Regulation of AMPA Receptors in Spinal Nociception由刀豆文库小编整理，希望给你工作、学习、生活带来方便，猜你可能喜欢“软件模拟spi同步串口”。
09年会麻醉学基础研究
Regulation of AMPA Receptors in Spinal Nociception
1首都医科大学附属北京朝阳医院麻醉科
2美国德克萨斯大学医学部神经外科、神经科学与细胞生物学系
王云1 岳云1 方力2 Introduction Glutamate synapses are involved in most excitatory neurotransmiion in the central nervous system(CNS).The major glutamate receptor subtypes at glutamatergic synapses are currently subdivided into ionotropic glutamate receptors(ion channel forming)and metabotropic glutamate receptors(G-protein coupled).The former may include N-methyl-D-aspartate(NMDA)receptor and non-NMDA receptors, such as AMPA and kainite receptors.Cumulative evidence suggests that activity-dependent changes in the efficacy of glutamatergic synapses in pain transmiion pathways greatly contribute to chronic pain caused by tiue damage or nerve injury.A great number of studies have addreed the role of NMDA receptors and metabotropic glutamate receptors in synapses between primary afferent fibers and spinal neurons.It has been demonstrated that the activation of NMDA receptors and metrabotropic glutamate receptors critically contributed to the development of chronic nociceptive hypersensitivity following peripheral tiue damage or nerve injury.In contrast, the glutamate receptors of the AMPA type were originally thought to mediate rapid excitatory neurotransmiion in the CNS.Recently, evidence has accumulated that supports the critical contributions of spinal AMPA receptors in the development of both acute and chronic persistent pain.Regulation of the number of spinal AMPA receptors in postsynaptic membrane through the receptor trafficking evoked by painful stimuli The trafficking of AMPA receptors has been well studied in glutamatergic synapses of hippocampus.These studies have shown that AMPA receptors present some distinctive characteristics in receptors trafficking from cytosol to postsynaptic membrane, compared with NMDA receptors(Fig.1).On one hand, AMPA receptors may rapidly and constitutively cycle between intracellular stores and the cell surface.On the other hand, AMPA receptors in plasma membrane may exchange between the extrasynaptic and synaptic membrane in a manner of lateral diffusion.The receptor-cycling event and lateral diffusion can influence the number of AMPA receptors in synapses and further changes the synapse strength.It has been demonstrated that the regulation of AMPA receptors cycling and surface trafficking plays a critical role in the induction of LTP in hippocampal slice culture.In contrast to the wealth of information on the mechanisms and regulation of the AMPA receptor’s trafficking in hippocampal neurons, much le is known about AMPA receptor’s trafficking in spinal nociceptive neurons in response to painful stimuli.Acumulative evidence suggests that spinal cord central sensitization resembles LTP in the hippocampus.A part of mechanisms underlying the LTP in the hippocampus may also apply to the spinal cord.For example, spinal cord central sensitization and the early phase of hippocampal LTP share two general mechanisms:(i)phosphorylation of synaptic receptors and(ii)insertion of new AMPA receptors into the postsynaptic membrane.In hippocampus, GluR2/3 subunits are constitutively cycled in and out the plasma membrane to maintain normal transmiion rather than to enhance the synaptic strength.However, trafficking of GluR1 AMPA subunits is induced by LTP and is a tightly regulated proce in hippocampus.The current evidence supports the involvement of the trafficking of AMPA receptors in spinal nociception.In a mice model of visceral hyperalgesia, Galan et al.demonstrated that trafficking of GluR1 AMPA subunits from cytosol to plasma membrane in spinal neurons was evoked by painful visceral stimuli.Nagy et al.showed that some GluR1 subunits in the dorsal horn were phosphorylated at Ser845 site following noxious stimulation.Since phosphorylation at this site is neceary for insertion of GluR1-containing receptors, this further provides the evidence that noxious stimulation may induce insertion of GluR1-containing receptors.The signaling pathways that drive the insertion of GluR1 subunits into the plasma membrane during LTP in vitro require the activity of CaMKII.In spinal neurons, intrathecal