Protein Transnitrosylation Signaling Networks Contribute to Inflammaging and Neurodegenerative Disorders

Abstract

Significance: Physiological concentrations of nitric oxide (NO^•) and related reactive nitrogen species (RNS) mediate multiple signaling pathways in the nervous system. During inflammaging (chronic low-grade inflammation associated with aging) and in neurodegenerative diseases, excessive RNS contribute to synaptic and neuronal loss. "NO signaling" in both health and disease is largely mediated through protein S-nitrosylation (SNO), a redox-based posttranslational modification with "NO" (possibly in the form of nitrosonium cation [NO⁺]) reacting with cysteine thiol (or, more properly, thiolate anion [R-S^-]). Recent Advances: Emerging evidence suggests that S-nitrosylation occurs predominantly via transnitros(yl)ation. Mechanistically, the reaction involves thiolate anion, as a nucleophile, performing a reversible nucleophilic attack on a nitroso nitrogen to form an SNO-protein adduct. Prior studies identified transnitrosylation reactions between glyceraldehyde-3-phosphate dehydrogenase (GAPDH)-nuclear proteins, thioredoxin-caspase-3, and X-linked inhibitor of apoptosis (XIAP)-caspase-3. Recently, we discovered that enzymes previously thought to act in completely disparate biochemical pathways can transnitrosylate one another during inflammaging in an unexpected manner to mediate neurodegeneration. Accordingly, we reported a concerted tricomponent transnitrosylation network from Uch-L1-to-Cdk5-to-Drp1 that mediates synaptic damage in Alzheimer's disease. Critical Issues: Transnitrosylation represents a critical chemical mechanism for transduction of redox-mediated events to distinct subsets of proteins. Although thousands of thiol-containing proteins undergo S-nitrosylation, how transnitrosylation regulates a myriad of neuronal attributes is just now being uncovered. In this review, we highlight recent progress in the study of the chemical biology of transnitrosylation between proteins as a mechanism of disease. Future Directions: We discuss future areas of study of protein transnitrosylation that link our understanding of aging, inflammation, and neurodegenerative diseases. Antioxid. Redox Signal. 35, 531-550.

Keywords: S-nitrosylation; neurodegenerative diseases; nitric oxide; transnitrosylation.

FIG. 1.

Trx mediates protein transnitrosylation and denitrosylation reactions. (A) Trx that is oxidized or reduced at its active site catalyzes transnitrosylation reactions through the nonactive-site cysteine Cys62 or Cys73. Under nitrosative conditions, Trx is S-nitrosylated at Cys62 or Cys73, which in turn can serve as an “NO donor” to transnitrosylate a thiol/thiolate group in other proteins such as caspase-3 and Trx itself. (B) Trx can also catalyze the transnitrosylation and denitrosylation of the same protein, such as caspase-3, although the forward rate reaction for S-nitrosylation has been reported to predominate (102, 103). Trx-mediated denitrosylation of the active-site cysteine of caspase-3 promotes activation of caspase-3, and thus it is proapoptotic. Transnitrosylation of the same cysteine of caspase-3 by Trx (Cys73) is antiapoptotic because it attenuates caspase-3 activation. Trx, thioredoxin.

FIG. 2.

SNO-Caspase-mediated transnitrosylation of XIAP contributes to caspase-dependent cell death. Through direct binding to caspases, XIAP antagonizes caspase activity. XIAP also serves as a ubiquitin E3 ligase that targets caspases for proteasomal degradation. Several caspases are known to be S-nitrosylated under basal conditions, inhibiting their proapoptotic protease activity. However, during apoptotic cell death, transnitrosylation from constitutively S-nitrosylated caspases (e.g., caspase-3 and caspase-9) to XIAP occurs. The resulting formation of S-nitrosylated XIAP (forming SNO-XIAP) at a cysteine residue in the RING domain decreases XIAP ubiquitination activity. This diminishes caspase degradation, leading to an increase in caspase activity, thus promoting caspase-dependent neuronal cell death. SNO, S-nitrosylation; XIAP, X-linked inhibitor of apoptosis.

