Synthesis and assembly of a catalytically active lysosomal enzyme, beta-hexosaminidase B, in a cell-free system

Multi-system disorders of glycosphingolipid and ganglioside metabolism

Glycosphingolipids (GSLs) and gangliosides are a group of bioactive glycolipids that include cerebrosides, globosides, and gangliosides. These lipids play major roles in signal transduction, cell adhesion, modulating growth factor/hormone receptor, antigen recognition, and protein trafficking. Specific genetic defects in lysosomal hydrolases disrupt normal GSL and ganglioside metabolism leading to their excess accumulation in cellular compartments, particularly in the lysosome, i.e., lysosomal storage diseases (LSDs). The storage diseases of GSLs and gangliosides affect all organ systems, but the central nervous system (CNS) is primarily involved in many. Current treatments can attenuate the visceral disease, but the management of CNS involvement remains an unmet medical need. Early interventions that alter the CNS disease have shown promise in delaying neurologic involvement in several CNS LSDs. Consequently, effective treatment for such devastating inherited diseases requires an understanding of the early developmental and pathological mechanisms of GSL and ganglioside flux (synthesis and degradation) that underlie the CNS diseases. These are the focus of this review.

Purification, Characterization, and Cloning of a Spodoptera frugiperda Sf9 β-N-Acetylhexosaminidase That Hydrolyzes Terminal N-Acetylglucosamine on the N-Glycan Core

Paucimannosidic glycans are often predominant in N-glycans produced by insect cells. However, a beta-N-acetylhexosaminidase responsible for the generation of paucimannosidic glycans in lepidopteran insect cells has not been identified. We report the purification of a beta-N-acetylhexosaminidase from the culture medium of Spodoptera frugiperda Sf9 cells (Sfhex). The purified Sfhex protein showed 10 times higher activity for a terminal N-acetylglucosamine on the N-glycan core compared with tri-N-acetylchitotriose. Sfhex was found to be a homodimer of 110 kDa in solution, with a pH optimum of 5.5. With a biantennary N-glycan substrate, it exhibited a 5-fold preference for removal of the beta(1,2)-linked N-acetylglucosamine from the Man alpha(1,3) branch compared with the Man alpha(1,6) branch. We isolated two corresponding cDNA clones for Sfhex that encode proteins with >99% amino acid identity. A phylogenetic analysis suggested that Sfhex is an ortholog of mammalian lysosomal beta-N-acetylhexosaminidases. Recombinant Sfhex expressed in Sf9 cells exhibited the same substrate specificity and pH optimum as the purified enzyme. Although a larger amount of newly synthesized Sfhex was secreted into the culture medium by Sf9 cells, a significant amount of Sfhex was also found to be intracellular. Under a confocal microscope, cellular Sfhex exhibited punctate staining throughout the cytoplasm, but did not colocalize with a Golgi marker. Because secretory glycoproteins and Sfhex are cotransported through the same secretory pathway and because Sfhex is active at the pH of the secretory compartments, this study suggests that Sfhex may play a role as a processing beta-N-acetylhexosaminidase acting on N-glycans from Sf9 cells.

Bicistronic lentiviral vector corrects beta-hexosaminidase deficiency in transduced and cross-corrected human Sandhoff fibroblasts

Sandhoff disease is an autosomal recessive neurodegenerative disease characterized by a GM2 ganglioside intralysosomal accumulation. It is due to mutations in the beta-hexosaminidases beta-chain gene, resulting in a beta-hexosaminidases A (alphabeta) and B (betabeta) deficiency. Mono and bicistronic lentiviral vectors containing the HEXA or/and HEXB cDNAs were constructed and tested on human Sandhoff fibroblasts. The bicistronic SIV.ASB vector enabled a massive restoration of beta-hexosaminidases activity on synthetic substrates and a 20% correction on the GM2 natural substrate. Metabolic labeling experiments showed a large reduction of ganglioside accumulation in SIV.ASB transduced cells, demonstrating a correct recombinant enzyme targeting to the lysosomes. Moreover, enzymes secreted by transduced Sandhoff fibroblasts were endocytosed in deficient cells via the mannose 6-phosphate pathway, allowing GM2 metabolism restoration in cross-corrected cells. Therefore, our bicistronic lentivector supplying both alpha- and beta-subunits of beta-hexosaminidases may provide a potential therapeutic tool for the treatment of Sandhoff disease.

