Prokineticin 2: native-cell receptor attribution

1) Ask which receptor causes the response

A native-cell response to PROK2 is evidence of ligand sensitivity, not receptor identity. Both PROKR1 and PROKR2 can connect to intracellular calcium and kinase pathways, and downstream amplification allows a small receptor population to produce a large signal. Transcript detection alone is also insufficient: RNA abundance does not establish surface delivery, coupling competence, or which receptor dominates under the assay conditions.

Primary mouse astrocytes illustrate the problem. A 2018 study reported substantially more Prokr1 than Prokr2 transcript, different apparent cellular localization of the two receptor proteins, and dose-dependent calcium mobilization after recombinant PROK2 exposure.^[1] Those observations support a prokineticin-responsive preparation, but a calcium trace cannot independently assign the effect to one subtype. The quantitative question is whether removing one receptor changes PROK2 potency, efficacy, kinetics, or only a later phenotype such as migration.

Pre-register the causal claim. “PROK2 increases intracellular calcium” requires a different control set from “PROKR1 mediates the calcium response” or “PROKR2-dependent arrestin scaffolding sustains ERK.” Specify the receptor, readout, time window, cell state, species, and acceptance threshold before collecting data. This prevents a convenient endpoint from being substituted after the receptor hypothesis fails.

2) Qualify the ligand and the concentration that reaches cells

PROK2 is a compact secreted protein with multiple cysteines. Correct nominal mass does not prove native disulfide connectivity, monomeric state, or biological equivalence across recombinant and synthetic preparations. Record species sequence, expression or synthesis method, terminal processing, carrier proteins, counterion, purity method, peptide-content basis, lot, and freeze–thaw history. Use intact-mass analysis together with a stability-indicating chromatographic method; add disulfide mapping when the experimental claim depends on structure–activity relationships.

Measure recovery in the actual assay matrix. Sample time-zero medium, cell-free incubations, and cell-containing wells over the response interval. Low-binding vessels and a carrier condition can test adsorption, while orthogonal chromatography or LC–MS distinguishes disappearance of intact PROK2 from dilution error. Report concentration on a peptide-content basis rather than vial mass when water, salts, or excipients contribute to the stated mass.

Native-cell assays introduce extracellular proteases, matrix binding, and uptake that may differ across activation states. A right-shifted concentration-response curve can therefore reflect lower exposure rather than lower receptor abundance. Run peptide-recovery measurements alongside receptor perturbations so that a knockout-associated change in cell state is not mistaken for a direct receptor effect.

3) Establish recombinant PROKR1 and PROKR2 reference systems

Build matched PROKR1, PROKR2, and empty-vector cell lines in one parental background. Quantify surface receptor abundance using the same validated method for both subtypes. Equal plasmid input or total lysate signal does not establish equal surface expression. Use more than one expression level because receptor reserve can compress potency and efficacy differences at high abundance.

For rapid signaling, collect full PROK2 concentration-response curves with a kinetic calcium assay. Report baseline, peak amplitude, area under the curve, time to peak, assay window, fitted potency, maximum response, slope, and independent biological replicates. Include a receptor-independent calcium control to show that a silent clone retains intact detection machinery. Empty-vector responses reveal endogenous prokineticin receptors or non-receptor artifacts.

Pair calcium with a time-resolved ERK assay. Calcium typically samples an early G-protein-dominated event, whereas ERK can integrate G proteins, receptor trafficking, arrestin scaffolds, and secondary signaling. Use multiple early and late time points rather than one convenient endpoint. Pathway inhibitors are useful perturbations only after their activity and off-target effects are checked in the same cells.

Human PROKR2 variants provide useful assay-stress controls. In a 2008 functional study, identified PROKR2 variants were tested for calcium mobilization, MAPK signaling, and receptor expression; different alleles impaired these dimensions to different degrees.^[2] The general lesson is that a weak calcium response may arise from poor surface expression, impaired coupling, or both. A surface-normalized signaling measurement is more informative than a single endpoint.

