What is the Vasoactive Intestinal Polypeptide Receptor in Carassius auratus?

The Vasoactive Intestinal Polypeptide Receptor (vipr1) in Carassius auratus (goldfish) belongs to the class II G protein-coupled receptor family. It binds VIP, a neuropeptide involved in multiple physiological functions including immune modulation and is part of the VPAC1 receptor classification in vertebrates . Notably, goldfish possess duplicated VPAC1 receptors, a characteristic shared with other teleost fish, suggesting a teleost-specific gene duplication event that occurred prior to the divergence of the Cypriniformes and Tetraodontiformes .

Where is vipr1 expressed in Carassius auratus tissues?

VIP-like immunoreactive cells have been observed in the second segment of some renal proximal tubules of Carassius auratus. These cells display a distinctive morphology resembling an overturned flask with the globe-like portion facing the tubular lumen. The immunoreactivity shows a granular appearance distributed throughout the cells . No VIP-like immunoreactivity has been detected in other nephron sections or interstitial cells . While the provided research doesn't explicitly map all tissue expression patterns, these findings suggest specialized functions for VIP signaling in goldfish renal physiology.

How does goldfish vipr1 compare structurally to mammalian VPAC1 receptors?

Goldfish VPAC1 receptor differs structurally from mammalian counterparts primarily in the N-terminal region. Unlike mammalian VPAC1R, which contains a signal peptide in the first exon, this exon is absent in goldfish VPAC1R, though the receptor remains functional . Despite this difference, goldfish VPAC1R effectively stimulates cAMP production when expressed in mammalian cell lines, indicating that the amino acids encoded by the first exon are not essential for receptor activity . The receptor otherwise maintains the characteristic features and signature motifs typical of class II GPCR family members found across vertebrates.

What are the recommended methodologies for cloning recombinant Carassius auratus vipr1?

For cloning recombinant Carassius auratus vipr1, researchers should consider the following methodological approach:

• Primer Design: Design degenerate primers based on conserved sequence motifs identified through multiple sequence alignment of VPAC/PAC family members. This approach successfully identified duplicate VPAC1R cDNAs in goldfish and salmon .

• cDNA Library Screening: Construct a cDNA library from goldfish tissues (particularly kidney tissue where VIP-immunoreactive cells have been documented) and screen using low-stringency hybridization (50°C hybridization temperature in Church Gilbert buffer followed by room temperature washing) .

• PCR Amplification: Use reverse transcription PCR (RT-PCR) on total RNA extracted from goldfish tissues, as this method has successfully amplified two partial cDNAs for VPAC1R in goldfish .

• Sequence Verification: Confirm the identity of cloned sequences through phylogenetic analysis comparing them with known VPAC1R sequences from other vertebrates .

Remember that goldfish likely possess two distinct VPAC1R genes due to teleost-specific genome duplication, so multiple isoforms may be identified .

What expression systems are optimal for producing functional recombinant Carassius auratus vipr1?

Based on previous successful approaches with goldfish VPAC1R:

• Mammalian Cell Lines: Despite structural differences from mammalian receptors, goldfish VPAC1R has been successfully expressed and shown to be functional in mammalian cell expression systems. These systems demonstrated that goldfish VPAC1R can stimulate cAMP production, confirming receptor functionality even without the signal peptide encoded by the first exon found in mammalian homologs .

• Methodology Considerations:

  • Transfection efficiency can be optimized using lipid-based transfection reagents

  • Selection of stable transfectants may require antibiotic resistance markers

  • Confirmation of expression through Western blotting with appropriate antibodies

  • Functional validation through cAMP assays as demonstrated in previous research

• Potential Challenges: Researchers should be aware that the absence of a conventional signal peptide in goldfish VPAC1R might affect membrane integration, though existing evidence suggests the receptor remains functional despite this structural difference .

How can one assess the binding affinity of ligands to recombinant Carassius auratus vipr1?

To assess binding affinities of ligands to recombinant Carassius auratus vipr1, researchers should consider these methodological approaches:

• Competitive Binding Assays: Using radiolabeled VIP or synthetic analogs to measure displacement by test ligands. This approach can determine relative binding affinities of different peptides.

