Involvement of myogenic regulator factors during fusion in the cell line C2C12

The myogenic factors, MyoD, myogenin, Myf5 and MRF4, can activate skeletal muscle differentiation when overexpressed in non-muscular cells. Gene targeting experiments have provided much insight into the in vivo functions of MRF and have defined two functional groups of MRFs. MyoD and Myf5 may be necessary for myoblast determination while myogenin and MRF4 may be required later during differentiation. However, the specific role of these myogenic factors has not been clearly defined during one important stage of myogenesis: the fusion of myoblasts. Using cultured C2C12 mouse muscular cells, the time-course of these proteins was analyzed and a distinct expression pattern in fusing cells was revealed. In an attempt to clarify the role of each of these regulators during myoblast fusion, an antisense strategy using oligonucleotides with phosphorothioate backbone modification was adoped. The results showed that the inhibition of myogenin and Myf5 activity is capable of significantly preventing fusion. Furthermore, the inhibition of MyoD can wholly arrest the engaged fusion process in spite of high endogenous expression of both myogenin and Myf5. Consequently, each MRF seems to have, at this defined step of myogenesis, a specific set of functions that can not be substituted for by the others and therefore may regulate a distinct subset of muscle-specific genes at the onset of fusion.     

高表达FoxO1抑制猪骨骼肌成肌细胞的分化

FoxO1(Forkhead box O1)是调控肌肉生长、代谢和细胞分化的重要转录因子,但其在成肌细胞分化中的作用还不甚清楚。为了研究FoxO1对哺乳动物成肌细胞分化的影响,以原代培养的长白仔猪成肌细胞作为实验材料,用2%马血清诱导分化,采用实时荧光定量PCR、Western blotting和脂质体转染等方法检测FoxO1及早期和晚期生肌调节因子MyoD和myogenin在猪成肌细胞分化过程中的表达变化。结果显示,在猪成肌细胞分化过程中,FoxO1mRNA表达量显著增加,但总蛋白量变化不显著,其磷酸化水平显著上调。同时,高表达FoxO1的猪成肌细胞中,生肌调节因子MyoD和myogenin mRNA表达受到显著抑制,而MyoD蛋白变化不显著,myogenin却显著下调(P0.05)。以上结果表明,FoxO1能够推迟猪成肌细胞的分化时间并抑制分化;同时推测,FoxO1可能通过抑制生肌调节因子的表达控制骨骼肌纤维类型的终末分化。     

The homeodomain protein Barx2 promotes myogenic differentiation and is regulated by myogenic regulatory factors

The homeobox protein Barx2 is expressed in both smooth and skeletal muscle and is up-regulated during differentiation of skeletal myotubes. Here we use antisense-oligonucleotide inhibition of Barx2 expression in limb bud cell culture to show that Barx2 is required for myotube formation. Moreover, overexpression of Barx2 accelerates the fusion of MyoD-positive limb bud cells and C2C12 myoblasts. However, overexpression of Barx2 does not induce ectopic MyoD expression in either limb bud cultures or in multipotent C3H10T1/2 mesenchymal cells, and does not induce fusion of C3H10T1/2 cells. These results suggest that Barx2 acts downstream of MyoD. To test this hypothesis, we isolated the Barx2 gene promoter and identified DNA regulatory elements that might control Barx2 expression during myogenesis. The proximal promoter of the Barx2 gene contained binding sites for several factors involved in myoblast differentiation including MyoD, myogenin, serum response factor, and myocyte enhancer factor 2. Co-transfection experiments showed that binding sites for both MyoD and serum response factor are necessary for activation of the promoter by MyoD and myogenin. Taken together, these studies indicate that Barx2 is a key regulator of myogenic differentiation that acts downstream of muscle regulatory factors.