Recombinant Mouse Succinate receptor 1 (Sucnr1)

What is the molecular structure of Mouse Sucnr1 and how does it compare to human SUCNR1?

Mouse Sucnr1 (also known as GPR91) is a G-protein coupled receptor with seven transmembrane domains connected by three hydrophilic extracellular loops. The receptor contains specific N-glycosylation sites and phosphorylation sites that regulate its function and stability. Recent structural studies have revealed detailed insights into the SUCNR1-Gi complex at near-atomic resolution (approximately 3 Å) .

The binding pocket of Sucnr1 is relatively large compared to the size of succinate . Key residues involved in succinate recognition and signaling have been identified through site-directed mutagenesis, including R281 (position 7.39), which is essential for G protein activation. Additional important residues include Y30 (position 1.39), Y83 (position 2.64), and F175 in the extracellular loop 2 (ECL2) . These structural features are largely conserved between mouse and human SUCNR1, though specific amino acid variations may influence ligand specificity and signaling efficiency.

Methodologically, researchers studying Sucnr1 structure should consider employing a combination of cryo-electron microscopy, X-ray crystallography, and computational modeling approaches to fully characterize species-specific structural features.

What expression systems are optimal for producing functional recombinant mouse Sucnr1?

Multiple expression systems have been successfully employed to produce recombinant mouse Sucnr1, each offering distinct advantages for different research applications:

• Cell-free expression systems provide rapid protein production with minimal cellular interference .

• E. coli, yeast, baculovirus, and mammalian cell systems offer varying degrees of post-translational modifications and protein yields .

• For structural studies requiring purified receptor-G protein complexes, insect cell (Sf9) expression systems have proven effective, as demonstrated in recent structural investigations .

• Mammalian expression systems (HEK293, CHO cells) are preferred for functional studies as they provide appropriate G-protein coupling machinery.

For optimal results, researchers should consider:

• Including appropriate affinity tags (such as N-terminal FLAG tags) for purification

• Co-expressing the receptor with relevant G proteins when studying complex formation

• Adding specific ligands (succinate or cis-epoxysuccinate) during purification to stabilize active conformations

• Employing protein engineering strategies to improve expression levels while maintaining function

The purity of recombinant Sucnr1 preparations is typically assessed by SDS-PAGE, with commercially available preparations achieving ≥85% purity .

What methodologies are most effective for validating recombinant mouse Sucnr1 activity?

Comprehensive validation of recombinant mouse Sucnr1 activity requires multiple complementary approaches:

Data from functional assays should be expressed as percentages (mean ± SEM) relative to maximal response levels, based on at least three independent experiments . Proper statistical analysis of EC50 values extracted from dose-response curves is essential for rigorous characterization.

Combining these methodologies provides robust validation of recombinant Sucnr1 activity across different aspects of receptor function.

How do succinate and synthetic agonists differ in their activation of mouse Sucnr1?

Succinate (the natural ligand) and synthetic agonists like cis-epoxysuccinate exhibit distinct properties in activating mouse Sucnr1:

The natural agonist succinate binds to the transmembrane pocket of Sucnr1, interacting with positively charged residues that accommodate its negatively charged carboxylate groups. This interaction triggers specific conformational changes that propagate to the intracellular domains, creating an interface for G protein binding .

Synthetic non-metabolic agonists like cis-epoxysuccinate have been developed to provide more stable alternatives for structural and functional studies. Recent cryo-EM structures have compared succinate-SUCNR1-Gi complex (2.97 Å resolution) with epoxysuccinate-SUCNR1-Gi complex (3.15 Å resolution) , revealing subtle differences in receptor conformation that may influence signaling outcomes.

When designing experiments, researchers should consider that:

• Natural and synthetic agonists may exhibit different binding kinetics and potencies

• Metabolic stability varies significantly between succinate and synthetic analogs

• Biased signaling properties may emerge with synthetic compounds

• Species-specific differences in ligand recognition should be accounted for when translating between mouse and human systems

These considerations are particularly important when developing SUCNR1-targeting compounds with therapeutic potential.

What are the primary signaling pathways activated by mouse Sucnr1 and how are they regulated?

Mouse Sucnr1 activates multiple signaling pathways that vary across different cell types and physiological contexts:

• G protein-dependent pathways:

  • Primarily couples to Gi proteins, leading to inhibition of adenylyl cyclase and decreased cAMP levels

  • Activation of MAPK-ERK1/2 signaling pathways

  • Release of Gβγ subunits that can activate phospholipase C

• Cell-type specific responses:

  • In retinal ganglion cells, Sucnr1 activation increases VEGF and PGE2 release through MAPK-ERK1/2 signaling, promoting vascularization

  • In neural stem cells, Sucnr1 signaling induces PGE2 secretion and extracellular succinate scavenging, contributing to resolution of neuroinflammation

  • In macrophages, Sucnr1 can promote anti-inflammatory phenotypes under specific conditions

Regulation of these pathways involves multiple mechanisms including receptor desensitization, internalization, and cross-talk with other signaling systems. The phosphorylation site (analogous to Ser326 in human SUCNR1) likely plays a role in regulating receptor activity and trafficking .

Methodologically, researchers investigating these pathways should employ pathway-specific inhibitors, siRNA knockdowns, and phosphoproteomic analyses to delineate the precise signaling networks in each cellular context.

How does the molecular mechanism of Sucnr1-mediated G protein activation inform structure-based drug design?

