Molecular heterogeneities of adipose depots - potential effects on adipose-muscle cross-talk in humans, mice and farm animals

White Adipose Tissue Heterogeneity in the Single-Cell Era: From Mice and Humans to Cattle

Adipose tissue is a major modulator of metabolic function by regulating energy storage and by acting as an endocrine organ through the secretion of adipokines. With the advantage of next-generation sequencing-based single-cell technologies, adipose tissue has been studied at single-cell resolution, thus providing unbiased insight into its molecular composition. Recent single-cell RNA sequencing studies in human and mouse models have dissected the transcriptional cellular heterogeneity of subcutaneous (SAT), visceral (VAT), and intramuscular (IMAT) white adipose tissue depots and revealed unique populations of adipose tissue progenitor cells, mature adipocytes, immune cell, vascular cells, and mesothelial cells that play direct roles on adipose tissue function and the development of metabolic disorders. In livestock species, especially in bovine, significant gaps of knowledge remain in elucidating the roles of adipose tissue cell types and depots on driving the pathogenesis of metabolic disorders and the distinct fat deposition in VAT, SAT, and IMAT in meat animals. This review summarizes the current knowledge on the transcriptional and functional cellular diversity of white adipose tissue revealed by single-cell approaches and highlights the depot-specific function of adipose tissue in different mammalian species, with a particular focus on recent findings and future implications in cattle.

IMFSegNet: Cost-effective and objective quantification of intramuscular fat in histological sections by deep learning

The assessment of muscle condition is of great importance in various research areas. In particular, evaluating the degree of intramuscular fat (IMF) in tissue sections is a challenging task, which today is still mostly performed qualitatively or quantitatively by a highly subjective and error-prone manual analysis. We here realize the mission to make automated IMF analysis possible that (i) minimizes subjectivity, (ii) provides accurate and quantitative results quickly, and (iii) is cost-effective using standard hematoxylin and eosin (H&E) stained tissue sections. To address all these needs in a deep learning approach, we utilized the convolutional encoder-decoder network SegNet to train the specialized network IMFSegNet allowing to accurately quantify the spatial distribution of IMF in histological sections. Our fully automated analysis was validated on 17 H&E-stained muscle sections from individual sheep and compared to various state-of-the-art approaches. Not only does IMFSegNet outperform all other approaches, but this neural network also provides fully automated and highly accurate results utilizing the most cost-effective procedures of sample preparation and imaging. Furthermore, we shed light on the opacity of black-box approaches such as neural networks by applying an explainable artificial intelligence technique to clarify that the success of IMFSegNet actually lies in identifying the hard-to-detect IMF structures. Embedded in our open-source visual programming language JIPipe that does not require programming skills, it can be expected that IMFSegNet advances muscle condition assessment in basic research across multiple areas as well as in research fields focusing on translational clinical applications.

Recent developments in adipose tissue-secreted factors and their target organs

The white adipose tissue's primary roles are to store and mobilise energy, which is very different from the brown adipose tissue's function of using fuel to generate heat and maintain the body temperature. The adipose tissues (ATs), co-ordinately with the other organs, sense energetic demands and inform of their reserves before embarking on energetically demanding physiological functions. It is not surprising that ATs exhibit highly integrated regulatory mechanisms mediated by a diversified secretome, including adipokines, lipokines, metabolites and a repertoire of extracellular miRNAs that contribute to integrating the function of the AT niche and connect the AT through paracrine and endocrine effects with the whole organism. Characterising the adipose secretome, its changes in health and disease, regulation by ageing and gender and their contribution to energy homoeostasis is necessary to optimise its use for personalised strategies to prevent or reverse metabolic diseases.

Developmental programming: Adipose depot-specific regulation of non-coding RNAs and their relation to coding RNA expression in prenatal testosterone and prenatal bisphenol-A -treated female sheep

Inappropriate developmental exposure to steroids is linked to metabolic disorders. Prenatal testosterone excess or bisphenol A (BPA, an environmental estrogen mimic) leads to insulin resistance and adipocyte disruptions in female lambs. Adipocytes are key regulators of insulin sensitivity. Metabolic tissue-specific differences in insulin sensitivity coupled with adipose depot-specific changes in key mRNAs, were previously observed with prenatal steroid exposure. We hypothesized that depot-specific changes in the non-coding RNA (ncRNA) - regulators of gene expression would account for the direction of changes seen in mRNAs. Non-coding RNA (lncRNA, miRNA, snoRNA, snRNA) from various adipose depots of prenatal testosterone and BPA-treated animals were sequenced. Adipose depot-specific changes in the ncRNA that are consistent with the depot-specific mRNA expression in terms of directionality of changes and functional implications in insulin resistance, adipocyte differentiation and cardiac hypertrophy were found. Importantly, the adipose depot-specific ncRNA changes were model-specific and mutually exclusive, suggestive of different regulatory entry points in this regulation.

Mutation c.-379 C>T in DGAT1 affects intramyocellular lipid content by altering MYOD1 binding affinity

Intramuscular fat (IMF) is one of the most important indexes of pork taste quality. Diacylglycerol acyltransferase 1 (DGAT1), belonging to the acyl-coenzyme A: DGAT enzymes family, is a rate-limiting enzyme responsible for the final step of triglyceride (TG) synthesis. It is involved in TG storage in skeletal muscle; however, the underlying mechanism is not well understood. This study aimed to uncover functional mutations that can influence DGAT1 expression and consequently affect IMF deposition in pork. Two experimental groups containing individuals with high and low IMF content (6.23 ± 0.20 vs. 1.25 ± 0.05, p < 0.01) were formed from 260 Duroc × Large White × Yorkshire (D × L × Y) cross-bred pigs. A novel SNP c.-379 C>T was uncovered in the DGAT1 gene using comparative sequencing with pool DNA of high- and low-IMF groups. The IMF content of CT genotype individuals (3.19 ± 0.11%) was higher than that of CC genotype individuals (2.86 ± 0.11%) when analyzing 260 D × L × Y pigs (p < 0.05). The DGAT1 expression levels revealed a significant positive correlation with IMF content (r = 0.33, p < 0.01). Luciferase assay revealed that the DGAT1 promoter with the c.-379 T allele has a higher transcription activity than that bearing the C allele in C2C12 myoblast cells, but not in 3T3-L1 pre-adipocytes. Online prediction followed by chromatin immunoprecipitation-polymerase chain reaction assay confirmed that myogenic determination factor 1 (MYOD1) binds to the DGAT1 promoter with the c.-379 C allele but not the T allele. In vitro experiments demonstrated that MYOD1 represses DGAT1 transcription and lipogenesis. As a muscle-specific transcription factor, MYOD1 can inhibit the transcription of DGAT1 with the c.-379 C allele in muscle cells. However, in the absence of MYOD1 binding to the mutated DGAT1 promoter with the c.-379 T allele, DGAT1 expresses at a higher level in the muscle cells of the c.-379 T genotype, leading to more intramyocellular lipid accumulation than in the muscle cells of the c.-379 C genotype. The SNP c.-379 C>T in the promoter region of the DGAT1 gene provides a promising molecular marker for improving pork IMF content without affecting other fat depots.

