Recombinant Rat Cysteine-rich with EGF-like domain protein 1 (Creld1)

What is the basic structure of rat Creld1 protein?

Rat Creld1 is a transmembrane protein characterized by several conserved domains. The protein contains a signal peptide, an N-terminal region rich in glutamic acid and tryptophan residues called a DUF3456 or WE domain, and three EGF-like domains. There are two major isoforms: CRELD1A and CRELD1B. CRELD1A ends with two predicted transmembrane domains, while CRELD1B ends with a KDEL sequence, which functions as an endoplasmic reticulum (ER) retention signal . This modular organization is highly conserved throughout evolution across CRELD proteins. The molecular mass of recombinant rat Creld1 is approximately 41 kDa, though the apparent molecular mass in SDS-PAGE can be around 45 kDa due to post-translational modifications .

What is the expression pattern of Creld1 during development?

Creld1 is ubiquitously expressed in multiple adult and embryonic tissues. Immunohistochemical studies show CRELD1 protein expression in various embryonic tissues, including the developing heart, placenta, and other organs . In mouse embryos, CRELD1 can be detected in the developing heart as early as embryonic day E9.5-10.5, with expression in the endocardial cushions, which are critical for proper heart septation . CRELD1 expression continues in adult tissues, including the heart, placenta, and immune cells, indicating its importance throughout the lifespan .

What is the role of Creld1 in cardiovascular development?

Creld1 plays a critical role in heart development, particularly in the formation of endocardial cushions and subsequent septation of the heart chambers. Studies using Creld1-knockout mice demonstrated that complete loss of Creld1 (Creld1(-/-)) results in embryonic lethality by day E12.5. These embryos exhibit malformations including cardiovascular and endocardial cushion defects . Histological analyses of E9.5 and E10.5 Creld1(-/-) mouse hearts show abnormalities in the atrioventricular canal and outflow tract, which are crucial for proper heart septation . Mutations in human CRELD1 are associated with atrioventricular septal defects (AVSD), confirming its importance in heart development across species.

The following table summarizes key cardiac phenotypes observed in Creld1-deficient models:

How does Creld1 function in the immune system?

Recent research has revealed an unexpected role for Creld1 in T cell homeostasis. Studies by Bonaguro et al. demonstrated that Creld1 modulates NFAT and Wnt signaling in T cells and is required for maintaining T cell homeostasis . Analysis of datasets from healthy humans showed that individuals with low CRELD1 expression had significantly reduced numbers of naïve peripheral CD4+ T cells and a trend toward reduced numbers of naïve CD8+ T cells. The transcriptome expression signature of Creld1-low cells resembled a previously described expression signature associated with immunological aging .

In mice with T cell-specific Creld1 deficiency, all T cell subsets were reduced in middle-aged animals (>3 months), and serum concentrations of T cell cytokines were lower, suggesting accelerated immunological aging. Furthermore, Creld1-deficient CD4+ T cells were more apoptotic despite differentiating more effectively to TH1 and TH17 lineages upon ex vivo culture . These findings indicate that Creld1 plays a crucial role in T cell survival and homeostasis.

What molecular pathways does Creld1 participate in?

Creld1 participates in several molecular pathways:

• VEGF Signaling: Research has identified a genetic interaction between CRELD1 and VEGF (Vascular Endothelial Growth Factor). Creld1-deficient tissues show altered response to VEGF, and the combination of Creld1 deficiency with the VEGF -634C polymorphism (associated with increased VEGF expression) creates a "two-hit" scenario that may contribute to heart defects .

• NFAT and Wnt Signaling in T cells: Creld1 modulates both NFAT and Wnt signaling pathways in T cells, which are critical for T cell activation, differentiation, and homeostasis .

• ER Function and Protein Folding: The CRELD1A isoform's localization to the ER membrane suggests a role in protein folding or quality control. Studies of the C. elegans ortholog (crld-1) indicate it may function as a protein disulfide isomerase (PDI) that assists in the assembly of acetylcholine receptors , though this specific function needs confirmation in mammals.

What expression systems are optimal for producing recombinant rat Creld1?

For producing functional recombinant rat Creld1, mammalian expression systems are generally preferred due to the protein's complex structure and potential post-translational modifications. HEK293 cells have been successfully used to express rat TDGF1 (another protein with EGF-like domains) and would be suitable for Creld1 as well. The search results indicate that for rat Creld1, a construct encoding Met 1-Cys 143 fused with an Fc tag at the C-terminus yields protein that can be purified to >90% purity as determined by SDS-PAGE .

