Neuronal-glial interactions in central nervous system neurogenesis: the neural stem cell perspective

Essentially, three neuroectodermal-derived cell types make up the complex architecture of the adult CNS: neurons, astrocytes and oligodendrocytes. These elements are endowed with remarkable morphological, molecular and functional heterogeneity that reaches its maximal expression during development when stem/progenitor cells undergo progressive changes that drive them to a fully differentiated state. During this period the transient expression of molecular markers hampers precise identification of cell categories, even in neuronal and glial domains. These issues of developmental biology are recapitulated partially during the neurogenic processes that persist in discrete regions of the adult brain. The recent hypothesis that adult neural stem cells (NSCs) show a glial identity and derive directly from radial glia raises questions concerning the neuronal-glial relationships during pre- and post-natal brain development. The fact that NSCs isolated in vitro differentiate mainly into astrocytes, whereas in vivo they produce mainly neurons highlights the importance of epigenetic signals in the neurogenic niches, where glial cells and neurons exert mutual influences. Unravelling the mechanisms that underlie NSC plasticity in vivo and in vitro is crucial to understanding adult neurogenesis and exploiting this physiological process for brain repair. In this review we address the issues of neuronal/glial cell identity and neuronal-glial interactions in the context of NSC biology and NSC-driven neurogenesis during development and adulthood in vivo, focusing mainly on the CNS. We also discuss the peculiarities of neuronal-glial relationships for NSCs and their progeny in the context of in vitro systems.     

Age-dependent remodelling of ionotropic signalling in cortical astroglia

Cortical astrocytes express fast ionotropic receptors for glutamate and ATP, although their role in neurone-glia communication remains controversial. Stimulation of neuronal afferents in mice neocortex triggers complex glial synaptic currents (GSCs) mediated by NMDA, P2X and AMPA receptors and glutamate transporters. In addition, astrocytes demonstrate spontaneous 'miniature' GSCs resulting from quantal release of neurotransmitters. Here, we demonstrate that maturation and aging of the brain of mice (from 1 to 21 months) affect the density of ionotropic receptors in astrocytes and their role in GSCs generation. The AMPA-receptor-mediated component is the largest in young animals and progressively declines with age. The P2X and NMDA components of GSC are smallest in young, maximal in adult (3 and 6 months old) and once more decrease in old mice, probably reflecting the remodelling of neuronal-glial circuitry. Our results demonstrate that fast synaptic transmission between neurones and astrocytes in neocortex that may be involved in information processing in neuronal-glial networks undergoes remodelling during brain maturation and aging.     

Diet, central nervous system, and aging.

A variety of morphological, structural, and chemical changes have been described in the central nervous systems of aging humans and animals. Brain size and volume decline during senescence, and the brain atrophy is accompanied by changes in the number, size, and ultrastructural characteristics of nerve and glial cells. Moreover, recent evidence suggests that the ability of central nervous system cells to communicate with one another via the release of neurotransmitter compounds might be impaired in the elderly. Nutritional factors may play important roles in the aging process of the central nervous system by influencing brain neurotransmission, or by accelerating or retarding geriatric changes in central nervous system structure.     

Segregation of postembryonic neuronal and glial lineages inferred from a mosaic analysis of the Drosophila larval brain

Due to its intermediate complexity and its sophisticated genetic tools, the larval brain of Drosophila is a useful experimental system to study the mechanisms that control the generation of cell diversity in the CNS. In order to gain insight into the neuronal and glial lineage specificity of neural progenitor cells during postembryonic brain development, we have carried an extensive mosaic analysis throughout larval brain development. In contrast to embryonic CNS development, we have found that most postembryonic neurons and glial cells of the optic lobe and central brain originate from segregated progenitors. Our analysis also provides relevant information about the origin and proliferation patterns of several postembryonic lineages such as the superficial glia and the medial-anterior Medulla neuropile glia. Additionally, we have studied the spatio-temporal relationship between gcm expression and gliogenesis. We found that gcm expression is restricted to the post-mitotic cells of a few neuronal and glial lineages and it is mostly absent from postembryonic progenitors. Thus, in contrast to its major gliogenic role in the embryo, the function of gcm during postembryonic brain development seems to have evolved to the specification and differentiation of certain neuronal and glial lineages.     

