Catherine Jurie

Intramuscular fat content in meat-producing animals: development, genetic and nutritional control, and identification of putative markers.

Intramuscular fat (IMF) content plays a key role in various quality traits of meat. IMF content varies between species, between breeds and between muscle types in the same breed. Other factors are involved in the variation of IMF content in animals, including gender, age and feeding. Variability in IMF content is mainly linked to the number and size of intramuscular adipocytes. The accretion rate of IMF depends on the muscle growth rate. For instance, animals having a high muscularity with a high glycolytic activity display a reduced development of IMF. This suggests that muscle cells and adipocytes interplay during growth. In addition, early events that influence adipogenesis inside the muscle (i.e proliferation and differentiation of adipose cells, the connective structure embedding adipocytes) might be involved in interindividual differences in IMF content. Increasing muscularity will also dilute the final fat content of muscle. At the metabolic level, IMF content results from the balance between uptake, synthesis and degradation of triacylglycerols, which involve many metabolic pathways in both adipocytes and myofibres. Various experiments revealed an association between IMF level and the muscle content in adipocyte-type fatty acid-binding protein, the activities of oxidative enzymes, or the delta-6-desaturase level; however, other studies failed to confirm such relationships. This might be due to the importance of fatty acid fluxes that is likely to be responsible for variability in IMF content during the postnatal period rather than the control of one single pathway. This is evident in the muscle of most fish species in which triacylglycerol synthesis is almost zero. Genetic approaches for increasing IMF have been focused on live animal ultrasound to derive estimated breeding values. More recently, efforts have concentrated on discovering DNA markers that change the distribution of fat in the body (i.e. towards IMF at the expense of the carcass fatness). Thanks to the exhaustive nature of genomics (transcriptomics and proteomics), our knowledge on fat accumulation in muscles is now being underpinned. Metabolic specificities of intramuscular adipocytes have also been demonstrated, as compared to other depots. Nutritional manipulation of IMF independently from body fat depots has proved to be more difficult to achieve than genetic strategies to have lipid deposition dependent of adipose tissue location. In addition, the biological mechanisms that explain the variability of IMF content differ between genetic and nutritional factors. The nutritional regulation of IMF also differs between ruminants, monogastrics and fish due to their digestive and nutritional particularities.

Meat quality and composition of three muscles from French cull cows and young bulls

The quality of grilled steaks was assessed by experienced panellists in longissimus thoracis (LT), semitendinosus (St) and triceps brachii (TB) muscles of Aubrac, Charolais, Limousin and Salers breeds raised in two production systems: 15-, 19- and 24-month-old bulls and 4-, 6- and 8-year-old cull cows. Scores for sensory ‘initial tenderness’, ‘overall tenderness’, ‘juiciness’, ‘residue after mastication’ and ‘flavour intensity’ for all 497 meats were pooled to derive three eating quality classes. Meats from the bulls and cows and from the four breeds were evenly distributed among the three eating quality classes. The highest quality class, representing one third of all the meats, contained 45% of the LT, 35% of the TB and 21% of the St muscles and one third of the meats from the 8-year-old cull cows. The meats in this class tended to have finer fibres, a greater proportion of slow oxidative fibres, slower post-mortem glycolysis, lower connective tissue and higher fat contents than those in the lower classes. Lipid content accounted for proportionately 0·56 of the variation in flavour intensity and pH at 3 h post mortem, 0·52 of the variation in tenderness due to muscle and slaughter age. Considering both young bulls and cull cows together, tenderness was highest in the meats from 15-month-old bulls and low in the meats from the intermediate age groups, and flavour and juiciness was highest in the meats from the oldest animals from each production system.

Classification of bovine muscle fibres by different histochemical techniques

The classification of bovine muscle fibres is of particular interest for the food industry because meat tenderness depends in part on the proportion of the different types of fibres. It is, therefore, important to define reliable methods for classifying fibre types. There are several classification systems. One is based on contractile type alone, as revealed by myofibrillar ATPase activity or with antibodies against myosin heavy chain. Others take both contractile and metabolic types into account. In this study, the classifications of fibres obtained by these three systems were compared on the semitendinosus and longissimus thoracis muscles of 35 Charolais bulls. Only the use of antibodies allowed the identification of a proportion of hybrid fibres containing two isoforms of fast myosin heavy chain (2a and 2b). In addition, the combination of metabolic types showed that the metabolism of these hybrid fibres differed according to the muscle.

Functional analysis of beef tenderness.

Meat tenderness represents a complex assembly of different cellular pathways. As a consequence, genomics studies have revealed many different proteins considered as tenderness markers. So it is difficult to have an overview of tenderness in terms of cellular pathways. In this work cellular pathways of tenderness were analyzed, an interactome of 330 proteins was designed, and explanations of tenderization processes were proposed.

