Histone acetylation is one of the most researched mechanisms among the numerous forms of histone modifications, and studies have demonstrated the significance of these alterations for brain development and function (Keverne, 2014; Sen, 2015). This procedure involves attaching an acetyl group to a lysine at the nucleosome’s N-terminal tail, which is the basic unit of DNA packing in eukaryotic cells. Histone acetyltransferases (HATs) and histone deacetylases (HDACs) are enzymes that catalyze the acetylation of histones. By removing acetyl groups from histone tails, HDACs reverse the activity of HATs and inhibit transcription (Berndsen and Denu, 2008; Wu and Zhao, 2018).

It is commonly believed that histone acetylation is advantageous for memory and cognition, since it is frequently linked to an increase in the expression of many genes that play significant roles in synaptic plasticity, learning, and memory in the brain (Kim and Kaang, 2017). Recent studies have shown long-term developmental cognition deficits and changes in histone acetylation in rodents exposed to anesthetics during the prenatal or neonatal phase. For instance, adult mice that were exposed to sevoflurane during the neonatal period had higher hippocampal levels of HDAC3 and HDAC8 but not HDAC1 and HDAC2. Additionally, rats exposed to sevoflurane had lower levels of acetylated H3K14/18 and H4K5/12 in the hippocampus (Chai et al., 2022) and lower levels of many genes involved in neurodevelopment and neuroplasticity, such as c-Fos, brain-derived neurotrophic factor (BDNF), and postsynaptic density protein 95 (PSD-95) (Wu and Zhao, 2018). It is important to note that sevoflurane exposure is linked to alterations in specific regions of the brain. For example, H3K9/14 and H4K12 acetylation were decreased in the hippocampal CA1 region, and H3K14 acetylation was decreased in both the CA1 and dentate gyrus (DG) regions of the hippocampus (Jia et al., 2016). While some studies have shown that histone hypoacetylation is controlled by an increase in HDACs, some studies have demonstrated opposing results. These studies have indicated that this process involves a decrease in histone acetylation and is a key factor in the memory and cognitive impairments caused by neonatal anesthesia. Through the use of the HDAC inhibitors trichostatin A (TSA) and sodium butyrate, researchers were able to successfully reduce the neurocognitive impairment induced by general anesthesia as well as the dysregulation of histone acetylation in the hippocampus (Zhong et al., 2015; Jia et al., 2016).

The most frequent posttranslational change in eukaryotes is protein phosphorylation. The creation of phosphodiester bonds can occur on the side chains of serine, threonine and tyrosine (often referred to as “residues”). Numerous protein kinases, protein phosphatases, and phosphorylated proteins are found in neurons, and many of these are crucial for controlling neuronal morphology and for a variety of cell processes, including membrane excitability, secretory procedures, cytoskeletal organization, and cellular metabolism (Li et al., 2021).

Histone phosphorylation is another PTM that offers a distinctive epigenetic signature to control chromatin dynamics (Graff and Mansuy, 2008). The “writer” enzymes responsible for histone phosphorylation are kinases, which mediate the transfer of a phosphate moiety from the high-energy molecule ATP. In contrast, the “eraser” enzymes that remove these phosphate groups are protein phosphatase 1 (PP1) and MAPK phosphatase 1 (MKP1) (Torres-Perez et al., 2021). Due to its link to chromosome condensation during mitosis, histone H3 phosphorylation has attracted considerable attention (Bradbury et al., 1973; Gurley et al., 1974; Gurley et al., 1978). Interestingly, the activation of mitogenic signaling pathways is the origin of H3 phosphorylation (Mahadevan et al., 1991). For example, ribosomal protein S6 kinase 2 (RSK2) is downstream of several other kinases, including extracellular signal-regulated kinase (ERK), aurora kinase family member increase in ploidy 1 (IPL1), and mitogen- and stress-activated protein kinase 1 (MSK1), and RSK2 mediates the phosphorylation of histone H3 on the serine 10 residue (Sassone-Corsi et al., 1999; Thomson et al., 1999; Hsu et al., 2000). In addition, H3 phosphorylation controls the binding of the transcriptional apparatus to chromatin molecules, which modulates key biological processes in conjunction with other histone modifications.

MSK1 is crucial in activating the histone phosphorylation process. This is supported by the fact that long-term spatial and contextual fear memory development is impaired by MSK1 germline deletion (Chwang et al., 2007). In addition to MSK1, the IB kinase (IKK) complex also controls histone phosphorylation in the hippocampal nucleus and memory formation (Lubin and Sweatt, 2007). The above studies show that histone phosphorylation is crucial for the development of memories. However, studies investigating the involvement of histone hypophosphorylation in sevoflurane-induced cognitive impairment have not been reported to date. Perhaps this will be investigated further and tested in the near future.