Fibroadipogenic progenitor cell response peaks prior to progressive fatty infiltration after rotator cuff tendon tear

Fibroadipogenic progenitor (FAP) cells are implicated as a major source of fatty infiltration (FI) in murine rotator cuff (RC) injury, but FAP cell response after RC tear in a rabbit model is unknown. This study determined whether changes in FAP cell count after an RC tear predate muscle degeneration in a clinically relevant rabbit model. We hypothesized increases in FAP cell count correlate temporally with RC degeneration. New Zealand white rabbits (n = 26) were evaluated at 1, 2, 4, and 6 weeks after unilateral full-thickness tenotomy of supraspinatus and infraspinatus tendons. FI area and adipocyte size were histologically analyzed, muscle density was measured by computerized tomography, and quantification of FAP cells was measured by flow cytometry and immunohistochemistry. The percentage of intrafascicular adipocyte area increased over time in supraspinatus muscle samples (p = 0.03), significantly between 1- and 6-week samples (p = 0.04). There were no differences in perifascicular adipocyte area percentages between time points. Peak increase in FAP cell count occurred at 1-week (p = 0.03), with a decrease in the following weeks. There was a negative correlation between supraspinatus adipocyte area and FAP cell count (p < 0.05). On computed tomography (CT) scan, maximal decrease in muscle density was observed in the 4th to 6th weeks. In summary, FAP cell response occurred early after tenotomy and did not correlate temporally with increases in FI. This suggests that FAP cell response may predate degenerative changes, and early targeting of FAPs before adipocyte maturation could blunt FI after RC tear.

Comparison of multifidus degeneration between scoliosis and lumbar disc herniation

Objective

To assess and compare the pathological and radiological outcomes of multifidus degeneration in scoliosis and lumbar disc herniation patients.

Methods

We performed a retrospective review on 24 patients with scoliosis and 26 patients with lumbar disc herniation (LDH) in the Third Hospital of Hebei Medical University from January 2017 to March2021. The patients were divided into scoliosis group and LDH group according to the treatment. The MRI fatty infiltration rate (FIR) of multifidus and strength of back muscle were calculated to evaluate muscle condition. Multifidus biopsy samples were obtained during surgery in the affected side at L4 or L5 segment in LDH group and on the concavity side of apical vertebrae in scoliosis group. The biopsy fatty infiltration degree (FID) and FIR in two groups, the FIR of affected and unaffected side in LDH group, and the FIR of concavity and convexity side in scoliosis group were compared. The correlation between concavity-convexity FIR difference and cobb angle in scoliosis group, back muscle strength and FIR in LDH group, FID and FIR in both groups was calculated respectively.

Results

The FIR was higher in scoliosis group than in LDH group, higher in concavity side than convexity side in scoliosis group (both P < 0.05). The FID was higher in scoliosis group than in LDH group (P < 0.05). No significant difference was found between affected and unaffected side in LDH group (P > 0.05). There was a positive correlation between concavity-convexity FIR difference and cobb angle, FIR and FID (both P < 0.01). There was a negative correlation between back muscle strength and FIR (P < 0.01). The biopsy staining results showed that both two groups were found the existence of rimmed vacuoles, nuclear aggregation, and abnormal enzyme activity, indicating that the scoliosis and LDH may be associated with myogenic diseases.

Conclusion

The scoliosis patients showed more serious fatty infiltration than LDH patients and rare pathological findings were found in both diseases.

Obesity impairs skeletal muscle repair through NID-1 mediated extracellular matrix remodeling by mesenchymal progenitors

Obesity triggers skeletal muscle physio-pathological alterations. However, the crosstalk between adipose tissue and myogenic cells remains poorly understood during obesity. We identified NID-1 among the adipose tissue secreted factors impairing myogenic potential of human myoblasts and murine muscle stem cells in vitro. Mice under High Fat Diet (HFD) displayed increased NID-1 expression in the skeletal muscle endomysium associated with intramuscular fat adipose tissue expansion and compromised muscle stem cell function. We show that NID-1 is highly secreted by skeletal muscle fibro-adipogenic/mesenchymal progenitors (FAPs) during obesity. We demonstrate that increased muscle NID-1 impairs muscle stem cells proliferation and primes the fibrogenic differentiation of FAPs, giving rise to an excessive deposition of extracellular matrix. Finally, we propose a model in which obesity leads to skeletal muscle extracellular matrix remodeling by FAPs, mediating the alteration of myogenic function by adipose tissue and highlighting the key role of NID-1 in the crosstalk between adipose tissue and skeletal muscle.

FATP1 Exerts Variable Effects on Adipogenic Differentiation and Proliferation in Cells Derived From Muscle and Adipose Tissue

In livestock, intramuscular adipose tissue is highly valued whereas adipose tissue in other depots is considered as waste. Thus, genetic factors that favor fat deposition in intramuscular compartments over that in other adipose depots are highly desirable in meat-producing animals. Fatty acid transport 1 (FATP1) has been demonstrated to promote cellular fatty acid uptake and metabolism; however, whether it also influences cellular lipid accumulation remains unclear. In the present study, we investigated the effects of FATP1 on the differentiation and proliferation of adipocytes in five types of cells derived from muscle and adipose tissue and estimated the effects of FATP1 on intramuscular fat (IMF) deposition. We showed that FATP1 is mainly expressed in heart and muscle tissue in buffaloes as well as cells undergoing adipogenic differentiation. Importantly, we found that FATP1 promoted the adipogenic differentiation of muscle-derived cells (buffalo myocytes and intramuscular preadipocytes and mouse C2C12 cells) but did not affect, or even inhibited, that of adipose-derived cells (buffalo subcutaneous preadipocytes and mouse 3T3-L1 cells, respectively). Correspondingly, our results further indicated that FATP1 promotes IMF deposition in mice in vivo. Meanwhile, FATP1 was found to enhance the proliferative activity of all the assessed cells, except murine 3T3-L1 cells. These results provide new insights into the potential effects of FATP1 on IMF deposition, especially regarding its positive effects on meat quality in buffaloes and other livestock.

