Recombinant Rat Succinate receptor 1 (Sucnr1)

What is Sucnr1 and what is its structural characterization?

Sucnr1, initially classified as the orphan receptor GPR91, is a G-protein coupled receptor (GPCR) with succinate identified as its cognate ligand in 2004. Structurally, Sucnr1 contains seven transmembrane domains connected by three hydrophilic extracellular loops, two N-glycosylation sites (Asn4 and Asn164), and one phosphorylation site (Ser326). These post-translational modification sites may regulate receptor function and stability, though their precise roles across different cell types remain incompletely characterized . Recent structural studies using cryo-electron microscopy have revealed SUCNR1-Gi complexes with succinate and its non-metabolite derivative epoxysuccinate, providing deeper insights into ligand binding mechanisms .

What is the tissue expression pattern of rat Sucnr1?

Rat Sucnr1 exhibits a distinct tissue expression pattern that varies by metabolic state. It is expressed in the pancreas, though at lower levels compared to white adipose tissue and liver. Within the pancreas, immunohistochemical analysis reveals higher SUCNR1 protein abundance in islets than in exocrine tissue, with specific expression in β cells from rat islets. Interestingly, Sucnr1 expression tends to increase in metabolic disorders, with elevated levels observed in diet-induced obese mice and further increases in diabetic (db/db) mice .

How pure are typical recombinant rat Sucnr1 preparations?

Commercial recombinant rat Sucnr1 preparations typically achieve ≥85% purity as determined by SDS-PAGE analysis. These preparations are produced using various expression systems including cell-free expression systems, E. coli, yeast, baculovirus, or mammalian cell platforms, with the choice of system potentially affecting protein folding, post-translational modifications, and functional properties .

What intracellular signaling pathways are activated by Sucnr1?

As a GPCR, Sucnr1's signaling depends on its coupling to different G-proteins, particularly the associated α subunit type. The primary signaling pathways include:

• MAPK-ERK1/2 pathway activation, demonstrated in rodent retinal ganglion cells where Sucnr1 activation leads to increased release of vascular endothelial growth factor (VEGF) and prostaglandin E2 (PGE2)

• Inhibition of cAMP production, which can be measured using time-resolved fluorescence resonance energy transfer (TR-FRET) assays after stimulating cells with varying concentrations of succinate or epoxysuccinate in the presence of forskolin

• Modulation of inflammatory and angiogenic responses, including increased expression of pro-angiogenic factors (Vegf, Ang1, Ang2) and pro-inflammatory cytokines (IL-1β and IL-6) in rat astrocytes

How do I optimize transfection conditions for recombinant rat Sucnr1 expression?

For optimal expression of recombinant rat Sucnr1 in mammalian cells, current methodologies suggest:

• Cell selection: HeLa cells have been successfully used for both wild-type and mutant Sucnr1 expression

• Transfection timing: Harvest cells approximately 24 hours post-transfection

• Buffer composition: Use HBSS containing 5 mM HEPES, 0.1% BSA (w/v), and 0.5 mM 3-isobutyl-1-methylxanthine for functional assays

• Expression verification: Confirm expression using Western blot or immunofluorescence with specific anti-Sucnr1 antibodies

What are validated methods for measuring Sucnr1 activation in vitro?

Several established methods can reliably measure Sucnr1 activation:

• cAMP inhibition assay:

  • Stimulate transfected cells with succinate or epoxysuccinate in the presence of 2.5 μM forskolin for 30 minutes

  • Measure intracellular cAMP levels using commercially available kits (e.g., LANCE Ultra cAMP kit)

  • Detect signals using time-resolved fluorescence resonance energy transfer (TR-FRET)

• ERK1/2 phosphorylation assay:

  • Treat cells expressing Sucnr1 with succinate

  • Measure MAPK-ERK1/2 pathway activation through phospho-specific antibodies

  • Quantify using Western blot or ELISA techniques

How does Sucnr1 contribute to glucose homeostasis and what are the implications for metabolic research?

Sucnr1 plays a crucial role in glucose homeostasis through several mechanisms:

• Expression and regulation: Sucnr1 is expressed in pancreatic β cells and is upregulated during hyperglycemia, suggesting a compensatory mechanism in metabolic disorders

• Insulin secretion: Sucnr1 is essential for maintaining insulin secretion during diet-induced insulin resistance, as Sucnr1 deficiency leads to impaired glucose tolerance and reduced insulin secretion on high-fat diets

• Prediabetic states: The succinate/SUCNR1 axis is activated by high glucose and represents a GPCR-mediated amplifying pathway for insulin secretion that is particularly relevant to the hyperinsulinemia observed in prediabetic states

• Correlation with obesity: Human islet SUCNR1 levels positively correlate with body mass index (BMI), indicating its potential involvement in obesity-related metabolic adaptations

These findings suggest that pharmacological targeting of Sucnr1 could offer therapeutic potential for metabolic disorders, particularly in preserving β-cell function during insulin resistance development.

What are the challenges in generating knockout or knockdown models of Sucnr1 for in vivo studies?

Researchers face several technical challenges when developing Sucnr1 knockout or knockdown models:

• Tissue-specific expression: Since Sucnr1 is expressed in multiple tissues with varying levels (pancreas, adipose tissue, liver, retina, neural tissue), cell-specific knockouts may be necessary to avoid confounding effects

• Compensatory mechanisms: Complete deletion may trigger adaptive responses through related metabolic pathways

• Phenotyping complexity: As Sucnr1 functions in diverse processes (glucose homeostasis, angiogenesis, inflammation), comprehensive phenotyping requires multidisciplinary approaches

• Model validation: Confirmation of knockout efficiency requires careful examination of both mRNA and protein levels across relevant tissues

The siRNA-mediated retinal down-regulation of Sucnr1 in wild-type rats has been demonstrated to abolish neovascularization in the presence of succinate, demonstrating one successful approach to tissue-specific manipulation .

