User:Nvirani3/sandbox

Description
Stem cells are characterized by their capability to differentiate into multiple cell types via exogenous stimuli from their environment (Figure 1). They undergo symmetric cell division that gives rise to one undifferentiated daughter cell and one daughter cell committed to a specific cell fate. Neural stem cells are specialized cells that are limited to neural cell fate differentiation, such as neurons, astrocytes, and oligodendrocytes. There are two identified neural stem cell populations within the central nervous system, ependymal cells and subventricular zone astrocytes.

Neural stem cells (NSCs) are the self-renewing, multipotent cells that generate the main phenotypes of the nervous system. In 1989, Sally Temple described multipotent, self-renewing progenitor and stem cells in the subventricular zone of the mouse brain (Temple, S, Nature, 1989). In 1992, Brent A. Reynolds and Samuel Weiss were the first to isolate neural progenitor and stem cells from the adult striatal tissue, including the subventricular zone — one of the neurogenic areas — of adult mice brain tissue.[1] In the same year the team of Constance Cepko and Evan Y. Snyder were the first to isolate multipotent cells from the mouse cerebellum and stable transfected them with the oncogene v-myc.[2] Interestingly this molecule is one of the genes widely used now to reprogram adult non-stem cells into pluripotent stem cells. Since then, neural progenitor and stem cells have been isolated from various areas of the adult brain,[3]including non-neurogenic areas, such as the spinal cord, and from various species including human.[4]

Origin
The main distinction between an adult stem cell and an embryonic stem cell is that embryonic stem cells are totipotent. This means that they are not limited to a particular cell fate; rather they have the capability to differentiate into any chosen cell type. Embryonic stem cells are derived from the inner cell mass of the blastocyst with the ability to self-replicate multiple times. Neural stem cells are considered adult stem cells because they can only differentiate into other neural cell fates. Since neurons do not divide within the central nervous system, neural stem cells can be differentiated to replace lost or injured neurons or in some cases even glial cells. Adult neural stem cells are distinguished from other cells in the brain because of the formation of plutipotent neurospheres which have the ability to form secondary spheres. These neurospheres are what differentiate to form the specified neurons, glia, and oligodendrocytes.

Communication
Neural stem cells are stimulated to begin differentiation via exogenous cues from the environment. This capability of the neural stem cells to regenerate lost or damaged neural cells is called neurogenesis. Neurogenesis occurs in the subventricular zone (SVZ) and in the hippocampaldentral gyrus. Some neural cells are migrated from the SVZ along the rostral migratory stream which contains a marrow-like structure with ependymal cells and astrocytes when stimulated. The ependymal cells and astrocytes form glial tubes used by migrating neuroblasts. The astrocytes in the tubes provide support for the migrating cells as well as insulation from electrical and chemical signals released from surrounding cells. The astrocytes in the SVZ are the primary precursors for rapid cell amplification. The neuroblasts form tight chains and migrate towards the specified site of cell damage to repair or replace neural cells. One example is neuroblast migrating towards the olfactory bulb to differential to periglomercular or granule neurons which have a radial migration patter rather than a tangential one.

On the other hand, the hippocampaldentate gyrus neural stem cells produce excitatory granule neurons which are involved in learning and memory. One example is the learning and memory involved in pattern separation which is a cognitive process to distinguish similar inputs.

Functions
Epidermal growth factor (EGF) and fibroblast growth factor (FGF) are mitogens that promote neural progenitor and stem cell growth in vitro, though other factors synthesized by the neural progenitor and stem cell populations are also required for optimal growth.[5] It is hypothesized that neurogenesis in the adult brain originates from NSCs. The origin and identity of NSCs in the adult brain remain to be defined.

Neural Stem Cell Differentiation
The most widely accepted model of an adult neural stem cell is a radial, astrocytelike, GFAP-positive cell. Quiescent stem cells are Type B that are able to remain in the quiescent state due to the renewable tissue provided by the specific niches composed of blood vessels, astrocytes, microglia, espendymal cells, and extracellular matrix present within the brain. These niches provide nourishment, structural support, and protection for the stem cells until they are actived by external stimuli. Once activated, the Type B cells develop into Type C cells, active proliferating intermediate cells, which then divide into neuroblasts consisting of Type A cells. The undifferentiated neuroblasts form chains that migrate and develop into mature neurons. In the olfactory bulb, they mature into GABAergic granule neurons, while in the hippocampus they mature into dentate granule cells.

Function of neural stem cells (NSC) during disease
NSCs have an important role during development producing the enormous diversity of neurons, astrocytes and oligodendrocytes in the developing CNS. They also have important role in adult animals, for instance in learning and hippocampal plasticity in the adult mice in addition to supplying neurons to the olfactory bulb in mice.

Notably the role of NSCs during diseases is now being elucidated by several research groups around the world. The responses during stroke, multiple sclerosis, parkinson's disease in humans and in model of these diseases is part of the current investigation. The results of this ongoing investigation may have future applications to treat human neurological diseases.

