for longer periods of time than in control cells and colocalized with accumulated MUC1 and beta(1) integrin in Rab7/lysobisphosphatidic acid(+) vesicles displaying features of late endosomes. The phenotype of BG-treated cells is reminiscent of that observed in lysosomal storage disorders. Sucrose induced a BG-like, lysosomal storage disease-like phenotype without affecting sialylation, indicating that undersialylation is not a requisite for the intracellular accumulation of membrane glycoproteins. Our findings strongly support the notion that the effects observed in BG-treated cells result from the accumulation of BG-derived metabolites and from defects in the endosomal pathway. We propose that abnormal subcellular distribution of membrane glycoproteins involved in cellular communication and/or signaling may also take place in lysosomal storage disorders and may contribute to their pathogenesis.

The promoter of a lysosomal membrane transporter gene, CTNS, binds Sp-1, shares sequences with the promoter of an adjacent gene, CARKL, and causes cystinosis if mutated in a critical region

Although >55 CTNS mutations occur in patients with the lysosomal storage disorder cystinosis, no regulatory mutations have been reported, because the promoter has not been defined. Using CAT reporter constructs of sequences 5' to the CTNS coding sequence, we identified the CTNS promoter as the region encompassing nucleotides -316 to +1 with respect to the transcription start site. This region contains an Sp-1 regulatory element (GGCGGCG) at positions -299 to -293, which binds authentic Sp-1, as shown by electrophoretic-mobility-shift assays. Three patients exhibited mutations in the CTNS promoter. One patient with nephropathic cystinosis carried a -295 G-->C substitution disrupting the Sp-1 motif, whereas two patients with ocular cystinosis displayed a -303 G-->T substitution in one case and a -303 T insertion in the other case. Each mutation drastically reduced CAT activity when inserted into a reporter construct. Moreover, each failed either to cause a mobility shift when exposed to nuclear extract or to compete with the normal oligonucleotide's mobility shift. The CTNS promoter region shares 41 nucleotides with the promoter region of an adjacent gene of unknown function, CARKL, whose start site is 501 bp from the CTNS start site. However, the patients' CTNS promoter mutations have no effect on CARKL promoter activity. These findings suggest that the CTNS promoter region should be examined in patients with cystinosis who have fewer than two coding-sequence mutations.

Ultrasound assessment in a case of sialic acid storage disease

A woman was referred to our unit at 25 weeks' gestation because of fetal ascites. Conventional and three-dimensional ultrasound examinations revealed coarse facies and micromelia which strongly suggested storage disease, despite the absence of an index familial case. Amniocentesis was performed and, in view of the poor prognosis, the pregnancy was terminated. Autopsy confirmed all the sonographic features and the cultured amniocytes confirmed the diagnosis of infantile sialic acid storage disease.

Identification and characterization of a lysosomal transporter for small neutral amino acids

In eukaryotic cells, lysosomes represent a major site for macromolecule degradation. Hydrolysis products are eventually exported from this acidic organelle into the cytosol through specific transporters. Impairment of this process at either the hydrolysis or the efflux step is responsible of several lysosomal storage diseases. However, most lysosomal transporters, although biochemically characterized, remain unknown at the molecular level. In this study, we report the molecular and functional characterization of a lysosomal amino acid transporter (LYAAT-1), remotely related to a family of H+-coupled plasma membrane and synaptic vesicle amino acid transporters. LYAAT-1 is expressed in most rat tissues, with highest levels in the brain where it is present in neurons. Upon overexpression in COS-7 cells, the recombinant protein mediates the accumulation of neutral amino acids, such as gamma-aminobutyric acid, l-alanine, and l-proline, through an H+/amino acid symport. Confocal microscopy on brain sections revealed that this transporter colocalizes with cathepsin D, an established lysosomal marker. LYAAT-1 thus appears as a lysosomal transporter that actively exports neutral amino acids from lysosomes by chemiosmotic coupling to the H+-ATPase of these organelles. Homology searching in eukaryotic genomes suggests that LYAAT-1 defines a subgroup of lysosomal transporters in the amino acid/auxin permease family.

