**4. Discussion**

In the present study, our model showed that astrocytes regulate enzymatic and protein activity from the genomic to the protein level, considering the protein functional modulation at different molecular levels. Additionally, during normal conditions, the enzymatic activity of hydrolase, transferase, peroxidase, oxidoreductase, isomerase and lyase/ligase were identified as constantly regulated due to elevated metabolic rates and plasticity in astrocytes [48, 49]. In this case, metabolic maintenance and support are not permanently regulated by epigenetic processes due to a dynamic environmental-dependent mechanism in astrocytes. Nevertheless, the presence of metabolic processes in bivalent regions implies the presence of highly active metabolic processes that change across time due to the fact that a genomic region can present both marks and become active or repressed [43, 50]. In terms of astrocyte-neuron interaction, we identified the presence of antioxidant activity associated with glutathione biosynthetic processes, reductase activity, as well as the activity of structural constituents of myelin sheath [51, 52]. Relationship between astrocytes and neurons in the context of antioxidant defense to ensuring neuronal well-being during pathological conditions play a significant role in metabolic support by neuroprotective capacity from oxidative stress, supply of glutathione to neurons, modulation of the extracellular matrix assembly, among others [52, 53].

To examine the molecular response to PA or APAR mechanism in astrocytes, we integrated the epigenetic data with the transcriptomic data from NHA to elucidate the potential damaging conditions by the PA activity in the brain. The shared TAD regions from both cerebellum and spinal cord astrocyte Hi-C data were compared to each other in order to establish the differences and possible considerations associated with tissue-specific stimuli. Thus, our multi-omic model showed that during PA lipotoxicity in astrocytes, inflammatory and stress responses are overexpressed. Our results also indicated that lipid droplets are epigenetically regulated in order to respond to free fatty acid concentrations in homeostatic conditions by the presence of apolipoprotein-E (APOE) gene in euchromatin regions [4, 40]. For instance, recent evidence has shown that maintenance of the homeostasis between astrocytes and neurons mitigate the lipotoxic effects of fatty acids as well as modulating APOE-lipid particles becomes of vital importance [54].

The presence of PA is associated with the overexpression of biological processes such as response to cellular lipid metabolism, which can lead to disease [5, 55]. Moreover, high concentrations of PA induce the expression of markers involved in pro-inflammatory response where the secretion of IL-1 activates endothelial cells and astrocytes to propagate the inflammatory signals in CNS [56, 57]. Overall, IL-1 is a typical biomarker associated with lipotoxicity and inflammation in astrocytes, as LC3-II, p62, or TLR2 have been directly linked to the astrocytic response to PA [5, 11, 58]. Likewise, IL-1 supports mechanisms as extracellular matrix binding modulation and regulation obtained in experimental studies that are essential for the response to mechanical stimuli in astrocytes [41]. In this sense, our results support the involvement of epigenetic regulation over cellular functional determinants in astrocytes during neurodegeneration but are necessary to develop more precise algorithms associated with gene screening [4].

Moreover, our model shows and support evidence from experimental studies, highlighting the expression and regulation of transporters such as the glutamate and lactate shuttle, redox stress reduction, transfer mitochondrial, among others, which are associated with the APAR mechanisms. Many of these biological functions associated with the response of astrocytes seem to be regulated by some of the tested histone modifications. Also, the response to external stimulus can be associated with the presence of neurotransmitter receptors, evidencing the neuron-astrocyte interaction beyond the metabolic support. Interestingly, we also report the presence of genes involved in the biosynthetic process of glutathione in the euchromatin regions, meaning a recurrent antioxidant activity process in astrocytes. Glutathione biosynthesis and release have been associated as a strategy for the balance and detoxify of the neural activity mediated by mitochondrial reactive oxygen species (ROS) in neurons linked to neurotransmission, neuroinflammation, neural disease etiology and progression [59, 60]. Glutathione biosynthesis is related to astrocytes antioxidant defense activity during pathological and non-pathological conditions.

Transcriptomic data, epigenetic landscape of TDAs, and histone modification regions data allowed the identification of APAR genes in the transcriptomic dataset and their localization (bivalent activation) [61]. TNFRSF1B, IL1R2, IL18RAP, IL1A, IL5RA, CXCL10, IL5, PIK3CG, IL10RA, and CCL8 genes were identified and associated with APAR mechanisms in astrocytes. Recently it has been demonstrated that during non-stimulating conditions, astrocytes secrete cytokines such as GM-CSF, CXCL1, CCL2, CXCL8, IL-6, and IL-8, all of those displayed at different levels [22, 37]. Moreover, administration of IL-1B and TNF activates astrocytes response with the production of cytokines IL-1B, IL-1RA, TNFA, CXCL10, CCL3, CCL5 and IL6 [62–64], being IL-6 response more efficient at higher concentration [65, 66].

