GO:1990661 S100A8 complex: Components, Assembly and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:1990661 (S100A8 complex) is a cellular_component defined as a protein complex composed of an S100A8 dimer that is capable of binding to toll-like receptor 4 (TLR4).
• The S100A8 complex is the homodimeric form of the alarmin S100A8; it can also associate with S100A9 to form the widely studied S100A8/A9 heterocomplex (calprotectin).
• S100A8/A9 signaling drives mitochondrial dysfunction and cell death in endothelial cells and cardiomyocytes through TLR4 and mitochondrial complex I deficiency.
• The complex is secreted through gasdermin D pores and acts as a damage-associated molecular pattern (DAMP) in autoinflammatory and cardiovascular disease.
• S100A8/A9 is a druggable alarmin axis in bone-marrow stromal progenitors and a mediator of myeloid-derived suppressor cell (MDSC) immunosuppression in melanoma.
• CRISPR knockout, knock-in, point-mutation and overexpression models, combined with proteomics and single-cell RNA sequencing, are key tools for dissecting S100A8 complex biology.
Description
GO:1990661, the S100A8 complex, is a Gene Ontology cellular_component term describing a protein complex composed of an S100A8 dimer that is capable of binding to toll-like receptor 4 (TLR4). S100A8 belongs to the S100 family of calcium-binding proteins and functions as an alarmin, a host-derived danger signal released during inflammation. The homodimeric S100A8 complex is the basic structural unit that can further assemble with S100A9 into the S100A8/A9 heterocomplex, also known as calprotectin, which is one of the most abundant cytosolic proteins in neutrophils and monocytes. Because the S100A8 complex engages TLR4, it sits at the interface between innate immune sensing and tissue injury, making it a central node in inflammatory, cardiovascular and oncologic research. Researchers study GO:1990661 to understand how alarmin dimerization, secretion and receptor engagement propagate sterile inflammation and to identify therapeutic entry points in diseases such as sepsis, myocardial infarction, familial Mediterranean fever and cancer. The term is therefore relevant to immunologists, cardiologists, oncologists and cell biologists who use CRISPR-based models to test causality of S100A8-driven signaling.
S100A8 complex At A Glance
| GO ID | GO:1990661 |
|---|---|
| GO term | S100A8 complex |
| Ontology | cellular_component |
| Synonym | S100A8 homodimer |
| Definition | A protein complex composed of a S100A8 dimer and capable of binding to toll-like receptor 4 (TLR4). |
| Major function | Alarmin signaling through TLR4; component of the S100A8/A9 heterocomplex (calprotectin). |
| Complex composition | S100A8 homodimer; can associate with S100A9 to form S100A8/A9. |
| Secretion route | Released via gasdermin D pores in autoinflammation. |
| Disease relevance | Sepsis, ischemia/reperfusion injury, familial Mediterranean fever, cancer cachexia, melanoma, myelofibrosis. |
What Is GO:1990661?
In the Gene Ontology, GO:1990661 (S100A8 complex) is a cellular_component defined as a protein complex composed of an S100A8 dimer and capable of binding to toll-like receptor 4 (TLR4). The synonym S100A8 homodimer reflects that the minimal unit is a dimer of two S100A8 polypeptides. The complex is not merely a static structural entity; its ability to bind TLR4 places it functionally upstream of innate immune receptor signaling. The S100A8 complex can also participate in higher-order assemblies, notably the S100A8/A9 heterocomplex, which is secreted and acts as an alarmin in multiple inflammatory settings.
Why Is S100A8 complex Important in Cell Biology?
The S100A8 complex is important because it converts a cytosolic calcium-binding protein into an extracellular alarmin signal that engages TLR4 and amplifies inflammation. This signaling axis has been causally linked to mitochondrial dysfunction and cell death in endothelial cells during sepsis and in cardiomyocytes after ischemia/reperfusion injury, making it a direct mediator of organ damage. In autoinflammatory disease, S100A8/A9 secretion through gasdermin D pores exacerbates familial Mediterranean fever, linking the complex to inflammasome biology. In cancer, S100A8/A9 supports tumor progression by driving MDSC-mediated immunosuppression through TLR4 and by promoting cachexia-associated metabolic changes. The complex is also a druggable alarmin axis in bone-marrow stromal progenitors that drive myelofibrosis, underscoring its translational value. Because the S100A8 complex is genetically tractable, CRISPR models can be used to test whether S100A8 or its partner S100A9 is causally required in each disease context.
