Migratory Neural Crest Cells Phagocytose Dead Cells in the Developing Nervous System

Single-Cell Mapping of Brain Myeloid Cell Subsets Reveals Key Transcriptomic Changes Favoring Neuroplasticity after Ischemic Stroke

Interactions between brain-resident and peripheral infiltrated immune cells are thought to contribute to neuroplasticity after cerebral ischemia. However, conventional bulk sequencing makes it challenging to depict this complex immune network. Using single-cell RNA sequencing, we mapped compositional and transcriptional features of peri-infarct immune cells. Microglia were the predominant cell type in the peri-infarct region, displaying a more diverse activation pattern than the typical pro- and anti-inflammatory state, with axon tract-associated microglia (ATMs) being associated with neuronal regeneration. Trajectory inference suggested that infiltrated monocyte-derived macrophages (MDMs) exhibited a gradual fate trajectory transition to activated MDMs. Inter-cellular crosstalk between MDMs and microglia orchestrated anti-inflammatory and repair-promoting microglia phenotypes and promoted post-stroke neurogenesis, with SOX2 and related Akt/CREB signaling as the underlying mechanisms. This description of the brain's immune landscape and its relationship with neurogenesis provides new insight into promoting neural repair by regulating neuroinflammatory responses.

Glial plasticity at nervous system transition zones

The central and peripheral nervous systems (CNS and PNS, respectively) are two separate yet connected domains characterized by molecularly distinct cellular components that communicate via specialized structures called transition zones to allow information to travel from the CNS to the periphery, and vice versa. Until recently, nervous system transition zones were thought to be selectively permeable only to axons, and the establishment of the territories occupied by glial cells at these complex regions remained poorly described and not well understood. Recent work now demonstrates that transition zones are occupied by dynamic glial cells and are precisely regulated over the course of nervous system development. This review highlights recent work on glial cell migration in and out of the spinal cord, at motor exit point (MEP) and dorsal root entry zone (DREZ) transition zones, in the physiological and diseased nervous systems. These cells include myelinating glia (oligodendrocyte lineage cells, Schwann cells and motor exit point glia), exit glia, perineurial cells that form the perineurium along spinal nerves, as well as professional and non-professional phagocytes (microglia and neural crest cells).

Neural crest E-cadherin loss drives cleft lip/palate by epigenetic modulation via pro-inflammatory gene-environment interaction

Gene-environment interactions are believed to play a role in multifactorial phenotypes, although poorly described mechanistically. Cleft lip/palate (CLP), the most common craniofacial malformation, has been associated with both genetic and environmental factors, with little gene-environment interaction experimentally demonstrated. Here, we study CLP families harbouring CDH1/E-Cadherin variants with incomplete penetrance and we explore the association of pro-inflammatory conditions to CLP. By studying neural crest (NC) from mouse, Xenopus and humans, we show that CLP can be explained by a 2-hit model, where NC migration is impaired by a combination of genetic (CDH1 loss-of-function) and environmental (pro-inflammatory activation) factors, leading to CLP. Finally, using in vivo targeted methylation assays, we demonstrate that CDH1 hypermethylation is the major target of the pro-inflammatory response, and a direct regulator of E-cadherin levels and NC migration. These results unveil a gene-environment interaction during craniofacial development and provide a 2-hit mechanism to explain cleft lip/palate aetiology.

Abnormal chondrocyte development in a zebrafish model of cblC syndrome restored by an MMACHC cobalamin binding mutant

