Increasing Energetic Demands on Photoreceptors in Diabetes Corrects Retinal Lipid Dysmetabolism and Reduces Subsequent Microvascular Damage

Mechanisms responsible for the pathogenesis of diabetic retinal disease remain incompletely understood, but they likely involve multiple cellular targets, including photoreceptors. Evidence suggests that dysregulated de novo lipogenesis in photoreceptors is a critical early target of diabetes. Following on this observation, the present study aimed to determine whether two interventions shown to improve diabetic retinopathy in mice-pharmacologic visual cycle inhibition and prolonged dark adaptation-reduce photoreceptor anabolic lipid metabolism. Elevated retinal lipid biosynthetic signaling was observed in two mouse models of diabetes, with both models showing reduced retinal AMP-activated kinase (AMPK) signaling, elevated acetyl CoA carboxylase (ACC) signaling, and increased activity of fatty acid synthase, which promotes lipotoxicity in photoreceptors. Although retinal AMPK-ACC axis signaling was dependent on systemic glucose fluctuations in healthy animals, mice with diabetes lacked such regulation. Visual cycle inhibition and prolonged dark adaptation reversed abnormal retinal AMPK-ACC signaling in mice with diabetes. Although visual cycle inhibition reduced the severity of diabetic retinopathy in control mice, as assessed by retinal capillary atrophy, this intervention was ineffective in fatty acid synthase gain-of-function mice. These results suggest that early diabetic retinopathy is characterized by glucose-driven elevations in retinal lipid biosynthetic activity, and that two interventions known to increase photoreceptor glucose demands alleviate disease by reversing these signals.

Asymmetric inheritance of centrosomes maintains stem cell properties in human neural progenitor cells

During human forebrain development, neural progenitor cells (NPCs) in the ventricular zone (VZ) undergo asymmetric cell divisions to produce a self-renewed progenitor cell, maintaining the potential to go through additional rounds of cell divisions, and differentiating daughter cells, populating the developing cortex. Previous work in the embryonic rodent brain suggested that the preferential inheritance of the pre-existing (older) centrosome to the self-renewed progenitor cell is required to maintain stem cell properties, ensuring proper neurogenesis. If asymmetric segregation of centrosomes occurs in NPCs of the developing human brain, which depends on unique molecular regulators and species-specific cellular composition, remains unknown. Using a novel, recombination-induced tag exchange-based genetic tool to birthdate and track the segregation of centrosomes over multiple cell divisions in human embryonic stem cell-derived regionalised forebrain organoids, we show the preferential inheritance of the older mother centrosome towards self-renewed NPCs. Aberration of asymmetric segregation of centrosomes by genetic manipulation of the centrosomal, microtubule-associated protein Ninein alters fate decisions of NPCs and their maintenance in the VZ of human cortical organoids. Thus, the data described here use a novel genetic approach to birthdate centrosomes in human cells and identify asymmetric inheritance of centrosomes as a mechanism to maintain self-renewal properties and to ensure proper neurogenesis in human NPCs.

The CUL4B-based E3 ubiquitin ligase regulates mitosis and brain development by recruiting phospho-specific DCAFs

The paralogs CUL4A and CUL4B assemble cullin-RING E3 ubiquitin ligase (CRL) complexes regulating multiple chromatin-associated cellular functions. Although they are structurally similar, we found that the unique N-terminal extension of CUL4B is heavily phosphorylated during mitosis, and the phosphorylation pattern is perturbed in the CUL4B-P50L mutation causing X-linked intellectual disability (XLID). Phenotypic characterization and mutational analysis revealed that CUL4B phosphorylation is required for efficient progression through mitosis, controlling spindle positioning and cortical tension. While CUL4B phosphorylation triggers chromatin exclusion, it promotes binding to actin regulators and to two previously unrecognized CUL4B-specific substrate receptors (DCAFs), LIS1 and WDR1. Indeed, co-immunoprecipitation experiments and biochemical analysis revealed that LIS1 and WDR1 interact with DDB1, and their binding is enhanced by the phosphorylated N-terminal domain of CUL4B. Finally, a human forebrain organoid model demonstrated that CUL4B is required to develop stable ventricular structures that correlate with onset of forebrain differentiation. Together, our study uncovers previously unrecognized DCAFs relevant for mitosis and brain development that specifically bind CUL4B, but not the CUL4B-P50L patient mutant, by a phosphorylation-dependent mechanism.

Chronic in vivo imaging defines age-dependent alterations of neurogenesis in the mouse hippocampus

Neural stem cells (NSCs) generate new neurons throughout life in the mammalian hippocampus1. Advancing age leads to a decline in neurogenesis, which is associated with impaired cognition2,3. The cellular mechanisms causing reduced neurogenesis with advancing age remain largely unknown. We genetically labeled NSCs through conditional recombination driven by the regulatory elements of the stem-cell-expressed gene GLI family zinc finger 1 (Gli1) and used chronic intravital imaging to follow individual NSCs and their daughter cells over months within their hippocampal niche4,5. We show that aging affects multiple steps, from cell cycle entry of quiescent NSCs to determination of the number of surviving cells, ultimately causing reduced clonal output of individual NSCs. Thus, we here define the developmental stages that may be targeted to enhance neurogenesis with the aim of maintaining hippocampal plasticity with advancing age.