• Defines the minimal homodimeric unit of the alarmin S100A8 that binds TLR4.
• Serves as a building block for the S100A8/A9 heterocomplex (calprotectin), a major neutrophil-derived DAMP.
• Drives mitochondrial complex I deficiency and PANoptosis in endothelial cells during sepsis.
• Causes mitochondrial dysfunction and cardiomyocyte death after ischemia/reperfusion injury.
• Is secreted via gasdermin D pores and exacerbates autoinflammation in familial Mediterranean fever.
• Promotes cancer cachexia by sustaining food intake and preventing adipose tissue loss in mice.
• Mediates MDSC-mediated immunosuppression in melanoma through TLR4 signaling.
• Marks a druggable alarmin axis in bone-marrow stromal progenitors driving myelofibrosis.
• Is a target for single-cell RNA sequencing studies of inflammatory macrophage subsets in acute kidney injury.
• Provides a genetically tractable node for CRISPR knockout, knock-in and overexpression studies.
What Happens During S100A8 complex?
Dimerization and complex assembly
In simple terms: Two S100A8 proteins join together to form the basic S100A8 complex.
The S100A8 complex is defined as a dimer of S100A8 that can bind TLR4. S100A8 is a calcium-binding protein of the S100 family, and dimerization is the structural prerequisite for its alarmin activity. The homodimer can further associate with S100A9 to form the S100A8/A9 heterocomplex, which is the predominant secreted form in neutrophils and monocytes. This assembly step is regulated by calcium availability and by the cellular redox environment, although the precise structural transitions remain an active area of research.
Secretion through gasdermin D pores
In simple terms: The complex leaves the cell through pores made by gasdermin D.
Unlike classical cytokines, S100A8/A9 lacks a signal peptide and is secreted through unconventional routes. In familial Mediterranean fever, S100A8/A9 secretion occurs via gasdermin D pores, linking the complex to inflammasome-driven pyroptosis. This secretion route places the S100A8 complex downstream of inflammasome activation and upstream of extracellular TLR4 engagement, creating a feed-forward inflammatory loop.
TLR4 binding and receptor activation
In simple terms: The complex docks onto TLR4 and switches on inflammatory signaling.
The defining functional property of GO:1990661 is its capacity to bind toll-like receptor 4 (TLR4). TLR4 engagement by S100A8/A9 activates NF-kB and MAPK pathways, leading to cytokine production and immune cell recruitment. In melanoma, S100A9 and HMGB1 orchestrate MDSC-mediated immunosuppression through TLR4 signaling, demonstrating that the S100A8/A9 axis can reprogram the tumor microenvironment. The homodimeric S100A8 complex is therefore a direct ligand for an innate immune receptor.
Mitochondrial dysfunction and cell death
In simple terms: Downstream of TLR4, the complex damages mitochondria and can kill cells.
S100A8/A9 signaling causes mitochondrial dysfunction and cardiomyocyte death after ischemia/reperfusion injury. In sepsis, S100A8/A9(hi) neutrophils induce mitochondrial complex I deficiency and PANoptosis in endothelial cells. These findings establish the S100A8 complex as an upstream trigger of mitochondrial failure and regulated cell death, providing a mechanistic link between alarmin signaling and organ damage.
Metabolic and systemic effects
In simple terms: The complex also influences appetite and fat tissue during cancer.
Beyond local inflammation, the S100A8/A9 complex promotes food intake and prevents adipose tissue loss during cancer cachexia in mice. This systemic metabolic role expands the functional repertoire of the S100A8 complex beyond classical innate immunity and suggests that TLR4-dependent alarmin signaling can modulate whole-body metabolism.
Key Genes Involved in GO:1990661 S100A8 complex
The following genes and proteins are directly implicated in the assembly, secretion, receptor engagement and downstream signaling of the S100A8 complex.
