The Importance of Peripheral Nerves in Adipose Tissue for the Regulation of Energy Balance

The nervous system and adipose tissues: a tale of dialogues

White, beige, and brown adipose tissues play a crucial role in maintaining energy homeostasis. Due to the heterogeneous and diffuse nature of fat pads, this balance requires a fine and coordinated control of many actors and therefore permanent dialogues between these tissues and the central nervous system. For about two decades, many studies have been devoted to describe the neuro-anatomical and functional complexity involved to ensure this dialogue. Thus, if it is now clearly demonstrated that there is an efferent sympathetic innervation of different fat depots controlling plasticity as well as metabolic functions of the fat pad, the crucial role of sensory innervation capable of detecting local signals informing the central nervous system of the metabolic state of the relevant pads is much more recent. The purpose of this review is to provide the current state of knowledge on this subject.

High-intensity interval training improves fatty infiltration in the rotator cuff through the β3 adrenergic receptor in mice

Aims

Rotator cuff muscle atrophy and fatty infiltration affect the clinical outcomes of rotator cuff tear patients. However, there is no effective treatment for fatty infiltration at this time. High-intensity interval training (HIIT) helps to activate beige adipose tissue. The goal of this study was to test the role of HIIT in improving muscle quality in a rotator cuff tear model via the β3 adrenergic receptor (β3AR).

Methods

Three-month-old C57BL/6 J mice underwent a unilateral rotator cuff injury procedure. Mice were forced to run on a treadmill with the HIIT programme during the first to sixth weeks or seventh to 12th weeks after tendon tear surgery. To study the role of β3AR, SR59230A, a selective β3AR antagonist, was administered to mice ten minutes before each exercise through intraperitoneal injection. Supraspinatus muscle, interscapular brown fat, and inguinal subcutaneous white fat were harvested at the end of the 12th week after tendon tear and analyzed biomechanically, histologically, and biochemically.

Results

Histological analysis of supraspinatus muscle showed that HIIT improved muscle atrophy, fatty infiltration, and contractile force compared to the no exercise group. In the HIIT groups, supraspinatus muscle, interscapular brown fat, and inguinal subcutaneous white fat showed increased expression of tyrosine hydroxylase and uncoupling protein 1, and upregulated the β3AR thermogenesis pathway. However, the effect of HIIT was not present in mice injected with SR59230A, suggesting that HIIT affected muscles via β3AR.

Conclusion

HIIT improved supraspinatus muscle quality and function after rotator cuff tears by activating systemic sympathetic nerve fibre near adipocytes and β3AR.

Appetite, Energy Expenditure, and the Regulation of Energy Balance

At usual weight, energy intake and expenditure are coupled and covary to maintain body weight (energy stores). A change in energy balance, especially weight loss, invokes discoordinated effects on energy intake and output that favor return to previous weight. These regulatory systems reflect physiological changes in systems regulating energy intake and expenditure rather than a lack of resolve. The biological and behavioral physiology of dynamic weight change are distinct from those of attempts at static weight maintenance of an altered body weight. This suggests that optimal therapeutic approaches to losing or gaining vs. sustaining weight changes are different for most individuals.

Somatosensory innervation of adipose tissues

The increasing prevalence of obesity and type 2 diabetes has led to a greater interest in adipose tissue physiology. Adipose tissue is now understood as an organ with endocrine and thermogenic capacities in addition to its role in fat storage. It plays a critical role in systemic metabolism and energy regulation, and its activity is tightly regulated by the nervous system. Fat is now recognized to receive sympathetic innervation, which transmits information from the brain, as well as sensory innervation, which sends information into the brain. The role of sympathetic innervation in adipose tissue has been extensively studied. However, the extent and the functional significance of sensory innervation have long been unclear. Recent studies have started to reveal that sensory neurons robustly innervate adipose tissue and play an important role in regulating fat activity. This brief review will discuss both historical evidence and recent advances, as well as important remaining questions about the sensory innervation of adipose tissue.

Schwann cells contribute to demyelinating diabetic neuropathy and nerve terminal structures in white adipose tissue

Peripheral neuropathy, which can include axonal degeneration and/or demyelination, impacts adipose tissues with obesity, diabetes, and aging. However, the presence of demyelinating neuropathy had not yet been explored in adipose. Both demyelinating neuropathies and axonopathies implicate Schwann cells (SCs), a glial support cell that myelinates axons and contributes to nerve regeneration after injury. We performed a comprehensive assessment of SCs and myelination patterns of subcutaneous white adipose tissue (scWAT) nerves, and changes across altered energy balance states. We found that mouse scWAT contains both myelinated and unmyelinated nerves and is populated by SCs, including SCs that were associated with synaptic vesicle-containing nerve terminals. BTBR ob/ob mice, a model of diabetic peripheral neuropathy, exhibited small fiber demyelinating neuropathy and alterations in SC marker gene expression in adipose that were similar to obese human adipose. These data indicate that adipose SCs regulate the plasticity of tissue nerves and become dysregulated in diabetes.

