Generation and Characterization of an Nxf7 Knockout Mouse to Study NXF5 Deficiency in a Patient with Intellectual Disability

Members of the Nuclear eXport Factor (NXF) family are involved in the export of mRNA from the nucleus to the cytoplasm, or hypothesized to play a role in transport of cytoplasmic mRNA. We previously reported on the loss of NXF5 in a male patient with a syndromic form of intellectual disability. To study the functional role of NXF5 we identified the mouse counterpart. Based on synteny, mouse Nxf2 is the ortholog of human NXF5. However, we provide several lines of evidence that mouse Nxf7 is the actual functional equivalent of NXF5. Both Nxf7 and NXF5 are predominantly expressed in the brain, show cytoplasmic localization, and present as granules in neuronal dendrites suggesting a role in cytoplasmic mRNA metabolism in neurons. Nxf7 was primarily detected in the pyramidal cells of the hippocampus and in layer V of the cortex. Similar to human NXF2, mouse Nxf2 is highly expressed in testis and shows a nuclear localization. Interestingly, these findings point to a different evolutionary path for both NXF genes in human and mouse. We thus generated and validated Nxf7 knockout mice, which were fertile and did not present any gross anatomical or morphological abnormalities. Expression profiling in the hippocampus and the cortex did not reveal significant changes between wild-type and Nxf7 knockout mice. However, impaired spatial memory was observed in these KO mice when evaluated in the Morris water maze test. In conclusion, our findings provide strong evidence that mouse Nxf7 is the functional counterpart of human NXF5, which might play a critical role in mRNA metabolism in the brain.

核输出因子（Nuclear eXport Factor, NXF）家族成员参与信使RNA（messenger RNA，mRNA）从细胞核向细胞质的转运过程，或被推测在细胞质mRNA的运输中发挥功能。我们此前曾报道，1例伴综合征性智力障碍的男性患者存在NXF5基因缺失。为研究NXF5的功能角色，我们鉴定了其小鼠同源基因。基于共线性分析，小鼠Nxf2曾被认为是人类NXF5的直系同源基因（ortholog），但本研究通过多组实验证据表明，小鼠Nxf7才是NXF5真正的功能等价物。Nxf7与NXF5均主要在大脑组织中表达，定位于细胞质，并在神经元树突中呈颗粒状分布，提示二者在神经元细胞质mRNA代谢过程中发挥作用。Nxf7主要在海马锥体神经元以及大脑皮层第五层神经元中被检测到表达。与人类NXF2类似，小鼠Nxf2在睾丸中高表达且定位于细胞核。有趣的是，上述发现提示人类与小鼠的NXF家族基因存在不同的进化路径。我们因此构建并验证了Nxf7基因敲除小鼠，该小鼠可育且未出现明显的大体解剖或形态学异常。对海马与皮层组织的表达谱分析未发现野生型与Nxf7敲除小鼠存在显著差异。但在Morris水迷宫（Morris water maze）实验中，该敲除小鼠表现出空间记忆受损。综上，本研究的多项证据表明，小鼠Nxf7是人类NXF5的功能等价物，二者可能在大脑mRNA代谢过程中发挥关键作用。