Myogenesis (2024)

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MYOGENESIS & MUSCLE REGENERATION

General mechanisms
Early myogenesis:
Intrinsic programs
Secreted factors
Other molecules
Muscle development

Myogenesis
Stem cells
Myogenic progenitors
Satellite cells
Side population cells
Myoblasts
Myotubes
Regeneration
Membrane repair
Muscle growth

bHLH transcription factors
Bone morphogenetic proteins (Bmp)
Calcineurin
c-met
Desmin
Fibroblast growth factors
Follistatin
GRB2
Laminins
Lbx1
M-cadherin
MEF2 family
Midkine
Mnf (FOXK1)
Mox2
Msx1

Myf5
Myf-6
MyoD
Myogenic determination factors
Myogenin (Myf4)
Myomaker (TMEM8C)
Myomixer
Myostatin
Noggin
Pax3
Pax3 associated (Eya; SIX)
Pax7
Sonic hedgehog (SHH)
TBX1
TGFβ
VCAM1

From: A Kornberg

Stages in Muscle Development4

Myogenesis

Stage	Associated genetic factors	Mutant effects
Delamination	Pax3; c-met	Pax3 mutant: No c-met expression; No lateral migration
Migration	c-met/HGF; Lbx1	Migration does not occur
Proliferation	? Pax3; c-met; Mox2; Msx1; Six; (Myf5; MyoD)	No proliferation
Determination	Myf5; MyoD	Myf5 + MyoD muations: Myogenic cell adopts non-muscle phenotype
Differentiation	Myogenin; Mcf2; Six; (MyoD; Myf6)	Mutants remain as myocytes
Specific muscle formation	Lbx1; Mox2	Lbx1 mutants: Extensor & Hindlimb muscle Mox-2 mutants: Forelimb & Hindlimb muscle
Satellite cells	Pax7	Satellite cells absent; No postnatal muscle growth

Mesodermal cells
Dorsal

Ventral
Sclerotome
Mesenchymal cells
Location: Ventral somite
Derivatives: Ribs & Vertebrae
Dermomyotome
Epithelial structure
Forms from somites
Location: Dorsal somite
Derivatives: Skin & Skeletal muscles
Epaxial: Back muscles
Hypaxial: Trunk & Limb muscles
Myotome
Origin: Dermomyotome
Derivative: Trunk muscle
DRG: Dorsal root ganglion
See Also
Frontiers | Salidroside Inhibits Myogenesis by Modulating p-Smad3-Induced Myf5 Transcription
- Location: Paraxial
- Cells not committed to specific muscle lineage
Myogenic progenitors
- Definition of cell type
  - Normal developmental fate is muscle
  - Not terminally differentiated
- Location: Cephalic mesenchyme & Prechordal plate
  - Precursors for: Head muscles
- Location: Somites
  - Epaxial lineage
    - Origin: Dorsomedial somites
    - Precursor for: Back (Dorsal) muscles
  - Hypaxial lineage
    - Origin: Dorsolateral somites
    - Precursor for
      - Body wall ventral muscles
        Via extension of myotome
      - Appendicular & Tongue muscles
        Via migration of precursor cells
    - Formation regulated by
      - Lateral-plate mesoderm
      - Dorsal ectoderm
        Provides signals that stimulate myogenesis & activate expression of MyoD
        Signals from dorsal ectoderm can be mimicked by Wnt-4, Wnt-6 & Wnt-7a
        Wnt-7a activates expression of MyoD: Leads to co-expression of MyoD and Myf5
        Soluble Frizzled-related proteins (sFRPs) block Wnt signalling: Inhibit myogenesis
- Migration
  - Components: Delamination & Movement into limb
  - Positional cues for progenitor cells: Mesenchymal cells of limb
  - Dependent on: c-met
- Genes expressed during early myogenesis in migrating precursor cells
  - Pax3
  - Mox-2
  - Lady Bird hom*olog (LBX1):
  - Basic helix-loop-helix transcription factors
    - Molecules
      - Myogenin (Myf-4)
      - Myf-5
      - Myf-6
      - MyoD
    - Expression: When progenitor cells reach limb
  - Mnf (FOXK1)
  - Msx1
- Secreted factors regulating embryonic muscle development
  - Sonic hedgehog (SHH)
  - Bone morphogenetic protein (Bmp) family
  - Noggin
  - Follistatin

