The vasculature as a neural stem cell niche

Glial Populations in the Human Brain Following Ischemic Injury

There is a growing interest in glial cells in the central nervous system due to their important role in maintaining brain homeostasis under physiological conditions and after injury. A significant amount of evidence has been accumulated regarding their capacity to exert either pro-inflammatory or anti-inflammatory effects under different pathological conditions. In combination with their proliferative potential, they contribute not only to the limitation of brain damage and tissue remodeling but also to neuronal repair and synaptic recovery. Moreover, reactive glial cells can modulate the processes of neurogenesis, neuronal differentiation, and migration of neurons in the existing neural circuits in the adult brain. By discovering precise signals within specific niches, the regulation of sequential processes in adult neurogenesis holds the potential to unlock strategies that can stimulate the generation of functional neurons, whether in response to injury or as a means of addressing degenerative neurological conditions. Cerebral ischemic stroke, a condition falling within the realm of acute vascular disorders affecting the circulation in the brain, stands as a prominent global cause of disability and mortality. Extensive investigations into glial plasticity and their intricate interactions with other cells in the central nervous system have predominantly relied on studies conducted on experimental animals, including rodents and primates. However, valuable insights have also been gleaned from in vivo studies involving poststroke patients, utilizing highly specialized imaging techniques. Following the attempts to map brain cells, the role of various transcription factors in modulating gene expression in response to cerebral ischemia is gaining increasing popularity. Although the results obtained thus far remain incomplete and occasionally ambiguous, they serve as a solid foundation for the development of strategies aimed at influencing the recovery process after ischemic brain injury.

Exploring the origin of the cancer stem cell niche and its role in anti-angiogenic treatment for glioblastoma

Cancer stem cells are thought to be the main drivers of tumorigenesis for malignancies such as glioblastoma (GBM). They are maintained through a close relationship with the tumor vasculature. Previous literature has well-characterized the components and signaling pathways for maintenance of this stem cell niche, but details on how the niche initially forms are limited. This review discusses development of the nonmalignant neural and hematopoietic stem cell niches in order to draw important parallels to the malignant environment. We then discuss what is known about the cancer stem cell niche, its relationship with angiogenesis, and provide a hypothesis for its development in GBM. A better understanding of the mechanisms of development of the tumor stem cell niche may provide new insights to potentially therapeutically exploit.

A fly's eye view of quiescent neural stem cells

The balance between proliferation and quiescence of stem cells is crucial in maintaining tissue homeostasis. Neural stem cells (NSCs) in the brain have the ability to be reactivated from a reversible quiescent state to generate new neurons. However, how NSCs transit between quiescence and reactivation remains largely elusive. Drosophila larval brain NSCs, also known as neuroblasts, have emerged as an excellent in vivo model to study molecular mechanisms underlying NSC quiescence and reactivation. Here, we discuss our current understanding of the molecular mechanisms underlying the reactivation of quiescent NSCs in Drosophila. We review the most recent advances on epigenetic regulations and microtubule cytoskeleton in Drosophila quiescent NSCs and their cross-talk with signaling pathways that are required in regulating NSC reactivation.

The Fly Blood-Brain Barrier Fights Against Nutritional Stress

In the wild, animals face different challenges including multiple events of food scarcity. How they overcome these conditions is essential for survival. Thus, adaptation mechanisms evolved to allow the development and survival of an organism during nutrient restriction periods. Given the high energy demand of the nervous system, the molecular mechanisms of adaptation to malnutrition are of great relevance to fuel the brain. The blood-brain barrier (BBB) is the interface between the central nervous system (CNS) and the circulatory system. The BBB mediates the transport of macromolecules in and out of the CNS, and therefore, it can buffer changes in nutrient availability. In this review, we collect the current evidence using the fruit fly, Drosophila melanogaster, as a model of the role of the BBB in the adaptation to starvation. We discuss the role of the Drosophila BBB during nutrient deprivation as a potential sensor for circulating nutrients, and transient nutrient storage as a regulator of the CNS neurogenic niche.

