Recombinant Mouse Tetraspanin-18 (Tspan18)

What is Tetraspanin-18 (Tspan18) and what are its primary functions?

Tspan18 is a member of the tetraspanin superfamily, which consists of transmembrane proteins that interact with specific partner proteins to regulate their trafficking and clustering. Tspan18 functions primarily as a novel regulator of the calcium channel Orai1 and von Willebrand factor (vWF) release in response to inflammatory stimuli. Unlike other tetraspanins, Tspan18 is relatively unique in sequence and appears to be specialized for regulating calcium signaling pathways, particularly in endothelial cells .

The protein contains four transmembrane regions with a characteristic structure that recent crystallography studies of related tetraspanins suggest may include a cholesterol-binding cavity. Molecular dynamics simulations indicate that tetraspanins like Tspan18 may function as "molecular switches" that undergo conformational changes to regulate partner protein function .

Which cell types express Tspan18 and at what levels?

Tspan18 is predominantly expressed in endothelial cells at several-fold higher levels than most other cell types. Transcriptomic analyses and qPCR studies show the following expression pattern:

• Tissue distribution: Highest expression in lung, with lower levels in other tissues

• Cellular distribution: Primary expression in endothelial cells compared to other lung cell types

• Specific cell types: Expressed in primary human umbilical vein endothelial cells (HUVECs) and human microvascular endothelial HMEC-1 cell line

• Other cell types: Peripheral blood leukocytes express comparable levels to HUVECs, while most other cell types show low or absent expression

Transcriptomic data from the Human Protein Atlas indicates that while Tspan18 is expressed by most human tissues at a level between 10-70 tags per million, only HUVECs and 8 out of 64 other analyzed cell lines expressed Tspan18 at levels of 10 or more tags per million .

How does Tspan18 regulate calcium signaling in endothelial cells?

Tspan18 regulates calcium signaling through its interaction with the store-operated calcium entry (SOCE) channel Orai1. The mechanism involves:

• Surface expression regulation: Tspan18 interacts with Orai1 and facilitates its trafficking to the cell surface. In Tspan18-knockdown endothelial cells, Orai1 cell surface localization is reduced by approximately 70% .

• Calcium mobilization: Tspan18-knockdown in primary HUVECs results in 55-70% decreased Ca²⁺ mobilization when stimulated with inflammatory mediators like thrombin or histamine, similar to the reduction seen with direct Orai1 knockdown .

• NFAT signaling activation: When overexpressed in lymphocyte model cell lines, Tspan18 induces 20-fold activation of Ca²⁺-responsive nuclear factor of activated T cell (NFAT) signaling in an Orai1-dependent manner .

Importantly, Tspan18-induced NFAT/AP-1 activation is independent of IP₃ receptors but requires extracellular calcium and is blocked by cyclosporin A (which prevents NFAT translocation) or by dominant-interfering forms of Orai1, confirming the specificity of the Tspan18-Orai1 pathway .

What phenotypes are observed in Tspan18-knockout mice?

• Impaired hemostasis: Tspan18-knockout mice lose on average 6-fold more blood in tail-bleed assays compared to wild-type littermates, indicating disrupted hemostasis .

• Thrombus formation: These mice have approximately 60% reduced thrombus size in a deep vein thrombosis model, with 4 out of 9 knockout mice failing to develop a thrombus compared to 100% thrombus formation in wild-type mice .

• Reduced platelet deposition: Following myocardial ischemia-reperfusion injury, platelet deposition and aggregate size in the microcirculation are reduced by approximately 50% in Tspan18-knockout mice .

• Impaired vWF release: Histamine-induced increase of plasma vWF is reduced by 45% in Tspan18-knockout mice, although basal plasma vWF levels remain normal .

• Cell type specificity: The hemostasis defect is due to Tspan18 deficiency in non-hematopoietic cells (likely endothelial cells), as demonstrated using chimeric mice studies .

What are the molecular mechanisms by which Tspan18 regulates Orai1 function?

