Aspartylglucosaminuria. II. Biochemical studies on brain, liver, kidney and spleen

Crystallographic snapshot of a productive glycosylasparaginase-substrate complex

Glycosylasparaginase (GA) plays an important role in asparagine-linked glycoprotein degradation. A deficiency in the activity of human GA leads to a lysosomal storage disease named aspartylglycosaminuria. GA belongs to a superfamily of N-terminal nucleophile hydrolases that autoproteolytically generate their mature enzymes from inactive single chain protein precursors. The side-chain of the newly exposed N-terminal residue then acts as a nucleophile during substrate hydrolysis. By taking advantage of mutant enzyme of Flavobacterium meningosepticum GA with reduced enzymatic activity, we have obtained a crystallographic snapshot of a productive complex with its substrate (NAcGlc-Asn), at 2.0 A resolution. This complex structure provided us an excellent model for the Michaelis complex to examine the specific contacts critical for substrate binding and catalysis. Substrate binding induces a conformational change near the active site of GA. To initiate catalysis, the side-chain of the N-terminal Thr152 is polarized by the free alpha-amino group on the same residue, mediated by the side-chain hydroxyl group of Thr170. Cleavage of the amide bond is then accomplished by a nucleophilic attack at the carbonyl carbon of the amide linkage in the substrate, leading to the formation of an acyl-enzyme intermediate through a negatively charged tetrahedral transition state.

Toward understanding the neuronal pathogenesis of aspartylglucosaminuria: expression of aspartylglucosaminidase in brain during development

The deficiency of a lysosomal enzyme, aspartylglucosaminidase, results in a lysosomal storage disorder, aspartylglucosaminuria, manifesting as progressive mental retardation. To understand tissue pathogenesis and disease progression we analyzed the developmental expression of the enzyme, especially in brain, which is the major source of the pathological symptoms. Highest mRNA levels in brain were detected during embryogenesis, the levels decreased neonatally and started to increase again from Day 7 on. In Western analyses, a defective processing of aspartylglucosaminidase was observed in brain as compared to other tissues, resulting in very low levels of the mature, active form of the enzyme. Interestingly immunohistochemical analyses of mouse brain revealed that aspartylglucosaminidase immunoreactivity closely mimicked the myelin basic protein immunostaining pattern. The only evident neuronal staining was observed in the developing Purkinje cells of the cerebellum from Days 3 to 10, reflecting well the mRNA expression. In human infant brain, the immunostaining was also present in myelinated fibers as well as in the Purkinje cells and, additionally, in the soma and extensions of other neurons. In the adult human brain neurons and oligodendrocytes displayed immunoreactivity whereas myelinated fibers were not stained. Our results of aspartylglucosaminidase immunostaining in myelinated fibers of infant brain might imply the involvement of aspartylglucosaminidase in the early myelination process. This is consistent with previous magnetic resonance imaging findings in the brains of aspartylglucosaminuria patients, revealing delayed myelination in childhood.

Expression and endocytosis of lysosomal aspartylglucosaminidase in mouse primary neurons

Aspartylglucosaminuria (AGU) is a neurodegenerative lysosomal storage disease that is caused by mutations in the gene encoding for a soluble hydrolase, aspartylglucosaminidase (AGA). In this study, we have used our recently developed mouse model for AGU and analyzed processing, intracellular localization, and endocytosis of recombinant AGA in telencephalic AGU mouse neurons in vitro. The processing steps of AGA were found to be similar to the peripheral cells, but both the accumulation of the inactive precursor molecule and delayed lysosomal processing of the enzyme were detected. AGA was distributed to the cell soma and neuronal processes but was not found in the nerve terminals. Endocytotic capability of cultured telencephalic neurons was comparable to that of fibroblasts, and endocytosis of AGA was blocked by free mannose-6-phosphate (M6P), indicating that uptake of the enzyme was mediated by M6P receptors (M6PRs). Uptake of extracellular AGA was also studied in the tumor-derived cell lines rat pheochromocytoma (PC12) and mouse neuroblastoma cells (N18), which both endocytosed AGA poorly as compared with cultured primary neurons. Expression of cation-independent M6PRs (CI-M6PRs) in different cell lines correlated well with the endocytotic capability of these cells. Although a punctate expression pattern of CI-M6PRs was found in fibroblasts and cultured primary neurons, the expression was beyond the detection limit in PC12 and N18 cells. This indicates that PC12 and N18 are not feasible cell lines to describe neuronal uptake of mannose-6-phosphate-tagged proteins. This in vitro data will form an important basis for the brain-targeted therapy of AGU.

