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Galectin-8 senses phagosomal damage and recruits selective autophagy adapter TAX1BP1 to control Mycobacterium tuberculosis infection in macrophages

Samantha L. Bell, Kayla L. Lopez, Jeffery S. Cox, Kristin L. Patrick, Robert O. Watson

Preprint posted on July 01, 2020 https://www.biorxiv.org/content/10.1101/2020.06.30.180877v1

Guardians from the galectins: Danger sensor galectin-8 promotes degradation of M. tuberculosis using selective autophagy

Selected by Josie Gibson, Zhang-He Goh

Categories: microbiology

preLight Authors’ note

This is the second preLight of a three-part series on airway inflammation, infection, autophagy and its mediators. Links to the first and third preLights.


Background:

Mycobacterium tuberculosis is an important human pathogen which causes tuberculosis, a disease which results in a large mortality worldwide [1]. M. tuberculosis antibiotic resistance is increasing and present treatments are not ideal. Therefore, there is great interest in understanding host-pathogen interactions in order to identify new therapeutic targets for treatment of tuberculosis.

Autophagy is a cellular self-degradation pathway used to recycle damaged or unwanted cellular components, accomplished through lysosomal degradation. In some cases, autophagy can be used to target specific components, termed selective autophagy, completed using autophagy adaptors which bind to both autophagy pathway proteins and ubiquitinated cargo. Autophagy adaptors are present in the cytosol and are able to target ubiquitinated pathogens which have escaped the phagosome and are accessible to cytosolic proteins [2].

M. tuberculosis is phagocytosed by macrophages; it has been reported to damage the vesicle it resides within, in order to escape degradation and enable continued infection. The authors have previously demonstrated that 30% of M. tuberculosis escaped to the cytosol become ubiquitinated [3], suggesting that M. tuberculosis can be targeted by autophagy adaptors. The mechanism of how selective autophagy may be activated in M. tuberculosis infection is not fully understood, so the authors choose to examine a potential role of galectins, which are danger sensors. Cytosolic galectins act to recognise and bind to sites of lysosomal or endosomal membrane damage and are known to interact with other intracellular pathogens as well as autophagy adaptors [4]. In this study, the authors examine how macrophages sense and respond to M. tuberculosis within damaged phagosomes, with a focus on the mechanistic roles of galectins and selective autophagy (Fig. 1).

Figure 1. Summary of preprint findings.

Key findings:

The initial important observation in this study is that M. tuberculosis targeted by selective autophagy machinery within macrophages, are also associated with galectin-8. The authors first demonstrate that three different cytosolic galectins, galectin-3, -8 and -9, can be recruited to M. tuberculosis within damaged phagosomes, by expressing fluorescently tagged galectin proteins within macrophages, and clearly visualising these co-localising with M. tuberculosis. Next, further imaging shows that M. tuberculosis (in addition to galectin-8) co-localises with ubiquitin, p62 and LC3, all key proteins involved in selective autophagy.

In order to determine whether galectins are required for autophagy protein recruitment, macrophages deficient in galectins-3, -8 or -9 were generated. Galectin-8 deficient macrophages had reduced LC3 recruitment to M. tuberculosis, as well as an increase in bacterial replication, suggesting that galectin-8 promotes M. tuberculosis targeting to autophagic machinery in macrophages.

The next key finding is that galectin-8 can interact with ubiquitin and a known selective autophagy receptor TAX1BP1. This led the authors to hypothesise that galectin-8 binding with TAX1BP1 increases selective autophagy at sites of membrane damage. TAX1BP was next shown to co-localise with galectin-8-positive M. tuberculosis in macrophages, but this did not occur using an M. tuberculosis strain which was unable to cause membrane damage. TAX1BP1 recruitment to M. tuberculosis was also reduced in cells deficient in galectin-8. Finally, galectin-8 was overexpressed in macrophages, leading to an increase in LC3 recruitment to M. tuberculosis. Together these data suggest that galectin-8 promotes selective autophagy adaptor recruitment to damaged M. tuberculosis phagosomes helping to control infection.