The molecular mechanisms of Tspan18-mediated Orai1 regulation involve multiple levels of interaction and functional modification:

• Direct protein interaction: Co-immunoprecipitation experiments using 1% digitonin (a stringent detergent) show that FLAG-tagged Tspan18 specifically co-immunoprecipitates with Myc-tagged Orai1, while five other control tetraspanins do not. This suggests a specific molecular interaction between Tspan18 and Orai1 .

• Co-localization: Immunofluorescence studies demonstrate that Tspan18 and Orai1 co-localize when expressed in HeLa cells, with approximately 90% pixel co-localization when assessed using the Manders' coefficient .

• Trafficking regulation: Tspan18 interacts with Orai1 in the endoplasmic reticulum and promotes its trafficking to the cell surface, consistent with the established role of tetraspanins. This is evidenced by the 70% reduction in Orai1 surface expression in Tspan18-knockdown endothelial cells .

• Clustering potential: Based on our understanding that tetraspanins can exist as nanodomains of approximately ten tetraspanins of a single type, Tspan18 may cluster Orai1 into pre-formed nanodomains. This could modulate Orai1 lattice formation by STIM1, providing a mechanism by which endothelial cells fine-tune SOCE and downstream functional responses .

• Specificity within the Orai family: The research shows Tspan18 primarily regulates Orai1 in endothelial cells, with no significant role identified for Orai2 and Orai3 in inflammatory mediator-induced HUVEC Ca²⁺ mobilization, consistent with other studies .

How does Tspan18 affect von Willebrand factor release from endothelial cells?

Tspan18 plays a critical role in regulated von Willebrand factor (vWF) release from endothelial cells through the following mechanisms:

• In vitro effects: Histamine- or thrombin-induced vWF release from endothelial cells is reduced by approximately 90% following Tspan18-knockdown, demonstrating its essential role in stimulus-induced vWF secretion .

• In vivo regulation: In Tspan18-knockout mice, histamine-induced increase of plasma vWF is reduced by 45% compared to wild-type controls, confirming the physiological relevance of this pathway .

• Selective regulation of stimulated release: Importantly, basal plasma vWF levels remain normal in Tspan18-knockout mice, indicating that Tspan18 is specifically required for regulated, but not constitutive, vWF release .

• Calcium-dependence: The mechanism likely involves Tspan18's regulation of Orai1 and subsequent calcium signaling, as calcium influx is essential for Weibel-Palade body exocytosis and vWF release in response to inflammatory mediators .

• Functional consequences: The impaired vWF release in Tspan18-deficient mice contributes to reduced severity in thrombo-inflammatory models, including deep vein thrombosis and myocardial ischemia-reperfusion injury, while not affecting platelet-driven arterial thrombosis models .

What methodologies are optimal for investigating Tspan18-Orai1 interactions?

Due to the complex nature of tetraspanin biology and the challenges in studying transmembrane protein interactions, several specialized methodologies are recommended:

• Co-immunoprecipitation with appropriate detergents: Use 1% digitonin, a stringent detergent that preserves tetraspanin-partner protein interactions. This approach successfully demonstrated the specific interaction between Tspan18 and Orai1 when other control tetraspanins showed no interaction .

• Epitope tagging strategies: Due to the lack of effective antibodies against many tetraspanins including Tspan18, epitope tagging (such as FLAG for Tspan18 and Myc for Orai1) is essential for detection in biochemical and imaging studies .

• Co-localization analysis: Quantitative co-localization studies using the Manders' coefficient can assess the degree of spatial overlap between Tspan18 and Orai1, with values approaching 90% indicating strong co-localization .

• Functional reporter assays: NFAT/AP-1 transcriptional luciferase reporter assays provide a sensitive readout for Ca²⁺ signaling downstream of Tspan18-Orai1 interaction. This system allowed demonstration that Tspan18 specifically activates this pathway in an Orai1-dependent manner .

• Surface biotinylation: To quantify Orai1 trafficking to the cell surface, surface biotinylation followed by streptavidin pulldown and western blotting can effectively measure the impact of Tspan18 manipulation on Orai1 surface expression .

