Alterations in neuronal metabolic pathways contribute to human cognitive aging and are exacerbated in Alzheimer's disease

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PROJECT SUMMARY To find therapies for altering the course of age-related diseases, experimental methods that discriminate between normal and pathological aging processes are needed. Nowhere is this need more urgent than in the quest for effective treatments for Alzheimer’s disease (AD). Despite decades of research, AD remains a debilitating, progressive, and ultimately fatal dementia with few and ineffective disease-modifying treatment options. The vast majority of cases (~95%) are sporadic, with no known cause aside from advanced age. As the population over age 65 grows, the burden of AD is destined to grow in lockstep. Avoiding this fate requires rethinking why existing efforts have been ineffective. Although AD is a disease of aged human brains, many AD studies fail to parse aging from disease. We have developed a system that lets us model aspects of human age in vitro: induced neurons (iNs). Once matured, neurons of the brain never again replicate their genome or divide. As such, post-mitotic neurons downregulate cell-cycle mediated metabolic pathways used for the synthesis of DNA building blocks, deoxyribonucleotides (dNTs) and become reliant on the salvage of ribonucleotides (rNTs). I asked if these fundamentally important homeostatic processes are maintained over the human lifespan or in AD. Multiple experimental approaches on iNs derived from 13 sporadic AD (sAD) patients against 13 cognitively normal, age-match individuals (CN), revealed dramatic differences in NT pools, with a significant increase in rNTs in CNs relative to young healthy iNs. This effect was even further exacerbated in AD iNs. I ask here if the dramatic increases in rNTs result in increased ribo-substitution in nuclear and mitochondrial DNA, thus promoting increased DNA damage and bioenergetic dysfunction in age and AD. Further, I observed significant differential expression of key cell cycle and metabolic genes such as CDK1 and RRM1, which I have confirmed at the protein level. Our proposal, for the first time, incorporates long noted pathological observations of chromosomal instability and presence of cell-cycle markers in AD into a testable model system that provides a mechanistic basis for the development of sporadic AD. In addition to the critical scientific advances this proposal aims to achieve, I will also contribute new powerful new tools and datasets to the fields of aging and AD. I have adapted our recently developed repair-seq technology to quantify ribo-substitution in aged and AD iNs. I will further map epigenetic drift over the human lifespan in neurons and quantify the neuronal capacity to generate new methyl groups for non-genetic regulation of transcription.