The X-ray crystal structure of human beta-hexosaminidase B provides new insights into Sandhoff disease

Human lysosomal beta-hexosaminidases are dimeric enzymes composed of alpha and beta-chains, encoded by the genes HEXA and HEXB. They occur in three isoforms, the homodimeric hexosaminidases B (betabeta) and S (alphaalpha), and the heterodimeric hexosaminidase A (alphabeta), where dimerization is required for catalytic activity. Allelic variations in the HEXA and HEXB genes cause the fatal inborn errors of metabolism Tay-Sachs disease and Sandhoff disease, respectively. Here, we present the crystal structure of a complex of human beta-hexosaminidase B with a transition state analogue inhibitor at 2.3A resolution (pdb 1o7a). On the basis of this structure and previous studies on related enzymes, a retaining double-displacement mechanism for glycosyl hydrolysis by beta-hexosaminidase B is proposed. In the dimer structure, which is derived from an analysis of crystal packing, most of the mutations causing late-onset Sandhoff disease reside near the dimer interface and are proposed to interfere with correct dimer formation. The structure reported here is a valid template also for the dimeric structures of beta-hexosaminidase A and S.

Characterization of two Turkish beta-hexosaminidase mutations causing Tay-Sachs disease

Two homoallelic mutations have recently been identified in the alpha-subunit of hexosaminidase A (EC 3.2.1.52) causing the infantile form of Tay-Sachs disease in Turkish patients. Both of these mutations, a 12 bp deletion (1096-1107 or 1098-1108 or 1099-1109) in exon 10 and a point mutation (G1362 to A, Gly454 to Asp) in exon 12, are located in the catalytic domain of the hexosaminidase alpha-chain. In order to determine whether these mutations affect the function of the catalytic domain or result in an instable protein, both mutant cDNAs were overexpressed in COS-1 cells. As judged by Western blotting, transfections of wild-type cDNA produced pro-alpha-chain and mature alpha-chain in parallel with a fivefold increase in cellular hexosaminidase activity using the synthetic substrate 4-methylumbelliferyl beta-N-acetylglucosamine 6-sulfate (MUGS). However, both mutants produced only pro-alpha-chains, although no mature form or detectable hexosaminidase activity towards two different synthetic substrates was observed. These data are consistent with the biochemical phenotype of infantile Tay-Sachs disease. We conclude that the overexpressed mutant pro-alpha-chains were misfolded and could not undergo further proteolytic processing to the active form of the enzyme in the lysosome.

Posttranslational regulation of acid sphingomyelinase in niemann-pick type C1 fibroblasts and free cholesterol-enriched chinese hamster ovary cells

Niemann-Pick type C disease is characterized by the accumulation of cholesterol and other lipids within the lysosomal compartment, a process that is often accompanied by a reduction in acid sphingomyelinase activity. These studies demonstrate that a CHO cell mutant (CT-60), which accumulates lysosomal cholesterol because of a defective NP-C1 protein, has approximately 5-10% of the acid sphingomyelinase activity of its parental cell line (25-RA) or wild type (CHO-K1) cells. The cholesterol-induced reduction in acid sphingomyelinase activity can be reproduced in CHO-K1 cells by incubation in the presence of low density lipoprotein (LDL) and progesterone, which impairs the normal egress of LDL-derived cholesterol from the lysosomal compartment. Kinetic analysis of sphingomyelin hydrolysis in cell extracts suggests that the CT60 cells have a reduced amount of functional acid sphingomyelinase as indicated by a 10-fold reduction in the apparent V(max). Western blot analysis using antibodies generated to synthetic peptides corresponding to segments within the carboxyl-terminal region of acid sphingomyelinase demonstrate that both the CT60 and the LDL/progesterone-treated CHO-K1 cells possess near normal levels of acid sphingomyelinase protein. Likewise, Niemann-Pick type C fibroblasts also displayed normal acid sphingomyelinase protein but negligible levels of acid sphingomyelinase activity. These data suggest that cholesterol-induced inhibition is a posttranslational event, perhaps involving cofactor mediated modulation of enzymatic activity or alterations in acid sphingomyelinase protein trafficking and maturation.

