The subependymal layer in rodents: a site of structural plasticity and cell migration in the adult mammalian brain

Mechanisms Involved in Neuroprotective Effects of Transcranial Magnetic Stimulation

Transcranial Magnetic Stimulation (TMS) is widely used in neurophysiology to study cortical excitability. Research over the last few decades has highlighted its added value as a potential therapeutic tool in the treatment of a broad range of psychiatric disorders. More recently, a number of studies have reported beneficial and therapeutic effects for TMS in neurodegenerative conditions and strokes. Yet, despite its recognised clinical applications and considerable research using animal models, the molecular and physiological mechanisms through which TMS exerts its beneficial and therapeutic effects remain unclear. They are thought to involve biochemical-molecular events affecting membrane potential and gene expression. In this aspect, the dopaminergic system plays a special role. This is the most directly and selectively modulated neurotransmitter system, producing an increase in the flux of dopamine (DA) in various areas of the brain after the application of repetitive TMS (rTMS). Other neurotransmitters, such as glutamate and gamma-aminobutyric acid (GABA) have shown a paradoxical response to rTMS. In this way, their levels increased in the hippocampus and striatum but decreased in the hypothalamus and remained unchanged in the mesencephalon. Similarly, there are sufficient evidence that TMS up-regulates the gene expression of BDNF (one of the main brain neurotrophins). Something similar occurs with the expression of genes such as c-Fos and zif268 that encode trophic and regenerative action neuropeptides. Consequently, the application of TMS can promote the release of molecules involved in neuronal genesis and maintenance. This capacity may mean that TMS becomes a useful therapeutic resource to antagonize processes that underlie the previously mentioned neurodegenerative conditions.

Maternal High-Energy Diet during Pregnancy and Lactation Impairs Neurogenesis and Alters the Behavior of Adult Offspring in a Phenotype-Dependent Manner

Obesity is one of the biggest and most costly health challenges the modern world encounters. Substantial evidence suggests that the risk of metabolic syndrome or obesity formation may be affected at a very early stage of development, in particular through fetal and/or neonatal overfeeding. Outcomes from epidemiological studies indicate that maternal nutrition during pregnancy and lactation has a profound impact on adult neurogenesis in the offspring. In the present study, an intergenerational dietary model employing overfeeding of experimental mice during prenatal and early postnatal development was applied to acquire mice with various body conditions. We investigated the impact of the maternal high-energy diet during pregnancy and lactation on adult neurogenesis in the olfactory neurogenic region involving the subventricular zone (SVZ) and the rostral migratory stream (RMS) and some behavioral tasks including memory, anxiety and nociception. Our findings show that a maternal high-energy diet administered during pregnancy and lactation modifies proliferation and differentiation, and induced degeneration of cells in the SVZ/RMS of offspring, but only in mice where extreme phenotype, such as significant overweight/adiposity or obesity is manifested. Thereafter, a maternal high-energy diet enhances anxiety-related behavior in offspring regardless of its body condition and impairs learning and memory in offspring with an extreme phenotype.

Relationship between Blood Vessels and Migration of Neuroblasts in the Olfactory Neurogenic Region of the Rodent Brain

Neural precursors originating in the subventricular zone (SVZ), the largest neurogenic region of the adult brain, migrate several millimeters along a restricted migratory pathway, the rostral migratory stream (RMS), toward the olfactory bulb (OB), where they differentiate into interneurons and integrate into the local neuronal circuits. Migration of SVZ-derived neuroblasts in the adult brain differs in many aspects from that in the embryonic period. Unlike in that period, postnatally-generated neuroblasts in the SVZ are able to divide during migration along the RMS, as well as they migrate independently of radial glia. The homophilic mode of migration, i.e., using each other to move, is typical for neuroblast movement in the RMS. In addition, it has recently been demonstrated that specifically-arranged blood vessels navigate SVZ-derived neuroblasts to the OB and provide signals which promote migration. Here we review the development of vasculature in the presumptive neurogenic region of the rodent brain during the embryonic period as well as the development of the vascular scaffold guiding neuroblast migration in the postnatal period, and the significance of blood vessel reorganization during the early postnatal period for proper migration of RMS neuroblasts in adulthood.

Adult Neurogenesis: A Story Ranging from Controversial New Neurogenic Areas and Human Adult Neurogenesis to Molecular Regulation

The generation of new neurons in the adult brain is a currently accepted phenomenon. Over the past few decades, the subventricular zone and the hippocampal dentate gyrus have been described as the two main neurogenic niches. Neurogenic niches generate new neurons through an asymmetric division process involving several developmental steps. This process occurs throughout life in several species, including humans. These new neurons possess unique properties that contribute to the local circuitry. Despite several efforts, no other neurogenic zones have been observed in many years; the lack of observation is probably due to technical issues. However, in recent years, more brain niches have been described, once again breaking the current paradigms. Currently, a debate in the scientific community about new neurogenic areas of the brain, namely, human adult neurogenesis, is ongoing. Thus, several open questions regarding new neurogenic niches, as well as this phenomenon in adult humans, their functional relevance, and their mechanisms, remain to be answered. In this review, we discuss the literature and provide a compressive overview of the known neurogenic zones, traditional zones, and newly described zones. Additionally, we will review the regulatory roles of some molecular mechanisms, such as miRNAs, neurotrophic factors, and neurotrophins. We also join the debate on human adult neurogenesis, and we will identify similarities and differences in the literature and summarize the knowledge regarding these interesting topics.

Effects of early life exposure to traffic-related air pollution on brain development in juvenile Sprague-Dawley rats

Epidemiological studies link traffic-related air pollution (TRAP) to increased risk for various neurodevelopmental disorders (NDDs); however, there are limited preclinical data demonstrating a causal relationship between TRAP and adverse neurodevelopmental outcomes. Moreover, much of the preclinical literature reports effects of concentrated ambient particles or diesel exhaust that do not recapitulate the complexity of real-world TRAP exposures. To assess the developmental neurotoxicity of more realistic TRAP exposures, we exposed male and female rats during gestation and early postnatal development to TRAP drawn directly from a traffic tunnel in Northern California and delivered to animals in real-time. We compared NDD-relevant neuropathological outcomes at postnatal days 51-55 in TRAP-exposed animals versus control subjects exposed to filtered air. As indicated by immunohistochemical analyses, TRAP significantly increased microglial infiltration in the CA1 hippocampus, but decreased astrogliosis in the dentate gyrus. TRAP exposure had no persistent effect on pro-inflammatory cytokine levels in the male or female brain, but did significantly elevate the anti-inflammatory cytokine IL-10 in females. In male rats, TRAP significantly increased hippocampal neurogenesis, while in females, TRAP increased granule cell layer width. TRAP had no effect on apoptosis in either sex. Magnetic resonance imaging revealed that TRAP-exposed females, but not males, also exhibited decreased lateral ventricular volume, which was correlated with increased granule cell layer width in the hippocampus in females. Collectively, these data indicate that exposure to real-world levels of TRAP during gestation and early postnatal development modulate neurodevelopment, corroborating epidemiological evidence of an association between TRAP exposure and increased risk of NDDs.

