Diabetic mice exhibited a peculiar alteration in body composition with exaggerated ectopic fat deposition after muscle injury due to anomalous cell differentiation

Integrating spatial transcriptomics and single-nucleus RNA-seq revealed the specific inhibitory effects of TGF-β on intramuscular fat deposition

Intramuscular fat (IMF) is a complex adipose tissue within skeletal muscle, appearing specially tissue heterogeneous, and the factors influencing its formation remain unclear. In conditions such as diabetes, aging, and muscle wasting, IMF was deposited in abnormal locations in skeletal muscle, damaged the normal physiological functions of skeletal muscle. Here, we used Longissimus dorsi muscles from pigs with different IMF contents as samples and adopted a method combining spatial transcriptome (ST) and single-nucleus RNA-seq to identify the spatial heterogeneity of IMF. ST revealed that genes involved in TGF-β signaling pathways were specifically highly enriched in IMF. In lean pigs, IMF autocrine produces more TGF-β2, while in obese pigs, IMF received more endothelial-derived TGF-β1. In vitro experiments have proven that porcine endothelial cells in a simulated high-fat environment released more TGF-β1 than TGF-β2. Moreover, under obesity mice, the addition of TGF-β after muscle injury abolished IMF production and slowed muscle repair, whereas TGF-β inhibition accelerated muscle repair. Our findings demonstrate that the TGF-β pathway specifically regulates these processes, suggesting it as a potential therapeutic target for managing muscle atrophy in obese patients and enhancing muscle repair while reducing IMF deposition.

Impaired skeletal muscle regeneration in diabetes: From cellular and molecular mechanisms to novel treatments

Diabetes represents a major public health concern with a considerable impact on human life and healthcare expenditures. It is now well established that diabetes is characterized by a severe skeletal muscle pathology that limits functional capacity and quality of life. Increasing evidence indicates that diabetes is also one of the most prevalent disorders characterized by impaired skeletal muscle regeneration, yet underlying mechanisms and therapeutic treatments remain poorly established. In this review, we describe the cellular and molecular alterations currently known to occur during skeletal muscle regeneration in people with diabetes and animal models of diabetes, including its associated comorbidities, e.g., obesity, hyperinsulinemia, and insulin resistance. We describe the role of myogenic and non-myogenic cell types on muscle regeneration in conditions with or without diabetes. Therapies for skeletal muscle regeneration and gaps in our knowledge are also discussed, while proposing future directions for the field.

Fibro-adipogenic progenitors in physiological adipogenesis and intermuscular adipose tissue remodeling

Excessive accumulation of intermuscular adipose tissue (IMAT) is a common pathological feature in various metabolic and health conditions and can cause muscle atrophy, reduced function, inflammation, insulin resistance, cardiovascular issues, and unhealthy aging. Although IMAT results from fat accumulation in muscle, the mechanisms underlying its onset, development, cellular components, and functions remain unclear. IMAT levels are influenced by several factors, such as changes in the tissue environment, muscle type and origin, extent and duration of trauma, and persistent activation of fibro-adipogenic progenitors (FAPs). FAPs are a diverse and transcriptionally heterogeneous population of stromal cells essential for tissue maintenance, neuromuscular stability, and tissue regeneration. However, in cases of chronic inflammation and pathological conditions, FAPs expand and differentiate into adipocytes, resulting in the development of abnormal and ectopic IMAT. This review discusses the role of FAPs in adipogenesis and how they remodel IMAT. It highlights evidence supporting FAPs and FAP-derived adipocytes as constituents of IMAT, emphasizing their significance in adipose tissue maintenance and development, as well as their involvement in metabolic disorders, chronic pathologies and diseases. We also investigated the intricate molecular pathways and cell interactions governing FAP behavior, adipogenesis, and IMAT accumulation in chronic diseases and muscle deconditioning. Finally, we hypothesize that impaired cellular metabolic flexibility in dysfunctional muscles impacts FAPs, leading to IMAT. A deeper understanding of the biology of IMAT accumulation and the mechanisms regulating FAP behavior and fate are essential for the development of new therapeutic strategies for several debilitating conditions.

Fat infiltration in skeletal muscle: Influential triggers and regulatory mechanism

Fat infiltration in skeletal muscle (also known as myosteatosis) is now recognized as a distinct disease from sarcopenia and is directly related to declining muscle capacity. Hence, understanding the origins and regulatory mechanisms of fat infiltration is vital for maintaining skeletal muscle development and improving human health. In this article, we summarized the triggering factors such as aging, metabolic diseases and metabolic syndromes, nonmetabolic diseases, and muscle injury that all induce fat infiltration in skeletal muscle. We discussed recent advances on the cellular origins of fat infiltration and found several cell types including myogenic cells and non-myogenic cells that contribute to myosteatosis. Furthermore, we reviewed the molecular regulatory mechanism, detection methods, and intervention strategies of fat infiltration in skeletal muscle. Based on the current findings, our review will provide new insight into regulating function and lipid metabolism of skeletal muscle and treating muscle-related diseases.