Variants in the MMACHC gene cause combined methylmalonic acidemia and homocystinuria cblC type, the most common inborn error of intracellular cobalamin (vitamin B12) metabolism. cblC is associated with neurodevelopmental, hematological, ocular, and biochemical abnormalities. In a subset of patients, mild craniofacial dysmorphia has also been described. Mouse models of Mmachc deletion are embryonic lethal but cause severe craniofacial phenotypes such as facial clefts. MMACHC encodes an enzyme required for cobalamin processing and variants in this gene result in the accumulation of two metabolites: methylmalonic acid (MMA) and homocysteine (HC). Interestingly, other inborn errors of cobalamin metabolism, such as cblX syndrome, are associated with mild facial phenotypes. However, the presence and severity of MMA and HC accumulation in cblX syndrome is not consistent with the presence or absence of facial phenotypes. Thus, the mechanisms by which mutations in MMACHC cause craniofacial defects are yet to be completely elucidated. Here we have characterized the craniofacial phenotypes in a zebrafish model of cblC (hg13) and performed restoration experiments with either a wildtype or a cobalamin binding deficient MMACHC protein. Homozygous mutants did not display gross morphological defects in facial development but did have abnormal chondrocyte nuclear organization and an increase in the average number of neighboring cell contacts, both phenotypes were fully penetrant. Abnormal chondrocyte nuclear organization was not associated with defects in the localization of neural crest specific markers, sox10 (RFP transgene) or barx1. Both nuclear angles and the number of neighboring cell contacts were fully restored by wildtype MMACHC and a cobalamin binding deficient variant of the MMACHC protein. Collectively, these data suggest that mutation of MMACHC causes mild to moderate craniofacial phenotypes that are independent of cobalamin binding.

A paradox promoted by microglia cannibalism shortens the lifespan of developmental microglia

The overproduction of cells and subsequent production of debris is a universal principle of neurodevelopment. Here we show an additional feature of the developing nervous system that causes neural debris - promoted by the sacrificial nature of embryonic microglia that irreversibly become phagocytic after clearing other neural debris. Described as long-lived, microglia colonize the embryonic brain and persist into adulthood. Using transgenic zebrafish to investigate the microglia debris during brain construction, we identified that unlike other neural cell-types that die in developmental stages after they have expanded, necroptotic-dependent microglial debris is prevalent when microglia are expanding in the zebrafish brain. Time-lapse imaging of microglia demonstrates that this debris is cannibalized by other microglia. To investigate features that promote microglia death and cannibalism, we used time-lapse imaging and fate-mapping strategies to track the lifespan of individual developmental microglia. These approaches revealed that instead of embryonic microglia being long-lived cells that completely digest their phagocytic debris, once most developmental microglia in zebrafish become phagocytic they eventually die, including ones that are cannibalistic. These results establish a paradox -- which we tested by increasing neural debris and manipulating phagocytosis -- that once most microglia in the embryo become phagocytic, they die, create debris and then are cannibalized by other microglia, resulting in more phagocytic microglia that are destined to die.

Self-organizing in vitro mouse neural tube organoids mimic embryonic development

The embryonic neural tube is the origin of the entire adult nervous system, and disturbances in its development cause life-threatening birth defects. However, the study of mammalian neural tube development is limited by the lack of physiologically realistic three-dimensional (3D) in vitro models. Here, we report a self-organizing 3D neural tube organoid model derived from single mouse embryonic stem cells that exhibits an in vivo-like tissue architecture, cell type composition and anterior-posterior (AP) patterning. Moreover, maturation of the neural tube organoids showed the emergence of multipotent neural crest cells and mature neurons. Single-cell transcriptome analyses revealed the sequence of transcriptional events in the emergence of neural crest cells and neural differentiation. Thanks to the accessibility of this model, phagocytosis of migrating neural crest cells could be observed in real time for the first time in a mammalian model. We thus introduce a tractable in vitro model to study some of the key morphogenetic and cell type derivation events during early neural development.

Transforming growth factor-beta signaling modulates perineurial glial bridging following peripheral spinal motor nerve injury in zebrafish

Spinal motor nerves are necessary for organismal locomotion and survival. In zebrafish and most vertebrates, these peripheral nervous system structures are composed of bundles of axons that naturally regenerate following injury. However, the cellular and molecular mechanisms that mediate this process are still only partially understood. Perineurial glia, which form a component of the blood-nerve barrier, are necessary for the earliest regenerative steps by establishing a glial bridge across the injury site as well as phagocytosing debris. Without perineurial glial bridging, regeneration is impaired. In addition to perineurial glia, Schwann cells, the cells that ensheath and myelinate axons within the nerve, are essential for debris clearance and axon guidance. In the absence of Schwann cells, perineurial glia exhibit perturbed bridging, demonstrating that these two cell types communicate during the injury response. While the presence and importance of perineurial glial bridging is known, the molecular mechanisms that underlie this process remain a mystery. Understanding the cellular and molecular interactions that drive perineurial glial bridging is crucial to unlocking the mechanisms underlying successful motor nerve regeneration. Using laser axotomy and in vivo imaging in zebrafish, we show that transforming growth factor-beta (TGFβ) signaling modulates perineurial glial bridging. Further, we identify connective tissue growth factor-a (ctgfa) as a downstream effector of TGF-β signaling that works in a positive feedback loop to mediate perineurial glial bridging. Together, these studies present a new signaling pathway involved in the perineurial glial injury response and further characterize the dynamics of the perineurial glial bridge.