Deep Proteome Profiling of White Adipose Tissue Reveals Marked Conservation and Distinct Features Between Different Anatomical Depots

White adipose tissue is deposited mainly as subcutaneous adipose tissue (SAT), often associated with metabolic protection, and abdominal/visceral adipose tissue, which contributes to metabolic disease. To investigate the molecular underpinnings of these differences, we conducted comprehensive proteomics profiling of whole tissue and isolated adipocytes from these two depots across two diets from C57Bl/6J mice. The adipocyte proteomes from lean mice were highly conserved between depots, with the major depot-specific differences encoded by just 3% of the proteome. Adipocytes from SAT (SAdi) were enriched in pathways related to mitochondrial complex I and beiging, whereas visceral adipocytes (VAdi) were enriched in structural proteins and positive regulators of mTOR presumably to promote nutrient storage and cellular expansion. This indicates that SAdi are geared toward higher catabolic activity, while VAdi are more suited for lipid storage. By comparing adipocytes from mice fed chow or Western diet (WD), we define a core adaptive proteomics signature consisting of increased extracellular matrix proteins and decreased fatty acid metabolism and mitochondrial Coenzyme Q biosynthesis. Relative to SAdi, VAdi displayed greater changes with WD including a pronounced decrease in mitochondrial proteins concomitant with upregulation of apoptotic signaling and decreased mitophagy, indicating pervasive mitochondrial stress. Furthermore, WD caused a reduction in lipid handling and glucose uptake pathways particularly in VAdi, consistent with adipocyte de-differentiation. By overlaying the proteomics changes with diet in whole adipose tissue and isolated adipocytes, we uncovered concordance between adipocytes and tissue only in the visceral adipose tissue, indicating a unique tissue-specific adaptation to sustained WD in SAT. Finally, an in-depth comparison of isolated adipocytes and 3T3-L1 proteomes revealed a high degree of overlap, supporting the utility of the 3T3-L1 adipocyte model. These deep proteomes provide an invaluable resource highlighting differences between white adipose depots that may fine-tune their unique functions and adaptation to an obesogenic environment.

Genome-wide screening for genetic variants in polyadenylation signal (PAS) sites in mouse selection lines for fatness and leanness

Alternative polyadenylation (APA) determines mRNA stability, localisation, translation and protein function. Several diseases, including obesity, have been linked to APA. Studies have shown that single nucleotide polymorphisms in polyadenylation signals (PAS-SNPs) can influence APA and affect phenotype and disease susceptibility. However, these studies focussed on associations between single PAS-SNP alleles with very large effects and phenotype. Therefore, we performed a genome-wide screening for PAS-SNPs in the polygenic mouse selection lines for fatness and leanness by whole-genome sequencing. The genetic variants identified in the two lines were overlapped with locations of PAS sites obtained from the PolyASite 2.0 database. Expression data for selected genes were extracted from the microarray expression experiment performed on multiple tissue samples. In total, 682 PAS-SNPs were identified within 583 genes involved in various biological processes, including transport, protein modifications and degradation, cell adhesion and immune response. Moreover, 63 of the 583 orthologous genes in human have been previously associated with human diseases, such as nervous system and physical disorders, and immune, endocrine, and metabolic diseases. In both lines, PAS-SNPs have also been identified in genes broadly involved in APA, such as Polr2c, Eif3e and Ints11. Five PAS-SNPs within 5 genes (Car, Col4a1, Itga7, Lat, Nmnat1) were prioritised as potential functional variants and could contribute to the phenotypic disparity between the two selection lines. The developed PAS-SNPs catalogue presents a key resource for planning functional studies to uncover the role of PAS-SNPs in APA, disease susceptibility and fat deposition.

Increased uncoupling protein 1 levels are associated with attention deficit and hyperactivity disorder in medication-free children

OBJECTIVE

This study aimed to investigate serum visfatin, irisin, and uncoupling protein 1 (UCP1) levels between children with attention deficit and hyperactivity disorder (ADHD) and healthy controls and to discuss how performance on the Stroop Color Test and Serial Digit Learning Test changes with these adipokines.

METHODS

A total of 45 medication-free children with ADHD and 43 controls aged 8-12 years were enrolled in this study. The serum levels of visfatin, UCP1, and irisin were measured using enzyme-linked immunosorbent assay kits.

RESULTS

As a result, in our study, a statistically significant difference was found in UCP1 in the ADHD group compared with the control group, but no significant difference was found in visfatin and irisin levels. An analysis of covariance was also performed for the whole sample, and when controlling for potential confounders, including body mass index, age, and gender, the results did not change. In addition, it was determined that adipokines did not correlate with neuropsychological tests.

CONCLUSION

These findings suggest that UCP1 might be associated with childhood ADHD.

Adipokines: Deciphering the cardiovascular signature of adipose tissue

Obesity and hypertension are intimately linked due to the various ways that the important cell types such as vascular smooth muscle cells (VSMC), endothelial cells (EC), immune cells, and adipocytes, communicate with one another to contribute to these two pathologies. Adipose tissue is a very dynamic organ comprised primarily of adipocytes, which are well known for their role in energy storage. More recently adipose tissue has been recognized as the largest endocrine organ because of its ability to produce a vast number of signaling molecules called adipokines. These signaling molecules stimulate specific types of cells or tissues with many adipokines acting as indicators of adipocyte healthy function, such as adiponectin, omentin, and FGF21, which show anti-inflammatory or cardioprotective effects, acting as regulators of healthy physiological function. Others, like visfatin, chemerin, resistin, and leptin are often altered during pathophysiological circumstances like obesity and lipodystrophy, demonstrating negative cardiovascular outcomes when produced in excess. This review aims to explore the role of adipocytes and their derived products as well as the impacts of these adipokines on blood pressure regulation and cardiovascular homeostasis.

Injectable Prevascularized Mature Adipose Tissues (iPAT) to Achieve Long-Term Survival in Soft Tissue Regeneration

Soft tissue regeneration remains a challenge in reconstructive surgery. So far, both autologous fat implantations and artificial implants methods used in clinical applications lead to various disadvantages and limited lifespan. To overcome these limitations and improve the graft volume maintenance, reproducing a mature adipose tissue already including vasculature structure before implantation can be the solution. Therefore, injectable prevascularized adipose tissues (iPAT) are made from physiological collagen microfibers mixed with human mature adipocytes, adipose-derived stem cells, and human umbilical vein endothelial cells, embedded in fibrin gel. Following murine subcutaneous implantation, the iPAT show a higher cell survival (84% ± 6% viability) and volume maintenance after 3 months (up to twice heavier) when compared to non-prevascularized balls and liposuctioned fat implanted controls. This higher survival can be explained by the greater amount of blood vessels found (up to 1.6-fold increase), with balanced host anastomosis (51% ± 1% of human/mouse lumens), also involving infiltration by the lymphatic and neural vasculature networks. Furthermore, with the cryopreservation possibility enabling their later reinjection, the iPAT technology has the merit to allow noninvasive soft tissue regeneration for long-term outcomes.

