Skeletal progenitors preserve proliferation and self-renewal upon inhibition of mitochondrial respiration by rerouting the TCA cycle

A functional electron transport chain (ETC) is crucial for supporting bioenergetics and biosynthesis. Accordingly, ETC inhibition decreases proliferation in cancer cells but does not seem to impair stem cell proliferation. However, it remains unclear how stem cells metabolically adapt. In this study, we show that pharmacological inhibition of complex III of the ETC in skeletal stem and progenitor cells induces glycolysis side pathways and reroutes the tricarboxylic acid (TCA) cycle to regenerate NAD⁺ and preserve cell proliferation. These metabolic changes also culminate in increased succinate and 2-hydroxyglutarate levels that inhibit Ten-eleven translocation (TET) DNA demethylase activity, thereby preserving self-renewal and multilineage potential. Mechanistically, mitochondrial malate dehydrogenase and reverse succinate dehydrogenase activity proved to be essential for the metabolic rewiring in response to ETC inhibition. Together, these data show that the metabolic plasticity of skeletal stem and progenitor cells allows them to bypass ETC blockade and preserve their self-renewal.

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Skeletal stem/progenitor cells can proliferate upon electron transport chain blockade

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Succinate dehydrogenase is reversed with fumarate functioning as electron acceptor

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Pyruvate and aspartate are critical for NAD⁺ regeneration and proliferation

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Metabolic changes prevent DNA demethylation and preserve self-renewal

A resazurin-based, nondestructive assay for monitoring cell proliferation during a scaffold-based 3D culture process

Development of viable cell estimation method without sacrificing proliferation and functions of cells cultured on regenerative biomaterials is essential for regenerative engineering. Cytotoxicity and depletion of resazurin are critical but often overlooked limitations that hindered applications of resazurin in viable cell estimation. The present work found that cytotoxicity and depletion of resazurin depended on cell concentration, resazurin concentration and resazurin incubation time. A simple strategy which only allowed cells to incubate with resazurin during each measurement was developed to eliminate negative effects of resazurin. This strategy was verified by monitoring proliferation of MC3T3-E1 preosteoblasts on poly(d,l-lactic acid) scaffold during a continuous 3D culture process for up to 21 days, comparing the accuracy with MTT assay which is a destructive assay with high sensitivity and accuracy and commonly used in regenerative engineering and comparing viability, proliferation and differentiation functions of MC3T3-E1, which were treated with/without this strategy for nondestructive evaluation. This method showed comparable linearity of standard curve and characteristics of growth curve to MTT assay. No major negative effects of this method on MC3T3-E1 viability and functions were found. Our work highlighted the importance of the concentration and incubation time of resazurin in designing application-specific nondestructive viability assay and would be helpful in improving the implanted medical devices as well as in regenerative engineering.