• In Silico Docking Analysis: Computational methods similar to those used for other VIP receptors can predict binding affinities. Research on human VIP receptors has successfully employed docking score analysis to predict relative affinities of VIP analogs . These methods could be adapted for goldfish vipr1, particularly since structural features of VIP receptors are conserved across vertebrates .

• Functional Response Measurement: Measuring downstream signaling (primarily cAMP production) in response to various concentrations of ligands, as demonstrated in previous goldfish VPAC1R functional studies .

• AlphaFold or Similar Structure Prediction: Advanced structural prediction tools can model ligand-receptor interactions, as demonstrated for human VIP receptors where "AlphaFold 3 and ChimeraX predictions are consistent with cryogenic electron microscopy studies" .

What are the phylogenetic relationships between goldfish vipr1 and other vertebrate VIP receptors?

Phylogenetic analysis has revealed significant insights about the evolutionary relationships of goldfish vipr1:

• Teleost-Specific Duplication: Goldfish possess two distinct VPAC1R genes as a result of teleost-specific gene duplication events that occurred prior to the divergence of Cypriniformes and Tetraodontiformes .

• Clustering Pattern: When compared using both Neighbour Joining and Maximum Likelihood phylogenetic methods, the goldfish VPAC1R sequences cluster with their orthologous counterparts from other teleosts like Fugu, rather than forming species-specific clusters. Specifically, one goldfish VPAC1R clusters with Fugu VPAC1RA while the other clusters with Fugu VPAC1RB .

• Evolutionary Timing: The duplication of these receptors appears to have occurred prior to the branch that gave rise to mammals, supporting the theory of teleost-specific genome duplication .

• Conserved Synteny: Analysis of linked genes shows patterns of conserved synteny between teleost VPAC receptor genes and mammalian counterparts, particularly with genes on human chromosome region 3p .

Table 1: Comparison of Key Features Between Teleost and Mammalian VPAC1 Receptors

How can structure-function relationships of Carassius auratus vipr1 be investigated using site-directed mutagenesis?

Structure-function analysis of Carassius auratus vipr1 through site-directed mutagenesis should focus on key domains identified through comparative analysis. Based on research approaches used for related receptors:

• N-terminal Domain: Given the unique feature of goldfish VPAC1R lacking the conventional signal peptide, investigating alternative sequences that may function in membrane targeting would be valuable. Create systematic mutations in the N-terminal region to identify critical residues for membrane integration and ligand binding .

• Transmembrane Domains: Target conserved residues in transmembrane regions that show high sequence similarity across species. Research on VIP receptors indicates that "TM domains" share high sequence similarity across species , making them prime targets for mutagenesis studies.

• Intracellular Loops: Investigate residues involved in G-protein coupling by creating mutations in intracellular loops, particularly those predicted to interact with signal transduction machinery.

• Methodology:

  • Generate mutations using PCR-based site-directed mutagenesis

  • Express mutant receptors in mammalian cell lines as previously demonstrated

  • Assess function through cAMP production assays

  • Compare binding affinities of wild-type and mutant receptors

What approaches can be used to investigate the physiological roles of vipr1 in Carassius auratus?

To investigate the physiological roles of vipr1 in Carassius auratus, researchers should consider:

• Tissue-Specific Expression Analysis:

  • Expand on existing immunohistochemistry data that has identified VIP-like immunoreactive cells in renal proximal tubules

  • Use quantitative PCR to map expression patterns across different tissues and developmental stages

  • Employ in situ hybridization to precisely localize receptor expression

• Functional Inhibition Studies:

  • Apply receptor antagonists similar to the VIP-ANTs described in recent research

  • Design goldfish-specific VIP receptor antagonists based on structural predictions

  • Use CRISPR-Cas9 gene editing to generate receptor knockdown models

• Ex Vivo Tissue Studies:

  • Analyze isolated renal tubule segments, where VIP-immunoreactive cells have been identified

  • Measure physiological responses (ion transport, fluid secretion) to VIP stimulation or inhibition

  • Correlate receptor activation with cellular signaling cascades (cAMP, CREB phosphorylation)

• In Vivo Physiological Measurements:

  • Examine effects of VIP and antagonists on renal function in intact goldfish

  • Investigate possible immune modulatory functions, given VIP's established role in immune regulation

  • Monitor potential effects on other systems where VIP signaling is important in vertebrates

What are common pitfalls when expressing recombinant Carassius auratus vipr1, and how can they be addressed?