Characterization of the ventricular-subventricular stem cell niche during human brain development

Human brain development proceeds via a sequentially transforming stem cell population in the ventricular-subventricular zone (V-SVZ). An essential, but understudied, contributor to V-SVZ stem cell niche health is the multi-ciliated ependymal epithelium, which replaces stem cells at the ventricular surface during development. However, reorganization of the V-SVZ stem cell niche and its relationship to ependymogenesis has not been characterized in the human brain. Based on comprehensive comparative spatiotemporal analyses of cytoarchitectural changes along the mouse and human ventricle surface, we uncovered a distinctive stem cell retention pattern in humans as ependymal cells populate the surface of the ventricle in an occipital-to-frontal wave. During perinatal development, ventricle-contacting stem cells are reduced. By 7 months few stem cells are detected, paralleling the decline in neurogenesis. In adolescence and adulthood, stem cells and neurogenesis are not observed along the lateral wall. Volume, surface area and curvature of the lateral ventricles all significantly change during fetal development but stabilize after 1 year, corresponding with the wave of ependymogenesis and stem cell reduction. These findings reveal normal human V-SVZ development, highlighting the consequences of disease pathologies such as congenital hydrocephalus.

Clusters of DCX+ cells "trapped" in the subcortical white matter of early postnatal Cetartiodactyla (Tursiops truncatus, Stenella coeruloalba and Ovis aries)

The cytoskeletal protein doublecortin (DCX) is a marker for neuronal cells retaining high potential for structural plasticity, originating from both embryonic and adult neurogenic processes. Some of these cells have been described in the subcortical white matter of neonatal and postnatal mammals. In mice and humans it has been shown they are young neurons migrating through the white matter after birth, reaching the cortex in a sort of protracted neurogenesis. Here we show that DCX+ cells in the white matter of neonatal and young Cetartiodactyla (dolphin and sheep) form large clusters which are not newly generated (in sheep, and likely neither in dolphins) and do not reach the cortical layers, rather appearing "trapped" in the white matter tissue. No direct contact or continuity can be observed between the subventricular zone region and the DCX+ clusters, thus indicating their independence from any neurogenic source (in dolphins further confirmed by the recent demonstration that periventricular neurogenesis is inactive since birth). Cetartiodactyla include two orders of large-brained, relatively long-living mammals (cetaceans and artiodactyls) which were recognized as two separate monophyletic clades until recently, yet, despite the evident morphological distinctions, they are monophyletic in origin. The brain of Cetartiodactyla is characterized by an advanced stage of development at birth, a feature that might explain the occurrence of "static" cell clusters confined within their white matter. These results further confirm the existence of high heterogeneity in the occurrence, distribution and types of structural plasticity among mammals, supporting the emerging view that multiple populations of DCX+, non-newly generated cells can be abundant in large-brained, long-living species.

Home sweet home: the neural stem cell niche throughout development and after injury

Neural stem cells and their progeny reside in two distinct neurogenic niches within the mammalian brain: the subventricular zone and the dentate gyrus. The interplay between the neural stem cells and the niche in which they reside can have significant effects on cell kinetics and neurogenesis. A comprehensive understanding of the changes to the niche that occur through postnatal development and aging, as well as following injury, is relevant for developing therapeutics and interventions to promote neural repair. We discuss changes that occur within the neural stem and progenitor cell populations, the vasculature, extracellular matrix, microglia, and secreted proteins through aging which impact cell behavior within the neurogenic niches. We examine neural precursor cell and niche responses to injury in neonatal hypoxia-ischemia, juvenile cranial irradiation, and adult stroke. This review examines the interplay between the niche and stem cell behavior through aging and following injury as a means to understand intrinsic and extrinsic factors that regulate neurogenesis in vivo.

Fourth Ventricular Lesions in Metastatic Gliomas: A Rare Predilection?

Over the course of five years, a total of ten cases were collected of glioma patients in whom a distant lesion at the fourth ventricle was noted. A ‘distant lesion’ was defined as a lesion with a normal appearing tissue bridge at imaging between the primary and secondary locations. Previous imaging of these patients was reviewed along with clinical history, course of therapy, and available histology. A review of the literature was performed with respect to present knowledge on patterns of glioma proliferation and dissemination. This case series is the first to describe the fourth ventricle as a location that may be prone to secondary lesions in glioma patients. Further investigation on this subject may yield deeper insights into the mechanisms by which glial tumors spread within the brain, with the hope of developing or improving therapeutic targets.

Perinatal Exposure to Glufosinate Ammonium Herbicide Impairs Neurogenesis and Neuroblast Migration through Cytoskeleton Destabilization

Neurogenesis, a process of generating functional neurons from neural precursors, occurs throughout life in restricted brain regions such as the subventricular zone (SVZ). During this process, newly generated neurons migrate along the rostral migratory stream to the olfactory bulb to replace granule cells and periglomerular neurons. This neuronal migration is pivotal not only for neuronal plasticity but also for adapted olfactory based behaviors. Perturbation of this highly controlled system by exogenous chemicals has been associated with neurodevelopmental disorders. We reported recently that perinatal exposure to low dose herbicide glufosinate ammonium (GLA), leads to long lasting behavioral defects reminiscent of Autism Spectrum Disorder-like phenotype in the offspring (Laugeray et al., 2014). Herein, we demonstrate that perinatal exposure to low dose GLA induces alterations in neuroblast proliferation within the SVZ and abnormal migration from the SVZ to the olfactory bulbs. These disturbances are not only concomitant to changes in cell morphology, proliferation and apoptosis, but are also associated with transcriptomic changes. Therefore, we demonstrate for the first time that perinatal exposure to low dose GLA alters SVZ neurogenesis. Jointly with our previous work, the present results provide new evidence on the link between molecular and cellular consequences of early life exposure to the herbicide GLA and the onset of ASD-like phenotype later in life.

