Axonal branch shapes

Transduction of Craniofacial Motoneurons Following Intramuscular Injections of Canine Adenovirus Type-2 (CAV-2) in Rhesus Macaques

Reliable viral vector-mediated transgene expression in primate motoneurons would improve our ability to anatomically and physiologically interrogate motor systems. We therefore investigated the efficacy of replication defective, early region 1-deleted canine adenovirus type-2 (CAV-2) vectors for mediating transgene expression of fluorescent proteins into brainstem motoneurons following craniofacial intramuscular injections in four rhesus monkeys (Macaca mulatta). Vector injections were placed into surgically identified and isolated craniofacial muscles. After a 1- to 2-month survival time, animals were sacrificed and transgene expression was assessed with immunohistochemistry in the corresponding motoneuronal populations. We found that injections of CAV-2 into individual craniofacial muscles at doses in the range of ∼10¹⁰ to 10¹¹ physical particles/muscle resulted in robust motoneuronal transduction and expression of immunohistochemically identified fluorescent proteins across multiple animals. By using different titers in separate muscles, with the resulting transduction patterns tracked via fluorophore expression and labeled motoneuron location, we established qualitative dose-response relationships in two animals. In one animal that received an atypically high titer (5.7 × 10¹¹ total CAV-2 physical particles) distributed across numerous injection sites, no transduction was detected, likely due to a retaliatory immune response. We conclude that CAV-2 vectors show promise for genetic modification of primate motoneurons following craniofacial intramuscular injections. Our findings warrant focused attention toward the use of CAV-2 vectors to deliver opsins, DREADDs, and other molecular probes to improve genetics-based methods for primate research. Further work is required to optimize CAV-2 transduction parameters. CAV-2 vectors encoding proteins could provide a new, reliable route for modifying activity in targeted neuronal populations of the primate central nervous system.

Developmental regulation of axon branching in the vertebrate nervous system

During nervous system development, axons generate branches to connect with multiple synaptic targets. As with axon growth and guidance, axon branching is tightly controlled in order to establish functional neural circuits, yet the mechanisms that regulate this important process are less well understood. Here, we review recent advances in the study of several common branching processes in the vertebrate nervous system. By focusing on each step in these processes we illustrate how different types of branching are regulated by extracellular cues and neural activity, and highlight some common principles that underlie the establishment of complex neural circuits in vertebrate development.

Gallium nitride induces neuronal differentiation markers in neural stem/precursor cells derived from rat cerebral cortex

In the present study, gallium nitride (GaN) was used as a substrate to culture neural stem/precursor cells (NSPCs), isolated from embryonic rat cerebral cortex, to examine the effect of GaN on the behavior of NSPCs in the presence of basic fibroblast growth factor (bFGF) in serum-free medium. Morphological studies showed that neurospheres maintained their initial shape and formed many long and thick processes with the fasciculate feature on GaN. Immunocytochemical characterization showed that GaN could induce the differentiation of NSPCs into neurons and astrocytes. Compared to poly-d-lysine (PDL), the most common substrate used for culturing neurons, there was considerable expression of synapsin I for differentiated neurons on GaN, suggesting GaN could induce the differentiation of NSPCs towards the mature differentiated neurons. Western blot analysis showed that the suppression of glycogen synthase kinase-3beta (GSK-3beta) activity was one of the effects of GaN-promoted NSPC differentiation into neurons. Finally, compared to PDL, GaN could significantly improve cell survival to reduce cell death after long-term culture. These results suggest that GaN potentially has a combination of electric characteristics suitable for developing neuron and/or NSPC chip systems.

Analysis of neuritic outgrowth from severed giant axons in Lumbricus terrestris

This study analyzes the detailed morphometric pattern at various postoperative times of neuritic outgrowths from the proximal and distal stumps of two uniquely identifiable axons. Morphological patterns of neuritic outgrowths from stumps of severed axons were compared for medial and lateral giant axons in the central nervous system of the earthworm Lumbricus terrestris. Outgrowths from proximal and distal stumps were labeled by injection of fluorescent dye into axonal stumps and assessed according to morphometric parameters. Outgrowths from axonal stumps of severed giant axons were statistically indistinguishable for most morphometric measures of neuritic quantity, shape, direction, and location. There were two exceptions to this general rule: 1) proximal stumps of medial giant axons produced significantly more neurites than distal stumps of medial giant axons, and 2) proximal stumps of lateral giant axons produced significantly longer neurites than proximal stumps of medial giant axons. No measure of neuritic outgrowth showed a significant change from the second through seventh postoperative week, suggesting that most outgrowth occurred in the first two postoperative weeks and that neuritic morphology remained stable through the seventh postoperative week. Neurites grew across the lesion site in relatively straight trajectories parallel to the longitudinal axis of the ventral nerve cord and often grew alongside the appropriate axonal stump across the lesion site. The length of neurites growing in close apposition to appropriate axonal stumps or giant axons was much greater than expected, had outgrowth been randomly directed. These data provide a basis for future investigations of the mechanisms that regulate neuritic outgrowth.