Adipose mTORC2 is essential for sensory innervation in white adipose tissue and whole-body energy homeostasis

OBJECTIVE

Adipose tissue, via sympathetic and possibly sensory neurons, communicates with the central nervous system (CNS) to mediate energy homeostasis. In contrast to the sympathetic nervous system, the morphology, role and regulation of the sensory nervous system in adipose tissue are poorly characterized.

METHODS AND RESULTS

Taking advantage of recent progress in whole-mount three-dimensional imaging, we identified a network of calcitonin gene-related protein (CGRP)-positive sensory neurons in murine white adipose tissue (WAT). We found that adipose mammalian target of rapamycin complex 2 (mTORC2), a major component of the insulin signaling pathway, is required for arborization of sensory neurons, but not of sympathetic neurons. Time course experiments revealed that adipose mTORC2 is required for maintenance of sensory neurons. Furthermore, loss of sensory innervation in WAT coincided with systemic insulin resistance. Finally, we established that neuronal protein growth-associated protein 43 (GAP43) is a marker for sensory neurons in adipose tissue.

CONCLUSION

Our findings indicate that adipose mTORC2 is necessary for sensory innervation in WAT. In addition, our results suggest that WAT may affect whole-body energy homeostasis via sensory neurons.

Importance of the Microenvironment and Mechanosensing in Adipose Tissue Biology

The expansion of adipose tissue is an adaptive mechanism that increases nutrient buffering capacity in response to an overall positive energy balance. Over the course of expansion, the adipose microenvironment undergoes continual remodeling to maintain its structural and functional integrity. However, in the long run, adipose tissue remodeling, typically characterized by adipocyte hypertrophy, immune cells infiltration, fibrosis and changes in vascular architecture, generates mechanical stress on adipose cells. This mechanical stimulus is then transduced into a biochemical signal that alters adipose function through mechanotransduction. In this review, we describe the physical changes occurring during adipose tissue remodeling, and how they regulate adipose cell physiology and promote obesity-associated dysfunction in adipose tissue.

Inhaled turmerones can be incorporated in the organs via pathways different from oral administration and can affect weight-gain of mice

Turmerones (α-turmerone, β-turmerone, and ar-turmerone) are the major volatile compounds in turmeric (Curcuma longa), a perennial herb of the ginger family. We previously reported that inhaled volatile turmerones could be transferred in the blood and organs. However, the difference between the two pathways, oral administration and inhalation, and the effect of inhaled turmerones on biological activities remain unknown. In this study, we compared the distribution patterns of turmerones after oral administration and inhalation. The relative levels (concentrations of turmerones in each organ/serum) in the lung, olfactory bulb, brain, heart, kidney, and epididymal fat in the inhalation group tended to be, or are significantly, higher than in the oral administration group. The relative levels of brown adipose tissue in the inhalation group were lower than in the oral administration group. Long-term (50 days) inhalation to volatile turmerones suppressed weight gain and hypertrophy of adipocytes in the epididymal fat of mice fed a high-fat diet. These results suggest that inhaled turmerones can be incorporated into the organs of mice via different pathway from as to those from oral administration and can affect the biological function of the organs under certain conditions.

Orexin receptor type 2 agonism inhibits thermogenesis in brown adipose tissue by attenuating afferent innervation

Orexin signaling has been associated with energy expenditure and brown adipose tissue (BAT) function. However, conflicting data exist in the field about how orexin signaling regulates BAT thermogenesis. In this study, we show that a specific orexin receptor type 2 (OX2R) agonist [Ala11, D-Leu15]-OxB (OB-Ala) inhibited intrascapular brown adipose tissue (iBAT) thermogenesis by reducing sympathetic output to iBAT. This effect is mediated by OX2Rs located on afferent nerve endings innervating iBAT instead of brown adipocyte itself. Microinjection of OB-Ala into iBAT inhibited iBAT thermogenesis in mice upon cold exposure and neuronal activity in the paraventricular nucleus. Findings suggest that OB-Ala could inhibit iBAT thermogenesis by attenuating sensory input thereby inhibiting the sympathetic-sensory iBAT feedback loop. Our study uncovers a novel primary action site of orexin in the regulation of energy balance.

Adipose Tissue Myeloid-Lineage Neuroimmune Cells Express Genes Important for Neural Plasticity and Regulate Adipose Innervation

