New regulators in adult neurogenesis and their potential role for repair

Adult neural stem cells hold great promise for repair because of their unique location within the central nervous system, their potential to proliferate and to differentiate into all major neural lineages, and their ability to incorporate functionally into the existing neuronal circuitry. However, recruitment of these cells for repair is hampered by the lack of knowledge about the signals that control the generation of a functional neuron from adult neural stem cells. Here, we discuss recent findings on the regulatory mechanisms that underlie neurogenesis from neural stem cells in the adult hippocampus and the implications of these findings for future stem-cell-based repair strategies.     

Secretome profiling of differentiated neural mes-c-myc A1 cell line endowed with stem cell properties

Neural stem cell proliferation and differentiation play a crucial role in the formation and wiring of neuronal connections forming neuronal circuits. During neural tissues development, a large diversity of neuronal phenotypes is produced from neural precursor cells. In recent years, the cellular and molecular mechanisms by which specific types of neurons are generated have been explored with the aim to elucidate the complex events leading to the generation of different phenotypes via distinctive developmental programs that control self-renewal, differentiation, and plasticity. The extracellular environment is thought to provide instructive influences that actively induce the production of specific neuronal phenotypes.     

Generation of diverse neuronal subtypes in cloned populations of stem-like cells

Background  

The central nervous tissue contains diverse subtypes of neurons with characteristic morphological and physiological features and different neurotransmitter phenotypes. The generation of neurons with defined neurotransmitter phenotypes seems to be governed by factors differently expressed along the anterior-posterior and dorsal-ventral body axes. The mechanisms of the cell-type determination, however, are poorly understood. Selected neuronal phenotypes had been generated from embryonic stem (ES) cells, but similar results were not obtained on more restricted neural stem cells, presumably due to the lack of homogeneous neural stem cell populations as a starting material.     

Glial influences on neural stem cell development: cellular niches for adult neurogenesis

Neural stem cells continually generate new neurons in very limited regions of the adult mammalian central nervous system. In the neurogenic regions there are unique and highly specialized microenvironments (niches) that tightly regulate the neuronal development of adult neural stem cells. Emerging evidence suggests that glia, particularly astrocytes, have key roles in controlling multiple steps of adult neurogenesis within the niches, from proliferation and fate specification of neural progenitors to migration and integration of the neuronal progeny into pre-existing neuronal circuits in the adult brain. Identification of specific niche signals that regulate these sequential steps during adult neurogenesis might lead to strategies to induce functional neurogenesis in other brain regions after injury or degenerative neurological diseases.     

The Path from Skin to Brain: Generation of Functional Neurons from Fibroblasts

Cell fate reprogramming makes possible the generation of new cell types from healthy adult cells to replace those lost or damaged in disease. Additionally, reprogramming patient cells into specific cell types allows for drug screening and the development of new therapeutic tools. Generation of new neurons is of particular interest because of the potential to treat diseases of the nervous system, such as neurodegenerative disorders and spinal cord injuries, with cell replacement therapy. Recent advances in cell fate reprogramming have led to the development of novel methods for the direct conversion of fibroblasts into neurons and neural stem cells. This review will highlight the advantages of these new methods over neuronal induction from embryonic stem cells and induced pluripotent stem cells, as well as outline the limitations and the potential for future applications.     

Heparan sulfate proteoglycans in the nervous system: their diverse roles in neurogenesis, axon guidance, and synaptogenesis

Development of the mammalian nervous system involves generation of neurons from neural stem cells, migration of generated neurons toward genetically determined locations, extension of axons and dendrites, and establishment of neuronal connectivity. Recent progresses revealed diverse role of heparan sulfate proteoglycans in these processes. This article reviews our current knowledge about the functional roles of heparan sulfate proteoglycans in three critical events in mammalian neural development, namely neurogenesis, axon guidance, and synapse development.     

Early neural development in vertebrates is also a matter of calcium

The calcium (Ca²⁺) signaling pathways have crucial roles in development from fertilization through differentiation to organogenesis. In the nervous system, Ca²⁺ signals are important regulators for various neuronal functions, including formation and maturation of neuronal circuits and long-term memory. However, Ca²⁺ signals are mainly involved in the earliest steps of nervous system development including neural induction, differentiation of neural progenitors into neurons, and the neuro-glial switch. This review examines when and how Ca²⁺ signals are generated during each of these steps with examples taken from in vivo studies in vertebrate embryos and from in vitro assays using embryonic and neural stem cells. Also discussed is the highly specific nature of the Ca²⁺ signaling pathway and its interaction with the other signaling pathways involved in early neural development.     