| Gene | Major Role | Research Relevance |
|---|---|---|
| S100A8 | Core subunit of the S100A8 complex; forms the homodimer that binds TLR4 | Primary target for knockout and knock-in studies of alarmin function |
| S100A9 | Partner subunit forming the S100A8/A9 heterocomplex (calprotectin) | Frequently co-targeted with S100A8 in inflammation and cancer models |
| TLR4 | Receptor for the S100A8 complex; initiates NF-kB and MAPK signaling | Key node for testing receptor-dependent effects of S100A8/A9 |
| GSDMD | Forms pores that mediate S100A8/A9 secretion | Links inflammasome activation to alarmin release |
| NLRP3 | Inflammasome sensor upstream of gasdermin D activation | Context for autoinflammatory secretion of S100A8/A9 |
| HMGB1 | Alarmin that cooperates with S100A9 in TLR4 signaling | Studied in MDSC-mediated immunosuppression |
| NFKB1 | Transcription factor downstream of TLR4 | Readout of S100A8/A9-induced inflammatory signaling |
| MAPK1 | Kinase downstream of TLR4 | Readout of S100A8/A9-induced signaling |
| MT-CO1 | Mitochondrial complex I component affected by S100A8/A9 | Marker of mitochondrial dysfunction in sepsis models |
| CASP3 | Executioner caspase in apoptosis | Cell death readout in cardiomyocyte and endothelial models |
| CASP8 | Initiator caspase in extrinsic apoptosis and PANoptosis | Cell death readout in sepsis models |
| RIPK3 | Kinase in necroptosis and PANoptosis | Cell death readout in sepsis models |
| MLKL | Executioner of necroptosis | Cell death readout in sepsis models |
| GZMB | Granzyme B, a pyroptosis-related effector | PANoptosis readout in endothelial cells |
| IL1B | Cytokine downstream of inflammasome and TLR4 | Inflammatory readout in autoinflammation and sepsis |
| IL6 | Cytokine downstream of TLR4 | Inflammatory readout in sepsis and cancer models |
| TNF | Cytokine downstream of TLR4 | Inflammatory readout in sepsis and cancer models |
| ARG1 | Arginase 1, MDSC effector | Immunosuppression readout in melanoma |
How Is S100A8 complex Regulated?
The S100A8 complex is regulated at multiple levels. Its assembly depends on calcium binding and the availability of S100A9, which determines whether the homodimer or the S100A8/A9 heterocomplex predominates. Secretion is controlled by gasdermin D pores downstream of inflammasome activation, linking release to pyroptotic cell death. Extracellular activity is regulated by TLR4 availability and by the presence of cooperating alarmins such as HMGB1. In bone-marrow stromal progenitors, the alarmin axis is druggable, indicating that microenvironmental cues regulate S100A8/A9 expression and function. Single-cell RNA sequencing has identified inflammatory macrophage subsets that express S100A8/A9 in acute kidney injury, suggesting that cell-state transitions regulate the complex in vivo.
S100A8 complex and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| S100A8 | Sepsis-associated endothelial PANoptosis | Endothelial cell knockout and overexpression models |
| S100A8 | Myocardial ischemia/reperfusion injury | Cardiomyocyte knockout and knock-in models |
| S100A9 | Familial Mediterranean fever autoinflammation | Macrophage knockout and gasdermin D pore assays |
| S100A8/A9 | Cancer cachexia and adipose tissue loss | Mouse knockout and overexpression models |
| S100A9 | Melanoma MDSC-mediated immunosuppression | Tumor co-culture and TLR4 knockout models |
| S100A8/A9 | Myelofibrosis alarmin axis | Bone-marrow stromal progenitor knockout models |
Sepsis and endothelial injury
In sepsis, S100A8/A9(hi) neutrophils induce mitochondrial complex I deficiency and PANoptosis in endothelial cells. This places the S100A8 complex upstream of mitochondrial failure and regulated cell death in the vasculature, making it a candidate target for protecting endothelial barriers during systemic inflammation.
Myocardial ischemia/reperfusion injury
S100A8/A9 signaling causes mitochondrial dysfunction and cardiomyocyte death in response to ischemic/reperfusion injury. The complex therefore contributes directly to myocardial damage after infarction, and TLR4-dependent signaling is a mechanistic node for cardioprotection.
Autoinflammation and familial Mediterranean fever
Complex regulation of alarmins S100A8/A9 and their secretion via gasdermin D pores exacerbates autoinflammation in familial Mediterranean fever. This links the S100A8 complex to inflammasome-driven diseases and suggests that blocking alarmin release could reduce autoinflammatory flares.
Cancer cachexia and immunosuppression
The S100A8/A9 complex promotes food intake and prevents adipose tissue loss during cancer cachexia in mice, while S100A9 and HMGB1 orchestrate MDSC-mediated immunosuppression in melanoma through TLR4 signaling. These findings connect the complex to metabolic and immune reprogramming in cancer.
