Embryonic (PSA) N-CAM reveals chains of migrating neuroblasts between the lateral ventricle and the olfactory bulb of adult mice

Intrathecal injection of epidermal growth factor and fibroblast growth factor 2 promotes proliferation of neural precursor cells in the spinal cords of mice with mutant human SOD1 gene

We investigated three steps of neural precursor cell activation--proliferation, migration, and differentiation--in amyotrophic lateral sclerosis spinal cord treated with intrathecal infusion of epidermal growth factor (EGF) and fibroblast growth factor 2 (FGF2) into the lumbar spinal cord region of normal and symptomatic transgenic (Tg) mice with a mutant human Cu/Zn superoxide dismutase (SOD1) gene. We observed that 5-bromodeoxyuridine (BrdU) + nestin double-labeled neural precursor cells increased in the spinal cords of Tg mice compared with non-Tg mice, with a much greater increase produced by EGF and FGF2 treatment. The number of BrdU + nestin double-labeled cells was larger than that of BrdU + ionized calcium-binding adapter molecule-1 (Iba1), BrdU + glial fibrillary acidic protein (GFAP), or BrdU + highly polysialylated neural cell adhesion molecule (PSA-NCAM) double-labeled cells, but none expressed neuronal nuclear antigen (NeuN). On further analysis of the gray matter of Tg mice, the number of BrdU + nestin and BrdU + PSA-NCAM double-labeled cells increased more in the ventral horns than the dorsal horns, which was again greatly enhanced by EGF and FGF2 treatment. Because neural precursor cells reside close to the ependyma of central canal, the present study suggests that proliferation and migration of neural precursor cells to the ventral horns is greatly activated in symptomatic Tg mice and is further enhanced by EGF and FGF2 treatment and, furthermore, that the neural precursor cells preferentially differentiate into neuronal precursor cells instead of astrocytes in Tg mice with EGF and FGF2 treatment.

Cellular composition and cytoarchitecture of the rabbit subventricular zone and its extensions in the forebrain

Persistent neurogenic sites, harboring neurogenic progenitor cells, which give rise to neuronal precursors throughout life, occur in different mammals, including humans. The telencephalic subventricular zone (SVZ) is the most active adult neurogenic site. Despite remarkable knowledge of its anatomical and cellular composition in rodents, detailed arrangement of SVZ in other mammals is poorly understood, yet comparative studies suggest that differences might exist. Here, by analyzing the cellular composition/arrangement in the SVZ of postnatal, young, and adult rabbits, we found a remarkably heterogeneous distribution of its chain and glia compartments. Starting from postnatal stages, this heterogeneity leads to a distinction between a ventricular SVZ and an abventricular SVZ, whereby the former contains small chains and isolated neuroblasts and the latter is characterized by large chains and a loose astrocytic meshwork. In addition to analysis of the SVZ proper, attention has been focused on its extensions, called parenchymal chains. Anterior parenchymal chains are compact chains surrounded by axon bundles and frequently establish direct contact with blood vessels. Posterior parenchymal chains are less compact, being squeezed between gray and white matter. In the shift from neonatal to adult rabbit SVZ, chains occur very early, both in the SVZ and within the brain parenchyma. Comparison of these results with the pattern in rodents reveals different types of chains, displaying a variety of relationships with glia or other substrates in vivo, an issue that might be important in understanding differences in the adaptation of persistent germinative layers to different mammalian brain anatomies.

The fate of neural progenitor cells expressing astrocytic and radial glial markers in the postnatal rat dentate gyrus

In the dentate gyrus neurons continue to be generated from late embryonic to adult stage. Recent extensive studies have unveiled several key aspects of the adult neurogenesis, but only few attempts have so far been made on the analysis of the early postnatal neurogenenesis, a transition state between the embryonic and adult neurogenesis. Here, we focus on the early postnatal neurogenesis and examine the nature and development of neural progenitor cells in Wistar rats. Immunohistochemistry for Ki67, a cell cycle marker, and 5-bromo-2-deoxyuridine (BrdU) labelling show that cell proliferation occurs mainly in the hilus and partly in the subgranular zone. A majority of the proliferating cells express S100beta and astrocyte-specific glutamate transporter (GLAST) and the subpopulation are also positive for glial fibrillary acidic protein (GFAP) and nestin. Tracing with BrdU and our modified retrovirus vector carrying enhanced green fluorescent protein (GFP) indicate that a substantial population of the proliferating cells differentiate into proliferative neuroblasts and immature neurons in the hilus, which then migrate to the granule cell layer (66.8%), leaving a long axon-like process behind in the hilus, and the others mainly become star-shaped astrocytes (12.0%) and radial glia-like cells (4.7%) in the subgranular zone. These results suggest that the progenitors of the granule cells expressing astrocytic and radial glial markers, proliferate and differentiate into neurons mainly in the hilus during the early postnatal period.

The repair of complex neuronal circuitry by transplanted and endogenous precursors

During the past three decades, research exploring potential neuronal replacement therapies has focused on replacing lost neurons by transplanting cells or grafting tissue into diseased regions of the brain. However, in the last decade, the development of novel approaches has resulted in an explosion of new research showing that neurogenesis, the birth of new neurons, normally occurs in two limited and specific regions of the adult mammalian brain, and that there are significant numbers of multipotent neural precursors in many parts of the adult mammalian brain. Recent advances in our understanding of related events of neural development and plasticity, including the role of radial glia in developmental neurogenesis, and the ability of endogenous precursors present in the adult brain to be induced to produce neurons and partially repopulate brain regions affected by neurodegenerative processes, have led to fundamental changes in the views about how the brain develops, as well as to approaches by which transplanted or endogenous precursors might be used to repair the adult brain. For example, recruitment of new neurons can be induced in a region-specific, layer-specific, and neuronal type-specific manner, and, in some cases, newly recruited neurons can form long-distance connections to appropriate targets. Elucidation of the relevant molecular controls may both allow control over transplanted precursor cells and potentially allow for the development of neuronal replacement therapies for neurodegenerative disease and other CNS injuries that might not require transplantation of exogenous cells.

Expression of collapsin response mediator proteins 1, 2 and 5 is differentially regulated in newly generated and mature neurons of the adult olfactory system

Collapsin-response mediator proteins (CRMPs) are highly expressed in the developing brain where they take part in several aspects of neuronal differentiation. CRMPs are still present postnatally, but their function remains speculative in the adult brain. We studied the expression and localization of CRMP1, CRMP2 and CRMP5 in two areas of the nervous system with persistent neurogenesis in adult mice, the olfactory mucosa and the olfactory bulb. In the olfactory mucosa, we have established that CRMP expression is restricted to postmitotic cells of the olfactory neurons lineage. CRMP5 is coexpressed with growth associated protein of 43 kDa (GAP43) in immature olfactory neurons and is down-regulated in olfactory marker protein-positive mature neurons. In contrast, CRMP1 and CRMP2 persist at all stages of differentiation from immature GAP43-positive to fully mature olfactory neurons. In the olfactory bulb, CRMP1, CRMP2 and CRMP5 are abundant in neuronal progenitors of the subependymal layer and in differentiating interneurons. In both areas, the subcellular distribution of CRMP1 or CRMP2 is different in mature vs. immature neurons, suggesting that these proteins are sequentially involved in various cellular events during neuronal lifetime. The variations of CRMP expression following axotomy are consistent with their differential localization and functional involvement in immature vs. mature neurons of the olfactory system. Our data bring new insight to the putative functions of CRMPs within areas of the adult nervous system with permanent neurogenesis, some related to differentiation of newly generated neurons but others occurring in mature neurons with a limited lifespan.

