Research reveals that microbiota-gut-brain axis signaling plays a fundamental role in mood disorders. Additionally, microRNAs (miRNAs)—which are small, single-stranded RNAs of about 22 nucleotides in length—also have vital roles in regulating gene expression at the post-transcriptional level and are highly implicated in the development of certain human diseases.

In a recent publication in the journal Scientific Reports, researchers theorized that some miRNAs are correlated with specific bacteria in the fecal samples in patients with MDD, and these miRNAs would show enrichment in pathways correlated with MDD.

The study recruited MDD patients and healthy control patients and collected fecal samples. The researchers performed 16S ribosome RNA sequence utilizing the Illumina MiSeq sequencers and analysis of 798 fecal miRNAs utilizing the nCounter Human-v2 miRNA Panel in 20 subjects. They calculated the Spearman correlation coefficient for bacteria abundance and miRNA expressions and evaluated the predicted miRNA pathways by enrichment analysis with false-discovery correction.

The results revealed that a total of 270 genera and 798 miRNAs were detected in the fecal samples. Seven genera (i.e., Anaerostipes, Bacteroides, Bifidobacterium, Clostridium, Collinsella, Dialister, and Roseburia) had fold changes greater than one and were present in over 90% of all fecal samples. In particular, Bacteroides and Dialister significantly differed between the MDD and control groups (P <.05).

The correlation coefficients between the seven genera and miRNAs in patients with MDD revealed 48 pairs of positive correlations and 36 negative correlations (P <.01). With regard to miRNA predicted functions, there were 57 predicted pathways with a P-value <.001, including MDD-associated pathways, axon guidance, circadian rhythm, dopaminergic synapse, focal adhesion, long-term potentiation, and neurotrophin-signaling pathway.

The authors concluded that in this pilot study, their findings indicate specific genera highly correlated with the predicted miRNA functions, which may offer clues for the interaction between host factors and gut microbiota via the microbiota-gut-brain axis. They also noted that it is critical to examine the roles between gut microbiota and miRNAs for depression by conducting follow-up studies with larger sample sizes and refined experimental design.

In conclusion, the authors wrote, "Overall, these findings offer clues to bridge the gaps in existing knowledge related to the link between gut microbiota and the brain, though further studies are required to elucidate the causal relationship and underlying mechanisms between the two."

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