<p> Skeletal muscle generates contractile forces that allow the body to execute movements for walking, speaking and breathing. Although we understand a great deal about the steps of muscle formation, the mechanisms that control muscle size are poorly understood. Even less is known about how muscles interact with skeletal elements, including connective tissue, tendon and bone. This dissertation describes a novel role for Abelson tyrosine-protein kinase 2, a non-receptor tyrosine kinase, during muscle development. First, I characterize the defects in skeletal muscle of <i>abl2</i> mutant mice and show that muscle fibers in the diaphragm and other muscles are extraordinarily long in <i>abl2</i> mutant mice. As a consequence of expansion of the diaphragm muscle, the central tendon of the diaphragm is proportionally reduced in size. Second, I demonstrate that <i>abl2</i> controls muscle size by regulating myoblast proliferation. Third, I show that Abl2 acts in myoblasts to attenuate their proliferation, thereby limiting myoblast fusion and muscle fiber size. Fourth, I show that the exercise endurance of <i> abl2</i> mutant mice is diminished, likely due to the compensatory reduction in size of the diaphragm central tendon. Finally, I provide evidence for signaling between muscle cells and tendon cells that induces tendon cell differentiation. </p><p>

<p> Jun N-terminal kinase (JNK) signaling is a key modulator of development and disease in all multicellular organisms. One process in which the consequences of both gain and loss of JNK signaling can be monitored is embryonic dorsal closure (DC) in the fruit fly, <i>Drosophila melanogaster.</i> DC occurs midway through embryogenesis; it is the process by which the lateral epidermis expands bilaterally to meet and fuse at the dorsal midline, thereby encasing the entire embryo in epidermis. JNK signaling in leading edge (LE) cells (the dorsal-most row of epidermis) initiates closure. My studies of a novel but conserved JNK signaling antagonist, Raw, have provided several unique insights into: 1) Jun function as a component of the AP-1 transcription factor, and 2) the role of the epidermis as a signaling template mediating development of the epidermis and adjacent tissues.</p><p> My graduate work has built upon the demonstration that <i>raw</i> is required to prevent promiscuous JNK signaling in the embryonic epidermis just prior to DC. I have shown that <i>raw</i> is necessary for proper accumulation of Jun in LE cells required to define the LE, which functions as a signaling center required for epidermal closure as well as for underlying heart development. I have gone on to show that Jun accumulates at previously unrecognized sites in the embryonic epidermis, including tracheal pits and solitary epidermal cells lying directly above the peripheral nervous system (PNS). Jun activity is required for tracheal and nervous system defects observed in mutants of two JNK signaling antagonists, <i>raw</i> and <i> rib,</i> and indicates that cell signals within and to an adjacent tissue are integral to proper development. I have found that the epidermis plays an instructive role during development, and results from my work have led to insights into how JNK signaling centers in the epidermis coordinate morphological processes.</p><p> As Raw is a novel but conserved JNK signaling antagonist, I have built and tested models of its molecular mechanism of action as well. Bolstering conclusions of previous studies of mammalian c-Jun in cell culture, my data indicate that N-terminal phosphorylation is not an on/off switch, but rather it increases Jun stability for its activity as a component of the AP-1 transcription factor. <i>raw</i> mutants exhibit both high levels of Jun protein and an accumulation of phosphorylated Jun (P-Jun), and my data point to a role for Raw in effecting the Jun:P-Jun ratio via mediation of Jun degradation. In deciphering the mechanism of Raw function, we are gaining significant new insights into previously unrecognized mechanisms of JNK signaling regulation. Understanding these mechanisms will be important for dissecting the etiology of developmental abnormalities and diseases, such as cancer, which hinge on the Goldilocks effect, having just the right amount of signaling at just the right time.</p>