TRPV4 is a regulator of adipose oxidative metabolism, inflammation, and energy homeostasis

Transient Receptor Potential Channels and Metabolism

Transient receptor potential (TRP) channels are nonselective cationic channels, conserved among flies to humans. Most TRP channels have well known functions in chemosensation, thermosensation, and mechanosensation. In addition to being sensing environmental changes, many TRP channels are also internal sensors that help maintain homeostasis. Recent improvements to analytical methods for genomics and metabolomics allow us to investigate these channels in both mutant animals and humans. In this review, we discuss three aspects of TRP channels, which are their role in metabolism, their functional characteristics, and their role in metabolic syndrome. First, we introduce each TRP channel superfamily and their particular roles in metabolism. Second, we provide evidence for which metabolites TRP channels affect, such as lipids or glucose. Third, we discuss correlations between TRP channels and obesity, diabetes, and mucolipidosis. The cellular metabolism of TRP channels gives us possible therapeutic approaches for an effective prophylaxis of metabolic syndromes.

Introducing a mammalian nerve-muscle preparation ideal for physiology and microscopy, the transverse auricular muscle in the ear of the mouse

A new mammalian neuromuscular preparation is introduced for physiology and microscopy of all sorts: the intrinsic muscle of the mouse ear. The great utility of this preparation is demonstrated by illustrating how it has permitted us to develop a wholly new technique for staining muscle T-tubules, the critical conductive-elements in muscle. This involves sequential immersion in dilute solutions of osmium and ferrocyanide, then tannic acid, and then uranyl acetate, all of which totally blackens the T-tubules but leaves the muscle pale, thereby revealing that the T-tubules in mouse ear-muscles become severely distorted in several pathological conditions. These include certain mouse-models of muscular dystrophy (specifically, dysferlin-mutations), certain mutations of muscle cytoskeletal proteins (specifically, beta-tubulin mutations), and also in denervation-fibrillation, as observed in mouse ears maintained with in vitro tissue-culture conditions. These observations permit us to generate the hypothesis that T-tubules are the "Achilles' heel" in several adult-onset muscular dystrophies, due to their unique susceptibility to damage via muscle lattice-dislocations. These new observations strongly encourage further in-depth studies of ear-muscle architecture, in the many available mouse-models of various devastating human muscle-diseases. Finally, we demonstrate that the delicate and defined physical characteristics of this 'new' mammalian muscle are ideal for ultrastructural study, and thereby facilitate the imaging of synaptic vesicle membrane recycling in mammalian neuromuscular junctions, a topic that is critical to myasthenia gravis and related diseases, but which has, until now, completely eluded electron microscopic analysis. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

Annexins in Adipose Tissue: Novel Players in Obesity

Obesity and the associated comorbidities are a growing health threat worldwide. Adipose tissue dysfunction, impaired adipokine activity, and inflammation are central to metabolic diseases related to obesity. In particular, the excess storage of lipids in adipose tissues disturbs cellular homeostasis. Amongst others, organelle function and cell signaling, often related to the altered composition of specialized membrane microdomains (lipid rafts), are affected. Within this context, the conserved family of annexins are well known to associate with membranes in a calcium (Ca²⁺)- and phospholipid-dependent manner in order to regulate membrane-related events, such as trafficking in endo- and exocytosis and membrane microdomain organization. These multiple activities of annexins are facilitated through their diverse interactions with a plethora of lipids and proteins, often in different cellular locations and with consequences for the activity of receptors, transporters, metabolic enzymes, and signaling complexes. While increasing evidence points at the function of annexins in lipid homeostasis and cell metabolism in various cells and organs, their role in adipose tissue, obesity and related metabolic diseases is still not well understood. Annexin A1 (AnxA1) is a potent pro-resolving mediator affecting the regulation of body weight and metabolic health. Relevant for glucose metabolism and fatty acid uptake in adipose tissue, several studies suggest AnxA2 to contribute to coordinate glucose transporter type 4 (GLUT4) translocation and to associate with the fatty acid transporter CD36. On the other hand, AnxA6 has been linked to the control of adipocyte lipolysis and adiponectin release. In addition, several other annexins are expressed in fat tissues, yet their roles in adipocytes are less well examined. The current review article summarizes studies on the expression of annexins in adipocytes and in obesity. Research efforts investigating the potential role of annexins in fat tissue relevant to health and metabolic disease are discussed.

Corneal pain and experimental model development

The cornea is a valuable tissue for studying peripheral sensory nerve structure and regeneration due to its avascularity, transparency, and dense innervation. Somatosensory innervation of the cornea serves to identify changes in environmental stimuli at the ocular surface, thereby promoting barrier function to protect the eye against injury or infection. Due to regulatory demands to screen ocular safety of potential chemical exposure, a need remains to develop functional human tissue models to predict ocular damage and pain using in vitro-based systems to increase throughput and minimize animal use. In this review, we summarize the anatomical and functional roles of corneal innervation in propagation of sensory input, corneal neuropathies associated with pain, and the status of current in vivo and in vitro models. Emphasis is placed on tissue engineering approaches to study the human corneal pain response in vitro with integration of proper cell types, controlled microenvironment, and high-throughput readouts to predict pain induction. Further developments in this field will aid in defining molecular signatures to distinguish acute and chronic pain triggers based on the immune response and epithelial, stromal, and neuronal interactions that occur at the ocular surface that lead to functional outcomes in the brain depending on severity and persistence of the stimulus.

Transient receptor potential ion-channel subfamily V member 4: a potential target for cancer treatment

The transient receptor potential ion-channel superfamily consists of nonselective cation channels located mostly on the plasma membranes of numerous animal cell types, which are closely related to sensory information transmission (e.g., vision, pain, and temperature perception), as well as regulation of intracellular Ca2+ balance and physiological activities of growth and development. Transient receptor potential ion channel subfamily V (TRPV) is one of the largest and most diverse subfamilies, including TRPV1-TRPV6 involved in the regulation of a variety of cellular functions. TRPV4 can be activated by various physical and chemical stimuli, such as heat, mechanical force, and phorbol ester derivatives participating in the maintenance of normal cellular functions. In recent years, the roles of TRPV4 in cell proliferation, differentiation, apoptosis, and migration have been extensively studied. Its abnormal expression has also been closely related to the onset and progression of multiple tumors, so TRPV4 may be a target for cancer diagnosis and treatment. In this review, we focused on the latest studies concerning the role of TRPV4 in tumorigenesis and the therapeutic potential. As evidenced by the effects on cancerogenesis, TRPV4 is a potential target for anticancer therapy.

Caffeine exposure induces browning features in adipose tissue in vitro and in vivo

Brown adipose tissue (BAT) is able to rapidly generate heat and metabolise macronutrients, such as glucose and lipids, through activation of mitochondrial uncoupling protein 1 (UCP1). Diet can modulate UCP1 function but the capacity of individual nutrients to promote the abundance and activity of UCP1 is not well established. Caffeine consumption has been associated with loss of body weight and increased energy expenditure, but whether it can activate UCP1 is unknown. This study examined the effect of caffeine on BAT thermogenesis in vitro and in vivo. Stem cell-derived adipocytes exposed to caffeine (1 mM) showed increased UCP1 protein abundance and cell metabolism with enhanced oxygen consumption and proton leak. These functional responses were associated with browning-like structural changes in mitochondrial and lipid droplet content. Caffeine also increased peroxisome proliferator-activated receptor gamma coactivator 1-alpha expression and mitochondrial biogenesis, together with a number of BAT selective and beige gene markers. In vivo, drinking coffee (but not water) stimulated the temperature of the supraclavicular region, which co-locates to the main region of BAT in adult humans, and is indicative of thermogenesis. Taken together, these results demonstrate that caffeine can promote BAT function at thermoneutrality and may have the potential to be used therapeutically in adult humans.

