Membrane potential can be determined in individual cells from the nernstian distribution of cationic dyes

Towards optical measurements of membrane potential values in Bacillus subtilis using fluorescence lifetime

Membrane potential (MP) changes can provide a simple readout of bacterial functional and metabolic state or stress levels. While several optical methods exist for measuring fast changes in MP in excitable cells, there is a dearth of such methods for absolute and precise measurements of steady-state membrane potentials (MPs) in bacterial cells. Conventional electrode-based methods for the measurement of MP are not suitable for calibrating optical methods in small bacterial cells. While optical measurement based on Nernstian indicators have been successfully used, they do not provide absolute or precise quantification of MP or its changes. We present a novel, calibrated MP recording approach to address this gap. In this study, we used a fluorescence lifetime-based approach to obtain a single-cell resolved distribution of the membrane potential and its changes upon extracellular chemical perturbation in a population of bacterial cells for the first time. Our method is based on (i) a unique VoltageFluor (VF) optical transducer, whose fluorescence lifetime varies as a function of MP via photoinduced electron transfer (PeT) and (ii) a quantitative phasor-FLIM analysis for high-throughput readout. This method allows MP changes to be easily visualized, recorded and quantified. By artificially modulating potassium concentration gradients across the membrane using an ionophore, we have obtained a Bacillus subtilis-specific MP versus VF lifetime calibration and estimated the MP for unperturbed B. subtilis cells to be -65 mV and that for chemically depolarized cells as -14 mV. We observed a population level MP heterogeneity of ~6-10 mV indicating a considerable degree of diversity of physiological and metabolic states among individual cells. Our work paves the way for deeper insights into bacterial electrophysiology and bioelectricity research.

MitoTracker Red for isolation of zone-specific hepatocytes and characterization of hepatic sublobular metabolism

BACKGROUND

The liver lobule is divided into three zones or regions: periportal (PP or Zone 1) that is highly oxidative and active in ureagenesis, pericentral (PC or Zone 3) that is more glycolytic, and midzonal (MZ or Zone 2) with intermediate characteristics.

AIM

Our goal was to isolate and metabolically characterize hepatocytes from specific sublobular zones.

METHODS

Mice were administered rhodamine123 (Rh123) or MitoTracker Red (MTR) prior to intravital imaging, liver fixation, or hepatocyte isolation. After in vivo MTR, hepatocytes were isolated and sorted based on MTR fluorescence intensity. Alternatively, E-cadherin (Ecad) and cytochrome P450 2E1 (CYP2E1) immunolabeling was performed in fixed liver slices. Ecad and CYP2E1 gene expression in sorted hepatocytes was assessed by qPCR. Oxygen consumption rates (OCR) of sorted hepatocytes were also assessed.

RESULTS

Multiphoton microscopy showed Rh123 and MTR fluorescence distributed zonally, decreasing from PP to PC in a flow-dependent fashion. In liver cross-sections, Ecad was expressed periportally and CYP2E1 pericentrally in association with high and low MTR labeling, respectively. Based on MTR fluorescence, hepatocytes were sorted into PP, MZ, and PC populations with PP and PC hepatocytes enriched in Ecad and CYP2E1, respectively. OCR of PP hepatocytes was ∼4 times that of PC hepatocytes.

CONCLUSIONS

MTR treatment in vivo delineates sublobular hepatic zones and can be used to sort hepatocytes zonally. PP hepatocytes have substantially greater OCR compared to PC and MZ. The results also indicate a sharp midzonal demarcation between hepatocytes with PP characteristics (Ecad) and those with PC features (CYP2E1). This new method to sort hepatocytes in a zone-specific fashion holds the potential to shed light on sublobular hepatocyte metabolism and regulatory pathways in health and disease.

cGMP compliant one-step, one-pot automated [18F]FBnTP production for clinical imaging of mitochondrial activity

BACKGROUND

4-[18F]fluorobenzyl-triphenylphosphonium ([18F]FBnTP) is a lipophilic cation PET tracer. The cellular uptake of [18F]FBnTP is correlated with oxidative phosphorylation by mitochondria, which has been associated with multiple critical diseases. To date, [18F]FBnTP has been successfully applied for imaging myocardial perfusion, assessment of severity of coronary artery stenosis, delineation of the ischemic area after transient coronary occlusion, and detection/quantification of apoptosis in various animal models. Recent preclinical and clinical studies have also expanded the possibilities of using [18F]FBnTP in oncological diagnosis and therapeutic monitoring. However, [18F]FBnTP is typically prepared through a tediously lengthy four-step, three-pot reaction and required multiple synthesizer modules; Thus, such an approach remains a challenge for this promising radiopharmaceutical to be implemented for routine clinical studies. Herein, we report an optimized one-step, one-pot automated approach to produce [18F]FBnTP through a single standard commercially-available radiosynthesizer that enables centralized production for clinical use.

RESULTS

The fully automated production of [18F]FBnTP took less than 55 min with radiochemical yields ranging from 28.33 ± 13.92% (non-decay corrected), apparent molar activity of 69.23 ± 45.62 GBq/µmol, and radiochemical purities of 99.79 ± 0.41%. The formulated [18F]FBnTP solution was determined to be sterile and colorless with a pH of 4.0-6.0. Our data has indicated no observable radiolysis after 8 h from the time of final product formulation and maximum assay of 7.88 GBq.

CONCLUSIONS

A simplified and cGMP-compliant radiosynthesis of [18F]FBnTP has been established on the commercially available synthesizer in high activity concentration and radiochemical purity. While the preclinical and clinical studies using [18F]FBnTP PET are currently underway, the automated approaches reported herein facilitate clinical adoption of this radiotracer and warrant centralized production of [18F]FBnTP for imaging multiple patients.

Aberrant ER-mitochondria communication is a common pathomechanism in mitochondrial disease

Genetic mutations causing primary mitochondrial disease (i.e those compromising oxidative phosphorylation [OxPhos]) resulting in reduced bioenergetic output display great variability in their clinical features, but the reason for this is unknown. We hypothesized that disruption of the communication between endoplasmic reticulum (ER) and mitochondria at mitochondria-associated ER membranes (MAM) might play a role in this variability. To test this, we assayed MAM function and ER-mitochondrial communication in OxPhos-deficient cells, including cybrids from patients with selected pathogenic mtDNA mutations. Our results show that each of the various mutations studied indeed altered MAM functions, but notably, each disorder presented with a different MAM "signature". We also found that mitochondrial membrane potential is a key driver of ER-mitochondrial connectivity. Moreover, our findings demonstrate that disruption in ER-mitochondrial communication has consequences for cell survivability that go well beyond that of reduced ATP output. The findings of a "MAM-OxPhos" axis, the role of mitochondrial membrane potential in controlling this process, and the contribution of MAM dysfunction to cell death, reveal a new relationship between mitochondria and the rest of the cell, as well as providing new insights into the diagnosis and treatment of these devastating disorders.

Messenger RNA transport on lysosomal vesicles maintains axonal mitochondrial homeostasis and prevents axonal degeneration

In neurons, RNA granules are transported along the axon for local translation away from the soma. Recent studies indicate that some of this transport involves hitchhiking of RNA granules on lysosome-related vesicles. In the present study, we leveraged the ability to prevent transport of these vesicles into the axon by knockout of the lysosome-kinesin adaptor BLOC-one-related complex (BORC) to identify a subset of axonal mRNAs that depend on lysosome-related vesicles for transport. We found that BORC knockout causes depletion of a large group of axonal mRNAs mainly encoding ribosomal and mitochondrial/oxidative phosphorylation proteins. This depletion results in mitochondrial defects and eventually leads to axonal degeneration in human induced pluripotent stem cell (iPSC)-derived and mouse neurons. Pathway analyses of the depleted mRNAs revealed a mechanistic connection of BORC deficiency with common neurodegenerative disorders. These results demonstrate that mRNA transport on lysosome-related vesicles is critical for the maintenance of axonal homeostasis and that its failure causes axonal degeneration.

Exploring mitochondrial targeting: an innovative fluorescent probe reveals Nernstian potential and partitioning combination

This study introduces a paradigm-shifting approach to optimize mitochondrial targeting. Employing a new fluorescent probe strategy, we unravel a combined influence of both Nernst potential (Ψ) and partitioning (P) contributions. Through the synthesis of new benz[e]indolinium-derived probes, our findings redefine the landscape of mitochondrial localization by optimizing the efficacy of mitochondrial probe retention in primary cortical neurons undergoing normoxia and oxygen-glucose deprivation. This methodology not only advances our understanding of subcellular dynamics, but also holds promise for transformative applications in biomedical research and therapeutic development.