 

Why we chose this preprint:

We have chosen this study because this preprint examines host-pathogen interactions leading to potential therapeutic targets for treatment of tuberculosis, similar to this study recently highlighted. This study points to host cell danger sensors—galectins—as potential therapeutic targets to promote the clearance of M. tuberculosis. In addition, the imaging of multiple protein interactions with M. tuberculosis in macrophages used in this study clearly demonstrated the extent of protein recruitment.

                                                                                                                                                                                                                                                     

Questions to the authors:

1. Do you think galectins-3 and/or -9 may play alternative roles in promotion of selective autophagy or alternative macrophage antimicrobial defence mechanisms?
2. Would you expect to find a similar role of galectin-8 and selective autophagy in tuberculosis infection in other phagocytes?
3. Do you think galectin-8 and selective autophagy may be important for control of other intracellular pathogens which gain access to the cytosol?

 

References

  1. World Health Organisation. Global tuberculosis report 2019. World Health Organization; 2019.
  2. Farré J-C, Subramani S. Mechanistic insights into selective autophagy pathways: lessons from yeast. Nat Rev Mol Cell Biol. 2016 Sep 6;17(9):537–52.
  3. Watson RO, Manzanillo PS, Cox JS. Extracellular M. tuberculosis DNA targets bacteria for autophagy by activating the host DNA-sensing pathway. Cell. 2012 Aug 17;150(4):803–15.
  4. Thurston TLM, Wandel MP, Muhlinen N von, Foeglein Á, Randow F. Galectin-8 targets damaged vesicles for autophagy to defend cells against bacterial invasion. Nature. 482(7385):414.

Tags: galectins, macrophage, selective autophagy, tb

Posted on: 25th July 2020

doi: https://doi.org/10.1242/prelights.23220

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Author's response

Samantha Bell shared

  1. That’s definitely possible. It’s interesting that galectins-3 and-9 are recruited to the Mtb-containing phagosome but are dispensable for targeting Mtb to selective autophagy and controlling replication. It’s possible that galectins-3 and -9 participate in other antimicrobial pathways that are ineffective at controlling Mtb replication. For example, new work from the Deretic lab suggests galectin-3 can help repair damaged membranes (Jia, et al., Developmental Cell, 2020), and work from the Coers lab (Feeley, et al., PNAS, 2017) showed that galectin-3 can recruit antimicrobial guanylate binding proteins (GBPs) to pathogen-containing vacuoles. Far less is known about how galectin-9 functions in innate immune responses, but we detected an interaction between galectin-9 and the selective autophagy adapter OPTN, which suggests galectin-9 might recruit this adapter to help with targeting to selective autophagy. Because galectin-3-/- and -9-/- macrophages were able to target and control Mtb, it seems that these other functions might be inferior for controlling Mtb in macrophages. Regardless, we are continuing to investigate what other innate immune pathways all of these galectins might participate in and how such pathways impact Mtb infection and host responses.
  2. Yes. We used RAW 264.7 macrophages, a genetically tractable murine macrophage cell line, which enabled us to perform experiments using complex cell lines (triple knockouts; knockouts also expressing protein variants) and draw important conclusions about the biology of galectins and TAX1BP1 during Mtb infection. Our previous studies (including Watson, et al., Cell, 2012; Watson & Bell, et al., Cell Host & Microbe, 2015) have found that primary macrophages like bone-marrow derived macrophages target Mtb to selective autophagy with similar efficiency and temporal dynamics as RAW 264.7 cells. While we currently have no reason to suspect major differences between phagocyte types, we are interested in studying this pathway in tissue-resident macrophages (like alveolar macrophages) in the future.
  3. Yes, absolutely. Several groups have observed colocalization of galectin-8 with various intracellular pathogens (including Salmonella, Listeria, Legionella, Group A Streptococcus, and even adenovirus), and in many cases, galectin recruitment has been linked to cells’ ability to control pathogen replication. Interestingly, different pathogens have different proportions of galectin-8+ (and galectin-3 and -9+) populations, which might be related to the degree to which pathogens damage the phagosome. Rather than galectins always serving the same purpose in every cell type with every pathogen, there appears to be really interesting cell- and pathogen-specific roles for galectins and downstream pathways like selective autophagy. We suspect these differences may stem in part from the ability of pathogens to modulate host recognition and response pathways, so we are very interested in identifying how Mtb in particular might modulate recognition of Mtb-containing phagosomes and selective autophagy.

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