Morphological Changes in Subcutaneous White Adipose Tissue After Severe Burn Injury

Single-nuclei RNA Profiling Reveals Disruption of Adipokine and Inflammatory Signaling in Adipose Tissue of Burn Patients

OBJECTIVE

We conducted a large-scale investigation of the systemic and adipose tissue-specific alterations in a clinical population of burn patients to identify factors that may influence hypermetabolism.

BACKGROUND

Previous research has identified chronic disturbances in adipose tissue inflammation, lipolysis, and browning, which may drive the perpetuation of hypermetabolism following the severe adrenergic stress of a burn injury. Given that adipose tissue is thought to be a central node in the regulation of systemic metabolism, we believe that systematically delineating the pathologic role of adipose tissue postburn, will lead to the identification of novel interventions to mitigate morbidity and mortality from severe burns.

METHODS

This was a single-institution cohort study, which obtained plasma and subcutaneous adipose tissue samples from severely burn adult patients over various time points during acute hospitalization. Whole-body clinical, metabolic, and inflammatory mediators were assessed in plasma, while genetic analyses through RT-qPCR and single-nuclei RNA sequencing were conducted in adipose tissue.

RESULTS

Systemic inflammation and adrenergic stress increase IL-6 signaling, lipolysis, browning, and adipokine dysfunction in the adipose tissue of adult burn patients, which may further propagate the long-term hypermetabolic response. Moreover, using single-nuclei RNA sequencing, we provide the first comprehensive characterization of alterations in the adipose tissue microenvironment occurring at acute and chronic stages postburn.

CONCLUSION

We provide novel insight toward the effect of burns on adipokine release, inflammatory signaling pathways, and adipose heterogeneity over the trajectory of acute and chronic stages.

Modulation of Burn Hypermetabolism in Preclinical Models

Severe burns elicit a state of physiological stress and increased metabolism to help the body compensate for the changes associated with the traumatic injury. However, this hypermetabolic state is associated with increased insulin resistance, cardiovascular dysfunction, skeletal muscle catabolism, impaired wound healing, and delayed recovery. Several interventions were attempted to modulate burn hypermetabolism, including nutritional support, early excision and grafting, and growth hormone application. However, burn hypermetabolism still imposes significant morbidity and mortality in burn patients. Due to the limitations of in vitro models, animal models are indispensable in burn research. Animal models provide researchers with invaluable tools to test the safety and efficacy of novel treatments or advance our knowledge of previously utilized agents. Several animal studies evaluated novel therapies to modulate burn hypermetabolism in the last few years, including recombinant human growth hormone, erythropoietin, acipimox, apelin, anti-interleukin-6 monoclonal antibody, and ghrelin therapies. Results from these studies are promising and may be effectively translated into human studies. In addition, other studies revisited drugs previously used in clinical practice, such as insulin and metformin, to further investigate their underlying mechanisms as modulators of burn hypermetabolism. This review aims to update burn experts with the novel therapies under investigation in burn hypermetabolism with a focus on applicability and translation. Furthermore, we aim to guide researchers in selecting the correct animal model for their experiments by providing a summary of the methodology and the rationale of the latest studies.

Adipose tissue: a neglected organ in the response to severe trauma?

Despite the manifold recent efforts to improve patient outcomes, trauma still is a clinical and socioeconomical issue of major relevance especially in younger people. The systemic immune reaction after severe injury is characterized by a strong pro- and anti-inflammatory response. Besides its functions as energy storage depot and organ-protective cushion, adipose tissue regulates vital processes via its secretion products. However, there is little awareness of the important role of adipose tissue in regulating the posttraumatic inflammatory response. In this review, we delineate the local and systemic role of adipose tissue in trauma and outline different aspects of adipose tissue as an immunologically active modifier of inflammation and as an immune target of injured remote organs after severe trauma.

