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.

Fatty infiltration of gastrocnemius-soleus muscle complex: Considerations for myosteatosis rehabilitation

Although previous studies have reported fatty infiltration of the gastrocnemius-soleus complex, little is known about the volumetric distribution and patterns of fatty infiltration. The purpose of this anatomical study was to document and quantify the frequency, distribution, and pattern of fatty infiltration of the gastrocnemius-soleus complex. One hundred formalin-embalmed specimens (mean age 78.1 ± 12.3 years; 48F/52M) were serially dissected to document the frequency, distribution, and pattern of fatty infiltration in the medial and lateral heads of gastrocnemius and soleus muscles. Fatty infiltration was found in 23% of specimens, 13 unilaterally (8F/5M) and 10 (5M/5F) bilaterally. The fatty infiltration process was observed to begin medially from the medial aspect of the medial head of gastrocnemius and medial margin of soleus and then progressed laterally throughout the medial head of gastrocnemius and the marginal, anterior, and posterior soleus. The lateral head of gastrocnemius remained primarily muscular in all specimens. Microscopically, the pattern of infiltration was demonstrated as intramuscular with intact aponeuroses, and septa. The remaining endo-, peri-, and epimysium preserved the overall contour of the gastrocnemius-soleus complex, even in cases of significant fatty replacement. Since the external contour of the calf is preserved, the presence of fatty infiltration may be underdiagnosed in the clinic without imaging. Myosteatosis is associated with gait and balance challenges in the elderly, which can impact quality of life and result in increased risk of falling. The findings of the study have implications in the rehabilitation management of elderly patients with sarcopenia and myosteatosis.

Effects of high-intensity training on fatty infiltration in paraspinal muscles in elderly males with osteosarcopenia - the randomized controlled FrOST study

BACKGROUND

Osteosarcopenia is a common geriatric syndrome with an increasing prevalence with age, leading to secondary diseases and complex consequences such as falls and fractures, as well as higher mortality and frailty rates. There is a great need for prevention and treatment strategies.

METHODS

In this analysis, we used magnetic resonance imaging (MRI) data from the randomised controlled FrOST trial, which enrolled community-dwelling osteosarcopenic men aged > 72 years randomly allocated to 16 months of twice-weekly high-intensity resistance training (HIRT) or a non-training control group. MR Dixon imaging was used to quantify the effects of HIRT on muscle fat infiltration in the paraspinal muscles, determined as changes in muscle tissue, fat faction and intermuscular adipose tissue (IMAT) in the erector spinae and psoas major muscles. Intention-to-treat analysis with multiple imputation was used to analyse the data set.

RESULTS

After 16 months of intervention, 15 men from the HIRT and 16 men from the CG were included in the MRI analysis. In summary, no positive effects on the fat infiltration of the erector spinae and psoas major muscles were observed.

CONCLUSIONS

The previously reported positive effects on lumbar spine bone mineral density (BMD) suggest that mechanotransduction induces tropic effects on bone, but that fat infiltration of the erector spinae and psoas major muscles are either irreversible or, for some unknown reason, resistant to exercise. Because of the beneficial effects on spinal BMD, HIRT is still recommended in osteosarcopenic older men, but further research is needed to confirm appropriate age-specific training exercises for the paraspinal muscles. The potential of different MRI sequences to quantify degenerative and metabolic changes in various muscle groups must be better characterized.

TRIAL REGISTRATIONS

FrOST was approved by the University Ethics Committee of the Friedrich-Alexander University of Erlangen-Nürnberg (number 67_15b and 4464b) and the Federal Office for Radiation Projection (BfS, number Z 5-2,246,212 - 2017-002). Furthermore, it fully complies with the Declaration of Helsinki and is registered at ClinicalTrials.gov: NCT03453463 (05/03/2018). JAMA 310:2191-2194, 2013.

Cross-sectional area and fat infiltration of the lumbar spine muscles in patients with back disorders: a deep learning-based big data analysis

PURPOSE

Validated deep learning models represent a valuable option to perform large-scale research studies aiming to evaluate muscle quality and quantity of paravertebral lumbar muscles at the population level. This study aimed to assess lumbar spine muscle cross-sectional area (CSA) and fat infiltration (FI) in a large cohort of subjects with back disorders through a validated deep learning model.

