Pathfinding by Identified Zebrafish Motoneurons in the Absence of Muscle Pioneers

Abstract

To identify the cellular cues that guide zebrafish neuronal growth cones to their targets, we examined interactions between identified motor growth cones and identified muscle fibers and tested whether these fibers were required for growth cone navigation. Caudal primary motoneurons (CaPs) and middle primary motoneurons (MiPs) are identified motoneurons that innervate cell-specific regions of the myotome. Growth cones of both cells initially extend along a common pathway and then pause at a set of identified muscle fibers, called muscle pioneers, before diverging along cell-specific pathways. Muscle pioneers are intermediate targets of both CaP and MiP Liu and Westerfield, 1990); both motoneurons extend their growth cones directly to the muscle pioneers on which the first functional neuromuscular contacts form, suggesting that muscle pioneers may provide guidance information to these growth cones. We tested this idea by ablating muscle pioneers and observing the resulting motor axonal trajectories. Both CaP and MiP ultimately formed normal axonal arbors after muscle pioneer ablation, showing that muscle pioneers are unnecessary for formation of correct axonal trajectories; however, although final cellular morphology was correct in the absence of muscle pioneers, MiP growth cones branched abnormally or extended ventrally beyond the common pathway. Ablation of CaP and the muscle pioneers together increased the aberrant behavior of the MiP growth cone. Our results provide evidence that an intermediate target, the muscle pioneers, affects motor axonal extension without altering target choice, suggesting that other cues also contribute to proper pathway navigation. Key words: acetylcholine receptors; neuromuscular junctions; axogenesis; zebrafish motoneurons; muscle pioneers; pathway navigationThe environment through which growth cones navigate contains cues that regulate neuronal pathfinding (Frank and Wenner, 1993;Goodman and Shatz, 1993;Keynes and Cook, 1995). En route to their synaptic targets, neurons may project axons to intermediate targets that influence subsequent pathway choice . For example, some grasshopper pioneer neurons project to guidepost cells (Bentley and Keshishian, 1982) that are required for normal pathfinding (Bentley and Caudy, 1983). Chick hindlimb motoneurons (Lance-Jones and Landmesser, 1981; Tosney and Landmesser, 1985a,b;Landmesser, 1992) and spinal commissural neurons (Bovolenta and Dodd, 1991) also project to specific intermediate targets, which appear to influence extension. These results suggest that intermediate targets serve as choice points for pathway selection.We examined the role of potential intermediate targets in the pathway choices of zebrafish primary motoneurons. We focused on the first two primary motoneurons to extend growth cones out of the spinal cord: C aP and MiP (see Fig. 1) Myers et al., 1986). The C aP growth cone pioneers a common pathway to the nascent horizontal myoseptum, where it pauses before selecting its cell-specific pathway along ventral myotome Myers e...

A functional analysis of myotomal muscle-fibre reorientation in developing zebrafishDanio rerio

“…At 18·h.p.f., the time of first movements, still only a medial portion of the somite contains fibres with an orientation in a preferential direction, with all fibres oriented more or less parallel to the notochord (Fig.·7C). These muscle pioneer cells perform the initial contractions (Melancon et al, 1997). More laterally located muscle fibres do not show a preferential direction at this stage (Fig.·7D).…”

Nicotinic Receptors Mediate Changes in Spinal Motoneuron Development and Axonal Pathfinding in Embryonic Zebrafish Exposed to Nicotine

“…rons Melancon et al, 1997). Taking advantage of the recently well characterized secondary motoneuron-specific islet-1 promoter-driven GFP transgenic fish, we show that nicotine affects secondary motoneuron development in zebrafish.…”

Visualization, characterization and modulation of calcium signaling during the development of slow muscle cells in intact zebrafish embryos

“…It has been previously reported that in zebrafish embryos, the trunk muscles start to show spontaneous contractile activity at 17 hpf and these movements are generated by acetylcholine (ACh) release from motor nerve terminals (Grunwald et al, 1988;Melançon et al, 1997). When acetylcholine binds to nicotinic acetylcholine receptors (nAChRs) expressed by the muscle cells, this binding is reported to activate dihydropyridine receptors (DHPRs) located on the surface membrane or on T-tubules, which in turn triggers the release of Ca 2+ from intracellular stores via RyRs, and hence contraction occurs (Schneider and Chandler, 1973).…”