Multi-layered transcriptional control of cranial neural crest development

The neural crest (NC) is an emblematic population of embryonic stem-like cells with remarkable migratory ability. These distinctive attributes have inspired the curiosity of developmental biologists for over 150 years, however only recently the regulatory mechanisms controlling the complex features of the NC have started to become elucidated at genomic scales. Regulatory control of NC development is achieved through combinatorial transcription factor binding and recruitment of associated transcriptional complexes to distal cis-regulatory elements. Together, they regulate when, where and to what extent transcriptional programmes are actively deployed, ultimately shaping ontogenetic processes. Here, we discuss how transcriptional networks control NC ontogeny, with a special emphasis on the molecular mechanisms underlying specification of the cephalic NC. We also cover emerging properties of transcriptional regulation revealed in diverse developmental systems, such as the role of three-dimensional conformation of chromatin, and how they are involved in the regulation of NC ontogeny. Finally, we highlight how advances in deciphering the NC transcriptional network have afforded new insights into the molecular basis of human diseases.

Cd59 and inflammation regulate Schwann cell development

Efficient neurotransmission is essential for organism survival and is enhanced by myelination. However, the genes that regulate myelin and myelinating glial cell development have not been fully characterized. Data from our lab and others demonstrates that cd59, which encodes for a small GPI-anchored glycoprotein, is highly expressed in developing zebrafish, rodent, and human oligodendrocytes (OLs) and Schwann cells (SCs), and that patients with CD59 dysfunction develop neurological dysfunction during early childhood. Yet, the function of Cd59 in the developing nervous system is currently undefined. In this study, we demonstrate that cd59 is expressed in a subset of developing SCs. Using cd59 mutant zebrafish, we show that developing SCs proliferate excessively and nerves may have reduced myelin volume, altered myelin ultrastructure, and perturbed node of Ranvier assembly. Finally, we demonstrate that complement activity is elevated in cd59 mutants and that inhibiting inflammation restores SC proliferation, myelin volume, and nodes of Ranvier to wildtype levels. Together, this work identifies Cd59 and developmental inflammation as key players in myelinating glial cell development, highlighting the collaboration between glia and the innate immune system to ensure normal neural development.

Temporal cell fate determination in the spinal cord is mediated by the duration of Notch signalling

During neural development, progenitor cells generate different types of neurons in specific time windows. Despite the characterisation of many of the transcription factor networks involved in these differentiation events, the mechanism behind their temporal regulation is poorly understood. To address this question, we studied the temporal differentiation of the simple lateral floor plate (LFP) domain in the zebrafish spinal cord. LFP progenitors generate both early-born Kolmer-Agduhr" (KA") interneuron and late-born V3 interneuron populations. Analysis using a Notch signalling reporter demonstrates that these cell populations have distinct Notch signalling profiles. Not only do V3 progenitors receive higher total levels of Notch response, but they collect this response over a longer duration compared to KA" progenitors. To test whether the duration of Notch signalling determines the temporal cell fate specification, we combined a transgene that constitutively activates Notch signalling in the ventral spinal cord with a heat shock inducible Notch signalling terminator to switch off Notch response at any given time. Sustained Notch signalling results in expanded LFP progenitors while KA" and V3 interneurons fail to specify. Early termination of Notch signalling leads to exclusively KA" cell fate, despite the high level of Notch signalling, whereas late attenuation of Notch signalling drives only V3 cell fate. This suggests that the duration of Notch signalling, not simply the level, mediates cell fate specification. Interestingly, knockdown experiments reveal a role for the Notch ligand Jag2b in maintaining LFP progenitors and limiting their differentiation into KA" and V3 interneurons. Our results indicate that Notch signalling is required for neural progenitor maintenance while a specific attenuation timetable defines the fate of the postmitotic progeny.

