Background/Purpose:

We aimed to compare the nociceptive innervation of healthy and osteoarthritic mouse knees.

Methods:

Destabilization of the medial meniscus (DMM) or sham surgery was performed in the right knee of 10-week old male C57BL/6 Na_V1.8-tdTomato mice. These mice express a bright red fluorescent tomato reporter in all neurons that express the voltage-gated sodium channel, Na_V1.8, which is expressed by approximately 75% of dorsal root ganglion (DRG) sensory neurons, including >90% of C-nociceptors (pain-sensing neurons) and C-low-threshold mechanoreceptors, as well as a lower percentage of Aδ-nociceptors and Aβ afferents. Sixteen weeks after surgery, mice were perfused transcardially with paraformaldehyde, knees were collected, post fixed and decalcified (DMM, sham, and 26-week-old naïve mice, n=5/group). Twenty-μm thick sections were cryosectioned and collected at mid-joint level. Na_V1.8-tdTomato signal was quantified using ImageJ. In order to confirm innervation patterns, we also used (1) Wild type C57BL/6 mice (n=3/group) immunostained for PGP9.5, and (2) C57BL/6-Pirt-GCaMP3 mice (n=3/group). These mice express the green fluorescent calcium indicator, GCaMP3, in ~90% of all sensory DRG neurons through the Pirt promoter.

Results:

In 26-week old naïve knees, Na_V1.8 expressing sensory neurons were observed in the lateral synovium and insertions of the cruciate ligaments, while medial synovium showed less innervation. Sixteen weeks after DMM surgery, mice had severe OA joint damage (cartilage degeneration score = 12.8 ± 1.9, compared to 0.4±0.49 after sham surgery). DMM, but not sham-operated knees showed profound neuroanatomical changes in the nociceptive innervation. We detected an increase in Na_V1.8 innervation of the medial synovium compared to age-matched naïve and sham-operated controls. The lateral synovium and the cruciate ligaments showed no changes. We also observed changes in the subchondral bone (SCB) innervation of the tibia and femur in the medial compartment, characterized by the presence of channel-like structures containing Na_V1.8 expressing fibers pointing toward the calcified cartilage. These nerves were also observed to a lesser extent in sham and naïve knees. Finally, Na_V1.8 signal was detected in osteophytes. Findings in Pirt-GCaMP3 mice and PGP9.5 staining in WT mice confirmed the above findings.

Conclusion:

The Na_V1.8 nociceptive innervation of the mouse knee profoundly changes at late stage experimental osteoarthritis, most notably an increase in the medial synovium and the presence of Na_V1.8 neurons in the SCB of the medial compartment. Ongoing studies are aimed at characterization of these nerves. The functional significance of these neuroanatomical changes associated with joint damage needs to be further investigated.

« Back to 2018 ACR/ARHP Annual Meeting

ACR Meeting Abstracts - https://acrabstracts.org/abstract/profound-changes-in-the-nav1-8-nociceptive-innervation-of-the-murine-knee-joint-after-destabilization-of-the-medial-meniscus/