Quantitative determination of rare mRNA species by PCR and solid-phase minisequencing

We present a new method for quantification of mRNA, in which the limitations of the current quantitative PCR methods can be overcome. A known amount of a synthetic RNA standard differing from the mRNA to be quantified by a single nucleotide is reverse-transcribed and amplified together with the mRNA template using a biotinylated primer. The biotinylated PCR product is immobilized on a streptavidin-coated solid support and denatured. The ratio between the two amplified sequences is determined by separate "mini-sequencing" reactions, in which a detection step primer annealing immediately adjacent to the site of the variable nucleotide is elongated by a single labeled dNTP complementary to the nucleotide at the variable site. The ratio between the incorporated labels accurately determines the ratio between the two sequences in the original RNA sample. We applied this method to quantify the mRNA of human aspartylglucosaminidase (AGA) in tissues and cultured cells. AGA is a lysosomal enzyme participating in the degradation of glycoproteins. A mutation in the AGA gene abolishes the enzyme activity and leads to aspartylglucosaminuria (AGU), a recessively inherited metabolic disorder. The mRNA quantification revealed that the normal and mutant genes are expressed at similar levels in kidney, liver, and cultured fibroblast, whereas the amount of AGA mRNA in normal placenta and brain is significantly higher than that found in the corresponding samples from AGU patients. The method presented here is generally applicable for PCR-based quantification of rare mRNAs and DNA as well.

Isolation of a human hepatic 60 kDa aspartylglucosaminidase consisting of three non-identical polypeptides

We have characterized the properties of human aspartylglucosaminidase (EC 3.5.1.26), the lysosomal enzyme which is deficient in the human inherited disease aspartylglucosaminuria. The purification procedure from human liver included affinity chromatography, gel filtration, strong-anion- and strong-cation-exchange h.p.l.c., chromatofocusing and reverse-phase h.p.l.c. In a denaturing SDS/polyacrylamide-gel electrophoresis, the 6600-fold purified enzyme was shown to be composed of three non-identical inactive polypeptide chains of molecular masses 24, 18 and 17 kDa. In a native polyacrylamide-gel electrophoresis, these polypeptide chains ran as one active enzyme complex. As judged from the elution position of the native enzyme in a Biogel P-100 gel filtration, the approximate molecular mass of this complex was 60 kDa. The enzyme had a pI of 5.7, a pH optimum at 6, of 0.48 mM and a specific activity of 200,000 nkat for the substrate 2-acetamido-1-beta-(L-aspartamido)-1,2-dideoxy-D-glucose. The enzyme showed a 57% loss of activity at 60 degrees C after 45 h but was practically inactive after incubation at 72 degrees C for a few minutes. The molecular structure, Km and specific activity as well as the thermostability of the enzyme described here are different from those reported previously for human aspartylglucosaminidase.

Prenatal diagnosis and fetal pathology of aspartylglucosaminuria

The prenatal diagnosis of aspartylglucosaminuria (AGU), a lysosomal storage disorder of glycoprotein degradation, was made by demonstrating the deficiency of N-aspartylglucosaminidase on cultured cells from a midterm amniotic fluid sample. Four other amniotic fluid studies from at-risk pregnancies gave a normal or a heterozygote level of enzyme activity. These pregnancies have gone to term and the delivery of healthy babies. The pregnancy with the affected fetus was terminated and the prenatal diagnosis was verified by enzyme assays on cord blood lymphocytes, cultured cells from skin biopsy, and from placental villi. Electron microscopic evidence of lysosomal storage was seen in several organs of the fetus with the notable exception of the central nervous system. The undifferentiated mesenchymal fibroblasts particularly were heavily loaded with cytoplasmic inclusions in skin, liver, kidney, and placenta.

Aspartylglucosaminuria in the United States

Aspartylglucosaminuria (AGU) was diagnosed in two unrelated males with progressive mental retardation, coarse facies and skeletal abnormalities. Until now, this disorder has been described in predominantly Finnish populations with only one previous case reported in the U.S. We conclude that AGU may be more common in non-Finnish populations than the number of reported cases would indicate and should be included in the differential diagnosis in patients with suspected lysosomal storage disorders regardless of their geographical or ethnic backgrounds.

