Chick muscle expresses various ARIA isoforms: regulation during development, denervation, and regeneration

Acetylcholine receptor inducing activity (ARIA) is a glycoprotein released from the motor neuron to stimulate the synthesis of acetylcholine receptors (AChRs) on the postsynaptic muscle fiber. Transcripts encoding ARIA were detected not only in brain but also in muscle, and immunohistochemical staining showed that muscle-derived ARIA was restricted to the neuromuscular junctions. RT-PCR analysis revealed three biological active isoforms of ARIA in chick muscle, namely ARIA beta 1, ARIA alpha 2, and ARIA beta 2 that were classified based on their variation in the carboxylterminus of the EGF-like domain. The expression of these ARIA isoforms in muscle change during development denervation, and nerve regeneration. ARIA beta 1, ARIA alpha 2, and ARIA beta 2 were expressed in embryonic and young chick muscles, while ARIA beta 1 was the major isoform expressed in adult chicken. The embryonic-like expression of ARIA alpha 2 and ARIA beta 2 was induced after nerve injury in adult chicken. However, the prominent expression of ARIA beta 1 in adult-like profile was restored after nerve regeneration. A splicing variation in the region between Ig-like and EGF-like domains of ARIA was also revealed; a zero-amino acid insertion (ARIASP0), a 17-amino acid insertion (ARIASP17), or a 34-amino acid insertion (ARIASP34) were identified. Unlike ARIASP0, the expression of ARIASP17 and ARIASP34 was found in muscle and sciatic nerve only. The expression of ARIASP0, ARIASP17, and ARIASP34 in chick muscle remained unchanged during development and after nerve injury. Moreover, the specific expression of these ARIA isoforms in cultured myotubes was not affected by drug treatments or by coculturing with neurons. Our findings provide strong evidence that muscle ARIA may play an important role in the formation of neuromuscular junctions.