The dynamics of the extinction of brain cellular biosynthesis at different times after death]

The paper presents experimental data on RNA synthesis in neuronal, glial, endothelial cells of a dying organism. Under certain conditions it is possible to define labelled cells in the majority of the brain cells within post-death hours 1-4. This fact proves that genome is still functioning. Endothelial cells possess higher vital capacity. CNS neurons are capable of synthesizing RNA within the period of up to 4 hours after death of the organism.     

Radial glia serve as neuronal progenitors in all regions of the central nervous system

Radial glial cells function during CNS development as neural progenitors, although their precise contribution to neurogenesis remains controversial. Recent work has argued that regional differences may exist regarding the neurogenic potential of radial glia. Here, we show that the vast majority of neurons in all brain regions derive from radial glia. Cre/loxP fate mapping and clonal analysis demonstrate that radial glia throughout the CNS serve as neuronal progenitors and that radial glia within different regions of the CNS pass through their neurogenic stage of development at distinct time points. Thus, radial glial populations within different CNS regions are not heterogeneous with regard to their potential to generate neurons versus glia.     

Neuronal-glial remodeling: a structural basis for neuronal-glial interactions in the adult hypothalamus.

Increasing evidence is establishing that adult neurons and their associated glia can undergo state-dependent changes in their morphology and in consequence, in their relationships and functional interactions. A neuronal system that illustrates this kind of neuronal-glial plasticity in an exemplary fashion is that responsible for the secretion of the neurohormone oxytocin (OT). As shown by comparative ultrastructural analysis, during physiological conditions like lactation and dehydration, which result in enhanced peripheral and central release of the peptide, astrocytic coverage of OT neurons is markedly reduced and their surfaces are left directly juxtaposed. Such reduced glial coverage is of consequence to neuronal activity since it modifies extracellular ionic homeostasis and glutamate neurotransmission. In addition, it is probably prerequisite to the synaptic remodeling that occurs concurrently, and results in an enhanced number of inhibitory (GABAergic) and excitatory (glutamatergic, noradrenergic) synapses, thus further affecting neuronal function. The neuronal-glial and synaptic changes occur rapidly, within a matter of hours, and are reversible with termination of stimulation. The adult OT system retains many juvenile molecular features that may allow such plasticity, including expression of cell adhesion molecules implicated in neuronal-glial interactions during development, like polysialylated NCAM, F3/contactin and its ligand, the matrix glycoprotein, tenascin-C. On the other hand, OT itself can induce the changes since in vivo (ventricular microinfusion) or in vitro (on acute hypothalamic slices) application leads to glial and neuronal transformations similar to those induced by physiological stimuli.     

Modulation of excitatory neurotransmission by neuronal/glial signalling molecules: interplay between purinergic and glutamatergic systems

Glutamate is the main excitatory neurotransmitter of the central nervous system (CNS), released both from neurons and glial cells. Acting via ionotropic (NMDA, AMPA, kainate) and metabotropic glutamate receptors, it is critically involved in essential regulatory functions. Disturbances of glutamatergic neurotransmission can be detected in cognitive and neurodegenerative disorders. This paper summarizes the present knowledge on the modulation of glutamate-mediated responses in the CNS. Emphasis will be put on NMDA receptor channels, which are essential executive and integrative elements of the glutamatergic system. This receptor is crucial for proper functioning of neuronal circuits; its hypofunction or overactivation can result in neuronal disturbances and neurotoxicity. Somewhat surprisingly, NMDA receptors are not widely targeted by pharmacotherapy in clinics; their robust activation or inhibition seems to be desirable only in exceptional cases. However, their fine-tuning might provide a promising manipulation to optimize the activity of the glutamatergic system and to restore proper CNS function. This orchestration utilizes several neuromodulators. Besides the classical ones such as dopamine, novel candidates emerged in the last two decades. The purinergic system is a promising possibility to optimize the activity of the glutamatergic system. It exerts not only direct and indirect influences on NMDA receptors but, by modulating glutamatergic transmission, also plays an important role in glia-neuron communication. These purinergic functions will be illustrated mostly by depicting the modulatory role of the purinergic system on glutamatergic transmission in the prefrontal cortex, a CNS area important for attention, memory and learning.     

