NMNAT2:HSP90 Complex Mediates Proteostasis in Proteinopathies

Abstract

Nicotinamide mononucleotide adenylyl transferase 2 (NMNAT2) is neuroprotective in numerous preclinical models of neurodegeneration. Here, we show that brain nmnat2 mRNA levels correlate positively with global cognitive function and negatively with AD pathology. In AD brains, NMNAT2 mRNA and protein levels are reduced. NMNAT2 shifts its solubility and colocalizes with aggregated Tau in AD brains, similar to chaperones, which aid in the clearance or refolding of misfolded proteins. Investigating the mechanism of this observation, we discover a novel chaperone function of NMNAT2, independent from its enzymatic activity. NMNAT2 complexes with heat shock protein 90 (HSP90) to refold aggregated protein substrates. NMNAT2’s refoldase activity requires a unique C-terminal ATP site, activated in the presence of HSP90. Furthermore, deleting NMNAT2 function increases the vulnerability of cortical neurons to proteotoxic stress and excitotoxicity. Interestingly, NMNAT2 acts as a chaperone to reduce proteotoxic stress, while its enzymatic activity protects neurons from excitotoxicity. Taken together, our data indicate that NMNAT2 exerts its chaperone or enzymatic function in a context-dependent manner to maintain neuronal health.

Loss of Nardilysin, a Mitochondrial Co-chaperone for α-Ketoglutarate Dehydrogenase, Promotes mTORC1 Activation and Neurodegeneration

“…Indeed, overexpression of dNrd1 in S2 cells significantly increases DOGDH-Rluc activity (Figure 5G), suggesting that NRD1 may help fold or stabilize OGDH. To test whether NRD1 is capable of protecting OGDH from stress (holdase activity) or refolding denatured OGDH (foldase activity) (Ali et al, 2016). we incubated S2 cells in which DOGDH-Rluc is expressed with or without DRND1 at 42°C for 15 min, and retur ned the cells to 21°C for 3 hrs.…”

PAM forms an atypical SCF ubiquitin ligase complex that ubiquitinates and degrades NMNAT2

“…NAD is an essential co-enzyme for several biological process, such as cell death, energy metabolism and calcium mobilization. NMNAT2 is highly expressed in the brain, and along with axon protection following injury (44)(45)(46)(47)(48)(49) is implicated in neurodegenerative disease (50)(51)(52)(53). While functional genetics indicate PHR proteins can inhibit NMNAT2, the biochemical mechanism by which this occurs remains untested.…”

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

“…Our observation also contrasts previous research demonstrating that injection of NMNAT2 but not NMNAT3 siRNA to superior cervical ganglion neurons caused neurite degeneration when examined 72 h posttransfection 39. These differences may be attributed to the timing of observation, type of neuron, and duration of siRNA exposure, given that neuronal degeneration and processes fragmentation in our cultures were not observed until 6 days postinfection, and viral‐mediated transduction of shRNA produces a more wide spread and long‐lasting level of siRNA expression when compared to direct siRNA injection 40. Although we cannot conclusively exclude the possibility that neuronal exposure and infection with our shNMNAT3 viral vector negatively affect other survival genes, we did not observe changes in the endogenous expression of NMNAT1 and NMNAT2 (Fig.…”

“…Interestingly, increases in the expression of NMNAT prevented SARM1‐dependent NAD+ depletion without increasing NAD+ production 54. In addition, NMNATs have been recently shown to have chaperon function in animal models of taupathies 40, 55. The above findings suggest that NMNATs have a broader and more complex neuroprotective function and therefore, targeting the mechanisms that affect the endogenous expression of these proteins may offer novel and more wide spread neurotherapeutic approaches.…”