Long-distance neuronal migration in the adult mammalian brain

Olfactory bulb recovery after early sensory deprivation

Olfactory bulbs retain the ability to acquire new neurons throughout life. Unilateral olfactory deprivation during the first postnatal month in rats results in a dramatic reduction in the size of the experimental olfactory bulb. Part of this reduction is attributable to the death of neurons and glia. To examine the regenerative capacity of the juvenile olfactory bulb, we developed a technique for reversible olfactory deprivation. Reversible blockade from postnatal day 1 (P1) to P20 or P30 results in reduced bulb volume and tyrosine hydroxylase immunostaining, and decreased depth in the olfactory mucosa. In another experiment, normal stimulation was restored for varying periods of time, and experimental and control bulb volumes were measured. Recovery of bulb size occurs after 40 d of normal stimulation. Rats injected with a thymidine analog to label dividing cells during the recovery period revealed that rescue results at least in part from the addition of new neurons and glia. Thus, cells born after the return of normal levels of environmental stimulation can replace some of the neurons and glia that are lost during olfactory deprivation. This system can be used to study mechanisms that underlie neuronal regeneration in the maturing mammalian brain.

Multipotent neural precursors can differentiate toward replacement of neurons undergoing targeted apoptotic degeneration in adult mouse neocortex

Neurons undergoing targeted photolytic cell death degenerate by apoptosis. Clonal, multipotent neural precursor cells were transplanted into regions of adult mouse neocortex undergoing selective degeneration of layer II/III pyramidal neurons via targeted photolysis. These precursors integrated into the regions of selective neuronal death; 15 +/- 7% differentiated into neurons with many characteristics of the degenerated pyramidal neurons. They extended axons and dendrites and established afferent synaptic contacts. In intact and kainic acid-lesioned control adult neocortex, transplanted precursors differentiated exclusively into glia. These results suggest that the microenvironmental alterations produced by this synchronous apoptotic neuronal degeneration in adult neocortex induced multipotent neural precursors to undergo neuronal differentiation which ordinarily occurs only during embryonic corticogenesis. Studying the effects of this defined microenvironmental perturbation on the differentiation of clonal neural precursors may facilitate identification of factors involved in commitment and differentiation during normal development. Because photolytic degeneration simulates some mechanisms underlying apoptotic neurodegenerative diseases, these results also suggest the possibility of neural precursor transplantation as a potential cell replacement or molecular support therapy for some diseases of neocortex, even in the adult.

Epidermal growth factor and fibroblast growth factor-2 have different effects on neural progenitors in the adult rat brain

Neurons and glia are generated throughout adulthood from proliferating cells in two regions of the rat brain, the subventricular zone (SVZ) and the hippocampus. This study shows that exogenous basic fibroblast growth factor (FGF-2) and epidermal growth factor (EGF) have differential and site-specific effects on progenitor cells in vivo. Both growth factors expanded the SVZ progenitor population after 2 weeks of intracerebroventricular administration, but only FGF-2 induced an increase in the number of newborn cells, most prominently neurons, in the olfactory bulb, the normal destination for neuronal progenitors migrating from the SVZ. EGF, on the other hand, reduced the total number of newborn neurons reaching the olfactory bulb and substantially enhanced the generation of astrocytes in the olfactory bulb. Moreover, EGF increased the number of newborn cells in the striatum either by migration of SVZ cells or by stimulation of local progenitor cells. No evidence of neuronal differentiation of newborn striatal cells was found by three-dimensional confocal analysis, although many of these newborn cells were associated closely with striatal neurons. The proliferation of hippocampal progenitors was not affected by either growth factor. However, EGF increased the number of newborn glia and reduced the number of newborn neurons, similar to the effects seen in the olfactory bulb. These findings may be useful for elucidating the in vivo role of growth factors in neurogenesis in the adult CNS and may aid development of neuronal replacement strategies after brain damage.

Selective expression of insulin-like growth factor II in the songbird brain

Neuronal replacement occurs in the forebrain of juvenile and adult songbirds. To address the molecular processes that govern this replacement, we cloned the zebra finch insulin-like growth factor II (IGF-II) cDNA, a factor known to regulate neuronal development and survival in other systems, and examined its expression pattern by in situ hybridization and immunocytochemistry in juvenile and adult songbird brains. The highest levels of IGF-II mRNA expression occurred in three nuclei of the song system: in the high vocal center (HVC), in the medial magnocellular nucleus of the neostriatum (mMAN), which projects to HVC, and to a lesser extent in the robust nucleus of the archistriatum (RA), which receives projections from HVC. IGF-II mRNA expression was developmentally regulated in zebra finches. In canary HVC, monthly changes in IGF-II mRNA expression covaried with previously reported monthly differences in neuron incorporation. Combining retrograde tracers with in situ hybridization and immunocytochemistry, we determined that the HVC neurons that project to area X synthesize the IGF-II mRNA, whereas the adjacent RA-projecting neurons accumulate the IGF-II peptide. Our findings raise the possibility that within HVC IGF-II acts as a paracrine signal between nonreplaceable area X-projecting neurons and replaceable RA-projecting neurons, a mode of action that is compatible with the involvement of IGF-II with the replacement of neurons. Additional roles for IGF-II expression in songbird brain are likely, because expression also occurs in some brain areas outside the song system, among them the cerebellar Purkinje cells in which neurogenesis is not known to occur.