Dominant inheritance of sialuria, an inborn error of feedback inhibition

"French type" sialuria, a presumably dominant disorder that, until now, had been documented in only five patients, manifests with mildly coarse facies, slight motor delay, and urinary excretion of large quantities (>1 g/d) of free N-acetylneuraminic acid (NeuAc). The basic defect consists of the very rare occurrence of failed feedback inhibition of a rate-limiting enzyme, in this case uridinediphosphate-N-acetylglucosamine (UDP-GlcNAc) 2-epimerase, by a downstream product, in this case cytidine monophosphate (CMP)-NeuAc. We report a new patient with sialuria who has a heterozygous G-->A substitution in nucleotide 848 of the epimerase gene, which results in an R266Q change. The proband's other allele, as expected, had no mutation. However, the heterozygous R266Q mutation was detected in the patient's mother, who has similarly increased urinary levels of free NeuAc, thereby confirming, for the first time, the dominant mode of inheritance of this inborn error. The biochemical diagnosis of the proband was verified by the greatly increased level of free NeuAc in his cultured fibroblasts, the NeuAc distribution, mainly (59%) in the cytoplasm, and by the complete failure of 100 microM CMP-NeuAc to inhibit UDP-GlcNAc 2-epimerase activity in the mutant cells. These findings call for expansion of the phenotype to include adults and for more-extensive assaying of free NeuAc in the urine of children with mild developmental delay. The prevalence of sialuria is probably grossly underestimated.

Clinical course and biochemistry of sialuria

Sialuria is a rare inborn error of metabolism in which excessive free sialic acid (N-acetylneuraminic acid, NeuAc) is synthesized. A defect in the feedback inhibition of UDP-N-acetylglucosamine (UDP-GlcNAc) 2-epimerase by the end-product of the sialic acid synthetic pathway, CMP-NeuAc, is the mechanism underlying this overproduction. Recent evidence suggests that sialuria is an autosomal dominant disorder. Only five patients have been documented to have such an enzymatic defect. We report a longitudinal study of one of the original sialuria patients, to age 11 years. Although he has coarse features and massive hepatomegaly, he has shown normal growth and relatively normal development. Pulmonary function testing showed minimal small airway obstruction. At 11 years, he developed intermittent abdominal pain and transient transaminase elevation above his baseline. Sialuria should be considered in the differential diagnosis of a patient with a phenotype suggestive of a mucopolysaccharidosis or oligosaccharidosis in the absence of developmental regression or prominent dysostosis multiplex. We recommend close monitoring of liver and pulmonary function in sialuria patients.

Prenatal detection of free sialic acid storage disease: genetic and biochemical studies in nine families

Sialic acid storage disorders, Salla disease (SD) and a severe infantile form of disease (ISSD), are recessively inherited allelic lysosomal storage disorders due to impaired egress of free sialic acid from lysosomes. Fourteen pregnancies at risk of adult-type free sialic acid storage disease, SD, were monitored by sialic acid assays, genetic linkage or mutation detection analyses using chorionic villus samples. Three affected and 12 unaffected fetuses were identified. The first studies were based on the sialic acid assays alone, but the location of the gene enabled the use of genetic linkage analysis and, more recently, the identification of the SLC17A5 gene and disease-causing mutations added yet another possibility for prenatal studies. A missense mutation 115C-->T (R39C) is present in 95% of all Finnish SD alleles, providing an easy and reliable means of diagnostic studies. Both molecular and biochemical (sialic acid assay) studies can be used for prenatal diagnosis of free sialic acid storage diseases.

