MPTP-induced parkinsonism in mice alters striatal and nigral xCT expression but is unaffected by the genetic loss of xCT

Implication of system xc- in neuroinflammation during the onset and maintenance of neuropathic pain

BACKGROUND

Despite the high prevalence of neuropathic pain, treating this neurological disease remains challenging, given the limited efficacy and numerous side effects associated with current therapies. The complexity in patient management is largely attributed to an incomplete understanding of the underlying pathological mechanisms. Central sensitization, that refers to the adaptation of the central nervous system to persistent inflammation and heightened excitatory transmission within pain pathways, stands as a significant contributor to persistent pain. Considering the role of the cystine/glutamate exchanger (also designated as system xc-) in modulating glutamate transmission and in supporting neuroinflammatory responses, we investigated the contribution of this exchanger in the development of neuropathic pain.

METHODS

We examined the implication of system xc- by evaluating changes in the expression/activity of this exchanger in the dorsal spinal cord of mice after unilateral partial sciatic nerve ligation. In this surgical model of neuropathic pain, we also examined the consequence of the genetic suppression of system xc- (using mice lacking the system xc- specific subunit xCT) or its pharmacological manipulation (using the pharmacological inhibitor sulfasalazine) on the pain-associated behavioral responses. Finally, we assessed the glial activation and the inflammatory response in the spinal cord by measuring mRNA and protein levels of GFAP and selected M1 and M2 microglial markers.

RESULTS

The sciatic nerve lesion was found to upregulate system xc- at the spinal level. The genetic deletion of xCT attenuated both the amplitude and the duration of the pain sensitization after nerve surgery, as evidenced by reduced responses to mechanical and thermal stimuli, and this was accompanied by reduced glial activation. Consistently, pharmacological inhibition of system xc- had an analgesic effect in lesioned mice.

CONCLUSION

Together, these observations provide evidence for a role of system xc- in the biochemical processes underlying central sensitization. We propose that the reduced hypersensitivity observed in the transgenic mice lacking xCT or in sulfasalazine-treated mice is mediated by a reduced gliosis in the lumbar spinal cord and/or a shift in microglial M1/M2 polarization towards an anti-inflammatory phenotype in the absence of system xc-. These findings suggest that drugs targeting system xc- could contribute to prevent or reduce neuropathic pain.

Research trends and hotspots of ferroptosis in neurodegenerative diseases from 2013 to 2023: A bibliometrics study

Background

With the aging population, the incidence of neurodegenerative diseases increases yearly, seriously impacting human health. Various journals have published studies on the pathogenesis of ferroptosis in neurodegenerative diseases. However, bibliometric analysis in this field is still lacking. The study aims to visually analyze global research trends in this field over the past decade.

Methods

The articles and reviews regarding ferroptosis in neurodegenerative diseases were retrieved from the Web of Science on September 1, 2023. Citespace [version 6.2. R4 (64-bit)] and VOSviewer (version 1.6.18) were used to conduct the bibliometric and knowledge-map analysis.

Results

In total, 370 studies were included in the paper and ranked by their citation frequency. Many articles on ferroptosis in neurodegenerative diseases have been published in the past decade. The country, institution, author, and journal with the highest publications were China, Guangzhou Medical University, Maher, Pamela, and Free Radical Biology And Medicine, respectively. The analysis of keyword co-occurrence indicated that research frontiers were molecular mechanisms of ferroptosis in neurodegenerative diseases, especially a few key pathways that triggered ferroptosis in these diseases, including lipid peroxidation signaling, iron metabolism, and GSH/GPX4 signaling. In addition, ferroptosis inhibitors such as liproxstatins and ferrostatins had protective effects in animal models of neurodegenerative diseases. Therefore, future attention should also be focused on therapeutic drugs that target ferroptosis.