Vascularization of iNSC spheroid in a 3D spheroid-on-a-chip platform enhances neural maturation

In vitro platforms for studying the human brain have been developed, and brain organoids derived from stem cells have been studied. However, current organoid models lack three-dimensional (3D) vascular networks, limiting organoid proliferation, differentiation, and apoptosis. In this study, we created a 3D model of vascularized spheroid cells using an injection-molded microfluidic chip. We cocultured spheroids derived from induced neural stem cells (iNSCs) with perfusable blood vessels. Gene expression analysis and immunostaining revealed that the vascular network greatly enhanced spheroid differentiation and reduced apoptosis. This platform can be used to further study the functional and structural interactions between blood vessels and neural spheroids, and ultimately to simulate brain development and disease.

NSCs Under Strain-Unraveling the Mechanoprotective Role of Differentiating Astrocytes in a Cyclically Stretched Coculture With Differentiating Neurons

The neural stem cell (NSC) niche is a highly vascularized microenvironment that supplies stem cells with relevant biological and chemical cues. However, the NSCs’ proximity to the vasculature also means that the NSCs are subjected to permanent tissue deformation effected by the vessels’ heartbeat-induced pulsatile movements. Cultivating NSCs under common culture conditions neglects the—yet unknown—influence of this cyclic mechanical strain on neural stem cells. Under the hypothesis that pulsatile strain should affect essential NSC functions, a cyclic uniaxial strain was applied under biomimetic conditions using an in-house developed stretching system based on cross-linked polydimethylsiloxane (PDMS) elastomer. While lineage commitment remained unaffected by cyclic deformation, strain affected NSC quiescence and cytoskeletal organization. Unexpectedly, cyclically stretched stem cells aligned in stretch direction, a phenomenon unknown for other types of cells in the mammalian organism. The same effect was observed for young astrocytes differentiating from NSCs. In contrast, young neurons differentiating from NSCs did not show mechanoresponsiveness. The exceptional orientation of NSCs and young astrocytes in the stretch direction was blocked upon RhoA activation and went along with a lack of stress fibers. Compared to postnatal astrocytes and mature neurons, NSCs and their young progeny displayed characteristic and distinct mechanoresponsiveness. Data suggest a protective role of young astrocytes in mixed cultures of differentiating neurons and astrocytes by mitigating the mechanical stress of pulsatile strain on developing neurons.

Msps governs acentrosomal microtubule assembly and reactivation of quiescent neural stem cells

The ability of stem cells to switch between quiescence and proliferation is crucial for tissue homeostasis and regeneration. Drosophila quiescent neural stem cells (NSCs) extend a primary cellular protrusion from the cell body prior to their reactivation. However, the structure and function of this protrusion are not well established. Here, we show that in the protrusion of quiescent NSCs, microtubules are predominantly acentrosomal and oriented plus‐end‐out toward the tip of the primary protrusion. We have identified Mini Spindles (Msps)/XMAP215 as a key microtubule regulator in quiescent NSCs that governs NSC reactivation via regulating acentrosomal microtubule growth and orientation. We show that quiescent NSCs form membrane contact with the neuropil and E‐cadherin, a cell adhesion molecule, localizes to these NSC‐neuropil junctions. Msps and a plus‐end directed motor protein Kinesin‐2 promote NSC cell cycle re‐entry and target E‐cadherin to NSC‐neuropil contact during NSC reactivation. Together, this work establishes acentrosomal microtubule organization in the primary protrusion of quiescent NSCs and the Msps‐Kinesin‐2 pathway that governs NSC reactivation, in part, by targeting E‐cad to NSC‐neuropil contact sites.

To Better Generate Organoids, What Can We Learn From Teratomas?