The Metabolic Significance of Intermuscular Adipose Tissue: Is IMAT a Friend or a Foe to Metabolic Health?

Adipose tissues are not homogeneous and show site-specific properties. An elusive and understudied adipose tissue depot, most likely due to its limited accessibility, is the intermuscular adipose tissue (IMAT) depot. Adipose tissue is a pliable organ with the ability to adapt to its physiological context, yet whether that adaptation is harmful or beneficial in the IMAT depot remains to be explored in humans. Potential reasons for IMAT accumulation in humans being deleterious or beneficial include 1) sex and related circulating hormone levels, 2) race and ethnicity, and 3) lifestyle factors (e.g., diet and physical activity level). IMAT quantity per se may not be the driving factor in the etiology of insulin resistance and type 2 diabetes, but rather the quality of the IMAT itself is the true puppeteer. Adipose tissue quality likely influences its secreted factors, which are also likely to influence metabolism of surrounding tissues. The advent of molecular assessments such as transcriptome sequencing (RNAseq), assay for transposase-accessible chromatin using sequencing (ATACseq), and DNA methylation at the single-cell and single-nucleus levels, as well as the potential for ultrasound-guided biopsies specifically for IMAT, will permit more sophisticated investigations of human IMAT and dramatically advance our understanding of this enigmatic adipose tissue.

Mouse model of the adipose organ: the heterogeneous anatomical characteristics

Adipose tissue plays a pivotal role in energy storage, hormone secretion, and temperature control. Mammalian adipose tissue is largely divided into white adipose tissue and brown adipose tissue, although recent studies have discovered the existence of beige adipocytes. Adipose tissues are widespread over the whole body and each location shows distinctive metabolic features. Mice are used as a representative experimental model system in metabolic studies due to their numerous advantages. Importantly, the adipose tissues of experimental animals and humans are not perfectly matched, and each adipose tissue exhibits both similar and specific characteristics. Nevertheless, the diversity and characteristics of mouse adipose tissue have not yet been comprehensively summarized. This review summarizes diverse information about the different types of adipose tissue being studied in mouse models. The types and characteristics of adipocytes were described, and each adipose tissue was classified by type, and features such as its distribution, origin, differences from humans, and metabolic characteristics were described. In particular, the distribution of widely studied adipose tissues was illustrated so that researchers can comprehensively grasp its location. Also, the adipose tissues misused or confusingly used among researchers were described. This review will provide researchers with comprehensive information and cautions needed to study adipose tissues in mouse models.

Dissecting the Gene Expression Networks Associated with Variations in the Major Components of the Fatty Acid Semimembranosus Muscle Profile in Large White Heavy Pigs

Simple Summary

The amount and fatty acid composition of intramuscular fat are important features for the qualitative characteristics of processed and fresh meat products, but the knowledge of the key molecular drivers controlling these traits is still scant. To this aim, the present study investigated the co-expression networks of genes related to variations in the major fatty acids deposited in pig Semimembranosus muscle. Palmitic and palmitoleic acid contents were associated with a downregulation of genes involved in autophagy, mitochondrial fusion, and mitochondrial activity, suggesting that the deposition of these fatty acids may be enhanced in muscles with a reduced mitochondrial function. A higher proportion of oleic acid and a reduction in the percentages of n-6 and n-3 polyunsaturated fatty acids were related to changes in the mRNA levels of genes involved in Mitogen-Activated Protein Kinase (MAPK) signaling. The obtained results indicated gene expression networks and new candidate genes associated with the studied traits. Further studies are needed to confirm our results and identify in the discussed genes molecular markers for future selection schemes aimed at improving pork nutritional and technological quality. Furthermore, as pigs are considered reliable animal models for several human conditions, the obtained results may also be of interest for improving the knowledge of the molecular pathways associated with obesity and diabetes.

Abstract

To date, high-throughput technology such as RNA-sequencing has been successfully applied in livestock sciences to investigate molecular networks involved in complex traits, such as meat quality. Pork quality depends on several organoleptic, technological, and nutritional characteristics, and it is also influenced by the fatty acid (FA) composition of intramuscular fat (IMF). To explore the molecular networks associated with different IMF FA compositions, the Semimembranosus muscle (SM) from two groups of Italian Large White (ILW) heavy pigs divergent for SM IMF content was investigated using transcriptome analysis. After alignment and normalization, the obtained gene counts were used to perform the Weighted Correlation Network Analysis (WGCNA package in R environment). Palmitic and palmitoleic contents showed association with the same gene modules, comprising genes significantly enriched in autophagy, mitochondrial fusion, and mitochondrial activity. Among the key genes related to these FAs, we found TEAD4, a gene regulating mitochondrial activity that seems to be a promising candidate for further studies. On the other hand, the genes comprised in the modules associated with the IMF contents of oleic, n-6, and n-3 polyunsaturated FAs (PUFAs) were significantly enriched in Mitogen-Activated Protein Kinase (MAPK) signaling, in agreement with previous studies suggesting that several MAPK players may have a primary role in regulating lipid deposition. These results give an insight into the molecular cascade associated with different IMF FA composition in ILW heavy pigs. Further studies are needed to validate the results and confirm whether some of the identified key genes may be effective candidates for pork quality.

Comparative transcriptome analysis reveals that PCK1 is a potential gene affecting IMF deposition in buffalo

BACKGROUND

In China, although buffaloes are abundant, beef is mainly obtained from cattle, and this preference is mainly attributed to the low intramuscular fat (IMF) content of buffalo. Genetic factors are an important driver that affects IMF deposition.

RESULTS

To reveal the intrinsic factors responsible for the low IMF content of buffalo, mRNA expression patterns in muscle and adipose tissue between buffalo and cattle were characterized by RNA sequencing analysis. The IMF content in Nanyang cattle was higher than that in Xinyang buffalo. A total of 1566 mRNAs expressed in adipose tissue showed differential expression between the longissimus dorsi muscles of buffalo and cattle. Functional annotation suggested a difference in the glycolysis/gluconeogenesis pathway between the two species. The results of RT-qPCR analysis and gain-of-function experiments confirmed the positive association between the IMF content and phosphoenolpyruvate carboxykinase 1 (PCK1) expression in buffalo. In both mouse C2C12 cells and cultured bovine myocytes, the activity of the PCK1 promoter in buffalo is lower than that in cattle. However, in mouse 3T3-L1 adipocytes and cultured bovine adipocytes, the activity of PCK1 in buffalo promoter is higher than that in cattle.

