p38alpha (MAPK14), c-Jun, mTOR, CARM1, PI3K cat class IA, MEK6(MAP2K6), Calcineurin B (regulatory), MAP3K3, PI3K reg class IA, NCOA2 (GRIP1/TIF2), AKT(PKB), Ca('2+) cytosol, HDAC9, ERK5 (MAPK7), <extracellular region> Ca('2+) = <cytosol> Ca('2+), MEF2A, CaMKK, Ca('2+) extracellular region, AMP deaminase 1, MYF6, p300, 14-3-3, MEF2D, HDAC4, 188.8.131.52, IGF-2, IGF-1, CPT-1B, PGAM2, MYOG, p70 S6 kinase1, PCAF, Tuberin, NF-AT1(NFATC2), MEF2C, p38beta (MAPK11), HDAC5, CaMK IV, MAP3K2 (MEKK2), Calcineurin A (catalytic), Calmodulin, HDAC7, L-type Ca(II) channel, alpha 1C subunit, PtdIns(3,4,5)P3, PtdIns(4,5)P2, MEF2, MAP2K5 (MEK5), IRS-1, RHEB2, MEF2B, MYOD, p21, IGF-1 receptor, PDK (PDPK1)
Role of HDAC and calcium/calmodulin-dependent kinase (CaMK) in control of skeletal myogenesis
There are two families of transcription factors that play pivotal roles during mammalian skeletal muscle differentiation. One of them includes MyoD family proteins (also called myogenic regulatory factors or MRFs), with four members Myf5, Myogenic differentiation 1 ( MYOD ), Myogenin ( MYOG ), and Myogenic factor 6 ( MYF6 ) that are exclusively expressed in skeletal muscles. The other group consists of Myocyte enhancer factors 2 ( MEF2 ): MEF2A, MEF2B, MEF2C, and MEF2D. The latter can form homo- and heterodimers that constitutively bind to the promoters or enhancers of the majority of the muscle-specific genes. Additionally, MRF and MEF2 members can physically interact with each other to synergistically activate many muscle-specific genes , .
The MEF2 activity is subjected to the complex regulation. MEF2 associate with a variety of regulating proteins: K(lysine) acetyltransferase 2B ( PCAF ), Binding protein p300 ( p300 ), Nuclear factor of activated T-cells, cytoplasmic, calcineurin-dependent 2 ( NF-AT1(NFATC2) ), Nuclear receptor coactivator 2 ( NCOA2 (GRIP1/TIF2) ), MYOD, 14-3-3, Mitogen-activated protein kinase 7 ( ERK5 (MAPK7) ), Histone deacetylases 4, 5 7 and 9 ( HDAC4, HDAC5, HDAC7, HDAC9 ). It is regulated by the MAP kinase cascades and calcium signaling.
Association of MEF2 with HDAC4, HDAC5, HDAC7 and HDAC9 results in deacetylation of nucleosomal histones surrounding MEF2 DNA-binding sites leading to subsequent suppression of MEF2 -dependent genes. Calcium/calmodulin-dependent protein kinase IV ( CaMK IV ) phosphorylates HDACs and creates docking sites for the chaperone protein 14-3-3. Upon binding of 14-3-3, HDACs are released from MEF2 and transported (except HDAC9 ) to the cytoplasm via a C-terminal nuclear export sequence. Once released from associated repressors, MEF2 binds the p300 co-activator .
The calcium-bound Calmodulin 2 ( Calmodulin ) also binds to Protein phosphatase 3 (formerly 2B), catalytic subunits ( Calcineurin A (catalytic) ) and activates it. Calcineurin A (catalytic) dephosphorylates the NFAT family of transcription factors, leading to their translocation to the nucleus. In the nucleus, the NF-AT1(NFATC2) directly associates with MEF2A and MEF2D and recruits the p300 co-activator to MEF2 target genes. .
p300 and PCAF are histone acetyltransferases (HATs). They acetylate histone tails. This leads to relaxation of the chromatin surrounding MEF2 target sites and subsequent stimulation of MEF2 target genes .
MAPK s couple MEF2 to multiple signaling pathways involved in the cell growth and differentiation. It was shown that Mitogen activated protein kinases 14 and 11 ( p38alpha (MAPK14), p38beta (MAPK11) ) phosphorylate and activate MEF2A and MEF2C, whereas ERK5(MAPK7) can phosphorylate and activate MEF2A, MEF2C and MEF2D , .
ERK5(MAPK7) can also function as a transcriptional co-activator by recruiting basal transcriptional machinery . ERK5(MAPK7) in turn is phosphorylated and activated by Mitogen-activated protein kinase kinase kinase2 and 3 ( MAP3K2 (MEKK2) and MAP2K3 ) .
Two members of the MRFs family are shown to be target genes of MEF2. These targer proteins are MYF6, MYOG, and muscle-specific enzymes Carnitine palmitoyltransferase 1B ( CPT-1B ) and, possibly, Adenosine monophosphate deaminase 1 ( AMP deaminase 1 ) and Phosphoglycerate mutase 2 ( PGAM2 ) , , , , .
Additionally, in response to p38alpha (MAPK14), p38beta (MAPK11) and ERK5(MAPK7) MEF2 activates the transcription factor Jun oncogene ( c-Jun ) involved in the control of duration of the myoblast proliferation , .
MYOD, a- coactivator of MEF2, besides activating muscle-specific transcription, induces permanent cell cycle arrest by up-regulating Cyclin-dependent kinase inhibitor 1A ( p21 ) .
Insulin-like growth factors ( IGF s) were shown to stimulate myogenesis in addition to extracellular stimuli that lead to activation of MEF2 via p38alpha (MAPK14), p38beta (MAPK11) and ERK5(MAPK7) kinase. It has been demonstrated that Phosphoinositide-3-kinase ( PI3K ) mediates the stimulatory effect of IGFs in muscle differentiation. PI3K converts phosphatidylinositol 4,5-bisphosphate ( PtdIns(4,5)P2 ) to phosphatidylinositol 3,4,5-trisphosphate ( PtdIns(3,4,5)P3 ), therefore leading to activation of the 3-phosphoinositide dependent protein kinase-1 ( PDK (PDPK1) )and v-akt murine thymoma viral oncogene homolog ( AKT(PKB) ). AKT(PKB) activates transcription MYOG via Ribosomal protein S6 kinase, 70kDa, polypeptide 1 ( p70 S6 kinase1 ), but exact mechanism of this regulation is not known .