Uptake of glutamate into synaptic vesicles by an inorganic phosphate transporter

Previous work has identified two families of proteins that transport classical neurotransmitters into synaptic vesicles, but the protein responsible for vesicular transport of the principal excitatory transmitter glutamate has remained unknown. We demonstrate that a protein that is unrelated to any known neurotransmitter transporters and that was previously suggested to mediate the Na(+)-dependent uptake of inorganic phosphate across the plasma membrane transports glutamate into synaptic vesicles. In addition, we show that this vesicular glutamate transporter, VGLUT1, exhibits a conductance for chloride that is blocked by glutamate.

GalNAc-alpha -O-benzyl inhibits sialylation of de Novo synthesized apical but not basolateral sialoglycoproteins and blocks lysosomal enzyme processing in a post-trans-Golgi network compartment

Glycosylation plays an important role in glycoprotein traffic. Our previous work has shown that long term treatment of mucus-secreting HT-29 cells with GalNAc-alpha-O-benzyl reversibly inhibits sialylation and causes the accumulation of apical glycoproteins in cytoplasmic vesicles. We have analyzed at the biochemical level the effects of GalNAc-alpha-O-benzyl on glycoprotein processing. Both apical and basolateral membrane glycoproteins were sialylated, but GalNAc-alpha-O-benzyl selectively inhibited the sialylation of apical glycoproteins. In addition, lysosomal alpha-glucosidase, which is partially targeted to the apical membrane, was abnormally processed leading to the accumulation of an immature molecular species. Several findings support the conclusion that accumulation of this protein occurs in a post-trans-Golgi network (TGN) compartment: 1) it is partially sialylated; 2) it does not occur when glycoprotein exit from the TGN is blocked at 20 degrees C; 3) upon Triton X-114 partition, it distributes to the aqueous phase, a characteristic that is acquired in a post-TGN compartment; and 4) its appearance is inhibited when cells are cultured in the presence of NH(4)Cl. The processing of cathepsin D was also found to be affected by GalNAc-alpha-O-benzyl treatment. In conclusion, GalNAc-alpha-O-benzyl selectively inhibits sialylation of apical glycoproteins and perturbs lysosomal enzyme processing; these effects occur in a post-TGN acidic compartment and are reminiscent of the alterations found in sialic acid storage diseases.

Accumulation of sialic acid in endocytic compartments interferes with the formation of mature lysosomes. Impaired proteolytic processing of cathepsin B in fibroblasts of patients with lysosomal sialic acid storage disease

The impact of an altered endocytic environment on the biogenesis of lysosomes was studied in fibroblasts of patients suffering from sialic acid storage disease (SASD). This inherited disorder is characterized by the accumulation of acidic monosaccharides in lysosomal compartments and a concomitant decrease of their buoyant density. We demonstrate that C-terminal trimming of the lysosomal cysteine proteinase cathepsin B is inhibited in SASD fibroblasts. This late event in the biosynthesis of cathepsin B normally takes place in mature lysosomes, suggesting an impaired biogenesis of these organelles in SASD cells. When normal fibroblasts are loaded with sucrose, which inhibits transport from late endosomes to lysosomes, C-terminal cathepsin B processing is prevented to the same extent. Further characterization of the terminal endocytic compartments of SASD cells revealed properties usually associated with late endosomes/prelysosomes. In addition to a decreased buoyant density, SASD "lysosomes" show a reduced acidification capacity and appear smaller than their normal counterparts. We conclude that the accumulation of small non-diffusible compounds within endocytic compartments interferes with the formation of mature lysosomes and that the acidic environment of the latter organelles is a prerequisite for C-terminal processing of lysosomal hydrolases.