Chain formation and glial tube assembly in the shift from neonatal to adult subventricular zone of the rodent forebrain

The subventricular zone (SVZ) is regarded as an embryonic germinal layer persisting at the end of cerebral cortex neurogenesis and capable of generating neuronal precursors throughout life. The two distinct compartments of the adult rodent forebrain SVZ, astrocytic glial tubes and chains of migrating cells, are not distinguishable in the embryonic and early postnatal counterpart. In this study we analyzed the SVZ of mice and rats around birth and throughout different postnatal stages, describing molecular and morphological changes which lead to the typical structural arrangement of adult SVZ. In both species studied, most changes occurred during the first month of life, the transition being slightly delayed in mice, in spite of their earlier development. Important modifications affected the glial cells, eventually leading to glial tube assembly. These changes involved an overall reorganization of glial processes and their mutual relationships, as well as gliogenesis occurring within the SVZ which gives rise to glial cell subpopulations. The neuroblast cell population remained qualitatively quite homogeneous throughout all the stages investigated, changes being restricted to the relationships among cells and consequent formation of chains at about the third postnatal week. Electron microscopy showed that chain formation is not directly linked to glial tube assembly, generally preceding the occurrence of complete glial ensheathment. Moreover, chain and glial tube formation is asymmetric in the medial/lateral aspect of the SVZ, being inversely related. The attainment of an adult SVZ compartmentalization, on the other hand, seems linked to the pattern of expression of adhesion and extracellular matrix molecules.

Adult neurogenesis and repair of the adult CNS with neural progenitors, precursors, and stem cells

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

Long-term survival of dopamine neurons derived from parthenogenetic primate embryonic stem cells (cyno-1) after transplantation

Dopamine (DA) neurons can be derived from human and primate embryonic stem (ES) cells in vitro. An ES cell-based replacement therapy for patients with Parkinson's disease requires that in vitro-generated neurons maintain their phenotype in vivo. Other critical issues relate to their proliferative capacity and risk of tumor formation, and the capability of migration and integration in the adult mammalian brain. Neural induction was achieved by coculture of primate parthenogenetic ES cells (Cyno-1) with stromal cells, followed by sequential exposure to midbrain patterning and differentiation factors to favor DA phenotypic specification. Differentiated ES cells were treated with mitomycin C and transplanted into adult immunosuppressed rodents and into a primate (allograft) with out immunosuppression. A small percentage of DA neurons survived in both rodent and primate hosts for the entire term of the study (4 and 7 months, respectively). Other neuronal and glial populations derived from Cyno-1 ES cells showed, in vivo, phenotypic characteristics and growth and migration patterns similar to fetal primate transplants, and a majority of cells (>80%) expressed the forebrain transcription factor brain factor 1. No teratoma formation was observed. In this study, we demonstrate long-term survival of DA neurons obtained in vitro from primate ES cells. Optimization of differentiation, cell selection, and cell transfer is required for functional studies of ES-derived DA neurons for future therapeutic applications.

Constitutive and induced neurogenesis in the adult mammalian brain: manipulation of endogenous precursors toward CNS repair

Over most of the past century of modern neuroscience, it was thought that the adult brain was completely incapable of generating new neurons. During the past 3 decades, research exploring potential neuronal replacement therapies has focused on replacing lost neurons by transplanting cells or grafting tissue into diseased regions of the brain. However, in the last decade, the development of new techniques has resulted in an explosion of new research showing that neurogenesis, the birth of new neurons, normally occurs in two limited and specific regions of the adult mammalian brain and that there are significant numbers of multipotent neural precursors in many parts of the adult mammalian brain. Recent advances in our understanding of related events of neural development and plasticity, including the role of radial glia in developmental neurogenesis and the ability of endogenous precursors present in the adult brain to be induced to produce neurons and partially repopulate brain regions affected by neurodegenerative processes, have led to fundamental changes in the views about how the brain develops as well as to approaches by which endogenous precursors might be recruited to repair the adult brain. Recruitment of new neurons can be induced in a region-specific, layer-specific and neuronal-type-specific manner, and, in some cases, newly recruited neurons can form long-distance connections to appropriate targets. Elucidation of the relevant molecular controls may both allow control over transplanted precursor cells and potentially allow the development of neuronal replacement therapies for neurodegenerative disease and other CNS injuries that do not require transplantation of exogenous cells.

Adult neurogenesis: from precursors to network and physiology

The discovery that the adult mammalian brain creates new neurons from pools of stemlike cells was a breakthrough in neuroscience. Interestingly, this particular new form of structural brain plasticity seems specific to discrete brain regions, and most investigations concern the subventricular zone (SVZ) and the dentate gyrus (DG) of the hippocampal formation (HF). Overall, two main lines of research have emerged over the last two decades: the first aims to understand the fundamental biological properties of neural stemlike cells (and their progeny) and the integration of the newly born neurons into preexisting networks, while the second focuses on understanding its relevance in brain functioning, which has been more extensively approached in the DG. Here, we propose an overview of the current knowledge on adult neurogenesis and its functional relevance for the adult brain. We first present an analysis of the methodological issues that have hampered progress in this field and describe the main neurogenic sites with their specificities. We will see that despite considerable progress, the levels of anatomic and functional integration of the newly born neurons within the host circuitry have yet to be elucidated. Then the intracellular mechanisms controlling neuronal fate are presented briefly, along with the extrinsic factors that regulate adult neurogenesis. We will see that a growing list of epigenetic factors that display a specificity of action depending on the neurogenic site under consideration has been identified. Finally, we review the progress accomplished in implicating neurogenesis in hippocampal functioning under physiological conditions and in the development of hippocampal-related pathologies such as epilepsy, mood disorders, and addiction. This constitutes a necessary step in promoting the development of therapeutic strategies.

Expression of the secreted factors noggin and bone morphogenetic proteins in the subependymal layer and olfactory bulb of the adult mouse brain

The antagonism between noggin and the bone morphogenetic proteins (BMPs) plays a key role during CNS morphogenesis and differentiation. Recent studies indicate that these secreted factors are also widely expressed in the postnatal and adult mammalian brain in areas characterized by different types of neural plasticity. In particular, significant levels of noggin and BMP expression have been described in the rodent olfactory system. In the mammalian forebrain, the olfactory bulb (OB) and associated subependymal layer (SEL) are documented as sites of adult neurogenesis. Here, using multiple approaches, including the analysis of noggin-LacZ heterozygous mice, we report the expression of noggin and two members of the BMP family, BMP4 and BMP7, in these regions of the adult mammalian forebrain. We observe that along the full extent of the SEL, from the lateral ventricle to the olfactory bulb, noggin and BMP4 and 7 are mainly associated with the astrocytic glial compartment. In the OB, BMP4 and 7 proteins remain primarily associated with the SEL while strong noggin expression was also found in cells located in different OB layers (i.e. granule, external plexiform, glomerular layers). Taken together our data lead us to hypothesize that within the SEL the antagonism between noggin and BMPs, both produced by the glial tubes, act through autocrine/paracrine inductive mechanisms to maintain a neurogenetic environment all the way from the lateral ventricle to the olfactory bulb. In the OB, their expression patterns suggest multiple regulatory roles on the unusual neural plasticity exhibited by this region.