1H-NMR Metabolomics Analysis of the Effects of Sulfated Polysaccharides from Masson Pine Pollen in RAW264.7 Macrophage Cells

Many polysaccharides have been shown to be bioactive, with the addition of sulfate often enhancing or altering this bioactivity. In previous studies, masson pine pollen polysaccharides, to include a sulfate derivative, have been shown to promote macrophage proliferation similarly to LPS. However, the exact metabolic mechanisms promoting this proliferation remain unclear. In this study, RAW264.7 macrophage cells were treated with a purified masson pine pollen polysaccharide (PPM60-D), a sulfate derivative (SPPM60-D), or LPS. Proliferation levels at a variety of concentrations were examined using MTT assay, with optimal concentration used when performing metabolomic analysis via ¹H nuclear magnetic resonance (¹H-NMR). This process resulted in the identification of thirty-five intracellular metabolites. Subsequent multivariate statistical analysis showed that both LPS and SPPM60-D promote RAW264.7 proliferation by promoting aerobic respiration processes and reducing processes associated with glycolysis. While some insight was gained regarding the mechanistic differences between SPPM60-D and LPS, the specific mechanisms governing the effect of SPPM60 on RAW264.7 cells will require further elucidation. These findings show that both LPS and SPPM60-D effectively promote RAW264.7 proliferation and may have beneficial uses in maintaining cellular vitality or inhibiting cancer.

TRPV4 inhibition prevents increased water diffusion and blood-retina barrier breakdown in the retina of streptozotocin-induced diabetic mice

A better understanding of the molecular and cellular mechanisms involved in retinal hydro-mineral homeostasis imbalance during diabetic macular edema (DME) is needed to gain insights into retinal (patho-)physiology that will help elaborate innovative therapies with lower health care costs. Transient receptor potential cation channel subfamily vanilloid member 4 (TRPV4) plays an intricate role in homeostatic processes that needs to be deciphered in normal and diabetic retina. Based on previous findings showing that TRPV4 antagonists resolve blood-retina barrier (BRB) breakdown in diabetic rats, we evaluated whether TRPV4 channel inhibition prevents and reverts retinal edema in streptozotocin(STZ)-induced diabetic mice. We assessed retinal edema using common metrics, including retinal morphology/thickness (histology) and BRB integrity (albumin-associated tracer), and also by quantifying water mobility through apparent diffusion coefficient (ADC) measures. ADC was measured by diffusion-weighted magnetic resonance imaging (DW-MRI), acquired ex vivo at 4 weeks after STZ injection in diabetes and control groups. DWI images were also used to assess retinal thickness. TRPV4 was genetically ablated or pharmacologically inhibited as follows: left eyes were used as vehicle control and right eyes were intravitreally injected with TRPV4-selective antagonist GSK2193874, 24 h before the end of the 4 weeks of diabetes. Histological data show that retinal thickness was similar in nondiabetic and diabetic wt groups but increased in diabetic Trpv4^-/- mice. In contrast, DWI shows retinal thinning in diabetic wt mice that was absent in diabetic Trpv4^-/- mice. Disorganized outer nuclear layer was observed in diabetic wt but not in diabetic Trpv4^-/- retinas. We further demonstrate increased water diffusion, increased distances between photoreceptor nuclei, reduced nuclear area in all nuclear layers, and BRB hyperpermeability, in diabetic wt mice, effects that were absent in diabetic Trpv4^-/- mice. Retinas of diabetic mice treated with PBS showed increased water diffusion that was not normalized by GSK2193874. ADC maps in nondiabetic Trpv4^-/- mouse retinas showed restricted diffusion. Our data provide evidence that water diffusion is increased in diabetic mouse retinas and that TRPV4 function contributes to retinal hydro-mineral homeostasis and structure under control conditions, and to the development of BRB breakdown and increased water diffusion in the retina under diabetes conditions. A single intravitreous injection of TRPV4 antagonist is however not sufficient to revert these alterations in diabetic mouse retinas.

Small molecules for fat combustion: targeting obesity

Obesity is increasing in an alarming rate worldwide, which causes higher risks of some diseases, such as type 2 diabetes, cardiovascular diseases, and cancer. Current therapeutic approaches, either pancreatic lipase inhibitors or appetite suppressors, are generally of limited effectiveness. Brown adipose tissue (BAT) and beige cells dissipate fatty acids as heat to maintain body temperature, termed non-shivering thermogenesis; the activity and mass of BAT and beige cells are negatively correlated with overweight and obesity. The existence of BAT and beige cells in human adults provides an effective weight reduction therapy, a process likely to be amenable to pharmacological intervention. Herein, we combed through the physiology of thermogenesis and the role of BAT and beige cells in combating with obesity. We summarized the thermogenic regulators identified in the past decades, targeting G protein-coupled receptors, transient receptor potential channels, nuclear receptors and miscellaneous pathways. Advances in clinical trials were also presented. The main purpose of this review is to provide a comprehensive and up-to-date knowledge from the biological importance of thermogenesis in energy homeostasis to the representative thermogenic regulators for treating obesity. Thermogenic regulators might have a large potential for further investigations to be developed as lead compounds in fighting obesity.

Volume expansion and TRPV4 activation regulate stem cell fate in three-dimensional microenvironments

For mesenchymal stem cells (MSCs) cultured in three dimensional matrices, matrix remodeling is associated with enhanced osteogenic differentiation. However, the mechanism linking matrix remodeling in 3D to osteogenesis of MSCs remains unclear. Here, we find that MSCs in viscoelastic hydrogels exhibit volume expansion during cell spreading, and greater volume expansion is associated with enhanced osteogenesis. Restriction of expansion by either hydrogels with slow stress relaxation or increased osmotic pressure diminishes osteogenesis, independent of cell morphology. Conversely, induced expansion by hypoosmotic pressure accelerates osteogenesis. Volume expansion is mediated by activation of TRPV4 ion channels, and reciprocal feedback between TRPV4 activation and volume expansion controls nuclear localization of RUNX2, but not YAP, to promote osteogenesis. This work demonstrates the role of cell volume in regulating cell fate in 3D culture, and identifies TRPV4 as a molecular sensor of matrix viscoelasticity that regulates osteogenic differentiation.

For mesenchymal stem cells (MSCs), matrix remodeling is associated with enhanced osteogenic differentiation. Here authors find that MSCs in viscoelastic hydrogels exhibit volume expansion during cell spreading, and greater volume expansion is associated with enhanced osteogenesis.

Could de-stressing the brain be the solution for long-term weight loss?

The obese brain is stressed and inflamed. This is mainly at the level of neurons and glial cells in the hypothalamus: a brain region where the adipokine leptin acts to control feeding and body weight. Relieving hypothalamic neuronal endoplasmic reticulum (ER) stress with the natural small molecule drugs celastrol or withaferin-A reverses the leptin resistance commensurate with obesity, producing a degree of weight loss found only with bariatric surgery. Here, recent evidence from rodent models of vertical sleeve gastrectomy (VSG) is brought to the fore which suggests that this particular bariatric surgical procedure may work in a similar fashion to celastrol and withaferin-A alongside remedying hypothalamic inflammation and gliosis. Thus, restoring and preserving healthy hypothalamic neuronal and glial cell function, be it by pharmacological or surgical means, ensures a negative energy balance in an environment constructed to promote a one - possibly through re-establishing communication between adipose tissue and the brain.

Role of Thermo-Sensitive Transient Receptor Potential Channels in Brown Adipose Tissue

Brown and beige adipocytes are a major site of mammalian non-shivering thermogenesis and energy dissipation. Obesity is caused by an imbalance between energy intake and expenditure and has become a worldwide health problem. Therefore modulation of thermogenesis in brown and beige adipocytes could be an important application for body weight control and obesity prevention. Over the last few decades, the involvement of thermo-sensitive transient receptor potential (TRP) channels (including TRPV1, TRPV2, TRPV3, TRPV4, TRPM4, TRPM8, TRPC5, and TRPA1) in energy metabolism and adipogenesis in adipocytes has been extensively explored. In this review, we summarize the expression, function, and pathological/physiological contributions of these TRP channels and discuss their potential as future therapeutic targets for preventing and combating human obesity and obesity-related metabolic disorders.

