The roles of RNA binding protein Celf4 and ribosomal protein Rpl13a in the development of mammalian neocortex
Creators: name: Salamon, Iva
Publisher: name: No Publisher Supplied
Year: 2023

description: The neocortex, named for its evolutionary novelty, is the site for sensorimotor and cognitive abilities unique to mammals. The neocortex’s intricate architecture is achieved through sequential developmental events that shape its six unique cellular layers, each composed of distinct neuronal populations with varying morpho-physiological traits and connectivity patterns. One of the biggest concerns in developmental neuroscience involves sorting out the molecular mechanisms responsible for the highly regulated gene expression programs that promote protein diversity governing neocorticogenesis. Any perturbation in these prenatal processes can cause gross as well as subtle changes that eventually can lead to various neurodevelopmental disorders (NDDs) and ribosomopathies causing NDDs. Recent studies have highlighted post-transcriptional regulatory programs involving translational control as one of the key nodes underlying neocortical formation. Building on an ever-growing body of experimental work, this dissertation elucidates novel roles of two post-transcriptional regulators, RNA binding protein CUGBP Elav-Like Family Member 4 (Celf4) and core 60S Ribosomal protein L13a (Rpl13a), in neocortical development. To better understand the complexity of translation regulation that controls various aspects of human neurodevelopment, we intersected the polysome profiling RNA seq with single nucleus RNAseq analyses of human developing prefrontal neocortices. This approach allowed for the identification of a set of human mRNAs undergoing active translation in distinct cell populations, which include mRNAs encoding synaptic proteins. Among those genes highly expressed in the deep-layer and subplate neurons – the sites of initial synaptogenesis, we found that high-confidence autism spectrum disorder (ASD) gene, CELF4, is the top RNA binding protein that targets a group of synaptic mRNAs in both human and murine prenatal cortices. We further discovered that selective Celf4 deletion from mouse cortical progenitors and postmitotic neurons promoted translational derepression of specific synaptogenesis-related mRNAs and, unexpectedly, sex-dependently altered the synapse density. Our findings confirm the crucial role of translational control in neocortical development and shed light on the specific molecular mechanisms underlying prenatal synaptogenesis. This dissertation also outlines a novel discovery for Rpl13a in neuronal survival and maintenance of the early post-mitotic neuronal state. Genetic deletion of Rpl13a from mouse postmitotic neurons triggered a pronounced cell death in the upper- and deep-layer neuronal subtypes that displayed the most prominent single-nucleus transcriptomic changes. This effect correlated with an increased association of cell death-inducing mRNAs with actively translating polysomes based on our polysome profiling RNAseq. Massive neuronal apoptosis further contributed to other neuroanatomical brain defects manifesting as expansion of neurogenic VZ and SVZ, defective axonal projections, agenesis of all three commissures and microcephaly, suggesting Rpl13a may be a novel candidate for microcephaly in humans. Taken together, this dissertation shows the importance of honing transcriptional output by translational control to expand the regulatory flexibility during prenatal development of the brain. The translational “fine-tuning” exerted by post-transcriptional regulators Celf4 and Rpl13a are fundamental to proper neocortical development and highlight the underlying role of aberrant translation-dependent control pathways in the pathophysiology of NDDs, such as autism spectrum disorders (ASD), epilepsy, and depression.