Vollum Institute, Oregon Health and Science University, Portland, Oregon 97239, USA.
J Neurosci. 2012 Jun 13;32(24):8127-37. doi: 10.1523/JNEUROSCI.6034-11.2012.
Glutamatergic synapses in early postnatal development transiently express calcium-permeable AMPA receptors (CP-AMPARs). Although these GluA2-lacking receptors are essential and are elevated in response to brain-derived neurotrophic factor (BDNF), little is known regarding molecular mechanisms that govern their expression and synaptic insertion. Here we show that BDNF-induced GluA1 translation in rat primary hippocampal neurons requires the activation of mammalian target of rapamycin (mTOR) via calcium calmodulin-dependent protein kinase kinase (CaMKK). Specifically, BDNF-mediated phosphorylation of threonine 308 (T308) in AKT, a known substrate of CaMKK and an upstream activator of mTOR-dependent translation, was prevented by (1) pharmacological inhibition of CaMKK with STO-609, (2) overexpression of a dominant-negative CaMKK, or (3) short hairpin-mediated knockdown of CaMKK. GluA1 surface expression induced by BDNF, as assessed by immunocytochemistry using an extracellular N-terminal GluA1 antibody or by surface biotinylation, was impaired following knockdown of CaMKK or treatment with STO-609. Activation of CaMKK by BDNF requires transient receptor potential canonical (TRPC) channels as SKF-96365, but not the NMDA receptor antagonist d-APV, prevented BDNF-induced GluA1 surface expression as well as phosphorylation of CaMKI, AKT(T308), and mTOR. Using siRNA we confirmed the involvement of TRPC5 and TRPC6 subunits in BDNF-induced AKT(T308) phosphorylation. The BDNF-induced increase in mEPSC was blocked by IEM-1460, a selected antagonist of CP-AMPARs, as well as by the specific repression of acute GluA1 translation via siRNA to GluA1 but not GluA2. Together these data support the conclusion that newly synthesized GluA1 subunits, induced by BDNF, are readily incorporated into synapses where they enhance the expression of CP-AMPARs and synaptic strength.
在出生后早期发育过程中，谷氨酸能突触会短暂表达钙通透性 AMPA 受体 (CP-AMPAR)。虽然这些缺乏 GluA2 的受体是必不可少的，并且会在脑源性神经营养因子 (BDNF) 的作用下升高，但对于控制其表达和突触插入的分子机制知之甚少。在这里，我们表明，在原代大鼠海马神经元中，BDNF 诱导的 GluA1 翻译需要钙调蛋白依赖性蛋白激酶激酶 (CaMKK) 通过哺乳动物雷帕霉素靶蛋白 (mTOR) 的激活。具体来说，BDNF 介导的 AKT 丝氨酸 308 (T308) 磷酸化，AKT 是 CaMKK 的已知底物，也是 mTOR 依赖性翻译的上游激活物，可通过以下方式阻止：(1) 用 STO-609 进行 CaMKK 的药理学抑制，(2) 过表达显性失活的 CaMKK，或 (3) 通过短发夹 RNA 敲低 CaMKK。使用细胞外 N 端 GluA1 抗体进行免疫细胞化学评估或通过表面生物素化评估的 BDNF 诱导的 GluA1 表面表达，在 CaMKK 敲低或 STO-609 处理后受损。BDNF 通过瞬时受体电位经典 (TRPC) 通道激活 CaMKK，而不是 NMDA 受体拮抗剂 d-APV，可防止 BDNF 诱导的 GluA1 表面表达以及 CaMKI、AKT(T308) 和 mTOR 的磷酸化。通过 siRNA，我们证实了 TRPC5 和 TRPC6 亚基在 BDNF 诱导的 AKT(T308)磷酸化中的参与。BDNF 诱导的 mEPSC 增加被 CP-AMPAR 的选择性拮抗剂 IEM-1460 以及通过 siRNA 特异性抑制急性 GluA1 翻译所阻断，但不影响 GluA2。这些数据共同支持这样的结论，即 BDNF 诱导的新合成的 GluA1 亚基很容易被整合到突触中，从而增强 CP-AMPAR 的表达和突触强度。
