Roles of the Insulin-like Growth Factor-II/Cation-independent Mannose 6-phosphate Receptor and Cathepsin D in Alzheimer’s Disease Pathology

• Author / Creator

  Wang, Yanlin

• Alzheimer’s disease (AD) is the most common type of senile dementia affecting the elderly. A critical contributing factor to the neurodegeneration and development of AD pathology stems from the processing of amyloid precursor protein (APP), leading to the generation of β-amyloid (Aβ) peptides. The endosomal-lysosomal (EL) system acts as an important site for Aβ generation and its dysfunction has been linked to increased Aβ production and neuronal loss in AD brains. Since insulin-like growth factor-II/cation-independent mannose 6-phosphate (IGF-II/M6P) receptor plays a critical role in the transport of lysosomal enzymes from the trans-Golgi network to endosomes, it is likely that the receptor may have a role in regulating APP processing, Aβ metabolism and Aβ-mediated toxicity in AD pathology. To test this hypothesis, we evaluated the effect of IGF-II/M6P receptor overexpression on the transcription of genes related to AD pathology, APP processing and Aβ metabolism in well-characterized mouse fibroblast cells overexpressing human IGF-II/M6P receptors. We also evaluated the levels and subcellular distribution of the IGF-II/M6P receptor and the lysosomal enzyme cathepsin D in cultured neurons, two lines of transgenic mouse models of AD (TgCRND8 and 5xFAD), as well as post-mortem AD brains, and characterized the involvement of these two enzymes with respect to neurodegeneration. Our results reveal that an elevation in IGF-II/M6P receptor levels alters the expression profiles of various genes and proteins related to APP processing, Aβ levels and toxicity, as well as molecules regulating EL system function and cholesterol metabolism in AD pathology. At the cellular level, IGF-II/M6P receptor overexpression triggers altered subcellular localization of APP and its processing enzyme β-site APP cleaving enzyme 1, an increase of lipid-raft components, and the redistribution of APP within the raft domain. The increased Aβ production resulting from this enhanced APP processing subsequently renders these cells more susceptible to staurosporine-induced cytotoxicity. Finally, we reveal that an increase in levels, activity and cytosolic release of cathepsin D is associated with Aβ-induced toxicity via both caspase-dependent and caspase-independent pathways in mouse cortical cultured neurons, and that these effects can be prevented with the cathepsin D inhibitor, pepstatin A. We further confirm that the aforementioned changes involving cathepsin D are associated with the affected frontal cortex but not the relatively spared cerebellar region of mutant APP transgenic mice overexpressing Aβ peptide (i.e. 5xFAD mice) and post-mortem AD brains. However, the cellular levels of the IGF-II/M6P receptor generally remain unchanged in the frontal cortex and cerebellum of 5xFAD mice and AD brains. Collectively, these results suggest that an elevation in IGF-II/M6P receptor levels can influence the development of AD by regulating APP processing and Aβ metabolism. In addition, the cytosolic release of cathepsin D can play important role in the neurodegeneration in AD pathology. However, it is critical to verify the effects of the IGF-II/M6P receptor on APP processing and Aβ production in human neuronal cell cultures in future studies. Likewise, it is well worth further exploring the potential role of cathepsin D in determining neuronal vulnerability, as well as the possibility of its inhibition as a therapeutic strategy for the treatment of AD pathology.

• Subjects / Keywords

  • Β-amyloid
  • Lysosome
  • Amyloid precursor protein
  • Cation-independent Mannose 6-phosphate receptor
  • Insulin-like growth factor II receptor
  • Cathepsin D
  • Endosomal– lysosomal system
  • Alzheimer's disease

• Graduation date

  Spring 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3BG2HR4J

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.