The goal of this study is to determine how PH domain Leucine-rich Protein Phosphatases (PHLPP) contribute to joint pain in PTOA. We explore how PHLPP inhibitors affect joint innervation and the role of Phlpp1 in neurons and satellite glial cells (SGC) within the dorsal root ganglia (DRG) in mice.

PTOA accounts for ~12% of all cases of OA. Na_V1.8 (SCN10A) expression is associated with heightened pain sensitivity in PTOA. PHLPP1 suppresses chondrocyte maturation and contributes to OA progression and pain in mouse models. Intra-articular injection of PHLPP inhibitors prevent cartilage degradation and pain-related behaviors.

We tested the effect of PHLPP inhibitors (NSC117079) and genetic deletion of Phlpp1 in the murine destabilization of the medial meniscus (DMM) model. Na_V1.8-tdTomato reporter mice and a bone clearing method were used to track nociceptor sprouting in the subchondral bone. Publicly available single-cell RNA datasets of the DRG were used to assess PHLPP expression. To examine SGC involvement, conditional knockout mice (Fabp7-creER; Phlpp1^f/f) underwent destabilization of the medial meniscus (DMM) surgery. Cartilage degeneration was evaluated using the Osteoarthritis Research Society International (OARSI) scoring system. Pain behaviors were assessed by open field activity chamber, von Frey, and PAM (Pressure Application Measurement) assays at baseline, +6, +8, and endpoint for male and female mice.

PHLPP inhibition decreased Na_V1.8-positive fiber branching into the subchondral bone. Phlpp1 was in enriched in SGCs and in neuronal subtypes. SGC-specific Phlpp1 deletion did not alter pain behaviors or cartilage damage in either sex, suggesting that other cellular sources within the joint contribute to Phlpp1-mediated OA pain.

PHLPP signaling contributes to pathological sensory innervation in PTOA. SGCs do not contribute to Phlpp1-mediated OA pain or cartilage preservation. Defining mechanisms by which Phlpp1 drives OA pain may uncover translational strategies to improve patient outcomes.