Spinning Disk Confocal Microscopy for Optimized and Quantified Live Imaging of 3D Mitochondrial Network

Mitochondria are the energy factories of a cell, and depending on the metabolic requirements, the mitochondrial morphology, quantity, and membrane potential in a cell change. These changes are frequently assessed using commercially available probes. In this study, we tested the suitability of three commercially available probes-namely 5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolo-carbocyanine iodide (JC-1), MitoTracker Red CMX Rox (CMXRos), and tetramethylrhodamine methyl ester (TMRM)-for assessing the mitochondrial quantity, morphology, and membrane potential in living human mesoangioblasts in 3D with confocal laser scanning microscope (CLSM) and scanning disk confocal microscope (SDCM). Using CLSM, JC-1, and CMXRos-but not TMRM-uncovered considerable background and variation. Using SDCM, the background signal only remained apparent for the JC-1 monomer. Repetitive imaging of CMXRos and JC-1-but not TMRM-demonstrated a 1.5-2-fold variation in signal intensity between cells using CLSM. The use of SDCM drastically reduced this variation. The slope of the relative signal intensity upon repetitive imaging using CLSM was lowest for TMRM (-0.03) and highest for CMXRos (0.16). Upon repetitive imaging using SDCM, the slope varied from 0 (CMXRos) to a maximum of -0.27 (JC-1 C1). Conclusively, our data show that TMRM staining outperformed JC-1 and CMXRos dyes in a (repetitive) 3D analysis of the entire mitochondrial quantity, morphology, and membrane potential in living cells.

Performance of TMRM and Mitotrackers in mitochondrial morphofunctional analysis of primary human skin fibroblasts

Mitochondrial membrane potential (Δψ) and morphology are considered key readouts of mitochondrial functional state. This morphofunction can be studied using fluorescent dyes ("probes") like tetramethylrhodamine methyl ester (TMRM) and Mitotrackers (MTs). Although these dyes are broadly used, information comparing their performance in mitochondrial morphology quantification and Δψ-sensitivity in the same cell model is still scarce. Here we applied epifluorescence microscopy of primary human skin fibroblasts to evaluate TMRM, Mitotracker Red CMXros (CMXros), Mitotracker Red CMH2Xros (CMH2Xros), Mitotracker Green FM (MG) and Mitotracker Deep Red FM (MDR). All probes were suited for automated quantification of mitochondrial morphology parameters when Δψ was normal, although they did not deliver quantitatively identical results. The mitochondrial localization of TMRM and MTs was differentially sensitive to carbonyl cyanide-4-phenylhydrazone (FCCP)-induced Δψ depolarization, decreasing in the order: TMRM ≫ CHM2Xros = CMXros = MDR > MG. To study the effect of reversible Δψ changes, the impact of photo-induced Δψ "flickering" was studied in cells co-stained with TMRM and MG. During a flickering event, individual mitochondria displayed subsequent TMRM release and uptake, whereas this phenomenon was not observed for MG. Spatiotemporal and computational analysis of the flickering event provided evidence that TMRM redistributes between adjacent mitochondria by a mechanism dependent on Δψ and TMRM concentration. In summary, this study demonstrates that: (1) TMRM and MTs are suited for automated mitochondrial morphology quantification, (2) numerical data obtained with different probes is not identical, and (3) all probes are sensitive to FCCP-induced Δψ depolarization, with TMRM and MG displaying the highest and lowest sensitivity, respectively. We conclude that TMRM is better suited for integrated analysis of Δψ and mitochondrial morphology than the tested MTs under conditions that Δψ is not substantially depolarized.

Shedding light on mitochondrial outer-membrane permeabilization and membrane potential: State of the art methods and biosensors

Membrane structural integrity is essential for optimal mitochondrial function. These organelles produce the energy needed for all vital processes, provided their outer and inner membranes are intact. This prevents the release of mitochondrial apoptogenic factors into the cytosol and ensures intact mitochondrial membrane potential (ΔΨm) to sustain ATP production. Cell death by apoptosis is generally triggered by outer mitochondrial membrane permeabilization (MOMP), tightly coupled with loss of ΔΨ m. As these two processes are essential for both mitochondrial function and cell death, researchers have devised various techniques to assess them. Here, we discuss current methods and biosensors available for detecting MOMP and measuring ΔΨ m, focusing on their advantages and limitations and discuss what new imaging tools are needed to improve our knowledge of mitochondrial function.

How and when to measure mitochondrial inner membrane potentials

The scientific literature on mitochondria has increased significantly over the years due to findings that these organelles have widespread roles in the onset and progression of pathological conditions such as metabolic disorders, neurodegenerative and cardiovascular diseases, inflammation, and cancer. Researchers have extensively explored how mitochondrial properties and functions are modified in different models, often using fluorescent inner mitochondrial membrane potential (ΔΨm) probes to assess functional mitochondrial aspects such as protonmotive force and oxidative phosphorylation. This review provides an overview of existing techniques to measure ΔpH and ΔΨm, highlighting their advantages, limitations, and applications. It discusses drawbacks of ΔΨm probes, especially when used without calibration, and conditions where alternative methods should replace ΔΨm measurements for the benefit of the specific scientific objectives entailed. Studies investigating mitochondria and their vast biological roles would be significantly advanced by the understanding of the correct applications as well as limitations of protonmotive force measurements and use of fluorescent ΔΨm probes, adopting more precise, artifact-free, sensitive, and quantitative measurements of mitochondrial functionality.

Intermittent proton bursts of single lactic acid bacteria

Lactic acid bacteria are a kind of probiotic microorganisms that efficiently convert carbohydrates to lactic acids, thus playing essential roles in fermentation and food industry. While conventional wisdom often suggests continuous release of protons from bacteria during acidification, here we developed a methodology to measure the dynamics of proton release at the single bacteria level, and report on the discovery of a proton burst phenomenon, i.e., the intermittent efflux of protons, of single Lactobacillus plantarum bacteria. When placing an individual bacterium in an oil-sealed microwell, efflux and accumulation of protons consequently reduced the pH in the confined extracellular medium, which was monitored with fluorescent pH indicators in a high-throughput and real-time manner. In addition to the slow and continuous proton release behavior (as expected), stochastic and intermittent proton burst events were surprisingly observed with a typical timescale of several seconds. It was attributed to the regulatory response of bacteria by activating H+-ATPase to compensate the stochastic and transient depolarizations of membrane potential. These findings not only revealed an unprecedented proton burst phenomenon in lactic acid bacteria, but also shed new lights on the intrinsic roles of H+-ATPase in membrane potential homeostasis, with implications for both fermentation industry and bacterial electrophysiology.

Design and Application of Fluorescent Probes to Detect Cellular Physical Microenvironments

The microenvironment is indispensable for functionality of various biomacromolecules, subcellular compartments, living cells, and organisms. In particular, physical properties within the biological microenvironment could exert profound effects on both the cellular physiology and pathology, with parameters including the polarity, viscosity, pH, and other relevant factors. There is a significant demand to directly visualize and quantitatively measure the fluctuation in the cellular microenvironment with spatiotemporal resolution. To satisfy this need, analytical methods based on fluorescence probes offer great opportunities due to the facile, sensitive, and dynamic detection that these molecules could enable in varying biological settings from in vitro samples to live animal models. Herein, we focus on various types of small molecule fluorescent probes for the detection and measurement of physical parameters of the microenvironment, including pH, polarity, viscosity, mechanical force, temperature, and electron potential. For each parameter, we primarily describe the chemical mechanisms underlying how physical properties are correlated with changes of various fluorescent signals. This review provides both an overview and a perspective for the development of small molecule fluorescent probes to visualize the dynamic changes in the cellular environment, to expand the knowledge for biological process, and to enrich diagnostic tools for human diseases.

Cloflucarban Illuminates Specificity and Context-Dependent Activation of the PINK1-Parkin Pathway by Mitochondrial Complex Inhibition

The PTEN-induced kinase 1 (PINK1)-Parkin pathway plays a vital role in maintaining a healthy pool of mitochondria in higher eukaryotic cells. While the downstream components of this pathway are well understood, the upstream triggers remain less explored. In this study, we conducted an extensive analysis of inhibitors targeting various mitochondrial electron transport chain (ETC) complexes to investigate their potential as activators of the PINK1-Parkin pathway. We identified cloflucarban, an antibacterial compound, as a novel pathway activator that simultaneously inhibits mitochondrial complexes III and V, and V. RNA interference (RNAi) confirmed that the dual inhibition of these complexes activates the PINK1-Parkin pathway. Intriguingly, we discovered that albumin, specifically bovine serum albumin (BSA) and human serum albumin (HSA) commonly present in culture media, can hinder carbonyl cyanide m-chlorophenyl hydrazone (CCCP)-induced pathway activation. However, cloflucarban's efficacy remains unaffected by albumin, highlighting its reliability for studying the PINK1-Parkin pathway. This study provides insights into the activation of the upstream PINK1-Parkin pathway and underscores the influence of culture conditions on research outcomes. Cloflucarban emerges as a promising tool for investigating mitochondrial quality control and neurodegenerative diseases.