4) Triangulate receptor identity in native cells

Begin with expression, but keep it quantitative. Measure PROKR1 and PROKR2 transcripts with validated assays and protein with specificity-tested reagents. Antibody validation should include receptor-null material and, ideally, rescue. Surface abundance matters more than total abundance for an extracellular ligand. Record passage, culture duration, confluence, differentiation or activation state, and donor or animal because these variables can change receptor balance.

Then perform individual receptor knockouts or knockdowns, a double perturbation, and expression-matched rescue. Confirm editing independently and test an unrelated Gq-coupled receptor to demonstrate preserved calcium machinery. A receptor-specific causal result has four parts: loss of the PROK2 response after perturbation, retention of general pathway competence, restoration by the intended receptor, and no restoration by a signaling-defective or surface-delivery-defective control.

Use native physiology as a separate validation layer. In dissociated rat subfornical-organ neurons, PROK2 altered membrane excitability and intracellular calcium in only a subset of cells. Pharmacological and electrophysiological experiments indicated that the measured calcium rise depended on extracellular calcium and action-potential activity rather than being a simple direct copy of recombinant receptor calcium release.^[3] This is a warning against assuming that identical fluorescent calcium traces share the same mechanism.

For heterogeneous cultures, combine single-cell calcium imaging with post hoc cell identity and receptor measurement. Population averages can merge a large response in a rare receptor-high subset with a small response across most cells. Mixed-effects analysis should retain culture, donor or animal, experimental day, and cell as nested sources of variation rather than treating every imaged cell as an independent biological replicate.

5) Separate G-protein activation from β-arrestin-2 regulation

PROKR2 can engage β-arrestin-2, adding a regulatory and signaling dimension that calcium alone misses. A 2024 study using human PROKR2 mutants reported constitutive receptor association with β-arrestin-2, identified a requirement for the third intracellular loop in that interaction, and found that the receptor C terminus contributed to ligand-dependent complex stability and downstream ERK activation.^[4] These data motivate direct measurement of receptor–arrestin interaction rather than inferring it from ERK.

Use a validated complementation, energy-transfer, co-immunoprecipitation, or imaging assay with untagged or minimally perturbed controls. Demonstrate that tagged PROKR2 retains surface expression and PROK2-responsive calcium signaling. Measure basal association, recruitment kinetics after ligand, washout, and ERK in the same receptor background. Include β-arrestin-2 depletion and rescue, plus receptor mutants whose expression is normalized at the surface.

Do not equate association with endocytosis. Measure surface receptor loss and intracellular redistribution directly, with a trafficking-positive GPCR showing that the assay can detect internalization. Likewise, ERK remaining after calcium blockade is not automatically arrestin-dependent; establish dependence genetically. Comparing wild-type, receptor-null, arrestin-null, and rescued cells provides a cleaner pathway map than combining several partially selective inhibitors.

6) An auditable PROK2 attribution workflow

1. Define the claim. Name the receptor, endpoint, cell state, and time window.
2. Qualify PROK2. Verify molecular form, integrity, and recovery in the complete matrix.
3. Calibrate recombinant references. Compare PROKR1 and PROKR2 at measured surface abundance.
4. Resolve rapid and integrated signals. Pair calcium kinetics with a multi-time-point ERK assay.
5. Establish causality. Use individual and combined receptor loss with expression-matched rescue.
6. Interrogate regulation. Measure β-arrestin-2 association and trafficking directly.
7. Validate in native cells. Preserve heterogeneity and test whether calcium reflects receptor release, calcium entry, or altered excitability.

The interpretable experimental unit is a chemically qualified PROK2 lot tested in expression-matched recombinant references and receptor-edited native cells, with intact-ligand recovery, surface-expression, kinetic signaling, and rescue data. This design distinguishes true PROKR1 or PROKR2 dependence from receptor reserve, altered exposure, mixed-cell averaging, downstream amplification, and an indirect calcium response.