Effects of unilateral olfactory deprivation in the developing opossum, Monodelphis domestica

Unilateral naris closure in young rodents leads to striking alterations in the development of the ipsilateral olfactory system. One of the most pronounced effects is a 25% reduction in the size of the experimental olfactory bulb, a change that stems in part from decreased cell survival. Since naris occlusion in rodents alters the system more during development than in adulthood, we investigated the consequences of olfactory deprivation in a species that is born in a very immature state, Monodelphis domestica. In this pouchless marsupial, offspring are born after a short 14-day gestation. In the present study, the thymidine analogue bromodeoxyuridine was used to examine early postnatal neurogenesis in the olfactory bulb. Unlike rats and mice, neurogenesis of the main output neurons (the mitral cells) continues into postnatal life. Unilateral naris closure was begun on postnatal day 4 (P4) or P5 in Monodelphis and continued for 30 or 60 days. Laminar volume measurements revealed a significant reduction in the size of the experimental bulb following 60, but not 30, days of early olfactory deprivation. Mitral cell number estimates indicated a significant reduction after both 30 and 60 days of naris closure. The immaturity of Monodelphis offspring may render the population of mitral cells susceptible to the effects of olfactory deprivation. These findings suggest that afferent activity plays a role in the survival of all bulb neurons, irrespective of cell class.

Nestin expression in reactive astrocytes following focal cerebral ischemia in rats

During central nervous system (CNS) development, intermediate filaments are subjected to a sequential remodelling process. Nestin is a distinct intermediate filament which is transiently expressed in proliferating neuroepithelial stem cells during the neurulation stage of development. Nestin re-expression in the adult rat was studied following transient (2 h) middle cerebral artery occlusion. Seven days after the ischemic insult, nestin reactive astrocytes were found in the border zone surrounding cerebral infarction. Nestin immunoreactivity delineated a zone between infarction and the surrounding intact cerebral parenchyma. In situ hybridization for nestin mRNA showed early changes in small cells in the surround of the ischemic lesion. These results with nestin, along with other stem cell markers expressed by reactive astrocytes, suggest an embryonic reversion of the mature cytoskeleton as a response of astrocytes to cerebral injury.

The cytosolic phosphoprotein stathmin is expressed in the olfactory system of the adult rat

Stathmin is a cytosolic protein expressed particularly in the developing nervous system, whose phosphorylation is correlated with the action of multiple extracellular stimuli regulating cell proliferation and differentiation. In this study, we used an antibody that specifically recognizes the carboxyterminal region of stathmin to analyze the distribution of this protein in the olfactory system of adult rats, and found a high and selective immunoreactivity in immature olfactory receptors of the olfactory neuroepithelium and in cells of the rostral migratory stream. These results reveal an expression of stathmin in regions of the adult nervous system characterized by striking structural plasticity and cell renewal, suggesting that this protein could play a role in the differentiation of newly generated cell populations.

Myelination following transplantation of EGF-responsive neural stem cells into a myelin-deficient environment

Epidermal growth factor (EGF)-responsive stem cells have been identified in the murine central nervous system. These cells can be isolated from the brain and maintained in an undifferentiated state in vitro in the presence of EGF. After removing EGF, the cells cease mitosis and can be induced to differentiate into neurons, astrocytes, and oligodendrocytes. We demonstrate that when the undifferentiated stem cells (nestin-positive) are injected into the myelin-deficient rat spinal cord, they respond to cues within the mutant CNS and differentiate into myelinating oligodendrocytes, in contrast to their behavior in vitro, where they mainly form astrocytes. The cells provide a valuable model system for the study of the development of early oligodendrocytes from multipotent neural stem cells. Because these cells are influenced to divide using growth factors, rather than oncogenes, and because they appear to make appropriate lineage decisions when transplanted into a mutant environment, they may provide an excellent source of cells for a variety of future therapies using cellular transplantation.

Visualization of beta-galactosidase by enzyme and immunohistochemistry in the olfactory bulb of transgenic mice carrying the LacZ transgene

In the olfactory bulb (OB) of a transgenic mouse line that carries the bacterial LacZ gene under the control of the 5'-regulatory region of the GAD67 gene, expression of the beta-galactosidase was confined almost exclusively to the non-GABAergic mitral and tufted cells. By light microscopy, enzyme histochemistry showed strong staining in the cell bodies and faint diffuse staining in the axons and dendrites. With immunohistochemistry for beta-galactosidase the entire cytoplasm, including the axons and dendrites, was strongly stained. By electron microscopy, beta-galactosidase enzyme histochemistry resulted in a submicroscopic reaction product that was diffusely distributed in the cytoplasm of neurons. In addition, large deposits of the reaction product were also seen attached to the cytoplasmic side of the membranes. In contrast, when the intracellular localization of beta-galactosidase was determined by immunohistochemistry, homogeneous cytoplasmic staining was obtained that filled the entire cytoplasm including the terminal dendrites and fine axons. Therefore, synaptic contacts of the beta-galactosidase-positive output neurons with other beta-galactosidase-negative neuronal cells were readily recognized in the OB. As we demonstrated, transgenic mouse lines expressing the LacZ reporter gene in a well-defined neuronal subpopulation can be used to follow beta-galactosidase-positive neurons and to directly identify their synaptic connections.

Postnatal neurogenesis in the telencephalon of turtles: evidence for nonradial migration of new neurons from distant proliferative ventricular zones to the olfactory bulbs

Postnatal neurogenesis in the the turtle telencephalon was investigated by using bromodeoxyuridine immunocytochemistry and [3H]thymidine autoradiography. Red-eared slider turtles Trachemys scripta elegans (Cryptodira, Emydidae) 2-3 months old were injected with the thymidine analogue 5'-bromodeoxyuridine (BrdU) and allowed to survive for 7, 30, 90, and 180 days. Results indicate that cells in the walls of the lateral ventricles continue to proliferate postnatally. Shortly after BrdU treatment (seven days) most labelled cells were found in the walls of the lateral ventricles (ventricular zone: VZ). Labelled cells were particularly abundant in and around the ventricular sulci. The same pattern of labelling was found in the telencephalon of juvenile turtles (> two years old) injected with BrdU and killed seven day later, suggesting that the proliferative activity continues in the telencephalic VZ of turtles during juvenile stages of life and possibly into adulthood. With longer survival periods after BrdU administration (30, 90, and 180 days), the VZ of the telencephalon showed a similar pattern of labelling to that found at seven days. Furthermore, with survival periods of 90 and 180 days labelled cells resembling neurons were found in most telencephalic regions. The largest numbers of these putative neurons were found in the olfactory bulbs. By using [3H]thymidine autoradiography combined with electron microscopy these postnatally generated cells were confirmed as neurons. We conclude that postnatal neurogenesis occurs in the turtle telencephalon. This process is most prominent in the olfactory bulbs. From the pattern of proliferation of neuronal precursors in the VZ we infer that neurons recruited postnatally into the olfactory bulbs come from distant proliferative VZs in the walls of the lateral ventricles.

