Sokolov Aidan M, Seluzicki Caitlin M, Morton Mary C, Feliciano David M
Department of Biological Sciences, Clemson University, Clemson, SC 29634-0314, USA.
Neurosci Lett. 2018 Apr 3;671:140-147. doi: 10.1016/j.neulet.2018.02.021. Epub 2018 Feb 12.
Ras homology enriched in brain (Rheb) is a GTPase that activates the protein kinase mammalian Target of Rapamycin (mTOR). Rheb mutations cause intellectual delay and megalencephaly. mTOR hyperactivation causes a constellation of neurodevelopmental disorders called "mTOR-opathies" that are frequently accompanied by hyperexcitable cortical malformations. Cortical malformations within the anterior cingulate cortex (ACC) and somatosensory cortex (SSC) frequently colocalize with hyperexcitability. Although Rheb and mTOR are implicated in the formation of cortical lesions, seizure activity, and defects in neuronal migration, the contribution of Rheb to changes in neuron size and dendrite morphology is not well established. Here, in utero electroporation of the developing embryonic brain was used to assess soma and dendrite growth in ACC and SCC layer II/III neurons. We found that between P0 and P21, neuronal soma size increased by 50 and 122 percent in the ACC and SSC, respectively. The increased size was accompanied by an increase in the number of basal dendrites and enhanced dendrite complexity. As an indicator of the involvement of the mTOR pathway in neuron maturation, phosphorylation of the mammalian target of rapamycin (mTOR) substrate S6 was identified in migrating cortical neuroblasts and maturing neurons. Notably, ectopic expression of Rheb caused cortical malformations comprised of ectopically positioned cytomegalic neurons with dendrite hypertrophy. This study provides a direct comparison of neuron maturation across two cortical regions during development, provides evidence for mTOR pathway activity during neuron maturation, and demonstrates that ectopic Rheb expression without mutation is sufficient to induce cortical malformations with cytomegaly and dendrite hypertrophy.
脑中富含的Ras同源物（Rheb）是一种激活蛋白激酶哺乳动物雷帕霉素靶蛋白（mTOR）的GTP酶。Rheb突变会导致智力发育迟缓及巨脑症。mTOR的过度激活会引发一系列被称为“mTOR病”的神经发育障碍，这些障碍常伴有皮质过度兴奋的畸形。前扣带回皮质（ACC）和躯体感觉皮质（SSC）内的皮质畸形常与过度兴奋共定位。尽管Rheb和mTOR与皮质病变的形成、癫痫活动及神经元迁移缺陷有关，但Rheb对神经元大小和树突形态变化的作用尚未明确。在此，利用发育中胚胎脑的子宫内电穿孔来评估ACC和SCC层II/III神经元的胞体和树突生长。我们发现，在出生后第0天（P0）至第21天（P21）期间，ACC和SSC中神经元胞体大小分别增加了50%和122%。胞体增大伴随着基底树突数量的增加和树突复杂性的增强。作为mTOR通路参与神经元成熟的一个指标，在迁移的皮质神经母细胞和成熟神经元中发现了雷帕霉素靶蛋白（mTOR）底物S6的磷酸化。值得注意的是，Rheb的异位表达导致了由异位定位的巨细胞神经元和树突肥大组成的皮质畸形。本研究直接比较了发育过程中两个皮质区域的神经元成熟情况，为神经元成熟过程中mTOR通路的活性提供了证据，并表明无突变的Rheb异位表达足以诱导伴有细胞肿大和树突肥大的皮质畸形。
