Objective: To determine age-associated changes in the size, density, movement, and dynamics of mitochondria as a function of sex, using microRNA-induced neurons derived from primary human fibroblasts.

Background: Mitochondrial dysfunction is common in Parkinson’s disease, stemming from either intrinsic mitochondrial damage due to genetic mutations, or from accumulation and lack of degradation of injured mitochondria. How the process of aging contributes to mitochondrial changes and thus predisposes individuals to age-related sporadic diseases is less well understood, as evaluating living neurons over the human lifespan is technically challenging. Identifying age- and sex- associated changes in mitochondrial behavior can help identify shifts between healthy aging and neurodegenerative disease.

Method: Neurons were directly differentiated from human donor fibroblasts using microRNAs [1]. This process preserves the molecular and epigenetic signatures of the donor cells, permitting assessment of characteristics that vary with the age of the donor [2-4]. For our study, these microRNA-induced neurons (miNs) were cultured for 30 days and then either fixed for confocal analysis or imaged alive using epifluorescence microscopy. Mitochondria in miNs from male and female donors of various ages ranging from newborn to 89 years old were analyzed for multiple features, including size, density, motility, dynamics, and incorporation into autophagosomes. Isogenic lines were used when possible to identify changes over 15 years within the same individual.

Results: Mitochondrial dynamic events, both fission and fusion, increase significantly as a function of age. This is true in miNs from both men and women. Mitochondria from male donors had a trend towards less motility than those from female donors, which was more apparent in miNs from young individuals. Despite the increase in both fission and fusion, no change was seen in overall mitochondrial length or density along the neurite as a function of age or sex. Furthermore, no change was observed in the percent of overall autophagosomes which contained mitochondrial fragments as a function of either age or sex.

Conclusion: This work provides insight into changes in the behavior of mitochondria during healthy aging, and highlights sex-related differences which may help clarify gender-related susceptibility to different neurogenerative conditions including Parkinson’s disease.

References: 1. Yoo AS et al. MicroRNA-mediated conversion of human fibroblasts to neurons. Nature 476, 228–231 (2011).
2. Huh CJ et al. Maintenance of age in human neurons generated by microRNA-based neuronal conversion of fibroblasts. Elife 5, (2016).
3. Cates K et al. Deconstructing Stepwise Fate Conversion of Human Fibroblasts to Neurons by MicroRNAs. Cell Stem Cell 28, 127-140.e9 (2021).
4. Capano LS et al. Recapitulation of endogenous 4R tau expression and formation of insoluble tau in directly reprogrammed human neurons. Cell Stem Cell 29, 918-932.e8 (2022).

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MDS Abstracts - https://www.mdsabstracts.org/abstract/age-and-sex-associated-changes-in-the-behavior-of-neuronal-mitochondria-during-healthy-aging/