Cellular mechanisms of dendrite pruning inDrosophila: insights from in vivo time-lapse of remodeling dendritic arborizing sensory neurons

Abstract

Regressive events that refine exuberant or inaccurate connections are critical in neuronal development. We used multi-photon, time-lapse imaging to examine how dendrites of Drosophila dendritic arborizing (da) sensory neurons are eliminated during early metamorphosis, and how intrinsic and extrinsic cellular mechanisms control this deconstruction. Removal of the larval dendritic arbor involves two mechanisms: local degeneration and branch retraction. In local degeneration, major branch severing events entail focal disruption of the microtubule cytoskeleton, followed by thinning of the disrupted region, severing and fragmentation. Retraction was observed at distal tips of branches and in proximal stumps after severing events. The pruning program of da neuron dendrites is steroid induced; cell-autonomous dominantnegative inhibition of steroid action blocks local degeneration, although retraction events still occur. Our data suggest that steroid-induced changes in the epidermis may contribute to dendritic retraction. Finally, we find that phagocytic blood cells not only engulf neuronal debris but also attack and sever intact branches that show signs of destabilization.

Drosophila IKK-related kinase Ik2 and Katanin p60-like 1 regulate dendrite pruning of sensory neuron during metamorphosis

“…In addition, marking the microtubules and membranes of ddaC neurons by antibody double staining at 5 h APF, we found that the dendritic membranes were still connected to the soma in some regions of proximal dendrites where the microtubules were already disrupted or absent (Fig. 3 G-I) (12). Thus, the microtubule breakage in the proximal dendrites appears to precede the physical separation of dendrite membrane from the cell body.…”

“…Overexpression of dominant negative ecdysone receptor (EcR-DN) efficiently suppressed dendrite pruning in ddaC neurons (12,13), and eliminated any sign of microtubule disruption in the dendrites at 6 h APF (Fig. 3J).…”

“…Neuronal pruning thus allows the larval neuronal processes and connectivity to be eliminated before the formation of adult-specific branches and connection, in both the central nervous system (CNS) (10,11) and the peripheral nervous system (12,13). Most of the larval peripheral neurons die during metamorphosis, but a subset including some of the class IV dendritic arborization (da) neurons survive through metamorphosis and undergo dendrite pruning, starting with severing of the proximal dendrites and followed with blebbing, and degeneration of disconnected dendrites (12,13). After removal of the severed dendrites by phagocytes (12), 2 of the pruned class IV da neurons regenerate their adult dendrites and 1 disappears presumably via apoptosis (13).…”

“…Ecdysone signaling gives class IV da neurons the competence to commence the whole pruning processes (12,13), executed by the ubiquitin-proteasome system (13), and involving both the matrix metalloproteases and the caspase activity (13,15,16). However, little is known about the underlying molecular mechanisms by which the dendrites of class IV neurons are severed at the early step of dendrite pruning.…”

“…In this study, we show that the Ik2 kinase plays an essential role to initiate dendrite pruning and sever the proximal dendrites of class IV neurons. Whereas microtubule disruption is one of the earliest events observed during neuronal pruning in many systems, including class IV da neurons (2,5,12), the molecular machinery that breaks microtubules is unknown. We found a gene, katanin p60-like 1 (kat-60L1) encoding a protein containing an AAA ATPase domain, essential for microtubule disruption in the proximal dendrites and efficient severing of the dendrites.…”