Histone methylation is another mechanism that controls chromatin structure; methyl groups can be added by histone methyltransferases (HMTs) or removed by FAD-dependent amine oxidase (KDM1 or LSD1) (Torres-Perez et al., 2021). In addition, histone methylation can cause transcriptional activation, even though methylation is typically thought of as a signal for transcriptional repression. When histone H3 is methylated at lysine 9 (H3K9), it is related to transcriptional repression. However, when it is methylated at lysine 4 (H3K4), it is related to transcriptional activation (Vermeulen et al., 2007). The role of histone methylation in long-term memory storage has been the subject of recent research (Gupta et al., 2010). Researchers have found that hippocampal H3K4 is trimethylated and hippocampal H3K9 is dimethylated throughout the process of memory formation, indicating that gene expression alterations exist during this process (Gupta et al., 2010). After surgery under general anesthesia for 24 h, mice showed a substantial increase in H3K9me3 expression and its binding to the BDNF exon IV promoter, while BDNF expression was downregulated (Wu et al., 2019). Although middle-aged mice were used as the study model, these findings enhance our understanding of neonatal anesthetic neurotoxicity, especially in light of the bidirectional regulation of this change and the paucity of in-depth research.

HMTs and histone demethylases are enzymes that regulate histone methylation. SET-domain HMTs, non-SET-domain DOT1/DOT1L methyltransferases, and PRMT1 arginine methyltransferases are the three enzyme families that compose the HMTs (Jarome and Lubin, 2013). Arginine and lysine residues may undergo histone methylation. Lysine methylation on histones H3 and H4 is much more extensively studied. Histone methylation is improved in the hippocampus after treatment with sodium butyrate, an HDAC inhibitor. This implies a functional connection between histone methylation and acetylation during the consolidation of terror memories (Gupta et al., 2010). Additionally, research has shown that mice lacking the histone methyltransferase myeloid/lymphoid or mixed-lineage leukemia 2 (MLL2/KMT2B) gene in adult excitatory neurons display memory deficit tasks that depend on the hippocampus (Kerimoglu et al., 2013). By correcting the transcriptional imbalance of memory-related genes, it is feasible to use histone methylation as a therapeutic target for sevoflurane-induced cognitive impairment.

Phosphorylation plays a crucial role in regulating the physiological functions of Tau, including its binding to microtubules, and therefore regulates the stabilization and assembly of microtubules themselves. Hyperphosphorylation of Tau may induce brain pathology and is one of the earliest events in the development of AD, playing an important role in the impairment of neuronal function, memory, and cognition. Tau phosphorylation provides a new perspective on sevoflurane-induced cognitive dysfunction in neonatal mice (Yu et al., 2022). Sevoflurane-induced cognitive impairment in neonatal mice is altered by Tau phosphorylation.

Tau protein was first identified in 1975 as a microtubule-associated protein (Weingarten et al., 1975) that is restricted to the axonal compartment of neurons in the healthy brain (Binder et al., 1985). Tau proteins are a family of microtubule-associated proteins that play a role in microtubule assembly, support axonal integrity in mature neurons (Buée et al., 2000; Sergeant et al., 2008), and perform functions at the dendrites in neurons (Ittner et al., 2010; Sultan et al., 2011). The main components of the intraneuronal paired helical filaments (PHFs) seen in AD are hyperphosphorylated and abnormally phosphorylated forms of Tau. They have also been observed in Tauopathies, which are other neurodegenerative diseases featuring filaments of a similar type (Buée et al., 2000; Buee et al., 2010; Iqbal et al., 2010). Excessive Tau phosphorylation encourages the creation of intractable Tau clumps. Once the aggregate has been produced, it can leave the original cell, come into contact with a connected cell, and then enter the cell to produce more aggregation by altering the conformation of the template (Holmes and Diamond, 2014). In addition, Tau phosphorylation can impair cognitive performance in mice (Faraco et al., 2019).

