Short Chain Fatty Acid Supplementation After Traumatic Brain Injury Attenuates Neurologic Injury Via the Gut-Brain-Microglia Axis.

Background: Traumatic brain injury (TBI) is an underrecognized public health threat. There are limited therapeutic options for TBI, and supportive care remains the mainstay of treatment. Our previously published data demonstrate that post-TBI fecal microbiome transplantation (FMT) can reverse TBI-induced depletion of commensal bacteria, preserve white matter connectivity and neurocognition, and decrease cortical volume loss in mice after TBI.

Hypothesis: We hypothesized that post-TBI supplementation with Short Chain Fatty Acids (SCFAs), metabolites of commensal gut bacteria, would attenuate neurologic injury after TBI in mice.

Methods: 14-week-old male C57BL/6 mice (n=52) underwent TBI via a controlled cortical impact vs. sham injury. Post-TBI, each group was treated with the SCFAs acetate, butyrate, and propionate vs. molar equivalent sodium chloride vehicle via free access to drinking water for four weeks post-TBI. The stool was collected three days pre-and sixty days post-TBI to assess the gut microbial community structure via 16s ribosomal RNA gene amplicon sequencing. Neurocognitive testing was performed with open-field and zero-maze testing. Ventricular volume and white matter connectivity were measured with 3D, contrast-enhanced MRI. Lastly, the transcriptional response of microglia was assessed with single-cell RNA sequencing (scRNAseq).

Results: SCFA supplementation decreased TBI-induced microbial loss, attenuated ventricular volume loss, preserved white matter connectivity, and altered the transcriptional profile of microglia after TBI. Post-TBI SCFA supplementation preserved the abundance of the butyrate-producing taxa Firmicutes, Clostridia, Ruminoccacaceae, and Peptoccacaceae (p=0.01). SCFA also reduced the TBI-induced increase in Clostridiales and Bacteroidales compared to the salt vehicle group (p=0.05). We also observed the preservation of non-TBI murine anxiety-like behavior in SCFA-treated TBI mice compared to vehicle-treated TBI mice in zero-maze (152.3 ± 101.8 cm vs. 147.5 ± 60.0 cm, p=0.006). These results were recapitulated with open field testing (11.7 ± 3%-time in the center in SCFA-treated TBI mice vs. 15.0 ± 6% %-time in the center of the field in vehicle-treated mice; p=0.002). Lastly, we observed upregulation of transcripts for the neuroprotective heat shock family of proteins and downregulation of neurodegeneration-associated transcripts, indicating an overall neuroprotective phenotype in microglia after SCFA supplementation post-TBI.

Conclusions: We hypothesized that SCFA supplementation would attenuate neurologic injury after TBI in mice. SCFA supplementation attenuated neurocognitive deficits, reduced cortical volume loss, preserved white matter connectivity, and decreased neuroinflammation. These benefits may result from the direct replacement of SCFAs. However, there may also be secondary mechanisms related to commensal refeeding of butyrate-producing bacteria within the gut microbial community, a neuroprotective heat shock response, and a decrease in the expression of genes associated with neurodegeneration. The current study highlights the role of SCFAs in microbiome homeostasis and the potential of dietary intervention as a novel therapy in TBI.