TLR4 antagonists as a therapeutic treatment for APOE-modulated neuroinflammation in vivo Increased levels of the peptide amyloid beta (Abeta), particularly as soluble oligomeric Abeta42 (oAbeta), are considered the proximal cause of Alzheimer Disease (AD). oAbeta can induce direct neurotoxicity, and both oAbeta and insoluble plaques containing Abeta42 can induce indirect neurotoxicity via pro-inflammatory cytokine release from glia. In addition, a prolonged pro-inflammatory response may result in decreased phagocytic clearance of Abeta by glia. Toll like receptor 4 (TLR4) is fundamental for innate immunity, is expressed by neurons and glia, is activated by oAbeta, and therefore may a significant role in the neuroinflammation characteristic of AD. However, in vitro and in vivo data are conflicting as to whether inhibiting TLR4 is beneficial or detrimental for AD pathogenesis. Overall concern centers on whether TLR4 inhibition will prevent A?-induced neuroinflammation and reduce Abeta levels or increase Abeta levels and neuroinflammation. However, there currently are no data on the efficacy of TLR4 antagonists as a therapeutic strategy for neuroinflammation. The primary genetic risk factor for AD is inheritance of the APOE4 gene for apolipoprotein E (apoE), increasing the risk approximately 4- and 15-fold with a single or double allele, compared to APOE3. APOE4 may increase AD risk through mechanisms involving Abeta accumulation and neuroinflammation, both modulated by TLR4 antagonism. Our preliminary data demonstrate that LPS-induced neuroinflammation is increased with APOE4 compared to APOE3 in vivo and in vitro. In vitro, the induction of cytokine secretion by oAbeta is greater with apoE4 than apoE3, a response inhibited by TLR4-antagonism. However, the in vivo effects of TLR4 inhibition are unknown in a transgenic mouse model expressing familial AD mutations (FAD-Tg). In this proposal we will utilize novel TLR4 antagonists and identify the most efficacious candidate in vitro (Aim 1) to test in vivo (Aim 2). Because of the increased prevalence of APOE4 in AD patients and their differential response to therapeutic interventions compared to APOE3, it is critical to test therapeutics in FAD-Tg mice that also expresses human APOE. We have developed this new preclinical model, the novel EFAD-Tg mice, and demonstrate that compared to E3FAD, E4FAD mice have a greater cognitive impairment, decreased synaptic plasticity, and increased neuroinflammation and Abeta pathology, importantly, an increase in soluble oAbeta. Therefore, Aim 2 will test our hypothesis that TLR4 antagonists inhibit Abeta-meditated APOE-modulated neuroinflammation using EFAD mice treated with a TLR4 antagonist in both prevention (4-6 months) and treatment (6-7 months) paradigms, and compared to a TLR4 agonist. Overall this first study, will provide evidence for a novel mechanism (TLR4) underlying both Abeta-induced neuroinflammation and APOE4-induced AD risk. If TLR4 inhibition, which has not received sufficient attention for CNS conditions, is a valid target, the pharmacokinetic, pharmacodynamic, target engagement and efficacy read-outs identified both in vitro and in EFAD mice will enable future hit-finding or hit-o-lead TLR4 drug discovery projects for AD.
Alzheimer's disease (AD) is the leading cause of dementia in the elderly and a rapidly growing epidemic in the United States that currently costs $100 BILLION annually for the care for AD patients (estimated by the National Institute of Aging), and currently, there is no cure for AD as available therapy consists of limited symptomatic relief. APOE4 is the greatest genetic risk factor for AD, and APOE4 carriers also respond differentially to therapeutics, often negatively, often leading to their omission from clinical trials. Thus, in tis proposal, we will determine whether therapeutics that target a novel AD-relevant pathway will benefit AD patients, and whether they will be particularly effective for APOE4 carriers, utilizing novel transgenic mouse model.
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