Axon fasciculation and differences in midline kinetics between pioneer and follower axons within commissural fascicles

Abstract

Early neuronal scaffold development studies suggest that initial neurons and their axons serve as guides for later neurons and their processes. Although this arrangement might aid axon navigation, the specific consequence(s) of such interactions are unknown in vivo. We follow forebrain commissure formation in living zebrafish embryos using timelapse fluorescence microscopy to examine quantitatively commissural axon kinetics at the midline:a place where axon interactions might be important. Although it is commonly accepted that commissural axons slow down at the midline, our data show this is only true for leader axons. Follower axons do not show this behavior. However, when the leading axon is ablated, follower axons change their midline kinetics and behave as leaders. Similarly, contralateral leader axons change their midline kinetics when they grow along the opposite leading axon across the midline. These data suggest a simple model where the level of growth cone exposure to midline cues and presence of other axons as a substrate shape the midline kinetics of commissural axons.

The Influence of Pioneer Neurons on a Growing Motor Nerve inDrosophilaRequires the Neural Cell Adhesion Molecule Homolog FasciclinII

“…However, cell ablations also remove cues displayed by the ablated pioneer neurons that are likely to influence the growth cones of followers. Consistent with this idea, filopodia of follower growth cones have in some contexts been shown to be closely attached to pioneer axons but changed shape after pioneer ablation (Kim et al, 1991;Bak and Fraser, 2003). Which are the molecules required for such influence on follower growth cones?…”

“…Experiments testing the requirement of pioneer neurons for correct growth of follower neurons led to three different scenarios: (1) pioneer neurons appear unnecessary for guidance of later outgrowing neurons (Keshishian and Bentley, 1983;Eisen et al, 1990); (2) pioneer neurons facilitate but are unnecessary for pathfinding by later outgrowing neurons (Pike et al, 1992;Lin et al, 1995;Bak and Fraser, 2003); (3) pioneer neurons appear to be absolutely required for normal pathfinding by later outgrowing neurons (Raper et al, 1984;Kuwada, 1986;Klose and Bentley, 1989;Ghosh et al, 1990;Pike et al, 1992;Hidalgo and Brand, 1997;Williams and Shepherd, 2002). These observations were mostly based on studies of follower neurons under conditions in which their pioneer neurons had been ablated.…”

Using a histone yellow fluorescent protein fusion for tagging and tracking endothelial cells in ES cells and mice

“…For example, endothelial-specific green fluorescent protein (GFP) reporters have proven useful for noninvasive observation of vascular development in zebrafish (Isogai et al, 2003;Lawson and Weinstein, 2002;Motoike et al, 2000). The uniform expression of GFP within the endothelial cells of these animals limits their utility in following cell movements: the position of a given cell can be tracked in three dimensions (3D) over time (i.e., in 4D) only if it is present among a group of nonexpressing cells, as in a mosaic or mixed-lineage population (e.g., see Anderson et al, 2000;Bak and Fraser, 2003;Hadjantonakis et al, 2001;Srinivas et al, 2004;Tam and Rossant, 2003). Ultimately, it will be desirable to construct anatomical models for analysis of normal, mutant, and pathological vascular development.…”

Differences in protein mobility between pioneer versus follower growth cones

“…Wiring this initial network of nerve cells involves guided extension of axons, led by their navigating tips called growth cones, toward their final targets (4). In many cases, leader growth cones (pioneers) trail-blaze the initial paths actively sensing guidance cues as they grow; follower axons largely progress along the leaders' tracks, simplifying their navigation challenges to more constrained explorations of their local environments (5). The different navigation schemes used by pioneers and followers are reflected in the differences in their external morphologies and behavior (5)(6)(7)(8)(9).…”

“…In many cases, leader growth cones (pioneers) trail-blaze the initial paths actively sensing guidance cues as they grow; follower axons largely progress along the leaders' tracks, simplifying their navigation challenges to more constrained explorations of their local environments (5). The different navigation schemes used by pioneers and followers are reflected in the differences in their external morphologies and behavior (5)(6)(7)(8)(9). However, we do not know whether similar differences might also exist inside the growth cones, reflected in differences in protein mobility.…”