STING mediates immune responses in the closest living relatives of animals
Curation statements for this article:-
Curated by eLife
Evaluation Summary:
The authors addressed the role of a STING ortholog in antimicrobial defense of choanoflagellates. The analysis of the response of the choanoflagellate Monosiga brevicollis to a variety of bacterial species revealed that exposure of M. brevicollis to Pseudomonas aeruginosa conditioned medium results in choanoflagellate death and the authors found that this is dependent on the newly discovered ortholog of STING. Characterization reveals that the STING response can be induced by 2'3' cGAMP, which parallels the activation of STING in diverse species. In addition, the finding that cyclic dinucleotide treatment induces autophagy also has parallels with the effector pathways observed in other organisms. There are a number of strengths as outlined by the Reviewers. First, the development of a Choanaoflagellate model system to study innate immunity, second with the development of genetics for M. brevicollis, third, the demonstration of a functional STING system in one of the closest relatives to animals and fourth, that cell death occurs in response to cyclic dinucleotides.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewer #2 agreed to share their name with the authors.)
This article has been Reviewed by the following groups
Listed in
- Evaluated articles (eLife)
- Evaluated articles (preLights)
- Microbiology and Infectious Disease (eLife)
Abstract
Animals have evolved unique repertoires of innate immune genes and pathways that provide their first line of defense against pathogens. To reconstruct the ancestry of animal innate immunity, we have developed the choanoflagellate Monosiga brevicollis , one of the closest living relatives of animals, as a model for studying mechanisms underlying pathogen recognition and immune response. We found that M. brevicollis is killed by exposure to Pseudomonas aeruginosa bacteria. Moreover, M. brevicollis expresses STING, which, in animals, activates innate immune pathways in response to cyclic dinucleotides during pathogen sensing. M. brevicollis STING increases the susceptibility of M. brevicollis to P. aeruginosa -induced cell death and is required for responding to the cyclic dinucleotide 2’3’ cGAMP. Furthermore, similar to animals, autophagic signaling in M. brevicollis is induced by 2’3’ cGAMP in a STING-dependent manner. This study provides evidence for a pre-animal role for STING in antibacterial immunity and establishes M. brevicollis as a model system for the study of immune responses.
Article activity feed
-
-
-
Evaluation Summary:
The authors addressed the role of a STING ortholog in antimicrobial defense of choanoflagellates. The analysis of the response of the choanoflagellate Monosiga brevicollis to a variety of bacterial species revealed that exposure of M. brevicollis to Pseudomonas aeruginosa conditioned medium results in choanoflagellate death and the authors found that this is dependent on the newly discovered ortholog of STING. Characterization reveals that the STING response can be induced by 2'3' cGAMP, which parallels the activation of STING in diverse species. In addition, the finding that cyclic dinucleotide treatment induces autophagy also has parallels with the effector pathways observed in other organisms. There are a number of strengths as outlined by the Reviewers. First, the development of a Choanaoflagellate model system …
Evaluation Summary:
The authors addressed the role of a STING ortholog in antimicrobial defense of choanoflagellates. The analysis of the response of the choanoflagellate Monosiga brevicollis to a variety of bacterial species revealed that exposure of M. brevicollis to Pseudomonas aeruginosa conditioned medium results in choanoflagellate death and the authors found that this is dependent on the newly discovered ortholog of STING. Characterization reveals that the STING response can be induced by 2'3' cGAMP, which parallels the activation of STING in diverse species. In addition, the finding that cyclic dinucleotide treatment induces autophagy also has parallels with the effector pathways observed in other organisms. There are a number of strengths as outlined by the Reviewers. First, the development of a Choanaoflagellate model system to study innate immunity, second with the development of genetics for M. brevicollis, third, the demonstration of a functional STING system in one of the closest relatives to animals and fourth, that cell death occurs in response to cyclic dinucleotides.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewer #2 agreed to share their name with the authors.)
-
Reviewer #1 (Public Review):
The authors initially showed that M. brevicollis were killed by Pseudomonas aeruginosa and avoided ingesting them, as opposed to other bacteria, like E. coli. The next sought to ask why this was occurring and found that M. brevicollis STING was upregulated in the presence of Pseudomonas aeruginosa but not bacteria that do not kill M. brevicollis. While previous studies have shown that STING orthologs exist in choanoflagellates, the authors next sought to determine how M. brevicollis STING functions, especially with regard to the presence of pathogenic bacteria. The authors achieved this goal through the generation of STING-null M. brevicollis and showed that these mutants were less susceptible to Pseudomonas-induced killing. They next found that M. brevicollis STING induces autophagy, which the authors …
Reviewer #1 (Public Review):
The authors initially showed that M. brevicollis were killed by Pseudomonas aeruginosa and avoided ingesting them, as opposed to other bacteria, like E. coli. The next sought to ask why this was occurring and found that M. brevicollis STING was upregulated in the presence of Pseudomonas aeruginosa but not bacteria that do not kill M. brevicollis. While previous studies have shown that STING orthologs exist in choanoflagellates, the authors next sought to determine how M. brevicollis STING functions, especially with regard to the presence of pathogenic bacteria. The authors achieved this goal through the generation of STING-null M. brevicollis and showed that these mutants were less susceptible to Pseudomonas-induced killing. They next found that M. brevicollis STING induces autophagy, which the authors conclude is the mechanism for Pseudomonas-induced, STING-mediated killing of M. brevicollis.