CONCLUSIONS

These results indicate the important role of PCK1 in buffalo IMF deposition and illustrate the differences between buffalo and cattle promoter activity that drive PCK1 expression. This research helps to establish a foundation for further studies investigating IMF deposition in buffalo.

The whole-transcriptome landscape of muscle and adipose tissues reveals the ceRNA regulation network related to intramuscular fat deposition in yak

Background

The Intramuscular fat (IMF) content in meat products, which is positively correlated with meat quality, is an important trait considered by consumers. The regulation of IMF deposition is species specific. However, the IMF-deposition-related mRNA and non-coding RNA and their regulatory network in yak (Bos grunniens) remain unknown. High-throughput sequencing technology provides a powerful approach for analyzing the association between transcriptome-related differences and specific traits in animals. Thus, the whole transcriptomes of yak muscle and adipose tissues were screened and analyzed to elucidate the IMF deposition-related genes. The muscle tissues were used for IMF content measurements.

Results

Significant differences were observed between the 0.5- and 2.5-year-old yaks. Several mRNAs, miRNAs, lncRNAs and circRNAs were generally expressed in both muscle and adipose tissues. Between the 0.5- and 2.5-year-old yaks, 149 mRNAs, 62 miRNAs, 4 lncRNAs, and 223 circRNAs were differentially expressed in muscle tissue, and 72 mRNAs, 15 miRNAs, 9 lncRNAs, and 211 circRNAs were differentially expressed in adipose tissue. KEGG annotation revelved that these differentially expressed genes were related to pathways that maintain normal biological functions of muscle and adipose tissues. Moreover, 16 mRNAs, 5 miRNAs, 3 lncRNAs, and 5 circRNAs were co-differentially expressed in both types of tissue. We suspected that these co-differentially expressed genes were involved in IMF-deposition in the yak. Additionally, LPL, ACADL, SCD, and FASN, which were previously shown to be associated with the IMF content, were identified in the competing endogenous RNA (ceRNA) regulatory network that was constructed on the basis of the IMF deposition-related genes. Three ceRNA subnetworks also revealed that TCONS-00016416 and its target SIRT1 “talk” to each other through the same miR-381-y and miR-208 response elements, whereas TCONS-00061798 and its target PRKCA, and TCONS-00084092 and its target LPL “talk” to each other through miR-122-x and miR-499-y response elements, respectively.

Conclusion

Taken together, our results reveal the potential mRNA and noncoding RNAs involved in IMF deposition in the yak, providing a useful resource for further research on IMF deposition in this animal species.

Impact of Skeletal Muscle Mass on Metabolic Health

Skeletal muscle is regarded as an endocrine and paracrine organ. Muscle-derived secretory proteins, referred to as myokines, mediate interactions between skeletal muscle mass and other organs such as the liver, adipose tissue, pancreas, bone, and the cardiovascular system. As individuals age, reduced levels of physical activity and sarcopenia (loss of skeletal muscle mass and strength) are associated with physical frailty and disability. Recently, several studies have suggested that the loss of skeletal muscle mass may contribute to metabolic disease. Therefore, herein, we focus on the relationships between skeletal muscle mass and metabolic diseases, including metabolic syndrome and non-alcoholic fatty liver disease.

Neudesin Neurotrophic Factor Promotes Bovine Preadipocyte Differentiation and Inhibits Myoblast Myogenesis

Simple Summary

Neudesin neurotrophic factor (NENF) is a secreted protein that was significantly inhibited in the fat-muscle co-culture system in our previous study. However, studies on NENF regulation of bovine muscle development and involvement in the cross-talk between adipose tissue and skeletal muscle have not been reported. Hence, the aim of this study was to clarify the roles of NENF in bovine myoblast and preadipocyte differentiation. In this study, we first examined the spatial expression patterns of NENF in different tissues and found that NENF was highly expressed in the muscle of four-day-old and 24-month-old Qinchuan cattle. Compared with 4-day-old Qinchuan cattle, the expression level of NENF was significantly up-regulated in 24-month-old bovine adipose tissue. Then, we detected the expression pattern of the NENF gene in bovine preadipocyte and myoblast differentiation and found that the expression of NENF mRNA peaks at day 6 during preadipocyte differentiation and peaks at day 4 during myoblast differentiation. Furthermore, we found that the endogenous knockdown of NENF inhibited the differentiation of preadipocytes and promoted the differentiation of myoblasts. These findings not only lay the foundation for the construction of regulatory pathways during fat and muscle differentiation but also provide a theoretical basis for molecular breeding of beef cattle.

Abstract

Neudesin neurotrophic factor (NENF) is a secreted protein that is essential in multiple biological processes, including neural functions, adipogenesis, and tumorigenesis. In our previous study, NENF was significantly inhibited in the bovine adipocytes-myoblasts co-culture system. However, studies on NENF regulation of bovine muscle development and involvement in the cross-talk between adipose tissue and skeletal muscle have not been reported. Hence, the aim of this study was to clarify the functional roles of NENF in bovine preadipocytes and myoblasts. Real-time quantitative PCR (RT-qPCR) was performed to examine the spatial expression patterns of NENF in different tissues, and the results showed that NENF was highly expressed in the muscle of four-day-old and 24-month-old Qinchuan cattle. Compared with four-day-old Qinchuan cattle, the expression level of NENF was significantly up-regulated in 24-month-old bovine adipose tissue. To explore the roles of NENF in bovine myoblast and preadipocyte differentiation, small interfering RNA (siRNA) targeting the NENF gene were transfected into bovine preadipocytes and myoblasts to knock down the expression of the NENF gene. The results showed that the knockdown of NENF in differentiating adipocytes attenuated lipid accumulation, decreased the mRNA expression of adipogenic key marker genes PPARγ, CEBPα, CEBPβ, FASN, and SCD1, and decreased the protein expression of PPARγ, CEBPα, and FASN. The formation of myotubes was significantly accelerated, and the mRNA expression levels of myogenic marker genes MYOD1, MYF5, MYF6, MEF2A, MEF2C, and CKM, and the protein expression levels of MYOD1, MYF6, MEF2A, and CKM were up-regulated in myoblasts where NENF was knocked down. In short, the knockdown of NENF inhibited preadipocyte differentiation and promoted myoblast myogenesis.

miRNA transcriptome comparison between muscle and adipose tissues indicates potential miRNAs associated with intramuscular fat in Chinese swamp buffalo

