Comment on: miR-129-5p Modulates Fndc5 Expression in Adipose Tissue of Diabetic Mice.

A study demonstrated that increased miR-129-5p expression is associated with reduced Fndc5 expression in adipose tissue of a type 2 diabetes mellitus (T2DM) mouse model, suggesting a post-transcriptional regulatory mechanism contributing to metabolic dysfunction. This Letter discusses the strengths and limitations of these findings. Although the dual-luciferase assay provided strong evidence of a direct interaction between miR-129-5p and the Fndc5 3'UTR, the absence of in vivo loss-of-function or gain-of-function experiments limits causal interpretation. In addition, downstream metabolic consequences of reduced FNDC5/irisin signaling, including insulin signaling pathways, GLUT4 regulation, and thermogenic activity, were not comprehensively evaluated. Methodological concerns, including sample size and the lack of reported blinding procedures, may also affect reproducibility. Future studies integrating miRNA manipulation strategies, circulating irisin measurement, and pathway-based metabolic analyses are needed to strengthen the biological and translational relevance of the miR-129-5p-FNDC5 axis in T2DM.