Reconstructing human brown fat developmental trajectory in vitro.

Brown adipocytes (BAs) represent a specialized cell type that is able to uncouple nutrient catabolism from ATP generation to dissipate energy as heat. In humans, the brown fat tissue is composed of discrete depots found throughout the neck and trunk region. BAs originate from a precursor common to skeletal muscle, but their developmental trajectory remains poorly understood. Here, we used single-cell RNA sequencing to characterize the development of interscapular brown fat in mice. Our analysis identified a transient stage of BA differentiation characterized by the expression of the transcription factor GATA6. We show that recapitulating the sequence of signaling cues identified in mice can lead to efficient differentiation of BAs in vitro from human pluripotent stem cells. These precursors can in turn be efficiently converted into functional BAs that can respond to signals mimicking adrenergic stimuli by increasing their metabolism, resulting in heat production.

MuscleJ2: a rebuilding of MuscleJ with new features for high-content analysis of skeletal muscle immunofluorescence slides.

Histological analysis of skeletal muscle is of major interest for understanding its behavior in different pathophysiological conditions, such as the response to different environments or myopathies. In this context, many software programs have been developed to perform automated high-content analysis. We created MuscleJ, a macro that runs in ImageJ/Fiji on batches of images. MuscleJ is a multianalysis tool that initially allows the analysis of muscle fibers, capillaries, and satellite cells. Since its creation, it has been used in many studies, and we have further developed the software and added new features, which are presented in this article. We converted the macro into a Java-language plugin with an improved user interface. MuscleJ2 provides quantitative analysis of fibrosis, vascularization, and cell phenotype in whole muscle sections. It also performs analysis of the peri-myonuclei, the individual capillaries, and any staining in the muscle fibers, providing accurate quantification within regional sublocalizations of the fiber. A multicartography option allows users to visualize multiple results simultaneously. The plugin is freely available to the muscle science community.

NR1D1 controls skeletal muscle calcium homeostasis through myoregulin repression.

The sarcoplasmic reticulum (SR) plays an important role in calcium homeostasis. SR calcium mishandling is described in pathological conditions, such as myopathies. Here, we investigated whether the nuclear receptor subfamily 1 group D member (NR1D1, also called REV-ERBα) regulates skeletal muscle SR calcium homeostasis. Our data demonstrate that NR1D1 deficiency in mice impaired sarco/endoplasmic reticulum calcium ATPase-dependent (SERCA-dependent) SR calcium uptake. NR1D1 acts on calcium homeostasis by repressing the SERCA inhibitor myoregulin through direct binding to its promoter. Restoration of myoregulin counteracted the effects of NR1D1 overexpression on SR calcium content. Interestingly, myoblasts from patients with Duchenne muscular dystrophy displayed lower NR1D1 expression, whereas pharmacological NR1D1 activation ameliorated SR calcium homeostasis and improved muscle structure and function in dystrophic mdx/Utr+/- mice. Our findings demonstrate that NR1D1 regulates muscle SR calcium homeostasis, pointing to its therapeutic potential for mitigating myopathy.

Uncovering the Role of Glycogen in Brown Adipose Tissue.

The presence of glycogen in the brown adipose tissue (BAT) has been described 60 years ago. However, the role of this energetic storage in brown adipocytes has been long time underestimated. We have recently shown that during brown adipocyte differentiation in the embryo, glycogen accumulates and is degraded by glycophagy, a dynamic essential for lipid droplets biogenesis. Recent studies have shown that the storage and degradation of triglycerides in BAT are not essential for the activation of BAT in response to cold exposure in adults, and that glycogen can compensate for their absence. In this review, we report the recent advances related to the importance of glycogen in brown adipocytes.

Influenza infection rewires energy metabolism and induces browning features in adipose cells and tissues.

Like all obligate intracellular pathogens, influenza A virus (IAV) reprograms host cell's glucose and lipid metabolism to promote its own replication. However, the impact of influenza infection on white adipose tissue (WAT), a key tissue in the control of systemic energy homeostasis, has not been yet characterized. Here, we show that influenza infection induces alterations in whole-body glucose metabolism that persist long after the virus has been cleared. We report depot-specific changes in the WAT of IAV-infected mice, notably characterized by the appearance of thermogenic brown-like adipocytes within the subcutaneous fat depot. Importantly, viral RNA- and viral antigen-harboring cells are detected in the WAT of infected mice. Using in vitro approaches, we find that IAV infection enhances the expression of brown-adipogenesis-related genes in preadipocytes. Overall, our findings shed light on the role that the white adipose tissue, which lies at the crossroads of nutrition, metabolism and immunity, may play in influenza infection.

[The muscle biological clock]. / L'horloge biologique du muscle.

