Mitoflash biogenesis and its role in the autoregulation of mitochondrial proton electrochemical potential

Integration of Mitoflash and Time-Series Transcriptomics Facilitates Energy Dynamics Tracking and Substrate Supply Analysis of Floral Thermogenesis in Lotus

The high biosynthetic and energetic demands of floral thermogenesis render thermogenic plants the ideal systems to characterize energy metabolism in plants, but real-time tracking of energy metabolism in plant cells remains challenging. In this study, a new method was developed for tracking the mitochondrial energy metabolism at the single mitochondria level by real-time imaging of mitochondrial superoxide production (i.e., mitoflash). Using this method, we observed the increased mitoflash frequencies in the receptacles of Nelumbo nucifera Gaertn. at the thermogenic stages. This increase, combined with the higher expression of antioxidant response-related genes identified through time-series transcriptomics at the same stages, shows us a new regulatory mechanism for plant redox balance. Furthermore, we found that the upregulation of respiratory metabolism-related genes during the thermogenic stages not only correlates with changes in mitoflash frequency but also underscores the critical roles of these pathways in ensuring adequate substrate supply for thermogenesis. Metabolite analysis revealed that sugars are likely one of the substrates for thermogenesis and may be transported over long distances by sugar transporters. Taken together, our findings demonstrate that mitoflash is a reliable tool for tracking energy metabolism in thermogenic plants and contributes to our understanding of the regulatory mechanisms underlying floral thermogenesis.

Calcium transients on the ER surface trigger liquid-liquid phase separation of FIP200 to specify autophagosome initiation sites

The mechanism that initiates autophagosome formation on the ER in multicellular organisms is elusive. Here, we showed that autophagy stimuli trigger Ca²⁺ transients on the outer surface of the ER membrane, whose amplitude, frequency, and duration are controlled by the metazoan-specific ER transmembrane autophagy protein EPG-4/EI24. Persistent Ca²⁺ transients/oscillations on the cytosolic ER surface in EI24-depleted cells cause accumulation of FIP200 autophagosome initiation complexes on the ER. This defect is suppressed by attenuating ER Ca²⁺ transients. Multi-modal SIM analysis revealed that Ca²⁺ transients on the ER trigger the formation of dynamic and fusion-prone liquid-like FIP200 puncta. Starvation-induced Ca²⁺ transients on lysosomes also induce FIP200 puncta that further move to the ER. Multiple FIP200 puncta on the ER, whose association depends on the ER proteins VAPA/B and ATL2/3, assemble into autophagosome formation sites. Thus, Ca²⁺ transients are crucial for triggering phase separation of FIP200 to specify autophagosome initiation sites in metazoans.

EFHD1 ablation inhibits cardiac mitoflash activation and protects cardiomyocytes from ischemia

Altered levels of intracellular calcium (Ca²⁺) are a highly prevalent feature in different forms of cardiac injury, producing changes in contractility, arrhythmias, and mitochondrial dysfunction. In cardiac ischemia-reperfusion injury, mitochondrial Ca²⁺ overload leads to pathological production of reactive oxygen species (ROS), activates the permeability transition, and cardiomyocyte death. Here we investigated the cardiac phenotype caused by deletion of EF-hand domain-containing protein D1 (Efhd1^-/-), a Ca²⁺-binding mitochondrial protein whose function is poorly understood. Efhd1^-/- mice are viable and have no adverse cardiac phenotypes. They feature reductions in basal ROS levels and mitoflash events, both important precursors for mitochondrial injury, though cardiac mitochondria have normal susceptibility to Ca²⁺ overload. Notably, we also find that Efhd1^-/- mice and their cardiomyocytes are resistant to hypoxic injury.

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.

BacFlash signals acid-resistance gene expression in bacteria

Intracellular pH (pHi) homeostasis is crucial for cellular functions and signal transduction across all kingdoms of life. In particular, bacterial pHi homeostasis is important for physiology, ecology, and pathogenesis. Here we report an exquisite bacterial acid-resistance (AR) mechanism in which proton leak elicits a pre-emptive AR response. A single bacterial cell undergoes quantal electrochemical excitation, termed "BacFlash", which consists of membrane depolarization, transient pHi rise, and bursting production of reactive oxygen species. BacFlash ignition is dictated by acid stress in the form of proton leak across the plasma membrane and the rate of BacFlash occurrence is reversely correlated with the pHi buffering capacity. Through genome-wide screening, we further identify the ATP synthase Fo complex subunit a as the putative proton sensor for BacFlash biogenesis. Importantly, persistent BacFlash hyperactivity activates transcription of a panel of key AR genes and predisposes the cells to survive imminent extreme acid stress. These findings demonstrate a prototypical coupling between electrochemical excitation and nucleoid gene expression in prokaryotes.

