Assessing Spatiotemporal and Functional Organization of Mitochondrial Networks

Fractal dynamics of individual mitochondrial oscillators measure local inter-mitochondrial coupling

Mitochondrial inner membrane potentials in cardiomyocytes may oscillate in cycles of depolarization/repolarization when the mitochondrial network is exposed to metabolic or oxidative stress. The frequencies of such oscillations are dynamically changing while clusters of weakly coupled mitochondrial oscillators adjust to a common phase and frequency. Across the cardiac myocyte, the averaged signal of the mitochondrial population follows self-similar or fractal dynamics; however, fractal properties of individual mitochondrial oscillators have not yet been examined. We show that the largest synchronously oscillating cluster exhibits a fractal dimension, D, that is indicative of self-similar behavior with D=1.27±0.11, in contrast to the remaining network mitochondria whose fractal dimension is close to that of Brownian noise, D=1.58±0.10. We further demonstrate that fractal behavior is correlated with local coupling mechanisms, whereas it is only weakly linked to measures of functional connections between mitochondria. Our findings suggest that individual mitochondrial fractal dimensions may serve as a simple measure of local mitochondrial coupling.

Automated Quantification and Network Analysis of Redox Dynamics in Neuronal Mitochondria

Mitochondria are complex organelles with multifaceted roles in cell biology, acting as signaling hubs that implicate them in cellular physiology and pathology. Mitochondria are both the target and the origin of multiple signaling events, including redox processes and calcium signaling which are important for organellar function and homeostasis. One way to interrogate mitochondrial function is by live cell imaging. Elaborated approaches perform imaging of single mitochondrial dynamics in living cells and animals. Imaging mitochondrial signaling and function can be challenging due to the sheer number of mitochondria, and the speed, propagation, and potential short half-life of signals. Moreover, mitochondria are organized in functionally coupled interorganellar networks. Therefore, advanced analysis and postprocessing tools are needed to enable automated analysis to fully quantitate mitochondrial signaling events and decipher their complex spatiotemporal connectedness. Herein, we present a protocol for recording and automating analyses of signaling in neuronal mitochondrial networks.

Modeling Cell Energy Metabolism as Weighted Networks of Non-autonomous Oscillators

Networks of oscillating processes are a common occurrence in living systems. This is as true as anywhere in the energy metabolism of individual cells. Exchanges of molecules and common regulation operate throughout the metabolic processes of glycolysis and oxidative phosphorylation, making the consideration of each of these as a network a natural step. Oscillations are similarly ubiquitous within these processes, and the frequencies of these oscillations are never truly constant. These features make this system an ideal example with which to discuss an alternative approach to modeling living systems, which focuses on their thermodynamically open, oscillating, non-linear and non-autonomous nature. We implement this approach in developing a model of non-autonomous Kuramoto oscillators in two all-to-all weighted networks coupled to one another, and themselves driven by non-autonomous oscillators. Each component represents a metabolic process, the networks acting as the glycolytic and oxidative phosphorylative processes, and the drivers as glucose and oxygen supply. We analyse the effect of these features on the synchronization dynamics within the model, and present a comparison between this model, experimental data on the glycolysis of HeLa cells, and a comparatively mainstream model of this experiment. In the former, we find that the introduction of oscillator networks significantly increases the proportion of the model's parameter space that features some form of synchronization, indicating a greater ability of the processes to resist external perturbations, a crucial behavior in biological settings. For the latter, we analyse the oscillations of the experiment, finding a characteristic frequency of 0.01–0.02 Hz. We further demonstrate that an output of the model comparable to the measurements of the experiment oscillates in a manner similar to the measured data, achieving this with fewer parameters and greater flexibility than the comparable model.

Propagation of Mitochondria-Derived Reactive Oxygen Species within the Dipodascus magnusii Cells

Mitochondria are considered to be the main source of reactive oxygen species (ROS) in the cell. It was shown that in cardiac myocytes exposed to excessive oxidative stress, ROS-induced ROS release is triggered. However, cardiac myocytes have a network of densely packed organelles that do not move, which is not typical for the majority of eukaryotic cells. The purpose of this study was to trace the spatiotemporal development (propagation) of prooxidant-induced oxidative stress and its interplay with mitochondrial dynamics. We used Dipodascus magnusii yeast cells as a model, as they have advantages over other models, including a uniquely large size, mitochondria that are easy to visualize and freely moving, an ability to vigorously grow on well-defined low-cost substrates, and high responsibility. It was shown that prooxidant-induced oxidative stress was initiated in mitochondria, far preceding the appearance of generalized oxidative stress in the whole cell. For yeasts, these findings were obtained for the first time. Preincubation of yeast cells with SkQ1, a mitochondria-addressed antioxidant, substantially diminished production of mitochondrial ROS, while only slightly alleviating the generalized oxidative stress. This was expected, but had not yet been shown. Importantly, mitochondrial fragmentation was found to be primarily induced by mitochondrial ROS preceding the generalized oxidative stress development.