Late-Stage Functionalization of Living Organisms: Rethinking Selectivity in Biology

With unlimited selectivity, full post-translational chemical control of biology would circumvent the dogma of genetic control. The resulting direct manipulation of organisms would enable atomic-level precision in "editing" of function. We argue that a key aspect that is still missing in our ability to do this (at least with a high degree of control) is the selectivity of a given chemical reaction in a living organism. In this Review, we systematize existing illustrative examples of chemical selectivity, as well as identify needed chemical selectivities set in a hierarchy of anatomical complexity: organismo- (selectivity for a given organism over another), tissuo- (selectivity for a given tissue type in a living organism), cellulo- (selectivity for a given cell type in an organism or tissue), and organelloselectivity (selectivity for a given organelle or discrete body within a cell). Finally, we analyze more traditional concepts such as regio-, chemo-, and stereoselective reactions where additionally appropriate. This survey of late-stage biomolecule methods emphasizes, where possible, functional consequences (i.e., biological function). In this way, we explore a concept of late-stage functionalization of living organisms (where "late" is taken to mean at a given state of an organism in time) in which programmed and selective chemical reactions take place in life. By building on precisely analyzed notions (e.g., mechanism and selectivity) we believe that the logic of chemical methodology might ultimately be applied to increasingly complex molecular constructs in biology. This could allow principles developed at the simple, small-molecule level to progress hierarchically even to manipulation of physiology.

Staphylococcus epidermidis bacteriocin A37 kills natural competitors with a unique mechanism of action

Many bacteria produce antimicrobial compounds such as lantibiotics to gain advantage in the competitive natural environments of microbiomes. Epilancins constitute an until now underexplored family of lantibiotics with an unknown ecological role and unresolved mode of action. We discovered production of an epilancin in the nasal isolate Staphylococcus epidermidis A37. Using bioinformatic tools, we found that epilancins are frequently encoded within staphylococcal genomes, highlighting their ecological relevance. We demonstrate that production of epilancin A37 contributes to Staphylococcus epidermidis competition specifically against natural corynebacterial competitors. Combining microbiological approaches with quantitative in vivo and in vitro fluorescence microscopy and cryo-electron tomography, we show that A37 enters the corynebacterial cytoplasm through a partially transmembrane-potential-driven uptake without impairing the cell membrane function. Upon intracellular aggregation, A37 induces the formation of intracellular membrane vesicles, which are heavily loaded with the compound and are essential for the antibacterial activity of the epilancin. Our work sheds light on the ecological role of epilancins for staphylococci mediated by a mode of action previously unknown for lantibiotics.

A Ru(II)-Strained Complex with 2,9-Diphenyl-1,10-phenanthroline Ligand Induces Selective Photoactivatable Chemotherapeutic Activity on Human Alveolar Carcinoma Cells via Apoptosis

[Ru(bipy)2(dpphen)]Cl2 (where bipy = 2,2'-bipyridine and dpphen = 2,9-diphenyl-1,10-phenanthroline) (complex 1) is a sterically strained compound that exhibits promising in vitro photocytotoxicity on an array of cell lines. Since lung adenocarcinoma cancer remains the most common lung cancer and the leading cause of cancer deaths, the current study aims to evaluate the plausible effect and uptake of complex 1 on human alveolar carcinoma cells (A549) and mesenchymal stem cells (MSC), and assess its cytotoxicity in vitro while considering its effect on cell morphology, membrane integrity and DNA damage. MSC and A549 cells showed similar rates of complex 1 uptake with a plateau at 12 h. Upon photoactivation, complex 1 exhibited selective, potent anticancer activity against A549 cells with phototoxicity index (PI) values of 16, 25 and 39 at 24, 48 and 72 h, respectively. This effect was accompanied by a significant increase in A549-cell rounding and detachment, loss of membrane integrity and DNA damage. Flow cytometry experiments confirmed that A549 cells undergo apoptosis when treated with complex 1 followed by photoactivation. In conclusion, this present study suggests that complex 1 might be a promising candidate for photochemotherapy with photoproducts that possess selective anticancer effects in vitro. These results are encouraging to probe the potential activity of this complex in vivo.

Thioflavin T indicates mitochondrial membrane potential in mammalian cells

The fluorescent benzothiazole dye thioflavin T (ThT) is widely used as a marker for protein aggregates, most commonly in the context of neurodegenerative disease research and diagnosis. Recently, this same dye was shown to indicate membrane potential in bacteria due to its cationic nature. This finding prompted a question whether ThT fluorescence is linked to the membrane potential in mammalian cells, which would be important for appropriate utilization of ThT in research and diagnosis. Here, we show that ThT localizes into the mitochondria of HeLa cells in a membrane-potential-dependent manner. Specifically, ThT colocalized in cells with the mitochondrial membrane potential indicator tetramethylrhodamine methyl ester (TMRM) and gave similar temporal responses as TMRM to treatment with a protonophore, carbonyl cyanide-4-(trifluoromethoxy) phenylhydrazone (FCCP). Additionally, we found that presence of ThT together with exposure to blue light (λ = 405 nm), but neither factor alone, caused depolarization of mitochondrial membrane potential. This additive effect of the concentration and blue light was recapitulated by a mathematical model implementing the potential-dependent distribution of ThT and its effect on mitochondrial membrane potential through photosensitization. These results show that ThT can act as a mitochondrial membrane potential indicator in mammalian cells, when used at low concentrations and with low blue light exposure. However, it causes dissipation of the mitochondrial membrane potential depending additively on its concentrations and blue light exposure. This conclusion motivates a re-evaluation of ThT's use at micromolar range in live-cell analyses and indicates that this dye can enable future studies on the potential connections between mitochondrial membrane potential dynamics and protein aggregation.

Mitophagy in Astrocytes Is Required for the Health of Optic Nerve

Mitochondrial dysfunction in astrocytes has been implicated in the development of various neurological disorders. Mitophagy, mitochondrial autophagy, is required for proper mitochondrial function by preventing the accumulation of damaged mitochondria. The importance of mitophagy, specifically in the astrocytes of the optic nerve (ON), has been little studied. We introduce an animal model in which two separate mutations act synergistically to produce severe ON degeneration. The first mutation is in Cryba1, which encodes βA3/A1-crystallin, a lens protein also expressed in astrocytes, where it regulates lysosomal pH. The second mutation is in Bckdk, which encodes branched-chain ketoacid dehydrogenase kinase, which is ubiquitously expressed in the mitochondrial matrix and involved in the catabolism of the branched-chain amino acids. BCKDK is essential for mitochondrial function and the amelioration of oxidative stress. Neither of the mutations in isolation has a significant effect on the ON, but animals homozygous for both mutations (DM) exhibit very serious ON degeneration. ON astrocytes from these double-mutant (DM) animals have lysosomal defects, including impaired mitophagy, and dysfunctional mitochondria. Urolithin A can rescue the mitophagy impairment in DM astrocytes and reduce ON degeneration. These data demonstrate that efficient mitophagy in astrocytes is required for ON health and functional integrity.

Can Single Cell Respiration be Measured by Scanning Electrochemical Microscopy (SECM)?

Ultramicroelectrode (UME), or, equivalently, microelectrode, probes are increasingly used for single-cell measurements of cellular properties and processes, including physiological activity, such as metabolic fluxes and respiration rates. Major challenges for the sensitivity of such measurements include: (i) the relative magnitude of cellular and UME fluxes (manifested in the current); and (ii) issues around the stability of the UME response over time. To explore the extent to which these factors impact the precision of electrochemical cellular measurements, we undertake a systematic analysis of measurement conditions and experimental parameters for determining single cell respiration rates via the oxygen consumption rate (OCR) in single HeLa cells. Using scanning electrochemical microscopy (SECM), with a platinum UME as the probe, we employ a self-referencing measurement protocol, rarely employed in SECM, whereby the UME is repeatedly approached from bulk solution to a cell, and a short pulse to oxygen reduction reaction (ORR) potential is performed near the cell and in bulk solution. This approach enables the periodic tracking of the bulk UME response to which the near-cell response is repeatedly compared (referenced) and also ensures that the ORR near the cell is performed only briefly, minimizing the effect of the electrochemical process on the cell. SECM experiments are combined with a finite element method (FEM) modeling framework to simulate oxygen diffusion and the UME response. Taking a realistic range of single cell OCR to be 1 × 10-18 to 1 × 10-16 mol s-1, results from the combination of FEM simulations and self-referencing SECM measurements show that these OCR values are at, or below, the present detection sensitivity of the technique. We provide a set of model-based suggestions for improving these measurements in the future but highlight that extraordinary improvements in the stability and precision of SECM measurements will be required if single cell OCR measurements are to be realized.