When designing expression constructs, researchers should consider:

• Including the native signal peptide or a heterologous one for proper targeting

• Choosing an appropriate tag (e.g., Fc, His, FLAG) based on experimental needs

• Determining whether to express full-length protein (including transmembrane domains) or soluble domains only

• Selecting the appropriate isoform (CRELD1A or CRELD1B) based on research questions

What purification strategies work best for recombinant rat Creld1?

Purification of recombinant rat Creld1 should be tailored to the expression system and tags used. For Fc-tagged Creld1, Protein A or Protein G affinity chromatography would be the primary purification step. Based on similar recombinant protein production approaches, a multi-step purification protocol is recommended:

• Affinity chromatography using the appropriate resin for the chosen tag

• Size exclusion chromatography to remove aggregates and improve purity

• Endotoxin removal step if the protein will be used for cell culture or in vivo experiments

• Quality control including SDS-PAGE, Western blotting, and functional assays

The final product should be lyophilized from a sterile buffer (such as PBS, pH 7.4) for stability during storage and shipping . Researchers should verify protein purity (>90% by SDS-PAGE) and confirm low endotoxin levels (<1.0 EU per μg of protein) using the LAL method .

What techniques are most effective for studying Creld1 function in heart development?

Multiple complementary approaches are valuable for studying Creld1's role in heart development:

• Genetic Models: Constitutive and conditional knockout mice have been informative. The constitutive Creld1(-/-) mouse is embryonic lethal but demonstrates the essential role of Creld1 in heart development . Conditional knockouts using tissue-specific Cre lines (e.g., Nkx2.5-Cre for cardiac-specific deletion) would help distinguish between direct cardiac effects and secondary effects.

• Ex Vivo Assays: Heart explant assays have been successfully used to study the effects of Creld1 deficiency on endocardial cushion development. These assays allow for controlled manipulation of factors like VEGF concentration and evaluation of cellular responses .

• Histological Analyses: Multiple staining techniques provide valuable information:

  • H&E staining for general morphology

  • Alcian Blue and Nuclear Fast Red staining to visualize endocardial cushions

  • Immunohistochemistry for CRELD1 expression patterns

  • PECAM staining to visualize endothelial cells and vascular development

  • TUNEL staining to assess apoptosis

  • BrdU proliferation assays

• Molecular Interaction Studies: Co-immunoprecipitation, proximity ligation assays, and other protein-protein interaction methods can identify Creld1 binding partners in the developing heart.

How can researchers investigate the CRELD1-VEGF interaction in heart development?

The interaction between CRELD1 and VEGF signaling represents an important area for investigation, given the evidence for genetic interaction between CRELD1 mutations and VEGF polymorphisms. Several approaches can be used:

• Heart Explant Assays: These have already demonstrated that Creld1-deficient explants respond differently to VEGF addition compared to wild-type explants. The developing endocardial cushions are sensitive to VEGF dosing, with both too little and too much VEGF being detrimental .

• Genetic Interaction Studies: Creating compound mutant mice (e.g., Creld1+/- with VEGF modifications) can help elucidate how these genes interact in vivo.

• Signaling Pathway Analysis: Investigating how Creld1 deficiency affects VEGF-induced signaling pathways (e.g., VEGFR2 phosphorylation, downstream MAPK or PI3K activation) would provide mechanistic insights.

• Transcriptome Analysis: RNA-seq of Creld1-deficient hearts with or without VEGF manipulation can identify gene expression changes that might explain the defects.

The research findings suggest a "two-hit" model where a CRELD1 deficiency combined with the VEGF -634C polymorphism (associated with increased VEGF expression) may contribute to heart defects. This model could be further tested using appropriate animal models and human genetic studies .

What are the methodological considerations for studying Creld1 in T cell biology?

For investigating Creld1's role in T cell biology, researchers should consider:

• Cell-Specific Knockout Models: Using Lck-Cre or CD4-Cre to generate T cell-specific Creld1 knockout mice allows for studying T cell development and function without confounding embryonic lethality issues.

• Aging Studies: Since Creld1 deficiency appears to accelerate immunological aging, longitudinal studies of T cell parameters in young versus middle-aged or old mice are important.

