Nerve Growth Factor Mediates a Switch in Intracellular Signaling for PGE2-Induced Sensitization of Sensory Neurons from Protein Kinase A to Epac

Abstract

We examined whether nerve growth factor (NGF), an inflammatory mediator that contributes to chronic hypersensitivity, alters the intracellular signaling that mediates the sensitizing actions of PGE2 from activation of protein kinase A (PKA) to exchange proteins directly activated by cAMP (Epacs). When isolated sensory neurons are grown in the absence of added NGF, but not in cultures grown with 30 ng/ml NGF, inhibiting protein kinase A (PKA) activity blocks the ability of PGE2 to augment capsaicin-evoked release of the neuropeptide CGRP and to increase the number of action potentials (APs) evoked by a ramp of current. Growing sensory neurons in culture in the presence of increasing concentrations of NGF increases the expression of Epac2, but not Epac1. An intradermal injection of complete Freund's adjuvant into the rat hindpaw also increases the expression of Epac2, but not Epac1 in the dorsal root ganglia and spinal cord: an effect blocked by intraplantar administration of NGF antibodies. Treating cultures grown in the presence of 30 ng/ml NGF with Epac1siRNA significantly reduced the expression of Epac1, but not Epac2, and did not block the ability of PGE2 to augment capsaicin-evoked release of CGRP from sensory neurons. Exposing neuronal cultures grown in NGF to Epac2siRNAreduced the expression of Epac2, but not Epac1 and prevented the PGE2-induced augmentation of capsaicin and potassium-evoked CGRP release in sensory neurons and the PGE2-induced increase in the number of APs generated by a ramp of current. In neurons grown with no added NGF, Epac siRNAs did not attenuate PGE2-induced sensitization. These results demonstrate that NGF, through increasing Epac2 expression, alters the signaling cascade that mediates PGE2-induced sensitization of sensory neurons, thus providing a novel mechanism for maintaining PGE2-induced hypersensitivity during inflammation.

Critical role for Epac1 in inflammatory pain controlled by GRK2-mediated phosphorylation of Epac1

“…Conversely, intraplantar administration of the Epac agonist 8-pCPT induces mechanical hyperalgesia (6,7,16,27). In vitro, ESI-09 inhibits both Epac1 and Epac2 (27,28), and there is evidence that both Epac1 (7) and Epac2 (29) contribute to sensitization of painsensing neurons. We demonstrate here that Epac1 −/− mice are protected from mechanical hyperalgesia in the CFA model of inflammatory pain and that these mice are also protected against neuropathic pain (7).…”

Divergent cAMP signaling differentially regulates serotonin-induced spinal motor plasticity

“…Different cAMP affinities and activation paradigms suggest that stimulation of Gs protein coupled receptors will initially activate PKA with EPAC signaling following only with greater/stronger receptor activation. However, certain growth/trophic factors can change the relative cAMP sensitivity of PKA and EPAC, shifting activation thresholds in favor of EPAC signaling (Vasko et al, 2014). For example, although nerve growth factor-1 (NGF-1) does not affect plasticity expression in a well-studied model of sensory hypersensitivity, it does convert the plasticity from PKA- to EPAC-dependence (Vasko et al, 2014).…”

“…However, certain growth/trophic factors can change the relative cAMP sensitivity of PKA and EPAC, shifting activation thresholds in favor of EPAC signaling (Vasko et al, 2014). For example, although nerve growth factor-1 (NGF-1) does not affect plasticity expression in a well-studied model of sensory hypersensitivity, it does convert the plasticity from PKA- to EPAC-dependence (Vasko et al, 2014). Similar effects could shift the PKA/EPAC balance downstream from 5-HT 7 receptors, enable 5-HT 7 receptors to contribute rather than constrain serotonin-induced plasticity, potentially explaining enhanced serotonin-dependent plasticity observed with preconditioning experiences known to increase growth/trophic factor expression (Kinkead et al, 1998; Johnson et al, 2000; Ling et al, 2001; Wilkerson and Mitchell, 2009).…”

Epac activation sensitizes rat sensory neurons through activation of Ras

“…Previous studies have shown that activation of Epacs enhances nociceptive behaviors (Hucho et al, 2005) and mediates the prostaglandin-induced sensitization of sensory neurons after inflammation or exposure to NGF (Vasko et al, 2014). To determine whether direct activation of Epacs sensitizes peptidergic sensory neurons, we assessed whether treatment with the selective Epac agonist, 8CPT-AM (Holz et al, 2008) augmented the evoked release of iCGRP from sensory neurons.…”

“…For example, acute exposure to prostaglandin E2 (PGE2) produces hyperalgesia and an increase in the excitability of sensory neurons, which are both mediated by increases in cAMP and activation of protein kinase A (PKA) (Aley and Levine, 1999; England et al, 1996; Hingtgen et al, 1995; Lopshire and Nicol, 1998; Sachs et al, 2009). With inflammation, or when sensory neurons are maintained in the presence of the inflammatory mediator, nerve growth factor (NGF), the sensitization induced by subsequent administration of PGE2 shifts from using PKA as the primary effector to activation of Epacs (Eijkelkamp et al, 2013; Hucho et al, 2005; Vasko et al, 2014; Wang et al, 2007). …”