The biological clock plays an essential role in the control of muscle activity, by dissociating temporally the metabolic functions of skeletal muscle. Exercise capacity also displays a circadian rhythm. Alterations in biological rhythm, as in shift workers, alter muscle function and are associated with the development of sarcopenia.

TITLE: L'horloge biologique du muscle. ABSTRACT: L'horloge biologique joue un rôle essentiel dans le contrôle de l'activité musculaire, en dissociant temporellement les fonctions métaboliques du muscle squelettique. Les capacités musculaires en réponse à l'exercice sont également circadiennes. Des perturbations des rythmes biologiques, telles que celles retrouvées chez les travailleurs postés affectent la fonction musculaire et sont associées au développement de la sarcopénie.

Glycogen Dynamics Drives Lipid Droplet Biogenesis during Brown Adipocyte Differentiation.

Browning induction or transplantation of brown adipose tissue (BAT) or brown/beige adipocytes derived from progenitor or induced pluripotent stem cells (iPSCs) can represent a powerful strategy to treat metabolic diseases. However, our poor understanding of the mechanisms that govern the differentiation and activation of brown adipocytes limits the development of such therapy. Various genetic factors controlling the differentiation of brown adipocytes have been identified, although most studies have been performed using in vitro cultured pre-adipocytes. We investigate here the differentiation of brown adipocytes from adipose progenitors in the mouse embryo. We demonstrate that the formation of multiple lipid droplets (LDs) is initiated within clusters of glycogen, which is degraded through glycophagy to provide the metabolic substrates essential for de novo lipogenesis and LD formation. Therefore, this study uncovers the role of glycogen in the generation of LDs.

MuscleJ: a high-content analysis method to study skeletal muscle with a new Fiji tool.

BACKGROUND: Skeletal muscle has the capacity to adapt to environmental changes and regenerate upon injury. To study these processes, most experimental methods use quantification of parameters obtained from images of immunostained skeletal muscle. Muscle cross-sectional area, fiber typing, localization of nuclei within the muscle fiber, the number of vessels, and fiber-associated stem cells are used to assess muscle physiology. Manual quantification of these parameters is time consuming and only poorly reproducible. While current state-of-the-art software tools are unable to analyze all these parameters simultaneously, we have developed MuscleJ, a new bioinformatics tool to do so. METHODS: Running on the popular open source Fiji software platform, MuscleJ simultaneously analyzes parameters from immunofluorescent staining, imaged by different acquisition systems in a completely automated manner. RESULTS: After segmentation of muscle fibers, up to three other channels can be analyzed simultaneously. Dialog boxes make MuscleJ easy-to-use for biologists. In addition, we have implemented color in situ cartographies of results, allowing the user to directly visualize results on reconstituted muscle sections. CONCLUSION: We report here that MuscleJ results were comparable to manual observations made by five experts. MuscleJ markedly enhances statistical analysis by allowing reliable comparison of skeletal muscle physiology-pathology results obtained from different laboratories using different acquisition systems. Providing fast robust multi-parameter analyses of skeletal muscle physiology-pathology, MuscleJ is available as a free tool for the skeletal muscle community.

Endospanin-2 enhances skeletal muscle energy metabolism and running endurance capacity.

Metabolic stresses such as dietary energy restriction or physical activity exert beneficial metabolic effects. In the liver, endospanin-1 and endospanin-2 cooperatively modulate calorie restriction-mediated (CR-mediated) liver adaptations by controlling growth hormone sensitivity. Since we found CR to induce endospanin protein expression in skeletal muscle, we investigated their role in this tissue. In vivo and in vitro endospanin-2 triggers ERK phosphorylation in skeletal muscle through an autophagy-dependent pathway. Furthermore, endospanin-2, but not endospanin-1, overexpression decreases muscle mitochondrial ROS production, induces fast-to-slow fiber-type switch, increases skeletal muscle glycogen content, and improves glucose homeostasis, ultimately promoting running endurance capacity. In line, endospanin-2-/- mice display higher lipid peroxidation levels, increased mitochondrial ROS production under mitochondrial stress, decreased ERK phosphorylation, and reduced endurance capacity. In conclusion, our results identify endospanin-2 as a potentially novel player in skeletal muscle metabolism, plasticity, and function.

Nuclear Receptor Subfamily 1 Group D Member 1 Regulates Circadian Activity of NLRP3 Inflammasome to Reduce the Severity of Fulminant Hepatitis in Mice.