Embryonic stem cells: taming the fountain of youth

Embryonic stem cells readily generate neurons in vitro, but steering their differentiation into specific neuronal subtypes is a major challenge. It has now been shown that mechanisms that regulate neuronal specification during development can be applied to embryonic stem cells in vitro; this may lead to new ways of generating cells for therapy.     

Culture of mouse neural stem cell precursors

Primary neural stem cell cultures are useful for studying the mechanisms underlying central nervous system development. Stem cell research will increase our understanding of the nervous system and may allow us to develop treatments for currently incurable brain diseases and injuries. In addition, stem cells should be used for stem cell research aimed at the detailed study of mechanisms of neural differentiation and transdifferentiation and the genetic and environmental signals that direct the specialization of the cells into particular cell types. This video demonstrates a technique used to disaggregate cells from the embryonic day 12.5 mouse dorsal forebrain. The dissection procedure includes harvesting E12.5 mouse embryos from the uterus, removing the "skin" with fine dissecting forceps and finally isolating pieces of cerebral cortex. Following the dissection, the tissue is digested and mechanically dissociated. The resuspended dissociated cells are then cultured in "stem cell" media that favors growth of neural stem cells.     

Glial cells more than support cells?

Glial cells are the most abundant cells in the human brain and have long been considered as passive supporting cells for neurons. In contrast to the extensive studies on various neuronal functions in the nervous system, we still have limited knowledge about glial cells. Recently a number of pioneering studies have provided convincing evidence that glia play active roles in development and function of the central nervous system. This review discusses recent advances in our understanding of the molecular mechanisms underlying glial cell differentiation. We then highlight some of the novel findings about glial function, i.e. the role of glia in synaptogenesis and the intricate relationship between astrocytes and adult neural stem cells. Finally, we summarize the emerging studies that implicate abnormalities in the formation or maintenance of glia leading to severe brain diseases, such as Alexander disease, glioblastoma and multiple sclerosis, and potential therapeutic strategies to tackle these diseases.     

Extracellular matrix: functions in the nervous system

An astonishing number of extracellular matrix glycoproteins are expressed in dynamic patterns in the developing and adult nervous system. Neural stem cells, neurons, and glia express receptors that mediate interactions with specific extracellular matrix molecules. Functional studies in vitro and genetic studies in mice have provided evidence that the extracellular matrix affects virtually all aspects of nervous system development and function. Here we will summarize recent findings that have shed light on the specific functions of defined extracellular matrix molecules on such diverse processes as neural stem cell differentiation, neuronal migration, the formation of axonal tracts, and the maturation and function of synapses in the peripheral and central nervous system.     

Mechanisms and molecules in motor neuron specification and axon pathfinding.

The vertebrate nervous system performs the most complex functions of any organ system. This feat is mediated by dedicated assemblies of neurons that must be precisely connected to one another and to peripheral tissues during embryonic development. Motor neurons, which innervate muscle and regulate autonomic functions, form an integral part of this neural circuitry. The first part of this review describes the remarkable progress in our understanding of motor neuron differentiation, which is arguably the best understood model of neuronal differentiation to date. During development, the coordinate actions of inductive signals from adjacent non-neural tissues initiate the differentiation of distinct motor neuron subclasses, with specific projection patterns, at stereotypical locations within the neural tube. Underlying this specialisation is the expression of specific homeodomain proteins, which act combinatorially to confer motor neurons with both their generic and subtype-specific properties. Ensuring that specific motor neuron subtypes innervate the correct target structure, however, requires precise motor axon guidance mechanisms. The second half of this review focuses on how distinct motor neuron subtypes pursue highly specific projection patterns by responding differentially to spatially discrete attractive and repulsive molecular cues. The tight link between motor neuron specification and axon pathfinding appears to be established by the dominant role of homeodomain proteins in dictating the ways that navigating motor axons interpret the plethora of guidance cues impinging on growth cones.     

Direct reprogramming of human astrocytes into neural stem cells and neurons

Generating neural stem cells and neurons from reprogrammed human astrocytes is a potential strategy for neurological repair. Here we show dedifferentiation of human cortical astrocytes into the neural stem/progenitor phenotype to obtain progenitor and mature cells with a neural fate. Ectopic expression of the reprogramming factors OCT4, SOX2, or NANOG into astrocytes in specific cytokine/culture conditions activated the neural stem gene program and induced generation of cells expressing neural stem/precursor markers. Pure CD44+ mature astrocytes also exhibited this lineage commitment change and did not require passing through a pluripotent state. These astrocyte-derived neural stem cells gave rise to neurons, astrocytes, and oligodendrocytes and showed in vivo engraftment properties. ASCL1 expression further promoted neuronal phenotype acquisition in vitro and in vivo. Methylation analysis showed that epigenetic modifications underlie this process. The restoration of multipotency from human astrocytes has potential in cellular reprogramming of endogenous central nervous system cells in neurological disorders.