The distribution of hydrolytic enzyme activities in human fibroblast cultures and their intercellular transfer

Characterization of inducible models of Tay-Sachs and related disease

Tay-Sachs and Sandhoff diseases are lethal inborn errors of acid β-N-acetylhexosaminidase activity, characterized by lysosomal storage of GM2 ganglioside and related glycoconjugates in the nervous system. The molecular events that lead to irreversible neuronal injury accompanied by gliosis are unknown; but gene transfer, when undertaken before neurological signs are manifest, effectively rescues the acute neurodegenerative illness in Hexb−/− (Sandhoff) mice that lack β-hexosaminidases A and B. To define determinants of therapeutic efficacy and establish a dynamic experimental platform to systematically investigate cellular pathogenesis of GM2 gangliosidosis, we generated two inducible experimental models. Reversible transgenic expression of β-hexosaminidase directed by two promoters, mouse Hexb and human Synapsin 1 promoters, permitted progression of GM2 gangliosidosis in Sandhoff mice to be modified at pre-defined ages. A single auto-regulatory tetracycline-sensitive expression cassette controlled expression of transgenic Hexb in the brain of Hexb−/− mice and provided long-term rescue from the acute neuronopathic disorder, as well as the accompanying pathological storage of glycoconjugates and gliosis in most parts of the brain. Ultimately, late-onset brainstem and ventral spinal cord pathology occurred and was associated with increased tone in the limbs. Silencing transgenic Hexb expression in five-week-old mice induced stereotypic signs and progression of Sandhoff disease, including tremor, bradykinesia, and hind-limb paralysis. As in germline Hexb−/− mice, these neurodegenerative manifestations advanced rapidly, indicating that the pathogenesis and progression of GM2 gangliosidosis is not influenced by developmental events in the maturing nervous system.

Sandhoff and Tay-Sachs disease are devastating neurological diseases associated with developmental regression, blindness, seizures, and death in infants and young children. These disorders are caused by mutations in β-hexosaminidase genes, which result in neuronal accumulation of certain lipids, glycosphingolipids, inside the lysosomes of neurons. It is not yet known how accumulation of lipids affects neuronal function, and although promising treatments such as gene therapy are in development, currently none has been clinically approved. We aimed to develop genetic models that allow manipulation of β-hexosaminidase expression over time. Two inducible strains of mice were created in which acute Sandhoff disease could be “turned on” by the addition of doxycycline in the diet. Once induced in the adult mouse, the disease progressed relentlessly and was apparently independent of the rapid developmental processes that occur in the fetal and neonatal brain, resembling disease course in the germline Hexb−/− mouse. These transgenic inducible strains of Sandhoff disease mice provide a dynamic platform with which to explore the pathophysiological sequelae immediately after loss of neuronal lysosomal β-hexosaminidase activity.

Insights into post-translational processing of beta-galactosidase in an animal model resembling late infantile human G-gangliosidosis

G_M1-gangliosidosis is a lysosomal storage disorder caused by a deficiency of ß-galactosidase activity. Human GM1-gangliosidosis has been classified into three forms according to the age of clinical onset and specific biochemical parameters. In the present study, a canine model for type II late infantile human GM1-gangliosidosis was investigated ‘in vitro’ in detail. For a better understanding of the molecular pathogenesis underlying G_M1-gangliosidosis the study focused on the analysis of the molecular events and subsequent intracellular protein trafficking of β-galactosidase. In the canine model the genetic defect results in exclusion or inclusion of exon 15 in the mRNA transcripts and to translation of two mutant precursor proteins. Intracellular localization, processing and enzymatic activity of these mutant proteins were investigated. The obtained results suggested that the β-galactosidase C-terminus encoded by exons 15 and 16 is necessary for correct C-terminal proteolytic processing and enzyme activity but does not affect the correct routing to the lysosomes. Both mutant protein precursors are enzymatically inactive, but are transported to the lysosomes clearly indicating that the amino acid sequences encoded by exons 15 and 16 are necessary for correct folding and association with protective protein/cathepsin A, whereas the routing to the lysosomes is not influenced. Thus, the investigated canine model is an appropriate animal model for the human late infantile form and represents a versatile system to test gene therapeutic approaches for human and canine G_M1-gangliosidosis.