From S100A8 complex-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| Is S100A8 required for TLR4-dependent inflammatory signaling? | S100A8 knockout cell line with TLR4 reporter assay |
| Does the S100A8 homodimer versus heterocomplex differ in function? | Point-mutation knock-in of dimer interface residues |
| How is S100A8/A9 secreted? | Gasdermin D knockout and pore-formation assays |
| Does S100A8/A9 drive mitochondrial dysfunction? | Knockout cardiomyocytes and endothelial cells with mitochondrial complex I readouts |
| Can S100A8/A9 be targeted in myelofibrosis? | Bone-marrow stromal progenitor knockout and drug treatment models |
| Which immune subsets express S100A8/A9 in injury? | Single-cell RNA sequencing of inflammatory macrophages |
How to Study the S100A8 complex Process
| Method | What It Measures | Typical Application |
|---|---|---|
| Affinity purification mass spectrometry | Protein composition of the S100A8 complex | Identifying S100A9 and TLR4 interactions |
| Single-cell RNA sequencing | Cell-type-specific expression of S100A8/A9 | Mapping inflammatory macrophage subsets in injury |
| Mitochondrial complex I activity assay | Mitochondrial function | Sepsis and ischemia/reperfusion models |
| PANoptosis marker panel | Cell death pathway activation | Endothelial cell injury in sepsis |
| Gasdermin D pore assay | Unconventional secretion | Familial Mediterranean fever models |
| TLR4 reporter assay | Receptor activation by S100A8/A9 | Melanoma MDSC and macrophage studies |
| Mouse cachexia model | Food intake and adipose tissue loss | Cancer cachexia studies |
| Bone-marrow stromal progenitor assay | Alarmin axis activity | Myelofibrosis research |
Proteomics and complex purification
Affinity purification coupled to mass spectrometry can isolate the S100A8 complex and its interacting partners, including S100A9 and TLR4. This approach defines the composition of the homodimer versus the heterocomplex and identifies post-translational modifications that regulate assembly.
Single-cell RNA sequencing
Single-cell RNA sequencing has been used to identify a unique inflammatory macrophage subset expressing S100A8/A9 as a druggable target in acute kidney injury. This method resolves which cell types produce the complex in complex tissues and how expression changes during disease progression.
Cell death and mitochondrial assays
Mitochondrial complex I activity, PANoptosis markers and cardiomyocyte viability assays are used to measure downstream effects of S100A8/A9 signaling. These readouts connect the S100A8 complex to organ-level injury and can be combined with TLR4 inhibition.
In vivo disease models
Mouse models of sepsis, ischemia/reperfusion, familial Mediterranean fever and cancer cachexia are used to test the causal role of S100A8/A9 in vivo. Genetic knockout and overexpression models allow separation of homodimer and heterocomplex functions.
How CRISPR Can Be Used to Study GO:1990661 S100A8 complex
Knockout
CRISPR knockout of S100A8 or S100A9 eliminates the S100A8 complex and its heterocomplex, allowing researchers to test whether TLR4-dependent inflammatory signaling, mitochondrial dysfunction and cell death require the alarmin. Knockout models are also used to validate druggable alarmin axes in myelofibrosis and melanoma.
Point Mutation
Point mutations at the S100A8 dimer interface or TLR4-binding surface can dissociate homodimer assembly from receptor engagement. Such knock-in models help define which structural features of GO:1990661 are required for signaling and which are dispensable.
Knock-in
Knock-in of epitope tags or fluorescent reporters into the endogenous S100A8 locus enables tracking of complex assembly, secretion and localization in live cells. Tagged knock-in models are particularly useful for studying gasdermin D-dependent secretion and extracellular release.
Overexpression
Overexpression of S100A8, alone or with S100A9, can drive TLR4-dependent inflammatory signaling and mitochondrial dysfunction in recipient cells. Overexpression models are used to test sufficiency of the complex in endothelial, cardiomyocyte and immune cell contexts.
How EDITGENE Supports S100A8 complex Research
Researchers studying S100A8 complex-related genes often need to determine whether a candidate gene is causally involved in alarmin assembly, secretion or TLR4-dependent signaling. EDITGENE provides CRISPR-based cell models and screening services that allow this causality to be tested directly in relevant cell types.
Contact EDITGENE today to design your custom CRISPR model for S100A8 complex research.
Frequently Asked Questions About S100A8 complex
What is the S100A8 complex (GO:1990661)?
GO:1990661 is a cellular_component term describing a protein complex composed of an S100A8 dimer that is capable of binding to toll-like receptor 4 (TLR4). It is also known as the S100A8 homodimer.
What genes are involved in the S100A8 complex?
