InteractiveFly: GeneBrief

Glucosidase 1: Biological Overview | References

In order to respond to infection, hosts must distinguish pathogens from their own tissues. This allows for the precise targeting of immune responses against pathogens and also ensures self-tolerance, the ability of the host to protect self tissues from immune damage. One way to maintain self-tolerance is to evolve a self signal and suppress any immune response directed at tissues that carry this signal. This study characterizes the Drosophila tuSz mutant strain, which mounts an aberrant immune response against its own fat body. This study demonstrates that this autoimmunity is the result of two mutations: 1) a mutation in the Glucosidase 1/GCS1 gene that disrupts N-glycosylation of extracellular matrix proteins covering the fat body, and 2) a mutation in the Drosophila Janus Kinase ortholog that causes precocious activation of hemocytes. Data indicate that N-glycans attached to extracellular matrix proteins serve as a self signal and that activated hemocytes attack tissues lacking this signal. The simplicity of this invertebrate self-recognition system and the ubiquity of its constituent parts suggests it may have functional homologs across animals (Mortimer, 2021).

This work has investigated the Drosophila tuSz¹ mutant strain. tuSz¹ is a temperature-sensitive mutant, and at the restrictive temperature, posterior fat body tissue is melanotically encapsulated by hemocytes in a reaction similar to the antiparasitoid immune response. The tuSz¹ phenotype is caused by two tightly linked mutations: a nonconditional, dominant gain-of-function mutation in hop that leads to ectopic immune activation and a temperature-sensitive, recessive mutation in GCS1 that leads to loss of protein N-glycosylation of the ECM overlaying the posterior fat body. These data lead to a a proposal of a two-step model in which immune activation and the loss of SAMP presentation/recognition are both necessary for the breakdown of self-tolerance. In a naïve wild-type larva, neither condition is met and self-tolerance is maintained. In the case of the tuSz¹ mutant, the posterior fat body lacks appropriate ECM protein N-glycosylation and is targeted by constitutively activated hemocytes for encapsulation. This two-step model is also reflected in the hop^Tum mutant background, in which the simultaneous disruption of N-glycosylation in this immune-activated background results in tissue self-encapsulation similar to the tuSz¹ mutant (Mortimer, 2021).

Models describing the necessity for two independent signals in fly encapsulation responses. (A) Homeostasis is maintained in naive wild-type larvae. (B) In tuSz¹ mutant larvae, immune cells are inappropriately activated by JAK-STAT pathway activation due to the hop^Sz gain-of-function mutation. The loss of protein N-glycosylation in posterior fat body tissue due to the GCS1^Sz mutation leads to loss of self-tolerance and tissue encapsulation. (C) In the model of self-tolerance described in a previous study, the coupled phenotypes of loss of cell integrity and loss of ECM integrity are sufficient to disrupt self-tolerance. (D) Immune cells are activated following parasitoid wasp infection, presumably due to the wound-mediated activation of JAK-STAT signaling. SAMP-presenting host tissues are protected from encapsulation, and wasp eggs may be targeted for encapsulation because they are missing the ECM N-glycosylation SAMP (Mortimer, 2021).

Interestingly, previous work also documented the necessity of at least two signals for self-encapsulation in Drosophila. In that case, both the loss of the ECM (with its glycosylated proteins) and the disrupted positional integrity of the underlying fat body cells (potentially mimicking a wound) were required for immune cells to become activated and encapsulate the self tissue. A similar loss of ECM and underlying cell integrity was also found in the classical melanotic tumor mutant tu(2)W. This model, in which at least two factors are required for self-encapsulation, may explain why the several classically described self-encapsulation mutants, unlike virtually all other types of visible Drosophila mutants, were never successfully mapped (Mortimer, 2021).

The disruption of either factor in the two-step model in isolation is not sufficient to cause self-encapsulation. This can be seen in parasitoid wasp infected larvae; the wounding associated with parasitoid infection leads to immune activation, but in the absence of SAMP disruption, the fly is able to specifically encapsulate the parasitoid egg while protecting against self-encapsulation. Conversely, while internal tissue damage in a naive larva does attract hemocyte interactions, in the absence of an immune stimulus, this does not lead to self-encapsulation, but rather the hemocytes attempt to repair the damaged tissue. That blood cells err on the side of fixing disrupted self tissue rather than treating it as pathogenic and encapsulating it unless another stress signal is also present suggests that flies may have evolved a multi-input system to safeguard against spurious encapsulation (Mortimer, 2021).

D. melanogaster immune responses have proven to be an excellent model for understanding the mechanisms underlying conserved innate immune responses, including those of humans. Findings on Drosophila self-tolerance may also be relevant to human innate self-tolerance. Indeed, data from a range of studies are consistent with the idea that protein glycosylation is a mediator of vertebrate immune responses, and cell-surface glycans have been proposed as candidate SAMPs for the innate immune response to distinguish healthy self tissues from aberrant or foreign tissues even if the mechanisms are not entirely understood. Protein-linked sugar groups should presumably fit this role well, as they can take on diverse combinations of sugar residues and branching patterns (Mortimer, 2021).