Switching on mTORC1 induces neurogenesis but not proliferation in neural stem cells of young mice

Recent evidence reported that activation of the mechanistic target of rapamycin complex 1 (mTORC1) induces terminal differentiation of neural stem cells (NSCs) in the neonatal subventricular zone (SVZ), but did not affect their proliferation. Here, we investigated whether such an effect of hyperactive mTORC1 would be recapitulated in young adults following removal of the negative mTORC1 regulator TSC1as seen in the neurological disorder tuberous sclerosis complex, TSC. Conditional mTORC1 activation in NSCs of 3-4 weeks old mice resulted in the generation of proliferative (Ki67+) cells and newborn neuroblasts. However, hyperactive mTORC1 did not induce NSCs to proliferate, consistent with the findings that mTORC1 induces symmetric division and differentiation of slow-cycling NSCs into proliferative daughter cells. Taken together these data suggest that hyperactivity of mTORC1 could lead to the progressive loss of NSCs over time.

The aged brain: genesis and fate of residual progenitor cells in the subventricular zone

Neural stem cells (NSCs) persist in the adult mammalian brain through life. The subventricular zone (SVZ) is the largest source of stem cells in the nervous system, and continuously generates new neuronal and glial cells involved in brain regeneration. During aging, the germinal potential of the SVZ suffers a widespread decline, but the causes of this turn down are not fully understood. This review provides a compilation of the current knowledge about the age-related changes in the NSC population, as well as the fate of the newly generated cells in the aged brain. It is known that the neurogenic capacity is clearly disrupted during aging, while the production of oligodendroglial cells is not compromised. Interestingly, the human brain seems to primarily preserve the ability to produce new oligodendrocytes instead of neurons, which could be related to the development of neurological disorders. Further studies in this matter are required to improve our understanding and the current strategies for fighting neurological diseases associated with senescence.

The Origin, Development and Molecular Diversity of Rodent Olfactory Bulb Glutamatergic Neurons Distinguished by Expression of Transcription Factor NeuroD1

Production of olfactory bulb neurons occurs continuously in the rodent brain. Little is known, however, about cellular diversity in the glutamatergic neuron subpopulation. In the central nervous system, the basic helix-loop-helix transcription factor NeuroD1 (ND1) is commonly associated with glutamatergic neuron development. In this study, we utilized ND1 to identify the different subpopulations of olfactory bulb glutamategic neurons and their progenitors, both in the embryo and postnatally. Using knock-in mice, transgenic mice and retroviral transgene delivery, we demonstrate the existence of several different populations of glutamatergic olfactory bulb neurons, the progenitors of which are ND1⁺ and ND1^- lineage-restricted, and are temporally and regionally separated. We show that the first olfactory bulb glutamatergic neurons produced – the mitral cells – can be divided into molecularly diverse subpopulations. Our findings illustrate the complexity of neuronal diversity in the olfactory bulb and that seemingly homogenous neuronal populations can consist of multiple subpopulations with unique molecular signatures of transcription factors and expressing neuronal subtype-specific markers.

GABAergic striatal neurons project dendrites and axons into the postnatal subventricular zone leading to calcium activity

GABA regulates the behavior of neuroblasts and neural progenitor cells in the postnatal neurogenic subventricular zone (SVZ) through GABAA receptor (GABAAR)-mediated calcium increases. However, the source of GABA necessary for sufficient GABAAR-mediated depolarization and calcium increase has remained speculative. Here, we explored whether GABAergic striatal neurons functionally connect with SVZ cells. Using patch clamp recordings or single cell electroporation, striatal neurons along the SVZ were filled with a fluorescent dye revealing that they send both dendrites and axons into the SVZ. About 93% of the recorded neurons were medium spiny or aspiny GABAergic neurons and each neuron sent 3-4 processes into the SVZ covering ~56 μm. Using calcium imaging, we found that depolarization of striatal neurons led to increased calcium activity in SVZ cells that were mediated by GABAAR activation. Collectively, these findings undercover a novel mode of signaling in the SVZ providing a mechanism of brain activity-mediated regulation of postnatal neurogenesis through GABAergic striatal activity.

Developmentally regulated collagen/integrin interactions confer adhesive properties to early postnatal neural stem cells

BACKGROUND

It is becoming increasingly apparent that the extracellular matrix acts as an important regulator of the neural stem niche. Previously we found that neural stem and progenitor cells (NSPCs) derived from the early postnatal subventricular zone of mice adhere to a collagen/hyaluronan hydrogel, whereas NSPCs from the adult and embryonic brain do not.

METHODS

To examine the specific adhesive properties of young stem cells in more detail, NSPCs isolated from embryonic, postnatal day 6 (P6), and adult mouse brains were cultured on collagen I.

RESULTS

Early postnatal NSPCs formed paxillin-positive focal adhesions on collagen I, and these adhesions could be prevented by an antibody that blocked integrin β1. Furthermore, we found the corresponding integrin alpha subunits α2 and α11 levels to be highest at the postnatal stage. Gene ontology analysis of differentially expressed genes showed higher expression of transcripts involved in vasculature development and morphogenesis in P6 stem cells, compared to adult.

CONCLUSIONS

The ability to interact with the extracellular matrix differs between postnatal and adult NSPCs.

GENERAL SIGNIFICANCE

Our observations that the specific adhesive properties of early postnatal NSPCs, which are lost in the adult brain, can be ascribed to the integrin subunits expressed by the former furthering our understanding of the developing neurogenic niche. This article is part of a Special Issue entitled Matrix-mediated cell behaviour and properties.

Adult neural stem cells from the subventricular zone: a review of the neurosphere assay

The possibility of obtaining large numbers of cells with potential to become functional neurons implies a great advance in regenerative medicine. A source of cells for therapy is the subventricular zone (SVZ) where adult neural stem cells (NSCs) retain the ability to proliferate, self-renew, and differentiate into several mature cell types. The neurosphere assay, a method to isolate, maintain, and expand these cells has been extensively utilized by research groups to analyze the biological properties of aNSCs and to graft into injured brains from animal models. In this review we briefly describe the neurosphere assay and its limitations, the methods to optimize culture conditions, the identity and the morphology of aNSC-derived neurospheres (including new ultrastructural data). The controversy regarding the identity and "stemness" of cells within the neurosphere is revised. The fine morphology of neurospheres, described thoroughly, allows for phenotypical characterization of cells in the neurospheres and may reveal slight changes that indirectly inform about cell integrity, cell damage, or oncogenic transformation. Along this review we largely highlight the critical points that researchers have to keep in mind before extrapolating results or translating experimental transplantation of neurosphere-derived cells to the clinical setting.

