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Protein S-nitros(yl)ation (SNO) is a posttranslational modification involved in diverse processes in health and disease and can contribute to synaptic damage in Alzheimer’s disease (AD). To identify SNO proteins in AD brains, we used triaryl phosphine ( SNO TRAP) combined with mass spectrometry (MS). We detected 1449 SNO proteins with 2809 SNO sites, representing a wide range of S-nitrosylated proteins in 40 postmortem AD and non-AD human brains from patients of both sexes. Integrative protein ranking revealed the top 10 increased SNO proteins, including complement component 3 (C3), p62 (SQSTM1), and phospholipase D3. Increased levels of S-nitrosylated C3 were present in female over male AD brains. Mechanistically, we show that formation of SNO-C3 is dependent on falling β-estradiol levels, leading to increased synaptic phagocytosis and thus synapse loss and consequent cognitive decline. Collectively, we demonstrate robust alterations in the S-nitrosoproteome that contribute to AD pathogenesis in a sex-dependent manner.

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. Science , this issue p. eaaw0843