Neuronal and mixed neuronal glial tumors associated to epilepsy. A heterogeneous and related group of tumours

The group of brain tumors with mature components encompasses several pathological entities including: the ganglioneuroma; the gangliocytoma; the ganglioglioma; the desmoplastic ganglioglioma; the neurocitoma and a group of glioneuronal hamartomatous tumorous lesions, such as meningoangiomatosis. The dysembryoplastic neuroepithelial tumor is characterized by the presence of multiple cortical nodules made up of small, oligo-like cells and a myxoid pattern rich in mucopolysaccharides. Mature neuronal cells are frequently detected throughout the tumor. Most of them are associated with microhamartias in the adjacent brain and pharmacoresistant epilepsy. The excellent prognosis of the majority of these tumors and the potential for malignant transformation of the glial component in the ganglioglioma are the two most remarkable findings. Histological signs of anaplasia and greater mitotic and proliferative activities are associated with local recurrences. Atypical neurocytomas occur only exceptionally. Treatment choices are surgical resectioning and, in those cases presenting greater proliferative activity and cytological atypia, postoperative radiotherapy may be recommended. This paper reviews this heterogeneous group of neoplasms and hamartomatous lesions, pointing out presumable transitions among the different types of mixed neuronal and glial brain tumors. A single term of "mixed neuronal-glial tumors" is defended, distinguishing different subgroups of tumors, depending on the predominant cellular component.     

A closer look at astrocyte morphology: Development,heterogeneity, and plasticity at astrocyte leaflets

Astrocytes represent an abundant type of glial cell involved in nearly every aspect of central nervous system (CNS) function, including synapse formation and maturation, ion and neurotransmitter homeostasis, blood–brain barrier maintenance, as well as neuronal metabolic support. These various functions are enabled by the morphological complexity that astrocytes adopt. Recent experimental advances in genetic and viral labeling, lineage tracing, and live- and ultrastructural imaging of miniscule astrocytic sub-compartments reveal a complex morphological heterogeneity that is based on the origin, local function, and environmental context in which astrocytes reside. In this minireview, we highlight recent findings that reveal the plastic nature of astrocytes in the healthy brain, particularly at the synapse, and emerging technologies that have advanced our understanding of these morphologically complex cells.     

Neuronal and Glial Properties Coexist in a Novel Mouse CNS Immortalized Cell Line

A mes-c-myc A1 (A1) cell line was generated by retroviral infection of cultured embryonic mesencephalic cells and selected by neomycin resistance. A1 cells cease to divide and undergo morphological differentiation after serum withdrawal or addition of c-AMP. Proliferating or morphologically differentiated A1 cells are all positive for vimentin and nestin, a marker of neural precursor, and show neuronal markers such as microtubule-associated protein 1, neuron-specific enolase and peripherin, and the glial marker glial fibrillary acidic protein. Neuronal and glial markers coexist in single cells. Furthermore, A1 cells show presence of glutamic acid decarboxylase 67 mRNA and its embryonic form EP10 and accumulate the neurotransmitter GABA. Electrophysiological studies demonstrate that morphologically differentiated A1 cells display voltage-gated sodium and potassium channels in response to depolarizing stimuli. A1 cells thus represent a novel, bipotent neural cell line useful for studying CNS differentiation and plasticity, as well as the molecular mechanisms underlying development of GABAergic neurotransmission.     

Determinants of regional and local diversity within the astroglial lineage of the normal central nervous system

Astrocytes are a major component of the resident non-neuronal glial cell population of the CNS. They are ubiquitously distributed throughout the brain and spinal cord, where they were initially thought to function in both structural and homeostatic capacities, providing the framework and environment in which neurons performed their parenchymal duties. However, this stroma-like view of astrocytes is no longer satisfactory. Mounting evidence particularly within the last decade indicates that astrocytes do not simply support neuronal activity but directly contribute to it. Congruent with this evolving view of astrocyte function in information processing is the emergent notion that these glial cells are not a homogeneous population of cells. Thus, astrocytes in various anatomically distinct regions of the normal CNS possess unique phenotypic characteristics that may directly influence the particular neuronal activities that define these regions. Remarkably, regional populations of astrocytes appear to exhibit local heterogeneity as well. Many phenotypic traits of the astrocyte lineage are responsive to local environmental cues (i.e., are adaptable), suggesting that plasticity contributes to this diversity. However, compelling evidence suggests that astrocytes arise from multiple distinct progenitor pools in the developing CNS, raising the intriguing possibility that some astrocyte heterogeneity may result from intrinsic differences between these progenitors. The purpose of this review is to explore the evidence for and mechanistic determinants of regional and local astrocyte diversity.     