Dietary polyphenols and their roles in fat browning

Discovery of the presence of brown adipose tissue (BAT) in newborn babies and adult humans, especially constitutively active brown fat or inducible beige fat, has led to the investigation of strategies employing BAT aimed at the development of novel therapeutic avenues for combating obesity and diabetes. Such antiobesity therapeutic tools include pharmaceutical and nutraceutical dietary polyphenols. Although there have been emerging notable advances in knowledge of and an increased amount of research related to brown and beige adipocyte developmental lineages and transcriptional regulators, current knowledge regarding whether and how food factors and environmental modifiers of BAT influence thermogenesis has not been extensively investigated. Therefore, in this review, we summarized recent updates on the exploration of dietary polyphenols while paying attention to the activation of BAT and thermogenesis. Specifically, we summarized findings pertaining to BAT metabolism, white adipose tissue (WAT) browning and thermogenic function of polyphenols (e.g., flavan-3-ols, green tea catechins, resveratrol, capsaicin/capsinoids, curcumin, thymol, chrysin, quercetin and berberine) that may foster a relatively safe and effective therapeutic option to improve metabolic health. We also deciphered the underlying proposed mechanisms through which these dietary polyphenols facilitate BAT activity and WAT browning. Characterization of thermogenic dietary factors may offer novel insight enabling revision of nutritional intervention strategies aimed at obesity and diabetes prevention and management. Moreover, identification of polyphenolic dietary factors among plant-derived natural compounds may provide information that facilitates nutritional intervention strategies against obesity, diabetes and metabolic syndrome.

Loss of DJ-1 promotes browning of white adipose tissue in diet-induced obese mice

The seminal discovery of browning of white adipose tissue (WAT) holds great promise for the treatment of obesity and metabolic syndrome. DJ-1 is evolutionarily conserved across species, and mutations in DJ-1 have been identified in Parkinson's disease. Higher levels of DJ-1 are associated with obesity, but the underlying mechanism is less understood. Here, we report the previously unappreciated role of DJ-1 in white adipocyte biology in mature models of obesity. We used DJ-1 knockout (KO) mouse models and wild-type littermates maintained on a normal diet or high-fat diet as well as in vitro cell models to show the direct effects of DJ-1 depletion on adipocyte phenotype, thermogenic capacity, fat metabolism, and microenvironment profile. Global DJ-1 KO mice show increased sympathetic input to WAT and β3-adrenergic receptor intracellular signaling, leading to a previously unrecognized compensatory mechanism through browning of WAT with associated characteristics, including high mitochondrial contents, reduced lipid accumulation, adequate vascularization and attenuated autophagy. DJ-1 KO mice had normal body weight, energy balance, and adiposity, which were associated with protective effects on healthy WAT expansion by hyperplasia. Our findings revealed that browning of inguinal WAT occurred in DJ-1 KO mice that do not show increased predisposition to obesity and suggest that such potential mechanism may overcome the adverse metabolic consequences of obesity independent of an effect on body weight. Here, we provide the first direct evidence that targeting DJ-1 in adipocyte metabolic health may offer a unique therapeutic strategy for the treatment of obesity.

Myeloid-specific Acat1 ablation attenuates inflammatory responses in macrophages, improves insulin sensitivity, and suppresses diet-induced obesity

Macrophages are phagocytes that play important roles in health and diseases. Acyl-CoA:cholesterol acyltransferase 1 (ACAT1) converts cellular cholesterol to cholesteryl esters and is expressed in many cell types. Unlike global Acat1 knockout (KO), myeloid-specific Acat1 KO ( Acat1^-) does not cause overt abnormalities in mice. Here, we performed analyses in age- and sex-matched Acat1^-M/-M and wild-type mice on chow or Western diet and discovered that Acat1^-M/-M mice exhibit resistance to Western diet-induced obesity. On both chow and Western diets, Acat1^-M/-M mice display decreased adipocyte size and increased insulin sensitivity. When fed with Western diet, Acat1^-M/-M mice contain fewer infiltrating macrophages in white adipose tissue (WAT), with significantly diminished inflammatory phenotype. Without Acat1, the Ly6C^hi monocytes express reduced levels of integrin-β₁, which plays a key role in the interaction between monocytes and the inflamed endothelium. Adoptive transfer experiment showed that the appearance of leukocytes from Acat1^-M/-M mice to the inflamed WAT of wild-type mice is significantly diminished. Under Western diet, Acat1^-M/-M causes suppression of multiple proinflammatory genes in WAT. Cell culture experiments show that in RAW 264.7 macrophages, inhibiting ACAT1 with a small-molecule ACAT1-specific inhibitor reduces inflammatory responses to lipopolysaccharide. We conclude that under Western diet, blocking ACAT1 in macrophages attenuates inflammation in WAT. Other results show that Acat1^-M/-M does not compromise antiviral immune response. Our work reveals that blocking ACAT1 suppresses diet-induced obesity in part by slowing down monocyte infiltration to WAT as well as by reducing the inflammatory responses of adipose tissue macrophages.

Transient receptor potential (TRP) channels: a metabolic TR(i)P to obesity prevention and therapy

Cellular transport of ions, especially by ion channels, regulates physiological function. The transient receptor potential (TRP) channels, with 30 identified so far, are cation channels with high calcium permeability. These ion channels are present in metabolically active tissues including adipose tissue, liver, gastrointestinal tract, brain (hypothalamus), pancreas and skeletal muscle, which suggests a potential role in metabolic disorders including obesity. TRP channels have potentially important roles in adipogenesis, obesity development and its prevention and therapy because of their physiological properties including calcium permeability, thermosensation and taste perception, involvement in cell metabolic signalling and hormone release. This wide range of actions means that organ-specific actions are unlikely, thus increasing the possibility of adverse effects. Delineation of responses to TRP channels has been limited by the poor selectivity of available agonists and antagonists. Food constituents that can modulate TRP channels are of interest in controlling metabolic status. TRP vanilloid 1 channels modulated by capsaicin have been the most studied, suggesting that this may be the first target for effective pharmacological modulation in obesity. This review shows that most of the TRP channels are potential targets to reduce metabolic disorders through a range of mechanisms.

Retinal Detachment-Induced Müller Glial Cell Swelling Activates TRPV4 Ion Channels and Triggers Photoreceptor Death at Body Temperature

Using region-specific injection of hyaluronic acid, we developed a mouse model of acute retinal detachment (RD) to investigate molecular mechanisms of photoreceptor cell death triggered by RD. We focused on the transient receptor potential vanilloid 4 (TRPV4) ion channel, which functions as a thermosensor, osmosensor, and/or mechanosensor. After RD, the number of apoptotic photoreceptors was reduced by ∼50% in TRPV4KO mice relative to wild-type mice, indicating the possible involvement of TRPV4 activation in RD-induced photoreceptor cell death. Furthermore, TRPV4 expressed in Müller glial cells can be activated by mechanical stimuli caused by RD-induced swelling of these cells, resulting in release of the cytokine MCP-1, which is reported as a mediator of Müller glia-derived strong mediator for RD-induced photoreceptor death. We also found that the TRPV4 activation by the Müller glial swelling was potentiated by body temperature. Together, our results suggest that RD adversely impacts photoreceptor viability via TRPV4-dependent cytokine release from Müller glial cells and that TRPV4 is part of a novel molecular pathway that could exacerbate the effects of hypoxia on photoreceptor survival after RD.

SIGNIFICANCE STATEMENT Identification of the mechanisms of photoreceptor death in retinal detachment is required for establishment of therapeutic targets for preventing loss of visual acuity. In this study, we found that TRPV4 expressed in Müller glial cells can be activated by mechanical stimuli caused by RD-induced swelling of these cells, resulting in release of the cytokine MCP-1, which is reported as a mediator of Müller glia-derived strong mediator for RD-induced photoreceptor death. We also found that the TRPV4 activation by the Müller glial swelling was potentiated by body temperature. Hence, TRPV4 inhibition could suppress cell death in RD pathological conditions and suggests that TRPV4 in Müller glial cells might be a novel therapeutic target for preventing photoreceptor cell death after RD.