• Dominant-interfering constructs: Using dominant-interfering forms of Orai1 (e.g., E106Q mutant) that multimerize with endogenous Orai1 to yield non-functional channels can help establish the specificity of Tspan18's effects on Orai1-mediated calcium entry .

How can researchers effectively study Tspan18 function in vivo?

For investigating Tspan18 function in vivo, several specialized models and approaches are recommended:

• Tspan18-knockout mouse models: Tspan18-knockout mice, such as those from Genentech/Lexicon Pharmaceuticals, provide a valuable tool for studying Tspan18 function in vivo. These mice are viable and breed successfully as homozygote knockouts, facilitating experimental design .

• Chimeric mouse studies: To determine the cell type responsible for Tspan18-dependent phenotypes, chimeric mice can be generated. This approach revealed that the hemostasis defect in Tspan18-knockout mice is due to Tspan18 deficiency in non-hematopoietic cells rather than hematopoietic cells .

• Hemostasis assessment: Tail-bleed assays provide a straightforward method to evaluate hemostasis function in Tspan18-knockout mice. Researchers should be aware that the bleeding phenotype shows some variability between individual Tspan18-knockout mice .

• Thrombo-inflammatory models:

  • Deep vein thrombosis model: This vWF-dependent model shows reduced thrombus formation in Tspan18-knockout mice

  • Myocardial ischemia-reperfusion injury model: This vWF-dependent model demonstrates reduced platelet deposition in Tspan18-knockout mice

  • Arterial thrombosis models: These platelet-driven models show no defect in Tspan18-knockout mice

• vWF release assessment: In vivo vWF release can be evaluated by:

  • Intraperitoneal histamine injection followed by plasma vWF ELISA, which demonstrated a 45% reduction in stimulated vWF release in Tspan18-knockout mice

  • Comparison of basal and stimulated vWF levels to distinguish between constitutive and regulated secretion pathways

What are the key considerations for designing Tspan18 knockdown/knockout experiments?

When designing experiments to manipulate Tspan18 expression, researchers should consider:

• Antibody limitations: Due to the lack of effective antibodies against Tspan18, verification of knockdown/knockout is best achieved through qPCR rather than protein detection. This challenge is common in the tetraspanin field due to their relatively small size, high degree of sequence conservation, and compact 4-transmembrane structure .

• Expression profiling: qPCR is the recommended method for determining Tspan18 expression across different tissues and cell types. Transcriptomic data analysis can complement this approach to gain a comprehensive understanding of expression patterns .

• Control selection: When studying the function of Tspan18, include other tetraspanins as controls to demonstrate specificity. In published studies, CD9, CD63, CD151, Tspan32, and Tspan9 have been used as controls representing diverse tetraspanin subfamilies .

• Functional readouts: For endothelial Tspan18 function, key readouts include:

  • Ca²⁺ mobilization in response to thrombin or histamine stimulation

  • Orai1 surface localization

  • vWF release in response to inflammatory mediators

• Rescue experiments: To confirm specificity of knockdown effects, rescue experiments with expression of knockdown-resistant Tspan18 constructs should be performed. This is particularly important given the potential for off-target effects in RNAi-based approaches .

• Compensation assessment: When working with knockout models, assess possible compensation by other tetraspanins or alternative calcium signaling pathways, as this may explain variability in phenotypes or partial effects .

How does Tspan18 contribute to thrombosis and inflammation models?

Tspan18 plays distinct roles in different thrombosis and inflammation models, with clear contributions to thrombo-inflammatory conditions:

• Deep vein thrombosis: In this thrombo-inflammatory model that depends on endothelial vWF, Tspan18-knockout mice show approximately 60% reduced thrombus length and weight compared to wild-type littermates. Notably, 44% (4/9) of Tspan18-knockout mice completely failed to develop a thrombus compared to 100% thrombus formation in wild-type mice .