The core gene is S100A8, which forms the homodimer. S100A9 partners with S100A8 to form the S100A8/A9 heterocomplex, and TLR4, GSDMD, NLRP3 and HMGB1 are key interacting or signaling genes.
How is the S100A8 complex secreted?
S100A8/A9 is secreted through gasdermin D pores in autoinflammatory conditions such as familial Mediterranean fever, linking its release to inflammasome activation.
What diseases are linked to the S100A8 complex?
The complex has been linked to sepsis, myocardial ischemia/reperfusion injury, familial Mediterranean fever, cancer cachexia, melanoma and myelofibrosis.
Does the S100A8 complex signal through TLR4?
Yes. The GO definition states that the S100A8 complex is capable of binding TLR4, and TLR4-dependent signaling has been demonstrated in melanoma MDSC and inflammatory models.
How does S100A8/A9 cause cell death?
S100A8/A9 signaling causes mitochondrial complex I deficiency and PANoptosis in endothelial cells during sepsis and mitochondrial dysfunction in cardiomyocytes after ischemia/reperfusion injury.
What is the difference between S100A8 homodimer and S100A8/A9 heterocomplex?
The S100A8 complex is the homodimeric form defined by GO:1990661, while the S100A8/A9 heterocomplex, also called calprotectin, includes S100A9 and is the predominant secreted alarmin in many inflammatory settings.
How can I study the S100A8 complex with CRISPR?
CRISPR knockout, point mutation, knock-in and overexpression models can be used to test the causal role of S100A8, S100A9 and TLR4 in alarmin assembly, secretion and downstream signaling.
Is the S100A8 complex a drug target?
The S100A8/A9 alarmin axis has been described as druggable in bone-marrow stromal progenitors driving myelofibrosis, and TLR4-dependent effects are being explored in melanoma and cardiovascular disease.
What methods are used to study the S100A8 complex?
Common methods include affinity purification mass spectrometry, single-cell RNA sequencing, mitochondrial complex I assays, PANoptosis marker panels, gasdermin D pore assays and TLR4 reporter assays.
Conclusion
GO:1990661 (S100A8 complex) defines the homodimeric alarmin unit that binds TLR4 and sits at the center of sterile inflammation, mitochondrial dysfunction and cell death. Its roles in sepsis, ischemia/reperfusion injury, autoinflammation, cancer cachexia and myelofibrosis make it a high-value target for mechanistic and translational research. CRISPR-based knockout, knock-in, point-mutation and overexpression models, combined with proteomics and single-cell RNA sequencing, provide the tools needed to dissect how the S100A8 complex is assembled, secreted and sensed in disease.
References
- 1. Wang Y et al.. 2024. S100A8/A9(hi) neutrophils induce mitochondrial dysfunction and PANoptosis in endothelial cells via mitochondrial complex I deficiency during sepsis.. Cell Death Dis 15(6):462 PMID: 38942784
- 2. Li Y et al.. 2019. S100a8/a9 Signaling Causes Mitochondrial Dysfunction and Cardiomyocyte Death in Response to Ischemic/Reperfusion Injury.. Circulation 140(9):751-764 PMID: 31220942
- 3. Jorch SK et al.. 2023. Complex regulation of alarmins S100A8/A9 and secretion via gasdermin D pores exacerbates autoinflammation in familial Mediterranean fever.. J Allergy Clin Immunol 152(1):230-243 PMID: 36822481
- 4. Gao L et al.. 2026. The S100A8/A9 complex promotes food intake and prevents adipose tissue loss during cancer cachexia in mice.. Cell Metab 38(5):909-928.e8 PMID: 41780522
- 5. Cozac DA et al.. 2025. The alarmin tandem: unraveling the complex effect of S100A8/A9 - from atherosclerosis to cardiac arrhythmias.. Front Immunol 16:1630410 PMID: 40948795
- 6. Özbay Kurt FG et al.. 2024. S100A9 and HMGB1 orchestrate MDSC-mediated immunosuppression in melanoma through TLR4 signaling.. J Immunother Cancer 12(9) PMID: 39266214
- 7. Yao W et al.. 2022. Single Cell RNA Sequencing Identifies a Unique Inflammatory Macrophage Subset as a Druggable Target for Alleviating Acute Kidney Injury.. Adv Sci (Weinh) 9(12):e2103675 PMID: 35112806
- 8. Leimkühler NB et al.. 2021. Heterogeneous bone-marrow stromal progenitors drive myelofibrosis via a druggable alarmin axis.. Cell Stem Cell 28(4):637-652.e8 PMID: 33301706