Information processing in the mammalian olfactory system

Recently, modern neuroscience has made considerable progress in understanding how the brain perceives, discriminates, and recognizes odorant molecules. This growing knowledge took over when the sense of smell was no longer considered only as a matter for poetry or the perfume industry. Over the last decades, chemical senses captured the attention of scientists who started to investigate the different stages of olfactory pathways. Distinct fields such as genetic, biochemistry, cellular biology, neurophysiology, and behavior have contributed to provide a picture of how odor information is processed in the olfactory system as it moves from the periphery to higher areas of the brain. So far, the combination of these approaches has been most effective at the cellular level, but there are already signs, and even greater hope, that the same is gradually happening at the systems level. This review summarizes the current ideas concerning the cellular mechanisms and organizational strategies used by the olfactory system to process olfactory information. We present findings that exemplified the high degree of olfactory plasticity, with special emphasis on the first central relay of the olfactory system. Recent observations supporting the necessity of such plasticity for adult brain functions are also discussed. Due to space constraints, this review focuses mainly on the olfactory systems of vertebrates, and primarily those of mammals.

Members of the NF-kappaB family expressed in zones of active neurogenesis in the postnatal and adult mouse brain

The Rel/NF-kappaB family of transcription factors is implicated in cell proliferation, cell death, cell migration and cell interactions. Here, we examined by immunohistochemistry the expression pattern of various members of this family during postnatal telencephalon development and during adulthood, and we used neuronal and glial markers to identify the cells types where they are expressed. Distinct Rel/NF-kappaB proteins are highly expressed postnatally in the subventricular zone and in the rostral migratory stream. In particular, Rel A and p50 are expressed in radial glial cells, in migrating neuron precursors and in a population belonging to the astrocytic lineage. Rel B, on the other hand, is only expressed in migrating neuron precursors, whereas c-Rel is present in a few cells located at the edges of the rostral migratory stream. The expression of Rel A and p50 persists into adulthood, particularly in subventricular zone astrocyte-like cells and in migrating neuron precursors, respectively. The selective expression of NF-kappaB members in the postnatal subventricular zone and rostral migratory stream and their persistence into adulthood in regions of ongoing neurogenesis suggests possible mechanisms linking NF-kappaB expression with cell proliferation and migration. Their presence in actively proliferating progenitor cells, detected by BrdU staining, further suggests that NF-kappaB may be part of a signaling pathway that is important for neurogenesis.

Transient increase of TUNEL-positive cells on postnatal day 20 in the developing rat olfactory bulb

In the developing central nervous system, apoptosis plays an important role in the normal organization of the neuronal circuit. The timing of neurogenesis, proliferation, and migration of the neurons in the developing olfactory bulb (OB) is well studied; however, the involvement of apoptosis in this process is not fully understood. In this study, we examined the changes in the distribution and the number of apoptotic cells in the rat OB during embryonic and postnatal periods, by using terminal deoxynucleotidyl transferase-mediated dUTP-digoxigenin nick end-labeling (TUNEL) staining. Although the number of TUNEL-positive cells was relatively small during the embryonic period, it gradually increased after birth, and peaked on postnatal day 20 with statistical significance, especially in the granule cell layer of the main OB. This transient increase of TUNEL-positive cells on postnatal day 20 may be involved in a critical event during maturation of the OB.

Neural stem cells in the adult nervous system

The concept of the immutability of the nervous tissue has recently been replaced with the new idea that a continuous neurogenic turnover does occur in some limited areas of the central nervous system (CNS). At least two neurogenic regions of the adult mammalian CNS are involved in this process: the subventricular zone of the forebrain and the dentate gyrus of the hippocampus, which are considered to be a reservoir of new neural cells. Neural stem cells (NSCs) are multipotential progenitors that have self-renewal capability. While in vivo endogenous NSCs seem able to produce almost exclusively neurons, a single NSC in vitro is competent to generate neurons, astrocytes, and oligodendrocytes. NSCs lack a specific morphology and unambiguous surface markers that could allow their identification. For this reason, one of the major difficulties in identifying stem cells is that they are defined in terms of their functional capabilities, the determination of which might alter the cells' nature. The purpose of this review is to describe the characteristics of the NSCs of the adult mammalian CNS, their potentiality in terms of proliferation and differentiation capabilities, as well as their stability in long-term culture, all attributes that make them a good tool for tissue replacement therapies.

Local neurons play key roles in the mammalian olfactory bulb

Over the past few decades, research exploring how the brain perceives, discriminates, and recognizes odorant molecules has received a growing interest. Today, olfaction is no longer considered a matter of poetry. Chemical senses entered the biological era when an increasing number of scientists started to elucidate the early stages of the olfactory pathway. A combination of genetic, biochemical, cellular, electrophysiological and behavioral methods has provided a picture of how odor information is processed in the olfactory system as it moves from the periphery to higher areas of the brain. Our group is exploring the physiology of the main olfactory bulb, the first processing relay in the mammalian brain. From different electrophysiological approaches, we are attempting to understand the cellular rules that contribute to the synaptic transmission and plasticity at this central relay. How olfactory sensory inputs, originating from the olfactory epithelium located in the nasal cavity, are encoded in the main olfactory bulb remains a crucial question for understanding odor processing. More importantly, the persistence of a high level of neurogenesis continuously supplying the adult olfactory bulb with newborn local neurons provides an attractive model to investigate how basic olfactory functions are maintained when a large proportion of local neurons are continuously renewed. For this purpose, we summarize the current ideas concerning the molecular mechanisms and organizational strategies used by the olfactory system to encode and process information in the main olfactory bulb. We discuss the degree of sensitivity of the bulbar neuronal network activity to the persistence of this high level of neurogenesis that is modulated by sensory experience. Finally, it is worth mentioning that analyzing the molecular mechanisms and organizational strategies used by the olfactory system to transduce, encode, and process odorant information in the olfactory bulb should aid in understanding the general neural mechanisms involved in both sensory perception and memory. Due to space constraints, this review focuses exclusively on the olfactory systems of vertebrates and primarily those of mammals.

Neurogenesis in the adult brain: new strategies for central nervous system diseases

New cells are continuously generated from immature proliferating cells throughout adulthood in many organs, thereby contributing to the integrity of the tissue under physiological conditions and to repair following injury. In contrast, repair mechanisms in the adult central nervous system (CNS) have long been thought to be very limited. However, recent findings have clearly demonstrated that in restricted areas of the mammalian brain, new functional neurons are constantly generated from neural stem cells throughout life. Moreover, stem cells with the potential to give rise to new neurons reside in many different regions of the adult CNS. These findings raise the possibility that endogenous neural stem cells can be mobilized to replace dying neurons in neurodegenerative diseases. Indeed, recent reports have provided evidence that, in some injury models, limited neuronal replacement occurs in the CNS. Here, we summarize our current understanding of the mechanisms controlling adult neurogenesis and discuss their implications for the development of new strategies for the treatment of neurodegenerative diseases.

Multiple roles for slits in the control of cell migration in the rostral migratory stream

The subventricular zone (SVZ) contains undifferentiated cells, which proliferate and generate olfactory bulb (OB) interneurons. Throughout life, these cells leave the SVZ and migrate along the rostral migratory stream (RMS) to the OB where they differentiate. In vitro, the septum and the choroid plexus (CP) secrete repulsive factors that could orient the migration of OB precursors. Slit1 and Slit2, two known chemorepellents for developing axons, can mimic this effect. We show here that the Slit receptors Robo2 and Robo3/Rig-1 are expressed in the SVZ and the RMS and that Slit1 and Slit2 are still present in the adult septum. Using Slit1/2-deficient mice, we found that Slit1 and Slit2 are responsible for both the septum and the CP repulsive activity in vitro. In adult mice lacking Slit1, small chains of SVZ-derived cells migrate caudally into the corpus callosum, supporting a role for Slits in orienting the migration of SVZ cells. Surprisingly, in adult mice, Slit1 was also expressed by type A and type C cells in the SVZ and RMS, suggesting that Slit1 could act cell autonomously. This hypothesis was tested using cultures of SVZ explants or isolated neurospheres from Slit1-/- or Slit1+/- mice. In both types of cultures, the migration of SVZ cells was altered in the absence of Slit1. This suggests that the regulation of the migration of OB precursors by Slit proteins is complex and not limited to repulsion.

