Recombinant Mouse Prokineticin receptor 1 (Prokr1)

What is the molecular structure and classification of Prokineticin receptor 1?

Prokineticin Receptor 1 (PKR1), also called ZAQ or GPR73a (G-protein coupled receptor 73a), is a 7-transmembrane glycoprotein of the GPCR family. The extracellular portions of human PKR1 share 81% amino acid identity with corresponding portions of mouse PKR1 and 78% with human PKR2, with non-identity mainly in the N-terminal sequences . Both PKR1 and PKR2 receptors recognize prokineticins 1 and 2 as ligands with similar binding affinities, suggesting that physiological responses are likely determined by the localized expression patterns of receptors rather than binding selectivity .

What signaling pathways are activated by Prokr1?

Prokr1 activates multiple signaling cascades, primarily through G protein-coupled mechanisms. Functional enrichment analysis of differentially expressed genes reveals that PROKR1 primarily activates Gq-mediated PI3K/AKT and MAPK/ERK signaling pathways in skeletal muscle cells . In cultured cells expressing PKR1, calcium mobilization occurs in a dose-dependent manner upon stimulation with Prokineticin 2 (PROK2), indicating activation of calcium-dependent pathways . This signaling versatility allows Prokr1 to regulate diverse cellular processes including metabolism, angiogenesis, and inflammatory responses.

How is Prokr1 expression regulated in different tissues?

Prokr1 expression varies significantly across tissues and developmental stages. In mouse models, Prokr1 shows dynamic regulation in uteroplacental tissues preceding parturition, with dramatic upregulation of Prok1 mRNA in fetal membranes on day 18 of pregnancy . In adult mice, Prokr1 is expressed in various tissues including skeletal muscle, cardiac tissue, kidneys, and adipose tissue . Expression levels significantly decrease in skeletal muscle and white adipose tissue of diet-induced obese mice, suggesting metabolic regulation of receptor expression . Similarly, human skeletal muscle cell-derived myotubes show decreased PROKR1 protein under insulin resistance conditions, indicating pathophysiological regulation .

What are effective in vivo models for studying Prokr1 function?

Several genetic mouse models have proven valuable for investigating Prokr1 function:

• Endothelium-specific PKR1 knockout (ec-PKR1−/−): This model effectively demonstrates the role of endothelial PKR1 in cardiovascular, renal, and metabolic homeostasis. These mice exhibit impaired capillary formation, defective transendothelial insulin delivery, and insulin resistance despite having a lean phenotype .

• Cardiomyocyte-specific PKR1 overexpression: Transgenic mice overexpressing PKR1 in cardiomyocytes show altered cardiac function and provide insights into paracrine signaling between cardiomyocytes and cardiac fibroblast progenitors .

• CFP-specific PKR1 knockout mice: Tcf21ERT-cre™-derived cardiac fibroblast progenitor (CFP)-specific PKR1 knockout mice (PKR1tcf−/−) help elucidate the role of PKR1 in adipogenesis and vasculogenesis within cardiac tissue .

Additionally, diet-induced obesity models can be used to study how metabolic stress affects Prokr1 expression and function, as significant decreases in Prokr1 are observed in tissues of obese mice .

Multiple complementary approaches can be employed to measure Prokr1 activation:

• Calcium mobilization assays: PROK2 induces calcium mobilization in a dose-dependent manner in PROKR1-expressing cells, making calcium flux a reliable indicator of receptor activation .

• Phosphorylation status: Measuring phosphorylation of downstream effectors (AKT, ERK) by western blotting provides quantitative assessment of signaling pathway activation .

• Transcriptional profiling: RNA sequencing or qPCR analysis of differentially expressed genes following receptor stimulation. PROKR1 activation alters the expression of 578 genes in PROKR1-overexpressed HEK293T cells .

• Functional readouts: Tissue-specific functional responses such as GLUT4 translocation in muscle cells or adipogenic/vasculogenic differentiation in cardiac fibroblasts provide contextual measures of activation .

• Ligand binding assays: Using labeled prokineticins to measure receptor occupancy and binding kinetics.

How does Prokr1 signaling interact with insulin pathways in metabolic research?

Prokr1 significantly influences insulin signaling and glucose metabolism through multiple mechanisms:

• Direct enhancement of insulin signaling: Prokr1 activation significantly enhances PI3K/AKT signaling in myotubes derived from C2C12 and satellite cells, both with and without insulin stimulation .

• GLUT4 translocation: Prokr1 promotes the translocation of glucose transporter 4 (GLUT4) to the plasma membrane, facilitating glucose uptake independent of insulin .

• Rescue of insulin resistance: In palmitate-induced insulin-resistant myotubes, Prokr1 activation enhances insulin-stimulated AKT phosphorylation, GLUT4 translocation, and glucose uptake, suggesting therapeutic potential .

• Endothelial-mediated insulin delivery: Endothelial PKR1 regulates transcapillary insulin passage into metabolically active tissues. Mice with endothelium-specific PKR1 deletion (ec-PKR1−/−) show impaired insulin delivery and peripheral insulin resistance despite displaying a lean phenotype .

