Short-term starvation is a strategy to unravel the cellular capacity of oxidizing specific exogenous/endogenous substrates in mitochondria

Organ chips with integrated multifunctional sensors enable continuous metabolic monitoring at controlled oxygen levels

Despite remarkable advances in Organ-on-a-chip (Organ Chip) microfluidic culture technology, recreating tissue-relevant physiological conditions, such as the region-specific oxygen concentrations, remains a formidable technical challenge, and analysis of tissue functions is commonly carried out using one analytical technique at a time. Here, we describe two-channel Organ Chip microfluidic devices fabricated from polydimethylsiloxane and gas impermeable polycarbonate materials that are integrated with multiple sensors, mounted on a printed circuit board and operated using a commercially available Organ Chip culture instrument. The novelty of this system is that it enables the recreation of physiologically relevant tissue-tissue interfaces and oxygen tension as well as non-invasive continuous measurement of transepithelial electrical resistance, oxygen concentration and pH, combined with simultaneous analysis of cellular metabolic activity (ATP/ADP ratio), cell morphology, and tissue phenotype. We demonstrate the reliable and reproducible functionality of this system in living human Gut and Liver Chip cultures. Changes in tissue barrier function and oxygen tension along with their functional and metabolic responses to chemical stimuli (e.g., calcium chelation, oligomycin) were continuously and noninvasively monitored on-chip for up to 23 days. A physiologically relevant microaerobic microenvironment that supports co-culture of human intestinal cells with living Lactococcus lactis bacteria also was demonstrated in the Gut Chip. The integration of multi-functional sensors into Organ Chips provides a robust and scalable platform for the simultaneous, continuous, and non-invasive monitoring of multiple physiological functions that can significantly enhance the comprehensive and reliable evaluation of engineered tissues in Organ Chip models in basic research, preclinical modeling, and drug development.

Cellular response during cellular starvation: A battle for cellular survivability

Cellular starvation occurs when a cell is deprived of nutrition and oxygen availability. The genesis of this state of deprivation is exclusively contingent upon the inadequacy in the supply of essential components, namely amino acids, glucose, and oxygen. Consequently, the impact of this altered condition manifests in the regulation of cellular respiratory, metabolic, and stress responses. Subsequently, as a reactive outcome, cell death may transpire through mechanisms such as autophagy or apoptosis, particularly under prolonged circumstances. However, the cell combats such situations by evolving altered activity in their metabolic and protein level. Modulated signaling cascades help them to conquer starvation. But as in a prolonged condition, the battle that a cell has to evolve will come into and result in the form of cellular death. Therefore, in cancer therapy, cellular starvation may also act as a possible way out so that the cancer cell can undergo its death pathway in an induced starved condition. This review has collectively depicted the mechanism of cellular starvation. Besides this, the cellular response in this starved condition has also been summarized. Gaining such knowledge of the causation of cell starvation and cellular response during starvation not only generates new insight into the mechanism of cell survivability but also may act as a beneficial role in combating cellular diseases like cancer.

Kinetic modelling of the cellular metabolic responses underpinning in vitro glycolysis assays

This study aims to demonstrate the benefits of augmenting commercially available, real-time, in vitro glycolysis assays with phenomenological rate equation-based kinetic models, describing the contributions of the underpinning metabolic pathways. To this end, a commercially available glycolysis assay, sensitive to changes in extracellular acidification (extracellular pH), was used to derive the glycolysis pathway kinetics. The pathway was numerically modelled using a series of ordinary differential rate equations, to simulate the obtained experimental results. The sensitivity of the model to the key equation parameters was also explored. The cellular glycolysis pathway kinetics were determined for three different cell-lines, under nonmodulated and modulated conditions. Over the timescale studied, the assay demonstrated a two-phase metabolic response, representing the differential kinetics of glycolysis pathway rate as a function of time, and this behaviour was faithfully reproduced by the model simulations. The model enabled quantitative comparison of the pathway kinetics of three cell lines, and also the modulating effect of two known drugs. Moreover, the modelling tool allows the subtle differences between different cell lines to be better elucidated and also allows augmentation of the assay sensitivity. A simplistic numerical model can faithfully reproduce the differential pathway kinetics for three different cell lines, with and without pathway-modulating drugs, and furthermore provides insights into the cellular metabolism by elucidating the underlying mechanisms leading to the pathway end-product. This study demonstrates that augmenting a relatively simple, real-time, in vitro assay with a model of the underpinning metabolic pathway provides considerable insights into the observed differences in cellular systems.