The genomic profile of animal models is not completely matched with the genomic profile of humans, and 2D cultures do not represent the cellular heterogeneity and tissue architecture found in tissues of their origin. Derived from 3D culture systems, organoids establish a crucial bridge between 2D cell cultures and in vivo animal models. Organoids have wide and promising applications in developmental research, disease modeling, drug screening, precision therapy, and regenerative medicine. However, current organoids represent only single or partial components of a tissue, which lack blood vessels, native microenvironment, communication with near tissues, and a continuous dorsal-ventral axis within 3D culture systems. Although efforts have been made to solve these problems, unfortunately, there is no ideal method. Teratoma, which has been frequently studied in pathological conditions, was recently discovered as a new in vivo model for developmental studies. In contrast to organoids, teratomas have vascularized 3D structures and regions of complex tissue-like organization. Studies have demonstrated that teratomas can be used to mimic multilineage human development, enrich specific somatic progenitor/stem cells, and even generate brain organoids. These results provide unique opportunities to promote our understanding of the vascularization and maturation of organoids. In this review, we first summarize the basic characteristics, applications, and limitations of both organoids and teratomas and further discuss the possibility that in vivo teratoma systems can be used to promote the vascularization and maturation of organoids within an in vitro 3D culture system.

Biomaterial Strategies to Bolster Neural Stem Cell-Mediated Repair of the Central Nervous System

Stem cell therapies have the potential to not only repair, but to regenerate tissue of the central nervous system (CNS). Recent studies demonstrate that transplanted stem cells can differentiate into neurons and integrate with the intact circuitry after traumatic injury. Unfortunately, the positive findings described in rodent models have not been replicated in clinical trials, where the burden to maintain the cell viability necessary for tissue repair becomes more challenging. Low transplant survival remains the greatest barrier to stem cell-mediated repair of the CNS, often with fewer than 1-2% of the transplanted cells remaining after 1 week. Strategic transplantation parameters, such as injection location, cell concentration, and transplant timing achieve only modest improvements in stem cell transplant survival and appear inconsistent across studies. Biomaterials provide researchers with a means to significantly improve stem cell transplant survival through two mechanisms: (1) a vehicle to deliver and protect the stem cells and (2) a substrate to control the cytotoxic injury environment. These biomaterial strategies can alleviate cell death associated with delivery to the injury and can be used to limit cell death after transplantation by limiting cell exposure to cytotoxic signals. Moreover, it is likely that control of the injury environment with biomaterials will lead to a more reliable support for transplanted cell populations. This review will highlight the challenges associated with cell delivery in the CNS and the advances in biomaterial development and deployment for stem cell therapies necessary to bolster stem cell-mediated repair.

Hair follicle stem cells as a skin-organizing signaling center during adult homeostasis

Stem cells are the essential source of building blocks for tissue homeostasis and regeneration. Their behavior is dictated by both cell-intrinsic cues and extrinsic cues from the microenvironment, known as the stem cell niche. Interestingly, recent work began to demonstrate that hair follicle stem cells (HFSCs) are not only passive recipients of signals from the surroundings, but also actively send out signals to modulate the organization and function of their own niches. Here, we discuss recent findings, and briefly refer to the old, on the interaction of HFSCs and their niches with the emphasis on the outwards signals from HFSCs toward their niches. We also highlight recent technology advancements that further promote our understanding of HFSC niches. Taken together, the HFSCs emerge as a skin-organizing center rich in signaling output for niche remodeling during various stages of adult skin homeostasis. The intricate crosstalk between HFSCs and their niches adds important insight to skin biology that will inform clinical and bioengineering fields aiming to build complete and functional 3D organotypic cultures for skin replacement therapies.

Histone lysine methyltransferase Pr-set7/SETD8 promotes neural stem cell reactivation

The ability of neural stem cells (NSCs) to switch between quiescence and proliferation is crucial for brain development and homeostasis. Increasing evidence suggests that variants of histone lysine methyltransferases including KMT5A are associated with neurodevelopmental disorders. However, the function of KMT5A/Pr-set7/SETD8 in the central nervous system is not well established. Here, we show that Drosophila Pr-Set7 is a novel regulator of NSC reactivation. Loss of function of pr-set7 causes a delay in NSC reactivation and loss of H4K20 monomethylation in the brain. Through NSC-specific in vivo profiling, we demonstrate that Pr-set7 binds to the promoter region of cyclin-dependent kinase 1 (cdk1) and Wnt pathway transcriptional co-activator earthbound1/jerky (ebd1). Further validation indicates that Pr-set7 is required for the expression of cdk1 and ebd1 in the brain. Similar to Pr-set7, Cdk1 and Ebd1 promote NSC reactivation. Finally, overexpression of Cdk1 and Ebd1 significantly suppressed NSC reactivation defects observed in pr-set7-depleted brains. Therefore, Pr-set7 promotes NSC reactivation by regulating Wnt signaling and cell cycle progression. Our findings may contribute to the understanding of mammalian KMT5A/PR-SET7/SETD8 during brain development.