Tolerance to Glucose and Lipid High Metabolic Reactions After Burns in an Obese Rat Model

The goal of this study was to determine what effect obese body weight and a burn injury can have on the metabolism of glucose and lipids in rats. We used a 3 * 3 factorial model design to provide basic glucose and lipid metabolic data characterizing the interaction between different weight and burn injury groups. Two hundred Sprague Dawley rats were categorized into three weight groups (normal, overweight, obese) and then further divided into control, second-degree, and third-degree burn groups. Our model compared interactions between weight and burn injury factors according to the above groups. Blood glucose and lipid metabolism indicators were monitored on the 1st, 3rd, 7th, and 14th days after burn injury occurred, and burned skin and blood samples were collected for testing. Compared with the normal weight group, the overweight group's fast blood glucose, fast insulin, and homeostasis model assessment of insulin resistance were higher (P < .05), and FBG in the obese group was higher than the normal weight group (P < .05). Burn injuries combined with obese body weight had an interactive effect on fast blood glucose, fast insulin, and homeostasis model assessment of insulin resistance after burn injury (P < .05). Burn injury combined with obese body weight had an interaction on low-density lipoprotein cholesterol on the 3rd day after burn injury (P < .05). Burn injury combined with obese weight had no interaction on triglyceride, total cholesterol, and high-density lipoprotein cholesterol (P > .05). Rats in the overweight and obese weight groups were observed to develop an adaptation and tolerance to a higher metabolic rate after burn injuries occurred.

The angiogenic potential of CD271+ human adipose tissue-derived mesenchymal stem cells

BACKGROUND

Autologous fat grafting is often a crucial aspect of reconstructive and aesthetic surgeries, yet poor graft retention is a major issue with this technique. Enriching fat grafts with adipose tissue-derived mesenchymal stem cells (AD-MSCs) improves graft survival-however, AD-MSCs represent a heterogeneous population. Selection of subpopulations of AD-MSCs would allow the targeting of specific AD-MSCs that may benefit fat graft survival more than the general AD-MSC population.

METHODS

Human AD-MSCs were selected for the surface marker CD271 using magnetic-activated cell sorting and compared to the CD271 negative phenotype.  These subpopulations were analysed for gene expression using Real-Time qPCR and RNA sequencing; surface marker characteristics using immunostaining; ability to form tubules when cultured with endothelial cells; and gene and protein expression of key angiogenic mediators when cultured with ex-vivo adipose tissue.

RESULTS

Human AD-MSCs with the surface marker CD271 express angiogenic genes at higher levels, and inflammatory genes at lower levels, than the CD271- AD-MSC population. A greater proportion of CD271+ AD-MSCs also possess the typical complement of stem cell surface markers and are more likely to promote effective neoangiogenesis, compared to CD271- AD-MSCs.

CONCLUSION

Enriching grafts with the CD271+ AD-MSC subpopulation holds potential for the improvement of reconstructive and aesthetic surgeries involving adipose tissue.

Experimental Study of Burn Damage Progression in a Human Composite Tissue Model

Simple Summary

Patients with severe burn injuries undergo surgical procedures to remove dead skin and underlying tissue. The study of tissue damage inflicted by burns is vital for further improving current treatments. Numerous animal burn models have been employed to inflict burns at different temperatures and study the local reaction of harmed tissue and its consequences for the whole organism. Because human skin tissue is different than that of animals, we used human skin tissue discarded from surgeries and exposed the tissue to different burn injuries. The samples were maintained for seven days to study the course of burn damage and activation of different cells in human skin tissue. Our study shows that milder burns result in slowly expanding damage, while severe burns cause deep persistent damage to the skin and underlying fat tissue. These different patterns were also reflected by the activation of immune and repair cells within the fat tissue. Our study suggests that fat tissue adjacent to burn wounds can play a crucial role in tissue repair. Thus, examination of fat tissue vitality before surgery may improve outcomes.