Outer mitochondrial membrane E3 Ub ligase MARCH5 controls mitochondrial steps in peroxisome biogenesis

Peroxisome de novo biogenesis requires yet unidentified mitochondrial proteins. We report that the outer mitochondrial membrane (OMM)-associated E3 Ub ligase MARCH5 is vital for generating mitochondria-derived pre-peroxisomes. MARCH5 knockout results in accumulation of immature peroxisomes and lower expression of various peroxisomal proteins. Upon fatty acid-induced peroxisomal biogenesis, MARCH5 redistributes to newly formed peroxisomes; the peroxisomal biogenesis under these conditions is inhibited in MARCH5 knockout cells. MARCH5 activity-deficient mutants are stalled on peroxisomes and induce accumulation of peroxisomes containing high levels of the OMM protein Tom20 (mitochondria-derived pre-peroxisomes). Furthermore, depletion of peroxisome biogenesis factor Pex14 leads to the formation of MARCH5- and Tom20-positive peroxisomes, while no peroxisomes are detected in Pex14/MARCH5 dko cells. Reexpression of WT, but not MARCH5 mutants, restores Tom20-positive pre-peroxisomes in Pex14/MARCH5 dko cells. Thus, MARCH5 acts upstream of Pex14 in mitochondrial steps of peroxisome biogenesis. Our data validate the hybrid, mitochondria-dependent model of peroxisome biogenesis and reveal that MARCH5 is an essential mitochondrial protein in this process.

Summary

The authors found that mitochondrial E3 Ub ligase MARCH5 controls the formation of mitochondria-derived pre-peroxisomes. The data support the hybrid, mitochondria-dependent model of peroxisome biogenesis and reveal that MARCH5 is an essential mitochondrial protein in this process.

Serum starvation affects mitochondrial metabolism of adipose-derived stem/stromal cells

BACKGROUND AIMS

A large part of mesenchymal stromal cell (MSC) regenerative and immunomodulatory action is mediated by paracrine signaling. Hence, an increasing body of evidence acknowledges the potential of MSC secretome in a variety of preclinical and clinical scenarios. Mid-term serum deprivation is a common approach in the pipeline of MSC secretome production. Nevertheless, up to now, little is known about the impact of this procedure on the metabolic status of donor cells.

METHODS

Here, through untargeted differential metabolomics, we revealed an impairment of mitochondrial metabolism in adipose-derived MSCs exposed for 72 h to serum deprivation.

RESULTS

This evidence was further confirmed by the significant accumulation of reactive oxygen species and the reduction of succinate dehydrogenase activity. Probably as a repair mechanism, an upregulation of mitochondrial superoxide dismutase was also induced.

CONCLUSIONS

Of note, the analysis of mitochondrial functionality indicated that, despite a significant reduction of basal respiration and ATP production, serum-starved MSCs still responded to changes in energy demand. This metabolic phenotype correlates with the obtained evidence of mitochondrial elongation and branching upon starvation.

