Additional biochemical findings in a patient and fetal sibling with a genetic defect in the sphingolipid activator protein (SAP) precursor, prosaposin. Evidence for a deficiency in SAP-1 and for a normal lysosomal neuraminidase

Abstract

It has been shown that sphingolipid activator proteins (SAPs) 1 and 2 are encoded on the same gene along with two other putative activator proteins [Fürst, Machleidt & Sandhoff (1988) Biol. Chem. Hoppe-Seyler 369, 317-328 and O'Brien, Kretz, Dewji, Wenger, Esch & Fluharty (1988) Science 241, 1098-1101]. We have undertaken further biochemical investigations on a patient and fetal sibling, who were previously shown to have a unique sphingolipid storage disorder associated with an SAP-2 deficiency [Harzer, Paton, Poulos, Kustermann-Kuhn, Roggendorf, Grisar & Popp (1989) Eur. J. Pediatr. 149, 31-39]. The severity of their disorder suggested that other products of the SAP precursor or prosaposin gene may also be deficient. The turnover of cerebroside sulphate and globotriaosylceramide were investigated and were both impaired in fibroblasts from the patient and fetus. However, the activities of cerebroside sulphate sulphatase and globotriaosylceramide alpha-galactosidase in vitro were normal in cells from the fetus and patient respectively. In addition, there was an increase in cerebroside sulphate concentration in the kidney of the affected fetus. These results indicate that, in addition to the SAP-2 deficiency, there was a defect in SAP-1 function in this disorder. Additional increases in the concentration of monohexosyl- and dihexosyl-ceramide in the fetal kidney probably reflect the deficiency of SAP-2 in the case of monohexosylceramides, and the combined activator deficiency in the case of dihexosylceramides. Lactosylceramide-loading studies confirmed that there was a defect in the turnover of this lipid in fibroblasts from the affected patient and fetus but not from a patient with an isolated SAP-1 deficiency, or from patients with Krabbe disease, GM1 gangliosidosis or galactosialidosis. It has been suggested [Potier, Lamontagne, Michaud & Tranchemontagne (1990) Biochem. Biophys. Res. Commun. 173, 449-456] that the prosaposin gene also codes for lysosomal neuroaminidase. However, we found normal neuraminidase activity in fibroblasts from our patient, using assay conditions which are diagnostic for sialidosis patients. The role of prosaposin gene products in sphingolipid metabolism is discussed in view of our biochemical findings in this genetic disorder.

Hydrolysis of lactosylceramide by human galactosylceramidase and GM1‐β‐galactosidase in a detergent‐free system and its stimulation by sphingolipid activator proteins, sap‐B and sap‐C Activator proteins stimulate lactosylceramide hydrolysis

“…This indicates that both enzymes participate in the in vivo degradation of lactosylceramide, galactosylceramidase presumably to a higher percentage than GM1-P-galactosidase. Quantitative data, however, on their participation in vivo cannot be calculated at present, since galactosylceramidase activity has been assayed in tissue extracts so far only in the presence of detergent mixtures [13, 161. Our in vitro data support results of loading studies of cultured skin fibroblasts [35]. Cells deficient in GM1-P-galactosidase (obtained from GM1-gangliosidosis and galactosialidosis patients) or galactosylceramidase (obtained from patients with Krabbe disease), or sup-B (obtained from a patient with a variant of metachromatic leukodystrophy) showed normal turnover of lactosylceramide.…”

“…A strongly reduced turnover of this glycolipid was observed, however, in fibroblasts obtained from a patient with a combined activator (sup-B and sup-C) deficiency. Our in vitro results and the feeding studies in situ [35] therefore indicate that lysosomal lactosylceramide turnover is maintained by both sup-C-stimulated hydrolysis by galactosylceramidase and sup-B-stimulated hydrolysis by GM1-P-galactosidase. The rate of complex formation is assumed to be proportional to the density of substrate molecules on liposomal surfaces : The complex can dissociate in two different ways.…”

Saposin C Is Required for Normal Resistance of Acid β-Glucosidase to Proteolytic Degradation

“…The proteolytic processing of prosaposin to the individual saposins occurs predominantly in acidified compartments including the lysosome (5,6). The physiological importance of this locus has been demonstrated by the genetic deficiencies of individual saposins or prosaposin that lead to various glycosphingolipid storage diseases (7)(8)(9)(10). For example, saposin B deficiency leads to sulfatide accumulation and a metachromatic leukodystrophy-like disease (11) that is similar to the deficiency of arylsulfatase A, the cognate enzyme.…”

Conditional expression of human acid β-glucosidase improves the visceral phenotype in a Gaucher disease mouse model

“…) leads to a decrease of GCase activity in selected tissues of affected humans and mice due to the loss of the proteolytic protective effects of saposin C (15)(16)(17)(18)(19).…”