Abstract
Intestinal epithelial cells (IEC) have important functions in nutrient absorption, barrier integrity, regeneration, pathogen-sensing, and mucus secretion. Goblet cells are a specialized cell type of IEC that secrete Trefoil factor 3 (TFF3) to regulate mucus viscosity and wound healing, but whether TFF3-responsiveness requires a receptor is unclear. Here, we show that leucine rich repeat receptor and nogo-interacting protein 2 (LINGO2) is essential for TFF3-mediated functions. LINGO2 immunoprecipitates with TFF3, co-localizes with TFF3 on the cell membrane of IEC, and allows TFF3 to block apoptosis. We further show that TFF3-LINGO2 interactions disrupt EGFR-LINGO2 complexes resulting in enhanced EGFR signaling. Excessive basal EGFR activation in Lingo2 deficient mice increases disease severity during colitis and augments immunity against helminth infection. Conversely, TFF3 deficiency reduces helminth immunity. Thus, TFF3-LINGO2 interactions de-repress inhibitory LINGO2-EGFR complexes, allowing TFF3 to drive wound healing and immunity.
Subject terms: Growth factor signalling, Mechanisms of disease, Parasitic infection, Chronic inflammation
TFF3 secretion by goblet cells regulates mucus viscosity and wound healing, but a receptor for TFF3 has not been identified. Here, the authors show that TFF3 binds LINGO2 to de-repress and enhance EGFR signaling that drives wound healing and immunity against helminths.
Introduction
Gastrointestinal (GI) tissue undergoes regenerative and injury-induced repair through diverse pathways that span arachidonic acid metabolites (e.g., prostaglandins, leukotrienes), growth factors (e.g., KGF, EGF), regulators of development (e.g., Notch, Wnts, Hippo), and reparative cytokines (e.g., Trefoil factor family proteins (TFF1-3))1–8. TFF3 is a small (6 kDa) secreted glycoprotein produced by goblet cells that drives wound healing throughout the respiratory, ocular, biliary, genitourinary and GI mucosa9–11. Despite identification over 30 years ago, and numerous studies implicating TFF3 in human diseases, the existence and/or identity of a bona-fide receptor responsible for TFF3-mediated cellular migration, anti-apoptotic activity, cell adhesion control, and/or immunoregulation remains unknown12–15. Autoradiography and chemical cross linking-based studies have identified TFF binding sites along the GI tract of rodents and most data suggests that TFFs can induce MAPK, beta catenin, and/or EGFR activation16–19. Still, whether a TFF3 receptor exists has been controversial because of evidence that TFF3 (along with TFF2) increase the rheological properties of mucus20,21. Given the implication for TFF3 (and TFF2) as a regulator of GI diseases such as Ulcerative colitis and Crohn’s disease, there is a clear need for a better understanding of TFF biology.
Evidence shows that pathways of tissue repair/regeneration and immunity functionally intersect22. Although TFF3 was previously considered solely a regulator of re-epithelialization, TFF3 has been reported to exert both pro and anti-inflammatory activities23–27. TFF3 undergoes auto-induction at its promoter, which is dependent upon STAT3 activity28 and TFF3 treatment can induce both pSTAT3 and pEGFR activation14,29, but there are no data that show either gp130 or EGFR directly bind to TFF330. EGFR activation serves an important role in host immunity, particularly against GI helminths such as the rodent hookworms Heligosomoides polygyrus bakeri and Nippostrongylus brasiliensis (N.b.). Amphiregulin, an EGFR ligand, promotes Type 2 inflammation and worm expulsion through EGFR dependent signals31. Type 2 immune responses can be synergistic with tissue repair, therefore we reasoned that the reparative and immunoregulatory effects of TFF3 would be evident during helminth infection or colitic inflammation.
This work ventures into the major gap of Trefoil factor family biology by searching for a TFF3 receptor. An approach was used called TRICEPS that employs ligand-guided covalent binding and capture of glycoproteins on the cell surface to identify low-affinity ligand-receptor interactions32. This approach led us to discover Leucine-rich repeat and immunoglobulin-like domain-containing nogo receptor-interacting protein 2 (LINGO2), as a critical component of TFF3 cellular responsiveness. LINGO2 is required for TFF3-mediated EGFR and STAT3 activation in intestinal epithelial cells (IEC). Paradoxically, LINGO2 deficiency in mice results in a phenotype opposite of TFF3 deficiency due to repressive EGFR-LINGO2 interactions. In support of this hypothesis, the increased disease severity of DSS colitis in LINGO2 deficient mice and their resistance to N.b. infection is both reversed following treatment with the EGFR antagonist Gefitinib. Taken together, these data suggest a model wherein TFF3 binding to LINGO2 reverses tonic inhibition of EGFR by LINGO2, resulting in heightened EGFR activation.
Results
LINGO2 expression facilitates responsiveness to TFF3
Anti-inflammatory activities of TFF3 suggested that leukocytes may directly respond to TFF325,27. Given the cell number requirement for the TRICEPS screen (109/sample) and the apparent lack of success with epithelial cell lines, we used a human macrophage/monocyte cell line (U937) to search for a potential TFF3 receptor. TFF3-induced interleukin 10 (IL-10) production occurred over several orders of magnitude in these cells, but was lost at higher doses of recombinant human TFF3 (rhTFF3) (Fig. 1a). This bi-modal response seemed similar to the activity range for cytokine and growth factor receptors33, implying that TFF3 was acting through a conventional ligand-receptor interaction.