Reconstituting neurovascular unit with primary neural stem cells and brain microvascular endothelial cells in three-dimensional matrix

Neurovascular dysfunction is a primary or secondary cause in the pathogenesis of several cerebrovascular and neurodegenerative disorders, including stroke. Therefore, the overall protection of the neurovascular unit (NVU) is a promising therapeutic strategy for various neurovascular diseases. However, the complexity of the NVU limits the study of the pathological mechanisms of neurovascular dysfunction. Reconstituting the in vitro NVU is important for the pathological study and drug screening of neurovascular diseases. In this study, we generated a spontaneously assembled three‐dimensional NVU (3D NVU) by employing the primary neural stem cells and brain microvascular endothelial cells in a Matrigel extracellular matrix platform. This novel model exhibits the fundamental structures and features of the NVU, including neurons, astrocytes, oligodendrocytes, vascular‐like structures, and blood–brain barrier‐like characteristics. Additionally, under oxygen‐glucose deprivation, the 3D NVU exhibits the neurovascular‐ or oxidative stress‐related pathological characteristics of cerebral ischemia and the injuries can be mitigated, respectively, by supplementing with the vascular endothelial growth factor or edaravone, which demonstrated that the availability of 3D NVU in ischemic stroke modeling. Finally, the 3D NVU promoted the angiogenesis and neurogenesis in the brain of cerebral ischemia rats. We expect that the proposed in vitro 3D NVU model will be widely used to investigate the relationships between angiogenesis and neurogenesis and to study the pathology and pharmacology of neurovascular diseases.

Neuroregeneration and functional recovery after stroke: advancing neural stem cell therapy toward clinical application

Stroke is a main cause of death and disability worldwide. The ability of the brain to self-repair in the acute and chronic phases after stroke is minimal; however, promising stem cell-based interventions are emerging that may give substantial and possibly complete recovery of brain function after stroke. Many animal models and clinical trials have demonstrated that neural stem cells (NSCs) in the central nervous system can orchestrate neurological repair through nerve regeneration, neuron polarization, axon pruning, neurite outgrowth, repair of myelin, and remodeling of the microenvironment and brain networks. Compared with other types of stem cells, NSCs have unique advantages in cell replacement, paracrine action, inflammatory regulation and neuroprotection. Our review summarizes NSC origins, characteristics, therapeutic mechanisms and repair processes, then highlights current research findings and clinical evidence for NSC therapy. These results may be helpful to inform the direction of future stroke research and to guide clinical decision-making.

Neurogenesis From Embryo to Adult - Lessons From Flies and Mice

The human brain is composed of billions of cells, including neurons and glia, with an undetermined number of subtypes. During the embryonic and early postnatal stages, the vast majority of these cells are generated from neural progenitors and stem cells located in all regions of the neural tube. A smaller number of neurons will continue to be generated throughout our lives, in localized neurogenic zones, mainly confined at least in rodents to the subependymal zone of the lateral ventricles and the subgranular zone of the hippocampal dentate gyrus. During neurogenesis, a combination of extrinsic cues interacting with temporal and regional intrinsic programs are thought to be critical for increasing neuronal diversity, but their underlying mechanisms need further elucidation. In this review, we discuss the recent findings in Drosophila and mammals on the types of cell division and cell interactions used by neural progenitors and stem cells to sustain neurogenesis, and how they are influenced by glia.