Conclusion

This study comprehensively analyzed the publications on ferroptosis in neurodegenerative diseases from a bibliometric perspective. Research on this topic is currently expanding at a rapid pace, and the China holds a leading position in this field by its scientific achievements and productivity. Moreover, the research frontiers were molecular mechanisms of ferroptosis in neurodegenerative diseases and developing targeted therapeutic drugs. In summary, our results showed an all-sided overview of the knowledge atlas and a valuable reference for the future research in this field.

Parkinson's disease-derived α-synuclein assemblies combined with chronic-type inflammatory cues promote a neurotoxic microglial phenotype

Parkinson's disease (PD) is a common age-related neurodegenerative disorder characterized by the aggregation of α-Synuclein (αSYN) building up intraneuronal inclusions termed Lewy pathology. Mounting evidence suggests that neuron-released αSYN aggregates could be central to microglial activation, which in turn mounts and orchestrates neuroinflammatory processes potentially harmful to neurons. Therefore, understanding the mechanisms that drive microglial cell activation, polarization and function in PD might have important therapeutic implications. Here, using primary microglia, we investigated the inflammatory potential of pure αSYN fibrils derived from PD patients. We further explored and characterized microglial cell responses to a chronic-type inflammatory stimulation combining PD patient-derived αSYN fibrils (FPD), Tumor necrosis factor-α (TNFα) and prostaglandin E2 (PGE2) (TPFPD). We showed that FPD hold stronger inflammatory potency than pure αSYN fibrils generated de novo. When combined with TNFα and PGE2, FPD polarizes microglia toward a particular functional phenotype departing from FPD-treated cells and featuring lower inflammatory cytokine and higher glutamate release. Whereas metabolomic studies showed that TPFPD-exposed microglia were closely related to classically activated M1 proinflammatory cells, notably with similar tricarboxylic acid cycle disruption, transcriptomic analysis revealed that TPFPD-activated microglia assume a unique molecular signature highlighting upregulation of genes involved in glutathione and iron metabolisms. In particular, TPFPD-specific upregulation of Slc7a11 (which encodes the cystine-glutamate antiporter xCT) was consistent with the increased glutamate response and cytotoxic activity of these cells toward midbrain dopaminergic neurons in vitro. Together, these data further extend the structure-pathological relationship of αSYN fibrillar polymorphs to their innate immune properties and demonstrate that PD-derived αSYN fibrils, TNFα and PGE2 act in concert to drive microglial cell activation toward a specific and highly neurotoxic chronic-type inflammatory phenotype characterized by robust glutamate release and iron retention.

Astrocytic Glutamatergic Transmission and Its Implications in Neurodegenerative Disorders

Several neurodegenerative disorders involve impaired neurotransmission, and glutamatergic neurotransmission sets a prototypical example. Glutamate is a predominant excitatory neurotransmitter where the astrocytes play a pivotal role in maintaining the extracellular levels through release and uptake mechanisms. Astrocytes modulate calcium-mediated excitability and release several neurotransmitters and neuromodulators, including glutamate, and significantly modulate neurotransmission. Accumulating evidence supports the concept of excitotoxicity caused by astrocytic glutamatergic release in pathological conditions. Thus, the current review highlights different vesicular and non-vesicular mechanisms of astrocytic glutamate release and their implication in neurodegenerative diseases. As in presynaptic neurons, the vesicular release of astrocytic glutamate is also primarily meditated by calcium-mediated exocytosis. V-ATPase is crucial in the acidification and maintenance of the gradient that facilitates the vesicular storage of glutamate. Along with these, several other components, such as cystine/glutamate antiporter, hemichannels, BEST-1, TREK-1, purinergic receptors and so forth, also contribute to glutamate release under physiological and pathological conditions. Events of hampered glutamate uptake could promote inflamed astrocytes to trigger repetitive release of glutamate. This could be favorable towards the development and worsening of neurodegenerative diseases. Therefore, across neurodegenerative diseases, we review the relations between defective glutamatergic signaling and astrocytic vesicular and non-vesicular events in glutamate homeostasis. The optimum regulation of astrocytic glutamatergic transmission could pave the way for the management of these diseases and add to their therapeutic value.