We postulated that TFF3 interacted with its receptor through low-affinity interactions reliant upon carbohydrates because TFF3 glycosylation has been shown critical for biological activity34,35, therefore U937 cells were subjected to the TRICEPSTM protocol as a biochemical screening strategy to identify glycosylated transmembrane protein(s) on the cell surface32,36. As bait, rhTFF3 was covalently linked to the TRICEPS probe and incubated with PMA-treated U937 that had been treated with sodium periodate. Cell pellets were subjected to glycosidase digestion and peptides generated using mass spectrometry. As a positive control to account for enrichment efficiency, recombinant human insulin was used as bait for the insulin receptor. Whereas the TFF3-probe induced an 8-fold enrichment of LINGO2 (LIGO2) peptide, the insulin-probe induced a 7.76-fold-enrichment in INSR peptide (Fig. 1b). No other enriched peptides in this screen met this level of enrichment or were derived from proteins that satisfied our selection criterion (e.g., extracellular domain, transmembrane region, cytoplasmic tail).
To further explore functional interactions between TFF3 and LINGO2, experiments were performed that employed anti-human LINGO2 Mab treatment of U937. At high doses, anti-LINGO2mAb blocked TFF3-induced IL-10 production (Fig. 1c) and resulted in a dose-dependent neutralization of TFF3-mediated TNF-alpha suppression (Supplementary Fig. 1A). Next, to ask whether LINGO2 and TFF3 were physical binding partners, we performed co-immunoprecipitation experiments utilizing murine TFF3-Fc vs. empty vector Fc incubated with Flag-tagged LINGO2, which revealed that TFF3 could pull down LINGO2 protein (Fig. 1d), indicating that TFF3 could bind LINGO2.
LINGO2 and TFF3 interact on the epithelial cell surface
Epithelial cell lineages are the major source and target of TFF337–39, therefore we focused on whether LINGO2 could capture TFF3 at the epithelial cell surface. HEK293 cells were double transiently transfected with constructs encoding LINGO2-GFP and TFF3-RFP, fixed, permeabilized, and imaged using confocal microscopy. Whereas TFF3-RFP single transfectants had multiple fluorescent puncta throughout the cytoplasm of the positive cells (Fig. 1e), TFF3-RFP transfectants that co-expressed LINGO2-GFP had clear yellow signal at the cell membrane, suggesting co-localization (Fig. 1f and Supplementary Fig. 1B). To confirm the LINGO2 extracellular domain was necessary for the TFF3 interaction, we co-transfected TFF3-RFP with a NH2-terminal LINGO2-Flag truncation mutant (Δ350 AA), which abrogated co-localization and resulted in TFF3 red fluorescent puncta, similar to TFF3-RFP single transfectants (Fig. 1g). These data suggested that the leucine-rich-repeat (LRR) ectodomain region was responsible for ligand binding. As a control and to address reports suggesting that TFF3 may also signal through the SDF-1 receptor CXCR440, CXCR4-GFP was co-transfected with TFF3-RFP or SDF1-RFP. Data show TFF3-RFP/CXCR4-GFP transfectants had predominantly red puncta away from the cell membrane suggesting no co-localization, whereas SDF1-RFP transfectants that co-expressed CXCR4-GFP demonstrated yellow signal providing evidence of membrane co-localization (Fig. 1h, i and Supplementary Fig. 1B).
To ensure that co-localization in our double transfection over-expression HEK system was not due to intracellular interactions, we changed the approach to a flow cytometry strategy involving exposure of LINGO2-GFP transfectants to soluble rTFF3 labeled with Alexa-Fluor 647 (TFF3-647). A flow cytometry gating strategy was devised (Supplementary Fig. 1C) to determine the portion of cells that were doubly fluorescent for GFP and AF-647 (Fig. 1j). Compared to incubation with PBS, or IL-2-647, (Fig. 1k, l) LINGO2-GFP transfectants had a 12-fold increase in double-positive cells following incubation with TFF3-647 (Fig. 1m). TFF3-647 did not bind to other LRR receptors such as TLR2 (Fig. 1n) and SDF-1-647 did not bind LINGO2 (Fig. 1o). TFF3 showed moderate binding to CXCR4 transfectants (Fig. 1p).
Next, to determine whether endogenous TFF3 production and LINGO2 expression occurred in a similar intestinal microenvironment to allow biologically relevant interactions in situ, normal human rectal tissues were immunostained with TFF3 and LINGO2 Ab to assess their spatial patterns. Compared to control Ab (Fig. 1q), LINGO2 was expressed in both enterocytes and sparsely along crypt epithelia, whereas TFF3 stained goblet cells located within crypts (Fig. 1r, s). In some instances, patches of cell-free TFF3 localized to the apical surface of LINGO2 positive enterocytes (Fig. 1r). Furthermore, distance between TFF3 and LINGO2 was determined using proximity ligation assay (PLA), which tests whether two Ab-bound proteins are within the 40 nm required for rolling circle amplification to produce fluorescent punctae41. Compared to single stained anti-LINGO2 mAb (Supplementary Fig. 1D and F), combined staining with TFF3 and LINGO2 Ab revealed puncta within intestinal lamina propria and crypt epithelia (Supplementary Fig. 1E and G). Collectively, these data suggested that TFF3-LINGO2 interactions occurred within 40 nm on the cell membrane.
TFF3 requires LINGO2 to activate EGFR and STAT3
TFF3-mediated cytoprotective functions are associated with EGFR and ERK activation13,42,43,15. To determine if TFF3 impacted epithelial EGFR signaling through LINGO2, we used a MC38 cells (murine intestinal adenocarcinoma) that expresses all four LINGO family member genes (Fig. 2a). A Lingo2KO subclone of MC38 (Δ-LINGO2) was generated using CRISPR-mediated insertion of a GFP-H2-Kk knock in construct followed by FACS sorting (Fig. 2b) showing a 20-fold reduction in LINGO2 mRNA expression (Fig. 2c). To directly test whether TFF3 protected against cytotoxicity in a LINGO2-dependent manner, we compared the parental MC38 vs. Δ-LINGO2 MC38 lines in cell-death assays using the apoptotic agent, staurosporine44. Whereas TFF3-Fc pre-treatment significantly reduced cytotoxicity of parental MC38, protection was abrogated in Δ-LINGO2 cells (Fig. 2d). Given the anti-apoptotic role for STAT3 and evidence that TFF3 regulates STAT3 at the promoter level, we asked whether TFF3-driven STAT3 activation required LINGO2. TFF3-Fc (1ug/ml) induced STAT3 phosphorylation (pStat3 705) within 5 min in parental MC38 that slowly dissipated over 30 min (Supplementary Fig. 2A, B). However, TFF3-Fc-treated Δ-LINGO2 cells lacked pSTAT3 to this extent (0.5 fold after 5 min treatment) and showed reduced total STAT3 levels compared to loading controls (Supplementary Fig. 2A, B). Moreover, TFF3-Fc-induced pEGFR within parental MC38, but only a marginal pEGFR increase in Δ-LINGO2 cells (Fig. 2e). Thus, LINGO2 facilitated TFF3 mediated cytoprotection, STAT3 induction, and EGFR activation.
