Neurodegeneration can be triggered by a variety of genetic, epigenetic, and environmental factors. Healthy neurons are able to maintain their integrity throughout the life of an organism, suggesting the existence of a maintenance mechanism that allows neurons to sustain, mitigate or even repair damage. Recently, we have identified a neuronal maintenance factor NMNAT in a forward genetic screen in Drosophila. Loss of nmnat causes rapid and severe neurodegeneration, whereas over-expression of NMNAT protein offers protection against neurodegeneration. These findings suggest that normal level of NMNAT maintains neuronal homeostasis, and increased level offers protection. NMNAT is a highly conserved housekeeping enzyme, and the neuroprotective function of NMNAT has also been implicated in a mouse model of slow Wallerian Degeneration. Currently, the detailed mechanisms of this maintenance function and the protective capability of NMNAT in mammalian neurons are unclear. Our preliminary experiments suggest that in addition to its NAD synthesis activity, NMNAT has a chaperone function that is involved in regulating protein misfolding and degradation. We hypothesize that like other chaperones, NMNAT is up-regulated under stress, reduces protein aggregation, and thus protects neurons from degenerative conditions. In the proposed research, we will characterize the biochemical and cellular mechanisms underlying the protective process mediated by NMNAT using both Drosophila and mammalian primary neuronal models.
In Specific Aim 1, we will use structure- function analysis to define the protein domains that are required for chaperone function, and characterize the transcriptional regulation of NMNAT under stress.
In Specific Aim 2, we will first determine the neuroprotective activity of mammalian NMNAT isoforms in primary neurons, and then characterize the role of NMNAT in reducing protein aggregation-induced neurotoxicity.
In Specific Aim 3, we will test whether NMNAT proteins can exert protective activity when their expression is induced after the onset of degeneration. For this last study, we will take advantage of the Drosophila genetic system and control the expression of NMNAT using a heat-inducible promoter. In summary, our proposed research in both Drosophila and mammalian model systems will help unmask the function of NMNAT and its regulation as a molecular chaperone, determine the neuroprotective properties of human NMNAT in primary neurons, and reveal the repair potential of NMNAT in neural regeneration after neuronal damage.
Neurodegenerative conditions are among the most intractable of diseases and therefore present an urgent need for developing effective treatments. Our studies on the neuronal maintenance process suggest that neurons have a self-defense system that can be augmented to protect against neurodegeneration. The experiments in this proposal will help us better understand the molecular events of protein folding and aggregation in neurodegenerative conditions, reveal potential neuroprotective mechanisms in mammalian neurons, and aid in the design of therapeutic treatments.
|Ruan, Kai; Zhu, Yi; Li, Chong et al. (2015) Alternative splicing of Drosophila Nmnat functions as a switch to enhance neuroprotection under stress. Nat Commun 6:10057|
|Ali, Yousuf O; Li-Kroeger, David; Bellen, Hugo J et al. (2013) NMNATs, evolutionarily conserved neuronal maintenance factors. Trends Neurosci 36:632-40|
|Zang, Shaoyun; Ali, Yousuf O; Ruan, Kai et al. (2013) Nicotinamide mononucleotide adenylyltransferase maintains active zone structure by stabilizing Bruchpilot. EMBO Rep 14:87-94|
|Wen, Yuhui; Zhai, R Grace; Kim, Michael D (2013) The role of autophagy in Nmnat-mediated protection against hypoxia-induced dendrite degeneration. Mol Cell Neurosci 52:140-51|
|Kitay, Brandon M; McCormack, Ryan; Wang, Yunfang et al. (2013) Mislocalization of neuronal mitochondria reveals regulation of Wallerian degeneration and NMNAT/WLD(S)-mediated axon protection independent of axonal mitochondria. Hum Mol Genet 22:1601-14|
|Ali, Yousuf O; Ruan, Kai; Zhai, R Grace (2012) NMNAT suppresses tau-induced neurodegeneration by promoting clearance of hyperphosphorylated tau oligomers in a Drosophila model of tauopathy. Hum Mol Genet 21:237-50|
|Ouyang, Hui; Ali, Yousuf O; Ravichandran, Mani et al. (2012) Protein aggregates are recruited to aggresome by histone deacetylase 6 via unanchored ubiquitin C termini. J Biol Chem 287:2317-27|
|Ljungberg, M Cecilia; Ali, Yousuf O; Zhu, Jie et al. (2012) CREB-activity and nmnat2 transcription are down-regulated prior to neurodegeneration, while NMNAT2 over-expression is neuroprotective, in a mouse model of human tauopathy. Hum Mol Genet 21:251-67|
|Ali, Yousuf O; Escala, Wilfredo; Ruan, Kai et al. (2011) Assaying locomotor, learning, and memory deficits in Drosophila models of neurodegeneration. J Vis Exp :|
|Ali, Yousuf O; McCormack, Ryan; Darr, Andrew et al. (2011) Nicotinamide mononucleotide adenylyltransferase is a stress response protein regulated by the heat shock factor/hypoxia-inducible factor 1alpha pathway. J Biol Chem 286:19089-99|
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