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Objectives: Urine free sialic acid (UFSA) is an important diagnostic biomarker for sialuria ( GNE variants) and infantile sialic acid storage disease/Salla disease ( SLC17A5 variants). Traditionally, UFSA has been measured using specific single-plex methodology in relatively small cohorts of patients with clinical symptoms suggestive of these disorders. The use of multiplex tandem mass spectrometry urine screening (UMSMS) has meant that UFSA can be measured semi-quantitatively in a much larger cohort of patients being investigated for suspected metabolic disorders. We hypothesised that the neuraminidase of Streptococcus pneumoniae may release free sialic acid from endogenous sialylated glycoconjugates and result in increased UFSA levels., Methods: We conducted a retrospective review of clinical records of patients who were identified as having S. pneumoniae infection and who also had UMSMS at the time of their acute infection., Results: We identified three cases of increased UFSA detected by UMSMS screening that were secondary to S. pneumoniae sepsis. Additional testing ruled out genetic causes of increased UFSA in the first patient. All three patients had overwhelming sepsis with multiorgan dysfunction which was fatal. Glycosylation abnormalities consistent with the removal of sialic acid were demonstrated in serum transferrin patterns in one patient., Conclusions: We have demonstrated in a retrospective cohort that elevation of UFSA levels have been observed in cases of S. pneumoniae sepsis. This expands our knowledge of UFSA as a biomarker in human disease. This research demonstrates that infection with organisms with neuraminidase activity should be considered in patients with unexplained increases in UFSA., (© 2022 Walter de Gruyter GmbH, Berlin/Boston.)

Lysosomal free sialic acid storage disorder (FSASD) is an extremely rare, autosomal recessive, neurodegenerative, multisystemic disorder caused by defects in the lysosomal sialic acid membrane exporter SLC17A5 (sialin). SLC17A5 defects cause free sialic acid and some other acidic hexoses to accumulate in lysosomes, resulting in enlarged lysosomes in some cell types and 10-100-fold increased urinary excretion of free sialic acid. Clinical features of FSASD include coarse facial features, organomegaly, and progressive neurodegenerative symptoms with cognitive impairment, cerebellar ataxia and muscular hypotonia. Central hypomyelination with cerebellar atrophy and thinning of the corpus callosum are also prominent disease features. Around 200 FSASD cases are reported worldwide, with the clinical spectrum ranging from a severe infantile onset form, often lethal in early childhood, to a mild, less severe form with subjects living into adulthood, also called Salla disease. The pathobiology of FSASD remains poorly understood and FSASD is likely underdiagnosed. Known patients have experienced a diagnostic delay due to the rarity of the disorder, absence of routine urine sialic acid testing, and non-specific clinical symptoms, including developmental delay, ataxia and infantile hypomyelination. There is no approved therapy for FSASD. We initiated a multidisciplinary collaborative effort involving worldwide academic clinical and scientific FSASD experts, the National Institutes of Health (USA), and the FSASD patient advocacy group (Salla Treatment and Research [S.T.A.R.] Foundation) to overcome the scientific, clinical and financial challenges facing the development of new treatments for FSASD. We aim to collect data that incentivize industry to further develop, obtain approval for, and commercialize FSASD treatments. This review summarizes current aspects of FSASD diagnosis, prevalence, etiology, and disease models, as well as challenges on the path to therapeutic approaches for FSASD., (Copyright © 2021. Published by Elsevier B.V.)

