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

EphA4 preserves postnatal and adult neural stem cells in an undifferentiated state in vivo

In the postnatal brain, new neurons continue to be generated in two neurogenic areas, the subventricular zone of the lateral ventricles (SVZ) and the subgranular zone of the hippocampus. There is evidence that ephrins and their Eph receptors belong to a signaling network that regulates neurogenesis. On the basis of previous data, we have identified Eph receptor A4 (EphA4) as a potential regulator of neurogenesis. We showed by immunohistochemistry that in adult neurogenic niches EphA4 is expressed only by neural stem cells (NSCs). Using in vitro and in vivo assays, we demonstrated that EphA4 expression maintains NSCs in an undifferentiated state. Specifically, in neurosphere cultures Epha4 knockdown resulted in a decrease of NSC proliferation and premature differentiation. In postnatal and adult brain, Epha4 knockdown caused a decrease in NSCs in the SVZ, eventually resulting in a reduced number of postnatally generated neuroblasts. Both in vitro and in vivo effects were rescued by co-infection with a modified EphA4 that was resistant to Epha4 shRNA.

Visual activity regulates neural progenitor cells in developing xenopus CNS through musashi1

Regulation of progenitor cell fate determines the numbers of neurons in the developing brain. While proliferation of neural progenitors predominates during early central nervous system (CNS) development, progenitor cell fate shifts toward differentiation as CNS circuits develop, suggesting that signals from developing circuits may regulate proliferation and differentiation. We tested whether activity regulates neurogenesis in vivo in the developing visual system of Xenopus tadpoles. Both cell proliferation and the number of musashi1-immunoreactive progenitors in the optic tectum decrease as visual system connections become stronger. Visual deprivation for 2 days increased proliferation of musashi1-immunoreactive radial glial progenitors, while visual experience increased neuronal differentiation. Morpholino-mediated knockdown and overexpression of musashi1 indicate that musashi1 is necessary and sufficient for neural progenitor proliferation in the CNS. These data demonstrate a mechanism by which increased brain activity in developing circuits decreases cell proliferation and increases neuronal differentiation through the downregulation of musashi1 in response to circuit activity.

When neurogenesis encounters aging and disease

In this review, we consider the evidence that a reduction in neurogenesis underlies aging-related cognitive deficits and impairments in disorders such as Alzheimer's disease (AD). The molecular and cellular alterations associated with impaired neurogenesis in the aging brain are discussed. Dysfunction of presenilin-1, misprocessing of amyloid precursor protein and toxic effects of hyperphosphorylated tau and β-amyloid probably contribute to impaired neurogenesis in AD. Because factors such as exercise, environmental enrichment and dietary energy restriction enhance neurogenesis, and protect against age-related cognitive decline and AD, knowledge of the underlying neurogenic signaling pathways could lead to novel therapeutic strategies for preserving brain function. In addition, manipulation of endogenous neural stem cells and stem cell transplantation, as stand-alone or adjunct treatments, seems promising.

IGF-I stimulates neurogenesis in the hypothalamus of adult rats

In the brain of adult rats neurogenesis persists in the subventricular zone of the lateral ventricles and in the dentate gyrus of the hippocampus. By contrast, low proliferative activity was observed in the hypothalamus. We report here that, after intracerebroventricular treatment with insulin-like growth factor I (IGF-I), cell proliferation significantly increased in both the periventricular and the parenchymal zones of the whole hypothalamus. Neurons, astrocytes, tanycytes, microglia and endothelial cells of the local vessels were stained with the proliferative marker 5-bromo-2'-deoxyuridine (BrdU) in response to IGF-I. Conversely, we never observed BrdU-positive ciliated cubic ependymal cells. Proliferation was intense in the subventricular area of a distinct zone of the mid third ventricle wall limited dorsally by ciliated cubic ependyma and ventrally by tanycytic ependyma. In this area, we saw a characteristic cluster of proliferating cells. This zone of the ventricular wall displayed three cell layers: ciliated ependyma, subependyma and underlying tanycytes. After IGF-I treatment, proliferating cells were seen in the subependyma and in the layer of tanycytes. In the subependyma, proliferating glial fibrillary acidic protein-positive astrocytes contacted the ventricle by an apical process bearing a single cilium and there were many labyrinthine extensions of the periventricular basement membranes. Both features are typical of neurogenic niches in other brain zones, suggesting that the central overlapping zone of the rat hypothalamic wall could be considered a neurogenic niche in response to IGF-I.

Stem cells and neuroprotection: understanding the players

The use of neuroprotective therapies begs the question of how such therapies could affect preexisting stem cell populations within the host, as well as those introduced through cell-replacement therapy. Multiple mechanisms may mediate stem cell responses to neuroprotectants such as host/donor age and gender, cellular lineage/differentiation status, and mitochondrial dynamics. Current therapeutic sources for stem cells are embryonic, somatic, or induced pluripotent, with very little known about the effects of gender, age, cell type, and mitochondrial dynamics. With the advent of therapies to stimulate and recruit endogenous stem cells or transplant donor cells into damage areas in the hopes of recuperative regeneration of lost neurons, it is important to discuss mechanisms that dictate the winning players in the neuroprotection game. This review will focus on our current understanding of the characteristics of renewing stem cells that may affect neuroprotection.

(+)-Cholesten-3-one induces differentiation of neural stem cells into dopaminergic neurons through BMP signaling

To identify small molecules that induce dopaminergic neurons from neural stem cells (NSCs) is promising for therapy of Parkinson's disease. Here we report the results of analyzing structurally related steroids in traditional Chinese medicine to identify agents that enhance dopaminergic differentiation of NSCs. Using P19 cells transfected by tyrosine hydroxylase (TH) promoter reporter construct, (+)-Cholesten-3-one with carbonyl, but not cholesterol and cholesterol myristate can effectively promote the activity of TH promoter. This effect depends on bone morphogenetic protein (BMP) signaling. Phenotypic cellular analysis indicated that (+)-Cholesten-3-one induces differentiation of NSCs to dopaminergic neurons with increased expression of specific dopaminergic markers including TH, dopamine transporter, dopa decarboxylase and higher level of dopamine secretion. (+)-Cholesten-3-one significantly increases the expression of BMPR IB, but not BMPR IA or BMPR II; p-Smad1/5/8 positive nuclei and expression of p-Smad1/5/8 were detected in NSCs treated with (+)-Cholesten-3-one, indicating that (+)-Cholesten-3-one may activate the BMP signaling. Moreover, overexpression of BMP4 or inhibition of BMP affects the effect of (+)-Cholesten-3-one on the dopaminergic phenotype. These findings may contribute to efficient production of dopaminergic neurons from NSCs culture for many applications and raise interesting questions about the role of (+)-Cholesten-3-one in neurogenesis.