The amount of intramuscular fat (IMF) affects the tenderness and juiciness of beef and is an important indicator of beef quality. A few miRNAs involved in IMF deposition have been identified in other livestock. However, in the buffalo, the association between miRNA and IMF has not been reported and the miRNA expression profile remains poorly understood. In this study, small RNA sequencing was performed to characterize the miRNA expression pattern in muscle and adipose tissues using the Illumina platform. A total of 108 differentially expressed (DE) miRNAs were identified, including 98 known miRNAs and 10 novel miRNAs. A qRT-PCR experiment confirmed the quality of the DE analysis. Eight DE miRNAs showed high expression in adipose tissue and a considerable expression level in muscle tissue. Functional enrichment indicated that bta-miR-148a, bta-miR-143, bta-miR-10b, bta-let-7i, bta-let-7f, bta-let-7b, bta-miR-30a-5p, and bta-miR-100 were significantly associated with adipogenesis, suggesting these as candidate regulators for IMF deposition in buffalo. However, further functional validation is required. This is the first characterization of the miRNA expression profile in the muscle and adipose tissues of buffalo. These results provide information for the identification of miRNAs with potential effects on IMF deposition in buffalo.

Brown Fat Promotes Muscle Growth During Regeneration

Accumulation of adipose tissue around and within muscles is highly correlated with reduced strength, functional limitations, and poor rehabilitative outcomes. Given the intimate physical contact between these tissues, paracrine cross-talk is a likely mediator of this association. The recent discovery that muscle-associated adipose tissue exhibits features of beige fat has suggested that this cross-talk may be modifiable, as beige fat can be stimulated to assume features of brown fat. In this work, we describe a novel intermuscular fat transplant model in the mouse rotator cuff to investigate cross-talk between muscle and adipose tissue. Specifically, we examine the role of transplanted fat phenotype on muscle regeneration by transplanting pieces of classical brown (interscapular), beige (inguinal), or white (epididymal) adipose tissue in conjunction with cardiotoxin injection to the adjacent supraspinatus muscle. Transplantation of brown fat, but not beige or white, significantly increased muscle mass, fiber cross-sectional area and contractile force production compared with sham injury. This effect was not seen when cardiotoxin was delivered to a distant muscle, or when adjacent muscles were injected with saline indicating that the effect is localized and specifically targeting the regenerative process. Thus, we conclude that local signaling between fat and muscle varies by phenotype and that brown fat supports regeneration. Clinical significance: Our findings suggest that the phenotype of muscle-associated fat could be a novel therapeutic target to modulate fat-muscle signaling. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1817-1826, 2019.

Intermuscular Adipose Tissue: A Brief Review of Etiology, Association With Physical Function and Weight Loss in Older Adults

Adipose tissue redistributes during aging resulting in increased intermuscular adipose tissue (IMAT), intramuscular, and intramyocellular lipid while subcutaneous fat decreases. IMAT has been associated with lower muscle strength, power, and quality, chronic inflammation, impaired glucose tolerance, and elevated total cholesterol in older adults. This review focused on trials investigating the role of age, physical activity and diet on IMAT. The studies agreed that IMAT increases with age and seems to be responsive to physical activity, particularly the combination of aerobic and resistance exercise. However, some reported this could occur with or without weight loss, and some reported that high IMAT at baseline may blunt the muscle quality adaptive response to physical training. Larger and longer trials are needed to differentiate the independent or synergistic effects of resistance and/or aerobic training, and obesity and weight loss combined with resistance, aerobic or combination of aerobic and resistance training on IMAT.

Anatomy-based MRI assessment of the iliopsoas muscle complex after pertrochanteric femoral fracture

OBJECTIVE

To evaluate the quality of the iliopsoas muscle complex after pertrochanteric femoral fracture, using MRI; to propose an anatomy-based evaluation of the iliopsoas muscle complex; and to determine the inter-reader reliability of two classifications of fatty muscle degeneration.

MATERIALS AND METHODS

We included adult patients with a displaced lesser trochanter following pertrochanteric femoral fracture. Muscle quality was evaluated using the Goutallier and Slabaugh classifications at three levels (L4/L5, L5/S1, and the anterior inferior iliac spine). Two radiologists independently reviewed the MRIs, and force measurement was performed on both hips. Linear mixed-effects models were used to determine the effect of fracture on muscle quality and strength, and Cohen's kappa statistic was used to assess inter-reader agreement.

RESULTS

In the 18 patients included, the iliopsoas muscle complex showed higher grades of fatty muscle degeneration on the fractured side than on the non-fractured side. The mean difference between muscle strength on the fractured vs the non-fractured side was -12 N (p > 0.05). Inter-reader agreement for the Goutallier and Slabaugh classifications was good and very good respectively (weighted K = 0.78 and 0.85 respectively).

CONCLUSION

Fatty muscle degeneration of the iliopsoas muscle complex after pertrochanteric femoral fracture was evident using both classification systems; however, fatty muscle degeneration resulted in only a minimal reduction of muscle strength. To provide a thorough assessment of iliopsoas muscle complex quality, we suggest evaluating it at different anatomical levels. Regarding inter-reader agreement, the Slabaugh classification was superior to the Goutallier classification.

Dietary sn-2 palmitic triacylglycerols reduced faecal lipids, calcium contents and altered lipid metabolism in Sprague-Dawley rats

In this study, the impact of dietary sn-2 palmitic triacylglycerol (sn-2 PTAG) on faecal lipids, calcium excretion and lipid metabolic alternation was investigated in Sprague-Dawley (SD) rats fed with high-fat diet containing either palm olein (PO, sn-2 palmitic acid (PA) of 14.8%), sn-2 PTAG50 (sn-2 PA of 56.4%) or sn-2 PTAG70 (sn-2 PA of 72.4%), respectively. After 4-week feeding period, SD rats fed with sn-2 PTAGs showed reduced faecal soap fatty acids, neutral lipid and calcium excretion compared to those of PO-fed rats, whereas a significant difference was only observed for the sn-2 PTAG70-fed rats (p < .05). Moreover, dietary sn-2 PTAG70 also showed a significant effect on decreasing serum triacylglycerol (TAG) level, reducing perirenal adipocyte size and regulating lipid metabolism in small intestine and perirenal adipose tissue of SD rats. Significantly increased mRNA levels of genes involved in intestinal lipid anabolism as well as lipid catabolism were both observed in the sn-2 PTAG70-fed rats (p < .05). Meanwhile, dietary sn-2 PTAG70 also significantly up-regulated lipolysis, mitochondrial fatty acid oxidation and thermogenesis-related gene and protein levels in perirenal adipose tissue, which might be correlated with the reduced perirenal adipocyte size. Taken together, our findings indicated that sn-2 PTAG70 may have some beneficial effects on intestinal lipid utilisation and lipid metabolic activity for energy supply in visceral adipose tissue.