Aged xCT-Deficient Mice Are Less Susceptible for Lactacystin-, but Not 1-Methyl-4-Phenyl-1,2,3,6- Tetrahydropyridine-, Induced Degeneration of the Nigrostriatal Pathway

The astrocytic cystine/glutamate antiporter system x _c ^- (with xCT as the specific subunit) imports cystine in exchange for glutamate and has been shown to interact with multiple pathways in the brain that are dysregulated in age-related neurological disorders, including glutamate homeostasis, redox balance, and neuroinflammation. In the current study, we investigated the effect of genetic xCT deletion on lactacystin (LAC)- and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced degeneration of the nigrostriatal pathway, as models for Parkinson's disease (PD). Dopaminergic neurons of adult xCT knock-out mice (xCT^-/-) demonstrated an equal susceptibility to intranigral injection of the proteasome inhibitor LAC, as their wild-type (xCT^+/+) littermates. Contrary to adult mice, aged xCT^-/- mice showed a significant decrease in LAC-induced degeneration of nigral dopaminergic neurons, depletion of striatal dopamine (DA) and neuroinflammatory reaction, compared to age-matched xCT^+/+ littermates. Given this age-related protection, we further investigated the sensitivity of aged xCT^-/- mice to chronic and progressive MPTP treatment. However, in accordance with our previous observations in adult mice (Bentea et al., 2015a), xCT deletion did not confer protection against MPTP-induced nigrostriatal degeneration in aged mice. We observed an increased loss of nigral dopaminergic neurons, but equal striatal DA denervation, in MPTP-treated aged xCT^-/- mice when compared to age-matched xCT^+/+ littermates. To conclude, we reveal age-related protection against proteasome inhibition-induced nigrostriatal degeneration in xCT^-/- mice, while xCT deletion failed to protect nigral dopaminergic neurons of aged mice against MPTP-induced toxicity. Our findings thereby provide new insights into the role of system x _c ^- in mechanisms of dopaminergic cell loss and its interaction with aging.

Corticostriatal dysfunction and social interaction deficits in mice lacking the cystine/glutamate antiporter

The astrocytic cystine/glutamate antiporter system x_c^- represents an important source of extracellular glutamate in the central nervous system, with potential impact on excitatory neurotransmission. Yet, its function and importance in brain physiology remain incompletely understood. Employing slice electrophysiology and mice with a genetic deletion of the specific subunit of system x_c^-, xCT (xCT^-/- mice), we uncovered decreased neurotransmission at corticostriatal synapses. This effect was partly mitigated by replenishing extracellular glutamate levels, indicating a defect linked with decreased extracellular glutamate availability. We observed no changes in the morphology of striatal medium spiny neurons, the density of dendritic spines, or the density or ultrastructure of corticostriatal synapses, indicating that the observed functional defects are not due to morphological or structural abnormalities. By combining electron microscopy with glutamate immunogold labeling, we identified decreased intracellular glutamate density in presynaptic terminals, presynaptic mitochondria, and in dendritic spines of xCT^-/- mice. A proteomic and kinomic screen of the striatum of xCT^-/- mice revealed decreased expression of presynaptic proteins and abnormal kinase network signaling, that may contribute to the observed changes in postsynaptic responses. Finally, these corticostriatal deregulations resulted in a behavioral phenotype suggestive of autism spectrum disorder in the xCT^-/- mice; in tests sensitive to corticostriatal functioning we recorded increased repetitive digging behavior and decreased sociability. To conclude, our findings show that system x_c^- plays a previously unrecognized role in regulating corticostriatal neurotransmission and influences social preference and repetitive behavior.

