F1000Research 2019, 8(F1000 Faculty Rev):1415 Last updated: 12 AUG 2019 REVIEW Converging physiological roles of the anthrax toxin receptors [version 1; peer review: 3 approved] Oksana A. Sergeeva , F. Gisou van der Goot Global Health Institute, School of Life Sciences, EPFL, Lausanne, Switzerland First published: 12 Aug 2019, 8(F1000 Faculty Rev):1415 ( Open Peer Review v1 https://doi.org/10.12688/f1000research.19423.1) Latest published: 12 Aug 2019, 8(F1000 Faculty Rev):1415 ( https://doi.org/10.12688/f1000research.19423.1) Reviewer Status Abstract Invited Reviewers The anthrax toxin receptors—capillary morphogenesis gene 2 (CMG2) and 1 2 3 tumor endothelial marker 8 (TEM8)—were identified almost 20 years ago, although few studies have moved beyond their roles as receptors for the version 1 anthrax toxins to address their physiological functions. In the last few years, published insight into their endogenous roles has come from two rare diseases: 12 Aug 2019 hyaline fibromatosis syndrome, caused by mutations in CMG2, and growth retardation, alopecia, pseudo-anodontia, and optic atrophy (GAPO) syndrome, caused by loss-of-function mutations in TEM8. Although CMG2 F1000 Faculty Reviews are written by members of and TEM8 are highly homologous at the protein level, the difference in the prestigious F1000 Faculty. They are disease symptoms points to variations in the physiological roles of the two commissioned and are peer reviewed before anthrax receptors. Here, we focus on the similarities between these publication to ensure that the final, published version receptors in their ability to regulate extracellular matrix homeostasis, angiogenesis, cell migration, and skin elasticity. In this way, we shed light is comprehensive and accessible. The reviewers on how mutations in these two related proteins cause such seemingly who approved the final version are listed with their different diseases and we highlight the existing knowledge gaps that could names and affiliations. form the focus of future studies. Keywords 1 Michael S Rogers, Harvard Medical School, anthrax toxin receptors, CMG2, TEM8, HFS, ISH, JHF, GAPO Boston, USA 2 James G Bann, Wichita State University, Wichita, USA 3 Brad St. Croix, National Institutes of Health, Frederick, USA Any comments on the article can be found at the end of the article. Page 1 of 10 F1000Research 2019, 8(F1000 Faculty Rev):1415 Last updated: 12 AUG 2019 Corresponding authors: Oksana A. Sergeeva ([email protected]), F. Gisou van der Goot ([email protected]) Author roles: Sergeeva OA: Conceptualization, Visualization, Writing – Original Draft Preparation, Writing – Review & Editing; van der Goot FG: Conceptualization, Funding Acquisition, Writing – Original Draft Preparation, Writing – Review & Editing Competing interests: No competing interests were disclosed. Grant information: This work was funded by Swiss National Science Foundation grant #310030B_176393. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Copyright: © 2019 Sergeeva OA and van der Goot FG. This is an open access article distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. How to cite this article: Sergeeva OA and van der Goot FG. Converging physiological roles of the anthrax toxin receptors [version 1; peer review: 3 approved] F1000Research 2019, 8(F1000 Faculty Rev):1415 (https://doi.org/10.12688/f1000research.19423.1) First published: 12 Aug 2019, 8(F1000 Faculty Rev):1415 (https://doi.org/10.12688/f1000research.19423.1) Page 2 of 10 F1000Research 2019, 8(F1000 Faculty Rev):1415 Last updated: 12 AUG 2019 Introduction Anthrax toxin receptor structure and epistructure In the early 2000s, two newly discovered proteins—capillary Both CMG2 and TEM8 are type I transmembrane proteins con- morphogenesis gene 2 (CMG2) and tumor endothelial marker sisting of an extracellular von Willebrand factor type A (vWA) 8 (TEM8)—were demonstrated to be anthrax toxin recep- that is also found in integrins and participates in receptor– tors (ANTXRs)1,2. Since then, much research has focused on ligand interactions, an extracellular immunoglobulin-like their toxin-related pathogenic roles (most recently reviewed (Ig-like) domain, and a long cytoplasmic tail that is predicted by Friebe et al.3 and Sun and Jacquez4). However, both to be largely unstructured and contains an actin–cytoskeleton CMG2, encoded by the ANTXR2 gene, and TEM8, encoded interacting domain (Figure 1). The structure of the extracellular by the ANTXR1 gene, also possess physiological roles in ver- domains was analyzed by low-resolution cryo-electron micro- tebrates, the study of which has not received nearly the same scopy, onto which the x-ray structure