Brain organoids: the quest to decipher human-specific features of brain development

Shaping the Neurovascular Unit Exploiting Human Brain Organoids

Brain organoids, three-dimensional cell structures derived from pluripotent stem cells, closely mimic key aspects of the human brain in vitro, providing a powerful tool for studying neurodevelopment and disease. The neuroectodermal induction protocol employed for brain organoid generation primarily gives rise to the neural cellular component but lacks the vital vascular system, which is crucial for the brain functions by regulating differentiation, migration, and circuit formation, as well as delivering oxygen and nutrients. Many neurological diseases are caused by dysfunctions of cerebral microcirculation, making vascularization of human brain organoids an important tool for pathogenetic and translational research. Experimentally, the creation of vascularized brain organoids has primarily focused on the fusion of vascular and brain organoids, on organoid transplantation in vivo, and on the use of microfluidic devices to replicate the intricate microenvironment of the human brain in vitro. This review summarizes these efforts and highlights the importance of studying the neurovascular unit in a forward-looking perspective of leveraging their use for understanding and treating neurological disorders.

Isogenic patient-derived organoids reveal early neurodevelopmental defects in spinal muscular atrophy initiation

Whether neurodevelopmental defects underlie postnatal neuronal death in neurodegeneration is an intriguing hypothesis only recently explored. Here, we focus on spinal muscular atrophy (SMA), a neuromuscular disorder caused by reduced survival of motor neuron (SMN) protein levels leading to spinal motor neuron (MN) loss and muscle wasting. Using the first isogenic patient-derived induced pluripotent stem cell (iPSC) model and a spinal cord organoid (SCO) system, we show that SMA SCOs exhibit abnormal morphological development, reduced expression of early neural progenitor markers, and accelerated expression of MN progenitor and MN markers. Longitudinal single-cell RNA sequencing reveals marked defects in neural stem cell specification and fewer MNs, favoring mesodermal progenitors and muscle cells, a bias also seen in early SMA mouse embryos. Surprisingly, SMN2-to-SMN1 conversion does not fully reverse these developmental abnormalities. These suggest that early neurodevelopmental defects may underlie later MN degeneration, indicating that postnatal SMN-increasing interventions might not completely amend SMA pathology in all patients.

The Promise and Potential of Brain Organoids

Brain organoids are 3D in vitro culture systems derived from human pluripotent stem cells that self-organize to model features of the (developing) human brain. This review examines the techniques behind organoid generation, their current and potential applications, and future directions for the field. Brain organoids possess complex architecture containing various neural cell types, synapses, and myelination. They have been utilized for toxicology testing, disease modeling, infection studies, personalized medicine, and gene-environment interaction studies. An emerging concept termed Organoid Intelligence (OI) combines organoids with artificial intelligence systems to generate learning and memory, with the goals of modeling cognition and enabling biological computing applications. Brain organoids allow neuroscience studies not previously achievable with traditional techniques, and have the potential to transform disease modeling, drug development, and the understanding of human brain development and disorders. The aspirational vision of OI parallels the origins of artificial intelligence, and efforts are underway to map a roadmap toward its realization. In summary, brain organoids constitute a disruptive technology that is rapidly advancing and gaining traction across multiple disciplines.

Pathological Deficit of Cystatin B Impairs Synaptic Plasticity in EPM1 Human Cerebral Organoids

Cystatin B (CSTB) is a small protease inhibitor protein being involved in cell proliferation and neuronal differentiation. Loss-of-function mutations in CSTB gene cause progressive myoclonic epilepsy 1 (EPM1). We previously demonstrated that CSTB is locally synthesized in synaptic nerve terminals from rat brain and secreted into the media, indicating its role in synaptic plasticity. In this work, we have further investigated the involvement of CSTB in synaptic plasticity, using synaptosomes from human cerebral organoids (hCOs) as well as from rodents' brain. Our data demonstrate that CSTB is released from synaptosomes in two ways: (i) as a soluble protein and (ii) in extracellular vesicles-mediated pathway. Synaptosomes isolated from hCOs are enriched in pre-synaptic proteins and contain CSTB at all developmental stages analyzed. CSTB presence in the synaptic territories was also confirmed by immunostaining on human neurons in vitro. To investigate if the depletion of CSTB affects synaptic plasticity, we characterized the synaptosomes from EPM1 hCOs. We found that the levels of presynaptic proteins and of an initiation factor linked to local protein synthesis were both reduced in EPM1 hCOs and that the extracellular vesicles trafficking pathway was impaired. Moreover, EPM1 neurons displayed anomalous morphology with longer and more branched neurites bearing higher number of intersections and nodes, suggesting connectivity alterations. In conclusion, our data strengthen the idea that CSTB plays a critical role in the synapse physiology and reveal that pathologically low levels of CSTB may affect synaptic plasticity, leading to synaptopathy and altered neuronal morphology.

