Mutation of ataxia–telangiectasia mutated is associated with dysfunctional glutathione homeostasis in cerebellar astroglia

Abstract

Astroglial dysfunction plays an important role in neurodegenerative diseases otherwise attributed to neuronal loss of function. Here we focus on the role of astroglia in ataxia–telangiectasia (A–T), a disease caused by mutations in the ataxia–telangiectasia mutated (ATM) gene. A hallmark of A–T pathology is progressive loss of cerebellar neurons, but the mechanisms that impact neuronal survival are unclear. We now provide a possible mechanism by which A–T astroglia affect the survival of cerebellar neurons. As astroglial functions are difficult to study in an in vivo setting, particularly in the cerebellum where these cells are intertwined with the far more numerous neurons, we conducted in vitro coculture experiments that allow for the generation and pharmacological manipulation of purified cell populations. Our analyses revealed that cerebellar astroglia isolated from Atm mutant mice show decreased expression of the cystine/glutamate exchanger subunit xCT, glutathione (GSH) reductase, and glutathione-S-transferase. We also found decreased levels of intercellular and secreted GSH in A–T astroglia. Metabolic labeling of L-cystine, the major precursor for GSH, revealed that a key component of the defect in A–T astroglia is an impaired ability to import this rate-limiting precursor for the production of GSH. This impairment resulted in suboptimal extracellular GSH supply, which in turn impaired survival of cerebellar neurons. We show that by circumventing the xCT-dependent import of L-cystine through addition of N-acetyl-L-cysteine (NAC) as an alternative cysteine source, we were able to restore GSH levels in A–T mutant astroglia providing a possible future avenue for targeted therapeutic intervention.

Dexamethasone improves redox state in ataxia telangiectasia cells by promoting an NRF2‐mediated antioxidant response

“…Suitable samples were extracted from LCLs with or without DEXA administration. In agreement with other studies [41][42][43][44][45], we found that basal levels of GSH do not differ significantly in A-T cells compared to WT cells (Fig. 1A).…”

“…Meredith et al [45] suggested an impaired cystine uptake in A-T fibroblasts but, as already noted, these findings were questioned by Dean [42]. More recently, Campbell et al [41] reported a decreased expression of XCT transporter in astroglia culture from A-T mice. We found intracellular cysteine levels to be similar in WT and A-T lymphoblastoid cells, and slightly increased in both after DEXA administration (data not shown), although cysteine transporter transcription was unaffected.…”

Cerebellar Astrocytes: Much More Than Passive Bystanders In Ataxia Pathophysiology

“…Specifically, stressed cerebellar astrocytes isolated from ATM-mutant mice showed decreased expression of the cystine/glutamate exchanger subunit xCT and of GSH reductase, which translated into reduced levels of both intracellular and secreted GSH. Consequently, when co-cultured with healthy neurons, these astrocytes provided insufficient GSH levels to support neuronal survival [114]. Nevertheless, the absence of neurodegeneration in AT experimental models did not allow so far to clarify whether this mechanism also holds true for astrocytes in vivo.…”

“…Stressed and senescent astrocytes, in turn, failed in providing their antioxidant support on neurons that eventually degenerated. Recently, the mechanism underlying this suboptimal antioxidant support of astrocytes was found in their impaired ability to import l-cystine and produce reduced glutathione (GSH), which is normally secreted by healthy astroglia in response to oxidative stress to provide additional support to neurons [114]. Specifically, stressed cerebellar astrocytes isolated from ATM-mutant mice showed decreased expression of the cystine/glutamate exchanger subunit xCT and of GSH reductase, which translated into reduced levels of both intracellular and secreted GSH.…”

“…Antioxidant therapies likely represent another valid approach to effectively ameliorate at least AT and FA, by reducing oxidative stress and supporting astrocytes' functions in neuronal survival. Indeed, in AT in vitro models, NAC treatments were able not only to suppress the activation of stress and senescence pathways in mutant astrocytes [112] but also to restore normal GSH levels by circumventing the defects in l-cystine import in these cells, which were, therefore, again capable of supporting neuronal survival and neurite outgrowth [114]. Another approach to normalize GSH levels and overcome neuronal death in AT may be the drug-induced activation of the cystine/glutamate exchanger subunit xCT, already suggested for Parkinson's disease and amyotrophic lateral sclerosis [120,121].…”