Irritable Bowel Syndrome (IBS) is a functional gastrointestinal disorder, newly called “disorders of the gut-brain interaction,” which was characterized by recurrent episodes of abdominal pain/discomfort and bowel habit changes (Botschuijver et al., 2019; Han et al., 2022; Brierley et al., 2023), with high prevalence both in childhood and adulthood worldwide (Sperber et al., 2021). With recurrent symptoms and without particularly effective treatments, IBS significantly affects the quality of life, and mental and physical health of patients (Black and Ford, 2020; Sperber et al., 2021; Li et al., 2023). Despite the major global effort, the mechanism underlying the pathogenesis of IBS remains unknown (Mishima and Ishihara, 2021). Brain-gut axis dysfunction and visceral hypersensitivity are two of the main characteristics of IBS, while intestinal hyperpermeability, abnormal gastrointestinal motility, activation of the intestinal mucosal immune response, low-grade intestinal inflammation, and somatic and psychological disorders may also be involved in the pathophysiological processes (Xiao et al., 2021; Tesfaye et al., 2023). In recent years, more attention has been focused on the role of early life stress (ELS) in the pathogenesis of IBS. A large number of pre-clinical and clinical studies have shown that ELS can result in persistent changes in the central stress response systems, heightened visceral hypersensitivity, enhanced intestinal motility, shifts in gut microbiota composition, elevated anxiety-and depressive-like behaviors, and increase predisposition to developing IBS in adulthood (Riba et al., 2018; Ju et al., 2020; Low et al., 2020; Rincel and Darnaudéry, 2020; Collins et al., 2023; Petitfils et al., 2023; Lee and Jung, 2024). Early life is an important period for the development of the central nervous system (CNS), gut, and gut microbiota (Osadchiy et al., 2019; Ratsika et al., 2021). ELS can disrupt this critical period and may contribute to the etiology of several neurodevelopmental disorders, such as IBS (Osadchiy et al., 2019; Tao et al., 2022b). Accordingly, ELS may impact the brain-gut-microbiota axis before adulthood. However, whether ELS can result in IBS in children and adolescents is not yet understood.

Maternal separation (MS) is a classic animal model of IBS, which effectively mimics ELS (Riba et al., 2018; Wong et al., 2019; Huang S. T. et al., 2021; Tao et al., 2022a). Using MS model, some of the pathogeneses of IBS were widely studied, such as visceral hypersensitivity (Wu et al., 2020; Huang S. T. et al., 2021; Wang et al., 2022; Tao et al., 2022a), intestinal hyperpermeability (Kuti et al., 2020; Torres-Maravilla et al., 2022), intestinal dysmotility (Bülbül and Sinen, 2021), intestinal dysbiosis (Rincel and Darnaudéry, 2020; Park et al., 2021), and anxiety-like and depressive-like behaviors (Zhou et al., 2022; Favoretto et al., 2023). Also, Riba et al. (Riba et al., 2018) systematically studied the influence of MS on the function of the intestine, mimicking IBS’s main features, including intestinal hyperpermeability, visceral hypersensitivity, microbiota dysbiosis, bile acid malabsorption, and low grade inflammation in the intestine. Results suggested that MS is a suitable model for IBS. These studies mainly focused on the effect of MS on adult rodents; however, few studies paid attention to the influence of MS on young rodents. One study reported that MS rats showed significant visceral hypersensitivity from the post-weaning period to adult (Yi et al., 2017). Moreover, our previous study using a novel distention balloon to evaluate visceral hypersensitivity found that MS induced visceral hypersensitivity in post-weaning mice (Tao et al., 2022a). Together, these results suggested that visceral hypersensitivity in the early life, such as post-weaning period, might play a more meaningful pathophysiologic role in the formation of adult IBS. Therefore, to dynamically explore the potential effect of MS on early life to adulthood may provide a new vision of the pathogenesis of IBS, and thus may develop new therapeutic targets for IBS.

