IL6ST Human

What is IL6ST and what is its primary function in cellular signaling?

IL6ST encodes the GP130 protein which functions as a critical signal transducer for the IL-6 cytokine family. It serves as a shared receptor component that transduces proinflammatory signaling through the Janus kinase signal transducers and activators of transcription (JAK/STAT) pathway, primarily leading to STAT3 activation. The IL-6 family includes over 10 proteins, including IL-11, IL-27, leukemia inhibitory factor (LIF), cardiotrophin-1, and oncostatin M (OSM), which participate not only in inflammatory processes but also in numerous other biological functions . When cytokines bind to their specific receptors, they form a hexameric complex—in the case of IL-6, consisting of two IL-6, two IL-6Rα, and two GP130 molecules—that initiates downstream signaling .

How do IL-11 and IL-6 differ in their interactions with GP130?

IL-11 and IL-6 both activate signaling through assembly with the GP130 receptor, but they exhibit different gp130-binding interfaces that may drive distinct signaling outcomes. Recent cryoEM structural studies have revealed how these cytokines engage with the receptor and how these differences in binding geometry may influence signal transmission across the cell membrane . The specificity of the interaction is determined by the unique cytokine-receptor binding parameters that are sensed by cells to initiate specific signaling programs. These findings support the concept that cytokine receptors can function as "dimmer switches," adjusting their signaling profiles based on different environmental cues and binding geometries .

What are the key structural domains of GP130 that mediate cytokine signaling?

GP130 contains multiple extracellular domains that participate in cytokine binding and signal transduction, a transmembrane domain, and an intracellular domain that interacts with JAK proteins. Recent cryoEM studies have provided detailed insights into the structure of IL-11 receptor recognition complex, revealing how the extracellular domains interact with cytokines and how these interactions may influence receptor configuration . Molecular dynamics simulations have shown that flexibility within these domains, particularly the cytokine-binding core, plays a crucial role in signaling. Notably, the distances between extracellular domains are minimized as the transmembrane helix exits the membrane, suggesting a specific geometric requirement for effective signal transduction .

What types of IL6ST mutations have been identified and what are their molecular consequences?

Several types of IL6ST mutations have been identified with distinct molecular consequences:

• Biallelic loss-of-function (LOF) variants: These cause autosomal recessive hyper-IgE recurrent infection syndrome (MIM 618523), characterized by recurrent respiratory, skin, and eye infections, as well as skeletal abnormalities including craniosynostosis, scoliosis, hip dislocation, and contractures .

• Recessive mutations affecting LIF signaling: These cause a lethal variant of Stuve–Wiedemann syndrome, presenting with skeletal dysplasia, lung dysfunction, congenital thrombocytopenia, dermatitis, renal abnormalities, and defective acute-phase response .

• Somatic gain-of-function (GOF) mutations: Small monoallelic in-frame deletions, particularly IL6ST Ser187_Tyr190del, are established drivers of inflammatory hepatocellular tumors. These mutations lead to constitutive activation of IL6 signaling in the absence of ligand binding .

• Constitutional mosaic GOF mutations: A case has been described with the IL6ST Tyr186_Tyr190del variant present constitutively in a mosaic pattern (15-40% of cells), leading to a novel syndrome of neonatal onset immunodeficiency with autoinflammation and dysmorphy .

How can one distinguish between IL6ST-related disorders and other immune dysregulation syndromes?

Distinguishing IL6ST-related disorders from other immune dysregulation syndromes requires careful clinical evaluation and molecular analysis:

• IL6ST LOF syndrome vs. other hyper-IgE syndromes:

  • IL6ST LOF syndrome presents with increased serum IgE, intermittent eosinophilia, impaired IL6 signaling, and dysregulated acute-phase response.

  • Distinctive features include specific skeletal abnormalities and distinctive patterns of infections .

• IL6ST GOF syndrome vs. STAT3 GOF syndrome:

  • Both show lymphoproliferation with adenopathy, immunodeficiency, interstitial lung disease, thyroiditis, joint laxity, and postnatal growth failure.

  • Important differences: IL6ST GOF patients typically lack autoimmune cytopenias, enteropathy, diabetes, atopic dermatitis, and arthritis that are common in STAT3 GOF syndrome. Additionally, dysmorphic features are more prominent in IL6ST GOF syndrome .