Gastric inhibitory polypeptide receptor antagonism suppresses intramuscular adipose tissue accumulation and ameliorates sarcopenia

BACKGROUND

Intramuscular adipose tissue (IMAT) formation derived from muscle fibro-adipogenic progenitors (FAPs) has been recognized as a pathological feature of sarcopenia. This study aimed to explore whether genetic and pharmacological gastric inhibitory polypeptide (GIP) receptor antagonism suppresses IMAT accumulation and ameliorates sarcopenia in mice.

METHODS

Whole body composition, grip strength, skeletal muscle weight, tibialis anterior (TA) muscle fibre cross-sectional area (CSA) and TA muscle IMAT area were measured in young and aged male C57BL/6 strain GIP receptor (Gipr)-knockout (Gipr-/- ) and wild-type (Gipr+/+ ) mice. FAPs isolated from lower limb muscles of 12-week-old Gipr+/+ mice were cultured with GIP, and their differentiation into mature adipocytes was examined. Furthermore, TA muscle IMAT area and fibre CSA were measured in untreated Gipr-/- mice and GIP receptor antagonist-treated Gipr+/+ mice after glycerol injection into the TA muscles.

RESULTS

Body composition analysis revealed that 104-week-old Gipr-/- mice had a greater proportion of lean tissue mass (73.7 ± 1.2% vs. 66.5 ± 2.7%, P < 0.05 vs. 104-week-old Gipr+/+ mice) and less adipose tissue mass (13.1 ± 1.3% vs. 19.4 ± 2.6%, P < 0.05 vs. 104-week-old Gipr+/+ mice). Eighty-four-week-old Gipr-/- mice exhibited increases in grip strength (P < 0.05), weights of TA (P < 0.05), soleus (P < 0.01), gastrocnemius (P < 0.05) and quadriceps femoris (P < 0.01) muscles, and average TA muscle fibre CSA (P < 0.05) along with a reduction in TA muscle IMAT area assessed by the number of perilipin-positive cells (P < 0.0001) compared with 84-week-old Gipr+/+ mice. Oil Red O staining analysis revealed 1.6- and 1.7-fold increased adipogenesis in muscle FAPs cultured with 10 and 100 nM of GIP (P < 0.01 and P < 0.001 vs. 0 nM of GIP, respectively). Furthermore, both untreated Gipr-/- mice and GIP receptor antagonist-treated Gipr+/+ mice for 14 days after glycerol injection into the TA muscles at 12 weeks of age showed reduced TA muscle IMAT area (1.39 ± 0.38% and 2.65 ± 0.36% vs. 6.54 ± 1.30%, P < 0.001 and P < 0.01 vs. untreated Gipr+/+ mice, respectively) and increased average TA muscle fibre CSA (P < 0.01 and P < 0.05 vs. untreated Gipr+/+ mice, respectively).

CONCLUSIONS

GIP promotes the differentiation of muscle FAPs into adipocytes and its receptor antagonism suppresses IMAT accumulation and promotes muscle regeneration. Pharmacological GIP receptor antagonism may serve as a novel therapeutic approach for sarcopenia.

Characterizing conventional ankle MRI findings of nerve and muscle changes in diabetic patients: a case-control study

BACKGROUND

Foot and ankle amputation is a feared complication of diabetic neuropathy and diabetes mellitus (DM) accounts for 80% of all in-hospital amputations. Magnetic resonance neurography is an effective tool in characterizing neuromuscular sequelae of the disease. However, conventional ankle MRI is more commonly performed and has not been studied to assess neuromuscular changes of DM.

OBJECTIVE

The objective is to characterize neuromuscular changes of diabetic patients in a case-control study using conventional ankle MRI.

METHODS

Between November 2019 and July 2021, 110 consecutive ankle MRI scans (n = 102 patients) at our county hospital were reviewed and met the inclusion criteria. Patients were divided into two cohorts, diabetic (N = 63) and non-diabetic (N = 39). Demographics, HgbA1c, and reason for MRI study were collected via retrospective chart review. The presence of intramuscular edema-like signal, pattern of the edema, muscle fatty infiltration, and measurements of the cross-sectional area of the posterior, medial, and lateral tibial nerves (PTN, MPN, and LPN) was recorded blinded to the clinical findings by two readers.

RESULTS

Muscle edema-like signal was much more likely to be found in DM (odds ratio 19.5, 95% CI 7.0-54.6, p < 0.001). DM also showed increase of 0.87 in the mean grade of muscle fatty infiltration (p < 0.001). There were higher rates of nerve T2 hyperintensity (odds ratio 14.0, 95% CI 3.1-62.7, p < 0.001) and the measured areas of the PTN, MPN, and LPN were also larger in DM compared to their non-diabetic counterparts (PTN: 0.16 cm² vs. 0.10 cm², p < 0.01; MPN: 0.09 cm² vs. 0.05 cm², p < 0.01; LPN: 0.07 cm² vs. 0.04 cm², p < 0.05).

