chemical reprogramming is associated with a toxic accumulation of lipid droplets hindering rejuvenation.

Partial reprogramming has emerged as a promising strategy to reset the epigenetic landscape of aged cells towards more youthful profiles. Recent advancements have included the development of chemical reprogramming cocktails that can lower the epigenetic and transcriptomic age of cells and upregulate mitochondrial biogenesis and oxidative phosphorylation. However, the ability for these cocktails to affect biological age in a mammalian aging model has yet to be tested. Here, we have analyzed the effects of partial chemical reprogramming on mitochondrial structure in aged mouse fibroblasts and tested its efficacy in genetically diverse male UM-HET3 mice. This approach increases the size of mitochondria, alters cristae morphology, causes an increased fusing of mitochondrial networks, and speeds up movement velocity. We also discover that partial chemical reprogramming upregulates the formation of intracellular lipid droplets. At lower doses, the chemical reprogramming cocktail can be safely administered to middle-aged mice using implantable osmotic pumps, albeit with no effect on the transcriptomic age of kidney or liver tissues, and only a modest effect on the expression of OXPHOS complexes. However, at higher doses, the cocktail causes a drastic reduction in body weight and body condition scores. In the livers and kidneys of these animals, we observe significant increases in oil red o staining indicative of excessive lipid droplet accumulation in these organs. Thus, the upregulation of lipid droplet formation during partial chemical reprogramming may cause toxicity hindering the rejuvenation of cells and tissues in aged mammals.