Cellular cholesterol storage in the Niemann-Pick disease type C mouse is associated with increased expression and defective processing of apolipoprotein D.

Abstract

Apolipoprotein D (apoD), a member of the lipocalin superfamily of ligand transporters, has been implicated in the transport of several small hydrophobic molecules including sterols and steroid hormones. We have previously established that apoD is a secreted protein from cultured mouse astrocytes and that treatment with the oxysterol 25-hydroxycholesterol markedly stimulates apoD release. Here, we have investigated expression and cellular processing of apoD in the Niemann-Pick type C (NPC) mouse, an animal model of human NPC, which is a genetic disorder affecting cellular cholesterol transport. NPC is phenotypically characterized by symptoms of chronic progressive neurodegeneration. ApoD gene expression was up-regulated in cultured NPC astrocytes and in NPC brain. ApoD protein levels were also increased in NPC brain with up to 30-fold higher apoD content in the NPC cerebellum compared with control mice. Subcellular fractionation of NPC brain homogenates revealed that most of the apoD was associated with the myelin fraction. ApoD was found to be a secreted protein from cultured normal astrocytes and treatment with the oxysterol, 25-hydroxycholesterol, markedly stimulated apoD release (by five- to 10-fold). By contrast, secretion of apoD from NPC astrocytes was markedly reduced and could not be stimulated by oxysterol treatment. Secretion of apoE, another apolipoprotein normally produced by astrocytes, was similar in NPC and control cells. Furthermore, apoE secretion was not potentiated by oxysterol treatment in either cell type. Plasma levels of apoD were sixfold higher in NPC, whereas hepatic levels were substantially reduced compared with controls, possibly reflecting reduced hepatic clearance of the circulating protein. These results reveal hitherto unrecognized defects in apoD metabolism in NPC that appear to be linked to the known defects in cholesterol homeostasis in this disorder.