A molecular and cellular perspective on human brain evolution and tempo

The evolution of the modern human brain was accompanied by distinct molecular and cellular specializations, which underpin our diverse cognitive abilities but also increase our susceptibility to neurological diseases. These features, some specific to humans and others shared with related species, manifest during different stages of brain development. In this multi-stage process, neural stem cells proliferate to produce a large and diverse progenitor pool, giving rise to excitatory or inhibitory neurons that integrate into circuits during further maturation. This process unfolds over varying time scales across species and has progressively become slower in the human lineage, with differences in tempo correlating with differences in brain size, cell number and diversity, and connectivity. Here we introduce the terms 'bradychrony' and 'tachycrony' to describe slowed and accelerated developmental tempos, respectively. We review how recent technical advances across disciplines, including advanced engineering of in vitro models, functional comparative genetics and high-throughput single-cell profiling, are leading to a deeper understanding of how specializations of the human brain arise during bradychronic neurodevelopment. Emerging insights point to a central role for genetics, gene-regulatory networks, cellular innovations and developmental tempo, which together contribute to the establishment of human specializations during various stages of neurodevelopment and at different points in evolution.

Human pluripotent stem cells as a translational toolkit in psychedelic research in vitro

Psychedelics, recognized for their impact on perception, are resurging as promising treatments with rapid onset for mood and substance use disorders. Despite increasing evidence from clinical trials, questions persist about the cellular and molecular mechanisms and their precise correlation with treatment outcomes. Murine neurons and immortalized non-neural cell lines harboring overexpressed constructs have shed light on neuroplastic changes mediated by the serotonin 2A receptor (5-HT2AR) as the primary mechanism. However, limitations exist in capturing human- and disease-specific traits. Here, we discuss current accomplishments and prospects for incorporating human pluripotent stem cells (PSCs) to complement these models. PSCs can differentiate into various brain cell types, mirroring endogenous expression patterns and cell identities to recreate disease phenotypes. Brain organoids derived from PSCs resemble cell diversity and patterning, while region-specific organoids simulate circuit-level phenotypes. PSC-based models hold significant promise to illuminate the cellular and molecular substrates of psychedelic-induced phenotypic recovery in neuropsychiatric disorders.

A human-specific enhancer fine-tunes radial glia potency and corticogenesis

Humans evolved an extraordinarily expanded and complex cerebral cortex, associated with developmental and gene regulatory modifications 1-3 . Human accelerated regions (HARs) are highly conserved genomic sequences with human-specific nucleotide substitutions. Although there are thousands of annotated HARs, their functional contribution to human-specific cortical development is largely unknown 4,5 . HARE5 is a HAR transcriptional enhancer of the WNT signaling receptor Frizzled8 (FZD8) active during brain development 6 . Here, using genome-edited mouse and primate models, we demonstrate that human (Hs) HARE5 fine-tunes cortical development and connectivity by controlling the proliferative and neurogenic capacity of neural progenitor cells (NPCs). Hs-HARE5 knock-in mice have significantly enlarged neocortices containing more neurons. By measuring neural dynamics in vivo we show these anatomical features correlate with increased functional independence between cortical regions. To understand the underlying developmental mechanisms, we assess progenitor fate using live imaging, lineage analysis, and single-cell RNA sequencing. This reveals Hs-HARE5 modifies radial glial progenitor behavior, with increased self-renewal at early developmental stages followed by expanded neurogenic potential. We use genome-edited human and chimpanzee (Pt) NPCs and cortical organoids to assess the relative enhancer activity and function of Hs-HARE5 and Pt-HARE5. Using these orthogonal strategies we show four human-specific variants in HARE5 drive increased enhancer activity which promotes progenitor proliferation. These findings illustrate how small changes in regulatory DNA can directly impact critical signaling pathways and brain development. Our study uncovers new functions for HARs as key regulatory elements crucial for the expansion and complexity of the human cerebral cortex.

A family portrait of human brain cells

A cell census provides information on the source of human brain specialization.

Cortical interneuron specification and diversification in the era of big data

Inhibition in the mammalian cerebral cortex is mediated by a small population of highly diverse GABAergic interneurons. These largely local neurons are interspersed among excitatory projection neurons and exert pivotal regulation on the formation and function of cortical circuits. We are beginning to understand the extent of GABAergic neuron diversity and how this is generated and shaped during brain development in mice and humans. In this review, we summarise recent findings and discuss how new technologies are being used to further advance our knowledge. Understanding how inhibitory neurons are generated in the embryo is an essential pre-requisite of stem cell therapy, an evolving area of research, aimed at correcting human disorders that result in inhibitory dysfunction.