Pharmacology/Drug Development

Proteases sustain hyperexcitability and pain by cleaving protease-activated receptor-2 (PAR2) on nociceptors through distinct mechanisms. Whereas trypsin induces PAR2 coupling to Gαq, Gαs, and β-arrestins, cathepsin-S (CS) and neutrophil elastase (NE) cleave PAR2 at distinct sites and activate it by biased mechanisms that induce coupling to Gαs, but not to Gαq or β-arrestins. Since proteases activate PAR2 by irreversible cleavage, and activated PAR2 is degraded in lysosomes, sustained extracellular protease-mediated signaling requires mobilization of intact PAR2 from the Golgi apparatus or de novo synthesis of new receptors by incompletely understood mechanisms. We found here that trypsin, CS, and NE stimulate PAR2-dependent activation of protein kinase D (PKD) in the Golgi of HEK293 cells, in which PKD regulates protein trafficking. The proteases stimulated translocation of the PKD activator Gβγ to the Golgi, coinciding with PAR2 mobilization from the Golgi. Proteases also induced translocation of a photo-converted PAR2-Kaede fusion protein from the Golgi to the plasma membrane of KNRK cells. After incubation of HEK293 cells and dorsal root ganglia neurons with CS, NE, or trypsin, PAR2 responsiveness initially declined, consistent with PAR2 cleavage and desensitization, and then gradually recovered. Inhibitors of PKD, Gβγ and protein translation inhibited recovery of PAR2 responsiveness. PKD and Gβγ inhibitors also attenuated protease-evoked mechanical allodynia in mice. We conclude that proteases that activate PAR2 by canonical and biased mechanisms stimulate PKD in the Golgi; PAR2 mobilization and de novo synthesis repopulate the cell surface with intact receptors and sustain nociceptive signaling by extracellular proteases.

Oxycodone and morphine are two opioid drugs commonly used for the treatment of moderate to severe pain. However, their use in the management of noncancer pain remains a controversial issue and, in this respect, the evidence on their effectiveness and safety, particularly in osteoarthritis, is being questioned. In order to analyse their analgesic profile, two different pain models in rats were used: the formalin-induced inflammatory pain and the monosodium iodoacetate (MIA)-induced knee osteoarthritic pain. Drugs were administered systemically (i.p.) and their antinociceptive effect and potency were assessed. In the formalin test, both morphine and oxycodone produced a dose-dependent antinociceptive effect, but oxycodone outdid morphine in terms of effectiveness and potency (nearly two times) in the early (acute nociceptive) as in the late phase (inflammatory). In the osteoarthritis model, both drugs reduced movement-evoked pain (knee-bend test), mechanical allodynia (von Frey test) and heat hyperalgesia (Plantar test). Pretreatment with naloxone and naloxone methiodide reduced morphine and oxycodone effects. Peripheral mu-opioid receptors play a crucial role in the antinociceptive effect of both drugs on movement-evoked pain and heat hyperalgesia, but not on tactile allodynia. The main finding of our study is that oxycodone has a better antinociceptive profile in the inflammatory and osteoarthritic pain, being more effective than morphine at 14 days post-MIA injection (phase with neuropathic pain); it overcame the morphine effect by improving the movement-induced pain, tactile allodynia and heat hyperalgesia. Therefore, oxycodone could be an interesting option to treat patients suffering from knee osteoarthritis when opioids are required.

Src Homology 2 domain-containing protein tyrosine phosphatase 1 (SHP1) interacts specifically with GluN2A subunit of N-methyl-D-aspartate (NMDA) subtype of glutamate receptors in spinal cord dorsal horn. This molecular interaction is involved in the development of GluN2A-dependent spinal sensitization of nociceptive behaviors. Intrathecal application of a GluN2A-derived polypeptide (short for pep-GluN2A) has been shown to disturb spinal GluN2A/SHP1 interaction and inhibit inflammatory pain. Here we found that SHP1 was also located at dorsal root ganglion (DRG) neurons and formed complexes with GluN2A subunit. Peripheral inflammation activated SHP1 in DRG neurons, which promoted GluN2A tyrosine phosphorylation. The SHP1 binding to GluN2A facilitated the glutamate release from primary afferent fibers and exaggerated nociceptive synaptic transmission onto postsynaptic spinal cord neurons. Our data showed that intradermal application of pep-GluN2A disrupted GluN2A/SHP1 interaction in DRG neurons, attenuated the ability of GluN2A subunit-containing NMDA receptors to regulate the presynaptic glutamate release and more importantly, alleviated the pain hypersensitivity caused by carrageenan, complete Freund's adjuvant and formalin. The neuropathic pain induced by spared nerve injury was also ameliorated by intradermal pep-GluN2A application. These data suggested that disruption of GluN2A/SHP1 interaction in DRG neurons generated an effective analgesic action against pathological pain.