Sialuria in a Portuguese girl: clinical, biochemical, and molecular characteristics

Sialuria, a disorder of sialic acid (NeuAc) metabolism characterized by increased free NeuAc in the cytoplasm of cells, is due to failure of CMP-Neu5Ac to feedback inhibit UDP-N-acetylglucosamine (UDP-GlcNAc) 2-epimerase. We now describe the fifth patient in the world with sialuria, a 7-year-old Portuguese girl with developmental delay, hepatomegaly, coarse facies, and urinary excretion of 19 micromol of free NeuAc/mg creatinine. The patient's fibroblasts stored excess free NeuAc in the cytosolic fraction, and fibroblast UDP-GlcNAc 2-epimerase activity was only 26% inhibited by 100 microM CMP-Neu5Ac (normal, 79%). The patient's UDP-GlcNAc 2-epimerase gene displayed an R266Q mutation in only one allele, consistent with known sialuria mutations and with the proposed dominant nature of this disorder. Extensive description of sialuria patients will help to define the clinical and biochemical spectrum of this disease.

Mutations in the human UDP-N-acetylglucosamine 2-epimerase gene define the disease sialuria and the allosteric site of the enzyme

Sialuria is a rare inborn error of metabolism characterized by cytoplasmic accumulation and increased urinary excretion of free N-acetylneuraminic acid (NeuAc, sialic acid). Overproduction of NeuAc is believed to result from loss of feedback inhibition of uridinediphosphate-N-acetylglucosamine 2-epimerase (UDP-GlcNAc 2-epimerase) by cytidine monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac). We report the cloning and characterization of human UDP-GlcNAc 2-epimerase cDNA, with mutation analysis of three patients with sialuria. Their heterozygote mutations, R266W, R266Q, and R263L, indicate that the allosteric site of the epimerase resides in the region of codons 263-266. The heterozygous nature of the mutant allele in all three patients reveals a dominant mechanism of inheritance for sialuria.

Cytosol-to-lysosome transport of free polymannose-type oligosaccharides. Kinetic and specificity studies using rat liver lysosomes

In hepatocellular carcinoma HepG2 cells, free polymannose-type oligosaccharides appearing in the cytosol during the biosynthesis and quality control of glycoproteins are rapidly translocated into lysosomes by an as yet poorly defined process (Saint-Pol, A., Bauvy, C., Codogno, P., and Moore, S. E. H. (1997) J. Cell Biol. 136, 45-59). Here, we demonstrate an ATP-dependent association of [2-3H]mannose-labeled Man5GlcNAc with isolated rat liver lysosomes. This association was only observed in the presence of swainsonine, a mannosidase inhibitor, which was required for the protection of sedimentable, but not nonsedimentable, Man5GlcNAc from degradation, indicating that oligosaccharides were transported into lysosomes. Saturable high affinity transport (Kuptake, 22.3 microM, Vmax, 7.1 fmol/min/unit of beta-hexosaminidase) was dependent upon the hydrolysis of ATP but independent of vacuolar H+/ATPase activity. Transport was inhibited strongly by NEM and weakly by vanadate but not by sodium azide, and, in addition, the sugar transport inhibitors phloretin, phloridzin, and cytochalasin B were without effect on transport. Oligosaccharide import did not show absolute specificity but was selective toward partially demannosylated and dephosphorylated oligosaccharides, and, furthermore, inhibition studies revealed that the free reducing GlcNAc residue of the oligosaccharide was of critical importance for its interaction with the transporter. These results demonstrate the presence of a novel lysosomal free oligosaccharide transporter that must work in concert with cytosolic hydrolases in order to clear the cytosol of endoplasmic reticulum-generated free oligosaccharides.

Cysteine transport in melanosomes from murine melanocytes

The synthesis of pheomelanin requires the incorporation of thiol-containing compound(s) during the process of mammalian melanogenesis. Since melanins are produced only in specialized, membrane-bound organelles, known as melanosomes, such thiol donor(s) must cross the membrane barrier from the cytosol to the melanosome interior. Cysteine and/or glutathione (GSH) were proposed as suitable thiol donors, although uptake of these compounds into melanosomes was not previously characterized. In this study, we show that cysteine is transported, in a temperature- and concentration-dependent manner, across membranes of melanosomes derived from murine melanocytes. Additional proof that cysteine uptake results from a carrier-mediated process and is not due to simple diffusion or to a membrane channel, was obtained in countertransport experiments, in which melanosomes preloaded with cysteine methyl ester took up significantly more [35S]cysteine than did unloaded controls. In contrast, we were unable to detect any significant uptake of [35S]GSH over a wide concentration range, in the presence or in the absence of reducing agent. This study is the first demonstration of melanosomal membrane transport of cysteine, and it strongly suggests that free cysteine is the thiol source utilized for pheomelanin synthesis in mammalian melanocytes.

