Independent Functions Of Clueless In Differential Integrin Secretion And Mitochondrial Quality Control In Drosophila Muscle
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Drosophila Clueless (Clu) and its conserved orthologs are known for their role in the prevention of mitochondrial clustering. Here, we uncover its independent roles in the delivery of integrin subunits and the regulation of mitochondrial quality control (MQC) in muscles. In clu mutants, αPS2, but not βPS, integrin abnormally accumulates in the perinuclear endoplasmic reticulum (ER), phenocopying depletion of the Drosophila Golgi reassembly and stacking protein GRASP55/65 (dGRASP). Clu binds dGRASP and regulates accumulation of dGRASP at ER exit sites (ERES). Loss of Clu leads to increased ER stress and destabilized ERES protein Sec16, which, together with defects in αPS2 delivery, can be restored by administration of chemical chaperones. Thus, Clu together with dGRASP prevents ER stress and therefore maintains Sec16 stability essential for the functional organization of perinuclear ERES, required by normal αPS2 integrin transport. Significantly, this role of Clu is separable from its mitochondrial function. PINK1/Parkin-mediated MQC requires valosin-containing protein (VCP)-dependent Mitofusin/Marf proteasomal degradation to prevent damaged mitochondria from fusing with healthy ones, facilitating their clearance by autophagy (mitophagy). clu was found to genetically interact with PINK1 and parkin to regulate mitochondrial clustering in germ cells. However, whether Clu acts in MQC remains unknown. Here, we show that Clu is downstream of PINK1, but not Parkin. Loss of clu leads to depolarized swollen mitochondria and mitochondrial recruitment of markers for mitophagy, including Parkin, VCP/p97, p62/Ref(2)P, and Atg8a. However, clearance of damaged mitochondria is impeded. This paradox is resolved by the findings that Clu is upstream of and binds VCP and is required for VCP-dependent Marf degradation. Marf accumulates in clu-deficient flies and is destabilized upon Clu overexpression. Furthermore, excessive mitochondrial fission alleviates impaired mitophagy caused by clu depletion. Thus, Clu is essential for mitochondrial homeostasis and functions in concert with Parkin and VCP for Marf degradation in MQC. Last, we will also discuss the advantages and disadvantages of the study on mitochondrial morphology and mobility using Drosophila larval muscle as a model. In summary, using the Drosophila muscle model, we found that Clu has independent functions required for both differential integrin subunit ER export and mitochondrial quality control.
Table of Contents
General introduction of drosophila myogenesis, the clueless gene and mitochondrial dynamics -- Loss of a cluelss-DGrasp complex results in ER stress and blocks integrin exit from the perinuclear endoplasmic reticulum in drpsphila larval muscle -- Parkin-dependent mitophagy requires clueless to promote VCP-mediated MARF degration in drosophila -- Analysis of mitochondrial morphology in the drosophila larval musculature -- Discussion -- Materials and methods
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Ph.D.