Sialic acids are important components of glycoproteins and glycolipids essential for cellular communication, infection, and metastasis. The importance of sialic acid biosynthesis in human physiology is well illustrated by the severe metabolic disorders in this pathway. However, the biological role of sialic acid catabolism in humans remains unclear. Here, we present evidence that sialic acid catabolism is important for heart and skeletal muscle function and development in humans and zebrafish. In two siblings, presenting with sialuria, exercise intolerance/muscle wasting, and cardiac symptoms in the brother, compound heterozygous mutations [chr1:182775324C>T (c.187C>T; p.Arg63Cys) and chr1:182772897A>G (c.133A>G; p.Asn45Asp)] were found in the N-acetylneuraminate pyruvate lyase gene (NPL). In vitro, NPL activity and sialic acid catabolism were affected, with a cell-type-specific reduction of N-acetyl mannosamine (ManNAc). A knockdown of NPL in zebrafish resulted in severe skeletal myopathy and cardiac edema, mimicking the human phenotype. The phenotype was rescued by expression of wild-type human NPL but not by the p.Arg63Cys or p.Asn45Asp mutants. Importantly, the myopathy phenotype in zebrafish embryos was rescued by treatment with the catabolic products of NPL: N-acetyl glucosamine (GlcNAc) and ManNAc; the latter also rescuing the cardiac phenotype. In conclusion, we provide the first report to our knowledge of a human defect in sialic acid catabolism, which implicates an important role of the sialic acid catabolic pathway in mammalian muscle physiology, and suggests opportunities for monosaccharide replacement therapy in human patients.

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., (© 2018 Elsevier Inc. All rights reserved.)

Sialuria is a rare autosomal dominant disorder of mammalian metabolism, caused by defective feedback inhibition of the UDP-N-acetylglucosamine-2-epimerase N-acetylmannosamine kinase (GNE), the key enzyme of sialic acid biosynthesis. Sialuria is characterized by overproduction of free sialic acid in the cell cytoplasm. Patients exhibit vastly increased urinary excretion of sialic acid and show differently pronounced developmental delays. The physiopathology of sialuria is not well understood. Here we established a transgenic mouse line that expresses GNE containing the sialuria mutation R263L, in order to investigate the influence of an altered sialic acid concentration on the organism. The transgenic mice that expressed the mutated RNA excreted up to 400 times more N-acetylneuraminic acid than wild-type mice. Additionally, we found higher sialic acid concentration in the brain cytoplasm. Analyzing the (poly)sialylation of neural cell adhesion molecule (NCAM) revealed increased polysialylation in brains of transgenic mice compared to wild-type. However, we found only minor changes in membrane-bound sialylation in various organs but, surprisingly, a significant increase in surface sialylation on leukocytes. Our results suggest that the intracellular sialic acid concentration regulates polysialylation on NCAM in vivo; this could play a role in the manifestation of the developmental delays in sialuria patients., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We describe here a 34 months child, practically asymptomatic which presented with high levels of free sialic acid in urine by biochemical detection in second-tier tests newborn screening and with two disease causing mutations in SLC17A5 gene. SLC17A5 mutation analysis showed p.Tyr306* previously described and the novel mutation p.Leu167Pro. This early onset diagnosis allowed us to perform a fast and accurate genetic counseling to the family, helped to better understanding the natural history of this rare disease and probably it could promote cost reduction in future diagnostic tests in the hypothetic case of starting symptoms without diagnosis established. Moreover, an early diagnosis could save family from a long period of time until achieving a definitive diagnostic and to develop an early symptomatic and supportive management of patient to attenuate, as much as possible, disease complications. But, above all, this case illustrates the huge ethical dilemma which arises from any secondary finding (second tier) in newborn screening., (Copyright © 2014 Elsevier Masson SAS. All rights reserved.)

In vivo recycling of nitrate (NO(3)(-)) and nitrite (NO(2)(-)) is an important alternative pathway for the generation of nitric oxide (NO) and maintenance of systemic nitrate-nitrite-NO balance. More than 25% of the circulating NO(3)(-) is actively removed and secreted by salivary glands. Oral commensal bacteria convert salivary NO(3)(-) to NO(2)(-), which enters circulation and leads to NO generation. The transporters for NO(3)(-) in salivary glands have not yet been identified. Here we report that sialin (SLC17A5), mutations in which cause Salla disease and infantile sialic acid storage disorder (ISSD), functions as an electrogenic 2NO(3)(-)/H(+) cotransporter in the plasma membrane of salivary gland acinar cells. We have identified an extracellular pH-dependent anion current that is carried by NO(3)(-) or sialic acid (SA), but not by Br(-), and is accompanied by intracellular acidification. Both responses were reduced by knockdown of sialin expression and increased by the plasma membrane-targeted sialin mutant (L22A-L23A). Fibroblasts from patients with ISSD displayed reduced SA- and NO(3)(-)-induced currents compared with healthy controls. Furthermore, expression of disease-associated sialin mutants in fibroblasts and salivary gland cells suppressed the H(+)-dependent NO(3)(-) conductance. Importantly, adenovirus-dependent expression of the sialinH183R mutant in vivo in pig salivary glands decreased NO(3)(-) secretion in saliva after intake of a NO(3)(-)-rich diet. Taken together, these data demonstrate that sialin mediates nitrate influx into salivary gland and other cell types. We suggest that the 2NO(3)(-)/H(+) transport function of sialin in salivary glands can contribute significantly to clearance of serum nitrate, as well as nitrate recycling and physiological nitrite-NO homeostasis.

