Nogo-a Pathway Contributes to Neural Injury and Pain in Sickle Cell Disease

Sickle cell disease (SCD) is characterized by chronic pain and bouts of extreme acute pain from vasoocclusive crises (VOC). Sickle pain has both neuropathic and inflammatory features (Tran et al., Blood 2017). Mechanisms underlying neural injury remain unknown in SCD. Neurite outgrowth inhibitor (NOGO-A/reticulon-4) and its receptor NGR1 contribute to pain, neuronal damage, and inhibition of neurite outgrowth (Hu et al., FASEB J 2019). We examined if NOGO-A pathway is activated in a sickle microenvironment and if its inhibition will ameliorate hyperalgesia in BERK sickle mice. We used Rho kinase activity (ROCK) downstream of NGR1 as a readout of activation of NOGO-A/NGR1 pathway. We observed increased expression of NOGO-A (~260%, p<0.05) and NGR1 (~180%, p<0.05) in the dorsal root ganglia, and increased NOGO-A and ROCK activity in spinal cords of sickle mice compared to control mice expressing normal human hemoglobin A. Earlier, we found that an endogenous cannabinoid, palmitoylethanolamide (PEA) inhibits spinal NOGO-A expression and ROCK activity in sickle mice (Argueta et al., ASH 2020 #225). We hypothesized that sickle microenvironment with cell-free heme and inflammation activates NOGO-A/NGR1-ROCK pathway leading to nerve injury and pain, and inhibition of this pathway will ameliorate hyperalgesia in sickle mice.

Using terminally differentiated rat pheochromocytoma (PC12) cells, we simulated a sickle microenvironment with hemin (40 µM) and TNFα (1 ng/ml) (H+T). H+T elevated ROCK activity compared to vehicle (~40%, p<0.05). PEA (30 uM) and 2 µM NEP (1-40), a competitive antagonist of NGR1, attenuated H+T-induced ROCK activity (both p<0.01); co-treatment had no additive effect, indicating a common pathway. As well, siRNA (10 nM) knockdown of NGR1 reversed H+T-induced ROCK activity (p<0.001), which was equally effective with 30 µM PEA co-treatment. Functionally, treatment with 30 µM PEA or 2 µM NEP (1-40) enhanced neurite outgrowth in H+T-treated PC-12 cells (~120%, p<0.001). NEP (1-40) at 5 mg/kg reduced mechanical and cold (both ~50%, p<0.001) hyperalgesia in sickle mice compared to baseline (BL) and/or vehicle treatment. Together, these data demonstrate that NOGO-A/NGR1 pathway activation may underlie nerve injury, and inhibition of this pathway with a NGR1 antagonist or PEA promotes neurite outgrowth and reduces hyperalgesia in a sickle microenvironment.

We next examined the contribution of exogenously administered and endogenously produced PEA in ameliorating hyperalgesia. Mass spectrometry revealed that spinal PEA is reduced in female (p<0.05) and male (p<0.001) sickle mice compared to age/sex-matched control mice. Exogenous PEA (i.p. 20 mg/kg/d) reduced cold avoidance over a 3-day treatment period, showing significantly more time in the cold chamber compared to BL or vehicle at 1 h, 24 h, and 72 h (p<0.05). The analgesic effect of PEA was maintained for 9 days of treatment without developing tolerance. We next increased endogenous PEA by inhibiting its degradative enzyme, N-acylethanolamine acid amidase, with ARN19702 (i.p. 3, 10, & 30 mg/kg/d), which reduced mechanical (~50%, p<0.001) and cold hyperalgesia (~40%, p<0.001) over 72 hours in a dose-dependent manner. Since hypoxia and ischemia reperfusion injury contribute to acute VOC pain, we incited hypoxia-reoxygenation (HR; 3 h @ 8% O ₂, 92% N ₂, followed by 1 h @ normoxia) to simulate acute VOC pain. We observed that 5-day pretreatment with PEA (i.p. 20 mg/kg/d) before HR prevented mechanical and cold hyperalgesia following HR in sickle mice. Moreover, treatment with PEA after HR incitement significantly reduced hyperalgesia for 24 h after HR compared to BL (~30%, p<0.001) and vehicle treated (~66%, p<0.001) sickle mice.

NGR1 antagonism reduces spinal microglial injury/activation. Heme and TNFα have been shown to cause microglial injury in vitro, while spinal microglial activation has been demonstrated in sickle mice (Lei et al., Antioxid Redox Signal 2021). Thus, NGR1/ROCK cascade may contribute to both neuronal injury and inflammation in the central nervous system leading to neuropathic pain. Our data suggest that PEA and targeting NOGO-A pathway may prevent/reduce chronic and acute hyperalgesia in sickle mice. We speculate that interventions targeting NOGO-A pathway may prevent/reduce neuropathic pain and that PEA has the translational potential for the treatment of chronic and acute pain in SCD.