CTNS mutations in an American-based population of cystinosis patients

Nephropathic cystinosis is an autosomal recessive lysosomal storage disease characterized by renal failure at 10 years of age and other systemic complications. The gene for cystinosis, CTNS, has 12 exons. Its 2.6-kb mRNA codes for a 367-amino-acid putative cystine transporter with seven transmembrane domains. Previously reported mutations include a 65-kb "European" deletion involving marker D17S829 and 11 small mutations. Mutation analysis of 108 American-based nephropathic cystinosis patients revealed that 48 patients (44%) were homozygous for the 65-kb deletion, 2 had a smaller major deletion, 11 were homozygous and 3 were heterozygous for 753G-->A (W138X), and 24 had 21 other mutations. In 20 patients (19%), no mutations were found. Of 82 alleles bearing the 65-kb deletion, 38 derived from Germany, 28 from the British Isles, and 4 from Iceland. Eighteen new mutations were identified, including the first reported missense mutations, two in-frame deletions, and mutations in patients of African American, Mexican, and Indian ancestry. CTNS mutations are spread throughout the leader sequence, transmembrane, and nontransmembrane regions. According to a cystinosis clinical severity score, homozygotes for the 65-kb deletion and for W138X have average disease, whereas mutations involving the first amino acids prior to transmembrane domains are associated with mild disease. By northern blot analysis, CTNS was not expressed in patients homozygous for the 65-kb deletion but was expressed in all 15 other patients tested. These data demonstrate the origins of CTNS mutations in America and provide a basis for possible molecular diagnosis in this population.

Tyrosine transport into melanosomes is increased following stimulation of melanocyte differentiation

A variety of physiological factors can stimulate differentiation of melanocytes to increase pigmentation, and critical to this process is the transport of the melanogenic substrate (tyrosine) into melanosomes. In this study, we examined whether stimulation of melanogenesis affects melanosomal tyrosine transport. Tyrosine uptake increased almost 2-fold in melanosomes derived from melanocytes treated with melanocyte-stimulating hormone (MSH), which acts to increase intracellular cAMP levels, resulting in the up-regulation of many genes involved in melanogenesis. Stimulation of melanoma cells with dibutyryl cAMP increased melanosomal tyrosine transport 2- to 3-fold after 24 to 48 hrs, with peak levels occurring after 3 to 5 days of treatment, suggesting that de novo gene expression may be required. The cAMP-induced increase in melanosomal tyrosine transport could be effectively competed with phenylalanine or tryptophan, but not with dopamine or proline, suggesting either that a pool of transporters with greater tyrosine transporting ability pre-exists, or that a greater number of tyrosine transporters reside within the melanosomal membrane. These results illustrate a rare example of hormonal plasma membrane stimulation which transduces a signal for increased vesicular transport of an amino acid.

