p38/Sp1/Sp4/HDAC4/BDNF Axis Is a Novel Molecular Pathway of the Neurotoxic Effect of the Methylmercury

Abstract

The molecular pathways involved in methylmercury (MeHg)-induced neurotoxicity are not fully understood. Since pan-Histone deacetylases (HDACs) inhibition has been found to revert the neurodetrimental effect of MeHg, it appeared of interest to investigate whether the pattern of HDACs isoform protein expression is modified during MeHg-induced neurotoxicity and the transcriptional/transductional mechanisms involved. SH-SY5Y neuroblastoma cells treated with MeHg 1 μM for 12 and 24 h showed a significant increase of HDAC4 protein and gene expression, whereas the HDACs isoforms 1–3, 5, and 6 were unmodified. Furthermore, MeHg-induced HDAC4 increase was reverted when cells were transfected with siRNAs against specificity protein 1 (Sp1) and Sp4, that were both increased during MeHg exposure. Next we studied the role of extracellular-signal-regulated kinases 1/2 (ERK1/2), c-Jun N-terminal kinases (JNK), and p38 mitogen-activated protein kinases (MAPKs) in MeHg—induced increase of Sp1, Sp4, and HDAC4 expression. As shown by Western Blot analysis MeHg exposure increased the phosphorylation of p38, but not of ERK and JNK. Notably, when p38 was pharmacologically blocked, MeHg-induced Sp1, Sp4 protein expression, and HDAC4 protein and gene expression was reverted. In addition, MeHg exposure increased the binding of HDAC4 to the promoter IV of the Brain-derived neurotrophic factor (BDNF) gene, determining its mRNA reduction, that was significantly counteracted by HDAC4 knocking down. Furthermore, rat cortical neurons exposed to MeHg (1 μM/24 h) showed an increased phosphorylation of p38, in parallel with an up-regulation of Sp1, Sp4, and HDAC4 and a down-regulation of BDNF proteins. Importantly, transfection of siRNAs against p38, Sp1, Sp4, and HDAC4 or transfection of vector overexpressing BDNF significantly blocked MeHg-induced cell death in cortical neurons. All these results suggest that p38/Sp1-Sp4/HDAC4/BDNF may represent a new pathway involved in MeHg-induced neurotoxicity.

The transcription factor Nurr1 is up-regulated in amyotrophic lateral sclerosis patients and SOD1-G93A mice

“…The chromatin immunoprecipitation assay and quantitative real-time polymerase chain reaction (qPCR) quantification were performed as previously described (Valsecchi et al, 2013;Guida et al, 2017;, Iannello et al, 2019. In particular, tissues were cross-linked with 1% formaldehyde in PBS for 10 min at 37°C.…”

Methylmercury Epigenetics

“…Experiments in human-derived SH-SY5Y neuroblastoma cells and rat cortical neurons detected increased histone deacetylase 4 (HDAC4) protein resultant to MeHg exposure [23]. Moreover, this correlated with increased HDAC4 binding to the brain-derived neurotrophic factor (BDNF) promoter [23] and decreased global histone 4 (H4) acetylation in SH-SY5Y cells [24].…”

Molecular Pathways Associated With Methylmercury-Induced Nrf2 Modulation

“…In contrast, MeHg exposure SH-SY5Y cells has been shown to induce Sp1 activation, which was associated with increased p38 MAPK phosphorylation and HDAC4 expression. Furthermore, the knockdown of p38 MAPK , Sp1, and HDAC4 afforded neuroprotection against MeHg toxicity in cortical primary neurons ( Guida et al, 2017 ). Thus, it seems that a decrease in Sp1 function was associated with a cytoprotective response in astrocytes (Nrf2 activation), but the activation of this transcription factor might be associated with neuronal cell death.…”

“…MAPKs are a group of serine–threonine kinases associated with a various range of cell processes, such as inflammation, cellular survival, differentiation, or death ( Cargnello and Roux, 2011 ; Kim and Choi, 2015 ). MeHg has been shown to activate members of the MAPK kinase family, especially the extracellular signal-regulated kinases 1/2 (ERK1/2) ( Sakaue et al, 2009 ; Lu et al, 2011 ), c-Jun N-terminal kinases (JNKs) ( Fujimura et al, 2009 ), and p38 MAPK ( Guida et al, 2017 ; Heimfarth et al, 2018a ). MeHg has also been reported to down-regulate MAPK.…”