Ontogenesis of muscle and adipose tissues and their interactions in ruminants and other species

The lean-to-fat ratio, that is, the relative masses of muscle and adipose tissue, is a criterion for the yield and quality of bovine carcasses and meat. This review describes the interactions between muscle and adipose tissue (AT) that may regulate the dynamic balance between the number and size of muscle v. adipose cells. Muscle and adipose tissue in cattle grow by an increase in the number of cells (hyperplasia), mainly during foetal life. The total number of muscle fibres is set by the end of the second trimester of gestation. By contrast, the number of adipocytes is never set. Number of adipocytes increases mainly before birth until 1 year of age, depending on the anatomical location of the adipose tissue. Hyperplasia concerns brown pre-adipocytes during foetal life and white pre-adipocytes from a few weeks after birth. A decrease in the number of secondary myofibres and an increase in adiposity in lambs born from mothers severely underfed during early pregnancy suggest a balance in the commitment of a common progenitor into the myogenic or adipogenic lineages, or a reciprocal regulation of the commitment of two distinct progenitors. The developmental origin of white adipocytes is a subject of debate. Molecular and histological data suggested a possible transdifferentiation of brown into white adipocytes, but this hypothesis has now been challenged by the characterization of distinct precursor cells for brown and white adipocytes in mice. Increased nutrient storage in fully differentiated muscle fibres and adipocytes, resulting in cell enlargement (hypertrophy), is thought to be the main mechanism, whereby muscle and fat masses increase in growing cattle. Competition or prioritization between adipose and muscle cells for the uptake and metabolism of nutrients is suggested, besides the successive waves of growth of muscle v. adipose tissue, by the inhibited or delayed adipose tissue growth in bovine genotypes exhibiting strong muscular development. This competition or prioritization occurs through cellular signalling pathways and the secretion of proteins by adipose tissue (adipokines) and muscle (myokines), putatively regulating their hypertrophy in a reciprocal manner. Further work on the mechanisms underlying cross-talk between brown or white adipocytes and muscle fibres will help to achieve better understanding as a prerequisite to improving the control of body growth and composition in cattle.     

Prenatal developmental changes in glucose transporters, intermediary metabolism and hormonal receptors related to the IGF/insulin-glucose axis in the heart and adipose tissue of bovines

Glucose transporter ontogenesis is likely to play a key role in glucose uptake by foetal tissues in order to satisfy their energy requirements. We thus investigated developmental changes in the bovine heart and perirenal adipose tissue in two glucose transporter isoforms, namely GLUT1 and GLUT4, the latter being responsible for the regulation of glucose uptake by insulin. Other key players of the glucose/insulin axis were also assessed. Plasma glucose concentration in the foetus was lower at 8 and 8.5 months of age than previously. In the heart, GLUT1 protein level markedly decreased between 3 and 4 months of age, whereas the number of insulin and IGF-I binding sites continually decreased, especially between 7 and 8 or 8.5 months of age. On the contrary, the GLUT4 level increased until 8 months of age and remained high until 2 weeks after birth. The activities of enzymes of glucose metabolism (namely phosphofructokinase [PFK] and lactate dehydrogenase [LDH]) increased throughout gestation and reached a plateau at 6 and 8.5 months of age for PFK and LDH, respectively. The activities of enzymes involved in fatty acid metabolism increased especially at birth. In perirenal adipose tissue, high mitochondrial activity was detected before birth which is a characteristic of brown adipose tissue. Furthermore, lipoprotein lipase activity and GLUT4 protein level markedly increased to reach a maximum at 6-7 and 8 months of age, and sharply decreased thereafter, whereas GLUT1 protein level increased between 6 and 7 months of age. In conclusion, considerable changes in the regulation of the insulin/glucose axis were observed from 6 months onwards of foetal development in both the heart and adipose tissue of cattle, which probably alters the potential of these tissues to use glucose or fat as energy sources.     

Enhanced muscle by myostatin propeptide increases adipose tissue adiponectin, PPAR-alpha, and PPAR-gamma expressions

Muscle tissue utilizes a large portion of metabolic energy for its growth and maintenance. Previously, we demonstrated that transgenic over-expression of myostatin propeptide in mice fed a high-fat diet enhanced muscle mass and circulating adiponectin while the wild-type mice developed obesity and insulin resistance. To understand the effects of enhanced muscle growth on adipose tissue metabolism, we analyzed adiponectin, PPAR-α, and PPAR-γ mRNA expressions in several fat tissues. Results indicated muscled transgenic mice fed a high-fat diet displayed increased epididymal adiponectin mRNA expression by 12 times over wild-type littermates. These transgenic mice fed either a high or normal fat diet also displayed significantly high levels of PPAR-α and PPAR-γ expressions above their wild-type littermates in epididymal fat while their expressions in mesenteric fats were not significantly different between transgenic mice and their littermates. This study demonstrates that enhanced muscle growth has positive effects on fat metabolisms through increasing adiponectin expression and its regulations.     