Nitric oxide synthesis inhibition increases proliferation of neural precursors isolated from the postnatal mouse subventricular zone

The subventricular zone (SVZ) of rodents retains the capacity to generate new neurons throughout the entire life of the animal. Neural progenitors of the SVZ survive and proliferate in vitro in the presence of epidermal growth factor (EGF). Nitric oxide (NO) has been shown to participate in neural tissue formation during development and to have antiproliferative actions, mediated in part by inhibition of the EGF receptor. Based on these findings, we have investigated the possible effects of endogenously produced and exogenously added NO on SVZ cell proliferation and differentiation. Explants were obtained from postnatal mouse SVZ and cultured in the presence of EGF. Cells migrated out of the explants and proliferated in culture, as assessed by bromodeoxyuridine (BrdU) incorporation. After 72 h in vitro, the colonies formed around the explants were constituted by cells of neuronal or glial lineages, as well as undifferentiated progenitors. Immunoreactivity for the neuronal isoform of NO synthase was observed in neuronal cells with long varicose processes. Cultures treated with the NOS inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) showed an increase in the percentage of BrdU-immunoreactive cells, whereas treatment with the NO donor diethylenetriamine-nitric oxide adduct (DETA-NO) led to a decrease in cell proliferation, without affecting apoptosis. The differentiation pattern was also altered by L-NAME treatment resulting in an enlargement of the neuronal population. The results suggest that endogenous NO may contribute to postnatal neurogenesis by modulating the proliferation and fate of SVZ progenitor cells.

Effects of progesterone derivatives, dihydroprogesterone and tetrahydroprogesterone, on the subependymal layer of the adult rat

Indirect evidence suggests that in the subependymal layer (SEL) steroid hormones could be partially involved in the modulation of neurogenesis, but little or nothing is known about a direct effect of these molecules on this cellular system. The possible effect of progesterone (P) and/or its neuroactive metabolites, dihydroprogesterone (DHP) and tetrahydroprogesterone (THP), on the two cellular components of the SEL (i.e., proliferating/migrating neuroblasts and protoplasmic astrocytes) has been analyzed in adult male rat. P, DHP, and THP were administered by intraventricular injections and after 2 days the SEL was analyzed by immunohistochemistry by using anti-glial fibrillary acidic protein (GFAP) and anti-vimentin antibodies, to label the glial compartment, anti-polysialylated form of the neural cell adhesion molecule (PSA-NCAM), anti-Stathmin, and anti-beta III Tubulin antibodies to label the migrating neuroblasts. Furthermore, the newly formed cells were identified by using intraventricular injections of 5-bromo-2'-deoxyuridine (BrdU) detected immunohistochemically. Our results demonstrate that DHP and THP treatments drastically decrease the number of BrdU-labeled cells within the SEL. THP, DHP, and to a lesser extent P, administrations also induce molecular and structural modifications of the SEL glial compartment. On the whole, the present results indicate that neuroactive derivatives of P (i.e., DHP and THP) exert direct effects on adult neurogenesis, strongly affecting both neuroblasts and astrocytes of the SEL.

Chronic restraint stress and chronic corticosterone treatment modulate differentially the expression of molecules related to structural plasticity in the adult rat piriform cortex

Stress and stress-related hormones induce structural changes in neurons of the adult CNS. Neurons in the hippocampus, the amygdala and the prefrontal cortex undergo neurite remodeling after chronic stress. In the hippocampus some of these effects can be mimicked with chronic administration of adrenal steroids. These changes in neuronal structure may be mediated by certain molecules related to plastic events such as the polysialylated form of the neural cell adhesion molecule (PSA-NCAM). The expression of PSA-NCAM persists in the adult hippocampus and it is up-regulated after chronic stress. The piriform cortex also displays considerable levels of PSA-NCAM during adulthood and indirect evidence suggests that it may also be the target of stress and stress related-hormones. Using immunohistochemistry we have studied the expression of PSA-NCAM and doublecortin (DCX; another protein implicated in neuronal structural plasticity) in the piriform cortex of adult rats subjected either to 21 days of chronic restraint stress or to oral corticosterone administration during the same period. Our results indicate that chronic stress and chronic corticosterone administration have differential effects on the expression of PSA-NCAM and DCX. While chronic stress increases the number of PSA-NCAM- and DCX-immunoreactive cells in the piriform cortex layer II, chronic corticosterone administration decreases these numbers. These findings indicate that stress and adrenal steroids affect the piriform cortex and suggest that in this region, as in the hippocampus, they may induce structural changes. This is a potential mechanism by which stress and corticosterone modulate functions of this limbic region, such as its participation in olfactory memory.

Increased hippocampal neurogenesis in Alzheimer's disease

Neurogenesis, which persists in the adult mammalian brain, may provide a basis for neuronal replacement therapy in neurodegenerative diseases like Alzheimer's disease (AD). Neurogenesis is increased in certain acute neurological disorders, such as ischemia and epilepsy, but the effect of more chronic neurodegenerations is uncertain, and some animal models of AD show impaired neurogenesis. To determine how neurogenesis is affected in the brains of patients with AD, we investigated the expression of immature neuronal marker proteins that signal the birth of new neurons in the hippocampus of AD patients. Compared to controls, Alzheimer's brains showed increased expression of doublecortin, polysialylated nerve cell adhesion molecule, neurogenic differentiation factor and TUC-4. Expression of doublecortin and TUC-4 was associated with neurons in the neuroproliferative (subgranular) zone of the dentate gyrus, the physiological destination of these neurons (granule cell layer), and the CA1 region of Ammon's horn, which is the principal site of hippocampal pathology in AD. These findings suggest that neurogenesis is increased in AD hippocampus, where it may give rise to cells that replace neurons lost in the disease, and that stimulating hippocampal neurogenesis might provide a new treatment strategy.

Cell migration in the forebrain

The forebrain comprises an intricate set of structures that are required for some of the most complex and evolved functions of the mammalian brain. As a reflection of its complexity, cell migration in the forebrain is extremely elaborated, with widespread dispersion of cells across multiple functionally distinct areas. Two general modes of migration are distinguished in the forebrain: radial migration, which establishes the general cytoarchitectonical framework of the different forebrain subdivisions; and tangential migration, which increases the cellular complexity of forebrain circuits by allowing the dispersion of multiple neuronal types. Here, we review the cellular and molecular mechanisms underlying each of these types of migrations and discuss how emerging concepts in neuronal migration are reshaping our understanding of forebrain development in normal and pathological situations.

Phenotypic and molecular identity of cells in the adult subventricular zone

We have studied the molecular identity of adult mouse SVZ cells in situ, and after isolation and expansion as neurospheres in vitro. The gene and protein expression patterns of the adult cells have been compared to that of the cells from the lateral ganglionic eminence (LGE), their putative embryonic counterparts. The LGE gives rise to both striatal projection neurons and olfactory bulb interneurons via spatially and molecularly distinct progenitor populations present in the SVZ of the LGE. These two populations are thought to have a common origin in the GSH2 expressing cells of the embryonic LGE ventricular zone. We found that a significant number of cells in the adult SVZ, and in the in vitro expanded neurospheres, derived from the adult SVZ express GSH2. However, under normal conditions, GSH2-expressing cells in the adult SVZ and in the in vitro expanded neurospheres appear to specify only olfactory bulb progenitors and not striatal progenitors.