Myoblasts
- Developmental features
  - Proliferating cells
  - Fully determined to myogenic phenotype
  - Capable of terminal differentiation
  - Fusion to become multinucleated cells: Steps8
    - Adhesion: Cell adhesion molecules (CAMs); Kirrel3, Nephrin,JAM2,JAM3
    - Membrane apposition & invasion: Actin propelled; Dock1, Dock5, Rac1, WASL, PLEKHO1
    - Destabilization of lipid bilayers: Makes cells prone to fusion; Forms fusion pore; ADGRB1, ADGRB3
    - Other: TMEM8C (Myomaker)
- Subtypes
  - Embryonic
    - Derived from somites
    - Oligoclonal founders form muscle fibers
  - Fetal: May have endothelial origin
  - Adult (Satellite cells)
- Molecular features: Usually express myogenic determination factor genes
  - MyoD
  - Myf-5
  - Myogenin (Myf-4)
  - Mrf4 (Myf-6, Herculin)
- Other molecules in late myogenesis (after specification)
  - Fibroblast growth factors
  - Myostatin
  - Growth factor receptor-bound protein 2 (GRB2) molecules
    - Growth factor receptor-bound protein 2 (GRB2)
    - Hepatocyte growth factor (HGF)
    - c-met: HGF receptor
  - Calcineurin pathway
    - Calcineurin
    - Nuclear factor of activated T-cells calcium responsive isoform 3 (NFATC3)
  - MEF2 family
  - TGFβ
  - Laminins

Myotubes
- Terminally differentiated muscle cell
- Multinuclear
- Two waves of formation
  - Primary myotubes: Earlier migration; Provide scaffold
  - Secondary myotubes: Later migration
- Grow & elongate by cell fusion
Muscle fibers
- Mature myotubes
  - Exhibit cross-striated myofibrils
  - Surrounded by basal lamina

Head & Neck musculature6

Features distinct from somite-derived muscles
- Muscle derivation: Branchiomeric & other
- Innervated by Phox2b-dependent visceral motor neurons
- May express myosin isoforms not found in other skeletal muscles (Masseter)
Branchiomeric muscle derivation: Muscles derived from progenitor cells in branchial arches
- General
- Mastication: Jaw opening & closing
  - Origination: 1st branchial (Mandibular arch)
  - Muscles: Temporalis; Masseter; Digastric
  - Absent masticatory muscles
    - Mouse model: Knockout of both Tcf21 & Msc
- Facial expression
  - Origination: 2nd branchial (Hyoid arch)
  - Muscles: Orbicularis oris & oculi; Frontalis; Mentalis
- Pharyngeal function
  - Origin: 3rd branchial
Non-branchiomeric muscle derivation
- Extra-ocular muscles: Derived from anterior-most paraxial & pre-chordal mesoderm
- Tongue muscles: Hypoglossal cord; Anterior somites
- Ventral neck muscles: Somites
Molecular associations
- Upstream regulators of branchiomeric myogenesis
  - TBX1
    - T-box containing transcription factor
    - Required for: Myogenic regulatory factor (MRF) gene activation during branchiomeric myogenesis
    - Regulates
      - Onset of branchiomeric myogenesis
      - Transcriptional activation of myogenic determination genes (Myf5 & MyoD) in the mandibular arch
    - Deletion
      - Failure of caudal pharyngeal morphogenesis
      - Craniofacial & cardiovascular defects
      - Reduced expression in pharyngeal mesoderm
        Myogenic factors: Myf5 & MyoD
        Tlx1 & Fgf10
      - No effects on cell migration factors: Capsulin; MyoR
  - Cell migration factors
    - Tcf21 (Capsulin)
    - Msc (MyoR; Musculin)
  - Pitx2
    - Homeobox-containing transcription factor
    - Controls core mesoderm survival & Myf5 activation
    - Acts through Msc to control cell proliferation & survival in 1st arch
    - Expressed in somites after initial myogenic differentiation
  - Cardiac muscle cells have similar pattern of regulatory molecules
- Myf5
  - Earliest expressed myogenic regulatory factor gene
  - Activation regulated by separate cis-acting elements from trunk musculature
- Fibroblast growth factor 8 (FGF-8): Promotes branchiomeric versus eye muscle fate in 1st arch
- Negative signaling molecules
  - Bone morphogenetic proteins (BMP): BMP4 & BMP7
  - Wnt proteins
  - Arise from pharyngeal epithelium
- No expression or prominent role in developing head muscles
  - Pax3
  - Mrf4
Disease associations
- del22q11.2 Syndromes
  - DiGeorge Syndrome
  - Velo-Cardio-Facial Syndrome