Accordingly, we conducted separated cohort studies of mice to investigate the hypothesis that ELS induced prime phenotypes of IBS, such as visceral hypersensitivity, intestinal hyperpermeability, abnormal gastrointestinal motility, intestinal dysbiosis, and anxiety-like behavior, from childhood to adulthood.

This study investigated the impact of ELS on the main phenotypes of IBS, such as visceral hypersensitivity, hyperpermeability, intestinal dysmotility, intestinal dysbiosis, and anxiety-like behavior from childhood to adulthood. Most importantly, this study showed for the first time, that ELS induced IBS from early life to adult in mice.

Numerous previous basic studies reported that neonatal pups experience of ELS was predisposed to IBS in adulthood (Rincel and Darnaudéry, 2020; Tao et al., 2022b). However, seldom study focused on the impact of ELS on the early life. A large number of clinical evidence indicated that early life adverse event increased the risk for IBS in adult (Burke et al., 2017; Ju et al., 2020; Low et al., 2020). Chang et al. investigated different types of early adverse life events before age 18 years and their association with IBS and demonstrated that IBS patients had a higher prevalence of general trauma, physical punishment, emotional abuse, and sexual events, compared with controls (Bradford et al., 2012). Furthermore, ELS was even correlated with symptom severity of IBS (Park et al., 2016). These clinical studies indicated that ELS play a crucial role in the initiation and progression of IBS. One cannot help but wonder when did ELS cause IBS? Gut microbiota and brain development begin during the prenatal period and continue throughout adulthood, particularly the first 3 years of life representing a critical developmental period. Disruptions in development can influence interaction between these two systems and may contribute to the pathogenesis of neurodevelopmental disorders such as IBS (Borre et al., 2014; Ratsika et al., 2021; Chen et al., 2022). Therefore, there is reason to believe that ELS will disrupt the vital developmental window and may present IBS-like alterations in early life. Indeed, one study supported the idea that ELS caused visceral hypersensitivity from the post-weaning period to adult in rats (Yi et al., 2017). Furthermore, our previous study reported that ELS induced visceral hypersensitivity in post-weaning mice (Tao et al., 2022a). The results indicated that ELS caused IBS-like phenotype in early life. However, beyond these two studies, no other documented study focused on the influence of ELS on IBS-like phenotypes in early life. Accordingly, the profound significance of our study was that it was the first study to comprehensively investigate the influence of ELS on the prime IBS-like phenotypes in early life. Also, our study provided convincing evidence that ELS induced IBS from early life to adult.