LINGO2 binds to EGFR and TFF3 reverses that interaction
Evidence suggesting that LINGO1 (a LINGO2 paralog) binds to and blocks EGFR activation45 prompted us to investigate whether LINGO2 directly interacted with EGFR. Reciprocal co-immunoprecipitation experiments demonstrated that Flag-LINGO2 and GFP-EGFR bound to each other (Fig. 2f) and confocal microscopy experiments demonstrated that EGFR-GFP and LINGO2-RFP co-localized on co-transfected cells (Supplementary Fig. 2C). Because TFF3 required LINGO2 to induce pEGFR (Fig. 2e), LINGO2 could pull-down and co-localize with EGFR, and LINGO1 inhibits EGFR, we surmised that a constitutively bound LINGO2-EGFR complex may be disrupted by TFF3 binding to LINGO2 thereby sequestering LINGO2 away from EGFR to facilitate pEGFR activation. Consistent with this hypothesis, Δ-LINGO2 cells expressed levels of the EGFR-responsive genes Areg and Egf, that were 4–5-fold increased compared to parental MC38 following exposure to EGF (Fig. 2g, h). Furthermore EGF-treatment induced stronger pEGFR accumulation in Δ-LINGO2 cells compared to parental MC38 (Fig. 2i).
To directly test whether TFF3 disrupted a constitutively bound LINGO2-EGFR complex, we devised a strategy wherein LINGO2-EGFR protein complexes were co-incubated with either media, an irrelevant cytokine (rIL-2) or rTFF3 in solution. Complexes were immunoprecipitated to detect Flag-LINGO2 and immunoblotted for GFP to detect EGFR. Data show complex formation remained intact following media only or rIL-2 treatment, but rTFF3 reduced the abundance of LINGO2-EGFR complexes (Fig. 2j). Lastly, we noticed the doubling rate of Δ-LINGO2 cells was consistently higher than the MC38 parental line which, if our model was correct, relied upon constitutive pEGFR. Indeed, treatment with the EGFR tyrosine kinase inhibitor Gefitinib significantly reduced the doubling rate of Δ-LINGO2 cells over a three-day period, but had minimal impact on the parental MC38 line (Fig. 2k). Altogether, this indicated a model wherein an otherwise constitutive LINGO2-EGFR repressive complex was disrupted by TFF3, allowing EGFR activation through ligand dependent or independent mechanisms, explaining how TFF3 could drive EGFR activation without being an EGFR ligand46.
LINGO2 deficient mice have excess intestinal EGFR activation
LINGO2 knockout mice (Lingo2KO) were generated through CRISPR/CAS9 gene-editing using gRNAs for gene disruption through non-homologous end joining (NHEJ). Of the several Lingo2KO founder lines created, we selected one strain with a 200 bp deletion immediately downstream of the transcriptional start site (Supplementary Fig. 3A). For initial characterization, intestinal tissues were subjected to LINGO2pAb immunostaining which revealed lack of LINGO2 on small intestinal villi in Lingo2KO mice compared to WT C57BL/6 controls and isotype matched Ab (Supplementary Fig. 3B–D).
To test our model that predicted LINGO2 constitutively suppressed EGFR, whole colonic tissues were isolated from WT and Lingo2KO mice and several animals were compared for their total vs. activated pEGFR levels under naive, steady-state conditions. Colonic tissues from Lingo2KO mice had no difference in colon length vs WT (Supplementary Fig. 3E), but higher basal levels of pEGFR compared to WT tissue (Fig. 3a). Consistent with enhanced EGFR activity, despite macroscopically similar appearing colonic crypts and colon lengths (Fig. 3b), quantification of S-phase crypt epithelial cells following anti-E-cadherin and anti-Ki-67 mAb co-immunostaining revealed a higher basal number of Ki-67 positive cells in the Lingo2KO compared to the WT (Fig. 3c). Also, Lingo2KO mice displayed impaired mucosal barrier function, demonstrated by 4-fold higher levels of FITC-dextran (4 kDa) in the peripheral blood at 4 h. post intragastric administration as compared to similarly treated WT mice (Fig. 3d). However, 16srDNA sequencing to determine fecal microbiome composition did not reveal any substantial differences between strains (Supplementary Fig. 3F). Thus, in mice Lingo2 deficiency resulted in heightened pEGFR activity and increased permeability of the GI tract at the steady-state.
DSS-induced colitis in Lingo2KO mice is EGFR-dependent
To determine the biological consequence of excess EGFR activation on intestinal injury and repair, Lingo2KO and co-housed WT littermates were orally administered dextran sodium sulfate (DSS) to induce transient colitic tissue injury (Fig. 3e). At 5 days following cessation of 2.5% DSS treatment, Lingo2KO mice had severe colitis marked by significantly greater weight loss and disease activity as compared to WT controls (Fig. 3f, g). Colon immunopathology was more severe in Lingo2KO mice, with greater degree of submucosal edema, ulceration, smooth muscle hyperproliferation and inflammatory infiltration as compared to WT controls (Fig. 3h). These experiments were then done with Gefitinib treatment to determine if inhibition of EGFR would reverse colitic disease in Lingo2KO mice. Indeed, weight loss, colon inflammation clinical scores, and colon shortening were all significantly reduced by Gefitinib treatment of Lingo2KO with negligible effect in the WT (Fig. 3i–l).
