Author response: TMEM79/MATTRIN defines a pathway for Frizzled regulation and is required for Xenopus embryogenesis [peer_review]

Maorong Chen, Nathalia Amado, Jieqiong Tan, Alice Reis, Mengxu Ge, Jose Garcia Abreu, Xi He
2020 unpublished
29 Wnt signaling through the Frizzled (FZD) family of serpentine receptors is essential for 30 embryogenesis and homeostasis, and stringent control of the FZD protein level is critical 31 for stem cell regulation. Through CRISPR/Cas9 genome-wide screening in human cells, 32 we identified TMEM79/MATTRIN, an orphan multi-span transmembrane protein, as a 33 specific inhibitor of Wnt/FZD signaling. TMEM79 interacts with FZD during biogenesis 34 and promotes FZD degradation independent of
more » ... ndent of ZNRF3/RNF43 ubiquitin ligases (R-35 spondin receptors). TMEM79 interacts with ubiquitin-specific protease 8 (USP8), whose 36 activating mutations underlie human tumorigenesis. TMEM79 specifically inhibits USP8 37 deubiquitination of FZD, thereby governing USP8 substrate specificity and promoting 38 FZD degradation. Tmem79 and Usp8 genes have a pre-bilaterian origin, and Tmem79 39 inhibition of Usp8 and Wnt signaling is required for anterior neural development and 40 gastrulation in Xenopus embryos. TMEM79 is a predisposition gene for Atopic dermatitis, 41 suggesting deregulation of Wnt/FZD signaling a possible cause for this most common 42 yet enigmatic inflammatory skin disease. 43 65 Oncogenic activation of USP8 by missense mutations or chromosomal translocation has 66 been linked to pituitary adenoma (Cushing's Disease) (Ma et al. 2015; Reincke et al. 67 2015) and leukemia (Janssen et al. 1998), respectively, and its expression is associated 68 with poor prognosis of lung cancer cases (Kim et al. 2017), consistent with its role in 69 4 stabilizing receptors for multiple growth factors. It is unknown, however, if or how the 70 broad substrate specificity of USP8, or for that matter of other USPs (Leznicki and 71 Kulathu 2017), is governed. 72 Regulation of Wnt signaling has critical roles in vertebrate embryogenesis. Most 73 notably the dorsal Spemann-Mangold organizer of Xenopus embryos produces a 74 plethora of secreted or membrane bound Wnt antagonists, in addition to secreted BMP 75 (bone morphogenetic protein) antagonists (De Robertis and Kuroda 2004; Niehrs 2004). 76 A prevailing view is that Organizer-derived BMP antagonists serve as neural inducers 77 that neuralize the overlying native ectoderm to form anterior CNS, while Organizer-78 derived Wnt antagonists ensure anterior patterning and head formation (De Robertis and 79 Kuroda 2004; Niehrs 2004). Recently Notum, a secreted Wnt antagonist/deacylase 80 expressed in naïve ectoderm was shown to be required for neuralization and anterior 81 patterning of Xenopus embryos, revealing a previously unrecognized role of a Wnt 82 antagonist acting within naive ectoderm as a prerequisite for CNS formation (Zhang et al. 83 2015). This requirement appears to be distinct from the requirement for Organizer-84 derived BMP and Wnt antagonists, and may represent a "double insurance" mechanism 85 envisioned by Spemann for explaining the robustness of induction (Spemann 1938). The 86 role of Naïve ectoderm-derived Wnt antagonists in anterior patterning and neuralization 87 remains to be further substantiated. 88 Here we identify, via CRISPR/Cas9 genome-wide loss-of function screening, an 89 orphan five-pass transmembrane protein TMEM79 (also called MATTRIN), as a specific 90 antagonist of Wnt signaling. TMEM79 down-regulates the FZD protein level through 91 promoting FZD ubiquitination and degradation during biogenesis. TMEM79 acts in a 92 pathway that is independent of ZNRF3/RNF43 ubiquitin ligases via associating with 93 USP8 and inhibits USP8 deubiquitination of FZD specifically. We show that Tmem79 is 94 essential for Xenopus embryogenesis. Tmem79 depletion in naïve ectoderm leads to 95 anterior patterning, neuralization and neural crest defects, which are rescued by co-96 depletion of Usp8 or β-catenin, corroborating its molecular role in antagonizing Wnt/β-97 catenin signaling and underscoring critical roles of Wnt antagonists in naïve ectoderm for 98 CNS development. Tmem79 also plays a role in FZD/planar cell polarity (PCP) signaling 99 and regulates gastrulation movements in axial mesoderm. Our findings uncover 100 TMEM79 as a novel antagonist of Wnt/FZD signaling and shed insights into USP 101 regulation and vertebrate embryogenesis. Interestingly TMEM79 is a predisposition gene 102 for atopic dermatitis (AD) (Sasaki et al. 2013; Saunders et al. 2013), the most common 103 chronic and relapsing inflammatory skin disease that affects 20% children and 5-10%
doi:10.7554/elife.56793.sa2 fatcat:5vlu7ysngzhjtk3pctv7jdxgka