Endogenous interferon gamma directly regulates neural precursors in the non-inflammatory brain

Although a number of growth factors have been shown to be involved in neurogenesis, the role of inflammatory cytokines remains relatively unexplored in the normal brain. Here we investigated the effect of interferon gamma (IFNgamma) in the regulation of neural precursor (NP) activity in both the developing and the adult mouse brain. Exogenous IFNgamma inhibited neurosphere formation from the wild-type neonatal and adult subventricular zone (SVZ). More importantly, however, these effects were mirrored in vivo, with mutant mice lacking endogenous IFNgamma displaying enhanced neurogenesis, as demonstrated by an increase in proliferative bromodeoxyuridine-labeled cells in the SVZ and an increased percentage of newborn neurons in the olfactory bulb. Furthermore, NPs isolated from IFNgamma null mice exhibited an increase in self-renewal ability and in the capacity to produce differentiated neurons and oligodendrocytes. These effects resulted from the direct action of IFNgamma on the NPs, as determined by single-cell assays and the fact that nearly all the neurospheres were derived from cells positive for major histocompatibility complex class I antigen, a downstream marker of IFNgamma-mediated activation. Moreover, the inhibitory effect was ameliorated in the presence of SVZ-derived microglia, with their removal resulting in almost complete inhibition of NP proliferation. Interestingly, in contrast to the results obtained in the adult, exogenous IFNgamma treatment stimulated neurosphere formation from the embryonic brain, an effect that was mediated by sonic hedgehog. Together these findings provide the first direct evidence that IFNgamma acts as a regulator of the active NP pool in the non-inflammatory brain.

Neuronal network formation from reprogrammed early postnatal rat cortical glial cells

In the subependymal zone and the dentate gyrus of the adult brain of rodents, neural stem cells with glial properties generate new neurons in a life-long process. The identification of glial progenitors outside the neurogenic niches, oligodendrocyte precursors in the healthy brain, and reactive astrocytes after cortical injury led to the idea of using these cells as endogenous cell source for neural repair in the cerebral cortex. Recently, our group showed that proliferating astroglia from the cerebral cortex can be reprogrammed into neurons capable of action potential firing by forced expression of neurogenic fate determinants but failed to develop synapses. Here, we describe a maturation profile of cultured reprogrammed NG2+ and glial fibrillary acidic protein+ glia cells of the postnatal rat cortex that ends with the establishment of a glutamatergic neuronal network. Within 3 weeks after viral expression of the transcription factor neurogenin 2 (Ngn2), glia-derived neurons exhibit network-driven, glutamate receptor-dependent oscillations in Ca(2+) and exhibit functional pre- and postsynaptic specialization. Interestingly, the Ngn2-instructed glutamatergic network also supports the maturation of a γ-aminobutyric acid (GABA)ergic input via GABA(A) receptors in a non-cell autonomous manner. The "proof-of-principle" results imply that a single transcription factor may be sufficient to instruct a neuronal network from a glia-like cell source.

LRP2 in ependymal cells regulates BMP signaling in the adult neurogenic niche

The microenvironment of growth factors in the subependymal zone (SEZ) of the adult brain provides the instructive milieu for neurogenesis to proceed in this germinal niche. In particular, tight regulation of bone morphogenetic protein (BMP) signaling is essential to balance proliferative and non-proliferative cell fate specification. However, the regulatory pathways that control BMP signaling in the SEZ are still poorly defined. We demonstrate that LRP2, a clearance receptor for BMP4 is specifically expressed in ependymal cells of the lateral ventricles in the adult brain. Intriguingly, expression is restricted to the ependyma that faces the stem cell niche. Expression is not seen in ependyma elsewhere in the lateral ventricles or in the dentate gyrus, the second major neurogenic zone of the adult brain. We further show that lack of LRP2 expression in adult mice results in impaired proliferation of neural precursor cells in the SEZ resulting in decreased numbers of neuroblasts reaching the olfactory bulb. Reduced neurogenesis coincides with increased BMP4 expression and enhanced activation of downstream mediators phospho-SMAD1/5/8 and ID3 in the stem cell niche. Our findings suggest a novel mechanism whereby LRP2-mediated catabolism of BMP4 in the ependyma modulates the microenvironment of the SEZ and enables adult neurogenesis to proceed.

Impact of actin filament stabilization on adult hippocampal and olfactory bulb neurogenesis

Rearrangement of the actin cytoskeleton is essential for dynamic cellular processes. Decreased actin turnover and rigidity of cytoskeletal structures have been associated with aging and cell death. Gelsolin is a Ca(2+)-activated actin-severing protein that is widely expressed throughout the adult mammalian brain. Here, we used gelsolin-deficient (Gsn(-/-)) mice as a model system for actin filament stabilization. In Gsn(-/-) mice, emigration of newly generated cells from the subventricular zone into the olfactory bulb was slowed. In vitro, gelsolin deficiency did not affect proliferation or neuronal differentiation of adult neural progenitors cells (NPCs) but resulted in retarded migration. Surprisingly, hippocampal neurogenesis was robustly induced by gelsolin deficiency. The ability of NPCs to intrinsically sense excitatory activity and thereby implement coupling between network activity and neurogenesis has recently been established. Depolarization-induced [Ca(2+)](i) increases and exocytotic neurotransmitter release were enhanced in Gsn(-/-) synaptosomes. Importantly, treatment of Gsn(-/-) synaptosomes with mycotoxin cytochalasin D, which, like gelsolin, produces actin disassembly, decreased enhanced Ca(2+) influx and subsequent exocytotic norepinephrine release to wild-type levels. Similarly, depolarization-induced glutamate release from Gsn(-/-) brain slices was increased. Furthermore, increased hippocampal neurogenesis in Gsn(-/-) mice was associated with a special microenvironment characterized by enhanced density of perfused vessels, increased regional cerebral blood flow, and increased endothelial nitric oxide synthase (NOS-III) expression in hippocampus. Together, reduced filamentous actin turnover in presynaptic terminals causes increased Ca(2+) influx and, subsequently, elevated exocytotic neurotransmitter release acting on neural progenitors. Increased neurogenesis in Gsn(-/-) hippocampus is associated with a special vascular niche for neurogenesis.