application of CaMKII inhibitor, KN-93, before the painful visceral stimulus, apparently inhibits the GluR1 accumulation in the plasma membrane fraction.This suggests that the painful visceral stimulus drives synaptic delivery of GluR1-containing receptors in a CaMKII-dependent manner.Laron et al.showed that a model of acute inflammatory hyperalgesia was aociated with an elevated density of GluR1-containing AMPA receptors as well as an increased synaptic ratio of GluR1 to GluR2/3 subunits at synapses established by C-fibers that lack the neuropeptide substance P, suggesting the membrane translocation of GluR1-containing AMPA receptors to a spinal nociceptive synapse during acute noxious stimulation.Functional regulation of spinal AMPA receptor by subunits’ phosphorylation A variety of intracellular signals trigger cellular and molecular changes at transcriptional, translational, or posttranslational levels, and theses changes contribute to the central sensitization induced by peripheral noxious stimulation.Membrane receptor phosphorylation is an important posttranslational mechanism underlying synaptic plasticity in nervous systems in pain proceing.Strong noxious stimulation in the periphery may activate several protein kinases such as CaMKII, PKA, PKC, and PKG, which play an important role in the phosphorylation of glutamate receptors in spinal nociceptive neurons.The increased sensitivity of glutamate receptors through the phosphorylation regulated by protein kinases may contribute to the enhanced responsivene of dorsal horn neurons during central sensitization.As an important cla of glutamate receptor, phosphorylation of AMPA receptor subunits has been widely investigated in relation to procees of synaptic plasticity in different brain regions.It has been demonstrated that phosphorylation of AMPA receptor subunits may potentiate their activity, influence their interaction with intracellular partner proteins, and promote their expreion at the plasma membrane during synaptic plasticity.All these intracellular cascades triggered by phosphorylation of AMPA receptor subunits may contribute to the enhanced efficacy of glutamatergic synapses(Fig.2).In spinal neurons, accumulating evidence supports the important role of the regulation of AMPA receptors by phosphorylation in spinal nociception.The intracellular C-terminal domains of AMPA receptor subunits allow subunit-specific regulation by phosphorylation.There are several protein phosphorylation sites located on the C-terminal region, which are working targets of PKA, PKC, and CaMKII.In vitro studies on hippocampal slices show that AMPA receptors can be directly phosphorylated on at least 12 distinct phosphorylation sites.Site-directed mutagenesis and phosphopeptide analysis has identified the two major phosphorylation sites on GluR1: Ser845, which is phosporylated by PKA, and Ser 831, which is phosphorylated by PKC.The phosphorylation of Ser845 in GluR1 by PKA regulates the open-channel probability of AMPA receptors;whereas the phosphorylation of Ser831 by PKC changes channel conductance.CaMKII was also found to phosphorylate Ser831 in GluR1 and contributes to the single-channel conductance of the receptor and poibly increases AMPA receptor conductance during LTP.In spinal neurons, our group has shown that PKA mediates the phosphorylation of serine at the Ser845 site, and PKC targets the Ser831 site following noxious stimulation.Further, we have demonstrated that AMPA receptors showed enhanced responsivene to nociceptive stimulation through this phosphorylation step during central sensitization.More specifically, CaMKII mediates the phosphorylation of GluR1 subunit of AMPA receptor at both Ser831 and Ser845 sites in neurons in the spinal cord after strong noxious peripheral stimulation.Phosphorylation of GluR1 at Ser831 by CaMKII in central sensitization is consistent with the results of studies of LTP in the hippocampus.CaMKII inhibitor, KN-93, partially blocked the phosphorylation of GluR1 at the Ser845 site, which is a PKA phosphorylation site.CaMKII may indirectly mediate the phosphorylation of GluR1 at the Ser845 site through adenylate cyclase and PKA, since the Ca2+-calmodulin complex can stimulate adenylate cyclase, which produces more cAMP and PKA.Lu et al