CONCLUSION

Conventional ankle MRIs can be used to detect DM-related neuromuscular changes.

The involvement of FATP1 regulating skeletal muscle fat deposition in stressed broilers was affected by fatty acid substrates

Fatty acid transport protein 1 (FATP1), plays a major role in the transport and uptake of fatty acids into cells. The effect of FATP1 on the regulation of skeletal muscle fat uptake and deposition in stressed broiler chickens was investigated both in vivo and in vitro, and the effect of different fatty acid substrates were also included. Dexamethasone (DEX), a synthetic glucocorticoid (GCs), was employed to induce a hyper glucocorticoid milieu and simulate stress. The in vivo results showed that DEX would increase the mRNA expression of FATP1 and fat deposition in muscle tissues (P < 0.05), the very-low-density lipoprotein (VLDL) and insulin (INS) levels were significantly increased in the plasma by DEX (P < 0.05), and the mRNA levels of the glucocorticoid receptor (GR), adiponectin receptor (ADPNR) and peroxisomal proliferator-activated receptor α (PPARα) in thigh were also up-regulated by DEX (P < 0.05). In vitro experiment, DEX did not affect the myoblast fat deposition and PPARα and FATP1 expressions without the external fatty acid (P > 0.05). Under PA pre-treatment, both myoblast fatty acid uptake and fat deposition were promoted by DEX treatment (P < 0.05), and the effects of DEX on the gene expressions of GR, ADPNR, PPARα and FATP1 were upregulated first and then downregulated as the dose of DEX increases; while under OA pre-treatment, the myoblast fat deposition was not affected by DEX (P > 0.05), the fatty acid uptake was decreased by DEX at 500 nM compared to control (P < 0.05). When GR and PPARα were, respectively inhibited by specific inhibitors RU486 and GW6471, the effects of DEX on fatty acid uptake were reversed for PA pre-treated myoblasts (P < 0.05) but not for OA pre-treated myoblasts (P > 0.05). These results indicate that FATP1 regulation by GCs was affected by fatty acid substrate - saturated fatty acids were favorable for fat uptake and deposition, while unsaturated fatty acids were not. GCs may affect the ADPNR-PPARα-FATP1 pathway by binding to its receptors, thus regulating the uptake of saturated fatty acids into myoblasts.

Juzentaihoto Suppresses Muscle Atrophy in KKAy Mice

In obese patients with type 2 diabetes, reduced insulin sensitivity, increased production of inflammatory cytokines, and increased oxidative stress were observed, which lead to decreased protein synthesis and increased proteolysis in the skeletal muscles. Juzentaihoto (JTT) is herbal medicine and we have previously reported that the administration of JTT hot water extract alleviates skeletal muscle atrophy in a mouse model with streptozotocin-induced type 1 diabetes. In this study, we evaluated the inhibitory effects of JTT on muscle atrophy in a mouse model with obesity and type 2 diabetes. JTT was administered to KKAy mice with type 2 diabetic obesity and its effects on the skeletal muscles were evaluated. After JTT administration in KKAy mice, the wet weight and muscle fibre cross-sectional area of gastrocnemius increased and the time duration of exercise in the rotarod test improved. In addition, the serum levels of tumour necrosis factor-α and interleukin-6 decreased, adiponectin levels increased, and homeostasis model assessment for insulin resistance improved. Furthermore, JTT administration decreased the mRNA levels of ubiquitin ligase (atrogin-1, muscle RING-finger protein-1), increased the mRNA levels of Sirtuin1 in gastrocnemius. Our results suggest that JTT improves insulin resistance, suppresses inflammation, and reduces oxidative stress in KKAy mice, thereby suppressing skeletal muscle atrophy. JTT administration in clinical practice is expected to improve muscle atrophy in patients with obesity and type 2 diabetes.

Galectin-3 promotes the adipogenic differentiation of PDGFRα+ cells and ectopic fat formation in regenerating muscle

Worldwide prevalence of obesity is associated with the increase of lifestyle-related diseases. The accumulation of intermuscular adipose tissue (IMAT) is considered a major problem whereby obesity leads to sarcopenia and metabolic disorders and thus is a promising target for treating these pathological conditions. However, whereas obesity-associated IMAT is suggested to originate from PDGFRα+ mesenchymal progenitors, the processes underlying this adipogenesis remain largely unexplored. Here, we comprehensively investigated intra- and extracellular changes associated with these processes using single-cell RNA sequencing and mass spectrometry. Our single-cell RNA sequencing analysis identified a small PDGFRα+ cell population in obese mice directed strongly toward adipogenesis. Proteomic analysis showed that the appearance of this cell population is accompanied by an increase in galectin-3 in interstitial environments, which was found to activate adipogenic PPARγ signals in PDGFRα+ cells. Moreover, IMAT formation during muscle regeneration was significantly suppressed in galectin-3 knockout mice. Our findings, together with these multi-omics datasets, could unravel microenvironmental networks during muscle regeneration highlighting possible therapeutic targets against IMAT formation in obesity.