Clinical evaluation of serum N-acetyl-beta-D-glucosaminidase as a liver function test

Serum N-acetyl-beta-D-glucosaminidase has been shown to be a sensitive indicator of liver function. The enzyme activity in serum from patients with different forms of hepatic disease was found to be elevated. A comparison is also made with routine liver parameters. The frequencies of pathological serum levels of the routine measurements made are in relatively good agreement with earlier reports.

Total fasting serum bile acids and beta-hexosaminidase in alcoholic liver disease

Total serum bile acid levels and beta-hexosaminidase activity were studied in 22 normal subjects, 35 non-cirrhotic patients with acute alcohol intoxication, 45 patients with alcoholic liver cirrhosis and 11 patients with alcoholic liver cirrhosis and surgical portal-systemic shunts. Comparison was made with traditional liver function tests. beta-Hexosaminidase was most frequently elevated in acute alcohol intoxication (94%) while total serum bile acids were elevated in all patients with alcoholic liver cirrhosis. Total serum bile acid levels were found to discriminate most efficiently between acute alcohol intoxication and liver cirrhosis. The combined determination of serum beta-hexosaminidase and total serum bile acids is proposed for evaluating alcoholic liver disease.

Direct enzyme transfer from lymphocytes is specific

Lymphocytes are known to interact directly with other cells in vivo and in vitro, and have recently been shown to transfer the lysosomal enzyme, beta-glucuronidase, to fibroblasts from patients with an inherited deficiency of the enzyme. This process requires cell-cell contact, is unaffected by inhibitors of 'classical' receptor-mediated endocytosis and is abolished by inhibitors of protein synthesis. Although it is not yet known to what extent the transfer of enzymes by direct cellular interaction is a general phenomenon, a similar mechanism could possibly be involved in the transfer of other lysosomal enzymes in vivo and in the exchange of protein in vitro. We show here that the direct transfer of enzymes from lymphocytes to fibroblasts is restricted to only certain lysosomal enzymes.

Receptor-mediated endocytosis of fibroblast beta-glucuronidase by peritoneal macrophages

beta-Glucuronidase secreted by mouse 3T3 fibroblasts in vitro was taken up into mouse peritoneal macrophages and into human fibroblasts by a process which was rapid and saturable. High concentrations of mannose-containing compounds inhibited uptake into macrophages but had no effect on uptake into fibroblasts. Mannose-6-phosphate inhibited uptake into both types of cell, reducing uptake into macrophages by 34% and abolishing uptake into fibroblasts completely at a concentration of 5 mM. Fructose-1-phosphate was almost equally as effective at inhibiting uptake into fibroblasts but had no effect on macrophages. Pre-treatment of beta-glucuronidase with alkaline phosphatase totally prevented its uptake into fibroblasts but had no effect on its uptake into macrophages. These results indicate that fibroblasts can secrete a lysosomal enzyme in a form recognised as a high uptake ligand not only by other fibroblasts but also by peritoneal macrophages and that endocytosis appears to be mediated by different receptors present on each type of cell. This has important implications for the potential treatment of mucopolysaccharidoses by fibroblast transplants.

The expression of hex A and hex B isozymes of hexosaminidase in parental and experimental human fibroblast cells and their components

The expression of the two major isozyme forms of hexosaminidase (EC 3.2.1.30), hexosaminidase A and hexosaminidase B, has been examined. The parental cells and/or cellular components of parental cells are individually fused using inactivated Sendai virus with the aid of a micromanipulator. The progeny cells produced from such hybrids are subjected to a microenzymatic assay which allows measurements at the single cell level. The lysosomal-deficient cells used in this study are Tay-Sachs and Sandhoff fibroblasts, and the normal cells used are WI-38 (fetal lung fibroblasts), amniotic fluid cells (GM 473), and JASD3 (normal human foreskin). The results show that the ratio of cell components which are fused to form the experimental cell affects the percentage of hexosaminidase A expressed in the progeny cells. Furthermore, our results imply the presence of a "factor" in the Sandhoff cell's cytoplasm which, together with the Tay-Sachs nucleus, is necessary for hexosaminidase A expression in the experimental cell's progeny.