Recent structural insights into SUCNR1-Gi complexes provide a foundation for structure-based drug design targeting Sucnr1:

The high-resolution structures of succinate-SUCNR1-Gi complex (2.97 Å) and epoxysuccinate-SUCNR1-Gi complex (3.15 Å) reveal the precise arrangement of the transmembrane binding pocket and G protein interaction interface. Functional validation through mutagenesis has identified R281 (position 7.39) as essential for G protein activation, while Y30 (position 1.39), Y83 (position 2.64), and F175 (ECL2) contribute moderately to signal transduction .

These structural and functional insights inform several approaches to drug design:

• Virtual screening strategies can now use the actual receptor structure rather than homology models based on the P2Y1 receptor (27% homology)

• Structure-activity relationship studies can focus on optimizing interactions with key binding pocket residues

• Biased ligand development can target specific receptor conformations that preferentially activate beneficial signaling pathways

• Species-selective compounds can be designed based on structural differences between mouse and human receptors

To validate candidate compounds, researchers should employ a combination of in silico binding predictions, in vitro functional assays (cAMP inhibition, G protein dissociation), and mutagenesis studies targeting key binding residues .

What is the role of mouse Sucnr1 in neuroinflammation and how can recombinant Sucnr1 be used in CNS disease models?

Mouse Sucnr1 plays complex roles in neuroinflammation, with both pro- and anti-inflammatory effects depending on context:

Elevated succinate levels have been detected in the cerebrospinal fluid (not peripheral blood) during experimental autoimmune encephalomyelitis (EAE), a mouse model of multiple sclerosis . This CSF succinate, likely released by pro-inflammatory macrophages and microglia, signals through Sucnr1 on neural stem cells (NSCs) to initiate anti-inflammatory responses. Specifically, NSCs secrete prostaglandin E2 and scavenge extracellular succinate, contributing to resolution of neuroinflammation .

Myeloid cell-specific Sucnr1 conditional knockout mice (LysMCreSucnr1fl/fl) have confirmed the anti-inflammatory role of SUCNR1 in vivo, showing that Sucnr1 in mouse macrophages can induce a predominantly anti-inflammatory phenotype .

Recombinant Sucnr1 can be applied in CNS disease research through:

• Development of binding assays for screening CNS-permeable therapeutic compounds

• Generation of specific antibodies for monitoring receptor expression in different CNS cell types

• Production of soluble decoy receptors to modulate extracellular succinate levels

• Structure-guided design of Sucnr1 modulators with improved CNS permeability

When designing experiments for neuroinflammation models, researchers must consider the cell- and context-specific effects of Sucnr1 signaling and the CNS-permeability of any prospective succinate analogs .

How does Sucnr1 contribute to vascular regulation in mouse models?

Sucnr1 plays significant roles in vascular regulation through multiple mechanisms:

In rodent retinal ganglion cells, activation of the succinate-Sucnr1 axis increases the release of vascular endothelial growth factor (VEGF) and prostaglandin E2 (PGE2), promoting vascularization through the MAPK-ERK1/2 signaling pathway . This has been demonstrated both in vitro and in vivo, where siRNA-mediated retinal down-regulation of Sucnr1 in wild-type rats abolishes neovascularization in the presence of succinate .

This vascular regulatory function is particularly relevant in pathological settings such as hypoxic-ischemic injury, where succinate can accumulate due to altered metabolism . While Sucnr1-mediated angiogenesis can be beneficial for tissue repair, excessive activation may contribute to pathological neovascularization in certain contexts.

Methodologically, researchers studying Sucnr1 in vascular regulation should consider:

• Using tissue-specific conditional knockout models to isolate vascular effects

• Employing in vivo imaging techniques to monitor vascular changes following Sucnr1 modulation

• Measuring local succinate concentrations in tissues undergoing vascular remodeling

• Combining pharmacological and genetic approaches to confirm receptor specificity

These approaches can help elucidate the therapeutic potential of targeting Sucnr1 in vascular disorders.

What strategies can overcome challenges in developing selective modulators of mouse Sucnr1?

Developing selective modulators of mouse Sucnr1 presents several challenges that require strategic approaches:

Recent progress has included the development of synthetic non-metabolic agonists like cis-epoxysuccinate and antagonists such as NF-56-EJ40 . Virtual screening approaches based on receptor structure have identified non-metabolite agonists .

For rigorous validation of new compounds, researchers should employ dose-response analyses using the "log[agonist] vs. response" equation and express results as percentages relative to maximal response levels .

How can single-cell techniques advance our understanding of heterogeneous Sucnr1 responses in complex tissues?

Single-cell approaches offer powerful tools to unravel the context-dependent signaling of Sucnr1 in heterogeneous tissues:

• Single-cell transcriptomics can map Sucnr1 expression across diverse cell populations within complex tissues like the brain or immune system. This can reveal how Sucnr1 expression correlates with specific cellular states and help resolve apparently contradictory findings in different cell types.

• Single-cell proteomics and phosphoproteomics can profile how the same ligand (succinate) induces different signaling patterns in distinct cell types. This is particularly relevant given the diverse outcomes of Sucnr1 activation observed in different contexts, from pro-inflammatory to anti-inflammatory effects .

• Live-cell imaging with fluorescent biosensors for second messengers (cAMP, Ca²⁺) and kinase activities can track Sucnr1 signaling dynamics in real-time at the single-cell level, revealing temporal aspects that may be missed in population-level analyses.

• CRISPR-based genetic screens in primary cell populations can identify cell-type specific regulators and effectors of Sucnr1 signaling, helping to explain differential responses across cell types.

These approaches can help resolve the complexity of Sucnr1 signaling seen in various studies, such as its seemingly contradictory roles in inflammation versus resolution , by revealing how cellular context shapes receptor signaling outcomes.