Studies on the defect underlying the lysosomal storage of sialic acid in Salla disease. Lysosomal accumulation of sialic acid formed from N-acetyl-mannosamine or derived from low density lipoprotein in cultured mutant fibroblasts.

Abstract

IntroductionSalla disease is a lysosomal storage disorder characterized by mental retardation and disturbed sialic acid metabolism. To study the metabolism of extracellular sialic acid, lowdensity lipoprotein (LDL) was labeled in the sialic acid moiety (periodate-NaB3H4) or in the protein moiety (125I). Binding, interialization, lysosomal degradation, and exit of products of protein catabolism were similar in normal and mutant fibroblasts. Upon incubation with LDL labeled in the sialic acid moiety, mutant cells accumulated 2-3 times more free sialic acid radioactivity than normal fibroblasts, mostly in the lysosomal fraction. After a 24-h chase incubation, radioactivity in free sialic acid decreased by 70-80% in normal but only by 10-30% in mutant cells. In mutant fibroblasts, 40% of the radioactivity remained in lysosomes, whereas no labeled free sialic acid was detected in lysosomes from normal fibroblasts.We conclude that in Salla disease, fibroblast endogenous synthesis of sialic acid and lysosomal cleavage of exogenous glycoconjugates is normal, but free sialic acid cannot leave the lysosome. These findings suggest that the basic defect in Salla disease is deficient transport of free sialic acid through the lysosomal membrane.

Defective Sialic Acid Egress from Isolated Fibroblast Lysosomes of Patients with Salla Disease

“…[3H]NANA bound to low-density lipoprotein (LDL) was taken up normally by the lysosomes of Salla disease fibroblasts but was cleared at a much slower rate than in normal cells (6). In these studies the specific radioactivity of the labeled free NANA was diluted by large amounts of nonradioactive NANA present in the Salla cells but not in the normal cells.…”

“…Normal fibroblasts exposed to ManNAc yielded granular fractions with up to 88 pmol of NANA per unit of hexosaminidase (no exposure: 2 pmol per unit of hexosaminidase); Salla disease cells exposed to 30 mM ManNAc produced granular fractions that had acquired up to 496 pmol of NANA per unit of hexosaminidase more than the endogenous level offree NANA in the granular fractions (see Table 1). Since lysosomes accumulate NANA in Salla disease (5,6), this suggested that the lysosomes within the granular fractions were being loaded by the ManNAc exposure. Substantial amounts of free NANA were found in other compartments of the normal cells but were removed during isolation of the lysosome-rich fractions.…”

Sialic Acid Storage Disease and Related Disorders

“…The results led to the conclusion that NANA is released from the lysosomes in mutant cells at a slower rate than from control cells (Renlund et al, 1986a). Although SD occurs predominantly in Finland, sporadic cases from other countries have been reported.…”

Accumulation of Sialic Acid in Endocytic Compartments Interferes with the Formation of Mature Lysosomes

“…It should be pointed out that most degradative processes, which have been classically attributed to lysosomes, can proceed in late endosomes (5). Hence, it is not surprising that the capacity of SASD fibroblasts to degrade internalized macromolecules is not significantly affected by the unique phenotype of their lysosomes (66).…”