Immunohistochemical evidence for the presence of synaptic connections of nitrergic neurons in the rat rostral migratory stream

The rostral migratory stream (RMS) is a migration route for neuroblasts originating in the richest neurogenic niche of the adult mammalian brain-the subventricular zone. Most studies are focused on cellular dynamics of migrating neuroblasts and interactions between neuroblasts and astrocytes which both represent the major cellular component of the RMS. Our previous experiments have brought evidence about the existence of a small population of mature neurons in the adult rat RMS with capacity to produce nitric oxide (NO). In order to further support functional significance of nitrergic cells, the aim of the present study was to determine whether NO producing neurons could form synapses. Sagittal sections from the adult rat brain were processed for simultaneous immunohistochemical detection of neuronal nitric oxide synthase (nNOS), the enzyme present in NO producing cells and synaptophysin, a glycoprotein found in synaptic vesicles. Synaptophysin positivity in the RMS was significantly lower in comparison with other brain areas, but its colocalization with nNOS-positive neurons was obvious. Our results suggest that nitrergic neurons in the RMS could be involved in a neuronal circuitry with potential impact on regulation of neurogenesis in the RMS.

Neurogenesis and progenitor cells in the adult human brain: a comparison between hippocampal and subventricular progenitor proliferation

For more than a decade, we have known that the human brain harbors progenitor cells capable of becoming mature neurons in the adult human brain. Since the original landmark article by Eriksson et al. in 1998 (Nat Med 4:1313-1317), there have been many studies investigating the effect that depression, epilepsy, Alzheimer's disease, Huntington's disease, and Parkinson's disease have on the germinal zones in the adult human brain. Of particular interest is the demonstration that there are far fewer progenitor cells in the hippocampal subgranular zone (SGZ) compared with the subventricular zone (SVZ) in the human brain. Furthermore, the quantity of progenitor cell proliferation in human neurodegenerative diseases differs from that of animal models of neurodegenerative diseases; there is minimal progenitor proliferation in the SGZ and extensive proliferation in the SVZ in the human. In this review, we will present the data from a range of human and rodent studies from which we can compare the amount of proliferation of cells in the SVZ and SGZ in different neurodegenerative diseases.

Photoperiod mediated changes in olfactory bulb neurogenesis and olfactory behavior in male white-footed mice (Peromyscus leucopus)

Brain plasticity, in relation to new adult mammalian neurons generated in the subgranular zone of the hippocampus, has been well described. However, the functional outcome of new adult olfactory neurons born in the subventricular zone of the lateral ventricles is not clearly defined, as manipulating neurogenesis through various methods has given inconsistent and conflicting results in lab mice. Several small rodent species, including Peromyscus leucopus, display seasonal (photoperiodic) brain plasticity in brain volume, hippocampal function, and hippocampus-dependent behaviors; plasticity in the olfactory system of photoperiodic rodents remains largely uninvestigated. We exposed adult male P. leucopus to long day lengths (LD) and short day lengths (SD) for 10 to 15 weeks and then examined olfactory bulb cell proliferation and survival using the thymidine analog BrdU, olfactory bulb granule cell morphology using Golgi-Cox staining, and behavioral investigation of same-sex conspecific urine. SD mice did not differ from LD counterparts in granular cell morphology of the dendrites or in dendritic spine density. Although there were no differences due to photoperiod in habituation to water odor, SD mice rapidly habituated to male urine, whereas LD mice did not. In addition, short day induced changes in olfactory behavior were associated with increased neurogenesis in the caudal plexiform and granule cell layers of the olfactory bulb, an area known to preferentially respond to water-soluble odorants. Taken together, these data demonstrate that photoperiod, without altering olfactory bulb neuronal morphology, alters olfactory bulb neurogenesis and olfactory behavior in Peromyscus leucopus.

Neural stem cells: mechanisms and modeling

In the adult brain, neural stem cells have been found in two major niches: the dentate gyrus and the subventricular zone [corrected]. Neurons derived from these stem cells contribute to learning, memory, and the autonomous repair of the brain under pathological conditions. Hence, the physiology of adult neural stem cells has become a significant component of research on synaptic plasticity and neuronal disorders. In addition, the recently developed induced pluripotent stem cell technique provides a powerful tool for researchers engaged in the pathological and pharmacological study of neuronal disorders. In this review, we briefly summarize the research progress in neural stem cells in the adult brain and in the neuropathological application of the induced pluripotent stem cell technique.

Subventricular zone-derived neuroblasts use vasculature as a scaffold to migrate radially to the cortex in neonatal mice

Neurons continue to be generated in the subventricular zone (SVZ) throughout postnatal development and adulthood in rodents. Whereas in adults, virtually all neuroblasts migrate tangentially to the olfactory bulb via the rostral migratory stream (RMS), in neonates, a substantial fraction migrate radially through the corpus callosum (CC) to the cortex. Mechanisms of radial cortical migration have remained unknown. We investigated this by taking recourse to enhanced green fluorescent protein (EGFP)-labeled neuroblasts in the CC and deep cortical layers of neonatal mice and found that they are frequently located adjacent to vasculature. Using time-lapse 2-photon microscopy in acute brain slices, we demonstrate that EGFP-labeled neuroblasts migrate along blood vessels. Although in close proximity to blood vessels, migrating neuroblasts are separated from endothelial cells by 1-2 layers of astrocytic processes, as revealed by electron microscopal studies of retrovirally labeled postnatally born cells. We propose that 2 factors could contribute to the decline of radial migration to the cortex during postnatal development, namely the establishment of a glial sheath delineating the RMS and a gradual decrease in the density of blood vessels in the CC. Together, our data provide evidence for a new mode of radial cortical migration of SVZ-generated neurons involving vasculature and astrocytes.

Delayed maturation and altered proliferation within the rat rostral migratory stream following maternal deprivation

The objective of this study was to investigate whether stressful experience during early postnatal period may influence morphological characteristics of the rat neurogenic pathway – the rostral migratory stream (RMS) and proliferation of neuronal precursors in three successive areas of the RMS: in the vertical arm, the elbow and the horizontal arm. To induce stress, the pups were subjected to repeated maternal deprivation during the first postnatal week after birth. Brains were analyzed at the seventh postnatal day. The controls matched the age of maternally deprived animals. Observation of hematoxylin-eosin stained sections showed that maternal deprivation did not affect the general morphological appearance of the RMS. The shape of the RMS of maternally deprived rats resembles the RMS of control animals. Maternal deprivation caused slight, not significant increase in the RMS thickness in comparison with control rats. Significant difference between the control and maternally deprived rats concerns the olfactory ventricle. While in seven days old control rats the olfactory ventricle is completely closed, in maternally deprived rats of the same age the olfactory ventricle was regularly visible as a narrow lumen at the axis of the RMS horizontal arm. This finding indicates delayed maturation of the migratory pathway as a consequence of stress. Proliferation activity has been assessed by immunoreactivity of the endogenous cell cycle protein Ki-67. The results of Ki-67 immunohistochemistry showed that seven days' maternal separation for 3 h daily induces significant quantitative changes in the number of proliferating cells within the RMS. The response of Ki-67-positive cells to stress differed in individual part of the RMS, with a marked decrease in the vertical arm and a significant increase in the elbow, suggesting heterogeneity of neural stem cells along the RMS; while in the RMS vertical arm the number of dividing cells significantly decreased, there was a marked increase of Ki-67-positive cells in the RMS elbow. This suggests heterogeneity of neural stem cells along the RMS.

