Neural organoids have the potential to improve our understanding of human brain development and neurological disorders. However, it remains to be seen whether these tissues can model circuit formation with functional neuronal output. Here, we have adapted air-liquid interface culture to cerebral organoids leading to improved neuronal survival and axon outgrowth. The resulting thick axon tracts display various morphologies including long-range projection within and away from the organoid, growth cone turning, and decussation. Single-cell RNA-sequencing reveals various cortical neuronal identities, and retrograde tracing demonstrates tract morphologies that match proper molecular identities. These cultures exhibit active neuronal networks, and extracortical projecting tracts can innervate mouse spinal cord and evoke contractions of adjacent muscle in a manner dependent on intact organoid-derived innervating tracts. Overall, these results reveal a remarkable self-organization of corticofugal and callosal tracts with a functional output, providing new opportunities to examine relevant aspects of human CNS development and disease.

Summary The human brain has undergone rapid expansion since humans diverged from other great apes, but the mechanism of this human-specific enlargement is still unknown. Here, we use cerebral organoids derived from human, gorilla, and chimpanzee cells to study developmental mechanisms driving evolutionary brain expansion. We find that neuroepithelial differentiation is a protracted process in apes, involving a previously unrecognized transition state characterized by a change in cell shape. Furthermore, we show that human organoids are larger due to a delay in this transition, associated with differences in interkinetic nuclear migration and cell cycle length. Comparative RNA sequencing (RNA-seq) reveals differences in expression dynamics of cell morphogenesis factors, including ZEB2, a known epithelial-mesenchymal transition regulator. We show that ZEB2 promotes neuroepithelial transition, and its manipulation and downstream signaling leads to acquisition of nonhuman ape architecture in the human context and vice versa, establishing an important role for neuroepithelial cell shape in human brain expansion.

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Neurons continuously adapt to external cues and challenges, including stimulation, plasticity-inducing signals and aging. These adaptations are critical for neuronal physiology and extended survival. Proteostasis is the process by which cells adjust their protein content to achieve the specific protein repertoire necessary for cellular function. Due to their complex morphology and polarized nature, neurons possess unique proteostatic requirements. Proteostatic control in axons and dendrites must be implemented through regulation of protein synthesis and degradation in a decentralized fashion, but at the same time, it requires integration, at least in part, in the soma. Here, we discuss current understanding of neuronal proteostasis, as well as open questions and future directions requiring further exploration. The challenge of regulating a distant proteomeMost catalyzed chemical reactions inside cells depend on protein levels, and fine-tuning protein concentrations is key to ensure proper cellular function. Because throughout the cellular lifespan proteins accumulate damage, become dysfunctional and need to be replaced continually, protein synthesis and protein turnover are central to cellular physiology and function. The dynamic regulation of a balanced and functional proteome (i.e., proteostasis) concerns all proteins whose levels need to be adjusted in space and time in response to intracellular and extracellular cues. Thus, the specific parameters of cellular proteostasis vary across cell types and states. However, cellular proteostasis invariably relies on the precise control of protein synthesis, folding and conformational maintenance, post-translational modifications (PTMs), degradation, and secretion [1]. Precise control of these parameters is already challenging in cells that have little or no polarity, but becomes a particularly impressive feat for highly polarized cells, such as neurons.Neurons stand out from all other cell types in their unique morphology and high degree of compartmentalization; characteristics that are central to neuronal computation. Often, extrinsic signals are spatially localized so that only a confined portion of the neuron receives a certain signal, with the neuronal portion being, for instance, a cluster of dendrites (influenced by a neuromodulator), a single dendritic branch, a synaptic neighborhood, or even an individual synapse. How these local signals are transmitted to the somata remains largely unknown. Work over the last two decades has shown that neurons have the capacity to tune their proteome locally through regulation of local protein synthesis, degradation, and PTMs to regulate multiple aspects of dendritic and axonal biology [2]. Nevertheless, to date, we still lack a comprehensive understanding of how different cellular degradation pathways interact with the protein synthesis machinery to shape and maintain the local proteome.In this review, we discuss current understanding of neuronal local proteostatic regulation, and key knowledge gaps in the field. We also com...