The conundrum of hot mitochondria

Natural convection in the cytoplasm: Theoretical predictions of buoyancy-driven flows inside a cell

The existence of temperature gradients within eukaryotic cells has been postulated as a source of natural convection in the cytoplasm, i.e. bulk fluid motion as a result of temperature-difference-induced density gradients. Recent computations have predicted that a temperature differential of ΔT ≈ 1 K between the cell nucleus and the cell membrane could be strong enough to drive significant intracellular material transport. We use numerical computations and theoretical calculations to revisit this problem in order to further understand the impact of temperature gradients on flow generation and advective transport within cells. Surprisingly, our computations yield flows that are an order of magnitude weaker than those obtained previously for the same relative size and position of the nucleus with respect to the cell membrane. To understand this discrepancy, we develop a semi-analytical solution of the convective flow inside a model cell using a bi-spherical coordinate framework, for the case of an axisymmetric cell geometry (i.e. when the displacement of the nucleus from the cell centre is aligned with gravity). We also calculate exact solutions for the flow when the nucleus is located concentrically inside the cell. The results from both theoretical analyses agree with our numerical results, thus providing a robust estimate of the strength of cytoplasmic natural convection and demonstrating that these are much weaker than previously predicted. Finally, we investigate the ability of the aforementioned flows to redistribute solute within a cell. Our calculations reveal that, in all but unrealistic cases, cytoplasmic convection has a negligible contribution toward enhancing the diffusion-dominated mass transfer of cellular material.

Mild Hyperthermia-Induced Thermogenesis in the Endoplasmic Reticulum Defines Stress Response Mechanisms

Previous studies reported that a mild, non-protein-denaturing, fever-like temperature increase induced the unfolded protein response (UPR) in mammalian cells. Our dSTORM super-resolution microscopy experiments revealed that the master regulator of the UPR, the IRE1 (inositol-requiring enzyme 1) protein, is clustered as a result of UPR activation in a human osteosarcoma cell line (U2OS) upon mild heat stress. Using ER thermo yellow, a temperature-sensitive fluorescent probe targeted to the endoplasmic reticulum (ER), we detected significant intracellular thermogenesis in mouse embryonic fibroblast (MEF) cells. Temperatures reached at least 8 °C higher than the external environment (40 °C), resulting in exceptionally high ER temperatures similar to those previously described for mitochondria. Mild heat-induced thermogenesis in the ER of MEF cells was likely due to the uncoupling of the Ca2+/ATPase (SERCA) pump. The high ER temperatures initiated a pronounced cytosolic heat-shock response in MEF cells, which was significantly lower in U2OS cells in which both the ER thermogenesis and SERCA pump uncoupling were absent. Our results suggest that depending on intrinsic cellular properties, mild hyperthermia-induced intracellular thermogenesis defines the cellular response mechanism and determines the outcome of hyperthermic stress.

Microwave radiation and thermal effects on the bioenergetics of isolated mitochondria

AIMS

This study proposes to investigate the effects of microwave radiation and its thermal effects, compared to thermal effects alone, on the bioenergetics of mitochondria isolated from mouse liver.

METHODS

The main parameters investigated in this study are mitochondrial respiration (coupled states: S3 and S4; uncoupled state), using a high-resolution respirometer, and swelling, using a spectrophotometer.

RESULTS

Mitochondria irradiated at 2.45 GHz microwave with doses 0.085, 0.113 and 0.141 kJ/g, presented a decrease in S3 and uncoupled state, but an increase in S4. Conversely, mitochondria thermally treated at 40, 44 and 50 °C presented an increasing in S3 and S4, while uncoupled state was unaltered. Mitochondrial swelling increases as a function of the dose or temperature, indicating membrane damages in both cases.

CONCLUSION

Microwave radiation and thermal effect alone indicated different bioenergetics mitochondria response. These results imply that the effects due to microwave in medical treatment are not exclusively due to the increase in temperature, but a combination of electromagnetic and thermal effects.