The nuclear receptor tailless induces long-term neural stem cell expansion and brain tumor initiation

Malignant gliomas are the most common primary brain tumors, and are associated with frequent resistance to therapy as well as poor prognosis. Here we demonstrate that the nuclear receptor tailless (Tlx), which in the adult is expressed exclusively in astrocyte-like B cells of the subventricular zone, acts as a key regulator of neural stem cell (NSC) expansion and brain tumor initiation from NSCs. Overexpression of Tlx antagonizes age-dependent exhaustion of NSCs in mice and leads to migration of stem/progenitor cells from their natural niche. The increase of NSCs persists with age, and leads to efficient production of newborn neurons in aged brain tissues. These cells initiate the development of glioma-like lesions and gliomas. Glioma development is accelerated upon loss of the tumor suppressor p53. Tlx-induced NSC expansion and gliomagenesis are associated with increased angiogenesis, which allows for the migration and maintenance of brain tumor stem cells in the perivascular niche. We also demonstrate that Tlx transcripts are overexpressed in human primary glioblastomas in which Tlx expression is restricted to a subpopulation of nestin-positive perivascular tumor cells. Our study clearly demonstrates how NSCs contribute to brain tumorgenesis driven by a stem cell-specific transcription factor, thus providing novel insights into the histogenesis and molecular pathogenesis of primary brain tumors.

Neuronal Ras activation inhibits adult hippocampal progenitor cell division and impairs spatial short-term memory

A large number of endogenous and exogenous factors have been identified to upregulate and downregulate proliferation, differentiation and/or survival of newborn cells in the adult hippocampus. For studying neuronal mechanisms mediating the impact of those factors, we used a transgenic synRas mouse model expressing constitutively activated Valin12-Harvey Ras selectively in differentiated neurons. BrdU injections showed significantly reduced proliferation of new cells within the adult hippocampus of transgenic animals compared with their wild-type siblings. In contrast, the relative survival of newborn cells was increased in synRas mice, although this effect did not fully compensate for diminished proliferation. Inhibition of progenitor cell proliferation and enhancement of cellular survival were more pronounced in males compared with females. Double labelling and doublecortin immunostaining verified that specifically newborn neurons were decreased in synRas mice. Reduced cell generation was observed already 2 h after BrdU pulse injections, identifying an early precursor cell population as target of the inhibitory transgene effect. Differences in proliferation remained stable after 24 h and were specific for the subgranular zone of the dentate gyrus, as subventricular cell generation was not affected supporting a non-cell autonomous effect on neural hippocampal progenitors. Transgene expression only starts with synaptic differentiation and therefore reduced proliferation must represent an indirect secondary consequence of synRas activity in differentiated neurons. This was associated with impaired spatial short-term memory capacities as observed in a radial maze paradigm. Our data suggest that constantly high Ras activity in differentiated neurons downregulates hippocampal precursor cell generation in the neuronal lineage, but is modulated by sex-dependent factors.

Cell cycle control of mammalian neural stem cells: putting a speed limit on G1

The potential to increase unlimitedly in number and to generate differentiated cell types is a key feature of somatic stem cells. Within the nervous system, cellular and environmental determinants tightly control the expansion and differentiation of neural stem cells. Importantly, a number of studies have indicated that changes in cell cycle length can influence development and physiopathology of the nervous system, and might have played a role during evolution of the mammalian brain. Specifically, it has been suggested that the length of G1 can directly influence the differentiation of neural precursors. This has prompted the proposal of a model to explain how manipulation of G1 length can be used to expand neural stem cells. If validated in non-neural systems, this model might provide the means to control the proliferation vs. differentiation of somatic stem cells, which will represent a significant advance in the field.

Signaling involved in neurite outgrowth of postnatally born subventricular zone neurons in vitro

BACKGROUND

Neurite outgrowth is a key process during neuronal migration and differentiation. Complex intracellular signaling is involved in the initiation of neurite protrusion and subsequent elongation. Although, in general many constituents of the machinery involved in this multi-stage process are common for neurons in distinct brain areas, there are notable differences between specific neuronal subtypes.

RESULTS

We analyzed key intracellular components of neurite outgrowth signaling in postnatally born subventricular zone (SVZ) neurons in vitro. We showed that inhibitors of PI3K, Akt1, PKCzeta and small GTPases significantly reduced neurite outgrowth. Transfection of SVZ-derived neurons with inactive forms of Rac1 or Cdc42 also decreased neurite length whereas transfection with constitutively active forms of Rac1, Cdc42 or Akt1 as well as with full-length PI3K or PKCzeta increased neurite length. PI3K, Akt1 and PKCzeta acted upstream of the small GTPases Rac1 and Cdc42, which in turn modulate lamellipodia formation of SVZ-derived neurons.

CONCLUSION

We showed the involvement of PI3K/Akt1/PKCzeta/Rac1/Cdc42 pathway in neurite outgrowth of postnatally born SVZ neurons.

A mesenchymal-like ZEB1(+) niche harbors dorsal radial glial fibrillary acidic protein-positive stem cells in the spinal cord

In humans and rodents the adult spinal cord harbors neural stem cells located around the central canal. Their identity, precise location, and specific signaling are still ill-defined and controversial. We report here on a detailed analysis of this niche. Using microdissection and glial fibrillary acidic protein (GFAP)-green fluorescent protein (GFP) transgenic mice, we demonstrate that neural stem cells are mostly dorsally located GFAP(+) cells lying ependymally and subependymally that extend radial processes toward the pial surface. The niche also harbors doublecortin protein (Dcx)(+) Nkx6.1(+) neurons sending processes into the lumen. Cervical and lumbar spinal cord neural stem cells maintain expression of specific rostro-caudal Hox gene combinations and the niche shows high levels of signaling proteins (CD15, Jagged1, Hes1, differential screening-selected gene aberrative in neuroblastoma [DAN]). More surprisingly, the niche displays mesenchymal traits such as expression of epithelial-mesenchymal-transition zinc finger E-box-binding protein 1 (ZEB1) transcription factor and smooth muscle actin. We found ZEB1 to be essential for neural stem cell survival in vitro. Proliferation within the niche progressively ceases around 13 weeks when the spinal cord reaches its final size, suggesting an active role in postnatal development. In addition to hippocampus and subventricular zone niches, adult spinal cord constitutes a third central nervous system stem cell niche with specific signaling, cellular, and structural characteristics that could possibly be manipulated to alleviate spinal cord traumatic and degenerative diseases.

