Comparison of neurotrophin and repellent sensitivities of early embryonic geniculate and trigeminal axons

Geniculate (gustatory) and trigeminal (somatosensory) afferents take different routes to the tongue during rat embryonic development. To learn more about the mechanisms controlling neurite outgrowth and axon guidance, we are studying the roles of diffusible factors. We previously profiled the in vitro sensitivity of trigeminal axons to neurotrophins and target-derived diffusible factors and now report on these properties for geniculate axons. GDNF, BDNF, and NT-4, but not NT-3 or NGF, stimulate geniculate axon outgrowth during the ages investigated, embryonic days 12-14. Sensitivity to effective neurotrophins is developmentally regulated and different from that of the trigeminal ganglion. In vitro coculture studies revealed that geniculate axons were repelled by branchial arch explants that were previously shown to be repellent to trigeminal axons (Rochlin and Farbman [1998] J Neurosci 18:6840-6852). In addition, some branchial arch explants and untransfected COS7 cells repelled geniculate but not trigeminal axons. Sema3A, a ligand for neuropilin-1, is effective in repelling geniculate and trigeminal axons, and antineuropilin-1, but not antineuropilin-2, completely blocks the repulsion by arch explants that repel axon outgrowth from both ganglia. Sema3A mRNA is concentrated in branchial arch epithelium at the appropriate time to mediate the repulsion. In Sema3A knockout mice, geniculate and trigeminal afferents explore medial regions of the immature tongue and surrounding territories not explored in heterozygotes, supporting our previous hypothesis that Sema3A-based repulsion mediates the early restriction of sensory afferents away from midline structures

Transient downregulation of Sema3A mRNA in a rat model for temporal lobe epilepsy - A novel molecular event potentially contributing to mossy fiber sprouting

Mossy fiber sprouting (MFS) in the hippocampal dentate gyrus is thought to play a critical role in the hyperexcitability of the hippocampus in temporal lobe epilepsy patients. The composition of molecular signals that is needed to direct this sprouting response has not yet been elucidated to a great extent. In the present study we investigated the expression profile of Sema3A mRNA and the axonal growth-associated protein GAP-43 mRNA during the process of electrically induced epileptogenesis in rats. Sema3A is an axon guidance molecule with repellent activity on dentate granule cell axons. It is produced by neurons in the entorhinal cortex, which synapse on the dendrites of dentate granule cells. Upregulation of GAP-43 expression in granule cells has often been reported in conjunction with MFS. After induction of status epilepticus, the expression of Sema3A mRNA was temporarily downregulated in the entorhinal cortex concomitantly with an upregulation of GAP-43 mRNA in dentate granule cells. In the following days, robust MFS into the dentate molecular layer was observed. When the induction of status epilepticus was incomplete the two responses appeared to dissociate, i.e., the downregulation of Sema3A mRNA did not occur, while upregulation of GAP-43 mRNA in dentate granule cells was still displayed. However, in these rats no significant MFS was observed. These findings indicate that Sema3A mRNA downregulation is temporarily correlated with MFS, while GAP-43 upregulation per se is not, and suggest that a loss of Sema3A in the molecular layer of the dentate gyrus could facilitate MFS into this area during epilepsy. © 2003 Elsevier Science (USA). All rights reserved

Adenoviral vector-mediated expression of B-50/GAP-43 induces alterations in the membrane organization of olfactory axon terminals in vivo

B-50/GAP-43 is an intraneuronal membrane-associated growth cone protein with an important role in axonal growth and regeneration. By using adenoviral vector-directed expression of B-50/GAP-43 we studied the morphogenic action of B-50/GAP-43 in mature primary olfactory neurons that have established functional synaptic connections. B-50/GAP-43 induced gradual alterations in the morphology of olfactory synapses. In the first days after overexpression, small protrusions originating from the preterminal axon shaft and from the actual synaptic bouton were formed. With time the progressive formation of multiple ultraterminal branches resulted in axonal labyrinths composed of tightly packed sheaths of neuronal membrane. Thus, B-50/GAP-43 is a protein that can promote neuronal membrane expansion at synaptic boutons. This function of B-50/GAP-43 suggests that this protein may subserve an important role in ongoing structural synaptic plasticity in adult neurons and in neuronal membrane repair after injury to synaptic fields

Injury-induced class 3 semaphorin expression in the rat spinal cord

In this study we evaluate the expression of all members of the class 3 semaphorins and their receptor components following complete transection and contusion lesions of the adult rat spinal cord. Following both types of lesions the expression of all class 3 semaphorins is induced in fibroblast in the neural scar. The distribution of semaphorin-positive fibroblasts differs markedly in scars formed after transection or contusion lesion. In contusion lesions semaphorin expression is restricted to fibroblasts of the meningeal sheet surrounding the lesion, while after transection semaphorin-positive fibroblast penetrate deep into the center of the lesion. Two major descending spinal cord motor pathways, the cortico- and rubrospinal tract, continue to express receptor components for class 3 semaphorins following injury, rendering them potentially sensitive to scar-derived semaphorins. In line with this we observed that most descending spinal cord fibers were not able to penetrate the semaphorin positive portion of the neural scar formed at the lesion site. These results suggest that the full range of secreted semaphorins contributes to the inhibitory nature of the neural scar and thereby may inhibit successful regeneration in the injured spinal cord. Future studies will focus on the neutralization of class 3 semaphorins, in order to reveal whether this creates a more permissive environment for regeneration of injured spinal cord axons