Newborn interneurons in the accessory olfactory bulb promote mate recognition in female mice

In the olfactory bulb of adult rodents, local interneurons are constantly replaced by immature precursors derived from the subventricular zone. Whether any olfactory sensory process specifically relies on this cell renewal remains largely unclear. By using the well known model of mating-induced imprinting to avoid pregnancy block, which requires accessory olfactory bulb (AOB) function, we demonstrate that this olfactory memory formation critically depends on the presence of newborn granule neurons in this brain region. We show that, in adult female mice, exposure to the male urine compounds involved in mate recognition increases the number of new granule cells surviving in the AOB. This process is modulated by male signals sensed through the vomeronasal organ and, in turn, changes the activity of the downstream amygdaloid and hypothalamic nuclei involved in the pregnancy block response. Chemical depletion of newly generated bulbar interneurons causes strong impairment in mate recognition, thus resulting in a high pregnancy failure rate to familiar mating male odors. Taken together, our results indicate that adult neurogenesis is essential for specific brain functions such as persistent odor learning and mate recognition.

Neurogenesis in humans

Since 1944 increasing evidence has been emerging that the adult human brain harbours progenitor cells with the potential to produce neuroblasts. However, it was not until 1998 that this fact was confirmed in the adult human brain. With the purpose of human neurogenesis being hotly debated, many research groups have focussed on the effect of neurodegenerative diseases in the brain to determine the strength of the endogenous regenerative response. Although most of the human studies have focussed on the hippocampus, there is a groundswell of evidence that there is greater plasticity in the subventricular zone and in the ventriculo-olfactory neurogenic system. In this review, we present the evidence for increased or decreased plasticity and neurogenesis in different diseases and with different drug treatments in the adult human brain. Whilst there is a paucity of studies on human neurogenesis, there are sufficient to draw some conclusions about the potential of plasticity in the human brain.

Hepatocyte growth factor acts as a mitogen and chemoattractant for postnatal subventricular zone-olfactory bulb neurogenesis

Neural progenitor cells persist throughout life in the forebrain subventricular zone (SVZ). They generate neuroblasts that migrate to the olfactory bulb and differentiate into interneurons, but mechanisms underlying these processes are poorly understood. Hepatocyte growth factor/scatter factor (HGF/SF) is a pleiotropic factor that influences cell motility, proliferation and morphogenesis in neural and non-neural tissues. HGF and its receptor, c-Met, are present in the rodent SVZ-olfactory bulb pathway. Using in vitro neurogenesis assays and in vivo studies of partially HGF-deficient mice, we find that HGF promotes SVZ cell proliferation and progenitor cell maintenance, while slowing differentiation and possibly altering cell fate choices. HGF also acts as a chemoattractant for SVZ neuroblasts in co-culture assays. Decreased HGF signaling induces ectopic SVZ neuroblast migration and alters the timing of migration to the olfactory bulb. These results suggest that HGF influences multiple steps in postnatal forebrain neurogenesis. HGF is a mitogen for SVZ neural progenitors, and regulates their differentiation and olfactory bulb migration.

Paced-mating increases the number of adult new born cells in the internal cellular (granular) layer of the accessory olfactory bulb

The continuous production and addition of new neurons during life in the olfactory bulb is well accepted and has been extensively studied in rodents. This process could allow the animals to adapt to a changing environment. Olfactory neurogenesis begins in the subventricular zone where stem cells proliferate and give rise to young undifferentiated neuroblasts that migrate along the rostral migratory stream to the olfactory bulb (OB). Olfaction is crucial for the expression of sexual behavior in rodents. In female rats, the ability to control the rate of sexual interactions (pacing) has important physiological and behavioral consequences. In the present experiment we evaluated if pacing behavior modifies the rate of new cells that reach the main and accessory olfactory bulb. The BrdU marker was injected before and after different behavioral tests which included: females placed in a mating cage (control), females allowed to pace the sexual interaction, females that mated but were not able to control the rate of the sexual interaction and females exposed to a sexually active male. Subjects were sacrificed fifteen days after the behavioral test. We observed a significant increase in the density of BrdU positive cells in the internal cellular layer of the accessory olfactory bulb when females paced the sexual interaction in comparison to the other 3 groups. No differences in the cell density in the main olfactory bulb were found. These results suggest that pacing behavior promotes an increase in density of the new cells in the accessory olfactory bulb.

From progenitors to integrated neurons: role of neurotransmitters in adult olfactory neurogenesis

Adult neurogenesis is due to the persistence of pools of constitutive stem cells able to give rise to a progeny of proliferating progenitors. In rodents, adult neurogenic niches have been found in the subventricular zone (SVZ) along the lateral ventricles and in the subgranular zone of the dentate gyrus in the hippocampus. SVZ progenitors undergo a unique process of tangential migration from the lateral ventricle to the olfactory bulb (OB) where they differentiate mainly into GABAergic interneurons in the granule and glomerular layers. SVZ progenitor proliferation, migration and differentiation into fully integrated neurons, are strictly related processes regulated by complex interactions between cell intrinsic and extrinsic influences. Numerous observations demonstrate that neurotrasmitters are involved in all steps of the adult neurogenic process, but the understanding of their role is hampered by their intricate mechanism of action and by the highly complex network in which neurotransmitters work. By considering the three main steps of olfactory adult neurogenesis (proliferation, migration and integration), this review will discuss recent advances in the study of neurotransmitters, highlighting the regulatory mechanisms upstream and downstream their action.

EGF induces CREB and ERK activation at the wall of the mouse lateral ventricles

The subependymal zone at the lateral ventricular wall represents a major neurogenic niche of the adult mammalian brain and continuously provides new neurons for the olfactory bulb. A mosaic of stem and progenitor cells in this niche has the potential to respond to multiple signals including growth factors such as EGF. Recent studies using long-term ventricular infusion of EGF demonstrate intense cell proliferation around the ventricular wall, implicating the presence of EGF-reactive cells also outside the classical neurogenic lateral niche. Here we show that intraventricular injection of EGF induces within minutes CREB and ERK phosphorylation in astrocyte-like progenitor cells (type B cells) and EGF receptor-expressing transit-amplifying progenitor cells-both in the striatal and septal ventricular walls. EGF infusion for 6 days induced continued CREB and ERK activation in nestin+ cells paralleled by intense periventricular cell proliferation. In addition, the ependyma became EGF receptor-immunoreactive, revealed intense CREB phosphorylation and underwent partial de-differentiation. Our results demonstrate that intraventricular application of EGF induces CREB and ERK phosphorylation along the entire ventricular walls and thus permits a direct identification of EGF-responsive cell types. They further support the notion that not only the striatal ventricular wall where the SEZ is located but also the septal ventricular wall carries latent potential for the formation of neurons and glial cells.