Cellular composition and three-dimensional organization of the subventricular germinal zone in the adult mammalian brain

The adult mammalian subventricular zone (SVZ) contains stem cells that give rise to neurons and glia. In vivo, SVZ progeny migrate 3-8 mm to the olfactory bulb, where they form neurons. We show here that the SVZ of the lateral wall of the lateral ventricles in adult mice is composed of neuroblasts, glial cells, and a novel putative precursor cell. The topographical organization of these cells suggests how neurogenesis and migration are integrated in this region. Type A cells had the ultrastructure of migrating neuronal precursors. These cells were arranged as chains parallel to the walls of the ventricle and were polysialylated neural adhesion cell molecule- (PSA-NCAM), TuJ1- (beta-tubulin), and nestin-positive but GFAP- and vimentin-negative. Chains of Type A cells were ensheathed by two ultrastructurally distinct astrocytes (Type B1 and B2) that were GFAP-, vimentin-, and nestin-positive but PSA-NCAM- and TuJ1-negative. Type A and B2 (but not B1) cells incorporated [3H]thymidine. The most actively dividing cell in the SVZ corresponded to Type C cells, which had immature ultrastructural characteristics and were nestin-positive but negative to the other markers. Type C cells formed focal clusters closely associated with chains of Type A cells. Whereas Type C cells were present throughout the SVZ, they were not found in the rostral migratory stream that links the SVZ with the olfactory bulb. These results suggest that chains of migrating neuroblasts in the SVZ may be derived from Type C cells. Our results provide a topographical model for the adult SVZ and should serve as a basis for the in vivo identification of stem cells in the adult mammalian brain.

Continuous neurogenesis in the olfactory brain of adult shore crabs, Carcinus maenas

To scrutinize the common belief that the number of neurons in the CNS of adult decapod crustaceans stays constant, in spite of their dramatic postlarval increase in size, I counted olfactory projection neurons (OPNs) in the brains of differently-sized postlarval shore crabs, Carcinus maenas, and performed in vivo labeling of proliferating cells with 5-bromo-2'-deoxyuridine (BrdU) on brains of adults. The number of OPNs increases continuously throughout the postlarval life of shore crabs and approximately doubles from the very young to the oldest animals. Brain sections from adult crabs labeled with BrdU revealed ongoing proliferation of cells in the lateral soma cluster, which consists of OPN cell bodies, and in the cluster of somata of hemiellipsoid body local interneurons, which are the targets of the OPNs. Post-injection survival times from 5.5 to 120 h revealed a small but relatively constant number of labeled nuclei with neuronal morphology in both soma clusters of all specimens (31.3 +/- 9.5 S.D. nuclei per lateral cluster, n = 29; 20.1 +/- 4.5 S.D. nuclei per hemiellipsoid body cluster, n = 10). The labeled nuclei were located in a distinct proliferative zone in each cluster. There were significantly more labeled nuclei in both soma clusters after a prolonged post-injection survival time of 1 month (71.3 +/- 7.8 S.D. nuclei per lateral cluster, n = 4; 38.2 +/- 7.1 nuclei per hemiellipsoid body cluster, n = 6). In both soma clusters the labeled nuclei formed a compact group that was dislocated from the proliferation zone towards the outer edge of the cluster. In the proliferation zone of the lateral cluster histological stainings revealed cell bodies of typical neuronal shape that are slightly smaller and more intensely stained than the surrounding OPN somata. Some of these cell bodies were captured in various stages of mitosis. Collectively, these data indicate that continuous neurogenesis occurs in the central olfactory pathway of the brain of shore crabs throughout their entire adult life. This unexpected structural plasticity may enable long-lived decapod crustaceans to adapt to ever-changing olfactory environments.

Extensive migration and target innervation by striatal precursors after grafting into the neonatal striatum

Embryonic striatal precursors grafted into the lesioned adult host striatum show limited integration with little migration and restricted efferent projections. In the present study, the influence of an immature striatal environment on the integrative capacity of grafted neuroblasts was examined after transplantation of striatal progenitors into the striatum at different stages of postnatal development. Mouse progenitors, derived from embryonic day 13.5-14 lateral or medial ganglionic eminence or the cerebellar primordium, were transplanted as a single cell suspension into the developing postnatal day 1, 7 and 21 rat striatum. The grafted cells and their axonal projections were visualized using antibodies raised against the mouse-specific neural markers, M6 and M2. Cells from the lateral (but not the medial) ganglionic eminence showed a remarkable capacity to innervate selectively the striatal target structures, globus pallidus, entopeduncular nucleus and substantia nigra, reminiscent of endogenous striatal neurons, which is not observed after grafting into adult hosts. M6 and M2-immunopositive cellular profiles from both the lateral and medial ganglionic eminences were observed to have migrated extensively away from the injection site, in contrast to the cerebellar precursors which remained clustered at the implantation site. Cells from the lateral ganglionic eminence were largely confined within the striatal complex where they developed striatal characteristics, displaying expression of DARPP-32, the 32,000 mol. wt dopamine- and cyclic AMP-regulated phosphoprotein, whereas cells from the medial ganglionic eminence had migrated caudally along the internal capsule and were observed predominantly in the globus pallidus and thalamus, in addition to the striatum. The cells located outside the striatum were all DARPP-32 negative. The improved integration and increased projection capacity of the lateral ganglionic eminence precursors grafted into postnatal day 1 hosts gradually declined as the host advanced into later stages of development (postnatal day 7), and in postnatal day 21 hosts the grafted striatal precursors behaved similarly to grafts implanted into adult recipients. These results demonstrate the specific capacity of embryonic striatal progenitors to integrate into the developing basal ganglia circuitry during early postnatal development, and that the extent of neuronal and glial integration and graft host connectivity declines when the host has developed beyond the first postnatal week.

