Neuregulins and the neuromuscular system: 10 years of answers and questions

The neuregulins were originally discovered in searches for the acetylcholine receptor-inducing activity (ARIA), glial growth factor (GGF), and a ligand for the oncogene neu (ErbB2/HER2). Neuregulin1 (NRG1)-mediated cell communication is critical in the central and peripheral nervous system, heart, breast, and other organ systems. This review will focus on the functions of NRG1s in the development and maintenance of the neuromuscular system and on the regulation of NRG1 signaling within this system. The roles of NRG1 signaling in the neuromuscular system are far more pervasive than contemplated when neuregulins were discovered 10 years ago. In fact, neuregulin-mediated cell communication plays an essential role in the biology of most components of the neuromuscular system-including motor and sensory neurons, muscle fibers, Schwann cells, and major specializations (neuromuscular synapses, muscle spindles, Golgi tendon organs, and peripheral nerves). It is argued here that while NRG1 proteins are indeed "ARIA" and "GGF", their involvement in regulating synapse-specific transcription and Schwann cell development is more complex than originally proposed. It is also argued that NRG1 isoforms differ in their signaling properties and that these differences tailor specific isoforms for specific signaling tasks; for example, some NRG1 isoforms may be specialized for paracrine signaling and others for juxtacrine signaling. In the first 10 years of neuregulin research there has been much progress in understanding the actions of neuregulins in shaping and maintaining the neuromuscular system. However, major questions, old and new, remain unanswered; and the second 10 years promises to be at least as exciting as the first.