Neuron-Astrocyte Interactions in Parkinson's Disease

Parkinson’s disease (PD) is the second most common neurodegenerative disease. PD patients exhibit motor symptoms such as akinesia/bradykinesia, tremor, rigidity, and postural instability due to a loss of nigrostriatal dopaminergic neurons. Although the pathogenesis in sporadic PD remains unknown, there is a consensus on the involvement of non-neuronal cells in the progression of PD pathology. Astrocytes are the most numerous glial cells in the central nervous system. Normally, astrocytes protect neurons by releasing neurotrophic factors, producing antioxidants, and disposing of neuronal waste products. However, in pathological situations, astrocytes are known to produce inflammatory cytokines. In addition, various studies have reported that astrocyte dysfunction also leads to neurodegeneration in PD. In this article, we summarize the interaction of astrocytes and dopaminergic neurons, review the pathogenic role of astrocytes in PD, and discuss therapeutic strategies for the prevention of dopaminergic neurodegeneration. This review highlights neuron-astrocyte interaction as a target for the development of disease-modifying drugs for PD in the future.

Synthesis and evaluation of a novel 'off-on' chemical sensor based on rhodamine B and the 2,5-pyrrolidinedione moiety for selective discrimination of glutathione and its bioimaging in living cells

A new "turn-on" fluorescent probe, RDMBM, based on the rhodamine B dye and the 2,5-pyrrolidinedione moiety was synthesized and characterized. Its sensing behavior toward various amino acids was evaluated via UV-vis and fluorescence spectroscopic techniques. The observed spectral changes showed that RDMBM displays high selectivity and sensitivity toward GSH in MeOH/H₂O (1:2, v/v, pH 7.40, Tris-HCl buffer, 1 mM) solution and that it undergoes 1:1 covalent binding with GSH. More importantly, the hydrogenation and ring-opening of the nitrogen atom in the spirane structure of rhodamine B derivatives were tightly bound to the induction effects of different groups. Furthermore, fluorescence imaging applications demonstrated that RDMBM can be successfully used for the detection of GSH in human breast cancer cells MCF-7.

Redox-based therapeutics in neurodegenerative disease

This review describes recent developments in the search for effective therapeutic agents that target redox homeostasis in neurodegenerative disease. The disruption to thiol redox homeostasis in Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and multiple sclerosis is discussed, together with the experimental strategies that are aimed at preventing, or at least minimizing, oxidative damage in these diseases. Particular attention is given to the potential of increasing antioxidant capacity by targeting the Nrf2 pathway, the development of inhibitors of NADPH oxidases that are likely candidates for clinical use, together with strategies to reduce nitrosative stress and mitochondrial dysfunction. We describe the shortcomings of compounds that hinder their progression to the clinic and evaluate likely avenues for future research.

LINKED ARTICLES

This article is part of a themed section on Redox Biology and Oxidative Stress in Health and Disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.12/issuetoc.

Proteomic change by Korean Red Ginseng in the substantia nigra of a Parkinson's disease mouse model

Background

Recent studies have shown that Korean Red Ginseng (KRG) successfully protects against dopaminergic neuronal death in the nigrostriatal pathway of a Parkinson's disease (PD) mouse model induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration; however, the mechanism has yet to be identified. Therefore, in this study we used two-dimensional electrophoresis to investigate the effects of KRG on the changes in protein expression in the substantia nigra (SN) of MPTP-treated mice.

Methods

Male C57BL/6 mice (9 wk old) were intraperitoneally administered MPTP (20 mg/kg) four times at 2-h intervals, after which KRG (100 mg/kg) was orally administered once a day for 5 d. Two hours after the fifth KRG administration, a pole test was conducted to evaluate motor function, after which the brains were immediately collected. Survival of dopaminergic neurons was measured by immunohistochemistry, and protein expression was measured by two-dimensional electrophoresis and Western blotting.