Muscle Growth

Primary muscle fibers
- First fibers that form in muscle
- Form from primary myoblasts
- Tend to become slow muscle fibers
Secondary muscle fibers
- Form around primary fibers
- Generated near time of innervation
- Form from secondary myoblasts
- Tend to acquire features of fast muscle fibers
Later muscle fibers
- Progenitors: Satellite cells

Membrane Repair Pathways10

Ca⁺⁺-triggered fusion of nearby vesicles
- Provides source of membrane for repair
- Mediated in skeletal muscle by dysferlin
- Other related molecules: Calpains
Severing of transmembrane-cytoskeletal tethers: Via
- Locally activated calpain
- Rho GTPases to enable vesicular transit
Wound closure: Via acto-myosin ring
Lysosomal exocytosis
- Extrudes enzymes & factors
- Augments repair from extracellular face of injured membrane
Disease associations

Muscle Fiber Regeneration5

Pluripotent stem cell2
- Functions
  - Self-renewal: Capable of high proliferation
  - Multipotential: May differentiate into different lineages
    - Muscle, Neural, Bone, Fat, Hematopoietic or Cartilage
- Location: Beneath basal lamina of normal myofibers
- Molecular markers
  - Scamp1
  - Bcl-2
  - CD34: +/-
  - Negative for all myogenic regulatory factors (MRFs), MHC1 & Pax7
- Functional characteristics
  - Commonly remain quiescent
  - Bmp-2: Stimulates conversion to bone pathway
  - Transplantation: More effective at restoring dystrophin than satellite cells
  - Immunogenicity: Relatively low
  - May provide muscle fiber precursors during regeneration
Satellite cells: General11
- Tissue specific stem cell in mammalian limb skeletal muscle
- Location: Periphery of muscle fibers; Close to plasma membrane
- Developmental origin: Dermomyotome
- Molecular label: Pax7
- Limb muscles
  - Satellite cells quiescent
  - Activated by exercise or damage
- General actions
  - Muscle growth: Fuse to muscle fibers longitudinally &amp radially
  - Contribute new muscle nuclei
  - Craniofacial & Extraocular muscles
    - Satellite cells, less quiescent: Actively contribute to homeostasis
Satellite cell: Quiescent
- Definition
  - Quiescent myoblast
  - Location
    - Adjacent to muscle fiber sarcolemma
    - Beneath basal lamina
  - Embryonic origin unclear
    - Contain endothelial cell markers
    - May originate from somite or cells associated with embryonic vasculature
  - Adult origins
    - Other satellite cells
    - Bone marrow cells
    - SP cells
- Function: Repair & Regeneration of muscle
- Other features
  - Multipotent
    - May differentiate into: Muscle, Bone, Fibroblast or Fat
    - Aged satellite cells: More likely to differentiate outside myogenic lineage
  - Cell division may be asymmetric
    - May generate daughter cells for renewal or differentiation
    - Associated with Numb protein
  - Limited capacity for replication
  - SC in Aging muscle
    - SC numbers
      - Variably maintained or reduced
      - More loss: Females
      - Less loss: Exercise
    - SC properties
      - Formation of myogenic colonies: Reduced
      - Ability to activate & proliferate: Decreased
      - Capacity to differentiate & fuse into myotubes: Impaired
      - Apoptosis: More likely
      - Increased propensity to exit quiescent state
      - FGF2 signal: Increased
- Molecules expressed
  - Pax7
    - PAX7 protein
      - Subcellular location: Nuclear
      - Can bind as heterodimer to Pax3
      - Commits pluripotent stem cells to myogenic lineage
      - May function in specification of satellite cells upstream of MRFs
    - Animal model: Pax7 mutations
      - Satellite cells: Absent
      - Embryonic muscle development: Normal