Ablation of soluble epoxide hydrolase reprogram white fat to beige-like fat through an increase in mitochondrial integrity, HO-1-adiponectin in vitro and in vivo

We have shown that epoxyeicosatrienoic acids (EETs), specifically 11,12- and 14,15-EETs, reduce adipogenesis in human mesenchymal stem cells and mouse preadipocytes (3T-3L1). In this study, we explore the effects of soluble epoxide hydrolase (sEH) deletion on various aspects of adipocyte-function, including programing for white vs. beige-like fat, and mitochondrial and thermogenic gene-expressions. We further hypothesize that EETs and heme-oxygenase 1 (HO-1) form a synergistic, functional module whose effects on adipocyte and vascular function is greater than the effects of sEH deletion alone. In in vitro studies, we examined the effect of sEH inhibitors on MSC-derived adipocytes. MSC-derived adipocytes exposed to AUDA, an inhibitor of sEH, exhibit an increased number of small and healthy adipocytes, an effect reproduced by siRNA for sEH. in vivo studies indicate that sEH deletion results in a significant decrease in adipocyte size, inflammatory adipokines NOV, TNFα, while increasing adiponectin (p < 0.05). These findings are associated with a decrease in body weight (p < 0.05), and visceral fat (p < 0.05). Importantly, sEH deletion was associated with a significant increase in Mfn1, COX 1, UCP1 and adiponectin (p < 0.03). sEH deletion was manifested by a significant increase in EETs isomers 5,6-EET, 8,9-EET, 11,12-EET, and 14,15-EET and an increased EETs/DHETEs ratio. Notably, activation of HO-1 gene expression further increased the levels of EETs, suggesting that the antioxidant HO-1 system protects EETs from degradation by ROS. These results are novel in that sEH deletion, while increasing EET levels, resulted in reprograming of white fat to express mitochondrial and thermogenic genes, a phenotype characteristic of beige-fat. Thus, EETs agonist(s) and sEH inhibitors may have therapeutic potential in the treatment of metabolic syndrome and obesity.

TRPV4 Channel-Regulated ATP Release Contributes to γ-Irradiation-Induced Production of IL-6 and IL-8 in Epidermal Keratinocytes

External stimuli, such as radiation, induce inflammatory cytokine and chemokine production in skin, but the mechanisms involved are not completely understood. We previously showed that the P2Y11 nucleotide receptor, p38 mitogen-activated protein kinase (MAPK) and nuclear factor-kappa B (NF-κB) all participate in interleukin (IL)-6 production induced by γ-irradiation. Here, we focused on the transient receptor potential vanilloid 4 (TRPV4) channel, which is expressed in skin keratinocytes and has been reported to play a role in inflammation. We found that irradiation of human epidermal keratinocytes HaCaT cells with 5 Gy of γ-rays (¹³⁷Cs: 0.75 Gy/min) induced IL-6 and IL-8 production. HaCaT cells treated with TRPV4 channel agonist GSK1016790A also showed increased IL-6 and IL-8 production. In both cases, IL-6/IL-8 production was not increased at 24 h after stimulation, but was increased at 48 h. ATP was released from cells exposed to γ-irradiation or TRPV4 channel agonist, and the release was suppressed by TRPV4 channel inhibitors. The γ-irradiation-induced increase in IL-6 and IL-8 production was suppressed by apyrase (ecto-nucleotidase), NF157 (selective P2Y11 receptor antagonist) and SB203580 (p38 MAPK inhibitor). GSK1016790A-induced inhibitor of kappa B-alpha (IκBα) decomposition, which causes NF-κB activation was suppressed by NF157 and SB203580, and γ-irradiation-induced IκBα decomposition was suppressed by TRPV4 channel inhibitors. Our results suggest that γ-irradiation of keratinocytes induces ATP release via activation of the TRPV4 channel, and then ATP activates P2Y11 receptor and p38 MAPK-NF-κB signaling, resulting in IL-6/IL-8 production.

Chinese Medicine in the Battle Against Obesity and Metabolic Diseases

Obesity is a multi-factor chronic disease caused by the mixed influence of genetics, environments and an imbalance of energy intake and expenditure. Due to lifestyle changes, modern society sees a rapid increase in obesity occurrence along with an aggravated risk of metabolic syndromes in the general population, including diabetes, hepatic steatosis, cardiovascular diseases and certain types of cancer. Although obesity has become a serious worldwide public health hazard, effective and safe drugs treating obesity are still missing. Traditional Chinese medicine (TCM) has been implicated in practical use in China for thousands of years and has accumulated substantial front line experience in treating various diseases. Compared to western medicine that features defined composition and clear molecular mechanisms, TCM is consisted with complex ingredients from plants and animals and prescribed based on overall symptoms and collective experience. Because of their fundamental differences, TCM and western medicine were once considered irreconcilable. However, nowadays, sophisticated isolation technologies and deepened molecular understanding of the active ingredients of TCM are gradually bridging the gap between the two, enabling the identification of active TCM components for drug development under the western-style paradigms. Thus, studies on TCM open a new therapeutic avenue and show great potential in the combat against obesity, though challenges exist. In this review, we highlight six key candidate substances derived from TCM, including artemisinin, curcumin, celastrol, capsaicin, berberine and ginsenosides, to review their recent discoveries in the metabolic field, with special focus on their therapeutic efficacy and molecular mechanisms in treating obesity and metabolic diseases. In addition, we discuss the translational challenges and perspectives in implementing modern Chinese medicine into the western pharmaceutical industry.

Brown adipose tissue plays a central role in systemic inflammation-induced sleep responses

We previously identified brown adipose tissue (BAT) as a source of sleep-inducing signals. Pharmacological activation of BAT enhances sleep while sleep loss leads to increased BAT thermogenesis. Recovery sleep after sleep loss is diminished in mice that lack uncoupling protein 1 (UCP-1), and also in wild-type (WT) mice after sensory denervation of the BAT. Systemic inflammation greatly affects metabolism and the function of adipose tissue, and also induces characteristic sleep responses. We hypothesized that sleep responses to acute inflammation are mediated by BAT-derived signals. To test this, we determined the effects of systemic inflammation on sleep and body temperature in UCP-1 knockout (KO) and WT mice. Intraperitoneal injections of lipopolysaccharide, tumor necrosis factor-α, interleukin-1 beta and clodronate containing liposomes were used to induce systemic inflammation. In WT animals, non-rapid-eye movement sleep (NREMS) was elevated in all four inflammatory models. All NREMS responses were completely abolished in UCP-1 KO animals. Systemic inflammation elicited an initial hypothermia followed by fever in WT mice. The hypothermic phase, but not the fever, was abolished in UCP-1 KO mice. The only recognized function of UCP-1 is to promote thermogenesis in brown adipocytes. Present results indicate that the presence of UCP-1 is necessary for increased NREMS but does not contribute to the development of fever in systemic inflammation.

Reactive oxygen species induced Ca2+ influx via TRPV4 and microvascular endothelial dysfunction in the SU5416/hypoxia model of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a lethal disease characterized by elevations in pulmonary arterial pressure, in part due to formation of occlusive lesions in the distal arterioles of the lung. These complex lesions may comprise multiple cell types, including endothelial cells (ECs). To better understand the molecular mechanisms underlying EC dysfunction in PAH, lung microvascular endothelial cells (MVECs) were isolated from normoxic rats (N-MVECs) and rats subjected to SU5416 plus hypoxia (SuHx), an experimental model of PAH. Compared with N-MVECs, MVECs isolated from SuHx rats (SuHx-MVECs) appeared larger and more spindle shaped morphologically and expressed canonical smooth muscle cell markers smooth muscle-specific α-actin and myosin heavy chain in addition to endothelial markers such as Griffonia simplicifolia and von Willebrand factor. SuHx-MVEC mitochondria were dysfunctional, as evidenced by increased fragmentation/fission, decreased oxidative phosphorylation, and increased reactive oxygen species (ROS) production. Functionally, SuHx-MVECs exhibited increased basal levels of intracellular calcium concentration ([Ca2+]i) and enhanced migratory and proliferative capacity. Treatment with global (TEMPOL) or mitochondria-specific (MitoQ) antioxidants decreased ROS levels and basal [Ca2]i in SuHx-MVECs. TEMPOL and MitoQ also decreased migration and proliferation in SuHx-MVECs. Additionally, inhibition of ROS-induced Ca2+ entry via pharmacologic blockade of transient receptor potential vanilloid-4 (TRPV4) attenuated [Ca2]i, migration, and proliferation. These findings suggest a role for mitochondrial ROS-induced Ca2+ influx via TRPV4 in promoting abnormal migration and proliferation in MVECs in this PAH model.

