Beta1 integrin expression during normal and low score normal avian myogenesis

Extracellular matrix proteoglycan decorin-mediated myogenic satellite cell responsiveness to transforming growth factor-beta1 during cell proliferation and differentiation Decorin and transforming growth factor-beta1 in satellite cells

Transforming growth factor-beta1 (TGF-beta1) is a potent inhibitor of muscle cell proliferation and differentiation. Decorin, a small proteoglycan in the extracellular matrix, binds to TGF-beta1 and modulates the activity of TGF-beta1 during muscle cell growth and development. However, its interaction with TGF-beta1 and involvement in myogenesis is not well characterized. In the present study, chicken myogenic satellite cells, myogenic precursors for muscle growth and repair, were isolated from the pectoralis major muscle and used to investigate the biological function of TGF-beta1 and decorin during myogenesis. The over-expression of decorin in satellite cells significantly increased cell proliferation, compared to the control cells. Consistent with this result, reducing decorin expression decreased cell proliferation, which suggests a decorin-mediated mechanism is involved in the regulation of myogenic satellite cell proliferation. Satellite cells over-expressing decorin were less sensitive to TGF-beta1 during proliferation, which indicates that decorin may sequester TGF-beta1 leading to increased proliferation. During satellite cell differentiation, the over-expression of decorin induced differentiation by increasing the muscle specific creatine kinase concentration. However, the addition of TGF-beta1 diminished decorin-mediated cell responsiveness to TGF-beta1 during differentiation. Taken together, these results suggest that decorin induces myogenic satellite cell proliferation and differentiation by regulating cellular responsiveness to TGF-beta1. An alternative TGF-beta1-independent pathway may be involved in the regulation of satellite cells by decorin.

Effect of transforming growth factor-beta on decorin and beta1 integrin expression during muscle development in chickens

Myoblast-extracellular matrix interactions play a pivotal role in skeletal muscle development. Transforming growth factor-beta (TGF-beta) is a key regulator of muscle cell proliferation and differentiation. The level of TGF-beta expressed will affect the concentration of the extracellular matrix proteoglycan decorin and the cell surface beta1 integrin subunit. The decorin proteoglycan is a regulator of cell growth as well as the organization of the extracellular matrix. The beta1 integrin plays a role in muscle cell attachment, migration, and the formation of multinucleated myotubes. In the current study, chicken myogenic satellite cells isolated from the pectoralis major muscle from the chicken genetic muscle weakness, low score normal (LSN), and normal pectoralis major muscle were used to investigate TGF-beta expression as it relates to decorin and beta1 integrin mRNA expression. The LSN muscle defect is characterized by altered myotube formation and sarcomere structure, and the satellite cells have reduced proliferation and differentiation. The mRNA expression was measured by real-time quantitative reverse transcription PCR. The LSN condition has elevated expression of TGF-beta2 and TGF-beta4 with increased expression of decorin and decreased beta1 integrin during myogenic satellite cell proliferation and differentiation. Normal satellite cell cultures were treated with the addition of exogenous TGF-beta during differentiation to determine if the altered expression of LSN decorin and beta1 integrin was associated with TGF-beta expression. The addition of exogenous TGF-beta decreased decorin expression during differentiation and reduced beta1 integrin expression at 24 and 48 h of differentiation. These results suggested that alteration of decorin expression in the LSN myogenic satellite cells may occur by a mechanism involving factors in addition to TGF-beta, but the addition of exogenous TGF-beta did affect both decorin and beta1 integrin expression. These data, therefore, suggested that TGF-beta might play a pivotal role in chicken skeletal muscle formation through modulation of the expression of both extracellular matrix molecules and cellular receptors important in the control of cell migration and growth regulation.

β1 integrin mediation of myogenic differentiation: implications for satellite cell differentiation

Myoblast-extracellular matrix interactions mediated by integrin receptors have been shown to play a pivotal role in skeletal muscle development. In the current study, the avian genetic muscle weakness, low score normal (LSN), which exhibits modified myotube and sarcomere structure and a reduction in beta1 integrin expression during satellite cell differentiation, was used as a model system to further investigate the role of beta1 integrins in myogenic differentiation. During normal satellite cell differentiation, the beta1 integrin was localized at areas of pseudopodial activity and points of cell contact. In contrast, in LSN satellite cell cultures the beta1 integrin was predominantly observed in clusters not associated with points of cell to cell contact. The LSN satellite cells underwent apoptosis at the same time normal satellite cells were fusing to form multinucleated myotubes. During muscle development, the beta1 integrin is regulated by alternative splicing from a beta1A to beta1D form. During both embryonic and posthatch pectoralis major muscle development, regulation of the beta1A and beta1D variants was altered in the LSN. Normal and LSN myogenic satellite cell cultures did not express the beta1D integrin, but the LSN satellite cells expressed higher concentrations of the beta1A integrin than did normal satellite cells. Taken together, these data further demonstrate the importance of the beta1 integrin subunit in the regulation of cell-extracellular matrix signal transduction pathways during muscle growth and differentiation.

