p75NTR in pain: from TRPV1 modification to bone cancer pain enhancement

Adequate management of pain is an unmet clinical problem. Available therapeutics are ineffective for certain types of pain and often come with negative side effects. A stronger knowledge of pain signaling mechanisms will facilitate the development of more effective treatments with potentially fewer side effects. Neurotrophins and their receptors are known to play critical roles in pain signaling in addition to their well-established influences on neuron development. The neurotrophin NGF is upregulated in many pain conditions and can modulate sensory neuron sprouting as well as the activity and expression of sensory ion channels, such as TRPV1. Through activation of its receptor, TrkA, NGF can sensitize TRPV1 activity, thus contributing to heightened cell excitability during pain conditions. Accordingly, NGF is a novel target of pain therapeutics in humans. TRPV1 is a target of pain therapeutics as well, however much remains unknown about its function. It is also unclear whether the other receptor for NGF, p75NTR, is necessary for pain sensation or if it participates in modulation of TRPV1 activity. The data presented in this thesis aimed 1) to elucidate the role of p75NTR in certain types of pain, 2) to understand better the mechanisms behind NGF-induced TRPV1 sensitization, and 3) to develop a novel imaging tool which will aid in future studies of TRPV1. Using a thorough genetic approach, we found that p75NTR does not mediate acute thermal sensation and that an observed deficit in acute mechanical sensation in mice with germline p75NTR deletion is likely due to a severe developmental loss of dorsal root ganglia (DRG) neurons in these animals. Indeed, mice with neuronal-specific conditional deletion of p75NTR had normal thermal and mechanical sensitivities and no developmental loss of DRG neurons. Additionally, p75NTR does not contribute to complete Freund's adjuvant (CFA)-induced inflammatory pain. Tamoxifen-induced conditional p75NTR deletion in the adult animal did not inhibit acute or CFA-induced inflammatory pain as well. Intriguingly, tamoxifen-induced p75NTR deletion in the adult significantly enhanced mechanical allodynia in a model of metastatic bone cancer pain, indicating p75NTR may play an inhibitory role in bone cancer pain. In order to examine the role of p75NTR in NGF-induced TRPV1 sensitization, we created and sub-cloned a PC12 cell line stably expressing TRPV1. We demonstrated that one sub-clone contains a subpopulation that undergoes NGF-induced potentiation of TRPV1 capsaicin responses, indicating that PC12 cells contain the necessary components to mediate this effect. Additionally, we created a novel imaging tool, TRPV1-G.GECO, which maintains normal TRPV1 function and acts as a sensitive sensor for local changes in calcium concentration through TRPV1. We show that p75NTR is dispensable for certain types of pain, as well as novel evidence that p75NTR may inhibit bone cancer pain. As many other aspects of pain signaling are unresolved, we also created a stable cell line system which will aid in our…