Protein S-Nitrosylation as a Therapeutic Target for Neurodegenerative Diseases

Abstract

At physiological levels, nitric oxide (NO) contributes to the maintenance of normal neuronal activity and survival, thus serving as an important regulatory mechanism in the central nervous system. By contrast, accumulating evidence suggests that exposure to environmental toxins or the normal aging process can trigger excessive production of reactive oxygen/nitrogen species (such as NO), contributing to the etiology of several neurodegenerative diseases. We highlight here protein S-nitrosylation, resulting from covalent attachment of an NO group to a cysteine thiol of the target protein, as a ubiquitous effector of NO signaling in both health and disease. We review our current understanding of this redox-dependent post-translational modification under neurodegenerative conditions, and evaluate how targeting dysregulated protein S-nitrosylation can lead to novel therapeutics.

Keywords: Alzheimer's disease; NitroMemantine; Parkinson's disease; denitrosylation; protein S-nitrosylation; transnitrosylation.

Figure 1

NO/S-nitrosylation signaling under physiological and pathological conditions in the central nervous system. Physiological activation of NMDA receptors localized at synapses triggers calcium influx through the NMDA receptor-associated ion channel and stimulates nNOS tethered to the NMDA receptor protein complex. Physiological (basal) levels of NO thus produced contribute to normal neuronal functions, e.g., via S-nitrosylation of NMDA receptors (SNO-NMDAR) to prevent their overactivation. Under pathological (neurodegenerative) conditions, excessive activation of extrasynaptic the NMDAR-nNOS pathway (or iNOS expression in glia cells) can lead to overproduction of NO. Under these conditions, excessive generation of ROS can also occur. These pathways cause aberrant SNO-protein formation, such as SNO-GAPDH and SNO-PDI, augmenting pathological processes; in some cases nitrosothiol formation is followed by reaction of the same cysteine residue with ROS to form –SOxH adducts (with × = 1–3).

Figure 2

Biochemical mechanisms of reversible protein S-nitrosylation. (1) Nitrosonium cation [NO⁺], potentially generated from ^•NO by metal ion acceptance of the electron, reacts with thiolate anion (R-S⁻) to generate R-SNO. Note that R-SNO denotes an S-nitrosylated protein (SNO-protein) or S-nitrosothiol (e.g., GSNO and S-nitrosocysteine). (2) Radical recombination of ^•NO with thiyl radical (RS^•) may also produce R-SNO. (3) Transnitrosylation (i.e., transfer of an NO group between two thiol groups). (4) Enzymatic denitrosylation of R-SNO by GSNOR or the Trx system counterbalances R-SNO formation.

Figure 3

NitroMemantine-mediated inhibition of hyperactivated NMDA receptors. NitroMemantine was synthesized by addition of a nitro group (-NO₂) to the memantine moiety. This allows NitroMemantine to antagonize excessively activated NMDA receptors via two sites of action: 1) the ion channel where memantine itself binds, and 2) an extracellular redox-sensitive cysteine thiol groups of the receptor where the nitro group reacts to inhibit NMDAR activity (forming –SNO or –SNO₂). Thus, in this scenario, NitroMemantine serves as an NO (or, more precisely, a nitro group) donor specifically targeting NMDA receptors. This figure also shows that the NMDA receptor is a heterodimer, composed of two GluN1 and two GluN2 subunits. Excessive concentrations of glycine (Gly) and glutamate (Glu), co-agonists of the receptor, can trigger pathological activation of the NMDA receptor.

Figure 4

CGP3466B exerts its neuroprotective effect via inhibition of SNO-GAPDH formation. S-Nitrosylation of GAPDH enhances its interaction with Siah1 (bearing a nuclear localization signal) and promotes its translocation to the nucleus, whereas SNO-GAPDH triggers cellular events leading to neurodegeneration. CGP3466B (TCH346 or Omigapil) potently and selectively blocks GAPDH S-nitrosylation and thereby Siah binding.