Clearing Your Mind: Mechanisms of Debris Clearance After Cell Death During Neural Development

Neurodevelopment and efferocytosis have fascinated scientists for decades. How an organism builds a nervous system that is precisely tuned for efficient behaviors and survival and how it simultaneously manages constant somatic cell turnover are complex questions that have resulted in distinct fields of study. Although neurodevelopment requires the overproduction of cells that are subsequently pruned back, very few studies marry these fields to elucidate the cellular and molecular mechanisms that drive nervous system development through the lens of cell clearance. In this review, we discuss these fields to highlight exciting areas of future synergy. We first review neurodevelopment from the perspective of overproduction and subsequent refinement and then discuss who clears this developmental debris and the mechanisms that control these events. We then end with how a more deliberate merger of neurodevelopment and efferocytosis could reframe our understanding of homeostasis and disease and discuss areas of future study. Expected final online publication date for the Annual Review of Neuroscience, Volume 45 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

A novel gene trap line for visualization and manipulation of erbb3b+ neural crest and glial cells in zebrafish

Glia are a diverse and essential cell type in the vertebrate nervous system. Transgenic tools and fluorescent reporter lines are critical resources to investigate how glial subtypes develop and function. However, despite the many lines available in zebrafish, the community still lacks the ability to label all unique stages of glial development and specific subpopulations of cells. To address this issue, we screened zebrafish gene and enhancer trap lines to find a novel reporter for peripheral glial subtypes. From these, we generated the gSAIzGFFD37A transgenic line that expresses GFP in neural crest cells and central and peripheral glia. We found that the gene trap construct is located within an intron of erbb3b, a gene essential for glial development. Additionally, we confirmed that GFP+ ​cells express erbb3b along with sox10, a known glial marker. From our screen, we have identified the gSAIzGFFD37A line as a novel and powerful tool for studying glia in the developing zebrafish, as well as a new resource to manipulate erbb3b+ ​cells.

The crucial role of model systems in understanding the complexity of cell signaling in human neurocristopathies

Animal models are useful to study the molecular, cellular, and morphogenetic mechanisms underlying normal and pathological development. Cell-based study models have emerged as an alternative approach to study many aspects of human embryonic development and disease. The neural crest (NC) is a transient, multipotent, and migratory embryonic cell population that generates a diverse group of cell types that arises during vertebrate development. The abnormal formation or development of the NC results in neurocristopathies (NCPs), which are characterized by a broad spectrum of functional and morphological alterations. The impaired molecular mechanisms that give rise to these multiphenotypic diseases are not entirely clear yet. This fact, added to the high incidence of these disorders in the newborn population, has led to the development of systematic approaches for their understanding. In this article, we have systematically reviewed the ways in which experimentation with different animal and cell model systems has improved our knowledge of NCPs, and how these advances might contribute to the development of better diagnostic and therapeutic tools for the treatment of these pathologies. This article is categorized under: Congenital Diseases > Genetics/Genomics/Epigenetics Congenital Diseases > Stem Cells and Development Congenital Diseases > Molecular and Cellular Physiology Neurological Diseases > Genetics/Genomics/Epigenetics.

Met is required for oligodendrocyte progenitor cell migration in Danio rerio

During vertebrate central nervous system development, most oligodendrocyte progenitor cells (OPCs) are specified in the ventral spinal cord and must migrate throughout the neural tube until they become evenly distributed, occupying non-overlapping domains. While this process of developmental OPC migration is well characterized, the nature of the molecular mediators that govern it remain largely unknown. Here, using zebrafish as a model, we demonstrate that Met signaling is required for initial developmental migration of OPCs, and, using cell-specific knock-down of Met signaling, show that Met acts cell-autonomously in OPCs. Taken together, these findings demonstrate in vivo, the role of Met signaling in OPC migration and provide new insight into how OPC migration is regulated during development.

