The Role of Cell-specific TLR-4 Signaling in Developing Chronic Pain

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 DESCRIPTION (provided by applicant): More Americans are affected by pain than heart disease, stroke, and cancer combined. In particular, neuropathic pain is responsible for a large percentage of these incidences of pain. The mechanisms that underlay the development of neuropathic pain are highly sought after, because current therapeutics fails to offer relief in ove half of patients. Neuronal injury or stimulation leading to maladaptive hyperexcitability appears to play a huge role in the development of neuropathic pain and the identification of therapeutic targets that can modulate neuronal action will be important as viable treatment options. Interestingly, TLR4 has been shown to be expressed on peripheral sensory neurons, which suggests neurons are activated directly by TLR4 agonists (LPS, HMGB1, free fatty acids etc.) to play an active role in peripheral nervous system plasticity leading to neuropathic pain. This study will focus on the peripheral nervous system bringing genetic tools to clarify the functional role of DRG nociceptive neurons versus macrophage/microglia TLR4 in developing neuropathic pain. Importantly, downstream signaling events connected to TLR4 activation in neurons are largely unknown and provide an additional layer of therapeutic focus. The Price lab has generated evidence that translation control signaling mechanisms via the eukaryotic translation initiation factor-4E (eIF4E) are required for the development and maintenance of neuropathic pain; and other groups have recently shown that TLR4 may activate the MAPK pathway that modulates eIF4E. A key feature of this proposal is that we will, for the first time, study the physiological relevance of TLR4 in specific cell populations in vivo to determine whether nociceptor or macrophage/microglia TLR4 is necessary and/or sufficient for the development of neuropathic pain. We will take advantage of the cre-lox transgenic system offering cell-specific ablation or reactivation of TLR4 using newly developed floxed mice and continued development of null-reactivatable mice (Joel Elmquist). We will cross these animals with Nav1.8-cre (sensory neuron) and LysM-cre (macrophage/microglia) animals to assess these particular populations in developing neuropathic pain. Our research is aimed at understanding mechanisms driving chronic pain and moving these molecular insights toward new therapeutic strategies for pain alleviation. To accomplish these goals we propose the following specific aims: 1) Determine the role of cell-specific TLR4-eIF4E signaling to develop neuropathic pain during the mentored phase, and 2) Determine the contribution of TLR4 expression in nociceptors vs. microglia to neuropathic pain in phase II. These studies will hopefully suggest novel therapeutic avenues for the treatment and prevention of chronic pain by targeting nociceptor and microglial TLR4 signaling, translational control, and phenotype switching. Drs. Theodore Price and Joel Elmquist are ideally suited as mentors for this project with their complementary expertise in chronic pain development and mouse genetics, respectively.