Differential accumulation of cholesterol in Golgi compartments of normal and Niemann-Pick type C fibroblasts incubated with LDL: a cytochemical freeze-fracture study.

Cholesterol accumulation in the Golgi of normal and Niemann-Pick Type C (NP-C) fibroblasts was shown by freeze-fracture electron microscopy using filipin as a probe for unesterified cholesterol. The specific distribution of cholesterol within individual Golgi compartments could be examined because membrane cholesterol forms complexes with filipin that are visible as membrane deformations (pits and protuberances) in freeze-fracture replicas. The density of filipin-cholesterol deformations, quantitated for cis, medial, and trans Golgi cisternae and trans Golgi vacuoles, was shown to increase in a cis to trans direction. After addition of low density lipoproteins (LDL) to cultured fibroblasts for 24 h, the cholesterol content increased within specific compartments of the Golgi. Normal cells showed an increase in filipin-cholesterol deformations in membranes of cis/medial cisternae and trans Golgi vacuoles, whereas NP-C cells showed only an increase in membranes of trans Golgi cisternae. LDL uptake by cells appears to induce a disparate cholesterol enrichment of Golgi compartments of normal and mutant cells. The ability of cells to process endocytosed cholesterol may in part depend on modulation of cholesterol-enriched membrane transport through the Golgi, a function which appears to be defective in NP-C cells.