The strengths of this manuscript are that the authors convincingly show evolutionary conservation of STING function in a choanoflagellate, the closest relative to animals. Furthermore, through their research they developed novel methods for genetic editing in M. brevicollis that is useful to the scientific community who use similar organisms, or for the development of these techniques in other organisms where they have yet to be used. A weakness of the study is that some gaps remain with the full story. From the start, why does Pseudomonas aeruginosa, but not other bacteria, induce STING and the generation of cyclic dinucleotides? What is the source of these STING-inducing cyclic dinucleotides? And in the end, is the STING-mediated induction of autophagy (Atg8 lipidation) the cause of M. brevicollis death? Overall, this is a nice study by Woznica et al. that is of interest to the broad scientific community.
-
Reviewer #2 (Public Review):
In this novel and interesting manuscript by Woznica, et al. the authors show that STING signaling, known in mammals as a key regulator of the interferon response, is found in the evolutionarily closest precursor to mammals, choanoflagellates. The authors show that there appears to be a choanoflagellate cell death pathway that is a specific response to Pseudomonas aeruginosa, although it is not clear how this is related to pathogen protection. The authors clearly and convincingly show that the STING-mediated response is dependent on the endogenously produced 2'3' cGAMP, presumably synthesized by the cGAS ortholog encoded by M. brevicollis. In contrast, microbial cyclic dinucleotides were not inducers of this system. By developing a system for genetic manipulation of M. brevicollis, they were able to evaluate …
Reviewer #2 (Public Review):
In this novel and interesting manuscript by Woznica, et al. the authors show that STING signaling, known in mammals as a key regulator of the interferon response, is found in the evolutionarily closest precursor to mammals, choanoflagellates. The authors show that there appears to be a choanoflagellate cell death pathway that is a specific response to Pseudomonas aeruginosa, although it is not clear how this is related to pathogen protection. The authors clearly and convincingly show that the STING-mediated response is dependent on the endogenously produced 2'3' cGAMP, presumably synthesized by the cGAS ortholog encoded by M. brevicollis. In contrast, microbial cyclic dinucleotides were not inducers of this system. By developing a system for genetic manipulation of M. brevicollis, they were able to evaluate the role of mutations, perform complementation analysis, and localize fluorescent proteins after transfections. Most convincingly, they showed a connection between the evolutionarily ancient autophagy pathway, STING and 2'3' cGAMP, showing a strong connection between this system and mammalian cell autophagy. Whether the autophagic response is the key link to pathogen protection, or host cell death to bacterial products is the critical link to interfering with spread and host disease is not clear. Also of unknown relevance is the observation that there is phagocytosis resistance toward P. aeruginosa, as it was not shown to be directly linked to the STING regulation, and it wasn't directly demonstrated that phagocytosis avoidance was a choanoflagellate activity or a bacterial activity. The fact that there also appears to be an LPS-responsive pathway leading to death indicates that the organism may have multiple pattern recognition receptors yet to be identified.
-
Reviewer #3 (Public Review):
In general, this is a well-performed study. The experiments are well-controlled and interpreted. There were a number of strengths, including (i) the development of a Choanaoflagellate model system to study innate immunity, (ii) the development of genetics for M. brevicollis, and (iii) the demonstration of a functional STING system in one of the closest relatives to animals.
The weaknesses are that as of now (i) these are really two unrelated stories that are each incomplete. It is unknown what P. aeruginosa factor is affecting M. brevicollis growth and whether this factor requires STING. Likewise, it is unknown what cGAS/STING is responding to in M. brevicollis. Is it a virus? (ii) M. brevecollis is 2 of >20 Choaoflagellates with a predicted intact cGAS-STING conservation. As mentioned by the authors, there …
Reviewer #3 (Public Review):
In general, this is a well-performed study. The experiments are well-controlled and interpreted. There were a number of strengths, including (i) the development of a Choanaoflagellate model system to study innate immunity, (ii) the development of genetics for M. brevicollis, and (iii) the demonstration of a functional STING system in one of the closest relatives to animals.
The weaknesses are that as of now (i) these are really two unrelated stories that are each incomplete. It is unknown what P. aeruginosa factor is affecting M. brevicollis growth and whether this factor requires STING. Likewise, it is unknown what cGAS/STING is responding to in M. brevicollis. Is it a virus? (ii) M. brevecollis is 2 of >20 Choaoflagellates with a predicted intact cGAS-STING conservation. As mentioned by the authors, there is evidence of horizontal transfer of cGAS/STING, which may have occurred after the evolutionary fork to animals. Is it true that this represents an evolutionary precursor of animal innate immunity?
-
Excerpt
Evolution of STING – Ancient Functions of a Critical Immune Effector
-