# In Vivo Chemical Reprogramming Is Associated With a Toxic Accumulation of Lipid Droplets Hindering Rejuvenation

## TL;DR

Chemical reprogramming of aged cells can rejuvenate mitochondria but causes toxic lipid buildup in mice.

## Contribution

The study reveals that partial chemical reprogramming induces mitochondrial stress and lipid droplet accumulation in aged mammals.

## Key findings

- Chemical reprogramming increases mitochondrial size and fusing but causes lipid droplet accumulation.
- Higher doses of the cocktail lead to acute toxicity and reduced body weight in mice.
- Mitochondrial stress and TCA cycle changes are linked to lipid accumulation and kidney injury.

## Abstract

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 of these cocktails to affect biological age in a mammalian aging model has yet to be tested. Here, we have characterized the effects of partial chemical reprogramming on mitochondrial structure and function in aged mouse fibroblasts and tested its in vivo 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. 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 necessitating euthanasia. In the livers and kidneys of these animals, we observe significant increases in lipid droplet accumulation, as well as changes in mitochondrial morphology in the livers that are associated with mitochondrial stress. Thus, partial chemical reprogramming may induce mitochondrial stress and lead to significant lipid accumulation, which may cause toxicity and hinder the rejuvenation of cells and tissues in aged mammals.

Partial chemical reprogramming significantly increases mitochondrial interaction networks, affects mitochondrial morphology, speeds up mitochondrial movement dynamics, increases mitochondrial transmembrane potential, and induces TCA cycle stress. These changes in mitochondrial function contribute to lipid droplet accumulation, acute kidney injury, and toxicity in vivo.