Following acute nerve injuries (e.g., avulsion or other injury to a section of a peripheral nerve) there is a complex sequence of biological processes that results in degeneration of the nervous tissue distal to the injury, followed by new nerve fiber outgrowth to the original target (Navarro et al., 2007). A part of the reinnervation process relies on the upregulation of a set of proteins to guide the regenerating axons from the proximal nerve end toward the denervated regions (Bradke et al., 2012, Brushart, 2011, Gordon, 2009). Despite the dramatic changes in expression of these neural guidance proteins during nerve regeneration, while the end-organs are reinnervated, reinnervation is non-selective and vital functions are never fully restored (Crumley, 2000, Crumley and McCabe, 1982, Iorio, 2019, Salminger et al., 2019, Skouras et al., 2011). In addition, during embryogenesis and in postnatal life, neural guidance proteins play essential roles in the formation and maintenance of the central (CNS) and peripheral nervous systems (PNS) (Bonanomi and Pfaff, 2010, Harrington and Ginty, 2013, Tessier-Lavigne and Goodman, 1996).

In the early 1900 s, it was first postulated that a specific set of molecules may act as axon chemoattractants during nerve formation and after traumatic injury of the nerves (Ramón y Cajal, 1914). This hypothesis was supported in 1956 when the first neurotrophic factor was isolated from snake venom and described as a protein that promotes cell survival and the extension of neurites in cell culture. This protein was named nerve growth factor (NGF) (Levi-Montalcini and Cohen, 1956). Following this breakthrough, significant progress was made in identification of proteins that function as trophic factors to neural cells as well as guidance cues for migrating neural cell bodies and their axons (Nomdedeu-Sancho and Alsina, 2023, Onesto et al., 2021, Tessier-Lavigne and Goodman, 1996).

Originally, only one family of axon guidance proteins, called neurotrophins, was described. They included several members such as NGF, brain-derived nerve growth factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), and Neurotrophin-3 (NT-3) (Huang and Reichardt, 2001). The description of the neurotrophins was followed by the identification of other protein families also serving as chemoattractants for axons, such as semaphorins, neuropilins, and ephrins, while others, such as slits or netrins, functioned as chemorepellents (Allen et al., 2013, Brose et al., 1999, Caton et al., 2000, Fitzgerald et al., 1993, Guthrie, 2007, Kim et al., 2015, Kingham and Terenghi, 2006, Murray et al., 2010). Demonstrating an additional level of regulation, several studies showed a third group of neural guidance proteins could serve as either chemoattractant or chemorepellent cues during CNS and PNS formation depending on expression of variable receptors in the growth cone of the axon (Poliak et al., 2015, Song and Poo, 2001).

One of the most relevant neural guidance proteins is Netrin-1, which plays important roles in the formation of the cranial nerve nuclei in the hindbrain, axonal extension within the CNS, and peripheral sensory and motor axons (Burgess et al., 2006, Chandrasekhar, 2004, Colamarino, 1995, Kim et al., 2015, Livesey and Hunt, 1997, Ratcliffe et al., 2006, Serafini et al., 1996, Varadarajan et al., 2017). Netrin-1 is expressed in muscle fibers during motor innervation of structures such as the larynx, extraocular, and facial muscles in both development and after nerve injury (Chandrasekhar, 2004, Guthrie and Pini, 1995, Hernandez-Morato et al., 2016a, Hernandez-Morato et al., 2017, Varela-Echavarría et al., 1997). Netrin-1 has dual chemoattractive and chemorepulsive properties in multiple systems, making it particularly interesting when investigating nerve regeneration therapies (Colamarino and Tessier-Lavigne, 1995, Dillon et al., 2007, Finci et al., 2014, Hong et al., 1999, Kang et al., 2004, Madison et al., 2000, Poliak et al., 2015, Ratcliffe et al., 2008, Shewan et al., 2002). This is particularly true in the larynx, where dysfunction after nerve injury is due to misguided synkinetic reinnervation and where peripheral manipulation of its expression has been shown to alter the pattern of laryngeal reinnervation (Hernandez-Morato et al., 2017). Further understanding of Netrin-1 will aid research with the goal of therapeutic manipulation of guidance proteins in the treatment of nerve injury. This review summarizes and discusses the roles of Netrin-1 in general development, axon guidance, and formation and maintenance of neuromuscular synapses during primary innervation, as well as its specific contribution to laryngeal muscle reinnervation after nerve injury.