Glia-independent chains of neuroblasts through the subcortical parenchyma of the adult rabbit brain

In the brains of adult mammals long-distance cell migration of neuronal precursors is known to occur in the rostral migratory stream, involving chains of cells sliding into astrocytic glial tubes. By combining immunocytochemistry for polysialylated neural cell adhesion molecule (PSA-NCAM), neuronal and glial antigens, endogenous and exogenously administered cell-proliferation markers, and light and electron microscopy 3D reconstructions, we show that chains of newly generated neuroblasts exist both inside and outside the subventricular zone of adult rabbits. Two groups of chains were detectable within the mature brain parenchyma: anterior chains, into the anterior forceps of the corpus callosum, and posterior chains, close to the external capsule. Parenchymal chains were not associated with any special glial structures, thus coming widely in contact with the mature nervous tissue, including unmyelinated/myelinated fibers, astrocytes, neurons, and oligodendrocytes. These chains of cells, unlike those in the subventricular zone, do not display cell proliferation, but they contain BrdUrd administered several weeks before. Telencephalic areas, such as the putamen, amygdala, claustrum, and cortex, adjacent to the chains harbor numerous PSA-NCAM-positive cells. The counting of newly generated cells in these areas shows small differences in comparison with others, and a few cells double-labeled for BrdUrd/PSA-NCAM (after 1-month survival) and for BrdUrd/NeuN (after 2 months) were detectable. These results demonstrate the occurrence of glial-independent chains of migrating neuroblasts, which directly contact the mature brain parenchyma of adult mammals. These chains could provide a possible link between the adult germinative layers and a very low-rate/long-term process of cell addition in the telencephalon.

Neuronal migration from the forebrain to the olfactory bulb requires a new attractant persistent in the olfactory bulb

Interneurons in the olfactory bulb (OB) are generated not only in the developing embryo but also throughout the postnatal life of mammals from neuronal precursor cells migrating from the anterior subventricular zone (SVZa) of the mammalian forebrain. We discovered that the OB secretes a diffusible activity that attracts these neuronal precursor cells. The attractive activity is present in specific layers in the OB, including the glomerular layer but not the granule cell layer. The attractive activity and the neuronal responsiveness persist from embryonic through neonatal to adult stages. Removal of the rostral OB significantly reduces SVZa migration toward the OB, an effect that can be rescued by a transplant of the OB but not by that of the neocortex. The activity in the OB is not mimicked by the known attractants. These results provide an explanation for the continuous migration of SVZa neurons toward the OB, demonstrate an important role of the OB in neuronal migration, and reveal the existence of a new chemoattractant.

Olfactory processing in a changing brain

The perception of odorant molecules provides the essential information that allows animals to explore their surrounding. We describe here how the external world of scents may sculpt the activity of the first central relay of the olfactory system, i.e., the olfactory bulb. This structure is one of the few brain areas to continuously replace one of its neuronal populations: the local GABAergic interneurons. How the newly generated neurons integrate into a pre-existing neural network and how basic olfactory functions are maintained when a large percentage of neurons are subjected to continuous renewal, are important questions that have recently received new insights. Furthermore, we shall see how the adult neurogenesis is specifically subjected to experience-dependent modulation. In particular, we shall describe the sensitivity of the bulbar neurogenesis to the activity level of sensory inputs from the olfactory epithelium and, in turn, how this neurogenesis may adjust the neural network functioning to optimize odor information processing. Finally, we shall discuss the behavioral consequences of the bulbar neurogenesis and how it may be appropriate for the sense of smell. By maintaining a constitutive turnover of bulbar interneurons subjected to modulation by environmental cues, we propose that adult ongoing neurogenesis in the olfactory bulb is associated with improved olfactory memory. These recent findings not only provide new fuel for the molecular and cellular bases of sensory perception but should also shed light onto cellular bases of learning and memory.

Prenatal development of the rodent rostral migratory stream

The aim of this study was to elucidate the embryological origins of the unique neuronal progenitor cells that form the rostral migratory stream (RMS), the path traversed by cells from the anterior part of the forebrain subventricular zone (SVZa) en route to the olfactory bulb. To determine when and where cells constituting the RMS initially exhibit their characteristic neuronal phenotype and high mitotic capacity, we analyzed the cells of the rat forebrain between embryonic day 14 (E14) and postnatal day 2 (P2). At E14, cells with a neuronal phenotype were observed within the ventricular zone in close proximity to the mantle layer of the future olfactory bulb. By E15, cells expressing neuronal markers are also PSA-NCAM immunoreactive and become aligned in chains of similarly oriented cells, a hallmark of the postnatal RMS. The cells that form chains organize into a patch that enlarges in the anterior-posterior and medial-lateral dimensions from E16 to E22 (birth). In comparing the forebrain cytoarchitecture to the pattern of cell type-specific staining, the patch constitutes only the central part of the proximal RMS. Early during development, the region of the RMS surrounding the patch expresses low levels of PSA-NCAM and neuron-specific markers. The proliferative activity of cells forming the patch vs. nonpatch regions of the RMS was analyzed following a short bromodeoxyuridine (BrdU) exposure. Between E15 and E22, the patch can be recognized by the mitotic activity of its cells; the cells of the patch incorporate less BrdU than the nonpatch portion of the RMS. The time course of appearance of cells forming the RMS indicates that the RMS arises in advance and independently of the cortical SVZ. Although the patch and the nonpatch regions of the embryonic RMS appear to merge postnatally, the two regions may originate separately under the influence of distinct intrinsic and extrinsic factors.

Deprivation of sensory inputs to the olfactory bulb up-regulates cell death and proliferation in the subventricular zone of adult mice

The main olfactory bulb (MOB) is the first relay on the olfactory sensory pathway and the target of the neural progenitor cells generated in the subventricular zone (SVZ) lining the lateral ventricles and which migrate along the rostral extension of the SVZ, also called the rostral migratory stream (RMS). Within the MOB, the neuroblasts differentiate into granular and periglomerular interneurons. A reduction in the number of granule cells during sensory deprivation suggests that neurogenesis may be influenced by afferent activity. Here, we show that unilateral sensory deafferentation of the MOB by axotomy of the olfactory receptor neurons increases apoptotic cell death in the SVZ and along the rostro-caudal extent of the RMS. The vast majority of dying cells in the RMS are migrating neuroblasts as indicated by double Terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick-end labeling/PSA-NCAM labeling. Counting bromodeoxyuridine-labeled cells in animals killed immediately or 4 days after tracer administration showed a bilateral increase in proliferation in the SVZ and RMS which was balanced by cell death on the operated side. These data suggest that olfactory inputs are required for the survival of newborn neural progenitors. The greatest enhancement in proliferation occurred in the extension of the RMS located in the MOB, revealing a population of local precursors mitotically stimulated following axotomy. Together, these findings indicate that olfactory inputs may strongly modulate the balance between neurogenesis and apoptosis in the SVZ and RMS and provide a model for further investigation of the underlying molecular mechanisms of this activity-dependent neuronal plasticity.