Determinants of thermal homeostasis in the preimplantation embryo: a role for the embryo's central heating system?

A number of factors may impinge on thermal homeostasis in the early embryo. The most obvious is the ambient temperature in which development occurs. Physiologically, the temperature in the lumen of the female tract is typically lower than the core body temperature, yet rises at ovulation in the human, while in an IVF setting, embryos are usually maintained at core body temperature. However, internal cellular developmental processes may modulate thermal control within the embryo itself, especially those occurring in the mitochondria which generate intracellular heat through proton leak and provide the embryo with its own 'central heating system'. Moreover, mitochondrial movements may serve to buffer high local intracellular temperatures. It is also notable that the preimplantation stages of development would generate proportionally little heat within their mitochondria until the blastocyst stage as mitochondrial metabolism is comparatively low during the cleavage stages. Despite these data, the specific notion of thermal control of preimplantation development has received remarkably scant consideration. This opinion paper illustrates the lack of reliable quantitative data on these markers and identifies a major research agenda which needs to be addressed with urgency in view of laboratory conditions in which embryos are maintained as well as climate change-derived heat stress which has a negative effect on numerous clinical markers of early human embryo development.

Response strategies and biological applications of organic fluorescent thermometry: cell- and mitochondrion-level detection

Temperature homeostasis is critical for cells to perform their physiological functions. Among the diverse methods for temperature detection, fluorescent temperature probes stand out as a proven and effective tool, especially for monitoring temperature in cells and suborganelles, with a specific emphasis on mitochondria. The utilization of these probes provides a new opportunity to enhance our understanding of the mechanisms and interconnections underlying various physiological activities related to temperature homeostasis. However, the complexity and variability of cells and suborganelles necessitate fluorescent temperature probes with high resolution and sensitivity. To meet the demanding requirements for intracellular/subcellular temperature detection, several strategies have been developed, offering a range of options to address this challenge. This review examines four fundamental temperature-response strategies employed by small molecule and polymer probes, including intramolecular rotation, polarity sensitivity, Förster resonance energy transfer, and structural changes. The primary emphasis was placed on elucidating molecular design and biological applications specific to each type of probe. Furthermore, this review provides an insightful discussion on factors that may affect fluorescent thermometry, providing valuable perspectives for future development in the field. Finally, the review concludes by presenting cutting-edge response strategies and research insights for mitigating biases in temperature sensing.

Deciphering the relationship between temperature and immunity

Fever is a hallmark symptom of disease across the animal kingdom. Yet, despite the evidence linking temperature fluctuation and immune response, much remains to be discovered about the molecular mechanisms governing these interactions. In patients with rheumatoid arthritis, for instance, it is clinically accepted that joint temperature can predict disease progression. But it was only recently demonstrated that the mitochondria of stimulated T cells can rise to an extreme 50°C, potentially indicating a cellular source of these localized 'fevers'. A challenge to dissecting these mechanisms is a bidirectional interplay between temperature and immunity. Heat shock response is found in virtually all organisms, activating protective pathways when cells are exposed to elevated temperatures. However, the temperature threshold that activates these pathways can vary within the same organism, with human immune cells, in particular, demonstrating differential sensitivity to heat. Such inter-cellular variation may be clinically relevant given the small but significant temperature differences seen between tissues, ages, and sexes. Greater understanding of how such small temperature perturbations mediate immune responses may provide new explanations for persistent questions in disease such as sex disparity in disease prevalence. Notably, the prevalence and severity of many maladies are rising with climate change, suggesting temperature fluctuations can interact with disease on multiple levels. As global temperatures are rising, and our body temperatures are falling, questions regarding temperature-immune interactions are increasingly critical. Here, we review this aspect of environmental interplay to better understand temperature's role in immune variation and subsequent risk of disease.