Aspartylglycosaminuria: an inborn error of glycoprotein catabolism

Aspartylglycosaminuria (AGU, McKusick 20840) is a metabolic disorder affecting the catabolism of glycoproteins. It was first described in 1967, by Jenner and Pollitt, in two mentally retarded English siblings. Subsequently several cases were reported from Finland (Palo and Mattsson, 1970; Autio, 1972; Autio et al., 1973). Today the number of known cases is about 140, most of them Finnish or of Finnish origin (Aula et al., 1980). The incidence of AGU in Finland has been estimated to be approximately 1:26000 and the disease is inherited as an autosomal recessive trait (Autio et al., 1973). Clinical manifestations include progressive mental retardation, coarse gargoyle-like facial features, skeletal abnormalities and recurrent infections. Early development of the patients is usually normal, but by the age of 5-15 years they are already severely retarded (Autio, 1972; Autio et al., 1973). Morphologically AGU is a generalized storage disease (Haltia et al., 1975). Affected tissues show enlarged lysosomes. Vacuolization is a prominent feature of liver and nerve cells (Haltia et al., 1975) and of peripheral lymphocytes (Aula et al., 1975).

Regional distribution of glycoasparagine storage material in the brain in aspartylglycosaminuria

We have studied the regional distribution of glycoasparagine storage material in the brain in aspartylglycosaminuria, a condition characterized by inherited deficiency of lysosomal N-aspartyl-beta-N-acetylglucosamine amidohydrolase. Gaschromatographic measurements of the main accumulating glycoprotein-derived metabolite, N-acetylglucosaminyl-asparagine (GlcNAc-Asn), in 12 defined cerebral areas showed that GlcNAc-Asn is rather evenly distributed in the brain. The mean concentrations ranged from 0.454 mg/g wet tissue (corpus callosum) to 0.0610 mg/g (pons). The GlcNAc-Asn concentrations tended to be higher in grey matter areas than in white matter areas. GlcNAc-Asn was identified in the isolated neuronal fraction, but not in the myelin fraction, by mass-fragmentographic techniques. Electron-microscopic reexamination of a brain biopsy specimen revealed, in addition to the abundant presence of storage lysosomes in the neuronal perikarya, numerous cytoplasmic inclusions in brain capillary endothelial cells and pericytes as well as in occasional macrophages. The results indicate that the glycoasparagine storage material is not limited to expected cortical areas in aspartylglycosaminuria, but is distributed in a rather constant fashion in all cerebral grey and white matter areas studied.

Urinary sialic acid levels in aspartylglycosaminuria

Urinary sialoglycoconjugates were studied in 22 patients with inherited deficiency of 1-aspartamido-beta-N-acetylglucosamine amidohydrolase (aspartylglycosaminuria), in eight obligate heterozygotes, and in age- and sex-matched control subjects. Total sialic acid excretion was significantly higher in the patients (38.3 +/- 17.7 mumol/mmol creatinine, mean +/- S.D.) than in the matched controls (17.7 +/- 7.3 mumol/mmol creatinine, p less than 0.001). The sialic acid output in the heterozygotes did not differ from that of the controls. Gel filtration studies revealed that the increase in urinary sialic acid in aspartylglycosaminuria is of bound type and confined to the low molecular mass region. A linear positive correlation was found between the output of sialic acid and glycoasparagine in the individual patients (r = 0.77, p less than 0.001). The amount of sialylated metabolites excreted in urine did not correlate with the severity of clinical manifestations in aspartyl-glycosaminuria.

N-Acetylglucosamine-asparagine levels in tissues of patients with aspartylglycosaminuria

The levels of the main glycoprotein-derived storage compound, N-acetylglucosamine-asparagine, in various post mortem tissues of three adult patients with inherited deficiency of lysosomal 1-aspartamido-beta-N-acetylglucosamine amidohydrolase (aspartylglycosaminuria) were measured by gas-liquid chromatography. All aspartylglycosaminuria tissues studied contained significant amounts of N-acetylglucosamine-asparagine, whereas none of the corresponding control tissues contained detectable amounts of this compound. High levels of N-acetylglucosamine-asparagine were found in the liver (3.65 mg/g wet weight), spleen (2.24) and thyroid (2.18), and lower levels in the kidney (0.89), brain (0.53), spinal cord (0.32), sciatic nerve (0.34) and skeletal muscle (0.16). The results show that N-acetylglucosamine-asparagine accumulates chiefly in tissues with important functions in glycoprotein metabolism and/or high endocytic activity. Correlation of the results to the clinical manifestations of aspartylglycosaminuria did not reveal a direct relationship between the amount of N-acetylglucosamine-asparagine stored and the degree of organ dysfunction.

