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Malnar, Martina; Košiček, Marko; Lisica, Ana; Posavec, Melanija; Krolo, Ana; Njavro, Jasenka; Omerbašić, Damir; Tahirović, Sabina; Hećimović, Silva (2012) Cholesterol-depletion corrects APP and BACE1 misstrafficking in NPC1-deficient cells. Biochimica et biophysica acta-molecular basis of disease, 1822 (8). Abstract Cholesterol accumulation in Niemann-Pick type C disease (NPC) causes increased levels of the amyloid-precursor-protein C-terminal fragments (APP-CTFs) and intracellular amyloid-B peptide (AB), the two central molecules in Alzheimer’s disease (AD) pathogenesis. We previously reported that cholesterol accumulation in NPC-cells leads to cholesterol-dependent increased APP processing by B-secretase (BACE1) and decreased APP expression at the cell surface (Malnar et al. Biochim Biophys Acta. We hypothesized that increased formation of APP-CTFs and AB in NPC disease is due to cholesterol-mediated altered endocytic trafficking of APP and/or BACE1. Here, we show that APP endocytosis is prerequisite for enhanced AB levels in NPC-cells. Moreover, we observed that NPC cells show cholesterol dependent sequestration and colocalization of APP and BACE1 within enlarged early/recycling endosomes which can lead to increased B-secretase processing of APP. We demonstrated that increased endocytic localization of APP in NPC-cells is likely due to both its increased internalization and its decreased recycling to the cell surface. Our findings suggest that increased cholesterol levels, such as in NPC disease and sporadic AD, may be the upstream effector that drives amyloidogenic APP processing characteristic for Alzheimer's disease by altering endocytic trafficking of APP and BACE1.
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Malnar, Martina (2012) The mechanism(s) of cholesterol-effect on processing and transport of APP protein. Doctoral thesis, Sveučilište u Zagrebu, Prirodoslovno-matematički fakultet. Abstract Peptid amiloid-B (AB), središnja molekula u patogenezi Alzheimerove bolesti, nastaje amiloidogenim cijepanjem proteina APP B-sekretazom u endosomima, dok cijepanje proteina APP na staničnoj membrani a-sekretazom sprečava nastanak ovog patološkog peptida. Nakupljanje peptida AB u mozgu otkriveno je ne samo u Alzheimerovoj bolesti nego i u bolesti Niemann-Pick tipa C (NPC), nasljednoj neurodegenerativnoj bolesti nakupljanja lipida. U ovom radu smo pokazali da je povećana razina peptida AB u stanicama NPC posljedica povećanog cijepanja proteina APP enzimom B-sekretaza uzrokovanog nakupljanjem kolesterola. Nadalje, pokazali smo da je internalizacija proteina APP nužna za nastanak peptida AB, te da u stanicama NPC zbog nakupljanja kolesterola u kasnim endosomima / lizosomima dolazi do poremećaja u transportu proteina APP u endocitoznom putu. Naime, u stanicama NPC smo detektirali ubrzanu internalizaciju i usporeno recikliranje proteina APP na površinu stanice što uzrokuje njegovo nakupljanje u ranim / reciklirajućim endosomima, centralnom mjestu stvaranja peptida AB. Ovim istraživanjem smo pokazali da povišena razina kolesterola može biti pokretač patoloških procesa u Alzheimerovoj bolesti jer potiče smještaj proteina APP u endosomima što rezultira njegovim povećanim amiloidogenim cijepanjem i nastankom peptida AB.