Nicotinamide Mononucleotide Adenylyltransferase Is a Stress Response Protein Regulated by the Heat Shock Factor/Hypoxia-inducible Factor 1α Pathway

Abstract

Stress responses are cellular processes essential for maintenance of cellular integrity and defense against environmental and intracellular insults. Neurodegenerative conditions are linked with inadequate stress responses. Several stress-responsive genes encoding neuroprotective proteins have been identified, and among them, the heat shock proteins comprise an important group of molecular chaperones that have neuroprotective functions. However, evidence for other critical stressresponsive genes is lacking. Recent studies on the NAD synthesis enzyme nicotinamide mononucleotide adenylyltransferase (NMNAT) have uncovered a novel neuronal maintenance and protective function against activity-, injury-, or misfolded protein-induced degeneration in Drosophila and in mammalian neurons. Here, we show that NMNAT is also a novel stress response protein required for thermotolerance and mitigation of oxidative stress-induced shortened lifespan. NMNAT is transcriptionally regulated during various stress conditions including heat shock and hypoxia through heat shock factor (HSF) and hypoxia-inducible factor 1␣ in vivo. HSF binds to nmnat promoter and induces NMNAT expression under heat shock. In contrast, under hypoxia, HIF1␣ up-regulates NMNAT indirectly through the induction of HSF. Our studies provide an in vivo mechanism for transcriptional regulation of NMNAT under stress and establish an essential role for this neuroprotective factor in cellular stress response.When faced with abnormal conditions, such as heat, oxidative stress, hypoxia, or accumulation of aberrant proteins, cells implement a stress response program to protect themselves and ensure survival. Stress conditions can cause protein unfolding, misfolding, or aggregation, and the consequent inadequate response to stress can lead to developmental defects, shortened lifespan, and neurodegenerative conditions (for review, see Ref.2). The increased synthesis of molecular chaperone heat shock proteins (HSPs) 2 is central to the stress response because they function to prevent protein misfolding and aggregation to maintain protein homeostasis (1, 2). It is thought that elevated expression of HSPs is sufficient to protect cells from a wide range of cytotoxic conditions (1, 3, 4). However, it is unclear whether the stress response network includes essential genes other than HSPs. Although a few metabolic enzymes have been found to be up-regulated upon stress (5, 6), it is unclear whether metabolic enzymes are an integral part of the stress response network.Heat shock factors (HSFs) are the master stress transcription factors of heat shock response, with one HSF in invertebrates and multiple HSFs in plants and vertebrates (7-9). Through their roles in mediating transcriptional activation of HSP genes, HSFs function in maintaining protein homeostasis and integrating cellular response to stress and development (10). Upregulation of HSPs by HSF1 is triggered by a variety of acute and chronic stress conditions and disease states (11). When faced with other stress conditio...

The NAD+ Synthesis Enzyme Nicotinamide Mononucleotide Adenylyltransferase (NMNAT1) Regulates Ribosomal RNA Transcription

“…Recent studies suggest that NMNAT1 is a stress-responsive gene and is inducible by heat shock, hypoxia, and oxidative stress in Drosophila (24). Our results showed that DNA damage also significantly induces NMNAT1 expression, which reduces the level of cell death.…”

NMNAT3 is protective against the effects of neonatal cerebral hypoxia‐ischemia

“…In addition, we find that endogenous NMNAT3 levels are increased during the first 72 h following neonatal H‐I. This effect is congruent with previous observations demonstrating that NMNATs are inducible under conditions of anoxic cellular stress 21, 3221.…”

“…This effect is congruent with previous observations demonstrating that NMNATs are inducible under conditions of anoxic cellular stress 21, 3221. Interestingly, NMNAT1 and HIF α peripheral blood mRNA levels are also increased in people living at high altitudes suggesting that the expression of HIF α and NMNATs is also adaptive to low oxygen conditions in humans 33, 34.…”

“…Therefore, increases in the endogenous expression of NMNAT3 in response to neonatal cerebral hypoxia and ischemia may represent an internal mechanism by which the immature neuron attempts to protect itself from the degenerative effects brought on by excessive metabolic stress. This hypothesis is supported by the observation that NMNAT3 upregulation in the fly brain is capable of enhancing the life span of flies in response to oxidative stress 21. Similarly, NMNAT3 overexpression reduced the degeneration of DRG axons after exposure to exogenous toxic oxidants such as rotenone and hydrogen peroxide,30 and increases in Drosophila NMNAT prevented hypoxia‐induced degeneration of neuronal dendrites 20, 36.…”

“…Furthermore, endogenous NMNAT levels are known to increase in response to cellular stress including hypoxia indicating that NMNAT proteins are potentially adaptive molecules that may provide neuroprotective actions in response to brain injury 20, 21. Therefore, we investigated whether endogenous NMNAT3 levels in the immature mouse brain are affected by in vivo cerebral hypoxia and/or ischemia.…”