Preventing Ca2+-mediated nitrosative stress in neurodegenerative diseases: possible pharmacological strategies

Abstract

Overactivation of the NMDA-subtype of glutamate receptor is known to trigger excessive calcium influx, contributing to neurodegenerative conditions. Such dysregulation of calcium signaling results in generation of excessive free radicals, including reactive oxygen and nitrogen species (ROS/RNS), including nitric oxide (NO). In turn, we and our colleagues have shown that these free radicals trigger pathological production of misfolded proteins, mitochondrial dysfunction, and apoptotic pathways in neuronal cells. Here, we discuss emerging evidence that excessive calcium-induced NO production can contribute to the accumulation of misfolded proteins, specifically by S-nitrosylation of the ubiquitin E3 ligase, parkin, and the chaperone enzyme for nascent protein folding, protein-disulfide isomerase. Additionally, excessive calcium-induced NO generation leads to the formation of S-nitrosylated dynamin-related protein 1, which causes abnormal mitochondrial fragmentation and resultant synaptic damage. In this review, we also discuss how two novel classes of pharmacological agents hold promise to interrupt these pathological processes. Firstly, the NMDA receptor antagonists, Memantine and NitroMemantine, block excessive extrasynaptic glutamate excitation while maintaining synaptic transmission, thereby limiting excessive calcium influx and production of ROS/RNS. Secondly, therapeutic pro-electrophiles are activated in the face of oxidative insult, thus protecting cells from calcium-induced oxidative stress via the Keap1/Nrf2 transcriptional pathway.

Fig. 1

Possible mechanisms whereby Ca²⁺ signaling contributes to NO generation in neurodegenerative conditions. Hyperactivation of NMDA receptors (NMDARs) by glutamate (Glu) and glycine (Gly) induces excessive Ca²⁺ influx and activation of neuronal NO synthase (nNOS). nNOS produces NO from l-arginine. In Alzheimer’s disease (AD), soluble oligomers of Aβ peptide, thought to be a key mediator in AD pathogenesis, can facilitate neuronal NO production in both NMDAR-dependent and -independent manners. In Parkinson’s disease (PD), mitochondrial dysfunction caused by pesticides or other environmental toxins can trigger NO production possibly via mitochondrial and NMDAR/Ca²⁺ influx pathways. Note that in addition to RNS, ROS are also produced in response to Aβ and pesticides

Fig. 2

Overstimulation of NMDA receptors (NMDARs) by glutamate (Glu) and glycine (Gly) induces excessive Ca²⁺ influx, activation of neuronal NO synthase (nNOS), and subsequent formation of SNO proteins. nNOS produces NO from l-arginine, and NO reacts with sulfhydryl groups to form S-nitrosylated proteins. Physiological levels of NO mediate neuroprotective effects, at least in part, by S-nitrosylating the NMDAR and caspases, thus inhibiting their activity. In contrast, we postulate that overproduction of NO can be neurotoxic via S-nitrosylation of Parkin (forming SNO-PARK), PDI (forming SNO-PDI), GAPDH, MMP-2/9, PrxII, and COX-2. S-Nitrosylated parkin and PDI contribute to neuronal cell injury by triggering accumulation of misfolded proteins. S-Nitrosylation of Drp1 (forming SNO-Drp1) causes excessive mitochondrial fragmentation in neurodegenerative conditions.

Fig. 3

Memantine and NitroMemantine preferentially block excessive extrasynaptic NMDA receptor activity, while relatively sparing synaptic receptors. (Top) Normal (physiological/synaptic) activity of the NMDAR is required for synaptic function and neuronal survival. Many NMDAR antagonists, such as MK-801, completely block receptor activity, including physiological synaptic activity, and thus result in severe side effects and clinical intolerability. (Bottom) Excessive activation of the NMDAR, predominantly at extrasynaptic sites, is thought to induce neuronal cell injury and death, and is associated with the accumulation of misfolded proteins. Memantine (Mem) and the newer NitroMemantine drugs (NitroMem) preferentially block excessive (pathological) extrasynaptic NMDA receptor activity, while relatively sparing normal (physiological) synaptic activity [14].