METHODS

T2 axial MRI images of 4434 patients (n = 2609 females, n = 1825 males; mean age: 56.7 ± 16.8) with back disorders, such as fracture, spine surgery or herniation, were retrospectively collected from a clinical database and automatically segmented. CSA, expressed as the ratio between total muscle area (TMA) and the vertebral body area (VBA), and FI, in percentages, of psoas major, quadratus lumborum, erector spinae, and multifidus were analyzed as primary outcomes.

RESULTS

Male subjects had significantly higher CSA (6.8 ± 1.7 vs. 5.9 ± 1.5 TMA/VBA; p < 0.001) and lower FI (21.9 ± 8.3% vs. 15.0 ± 7.3%; p < 0.001) than females. Multifidus had more FI (27.2 ± 10.6%; p < 0.001) than erector spinae (22.2 ± 9.7%), quadratus lumborum (17.5 ± 7.0%) and psoas (13.7 ± 5.8%) whereas CSA was higher in erector spinae than other lumbar muscles. A high positive correlation between age and total FI was detected (rs = 0.73; p < 0.001) whereas a negligible negative correlation between total CSA and age was observed (rs =  - 0.24; p < 0.001). Subjects with fractures had lower CSA and higher FI compared to those with herniations, surgery and with no clear pathological conditions.

CONCLUSION

CSA and FI values of paravertebral muscles vary a lot in accordance with subjects' sex, age and clinical conditions. Given also the large inter-muscle differences in CSA and FI, the choice of muscles needs to be considered with attention by spine surgeons or physiotherapists when investigating changes in lumbar muscle morphology in clinical practice.

Muscle quality and not quantity as a predictor of survival in head and neck squamous cell carcinoma

BACKGROUND

Sarcopenia is frequent in head and neck squamous cell carcinoma (HNSCC), as a consequence of malnutrition related to risk factors or tumoral mass. Treatment is associated with toxicities that lead to reduced calories intake and muscle mass wasting. Sarcopenia has been negatively associated with tumor control and survival outcomes.

PURPOSE

Our aim is to evaluate the prognostic impact of sarcopenia on overall survival (OS) and progression free survival (PFS) in HNSCC patients undergoing chemoradiation therapy within a prospective clinical trial of chemoradiation vs induction chemotherapy followed by radiation and cetuximab (INTERCEPTOR).

MATERIALS AND METHODS

On baseline CT or MRI, we investigated the association between OS and PFS with radiological markers of sarcopenia, measured at the third cervical vertebra level. We studied paravertebral skeletal muscles area (cm2), muscle density (HU), muscle index (cm2/m2), and intermuscular adipose tissue (IMAT) area (cm2).

RESULTS

Imaging of 128 patients was evaluable. We found out that higher body mass index (BMI) was associated with better OS (p = 0.02), and PFS (p = 0.04). Skeletal muscle area (p = 0.02), and IMAT (p = 0.02) were negatively associated with PFS. IMAT was positively correlated with muscle area (Correlation coefficient 0.6, CI95% 0.47-0.7), and negatively associated with muscle density (Correlation coefficient -0.37, CI95% -0.53 - -0.18).

CONCLUSIONS

IMAT can be used as predictor of PFS in HNC patients undergoing chemoradiation therapy. The amount of intermuscular fat deposits induces alterations of muscle quality, without alterations of muscle quantity, influencing patients' prognosis.

Four irradiation and three positioning techniques for whole-breast radiotherapy: Is sophisticated always better?

PURPOSE

We report on a dosimetrical study of three patient positions (supine, prone dive, and prone crawl) and four irradiation techniques for whole-breast irradiation (WBI): wedged-tangential fields (W-TF), tangential-field intensity-modulated radiotherapy (TF-IMRT), multi-beam IMRT (MB-IMRT), and intensity-modulated arc therapy (IMAT). This is the first study to evaluate prone crawl positioning in WBI and the first study to quantify dosimetrical and anatomical differences with prone dive positioning.

METHODS

We analyzed five datasets with left- and right-sided patients (n = 51). One dataset also included deep-inspiration breath hold (DIBH) data. A total of 252 new treatment plans were composed. Dose-volume parameters and indices of conformity were calculated for the planning target volume (PTV) and organs-at-risk (OARs). Furthermore, anatomical differences among patient positions were quantified to explain dosimetrical differences.

RESULTS

Target coverage was inferior for W-TF and supine position. W-TF proved overall inferior, and IMAT proved foremost effective in supine position. TF-IMRT proved competitive to the more demanding MB-IMRT and IMAT in prone dive, but not in prone crawl position. The lung-sparing effect was overall confirmed for both prone dive and prone crawl positioning and was largest for prone crawl. For the heart, no differences were found between prone dive and supine positioning, whereas prone crawl showed cardiac advantages, although minor compared to the established heart-sparing effect of DIBH. Dose differences for contralateral breast were minor among the patient positions. In prone crawl position, the ipsilateral breast sags deeper and the PTV is further away from the OARs than in prone dive position.