Cell population dynamics in the course of adult hippocampal neurogenesis: Remaining unknowns

Neural stem cells (NSCs) generate new neurons throughout life in the mammalian hippocampus. The distinct developmental steps in the course of adult neurogenesis, including NSC activation, expansion, and neuronal integration, are increasingly well characterized down to the molecular level. However, substantial gaps remain in our knowledge about regulators and mechanisms involved in this biological process. This review highlights three long-standing unknowns. First, we discuss potency and identity of NSCs and the quest for a unifying model of short- and long-term self-renewal dynamics. Next, we examine cell death, specifically focusing on the early demise of newborn cells. Then, we outline the current knowledge on cell integration dynamics, discussing which (if any) neurons are replaced by newly added neurons in the hippocampal circuits. For each of these unknowns, we summarize the trajectory of studies leading to the current state of knowledge. Finally, we offer suggestions on how to fill the remaining gaps by taking advantage of novel technology to reveal currently hidden secrets in the course of adult hippocampal neurogenesis.

Human neural progenitors establish a diffusion barrier in the endoplasmic reticulum membrane during cell division

Asymmetric segregation of cellular components regulates the fate and behavior of somatic stem cells. Similar to dividing budding yeast and precursor cells in Caenorhabditis elegans, it has been shown that mouse neural progenitors establish a diffusion barrier in the membrane of the endoplasmic reticulum (ER), which has been associated with asymmetric partitioning of damaged proteins and cellular age. However, the existence of an ER diffusion barrier in human cells remains unknown. Here, we used fluorescence loss in photobleaching (FLIP) imaging to show that human embryonic stem cell (hESC)- and induced pluripotent stem cell (iPSC)-derived neural progenitor cells establish an ER diffusion barrier during cell division. The human ER diffusion barrier is regulated via lamin-dependent mechanisms and is associated with asymmetric segregation of mono- and polyubiquitylated damaged proteins. Further, forebrain regionalized organoids derived from hESCs were used to show the establishment of an ER membrane diffusion barrier in more naturalistic tissues, mimicking early steps of human brain development. Thus, the data provided here show that human neural progenitors establish a diffusion barrier during cell division in the membrane of the ER, which may allow for asymmetric segregation of cellular components, contributing to the fate and behavior of human neural progenitor cells.

Summary: Asymmetric segregation of cellular components, which may contribute to the fate and behavior of human neural progenitors, is allowed by a diffusion barrier in the endoplasmic reticulum membrane during cell division.

Injection and electroporation of plasmid DNA into human cortical organoids

Pluripotent stem cell-derived human cortical organoids allow for the analysis of stem cell behavior and neurogenesis in neural tissues. Delivery of plasmid DNA into organoids permits visualization of individual cells, characterization of cellular components, and manipulation of gene expression. We describe a protocol to transfect cells inside organoids with plasmid DNA using micro-injection and electroporation, allowing for DNA delivery to cells within cortical units. This protocol was optimized for cortical organoids; however, it may be adapted to other organoid models. For complete details on the use and execution of this protocol, please refer to Denoth-Lippuner et al. (2021).

Characterization of the neurogenic niche in the aging dentate gyrus using iterative immunofluorescence imaging

Advancing age causes reduced hippocampal neurogenesis, associated with age-related cognitive decline. The spatial relationship of age-induced alterations in neural stem cells (NSCs) and surrounding cells within the hippocampal niche remains poorly understood due to limitations of antibody-based cellular phenotyping. We established iterative indirect immunofluorescence imaging (4i) in tissue sections, allowing for simultaneous detection of 18 proteins to characterize NSCs and surrounding cells in 2-, 6-, and 12-month-old mice. We show that reorganization of the dentate gyrus (DG) niche already occurs in middle-aged mice, paralleling the decline in neurogenesis. 4i-based tissue analysis of the DG identifies changes in cell-type contributions to the blood-brain barrier and microenvironments surrounding NSCs to play a pivotal role to preserve neurogenic permissiveness. The data provided represent a resource to characterize the principles causing alterations of stem cell-associated plasticity within the aging DG and provide a blueprint to analyze somatic stem cell niches across lifespan in complex tissues.

Visualization of individual cell division history in complex tissues using iCOUNT

The division potential of individual stem cells and the molecular consequences of successive rounds of proliferation remain largely unknown. Here, we developed an inducible cell division counter (iCOUNT) that reports cell division events in human and mouse tissues in vitro and in vivo. Analyzing cell division histories of neural stem/progenitor cells (NSPCs) in the developing and adult brain, we show that iCOUNT can provide novel insights into stem cell behavior. Further, we use single-cell RNA sequencing (scRNA-seq) of iCOUNT-labeled NSPCs and their progenies from the developing mouse cortex and forebrain-regionalized human organoids to identify functionally relevant molecular pathways that are commonly regulated between mouse and human cells, depending on individual cell division histories. Thus, we developed a tool to characterize the molecular consequences of repeated cell divisions of stem cells that allows an analysis of the cellular principles underlying tissue formation, homeostasis, and repair.

