Ephrin-A3 specifies slow muscle fiber type

描述（由申请人提供）：骨骼肌的发育是一个精细协调的过程，需要在时间和空间上整合多个细胞时间来形成成年哺乳动物中发现的200多个肌肉，每个肌肉在其大小，形状和位置以及与其他器官系统（如神经和肌腱，神经支配和收缩特性）形成的功能连接方面都是独一无二的。当肌肉的完整性被急性损伤或疾病破坏时，骨骼肌不仅具有快速有效地再生失去的细胞数量和质量的能力，而且在一定程度上再现了每块肌肉固有的原有模式。对任何特定肌肉的特定功能至关重要的一个方面是快速和慢速肌纤维的独特比例和排列，以提供必要的力量和耐力平衡。肢体肌肉中存在不同类型的肌纤维，可根据其糖酵解与氧化能力、抽搐力产生、运动神经元连通性和肌球蛋白重链基因表达来定义，大致定义为“慢”（表达I型MyHC）和“快”（表达II型MyHC）。50多年来，在肌源性细胞之间以及在肌源性细胞与其他类型细胞之间，已经观察到基于起源或与快肌或慢肌相关的优先细胞-细胞关联，但尚未发现任何发生这种情况的分子解释。最近的数据显示，Eph/ephrin家族成员的经典排斥引导信号可以改变骨骼肌中干细胞的行为，这导致了一种假设，即它可能作为一种保持纤维类型身份的机制，在肌肉形成、神经支配和再生过程中负责细胞分选。一个单一的排斥引导配体，ephrin-A3，在成人肢体肌肉的所有慢肌纤维上表达，而在任何快肌纤维上都不表达。两种ephrin-A3受体EphA3和EphA8的表达呈互补模式，与所有且仅与快速肌纤维相关，而与慢速肌纤维无关，似乎分别识别成肌细胞和神经元细胞。由于Eph-ephrin相互作用的通常效果是细胞去粘附和排斥，因此表达数据将支持一种模型，即通过特定的Eph/ephrin相互作用，阻止“快”细胞与慢肌纤维相互作用。该模型将在最初的肌肉发育和模式以及成年肌肉或神经损伤期间进行测试，主要是通过比较野生型、ephrin-A3功能丧失和ephrin-A3功能获得肌肉的纤维类型特征。由于纤维类型特异性肌肉群的损失与衰老和代谢或神经肌肉疾病中的肌肉功能障碍有关，这种理解纤维类型模式如何最初指定然后重述的新方法将提供新的靶点，可能会原位修改肌纤维类型以保持肌肉质量和功能。

DESCRIPTION (provided by applicant): Skeletal muscle development is a finely orchestrated process requiring integration of multiple cell times in time and space to form each of the 200+ muscles found in adult mammals, each of which is unique in its size, shape, and location as well as in the functional connections it forms with other organ systems such as nerves and tendons, innervation, and contractile properties. When the integrity of a muscle is disrupted by acute damage or disease, skeletal muscle is remarkable not only in its capacity to rapidly and efficiently regenerate lost cell number and mass, but in the extent to which the pre-existing patterns intrinsic to each muscle are recapitulated. One aspect of patterning that is critical for the specific function of any given muscle is the unique proportion and arrangement of fast and slow myofibers to provide the necessary balance of force and endurance. Distinct muscle fiber types are present in limb muscles, which can be defined on the basis of their glycolytic vs. oxidative capacity, twitch force generation, motor neuron connectivity, and myosin heavy chain gene expression and are broadly defined as 'slow' (expressing Type I MyHC) and 'fast' (expressing Type II MyHC). Preferential cell-cell associations based on origin or associations with fast vs. slow muscle have been observed both among myogenic cells and between myogenic cells and other cell types for more than fifty years, but no molecular explanation for any of the cases where it occurs have been found. Recent data showing that classical repulsive guidance signaling by Eph/ephrin family members modifies stem cell behavior in skeletal muscle led to the hypothesis that it could be responsible for cell sorting during myogenesis, innervation, and regeneration as a mechanism to preserve fiber type identity. A single repulsive guidance ligand, ephrin-A3, is expressed on all and every slow myofiber in adult limb muscle, and is not expressed by any fast myofiber. Expression of two ephrin-A3 receptors, EphA3 and EphA8, are found in a complementary pattern, associated with all and only fast myofibers and no slow myofibers and appear to identify myoblasts and neuronal cells, respectively. Because the usual effect of Eph-ephrin interactions is cellular de-adhesion and repulsion, expression data would support a model in which 'fast' cells are prevented from interacting with slow myofibers via specific Eph/ephrin interactions. This model will be tested both during initial muscle development and patterning and following injury to adult muscle or nerve, primarily by comparing fiber type profiles in wild type, ephrin-A3 loss-of-function and ephrin-A3 gain-of-function muscles in vivo. Because fiber type-specific loss of muscle groups is associated with both frailty due to aging and muscle dysfunction in metabolic or neuromuscular disease, this novel approach to understanding how fiber type patterns are initially specified then recapitulated will provide novel targets that may be modified muscle fiber types in situ to preserve muscle mass and function.