Temporal depolarization of mitochondria during M phase

Regulation of proteostasis and innate immunity via mitochondria-nuclear communication

Mitochondria are perhaps best known as the "powerhouse of the cell" for their role in ATP production required for numerous cellular activities. Mitochondria have emerged as an important signaling organelle. Here, we first focus on signaling pathways mediated by mitochondria-nuclear communication that promote protein homeostasis (proteostasis). We examine the mitochondrial unfolded protein response (UPRmt) in C. elegans, which is regulated by a transcription factor harboring both a mitochondrial- and nuclear-targeting sequence, the integrated stress response in mammals, as well as the regulation of chromatin by mitochondrial metabolites. In the second section, we explore the role of mitochondria-to-nuclear communication in the regulation of innate immunity and inflammation. Perhaps related to their prokaryotic origin, mitochondria harbor molecules also found in viruses and bacteria. If these molecules accumulate in the cytosol, they elicit the same innate immune responses as viral or bacterial infection.

Potassium Ions Decrease Mitochondrial Matrix pH: Implications for ATP Production and Reactive Oxygen Species Generation

Potassium (K+) is the most abundant cation in the cytosol and is maintained at high concentrations within the mitochondrial matrix through potassium channels. However, many effects of K+ at such high concentrations on mitochondria and the underlying mechanisms remain unclear. This study aims to elucidate these effects and mechanisms by employing fluorescence imaging techniques to distinguish and precisely measure signals inside and outside the mitochondria. We stained the mitochondrial matrix with fluorescent dyes sensitive to K+, pH, reactive oxygen species (ROS), and membrane potential in plasma membrane-permeabilized C6 cells and isolated mitochondria from C6 cells. Fluorescence microscopy facilitated the accurate measurement of fluorescence intensity inside and outside the matrix. Increasing extramitochondrial K+ concentration from 2 mM to 127 mM led to a reduction in matrix pH and a decrease in the generation of highly reactive ROS. In addition, elevated K+ levels electrically polarized the inner membrane of the mitochondria and promoted efficient ATP synthesis via FoF1-ATPase. Introducing protons (H+) into the matrix through phosphate addition led to further mitochondrial polarization, and this effect was more pronounced in the presence of K+. K+ at high concentrations, reaching sub-hundred millimolar levels, increased H+ concentration within the matrix, suppressing ROS generation and boosting ATP synthesis. Although this study does not elucidate the role of specific types of potassium channels in mitochondria, it does suggest that mitochondrial K+ plays a beneficial role in maintaining cellular health.

Effects of proton pumping on the structural rigidity of cristae in mitochondria

Mitochondria change their morphology and inner membrane structure depending on their activity. Since mitochondrial activity also depends on their structure, it is important to elucidate the interrelationship between the activity and structure of mitochondria. However, the mechanism by which mitochondrial activity affects the structure of cristae, the folded structure of the inner membrane, is not well understood. In this study, the effect of the mitochondrial activity on the cristae structure was investigated by examining the structural rigidity of cristae. Taking advantage of the fact that unfolding of cristae induces mitochondrial swelling, we investigated the relationship between mitochondrial activity and the susceptibility to swelling. The swelling of individual isolated mitochondria exposed to a hypotonic solution was observed with an optical microscope. The presence of respiratory substrates (malate and glutamate) increased the percentage of mitochondria that underwent swelling, and the further addition of rotenone or KCN (inhibitors of proton pumps) reversed the increase. In the absence of respiratory substrates, acidification of the buffer surrounding the mitochondria also increased the percentage of swollen mitochondria. These observations suggest that acidification of the outer surface of inner membranes, especially intracristal space, by proton translocation from the matrix to the intracristal space, decreases the structural rigidity of the cristae. This interpretation was verified by the observation that ADP or CCCP, which induces proton re-entry to the matrix, suppressed the mitochondrial swelling in the presence of respiratory substrates. The addition of CCCP to the cells induced a morphological change in mitochondria from an initial elongated structure to a largely curved structure at pH 7.4, but there were no morphological changes when the pH of the cytosol dropped to 6.2. These results suggest that a low pH in the intracristal space may be helpful in maintaining the elongated structure of mitochondria. The present study shows that proton pumping by the electron transfer chain is the mechanism underlying mitochondrial morphology and the flexibility of cristae structure.

