The mechanism of Annexin A1 to modulate TRPV1 and nociception in dorsal root ganglion neurons

Background Annexin A1 (ANXA1) exerts anti-nociceptive effect through ANXA1 receptor formyl peptide receptor 2 (FPR2/ALX (receptor for lipoxin A4), FPR2) at the dorsal root ganglia (DRG) level. However, the mechanisms remain elucidated. By using radiant heat, hot/cold plate, tail flick, von Frey, and Randall-Selitto tests to detect nociception in intact and chemical (capsaicin, menthol, mustard oil, formalin or CFA) injected AnxA1 conditional knockout ( AnxA1 −/− ) mice, applying calcium imaging and patch clamp recordings in cultured DRG neurons to measure neuronal excitability, conducting immunofluorescence and western blotting to detect the protein levels of TRPV1, FPR2 and its downstream molecules, and performing double immunofluorescence and co-immunoprecipitation to investigate the interaction between Calmodulin (CaM) and TRPV1; we aim to uncover the molecular and cellular mechanisms of ANXA1’s role in antinociception. Results AnxA1 −/− mice exhibited significant sensitivity to noxious heat (mean ± SD, 6.2 ± 1.0 s vs . 9.9 ± 1.6 s in Hargreaves test; 13.6 ± 1.5 s vs . 19.0 ± 1.9 s in hot plate test; n = 8; P < 0.001), capsaicin (101.0 ± 15.3 vs . 76.2 ± 10.9; n = 8; P < 0.01), formalin (early phase: 169.5 ± 32.8 s vs . 76.0 ± 21.9 s; n = 8; P < 0.05; late phase: 444.6 ± 40.1 s vs . 320.4 ± 33.6 s; n = 8; P < 0.01) and CFA (3.5 ± 0.8 s vs . 5.9 ± 1.4 s; n = 8; P < 0.01). In addition, we found significantly increased capsaicin induced Ca 2+ response, TRPV1 currents and neuronal firing in AnxA1 deficient DRG neurons. Furthermore, ANXA1 mimic peptide Ac2-26 robustly increased intracellular Ca 2+ , inhibited TRPV1 current, activated PLCβ and promoted CaM-TRPV1 interaction. And these effects of Ac2-26 could be attenuated by FPR2 antagonist Boc2. Conclusions Selective deletion of AnxA1 in DRG neurons enhances TRPV1 sensitivity and deteriorates noxious heat or capsaicin induced nociception, while ANXA1 mimic peptide Ac2-26 desensitizes TRPV1 via FPR2 and the downstream PLCβ-Ca 2+ -CaM signal. This study may provide possible target for developing new analgesic drugs in inflammatory pain.