pH-Responsive Upconversion Mesoporous Silica Nanospheres for Combined Multimodal Diagnostic Imaging and Targeted Photodynamic and Photothermal Cancer Therapy

Photodynamic therapy (PDT) and photothermal therapy (PTT) have gained considerable attention as potential alternatives to conventional cancer treatments. However, these approaches remain limited by low solubility, poor stability, and inefficient targeting of many common photosensitizers (PSs) and photothermal agents (PTAs). To overcome the aforementioned limitations, we engineered biocompatible and biodegradable tumor-targeted upconversion nanospheres with imaging capabilities. The multifunctional nanospheres consist of a sodium yttrium fluoride core doped with lanthanides (ytterbium, erbium, and gadolinium) and the PTA bismuth selenide (NaYF4:Yb/Er/Gd,Bi2Se3) enveloped in a mesoporous silica shell that encapsulates a PS, chlorin e6 (Ce6), within its pores. NaYF4:Yb/Er converts deeply penetrating near-infrared (NIR) light to visible light, which excites Ce6 to generate cytotoxic reactive oxygen species (ROS), while Bi2Se3 efficiently converts absorbed NIR light to heat. Additionally, Gd enables magnetic resonance imaging of the nanospheres. The mesoporous silica shell is coated with DPPC/cholesterol/DSPE-PEG to retain the encapsulated Ce6 and prevent serum protein adsorption and macrophage recognition that hinder tumor targeting. Finally, the coat is conjugated to the acidity-triggered rational membrane (ATRAM) peptide, which promotes specific and efficient internalization into malignant cells in the mildly acidic microenvironment of tumors. The nanospheres facilitated tumor magnetic resonance and thermal and fluorescence imaging and exhibited potent NIR laser light-induced anticancer effects in vitro and in vivo via combined ROS production and localized hyperthermia, with negligible toxicity to healthy tissue, hence markedly extending survival. Our results demonstrate that the ATRAM-functionalized, lipid/PEG-coated upconversion mesoporous silica nanospheres (ALUMSNs) offer multimodal diagnostic imaging and targeted combinatorial cancer therapy.

High-resolution bioenergetics correlates the length of continuous protonatable diaminoethane motif of four-armed oligo(ethanamino)amide transfectants to cytotoxicity

Recent clinical success with Onpattro and cationic ionizable lipid nanoparticle-based mRNA vaccines has rejuvenated research in the design and engineering of broader synthetic cationic vectors for nucleic acid compaction and transfection. However, perturbation of metabolic processes and cytotoxicity are still of concern with synthetic cationic vectors. Here, through an integrated bioenergetic and biomembrane integrity probing in three different human cell lines we reveal the dynamic effect of a library of sequence-defined four-arm oligo(ethanamino)amide transfectant on cell homeostasis, and identify metabolically safe building units over wide concentration ranges. The results show differential effects of the oligo(ethanamino)amide structure of comparable molecular weight on cell energetics. The severity of polycation effect on bioenergetic crisis follows with the length of continuous protonatable diaminoethane motif in the ascending order of glutaryl-triethylene tetramine, succinyl-tetraethylene pentamine and succinyl-pentaethylene hexamine. We further identify oligomeric structures that do not induce bioenergetic crisis even at high concentrations. Finally, transfection studies with a library of polyplexes carrying a reporter gene show no correlation between transfection efficiency and cytotoxicity. These observations demonstrate the usefulness of integrated high-resolution respirometry and plasma membrane integrity probing as a highly sensitive medium-throughput screening strategy for identification and selection of safe building units for transfectant engineering.

RNA-binding protein complex AMG-1/SLRP-1 mediates germline development and spermatogenesis by maintaining mitochondrial homeostasis in Caenorhabditis elegans

The mechanisms of RNA-binding proteins (RBPs)-mediated post-transcriptional regulation of pre-existing mRNAs, which is essential for spermatogenesis, remain poorly understood. In this study, we identify that a germline-specific mitochondrial RBP AMG-1(abnormal mitochondria in germline 1), a homolog of mammalian leucine-rich PPR motif-containing protein (LRPPRC), is required for spermatogenesis in Caenorhabditis elegans. The amg-1 mutation hinders germline development without affecting somatic development and leads to the aberrant mitochondrial morphology and structure associated with mitochondrial dysfunctions specifically in the germline. We demonstrate that AMG-1 is most frequently bound to mtDNA-encoded 12S and 16S ribosomal RNA, the essential components of mitochondrial ribosomes, and that 12S rRNA expression mediated by AMG-1 is crucial for germline mitochondrial protein homeostasis. Furthermore, steroid receptor RNA activator (SRA) stem loop interacting RNA binding protein (SLRP-1), a homolog of mammalian SRA stem loop interacting RNA binding protein (SLIRP) in C. elegans, interacts with AMG-1 genetically to regulate germline development and reproductive success in C. elegans. Overall, these findings reveal the novel function of mtRBP, specifically in regulating germline development.

Fluorescence microscopy imaging of mitochondrial metabolism in cancer cells

Mitochondrial metabolism is an important contributor to cancer cell survival and proliferation that coexists with enhanced glycolytic activity. Measuring mitochondrial activity is useful to characterize cancer metabolism patterns, to identify metabolic vulnerabilities and to identify new drug targets. Optical imaging, especially fluorescent microscopy, is one of the most valuable tools for studying mitochondrial bioenergetics because it provides semiquantitative and quantitative readouts as well as spatiotemporal resolution of mitochondrial metabolism. This review aims to acquaint the reader with microscopy imaging techniques currently used to determine mitochondrial membrane potential (ΔΨm), nicotinamide adenine dinucleotide (NADH), ATP and reactive oxygen species (ROS) that are major readouts of mitochondrial metabolism. We describe features, advantages, and limitations of the most used fluorescence imaging modalities: widefield, confocal and multiphoton microscopy, and fluorescent lifetime imaging (FLIM). We also discus relevant aspects of image processing. We briefly describe the role and production of NADH, NADHP, flavins and various ROS including superoxide and hydrogen peroxide and discuss how these parameters can be analyzed by fluorescent microscopy. We also explain the importance, value, and limitations of label-free autofluorescence imaging of NAD(P)H and FAD. Practical hints for the use of fluorescent probes and newly developed sensors for imaging ΔΨm, ATP and ROS are described. Overall, we provide updated information about the use of microscopy to study cancer metabolism that will be of interest to all investigators regardless of their level of expertise in the field.

Label-Free Optical Metabolic Imaging in Cells and Tissues

Over the last half century, the autofluorescence of the metabolic cofactors NADH (reduced nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide) has been quantified in a variety of cell types and disease states. With the spread of nonlinear optical microscopy techniques in biomedical research, NADH and FAD imaging has offered an attractive solution to noninvasively monitor cell and tissue status and elucidate dynamic changes in cell or tissue metabolism. Various tools and methods to measure the temporal, spectral, and spatial properties of NADH and FAD autofluorescence have been developed. Specifically, an optical redox ratio of cofactor fluorescence intensities and NADH fluorescence lifetime parameters have been used in numerous applications, but significant work remains to mature this technology for understanding dynamic changes in metabolism. This article describes the current understanding of our optical sensitivity to different metabolic pathways and highlights current challenges in the field. Recent progress in addressing these challenges and acquiring more quantitative information in faster and more metabolically relevant formats is also discussed.

The natural chalcone cardamonin selectively induces apoptosis in human neuroblastoma cells

Neuroblastoma is the most common extracranial malignant tumor in childhood. Approximately 60% of all patients are classified as high-risk and require intensive treatment including non-selective chemotherapeutic agents leading to severe side effects. Recently, phytochemicals like the natural chalcone cardamonin (CD) have gained attention in cancer research. For the first time, we investigated the selective anti-cancer effects of CD in SH-SY5Y human neuroblastoma cells compared to healthy (normal) fibroblasts (NHDF). Our study revealed selective and dose-dependent cytotoxicity of CD in SH-SY5Y. The natural chalcone CD specifically altered the mitochondrial membrane potential (ΔΨm), as an early marker of apoptosis, in human neuroblastoma cells. Caspase activity was also selectively induced and the amount of cleaved caspase substrates such as PARP was thus increased in human neuroblastoma cells. CD-mediated apoptotic cell death was rescued by pan caspase inhibitor Z-VAD-FMK. The natural chalcone CD selectively induced apoptosis, the programmed cell death, in SH-SY5Y human neuroblastoma cells whereas NHDF being a model for normal (healthy) cells were unaffected. Our data indicates a clinical potential of CD in the more selective and less harmful treatment of neuroblastoma.

pH-responsive upconversion mesoporous silica nanospheres for combined multimodal diagnostic imaging and targeted photodynamic and photothermal cancer therapy

