Human lysosomal protective protein. Glycosylation, intracellular transport, and association with beta-galactosidase in the endoplasmic reticulum

Lysosomal sialidase NEU1, its intracellular properties, deficiency, and use as a therapeutic agent

Neuraminidase 1 (NEU1) is a lysosomal sialidase that cleaves terminal α-linked sialic acid residues from sialylglycans. NEU1 is biosynthesized in the rough endoplasmic reticulum (RER) lumen as an N-glycosylated protein to associate with its protective protein/cathepsin A (CTSA) and then form a lysosomal multienzyme complex (LMC) also containing β-galactosidase 1 (GLB1). Unlike other mammalian sialidases, including NEU2 to NEU4, NEU1 transport to lysosomes requires association of NEU1 with CTSA, binding of the CTSA carrying terminal mannose 6-phosphate (M6P)-type N-glycan with M6P receptor (M6PR), and intralysosomal NEU1 activation at acidic pH. In contrast, overexpression of the single NEU1 gene in mammalian cells causes intracellular NEU1 protein crystallization in the RER due to self-aggregation when intracellular CTSA is reduced to a relatively low level. Sialidosis (SiD) and galactosialidosis (GS) are autosomal recessive lysosomal storage diseases caused by the gene mutations of NEU1 and CTSA, respectively. These incurable diseases associate with the NEU1 deficiency, excessive accumulation of sialylglycans in neurovisceral organs, and systemic manifestations. We established a novel GS model mouse carrying homozygotic Ctsa IVS6 + 1 g/a mutation causing partial exon 6 skipping with simultaneous deficiency of Ctsa and Neu1. Symptoms developed in the GS mice like those in juvenile/adult GS patients, such as myoclonic seizures, suppressed behavior, gargoyle-like face, edema, proctoptosis due to Neu1 deficiency, and sialylglycan accumulation associated with neurovisceral inflammation. We developed a modified NEU1 (modNEU1), which does not form protein crystals but is transported to lysosomes by co-expressed CTSA. In vivo gene therapy for GS and SiD utilizing a single adeno-associated virus (AAV) carrying modNEU1 and CTSA genes under dual promoter control will be created.

Reversal of neuroinflammation in novel GS model mice by single i.c.v. administration of CHO-derived rhCTSA precursor protein

Galactosialidosis (GS) is a lysosomal cathepsin A (CTSA) deficiency. It associates with a simultaneous decrease of neuraminidase 1 (NEU1) activity and sialylglycan storage. Central nervous system (CNS) symptoms reduce the quality of life of juvenile/adult-type GS patients, but there is no effective therapy. Here, we established a novel GS model mouse carrying homozygotic Ctsa IVS6+1g→a mutation causing partial exon 6 skipping with concomitant deficiency of Ctsa/Neu1. The GS mice developed juvenile/adult GS-like symptoms, such as gargoyle-like face, edema, proctoprosia due to sialylglycan accumulation, and neurovisceral inflammation, including activated microglia/macrophage appearance and increase of inflammatory chemokines. We produced human CTSA precursor proteins (proCTSA), a homodimer carrying terminal mannose 6-phosphate (M6P)-type N-glycans. The CHO-derived proCTSA was taken up by GS patient-derived fibroblasts via M6P receptors and delivered to lysosomes. Catalytically active mature CTSA showed a shorter half-life due to intralysosomal proteolytic degradation. Following single i.c.v. administration, proCTSA was widely distributed, restored the Neu1 activity, and reduced the sialylglycans accumulated in brain regions. Moreover, proCTSA suppressed neuroinflammation associated with reduction of activated microglia/macrophage and up-regulated Mip1α. The results show therapeutic effects of intracerebrospinal enzyme replacement utilizing CHO-derived proCTSA and suggest suppression of CNS symptoms.

The Elastin Receptor Complex: An Emerging Therapeutic Target Against Age-Related Vascular Diseases

The incidence of cardiovascular diseases is increasing worldwide with the growing aging of the population. Biological aging has major influence on the vascular tree and is associated with critical changes in the morphology and function of the arterial wall together with an extensive remodeling of the vascular extracellular matrix. Elastic fibers fragmentation and release of elastin degradation products, also known as elastin-derived peptides (EDPs), are typical hallmarks of aged conduit arteries. Along with the direct consequences of elastin fragmentation on the mechanical properties of arteries, the release of EDPs has been shown to modulate the development and/or progression of diverse vascular and metabolic diseases including atherosclerosis, thrombosis, type 2 diabetes and nonalcoholic steatohepatitis. Most of the biological effects mediated by these bioactive peptides are due to a peculiar membrane receptor called elastin receptor complex (ERC). This heterotrimeric receptor contains a peripheral protein called elastin-binding protein, the protective protein/cathepsin A, and a transmembrane sialidase, the neuraminidase-1 (NEU1). In this review, after an introductive part on the consequences of aging on the vasculature and the release of EDPs, we describe the composition of the ERC, the signaling pathways triggered by this receptor, and the current pharmacological strategies targeting ERC activation. Finally, we present and discuss new regulatory functions that have emerged over the last few years for the ERC through desialylation of membrane glycoproteins by NEU1, and its potential implication in receptor transactivation.

Effects of Genetic Polymorphisms of Cathepsin A on Metabolism of Tenofovir Alafenamide

Cathepsin A (CatA) is important as a drug-metabolizing enzyme responsible for the activation of prodrugs, such as the anti-human immunodeficiency virus drug Tenofovir Alafenamide (TAF). The present study was undertaken to clarify the presence of polymorphisms of the CatA gene in healthy Japanese subjects and the influence of gene polymorphism on the expression level of CatA protein and the drug-metabolizing activity. Single-strand conformation polymorphism method was used to analyze genetic polymorphisms in healthy Japanese subjects. Nine genetic polymorphisms were identified in the CatA gene. The polymorphism (85_87CTG>-) in exon 2 was a mutation causing a deletion of leucine, resulting in the change of the leucine 9-repeat (Leu9) to 8-repeat (Leu8) in the signal peptide region of CatA protein. The effect of Leu8 on the expression level of CatA protein was evaluated in Flp-In-293 cells with a stably expressed CatA, resulting in the expression of CatA protein being significantly elevated in variant 2 with Leu8 compared with Leu9. Higher concentrations of tenofovir alanine (TFV-Ala), a metabolite of TAF, were observed in the Leu8-expressing cells than in the Leu9-expressing cells using LC/MS/MS. Our findings suggest that the drug metabolic activity of CatA is altered by the genetic polymorphism.