Investigation of Retinal Metabolic Function in Type 1 Diabetic Akita Mice

Diabetic retinopathy (DR) is the leading cause of vision loss in working age adults. Understanding the retinal metabolic response to circulating high glucose levels in diabetic patients is critical for development of new therapeutics to treat DR. Measuring retinal metabolic function using the Seahorse analyzer is a promising technique to investigate the effect of hyperglycemia on retinal glycolysis and mitochondrial respiration. Here, we analyzed the retinal metabolic function in young and old diabetic and control mice. We also compared the expression of key glycolytic enzymes between the two groups. The Seahorse XF analyzer was used to measure the metabolic function of retina explants from young and old type 1 diabetic Akita (Ins2^Akita) mice and their control littermates. Rate-limiting glycolytic enzymes were analyzed in retina lysates from the two age groups by Western blotting. Retinas from young adult Akita mice showed a decreased glycolytic response as compared to control littermates. However, this was not observed in the older mice. Western blotting analysis showed decreased expression of the glycolytic enzyme PFKFB3 in the young Akita mice retinas. Measurement of the oxygen consumption rate showed no difference in retinal mitochondrial respiration between Akita and WT littermates under normal glucose conditions ex vivo despite mitochondrial fragmentation in the Akita retinas as examined by electron microscopy. However, Akita mice retinas showed decreased mitochondrial respiration under glucose-free conditions. In conclusion, diabetic retinas display a decreased glycolytic response during the early course of diabetes which is accompanied by a reduction in PFKFB3. Diabetic retinas exhibit decreased mitochondrial respiration under glucose deprivation.

Evaluation of long-chain fatty acid respiration in neonatal mouse cardiomyocytes using SeaHorse instrument

Metabolic switches play a critical role in the pathophysiology of cardiac diseases, including heart failure. Here, we describe an assay for long-chain fatty acid oxidation in neonatal mouse cardiomyocytes by using a SeaHorse Flux Analyzer (Agilent). This protocol is a simplified but robust adaptation of the standard protocol that enables metabolic measurements in cells isolated from transgenic mouse models, which can be timesaving and informative. Cell isolation and culture represent a critical point that may require bench optimization.

For complete details on the use and execution of this protocol, please refer to Angelini et al. (2021).

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Isolation and culture of neonatal mouse cardiomyocytes

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Measurement of long-chain fatty acid oxidation using SeaHorse Flux Analyzer

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Guidance on quantification of the generated SeaHorse dataset

Publisher's note: Undertaking any experimental protocol requires adherence to local institutional guidelines for laboratory safety and ethics.

Protein Corona Inhibits Endosomal Escape of Functionalized DNA Nanostructures in Living Cells

DNA nanostructures (DNs) can be designed in a controlled and programmable manner, and these structures are increasingly used in a variety of biomedical applications, such as the delivery of therapeutic agents. When exposed to biological liquids, most nanomaterials become covered by a protein corona, which in turn modulates their cellular uptake and the biological response they elicit. However, the interplay between living cells and designed DNs are still not well established. Namely, there are very limited studies that assess protein corona impact on DN biological activity. Here, we analyzed the uptake of functionalized DNs in three distinct hepatic cell lines. Our analysis indicates that cellular uptake is linearly dependent on the cell size. Further, we show that the protein corona determines the endolysosomal vesicle escape efficiency of DNs coated with an endosome escape peptide. Our study offers an important basis for future optimization of DNs as delivery systems for various biomedical applications.

Circumventing the Crabtree effect in cell culture: A systematic review

Metabolic reprogramming and mitochondrial dysfunction are central elements in a broad variety of physiological and pathological processes. While cell culture established itself as a versatile technique for the elaboration of physiology and disease, studying metabolism using standard cell culture protocols is profoundly interfered by the Crabtree effect. This phenomenon refers to the adaptation of cultured cells to a glycolytic phenotype, away from oxidative phosphorylation in glucose-containing medium, and questions the applicability of cell culture in certain fields of research. In this systematic review we aim to provide a comprehensive overview and critical appraisal of strategies reported to circumvent the Crabtree effect.

