Departamento de Bioquímica, Facultad de Veterinaria, Universidad Complutense, 28040 Madrid, Spain; Instituto Universitario de Investigación en Neuroquímica (IUIN), Universidad Complutense, 28040 Madrid, Spain; Instituto de Investigación Sanitaria del Hospital Clínico San Carlos, 28040 Madrid, Spain.
Departamento de Bioquímica, Facultad de Veterinaria, Universidad Complutense, 28040 Madrid, Spain; Instituto de Investigación Sanitaria del Hospital Clínico San Carlos, 28040 Madrid, Spain.
Neurobiol Dis. 2019 Oct;130:104482. doi: 10.1016/j.nbd.2019.104482. Epub 2019 May 23.
In fragile X syndrome, the absence of Fragile X Mental Retardation Protein (FMRP) is known to alter postsynaptic function, although alterations in presynaptic function also occur. We found that the potentiation of glutamate release induced by the β adrenergic receptor (βAR) agonist isoproterenol is absent in cerebrocortical nerve terminals (synaptosomes) from mice lacking FMRP (Fmr1 KO), despite the normal cAMP generation. The glutamate release induced by moderate stimulation of synaptosomes with 5 mM KCl was not potentiated in Fmr1 KO synaptosomes by isoproterenol, nor by stimulating the receptor associated signaling pathway with the adenylyl cyclase activator forskolin or with the Epac activator 8-pCPT. Hence, the impairment in the pathway potentiating release is distal to βARs. Electron microscopy shows that Fmr1 KO cortical synapses have more docked vesicles than WT synapses, consequently occluding the isoproterenol response through which more SVs approach the active zone (AZ) of the plasma membrane. Weak stimulation of synaptosomes with the Ca ionophore ionomycin recovered the release potentiation driven by forskolin and 8-pCPT but not with isoproterenol, revealing an impairment in the efficiency of receptor generated cAMP to activate the release potentiation pathway. Indeed, inhibiting cyclic nucleotide phosphodiesterase PDE2A with BAY 60-7550 reestablished isoproterenol mediated potentiation in Fmr1 KO synaptosomes. Thus, the lack of β-AR mediated potentiation of glutamate release appears to be the consequence of an impaired capability of the receptor to mobilize SVs to the AZ and because of a decreased efficiency of cAMP to activate the signaling pathway that enhances neurotransmitter release.
在脆性 X 综合征中，已知缺乏脆性 X 智力迟钝蛋白 (FMRP) 会改变突触后功能，尽管突触前功能也会发生改变。我们发现，缺乏 FMRP (Fmr1 KO) 的大脑皮质神经末梢 (突触体) 中，β 肾上腺素能受体 (βAR) 激动剂异丙肾上腺素诱导的谷氨酸释放增强作用缺失，尽管 cAMP 正常产生。用 5mM KCl 适度刺激突触体诱导的谷氨酸释放，在 Fmr1 KO 突触体中，异丙肾上腺素、激活腺苷酸环化酶的激动剂 forskolin 或激活 Epac 的激动剂 8-pCPT 均不能增强其释放。因此，增强释放的途径受损发生在 βAR 之后。电子显微镜显示，Fmr1 KO 皮质突触比 WT 突触具有更多的停靠囊泡，从而通过更多 SV 接近质膜的活性区 (AZ) 来阻断异丙肾上腺素反应。用 Ca 离子载体离子霉素弱刺激突触体，恢复了由 forskolin 和 8-pCPT 驱动的释放增强作用，但不能恢复异丙肾上腺素的作用，这表明受体产生的 cAMP 激活释放增强途径的效率受损。事实上，用 BAY 60-7550 抑制环核苷酸磷酸二酯酶 PDE2A，可重建 Fmr1 KO 突触体中的异丙肾上腺素介导的增强作用。因此，β-AR 介导的谷氨酸释放增强作用的缺失似乎是由于受体动员 SV 到 AZ 的能力受损，以及由于 cAMP 激活增强神经递质释放的信号通路的效率降低所致。