Results

KRG alleviated MPTP-induced behavioral dysfunction and neuronal toxicity in the SN. Additionally, the expression of eight proteins related to neuronal formation and energy metabolism for survival were shown to have changed significantly in response to MPTP treatment or KRG administration. KRG alleviated the downregulated protein expression following MPTP administration, indicating that it may enhance neuronal development and survival in the SN of MPTP-treated mice.

Conclusion

These findings indicate that KRG may have therapeutic potential for the treatment of patients with PD.

The role of system Xc- in methamphetamine-induced dopaminergic neurotoxicity in mice

The cystine/glutamate antiporter (system Xc^-, Sxc) transports cystine into cell in exchange for glutamate. Since xCT is a specific subunit of Sxc, we employed xCT knockout mice and investigated whether this antiporter affected methamphetamine (MA)-induced dopaminergic neurotoxicity. MA treatment significantly increased striatal oxidative burdens in wild type mice. xCT inhibitor [i.e., S-4-carboxy-phenylglycine (CPG), sulfasalazine] or an xCT knockout significantly protected against these oxidative burdens. MA-induced increases in Iba-1 expression and Iba-1-labeled microglial immunoreactivity (Iba-1-IR) were significantly attenuated by CPG or sulfasalazine administration or xCT knockout. CPG or sulfasalazine significantly attenuated MA-induced TUNEL-positive cell populations in the striatum of Taconic ICR mice. The decrease in excitatory amino acid transporter-2 (or glutamate transporter-1) expression and increase in glutamate release were attenuated by CPG, sulfasalazine or xCT knockout. In addition, CPG, sulfasalazine or xCT knockout significantly protected against dopaminergic loss (i.e., decreases in tyrosine hydroxylase expression and immunoreactivity, and an increase in dopamine turnover rate) induced by MA. However, CPG, sulfasalazine or xCT knockout did not significantly affect the impaired glutathione system [i.e., decrease in reduced glutathione (GSH) and increase in oxidized glutathione (GSSG)] induced by MA. Our results suggest that Sxc mediates MA-induced neurotoxicity via facilitating oxidative stress, microgliosis, proapoptosis, and glutamate-related toxicity.

Main path and byways: non-vesicular glutamate release by system xc(-) as an important modifier of glutamatergic neurotransmission

System xc(-) is a cystine/glutamate antiporter that exchanges extracellular cystine for intracellular glutamate. Cystine is intracellularly reduced to cysteine, a building block of GSH. As such, system xc(-) can regulate the antioxidant capacity of cells. Moreover, in several brain regions, system xc(-) is the major source of extracellular glutamate. As such this antiporter is able to fulfill key physiological functions in the CNS, while evidence indicates it also plays a role in certain brain pathologies. Since the transcription of xCT, the specific subunit of system xc(-), is enhanced by the presence of reactive oxygen species and inflammatory cytokines, system xc(-) could be involved in toxic extracellular glutamate release in neurological disorders that are associated with increased oxidative stress and neuroinflammation. System xc(-) has also been reported to contribute to the invasiveness of brain tumors and, as a source of extracellular glutamate, could participate in the induction of peritumoral seizures. Two independent reviews (Pharmacol. Rev. 64, 2012, 780; Antioxid. Redox Signal. 18, 2013, 522), approached from a different perspective, have recently been published on the functions of system xc(-) in the CNS. In this review, we highlight novel achievements and insights covering the regulation of system xc(-) as well as its involvement in emotional behavior, cognition, addiction, neurological disorders and glioblastomas, acquired in the past few years. System xc(-) constitutes an important source of extrasynaptic glutamate in the brain. By modulating the tone of extrasynaptic metabotropic or ionotropic glutamate receptors, it affects excitatory neurotransmission, the threshold for overexcitation and excitotoxicity and, as a consequence, behavior. This review describes the current knowledge of how system xc(-) is regulated and involved in physiological as well as pathophysiological brain functioning.