engleski

Retromer (dys)function in Niemann-Pick type C disease models

Niemann-Pick type C (NPC) disease is a rare childhood neurodegenerative disorder caused by mutations in cholesterol transport genes NPC1 or NPC2. Interestingly, NPC disease shares several features with Alzheimer's disease (AD), including dementia, neurodegeneration, endosome/lysosome dysfunction, and increased levels of the amyloid-β peptide (Aβ) and of the hyperphosphorylated tau protein. Previously, we have shown that increased Aβ in NPC1-null cells involves sequestration of the Alzheimer's amyloid-β precursor protein (APP) and BACE1 within endocytic pathway. We hypothesized that dysfunction of retromer, a protein complex involved in recycling of proteins, including APP, from early endosomes to the trans-Golgi network or plasma membrane, is responsible for mistrafficking of APP and BACE1 in NPC1-null cells. The goal of this work is to elucidate whether retromer dysfunction is responsible for an Alzheimer's-like phenotype upon cholesterol accumulation in NPC disease models. To determine the effect of cholesterol levels on retromer function we used cellular and mouse models of NPC disease. For subcellular localization of retromer, we analyzed Chinese Hamster Ovary cells (CHOwt) and CHO NPC1-null cells in which NPC1 gene has been deleted. To determine the levels and distribution of retromer proteins in different brain regions, we analyzed hippocampi, cerebella and cortices of NPC1+/+ (wt) and NPC1-/- mice before onset of the disease (4-weeks old), after behavioral deficits started (7-weeks old) and in the terminal stage (10-weeks old). The levels of retromer proteins Vps26, Vps35 and retromer receptor sorLA were analyzed by western blotting. Subcellular and regional distributions of retromer proteins were analyzed by immunocyto(histo)chemistry. The levels of retromer components are not altered between NPC and wt disease models (NPC1-null cells and NPC1-/- mouse brains). However, upon cholesterol accumulation in NPC1- null cells, retromer was retained in early endosomes where it co-localized with APP. These results were supported by our immunohistochemistry studies of NPC1-/- mouse brains which showed accumulation of Vps35 in the neuronal cell soma in all three brain regions analyzed. We also observed decreased immunostaining of sorLA already in 4-weeks old NPC1-/- vs. wt mouse brains. Our studies on NPC disease cellular model show that retromer (dys)function is dependent on cholesterol levels. Spatial and temporal distribution of retromer components is altered in NPC1-/- mouse brains. Decreased immunostaining of sorLA observed already at the very early stage of the disease (when no symptoms are detected) suggests that alterations in sorLA may be a trigger of retromer dysfunction upon cholesterol accumulation in NPC disease models. Altogether, dysfunction of retromer complex in NPC disease may be responsible for an AD-like phenotype previously observed in NPC disease.

Alzheimer’s disease, APP, cholesterol, Niemann-Pick type C disease, retromer

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