Sonic hedgehog is a polarized signal for motor neuron regeneration in adult zebrafish

In contrast to mammals, the spinal cord of adult zebrafish has the capacity to reinitiate generation of motor neurons after a lesion. Here we show that genes involved in motor neuron development, i.e., the ventral morphogen sonic hedgehog a (shha), as well as the transcription factors nkx6.1 and pax6, together with a Tg(olig2:egfp) transgene, are expressed in the unlesioned spinal cord of adult zebrafish. Expression is found in ependymoradial glial cells lining the central canal in ventrodorsal positions that match expression domains of these genes in the developing neural tube. Specifically, Tg(olig2:egfp)(+) ependymoradial glial cells, the adult motor neuron progenitors (pMNs), coexpress Nkx6.1 and Pax6, thus defining an adult pMN-like zone. shha is expressed in distinct ventral ependymoradial glial cells. After a lesion, expression of all these genes is strongly increased, while relative spatial expression domains are maintained. In addition, expression of the hedgehog (hh) receptors patched1 and smoothened becomes detectable in ependymoradial glial cells including those of the pMN-like zone. Cyclopamine-induced knock down of hh signaling significantly reduces ventricular proliferation and motor neuron regeneration. Expression of indicator genes for the FGF and retinoic acid signaling pathways was also increased in the lesioned spinal cord. This suggests that a subclass of ependymoradial glial cells retain their identity as motor neuron progenitors into adulthood and are capable of reacting to a sonic hedgehog signal and potentially other developmental signals with motor neuron regeneration after a spinal lesion.

Transforming Growth Factor type beta and Smad family signaling in stem cell function

Ligands of the Transforming Growth Factor type beta (TGFbeta) family exert multiple and sometimes opposite effects on most cell types in vivo depending on cellular context, which mainly includes the stage of the target cell, the local environment of this cell or niche, and the identity and the dosage of the ligand. Significant progress has been made in the molecular dissection of the regulation of the activity of the ligands and their intracellular signal transduction pathways, including via the canonical Smad pathway where Smads interact with many transcription factors. This knowledge together with results from functional studies within the embryology and stem cell research fields is giving us insight in the role of individual ligands and other components of this signaling system and where and how it regulates many properties of embryonic and adult stem/progenitor cells, which is anticipated to contribute to successful cell-based therapy in the future. We review and discuss recent progress on the effects of Nodal/Activin and Bone Morphogenetic Proteins (BMPs) and their canonical signaling in cells with stem cell properties. We focus on embryonic stem cells and their maintenance and pluripotency, and conversion into selected cell types of neuroectoderm, mesoderm and endoderm, on induced pluripotent cells and on neurogenic cells in the adult brain.

Intact and injured endothelial cells differentially modulate postnatal murine forebrain neural stem cells

Neural stem cells (NSCs) persist in the forebrain subventricular zone (SVZ) within a niche containing endothelial cells. Evidence suggests that endothelial cells stimulate NSC expansion and neurogenesis. Experimental stroke increases neurogenesis and angiogenesis, but how endothelial cells influence stroke-induced neurogenesis is unknown. We hypothesized intact or oxygen-glucose deprived (OGD) endothelial cells secrete factors that enhance neurogenesis. We co-cultured mouse SVZ neurospheres (NS) with endothelial cells, or differentiated NS in endothelial cell-conditioned medium (ECCM). NS also were expanded in ECCM from OGD-exposed (OGD-ECCM) endothelial cells to assess injury effects. ECCM significantly increased NS production. NS co-cultured with endothelial cells or ECCM generated more immature-appearing neurons and oligodendrocytes, and astrocytes with radial glial-like/reactive morphology than controls. OGD-ECCM stimulated neuroblast migration and yielded neurons with longer processes and more branching. These data indicate that intact and injured endothelial cells exert differing effects on NSCs, and suggest targets for stimulating regeneration after brain insults.

A synergistic approach for neural repair: cell transplantation and induction of endogenous precursor cell activity

Stem cell research offers enormous potential for treating many diseases of the nervous system. At present, therapeutic strategies in stem cell research segregate into two approaches: cell transplantation or endogenous cell stimulation. Realistically, future cell therapies will most likely involve a combination of these two approaches, a theme of our current research. Here, we propose that there exists a 'synergy' between exogenous (transplanted) and endogenous stem cell actions that can be utilized to achieve therapeutic ends. Elucidating mechanisms underlying this exogenous-endogenous stem cell synergism may lead to the development of optimal cell therapies for neural disorders.

DISC1 regulates new neuron development in the adult brain via modulation of AKT-mTOR signaling through KIAA1212

Disrupted-in-schizophrenia 1 (DISC1), a susceptibility gene for major mental illnesses, regulates multiple aspects of embryonic and adult neurogenesis. Here, we show that DISC1 suppression in newborn neurons of the adult hippocampus leads to overactivated signaling of AKT, another schizophrenia susceptibility gene. Mechanistically, DISC1 directly interacts with KIAA1212, an AKT binding partner that enhances AKT signaling in the absence of DISC1, and DISC1 binding to KIAA1212 prevents AKT activation in vitro. Functionally, multiple genetic manipulations to enhance AKT signaling in adult-born neurons in vivo exhibit similar defects as DISC1 suppression in neuronal development that can be rescued by pharmacological inhibition of mammalian target of rapamycin (mTOR), an AKT downstream effector. Our study identifies the AKT-mTOR signaling pathway as a critical DISC1 target in regulating neuronal development and provides a framework for understanding how multiple susceptibility genes may functionally converge onto a common pathway in contributing to the etiology of certain psychiatric disorders.

