Brain micro-ecologies: neural stem cell niches in the adult mammalian brain

Neurogenesis persists in two germinal regions in the adult mammalian brain, the subventricular zone of the lateral ventricles and the subgranular zone in the hippocampal formation. Within these two neurogenic niches, specialized astrocytes are neural stem cells, capable of self-renewing and generating neurons and glia. Cues within the niche, from cell-cell interactions to diffusible factors, are spatially and temporally coordinated to regulate proliferation and neurogenesis, ultimately affecting stem cell fate choices. Here, we review the components of adult neural stem cell niches and how they act to regulate neurogenesis in these regions.     

Cellular and molecular attributes of neural stem cell niches in adult zebrafish brain

Adult neurogenesis is a complex, presumably conserved phenomenon in vertebrates with a broad range of variations regarding neural progenitor/stem cell niches, cellular composition of these niches, migratory patterns of progenitors and so forth among different species. Current understanding of the reasons underlying the inter‐species differences in adult neurogenic potential, the identification and characterization of various neural progenitors, characterization of the permissive environment of neural stem cell niches and other important aspects of adult neurogenesis is insufficient. In the last decade, zebrafish has emerged as a very useful model for addressing these questions. In this review, we have discussed the present knowledge regarding the neural stem cell niches in adult zebrafish brain as well as their cellular and molecular attributes. We have also highlighted their similarities and differences with other vertebrate species. In the end, we shed light on some of the known intrinsic and extrinsic factors that are assumed to regulate the neurogenic process in adult zebrafish brain. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1188–1205, 2017     

Embryonic stem cell neurogenesis and neural specification

The prospect of using embryonic stem cell (ESC)‐derived neural progenitors and neurons to treat neurological disorders has led to great interest in defining the conditions that guide the differentiation of ESCs, and more recently induced pluripotent stem cells (iPSCs), into neural stem cells (NSCs) and a variety of neuronal and glial subtypes. Over the past decade, researchers have looked to the embryo to guide these studies, applying what we know about the signaling events that direct neural specification during development. This has led to the design of a number of protocols that successfully promote ESC neurogenesis, terminating with the production of neurons and glia with diverse regional addresses and functional properties. These protocols demonstrate that ESCs undergo neural specification in two, three, and four dimensions, mimicking the cell–cell interactions, patterning, and timing that characterizes the in vivo process. We therefore propose that these in vitro systems can be used to examine the molecular regulation of neural specification. J. Cell. Biochem. 111: 535–542, 2010. © 2010 Wiley‐Liss, Inc.     

Comparison of adult stem cells derived from multiple stem cell niches

Objectives

Adult stem cells (ASCs) have great potential for tissue regeneration; however, comparative studies of ASCs from different niches are required to understand the characteristics of each population for their potential therapeutic uses.

Results

We compared the proliferation, stem cell marker expression, and differentiation potential of ASCs from bone marrow, skin dermis, and adipose tissue. ASCs from bone marrow and skin dermis showed 50–100 % increased proliferation in comparison to the ASCs from adipose tissues. Furthermore, ASCs from each stem cell niche showed differential expression of stem cell marker genes, and preferentially differentiated into cell types of their tissue of origin.

Conclusion

Different characters of each ASC might be major factors for their effective use for therapeutics and tissue regeneration.

     

Ogt controls neural stem/progenitor cell pool and adult neurogenesis through modulating Notch signaling

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p57 controls adult neural stem cell quiescence and modulates the pace of lifelong neurogenesis

Throughout life, neural stem cells (NSCs) in the adult hippocampus persistently generate new neurons that modify the neural circuitry. Adult NSCs constitute a relatively quiescent cell population but can be activated by extrinsic neurogenic stimuli. However, the molecular mechanism that controls such reversible quiescence and its physiological significance have remained unknown. Here, we show that the cyclin‐dependent kinase inhibitor p57^kip2 (p57) is required for NSC quiescence. In addition, our results suggest that reduction of p57 protein in NSCs contributes to the abrogation of NSC quiescence triggered by extrinsic neurogenic stimuli such as running. Moreover, deletion of p57 in NSCs initially resulted in increased neurogenesis in young adult and aged mice. Long‐term p57 deletion, on the contrary, led to NSC exhaustion and impaired neurogenesis in aged mice. The regulation of NSC quiescence by p57 might thus have important implications for the short‐term (extrinsic stimuli‐dependent) and long‐term (age‐related) modulation of neurogenesis.     

