Dendrite growth and the effect of ectopic Rheb expression on cortical neurons.

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.