Cerebral organoid and mouse models reveal a RAB39b–PI3K–mTOR pathway-dependent dysregulation of cortical development leading to macrocephaly/autism phenotypes

Abstract

Dysregulation of early neurodevelopment is implicated in macrocephaly/autism disorders. However, the mechanism underlying this dysregulation, particularly in human cells, remains poorly understood. Mutations in the small GTPase gene RAB39b are associated with X-linked macrocephaly, autism spectrum disorder (ASD), and intellectual disability. The in vivo roles of RAB39b in the brain remain unknown. We generated Rab39b knockout (KO) mice and found that they exhibited cortical neurogenesis impairment, macrocephaly, and hallmark ASD behaviors, which resembled patient phenotypes. We also produced mutant human cerebral organoids that were substantially enlarged due to the overproliferation and impaired differentiation of neural progenitor cells (NPCs), which resemble neurodevelopmental deficits in KO mice. Mechanistic studies reveal that RAB39b interacts with PI3K components and its deletion promotes PI3K-AKT-mTOR signaling in NPCs of mouse cortex and cerebral organoids. The mTOR activity is robustly enhanced in mutant outer radial glia cells (oRGs), a subtype of NPCs barely detectable in rodents but abundant in human brains. Inhibition of AKT signaling rescued enlarged organoid sizes and NPC overproliferation caused by RAB39b mutations. Therefore, RAB39b mutation promotes PI3K-AKT-mTOR activity and alters cortical neurogenesis, leading to macrocephaly and autistic-like behaviors. Our studies provide new insights into neurodevelopmental dysregulation and common pathways associated with ASD across species.

RAB39B Deficiency Impairs Learning and Memory Partially Through Compromising Autophagy

“…One recent study found that RAB39B regulates the PI3K-AKT-mTOR signaling (Zhang et al, 2020). Here we also found that phosphorylated S6 levels were increased in the hippocampus, cortex, and midbrain of Rab39b KO mice compared to WT controls ( Figure 4A and Supplementary Figure 6), corroborating an upregulation of the PI3K-AKT-mTOR signaling upon loss of RAB39B.…”

Electrophysiological Maturation of Cerebral Organoids Correlates with Dynamic Morphological and Cellular Development

“…Importantly, our results highlight the translational potential of COs to model neurodevelopmental disorders that arise from aberrant neural network formation. To date, COs have provided critical insights into the structural etiology of several neurodevelopmental disorders, including autism ( Hou et al., 2020 ; Mariani et al., 2015 ; Papariello and Newell-Litwa, 2020 ; Wang et al., 2020 ; Zhang et al., 2020 ) and epilepsy ( Niu and Parent, 2020 ; Sun et al., 2019 ). However, the characterization of neural network activity is indispensable in several of these disorders, including epilepsy.…”

A novel RAB39B mutation and concurrent de novo NF1 mutation in a boy with neurofibromatosis type 1, intellectual disability, and autism: a case report

“…In mouse brain, Rab39b is expressed in cortical and hippocampal neurons, as well as in dopaminergic neurons of the substantia nigra, concordant with its association with parkinsonism and cognitive impairment in humans [ 4 ]. Rab39b knockout mice showed reduced cortical neurogenesis, macrocephaly, and autistic behaviors, similarly to patients with mutations in RAB39B [ 5 ]. The Ras-related protein Rab-39B is a small neuron-specific GTPase that contributes to synapse formation and maintenance by regulating organization and dynamics of intracellular membranes and vesicular membrane traffic [ 6 ].…”