Spatiotemporal analysis of the UPR transition induced by methylmercury in the mouse brain

Abstract

Methylmercury (MeHg), an environmental toxicant, induces neuronal cell death and injures a specific area of the brain. MeHg-mediated neurotoxicity is believed to be caused by oxidative stress and endoplasmic reticulum (ER) stress, but the mechanism by which those stresses lead to neuronal loss is unclear. Here, by utilizing the ER stress-activated indicator (ERAI) system, we investigated the signaling alterations in the unfolded protein response (UPR) prior to neuronal apoptosis in the mouse brain. In ERAI transgenic mice exposed to MeHg (25 mg/kg, S.C.), the ERAI signal, which indicates activation of the cytoprotective pathway of the UPR, was detected in the brain. Interestingly, detailed ex vivo analysis showed that the ERAI signal was localized predominantly in neurons. Time course analysis of MeHg exposure (30 ppm in drinking water)showed that whereas the ERAI signal was gradually attenuated at the late phase after increasing at the early phase, activation of the apoptotic pathway of the UPR was enhanced in proportion to the exposure time. These results suggest that MeHg induces not only ER stress but also neuronal cell death via a UPR shift. UPR modulation could be a therapeutic target for treating neuropathy caused by electrophiles similar to MeHg.

Alterations in UPR Signaling by Methylmercury Trigger Neuronal Cell Death in the Mouse Brain

“…However, few reports have demonstrated differences in MeHg sensitivity among these regions. We have previously shown that ER stress is elicited in a neuron-specific manner in the damaged regions by MeHg (somatosensory cortex, motor cortex, visual cortex, auditory cortex, and striatum), but the sensitivity differs among these regions [ 22 ]. In a previous study, the site-specific toxicity of MeHg was indicated to arise from the expression level of antioxidant proteins [ 23 ].…”

“…Upon sensing ER stress, IRE1α is activated via autophosphorylation and induces the expression of molecular chaperones and ERAD-related proteins by splicing XBP1, thereby relieving ER stress [ 24 , 25 ]. We have previously shown that 30 ppm MeHg exposure transiently upregulates the expression of HRD1, a downstream factor of the IRE1α-XBP1 axis, similar to the ERAI signaling pattern [ 22 ]. Therefore, we examined the changes in IRE1α activity and downstream HRD1 expression by 50 ppm MeHg exposure.…”

“…When ER stress is not resolved, cells induce expression of the ER stress-mediated apoptosis-inducing factor, CHOP, via the PERK or ATF6 pathway, thereby promoting apoptosis [ 26 ]. We have previously shown that the number of CHOP-positive cells increases in a time-dependent manner in neurons [ 22 ]. Therefore, to clarify the contribution of ER stress and the PERK pathway to MeHg-induced neuronal cell death, we investigated the time-dependent induction of CHOP expression and activation of the PERK branch.…”

“…Indeed, an increase in apoptotic cells was observed in the mouse brain during the late phase of MeHg exposure ( Figure 5 ). We have previously shown that the ERAI signal was observed predominantly in NeuN-positive neurons in the somatosensory cortex [ 22 ], suggesting that MeHg-mediated ER stress induction and alterations in UPR signaling are involved in neuronal cell death.…”

Cellular Conditions Responsible for Methylmercury-Mediated Neurotoxicity

“…These results suggest that MeHg induces neuronal apoptosis through ROS-mediated ER stress and mitochondrial apoptosis pathways. Furthermore, a recent study showed site-and cell type-specific ER stress in the MeHg-exposed brains of ER stress-activated indicator (ERAI) transgenic mice [49]. The transgenic ERAI was constructed by fusing the gene encoding a variant of the green fluorescent protein Venus as a reporter downstream of a partial sequence of the human X-box binding protein 1 (XBP1), including an ER stress-specific intron [50].…”