Responsiveness to neurturin of subpopulations of embryonic rat spinal motoneuron does not correlate with expression of GFR alpha 1 or GFR alpha 2

Glial cell-line derived neurotrophic factor (GDNF) and its relative neurturin (NTN) are both potent trophic factors for motoneurons. They exert their biological effects by activating the RET tyrosine kinase in the presence of a GPI-linked coreceptor, either GFR alpha 1 (considered to be the favored coreceptor for GDNF) or GFR alpha 2 (the preferred NTN coreceptor). By whole-mount in situ hybridization on embryonic rat spinal cord, we demonstrate that, whereas Ret is expressed by nearly all motoneurons, Gfra1 and Gfra2 exhibit complementary and sometimes overlapping patterns of expression. In the brachial and sacral regions, the majority of motoneurons express Gfra1 but only a minority express Gfra2. Accordingly, most motoneurons purified from each region are kept alive in culture by GDNF. However, brachial motoneurons respond poorly to NTN, whereas NTN maintains as many sacral motoneurons as does GDNF. Thus, spinal motoneurons are highly heterogeneous in their expression of receptors for neurotrophic factors of the GDNF family, but their differing responses to NTN are not correlated with expression levels of Gfra1 or Gfra2.

GFRalpha 1 is required for development of distinct subpopulations of motoneuron

Glial cell-line derived neurotrophic factor (GDNF) and its relative neurturin (NTN) are potent trophic factors for motoneurons. They exert their biological effects by activating the RET tyrosine kinase in the presence of a glycosyl-phosphatidylinositol-linked co-receptor, either GFRalpha1 or GFRalpha2. By whole-mount in situ hybridization on embryonic mouse spinal cord, we demonstrate that whereas Ret is expressed by nearly all motoneurons, Gfra1 and Gfra2 exhibit complex and distinct patterns of expression. Most motoneurons purified from Gfra1 null mutant mice had lost their responsiveness to both GDNF and NTN. However, a minority of them ( approximately 25%) retained their ability to respond to both factors, perhaps because they express GFRalpha2. Surprisingly, Gfra2(-/-) motoneurons showed normal survival responses to both GDNF and NTN. Thus, GFRalpha1, but not GFRalpha2, is absolutely required for the survival response of a majority of motoneurons to both GDNF and NTN. In accordance with the phenotype of the mutant motoneurons observed in culture we found the loss of distinct groups of motoneurons, identified by several markers, in the Gfra1(-/-) spinal cords but no gross defects in the Gfra2(-/-) mutant. During their natural programmed cell death period, motoneurons in the Gfra1(-/-) mutant mice undertook increased apoptosis. Taken together these findings support the existence of subpopulations of motoneuron with different trophic requirements, some of them being dependent on the GDNF family.

The branchial arches and HGF are growth-promoting and chemoattractant for cranial motor axons

During development, cranial motor neurons extend their axons along distinct pathways into the periphery. For example, branchiomotor axons extend dorsally to leave the hindbrain via large dorsal exit points. They then grow in association with sensory ganglia, to their targets, the muscles of the branchial arches. We have investigated the possibility that pathway tissues might secrete diffusible chemorepellents or chemoattractants that guide cranial motor axons, using co-cultures in collagen gels. We found that explants of dorsal neural tube or hindbrain roof plate chemorepelled cranial motor axons, while explants of cranial sensory ganglia were weakly chemoattractive. Explants of branchial arch mesenchyme were strongly growth-promoting and chemoattractive for cranial motor axons. Enhanced and oriented axon outgrowth was also elicited by beads loaded with Hepatocyte Growth Factor (HGF); antibodies to this protein largely blocked the outgrowth and orientation effects of the branchial arch on motor axons. HGF was expressed in the branchial arches, whilst Met, which encodes an HGF receptor, was expressed by subpopulations of cranial motor neurons. Mice with targetted disruptions of HGF or Met showed defects in the navigation of hypoglossal motor axons into the branchial region. Branchial arch tissue may thus act as a target-derived factor that guides motor axons during development. This influence is likely to be mediated partly by Hepatocyte Growth Factor, although a component of branchial arch-mediated growth promotion and chemoattraction was not blocked by anti-HGF antibodies.