Neurogenesis and Alzheimer's disease: at the crossroads

While a massive and progressive neuronal loss in specific areas such as the hippocampus and cortex unequivocally underlies cognitive deterioration and memory loss in Alzheimer's disease, noteworthy alterations take place in the neurogenic microenvironments, namely, the subgranule layer of the dentate gyrus and the subventricular zone. Compromised neurogenesis presumably takes place earlier than onset of hallmark lesions or neuronal loss, and may play a role in the initiation and progression of neuropathology in Alzheimer's disease. Neurogenesis in the adult brain is thought to play a role in numerous forms and aspects of learning and memory and contribute to the plasticity of the hippocampus and olfactory system. Misregulated or impaired neurogenesis on the other hand, may compromise plasticity and neuronal function in these areas and exacerbate neuronal vulnerability. Interestingly, increasing evidence suggests that molecular players in Alzheimer's disease, including PS1, APP and its metabolites, play a role in adult neurogenesis. In addition, recent studies suggest that alterations in tau phosphorylation are pronounced in neurogenic areas, and may interfere with the potential central role of tau proteins in neuronal maturation and differentiation. On the other hand, numerous neurogenic players, such as Notch-1, ErbB4 and L1 are substrates of alpha- beta- and gamma- secretase that play a major role in Alzheimer's disease. This review will discuss current knowledge concerning alterations of neurogenesis in Alzheimer's disease with specific emphasis on the cross-talk between signaling molecules involved in both processes, and the ways by which familial Alzheimer's disease-linked dysfunction of these signaling molecules affect neurogenesis in the adult brain.

Adult neural stem cells for the treatment of neuroinflammation. Fondazione Centro San Raffaele del Monte Tabor: WO2007015173

BACKGROUND

The application is in the field of stem cells and their therapeutic application for the treatment of inflammation.

OBJECTIVE

It aims at characterizing the potential of adult-derived neural progenitor and stem cells for the treatment of inflammation of the central nervous system (CNS).

METHODS

Neural progenitor and stem cells were isolated and expanded from the subventricular zone (SVZ) of adult mice (aNSCs). They were administered intravenously in an animal model of multiple sclerosis (MS).

RESULTS

Mice transplanted either at the disease onset or at the onset of the first relapse show clinical signs of improvements. Adult NSCs exert their therapeutical activity by reducing neuroinflammation.

CONCLUSION

The application claims the use of aNSCs and multipotent somatic stem cells for the treatment of inflammation, associated with neurological diseases, disorders and injuries particularly, and for inducing tolerance to the immune central and/or peripheral system.

Major signaling pathways in migrating neuroblasts

Neuronal migration is a key process in the developing and adult brain. Numerous factors act on intracellular cascades of migrating neurons and regulate the final position of neurons. One robust migration route persists postnatally – the rostral migratory stream (RMS). To identify genes that govern neuronal migration in this unique structure, we isolated RMS neuroblasts by making use of transgenic mice that express EGFP in this cell population and performed microarray analysis on RNA. We compared gene expression patterns of neuroblasts obtained from two sites of the RMS, one closer to the site of origin, the subventricular zone, and one closer to the site of the final destination, the olfactory bulb (OB). We identified more than 400 upregulated genes, many of which were not known to be involved in migration. These genes were grouped into functional networks by bioinformatics analysis. Selecting a specific upregulated intracellular network, the cytoskeleton pathway, we confirmed by functional in vitro and in vivo analysis that the identified genes of this network affected RMS neuroblast migration. Based on the validity of this approach, we chose four new networks and tested by functional in vivo analysis their involvement in neuroblast migration. Thus, knockdown of Calm1, Gria1 (GluA1) and Camk4 (calmodulin-signaling network), Hdac2 and Hsbp1 (Akt1-DNA transcription network), Vav3 and Ppm1a (growth factor signaling network) affected neuroblast migration to the OB.

Coordinate pathways for nucleotide and EGF signaling in cultured adult neural progenitor cells

The adult subventricular zone (SVZ) contains astrocyte-like stem cells capable of generating new neurons for the olfactory bulb. Adult neurogenesis is driven by a variety of signal systems that can induce synergistic or opposing cellular responses. It is therefore important to gain insight into the underlying downstream signaling pathways. We have previously shown that the nucleotides ADPbetaS and UTP induce rapid Ca2+ transients in cultured SVZ-derived adult neural progenitors and augment growth-factor-mediated progenitor cell proliferation. Here, we investigated signaling pathways elicited by ADPbetaS, UTP and epidermal growth factor (EGF). All three agonists elicit ERK1/2 and CREB phosphorylation but the temporal characteristics differ between the nucleotides and EGF. Differentiation of the progenitors alters the receptor profile. Oligodendrocytes and young neurons, but not astrocytes, lose responsiveness to the agonists. Inhibition experiments are indicative of an ADPbetaS-elicited EGF receptor transactivation. Whereas UTP acts via the P2Y2 receptor, ADPbetaS exerts its function via the P2Y1 receptor and the P2Y13 receptor. Our data demonstrate that nucleotides and EGF induce converging, but also differential, intracellular signaling pathways and suggest that they carry the potential to act synergistically in the control of cell proliferation and cell survival in adult neurogenesis.

Early embryonic brain development in rats requires the trophic influence of cerebrospinal fluid

Cerebrospinal fluid has shown itself to be an essential brain component during development. This is particularly evident at the earliest stages of development where a lot of research, performed mainly in chick embryos, supports the evidence that cerebrospinal fluid is involved in different mechanisms controlling brain growth and morphogenesis, by exerting a trophic effect on neuroepithelial precursor cells (NPC) involved in controlling the behaviour of these cells. Despite it being known that cerebrospinal fluid in mammals is directly involved in corticogenesis at fetal stages, the influence of cerebrospinal fluid on the activity of NPC at the earliest stages of brain development has not been demonstrated. Here, using "in vitro" organotypic cultures of rat embryo brain neuroepithelium in order to expose NPC to or deprive them of cerebrospinal fluid, we show that the neuroepithelium needs the trophic influence of cerebrospinal fluid to undergo normal rates of cell survival, replication and neurogenesis, suggesting that NPC are not self-sufficient to induce their normal activity. This data shows that cerebrospinal fluid is an essential component in chick and rat early brain development, suggesting that its influence could be constant in higher vertebrates.