In this study, a significant observation was the impact of ELS on the structural integrity of intestinal mitochondria. The mitochondria exhibited evident disruptions in their usual architecture, which included disorganized arrangement, irregular variations in size and shape, and notably, the presence of cristae vacuolation (Figure 4). This significant result indicated that intestinal mitochondrial dysfunction may play an important role in the prosses of ELS induced IBS. Normal mitochondrial function is essential for intestinal epithelial cell homeostasis. Mitochondrial function emerges as a key player in cell fate decisions and in coordinating cellular metabolism, immunity, stress responses and apoptosis (Rath et al., 2018; Guerbette et al., 2022). Preclinical evidence demonstrated that alterations in mitochondrial function and structure are linked to both early stress and systemic biological dysfunction. In addition, early clinical studies supported that increased mitochondrial DNA content and altered cellular energy demands may be present in individuals with a history of ELS (Zitkovsky et al., 2021). Evidence from rodent models suggested that mitochondria exhibited structural and functional changes, such as decreased respiratory enzymatic activity or mitochondrial membrane potential, was associated with long-term or excessive exposure to stress, resulting in an impaired capacity for energy production (Picard and McEwen, 2018; Zitkovsky et al., 2021). Furthermore, chronic psychosocial stress induced epithelial hyperpermeability and visceral hypersensitivity and disturbing mitochondrial activity throughout the intestine (Vicario et al., 2012). Therefore, the ELS induced visceral hypersensitivity (Figure 2), and intestinal hyperpermeability (Figure 3) in early life at the present study may be associated with the dysfunction of mitochondria in early life in intestinal epithelium (Figure 4). Further studies specifically investigating these interactions are warranted. Notably, fluorescence intensity of FITC-D in MS was higher than NS at PND 27, but the difference had no statistical significance (Figure 3A). Subsequent TEM revealed that epithelial tight junctions were loosened and the gap widened in MS compared to NS at PND 30, but the differences were not statistically significant (Figures 3B,C). These results implied that intestinal paracellular permeability was mild increased caused by ELS, but not increased enough to allow biological macromolecules such as FITC-D, entrance. These results also indicated that although the dysfunction of mitochondria was induced by ELS in childhood, the impairment of intestinal barrier function is not severe. On the other hand, work in animal models supported a causal association between mitochondrial dysfunction and changes representative of psychopathology, such as anxiety or depressive-like behaviors (Hollis et al., 2015; Kasahara et al., 2016). MS induced behavioral abnormalities in rats were associated with mitochondrial dysfunction in brain (Khorjahani et al., 2020). Recently, clinical investigations have revealed that significant interactions of mitochondrial respiratory and the inflammatory in the development of anxiety and depression (Liu et al., 2023). We found that MS mice presented anxiety-like behavior at adolescence and adulthood, but not at childhood (Figure 5). These results suggest that behavioral abnormalities induced by ELS may be relatively mild during the early stages of life. Furthermore, the association between abnormal mitochondrial function in the intestines and behavioral anomalies implies that the impact of ELS on intestinal mitochondrial function during early life, such as childhood, might also be subtle. This would also imply that the impact of ELS on intestinal function in early life might be reversed by effective treatment. Therefore, further investigations are much warranted to determine whether early treatment, such as restoring mitochondrial function, can reverse ELS induced IBS-like alterations. However, confirming these hypotheses would require further research.