Proteolytic activation of human cathepsin A

Galactosialidosis is a human lysosomal storage disease caused by deficiency in the multifunctional lysosomal protease cathepsin A (also known as protective protein/cathepsin A, PPCA, catA, HPP, and CTSA; EC 3.4.16.5). Previous structural work on the inactive precursor human cathepsin A (zymogen) led to a two-stage model for activation, where proteolysis of a 1.6-kDa excision peptide is followed by a conformational change in a blocking peptide occluding the active site. Here we present evidence for an alternate model of activation of human cathepsin A, needing only cleavage of a 3.3-kDa excision peptide to yield full enzymatic activity, with no conformational change required. We present x-ray crystallographic, mass spectrometric, amino acid sequencing, enzymatic, and cellular data to support the cleavage-only activation model. The results clarify a longstanding question about the mechanism of cathepsin A activation and point to new avenues for the design of mechanism-based inhibitors of the enzyme.

Fluorous iminoalditols act as effective pharmacological chaperones against gene products from GLB₁ alleles causing GM1-gangliosidosis and Morquio B disease

Unlike replacement therapy by infusion of exogenous recombinant lysosomal enzymes, pharmacological chaperones aim at a gain of function of endogenous gene products. Deficits resulting from missense mutations may become treatable by small, competitive inhibitors binding to the catalytical site and thus correcting the erroneous conformation of mutant enzymes. This may prevent their premature degradation and normalize intracellular trafficking as well as biological half-life. A major limitation currently arises from the huge number of individual missense mutations and the lack of knowledge on the structural requirements for specific interaction with mutant protein domains. Our previous work on mutations of the β-galactosidase (β-gal) gene, causing GM1 gangliosidosis (GM1) and Morquio B disease (MBD), respectively, characterized clinical phenotypes as well as biosynthesis, intracellular transport and subcellular localization of mutants. We recently identified an effective chaperone, DL-HexDGJ (Methyl 6-{[N(2)-(dansyl)-N(6)-(1,5-dideoxy-D-galactitol-1,5-diyl)- L-lysyl]amino} hexanoate), among a series of N-modified 1-deoxygalactonojirimycin derivatives carrying a dansyl group in its N-acyl moiety. Using novel and flexible synthetic routes, we now report on the effects of two oligofluoroalkyl-derivatives of 1-deoxygalactonojirimycin, Ph(TFM)(2)OHex-DGJ (N-(α,α-di-trifluoromethyl) benzyloxyhexyl-1,5-dideoxy-1,5-imino-D: -galactitol) and (TFM)(3)OHex-DGJ (N-(Nonafluoro-tert-butyloxy)hexyl-1,5-dideoxy-1,5-imino-D: -galactitol) on the β-gal activity of GM1 and MBD fibroblasts. Both compounds are competitive inhibitors and increase the residual enzyme activities up to tenfold over base line activity in GM1 fibroblasts with chaperone-sensitive mutations. Western blots showed that this was due to a normalization of protein transport and intralysosomal maturation. The fact that the novel compounds were effective at very low concentrations (0.5-10 μM) in the cell culture medium as well as their novel chemical character suggest future testing in animal models. This may contribute to new aspects for efficient and personalized small molecule treatment of lysosomal storage diseases.

NEU1 sialidase expressed in human airway epithelia regulates epidermal growth factor receptor (EGFR) and MUC1 protein signaling

Epithelial cells (ECs) lining the airways provide a protective barrier between the external environment and the internal host milieu. These same airway epithelia express receptors that respond to danger signals and initiate repair programs. Because the sialylation state of a receptor can influence its function and is dictated in part by sialidase activity, we asked whether airway epithelia express catalytically active sialidase(s). Human primary small airway and A549 ECs expressed NEU1 sialidase at the mRNA and protein levels, and NEU1 accounted for >70% of EC sialidase activity. Blotting with Maackia amurensis and peanut agglutinin lectins established epidermal growth factor receptor (EGFR) and MUC1 as in vivo substrates for NEU1. NEU1 associated with EGFR and MUC1, and NEU1-EGFR association was regulated by EGF stimulation. NEU1 overexpression diminished EGF-stimulated EGFR Tyr-1068 autophosphorylation by up to 44% but enhanced MUC1-dependent Pseudomonas aeruginosa adhesion by 1.6-1.7-fold and flagellin-stimulated ERK1/2 activation by 1.7-1.9-fold. In contrast, NEU1 depletion increased EGFR activation (1.5-fold) and diminished MUC1-mediated bacterial adhesion (38-56%) and signaling (73%). These data indicate for the first time that human airway epithelia express catalytically active NEU1 sialidase that regulates EGFR- and MUC1-dependent signaling and bacterial adhesion. NEU1 catalytic activity may offer an additional level of regulation over the airway epithelial response to ligands, pathogens, and injurious stimuli.

Three novel beta-galactosidase gene mutations in Han Chinese patients with GM1 gangliosidosis are correlated with disease severity

BACKGROUND

GM1 gangliosidosis (GM1) is an autosomal recessive lysosomal storage disease caused by deficiency of acid beta-galactosidase (GLB1; EC3.2.1.23). Here, we identify three novel mutations in the GLB1 gene from two Han Chinese patients with GM1 that appear correlated with clinical phenotype.

METHODS

One of the two Han Chinese patients with GM1 presented with the juvenile form, and the other with the infantile form with cardiac involvement. Sequencing of the entire GLB1 gene revealed three novel mutations (p.H102 D, p.G494V, c.495_497delTCT), which were absent in 94 normal controls. Transient expression of cDNA encoding these variants was performed in COS-1 cells to evaluate β-galactosidase activities.