Early stress affects neurogenesis in the rat rostral migratory stream

Stressful experience during the early postnatal period may influence processes associated with neurogenesis (i.e. proliferation, cell death, appearance of astrocytes or cell differentiation) in the neonatal rat rostral migratory stream (RMS). To induce stress, pups were subjected to maternal deprivation daily for three hours, starting from the first postnatal day till the seventh postnatal day. Immunohistochemical methods were used to visualize proliferating cells and astrocytes; dying cells and nitrergic cells were visualized using histochemical staining. Quantitative analysis showed that maternal deprivation decreased the number of proliferating cells and significantly increased the number of dying cells in the RMS. Maternal deprivation did not influence the appearance of astrocytes in the RMS, but caused premature differentiation of nitrergic cells. In control rats, nitrergic cells can be observed in the RMS as early as the tenth postnatal day. In maternally deprived pups, these cells were detected as early as the seventh postnatal day. The observed earlier appearance of nitrergic cells in the RMS was associated with altered proliferation and increased cell dying and this observation supports the hypothesis that nitric oxide has an anti-proliferative role in the RMS. Our study demonstrates that maternal deprivation represents a stressful condition with a profound impact on early postnatal neurogenesis.

Long-lasting changes in the anatomy of the olfactory bulb after ionizing irradiation and bone marrow transplantation

The adult brain is considered to be a radioresistant organ since it is mainly composed of non-dividing cells. However, in adult animals there are a few neurogenic brain areas that are affected by ionizing radiation whose plasticity and capacity for recovery are still unclear. Here, mice were irradiated with a minimal lethal dose of radiation in order to determine its effects on the subventricular zone (SVZ), the rostral migratory stream (RMS), and the olfactory bulb (OB). These regions underwent a dramatic reduction in cell proliferation and ensuing morphological alterations, accompanied by a patent reactive gliosis. Bone marrow stem cell (BMSC) transplants were also performed after the radiation treatment to allow the mouse survival with a view to analyzing long-term effects. Normal proliferation rates were not recovered over time and although bone marrow-derived cells reached the brain, they were not incorporated into the SVZ-RMS-OB pathway in an attempt to rescue the damaged regions. Since neurogenesis produces new interneurones in the OB, thus feeding cell turnover, the volume and lamination of the OB were analyzed. The volume of the OB proved to be dramatically reduced at postnatal day 300 (P300), and this shrinkage affected the periependymal white matter, the granule cell layer, the external plexiform layer, and the glomerular layer. These results should be taken into account in cell therapies employing BMSC, since such cells reach the encephalon, although they cannot restore the damage produced in neurogenic areas. This study thus provides new insight into the long-term effects of ionizing radiation, widely employed in animal experimentation and even in clinical therapies for human beings.

Glutamine synthetase deficiency in murine astrocytes results in neonatal death

Glutamine synthetase (GS) is a key enzyme in the "glutamine-glutamate cycle" between astrocytes and neurons, but its function in vivo was thus far tested only pharmacologically. Crossing GS(fl/lacZ) or GS(fl/fl) mice with hGFAP-Cre mice resulted in prenatal excision of the GS(fl) allele in astrocytes. "GS-KO/A" mice were born without malformations, did not suffer from seizures, had a suckling reflex, and did drink immediately after birth, but then gradually failed to feed and died on postnatal day 3. Artificial feeding relieved hypoglycemia and prolonged life, identifying starvation as the immediate cause of death. Neuronal morphology and brain energy levels did not differ from controls. Within control brains, amino acid concentrations varied in a coordinate way by postnatal day 2, implying an integrated metabolic network had developed. GS deficiency caused a 14-fold decline in cortical glutamine and a sevenfold decline in cortical alanine concentration, but the rising glutamate levels were unaffected and glycine was twofold increased. Only these amino acids were uncoupled from the metabolic network. Cortical ammonia levels increased only 1.6-fold, probably reflecting reduced glutaminolysis in neurons and detoxification of ammonia to glycine. These findings identify the dramatic decrease in (cortical) glutamine concentration as the primary cause of brain dysfunction in GS-KO/A mice. The temporal dissociation between GS(fl) elimination and death, and the reciprocal changes in the cortical concentration of glutamine and alanine in GS-deficient and control neonates indicate that the phenotype of GS deficiency in the brain emerges coincidentally with the neonatal activation of the glutamine-glutamate and the associated alanine-lactate cycles.

Removal of polysialic acid triggers dispersion of subventricularly derived neuroblasts into surrounding CNS tissues

Cells generated in the subventricular zone give rise to neuroblasts that migrate to the olfactory bulb (OB) along the rostral migratory stream (RMS). The polysialylated form of neural cell adhesion molecule (PSA-NCAM) is expressed by these cells, and has been shown to both promote their migration and suppress differentiation induced by NCAM. In the present study, enzymatic removal of PSA from these neuroblasts using PSA-specific endoneuraminidase has been found not only to disrupt the tangential migration and cellular organization of the RMS, but also to cause a massive dispersion of BrdU (5-bromo-2'-deoxyuridine)-labeled neuroblasts into surrounding brain regions, including cortex and striatum. These dispersed cells are capable of differentiation, some into mature neurons, and could potentially be of value in the repair of CNS injury. Although the removal of PSA by genetic deletion of NCAM also results in a smaller OB and a swollen RMS, the cells do not escape the RMS in large numbers. These findings suggest that the presence of NCAM without PSA plays a role in the dispersion process, possibly by inducing a new pattern of migration associated with NCAM-dependent differentiation.

Developmental neurotoxicity study of dietary bisphenol A in Sprague-Dawley rats

This study was conducted to determine the potential of bisphenol A (BPA) to induce functional and/or morphological effects to the nervous system of F(1) offspring from dietary exposure during gestation and lactation according to the Organization for Economic Cooperation and Development and U.S. Environmental Protection Agency guidelines for the study of developmental neurotoxicity. BPA was offered to female Sprague-Dawley Crl:CD (SD) rats (24 per dose group) and their litters at dietary concentrations of 0 (control), 0.15, 1.5, 75, 750, and 2250 ppm daily from gestation day 0 through lactation day 21. F(1) offspring were evaluated using the following tests: detailed clinical observations (postnatal days [PNDs] 4, 11, 21, 35, 45, and 60), auditory startle (PNDs 20 and 60), motor activity (PNDs 13, 17, 21, and 61), learning and memory using the Biel water maze (PNDs 22 and 62), and brain and nervous system neuropathology and brain morphometry (PNDs 21 and 72). For F(1) offspring, there were no treatment-related neurobehavioral effects, nor was there evidence of neuropathology or effects on brain morphometry. Based on maternal and offspring body weight reductions, the no-observed-adverse-effect level (NOAEL) for systemic toxicity was 75 ppm (5.85 and 13.1 mg/kg/day during gestation and lactation, respectively), with no treatment-related effects at lower doses or nonmonotonic dose responses observed for any parameter. There was no evidence that BPA is a developmental neurotoxicant in rats, and the NOAEL for developmental neurotoxicity was 2250 ppm, the highest dose tested (164 and 410 mg/kg/day during gestation and lactation, respectively).