Distribution of NADPH-diaphorase activity in the central nervous system of the young and adult land snail Megalobulimus abbreviatus

Nitric oxide (NO) is a gas produced through the action of nitric oxide synthase that acts as a neurotransmitter in the central nervous system (CNS) of adult gastropod mollusks. There are no known reports of the presence of NOS-containing neurons and glial cells in young and adult Megalobulimus abbreviatus. Therefore, NADPH-d histochemistry was employed to map the nitrergic distribution in the CNS of young and adult snails in an attempt to identify any transient enzymatic activity in the developing CNS. Reaction was observed in neurons and fibers in all CNS ganglia of both age groups, but in the pedal and cerebral ganglia, positive neurons were more intense than in other ganglia, forming clusters symmetrically located in both paired ganglia. However, neuronal NADPH-d activity in the mesocerebrum and pleural ganglia decreased from young to adult animals. In both age groups, positive glial cells were located beneath the ganglionic capsule, forming a network and surrounding the neuronal somata. The trophospongium of large and giant neurons was only visualized in young animals. Our results indicate the presence of a nitrergic signaling system in young and adult M. abbreviatus, and the probable involvement of glial cells in NO production.     

Phenotypic and gene expression modification with normal brain aging in GFAP-positive astrocytes and neural stem cells

Astrocytes secrete growth factors that are both neuroprotective and supportive for the local environment. Identified by glial fibrillary acidic protein (GFAP) expression, astrocytes exhibit heterogeneity in morphology and in the expression of phenotypic markers and growth factors throughout different adult brain regions. In adult neurogenic niches, astrocytes secrete vascular endothelial growth factor (VEGF) and fibroblast growth factor-2 (FGF-2) within the neurogenic niche and are also a source of special GFAP-positive multipotent neural stem cells (NSCs). Normal aging is accompanied by a decline in CNS function and reduced neurogenesis. We asked whether a decreased availability of astrocyte-derived factors may contribute to the age-related decline in neurogenesis. Determining alterations of astrocytic activity in the aging brain is crucial for understanding CNS homeostasis in aging and for assessing appropriate therapeutic targets for an aging population. We found region-specific alterations in the gene expression of GFAP, VEGF, and FGF-2 and their receptors in the aged brain corresponding to changes in astrocytic reactivity, supporting astrocytic heterogeneity and demonstrating a differential aging effect. We found that GFAP-positive NSCs uniquely coexpress both VEGF and its key mitotic receptor Flk-1 in both young and aged hippocampus, indicating a possible autocrine/paracrine signaling mechanism. VEGF expression is lost once NSCs commit to a neuronal fate, but Flk-1-mediated sensitivity to VEGF signaling is maintained. We propose that age-related astrocytic changes result in reduced VEGF and FGF-2 signaling, which in turn limits NSC and progenitor cell maintenance and contributes to decreased neurogenesis.     

Antioxidant enzymatic systems in neuronal and glial cell-enriched fractions of rat brain during aging

The activities of Cu,Zn superoxide dismutase, glutathione peroxidase, catalase and glutathione reductase in neuronal and glial cell-enriched fractions obtained from the cerebral cortex of rat brain during aging (15, 30, 90, 350, 750 days of age) were assayed. Our results showed that glutathione peroxidase, catalase and glutathione reductase activities varied little during the examined periods. Only the Cu,Zn superoxide dismutase activity decreased notably from 15th to 750th day of age in both neuronal and glial cells, moreover the activities of all enzymes studied were always detected at lower levels in neuronal cells with respect to glial cells. In agreement with diminished SOD activity, the lipid peroxidation showed an elevated increase with aging; this fact is more evident in neuronal than in glial cells. In conclusion our data show that Cu,Zn superoxide dismutase is the most affected antioxidant enzymatic system of brain aging and it could be responsible for the increased lipid peroxidation in both cell types examined.A preliminary report of these results was presented at the 19th Meeting F.E.B.S. Rome July 2–7, 1989.     

Do neurons in the vertebrate CNS migrate on laminin?

