Interactions between excitotoxicity and lysosomal inhibition: Implications for Alzheimer's disease pathogenesis

grantor: University of TorontoThe peptide ß-amyloid (Aß) is believed to be the crucial element in Alzheimer's disease (AD) pathogenesis, being terminally responsible for neuronal death and ensuing dementia. Lysosomal inhibition alters the processing of the ß-amyloid precursor protein (ßAPP), from which Aß is derived, such that potentially amyloidogenic fragments (fßAPP) accumulate intracellularly, largely within lysosomes. Although it has been hypothesized that the lysosomal dysfunction accompanying normal aging (and modeled with lysosomal inhibitors) may be connected to AD, the consequences of this manipulation on the CNS are not known, nor is the potential importance of intracellular Aß (iAß) understood. Hence, monkeys, rats and ßAPP transgenic mice received direct brain infusions (i.c.v.) of the lysosomal inhibitors leupeptin or chloroquine (CHL) for up to 2 months. Both chemicals caused an increase in iAß, but did not cause significant neuronal death. Both also caused a strong astrocytic response, and CHL caused blood-brain barrier (BBB) dysfunction. These changes do not resemble major aspects of AD pathology. It has been shown that AD is associated with neurotrauma, such as head injury, and that excitotoxic processes cause similar degenerative changes. Also, Aß renders cultured neurons vulnerable to excitotoxic damage. Therefore, if dysfunctional lysosomes and iAß represent a risk factor for AD, excitotoxins such as kainic acid (KA) may combine synergistically with lysosomal inhibitors. Hence, CHL (over a range of systemic doses and i.c.v.; 1 week, mice and rats) was combined with KA followed by a 1 week survival period; multiple behavioral and histological analyses were conducted at several time points. Accumulation of iAß was evident at high doses, as was astrocytic activation. No deficits in sensorimotor ability (balance, muscle strength, vision, exploratory behavior) were detected. However, animals given KA and doses of CHL sufficient to cause iAß accumulation were markedly impaired on the Morris watermaze test of spatial learning and exhibited increased limbic system damage, particularly in the amygdala. These results indicate that although iAß may not be immediately harmful, it may nevertheless be coupled to a pathogenic process relevant to the development of AD and should be further examined in the context of the development of appropriate treatment strategies.Ph.D