Peripheral nerves allow a bidirectional communication between brain and adipose tissues, and many studies have clearly demonstrated that a loss of the adipose nerve supply results in tissue dysfunction and metabolic dysregulation. Neuroimmune cells closely associate with nerves in many tissues, including subcutaneous white adipose tissue (scWAT). However, in scWAT, their functions beyond degrading norepinephrine in an obese state remain largely unexplored. We previously reported that a myeloid-lineage knockout (KO) of brain-derived neurotrophic factor (BDNF) resulted in decreased innervation of scWAT, accompanied by an inability to brown scWAT after cold stimulation, and increased adiposity after a high-fat diet. These data underscored that adipose tissue neuroimmune cells support the peripheral nerve supply to adipose and impact the tissue's metabolic functions. We also reported that a subset of myeloid-lineage monocyte/macrophages (Ly6c+CCR2+Cx3cr1+) is recruited to scWAT in response to cold, a process known to increase neurite density in adipose and promote metabolically healthy processes. These cold-induced neuroimmune cells (CINCs) also expressed BDNF. Here we performed RNAseq on CINCs from cold-exposed and room temperature-housed mice, which revealed a striking and coordinated differential expression of numerous genes involved in neuronal function, including neurotrophin signaling and axonal guidance, further supporting that CINCs fulfill a nerve-supporting role in adipose. The increased expression of leukocyte transendothelial migration genes in cold-stimulated CINCs also confirms prior evidence that they are recruited to scWAT and are not tissue resident. We now provide whole-depot imaging of scWAT from LysM-BDNF KO mice, revealing a striking reduction of innervation across the depot fitting with their reduced energy expenditure phenotype. By contrast, Cx3cr1-BDNF KO mice (a macrophage subset of LysM+ cells) exhibited increased thermogenesis and energy expenditure, with compensatory increased food intake and no change in adiposity or body weight. While these KO mice also exhibit a significantly reduced innervation of scWAT, especially around the subiliac lymph node, they displayed an increase in small fiber sympathetic neurite branching, which may underlie their increased thermogenesis. We propose a homeostatic role of scWAT myeloid-lineage neuroimmune cells together in nerve maintenance and neuro-adipose regulation of energy expenditure.

The sympathies of the body: functional organization and neuronal differentiation in the peripheral sympathetic nervous system

During the last 30 years, our understanding of the development and diversification of postganglionic sympathetic neurons has dramatically increased. In parallel, the list of target structures has been critically extended from the cardiovascular system and selected glandular structures to metabolically relevant tissues such as white and brown adipose tissue, lymphoid tissues, bone, and bone marrow. A critical question now emerges for the integration of the diverse sympathetic neuron classes into neural circuits specific for these different target tissues to achieve the homeostatic regulation of the physiological ends affected.

Thermal Stress Induces Long-Term Remodeling of Adipose Tissue and Is Associated with Systemic Dysfunction

ABSTRACT

Severe burns are characterized by the magnitude and duration of the hypermetabolic response thereafter, and demarcated by the loss of lean body mass and catabolism of fat stores. The aim of the present study was to delineate the temporal and location-specific physiological changes to adipose depots and downstream consequences post-burn in a murine model of thermal injury. C57BL/6 mice were subjected to a 30% total body surface area burn and body mass, food intake, and tissue mass were monitored for various time points up until 60 days postinjury. Mitochondrial respirometry was performed using a Seahorse XF96 analyzer. Lipolytic markers and browning markers were analyzed via Western blotting and histology. A severe burn results in a futile cycle of lipolysis and white adipose tissue (WAT) browning, the sequelae of which include fat catabolism, hepatomegaly, and loss of body mass despite increased food intake. A dynamic remodeling of epididymal WAT was observed with acute and chronic increases in lipolysis. Moreover, we demonstrate that pathological browning of inguinal WAT persists up to 60 days post-burn, highlighting the magnitude of the β-adrenergic response to thermal injury. Our data suggests that adipose depots have a heterogeneous response to burns and that therapeutic interventions targeting these physiological changes can improve outcomes. These data may also have implications for treating catabolic conditions such as cancer cachexia as well as developing treatments for obesity and type II diabetes.

Visualization and analysis of whole depot adipose tissue neural innervation

Little is known about the diversity and function of adipose tissue nerves, due in part to the inability to effectively visualize the tissue’s diverse nerve subtypes and the patterns of innervation across an intact depot. The tools to image and quantify adipose tissue innervation are currently limited. Here, we present a method of tissue processing that decreases tissue thickness in the z-axis while leaving cells intact for subsequent immunostaining. This was combined with autofluorescence quenching techniques to permit intact whole tissues to be mounted on slides and imaged by confocal microscopy, with a complementary means to perform whole tissue neurite density quantification after capture of tiled z-stack images. Additionally, we demonstrate how to visualize nerve terminals (the neuro-adipose nexus) in intact blocks of adipose tissue without z-depth reduction. We have included examples of data demonstrating nerve subtypes, neurovascular interactions, label-free imaging of collagen, and nerve bundle digital cross-sections.

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Whole depot adipose tissue innervation was imaged and quantified by a novel method

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Numerous aspects of adipose nerve heterogeneity were observed by microscopy

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We have identified a nerve terminal in adipose, the neuro-adipose nexus

Adipocytes and Stromal Cells Regulate Brown Adipogenesis Through Secretory Factors During the Postnatal White-to-Brown Conversion of Adipose Tissue in Syrian Hamsters

Brown adipose tissue (BAT) is a specialized tissue that regulates non-shivering thermogenesis. In Syrian hamsters, interscapular adipose tissue is composed primarily of white adipocytes at birth, which is converted to BAT through the proliferation and differentiation of brown adipocyte progenitors and the simultaneous disappearance of white adipocytes. In this study, we investigated the regulatory mechanism of brown adipogenesis during postnatal BAT formation in hamsters. Interscapular adipose tissue of a 10-day-old hamster, which primarily consists of brown adipocyte progenitors and white adipocytes, was digested with collagenase and fractioned into stromal–vascular (SV) cells and white adipocytes. SV cells spontaneously differentiated into brown adipocytes that contained multilocular lipid droplets and expressed uncoupling protein 1 (Ucp1), a marker of brown adipocytes, without treatment of adipogenic cocktail such as dexamethasone and insulin. The spontaneous differentiation of SV cells was suppressed by co-culture with adipocytes or by the addition of white adipocyte-conditioned medium. Conversely, the addition of SV cell-conditioned medium increased the expression of Ucp1. These results indicate that adipocytes secrete factors that suppress brown adipogenesis, whereas SV cells secrete factors that promote brown adipogenesis. Transcriptome analysis was conducted; however, no candidate suppressing factors secreted from adipocytes were identified. In contrast, 19 genes that encode secretory factors, including bone morphogenetic protein (BMP) family members, BMP3B, BMP5, and BMP7, were highly expressed in SV cells compared with adipocytes. Furthermore, the SMAD and MAPK signaling pathways, which represent the major BMP signaling pathways, were activated in SV cells, suggesting that BMPs secreted from SV cells induce brown adipogenesis in an autocrine manner through the SMAD/MAPK signaling pathways. Treatment of 5-day-old hamsters with type I BMP receptor inhibitor, LDN-193189, for 5 days reduced p38 MAPK phosphorylation and drastically suppressed BAT formation of interscapular adipose tissue. In conclusion, adipocytes and stromal cells regulate brown adipogenesis through secretory factors during the postnatal white-to-brown conversion of adipose tissue in Syrian hamsters.

