Research published in Translational Psychiatry recently explored the connection between the chromodomain helicase DNA-binding protein (CHD8) gene and autism-related gastrointestinal (GI) manifestations.
Additionally, the study investigated the potential relationship between CHD8 haploinsufficiency and autism-related behavioral phenotypes.
Study: CHD8 regulates gut epithelial cell function and affects autism-related behaviors through the gut-brain axis. Image Credit: Africa Studio/Shutterstock.com
Previous studies have established that individuals with autism may exhibit behavioral deficits, and there is a correlation between autism severity and the likelihood of developing GI symptoms.
Interestingly, approximately 80% of individuals with severe CHD8 mutations experience a wide range of GI symptoms, notably including constipation and altered intestinal permeability. However, how CHD8 impacts gut function remains unclear and to whether these GI abnormalities contribute to autism-related behavioral traits.
About the study
In this study, researchers utilized an experimental murine model that carried the large isoform of CHD8 (CHD8L), a gene located on chromosome 14q11.2 known for its role in regulating the Wnt signaling pathway, which plays a crucial role in neurodevelopment.
Specifically, eight-week-old C57BL/6 Chd8L+/− male mice lacking CHD8 gene exons 11–13, were used. These mice exhibited increased expression of CHD8 exon 1 to compensate for the loss of exons and showed improved brain weight and volume compared to wild-type (WT) mice, which served as controls.
The researchers employed a Cre-lox system to knock out the Chd8 gene in gut epithelial cells, inducing anxiety-like behavior in some mice referred to as Chd8 gut epithelial haploinsufficient mice (CHD8+/ΔIEC mice). Experiments with these mice allowed for investigating the role of gut dysfunction in autism-related anxiety-like behavior.
Immunohistochemical staining was performed to detect tuft cells in the GI epithelium of Chd8+/ΔIEC mice. The researchers also administered fluorescein isothiocyanate-conjugated dextran (FITC-dextran) to Chd8L+/− male mice to assess intestinal permeability. Additionally, they used 16S ribosomal deoxyribonucleic acid (rDNA) sequencing to compare the fecal microbiome compositions of Chd8L+/− and WT controls. The study revealed variations in the abundance of three bacterial taxa between the two groups.
Behavioral testing, conducted in different environments, involved habituating the mice to the testing room for at least an hour to minimize stress and anxiety. Each test was performed on a separate day with a one-day interval between tests.
Cameras tracked animal movement, and "ethovision" software automatically scored animal behavior. The study included open-field, elevated plus maze (EPM), light/dark box, self-grooming, social interaction, and marble-burying behavioral tests. Animals of the same genotype were randomly assigned to different experimental groups, such as those receiving antibiotics versus those without antibiotics.
Similar to individuals with autism, eight-week-old Chd8L+/− mice displayed increased intestinal permeability. A decrease in goblet cell numbers in their gut potentially reduced the thickness of the colon's mucus layer, contributing to higher intestinal permeability. However, the analysis did not elucidate how the goblet cell population affected mucus levels.
Unlike previous studies that assessed microflora diversity through 16S sequencing without quantifying bacterial load, this study revealed that Chd8L+/− mice had a higher bacterial load and increased microbiome richness (greater alpha diversity).
Transcriptomic and reverse transcriptase-polymerase chain reaction (RT-PCR) analyses also indicated higher expression of regenerating islet-derived protein (Reg)3β and Reg3γ in the gut epithelial cells of Chd8L+/− mice. Further research is needed to investigate the functional implications of this greater alpha diversity.
Additionally, Chd8L+/− mice exhibited a substantial increase in A. muciniphila, a bacterial taxon associated with several neurological conditions, including autism spectrum disorders. Nevertheless, further research is required to establish the connection between these bacteria and neurological pathogenesis.
RT-PCR and transcriptomic analyses identified the upregulation of more than 900 differentially expressed immune system-related genes in the gut of Chd8L+/− mice compared to WT mice. Conversely, these analyses revealed a significant downregulation of tuft cell marker genes in these mice.
Only around 30% of CHD8+/ΔIEC mice exhibited increased anxiety-like behaviors. This suggests that factors like antibiotic treatment or other cell populations like gut immune cells may have driven these behavioral deficits.
CHD8+/ΔIEC mice also displayed fewer tuft cells in the colon and gut than WT mice. The study suggests systemic CHD8 deficiency likely overshadowed the effects of epithelial cell Chd8 haploinsufficiency regarding tuft cell counts in the colon of CHD8+/ΔIEC mice.
In conclusion, Chd8L haploinsufficiency induced alterations in GI morphology, tuft cell counts, intestinal permeability, bacterial load, and alpha diversity in mice.
Specifically, CHD8+/ΔIEC mice exhibited anxiety-like behaviors but showed no changes in social behavior, suggesting that GI abnormalities played a significant role in the symptomology and behavioral phenotypes associated with autism.
Considering the substantial variation in the underlying causes of autism among individuals, further research should focus on responses within subgroups characterized by specific genetic backgrounds or behavioral phenotypes.