In vitro assembly of the component chains of fibrinogen requires endoplasmic reticulum factors

Human fibrinogen (340 kDa) is a dimer, with each identical half-molecule composed of three different polypeptides (Aalpha, 66 kDa; Bbeta, 55 kDa; and gamma, 48 kDa). To understand the mechanisms of chain assembly, a coupled in vitro transcription translation system capable of assembling fibrinogen chains was developed. Fibrinogen chain assembly was assayed in an expression system coupled to rabbit reticulocyte lysate in the presence or absence of dog pancreas microsomal membranes. Fibrinogen chain assembly required microsomal membranes and oxidized glutathione. Co-expression of two of the chains, Bbeta and gamma or Aalpha and gamma, yielded free chains and two-chain complexes. Unlike combinations of Aalpha with gamma and Bbeta with gamma, co-expression of Aalpha and Bbeta did not form a single two-chain complex but produced a mixture of two-chain complexes. Co-expression of all three chains yielded free chains, two-chain complexes, and higher molecular weight complexes that corresponded to a half-molecule and to fully formed fibrinogen. Upon treatment of this mixture with thrombin and factor XIIIa, a gamma.gamma dimer, similar to that obtained from cross-linked human fibrin, was produced, indicating that properly folded fibrinogen was formed in vitro. Molecular chaperones may participate in fibrinogen assembly, since antibodies to resident proteins of the endoplasmic reticulum (BiP, Hsp90, protein disulfide isomerase, and calnexin) co-precipitated the chaperones together with nascent fibrinogen chains and complexes.

Identification of domains in human beta-hexosaminidase that determine substrate specificity

The lysosomal beta-hexosaminidases are dimers composed of alpha and beta subunits. beta-Hexosaminidase A (alphabeta) is a heterodimer, whereas hexosaminidase B (betabeta) and S (alphaalpha) are homodimers. Although containing a high degree of amino acid identity, each subunit expresses a unique active site that can be distinguished by a differential ability to hydrolyze charged substrates. The site on the beta-subunit primarily degrades neutral substrates, whereas the alpha-subunit site is, in addition, active against sulfated substrates. Isozyme specificity is also exhibited with glycolipid substrates. Among human isozymes, only beta-hexosaminidase A together with the GM2 activator protein can degrade the natural substrate, GM2 ganglioside, at physiologically significant rates. To identify the domains of the human beta-hexosaminidase subunits that determine substrate specificity, we have generated chimeric subunits containing both alpha- and beta-subunit sequences. The chimeric constructs were expressed in HeLa cells to screen for activity and then selected constructs were produced in the baculovirus expression system to assess their ability to degrade GM2 ganglioside in the presence of GM2 activator protein. Generation of activity against the sulfated substrate required the substitution of two noncontinuous alpha-subunit sequences (amino acids 1-191 and 403-529) into analogous positions of the beta-subunit. Chimeric constructs containing only one of these regions linked to the beta-subunit sequence showed either neutral substrate activity only (amino acids 1-191) or lacked enzyme activity entirely (amino acids 403-529). Neither the chimeras nor the wild-type subunits displayed activator-dependent GM2-hydrolyzing activity when expressed alone. However, one chimeric subunit containing alpha amino acids 1-191 fused with beta amino acids 225 to 556, when co-expressed with the wild-type alpha-subunit, showed activity comparable with that of recombinant beta-hexosaminidase A formed by the co-expression of the alpha- and beta-subunits. This result indicates that the beta-subunit amino acids 225-556 contribute an essential function in the GM2-hydrolyzing activity of beta-hexosaminidase A.