Dlx-1 and Dlx-2 expression in the adult mouse brain: relationship to dopaminergic phenotypic regulation

Expression of the homeodomain-containing transcription factors Dlx-1 and Dlx-2 in the lateral (LGE) and medial (MGE) ganglionic eminences, subpallial embryonic structures, is required for generation of telencephalic interneurons. LGE- and MGE-derived progenitors migrate and populate a number of forebrain structures, including the cortex, hippocampus, and olfactory bulb (OB). Previous reports focusing on embryogenesis of telencephalic neurons in Dlx-1 and Dlx-2 null mice suggested a specific role for these genes in expression of the OB dopamine (DA) phenotype. We have investigated whether these genes also are expressed in adult brain, especially in those pallial derivatives, such as the OB, hippocampus, and possibly cortex, where neurogenesis continues in adults. With a highly sensitive, nonradioactive in situ hybridization technique and both DLX-2 and pan DLX antisera, widespread expression of both genes was found in adult mouse fore- but not mid- or hindbrain. The adult unilateral naris closure paradigm was employed to establish a causative role for Dlx in regulating tyrosine hydroxylase (TH) expression; TH is the first enzyme in DA biosynthesis. TH mRNA, but not Dlx expression, was significantly down-regulated in the OB ipsilateral to closure. These findings suggest that Dlx-1 and -2 do not play a direct role in DA phenotypic differentiation and TH gene regulation in adult OB. The widespread expression of Dlx mRNA and protein in the adult brain suggests that these genes may have additional roles in mature animals.

Morphological basis of nitric oxide production and its correlation with the polysialylated precursor cells in the dentate gyrus of the adult guinea pig hippocampus

Neurogenesis in the hippocampus persist throughout life and precursors of neurons reside in the granule cell layer of the dentate gyrus. Until now, the role of nitric oxide (NO) in the phenomenon has been unclear. By using specific antibodies and a confocal laser scanning microscope, the localization of NO synthase (NOS) was examined in the dentate gyrus of the adult guinea pig in relation with the neuronal precursor marker highly polysialylated neural cell adhesion molecule (PSA-N-CAM). Observation of single immunolabeled sections has revealed that both the PSA-N-CAM- and most NOS-positive cells were localized in the granule cell layer of the dentate gyrus. The former were small in size and showed a punctate, clustered immunoreaction with an irregular cellular margin, whereas the latter showed somewhat diverse cellular profiles. Some NOS-positive neurons had elliptical-like morphology with elongated dendrites, whereas others were small, irregularly shaped and mostly lacking dendritic spines. Double immunolabeling has revealed that NOS-immunoreactivity intermingled, as well as colocalized, with that of PSA-N-CAM, particulary in the granule cell layer. The doubly stained cells were morphologically indistinguishable from PSA-N-CAM single positive cells. These results not only suggest the role of NO production in adult hippocampal neurogenesis, but also indicate that some PSA-N-CAM-expressing neuronal precursors produce NO.

Migration and differentiation of adult rat subventricular zone progenitor cells transplanted into the adult rat striatum

Adult brain subventricular zone progenitor cells undergo neurogenesis in the olfactory bulb. We tested the hypothesis that cultured adult subventricular zone progenitor cells migrate and differentiate into neurons when transplanted into the adult striatum. Cells in the adult rat subventricular zone were isolated and cultured for 8 days in medium containing basic fibroblast growth factor. These cells proliferated as assayed by bromodeoxyuridine immunostaining, and the majority of them were neuron-specific class III beta-tubulin (TuJ1) immunoreactive at 8 days of culture. These cultured cells were labeled in vitro with bromodeoxyuridine or with lipophilic dye-coated particles and were transplanted into the adult rat striatum. Twenty-eight days after transplantation, the cells migrated 0.5-1.5 mm from the midline of the graft to the surrounding host striatum. Migration of grafted cells in the host striatum was also detected on magnetic resonance imaging in living rats. Morphological analysis revealed that many of these migrated cells exhibited multibranched processes from the cell soma resembling host medium-size striatal projection neurons. Only a few astrocyte-like cells were detected. Double immunostaining showed that many bromodeoxyuridine immunoreactive cells were microtubule-associated protein 2 or immunoreactive with a mouse monoclonal antibody against neuronal nuclear protein, whereas only a few bromodeoxyuridine immunoreactive cells had glial fibrillary acidic protein immunoreactivity. Morphology of bromodeoxyuridine and microtubule-associated protein 2 immunoreactive cells was similar to those of host microtubule-associated protein 2 immunoreactive cells. These results suggest that transplanted cultured adult subventricular zone progenitor cells can migrate and differentiate in response to guidance cues within the adult striatum.

Distribution of p120 catenin during rat brain development: potential role in regulation of cadherin-mediated adhesion and actin cytoskeleton organization

p120 catenin (p120ctn) is implicated in the regulation of cadherin-mediated adhesion and actin cytoskeleton remodeling. The interaction of cytoplasmic p120ctn with the guanine exchange factor Vav2 is one of the signaling pathways implicated in cytoskeleton dynamics. We show here that p120ctn is regulated during rat brain development and is distributed at the membrane and within the cytoplasm where it associates with N-cadherin and Vav2, respectively. p120ctn shifts progressively from an axonal expression to a punctuate staining localized to a subset of synapses. In cultured hippocampal neurons, p120ctn redistributes from growth cones to synapses, where it partly colocalizes with N-cadherin or Vav2 and filamentous actin. In the adult forebrain, we show that p120ctn and Vav2 are highly expressed by neuroblasts migrating from the lateral subventricular zone to the olfactory bulb. The dynamic expression pattern of p120ctn and the biochemical evidences of its association with N-cadherin and Vav2 strongly suggest that p120ctn plays a major role in neuronal migration, neurite outgrowth and synapse formation, and plasticity.

Cerebrospinal fluid-contacting neurons in the rat spinal cord, a gamma-aminobutyric acidergic system expressing the P2X2 subunit of purinergic receptors, PSA-NCAM, and GAP-43 immunoreactivities: light and electron microscopic study

Cerebrospinal fluid-contacting neurons (CSFcNs) occur in various brain regions of lower vertebrates. In mammals, they are restricted to medullospinal areas, and little is known about their projection sites. In the present work, we investigated some morphofunctional characteristics of such neurons in the rat spinal cord by light and electron microscopic immunocytochemistry. CSFcNs expressing the P2X(2) subunit of purinergic receptors were present throughout the spinal cord, though more numerous at lower thoracolumbar and sacral levels. These neurons coexpressed GAD and the polysialylated neural cell adhesion molecule (PSA-NCAM), a marker of cellular plasticity. From low thoracic levels downward, tiny amyelinic axons (less than 200 nm in diameter) were tightly packed in bundles, which ran along the ependyma and extended ventrally, eventually concentrating against the walls of the ventral median fissure. In addition to P2X(2), GAD, gamma-aminobutyric acid (GABA), and PSA, these axons expressed GAP-43 immunoreactivity. Moreover, they were labelled along their entire lengths with antibodies against synaptotagmin and synaptophysin, but these failed to reveal intraspinal terminal fields. Taken together, our observations indicate the presence in the rat spinal cord of a highly plastic system of GABAergic CSFcNs that express the P2X(2) subunit of purinergic receptors. The function of this original system remains open to question. In these neurons, the P2X(2) receptors may confer a sensitivity to ATP either present in the CSF or released by nearby neurons of the central autonomic area.

Roles of the mammalian subventricular zone in brain development

There has been enormous progress in uncovering the contributions of the subventricular zone (SVZ) to the developing brain. Here, we review the roles of four anatomically defined embryologic divisions of the SVZ of the mammalian brain: the lateral ganglionic eminence (LGE), the medial ganglionic eminence (MGE), the caudal ganglionic eminence (CGE), and the fetal neocortical SVZ (SVZn), as well as the roles of the two major anatomically defined regions of the postnatal SVZ, the anterior SVZ (SVZa) and the dorsolateral SVZ (SVZdl). We describe the types of cells within each subdivision of the SVZ, the types of brain cells that they generate during embryonic, fetal, and perinatal development, and when known the mechanisms that regulate their differentiation. This review provides a critical analysis of the literature, from which current and future studies on the SVZ can be formulated and evaluated.