Sympathetic Innervation of White Adipose Tissue: to Beige or Not to Beige?

Obesity research progresses in understanding neuronal circuits and adipocyte biology to regulate metabolism. However, the interface of neuro-adipocyte interaction is less studied. We summarize the current knowledge of adipose tissue innervation and interaction with adipocytes and emphasize adipocyte transitions from white to brown adipocytes and vice versa. We further highlight emerging concepts for the differential neuronal regulation of brown/beige versus white adipocyte and the interdependence of both for metabolic regulation.

Single-Cell RNA Profiling Reveals Adipocyte to Macrophage Signaling Sufficient to Enhance Thermogenesis

Adipocytes deficient in fatty acid synthase (iAdFASNKO) emit signals that mimic cold exposure to enhance the appearance of thermogenic beige adipocytes in mouse inguinal white adipose tissues (iWATs). Both cold exposure and iAdFASNKO upregulate the sympathetic nerve fiber (SNF) modulator Neuregulin 4 (Nrg4), activate SNFs, and require adipocyte cyclic AMP/protein kinase A (cAMP/PKA) signaling for beige adipocyte appearance, as it is blocked by adipocyte Gsα deficiency. Surprisingly, however, in contrast to cold-exposed mice, neither iWAT denervation nor Nrg4 loss attenuated adipocyte browning in iAdFASNKO mice. Single-cell transcriptomic analysis of iWAT stromal cells revealed increased macrophages displaying gene expression signatures of the alternately activated type in iAdFASNKO mice, and their depletion abrogated iWAT beiging. Altogether, these findings reveal that divergent cellular pathways are sufficient to cause adipocyte browning. Importantly, adipocyte signaling to enhance alternatively activated macrophages in iAdFASNKO mice is associated with enhanced adipose thermogenesis independent of the sympathetic neuron involvement this process requires in the cold.

Henriques et al. show an alternative pathway to enhance thermogenesis through an adipocyte cAMP/PKA axis in denervated iWAT. Signals emanating from this pathway generate M2-type macrophages associated with iWAT browning.

Adipocyte integrin-linked kinase plays a key role in the development of diet-induced adipose insulin resistance in male mice

Objective

Increased deposition of the extracellular matrix (ECM) in adipose tissue (AT) during obesity contributes to insulin resistance. The integrin receptors transmit changes in the extracellular environment causing corresponding intracellular adaptations. Integrin-linked kinase (ILK), an adaptor protein, is a central hub for intracellular signaling of integrins. This study determined the role of ILK in adipose function and insulin resistance.

Methods

The pathogenic role of ILK in obesity and insulin resistance was studied in human adipose tissue and adipocyte-specific ILK-deficient mice (ILK^lox/loxAdCre). ILK^lox/loxAdCre mice together with wild-type littermates (ILK^lox/lox) were fed a chow diet or 60% high-fat (HF) diet for 16 weeks. In vivo insulin sensitivity was determined by hyperinsulinemic-euglycemic clamps.

Results

AT ILK expression was increased by HF diet feeding in mice and increased in visceral fat of morbidly obese humans. The HF-fed ILK^lox/loxAdCre mice displayed reduced fat mass and improved glucose tolerance relative to the HF-fed ILK^lox/lox mice. During a hyperinsulinemic-euglycemic clamp, the HF-fed ILK^lox/loxAdCre mice exhibited partially improved insulin resistance in AT. Lipolysis was suppressed to a greater extent by insulin and glucose uptake in brown AT (BAT) increased. Increased inhibition of lipolysis may have been attributed to increased vascularization in white AT, while increased glucose uptake in BAT was associated with increased Akt phosphorylation and P38/JNK dephosphorylation. Notably, AT insulin sensitivity in lean mice was not affected by ILK deletion. Moreover, reduced fat mass in the HF-fed ILK^lox/loxAdCre mice may have been attributed to decreased free fatty acid uptake into adipocytes via the downregulation of CD36 gene expression. Consistent with the results in the mice, knockdown and knockout of ILK in 3T3-L1 cells decreased lipid accumulation and CD36 gene expression during adipogenesis.

Conclusions

These data show that adipocyte ILK is important for regulating HF diet-mediated insulin resistance in AT in a manner consistent with AT function.

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ILK protein increased in visceral adipose tissue of obese humans and mice.

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Mice lacking adipocyte ILK had less fat and improved glucose tolerance in obesity.

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Adipocyte ILK deletion improved anti-lipolytic action of insulin in obese mice.

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Adipocyte ILK deletion stimulated brown adipose tissue glucose uptake in obese mice.

