Astrocytes resolve ER stress through mitochondrial fusion facilitated by biotin availability.

Structures of cellular organelles are intertwined with their functions that undergo alterations once the organelles are stressed. Since organelle functions are dependent on each other, an organelle-specific stress possibly influences the structure and function of its associated organelles. In this perspective, our study demonstrated that endoplasmic reticulum (ER)-specific stress induced by tunicamycin in primary astroglial culture is associated with altered mitochondrial dynamics and matched with the changes as observed in the aging rat brain. However, the exogenous addition of biotin, a highly lipogenic and mitochondrial vitamin, ameliorates ER stress even though its direct targets are not known within ER. Alternatively, the increased biotinylation of mitochondrial carboxylases preserves its basal respiratory capacity by upregulating mitofusin 2 (Mfn2) and, possibly, its associated role on mitochondrial fusion. Furthermore, the Mfn2 increase by biotin augments physical interaction between ER and functional mitochondria to exchange biomolecules as a part of ER stress resolution. This suggests an increased demand for micronutrient biotin under ER stress resolves the same by undergoing appropriate structural and metabolic contacts between ER and mitochondria. These findings provide a paradigm to resolve stress in one organelle by sustaining the metabolic commitments of another interdependent organelle. The findings also highlight a novel role of biotin in inducing Mfn2 expression and localization under ER stress in addition to its known role as a co-enzyme.