Limitations of Bulk Diamond Sensors for Single-Cell Thermometry

The present paper reports on a Finite Element Method (FEM) analysis of the experimental situation corresponding to the measurement of the temperature variation in a single cell plated on bulk diamond by means of optical techniques. Starting from previous experimental results, we have determined-in a uniform power density approximation and under steady-state conditions-the total heat power that has to be dissipated by a single cell plated on a glassy substrate in order to induce the typical maximum temperature increase ΔTglass=1 K. While keeping all of the other parameters constant, the glassy substrate has been replaced by a diamond plate. The FEM analysis shows that, in this case, the maximum temperature increase is expected at the diamond/cell interface and is as small as ΔTdiam=4.6×10-4 K. We have also calculated the typical decay time in the transient scenario, which resulted in τ≈ 250 μs. By comparing these results with the state-of-the-art sensitivity values, we prove that the potential advantages of a longer coherence time, better spectral properties, and the use of special field alignments do not justify the use of diamond substrates in their bulk form.

Warm Cells, Hot Mitochondria: Achievements and Problems of Ultralocal Thermometry

Temperature is a crucial regulator of the rate and direction of biochemical reactions and cell processes. The recent data indicating the presence of local thermal gradients associated with the sites of high-rate thermogenesis, on the one hand, demonstrate the possibility for the existence of "thermal signaling" in a cell and, on the other, are criticized on the basis of thermodynamic calculations and models. Here, we review the main thermometric techniques and sensors developed for the determination of temperature inside living cells and diverse intracellular compartments. A comparative analysis is conducted of the results obtained using these methods for the cytosol, nucleus, endo-/sarcoplasmic reticulum, and mitochondria, as well as their biological consistency. Special attention is given to the limitations, possible sources of errors and ambiguities of the sensor's responses. The issue of biological temperature limits in cells and organelles is considered. It is concluded that the elaboration of experimental protocols for ultralocal temperature measurements that take into account both the characteristics of biological systems, as well as the properties and limitations of each type of sensor is of critical importance for the generation of reliable results and further progress in this field.

Luminescence Thermometry with Nanoparticles: A Review

Luminescence thermometry has emerged as a very versatile optical technique for remote temperature measurements, exhibiting a wide range of applicability spanning from cryogenic temperatures to 2000 K. This technology has found extensive utilization across many disciplines. In the last thirty years, there has been significant growth in the field of luminous thermometry. This growth has been accompanied by the development of temperature read-out procedures, the creation of luminescent materials for very sensitive temperature probes, and advancements in theoretical understanding. This review article primarily centers on luminescent nanoparticles employed in the field of luminescence thermometry. In this paper, we provide a comprehensive survey of the recent literature pertaining to the utilization of lanthanide and transition metal nanophosphors, semiconductor quantum dots, polymer nanoparticles, carbon dots, and nanodiamonds for luminescence thermometry. In addition, we engage in a discussion regarding the benefits and limitations of nanoparticles in comparison with conventional, microsized probes for their application in luminescent thermometry.

Simultaneous Nanorheometry and Nanothermometry Using Intracellular Diamond Quantum Sensors

The viscoelasticity of the cytoplasm plays a critical role in cell morphology, cell division, and intracellular transport. Viscoelasticity is also interconnected with other biophysical properties, such as temperature, which is known to influence cellular bioenergetics. Probing the connections between intracellular temperature and cytoplasmic viscoelasticity provides an exciting opportunity for the study of biological phenomena, such as metabolism and disease progression. The small length scales and transient nature of changes in these parameters combined with their complex interdependencies pose a challenge for biosensing tools, which are often limited to a single readout modality. Here, we present a dual-mode quantum sensor capable of performing simultaneous nanoscale thermometry and rheometry in dynamic cellular environments. We use nitrogen-vacancy centers in diamond nanocrystals as biocompatible sensors for in vitro measurements. We combine subdiffraction resolution single-particle tracking in a fluidic environment with optically detected magnetic resonance spectroscopy to perform simultaneous sensing of viscoelasticity and temperature. We use our sensor to demonstrate probing of the temperature-dependent viscoelasticity in complex media at the nanoscale. We then investigate the interplay between intracellular forces and the cytoplasmic rheology in live cells. Finally, we identify different rheological regimes and reveal evidence of active trafficking and details of the nanoscale viscoelasticity of the cytoplasm.