RESULTS

The first case (patient 1) with the juvenile form contained two missense mutations, p.H102 D and p.A301V. Patient 2 diagnosed with the infantile form of the disease with cardiac involvement was compound heterozygous for p.G494V and c.495_497delTCT mutations. All mutant beta-galactosidases exhibited significantly reduced activity (12%, 0%, 0%, and 0% for p.H102 D, p.A301V, p.G494V, and c.495_497delTCT), compared with the wild-type beta-galactosidase cDNA clone. The mutations identified in patient 2 with cardiomyopathy were localized in the GLB1 gene region common to both lysosomal beta-galactosidase and elastin binding protein (EBP), and caused a deletion in the elastin-binding domain of EBP.

CONCLUSIONS

All four mutations identified in Han Chinese patients induce significant suppression of β-galactosidase activity, correlating with severity of disease and presence of cardiomyopathy.

Neuraminidase-1 is required for the normal assembly of elastic fibers

The assembly of elastic fibers in tissues that undergo repeated cycles of extension and recoil, such as the lungs and blood vessels, is dependent on the proper interaction and alignment of tropoelastin with a microfibrillar scaffold. Here, we describe in vivo histopathological effects of neuraminidase-1 (Neu1) deficiency on elastin assembly in the lungs and aorta of mice. These mice exhibited a tight-skin phenotype very similar to the Tsk mouse. Normal septation of Neu1-null mice did not occur in neonatal mice, resulting in enlarged alveoli that were maintained in adults. The abnormal development of elastic fibers was remarkable under electron microscopy and confirmed by the overlapping distribution of elastin, fibrillin-1, fibrillin-2, and fibulin-5 (Fib-5) by the light microscopy immunostainings. Fib-5 fibers appeared diffuse and unorganized around the alveolar walls and the apex of developing secondary septal crests. Fibrillin-2 deposition was also abnormal in neonatal and adult lungs. Dispersion of myofibroblasts appeared abnormal in developing lungs of Neu1-null mice, with a random distribution of myofibroblast around the alveolar walls, rather than concentrating at sites of elastin synthesis. The elastic lamellae in the aorta of the Neu1-null mice were thinner and separated by hypertrophic smooth muscle cells that were surrounded by an excess of the sialic acid-containing moieties. The concentration of elastin, as measure by desmosine levels, was significantly reduced in the aorta of Neu1-null mice. Message levels for tropoelastin and Fib-5 were normal, suggesting the elastic fiber defects in Neu1-null mice result from impaired extracellular assembly.

Expression and characterization of 14 GLB1 mutant alleles found in GM1-gangliosidosis and Morquio B patients

GM1-gangliosidosis and Morquio B disease are lysosomal storage disorders caused by beta-galactosidase deficiency attributable to mutations in the GLB1 gene. On reaching the endosomal-lysosomal compartment, the beta-galactosidase protein associates with the protective protein/cathepsin A (PPCA) and neuraminidase proteins to form the lysosomal multienzyme complex (LMC). The correct interaction of these proteins in the complex is essential for their activity. More than 100 mutations have been described in GM1-gangliosidosis and Morquio B patients, but few have been further characterized. We expressed 12 mutations suspected to be pathogenic, one known polymorphic change (p.S532G), and a variant described as either a pathogenic or a polymorphic change (p.R521C). Ten of them had not been expressed before. The expression analysis confirmed the pathogenicity of the 12 mutations, whereas the relatively high activity of p.S532G is consistent with its definition as a polymorphism. The results for p.R521C suggest that this change is a low-penetrant disease-causing allele. Furthermore, the effect of these beta-galactosidase changes on the LMC was also studied by coimmunoprecipitations and Western blotting. The alteration of neuraminidase and PPCA patterns in several of the Western blotting analyses performed on patient protein extracts indicated that the LMC is affected in at least some GM1-gangliosidosis and Morquio B patients.

Cathepsin A is the major hydrolase catalyzing the intracellular hydrolysis of the antiretroviral nucleotide phosphonoamidate prodrugs GS-7340 and GS-9131

GS-7340 and GS-9131 {9-[(R)-2-[[(S)-[[(S)-1-(isopropoxycarbonyl)ethyl]amino]phenoxyphosphinyl]methoxy]-propyl]adenine and 9-(R)-4'-(R)-[[[(S)-1-[(ethoxycarbonyl)ethyl]amino]phenoxyphosphinyl]methoxy]-2'-fluoro-1'-furanyladenine, respectively} are novel alkylalaninyl phenyl ester prodrugs of tenofovir {9-R-[(2-phosphonomethoxy)propyl]adenine} (TFV) and a cyclic nucleotide analog, GS-9148 (phosphonomethoxy-2'-fluoro-2', 3'-dideoxydidehydroadenosine), respectively. Both prodrugs exhibit potent antiretroviral activity against both wild-type and drug-resistant human immunodeficiency virus type 1 strains and excellent in vivo pharmacokinetic properties. In this study, the main enzymatic activity responsible for the initial step in the intracellular activation of GS-7340 and GS-9131 was isolated from human peripheral blood mononuclear cells and identified as lysosomal carboxypeptidase A (cathepsin A [CatA]; EC 3.4.16.5). Biochemical properties of the purified hydrolase (native complex and catalytic subunit molecular masses of 100 and 29 kDa, respectively; isoelectric point [pI] of 5.5) matched those of CatA. Recombinant CatA and the isolated prodrug hydrolase displayed identical susceptibilities to inhibitors and identical substrate preferences towards a panel of tenofovir phosphonoamidate prodrugs. Incubation of both enzymes with 14C-labeled GS-7340 or [3H]difluorophosphonate resulted in the covalent labeling of identical 29-kDa catalytic subunits. Finally, following a 4-h incubation with GS-7340 and GS-9131, the intracellular concentrations of prodrug metabolites detected in CatA-negative fibroblasts were approximately 7.5- and 3-fold lower, respectively, than those detected in normal control fibroblasts. Collectively, these data demonstrate the key role of CatA in the intracellular activation of nucleotide phosphonoamidate prodrugs and open new possibilities for further improvement of this important class of antiviral prodrugs.

