The inside-out of Alzheimer’s disease

Abstract

Every three seconds someone is diagnosed with dementia, and Alzheimer’s disease (AD) constitutes most of these cases. In the absence of a cure for AD, testing hypotheses regarding disease onset and spread, and developing methods for early diagnosis and treatment of the disease remain an urgent priority. Researchers have made tremendous strides in research to understand the origin of AD, and we now know that the neurons in lateral entorhinal cortex (LEC) layer II (a small brain region in the temporal lobes) are at selective risk for neurodegeneration in patients going through transition stages to AD (referred to as mild cognitive impairment [MCI]), and during this transition stage as much as half of the cell population is lost. The disease progression in human AD has been well-characterized by biomarker studies to assess pathological hallmarks at various stages of the disease. Biomarkers in AD can be elucidated by cerebrospinal fluid (CSF) analysis (commonly used biomarkers include decreased amyloid-β (Aβ)42, increased total tau, and increased phosphorylated tau) or neuroimaging markers of disease, such as positron emission tomography (PET) revealing amyloid plaques and tau pathology. Despite these research efforts, we still do not know how to hinder cognitive decline in patients with AD, and one reason may be the poor translatability between preclinical models and patients. Before any new hypotheses and treatments can reach the clinic, they first need to be tested in preclinical models using basic research strategies. We therefore need robust preclinical models that can help us achieve improved comparability with the human condition, and thereby improved translatability into the clinic. Here we used a highly clinically comparable rodent model of AD, the 3xTg AD mouse, which contains human genetic mutations recapitulating neuropathology observed in patients. First, we developed and applied our modified microdialysis method for repeated, longitudinal in vivo CSF collection and characterized biomarker protein changes in mice along the entire AD disease progression (Paper II). We subsequently applied our optimized microdialysis method to administer repurposed drugs aimed at attenuating AD-related Aβ and tau neuropathology (Paper III). In another set of experiments, we overexpressed human tau pathology in LEC layer II in mice using a viral vector delivery system, and then monitored tau spread from LEC layer II to its projection targets in the hippocampus (Paper IV). Lastly, we chemogenetically silenced neurons in LEC layer II and monitored the effect on intraneuronal Aβ levels in LEC and in downstream hippocampus (paper V). Using our modified microdialysis protocol, we found that the concentrations of CSF Aβ and tau proteins in mouse models of AD are comparable to changes observed along the disease cascade in patients (Paper II). Repurposed drugs not only attenuated neuropathology at the molecular, but also at the functional level when administered in combination using our microdialysis protocol at various therapeutic time windows (Paper III). Moreover, we found that the presence and spread of tau from LEC layer II to the hippocampus increased with age and was affected by the presence of endogenous tau load (Paper IV). We also show a correlation between intraneuronal Aβ levels and neuronal activity, and that chemogenetic silencing of LEC layer II neurons led to reduced early intraneuronal Aβ in LEC and in the downstream hippocampus (Paper V). In summary, I have (i) characterized CSF biomarkers along the entire disease cascade in a mouse model of AD and have validated the translational value of this model to patients. (ii) My findings lend support to the application of repurposed drugs to attenuate AD neuropathology at various therapeutic time windows. (iii) I have shown that tau spread from its site of anatomical origin depends upon aging and the pre-existence of tau. (iv) Lastly, I have demonstrated that the activity of LEC layer II neurons affects early intraneuronal Aβ build-up. In line with previous research, I have shown the vulnerability of EC layer II for developing ADrelated neuropathology, obtained insights into the origins and mechanisms of neuropathological spread, and shown that experimental animal models and molecular techniques are invaluable tools for answering fundamental questions within the field. Ultimately, I have aimed to develop a springboard for future integration of basic research findings to the clinic and AD patients.