Photoaffinity labeling of human lysosomal beta-hexosaminidase B. Identification of Glu-355 at the substrate binding site

The carbene precursor 3-azi-1-[([6-3H]-2-acetamido-2-deoxy-1-beta-D-galactopyranosyl)thi o -butane (also designated [3H]-1-ATB-GalNAc) has been used as a photoaffinity label for human lysosomal beta-hexosaminidase B (Hex B, EC 3.2.1.52) purified to apparent homogeneity from postmortal liver. [3H]-1-ATB-GalNAc behaved as an active site-directed inhibitor, which bound covalently to Hex B upon photolysis at 350 nm and resulted in 15% inactivation of enzyme activity. Up to 75% of the inactivation of Hex B was prevented by including the competitive inhibitor 2-acetamido-2-deoxy-D-glucono-1,5-lactone in the photoaffinity experiment. Incubation of [3H]-1-ATB-GalNAc with the enzyme followed by irradiation and subsequent separation of the three polypeptides composing the beta-subunit led mainly to labeling of the beta a-polypeptide. Subsequent proteolysis of beta a with trypsin and separation of the resulting peptides by high pressure liquid chromatography yielded one prominently labeled peptide fraction. Edman degradation resulted in the sequence E339ISEVFPDQFIHLGGD-EVEFK359. However, no modified amino acid was detected, indicating that the photoaffinity label was presumably bound to the peptide by a labile ester linkage. This was proven when the radiolabel was almost completely released from the peptide by treatment with aqueous ammonium hydroxide. Simultaneously, Glu-355 was converted into Gln-355, which is located within a region of Hex B that shows considerable homology with the alpha-subunit of human hexosaminidase A and other hexosaminidases from various species.

Post-translational processing and Thr-206 are required for glycosylasparaginase activity

Lysosomal glycosylasparaginase is encoded as a 36.5 kDa polypeptide that is post-translationally processed to subunits of 19.5 kDa (heavy) and 15 kDa (light). Recombinant glycosylasparaginase has been expressed in Spodoptera frugiperda insect cells enabling the precursor and processed forms to be isolated and their catalytic potential determined. Only the subunit conformation was functional indicating glycosylasparaginase is encoded as an inactive zymogen. The newly created amino terminal residue of the light subunit following maturation, Thr-206, is believed to be involved in the catalytic mechanism [1992, J. Biol. Chem. 267, 6855-6858]. Here we have constructed two amino acid substitution mutants replacing Thr-206 with Ala-206 or Ser-206 and demonstrate that both destroy enzyme activity.

Crystallization of human beta-hexosaminidase B

beta-Hexosaminidase is a lysosomal hydrolase that is important in the metabolism of sphingoglycolipids. beta-Hexosaminidase B and beta-hexosaminidase A are the major isozymes in normal human tissue. beta-Hexosaminidase B is a homodimer of beta subunits, and beta-hexosaminidase A is a heterodimer composed of an alpha and a beta subunit. Crystals of beta-hexosaminidase B (M(r) 112,000) have been grown using the handling drop technique. They are elongated hexagonal prisms with maximum dimensions of 0.2 mm x 0.2 mm x 0.7 mm. The space group is P6(1)22 (or enantiomorph); the unit cell dimensions are a = b = 114.2 A, c = 402.2 A, alpha = beta = 90 degrees, gamma = 120 degrees. The molecular mass and cell dimensions suggest that there is one dimer per asymmetric unit. Crystals diffract to 3.2 A resolution.