Seebeck Coefficient of Liquid Water from Equilibrium Molecular Dynamics

The application of a temperature gradient to an extended system generates an electromotive force that induces an electric current in conductors and macroscopic polarization in insulators. The ratio of the electromotive force to the temperature difference, usually referred to as the Seebeck coefficient, is often computed using nonequilibrium techniques, such as nonequilibrium molecular dynamics (NEMD). In this article, we argue that thermo-polarization effects in insulating fluids can be conveniently treated by standard equilibrium thermodynamics and devise a protocol─based on a combination of equilibrium molecular dynamics and Bayesian inference methods─that allows one to compute the Seebeck coefficient in these systems along with a rigorous estimate of the resulting statistical accuracy. The application of our methodology to liquid SPC/E water results in good agreement with previous studies─based on more elaborate NEMD simulations─and in a more reliable estimate of the statistical accuracy of the results.

Trans-scale thermal signaling in biological systems

Biochemical reactions in cells serve as the endogenous source of heat, maintaining a constant body temperature. This process requires proper control; otherwise, serious consequences can arise due to the unwanted but unavoidable responses of biological systems to heat. This review aims to present a range of responses to heat in biological systems across various spatial scales. We begin by examining the impaired thermogenesis of malignant hyperthermia in model mice and skeletal muscle cells, demonstrating that the progression of this disease is caused by a positive feedback loop between thermally driven Ca2+ signaling and thermogenesis at the subcellular scale. After we explore thermally driven force generation in both muscle and non-muscle cells, we illustrate how in vitro assays using purified proteins can reveal the heat-responsive properties of proteins and protein assemblies. Building on these experimental findings, we propose the concept of 'trans-scale thermal signaling'.

Joule heating involving ion currents through channel proteins

Ion currents associated with channel proteins in the presence of membrane potential are ubiquitous in cellular and organelle membranes. When an ion current occurs through a channel protein, Joule heating should occur. However, this Joule heating seems to have been largely overlooked in biology. Here we show theoretical investigation of Joule heating involving channel proteins in biological processes. We used electrochemical potential to derive the Joule's law for an ion current through an ion transport protein in the presence of membrane potential, and we suggest that heat production and absorption can occur. Simulation of temperature distribution around a single channel protein with the Joule heating revealed that the temperature increase was as small as <10-3 K, although an ensemble of channel proteins was suggested to exhibit a noticeable temperature increase. Thereby, we theoretically investigated the Joule heating of systems containing ensembles of channel proteins. Nerve is known to undergo rapid heat production followed by heat absorption during the action potential, and our simulation of Joule heating for a squid giant axon combined with the Hodgkin-Huxley model successfully reproduced the feature of the heat. Furthermore, we extended the theory of Joule heating to uncoupling protein 1 (UCP1), a solute carrier family transporter, which is important to the non-shivering thermogenesis in brown adipose tissue mitochondria (BATM). Our calculations showed that the Joule heat involving UCP1 was comparable to the literature calorimetry data of BATM. Joule heating of ion transport proteins is likely to be one of important mechanisms of cellular thermogenesis.

