Erythropoietin-induced neuroprotection requires cystine glutamate exchanger activity

Does erythropoietin affect the outcome and complication rates of patient with traumatic brain injury? A pooled-analysis

OBJECTIVE

The aim of this meta-analysis was to review the scientific literature published until April 18, 2021, to summarize existing knowledge on the efficacy and safety of erythropoietin (EPO) for traumatic brain injury (TBI).

METHODS

This systematic review followed PRISMA guidelines. Randomized controlled trials (RCTs) reporting on the efficacy and safety of EPO in the treatment of TBI were systematically searched in relevant electronic databases according to a pre-designed search strategy. The primary outcomes are the mortality; and secondary outcomes are the good functional outcome (GFO) and adverse events (AEs).

RESULTS

A total of 10 RCTs involving 2,402 participants fulfilled the inclusion criteria. The results showed that there is a significant difference in terms of the mortality (RR = 0.67, 95% CI = 0.54-0.84, P = 0.0003) and seizure rate (RR = 0.52, 95% CI = 0.29-0.96, P = 0.04) between the EPO groups compared to those in the control groups. However, compared with the control groups, the GFO in the EPO groups was not statistically significant (RR = 1.18, 95% CI = 0.93-1.48, P = 0.17).

CONCLUSIONS

Findings of the present meta-analysis suggest that the use of EPO could reduce mortality rate in patients with TBI, without increasing the incidence of AEs. EPO has potential research and application value in the treatment of TBI.

Whether erythropoietin can be a neuroprotective agent against premature brain injury: cellular mechanisms and clinical efficacy

Preterm infants are at high risk of brain injury. With more understanding of the preterm brain injury's pathogenesis, neuroscientists are looking for more effective methods to prevent and treat it, among which erythropoietin (Epo) is considered as a prime candidate. This review tries to clarify the possible mechanisms of Epo in preterm neuroprotection and summarize updated evidence considering Epo as a pharmacological neuroprotective strategy in animal models and clinical trials. To date, various animal models have validated that Epo is an anti-apoptotic, anti-inflammatory, anti-oxidant, anti-excitotoxic, neurogenetic, erythropoietic, angiogenetic, and neurotrophic agent, thus preventing preterm brain injury. However, although the scientific rationale and preclinical data for Epo's neuroprotective effect are promising, when translated to bedside, the results vary in different studies, especially in its long-term efficacy. Based on existing evidence, it is still too early to recommend Epo as the standard treatment for preterm brain injury.

Astrocytes release glutamate via cystine/glutamate antiporter upregulated in response to increased oxidative stress related to sporadic amyotrophic lateral sclerosis

A vast body of evidence implicates increased oxidative stress and extracellular glutamate accumulation in the pathomechanism of sporadic amyotrophic lateral sclerosis (ALS). Cystine/glutamate antiporter (xCT) carries extracellular cystine uptake and intracellular glutamate release (cystine/glutamate exchange) in the presence of oxidative stress. The aim of the present study was to determine the involvement of xCT in ALS. Immunohistochemical observations in the spinal cord sections demonstrated that xCT was mainly expressed in astrocytes, with staining more intense in 12 sporadic ALS patients as compared to 12 age-matched control individuals. Western blot and densitometric analyses of the spinal cord samples revealed that the relative value of xCT/β-actin optical density ratio was significantly higher in the ALS group as compared to the control group. Next, we conducted cell culture experiments using a human astrocytoma-derived cell line (1321N1) and a mouse motor neuron/neuroblastoma hybrid cell line (NSC34). In 1321N1 cells, the normalized xCT expression levels in cell lysates were significantly increased by H₂ O₂ treatment. Glutamate concentrations in 1321 N1 cell culture-conditioned media were significantly elevated by H₂ O₂ treatment, and the H₂ O₂ -driven elevations were completely canceled by the xCT inhibitor erastin pretreatment. In motor neuron-differentiated NSC34 cells (NSC34d cells), both the normalized xCT expression levels in the cell lysates and glutamate concentrations in the cell-conditioned media were constant with or without H₂ O₂ treatment. The present results provide in vivo and in vitro evidence that astrocytes upregulate xCT expression to release glutamate in response to increased oxidative stress associated with ALS, contributing to extracellular glutamate accumulation.