Gene regulation in the formation of the central nervous system

In this review recent advances in our understanding of the genetic control of central nervous system development will be discussed. Stem and/or progenitor cells in the neuroepithelium of the neural tube differentiate into neurons and glial cells in the brain and spinal cord. There is an emerging picture of how key regulatory genes act to control various steps in this differentiation pathway, including organization along the anterio-posterior and dorso-ventral body axes. Examples from our own research on nestin and Notch genes will also be presented. It is our hope that information about the biology of these genes may shed light on the nature of the central nervous system stem cell and its developmental decisions.

Neuronal precursors of the adult rat subependymal zone persist into senescence, with no decline in spatial extent or response to BDNF

The adult mammalian brain continues to harbor ependymal/subependymal zone (SZ) precursor cells, which can give rise to neurons in vitro. In adult rats, explants of the rostral 6-7 mm of the SZ give rise to neurons in vitro, and over this entire expanse, neuronal survival is supported specifically by brain-derived neurotrophic factor (BDNF). We asked whether either the (a) spatial distribution, (b) abundance, or (c) BDNF responsiveness of the neuronal precursor population was affected by age. Explants of three rostrocaudally defined regions were taken from both young and old rats (3 and 20 months old, respectively), and cultured in 2% fetal bovine serum-containing media with or without added BDNF (20 ng/ml). The extent of neuronal production by these explants varied only minimally with their level of derivation, such that substantial outgrowth was observed at each level tested. Neuronal outgrowth was marginally higher and more rapid in achieving its maximal extent in the 3-month-old rats compared with their aged counterparts, but neuronal outgrowth was robust at each age tested. The duration of survival of SZ-derived neurons did not differ between the young and old rats. At both ages, BDNF supported the survival of these new adult neurons. The extent of BDNF's influence was independent of both the age of the donor rat and the rostrocaudal level at which the parent SZ explant was taken. Thus, the neuronal precursors of the rat brain persist into senescence; the size of the precursor pool attenuates minimally with age, and its spatial extent remains constant. The neurons generated from these precursors can respond to BDNF throughout life.

Survival, integration, and differentiation of neural stem cell lines after transplantation to the adult rat striatum

The in vivo properties of four different neural stem cell lines, generated from embryonic striatum or hippocampus by immortalization with the temperature-sensitive (s) A58/U19 allele of the SV40 Large T-antigen, have been studied with respect to their ability to survive, differentiate, and integrate after transplantation to the adult rat striatum. The cells were labeled with [3H]thymidine prior to grafting, and combined autoradiography and immunohistochemistry was used to characterize their phenotypic differentiation within the adult brain environment. The results show that all four types of cells survived well, up to at least 1.5-6 months postgrafting, without any signs of tissue perturbation or tumor formation. The cells underwent, on average, 2-3 cell divisions during the first 5 days after implantation and exhibited extensive migration over a distance of 1-1.5 mm from the injection site to become morphologically integrated with the surrounding host striatum. The cell number and tissue distribution attained by 2 weeks remained stable for up to 6 months postgrafting with the exception of one cell line, which showed a 40% loss of cells between 2 and 6 weeks. Twice the number of [3H]thymidine-labeled cells were recovered when the cells were grafted into a 1-week-old excitotoxic striatal lesion, probably due to an increased proliferation of the cells in response to the neuron-depleting depleting lesion. The immortalized cells behaved as multipotent neural progenitors. The vast majority of the cells developed a glial-like morphology, 6-14% being clearly GFAP-positive; however, a small but consistent proportion of them (1-3%) expressed MAP-2 and exhibited neuron-like morphology. In mature transplants about 75-80% of the grafted cells were located in the striatal grey matter, and 10-15% in white matter, some of which are proposed to have differentiated into oligodendrocytes. Remaining 5-10% occurred around small blood vessels (resembling pericytes) and in the subventricular zone underneath the ependyma of the lateral ventricle. It is concluded that the ts cell lines are highly suitable for intracerebral transplantation and that they allow the creation of a regionally confined cellular chimeras where the graft-derived glial cells become stably integrated with the resident glial cell matrix.

Direct evidence for homotypic, glia-independent neuronal migration

Neuronal precursors born in the subventricular zone (SVZ) of the neonatal and adult rodent brain migrate 3-8 mm from the walls of the lateral ventricle into the olfactory bulb. This tangentially oriented migration occurs without the guidance of radial glia or axonal processes. The cells move closely associated, forming elongated aggregates called chains, which are ensheathed by astrocytes. We have developed a culture system in which postnatal mouse SVZ neuronal precursors assemble into chains with ultrastructural and immunocytochemical characteristics equivalent to those in vivo but without the astrocytic sheath. Time-lapse videomicrography revealed that individual cells migrate along the chains very rapidly (approximately 122 microm/hr) in both directions. Periods of cell body translocation were interspersed with stationary periods. This saltatory behavior was similar to radial glia-guided migration but approximately 4 times faster. Neuronal precursors isolated from embryonic cortical ventricular zone or cerebellar external granule layer did not form chains under these conditions, suggesting that chain migration is characteristic of SVZ precursors. This study directly demonstrates that SVZ neuronal precursors migrate along each other without the assistance of astrocytes or other cell types. (Additional data are presented in www.cell.com).

