Implication of the glutamate-cystine antiporter xCT in schizophrenia cases linked to impaired GSH synthesis

Abstract

xCT is the specific chain of the cystine/glutamate antiporter, which is widely reported to support anti-oxidant defenses in vivo. xCT is therefore at the crossroads between two processes that are involved in schizophrenia: oxidative stress and glutamatergic neurotransmission. But data from human studies implicating xCT in the illness and clarifying the upstream mechanisms of xCT imbalance are still scarce. Low glutathione (GSH) levels and genetic risk in GCLC (Glutamate-Cysteine Ligase Catalytic subunit), the gene of limiting synthesizing enzyme for GSH, are both associated with schizophrenia. In the present study, we aimed at determining if xCT regulation by the redox system is involved in schizophrenia pathophysiology. We assessed whether modulating GCLC expression impact on xCT expression and activity (i) in fibroblasts from patients and controls with different GCLC genotypes which are known to affect GCLC regulation and GSH levels; (ii) in rat brain glial cells, i.e., astrocytes and oligodendrocytes, with a knock-down of GCLC. Our results highlight that decreased GCLC expression leads to an upregulation of xCT levels in patients' fibroblasts as well as in astrocytes. These results support the implication of xCT dysregulation in illness pathophysiology and further indicate that it can result from redox changes. Additionally, we showed that these anomalies may already take place at early stages of psychosis and be more prominent in a subgroup of patients with GCLC high-risk genotypes. These data add to the existing evidence identifying the inflammatory/redox systems as important targets to treat schizophrenia already at early stages.

Schizophrenia: ANTIOXIDANT DEFICIT INCREASES A KEY NEUROTRANSMITTER TRANSPORTER: Deficit of antioxidant synthesis in schizophrenia leads to oxidative stress and changes in neurotransmitter transporter. Led by Kim Do, a team of researchers from Lausanne University in Switzerland investigated the role of the cell-surface transport protein xCT in schizophrenia. They found that an enzyme responsible for antioxidant production is disturbed in patients. This leads to decreased antioxidant levels and consequently to oxidative stress-i.e. the accumulation of reactive oxygen molecules, damaging the cells component and impairing cell functioning-which in turn affects the functioning of the antioxidant pathway, including xCT. xCT, which exports the neurotransmitter glutamate, is thus overproduced in schizophrenia. The resulting increase of neurotransmitter activity, alongside the increase in oxidative stress, is thought to play a major role in the pathophysiology of schizophrenia, including at early stages of the disease.

Fig. 1

SLC7A11 expression in skin fibroblasts with GCLC high-risk or GCLC low-risk genotypes: Top-5 genes up-regulated in fibroblasts treated by tBHQ versus vehicle (a), and up-regulated in fibroblasts with GCLC high-risk versus GCLC low-risk genotypes (b) both in patients and controls. FC fold of change, FDR false discovery rate (paired comparisons). c Plot illustrating microarray data for SLC7A11. Data are represented as mean ± standard error of the mean

Fig. 2

Cystine uptake by skin fibroblasts: Cells were treated (a) with vehicle (0.05% DMSO) or (b) with tBHQ (50uM for 18 h) which induces the anti-oxidant response. Internalized Cystine was assessed after 5 min of uptake. Data are represented as mean ± standard error of the mean; *p-value < 0.05

Fig. 3

Uptake of cystine by OPCs was evaluated after 15 min (a) and 30 min (b). Three conditions were compared: without shRNA (black), transduced with a scrambled shRNA (gray), transduced with GCLC knock-down (white). Background level of cystine uptake was evaluated in the presence of glutamate (red). Data are represented as mean ± standard error of the mean (n = 4 per group); *p-value < 0.05

Fig. 4

Uptake of cystine by astrocytes: a Decrease of GCLC mRNA assessed by quantitative PCR in astrocytes transduced with GCLC shRNA is expressed as percentage of the condition with scrambled shRNA. b Uptake of cystine in dividing astrocytes transduced with scrambled (black) or GCLC shRNA (white). c Uptake of cystine in dbcAMP-treated astrocytes transduced with scrambled (black) or GCLC shRNA (white). Data are represented as mean ± standard error of the mean (n = 3 per group); *p-value < 0.05