• Myocardial ischemia-reperfusion injury: In this vWF-dependent model, Tspan18-knockout mice exhibit approximately 50% reduced platelet deposition and aggregate size in the microcirculation, demonstrating a significant role for Tspan18 in cardiac thrombo-inflammatory responses .

• Arterial thrombosis models: In contrast to the thrombo-inflammatory models, Tspan18-knockout mice show no defect in platelet-driven arterial thrombosis models, including:

  • Aorta injury model: No difference in time to complete occlusion

  • FeCl₃-induced mesenteric arteriole thrombosis: No defect in thrombus formation

This pattern of results suggests that Tspan18 primarily influences thrombosis in contexts where inflammation and endothelial vWF play major roles, rather than in primarily platelet-driven arterial thrombosis .

What are the technical considerations for assessing calcium signaling in Tspan18 studies?

When investigating calcium signaling in the context of Tspan18 research, several technical considerations should be addressed:

• Ca²⁺ mobilization assays: For endothelial cells, stimulation with inflammatory mediators such as thrombin or histamine (typically 1 U/mL and 100 μM, respectively) followed by measurement of intracellular Ca²⁺ using fluorescent indicators provides a reliable assessment of Tspan18's impact on calcium signaling .

• Store-operated calcium entry (SOCE) isolation: To specifically assess SOCE, researchers can:

  • Use thapsigargin to deplete ER Ca²⁺ stores

  • Chelate extracellular Ca²⁺ with EGTA followed by Ca²⁺ re-addition

  • Apply specific SOCE inhibitors as controls

• Downstream signaling assessment: NFAT/AP-1 transcriptional reporter assays provide a sensitive readout for Ca²⁺ signaling downstream of Tspan18-Orai1. For optimal results:

  • Use combined PMA stimulation (activating the MAPK pathway) with Tspan18 expression to observe synergistic NFAT/AP-1 activation

  • Include controls with ionomycin and combined PMA/ionomycin stimulation

  • Use cyclosporin A to confirm NFAT dependence

• Channel specificity determination: To establish which calcium channels are regulated by Tspan18:

  • Use dominant-interfering channel constructs (e.g., Orai1-E106Q)

  • Test IP₃ receptor independence using IP₃ receptor-knockout cells

  • Perform rescue experiments with constitutively active downstream components (e.g., active calcineurin) to confirm intact signaling pathways

How might recombinant Tspan18 be used as a research tool?

Recombinant mouse Tspan18 has several potential applications as a research tool:

• Structure-function studies: Recombinant Tspan18 with various tags or mutations can help identify:

  • Critical domains for Orai1 interaction

  • Regions essential for trafficking versus functional regulation

  • Structure-based differences from other tetraspanins that explain its unique role in calcium signaling

• Binding partner identification: Purified recombinant Tspan18 can be used in:

  • Pull-down assays to identify novel binding partners

  • Surface plasmon resonance (SPR) to determine binding affinities

  • Cross-linking studies to capture transient interactions

• Reconstitution experiments: Recombinant Tspan18 can be used to:

  • Rescue function in Tspan18-knockout or knockdown systems

  • Test sufficiency for Orai1 regulation in heterologous expression systems

  • Explore dominant-negative or enhancing effects of mutant forms

• Tetraspanin web manipulation: Since tetraspanins form specialized membrane domains, recombinant Tspan18 could be used to:

  • Disrupt existing tetraspanin webs

  • Create artificial tetraspanin-enriched microdomains

  • Study competitive interactions between different tetraspanins

• Development of blocking or activating reagents: Based on understanding Tspan18 structure and function, researchers could develop:

  • Antibodies or peptides that specifically target functional domains

  • Small molecules that modulate Tspan18-Orai1 interactions

  • Tools to selectively manipulate endothelial calcium signaling in complex systems

How does Tspan18 compare to other tetraspanins in calcium channel regulation?

Tspan18 appears to have a specialized role in calcium channel regulation that distinguishes it from other tetraspanins:

• Sequence uniqueness: Tspan18 is not particularly related to any of the other 32 mammalian tetraspanins based on sequence analysis, suggesting it may be unique amongst tetraspanins in regulating Orai1 .

