Dynamics of transcriptome and chromatin accessibility revealed sequential regulation of potential transcription factors during the brown adipose tissue whitening in rabbits

Brown adipose tissue (BAT) represents a valuable target for treating obesity in humans. BAT losses of thermogenic capacity and gains a “white adipose tissue-like (WAT-like)” phenotype (BAT whitening) under thermoneutral environments, which could lead to potential low therapy responsiveness in BAT-based obesity treatments. However, the epigenetic mechanisms of BAT whitening remain largely unknown. In this study, BATs were collected from rabbits at day0 (D0), D15, D85, and 2 years (Y2). RNA-sequencing (RNA-seq) and the assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) were performed to investigate transcriptome and chromatin accessibility of BATs at the four whitening stages, respectively. Our data showed that many genes and chromatin accessible regions (refer to as “peaks”) were identified as significantly changed during BAT whitening in rabbits. The BAT-selective genes downregulated while WAT-selective genes upregulated from D0 to Y2, and the de novo lipogenesis-related genes reached the highest expression levels at D85. Both the highly expressed genes and accessible regions in Y2 were significantly enriched in immune response-related signal pathways. Analysis of different relationships between peaks and their nearby genes found an increased proportion of the synchronous changes between chromatin accessibility and gene expression during BAT whitening. The synergistic changes between the chromatin accessibility of promoter and the gene expression were found in the key adipose genes. The upregulated genes which contained increased peaks were significantly enriched in the PI3K-Akt signaling pathway, steroid biosynthesis, TGF-beta signaling pathway, osteoclast differentiation, and dilated cardiomyopathy. Moreover, the footprinting analysis suggested that sequential regulation of potential transcription factors (TFs) mediated the loss of thermogenic phenotype and the gain of a WAT-like phenotype of BAT. In conclusion, our study provided the transcriptional and epigenetic frameworks for understanding BAT whitening in rabbits for the first time and might facilitate potential insights into BAT-based obesity treatments.

Endocrinology of thermoregulation in birds in a changing climate

The ability to maintain a (relatively) stable body temperature in a wide range of thermal environments by use of endogenous heat production is a unique feature of endotherms such as birds. Endothermy is acquired and regulated via various endocrine and molecular pathways, and ultimately allows wide aerial, aquatic, and terrestrial distribution in variable environments. However, due to our changing climate, birds are faced with potential new challenges for thermoregulation, such as more frequent extreme weather events, lower predictability of climate, and increasing mean temperature. We provide an overview on thermoregulation in birds and its endocrine and molecular mechanisms, pinpointing gaps in current knowledge and recent developments, focusing especially on non-model species to understand the generality of, and variation in, mechanisms. We highlight plasticity of thermoregulation and underlying endocrine regulation, because thorough understanding of plasticity is key to predicting responses to changing environmental conditions. To this end, we discuss how changing climate is likely to affect avian thermoregulation and associated endocrine traits, and how the interplay between these physiological processes may play a role in facilitating or constraining adaptation to a changing climate. We conclude that while the general patterns of endocrine regulation of thermogenesis are quite well understood, at least in poultry, the molecular and endocrine mechanisms that regulate, e.g. mitochondrial function and plasticity of thermoregulation over different time scales (from transgenerational to daily variation), need to be unveiled. Plasticity may ameliorate climate change effects on thermoregulation to some extent, but the increased frequency of extreme weather events, and associated changes in resource availability, may be beyond the scope and/or speed for plastic responses. This could lead to selection for more tolerant phenotypes, if the underlying physiological traits harbour genetic and individual variation for selection to act on - a key question for future research.

Thyroid hormones and the mechanisms of adaptation to cold

The thyroid gland plays a crucial role in the regulation of metabolism, oxygen consumption, and the release of energy in the form of heat to maintain the body. Even at rest, these processes are sensitive to changes in thyroid function. This means that along with the adrenergic system, thyroid function determines the organism's ability to adapt to cold. Cold adaptation causes deiodination of thyroxine (T₄) and thus promotes an increase in blood triiodothyronine (T₃) levels in humans and animals. Triiodothyronine is an inductor of iodothyronine deiodinase expression in brown fat, liver, and kidney. Iodothyronine deiodinase plays an important role in adaptation of the organism to cold by contributing to high adrenergic reactivity of brown fat. T₃ also leads to an increase in expression of uncoupling proteins and uncoupling oxidative phosphorylation and an increase in heat production. The aim of this article is to review the available literature regarding the role of thyroid hormones in adaptation to cold and to present the current knowledge of the understanding of the molecular mechanism underlying their action during cold adaptation.