Total Cellular ATP Production Changes With Primary Substrate in MCF7 Breast Cancer Cells

Cancer growth is predicted to require substantial rates of substrate catabolism and ATP turnover to drive unrestricted biosynthesis and cell growth. While substrate limitation can dramatically alter cell behavior, the effects of substrate limitation on total cellular ATP production rate is poorly understood. Here, we show that MCF7 breast cancer cells, given different combinations of the common cell culture substrates glucose, glutamine, and pyruvate, display ATP production rates 1.6-fold higher than when cells are limited to each individual substrate. This increase occurred mainly through faster oxidative ATP production, with little to no increase in glycolytic ATP production. In comparison, non-transformed C2C12 myoblast cells show no change in ATP production rate when substrates are limited. In MCF7 cells, glutamine allows unexpected access to oxidative capacity that pyruvate, also a strictly oxidized substrate, does not. Pyruvate, when added with other exogenous substrates, increases substrate-driven oxidative ATP production, by increasing both ATP supply and demand. Overall, we find that MCF7 cells are highly flexible with respect to maintaining total cellular ATP production under different substrate-limited conditions, over an acute (within minutes) timeframe that is unlikely to result from more protracted (hours or more) transcription-driven changes to metabolic enzyme expression. The near-identical ATP production rates maintained by MCF7 and C2C12 cells given single substrates reveal a potential difficulty in using substrate limitation to selectively starve cancer cells of ATP. In contrast, the higher ATP production rate conferred by mixed substrates in MCF7 cells remains a potentially exploitable difference.

Glucose Favors Lipid Anabolic Metabolism in the Invasive Breast Cancer Cell Line MDA-MB-231

Metabolic reprogramming in tumor cells is considered one of the hallmarks of cancer. Many studies have been carried out in order to elucidate the effects of tumor cell metabolism on invasion and tumor progression. However, little is known about the immediate substrate preference in tumor cells. In this work, we wanted to study this short-time preference using the highly invasive, hormone independent breast cancer cell line MDA-MB-231. By means of Seahorse and uptake experiments, our results point to a preference for glucose. However, although both glucose and glutamine are required for tumor cell proliferation, MDA-MB-231 cells can survive two days in the absence of glucose, but not in the absence of glutamine. On the other hand, the presence of glucose increased palmitate uptake in this cell line, which accumulates in the cytosol instead of going to the plasma membrane. In order to exert this effect, glucose needs to be converted to glycerol-3 phosphate, leading to palmitate metabolism through lipid synthesis, most likely to the synthesis of triacylglycerides. The effect of glucose on the palmitate uptake was also found in other triple-negative, invasive breast cancer cell lines, but not in the non-invasive ones. The results presented in this work suggest an important and specific role of glucose in lipid biosynthesis in triple-negative breast cancer.

Amyloid-β Causes Mitochondrial Dysfunction via a Ca2+-Driven Upregulation of Oxidative Phosphorylation and Superoxide Production in Cerebrovascular Endothelial Cells

Cerebrovascular pathology is pervasive in Alzheimer's disease (AD), yet it is unknown whether cerebrovascular dysfunction contributes to the progression or etiology of AD. In human subjects and in animal models of AD, cerebral hypoperfusion and hypometabolism are reported to manifest during the early stages of the disease and persist for its duration. Amyloid-β is known to cause cellular injury in both neurons and endothelial cells by inducing the production of reactive oxygen species and disrupting intracellular Ca2+ homeostasis. We present a mechanism for mitochondrial degeneration caused by the production of mitochondrial superoxide, which is driven by increased mitochondrial Ca2+ uptake. We found that persistent superoxide production injures mitochondria and disrupts electron transport in cerebrovascular endothelial cells. These observations provide a mechanism for the mitochondrial deficits that contribute to cerebrovascular dysfunction in patients with AD.

