Disease relevance of CTSA

• Human lysosomal N-acetyl-alpha-neuraminidase is deficient in two lysosomal storage disorders, sialidosis, caused by structural mutations in the neuraminidase gene, and galactosialidosis, in which a primary defect of protective protein/cathepsin A (PPCA) leads to a combined deficiency of neuraminidase and beta-D-galactosidase [1].
• To investigate this process, we have used the baculovirus expression system to co-express neuraminidase and PPCA precursors in insect cells, which resulted in high enzymatic activity of neuraminidase [2].
• Galactosialidosis is a recessively inherited lysosomal storage disease characterized by the combined deficiency of neuraminidase and beta-galactosidase secondary to the genetic deficiency of cathepsin A/protective protein [3].
• Expression of lysosomal protective protein/cathepsin A in a stably transformed human neuroblastoma cell line during bi-directional differentiation into neuronal and Schwannian cells [4].
• Cathepsin A was, however, increased in muscles moderately affected by muscular dystrophy and denervating diseases [5].

High impact information on CTSA

• 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 [6].
• The human lysosomal enzyme occurs in complex with beta-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 [6].
• Immediately after infection, APCs increase the endogenous GSL synthesis and stimulate GSL-specific T cells in a CD1- and T cell receptor (TCR)-dependent manner [7].
• These properties and the substrate specificity distinguish PCP from cathepsin A, which is also in fibroblasts [8].
• Cathepsin A regulates chaperone-mediated autophagy through cleavage of the lysosomal receptor [9].

Biological context of CTSA

• The gene encoding human protective protein (PPGB) is on chromosome 20 [10].
• To explain the unusual need of neuraminidase for an auxiliary protein, we examined, in transfected COS-1 cells, the effect of PPCA expression on post-translational modification, turnover and intracellular localization of neuraminidase [1].
• New dinucleotide repeat polymorphisms associated with the D20S17, PPGB, and ADA loci have been identified and mapped [11].
• The previously unmapped PPGB marker is closely linked to D20S17, with a two-point lod score of 50.53 at theta = .005 [11].
• Molecular pathology of galactosialidosis in a patient affected with two new frameshift mutations in the cathepsin A/protective protein gene [3].

Anatomical context of CTSA

• Transport of human lysosomal neuraminidase to mature lysosomes requires protective protein/cathepsin A [1].
• Gel filtration analysis of fibroblast extracts of patients deficient in either beta-galactosidase (beta-galactosidosis) or cathepsin A (galactosialidosis), which accumulate KS, demonstrates that the 1.27-MDa complex is disrupted and that GALNS is present only in free homodimeric form [12].
• The lysosomal carboxypeptidase, cathepsin A, which forms a complex with and activates Neu1 in the lysosome, is sorted to the plasma membrane of the differentiating cells similarly to Neu1 [13].
• Protective protein/cathepsin A loss in cultured cells derived from an early-infantile form of galactosialidosis patients homozygous for the A1184-G transition (Y395C mutation) [14].
• The acid carboxypeptidase (cathepsin A) and lysosomal neuraminidase activities were markedly decreased in cultured fibroblasts and chorionic villus cells derived from the patients, although the decrease in beta-galactosidase activity was less [14].

Associations of CTSA with chemical compounds

• Association of N-acetylgalactosamine-6-sulfate sulfatase with the multienzyme lysosomal complex of beta-galactosidase, cathepsin A, and neuraminidase. Possible implication for intralysosomal catabolism of keratan sulfate [12].
• PPCA associates with lamp2a on the lysosomal membrane and cleaves lamp2a near the boundary between the luminal and transmembrane domains [9].
• The latter is homologous to serine carboxypeptidases and has cathepsin A-like activity which is distinct from its protective function towards the two glycosidases [15].
• A structural model of the mutant PPCA was constructed by amino acid substitution of 453glutamic acid for lysine in the crystal structure of the wild type PPCA precursor reported [16].
• Invasion is a complex process controlled by secretion and activation of proteases, alteration of integrin levels and GSL (glycosphingolipid) patterns [17].

Physical interactions of CTSA

• Anti-human cathepsin A rabbit antiserum coprecipitates GALNS together with cathepsin A, beta-galactosidase, and alpha-neuraminidase in both a purified preparation of the 1 [12].
• New mutations in the PPBG gene lead to loss of PPCA protein which affects the level of the beta-galactosidase/neuraminidase complex and the EBP-receptor [18].
• Galactosialidosis, a clinically similar disorder, is caused by the secondary Neu1 deficiency because of genetic defects in cathepsin A that form a complex with Neu1 and activate it [19].

Other interactions of CTSA

• GALNS copurifies with the complex by different chromatography techniques: affinity chromatography on both cathepsin A-binding and beta-galactosidase-binding columns, gel filtration, and chromatofocusing [12].
• The reduction in GLB1 and EBP confirms that PPCA is essential for their integrity [18].
• Bovine protective protein homologous to the human protein had a molecular weight of 48 kDa on SDS-PAGE and cathepsin A activity optimum around pH 6 [20].
• PGCP was co-purified from human placenta with lysosomal carboxypeptidase, cathepsin A, lysosomal endopeptidase, cathepsin D, and a gamma-interferon-inducible protein, IP-30, using an affinity chromatography on a Phe-Leu-agarose column [21].
• Lysosomal protective protein/cathepsin A is a serine carboxypeptidase that forms a complex with beta-galactosidase and neuraminidase [22].

Analytical, diagnostic and therapeutic context of CTSA

• The deleterious effect of such mutations was confirmed by the complete absence of the PPCA protein on Western blots [18].
• We determined the main forms of the supramolecular organization of human placental cathepsin A and the quantitative relationship between them, using an affinity chromatography on agarose-Phe-Leu for direct purification of cathepsin A [23].
• Northern blot analysis of RNA isolated from the patients' cultured fibroblasts showed substantially decreased levels of the PPCA transcript, which nevertheless had the correct size of 2 kb [24].
• All mutants showed a reticular immunofluorescence distribution when coexpressed with the PPCA gene in COS-1 cells, suggesting that the gene products were retained in the endoplasmic reticulum/Golgi area or rapidly degraded in the lysosomes [25].
• Immunoprecipitation using an anti beta-galactosidase antibody demonstrated that cathepsin A is a component of both the sialidase and beta-galactosidase complexes [20].

References