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.

Monitoring the CNS pathology in aspartylglucosaminuria mice.

Aspartylglucosaminuria (AGU) is a recessively inherited lysosomal storage disorder caused by the deficiency of the aspartylglucosaminidase (AGA) enzyme. The hallmark of AGU is slowly progressing mental retardation but the progression of brain pathology has remained uncharacterized in humans. Here we describe the long-term follow-up of mice carrying a targeted AGU-mutation in both alleles. Immunohistochemistry, histology, electron microscopy, quantitative magnetic resonance imaging (MRI) and behavioral studies were carried out to evaluate the CNS affection of the disease during development. The lysosomal storage vacuoles of the AGA -/- mice were most evident in central brain regions where MRI also revealed signs of brain atrophy similar to that seen in the older human patients. By immunohistochemistry and MRI examinations, a subtle delay of myelination was observed in AGA -/- mice. The life span of the AGA -/- mice was not shortened. Similar to the slow clinical course observed in human patients, the AGA -/- mice have behavioral symptoms that emerge at older age. Thus, the AGU knock-out mice represent an accurate model for AGU, both histopathologically and phenotypically.

Mice with an aspartylglucosaminuria mutation similar to humans replicate the pathophysiology in patients.

Aspartyglucosaminuria (AGU) is a lysosomal storage disease with autosomal recessive inheritance that is caused by deficient activity of aspartylglucosaminidase (AGA), a lysosomal enzyme belonging to the newly described enzyme family of N-terminal hydrolases. An AGU mouse model was generated by targeted disruption of the AGA gene designed to mimic closely one human disease mutation. These homozygous mutant mice have no detectable AGA activity and excrete aspartylglucosamine in their urine. Analogously to the human disease, the affected homozygous animals showed storage in lysosomes in all analyzed tissues, including the brain, liver, kidney and skin, and lysosomal storage was already detected in fetuses at 19 days gestation. Electron microscopic studies of brain tissue samples demonstrated lysosomal storage vacuoles in the neurons and glia of the neocortical and cortical regions. Magnetic resonance images (MRI) facilitating monitoring of the brains of living animals indicated cerebral atrophy and hypointensity of the deep gray matter structures of brain-findings similar to those observed in human patients. AGU mice are fertile, and up to 11 months of age their movement and behavior do not differ from their age-matched littermates. However, in the Morris water maze test, a slow worsening of performance could be seen with age. The phenotype mimics well AGU in humans, the patients characteristically showing only slowly progressive mental retardation and relatively mild skeletal abnormalities.

Expression and regulation of the human and mouse aspartylglucosaminidase gene.

Aspartylglucosaminidase (AGA) is a lysosomal enzyme that catalyzes one of the final steps in the degradation of N-linked glycoproteins. Here we have analyzed the tissue-specific expression and regulation of the human and mouse AGA genes. We isolated and characterized human and mouse AGA 5'-flanking sequences including the promoter regions. Primer extension assay revealed multiple transcription start sites in both genes, characteristic of a housekeeping gene. The cross-species comparison studies pinpointed an approximately 450-base pair (bp) homologous region in the distal promoter. In the functional analysis of human AGA 5' sequence, the critical promoter region was defined, and an additional upstream region of 181 bp exhibiting an inhibitory effect on transcription was identified. Footprinting and gel shift assays indicated protein binding to the core promoter region consisting of two Sp1 binding sites, which were sufficient to produce basal promoter activity in the functional studies. The results also suggested the binding of a previously uncharacterized transcription factor to a 23-bp stretch in the inhibitory region.

Molecular cloning, chromosomal assignment, and expression of the mouse aspartylglucosaminidase gene.

Aspartylglucosaminidase (AGA) is a lysosomal enzyme, the deficiency of which leads to human lysosomal storage disease aspartylglucosaminuria. Here, we describe isolation, chromosomal location, genomic structure, and tissue-specific expression of the mouse Aga gene as well as the intracellular processing of the mouse Aga polypeptide and compare these characteristics to human AGA. The mouse Aga gene was localized to the central area of the B region of chromosome 8, which represents the synteny group in the human chromosome 4q telomeric region where the human AGA gene is located. The mouse gene spans an 11-kb genomic region and contains nine exons and eight introns, which is analogous to the human gene. Furthermore, the exon-intron boundaries of the mouse and human genes are identically positioned. The nucleotide sequence identity of the cDNA and deduced amino acid sequence identity of the protein are 84.4 and 82.4%, respectively. However, the mouse Aga cDNA contains untranslated regions that are shorter than those in the human cDNA, and only one 1.2-kb mRNA transcript is produced in mouse versus two transcripts in human. Expression of the mouse Aga cDNA in COS-1 cells showed that the mouse Aga polypeptide was processed similarly to the human counterpart.