Local endocytosis triggers dendritic thinning and pruning in Drosophila sensory neurons.

Takahiro Kanamori,Kazuo Emoto,Yusuke Dairyo,Jiro Yoshino,Kei-Ichiro Yasunaga

doi:10.1038/ncomms7515

Abstract

The refinement of neural circuits involves dendrite pruning, a process that removes inappropriate projections that are formed during development. In Drosophila sensory neurons, compartmentalized calcium (Ca(2+)) transients in dendrites act as spatiotemporal cues to trigger pruning, yet how neurons define the dendrites with Ca(2+) transients remains elusive. Here we report that local elevation of endocytic activity contributes to defining dendrites that generate Ca(2+) transients, triggering pruning. In vivo imaging of single dendrites reveals an increase of endocytosis in proximal dendrites that spatially and temporally correlates with dendrite thinning, an early step in pruning tightly coupled with compartmentalized Ca(2+) transients. Two GTPases, Rab5 and dynamin, are required for both the increased endocytic activity and compartmentalized Ca(2+) transients. Further genetic analyses suggest that local endocytosis in proximal dendrites functions cooperatively with global endocytosis-mediated protein degradation pathways to promote dendrite pruning.

Highlights

The refinement of neural circuits involves dendrite pruning, a process that removes inappropriate projections that are formed during development
We reported that Ca2 þ transients occur in a compartmentalized manner within dendritic branches during early metamorphosis and act as a trigger for dendrite pruning in Drosophila C4da sensory neurons[19]
Fluorescence recovery after photobleaching (FRAP) experiments demonstrated that protein diffusion (green fluorescent protein (GFP) in this case) was inhibited from the branch unit into non-branch units (Supplementary Fig. 2). These data indicate that dendrite thinning provides bidirectional diffusion barriers for cytosolic components at the boundaries between branch units and nonbranch units, and raise a possibility that dendrite thinning might contribute to compartmentalization of dendritic Ca2 þ transients (Fig. 1g). To further test this possibility, we explored the relationship between dendrite thinning and elevation of branch excitability, the first sign of compartmentalization of pruning dendrites, which is detected before the initiation of Ca2 þ transients and presumably facilitates compartmentalized Ca2 þ transients via the voltage-gated calcium channels (VGCCs)[19]

Summary

Introduction

The refinement of neural circuits involves dendrite pruning, a process that removes inappropriate projections that are formed during development. Spontaneous Ca2 þ transients commence in a compartmentalized manner in groups of C4da dendrites before pruning in early metamorphosis, and dendrites are eventually pruned away in the temporal order in which they experience the Ca2 þ transients, in part through the activation of the Ca2 þ dependent protease calpains. These data demonstrate that the compartmentalized Ca2 þ transients provide spatial and temporal information about dendrite pruning. We further provide genetic evidence that the local endocytosis in the proximal dendrites functions cooperatively with the global endocytosis-mediated protein degradation mechanism to efficiently promote dendrite pruning

Methods

Results

Conclusion