LINGO2 mRNA expression was detected in IEC, but also in myeloid cells, B cells, and CD4+T cells (Fig. 4a), therefore irradiation bone marrow chimeras were generated to test the contribution of hematopoietic vs. non hematopoietic cells (Supplementary Fig. 4A-D). Compared to WT-WT chimeras, both WT-KO and KO-WT groups showed worse colitis, as defined by weight loss, disease activity, and clinical pathology, yet neither was as severe as the KO-KO group (Fig. 4b–e). There were no differences in colon length (Supplementary Fig. 4E). Surprisingly, despite a report that Tff3KO mice are highly sensitive to (DSS)-induced colitis47, we found that Tff3KO were not more susceptible to colitis than WT (Fig. 5a–c). This unexpected outcome was likely due to EGFR activity because Gefitinib treatment of Tff3KO mice, but not of WT mice, exacerbated colitic disease (Fig. 5d–g). Indeed, Egf mRNA transcripts were significantly higher in DSS-treated Tff3KO mice compared to WT or compared to either strain following Gefitinib treatment (Fig. 5h). Collectively, this suggested that EGFR activity, perhaps due to increased availability of EGFR ligand, protected Tff3KO mice from an otherwise increased susceptibility to DSS-induced colitis.
LINGO2 deficiency protects against helminths via EGFR
EGFR signaling drives host protective immune responses against different GI nematode species48. Goblet cells secrete substances toxic to GI helminths including Muc2, Muc5ac, and RELM-beta and also secrete immunoregulatory molecules such as TFF249–52. Mice were inoculated with the infective larval stage (L3) of Nippostrongylus brasiliensis (N.b.). In this system, L3 migrate from the skin to the lung, down the esophagus, and into the jejunum within 3d where they develop into fecund adults that are expelled between 9–12 days post-infection via suppression of Type 1 (IL-12,IFN-γ) and induction of Type 2 cytokine production (e.g., Areg, interleukins 4, 5, 9, 13, 25, and 33)51,53 (Fig. 6a). Tff3 mRNA jejunal transcripts peaked between 5–7d post infection (Fig. 6b). Importantly, N.b.-infected Tff3KO mice harbored significantly more adult worms than WT mice at 9d (Fig. 6c). Increased susceptibility in Tff3KO mice was associated with increased IFN-γ production from mesenteric lymph node cells, without differences in Type 2 cytokines (Fig. 6d). At baseline, prior to infection, there were no differences in CD3 stimulated Type 1 vs. Type 2 cytokines between naïve WT and Lingo2KO mice (Supplementary Fig. 4F). Strikingly, N.b.-infected Lingo2KO mice had early reduced worm numbers by d4 (Fig. 6e) and reduced fecal egg numbers compared to WT between d7–8 post-infection (Fig. 6f). This protective phenotype occurred regardless of co-housing or the sex of the animals. Curiously, Type 2 cytokines (IL-9 and IL-13) were significantly reduced in Lingo2KO MLN cells compared to WT (Fig. 6g, h). Host resistance in Lingo2KO mice was primarily due to non-hematopoietic cells, as irradiation BM chimeras demonstrated that transfer of WT BM into KO mice resulted in significantly less fecal egg output as compared to the KO−WT chimeras (Fig. 6i). Consistent with the notion that enhanced EGFR activity occurred in the absence of LINGO2, we found higher amounts of pEGFR in intestinal homogenates of Lingo2KO compared to WT (Fig. 6j, k). As our model predicted, Gefitinib treatment during N.b. infection reversed resistance in Lingo2KO mice resulting in significantly higher intestinal worms compared to vehicle-treated controls (Fig. 6l). This indicated that Lingo2 deficiency protected against helminth infection, at least in part, due to EGFR activity.
Discussion
We report a pathway operating at the GI mucosal interface controlled by LINGO2, a leucine rich repeat and Ig domain containing receptor. Our data support a model wherein TFF3 produced by goblet cells can bind the LINGO2 extracellular domain (NH2 terminal 350 AA) to de-repress inhibitory LINGO2-EGFR complexes, thereby allowing TFF3 to help maintain mucosal barrier integrity, suppress colitis, and promote immunity against GI helminths. TFF3-mediated disruption of LINGO2/EGFR complexes seems to operate similar to a rheostat tuning the extent of EGFR activity inasmuch as total LINGO2 deficiency was deleterious due to excessive basal EGFR activity, but loss of TFF3 was somewhat protective against colitis (Fig. 5d, e) and also impaired worm immunity. Inhibition of EGFR in Lingo2KO mice reversed the phenotype in models of colitis and helminth infection. It is likely this EGFR activity was due to activation by other EGF ligands (e.g., Areg, EGF) or TFF members (i.e., TFF2). While there are certainly other endogenous negative regulators of excessive other EGFR activity such as RALT, LRIG, SOCS4 and SOCS554–56, this is the first demonstration that: (1) TFF family members activate EGFR through de-repression, (2) LINGO2 regulates mucosal repair and immunity, and (3) TFF3 exerts its function(s), at least partially, through LINGO2.