A niche for adult neurogenesis in social behavior

The structural and functional changes occurring into the brain is the hallmark of its tremendous capacity for dealing with the complexity that we are facing throughout life. It is also the hallmark of what neuroscientists refer as neuroplasticity. The continuous generation of cohorts of new neurons in some discrete regions of the adult brain, including the olfactory system, is a newly recognized form of neuroplasticity that has been recently the focus of neuroscience studies. Several lines of evidence indicate that this recruitment of newly-generated neurons is extremely sensitive to the overall neuronal activity of the host circuits. Therefore, adult neurogenesis represents, not only a constitutive replacement mechanism for lost neurons, but also a process supporting a capacity of neural plasticity in response to specific experience throughout life. The remarkable complexity of the social life offers a host of daily challenges that require a diversity of brain mechanism to make sense of the ever-changing social world. This review describes some recent findings which have begun to define reciprocal relationships between the production and integration of newborn neurons in the adult brain and social behavior. These studies demonstrate how this domain of research has the potential to address issues in the functional contribution of adult neurogenesis in the expression of some social traits as well in the role of some social contexts to finely regulate the production, survival and integration of adult newborn neurons.

Postinjury niches induce temporal shifts in progenitor fates to direct lesion repair after spinal cord injury

Progenitors that express NG2-proteoglycan are the predominant self-renewing cells within the CNS. NG2 progenitors replenish oligodendrocyte populations within the intact stem cell niche, and cycling NG2 cells are among the first cells to react to CNS insults. We investigated the role of NG2 progenitors after spinal cord injury and how bone morphogen protein signals remodel the progressive postinjury (PI) niche. Progeny labeled by an NG2-specific reporter virus undergo a coordinated shift in differentiation profile. NG2 progeny born 24 h PI produce scar-forming astrocytes and transient populations of novel phagocytic astrocytes shown to contain denatured myelin within cathepsin-D-labeled endosomes, but NG2 progenitors born 7 d PI differentiate into oligodendrocytes and express myelin on processes that wrap axons. Analysis of spinal cord mRNA shows a temporal shift in the niche transcriptome of ligands that affect PI remodeling and direct progenitor differentiation. We conclude that NG2 progeny are diverse lineages that obey progressive cues after trauma to replenish the injured niche.

Analysis of neuronal proliferation, migration and differentiation in the postnatal brain using equine infectious anemia virus-based lentiviral vectors

Ongoing neurogenesis in discrete sectors of the adult central nervous system depends on the mitotic activity of an elusive population of adult stem cells. The existence of adult neural stem cells provides an alternative approach to transplantation of embryonic stem cells in cell-based therapies. Owing to the limited intrinsic fate of adult stem cells and inhibitory nature of the adult brain for neurogenesis, accommodation for circuit replacement in the brain will require genetic and epigenetic manipulation. Here, we show that a replication-incompetent Equine Infectious Anemia Virus (EIAV) is highly suitable for stable and persistent gene transfer to adult neural stem cells. The transduced regions were free of long-lasting neuroimmune responses to EIAV. Transduction in the subventricular zone was specific to the stem cell niche, but spared the progeny of adult neural stem cells that includes transit amplifying progenitors (TAPs) and migrating neuroblasts. With time, EIAV-transduced stem cells passed on the transgene to TAPs and migrating neuroblasts, which ultimately differentiated into neurons in the olfactory bulbs. We show that EIAV is highly suitable for discovery and assessment of mechanisms that regulate proliferation, migration and differentiation in the postnatal brain.

Human neural progenitor and stem cells: US20080107633A1

BACKGROUND

The application is in the field of neural stem cells (NSCs) and cellular therapy.

OBJECTIVE

It aims at establishing conditions for the isolation and propagation of neural progenitor and stem cells from human fetal tissue, with high rate of growth and high yields of differentiation into the neuronal, astroglial and oligodendroglial pathways.

METHODS

Neural progenitor and stem cells were isolated from fetal forebrain tissue and propagated as neurospheres, in defined medium in the presence of leukemia inhibitory factor (LIF). Three protocols were designed to differentiate human fetal neural progenitor and stem cells into their progenies.

RESULTS

The application claims the generation of human fetal neural progenitor and stem cells with a doubling rate between 5-10 days. It claims the differentiation of the neural progenitor and stem cells in vitro, into neurons, astrocytes and oligodendrocytes with high yields, e.g., 20 to 35% for neuronal cells.

CONCLUSION

The establishment of human neural progenitor and stem cells with high rate of growth and high yields of differentiation provides a source of cells for therapy, particularly for the treatment of neurodegenerative diseases, like Alzheimer's and Parkinson's diseases.

Magnetic resonance imaging for monitoring neurogenesis in the adult hippocampus

BACKGROUND

The application is in the field of bioimaging and adult neurogenesis.

OBJECTIVE

It aims at correlating the volume of cerebral blood (CBV) in the dentate gyrus (DG) of the human hippocampus, determined by magnetic resonance imaging (MRI), with neurogenesis in the brain of adult rodents.

METHODS

Adult mice were submitted to voluntary exercise or administration of fluoxetine or valproic acid (VPA). The CBV of DG was determined by MRI and neurogenesis was quantified by immunohistofluorescence. The CBV in human subjects selected and grouped according to their fitness activity was determined by MRI in the DG.

RESULTS

A selective increase in the CBV of the DG is observed in rodents housed in activity cages or administered with fluoxetine and VPA. A selective increase in the CBV of the DG is also observed in exercising humans. The selective increase of the CBV in the human DG correlates with the selective increase of the CBV in the DG and neurogenesis induced by exercise or fluoxetine and VPA in rodents.

CONCLUSION

This indicates that neurogenesis is increased in the DG of exercising humans. The application claims the imaging of the DG of patients by MRI as a paradigm to monitor neurogenesis and identify treatments involving stimulation of neurogenesis.