Aspartate, an excitatory amino acid, is known to be stored in synaptic vesicles and exocytosed from some neurons to perform aspartergic neurotransmission. Through in vitro reconstitution, we found that sialin, a lysosomal sialic acid exporter, is responsible for the vesicular storage of aspartate in hippocampal neurons and pinealocytes. Mutations found in Salla disease cause decreased aspartate transport activity without affecting sialic acid transport. Thus, sialin is a multifunctional transporter. It is possible that people with Salla disease lose the ability of aspartergic neurotransmission, and this could explain why Salla disease involves severe neurological defects.

Sialic acid storage disorder, known as Salla disease, is a rare autosomal recessive lysosomal disorder produced by a defect of a proton-driven carrier that is responsible for the efflux of sialic acid from the lysosomal compartment. We report two patients with Salla disease: a two-year-old girl, presented with hypotonia, inability to speak and walk, bilateral optic atrophies, defective myelination, cerebellar atrophy, and thinning of the corpus callosum on magnetic resonance imaging (MRI), who was classified as intermediate severe Salla disease; and a four-year-old girl, presented with relatively late-onset, slight hypotonia, and delayed language and mobility development, and supported by relatively protected MRI findings, who was classified as conventional Salla disease. Diagnosis of Salla disease was confirmed by accumulation of sialic acid in fibroblast culture: 15.1 and 13.2 nmol/mg protein in the first and second patient, respectively. Optic atrophy observed in the first case may be an additional feature besides the characteristic manifestations of Salla disease.

Background Information: Free sialic acid storage diseases are caused by mutations of a lysosomal sialic acid transporter called sialin. We showed recently that the milder clinical form, Salla disease, and a related non-Finish case, are characterized by residual transport, whereas sialin mutants found in lethal infantile cases are inactive. In the present study, we have characterized the molecular effects of a putative polymorphism (M316I) and of four pathogenic mutations associated with either infantile (G127E and R57C) or Salla-like (G409E) phenotypes, or both (G328E). The transport activity of human sialin was analysed using a novel assay that was based on a construct without the functional lysosomal sorting motif, which is expressed at the plasma membrane., Results: The lysosomal localization of human sialin was not (M316I and G328E) or only partially (R57C, G127E and G409E) affected by the missense mutations. In contrast, all pathogenic mutations abolished transport, whereas the putative M316I polymorphism induced an approx. 5-fold decrease of sialic acid transport., Conclusions: The molecular effects of the R57C and G127E mutations strengthen the conclusion that the infantile phenotype is caused by loss-of-function mutations. On the other hand, the milder severity of the heterozygous G409E patient may reflect an incomplete expression of the splicing mutation present on the second allele. In the case of the G328E mutation, found in the homozygous state in a clinically heterogeneous family, the apparent severity of the transport phenotype suggests that the genetic or environmental factors underlying this clinical heterogeneity might be protective.

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.

Two disease-associated missense mutations in the sialin gene (G328E and G409E) have recently been identified in patients with lysosomal free sialic acid storage disease. We have assessed the effect of these mutations and find complete loss of measurable transport activity with both and impaired trafficking of the G409E protein. These results suggest that the two residues are important for proper function of sialin and confirm the association of loss of transport with disease causative mutations.

