SUMMARY The regenerative capacity of the injured CNS in adult mammals is severely limited, yet axons in the peripheral nervous system (PNS) regrow, albeit to a limited extent, after injury. We reasoned that coordinate regulation of gene expression in injured neurons involving multiple pathways was central to PNS regenerative capacity. To provide a framework for revealing pathways involved in PNS axon regrowth after injury, we applied a comprehensive systems biology approach, starting with gene expression profiling of dorsal root ganglia (DRGs) combined with multi-level bioinformatic analyses and experimental validation of network predictions. We used this rubric to identify a drug that accelerates DRG neurite outgrowth in vitro and optic nerve outgrowth in vivo by inducing elements of the identified network. The work provides a functional genomics foundation for understanding neural repair and proof of the power of such approaches in tackling complex problems in nervous system biology.

Acute and chronic pain complaints, while very common, are generally poorly served by existing therapies. The unmet clinical need reflects the failure in developing novel classes of analgesics with superior efficacy, diminished adverse effects and a lower abuse liability than those currently available. Reasons for this include the heterogeneity of clinical pain conditions, the complexity and diversity of underlying pathophysiological mechanisms coupled with the unreliability of some preclinical pain models. However, recent advances in our understanding of the neurobiology of pain are beginning to offer opportunities to develop new therapeutic strategies and revisit existing targets, including modulating ion channels, enzymes and GPCRs.

Numerous G protein-coupled receptors (GPCRs) have been shown to form heteromeric receptors in cell-based assays. Among the many heteromers reported in the opioid receptor family are μ/κ, κ/δ, and μ/δ. However, the in vivo physiological and behavioral relevance for the proposed heteromers have not yet been established. Here we report a unique example of a ligand, N-naphthoyl-β-naltrexamine (NNTA) that selectively activates heteromeric μ/κ-opioid receptors in HEK-293 cells and induces potent antinociception in mice. NNTA was an exceptionally potent agonist in cells expressing μ/κ-opioid receptors. Intriguingly, it was found to be a potent antagonist in cells expressing only μ-receptors. In the mouse tail-flick assay, intrathecal (i.t.) NNTA produced antinociception that was ∼100-fold greater than by intracerebroventricular (i.c.v.) administration. The κ-antagonist, norBNI, decreased the i.t. potency, and the activity was virtually abolished in μ-opioid receptor knockout mice. No tolerance was induced i.t., but marginal tolerance (3-fold) was observed via the i.c.v. route. Moreover, NNTA produced neither significant physical dependence nor place preference in the ED 50 dose range. Taken together, this work provides an important pharmacologic tool for investigating the in vivo functional relevance of heteromeric μ/κ-opioid receptors and suggests an approach to potent analgesics with fewer deleterious side effects.

Chemotherapy-induced peripheral neuropathic pain (CIPNP) is a severe dose-and therapy-limiting side effect of widely used cytostatics that is particularly difficult to treat. Here, we report increased expression of the cytochrome-P 450 -epoxygenase CYP2J6 and increased concentrations of its linoleic acid metabolite 9,10-EpOME (9,10-epoxy-12Z-octadecenoic acid) in dorsal root ganglia (DRGs) of paclitaxeltreated mice as a model of CIPNP. The lipid sensitizes TRPV1 ion channels in primary sensory neurons and causes increased frequency of spontaneous excitatory postsynaptic currents in spinal cord nociceptive neurons, increased CGRP release from sciatic nerves and DRGs, and a reduction in mechanical and thermal pain hypersensitivity. In a drug repurposing screen targeting CYP2J2, the human ortholog of murine CYP2J6, we identified telmisartan, a widely used angiotensin II receptor antagonist, as a potent inhibitor. In a translational approach, administration of telmisartan reduces EpOME concentrations in DRGs and in plasma and reverses mechanical hypersensitivity in paclitaxeltreated mice. We therefore suggest inhibition of CYP2J isoforms with telmisartan as a treatment option for paclitaxel-induced neuropathic pain.chemotherapy-induced neuropathy | neuropathic pain | TRPV1 | telmisartan | oxidized lipids R ecent studies identified members of the transient receptor potential-family of ion channels (TRPV1, TRPA1, and TRPV4) as contributors to both mechanical and cold allodynia during oxaliplatin and paclitaxel-induced neuropathy (1-5). Activation or sensitization of TRPV1 and TRPA1 can lead to enhanced release of CGRP and substance P, both of which can cause neurogenic inflammation and recruitment of T cells (6, 7).However, it remains unclear which endogenous mediators are involved in paclitaxel-dependent activation or sensitization of TRP channels, as paclitaxel cannot directly activate TRP channels (4,5,8). Interestingly, paclitaxel is an inducer of some Cytochrome-P 450 epoxygenases (e.g., CYP2C8, CYP2C9) (9). CYP epoxygenases can metabolize ω-6 fatty acids, such as arachidonic acid (AA) and linoleic acid (LA), generating either lipid epoxides such as EETs (epoxyeicosatrienoid acids) or ω-hydroxides such as 20-hydroxyeicosatetraenoic acid (20-HETE) (10, 11).Although therapeutic alternatives exist, paclitaxel is still the preferred first line of therapy for metastatic breast cancer (12), causing severe CIPNP in many treated patients. Here, we performed liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based lipid profiling of sciatic nerve, dorsal root ganglion (DRG), and dorsal horn tissue from paclitaxel-treated mice.We identified 9,10-EpOME (9,10-epoxy-12Z-octadecenoic acid), a CYP metabolite of LA, to be strongly synthesized in DRGs 24 h and 8 d after paclitaxel injection in mice. 9,10-EpOME is capable of sensitizing TRPV1 at submicromolar concentrations via a cAMP-PKA-dependent mechanism, causing enhanced frequency of spontaneous excitatory postsynaptic currents (sEPSCs) in lamina II neurons of the spin...

