Alzheimer’s disease (AD) is a devastating neurodegenerative disease that affects millions of peoples’ lives worldwide. While the consequences of AD are recognizable, the etiology is unclear. Gene-environment interactions have been implicated in the development of the disease, and exposure to organophosphorus (OPs) compounds is one of the environmental factors associated with AD. Evidence links exposure to levels of OPs encountered in agriculture, horticulture, and other work places with neurodegenerative disease, psychiatric illness, and sensorimotor deficits. Unfortunately, the mechanisms underlying these effects have yet to be established. Here, we set out to examine the long-term consequences of exposure to a commonly applied OP insecticide, chlorpyrifos (CPF), in an attempt to identify a causal link between occupational exposures and chronic illnesses. We exposed a transgenic rodent model of AD, TgF344-AD, to levels of CPF representing occupational exposures and examined ensuing behaviors and neuropathologies. We observed a sex-specific, biphasic response in CPF-exposed animals, including acute neurotoxicities, followed by intermediate recovery, and finally, chronic cognitive impairments. CPF exposure exacerbated neuronal damage in brain regions critical to the impaired behaviors, and neuroinflammatory pathways were identified as facilitators of this damage. This work emphasizes the long-term consequences of early life repeated exposures to OPs and identifies dysregulated microglia as a potential deleterious modifier of disease.
Additionally, we investigated the efficacy of a neuroprotective compound, (-)-P7C3-S243 in TgF344-AD rats. P7C3 compounds exert protection by preventing young hippocampal neurons from dying prematurely and also enhancing flux of nicotinamide adenine dinucleotide (NAD), thereby aiding in neuron survival under conditions that normally cause axon degeneration and cell death. These compounds have proven effective in preclinical models of Parkinson’s disease, amyotrophic lateral sclerosis, and traumatic brain injury. Thus, we sought to investigate the neuroprotective efficacy of P7C3 compounds in AD, as well. (-)-P7C3-S243 was administered to wild-type and transgenic male and female rats daily for 9 and 18 months, and classic hallmarks of the disease were assessed. Transgenic rats developed a spectrum of AD pathologies and behaviors, as expected, and (-)-P7C3-S243 ameliorated early depression-like behaviors, late learning and memory deficits, and progressive neuronal damage in this model, without influencing amyloid plaque deposition, tauopathies, or neuroinflammation. This data suggests that targeting neuronal cell death pathways is a promising treatment strategy in AD.
Taken together, the research presented here expands our current understanding of pathways of regulation in Alzheimer’s disease—organophosphates are capable of exacerbating the severity of AD, while P7C3 compounds are promising therapeutic candidates for neuronal death in the disease. Given the overlapping molecular pathways of modulation in CPF-induced toxicity and (-)-P7C3-S243 neuroprotection in AD, future studies will investigate the efficacy of (-)-P7C3-S243 in cognitive deficits induced by CPF exposure. Ultimately, this body of work highlights the plasticity of neuronal cell death and cognitive impairment in AD, thus indicating a better understanding of these pathways could facilitate vastly improved intervention strategies in Alzheimer’s disease.