Intestinal dysbiosis plays an important role the pathogenesis of IBS (Canakis et al., 2020; Petitfils et al., 2023; Zhou et al., 2023). Early life is the critical window for the development of gut microbiota, gut and brain (Laursen et al., 2021; Ratsika et al., 2021). Therefore, disturbance of this process, such as caused by ELS, may have a high predisposition to the development of IBS in adulthood (Wu et al., 2020). We found that MS affected the composition of both ileal and fecal microbiota (Figures 6, 7), suggesting MS affected intestinal microbiota community from early life to adulthood. Importantly, MS significantly reduced the ileal genus abundance of beneficial bacteria, such as Lactobacillus (Zhang Q. et al., 2021), Parvibacter (Liu et al., 2021), Enterorhabdus (Pagliai et al., 2020), Dubosiella (Yuan et al., 2021), and remarkably increased the genus increment of harmful bacteria, such as Burkholderia-Caballeronia-Paraburkholderia, at PND 30 (Figures 6A,B). Lactobacillus and Dubosiella were short-chain-fatty acids (SCFAs) producing bacteria. The reduction in abundance of Lactobacillus and Dubosiella may indicate the decreased synthesis of SCFAs level in ileum. SCFAs were important fuels for intestinal epithelial cells (IEC) and regulate IEC functions through different mechanisms to modulate their proliferation, differentiation as well as functions of subpopulations such as enteroendocrine cells, to impact gut motility and to strengthen the gut barrier functions as well as host metabolism (Yao et al., 2020; Martin-Gallausiaux et al., 2021). Stanton et al. found that MS rats had significantly lower ratios of SCFAs producers (Egerton et al., 2020). Additionally, animal exposure to prolonged restraint significantly reduced SCFAs, and Lactobacillus in the gut was significantly reduced (Maltz et al., 2018). Furthermore, low SCFAs were reported to be associated with visceral hypersensitivity in rats (Zhang J. D. et al., 2021). Therefore, the intestinal dysbiosis in early life observed in our MS mice might play an important role in the initiation of IBS. Thus, further studies were warranted. On the other hand, the relative abundance of genus Methylobacterium in ileum displayed significantly higher in MS than NS at PND 75 (Figures 6C,D). Methylobacterium was shown to be related to constipation (Cao et al., 2017). Matsumoto, et al. indicated that genera of Methylobacterium was significantly higher in the constipation-predominant IBS compared to diarrhea-predominant IBS (Matsumoto et al., 2021). The pathogenic mechanism by which Methylobacterium leads to IBS may be related to its ability to trigger intestinal immune and inflammatory responses (Sun et al., 2021). The role of Methylobacterium in the implication of MS induced IBS requires further exploration. In addition to ileum dysbiosis caused by MS, the microbial community of fecal samples was also disturbed from the early life to adulthood (Figure 7). For example, the abundance of oxyphotobacteria_unclassified was upregulated in MS compared to NS. Moreover, the abundance of Lachnospiraceae_UCG-006, Oscillibacter, Ruminiclostridium, GCA-900066575 was downregulated in MS compared to CT at PND 30. Besides, various harmful bacteria were enrichment in fecal samples at PND 45, such as Candidatus_Saccharimonas (Cruz et al., 2020), Prevotellaceae_UCG-001(Ibrahim et al., 2019), Rikenellaceae RC9 gut group (Emami et al., 2021), Tyzzerella (Huang J. et al., 2021), Paraprevotella (Yoon et al., 2021), Eubacterium_coprostanoligenes_group (Wei et al., 2021), and the acetic acid production bacteria, such as Alistipes (Xu et al., 2021). It reported that Alistipes was enriched in post inflammatory irritable bowel syndrome (PI-IBS) (Song et al., 2020). In pediatric patients with IBS, a greater frequency of pain correlated with an increased abundance of the genus Alistipes (Saulnier et al., 2011). The results demonstrated that Alistipes may play a role in MS induced IBS in early life. Moreover, the abundance of genus Erysipelotrichaceae_unclassified was also higher in MS than NS, which was correlation with obesity (Oh et al., 2021). By contrast, some beneficial bacteria, including Gastranaerophilales_unclassified (Wu et al., 2021), Dehalobacterium (Gu et al., 2020), Christensenellaceae (Brandsma et al., 2019), Blautia (Wang et al., 2019), Muribaculum (McNamara et al., 2021), were reduced in MS compared to NS. Gastranaerophilales have been identified as the primary producers of indole, which can subsequently be converted into indolepropionic acid. Indolepropionic acid is recognized for its anti-inflammatory properties in both the gastrointestinal tract and the peripheral system (Rosario et al., 2021). A reduction in Dehalobacterium has been linked to the development of inflammation (Chen et al., 2023). The aboundance of Christensenellaceae has shown a positive association with microbial metabolites related to intestinal barrier function (Zhao et al., 2022). The role of Blautia, or its products, in modulating intestinal epithelium health has been highlighted (Rashidi et al., 2022). Moreover, recent research indicated that Muribaculum exhibited a negative correlation with plasma TNF-α and colon inflammatory gene expression (TNF-α), while showing a positive correlation with colon tight junction genes (OCLN and CLDN1) (Yang et al., 2023). Likewise, MS significantly affect the composition of microbiota in fecal samples at PND 75. At the genus level, the beneficial bacteria, including Prevotellaceae_UCG-001 (Hu et al., 2020) and Lachnospiraceae_UCG-010 (Han et al., 2021), were downregulated, while the bacteria, such as Kineothrix (Xie et al., 2021), Blautia (Wang et al., 2019), Bifidobacterium (Liu et al., 2021), Duncaniella (Chang et al., 2021) were upregulated. Prevotellaceae_UCG-001 was widely known as a probiotic for superior SCFAs production capacity. A decrease in its abundance has been linked to depression (Zhang Z.-W. et al., 2022) and intestinal inflammation (Wu et al., 2023). On the other hand, the abundance of Lachnospiraceae_UCG-010 was found to be significantly reduced in the fecal samples of patients with IBS, while it was observed to be increased in healthy individuals (Zhuang et al., 2018). Study revealed that higher levels of Lachnospiraceae_UCG-010 were associated with improved intestinal barrier function and a reduction in intestinal lesion scores (Kan et al., 2021). Taken together, MS leads to dysbiosis in fecal samples from early life to adulthood in mice. These findings suggest that early-life exposure to MS disrupts the balance of intestinal microbiota. The observed dysbiosis in the gut microbiota during early life could potentially play a role in the onset and development of MS-induced IBS. Further studies are required to fully elucidate the mechanisms underlying this relationship.

