A human cell contains hundreds to thousands of mitochondrial DNA (mtDNA) packaged into nucleoids, and their proper distribution in the mitochondrial network is essential for mtDNA maintenance and proper cellular functions^{1, 2}. Currently, the segregation and allocation of mitochondrial nucleoids are thought to be passively determined by mitochondrial fusion and division^{3, 4, 5, 6}, but whether mitochondrial nucleoids undergo active transport, and if so how it contributes to the nucleoid distribution remains elusive.

　　On September 8^th, 2020, Nature Communications published ‘ER-mitochondria contacts promote mtDNA nucleoids active transportation via mitochondrial dynamic tubulation’ from the laboratories of Yujie Sun and Dong Li. In this study, they identified an active transportation mechanism of nucleoids.

　　Using grazing incidence structured illumination microscopy (GI-SIM), a super-resolution technique that has multicolor, high-speed, long-term and live-cell imaging capabilities, the research team characterized the dynamic interplay between mitochondria and the nucleoids. They observed that during mitochondrial dynamic tubulation, a process that thin, highly dynamic tubules are pulled out of mitochondria by the motor protein KIF5B⁷, a considerable amount of the nucleoids could move into and traverse to the tip of the mitochondrial tubules, and that the motion of the nucleoids and mitochondrial dynamic tubulation are well-coupled. They also found that, in some cases, mitochondrial nucleoids are moving faster than the tubular tip. Thus, it appears that mitochondrial nucleoids can be actively transported within the thin tubules, both as hitchhikers and self-movers (Fig. 1).

　　Fig. 1 Mitochondrial DNA nucleoids are transported via mitochondrial dynamic tubulation　　

　　Moreover, mitochondrial dynamic tubulation activities were observed to frequently occur at the endoplasmic reticulum (ER)-mitochondria contacts, the major sites where mtDNA is localized and synthesized (Fig. 2). Collectively, these findings suggest that mitochondrial nucleoids can be actively transported via KIF5B-driven mitochondrial dynamic tubulation activities occurring predominantly at the ER-mitochondria contact sites.

　　Fig. 2 ER tubules mark the initiation sites of mitochondrial dynamic tubulation

　　The research team next investigated the molecular basis of mitochondrial dynamic tubulation-dependent active transportation of the nucleoids. Imaging and co-immunoprecipitation studies collectively demonstrated that Mic60, a mitochondrial inner membrane protein that interacts with the nucleoids, can also interact with Miro1, a KIF5B receptor on the mitochondrial outer membrane, at the ER-mitochondria contacts. This suggests that KIF5B drives mitochondrial dynamic tubulation-dependent active transportation of the nucleoids through a protein complex containing both Mic60 and Miro1. Consistent with this possibility, knockdown of Mic60 reduced the endogenous level of Miro1 and caused the nucleoids to form large aggregates, while destabilizing the ER–mitochondria contacts. Altogether, these findings indicate that mitochondrial nucleoids at the ER-mitochondria contacts are linked to KIF5B through both Mic60 and Miro1, and that these interactions contribute to the observed nucleoid movement during mitochondrial dynamic tabulation (Fig. 3).

　　Finally, the researchers investigated whether the observed nucleoid active transportation plays a role in establishing the intracellular distribution of mitochondrial nucleoids. Knockdown of Mic60 in cells abolished nucleoid motility and prevented localization of the nucleoids to the periphery of mitochondria. These findings suggest that active transportation is essential for the distribution of nucleoids within the mitochondrial network.

　　Fig. 3 The role of mitochondrial dynamic tubulation in the partitioning of mitochondrial DNA nucleoids

　　In summary, the work unveils a novel nucleoid transportation mechanism based on a Mic60-Miro1 protein complex that links mitochondrial nucleoids in the inner mitochondrial matrix to the motor protein KIF5B at the ER-mitochondria contact sites. These findings should greatly advance our knowledge of the mechanisms of nucleoid distribution in the cell and have significant implications for understanding the role of mtDNA segregation and transmission in disease and aging.

　　Dr. Jinshan Qin is the first author of this paper. Prof. Yujie Sun at BIOPIC of Peking University and Prof. Dong Li at Institute of Biophysics, Chinese Academy of Sciences are the co-correspondence authors. Dr. Yuting Guo at Institute of Biophysics, Chinese Academy of Sciences, Dr. Boxin Xue, Peng Shi, Prof. Yang Chen, Dr. Huiwen Hao, Dr Shujuan Zhao, Prof. Congying Wu at Peking University, Dr. Qian Peter Su at the University of Technology Sydney and Prof. Li Yu at Tsinghua University contributed to this work.

　　Qin, J., Guo, Y., Xue, B. et al.ER-mitochondria contacts promote mtDNA nucleoids active transportation via mitochondrial dynamic tubulation. Nat Commun 11, 4471 (2020). https://doi.org/10.1038/s41467-020-18202-4　

　　References

　　1.    Friedman JR, Nunnari J. Mitochondrial form and function. Nature 505, 335-343 (2014).　

　　2.    Nunnari J, Suomalainen A. Mitochondria: In Sickness and in Health. Cell 148, 1145-1159 (2012).　　

　　3.    Iborra FJ, Kimura H, Cook PR. The functional organization of mitochondrial genomes in human cells. BMC Biol 2, 9 (2004).

　　4.    Chen H, et al. Mitochondrial Fusion Is Required for mtDNA Stability in Skeletal Muscle and Tolerance of mtDNA Mutations. Cell 141, 280-289 (2010).

　　5.    Chen H, McCaffery JM, Chan DC. Mitochondrial fusion protects against neurodegeneration in the cerebellum. Cell 130, 548-562 (2007).　

　　6.    Ban-Ishihara R, Ishihara T, Sasaki N, Mihara K, Ishihara N. Dynamics of nucleoid structure regulated by mitochondrial fission contributes to cristae reformation and release of cytochrome c. Proc Natl Acad Sci U S A 110, 11863-11868 (2013).　

　　7.    Wang C, et al. Dynamic tubulation of mitochondria drives mitochondrial network formation. Cell Res 25, 1108-1120 (2015).