LINGO2 is a 606aa protein with a large extracellular portion (ectodomain), a transmembrane domain, and a short cytoplasmic tail. LINGO2 belongs to the family of leucine-rich repeat and IgG-like domain proteins that includes LRIG157 that is known for ErbB receptor family regulation. LRIG1, a tumor suppressor and stem cell marker, induces proteasomal degradation of EGFR (and its other three ErbB family members) via polyubiquination58. LINGO1 is a component of a heterotrimeric signaling complex comprised of the Nogo-66 receptor and the p75 neurotrophin receptor that directly binds EGFR45 and inhibits EGFR signaling and axon regeneration59. In oligodendrocytes, LINGO1 suppresses signaling of the ErbB2 receptor by preventing lipid raft translocation where it can be phosphorylated60. Similar to other leucine-rich repeat and Ig-like domain proteins, we observed an ErbB receptor family negative regulatory function for LINGO2. Our data suggest that LINGO2 binds with EGFR constitutively, suppressing its ability to undergo phosphorylation. Whether this occurs through catalytic inhibition or receptor downmodulation remains unclear. Whether LINGO2 operates in a signaling complex similar to LINGO159 is also unknown. Regardless, our data support a mechanism wherein upon injury, TFF3 gains access to LINGO2 in a manner requiring the NH2 terminus, sequestering it away from EGFR, thereby raising the level of EGFR activation at times when the mucosa needs to repair tissue or expel pathogens.
Contrary to the initial report of DSS-treatment of the Tff3KO mouse strain47 and in contrast to the prevailing notion that TFF3 is essential for protection of the GI mucosa61, we did not find that Tff3KO mice were more susceptible to DSS colitis. There may be several explanations for this discrepancy. The original study used mice on a mixed background (C57BL/6 × 129/SVJ), whereas our strain was backcrossed to C57BL/6 for >8 generations. Also, that report was published before recognition that microbial flora is a major determinant in GI inflammatory disease and did not take into consideration the importance of cohousing, whereas our studies were all completed using cohoused mice. Lastly, our protocol was 6 days on DSS followed by a return to normal drinking water in order to evaluate the wound healing response, whereas that study continuously treated mice with DSS for 9 days47. Notably, IEC-specific EGFR deletion in mice did not lead to any change in colitic disease severity indicating that normally the loss of EGFR on intestinal epithelia is not deleterious62.
EGFR expression on Th2 cells is critical to their production of Type 2 cytokines that drive worm expulsion31. IEC also serve a host protective role against helminth infection49,51,52. In addition to secretion of toxic factors by IEC, an increased epithelial cell turnover provides host protection against helminths via an “elevator mechanism” that dislodges parasites from their niche through sloughing into the lumen63. Lingo2KO mice had higher basal numbers of proliferating epithelial cells in the crypt compared to WT, which when combined with the enhanced basal pEGFR activity, provides a plausible explanation for host resistance in Lingo2KO mice despite a reduced Type 2 cytokine response. Although there was no impact in the context of DSS colitis, N.b.-infected Tff3KO mice were more susceptible, which was perhaps due to the increased IFN-γ production, known to block immunity against N.b. infection64. TFF3 may serve an additional role as an immunomodulatory cytokine similar to TFF2, which also suppresses Type 1 responses65.
In conclusion, our results provide supportive evidence for a model that predicts LINGO2 binds to and inhibits EGFR activity and that TFF3 interactions with LINGO2 remove this negative regulation, allowing for enhanced EGFR activity. LINGO2 deficiency basally elevates EGFR activity, susceptibility to colitis and resistance to helminth infection. Genome wide association study (GWAS) data show LINGO2 genetic polymorphisms in human patients with cancer66, asthma67, chronic obstructive pulmonary disease68, Parkinson’s, and Inflammatory bowel disease69. In sum, this complex interplay among TFF3, LINGO2, and EGFR deserves greater attention to decipher how this triad controls mucosal repair and immunity.
Methods
Mice
All animal procedures were approved by the Institutional Animal Care and Use Committee at University of California, San Francisco and at University of Pennsylvania. Super-ovulated female C57BL/6 mice (4 weeks old) were mated to C57BL/6 stud males. Fertilized zygotes were collected from oviducts and injected with Cas9 protein (30 ng/μl), sgRNA (15 ng/μl) into pronucleus of fertilized zygotes. Injected zygotes were implanted into oviducts of pseudo pregnant CD1 female mice. Animals were housed under specific-pathogen free barriers in vivarium at San Francisco General Hospital or University of Pennsylvania. Irradiation bone marrow chimeras were generated as previously described70. The recipients were irradiated with 10 Gy administered as a split-dose and were retro-orbitally inoculated with 2–5 × 106 total bone marrow cells isolated from the femurs of the donor animals within one hour following the second dose of irradiation. All irradiated recipients were administered 1% enrofloxacin for 2 weeks post-irradiation and were evaluated for chimerism efficiency at 6–8 weeks post transfer. Only animals showing >90% donor chimerism were used for subsequent data analysis as determined by anti-CD45.1 and anti CD45.2 mAb combined with lineage specific Ab for CD4, B220 and CD11b and CD11c70. All the procedures complied with all relevant ethical regulations for animal testing and research and were reviewed and approved by IACUC at University of California at San Francisco (protocol #AN109782-01) and University of Pennsylvania (protocol #805911).
DSS colitis and Nippostrongylus brasiliensis mouse models
Colitis was induced in mice using 2.5% DSS (w/v, 0216011080, MP-Biomedicals LLC Solon, OH) in autoclaved drinking water for 6 days. Normal water was supplied on Day 6. Mice were assessed for body weight, appearance, fecal occult blood (Fisherbrand™ Sure-Vue™ Fecal Occult Blood Slide Test System, Thermo Fisher Scientific, Inc., Waltham, MA, USA), stool consistency, and diarrhea during the entire course of colitis. Colon length was determined using a ruler at necropsy. Disease activity index (DAI) a cumulative score with a maximum of 12, was derived as follows: every mouse was assigned a score. 1 was for each display of lack of grooming, piloerection, awkward gait, hunched posture, and lack of mobility. Diarrhea score (0, Normal stool, 1 soft stool, 2 loose formed stools and 3, watery fecal matter), Rectal Bleeding was scored as: 0 no blood, 1 minor bleeding and 2 gross rectal bleeding). Fecal occult blood test was scored as: 0, Negative, 1 Mild amount of blood and 2 excessive presence of blood. Pharmacological inhibition of EGFR was achieved by administering Gefitinib (10 mg/Kg body weight i.p., G-4408 LC Laboratories, Woburn, MA) in DMSO. Gefitinib is an inhibitor of epidermal growth factor receptor (EGFR) tyrosine kinase domain that acts through binding to the adenosine triphosphate (ATP)-binding site of the enzyme. Parasites were maintained on copro-cultures incubated at 25C. Larvae from 7 to 14-day old cultures were collected and washed 3× in PBS with 1% Penicillin/Streptomycin. To infect with Nippostrongylus brasiliensis, 650–750 L3 per mouse were injected subcutaneously using procedures previously established51. Mice were evaluated between d4–d9 for intestinal worm burden using the Baermann technique. On d6–d8 fecal pellets were collected from each mouse to assess egg counts. On designated days, mice were euthanized and the proximal half of small intestines were removed, opened longitudinally, and incubated in PBS for 2 h before collecting and counting adult worms.
