SUMMARY We report a novel mechanism of ribonucleoprotein (RNP) nucleocytoplasmic export by nuclear envelope budding. During development of Drosophila synapses, a fragment of the Wnt-1 receptor, DFrizzled2, is imported into postsynaptic nuclei where it forms prominent foci. We now show these foci to be composed of large RNP granules harboring synaptic protein transcripts. These RNPs exit the nucleus via a budding mechanism akin to nuclear egress of Herpes-type viruses, a process previously thought to be exclusive to these viruses. During this mechanism, RNP granules bud into the space between the inner and the outer nuclear membranes (the perinuclear space), in a manner dependent on Lamin C, a nuclear protein linked to muscular dystrophies. Like herpes virus nuclear egress, this process requires protein kinase C, which is known to disrupt the lamin through phosphorylation. We suggest that nuclear budding is an endogenous nuclear export pathway for large RNP granules.

SUMMARY A previously unrecognized mechanism by which large ribonucleoprotein (megaRNP) granules exit the nucleus is by budding through the nuclear envelope (NE). This mechanism is akin to the nuclear egress of Herpes-type viruses and is essential for proper synapse development. However, the molecular machinery required to remodel the NE during this process is unknown. Here we identify Torsin, a AAA-ATPase that in humans is linked to dystonia, as a major mediator of primary megaRNP envelopment during NE-budding. In torsin mutants, megaRNPs accumulate within the perinuclear space and the mRNAs contained within fail to reach synaptic sites, preventing normal synaptic protein synthesis, and thus proper synaptic bouton development. These studies begin to establish the cellular machinery underlying the exit of megaRNPs via budding, offer an explanation to the “nuclear blebbing” phenotype found in dystonia models and provide an important link between Torsin and synaptic phenotypes observed in dystonia.

The number of disease risk genes and loci identified through human genetic studies far outstrips the capacity to systematically study their functions. We applied a scalable genetic screening approach, in vivo Perturb-Seq, to functionally evaluate 35 autism spectrum disorder/neurodevelopmental delay (ASD/ND) de novo loss-of-function risk genes. Using CRISPR-Cas9, we introduced frameshift mutations in these risk genes in pools, within the developing mouse brain in utero, followed by single-cell RNA-sequencing of perturbed cells in the postnatal brain. We identified cell type–specific and evolutionarily conserved gene modules from both neuronal and glial cell classes. Recurrent gene modules and cell types are affected across this cohort of perturbations, representing key cellular effects across sets of ASD/ND risk genes. In vivo Perturb-Seq allows us to investigate how diverse mutations affect cell types and states in the developing organism.

Graphical AbstractHighlights d Individual oligodendrocytes show bias for inhibitory axons in the neocortex d Interneuron sub-classes present different profiles of myelination d Class-specific myelin distribution patterns are set up from the onset of myelination SUMMARY Reciprocal communication between neurons and oligodendrocytes is essential for the generation and localization of myelin, a critical feature of the CNS. In the neocortex, individual oligodendrocytes can myelinate multiple axons; however, the neuronal origin of the myelinated axons has remained undefined and, while largely assumed to be from excitatory pyramidal neurons, it also includes inhibitory interneurons. This raises the question of whether individual oligodendrocytes display bias for the class of neurons that they myelinate. Here, we find that different classes of cortical interneurons show distinct patterns of myelin distribution starting from the onset of myelination, suggesting that oligodendrocytes can recognize the class identity of individual types of interneurons that they target. Notably, we show that some oligodendrocytes disproportionately myelinate the axons of inhibitory interneurons, whereas others primarily target excitatory axons or show no bias. These results point toward very specific interactions between oligodendrocytes and neurons and raise the interesting question of why myelination is differentially directed toward different neuron types.

Cellular effects of visual deprivation Myelination speeds the progress of action potentials along neuronal axons. Yang et al. studied changes in myelination in the mouse visual cortex in response to visual experience (see the Perspective by Yalçin and Monje). With normal vision, myelination is continuously remodeled. As ocular dominance shifts in response to monocular deprivation, myelination patterns change on certain inhibitory interneurons but not on excitatory callosal projection neurons. Myelin sheaths are both added and subtracted, segments of myelin elongate and contract, and preexisting oligodendrocytes make new myelin sheaths. This adaptive myelination helps to diversify neuronal function and remodel neuronal circuits in response to sensory experience. Science , this issue p. eabd2109 ; see also p. 1414