A dynamic regulation of GDNF-family receptors correlates with a specific trophic dependency of cranial motor neuron subpopulations during development

Glial cell line-derived neurotrophic factor (GDNF) family ligands promote the survival of developing motor neurons in vivo and in vitro. However, not all neurons survive with any single ligand in culture and GDNF null mutant mice display only a partial motor neuron loss. An interesting possibility is that subpopulations of motor neurons based on their function and/or their myotopic organization require distinct members of GDNF family ligands. Because responsiveness to the different ligands depends on the expression of their cognate ligand-binding receptor we have herein addressed this issue by examining the expression of GDNF-family receptors (gfr) during development and in the adult in cranial motor nuclei subpopulations. We have furthermore examined the in vivo role of GDNF for cranial motor neuron subpopulations. The shared ret receptor was expressed in all somatic, branchial and visceral cranial embryonic motor nuclei examined, showing that they are all competent to respond to GDNF family ligands during development. At early stages of development both the GDNF receptor, gfralpha1, and the neurturin (NTN) receptor, gfralpha2, were expressed in the oculomotor, facial and spinal accessory, and only gfralpha1 in the trochlear, superior salivatory, trigeminal, hypoglossal and weakly in the dorsal motor nucleus of the vagus and the ambiguous nucleus. The abducens nucleus was negative for both gfralpha1 and gfralpha2. The artemin (ART) receptor, gfralpha3, was expressed only in the superior salivatory nucleus. A motor neuron subnuclei-specific expression of gfralpha1 and gfralpha2 was seen in the facial and trigeminal nuclei which corresponded to their dependence on GDNF in null mutant mice. We found that the expression was dynamic in these nuclei, which may reflect developmental changes in their trophic factor dependency. Analysis of GDNF null mutant mice revealed that the dynamic receptor expression is regulated by the ligand in vivo, indicating that the attainment of changes in dependency could be ligand induced. Our results indicate that specific GDNF family ligands support selective muscle-motor neuron circuits during development.

Role of neurotrophic factors in motoneuron development

More than 10 factors from different gene families are now known to enhance motoneuron survival, and to be expressed in a manner consistent with a role in regulating motoneuron numbers during development. We provide evidence that: a) different factors may act on different sub-populations of motoneurons; b) different factors may act in synergy on a given motoneuron. Thus, the functional diversity of motoneurons, and the cellular complexity of their environment, may be reflected in the mechanisms that have evolved to keep them alive.

Hepatocyte growth factor (HGF/SF) is a muscle-derived survival factor for a subpopulation of embryonic motoneurons

Muscle-derived factors are known to be important for the survival of developing spinal motoneurons, but the molecules involved have not been characterized. Hepatocyte growth factor/scatter factor (HGF/SF) plays an important role in muscle development and motoneuron axon outgrowth. We show that HGF/SF has potent neurotrophic activity (EC50=2 pM) for a subpopulation (40%) of purified embryonic rat motoneurons. Moreover, HGF/SF is an essential component of muscle-derived support for motoneurons, since blocking antibodies to HGF/SF specifically inhibited 65% of the trophic activity of media conditioned by C2/C7 skeletal myotubes, but did not inhibit the trophic activity secreted by Schwann cell lines. High levels of expression of the HGF/SF receptor c-Met in the spinal cord are restricted to subsets of motoneurons, mainly in limb-innervating segments. Consistent with this distribution, cultured motoneurons from limb-innervating brachial and lumbar segments showed a more potent response to HGF/SF than did thoracic motoneurons. By the end of the period of motoneuron cell death, levels of c-Met mRNA in motoneurons were markedly reduced, suggesting that the effects of HGF/SF may be limited to the period of motoneuron cell death. HGF/SF may play an important role during motoneuron development as a muscle-derived survival factor for a subpopulation of limb-innervating motoneurons.