Regulation of mitochondrial dynamics and bioenergetics : implications of circadian clock and neurosteroids in health and diseases

The cellular metabolism is a highly dynamic process where mitochondria network is a prominent actor in regulation of both energy metabolism and apoptotic pathways. To preserve the integrity of a healthy mitochondrial population within the cell but also the integrity of the cell itself, mitochondrial networks come in varied shapes and ultrastructures to ensure the main energy supply, stored in the form of adenosine triphosphate (ATP), by oxidative reactions from nutritional sources. Therefore, alterations in both mitochondrial dynamics and metabolism are often related to each other as early and prominent events in the pathogenesis of several age-related disorders including Alzheimer’s disease (AD). Advances in the understanding of the mechanisms underlying the coordination between mitochondrial dynamics and the functional state of mitochondria in health are essential for the characterization of disease-related changes of mitochondria in the course of neurodegenerative disorders.
The purpose of this thesis was therefore to pinpoint the mechanistic processes that are involved in the regulation of mitochondrial bioenergetics and dynamics. To better understand (I) the tight equilibrium between mitochondrial morphology and function in physiological state and (II) its impact on abnormal mitochondrial, the thesis was divided in two main parts:
I. The first aim of the thesis was to investigate the potential influence of (A) the circadian clock and (B) neurosteroids on the maintenance of mitochondrial homeostasis.
(A) Since biological clocks are tightly connected to metabolic processes within the cell, we first determined whether mitochondrial dynamics and metabolism are coupled events that are coordinated by the circadian system.
(B) Considering compelling evidence that highlighted neuroprotective effects of steroids in the brain, we examined whether different neurosteroids are able to improve mitochondrial bioenergetics to prevent age-related mitochondrial alterations which eventually lead to neurodegeneration.
II. In the second part (C), we determined whether amyloid-beta impacts the integrity of the mitochondrial structure–function relationship since both mitochondrial dynamics and bioenergetics are hallmarks of Aβ-induced neuronal toxicity in AD.
I. (A) The circadian clock is a hierarchical network of oscillators that coordinate a wide variety of daily biological functions, including metabolic functions, to the optimal time of day anticipating the periodical changes of the external environment for all living organisms. Mitochondria are dynamic organelles at the crossroad of the cellular metabolism that fuse and divide continuously to fulfill their role in the maintenance of the cellular bioenergetic homeostasis. While it is well known that metabolism is a complex biochemical network that is tightly intertwined with the circadian clock through reciprocal regulation from metabolites to transcription factors, the mechanistic connections between the biological clock and the mitochondrial network remain mostly elusive. We therefore addressed the questions whether and how the circadian clock intervenes in the coordination between mitochondrial dynamics and functions and whether the coupled mitochondrial network- metabolism may be able to influence the circadian clock.
We demonstrated in vitro and in vivo that mitochondrial fission-fusion dynamics were strongly clock-controlled, as well as all other aspects of mitochondrial metabolic flux, including oxidative phosphorylation, generation of ATP and reactive oxygen species (ROS). The changes in cell cycle-based mitochondrial morphology required the circadian phosphorylation of the key protein, dynamin-related protein 1 (DRP1), the major protein involved in mitochondrial fission. Genetic or pharmacological abrogation of DRP1 activity abolished circadian mitochondrial network dynamics and mitochondrial respiratory activity, as well as eliminated circadian ATP production. The disruption of circadian mitochondrial dynamics furthermore feeds back to impair the core circadian clock.
Overall, our findings are consistent with the existence of a crosstalk between the clock and the mitochondrial network that maintains bioenergetic homeostasis in response to circadian metabolic changes.
I. (B) We aimed to investigate the potential role of different neurosteroids on mitochondrial bioenergetics and redox homeostasis in neuronal cells. In contrast to steroid hormones produced by endocrine glands, neurosteroids are synthetized within the nervous system itself and are defined as neuroactive molecules acting on the nervous system in an auto/paracrine manner. Neurosteroids exhibit several biological functions that are essential during brain development as well as in the adult brain. Moreover, progressive depletion in neurosteroid content might contribute to an age-related neuronal decline that eventually leads to the development of neurodegenerative disorders including AD. Although compelling evidence has shown that estradiol interacts with mitochondria to counteract oxidative stress occurring in age-related diseases such as AD, the potential role of other neurosteroids on mitochondria is rather poorly investigated and understood.