Highly Glycolytic Immortalized Human Dermal Microvascular Endothelial Cells are Able to Grow in Glucose-Starved Conditions

Endothelial cells form the inner lining of blood vessels, in a process known as angiogenesis. Excessive angiogenesis is a hallmark of several diseases, including cancer. The number of studies in endothelial cell metabolism has increased in recent years, and new metabolic targets for pharmacological treatment of pathological angiogenesis are being proposed. In this work, we wanted to address experimental evidence of substrate (namely glucose, glutamine and palmitate) dependence in immortalized dermal microvascular endothelial cells in comparison to primary endothelial cells. In addition, due to the lack of information about lactate metabolism in this specific type of endothelial cells, we also checked their capability of utilizing extracellular lactate. For fulfilling these aims, proliferation, migration, Seahorse, substrate uptake/utilization, and mRNA/protein expression experiments were performed. Our results show a high glycolytic capacity of immortalized dermal microvascular endothelial cells, but an early independence of glucose for cell growth, whereas a total dependence of glutamine to proliferate was found. Additionally, in contrast with reported data in other endothelial cell lines, these cells lack monocarboxylate transporter 1 for extracellular lactate incorporation. Therefore, our results point to the change of certain metabolic features depending on the endothelial cell line.

Systems biology identifies preserved integrity but impaired metabolism of mitochondria due to a glycolytic defect in Alzheimer's disease neurons

Mitochondrial dysfunction is implicated in most neurodegenerative diseases, including Alzheimer's disease (AD). We here combined experimental and computational approaches to investigate mitochondrial health and bioenergetic function in neurons from a double transgenic animal model of AD (PS2APP/B6.152H). Experiments in primary cortical neurons demonstrated that AD neurons had reduced mitochondrial respiratory capacity. Interestingly, the computational model predicted that this mitochondrial bioenergetic phenotype could not be explained by any defect in the mitochondrial respiratory chain (RC), but could be closely resembled by a simulated impairment in the mitochondrial NADH flux. Further computational analysis predicted that such an impairment would reduce levels of mitochondrial NADH, both in the resting state and following pharmacological manipulation of the RC. To validate these predictions, we utilized fluorescence lifetime imaging microscopy (FLIM) and autofluorescence imaging and confirmed that transgenic AD neurons had reduced mitochondrial NAD(P)H levels at rest, and impaired power of mitochondrial NAD(P)H production. Of note, FLIM measurements also highlighted reduced cytosolic NAD(P)H in these cells, and extracellular acidification experiments showed an impaired glycolytic flux. The impaired glycolytic flux was identified to be responsible for the observed mitochondrial hypometabolism, since bypassing glycolysis with pyruvate restored mitochondrial health. This study highlights the benefits of a systems biology approach when investigating complex, nonintuitive molecular processes such as mitochondrial bioenergetics, and indicates that primary cortical neurons from a transgenic AD model have reduced glycolytic flux, leading to reduced cytosolic and mitochondrial NAD(P)H and reduced mitochondrial respiratory capacity.

Osmolyte Induced Tumorigenesis and Metastasis: Interactions With Intrinsically Disordered Proteins