Cell migration in the normal and pathological postnatal mammalian brain

In the developing brain, cell migration is a crucial process for structural organization, and is therefore highly regulated to allow the correct formation of complex networks, wiring neurons, and glia. In the early postnatal brain, late developmental processes such as the production and migration of astrocyte and oligodendrocyte progenitors still occur. Although the brain is completely formed and structured few weeks after birth, it maintains a degree of plasticity throughout life, including axonal remodeling, synaptogenesis, but also neural cell birth, migration and integration. The subventricular zone (SVZ) and the dentate gyrus (DG) of the hippocampus are the two main neurogenic niches in the adult brain. Neural stem cells reside in these structures and produce progenitors that migrate toward their ultimate location: the olfactory bulb and granular cell layer of the DG respectively. The aim of this review is to synthesize the increasing information concerning the organization, regulation and function of cell migration in a mature brain. In a normal brain, proteins involved in cell-cell or cell-matrix interactions together with secreted proteins acting as chemoattractant or chemorepellant play key roles in the regulation of neural progenitor cell migration. In addition, recent data suggest that gliomas arise from the transformation of neural stem cells or progenitor cells and that glioma cell infiltration recapitulates key aspects of glial progenitor migration. Thus, we will consider glioma migration in the context of progenitor migration. Finally, many observations show that brain lesions and neurological diseases trigger neural stem/progenitor cell activation and migration toward altered structures. The factors involved in such cell migration/recruitment are just beginning to be understood. Inflammation which has long been considered as thoroughly disastrous for brain repair is now known to produce some positive effects on stem/progenitor cell recruitment via the regulation of growth factor signaling and the secretion of a number of chemoattractant cytokines. This knowledge is crucial for the development of new therapeutic strategies. One of these strategies could consist in increasing the mobilization of endogenous progenitor cells that could replace lost cells and improve functional recovery.

Mobilization of neural stem cells and generation of new neurons in 6-OHDA-lesioned rats by intracerebroventricular infusion of liver growth factor

Neural stem cells with self-renewal and multilineage potential persist in the subventricular zone of the adult mammalian forebrain. These cells remain relatively quiescent but, under certain conditions, can be stimulated, giving rise to new neurons. Liver growth factor (LGF) is a mitogen for liver cells that shows biological activity in extrahepatic sites and is useful for neuroregenerative therapies. The aim of this study was to investigate the potential neurogenic activity of LGF in the 6-hydroxydopamine rat model of Parkinson's disease. Proliferation was significantly increased in the subventricular zone and denervated striatum of rats receiving ICV LGF infusions, and 25% of the proliferating cells were doublecortin-positive neurons. Doublecortin-positive cells with the morphology of migrating neuroblasts were also observed in the dorsal and ventral regions of the striatum of LGF-infused animals. Moreover, some newly generated cells were neuronal nuclei-positive mature neurons. LGF also stimulated microglia and induced astrogliosis, both phenomena associated with generation and migration of new neurons in the adult brain. In summary, our study shows that LGF stimulates neurogenesis when applied intraventricularly in 6-hydroxydopamine-lesioned rats. Considering that this factor also promotes neuronal migration into damaged tissue, we propose LGF as a novel factor useful for neuronal replacement in neurodegenerative diseases.

Nitric oxide regulates activity-dependent neuroprotective protein (ADNP) in the dentate gyrus of the rodent model of kainic acid-induced seizure

The dentate gyrus (DG) of the normal rat brain contains activity-dependent neuroprotective protein (ADNP) which is widely distributed in the cytoplasm of neurons and astrocytes. Treatment with nitric oxide (NO) synthase (NOS) inhibitor N(G)-nitro-L: -arginine methyl ester (L: -NAME) caused a decrease in ADNP expression in granule cells which persisted 3 days post-treatment. However, treatment with neuronal-specific NOS inhibitor, 7-nitroindazole (7-NI), or soluble guanylyl cyclase inhibitor, ODQ, did not change ADNP expression in the DG. We have previously shown that kainic acid (KA)-induced seizure increases neuronal NOS in neurons and inducible NOS in glia cells and suppresses ADNP in the hippocampus (Cosgrave et al., Neurobiol Dis 30(3):281-292, 2008). In the DG, L: -NAME treatment prior to KA causes ADNP synthesis in granule cells by 3 h which was later restricted to the subgranular zone by 3 days. 7-NI and ODQ had no effect. Double immunostaining for neuronal marker NeuN and ADNP revealed a significant decrease of both ADNP(+) neurons and of total neuron numbers (NeuN(+)) in the hilus of animals having KA-induced seizure that had been pretreated with L: -NAME implying that NO and ADNP may act together to protect hilar neurons. Overall, these observations suggest that NO regulates ADNP in the DG under both basal and pathophysiological conditions.

Regulation of neural progenitor cell development in the nervous system

The mammalian telencephalon, which comprises the cerebral cortex, olfactory bulb, hippocampus, basal ganglia, and amygdala, is the most complex and intricate region of the CNS. It is the seat of all higher brain functions including the storage and retrieval of memories, the integration and processing of sensory and motor information, and the regulation of emotion and drive states. In higher mammals such as humans, the telencephalon also governs our creative impulses, ability to make rational decisions, and plan for the future. Despite its massive complexity, exciting work from a number of groups has begun to unravel the developmental mechanisms for the generation of the diverse neural cell types that form the circuitry of the mature telencephalon. Here, we review our current understanding of four aspects of neural development. We first begin by providing a general overview of the broad developmental mechanisms underlying the generation of neuronal and glial cell diversity in the telencephalon during embryonic development. We then focus on development of the cerebral cortex, the most complex and evolved region of the brain. We review the current state of understanding of progenitor cell diversity within the cortical ventricular zone and then describe how lateral signaling via the Notch-Delta pathway generates specific aspects of neural cell diversity in cortical progenitor pools. Finally, we review the signaling mechanisms required for development, and response to injury, of a specialized group of cortical stem cells, the radial glia, which act both as precursors and as migratory scaffolds for newly generated neurons.