Ventricular myosin heavy chain isoform expression is altered in vitro in low score normal chickens

The low score normal (LSN) chicken exhibits a genetic muscle weakness and altered in vitro myogenesis compared to the normal White Leghorn chicken. The ventricular myosin heavy chain isoform has been reported to be the initial muscle-specific contractile protein expressed during myogenesis. The goals of this study were to determine whether altered myogenesis of the LSN satellite cells in culture was accompanied by delayed ventricular myosin heavy chain expression and to further characterize the altered myogenic events exhibited by the LSN chicken. Immunocytochemical and ELISA analyses were employed to document the temporal expression of the ventricular myosin heavy chain during LSN chicken myogenesis. Satellite cells derived from the LSN chicken pectoralis major exhibited lower (P </= 0.05) expression of ventricular myosin heavy chain during proliferation and differentiation in culture than did satellite cells derived from White Leghorn chickens. Low score normal cells failing to express the ventricular myosin heavy chain generally remained mononucleated and unfused, whereas cells that were multinucleated appeared to express ventricular myosin heavy chain regardless of the avian source. These results suggest that the altered myogenesis observed in LSN chickens is associated with delayed ventricular myosin heavy chain expression.

The structure and functional significance of variations in the connective tissue within muscle

The amount of intramuscular connective tissue (IMCT) and its morphological distribution is highly variable between muscles of differing function. The functional roles of this component of muscle have been poorly understood, but a picture is gradually emerging of the central role this component has in growth, transmission of mechanical signals to muscle cells and co-ordination of forces between fibres within a muscle. The aim of this review is to highlight recent advances that begin to show the functional significance of some of the variability in IMCT. IMCT has a number of clearly defined roles. It patterns muscle development and innervation, and mechanically integrates the tissue. In developing muscles, proliferation and growth of muscle cells is stimulated and guided by cell-matrix interactions. Recent work has shown that the topography of collagen fibres is an important signal. The timing and rates of expression of connective tissue proteins also show differences between muscles. Discussion of mechanical roles for IMCT has traditionally been limited to the passive elastic response of muscle. However, it is now clear that IMCT provides a matrix to integrate the contractile function of the whole tissue. Mechanical forces are co-ordinated and passed between adjacent muscle cells via cell-matrix interactions and the endomysial connective tissue that links the cells together. An emerging concept is that division of a muscle into fascicles by the perimysial connective tissue is related to the need to accommodate shear strains as muscles change shape during contraction and extension.

Mode of inheritance of the low score normal condition in chickens

The abnormal muscle condition termed low score normal (LSN) was first detected in an outcross of chickens with hereditary muscular dystrophy (MD) to a commercial White Leghorn stock. At 2 to 3 mo of age, normal birds can right themselves between 15 and 20 times (exhaustion score) when placed on their back on a flat surface, whereas birds with MD cannot right themselves under similar conditions. Birds classified as LSN are intermediate to these extremes. The inheritance of the LSN abnormality has not been established. In order to determine if the LSN condition was controlled by a single gene, the LSN line was reciprocally crossed with a White Leghorn line to produce two F1 populations. Two F2 populations were produced by randomly mating individuals within each F1 population. Each F1 population was backcrossed to the White Leghorn line. When birds with an exhaustion score of 6 or less were considered LSN, ratios obtained in the F1 population were 3 normal and 134 LSN individuals for the cross of the LSN line males and White Leghorn line females, and 8 normal and 118 LSN for the reciprocal cross, suggesting that the LSN condition was influenced by a dominant gene with incomplete penetrance. The frequencies did not differ between sexes in either F1 cross, suggesting autosomal inheritance. In general, ratios of normal to LSN individuals in the two F2 populations and in the two backcross populations supported the hypothesis that the LSN trait was controlled by a dominant autosomal gene. However, there was an excess of normal females relative to that expected in the cross of the White Leghorn line males and LSN line females. Heritability (h2) of the LSN trait was estimated by regression of F2 offspring on F1 parents. The h2 estimates based on regressions were higher (range = 0.520 to 1.107) in the LSN line male x White Leghorn line female cross than in the reciprocal cross (range = 0.161 to 0.621). The h2 estimates based on regression of offspring on dams were higher for male offspring than for female offspring, suggesting the presence of sex-linked effects. It was concluded that the LSN trait was influenced primarily by an autosomal dominant gene but was also influenced by other genes, some of which were on the sex chromosome.