The hypothermic nature of fungi

Fungi play essential roles in global health, ecology, and economy, but their thermal biology is relatively unexplored. Mushrooms, the fruiting body of mycelium, were previously noticed to be colder than surrounding air through evaporative cooling. Here, we confirm those observations using infrared thermography and report that this hypothermic state is also observed in mold and yeast colonies. The relatively colder temperature of yeasts and molds is also mediated via evaporative cooling and associated with the accumulation of condensed water droplets on plate lids above colonies. The colonies appear coldest at their center and the surrounding agar appears warmest near the colony edges. The analysis of cultivated Pleurotus ostreatus mushrooms revealed that the hypothermic feature of mushrooms can be observed throughout the whole fruiting process and at the level of mycelium. The mushroom's hymenium was coldest, and different areas of the mushroom appear to dissipate heat differently. We also constructed a mushroom-based air-cooling prototype system capable of passively reducing the temperature of a semiclosed compartment by approximately 10 °C in 25 min. These findings suggest that the fungal kingdom is characteristically cold. Since fungi make up approximately 2% of Earth's biomass, their evapotranspiration may contribute to cooler temperatures in local environments.

Ectotherm mitochondrial economy and responses to global warming

Temperature is a key abiotic factor affecting ecology, biogeography, and evolution of species. Alterations of energy metabolism play an important role in adaptations and plastic responses to temperature shifts on different time scales. Mitochondrial metabolism affects cellular bioenergetics and redox balance making these organelles an important determinant of organismal performances such as growth, locomotion, or development. Here I analyze the impacts of environmental temperature on the mitochondrial functions (including oxidative phosphorylation, proton leak, production of reactive oxygen species(ROS), and ATP synthesis) of ectotherms and discuss the mechanisms underlying negative shifts in the mitochondrial energy economy caused by supraoptimal temperatures. Owing to the differences in the thermal sensitivity of different mitochondrial processes, elevated temperatures (beyond the species- and population-specific optimal range) cause reallocation of the electron flux and the protonmotive force (Δp) in a way that decreases ATP synthesis efficiency, elevates the relative cost of the mitochondrial maintenance, causes excessive production of ROS and raises energy cost for antioxidant defense. These shifts in the mitochondrial energy economy might have negative consequences for the organismal fitness traits such as the thermal tolerance or growth. Correlation between the thermal sensitivity indices of the mitochondria and the whole organism indicate that these traits experience similar selective pressures but further investigations are needed to establish whether there is a cause-effect relationship between the mitochondrial failure and loss of organismal performance during temperature change.

How hot can mitochondria be? Incubation at temperatures above 43 °C induces the degradation of respiratory complexes and supercomplexes in intact cells and isolated mitochondria

Mitochondrial function generates an important fraction of the heat that contributes to cellular and organismal temperature maintenance, but the actual values of this parameter reached in the organelles is a matter of debate. The studies addressing this issue have reported divergent results: from detecting in the organelles the same temperature as the cell average or the incubation temperature, to increasing differences of up to 10 degrees above the incubation value. Theoretical calculations based on physical laws exclude the possibility of relevant temperature gradients between mitochondria and their surroundings. These facts have given rise to a conundrum or paradox about hot mitochondria. We have examined by Blue-Native electrophoresis, both in intact cells and in isolated organelles, the stability of respiratory complexes and supercomplexes at different temperatures to obtain information about their tolerance to heat stress. We observe that, upon incubation at values above 43 °C and after relatively short periods, respiratory complexes, and especially complex I and its supercomplexes, are unstable even when the respiratory activity is inhibited. These results support the conclusion that high temperatures (>43 °C) cause damage to mitochondrial structure and function and question the proposal that these organelles can physiologically work at close to 50 °C.

A Guide to Plant Intracellular Temperature Imaging using Fluorescent Thermometers

All aspects of plant physiology are influenced by temperature. Changes in environmental temperature alter the temperatures of plant tissues and cells, which then affect various cellular activities, such as gene expression, protein stability and enzyme activities. In turn, changes in cellular activities, which are associated with either exothermic or endothermic reactions, can change the local temperature in cells and tissues. In the past 10 years, a number of fluorescent probes that detect temperature and enable intracellular temperature imaging have been reported. Intracellular temperature imaging has revealed that there is a temperature difference >1°C inside cells and that the treatment of cells with mitochondrial uncoupler or ionomycin can cause more than a 1°C intracellular temperature increase in mammalian cultured cells. Thermogenesis mechanisms in brown adipocytes have been revealed with the aid of intracellular temperature imaging. While there have been no reports on plant intracellular temperature imaging thus far, intracellular temperature imaging is expected to provide a new way to analyze the mechanisms underlying the various activities of plant cells. In this review, I will first summarize the recent progress in the development of fluorescent thermometers and their biological applications. I will then discuss the selection of fluorescent thermometers and experimental setup for the adaptation of intracellular temperature imaging to plant cells. Finally, possible applications of intracellular temperature imaging to investigate plant cell functions will be discussed.