The early and late processing of lysosomal enzymes: proteolysis and compartmentation

Lysosomal enzymes are subjected to a number of modifications including carbohydrate restructuring and proteolytic maturation. Some of these reactions support lysosomal targeting, others are necessary for activation or keeping the enzyme inactive before being segregated, while still others may be adventitious. The non-segregated fraction of the enzyme is secreted and can be isolated from the medium. It is considered that the secreted lysosomal enzymes fulfill certain physiological and pathophysiological roles. By comparing the secreted and the intracellular enzymes it is possible to distinguish between the reactions that occur before and after the segregation. In this review the reactions that may influence the segregation are referred to as the early processing and those characteristic for the enzymes isolated from lysosomal compartments as the late processing. The early processing is characterized mainly by modifications of carbohydrate side chains. In the late processing, proteolytic fragmentation represents the most conspicuous changes. The review focuses on the compartmentation of the reactions and the proteolytic fragmentation of lysosomal enzyme precursors. While a plethora of proteolytic reactions are involved, our knowledge of the proteinases responsible for the particular maturation reactions remains very limited. The review points also to work with cells from patients affected with lysosomal storage disorders, which contributed to our understanding of the lysosomal apparatus.

Biochemistry and genetics of Tay-Sachs disease

Tay-Sachs disease is one of the few neurodegenerative diseases of known causes. It results from mutations of the HEXA gene encoding the alpha subunit of beta-hexosaminidase, producing a destructive ganglioside accumulation in lysosomes, principally in neurons. With the determination of the protein sequence of the alpha and beta subunits, deduced from cDNA sequences, the complex pathway of subcellular and lysosomal processing of the enzyme has been determined. More recently, detailed knowledge of the gene structure has allowed the determination of specific mutations causing Tay-Sachs disease. The high incidence of the disease in Ashkenazi Jews is attributed predominantly to three mutations present in high frequency, while in non-Jews some two dozen mutations have been identified thus far. The cataloguing of mutations has important implications for carrier screening and prenatal diagnosis for Tay-Sachs disease.

Synthesis of a human lysosomal enzyme, beta-hexosaminidase B, using the baculovirus expression system

Human lysosomal beta-hexosaminidase exists in two major forms: the A isoform is composed of both alpha and beta chains, while the B form is a homopolymer of beta chains. Deficiency of beta-hexosaminidase underlies the GM2 gangliosidoses. We have produced active beta-hexosaminidase B in cultured insect (Sf9) cells by isolation of a recombinant insect virus (baculovirus) containing the cDNA for the beta chain within the viral polyhedron gene and infection of Sf9 cells with this construct. That portion of the enzyme secreted into the medium, 50%, was purified with concanavalin A Sepharose and subsequent affinity chromatography to yield beta-hexosaminidase B that is 75% pure. The product has an N-terminal amino acid sequence, specific activity, and size (M(r) 62,000) similar to that of the enzyme present in cultured human fibroblasts. However, endo H sensitivity studies revealed that the oligosaccharide structures present on recombinant beta-hexosaminidase B differ from those found on the enzyme synthesized in the human system. In addition, these structures lack the mannose 6-phosphate recognition marker that targets degradative hydrolases to lysosomes. Despite these differences, recombinant beta-hexosaminidase B does serve as a specific substrate for the mannose phosphorylating enzyme, N-acetylglucosaminyl phosphotransferase. Furthermore, the oligosaccharide moieties phosphorylated in vitro match those phosphorylated in vivo, pointing to the conformational integrity of the recombinant enzyme. Generous amounts of easily obtained, easily purified, and properly folded beta-hexosaminidase B will facilitate physical structural analysis of the enzyme.

Expression of human lysosomal beta-hexosaminidase in yeast vacuoles

The yeast Saccharomyces cerevisiae was tested as a recipient for functional expression of a mammalian lysosomal enzyme. The beta chain of human beta-hexosaminidase formed active dimeric enzyme, HexB, in transformants. HexB activity was localized to the vacuole, the yeast counterpart to the lysosome. A simple in situ enzyme assay was developed, which could be useful in expressing other lysosomal cDNAs.