S-Sulfocysteine Induces Seizure-Like Behaviors in Zebrafish

Abstract

Sulfite is a neurotoxin, which is detoxified by the molybdenum cofactor (Moco)-dependent enzyme sulfite oxidase (SOX). In humans, SOX deficiency causes the formation of the glutamate analog S-Sulfocysteine (SSC) resulting in a constant overstimulation of ionotropic glutamatergic receptors. Overstimulation leads to seizures, severe brain damage, and early childhood death. SOX deficiency may be caused either by a mutated sox gene or by mutations in one of the genes of the multi-step Moco biosynthesis pathway. While patients affected in the first step of Moco biosynthesis can be treated by a substitution therapy, no therapy is available for patients affected either in the second or third step of Moco biosynthesis or with isolated SOX deficiency. In the present study, we used a combination of behavior analysis and vital dye staining to show that SSC induces increased swimming, seizure-like movements, and increased cell death in the central nervous system of zebrafish larvae. Seizure-like movements were fully revertible upon removal of SSC or could be alleviated by a glutamatergic receptor antagonist. We conclude that in zebrafish SSC can chemically induce phenotypic characteristics comparable to the disease condition of human patients lacking SOX activity.

Case Report: Compound Heterozygous Variants in MOCS3 Identified in a Chinese Infant With Molybdenum Cofactor Deficiency

“…The structure of S-sulfocysteine highly resembled the excitatory neurotransmitter glutamate that could bind and stimulate glutamatergic receptors. It has been proved in a zebrafish model that S-sulfocysteine could stimulate glutamatergic receptors, induce seizure-like movements, and increase cell death in the central nervous system (Zaki et al, 2016 ; Plate et al, 2019 ). Accumulation of S-sulphocysteine and thiosulfate would activate the N-methyl-D-aspartate receptor and increase intracellular magnesium and calcium.…”

“…Accumulation of S-sulphocysteine and thiosulfate would activate the N-methyl-D-aspartate receptor and increase intracellular magnesium and calcium. This could enhance the permeability of the mitochondria, compromise the mitochondrial energy supply, and reduce the cell viability in cerebral cortex of rat brain (Grings et al, 2014 ; Marelja et al, 2018 ; Bender et al, 2019 ; Plate et al, 2019 ). All these processes may be the possible mechanisms underlying the neurological dysfunction observed in our patient.…”

Fosdenopterin: a First-in-class Synthetic Cyclic Pyranopterin Monophosphate for the Treatment of Molybdenum Cofactor Deficiency Type A

“…42 In addition, a specific pathomechanism has been proposed that can explain the apparent peracute neuronal failure after birth: SSC is a strong N-methyl-D-aspartate (NMDA) receptor agonist, 43,44 and leads to excitotoxic neuronal cell death and seizures. [45][46][47] It is therefore likely that, in MoCD, the encephalopathic crisis after birth is caused by an excitotoxic storm that leads to widespread neuronal necrosis.…”

A zebrafish-centric approach to antiepileptic drug development

“…We also learned that, by itself, a simple locomotion-based assay yielded ‘false positive’ results of drugs that reduced seizure-like behavior but did not change electrographic events because they are sedatives or muscle-relaxants. It is worth a strong note of caution that many subsequent screening efforts that have adapted our larval zebrafish seizure models largely relied upon locomotion-based outcomes but lack any electrophysiological confirmation of anti-seizure activity, thereby adding putative ‘hit’ compounds to the literature that are likely to be false positives ( Sharma et al, 2020 ; Shen et al, 2020 ; Gong et al, 2020 ; Baxendale et al, 2012 ; Jones et al, 2020 ; Thornton et al, 2020 ; Brillatz et al, 2020 ; Li et al, 2020 ; Decui et al, 2020 ; Plate et al, 2019 ; Brueggeman et al, 2019 ; Aourz et al, 2019 ; Bertoncello et al, 2018 ; Moradi-Afrapoli et al, 2017 ; Li et al, 2015 ; Siebel et al, 2015 ; Rahn et al, 2014 ; Buenafe et al, 2013 ). Specifically, acute larval PTZ-induced seizure assays coupled with electrophysiological confirmation can accurately predict AED efficacy ( Baraban et al, 2005 ; Afrikanova et al, 2013 ; Berghmans et al, 2007 ), whereas behavior-only discoveries lacking this crucial electrophysiology assay are difficult to interpret.…”