Identification and analysis of lncRNA, miRNA and mRNA related to subcutaneous and intramuscular fat in Laiwu pigs

BACKGROUND

Long non-coding RNAs (lncRNAs) regulate adipocyte differentiation and metabolism, However, their function on subcutaneous and intramuscular adipose tissues in pigs is unclear. Intramuscular fat (IMF) is an important indicator for evaluating meat quality. Breeds with high IMF content are often accompanied by high subcutaneous fat (SCF), which severely affects the meat rate of pigs. It is of great significance for porcine breeding to study the mechanism of lncRNA related to adipogenesis and lipid metabolism.

METHODS

We identified differentially expressed lncRNAs, miRNAs and mRNAs in subcutaneous and intramuscular adipose tissues in three female Laiwu pigs by deep RNA-sequencing(|log2foldchange|≥1, P_value ≤ 0.05). The gene expression profiles of IMF and SCF in Laiwu pigs were comparatively analyzed by Bioinformatics methods to identify key lncRNAs, miRNAs, and mRNAs associated with lipid metabolism and adipogenesis.

RESULTS

A total of 1209 lncRNAs (DElncRNAs), 286 miRNAs (DEmiRNAs), and 1597 mRNAs (DEgenes) were differentially expressed between two types of adipose. Among them, 17 DElncRNAs and 103 target genes play a role in the co-expression network, as well as 59 DElncRNAs, 44 DEmiRNAs, and 88 DEgenes involved in ceRNA network. In GO(Gene Ontology) and KEGG (Kyoto Encyclopedia of Genes and Genomes) pathway analysis of DElncRNAs their target genes involved in many adipogenesis and lipid metabolism biological processes and signaling pathways, such as PPAR signaling pathway, Wnt signaling pathway, MAPK signaling pathway.

CONCLUSIONS

By constructing co-expression and ceRNAs network we found that Wnt signaling pathway play a critical regulatory role in intramuscular adipogenesis and lipid accumulation in Laiwu pigs. TCONS_00006525, TCONS_00046551 and TCONS_00000528 may target WNT5A, WNT10B and FDZ3 in co-expression network, TCONS_00026517 and other lncRNAs regulate the expression of PPARG, RXRG and SCD in ceRNA network, and were involved in Wnt signaling pathway. This study provides a theoretical basis for further understanding the post-transcriptional regulation mechanism of meat quality formation, predicting and treating diseases caused by ectopic fat.

Fibro-adipogenic progenitors in skeletal muscle homeostasis, regeneration and diseases

Skeletal muscle possesses a remarkable regenerative capacity that relies on the activity of muscle stem cells, also known as satellite cells. The presence of non-myogenic cells also plays a key role in the coordination of skeletal muscle regeneration. Particularly, fibro-adipogenic progenitors (FAPs) emerged as master regulators of muscle stem cell function and skeletal muscle regeneration. This population of muscle resident mesenchymal stromal cells has been initially characterized based on its bi-potent ability to differentiate into fibroblasts or adipocytes. New technologies such as single-cell RNAseq revealed the cellular heterogeneity of FAPs and their complex regulatory network during muscle regeneration. In acute injury, FAPs rapidly enter the cell cycle and secrete trophic factors that support the myogenic activity of muscle stem cells. Conversely, deregulation of FAP cell activity is associated with the accumulation of fibrofatty tissue in pathological conditions such as muscular dystrophies and ageing. Considering their central role in skeletal muscle pathophysiology, the regulatory mechanisms of FAPs and their cellular and molecular crosstalk with muscle stem cells are highly investigated in the field. In this review, we summarize the current knowledge on FAP cell characteristics, heterogeneity and the cellular crosstalk during skeletal muscle homeostasis and regeneration. We further describe their role in muscular disorders, as well as different therapeutic strategies targeting these cells to restore muscle regeneration.

Pim1 knockout alleviates sarcopenia in aging mice via reducing adipogenic differentiation of PDGFRα+ mesenchymal progenitors

BACKGROUND

Sarcopenia widely exists in elderly people and triggers numerous age-related events. The essential pathologic change lies in the increased intramuscular adipose tissue after aging with no exception to non-obese objects. Pim1 appears to be associated with adipogenic differentiation in recent studies, inspiring us to explore whether it regulates adipogenesis in aging muscles and affects sarcopenia.