Tissue expression of the novel serine carboxypeptidase Scpep1

We previously identified a novel gene designated retinoid-inducible serine carboxypeptidase (RISC or Scpep1). Here we characterize a polyclonal antibody raised to Scpep1 and assess its localization in mouse cells and tissues. Western blot analysis revealed an immunospecific approximately 35-kDa protein corresponding to endogenous Scpep1. This protein is smaller than the predicted approximately 51-kDa, suggesting that Scpep1 is proteolytically cleaved to a mature enzyme. Immunohistochemical studies demonstrate Scpep1 expression in embryonic heart and vasculature as well as in adult aortic smooth muscle cells and endothelial cells. Scpep1 displays a broad expression pattern in adult tissues with detectable levels in epithelia of digestive tract and urinary bladder, islet of Langerhans, type II alveolar cells and macrophages of lung, macrophage-like cells of lymph nodes and spleen, Leydig cells of testis, and nerve fibers in brain and ganglia. Consistent with previous mRNA studies in kidney, Scpep1 protein is restricted to proximal convoluted tubular epithelium (PCT). Immunoelectron microscopy shows enriched Scpep1 within lysosomes of the PCT, and immunofluorescence microscopy colocalizes Scpep1 with lysosomal-associated membrane protein-2. These results suggest that Scpep1 is a widely distributed lysosomal protease requiring proteolytic cleavage for activity. The highly specific Scpep1 antibody characterized herein provides a necessary reagent for elucidating Scpep1 function.

Identification of Sites of Mannose 6-Phosphorylation on Lysosomal Proteins

Most newly synthesized soluble lysosomal proteins contain mannose 6-phosphate (Man-6-P), a specific carbohydrate modification that is recognized by Man-6-P receptors (MPRs) that direct targeting to the lysosome. A number of proteomic studies have focused on lysosomal proteins, exploiting the fact that Man-6-P-containing forms can be purified by affinity chromatography on immobilized MPRs. These studies have identified many known lysosomal proteins as well as many proteins not previously classified as lysosomal. The latter are of considerable biological interest with potential implications for lysosomal function and as candidates for lysosomal storage diseases of unknown etiology. However, a significant problem in interpreting the biological relevance of such proteins has been in distinguishing true Man-6-P glycoproteins from simple contaminants and from proteins associated with true Man-6-P glycoproteins (e.g. protease inhibitors and lectins). In this report, we describe a mass spectrometric approach to the verification of Man-6-phosphorylation based upon LC-MS of MPR-purified proteolytic glycopeptides. This provided a useful tool in validating novel MPR-purified proteins as true Man-6-P glycoproteins and also allowed identification of low abundance components not observed in the analysis of the total Man-6-P glycoprotein mixture. In addition, this approach allowed the global mapping of 99 Man-6-phosphorylation sites from 44 known lysosomal proteins purified from mouse and human brain. This information is likely to provide useful insights into protein determinants for this modification and may be of significant value in protein engineering approaches designed to optimize protein delivery to the lysosome in therapeutic applications such as gene and enzyme replacement therapies.

Identification of novel lysosomal matrix proteins by proteome analysis

The lysosomal matrix is estimated to contain about 50 different proteins. Most of the matrix proteins are acid hydrolases that depend on mannose 6-phosphate receptors (MPR) for targeting to lysosomes. Here, we describe a comprehensive proteome analysis of MPR-binding proteins from mouse. Mouse embryonic fibroblasts defective in both MPR (MPR 46-/- and MPR 300-/-) are known to secrete the lysosomal matrix proteins. Secretions of these cells were affinity purified using an affinity matrix derivatized with MPR46 and MPR300. In the protein fraction bound to the affinity matrix and eluted with mannose 6-phosphate, 34 known lysosomal matrix proteins, 4 candidate proteins of the lysosomal matrix and 4 non-lysosomal contaminants were identified by mass spectrometry after separation by two-dimensional gel electrophoresis or by multidimensional protein identification technology. For 3 of the candidate proteins, mammalian ependymin-related protein-2 (MERP-2), retinoid-inducible serine carboxypeptidase (RISC) and the hypothetical 66.3-kDa protein we could verify that C-terminally tagged forms bound in an M6P-dependent manner to an MPR-affinity matrix and were internalized via MPR-mediated endocytosis. Hence these 3 proteins are likely to represent hitherto unrecognized lysosomal matrix proteins.

Lysosomal Sialidase (Neuraminidase-1) Is Targeted to the Cell Surface in a Multiprotein Complex That Facilitates Elastic Fiber Assembly

We have established previously that the 67-kDa elastin-binding protein (EBP), identical to the spliced variant of beta-galactosidase, acts as a recyclable chaperone that facilitates secretion of tropoelastin. (Hinek, A., Keeley, F. W., and Callahan, J. W. (1995) Exp. Cell Res. 220, 312-324). We now demonstrate that EBP also forms a cell surface-targeted molecular complex with protective protein/cathepsin A and sialidase (neuraminidase-1), and provide evidence that this sialidase activity is a prerequisite for the subsequent release of tropoelastin. We found that treatment with sialidase inhibitors repressed assembly of elastic fibers in cultures of human skin fibroblasts, aortic smooth muscle cells, and ear cartilage chondrocytes and caused impaired elastogenesis in developing chick embryos. Fibroblasts derived from patients with congenital sialidosis (primary deficiency of neuraminidase-1) and galactosialidosis (secondary deficiency of neuraminidase-1) demonstrated impaired elastogenesis, which could be reversed after their transduction with neuraminidase-1 cDNA or after treatment with bacterial sialidase, which has a similar substrate specificity to human neuraminidase-1. We postulate that neuraminidase-1 catalyzes removal of the terminal sialic acids from carbohydrate chains of microfibrillar glycoproteins and other adjacent matrix glycoconjugates, unmasking their penultimate galactosugars. In turn, the exposed galactosugars interact with the galectin domain of EBP, thereby inducing the release of transported tropoelastin molecules and facilitating their subsequent assembly into elastic fibers.