• Key laboratory findings:

  • Constitutional IL6ST GOF shows persistent elevation of inflammatory markers (CRP and SAA)

  • Normal IgE levels (unlike in IL6ST LOF syndrome)

  • Constitutive STAT3 Tyr705 hyperphosphorylation can be detected in patient-derived cells .

What is the molecular basis for phenotypic variability in IL6ST mutations?

The phenotypic variability in IL6ST mutations stems from multiple factors:

• Type of mutation:

  • LOF mutations primarily affect immune function and skeletal development

  • GOF mutations tend to drive inflammation and affect growth and morphology

• Mosaicism:

  • In the case of the constitutional mosaic IL6ST Tyr186_Tyr190del, the variant was present in 15-40% of cells depending on the tissue, potentially contributing to the specific manifestation of symptoms

  • The percentage of cells carrying the mutation may vary across tissues, affecting tissue-specific phenotypes

• Signaling pathway specificity:

  • Some mutations may preferentially affect certain aspects of IL6ST signaling (e.g., LIF-specific defects cause Stuve-Wiedemann syndrome variant)

  • Different mutations can result in varying degrees of pathway activation or inhibition

• Interaction with other genetic and environmental factors:

  • While somatic IL6ST c.560_571del is a driver of inflammatory hepatocellular adenomas, constitutional presence of this mutation does not necessarily lead to liver tumors, suggesting that additional factors are required for tumor development

How does IL6ST transmit signals across the cell membrane to initiate JAK/STAT activation?

IL6ST transmits signals through a complex mechanism involving:

• Hexameric complex formation: When cytokines of the IL-6 family bind to their specific receptors, they form a hexameric complex. For IL-6 specifically, this consists of two IL-6, two IL-6Rα, and two GP130 molecules .

• Conformational changes: Recent cryoEM studies have revealed that cytokine binding induces specific conformational changes in the extracellular domains of GP130. These structural rearrangements are transmitted across the membrane .

• Transmembrane geometry: The structural data suggests that signaling requires a stringent geometry for effective signal transduction. Notably, the distances between extracellular domains are minimized as the transmembrane helix exits the membrane, supporting a "dimmer switch" mode of action .

• JAK recruitment and activation: The conformational changes in GP130 lead to appropriate positioning of the associated JAK proteins, allowing their trans-phosphorylation and activation.

• STAT3 phosphorylation: Activated JAKs then phosphorylate STAT3 at Tyr705, leading to its dimerization, nuclear translocation, and transcriptional activity .

In disease-associated variants of GP130, molecular dynamics simulations have shown increased flexibility within the cytokine-binding core and increased distance between extracellular domains, which may explain impaired signaling even when cytokine binding is preserved .

What experimental evidence supports the "dimmer switch" model of GP130 signaling?

The "dimmer switch" model of GP130 signaling is supported by multiple lines of experimental evidence:

• Structural studies: CryoEM structures of IL-11 and IL-6 receptor recognition complexes reveal specific conformational states that provide a mechanistic basis for how differences in binding interfaces may drive distinct signaling outcomes .

• Molecular dynamics simulations: These have demonstrated that disease-associated variants lead to increased flexibility within the cytokine-binding core and altered distances between extracellular domains, suggesting that the precise geometry of the receptor complex is critical for proper signal transduction .

• Patient-derived cell studies: B lymphoblasts from a patient with a mosaic IL6ST c.560_571del variant exhibited constitutive STAT3 Tyr705 hyperphosphorylation in the absence of cytokine stimulation, confirming that specific structural alterations in GP130 can modify signaling output .

• Differential responses to cytokines: Studies have shown that changes in cytokine-receptor binding parameters are sensed by cells to initiate specific signaling programs, suggesting that receptors can adjust their signaling profiles in response to different environmental cues .

• Inhibitor studies: JAK inhibitors (ruxolitinib and tofacitinib) were able to suppress the constitutive STAT3 activation in patient cells with GOF IL6ST mutations, demonstrating the downstream consequence of altered receptor configuration .

How do IL6ST disease-associated mutations affect receptor structure and downstream signaling?