Glucose-mediated de novo lipogenesis in photoreceptors drives early diabetic retinopathy

Diabetic retinopathy (DR) is an increasingly frequent cause of blindness across populations; however, the events that initiate pathophysiology of DR remain elusive. Strong preclinical and clinical evidence suggests that abnormalities in retinal lipid metabolism caused by diabetes may account for the origin of this disease. A major arm of lipid metabolism, de novo biosynthesis, is driven by elevation in available glucose, a common thread binding all forms of vision loss in diabetes. Therefore, we hypothesized that aberrant retinal lipid biogenesis is an important promoter of early DR. In murine models, we observed elevations of diabetes-associated retinal de novo lipogenesis ∼70% over control levels. This shift was primarily because of activation of fatty acid synthase (FAS), a rate-limiting enzyme in the biogenic pathway. Activation of FAS was driven by canonical glucose-mediated disinhibition of acetyl-CoA carboxylase, a major upstream regulatory enzyme. Mutant mice expressing gain-of-function FAS demonstrated increased vulnerability to DR, whereas those with FAS deletion in rod photoreceptors maintained preserved visual responses upon induction of diabetes. Excess retinal de novo lipogenesis—either because of diabetes or because of FAS gain of function—was associated with modestly increased levels of palmitate-containing phosphatidylcholine species in synaptic membranes, a finding with as yet uncertain significance. These findings implicate glucose-dependent increases in photoreceptor de novo lipogenesis in the early pathogenesis of DR, although the mechanism of deleterious action of this pathway remains unclear.

Isolation of adult mouse hippocampal neural stem cells for fluorescence loss in photobleaching assays

This protocol describes the isolation and culturing of primary neural stem cells (NSCs) from the adult mouse hippocampus, followed by the experimental approach for fluorescence loss in photobleaching assays, previously used to characterize the presence of an endoplasmic reticulum (ER) membrane diffusion barrier. The assay described here can be used to study live asymmetry in the ER membrane or other organelles that is established in dividing NSCs.

For complete details on the use and execution of this protocol, please refer to Clay et al. (2014); bin Imtiaz et al. (2021); Lee et al. (2016); Luedeke et al. (2005); Moore et al. (2015); Shcheprova et al. (2008).

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Protocol describes the isolation of hippocampal neural stem cells (NSCs)

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Approach to culture NSCs for fluorescence loss in photobleaching (FLIP) assays

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FLIP assays of NSCs explained step by step

Declining lamin B1 expression mediates age-dependent decreases of hippocampal stem cell activity

Neural stem cells (NSCs) generate neurons throughout life in the hippocampal dentate gyrus. With advancing age, levels of neurogenesis sharply drop, which has been associated with a decline in hippocampal memory function. However, cell-intrinsic mechanisms mediating age-related changes in NSC activity remain largely unknown. Here, we show that the nuclear lamina protein lamin B1 (LB1) is downregulated with age in mouse hippocampal NSCs, whereas protein levels of SUN-domain containing protein 1 (SUN1), previously implicated in Hutchinson-Gilford progeria syndrome (HGPS), increase. Balancing the levels of LB1 and SUN1 in aged NSCs restores the strength of the endoplasmic reticulum diffusion barrier that is associated with segregation of aging factors in proliferating NSCs. Virus-based restoration of LB1 expression in aged NSCs enhances stem cell activity in vitro and increases progenitor cell proliferation and neurogenesis in vivo. Thus, we here identify a mechanism that mediates age-related decline of neurogenesis in the mammalian hippocampus.

Long-term self-renewing stem cells in the adult mouse hippocampus identified by intravital imaging

Neural stem cells (NSCs) generate neurons throughout life in the mammalian hippocampus. However, the potential for long-term self-renewal of individual NSCs within the adult brain remains unclear. We used two-photon microscopy and followed NSCs that were genetically labeled through conditional recombination driven by the regulatory elements of the stem cell-expressed genes GLI family zinc finger 1 (Gli1) or achaete-scute homolog 1 (Ascl1). Through intravital imaging of NSCs and their progeny, we identify a population of Gli1-targeted NSCs showing long-term self-renewal in the adult hippocampus. In contrast, once activated, Ascl1-targeted NSCs undergo limited proliferative activity before they become exhausted. Using single-cell RNA sequencing, we show that Gli1- and Ascl1-targeted cells have highly similar yet distinct transcriptional profiles, supporting the existence of heterogeneous NSC populations with diverse behavioral properties. Thus, we here identify long-term self-renewing NSCs that contribute to the generation of new neurons in the adult hippocampus.

How stem cells remember their past

Somatic stem cells are required for tissue development, homeostasis, and repair. Recent data suggested that previous biographical experiences of individual stem cells influence their behavior in the context of tissue formation and govern stem cell responses to external stimuli. Here we provide a concise review how a cell's biography, for example, previous rounds of cell divisions or the age-dependent accumulation of cellular damage, is remembered in stem cells and how previous experiences affect the segregation of cellular components, thus guiding cellular behavior in vertebrate stem cells. Further, we suggest future directions of research that may help to unravel the molecular underpinnings of how past experiences guide future cellular behavior.

Miniaturization of Smart-seq2 for Single-Cell and Single-Nucleus RNA Sequencing

This protocol presents a plate-based workflow to perform RNA sequencing analysis of single cells/nuclei using Smart-seq2. We describe (1) the dissociation procedures for cell/nucleus isolation from the mouse brain and human organoids, (2) the flow sorting of single cells/nuclei into 384-well plates, and (3) the preparation of libraries following miniaturization of the Smart-seq2 protocol using a liquid-handling robot. This pipeline allows for the reliable, high-throughput, and cost-effective preparation of mouse and human samples for full-length deep single-cell/nucleus RNA sequencing. For complete details on the use and execution of this protocol, please refer to Bowers et al. (2020).