Mitoflash generated at the Qo site of mitochondrial Complex III

The previous research has shown that mitochondrial flash (mitoflash) genesis are functionally and mechanistically integrated with mitochondrial electron transport chain (ETC) energy metabolism. However, the response of mitoflash to superoxide is not entirely consistent with the response of MitoSOX Red. The generation mechanism of mitoflash is still unclear. Here, we investigated mitoflash activities, using the different combinations of ETC substrates and inhibitors, in permeabilized cardiomyocytes or hearts. We found that blocking the complete electron flow, from Complex I to IV, with any one of ETC inhibitors including rotenone (Rot), antimycin A (AntA), myxothiazol (Myxo), stigmatellin, and sodium cyanide, will lead to the abolishment of mitoflashes triggered by substrates in adult permeabilized cardiomyocytes. However, Myxo boosted mitoflashes triggered by the reverse electron of N,N,N',N'-tetramethyl-p-phenylenediamine/ascorbate. Moreover, Rot and AntA furtherly enhanced mitoflash activity rather than depressed it, suggesting that mitoflashes generated at the Complex III Qo site. Meanwhile, the inhibition of Complex III protein expression resulted in the activity of Complex III decrease, which decreased mitoflash frequency. The function defect (no change of protein level) of the Qo site of Complex III in aging hearts augmented mitoflash generation confirmed the Qo site function was critical to mitoflash genesis. Thus, our results indicate that mitoflash detected by circularly permuted yellow fluorescent protein is generated at the Qo site of Complex III.

Protocol for Imaging of Mitoflashes in Live Cardiomyocytes

We describe a protocol for imaging a mitochondrial fluorescence transient increase event (Mitoflash) in live cardiomyocytes using a confocal microscope. Mitoflash, detected by mitochondria-targeted circularly permuted fluorescent protein (mt-cpYFP), can be used to assess mitochondrial respiration function in situ. The protocol is also suitable for live-cell imaging of other adherent cells, including fibroblasts and hepatocytes. For complete details on the use and execution of this protocol, please refer to Gong et al. (2014) and Gong et al. (2015).

DnmA and FisA Mediate Mitochondria and Peroxisome Fission, and Regulate Mitochondrial Function, ROS Production and Development in Aspergillus nidulans

The dynamin-like protein Drp1 and its receptor Fis-1 are required for mitochondria and peroxisome fission in animal and yeast cells. Here, we show that in the fungus Aspergillus nidulans the lack of Drp1 and Fis-1 homologs DnmA and FisA has strong developmental defects, leading to a notable decrease in hyphal growth and asexual and sexual sporulation, with some of these defects being aggravated or partially remediated by different carbon sources. Although both DnmA and FisA, are essential for mitochondrial fission, participate in peroxisomal division and are fully required for H2O2-induced mitochondrial division, they also appear to play differential functions. Despite their lack of mitochondrial division, ΔdnmA and ΔfisA mutants segregate mitochondria to conidiogenic cells and produce viable conidia that inherit a single mitochondrion. During sexual differentiation, ΔdnmA and ΔfisA mutants develop fruiting bodies (cleistothecia) that differentiate excessive ascogenous tissue and a reduced number of viable ascospores. ΔdnmA and ΔfisA mutants show decreased respiration and notably high levels of mitochondrial reactive oxygen species (ROS), which likely correspond to superoxide. Regardless of this, ΔdnmA mutants can respond to an external H2O2 challenge by re-localizing the MAP kinase-activated protein kinase (MAPKAP) SrkA from the cytoplasm to the nuclei. Our results show that ROS levels regulate mitochondrial dynamics while a lack of mitochondrial fission results in lower respiration, increased mitochondrial ROS and developmental defects, indicating that ROS, mitochondrial division and development are critically interrelated processes.

Isolating a reverse-mode ATP synthase-dependent mechanism of mitoflash activation

Wei-LaPierre and Dirksen discuss new work investigating the molecular events underlying mitoflash biogenesis.