Molecular chaperones protect against JNK- and Nmnat-regulated axon degeneration in Drosophila

Abstract

SummaryAxon degeneration is observed at the early stages of many neurodegenerative conditions and this often leads to subsequent neuronal loss. We previously showed that inactivating the c-Jun N-terminal kinase (JNK) pathway leads to axon degeneration in Drosophila mushroom body (MB) neurons. To understand this process, we screened candidate suppressor genes and found that the Wallerian degeneration slow (Wld S ) protein blocked JNK axonal degeneration. Although the nicotinamide mononucleotide adenylyltransferase (Nmnat1) portion of Wld S is required, we found that its nicotinamide adenine dinucleotide (NAD + ) enzyme activity and the Wld S N-terminus (N70) are dispensable, unlike axotomy models of neurodegeneration. We suggest that Wld S -Nmnat protects against axonal degeneration through chaperone activity. Furthermore, ectopically expressed heat shock proteins (Hsp26 and Hsp70) also protected against JNK and Nmnat degeneration phenotypes. These results suggest that molecular chaperones are key in JNK-and Nmnat-regulated axonal protective functions.

NMNAT: It’s an NAD + synthase… It’s a chaperone… It’s a neuroprotector

“…Further supporting the chaperone function of NMNAT is the identification of its endogenous ‘client’ protein in Drosophila synapses, where Nmnat maintains active zone structure by directly interacting with the active zone protein, Bruchpilot (BRP) in an activity-dependent manner [28]. Perhaps a source of resistance to this hypothesis is that although enzyme-inactive mouse Nmnat proteins fail to protect axons [31–33], enzyme-inactive Drosophila Nmnat is sufficient to protect against activity-induced retinal degeneration, Wallerian degeneration, and axonal degeneration induced by loss of JNK (c-Jun N-terminal kinase); in these conditions NMNAT chaperone-dependent protection is perhaps less implicit since here the client for such a chaperone role is not apparent [34–36]. Continued advances in NMNAT overexpression-mediated protection in neurodegenerative models will be invaluable for identifying not only the underlying mechanisms of NMNAT’s protection, but also by extension, the primary triggers of neurodegeneration.…”

NMNATs, evolutionarily conserved neuronal maintenance factors

“…Enzyme-independent functions of NMNATs were first explored after the surprising observation that an enzymatically inactive form of d NMNAT protected photoreceptors from degeneration in Drosophila [28]. Subsequent studies provided additional support that NMNATs can act as chaperones [13, 28, 47, 48]. …”

“…However, when mitochondria were genetically ablated from axons, d NMNAT still provides neuroprotection, suggesting the presence of additional, mitochondria-independent mechanisms of d NMNAT-mediated axonal protection [61]. Furthermore, the NAD synthesizing activity has been shown to be dispensable for d NMNAT and Wld S -mediated protection during axonal degeneration induced by inactivation of the c-Jun N-terminal kinase (JNK) pathway in Drosophila mushroom body (MB) neurons [48]. …”

Characterization of Leber Congenital Amaurosis-associated NMNAT1 Mutants

“…Moreover, NMNAT enzymatic activity is essential and sufficient for axonal protection (14). In contrast, studies in Drosophila show that dNMNAT chaperone activity, but not enzymatic activity, is required to prevent degeneration of photoreceptors (15) or axons of mushroom body neurons after JNK inactivation (16). In this study, we used multiple assays to characterize a series of NMNAT1 mutants associated with LCA to gain insights into how these mutations lead to retinal degeneration.…”