One of the major challenges for the U.S. Department of Veterans Affairs is to extend the health-span of the veterans and their families as their physical and/or cognitive performance capabilities decline with age. Human neurodegenerative protein misfolding disorders or proteinopathies, are associated with abnormal protein depositions in brain neurons. They include polyglutamine (polyQ) disorders such as Huntington's disease and ?-synucleinopathies such as Parkinson's disease. Disclosing the basic molecular and metabolic alterations that occur during aging of post-mitotic cells such as neurons, under proteotoxic stress is crucial for understanding the etiology of neuro-proteinopathies. Metabolic and mitochondrial alterations are hallmarks of aging and neurodegeneration. Over the last decade, we and others have shown that enhancement of mitogenesis or overexpression of NMNAT/NMA1, an enzyme in the NAD+ biosynthetic salvage pathway, act as powerful suppressors of proteotoxicities in yeast, fly and mouse models. Although the mechanisms involved remain to be fully understood, our preliminary data suggest that the two mechanisms are independent and that NMNAT could act as a chaperone to promote clearance of misfolded proteins. Recent screens in yeast models in our lab allowed us to identify three additional enzymes of the NAD+ biosynthetic salvage pathway that perform similar functions in protection against proteotoxic stress: NADS/QNS1, NaPTRase/NPT1 and NDase/PNC1. These observations suggest the existence of an evolutionarily conserved strategy of `repurposing' (or `moonlighting') housekeeping enzymes under stress conditions, and further reveal the intricate balance of metabolic activity and stress response in neurons. The main objective of this proposal is to investigate the cytoprotective roles of NAD+ biosynthetic enzymes and mitochondria in normal and proteotoxic environments. We will test the hypothesis that NAD+ biosynthetic enzymes are a new class of chaperones that perform this function independently of their catalytic activities and without requiring mitochondrial functions. Furthermore, we will test whether neuroprotection offered by enhanced mitochondrial biogenesis is additive or synergistic to protection exerted by the chaperone function of NAD+ biosynthetic enzymes. These hypotheses will be investigated in yeast models of polyQ toxicity during yeast chronological lifespan (CLS), a model of neuronal aging. Furthermore, the results obtained will be validated in human striatal cells differentiated from induced pluripotent stem cells (iPSCs) derived from human fibroblasts.
Three aims will be pursued: First, we will establish the effectiveness of NAD+ biosynthetic proteins in cellular aging and protection against proteotoxicity. Second, we will perform structure-function correlation studies to define the domains in NAD+ biosynthetic proteins required for chaperone or enzymatic activity and their requirement for protection against proteotoxicity. Third, we will characterize the mechanism by which NAD+ biosynthetic proteins promote the clearance of misfolded/oligomerized proteins and its crosstalk with mitochondrial mechanisms of protection. Identifying and characterizing independent yet synergistic pathways of neuroprotection will reveal the complex network for neuroprotection and the intricate relationship between metabolism and neurodegeneration. The proposed research may lead to novel therapeutic approaches to modulate these pathways to counteract cellular toxicities and extend health-span. Finally, the ability to control stress- resistance mechanisms such as those against proteotoxic stress may provide molecular targets and tools to treat the Veterans and the general population to enhance their physical and cognitive performance and postpone their progressive deterioration with age.
Metabolic abnormalities and mitochondrial impairment have long and extensively been documented in patients suffering from neurodegenerative proteinopathies and are targets for neuroprotection. The goal of this application is to test the unconventional hypothesis that NAD+ biosynthetic enzymes possess additional functions as cytoprotective chaperones in the context of proteotoxic stress and cellular aging. We aim to understand the mechanism involved and reveal additive or synergistic interaction with neuroprotective mechanisms based on the enhancement of mitochondrial biogenesis. We believe the proposed studies are germane to the important mission of the VA. Understanding the fundamental molecular and metabolic mechanisms that offer protection against protein cytotoxicity is crucial for comprehending the etiology of age- related neurodegenerative diseases, prevalent in the Veterans and their families, and a prerequisite to devise therapeutic interventions aiming to neuroprotection, the ultimate goal of our project.
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