Abstract

Comparative studies of human tissue damage caused by burns are challenging because precise information regarding the temperature, time, and duration of the exposure is often missing. Animal models cannot be fully translated to the human system due to interspecies differences in cutaneous tissues. We used a human composite tissue model to compare tissue damage caused by thermal burns with different dynamics. Equal subcutaneous/cutaneous composite tissue samples from six donors were first exposed to either preheated steel (100 °C) or a precision flame burner (300 °C) and were then maintained in vitro for seven days. Histological and immunohistochemical analyses revealed that flame burns instantly caused deep and stable damage to the subcutaneous tissue, which stayed constant for seven days. By contrast, contact burns inflicted tissue damage that was initially superficial but then expanded deeper into the adipose tissue. This spatiotemporal expansion of tissue damage was essentially accompanied by macrophage and fibroblast activation, which points towards inflammation resolution and wound healing. Our study suggests that thermal differences in burns directly influence the course of tissue damage, the cellular response and, consequently, the likely dynamics of repair processes days after burn injuries.

White adipose tissue browning in critical illness: A review of the evidence, mechanisms and future perspectives

Observational studies suggest better clinical outcomes following critical illness in patients with overweight and obesity (obesity paradox). An understanding of the morphologic, physiologic and metabolic changes in adipose tissue in critical illness may provide an explanation. Recent studies have demonstrated the transformation of white to brown-like adipocytes due to the "browning process," which has been of interest as a potential novel therapy in obesity during the last decade. The characteristics of the browning of white adipose tissue (WAT) include the appearance of smaller, multilocular adipocytes, increased UCP1 mRNA expression, mitochondrial density and respiratory capacity. These changes have been identified in some critical illnesses, which specifically refers to burns, sepsis and cancer cachexia in this study. The pathophysiological nature of WAT browning, underlying mechanisms, main regulators and potential benefits and harms of this process are interesting new areas that warrants further investigations. In this review, we discuss emerging scientific discipline of adipose tissue physiology in metabolic stress, available data, gaps of knowledge and future perspectives. Future investigations in this field may provide insights into the underlying mechanisms and clinical aspects of browning that may further our understanding of the proposed obesity paradox following critical illness, which may in turn open up opportunities for novel therapies to save lives and improve recovery.

Leptin and fractalkine: novel subcutaneous cytokines in burn injury

Burn injury is a pathology underpinned by progressive and aberrant inflammation. It is a major clinical challenge to survival and quality of life. Although the complex local and disseminating pathological processes of a burn injury ultimately stem from local tissue damage, to date relatively few studies have attempted to characterise the local inflammatory mediator profile. Here, cytokine content and associated transcriptional changes were measured in rat skin for three hours immediately following induction of a scald-type (60°C, 2 min) burn injury model. Leptin (P=0.0002) and fractalkine (P=0.0478) concentrations were significantly elevated post-burn above pre-burn and control site values, coinciding with the development of burn site oedema and differential expression of leptin mRNA (P=0.0004). Further, gene sequencing enrichment analysis indicated cytokine-cytokine receptor interaction (P=1.45×10-6). Subsequent behavioural studies demonstrated that, following subcutaneous injection into the dorsum of the paw, both leptin and fractalkine induced mechanical allodynia, heat hyperalgesia and the recruitment of macrophages. This is the first report of leptin elevation specifically at the burn site, and the first report of fractalkine elevation in any tissue post-burn which, together with the functional findings, calls for exploration of the influence of these cytokines on pain, inflammation and burn wound progression. In addition, targeting these signalling molecules represents a therapeutic potential as early formative mediators of these pathological processes.