Microbial sequencing
Genomic DNA was extracted from fecal pellets of mice using Qiagen DNeasy Power Soil Kit. The V4 region of the 16S rRNA gene was amplified using barcoded primers for use on the Illumina platform32,33. Sequencing was performed using 250-base paired-end chemistry on an Illumina MiSeq instrument. QIIME v. 1.834 was used to process paired-end reads and to perform downstream analyses including taxonomy assignment and heat map generation.
Cloning/plasmid construction
The full-length mLingo2 coding region was obtained with KpnI and XhoI double digestion of Lingo2 cDNA plasmid (MR215769, Origene) and subcloned into pcDNA3:GFP vector (Addgene) or pcDNA3:mRFP vector (Addgene) to generate pcDNA3:Lingo2:GFP and pcDNA3:Lingo2:RFP constructs. A serial of deletion of Lingo2 functional domains (Ecto and cytoplasm tails) were generated by PCR amplification with the specific primers bearing KpnI and XhoI overhangs, and cloned into pcDNA3:GFP vector (Adgene) to get Lingo2 truncated constructs fused with GFP. The full-length mTFF3 coding region was amplified with mTFF3 forward primer with HindIII overhang and mTFF3 reverse primer with XhoI overhang using mTFF3 cDNA plasmid (MR200151, Origene) as the template. The digested TFF3 PCR product with HindIII and Xho enzymes was cloned into pcDNA3:mRFP vector (Addgene) to generate pcDNA3:mTFF3:RFP construct. Using the similar method, the full-length mTFF3 coding region was also cloned into pINFUSE-mIgG2b-Fc2 vector (InvivoGen) to generate pINFUSE-TFF3:mIgG2b-Fc2 construct. The full-length mCXCR4 coding region was amplified with mCXCR4 forward primer with HindIII overhang and mCXCR4 reverse primer with XhoI overhang using mCXCR4 cDNA plasmid (OMu23034D, Origene) as the template. The digested CXCR4 PCR product with HindIII and Xho enzymes was cloned into pcDNA3:GFP vector (Addgene) to generate pcDNA3:mCXCR4:GFP construct. The full-length SDF1/CXCR12 coding region was amplified with mSDF1 forward primer with EcoRI overhang and mSDF-1 reverse primer with XhoI overhang using SDF1 cDNA plasmid (MR227229, Origene) as the template. The digested SDF1 PCR product with EcoRI and Xho enzymes was cloned into the pcDNA3:mRFP vector (Addgene) to generate pcDNA3:SDF1:RFP construct. The sequences of constructs were confirmed by DNA sequencing in UPenn Sequencing facility. HEK293 cells were maintained in DMEM medium (Invitrogen) supplemented with 10% fetal bovine serum (Invitrogen) and 1% penicillin-streptomycin (Invitrogen) at 25 °C with 5% CO2. Before transfection, 2 ml of cells (0.2 × 106/ml) per well was seeded on in a well of Lab-Tek Chamber slide (sigma) for overnight culture. 1ug plasmids Lingo2:GFP and TFF3:RFP were co-transfected to the cells using X-tremeGENE HP DNA transfection reagent (Roche) based on the manufacturer’s manual. For co-localization of EGFR:GFP and Lingo2:RFP, pCMV3-mEGFR-GFPSpark(MG51091-ACG, Sino Biological Inc.) and pDNA3-Lingo2:RFP was transfected in the same method. After three-days culture, the cells were fixed with 4% paraformaldehyde in PBS for 10 min and stained with DAPI (0.5ug/ml) for 5 min. After mounting the cells with the coverslip, Lingo2:GFP and TFF3:RFP were visualized using the Leica TCS-NT confocal microscope.
Human tissue and immunostaining
Archived rectosigmoid biopsy samples were from the SCOPE cohort at the University of California, San Francisco (UCSF). The SCOPE cohort is an ongoing longitudinal study of over 1,500 HIV-infected and uninfected adults followed for research purposes. The UCSF Committee on Human Research reviewed and approved the SCOPE study (IRB# 10–01218), and all participants provided written informed consent. PLA starter Duolink® In Situ Orange Starter Kit Mouse/Goat was obtained from Sigma Aldrich containing the PLA probes anti-goat MINUS and anti-mouse PLUS with the detection reagent DuoLink® Orange. Fluorescence for proximity ligation was observed under a Cy3 filter on a Leica Inverted Microscope DMi8 S Platform Formalin fixed paraffin embedded (FFPE) tissues were stained with primary antibodies for anti-LINGO2 Ab was raised against Goat (R&D Systems; AF3679) and anti-TFF3 raised against mouse (eBioscience; 5uclnt3) were used to detect the LINGO2 and TFF3 interactions in situ and for PLAs that followed the manufacturers instruction (Sigma). Technical controls omitting either one of the antibodies served as a negative control as no circular DNA oligonucleotides were able to be amplified.