To expand our knowledge on the mechanisms behind the neuroprotective action of neurosteroids, a selection of sex-hormone-related neurosteroids, including progesterone, estradiol, estrone, testosterone, 3-alpha-androstanediol, dehydroepiandrosterone (DHEA) as well as allopregnolone, were tested on mitochondrial function. Using human SH-SY5Y neuroblastoma cells, we determined which of the neurosteroids exhibited the capacity to enhance mitochondrial metabolism by increasing ATP content along with an augmentation of mitochondrial membrane potential and mitochondrial respiration. Interestingly, particular bioenergetic profiles were found for each neurosteroid, which might be due to an involvement of different receptors. When the respective steroid receptors were blocked with specific inhibitors, ATP contents were entirely depleted confirming a receptor-specific mode of action of neurosteroids. Concomitantly with the enhanced mitochondrial metabolism, treatment with neurosteroids induced an augmentation of ROS levels in parallel to an up-regulation of antioxidant defenses suggesting a direct or indirect role of neurosteroids on redox homeostasis in neuronal cells.
Collectively, these novel findings demonstrate that neurosteroids are able to differentially improve mitochondrial function as well as to modulate redox homeostasis through distinct receptors. Because of the disparate effects of neurosteroids on mitochondrial metabolism, the underlying mechanisms have to be further elucidated in future studies, particularly the effect of neurosteroids on mitochondrial dynamics, as well as those in models of neurodegenerative diseases, such as AD.
II. (C) The aim of the second part of this thesis was to investigate the impact of amyloid-beta on the balance between mitochondrial structure and function since abnormalities in mitochondrial dynamics and bioenergetics are hallmarks of Aβ-induced neuronal toxicity in AD. For that purpose, we examined mitochondrial architecture and bioenergetics in cell-cycle controlled human primary fibroblast cultures treated with amyloid-beta 1-42 peptide compared to non-treated cells. We demonstrated that variations in mitochondrial respiration, ATP and ROS content coincided with the oscillations pattern of the mitochondrial network in physiological conditions in control cells confirming the existence of a direct link between the mitochondrial network and the metabolic state of mitochondria. Indeed, in between the switch from tubular to fragmented mitochondrial network, we observed an increase in ATP level which correlated with a higher oxygen consumption rate (OCR) in the basal respiration as well as in ATP turnover and maximal respiration. In contrast, amyloid-beta 1-42 almost completely dampened the oscillations of mitochondrial dynamics followed by a decline of mitochondrial metabolism including reduced ATP level and OCR. Furthermore, Aβ-induced mitochondrial defects provoked a drastic augmentation in mitochondrial ROS level which might participate, along with an imbalance in the NAD+/NADH ratio, in the generation of oxidative stress confirming the oxidative stress theory of aging and AD.
Hence, these new insights support the concept that mitochondrial bioenergetics is coordinated by mitochondrial architecture transitions and that Aβ induced a functional imbalance in the mitochondrial structure-function relationship, which might contribute already at an early disease state to AD pathogenesis.
Altogether, in the present thesis, we gained new insights on factors regulating mitochondrial dynamics and metabolism in health and disease states, e.g. AD. We firstly revealed that the circadian clock system, even in nondividing cells and tissues, regulates the phosphorylation of DRP1 resulting in cycles of fission and fusion that are essential for circadian oscillations in ATP production. These findings provide multiple implications into the understanding of metabolic homeostasis in human health as well as in disorders linked to impairments in circadian clock and/ or mitochondrial function. Secondly, we determined that neurosteroids are able to differentially modulate mitochondrial metabolism at the physiological state suggesting that these molecules might be considered as promising candidates in neuroprotective approaches to counterbalance mitochondrial deficiencies. Finally, we contributed to a better understanding of Aβ-induced neurotoxicity that is mediated by mitochondrial malfunctions further emphasizing the mitochondrial cascade hypothesis of AD.