Of telomeres and temperature: Measuring thermal effects on telomeres in ectothermic animals

Ectotherms are classic models for understanding life-history tradeoffs, including the reproduction-somatic maintenance tradeoffs that may be reflected in telomere length and their dynamics. Importantly, life-history traits of ectotherms are tightly linked to their thermal environment, with diverse or synergistic mechanistic explanations underpinning the variation. Telomere dynamics potentially provide a mechanistic link that can be used to monitor thermal effects on individuals in response to climatic perturbations. Growth rate, age and developmental stage are all affected by temperature, which interacts with telomere dynamics in complex and intriguing ways. The physiological processes underpinning telomere dynamics can be visualized and understood using thermal performance curves (TPCs). TPCs reflect the evolutionary history and the thermal environment during an individual's ontogeny. Telomere maintenance should be enhanced at or near the thermal performance optimum of a species, population and individual. The thermal sensitivity of telomere dynamics should reflect the interacting TPCs of the processes underlying them. The key processes directly underpinning telomere dynamics are mitochondrial function (reactive oxygen production), antioxidant activity, telomerase activity and telomere endcap protein status. We argue that identifying TPCs for these processes will significantly help design robust, repeatable experiments and field studies of telomere dynamics in ectotherms. Conceptually, TPCs are a valuable framework to predict and interpret taxon- and population-specific telomere dynamics across thermal regimes. The literature of thermal effects on telomeres in ectotherms is sparse and mostly limited to vertebrates, but our conclusions and recommendations are relevant across ectothermic animals.

The multiple facets of mitochondrial regulations controlling cellular thermogenesis

Understanding temperature production and regulation in endotherm organisms becomes a crucial challenge facing the increased frequency and intensity of heat strokes related to global warming. Mitochondria, located at the crossroad of metabolism, respiration, Ca²⁺ homeostasis, and apoptosis, were recently proposed to further act as cellular radiators, with an estimated inner temperature reaching 50 °C in common cell lines. This inner thermogenesis might be further exacerbated in organs devoted to produce consistent efforts as muscles, or heat as brown adipose tissue, in response to acute solicitations. Consequently, pathways promoting respiratory chain uncoupling and mitochondrial activity, such as Ca²⁺ fluxes, uncoupling proteins, futile cycling, and substrate supplies, provide the main processes controlling heat production and cell temperature. The mitochondrial thermogenesis might be further amplified by cytoplasmic mechanisms promoting the over-consumption of ATP pools. Considering these new thermic paradigms, we discuss here all conventional wisdoms linking mitochondrial functions to cellular thermogenesis in different physiological conditions.

Regulation of mitochondrial temperature in health and disease

Mitochondrial temperature is produced by various metabolic processes inside the mitochondria, particularly oxidative phosphorylation. It was recently reported that mitochondria could normally operate at high temperatures that can reach 50℃. The aim of this review is to identify mitochondrial temperature differences between normal cells and cancer cells. Herein, we discussed the different types of mitochondrial thermosensors and their advantages and disadvantages. We reviewed the studies assessing the mitochondrial temperature in cancer cells and normal cells. We shed the light on the factors involved in maintaining the mitochondrial temperature of normal cells compared to cancer cells.

