Diencephalic organoids - A key to unraveling development, connectivity, and pathology of the human diencephalon

The diencephalon, an integral component of the forebrain, governs a spectrum of crucial functions, ranging from sensory processing to emotional regulation. Yet, unraveling its unique development, intricate connectivity, and its role in neurodevelopmental disorders has long been hampered by the scarcity of human brain tissue and ethical constraints. Recent advancements in stem cell technology, particularly the emergence of brain organoids, have heralded a new era in neuroscience research. Although most brain organoid methodologies have hitherto concentrated on directing stem cells toward telencephalic fates, novel techniques now permit the generation of region-specific brain organoids that faithfully replicate precise diencephalic identities. These models mirror the complexity of the human diencephalon, providing unprecedented opportunities for investigating diencephalic development, functionality, connectivity, and pathophysiology in vitro. This review summarizes the development, function, and connectivity of diencephalic structures and touches upon developmental brain disorders linked to diencephalic abnormalities. Furthermore, it presents current diencephalic organoid models and their applications in unraveling the intricacies of diencephalic development, function, and pathology in humans. Lastly, it highlights thalamocortical assembloid models, adept at capturing human-specific aspects of thalamocortical connections, along with their relevance in neurodevelopmental disorders.

Generation of ventralized human thalamic organoids with thalamic reticular nucleus

Human brain organoids provide unique platforms for modeling several aspects of human brain development and pathology. However, current brain organoid systems mostly lack the resolution to recapitulate the development of finer brain structures with subregional identity, including functionally distinct nuclei in the thalamus. Here, we report a method for converting human embryonic stem cells (hESCs) into ventral thalamic organoids (vThOs) with transcriptionally diverse nuclei identities. Notably, single-cell RNA sequencing revealed previously unachieved thalamic patterning with a thalamic reticular nucleus (TRN) signature, a GABAergic nucleus located in the ventral thalamus. Using vThOs, we explored the functions of TRN-specific, disease-associated genes patched domain containing 1 (PTCHD1) and receptor tyrosine-protein kinase (ERBB4) during human thalamic development. Perturbations in PTCHD1 or ERBB4 impaired neuronal functions in vThOs, albeit not affecting the overall thalamic lineage development. Together, vThOs present an experimental model for understanding nuclei-specific development and pathology in the thalamus of the human brain.

Advanced in vitro models: Microglia in action

In the central nervous system (CNS), microglia carry out multiple tasks related to brain development, maintenance of brain homeostasis, and function of the CNS. Recent advanced in vitro model systems allow us to perform more detailed and specific analyses of microglial functions in the CNS. The development of human pluripotent stem cells (hPSCs)-based 2D and 3D cell culture methods, particularly advancements in brain organoid models, offers a better platform to dissect microglial function in various contexts. Despite the improvement of these methods, there are still definite restrictions. Understanding their drawbacks and benefits ensures their proper use. In this primer, we review current developments regarding in vitro microglial production and characterization and their use to address fundamental questions about microglial function in healthy and diseased states, and we discuss potential future improvements with a particular emphasis on brain organoid models.

Dyslexia associated gene KIAA0319 regulates cell cycle during human neuroepithelial cell development

Dyslexia, also known as reading disability, is defined as difficulty processing written language in individuals with normal intellectual capacity and educational opportunity. The prevalence of dyslexia is between 5 and 17%, and the heritability ranges from 44 to 75%. Genetic linkage analysis and association studies have identified several genes and regulatory elements linked to dyslexia and reading ability. However, their functions and molecular mechanisms are not well understood. Prominent among these is KIAA0319, encoded in the DYX2 locus of human chromosome 6p22. The association of KIAA0319 with reading performance has been replicated in independent studies and different languages. Rodent models suggest that kiaa0319 is involved in neuronal migration, but its role throughout the cortical development is largely unknown. In order to define the function of KIAA0319 in human cortical development, we applied the neural developmental model of a human embryonic stem cell. We knocked down KIAA0319 expression in hESCs and performed the cortical neuroectodermal differentiation. We found that neuroepithelial cell differentiation is one of the first stages of hESC differentiation that are affected by KIAA0319 knocked down could affect radial migration and thus differentiation into diverse neural populations at the cortical layers.

Human Down syndrome microglia are up for a synaptic feast

In this issue of Cell Stem Cell, Jin et al. report that human Down syndrome microglia exhibit enhanced synaptic engulfment and accelerated tau-induced cellular senescence in human-mouse chimeric brains. They show that inhibiting interferon signaling rescues both developmental and tau-associated phenotypes, rendering it a potential therapeutic target for Down syndrome.

