Microtubules in larval c4da neuron dendrites are oriented uniformly with their plus ends facing the soma (‘plus-end-in’ orientation) ( Rolls et al., 2007 Stone et al., 2008). This nomenclature reflects the fact that the rates of both growth and shrinkage are greater at the plus ends.
Microtubules are polar rods with so-called ‘plus’ and ‘minus’ ends. Furthermore, dendrite pruning also requires the microtubule-severing enzyme Katanin p60-like 1 (Kat-60L1) ( Lee et al., 2009), possibly also downstream of PAR-1 and Tau ( Herzmann et al., 2017). Microtubule disassembly requires the kinase PAR-1, which mediates inhibition of the microtubule-associated protein Tau ( Herzmann et al., 2017). Microtubule disassembly is therefore the earliest known local sign of dendrite pruning. Live imaging and genetic data suggest that local microtubule breakdown precedes membrane thinning ( Herzmann et al., 2017 Kanamori et al., 2015). Plasma membrane retrieval through increased local endocytosis also contributes to thinning of proximal dendrites ( Kanamori et al., 2015). In these regions, microtubules are disassembled locally ( Lee et al., 2009 Williams and Truman, 2005). First signs of dendrite pruning are visible at 2-3 h APF, when proximal dendrites take on an irregular appearance with beadings and thinnings. Severed dendrites are then fragmented and phagocytosed by the epidermal cells surrounding them ( Han et al., 2014). Dendrites are first severed at proximal sites close to the cell body between 5 and 10 h after puparium formation (APF). Dendrite pruning is induced in a cell-autonomous fashion by the steroid hormone ecdysone and proceeds in a stereotypical fashion ( Kuo et al., 2005 Williams and Truman, 2005). The sensory class IV dendritic arborization (c4da) neurons completely and specifically prune their long and branched larval dendrites at the onset of the pupal phase by a degenerative mechanism, while their axons remain intact ( Kuo et al., 2005). In the peripheral nervous system (PNS) of Drosophila, several types of sensory neurons undergo either apoptosis or prune their larval processes in an ecdysone-dependent manner. In holometabolous insects, the nervous system is remodeled on a large scale during metamorphosis. Local microtubule organization may therefore contribute spatial information for severing sites during dendrite pruning. Our data suggest that the local microtubule organization at branchpoints determines where microtubule breakdown occurs. Mutants in kinesin-1 and -2, which are required for uniform microtubule orientation, show defects in microtubule breakdown and dendrite pruning. In fly dendrites, microtubules are arranged in uniformly oriented arrays where all plus ends face towards the soma. Live imaging of fluorescently tagged tubulin shows that microtubule breakdown first occurs at proximal dendritic branchpoints, followed by breakdown at more distal branchpoints, suggesting that the process is triggered by a signal emanating from the soma. Here, we investigated spatial aspects of microtubule breakdown during dendrite pruning. Dendrite severing is preceded by local breakdown of dendritic microtubules through PAR-1-mediated inhibition of Tau. Pruning dendrites are severed in their proximal regions, but how this spatial information is encoded is not clear.
Drosophila sensory neurons prune their larval dendrites during metamorphosis. Large-scale neurite pruning is an important specificity mechanism during neuronal morphogenesis.
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