TRPV4 activates cytosolic phospholipase A2 via Ca2+ -dependent PKC/ERK1/2 signalling in controlling hypertensive contraction

Activation of TRPV4 (transient receptor potential vanilloid 4) has been reported to result in endothelium-dependent contraction in the aortae of hypertensive mice. This contraction involved increased cPLA2 (cytosolic phospholipase A2 ) activity. The mechanism by which TRPV4 regulates cPLA2 activity to induce contraction in hypertension, however, is unknown. Through measurements of arterial tension and protein level, we showed that high-salt diet induced hypertension increases activity of PKC (protein kinase C) and ERK1/2 (extracellular signal-regulated kinase 1/2). GSK1016790A, a TRPV4 agonist and ACh (acetylcholine) induced contractions were suppressed by Go6983, a PKC inhibitor and PD98059, an ERK1/2 inhibitor. TRPV4 activation increased activity of PKC and ERK1/2 in endothelial cells from hypertensive mice and this response was suppressed by HC067047, a TRPV4 inhibitor and BAPTA/AM, a Ca2+ chelator. PLA2 assay and western blotting showed that blocking of PKC or ERK1/2 inhibited TRPV4 or ACh-induced cPLA2 activity. Enzyme immunoassay showed that GSK1016790A or ACh triggered the release of PGF2α (prostaglandin F2α ) was reduced by inhibition of PKC or ERK1/2. These data further suggest Ca2+ /PKC/ERK1/2 axis as a novel mechanism for TRPV4 in the activation of cPLA2 in hypertension.

Selective inhibition of BCL-2 is a promising target in patients with high-risk myelodysplastic syndromes and adverse mutational profile

Somatic mutations in genes such as ASXL1, RUNX1, TP53 or EZH2 adversely affect the outcome of patients with myelodysplastic syndromes (MDS). Since selective BCL-2 inhibition is a promising treatment strategy in hematologic malignancies, we tested the therapeutic impact of ABT-199 on MDS patient samples bearing an adverse mutational profile. By gene expression, we found that the level of pro-apoptotic BIM significantly decreased during MDS disease progression in line with an acquired resistance to cell death. Supporting the potential for ABT-199 treatment in MDS, high-risk MDS patient samples specifically underwent cell death in response to ABT-199 even when harbouring mutations in ASXL1, RUNX1, TP53 or EZH2. ABT-199 effectively targeted the stem- and progenitor compartment in advanced MDS harbouring mutations in ASXL1, RUNX1, TP53 or EZH2 and even proved effective in patients harbouring more than one of the defined high-risk mutations. Moreover, we utilized the protein abundance of BCL-2 family members in primary patient samples using flow cytometry as a biomarker to predict ABT-199 treatment response. Our data demonstrate that ABT-199 effectively induces apoptosis in progenitors of high-risk MDS/sAML despite the presence of adverse genetic mutations supporting the notion that pro-apoptotic intervention will hold broad therapeutic potential in high-risk MDS patients with poor prognosis.

TRPV4: a Sensor for Homeostasis and Pathological Events in the CNS

Transient receptor potential vanilloid type 4 (TRPV4) was originally described as a calcium-permeable nonselective cation channel. TRPV4 is now recognized as a polymodal ionotropic receptor: it is a broadly expressed, nonselective cation channel (permeable to calcium, potassium, magnesium, and sodium) that plays an important role in a multitude of physiological processes. TRPV4 is involved in maintaining homeostasis, serves as an osmosensor and thermosensor, can be activated directly by endogenous or exogenous chemical stimuli, and can be activated or sensitized indirectly via intracellular signaling pathways. Additionally, TRPV4 is upregulated in a variety of pathological conditions. In this review, we focus on the role of TRPV4 in mediating homeostasis and pathological events in the central nervous system (CNS). This review is composed of three parts. Section 1 describes the role of TRPV4 in maintaining homeostatic processes, including the volume of body water, ionic concentrations, volume, and the temperature. Section 2 describes the effects of activation and inhibition of TRPV4 in the CNS. Section 3 focuses on the role of TRPV4 during pathological events in CNS.

Pharmacological inhibition of TRPV4 channel suppresses malignant biological behavior of hepatocellular carcinoma via modulation of ERK signaling pathway

TRPV4 (transient receptor potential vanilloid 4), a member of the TRP superfamily, has been reported to correlate with several different forms of cancers. However, the role of TRPV4 in human hepatocellular carcinoma (HCC) remains unclear. The present study demonstrated that elevated expression of TRPV4 was shown in HCC tumor tissues when compared with paired non-tumoral livers both in protein and mRNA levels. Furthermore, the enhanced expression of TRPV4 was highly associated with histological grade (P = 0.036) and the number of tumors (P = 0.045). Pharmacological inhibition of TRPV4 channels in HCC cells with the specific antagonist HC-067047 suppressed cell proliferation, induced apoptosis and decreased the migration capability by attenuating the epithelial-mesenchymal transition (EMT) process in vitro. The p-ERK expression was apparently repressed after treatment with the TRPV4 antagonist, further blockade of the ERK pathway with U0126 could significantly aggravate HCC cells apoptosis. In NOD-SCID mouse xenograft models, intraperitoneal injection of HC-067047 could obviously suppress tumor growth and induce apoptosis in vivo. Together, our studies showed that the antitumor effects caused by TRPV4 channel inhibition in HCC cell lines might be attributed to the suppression of EMT process and inactivation of p-ERK which induced subsequent cell apoptosis. Thus, pharmacological inhibition of TRPV4 channel may be an option for HCC treatment.

Adipose morphology and metabolic disease

Adipose morphology is defined as the number and size distribution of adipocytes (fat cells) within adipose tissue. Adipose tissue with fewer but larger adipocytes is said to have a 'hypertrophic' morphology, whereas adipose with many adipocytes of a smaller size is said to have a 'hyperplastic' morphology. Hypertrophic adipose morphology is positively associated with insulin resistance, diabetes and cardiovascular disease. By contrast, hyperplastic morphology is associated with improved metabolic parameters. These phenotypic associations suggest that adipose morphology influences risk of cardiometabolic disease. Intriguingly, monozygotic twin studies have determined that adipose morphology is in part determined genetically. Therefore, identifying the genetic regulation of adipose morphology may help us to predict, prevent and ameliorate insulin resistance and associated metabolic diseases. Here, we review the current literature regarding adipose morphology in relation to: (1) metabolic and medical implications; (2) the methods used to assess adipose morphology; and (3) transcriptional differences between morphologies. We further highlight three mechanisms that have been hypothesized to promote adipocyte hypertrophy and thus to regulate adipose morphology.

Cryo-EM and X-ray structures of TRPV4 reveal insight into ion permeation and gating mechanisms

The transient receptor potential (TRP) channel TRPV4 participates in multiple biological processes, and numerous TRPV4 mutations underlie several distinct and devastating diseases. Here we present the cryo-EM structure of Xenopus tropicalis TRPV4 at 3.8-Å resolution. The ion-conduction pore contains an intracellular gate formed by the inner helices, but lacks any extracellular gate in the selectivity filter, as observed in other TRPV channels. Anomalous X-ray diffraction analyses identify a single ion-binding site in the selectivity filter, thus explaining TRPV4 nonselectivity. Structural comparisons with other TRP channels and distantly related voltage-gated cation channels reveal an unprecedented, unique packing interface between the voltage-sensor-like domain and the pore domain, suggesting distinct gating mechanisms. Moreover, our structure begins to provide mechanistic insights to the large set of pathogenic mutations, offering potential opportunities for drug development.