METHODS

Wild-type and Pim1 knockout C57/BL6J mice were randomized into young and old groups. Histo-pathological and molecular biological methods were applied to assess the intramuscular adipose tissue content, the atrophy and regeneration, and the expressions of Pim1 and adipogenic transcription factors. PDGFRα+ mesenchymal progenitors were separated and their replicative aging model were established. Different time of adipogenic induction and different amounts of Pim1 inhibitor were applied, after which the adipogenic potency were evaluated. The expressions of Pim1 and adipogenic transcription factors were measured through western blotting.

RESULTS

The aging mice demonstrated decreased forelimb grip strength (P = 0.0003), hanging impulse (P < 0.0001), exhaustive running time (P < 0.0001), tetanic force (P = 0.0298), lean mass (P = 0.0008), and percentage of gastrocnemius weight in body weight (P < 0.0001), which were improved by Pim1 knockout (P = 0.0015, P = 0.0222, P < 0.0001, P = 0.0444, P = 0.0004, and P = 0.0003, respectively). To elucidate the mechanisms, analyses showed that Pim1 knockout decreased the fat mass (P = 0.0005) and reduced the intramuscular adipose tissue content (P = 0.0008) by inhibiting the C/EBPδ pathway (P = 0.0067) in aging mice, resulting in increased cross-sectional area of all and fast muscle fibres (P = 0.0017 and 0.0024 respectively), decreased levels of MuRF 1 and atrogin 1 (P = 0.0001 and 0.0329 respectively), and decreased content of Pax7 at the basal state (P = 0.0055). In vitro, senescent PDGFRα+ mesenchymal progenitors showed significantly increased the intracellular adipose tissue content (OD510) compared with young cells after 6 days of adipogenic induction (P < 0.0001). The Pim1 expression was elevated during adipogenic differentiation, and Pim1 inhibition significantly reduced the OD510 in senescent cells (P = 0.0040) by inhibiting the C/EBPδ pathway (P = 0.0047).

CONCLUSIONS

Pim1 knockout exerted protective effects in sarcopenia by inhibiting the adipogenic differentiation of PDGFRα+ mesenchymal progenitors induced by C/EBPδ activation and thus reducing the intramuscular adipose tissue content in aging mice. These results provide a potential target for the treatment of sarcopenia.

Obesity phenotypes are, in part, associated with physical activity in diabetic hemodialysis patients

PURPOSE

To investigate the prevalence of obesity phenotypes and their association with physical activity levels among diabetic hemodialysis patients.

METHODS

This is a cross-sectional study with 84 diabetic hemodialysis patients (63.5 ± 9.4 years, 54.8% of men). Obesity was diagnosed as high body fat (≥ 40% for male and ≥ 30% for female). Sarcopenic obesity was considered if low skeletal muscle mass (< 20.0 kg for males and < 15.0 kg for females) and obesity were combined. Dynapenic obesity was defined in the presence of low handgrip strength (< 27 kg for males and < 16 kg for females) and obesity. Muscle failure obesity was confirmed in the concomitant presence of obesity, sarcopenia, and dynapenia. Physical activity level was assessed by the Baecke questionnaire and patients were classified as low physical activity according to the first tertile for each of and total domains.

RESULTS

Fifty-four patients (64%) presented obesity. From these, 5 (6%), 19 (23%) and 8 (10%) were classified as sarcopenic obese, dynapenic obese, and muscle failure obese, respectively, and 22 (26%) were only obese. Patients with sarcopenic obesity and muscle failure obesity had lower leisure and locomotion physical activity scores than non-obese, whereas the total domain score did not differ across the groups. Muscle failure obesity was independently associated with low leisure physical activity (OR 10.8, 95% CI 1.3-88.1). Only sarcopenic obesity was independently associated with the locomotion and total physical activity domains (OR 15.4, 95% CI 1.4-90.2 and OR 17.0, 95% CI 1.5-95.4, respectively).

CONCLUSION

Our study found a lower prevalence of sarcopenic obesity compared to dynapenic obesity and muscle failure obesity among diabetic hemodialysis patients. Moreover, sarcopenic obesity and muscle failure obesity, but not dynapenic obesity, were associated with low physical activity levels.

Skeletal Lipocalin-2 Is Associated with Iron-Related Oxidative Stress in ob/ob Mice with Sarcopenia

Obesity and insulin resistance accelerate aging-related sarcopenia, which is associated with iron load and oxidative stress. Lipocalin-2 (LCN2) is an iron-binding protein that has been associated with skeletal muscle regeneration, but details regarding its role in obese sarcopenia remain unclear. Here, we report that elevated LCN2 levels in skeletal muscle are linked to muscle atrophy-related inflammation and oxidative stress in leptin-deficient ob/ob mice. RNA sequencing analyses indicated the LCN2 gene expression is enhanced in skeletal muscle of ob/ob mice with sarcopenia. In addition to muscular iron accumulation in ob/ob mice, expressions of iron homeostasis-related divalent metal transporter 1, ferritin, and hepcidin proteins were increased in ob/ob mice compared to lean littermates, whereas expressions of transferrin receptor and ferroportin were reduced. Collectively, these findings demonstrate that LCN2 functions as a potent proinflammatory factor in skeletal muscle in response to obesity-related sarcopenia and is thus a therapeutic candidate target for sarcopenia treatment.