• Flow Cytometry Panels: Comprehensive immunophenotyping should include markers for:

  • Naïve vs. memory T cells (CD44, CD62L)

  • T cell subsets (Th1, Th2, Th17, Treg)

  • Activation markers (CD69, CD25)

  • Apoptosis markers (Annexin V, caspase activity)

• Signaling Pathway Analysis: Since Creld1 modulates NFAT and Wnt signaling, assays to measure:

  • NFAT nuclear translocation

  • NFAT-dependent gene expression

  • Wnt pathway activation (β-catenin phosphorylation/localization)

  • TCR signaling components

• Human Correlation Studies: Leveraging datasets like the Human Functional Genomics Project (HFGP), the Correlated Expression & Disease Association Research (CEDAR) study, and the Immune Variation (ImmVar) project to correlate CRELD1 expression with immune parameters in humans .

How can researchers differentiate between the functions of CRELD1A and CRELD1B isoforms?

The two major Creld1 isoforms (membrane-associated CRELD1A and ER-resident CRELD1B) likely have distinct functions. To study them separately:

• Isoform-Specific Knockout Models: Using CRISPR-Cas9 to modify splice acceptor sites of specific exons can generate isoform-specific mutants, as demonstrated in C. elegans for crld-1 isoforms .

• Isoform-Specific Antibodies: Generating antibodies that recognize unique C-terminal regions of each isoform allows for distinction in immunohistochemistry, Western blotting, and immunoprecipitation experiments.

• Rescue Experiments: Complementation studies expressing either CRELD1A or CRELD1B in Creld1-null backgrounds can determine which isoform rescues specific phenotypes.

• Subcellular Localization Studies: Fluorescently tagged isoforms can confirm CRELD1A's membrane localization versus CRELD1B's ER localization and potentially reveal other unexpected locations.

• Proteomic Approaches: Proximity labeling techniques (BioID, APEX) using each isoform as bait can identify isoform-specific interaction partners.

How might Creld1 function in the endoplasmic reticulum?

The CRELD1B isoform contains a KDEL ER-retention signal, suggesting functions within the ER . Additionally, studies of the C. elegans ortholog crld-1 suggest that CRLD-1A functions as a membrane-associated ER-resident protein disulfide isomerase (PDI) . To investigate these ER functions in mammalian systems:

• Biochemical Assays: Test for PDI activity using standard reduction/oxidation assays with purified recombinant Creld1.

• ER Stress Response: Examine whether Creld1 deficiency affects the unfolded protein response (UPR) or ER stress pathways by measuring expression of UPR markers (BiP, CHOP, XBP1 splicing) under normal and stress conditions.

• Protein Folding Client Identification: Use co-immunoprecipitation followed by mass spectrometry to identify proteins that interact with Creld1 in the ER, potentially representing folding clients.

• ER Morphology Analysis: Assess whether Creld1 deficiency affects ER structure using electron microscopy or super-resolution fluorescence microscopy.

What approaches can determine whether Creld1's developmental roles are tissue-autonomous?

The embryonic lethality of Creld1(-/-) mice complicates the analysis of Creld1's tissue-specific functions. Additionally, the search results indicate that Creld1(-/-) embryos had both heart defects and placental abnormalities , raising questions about whether the heart defects are primary or secondary to placental insufficiency. To address this:

• Tissue-Specific Conditional Knockouts: Generate and compare heart-specific (e.g., Nkx2.5-Cre), placenta-specific (e.g., Tpbpa-Cre), and endothelial-specific (e.g., Tie2-Cre) Creld1 knockout models.

• Tetraploid Complementation: Wild-type tetraploid embryos can provide normal extraembryonic tissues (including placenta) for Creld1(-/-) embryonic cells, allowing assessment of whether normal placental function can rescue the embryonic phenotypes.

• Ex Vivo Culture Systems: Heart explant cultures, as already utilized in studies of Creld1 and VEGF interaction , can help determine direct effects on cardiac tissues independent of placental function.

• Chimeric Analysis: Creating chimeric embryos with mixed Creld1-positive and Creld1-negative cells can reveal tissue-autonomous requirements through analysis of which tissues tolerate Creld1-deficient cells.

What are the most promising therapeutic implications of Creld1 research?

Understanding Creld1 function has several potential therapeutic implications:

• Congenital Heart Disease: Insights into CRELD1-VEGF interactions might lead to prenatal interventions for high-risk pregnancies with identified CRELD1 mutations or VEGF polymorphisms.

• Immunosenescence: Given Creld1's role in preventing premature T cell aging , therapies targeting the Creld1 pathway might help address age-related immune decline.

• Regenerative Medicine: Understanding Creld1's role in cardiac development could inform approaches to direct stem cell differentiation for heart repair.