Vascularized human cortical organoids (vOrganoids) model cortical development in vivo

Modeling the processes of neuronal progenitor proliferation and differentiation to produce mature cortical neuron subtypes is essential for the study of human brain development and the search for potential cell therapies. We demonstrated a novel paradigm for the generation of vascularized organoids (vOrganoids) consisting of typical human cortical cell types and a vascular structure for over 200 days as a vascularized and functional brain organoid model. The observation of spontaneous excitatory postsynaptic currents (sEPSCs), spontaneous inhibitory postsynaptic currents (sIPSCs), and bidirectional electrical transmission indicated the presence of chemical and electrical synapses in vOrganoids. More importantly, single-cell RNA-sequencing analysis illustrated that vOrganoids exhibited robust neurogenesis and that cells of vOrganoids differentially expressed genes (DEGs) related to blood vessel morphogenesis. The transplantation of vOrganoids into the mouse S1 cortex resulted in the construction of functional human-mouse blood vessels in the grafts that promoted cell survival in the grafts. This vOrganoid culture method could not only serve as a model to study human cortical development and explore brain disease pathology but also provide potential prospects for new cell therapies for nervous system disorders and injury.

This study establishes a method to generate vascularized cortical organoids. This shows that in addition to reducing hypoxia and cell death, the vascular system promotes neural development in organoids. When transplanting these organoids into host mice, a graft-host vascular system could be reconstructed.

Single-Cell Analysis of Regional Differences in Adult V-SVZ Neural Stem Cell Lineages

The ventricular-subventricular zone (V-SVZ) harbors adult neural stem cells. V-SVZ neural stem cells exhibit features of astrocytes, have a regional identity, and depending on their location in the lateral or septal wall of the lateral ventricle, generate different types of neuronal and glial progeny. We performed large-scale single-cell RNA sequencing to provide a molecular atlas of cells from the lateral and septal adult V-SVZ of male and female mice. This revealed regional and sex differences among adult V-SVZ cells. We uncovered lineage potency bias at the single-cell level among lateral and septal wall astrocytes toward neurogenesis and oligodendrogenesis, respectively. Finally, we identified transcription factor co-expression modules marking key temporal steps in neurogenic and oligodendrocyte lineage progression. Our data suggest functionally important spatial diversity in neurogenesis and oligodendrogenesis in the adult brain and reveal molecular correlates of adult NSC dormancy and lineage specialization.

Mizrak et al. performed large-scale, single-cell RNA sequencing of the adult ventricular-subventricular zone neural stem cell niche. They identify regional differences between the lateral wall and septal wall, as well as sex differences in cell types and signaling pathways.

From Early to Late Neurogenesis: Neural Progenitors and the Glial Niche from a Fly's Point of View

Drosophila melanogaster is an important model organism used to study the brain development of organisms ranging from insects to mammals. The central nervous system in fruit flies is formed primarily in two waves of neurogenesis, one of which occurs in the embryo and one of which occurs during larval stages. In order to understand neurogenesis, it is important to research the behavior of progenitor cells that give rise to the neural networks which make up the adult nervous system. This behavior has been shown to be influenced by different factors including interactions with other cells within the progenitor niche, or local tissue microenvironment. Glial cells form a crucial part of this niche and play an active role in the development of the brain. Although in the early years of neuroscience it was believed that glia were simply scaffolding for neurons and passive components of the nervous system, their importance is nowadays recognized. Recent discoveries in progenitors and niche cells have led to new understandings of how the developing brain shapes its diverse regions. In this review, we attempt to summarize the distinct neural progenitors and glia in the Drosophila melanogaster central nervous system, from embryo to late larval stages, and make note of homologous features in mammals. We also outline the recent advances in this field in order to define the impact that glial cells have on progenitor cell niches, and we finally emphasize the importance of communication between glia and progenitor cells for proper brain formation.

