Ck2-Dependent Phosphorylation Is Required to Maintain Pax7 Protein Levels in Proliferating Muscle Progenitors

Abstract

Skeletal muscle regeneration and long term maintenance is directly link to the balance between self-renewal and differentiation of resident adult stem cells known as satellite cells. In turn, satellite cell fate is influenced by a functional interaction between the transcription factor Pax7 and members of the MyoD family of muscle regulatory factors. Thus, changes in the Pax7-to-MyoD protein ratio may act as a molecular rheostat fine-tuning acquisition of lineage identity while preventing precocious terminal differentiation. Pax7 is expressed in quiescent and proliferating satellite cells, while its levels decrease sharply in differentiating progenitors Pax7 is maintained in cells (re)acquiring quiescence. While the mechanisms regulating Pax7 levels based on differentiation status are not well understood, we have recently described that Pax7 levels are directly regulated by the ubiquitin-ligase Nedd4, thus promoting proteasome-dependent Pax7 degradation in differentiating satellite cells. Here we show that Pax7 levels are maintained in proliferating muscle progenitors by a mechanism involving casein kinase 2-dependent Pax7 phosphorylation at S201. Point mutations preventing S201 phosphorylation or casein kinase 2 inhibition result in decreased Pax7 protein in proliferating muscle progenitors. Accordingly, this correlates directly with increased Pax7 ubiquitination. Finally, Pax7 down regulation induced by casein kinase 2 inhibition results in precocious myogenic induction, indicating early commitment to terminal differentiation. These observations highlight the critical role of post translational regulation of Pax7 as a molecular switch controlling muscle progenitor fate.

Casein kinase 2-mediated phosphorylation of Brahma-related gene 1 controls myoblast proliferation and contributes to SWI/SNF complex composition

“…Activation of the caspase 3 protease contributes to Pax7 degradation via targeted cleavage of Pax7 ( 64 , 65 ). This process is impacted by CK2, which phosphorylates Pax7 in the vicinity of the caspase 3 cleavage site and sterically inhibits caspase 3 activity ( 64 , 66 , 67 ). CK2 phosphorylation of Pax7 therefore promotes myoblast proliferation as well as satellite cell renewal.…”

Protein kinase CK2 subunits exert specific and coordinated functions in skeletal muscle differentiation and fusogenic activity

“…CK2 kinase activity has been described to target members of both the MRF and paired box (Pax)3 and 7 families via either direct phosphorylation or phosphorylation of their transcriptional regulators and protein partners (24)(25)(26)(27)(28)(29)(30)(31) and to be mainly involved in promoting muscle precursor cell proliferation (27,28,(30)(31)(32). However, the role of CK2 in the progression of the myogenic program and skeletal muscle formation is still to be investigated.…”

CK2-Dependent Phosphorylation of the Brg1 Chromatin Remodeling Enzyme Occurs during Mitosis

“…Work from our group and many others have demonstrated that CK2 is implicated in myoblast function [17,[45][46][47][48][49][50][51][52][59][60][61]. Specifically, we demonstrated that CK2 modulates the ability of Brg1 to promote myoblast proliferation by inducing Pax7 expression [17].…”

“…CK2-mediated regulation of myoblasts occurs at multiple levels. For instance, transcription factors from the myogenic regulatory factor (MRF) family and paired box (Pax) 3 and 7 are directly or indirectly regulated by CK2 activity [17,[45][46][47][48][49][50][51][52]. These proteins are necessary for the proliferation and/or differentiation of muscle-specific precursor cells [46,[53][54][55][56][57][58].…”