Satellite cells (also called muscle stem cells) are responsible for muscle regeneration. Adult muscle satellite cells stay in a quiescent state under normal situation. At the onset of muscle injury, however, satellite cells will exit from the quiescent state to start proliferating. This process is called satellite cell activation. The majority of satellite cells will undergo proliferation for a few rounds followed by differentiation and fusion to form new myofibers. These myofibers are basic cellular units of skeletal muscles.

Paxbp1 is a nuclear protein recently identified in our laboratory as a direct binding partner of Pax3 and Pax7, two key transcription factors that play important roles in muscle stem cells. In addition to its role in facilitating Pax7 to recruit the histone 3 lys...[ Read more ]

Satellite cells (also called muscle stem cells) are responsible for muscle regeneration. Adult muscle satellite cells stay in a quiescent state under normal situation. At the onset of muscle injury, however, satellite cells will exit from the quiescent state to start proliferating. This process is called satellite cell activation. The majority of satellite cells will undergo proliferation for a few rounds followed by differentiation and fusion to form new myofibers. These myofibers are basic cellular units of skeletal muscles.

Paxbp1 is a nuclear protein recently identified in our laboratory as a direct binding partner of Pax3 and Pax7, two key transcription factors that play important roles in muscle stem cells. In addition to its role in facilitating Pax7 to recruit the histone 3 lysine 4 (H3K4) methyltransferase (HMT) complex, we found that knockdown of Paxbp1 by siRNA also inhibited myogenic differentiation in both primary myoblasts and an immortalized myogenic cell line. Meanwhile, in vivo Paxbp1 siRNA delivery also substantially delayed muscle regeneration process in a line of dystrophin-null mice (mdx). However, it remains unclear how Paxbp1 regulates myogenic differentiation.

Our mechanistic studies showed that knockdown of Paxbp1inhibited cell cycle withdraw, and repressed the transcriptional activities of key myogenic transcription factors including MyoD, MEF2 and Myf5. Among them, Myf5 is a poorly-studied transcription factor which executes significant functions during myoblast differentiation. Further analysis showed that Paxbp1 binds with both Myf5 and Brg-1, a core catalytic subunit of the SWI/SNF chromatin remodeling complex. Knockdown of Paxbp1 disrupted the interaction between Myf5 and Brg-1, which consequently weakened the transcriptional activity of Myf5. In summary, we proposed that pre-assembled Myf5-Paxbp1 complex directs recruitment of SWI/SNF to the myog loci, thus to initiate differentiation pathway.

To determine the target genes controlled by Paxbp1 during myogenic differentiation, we examined the transcriptomes of the differentiating C2C12 cells with or without Paxbp1-siRNA by microarray-based experiments. Among genes specifically regulated by Paxbp1, there was a sub-set of genes known to be involved in myogenic differentiation. It is also worth noting that there was another sub-set of potential target genes of Paxbp1 which behave not been associated with myogenic differentiation. We examined several transcription factors and confirmed that they are bona fide Paxbp1 target genes. In particular, we found that Peg3 was an indispensable transcription factor during myogenic differentiation. Moreover, we also proved Myf5-Paxbp1 transcription complex directly associated with the E-boxes on the Peg3 promoter to activate its expression, which further consolidated our previous model.

So far, we have uncovered a molecular mechanism of Paxbp1 during differentiation process in skeletal muscles. However, Paxbp1 is also ubiquitously expressed in other tissues such as the heart and blood system. Up to now, little is known about the role of this gene in other tissue types. It will be interesting to further investigate the general role of Paxbp1in other tissue systems via mass spectrometry analysis and Ch-IP sequencing technology.