Myogenesis
Myogenesis is the formation of
Muscle fibers generally form through the fusion of precursor myoblasts into multinucleated fibers called myotubes. In the early development of an embryo, myoblasts can either proliferate, or differentiate into a myotube. What controls this choice in vivo is generally unclear. If placed in cell culture, most myoblasts will proliferate if enough fibroblast growth factor (FGF) or another growth factor is present in the medium surrounding the cells. When the growth factor runs out, the myoblasts cease division and undergo terminal differentiation into myotubes. 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.[1]
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.
Novel genes and their protein products that are expressed during the process are under active investigation in many laboratories. They include:
- Myocyte enhancer factors (MEFs), which promote myogenesis.
- alpha-actin is also regulated by the androgen receptor; steroids can thereby regulate myogenesis.[3]
- Myogenic regulatory factors (MRFs): MyoD, Myf5, Myf6 and Myogenin.
Overview
There are a number of stages (listed below) of muscle development, or myogenesis.[4] Each stage has various associated genetic factors lack of which will result in muscular defects.
Stages
Stage | Associated Genetic Factors |
---|---|
Delamination | c-Met
|
Migration | c-met/HGF, LBX1 |
Proliferation | PAX3, c-Met, Mox2, Myf5, MyoD
|
Determination | Myf5 and MyoD |
Differentiation | Myf6
|
Specific Muscle Formation | Lbx1, Meox2 |
Satellite Cells | PAX7 |
Delamination
Associated Genetic Factors:
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
Migration
Associated Genetic Factors:
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.[4] 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.[4]
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.[4] 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.[4]
Proliferation
Associated Genetic Factors:
Mox2 (also referred to as MEOX-2) plays an important role in the induction of
Myf5 is required for proper myoblast proliferation.[4] Studies have shown that mice muscle development in the intercostal and paraspinal regions can be delayed by inactivating Myf-5.[4] 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.[4] These consequences further reveal the complexity of myogenesis and the importance of each genetic factor in proper muscle development.
Determination
Associated Genetic Factors:
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.[4]
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.
Differentiation
Associated genetic factors:
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.[4] 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.[4]
Myf-6 (also known as
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.[4] 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.[4]
Satellite cells
Associated genetic factors: PAX7
Mutations in Pax7 will prevent the formation of satellite cells and, in turn, prevent postnatal muscle growth.[4]
Skeletal muscle
During
There are a number of events that occur in order to propel the specification of muscle cells in the somite. For both the lateral and medial regions of the somite,
Regulation of myogenic differentiation is controlled by two pathways: the
Muscle fusion
Primary muscle fibers originate from primary myoblasts and tend to develop into slow muscle fibers.[4] Secondary muscle fibers then form around the primary fibers near the time of innervation. These muscle fibers form from secondary myoblasts and usually develop as fast muscle fibers. Finally, the muscle fibers that form later arise from satellite cells.[4]
Two genes significant in muscle fusion are
The
Protein synthesis and actin heterogeneity
There are 3 types of proteins produced during myogenesis.[5] Class A proteins are the most abundant and are synthesized continuously throughout myogenesis. Class B proteins are proteins that are initiated during myogenesis and continued throughout development. Class C proteins are those synthesized at specific times during development. Also 3 different forms of actin were identified during myogenesis.
Sim2, a BHLH-Pas transcription factor, inhibits transcription by active repression and displays enhanced expression in ventral limb muscle masses during chick and mouse embryonic development. It accomplishes this by repressing MyoD transcription by binding to the enhancer region, and prevents premature myogenesis.[14]
Techniques
The significance of alternative splicing was elucidated using microarrary analysis of differentiating C2C12 myoblasts.[16] 95 alternative splicing events occur during C2C12 differentiation in myogenesis. Therefore, alternative splicing is necessary in myogenesis.
Systems approach
Systems approach is a method used to study myogenesis, which manipulates a number of different techniques like
This approach, using cell based high-throughput transfection assay and whole-mount in situ hybridization, was used in identifying the myogenetic regulator RP58, and the tendon differentiation gene, Mohawk homeobox.[8]
References
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