Recombinant Rat Vasoactive intestinal polypeptide receptor 1 (Vipr1)

What is Vipr1 and what are its alternative nomenclatures in scientific literature?

Vipr1, primarily known as VPAC1 in scientific literature, is a G-protein coupled receptor belonging to class B1. This receptor binds both vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) with high affinity. In rats, this receptor is encoded by the Vipr1 gene (ID: 24875) . The receptor has multiple alternative names across scientific publications including VIP-R-1, VAPC1, PACAP-R-2, VPAC1R, and VPCAP1R . These various designations reflect its discovery history and functional characterization across different research groups.

The receptor has structural characteristics typical of class B1 GPCRs, with a large N-terminal extracellular domain that participates in ligand binding and seven transmembrane domains that facilitate signal transduction . This structure allows Vipr1 to couple with G proteins that activate adenylate cyclase, leading to cAMP production and subsequent cellular responses .

What is the tissue distribution pattern of Vipr1 in rats?

Vipr1 exhibits a distinct expression pattern across rat tissues, with particularly high expression in the central nervous system and immune system. Within the brain, significant Vipr1 expression has been detected in the hypothalamus, specifically in neurons of the paraventricular nucleus (PVN) and supraoptic nucleus (SON) . This localization suggests important roles in neuroendocrine functions.

In addition to neuronal expression, Vipr1 is also present in cerebral blood vessels. Studies have shown that VPAC1 receptors are localized on the surface of smooth muscle cells in cerebral blood vessels of rats, indicating vascular regulatory functions . This finding has been reproduced by multiple research groups using different VPAC1 antibodies .

Within the immune system, Vipr1 is highly expressed and modulates diverse T cell functions . This expression pattern suggests a significant role in immune regulation, potentially mediating interactions between the nervous and immune systems.

What are the primary physiological functions of Vipr1?

Vipr1 mediates several critical physiological functions across multiple organ systems:

• Smooth muscle relaxation: Vipr1 activation leads to relaxation of smooth muscle, particularly in vascular and bronchial tissues .

• Exocrine and endocrine secretion: The receptor regulates secretory processes in various glands and tissues .

• Epithelial water and ion flux: Vipr1 modulates water and ion transport across lung and intestinal epithelia, contributing to fluid homeostasis .

• Immune modulation: In the immune system, Vipr1 regulates diverse T cell functions, playing a role in immune responses and potentially in inflammatory conditions .

• Neurotransmission: The presence of Vipr1 in hypothalamic neurons suggests roles in neuroendocrine regulation and potentially in circadian rhythms .

These functions are mediated through G-protein coupling that activates adenylate cyclase, leading to increased cAMP production and subsequent activation of downstream signaling cascades .

What expression systems are optimal for producing recombinant rat Vipr1?

For research applications requiring functional recombinant rat Vipr1, the following expression systems should be considered:

The choice of expression system should be guided by the specific research application, whether structural studies, functional assays, or antibody production.

What immunological tools are available for detecting rat Vipr1?

Several antibodies are available for detecting and studying rat Vipr1 in various experimental contexts:

When using these antibodies, it's important to note that different detection systems may yield varying results. For instance, research has shown that when visualizing VPAC1 in mouse brain tissue, biotinylated tyramide amplification showed stronger staining in blood vessels, while the ENVISION detection system better revealed neuronal expression . This methodological consideration is crucial for accurately interpreting experimental results.

What methodological approaches are most effective for studying Vipr1-ligand interactions?

Multiple complementary methodological approaches have been successfully employed to study Vipr1-ligand interactions:

• Radioligand binding assays: 125I-labeled vasoactive intestinal peptide binding has been used to quantify VPAC-1 expression levels and receptor-ligand interactions . This approach allows for precise measurement of binding affinities and receptor densities.

• cAMP accumulation assays: Since Vipr1 primarily signals through G proteins that activate adenylate cyclase, measuring cAMP production provides a functional readout of receptor activation . This approach has been used to assess the impact of receptor modifications on signaling efficiency.

