Motor Axon Subpopulations Respond Differentially to the Chemorepellents Netrin-1 and Semaphorin D

Abstract

During development, growing motor axons are excluded from the ventral midline of the neural tube by diffusible chemorepellents emanating from this region. Molecular candidates for this chemorepellent activity include semaphorin D and netrin-1; the latter is known to repel trochlear motor axons. Qualitatively or quantitatively different responses to these molecules might underlie the initial deflection from the midline and subsequent segregation of motor axon trajectories. To test this idea, we have cocultured cell aggregates secreting netrin-1 or semaphorin D at a distance from tissue explants containing different motor neuron subpopulations, in collagen gels. Cranial motor axons that project dorsally in vivo such as those of the trigeminal, facial, and glossopharyngeal nuclei were repelled by both netrin-1 and semaphorin D. By contrast, ventrally projecting spinal motor axons and abducens axons were not affected by netrin-1. Spinal and abducens motor neurons also responded to semaphorin D. The ventrally projecting axons of oculomotor neurons were not repelled by netrin-1 or semaphorin D. Differential responsiveness to netrin-1 and semaphorin D could thus contribute to the generation of dorsal and ventral motor axon pathways during development.

Turning heads: Development of vertebrate branchiomotor neurons

“…Mouse trochlear (nIV) axons are repelled by netrin1-expressing cells in vitro (Colamarino and Tessier-Lavigne, 1995). Furthermore, BMN axon outgrowth from rat hindbrain explants is reduced in the presence of Netrin1 (Varela-Echavarria et al, 1997), and netrin receptors Unc5h1 and DCC are expressed in rat cranial motor neurons shortly after the onset of axon outgrowth (Barrett and Guthrie, 2001). In zebrafish, netrins are expressed in the ventral hindbrain (Lauderdale et al, 1997;Strahle et al, 1997) and genes encoding netrin receptors are expressed rather broadly in the hindbrain at the time of BMN axonogenesis (Hjorth et al, 2001;Shen et al, 2002).…”

“…Because a zebrafish homologue of the chemorepellant semaphorin plays a role in guiding BMN axons outside the hindbrain (Xiao et al, 2003;see below), semaphorins may also play a role in keeping BMN axons away from the floor plate. BMN axon outgrowth from rat hindbrain explants is reduced in the presence of Sem D (Sema3A) (Varela-Echavarria et al, 1997). Of interest, BMN axons exit normally from the hindbrain in mice defective for Neuropilin2, a Sema3A receptor that is expressed in BMN nuclei (Chen et al, , 2000Giger et al, 2000), and in mice defective for Sema3A, which is expressed in the ventral hindbrain Varela-Echavarria et al, 1997;Catalano et al, 1998).…”

“…BMN axon outgrowth from rat hindbrain explants is reduced in the presence of Sem D (Sema3A) (Varela-Echavarria et al, 1997). Of interest, BMN axons exit normally from the hindbrain in mice defective for Neuropilin2, a Sema3A receptor that is expressed in BMN nuclei (Chen et al, , 2000Giger et al, 2000), and in mice defective for Sema3A, which is expressed in the ventral hindbrain Varela-Echavarria et al, 1997;Catalano et al, 1998). However, because BMN axon outgrowth within the hindbrain was not specifically examined in the mutant embryos, defects in axon outgrowth at the ventral midline, and a specific role for semaphorins in this process, cannot be ruled out.…”

Cranial expression of class 3 secreted semaphorins and their neuropilin receptors

“…Among the many families of molecules that play a role in these processes are a group of repulsive molecules, the semaphorins (Raper, 2000). Semaphorin3A (Sema3A) is a secreted molecule that was initially demonstrated to cause the collapse of growth cones in vitro and subsequently shown to repel axons from a variety of neuronal types (Luo et al, 1993;Messersmith et al, 1995;Puschel et al, 1996;Varela-Echavarria et al, 1997). There are five other members of the class 3 family of semaphorins (Semaphorin Nomenclature Committee, 1999).…”

“…There is strong evidence that semaphorins and NPNs play a key role in guiding these nerves to their targets. Specific subpopulations of cultured cranial motor axons respond to Sema3A in vitro (Varela-Echavarria et al, 1997) and in the Sema3A mutant mouse there is marked defasciculation, with some aberrant projections, of the trigeminal, facial, glossopharyngeal, vagal, and accessory nerves . The same nerves are disorganised in mice lacking NPN1 (Kitsukawa et al, 1997), demonstrating the importance of this receptor-ligand interaction in the formation of these nerve pathways.…”