IBS is a highly prevalent, chronic disorder that significantly reduces patients’ quality of life (Lacy et al., 2021). At present, the pathogenesis of IBS remains elusive and the treatments for IBS are unsatisfactory (van Orten-Luiten et al., 2022; Gao et al., 2023). Therefore, other line of thought may be needed, namely to investigate the initiation and process of IBS. Our results suggested that MS induced IBS in early life. Therefore, when investigating the pathogenesis of IBS, it is necessary to extend the research timeline to the earlier stages of life, which also represents a crucial developmental window for the brain-gut-microbiota axis (Leyrolle et al., 2021). While we did not delve into the potential mechanisms underlying the induction of IBS by ELS, our findings suggest that mitochondrial dysfunction in the intestinal epithelium and dysbiosis of the gut microbiota might play crucial roles in ELS-induced IBS. The interplay between mitochondria and gut microbiota holds a vital significance in maintaining intestinal physiological balance. Under normal circumstances, intestinal epithelial cells plays a vital role in maintaining the hypoxic environment in the intestinal lumen, facilitating the prevalence of obligate anaerobic microbiota, which is dependent of mitochondrial oxidative phosphorylation (Litvak et al., 2018). On the other hand, these microbes contribute essential metabolites like SCFAs to supply nutrients for gut epithelial cells (Takihara and Okuda, 2023). In addition, SCFAs are potentially effective modulators for mitochondria function (Zhang Y. et al., 2022). Microbial communications with intestinal epithelial mitochondria might modify mitochondrial structural characteristics and metabolic capabilities. This abnormal interaction could trigger inflammasome activation, potentially disrupting epithelial hypoxia and affecting the structure of the gut microbiota (Zhang Y. et al., 2022). Interestingly, we found disrupted intestinal mitochondrial structure and the significant dysbiosis of intestinal microbiota in early life. Therefore, microbiota-mitochondria crosstalk dysfunction may be involved in the pathogenesis and initiation of IBS induced by ELS (Zhang Y. et al., 2022). Further research is needed to validate this hypothesis. The hypothetical mechanism of the potential role of microbiota-mitochondria crosstalk dysfunction in the pathogenesis of ELS-induced IBS is summarized in Figure 8.

In conclusion, this study shows for the first time that ELS induces IBS from early life to adulthood in mice. The disrupted intestinal mitochondrial structure and the significant dysbiosis of intestinal microbiota in early life may contribute to the initiation and progress of IBS from early life to adulthood. A noteworthy implication of our study is that it paves the way for new insights into pathogenetic investigation of IBS and contributes to develop novel therapeutic targets for IBS in future investigations.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found at: https://www.ncbi.nlm.nih.gov/, PRJNA804655.

The animal study was approved by Zhejiang University Ethics Committee for Animal Research. The study was conducted in accordance with the local legislation and institutional requirements.

ET: Conceptualization, Methodology Writing – original draft, Writing – review & editing. YW: Methodology, Resources, Writing – review & editing. CH: Methodology, Resources, Writing – review & editing. ZZ: Methodology, Resources, Writing – review & editing. DY: Data curation, Methodology, Writing – review & editing. GL: Data curation, Formal analysis, Writing – review & editing. BC: Writing – review & editing. RG: Writing – review & editing. XS: Data curation, Formal analysis, Writing – review & editing. WZ: Data curation, Formal analysis, Writing – review & editing. TZ: Formal analysis, Visualization, Writing – review & editing. XJ: Data curation, Formal analysis, Writing – review & editing. XD: Data curation, Formal analysis, Writing – review & editing. MF: Formal analysis, Methodology, Writing – review & editing. MJ: Funding acquisition, Supervision, Writing – original draft, Writing – review & editing.