The influence of intracellular lactate and H+ on cell volume in amphibian skeletal muscle

MicroRNA-Mediated Metabolic Reprograming in Renal Cancer

Metabolic reprogramming is one of the hallmarks of renal cell cancer (RCC). We hypothesized that altered metabolism of RCC cells results from dysregulation of microRNAs targeting metabolically relevant genes. Combined large-scale transcriptomic and metabolic analysis of RCC patients tissue samples revealed a group of microRNAs that contribute to metabolic reprogramming in RCC. miRNAs expressions correlated with their predicted target genes and with gas chromatography-mass spectrometry (GC-MS) metabolome profiles of RCC tumors. Assays performed in RCC-derived cell lines showed that miR-146a-5p and miR-155-5p targeted genes of PPP (the pentose phosphate pathway) (G6PD and TKT), the TCA (tricarboxylic acid cycle) cycle (SUCLG2), and arginine metabolism (GATM), respectively. miR-106b-5p and miR-122-5p regulated the NFAT5 osmoregulatory transcription factor. Altered expressions of G6PD, TKT, SUCLG2, GATM, miR-106b-5p, miR-155-5p, and miR-342-3p correlated with poor survival of RCC patients. miR-106b-5p, miR-146a-5p, and miR-342-3p stimulated proliferation of RCC cells. The analysis involving >6000 patients revealed that miR-34a-5p, miR-106b-5p, miR-146a-5p, and miR-155-5p are PanCancer metabomiRs possibly involved in global regulation of cancer metabolism. In conclusion, we found that microRNAs upregulated in renal cancer contribute to disturbed expression of key genes involved in the regulation of RCC metabolome. miR-146a-5p and miR-155-5p emerge as a key “metabomiRs” that target genes of crucial metabolic pathways (PPP (the pentose phosphate pathway), TCA cycle, and arginine metabolism).

SWELL1 is a glucose sensor regulating β-cell excitability and systemic glycaemia

Insulin secretion is initiated by activation of voltage-gated Ca²⁺ channels (VGCC) to trigger Ca²⁺-mediated insulin vesicle fusion with the β-cell plasma membrane. The firing of VGCC requires β-cell membrane depolarization, which is regulated by a balance of depolarizing and hyperpolarizing ionic currents. Here, we show that SWELL1 mediates a swell-activated, depolarizing chloride current (I_Cl,SWELL) in both murine and human β-cells. Hypotonic and glucose-stimulated β-cell swelling activates SWELL1-mediated I_Cl,SWELL and this contributes to membrane depolarization and activation of VGCC-dependent intracellular calcium signaling. SWELL1 depletion in MIN6 cells and islets significantly impairs glucose-stimulated insulin secretion. Tamoxifen-inducible β-cell-targeted Swell1 KO mice have normal fasting serum glucose and insulin levels but impaired glucose-stimulated insulin secretion and glucose tolerance; and this is further exacerbated in mild obesity. Our results reveal that β-cell SWELL1 modulates insulin secretion and systemic glycaemia by linking glucose-mediated β-cell swelling to membrane depolarization and activation of VGCC-triggered calcium signaling.

SWELL1 is a glucose sensor regulating β-cell excitability and systemic glycaemia

Insulin secretion is initiated by activation of voltage-gated Ca2+ channels (VGCC) to trigger Ca2+-mediated insulin vesicle fusion with the β-cell plasma membrane. The firing of VGCC requires β-cell membrane depolarization, which is regulated by a balance of depolarizing and hyperpolarizing ionic currents. Here, we show that SWELL1 mediates a swell-activated, depolarizing chloride current (ICl,SWELL) in both murine and human β-cells. Hypotonic and glucose-stimulated β-cell swelling activates SWELL1-mediated ICl,SWELL and this contributes to membrane depolarization and activation of VGCC-dependent intracellular calcium signaling. SWELL1 depletion in MIN6 cells and islets significantly impairs glucose-stimulated insulin secretion. Tamoxifen-inducible β-cell-targeted Swell1 KO mice have normal fasting serum glucose and insulin levels but impaired glucose-stimulated insulin secretion and glucose tolerance; and this is further exacerbated in mild obesity. Our results reveal that β-cell SWELL1 modulates insulin secretion and systemic glycaemia by linking glucose-mediated β-cell swelling to membrane depolarization and activation of VGCC-triggered calcium signaling.