Enzyme therapy for lysosomal storage disease: principles, practice, and prospects

Over the past three decades, enzyme therapy for lysosomal storage diseases has moved from an academic pursuit to direct delivery of effective clinical care for affected patients and families. This success is based on understanding the complexities of lysosomal biogenesis, lysosomal hydrolase sorting and hydrolytic requirements, and the target sites of pathology of these diseases. This article reviews these concepts and their application to the treatment of affected patients with Gaucher disease, Fabry disease, and mucopolysaccharidosis I. The principles, progress, and practice in these diseases provide prototypes for expansion of enzyme therapy to a growing set of these diseases.

Increase in macrophages in the testis of cathepsin a deficient mice suggests an important role for these cells in the interstitial space of this tissue

Cathepsin A (PPCA) is a lysosomal carboxypeptidase that functions as a protective protein for alpha-neuraminidase and beta-galactosidase in a multienzyme complex. In the present study, the testes of PPCA -/- mice from 2 to 10 months of age were compared with those of their wild type counterparts. While germ and Sertoli cells appeared comparable in appearance and distribution, the mean profile area of seminiferous tubules showed a significant decrease between wild type and PPCA -/- mice, suggesting changes to the seminiferous tubules and their contents. In addition, macrophages in the interstitial space (IS) of PPCA -/- mice were large, spherical, and filled with pale lysosomes, unlike those seen in wild type mice, and a quantitative analysis of their frequency per unit area of IS in PPCA -/- mice revealed a significant increase compared to that of wild type mice; this was also the case for their mean profile area. Absence of mitotic figures, cycling cells, or degenerating figures in the IS suggests that the major recruitment of macrophages appears to be from the circulation. In the IS, Leydig cells also showed an accumulation of large pale lysosomes in PPCA -/- mice, and their frequency also increased significantly as compared to wild type mice. In the electron microscope, a close association of Leydig cell microvilli with the surface of macrophages was pronounced in PPCA -/- mice. Since macrophages and Leydig cells interact by secreting various factors between each other, and considering the fact that Leydig cells show an accumulation of large pale lysosomes in PPCA -/- mice, it is suggested that macrophages accumulate as a result of abnormalities occurring in Leydig cells. Taken together, the data on increase in frequency of macrophages suggests important functions for these cells in both wild type and PPCA -/- mice.

Lysosomal multienzyme complex: biochemistry, genetics, and molecular pathophysiology

Lysosomal enzymes sialidase (alpha-neuraminidase), beta-galactosidase, and N-acetylaminogalacto-6-sulfate sulfatase are involved in the catabolism of glycolipids, glycoproteins, and oligosaccharides. Their functional activity in the cell depends on their association in a multienzyme complex with lysosomal carboxypeptidase, cathepsin A. We review the data suggesting that the integrity of the complex plays a crucial role at different stages of biogenesis of lysosomal enzymes, including intracellular sorting and proteolytic processing of their precursors. The complex plays a protective role for all components, extending their half-life in the lysosome from several hours to several days; and for sialidase, the association with cathepsin A is also necessary for the expression of enzymatic activity. The disintegration of the complex due to genetic mutations in its components results in their functional deficiency and causes severe metabolic disorders: sialidosis (mutations in sialidase), GM1-gangliosidosis and Morquio disease type B (mutations in beta-galactosidase), galactosialidosis (mutations in cathepsin A), and Morquio disease type A (mutations in N-acetylaminogalacto-6-sulfate sulfatase). The genetic, biochemical, and direct structural studies described here clarify the molecular pathogenic mechanisms of these disorders and suggest new diagnostic tools.

Mutations in sialidosis impair sialidase binding to the lysosomal multienzyme complex

Sialidosis is an autosomal recessive disease caused by the genetic deficiency of lysosomal sialidase, which catalyzes the catabolism of sialoglycoconjugates. The disease is associated with progressive impaired vision, macular cherry-red spots, and myoclonus (sialidosis type I) or with skeletal dysplasia, Hurler-like phenotype, dysostosis multiplex, mental retardation, and hepatosplenomegaly (sialidosis type II). We analyzed the effect of the missense mutations G68V, S182G, G227R, F260Y, L270F, A298V, G328S, and L363P, which are identified in the sialidosis type I and sialidosis type II patients, on the activity, stability, and intracellular distribution of sialidase. We found that three mutations, F260Y, L270F, and A298V, which are clustered in the same region on the surface of the sialidase molecule, dramatically reduced the enzyme activity and caused a rapid intralysosomal degradation of the expressed protein. We suggested that this region might be involved in sialidase binding with lysosomal cathepsin A and/or beta-galactosidase in the multienzyme lysosomal complex required for the expression of sialidase activity. Transgenic expression of mutants followed by density gradient centrifugation of cellular extracts confirmed this hypothesis and showed that sialidase deficiency in some sialidosis patients results from disruption of the lysosomal multienzyme complex.

Procathepsin L self-association as a mechanism for selective secretion

The lysosomal cysteine pro-protease procathepsin L was enriched in dense vesicles detectable when microsomes prepared from wild-type or transformed mouse fibroblasts were resolved on sucrose gradients. These dense vesicles did not comigrate with proteins characteristic of the endoplasmic reticulum, Golgi, endosomes or lysosomes. When gradient fraction vesicles were lysed at acidic pH in the presence of excess mannose 6-phosphate to prevent binding to mannose phosphate receptors, the majority of the procathepsin L was associated with the membrane, not the soluble, fraction. Immunogold labeling of procathepsin L in thin sections of cells or gradient fractions, using antibodies directed against the propeptide to avoid detection of the mature enzyme in dense lysosomes, revealed that the proenzyme was concentrated in dense cores localized in small vesicles near the plasma membrane and in multivesicular bodies. Consistent with the density of the gradient fraction and the electron density of the cores, yeast two-hybrid assays indicated the proenzyme could bind itself but could not interact with the aspartic proprotease procathepsin D. The data suggest that in mouse fibroblasts procathepsin L may self-associate into aggregates, initiating the formation of dense vesicles that could mediate the selective secretion of procathepsin L independent of mannose phosphate receptors.

