MyD88-deficient Hydra reveal an ancient function of TLR signaling in sensing bacterial colonizers Sören Franzenburga,1, Sebastian Fraunea,1,2, Sven Künzelb, John F. Bainesb,c, Tomislav Domazet-Losoa, and Thomas C. G. Boscha,2 aZoological Institute, Christian Albrechts University, 24098 Kiel, Germany; bMax Planck Institute for Evolutionary Biology, 24306 Plön, Germany; and cInstitute for Experimental Medicine, Christian Albrechts University, 24105 Kiel, Germany Edited by Nancy A. Moran, Yale University, West Haven, CT, and approved October 4, 2012 (received for review August 13, 2012) Toll-like receptor (TLR) signaling is one of the most important Vertebrate homologs of Toll, the TLRs, are receptors of the signaling cascades of the innate immune system of vertebrates. immune system. Vertebrate TLRs are involved in eliminating Studies in invertebrates have focused on the fruit fly Drosophila pathogens and controlling commensal colonization (1, 14, 15) by melanogaster and the nematode Caenorhabditis elegans,and recognizing conserved microbe-associated molecular patterns there is little information regarding the evolutionary origin and (MAMPs) including lipopolysaccharides, flagellin, and peptido- ancestral function of TLR signaling. In Drosophila, members of the glycans (1, 16). Therefore, it was proposed that immune function κ Toll-like receptor family are involved in both embryonic develop- of TLR signaling involving NF- B and MyD88 has evolved ment and innate immunity. In C. elegans, a clear immune function within the bilaterians (17). However, recent genome projects in of the TLR homolog TOL-1 is controversial and central components the nonbilaterians Hydra magnipapillata (18) and Nematostella vectensis (19) revealed the presence of TLRs, MyD88, and NF- of vertebrate TLR signaling including the key adapter protein my- κ eloid differentiation primary response gene 88 (MyD88) and the B (20, 21). Their role in bacterial recognition and innate im- transcription factor NF-κB are not present. In basal metazoans such munity, however, remains to be shown (22). Cnidaria are a sister as the cnidarians Hydra magnipapillata and Nematostella vectensis, group to the Bilateria (19) and one of the earliest branches in the animal tree of life (Fig. 1A). The recent genome project of the all components of the vertebrate TLR signaling cascade are present, cnidarian H. magnipapillata identified a conserved TLR-signaling but their role in immunity is unknown. Here, we use a MyD88 loss- cascade (21, 23) (Fig. 1B and Table S1), making Hydra a suitable of-function approach in Hydra to demonstrate that recognition of model for addressing questions of the ancestral function of TLR bacteria is an ancestral function of TLR signaling and that this signaling. Is the TLR pathway involved in the defense against process contributes to both host-mediated recolonization by com- bacterial pathogens or in maintaining specific host–microbe mensal bacteria as well as to defense against bacterial pathogens. interactions? Does it affect the mechanisms and routes by which functionally diverse bacteria colonize their host? Is it involved in oll-like receptors (TLRs) are conserved throughout animal developmental processes such as axis formation? To gain insight Tevolution but appear to serve different functions in different into these questions, we performed MyD88 loss-of-function ex- model organisms. TLRs are transmembrane receptors with ex- periments in Hydra vulgaris [AEP strain (24)]. We used a com- tracellular leucin-rich repeat (LRR) motifs and an intracellular bination of microarray-based gene expression screening and 16S Toll/interleukin-1 receptor (TIR) domain. Upon stimulation of rRNA-gene sequencing to detect changes in both the Hydra tran- TLRs, the key adapter protein MyD88 associates with the cyto- scriptome and the associated microbiota. Further, we investigated solic part of the TLR through a homophilic interaction of the TIR the role of TLR signaling in pathogen defense against Pseudo- domains and then recruits the IL-1R–associated kinase (IRAK), monas aeruginosa. The patterns of differentially regulated host which subsequently associates with the TNFR-associated factor genes as well as changes in the bacterial colonization process and (TRAF). TRAF recruits the TGF-β activated kinase 1 (TAK1). pathogen susceptibility in MyD88-knockdown polyps point to a The kinase TAK1 induces a phosphorylation cascade finally leading role of TLR signaling in sensing of bacteria, be it associated com- to the nuclear translocation of the transcription factors NF-κB via mensals or pathogens. Thus, this functional analysis clearly identi- the inhibitor of kappa B-kinase (IKK) signalosome or c-Jun via the fies a role of TLR signaling in