Relationships between brain activity, tryptophan-related gut metabolites, and autism symptomatology

Abstract

While it has been suggested that alterations in the composition of gut microbial metabolites may play a causative role in the pathophysiology of autism spectrum disorder (ASD), it is not known how gut microbial metabolites are associated with ASD-specific brain alterations. In this cross-sectional, case-control observational study, (i) fecal metabolomics, (ii) task-based functional magnetic resonance imaging (fMRI), and (iii) behavioral assessments were obtained from 43 ASD and 41 neurotypical (NT) children, aged 8-17. The fMRI tasks used socio-emotional and sensory paradigms that commonly reveal strong evoked brain differences in ASD participants. Our results show that fecal levels of specific tryptophan-related metabolites, including kynurenate, were significantly lower in ASD compared to NT, and were associated with: 1) alterations in insular and cingulate cortical activity previously implicated in ASD; and 2) ASD severity and symptoms (e.g., ADOS scores, disgust propensity, and sensory sensitivities). Moreover, activity in the mid-insula and mid-cingulate significantly mediated relationships between the microbial tryptophan metabolites (indolelactate and tryptophan betaine) and ASD severity and disgust sensitivity. Thus, we identify associations between gut microbial tryptophan metabolites, ASD symptoms, and brain activity in humans, particularly in brain regions associated with interoceptive processing.

Fig. 1. Metabolites within the tryptophan pathway.

Arrow thickness represents the strength of the pathway under normal conditions.

Fig. 2. Group difference in kynurenate (KA).

A contrast analysis within the framework of the GLM indicated significant differences between ASD (N = 43) and NT (N = 41) groups in kynurenate (KA; q = 0.02, two-tailed). NT: minimum = −1.15, 1st quartile = −0.23, median = 0.51, 3rd quartile = 1.00, maximum = 1.45. ASD: minimum = −2.68, 1st quartile = −0.98, median = −0.18, 3rd quartile = 0.75, maximum = 1.30. Source data are provided as a Source data file.

Fig. 3. Regions of Interest (ROIs) based on between-group differences in fMRI tasks.

See Supplemental Table 5 for peak MNI coordinates for each ROI. While our predominant focus was on subregions of the insula and cingulate due to their involvement in interoceptive and emotional processing, we additionally considered other ROIs with significant ASD vs. NT differences in our fMRI tasks and from prior ASD studies. Please see “Methods” for how ROIs were selected. R Right, L Left, insular subregions (dAI left dorsal anterior insula, vAI ventral anterior insula, MI mid-insula, PI posterior insula); cingulate subregions (pACC pregenual anterior cingulate cortex, MCC mid-cingulate cortex, dmPFC/aMCC dorsal medial prefrontal cortex/anterior mid-cingulate cortex); IFGop inferior frontal gyrus, pars opercularis, S1 primary somatosensory cortex.

Fig. 4. Significant mediation models in the ASD group.

Significant mediation models within the ASD group, with the brain as the mediator between specific metabolites and behavior. For each mediation model, the figure contains the standardized beta and standard error: Std. β (SE) for direct effects. The indirect effect between the metabolite and behavior is listed below the arrow in brackets. * indicates significant indirect effects. A Right mid-insula: disgusting foods vs. rest, indolelactate, and ADOS RRB. B Right mid-insula: disgusting foods vs. rest, indolelactate, and ADOS total score. C right mid-cingulate: non-emotional faces vs. rest, tryptophan betaine, and disgust sensitivity. ADOS Autism Diagnosis Observation Schedule, ADI-R Autism Diagnostic Interview-Revised, RRB Restricted and Repetitive Behaviors, DPSS-R Disgust Propensity and Sensitivity Scale - Revised, r_MI right mid-insula, MCC mid-cingulate cortex. Source data are provided as a Source data file.