Effects of Matrix pH on Spontaneous Transient Depolarization and Reactive Oxygen Species Production in Mitochondria

Reactive oxygen species (ROS) oxidize surrounding molecules and thus impair their functions. Since mitochondria are a major source of ROS, suppression of ROS overproduction in the mitochondria is important for cells. Spontaneous transient depolarization of individual mitochondria is a physiological phenomenon widely observed from plants to mammals. Mitochondrial uncoupling can reduce ROS production; therefore, it is conceivable that transient depolarization could reduce ROS production. However, transient depolarization has been observed with increased ROS production. Therefore, the exact contribution of transient depolarization to ROS production has not been elucidated. In this study, we examined how the spontaneous transient depolarization occurring in individual mitochondria affected ROS production. When the matrix pH increased after the addition of malate or exposure of the isolated mitochondria to a high-pH buffer, transient depolarization was stimulated. Similar stimulation by an increased matrix pH was also observed in the mitochondria in intact H9c2 cells. Modifying the mitochondrial membrane potential and matrix pH by adding K⁺ in the presence of valinomycin, a K⁺ ionophore, clarified that an increase in the matrix pH is a major cause of ROS generation. When we added ADP in the presence of oligomycin to suppress the transient depolarization without decreasing the matrix pH, we observed the suppression of mitochondrial respiration, increased matrix pH, and enhanced ROS production. Based on these results, we propose a model where spontaneous transient depolarization occurs during increased proton influx through proton channels opened by increased matrix pH, leading to the suppression of ROS production. This study improves our understanding of mitochondrial behavior.

The Phosphorylation Status of Drp1-Ser637 by PKA in Mitochondrial Fission Modulates Mitophagy via PINK1/Parkin to Exert Multipolar Spindles Assembly during Mitosis

Mitochondrial fission and fusion cycles are integrated with cell cycle progression. Here we first re-visited how mitochondrial ETC inhibition disturbed mitosis progression, resulting in multipolar spindles formation in HeLa cells. Inhibitors of ETC complex I (rotenone, ROT) and complex III (antimycin A, AA) decreased the phosphorylation of Plk1 T210 and Aurora A T288 in the mitotic phase (M-phase), especially ROT, affecting the dynamic phosphorylation status of fission protein dynamin-related protein 1 (Drp1) and the Ser637/Ser616 ratio. We then tested whether specific Drp1 inhibitors, Mdivi-1 or Dynasore, affected the dynamic phosphorylation status of Drp1. Similar to the effects of ROT and AA, our results showed that Mdivi-1 but not Dynasore influenced the dynamic phosphorylation status of Ser637 and Ser616 in Drp1, which converged with mitotic kinases (Cdk1, Plk1, Aurora A) and centrosome-associated proteins to significantly accelerate mitotic defects. Moreover, our data also indicated that evoking mito-Drp1-Ser637 by protein kinase A (PKA) rather than Drp1-Ser616 by Cdk1/Cyclin B resulted in mitochondrial fission via the PINK1/Parkin pathway to promote more efficient mitophagy and simultaneously caused multipolar spindles. Collectively, this study is the first to uncover that mito-Drp1-Ser637 by PKA, but not Drp1-Ser616, drives mitophagy to exert multipolar spindles formation during M-phase.

