Mutations in nitrogen permease regulator-like 3 (NPRL3), a component of the GATOR1 complex within the mechanistic target of rapamycin (mTOR) pathway, are associated with epilepsy and malformations of cortical development. Little is known about the effects of NPRL3 loss on neuronal mTOR signaling and morphology, or cerebral cortical development and seizure susceptibility. We report the clinical phenotypic spectrum of a founder NPRL3 pedigree (c.349delG, p.Glu117LysFS; n = 133) among Old Order Mennonites dating to 1727. Next, as a strategy to define the role of NPRL3 in cortical development, CRISPR/Cas9 Nprl3 knockout in Neuro2a cells in vitro and in fetal mouse brain in vivo was used to assess effects of Nprl3 knockout on mTOR activation, subcellular mTOR localization, nutrient signaling, cell morphology and aggregation, cerebral cortical cytoarchitecture, and network integrity. The NPRL3 pedigree exhibited an epilepsy penetrance of 28% and heterogeneous clinical phenotypes with a range of epilepsy semiologies i.e., focal or generalized onset, brain imaging abnormalities i.e., polymicrogyria, focal cortical dysplasia, or normal imaging, and EEG findings, e.g., focal, multi-focal, or generalized spikes, focal or generalized slowing. Whole exome analysis comparing a seizure-free group (n = 37) to those with epilepsy (n = 24) to search for gene modifiers for epilepsy did not identify a unique genetic modifier that explained the variability in seizure penetrance in this cohort. Nprl3 knockout in vitro caused mTOR pathway hyperactivation, cell soma enlargement, and the formation of cellular aggregates seen in time-lapse videos that were prevented with the mTOR inhibitors rapamycin or torin1. In Nprl3 KO cells, mTOR remained localized on the lysosome in a constitutively active conformation, as evidenced by phosphorylation of S6 and 4E-BP1 proteins, even under nutrient starvation (amino acid free) conditions, demonstrating that Nprl3 loss decouples mTOR activation from neuronal metabolic state. To model human malformations of cortical development associated with NPRL3 variants, we created a focal Nprl3 KO in fetal mouse cortex by in utero electroporation and found altered cortical lamination and white matter heterotopic neurons, effects which were prevented with rapamycin treatment. EEG recordings showed network hyperexcitability and reduced seizure threshold to pentylenetetrazol treatment. NPRL3 variants are linked to a highly variable clinical phenotype which we propose result from mTOR-dependent effects on cell structure, cortical development, and network organization.
Nitrogen Permease Regulator Like 3 (NPRL3) variants are associated with malformations of cortical development (MCD) and epilepsy. We report a large (n=133) founder NPRL3 (c.349delG, p.Glu117LysFS) pedigree dating to 1727, with heterogeneous epilepsy and MCD phenotypes. Whole exome analysis in individuals with and without seizures in this cohort did not identify a genetic modifier to explain the variability in seizure phenotype. Then as a strategy to investigate the developmental effects of NPRL3 loss in human brain, we show that CRISPR/Cas9 Nprl3 knockout (KO) in Neuro2a cells (N2aC) in vitro causes mechanistic target of rapamycin (mTOR) pathway hyperactivation, cell soma enlargement, and excessive cellular aggregation. Amino acid starvation caused mTOR inhibition and cytoplasmic mTOR localization in wildtype cells, whereas following Nprl3 KO, mTOR remained inappropriately localized on the lysosome and activated, evidenced by persistent ribosomal S6 and 4E-BP1 phosphorylation, demonstrating that Nprl3 loss decouples mTOR activation from metabolic state. Nprl3 KO by in utero electroporation in fetal (E14) mouse cortex resulted in mTOR-dependent cortical dyslamination with ectopic neurons in subcortical white matter. EEG recordings of these mice showed hyperexcitability in the electroporated hemisphere. NPRL3 variants are linked to a highly variable clinical phenotype likely as a consequence of mTOR-dependent effects on cell structure, cortical development, and network organization.
The kinase mTOR is a signaling hub for pathways that regulate cellular growth. In neurons, the subcellular localization of mTOR takes on increased significance. Here, we review findings on the localization of mTOR in axons and offer a perspective on how these may impact our understanding of nervous system development, function, and disease. We propose a model where mTOR accumulates in local foci we term mTOR outposts, which can be found in processes distant from a neuron’s cell body. In this model, pathways that funnel through mTOR are gated by local outposts to spatially select and amplify local signaling. The presence or absence of mTOR outposts in a segment of axon or dendrite may determine whether regional mTOR-dependent signals, such as nutrient and growth factor signaling, register toward neuron-wide responses. In this perspective, we present the emerging evidence for mTOR outposts in neurons, their putative roles as spatial gatekeepers of signaling inputs, and the implications of the mTOR outpost model for neuronal protein synthesis, signal transduction, and synaptic plasticity.
Current Cas9 reagents can target genomic loci with high specificity. However, when used for knockin, on-target outcomes are inherently imprecise, often leading to unintended knockout rather than intended edits. This restricts applications of genome editing to ex vivo approaches, where clonal selection is possible. Here we describe a workflow using iterative high-throughput in vitro and high-yield in vivo assays to evaluate and compare the performance of CRISPR knockin reagents for both editing efficiency and precision. We tested combinations of Cas9 and DNA donor template variants and determined that Cas9-CtIP with in situ linearized donors display fold-increases of editing precision in cell lines and in vivo in the mouse brain. Iterating this process, we generated novel compound fusions, including eRad18-Cas9-CtIP that showed further fold-increases in performance. Continued development of precision editing reagents with this platform holds promise for direct in vivo knockin across model organisms and future targeted gene therapies.
Neuroligin-3 is a postsynaptic adhesion molecule involved in development, function, and pathologies of synapses in the brain. It is a genetic cause of autism and a potent component of the tumor microenvironment in gliomas. There are four Neuroligins that operate at distinct synapse types, selectively interacting with presynaptic adhesion and postsynaptic scaffold proteins. We investigated the subcellular localization and scaffold specificities of synaptic Neuroligin-3 and demonstrate an unexpected pattern of localization to excitatory synapses in cortical areas, and inhibitory synapses in subcortical areas. Using phosphoproteomics, we identify Neuroligin-3-specific serine phosphorylation in cortex and hippocampus that obstructs a key binding site for inhibitory synapse scaffolds. Using in utero CRISPR/Cas9 knockout and replacement with phosphomimetic mutants, we demonstrate that phosphorylation at this site determines excitatory versus inhibitory synapse localization of Neuroligin-3 in vivo. Our data reveal a mechanism that differentially regulates the balance of Neuroligin-3 between excitatory and inhibitory synapses, adding to our emerging understanding of their role in the development of brain connectivity and associated pathologies.
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