The life and death of RNA across temperatures

Temperature is an environmental condition that has a pervasive effect on cells along with all the molecules and reactions in them. The mechanisms by which prototypical RNA molecules sense and withstand heat have been identified mostly in bacteria and archaea. The relevance of these phenomena is, however, broader, and similar mechanisms have been recently found throughout the tree of life, from sex determination in reptiles to adaptation of viral RNA polymerases, to genetic disorders in humans. We illustrate the temperature dependence of RNA metabolism with examples from the synthesis to the degradation of mRNAs, and review recently emerged questions. Are cells exposed to greater temperature variations and gradients than previously surmised? How do cells reconcile the conflicting thermal stability requirements of primary and tertiary structures of RNAs? To what extent do enzymes contribute to the temperature compensation of the reaction rates in mRNA turnover by lowering the energy barrier of the catalyzed reactions? We conclude with the ecological, forensic applications of the temperature-dependence of RNA degradation and the biotechnological aspects of mRNA vaccine production.

Opto-thermal technologies for microscopic analysis of cellular temperature-sensing systems

Could enzymatic activities and their cooperative functions act as cellular temperature-sensing systems? This review introduces recent opto-thermal technologies for microscopic analyses of various types of cellular temperature-sensing system. Optical microheating technologies have been developed for local and rapid temperature manipulations at the cellular level. Advanced luminescent thermometers visualize the dynamics of cellular local temperature in space and time during microheating. An optical heater and thermometer can be combined into one smart nanomaterial that demonstrates hybrid function. These technologies have revealed a variety of cellular responses to spatial and temporal changes in temperature. Spatial temperature gradients cause asymmetric deformations during mitosis and neurite outgrowth. Rapid changes in temperature causes imbalance of intracellular Ca2+ homeostasis and membrane potential. Among those responses, heat-induced muscle contractions are highlighted. It is also demonstrated that the short-term heating hyperactivates molecular motors to exceed their maximal activities at optimal temperatures. We discuss future prospects for opto-thermal manipulation of cellular functions and contributions to obtain a deeper understanding of the mechanisms of cellular temperature-sensing systems.

Fluorescent Benzothiadiazole Derivatives as Fluorescence Imaging Dyes: A Decade of New Generation Probes

The current review describes advances in the use of fluorescent 2,1,3-benzothiadiazole (BTD) derivatives after nearly one decade since the first description of bioimaging experiments using this class of fluorogenic dyes. The review describes the use of BTD-containing fluorophores applied as, inter alia, bioprobes for imaging cell nuclei, mitochondria, lipid droplets, sensors, markers for proteins and related events, biological processes and activities, lysosomes, plasma membranes, multicellular models, and animals. A number of physicochemical and photophysical properties commonly observed for BTD fluorogenic structures are also described.

Mitochondrially localized MPZL3 emerges as a signaling hub of mammalian physiology

MPZL3 is a nuclear-encoded, mitochondrially localized, immunoglobulin-like V-type protein that functions as a key regulator of epithelial cell differentiation, lipid metabolism, ROS production, glycemic control, and energy expenditure. Recently, MPZL3 has surfaced as an important modulator of sebaceous gland function and of hair follicle cycling, an organ transformation process that is also governed by peripheral clock gene activity and PPARγ. Given the phenotype similarities and differences between Mpzl3 and Pparγ knockout mice, we propose that MPZL3 serves as a signaling hub that is regulated by core clock gene products and/or PPARγ to translate signals from these nuclear transcription factors to the mitochondria to modulate circadian and metabolic regulation. Conservation between murine and human MPZL3 suggests that human MPZL3 may have similarly complex functions in health and disease. We summarize current knowledge and discuss future directions to elucidate the full spectrum of MPZL3 functions in mammalian physiology.