Effects of maternal behavior induction and pup exposure on neurogenesis in adult, virgin female rats

The states of pregnancy and lactation bring about a range of physiological and behavioral changes in the adult mammal that prepare the mother to care for her young. Cell proliferation increases in the subventricular zone (SVZ) of the female rodent brain during both pregnancy and lactation when compared to that in cycling, diestrous females. In the present study, the effects of maternal behavior induction and pup exposure on neurogenesis in nulliparous rats were examined in order to determine whether maternal behavior itself, independent of pregnancy and lactation, might affect neurogenesis. Adult, nulliparous, Sprague-Dawley, female rats were exposed daily to foster young in order to induce maternal behavior. Following the induction of maternal behavior each maternal subject plus females that were exposed to pups for a comparable number of test days, but did not display maternal behavior, and subjects that had received no pup exposure were injected with bromodeoxyuridine (BrdU, 90 mg/kg, i.v.). Brain sections were double-labeled for BrdU and the neural marker, NeuN, to examine the proliferating cell population. Increases in the number of double-labeled cells were found in the maternal virgin brain when compared with the number of double-labeled cells present in non-maternal, pup-exposed nulliparous rats and in females not exposed to young. No changes were evident in the dentate gyrus of the hippocampus as a function of maternal behavior. These data indicate that in nulliparous female rats maternal behavior itself is associated with the stimulation of neurogenesis in the SVZ.

A brain slice culture model for studies of endogenous and exogenous precursor cell migration in the rostral migratory stream

The rostral migratory stream (RMS) is the main pathway by which newly born subventricular zone (SVZ) cells reach the olfactory bulb (OB) in rodents. This migration has been well studied in vivo, but an organotypic in vitro model would facilitate more experimental investigations. Here we introduce a slice culture preparation of the rat forebrain including en suite the rostral part of the lateral ventricle, the RMS and the OB. The preparation was validated with regard to endogenous cell proliferation and migration by tracking bromodeoxyuridine (BrdU)-labelled cells in newly established and 3 and 6 week old cultures. For testing the migratory abilities of exogenous precursor cells, rat SVZ neurospheres and human neural (HNS1 cells) and mesenchymal (hMSC-TERT) stem cell lines were micrografted to the rostral SVZ of 1 and 7 day old cultures. Two weeks later graft derivatives were identified by immunohistochemical staining for human nuclei (HNS1/hMSC-TERT cells) and BrdU (HNS1 cells/neurospheres). Numerous HNS1 cells and BrdU-positive neurosphere cells were found in the RMS. Having reached the OB, subpopulations of the cells expressed the astroglial markers glial fibrillary acidic protein/hAM and the neuronal markers NeuN/tyrosine hydroxylase. Interestingly, the hMSC-TERT cells remained at the implantation site, demonstrating a diversity in migratory capability of different precursor cells. In conclusion, the RMS in rat forebrain slice cultures retains its ability to support migration of endogenous and exogenous neural precursors, making the cultures highly feasible for studies of conditions and factors regulating cell migration.

Ultrastructural and antigenic properties of neural stem cells and their progeny in adult rat subventricular zone

Neural stem cells (NSCs) in the subventricular zone (SVZ) continuously generate olfactory bulb interneurons in the adult rodent brain. Based on their ultrastructural and antigenic properties, NSCs, transient amplifying precursor cells, and neuroblasts (B, C, and A cells, respectively) have been distinguished in mouse SVZ. Here, we aimed to identify these cell types in rat SVZ ultrastructurally and at the light microscopy level, and to determine the antigenic properties of each cell type using gold and fluorescence immunolabeling. We found astrocytes with single cilia (NSCs, correspond to B cells) and neuroblasts (A cells). We also observed mitotic cells, ependymal cells, displaced ependymal cells, and mature astrocytes. In contrast, transient amplifying precursor cells (C cells) were not detected. The NSCs and neuroblasts had epidermal growth factor receptor (EGFR) and platelet-derived growth factor receptor alpha (PDGFRalpha) expressed on the ciliary apparatus and were the only cell types incorporating the proliferation marker BrdU. Throughout mitosis, EGFR and PDGFRalpha were associated with the microtubule of the mitotic spindle. Ependymal and displaced ependymal cells also expressed EGFR and PDGFRalpha on their cilia but did not incorporate BrdU. Our findings indicate that the NSCs in adult rat SVZ give rise directly to neuroblasts. During mitosis, the NSCs disassemble the primary cilium and symmetrically distribute EGFR and PDGFRalpha among their progeny.

Cellular immortality in brain tumours: an integration of the cancer stem cell paradigm

Brain tumours are a diverse group of neoplasms that continue to present a formidable challenge in our attempt to achieve curable intervention. Our conceptual framework of human brain cancer has been redrawn in the current decade. There is a gathering acceptance that brain tumour formation is a phenotypic outcome of dysregulated neurogenesis, with tumours viewed as abnormally differentiated neural tissue. In relation, there is accumulating evidence that brain tumours, similar to leukaemia and many solid tumours, are organized as a developmental hierarchy which is maintained by a small fraction of cells endowed with many shared properties of tissue stem cells. Proof that neurogenesis persists throughout adult life, compliments this concept. Although the cancer cell of origin is unclear, the proliferative zones that harbour stem cells in the embryonic, post-natal and adult brain are attractive candidates within which tumour-initiation may ensue. Dysregulated, unlimited proliferation and an ability to bypass senescence are acquired capabilities of cancerous cells. These abilities in part require the establishment of a telomere maintenance mechanism for counteracting the shortening of chromosomal termini. A strategy based upon the synthesis of telomeric repeat sequences by the ribonucleoprotein telomerase, is prevalent in approximately 90% of human tumours studied, including the majority of brain tumours. This review will provide a developmental perspective with respect to normal (neurogenesis) and aberrant (tumourigenesis) cellular turnover, differentiation and function. Within this context our current knowledge of brain tumour telomere/telomerase biology will be discussed with respect to both its developmental and therapeutic relevance to the hierarchical model of brain tumourigenesis presented by the cancer stem cell paradigm.

