Local translation of mRNA in primary afferent fibres: a novel mechanism for the control of pain and itch

We have all experienced pain. Pain, whether caused through injury, illness and disease or surgery, is usually temporary. As the body heals the pain diminishes, then disappears. However, for one in five adults pain persists and becomes a chronic pain state. For these people pain becomes the overriding complaint, regardless of the cause. Pain invades all aspects of everyday life. Not only does chronic pain have a huge impact on the sufferer but family, friends and carers face major life changes caused by the lifestyle and financial limitations imposed by chronic pain. Although there are many different pain-killers very few are wholly effective. Most people with chronic pain taking prescription medicines will routinely suffer from pain that these medicines cannot suppress. For these people there is no respite. We need better pain killing medicines to manage chronic pain. We believe the situation will be helped by finding new targets for the development of novel kinds of pain killing drugs. Identification of new drug targets depends on gaining a better understanding of how the nervous system translates injury into pain. Our recent experiments have provided exactly this sort of information. We discovered that proteins are not just made in the cell bodies of the nerves that transmit pain but also in their axons, the long processes that can be meters in length and which collect information about injury from the skin and carry it to the brain. This local ability to make proteins was however only found in axons that signal fast, pricking pain and we could reduce this pain by stopping the synthesis of protein in the skin with an anti-cancer drug called rapamycin. Surprisingly, we also found that the sensation of itch may be regulated by rapamycin in the same way. Chronic itch represents a significant clinical problem resulting from renal and liver diseases, as well as several serious skin conditions such as atopic dermatitis. We intend to find out more about the specific proteins that are made so far away from the cell body and regulate pain and itch. When we have identified these proteins we can design specific drugs that block their activity. By simply placing them over the area of painful or itchy skin or over the spinal cord we hope to be able to alleviate some of these very unpleasant and debilitating symptoms.

Local translation of mRNA in dendrites and axons, regulated by the mammalian target of rapamycin (mTOR), plays a critical role in the modulation of synaptic plasticity. We have previously shown that the necessary translational machinery is present in some myelinated, fast-conducting, cutaneous sensory fibers and that active mTOR-dependent pathways participate in maintaining their sensitivity. This population of myelinated nociceptors mediates secondary mechanical hyperalgesia. The mTOR inhibitor rapamycin reduced secondary mechanical hyperalgesia and attenuated mechanical sensitivity in a model of neuropathic pain. Taken together, our results showed that the sensitivity of a subset of sensory fibers is maintained by ongoing mTOR-mediated local protein synthesis and uncovered a novel target for the control of long-term pain states. This application is designed to extend this research in three ways. i) We will identify the mRNAs that are transported to peripheral sites of translation in nociceptive primary afferents. Using microarray analysis of sciatic nerve we have identified several transported mRNAs and confirmed some of these with RT-qPCR. We will extend this study and confirm other mRNAs as well as localize them to primary afferent fibres using non-radioactive in situ hybridization. We will regulate local translation of candidate mRNAs using cutaneous injections of rapamycin and confirm changes in protein levels using Western blotting or immunohistochemistry. Behavioural measures, and electromyographic and single nerve fibre recordings will be used to assess mechanical and thermal sensitivity after delivery of rapamycin or antisense DNA oligodeoxynucleotides, targeted to identified mRNAs, to cutaneous tissue ii) We will investigate the potential role of local translation in primary afferent C- fibres by recording in vivo from peripheral nerve. We will also examine the sensation of itch with behavioural techniques as preliminary evidence suggested that itch is regulated by local translation in a subset of C-fibres. iii) We will investigate the presence and role of local translation in the central processes of primary afferents using immunohistochemistry and single unit recording from deep dorsal horn neurons following intrathecal application of rapamycin or vehicle. We will further investigate mechanisms underlying chronic pain sensitivity by studying the effect of rapamycin on pain-related behaviours using a chronic pain model. Our aim is to identify candidate genes that regulate the sensitivity of primary afferent nociceptors and explore the use of antisense inhibitors and translational blockers to control itch and persistent pain states in animals and potentially human subjects.