IL6ST disease-associated mutations have distinct effects on receptor structure and signaling:

• Loss-of-function mutations:

  • Impair cytokine binding or signal transduction

  • Result in defective JAK/STAT pathway activation

  • Lead to impaired inflammatory responses and inadequate defense against infections

• Gain-of-function mutations (e.g., IL6ST c.560_571del/p.Ser187_Tyr190del):

  • Create a constitutively active receptor that signals independently of cytokine binding

  • Western blot analyses of patient-derived B lymphoblasts show constitutive STAT3 Tyr705 hyperphosphorylation in the absence of stimulation

  • Lead to persistent elevation of inflammatory markers (CRP and SAA)

• Structural implications of GOF mutations:

  • In-frame deletions in the IL6ST extracellular domain likely alter the conformation of the receptor

  • These structural changes mimic the active conformation that would normally be induced by cytokine binding

  • CryoEM studies of disease-associated variants have shown they lead to increased flexibility within the cytokine-binding core and altered distances between extracellular domains

• Tissue-specific effects:

  • While somatic IL6ST c.560_571del mutations drive inflammatory hepatocellular adenomas, constitutional presence of this mutation does not necessarily cause liver tumors

  • This suggests that the cellular context and potential additional factors influence the phenotypic manifestation of the mutation

What techniques are most effective for detecting and characterizing IL6ST mutations in patient samples?

Multiple techniques have proven effective for detecting and characterizing IL6ST mutations:

• Whole Exome Sequencing (WES):

  • Used to identify the IL6ST c.560_571del variant in a pediatric proband

  • Particularly useful for detecting novel variants or when the genetic cause is unknown

• Amplicon Deep Sequencing (ADS):

  • Essential for detecting and quantifying mosaicism

  • Successfully identified IL6ST c.560_571del variant in various tissues with different percentages:

    • Peripheral blood: 7.6% variant reads (15.2% mosaicism)

    • Urine sediment: 15% variant reads (30% mosaicism)

    • Hair bulbs: 18% variant reads (36% mosaicism)

    • Buccal swab: 21% variant reads (42% mosaicism)

• Capillary electrophoresis of fluorescently labeled PCR products:

  • Used to estimate the percentage of cells carrying the variant in cell lines

  • In the described case, approximately 95% of EBV-transformed patient's B cells had the c.560_571del variant

• Western blot analysis:

  • Critical for functional validation of mutations

  • Used to assess STAT3 Tyr705 phosphorylation status in patient-derived B lymphoblasts

  • Demonstrated constitutive STAT3 hyperphosphorylation in cells with GOF mutations

• Family studies:

  • Important for determining if variants are de novo or inherited

  • Can help establish pathogenicity of novel variants

How can patient-derived cells be used to study IL6ST signaling dysregulation?

Patient-derived cells provide valuable models for studying IL6ST signaling dysregulation:

• EBV-immortalized B lymphoblastoid cell lines:

  • Can be established from patient peripheral blood

  • Maintain the genetic alterations present in the patient

  • Used to evaluate IL6 signaling pathway activity through assessment of STAT3 Tyr705 phosphorylation

• Functional assays for signaling assessment:

  • Western blot analysis of STAT3 phosphorylation status (pSTAT3-Tyr705)

  • Assessment of downstream target gene expression

  • Cytokine production profiling

  • Response to exogenous cytokine stimulation

• Inhibitor studies:

  • Treatment of patient-derived cells with JAK inhibitors (e.g., ruxolitinib, tofacitinib)

  • Assessment of STAT3 phosphorylation inhibition

  • Provides insights into potential therapeutic approaches

• Comparative analysis:

  • Side-by-side comparison with cells from healthy controls

  • Reveals constitutive activation or impaired signaling in patient cells

  • Quantifies the degree of signaling dysregulation

• Genetic manipulation:

  • CRISPR-Cas9 correction of mutations in patient cells

  • Introduction of patient mutations in healthy donor cells

  • Allows direct assessment of mutation-specific effects

What are the current approaches for structural analysis of IL6ST complexes?

Current approaches for structural analysis of IL6ST complexes include:

• Cryo-electron microscopy (cryoEM):

  • Cutting-edge technique used to solve the structure of the IL-11 receptor recognition complex

  • Also used to solve structures of both IL-11 and IL-6 complexes with disease-associated gp130 variants

  • Provides high-resolution insights into receptor-cytokine interactions and conformational states

• Molecular dynamics simulations:

  • Computational approach that complements experimental structural data

  • Reveals dynamic aspects of receptor behavior

  • Demonstrated that disease-associated variants lead to increased flexibility within cytokine-binding core and altered distances between extracellular domains

• X-ray crystallography:

  • Traditional approach for protein structure determination

  • Has been used in previous studies to determine structures of cytokine-receptor complexes

• Hydrogen-deuterium exchange mass spectrometry (HDX-MS):

  • Provides information about protein dynamics and conformational changes

  • Can reveal regions of proteins that become more or less solvent-exposed upon binding or mutation

• Single-particle analysis:

  • Advanced technique used in conjunction with cryoEM

  • Allows visualization of different conformational states of receptor complexes

How might JAK inhibitors be used to treat IL6ST-related disorders?

