Pitx2c modulates Pax3+/Pax7+ cell populations and regulates Pax3 expression by repressing miR27 expression during myogenesis

Pitx2 is a paired-related homeobox gene that is expressed in muscle progenitors during myogenesis. We have previously demonstrated that overexpression of Pitx2c isoform in myoblasts maintained these cells with a high proliferative capacity and completely blocked terminal differentiation by inducing high Pax3 expression levels (Martinez et al., 2006). We now report that Pitx2c-mediated proliferation vs. differentiation effect is maintained during in vivo myogenesis. In vivo Pitx2c loss of function leads to a decrease in Pax3+/Pax7− cell population in the embryo accompanied by an increase of Pax3+/Pax7+ cells. Pitx2c transient-transfection experiments further supported the notion that Pitx2c can modulate Pax3/Pax7 expression. Pitx2c but not Pitx3 controls Pax3/Pax7 expression, although redundant roles are elicited at the terminal myoblast differentiation. Contrary to Pitx2c, Pitx3 does not regulate cell proliferation or Pax3 expression, demonstrating the specificity of Pitx2c mediating these actions in myoblasts. Furthermore we demonstrated that Pitx2c modulates Pax3 by repressing miR27 expression and that Pax3-miR-27 modulation mediated by Pitx2c is independent of Pitx2c effects on cell proliferation. Therefore, this study sheds light on previously unknown function of Pitx2c balancing the different myogenic progenitor populations during myogenesis.     

Pax3对神经细胞中的转录调控作用的实验研究

摘要 目的：探讨Pax3的过表达对Neuro-2a细胞中转录本的表达影响,初步分析Pax3对Neuro-2a细胞可能的转录调控作用。方法：反复冻融裂解法获取Pax3过表达腺病毒后将神经瘤母细胞系Neuro-2a传代培养,而后将Pax3过表达腺病毒和传代培养后的Neuro-2a细胞加入到同一培养皿中,蛋白质免疫印迹（Western blot）检测过表达Pax3蛋白的Neuro-2a细胞（Pax3过表达组）和对照组（NC组）Neuro-2a细胞的Pax3蛋白表达水平,实时荧光定量PCR（qRT-PCR）法检测Pax3过表达组和NC组Neuro-2a细胞的Pax3mRNA水平,Trizol法提取Pax3过表达组和NC组Neuro-2a细胞的总RNA,然后进行全转录本测序,最后将选出的有差异性的基因使用实时荧光定量PCR（qRT-PCR）验证。结果：与NC组相比,Pax3过表达组的Pax3蛋白和Pax3mRNA表达水平明显升高（P＜0.05）;Pax3过表达组中发现了1045个基因表达上调,1313个基因表达下调。通过qRT-PCR验证发现在Pax3过表达组中Nppb和Chrna5表达水平上升（P＜0.05）,Arhgap5、Rock1、Rif1、Brca2、Prkg2和Stag2表达水平下降（P＜0.05）。结论：Pax3过表达腺病毒感染Neuro-2a细胞后,其蛋白和mRNA表达水平均升高,Rock1、Rif1和Stag2可能作为Pax3的下游靶点参与调控Neuro-2a细胞周期和干细胞特性。     

Pax3 and Pax7 expression during myoblast differentiation in vitro and fast and slow muscle regeneration in vivo

In this report, we focused on Pax3 and Pax7 expression in vitro during myoblast differentiation and in vivo during skeletal muscle regeneration. We showed that Pax3 and Pax7 were present in EDL (extensor digitorum longus) and Soleus muscle derived cells. These cells express in vitro a similar level of Pax3 mRNA, however, differ in the levels of mRNA encoding Pax7. Analysis of Pax3 and Pax7 proteins showed that Soleus and EDL satellite cells differ in the level of Pax3/7 proteins and also in the number of Pax3/7 positive cells. Moreover, Pax3/7 expression was restricted to undifferentiated cells, and both proteins were absent at further stages of myoblast differentiation, indicating that Pax3 and Pax7 are down-regulated during myoblast differentiation. However, we noted that the population of undifferentiated Pax3/7 positive cells was constantly present in both in vitro cultured satellite cells of EDL and Soleus. In contrast, there was no significant difference in Pax3 and Pax7 during in vivo differentiation accompanying regeneration of EDL and Soleus muscle. We demonstrated that Pax3 and Pax7, both in vitro and in vivo, participated in the differentiation and regeneration events of muscle and detected differences in the Pax7 expression pattern during in vitro differentiation of myoblasts isolated from fast and slow muscles.     

A gradient of Shh establishes mutually repressing somitic cell fates induced by Nkx3.2 and Pax3

Wnt and Sonic Hedgehog (Shh) signals are known to pattern the somite into dermomyotomal, myotomal and sclerotomal cell fates. By employing explants of presomitic mesoderm cultured with constant levels of Wnt3a conditioned medium and increasing levels of Shh, we found that differing levels of Shh signaling elicit differing responses from somitic cells: the lowest level of Shh signaling allows dermomyotomal gene expression, intermediate levels induce loss of dermomyotomal markers and activation of myogenic differentiation, and higher levels induce loss of myotomal markers and activation of sclerotomal gene expression. In addition, we have found that in the presence of high levels of Wnt signaling, instead of inducing sclerotomal markers, Shh signals act to maintain the expression of dermomyotomal and myotomal markers. One of the sclerotomal genes induced by high levels of Shh signaling is Nkx3.2. Forced expression of Nkx3.2 blocks somitic expression of the dermomyotomal marker Pax3 both in vitro and in vivo. Conversely, forced expression of Pax3 in somites can block Shh-mediated induction of sclerotomal gene expression and chondrocyte differentiation in vitro. Thus we propose that varying levels of Shh signaling act in a morphogen-like manner to elicit differing responses from somitic cells, and that Pax3 and Nkx3.2 set up mutually repressing cell fates that promote either dermomyotome/myotome or sclerotome differentiation, respectively.