Burn-Induced Cardiac Mitochondrial Dysfunction via Interruption of the PDE5A-cGMP-PKG Pathway

Burn-induced heart dysfunction is a key factor for patient mortality. However, the molecular mechanisms are not yet fully elucidated. This study sought to understand whether burn-induced heart dysfunction is associated with cardiac mitochondrial dysfunction and interruption of the PDE5A-cGMP-PKG pathway. Sixty percent total body surface area (TBSA) scald burned rats (±sildenafil) were used in this study. A transmission electron microscope (TEM), real-time qPCR, O2K-respirometer, and electron transport chain assays were used to characterized molecular function. Cardiac mitochondrial morphological shapes were disfigured with a decline in mitochondrial number, area, and size, resulting in deficiency of cardiac mitochondrial replication. Burn induced a decrease in all mitDNA encoded genes. State 3 oxygen consumption was significantly decreased. Mitochondrial complex I substrate-energized or complex II substrate-energized and both of respiratory control ratio (RCRs) were decreased after burn. All mitochondrial complex activity except complex II were decreased in the burn group, correlating with decreases in mitochondrial ATP and MnSOD activity. Sildenafil, a inhibitor of the PDE5A-cGMP-PKG pathway, preserved the mitochondrial structure, respiratory chain efficiency and energy status in cardiac tissue. Furthermore, sildenafil treatment significantly restored ADP-conjugated respiration in burned groups. In conclusion, cardiac mitochondrial damage contributes to burn-induced heart dysfunction via the PDE5A-cGMP-PKG pathway.

The Effect of Burn Trauma on Lipid and Glucose Metabolism: Implications for Insulin Sensitivity

Severe burns represent a unique form of trauma in terms of the magnitude and persistence of the stress response they incur. Given advances in acute burn care in the last quarter of a century and the resultant reduction in mortality rates, even for those with massive burns, greater emphasis is now placed on understanding the metabolic stress response to severe burn trauma in order to devise strategies that promote recovery and reduce morbidity. Derangements in metabolism including protein and lipid redistribution and altered glucose handling are hallmarks of the pathophysiological response to burn trauma. In this review article, we aim to distill and discuss the c urrent literature concerning the effect of burn trauma on lipid and glucose metabolism. Furthermore, we will discuss the implications of altered lipid metabolism with regards to insulin sensitivity and glucose control, while discussing the utility of agents and strategies aimed at restoring normal lipid and glucose metabolism in burned patients.

Inducible Loss of the Aryl Hydrocarbon Receptor Activates Perigonadal White Fat Respiration and Brown Fat Thermogenesis via Fibroblast Growth Factor 21

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor highly expressed in hepatocytes. Researchers have employed global and liver-specific conditional Ahr knockout mouse models to characterize the physiological roles of the AHR; however, the gestational timing of AHR loss in these models can complicate efforts to distinguish the direct and indirect effects of post-gestational AHR deficiency. Utilizing a novel tamoxifen-inducible AHR knockout mouse model, we analyzed the effects of hepatocyte-targeted AHR loss in adult mice. The data demonstrate that AHR deficiency significantly reduces weight gain and adiposity, and increases multilocular lipid droplet formation within perigonadal white adipose tissue (gWAT). Protein and mRNA expression of fibroblast growth factor 21 (FGF21), an important hepatokine that activates thermogenesis in brown adipose tissue (BAT) and gWAT, significantly increases upon AHR loss and correlates with a significant increase of BAT and gWAT respiratory capacity. Confirming the role of FGF21 in mediating these effects, this phenotype is reversed in mice concomitantly lacking AHR and FGF21 expression. Chromatin immunoprecipitation analyses suggest that the AHR may constitutively suppress Fgf21 transcription through binding to a newly identified xenobiotic response element within the Fgf21 promoter. The data demonstrate an important AHR-FGF21 regulatory axis that influences adipose biology and may represent a “druggable” therapeutic target for obesity and its related metabolic disorders.