Role of Maternal Infections and Inflammatory Responses on Craniofacial Development

Pregnancy is a tightly regulated immunological state. Mild environmental perturbations can affect the developing fetus significantly. Infections can elicit severe immunological cascades in the mother's body as well as the developing fetus. Maternal infections and resulting inflammatory responses can mediate epigenetic changes in the fetal genome, depending on the developmental stage. The craniofacial development begins at the early stages of embryogenesis. In this review, we will discuss the immunology of pregnancy and its responsive mechanisms on maternal infections. Further, we will also discuss the epigenetic effects of pathogens, their metabolites and resulting inflammatory responses on the fetus with a special focus on craniofacial development. Understanding the pathophysiological mechanisms of infections and dysregulated inflammatory responses during prenatal development could provide better insights into the origins of craniofacial birth defects.

Glutamate Signaling via the AMPAR Subunit GluR4 Regulates Oligodendrocyte Progenitor Cell Migration in the Developing Spinal Cord

Oligodendrocyte progenitor cells (OPCs) are specified from discrete precursor populations during gliogenesis and migrate extensively from their origins, ultimately distributing throughout the brain and spinal cord during early development. Subsequently, a subset of OPCs differentiates into mature oligodendrocytes, which myelinate axons. This process is necessary for efficient neuronal signaling and organism survival. Previous studies have identified several factors that influence OPC development, including excitatory glutamatergic synapses that form between neurons and OPCs during myelination. However, little is known about how glutamate signaling affects OPC migration before myelination. In this study, we use in vivo, time-lapse imaging in zebrafish in conjunction with genetic and pharmacological perturbation to investigate OPC migration and myelination when the GluR4A ionotropic glutamate receptor subunit is disrupted. In our studies, we observed that gria4a mutant embryos and larvae displayed abnormal OPC migration and altered dorsoventral distribution in the spinal cord. Genetic mosaic analysis confirmed that these effects were cell-autonomous, and we identified that voltage-gated calcium channels were downstream of glutamate receptor signaling in OPCs and could rescue the migration and myelination defects we observed when glutamate signaling was perturbed. These results offer new insights into the complex system of neuron-OPC interactions and reveal a cell-autonomous role for glutamatergic signaling in OPCs during neural development.SIGNIFICANCE STATEMENT The migration of oligodendrocyte progenitor cells (OPCs) is an essential process during development that leads to uniform oligodendrocyte distribution and sufficient myelination for central nervous system function. Here, we demonstrate that the AMPA receptor (AMPAR) subunit GluR4A is an important driver of OPC migration and myelination in vivo and that activated voltage-gated calcium channels are downstream of glutamate receptor signaling in mediating this migration.

Cell guidance on peptide micropatterned silk fibroin scaffolds

Guiding neuronal cell growth is desirable for neural tissue engineering but is very challenging. In this work, a self-assembling ultra-short surfactant-like peptide I₃K which possesses positively charged lysine head groups, and hydrophobic isoleucine tails, was chosen to investigate its potential for guiding neuronal cell growth. The peptides were able to self-assemble into nanofibrous structures and interact strongly with silk fibroin (SF) scaffolds, providing a niche for neural cell attachment and proliferation. SF is an excellent biomaterial for tissue engineering. However neuronal cells, such as rat PC12 cells, showed poor attachment on pure regenerated SF (RSF) scaffold surfaces. Patterning of I₃K peptide nanofibers on RSF surfaces significantly improved cellular attachment, cellular density, as well as morphology of PC12 cells. The live / dead assay confirmed that RSF and I₃K have negligible cytotoxicity against PC12 cells. Atomic force microscopy (AFM) was used to image the topography and neurite formation of PC12 cells, where results revealed that self-assembled I₃K nanofibers can support the formation of PC12 cell neurites. Immunolabelling also demonstrated that coating of I₃K nanofibers onto the RSF surfaces not only increased the percentage of cells bearing neurites but also increased the average maximum neurite length. Therefore, the peptide I₃K could be used as an alternative to poly-l-lysine for cell culture and tissue engineering applications. As micro-patterning of neural cells to guide neurite growth is important for developing nerve tissue engineering scaffolds, inkjet printing was used to pattern self-assembled I₃K peptide nanofibers on RSF surfaces for directional control of PC12 cell growth. The results demonstrated that inkjet-printed peptide micro-patterns can effectively guide the cell alignment and organization on RSF scaffold surfaces, providing great potential for nerve regeneration applications.