Free sialic acid storage disorders, Salla disease (SD) and Infantile sialic acid storage disease (ISSD), are lysosomal storage diseases due to impaired function of a sialic acid transporter, sialin, at the lysosomal membrane. Several mutations of the sialin gene, SLC17A5, are known, leading either to the severe neonatal/infantile disease or to the milder, adult-type developmental disorder, Salla disease. Free sialic acid accumulation in lysosomes causes increased tissue concentration and consequently elevated urinary excretion. Prenatal diagnosis of SASD is possible either by determination of free sialic acid concentration or by mutation analysis of the SLC17A5 gene in fetal specimen, in chorionic villus biopsy particularly. Both techniques have been successfully applied in several cases, sialic acid assay more often in ISSD cases but mutation analysis preferentially in SD. Sialic acid assay of amniotic fluid supernatant or cultured amniotic fluid cells may give erroneous results and should not be used for prenatal diagnosis of these disorders. The present comments are mainly based on our experience of prenatal diagnosis of SD in Finnish families. A founder mutation in SLC17A5 gene, 115C-> T, represents 95% of the disease alleles in the Finnish SD patients, which provides a unique possibility to apply mutation analysis. Therefore, molecular studies have successfully been used in 17 families since the identification of the gene and the characterization of the SD mutations. Earlier, eight prenatal studies were performed by measuring the free sialic acid concentration in chorionic villus samples.

Sialuria is an inborn error of metabolism characterized by coarse face, hepatomegaly and recurrent respiratory tract infections. The genetic defect in this disorder results in a loss of feedback control of UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine-kinase by CMP-N-acetylneuraminic acid (CMP-NeuAc) resulting in a substantial overproduction of cytoplasmic free sialic acid. This study addresses fibroblast CMP-NeuAc levels and N- and O-glycan sialylation of serum proteins from Sialuria patients. CMP-NeuAc levels were measured with HPLC in fibroblasts. Isoelectric focusing (IEF) of serum transferrin and of apolipoprotein C-III (apoC-III) was performed on serum of three Sialuria patients. Isoforms of these proteins can be used as specific markers for the biosynthesis of N- and core 1 O-glycans. Furthermore, total N- and O-linked glycans from serum proteins were analyzed by HPLC. HPLC showed a clear overproduction of CMP-NeuAc in fibroblasts of a Sialuria patient. Minor changes were found for serum N-glycans and hypersialylation was found for core 1 O-glycans on serum apoC-III and on total serum O-glycans in Sialuria patients. HPLC showed an increased ratio of disialylated over monosialylated core 1 O-glycans. The hypersialylation of core 1 O-glycans is due to the increase of NeuAcalpha2,6-containing structures (mainly NeuAcalpha2-3Galbeta1-3[NeuAcalpha2-6]GalNAc). This may relate to KM differences between GalNAc-alpha2,6-sialyltransferase and alpha2,3-sialyltransferases. This is the first study demonstrating that the genetic defect in Sialuria results in a CMP-NeuAc overproduction. Subsequently, increased amounts of alpha2,6-linked NeuAc were found on serum core 1 O-glycans from Sialuria patients. N-glycosylation of serum proteins seems largely unaffected. Sialuria is the first metabolic disorder presenting with hypersialylated O-glycans.

Sialic acid (SA), a terminal monosaccharide of glycoconjugates, has a central role in human biological function. Various point mutations result in the malmetabolism of SA and inherited disorders: Defective SA synthesis causes sialuria and defective SA catabolism causes sialidosis and sialic acid storage disease (SASD). These inborn errors of metabolism are characterised by increased urinary free SA. This article reviews biochemical and clinical features that are distinct to each disorder. In view of recent evidence indicating a wide underestimation in the prevalence of sialic acid disorders, laboratory methods for determining urinary free SA and its implications for screening and prenatal diagnosis are evaluated.

