Mice with targeted deletion of Nxf7, the murine ortholog of the clinically implicated NXF5, display impaired hippocampal learning and neuroplasticity

Members of the Nuclear mRNA Export Factor (NXF) family are implicated in nuclear export and/or cytoplasmic transport of mRNAs in eukaryotic cells. We previously proposed NXF5 to be involved in cognitive development and aimed to study its functional role further using a mouse model. The syntenic region of the human Xcen-GLA-NXF5-NXF2-NXF3-BEX4-Xqter in the mouse is Xcen-Gla-Nxf2-Nxf7-Nxf3-Bex4-Xqter strongly indicating that mouse Nxf2 is the homolog of human NXF5. However, our functional analyses demonstrated that mouse Nxf7 is actually the functional homolog of NXF5. Both orthologs are expressed in brain, although at low levels, show predominant cytoplasmic localization, and present a granular staining in neuronal dendrites, all indicative for a role in cytoplasmic mRNA transport or metabolism. The mouse and human Nxf2/NXF2 proteins on the other hand, show highest expression in testis, and mostly nuclear staining with incorporation in the nuclear membrane, suggesting a predominant role in nuclear mRNA export. Based on these findings we generated an Nxf7 knockout mouse, which was viable and fertile and no gross anatomical changes or morphological brain abnormalities were detected. Detailed behavioral analysis demonstrated that Nxf7 KO mice displayed a deficit in hippocampus-dependent spatial learning and memory. At the cellular level, this was accompanied by impaired hippocampal long-term potentiation, but normal long-term depression, suggesting a severe functional imbalance in NMDA-dependent synaptic plasticity. In conclusion, the present findings indicate that NXF5, and its murine ortholog Nxf7, play a crucial role in neurocognitive (memory) functions, and explain why its deletion leads to intellectual disability. Two male wild type (WT1 and WT2) and two Nxf7 knockout (KO1 and KO2) mice were used in each hybridization experiment (hippocampus or cortex). A loop-design hybridization scheme (WT1/KO1, KO1/WT2, WT2/KO2 and KO2/WT1) was performed for each experiment so as to have each sample labeled once with Cy5 and once with Cy3, and hybridised to both samples of the other genotype (WT or KO). Hybridization was done on Agilent Mouse Whole Genome arrays according to the manufacturer's procedures.核mRNA输出因子（Nuclear mRNA Export Factor, NXF）家族成员参与真核细胞内mRNA的核输出及/或细胞质转运过程。我们此前曾提出NXF5可能参与认知发育，并计划借助小鼠模型进一步研究其功能角色。人类染色体X着丝粒区-GLA-NXF5-NXF2-NXF3-BEX4-Xqter区域在小鼠中的同线区域为X着丝粒区-Gla-Nxf2-Nxf7-Nxf3-Bex4-Xqter，这强烈提示小鼠Nxf2是人类NXF5的同源基因。然而，我们的功能分析证实，小鼠Nxf7才是NXF5的功能同源基因。两种直系同源基因均在脑组织中低水平表达，主要定位于细胞质，并在神经元树突中呈现颗粒状染色，所有这些特征均表明其可能参与细胞质mRNA的转运或代谢过程。 与之相对，小鼠和人类的Nxf2/NXF2蛋白则在睾丸中表达量最高，且主要呈现细胞核染色模式，并定位于核膜，这提示其主要功能为核mRNA输出。基于上述发现，我们构建了Nxf7基因敲除小鼠，该小鼠可正常存活且具有生育能力，未观察到明显的大体解剖学改变或脑部形态异常。详细的行为学分析显示，Nxf7敲除小鼠表现出海马依赖的空间学习与记忆功能缺陷。在细胞层面，这一表型伴随海马长时程增强受损，但长时程抑制正常，提示NMDA依赖的突触可塑性存在严重功能失衡。 综上，本研究结果表明，NXF5及其小鼠直系同源基因Nxf7在神经认知（记忆）功能中发挥关键作用，这也解释了该基因缺失为何会导致智力障碍。 本研究的每个杂交实验均使用2只雄性野生型（WT1、WT2）小鼠与2只Nxf7敲除（KO1、KO2）小鼠。每个实验均采用环路设计杂交方案：WT1/KO1、KO1/WT2、WT2/KO2及KO2/WT1，以使每个样本分别用Cy5和Cy3标记一次，并与另一基因型（野生型或敲除型）的两份样本进行杂交。杂交实验按照安捷伦小鼠全基因组芯片的制造商操作规程进行。