This study confirmed the beneficial effects of EE on MSD-induced cognitive deficits in CD-1 mice and its associated mechanisms. The results showed that EE exposure significantly improved the decline of hippocampal learning and memory in MSD-induced offspring. EE reversed the dysfunction of histone acetylation caused by MSD, as shown by increased expression of CBP, acetyl-H3K9, acetyl-H4K12, and decreased expression of HDAC2, in hippocampal tissues. Increased expression levels of BDNF and PSD-95 indicated that EE had beneficial effects on synaptic plasticity markers. It suggests that a housing environment could be a significant non-pharmacological method for improving cognitive dysfunction through the histone acetylation pathway.
An enriched environment improved the spatial cognition impairment resulting from maternal sleep deprivation
MSD, as a prenatal stress, affects the development of the fetal central nervous system, especially the hippocampus, through interfering with the intrauterine environment. The Morris water maze test was chosen as a robust and reliable test that is strongly correlated with spatial learning and memory function (Zhuang et al. 2021). The results showed that adult CD-1 offspring of male and female mice (PND90) whose mothers exposed to sleep deprivation by activity wheel for 6 h per day during the third trimester (gestational days 15-21), spent a significantly longer time locating a hidden platform and less time exploring the target quadrant than the control group. Similarly, our previous study demonstrated that adolescence CD-1 offspring mice (PND60) received the same treatment performed poorly during the Morris water maze test (Zhang et al. 2022). These results indicated that MSD caused spatial cognitive impairment in offspring that persists from adolescence to adult. Furthermore, male offspring Wistar rats (21-d-old) exposed to maternal sleep deprivation using a modified small-platform method on gestational day (GD) 18, but not on GD 4 and GD 9, for 72 h took significantly longer to find the hidden platform than the controls during the Morris water maze test (Zhao et al. 2014). These results appear to indicate that only sleep deprivation during late pregnancy could negatively affect offspring’s cognitive function. However, young adult offspring Sprague-Dawley rats exposed to sleep deprivation with gentle handling for 6 h per day during the first (gestational days 1-7), second (gestational days 8-14), and third trimester (gestational days 15-21) of pregnancy showed dramatic deficits in spatial learning and memory in the Morris water maze test (Peng et al. 2016). These inconsistent results regarding the effects of sleep deprivation at different stages of pregnancy on offspring’s cognitive ability may be attributed to different strains, methods, and duration of sleep deprivation. Notably, the above aforementioned results provide guidance that, no matter what stage of pregnancy sleep disorders occurs in, it is worthy of attention and prevention.
EE could improve impaired synaptic plasticity and cognitive dysfunction by increasing synaptic activity in the hippocampus through the exploration of novel objects and social communication. One study showed that a short-term exposure to EE alleviated spatial learning and object-place associative memory impairment in schizophrenia model induced by MK-801(Murueta-Goyena et al. 2019). Another study suggested that EE paradigm with a duration of 13 d reversed maternal separation-induced impairments of spatial learning and memory function (Joushi et al. 2021). These results are consistent with our study, showing that access to EE during adolescence improved MSD-induced cognitive decline in offspring from the MSD+EE group, as reflected by a shorter escape latency and more time spent in the training quadrant than the control group. Taken together, EE could be used as an intervention to improve spatial learning and memory impairment in offspring whose dams underwent sleep deprivation during the third trimester of pregnancy.