Sexual dimorphic stages affect both proliferation and serotonergic innervation in the adult rostral migratory stream

One of the sexual dimorphic differences in adult rodents is neural proliferation. Here we demonstrate that physiological hormone stages can modulate this proliferation in the adult forebrain. Female mice, both pregnant and synchronized in oestrus, exhibited higher proliferating cell percentages than males in both the rostral migratory stream (RMS) and the olfactory bulb (OB). Moreover, although the hormonal component also influenced the subventricular zone (SVZ), no differences in proliferation were observed in this region. In addition, both groups of females had higher numbers of serotonergic fibres in these regions. Serotonin may therefore be related to the mechanism of action by which hormones can affect cell proliferation of this brain region. We also evaluated cell death in the SVZ in males and females, finding that this was higher in the former. Taken together, our results support the idea that in female rodents more neuroblasts are able to reach the RMS and then proliferate, apoptosis being an additional mechanism affecting the low proliferation of cells in the RMS and OB in males. Thus, proliferation in the RMS is influenced by sexual dimorphism.

The astrocyte odyssey

Neurons have long held the spotlight as the central players of the nervous system, but we must remember that we have equal numbers of astrocytes and neurons in the brain. Are these cells only filling up the space and passively nurturing the neurons, or do they also contribute to information transfer and processing? After several years of intense research since the pioneer discovery of astrocytic calcium waves and glutamate release onto neurons in vitro, the neuronal-glial studies have answered many questions thanks to technological advances. However, the definitive in vivo role of astrocytes remains to be addressed. In addition, it is becoming clear that diverse populations of astrocytes coexist with different molecular identities and specialized functions adjusted to their microenvironment, but do they all belong to the umbrella family of astrocytes? One population of astrocytes takes on a new function by displaying both support cell and stem cell characteristics in the neurogenic niches. Here, we define characteristics that classify a cell as an astrocyte under physiological conditions. We will also discuss the well-established and emerging functions of astrocytes with an emphasis on their roles on neuronal activity and as neural stem cells in adult neurogenic zones.

CD11c/EYFP transgene illuminates a discrete network of dendritic cells within the embryonic, neonatal, adult, and injured mouse brain

The CD11c enhanced yellow fluorescent protein (EYFP) transgenic mouse was constructed to identify dendritic cells in the periphery (Lindquist et al. [2004] Nat. Immunol. 5:1243-1250). In this study, we used this mouse to characterize dendritic cells within the CNS. Our anatomic results showed discrete populations of EYFP(+) brain dendritic cells (EYFP(+) bDC) that colocalized with a small fraction of microglia immunoreactive for Mac-1, Iba-1, CD45, and F4/80 but not for NeuN, Dcx, NG2 proteoglycan, or GFAP. EYFP(+) bDC, isolated by fluorescent activated cell sorting (FACS), expressed mRNA for the Itgax (CD11c) gene, whereas FACS anlaysis of EYFP(+) bDC cultures revealed the presence of CD11c protein. Light microscopy studies revealed that EYFP(+) bDC were present in the embryonic CNS when the blood-brain barrier is formed and postnatally when brain cells are amenable to culturing. In adult male mice, EYFP(+) bDC distribution was prominent within regions of the CNS that 1) are subject to structural plasticity and neurogenesis, 2) receive sensory and humoral input from the external environment, and 3) lack a blood-brain barrier. Ultrastructural analysis of EYFP(+) bDC in adult neurogenic niches showed their proximity to developing neurons and a morphology characteristic of immune/microglia cells. Kainic acid-induced seizures revealed that EYFP(+) bDC responded to damage of the hippocampus and displayed morphologies similar to those described for seizure-activated EGFP(+) microglia in the hippocampus of cfms (CSF-1R) EGFP mice. Collectively, these findings suggest a new member of the dendritic cell family residing among the heterogeneous microglia population.

Doublecortin-expressing cells in the ischemic penumbra of a small-vessel stroke

A cortical lesion was induced by disrupting the medium-size pial vessels, which led to a cone-shaped cortical lesion and turned into a fluid-filled cavity surrounded by a glial acidic fibrillary protein-positive (GFAP(+)) glia limitans 21 days after injury. Therefore, it mimics conditions of lacunar infarctions, one of the most frequent human stroke pathologies. Doublecortin (DCX)-positive cells were present in the neocortex and corpus callosum at the base of the lesion. The number of DCX-positive cells in the corpus callosum was significantly increased from day 5 to day 14 compared with the control group. In contrast, there were no DCX-positive cells in neocortex of control animals; the DCX-positive cells appeared in the neocortex after lesioning and were maintained until 14 days postlesioning. Some of the DCX-positive cells were also immunoreactive for beta III-tubulin, another marker of immature neurons. They did not stain positively for markers of glia cells. The presence of these DCX-positive cells near the lesion might indicate a migratory pathway for developing neuroblasts from the subventricular zone (SVZ) through the corpus callosum to the lesion. SVZ cells were labeled with a lipophilic molecule, 5- (and 6-) carboxyfluorescein diacetate succinimidyl ester (CFSE) stereotaxical injections. Although rostral migratory stream and olfactory bulb were intensely labeled, no CFSE-containing cells were found in the cortex beneath the lesion. These results do not support the idea that the DCX-positive cells were originating from neural precursors of the SVZ, but they might be generated from local progenitor cells.

Hepatocyte growth factor regulates migration of olfactory interneuron precursors in the rostral migratory stream through Met-Grb2 coupling

The olfactory bulb is one of the few structures in the mammalian forebrain in which continuous neurogenesis takes place throughout life. Neuronal precursors originate from progenitors located in the subventricular zone (SVZ) of the lateral ventricles, move tangentially in chains through the rostral migratory stream (RMS), and reach the olfactory bulb (OB), where they finally differentiate into granule and glomerular interneurons. Multiple molecular factors are involved in controlling the various steps of this neurogenic process. Here, we show that hepatocyte growth factor (HGF) and its receptor Met protein are expressed in vivo in the OB and throughout the migratory pathway, implying that HGF might mediate migratory signals in this system. By using primary in vitro cultures, we demonstrate that HGF promotes migration of RMS neuroblasts, acting both as an inducer and attractant. HGF stimulation on RMS tissue explants selectively induces MAP kinase pathway activation. Furthermore, in vitro analysis of mice with a point mutation in the Met receptor that impairs signal transduction through the Ras/MAP kinase pathway (Met(Grb2/Grb2)) shows that without Met-Grb2 binding, neuroblast migration is reduced. Overall, these findings indicate that HGF signaling via Met-Grb2 coupling influences olfactory interneuron precursor migration along the RMS.