Astrocytes' Contribution to Adult Neurogenesis in Physiology and Alzheimer's Disease

Adult neurogenesis is one of the most drastic forms of brain plasticity in adulthood and there is a growing body of evidence showing that, in the hippocampus, this process contributes to mechanisms of memory as well as depression. Interestingly, adult neurogenesis is tightly regulated by the neurogenic niche, which provides a structural and molecular scaffold for stem cell proliferation and the differentiation and functional integration of new neurons. In this review, we highlight the role of astrocytes in the regulation of adult neurogenesis in the context of cognitive function. We also discuss how the changes in astrocytes function may dysregulate adult neurogenesis and contribute to cognitive impairment in the context of Alzheimer's disease.

Propofol Regulates Neural Stem Cell Proliferation and Differentiation via Calmodulin-Dependent Protein Kinase II/AMPK/ATF5 Signaling Axis

BACKGROUND

Propofol can cause degeneration of developing brain cells and subsequent long-term learning or memory impairment. However, at the early stage of embryonic development, the molecular mechanism of propofol-induced inhibition in neural stem cells (NSCs) neurogenesis is still unclear. The aim of this study was to determine the role of propofol in NSCs neurogenesis and, more importantly, to explore the underlying mechanism.

METHODS

First, a single intraperitoneal injection of propofol was performed in pregnant mice, and 6 hours after administration of propofol, the hippocampus RNA and the protein of the embryos' brains was extracted to analyze the expression of neuron-specific markers. Second, the primary NSCs were isolated from the hippocampus of mouse embryonic brain and then treated with propofol for cell viability, immunostaining, and transwell assays; more importantly, we performed RNA sequencing (RNA-seq) and q-reverse transcription polymerase chain reaction assays to identify genes regulated by propofol; the Western blot, small interfering RNA (SiRNA), and luciferase reporter assays were used to study the effects of propofol on calmodulin-dependent protein kinase (CaMk) II/5' adenosine monophosphate-activated protein kinase (AMPK)/activating transcription factor 5 (ATF5) signaling pathway.

RESULTS

Our results indicated that propofol treatment could inhibit the proliferation, migration, and differentiation of NSCs. The results of RNA-seq assays showed that propofol treatment resulted in downregulation of a group of Ca-dependent genes. The following mechanism studies showed that propofol regulates the proliferation, differentiation, and migration of NSCs through the CaMkII/phosphorylation of serine at amino acid position 485 (pS485)/AMPK/ATF5 signaling pathway.

CONCLUSIONS

The results from study demonstrated that propofol inhibits the proliferation, differentiation, and migration of NSCs, and these effects are partially mediated by CaMkII/pS485/AMPK/ATF5 signaling pathway.

Playing Well with Others: Extrinsic Cues Regulate Neural Progenitor Temporal Identity to Generate Neuronal Diversity

During neurogenesis, vertebrate and Drosophila progenitors change over time as they generate a diverse population of neurons and glia. Vertebrate neural progenitors have long been known to use both progenitor-intrinsic and progenitor-extrinsic cues to regulate temporal patterning. In contrast, virtually all temporal patterning mechanisms discovered in Drosophila neural progenitors (neuroblasts) involve progenitor-intrinsic temporal transcription factor cascades. Recent results, however, have revealed several extrinsic pathways that regulate Drosophila neuroblast temporal patterning: nutritional cues regulate the timing of neuroblast proliferation/quiescence and a steroid hormone cue that is required for temporal transcription factor expression. Here, we discuss newly discovered extrinsic cues regulating neural progenitor temporal identity in Drosophila, highlight conserved mechanisms, and raise open questions for the future.

Bipotent progenitors as embryonic origin of retinal stem cells

In lower vertebrates, retinal stem cells (RSCs) capable of producing all retinal cell types are a resource for retinal tissue growth throughout life. However, the embryonic origin of RSCs remains largely elusive. Using a Zebrabow-based clonal analysis, we characterized the RSC niche in the ciliary marginal zone of zebrafish retina and illustrate that blood vessels associated with RSCs are required for the maintenance of actively proliferating RSCs. Full lineage analysis of RSC progenitors reveals lineage patterns of RSC production. Moreover, in vivo lineage analysis demonstrates that these RSC progenitors are the direct descendants of a set of bipotent progenitors in the medial epithelial layer of developing optic vesicles, suggesting the involvement of the mixed-lineage states in the RSC lineage specification.