Soluble ligand binding assay
Purified murine IgG (RayBiotech), murine IL-2, murine recombinant TFF3 (Peprotech) and SDF1a (R&D systems) were labeled with Alexa Fluor 647 dye using the Alexa Fluor 647 protein labeling kit (A30009, Molecular Probes) per manufacturer’s directions. HEK293 cells were maintained in DMEM medium (Invitrogen) supplemented with 10% fetal bovine serum (Invitrogen) and 1% penicillin-streptomycin (Invitrogen) at 37 °C with 5% CO2. HEK293 cells were seeded at 1 × 106/well and incubated at 37 °C with 5% CO2 overnight before transfection with 3ug of plasmids pCMV3-mTLR2-GFP, pCMV3-mLINGO2-GFP or 6ug of pCMV3-mCXCR4-GFP using FuGENE HD DNA transfection reagent (Promega) based on manufacturer’s instructions. 24 h post transfection, cells were incubated with either PBS or 3 μg of Alexa Fluor 647 labeled soluble ligands for 16 h at 4 °C. Cells expressing GFP and/or Alexa Fluor 647 positive were identified by flow cytometry using a LSRII.
Co-immunoprecipitation
HEK293 or CHO cells were transfected with 2 μg pCMV6:Lingo2:Flag (MR215769, Origene) plasmid. After 72 h., the cells were washed and lysed with 0.5 ml Digitonin extraction buffer [1% digitonin; 150 mM NaCl; 1 mM MgCl2;10 mM TrisCl, pH 7.4, 1× Proteinase Inhibitor (Sigma)]. The protein concentration was measured using BCA assay kit (Invitrogen). A minimum of 0.6 mg total protein was incubated with 10 μg TFF3-Fc (Ray Biotech) or Fc (Ray Biotech) overnight and precipitated with 30 μl protein G agarose (Invitrogen) for 2 h at 4 °C. The IP complex was washed and eluted with 2× SDS loading buffer at 95 °C. The IP complex along with 5% input were immunoblotted with mouse anti-Flag antibody (1:5000) (Sigma) to detect the interaction between Lingo2:Flag and TFF3-Fc. In some cases, rabbit anti-FLAG followed by incubation with goat anti-rabbit IgG Cy5 and goat anti-mouse IgG Cy3 antibodies was used. To detect EGFR and Lingo2 interactions, HEK293 were co-transfected with 2 μg pEGFR:Flag (MG51091-CF, Sino Biological Inc.) and 2 μg pDNA3:Lingo2:GFP plasmids. Around 0.5 mg total protein was precipitated using 2 μg normal rabbit IgG (the negative control), 2 μg anti-GFP rabbit polyclonal antibody (Torrey Pines Biolabs), 2 μg normal mouse IgG (the negative control), 2ug anti-Flag mouse antibody (Sigma) and incubated with 30 μl protein A agarose (Invitrogen) for 2 h at 4 °C. The IP complex precipitated with anti-GFP antibody along with 5% input was immunoblotted with mouse anti-Flag antibody (1:5000) (Sigma) to detect the interaction between Lingo2:GFP and EGFR-Flag. The IP complex precipitated with anti-flag antibody along with 5% input was immunoblotted with rabbit anti-GFP antibody (1:5000) for the detection of reciprocal interaction. To determine if TFF3 disrupts the interaction between Lingo2 and EGFR, the mixture of 100ug Lingo2:GFP lysate and 50ug EGFR-flag lysate was incubated with either 2 μg TFF3 or 2 μg mIL2 (R& D System) proteins for two hours at 4 °C. Then, Co-IP was performed with either rabbit anti-GFP antibody or normal rabbit IgG as above. The IP complex was immunoblotted with mouse anti-Flag antibody (1:5000) (Sigma) to detect the interaction between Lingo2:GFP and EGFR-flag. IL-10 levels determined by ELISA according to the manufacturer’s protocol (EBioscience).
Cell lines
The U-937 histocytic cell line was obtained from ATCC (ATCC® CRL-1593.2) and grown according to the suggested culture conditions in RPMI-10. Prior to stimulation with 100 ng/ml E. coli lipopolysaccharide, cells, were treated overnight with PMA to induce macrophage morphology. Supernatants were collected after 24 h in the presence or absence of rhTFF3 (Peprotech) for measurement of IL-10 or TNF using the manufacturer’s suggestion. LINGO2 Knockout colon epithelial cell line (MC38) was generated based on HR (homologous recombination)-mediated CRISPR–Cas9 genome editing. Validated LINGO2 guide RNA obtained commercially (PNA Bio). Briefly, MC38 cells were maintained in DMEM medium (Invitrogen) supplemented with 10% fetal bovine serum (Invitrogen), 1XNEAA, 10 mM HEPES, 1 mM Sodium Pyruvate, 50ug/ml Gentamicin and 1% penicillin-streptomycin (Invitrogen) at 25 °C with 5% CO2. Before transfection, cells (1 × 105 per well) were seeded in six-well plates for overnight culture. 1ug Lingo2 sgRNA, 6 μg Lingo2 donor vector and 1ug Cas9 plasmid were co-transfected into the cells using X-tremeGENE HP DNA transfection reagent (Roche). After expanding the cells for two generations, H2-KK positive cells (Lingo2KO cells were identified by flow cytometry because Lingo2 donor vector contains a H2-KK gene cassette. Reduction of Lingo2 mRNA expression in H2-KK positive cells was confirmed by real-time QRT-PCR.
Western blotting
M38 parental or Lingo2 KO cell lines (1 × 105 per well) were seeded in 24-well plates and cultured for 48 h. in DMEM-10. Cells were then washed and cultured in serum-free medium for 24 h. Cells were treated with TFF3-Fc (1ug/ml) for 0, 5 min, 15 min, 30 min, and 1 h and were lysed with buffer containing [1% Triton X-100, 150 mM NaCl,10 mM Tris (pH 7.4), 1 mM EDTA, protease and phosphatase inhibitor cocktail (Sigma)]. Mouse colon tissues from WT or Lingo2 KO mice were lysed with RIPA buffer containing protease and phosphatase inhibitor cocktail (Sigma). 15ug samples were loaded and separated in 4–12% PAGE gel (Invitrogen). The following primary Abs were used for immunoblotting assays: mouse anti-Stat3 (Cell signaling,1:5000), rabbit anti-pStat3(Y705) (cell signaling, 1:1000), rabbit anti-EGFR (Millerpore,1:1000), rabbit anti-pEGFR (Y1068) (Abcam,1:1000); mouse anti-actin (Santa Cruz, 1:2500). Uncropped blots are shown in the Source Data File.