Region Specific Brain Organoids to Study Neurodevelopmental Disorders

Region specific brain organoids are brain organoids derived by patterning protocols using extrinsic signals as opposed to cerebral organoids obtained by self-patterning. The main focus of this review is to discuss various region-specific brain organoids developed so far and their application in modeling neurodevelopmental disease. We first discuss the principles of neural axis formation by series of growth factors, such as SHH, WNT, BMP signalings, that are critical to generate various region-specific brain organoids. Then we discuss various neurodevelopmental disorders modeled so far with these region-specific brain organoids, and findings made on mechanism and treatment options for neurodevelopmental disorders (NDD).

Expression of the transcription factor PU.1 induces the generation of microglia-like cells in human cortical organoids

Microglia play a role in the emergence and preservation of a healthy brain microenvironment. Dysfunction of microglia has been associated with neurodevelopmental and neurodegenerative disorders. Investigating the function of human microglia in health and disease has been challenging due to the limited models of the human brain available. Here, we develop a method to generate functional microglia in human cortical organoids (hCOs) from human embryonic stem cells (hESCs). We apply this system to study the role of microglia during inflammation induced by amyloid-β (Aβ). The overexpression of the myeloid-specific transcription factor PU.1 generates microglia-like cells in hCOs, producing mhCOs (microglia-containing hCOs), that we engraft in the mouse brain. Single-cell transcriptomics reveals that mhCOs acquire a microglia cell cluster with an intact complement and chemokine system. Functionally, microglia in mhCOs protect parenchyma from cellular and molecular damage caused by Aβ. Furthermore, in mhCOs, we observed reduced expression of Aβ-induced expression of genes associated with apoptosis, ferroptosis, and Alzheimer's disease (AD) stage III. Finally, we assess the function of AD-associated genes highly expressed in microglia in response to Aβ using pooled CRISPRi coupled with single-cell RNA sequencing in mhCOs. In summary, we provide a protocol to generate mhCOs that can be used in fundamental and translational studies as a model to investigate the role of microglia in neurodevelopmental and neurodegenerative disorders.

Brain connectivity inversely scales with developmental temperature in Drosophila

Variability of synapse numbers and partners despite identical genes reveals the limits of genetic determinism. Here, we use developmental temperature as a non-genetic perturbation to study variability of brain wiring and behavior in Drosophila. Unexpectedly, slower development at lower temperatures increases axo-dendritic branching, synapse numbers, and non-canonical synaptic partnerships of various neurons, while maintaining robust ratios of canonical synapses. Using R7 photoreceptors as a model, we show that changing the relative availability of synaptic partners using a DIPγ mutant that ablates R7's preferred partner leads to temperature-dependent recruitment of non-canonical partners to reach normal synapse numbers. Hence, R7 synaptic specificity is not absolute but based on the relative availability of postsynaptic partners and presynaptic control of synapse numbers. Behaviorally, movement precision is temperature robust, while movement activity is optimized for the developmentally encountered temperature. These findings suggest genetically encoded relative and scalable synapse formation to develop functional, but not identical, brains and behaviors.

Live Observation of Two Parallel Membrane Degradation Pathways at Axon Terminals

Neurons are highly polarized cells that require continuous turnover of membrane proteins at axon terminals to develop, function, and survive. Yet, it is still unclear whether membrane protein degradation requires transport back to the cell body or whether degradation also occurs locally at the axon terminal, where live observation of sorting and degradation has remained a challenge. Here, we report direct observation of two cargo-specific membrane protein degradation mechanisms at axon terminals based on a live-imaging approach in intact Drosophila brains. We show that different acidification-sensing cargo probes are sorted into distinct classes of degradative “hub” compartments for synaptic vesicle proteins and plasma membrane proteins at axon terminals. Sorting and degradation of the two cargoes in the separate hubs are molecularly distinct. Local sorting of synaptic vesicle proteins for degradation at the axon terminal is, surprisingly, Rab7 independent, whereas sorting of plasma membrane proteins is Rab7 dependent. The cathepsin-like protease CP1 is specific to synaptic vesicle hubs, and its delivery requires the vesicle SNARE neuronal synaptobrevin. Cargo separation only occurs at the axon terminal, whereas degradative compartments at the cell body are mixed. These data show that at least two local, molecularly distinct pathways sort membrane cargo for degradation specifically at the axon terminal, whereas degradation can occur both at the terminal and en route to the cell body.

The where, what, and when of membrane protein degradation in neurons

Membrane protein turnover and degradation are required for the function and health of all cells. Neurons may live for the entire lifetime of an organism and are highly polarized cells with spatially segregated axonal and dendritic compartments. Both longevity and morphological complexity represent challenges for regulated membrane protein degradation. To investigate how neurons cope with these challenges, an increasing number of recent studies investigated local, cargo-specific protein sorting, and degradation at axon terminals and in dendritic processes. In this review, we explore the current answers to the ensuing questions of where, what, and when membrane proteins are degraded in neurons. © 2017 The Authors Developmental Neurobiology Published by Wiley Periodicals, Inc. Develop Neurobiol 78: 283-297, 2018.