Photodynamic therapy (PDT) and photothermal therapy (PTT) have garnered considerable interest as non-invasive cancer treatment modalities. However, these approaches remain limited by low solubility, poor stability and inefficient targeting of many common photosensitizers (PSs) and photothermal agents (PTAs). To overcome these limitations, we have designed biocompatible and biodegradable tumor-targeted upconversion nanospheres with imaging capabilities. The multifunctional nanospheres consist of a sodium yttrium fluoride core doped with lanthanides (ytterbium, erbium and gadolinium) and bismuth selenide (NaYF 4 :Yb/Er/Gd,Bi 2 Se 3 ) within a mesoporous silica shell that encapsulates a PS, Chlorin e6 (Ce6), in its pores. NaYF 4 :Yb/Er converts deeply penetrating near-infrared (NIR) light to visible light, which excites the Ce6 to generate cytotoxic reactive oxygen species (ROS), while the PTA Bi 2 Se 3 efficiently converts absorbed NIR light to heat. Additionally, Gd enables magnetic resonance imaging (MRI) of the nanospheres. The mesoporous silica shell is coated with lipid/polyethylene glycol (DPPC/cholesterol/DSPE-PEG) to ensure retention of the encapsulated Ce6 and minimize interactions with serum proteins and macrophages that impede tumor targeting. Finally, the coat is functionalized with the acidity-triggered rational membrane (ATRAM) peptide, which promotes specific and efficient internalization into cancer cells within the mildly acidic tumor microenvironment. Following uptake by cancer cells in vitro , NIR laser irradiation of the nanospheres caused substantial cytotoxicity due to ROS production and hyperthermia. The nanospheres facilitated tumor MRI and thermal imaging, and exhibited potent NIR laser light-induced antitumor effects in vivo via combined PDT and PTT, with no observable toxicity to healthy tissue, thereby substantially prolonging survival. Our results demonstrate that the ATRAM-functionalized, lipid/PEG-coated upconversion mesoporous silica nanospheres (ALUMSNs) offer multimodal diagnostic imaging and targeted combinatorial cancer therapy.

Membrane Targeted Azobenzene Drives Optical Modulation of Bacterial Membrane Potential

Recent studies have shown that bacterial membrane potential is dynamic and plays signaling roles. Yet, little is still known about the mechanisms of membrane potential dynamics regulation-owing to a scarcity of appropriate research tools. Optical modulation of bacterial membrane potential could fill this gap and provide a new approach for studying and controlling bacterial physiology and electrical signaling. Here, the authors show that a membrane-targeted azobenzene (Ziapin2) can be used to photo-modulate the membrane potential in cells of the Gram-positive bacterium Bacillus subtilis. It is found that upon exposure to blue-green light (λ = 470 nm), isomerization of Ziapin2 in the bacteria membrane induces hyperpolarization of the potential. To investigate the origin of this phenomenon, ion-channel-deletion strains and ion channel blockers are examined. The authors found that in presence of the chloride channel blocker idanyloxyacetic acid-94 (IAA-94) or in absence of KtrAB potassium transporter, the hyperpolarization response is attenuated. These results reveal that the Ziapin2 isomerization can induce ion channel opening in the bacterial membrane and suggest that Ziapin2 can be used for studying and controlling bacterial electrical signaling. This new optical tool could contribute to better understand various microbial phenomena, such as biofilm electric signaling and antimicrobial resistance.

Fluorescence Imaging of Cell Membrane Potential: From Relative Changes to Absolute Values

Membrane potential is a fundamental property of biological cells. Changes in membrane potential characterize a vast number of vital biological processes, such as the activity of neurons and cardiomyocytes, tumorogenesis, cell-cycle progression, etc. A common strategy to record membrane potential changes that occur in the process of interest is to utilize organic dyes or genetically-encoded voltage indicators with voltage-dependent fluorescence. Sensors are introduced into target cells, and alterations of fluorescence intensity are recorded with optical methods. Techniques that allow recording relative changes of membrane potential and do not take into account fluorescence alterations due to factors other than membrane voltage are already widely used in modern biological and biomedical studies. Such techniques have been reviewed previously in many works. However, in order to investigate a number of processes, especially long-term processes, the measured signal must be corrected to exclude the contribution from voltage-independent factors or even absolute values of cell membrane potential have to be evaluated. Techniques that enable such measurements are the subject of this review.

Sensitive bacterial Vm sensors revealed the excitability of bacterial Vm and its role in antibiotic tolerance

As an important free energy source, the membrane voltage (V_m) regulates many essential physiological processes in bacteria. However, in comparison with eukaryotic cells, knowledge of bacterial electrophysiology is very limited. Here, we developed a set of novel genetically encoded bacterial V_m sensors which allow single-cell recording of bacterial V_m dynamics in live cells with high temporal resolution. Using these new sensors, we reveal the electrically "excitable" and "resting" states of bacterial cells dependent on their metabolic status. In the electrically excitable state, frequent hyperpolarization spikes in bacterial V_m are observed, which are regulated by Na⁺/K⁺ ratio of the medium and facilitate increased antibiotic tolerance. In the electrically resting state, bacterial V_m displays significant cell-to-cell heterogeneity and is linked to the cell fate after antibiotic treatment. Our findings demonstrate the potential of our newly developed voltage sensors to reveal the underpinning connections between bacterial V_m and antibiotic tolerance.

Mitochondrial depolarization after acute ethanol treatment drives mitophagy in living mice

Ethanol increases hepatic mitophagy driven by unknown mechanisms. Type 1 mitophagy sequesters polarized mitochondria for nutrient recovery and cytoplasmic remodeling. In Type 2, mitochondrial depolarization (mtDepo) initiates mitophagy to remove the damaged organelles. Previously, we showed that acute ethanol administration produces reversible hepatic mtDepo. Here, we tested the hypothesis that ethanol-induced mtDepo initiates Type 2 mitophagy. GFP-LC3 transgenic mice were gavaged with ethanol (2-6 g/kg) with and without pre-treatment with agents that decrease or increase mtDepo-Alda-1, tacrolimus, or disulfiram. Without ethanol, virtually all hepatocytes contained polarized mitochondria with infrequent autophagic GFP-LC3 puncta visualized by intravital microscopy. At ~4 h after ethanol treatment, mtDepo occurred in an all-or-none fashion within individual hepatocytes, which increased dose dependently. GFP-LC3 puncta increased in parallel, predominantly in hepatocytes with mtDepo. Mitochondrial PINK1 and PRKN/parkin also increased. After covalent labeling of mitochondria with MitoTracker Red (MTR), GFP-LC3 puncta encircled MTR-labeled mitochondria after ethanol treatment, directly demonstrating mitophagy. GFP-LC3 puncta did not associate with fat droplets visualized with BODIPY558/568, indicating that increased autophagy was not due to lipophagy. Before ethanol administration, rhodamine-dextran (RhDex)-labeled lysosomes showed little association with GFP-LC3. After ethanol treatment, TFEB (transcription factor EB) translocated to nuclei, and lysosomal mass increased. Many GFP-LC3 puncta merged with RhDex-labeled lysosomes, showing autophagosomal processing into lysosomes. After ethanol treatment, disulfiram increased, whereas Alda-1 and tacrolimus decreased mtDepo, and mitophagy changed proportionately. In conclusion, mtDepo after acute ethanol treatment induces mitophagic sequestration and subsequent lysosomal processing.Abbreviations : AcAld, acetaldehyde; ADH, alcohol dehydrogenase; ALDH, aldehyde dehydrogenase; ALD, alcoholic liver disease; Alda-1, N-(1,3-benzodioxol-5-ylmethyl)-2,6-dichlorobenzamide; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; GFP, green fluorescent protein; LAMP1, lysosomal-associated membrane protein 1; LMNB1, lamin B1; MAA, malondialdehyde-acetaldehyde adducts; MAP1LC3/LC3, microtubule-associated protein 1 light chain 3; MPT, mitochondrial permeability transition; mtDAMPS, mitochondrial damage-associated molecular patterns; mtDepo, mitochondrial depolarization; mtDNA, mitochondrial DNA; MTR, MitoTracker Red; PI, propidium iodide; PINK1, PTEN induced putative kinase 1; PRKN, parkin; RhDex, rhodamine dextran; TFEB, transcription factor EB; Tg, transgenic; TMRM, tetramethylrhodamine methylester; TOMM20, translocase of outer mitochondrial membrane 20; VDAC, voltage-dependent anion channel.

