Quantitative analysis of synaptic pathology and neuroinflammation: an initial study in a female rhesus monkey model of the “synaptic” phase of Alzheimer’s disease

Background Soluble oligomers of the Aβ peptide (AβOs) are toxins that target and disrupt synapses. Generation of AβOs has been recently recognized as a probable initiating event in Alzheimer’s disease (AD), leading to cognitive impairment. There is a translational gap in AD studies, with promising drugs developed based on work in rodent models failing in AD patients in clinical trials. Additionally, although women have a two-fold greater lifetime risk of developing AD compared to men, females have not been a focus of preclinical studies. Thus, we sought to develop a model of AβO toxicity in female rhesus monkeys, to take advantage of the more highly differentiated cortical structure in this species as well as the similarities in the endocrine system between rhesus monkeys and humans. Methods Repeated intracerebroventricular (i.c.v) injections of AβOs were performed in adult female rhesus monkeys. Controls were unoperated aged matched monkeys. High-resolution confocal microscopy and morphometric analysis of Alexa 568 (A568) filled neurons were used to evaluate synaptic, neuronal, and glial markers in the dorsolateral prefrontal cortex (dlPFC) and hippocampus after AβO injections. Cerebrospinal fluid (CSF) and brain tissue were also collected and analyzed for biomarkers of AD pathology, including: phosphorylated Tau protein (pTau), total Tau, Aβ 1-42 , Aβ 1-40 and TNF-α levels. Results Here, we report that AβO injection into the lateral ventricle of the brain induces loss of 37% of thin spines in targeted dlPFC neurons, an area highly vulnerable in AD and aging. Further, AβOs associate with the synaptic marker PSD95, inducing loss of more than 60% of local excitatory synapses. AβOs induce a robust neuroinflammatory response in the hippocampus, far from the injection site, with numerous activated ameboid microglia and TNF-α release. Finally, AβOs increased CSF levels of Aβ 1-42 , pTau Ser396 and pTau Ser199, but not Aβ 1-40 or total Tau. Conclusions These initial findings from detailed quantitative analysis of effects of AβO administration on synapses in a female nonhuman primate model are a very promising step toward understanding the mechanism of early AD pathogenesis in the primate brain, and may help develop an effective disease-modifying therapy of high relevance to women’s health.