Early postnatal interactions between beige adipocytes and sympathetic neurites regulate innervation of subcutaneous fat

While beige adipocytes have been found to associate with dense sympathetic neurites in mouse inguinal subcutaneous white fat (iWAT), little is known about when and how this patterning is established. Here, we applied whole-tissue imaging to examine the development of sympathetic innervation in iWAT. We found that parenchymal neurites actively grow between postnatal day 6 (P6) and P28, overlapping with early postnatal beige adipogenesis. Constitutive deletion of Prdm16 in adipocytes led to a significant reduction in early postnatal beige adipocytes and sympathetic density within this window. Using an inducible, adipocyte-specific Prdm16 knockout model, we found that Prdm16 is required for guiding sympathetic growth during early development. Deleting Prdm16 in adult animals, however, did not affect sympathetic structure in iWAT. Together, these findings highlight that beige adipocyte-sympathetic neurite communication is crucial to establish sympathetic structure during the early postnatal period but may be dispensable for its maintenance in mature animals.

Subcutaneous and intramuscular fat transcriptomes show large differences in network organization and associations with adipose traits in pigs

Subcutaneous fat (SCF) and intramuscular fat (IMF) deposition is relevant to health in humans, as well as meat production and quality in pigs. In this study, we generated RNA sequence data for 122 SCF, 120 IMF, and 87 longissimus dorsi muscle (LDM) samples using 155 F₆ pigs from a specially designed heterogeneous population generated by intercrossing four highly selected European commercial breeds and four indigenous Chinese pig breeds. The phenotypes including waist back fat thickness and intramuscular fat content were also measured in the 155 F₆ pigs. We found that the genes in SCF and IMF differed largely in both expression levels and network connectivity, and highlighted network modules that exhibited strongest gain of connectivity in SCF and IMF, containing genes that were associated with the immune process and DNA double-strand repair, respectively. We identified 215 SCF genes related to kinase inhibitor activity, mitochondrial fission, and angiogenesis, and 90 IMF genes related to lipolysis and fat cell differentiation, displayed a tissue-specific association with back fat thickness and IMF content, respectively. We found that cis-expression QTL for trait-associated genes in the two adipose tissues tended to have tissue-dependent predictability for the two adipose traits. Alternative splicing of genes was also found to be associated with SCF or IMF deposition, but the association was much less extensive than that based on expression levels. This study provides a better understanding of SCF and IMF gene transcription and network organization and identified critical genes and network modules that displayed tissue-specific associations with subcutaneous and intramuscular fat deposition. These features are helpful for designing breeding programs to genetically improve the two adipose traits in a balanced way.

Hip preservation surgery and the acetabular fossa

As our understanding of hip function and disease improves, it is evident that the acetabular fossa has received little attention, despite it comprising over half of the acetabulum’s surface area and showing the first signs of degeneration. The fossa’s function is expected to be more than augmenting static stability with the ligamentum teres and being a templating landmark in arthroplasty. Indeed, the fossa, which is almost mature at 16 weeks of intrauterine development, plays a key role in hip development, enabling its nutrition through vascularization and synovial fluid, as well as the influx of chondrogenic stem/progenitor cells that build articular cartilage. The pulvinar, a fibrofatty tissue in the fossa, has the same developmental origin as the synovium and articular cartilage and is a biologically active area. Its unique anatomy allows for homogeneous distribution of the axial loads into the joint. It is composed of intra-articular adipose tissue (IAAT), which has adipocytes, fibroblasts, leucocytes, and abundant mast cells, which participate in the inflammatory cascade after an insult to the joint. Hence, the fossa and pulvinar should be considered in decision-making and surgical outcomes in hip preservation surgery, not only for their size, shape, and extent, but also for their biological capacity as a source of cytokines, immune cells, and chondrogenic stem cells.

Cite this article: Bone Joint Res 2020;9(12):857–869.

Distinctive Features of Orbital Adipose Tissue (OAT) in Graves' Orbitopathy

Depot specific expansion of orbital-adipose-tissue (OAT) in Graves' Orbitopathy (GO) is associated with lipid metabolism signaling defects. We hypothesize that the unique adipocyte biology of OAT facilitates its expansion in GO. A comprehensive comparison of OAT and white-adipose-tissue (WAT) was performed by light/electron-microscopy, lipidomic and transcriptional analysis using ex vivo WAT, healthy OAT (OAT-H) and OAT from GO (OAT-GO). OAT-H/OAT-GO have a single lipid-vacuole and low mitochondrial number. Lower lipolytic activity and smaller adipocytes of OAT-H/OAT-GO, accompanied by similar essential linoleic fatty acid (FA) and (low) FA synthesis to WAT, revealed a hyperplastic OAT expansion through external FA-uptake via abundant SLC27A6 (FA-transporter) expression. Mitochondrial dysfunction of OAT in GO was apparent, as evidenced by the increased mRNA expression of uncoupling protein 1 (UCP1) and mitofusin-2 (MFN2) in OAT-GO compared to OAT-H. Transcriptional profiles of OAT-H revealed high expression of Iroquois homeobox-family (IRX-3&5), and low expression in HOX-family/TBX5 (essential for WAT/BAT (brown-adipose-tissue)/BRITE (BRown-in-whITE) development). We demonstrated unique features of OAT not presented in either WAT or BAT/BRITE. This study reveals that the pathologically enhanced FA-uptake driven hyperplastic expansion of OAT in GO is associated with a depot specific mechanism (the SLC27A6 FA-transporter) and mitochondrial dysfunction. We uncovered that OAT functions as a distinctive fat depot, providing novel insights into adipocyte biology and the pathological development of OAT expansion in GO.

Peripheral Innervation in the Regulation of Glucose Homeostasis

Precise regulation of circulating glucose is crucial for human health and ensures a sufficient supply to the brain, which relies almost exclusively on glucose for metabolic energy. Glucose homeostasis is coordinated by hormone-secreting endocrine cells in the pancreas, as well as glucose utilization and production in peripheral metabolic tissues including the liver, muscle, and adipose tissue. Glucose-regulatory tissues receive dense innervation from sympathetic, parasympathetic, and sensory fibers. In this review, we summarize the functions of peripheral nerves in glucose regulation and metabolism. Dynamic changes in peripheral innervation have also been observed in animal models of obesity and diabetes. Together, these studies highlight the importance of peripheral nerves as a new therapeutic target for metabolic disorders.