• Immunohistochemistry with specialized detection: Different amplification systems (tyramide-based vs. ENVISION-based) have been employed to visualize Vipr1 in different cellular compartments (blood vessels vs. neurons) . The choice of detection system should be guided by the specific cellular localization being investigated.

• Three-dimensional reconstruction and colocalization analysis: Advanced imaging techniques, including Z-stack acquisition and 3D reconstruction, have been used to determine close appositions between VIP fibers and VPAC1-expressing neurons . This approach provides insights into the physiological context of receptor-ligand interactions.

• Electrophoretic mobility shift assays (EMSA): Although not directly assessing ligand binding, this technique has been used to study factors like Ikaros that regulate Vipr1 expression .

For comprehensive characterization of Vipr1-ligand interactions, combining multiple methodological approaches is recommended.

How is Vipr1 gene expression regulated at the transcriptional level?

The transcriptional regulation of Vipr1 involves several specific mechanisms, with particular importance of Ikaros (IK) transcription factors:

The human VPAC-1 5'-flanking region (1.4 kb) contains four high-affinity Ikaros consensus sequences . Research has demonstrated that both Ikaros native protein from T cell nuclear extracts and recombinant IK-1 and IK-2 proteins specifically recognize an IK high-affinity binding motif in the VPAC-1 promoter through sequence-specific mechanisms .

Functional studies using stable NIH-3T3 clones overexpressing either IK-1 or IK-2 isoforms have shown that both isoforms significantly suppress endogenous VPAC-1 expression, reducing levels by 50-93% as measured by both mRNA quantification and receptor binding assays . This suppression occurs at the transcriptional level, as evidenced by decreased activity of VPAC-1 luciferase reporter constructs when transiently transfected into IK-2-expressing clones .

Two major IK-2 binding domains have been identified in the VPAC-1 promoter: one located at -1076 to -623 bp and another at -222 to -35 bp . The presence of these regulatory elements suggests that Ikaros-mediated suppression may be a dominant determinant of VPAC-1 expression in immune responses, as both Ikaros and VPAC-1 are highly expressed in T cells .

What are the critical structural features of Vipr1 that determine ligand binding and receptor activation?

While the search results don't provide specific structural details for rat Vipr1, insights can be gained from studies of the related VIP2R (VPAC2) receptor which shares significant homology with Vipr1. Both belong to class B1 GPCRs and bind similar ligands (VIP and PACAP) .

A distinctive feature revealed in the cryo-electron microscopy structure of human VIP2R is that its N-terminal α-helix adopts a unique conformation that deeply inserts into a cleft between the peptide ligand (PACAP27) and the extracellular loop 1, thereby stabilizing the peptide-receptor interface . Functional studies demonstrated that truncation or extension of this N-terminal α-helix significantly decreased receptor-mediated cAMP accumulation .

Given the homology between Vipr1 and VIP2R, similar structural elements likely play crucial roles in Vipr1 function:

• The N-terminal extracellular domain, which provides the initial binding site for peptide ligands

• The extracellular loops, particularly ECL1, which interact with the bound peptide

• The transmembrane domains, which undergo conformational changes upon ligand binding to activate G protein coupling

For researchers working with recombinant rat Vipr1, preserving these structural elements is essential for maintaining functional integrity.

How do post-translational modifications affect Vipr1 function and signaling?

Although the search results don't specifically address post-translational modifications of rat Vipr1, as a class B1 GPCR, several modifications likely influence its function and signaling properties:

• Glycosylation: N-linked glycosylation of the extracellular domain can affect receptor folding, trafficking to the cell surface, ligand binding affinity, and resistance to proteolytic degradation. The variable detection of Vipr1 in different tissues using different amplification methods may partly reflect differences in glycosylation states.

• Phosphorylation: Receptor phosphorylation, particularly of the intracellular loops and C-terminal tail, regulates receptor desensitization, internalization, and interaction with signaling partners. This modification likely influences the duration and specificity of Vipr1 signaling.