Dexmedetomidine alleviates sevoflurane-induced neurotoxicity via mitophagy signaling

Dexmedetomidine, a class of α2-adrenergic agonist, was reported to exert a neuroprotective effect on sevoflurane-induced neurotoxicity. However, the specific mechanisms have not been fully clarified yet. The aim of our study is to uncover the role of dexmedetomidine in sevoflurane-induced neurotoxicity. The rats pretreated with dexmedetomidine and/or Rapamycin 3-Methyladenine were housed in a box containing 30% O₂, 68% N₂ and 2% sevoflurane for 4 h for anesthesia. 24 h after drug injection, Morris water maze test was used to evaluate rats' learning and memory ability. Hematoxylin & eosin (H&E) staining was adopted to analyze the pathological changes of hippocampus. TUNEL assay was performed to measure cell apoptosis in hippocampus. Immunofluorescent assay was utilized to detect HSP60 level. The protein levels of LC3I, LC3II, Beclin-1, CypD, VDAC1 and Tom20 were examined by western blot. 5 weeks after drug injection, Morris water maze test was used to evaluate rats' learning and memory ability again. Dexmedetomidine alleviated sevoflurane-induced nerve injury and the impairment of learning and memory abilities. Additionally, dexmedetomidine inhibited sevoflurane-induced cell apoptosis in hippocampus. In mechanism, dexmedetomidine activated mitophagy to mitigate neurotoxicity by enhancing LC3II/LC3I ratio, HSP60, Beclin-1, CypD, VDAC1 and Tom20 protein levels in hippocampus. Dexmedetomidine alleviates sevoflurane-induced neurotoxicity via mitophagy signaling, offering a potential strategy for sevoflurane-induced neurotoxicity treatment.

Diabetes Increases the Vulnerability of the Cardiac Mitochondrial Network to Criticality

Mitochondrial criticality describes a state in which the mitochondrial cardiac network under intense oxidative stress becomes very sensitive to small perturbations, leading from local to cell-wide depolarization and synchronized oscillations that may escalate to the myocardial syncytium generating arrhythmias. Herein, we describe the occurrence of mitochondrial criticality in the chronic setting of a metabolic disorder, type 1 diabetes (T1DM), using a streptozotocin (STZ)-treated guinea pig (GP) animal model. Using wavelet analysis of mitochondrial networks from two-photon microscopy imaging of cardiac myocytes loaded with a fluorescent probe of the mitochondrial membrane potential, we show that cardiomyocytes from T1DM GPs are closer to criticality, making them more vulnerable to cell-wide mitochondrial oscillations as can be judged by the latency period to trigger oscillations after a laser flash perturbation, and their propensity to oscillate. Insulin treatment of T1DM GPs rescued cardiac myocytes to sham control levels of susceptibility, a protective condition that could also be attained with interventions leading to improvement of the cellular redox environment such as preincubation of diabetic cardiac myocytes with the lipid palmitate or a cell-permeable form of glutathione, in the presence of glucose.

Intrafibrillar and perinuclear mitochondrial heterogeneity in adult cardiac myocytes

Mitochondria are involved in multiple cellular functions, in addition to their core role in energy metabolism. Mitochondria localized in different cellular locations may have different morphology, Ca²⁺ handling and biochemical properties and may interact differently with other intracellular structures, causing functional specificity. However, most prior studies have utilized isolated mitochondria, removed from their intracellular environment. Mitochondria in cardiac ventricular myocytes are highly organized, with a majority squeezed between the myofilaments in longitudinal chains (intrafibrillar mitochondria, IFM). There is another population of perinuclear mitochondria (PNM) around and between the two nuclei typical in myocytes. Here, we take advantage of live myocyte imaging to test for quantitative morphological and functional differences between IFM and PNM with respect to calcium fluxes, membrane potential, sensitivity to oxidative stress, shape and dynamics. Our findings show higher mitochondrial Ca²⁺ uptake and oxidative stress sensitivity for IFM vs. PNM, which may relate to higher local energy demand supporting the contractile machinery. In contrast to IFM which are remarkably static, PNM are relatively mobile, appear to participate readily in fission/fusion dynamics and appear to play a central role in mitochondrial genesis and turnover. We conclude that while IFM may be physiologically tuned to support local myofilament energy demands, PNM may be more critical in mitochondrial turnover and regulation of nuclear function and import/export. Thus, important functional differences are present in intrafibrillar vs. perinuclear mitochondrial subpopulations.