TRICEPS mass spectrometry
The CaptiRec samples were analyzed on a Thermo LTQ Orbitrap XL spectrometer fitted with an electrospray ion source. The samples were measured in data dependent acquisition mode in a 40 min gradient using a 10 cm C18. The 12 individual samples in the CaptiRec dataset were analyzed with a statistical ANOVA model. This model assumes that the measurement error follows Gaussian distribution and views individual features as replicates of a protein’s abundance and explicitly accounts for this redundancy. It tests each protein for differential abundance in all pairwise comparisons of ligand and control samples and reports the p-values. Next, p-values are adjusted for multiple comparisons to control the experiment-wide false discovery rate (FDR). The adjusted p-value obtained for every protein is plotted against the magnitude of the fold enrichment between the two experimental conditions. The area in the volcano plot that is limited by an enrichment factor of twofold or greater and an FDR-adjusted p-value less than or equal to 0.01 is defined as the receptor candidate space. To allow for statistical analysis, the experiment was done in biochemical triplicates. The samples enriched in glycopeptides were analyzed on a Thermo LTQ Orbitrap XL spectrometer. Peptide identifications were filtered to a false-discovery rate of < =1% and quantified using an MS1-based label-free approach.
Histology
Mice were eviscerated, and colons extracted. A small biopsy of the intact distal colon was excised, fixed overnight at 4 °C in neutral 10% formaldehyde, further dehydrated and embedded in paraffin for histological analysis. Another small biopsy in the distal region was snap frozen in liquid N2 prior to analysis. Remaining length of the colon was embedded as “swiss rolls” in OCT. Pathological assessment of colitis was performed blindly by certified pathologists as per established criteria for the Penn Vet Comparative pathology core.
Primers (Forward-Reverse)
mTTF3-5′ GCC CTC TGG CTA ATG CTG TT 3′ and 5′ TTG GGA TAC TGG AGT CAA AGC 3′
mLINGO1-5′ GAA CAA GAT CGT CAT CCT GC 3′and 5′ GAT GTT GAG ATG TCG TAG CC 3′
mLINGO2-5′ GCC CGT AAC CAA GGT GTA GAC-3′ and 5′ TTA AGA GTA ACA CCA CAG CCA GAC 3′
mLINGO3-5′ CTG CAC TTG CTG CTG CT 3′ and 5′ CTC CAG CAT GCG AGA TTC G 3′
mLINGO4-5′ AGA TGC AGC GCC CAA CTT GA 3′and 5′ CGG GAA TAG TGT CCA GTC G 3′
mGAPDH-5′ TGT GTC CGT CGT GGA TCT GA-3′and 5′ CCT GCT TCA CCA CCT TCT TGA 3′
Areg-5′ GCA GAT ACA TCG AGA ACC TGG 3′ and 5′ CTG CAA TCT TGG ATA GGT CCT 3′
Ereg-5′ CAC CGA GAA AGA AGG ATG GA 3′ and 5′ TCA CGG TTG TGC TGA TAA CTG 3′
mCXCR4-5′ CCA CCC AGG ACA GTG TGA CTC 3′ and 5′ GAT GGG ATT TCT GTA TGA GGA 3′
RNA isolation and real-time qPCR
Approximately 0.5 cm of duodenum was removed for RNA extraction using NucleoSpin RNA Plus kit (Macherey-Negel, Dueren, Germany). Five hundred nano-grams of total RNA were used to generate cDNA with Super Script II (Invitrogen). Quantitative real-time PCR was performed on the CFX96 platform (Bio-Rad, Hercules, CA). Gene expression levels were normalized to (Gapdh). Intestinal transcripts from colon biopsies were isolated and analyzed as previously described51.
Statistics
Statistical analyses were performed using GraphPad Prism version 7.0 (GraphPad, La Jolla, CA). A two-tailed Student’s T test or ANOVA was used where appropriate wth Kruskal Wallis used for non-parametric data (DAI scores).
Reporting Summary
Further information on research design is available in the Nature Research Reporting Summary linked to this article.
Supplementary information
Acknowledgements
We appreciate Anil Rustgi and Mike McCune for their critical evaluation of this manuscript.
Author contributions
N.B., Y.J., Y.W., J.P., LY.H., K.Z., T.Y., K.H., T.O., C.P., S.S., and W.N. conducted experiments, M.S. contributed valuable reagents and samples, N.B., Y.J., Y.W., and J.P. helped to write the manuscript and D.R.H. conceived the study and wrote the manuscript.
Data availability
The authors declare that the data supporting the findings of this study are available from the authors upon request. The source data underlying Figs. 1a, c, d, 2a, c–k, 3a, c, d, f–g, i–j, l, 4a–d, 5a–I, 6b–l and Supplementary Fig. 2a–b is available as a Source Data file.
Competing interests
The authors declare no competing interests.
Footnotes
Peer review information Nature Communications thanks Marisa Martin-Fernandez and other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
These authors contributed equally: Yingbiao Ji, Karl Herbine, Yun Wei, JoonHyung Park.
Supplementary information
Supplementary Information accompanies this paper at 10.1038/s41467-019-12315-1.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Data Availability Statement
The authors declare that the data supporting the findings of this study are available from the authors upon request. The source data underlying Figs. 1a, c, d, 2a, c–k, 3a, c, d, f–g, i–j, l, 4a–d, 5a–I, 6b–l and Supplementary Fig. 2a–b is available as a Source Data file.