AMP-Activated Protein Kinase (AMPK) Regulates Energy Metabolism through Modulating Thermogenesis in Adipose Tissue

Obesity occurs when excess energy accumulates in white adipose tissue (WAT), whereas brown adipose tissue (BAT), which is specialized in dissipating energy through thermogenesis, potently counteracts obesity. White adipocytes can be converted to thermogenic "brown-like" cells (beige cells; WAT browning) under various stimuli, such as cold exposure. AMP-activated protein kinase (AMPK) is a crucial energy sensor that regulates energy metabolism in multiple tissues. However, the role of AMPK in adipose tissue function, especially in the WAT browning process, is not fully understood. To illuminate the effect of adipocyte AMPK on energy metabolism, we generated Adiponectin-Cre-driven adipose tissue-specific AMPK α1/α2 KO mice (AKO). These AKO mice were cold intolerant and their inguinal WAT displayed impaired mitochondrial integrity and biogenesis, and reduced expression of thermogenic markers upon cold exposure. High-fat-diet (HFD)-fed AKO mice exhibited increased adiposity and exacerbated hepatic steatosis and fibrosis and impaired glucose tolerance and insulin sensitivity. Meanwhile, energy expenditure and oxygen consumption were markedly decreased in the AKO mice both in basal conditions and after stimulation with a β3-adrenergic receptor agonist, CL 316,243. In contrast, we found that in HFD-fed obese mouse model, chronic AMPK activation by A-769662 protected against obesity and related metabolic dysfunction. A-769662 alleviated HFD-induced glucose intolerance and reduced body weight gain and WAT expansion. Notably, A-769662 increased energy expenditure and cold tolerance in HFD-fed mice. A-769662 treatment also induced the browning process in the inguinal fat depot of HFD-fed mice. Likewise, A-769662 enhanced thermogenesis in differentiated inguinal stromal vascular fraction (SVF) cells via AMPK signaling pathway. In summary, a lack of adipocyte AMPKα induced thermogenic impairment and obesity in response to cold and nutrient-overload, respectively, whereas chronic AMPK activation by A-769662 promoted WAT browning in inguinal WAT and protected against HFD-induced obesity and related metabolic dysfunction. These findings reveal a vital role for adipocyte AMPK in regulating the browning process in inguinal WAT and in maintaining energy homeostasis, which suggests that the targeted activation of adipocyte AMPK may be a promising strategy for anti-obesity therapy.

Transient receptor potential vanilloid 4 (TRPV4) activation by arachidonic acid requires protein kinase A-mediated phosphorylation

Transient receptor potential vanilloid 4 (TRPV4) is a Ca²⁺-permeable channel of the transient receptor potential (TRP) superfamily activated by diverse stimuli, including warm temperature, mechanical forces, and lipid mediators such as arachidonic acid (AA) and its metabolites. This activation is tightly regulated by protein phosphorylation carried out by various serine/threonine or tyrosine kinases. It remains poorly understood how phosphorylation differentially regulates TRPV4 activation in response to different stimuli. We investigated how TRPV4 activation by AA, an important signaling process in the dilation of coronary arterioles, is affected by protein kinase A (PKA)-mediated phosphorylation at Ser-824. Wildtype and mutant TRPV4 channels were expressed in human coronary artery endothelial cells (HCAECs). AA-induced TRPV4 activation was blunted in the S824A mutant but was enhanced in the phosphomimetic S824E mutant, whereas the channel activation by the synthetic agonist GSK1016790A was not affected. The low level of basal phosphorylation at Ser-824 was robustly increased by the redox signaling molecule hydrogen peroxide (H₂O₂). The H₂O₂-induced phosphorylation was accompanied by an enhanced channel activation by AA, and this enhanced response was largely abolished by PKA inhibition or S824A mutation. We further identified a potential structural context dependence of Ser-824 phosphorylation-mediated TRPV4 regulation involving an interplay between AA binding and the possible phosphorylation-induced rearrangements of the C-terminal helix bearing Ser-824. These results provide insight into how phosphorylation specifically regulates TRPV4 activation. Redox-mediated TRPV4 phosphorylation may contribute to pathologies associated with enhanced TRPV4 activity in endothelial and other systems.

Cinnamaldehyde induces fat cell-autonomous thermogenesis and metabolic reprogramming

OBJECTIVE

Cinnamaldehyde (CA) is a food compound that has previously been observed to be protective against obesity and hyperglycemia in mouse models. In this study, we aimed to elucidate the mechanisms behind this protective effect by assessing the cell-autonomous response of primary adipocytes to CA treatment.

METHODS

Primary murine adipocytes were treated with CA and thermogenic and metabolic responses were assessed after both acute and chronic treatments. Human adipose stem cells were differentiated and treated with CA to assess whether the CA-mediated signaling is conserved in humans.

RESULTS

CA significantly activated PKA signaling, increased expression levels of thermogenic genes and induced phosphorylation of HSL and PLIN1 in murine primary adipocytes. Inhibition of PKA or p38 MAPK enzymatic activity markedly inhibited the CA-induced thermogenic response. In addition, chronic CA treatment regulates metabolic reprogramming, which was partially diminished in FGF21KO adipocytes. Importantly, both acute and chronic effects of CA were observed in human adipose stem cells isolated from multiple donors of different ethnicities and ages and with a variety of body mass indexes (BMI).

CONCLUSIONS

CA activates thermogenic and metabolic responses in mouse and human primary subcutaneous adipocytes in a cell-autonomous manner, giving a mechanistic explanation for the anti-obesity effects of CA observed previously and further supporting its potential metabolic benefits on humans. Given the wide usage of cinnamon in the food industry, the notion that this popular food additive, instead of a drug, may activate thermogenesis, could ultimately lead to therapeutic strategies against obesity that are much better adhered to by participants.

MiR-29a Suppresses Spermatogenic Cell Apoptosis in Testicular Ischemia-Reperfusion Injury by Targeting TRPV4 Channels

Background: MicroRNAs (miRNAs) have emerged as gene expression regulators in the progression of ischemia-reperfusion injury (IRI). Accumulating evidences have indicated miR-29a play roles in myocardial and cerebral IRI. However, the role of miR-29a in testicular IRI has not been elucidated.

Methods: Changes in expression of miR-29a and Transient Receptor Potential Vanilloid 4 (TRPV4) in animal samples and GC-1 spermatogenic cells were examined. The effects of miR-29a on spermatogenic cell apoptosis in testicular IRI were analyzed both in vitro and in vivo.

Results: The expression of MiR-29a was negatively correlated with the expression of TRPV4 and significantly downregulated in animal samples and GC-1 cells as testicular IRI progressed. Further studies revealed TRPV4 as a downstream target of miR-29a. Inhibition of miR-29a expression increased the expression of TRPV4 and promoted spermatogenic cell apoptosis, whereas overexpression of miR-29a downregulated TRPV4 expression and suppressed spermatogenic cell apoptosis caused by testicular IRI in vitro and in vivo.

Conclusion: Our results suggest that miR-29a suppresses apoptosis induced by testicular IRI by directly targeting TRPV4.

The role of TRPV4 in fibrosis

The transient receptor potential vanilloid 4 (TRPV4) is a highly Ca²⁺-permeable non-selective cation channel in TRPV family. Accumulating evidence hints that TRPV4 play a significant role in a wide diversity of pathologic changes. Fibrosis is a kind of chronic disease which was characterized by the formation of excessive accumulation of extracellular matrix (ECM) components in tissues and organs. In recent years, a growing body of studies showed that TRPV4 acted as a crucial regulator in the progression of fibrosis including myocardial fibrosis, cystic fibrosis, pulmonary fibrosis, hepatic fibrosis and pancreatic fibrosis, suggesting TRPV4 may be a potential therapeutic vehicle in fibrotic diseases. However, the mechanisms by which TRPV4 regulates fibrosis are still undefined. In this review, firstly, we intend to sum up the collective knowledge of TRPV4. Then we provided the latent mechanism between TRPV4 and fibrosis. We also elaborated the distinct signaling pathways focus on TRPV4 with fibrosis. Finally, we discussed its potential as a novel therapeutic target for fibrosis.