Establishment and characterization of an Epstein-Barr virus-transformed B cell line, KM/C8, from a patient with infantile sialic acid storage disease

Nakano et al. have recently reported a Japanese case of infantile sialic acid storage disease [C. Nakano, Y. Hirabayashi, K. Ohno, T. Yano, T. Mito, M. Sakurai, Brain Dev., 18 (1996) 153-156]. For further etiological analysis of this disease, we prepared the Epstein-Barr virus (EBV)-transformed cell line (LCL) from the peripheral lymphocytes of this patient and performed initial characterization of the cells. Electron microscopy of the cells showed that the cells contained many vacuoles and swelled lysosomes. Cytochemical staining with sialic acid-specific lectin, Limax flavus agglutinin (LFA), showed strong staining on membranes and subcellular organelles on the patient-derived cells, whereas LCL from a normal person was only weakly stained. The cells from the patient contained 5.5-7.3 nmol/107 cells of free N-acetyl neuraminic acid, whereas three strains of LCLs derived from normal persons contained 1 nmol/107 cells. The culture supernatant of LCL from the patient contained 144 nmol/ml of free N-acetyl neuraminic acid, whereas the LCL culture supernatant from normal persons contained 57-73 nmol/ml of free sialic acid, which was the same or only at a slightly higher level than the fresh medium. In addition, cellular acidic sialidase measured as 4-methylumbelliferyl sialidase was elevated (107 nmol 4-methylumbelliferon released/mg cellular protein/60 min). The EBV-LCL from an ISSD patient is considered to remain as the abnormality of the cell donor.

Chapter 5 Role of lysosomes in cell injury

Lysosomes are acidic intracellular vacuoles of heterogeneous shape, size, and content. Lysosomes contain hydrolytic enzymes that degrade proteins, lipids, carbohydrates, and nucleic acids derived from intracellular (through autophagy) and extracellular (through heterophagy) sources. Lysosomal degradation regulates several physiological cell functions. These include turnover of cellular organelles and extracellular constituents; amino acid and glucose homeostasis; processing of proteins; lipid metabolism; cell growth, differentiation, and involution; host defenses against microorganisms and other pathogens; and removal of necrotic and foreign material from the circulation and from tissues.

Lysosomal degradation also plays an important role in the pathophysiology of acute and chronic cell injury, inflammation and repair, and tumor growth and metastasis. The participation of the lysosomes in the specific types of cell injury we have discussed is due to altered regulation of one or more of the following processes: turnover of cellular organelles by autophagic degradation; levels and activities of lysosomal hydrolases; levels of intracellular and extracellular lysosomal hydrolase inhibitors; transport of degradation products from the lysosomal matrix to the cytosol; permeability of the lysosomal membrane to hydrolases; lysosomal vacuolar acidification; transport of degradable substrates and of pathogens to the lysosomes; transport and processing of secretory proteins and lysosomal hydrolases during biogenesis; traffic and fusion of lysosomal vacuoles and vesicles; secretion of lysosomal hydrolases; and accumulation of metals, particularly iron, acidotropic agents, and undegraded and/or undegradable materials in lysosomes.

A Japanese case of infantile sialic acid storage disease

We report a 4-year-old Japanese girl with infantile sialic acid storage disease. She presented with failure to thrive, coarse facial features, hepatosplenomegaly, severe mental retardation and spastic quadriplegia. Electron microscopic examination of cultured skin fibroblasts revealed multiple vacuoles and inclusion material representing distended lysosomes, thus suggesting a lysosomal storage disorder. A high concentration of free sialic acid was present in the urine and cultured fibroblasts, but bound sialic acid was not increased. The activity of a variety of lysosomal enzymes was not diminished. The MRI findings included brain atrophy and a diffuse high signal in the cerebral white matter and low signal in the basal ganglia on T2-weighted images. To our knowledge, this is the first case of infantile sialic acid storage disease described in a non-Caucasian family.

