Rise of an Epitranscriptomic Era in Understanding Neurodevelopment and Neuropsychiatric Diseases

Background and Aim: A broad spectrum of developmental andpsychiatric disorders, ranging from autism and intellectual disability toschizophrenia, can be connected by commonly underlying structuraland genetic causes. In the past 30 years, evidence provided by theneuroepigenetics field has made an immense impact on understandingneurodevelopmental and psychiatric disorders that were previouslyassumed to be irreversible. Recently a new field of epitranscriptomicshas risen with new promises in connecting the dysfunction of neuronalcircuits and cognitive disorders. In this talk, I will review recent evidencesuggesting a potential link between epitranscriptomic regulation andneurodevelopmental and psychiatric diseases.Methods: To understand how epitranscriptome affects synaptic functionand contribute to human diseases, our group focus on N6-methyladenosine(m6A), one of the most abundant internal messenger RNAmodifications, and study the function of this cellular pathway in developingneurons. Using an in vitro neuronal culture system, we introducedgenetic manipulations using short-hairpin RNAs and CRISPR/Cas9system, to specifically reduce expression levels of m6A reader proteins,or the proteins that recognize m6A RNA species and direct them intodistinct metabolic pathways. Furthermore, enhanced expression of m6Areader proteins was induced in neurons using strong promoters. The effectof such genetic manipulations was examined using cytohistochemistry,dynamic fluorescent imaging, and electrophysiology. Key neuronaldevelopmental steps such as polarization, axon specification, dendriticdevelopment, and synapse maturation were examined. Sholl analysisand morphological classifications were conducted to evaluate neurondevelopment. Furthermore, immunocytochemistry under cell-membraneimpermeable conditions was carried out to assess glutamate receptorstrafficking to the cell membrane. All these experiments were carried outin a blind manner where experimenters were blinded with the geneticmanipulations.Results: Structural dendritic and synaptic deficits were detected inassociation with the interruption of m6A recognition pathways. Multiplestructural deficits at extending axons and growth cones, dendriticatrophy, and immature spine phenotypes were observed. Lack ofpost-synaptic density scaffold protein clusters at the immature spineswas observed in the combination of reduced spontaneous synaptictransmission. The structural and functional deficits are consistent withRNA deep sequencing results in adult healthy mouse forebrain, as m6AmodifiedRNA as a cohort was enriched in the human phenotype of intellectual disability and in categories of genes regulating synapseassembly, organization, maturation, and modulation. Furthermore,the spine immaturity deficits (elongated spine neck and smaller spinehead) were reminiscent to the previously reported spine phenotypes inhuman infants with mental retardation. A microtubule plus-end bindingprotein APC has been shown to associate with spine morphology, growthcone dynamics, and axon extension. In neurons where m6A-mediatedregulation was interrupted via knockdown of readers in hippocampalneurons, APC protein level decreased in addition to other microtubuleregulatory proteins. Altered APC protein expression may partially explainthe altered phenotypes of developing neurons and deficits in moleculartrafficking over a long distance in affected neurons.Conclusion: Our findings indicate a structural and functional link betweenRNA methylation and neurodevelopmental and neuropsychiatricdiseases. Failed recognition of this RNA modification affects neurondevelopment at different stages and in all neuronal compartments suchas axons, dendrites, and dendritic spines. We speculate that activitydependentstructural maturation of neurons requires spatiotemporallyorchestrated gene expression, partially through regulating neuronalcytoskeleton development.

کلیدواژه ها:

Microtubule ، Spine ، Epitranscriptomics ، Neuroepigenetics ، Neuron development ، Intellectual disability