The complex roles of STAT3 and STAT5 in maintaining redox balance: Lessons from STAT-mediated xCT expression in cancer cells

STAT3 and STAT5 mediate diverse cellular processes, transcriptionally regulating gene expression and interacting with cytoplasmic proteins. Their canonical activity is stimulated by cytokines/growth factors through JAK-STAT signaling. As targets of oncogenes with intrinsic tyrosine kinase activity, STAT3 and STAT5 become constitutively active in hematologic neoplasms and solid tumors, promoting cell proliferation and survival and modulating redox homeostasis. This review summarizes reactive oxygen species (ROS)-regulated STAT activation and how STATs influence ROS production. ROS-induced effects on post-translational modifications are presented, and STAT3/5-mediated regulation of xCT, a redox-sensitive target up-regulated in numerous cancers, is discussed with regard to transcriptional cross-talk.

Erythropoietin: Endogenous Protection of Ischemic Brain

The human brain requires uninterrupted delivery of blood-borne oxygen and nutrients to sustain its function. Focal ischemia, particularly, ischemic stroke, and global ischemia imposed by cardiac arrest disrupt the brain's fuel supply. The resultant ATP depletion initiates a complex injury cascade encompassing intracellular Ca²⁺ overload, glutamate excitotoxicity, oxido-nitrosative stress, extracellular matrix degradation, and inflammation, culminating in neuronal and astroglial necrosis and apoptosis, neurocognitive deficits, and even death. Unfortunately, brain ischemia has proven refractory to pharmacological intervention. Many promising treatments afforded brain protection in animal models of focal and global ischemia, but failed to improve survival and neurocognitive recovery of stroke and cardiac arrest patients in randomized clinical trials. The culprits are the blood-brain barrier (BBB) that limits transferral of medications to the brain parenchyma, and the sheer complexity of the injury cascade, which presents a daunting array of targets unlikely to respond to monotherapies. Erythropoietin is a powerful neuroprotectant capable of interrupting multiple aspects of the brain injury cascade. Preclinical research demonstrates erythropoietin's ability to suppress glutamate excitotoxicity and intracellular Ca²⁺ overload, dampen oxidative stress and inflammation, interrupt the apoptotic cascade, and preserve BBB integrity. However, the erythropoietin dosages required to traverse the BBB and achieve therapeutically effective concentrations in the brain parenchyma impose untoward side effects. Recent discoveries that hypoxia induces erythropoietin production within the brain and that neurons, astroglia, and cerebrovascular endothelium harbor membrane erythropoietin receptors, raise the exciting prospect of harnessing endogenous erythropoietin to protect the brain from the ravages of ischemia-reperfusion.

Erythropoietin-induced cytoprotection in intestinal epithelial cells is linked to system Xc<sup/>

Necrotizing enterocolitis is a common but serious complication among premature babies. Currently, there are limited treatment options. These include intensive supportive care and surgical intervention. In this study, we hypothesize that erythropoietin (Epo) could be protective against cell necrosis by increasing the levels of glutathione. This can be regulated by increasing the activity of system xC^-. This was demonstrated using intestinal epithelial cells (IEC-6) as a model system. S4-CPG and sulfasalazine pharmacologically inhibit xCT, which induced cell death. Our data showed a dose dependent decrease in cell viability when treated with both inhibitors. In addition, the IEC-6 cells displayed a dose dependent increase when treated with Epo. In conclusion, Epo can be protective against cell death and ultimately be considered as a treatment option for intestinal epithelial cell death.