Neurogenesis in postnatal rat spinal cord: a study in primary culture

Spinal cord injuries result in paralysis, because when damaged neurons die they are not replaced. Neurogenesis of electrophysiologically functional neurons occurred in spinal cord cultured from postnatal rats. In these cultures, the numbers of immunocytochemically identified neurons increased over time. Additionally, neurons identified immunocytochemically or electrophysiologically incorporated bromodeoxyuridine, confirming they had differentiated from mitotic cells in vitro. These findings suggest that postnatal spinal cord retains the capacity to generate functional neurons. The presence of neuronal precursor cells in postnatal spinal cord may offer new therapeutic approaches for restoration of function to individuals with spinal cord injuries.

Stem cells in the central nervous system

In the vertebrate central nervous system, multipotential cells have been identified in vitro and in vivo. Defined mitogens cause the proliferation of multipotential cells in vitro, the magnitude of which is sufficient to account for the number of cells in the brain. Factors that control the differentiation of fetal stem cells to neurons and glia have been defined in vitro, and multipotential cells with similar signaling logic can be cultured from the adult central nervous system. Transplanting cells to new sites emphasizes that neuroepithelial cells have the potential to integrate into many brain regions. These results focus attention on how information in external stimuli is translated into the number and types of differentiated cells in the brain. The development of therapies for the reconstruction of the diseased or injured brain will be guided by our understanding of the origin and stability of cell type in the central nervous system.

Application of ex vivo gene therapy in the treatment of Parkinson's disease

Ex vivo gene therapy approaches hold great promise for the treatment of neurodegenerative diseases where there is currently no cure or adequate treatment for affected individuals. In this review we have focused on the use of ex vivo gene transfer techniques in Parkinson's disease models; however, the issues and approaches outlined are applicable to other neurodegenerative disorders. In utilizing the ex vivo strategy two considerations are critical for delivery of therapeutic levels of transgene product to the target: (i) the vector system and (ii) the cell type for grafting. We describe herein different vector systems that are currently available and briefly review the various cell types that have been transduced and grafted into the striatum of animals with experimental Parkinson's disease. The strategies for application of gene therapy techniques to a treatment for Parkinson's disease have expanded beyond the classical dopamine replacement toward the use of neurotrophic factors in enhancing cell function or preventing cell death. In addition, we explore the utility of CNS-derived neural progenitors as alternative cell types for ex vivo gene therapy in an animal model of Parkinson's disease.

Postmitotic neurons migrate tangentially in the cortical ventricular zone

Patterns of cell movement play a key role in the establishment of the brain's functional architecture during development. The migration of neuronal progenitor cells has been hypothesized to disperse clonally related cells among different areas of the developing cerebral cortex. To test this model, we explored the migratory patterns of cells in the proliferative zone of the intact cortex of the ferret. After focal injections of DiI, labeled cells migrated in all directions and over long distances within the ventricular and subventricular zones. These cells expressed the neuron-specific marker TuJ1 and did not incorporate BrdU after cumulative labeling. Our results reveal an extensive tangential dispersion of cortical cells mediated predominantly or exclusively by the non-radial migration of postmitotic neurons.

Developmental expression of platelet-derived growth factor alpha-receptor in neurons and glial cells of the mouse CNS

The synthesis of platelet-derived growth factor-alpha receptor (PDGF-alphaR) is commonly attributed to oligodendrocyte progenitors during late embryonic and postnatal development. However, we recently demonstrated that mature neurons could also synthesize PDGF-alphaR, emphasizing a larger role for this receptor than previously described. In the present study, to analyze the pattern of PDGF-alphaR expression during postnatal development of the mouse CNS, we used in situ hybridization and immunohistochemistry on brain and spinal cord tissue sections. We found that, in addition to immature cells of the oligodendrocyte lineage, neurons of various CNS regions express PDGF-alphaR transcripts and protein as early as postnatal day 1 (P1). Whereas neuronal expression was maintained at all ages, the oligodendroglial expression strongly decreased after P21. In the adult, PDGF-alphaR was detected in very few oligodendrocyte progenitors scattered in the cerebral cortex or in white matter tracts, thus suggesting the presence of PDGF-alphaR on O-2Aadult progenitors. In the mature CNS, PDGF-alphaR transcripts and protein were mainly localized in neurons of numerous structures, such as the olfactory bulb, cerebral cortex, hippocampus, and brainstem nuclei and in motor neurons of the ventral horn of the spinal cord. The differential expression of PDGF-alphaR in oligodendroglia and neurons argues in favor of several roles of PDGF during development.

Glial lineages and myelination in the central nervous system

Oligodendrocytes, derived from stem cell precursors which arise in subventricular zones of the developing central nervous system, have as their specialist role the synthesis and maintenance of myelin. Astrocytes contribute to the cellular architecture of the central nervous system and act as a source of growth factors and cytokines; microglia are bone-marrow derived macrophages which function as primary immunocompetent cells in the central nervous system. Myelination depends on the establishment of stable relationships between each differentiated oligodendrocyte and short segments of several neighbouring axons. There is growing evidence, especially from studies of glial cell implantation, that oligodendrocyte precursors persist in the adult nervous system and provide a limited capacity for the restoration of structure and function in myelinated pathways damaged by injury or disease.

Multipotent CNS stem cells are present in the adult mammalian spinal cord and ventricular neuroaxis

Neural stem cells in the lateral ventricles of the adult mouse CNS participate in repopulation of forebrain structures in vivo and are amenable to in vitro expansion by epidermal growth factor (EGF). There have been no reports of stem cells in more caudal brain regions or in the spinal cord of adult mammals. In this study we found that although ineffective alone, EGF and basic fibroblast growth factor (bFGF) cooperated to induce the proliferation, self-renewal, and expansion of neural stem cells isolated from the adult mouse thoracic spinal cord. The proliferating stem cells, in both primary culture and secondary expanded clones, formed spheres of undifferentiated cells that were induced to differentiate into neurons, astrocytes, and oligodendrocytes. Neural stem cells, whose proliferation was dependent on EGF+bFGF, were also isolated from the lumbar/sacral segment of the spinal cord as well as the third and fourth ventricles (but not adjacent brain parenchyma). Although all of the stem cells examined were similarly multipotent and expandable, quantitative analyses demonstrated that the lateral ventricles (EGF-dependent) and lumbar/sacral spinal cord (EGF+bFGF-dependent) yielded the greatest number of these cells. Thus, the spinal cord and the entire ventricular neuroaxis of the adult mammalian CNS contain multipotent stem cells, present at variable frequency and with unique in vitro activation requirements.

