脆性X综合征(FXS)是由单个基因的功能丧失突变引起的遗传性智力障碍的最著名形式。FMR1基因突变消除了脆性X信使核糖核蛋白(FMRP)的表达,调节许多突触蛋白的表达。死后FXS患者大脑中的皮质锥体神经元显示出异常高密度和未成熟的树突棘形态;该表型在Fmr1敲除(KO)小鼠中复制。虽然FMRP在树突中很好地定位以调节突触可塑性,有趣的体外和体内数据表明,嵌入Fmr1KO神经元或神经胶质网络中的野生型神经元表现出脊柱异常,就像Fmr1全局KO小鼠中的神经元一样。这提出了一个问题:FMRP是否以细胞自主的方式调节突触形态和动力学,或者突触表型来自异常的突触前输入?为了解决这个问题,我们结合病毒和小鼠遗传学方法,在出生后早期或成年期,从非常稀疏的皮层5层锥体神经元(L5PyrNs)中删除FMRP.然后,我们通过体内双光子显微镜跟踪了这些Fmr1KO神经元上树突棘的结构动力学。我们发现,而成年Fmr1全球KO小鼠中的L5PyrN具有异常高的细棘密度,成年期的单细胞Fmr1KO不影响脊柱密度,形态学,或动态。相反,具有新生儿FMRP缺失的神经元具有正常的脊柱密度,但在1月龄时脊柱形成升高,在Fmr1全局KO小鼠中复制表型。有趣的是,这些神经元表现出增加的薄脊柱密度,但脊柱总密度正常,成年。一起,我们的数据揭示了青春期皮质突触动力学的细胞自主FMRP调节,但成年期的脊柱缺陷也涉及非细胞自主因素。
Fragile X Syndrome (FXS) is the best-known form of inherited intellectual disability caused by the loss-of-function mutation in a single gene. The FMR1 gene mutation abolishes the expression of Fragile X Messenger Ribonucleoprotein (FMRP), which regulates the expression of many synaptic proteins. Cortical pyramidal neurons in postmortem FXS patient brains show abnormally high density and immature morphology of dendritic spines; this phenotype is replicated in the Fmr1 knockout (KO) mouse. While FMRP is well-positioned in the dendrite to regulate synaptic plasticity, intriguing in vitro and in vivo data show that wild type neurons embedded in a network of Fmr1 KO neurons or glia exhibit spine abnormalities just as neurons in Fmr1 global KO mice. This raises the question: does FMRP regulate synaptic morphology and dynamics in a cell-autonomous manner, or do the synaptic phenotypes arise from abnormal pre-synaptic inputs? To address this question, we combined viral and mouse genetic approaches to delete FMRP from a very sparse subset of cortical layer 5 pyramidal neurons (L5 PyrNs) either during early postnatal development or in adulthood. We then followed the structural dynamics of dendritic spines on these Fmr1 KO neurons by in vivo two-photon microscopy. We found that, while L5 PyrNs in adult Fmr1 global KO mice have abnormally high density of thin spines, single-cell Fmr1 KO in adulthood does not affect spine density, morphology, or dynamics. On the contrary, neurons with neonatal FMRP deletion have normal spine density but elevated spine formation at 1 month of age, replicating the phenotype in Fmr1 global KO mice. Interestingly, these neurons exhibit elevated thin spine density, but normal total spine density, by adulthood. Together, our data reveal cell-autonomous FMRP regulation of cortical synaptic dynamics during adolescence, but spine defects in adulthood also implicate non-cell-autonomous factors.