Septic predictor index: A novel platform to identify thermally injured patients susceptible to sepsis

BACKGROUND

During the past decades' sepsis has become the major cause of death in severely burned patients. Despite the importance of burn sepsis, its diagnosis, let alone its prediction, is difficult if not impossible. Recently, we have demonstrated burn patients have increased NLRP3 inflammasome activation in white adipose tissue. We aimed to delineate a unique immune profile that can be used to identify septic outcomes in severely burned patients.

METHODS

Adult burn patients (n = 37) admitted to our burn center between June 2013-2015 were enrolled in this study. White adipose tissue from the site of injury and plasma were collected from severely burned patients (>20% total body surface area) within 96 hours after thermal injury, indiscriminate of sex or age.

RESULTS

We found that patients exhibiting aberrantly high levels of proinflammatory interleukin-1β and decreased macrophages at the site of injury are highly susceptible to development of sepsis. Septic patients also had increased anti-inflammatory (interleukin-10, interleukin-1RA) cytokines in plasma. The Septic Predictor Index was generated as a quotient for the site of injury macrophage proportion and interleukin-1β production. All patients who eventually develop sepsis had septic predictor index values >0.5. Septic patients with Septic Predictor Index values >1 all had sepsis onset within 12 days post-injury, whereas patients with Septic Predictor Index values between 0.5-1 all had later onset (>12 days).

CONCLUSION

The Septic Predictor Index can determine sepsis onset accurately in thermally injured patients a priori and further enables surgeons to develop clinical studies and focused therapies specifically designed for septic cohorts.

Adipose tissue: between the extremes

Adipose tissue represents a critical component in healthy energy homeostasis. It fulfills important roles in whole-body lipid handling, serves as the body's major energy storage compartment and insulation barrier, and secretes numerous endocrine mediators such as adipokines or lipokines. As a consequence, dysfunction of these processes in adipose tissue compartments is tightly linked to severe metabolic disorders, including obesity, metabolic syndrome, lipodystrophy, and cachexia. While numerous studies have addressed causes and consequences of obesity-related adipose tissue hypertrophy and hyperplasia for health, critical pathways and mechanisms in (involuntary) adipose tissue loss as well as its systemic metabolic consequences are far less understood. In this review, we discuss the current understanding of conditions of adipose tissue wasting and review microenvironmental determinants of adipocyte (dys)function in related pathophysiologies.

Glutamine with probiotics attenuates intestinal inflammation and oxidative stress in a rat burn injury model through altered iNOS gene aberrant methylation

Severe burns may lead to intestinal inflammation and oxidative stress resulting in intestinal barrier damage and gut dysfunction. In the management of severe burns, therapies are needed to attenuate whole-body inflammatory responses and control the burden of oxidative stress. In this study, we evaluated the effects of oral glutamine (Gln) with probiotics on burn-induced intestinal inflammation and oxidative stress using a Wistar rat burn injury model. We then explored potential molecular mechanisms for the effects of glutamine and probiotics on intestinal tissue inflammation and oxidative stress. We found that glutamine and probiotics together significantly inhibited nitric oxide (NO) content; reduced levels of the inflammatory factors TNF-α, IL-6, and IL-8; and altered expression of oxidative stress factors including reactive oxygen species and superoxide dismutase. We found that the apoptotic proportion of intestinal epithelial cells in severely burned subjects was notably decreased following treatment with glutamine plus probiotics. We also found that glutamine and probiotics given together markedly reduced NO content by down-regulating the expression of iNOS in blood and intestinal tissue. These findings indicate that regulation of the iNOS gene plays a pivotal role in inflammation and oxidative stress in the response to severe burns in the Wistar rat. We then further investigated the mechanism by which combined therapy with glutamine and probiotics might reduce expression of iNOS and found that this treatment resulted in increased methylation of the iNOS gene. The methylation level of the iNOS gene was found to be regulated via differential expression of DNMT1 and Tet1. Collectively, our results suggest that combined therapy with glutamine and probiotics can markedly reduce the synthesis of NO, suppressing intestinal inflammation and oxidative stress in the Wistar rat burn injury model.