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.
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•A protocol for vThOs with discrete nuclei identities•vThOs exhibit cellular and functional features akin to the thalamic reticular nucleus•Similar molecular features between vThOs and the human ventral thalamus•Perturbation in nuclei-specific, disease-linked genes impairs neural functions in vThOs
Generation of organoid models for subregions of the human brain is challenging. Park and colleagues report a method to generate ventralized thalamic organoids, which recapitulate molecular and functional features, and cellular diversity of ventral thalamic nuclei and offer a model to dissect related brain disorders.
The telencephalon is the largest region of the brain and processes critical brain activity. Despite much progress, our understanding of the telencephalon’s function, development, and ...pathophysiological processes remains largely incomplete. Recently, 3-dimensional brain models, known as brain organoids, have attracted considerable attention in modern neurobiological research. Brain organoids have been proven to be valuable for studying the neurodevelopmental principles and pathophysiology of the brain, as well as for developing potential therapeutics. Brain organoids can change the paradigm of current research, replacing animal models. However, there are still limitations, and efforts are needed to improve brain organoid models. In this review, we provide an overview of the development and function of the telencephalon, as well as the techniques and scientific methods used to create fully developed telencephalon organoids. Additionally, we explore the limitations and challenges of current brain organoids and potential future advancements.
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.
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•Developmental temperature affects synapse numbers and partner availability in flies•Development at 18°C results in synaptic partnerships absent at 25°C•The same non-canonical synapses form at 18°C and in the absence of a main partner at 25°C•Adult behavioral activity is adapted for the temperature at which the fly developed
Different developmental temperatures lead to differently wired brains and behavioral adaptation to the temperature at which a fly developed. Kiral et al. propose that evolution has selected for a Drosophila genome that can develop functional, but non-identical, brains through scalable synapse formation based on relative availability of synaptic partners.
The daunting complexity of the brain emerges from the large number of neurons it contains and their compartmentalized synaptic interactions at axon terminals and dendrites. Generation of functional ...neuronal networks requires robust, unambiguous developmental processes to ensure synapse-specific neuronal partner choice and subsequent maintenance mechanisms to keep neurons and particularly synapses healthy and functional over a long time. Defects in wiring and maintenance mechanisms are associated with neuropsychiatric and neurodegenerative disorders.Having regard to the importance of protein quality control mechanism both during development and function of the nervous system, in this doctoral work, I investigated possible local roles of lysosomal degradation pathways including ubiquitous and neuron-specific endolysosomal degradation and autophagy at axon terminals. Using live imaging in intact Drosophila brains and novel acidification-sensing degradation probes, first, we reported a direct live observation of local protein degradation at axon terminals in large, acidified compartments. These acidic, degradative endocytic compartments undergo continuous flux of fusion and fission of smaller compartments that is reflected by their molecular composition at a given time. Therefore, we named these compartments ‘local hubs’ as they behave as sort-and-degrade stations for local protein turnover at axon terminals. Secondly, we reported differential, cargo-specific sorting of plasma and synaptic vesicle membrane proteins into distinct hubs via two molecularly distinct pathways. Although plasma membrane protein sorting and degradation depends on ubiquitous Rab GTPase, Rab7, synaptic vesicle membrane protein sorting and degradation is Rab7-independent and operated by previously characterized synaptic vesicle proteins V100 and n-Syb. V100, as a subunit of a proton pump, particularly affects acidification of synaptic vesicles hubs, whereas n-Syb is required for the delivery of golgi-derived microvesicles containing acidic hydrolases into synaptic vesicle hubs. Interestingly, autophagy does not overlap with any of these local degradation pathways. Following their formation at axon terminals, they enter in axons without engaging in any fusion/fission events, hence morphologically and dynamically distinct from local hub compartments.Despite several reports on formation of autophagosomes at axon terminals, potential physiological roles it may exert still remain largely unknown, especially during neural circuit assembly. Live imaging of developing Drosophila photoreceptor axon terminals with autophagosome markers revealed their formation at the tip of synaptogenic filopodia followed by destabilization of these structures. Consistent with this observation, loss of function analyses of autophagy in developing Drosophila photoreceptors revealed increased stability of synaptogenic filopodia and subsequent increase in synapse numbers. More importantly, autophagy-deficient neurons connect to several aberrant synaptic partners causing neuronal miswiring. Finally, adult flies with miswired brains due to loss of autophagy show distinct and predictable behavioral phenotypes such as prolonged, repetitive visual attention to objects. Interestingly, development at colder temperatures exerts similar effect on filopodial stability as in loss of autophagy where axonal filopodia slow down and stabilize more synaptogenic filopodia. This effect on filopodia stability further leads to increased synapse formation and recruitment of aberrant synaptic partners changing brain wiring pattern. Collectively, these results demonstrate that filopodia kinetics play an important role to restrict or facilitate synaptic partnerships between neurons in close proximity during brain wiring.
The Drosophila inner photoreceptors R7 and R8 are responsible for color vision and their differentiation starts at the third instar larval stage. Only a handful of genes with R7 or R8‐cell‐specific ...expression are known. We performed an enhancer‐trap screen using a novel piggyBac transposable element, pBGay, carrying a Gal4 sequence under the control of the P promoter to identify novel genes expressed specifically in R7 or R8 cells. From this screen, three lines were analyzed in detail: piggyBacAC109 and piggyBacAC783 are expressed in R8 cells and piggyBacAC887 is expressed in R7 cells at the third instar larval stage and pupal stages. Molecular analysis showed that the piggyBac elements were inserted into the first intron of CG14160 and CG7985 genes and the second intron of unzipped. We show the expression pattern in the developing eye imaginal disc, pupal retina as well as the adult retina. The photoreceptor‐specific expression of these genes is reported for the first time and we propose that these lines are useful tools for studying the development of the visual system.
The logic of enhancer‐trap screening. The enhancer‐trap element causes the expression of the transcription factor GAL4 that will bind to the UAS (upstream activating sequence) to express GFP. The GFP expression pattern reflects the expression of the nearby gene. The expression pattern of the enhancer‐trap line piggyBacAC109 in the Drosophila eye imaginal disc is shown.
Research Highlights
A novel piggyBac element for enhancer‐trap screening in Drosophila melanogaster has been developed and a screen for inner photoreceptor cell (PR)‐specific expression has been performed
Three novel genes, CG14160, CG7985, and unzipped, with putative roles in inner PR differentiation have been identified
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