Post-natal changes in the biological characteristics of Semitendinosus muscle in male Limousin cattle

Samples of semitendinosus muscle from 45 male Limousin cattle reared under similar conditions were taken at 1, 6 and 12 months of age (biopsy) and at 16 months (at slaughter). The development of biochemical (DNA, protein/DNA), enzymic (lactic dehydrogenase, isocitric dehydrogenase), and histochemical (ATPase, succunic dehydrogenase) characteristics was studied. Muscle fibres were classified according to their contractile and metabolic properties (SO = slow oxidative; FOG = fast oxidative glycolytic; FG = fast glycolytic). DNA accumulation or hyperplasia was observed up to the age of 16 months. The period of growth and development, between 1 and 12 months of age, was characterized by increased glycolytic metabolism, as evidenced by the conversion of FOG fibres into FG fibres, a decrease in ICDH activity and a rise in LDH activity. The period between 12 and 16 months, was characterized by a slowing down of the conversion of FOG fibres into FG fibres and an increase in ICDH activity. The presence of type IIC fibres at the four ages studied is discussed. Of the muscle characteristics studied, LDH activity seemed to be a discriminating factor between animals from an early age. It could thus be used as to predict animal muscle characteristics at birth.

In vivo proteome dynamics during early bovine myogenesis

Myogenesis is a complex process of which the underlying mechanisms are conserved between species, including birds and mammals. Despite a good understanding of the stages of myogenesis, many of the mechanisms involved in the regulation of proliferation of the successive myoblast generations, the cellular transitions cell proliferation/alignment of myoblasts/fusion of myoblasts into myotubes/differentiation of myofibres and the control of total myofibre number still remain unknown. An in vivo proteomic analysis of the semitendinosus muscle from Charolais foetuses, at three specific stages of myogenesis (60, 110 and 180 days postconception), was conducted using 2‐DE and MS. Expression profiles of more than 170 proteins were revealed and analysed using two way hierarchical clustering and statistical analysis. Our studies identify, for the first time, distinct proteins of varied biological functions and protein clusters with myogenic processes, such as the control of cell cycle activity and apoptosis, the establishment of cellular metabolism and muscle contractile properties and muscle cell reorganisation. These results are of fundamental interest to the field of myogenesis in general, and more specifically to the control of muscle development in meat producing animals.

Effect of the type of diet on muscle characteristics and meat palatability of growing Salers bulls

The effect of the type of diet (hay vs grass silage) on body composition and characteristics and palatability of semitendinosus (ST) and longissimus thoracis (LT) muscles of 16 month old Salers bulls fed at the same energy levels were studied. Animals fed hay had a lower daily weight gain and carcass weight and were leaner. There were no significant differences in the proportions of fibre types in the ST or LT due to diet. ST muscle of hay fed animals had a lower oxidative metabolism, but contained similar amounts of total and type I collagen and greater amounts, and proportions of soluble collagen and of type III collagen, than those of animals fed grass silage. ST muscles of hay-fed animals were more tender than those of silage-fed animals.

Variations in the abundance of 24 protein biomarkers of beef tenderness according to muscle and animal type.

Some proteins have been revealed as biomarkers for beef tenderness by previous studies. These markers could be used in immunological tests to predict beef tenderness, in living animals as well as in carcasses. It is well known that rearing practices modify the amounts of mRNA and proteins. Therefore, the reliability of protein tests could be affected by livestock and biological effects such as production systems, breed, muscle and animal type. This study analysed the effects of animal and muscle type on 24 proteins. The animals studied were 67 young bulls and 44 steers of the Charolais breed, and muscles were Longissimus thoracis and Semitendinosus. Protein amounts were determined by Dot blot, an immunological technique. Results showed that expressions of 20 proteins were influenced by animal and/or muscle type. These results could lead to modifications and adaptations of prediction tests according to rearing practice, bovine breed and beef cut.

Biochemical and transcriptomic analyses of two bovine skeletal muscles in Charolais bulls divergently selected for muscle growth.

This work aimed to investigate the consequences of muscle growth selection on muscle characteristics. An oxidative muscle (Rectus abdominis, RA) and a glycolytic one (Semitendinosus, ST) were studied in two groups of six extreme young Charolais bulls of high or low muscle growth. Mitochondrial activity was lower in muscles of bulls with high muscle growth. Transcriptomic studies allowed the identification of putatively differentially expressed genes. The differential expression between genetic types of two genes in RA (a heat shock protein and a thyroid receptor interacting protein) and of seven genes in ST (including LEU5, tropomyosin 2, and sarcosin) was confirmed by different statistical approaches or Northern blot analysis, as well as the differential expression of five genes (including PSMD4 and DPM synthase) between RA and ST. Both biochemical and transcriptomic results indicate that selection on muscle growth potential is associated with reduced slow-oxidative muscle characteristics. Further studies are required to understand the physiological importance of genes whose expression is changed by selection.

Muscle and meat quality characteristics of Holstein and Salers cull cows

Muscle characteristics and sensory rating of meat were determined in M. longissimus thoracis (LT), M. semimembranosus (SM), M. semitendinosus (ST) and M. triceps brachii (TB) from seven Holstein (HO, dairy breed) and six Salers (SA, beef breed) cull cows slaughtered at 6-7 years of age at the same fat score. Significant differences (P<0.001) among muscle types were observed: ST was the more glycolytic and TB the more oxidative; total collagen: ST>SM=TB>LT; initial and overall tenderness: LT>TB=SM>ST, juiciness: TB>LT=SM>ST. Flavour differed only between breeds: HO>SA (P<0.01). Three tenderness classes (high, intermediate, low) were determined from scores for sensory overall tenderness for all 52 meats: the lower total and insoluble collagen contents, the more oxidative metabolism, the more tender was the meat. Muscle type, and not breed explained most of the variability of meat quality from dairy and beef cull cows slaughtered at the same age and fat score.