Exosomes derived from human placental mesenchymal stem cells enhanced the recovery of spinal cord injury by activating endogenous neurogenesis

Background

Spinal cord injury (SCI) is a debilitating medical condition that can result in the irreversible loss of sensorimotor function. Current therapies fail to provide an effective recovery being crucial to develop more effective approaches. Mesenchymal stem cell (MSC) exosomes have been shown to be able to facilitate axonal growth and act as mediators to regulate neurogenesis and neuroprotection, holding great therapeutic potential in SCI conditions. This study aimed to assess the potential of human placental MSC (hpMSC)-derived exosomes on the functional recovery and reactivation of endogenous neurogenesis in an experimental animal model of SCI and to explore the possible mechanisms involved.

Methods

The hpMSC-derived exosomes were extracted and transplanted in an experimental animal model of SCI with complete transection of the thoracic segment. Functional recovery, the expression of neural stem/progenitor cell markers and the occurrence of neurogenesis, was assessed 60 days after the treatment. In vitro, neural stem cells (NSCs) were incubated with the isolated exosomes for 24 h, and the phosphorylation levels of mitogen-activated protein kinase kinase (MEK), extracellular signal-regulated kinases (ERK), and cAMP response element binding (CREB) proteins were assessed by western blot.

Results

Exosomes were successfully isolated and purified from hpMSCs. Intravenous injections of these purified exosomes significantly improved the locomotor activity and bladder dysfunction of SCI animals. Further study of the exosomes’ therapeutic action revealed that hpMSC-derived exosomes promoted the activation of proliferating endogenous neural stem/progenitor cells as denoted by the significant increase of spinal SOX2⁺GFAP⁺, PAX6⁺Nestin⁺, and SOX1⁺KI67⁺ cells. Moreover, animals treated with exosomes exhibited a significative higher neurogenesis, as indicated by the higher percentage of DCX⁺MAP 2⁺ neurons. In vitro, hpMSC-derived exosomes promoted the proliferation of NSCs and the increase of the phosphorylated levels of MEK, ERK, and CREB.

Conclusions

This study provides evidence that the use of hpMSC-derived exosomes may constitute a promising therapeutic strategy for the treatment of SCI.

Spinal cord precursors utilize neural crest cell mechanisms to generate hybrid peripheral myelinating glia

During development, oligodendrocytes and Schwann cells myelinate central and peripheral nervous system axons, respectively, while motor exit point (MEP) glia are neural tube-derived, peripheral glia that myelinate axonal territory between these populations at MEP transition zones. From which specific neural tube precursors MEP glia are specified, and how they exit the neural tube to migrate onto peripheral motor axons, remain largely unknown. Here, using zebrafish, we found that MEP glia arise from lateral floor plate precursors and require foxd3 to delaminate and exit the spinal cord. Additionally, we show that similar to Schwann cells, MEP glial development depends on axonally derived neuregulin1. Finally, our data demonstrate that overexpressing axonal cues is sufficient to generate additional MEP glia in the spinal cord. Overall, these studies provide new insight into how a novel population of hybrid, peripheral myelinating glia are generated from neural tube precursors and migrate into the periphery.

Engulfed by Glia: Glial Pruning in Development, Function, and Injury across Species

Phagocytic activity of glial cells is essential for proper nervous system sculpting, maintenance of circuitry, and long-term brain health. Glial engulfment of apoptotic cells and superfluous connections ensures that neuronal connections are appropriately refined, while clearance of damaged projections and neurotoxic proteins in the mature brain protects against inflammatory insults. Comparative work across species and cell types in recent years highlights the striking conservation of pathways that govern glial engulfment. Many signaling cascades used during developmental pruning are re-employed in the mature brain to "fine tune" synaptic architecture and even clear neuronal debris following traumatic events. Moreover, the neuron-glia signaling events required to trigger and perform phagocytic responses are impressively conserved between invertebrates and vertebrates. This review offers a compare-and-contrast portrayal of recent findings that underscore the value of investigating glial engulfment mechanisms in a wide range of species and contexts.