A Single Metabolite which Modulates Lipid Metabolism Alters Hematopoietic Stem/Progenitor Cell Behavior and Promotes Lymphoid Reconstitution

Fatty acid β-oxidation (FAO), the breakdown of lipids, is a metabolic pathway used by various stem cells. FAO levels are generally high during quiescence and downregulated with proliferation. The endogenous metabolite malonyl-CoA modulates lipid metabolism as a reversible FAO inhibitor and as a substrate for de novo lipogenesis. Here we assessed whether malonyl-CoA can be exploited to steer the behavior of hematopoietic stem/progenitor cells (HSPCs), quiescent stem cells of clinical relevance. Treatment of mouse HSPCs in vitro with malonyl-CoA increases HSPC numbers compared with nontreated controls and ameliorates blood reconstitution capacity when transplanted in vivo, mainly through enhanced lymphoid reconstitution. Similarly, human HSPC numbers also increase upon malonyl-CoA treatment in vitro. These data corroborate that lipid metabolism can be targeted to direct cell fate and stem cell proliferation. Physiological modulation of metabolic pathways, rather than genetic or pharmacological inhibition, provides unique perspectives for stem cell manipulations in health and disease.

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Modulating lipid metabolism with malonyl-CoA increases murine HSPCs in vitro

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Malonyl-CoA promotes lymphoid cell fate of HSPCs in vitro and in vivo

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The myeloid bias in aged HSPCs is partially recovered with malonyl-CoA exposure

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Malonyl-CoA increases the numbers of human HSPCs in vitro

FASN-Dependent Lipid Metabolism Links Neurogenic Stem/Progenitor Cell Activity to Learning and Memory Deficits

Altered neural stem/progenitor cell (NSPC) activity and neurodevelopmental defects are linked to intellectual disability. However, it remains unclear whether altered metabolism, a key regulator of NSPC activity, disrupts human neurogenesis and potentially contributes to cognitive defects. We investigated links between lipid metabolism and cognitive function in mice and human embryonic stem cells (hESCs) expressing mutant fatty acid synthase (FASN; R1819W), a metabolic regulator of rodent NSPC activity recently identified in humans with intellectual disability. Mice homozygous for the FASN R1812W variant have impaired adult hippocampal NSPC activity and cognitive defects because of lipid accumulation in NSPCs and subsequent lipogenic ER stress. Homozygous FASN R1819W hESC-derived NSPCs show reduced rates of proliferation in embryonic 2D cultures and 3D forebrain regionalized organoids, consistent with a developmental phenotype. These data from adult mouse models and in vitro models of human brain development suggest that altered lipid metabolism contributes to intellectual disability.

Palmitoylation of BMPR1a regulates neural stem cell fate

Neural stem cells (NSCs) generate neurons and glial cells throughout embryonic and postnatal brain development. The role of S-palmitoylation (also referred to as S-acylation), a reversible posttranslational lipid modification of proteins, in regulating the fate and activity of NSCs remains largely unknown. We used an unbiased screening approach to identify proteins that are S-acylated in mouse NSCs and showed that bone morphogenic protein receptor 1a (BMPR1a), a core mediator of BMP signaling, is palmitoylated. Genetic manipulation of S-acylated sites affects the localization and trafficking of BMPR1a and leads to altered BMP signaling. Strikingly, defective palmitoylation of BMPR1a modulates NSC function within the mouse brain, resulting in enhanced oligodendrogenesis. Thus, we identified a mechanism regulating the behavior of NSCs and provided the framework to characterize dynamic posttranslational lipid modifications of proteins in the context of NSC biology.

Sleep or deplete: how the choroid plexus helps to keep neural stem cells in balance

The activity of stem cells in the adult brain is controlled by various niche-dependent mechanisms. A new article by Lepko et al (2019) shows that proliferation of neural stem cells in the ventricular-subventricular zone is regulated by choroid plexus-derived miR-204, identifying a novel mechanism of how the delicate balance between stem cell quiescence and activation is controlled.

Mechanisms of cellular rejuvenation

Aging leads to changes on an organismal but also cellular level. However, the exact mechanisms of cellular aging in mammals remain poorly understood and the identity and functional role of aging factors, some of which have previously been defined in model organisms such as Saccharomyces cerevisiae, remain elusive. Remarkably, during cellular reprogramming most if not all aging hallmarks are erased, offering a novel entry point to study aging and rejuvenation on a cellular level. On the other hand, direct reprogramming of old cells into cells of a different fate preserves many aging signs. Therefore, investigating the process of reprogramming and comparing it to direct reprogramming may yield novel insights about the clearing of aging factors, which is the basis of rejuvenation. Here, we discuss how reprogramming might lead to rejuvenation of a cell, an organ, or even the whole organism.

Life Science Alliance, from the Academic Editors

Life Science Alliance is a community-driven journal that brings together research from diverse fields. The journal is first-of-its-kind in uniting not-for-profit publishers and employing the scientific community to change the way authors can publish their work.

