Axonal degeneration is blocked by nicotinamide mononucleotide adenylyltransferase (Nmnat) protein transduction into transected axons

Axon degeneration: molecular mechanisms of a self-destruction pathway

Axon degeneration is a characteristic event in many neurodegenerative conditions including stroke, glaucoma, and motor neuropathies. However, the molecular pathways that regulate this process remain unclear. Axon loss in chronic neurodegenerative diseases share many morphological features with those in acute injuries, and expression of the Wallerian degeneration slow (WldS) transgene delays nerve degeneration in both events, indicating a common mechanism of axonal self-destruction in traumatic injuries and degenerative diseases. A proposed model of axon degeneration is that nerve insults lead to impaired delivery or expression of a local axonal survival factor, which results in increased intra-axonal calcium levels and calcium-dependent cytoskeletal breakdown.

WldS but not Nmnat1 protects dopaminergic neurites from MPP+ neurotoxicity

BACKGROUND

The WldS mouse mutant ("Wallerian degeneration-slow") delays axonal degeneration in a variety of disorders including in vivo models of Parkinson's disease. The mechanisms underlying WldS -mediated axonal protection are unclear, although many studies have attributed WldS neuroprotection to the NAD+-synthesizing Nmnat1 portion of the fusion protein. Here, we used dissociated dopaminergic cultures to test the hypothesis that catalytically active Nmnat1 protects dopaminergic neurons from toxin-mediated axonal injury.

RESULTS

Using mutant mice and lentiviral transduction of dopaminergic neurons, the present findings demonstrate that WldS but not Nmnat1, Nmnat3, or cytoplasmically-targeted Nmnat1 protects dopamine axons from the parkinsonian mimetic N-methyl-4-phenylpyridinium (MPP+). Moreover, NAD+ synthesis is not required since enzymatically-inactive WldS still protects. In addition, NAD+ by itself is axonally protective and together with WldS is additive in the MPP+ model.

CONCLUSIONS

Our data suggest that NAD+ and WldS act through separate and possibly parallel mechanisms to protect dopamine axons. As MPP+ is thought to impair mitochondrial function, these results suggest that WldS might be involved in preserving mitochondrial health or maintaining cellular metabolism.

CREB-activity and nmnat2 transcription are down-regulated prior to neurodegeneration, while NMNAT2 over-expression is neuroprotective, in a mouse model of human tauopathy

Tauopathies, characterized by neurofibrillary tangles (NFTs) of phosphorylated tau proteins, are a group of neurodegenerative diseases, including frontotemporal dementia and both sporadic and familial Alzheimer's disease. Forebrain-specific over-expression of human tau(P301L), a mutation associated with frontotemporal dementia with parkinsonism linked to chromosome 17, in rTg4510 mice results in the formation of NFTs, learning and memory impairment and massive neuronal death. Here, we show that the mRNA and protein levels of NMNAT2 (nicotinamide mononucleotide adenylyltransferase 2), a recently identified survival factor for maintaining neuronal health in peripheral nerves, are reduced in rTg4510 mice prior to the onset of neurodegeneration or cognitive deficits. Two functional cAMP-response elements (CREs) were identified in the nmnat2 promoter region. Both the total amount of phospho-CRE binding protein (CREB) and the pCREB bound to nmnat2 CRE sites in the cortex and the hippocampus of rTg4510 mice are significantly reduced, suggesting that NMNAT2 is a direct target of CREB under physiological conditions and that tau(P301L) overexpression down-regulates CREB-mediated transcription. We found that over-expressing NMNAT2 or its homolog NMNAT1, but not NMNAT3, in rTg4510 hippocampi from 6 weeks of age using recombinant adeno-associated viral vectors significantly reduced neurodegeneration caused by tau(P301L) over-expression at 5 months of age. In summary, our studies strongly support a protective role of NMNAT2 in the mammalian central nervous system. Decreased endogenous NMNAT2 function caused by reduced CREB signaling during pathological insults may be one of underlying mechanisms for neuronal death in tauopathies.

Nicotinamide mononucleotide adenylyl transferase 1 protects against acute neurodegeneration in developing CNS by inhibiting excitotoxic-necrotic cell death

Hypoxic-ischemic (H-I) injury to the developing brain is a significant cause of morbidity and mortality in humans. Other than hypothermia, there is no effective treatment to prevent or lessen the consequences of neonatal H-I. Increased expression of the NAD synthesizing enzyme nicotinamide mononucleotide adenylyl transferase 1 (Nmnat1) has been shown to be neuroprotective against axonal injury in the peripheral nervous system. To investigate the neuroprotective role of Nmnat1 against acute neurodegeneration in the developing CNS, we exposed wild-type mice and mice overexpressing Nmnat1 in the cytoplasm (cytNmnat1-Tg mice) to a well-characterized model of neonatal H-I brain injury. As early as 6 h after H-I, cytNmnat1-Tg mice had strikingly less injury detected by MRI. CytNmnat1-Tg mice had markedly less injury in hippocampus, cortex, and striatum than wild-type mice as assessed by loss of tissue volume 7 d days after H-I. The dramatic protection mediated by cytNmnat1 is not mediated through modulating caspase3-dependent cell death in cytNmnat1-Tg brains. CytNmnat1 protected neuronal cell bodies and processes against NMDA-induced excitotoxicity, whereas caspase inhibition or B-cell lymphoma-extra large (Bcl-XL) protein overexpression had no protective effects in cultured cortical neurons. These results suggest that cytNmnat1 protects against neonatal HI-induced CNS injury by inhibiting excitotoxicity-induced, caspase-independent injury to neuronal processes and cell bodies. As such, the Nmnat1 protective pathway could be a useful therapeutic target for acute and chronic neurodegenerative insults mediated by excitotoxicity.

Reducing expression of NAD+ synthesizing enzyme NMNAT1 does not affect the rate of Wallerian degeneration

NAD(+) synthesizing enzyme NMNAT1 constitutes most of the sequence of neuroprotective protein Wld(S), which delays axon degeneration by 10-fold. NMNAT1 activity is necessary but not sufficient for Wld(S) neuroprotection in mice and 70 amino acids at the N-terminus of Wld(S), derived from polyubiquitination factor Ube4b, enhance axon protection by NMNAT1. NMNAT1 activity can confer neuroprotection when redistributed outside the nucleus or when highly overexpressed in vitro and partially in Drosophila. However, the role of endogenous NMNAT1 in normal axon maintenance and in Wallerian degeneration has not been elucidated yet. To address this question we disrupted the Nmnat1 locus by gene targeting. Homozygous Nmnat1 knockout mice do not survive to birth, indicating that extranuclear NMNAT isoforms cannot compensate for its loss. Heterozygous Nmnat1 knockout mice develop normally and do not show spontaneous neurodegeneration or axon pathology. Wallerian degeneration after sciatic nerve lesion is neither accelerated nor delayed in these mice, consistent with the proposal that other endogenous NMNAT isoforms play a principal role in Wallerian degeneration.