      - Postnatal muscle growth: None
      - Pluripotent stem cells still present
      - Disrupt function of embryonic, but not adult, stem cells
    - PAX7 disorders
      - Rhabdomyosarcoma 2, alveolar: Somatic mutation
      - Myopathy: Recessive
  - Heparan sulfate proteoglycans
    - Syndecan-3
    - Syndecan-4
  - Mnf: Responsible for timing of expression of genes for
    - Myogenic determination
    - Cell cycle progression
  - Muscle segment homeobox 1 (Msx-1; HOX7)
    - Subcellular location: Nuclear
    - Homeobox transcription factor
    - Expression
      - Lateral domain of dermomyotome
      - Migrating forelimb muscle progenitor cells
    - Actions
      - Down-regulates expression of MyoD & other muscle regulatory factors
      - May interact with Pax3
      - Promotes muscle cell de-differentiation & proliferation
      - Ensure correct expansion of myogenic precursors before arrest & differentiation
    - Mutations: Hypodontia; Witkop syndrome
  - ± MyoD
    - Promotes regenerative ability
    - Upregulated in dystrophin deficient & inflammatory myopathies
    - Mice deficient in dystrophin & MyoD: Severe phenotype
  - CD34 (Hematopoietic progenitor cell antigen): Truncated form
  - V-CAM1
  - c-met (Hepatocyte growth factor receptor)
  - Pax3
    - Present in satellite cells in some proximal muscles
    - Absent from limb muscle satellite cells
  - NCAM (CD56)
  - No myogenic regulatory factors
- Sub-populations
  - EP: Proliferate well
  - LP: Slow division; Lack of fusion; 1% of satellite cells
Satellite cell: Activated (Proliferating myogenic precursor)
- Stimuli to activation: Muscle
  - Injury
  - Stretch
  - Mechanical load
- Molecular markers
  - M-cadherin
  - Pax-7: Present on quiescent & activated satellite cells
  - c-Met (Hepatocyte growth factor receptor)
  - CD34 (Hematopoietic progenitor cell antigen): Full length
  - Myogenic regulatory factors
    - Myf5
    - MyoD
    - Myogenin: Some cells
  - Desmin
- Development
  - Arise during late stages of embryogenesis
  - Highly active during postnatal growth of muscle tissue
  - Provide most of myonuclei to adult muscles
- Regeneration: Stages
  - Satellite cells
    - Necessary for adult muscle regeneration
    - Not necessary for recovery of size after disuse
  - Activation: Associated with MyoD
  - Proliferation
    - Related to Myf-5
    - Midkine: Expressed by small regenerating muscle fibers
  - Fusion
- Muscle usage7
  - Muscle fiber hypertrophy
    - Normally occurs with fusion of satellite cells with muscle fibers
    - May occur without satellite cells: Increase of
      - Myonuclear domain size
      - Myonuclear size
    - Increase in muscle fiber size from myostatin inhibition or Akt overexpression
      - May occur without satellite cellproliferation
  - Addition of new small muscle fibers
    - Requires satellite cells
    - Fibers have immature myosin patterns
- Differentiated product: Myoblast
Myokines
Side population (SP) cells
- Cells with heterogeneous hematopoetic & myogenic capacities
- CD45 positive subpopulation can develop myogenic specificity: May express Pax7
- Injured muscle: SP cells
  - Stimulated by Wnt (Wingless) pathway
  - May replenish satellite cell population during muscle regeneration
Terminal differentiation
- Molecular markers
  - Myogenin
  - Myf-6 (MRF-4)
Myogenesis: Satellite cell vs Embryonic
- Regeneration more dependent than embryogenesis on MyoD
- Pax7 required for ontogeny of satellite cells but not embryonic precursors
Regenerating muscle fibers (After necrosis)(Arrow): Pathology
- Small
- Large nuclei
- NADH: Dark staining
- May occur individually or in clusters
NADH stain