A guide for membrane potential measurements in Gram-negative bacteria using voltage-sensitive dyes

Transmembrane potential is one of the main bioenergetic parameters of bacterial cells, and is directly involved in energizing key cellular processes such as transport, ATP synthesis and motility. The most common approach to measure membrane potential levels is through use of voltage-sensitive fluorescent dyes. Such dyes either accumulate or are excluded from the cell in a voltage-dependent manner, which can be followed by means of fluorescence microscopy, flow cytometry, or fluorometry. Since the cell's ability to maintain transmembrane potential relies upon low and selective membrane ion conductivity, voltage-sensitive dyes are also highly sensitive reporters for the activity of membrane-targeting antibacterials. However, the presence of an additional membrane layer in Gram-negative (diderm) bacteria complicates their use significantly. In this paper, we provide guidance on how membrane potential and its changes can be monitored reliably in Gram-negatives using the voltage-sensitive dye 3,3'-dipropylthiadicarbocyanine iodide [DiSC₃(5)]. We also discuss the confounding effects caused by the presence of the outer membrane, or by measurements performed in buffers rather than growth medium. We hope that the discussed methods and protocols provide an easily accessible basis for the use of voltage-sensitive dyes in Gram-negative organisms, and raise awareness of potential experimental pitfalls associated with their use.

Bacterial Homologs of Progestin and AdipoQ Receptors (PAQRs) Affect Membrane Energetics Homeostasis but Not Fluidity

Membrane potential homeostasis is essential for cell survival. Defects in membrane potential lead to pleiotropic phenotypes, consistent with the central role of membrane energetics in cell physiology. Homologs of the progestin and AdipoQ receptors (PAQRs) are conserved in multiple phyla of Bacteria and Eukarya. In eukaryotes, PAQRs are proposed to modulate membrane fluidity and fatty acid (FA) metabolism. The role of bacterial homologs has not been elucidated. Here, we use Escherichia coli and Bacillus subtilis to show that bacterial PAQR homologs, which we name “TrhA,” have a role in membrane energetics homeostasis. Using transcriptional fusions, we show that E. coli TrhA (encoded by yqfA) is part of the unsaturated fatty acid biosynthesis regulon. Fatty acid analyses and physiological assays show that a lack of TrhA in both E. coli and B. subtilis (encoded by yplQ) provokes subtle but consistent changes in membrane fatty acid profiles that do not translate to control of membrane fluidity. Instead, membrane proteomics in E. coli suggested a disrupted energy metabolism and dysregulated membrane energetics in the mutant, though it grew similarly to its parent. These changes translated into a disturbed membrane potential in the mutant relative to its parent under various growth conditions. Similar dysregulation of membrane energetics was observed in a different E. coli strain and in the distantly related B. subtilis. Together, our findings are consistent with a role for TrhA in membrane energetics homeostasis, through a mechanism that remains to be elucidated.

IMPORTANCE Eukaryotic homologs of the progestin and AdipoQ receptor family (PAQR) have been shown to regulate membrane fluidity by affecting, through unknown mechanisms, unsaturated fatty acid (FA) metabolism. The bacterial homologs studied here mediate small and consistent changes in unsaturated FA metabolism that do not seem to impact membrane fluidity but, rather, alter membrane energetics homeostasis. Together, the findings here suggest that bacterial and eukaryotic PAQRs share functions in maintaining membrane homeostasis (fluidity in eukaryotes and energetics for bacteria with TrhA homologs).

Lifespan Increase of Podospora anserina by Oleic Acid Is Linked to Alterations in Energy Metabolism, Membrane Trafficking and Autophagy

The maintenance of cellular homeostasis over time is essential to avoid the degeneration of biological systems leading to aging and disease. Several interconnected pathways are active in this kind of quality control. One of them is autophagy, the vacuolar degradation of cellular components. The absence of the sorting nexin PaATG24 (SNX4 in other organisms) has been demonstrated to result in impairments in different types of autophagy and lead to a shortened lifespan. In addition, the growth rate and the size of vacuoles are strongly reduced. Here, we report how an oleic acid diet leads to longevity of the wild type and a PaAtg24 deletion mutant (ΔPaAtg24). The lifespan extension is linked to altered membrane trafficking, which abrogates the observed autophagy defects in ΔPaAtg24 by restoring vacuole size and the proper localization of SNARE protein PaSNC1. In addition, an oleic acid diet leads to an altered use of the mitochondrial respiratory chain: complex I and II are bypassed, leading to reduced reactive oxygen species (ROS) production. Overall, our study uncovers multiple effects of an oleic acid diet, which extends the lifespan of P. anserina and provides perspectives to explain the positive nutritional effects on human aging.

Comparative analysis of vertebrates reveals that mouse primordial oocytes do not contain a Balbiani body

Oocytes spend the majority of their lifetime in a primordial state. The cellular and molecular biology of primordial oocytes is largely unexplored; yet, studying these is necessary to understand the mechanisms through which oocytes maintain cellular fitness for decades, and why they eventually fail with age. Here, we develop enabling methods for live-imaging based comparative characterization of Xenopus, mouse and human primordial oocytes. We show that primordial oocytes in all three vertebrate species contain active mitochondria, Golgi apparatus and lysosomes. We further demonstrate that human and Xenopus oocytes have a Balbiani body characterized by a dense accumulation of mitochondria in their cytoplasm. However, despite previous reports, we did not find a Balbiani body in mouse oocytes. Instead, we demonstrate what was previously used as a marker for the Balbiani body in mouse primordial oocytes is in fact a ring-shaped Golgi apparatus that is not functionally associated with oocyte dormancy. Our work provides the first insights into the organisation of the cytoplasm in mammalian primordial oocytes, and clarifies relative advantages and limitations of choosing different model organisms for studying oocyte dormancy.

Toward absolute viability measurements for bacteria

We aim to develop a quantitative viability method that distinguishes individual quiescent from dead cells and is measured in time (ns) as a referenceable, comparable quantity. We demonstrate that fluorescence lifetime imaging of an anionic, fluorescent membrane voltage probe fulfills these requirements for Streptococcus mutans. A random forest machine-learning model assesses whether individual S. mutans can be correctly classified into their original populations: stationary phase (quiescent), heat killed and inactivated via chemical fixation. We compare the results to intensity using three models: lifetime variables (τ₁ , τ₂ and p₁ ), phasor variables (G, S) or all five variables, with the five variable models having the most accurate classification. This initial work affirms the potential for using fluorescence lifetime of a membrane voltage probe as a viability marker for quiescent bacteria, and future efforts on other bacterial species and fluorophores will help refine this approach.

Biomedical Applications of Translational Optical Imaging: From Molecules to Humans

Light is a powerful investigational tool in biomedicine, at all levels of structural organization. Its multitude of features (intensity, wavelength, polarization, interference, coherence, timing, non-linear absorption, and even interactions with itself) able to create contrast, and thus images that detail the makeup and functioning of the living state can and should be combined for maximum effect, especially if one seeks simultaneously high spatiotemporal resolution and discrimination ability within a living organism. The resulting high relevance should be directed towards a better understanding, detection of abnormalities, and ultimately cogent, precise, and effective intervention. The new optical methods and their combinations needed to address modern surgery in the operating room of the future, and major diseases such as cancer and neurodegeneration are reviewed here, with emphasis on our own work and highlighting selected applications focusing on quantitation, early detection, treatment assessment, and clinical relevance, and more generally matching the quality of the optical detection approach to the complexity of the disease. This should provide guidance for future advanced theranostics, emphasizing a tighter coupling-spatially and temporally-between detection, diagnosis, and treatment, in the hope that technologic sophistication such as that of a Mars rover can be translationally deployed in the clinic, for saving and improving lives.

Tau phosphorylation and OPA1 proteolysis are unrelated events: Implications for Alzheimer's Disease

The neuropathological hallmarks of Alzheimer's Disease are plaques and neurofibrillary tangles. Yet, Alzheimer's is a complex disease with many contributing factors, such as energy-metabolic changes, which have been documented in autopsy brains from individuals with Alzheimer's and animal disease models alike. One conceivable explanation is that the interplay of age-related extracellular and intracellular alterations pertaining to Alzheimer's, such as cerebrovascular changes, protein aggregates and inflammation, evoke a mitochondrial response. However, it is not clear if and how mitochondria can contribute to Alzheimer's pathophysiology. This study focuses on one particular aspect of this question by investigating the functional interaction between the microtubule-associated protein tau and the mitochondrial inner membrane fusion machinery, which shows alterations in Alzheimer's brains. OPA1 is an essential inner membrane-fusion protein regulated by the two membrane proteases OMA1 and YME1L1. Assessment of OPA1 proteolysis-usually found in dividing mitochondria-and posttranslational tau modifications in mouse and human neuroblastoma cells under different experimental conditions clarified the relationship between these two pathways: OPA1 hydrolysis and phosphorylation or dephosphorylation of tau may coincide, but are not causally related. OPA1 cleavage did not alter tau's phosphorylation pattern. Conversely, tau's phosphorylation state did not induce nor correlate with OPA1 proteolysis. These results irrefutably demonstrate that there is no direct functional interaction between posttranslational tau modifications and the regulation of the OMA1-OPA1 pathway, which implies a common root cause modulating both pathways in Alzheimer's.