Retinoic acid signalling in fibro/adipogenic progenitors robustly enhances muscle regeneration

BACKGROUND

During muscle regeneration, excessive formation of adipogenic and fibrogenic tissues, from their respective fibro/adipogenic progenitors (FAPs), impairs functional recovery. Intrinsic mechanisms controlling the proliferation and differentiation of FAPs remain largely unexplored.

METHODS

Here, we investigated the role of retinoic acid (RA) signalling in regulating FAPs and the subsequent effects on muscle restoration from a cardiotoxin-induced injury. Blockage of retinoic acid receptor (RAR) signalling was achieved through dominant negative retinoic acid receptor α (RARα403) expression specific in PDGFRα+ FAPs in vivo and by BMS493 treatment in vitro. Effects of RAR-signalling on FAP cellularity and muscle regeneration were also investigated in a high-fat diet-induced obese mice model.

FINDINGS

Supplementation of RA increased the proliferation of FAPs during the early stages of regeneration while suppressing FAP differentiation and promoting apoptosis during the remodelling stage. Loss of RAR-signalling caused ectopic adipogenic differentiation of FAPs and impaired muscle regeneration. Furthermore, obesity disrupted the cellular transition of FAPs and attenuated muscle regeneration. Supplementation of RA to obese mice not only rescued impaired muscle fibre regeneration, but also inhibited infiltration of fat and fibrotic tissues during muscle repair. These beneficial effects were abolished after blocking RAR-signalling in FAPs of obese mice.

INTERPRETATION

These data suggest that RAR-signalling in FAPs is a critical therapeutic target for suppressing differentiation of FAPs and facilitating the regeneration of muscle and other tissues.

FUNDING

This study was supported by grants from the National Institutes of Health (R01-HD067449 and R21-AG049976) to M.D.

What are the factors associated with sarcopenia-related variables in adult women with severe obesity?

BACKGROUND

Understanding the association between sarcopenia-related variables and several risk factors may help to implement interventions aimed at preventing its occurrence by reducing or controlling the identified risk factors. Although changes in body composition occur in both sexes, in women, muscle loss is accentuated due to decreased estrogen levels following menopause. This study aims to determine the factors associated with sarcopenia-related parameters in middle-aged women identified with class II/III obesity (body mass index [BMI] ≥ 35 kg/m2).

METHODS

The study included 104 women with severe obesity (40.23 ± 8.49 years) with an average body fat percentage of 52.45 ± 4.14%. Sarcopenia was assessed using total appendicular skeletal muscle mass (ASMM), appendicular skeletal muscle mass index (ASMMI), and appendicular skeletal muscle mass adjusted by BMI (ASMM/BMI) as evaluated using dual energy X-ray absorptiometry (DXA). Hand grip strength (HGS) and HGS adjusted by BMI (HGS/BMI) were evaluated using dynamometry. Functional performance was assessed using the walking speed test (WS). The explanatory variables were age, lifestyle, comorbidities, food consumption, and metabolic parameters. A multivariate linear regression was performed.

RESULTS

Factors associated with sarcopenia-related variables in 104 severely obese women with a mean BMI of 43.85 kg/m2 were as follows: ASMMI negatively correlated with serum levels of tetraiodothyronine (T4) and tobacco use; ASMM/BMI negatively correlated with age, serum T4 levels, and diabetes; ASMM negatively correlated with T4 serum levels and diabetes; HGS negatively correlated with age and hypercholesterolemia, and positively correlated with low-density lipoprotein cholesterol (LDL-c); HGS/BMI negatively correlated with age and hypercholesterolemia and positively correlated with LDL-c; and WS negatively correlated with hypothyroidism and diabetes.

CONCLUSION

In severely obese women, muscle mass and function were inversely associated with age, smoking status, endocrine parameters, hypercholesterolemia, and comorbidities such as diabetes. Thus, the results of this investigation are relevant in supporting the development of clinical interventions to aid in the prevention of sarcopenia in adult women with severe obesity.

Uremic Sarcopenia: Clinical Evidence and Basic Experimental Approach

Sustained physical activity extends healthy life years while a lower activity due to sarcopenia can reduce them. Sarcopenia is defined as a decrease in skeletal muscle mass and strength due not only to aging, but also from a variety of debilitating chronic illnesses such as cancer and heart failure. Patients with chronic kidney disease (CKD), who tend to be cachexic and in frail health, may develop uremic sarcopenia or uremic myopathy due to an imbalance between muscle protein synthesis and catabolism. Here, we review clinical evidence indicating reduced physical activity as renal function deteriorates and explore evidence-supported therapeutic options focusing on nutrition and physical training. In addition, although sarcopenia is a clinical concept and difficult to recapitulate in basic research, several in vivo approaches have been attempted, such as rodent subtotal nephrectomy representing both renal dysfunction and muscle weakness. This review highlights molecular mechanisms and promising interventions for uremic sarcopenia that were revealed through basic research. Extensive study is still needed to cast light on the many aspects of locomotive organ impairments in CKD and explore the ways that diet and exercise therapies can improve both outcomes and quality of life at every level.