Age-associated expression of erythropoietin and its receptor in rat spiral ganglion neurons and its association with neuronal apoptosis and hearing alterations

The present study aimed to determine the expression of erythropoietin (EPO) and the EPO receptor (EPOR) in spiral ganglion neurons (SGNs) in the inner ear of rats of various ages, and the associated neuronal apoptosis and hearing alterations. A total of 15 healthy rats (n=30 ears), were divided into three groups: i) A nominated infant group at post-natal day (PND) 12–14, ii) an adult group at PND 60 and iii) a 3-year postnatal aged group. Auditory brainstem response (ABR) measurements were performed on all rats. EPO and EPOR expression in the inner ear was detected by immunohistochemistry. In situ terminal deoxynucleotidyl transferase dUTP nick end labeling assays were performed to detect the apoptosis of SGNs. The average hearing thresholds of the ABR (decibels above normal hearing level) were 5.625±4.955 in the infant, 15.000±8.498 in the adult and 23.500±13.134 in the aged groups. Hearing thresholds for aged and adult rats increased significantly compared with infant rats. However, the difference in latencies of peak I was not significant (P>0.05). EPO in SGNs was detected during different developmental periods without significant alterations, but were reduced compared with Corti's organ or the stria vascularis. EPOR expression increased significantly from infant to adult stage, and this increased expression was maintained in the aged group. An age-associated increase in the apoptosis of SGNs was detected in all three groups (P=0.0347). The potential neuroprotective effects of EPO in SGNs may not be revealed during the aging process under natural conditions, and may be associated with spontaneous neuronal apoptosis and consequently, hearing diminution. However, the age-associated increase in EPOR in SGNs may exert a role in neuroprotection when necessary, for example in presbycusis.

Regulation of System xc(-) by Pharmacological Manipulation of Cellular Thiols

The cystine/glutamate exchanger (system xc (-)) mediates the transport of cystine into the cell in exchange for glutamate. By releasing glutamate, system xc (-) can potentially cause excitotoxicity. However, through providing cystine to the cell, it regulates the levels of cellular glutathione (GSH), the main endogenous intracellular antioxidant, and may protect cells against oxidative stress. We tested two different compounds that deplete primary cortical cultures containing both neurons and astrocytes of intracellular GSH, L-buthionine-sulfoximine (L-BSO), and diethyl maleate (DEM). Both compounds caused significant concentration and time dependent decreases in intracellular GSH levels. However; DEM caused an increase in radiolabeled cystine uptake through system xc (-), while unexpectedly BSO caused a decrease in uptake. The compounds caused similar low levels of neurotoxicity, while only BSO caused an increase in oxidative stress. The mechanism of GSH depletion by these two compounds is different, DEM directly conjugates to GSH, while BSO inhibits γ-glutamylcysteine synthetase, a key enzyme in GSH synthesis. As would be expected from these mechanisms of action, DEM caused a decrease in intracellular cysteine, while BSO increased cysteine levels. The results suggest that negative feedback by intracellular cysteine is an important regulator of system xc (-) in this culture system.

Excitotoxicity triggered by neonatal monosodium glutamate treatment and blood-brain barrier function

It is likely that monosodium glutamate (MSG) is the excitotoxin that has been most commonly employed to characterize the process of excitotoxicity and to improve understanding of the ways that this process is related to several pathological conditions of the central nervous system. Excitotoxicity triggered by neonatal MSG treatment produces a significant pathophysiological impact on adulthood, which could be due to modifications in the blood-brain barrier (BBB) permeability and vice versa. This mini-review analyzes this topic through brief descriptions about excitotoxicity, BBB structure and function, role of the BBB in the regulation of Glu extracellular levels, conditions that promote breakdown of the BBB, and modifications induced by neonatal MSG treatment that could alter the behavior of the BBB. In conclusion, additional studies to better characterize the effects of neonatal MSG treatment on excitatory amino acids transporters, ionic exchangers, and efflux transporters, as well as the role of the signaling pathways mediated by erythropoietin and vascular endothelial growth factor in the cellular elements of the BBB, should be performed to identify the mechanisms underlying the increase in neurovascular permeability associated with excitotoxicity observed in several diseases and studied using neonatal MSG treatment.