Migration of small and large cells from the grafts of embryonic olfactory bulbs, transplanted into the anterior wall of the lateral ventricle

Embryonic olfactory bulbs (OBs) were transplanted into the anterior wall of the lateral ventricle (ALV) of juvenile and adult host mice. Neurons originated from the transplanted OBs were detected immunohistochemically by using mouse Thy-1 allelic system. Within the OBs of both host groups, a substantial number of Thy-1.2-positive small presumed granule cells were observed. Furthermore, Thy-1.2 labeled large cells, although far smaller in number, were encountered in some cortical areas, including the anterior olfactory nucleus of the juvenile host group, but not within the host OBs. These results indicated that both small and large cell precursors had the capacity to migrate from the host ALV, but only the small cell precursors could migrate tangentially into host OBs.

Neuronal progenitor cells derived from the anterior subventricular zone of the neonatal rat forebrain continue to proliferate in vitro and express a neuronal phenotype

A discrete area of the anterior part of the subventricular zone, or SVZa, of the postnatal forebrain is composed of progenitor cells that are dissimilar to those elsewhere in the CNS. In vivo SVZa progenitor cells retain the ability for division, even though they are phenotypically neurons. To characterize further the properties of SVZa cells, we have analyzed their characteristics in vitro using cell-type specific antibodies and their proliferative capacity by the incorporation of bromodeoxyuridine. At 2 h in vitro, as well as after 1 day in vitro, virtually all SVZa cells isolated from the neonatal forebrain express TuJ1, an antibody that recognizes neuron-specific tubulin, and are GFAP-negative. Likewise, the preponderance of SVZa cells express the neuron-specific markers N-CAM and MAP-2 when examined after 1 day in culture. The majority of SVZa cells cultured for as long as 8 days also possessed a neuronal phenotype. In addition, process-bearing TuJ1-positive SVZa cells continued to proliferate throughout the entire culture period. Thus, the neuronal progenitor cells of the SVZa constitute a unique cell population with characteristics distinct from the cells of other germinal zones.

Neuronal stem cells express vesicular monoamine transporter 2 immunoreactivity in the adult rat

In the adult CNS, proliferating cells persist only in the olfactory epithelium, olfactory bulb and subventricular zones. The cells of the subventricular zone are believed to constitute the cells in the adult mammalian brain, including the human brain, which can be stimulated to proliferate in response to epidermal growth factor or basic fibroblast growth factor. These cells are of particular interest, as they may be amenable to genetic engineering with markers such as tyrosine hydroxylase, and they may represent a long-term source of modified neurons suitable for transplantation therapy. Recent work by Lois and Alvarez-Buylla, in the mouse, has shown that labelled subventricular zone cells can migrate from the subventricular zone to the olfactory bulb, where they contribute to the granule cell population. In this study we have used an antibody we raised recently against the carboxy-terminal sequence of the vesicular monoamine transporter 2 (also known as the synaptic vesicle monoamine transporter) to detect vesicular monoamine transporter 2-like immunoreactive subventricular zone cells in the rat, and to visualize them as they migrate from the edge of the ventricle, through the olfactory bulb to locate them as differentiated neurons in the granule cell layer of the olfactory bulb. These data show that the subventricular zone cells express a vesicular monoamine transporter 2-like antigen and demonstrate that this protein may be a useful developmental marker for rat neuronal stem cells.

Adult nestin-expressing subependymal cells differentiate to astrocytes in response to brain injury

The adult brain contains a small population of central nervous system (CNS) cells in the subependyma which, like embryonic CNS progenitor cells, express the intermediate filament nestin. In this report, the differentiation capacity in vivo of these cells was analysed following a standardized trauma. Before the trauma, the subependymal cells expressed nestin but not the astrocytic and neuronal differentiation markers glial fibrillary acidic protein (GFAP) and neurofilament respectively. In response to injury, the majority of the subependymal cells coexpressed nestin and GFAP, but never nestin and neurofilament. Furthermore, cells coexpressing nestin and GFAP were found progressively further away from the subependyma and closer to the lesion at later time points after the injury, indicating that these cells migrate towards the lesion. Nestin was in addition re-expressed in reactive astrocytes near the lesion and in non-reactive astrocytes very far from the lesion throughout the ipsilateral cortex. In conclusion, our data indicate that the nestin-positive subependymal cells are an in vivo source for the generation of new astrocytes but not neurons after injury, and that nestin re-expression in astrocytes following traumatic stimuli can be used as a sensitive marker for astroglial activation.

Glial tubes in the rostral migratory stream of the adult rat

In the central nervous system cell migration is usually restricted to developmental periods and occurs mainly radially, following the radial glia. Nevertheless, in the subependymal layer of the adult rodent forebrain tangential migration of newly generated neuronal precursors directed to the olfactory bulb, which follow a well-defined pathway without dispersion, has been recently demonstrated. In the present study, by using light microscopic immunocytochemistry for glia-associated antigens (glial fibrillary acidic protein, S-100 and vimentin), and conventional electron microscopy, we observed a dense mesh-work of astrocytic cells and processes throughout the subependymal layer of the adult rat. These cells were organized to form tangentially oriented glial tubes in the subependymal layer of the lateral ventricle and in its rostral extension to the olfactory bulb. Glial tubes were particularly evident within the rostral extension and were widely intercommunicating. Using markers for the proliferating/ migrating cells of the rostral migratory stream (5-bromo-2'-deoxyuridine, PSA-NCAM, class III beta-tubulin), we provide evidence that long chains of PSA-NCAM/beta-tubulin-positive, newly generated cells are consistently observed inside the glial tubes. These results demonstrate the existence of a peculiar glial organization within the subependymal layer of the adult rat, consisting of long astrocytic tubes that likely represent a new type of glial guidance, accounting for the tangential migration of a high number of cells along their restricted pathway, to the olfactory bulb.