Common challenges and their solutions include:

• Low Expression Levels:

  • Problem: The absence of a conventional signal peptide in goldfish VPAC1R may result in inefficient membrane targeting

  • Solution: Add a heterologous signal sequence to the construct or optimize codon usage for the expression system

  • Solution: Test multiple cell lines, as some may better accommodate the unconventional processing of goldfish VPAC1R

• Difficulty in Functional Assessment:

  • Problem: Variable responses in cAMP assays

  • Solution: Include positive controls (such as forskolin) to verify assay functionality

  • Solution: Normalize data to receptor expression levels as determined by Western blot or flow cytometry

• Isoform Confusion:

  • Problem: Inadvertent amplification of both duplicated VPAC1R genes

  • Solution: Design isoform-specific primers based on divergent regions

  • Solution: Use phylogenetic analysis to clearly distinguish between the two goldfish VPAC1R paralogs

• Membrane Integration Issues:

  • Problem: Poor trafficking to cell membrane despite expression

  • Solution: Create fusion constructs with fluorescent tags to monitor localization

  • Solution: Consider chaperone co-expression to facilitate proper folding and trafficking

How can researchers verify the functionality of recombinant Carassius auratus vipr1 in heterologous expression systems?

To verify functionality of recombinant Carassius auratus vipr1:

• cAMP Assays: As the primary signaling pathway for VIP receptors, measuring cAMP production in response to VIP stimulation is essential. Previous research has confirmed goldfish VPAC1R functionality through cAMP production in transfected mammalian cells .

• Calcium Mobilization: Secondary signaling pathways can be assessed using calcium-sensitive dyes and imaging techniques.

• Binding Studies: Confirm ligand binding using radioligand binding assays with labeled VIP or related peptides.

• Phosphorylation Analysis: Monitor downstream effects such as CREB phosphorylation, which has been used as a readout for VIP receptor activation in other systems .

• Receptor Internalization: Track receptor responses to ligand exposure using fluorescently tagged constructs to observe internalization dynamics.

• Antagonist Response: Verify that known VIP receptor antagonists can block activation, potentially using antagonists like the VIP-ANTs described in recent research on human receptors .

How do the duplicated VPAC1 receptors in goldfish differ functionally from single VPAC1 receptors in mammals?

The functional differences between duplicated goldfish VPAC1 receptors and singular mammalian counterparts represent an important research area:

What insights does the study of teleost-specific duplicated vipr1 provide for understanding receptor evolution?

The teleost-specific duplication of vipr1 offers several valuable insights into receptor evolution:

• Sub-functionalization vs. Neo-functionalization: The maintenance of both paralogs suggests they may have undergone either sub-functionalization (division of ancestral functions between duplicates) or neo-functionalization (evolution of new functions in one duplicate).

• Evolutionary Pressure: Phylogenetic analysis indicates that the duplication of these receptors occurred prior to the divergence of Cypriniformes and Tetraodontiformes, suggesting both copies have been maintained under selective pressure for an extended evolutionary period .

• Genomic Architecture: Conserved synteny analysis shows that despite large-scale chromosomal rearrangements, linkage relationships between vipr1 and neighboring genes show some conservation between fish and mammals, providing insights into chromosome evolution .

• Receptor Specialization: The teleost-specific whole genome duplication event may have allowed for greater specialization of receptor function in fish compared to tetrapods, potentially contributing to their evolutionary success in diverse aquatic environments.

• Molecular Clock: The degree of divergence between the duplicated receptors can serve as a molecular clock, helping to time evolutionary events in teleost radiation.