Phosphoinositide 3-kinases upregulate system xc(-) via eukaryotic initiation factor 2α and activating transcription factor 4 - A pathway active in glioblastomas and epilepsy

AIMS

Phosphoinositide 3-kinases (PI3Ks) relay growth factor signaling and mediate cytoprotection and cell growth. The cystine/glutamate antiporter system xc(-) imports cystine while exporting glutamate, thereby promoting glutathione synthesis while increasing extracellular cerebral glutamate. The aim of this study was to analyze the pathway through which growth factor and PI3K signaling induce the cystine/glutamate antiporter system xc(-) and to demonstrate its biological significance for neuroprotection, cell growth, and epilepsy.

RESULTS

PI3Ks induce system xc(-) through glycogen synthase kinase 3β (GSK-3β) inhibition, general control non-derepressible-2-mediated eukaryotic initiation factor 2α phosphorylation, and the subsequent translational up-regulation of activating transcription factor 4. This pathway is essential for PI3Ks to modulate oxidative stress resistance of nerve cells and insulin-induced growth in fibroblasts. Moreover, the pathway is active in human glioblastoma cells. In addition, it is induced in primary cortical neurons in response to robust neuronal activity and in hippocampi from patients with temporal lobe epilepsy.

INNOVATION

Our findings further extend the concepts of how growth factors and PI3Ks induce neuroprotection and cell growth by adding a new branch to the signaling network downstream of GSK-3β, which, ultimately, leads to the induction of the cystine/glutamate antiporter system xc(-). Importantly, the induction of this pathway by neuronal activity and in epileptic hippocampi points to a potential role in epilepsy.

CONCLUSION

PI3K-regulated system xc(-) activity is not only involved in the stress resistance of neuronal cells and in cell growth by increasing the cysteine supply and glutathione synthesis, but also plays a role in the pathophysiology of tumor- and non-tumor-associated epilepsy by up-regulating extracellular cerebral glutamate.

Long-term moderate dose exogenous erythropoietin treatment protects from intermittent hypoxia-induced spatial learning deficits and hippocampal oxidative stress in young rats

Exposure to intermittent hypoxia (IH) is associated with cognitive impairments and oxidative stress in brain regions involved in learning and memory. In earlier studies, erythropoietin (EPO) showed a neuroprotective effect in large doses. The aim of the present study was to explore the effect of smaller doses of EPO, such as those used in the treatment of anemia, on IH-induced cognitive deficits and hippocampal oxidative stress in young rats. The effect of concurrent EPO treatment (500 and 1,000 IU/kg/day ip) on spatial learning and memory deficits induced by long-term exposure to IH for 6 weeks was tested using the Morris water maze (MWM) test and the elevated plus maze (EPM) test. Moreover, the effect on hippocampal glutamate and oxidative stress were assessed. Exposure to IH induced a significant impairment of spatial learning and cognition of animals in both MWM and EPM performance parameters. Moreover, hippocampal glutamate and thiobarbituric acid reactive substances (TBARS) increased while antioxidant defenses (GSH and GSH-Px) decreased. EPO in the tested doses significantly reduced the IH-induced spatial learning deficits in both MWM and EPM tests and dose-dependently antagonized the effects of IH on hippocampal glutamate, TBARS, GSH levels, and GSH-Px activity. Treatment with EPO in moderate doses that used for anemia, concurrently with IH exposure can antagonize IH-induced spatial learning deficits and protect hippocampal neurons from IH-induced lipid peroxidation and oxidative stress-induced damage in young rats, possibly through multiple mechanisms involving a potential antioxidative effect.