Quiescent Endothelial Cells Upregulate Fatty Acid β-Oxidation for Vasculoprotection via Redox Homeostasis

Little is known about the metabolism of quiescent endothelial cells (QECs). Nonetheless, when dysfunctional, QECs contribute to multiple diseases. Previously, we demonstrated that proliferating endothelial cells (PECs) use fatty acid β-oxidation (FAO) for de novo dNTP synthesis. We report now that QECs are not hypometabolic, but upregulate FAO >3-fold higher than PECs, not to support biomass or energy production but to sustain the tricarboxylic acid cycle for redox homeostasis through NADPH regeneration. Hence, endothelial loss of FAO-controlling CPT1A in CPT1A^ΔEC mice promotes EC dysfunction (leukocyte infiltration, barrier disruption) by increasing endothelial oxidative stress, rendering CPT1A^ΔEC mice more susceptible to LPS and inflammatory bowel disease. Mechanistically, Notch1 orchestrates the use of FAO for redox balance in QECs. Supplementation of acetate (metabolized to acetyl-coenzyme A) restores endothelial quiescence and counters oxidative stress-mediated EC dysfunction in CPT1A^ΔEC mice, offering therapeutic opportunities. Thus, QECs use FAO for vasculoprotection against oxidative stress-prone exposure.

A Fatty Acid Oxidation-Dependent Metabolic Shift Regulates Adult Neural Stem Cell Activity

Hippocampal neurogenesis is important for certain forms of cognition, and failing neurogenesis has been implicated in neuropsychiatric diseases. The neurogenic capacity of hippocampal neural stem/progenitor cells (NSPCs) depends on a balance between quiescent and proliferative states. Here, we show that the rate of fatty acid oxidation (FAO) regulates the activity of NSPCs. Quiescent NSPCs show high levels of carnitine palmitoyltransferase 1a (Cpt1a)-dependent FAO, which is downregulated in proliferating NSPCs. Pharmacological inhibition and conditional deletion of Cpt1a in vitro and in vivo leads to altered NSPC behavior, showing that Cpt1a-dependent FAO is required for stem cell maintenance and proper neurogenesis. Strikingly, manipulation of malonyl-CoA, the metabolite that regulates levels of FAO, is sufficient to induce exit from quiescence and to enhance NSPC proliferation. Thus, the data presented here identify a shift in FAO metabolism that governs NSPC behavior and suggest an instructive role for fatty acid metabolism in regulating NSPC activity.

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A metabolic shift defines NSPC quiescence versus proliferation

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Quiescent NSPCs require high levels of FAO

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Changing levels of a single metabolite is sufficient to induce NSPC proliferation

Human Adult Neurogenesis: Evidence and Remaining Questions

Renewed discussion about whether or not adult neurogenesis exists in the human hippocampus, and the nature and strength of the supporting evidence, has been reignited by two prominently published reports with opposite conclusions. Here, we summarize the state of the field and argue that there is currently no reason to abandon the idea that adult-generated neurons make important functional contributions to neural plasticity and cognition across the human lifespan.

Live imaging of neurogenesis in the adult mouse hippocampus

Neural stem and progenitor cells (NSPCs) generate neurons throughout life in the mammalian hippocampus. We used chronic in vivo imaging and followed genetically labeled individual NSPCs and their progeny in the mouse hippocampus for up to 2 months. We show that NSPCs targeted by the endogenous Achaete-scute homolog 1 (Ascl1) promoter undergo limited rounds of symmetric and asymmetric divisions, eliciting a burst of neurogenic activity, after which they are lost. Further, our data reveal unexpected asymmetric divisions of nonradial glia-like NSPCs. Cell fates of Ascl1-labeled lineages suggest a developmental-like program involving a sequential transition from a proliferative to a neurogenic phase. By providing a comprehensive description of lineage relationships, from dividing NSPCs to newborn neurons integrating into the hippocampal circuitry, our data offer insight into how NSPCs support life-long hippocampal neurogenesis.

De novo fatty acid synthesis by Schwann cells is essential for peripheral nervous system myelination

Montani et al. reveal that de novo fatty acid synthesis by Schwann cells, mediated by fatty acid synthase, contributes fundamentally to driving myelination in the peripheral nervous system. They identify lipogenic activation of the PPARγ transcriptional network as a putatively involved functional mechanism.

Myelination calls for a remarkable surge in cell metabolism to facilitate lipid and membrane production. Endogenous fatty acid (FA) synthesis represents a potentially critical process in myelinating glia. Using genetically modified mice, we show that Schwann cell (SC) intrinsic activity of the enzyme essential for de novo FA synthesis, fatty acid synthase (FASN), is crucial for precise lipid composition of peripheral nerves and fundamental for the correct onset of myelination and proper myelin growth. Upon FASN depletion in SCs, epineurial adipocytes undergo lipolysis, suggestive of a compensatory role. Mechanistically, we found that a lack of FASN in SCs leads to an impairment of the peroxisome proliferator-activated receptor (PPAR) γ–regulated transcriptional program. In agreement, defects in myelination of FASN-deficient SCs could be ameliorated by treatment with the PPARγ agonist rosiglitazone ex vivo and in vivo. Our results reveal that FASN-driven de novo FA synthesis in SCs is mandatory for myelination and identify lipogenic activation of the PPARγ transcriptional network as a putative downstream functional mediator.