PAX 7 stain

Satellite cells in duch*enne MD

Muscle Regeneration: Pathways

General mechanisms underlying muscle development1

Intrinsic programs in muscle precursor cells

Genes expressed during early myogenesis in migrating precursor cells
- Pax3
  - Pax molecule family structure: Contain homeobox & paired box
  - Subcellular location: Nucleus
  - Pax3 Function & Actions
    - Myogenic specification
      - Stimulates proliferation of precursors in different lineages
      - Induces activation of MyoD expression
      - Interacts with Msx1
      - Required for: Delamination of myogenic precursors from hypaxial dermomyotome
      - Mechanism: Mediates transcription of c-met gene
    - Schwann cell development
  - Expression
    - Highest during embryonic stages; Reduced during fetal life
    - Expressed in myotome precursor (dermomyotome) cells
    - Occurs in all migrating hypaxial cells
    - Not present during facial muscle development
  - Pax3 positive cells do not express myogenic regulatory factors (Mrf)
  - Pax3 associated molecules: Homeobox genes
    - Regulate proliferation of muscle premasses in somites
    - Drosophila 'dachshund' gene
    - Eya 2
    - Eya 4
    - 'sine oculis' (SIX 1) : Regulates embryonic myogenesis
    - SIX 4
  - Pax3 mutations
    - Waardenburg syndrome I
    - Waardenburg syndrome III
    - Craniofacial-Deafness-Hand syndrome (CDHS)
    - Rhabdomyosarcoma-2
      - Chromosomal translocation T(2;13)(Q35;Q14) involving PAX-3 & FOXO1A
      - Constitutionally active Pax3
    - Mouse knockout
      - Absent limb muscle: No embryonic muscle formation
      - Preserved head muscles
    - Splotch mice: Pax3-deficient
      - Defective lateral migration & reduced proliferation in dermomyotome
      - Limb & diaphragm muscles are not formed
- Mox-2 (MEOX-2; Gax)12
  - Homeobox containing transcription factor
  - Location: Sensory neuron nuclei
  - Functions
    - Role in mesoderm induction & regional specification
    - Development of bones, muscles, vasculature & dermatomes
    - Nociceptor modulation (Noxious heat, Acute, Inflammatory)
    - Deletion
      - Myogenic precursor proliferation in limb impaired
      - Limb muscle patterning abnormal
        Forelimbs: Some muscles absent
        Hindlimb muscles: Reduction in size
- Basic helix-loop-helix (bHLH) transcription factors
  - General
    - Subcellular localization: Nuclear
    - Bind DNA: Requires dimerization with second bHLH protein
    - bHLH domain
      - Required for DNA binding & dimerization with E-protein family of transcription factors
      - MRFE protein heterodimers & MRF monomers
        Bind to consensus E-box sequence CANNTG: Found in promoters of many muscle-specific genes
    - C-terminal amphipathic α-helix has discrete functions
      - MyoD
        Specification domain
        Initiates transcription from normally silent genes
      - Myogenin
        General transactivation domain
        Amplifies level of expression of genes that are already being expressed (with open chromatin structure)
    - General actions: Induce fibroblasts to differentiate into myoblasts
    - Myf-5 or MyoD is needed for determination & proliferation of skeletal myoblasts
    - Upregulated in regenerating muscle fibers
    - Pax3 & Pax7 act upstream of bHLH factors
    - bHLH Proteins
      - Myogenic regulatory factors (MRFs)
        Upstream: MyoD; Myf-5
        Downstream: Myogenin; Myf-6
      - Other: Twist; ITF-2
  - Myogenin (Myf-4)
    - Associated with terminal differentiation & fusion of myogenic precursor cells to new or existing fibers
    - Mouse knockout
      - Skeletal muscle mass: Loss, especially latero-ventral body wall