Lysosomal enzyme activities and RNA turnover rates in growing and nongrowing WI-38 and HeLa cells

Activities of three lysosomal enzymes--acid RNase. N-acetyl-beta-D-glucosaminidase and acid phosphatase--were determined during the growth cycles of WI-38 and HeLa cells, as well as in radiation-arrested WI-38 cells. In confluent and growth-arrested cultures of WI-38 cells, the lysosomal RNase increased six- to sevenfold; glucosaminidase, four- to fivefold; and phosphatase, two- to threefold. In HeLa cells, the lysosomal enzymes also increased in confluent cultures, but less than twofold; and the RNase level increased only transiently. In both WI-38 and HeLa cells, the rate of RNA breakdown also increased as cultures approached confluency. The rate of turnover of RNA, like the level of acid RNase, was higher in WI-38 cells than in HeLa cells (4 d half-life compared to 8 d). The increase in acid RNase could be prevented by incubation of cells in NH4Cl, but the rate of turnover in the presence of NH4Cl increased just as much when cells became confluent or stopped growth. The content of acid RNase could be changed more than 10-fold without altering the rate of RNA turnover. It is suggested that the increase in enzyme level is more important for possible autophagy or increased digestion of engulfed RNA, rather than for normal RNA turnover, when growth stops.

The stability and aggregation properties of human liver acid beta-D-galactosidase

1. A method is described for following continuously the action of beta-galactosidase on 4-methylumbelliferyl beta-D-galactoside at pH 4.5, in which 4-methylumbelliferone production is measured at fluorescence excitation and emission wavelengths of 324 and 444nm respectively. 2. Initial-rate studies show that the presence of salt activates beta-galactosidase up to 100 mM, but is inhibitory above that concentration. The enzyme is very unstable at 37 degrees C and low ionic strength, but stability increases with ionic strength. 3. The stability of the enzyme at 37 degrees C decreases markedly with rising pH in the range 5.9--8.0. 4. Gel-filtration patterns demonstrate that there is a marked tendency to polymerization with increasing ionic strength. The gel-filtration pattern shows decreasing amounts of dimer with increasing pH. 5. The correlation between activity, stability and molecular form of beta-galactosidase is discussed. It is suggested that the dimeric form of the enzyme is the most stable and active form. The implications of this finding for the assay of beta-galactosidase under physiological conditions for prenatal diagnosis are discussed. 6. Evidence for the possible occurrence of a 36 000-mol.wt. from of beta-galactosidase is presented. 7. A computer program for the calculation of initial rates has been deposited as Supplementary Publication SUP 50114 (4 pages) at the British Library Lending Division, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1981) 193, 5.

Molecular forms of beta-hexosaminidase and alpha-mannosidase in cell cultures from patients with mucolipidosis types II and III

Molecular forms of beta-hexosaminidase and alpha-mannosidase were studied using isoelectric focusing in fibroblasts and culture medium from patients with mucolipidosis Types II and III. Compared to controls, no significant aberrations could be found in the isofocusing isoenzyme pattern. Mannose-6-phosphate, which inhibits the uptake of lysosomal enzymes into the cells, was added to the cell cultures from controls without any significant effect on the isofocusing isoenzyme pattern.

Biochemistry and genetics of gangliosidoses

The gangliosidoses comprise an-ever increasing number of biochemically and phenotypically variant diseases. In most of them an autosomal recessive inherited deficiency of a lysosomal hydrolase results in the fatal accumulation of glucolipids (predominantly in the nervous tissue) and of oligosaccharides. The structure, substrate specificity, immunological properties of and genetic studies on the relevant glycosidases, ganglioside GM1 beta-galactosidase and beta-hexosaminidase isoenzymes, are reviewed in this paper. Contrary to general expectation, only a poor correlation is observed between the severity of the disease and residual activity of the defective enzyme when measured with synthetic or natural substrates in the presence of detergents. For the understanding of variant diseases and for their pre- and postnatal diagnosis, the necessity of studying the substrate specificity of normal and mutated enzymes under conditions similar to the in vivo situation, e.g., with natural substrates in the presence of appropriate activator proteins, is stressed. The possibility that detergents may have adverse affects on the substrate specificity of the enzymes is discussed for the beta-hexosaminidases. The significance of activator proteins for the proper interaction of lipid substrates and water-soluble hydrolases is illustrated by the fatal glycolipid storage resulting from an activator protein deficiency in the AB variant of GM2-gangliosidosis. Recent somatic complementation studies have revealed the existence of a presumably post-translational modification factor necessary for the expression of ganglioside GM1 beta-galactosidase activity. This factor is deficient in a group of variants of GM1-glangliosidosis. Among the possible reasons for the variability of enzyme activity levels in heterozygotes and patients, allelic mutations, formation of hybrid enzymes, and the existence of patients as compound heterozygotes are discussed. All these may result in the production of mutant enzymes with an altered specificity for a variety of natural substrates.