Protein N-glycosylation is a complex multistep process that begins with the addition of a presynthesized glycosyl precursor to the protein at an asparagine residue. This nascent glycan is then trimmed back to a core glycan structure, a process that is initiated by the activity of GCS1. The core glycan is then elaborated with the addition of multiple carbohydrate groups to give rise to a variety of final structures, with hybrid and complex type N-glycans among the most prevalent. Glycan elaboration begins with the activity of Mgat1, which leads to the production of hybrid type N-glycans. These hybrid N-glycans can be further processed by the α-mannosidase-I and -II family of enzymes to produce paucimannose N-glycans, which can serve as complex N-glycan precursors and are further elaborated by downstream enzymes to give rise to the final complex N-glycan structure. The current data suggest that disruption of any of the genes encoding key N-glycan-processing enzymes will be associated with the loss of self-tolerance in Drosophila. Similarly, the loss of the α-mannosidase-II (αM-II) gene in mice is linked with the development of an autoimmune phenotype that is likened to systemic lupus erythematosus. Like the tuSz¹ mutant, the αM-II mouse phenotype arises due to alterations in protein N-glycosylation in nonimmune tissues and is mediated by innate immune cells. Altered patterns of protein N-glycosylation are also observed in additional mouse models of autoimmune disease and have been linked to autoimmune disease in human patients (Mortimer, 2021).

The ECM is a conserved structure made up of numerous proteins, many of which are N-glycosylated, including laminin and collagen. A role for the ECM in mediating self-tolerance has been previously proposed: The encapsulation of self tissues in D. melanogaster is also observed following RNA interference (RNAi) knockdown of genes encoding the ECM proteins laminin and collagen, supporting the idea that SAMPs reside in the ECM. The role of the ECM in self-tolerance is further emphasized by tissue transplantation studies in Drosophila. Drosophila larvae are largely tolerant of conspecific tissue transplants, but this tolerance is abolished when tissues are first treated with collagenase to disrupt the ECM, leading to the specific encapsulation of treated tissues. Reactivity to ECM proteins is also associated with various forms of human autoimmune disease. Based on these data, it is proposed that the N-glycosylation of ECM proteins may serve as a conserved self-tolerance signal for innate immune mechanisms and that loss of ECM protein N-glycosylation may lead to loss of self-tolerance and, consequently, autoimmune disease in a diverse range of species (Mortimer, 2021).

An alternative means by which hosts can recognize pathogens is missing-self recognition. Instead of tracking pathogen diversity with numerous recognition receptors, as in nonself recognition, missing-self recognition does not rely on tracking pathogens at all, but only on specifically recognizing self and attacking tissues that lack the self signal. Further, unlike germ line-encoded forms of nonself recognition, missing-self recognition systems allow host species to respond to novel pathogen types that they have never encountered in their evolutionary history. Protein glycosylation plays an important role in the handful of identified missing-self recognition systems of vertebrates. The most well-known case of missing-self recognition involves the interaction between vertebrate NK cells and host MHC class I (MHCI) proteins. All vertebrate cells produce MHCI to display any possible antigens present in their cytoplasm to T cells, but intracellular pathogens often suppress host cell MHCI expression to prevent their molecules from being displayed. NK cells are lymphoid-type cells that induce cytolysis in infected host cells. In an uninfected state, recognition of properly glycosylated MHCI inhibits NK cell cytolysis of host cells, but in an infected state in which host cells are missing the MHCI self signal, the NK cell inhibitory receptors fail to recognize 'self,' and the infected host cells are lysed, an effective means of killing intracellular pathogens that have suppressed host MHC signaling (Mortimer, 2021).

As of yet, there are no examples of missing-self immune recognition systems in invertebrates, and it has been hypothesized that invertebrate immune systems rely largely on PRRs for nonself recognition of pathogens. Still, invertebrates do mount immune responses against a variety of inanimate objects like oil droplets, sterile nylon, and charged chromatography beads as well as tissue transplants from other insect species. All of these foreign bodies presumably lack distinct PAMPs, suggesting that invertebrates have some sort of missing-self recognition system. Additionally, while multiple antimicrobial PRRs have been identified in the Drosophila genome, PRRs targeting macroparasites like parasitoid wasps have not yet been discovered. The current model of self-recognition suggests that following parasitoid infection, activated immune cells assess all exposed tissue surfaces for the self-tolerance glycan signal and that the absence of this Drosophila SAMP on parasitoid wasp eggs might be the cue that targets them for melanotic encapsulation (Mortimer, 2021).

Search PubMed for articles about Drosophila GSC1

Mortimer, N. T., Fischer, M. L., Waring, A. L., Kr, P., Kacsoh, B. Z., Brantley, S. E., Keebaugh, E. S., Hill, J., Lark, C., Martin, J., Bains, P., Lee, J., Vrailas-Mortimer, A. D. and Schlenke, T. A. (2021). Extracellular matrix protein N-glycosylation mediates immune self-tolerance in Drosophila melanogaster. Proc Natl Acad Sci U S A 118(39). PubMed ID: 34544850

date revised: 11 August 2023

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