• Palmitoylation: Cysteine palmitoylation can affect receptor localization in membrane microdomains and interaction with signaling partners, potentially influencing Vipr1's coupling efficiency to G proteins.

For researchers working with recombinant rat Vipr1, the choice of expression system will significantly impact the post-translational modification profile of the receptor. Mammalian expression systems generally provide the most native-like modifications, while bacterial systems lack most post-translational modification capabilities. Careful characterization of these modifications should be considered when interpreting functional data from recombinant Vipr1 systems.

What are the key considerations for designing functional assays for recombinant rat Vipr1?

When designing functional assays for recombinant rat Vipr1, several critical factors should be considered:

What challenges might arise when expressing recombinant rat Vipr1 and how can they be addressed?

Expressing recombinant rat Vipr1 presents several potential challenges that researchers should anticipate:

• Low surface expression: GPCRs often exhibit limited trafficking to the cell surface due to misfolding or retention in intracellular compartments. This can be addressed by:

  • Including signal sequences optimized for the expression system

  • Co-expressing chaperone proteins to aid folding

  • Using receptor fusion constructs with partners that enhance surface expression

  • Culturing cells at lower temperatures (30-32°C) to promote proper folding

• Receptor instability: Class B GPCRs can be unstable when removed from their native membrane environment. Potential solutions include:

  • Adding ligands during expression and purification to stabilize active conformations

  • Incorporating stabilizing mutations identified through directed evolution or alanine scanning

  • Using detergents or lipid environments optimized for maintaining receptor structure

• Toxicity to host cells: Overexpression of GPCRs can sometimes lead to constitutive activity or toxicity. This can be mitigated by:

  • Using inducible expression systems to control expression timing and level

  • Selecting cell lines with higher tolerance for GPCR expression

  • Co-expressing proteins that regulate receptor activity or trafficking

• Post-translational modification variability: Different expression systems provide different post-translational modifications. Researchers should:

  • Characterize the glycosylation and other modification profiles of the expressed receptor

  • Compare functional properties with native receptor when possible

  • Consider testing multiple expression systems if modifications are critical for the research question

How can researchers verify the functional integrity of purified recombinant rat Vipr1?

Verifying the functional integrity of purified recombinant rat Vipr1 requires multiple complementary approaches:

• Ligand binding assays: Radioligand binding using 125I-labeled VIP or PACAP can confirm that the purified receptor retains its ligand-binding capability . Competition binding assays with unlabeled peptides can verify binding specificity and affinity.

• Structural characterization: Techniques such as circular dichroism spectroscopy, thermal stability assays, or limited proteolysis can assess whether the receptor maintains its native fold after purification.

• G protein coupling assays: In vitro G protein activation assays (e.g., [35S]GTPγS binding) can demonstrate that the purified receptor can still activate its cognate G proteins in response to ligand binding.

• Reconstitution into lipid environments: Functional reconstitution into proteoliposomes, nanodiscs, or other membrane mimetics followed by activity assays can confirm that the receptor regains its native function when provided with an appropriate lipid environment.

• Antibody recognition: Using conformation-specific antibodies that recognize properly folded epitopes can help distinguish correctly folded receptors from misfolded variants.

• Microscale thermophoresis or surface plasmon resonance: These techniques can measure ligand binding to purified receptors without requiring radioligands and can provide detailed binding kinetics.

For comprehensive validation, combining multiple approaches is recommended to ensure that the purified receptor maintains both structural and functional integrity.

What is the potential of Vipr1 as a therapeutic target in neurological and immunological disorders?

While the search results don't specifically address the therapeutic potential of rat Vipr1, they do indicate that VIP2R (a related receptor) shows "great potential as a therapeutic target for pulmonary arterial hypertension, chronic obstructive pulmonary disease (COPD), cancer, asthma, autoimmune and psychiatric disorders" . Given the functional similarities between Vipr1 and VIP2R, Vipr1 likely shares some of this therapeutic potential.