demonstrated that phosphorylated GluR1(phosphorylated GluR1-Ser845 and phosphorylated GluR1-Ser831)might play a role in the induction of inflammatory but not neuropathic pain.Regulation of the interactions of AMPA receptor subunits and partner proteins in spinal cord neurons A number of proteins interacting with the intracellular C-termini of postsynaptic AMPA receptor GluR1-4 subunits have been identified.These proteins are closely aociated with AMPA receptor subunits trafficking and the regulation of intracellular signal transduction.It has been demonstrated that the regulated synaptic insertion of AMPA receptor subunits through the interactions of subunits with protein partners can play a critical role in spinal dorsal horn sensitization(Fig.2).PDZ-domain–containing proteins, such as GRIP and ABP were found to interact with the xS/TxV motif at the extreme C-terminal of short forms of AMPA receptor subunits.The interaction of GluR2 and GRIP plays a role in clustering of AMPA receptors at excitatory synapses.It has been found that phosphorylation of GluR2 on Ser880 by PKC reduces the affinity of GluR2 for GRIP and releases GluR2 from GRIP and promotes the internalization of GluR2 in vitro and in transfected cells, suggesting that the interaction of GluR2 with GRIP is negatively regulated by GluR2 phosphorylation.The internalization of phosphorylated GluR2 subunit of AMPA by PKC may lead to an increased synaptic ratio of GluR1 to GluR2 subunit since the membrane translocation of phosporylated GluR1subunit of AMPA receptor by PKC or PKA to a spinal nociceptive synapse may occur during noxious stimulation.PICK1 has also been found to bind to the C-terminal of GluR2/3/4 subunits of AMPA receptors through its PDZ domain.PICK1 dimers may induce the aggregation of AMPA receptors in heterologous expreion systems.It has also been demonstrated that PICK1 dimers could bring PKCα to AMPA receptors and be involved in a mechanism for selectively phosphorylating AMPA receptor GluR2 on Ser880.Stargazin is one of a family of transmembrane AMPA receptor regulatory proteins(TARP), which binds to GluR1, 2, and 4(at a site other than their PDZ target motifs).Stargazin plays a critical role in regulating AMPA receptor trafficking between synaptic and extrasynaptic sites.Binding of the Stargazin C-terminal tail to the PDZ domains of a number of synaptic scaffolding proteins, such as PSD-95(an interacting protein aociated with NR2 subunits of the NMDA receptor)mediate the synaptic targeting of surface AMPA receptors.PSD-95 has been shown to play a crucial role in spinal mechanisms of sensitization.Thus, we may suggest that the Stargazin-mediated interaction of AMPA receptors with PSD-95 is poibly implicated in the spinal sensitization.AMPA receptors’ translocation from cytosol to plasma membrane also requires Stargazin.It has been demonstrated that the overexpreion of Stargazin increases the number of extrasynaptic AMPA receptors, but has no effect on AMPA receptor-mediated synaptic transmiion and that the overexpreion of PSD-95 enhances AMPA receptor-mediated synaptic responses, suggesting the critical role of Stargazin in the regulation of AMPA receptor trafficking from extrasynaptic to synaptic sites and in the mediated interaction of AMPA receptors with PSD-95.Tao et al.investigated the effect of knockdown of spinal stagazin on AMPA receptor-mediated pain sensitization after inflammation and demonstrated a key role for stagazin in central sensitization of inflammatory pain by interacting with AMPA receptors in the spinal cord.Concluding remark AMPA receptor subunits may present unique characteristics in the receptor trafficking, subunit phosphorylation, interaction with partner proteins, and composition changes in response to noxious stimuli.All of these procees are closely aociated with the development or maintenance of persistent pain.Of all, AMPA receptor subunit phosphorylation is the most important proce because it can affect the other procees such as receptor trafficking, interaction with partner proteins, and composition changes.ACKNOWLEDGMENTS:
This work was funded by Sealy Grant 2951-02, NIH P30/DK 56338-02/05, DE 15814, NS11255, 40723 to Li Fang and National Natural Science Foundation of China, 30801073/C160202 to Yun Wang.