Identification of 4-N-2-acetamido-2-deoxy-beta-D-glucopyranosyl-L-asparagine in biological materials by gas chromatography-mass spectrometry

A specific and sensitive method for the identification of 4-N-2-acetamido-2-deoxy-beta-D-glucopyranosyl-L-asparagine (GlcNAc-Asn) in urine in aspartylglycosaminuria and in hydrolysates of glycoproteins is described. The method involves permethylation of GlcNAc-Asn followed by gas chromatographic-mass spectrometric analysis of the methylated derivative. It can be used to confirm the diagnosis of aspartylglycosaminuria and to assess the excretion of GlcNAc-Asn in urine during various phases of the disease. The presence of an N-acetylglucosaminyl-asparagine type of carbohydrate-peptide linkage in a glycoprotein can be determined by applying the method to the partial acid hydrolysate of a proteolytically digested glycoprotein.

Measurement of 1-aspartamido-beta-N-acetylglucosamine amidohydrolase activity in human tissues

The activity of 1-aspartamido-beta-N-acetylglucosamine amidohydrolase (aspartylglucosylaminase, EC 3.5.1.26) was measured in normal and diseased human liver, brain and kidney. Organs from patients with aspartylglucosaminuria show very little activity. Crude homogenates of human organs show a reaction catalysed by a complex enzyme system. With homogenate, the formation of product was linear with time up to about 6 h. Reaction times longer than 6-7h resulted in a decrease in the total concentration of product. This phenomenon was not found with the partially purified enzyme fraction. Linearity of the enzyme activity with different protein concentrations was found, independent of the incubation time. Longer incubation of the crude homogenate resulted in the utilization of the product, N-acetylglucosamine. This phenomenon was not observed with the partially purified enzyme fraction. This amidase from human organs differs from that obtained from other sources and apparently represents a rather complex enzyme system.

Aspartylglucosaminuria: unique biochemical and ultrastructural characteristics

The observation of vacuolated lymphocytes in a coarsely featured two year old female with hepatosplenomegaly, mitral insufficiency, and mild psychomotor retardation led to the first diagnosed case of aspartylglucosaminuria in the United States. Although physical characteristics and bone roentgenograms were consistent with a mucopolysaccharide disorder, analysis of the urine showed no mucopolysaccharide elevation. The chromatographic, enzymatic, and ultrastructural studies confirming the diagnosis are presented.

Aspartylglucosaminuria: psychomotor retardation masquerading as a mucopolysaccharidosis

A 5-year-old girl with coarse facies, visceromegaly, and vacuolated lymphocytes is presented as the first case of aspartylglucosaminuria diagnosed in this country. This metabolic defect in glycoprotein catabolism can be clinically confused with other storage diseases such as the mucopolysaccharidoses and mucolipidoses. It is not diagnosed by routine laboratory screening methods. Special studies are required to confirm the diagnosis, but a thin-layer chromatography method for screening urine is presented for use when the diagnosis is suspected. The developmental potential in this inborn error of metabolism is documented.

Aspartylglycosaminuria: a generalized storage disease. Morphological and histochemical studies

Aspartylglycosaminuria (AGU) is a hereditary metabolic disorder characterized by slowly progressive mental deterioration from infancy, urinary excretion of large amounts of aspartylglycosamine, and decreased activity of the lysosomal enzyme aspartylglcosamine amido hydrolase in various body tissues and fluids. The nature and distribution of the morphological and histochemical alterations in AUG are described in the light of the first AGU patient investigated post mortem and brain and liver. Most nerve cells and hepatocytes contained large vacuoles without any histochemically demonstrable lipid or carbohydrate material. Ultrastructural studies revealed numerous electron-lucent vaculoles, limited by a single, membrane, in the cytoplasm of these cells. In addition to evenly disperesed finely granular or reticular material the vacuoles contained small electron-opaque "lipid" droplets and polymorphic membraneous or granular aggregates. Similar vacuoles were also seen in a number of other cell types, particularly in the kupffer cells and brain macrophages, as well as in the capillary pericytes. Biochemical studies suggest that the principal storage material consists of aspartylglycosamine itself; glycoasparagines of higher molecular weight are present as only minor components. Correlated morphological and biochemical studies thus definitely establish that AGU is a generalized storage disorder. The condition is apparently due to decreased activity of aspartylglycosamine amido hydrolase, with accumulation of products of flycoprotein carabolism in cytoplasmic vacuoles in both epithelial and mesenchymal cells.