Cathepsin A is expressed in a cell- and region-specific manner in the testis and epididymis and is not regulated by testicular or pituitary factors

The epithelial cells of the testis are involved in the production, differentiation, and sustenance of sperm, and those of the epididymis play a major role in sperm maturation, protection, and storage. These tissues express various proteins that respond differently to androgens. Cathepsin A is a multifunctional lysosomal carboxypeptidase that also functions as a protective and an activator protein for neuraminidase and beta-galactosidase. In this study, cathepsin A was immunolocalized by light and electron microscopy using a polyclonal affinity-purified antibody on the testis and epididymis of normal, orchidectomized with or without testosterone supplementation, efferent duct-ligated, and hypophysectomized adult rats. In normal rats, cathepsin A expression was noted in lysosomes of Sertoli and Leydig cells but not in germ cells of the testis, as well as nonciliated cells of the efferent ducts. In the epididymis, a cell- and region-specific distribution of cathepsin A was noted. In experimentally treated animals, no changes were noted in the expression of cathepsin A. Immunolabeling of tissues examined at the electron microscopic level revealed that lysosomes were reactive. These data indicate cell- and region-specific expression of cathepsin A in cells of the testis and epididymis and also indicate that cathepsin A expression is not regulated by testicular or pituitary factors.

beta-galactosidase gene mutations affecting the lysosomal enzyme and the elastin-binding protein in GM1-gangliosidosis patients with cardiac involvement

GM1-gangliosidosis is a lysosomal storage disorder caused by deficiency of acid beta-galactosidase (GLB1). We report five new beta-galactosidase gene mutations in nine Italian patients and one fetus, segregating in seven unrelated families. Six of the eight patients with the infantile, severe form of the disease presented cardiac involvement, a feature rarely associated with GM1-gangliosidosis. Molecular analysis of the patients' RNA and DNA identified two new RNA splicing defects, three new and three previously described amino acid substitutions. Interestingly, all patients with cardiac involvement were homozygous for one of these mutations: R59H, Y591C, Y591N, or IVS14-2A>G. In contrast, all other patients were compound heterozygous for one of the following mutations: R201H, R482H, G579D, IVS8+2T>C. Although we could not directly correlate the presence of cardiac abnormalities with specific genetic lesions, the mutations identified in patients with cardiomyopathy fell in the GLB1 cDNA region common to the lysosomal enzyme and the Hbeta-Gal-related protein, also known as the elastin binding protein (EBP). Consequently, both molecules are affected by the mutations, and they may contribute differently to the occurrence of specific clinical manifestations.

Correction of acid beta-galactosidase deficiency in GM1 gangliosidosis human fibroblasts by retrovirus vector-mediated gene transfer: higher efficiency of release and cross-correction by the murine enzyme

Mutations in the lysosomal acid beta-galactosidase (EC 3.2.1.23) underlie two different disorders: GM1 gangliosidosis, which involves the nervous system and visceral organs to varying extents, and Morquio's syndrome type B (Morquio B disease), which is a skeletal-connective tissue disease without any CNS symptoms. This article shows that transduction of human GM1 gangliosidosis fibroblasts with retrovirus vectors encoding the human acid beta-galactosidase cDNA leads to complete correction of the enzymatic deficiency. The newly synthesized enzyme is correctly processed and targeted to the lysosomes in transduced cells. Cross-correction experiments using retrovirus-modified cells as enzyme donors showed, however, that the human enzyme is transferred at low efficiencies. Experiments using a different retrovirus vector carrying the human cDNA confirmed this observation. Transduction of human GM1 fibroblasts and mouse NIH 3T3 cells with a retrovirus vector encoding the mouse beta-galactosidase cDNA resulted in high levels of enzymatic activity. Furthermore, the mouse enzyme was found to be transferred to human cells at high efficiency. Enzyme activity measurements in medium conditioned by genetically modified cells suggest that the human beta-galactosidase enzyme is less efficiently released to the extracellular space than its mouse counterpart. This study suggests that lysosomal enzymes, contrary to the generalized perception in the field of gene therapy, may differ significantly in their properties and provides insights for design of future gene therapy interventions in acid beta-galactosidase deficiency.

Human egasyn binds beta-glucuronidase but neither the esterase active site of egasyn nor the C terminus of beta-glucuronidase is involved in their interaction

Lysosomal beta-glucuronidase shows a dual localization in mouse liver, where a significant fraction is retained in the endoplasmic reticulum (ER) by interaction with an ER-resident carboxyl esterase called egasyn. This interaction of mouse egasyn (mEg) with murine beta-glucuronidase (mGUSB) involves binding of the C-terminal 8 residues of the mGUSB to the carboxylesterase active site of the mEg. We isolated the recombinant human homologue of the mouse egasyn cDNA and found that it too binds human beta-glucuronidase (hGUSB). However, the binding appears not to involve the active site of the human egasyn (hEg) and does not involve the C-terminal 18 amino acids of hGUSB. The full-length cDNA encoding hEg was isolated from a human liver cDNA library using full-length mEg cDNA as a probe. The 1941-bp cDNA differs by only a few bases from two previously reported cDNAs for human liver carboxylesterase, allowing the anti-human carboxylesterase antiserum to be used for immunoprecipitation of human egasyn. The cDNA expressed bis-p-nitrophenyl phosphate (BPNP)-inhibitable esterase activity in COS cells. When expressed in COS cells, it is localized to the ER. The intracellular hEg coimmunoprecipitated with full-length hGUSB and with a truncated hGUSB missing the C-terminal 18-amino-acid residue when extracts of COS cells expressing both proteins were treated with anti-hGUSB antibody. It did not coimmunoprecipitate with mGUSB from extracts of coexpressing COS cells. Unlike mEg, hEg was not released from the hEg-GUSB complex with BPNP. Thus, hEg resembles mEg in that it binds hGUSB. However, it differs from mEg in that (i) it does not appear to use the esterase active site for binding since treatment with BPNP did not release hEg from hGUSB and (ii) it does not use the C terminus of GUSB for binding, since a C-terminal truncated hGUSB (the C-terminal 18 amino acids are removed) bound as well as nontruncated hGUSB. Evidence is presented that an internal segment of 51 amino acids between 228 and 279 residues contributes to binding of hGUSB by hEg.