Research in the opioid field has relied heavily on the use of standard agonist ligands such as morphine, [d-Ala(2)-MePhe(4)-Glyol(5)]enkephalin (DAMGO), U69593, bremazocine, [d-Pen(2)d-Pen(5)]enkephalin (DPDPE), and deltorphin-II as tools for investigating the three major types of opioid receptors, MOP (μ), KOP (κ), and DOP (δ), that mediate antinociception. The functional selectivity of these ligands has been based on the assumption that opioid receptors exist as homomers. As numerous studies in cultured cells have suggested that opioid receptors can associate both as homomers and heteromers, we have investigated the selectivity of these standard ligands using intracellular calcium release and [(35)S]GTPγS assays in HEK-293 cells that contain singly and coexpressed opioid receptors. The present study reveals that morphine and DAMGO, traditionally classified as μ selective agonists, selectively activate μ-δ heteromeric opioid receptors with greater efficacy than homomeric opioid receptors. Moreover, standard ligands that have been widely employed as κ- and δ-selective agonists display little or no differences in the activation of homomeric and heteromeric opioid receptors. The far-reaching implications of these results are discussed.

Given that mu opioid (MOP) and canabinoid (CB1) receptors are co-localized in various regions of the CNS and have been reported to associate as heteromer (MOP-CB1) in cultured cells has raised the possibility of functional, endogenous MOP-CB1 in nociception and other pharmacologic effects. As a first step in investigating this possibility, we have synthesized a series of bivalent ligands 1-5 that contain both mu agonist and CB1 antagonist pharmacophores for use as tools to study the functional interaction between MOP and CB1 receptors in vivo. Immunofluorescent studies on HEK293 cells co-expressing both receptors suggested 5 (20-atom spacer) to be the only member of the series that bridges the protomers of the heteromer. Antinociceptive testing in mice revealed 5 to be the most potent member of the series. As neither a mixture of monovalent ligands 9 + 10 nor bivalents 2-5 produced tolerance in mice, MOR-CB1 apparently is not an important target for reducing tolerance.

In an effort to develop antagonists for κ-μ opioid receptor heterodimers, a series of bivalent ligands 3 - 6 containing κ- and μ-antagonist pharmacophores were designed and synthesized. Evaluation of the series in HEK-293 cells revealed 4 (KMN-21) to selectively antagonize the activation of κ-μ heterodimers, suggesting possible bridging of receptors when the bivalent ligand spacer contains 21 atoms.

Bivalent ligands that contain two pharmacophores linked by a spacer are promising tools to investigate the pharmacology of opioid receptor heteromers. Evidence for occupation of neighboring protomers by two phamacophores of a single bivalent ligand (bridging) has relied mainly on pharmacological data. In the present study, we have employed an immunocytochemical correlate to support in vivo biological studies that are consistent with bridging. We show that a bivalent mu agonist/delta antagonist (MDAN-21) that is devoid of tolerance due to possible bridging of mu and delta protomers, prevents endocytosis of the heteromeric receptors in HEK-293 cells. Conversely, a bivalent ligand (MDAN-16) with a short spacer, or monovalent mu agonist give rise to robust internalization. The data suggest that the immobilization of proximal mu and delta protomers is due to bridging by MDAN-21. The finding that MDAN-21 and its shorter spacer homologue MDAN-16 possess equivalent activity in HEK-293 cells, but produce dramatically divergent internalization of mu-delta heteromer, is relevant to the role of internalization and tolerance.