Hypoxia Modulates the Swelling-Activated Cl Current in Human Glioblastoma Cells: Role in Volume Regulation and Cell Survival

The malignancy of glioblastoma multiforme (GBM), the most common human brain tumor, correlates with the presence of hypoxic areas, but the underlying mechanisms are unclear. GBM cells express abundant Cl channels whose activity supports cell volume and membrane potential changes, ultimately leading to cell proliferation, migration, and escaping death. In non-tumor tissues Cl channels are modulated by hypoxia, which prompted us to verify whether hypoxia would also modulate Cl channels in GBM cells. Our results show that in GBM cell lines, acute application of a hypoxic solution activates a Cl current displaying the biophysical and pharmacological features of the swelling-activated Cl current (ICl,swell ). We also found that acute hypoxia increased the cell volume by about 20%, and a 30% hypertonic solution partially inhibited the hypoxia-activated Cl current, suggesting that cell swelling and the activation of the Cl current are sequential events. Notably, the hypoxia-induced cell swelling was followed by a regulatory volume decrease (RVD) mediated mainly by ICl,swell . Since, a hypoxia-induced prolonged cell swelling is usually regarded as a death insult, we hypothesized that the hypoxia-activated Cl current could limit cell swelling and prevent necrotic death of GBM cells under hypoxic conditions. In accordance, we found that the ICl,swell inhibitor DCPIB hampered the RVD process, and more importantly it sensibly increased the hypoxia-induced necrotic death in these cells. Taken together, these results suggest that Cl channels are strongly involved in the survival of GBM cells in a hypoxic environment, and may thus represent a new therapeutic target for this malignant tumor. J. Cell. Physiol. 232: 91-100, 2017. © 2016 Wiley Periodicals, Inc.

Control of cell volume in skeletal muscle

Regulation of cell volume is a fundamental property of all animal cells and is of particular importance in skeletal muscle where exercise is associated with a wide range of cellular changes that would be expected to influence cell volume. These complex electrical, metabolic and osmotic changes, however, make rigorous study of the consequences of individual factors on muscle volume difficult despite their likely importance during exercise. Recent charge-difference modelling of cell volume distinguishes three major aspects to processes underlying cell volume control: (i) determination by intracellular impermeant solute; (ii) maintenance by metabolically dependent processes directly balancing passive solute and water fluxes that would otherwise cause cell swelling under the influence of intracellular membrane-impermeant solutes; and (iii) volume regulation often involving reversible short-term transmembrane solute transport processes correcting cell volumes towards their normal baselines in response to imposed discrete perturbations. This review covers, in turn, the main predictions from such quantitative analysis and the experimental consequences of comparable alterations in extracellular pH, lactate concentration, membrane potential and extracellular tonicity. The effects of such alterations in the extracellular environment in resting amphibian muscles are then used to reproduce the intracellular changes that occur in each case in exercising muscle. The relative contributions of these various factors to the control of cell volume in resting and exercising skeletal muscle are thus described.

Skeletal Muscle Fatigue: Cellular Mechanisms

Repeated, intense use of muscles leads to a decline in performance known as muscle fatigue. Many muscle properties change during fatigue including the action potential, extracellular and intracellular ions, and many intracellular metabolites. A range of mechanisms have been identified that contribute to the decline of performance. The traditional explanation, accumulation of intracellular lactate and hydrogen ions causing impaired function of the contractile proteins, is probably of limited importance in mammals. Alternative explanations that will be considered are the effects of ionic changes on the action potential, failure of SR Ca2+release by various mechanisms, and the effects of reactive oxygen species. Many different activities lead to fatigue, and an important challenge is to identify the various mechanisms that contribute under different circumstances. Most of the mechanistic studies of fatigue are on isolated animal tissues, and another major challenge is to use the knowledge generated in these studies to identify the mechanisms of fatigue in intact animals and particularly in human diseases.