      - Differentiated muscle fibers: Loss, severe
      - Undifferentiated myoblasts: Normal
    - Rhabdomyosarcomas: Overexpression
  - Myf-5
    - Expression
      - Positively regulated by: Sonic hedgehog; Wnt1; MyoD
    - Function: Myoblast proliferation
    - Inactivation in mice
      - Delayed development
        Myotome
        Paraspinal, deep back & intercostal muscles
      - Normal limbs
    - Disorder: CFEOM
    - Combined MyoD and Myf5 knockout: Complete absence of skeletal muscle
  - Myf-6 (MRF4; Herculin)
    - Muscle determination factor
    - Helix--loop--helix domain: Common feature of myogenic factors
    - Skeletal muscle specific
    - May be involved in
      - Differentiation of myotubes
        Terminal differentiation & fusion of myogenic precursor cells: To new or existing fibers
      - Control of muscle fiber size
        With MEF2 factors
        Loss of MRF4: Hypertrophy of muscle fibers
    - Mutation may
      - Produce mild Centronuclear myopathy (CNM3)
      - Exacerbate Becker muscular dystrophy
  - MyoD1 (MYF3)
    - Forms heterodimers with ITF-2 (bHLH protein)
    - Inhibited by Twist protein (bHLH protein)
    - Target gene
      - Transcription factor Slug (Snai2)
    - Function
      - Induction of cell-cycle arrest & differentiation
    - Inactivation in mice
      - Delayed muscle development in limbs
      - Increased numbers of satellite cells
      - Deficient regenerative processes
    - Overlap: Inactivation of both MyoD & Myf-5 causes complete absence of myoblasts
    - Disease: MYODRIF
- Lady Bird hom*olog (LBX1)
  - Homeobox
  - Subcellular localization: ? Nuclear
  - Function in myogenesis
    - Acquisition of muscles in dorsal forelimb
    - Associated with movement of dorsal muscle masses into limb after delamination
  - Deletion: Loss of limb muscles
    - Hindlimb muscles: Very severely affected
    - Forelimb muscles: Cells migrate to ventral muscle mass; Flexor, but not extensor, muscles form
  - Disorders
- Mnf (FOXK1)
  - Transcriptional activator
  - Subcellular location: Nuclear.
  - Binds to the upstream enhancer region (CCAC box) of myoglobin gene
  - Roles
    - Myogenic differentiation
    - Remodeling of adult muscles in response to physiological stimuli
  - Inactivation in mice: Reduced muscle mass & regeneration
Other molecules in myogenesis
- Myostatin (GDF-8)13
  - Member of TGFβ superfamily
  - Structure: hom*odimer
  - Action: Negative regulator of skeletal muscle growth
    - Inhibits satellite cell differentiation
    - Receptor: Activin type II receptor B
    - After myostatin binding to receptor: Complex binds to ALK-4/5
    - 2nd messenger: SMAD 2/3
  - Negative regulators
    - Myostatin pro-peptide: Bound to myostatin in serum
    - Growth and differentiation factor-associated serum protein 1 (GASP-1)
    - Follistatin
    - Follistatin-related gene (FLRG)
  - Human disease
    - Myostatin mutation: Muscle hypertrophy
    - High levels with muscle wasting
      - HIV infection
      - Cirrhosis
      - Chronic obstreuctive pulmonary disease (COPD)
    - Low levels in muscle: Muscular dystrophy
  - Animal disease
    - Inactivation
      - Increase in muscle mass due to hyperplasia & hypertrophy
      - Double-muscled cows
    - Myostain blockade by antibody3
      - Increased muscle mass in normals
      - Improved strength in dystrophic muscle
    - Myostatin null
      - Muscle hypertrophy
      - Spindle excess
  - External links
    - GeneReviews
    - Wikipedia
- Growth factor receptor-bound protein 2 (GRB2) pathway
  - Molecules
    - Growth factor receptor-bound protein 2 (GRB2)