Cellular characterization of epidermal growth factor-expanded free-floating neurospheres

Neural stem cells proliferate in liquid culture as cell clusters (neurospheres). This study was undertaken to characterize the epidermal growth factor (EGF)-expanded free-floating neurospheres derived from rat fetal striatum. We examined the ultrastructural and antigenic characteristics of these spheres. They consisted of two cell types, electron-dense and electron-lucent cells. Lucent cells were immunopositive to actin, vimentin, and nestin, whereas dense cells were immunopositive to actin, weakly positive to vimentin, and nestin-negative. Neurospheres contained healthy, apoptotic, and necrotic cells. Healthy cells were attached to each other by adherens junctions. They showed many pseudopodia and occasionally a single cilium. Sphere cells showed phagocytic capability because healthy cells phagocytosed the cell debris derived from dead cells in a particular process that involves the engulfment of dying cells by cell processes from healthy cells. Sphere cells showed a cytoplasmic and a nuclear pool of fibroblast growth factor (FGF) receptors. They expressed E- and N-cadherin, alpha- and beta-catenin, EGF receptor, and a specific subset of FGF receptors. Because sphere cells expressed this factor in the absence of exogenous FGF-2, we propose that they are able to synthesize FGF-2.

Differential neurogenesis and gliogenesis by local and migrating neural stem cells in the olfactory bulb

The rostral migratory stream (RMS) is a unique forebrain structure that provides a long-distance migratory route for the neural stem cells of the periventricular region towards the olfactory bulb (OB). The purpose of the study presented here is to examine the extent of neurogenesis and gliogenesis by the neural stem cells of different origins (periventricular vs. intrabulbar) in the OB. After the RMS had been subjected to injury, the rats received intraperitoneal injections of 5-bromodeoxyuridine (BrdU) and were further reared for 2 weeks. Neuronal and glial differentiations of the BrdU(+) cells in the olfactory bulbar granule cell (OB-GCL) and the olfactory glomerular (OB-GL) layers were examined immunohistochemically using antibodies against neuronal (NeuN, neuronal nuclei) and glial (GFAP, glial fibrillary acidic protein) markers in the OBs with injured and uninjured (control) RMS. In the completely RMS-lesioned OB, where migration of the periventricular neural stem cells was inhibited, a small number of BrdU(+) NeuN(+) cells were found in both the OB-GCL and OB-GL. The BrdU(+) NeuN(+) cells accounted for a much higher percentage of the BrdU(+) cells on the control side (OB-GCL, 36.7%; OB-GL, 8.8%) than on the completely RMS-lesioned side (OB-GCL, 3.7%; OB-GL, 0.6%). The percentage of the BrdU(+) GFAP(+) cells relative to the BrdU(+) cells did not show any major difference between the control and completely RMS-lesioned sides. This study revealed differences in neurogenesis and gliogenesis between the local and migrating neural stem cells in the OB of the adult rodent.

PSA-NCAM immunocytochemistry in the cerebral cortex and other telencephalic areas of the lizard Podarcis hispanica: differential expression during medial cortex neuronal regeneration

The lizard medial cortex, a region homologous to the mammalian dentate gyrus, shows postnatal neurogenesis and the surprising ability to replace its neurons after being lesioned specifically with the neurotoxin 3-acetylpyridine. As the polysialylated form of the neural cell adhesion molecule (PSA-NCAM) is expressed during neuronal migration and differentiation, we have studied its distribution in adult lizards and also during the lesion-regeneration process. In the medial cortex of control animals, many labeled fusiform somata, presumably corresponding to migratory neuroblasts, appeared in the inner plexiform layer. There were also scattered immunoreactive granule neurons in the cell layer. Double immunocytochemistry with 5'-bromodeoxyuridine revealed that some of the PSA-NCAM-expressing cells in the inner plexiform and cell layers were generated recently. PSA-NCAM immunoreactivity was also present in the dorsomedial, dorsal, and lateral cortices, as well as in the dorsal ventricular ridge, the nucleus accumbens, and the nucleus sphericus. Twelve hours after the injection of 3-acetylpyridine, some medial cortex granule neurons appeared degenerated, although some of them still expressed PSA-NCAM. One to 2 days after the injection, most granule neurons appeared degenerated and no PSA-NCAM immunoreactivity was detected in the medial cortex cell layer. Four to 7 days after treatment, abundant labeled fusiform cells populated the inner plexiform layer and some immunoreactive somata were seen in the cell layer. Fifteen to 30 days after the neurotoxin injection, the number of PSA-NCAM expressing granule neurons augmented considerably and the level was still above control levels in lizards that survived 42 days. Our results show for the first time the expression of PSA-NCAM in a reptile brain, where it appears to participate in the migration and differentiation of granule neurons during adult neurogenesis and regeneration.

Growth factor treatment promotes mobilization of young but not aged adult subventricular zone precursors in response to demyelination

Precursor cells of the adult mouse subventricular zone (SVZ) are mobilized and recruited by a lysolecithin (LPC)-induced demyelination of the corpus callosum. Because age decreases the proliferation of the SVZ neural precursors as well as the potential for myelin repair of the adult central nervous system, we have compared the ability of young and aged adult neural precursors to respond to LPC-induced demyelination. With age, the SVZ cells lost their capacity to proliferate and to be recruited by the lesion. Whereas a single injection of fibroblast growth factor-2 or transforming growth factor-alpha stimulated the proliferation of SVZ and rostral migratory stream precursors in both groups of animals after demyelination, they favored recruitment at the lesion in young mice but not in aged ones. In vitro experiments using neurospheres derived from young and aged animals indicated that both populations have the same migratory performances. Our in vivo data thus suggest that aged neural precursors may loose their intrinsic capacities to respond to demyelination-induced signals. Alternatively, their function may be altered by modification of the aged extracellular environment.

Hippocampal adult neurogenesis occurs in a microenvironment provided by PSA-NCAM-expressing immature neurons

Neurons continue to be generated in the adult hippocampus. In the present study, the early developmental processes of newly generated neurons in the adult rat hippocampus were examined by confocal laser scanning microscopy using a combination of bromodeoxyuridine (BrdU) labeling and immunohistochemistry for highly polysialylated neural cell adhesion molecule (PSA-NCAM) and NeuroD, which are markers for immature neurons, and glial fibrillary acidic protein (GFAP). Rats were injected with BrdU and 2 hours, 1, 3, and 7 days after the injection, the hippocampus was processed for immunohistochemistry. One day after the injection, BrdU-labeled cells were found frequently in clusters consisting of dividing cells, putative undifferentiated cells, NeuroD-positive differentiated neurons, and GFAP-positive astrocytes. Three days later, BrdU-labeled cells were loosely aggregated and BrdU-positive fragmented nuclei were sometimes observed, suggesting that apoptosis occurred in the clusters. These BrdU-labeled nuclei were frequently associated in various ways with the processes of immature PSA-positive granule cells. They are positioned along PSA-positive apical and basal dendrites or surrounded by these processes. By 7 days after the injection, the number of the clusters was reduced and the BrdU-labeled cells had developed dendrites. These cell-to-cell associations support the hypothesis that the clustering and a microenvironment provided by the PSA-expressing immature neurons contribute to the early developmental events of adult neurogenesis, such as proliferation, differentiation, apoptosis, and neurophilic migration in the adult hippocampus.