Pyruvate transport in isolated cardiac mitochondria from two species of amphibian exhibiting dissimilar aerobic scope: Bufo marinus and Rana catesbeiana

Cardiac mitochondria were isolated from Bufo marinus and Rana catesbeiana, two species of amphibian whose cardiovascular systems are adapted to either predominantly aerobic or glycolytic modes of locomotion. Mitochondrial oxidative capacity was compared using VO2 max and respiratory control ratios in the presence of a variety of substrates including pyruvate, lactate, oxaloacetate, beta-hydroxybutyrate, and octanoyl-carnitine. B. marinus cardiac mitochondria exhibited VO2 max values twice that of R. catesbeiana cardiac mitochondria when oxidizing carbohydrate substrates. Pyruvate transport was measured via a radiolabeled-tracer assay in isolated B. marinus and R. catesbeiana cardiac mitochondria. Time-course experiments described both alpha-cyano-4-hydroxycinnamate-sensitive (MCT-like) and phenylsuccinate-sensitive pyruvate uptake mechanisms in both species. Pyruvate uptake by the MCT-like transporter was enhanced in the presence of a pH gradient, whereas the phenylsuccinate-sensitive transporter was inhibited. Notably, anuran cardiac mitochondria exhibited activities of lactate dehydrogenase and pyruvate carboxylase. The presence of both transporters on the inner mitochondrial membrane affords the net uptake of monocarboxylates including pyruvate, beta-hydroxybutyrate, and lactate; the latter potentially indicating the presence of a lactate/pyruvate shuttle allowing oxidation of extramitochondrial NADH. Intramitochondrial lactate dehydrogenase and pyruvate carboxylase enables lactate to be oxidized to pyruvate or converted to anaplerotic oxaloacetate. Kinetics of the MCT-like transporter differed significantly between the two species, suggesting differences in aerobic scope may be in part attributable to differences in mitochondrial carbohydrate utilization.

Temporal differences in the influence of ischemic factors and deformation on the metabolism of engineered skeletal muscle

Prolonged periods of tissue compression may lead to the development of pressure ulcers, some of which may originate in, for example, skeletal muscle tissue and progress underneath intact skin, representing deep tissue injury. Their etiology is multifactorial and the interaction between individual causal factors and their relative importance remain unknown. The present study addressed the relative contributions of deformation and ischemic factors to altered metabolism and viability. Engineered muscle tissue was prepared as previously detailed ( 14 ) and subjected to a combination of factors including 0% oxygen, lactic acid concentrations resulting in pH from 5.3 to 7.4, 34% compression, and low glucose levels. Deformation had an immediate effect on tissue viability {[3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] (MTT) assay}, which increased with time. By contrast, hypoxia evoked metabolic responses (glucose and lactate levels) within 24 h, but viability was only reduced after 48 h. In addition, lactic acidification downregulated tissue metabolism up to an acid concentration (∼23 mM) where metabolism was arrested and cell death enhanced. A similar tissue response was observed during glucose deprivation, which, at negligible concentration, resulted in both a cessation of metabolic activity and a reduction in cell viability. The combination of results suggests that in a short-term (<24 h) deformation, extreme acidification and glucose deprivation increased the level of cell death. By contrast, nonextreme acidification and hypoxia influenced tissue metabolism, but not the development of cell death. These data provide more insight into how compression-induced factors can lead to the onset of deep tissue injury.

Additive protective effects of the addition of lactic acid and adrenaline on excitability and force in isolated rat skeletal muscle depressed by elevated extracellular K+

During strenuous exercise, extracellular K(+) ([K(+)](o)) is increased, which potentially can reduce muscle excitability and force production. In addition, exercise leads to accumulation of lactate and H(+) and increased levels of circulating catecholamines. Individually, reduced pH and increased catecholamines have been shown to counteract the depressing effect of elevated K(+). This study examines (i) whether the effects of addition of lactic acid and adrenaline on the excitability of isolated muscles are caused by separate mechanisms and are additive and (ii) whether the effect of adding lactic acid or increasing CO(2) is related to a reduction of intra- or extracellular pH. Rat soleus muscles were incubated at a [K(+)](o) of 15 mM, which reduced tetanic force by 85%. Subsequent addition of 20 mM lactic acid or 10(-5) M adrenaline led to a small recovery of force, but when added together induced an almost complete force recovery. Compound action potentials showed that the force recovery was associated with recovery of muscle excitability. The improved excitability after addition of adrenaline was associated with increased Na(+)-K(+) pump activity resulting in hyperpolarization and an increase in the chemical Na(+) gradient. In contrast, addition of lactic acid had no effect on the membrane potential or the Na(+)-K(+) pump activity, but most likely increased excitability via a reduction in intracellular pH. It is concluded that the protective effects of acidosis and adrenaline on muscle excitability and force took place via different mechanisms and were additive. The results suggest that circulating catecholamines and development of acidosis during exercise may improve the tolerance of muscles to elevated [K(+)](o).