In mice at postnatal day 6, multiple exposures, but not a single exposure, to 3% sevoflurane for 2 hours daily led to Tau phosphorylation via GSK-3β activation. This phosphorylation increased IL-6 levels and decreased PSD-95 levels in the hippocampus, leading to cognitive impairment (Tao et al., 2014). Tau may play a key role in sevoflurane-induced neuroinflammation and synaptic impairments in mice since these side effects of the drug did not manifest in Tau knockout (Tau KO) mice. Because of reduced mitochondrial performance in newborn mice, ATP is decreased and NUAK family kinase 1 (NUAK1) is increased in the brain. According to Lasagna-Reeves et al., increasing NUAK1 may cause Tau phosphorylation at serine 356 and prevent Tau degradation (Lasagna-Reeves et al., 2016), causing more Tau to accumulate in the brain tissues of neonatal mice than in those of adult mice (Yu et al., 2020). Therefore, neonatal mice are more vulnerable than adult mice to the onset of Tau phosphorylation caused by sevoflurane anesthesia (Yu et al., 2022). Acute 1.5% sevoflurane exposure resulted in considerable, dose-dependent, and reversible hyperphosphorylation of Tau in the hippocampus of 5- to 6-month-old mice (Le Freche et al., 2012). Furthermore, previous research demonstrated that repeated exposure to 2.5% sevoflurane under normothermic conditions causes Tau to be persistently hyperphosphorylated at the Ser396/Ser404 and Thr181 phosphoepitopes (Le Freche et al., 2012). The Morris water maze (MWM) test was used to measure subsequent substantial deficits in spatial learning and memory in mice (Le Freche et al., 2012). These earlier investigations highlight a potential mechanism through which anesthetics may impair cognition and increase the risk of AD because hyperphosphorylated Tau is a prominent component of neurofibrillary lesions (Run et al., 2009). However, a direct link between the accumulation of phosphorylated Tau and cognitive impairment needs to be established. IL-6 production and cognitive dysfunction were shown to be the result of the ability of sevoflurane to cause Tau phosphorylation and the extracellular vesicle-associated trafficking of Tau from neurons to microglia (Dong et al., 2021) (Figure 2). As a result, these recent discoveries suggest that Tau phosphorylation may be a factor in sevoflurane-induced memory and cognitive impairments. Future research is undoubtedly needed to better understand how phosphorylated Tau contributes to anesthesia-induced memory and cognitive impairments.

Sex-dependent anesthetic-induced neurotoxicity has been reported in the developing brain (Lee et al., 2014; Ju et al., 2018). The role of sex hormones in these effects has been specifically determined (Yang et al., 2021; Jevtovic-Todorovic, 2022). For example, neonatal male mice had lower levels of brain testosterone and were more vulnerable to the neurotoxic effects of sevoflurane. Treatment with testosterone before sevoflurane anesthesia attenuated sevoflurane-induced Tau phosphorylation and cognitive impairment by decreasing the binding and interaction between Tau and GSK-3β, a kinase that phosphorylates Tau in neonatal male mice (Yang et al., 2021). These results suggest possible protective effects of testosterone on anesthetic-induced neurotoxicity. However, it is important to note that Tau levels in the hippocampus and cortex are higher in neonatal mice than in adult mice, which could account for the increased Tau phosphorylation in neonatal mice after sevoflurane anesthesia (Yu et al., 2020). Testosterone enhanced mitochondrial function in the rat brain by increasing mitochondrial reduced nicotinamide adenine dinucleotide-ubiquinone oxidoreductase chain 1 protein levels and mitigating oxidative damage (Yan et al., 2017). Thus, future work should determine whether neonatal male mice accumulate more Tau and phosphorylated Tau in the brain attributable to impaired mitochondrial function resulting from lower testosterone levels.

We should note that a single exposure of neonatal rats to sevoflurane led to significant behavioral abnormalities not only in exposed rats in adulthood but also in their adult male offspring that were never exposed to sevoflurane (Ju et al., 2018). In addition, anesthetic-induced abnormalities are greater in male animals (Lee et al., 2014; Ju et al., 2018). Male progeny, but not female progeny, of sevoflurane-exposed parents showed abnormalities in behavioral testing and expression of the KCC2 cotransporter, and the male F1 rats of two exposed parents showed impaired spatial memory and KCC2 expression (Ju et al., 2018). These results suggest that there could be sex-dependent differences in sevoflurane-induced neurotoxicity (Chastain-Potts et al., 2020). The neurobehavioral abnormalities induced by neonatal exposure to the general anesthetic sevoflurane can be transmitted to the next-generation in a complex, sex-specific manner through epigenetic mechanisms (Ju et al., 2018). The underlying mechanisms of sevoflurane-induced sex-specific effects across two generations are exciting and challenging topics for future studies. Researchers should make further efforts to elucidate how closely the observed changes in protein PTMs expression relate to sex-specific effects of sevoflurane.