From OCR and ECAR to energy: Perspectives on the design and interpretation of bioenergetics studies

Biological energy transduction underlies all physiological phenomena in cells. The metabolic systems that support energy transduction have been of great interest due to their association with numerous pathologies including diabetes, cancer, rare genetic diseases, and aberrant cell death. Commercially available bioenergetics technologies (e.g., extracellular flux analysis, high-resolution respirometry, fluorescent dye kits, etc.) have made practical assessment of metabolic parameters widely accessible. This has facilitated an explosion in the number of studies exploring, in particular, the biological implications of oxygen consumption rate (OCR) and substrate level phosphorylation via glycolysis (i.e., via extracellular acidification rate (ECAR)). Though these technologies have demonstrated substantial utility and broad applicability to cell biology research, they are also susceptible to historical assumptions, experimental limitations, and other caveats that have led to premature and/or erroneous interpretations. This review enumerates various important considerations for designing and interpreting cellular and mitochondrial bioenergetics experiments, some common challenges and pitfalls in data interpretation, and some potential "next steps" to be taken that can address these highlighted challenges.

Cardio-Oncology: Understanding the Intersections Between Cardiac Metabolism and Cancer Biology

An important priority in the cardiovascular care of oncology patients is to reduce morbidity and mortality, and improve the quality of life in cancer survivors through cross-disciplinary efforts. The rate of survival in cancer patients has improved dramatically over the past decades. Nonetheless, survivors may be more likely to die from cardiovascular disease in the long term, secondary, not only to the potential toxicity of cancer therapeutics, but also to the biology of cancer. In this context, efforts from basic and translational studies are crucial to understanding the molecular mechanisms causal to cardiovascular disease in cancer patients and survivors, and identifying new therapeutic targets that may prevent and treat both diseases. This review aims to highlight our current understanding of the metabolic interaction between cancer and the heart, including potential therapeutic targets. An overview of imaging techniques that can support both research studies and clinical management is also provided. Finally, this review highlights opportunities and challenges that are necessary to advance our understanding of metabolism in the context of cardio-oncology.

Optical Electrophysiology: Toward the Goal of Label-Free Voltage Imaging

Measuring and monitoring the electrical signals transmitted between neurons is key to understanding the communication between neurons that underlies human perception, information processing, and decision-making. While electrode-based electrophysiology has been the gold standard, optical electrophysiology has opened up a new area in the past decade. Voltage-dependent fluorescent reporters enable voltage imaging with high spatial resolution and flexibility to choose recording locations. However, they exhibit photobleaching as well as phototoxicity and may perturb the physiology of the cell. Label-free optical electrophysiology seeks to overcome these hurdles by detecting electrical activities optically, without the incorporation of exogenous fluorophores in cells. For example, electrochromic optical recording detects neuroelectrical signals via a voltage-dependent color change of extracellular materials, and interferometric optical recording monitors membrane deformations that accompany electrical activities. Label-free optical electrophysiology, however, is in an early stage, and often has limited sensitivity and temporal resolution. In this Perspective, we review the recent progress to overcome these hurdles. We hope this Perspective will inspire developments of label-free optical electrophysiology techniques with high recording sensitivity and temporal resolution in the near future.

Mitochondria Encapsulation in Hydrogel-Based Artificial Cells as ATP Producing Subunits

Artificial cells (ACs) aim to mimic selected structural and functional features of mammalian cells. In this context, energy generation is an important challenge to be addressed when self-sustained systems are desired. Here, mitochondria isolated from HepG2 cells are employed as natural subunits that facilitate chemically driven adenosine triphosphate (ATP) synthesis. The successful mitochondria isolation is confirmed by monitoring the preserved inner membrane potential, the respiration, and the ATP production ability. The encapsulation of the isolated mitochondria in gelatin-based hydrogels results in similar initial ATP production compared to mitochondria in solution with a sustained ATP production over 24 h. Furthermore, luciferase is coencapsulated with the mitochondria in gelatin-based particles to create ACs and employ the in situ produced ATP to drive the catalytic conversion of d-luciferin. The coencapsulation of luciferase-loaded liposomes with mitochondria in gelatin-based hydrogels is additionally explored where the encapsulation of mitochondria and liposomes resulted in clustering effects that are likely contributing to the functional performance of the active entities. Taken together, mitochondria show potential in cell mimicry to facilitate energy-dependent processes.

A Tunable Palette of Molecular Rotors Allows Multicolor, Ratiometric Fluorescence Imaging and Direct Mapping of Mitochondrial Heterogeneity

Environment-sensitive molecular probes offer the potential for a comprehensive mapping of the complex cellular milieu. We present here a radically new strategy of multiplexing highly sensitive, spectrally tuned fluorescent dyes for sensing cellular microenvironment. To achieve this multicolor, ratiometric cellular imaging, we first developed a series of highly sensitive, tunable molecular rotors for mitochondrial imaging, with emission wavelengths spanning the visible spectrum. These fluorogenic merocyanine dyes are all sensitive to solvent viscosity despite distinctive photophysical features. Our results show that merocyanine dyes can show a rotor-like behavior despite significant changes to the conventional donor-acceptor or push-pull scaffolds, thereby revealing conserved features of rotor dye chemistry. Developing closely related but spectrally separated dyes that have distinct response functions allows us to do ″two-color, two-dye″ imaging of the mitochondrial microenvironment. Our results with multidye, combinatorial imaging provide a direct visualization of the intrinsic heterogeneity of the mitochondrial microenvironment. The overall mitochondrial microenvironment (including contributions from local membrane order) as reported through two-color fluorescence ″ratio″ changes of multiplexed rotor dyes shows dynamic heterogeneity with distinct spatiotemporal signatures that evolve over time and respond to chemical perturbations. Our results offer a powerful illustration of how multiplexed dye imaging allows the quantitative imaging of mitochondrial membrane order and cellular microenvironment.

Inhibition of eIF5A hypusination pathway as a new pharmacological target for stroke therapy

In eukaryotes, the polyamine pathway generates spermidine that activates the hypusination of the translation factor eukaryotic initiation factor 5A (eIF5A). Hypusinated-eIF5A modulates translation, elongation, termination and mitochondrial function. Evidence in model organisms like drosophila suggests that targeting polyamines synthesis might be of interest against ischemia. However, the potential of targeting eIF5A hypusination in stroke, the major therapeutic challenge specific to ischemia, is currently unknown. Using in vitro models of ischemic-related stress, we documented that GC7, a specific inhibitor of a key enzyme in the eIF5A activation pathway, affords neuronal protection. We identified the preservation of mitochondrial function and thereby the prevention of toxic ROS generation as major processes of GC7 protection. To represent a thoughtful opportunity of clinical translation, we explored whether GC7 administration reduces the infarct volume and functional deficits in an in vivo transient focal cerebral ischemia (tFCI) model in mice. A single GC7 pre- or post-treatment significantly reduces the infarct volume post-stroke. Moreover, GC7-post-treatment significantly improves mouse performance in the rotarod and Morris water-maze, highlighting beneficial effects on motor and cognitive post-stroke deficits. Our results identify the targeting of the polyamine-eIF5A-hypusine axis as a new therapeutic opportunity and new paradigm of research in stroke and ischemic diseases.

Permeability-Controllable Potentiometric Fluorescent Probes Enable Visually Discriminating Near-Zero and Normal Situations of Cell Membrane Potential

The permeability-controllable potentiometric fluorescent probes that can visually discriminate near-zero and normal situations of cell membrane potential were reported for the first time. Different from traditional potentiometric probes that utilize fluorescence intensity to reflect membrane potential, CQ12 and CP12 have different localizations under the two situations of cell membrane potential. Thus, the two situations can be point-to-point indicated by two fluorescent images with an obvious difference, avoiding complex operations and calibration of conventional methods.

Spatio-temporal parameters for optical probing of neuronal activity

The challenge to understand the complex neuronal circuit functions in the mammalian brain has brought about a revolution in light-based neurotechnologies and optogenetic tools. However, while recent seminal works have shown excellent insights on the processing of basic functions such as sensory perception, memory, and navigation, understanding more complex brain functions is still unattainable with current technologies. We are just scratching the surface, both literally and figuratively. Yet, the path towards fully understanding the brain is not totally uncertain. Recent rapid technological advancements have allowed us to analyze the processing of signals within dendritic arborizations of single neurons and within neuronal circuits. Understanding the circuit dynamics in the brain requires a good appreciation of the spatial and temporal properties of neuronal activity. Here, we assess the spatio-temporal parameters of neuronal responses and match them with suitable light-based neurotechnologies as well as photochemical and optogenetic tools. We focus on the spatial range that includes dendrites and certain brain regions (e.g., cortex and hippocampus) that constitute neuronal circuits. We also review some temporal characteristics of some proteins and ion channels responsible for certain neuronal functions. With the aid of the photochemical and optogenetic markers, we can use light to visualize the circuit dynamics of a functioning brain. The challenge to understand how the brain works continue to excite scientists as research questions begin to link macroscopic and microscopic units of brain circuits.

