Translational regulation mediates hyperactive mTORC1-induced aberrations in cortical development.

摘要

Upregulated mTORC1 signaling is a molecular hallmark of many types of malformations of cortical development as well as several neurodevelopmental disorders that are characterized by cortical malformations, such as tuberous sclerosis. Patients with these disorders have overlapping neurological manifestations, including seizures and cognitive dysfunction. In addition to these specific neurological outcomes, several mTORC1-signaling disorders are among the most prevalent forms of syndromic autism. Many animal models have been developed to recapitulate the phenotypes of these disorders and to examine the dependence of those phenotyeps on mTORC1 signaling. However, there is still little known about the downstream effectors that regulate mTORC1-driven cortical malformations. mTORC1 regulates a host of cellular processes through independent pathways including autophagy, lysosomal function, and, canonically, protein translation. As any one of these pathways is critical to cell function, it is plausible that aberrant signaling can lead to developmental dysfunction through any single pathway or a combination thereof.;This dissertation used a focal model of mTORC1 upregulation--- in utero electroporation of a constitutively active Rheb construct to 1) more deeply characterize the effects of aberrant signaling on lamination of the cortex and integration of cortical pyramidal neurons, and 2) elucidate the role of translational regulation through 4EBP signaling in producing these phenotypes. The findings presented show that ectopic integration of neurons does not require dysregluated signaling in radial glia cells, which are both the progenitors and the migrational scaffold for the cortical pyramidal neurons. Furthermore, many ectopic neurons not only lose the molecular identity of the layer appropriate for their birthdate, but also acquire the identity of the deep layer into which they integrate. Even cells that integrate into layer 2/3 still exhibit disrupted integration as evidenced by morphological hypertrophy, decreased frequency of spontaneous excitatory postsynaptic currents, and aberrant hyperpolarization-activated current (Ih) expression. Most importantly, normalizing translation through 4EBP signaling largely restored normal lamination, somatic enlargement, frequency of synaptic events, and Ih expression, but only partially blocked dendritic hypertrophy. Finally, not only was 4EBP signaling necessary for mislamination, but recapitulating mTORC1 signaling through 4EBP2 was sufficient to induce ectopic integration of neurons in the cortex.;Taken together, these data point to the importance of translational regulation in producing many of the cortical developmental phenotypes driven by pathological mTORC1 signaling. The model used provides a path for future studies to further refine which translationally-regulated targets are the direct effectors of mTORC1 activity and how particular phenotypes contribute to cognitive outcomes. Understanding the downstream mechanistic underpinnings of mTORC1 -signaling disorders is critical for the development of therapeutic options that are more effective and efficient while producing fewer unintended effects than targeting mTOR directly.