Background: Sialic acid storage diseases (SASDs) are caused by the defective transport of free sialic acid outside the lysosome. Apart from the Salla presentation in Finland, SASD is a very rare form of lysosomal storage disease (LSD) with approximately 35 cases, all diagnosed after birth, having been reported worldwide. We report a series of 12 French patients with very early manifestations, including eight fetuses diagnosed in utero., Results: Ultrasound examination, fetal autopsy, or clinical examination showed prominent ascites, rarely progressing to complete hydrops, and highlighted the early severity of bone disease. Dramatic increase of free sialic acid in various biological samples confirmed the diagnosis in all cases. Storage staining affinities and storage distribution in placenta and fetal organs allowed differential diagnosis from other LSDs but cannot differentiate between SASD, sialidosis, and galactosialidosis. Fourteen different mutations were identified, showing the molecular heterogeneity of SASD in the French population. We found that the previously described p.Y306X mutation generated two different transcripts, and we identified seven novel mutations: three deletions (del exon 7, del exons10+11 and c.1296delT), one splice site mutation (c.1350+1G-->T) one nonsense mutation (p.W339X), and two missense mutations (p.R57C and p.G127E)., Conclusions: The severity of our patients' genotypes is in agreement with their phenotypes but not with the importance and early appearance of the very frequent in utero manifestations. Minimal fetal disease in some patients and a reported case of heterogeneity of fetal involvement within a family suggest that factors other than the genotype influence fetal manifestations.

Sialic acids are expressed as terminal sugars in many glycoconjugates and play an important role during development and regeneration, as they are involved as polysialic acid in a variety of cell-cell interactions mediated by the neural cell adhesion molecule NCAM. The key enzyme for the biosynthesis of sialic acid is the UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine-kinase (GNE). Mutations in the binding site of the feedback inhibitor CMP-sialic acid of the GNE leads to sialuria, a disease in which patients produce sialic acid in gram scale. Here, we report on the consequences after expression of a sialuria-mutated GNE. Expression of the sialuria-mutated GNE leads to a dramatic increase of both cellular sialic acid and polysialic acid on NCAM. This could also be achieved by application of the sialic acid precursor N-acetylmannosamine. Our data suggest that biosynthesis of sialic acid regulates and limits the synthesis of polysialic acid.

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.

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.

SLC17A5 encodes a lysosomal membrane protein, sialin, which transports sialic acid from lysosomes. Mutations in sialin result in neurodegenerative sialic acid storage disorders, Salla disease (SD) and infantile sialic acid storage disease (ISSD). Here we analyzed sialin in mouse central nervous system (CNS) and primary cortical and hippocampal neurons and glia. In the CNS, sialin was predominantly expressed in neurons, especially in the proliferative zone of the prospective neocortex and the hippocampus in developing brain. In nonneuronal cells and primary glial cell cultures, mouse sialin was localized into lysosomes but interestingly, in primary neuronal cultures sialin was not targeted into lysosomes but rather revealed a punctate staining along the neuronal processes and was also seen in the plasma membrane. These data demonstrate a nonlysosomal localization of sialin in neurons and would imply a role for sialin in the secretory processes of neuronal cells.

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., (Copyright 2003 Wiley-Liss, Inc.)

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.

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.

Salla disease (SD) and infantile sialic acid storage disease (ISSD) are recessively inherited, neuro-degenerative disorders caused by mutations in the SLC17A5 gene. The gene product, sialin, is a lysosomal membrane protein which transports free sialic acid across the membrane. Although the function of sialin is basically known, the details of biosynthesis and intracellular trafficking as well as functional consequences of disease mutations in the SLC17A5 gene are not characterized. Here we studied for the first time the expression, localization, and targeting of the wild-type sialin as well as two mutant polypeptides; one mimicking the Finnish founder mutation, R39C (Salla(FIN)), and the other a deletion (del268-272) found in ISSD patients using in vitro expression of the corresponding cDNA constructs. The wild-type sialin was targeted to lysosomes whereas a significant fraction of the Salla(FIN) polypeptides and the majority of the ISSD polypeptides remained in the Golgi compartment. Further, using a temperature block of intracellular transport, we observed that the rate of the trafficking of the mutant polypeptides to lysosomes is significantly slower than that of their wild-type counterpart. These findings are in line with the phenotypic differences between SD and ISSD, the former presenting mental retardation with long life span in contrast to the latter being an early fatal disorder.