Alterations in triad ultrastructure following repetitive stimulation and intracellular changes associated with exercise in amphibian skeletal muscle

This study used Rana temporaria sartorius muscles to examine the effect of fatiguing electrical stimulation on the gap between the T-tubular and sarcoplasmic reticular membranes (T-SR distance) and the T-tubule diameter and compared this with corresponding effects on resting fibres exposed to a range of extracellular conditions that each replicate one of the major changes associated with muscular activity: membrane depolarisation, isotonic volume increase, acidification and intracellular lactate accumulation. Following each treatment, muscles were immersed in isotonic fixative solution and processed for transmission electron microscopy (TEM). Mean T-SR distances were estimated from orthogonal intercepts to provide estimates of diffusion distances between T and SR membranes and T-tubule diameter was estimated by measuring its shortest axis in the sampled J-SR complexes. Measurements from muscles fatigued by low frequency intermittent stimulation showed significant (P << 0.05) reversible increases in both T-SR distance and T-tubule diameter from 15.97 ± 0.37 nm to 20.15 ± 0.56 nm and from 15.44 ± 0.60 nm to 22.26 ± 0.84 nm (n = 40, 30) respectively. Exposure to increasing concentrations of extracellular [K⁺] in the absence of Cl⁻ to produce membrane depolarisation without accompanying cell swelling reduced T-SR distance and increased T-tubule diameter, whilst comparable increases in [K⁺]_e in the presence of Cl⁻ suggested that isotonic cell swelling has the opposite effect. Acidification alone, produced by NH₄Cl addition and withdrawal, also decreased T-SR distance and T-tubule diameter. A similar reduction in T-SR distance occurred following exposure to extracellular Na-lactate where such acidification was accompanied by elevations of intracellular lactate, but these conditions produced a significant swelling of T-tubules attributable to movement of lactate from the cell into the T-tubules. This study thus confirms previous reports of significant increases in T-SR distance and T-tubule diameter following stimulation. However, of membrane depolarisation, isotonic cell swelling, intracellular acidification and lactate accumulation, only isotonic cell swelling increases T-SR distance whilst membrane depolarisation and intracellular lactate likely contribute to the observed increases in T-tubule diameter.

Alterations in calcium homeostasis reduce membrane excitability in amphibian skeletal muscle

The effects of alterations in intracellular calcium homeostasis on surface membrane excitability were investigated in resting Rana temporaria sartorius muscle. This was prompted by initial results from a fatiguing stimulation protocol study that demonstrated a fibre subpopulation in which action potential generation in response to a standard 1.5 V electrical stimulus failed despite mean membrane potentials [E (m), -69+/-2.3 mV (n=14)] compatible with spike firing in a control set of quiescent muscle fibres. Intracellular micro-electrode recordings showed a similar reversible loss of excitability, attributable to an increased threshold, despite only small (7.1+/-1.8 mV) positive changes in E (m) after approximately 60-min exposures to nominally 0 Ca(2+) Ringer solutions in which Ca(2+) was replaced by Mg(2+). This effect was not reproduced by addition of Mg(2+) to the Ringer solution and persisted under conditions of Cl(-) deprivation. The effects of three pharmacological agents, cyclopiazonic acid (CPA), caffeine and 4-chloro-m-cresol (4-CmC), each known to deplete store Ca(2+) and increase cytosolic Ca(2+) through contrasting mechanisms without influencing E (m), were then investigated. All three agents produced a more rapid, but nevertheless still reversible, loss of membrane excitability than in 0 Ca(2+) Ringer solution alone. This reduction in membrane excitability persisted in fibres studied in solutions containing a normal [Ca(2+)] following prior depletion of store Ca(2+) using CPA- and 4-CmC-containing solutions. These novel findings suggest that sarcoplasmic reticulum Ca(2+) content profoundly influences surface membrane excitability, thereby providing a potential mechanism by which spike firing fails in well-polarised fibres during fatigue.