Lack of PPCA expression only partially coincides with lysosomal storage in galactosialidosis mice: indirect evidence for spatial requirement of the catalytic rather than the protective function of PPCA

Protective protein/cathepsin A (PPCA) is a pleiotropic lysosomal enzyme that complexes with beta-galactosidase and neuraminidase, and possesses serine carboxypeptidase activity. Its deficiency in man results in the neurodegenerative lysosomal storage disorder galactosialidosis (GS). The mouse model of this disease resembles the human early onset phenotype and results in severe nephropathy and ataxia. To understand better the pathophysiology of the disease, we compared the occurrence of lysosomal PPCA mRNA and protein in normal adult mouse tissues with the incidence of lysosomal storage in PPCA(-/-) mice. PPCA expression was markedly variable among different tissues. Most sites that produced both mRNA and protein at high levels in normal mice showed extensive and overt storage in the knockout mice. However, this correlation was not consistent as some cells that normally expressed high levels of PPCA were unaffected in their storage capability in the PPCA(-/-) mice. In addition, some normally low expressing cells accumulated large amounts of undegraded products in the GS mouse. This apparent discrepancy may reflect a requirement for the catalytic rather than the protective function of PPCA and/or the presence of cell-specific substrates in certain cell types. A detailed map showing the cellular distribution of PPCA in nomal mouse tissues as well as the sites of lysosomal storage in deficient mice is critical for accurate assessment of the effects of therapeutic interventions.

The atomic model of the human protective protein/cathepsin A suggests a structural basis for galactosialidosis

Human protective protein/cathepsin A (PPCA), a serine carboxypeptidase, forms a multienzyme complex with beta-galactosidase and neuraminidase and is required for the intralysosomal activity and stability of these two glycosidases. Genetic lesions in PPCA lead to a deficiency of beta-galactosidase and neuraminidase that is manifest as the autosomal recessive lysosomal storage disorder galactosialidosis. Eleven amino acid substitutions identified in mutant PPCAs from clinically different galactosialidosis patients have now been modeled in the three-dimensional structure of the wild-type enzyme. Of these substitutions, 9 are located in positions likely to alter drastically the folding and stability of the variant protein. In contrast, the other 2 mutations that are associated with a more moderate clinical outcome and are characterized by residual mature protein appeared to have a milder effect on protein structure. Remarkably, none of the mutations occurred in the active site or at the protein surface, which would have disrupted the catalytic activity or protective function. Instead, analysis of the 11 mutations revealed a substantive correlation between the effect of the amino acid substitution on the integrity of protein structure and the general severity of the clinical phenotype. The high incidence of PPCA folding mutants in galactosialidosis reflects the fact that a single point mutation is unlikely to affect both the beta-galactosidase and the neuraminidase binding sites of PPCA at the same time to produce the double glycosidase deficiency. Mutations in PPCA that result in defective folding, however, disrupt every function of PPCA simultaneously.

Protective protein in the bovine lysosomal beta-galactosidase complex

Cathepsin A [EC 3.4.16.1], so called protective protein, occurs as an enzyme complex with lysosomal beta-galactosidase [3.2.1.23] and is involved in the stable enzymic expression of lysosomal sialidase [3.2.1.18]. In this study we investigated the enzymatic properties of cathepsin A in the bovine beta-galactosidase complex and how it is involved in the molecular multiplicities of the beta-galactosidase and sialidase complexes. 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 6.0. It hydrolyzed dipeptide substrates composed of hydrophobic amino acids much faster than any other type of substrate tested. This specificity was found to be conserved from human to a non-mammal, chicken. Immunoprecipitation using an anti beta-galactosidase antibody demonstrated that cathepsin A is a component of both the sialidase and beta-galactosidase complexes. The over 700 kDa sialidase complex depolymerized by a brief incubation at pH 7.5 and the sialidase was inactivated irreversibly via formation of an enzyme active smaller species of sialidase. The 669 kDa beta-galactosidase complex dissociated reversibly into a 120 kDa beta-galactosidase and a 170 kDa cathepsin A, but the 120 kDa beta-galactosidase, free from the cathepsin A, formed a 260 kDa aggregate under the same conditions. Inactivation of cathepsin A by heat treatment did not affect its complex forming activity. The 170 kDa protective protein dissociated into a 50 kDa one at pH 7.5, which no longer formed the complex. These findings indicate that the 170 kDa protective protein could be the minimum unit required for in vitro reconstitution of the complex, and that its complex forming activity is carried in a heat-stable domain. Both beta-galactosidase and cathepsin A activities were labile under the dissociated condition, indicating that it physiologically stabilizes not only beta-galactosidase but also itself by forming the complex.

Identification of a sialidase encoded in the human major histocompatibility complex

Mammalian sialidases are important in modulating the sialic acid content of cell-surface and intracellular glycoproteins. However, the full extent of this enzyme family and the physical and biochemical properties of its individual members are unclear. We have identified a novel gene, G9, in the human major histocompatibility complex (MHC), that encodes a 415-amino acid protein sharing 21-28% sequence identity with the bacterial sialidases and containing three copies of the Asp-block motif characteristic of these enzymes. The level of sequence identity between human G9 and a cytosolic sialidase identified in rat and hamster (28-29%) is much less than would be expected for analogous proteins in these species, suggesting that G9 is distinct from the cytosolic enzyme. Expression of G9 in insect cells has confirmed that it encodes a sialidase, which shows optimal activity at pH 4.6, but appears to have limited substrate specificity. The G9 protein carries an N-terminal signal sequence and immunofluorescence staining of COS7 cells expressing recombinant G9 shows localization of this sialidase exclusively to the endoplasmic reticulum. The location of the G9 gene, within the human MHC, corresponds to that of the murine Neu-1 locus, suggesting that these are analogous genes. One of the functions attributed to Neu-1 is the up-regulation of sialidase activity during T cell activation.

