Recombinant Danio rerio Tripartite motif-containing 13 (trim13)

What is the molecular structure of Danio rerio trim13 and how does it compare to mammalian TRIM13?

Zebrafish trim13 (also known as zgc:110578) is a protein-coding gene located on chromosome 9 of the Danio rerio genome . The full-length protein consists of 404 amino acids compared to 407 amino acids in the human TRIM13 . Like other TRIM family members, zebrafish trim13 contains a conserved modular tripartite motif structure consisting of:

• A RING finger domain at the N-terminus (responsible for E3 ubiquitin ligase activity)

• B-box-type zinc finger domain

• Predicted coiled-coil (CC) domain

What is the expression pattern of trim13 in zebrafish tissues?

Zebrafish trim13 shows a tissue-specific expression pattern. According to ZFIN gene database information, trim13 is predominantly expressed in:

• Brain

• Muscle

• Ovary

• Testis (with notably high expression)

This expression pattern is somewhat different from mammalian TRIM13, which in mice shows strongest expression in testis, but also low abundance in brain, lung, heart, liver, and kidney . The differential expression pattern suggests possible functional divergence between species.

How can recombinant Danio rerio trim13 be produced in prokaryotic expression systems?

Recombinant zebrafish trim13 protein can be effectively produced in E. coli expression systems using the following methodology:

• Gene cloning: The full-length coding sequence (1-404aa) should be PCR-amplified from zebrafish cDNA and cloned into an appropriate expression vector (e.g., pET vector system) with an N-terminal His-tag for purification .

• Expression conditions: Transform the recombinant plasmid into an E. coli expression strain (typically BL21(DE3)). Culture in LB medium and induce protein expression with 0.5 mM IPTG at lower temperatures (18°C) overnight with gentle shaking (120 rpm) to promote proper folding .

• Cell lysis: Harvest cells by centrifugation (4,500 rpm for 30 minutes) and lyse in appropriate buffer (100 mM sodium phosphate, pH 8.0, 600 mM NaCl, and 0.02% Tween-20) via sonication on ice .

• Purification: Purify using affinity chromatography with His-Tag magnetic beads or Ni-NTA columns. The lysate should be centrifuged at 15,000 rpm at 4°C for 20 minutes before loading onto the affinity matrix .

• Storage: Store the purified protein in Tris/PBS-based buffer with 6% trehalose at pH 8.0. For long-term storage, add glycerol (final concentration 5-50%) and store at -20°C/-80°C in aliquots to avoid repeated freeze-thaw cycles .

What are effective approaches to study trim13 function in zebrafish models?

Several methodological approaches can be used to investigate trim13 function in zebrafish:

• Morpholino knockdown: Design antisense morpholino oligonucleotides targeting the translation start site or splice junctions of trim13 mRNA. Microinject into zebrafish embryos at the single-cell stage (1-2 nl per embryo) to achieve transient knockdown .

• CRISPR-Cas9 gene editing: Design guide RNAs targeting exons of the trim13 gene, particularly the RING domain which is crucial for its E3 ligase activity. Co-inject with Cas9 mRNA or protein into single-cell embryos to generate stable mutant lines .

• Viral challenge models: Challenge zebrafish embryos or cell lines (such as ZBE3) with RNA viruses like RGNNV (Red-spotted grouper nervous necrosis virus) to assess trim13's role in antiviral immunity. Typically, viruses can be microinjected into the egg yolk (10^8 TCID50/ml, 1 nl per embryo) or used to infect cells at MOI=1 .

• siRNA knockdown in cell culture: For in vitro studies, design specific siRNAs targeting zebrafish trim13 and transfect into zebrafish cell lines using appropriate transfection reagents like Lipofectamine 3000 .

• Expression analysis: Monitor gene expression changes using qRT-PCR with primers specific to trim13 and downstream immune genes (e.g., interferon pathway components) .

How does trim13 regulate antiviral immune responses in fish models?

Based on studies with fish TRIM13 homologs, particularly in grouper (Epinephelus coioides), trim13 functions as a negative regulator of antiviral immunity:

• Interferon pathway regulation: Zebrafish trim13, like its mammalian counterpart, appears to negatively regulate type I interferon signaling. Overexpression of fish TRIM13 significantly decreases the expression of interferon-related factors including IRF3, IRF7, MDA5, MXI, and ISG15 .

