Date of Award
Doctor of Philosophy
Transient receptor potential (TRP) channels play an important role in sensory and nonsensory functions. TRPVanilloid 1 and TRPVanilloid 4 are proposed to be involved in inflammation-induced pain. TRPV1 is extensively studied and it is specifically involved in inflammatory thermal hypersensitivity. Mechanical hypersensitivity is one of the significant components of nociception. Several receptors have been proposed to underlie mechanosensation. The molecular entities responsible for mechanosensation are not fully understood. In this study, I have characterized the properties of TRPV4, a putative mechanosensitive ion channel expressed in dorsal root ganglion (DRG) neurons and nonsensory tissues. First, I have investigated the expression and function of TRPV4 and TRPV1 in the DRG neuronal cell bodies as well as their central terminals and determined the modulation by protein kinase C (PKC). Both TRPV4 and TRPV1 are expressed in DRG and laminae I and II of the spinal dorsal horn (DH). Ca2+ fluorescence imaging and whole-cell patch-clamp experiments showed that both capsaicin-induced TRPV1 response and 4alpha-phorbol 12, 13-didecanoate (4alpha-PDD)-induced TRPV4 response were observed in a proportion of the same DRG neurons, suggesting their co-expression. Incubation of DRG neurons with phorbol 12, 13-dibutyrate (PDBu), a PKC activator, resulted in a significantly greater potentiation of TRPV4 currents than TRPV1 currents. In HEK cells heterologously expressing TRPV4, PDBu potentiated TRPV4-mediated single-channel current activity. In patch-clamped DH neurons, the application of 4alpha-PDD at the first sensory synapse increased the frequency but not the amplitude of the miniature excitatory postsynaptic currents (mEPSCs), suggesting a presynaptic locus of action. 4alpha-PDD-induced increase in the frequency of mEPSC was further facilitated by PDBu. These results suggest that TRPV4 in the central terminals modulates synaptic transmission and is regulated by PKC. Second, I have studied the mechanosensitivity of TRPV4 in cell-attached patches by applying direct mechanical force via the patch pipette. In TRPV4 expressing HEK cells, the application of negative pressure evoked single-channel current activity in a reversible manner and the channel activity was enhanced after incubation with PDBu. TRPV4 has been shown to be activated by hypotonicity. Here I show that negative pressure exaggerated hypotonicity-induced single-channel current activity. However, in similar experimental conditions, cells expressing TRPV1 did not respond to mechanical force. TRP channels are also expressed in non-sensory regions and the role of these channels is not fully understood. Both TRPV4 and TRPV1 are expressed in the hippocampus. Using whole-cell patch-clamp techniques, I have found that 4alpha-PDD increased the frequency, but not the amplitude of mEPSCs in cultured hippocampal neurons, suggesting a presynaptic site of action. Interestingly, the application of capsaicin had no effect on synaptic transmission in hippocampal neuronal cultures. Finally, I have investigated the expression and function of TRP channels in diabetes because TRP channels have been shown to be involved in peripheral neuropathy as well as vascular complications in diabetes. ROS production plays a critical role in the progress of diabetes. I propose that lower levels of ROS up-regulate the expression TRP channels in the early stages of diabetes, leading to hyperalgesia, and higher levels of ROS or chronic exposure to ROS down-regulate TRP channels in the late stages of diabetes, resulting in hypoalgesia. I have found that the expression of TRPV1 and phospho p38 (p-p38) MAPK was increased in DRG of streptozotocin (STZ)-injected diabetic and non-diabetic hyperalgesic mice. An increase in TRPV1 and p-p38 MAPK levels was induced by STZ or H2O2 treatment in stably TRPV1 expressing HEK cells, suggesting the involvement of STZ-ROS-p38MAPK pathway. TRPV4 has been reported to be involved in vasodilatation by shear stress in blood vessels. Here, I have demonstrated that TRPV4 is expressed in lymphatic endothelial cells (LECs). Treatment with low concentration of H2O2 enhanced the expression of TRPV4 at mRNA and protein levels in LECs, suggesting that mild levels of ROS up-regulate TRPV4 expression. In diabetes, beta cell dysfunction is responsible for decreased insulin release. TRPV4 is expressed in RINm5F (beta cell line), islets and pancreas. It has been shown that hypotonicity induced insulin release in beta cell lines, which was mediated by activation of stretch-activated channels, raising the possibility of the involvement of TRPV4, a mechanosensitive channel. Therefore, I have studied the functional role of TRPV4 in beta cells. Incubation with 4alpha-PDD enhanced insulin release in RINm5F cells, suggesting TRPV4 regulates insulin secretion from pancreatic beta cells. Since TRPV4 expression levels are decreased in diabetes, insulin secretion from beta cells may be impaired. In summary, TRPV1, a thermosensitive channel, and TRPV4, a mechanosensitive channel, contribute to thermal and mechanical hyperalgesia, respectively in the early stage of DPN through their up-regulation by ROS-p38 MAPK and insulin/IGF-1 pathways. Due to the mechanical sensitivity of TRPV4 channel, the up-regulation in the early stage and down-regulation in the late stage may be involved in the development of vascular complications and regulation of insulin release in diabetes.
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