BACKGROUND & AIMS: The innate immune system responds not only to bacterial signals, but also to non-infectious danger-associated molecular patterns that activate the NLRP3 inflammasome complex after tissue injury. Immune functions vary over the course of the day, but it is not clear whether these changes affect the activity of the NLRP3 inflammasome. We investigated whether the core clock component nuclear receptor subfamily 1 group D member 1 (NR1D1, also called Rev-erbα) regulates expression, activity of the NLRP3 inflammasome, and its signaling pathway. METHODS: We collected naïve peritoneal macrophages and plasma, at multiple times of day, from Nr1d1-/- mice and their Nr1d1+/+ littermates (controls) and analyzed expression NLRP3, interleukin 1ß (IL1B, in plasma), and IL18 (in plasma). We also collected bone marrow-derived primary macrophages from these mice. Levels of NR1D1 were knocked down with small hairpin RNAs in human primary macrophages. Bone marrow-derived primary macrophages from mice and human primary macrophages were incubated with lipopolysaccharide (LPS) to induce expression of NLRP3, IL1B, and IL18; cells were incubated with LPS and adenosine triphosphate to activate the NLRP3 complex. We analyzed caspase 1 activity and cytokine secretion. NR1D1 was activated in primary mouse and human macrophages by incubation with SR9009; some of the cells were also incubated with an NLRP3 inhibitor or inhibitors of caspase 1. Nr1d1-/- mice and control mice were given intraperitoneal injections of LPS to induce peritoneal inflammation; plasma samples were isolated and levels of cytokines were measured. Nr1d1-/- mice, control mice, and control mice given injections of SR9009 were given LPS and D-galactosamine to induce fulminant hepatitis and MCC950 to specifically inhibit NLRP3; plasma was collected to measure cytokines and a marker of liver failure (alanine aminotransferase); liver tissues were collected and analyzed by quantitative polymerase chain reaction, immunohistochemistry, and flow cytometry. RESULTS: In peritoneal macrophages, expression of NLRP3 and activation of its complex varied with time of day (circadian rhythm)-this regulation required NR1D1. Primary macrophages from Nr1d1-/- mice and human macrophages with knockdown of NR1D1 had altered expression patterns of NLRP3, compared to macrophages that expressed NR1D1, and altered patterns of IL1B and 1L18 production. Mice with disruption of Nr1d1 developed more-severe acute peritoneal inflammation and fulminant hepatitis than control mice. Incubation of macrophage with the NR1D1 activator SR9009 reduced expression of NLRP3 and secretion of cytokines. Mice given SR9009 developed less-severe liver failure and had longer survival times than mice given saline (control). CONCLUSIONS: In studies of Nr1d1-/- mice and human macrophages with pharmacologic activation of NR1D1, we found NR1D1 to regulate the timing of NLRP3 expression and production of inflammatory cytokines by macrophages. Activation of NR1D1 reduced the severity of peritoneal inflammation and fulminant hepatitis in mice.

Rev-erb-α regulates atrophy-related genes to control skeletal muscle mass.

The nuclear receptor Rev-erb-α modulates hepatic lipid and glucose metabolism, adipogenesis and thermogenesis. We have previously demonstrated that Rev-erb-α is also an important regulator of skeletal muscle mitochondrial biogenesis and function, and autophagy. As such, Rev-erb-α over-expression in skeletal muscle or its pharmacological activation improved mitochondrial respiration and enhanced exercise capacity. Here, in gain- and loss-of function studies, we show that Rev-erb-α also controls muscle mass. Rev-erb-α-deficiency in skeletal muscle leads to increased expression of the atrophy-related genes (atrogenes), associated with reduced muscle mass and decreased fiber size. By contrast, in vivo and in vitro Rev-erb-α over-expression results in reduced atrogenes expression and increased fiber size. Finally, Rev-erb-α pharmacological activation blocks dexamethasone-induced upregulation of atrogenes and muscle atrophy. This study identifies Rev-erb-α as a promising pharmacological target to preserve muscle mass.

Circadian control of metabolism and pathological consequences of clock perturbations.

Most organisms have developed an autonomous time-keeping system that generates self-sustained daily fluctuations in behavior and physiological processes. These biological clocks are reset every day by light to adjust physiology to the day/night cycle generated by the rotation of the Earth. Clocks present in organs involved in glucose and lipid metabolism such as the liver, muscle, adipose tissue and pancreas are also reset by feeding cues which permits the local integration of systemic and nutritional signals to switch fuel production and utilization according to the feeding/fasting cycle. However, derangements in this finely tuned system can be induced by extended light exposure, 24/7 food availability and altered food intake patterns, repeated jet-lag and shift-working, promoting metabolic imbalances ranging from body weight gain to the development of insulin resistance and liver diseases. Here, we review recent findings on the link between the clock and metabolic fluxes to maintain whole-body homeostasis, and what clock disruption in mice has revealed about the role of the clock in metabolic regulation.

Endothelial cell specification in the somite is compromised in Pax3-positive progenitors of Foxc1/2 conditional mutants, with loss of forelimb myogenesis.