Transcriptome analysis of adipose tissue from pigs divergent in feed efficiency reveals alteration in gene networks related to adipose growth, lipid metabolism, extracellular matrix, and immune response

Adipose tissue is hypothesized to play a vital role in regulation of feed efficiency (FE; efficiency in converting energy and nutrients into tissue), of which improvement will simultaneously reduce environmental impact and feed cost per pig. The objective of the present study was to sequence the subcutaneous adipose tissue transcriptome in FE-divergent pigs (n = 16) and identify relevant biological processes underpinning observed differences in FE. We previously demonstrated that high-FE pigs were associated with lower fatness when compared to their counterparts. Here, ontology analysis of a total of 209 annotated genes that were differentially expressed at a p < 0.01 revealed establishment of a dense extracellular matrix and inhibition of capillary formation as one underlying mechanism to achieve suppressed adipogenesis. Moreover, mechanisms ensuring an efficient utilization of lipids in high-FE pigs might be orchestrated by upstream regulators including CEBPA and EGF. Consequently, high-FE adipose tissue could exhibit more efficient cholesterol disposal, whilst inhibition of inflammatory and immune response in high-FE pigs may be an indicator of an optimally functioning adipose tissue. Taken together, adipose tissue growth, extracellular matrix formation, lipid metabolism and inflammatory and immune response are key biological events underpinning the differences in FE. Further investigations focusing on elucidating these processes would assist the animal production industry in optimizing strategies related to nutrient utilization and product quality.

Method using a co-culture system with high-purity intramuscular preadipocytes and satellite cells from chicken pectoralis major muscle

Intramuscular fat is important in improving meat quality; however, the lack of high-purity intramuscular preadipocyte (IMP) in vitro has severely limited the in-depth research on the mutual regulation of myocytes and adipocytes in chicken. In this study, we establish a new method by combining the mature adipocyte ceiling method and the transwell co-culture system. Mature intramuscular adipocyte (MIA) and muscle satellite cell (MSC) were obtained from digested pectoralis major, and MIAs were transformed into IMPs by dedifferentiation with ceiling culture. MSCs were then purified by differential adhesion for 2 h. The results by inverted-microscope observation, MTT assay, Oil Red O staining, and q-PCR revealed that the de-differentiated cells from MIA were identified as the IMPs, and had the same the cellular morphology, the capacity on differentiation, proliferation and passage with the abdominal fat preadipocytes (P > 0.05). The applicability of the obtained IMPs in co-cultured experiment with the MSC revealed that it could meet the requirements of the experimental study. Finally, a co-culture system of IMPs and MSCs was established using a transwell chamber. The co-cultured results indicated that MSCs in the proliferative stage tend to accelerate the differentiation of IMPs to induce more fat content in co-cultured IMPs than in single-culture IMPs (P < 0.05), in the non-proliferative stage, the results tend to show the opposite (P < 0.05). The mRNA levels of related genes significantly changed in accordance with the fat content in cells. The results strongly supported the view that the established co-culture system was effective and feasible. In summary, we successfully found a new method to explore the interaction between myocytes and adipocytes of chicken. Our findings can deepen research on the regulation of chicken myocytes and adipocytes.

An integrative transcriptomic approach to identify depot differences in genes and microRNAs in adipose tissues from high fat fed mice

Obesity contributes to metabolic disorders such as diabetes and cardiovascular disease. Characterization of differences between the main adipose tissue depots, white (WAT) [including subcutaneous (SAT) and visceral adipose tissue (VAT)] and brown adipose tissue (BAT) helps to identify their roles in obesity. Thus, we studied depot-specific differences in whole transcriptome and miRNA profiles of SAT, VAT and BAT from high fat diet (HFD/45% of calories from fat) fed mice using RNA sequencing and small RNA-Seq. Using quantitative real-time polymerase chain reaction, we validated depot-specific differences in endoplasmic reticulum (ER) stress related genes and miRNAs using mice fed a HFD vs. low fat diet (LFD/10% of calories from fat). According to the transcriptomic analysis, lipogenesis, adipogenesis, inflammation, endoplasmic reticulum (ER) stress and unfolded protein response (UPR) were higher in VAT compared to BAT, whereas energy expenditure, fatty acid oxidation and oxidative phosphorylation were higher in BAT than in VAT of the HFD fed mice. In contrast to BAT, ER stress marker genes were significantly upregulated in VAT of HFD fed mice than the LFD fed mice. For the first time, we report depot specific differences in ER stress related miRNAs including; downregulation of miR-125b-5p, upregulation miR-143-3p, and miR-222-3p in VAT following HFD and upregulation of miR-30c-2-3p only in BAT following a HFD in mice than the LFD mice. In conclusion, HFD differentially regulates miRNAs and genes in different adipose depots with significant induction of genes related to lipogenesis, adipogenesis, inflammation, ER stress, and UPR in WAT compared to BAT.

TRIENNIAL GROWTH AND DEVELOPMENT SYMPOSIUM: Factors influencing bovine intramuscular adipose tissue development and cellularity

Appearance, distribution, and amount of intramuscular fat (IMF), often referred to as marbling, are highly variable and depend on environmental and genetic factors. On the molecular level, the concerted action of several drivers, including hormones, receptors, transcription factors, etc., determines where clusters of adipocytes arise. Therefore, the aim of future studies remains to identify such factors as biological markers of IMF to increase the ability to identify animals that deposit IMF early in age to increase efficiency of high-quality meat production. In an attempt to unravel the cellular development of marbling, we investigated the abundance of markers for adipogenic differentiation during fattening of cattle and the transcriptome of muscle and dissected IMF. Markers of different stages of adipogenic differentiation are well known from cell culture experiments. They are usually transiently expressed, such as delta-like homolog 1 (DLK1) that is abundant in preadipocytes and absent during differentiation to mature adipocytes. It is even a greater challenge to detect those markers in live animals. Within skeletal muscles, hyperplasia and hypertrophy of adipocytes can be observed throughout life. Therefore, development of marbling requires, on the cellular level, recruitment, proliferation, and differentiation of adipogenic cells to store excess energy in the form of lipids in new cells. In a recent study, we investigated the localization and abundance of early markers of adipogenic differentiation, such as DLK1, in bovine muscle tissue. An inverse relationship between IMF content and number of DLK1-positive cells in bovine muscle was demonstrated. Considering the cellular environment of differentiating adipocytes in muscle and the secretory action of adipocytes and myocytes, it becomes obvious that cross talk between cells via adipokines and myokines may be important for IMF development. Secreted proteins can act on other cells, inhibiting or stimulating their function via autocrine and paracrine actions. Such factors with potential influence on IMF, among them, agouti signaling protein and thrombospondin 4, were identified in transcriptome analyses and further investigated. Furthermore, results from transcriptome analysis indicate involvement of genes that are not directly related to adipogenesis and lipid metabolism, providing new candidates for future research.