Chromatin conformation in astrocytes has shown that PA response genes were located within shared TADs. During inflammation interleukin-1 receptor type II (IL1R2) has been described as a key receptor of which the expression reduces IL1A and IL1B activity [9]. On the other hand, the interleukin 18 receptor accessory protein (IL18RAP) that is associated with the pro-inflammatory response of IL18 by intracellular signaling was located in the same TAD region, suggesting that they share the same regulatory response when inflammatory processes occur in astrocyte [67]. Additionally, this TAD region also contains IL1R1 which is a key molecular mechanism associated with astrocytic response to inflammation by interaction with IL1A, IL1B and IL1R-agonists. Likewise, the TAD contains IL1RL2, and IL18R1, both interleukin receptors related to inflammatory cellular processes [5, 68, 69].

The coregulation of certain gene groups can also be associated with either master regulatory regions in TADs or architecture proximity regulation in the nucleus [70]. It is plausible that PA-lipotoxic responses in regulation of astrocytes by activating TAD regions depends upon extracellular signaling. This is possible because of the proximity of TAD to nucleus for cooperative organized regulation of genomic regions [44, 71]. Our results finally suggest that epigenetic modulation has an important role in the regulation of APAR mechanisms, yet further experiments are necessary to explore the TAD proximity involved in APAR regulation.

### **5. Conclusions**

We present the first comprehensive data integration of epigenetic involvement in the astrocytic response to PA through the analysis from Hi-C, ChIP-Seq, and transcriptomic data in a multi-omic level. We described the role of epigenetics as a key mechanism of astrocytic PA response within which we found histones markers with bivalent capacity associated with repression of genomic activity (H3K4me3 and H3K27me3). This finding determines the adaptability and response to environmental stress, provided through complex astrocyte metabolic plasticity

#### *Multi-Omic Epigenetic-Based Model Reveals Key Molecular Mechanisms Associated… DOI: http://dx.doi.org/10.5772/intechopen.100133*

networks. In addition, our results showed that markers as H3K4me3, H3K27ac, H3K9ac, H3K4me1, H4K20me1, H3K36me3 and H3K79me2 have regions associated with homeostatic processes linked to exchange processes, regulation of the extracellular matrix, protein maintenance and ion channels regulation. These processes were found in euchromatin regions, highlighting that it is associated with essential basal functions in astrocytes. Likewise, signaling pathways modulation (*i.e.,* PI3K/ AKT), antioxidant activity (a recurrent mechanism in astrocytes), among others, were associated with glutathione biosynthesis processes, glutamate transport and glutamatergic neuronal support, identified as active basal coding regions.

APAR mechanisms proved to be highly regulated by histone modifications along the genome which is essential for the response to PA. Additionally, our results revealed the presence of highly regulatory regions in the TADs associated with IL1R2 and IL18RAP. Moreover, the location of genes encoding to interleukins in the genome and chromatin conformation revealed the putative epigenetic regulation of the inflammatory response. In this sense, our results support the involvement of APAR mechanisms on the lipotoxic effect of PA in astrocytes. While integrating transcriptomics with epigenetics data was possible to identify associated genes with APAR mechanisms and genes in response to PA located inside the topologically associated, genes found in the TAD region that shared the regulator responses linked to inflammatory processes were likely modulated by lipotoxicity actions. Additionally, it is possible to suggest that additional epigenetic mechanisms such as lncRNA, miRNA and extracellular signaling could be involved in the astrocytic response to PA. Considering that deterministic mechanisms of expression are still unknown for astrocytes in lipotoxic conditions, we suggest that epigenetic modulation is essential for an efficient and dynamic cellular response. This work is a novel approach that involves epigenetic regulation in the cellular response to PA-lipotoxicity in astrocytes. Therefore, it should be emphasized that it is recommended the development of new methodologies and algorithms for more accurate analysis associated especially to genetic encryption. Finally, an accurate investigation of this new multiomic epigenetic-based model by integrating multiple underlying data sources about the cellular mechanisms of the response to PA-lipotoxicity in astrocytes, might help in the future to detect shared genetic patterns found in the TAD region among the neurodegenerative diseases, identifying biomarkers for differentiating disease states and thereby facilitating the decision-making process and treatment management.