Plant stem cell niches

Stem cells are required to support the indeterminate growth style of plants. Meristems are a plants stem cell niches that foster stem cell survival and the production of descendants destined for differentiation. In shoot meristems, stem cell fate is decided at the populational level. The size of the stem cell domain at the meristem tip depends on signals that are exchanged with cells of the organizing centre underneath. In root meristems, individual stem cells are controlled by direct interaction with cells of the quiescent centre that lie in the immediate neighbourhood. Analysis of the interactions and signaling processes in the stem cell niches has delivered some insights into the molecules that are involved and revealed that the two major niches for plant stem cells are more similar than anticipated.     

Signaling in adult neurogenesis: from stem cell niche to neuronal networks

The mechanisms that determine why neurogenesis is restricted to few regions of the adult brain in mammals, in contrast to its more widespread nature in other vertebrates such as zebrafish, remain to be fully understood. The local environment must provide key signals that instruct stem cell and neurogenic fate, because non-neurogenic progenitors can be instructed towards neurogenesis in this environment. Here, we discuss the recent progress in understanding key factors in the local stem cell niche of the adult mammalian brain, including surprising sources of new signals such as endothelial cells, complement factors and microglia. Moreover, new insights have been gained into how neuronal diversity is instructed in adult neurogenesis, prompting a new view of stem and progenitor cell heterogeneity in the adult mammalian brain.     

p73 is an essential regulator of neural stem cell maintenance in embryonal and adult CNS neurogenesis

The p53 family member p73 is essential for brain development, but its precise role and scope remain unclear. Global p73 deficiency determines an overt and highly penetrant brain phenotype marked by cortical hypoplasia with ensuing hydrocephalus and hippocampal dysgenesis. The ΔNp73 isoform is known to function as a prosurvival factor of mature postmitotic neurons. In this study, we define a novel essential role of p73 in the regulation of the neural stem cell compartment. In both embryonic and adult neurogenesis, p73 has a critical role in maintaining an adequate neurogenic pool by promoting self-renewal and proliferation and inhibiting premature senescence of neural stem and early progenitor cells. Thus, products of the p73 gene locus are essential maintenance factors in the central nervous system, whose broad action stretches across the entire differentiation arch from stem cells to mature postmitotic neurons.     

Neuronal-glial interactions in central nervous system neurogenesis: the neural stem cell perspective

Essentially, three neuroectodermal-derived cell types make up the complex architecture of the adult CNS: neurons, astrocytes and oligodendrocytes. These elements are endowed with remarkable morphological, molecular and functional heterogeneity that reaches its maximal expression during development when stem/progenitor cells undergo progressive changes that drive them to a fully differentiated state. During this period the transient expression of molecular markers hampers precise identification of cell categories, even in neuronal and glial domains. These issues of developmental biology are recapitulated partially during the neurogenic processes that persist in discrete regions of the adult brain. The recent hypothesis that adult neural stem cells (NSCs) show a glial identity and derive directly from radial glia raises questions concerning the neuronal-glial relationships during pre- and post-natal brain development. The fact that NSCs isolated in vitro differentiate mainly into astrocytes, whereas in vivo they produce mainly neurons highlights the importance of epigenetic signals in the neurogenic niches, where glial cells and neurons exert mutual influences. Unravelling the mechanisms that underlie NSC plasticity in vivo and in vitro is crucial to understanding adult neurogenesis and exploiting this physiological process for brain repair. In this review we address the issues of neuronal/glial cell identity and neuronal-glial interactions in the context of NSC biology and NSC-driven neurogenesis during development and adulthood in vivo, focusing mainly on the CNS. We also discuss the peculiarities of neuronal-glial relationships for NSCs and their progeny in the context of in vitro systems.     

Mimicking stem cell niches to increase stem cell expansion

Niches regulate lineage-specific stem cell self-renewal versus differentiation in vivo and are composed of supportive cells and extracellular matrix components arranged in a three-dimensional topography of controlled stiffness in the presence of oxygen and growth factor gradients. Mimicking stem cell niches in a defined manner will facilitate production of the large numbers of stem cells needed to realize the promise of regenerative medicine and gene therapy. Progress has been made in mimicking components of the niche. Immobilizing cell-associated Notch ligands increased the self-renewal of hematopoietic (blood) stem cells. Culture on a fibrous scaffold that mimics basement membrane texture increased the expansion of hematopoietic and embryonic stem cells. Finally, researchers have created intricate patterns of cell-binding domains and complex oxygen gradients.     