Cerebral cortex lesions decrease the number of bromodeoxyuridine-positive subventricular zone cells in mice

We previously showed that cortical lesions in rats increase the number of subventricular zone (SVZ) cells. Here, we examined the response of the SVZ to cortical lesions in mice from 6 h to 35 days later. Whereas the total number of cells did not change, the number of cells in S-phase (bromodeoxyuridine-positive) decreased in a biphasic manner (from 6 h to day 3, and again at days 25-35). In addition, there was a delayed (days 25-35) increase in immunoreactivity for polysialylated neural cell adhesion molecule, a marker of neuroblasts. The results suggest that in mice there are rapid as well as delayed responses in the SVZ to injury of the overlying cerebral cortex. They also show that the SVZ of different mammalian species can exhibit widely divergent responses to the same brain injury.

Origin and migration of interneurons of the olfactory bulb in the musk shrew, Suncus murinus

The olfactory bulb of the musk shrew, Suncus murinus, is characterized by the presence of various interneurons. Our previous report (Kakuta et al., 2001) demonstrated that positive immunoreactions for calretinin were observed in periglomerular and perinidal cells in the glomerular layer, small ovoid neurons in the external plexiform layer, and granule cells in the granule cell layer of the olfactory bulb in the musk shrew aged 1 to 5 weeks, in addition to calretinin-immunoreactive bipolar cells distributed in the anterior subependymal layer and in each layer of the olfactory bulb. To examine the origin and migration of interneurons of the olfactory bulb, we labeled generated cells by injecting 28-day-old musk shrews with 5-bromo-2'-deoxyuridine (BrdU), and detected the labeled progeny cells that survived after several intervals. BrdU-labeled cells originated in the subependymal layer around the anterior horn of the lateral ventricle, and rostrally migrated in the subependymal layer from the anterior wall of the lateral ventricle into the center of the olfactory bulb, where they radially migrated into the granule cell layer, external plexiform layer, and glomerular layer. It took 2 days to migrate rostrally in the subependymal layer from the anterior lateral ventricle to the center of the olfactory bulb, and 2 to 6 days to migrate radially from the bulbar subependymal layer into the three layers mentioned. The rate of rostralward migration of the labeled cells was estimated to be 38 microm/h, while that of radial migration, 7 to 25 microm/h. The present BrdU-labeling study, together with our previous immunohistochemical study (Kakuta et al., 2001), indicates that anterior subependymal cells differentiate into granule cells in the granule cell layer, into Van Gehuchten cells in the external plexiform layer, and into periglomerular and perinidal cells in the glomerular layer of the olfactory bulb in the musk shrew.

Deleted in colorectal carcinoma and differentially expressed integrins mediate the directional migration of neural precursors in the rostral migratory stream

Precursors of the olfactory interneurons migrate from the subventricular zone via the rostral migratory stream (RMS). To investigate the molecular mechanisms by which RMS cells migrate, we used a slice preparation, which allows the migrating cells to be imaged at very high temporal and spatial resolution in the presence of added inhibitors. Using immunohistochemistry, we first determined that the alpha1-, beta8-, and beta1-integrin subunits and the alpha5- and gamma1-laminin subunits are expressed during embryonic day 16 to the early postnatal stage. During early postnatal days, alpha(v)- and beta6-integrins appeared, and their expression persisted throughout adulthood. The migrating cells also expressed the netrin receptors neogenin and Deleted in Colorectal Carcinoma (DCC). Netrin-1 is expressed in olfactory mitral cells. Anti-integrin antibodies inhibited the production of protrusions as well as cellular translocation. In contrast, anti-DCC antibodies primarily altered the direction of the protrusions; consequently, the migration was no longer unidirectional, and the speed was reduced. Thus, the interaction of DCC, possibly through an interaction with netrin-1, contributes to the direction of migration by regulating the formation of directed protrusions. In contrast, the integrins function in production of protrusions and cellular translocation, with different integrins participating at different developmental stages.

Enriched odor exposure increases the number of newborn neurons in the adult olfactory bulb and improves odor memory

In the mammalian forebrain, most neurons originate from proliferating cells in the ventricular zone lining the lateral ventricles, including a discrete area of the subventricular zone (SVZ). In this region, neurogenesis continues into adulthood. Most of the cells generated in the SVZ are neuronal precursors with progeny that migrate rostrally along a pathway known as the rostral migratory stream before they reach the main olfactory bulb (MOB) where they differentiate into local interneurons. The olfactory system thus provides an attractive model to investigate neuronal production and survival, processes involving interplay between genetic and epigenetic influences. The present study was conducted to investigate whether exposure to an odor-enriched environment affects neurogenesis and learning in adult mice. Animals housed in either a standard or an odor-enriched environment for 40 d were injected intraperitoneally with bromodeoxyuridine (BrdU) to detect proliferation among progenitor cells and to follow their survival in the MOB. The number of BrdU-labeled neurons was not altered 4 hr after a single BrdU injection. In contrast, the number of surviving progenitors 3 weeks after BrdU injection was markedly increased in animals housed in an enriched environment. This effect was specific because enriched odor exposure did not influence hippocampal neurogenesis. Finally, we showed that adult mice housed in odor-enriched cages display improved olfactory memory without a change in spatial learning performance. By maintaining a constitutive turnover of granule cells subjected to modulation by environmental cues, ongoing bulbar neurogenesis could be associated with improved olfactory memory.

The proliferative ventricular zone in adult vertebrates: a comparative study using reptiles, birds, and mammals

Although evidence accumulated during the last decades has advanced our understanding of adult neurogenesis in the vertebrate brain, many aspects of this intriguing phenomenon remain controversial. Here we review the organization and cellular composition of the ventricular wall of reptiles, birds, and mammals in an effort to identify differences and commonalities among these vertebrate classes. Three major cell types have been identified in the ventricular zone of reptiles and birds: migrating (Type A) cells, radial glial (Type B) cells, and ependymal (Type E) cells. Cells similar anatomically and functionally to Types A, B, and E have also been described in the ventricular wall of mammals, which contains an additional cell type (Type C) not found in reptiles or birds. The bulk of the evidence points to a role of Type B cells as primary neural precursors (stem cells) in the three classes of living amniotic vertebrates. This finding may have implications for the development of strategies for the possible treatment of human neurological disorders.

Induction of neuronal type-specific neurogenesis in the cerebral cortex of adult mice: manipulation of neural precursors in situ

Over the past 3 decades, research exploring potential neuronal replacement therapies have focused on replacing lost neurons by transplanting cells or grafting tissue into diseased regions of the brain [Nat. Neurosci. 3 (2000) 67-78]. Over most of the past century of modern neuroscience, it was thought that the adult brain was completely incapable of generating new neurons. However, in the last decade, the development of new techniques has resulted in an explosion of new research showing that neurogenesis, the birth of new neurons, normally occurs in two limited and specific regions of the adult mammalian brain, and that there are significant numbers of multipotent neural precursors in many parts of the adult mammalian brain [Mol. Cell. Neurosci. 19 (1999) 474-486]. Recent findings from our laboratory demonstrate that it is possible to induce neurogenesis de novo in the adult mammalian brain, particularly in the neocortex where it does not normally occur, and that it may become possible to manipulate endogenous multipotent precursors in situ to replace lost or damaged neurons [Nature 405 (2000) 951-955; Neuron 25 (2000) 481-492]. Recruitment of new neurons can be induced in a region-specific, layer-specific, and neuronal type-specific manner, and newly recruited neurons can form long-distance connections to appropriate targets. Elucidation of the relevant molecular controls may both allow control over transplanted precursor cells and potentially allow the development of neuronal replacement therapies for neurodegenerative disease and other central nervous system injuries that do not require transplantation of exogenous cells.