Characterization of a melanosomal transport system in murine melanocytes mediating entry of the melanogenic substrate tyrosine

In this study, we identify a transport system for tyrosine, the initial precursor of melanin synthesis, in the melanosomes of murine melanocytes. Melanosomes preloaded with tyrosine demonstrated countertransport of 10 microM [3H]tyrosine, indicating carrier-mediated transport. Melanosomal tyrosine transport was saturable, with an apparent Km for tyrosine transport of 54 microM and a maximal velocity of 15 pmol of tyrosine/unit of hexosaminidase/min. Transport was temperature-dependent (Ea = 7.5 kcal/mol) and showed stereospecificity for the l-isomer of tyrosine. Aromatic, neutral hydrophobic compounds (such as tryptophan and phenylalanine), as well as the small, bulky neutral amino acids (such as leucine, isoleucine, and methionine) competed for tyrosine transport. Tyrosine transport was inhibited by the classical system L analogue, 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid and by monoiodotyrosine, but not by cystine, lysine, glutamic acid, or 2-(methylamino)-isobutyric acid. Tyrosine transport showed no dependence on Na+ or K+, and did not require an acidic environment or the availability of free thiols. These results demonstrate the existence of a neutral amino acid carrier in murine melanocyte melanosomes which resembles the rat thyroid FRTL-5 lysosomal system h. This transport system is critical to the function of the melanosome since tyrosine is the essential substrate required for the synthesis of the pigment melanin.

Melanosomal tyrosine transport in normal and pink-eyed dilution murine melanocytes

Tyrosine is the endogenous substrate for melanin production within melanosomes, but the method of tyrosine transport into the melanosome has not been investigated. In the mouse, melanogenesis is disrupted by mutations in the p gene resulting in the pink-eyed dilution phenotype; it has been suggested that the p gene codes for a tyrosine transport protein. We determined that normal (melan-a) melanosome-rich granular fractions take up 10 microns [3H]tyrosine at 21.1 +/- 6.1 (SEM, standard error of the mean) pmol/min/mg protein (N = 7) compared with 21.3 +/- 5.8 SEM pmol/min/mg protein (N = 5) for pink-eyed dilution, whose plasma membrane tyrosine transport was also normal (Km 89 microM; Vmax 302 pmol/min/mg cell protein). We also demonstrated that pink-eyed dilution melanosomes are immature by virtue of their low density, high hexosaminidase activity, and lack of pigment. These data indicate that a tyrosine transport system exists in the melanosomal membrane and that the p gene does not encode a tyrosine transporter of critical importance.

Infantile sialic acid storage disease: biochemical studies

Infantile free sialic acid storage disease (ISSD), is an inherited metabolic disorder characterized by hyperexcretion of free sialic acid in the urine and by its storage in the lysosomes of different tissues. In order to obtain more reliable data on the amount of total and free sialic acid, we analyzed the urine, brain, cerebellum, liver, spleen, and kidneys from a 3-month-old baby who died with a diagnosis of ISSD. The lysosomal nature of the disease was confirmed by an electron microscopic study of cells in culture. No significant abnormalities were found involving cholesterol, total phospholipids, glycolipids, and gangliosides in the tissues examined. However, differences in the tissue distribution of individual glycolipids and gangliosides were observed. The amount of free and total sialic acid was markedly increased, due to the storage of free sialic acid accompanied by its hyperexcretion in the urine. These results demonstrate and confirm that only acid monosaccharide transport from the lysosome compartment is involved in the pathogenesis of ISSD.

Lysosomal sulfate efflux following glycosaminoglycan degradation: measurements in enzyme-supplemented Maroteaux-Lamy syndrome fibroblasts and isolated lysosomes

Studies using lysosomal membrane vesicles have suggested that efflux of the sulfate that results from lysosomal glycosaminoglycan degradation is carrier-mediated. In this study, glycosaminoglycan degradation and sulfate efflux were examined using cultured skin fibroblasts and lysosomes deficient in the lysosomal enzyme N-acetylgalactosamine-4-sulfatase. Such fibroblasts store dermatan sulfate lysosomally, which could be labelled biosynthetically with Na2(35)SO4. The addition of recombinant N-acetylgalactosamine-4-sulfatase to the media of 35S labelled fibroblasts degraded up to 82% of the stored dermatan [35S] sulfate over a subsequent 96 h chase and released inorganic [35S] sulfate into the medium. In the presence of 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS), sulfate was reused to a minor extent in newly synthesized proteoglycan. Isolated granules from recombinant enzyme supplemented fibroblasts degraded stored dermatan [35S]sulfate to sulfate which was rapidly released into the medium at a rate that was reduced by the extra-lysosomal presence of the lysosomal sulfate transport inhibitors SITS, Na2SO4 and Na2MoO4. SITS also inhibited dermatan sulfate turnover, although it had no effect on the action of purified recombinant enzyme in vitro. These data imply that sulfate clearance occurred concomitantly with dermatan sulfate turnover in the lysosome even at high substrate loading, and that lysosome-derived sulfate, while available, is reutilized minimally in synthetic pathways.