of the vWA domain8,9 level of attention. What has helped to drive research into their and a model of the Ig-like domain10,11 were successfully endogenous roles is that both receptors are associated with docked. CMG2 and TEM8 have long cytosolic tails of 148 recessive autosomal diseases: mutations in CMG2 lead to hya- and 222 residues, respectively, which are predicted to be intrin- line fibromatosis syndrome (HFS), whereas mutations in sically unstructured like the cytoplasmic domains of many TEM8 result in growth retardation, alopecia, pseudo-anodontia, other signaling receptors12. These tails could allow sequential and optic atrophy (GAPO) syndrome5–7. The literature contains interaction with diverse partner molecules13 and were also reports of some 350 HFS patients of whom 112 have been found to be the site of various post-translational modifications, genotyped, possessing 56 different mutations. In parallel, such as S-palmitoylation14, ubiquitination14, and tyrosine- 70 GAPO patients have been reported, 21 of which have geno- phosphorylation15, all of which were found to be necessary for type information, comprising 14 different mutations. Even toxin-induced endocytosis3. The CMG2 and TEM8 tails are though these diseases have grossly different symptoms, CMG2 completely identical in certain segments but have no homol- and TEM8 share 62% sequence similarity, reasonably allowing ogy in others16, pointing toward similarities and differences, for some overlapping structure/function relationship and respectively, in interacting partners. The most highly conserved ensuring that a better understanding of the function of one portion between the two receptors, which is juxtamembrane, receptor can prompt progress on the other. Therefore, this has homology with the actin-regulating Wiskott–Aldrich syn- review aims to synthesize the latest information on these drome protein17. Relatedly, TEM8 was also consistently shown to ANTXRs with earlier established observations to better under- interact with actin18,19, although this interaction may be indirect stand their physiological functions and provide open pathways and require adaptor proteins, such as those observed for for future research. integrins20. Figure 1. TEM8 and CMG2 are similar in gene and protein structure: mutations in the former lead to GAPO and those in the latter lead to HFS. Tumor endothelial marker 8 (TEM8) (blue) and capillary morphogenesis gene 2 (CMG2) (green) have similar exon schemes and protein structures. Crystal structures of the von Willebrand factor type A (vWA) domain of TEM8 (Protein Data Bank [PDB]: 3N2N8) and CMG2 (PDB: 1TZN9) are shown aligned. The immunoglobulin (Ig)-like and transmembrane domains have been modelled on CMG2 and are shown in gray10. The cytosolic tails, longer for TEM8 than for CMG2, are intrinsically disordered with a conserved juxtamembranous actin- binding domain (ABD). The number of reported occurrences of mutations in growth retardation, alopecia, pseudo-anodontia, and optic atrophy (GAPO) is depicted next to TEM8, and the corresponding number in hyaline fibromatosis syndrome (HFS) is shown next to CMG2. The number of HFS patients with mutations in CMG2 is almost an order of magnitude higher than those of GAPO/TEM8. Page 3 of 10 F1000Research 2019, 8(F1000 Faculty Rev):1415 Last updated: 12 AUG 2019 Receptors for anthrax toxin which was named in 198625, and juvenile hyaline fibromatosis The function of CMG2 and TEM8 as ANTXRs has been (JHF), first described in 1873 by Murray26 and named in 197627. extensively studied. The surface residence time of ANTXRs Initially, these terms were thought to describe two different dis- is regulated by S-palmitoylation at three or four sites in their eases with overlapping symptoms, including subcutaneous cytoplasmic tails, as shown for TEM814, and under “resting” nodules, gingival hypertrophy, painful joint contractures, and conditions, TEM8 is associated with the actin cytoskeleton15,21 persistent infections. ISH is, however, more severe with death (Figure 2A). As opposed to how integrins concurrently inter- occurring before the age of two because of protein-losing enter- act with their ligand and the cytoskeleton, upon binding of a opathy. In the early 2000s, patients with both JHF and ISH ligand such as anthrax toxin protective antigen (PA), the inter- were found to have mutations in CMG25,6,28, pointing to the action of TEM8 with the actin cytoskeleton is released. Upon disease causality. This allowed the classification of a unified ligand binding, PA oligomerizes into a heptameric or octa- syndrome with a symptom grading system, placing the pre- meric complex, leading