CONCLUSIONS

The prone dive and prone crawl position are valid alternatives to the supine position in WBI, with largest advantages for lung structures. For the heart, differences are small, which establishes the role of DIBH in different patient positions. These results may be of particular interest to radiotherapy centers with limited technical resources.

Development of skeletal muscle atrophy and intermuscular adipose tissue in early breast cancer patients treated with chemotherapy

Chemotherapy is a common therapy to treat patients with breast cancer but also leads to skeletal muscle deconditioning. Skeletal muscle deconditioning is multifactorial and intermuscular adipose tissue (IMAT) accumulation is closely linked to muscle dysfunction. To date, there is no clinical study available investigating IMAT development through a longitudinal protocol and the underlying mechanisms remain unknown. Our study was dedicated to investigating IMAT content in patients with early breast cancer who were treated with chemotherapy and exploring the subsequent cellular mechanisms involved in its development. We included 13 women undergoing chemotherapy. Muscle biopsies and ultrasonography assessment were performed before and after chemotherapy completion. Histological and Western blotting analyses were conducted. We found a substantial increase in protein levels of three mature adipocyte markers (perilipin, +901%; adiponectin, +135%; FABP4, +321%; P < 0.05). These results were supported by an increase in oil red O-positive staining (+358%; P < 0.05). A substantial increase in PDGFRα protein levels was observed (+476%; P < 0.05) highlighting an increase in fibro-adipogenic progenitors (FAPs) content. The cross-sectional area of the vastus lateralis muscle fibers substantially decreased (−21%; P < 0.01), and muscle architecture was altered, as shown by a decrease in fascicle length (−15%; P < 0.05) and a decreasing trend in muscle thickness (−8%; P = 0.08). We demonstrated both IMAT development and muscle atrophy in patients with breast cancer who were treated with chemotherapy. FAPs, critical stem cells inducing both IMAT development and skeletal muscle atrophy, also increased, suggesting that FAPs likely play a critical role in the skeletal muscle deconditioning observed in patients with breast cancer who were treated with chemotherapy.

Quantifying the inflammatory secretome of human intermuscular adipose tissue

Adipose tissue secretes an abundance of lipid and protein mediators, and this secretome is depot-specific, with local and systemic effects on metabolic regulation. Intermuscular adipose tissue (IMAT) accumulates within the skeletal muscle compartment in obesity, and is associated with insulin resistance and metabolic disease. While the human IMAT secretome decreases insulin sensitivity in vitro, its composition is entirely unknown. The current study was conducted to investigate the composition of the human IMAT secretome, compared to that of the subcutaneous (SAT) and visceral adipose tissue (VAT) depots. IMAT, SAT, and VAT explants from individuals with obesity were used to generate conditioned media. Proteomics analysis of conditioned media was performed using multiplex proximity extension assays, and eicosanoid analysis using liquid chromatography-tandem mass spectrometry. Compared to SAT and/or VAT, IMAT secreted significantly more cytokines (IL2, IL5, IL10, IL13, IL27, FGF23, IFNγ and CSF1) and chemokines (MCP1, IL8, CCL11, CCL20, CCL25 and CCL27). Adipokines hepatocyte growth factor and resistin were secreted significantly more by IMAT than SAT or VAT. IMAT secreted significantly more eicosanoids (PGE2, TXB2 , 5-HETE, and 12-HETE) compared to SAT and/or VAT. In the context of obesity, IMAT is a distinct adipose tissue with a highly immunogenic and inflammatory secretome, and given its proximity to skeletal muscle, may be critical to glucose regulation and insulin resistance.

The effect of ageing on fat infiltration of thigh and paraspinal muscles in men

Background

Myosteatosis, skeletal muscle fat infiltration, is associated with inflammation and fibrosis. The age-related increase of myosteatosis is an important characteristic of sarcopenia and contributes to fragility.

Aims

To investigate the impact of healthy aging on intermuscular adipose tissue (IMAT) and muscle fat fraction (FF) in the thigh and the paraspinal muscles in males.

Methods

In 54 healthy males (age 20–70), all active hobby golfers, magnetic resonance imaging was performed to determine volume of IMAT, volume of muscle tissue (MT) and of percentage of FF.

