ACCUMULATION OF SPHINGOLIPIDS IN SAP-PRECURSOR (PROSAPOSIN)-DEFICIENTFIBROBLASTS OCCURS AS INTRALYSOSOMAL MEMBRANE STRUCTURES AND CAN BE COMPLETELY REVERSED BY TREATMENT WITH HUMAN SAP-PRECURSOR
Jk. Burkhardt et al., ACCUMULATION OF SPHINGOLIPIDS IN SAP-PRECURSOR (PROSAPOSIN)-DEFICIENTFIBROBLASTS OCCURS AS INTRALYSOSOMAL MEMBRANE STRUCTURES AND CAN BE COMPLETELY REVERSED BY TREATMENT WITH HUMAN SAP-PRECURSOR, European journal of cell biology, 73(1), 1997, pp. 10-18
The degradation of glycosphingolipids takes place in lysosomes iby act
ion of specific exohydrolases, with the assistance of sphingolipid act
ivator proteins (SAPs), Four of the SAPs, SAP-A to -D (also called sap
osins A to D), are synthesized from a single protein, the SAP-precurso
r (prosaposin). Deficiency in this precursor protein, a rape inherited
disease in humans, results in tile storage of sphingolipids with shor
t oligosaccharide head groups within the patients' tissues, and electr
on microscopy revealed the accumulation of large multivesicular storag
e organelles. In this study we analyze the multivesicular storage orga
nelles in cultivated fibroblasts from these patients. The results supp
ort our hypothesis that endocytosis of plasma membrane-derived lipids
occurs via small intraendosomal and intralysosomal vesicles and membra
ne structures that are then digested within the lysosomes (Sanehoff, K
., T. Kolter, Trends in Cell Biol, 6, 98-103 (1996). First, we show th
at the storage compartment consists of late endosomes and lysosomes by
immunogold labeling for marker proteins of these organelles, The tran
sport of endocytosed bovine serum albumin-colloidal gold or cationized
ferritin into the compartment occurs with tile timing expected for tr
ansport to late endocytic organeiles. Second, complementation of the m
edium of the SAP-precursor-deficient fibroblasts with only nanomolar c
oncentrations of purified SAP-precursor nearly completely reversed the
aberrant accumulation of multivesicular structures, thereby abolishin
g most of the intralysosomal membrane structures, Analysis of the sphi
ngolipid pattern of the cells after metabolic labeling with [C-14]seri
ne reveals that the cells' ability to degrade glycosphingolipids is co
mpletely restored by feeding of SAP-precursor at the same concentratio
ns. This is tile first demonstration in vivo that endocytosed SAP-prec
ursor is processed into functional active SAPs A,- B,- C, and D and th
at the degradation of the vesicular structures within the lysosomes de
pends on tile presence of the SAPs. Moreover, these studies suggest th
at a therapy program based on feeding purified SAP-precursor mag be va
luable in treating the entire family of diseases which result from the
loss of one or more of the SAPs.