Neural stem cells have been shown to engage in migration and replacement of dying neurons in classical experiments performed by Sanjay Magavi and Jeffrey Macklis.[6] Using a laser-induced damage of cortical layers, Magavi showed that SVZ neural progenitors expressing Doublecortin, a critical molecule for migration of neuroblasts, migrated long distances to the area of damage and differentiated into mature neurons expressing NeuN, a widely used neuronal marker. In addition Masato Nakafuku's group from Japan showed for the first time the role of hippocampal stem cells during stroke in mice.[7] These results demonstrated that NSCs can engage in the adult brain as a results of injury. Furthermore, In 2004 based on early work of Evan Snyder's group, that showed that NSCs migrate to brain tumors in a directed fashion, Jaime Imitola, M.D and colleagues from Harvard demonstrated for the first time, a molecular mechanism for the responses of NSCs to injury, they showed that chemokines released during injury such as SDF-1a were responsible for the directed migration of human and mouse NSCs to areas of injury in mice.[8] Since then other molecules have been found to participate in the responses of NSCs to Injury. All these results have been widely reproduced and expanded by other investigators joining the classical work of Altman and Sidman in 1960's as evidence of the responses of adult NSCs activities and neurogenesis during homeostasis and injury. The search for additional mechanisms that operate in the injury environment and how they influence the responses of NSCs during acute and chronic disease is matter of intense research.[9]

Regenerative therapy of the central nervous system
Cell death is the leading characteristic of central nervous system acute disorders as well as neurodegenerative disease. This phenomenon is amplified by the CNS’s lack of regenerative abilities for cell replacement and repair. One way to circumvent this is to use cell replacement therapy via regenerating neural stem cells. Neural stem cells can be cultured in vitro as neuroshperes. These neurospheres are composed of neural stem cells and progenitors (NSPCs) with growth factors such as EGF and FGF. The withdrawal of these growth factors active differentiation into neurons, astrocytes, or oligodendrocytes which can be transplanted within the brain at the site of injury. The benefits of this therapeutic approach have been examined in Parkinson disease, Huntington disease, and multiple sclerosis. NSPCs induce neural repair via intrinsic properties of neuroprotection and immunomodulation. Some possible routes of transplantation include intracerebral transplantation and xenotransplantation.

Generation of 3D in vitro models of the human CNS
Human midbrain-derived neural progenitor cells (hmNPC) have the ability to differentiate down multiple neural cell lineages that lead to neurospheres as well as multiple neural phenotypes. The hmNPC can be used to develop a 3D in vitro model of the human central nervous system. There are two ways to culture the hmNPCs including the adherent monolayer and the neurosphere culture system. The neurosphere culture system has previously been used to isolate and expand CNS stem cells by its ability to aggregate and proliferate NPCs under serum-free media conditions as well as with the presence of epidermal growth factor (EGF) and fibroblast growth factor-2 (FGF2). Initially, the hmNPCs was isolated and expanded before performing a 2D differentiation which was used to produce a single-cell suspension. This single-cell suspension helped achieve a homogenous 3D structure of uniform aggregate size. The 3D aggregation formed neurospheres which can be used to form an in vitro 3D CNS model.

Galectin-1 in Neural Stem Cells
Galectin-1 is expressed in adult neural stem cells and has been shown to have a physiological role in the treatment of neurological disorders in animal models. There are two approaches to using NSCs as a therapeutic treatment: (1) stimulate intrinsic NSCs to promote proliferation in order to replace injured tissue, and (2) transplant NSCs into the damaged brain area in order to allow the NSCs to restore the damaged tissue. A lentivirus vector was used to infect hNSCs with Galectin-1 which were later transplanted into the damaged tissue. The hGal-1-hNSCs induced better and faster brain recovery of the injured tissue as well as a reduction in motor and sensory deficits as compared to only hNSC transplantation.

Assays
Neural stem cells are routinely studied in vitro using a method referred to as the Neurosphere Assay (or Neurosphere culture system), first developed by Reynolds and Weiss.[1] Neurospheres are intrinsically heterogeneous cellular entities almost entirely formed by a small fraction (1 to 5%) of slowly dividing neural stem cells and by their progeny, a population of fast-dividing nestin-positive progenitor cells.[1][10][11] The total number of these progenitors determines the size of a neurosphere and, as a result, disparities in sphere size within different neurosphere populations may reflect alterations in the proliferation, survival and/or differentiation status of their neural progenitors. Indeed, Leone et al. (2005) have reported that loss of β1-integrin in a neurosphere culture does not significantly affect the capacity of β1-integrin deficient stem cells to form new neurospheres, but it influences the size of the neurosphere: β1-integrin deficient neurospheres were overall smaller due to increased cell death and reduced proliferation.[12] While the Neurosphere Assay has been the method of choice for isolation, expansion and even the enumeration of neural stem and progenitor cells, several recent publications have highlighted some of the limitations of the neurosphere culture system as a method for determining neural stem cell frequencies.[13] In collaboration with Reynolds, STEMCELL Technologies has developed a collagen-based assay, called the Neural Colony-Forming Cell (NCFC) Assay, for the quantification of neural stem cells. Importantly, this assay allows discrimination between neural stem and progenitor cells.[14]

Neural Stem Cell Institutes
The damaged central nervous system (CNS) tissue has very limited regenerative and repair capacity so that loss of neurological function is often chronic and progressive. Cell replacement from stem cells is being actively pursued as a therapeutic option. Recently in 2009, a research institute dedicated solely to translating neural stem research into therapies for patients was created outside of Albany, New York, The Neural Stem Cell Institute.