Increased proliferation of precursor cells in the adult rat brain after targeted lesioning

There have been a number of reports on the proliferation of a subset of precursor cells in the subventricular zone of the lateral ventricles in the adult mammalian brain. Here we report on studies that sought to ascertain whether these cells could respond to a targeted lesion of the adult brain by increasing their proliferative rate. We have lesioned the fimbria fornix, a major pathway of septal cholinergic fibers. Previous reports demonstrated that such a lesion results in the loss of neurons in both the basal forebrain and in the CA1 field of hippocampus, without direct injury to either tissue. Ten days after making such a lesion in adult rats, the animals were given serial injections of [3H]thymidine and sacrificed after a final injection. The brains were processed for both immunocytochemistry and autoradiography. Our data demonstrate a two-fold increase over the basal proliferative rate in animals that had received such a lesion. We used a panel of antibodies to ascertain the identity of the proliferating cells. The only clearly identifiable cells that were [3H]thymidine-positive were astrocytes, based on GFAP staining. The remainder of the cells were of a null phenotype.

Network of tangential pathways for neuronal migration in adult mammalian brain

Cells in the brains of adult mammals continue to proliferate in the subventricular zone (SVZ) throughout the lateral wall of the lateral ventricle. Here we show, using whole mount dissections of this wall from adult mice, that the SVZ is organized as an extensive network of chains of neuronal precursors. These chains are immunopositive to the polysialylated form of NCAM, a molecule present at sites of plasticity, and TuJ1, an early neuronal marker. The majority of the chains are oriented along the rostrocaudal axis and many join the rostral migratory stream that terminates in the olfactory bulb. Using focal microinjections of DII and transplantation of SVZ cells carrying a neuron-specific reporter gene, we demonstrate that cells originating at different rostrocaudal levels of the SVZ migrate rostrally and reach the olfactory bulb where they differentiate into neurons. Our results reveal an extensive network of pathways for the tangential chain migration of neuronal precursors throughout the lateral wall of the lateral ventricle in the adult mammalian brain.

A subset of clones in the chick telencephalon arranged in rostrocaudal arrays

BACKGROUND

Different areas of the vertebrate central nervous system appear to follow different rules during development for determining the position of sibling cells. For example, in the chick hindbrain, clones are frequently confined to a single functional unit that derives from a single rhombomere. In contrast, clones in the mammalian cerebral cortex often cross functional boundaries because of the extensive migration of sibling cells in orthogonal directions. We have investigated whether the pattern of clonal distribution in the chick telencephalon is similar to that of the hindbrain or to the more functionally analogous mammalian cerebral cortex. Progenitor cells in the chick telencephalon were marked using a retroviral library encoding alkaline phosphatase and over 10(5) distinct molecular tags. Patterns of dispersion were detected using alkaline phosphatase histochemistry, followed by the recovery and sequencing of the molecular tag. We also analyzed the phenotypes of cells that occurred within the clones.

RESULTS

A subset of progenitors gave rise to clones that were found in rostrocaudal arrays resembling tubes. Arrays were restricted in the mediolateral and dorsoventral planes but could span up to 4 mm in the rostrocaudal direction. They were found throughout the telencephalon and a single clone often spanned more than one telencephalic nucleus. Rostrocaudal clones comprised 60% of clones containing five or more cells and contained many different types of neurons, astrocytes, oligodendrocytes, or various combinations of these cell types.

CONCLUSIONS

Telencephalic progenitors are multipotent, producing progeny that become distinct cell types. Clonally related cells can migrate rostrocaudally within domains that are restrained in the mediolateral and dorsoventral directions. A subset of rostrocaudal clones resemble those seen in the mammalian cerebral cortex, with respect to the crossing of functional boundaries, but all rostrocaudal clones differ from the cerebral cortical clones in the pattern of spread of sibling cells, with the rostrocaudal clones being more constrained in the mediolateral and dorsoventral directions. A role for lineage in the patterning of the chick forebrain is supported by these observations. In addition, these data suggest a role for cues within the telencephalic marginal zone that serve to guide clones in their rostrocaudal migration.

Cerebrospinal fluid of newborn infants contains a deglycosylated form of the intermediate filament nestin

Nestin is an intermediate filament protein found in CNS progenitor cells. Nestin reappears in CNS tumor cells and reactive astrocytes after CNS injury. In this study we investigated whether nestin could be detected in the cerebrospinal fluid (CSF) of newborn infants and whether expression levels change with gestational age (GA) and/or brain injury. Using Western blot analysis, we examined the expression of nestin in the CSF of newborn infants (GA 25-42 wk) with asphyxia (n = 14), periventricular leukomalacia and peri(intra)ventricular hemorrhage (n = 7), and in a control group (n = 11). Protein extract from the periventricular brain tissue of a 1-wk-old infant was also analyzed. Nestin was detected in all the CSF samples and in the protein extract from the periventricular brain tissue. Although the CSF levels of nestin expression did not change with increasing GA, the asphyxia group had significantly lower levels of nestin in the CSF. An unexpected finding was that brain-derived nestin had an apparent molecular mass of approximately 240 kD, whereas all analyzed CSF samples contained two nestin-immunoreactive proteins at 200 and 220 kD. Experimental deglycosylation of the 240-kD form reduced the molecular mass to 220 kD, indicating that nestin undergoes a specific deglycosylation upon release into the CSF.