      - Associates with activated tyrosine-phosphorylated EGF receptors & PDGF receptors
      - Regulates differentiation
    - Hepatocyte growth factor (HGF)
      - Mitogenic growth factor
      - Stages
        Required for emigration of myoblasts from the somites into limb field
      - Role in secondary myogenesis
    - Requires heparan sulfate to transduce an intracellular signal
  - c-met (HGF receptor; MET)
    - Tyrosine kinase receptor
    - Ligand: Hepatocyte growth factor
    - Cell associations
      - Non-somitic mesodermal cells
      - Satellite cell marker
    - Associated myogenesis stages: Delamination; Migration; Proliferation
    - Mouse mutations
      - Impaired secondary myogenesis
      - No limb musculature
      - Precursor cells delamination & migration from somite: Normal
    - Disease: Papillary renal carcinoma 1 (RCCP1)
- Functions
  - Sustain survival & proliferation of myoblasts during migration
  - Role in proliferation of myoblasts from which 2° myotubes are formed
Calcineurin pathway
- Calcineurin
  - Found in all cells
  - Protein properties: Calcium-dependent, calmodulin stimulated protein phosphatase
  - Confers Ca⁺⁺ sensitivity
  - Action: Stimulates muscle hypertrophy
    - Stimulated by Ca⁺⁺
    - Stimulation causes association with GATA-2 transcription factor: Regulates gene expression
    - Associated with nuclear translocation of transcription factor NFATC1
  - Dominant-negative calcineurin mutant: Represses myocyte differentiation & hypertrophy
- GATA-2: Increased expression in muscle
  - Activated by: IGF-1; Muscle regeneration
  - Effects: Muscle hypertrophy; Reduced age-related decrease in muscle mass
- Nuclear factor of activated T-cells calcium responsive isoform 3 (NFATC3)
  - Downstream effector of calcineurin
  - Inactivation: Reduced muscle mass due to loss of primary myotubes
  - Calcineurin + NFATC3: Regulate expression of Myf5 in satellite cells
MEF2 family9
- Molecules
  - Myocyte-specific enhancer factor 2A
  - Myocyte-specific enhancer factor 2B
  - Myocyte-specific enhancer factor 2C
  - Myocyte-specific enhancer factor 2D
- General functions
  - Induction & maintenance of muscle differentiation
  - Activation of muscle-specific gene expression: Developing cardiac, skeletal & smooth muscle
  - Functions with the peroxisome proliferator-activated receptor-γ co-activator-1α (PGC-1α)
    - Directs slow-muscle-specific gene expression in adult muscle
    - PGC-1α
      - Upregulated during exercise
      - Targeted by calcineurin signaling to promote the specialization of adult muscle fibers
  - Control of muscle fiber size in adults: With MRF4
    - MEF2 factors: Promote hypertrophy
    - MEF2C: Implicated in cardiac & smooth muscle hypertrophy
    - Denervation atrophy: Reduced MEF2 transcriptional activity
    - Muscle fiber regeneration impairment
      - Triple knock-out of MEF2A, MEF2C & MEF2D: Impaired terminal differentiation & myoblast fusion
    - MRF4
      - Negative regulator of muscle growth: Related to histone deacetylases (HDAC)
      - Represses MEF2 growth-promotion
- Expression levels
  - Present in many tissues
  - Upregulated: Development; Regeneration
  - Downregulated: After birth
- Additive, or autoregulatory, action with bHLH (MRF) proteins
  - Expression of Myogenin & Mrf4 (Myf6) is dependent on both sets of transcription factors
TGFβ
- Inactivates myogenic bHLH factors
- Promotes fibrosis
- duch*enne muscular dystrophy: TGFβ increased in muscle
Notch pathway
- Temporal control of skeletal-muscle differentiation
- Mechanisms
  - Regulation of MyoD expression
  - Negatively regulates muscle differentiation

