Neuregulin-mediated ErbB 3 signaling is required for formation of zebrafish dorsal root ganglion neurons

INTRODUCTION Segmentally reiterated dorsal root ganglia (DRGs) are required for vertebrates to sense and respond to many environmental and proprioceptive signals (Scott, 1992). DRGs are located adjacent to the spinal cord, and their component neurons and glia arise from trunk neural crest (NC) cells (Le Douarin and Kalcheim, 1999; Kalcheim, 2000; Eisen and Weston, 1993). Neural crest is a transient, embryonic cell population that emigrates from the dorsal neuroepithelium, migrates along specific pathways and generates a variety of progeny, including neurons and glia of the peripheral nervous system, pigment cells and craniofacial cartilages. Trunk NC cells migrate on two pathways: a ventral (also called medial) pathway between the somites and neural tube; and a dorsolateral (also called lateral) pathway between the somites and overlying ectodermal epithelium (Weston and Butler, 1966; Serbedzija et al., 1989; Serbedzija et al., 1990; Erickson et al., 1992; Erickson and Weston, 1983; Raible et al., 1992). The ventral or medial migration pathway is restricted to a particular region of the somites (Le Douarin and Kalcheim, 1999; Kalcheim, 2000); in zebrafish, this pathway is restricted to the middle of the medial surface of each somite, half way between adjacent somite boundaries (Honjo and Eisen, 2005; Raible et al., 1992). Thus, zebrafish trunk NC cells migrate in a pattern of ‘streams’; a single stream underlies the middle of each somite. Zebrafish DRG neurons are derived from among the earliest NC cells to migrate on the medial pathway (Raible and Eisen, 1994). However, whether DRGs arise from a specified NC subpopulation, how the fates of individual NC cells are regulated during migration, and how NC cells recognize specific target locations appropriate for their eventual differentiation are currently not well understood. In amniote embryos, ErbB3 is one factor that has been implicated in NC migration. ErbB3 is a receptor tyrosine kinase proto-oncogene member of the epidermal growth factor (EGF) receptor family. Although ErbB3 can form homodimers, active receptors require heterodimerization with ErbB2 (Guy et al., 1994; Tzahar et al., 1996; Graus-Porta et al., 1997; Kim et al., 1998; Olayioye et al., 2000; Holbro et al., 2003). ErbB2/ErbB3 heterodimers bind to neuregulin (Nrg) 1 or Nrg2, members of the EGF-like polypeptide growth factor superfamily (Riese et al., 1995; Busfield et al., 1997; Carraway et al., 1997; Chang et al., 1997; Olayioye et al., 2000; Holbro et al., 2003). Nrg1 signaling via ErbB3 is important for several developmental processes, including glial migration and development (Riethmacher et al., 1997; Erickson et al., 1997; Garratt et al., 2000; Lyons et al., 2005), development of myelination (Chen et al., 2006), muscle acetylcholine receptor expression (Morris et al., 1999; Woldeyesus et al., 1999; Lin et al., 2000; Fall, 2003), and neural migration (Anton et al., 1997; Rio et al., 1997; Olayioye et al., 2000). The initial suggestion of a role for ErbB3 in NC migration came from mice with targeted mutations in Erbb2, Erbb3 or Nrg1 (Britsch et al., 1998). These mice lack sympathetic ganglia, presumably because without ErbB receptor-mediated neuregulin signaling, NC cells cannot migrate to the region lateral of the dorsal aorta, where sympathetic neuron differentiation occurs (Britsch et al., 1998). By contrast, DRGs, the progenitors of which do not have to migrate any significant distance after emigrating from the neural folds (Teillet et al., 1987), initially appear normal in Erbb3 mutant mice (Britsch et al., 1998). However, although NC cells form an ectopic cluster around the DRGs in Erbb3 mutant mice, these ectopic NC cells do not differentiate into DRG neurons or glia (Britsch et al., 1998); later, most DRG neurons reportedly die (Riethmacher et al., 1997). Erbb2 mutant mice die of cardiac defects too early to determine whether ErbB2 is required for DRG formation (Morris et al., 1999; Woldeyesus et al., 1999). Thus, the role of ErbB signaling in DRG formation remains unresolved. Here, we show that zebrafish mutants lacking function of either ErbB3b or ErbB2 do not form trunk NC-derived DRG or sympathetic neurons. By contrast, cranial NC-derived enteric neurons appear Neuregulin-mediated ErbB3 signaling is required for formation of zebrafish dorsal root ganglion neurons