Deletion of TrkB in adult progenitors alters newborn neuron integration into hippocampal circuits and increases anxiety-like behavior

New neurons in the adult dentate gyrus are widely held to incorporate into hippocampal circuitry via a stereotypical sequence of morphological and physiological transitions, yet the molecular control over this process remains unclear. We studied the role of brain-derived neurotrophic factor (BDNF)/TrkB signaling in adult neurogenesis by deleting the full-length TrkB via Cre expression within adult progenitors in TrkB(lox/lox) mice. By 4 weeks after deletion, the growth of dendrites and spines is reduced in adult-born neurons demonstrating that TrkB is required to create the basic organization of synaptic connections. Later, when new neurons normally display facilitated synaptic plasticity and become preferentially recruited into functional networks, lack of TrkB results in impaired neurogenesis-dependent long-term potentiation and cell survival becomes compromised. Because of the specific lack of TrkB signaling in recently generated neurons a remarkably increased anxiety-like behavior was observed in mice carrying the mutation, emphasizing the contribution of adult neurogenesis in regulating mood-related behavior.

BM88/Cend1 expression levels are critical for proliferation and differentiation of subventricular zone-derived neural precursor cells

Neural stem cells remain in two areas of the adult mammalian brain, the subventricular zone (SVZ) and the dentate gyrus of the hippocampus. Ongoing neurogenesis via the SVZ-rostral migratory stream pathway maintains neuronal replacement in the olfactory bulb (OB) throughout life. The mechanisms determining how neurogenesis is restricted to only a few regions in the adult, in contrast to its more widespread location during embryogenesis, largely depend on controlling the balance between precursor cell proliferation and differentiation. BM88/Cend1 is a neuronal lineage-specific regulator implicated in cell cycle exit and differentiation of precursor cells in the embryonic neural tube. Here we investigated its role in postnatal neurogenesis. Study of in vivo BM88/Cend1 distribution revealed that it is expressed in low levels in neuronal precursors of the adult SVZ and in high levels in postmitotic OB interneurons. To assess the functional significance of BM88/Cend1 in neuronal lineage progression postnatally, we challenged its expression levels by gain- and loss-of-function approaches using lentiviral gene transfer in SVZ-derived neurospheres. We found that BM88/Cend1 overexpression decreases proliferation and favors neuronal differentiation, whereas its downregulation using new-generation RNA interference vectors yields an opposite phenotype. Our results demonstrate that BM88/Cend1 participates in cell cycle control and neuronal differentiation mechanisms during neonatal SVZ neurogenesis and becomes crucial for the transition from neuroblasts to mature neurons when reaching high levels.

Expression and distribution of S100 protein in the nervous system of the adult zebrafish (Danio rerio)

S100 proteins are EF-hand calcium-binding protein highly preserved during evolution present in both neuronal and non-neuronal tissues of the higher vertebrates. Data about the expression of S100 protein in fishes are scarce, and no data are available on zebrafish, a common model used in biology to study development but also human diseases. In this study, we have investigated the expression of S100 protein in the central nervous system of adult zebrafish using PCR, Western blot, and immunohistochemistry. The central nervous system of the adult zebrafish express S100 protein mRNA, and contain a protein of approximately 10 kDa identified as S100 protein. S100 protein immunoreactivity was detected widespread distributed in the central nervous system, labeling the cytoplasm of both neuronal and non-neuronal cells. In fact, S100 protein immunoreactivity was primarily found in glial and ependymal cells, whereas the only neurons displaying S100 immunoreactivity were the Purkinje's neurons of the cerebellar cortex and those forming the deep cerebellar nuclei. Outside the central nervous system, S100 protein immunoreactivity was observed in a subpopulation of sensory and sympathetic neurons, and it was absent from the enteric nervous system. The functional role of S100 protein in both neurons and non-neuronal cells of the zebrafish central nervous system remains to be elucidated, but present results might serve as baseline for future experimental studies using this teleost as a model.

Neural stem cells: balancing self-renewal with differentiation

Stem cells are captivating because they have the potential to make multiple cell types yet maintain their undifferentiated state. Recent studies of Drosophila and mammalian neural stem cells have shed light on how stem cells regulate self-renewal versus differentiation and have revealed the proteins, processes and pathways that all converge to regulate neural progenitor self-renewal. If we can better understand how stem cells balance self-renewal versus differentiation, we will significantly advance our knowledge of embryogenesis, cancer biology and brain evolution, as well as the use of stem cells for therapeutic purposes.

Adult neurogenesis requires Smad4-mediated bone morphogenic protein signaling in stem cells

In the mammalian brain, neurogenesis continues only in few regions of the forebrain. The molecular signals governing neurogenesis in these unique neurogenic niches, however, are still ill defined. Here, we show that bone morphogenic protein (BMP)-mediated signaling is active in adult neural stem cells and is crucial to initiate the neurogenic lineage in the adult mouse subependymal zone. Conditional deletion of Smad4 in adult neural stem cells severely impairs neurogenesis, and this is phenocopied by infusion of Noggin, an extracellular antagonist of BMP. Smad4 deletion in stem, but not progenitor cells, as well as Noggin infusion lead to an increased number of Olig2-expressing progeny that migrate to the corpus callosum and differentiate into oligodendrocytes. Transplantation experiments further verified the cell-autonomous nature of this phenotype. Thus, BMP-mediated signaling via Smad4 is required to initiate neurogenesis from adult neural stem cells and suppress the alternative fate of oligodendrogliogenesis.

Genes, plasticity and mental retardation

Functional and structural plasticity is a fundamental property of the brain involved in diverse processes ranging from brain construction and repair to storage of experiences during lifetime. Our current understanding of different forms of brain plasticity mechanisms has advanced tremendously in the last decades, benefiting from studies of development and memory storage in adulthood and from investigations of diverse diseased conditions. In this review, we focus on the role of mental retardation (MR) genes and show how this developing area of research can enrich our knowledge of the cellular and molecular mechanisms of brain plasticity and cognitive functions, and of the dysfunctional mechanisms underlying MR. We describe two main groups of MR genes; those leading to dysfunctional neurodevelopmental programs and brain malformations, and those which rely on alterations in molecular mechanisms underlying synaptic organization and plasticity. We first explore the role of MR genes in key mechanisms of neurogenesis and neuronal migration during development and in the adult, such as actin and microtubule-cytoskeletal dynamics and signal transduction. We then define the contribution of MR genes to forms of activity-dependent synaptic modifications, such as those involved in molecular organization of the synapse, intracellular signaling regulating gene programs and neuronal cytoskeleton to control network remodeling. We trace the characteristics of MR genes playing key roles in many forms of brain plasticity mechanisms, and highlight specific MR genes that endorse distinct roles in different cell types or brain regions, and at various times of a brain lifetime.