FIG. 3.

GAPDH-mediated transnitrosylation. S-Nitrosylation of GAPDH promotes its binding to Siah1, facilitating translocation into the nucleus as a SNO-GAPDH/Siah1 complex. Complex formation stabilizes Siah1 protein, thus enhancing degradation of its nuclear targets. SNO-GAPDH can also bind and activate p300/CBP in the nucleus. Moreover, SNO-GAPDH can mediate transnitrosylation of other nuclear proteins, such as SIRT1, HDAC2, and DNAPK, altering their function. DNAPK, DNA-activated protein kinase; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; HDAC2, histone deacetylase 2; SIRT1, sirtuin 1.

FIG. 4.

Schema of SNO-DJ-1 contributing to neuronal survival via transnitrosylation of PTEN. SNO-DJ-1 inhibits PTEN activity via transnitrosylation (forming SNO-PTEN). SNO-PTEN prevents PTEN from antagonizing the action of PI3K and thus activates the PI3K/Akt pathway. Akt, protein kinase B; PI3K, phosphoinositide 3-kinase; PTEN, phosphatase and tensin homologue.

FIG. 5.

Schematic modeling of multiple transnitrosylation steps from Uch-L1 to Cdk5 to Drp1 in the pathogenesis of AD. In AD, oligomerized Aβ, other misfolded proteins, aging, and neuroinflammation can all increase intracellular NO^• levels in neurons. The increase in NO^• occurs via extrasynaptic (e)NMDAR-mediated nNOS activation in neurons and iNOS activation in glia cells. Excessive elevation of NO^• causes aberrant mitochondrial fragmentation, resulting in bioenergetic compromise with consequent neuronal damage and synaptic loss. This loss of synapses contributes to cognitive decline in AD. The aberrant transnitrosylation pathway that triggers the loss of synapses involves transfer of NO⁺ from SNO-Uch-L1 to Cdk5 to Drp1. Accounting for the mitochondrial damage, the formation of SNO-Drp1 activates its GTPase activity and increases mitochondrial fragmentation. This multistep transnitrosylation cascade represents a non-canonical signaling pathway distinct from the known enzymatic functions of Uch-L1, Cdk5, and Drp1. Aβ, amyloid β; AD, Alzheimer's disease; Cdk, cyclin-dependent kinase; Drp1, dynamin-related protein 1; iNOS, inducible nitric oxide synthase; NMDAR, N-methyl-d-aspartate-type glutamate receptor; nNOS, neuronal nitric oxide synthase; NO^•, nitric oxide; Uch-L1, ubiquitin carboxy-terminal hydrolase L1.

FIG. 6.

Transnitrosylation in Escherichia coli and immune cells. (A) Schematic of Hcp-mediated transnitrosylation. Nitrate reductase (NarGHI) contributes to production of NO-related species in E. coli during anaerobic respiration using nitrate. The iron/sulfur cluster of Hcp may promotes NO^• oxidation to NO⁺, facilitating formation of an SNO at an Fe-coordinating cysteine residue. Transnitrosylation from SNO-Hcp to interacting proteins, such as OxyR, GAPDH, and LpdA, regulates cellular metabolism and nitrosative stress in E. coli. (B) S100A9-mediated transnitrosylation in myeloid cells. Inflammatory stimuli induce iNOS expression, followed by assembly of a protein complex consisting of iNOS, S100A8, and S100A9. iNOS-driven S-nitrosylation of S100A9 at Cys3 (forming SNO-S100A9) transfers NO-related species to target proteins recruited to the complex by S100A8. These target proteins include GAPDH, annexin V, ezrin, moesin, and vimentin. The transnitrosylation events result in dysregulation of the GAIT (IFN-γ-activated inhibitor of translation) complex, contributing to prolonged expression of inflammatory genes. Hcp, hybrid cluster protein; IFN, interferon.