Axon Branch Removal at Developing Synapses by Axosome Shedding

Abstract

In many parts of the developing nervous system, the number of axonal inputs to each postsynaptic cell is dramatically reduced. This synapse elimination has been extensively studied at the neuromuscular junction, but how axons are lost is unknown. Here, we combine time-lapse imaging of fluorescently labeled axons and serial electron microscopy to show that axons at neuromuscular junctions are removed by an unusual cellular mechanism. As axons disappear, they shed numerous membrane bound remnants. These "axosomes" contain a high density of synaptic organelles and are formed by engulfment of axon tips by Schwann cells. After this engulfment, the axosome's contents mix with the cytoplasm of the glial cell. Axosome shedding might underlie other forms of axon loss and may provide a pathway for interactions between axons and glia.

Guidance Molecules in Axon Pruning and Cell Death

“…There has been no evidence that glial cells are involved in the IPB mossy fiber pruning, but a recent in vitro time-lapse imaging shows that EphB-containing vesicles on the hippocampal neurons are pinched off by neighboring glial cells expressing Ephrin-Bs (Lauterbach and Klein 2006). These results suggest that the EphB/Ephrin-B mediated IPB pruning might be facilitated by glial cells, as has been clearly shown in the small-scale pruning in vertebrates and the pruning in Drosophila (Awasaki and Ito 2004;Bishop et al 2004;Watts et al 2004). In addition, it remains to be clarified whether semaphorin signaling involves similar downstream pathways, and how it coordinates with Ephrin-B/EphB reverse signaling to mediate the stereotyped pruning of the IPB.…”

“…Terminal arbors are removed by axon and synapse elimination, rather than by cell death (Riley 1981;Lichtman and Colman 2000;Sanes and Lichtman 1999). Recent data indicate that these arbors are pruned by axosome shedding (Bishop et al 2004), which is associated with lysosomal activities (Song et al 2008). The pruning of the climbing fibers also involves elimination of competing arbors and synapses (Hashimoto and Kano 2003;Hashimoto et al 2009;Lohof et al 1996;Mason and Gregory 1984).…”

Axotomy or compression is required for axonal sprouting following end-to-side neurorrhaphy

“…The most direct evidence for pruning occurs during development where axons retract from previously innervated motor endplates (Keller-Peck, et al 2001) probably by shedding "axosomes" that are morphologically discrete from the products of Wallerian degeneration (Bishop, et al 2004). Following traumatic injury, motor endplates are initially polyneuronally innervated before reestablishing a mononeuronal relationship (Gillingwater and Ribchester 2003).…”

“…This study suggested that axons could cross intact epineurium and that multiple projections from the same neuron would differentially innervate more than one target (Hess, et al 2005). Developmental studies, however, have shown that although a single neuron can initially extend numerous axonal projections that differentially respond to local guidance cues (Dickson 2002, Liu andHalloran 2005), one-to-one relationships between neurons and motor units are established after synapse elimination (Bishop, et al 2004, ).…”

Multimodal imaging of mouse development: Tools for the postgenomic era

“…As one axon contracted, another expanded to fill in its space. More recent studies have shown that axons are eliminated from the synapse by shedding membrane-bound clusters called axosomes (Bishop et al, 2004).…”