Oxidative Stress Produced by Hyperthyroidism Status Induces the Antioxidant Enzyme Transcription through the Activation of the Nrf-2 Factor in Lymphoid Tissues of Balb/c Mice

Hyperthyroidism is an endocrine disorder characterized by excessive secretion of thyroid hormones T3 and T4. Thyroid hormones exert pleiotropic actions on numerous tissues and induce an overall increase in metabolism, with an increase in energy demand and oxygen consumption. Therefore, the purpose of this study was to investigate the effects of hyperthyroidism on the production of reactive oxygen species (ROS) in lymph node and spleen cells of euthyroid and hyperthyroid mice, analyzing antioxidant mechanisms involved in the restitution of the cellular redox state. For this, thirty female Balb/c (H-2d) mice were randomly divided into two groups: euthyroid (by treatment with placebo) and hyperthyroid (by treatment with 12 mg/l of T4 in drinking water for 30 days). We found a significant increase in ROS and an increase in the genomic and protein expression of the antioxidant enzymes catalase (CAT) and glutathione peroxidase-1 (GPx-1) in lymph node and spleen cells of hyperthyroid mice. In vitro treatment with H₂O₂ (250 μM) of the lymphoid cells of euthyroid mice increased the expression levels of CAT and GPx-1. The hyperthyroidism increased the phosphorylation levels of Nrf2 (nuclear factor erythroid 2-related factor) and the kinase activity of protein kinase C (PKC) and extracellular signal-regulated kinase (ERK). Additionally, we found an increase in the expression of the classic isoenzymes of PKCα, β and γ. In conclusion, these results indicated that the increase in ROS found in the hyperthyroid state induces the antioxidant enzyme transcription through the activation of the Nrf-2 factor in lymphoid tissues. This shows the influence of hyperthyroidism on the regulation of the cellular antioxidant system.

Thyroid Hormone Action: The p43 Mitochondrial Pathway

The possibility that several pathways are involved in the multiplicity of thyroid hormone physiological influences led to searches for the occurrence of T3 extra nuclear receptors. The existence of a direct T3 mitochondrial pathway is now well established. The demonstration that TRα1 mRNA encodes not only a nuclear thyroid hormone receptor but also two proteins imported into mitochondria with molecular masses of 43 and 28 kDa has provided new clues to understand the pleiotropic influence of iodinated hormones.The use of a T3 photo affinity label derivative (T3-PAL) allowed detecting two mitochondrial T3 binding proteins. In association with western blots using antibodies raised against the T3 nuclear receptor TRα1, mitochondrial T3 receptors were identified as truncated TRα1 forms. Import and in organello transcription experiments performed in isolated mitochondria led to the conclusion that p43 is a transcription factor of the mitochondrial genome, inducing changes in the mitochondrial/nuclear crosstalk. In vitro experiments indicated that this T3 mitochondrial pathway affects cell differentiation, apoptosis, and transformation. Generation of transgenic mice demonstrated the involvement of this mitochondrial pathway in the determination of muscle phenotype, glucose metabolism, and thermogenesis.

Mitochondrial T3 receptor and targets

The demonstration that TRα1 mRNA encodes a nuclear thyroid hormone receptor and two proteins imported into mitochondria with molecular masses of 43 and 28 kDa has brought new clues to better understand the pleiotropic influence of iodinated hormones. If p28 activity remains unknown, p43 binds to T3 responsive elements occurring in the organelle genome, and, in the T3 presence, stimulates mitochondrial transcription and the subsequent synthesis of mitochondrial encoded proteins. This influence increases mitochondrial activity and through changes in the mitochondrial/nuclear cross talk affects important nuclear target genes regulating cell proliferation and differentiation, oncogenesis, or apoptosis. In addition, this pathway influences muscle metabolic and contractile phenotype, as well as glycaemia regulation. Interestingly, according to the process considered, p43 exerts opposite or cooperative effects with the well-known T3 pathway, thus allowing a fine tuning of the physiological influence of this hormone.