Abstract
Neuronal growth-associated protein 43 (GAP43) has crucial roles in the nervous system, and during development, regeneration after injury, and learning and memory. GAP43 is expressed in mouse skeletal muscle fibers and satellite cells, with suggested its involvement in intracellular Ca2+ handling. However, the physiological role of GAP43 in muscle remains unknown. Using a GAP43-knockout (GAP43−/−) mouse, we have defined the role of GAP43 in skeletal muscle. GAP43−/− mice showed low survival beyond weaning, reduced adult body weight, decreased muscle strength, and changed myofiber ultrastructure, with no significant differences in the expression of markers of satellite cell and myotube progression through the myogenic program. Thus, GAP43 expression is involved in timing of muscle maturation in-vivo. Intracellular Ca2+ measurements in-vitro in myotubes revealed GAP43 involvement in Ca2+ handling. In the absence of GAP43 expression, the spontaneous Ca2+ variations had greater amplitudes and higher frequency. In GAP43−/− myotubes, also the intracellular Ca2+ variations induced by the activation of dihydropyridine and ryanodine Ca2+ channels, resulted modified. These evidences suggested dysregulation of Ca2+ homeostasis. The emerging hypothesis indicates that GAP43 interacts with calmodulin to indirectly modulate the activities of dihydropyridine and ryanodine Ca2+ channels. This thus influences intracellular Ca2+ dynamics and its related intracellular patterns, from functional excitation-contraction coupling, to cell metabolism, and gene expression.
Highlights
Neuronal growth-associated protein 43 (GAP43) was originally considered to be a neuronspecific marker
GAP43 Affects Ca2+ in Skeletal Muscle conserved in all vertebrates, and where the first 10 amino acids are crucial for its binding to the nerve-terminal membrane (Zuber et al, 1989); and (iii) the domain, following the N-terminal region, that is known as the IQ domain, of 12–15 amino acids, and which binds calmodulin (CaM), a Ca2+-binding protein involved in cellular processes that are modulated by intracellular Ca2+ (Oestreicher et al, 1984; Jacobson et al, 1986; Alexander et al, 1987; Benowitz et al, 1987; Coggins and Zwiers, 1989; Zuber et al, 1989; Chapman et al, 1991)
As reported in other studies (Strittmatter et al, 1995; Metz and Schwab, 2004), more than 50% of the GAP43−/− mice died within 2 days of birth (P0–P2), and most of the remaining died between P14 and P21, with only about 5% that survived weaning
Summary
Neuronal growth-associated protein 43 (GAP43) was originally considered to be a neuronspecific marker. The IQ domain of GAP43 starts at the isoleucine and glycine that lie just prior to its Ser and it extends over about 12– 15 amino acids toward the C-terminus (Oestreicher et al, 1984; Jacobson et al, 1986; Alexander et al, 1987; Benowitz et al, 1987; Coggins and Zwiers, 1989; Zuber et al, 1989; Chapman et al, 1991) This Ser is crucial, because it is the only site that can be phosphorylated by protein kinase C (PKC) (Coggins and Zwiers, 1989; Chapman et al, 1991). The activation of one target protein, calcineurin, results in GAP43 de-phosphorylation and CaM re-association (Benowitz and Routtenberg, 1997)
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