Ontogeny of amino-acid metabolism-enzymes in peripheral tissues of developing rats

The activities of alanine, aspartate and tyrosine transaminase, adenylate deaminase, glutamate dehydrogenase and glutamine synthetase have been measured in hind leg striated muscle, lumbar adipose tissue and lumbar skin of developing rats from late foetal to weaning stage. In a general way, despite minor differences and different physiological r?les, the three peripheral tissues studied showed a concordant enzyme activity pattern with the r?les found for these enzymes in the adult. Muscle had a more constant pattern throughout development, with wider changes in skin and widest in adipose tissue. The results found agree with a marked "synthetic" mode in the tissues studied throughout all development studied. The patterns observed agree with a strict amino-acid conservation scheme during foetal life and lactation that progressively changes with weaning towards a frank degrading mode.     

Immunohistochemical analysis of bFGF,TGF-beta1 and catalase in rectus abdominis muscle from cattle foetuses at 180 and 260 days post-conception

The potential for muscle growth depends on myoblast proliferation, which occurs essentially during the first two thirds of the foetal period in cattle. Thereafter, myofibres acquire their contractile and metabolic properties. Proliferation is regulated by molecular growth factors and by the tissue oxidative activity. The aim of this study was the quantification by immunochemistry of basic fibroblast growth factor (bFGF) and transforming growth factor beta 1 (TGF-beta1) and also of enzyme catalase (CAT) activity in rectus abdominis muscle. Samples were collected from cattle foetuses of different growth potential at 180 and 260 days post-conception (dpc). One major conclusion from this work is that protein contents of the muscle tissue bFGF and, to a lower extent, CAT activity decreased with increasing age during the foetal life. No differences were found between the different genotypes of cattle. However, the CAT to bFGF ratio tended to be lower in fast-growing cattle and increased with foetal age. TGF-beta1 did not change with age and was localised mostly at the vascular bed. CAT was detected in smooth and rough reticulum in striated muscles at 180dpc, and additionally in mitochondria at 260dpc. In conclusion, the balance between intracellular growth factors (bFGF and TGF-beta1) and the activity of antioxidant enzyme CAT may participate in the regulation of the transition from myoblast proliferation to differentiation. Thus, increased ratio of CAT to bFGF might be a good index indicating initiation of muscle maturation in cattle foetus prior to birth.     

Lipomatous muscular ‘dystrophy’ of Piedmontese cattle

Lipomatous myopathy is a degenerative muscle pathology characterized by the substitution of muscle cells with adipose tissue, sporadically reported in cattle, pigs, and rarely in sheep, horses and dogs. This study investigated the pathology of this myopathy in 40 muscle samples collected from regularly slaughtered Piedmontese cattle living in Piedmont region (Italy). None of the animals showed clinical signs of muscular disease. Muscle specimens were submitted to histological and enzymatic investigations. Gross pathology revealed a different grade of infiltration of adipose tissue, involving multiple or single muscles. The most affected regions were the ventral abdomen and the shoulders, especially the cutaneous muscles and the muscles of the thoracic group. Morphological staining revealed an infiltration of adipose tissue varying in distribution and severity, changes in muscle fibre size and increased number of fibres with centrally located nuclei, suggesting muscle degeneration–regeneration. Necrosis and non-suppurative inflammatory cells were also seen. Furthermore, proliferation of connective tissue and non-specific myopathic changes were present. Chemical and physical characteristics of the affected tissue were also evaluated. The authors discuss about the aetiopathogenesis and classification of this muscle disorder whose histological lesions were similar to those reported in human dystrophies.     

