Cellular and molecular mechanisms of dendrite pruning in Drosophila

The nervous system needs to be accurately wired together to function
properly. During development nerve cells (neurons) often overgrow
forming many branches, each of which makes connections with many
different cells. Some of these branches are redundant or inappropriate
and need to be removed for neural circuits to work well. This removal
of branches is termed pruning. We have shown that during metamorphosis
fruit flies prune back the branches of their sensory neurons by a
local degeneration. This pruning looks very similar to degeneration
found in nerves following injury or that seen in diseases like
Alzheimer?s. Circulating blood cells remove branches by engulfing
(eating) them. Our preliminary data shows that the molecular machinery
normally used by a cell to commit ?cellular suicide? (apoptosis) is
activated locally in the branches that need to be removed. During
apoptosis a cell disassembles itself in an orderly manner and signals
to other cells in the body to eat it whole. In pruning neurons this
machinery appears to be tightly regulated to prevent the cell dying.
Blocking the machinery suppresses the removal of the branches,
suggesting that it is important for flagging branches to be eaten by
blood cells. The work described here will establish how the ?apoptotic
machinery? is switched on in these branches and how this tells the
blood cells to eat only those regions of the cell. We hope this will
also tell us something about way which neurons degenerate during
disease, which could ultimately help cure people. We will make our
results available to the public by publishing significant advances on
our website, through press releases and by giving talks.

The precise connectivity between appropriate sets of neurons is essential for the normal functioning of the nervous system. During development neurons often grow exuberantly, making inappropriate branches and synapses that are later pruned back. Large-scale pruning, the removal of relatively long processes, is observed during the maturation of circuits and is believed to take place by local degeneration. At present little is known about the cellular and molecular mechanisms that orchestrate this type of pruning. We have exploited the dendrites of Drosophila dendritic arborizing (da) sensory neurons that remodel during metamorphosis to study large-scale pruning. Pruning of da neurons is hormonally-gated and can be followed live with time-lapse microscopy in whole intact animals. Recently we have found that the apoptotic pathway is locally activated in pruning branches. During the early phase of pruning the cytoskeleton is destabilized as is manifest by the pronounced thinning of proximal dendritic regions and beading in neighbouring regions. Circulating phagocytic blood cells then become involved in the pruning process. We established that phagocytes both cut destabilized branches away from the cell body and remove the remains of the severed branches. Caspase activity appears tightly regulated, spatially and temporally, to prevent the cell dying. Blocking the pathway suppresses the removal of the dendritic arbor. This proposal addresses 3 major questions; which components of the apoptotic pathway are deployed, how the pathway is activated and what the downstream output of this activation is, in the context of a pruning neuron.