蛋白酶體活性與Wnt訊息傳導路徑調控果蠅神經元樹突修剪之研究

Abstract

Neuronal remodeling plays a critical role during development from invertebrates to vertebrates. Pruning, one of the neuronal remodeling mechanisms, is a process to selectively remove specific parts of neurons without causing cell death. In Drosophila, the class IV dendritic arborization (da) neurons, which undergo pruning process to eliminate larval dendrites during metamorphosis, is an ideal model system to study the underlying mechanisms of pruning. Previous studies showed that ubiquitin proteasome system (UPS) initiates the dendrite pruning. To address the dynamics of proteasome activity during pruning process, we generated a photoconvertible reporter system. The decrease of converted fluorescent signals over a period of time could be used to monitor the proteasome activity in vivo. Wingless-Int (Wnt) signaling is crucial for variety of biological events. It could be divided into three major pathways: canonical Wnt pathway, planar cell polarity (PCP) pathway, and calcium pathway. Previous studies reported a trophic role for Wnt-Ror kinase signaling to regulate neuronal pruning in C.elegans, thus highlighted the regulatory roles of Wnt signaling. Here, we identified dishevelled (dsh), a Wnt signaling molecule, as a candidate to regulate Drosophila dendrite pruning. Dsh is a cytoplasmic phosphoprotein, which is involved in both canonical Wnt and PCP pathways. To determine which pathway of Wnt signaling can regulate dendrite pruning, we examined the dendrite pruning of class IV da neurons in various dsh mutant flies, and found the canonical Wnt pathway is associated with dendrite pruning. Next, we identified which Wnt receptors might act upstream of dsh during dendrite pruning. Consistent with the fact that the redundant roles of fz and fz2 in canonical Wnt signaling, both fz and fz2 loss of function mutants showed pruning defects. Furthermore, a co-receptor arrow, which is required for canonical Wnt signaling, also showed some dendrite severing defects when it lost its function. Finally, the canonical Wnt-specific nuclear TCF caused critical severing defects when expressing its N-terminal deletion form. Taken together, these results demonstrated that the canonical Wnt pathway is essential for Drosophila dendrite pruning.