Dietary Options for Rodents in the Study of Obesity

Obesity and its associated metabolic diseases are currently a priority research area. The increase in global prevalence at different ages is having an enormous economic and health impact. Genetic and environmental factors play a crucial role in the development of obesity, and diet is one of the main factors that contributes directly to the obesogenic phenotype. Scientific evidence has shown that increased fat intake is associated with the increase in body weight that triggers obesity. Rodent animal models have been extremely useful in the study of obesity since weight gain can easily be induced with a high-fat diet. Here, we review the dietary patterns and physiological mechanisms involved in the dynamics of energy balance. We report the main dietary options for the study of obesity and the variables to consider in the use of a high-fat diet, and assess the progression of obesity and diet-induced thermogenesis.

Excess of ovarian nerve growth factor impairs embryonic development and causes reproductive and metabolic dysfunction in adult female mice

Nerve growth factor (NGF) is critical for the development and maintenance of the peripheral sympathetic neurons. NGF is also involved in the ovarian sympathetic innervation and in the development and maintenance of folliculogenesis. Women with the endocrine disorder, polycystic ovary syndrome (PCOS), have an increased sympathetic nerve activity and increased ovarian NGF levels. The role of ovarian NGF excess in the PCOS pathophysiology and in the PCOS-related features is unclear. Here, using transgenic mice overexpressesing NGF in the ovarian theca cells (17NF mice), we assessed the female embryonic development, and the reproductive and metabolic profile in adult females. Ovarian NGF excess caused growth restriction in the female fetuses, and a delayed gonocyte and primary oocyte maturation. In adulthood, the 17NF mice displayed irregular estrous cycles and altered ovarian expression of steroidogenic and epigenetic markers. They also exhibited an increased sympathetic output with increased circulating dopamine, and metabolic dysfunction reflected by aberrant adipose tissue morphology and function, impaired glucose metabolism, decreased energy expenditure, and hepatic steatosis. These findings indicate that ovarian NGF excess leads to adverse fetal development and to reproductive and metabolic complications in adulthood, mirroring common features of PCOS. This work provides evidence that NGF excess may be implicated in the PCOS pathophysiology.

Innervation of supraclavicular adipose tissue: A human cadaveric study

Functional brown adipose tissue (BAT) was identified in adult humans only in 2007 with the use of fluorodeoxyglucose positron emission tomography imaging. Previous studies have demonstrated a negative correlation between obesity and BAT presence in humans. It is proposed that BAT possesses the capacity to increase metabolism and aid weight loss. In rodents it is well established that BAT is stimulated by the sympathetic nervous system with the interscapular BAT being innervated via branches of intercostal nerves. Whilst there is evidence to suggest that BAT possesses beta-3 adrenoceptors, no studies have identified the specific nerve branch that carries sympathetic innervation to BAT in humans. The aim of this study was to identify and trace the peripheral nerve or nerves that innervate human BAT in the supraclavicular region. The posterior triangle region of the neck of cadaveric specimens were dissected in order to identify any peripheral nerve branches piercing and/or terminating in supraclavicular BAT. A previously undescribed branch of the cervical plexus terminating in a supraclavicular adipose depot was identified in all specimens. This was typically an independent branch of the plexus, from the third cervical spinal nerve, but in one specimen was a branch of the supraclavicular nerve. Histological analysis revealed the supraclavicular adipose depot contained tyrosine hydroxylase immunoreactive structures, which likely represent sympathetic axons. This is the first study that identifies a nerve branch to supraclavicular BAT-like tissue. This finding opens new avenues for the investigation of neural regulation of fat metabolism in humans.

The biology of lipid droplet-bound mitochondria

Proper regulation of cellular lipid storage and oxidation is indispensable for the maintenance of cellular energy homeostasis and health. Mitochondrial function has been shown to be a main determinant of functional lipid storage and oxidation, which is of particular interest for the adipose tissue, as it is the main site of triacylglyceride storage in lipid droplets (LDs). Recent studies have identified a subpopulation of mitochondria attached to LDs, peridroplet mitochondria (PDM) that can be separated from cytoplasmic mitochondria (CM) by centrifugation. PDM have distinct bioenergetics, proteome, cristae organization and dynamics that support LD build-up, however their role in adipose tissue biology remains largely unexplored. Therefore, understanding the molecular basis of LD homeostasis and their relationship to mitochondrial function and attachment in adipocytes is of major importance.

Effects of amylin on food intake and body weight via sympathetic innervation of the interscapular brown adipose tissue

Objective: Amylin acts on the lateral dorsal tegmental nucleus (LDT), resulting in anorexic and weight-loss effects and activates thermogenesis in the interscapular brown adipose tissue (IBAT). In addition, it induces neuronal nitric oxide synthase (nNOS) and choline acetyltransferase (ChAT)-mediated feeding. However, the influence of the intact sympathetic nervous system (SNS) in mediating amylin's effects has not been fully characterised. We investigated whether extracellular signal-regulated kinase (ERK), nNOS, and ChAT activities in the LDT are responsible for amylin's anorexigenic effects and whether this requires an intact SNS.Methods: C57BL/6J mice [wild-type (WT), sham, and sympathetic denervation of IBAT] were used. Food consumption, body weight, and distribution of pERK, nNOS, and ChAT positive neurons in the brain were examined following acute and chronic amylin administration.Results: Food intake was significantly decreased in WT and sham animals following acute amylin injection, but not in the denervated mice. Chronic amylin reduced body weight and serum glucose levels after 6 weeks, but increased insulin levels; no changes were observed in the denervated mice. Acute amylin increased the expression of nNOS, ChAT, and uncoupling protein-1 in the IBAT of WT and sham mice, while no changes were observed in the denervated mice and pERK from the above effect.Conclusions: Intact SNS of IBAT influences amylin-induced suppression of food intake and body weight, thus affecting nNOS and ChAT signalling in the LDT and locus coeruleus.

