Microtubules are a primary cytoskeletal constituent of axons and growth cones. In addition to serving as a scaffolding for axon assembly, they also provide a means of transport of organelles that are essential for outgrowth and maintenance of synaptic function. Pharmacological manipulations that disrupt net assembly of microtubules also interfere with growth cone advance and axon extension. Less is known after the effects of disrupting microtubule dynamics without affecting net assembly. To investigate this, we studied the effects of low doses of nocodazole on axon extension and microtubule organization in rat superior cervical ganglion neurons. We report that 165-330 nM nocodazole significantly reduces axon extension rate and increases axon diameter without affecting the rate of production of axoplasm or microtubule polymer, and without decreasing the average length or number of microtubules. Two observations suggested that microtubule dynamics were depressed by this dose of nocodazole. First, this treatment eliminated the highly divergent lengths and positions of microtubules characteristic of normal growth cones, inducing an array in which each microtubule terminated at roughly the same position in the proximal regions of the growth cone. Second, there was a decrease in the proportion of microtubule length containing mostly tyrosinated (newly assembled) alpha-tubulin and an increase in the proportion of microtubule length containing mostly acetylated (older, more stable) alpha-tubulin. Together, these data suggest that a decrease in dynamic instability of microtubules is sufficient to disrupt axon extension but does not interfere with axoplasm production.

We identify an actin-based protrusive structure in growth cones termed "intrapodium." Unlike filopodia, intrapodia are initiated exclusively within lamellipodia and elongate in a continuous (nonsaltatory) manner parallel to the plane of the dorsal plasma membrane causing a ridge-like protrusion. Intrapodia resemble the actin-rich structures induced by intracellular pathogens (e.g., Listeria) or by extracellular beads. Cytochalasin B inhibits intrapodial elongation and removal of cytochalasin B produced a burst of intrapodial activity. Electron microscopic studies revealed that lamellipodial intrapodia contain both short and long actin filaments oriented with their barbed ends toward the membrane surface or advancing end. Our data suggest an interaction between microtubule endings and intrapodia formation. Disruption of microtubules by acute nocodazole treatment decreased intrapodia frequency, and washout of nocodazole or addition of the microtubule-stabilizing drug Taxol caused a burst of intrapodia formation. Furthermore, individual microtubule ends were found near intrapodia initiation sites. Thus, microtubule ends or associated structures may regulate these actin-dependent structures. We propose that intrapodia are the consequence of an early step in a cascade of events that leads to the development of F-actin-associated plasma membrane specializations.

The trigeminal ganglion provides the somatosensory innervation for the anterior rat tongue. At early embryonic stages (embryonic day [E] 12-13) pre-tongue explants repel trigeminal axon outgrowth, and this is mediated by Sema3A (Rochlin and Farbman [1998] J. Neurosci. 18:6840-6852; Rochlin et al. [2000] J. Comp. Neurol. 422:579-593). Despite a decrease in repulsion by E14 and older tongue explants, Sema3A mRNA persists throughout the dorsal epithelium through E18, after axons have begun to penetrate papilla epithelium. We investigated the hypothesis that Sema3A continues to act as a repellent and that subpopulations of trigeminal axons that penetrate the epithelium become unresponsive to Sema3A. Sema3A repelled trigeminal axons in vitro regardless of the neurotrophic factor used to stimulate axon outgrowth, but the minimum level of Sema3A required to repel depended on the neurotrophic factor. Thus, in vitro, trigeminal axons are repelled by Sema3A when they would be penetrating the Sema3A-mRNA rich epithelium in vivo. Whereas dorsal epithelium on tongue explants dissected at stages preceding target contact (E15) repelled trigeminal axons in vitro, explants dissected at later stages (E18), after axons would have penetrated the epithelium in vivo, were not repellent. To determine whether Sema3A prevents premature target penetration in vivo, we assessed the timing of target contact by sensory axons in Sema3A-/minus; and +/+ mice. Contact of the epithelium occurs prematurely in Sema3A-/minus; mice, but not penetration. Taken together, our data imply that Sema3A acts as a short-range repellent that regulates the timing of target contact by 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.

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.