Quantitative effects of laminin concentration on neurite outgrowth in vitro

Recent studies indicate that mediation of neurite outgrowth by the glycoprotein laminin may be a significant factor in the outgrowth of neurites to their targets during embryogenesis. To further characterize the possible role of this extracellular matrix molecule during development, we have systematically measured several features of outgrowth by neonatal rat sympathetic neurons on different concentrations of laminin. Individual neurons, obtained by mechanical dissociation of superior cervical ganglia (SCG), were cultured at low density on laminin substrates ranging from 0.01 to 1.0 microgram/cm2. Outgrowth characteristics were subsequently analyzed for noninteracting cells in both fixed and live cultures. Data obtained from neurons fixed after 11 hr of culture showed approximately twofold increases in neurite initiation and outgrowth, and a twofold decrease in branching for a corresponding 100-fold increase in adsorbed laminin concentration. In time-lapse videomicroscopy observations, the root-mean square speed of growth cone movement increased from 60 to 90 microns/hr over the same range in concentration, while the persistence time remained constant at 0.10 hr. In general, neurite outgrowth parameters were relatively insensitive to changes in laminin concentration, supporting the idea that laminin is a permissive rather than an "instructive" substrate during development. Data obtained from fixed cultures were examined in terms of probability models to suggest possible mechanisms contributing to the dose-dependent effects observed.

Random-sequence stimulation of the G1 hair afferent unit

Impulse trains were recorded from the parent axon of the cat G1 hair afferent unit. Separate random (Poisson-like) trains of mechanical stimuli were applied to two coinnervated receptive field hairs individually or concurrently. The objective was to determine whether the parent axonal impulse train elicited by dual-hair stimulation was due to a temporal combining ("mixing"; Fukami, 1980) of the impulse trains elicited in the parent axons by the same stimulation to each hair alone. Both impulse rates and patterns were assessed. During single-hair random stimulation, impulse trains differed from stimulus trains, having lower mean rates and short-interval doublets. During dual-hair random stimulation, mean impulse frequencies were on average 36% less than those predicted for mixing. There were no correlations between stimulus amplitude and departures from mixing. As a further test of the mixing hypothesis, the two single-hair-elicited impulse trains were temporally merged (i.e., superimposed to form one impulse train). Such merged impulse trains were compared with the corresponding dual-hair-elicited impulse train. Dual-hair-elicited frequencies were typically less than those of the merged trains, despite the use of an absolute-refractory-period criterion during merging. The impulse patterns elicited by dual-hair stimulation usually differed from the merged-train patterns. Temporal coupling between stimuli and impulses was either variable or absent during single-hair random stimulation; such coupling was altered during dual-hair random stimulation. In summary, this work showed that the dual-hair responses could not be predicted from the single-hair responses. Limitations of the mixing hypothesis and possible biophysical mechanisms in the axonal arborization are discussed. The results are consistent with a general hypothesis of analog processing within the arborization of the parent axon.

Rat mesencephalic neurons in culture exhibit different morphological traits in the presence of media conditioned on mesencephalic or striatal astroglia

Embryonic rat mesencephalic neurons were plated at low density in a chemically defined medium (CDM) or in CDM conditioned on either mesencephalic or striatal astrocytes (CM Gmes and CM Gstr). It was found that "axon-like" neurites, in general long with few branching points, could be initiated in CM Gmes and CM Gstr, whereas "dendrite-like" neurites (shorter and with a high branching capacity) were preferentially initiated in CM Gmes. The effects of CM Gmes and CM Gstr on the morphology of mesencephalic neurons were abolished by protein denaturating treatments. Comparisons with basic FGF, laminin, or fibronectin demonstrated that these three molecules were also able to modify the morphological traits of the neurons. However the different morphologies observed in CM Gmes and CM Gstr could not be explained only by the presence of these proteins in the conditioned media. Our results therefore indicate that different factors may regulate the initiation of different categories of neurites and that in contrast to several molecules able to promote neurite elongation these "initiation" factors may show important regional specificity.

Projections and terminations of single respiratory axons in the cervical spinal cord of cat

Position, divergence, branching, and termination patterns of single, respiratory axons were studied in cat cervical spinal cord by injecting horseradish peroxidase (HRP) intra-axonally. We stained 12 axons which were characterized by their firing patterns and by electrical stimulation. Five axons discharged during inspiration (I); the remaining 7 discharged during expiration (E). No injected axon was evoked by stimulating ipsilateral phrenic nerve roots while 7 (4 I, 3 E) of 12 were excited at a short latency from stimulating at a medullary site (on the midline, 1-2 mm rostral to the obex, approximately 3 mm below the dorsal medullary surface) where many bulbospinal respiratory axons decussate. All injected stem axons were located in the ventral and ventrolateral funiculi, traversed in a rostrocaudal direction, and were stained for lengths ranging from 3.6 to 12.4 mm. Mean axonal diameter was 2.9 microns. In 6 axons (4 I, 2 E), 14 collaterals were stained: 1 on each E axon, 2 on one I axon, 3 each on 2 others and 4 on another I axon. Collaterals emerged perpendicularly from the descending stem axon and projected directly to the ventral horn. The average distance between neighboring collaterals was 1.0 mm (n = 7). Collaterals did not arborize until they were near or within the ventral horn. Both en passant and terminaux types of presynaptic boutons were found primarily within the rostrocaudal cylinder that defined the phrenic motor column. In addition, some boutons were located dorsomedial to the phrenic motor column. We conclude that I axons, presumably of medullary origin, have multiple collaterals which terminate primarily in the phrenic motor column. However, the same axon can have terminals in different regions of the ventral horn, which are known to contain dendrites of phrenic motoneurons.