Results

Between ages 20–70, at the thigh, IMAT/MT volume and MT FF increased annually by 2.9% and 1.3%, respectively. At the psoas IMAT/Psoas volume did not change with age. MT FF increased by 1.5% annually. At the erector spinae IMAT/Erector volume decreased by 0.3% and MT FF increased by 2.8% annually.

Discussion

With increasing age, in males, thigh muscle atrophied, muscle tissue was partly replaced by adipose tissue and remaining muscle tissue also contained more fat. Similar effects were observed in the erector spinae. The psoas muscle did not atrophy, although MT FF also increased with age. Overall correlations with age were weak to moderate with higher correlations observed in the paraspinal muscles.

Conclusions

Age-related increases of muscle fat infiltration were observed in the thigh and in the spine. Muscle atrophy did not occur in the psoas. In cross-sectional studies, an adjustment of volumetric parameters by muscle volume is advisable when comparing age-dependent results.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40520-022-02149-1.

A unique sarcopenic progression in the mouse rotator cuff

BACKGROUND

In response to chronic injury, the muscles of the rotator cuff (RC) experience a unique degeneration characterized by extensive fatty infiltration and loss of contractile function. Human studies suggest this degeneration is also a feature of RC sarcopenia and may precede RC injury. In this study, we investigated whether RC muscles exhibit a similar unique sarcopenia in the mouse.

METHODS

Male and female mice were subdivided into four age groups: 3, 9, 18, and 24 months. The supraspinatus (SS) and infraspinatus muscles of the RC and the tibialis anterior (TA) muscle of the hindlimb were assessed. Muscle mass, contractile function, fibre cross-sectional areas and numbers, fatty infiltration, and fibrosis were assessed at each time point. Targeted transcriptional analyses were performed to assess the role of metabolic and inflammatory derangement in the pathology.

RESULTS

The 24-month-aged female mice exhibited decreased mass (25% lower than at 9 and 18 months, P < 0.01) in all muscles tested. However, only RC muscles also exhibited decreased contractile tension at this time point (20% lower than at 18 months, P < 0.005). Similarly, only female RC muscles exhibited increased fatty infiltration at 24 months (20% higher than 9 months, P < 0.05) and had elevated transcriptional markers of adipogenesis (2.4-fold higher Pparg and 3.8-fold higher Adipoq expression compared with 9 months, P < 0.001). Unbiased metabolic transcriptional profiling identified up-regulation of the antigen presentation (Z scores of 2.3 and 1.9 for SS and TA, respectively) and cytokine and chemokine signalling (Z scores of 3.1 and 2.4 for SS and TA, respectively) pathways in 24 month female muscle compared with 9. Further transcriptional analysis supported increased expression of pro-adipogenic inflammatory signals (6.3-fold increase in Il6 and 5.0-fold increase in Anxa2, P < 0.01) and increased presence of fibro-adipogenic progenitors (2.5-fold) in the 24-month-aged female RC compared with 9 months that together exacerbate fatty infiltration.

CONCLUSIONS

These data indicate that female mice replicate the unique sarcopenic pathology in the ageing human RC. Furthermore, they suggest that the exacerbated fatty infiltration is due to an interaction between higher resident fibro-adipogenic progenitor numbers and an elevated systemic inflammation in aged female mice. We conclude that female mouse RC muscle is a novel system to study both human RC degeneration and the signals that regulate sarcopenic fatty infiltration in general, which is prevalent in humans but largely absent from the rodent hindlimb.

Growth hormone/IGF-I-dependent signaling restores decreased expression of the myokine SPARC in aged skeletal muscle

Skeletal muscle exerts many beneficial effects on the human body including the contraction-dependent secretion of peptides termed myokines. We have recently connected the myokine secreted protein acidic and rich in cysteine (SPARC) to the formation of intramuscular adipose tissue (IMAT) in skeletal muscle from aged mice and humans. Here, we searched for inducers of SPARC in order to uncover novel treatment approaches for IMAT. Endurance exercise in mice as well as forskolin treatment in vitro only modestly activated SPARC levels. However, through pharmacological treatments in vitro, we identified IGF-I as a potent inducer of SPARC expression in muscle cells, likely through a direct activation of its promoter via phosphatidylinositol 4,5-bisphospate 3-kinase (PI3K)-dependent signaling. We employed two different mouse models of growth hormone (GH)/IGF-I deficiency to solidify our understanding of the relationship between IGF-I and SPARC in vivo. GH administration robustly increased intramuscular SPARC levels (3.5-fold) in GH releasing hormone receptor-deficient mice and restored low intramuscular SPARC expression in skeletal muscle from aged mice. Intramuscular glycerol injections induced higher levels of adipocyte markers (adiponectin, perilipin) in aged compared to young mice, which was not prevented by GH treatment. Our study provides a roadmap for the study of myokine regulation during aging and demonstrates that the GH/IGF-I axis is critical for SPARC expression in skeletal muscle. Although GH treatment did not prevent IMAT formation in the glycerol model, targeting SPARC by exercise or by activation of IGF-I signaling might offer a novel therapeutic strategy against IMAT formation during aging. KEY MESSAGES : IGF-I regulates the myokine SPARC in muscle cells directly at the promoter level. GH/IGF-I is able to restore the decreased SPARC levels in aged skeletal muscle. The glycerol model induces higher adipocyte markers in aged compared to young muscle. GH treatment does not prevent IMAT formation in the glycerol model.