PDGFRα Regulated by miR-34a and FoxO1 Promotes Adipogenesis in Porcine Intramuscular Preadipocytes through Erk Signaling Pathway

Suitable intramuscular fat (IMF) content improves porcine meat quality. The vital genes regulating IMF deposition are necessary for the selection and breeding of an IMF trait. However, the effect and mechanism of PDGFRα on IMF deposition are still unclear. Here, PDGFRα is moderately expressed in porcine longissimus dorsi muscle (LD), whereas it highly expressed in white adipose tissue (WAT). Moreover, PDGFRα-positive cells were located in the gaps of LD fibers which there were IMF adipocytes. Compared with 180-day-old and lean-type pigs, the levels of PDGFRα were much higher in one-day-old and fat-type pigs. Meanwhile the levels of PDGFRα gradually decreased during IMF preadipocyte differentiation. Furthermore, PDGFRα promoted adipogenic differentiation through activating Erk signaling pathway. Based on PDGFRα upstream regulation analysis, we found that the knockdown of FoxO1 repressed lipogenesis by downregulating PDGFRα, and miR-34a inhibited adipogenesis through targeting PDGFRα. Collectively, PDGFRα is a positive regulator of IMF deposition. Therefore, we suggest that PDGFRα is a possible target to improve meat quality.

Muscle as a paracrine and endocrine organ

Skeletal muscle cells are highly abundant and metabolically active and are known to 'communicate' their energy demands to other organs through active secretion. Muscle-derived secretory proteins include a variety of cytokines and peptides collectively referred to as 'myokines' that exert autocrine, paracrine or endocrine effects. Analyses of the skeletal muscle secretome revealed that numerous myokines are secreted in response to contraction or strength training, and that these factors not only regulate energy demand but also contribute to the broad beneficial effects of exercise on cardiovascular, metabolic, and mental health. Herein we review recent studies on the myokines that regulate muscle function and those that mediate cross talk between skeletal muscle and other organs including adipose tissue, liver, pancreas, the cardiovascular system, brain, bones, and skin.

Effects of Low-Protein Diets Supplemented with Branched-Chain Amino Acid on Lipid Metabolism in White Adipose Tissue of Piglets

This study evaluated the effect of branched-chain amino acid (BCAA) supplementation in low-protein diets on lipid metabolism in dorsal subcutaneous adipose (DSA), abdominal subcutaneous adipose (ASA), and perirenal adipose (PRA) tissues. A total of 24 piglets were allotted to four treatments, and each group was fed the adequate protein (AP) diet, low-protein (LP) diet, LP diet supplemented with BCAA (LP + B), or LP diet supplemented with twice BCAA (LP + 2B). Serum concentrations of leptin in the BCAA-supplemented treatments were higher (P < 0.01) than in the AP treatment, but lower (P < 0.01) than in the LP treatment. In DSA, the mRNA and protein levels for lipogenic-related genes were highest in the LP treatment and lowest in the LP + 2B treatment. However, in ASA and PRA, the expression levels for those genes were significantly elevated in the LP + 2B treatment. In conclusion, BCAA supplementation could alter the body fat condition, and this effect was likely modulated by the expression of lipid metabolic regulators in DSA, ASA, and PRA in a depot-specific manner.

Invited review: Pre- and postnatal adipose tissue development in farm animals: from stem cells to adipocyte physiology

Both white and brown adipose tissues are recognized to be differently involved in energy metabolism and are also able to secrete a variety of factors called adipokines that are involved in a wide range of physiological and metabolic functions. Brown adipose tissue is predominant around birth, except in pigs. Irrespective of species, white adipose tissue has a large capacity to expand postnatally and is able to adapt to a variety of factors. The aim of this review is to update the cellular and molecular mechanisms associated with pre- and postnatal adipose tissue development with a special focus on pigs and ruminants. In contrast to other tissues, the embryonic origin of adipose cells remains the subject of debate. Adipose cells arise from the recruitment of specific multipotent stem cells/progenitors named adipose tissue-derived stromal cells. Recent studies have highlighted the existence of a variety of those cells being able to differentiate into white, brown or brown-like/beige adipocytes. After commitment to the adipocyte lineage, progenitors undergo large changes in the expression of many genes involved in cell cycle arrest, lipid accumulation and secretory functions. Early nutrition can affect these processes during fetal and perinatal periods and can also influence or pre-determinate later growth of adipose tissue. How these changes may be related to adipose tissue functional maturity around birth and can influence newborn survival is discussed. Altogether, a better knowledge of fetal and postnatal adipose tissue development is important for various aspects of animal production, including neonatal survival, postnatal growth efficiency and health.

Genetic Marker Discovery in Complex Traits: A Field Example on Fat Content and Composition in Pigs

Among the large number of attributes that define pork quality, fat content and composition have attracted the attention of breeders in the recent years due to their interaction with human health and technological and sensorial properties of meat. In livestock species, fat accumulates in different depots following a temporal pattern that is also recognized in humans. Intramuscular fat deposition rate and fatty acid composition change with life. Despite indication that it might be possible to select for intramuscular fat without affecting other fat depots, to date only one depot-specific genetic marker (PCK1 c.2456C>A) has been reported. In contrast, identification of polymorphisms related to fat composition has been more successful. For instance, our group has described a variant in the stearoyl-coA desaturase (SCD) gene that improves the desaturation index of fat without affecting overall fatness or growth. Identification of mutations in candidate genes can be a tedious and costly process. Genome-wide association studies can help in narrowing down the number of candidate genes by highlighting those which contribute most to the genetic variation of the trait. Results from our group and others indicate that fat content and composition are highly polygenic and that very few genes explain more than 5% of the variance of the trait. Moreover, as the complexity of the genome emerges, the role of non-coding genes and regulatory elements cannot be disregarded. Prediction of breeding values from genomic data is discussed in comparison with conventional best linear predictors of breeding values. An example based on real data is given, and the implications in phenotype prediction are discussed in detail. The benefits and limitations of using large SNP sets versus a few very informative markers as predictors of genetic merit of breeding candidates are evaluated using field data as an example.

