Myogenesis

Myoblast differentiation proceeds in stages. The first stage involves cell cycle exit and the commencement of expression of certain genes. The second stage of differentiation involves the alignment of the myoblasts with one another. Studies have shown that even rat and chick myoblasts can recognise and align with one another, suggesting evolutionary conservation of the mechanisms involved. The third stage is the actual cell fusion itself. In this stage, the presence of calcium ions is critical. Fusion in humans is aided by a set of metalloproteinases coded for by the ADAM12 gene, and a variety of other proteins. Fusion involves recruitment of actin to the plasma membrane, followed by close apposition and creation of a pore that subsequently rapidly widens.

Genes and their protein products that are expressed during the process include: myocyte enhancer factors, myogenic regulatory factors, and serum response factor. Expression of skeletal alpha-actin is also regulated by the androgen receptor; steroids can thereby regulate myogenesis.

Overview

There are a number of stages (listed below) of muscle development, or myogenesis. Each stage has various associated genetic factors lack of which will result in muscular defects.

Stages

Delamination

Associated Genetic Factors: PAX3 and c-Met
Mutations in PAX3 can cause a failure in c-Met expression. Such a mutation would result in a lack of lateral migration.

PAX3 mediates the transcription of c-Met and is responsible for the activation of MyoD expression—one of the functions of MyoD is to promote the regenerative ability of satellite cells (described below). PAX3 is generally expressed at its highest levels during embryonic development and is expressed at a lesser degree during the fetal stages; it is expressed in migrating hypaxial cells and dermomyotome cells, but is not expressed at all during the development of facial muscle. Mutations in Pax3 can cause a variety of complications including Waardenburg syndrome I and III as well as craniofacial-deafness-hand syndrome. Waardenburg syndrome is most often associated with congenital disorders involving the intestinal tract and spine, an elevation of the scapula, among other symptoms. Each stage has various associated genetic factors without which will result in muscular defects.

Migration

Associated Genetic Factors: c-Met/HGF and LBX1
Mutations in these genetic factors causes a lack of migration.

LBX1 is responsible for the development and organization of muscles in the dorsal forelimb as well as the movement of dorsal muscles into the limb following delamination. Without LBX1, limb muscles will fail to form properly; studies have shown that hindlimb muscles are severely affected by this deletion while only flexor muscles form in the forelimb muscles as a result of ventral muscle migration.

c-Met is a tyrosine kinase receptor that is required for the survival and proliferation of migrating myoblasts. A lack of c-Met disrupts secondary myogenesis and—as in LBX1—prevents the formation of limb musculature. It is clear that c-Met plays an important role in delamination and proliferation in addition to migration. PAX3 is needed for the transcription of c-Met.

Proliferation

Associated Genetic Factors: PAX3, c-Met, Mox2, MSX1, Six, Myf5, and MyoD

Mox2 (also referred to as MEOX-2) plays an important role in the induction of mesoderm and regional specification. Impairing the function of Mox2 will prevent the proliferation of myogenic precursors and will cause abnormal patterning of limb muscles. Specifically, studies have shown that hindlimbs are severely reduced in size while specific forelimb muscles will fail to form.

Myf5 is required for proper myoblast proliferation. Studies have shown that mice muscle development in the intercostal and paraspinal regions can be delayed by inactivating Myf-5. Myf5 is considered to be the earliest expressed regulatory factor gene in myogenesis. If Myf-5 and MyoD are both inactivated, there will be a complete absence of skeletal muscle. These consequences further reveal the complexity of myogenesis and the importance of each genetic factor in proper muscle development.

Determination

Associated Genetic Factors: Myf5 and MyoD
One of the most important stages in myogenesis determination requires both Myf5 and MyoD to function properly in order for myogenic cells to progress normally. Mutations in either associated genetic factor will cause the cells to adopt non-muscular phenotypes.

As stated earlier, the combination of Myf5 and MyoD is crucial to the success of myogenesis. Both MyoD and Myf5 are members of the myogenic bHLH (basic helix-loop-helix) proteins transcription factor family. Cells that make myogenic bHLH transcription factors (including MyoD or Myf5) are committed to development as a muscle cell. Consequently, the simultaneous deletion of Myf5 and MyoD also results in a complete lack of skeletal muscle formation. Research has shown that MyoD directly activates its own gene; this means that the protein made binds the myoD gene and continues a cycle of MyoD protein production. Meanwhile, Myf5 expression is regulated by Sonic hedgehog, Wnt1, and MyoD itself. By noting the role of MyoD in regulating Myf5, the crucial interconnectedness of the two genetic factors becomes clear.

Serum response factor is needed for myogenesis and muscle development. Interaction of SRF with other proteins, such as steroid hormone receptors, may contribute to regulation of muscle growth by steroids.

Differentiation

Associated genetic factors: Myogenin, Mcf2, Six, MyoD, and Myf6
Mutations in these associated genetic factors will prevent myocytes from advancing and maturing.

Myogenin (also known as Myf4) is required for the fusion of myogenic precursor cells to either new or previously existing fibers. In general, myogenin is associated with amplifying expression of genes that are already being expressed in the organism. Deleting myogenin results in nearly complete loss of differentiated muscle fibers and severe loss of skeletal muscle mass in the lateral/ventral body wall.

Myf-6 (also known as MRF4 or Herculin) is important to myotube differentiation and is specific to skeletal muscle. Mutations in Myf-6 can provoke disorders including centronuclear myopathy and Becker muscular dystrophy.

Specific muscle formation

Associated genetic factors: LBX1 and Mox2
In specific muscle formation, mutations in associated genetic factors begin to affect specific muscular regions. Because of its large responsibility in the movement of dorsal muscles into the limb following delamination, mutation or deletion of Lbx1 results in defects in extensor and hindlimb muscles. As stated in the Proliferation section, Mox2 deletion or mutation causes abnormal patterning of limb muscles. The consequences of this abnormal patterning include severe reduction in size of hindlimbs and complete absence of forelimb muscles.

Satellite cells

Associated genetic factors: PAX7
Mutations in Pax7 will prevent the formation of satellite cells and, in turn, prevent postnatal muscle growth.

Satellite cells are described as quiescent myoblasts and neighbor muscle fiber sarcolemma. They are crucial for the repair of muscle, but have a very limited ability to replicate. Activated by stimuli such as injury or high mechanical load, satellite cells are required for muscle regeneration in adult organisms. In addition, satellite cells have the capability to also differentiate into bone or fat. In this way, satellite cells have an important role in not only muscle development, but in the maintenance of muscle through adulthood.