In adult rat brain the extracellular matrix glycoprotein, laminin, is found only in basement membranes, but is transiently expressed by astrocytes after brain injury. Here, I show that laminin also appears in immature brain cells during CNS development, and that its presence coincides with phases of neuronal migration. In early embryos, laminin is seen throughout the whole thickness of the forming brain, and is apparently synthesized by the cells, as judged by its intracytoplasmic localization. As development proceeds, intracellular laminin becomes restricted to the periventricular regions while punctate deposits of laminin follow the course of vimentin-positive radial glial fibers. In most brain regions, the adult pattern of laminin expression is achieved by birth. In the post-natal rat cerebellum, however, laminin is detected in external granule cells, in Purkinje cells, and in punctate deposits along the radial Bergmann glial fibers. By day 24 after birth, when the migration of external granule cells is complete, all laminin immunoreactivity disappears from these structures. The transient expression of laminin in regions where neurons are migrating raises the possibility that laminin plays a role in neuronal migration during CNS development.     

Synthesis and secretion of GnRH

Comprehensive studies have provided a clear understanding of the effects of gonadal steroids on the secretion of gonadotropin releasing hormone (GnRH), but some inconsistent results exist with regard to effects on synthesis. It is clear that regulation of both synthesis and the secretion of GnRH are effected by neurotransmitter systems in the brain. Thus, steroid regulation of GnRH synthesis and secretion can be direct, but the predominant effects are transmitted through steroid-responsive neuronal systems in various parts of the brain. There is also emerging evidence of direct effects on GnRH cells. Overriding effects on synthesis and secretion of GnRH can be observed during aging, in undernutrition and under stressful situations; these involve various neuronal systems, which may have serial or parallel effects on GnRH cells. The effect of aging is accompanied by changes in GnRH synthesis, but comprehensive studies of synthesis during undernutrition and stress are less well documented. Altered GnRH and gonadotropin secretion that occurs in seasonal breeding animals and during the pubertal transition is not generally accompanied by changes in GnRH synthesis. Secretion of GnRH from the brain is a reflection of the inherent function of GnRH cells and the inputs that integrate all of the central regulatory elements. Ultimately, the pattern of secretion dictates the reproductive status of the organism. In order to fully understand the central mechanisms that control reproduction, more extensive studies are required on the neuronal circuitry that provides input to GnRH cells.     

Glycine neurotransmitter transporters: an update

Glycine accomplishes several functions as a transmitter in the central nervous system (CNS). As an inhibitory neurotransmitter, it participates in the processing of motor and sensory information that permits movement, vision, and audition. This action of glycine is mediated by the strychnine-sensitive glycine receptor, whose activation produces inhibitory post-synaptic potentials. In some areas of the CNS, glycine seems to be co-released with GABA, the main inhibitory amino acid neurotransmitter. In addition, glycine modulates excitatory neurotransmission by potentiating the action of glutamate at N-methyl-D-aspartate (NMDA) receptors. It is believed that the termination of the different synaptic actions of glycine is produced by rapid re-uptake through two sodium-and-chloride-coupled transporters, GLYT1 and GLYT2, located in the plasma membrane of glial cells or pre-synaptic terminals, respectively. Glycine transporters may become major targets for therapeutic of pathological alterations in synaptic function. This article reviews recent progress on the study of the molecular heterogeneity, localization, function, structure, regulation and pharmacology of the glycine transporter proteins.     

Glycine neurotransmitter transporters: an update

Glycine accomplishes several functions as a transmitter in the central nervous system(CNS). As an inhibitory neurotransmitter, it participates in the processing of motor and sensory information that permits movement, vision, and audition. This action of glycine is mediated by the strychnine-sensitive glycine receptor, whose activation produces inhibitory post-synaptic potentials. In some areas of the CNS, glycine seems to be co-released with GABA, the main inhibitory amino acid neurotransmitter. In addition, glycine modulates excitatory neurotransmission by potentiating the action of glutamate at N-methyl-D-aspartate (NMDA) receptors. It is believed that the termination of the different synaptic actions of glycine is produced by rapid reuptake through two sodium-and-chloride-coupled transporters, GLYT1 and GLYT2, located in the plasma membrane of glial cells or pre-synaptic terminals, respectively. Glycine transporters may become major targets for therapeutic of pathological alterations in synaptic function. This article reviews recent progress on the study of the molecular heterogeneity, localization, function, structure, regulation and pharmacology of the glycine transporter