Synthesis of a Unique Psammaplysin F Library and Functional Evaluation in Prostate Cancer Cells by Multiparametric Quantitative Single Cell Imaging

The spirooxepinisoxazoline alkaloid psammaplysin F (1) was selected as a scaffold for the generation of a unique screening library for both drug discovery and chemical biology research. Large-scale extraction and isolation chemistry was performed on a marine sponge (Hyattella sp.) collected from the Great Barrier Reef in order to acquire >200 mg of the desired bromotyrosine-derived alkaloidal scaffold. Parallel solution-phase semisynthesis was employed to generate a series of psammaplysin-based urea (2-9) and amide analogues (10-11) in low to moderate yields. The chemical structures of all analogues were characterized using NMR and MS data. The absolute configuration of psammaplysin F and all semisynthetic analogues was determined as 6R, 7R by comparison of ECD data with literature values. All compounds (1-11) were evaluated for their effect on cell cycle distribution and changes to cancer metabolism in LNCaP prostate cancer cells using a multiparametric quantitative single-cell imaging approach. These investigations identified that in LNCaP cells psammaplysin F and some urea analogues caused loss of mitochondrial membrane potential, fragmentation of the mitochondrial tubular network, chromosome misalignment, and cell cycle arrest in mitosis.

Distinct Kinetics in Electrophoretic Extraction of Cytoplasmic RNA from Single Cells

The physical fractionation of cytoplasmic versus nuclear components of cells is a key step for studying the subcellular localization of molecules. The application of an electric field is an emerging method for subcellular fractionation of proteins and nucleic acids from single cells. However, the multibiophysical process that involves electrical lysis of cytoplasmic membranes, electrophoresis, and diffusion of charged molecules remains unclear. Here we study RNA dynamics in single cells during the electrophoretic extraction via a microfluidic system that enables stringent fractionation of the subcellular components leveraging a focused electric field. We identified two distinct kinetics in the extraction of RNA molecules, which were respectively associated with soluble RNA and mitochondrial RNA. We show that the extraction kinetics of soluble RNA is dominated by electrophoresis over diffusion and has a time constant of 0.15 s. Interestingly, the extraction of mitochondrial RNA showed unexpected heterogeneity in the extraction with slower kinetics (3.8 s), while reproducibly resulting in the extraction of 98.9% ± 2% after 40 s. Together, we uncover that the microfluidic system uniquely offers length bias-free fractionation of RNA molecules for quantitative analysis of correlations among subcellular compartments by exploiting the homogeneous electrophoretic properties of RNA.

Photostable Fluorescent Tracker for Imaging Mitochondria with Super Resolution

The power factories in cells, mitochondria, play important roles in all physiological processes. It is reported that progressive mitochondrial swelling and outer mitochondrial membrane rupture could be induced by a wide variety of apoptotic and necrotic stimuli. Regrettably, although a variety of mitochondrial probes have been developed, most of them are based on the detection of active species in mitochondria. Probes that can monitor the status and distribution of mitochondria for a long time are still urgently needed. In this study, a fluorescent sensor with excellent properties, EtNBEn, is described. Outstanding performance allows it to be observed not only in cells but also in living Daphnia and zebrafish under confocal microscopy for a long time. Moreover, the swelling process of mitochondria under light stimulation is also visualized under super-resolution (SR) microscopy. All these results suggest that EtNBEn could be employed for tagging mitochondria in various physiological processes, which makes a great contribution to the cure of diseases.

A protonic biotransducer controlling mitochondrial ATP synthesis

In nature, protons (H⁺) play an important role in biological activities such as in mitochondrial ATP synthesis, which is driven by a H⁺ gradient across the inner membrane, or in the activation of acid sensing ion channels in neuron cells. Bioprotonic devices directly interface with the H⁺ concentration (pH) to facilitate engineered interactions with these biochemical processes. Here we develop a H⁺ biotransducer that changes the pH in a mitochondrial matrix by controlling the flow of H⁺ between a conductive polymer of sulfonated polyaniline and solution. We have successfully modulated the rate of ATP synthesis in mitochondria by altering the solution pH. Our H⁺ biotransducer provides a new way to monitor and modulate pH dependent biological functions at the interface between the electronic devices and biological materials.