The cystine/glutamate antiporter system x(c)(-) in health and disease: from molecular mechanisms to novel therapeutic opportunities

The antiporter system x(c)(-) imports the amino acid cystine, the oxidized form of cysteine, into cells with a 1:1 counter-transport of glutamate. It is composed of a light chain, xCT, and a heavy chain, 4F2 heavy chain (4F2hc), and, thus, belongs to the family of heterodimeric amino acid transporters. Cysteine is the rate-limiting substrate for the important antioxidant glutathione (GSH) and, along with cystine, it also forms a key redox couple on its own. Glutamate is a major neurotransmitter in the central nervous system (CNS). By phylogenetic analysis, we show that system x(c)(-) is a rather evolutionarily new amino acid transport system. In addition, we summarize the current knowledge regarding the molecular mechanisms that regulate system x(c)(-), including the transcriptional regulation of the xCT light chain, posttranscriptional mechanisms, and pharmacological inhibitors of system x(c)(-). Moreover, the roles of system x(c)(-) in regulating GSH levels, the redox state of the extracellular cystine/cysteine redox couple, and extracellular glutamate levels are discussed. In vitro, glutamate-mediated system x(c)(-) inhibition leads to neuronal cell death, a paradigm called oxidative glutamate toxicity, which has successfully been used to identify neuroprotective compounds. In vivo, xCT has a rather restricted expression pattern with the highest levels in the CNS and parts of the immune system. System x(c)(-) is also present in the eye. Moreover, an elevated expression of xCT has been reported in cancer. We highlight the diverse roles of system x(c)(-) in the regulation of the immune response, in various aspects of cancer and in the eye and the CNS.

Cystine glutamate exchanger upregulation by retinoic acid induces neuroprotection in neural stem cells

Oxidative stress and excitotoxic injury are commonly associated with several neurodegenerative diseases, such as Parkinson's disease, Alzheimer's disease, and periventricular leukomalacia. As cystine is imported into the cell, it is used in the synthesis of intracellular glutathione, an important antioxidant necessary for the defense of brain cells from oxidative stress and glutamate-mediated excitotoxicity. Recent studies have shown that retinoic acid increases the activity of glutathione synthesis and exhibits neuroprotective properties in brain cells. Previously, we have shown that the regulation of the cystine glutamate exchanger (system Xc(-)) also leads to neuroprotection. Here, we examined the effects of retinoic acid on the regulation of system Xc(-). Our results suggest that retinoic acid-induced neuroprotection is mediated through system Xc(-) by regulating glutathione biosynthesis.

Hypoxic preconditioning involves system Xc- regulation in mouse neural stem cells

In animals, hypoxic preconditioning has been used as a form of neuroprotection. The exact mechanism involved in neuroprotective hypoxic preconditioning has not been described, yet could be valuable for possible neuroprotective strategies. The overexpression of the cystine-glutamate exchanger, system Xc-, has been demonstrated as being neuroprotective (Shih, Erb et al. 2006). Here, using immunohistochemistry, we demonstrate that C57BL/6 mice exposed to hypoxia showed an increase in system Xc- expression, with the highest level of intensity in the hippocampus. Western Blot analysis also showed an almost 2-fold increase in system Xc- protein in hypoxia-exposed versus control mice. The mRNA for the regulatory subunit of system Xc-, xCT, and the xCT/actin ratio were also increased under hypoxic conditions. Experiments using hypoxia-inducible factor (HIF-1α) siRNA showed a statistically significant decrease in HIF-1α and system Xc- expression. Under hypoxic conditions, system Xc- activity, as determined by cystine uptake, increased 2-fold. Importantly, hypoxic preconditioning was attenuated in neural stem cells by pharmacological inhibition of system Xc- activity with S4-carboxyphenylglycine. These data provide the first evidence of hypoxic regulation of the cystine glutamate exchanger system Xc-.

