NMNATs, evolutionarily conserved neuronal maintenance factors

Abstract

Proper brain function requires neuronal homeostasis over a range of environmental challenges. Neuronal activity, injury, and aging stress the nervous system, and lead to neuronal dysfunction and degeneration. Nevertheless, most organisms maintain healthy neurons throughout life, implying the existence of active maintenance mechanisms. Recent studies have revealed a key neuronal maintenance and protective function for nicotinamide mononucleotide adenylyl transferases (NMNATs). We review evidence that NMNATs protect neurons through multiple mechanisms in different contexts, and highlight functions that either require or are independent of NMNAT catalytic activity. We then summarize data supporting a role for NMNATs in neuronal maintenance and raise intriguing questions on how NMNATs preserve neuronal integrity and facilitate proper neural function throughout life.

Keywords: NAD; NMNAT; axonopathy; chaperone; neurodegeneration; neuronal maintenance; proteinopathies; synapse.

Figure 1. Mammalian NMNATs have distinct subcellular localizations

NMNAT1 is a nuclear protein with a predicted nuclear localization signal (NLS) between Glu¹⁰⁷–Lys¹⁴⁶ [87]. NMNAT2 is localized to the Golgi apparatus via palmitoylation of Cys¹⁶⁴ and Cys¹⁶⁵ [4, 87]. In the mouse brain, NMNAT2 is enriched in numerous membrane compartments, including synaptic terminals and synaptic vesicles [4, 22]. NMNAT3 is predominantly localized in mitochondria and its first 25 residues encode a mitochondrial targeting sequence [50]. NAD⁺, the enzymatic product of NMNATs, is an essential cofactor for many metabolic processes, transcriptional regulation and several protein modification reactions. The NAD signaling pathway generates precursors of several intracellular calcium mobilizing agents including cADPR and NAADP. Abbreviations: ADPR, ADP-ribose; CAC, citric acid cycle; cADPR, cyclic ADP-ribose; NAADP, nicotinic acid adenine dinucleotide phosphate; NADase, bifunctional NAD glycohydrolase/ADP-ribosylcyclase; PARP, poly-ADP-ribose polymerase.

Figure 2. NMNAT interacts with Bruchpilot (BRP) to maintain active zone integrity in an activity-dependent manner

The T-bar is a dynamic structure undergoing continuous assembly and disassembly. Neural activity level affects the size of T-bars. Under normal conditions, NMNAT associates with BRP at the synapse and facilitates the assembly of T-bars. Neural activity enhances the NMNAT-BRP interaction to maintain T-bar integrity. When NMNAT is reduced below a critical threshold, BRP becomes ubiquitinated and aggregates, recruiting chaperones such as HSP70 and residual NMNAT, leading to a loss of T-bars at the synapse. Increased neuronal activity will aggravate this phenotype due to an increase in BRP aggregates.

Figure 3. Transcriptional regulation of NMNAT in Drosophila and mammals

Under normal conditions, an appropriate level of nmnat transcription is maintained. In Drosophila, there is one NMNAT gene, under the regulation of stress transcription factor HSF. During acute stress conditions, NMNAT is upregulated by direct binding of HSF to a HSE present in its promoter. In mice, NMNAT2 is involved in neuronal maintenance and its transcription is regulated by CREB. CREB can be activated by a variety of signaling cascades including activation through the AC–cAMP-PKA pathway and direct phosphorylation via activity-dependent increase in CaMKIV. AC activity can be triggered by calcium influx through NMDAR or voltage-gated calcium channels upon depolarization. However, in tauopathy, toxic Tau species (phosphorylated Tau oligomers) reduce synaptic NMDAR function, which could account for a reduction in pCREB levels. Alternatively, AKT activity and pGSK3 levels have also been shown to affect pCREB levels and these signaling cascades are disrupted in tauopathies. Abbreviations: AC, adenylyl cyclase; AKT, serine/threonine protein kinase AKT; CaMKIV, Ca²⁺/calmodulin-dependent protein kinase IV; cAMP, cyclic adenosine monophosphate; CRE, cAMP response element; CREB, cAMP response element-binding protein; FKBP5, FK506 binding protein 5; GSK3β, glycogen synthase kinase 3β; HSE, heat shock element; HSF, heat shock factor; HSP, heat shock proteins; NMDAR, N-methyl-D-aspartate receptor; PKA, protein kinase A.

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