Pro-inflammatory cytokines in the paraventricular nucleus mediate the adipose afferent reflex in rats

Our previous study showed that the adipose afferent reflex (AAR) induced by chemical stimulation of white adipose tissue (WAT) increased sympathetic outflow and blood pressure. We also found that pro-inflammatory cytokines (PICs) in the hypothalamic paraventricular nucleus (PVN) potentiate the cardiac sympathetic afferent reflex in rats. However, the role of PICs in the PVN in regulating the AAR is still not clear. This study determined whether PICs in the PVN mediate the AAR in rats. The AAR was evaluated based on renal sympathetic nerve activity and mean arterial blood pressure in response to capsaicin injection into inguinal WAT (iWAT). PIC levels were measured by ELISA. PVN microinjection with the PICs tumor necrosis factor (TNF)-α or interleukin (IL)-1β enhanced the AAR in a dose-dependent manner. Furthermore, pretreatment via the bilateral microinjection of the TNF-α-blocker etanercept or IL-1β blocker IL-1ra into the PVN attenuated the AAR. In rats pretreated with TNF-α or IL-1β, a sub-response dose of angiotensin II (Ang II) significantly enhanced the AAR. Moreover, delivery of the angiotensin II type 1(AT₁) receptor antagonist losartan into the PVN attenuated the effects of TNF-α or IL-1β on the AAR. In addition, stimulating either iWAT or retroperitoneal WAT with capsaicin increased TNF-α or IL-1β levels in the PVN, but the injection of capsaicin into the jugular vein, skeletal muscle, and skin had no effects on TNF-α or IL-1β levels in the PVN. These results suggest that TNF-α or IL-1β and Ang II in the PVN synergistically enhance the AAR in rats.

Adipocyte-progenitor cell communication that influences adipogenesis

Adipose tissue is located in discrete depots that are differentially associated with elevated risk of metabolic complications, with fat accretion in visceral depots being most detrimental to metabolic health. Currently, the regulation of specific adipose depot expansion, by adipocyte hypertrophy and hyperplasia and consequently fat distribution, is not well understood. However, a growing body of evidence from in vitro investigations indicates that mature adipocytes secrete factors that modulate the proliferation and differentiation of progenitor, adipose-derived stem cells (ADSCs). It is therefore plausible that endocrine communication between adipocytes and ADSCs located in different depots influences fat distribution, and may therefore contribute to the adverse health outcomes associated with visceral adiposity. This review will explore the available evidence of paracrine and endocrine crosstalk between mature adipocytes and ADSCs that affects adipogenesis, as a better understanding of the regulatory roles of the extracellular signalling mechanisms within- and between adipose depots may profoundly change the way we view adipose tissue growth in obesity and related comorbidities.

Lipokines and Thermogenesis

Adaptive thermogenesis is a catabolic process that consumes energy-storing molecules and expends that energy as heat in response to environmental changes. This process occurs primarily in brown and beige adipose tissue. Thermogenesis is regulated by many factors, including lipid derived paracrine and endocrine hormones called lipokines. Recently, technologic advances for identifying new lipid biomarkers of thermogenic activity have shed light on a diverse set of lipokines that act through different pathways to regulate energy expenditure. In this review, we highlight a few examples of lipokines that regulate thermogenesis. The biosynthesis, regulation, and effects of the thermogenic lipokines in several families are reviewed, including oloeylethanolamine, endocannabinoids, prostaglandin E2, and 12,13-diHOME. These thermogenic lipokines present potential therapeutic targets to combat states of excess energy storage, such as obesity and related metabolic disorders.

Neuropathy and neural plasticity in the subcutaneous white adipose depot

The difficulty in obtaining as well as maintaining weight loss, together with the impairment of metabolic control in conditions like diabetes and cardiovascular disease, may represent pathological situations of inadequate neural communication between the brain and peripheral organs and tissues. Innervation of adipose tissues by peripheral nerves provides a means of communication between the master metabolic regulator in the brain (chiefly the hypothalamus), and energy-expending and energy-storing cells in the body (primarily adipocytes). Although chemical and surgical denervation studies have clearly demonstrated how crucial adipose tissue neural innervation is for maintaining proper metabolic health, we have uncovered that adipose tissue becomes neuropathic (ie: reduction in neurites) in various conditions of metabolic dysregulation. Here, utilizing both human and mouse adipose tissues, we present evidence of adipose tissue neuropathy, or loss of proper innervation, under pathophysiological conditions such as obesity, diabetes, and aging, all of which are concomitant with insult to the adipose organ as well as metabolic dysfunction. Neuropathy is indicated by loss of nerve fiber protein expression, reduction in synaptic markers, and lower neurotrophic factor expression in adipose tissue. Aging-related adipose neuropathy particularly results in loss of innervation around the tissue vasculature, which cannot be reversed by exercise. Together with indications of neuropathy in muscle and bone, these findings underscore that peripheral neuropathy is not restricted to classic tissues like the skin of distal extremities, and that loss of innervation to adipose may trigger or exacerbate metabolic diseases. In addition, we have demonstrated stimulation of adipose tissue neural plasticity with cold exposure, which may ameliorate adipose neuropathy and be a potential therapeutic option to re-innervate adipose and restore metabolic health.