High-fat diet pre-conditioning improves microvascular remodelling during regeneration of ischaemic mouse skeletal muscle

AIM

Critical limb ischaemia (CLI) is characterized by inadequate angiogenesis, arteriolar remodelling and chronic myopathy, which are most severe in type 2 diabetic patients. Hypertriglyceridaemia, commonly observed in these patients, compromises macrovascular function. However, the effects of high-fat diet-induced increases in circulating lipids on microvascular remodelling are not established. Here, we investigated if high-fat diet would mimic the detrimental effect of type 2 diabetes on post-ischaemia vascular remodelling and muscle regeneration, using a mouse model of hindlimb ischaemia.

METHODS

Male C57Bl6/J mice were fed with normal or high-fat diets for 8 weeks prior to unilateral femoral artery ligation. Laser doppler imaging was used to assess limb perfusion recovery. Vascular recovery, inflammation, myofibre regeneration and fibrosis were assessed at 4 or 14 days post-ligation by histology and RNA analyses. Capillary-level haemodynamics were assessed by intravital microscopy of control and regenerating muscles 14 days post-ligation.

RESULTS

High-fat diet increased muscle succinate dehydrogenase activity and capillary-level oxygen supply. At 4 days post-ligation, no diet differences were detected in muscle damage, inflammatory infiltration or capillary activation. At 14 days post-ligation, high fat-fed mice displayed accelerated limb blood flow recovery, elevated capillary and arteriole densities as well as greater red blood cell supply rates and capillary-level oxygen supply. Regenerating muscles from high fat-fed mice displayed lower interstitial fat and collagen deposition.

CONCLUSION

The muscle-level adaptations to high-fat diet improved multiple aspects of muscle recovery in response to ischaemia and did not recapitulate the worse outcomes seen in diabetic CLI patients.

Role of Metabolic Stress and Exercise in Regulating Fibro/Adipogenic Progenitors

Obesity is a major public health concern and is associated with decreased muscle quality (i.e., strength, metabolism). Muscle from obese adults is characterized by increases in fatty, fibrotic tissue that decreases the force producing capacity of muscle and impairs glucose disposal. Fibro/adipogenic progenitors (FAPs) are muscle resident, multipotent stromal cells that are responsible for muscle fibro/fatty tissue accumulation. Additionally, they are indirectly involved in muscle adaptation through their promotion of myogenic (muscle-forming) satellite cell proliferation and differentiation. In conditions similar to obesity that are characterized by chronic muscle degeneration, FAP dysfunction has been shown to be responsible for increased fibro/fatty tissue accumulation in skeletal muscle, and impaired satellite cell function. The role of metabolic stress in regulating FAP differentiation and paracrine function in skeletal muscle is just beginning to be unraveled. Thus, the present review aims to summarize the recent literature on the role of metabolic stress in regulating FAP differentiation and paracrine function in skeletal muscle, and the mechanisms responsible for these effects. Furthermore, we will review the role of physical activity in reversing or ameliorating the detrimental effects of obesity on FAP function.

The effect of type 2 diabetes mellitus and obesity on muscle progenitor cell function

In addition to its primary function to provide movement and maintain posture, the skeletal muscle plays important roles in energy and glucose metabolism. In healthy humans, skeletal muscle is the major site for postprandial glucose uptake and impairment of this process contributes to the pathogenesis of type 2 diabetes mellitus (T2DM). A key component to the maintenance of skeletal muscle integrity and plasticity is the presence of muscle progenitor cells, including satellite cells, fibroadipogenic progenitors, and some interstitial progenitor cells associated with vessels (myo-endothelial cells, pericytes, and mesoangioblasts). In this review, we aim to discuss the emerging concepts related to these progenitor cells, focusing on the identification and characterization of distinct progenitor cell populations, and the impact of obesity and T2DM on these cells. The recent advances in stem cell therapies by targeting diabetic and obese muscle are also discussed.

Casting the net broader to confirm our imaginations: the long road to treating wasting disorders

Wasting embraces muscle and tissue wasting in sarcopenia and cachexia. This article describes recent advances in the field published in the Journal of Cachexia, Sarcopenia and Muscle concerning diagnostic tools, biomarker development, pathophysiology, and treatment. Studies discussed herein embrace those on sarcopenia and cachexia in heart failure, chronic obstructive pulmonary disease, and cancer including also animal models.

The effect of blood glucose regulation on sarcopenia parameters in obese and diabetic patients

Objectives

This study aims to evaluate the effect of blood glucose regulation on sarcopenia parameters in sarcopenic, obese, and poorly- regulated diabetic patients.