In spite of a great deal of work, the biochemical mechanisms underlying tumorigenesis and metastasis are not yet fully understood. Specifically regarding metastasis many authors consider that malignancy is caused by the accumulation of mutations. However, evidence is gathering to show that tumors are composed of heterogeneous cell populations subjected to selective pressures. In this micro evolutionary scenario, intra- and extra-cellular selective pressures will determine which subpopulations of tumor cells will thrive and be able to dissociate from the tumor as autonomous metastatic cells. We propose here that alteration of conformations of transcription factors confer novel non-canonical functions that may induce oncogenesis and metastasis in a mutation independent manner. We argue that the functional plasticity of transcription factors is due to intrinsically disordered domains (IDRs) of proteins. IDRs prevent spontaneous folding of proteins into well-defined three-dimensional structures. Because most transcription factors contain IDRs, each could potentially interact with many ligands. This high degree of functional pleiotropy would then be ultimately responsible for the metastatic phenotype. The conformations of proteins can be altered by chemical chaperones collectively known as osmolytes. Osmolytes are small organic molecules permeable through biological membranes that can accumulate in cells, increase the thermodynamic stability of proteins, modulate enzyme activity and prevent protein aggregation. Thus, by modifying IDRs, osmolytes could subvert the homeostatic regulatory network of cells. Untargeted metabolomic analysis of oral cancer cells showed that those with the greatest metastatic potential contained several osmolytes that were absent in the non-metastatic cells. We hypothesize that high concentrations of osmolytes might promote conformational alterations of transcription factors that favor metastatic behavior. This hypothesis is eminently testable by investigating whether: (a) the intracellular microenvironment of metastatic cells differs from non-metastatic cells and whether osmolytes are responsible for this change and (b) high intracellular concentrations of osmolytes are sufficient to induce structural modifications in regulatory protein so as to establish novel interactive networks that will constitute the metastatic phenotype. Synthetic cell penetrating peptides mimicking IDRs could act as sensitive probes. By exposing the peptides to the microenvironments of living tumor and metastatic tumor cells one should be able to compare the chemical shifts as revealed by spectra obtained by nuclear magnetic resonance (NMR).

Anaplerotic Role of Glucose in the Oxidation of Endogenous Fatty Acids during Dengue Virus Infection

Dengue virus infection is a major cause of human arbovirosis, for which clinical and experimental evidence supports the idea that liver dysfunction and lipid metabolism disorders are characteristics of severe disease. Analyzing mitochondrial bioenergetics, here we show that infection of hepatic cells with dengue virus favors the cellular capacity of metabolizing glucose, impairing the normal metabolic flexibility that allows the oxidative machinery to switch among the main energetic substrates. However, instead of being used as an energy source, glucose performs an anaplerotic role in the oxidation of endogenous fatty acids, which become the main energetic substrate during infection. Taken together, the results shed light on metabolic mechanisms that may explain the profound alterations in lipid metabolism for severe dengue patients, contributing to the understanding of dengue physiopathology.

Dengue virus (DENV) is among the most important human arboviruses and is clinically and experimentally associated with lipid metabolism disorders. Using high-resolution respirometry, we analyzed the metabolic switches induced by DENV in a human hepatic cell line. This experimental approach allowed us to determine the contribution of fatty acids, glutamine, glucose, and pyruvate to mitochondrial bioenergetics, shedding light on the mechanisms involved in DENV-induced metabolic alterations. We found that while infection strongly inhibits glutamine oxidation, it increases the cellular capacity of metabolizing glucose; remarkably, though, this substrate, instead being used as an energy source, performs an anaplerotic role in the oxidation of endogenous lipids. Fatty acids become the main energetic substrate in infected cell, and through the pharmacological modulation of β-oxidation we demonstrated that this pathway is essential for virus replication. Interestingly, infected cells were much less susceptible to the Crabtree effect, i.e., the glucose-mediated inhibition of mitochondrial oxygen consumption, suggesting that infection favors cellular respiration by increasing ADP availability.

IMPORTANCE Dengue virus infection is a major cause of human arbovirosis, for which clinical and experimental evidence supports the idea that liver dysfunction and lipid metabolism disorders are characteristics of severe disease. Analyzing mitochondrial bioenergetics, here we show that infection of hepatic cells with dengue virus favors the cellular capacity of metabolizing glucose, impairing the normal metabolic flexibility that allows the oxidative machinery to switch among the main energetic substrates. However, instead of being used as an energy source, glucose performs an anaplerotic role in the oxidation of endogenous fatty acids, which become the main energetic substrate during infection. Taken together, the results shed light on metabolic mechanisms that may explain the profound alterations in lipid metabolism for severe dengue patients, contributing to the understanding of dengue physiopathology.