The TRPC1 Ca2+-permeable channel inhibits exercise-induced protection against high-fat diet-induced obesity and type II diabetes

The transient receptor potential canonical channel-1 (TRPC1) is a Ca²⁺-permeable channel found in key metabolic organs and tissues, including the hypothalamus, adipose tissue, and skeletal muscle. Loss of TRPC1 may alter the regulation of cellular energy metabolism resulting in insulin resistance thereby leading to diabetes. Exercise reduces insulin resistance, but it is not known whether TRPC1 is involved in exercise-induced insulin sensitivity. The role of TRPC1 in adiposity and obesity-associated metabolic diseases has not yet been determined. Our results show that TRPC1 functions as a major Ca²⁺ entry channel in adipocytes. We have also shown that fat mass and fasting glucose concentrations were lower in TRPC1 KO mice that were fed a high-fat (HF) (45% fat) diet and exercised as compared with WT mice fed a HF diet and exercised. Adipocyte numbers were decreased in both subcutaneous and visceral adipose tissue of TRPC1 KO mice fed a HF diet and exercised. Finally, autophagy markers were decreased and apoptosis markers increased in TRPC1 KO mice fed a HF diet and exercised. Overall, these findings suggest that TRPC1 plays an important role in the regulation of adiposity via autophagy and apoptosis and that TRPC1 inhibits the positive effect of exercise on type II diabetes risk under a HF diet-induced obesity environment.

Dual contribution of TRPV4 antagonism in the regulatory effect of vasoinhibins on blood-retinal barrier permeability: diabetic milieu makes a difference

Breakdown of the blood-retinal barrier (BRB), as occurs in diabetic retinopathy and other chronic retinal diseases, results in vasogenic edema and neural tissue damage, causing vision loss. Vasoinhibins are N-terminal fragments of prolactin that prevent BRB breakdown during diabetes. They modulate the expression of some transient receptor potential (TRP) family members, yet their role in regulating the TRP vanilloid subtype 4 (TRPV4) remains unknown. TRPV4 is a calcium-permeable channel involved in barrier permeability, which blockade has been shown to prevent and resolve pulmonary edema. We found TRPV4 expression in the endothelium and retinal pigment epithelium (RPE) components of the BRB, and that TRPV4-selective antagonists (RN-1734 and GSK2193874) resolve BRB breakdown in diabetic rats. Using human RPE (ARPE-19) cell monolayers and endothelial cell systems, we further observed that (i) GSK2193874 does not seem to contribute to the regulation of BRB and RPE permeability by vasoinhibins under diabetic or hyperglycemic-mimicking conditions, but that (ii) vasoinhibins can block TRPV4 to maintain BRB and endothelial permeability. Our results provide important insights into the pathogenesis of diabetic retinopathy that will further guide us toward rationally-guided new therapies: synergistic combination of selective TRPV4 blockers and vasoinhibins can be proposed to mitigate diabetes-evoked BRB breakdown.

Temperature-activated ion channels in neural crest cells confer maternal fever-associated birth defects

Birth defects of the heart and face are common, and most have no known genetic cause, suggesting a role for environmental factors. Maternal fever during the first trimester is an environmental risk factor linked to these defects. Neural crest cells are precursor populations essential to the development of both at-risk tissues. We report that two heat-activated transient receptor potential (TRP) ion channels, TRPV1 and TRPV4, were present in neural crest cells during critical windows of heart and face development. TRPV1 antagonists protected against the development of hyperthermia-induced defects in chick embryos. Treatment with chemical agonists of TRPV1 or TRPV4 replicated hyperthermia-induced birth defects in chick and zebrafish embryos. To test whether transient TRPV channel permeability in neural crest cells was sufficient to induce these defects, we engineered iron-binding modifications to TRPV1 and TRPV4 that enabled remote and noninvasive activation of these channels in specific cellular locations and at specific developmental times in chick embryos with radio-frequency electromagnetic fields. Transient stimulation of radio frequency-controlled TRP channels in neural crest cells replicated fever-associated defects in developing chick embryos. Our data provide a previously undescribed mechanism for congenital defects, whereby hyperthermia activates ion channels that negatively affect fetal development.

Repairing Mitochondrial Dysfunction in Disease

Mitochondria are essential organelles for many aspects of cellular homeostasis, including energy harvesting through oxidative phosphorylation. Alterations of mitochondrial function not only impact on cellular metabolism but also critically influence whole-body metabolism, health, and life span. Diseases defined by mitochondrial dysfunction have expanded from rare monogenic disorders in a strict sense to now also include many common polygenic diseases, including metabolic, cardiovascular, neurodegenerative, and neuromuscular diseases. This has led to an intensive search for new therapeutic and preventive strategies aimed at invigorating mitochondrial function by exploiting key components of mitochondrial biogenesis, redox metabolism, dynamics, mitophagy, and the mitochondrial unfolded protein response. As such, new findings linking mitochondrial function to the progression or outcome of this ever-increasing list of diseases has stimulated the discovery and development of the first true mitochondrial drugs, which are now entering the clinic and are discussed in this review.

Cold-sensing TRPM8 channel participates in circadian control of the brown adipose tissue

Transient receptor potential (TRP) channels are known to regulate energy metabolism, and TRPM8 has become an interesting player in this context. Here we demonstrate the role of the cold sensor TRPM8 in the regulation of clock gene and clock controlled genes in brown adipose tissue (BAT). We investigated TrpM8 temporal profile in the eyes, suprachiasmatic nucleus and BAT; only BAT showed temporal variation of TrpM8 transcripts. Eyes from mice lacking TRPM8 lost the temporal profile of Per1 in LD cycle. This alteration in the ocular circadian physiology may explain the delay in the onset of locomotor activity in response to light pulse, as compared to wild type animals (WT). Brown adipocytes from TrpM8 KO mice exhibited a larger multilocularity in comparison to WT or TrpV1 KO mice. In addition, Ucp1 and UCP1 expression was significantly reduced in TrpM8 KO mice in comparison to WT mice. Regarding circadian components, the expression of Per1, Per2, Bmal1, Pparα, and Pparβ oscillated in WT mice kept in LD, whereas in the absence of TRPM8 the expression of clock genes was reduced in amplitude and lack temporal oscillation. Thus, our results reveal new roles for TRPM8 channel: it participates in the regulation of clock and clock-controlled genes in the eyes and BAT, and in BAT thermogenesis. Since disruption of the clock machinery has been associated with many metabolic disorders, the pharmacological modulation of TRPM8 channel may become a promising therapeutic target to counterbalance weight gain, through increased thermogenesis, energy expenditure, and clock gene activation.

The FGF21-CCL11 Axis Mediates Beiging of White Adipose Tissues by Coupling Sympathetic Nervous System to Type 2 Immunity

Type 2 cytokines are important signals triggering biogenesis of thermogenic beige adipocytes in white adipose tissue (WAT) during cold acclimation. However, how cold activates type 2 immunity in WAT remains obscure. Here we show that cold-induced type 2 immune responses and beiging in subcutaneous WAT (scWAT) are abrogated in mice with adipose-selective ablation of FGF21 or its co-receptor β-Klotho, whereas such impairments are reversed by replenishment with chemokine CCL11. Mechanistically, FGF21 acts on adipocytes in an autocrine manner to promote the expression and secretion of CCL11 via activation of ERK1/2, which drives recruitment of eosinophils into scWAT, leading to increases in accumulation of M2 macrophages, and proliferation and commitment of adipocyte precursors into beige adipocytes. These FGF21-elicited type 2 immune responses and beiging are blocked by CCL11 neutralization. Thus, the adipose-derived FGF21-CCL11 axis triggers cold-induced beiging and thermogenesis by coupling sympathetic nervous system to activation of type 2 immunity in scWAT.