JAK inhibitors show promise for treating IL6ST-related disorders, particularly those involving gain-of-function mutations:

• Experimental evidence:

  • Western blot studies demonstrated that STAT3 hyperphosphorylation in patient-derived cells with IL6ST c.560_571del could be inhibited with both ruxolitinib (JAK1 inhibitor) and tofacitinib (primarily JAK3 and to a lesser extent JAK1 inhibitor)

• Therapeutic rationale:

  • GOF IL6ST mutations lead to constitutive activation of JAK/STAT signaling

  • JAK inhibitors target the pathway downstream of the mutated receptor

  • Similar therapeutic approach has been successful in diseases caused by direct activation of STAT3 or STAT1

• Potential applications:

  • Constitutional IL6ST GOF syndrome with neonatal onset immunodeficiency, autoinflammation, and dysmorphy

  • Other disorders involving hyperactivation of IL6ST signaling

  • Potentially inflammatory hepatocellular adenomas caused by somatic IL6ST mutations

• Considerations for clinical use:

  • Dose optimization to balance efficacy with adverse effects

  • Monitoring for immunosuppression

  • Tailoring treatment based on patient-specific manifestations

• Emerging evidence:

  • Successful use of JAK inhibitors in related conditions (STAT3 GOF, STAT1 GOF)

  • Supports potential efficacy in IL6ST-related disorders

What are the challenges in developing targeted therapies for IL6ST-related disorders?

Developing targeted therapies for IL6ST-related disorders faces several challenges:

• Phenotypic heterogeneity:

  • Different mutations in IL6ST lead to distinct clinical manifestations

  • LOF and GOF mutations require opposite therapeutic approaches

  • Mosaicism adds complexity to treatment responses

• Pathway complexity:

  • IL6ST is involved in signaling for multiple cytokines (IL-6, IL-11, IL-27, LIF, etc.)

  • Targeting IL6ST signaling may affect multiple biological processes beyond the pathogenic pathway

  • Need to balance inhibition of pathogenic signaling while preserving beneficial functions

• Treatment timing and duration:

  • Developmental aspects of some manifestations (e.g., skeletal abnormalities, growth failure) may limit efficacy of treatments initiated after symptom onset

  • Unclear optimal duration of treatment for chronic manifestations

• Tissue penetration:

  • Ensuring therapeutic agents reach all affected tissues

  • Particularly challenging for neurological manifestations due to blood-brain barrier

• Safety concerns with JAK inhibitors:

  • Increased risk of infections

  • Potential hematological toxicity

  • Long-term safety data in pediatric populations is limited

  • Balancing immunosuppression with treating autoimmunity/autoinflammation

What future research directions might advance understanding of IL6ST biology and pathology?

Several promising research directions could advance understanding of IL6ST biology and pathology:

• Expanded structural studies:

  • Further cryoEM analyses of different cytokine-IL6ST complexes

  • Investigation of the full-length receptor including transmembrane and intracellular domains

  • Structural comparison of wild-type and mutant receptors in various signaling states

• Single-cell analysis:

  • Characterization of cell-specific responses to IL6ST mutations

  • Investigation of cellular heterogeneity in mosaic patients

  • Identification of particularly vulnerable cell populations

• Detailed signaling pathway mapping:

  • Comprehensive phosphoproteomic analysis of cells with different IL6ST mutations

  • Identification of differential pathway activation between IL-6, IL-11, and other cytokines

  • Characterization of feedback mechanisms regulating IL6ST signaling

• Development of improved model systems:

  • Patient-derived iPSCs differentiated into relevant cell types

  • Knock-in mouse models of human IL6ST mutations

  • Tissue-specific conditional expression of mutant IL6ST

• Therapeutic innovations:

  • Development of cytokine-specific signaling inhibitors

  • Structure-guided design of small molecules targeting specific IL6ST conformations

  • Cell-type specific delivery of therapeutics to minimize off-target effects

  • Exploration of gene editing approaches for correcting pathogenic mutations