Exosomal arrow (Arr)/lipoprotein receptor protein 6 (LRP6) in Drosophila melanogaster increases the extracellular level of Sol narae (Sona) in a Wnt-independent manner

Abstract

Wg/Wnt as a signaling protein binds to Frizzled (Fz) and Arrow (Arr), two Wg co-receptors essential for Wg signaling for cell proliferation, differentiation, and cell survival. Arr has a long extracellular region, a single transmembrane domain and an intracellular region. Here, we report that a new arrm7 mutant is identified in a genetic screen as a suppressor of lethality induced by overexpression of Sol narae (Sona), a secreted metalloprotease in ADAMTS family involved in Wg signaling. arrm7 allele has a premature stop codon, which encodes Arrm7 protein missing the intracellular region. arrm7 clones show cell death phenotype and overexpression of Arrm7 protein also induces cell death. Levels of extracellular Sona were decreased in both arrm7 and arr2 null clones, demonstrating that Arr increases the level of extracellular Sona. Indeed, Arr but not Arrm7, increased levels of Sona in cytoplasm and exosome fraction by inhibiting the lysosomal degradation pathway. Interestingly, Arr itself was identified in the exosome fraction, demonstrating that Arr is secreted to extracellular space. When Sona-expressing S2 cells were treated with exosomal Arr, the extracellular level of active Sona was increased. These results show that exosomal Arr dictates Sona-expressing cells to increase the level of extracellular Sona. This new function of Arr occurred in the absence of Wg because S2 cells do not express Wg. We propose that Arr plays two distinct roles, one as an exosomal protein to increase the level of extracellular Sona in a Wnt-independent manner and the other as a Wg co-receptor in a Wnt-dependent manner.

Anastral Spindle 3/Rotatin Stabilizes Sol narae and Promotes Cell Survival in Drosophila melanogaster

“…Interestingly, Wg-CTD but not full-length Wg induces Cyc D, which demonstrates that Sona is involved in intercellular communication to support the normal development of damaged tissues by regulating Wg signaling. Consistent with this, sona suppressors such as wntless, arrow, pou domain motif 3, and archipelago are related to Wg signaling (Han et al, 2020;Kim and Cho, 2020;Kim et al, 2016;Nam and Cho, 2020;Won et al, 2019).…”

“…Survival rates from two independent experiments are averaged (n > 100, each). Lethality by UAS-sona expression is similar to that by UAS-sona ; UAS-GFP , establishing that lethality caused by Sona overexpression is not affected by UAS gene dose ( Han et al, 2020 ). (B-H) The small wing phenotype by knockdown of ana3 is rescued by overexpression of Sona.…”

“…We have previously shown that Arrow (Arr) secreted via exosomes can stabilize Sona when it is added to the culture media of Sona-expressing S2 cells ( Han et al, 2020 ). Since both Ana3 and Arr stabilize exosomal Sona and are secreted via exosomes, we reasoned that exosomal Ana3 may also stabilize Sona.…”

“…3 ). It is worth noting that a degradation of Sona occurs in the lysosome but not in the proteasome complex, as well as that another sona suppressor Arr also stabilizes Sona ( Han et al, 2020 ). Since our original genetic screen was aimed at identifying suppressors that reduce Sona activity, it makes sense that both ana3 and arr mutants are identified as sona suppressors.…”

Dynein Heavy Chain 64C Differentially Regulates Cell Survival and Proliferation of Wingless-Producing Cells in Drosophila melanogaster

“…To understand the function of Sona, our laboratory previously carried out a genetic screen, and identified suppressors that overcome lethality induced by overexpressed Sona. Several suppressors that have mutations in wntless, pou domain factor 3 (pdm3), archipelago (ago), arrow (arr) and anastral spindle 3 genes have been reported [11,[13][14][15][16]. The focus of this study is m115, one of the suppressors that has a missense mutation in the Dhc64C gene.…”