Cooperative epithelial phagocytosis enables error correction in the early embryo

Errors in early embryogenesis are a cause of sporadic cell death and developmental failure^1,2. Phagocytic activity has a central role in scavenging apoptotic cells in differentiated tissues^3-6. However, how apoptotic cells are cleared in the blastula embryo in the absence of specialized immune cells remains unknown. Here we show that the surface epithelium of zebrafish and mouse embryos, which is the first tissue formed during vertebrate development, performs efficient phagocytic clearance of apoptotic cells through phosphatidylserine-mediated target recognition. Quantitative four-dimensional in vivo imaging analyses reveal a collective epithelial clearance mechanism that is based on mechanical cooperation by two types of Rac1-dependent basal epithelial protrusions. The first type of protrusion, phagocytic cups, mediates apoptotic target uptake. The second, a previously undescribed type of fast and extended actin-based protrusion that we call 'epithelial arms', promotes the rapid dispersal of apoptotic targets through Arp2/3-dependent mechanical pushing. On the basis of experimental data and modelling, we show that mechanical load-sharing enables the long-range cooperative uptake of apoptotic cells by multiple epithelial cells. This optimizes the efficiency of tissue clearance by extending the limited spatial exploration range and local uptake capacity of non-motile epithelial cells. Our findings show that epithelial tissue clearance facilitates error correction that is relevant to the developmental robustness and survival of the embryo, revealing the presence of an innate immune function in the earliest stages of embryonic development.

Apoptotic Bodies: Mechanism of Formation, Isolation and Functional Relevance

In the final stages of apoptosis, apoptotic cells can generate a variety of membrane-bound vesicles known as apoptotic extracellular vesicles (ApoEVs). Apoptotic bodies (ApoBDs), a major subset of ApoEVs, are formed through a process termed apoptotic cell disassembly characterised by a series of tightly regulated morphological steps including plasma membrane blebbing, apoptotic membrane protrusion formation and fragmentation into ApoBDs. To better characterise the properties of ApoBDs and elucidate their function, a number of methods including differential centrifugation, filtration and fluorescence-activated cell sorting were developed to isolate ApoBDs. Furthermore, it has become increasingly clear that ApoBD formation can contribute to various biological processes such as apoptotic cell clearance and intercellular communication. Together, recent literature demonstrates that apoptotic cell disassembly and thus, ApoBD formation, is an important process downstream of apoptotic cell death. In this chapter, we discuss the current understandings of the molecular mechanisms involved in regulating apoptotic cell disassembly, techniques for ApoBD isolation, and the functional roles of ApoBDs in physiological and pathological settings.

On the road again: Establishment and maintenance of stemness in the neural crest from embryo to adulthood

Unique to vertebrates, the neural crest (NC) is an embryonic stem cell population that contributes to a greatly expanding list of derivatives ranging from neurons and glia of the peripheral nervous system, facial cartilage and bone, pigment cells of the skin to secretory cells of the endocrine system. Here, we focus on what is specifically known about establishment and maintenance of NC stemness and ultimate fate commitment mechanisms, which could help explain its exceptionally high stem cell potential that exceeds the "rules set during gastrulation." In fact, recent discoveries have shed light on the existence of NC cells that coexpress commonly accepted pluripotency factors like Nanog, Oct4/PouV, and Klf4. The coexpression of pluripotency factors together with the exceptional array of diverse NC derivatives encouraged us to propose a new term "pleistopotent" (Greek for abundant, a substantial amount) to be used to reflect the uniqueness of the NC as compared to other post-gastrulation stem cell populations in the vertebrate body, and to differentiate them from multipotent lineage restricted stem cells. We also discuss studies related to the maintenance of NC stemness within the challenging context of being a transient and thus a constantly changing population of stem cells without a permanent niche. The discovery of the stem cell potential of Schwann cell precursors as well as multiple adult NC-derived stem cell reservoirs during the past decade has greatly increased our understanding of how NC cells contribute to tissues formed after its initial migration stage in young embryos.

The Neural Crest Pitches In to Remove Apoptotic Debris

In this issue of Cell, Zhu et al. show that in the developing zebrafish, neural crest cells can act as professional phagocytes and directionally approach apoptotic cells to clear the larval nervous system from cell debris.