To be or not to be a stem cell: dissection of cellular and molecular components of haematopoietic stem cell niches

Nature 481 7382, 457–462 (2012); published online January252012Recent studies have identified multiple cell types that regulate haematopoietic stem cells (HSCs); however, proof that a specific cell type produces a specific factor important for HSC function and maintenance is largely lacking. Ding et al (2012) reported recently that conditional deletion of stem cell factor (SCF) in Leptin receptor (Lepr) expressing perivascular cells or endothelial and haematopoietic cells resulted in significant reductions in number but less profound reduction in function of HSCs. Although the long-term fate of HSCs in these models is largely unexplored and an underlying mechanism for reduction in HSCs not yet reported, these findings further implicate the vascular niche in the functional maintenance of HSCs in vivo and also raise intriguing questions for future studies in this field.The haematopoietic stem cell (HSC) niche has traditionally been considered a discrete site within the bone marrow; however, recent studies have shown that numerous cell types are critically important for HSC regulation and maintenance (Wang and Wagers, 2011). Imaging studies have shown that phenotypic HSCs can be found adjacent to osteoblasts or osteoprogenitor cells on the inner surface of trabecular bone, and genetic studies have further shown that expansion of trabecular bone, leads to expansion of HSCs (Calvi et al, 2003; Zhang et al, 2003; Lo Celso et al, 2009; Xie et al, 2009). Other studies have found that phenotypic HSCs are frequently localized to the central marrow, specifically near endothelial or perivascular cells (Kiel et al, 2005). Recently, endothelial cells have been shown to support the ex vivo expansion of HSCs (Butler et al, 2010); however, it was so far not known whether endothelial or perivascular cells functionally maintain in vivo HSCs.Ding and colleagues used knockin reporter mice for Scf expression and found that stem cell factor (SCF) was produced predominantly by endothelial and perivascular cells but was not concentrated near the bone surface. To investigate which cellular sources of SCF are important for HSC maintenance, they conditionally deleted Scf specifically in haematopoietic cells, osteoblasts and Nestin-Cre expressing cells but found no significant effects on HSC maintenance. In contrast, conditional deletion in both haematopoietic and endothelial cells or in Leptin receptor (Lepr) expressing perivascular stromal cells significantly reduced phenotypic and, to a lesser extent, functional HSC frequency—thus further demonstrating that the vascular niche plays a role in functionally supporting HSCs (Figure 1). These findings underscore the complexity of the HSC niche and raise crucial future questions.Open in a separate windowFigure 1(A) HSCs reside in both osteoblastic and vascular niches. The vascular niche is juxtaposed with the osteoblastic niche and includes endothelial cells, CAR cells, Nestin⁺ cells, Lepr⁺ perivascular cells and other cell types. Ding et al show that SCF is predominantly provided by endothelial and perivascular cells. (B) Cell-specific deletion of Scf in endothelial and Lepr⁺ cells results in reduced HSCs; however, other HSCs are maintained, possibly from a quiescent reserved population that is less dependent on SCF, providing significant levels of haematopoiesis.The nature and specific identity of Lepr expressing cells is uncertain. This population appears to partially overlap with Nestin-Cre expressing cells, and it is not clear to what extent Lepr expressing cells might identify with Cxcl12-abundant reticular (CAR) cells, both of which have been previously identified as HSC niche components (Sugiyama et al, 2006; Mendez-Ferrer et al, 2010). Although phenotypic HSC frequency (determined by the cell-surface markers lineage⁻, Sca-1⁺, Kit⁺, CD150⁺, CD48⁻) is dramatically reduced, functional HSC frequency is only mildly compromised following conditional deletion of Scf either ubiquitously or in endothelial/perivascular cells. This indicates that other factors or sources of SCF maintain substantial numbers of HSCs independent of SCF produced by the vascular niche or elsewhere. Indeed, these results may be consistent with the coexistence of quiescent and active HSC populations—with the quiescent, reserved population serving as a backup HSC source to support life-long haematopoiesis, especially following the loss of active HSCs in response to stress (Li and Clevers, 2010). Considering the role of SCF in promoting proliferation (Broudy, 1997), it would be interesting to know the long-term effects of cell-specific deletion of Scf on HSC maintenance. Cell-specific deletion in Nestin-Cre expressing cells apparently did not affect HSC frequency long-term (5 months); however, such long-term data were not presented for osteoblast-specific knockout of Scf. It would also be interesting to know the mechanism for HSC loss following endothelial/perivascular-specific deletion of Scf. Interesting topics to address in the future are whether HSC quiescence is compromised, or whether apoptosis or differentiation is increased.As the authors note, multiple cell types is involved in HSC maintenance. Given the juxtaposition of endothelial and perivascular cells with the bone surface, the osteoblastic and vascular niche represent not always mutually exclusive entities (Lo Celso et al, 2009). We have recently proposed that stem cells may reside in special zones, where active stem cells may provide for the daily replenishment of tissues while quiescent, reserved stem cells serve as a backup sub-population to ensure life-long tissue maintenance and replenishment of the stem cell pool following stress (Li and Clevers, 2010). It remains for future studies to continue to determine which specific niche cells produce which particular factors for maintaining long-term quiescence versus those for supporting proliferation and survival of stem cells. The results published by Ding et al present a significant step towards this goal.     