These findings position Prokr1 as a potential therapeutic target for ameliorating insulin resistance, particularly in skeletal muscle, which is responsible for approximately 80% of insulin-stimulated glucose disposal.

What role does Prokr1 play in cardiovascular tissue homeostasis?

Prokr1 serves critical functions in cardiac tissue through multiple mechanisms:

• Regulation of cardiac progenitor differentiation: Prokineticin-2, acting through PKR1, controls cardiac fibroblast progenitor (CFP) transformation into either adipocytes or vasculogenic cells .

• Adipose tissue regulation: Lack of proper PKR1 signaling leads to excessive fat deposition in the atrioventricular groove, perivascular area, and pericardium when exposed to high-fat diet .

• Vascular network formation: PKR1 signaling is essential for proper vascular network development in cardiac tissue. PKR1tcf−/− mice display impaired vascular networks accompanied by cardiac dysfunction .

• Endothelial function: Endothelial PKR1 loss results in defective expression and activation of endothelial nitric oxide synthase in the aorta, diminishing endothelium-dependent relaxation .

This multifaceted role positions Prokr1 as a central regulator of cardiac tissue architecture and function, with potential implications for treating cardiometabolic diseases.

What contradictions exist in the current literature regarding Prokr1 function?

Several notable contradictions warrant consideration when designing Prokr1 research:

Researchers should carefully consider these contradictions when designing experiments and interpreting results, potentially through tissue-specific and time-resolved approaches.

What methodological approaches are most effective for studying Prokr1 in the context of metabolic diseases?

For metabolic disease research, several approaches have proven particularly informative:

• Discrete Choice Experiments (DCE): This methodology systematically generates choice sets to reveal preferences in targeting specific pathways. Though not specific to Prokr1, this approach can be adapted to optimize research design when studying receptor-mediated processes .

• Tissue-specific knockout models: Endothelium-specific and cardiac fibroblast progenitor-specific PKR1 knockout models effectively demonstrate the tissue-specific metabolic consequences of receptor deficiency .

• Palmitate-induced insulin resistance models: In vitro models using palmitate to induce insulin resistance in muscle cells provide a controlled system for studying how Prokr1 activation affects insulin signaling under pathophysiological conditions .

• Lipid accumulation assessment: Methods to measure ectopic lipid deposition in skeletal muscle, heart, and kidneys help evaluate how Prokr1 signaling affects lipid metabolism across tissues .

• Glucose uptake assays: Using 2-NBDG (2-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) amino)-2-deoxyglucose) to measure cellular glucose uptake provides functional assessment of Prokr1's metabolic effects .

The most robust experimental designs will integrate multiple methodological approaches and include appropriate controls that account for the complex signaling networks in which Prokr1 participates.

What are promising therapeutic approaches targeting Prokr1 for metabolic diseases?

Several therapeutic strategies targeting Prokr1 show promise:

• Receptor agonists: Development of selective Prokr1 agonists could enhance insulin sensitivity in skeletal muscle and improve metabolic function .

• Tissue-specific delivery systems: Given the differential effects of Prokr1 in various tissues, targeted delivery to skeletal muscle or cardiac tissue could maximize beneficial effects while minimizing unwanted responses.

• Combination approaches: Pairing Prokr1-targeted therapies with established insulin-sensitizing agents might produce synergistic effects by activating complementary signaling pathways.

• Prokineticin antagonists: PKRA 7, a potent prokineticin receptor antagonist that penetrates the blood-brain barrier, has shown antitumor effects and may have applications in conditions where excessive Prokr1 activation contributes to pathology .

• Gene therapy approaches: Viral vector-mediated restoration of Prokr1 expression could reverse metabolic defects in tissues where receptor levels are decreased, such as in diet-induced obesity .

As research progresses, therapeutic strategies will likely evolve to target specific aspects of Prokr1 signaling in a tissue-specific manner, potentially offering novel treatments for metabolic syndrome, type 2 diabetes, and cardiovascular complications.

What are the most critical knowledge gaps in Prokr1 research?

Despite significant advances, several important knowledge gaps remain:

• Receptor regulation mechanisms: The molecular mechanisms controlling Prokr1 expression under different physiological and pathological conditions remain poorly understood.

• Ligand selectivity: While both prokineticins 1 and 2 activate Prokr1, the conditions determining preferential signaling through specific ligand-receptor pairs in vivo require further clarification .

• Integration with other signaling networks: How Prokr1 signaling integrates with other metabolic and inflammatory pathways remains incompletely characterized.

• Translational validation: Confirmation of whether findings in mouse models accurately reflect human Prokr1 biology is essential for therapeutic development.

• Long-term effects: The consequences of chronic Prokr1 modulation on metabolism, cardiovascular function, and other systems require thorough investigation before therapeutic applications can advance.

Addressing these knowledge gaps will require innovative approaches combining molecular, cellular, and physiological methods with advanced genetic models and computational analyses.