Fatty Infiltration of Skeletal Muscle: Mechanisms and Comparisons with Bone Marrow Adiposity

Skeletal muscle and bone share common embryological origins from mesodermal cell populations and also display common growth trajectories early in life. Moreover, muscle and bone are both mechanoresponsive tissues, and the mass and strength of both tissues decline with age. The decline in muscle and bone strength that occurs with aging is accompanied in both cases by an accumulation of adipose tissue. In bone, adipocyte (AC) accumulation occurs in the marrow cavities of long bones and is known to increase with estrogen deficiency, mechanical unloading, and exposure to glucocorticoids. The factors leading to accumulation of intra- and intermuscular fat (myosteatosis) are less well understood, but recent evidence indicates that increases in intramuscular fat are associated with disuse, altered leptin signaling, sex steroid deficiency, and glucocorticoid treatment, factors that are also implicated in bone marrow adipogenesis. Importantly, accumulation of ACs in skeletal muscle and accumulation of intramyocellular lipid are linked to loss of muscle strength, reduced insulin sensitivity, and increased mortality among the elderly. Resistance exercise and whole body vibration can prevent fatty infiltration in skeletal muscle and also improve muscle strength. Therapeutic strategies to prevent myosteatosis may improve muscle function and reduce fall risk in the elderly, potentially impacting the incidence of bone fracture.

Skeletal muscle and pediatric bone development

PURPOSE OF REVIEW

The purpose of this review is to summarize the recent clinical findings surrounding the muscle-bone relationships in children, while considering muscle adiposity, endocrine factors, and lifestyle influences (i.e., diet and exercise) involved in pediatric musculoskeletal development.

RECENT FINDINGS

Positive relationships between cortical bone geometry and muscle mass, size and function have been reported. Prospective studies in particular have helped clarify some of the inconsistent relationships between muscle and cortical bone volumetric density. Muscle fat is associated with impaired glucose handling and muscular functionality, which may in turn have a downstream effect on cortical bone growth during adolescence. Lifestyle factors such as healthful diets and higher impact physical activities can promote optimal skeletal development by improving the muscular phenotype and endocrine profile.

SUMMARY

Muscle and bone are two intricately-related tissue types; however, factors such as sex, maturation, study design, and outcome measures studied can modify this relationship. Further research is warranted to understand the impact of muscle adiposity on cardiometabolic health, muscle function and, subsequently, pediatric musculoskeletal development and fracture risk. Following age-specific diet and physical activity recommendations should be a major focus in obtaining optimal muscle and bone development throughout maturation.

Identification of novel putative adipomyokines by a cross-species annotation of secretomes and expression profiles

Adipose tissue and skeletal muscle are organs that respond strongly to obesity and physical activity exhibiting high secretory activity. To identify novel putative adipomyokines, comparative expression studies of skeletal muscle and adipose tissue of lean (C57BL/6J) and obese (C57BL/6J on a high-fat diet and NZO) mice, of sedentary and endurance trained C57BL/6J mice and of cattle characterized by different amounts of intramuscular fat were combined with human secretome data and scored. In highly regulated transcripts, we identified 119 myokines, 79 adipokines and 22 adipomyokines. Network analysis of these candidates revealed remodelling of extracellular matrix and tissue fibrosis as relevant functions of several of these candidates. Given the pathophysiogical relevance of fibrosis for adipose-muscle-cross-talk in obesity and type 2 diabetes and its physiological role in exercise adaptation and meat quality of farm animals, they represent interesting candidates for further investigations in different research areas and species.

Intermuscular and intramuscular adipose tissues: Bad vs. good adipose tissues

Human studies of the influence of aging and other factors on intermuscular fat (INTMF) were reviewed. Intermuscular fat increased with weight loss, weight gain, or with no weight change with age in humans. An increase in INTMF represents a similar threat to type 2 diabetes and insulin resistance as does visceral adipose tissue (VAT). Studies of INTMF in animals covered topics such as quantitative deposition and genetic relationships with other fat depots. The relationship between leanness and higher proportions of INTMF fat in pigs was not observed in human studies and was not corroborated by other pig studies. In humans, changes in muscle mass, strength and quality are associated with INTMF accretion with aging. Gene expression profiling and intrinsic methylation differences in pigs demonstrated that INTMF and VAT are primarily associated with inflammatory and immune processes. It seems that in the pig and humans, INTMF and VAT share a similar pattern of distribution and a similar association of components dictating insulin sensitivity. Studies on intramuscular (IM) adipocyte development in meat animals were reviewed. Gene expression analysis and genetic analysis have identified candidate genes involved in IM adipocyte development. Intramuscular (IM) adipocyte development in human muscle is only seen during aging and some pathological circumstance. Several genetic links between human and meat animal adipogenesis have been identified. In pigs, the Lipin1 and Lipin 2 gene have strong genetic effects on IM accumulation. Lipin1 deficiency results in immature adipocyte development in human lipodystrophy. In humans, overexpression of Perilipin 2 (PLIN2) facilitates intramyocellular lipid accretion whereas in pigs PLIN2 gene expression is associated with IM deposition. Lipins and perilipins may influence intramuscular lipid regardless of species.

Locus characterization and gene expression of bovine FNDC5: is the myokine irisin relevant in cattle?

The transmembrane protein FNDC5 was recently characterized as precursor of an exercise induced myokine named irisin. Previous studies found a relationship between circulating irisin levels and muscle mass in humans. Consequently, we tested the hypothesis whether FNDC5/irisin is involved in the modulation of body composition in cattle. Since information on the bovine FNDC5 locus was scarce, we characterized the gene experimentally as prerequisite for these investigations. We provide here a revised and extended gene model for bovine FNDC5. Although similarly organized like the human and murine loci, a higher variability was observed at transcript level in the bovine locus. FNDC5 mRNA was abundant in bovine skeletal muscle and was detected at lower levels in adipose tissue and liver. There were no expression differences between two groups of bulls highly different in muscularity and adiposity. Full-length FNDC5 protein (25 kDa) was present in bovine skeletal muscle independent of muscularity. Neither FNDC5 nor its putatively secreted peptide irisin were found in circulation of bulls. In contrast, we demonstrated that FNDC5 (25 kDa) and irisin (12 kDa) were present in murine skeletal muscle and that irisin was circulating in murine serum. This indicates fundamental differences in the regulation of FNDC5 and irisin between rodents and cattle.