Age-related changes and location of types I, III, XII and XIV collagen during development of skeletal muscles from genetically different animals

The ontogenesis of total collagen and of different collagen types was studied in four muscle types from genetically different cattle. Hydroxyproline content was 1.2-fold higher in muscles from cross-bred foetuses with normal muscle growth compared to those of the other genetic types (pure bred with different growth rates, double-muscled breed). A similar tendency was observed for type III collagen content. In all muscles of each animal studied, type XII and XIV collagens were colocated in perimysium. Immunolabelling obtained for type XII collagen was higher during foetal life than after birth, while for type XIV collagen, the opposite result was obtained. Whatever the muscle studied, but especially in semitendinosus muscle, during the foetal and the post-natal period until 15 months of age, immunolabelling with antibody anti-type XIV collagen tended to be more intense in muscles of animals from fathers selected for a low muscle growth capacity compared to those from fathers selected for a high muscle growth capacity. In conclusion, this study shows, that during foetal life, selection according to muscle growth capacity has no significant effect on the contents of total hydroxyproline or type III collagen, but minor effects on collagen localization.     

Immunohistochemical analysis of bFGF, TGF-β1 and catalase in rectus abdominis muscle from cattle foetuses at 180 and 260 days post-conception

The potential for muscle growth depends on myoblast proliferation, which occurs essentially during the first two thirds of the foetal period in cattle. Thereafter, myofibres acquire their contractile and metabolic properties. Proliferation is regulated by molecular growth factors and by the tissue oxidative activity. The aim of this study was the quantification by immunochemistry of basic fibroblast growth factor (bFGF) and transforming growth factor beta 1 (TGF-β1) and also of enzyme catalase (CAT) activity in rectus abdominis muscle. Samples were collected from cattle foetuses of different growth potential at 180 and 260 days post-conception (dpc). One major conclusion from this work is that protein contents of the muscle tissue bFGF and, to a lower extent, CAT activity decreased with increasing age during the foetal life. No differences were found between the different genotypes of cattle. However, the CAT to bFGF ratio tended to be lower in fast-growing cattle and increased with foetal age. TGF-β1 did not change with age and was localised mostly at the vascular bed. CAT was detected in smooth and rough reticulum in striated muscles at 180 dpc, and additionally in mitochondria at 260 dpc. In conclusion, the balance between intracellular growth factors (bFGF and TGF-β1) and the activity of antioxidant enzyme CAT may participate in the regulation of the transition from myoblast proliferation to differentiation. Thus, increased ratio of CAT to bFGF might be a good index indicating initiation of muscle maturation in cattle foetus prior to birth.     

Muscle fibre ontogenesis in farm animal species

In farm animals (bovine, ovine, swine, rabbit and poultry), muscle fibre characteristics play a key role in meat quality. The present review summarises the knowledge on muscle fibre characteristics and ontogenesis in these species. Myofibre ontogenesis begins very early during embryonic life, with the appearance of two or three successive waves of myoblasts which constitute the origin of the different types of muscle fibres. In small animals (rodents and poultry), a primary and a secondary generation of fibres arise respectively during the embryonic and foetal stages of development. In the largest species (bovines, sheep, pigs) a third generation arises in the late foetal or early postnatal period. Following these two or three waves of myogenesis, the total number of fibres is fixed. This occurs during foetal life (bovines, ovines, pigs and poultry) or during the first postnatal month in rabbits. Contractile and metabolic differentiation proceed by steps in parallel to myogenesis and are partially linked to each other. In bovines and ovines, the main events occur during foetal life, whereas they occur soon after birth in the pig, poultry and rabbit, but some plasticity remains later in life in all species. This comparative survey shows that the cellular processes of differentiation are comparable between species, while their timing is usually species specific.     

Fatty acid synthesis from amino acids in sheep adipose tissue

The rates of incorporation of 14C from 14C labelled acetate, glucose, alanine, leucine, isoleucine and valine into fatty acids has been measured in perirenal adipose tissue from foetal lambs and 8-month-old sheep, and into both fatty acids and acylglycerol glycerol in adipose tissue from 3-year-old sheep and 220-240 g female rats. Rates of incorporation of 14C from amino acids into fatty acids were much lower in adipose tissue from sheep (at all three ages) than from rats, whereas rates of incorporation of 14C into acylglycerol glycerol were either greater in sheep adipose tissue or the same as in rat adipose tissue. The rate of incorporation of 14C from amino acids into fatty acids decreased in the order leucine greater than alanine greater than isoleucine greater than valine in adipose tissue from rats and foetal lambs, and in the order leucine greater than alanine = isoleucine greater than valine in adipose tissue from 8-month- and 3-year-old sheep. Amino acids make a very small contribution to fatty acid synthesis in adipose tissue from sheep at all stages of development examined while fatty acids are a minor product of amino acid metabolism in sheep adipose tissue. The study provides further evidence for an important role for ATP-citrate lyase in restricting the utilization of acetyl-CoA generated in the mitochondria for fatty acid synthesis.     

