mTOR Inhibition Mitigates Molecular and Biochemical Alterations of Vigabatrin-Induced Visual Field Toxicity in Mice

Abstract

BACKGROUND Gamma-vinyl–γ-aminobutyric acid (GABA) (vigabatrin) is an antiepileptic drug and irreversible GABA transaminase inhibitor associated with visual field impairment, which limits its clinical utility. We sought to relate altered visual evoked potentials associated with vigabatrin intake to transcriptional changes in the mechanistic target of rapamycin (mTOR) pathway and GABA receptors to expose further mechanisms of vigabatrin-induced visual field loss. METHODS Vigabatrin was administered to mice via an osmotic pump for two weeks to increase GABA levels. Visual evoked potentials were examined, eye samples were collected, and gene expression was measured by quantitative reverse transcription-polymerase chain reaction. Similarly, human retinal pigment epithelial cells (ARPE19) were exposed to vigabatrin and treated with mTOR inhibitors for mTOR pathway analysis and to assess alterations in organelle accumulation by microscopy. RESULTS Dysregulated expression of transcripts in the mTOR pathway, GABAA/B receptors, metabotropic glutamate (Glu) receptors 1/6, and GABA/glutamate transporters in the eye were found in association with visual evoked potential changes during vigabatrin administration. Rrag genes were upregulated in both mouse eye and ARPE19 cells. Immunoblot of whole eye revealed greater than three fold upregulation of a 200 kDa band when immunoblotted for ras-related guanosine triphosphate binding D. Microscopy of ARPE19 cells revealed selective reversal of vigabatrin-induced organelle accumulation by autophagy-inducing drugs, notably Torin 2. Changes in the mTOR pathway gene expression, including Rrag genes, were corrected by Torin 2 in ARPE19 cells. CONCLUSIONS Our studies, indicating GABA-associated augmentation of RRAG and mTOR signaling, support further preclinical evaluation of mTOR inhibitors as a therapeutic strategy to potentially mitigate vigabatrin-induced ocular toxicity.

Succinic semialdehyde dehydrogenase deficiency, a disorder of GABA metabolism: an update on pharmacological and enzyme‐replacement therapeutic strategies

“…We hypothesized that in SSADHD, postnatal seizures were caused by overexpression of NKCC1 and a continuing excitatory capacity of GABA A receptors after birth. If confirmed, this mechanism would explain the paradox of seizures in a hyperGABAergic condition (Vogel et al 2017c). Furthermore, it would also suggest that inhibition of NKCC1 might have positive therapeutic effects in SSADHD.…”

“…A number of gene expression changes correlated between the two animal models. Prkag1 was down-regulated in brain of both models (Vogel et al 2017b; 2017c). Prkag1 is a gamma regulatory subunit of the heterotrimeric AMP-activated protein kinase (AMPK), which also contains an alpha catalytic subunit and a non-catalytic beta subunit (Fig.…”

“…NKCC1 and KCC2 control transmembrane chloride gradient and determine GABA A receptor directional transmembrane chloride flux. In experimental SSADHD, the expression ratio of NKCC1 and KCC2 (NKCC1/KCC2) is elevated (Vogel, 2017c), suggesting that activation of GABA A receptors causes chloride efflux from brain cells, membrane depolarization and activation of neurotransmission (see Fig. 3 for further details).…”

Preclinical tissue distribution and metabolic correlations of vigabatrin, an antiepileptic drug associated with potential use‐limiting visual field defects

“…The role(s) of GABA, β‐alanine and 4‐GBA in the ocular toxicity of VGB remain unknown. Conversely, we and others have demonstrated that supraphysiological GABA impacts the mTOR pathway of autophagy, leading to mitochondrial accumulation and enhanced oxidative stress . β‐Alanine, the structural homologue of GABA, has GABAergic and glycinergic roles that are well‐described, but its role in VGB‐mediated toxicity has not been investigated.…”

“…Long‐term VGB intervention associates with peripheral atrophy of the retinal nerve fiber layer, and rodents similarly treated manifest disorganization of the photoreceptor nuclear layer and cone photoreceptor damage . It has been suggested that VGB‐induced elevation of ocular/retinal GABA induces excitotoxicity via GABAergic receptors, resulting in oxidative stress . Other studies have suggested globus pallidi and white matter anomalies associated with chronic VGB intake, and pathological roles for amino acids (ornithine, taurine) that share structural and biochemical properties of GABA have also been implicated in VGB‐associated ocular toxicity .…”