FGF-2-dependent signaling activated in aged human skeletal muscle promotes intramuscular adipogenesis

Aged skeletal muscle is markedly affected by fatty muscle infiltration, and strategies to reduce the occurrence of intramuscular adipocytes are urgently needed. Here, we show that fibroblast growth factor-2 (FGF-2) not only stimulates muscle growth but also promotes intramuscular adipogenesis. Using multiple screening assays upstream and downstream of microRNA (miR)-29a signaling, we located the secreted protein and adipogenic inhibitor SPARC to an FGF-2 signaling pathway that is conserved between skeletal muscle cells from mice and humans and that is activated in skeletal muscle of aged mice and humans. FGF-2 induces the miR-29a/SPARC axis through transcriptional activation of FRA-1, which binds and activates an evolutionary conserved AP-1 site element proximal in the miR-29a promoter. Genetic deletions in muscle cells and adeno-associated virus-mediated overexpression of FGF-2 or SPARC in mouse skeletal muscle revealed that this axis regulates differentiation of fibro/adipogenic progenitors in vitro and intramuscular adipose tissue (IMAT) formation in vivo. Skeletal muscle from human donors aged >75 y versus <55 y showed activation of FGF-2-dependent signaling and increased IMAT. Thus, our data highlights a disparate role of FGF-2 in adult skeletal muscle and reveals a pathway to combat fat accumulation in aged human skeletal muscle.

Effects of 16 months of high intensity resistance training on thigh muscle fat infiltration in elderly men with osteosarcopenia

Osteosarcopenia is characterized by a progressive decline in muscle function and bone strength and associated with muscle fat accumulation. This study aimed to determine the effect of long-term high intensity resistance training (HIRT) on thigh muscle fat infiltration in older men with osteosarcopenia. Forty-three community-dwelling men (72 years and older) were randomly assigned to either an exercise group (EG, n = 21) or an inactive control group (CG, n = 22). EG participants performed a supervised single-set exercise training with high effort two times per week. Participants of both groups were individually provided with dietary protein to reach a cumulative intake of 1.5-1.6 g/kg/day or 1.2-1.3 g/kg/day (EG/CG), respectively, and Up to 10,000 IE/week of Vitamin-D were supplemented in participants with 25 OH Vitamin-D 3 levels below 100 nmol/l. Magnetic resonance (MR) imaging was performed to determine muscle and adipose tissue volume and fat fraction of the thigh. At baseline, there were no significant differences between the two groups. After 16 month,, there were significant training effects of 15% (p = 0.004) on intermuscular adipose tissue (IMAT) volume, which increased in the CG (p = 0.012) and was stable in the EG. In parallel, fat fraction within the deep fascia of the thigh (Baseline, EG: 18.2 vs CG: 15.5, p = 0.16) significantly differed between the groups (Changes, EG: 0.77% vs. CG: 7.7%, p = 0.009). The study confirms the role of fat infiltration of the muscles as an advanced imaging marker in osteosarcopenia and the favorable effects of HIRT on adipose tissue volume of the thigh, in men with osteosarcopenia.

Infiltration of intramuscular adipose tissue impairs skeletal muscle contraction

KEY POINTS

Muscle infiltration with adipose tissue (IMAT) is common and associated with loss of skeletal muscle strength and physical function across a diverse set of pathologies. Whether the association between IMAT and muscle weakness is causative or simply correlative remains an open question that needs to be addressed to effectively guide muscle strengthening interventions in people with increased IMAT. In the present studies, we demonstrate that IMAT deposition causes decreased muscle strength using mouse models. These findings indicate IMAT is a novel therapeutic target for muscle dysfunction.

