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Mitochondrial mobility in differentiating muscle heterokaryons
U A Walker1, W F Walker, A F Miranda
1Department of Neurology, Columbia University, College of Physicians and Surgeons, NY 10032, USA.
Abstract:
Ragged-red fibers, a morphological hallmark of many patients with mitochondrial encephalomyopathies who harbor mitochondrial DNA (mtDNA) mutations, usually contain varying ratios of mutated and wild-type mtDNAs. Deficient respiratory function in muscle is almost invariably segmental. To investigate whether this observation may be explained by restricted lateral movement of mitochondria within myofibers, we studied the spatial and temporal behavior of two different mitochondrial populations within multinucleate myotubes. We co-cultured normal human and mouse myoblasts, allowed them to fuse into muscle heterokaryons and investigated whether the mitochondria remained segregated, or migrated and intermixed. Human and mouse nuclei were identified by their differential staining pattern with the dye Hoechst 33 258 and mitochondria were distinguished immunologically and by in situ hybridization. Although we observed some territoriality at very early time points after myoblast fusion, there was rapid intermixing of the mitochondrial populations, as early as 48 h after myoblast fusion. We conclude that mitochondria, unlike many other muscle components, lack territorial organization in cultured, differentiating heterokaryons.
Insights
Mitochondria rapidly intermix within muscle cells, challenging the idea that their restricted movement causes ragged-red fibers in mitochondrial diseases. This suggests other factors contribute to segmental respiratory dysfunction.
Area of Science:
- Cell Biology
- Mitochondrial Biology
- Muscle Physiology
Background:
- Ragged-red fibers, indicative of mitochondrial DNA (mtDNA) mutations in mitochondrial encephalomyopathies, exhibit segmental respiratory deficiency.
- This segmental dysfunction has been hypothesized to result from restricted lateral movement of mitochondria within myofibers.
Purpose of the Study:
- To investigate the spatial and temporal behavior of distinct mitochondrial populations within multinucleate myotubes.
- To determine if limited mitochondrial movement contributes to the observed segmental respiratory deficits in muscle.
Main Methods:
- Co-culture of normal human and mouse myoblasts to form muscle heterokaryons.
- Identification of human and mouse nuclei using Hoechst 33 258 dye.
- Distinguishing and tracking mitochondrial populations via immunological methods and in situ hybridization.
Main Results:
- Rapid intermixing of human and mouse mitochondrial populations was observed as early as 48 hours post-fusion.
- While some initial territoriality was noted, it was transient and followed by extensive mitochondrial migration.
Conclusions:
- Mitochondria demonstrate significant lateral mobility and do not exhibit territorial organization in cultured, differentiating muscle heterokaryons.
- The findings suggest that restricted mitochondrial movement is unlikely to be the primary cause of segmental respiratory dysfunction in ragged-red fibers.