# Artemin/GFRA3 axis and TRP channels: molecular insights from a feline model of osteoarthritis

## TL;DR

This study explores how the Artemin/GFRA3 pathway and TRP channels contribute to pain in feline osteoarthritis, using a cat model to find potential therapeutic targets.

## Contribution

The study provides first evidence of TRP channel function in feline DRG neurons and links the Artemin/GFRA3/TRP axis to DJD pain in cats.

## Key findings

- TRPV1, TRPV2, and TRPM8 showed increased expression in DJD cats, though not statistically significant.
- GFRA3 was expressed in cat DRG neurons but unchanged in DJD.
- Serum artemin levels correlated with radiographic DJD scores, suggesting a role in disease severity.

## Abstract

Degenerative Joint Disease (DJD) is a form of highly prevalent osteoarthritis in humans and animals, including cats, which causes significant pain and hypersensitivity. Despite its prevalence, the mechanisms underlying the DJD-associated pain in cats are poorly understood. While transient receptor potential (TRP) ion channels are expressed in the dorsal root ganglia (DRG) and are implicated in osteoarthritis pain (e.g., through Artemin/GFRA3-mediated changes to TRPV1 and TRPA1 electrical properties), there is currently only indirect evidence of TRP ion channel expression in the feline DRG. This study aims to address this knowledge gap.

Fura-2 based in vitro calcium imaging was used to confirm the functional expression of TRPV1, TRPV2, TRPA1, and TRPM8 in healthy cat DRG neurons. A quantitative reverse transcription-polymerase chain reaction (qRT-PCR) with SYBR green was used to confirm and compare mRNA expression of pain sensors including TRPV1, TRPV2, TRPV4, TRPA1, TRPM3, TRPM8, MRGPRD, TAC1, and GFRA3 in the DRG neurons of healthy cats and DJD group. Finally, serum artemin concentrations were quantified using enzyme linked-immunosorbent assay (ELISA) and compared between healthy and DJD cats.

Functional responses of TRPV1, TRPV2, TRPA1 and TRPM8 were determined via calcium imaging in DRG neurons obtained from healthy cats. Gene expression is further extended into healthy vs. DJD cats. While TRPV1, TRPV2, and TRPM8 showed a >1.5-fold increase in cats with DJD compared to healthy controls, MRGPRD mRNA expression showed a corresponding ∼1.5-fold decrease. However, these increases or decreases in fold-change did not reach statistical significance. GFRA3, a receptor for artemin, was found to be expressed in the cat DRG, though its levels remained unchanged in DJD-affected cats. Lastly, a significant association was found between serum artemin concentrations and radiographic DJD scores but not with veterinarian pain scores.

Our findings characterize functional expression of several pain and hypersensitivity-associated TRP ion channels in cat DRG neurons and identify the Artemin/GFRA3/TRP axis as a potential driver of chronic pain. The expression of channels, including TRPV1, TRPA1, and TRPM8 modulated by Artemin/GFRA3 pathway was confirmed. Bridging these cellular findings to DJD state, the observed correlation between serum artemin concentrations and radiographic DJD scores further implicates this pathway in disease severity. These results provide potential early evidence that the Artemin/GFRA3/TRP axis drives pain in feline DJD. This conservation is consistent with findings in other species, such as rodents and canines, suggesting translational relevance for therapeutic targeting.