ABSTRACT

Intramuscular adipose tissue (IMAT) is associated with deficits in strength and physical function across a wide array of conditions, from injury to ageing to metabolic disease. Due to the diverse aetiologies of the primary disorders involving IMAT and the strength of the associations, it has long been proposed that IMAT directly contributes to this muscle dysfunction. However, infiltration of IMAT and reduced strength could both be driven by muscle disuse, injury and systemic disease, making IMAT simply an 'innocent bystander.' Here, we utilize novel mouse models to evaluate the direct effect of IMAT on muscle contraction. First, we utilize intramuscular glycerol injection in wild-type mice to evaluate IMAT in the absence of systemic disease. In this model we find that, in isolation from the neuromuscular and circulatory systems, there remains a muscle-intrinsic association between increased IMAT volume and decreased contractile tension (r2  > 0.5, P < 0.01) that cannot be explained by reduction in contractile material. Second, we utilize a lipodystrophic mouse model which cannot generate adipocytes to 'rescue' the deficits. We demonstrate that without IMAT infiltration, glycerol treatment does not reduce contractile force (P > 0.8). Taken together, this indicates that IMAT is not an inert feature of muscle pathology but rather has a direct impact on muscle contraction. This finding suggests that novel strategies targeting IMAT may improve muscle strength and function in a number of populations.

Contribution of Protein Intake and Concurrent Exercise to Skeletal Muscle Quality with Aging

BACKGROUND

The use of magnetic resonance imaging (MRI) derived functional cross-sectional area (FCSA) and intramuscular adipose tissue (IMAT) to define skeletal muscle quality is of fundamental importance in order to understand aging and inactivity-related loss of muscle mass.

OBJECTIVES

This study examined factors associated with lower-extremity skeletal muscle quality in healthy, younger, and middle-aged adults.

DESIGN

Cross-sectional study.

SETTING AND PARTICIPANTS

Ninety-eight participants (53% female) were classified as younger (20-35 years, n=50) or middle-aged (50-65 years, n=48) as well as sedentary (≤1 day per week) or active (≥3 days per week) on self-reported concurrent exercise (aerobic and resistance).

MEASUREMENTS

All participants wore an accelerometer for seven days, recorded a three-day food diary, and participated in magnetic resonance imaging (MRI) of the lower limbs. Muscle cross-sectional area (CSA) was determined by tracing the knee extensors (KE) and plantar flexors, while muscle quality was established through the determination of FCSA and IMAT via color thresholding.

RESULTS

One-way analysis of variance and stepwise regression models were performed to predict FCSA and IMAT. KE-IMAT (cm2) was significantly higher among sedentary (3.74 ± 1.93) vs. active (1.85 ± 0.56) and middle-aged (3.14 ± 2.05) vs. younger (2.74 ± 1.25) (p < 0.05). Protein intake (g•kg•day-1) was significantly higher in active (1.63 ± 0.55) vs. sedentary (1.19 ± 0.40) (p < 0.05). Sex, age, concurrent exercise training status, and protein intake were significant predictors of KE FCSA (R2 = 0.71, p < 0.01), while concurrent exercise training status and light physical activity predicted 33% of the variance in KE IMAT (p < 0.01).

CONCLUSION

Concurrent exercise training, dietary protein intake, and light physical activity are significant determinants of skeletal muscle health and require further investigation to mitigate aging and inactivity-related loss of muscle quality.

Skewed macrophage polarization in aging skeletal muscle

Skeletal muscle aging is a major cause of disability and frailty in the elderly. The progressive impairment of skeletal muscle function with aging was recently linked to a disequilibrium between damage and repair. Macrophages participate in muscle tissue repair, first as pro-inflammatory M1 subtype and then as anti-inflammatory M2 subtype. However, information on the presence of macrophages in skeletal muscle is still sporadic and the effect of aging on macrophage phenotype remains unknown. In this study, we sought to characterize the polarization status of macrophages in skeletal muscle of persons across a wide range of ages. We found that most macrophages in human skeletal muscle are M2, and that this number increased with advancing age. On the contrary, M1 macrophages declined with aging, making the total number of macrophages invariant with older age. Notably, M2 macrophages colocalized with increasing intermuscular adipose tissue (IMAT) in aging skeletal muscle. Similarly, aged BALB/c mice showed increased IMAT and M2 macrophages in skeletal muscle, accompanied by slightly increased collagen protein production. Collectively, we report that polarization of macrophages to the major M2 subtype is associated with IMAT and propose that increased M2 in aged skeletal muscle may impact upon muscle metabolism associated with aging.

