Research Article

Noncanonical transnitrosylation network contributes to synapse loss in Alzheimer’s disease

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Science  15 Jan 2021:
Vol. 371, Issue 6526, eaaw0843
DOI: 10.1126/science.aaw0843

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A cascade of NO in Alzheimer's disease

One of the ill effects of the amyloid-β peptide that accumulates in Alzheimer's disease (AD) is the promotion of the production of nitric oxide (NO) and consequent nitrosylation of thiols in proteins such as dynamin-related protein 1 (Drp1), which can lead to loss of neuronal synapses. Nakamura et al. found that this S-nitrosylation occurs in an unusual way. They detected a series of transnitrosylation events in which an NO group is passed between at least three proteins. The deubiquinating enzyme Uch-L1 was S-nitrosylated in brains from human AD patients or in mouse models of AD. Uch-L1 could lead to S-nitrosylation of Drp1 after transferring the NO group to another enzyme, Cdk5 (cyclin-dependent kinase 5). The results implicate a mechanism in which unrelated enzymes might aberrantly function together to disrupt brain function.

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Structured Abstract

INTRODUCTION

Contributing to the pathogenesis of Alzheimer’s disease (AD), oligomerized amyloid-β peptide (Aβ), neuronal hyperexcitability, and aging-associated neuroinflammation trigger the production of excessive nitric oxide (NO) with resultant aberrant S-nitrosylation of multiple proteins. In this chemical redox reaction, NO [possibly in the form of nitrosonium cation (NO+)] reacts with cysteine thiol [or, more precisely, thiolate anion (R-S, where R is a protein)]. The reaction mechanism for transnitros(yl)ation (transfer of an NO+ group from one protein to another) involves thiolate anion, as a nucleophile, performing a reversible nucleophilic attack on a nitroso nitrogen to form an SNO-protein adduct. Such reactions are known to contribute to synaptic damage in AD. For example, S-nitrosylation of the mitochondrial fission enzyme Drp1 (forming SNO-Drp1) stimulates its activity, leading to excessive mitochondrial fragmentation and bioenergetic compromise, with consequent synapse loss. Because synapse loss is closely correlated with cognitive decline in AD, this nitrosylation reaction represents an important potential contributor to disease pathogenesis. Here we explore the origin of formation of SNO-Drp1, revealing a previously unknown network of transnitrosylation reactions in AD, culminating in synaptic damage and dementia.

RATIONALE

In general, biochemical reactions are thought to develop through evolutionary selection pressure that gives an advantage to the organism. However, in the case of S-nitrosylation of Drp1, this reaction is clearly deleterious. Although prior studies in our laboratory have demonstrated that S-nitrosylated Cdk5 is upstream from Drp1, transferring NO to Drp1 to form SNO-Drp1 via a reaction termed transnitrosylation, additional members of this pathological cascade are yet to be determined. Characterizing this aberrant series of NO-transfer reactions is important to assess its contribution to AD pathogenesis and to examine its potential therapeutic implications.

RESULTS

Using both in vitro and in vivo models of AD plus postmortem brains of humans with AD, we found that enzymes with disparate catalytic activities from different biochemical pathways can form an alternative biochemical network consisting of a concerted transnitrosylation cascade that contributes to synapse loss in AD. We demonstrate that the deubiquitinylating enzyme, Uch-L1, contributes to this transnitrosylation cascade, leading to transfer of an NO group from Uch-L1 to Cdk5 to Drp1. We show that these reactions are kinetically and thermodynamically favored. Moreover, we develop a quantitative method based on the Nernst equation for thermodynamic assessment of the reactions at steady state, as might be expected to occur in a chronic disease. This analysis revealed Gibbs free energies that predict the spontaneous forward reaction of the transnitrosylation cascade. Although other potential members of the cascade remain to be determined, this work shows that noncanonical pathways mediating a series of aberrant transnitrosylation reactions can contribute to the pathogenesis of neurodegenerative disorders. One reason that this second noncanonical function of transnitrosylation may occur is that the catalytic sites of many types of enzymes contain cysteine residues, and their thiol groups (thiolates) are nucleophiles that can perform a reversible nucleophilic attack on the nitroso nitrogen via an S-nitrosylation reaction scheme.

CONCLUSION

We conclude that enzymes with distinct primary reaction mechanisms can form a completely separate network for protein transnitrosylation. Because this network operates in the aging-associated postreproductive period that is fraught with neurodegenerative disorders, natural selection pressure may be lessened on this aberrant alternative activity. Moreover, aberrant redox signaling pathways, as exemplified by NO transfer from Uch-L1 to Cdk5 to Drp1, may represent a potential therapeutic target for AD and possibly other diseases associated with nitrosative stress.

Concerted transnitrosylation reactions in the pathophysiology of synapse loss in AD.

Oligomerized Aβ, neuronal hyperexcitability, and aging-associated neuroinflammation trigger NO production via neuronal or inducible NO synthase (nNOS or iNOS), resulting in S-nitrosylated Uch-L1 (SNO-Uch-L1). Transnitrosylation from Uch-L1 to Cdk5 to Drp1 results in aberrant SNO-Drp1 formation, causing excessive stimulation of Drp1 and consequent mitochondrial fragmentation. The resulting bioenergetic failure contributes to synaptic damage underlying cognitive decline. eNMDAR, extrasynaptic N-methyl-d-aspartate–type glutamate receptor.

Abstract

Here we describe mechanistically distinct enzymes (a kinase, a guanosine triphosphatase, and a ubiquitin protein hydrolase) that function in disparate biochemical pathways and can also act in concert to mediate a series of redox reactions. Each enzyme manifests a second, noncanonical function—transnitrosylation—that triggers a pathological biochemical cascade in mouse models and in humans with Alzheimer’s disease (AD). The resulting series of transnitrosylation reactions contributes to synapse loss, the major pathological correlate to cognitive decline in AD. We conclude that enzymes with distinct primary reaction mechanisms can form a completely separate network for aberrant transnitrosylation. This network operates in the postreproductive period, so natural selection against such abnormal activity may be decreased.

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