Significantly reduced, but balanced, rates of mitochondrial fission and fusion are sufficient to maintain the integrity of yeast mitochondrial DNA

Brett T. Wisniewski, Laura L. Lackner

Posted on: 30 August 2024

Preprint posted on 19 July 2024

Striking the Balance: equal mitochondrial fission and fusion safeguard mtDNA function

Selected by Leeba Ann Chacko

Categories: cell biology, genetics, microbiology

mtDNA is found at sites where mitochondrial fission and fusion occur, but it does not initiate or control these processes

Background

Mitochondria are essential organelles in eukaryotic cells [1], generating the energy needed for cellular activities [5]. These organelles are highly dynamic, constantly undergoing fission and fusion to maintain their integrity and meet the cell’s energy demands [6]. These processes are driven by specialized proteins that regulate the fission and fusion of mitochondrial membranes. Disruption or deletion of these proteins can drastically alter fission [4, 2] and fusion [7] rates, leading to changes in mitochondrial morphology and impaired function [3]. While the roles of individual fission and fusion proteins in mitochondrial health are fairly known, the impact of disrupting both processes simultaneously is less understood. Wisniewski and Lackner found that mitochondrial DNA (mtDNA) does not affect fission and fusion rates. However, while the complete loss of both fission and fusion machinery compromises mitochondrial function, partial impairment does not.

Key Findings

Presence of mtDNA at Mitochondrial Fission and Fusion Sites

Wisniewski and Lackner used fluorescently tagged mtDNA-binding proteins and a mitochondrial LacO-LacI probe to show that mtDNA is present at most mitochondrial fission and fusion sites. They further confirmed that in wild-type cells, the rates of fission and fusion are well-balanced, and that these rates remain unchanged even in cells lacking mtDNA, indicating that mtDNA is not required to maintain this balance.

Role of Fission and Fusion Machinery in mtDNA Function

The authors then demonstrated that the defects in mtDNA function were due to the overall absence of the fission and fusion machinery rather than the absence of specific fission or fusion proteins. They confirmed this by testing mtDNA function in cells lacking various combinations of inner and outer mitochondrial membrane fission and fusion proteins.

Preservation of mtDNA Function Despite Partial Loss of Fission Machinery

They also showed that mtDNA function can be preserved despite a partial loss of the fission ma- chinery. To do so, they used hypomorphic Dnm1-GFP alleles with reduced activity. These mutants displayed wild-type-like mitochondrial morphology and maintained lower, but balanced, fission and fusion rates, which rescued the mtDNA dysfunction observed in cells lacking fission and fusion machinery. This suggests that even minimal, balanced fission and fusion are sufficient to support mtDNA function.

Compensatory Mechanisms for mtDNA Loss-of-Function Phenotype

Finally, the authors discovered that increasing mtDNA copy number in cells lacking fission and fusion machinery could rescue the mtDNA loss-of-function phenotype. They achieved this increase by deleting proteins involved in regulating mtDNA copy number. Importantly, they confirmed that the observed differences in respiratory competency among the strains were not linked to variations in mtDNA copy number.

Importance of Mitochondrial Dynamics

Wisniewski and Lackner showed that balanced mitochondrial fission and fusion are essential for maintaining mtDNA function. They found that defects in mtDNA function result from the absence of fission and fusion machinery as a whole, rather than specific proteins, and that even minimal fission and fusion can sustain mtDNA function. Furthermore, increasing mtDNA copy number in cells lacking this machinery can rescue mtDNA dysfunction. This study highlights how mitochondrial dynamics help preserve mitochondrial function.

What I Like About This Preprint

For my PhD, I am studying how mitochondrial homeostasis is maintained in fission yeast cells. While investigating this, I used mutants with disrupted fission and fusion machinery and observed that these mutants grew slower than wild-type cells. I suspected that this might be related to mitochondrial function and reading the preprint by Wisniewski and Lackner has now reinforced this suspicion. I am now motivated to specifically test mitochondrial function in my mutants.

Questions for the Authors

(1) It’s interesting that fewer Yme2-GFP proteins localize to fission and fusion sites compared to the mito-LacO-LacI probe. Do the mito-LacO-LacI probe and Yme2-GFP fully co- localize within the cell?

(2) Considering that the mitochondrial morphology in mutants with simultaneous disruption of fission and fusion machinery resembles that of wild-type cells, are the rates of fission and fusion also comparable to wild-type? If so, does mtDNA in these cells localize to fission and fusion sites similarly to wild-type cells?

(3) It’s intriguing that mitochondria in mutants with simultaneous disruption of fission and fusion machinery show an increase in the collapsed phenotype. Do you think there could be a relationship between the fission-fusion machinery and the cortical anchor Num1?

(4) While you demonstrated that mtDNA copy number remains consistent in both wild-type and mutant strains, could there be variations in the number and/or size of mitochondrial nucleoids?

(5) Is there a way to test whether partial loss of function in the fusion machinery, such as in hypomorphic fzo1 or mgm1 mutants, results in a phenotype similar to that of the Dnm1 hypomorphic mutants?

(6) Are there other methods to assess mitochondrial function besides measuring petite frequency? Additionally, could the observed petite frequency phenotype be due to mutations in nuclear-encoded genes involved in oxidative phosphorylation rather than a direct loss of mitochondrial function?

References

[1] Sarah J. Annesley and Paul R. Fisher. Mitochondria in health and disease. Cells, 8, 7 2019.
[2] William Bleazard, J. Michael McCaffery, Edward J. King, Susan Bale, Amy Mozdy, Quinton Tieu, Jodi Nunnari, and Janet M. Shaw. The dynamin-related gtpase dnm1 regulates mitochondrial fission in yeast. Nature cell biology, 1:298, 1999.
[3] Fenfen Dong, Mengdan Zhu, Fan Zheng, and Chuanhai Fu. Mitochondrial fusion and fission are required for proper mitochondrial function and cell proliferation in fission yeast. The FEBS Journal, 289:262–278, 1 2022.
[4] Isabelle Jourdain, Yannick Gachet, and Jeremy S. Hyams. The dynamin related protein dnm1 fragments mito- chondria in a microtubule-dependent manner during the fission yeast cell cycle. Cell Motility and the Cytoskele- ton, 66:509–523, 8 2009.
[5] Jodi Nunnari and Anu Suomalainen. Mitochondria: In sickness and in health. Cell, 148:1145, 3 2012.
[6] Mitali Shah, Leeba Ann Chacko, Joel P. Joseph, and Vaishnavi Ananthanarayanan. Mitochondrial dynamics, positioning and function mediated by cytoskeletal interactions. Cellular and Molecular Life Sciences: CMLS, 78:3969, 4 2021.
[7] Rongmei Yuan, Xiang Ding, Xiumei Tan, and Yiling Hou. Loss of fzo1 gene results in changes of cell dynamics in fission yeast. International Journal of Molecular Medicine, 46:2194, 12 2020.

Tags: fission yeast, mitochondria, mtdna, pombe

doi: https://doi.org/10.1242/prelights.38243

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