Measurement of sialic acid in serum and urine: clinical applications and limitations

Many recent studies have examined the sialic acid content of serum or urine in various pathological states. We have briefly reviewed the substances which contribute to the observed total sialic acid concentration, and given an overview of assay methods used. Three major areas of clinical interest in sialic acid metabolism are discussed. Serum total sialic acid, 'lipid-bound' and 'protein bound' sialic acid have all been proposed as tumour markers; but the usefulness of any of these tests is severely limited by changes due to accompanying inflammatory processes. Serum total sialic acid is not a valuable simple marker of an acute phase response. Urinary free and bound sialic acid measurements should be included in screening protocols for inherited disorders of lysosomal metabolism. Current developments in research and potential applications within the clinical biochemistry laboratory are briefly discussed.

Salla disease variant in a Dutch patient. Potential value of polymorphonuclear leucocytes for heterozygote detection

A Dutch child with psychomotor retardation, impaired speech, ataxia, sialic acid storage and vacuolized skin fibroblasts and lymphocytes was diagnosed as having free sialic acid storage disease. Slight corneal opacities, pale optic disks at the fundus oculi and vertebral abnormalities, not earlier reported in Salla disease, were peculiar to this case. Free sialic acid was about tenfold increased in urine and cultured fibroblasts, without changes in the glycoconjugate-bound sialic acid pool. A subsequent pregnancy of the patient's mother was monitored by assay of sialic acid in chorionic villi and amniotic fluid. An unaffected foetus was predicted. Sialic acid was also assayed in peripheral blood total leucocytes, and in mononuclear and polymorphonuclear (PMN) leucocyte subpopulations. Each of these leucocyte fractions from the patient showed 10- to 30-fold increase in sialic acid content. The PMN subpopulation provided the most restricted range of control values and showed slightly increased values for the patient's parents. These results suggest that the assay of sialic acid in PMN might be useful for the identification of heterozygotes in sialic acid storage disease. Studies on a larger number of obligate heterozygotes are needed to confirm this observation.

Storage material from urine and tissues in the nephropathic phenotype of infantile sialic acid storage disease

We analysed urine and tissue specimens from two nephrotic infantile sialic acid storage disease patients (nISSD) for free and bound sialic acids in comparison to non-nephrotic ISSD patients (ISSD), patients with minimal change nephrosis (nControl) and normal controls (Control). No differences in the excretion of urinary free sialic acid could be detected between ISSD and nISSD urines. Sialyloligosaccharide fractions were only slightly elevated and of apparently normal composition. Owing to glomerular dysfunction, measurable quantities of protein-bound sialic acids were present in nISSD and nControl. In nISSD tissues, free sialic acid was elevated 18-100-fold above control and 3-12-fold above Niemann-Pick A (NPA) samples. The storage of membrane-bound sialic acid was slightly increased compared to control tissues, but equal to those from NPA, thus reflecting an unspecific increase of membranes due to lysosomal storage. According to these results no major biochemical differences were detectable between ISSD and nISSD. The nephrotic syndrome in nISSD could not be related to a general deficit in the sialylation of glycoproteins. Nevertheless, a cell membrane-specific alteration in sialoglycoproteins of glomerular cells might still be possible.