The localization of Vipr1 in specific brain regions and immune cells suggests several potential therapeutic applications:

• Neurological disorders: Vipr1's expression in hypothalamic nuclei (PVN and SON) suggests potential roles in disorders involving neuroendocrine dysregulation, stress responses, or circadian rhythm disturbances. Targeting Vipr1 might benefit conditions such as anxiety disorders, depression, or sleep disturbances.

• Immunological conditions: The high expression of Vipr1 in the immune system and its role in modulating T cell functions indicates potential applications in autoimmune disorders, inflammatory conditions, or transplant rejection. The negative regulation of Vipr1 by Ikaros transcription factors suggests complex roles in immune homeostasis that could be therapeutically exploited.

• Vascular disorders: Vipr1's presence in cerebral blood vessels points to potential applications in cerebrovascular disorders, including stroke or vascular dementia, where modulating vascular tone might be beneficial.

Developing Vipr1-targeted therapeutics would require careful consideration of specificity (distinguishing between Vipr1, VIP2R, and PAC1R) and tissue selectivity to minimize off-target effects.

How do Vipr1 knockout models advance our understanding of its physiological roles?

The search results mention VPAC1 knockout mice that had "exon 4–6 deletion making a functional knockout" , though it's noted that "a truncated part of the N-terminal receptor protein was still expressed" . This model has been used to investigate VPAC1 function, though the residual expression of a truncated protein complicates interpretation.

Knockout models provide valuable insights into physiological roles through several approaches:

• Phenotypic analysis: Comprehensive phenotyping of Vipr1 knockout rats could reveal abnormalities in systems where the receptor plays crucial roles. The high expression of Vipr1 in the immune system and hypothalamus suggests that immunological, neuroendocrine, and behavioral phenotypes should be carefully assessed.

• Challenge models: Exposing knockout animals to various challenges (immune stimulation, stress, altered circadian cycles) can unmask phenotypes not apparent under basal conditions, revealing context-dependent functions of Vipr1.

• Tissue-specific knockouts: Generating conditional knockout models with tissue-specific deletion of Vipr1 can help distinguish direct from indirect effects and avoid developmental compensations that might occur in global knockouts.

• Molecular profiling: Transcriptomic, proteomic, or metabolomic analysis of tissues from knockout animals can identify downstream molecular pathways affected by Vipr1 deletion, providing mechanistic insights into its function.

When interpreting data from knockout models, researchers should consider potential compensatory mechanisms, such as upregulation of related receptors (VIP2R/VPAC2 or PAC1R), which might mask the full impact of Vipr1 deletion.

What recent technological advances have enhanced our ability to study Vipr1 structure and function?

Although the search results don't specifically discuss technological advances for studying rat Vipr1, several recent methodologies have transformed GPCR research in general:

• Cryo-electron microscopy (cryo-EM): The search results mention the use of cryo-EM to determine the structure of human VIP2R bound to PACAP27 and the stimulatory G protein . This technology has revolutionized structural studies of GPCRs by allowing visualization of receptors in native-like environments without the need for crystallization. Similar approaches could be applied to rat Vipr1.

• Advanced imaging techniques: The search results describe the use of Z-stack acquisition (Z = 0.2 μm) with 100 images and 3D reconstruction to visualize close appositions between VIP fibers and VPAC1-expressing neurons . These approaches allow for detailed analysis of receptor localization in complex tissues.

• IMARIS colocalization module: This computational tool was mentioned as being used for determining possible synaptic appositions between VIP and VPAC1 , highlighting the role of advanced image analysis in receptor research.

• Computational methods: While not explicitly mentioned in the search results, recent advances in computational modeling, including molecular dynamics simulations and machine learning approaches, have enhanced our ability to predict ligand binding modes, receptor conformational changes, and signaling outcomes.

• Gene editing technologies: CRISPR-Cas9 and related technologies have facilitated the generation of knockout models, reporter lines, and tagged receptor variants, expanding the toolkit for studying receptor function in vitro and in vivo.

These technological advances provide researchers with unprecedented capabilities to investigate the structure, localization, dynamics, and signaling properties of Vipr1 at multiple scales from molecular to systems level.