• Viral replication effects: Ectopic expression of fish TRIM13 enhances the replication of RNA viruses such as RGNNV. This is evidenced by increased cytopathic effect progression and viral gene transcription in overexpression models .

• Molecular mechanism: The regulatory effect on virus replication is dependent on the RING domain, suggesting that the E3 ubiquitin ligase activity is essential for its function. Deletion of the RING domain significantly weakens the enhancing effect on virus replication .

• Inflammatory response modulation: trim13 also differentially regulates the transcription of pro-inflammatory factors including IL-6, IL-1β, and TNFα, suggesting a broader role in immune homeostasis .

• Evolutionary conservation: This negative regulatory function appears to be conserved from fish to mammals, as mammalian TRIM13 has been shown to negatively regulate MDA5-mediated interferon production .

What molecular interactions mediate trim13 function in innate immunity?

The molecular mechanisms of trim13 function involve several key interactions:

• RIG-I/MDA5 interaction: TRIM13 has been shown to interact with both MDA5 and RIG-I (cytosolic pattern recognition receptors) in vitro. This interaction is critical for its negative regulation of RLR signaling pathways .

• STING pathway modulation: In mammals, TRIM13 has been identified as a regulator of the STING pathway, which is critical for DNA virus sensing. TRIM13 inhibits STING-directed activation of IRF3 and NF-κB pathways .

• E3 ubiquitin ligase activity: The RING domain of trim13 confers E3 ubiquitin ligase activity, which is essential for its immunoregulatory function. This activity likely mediates the ubiquitination of key signaling molecules in the interferon pathway .

• Subcellular localization: Zebrafish trim13 is predicted to localize to the endoplasmic reticulum membrane, similar to mammalian TRIM13. This localization is important for its interaction with other components of the antiviral signaling machinery .

The experimental data table below summarizes the effect of TRIM13 on interferon pathway components:

How has the trim13 gene evolved in zebrafish compared to other teleosts and vertebrates?

The evolution of trim13 in zebrafish represents an interesting case of TRIM protein diversification:

• Evolutionary origin: TRIM proteins are ancient and have greatly diversified in vertebrates, especially in fish. The trim13 gene belongs to the class IV TRIMs, characterized by their C-terminal B30.2 domain structure .

• Genomic context: Unlike some other TRIM genes that have undergone extensive duplication in fish (such as finTRIMs with 84 genes and bloodthirsty-like TRIMs with 33 genes in zebrafish), trim13 appears to be maintained as a single-copy gene, suggesting functional conservation under selective pressure .

• Domain conservation: Sequence analysis shows that zebrafish trim13 contains the conserved RING finger and B-box domains that are characteristic of the TRIM family. These domains show high conservation across vertebrates, indicating functional importance .

• Functional divergence: While the basic structure is conserved, functional studies suggest some divergence in expression patterns and possibly in functional specificity between fish and mammals .

• Selective pressures: Unlike some fish-specific TRIM genes (like fintrim/ftr) that show evidence of positive selection in their B30.2 domains, trim13 appears to have evolved under more constrained selection, consistent with its role in fundamental cellular processes .

What are the technical challenges in designing CRISPR-Cas9 knockout models for zebrafish trim13?

Generating effective CRISPR-Cas9 knockout models for zebrafish trim13 involves several technical considerations:

How can protein-protein interaction studies be optimized for recombinant zebrafish trim13?

Investigating protein-protein interactions involving zebrafish trim13 requires specialized approaches:

• Co-immunoprecipitation techniques:

  • Express epitope-tagged versions of trim13 (e.g., Flag-tagged or Myc-tagged) in appropriate cell lines

  • For pull-down assays, use His-tagged recombinant trim13 purified from E. coli with magnetic beads

  • Cell lysates should be prepared in buffers that preserve protein interactions (typically containing 100-150 mM NaCl, 1% mild detergent, and protease inhibitors)

  • Interactions can be detected by Western blotting using antibodies against putative binding partners

• Domain mapping strategies:

  • Generate deletion mutants of trim13 lacking specific domains (RING domain, B-box, coiled-coil, or C-terminal region)

  • Express these mutants alongside potential interaction partners to determine which domains are essential for specific interactions

