The formation of skeletal muscle: from somite to limb

Age-related epigenetic changes and the immune system

The role of DNA methylation in immune function is discussed extensively in other papers in this issue. Many of these discussions assume that DNA methylation, a major mediator of epigenetic information, is fairly immutable and uniform in adult cells and tissues. There is, however, growing evidence that DNA methylation changes subtly with age. Normal aging cells and tissues show a progressive loss of 5-methylcytosine content, primarily within DNA repeated sequences, but also in potential gene regulatory areas. In parallel, selected genes show progressive age-related increases in promoter methylation, which, once a critical methylation density is reached, have the potential to permanently silence gene expression. These changes are highly mosaic within a given tissue and introduce a high degree of epigenetic variability in aging cells. Such epigenetic phenomena could impact immune response through masking/unmasking potential tissue antigens as well as by modulating the differentiation and response of immune effector cells. The contribution of epigenetic changes to the altered immune function observed in aging humans deserves careful investigation.

Megakaryoblastic leukemia-1/2, a transcriptional co-activator of serum response factor, is required for skeletal myogenic differentiation

Serum response factor (SRF) is required for the expression of a wide variety of muscle-specific genes that are expressed upon differentiation and is thus required for both striated and smooth muscle differentiation in addition to its role in regulating growth factor-inducible genes. A heart and smooth muscle-specific SRF co-activator, myocardin, has been shown to be required for cardiac development and smooth muscle differentiation. However, no such co-factors of SRF have been identified in the skeletal myogenic differentiation program. Myocardin and the related transcription factor megakaryoblastic leukemia-1 (MKL1/MAL/MRTF-A) can strongly potentiate the activity of SRF. Here we report the cloning of the third member of the myocardin/MKL family in humans, MKL2. MKL2 binds to and activates SRF similar to myocardin and MKL1. To determine the role of these factors in skeletal myogenic differentiation we used a dominant negative MKL2 to show that the MKL family of proteins is required for skeletal myogenic differentiation. Expression of the dominant negative protein in C2C12 skeletal myoblasts blocked the differentiation-induced expression of the SRF target genes skeletal alpha-actin and alpha-myosin heavy chain and blocked differentiation of the myoblasts to myotubes in vitro. C2C12 cells express both MKL1 and MKL2, but not myocardin, implicating MKL1 and/or MKL2 in the requirement for skeletal myogenic differentiation. MKL1 was predominantly cytoplasmic in C2C12 cells, with a small amount in the nucleus, however, no movement of MKL1 to the nucleus was observed upon differentiation.

Wnt signalling regulates myogenic differentiation in the developing avian wing

The limb musculature arises by delamination of premyogenic cells from the lateral dermomyotome. Initially the cells express Pax3 but, upon entering the limb bud, they switch on the expression of MyoD and Myf5 and undergo terminal differentiation into slow or fast fibres, which have distinct contractile properties that determine how a muscle will function. In the chick, the premyogenic cells express the Wnt antagonist Sfrp2, which is downregulated as the cells differentiate, suggesting that Wnts might regulate myogenic differentiation. Here, we have investigated the role of Wnt signalling during myogenic differentiation in the developing chick wing bud by gain- and loss-of-function studies in vitro and in vivo. We show that Wnt signalling changes the number of fast and/or slow fibres. For example, in vivo, Wnt11 decreases and increases the number of slow and fast fibres, respectively, whereas overexpression of Wnt5a or a dominant-negative Wnt11 protein have the opposite effect. The latter shows that endogenous Wnt11 signalling determines the number of fast and slow myocytes. The distinct effects of Wnt5a and Wnt11 are consistent with their different expression patterns, which correlate with the ultimate distribution of slow and fast fibres in the wing. Overexpression of activated calmodulin kinase II mimics the effect of Wnt5a, suggesting that it uses this pathway. Finally, we show that overexpression of the Wnt antagonist Sfrp2 and DeltaLef1 reduces the number of myocytes. In Sfrp2-infected limbs, the number of Pax3 expressing cells was increased, suggesting that Sfrp2 blocks myogenic differentiation. Therefore, Wnt signalling modulates both the number of terminally differentiated myogenic cells and the intricate slow/fast patterning of the limb musculature.

Looking back to the embryo: defining transcriptional networks in adult myogenesis

Skeletal muscle has an intrinsic capacity for regeneration following injury or exercise. The presence of adult stem cells in various tissues with myogenic potential provides new opportunities for cell-based therapies to treat muscle disease. Recent studies have shown a conserved transcriptional hierarchy that regulates the myogenic differentiation of both embryonic and adult stem cells. Importantly, the molecules and signalling pathways that induce myogenic determination in the embryo might be manipulated or mimicked to direct the differentiation of adult stem cells either in vivo or ex vivo.