Rat brain contains high levels of mannose-6-phosphorylated glycoproteins including lysosomal enzymes and palmitoyl-protein thioesterase, an enzyme implicated in infantile neuronal lipofuscinosis

Mannose 6-phosphate (Man-6-P) is a posttranslational carbohydrate modification typical of newly synthesized acid hydrolases that signals targeting from the Golgi apparatus to the lysosome via Man-6-P receptors (MPRs). Using iodinated cation independent MPR as a probe in a Western blot assay, we surveyed levels of Man-6-P glycoproteins in a number of different rat tissues. Considerable variation was observed with respect to total amounts and types of Man-6-P glycoproteins in the different tissues. Brain contained 2-8-fold more Man-6-P glycoproteins than other tissues, with relative abundance being brain >> testis approximately heart > lung approximately kidney approximately ovary approximately spleen > skeletal muscle approximately liver approximately serum. Analysis of 16 different lysosomal enzyme activities revealed that brain contains lower activities than other tissues which suggested that decreased removal of Man-6-P results in increased levels of Man-6-P glycoproteins. This was directly demonstrated by comparing activities of phosphorylated lysosomal enzymes, purified by immobilized MPR affinity chromatography, with total activities. The phosphorylated forms accounted for a considerable proportion of the MPR-targeted activities measured in brain (on average, 36.2%) but very little in lung, kidney, and liver (on average, 5.5, 2.3, and 0. 7%, respectively). Man-6-P glycoproteins were also isolated from rat brain by MPR affinity chromatography on a preparative scale. Of the 18 bands resolvable by SDS-polyacrylamide gel electrophoresis, seven bands were NH2-terminally sequenced and identified as the known lysosomal enzymes cathepsin L, cathepsin A, cathepsin D, alpha-galactosidase A, arylsulfatase A, and alpha-iduronidase. One of the major Man-6-P glycoproteins was identified as palmitoyl protein thioesterase, which was not previously thought to be lysosomal. This finding raises important questions about the cellular location and function of palmitoyl protein thioesterase, mutations in which result in the neurodegenerative disorder, infantile neuronal ceroid lipofuscinosis.

Proteolytic processing patterns of prosaposin in insect and mammalian cells

Prosaposin is a multifunctional protein encoded at a single locus in humans and mice. The precursor contains, in tandem, four glycoprotein activators or saposins, termed A, B, C, and D, that are essential for specific glycosphingolipid hydrolase activities. Prosaposin appears to be a potent neurotrophic factor. To explore the proteolytic processing from prosaposin to mature activator proteins, metabolic labeling was done with human prosaposin expressed in insect cells, human fibroblasts, neuronal stem cells (NT2) and retinoic acid-differentiated NT2 neurons. In all cell types, the major processing pathway was through a tetrasaposin, A-B-C-D, from which saposin A was then removed. In mammalian cells monosaposins were derived from the trisaposin B-C-D by cleavage to the disaposins, B-C and C-D, that were processed to monosaposins. In insect cells the major end products were the disaposins, with A-B and C-D derived from the tetrasaposin, A-B-C-D, or with B-C and C-D derived from the trisaposin, B-C-D. In insect and mammalian cells, the nonsignal NH2-terminal peptide preceding saposin A (termed Nter) was usually removed prior to saposin A cleavage. In NT2-derived differentiated neurons, precursor tetrasaposins containing A-B-C-D were secreted with and without Nter. Immunofluorescence studies using prosaposin-specific antisera showed large steady state amounts of uncleaved prosaposin in Purkinje cells, cortical neurons, and other specific cell types in adult mice. These studies indicate that prosaposin processing is highly regulated at a proteolytic level to produce prosaposin, tetrasaposins, or mature monosaposins in specific mammalian cells.

The beta-glucuronidase propeptide contains a serpin-related octamer necessary for complex formation with egasyn esterase and for retention within the endoplasmic reticulum

beta-Glucuronidase is retained within the endoplasmic reticulum (ER) via complex formation with esterase-22 (egasyn), which in turn has a COOH-terminal HTEL ER retention sequence. To identify the regions of glucuronidase that interact with egasyn, complex formation was assayed in COS cells cotransfected with egasyn cDNA and with either deletion constructs of glucuronidase or with constructs containing specific glucuronidase propeptide sequences appended to the carboxyl terminus of a rat secretory protein alpha 1-acid glycoprotein. The region of glucuronidase essential for complex formation is a linear octamer sequence at the COOH terminus of the propeptide. A portion of this octamer is similar to a sequence near the reactive site of serpins. This and associated data indicate that an interaction related to that between serine proteinases and their serpin inhibitors retains beta-glucuronidase within the ER. Further, attachment of this octamer sequence provides an alternative method of targeting proteins to the ER lumen of any cell that contains egasyn. These and related results demonstrate that complex formation with esterases/proteinases within the ER is important in the subcellular targeting and/or processing of certain proteins.

Adult-onset lysosomal storage disease in a Schipperke dog: clinical, morphological and biochemical studies

An adult-onset lysosomal storage disorder was diagnosed in a 5-year-old Schipperke dog with progressive cerebellar and central vestibular signs. It was characterized by cerebellar atrophy with extensive loss of Purkinje and granular cells, and hydrocephalus. Enlarged and vacuolated neurons were observed in spinal cord and brain; pancreatic centrolobular and islet cells were also vacuolated. Ultrastructurally, enlarged secondary lysosomes laden with lamellated membrane structures were present in neurons and empty enlarged vacuoles were found in pancreatic centroacinar, ductal, and islet cells. On frozen sections neurons stained with Ricinus communis agglutinin-I and wheat germ agglutinin. On paraffin sections neurons stained with luxol fast blue, periodic acid-Schiff, Concanavalia ensiformis agglutinin, and were autofluorescent. These findings indicate an accumulation of glycolipids containing terminal beta-galactosyl and alpha-sialyl residues, and N-linked oligosaccharides. Tissue activity of lysosomal beta-galactosidase was 50% of normal and the activity of beta-hexosaminidase was elevated. Brain lipid-bound sialic acid was twice normal, with a small increase of GM1-ganglioside, but there was a significant elevation of GM2 (GD2) and GM3 (GD3). In addition, significant elevations of sialylated and non-sialylated oligosaccharides were noted. These clinical, biochemical and pathological findings are similar to those observed in human patients with adult-onset galactosialidosis.