TRPV4 calcium-permeable channel is a novel regulator of oxidized LDL-induced macrophage foam cell formation

Cardiovascular disease is the number one cause of death in United States, and atherosclerosis, a chronic inflammatory arterial disease, is the most dominant underlying pathology. Macrophages are thought to orchestrate atherosclerosis by generating lipid-laden foam cells and by secreting inflammatory mediators. Emerging data support a role for a mechanical factor, e.g., matrix stiffness, in regulation of macrophage function, vascular elasticity, and atherogenesis. However, the identity of the plasma membrane mechanosensor and the mechanisms by which pro-atherogenic signals are transduced/maintained are unknown. We have obtained evidence that TRPV4, an ion channel in the transient receptor potential vanilloid family and a known mechanosensor, is the likely mediator of oxidized low-density lipoprotein (oxLDL)-dependent macrophage foam cell formation, a critical process in atherogenesis. Specifically, we found that: i) genetic ablation of TRPV4 or pharmacologic inhibition of TRPV4 activity by a specific antagonist blocked oxLDL-induced macrophage foam cell formation, and ii) TRPV4 deficiency prevented pathophysiological range matrix stiffness or scratch-induced exacerbation of oxLDL-induced foam cell formation. Mechanistically, we found that: i) plasma membrane localization of TRPV4 was sensitized to the increasing level of matrix stiffness, ii) lack of foam cell formation in TRPV4 null cells was not due to lack of expression of CD36, a major receptor for oxLDL, and iii) TRPV4 channel activity regulated oxLDL uptake but not its binding on macrophages. Altogether, these findings identify a novel role for TRPV4 in regulating macrophage foam cell formation by modulating uptake of oxLDL. These findings suggest that therapeutic targeting of TRPV4 may provide a selective approach to the treatment of atherosclerosis.

Involvement of thermosensitive TRP channels in energy metabolism

To date, 11 thermosensitive transient receptor potential (thermo-TRP) channels have been identified. Recent studies have characterized the mechanism of thermosensing by thermo-TRPs and the physiological role of thermo-TRPs in energy metabolism. In this review, we highlight the role of various thermo-TRPs in energy metabolism and hormone secretion. In the pancreas, TRPM2 and other TRPs regulate insulin secretion. TRPV2 expressed in brown adipocytes contributes to differentiation and/or thermogenesis. Sensory nerves that express TRPV1 promote increased energy expenditure by activating sympathetic nerves and adrenaline secretion. Here, we first show that capsaicin-induced adrenaline secretion is completely impaired in TRPV1 knockout mice. The thermogenic effects of TRPV1 agonists are attributable to brown adipose tissue (BAT) activation in mice and humans. Moreover, TRPA1- and TRPM8-expressing sensory nerves also contribute to potentiation of BAT thermogenesis and energy expenditure in mice. Together, thermo-TRPs are promising targets for combating obesity and metabolic disorders.

Physiological regulation and metabolic role of browning in white adipose tissue

Great progress has been made in our understanding of the browning process in white adipose tissue (WAT) in rodents. The recognition that i) adult humans have physiologically inducible brown adipose tissue (BAT) that may facilitate resistance to obesity and ii) that adult human BAT molecularly and functionally resembles beige adipose tissue in rodents, reignited optimism that obesity and obesity-related diabetes type 2 can be battled by controlling the browning of WAT. In this review the main cellular mechanisms and molecular mediators of browning of WAT in different physiological states are summarized. The relevance of browning of WAT in metabolic health is considered primarily through a modulation of biological role of fat tissue in overall metabolic homeostasis.

Dietary menthol-induced TRPM8 activation enhances WAT "browning" and ameliorates diet-induced obesity

Beige adipocytes are a new type of recruitable brownish adipocytes, with highly mitochondrial membrane uncoupling protein 1 expression and thermogenesis. Beige adipocytes were found among white adipocytes, especially in subcutaneous white adipose tissue (sWAT). Therefore, beige adipocytes may be involved in the regulation of energy metabolism and fat deposition. Transient receptor potential melastatin 8 (TRPM8), a Ca²⁺-permeable non-selective cation channel, plays vital roles in the regulation of various cellular functions. It has been reported that TRPM8 activation enhanced the thermogenic function of brown adiposytes. However, the involvement of TRPM8 in the thermogenic function of WAT remains unexplored. Our data revealed that TRPM8 was expressed in mouse white adipocytes at mRNA, protein and functional levels. The mRNA expression of Trpm8 was significantly increased in the differentiated white adipocytes than pre-adipocytes. Moreover, activation of TRPM8 by menthol enhanced the expression of thermogenic genes in cultured white aidpocytes. And menthol-induced increases of the thermogenic genes in white adipocytes was inhibited by either KT5720 (a protein kinase A inhibitor) or BAPTA-AM. In addition, high fat diet (HFD)-induced obesity in mice was significantly recovered by co-treatment with menthol. Dietary menthol enhanced WAT “browning” and improved glucose metabolism in HFD-induced obesity mice as well. Therefore, we concluded that TRPM8 might be involved in WAT “browning” by increasing the expression levels of genes related to thermogenesis and energy metabolism. And dietary menthol could be a novel approach for combating human obesity and related metabolic diseases.

Plasticity of Performance Curves Can Buffer Reaction Rates from Body Temperature Variation in Active Endotherms

Endotherms regulate their core body temperature by adjusting metabolic heat production and insulation. Endothermic body temperatures are therefore relatively stable compared to external temperatures. The thermal sensitivity of biochemical reaction rates is thought to have co-evolved with body temperature regulation so that optimal reaction rates occur at the regulated body temperature. However, recent data show that core body temperatures even of non-torpid endotherms fluctuate considerably. Additionally, peripheral temperatures can be considerably lower and more variable than core body temperatures. Here we discuss whether published data support the hypothesis that thermal performance curves of physiological reaction rates are plastic so that performance is maintained despite variable body temperatures within active (non-torpid) endotherms, and we explore mechanisms that confer plasticity. There is evidence that thermal performance curves in tissues that experience thermal fluctuations can be plastic, although this question remains relatively unexplored for endotherms. Mechanisms that alter thermal responses locally at the tissue level include transient potential receptor ion channels (TRPV and TRPM) and the AMP-activated protein kinase (AMPK) both of which can influence metabolism and energy expenditure. Additionally, the thermal sensitivity of processes that cause post-transcriptional RNA degradation can promote the relative expression of cold-responsive genes. Endotherms can respond to environmental fluctuations similarly to ectotherms, and thermal plasticity complements core body temperature regulation to increase whole-organism performance. Thermal plasticity is ancestral to endothermic thermoregulation, but it has not lost its selective advantage so that modern endotherms are a physiological composite of ancestral ectothermic and derived endothermic traits.

Browning deficiency and low mobilization of fatty acids in gonadal white adipose tissue leads to decreased cold-tolerance of transglutaminase 2 knock-out mice

During cold-exposure 'beige' adipocytes with increased mitochondrial content are activated in white adipose tissue (WAT). These cells, similarly to brown adipose tissue (BAT), dissipate stored chemical energy in the form of heat with the help of uncoupling protein 1 (UCP1). We investigated the effect of tissue transglutaminase (TG2) ablation on the function of ATs in mice. Although TG2^+/+ and TG2^-/- mice had the same amount of WAT and BAT, we found that TG2^+/+ animals could tolerate acute cold exposure for 4h, whereas TG2^-/- mice only for 3h. Both TG2^-/- and TG2^+/+ animals used up half of the triacylglycerol content of subcutaneous WAT (SCAT) after 3h treatment; however, TG2^-/- mice still possessed markedly whiter and higher amount of gonadal WAT (GONAT) as reflected in the larger size of adipocytes and lower free fatty acid levels in serum. Furthermore, lower expression of 'beige' marker genes such as UCP1, TBX1 and TNFRFS9 was observed after cold exposure in GONAT of TG2^-/- mice, paralleled with a lower level of UCP1 protein and a decreased mitochondrial content. The detected changes in gene expression of Resistin and Adiponectin did not provoke glucose intolerance in the investigated TG2^-/- mice, and TG2 deletion did not influence adrenaline, noradrenaline, glucagon and insulin production. Our data suggest that TG2 has a tissue-specific role in GONAT function and browning, which becomes apparent under acute cold exposure.