# mtDNA-Depleted Mitochondria Form Sites of Contact with the Nucleus and Alter the Cellular Epigenome

## TL;DR

This study shows that mitochondria without mtDNA form more contacts with the nucleus and change the cell's epigenetic state.

## Contribution

The study reveals that mtDNA-depleted mitochondria increase nuclear contacts and alter epigenetic marks via TSPO.

## Key findings

- mtDNA-depleted cells show increased NAM frequency compared to mtDNA-retaining cells.
- Loss of mtDNA leads to TSPO enrichment and evasion of mitophagy.
- mtDNA depletion causes reduced DNA methylation and chromatin decondensation.

## Abstract

Mitochondrial sites of contact with the nucleus, hereafter referred to as Nucleus-Associated Mitochondria (NAM), are specialised domains that enable communication, influencing cellular function. Previous studies have shown that these contacts can be stabilised by protein scaffolds acting as tethers to promote retrograde signalling, particularly during apoptotic stress. This is facilitated via the mitochondrial protein TSPO. In this study, we have investigated a mitochondrial DNA (mtDNA)-depleted (ρ0) 4T1 cell model to further inform the role of NAM in retrograde communication between corrupted mitochondria and the nucleus. Our data report an increase in NAM frequency in mtDNA-depleted cells compared to the mtDNA-retaining parental 4T1 line. Using a combination of cellular assays, transmission electron microscopy, and epigenetic profiling, we have found that under conditions of mtDNA loss, mitochondria become enriched in TSPO, evading mitophagic clearance and are prone to forming stable contacts with the nucleus. This coincides with an extreme reduction in DNA methylation, as well as histone modifications associated with chromatin decondensation.