Ultrastructural and immunocytochemical study of skin fibroblasts from normal and sialidosis patients

The objectives of this study were to analyze morphologically, morphometrically and immunocytochemically the lysosomal compartment of normal fibroblasts and of fibroblasts with neuraminidase deficiency. The immunocytochemical analyses consisted of quantifying the distribution of saposins and β-galactosidase in the lysosomes of these cells to test the hypothesis that neuraminisdase deficiency is associated with an impairment in the transport of these proteins to the lysosomal compartment. To test this idea, cultured skin fibroblasts of patients with or without sialidosis were prepared for electron microscopy and probed with antibodies against lysosomal β-galactosidase and lysosomal saposins. The lysosomes of the affected cells had an abnormal accumulation of incompletely digested membranes which was associated with a significant lowering in the density of antigenic sites per lysosome. However, due to a significant increase in the number of lysosomes per affected cell, the total number of antigenic sites in control and neuraminidase deficient cells was similar. This presumably compensatory effect indicates that although the rate of production of β-galactosidase and saposins remains unchanged, the transport of these molecules to the lysosomes is somehow affected. Our data also indicate that in the fibroblasts, lysosomes require a normal concentration of the three enzymes to maintain neuraminidase activity and sphingolipid degradation.