Effects of Tolerance-Induced Preconditioning on Mitochondrial Biogenesis in Undifferentiated and Differentiated Neuronal Cells

Abstract

Background: Mitochondrial biogenesis occurs in response to chronic stresses as an adaptation to the increased energy demands and often renders cells more refractive to subsequent injuries which is referred to as preconditioning. This phenomenon is observed in several nonneuronal cell types, but it is not yet fully established in neurons, although it is fundamentally important for neuroprotection and could be exploited for therapeutic purposes. Methods: This study was designed to examine whether the preconditioning treatment with hypoxia or nitric oxide could trigger biogenesis in undifferentiated and differentiated neuronal cells (rat PC12 and human NT2 cells) as well as in primary mouse cortical neurons. Results: The results showed that both preconditioning paradigms induced mitochondrial biogenesis in undifferentiated cell lines, as indicated by an increase of mitochondrial mass (measured by flow cytometry of NAO fluorescence) and increased expression of genes required for mitochondrial biogenesis (Nrf1, Nrf2, Tfam, Nfκb1) and function (Cox3, Hk1). All these changes translated into an increase in the organelle copy number from an average of 20-40 to 40-60 mitochondria per cell. The preconditioning treatments also rendered the cells significantly less sensitive to the subsequent oxidative stress challenge brought about by oxygen/glucose deprivation, consistent with their improved cellular energy status. Mitochondrial biogenesis was abolished when preconditioning treatments were performed in the presence of antioxidants (vitamin E or CoQ10), indicating clearly that ROS-signaling pathway(s) played a critical role in the induction of this phenomenon in undifferentiated cells. However, mitochondrial biogenesis could not be re-initiated by preconditioning treatments in any of the post-mitotic neuronal cells tested, i.e., neither rat PC12 cells differentiated with NGF, human NT2 cells differentiated with retinoic acid nor mouse primary cortical neurons. Instead, differentiated neurons had a much higher organelle copy number per cell than their undifferentiated counterparts (100-130 mitochondria per neuron vs. 20-40 in proliferating cells), and this feature was not altered by preconditioning. Conclusions: Our study demonstrates that mitochondrial biogenesis occurred during the differentiation process resulting in more beneficial energy status and improved tolerance to oxidative stress in neurons, putting in doubt whether additional enhancement of this phenomenon could be achieved and successfully exploited as a way for better neuroprotection.

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“…Nrf-2 interacts with mitochondrial transcription factor A ( TFAM ), which is a downstream target gene of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) and drives the transcription and replication of mitochondrial DNA [ 54 ]. According to Sandhu et al, the Nrf2/ARE and PGC-1α pathways interacted to promote mitochondrial biogenesis and neuroprotection [ 55 ]. During the dynamic mitochondrial process, fusion and fission constantly occur within mitochondria that continuously undergo morphological changes [ 56 , 57 ].…”