Role of Mitochondrial Metabolism in the Control of Early Lineage Progression and Aging Phenotypes in Adult Hippocampal Neurogenesis

Precise regulation of cellular metabolism is hypothesized to constitute a vital component of the developmental sequence underlying the life-long generation of hippocampal neurons from quiescent neural stem cells (NSCs). The identity of stage-specific metabolic programs and their impact on adult neurogenesis are largely unknown. We show that the adult hippocampal neurogenic lineage is critically dependent on the mitochondrial electron transport chain and oxidative phosphorylation machinery at the stage of the fast proliferating intermediate progenitor cell. Perturbation of mitochondrial complex function by ablation of the mitochondrial transcription factor A (Tfam) reproduces multiple hallmarks of aging in hippocampal neurogenesis, whereas pharmacological enhancement of mitochondrial function ameliorates age-associated neurogenesis defects. Together with the finding of age-associated alterations in mitochondrial function and morphology in NSCs, these data link mitochondrial complex function to efficient lineage progression of adult NSCs and identify mitochondrial function as a potential target to ameliorate neurogenesis-defects in the aging hippocampus.

Creating Age Asymmetry: Consequences of Inheriting Damaged Goods in Mammalian Cells

Accumulating evidence suggests that mammalian cells asymmetrically segregate cellular components ranging from genomic DNA to organelles and damaged proteins during cell division. Asymmetric inheritance upon mammalian cell division may be specifically important to ensure cellular fitness and propagate cellular potency to individual progeny, for example in the context of somatic stem cell division. We review here recent advances in the field and discuss potential effects and underlying mechanisms that mediate asymmetric segregation of cellular components during mammalian cell division.

Stem Cell-Mediated Regeneration of the Adult Brain

Acute or chronic injury of the adult mammalian brain is often associated with persistent functional deficits as its potential for regeneration and capacity to rebuild lost neural structures is limited. However, the discovery that neural stem cells (NSCs) persist throughout life in discrete regions of the brain, novel approaches to induce the formation of neuronal and glial cells, and recently developed strategies to generate tissue for exogenous cell replacement strategies opened novel perspectives how to regenerate the adult brain. Here, we will review recently developed approaches for brain repair and discuss future perspectives that may eventually allow for developing novel treatment strategies in acute and chronic brain injury.

Engineering of Adult Neurogenesis and Gliogenesis

Neural stem/progenitor cells (NSPCs) retain their ability to generate newborn neurons throughout life in the mammalian brain. Here, we describe how recently developed virus- and transgenesis-based techniques will help us (1) to understand the functional effects of neurogenesis in health and disease, (2) to design novel approaches to harness the potential for NSPC-associated endogenous repair, and (3) to induce the generation of neurons outside the main neurogenic niches in the adult brain.

Linking adult hippocampal neurogenesis with human physiology and disease

We here review the existing evidence linking adult hippocampal neurogenesis and human brain function in physiology and disease. Furthermore, we aim to point out where evidence is missing, highlight current promising avenues of investigation, and suggest future tools and approaches to foster the link between life-long neurogenesis and human brain function. Developmental Dynamics 245:702-709, 2016. © 2016 Wiley Periodicals, Inc.

mTORC1 Inhibition Corrects Neurodevelopmental and Synaptic Alterations in a Human Stem Cell Model of Tuberous Sclerosis

Hyperfunction of the mTORC1 pathway has been associated with idiopathic and syndromic forms of autism spectrum disorder (ASD), including tuberous sclerosis, caused by loss of either TSC1 or TSC2. It remains largely unknown how developmental processes and biochemical signaling affected by mTORC1 dysregulation contribute to human neuronal dysfunction. Here, we have characterized multiple stages of neurogenesis and synapse formation in human neurons derived from TSC2-deleted pluripotent stem cells. Homozygous TSC2 deletion causes severe developmental abnormalities that recapitulate pathological hallmarks of cortical malformations in patients. Both TSC2(+/-) and TSC2(-/-) neurons display altered synaptic transmission paralleled by molecular changes in pathways associated with autism, suggesting the convergence of pathological mechanisms in ASD. Pharmacological inhibition of mTORC1 corrects developmental abnormalities and synaptic dysfunction during independent developmental stages. Our results uncouple stage-specific roles of mTORC1 in human neuronal development and contribute to a better understanding of the onset of neuronal pathophysiology in tuberous sclerosis.

Neural repair in the adult brain

Acute or chronic injury to the adult brain often results in substantial loss of neural tissue and subsequent permanent functional impairment. Over the last two decades, a number of approaches have been developed to harness the regenerative potential of neural stem cells and the existing fate plasticity of neural cells in the nervous system to prevent tissue loss or to enhance structural and functional regeneration upon injury. Here, we review recent advances of stem cell-associated neural repair in the adult brain, discuss current challenges and limitations, and suggest potential directions to foster the translation of experimental stem cell therapies into the clinic.