Adult neurogenesis and cellular brain repair with neural progenitors, precursors and stem cells

Recent work in neuroscience has shown that the adult central nervous system (CNS) contains neural progenitors, precursors and stem cells that are capable of generating new neurons, astrocytes and oligodendrocytes. While challenging the previous dogma that no new neurons are born in the adult mammalian CNS, these findings bring with them the future possibilities for development of novel neural repair strategies. The purpose of this review is to present the current knowledge about constitutively occurring adult mammalian neurogenesis, highlight the critical differences between 'neurogenic' and 'non-neurogenic' regions in the adult brain, and describe the cardinal features of two well-described neurogenic regions-the subventricular zone/olfactory bulb system and the dentate gyrus of the hippocampus. We also provide an overview of presently used models for studying neural precursors in vitro, mention some precursor transplantation models and emphasize that, in this rapidly growing field of neuroscience, one must be cautious with respect to a variety of methodological considerations for studying neural precursor cells both in vitro and in vivo. The possibility of repairing neural circuitry by manipulating neurogenesis is an intriguing one, and, therefore, we also review recent efforts to understand the conditions under which neurogenesis can be induced in non-neurogenic regions of the adult CNS. This work aims towards molecular and cellular manipulation of endogenous neural precursors in situ, without transplantation. We conclude this review with a discussion of what might be the function of newly generated neurons in the adult brain, and provide a summary of present thinking about the consequences of disturbed adult neurogenesis and the reaction of neurogenic regions to disease.     

Engineering stem cell niches in bioreactors

Stem cells, including embryonic stem cells, induced pluripotent stem cells, mesenchymal stem cells and amniotic fluid stem cells have the potential to be expanded and differentiated into various cell types in the body. Efficient differentiation of stem cells with the desired tissue-specific function is critical for stem cell-based cell therapy, tissue engineering, drug discovery and disease modeling. Bioreactors provide a great platform to regulate the stem cell microenvironment, known as “niches”, to impact stem cell fate decision. The niche factors include the regulatory factors such as oxygen, extracellular matrix (synthetic and decellularized), paracrine/autocrine signaling and physical forces (i.e., mechanical force, electrical force and flow shear). The use of novel bioreactors with precise control and recapitulation of niche factors through modulating reactor operation parameters can enable efficient stem cell expansion and differentiation. Recently, the development of microfluidic devices and microbioreactors also provides powerful tools to manipulate the stem cell microenvironment by adjusting flow rate and cytokine gradients. In general, bioreactor engineering can be used to better modulate stem cell niches critical for stem cell expansion, differentiation and applications as novel cell-based biomedicines. This paper reviews important factors that can be more precisely controlled in bioreactors and their effects on stem cell engineering.     

The dynamic plant stem cell niches

Stem cells exist in specific locations called niches, where extracellular signals maintain stem cell division and prevent differentiation. In plants, the best characterised niches are within the shoot and root meristems. Networks of regulatory genes and intercellular signals maintain meristem structure in spite of constant cell displacement by division. Recent works have improved our understanding of how these networks function at the cellular and molecular levels, particularly in the control of the stem cell population in the shoot meristem. The meristem regulatory genes have been found to function partly through localised control of widely used signals such as cytokinin and auxin. The retinoblastoma protein has also emerged as a key regulator of cell differentiation in the meristems.     

In vitro recapitulation of neural development using embryonic stem cells: from neurogenesis to histogenesis

Embryonic stem (ES) cells have been successfully used over the past decade to generate specific types of neuronal cells. In addition to its value for regenerative medicine, ES cell culture also provides versatile experimental systems for analyzing early neural development. These systems are complimentary to conventional animal models, particularly because they allow unique constructive (synthetic) approaches, for example, step-wise addition of components. Here we review the ability of ES cells to generate not only specific neuronal populations but also functional neural tissues by recapitulating microenvironments in early mammalian development. In particular, we focus on cerebellar neurogenesis from mouse ES cells, and explain the basic ideas for positional information and self-formation of polarized neuroepithelium. Basic research on developmental signals has fundamentally contributed to substantial progress in stem cell technology. We also discuss how in vitro model systems using ES cells can shed new light on the mechanistic understanding of organogenesis, taking an example of recent progress in self-organizing histogenesis.