Myosin: Formation and maintenance of thick filaments

Skeletal muscle consists of bundles of myofibers containing millions of myofibrils, each of which is formed of longitudinally aligned sarcomere structures. Sarcomeres are the minimum contractile unit, which mainly consists of four components: Z‐bands, thin filaments, thick filaments, and connectin/titin. The size and shape of the sarcomere component is strictly controlled. Surprisingly, skeletal muscle cells not only synthesize a series of myofibrillar proteins but also regulate the assembly of those proteins into the sarcomere structures. However, authentic sarcomere structures cannot be reconstituted by combining purified myofibrillar proteins in vitro, therefore there must be an elaborate mechanism ensuring the correct formation of myofibril structure in skeletal muscle cells. This review discusses the role of myosin, a main component of the thick filament, in thick filament formation and the dynamics of myosin in skeletal muscle cells. Changes in the number of myofibrils in myofibers can cause muscle hypertrophy or atrophy. Therefore, it is important to understand the fundamental mechanisms by which myofibers control myofibril formation at the molecular level to develop approaches that effectively enhance muscle growth in animals.     

香猪、二花脸猪和大白猪肌纤维超微结构和肌肉组织化学特性的研究

电镜观察香猪、二花脸猪和大白猪肌纤维超微结构表明,肌原纤维直径、肌丝直径、肌节的长度和I带、A带的长度无明显的品种差异,肌纤维横截面单位面积内肌质网和肌原纤维所占的面积、肌质网终池、肌质和背最长肌中红肌纤维和白肌纤维的比例,肌原纤维间脂肪滴和糖原的含量、3个猪种间均有明显的差别。     

两段式冷却对牛肉食用品质的影响

为探讨两段式冷却对牛肉食用品质的影响,以20头杂交牛胴体（延边牛×西门塔尔牛）按试验设计进行两段式冷却（温度-13～-15℃、时间2 h,风速3 m/s,随后转入常规冷却间至24 h）和常规冷却处理（温度0～4℃、时间24 h）相比较,研究不同处理对牛肉食用品质的影响。表明：两段式冷却处理加快胴体温度下降速率,减缓pH值下降速率,显著降低胴体冷却质量损失,使肉的色泽鲜红,提高肉的剪切力,对背最长肌的保水性无影响。可以认为在肉牛屠宰加工流程中,单独采用两段式冷却处理能降低肉的嫩度,但随时间的增加,剪切力值会有所降低,建议应用时结合电刺激处理来同时改善肉的食用品质。     

姜曲海猪瘦肉型品系(零世代)早期背最长肌超微结构的研究

选择姜曲海猪瘦肉型品系(零世代)仔猪24头于0,15,30,45,60,75d进行屠宰,取背最长肌做超薄切片,对肌原纤维直径、肌节、A带、I带和H带进行测量分析。结果表明,肌原纤维直径从出生至15日龄期间生长迅速,15日龄时肌原纤维直径比初生时几乎增加了1.5倍,此后各日龄段增长缓慢,到75日龄已接近成熟;肌节长度到45日龄时已达到1649nm,基本分化成熟;A带长度从出生到60日龄呈逐渐增加趋势,60日龄时达到1346nm基本分化成熟;而I带和H带长度不随年龄增长而增加,说明I带和H带长度在胚胎期已基本成熟而稳定;研究发现各部分成熟的先后顺序为:I带(胚胎期)>肌节(45日龄)>A带(60日龄)>肌原纤维直径(75日龄)。     

Myosin substitution rate is affected by the amount of cytosolic myosin in cultured muscle cells

In striated muscles, approximately 300 myosin molecules form a single thick filament in myofibrils. Each myosin is continuously displaced by another myosin to maintain the thick filament structure. Our previous study using a fluorescence recovery after photobleaching (FRAP) technique showed that the myosin replacement rate is decreased by inhibition of protein synthesis, but myosin is still exchangeable. This result prompted us to examine whether myosin in the cytoplasm is involved in myosin replacement in myofibrils. To address this, FRAP was measured in green fluorescent protein (GFP)‐tagged myosin heavy chain 3 (Myh3) expressing myotubes that were treated with streptolysin‐O (SLO), which forms pores specifically in the plasma membrane to induce leakage of cytoplasmic proteins. Our biochemical data demonstrated that the cytoplasmic myosin content was reduced in SLO‐permeabilized semi‐intact myotubes. Furthermore, FRAP experiments showed a sluggish substitution rate of GFP‐Myh3 in SLO‐permeabilized myotubes. Taken together, these results demonstrate that the myosin substitution rate is significantly reduced by a decreased amount of myosin in the cytoplasm and that cytoplasmic myosin contributes to myosin replacement in myofibrils.     

