Characterization of Netrin-1 and Its Receptors UNC5B and Neogenin-1 in a Rat Rotator Cuff Tear Model: Associations with Inflammatory Mediators and Neurite Extension.

Rotator cuff tears are a leading cause of shoulder pain and dysfunction, yet the molecular mechanisms that link tendon injury to inflammation and nociceptive signaling remain poorly understood. Netrin-1, a classical axon guidance cue signaling through dependence receptors UNC5B and Neogenin-1, has been implicated in both neuronal plasticity and inflammatory processes, but its role in tendon pathology has not been explored. A rat supraspinatus tear model was employed to assess, in vivo, the expression of genes encoding netrin-1 (Ntn1) and its receptors (Unc5b and Neo1) at 0, 7, 14, 28, and 56 days post-injury (n = 10 per time point). Primary rat tenocytes isolated from rotator cuff tissue were treated in vitro with recombinant netrin-1, and transcriptional changes in genes encoding TNF-α (Tnfa), IL-6 (Il6), MMP-1 (Mmp1), and MMP-3 (Mmp3) were quantified by qRT-PCR. Separately, human iPSC-derived sensory neurons were exposed to netrin-1, and dose- and time-dependent effects on neurite outgrowth were measured at 4 and 14 days in culture. In injured tendons, Ntn1 mRNA increased significantly at day 14 (p = 0.010) and 28 (p = 0.042), Unc5b at day 7 (p = 0.002) and 14 (p < 0.001), and Neo1 at day 14 (p < 0.001) versus intact controls. Tenocyte exposure to 500 ng/mL netrin-1 induced transient upregulation of Tnfa (3 h, p = 0.023; 6 h, p = 0.009) and Il6 (3 h-24 h, all p < 0.013), as well as Mmp3 (3-24 h, p < 0.043) and Mmp1 (6 h-24 h, p < 0.024); no induction was observed at 50 ng/mL. In sensory neurons, 50 ng/mL of netrin-1 enhanced neurite extension at day 4 (p = 0.006) but not at 500 ng/mL or at day 14 for either dose. Netrin-1 and its receptors are upregulated in a rat rotator cuff tear model, and netrin-1 elicits distinct pro-inflammatory and matrix-remodeling responses in tenocytes while promoting early neurite growth in sensory neurons. These findings suggest netrin-1 as a key modulator of tendon inflammation, matrix turnover, and peripheral nerve plasticity following injury.