Epilepsy and Adult Neurogenesis

Seizure activity in the hippocampal region strongly affects stem cell-associated plasticity in the adult dentate gyrus. Here, we describe how seizures in rodent models of mesial temporal lobe epilepsy (mTLE) affect multiple steps in the developmental course from the dividing neural stem cell to the migrating and integrating newborn neuron. Furthermore, we discuss recent evidence indicating either that seizure-induced aberrant neurogenesis may contribute to the epileptic disease process or that altered neurogenesis after seizures may represent an attempt of the injured brain to repair itself. Last, we describe how dysfunction of adult neurogenesis caused by chronic seizures may play an important role in the cognitive comorbidities associated with mTLE.

A mechanism for the segregation of age in mammalian neural stem cells

Throughout life, neural stem cells (NSCs) generate neurons in the mammalian brain. Using photobleaching experiments, we found that during cell division in vitro and within the developing mouse forebrain, NSCs generate a lateral diffusion barrier in the membrane of the endoplasmic reticulum, thereby promoting asymmetric segregation of cellular components. The diffusion barrier weakens with age and in response to impairment of lamin-associated nuclear envelope constituents. Weakening of the diffusion barrier disrupts asymmetric segregation of damaged proteins, a product of aging. Damaged proteins are asymmetrically inherited by the nonstem daughter cell in embryonic and young adult NSC divisions, whereas in the older adult brain, damaged proteins are more symmetrically distributed between progeny. Thus, these data identify a mechanism of how damage that accumulates with age is asymmetrically distributed during somatic stem cell division.

SPOT14-positive neural stem/progenitor cells in the hippocampus respond dynamically to neurogenic regulators

Proliferation of neural stem/progenitor cells (NSPCs) in the adult brain is tightly controlled to prevent exhaustion and to ensure proper neurogenesis. Several extrinsic stimuli affect NSPC regulation. However, the lack of unique markers led to controversial results regarding the in vivo behavior of NSPCs to different stimuli. We recently identified SPOT14, which controls NSPC proliferation through regulation of de novo lipogenesis, selectively in low-proliferating NSPCs. Whether SPOT14-expressing (SPOT14+) NSPCs react in vivo to neurogenic regulators is not known. We show that aging is accompanied by a marked disappearance of SPOT14+ NSPCs, whereas running, a positive neurogenic stimulus, increases proliferation of SPOT14+ NSPCs. Furthermore, transient depletion of highly proliferative cells recruits SPOT14+ NSPCs into the proliferative pool. Additionally, we have established endogenous SPOT14 protein staining, reflecting transgenic SPOT14-GFP expression. Thus, our data identify SPOT14 as a potent marker for adult NSPCs that react dynamically to positive and negative neurogenic regulators.

Chemical conversion of human fibroblasts into functional Schwann cells

Direct transdifferentiation of somatic cells is a promising approach to obtain patient-specific cells for numerous applications. However, conversion across germ-layer borders often requires ectopic gene expression with unpredictable side effects. Here, we present a gene-free approach that allows efficient conversion of human fibroblasts via a transient progenitor stage into Schwann cells, the major glial cell type of peripheral nerves. Using a multikinase inhibitor, we transdifferentiated fibroblasts into transient neural precursors that were subsequently further differentiated into Schwann cells. The resulting induced Schwann cells (iSCs) expressed numerous Schwann cell-specific proteins and displayed neurosupportive and myelination capacity in vitro. Thus, we established a strategy to obtain mature Schwann cells from human postnatal fibroblasts under chemically defined conditions without the introduction of ectopic genes.

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Direct conversion of human fibroblasts into Schwann cells

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No introduction of ectopic genes

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Induction of conversion by multikinase inhibitor

Adult neurogenesis and its role in neuropsychiatric disease, brain repair and normal brain function

Neural stem/progenitor cells (NSPCs) in the mammalian brain retain the ability to generate new neurones throughout life in discrete brain regions, through a process called adult neurogenesis. Adult neurogenesis, a dramatic form of adult brain circuitry plasticity, has been implicated in physiological brain function and appears to be of pivotal importance for certain forms of learning and memory. In addition, failing or altered neurogenesis has been associated with a variety of brain diseases such as major depression, epilepsy and age-related cognitive decline. Here we review recent advances in our understanding of the basic biology underlying the neurogenic process in the adult brain, focusing on mechanisms that regulate quiescence, proliferation and differentiation of NSPCs. In addition, we discuss how neurogenesis influences normal brain function, and in particular its role in memory formation, as well as its contribution to neuropsychiatric diseases. Finally, we evaluate the potential of targeting endogenous NSPCs for brain repair.

Adult neurogenesis: bridging the gap between mice and humans

Neural stem/progenitor cells (NSPCs) generate new neurons in the mammalian brain throughout life. Over the past two decades, substantial progress has been made in deciphering the cellular and molecular mechanisms underlying adult neurogenesis and in understanding the role played by new neurons in brain function in animal models of health and disease. By contrast, knowledge regarding the extent and relevance of neurogenesis in the adult human brain remains scant. Here we review new concepts about how new neurons shape adult brain circuits, discuss fundamental, unanswered questions about stem cell-associated neural plasticity, and illustrate how the gap between the animal-based basic research and current efforts to analyze life-long neuronal development of the human brain may be overcome by using novel experimental strategies.

