Myology Laboratory, Institute of Biomedical Problems RAS, Moscow, Russia.
Faculty of Basic Medicine, Lomonosov Moscow State University, Moscow, Russia.
J Physiol. 2017 Dec 1;595(23):7123-7134. doi: 10.1113/JP275184. Epub 2017 Oct 25.
Inactivation of a skeletal muscle results in slow to fast myosin heavy chain (MyHC) shift. AMP-activated protein kinase (AMPK) can be implicated in the regulation of genes encoding the slow MyHC isoform. Here we report that AMPK dephosphorylation after 24 h of mechanical unloading can contribute to histone deacetylase (HDAC) nuclear translocation; activation of AMPK prevents HDAC4 nuclear accumulation after 24 h of unloading and AMPK dephosphorylation inhibits slow MyHC expression following 24 h of unloading. Our data indicate that AMPK dephosphorylation during the first 24 h of mechanical unloading has a significant impact on the expression of MyHC isoforms in rat soleus causing a decrease in MyHC I(β) pre-mRNA and mRNA expression as well as MyHC IIa mRNA expression.
One of the key events that occurs during skeletal muscle inactivation is a change in myosin phenotype, i.e. increased expression of fast isoforms and decreased expression of the slow isoform of myosin heavy chain (MyHC). It is known that calcineurin/nuclear factor of activated T-cells and AMP-activated protein kinase (AMPK) can regulate the expression of genes encoding MyHC slow isoform. Earlier, we found a significant decrease in phosphorylated AMPK in rat soleus after 24 h of hindlimb unloading (HU). We hypothesized that a decrease in AMPK phosphorylation and subsequent histone deacetylase (HDAC) nuclear translocation can be one of the triggering events leading to a reduced expression of slow MyHC. To test this hypothesis, Wistar rats were treated with AMPK activator (AICAR) for 6 days before HU as well as during 24 h of HU. We discovered that AICAR treatment prevented a decrease in pre-mRNA and mRNA expression of MyHC I as well as MyHC IIa mRNA expression. Twenty-four hours of hindlimb suspension resulted in HDAC4 accumulation in the nuclei of rat soleus but AICAR pretreatment prevented this accumulation. The results of the study indicate that AMPK dephosphorylation after 24 h of HU had a significant impact on the MyHC I and MyHC IIa mRNA expression in rat soleus. AMPK dephosphorylation also contributed to HDAC4 translocation to the nuclei of soleus muscle fibres, suggesting an important role of HDAC4 as an epigenetic regulator in the process of myosin phenotype transformation.
骨骼肌失活会导致肌球蛋白重链（MyHC）从慢肌向快肌转变。AMP 激活的蛋白激酶（AMPK）可调节慢肌 MyHC 同工型的基因表达。本研究报告，机械去负荷 24 小时后 AMPK 的去磷酸化可能导致组蛋白去乙酰化酶（HDAC）核转位；去负荷 24 小时后 AMPK 的激活可阻止 HDAC4 核内聚集，而 AMPK 的去磷酸化则抑制去负荷 24 小时后慢肌 MyHC 的表达。我们的数据表明，机械去负荷最初 24 小时内 AMPK 的去磷酸化对大鼠比目鱼肌 MyHC 同工型的表达有显著影响，导致 MyHC I（β）前体 mRNA 和 mRNA 表达以及 MyHC IIa mRNA 表达减少。
骨骼肌失活过程中的一个关键事件是肌球蛋白表型的改变，即快肌同工型表达增加和慢肌 MyHC 同工型表达减少。已知钙调神经磷酸酶/激活 T 细胞核因子和 AMP 激活的蛋白激酶（AMPK）可以调节慢肌 MyHC 基因的表达。我们之前发现，大鼠比目鱼肌去负荷 24 小时后 AMPK 的磷酸化显著降低。我们假设 AMPK 磷酸化的降低和随后的组蛋白去乙酰化酶（HDAC）核转位可能是导致慢肌 MyHC 表达减少的触发事件之一。为了验证这一假说，我们在去负荷前 6 天和去负荷 24 小时内用 AMPK 激活剂（AICAR）处理 Wistar 大鼠。结果发现，AICAR 处理可防止 MyHC I 的前体和 mRNA 表达以及 MyHC IIa mRNA 表达减少。24 小时的下肢悬吊导致大鼠比目鱼肌 HDAC4 在细胞核内积累，但 AICAR 预处理可防止这种积累。研究结果表明，去负荷 24 小时后 AMPK 的去磷酸化对大鼠比目鱼肌的 MyHC I 和 MyHC IIa mRNA 表达有显著影响。AMPK 的去磷酸化也导致 HDAC4 向比目鱼肌纤维细胞核内转位，提示 HDAC4 作为一种表观遗传调节剂在肌球蛋白表型转化过程中发挥重要作用。