Biochemical Studies of Mitochondrial Malate: Quinone Oxidoreductase from Toxoplasma gondii

Toxoplasma gondii is a protozoan parasite that causes toxoplasmosis and infects almost one-third of the global human population. A lack of effective drugs and vaccines and the emergence of drug resistant parasites highlight the need for the development of new drugs. The mitochondrial electron transport chain (ETC) is an essential pathway for energy metabolism and the survival of T. gondii. In apicomplexan parasites, malate:quinone oxidoreductase (MQO) is a monotopic membrane protein belonging to the ETC and a key member of the tricarboxylic acid cycle, and has recently been suggested to play a role in the fumarate cycle, which is required for the cytosolic purine salvage pathway. In T. gondii, a putative MQO (TgMQO) is expressed in tachyzoite and bradyzoite stages and is considered to be a potential drug target since its orthologue is not conserved in mammalian hosts. As a first step towards the evaluation of TgMQO as a drug target candidate, in this study, we developed a new expression system for TgMQO in FN102(DE3)TAO, a strain deficient in respiratory cytochromes and dependent on an alternative oxidase. This system allowed, for the first time, the expression and purification of a mitochondrial MQO family enzyme, which was used for steady-state kinetics and substrate specificity analyses. Ferulenol, the only known MQO inhibitor, also inhibited TgMQO at IC₅₀ of 0.822 μM, and displayed different inhibition kinetics compared to Plasmodium falciparum MQO. Furthermore, our analysis indicated the presence of a third binding site for ferulenol that is distinct from the ubiquinone and malate sites.

A new approach to precise mapping of local temperature fields in submicrometer aqueous volumes

Nanodiamonds hosting temperature-sensing centers constitute a closed thermodynamic system. Such a system prevents direct contact of the temperature sensors with the environment making it an ideal environmental insensitive nanosized thermometer. A new design of a nanodiamond thermometer, based on a 500-nm luminescent nanodiamond embedded into the inner channel of a glass submicron pipette is reported. All-optical detection of temperature, based on spectral changes of the emission of "silicon-vacancy" centers with temperature, is used. We demonstrate the applicability of the thermometric tool to the study of temperature distribution near a local heater, placed in an aqueous medium. The calculated and experimental values of temperatures are shown to coincide within measurement error at gradients up to 20 °C/μm. Until now, temperature measurements on the submicron scale at such high gradients have not been performed. The new thermometric tool opens up unique opportunities to answer the urgent paradigm-shifting questions of cell physiology thermodynamics.

Contextualizing the biological relevance of standardized high-resolution respirometry to assess mitochondrial function in permeabilized human skeletal muscle

Aim

This study sought to provide a statistically robust reference for measures of mitochondrial function from standardized high‐resolution respirometry with permeabilized human skeletal muscle (ex vivo), compare analogous values obtained via indirect calorimetry, arterial‐venous O₂ differences and ³¹P magnetic resonance spectroscopy (in vivo) and attempt to resolve differences across complementary methodologies as necessary.

Methods

Data derived from 831 study participants across research published throughout March 2009 to November 2019 were amassed to examine the biological relevance of ex vivo assessments under standard conditions, ie physiological temperatures of 37°C and respiratory chamber oxygen concentrations of ~250 to 500 μmol/L.

Results

Standard ex vivo‐derived measures are lower (Z ≥ 3.01, P ≤ .0258) en masse than corresponding in vivo‐derived values. Correcting respiratory values to account for mitochondrial temperatures 10°C higher than skeletal muscle temperatures at maximal exercise (~50°C): (i) transforms data to resemble (Z ≤ 0.8, P > .9999) analogous yet context‐specific in vivo measures, eg data collected during maximal 1‐leg knee extension exercise; and (ii) supports the position that maximal skeletal muscle respiratory rates exceed (Z ≥ 13.2, P < .0001) those achieved during maximal whole‐body exercise, e.g. maximal cycling efforts.

Conclusion

This study outlines and demonstrates necessary considerations when actualizing the biological relevance of human skeletal muscle respiratory control, metabolic flexibility and bioenergetics from standard ex vivo‐derived assessments using permeabilized human muscle. These findings detail how cross‐procedural comparisons of human skeletal muscle mitochondrial function may be collectively scrutinized in their relationship to human health and lifespan.