Patients and methods

Between June 2013 and December 2013, a total of 147 patients (64 males, 83 females; mean age 70.3±6.3 years; range, 60 to 90 years) who were diagnosed with sarcopenia according to the European Working Group on Sarcopenia in Older People (EWGSOP) criteria were included in the study. All patients were obese with a body mass index (BMI) of >30 kg/m2 and their glycated hemoglobin (HbA1c) levels were above 8%. Sarcopenia parameters including the gait speed, muscle strength, muscle mass, and handgrip strength were assessed. After a six-month treatment period, the patients were divided into two groups according to their HbA1c levels as having <8% or >8%. Sarcopenia parameters were evaluated before and after receiving treatment.

Results

The mean disease duration was 16±6.2 years. Sixty patients were found to have a HbA1c level of <8% and 45 patients with a HbA1c level of ≥8% at sixth months of follow-up. In better regulated group, sarcopenia parameters such as gait speed, muscle mass, and handgrip strength improved; however, only the change in the muscle mass was found to be statistically significant (p=0.041). There was no significant change in the parameters of sarcopenia in the patient group with a HbA1c level ≥8%. A negative correlation was found between the muscle mass and HbA1c levels in good- and poorly-regulated groups (p=0.039 r:-0.327 and p=0.044 r:-0.183, respectively).

Conclusion

Our study demonstrates that lowering HbA1c levels may have positive effects on the muscle mass even in diabetic and sarcopenic obese elderly individuals.

An analysis of the types of recently published research in the field of cachexia

Introduction Cachexia is a common complication of many and varied chronic disease processes, yet it has received very little attention as an area of clinical research effort until recently. We sought to survey the contemporary literature on published research into cachexia to define where it is being published and the proportion of output classified into the main types of research output. Methods I searched the PubMed listings under the topic research term "cachexia" and related terms for articles published in the calendar years of 2015 and 2016, regardless of language. Searches were conducted and relevant papers extracted by two observers, and disagreements were resolved by consensus. Results There were 954 publications, 370 of which were review articles or commentaries, 254 clinical observations or non-randomised trials, 246 original basic science reports and only 26 were randomised controlled trials. These articles were published in 478 separate journals but with 36% of them being published in a core set of 23 journals. The H-index of these papers was 25 and there were 147 papers with 10 or more citations. Of the top 100 cited papers, 25% were published in five journals. Of the top cited papers, 48% were review articles, 18% were original basic science, and 7% were randomised clinical trials. Discussion This analysis shows a steady but modest increase in publications concerning cachexia with a strong pipeline of basic science research but still a relative lack of randomised clinical trials, with none exceeding 1000 patients. Research in cachexia is still in its infancy, but the solid basic science effort offers hope that translation into randomised controlled clinical trials may eventually lead to effective therapies for this troubling and complex clinical disease process.

Diabetic mice exhibited a peculiar alteration in body composition with exaggerated ectopic fat deposition after muscle injury due to anomalous cell differentiation

BACKGROUND

Sarcopenic obesity, age-related muscle loss, which is compensated by an increase in fat mass, impairs quality of life in elderly people. Although the increase in intramuscular fat is associated with decreased insulin sensitivity and increased metabolic risk factors, the origin of diabetes-associated intramuscular fat has not been elucidated. Here, we investigated intramuscular fat deposition using a muscle injury model in type 2 diabetic mice.

METHODS

Male 8-week-old C57BL/6 and 8-week-old and 26-week-old KKAy underwent intramuscular injection of cardiotoxin (Ctx) (100 μL/10 μM) into the tibialis anterior (TA) muscles. After 2 weeks, the muscles were removed and evaluated.

RESULTS

KKAy exhibited impaired muscle regeneration and ectopic fat deposition. Such impairment was more marked in older KKAy. These changes were also observed in another diabetic mouse model, db/db and diet-induced obese mice but not in streptozocin-induced diabetic mice. Deposited fat was platelet-derived growth factor (PDGF) receptor alpha positive and its cytoskeleton was stained with Masson's trichrome, indicating it to be of fibro-adipocyte progenitor cell origin. Expression of a myogenic marker, myoD, was lower and that of PDGF receptor alpha and CCAAT/enhancer binding protein (CEBP) alpha was higher in Ctx-injured TA of KKAy compared with that of C57BL/6. Peroxisome proliferator-activated receptor γ (PPARγ) was highly expressed in fat-forming lesions in older KKAy. Treatment with all-trans retinoic acid prevented the formation of intramuscular fat; however, treatment with GW9662, a PPARγ antagonist, increased the fibrotic change in muscle.

CONCLUSIONS

Diabetic mice showed impaired muscle regeneration with fat deposition, suggesting that diabetes may enhance sarcopenic obesity through a mechanism involving anomalous fibro-adipocyte progenitor cell differentiation.