Secreted factors regulating embryonic muscle development

General: Environmental control signals influencing precursor cells
- Location of signals: Ectoderm & Other
- Ablation of ectoderm causes
  - Premature differentiation of muscle precursors in abnormal location
  - Underdevelopment of muscle
Sonic hedgehog (SHH)
- Member of TGFβ superfamily
- Receptors: Patched (PTCH1);Smoothened (SMOH)
- Regulates muscle development
  - Stimulates expansion of muscle precursor population
  - Represses terminal differentiation of early myogenic precursor cells
  - Induces Pax3: Acts indirectly by inducing BMP proteins
  - Upregulates Myf-5: Binds to its receptor Patched (Ptc)
    - De-represses Smoothened (Smo)
    - Activates Gli family of transcription factors
    - Gli binds to specific sites in Myf5 promoter: Activates expression of Myf5 in epaxial somite
  - Functions
    - Function in hypaxial muscle: Spatially restricted to early myoblasts in ventral muscles of the posterior limb
    - Acts as permissive, not inductive, signal for slow MyHC expression in myoblasts
    - No function as a myoblast mitogen
- Mouse knockout
  - Epaxial musculature: Absent
  - Hypaxial musculature: Reduced
- Disorders
  - Mutations: Holoprosencephaly
  - ? Oncogene: Overexpression in Basal cell nevus syndrome
- Also see: Desert hedgehog
- External links
  - Hedgehog proteins
  - Hedgehog database
Fibroblast growth factors: Actions
- Regulate growth & differentiation of early embryonic primary myoblasts: FGF-1
- ? Differentiation: Influences secondary & adult myoblasts
- Require heparan sulfate to transduce an intracellular signal
Bone morphogenetic protein (Bmp) family

Noggin (NOG)
- Subcellular location: Secreted
- Binds
  - TGFβ signalling proteins: Inactivation
  - BMP (BMP2, BMP4, BMP5, BMP7) ligands
  - Growth Differentiation Factor (GDF5, GDF6) ligands
- Control
  - Induced by: Activin (INHBA)
  - Wnt signalling
  - BMP-Smad signaling: Suppression
- Myo/Nog cells
  - Express MyoD
  - Secrete Noggin
  - BMP signal regulators
  - Myofibroblast progenitors
  - Phagocytes
- Muscle
  - Restricts proliferative action of BMP
  - Promotes differentiation
  - Noggin- embryos
    - BMP hypersignaling
    - Disorganized myofibers: Fail to align
    - Multinucleated myofibers: Reduced numbers
    - Muscle progenitor cells: Reduced numbers; Phenotypic shift towards adipocytes
  - Increased in: Cachexia
- Mutations
  - Proximal Symphalangism 1A
  - Tarsal-carpal coalition syndrome
  - Multiple-synostoses syndrome 1
  - Stapes ankylosis with broad thumbs & toes
  - Brachydactyly, type B2
Follistatin (FST)
- Secreted glycoprotein
- Binds to
  - Activin (INHBA)
  - Heparan sulfate proteoglycans
  - BMP7: ? Aids binding of Bmps to receptors
    - Stimulates muscle cell proliferation
    - Prevents Bmp-induced apoptosis
  - Myostatin
- May modulate
  - Several members of TGFβ family
  - Myostatin: Inhibition in vivo
- Overexpression in skeletal muscle: Increased muscle weight
- Knockout: Muscle loss in diaphragm & intercostal muscles

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References
1. Neuromuscular Disorders 2002;12:438446
2. Gene Therapy 2002;9:642647; J Cell Biology 2002;157:851864
3. Nature 2002;420:418-421; BBRC 2003;300:965-971
4. J Anat 2003;202:5968, EMBO reports 2003;4:855-860
5. Nature Rev Genet 2003;4:495-505
6. Trends Genet 2007 May 22
7. Development 2011;138:3657-3666
8. Curr Opin Genet Dev 2015;32:162-170
9. Curr Opin Clin Nutr Metab Care 2018 Jan 30
10. Am J Physiol Cell Physiol 2020 Apr 29
11. FASEB J 2021;35(10):e21893
12. FEBS J 2022 Jan 13
13. Adv Clin Chem 2022;106:181-234,Cell Biochem Funct 2024;42:e4106

8/15/2024