Relationship between intermuscular adipose tissue infiltration and myostatin before and after aerobic exercise training

Intermuscular adipose tissue (IMAT) is associated with impaired skeletal muscle contractile and metabolic function. Myostatin and downstream signaling proteins such as cyclin-dependent kinase 2 (CDK2) contribute to the regulation of adipose and skeletal muscle mass in cell culture and animals models, but this relationship remains incompletely understood in humans. The purpose of this study was to determine if the infiltration of IMAT was associated with skeletal muscle myostatin and downstream proteins before and after 12 wk of aerobic exercise training (AET) in healthy older women (OW; 69 ± 2 yr), older men (OM; 74 ± 3 yr), and young men (YM; 20 ± 1 yr). We found that the infiltration of IMAT was correlated with myostatin and phosphorylated CDK2 at tyrosine 15 [P-CDK2(Tyr15)]. IMAT infiltration was greater in the older subjects and was associated with lower skeletal muscle function and exercise capacity. After 12 wk of AET, there was no change in body weight. Myostatin and P-CDK2(Tyr15) were both decreased after AET, and the reduction in myostatin was associated with decreased IMAT infiltration. The decrease in myostatin and IMAT occurred concomitantly with increased exercise capacity, skeletal muscle size, and function after AET. These findings demonstrate that the reduction in IMAT infiltration after AET in weight stable individuals was accompanied by improvements in skeletal muscle function and exercise capacity. Moreover, the association between myostatin and IMAT was present in the untrained state and in response to exercise training, strengthening the potential regulatory role of myostatin on IMAT.

miR-17-5p Regulates Differential Expression of NCOA3 in Pig Intramuscular and Subcutaneous Adipose Tissue

Fat distribution affects economic value in pork production. Intramuscular adipose tissue (IMAT) improves meat quality, whereas subcutaneous adipose tissue (SCAT) is usually regarded as waste. In the present study, we analyzed IMAT/SCAT (I/S) ratios in each pig. Individuals selected from a population of 1200 Suhuai pigs were divided into two cohorts; those with high I/S ratios and those with low I/S ratios, and correlations between nuclear Receptor Co-activator 3 (NCOA3), a critical gene involved in regulating fat accumulation, and fat distribution were investigated. The ratio of IMAT NCOA3 to SCAT NCOA3 expression levels (NCOA3_I/NCOA3_S) was higher in the high I/S group compared with the low I/S group. The NCOA3 expression level in fat tissue was positively correlated with fat deposition. miR-17-5p was identified as a putative regulator of NCOA3 based on bioinformatics prediction analysis followed by gene expression analysis. The miR-17-5p_I/miR-17-5p_S ratio was negatively correlated with the NCOA3_I/NCOA3_S ratio. The predicted relationship between miR-17-5p and NCOA3 was further verified by dual luciferase activity assays, qPCR, and western blots. Overexpression of miR-17-5p in intramuscular preadipocytes inhibited NCOA3 expression and reduced preadipocyte differentiation. FABP4 and PPARG expression were also significantly decreased, as was triglyceride content. Meanwhile, knockdown of miR-17-5p significantly increased NCOA3 expression and promoted intramuscular preadipocyte differentiation. Based on these results, we propose that differential expression of NCOA3 in pig intramuscular and subcutaneous adipose tissue is regulated by miR-17-5p.

Macronutrient and alcohol intake is associated with intermuscular adipose tissue in a randomly selected group of younger and older men and women

BACKGROUND

Alcohol and macronutrient intake have been found to be related to general and central body fat distribution. Intermuscular adipose tissue (IMAT) is a small ectopic fat depot located within the muscle bundles. IMAT is important for muscle function, mobility and energy homeostasis and also associated with cardiovascular- and diabetes-related risk factors.

AIM

To test the hypothesis that macronutrient and alcohol intake is associated with IMAT.

METHODS

50 men and 50 women, randomly selected from the general population formed height- and weight-representative age groups of 50 younger (27-31 years) and 50 older (57-61 years) subjects. A dietary questionnaire was used to estimate habitual intake of foods and beverages. Body composition including IMAT was measured with computed tomography.

