ATP and its receptors in nerve injury and repair.
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Unlike the peripheral nervous system (PNS), adult neurons in the central nervous system (CNS) have limited regenerative capacity after injury. One interesting phenomenon observed nearly four decades ago was that lesion of a peripheral nerve can significantly enhance the regenerative capacity of the central axons of the corresponding dorsal root ganglion (DRG) neurons, termed a ‘conditioning lesion’, but the underlying mechanism is still not fully understood. Since ATP is released after nerve injury and extracellular ATP has a broad range of biological activities, we postulated that ATP might be the injury signalling molecule that triggers the regenerative machinery in the injured neurons. If that were the case, injection of ATP into a peripheral nerve should be able to mimic the effect of a conditioning lesion. To test this theory, we injected ATP into a peripheral (sciatic) nerve after a dorsal column transection and found that ATP injection did promote the regeneration of injured axons into the lesion cavity. We also found that ATP injection activated transcription factor STAT3 and increased the expression of growth associated protein 43 (GAP43) in the corresponding DRG neurons. ATP injection increased the concentrations of ciliary neurotrophic factor and interleukin-6 in sciatic nerve and DRG. These results indicate that intraneural injection of ATP can mimic conditioning lesion to a certain degree. Most interestingly, we found that a second injection of ATP one week after the first one markedly boosted the effects of the first injection as many more axons grew into or across the lesion compared with double saline injection or ATP plus saline injection. Double ATP injection is also more effective in sustaining the expression of phospho- STAT3 and GAP43. Immunohistochemical analysis showed ATP injection caused little Wallerian degeneration at the injection site. Behavioural tests showed no long-term adverse effects to the injected sciatic nerve. In order to explore the underlying mechanism of ATP induced elevation of the regeneration state of DRG neurons and look for more potent purinoceptor agonists to stimulate axonal regeneration, we first tried to identify the expression of purinoceptor subtypes in sciatic nerves using quantitative PCR and immunohistochemistry. We found that mRNAs for all the four P1 and fourteen P2 purinoceptor subtypes were expressed in the sciatic nerve, DRG or dissociated Schwann cells at various levels. Immunohistochemical analysis showed that purinoceptor subtypes are expressed by different types of cells. Due to the expression of nearly all purinoceptor subtypes in the sciatic nerve, it will be a big challenge to identify the receptor subtype(s) responsible for ATP induced axonal regeneration. We have set up a compartmented co-culture system to test various agonists/antagonists of purinoceptors. Taken together, we have shown that intraneural ATP injection can mimic conditioning lesion in promoting sensory axonal regeneration. Identification of the receptor subtype(s) and other molecules involved in the enhanced regeneration capacity of injured neurons may lead to the development of therapeutic agents to effectively promote the axonal regeneration of both peripheral and central neurons.
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