Pax3 and Foxc2 have been shown genetically to mutually repress each other in the mouse somite. Perturbation of this balance in multipotent cells of the dermomyotome influences cell fate; upregulation of Foxc2 favours a vascular fate, whereas higher levels of Pax3 lead to myogenesis. Foxc1 has overlapping functions with Foxc2. In Foxc1/2 double-mutant embryos, somitogenesis is severely affected, precluding analysis of somite derivatives. We have adopted a conditional approach whereby mutations in Foxc1 and Foxc2 genes were targeted to Pax3-expressing cells. Inclusion of a conditional reporter allele in the crosses made it possible to follow cells that had expressed Pax3. At the forelimb level, endothelial and myogenic cells migrate from adjacent somites into the limb bud. This population of endothelial cells is compromised in the double mutant, whereas excessive production of myogenic cells is observed in the trunk. However, strikingly, myogenic progenitors fail to enter the limbs, leading to the absence of skeletal muscle. Pax3-positive migratory myogenic progenitors, marked by expression of Lbx1, are specified in the somite at forelimb level, but endothelial progenitors are absent. The myogenic progenitors do not die, but differentiate prematurely adjacent to the somite. We conclude that the small proportion of somite-derived endothelial cells in the limb is required for the migration of myogenic limb progenitors.

Notch regulation of myogenic versus endothelial fates of cells that migrate from the somite to the limb.

Multipotent Pax3-positive (Pax3(+)) cells in the somites give rise to skeletal muscle and to cells of the vasculature. We had previously proposed that this cell-fate choice depends on the equilibrium between Pax3 and Foxc2 expression. In this study, we report that the Notch pathway promotes vascular versus skeletal muscle cell fates. Overactivating the Notch pathway specifically in Pax3(+) progenitors, via a conditional Pax3(NICD) allele, results in an increase of the number of smooth muscle and endothelial cells contributing to the aorta. At limb level, Pax3(+) cells in the somite give rise to skeletal muscles and to a subpopulation of endothelial cells in blood vessels of the limb. We now demonstrate that in addition to the inhibitory role of Notch signaling on skeletal muscle cell differentiation, the Notch pathway affects the Pax3:Foxc2 balance and promotes the endothelial versus myogenic cell fate, before migration to the limb, in multipotent Pax3(+) cells in the somite of the mouse embryo.

Prdm1 functions in the mesoderm of the second heart field, where it interacts genetically with Tbx1, during outflow tract morphogenesis in the mouse embryo.

Congenital heart defects affect at least 0.8% of newborn children and are a major cause of lethality prior to birth. Malformations of the arterial pole are particularly frequent. The myocardium at the base of the pulmonary trunk and aorta and the arterial tree associated with these great arteries are derived from splanchnic mesoderm of the second heart field (SHF), an important source of cardiac progenitor cells. These cells are controlled by a gene regulatory network that includes Fgf8, Fgf10 and Tbx1. Prdm1 encodes a transcriptional repressor that we show is also expressed in the SHF. In mouse embryos, mutation of Prdm1 affects branchial arch development and leads to persistent truncus arteriosus (PTA), indicative of neural crest dysfunction. Using conditional mutants, we show that this is not due to a direct function of Prdm1 in neural crest cells. Mutation of Prdm1 in the SHF does not result in PTA, but leads to arterial pole defects, characterized by mis-alignment or reduction of the aorta and pulmonary trunk, and abnormalities in the arterial tree, defects that are preceded by a reduction in outflow tract size and loss of caudal pharyngeal arch arteries. These defects are associated with a reduction in proliferation of progenitor cells in the SHF. We have investigated genetic interactions with Fgf8 and Tbx1, and show that on a Tbx1 heterozygote background, conditional Prdm1 mutants have more pronounced arterial pole defects, now including PTA. Our results identify PRDM1 as a potential modifier of phenotypic severity in TBX1 haploinsufficient DiGeorge syndrome patients.

Itm2a is a Pax3 target gene, expressed at sites of skeletal muscle formation in vivo.

The paired-box homeodomain transcription factor Pax3 is a key regulator of the nervous system, neural crest and skeletal muscle development. Despite the important role of this transcription factor, very few direct target genes have been characterized. We show that Itm2a, which encodes a type 2 transmembrane protein, is a direct Pax3 target in vivo, by combining genetic approaches and in vivo chromatin immunoprecipitation assays. We have generated a conditional mutant allele for Itm2a, which is an imprinted gene, by flanking exons 2-4 with loxP sites and inserting an IRESnLacZ reporter in the 3' UTR of the gene. The LacZ reporter reproduces the expression profile of Itm2a, and allowed us to further characterize its expression at sites of myogenesis, in the dermomyotome and myotome of somites, and in limb buds, in the mouse embryo. We further show that Itm2a is not only expressed in adult muscle fibres but also in the satellite cells responsible for regeneration. Itm2a mutant mice are viable and fertile with no overt phenotype during skeletal muscle formation or regeneration. Potential compensatory mechanisms are discussed.