The importance of subfragment 2 and C‐terminus of myosin heavy chain for thick filament assembly in skeletal muscle cells

In skeletal muscle cells, myofibrillar proteins are highly organized into sarcomeres in which thick filaments interdigitate with thin filaments to generate contractile force. The size of thick filaments, which consist mainly of myosin molecules, is strictly controlled. However, little is known about the mechanisms by which myosin molecules assemble into thick filaments. Here, we assessed the ability of each domain of myosin heavy chain (Myh) to form thick filaments. We showed that exogenously expressed subfragment 2 (S2) + light meromyosin (LMM) of Myh was efficiently incorporated into thick filaments in muscle cells, although neither solely expressed S2 nor LMM targeted to thick filaments properly. In nonmuscle COS7 cells, S2+LMM formed more enlarged filaments/speckles than LMM. These results suggest that Myh filament formation is induced by S2 accompanying LMM. We further examined the effects of Myh C‐terminus on thick filament assembly. C‐terminal deletion mutants were incorporated not into entire thick filaments but rather into restricted regions of thick filaments. Our findings suggest that the elongation of myosin filaments to form thick filaments is regulated by S2 as well as C‐terminus of LMM.     

在直流、交流脉冲电流刺激下马半腱肌的显微结构变化

利用电子显微镜技术研究马半腱肌在直流、交流脉冲电流刺激下肌节及纤维、Z 线等显微结构的变化。采用5个不同档次的电压对屠宰后20～60min 内的屠体,实施3min 的电刺激处理。实验结果表明:对照组与实验组显微结构差异明显,对照组肌节、肌纤维排列整齐。实验组中肌节、肌纤维、Z 线的显微形态发生了变化,肌节出现了强直收缩或延展,I 带和 Z 线处产生了撕裂并伴随产生了细胞溶解物。这一结果为电刺激对肌肉的嫩化作用提供了证据,同时也扩展了前人在这方面的研究。     

肉种鸡产蛋中期能量限饲对子代肉鸡生长性能、血液生化指标及肌节长度的影响

试验对产蛋中期AA肉种鸡进行能量限饲,探讨其对子代生长性能、血液生化指标及肌节长度的影响。选用产蛋中期肉种母鸡288羽,分为4个处理组,每组6个重复,每个重复12只。对照组饲喂基础饲粮,试验组日粮能量水平分别为基础饲粮能量的80%、70%、50%,对照组和实验组均限饲且饲喂量相同,于产蛋中期(40~42周龄)进行人工授精、孵化得到子代。每组选用90只肉仔鸡公雏,每组设6个重复,每个重复15只,自由采食同种日粮,试验期为49 d。结果表明,①生产性能:与对照组相比,50%、70%能量组仔鸡初生重显著降低(P<0.05),80%能量组仔鸡49日龄体重显著升高(P<0.05)。②血液生化指标:母体能量限饲显著降低了28日龄各组仔鸡血清中ALB的含量(P<0.05),显著提高了50%能量组IGF-1的水平(P<0.05)。③肌节长度:与对照组相比,50%、70%能量组1日龄肉鸡腿肌肌节长度显著降低(P<0.05),70%、80%能量组28日龄肉鸡腿肌肌节长度显著升高(P<0.05),50%能量组49日龄肉鸡腿肌肌节长度显著增大(P<0.05)。结果表明,肉种鸡产蛋中期能量限饲对子代生长发育有显著影响,并且子代在生长过程中表现出补偿生长作用。     

长白山野猪与莱芜猪杂交一代猪的肉品品质

为了利用野猪和地方品种猪的优良种质资源,生产安全优质绿色猪肉,以长白山野猪与山东省优良地方品种莱芜猪的杂交1代猪（简称野莱F1代猪）和长×大白猪（各18头,公母各半）为研究对象,利用不含任何添加剂的绿色玉米、豆粕、麦麸组成的日粮,在舍饲＋放牧的条件下饲养至90kg左右时屠宰,对试验猪的肌肉组织学性状、氨基酸、肌苷酸、矿物质和微量元素含量及氨基酸评分进行了比较全面的测定分析。结果表明：野菜F1代猪的肌纤维比长×大白猪细,肌小节比长×大白猪长（P〈0.01）;除脯氨酸外,F1代猪的其他17种氨基酸含量、氨基酸总量、必需＋半必需氨基酸含量,鲜味氨基酸和肌苷酸含量及氨基酸评分,均显著或极显著高于长×大白猪（P〈0.05或P〈0.01）;F1代猪肉的矿物质和微量元素含量较丰富,除K显著低于长×大白猪外,Ca、P、Fe、Zn、Mg和Cu的含量都显著或极显著高于长×大白猪（P〈0.05或P〈0.01）。结果提示：野莱F1代猪肉的肉品品质和营养价值比长×大白猪好,可以作为生产安全优质绿色猪肉的原料猪用。