Mitochondria, neurosteroids and biological rhythms : implications in health and disease states

Mitochondria are considered as the “powerhouses” of cells because they synthesize the universal source of energy under the form of adenosine triphosphate (ATP) molecules via oxidative phosphorylation from nutritional sources. Thus, impaired mitochondrial function, especially in neurons that have high energy requirements, lead inevitably to disease, ranging from subtle alterations in neuronal function to cell death and neurodegenerative diseases, such as Alzheimer’s disease (AD). The purpose of this PhD thesis was therefore to deepen our understanding of the regulation of mitochondrial function and to identify key factors that are critical in the control of mitochondrial bioenergetics and dynamics. To achieve this goal, the thesis was divided into two main parts:
1) Since a growing body of evidence suggests that neurosteroids have a strong neuroprotective potential, the first part is based on the hypothesis that neurosteroids may exert a determinant action against neurodegeneration by improving mitochondrial bioenergetics, (A) under “healthy” conditions as well as (B) under pathological conditions (AD);
2) In the second part (C), we determined whether the biological clock, which coordinates a whole range of daily behaviors and physiological processes, is involved in the endogenous regulation of mitochondrial dynamics and bioenergetics.
(A) In the first part of this thesis, we aimed to characterize the bioenergetic modulating profile of a panel of seven structurally diverse neurosteroids (progesterone, estradiol, estrone, testosterone, 3alpha-androstanediol, DHEA and allopregnanolone), known to be involved in brain function regulation. Our key findings were that: i) the majority of these steroids increased energy metabolism, mainly via an up-regulation of the mitochondrial activity and at least in part via receptor activation, and ii) neurosteroids regulated redox homeostasis by increasing the antioxidant activity as a compensatory mechanism to the reactive oxygen species (ROS) level enhancement which might result from the acceleration in oxygen consumption accompanied by a greater electron leakage from the electron transport chain. Additionally, each neurosteroid seems to have a specific bioenergetic profile.
Together, these first data indicated that neurosteroids were indeed able to boost mitochondrial function in a delicate balance, possibly by regulating the expression of genes involved in glycolysis and oxidative phosphorylation, but also the content and activity of mitochondrial respiratory complexes. Further investigations are required to determine the underlying molecular mechanisms.
(B) Based on these findings, we investigated in the next step whether neurosteroids were able to alleviate AD-related bioenergetic deficits. We distinguished the effects of several neurosteroids on ATP synthesis, mitochondrial membrane potential (MMP), mitochondrial respiration and glycolysis in two AD cellular models overexpressing either the amyloid precursor protein and amyloid-beta peptide (APP/Abeta) or the mutant form of tau producing abnormally hyperphosphorylated tau proteins, respectively. Key findings were that: i) APP/Abeta and mutant tau-overexpressing cells present distinct bioenergetic impairments, with APP/Abeta having the strongest deleterious effect on mitochondrial function; ii) the male steroid hormone, testosterone, was more efficient to alleviate mitochondrial deficits induced by APP/Abeta, whereas female steroid hormones, progesterone and estrogen, were more efficient to increase bioenergetic outcomes in our model of AD-related tauopathies. Together, our findings lend further evidence to the neuroprotective effects of neurosteroids in AD pathology and indicate that these molecules represent promising tools able to increase mitochondrial bioenergetics via enhanced mitochondrial respiration, in healthy and pathological conditions, respectively. Our results may open new avenues for drug development with regard to targeting mitochondria in neurodegeneration.
(C) The aim of the second part of this thesis was to investigate more closely how mitochondrial function is endogenously regulated within the cells. Since a growing body of evidences shows that the circadian clock and metabolic homeostasis are connected in numerous ways via reciprocal regulation, we asked whether mitochondrial bioenergetics and dynamics may exhibit circadian oscillations and whether mitochondria themselves may be able to influence the circadian clock. We found that mitochondrial bioenergetics, including mitochondrial respiration and, consequently, generation of the byproducts ATP and ROS, is directly coupled to mitochondrial network which is, at least in part, regulated by clock-controlled phosphorylation of Drp1, the main factor involved in mitochondrial fission. The time-dependent reorganization of mitochondrial architecture in turn regulates the clock through circadian oscillation of mitochondrial ATP which can act as input signal through activation of AMP-activated protein kinase (AMPK). Our findings highlight new insights in the understanding of the reciprocal temporal crosstalk that governs the molecular interplay between the coupling of mitochondrial dynamics and metabolism and circadian rhythms.
Overall, the studies performed in the present thesis importantly helped to deepen our knowledge about the modulation of mitochondrial function in health and disease states. Our findings could have multiple implications with regard to the regulation of metabolic homeostasis in health and disease states associated with mitochondrial impairments and / or circadian disruption.