  • For the RING domain, which is critical for E3 ligase activity, create point mutations in key cysteine residues rather than complete deletions

• Functional validation:

  • Use reporter gene assays (e.g., luciferase reporters driven by interferon promoters) to assess the functional consequences of interactions

  • Compare wild-type trim13 with domain mutants to establish structure-function relationships

  • Include appropriate positive and negative controls in all interaction studies

• In vivo validation:

  • Confirm interactions identified in vitro using zebrafish embryos or adult tissues

  • Consider proximity ligation assays or FRET-based approaches for detecting interactions in intact cells

  • Correlate interaction data with functional outcomes in response to immune stimuli or viral challenges

What are common challenges in expressing recombinant zebrafish trim13 and how can they be addressed?

Researchers often encounter several challenges when producing recombinant zebrafish trim13:

• Protein solubility issues:

  • Challenge: TRIM proteins often form inclusion bodies in E. coli due to their multiple domains.

  • Solution: Lower the induction temperature to 16-18°C, reduce IPTG concentration (0.1-0.5 mM), and use slower induction (overnight) to improve solubility .

  • Alternative: Consider using solubility-enhancing fusion tags like GST, MBP, or SUMO in addition to the His-tag.

• Maintaining E3 ligase activity:

  • Challenge: The RING domain, responsible for E3 ligase activity, can be sensitive to oxidation and improper folding.

  • Solution: Include reducing agents (DTT or β-mercaptoethanol) in purification buffers and add zinc chloride (10-50 μM) to stabilize zinc-finger domains .

• Protein degradation:

  • Challenge: Recombinant TRIM proteins may undergo self-ubiquitination, leading to degradation.

  • Solution: Include protease inhibitors throughout purification, consider using E. coli strains lacking specific proteases, and purify rapidly at 4°C .

• Low yield:

  • Challenge: Expression levels of full-length trim13 may be low.

  • Solution: Optimize codon usage for E. coli, consider using stronger promoters, or test multiple E. coli strains (BL21(DE3), Rosetta, Arctic Express) to identify optimal expression conditions .

• Proper folding verification:

  • Challenge: Ensuring the recombinant protein is properly folded.

  • Solution: Perform circular dichroism spectroscopy to assess secondary structure, and verify E3 ligase activity using in vitro ubiquitination assays with E1, E2 enzymes, and ubiquitin .

How can the immune functions of zebrafish trim13 be effectively studied in vivo?

To investigate the immune functions of trim13 in zebrafish, researchers should consider these methodological approaches:

• Infection models:

  • Viral challenge: Inject RGNNV (10^8 TCID50/ml) into the egg yolk of zebrafish embryos at the single-cell stage using a microinjector (1 nl per embryo) .

  • Monitor survival rates, viral loads by qRT-PCR targeting viral genes (e.g., RDRP), and expression of immune response genes.

  • Include appropriate controls (injection of culture medium without virus).

• Gene expression analysis:

  • Design qRT-PCR primers for zebrafish trim13 and downstream genes in the interferon pathway (IRF3, IRF7, MAVS, TRAF3, type I interferons, and ISGs).

  • Normalize expression to stable reference genes (e.g., 18S rRNA) .

  • Sample collection timepoints: 6, 12, 24, and 48 hours post-infection to capture the dynamics of the immune response.

• Morphant/mutant phenotyping:

  • For morpholino studies, use at least two different morpholinos (translation-blocking and splice-blocking) with appropriate controls.

  • For CRISPR mutants, establish homozygous lines and characterize them for developmental and immunological phenotypes.

  • Challenge both morphants and mutants with viruses or PAMPs (e.g., poly I:C) to assess immune response alterations .

• Pathway analysis using reporter systems:

  • Generate transgenic zebrafish lines with fluorescent reporters driven by interferon-responsive promoters.

  • Cross these reporter lines with trim13 mutants to visualize alterations in interferon signaling in vivo.

  • Alternatively, use dual-luciferase reporter assays in zebrafish cell lines to measure specific pathway activation .

• Rescue experiments:

  • Perform rescue experiments by injecting wild-type or domain-mutant trim13 mRNA into trim13-deficient embryos.

  • Assess whether specific domains (especially the RING domain) are essential for its immune regulatory functions .