## 7 Molecular Effects of Nanoplastics in Processes of Neurotoxicity
### Nanoplastics Affect the Transcriptomics and Epigenomic in Neuronal Models
Key factors controlling gene expression at the translational and post-translational levels include DNA methylation or hydroxymethylylation, phosphorylation, histone acetylation, and deacetylation, as well as the actions of histone variants, microRNAs (miRNAs), and long non-coding RNAs (IncRNAs). The emerging field of neuroepigenetics explores how epigenetic dysregulation contributes to neuronal death associated with neurodegenerative diseases [153; 154]. PS-NPs (44 nm mean size) have been shown to alter the transcriptional profile and DNA methylation of fibroblasts and human-induced pluripotent stem cells (hiPSCs). Exposure to PS-NPs resulted in the downregulation of the dopaminergic synapse signaling pathway, which is known to be related to amino acid metabolism and Parkinson's disease [155]. Additionally, enrichment analysis of transcriptome data from hiPSCs exposed to PS-NPs revealed the downregulation of genes associated with the alteration of molecular functions of metal ion transmembrane transporter activity and voltage-gated cation channel activity, both of which are key activities related to neuronal functions. As previously described (Section 4), one of the routes of NP exposure to humans is through epidermal exposure, with systemic transport allowing for deposition into various organs and tissues, including the nervous system. Thus, there is an urgent need to develop robust human cell models to investigate the potentially harmful effects of NPs on human health that consider the possible different routes of exposure, such as dermal contact. The use of fibroblasts and hiPSCs as cell models to identify epigenomic signatures following NPs exposure has been proposed to identify pathways involved in human diseases. Transcriptomic and epigenomic signatures can be revealed through RNA-seq and whole-genome methyl-seq [155]. Fibroblasts and hiPSCs exposed even to low concentrations of PS-NPs exhibited cellular responses that might trigger diseases such as cancer and neurodegeneration, with the ATG7 gene identified among them. Autophagy-related (ATG) proteins, including ATG7, play crucial roles in protecting against complex diseases like neurodegeneration, although the precise mechanism by which dysfunctional ATG7 contributes to neurodegenerative diseases in humans remains to be fully elucidated [156]. In both fibroblasts and hiPSCs, differentially methylated regions (DMRs) have been identified, with hypermethylated or hypomethylated regions. Gene ontology (GO) analysis for methylation data has revealed genes with the most prominent differentially methylated regions associated with differential gene expression. Fibroblasts exposed to NPs are affected in various GO terms related to axon guidance, cell adhesion, synapse assembly, and nervous system development, indicating potential impacts on neural development and function. Expanding our understanding to common processes and molecular markers in seemingly different diseases, like carcinogenesis and neurodegeneration, reveals intriguing insights and shared features. Techniques such as gene expression evaluations and the identification of different families of microRNAs have unveiled the carcinogenic potential of pristine PS-NPs [157]. Prolonged in vitro exposure (6 months) of mouse embryonic fibroblasts, which are sensitive to oxidative stress and prone to transformation progress (PTP), to 25 ug/mL of PS-NPs leads to stress-related gene expression and deregulation. Significant downregulation of genes crucial for neurodegenerative processes, such as _Sod1_, _Sod2_, and _Nr/2_, is observed. The _Nr/2_ gene product, a transcriptional factor, plays essential roles in regulating antioxidant defense mechanisms against oxidative stress and is implicated in neurodegenerative disorders [158]. The downregulation of these gene groups by PS-NPs supports their involvement in neurodegenerative processes.