Transmembrane Prolyl 4-Hydroxylase is a Novel Regulator of Calcium Signaling in Astrocytes

Prolyl 4-hydroxylases (P4Hs) have vital roles in regulating collagen synthesis and hypoxia response. A transmembrane P4H (P4H-TM) is a recently identified member of the family. Biallelic loss of function P4H-TM mutations cause a severe autosomal recessive intellectual disability syndrome in humans, but functions of P4H-TM are essentially unknown at cellular level. Our microarray data on P4h-tm ^-/- mouse cortexes where P4H-TM is abundantly expressed indicated expression changes in genes involved in calcium signaling and expression of several calcium sequestering ATPases was upregulated in P4h-tm ^-/- primary mouse astrocytes. Cytosolic and intraorganellar calcium imaging of P4h-tm ^-/- cells revealed that receptor-operated calcium entry (ROCE) and store-operated calcium entry (SOCE) and calcium re-uptake by mitochondria were compromised. HIF1, but not HIF2, was found to be a key mediator of the P4H-TM effect on calcium signaling. Furthermore, total internal reflection fluorescence (TIRF) imaging showed that calcium agonist-induced gliotransmission was attenuated in P4h-tm ^-/- astrocytes. This phenotype was accompanied by redistribution of mitochondria from distal processes to central parts of the cell body and decreased intracellular ATP content. Our data show that P4H-TM is a novel regulator of calcium dynamics and gliotransmission.

Mitophagy of polarized sperm-derived mitochondria after fertilization

Loss of membrane potential of sperm mitochondria has been regarded as the first step preceding mitophagy degradation after their entry into the C. elegans oocyte at fertilization. This is in line with the classical view of mitophagy of defective or abnormal mitochondria and could serve as a recognition signal for their specific and quick autophagy degradation. Here, using TMRE (tetramethylrhodamine ethyl ester) and live imaging we show that this is not the case. Instead, sperm inherited mitochondria show a stable labeling with TMRE before and at the time of autophagosomes formation. Interestingly, this labeling remains in late-stage-embryos of autophagy-defective-mutants suggesting that the loss of membrane potential occurs upon the entry of the mitochondria into the autophagy pathway. These stabilized and still polarized sperm mitochondria remain distinct but associated with the maternal-derived mitochondrial network suggesting a mechanism that prevents their fusion and represents an efficient additional protective system against fertilization-induced heteroplasmy.

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Oocyte-derived mitochondria become tubular only during the first zygotic division

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Sperm mitochondria can not fuse but associate with the oocyte-derived mitochondria

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Sperm mitochondria are still polarized when engulfed for autophagy degradation

Flavonoids mitigate neurodegeneration in aged Caenorhabditis elegans by mitochondrial uncoupling

Dietary supplementation of flavonoids has been shown to reduce the severity of neurodegenerative disorders such as dementia, Parkinson's disease, and Alzheimer's disease by their antioxidant effects. However, their low bioavailabilityin vivo raises the question of how much their antioxidant capacity actually contributes to the mitigating effects. The physicochemical properties of flavonoids suggest they could function as mitochondrial uncouplers. Moreover, mitochondrial uncoupling alleviated neurodegeneration in Caenorhabditis elegans during aging in previous research. Therefore, we investigated whether various flavonoids (fisetin, quercetin, apigenin, chrysin, catechin, and naringenin) could reduce neuronal defects by mitochondrial uncoupling in C. elegans. Both neuronal defects and mitochondrial membrane potential were reduced in aged worms in nearly all of the flavonoid treatments suggesting that flavonoids may reduce neurodegeneration in C. elegans. However, there was no significant reduction of neuronal defects in mitophagy-deficient pink-1/pdr-1 double mutants under flavonoid treatments. These results suggest that flavonoids could function as mitochondrial uncouplers to mitigate neurodegeneration in aged C. elegans, possibly via a PINK1/Parkin mitophagy process.

T lymphocytes from malignant hyperthermia-susceptible mice display aberrations in intracellular calcium signaling and mitochondrial function

Gain-of-function RyR1-p.R163C mutation in ryanodine receptors type 1 (RyR1) deregulates Ca²⁺ signaling and mitochondrial function in skeletal muscle and causes malignant hyperthermia in humans and mice under triggering conditions. We investigated whether T lymphocytes from heterozygous RyR1-p.R163C knock-in mutant mice (HET T cells) display measurable aberrations in resting cytosolic Ca²⁺ concentration ([Ca²⁺]_i), Ca²⁺ release from the store, store-operated Ca²⁺ entry (SOCE), and mitochondrial inner membrane potential (ΔΨ_m) compared with T lymphocytes from wild-type mice (WT T cells). We explored whether these variables can be used to distinguish between T cells with normal and altered RyR1 genotype. HET and WT T cells were isolated from spleen and lymph nodes and activated in vitro using phytohemagglutinin P. [Ca²⁺]_i and ΔΨ_m dynamics were examined using Fura 2 and tetramethylrhodamine methyl ester fluorescent dyes, respectively. Activated HET T cells displayed elevated resting [Ca²⁺]_i, diminished responses to Ca²⁺ mobilization with thapsigargin, and decreased rate of [Ca²⁺]_i elevation in response to SOCE compared with WT T cells. Pretreatment of HET T cells with ryanodine or dantrolene sodium reduced disparities in the resting [Ca²⁺]_i and ability of thapsigargin to mobilize Ca²⁺ between HET and WT T cells. While SOCE elicited dissipation of the ΔΨ_m in WT T cells, it produced ΔΨ_m hyperpolarization in HET T cells. When used as the classification variable, the amplitude of thapsigargin-induced Ca²⁺ transient showed the best promise in predicting the presence of RyR1-p.R163C mutation. Other significant variables identified by machine learning analysis were the ratio of resting cytosolic Ca²⁺ level to the amplitude of thapsigargin-induced Ca²⁺ transient and an integral of changes in ΔΨ_m in response to SOCE. Our study demonstrated that gain-of-function mutation in RyR1 significantly affects Ca²⁺ signaling and mitochondrial fiction in T lymphocytes, which suggests that this mutation may cause altered immune responses in its carrier. Our data link the RyR1-p.R163C mutation, which causes inherited skeletal muscle diseases, to deregulation of Ca²⁺ signaling and mitochondrial function in immune T cells and establish proof-of-principle for in vitro T cell-based diagnostic assay for hereditary RyR1 hyperfunction.

Macrophage migration inhibitory factor inhibition as a novel therapeutic approach against triple-negative breast cancer

Triple-negative breast cancer (TNBC), defined as loss of estrogen, progesterone, and Her2 receptors, is a subtype of highly aggressive breast cancer with worse prognosis and poor survival rate. Macrophage migration inhibitory factor (MIF) is a pleiotropic pro-inflammatory cytokine aberrantly expressed in many solid tumors and known to promote tumor progression and metastasis. However, its role in TNBC progression and metastasis is unexplored. Here we have shown that in TNBC patients, MIF expression was significantly enriched in the tumor compared to adjacent normal tissue. Using publically available patient datasets, we showed that MIF overexpression correlates with worse survival in TNBC compared to other hormonal status. Orthotopic implantation of TNBC cells into MIF knockout mice showed reduced tumor growth compared to wild-type mice. In addition, we have shown that MIF downregulation inhibits TNBC growth and progression in a syngeneic mouse model. We further showed that CPSI-1306, a small-molecule MIF inhibitor, inhibits the growth of TNBC cells in vitro. Mechanistic studies revealed that CPSI-1306 induces intrinsic apoptosis by alteration in mitochondrial membrane potential, cytochrome c (Cyt c) release, and activation of different caspases. In addition, CPSI-1306 inhibits the activation of cell survival and proliferation-related molecules. CPSI-1306 treatment also reduced the tumor growth and metastasis in orthotopic mouse models of mammary carcinoma. CPSI-1306 treatment of tumor-bearing mice significantly inhibited TNBC growth and pulmonary metastasis in a dose-dependent manner. Histological analysis of xenograft tumors revealed a higher number of apoptotic cells in CPSI-1306-treated tumors compared to vehicle controls. Our studies, for the first time, show that MIF overexpression in TNBC enhances growth and metastasis. Taken together, our results indicate that using small molecular weight MIF inhibitors could be a promising strategy to inhibit TNBC progression and metastasis.