• Functional specificity: In comparative studies, five control tetraspanins (CD9, CD63, CD151, Tspan32, and Tspan9) neither interacted with Orai1 nor induced Ca²⁺-responsive NFAT activation when overexpressed, despite higher expression levels than Tspan18 .

• Evolutionary conservation: The specific role of Tspan18 appears to be conserved, as it was previously studied in chick embryos where it stabilizes expression of cadherin 6B to maintain adherens junctions between premigratory epithelial cranial neural crest cells .

• Cell type expression pattern: Unlike many other tetraspanins that show broad expression patterns, Tspan18's relatively restricted expression in endothelial cells and few other cell types suggests specialized function .

• Domain-specific functions: The unique properties of Tspan18 may relate to specific structural features:

  • Recent crystal structure of tetraspanin CD81 revealed a cone-shaped structure with a cholesterol-binding cavity

  • Molecular dynamics simulations suggest cholesterol removal causes conformational changes in tetraspanins

  • Tspan18 may have unique structural features that enable specific Orai1 regulation

What are the unresolved questions regarding Tspan18 biology?

Despite significant advances in understanding Tspan18 function, several important questions remain unresolved:

• Mechanism of Orai1 trafficking enhancement: While Tspan18 clearly promotes Orai1 surface expression, the precise molecular mechanisms and trafficking pathways involved remain to be elucidated .

• Structural basis of interaction: The specific domains and residues mediating the Tspan18-Orai1 interaction have not been mapped, which would provide insights into the selectivity of this interaction .

• Regulation of Tspan18 expression and function: The factors controlling Tspan18 expression in endothelial cells and potential post-translational modifications affecting its function are largely unknown .

• Role in other Orai family members: Whether Tspan18 also regulates Orai2 and Orai3 in contexts outside of inflammatory mediator-induced endothelial Ca²⁺ mobilization requires further investigation .

• Potential role in pathological conditions: The contribution of Tspan18 dysregulation to vascular diseases, particularly those involving endothelial dysfunction, remains to be explored .

• Broader signaling network: How Tspan18-mediated calcium signaling integrates with other endothelial signaling pathways and whether Tspan18 has calcium-independent functions need further study .

• Compensation mechanisms: The variability in bleeding phenotype in Tspan18-knockout mice suggests potential compensation mechanisms that require clarification .

What experimental protocols are recommended for studying recombinant Tspan18?

For researchers working with recombinant mouse Tspan18, the following experimental protocols are recommended:

• Expression and purification:

  • Use mammalian expression systems (HEK293T cells) rather than bacterial systems to ensure proper folding and post-translational modifications

  • Include epitope tags (FLAG, HA, or His) at the C-terminus to avoid interfering with N-terminal protein processing

  • Apply stringent detergent conditions (1% digitonin) for extraction while preserving protein-protein interactions

• Functional assessment:

  • NFAT/AP-1 reporter assay in DT40 or Jurkat cells provides a sensitive readout for Tspan18 activity

  • Calcium imaging in transfected cells can directly measure the impact on calcium signaling

  • Surface biotinylation assays can quantify effects on Orai1 trafficking

• Interaction studies:

  • Co-immunoprecipitation in 1% digitonin to preserve specific interactions

  • Microscopy-based co-localization studies with quantitative analysis using Manders' coefficient

  • Proximity ligation assays can detect interactions in intact cells

• Structure-function analysis:

  • Site-directed mutagenesis targeting conserved tetraspanin residues versus Tspan18-specific residues

  • Domain swapping with other tetraspanins to identify regions conferring Orai1 regulation

  • Truncation constructs to map minimal functional domains

• In vivo delivery considerations:

  • For rescue experiments in Tspan18-knockout models, consider endothelial-specific expression systems

  • Adeno-associated virus (AAV) vectors with endothelial-specific promoters can provide targeted expression

  • For acute manipulation, consider membrane-permeable peptides derived from key Tspan18 interaction domains