How can structural information about Sucnr1-ligand binding inform drug discovery?

Recent structural biology advancements provide valuable insights for Sucnr1-targeted drug discovery:

• Binding pocket characterization: Cryo-EM structures of SUCNR1-Gi complexes with succinate and epoxysuccinate reveal detailed ligand-receptor interactions

• Structure-based design: These structures can guide rational design of novel agonists or antagonists with improved selectivity and pharmacokinetic properties

• Allosteric modulation: Understanding the conformational changes associated with receptor activation may identify allosteric sites for drug targeting

• Species differences: Comparing rat and human SUCNR1 structures helps identify conserved binding sites for translational drug development

The previously obtained crystal structure of rat SUCNR1 in the apo state and humanized rat SUCNR1 bound to the antagonist NF-56-EJ40 provide additional reference points for structure-based drug design approaches .

Why might functional assays with recombinant rat Sucnr1 show inconsistent results?

Inconsistent results in Sucnr1 functional assays could stem from several factors:

• Post-translational modifications: The two N-glycosylation sites (Asn4 and Asn164) and phosphorylation site (Ser326) may vary in their modification status depending on the expression system, potentially affecting ligand binding and signaling

• G-protein coupling variability: Sucnr1 couples to different G-proteins, and the predominant coupling may vary based on cell type or experimental conditions

• Expression level variations: Differences in transfection efficiency or protein stability can affect functional responses

• Ligand purity and preparation: Variations in succinate preparation or contamination with other TCA cycle intermediates may impact receptor activation

Standardizing expression systems, carefully validating protein expression levels, and using multiple complementary assays can help improve consistency.

How can I distinguish between direct Sucnr1-mediated effects and indirect metabolic effects of succinate?

Differentiating direct receptor-mediated effects from metabolic effects requires careful experimental design:

• Use non-metabolizable analogs: Epoxysuccinate, a non-metabolite derivative of succinate, activates Sucnr1 without entering cellular metabolism, allowing distinction between receptor-mediated and metabolic effects

• Employ receptor antagonists: Specific SUCNR1 antagonists like NF-56-EJ40 can block receptor-mediated effects while leaving metabolic effects intact

• Utilize Sucnr1 knockout controls: Compare responses in Sucnr1-deficient and wild-type cells or tissues to identify receptor-dependent components

• Perform time-course analyses: Receptor-mediated signaling typically occurs more rapidly than metabolic effects

What are the optimal storage conditions for maintaining recombinant rat Sucnr1 activity?

To maintain optimal activity of recombinant rat Sucnr1:

• Temperature: Store purified protein at -80°C for long-term storage or at -20°C for shorter periods

• Buffer composition: Include stabilizing agents such as glycerol (typically 10-20%) to prevent freeze-thaw damage

• Avoid repeated freeze-thaw cycles: Aliquot the protein before freezing to minimize degradation

• Consider receptor stabilization: Addition of specific ligands or lipids may enhance stability during storage

• Quality control: Periodically verify protein integrity by SDS-PAGE and functional activity using established assays

What is known about Sucnr1's role in immune cell function and its implications for immunological research?

Emerging evidence indicates Sucnr1 plays important roles in immune cell function:

• Myeloid cell regulation: The succinate-SUCNR1 axis guides divergent responses in immune cells and serves as an essential regulator of tissue homeostasis

• T cell interaction: SUCNR1 expression correlates with tumor-infiltrating lymphocytes and T cell exhaustion markers in certain cancers

• Mast cell reactivity: Activation of succinate receptor 1 boosts human mast cell reactivity, potentially influencing allergic and inflammatory responses

• Cancer immunotherapy: High SUCNR1 expression correlates with immune cell infiltration in ovarian cancer, suggesting potential roles in cancer immunotherapy approaches

These findings indicate that Sucnr1 may serve as a metabolic checkpoint in immune cell function, linking cellular metabolism with inflammatory responses.

How might genome-wide variation in SUCNR1 function influence experimental results across different rat strains?

Genetic variation in SUCNR1 across rat strains can significantly impact experimental outcomes:

• Receptor sensitivity: Polymorphisms may alter ligand binding affinity or efficacy

• Expression levels: Strain-specific regulatory elements could affect baseline and inducible expression levels

• Signaling bias: Genetic variations might influence preferential coupling to different G-protein subtypes

• Disease models: Strain-dependent Sucnr1 function could contribute to variable phenotypes in metabolic or inflammatory disease models

Recent findings of genome-wide variation in SUCNR1 function suggest that the receptor's activity may be a predisposing factor for various physiological and pathological conditions . When designing experiments, researchers should consider characterizing Sucnr1 expression and function in their specific rat strain or consider using multiple strains for validation.

What innovative approaches are being developed to study Sucnr1 dynamics and trafficking?

Cutting-edge approaches for investigating Sucnr1 dynamics include:

• Real-time receptor tracking: Fluorescent protein-tagged Sucnr1 constructs enable visualization of receptor localization and trafficking dynamics in living cells

• Super-resolution microscopy: Techniques like STORM or PALM provide nanoscale resolution of receptor clustering and membrane organization

• BRET/FRET biosensors: These approaches allow monitoring of conformational changes and protein-protein interactions in real-time

• Single-molecule tracking: This methodology can reveal the mobility and oligomerization state of individual receptor molecules

• Optogenetic approaches: Light-controlled receptor activation enables precise spatiotemporal control for studying signaling dynamics

These approaches offer unprecedented insights into how Sucnr1 responds to succinate stimulation, interacts with signaling partners, and undergoes internalization and recycling.