Adult-born granule cells mature through two functionally distinct states

Adult-born granule cells (ABGCs) are involved in certain forms of hippocampus-dependent learning and memory. It has been proposed that young but functionally integrated ABGCs (4-weeks-old) specifically contribute to pattern separation functions of the dentate gyrus due to their heightened excitability, whereas old ABGCs (>8 weeks old) lose these capabilities. Measuring multiple cellular and integrative characteristics of 3- 10-week-old individual ABGCs, we show that ABGCs consist of two functionally distinguishable populations showing highly distinct input integration properties (one group being highly sensitive to narrow input intensity ranges while the other group linearly reports input strength) that are largely independent of the cellular age and maturation stage, suggesting that ‘classmate’ cells (born during the same period) can contribute to the network with fundamentally different functions. Thus, ABGCs provide two temporally overlapping but functionally distinct neuronal cell populations, adding a novel level of complexity to our understanding of how life-long neurogenesis contributes to adult brain function.

DOI:http://dx.doi.org/10.7554/eLife.03104.001

Remembering what happened on different occasions involves a process in the brain called pattern separation, which allows us to separate and distinguish our memories. One part of the brain where pattern separation occurs is called the dentate gyrus, which sits in the hippocampus—the brain region that is in charge of certain forms of learning and memory.

Neurons called granule cells are thought to play a central role in hippocampal pattern separation. These cells, unlike the majority of nerve cells, can form at any time, and those that form in the mature brain are called adult born granule cells (ABGCs). Although it usually takes 10 weeks for these cells to fully mature, they are capable of communicating with each other about 3–4 weeks after being generated. Previously, it had been reported that while young, 4-week-old ABGCs are required for pattern separation, slightly older (8 week old) ABGCs are not.

What intrinsic properties make ABGCs capable of contributing to pattern separation? Is this property defined by the fate (i.e. a predetermined program) of the cell, or by the cell's experiences and activities?

To investigate these questions, Brunner et al. labeled ABGCs with a fluorescent tag when these neurons were born in adult male rats. Then, when the tagged cells were aged between 3 and 10 weeks old, the electrical properties of the labeled cells were measured from thin brain slices.

Brunner et al. found that ABGCs respond to input signals with two different levels of sensitivity. The youngest cells (3–5 weeks old) are exceptionally sensitive to a narrow range of input signal strengths, which is useful for pattern separation. The oldest investigated cells (10 weeks old), on the other hand, respond incrementally to a wide range of different input signal strengths. Under these experimental conditions, the cells changed how they respond to input signals some time between 5 and 9 weeks after being born. However, they either behaved like the youngest or like the oldest cells: no intermediate behavior was seen.

Unexpectedly, the switch is not directly related to the age of the cells: cells born at the same time don't necessarily change behavior at the same time, and cells born at different times may behave similarly. Thus, Brunner et al. suggest that it is the experience of the cells, and not their fate, that determines how they help the dentate gyrus function during the investigated period.

DOI:http://dx.doi.org/10.7554/eLife.03104.002

Adult neurogenesis: mechanisms and functional significance

New neurons are generated throughout life in distinct regions of the mammalian brain. This process, called adult neurogenesis, has been implicated in physiological brain function, and failing or altered neurogenesis has been associated with a number of neuropsychiatric diseases. Here, we provide an overview of the mechanisms governing the neurogenic process in the adult brain and describe how new neurons may contribute to brain function in health and disease.

Maturation and integration of adult born hippocampal neurons: signal convergence onto small Rho GTPases

Adult neurogenesis, restricted to specific regions in the mammalian brain, represents one of the most interesting forms of plasticity in the mature nervous system. Adult-born hippocampal neurons play important roles in certain forms of learning and memory, and altered hippocampal neurogenesis has been associated with a number of neuropsychiatric diseases such as major depression and epilepsy. Newborn neurons go through distinct developmental steps, from a dividing neurogenic precursor to a synaptically integrated mature neuron. Previous studies have uncovered several molecular signaling pathways involved in distinct steps of this maturational process. In this context, the small Rho GTPases, Cdc42, Rac1, and RhoA have recently been shown to regulate the morphological and synaptic maturation of adult-born dentate granule cells in vivo. Distinct upstream regulators, including growth factors that modulate maturation and integration of newborn neurons have been shown to also recruit the small Rho GTPases. Here we review recent findings and highlight the possibility that small Rho GTPases may act as central assimilators, downstream of critical input onto adult-born hippocampal neurons contributing to their maturation and integration into the existing dentate gyrus (DG) circuitry.

Temporal control of retroviral transgene expression in newborn cells in the adult brain

Neural stem/progenitor cells (NSPCs) generate new neurons throughout life in distinct areas of the adult mammalian brain. Besides classical transgenesis-based approaches, retrovirus-mediated genetic manipulation is frequently used to study mechanisms that regulate neurogenesis in the nervous system. Here, we show that fusion of a tamoxifen-regulatable estrogen receptor (ER^T2) motif to transcription factors (i.e., ASCL1 and NEUROD1) enables temporal control of transgene expression in adult mouse NSPCs in vitro and in vivo. Thus, the approach described here represents a versatile strategy for regulating gene expression to study gene function in dividing cells and their progeny.

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Inducible TF-ER^T2 vectors enable increased neuronal differentiation of NSPCs in vitro

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TF-ER^T2 vectors allow temporal control of TFs in transduced NSPCs in the adult brain

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Versatile strategy to genetically manipulate neurogenic cells in the adult brain