Trpc2 depletion protects red blood cells from oxidative stress-induced hemolysis

Transient receptor potential (TRP) channels Trpc2 and Trpc3 are expressed on normal murine erythroid precursors, and erythropoietin stimulates an increase in intracellular calcium ([Ca(2+)](i)) through TRPC2 and TRPC3. Because modulation of [Ca(2+)](i) is an important signaling pathway in erythroid proliferation and differentiation, Trpc2, Trpc3, and Trpc2/Trpc3 double knockout mice were utilized to explore the roles of these channels in erythropoiesis. Trpc2, Trpc3, and Trpc2/Trpc3 double knockout mice were not anemic, and had similar red blood cell counts, hemoglobins, and reticulocyte counts as wild-type littermate controls. Although the erythropoietin-induced increase in [Ca(2+)](i) was reduced, these knockout mice showed no defects in red cell production. The major phenotypic difference at steady state was that the mean corpuscular volume, mean corpuscular hemoglobin, and hematocrit of red cells were significantly greater in Trpc2 and Trpc2/Trpc3 double knockout mice, and mean corpuscular hemoglobin concentration was significantly reduced. All hematological parameters in Trpc3 knockout mice were similar to controls. When exposed to phenylhydrazine, unlike the Trpc3 knockouts, Trpc2 and Trpc2/Trpc3 double knockout mice showed significant resistance to hemolysis. This was associated with a significant reduction in hydrogen peroxide-induced calcium influx in erythroblasts. Although erythropoietin-induced calcium influx through TRPC2 or TRPC3 is not critical for erythroid production, these data demonstrate that TRPC2 plays an important role in oxidative stress-induced hemolysis, which may be related to reduced calcium entry in red cells in the presence of Trpc2 depletion.

The transient receptor potential (TRP) channel TRPC3 TRP domain and AMP-activated protein kinase binding site are required for TRPC3 activation by erythropoietin

Modulation of intracellular calcium ([Ca(2+)](i)) by erythropoietin (Epo) is an important signaling pathway controlling erythroid proliferation and differentiation. Transient receptor potential (TRP) channels TRPC3 and homologous TRPC6 are expressed on normal human erythroid precursors, but Epo stimulates an increase in [Ca(2+)](i) through TRPC3 but not TRPC6. Here, the role of specific domains in the different responsiveness of TRPC3 and TRPC6 to erythropoietin was explored. TRPC3 and TRPC6 TRP domains differ in seven amino acids. Substitution of five amino acids (DDKPS) in the TRPC3 TRP domain with those of TRPC6 (EERVN) abolished the Epo-stimulated increase in [Ca(2+)](i). Substitution of EERVN in TRPC6 TRP domain with DDKPS in TRPC3 did not confer Epo responsiveness. However, substitution of TRPC6 TRP with DDKPS from TRPC3 TRP, as well as swapping the TRPC6 distal C terminus (C2) with that of TRPC3, resulted in a chimeric TRPC6 channel with Epo responsiveness similar to TRPC3. Substitution of TRPC6 with TRPC3 TRP and the putative TRPC3 C-terminal AMP-activated protein kinase (AMPK) binding site straddling TRPC3 C1/C2 also resulted in TRPC6 activation. In contrast, substitution of the TRPC3 C-terminal leucine zipper motif or TRPC3 phosphorylation sites Ser-681, Ser-708, or Ser-764 with TRPC6 sequence did not affect TRPC3 Epo responsiveness. TRPC3, but not TRPC6, and TRPC6 chimeras expressing TRPC3 C2 showed significantly increased plasma membrane insertion following Epo stimulation and substantial cytoskeletal association. The TRPC3 TRP domain, distal C terminus (C2), and AMPK binding site are critical elements that confer Epo responsiveness. In particular, the TRPC3 C2 and AMPK site are essential for association of TRPC3 with the cytoskeleton and increased channel translocation to the cell surface in response to Epo stimulation.

Long-term treatment with suberythropoietic Epo is vaso- and neuroprotective in experimental diabetic retinopathy

BACKGROUND/AIMS

Diabetic retinopathy is characterized by pericyte loss and vasoregression both in the clinic and in animal models. A mild neurodegeneration with loss of ganglion cells is also described in the diabetic retina. Like VEGF-A, Epo is angioprotective and, in high doses, neuroprotective, however, without affecting vessel permeability. This study was to investigate the effect of a long-term suberythropoietic dose of Epo on vascular damage and neurodegeneration in a rat model of diabetic retinopathy.

