In Drosophila, ddaC sensory neurons have emerged as an attractive model system with which to elucidate the molecular and cellular mechanisms of dendrite-specific pruning during early metamorphosis. Thus, a complete understanding of cellular and molecular mechanisms of developmental pruning would shed some light on pathological neurodegeneration following neurological diseases and injury. The pruning event involves both local degeneration and retraction ( Luo and O'Leary, 2005), resembling neurodegeneration associated with brain injury and neurodegenerative diseases. In the peripheral nervous system (PNS), some dorsal dendritic arborization (da) neurons, ddaC, ddaD and ddaE, selectively eliminate their larval dendrites without affecting their axons ( Kuo et al., 2005 Williams and Truman, 2005), whereas ddaF neurons are apoptotic during early metamorphosis ( Williams and Truman, 2005). In the central nervous system (CNS), mushroom body (MB) γ neurons prune their dorsal and medial axon branches as well as entire dendrites ( Lee et al., 1999). In invertebrates, such as Drosophila, the nervous systems undergo drastic remodeling during metamorphosis, a transition stage from a larva to an adult fly ( Kanamori et al., 2015a Truman, 1990 Yu and Schuldiner, 2014). In vertebrates, many neurons in the neocortex, neuromuscular system and hippocampal dentate gyrus prune their unwanted neurites to control the proper wiring of the nervous systems ( Bagri et al., 2003 O'Leary and Koester, 1993 Tapia et al., 2012). Neuronal pruning is a conserved process widely occurring in both vertebrates and invertebrates. Subsequent removal of those exuberant or inappropriate neurites without causing the death of parental neurons, a process known as pruning, is crucial for the refinement of neural circuits at late developmental stages ( Luo and O'Leary, 2005 Riccomagno and Kolodkin, 2015 Schuldiner and Yaron, 2015). In the developing nervous systems, neurons often extend excessive neurites and form superfluous connections at early stages. Finally, we show that the Sec71/Arf1-mediated trafficking process is a prerequisite for Rab5-dependent endocytosis to facilitate endocytosis and degradation of the cell-adhesion molecule Neuroglian (Nrg). Arf1 and Sec71 are interdependent for their localizations on Golgi. Like Arf1, Sec71 is also important for dendrite pruning, but not for apoptosis, of sensory neurons. We further identified Sec71 as a guanine nucleotide exchange factor for Arf1 that preferentially interacts with its GDP-bound form. Analyses of the GTP- and GDP-locked forms of Arf1 indicate that the cycling of Arf1 between GDP-bound and GTP-bound forms is essential for dendrite pruning. Here, we show that the small GTPase Arf1, an important regulator of the secretory pathway, is specifically required for dendrite pruning of ddaC/D/E sensory neurons but dispensable for apoptosis of ddaF neurons. However, it is unknown whether the secretory pathway plays a role in dendrite pruning. In Drosophila, sensory neurons, ddaCs, selectively prune their larval dendrites without affecting their axons during metamorphosis. Pruning, whereby neurons eliminate their excess neurites, is central for the maturation of the nervous system.
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