Expression of hepatocyte growth factor/scatter factor, its receptor, c-met, and tissue-type plasminogen activator during development of the murine olfactory system

The expression of hepatocyte growth factor/scatter factor (HGF/SF) and its receptor, the c-met proto-oncogene product, was examined by in situ hybridization in the developing and adult murine olfactory system and compared with the expression of a known activator of HGF/SF, tissue-type plasminogen activator (tPA). In the developing olfactory canal, expression of both c-met and tPA was observed in the olfactory neuroepithelium, whereas HGF/SF expression appeared to be confined to the mucosa adjacent to the neuroepithelium. During development of the olfactory bulb, HGF/SF and tPA were expressed within the rostral migratory pathway leading to the olfactory bulb, whereas c-met expression was observed in the mitral cell layer (MCL) of the olfactory bulb and in the anterior olfactory nucleus. In the adult olfactory bulb, expression of HGF/SF was restricted to the periglomerular region of the glomerular layer, whereas c-met was expressed in the MCL and olfactory nerve fiber layers (ONL). tPA expression in the adult olfactory bulb was observed in the ONL, MCL, and granule cell layers. Therefore, tPA expression was relatively coincident with the expression of HGF/SF and/or c-met in the appropriate projection patterns of the developing and adult olfactory system. In addition, antibodies against tPA inhibited the olfactory bulb extract-mediated cleavage of single-chain HGF/SF. These results suggest that tPA may play a regulatory role in the development and maintenance of the olfactory system by activating HGF/SF in the immediate vicinity of its receptor.

Neurogenesis in the olfactory bulb of the frog Xenopus laevis shows unique patterns during embryonic development and metamorphosis

We determined the time of origin of neurons in the olfactory bulb of the South African clawed frog, Xenopus laevis. Tritiated thymidine injections were administered to frog embryos and tadpoles from gastrulation (stage 11/12) through metamorphosis (stage 65), paraffin sections were processed for autoradiography, and the distribution of heavily and lightly labeled cells was examined. In the ventral olfactory bulb, we observed that the mitral cells were born as early as stage 11/12 and continued to be generated through the end of metamorphosis. Interneurons (periglomerular and granule cells) were not born in the ventral bulb until stage 41, and birth of these cells also continued through metamorphosis. Labeled cells were observed in the accessory olfactory bulb, beginning at stage 41. In contrast, the cells of the dorsal olfactory bulb were not born until the onset of metamorphosis (stage 54); at this stage in the dorsal bulb, the genesis of mitral cells, interneurons, and glial cells completely overlapped. The results indicate that olfactory axon innervation is not necessary to induce early stages of neurogenesis in the ventral olfactory bulb. On the other hand, the results on the dorsal olfactory bulb are consistent with the hypothesis that innervation from new or transformed sensory neurons in the principal cavity induces neurogenesis in the dorsal bulb.

Dopaminergic and GABAergic interneurons of the olfactory bulb are derived from the neonatal subventricular zone

Earlier studies in our laboratory have demonstrated that a discrete region of the anterior part of the neonatal subventricular zone (SVZa) contains exclusively neuronal progenitor cells. The descendants of the SVZa progenitor cells are destined for the granule cell and glomerular layers of the olfactory bulb, where they differentiate into granule and periglomerular cells, the interneurons of the olfactory bulb, respectively. In the present set of experiments we examined the neurotransmitter phenotype of the SVZa-derived cells. In order to label SVZa-derived cells, the cell proliferation marker bromodeoxyuridine (BrdU) was injected into the SVZa of postnatal day 2 (P2) rats. After 3 weeks, by which time most of the SVZa-derived cells have migrated to their final destination in the bulb, the animals were perfused and their brains processed for immunohistochemistry. To identify the neurotransmitter phenotype of the SVZa-derived cells, sagittal sections of the forebrain, including the olfactory bulb, were double-labeled with an antibody to BrdU in conjunction with an antibody to gamma-amino-butyric acid (GABA) or tyrosine hydroxylase (TH), the rate limiting enzyme in the synthesis of dopamine. Using simultaneous indirect immunofluorescence to detect the presence of single- and double-labeled cells, we found that 59% and 51% of the BrdU-positive cells were immunoreactive for GABA in the granule cell and glomerular layers, respectively. In addition, 10% of the BrdU-positive periglomerular cells were immunoreactive for TH. The presence of double-labeled (BrdU-positive/GABA-positive and BrdU-positive/TH-positive) cells in the olfactory bulb, demonstrates that the SVZa is a source of the GABAergic and dopaminergic interneurons of the olfactory bulb during postnatal development.

Conditionally immortalized neural progenitor cell lines integrate and differentiate after grafting to the adult rat striatum. A combined autoradiographic and electron microscopic study

Neural progenitor cell lines, generated by conditional immortalization from the embryonic CNS, have previously been shown to survive and integrate after transplantation to the adult brain. The present study was designed to investigate the in vivo differentiation and morphological features of grafted neural progenitors using combined autoradiography and transmission electron microscopy of two temperature-sensitive neural progenitor cell lines, HiB5 and ST14A, labeled with 3H-thymidine prior to grafting. Two weeks after transplantation to the striatum the cells were found dispersed over an area extending about 1.5 mm from the injection site. Labeled cells located within the myelinated fiber bundles of the internal capsule were closely associated with myelinated axons and presented profiles similar to oligodendrocytes, while most of the grafted cells in the grey matter had morphological features of astroglia. Some labeled cells occurred also in close association with small blood vessels, morphologically resembling host pericytes. The results show that the immortalized neural progenitors can differentiate into mature glial cells, including astrocytes, oligodendrocytes and pericytes, after implantation into the adult striatum. The ability of the cells to become fully integrated with the resident glial population suggests that they will be highly useful as vehicles for intracerebral transgene expression in ex vivo gene transfer.