Adipose tissue development--impact of the early life environment

Increasing experimental and observational evidence in both animals and humans suggests that early life events are important in setting later fat mass. This includes both the number of adipocytes and the relative distribution of both brown and white adipose tissue. Brown adipose tissue is characterised as possessing a unique uncoupling protein (UCP)1 which enables the rapid generation of large amounts of heat and is most abundant in the newborn. In large mammals such as sheep and humans, brown fat that is located around the major internal organs, is largely lost during the postnatal period. However, it is retained in small and discrete areas into adulthood when it is sensitive to environmental cues such as changes in ambient temperature or day length. The extent to which brown adipose tissue is lost or replaced by white adipose tissue and/or undergoes a process of transdifferentiation remains controversial. Small amounts of UCP1 can also be present in skeletal muscle which now appears to share the same common precursor cell as brown adipose tissue. The functional consequences of UCP1 in muscle remain to be confirmed but it could contribute to dietary induced thermogenesis. Challenges in elucidating the primary mechanisms regulating adipose tissue development include changes in methylation status of key genes during development in different species, strains and adipose depots. A greater understanding of the mechanisms by which early life events regulate adipose tissue distribution in young offspring are likely to provide important insights for novel interventions that may prevent excess adiposity in later life.     

mRNA expression pattern and association study with growth traits of bovine vaspin gene

Visceral adipose tissue-derived serine protease inhibitor (vaspin) is an interesting novel adipocytokine with insulin-sensitizing effects. Some studies have suggested that vaspin could play an important role in the development of obesity and metabolic disorders. However, the tissue expression patterns in cattle and impact of vaspin gene variants on the growth traits has not been determined yet. Herein, we firstly investigated the tissue expression patterns of vaspin gene in new born and adult cattle. The results showed that vaspin was ubiquitously expressed in most tissues and strongly expressed in the heart, skeletal muscle and fat. Then, genetic variants within bovine vaspin gene were screened in 1235 individuals from five Chinese indigenous cattle breeds. Two novel mutations in coding region (NW_001494061: g.1124477 G>A and g.1118561 T>C) of bovine vaspin gene were identified using MspI PCR–RFLP and HhaI ACRS PCR–RFLP detection. Association analysis revealed both two mutations were significantly associated with bodyweight and chest girth at 24 months in cattle (P < 0.05). Therefore, the MspI and HhaI genetic variants of bovine vaspin gene were recommended as DNA markers related to growth traits through marker-assisted selection for genetics and breeding in cattle.     

Monocyte chemotactic protein-1 and its role in insulin resistance

PURPOSE OF REVIEW: In obesity, there is a strong link between increased adipose tissue mass and development of insulin resistance in tissues such as liver and muscle. Under these conditions, adipose tissue synthesizes various pro-inflammatory chemokines such as monocyte chemotactic protein-1. This review provides a summary of recent knowledge on the role of monocyte chemotactic protein-1 in adipose tissue inflammation and insulin resistance. RECENT FINDINGS: Monocyte chemotactic protein-1 is a proinflammatory adipokine that is believed to play a role in the pathogenesis of obesity and diabetes. New in-vitro data demonstrate that monocyte chemotactic protein-1 has the ability to induce insulin resistance in adipocytes and skeletal muscle cells. By using mice that either overexpress monocyte chemotactic protein-1 or are deficient in monocyte chemotactic protein-1 or its receptor, exciting new insights have been obtained into the role of monocyte chemotactic protein-1 in adipose tissue inflammation and insulin resistance. SUMMARY: Monocyte chemotactic protein-1 is an adipokine with insulin-resistance-inducing capacity that is related to increased adipose tissue mass in obesity and insulin resistance. It plays an important role in adipose tissue inflammation by recruiting macrophages into fat. Monocyte chemotactic protein-1 is thus a therapeutic target, and may represent an important factor linking adipose tissue inflammation, obesity and type 2 diabetes.     

