Neuraminidases (sialidases) have an essential role in the removal of terminal sialic acid residues from sialoglycoconjugates and are distributed widely in nature. The human lysosomal enzyme occurs in complex with 13-galactosidase and protective protein/cathepsin A (PPCA), and is deficient in two genetic disorders: sialidosis, caused by a structural defect in the neuraminidase gene, and galactosialidosis, in which the loss of neuraminidase activity is secondary to a deficiency of PPCA. We identified a full-length cDNA clone in the dbEST data base, of which the predicted amino acid sequence has extensive homology to other mammalian and bacterial neuraminidases, including the F(Y)RIP domain and "Asp-boxes." In situ hybridization localized the human neuraminidase gene to chromosome band 6p21, a region known to contain the HLA locus. Transient expression of the cDNA in deficient human fibroblasts showed that the enzyme is compartmentalized in lysosomes and restored neuraminidase activity in a PPCA-dependent manner. The authenticity of the cDNA was verified by the identification of three independent mutations in the open reading frame of the mRNA from clinically distinct sialidosis patients. Coexpression of the mutant cDNAs with PPCA failed to generate neuraminidase activity, confirming the inactivating effect of the mutations. These results establish the molecular basis of sialidosis in these patients, and clearly identify the cDNA-encoded protein as lysosomal neuraminidase.

SUMMARY Lysosomal exocytosis is a Ca2+-regulated mechanism that involves proteins responsible for cytoskeletal attachment and fusion of lysosomes with the plasma membrane. However, whether luminal lysosomal enzymes contribute to this process remains unknown. Here we show that neuraminidase Neu1 negatively regulates lysosomal exocytosis in hematopoietic cells by processing the sialic acids on the lysosomal membrane protein Lamp-1. In macrophages from Neu1-deficient mice, a model of the disease sialidosis, and in patients’ fibroblasts, oversialylated Lamp-1 enhances lysosomal exocytosis. Silencing of Lamp-1 reverts this phenotype by interfering with the docking of lysosomes at the plasma membrane. In Neu1-/- mice the excessive exocytosis of serine proteases in the bone niche leads to inactivation of extracellular serpins, premature degradation of VCAM-1, and loss of bone marrow retention. Our findings uncover an unexpected mechanism influencing lysosomal exocytosis and argue that exacerbations of this process form the basis for certain genetic diseases.

Lysosomal neuraminidase is the key enzyme for the intralysosomal catabolism of sialylated glycoconjugates and is deficient in two neurodegenerative lysosomal disorders, sialidosis and galactosialidosis. Here we report the identification of eight novel mutations in the neuraminidase gene of 11 sialidosis patients with various degrees of disease penetrance. Comparison of the primary structure of human neuraminidase with the primary and tertiary structures of bacterial sialidases indicated that most of the single amino acid substitutions occurred in functional motifs or conserved residues. On the basis of the subcellular distribution and residual catalytic activity of the mutant neuraminidases we assigned the mutant proteins to three groups: (i) catalytically inactive and not lysosomal; (ii) catalytically inactive, but localized in lysosome; and (iii) catalytically active and lysosomal. In general, there was a close correlation between the residual activity of the mutant enzymes and the clinical severity of disease. Patients with the severe infantile type II disease had mutations from group I, whereas patients with a mild form of type I disease had at least one mutation from group III. Mutations from the second group were mainly found in juvenile type II patients with intermediate clinical severity. Overall, our findings explain the clinical heterogeneity observed in sialidosis and may help in the assignment of existing or new allelic combinations to specific phenotypes.

The lysosomal storage disorder galactosialidosis results from a primary deficiency of the protective protein/cathepsin A (PPCA), which in turn affects the activities of p-galactosidase and neuraminidase. Mice homozygous for a null mutation at the PPCA locus present with signs of the disease shortly after birth and develop a phenotype closely resembling human patients with galactosialidosis. Most of their tissues show characteristic vacuolation of specific cells, attributable to lysosomal storage. Excessive excretion of sialyloligosaccharides in urine is diagnostic of the disease. Affected mice progressively deteriorate as a consequence of severe organ dysfunction, especially of the kidney. The deficient phenotype can be corrected by transplanting null mutants with bone marrow from a transgenic line overexpressing human PPCA in erythroid precursor cells. The transgenic bone marrow gives a more efficient and complete correction of the visceral organs than normal bone marrow. Our data demonstrate the usefulness of this animal model, very similar to the human disease, for experimenting therapeutic strategies aimed to deliver the functional protein or gene to affected organs. Furthermore, they suggest the feasibility of gene therapy for galactosialidosis and other disorders, using bone marrow cells engineered to overexpress and secrete the correcting lysosomal protein.

Glucocorticoids are universally used in the treatment of acute lymphoblastic leukemia (ALL), and leukemia cell resistant to glucocorticoids confers a poor prognosis. To elucidate mechanisms of glucocorticoid resistance, we determined the sensitivity to prednisolone of primary leukemia cells from 444 newly diagnosed ALL patients, revealing significantly higher expression of caspase 1 (CASP1) and its activator NLRP3 in glucocorticoid resistant leukemia cells, due to significantly lower somatic methylation of CASP1 and NLRP3 promoters. Over-expression of CASP1 resulted in cleavage of the glucocorticoid receptor, diminished glucocorticoid-induced transcriptional response and increased glucocorticoid resistance. Knockdown or inhibition of CASP1 significantly increased glucocorticoid receptor levels and mitigated glucocorticoid resistance in CASP1 overexpressing ALL. Our findings establish a new mechanism by which the NLRP3/CASP1 inflammasome modulates cellular levels of the glucocorticoid receptor and diminishes cell sensitivity to glucocorticoids. The broad impact on glucocorticoid transcriptional response suggests this mechanism could also modify glucocorticoid effects in other diseases.