RESULTS

Energy percent (E%) carbohydrates were negatively associated with IMAT in men (β: -0.6234, P < 0.05) and in younger subjects (β: -0.792, P < 0.05). E% alcohol was positively associated with IMAT in women (β: 2.3663, P < 0.01) and in older subjects (β: 1.232, P < 0.01). In younger individuals, E% protein was positively associated with IMAT (β: 1.840, P < 0.05). Centralized and general body fat distributions were positively associated with IMAT. S- (serum) cholesterol was positively associated with IMAT in men (β: 0.05177, P < 0.01) and younger individuals (β: 0.06701, P < 0.01).

CONCLUSIONS

These cross-sectional analyses showed associations between measures of body fat distribution and IMAT as well as associations between macronutrient- and alcohol intake and IMAT. Since IMAT is situated within the energy demanding striated muscles, our data could suggest that changes in dietary energy intake and macronutrient distribution may induce changes in IMAT in both normal weight and obese subjects.

A novel method for the quantification of fatty infiltration in skeletal muscle

BACKGROUND

Fatty infiltration of the skeletal muscle is a common but poorly understood feature of many myopathies. It is best described in human muscle, where non-invasive imaging techniques and representative histology have been optimized to view and quantify infiltrating fat. However, human studies are limited in their ability to identify cellular and molecular mechanisms regulating fatty infiltration, a likely prerequisite to developing targeted interventions. As mechanistic investigations move to small animals, studies may benefit from new or adapted imaging tools optimized for high resolution and whole muscle quantification.

RESULTS

Here, we describe a novel method to evaluate fatty infiltration, developed for use with mouse muscle. In this methodology, muscle cellular membranes and proteins are removed via decellularization, but fatty infiltrate lipid is spared, trapped in its native distribution in a transparent extracellular matrix construct. This lipid can then be stained with visible or fluorescent dyes and imaged. We present three methods to stain and evaluate lipid in decellularized muscles which can be used individually or combined: (1) qualitative visualization of the amount and 3D spatial distribution of fatty infiltration using visible lipid soluble dye Oil Red O (ORO), (2) quantitative analysis of individual lipid droplet metrics (e.g., volume) via confocal imaging of fluorescent lipid soluble dye boron-dipyrromethene (BODIPY), and (3) quantitative analysis of total lipid content by optical density reading of extracted stained lipid. This methodology was validated by comparing glycerol-induced fatty infiltration between two commonly used mouse strains: 129S1/SvlmJ (129S1) and C57BL/6J (BL/6J). All three methods were able to detect a significant increase in fatty infiltrate volume in the 129S1 muscle compared with that in BL/6J, and methods 1 and 2 additionally described a difference in the distribution of fatty infiltrate, indicating susceptibility to glycerol-induced fatty infiltration is strain-specific.

CONCLUSIONS

With more mechanistic studies of fatty infiltration moving to small animal models, having an alternative to expensive non-invasive imaging techniques and selective representative histology will be beneficial. In this work, we present a method that can quantify both individual adipocyte lipids and whole muscle total fatty infiltrate lipid.

Dosimetric verification for intensity-modulated arc therapy plans by use of 2D diode array, radiochromic film and radiosensitive polymer gel

Several tools are used for the dosimetric verification of intensity-modulated arc therapy (IMAT) treatment delivery. However, limited information is available for composite on-line evaluation of these tools. The purpose of this study was to evaluate the dosimetric verification of IMAT treatment plans using a 2D diode array detector (2D array), radiochromic film (RCF) and radiosensitive polymer gel dosimeter (RPGD). The specific verification plans were created for IMAT for two prostate cancer patients by use of the clinical treatment plans. Accordingly, the IMAT deliveries were performed with the 2D array on a gantry-mounting device, RCF in a cylindrical acrylic phantom, and the RPGD in two cylindrical phantoms. After the irradiation, the planar dose distributions from the 2D array and the RCFs, and the 3D dose distributions from the RPGD measurements were compared with the calculated dose distributions using the gamma analysis method (3% dose difference and 3-mm distance-to-agreement criterion), dose-dependent dose difference diagrams, dose difference histograms, and isodose distributions. The gamma passing rates of 2D array, RCFs and RPGD for one patient were 99.5%, 96.5% and 93.7%, respectively; the corresponding values for the second patient were 97.5%, 92.6% and 92.9%. Mean percentage differences between the RPGD measured and calculated doses in 3D volumes containing PTVs were -0.29 ± 7.1% and 0.97 ± 7.6% for the two patients, respectively. In conclusion, IMAT prostate plans can be delivered with high accuracy, although the 3D measurements indicated less satisfactory agreement with the treatment plans, mainly due to the dosimetric inaccuracy in low-dose regions of the RPGD measurements.