METHODS

We administered Epo 3x256 IU/kg body weight/week to streptozotocin-diabetic Wistar rats for up to 6 months. Leukostasis was analyzed by quantitation of CD45 positive cells adherent to the retinal microvasculature. VEGF-A levels were assessed by Elisa at 3 months of treatment. Vasoregression was quantified in retinal digest preparations after 6 months of Epo treatment. Neurodegeneration was analyzed from PAS stained retinal paraffin preparations.

RESULTS

Leukostasis was unaffected by treatment with Epo which significantly inhibited the loss of pericyte and the formation of acellular capillaries. Neurodegeneration in the diabetic retina was significantly reduced by Epo treatment. Increased VEGF-A levels in the diabetic retina were normalized by Epo treatment.

CONCLUSIONS

Suberythropoietic Epo is effective to protect microvascular and neuronal damage in the experimental diabetic retina.

The oxidative stress-inducible cystine/glutamate antiporter, system x (c) (-) : cystine supplier and beyond

The oxidative stress-inducible cystine/glutamate exchange system, system x (c) (-) , transports one molecule of cystine, the oxidized form of cysteine, into cells and thereby releases one molecule of glutamate into the extracellular space. It consists of two protein components, the 4F2 heavy chain, necessary for membrane location of the heterodimer, and the xCT protein, responsible for transport activity. Previously, system x (c) (-) has been regarded to be a mere supplier of cysteine to cells for the synthesis of proteins and the antioxidant glutathione (GSH). In that sense, oxygen, electrophilic agents, and bacterial lipopolysaccharide trigger xCT expression to accommodate with increased oxidative stress by stimulating GSH biosynthesis. However, emerging evidence established that system x (c) (-) may act on its own as a GSH-independent redox system by sustaining a redox cycle over the plasma membrane. Hallmarks of this cycle are cystine uptake, intracellular reduction to cysteine and secretion of the surplus of cysteine into the extracellular space. Consequently, increased levels of extracellular cysteine provide a reducing microenvironment required for proper cell signaling and communication, e.g. as already shown for the mechanism of T cell activation. By contrast, the enhanced release of glutamate in exchange with cystine may trigger neurodegeneration due to glutamate-induced cytotoxic processes. This review aims to provide a comprehensive picture from the early days of system x (c) (-) research up to now.

Erythropoietin 2nd cerebral protection after acute injuries: a double-edged sword?

Over the past 15 years, a large body of evidence has revealed that the cytokine erythropoietin exhibits non-erythropoietic functions, especially tissue-protective effects. The discovery of EPO and its receptors in the central nervous system and the evidence that EPO is made locally in response to injury as a protective factor in the brain have raised the possibility that recombinant human EPO (rhEPO) could be administered as a cytoprotective agent after acute brain injuries. This review highlights the potential applications of rhEPO as a neuroprotectant in experimental and clinical settings such as ischemia, traumatic brain injury, and subarachnoid and intracerebral hemorrhage. In preclinical studies, EPO prevented apoptosis, inflammation, and oxidative stress induced by injury and exhibited strong neuroprotective and neurorestorative properties. EPO stimulates vascular repair by facilitating endothelial progenitor cell migration into the brain and neovascularisation, and it promotes neurogenesis. In humans, small clinical trials have shown promising results but large prospective randomized studies failed to demonstrate a benefit of EPO for brain protection and showed unwanted side effects, especially thrombotic complications. Recently, regions have been identified within the EPO molecule that mediate tissue protection, allowing the development of non-erythropoietic EPO variants for neuroprotection conceptually devoid of side effects. The efficacy and the safety profile of these new compounds are still to be demonstrated to obtain, in patients, the benefits observed in experimental studies.