innate immunity in an animal at the c-Jun N-terminal kinase (JNK)/p38 branch of TLR signaling (1). base of metazoan evolution. The Toll pathway was initially identified to be essential in early embryonic development in Drosophila (2). In addition to its cru- Results – − cial role in the establishment of the dorsal ventral axis, Drosophila Generation of MyD88-Knockdown (MyD88 ) H. vulgaris (AEP). To ana- Toll was shown to be involved in muscle development (3) and lyze the function of TLR signaling in the basal metazoan Hydra, heart formation (4). Later on, it was discovered that Toll signaling we generated a stable transgenic H. vulgaris (AEP) line with in Drosophila also contributes to defense reactions against bac- drastically reduced expression levels of the universal adapter teria as well as to antifungal defense by regulating, among others, the expression of the antifungal peptide drosomycin in adult flies (5, 6). Further immunity functions have been identified for Toll- Author contributions: S. Franzenburg, S. Fraune, and T.C.G.B. designed research; S. Franzenburg 7 (7) and Toll-8 (8). Studies in the mosquito Aedes aegypti also and S. Fraune performed research; S.K. and J.F.B. contributed new reagents/analytic identified MyD88-dependent Toll signaling to mediate immune tools; S. Franzenburg, S. Fraune, and T.D.-L. analyzed data; and S. Franzenburg, S. Fraune, defenses against dengue viruses (9). One other invertebrate and T.C.G.B. wrote the paper. model system, the nematode Caenorhabditis elegans lacks central The authors declare no conflict of interest. proteins of the canonical TLR-signaling cascade (10). Only one This article is a PNAS Direct Submission. Toll homolog, termed TOL-1, was identified in C. elegans (10). Data deposition: The microarray data reported in this paper have been deposited in the The fact that TOL-1 mutants show strong developmental defects Gene Expression Omnibus (GEO) database, www.ncbi.nlm.nih.gov/geo (accession no. despite mutants for the putative signaling cascade displaying no GSE32383); and the 16S 454 data are deposited at the metagenome analysis server MG- developmental abnormalities led to the belief that TOL-1 in C. RAST (Project ID 1719). elegans might function as a cell–cell adhesion protein in neurons 1S. Franzenburg and S. Fraune contributed equally to this work. (10). Other reports state an additional involvement of TOL-1 in 2To whom correspondence may be addressed. E-mail: [email protected] or pathogen defense (11). In addition, a TIR-domain–containing [email protected]. protein called tir-1 is required for resistance to fungal and bac- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. terial infection in C. elegans (12, 13). 1073/pnas.1213110109/-/DCSupplemental. 19374–19379 | PNAS | November 20, 2012 | vol. 109 | no. 47 www.pnas.org/cgi/doi/10.1073/pnas.1213110109 Downloaded by guest on September 24, 2021 Neither line displayed any obvious developmental or behavioral abnormalities. Absence of Bacteria as Well as MyD88 Deficiency Influence Central Parts of the TLR-Signaling Cascade. To assess the transcriptional consequences of a MyD88 knockdown and identify potential downstream effector genes of the TLR-signaling cascade, we per- − formed microarray analyses. Expression levels of both MyD88 polyps as well as germ-free polyps (Fig. 1G) were compared with − control polyps. The MyD88 polyps combined with the germ- free polyps provided unique resources that allowed us to directly investigate the connection between TLR signaling and the reg- ulation of associated bacterial diversity. Statistical analysis was carried out by ANOVA with Student–Newman–Keuls (SNK) post hoc tests and false discovery rate (FDR) correction. The microarray data independently validate the successful MyD88 knockdown. Contig 11552, encoding for MyD88, shows a 4.29- − fold down-regulation (P < 0.001) in MyD88 polyps and is not differentially expressed in germ-free polyps (fold change 1.09, N.S.) (Table S1). To check for transcriptional changes of other putative members of the TLR cascade, the H. vulgaris (AEP) transcriptome (29) was screened for homologs of previously described members of the pathway. The majority of central cascade members are present in H. vulgaris (AEP) (Fig. 1A and Table S1). Various central components of the putative TLR cascade including members of the TRAF family of ubiquitin protein ligases, the kinase TAK1, MAP-kinase p38, and the JNK inhibitor JSP-1 show significantly decreased expression in germ-free and/or MyD88-deficient conditions (Table S1). We hypothesize therefore the existence of positive feedback loops of the putative effector transcription factors NF-κBandc-Jun on certain upstream pathway components, pointing toward a functional unity of