Cell death-inducing DFFA-like effector c (CIDEC/Fsp27) gene: molecular cloning, sequence characterization, tissue distribution and polymorphisms in Chinese cattles

Cell death-inducing DFFA-like effector c (CIDEC) protein, also known as fat specific protein 27 (Fsp27), is localized to lipid droplets. CIDEC protein is required for unilocular lipid droplet formation and optimal energy storage in addition to controlling lipid metabolism in adipocytes and hepatocytes. Research found that Ad-36 could induce lipid droplets in the cultured skeletal muscle cells and this process may be mediated by promoting CIDEC expression. The content of intermuscular fat is an important index for evaluation of beef quality, so the CIDEC gene appeared to be a candidate gene for regulation of intermuscular fat, however similar research for the bovine CIDEC gene is lacking. This paper examined the tissue expression profile of CIDEC gene in cattle using real-time RT-PCR to suggest that bovine CIDEC is highly expressed in adipose tissue. In addition, the Bovine CIDEC gene was cloned and inserted into the eukaryotic expression vector pET-28a(+), whereupon recombinant bovine CIDEC protein was induced and identified by Western-blot. A phylogenetic analysis showed that the animo acid sequence of bovine CIDEC was closer to mammalian CIDEC than rasorial CIDEC. We found ten single nucleotide polymorphisms sites (SNPs) in bovine CIDEC gene, of which SNP 2, 3, 4, 6 and 9, and SNP 8 and 10 were in complete linkage disequilibrium, respectively. SNP 1, 2 and 10 were used in further haplotype studies. Eight different haplotypes were identified in 973 cattle, of which haplotype 8 predominated with frequencies ranging from 42.90 to 54.30 %. This research provides a basis for future functional studies of CIDEC in cattle.     

Pigs fed saturated fat/cholesterol have a blunted hypothalamic-pituitary-adrenal function,are insulin resistant and have decreased expression of IRS-1, PGC1α and PPARα

The increasing incidence of insulin resistance has been linked to both increased intake of saturated fatty acids and disruption of the hypothalamic-pituitary-adrenal (HPA) axis. We tested the hypothesis that adding saturated fat/cholesterol to the diet of growing pigs would both disrupt HPA function and cause insulin resistance. Three-month-old pigs were fed either a control (13% energy from fat) or a high saturated fatty acid cholesterol (HSFC) diet (44% energy from fat; 2% cholesterol). After 10 weeks on the diets, intravenous ACTH, insulin and glucose challenges were performed, and after 12 weeks, tissue samples were taken for measurement of mRNA and for lipid-rich aortic lesions. Plasma total, HDL- and LDL-cholesterol were significantly increased in pigs fed the HSFC diet. Cortisol release during the ACTH challenge was suppressed in HSFC-fed pigs which were also more insulin resistant and glucose intolerant than controls. The HSFC diet decreased the expression of insulin receptor (IR) and insulin receptor substrate-1 in muscle and adipose tissue as well as adiponectin and adiponectin receptor 2 expression in fat. The HSFC diet decreased PGC-1α and PPARα expression in muscle but increased PPARα expression in liver. There was a trend for an increase in lipid-stained lesion frequency around the abdominal branches of the aorta in HSFC-fed pigs. We conclude that feeding increased saturated fat to pigs causes disruption in the HPA axis, insulin resistance and decreased muscle and adipose expression of genes controlling insulin signalling and mitochondrial oxidative capacity.     

Growth hormone receptor gene expression in the skeletal muscle of normal and double-muscled bovines during foetal development

The expression of the growth hormone receptor (GHR) gene was investigated in semitendinosus muscle during bovine foetal development in both normal and double-muscled Charolais foetuses which differ with respect to muscle development. Northern-blot analysis of foetal muscle RNA preparations with a GHR cDNA probe identified the 4.5 kb GHR mRNA as early as 130 days post-conception. In double-muscled animals, the expression of GHR mRNA increased from 130 to 210 days of gestation while it stayed stable in normal ones. It was significantly higher (P < 0.05) in double-muscled foetuses compared to normal ones from the second third of gestation. Northern-blot analysis of foetal muscle RNA preparations from both genotypes with a beta-actin cDNA probe, revealed lower beta-actin gene expression in double-muscled foetuses than in normal ones, suggesting a delay in the differentiation of muscle cells. In situ hybridisation revealed the localisation of specific GHR mRNA in muscle cells at all gestation stages analysed (130, 170, 210 days post-conception) but not in connective tissue surrounding the muscle cells. At the adult stage, the hybridisation signal was also very high and observed in muscle cells only. These results show the ontogeny of GHR mRNA in bovine muscle and demonstrate a difference between normal and double-muscled animals.