A new automated tool to quantify nucleoid distribution within mitochondrial networks

Abstract

Mitochondrial DNA (mtDNA) maintenance is essential to sustain a functionally healthy population of mitochondria within cells. Proper mtDNA replication and distribution within mitochondrial networks are essential to maintain mitochondrial homeostasis. However, the fundamental basis of mtDNA segregation and distribution within mitochondrial networks is still unclear. To address these questions, we developed an algorithm, Mitomate tracker to unravel the global distribution of nucleoids within mitochondria. Using this tool, we decipher the semi-regular spacing of nucleoids across mitochondrial networks. Furthermore, we show that mitochondrial fission actively regulates mtDNA distribution by controlling the distribution of nucleoids within mitochondrial networks. Specifically, we found that primary cells bearing disease-associated mutations in the fission proteins DRP1 and MYH14 show altered nucleoid distribution, and acute enrichment of enlarged nucleoids near the nucleus. Further analysis suggests that the altered nucleoid distribution observed in the fission mutants is the result of both changes in network structure and nucleoid density. Thus, our study provides novel insights into the role of mitochondria fission in nucleoid distribution and the understanding of diseases caused by fission defects.

The Role of Impaired Mitochondrial Dynamics in MFN2-Mediated Pathology

“…Whether or not the different types of fission events linked to either biogenesis or mitophagy ( Kleele et al, 2021 ) are also involved in mitochondrial axonal transport remains unknown. It is tempting to speculate that ‘biogenesis’ fission, which may be coupled to mtDNA replication ( Lewis et al, 2016 ), and is required for proper nucleoid distribution ( Ilamathi et al, 2021 ), would precede anterograde transport to help deliver “fresh” healthy mitochondria to the cell periphery. With respect to retrograde mitochondrial transport, one early idea was that dysfunctional mitochondria need to be transported to the soma for mitophagy.…”

Contact sites between endoplasmic reticulum sheets and mitochondria regulate mitochondrial DNA replication and segregation

“…5A ). Mitobulbs are present in the perinuclear region of DRP1 mutant cells ( 14 ) where ER sheets are present, but absent from the peripheral region characterised by RTN4-positive ER tubules ( Fig. S2 ).…”

“…Loss of DRP1 results in enlarged nucleoids (Fig. 6C) that are likely caused by nucleoid aggregation (14)(15)(16). To confirm the presence of multiple mtDNA copies in the enlarged nucleoids present within mitobulbs, we labelled dividing nucleoids with the nucleotide analogue EdU and observed multiple EdU-positive foci within mitobulbs (Fig.…”

“…Mitochondrial dynamics are also essential for mitochondrial DNA (mtDNA) maintenance, with defects in mitochondrial fusion affecting mtDNA integrity and copy number (11,12). On the other hand, while defective mitochondrial fission does not generally affect overall mtDNA content, it alters the distribution of nucleoids (mtDNA-protein complexes), leading to the formation of bulb-like mitochondrial structures termed mitobulbs (13)(14)(15)(16). Importantly, both mitochondrial fission and mtDNA replication are initiated at sites of contact between mitochondria and the endoplasmic reticulum (ERMCS) (17)(18)(19), indicating a crucial role for the ER in the regulation of mitochondrial structure and function.…”

Mitochondrial network structure controls cell-to-cell mtDNA variability generated by cell divisions

“…In yeast, mtDNA inheritance occurs with finer-than-random (binomial) control over the number of mitochondrial nucleoids [53], suggesting that physical mechanisms must exist to exert this control. As mtDNA resides in nucleoids that are physically distributed – potentially heterogeneously – throughout the mitochondria of the cell [54], we set out to explore how different physical structures of mitochondria may shape mtDNA inheritance.…”