Enriched environment reversed histone acetylation dysfunction resulting from maternal sleep deprivation
Accumulating evidence indicates that histone acetylation regulates the transcription of learning and memory-associated protein genes, including Homer, Arc, c-fos, and Creb, and abnormal histone acetylation is an important factor in the pathogenesis of cognitive impairment (Rudenko and Tsai 2014). CBP is one of the most well-studied histone acetylases and has been shown to be critical for long-term memory formation. Early research indicates that genetic mutations in CBP are linked to cognitive deficits in Rubinstein-Taybi syndrome (Oike et al. 1999). CBP could broadly affect the transcription of genes involved in neuronal proliferation and differentiation, and synaptic plasticity. During in vitro study, the cultured neurons with CBP ablation resulted in impaired outgrowth, immature spines, and deficient activity-dependent synaptic remodeling (Del Blanco et al. 2019). During in vivo studies, researchers tried to use different methods to knock out CBP to explore the action of CBP on cognitive function and its specific mechanisms. The CBP+/- heterozygous mice exhibit a severe defect in long-term LTP of the hippocampus, paralleling their deficits in long-term memory during fear conditioning and novel object recognition tasks. Moreover, CaMKIIα-cre/CBPf/f mice with the CBP gene deleted in the forebrain have impaired long-term recognition memory during novel object recognition test(Alarcón et al. 2004; Valor et al. 2011).Furthermore, CBP plays a dual role in CREB-mediated genes as a scaffolding protein to recruit CREB and as a histone acetylase activity (HAT) that can leave epigenetic marks on the chromatin. The CBPKIX/KIX mice with mutations in the CREB-binding (KIX) domain of the coactivator CBP show impaired long-term memory during contextual and cued fear conditioning (Wood et al. 2006), while CBP{HAT-} mice carrying a dominant-negative CBP transgene that specifically blocks HAT show deficits in spatial and recognition memory (Korzus et al. 2004). Even though these transgenic mice produce different performances in various learning and memory tasks owing to different gene knockout techniques, it is undeniable that CBP expression level is closely related to cognitive function. Many studies reported that the downregulation of CBP is associated with cognitive impairment in Alzheimer’s disease and Huntington’s disease a (Ettcheto et al. 2018; Giralt et al. 2012). The results of WB and RT-PCR showed that MSD decreased CBP expression as well as acetyal-H3K9 and acetyal-H4K12.
HDAC2 acts in an opposite role to CBP and inhibits learning and memory-associated gene transcription by repressing the interaction between transcription factor and chromatin. Previous study demonstrates that overexpression of HDAC2 decreases dendritic spine density, synapse number, synaptic plasticity, and associative learning and memory formation (Guan et al. 2009). Inversely, the knock-down of HDAC2 in the hippocampus of CK-p25 mice, a mouse model of Alzheimer’s disease, leads to a rescue of synaptic plasticity, and abolishes the impairment of neurodegeneration-associated memory (Gräff et al. 2012). The upregulation of HDAC2 and HDAC3 has been observed in the hippocampus of offspring exposed to sevoflurane during late-pregnancy on postnatal day 1 (P1) and P35 accompanied with long-term cognitive impairment. The current study demonstrated that mRNA and protein of HDAC2 were increased in offspring from the MSD group.
Substantial evidence suggests that histone deacetylase inhibitions, including Trichostatin A and sodium butyrate, have been shown to increase the level of histone acetylation to improve cognitive decline in different pathological models (Hsing et al. 2015; Topuz et al. 2020). Additionally, EE improved traumatic brain injury- and sevoflurane induced cognitive impairment by increasing CBP expression and decreasing HDAC2 expression, respectively (Wang et al. 2018; Yu et al. 2020). Furthermore, mice with CBP deficiency showed a strong defect in EE-induced neurogenesis and enhancement of spatial navigation (Lopez-Atalaya et al. 2011). In the present study, the results showed that EE treatment increased CBP and decreased HDAC2 in the offspring from the MSD+EE group. Taken together, EE treatment could reverse histone acetylation dysfunction induced by MSD.
An enriched environment reversed changes in synaptic plasticity markers resulting from maternal sleep deprivation
BDNF is an important neuroprotective factor associated with cognition and its transcription is controlled by histone acetylation modification. Three days of sleep deprivation reduced H3 and H4 acetylation levels in the promoters of Bdnf and significantly downregulated BDNF expression (Duan et al. 2016). In this study, the expression levels of acetyal-H3K9, acetyal-H4K12, and BDNF were decreased in MSD-induced offspring. Previous study report that exercise improves Bdnf gene transcription and decreases the binding of HDAC2 to the Bdnf promoter in the hippocampus, resulting in an increase in BDNF release (Sleiman et al. 2016). In the current study, EE treatment consistently increased the expression levels of acetyal-H3K9, acetyal-H4K12, and BDNF in the offspring from the MSD+EE group. Meanwhile, PSD-95 is an important postsynaptic protein associated with synaptic maturation and is involved in postsynaptic plasticity; MSD decreased the mRNA and protein expression of PSD-95 in the offspring hippocampus, while EE reversed it. These results are in accordance with previous studies showing that exposure to an EE protocol improves sepsis-associated cognitive impairment by upregulating the expression of PSD-95 (Córneo et al. 2022).
Our study has limitations. Firstly, histone acetylation is regulated by histone HATs and HDACs, but we only examined CBP and HDAC2. Secondly, we did not use chromatin immunoprecipitation technique to specifically detect histone acetylation levels in the promoter region of Bdnf and PSD-95. Finally, we did not use histone deacetylases inhibitors to reverse the cognitive impairment induced by MSD to further verify the origin of the cognitive decline is histone acetylation dysfunction.