Cecilia Hedin-Pereira

The Marginal Zone/Layer I as a Novel Niche for Neurogenesis and Gliogenesis in Developing Cerebral Cortex

The cellular diversity of the cerebral cortex is thought to arise from progenitors located in the ventricular zone and subventricular zone in the telencephalon. Here we describe a novel source of progenitors located outside these two major germinative zones of the mouse cerebral cortex that contributes to neurogenesis and gliogenesis. Proliferating cells first appear in the preplate of the embryonic cerebral cortex and further increase in the marginal zone during mid and late neurogenesis. The embryonic marginal zone progenitors differ in their molecular characteristics as well as the size and identity of their clonal progeny from progenitors isolated from the ventricular zone and subventricular zone. Time-lapse video microscopy and clonal analysis in vitro revealed that the marginal zone progenitor pool contains a large fraction of oligodendrocyte or astrocyte progenitors, as well as neuronal and bipotent progenitors. Thus, marginal zone progenitors are heterogenous in regard to their fate specification, as well as in regard to their region of origin (pallial and subpallial) as revealed by in vivo fate mapping. The local environment in the marginal zone tightly regulates the size of this novel progenitor pool, because both basement membrane defects in lamininγ1−/− mice or alterations in the cellular composition of the marginal zone in Pax6 Small Eye mutant mice lead to an increase in the marginal zone progenitor pool. In conclusion, we have identified a novel source of neuronal and glial progenitors in the marginal zone of the developing cerebral cortex with properties notably distinct from those of ventricular zone and subventricular zone progenitors.

Astrocyte heterogeneity in the brain: from development to disease

In the last decades, astrocytes have risen from passive supporters of neuronal activity to central players in brain function and cognition. Likewise, the heterogeneity of astrocytes starts to become recognized in contrast to the homogeneous population previously predicted. In this review, we focused on astrocyte heterogeneity in terms of their morphological, protein expression and functional aspects, and debate in a historical perspective the diversity encountered in glial progenitors and how they may reflect mature astrocyte heterogeneity. We discussed data that show that different progenitors may have unsuspected roles in developmental processes. We have approached the functions of astrocyte subpopulations on the onset of psychiatric and neurological diseases.

Callosal neurons in the cingulate cortical plate and subplate of human fetuses

Given the scarcity of data on the development of the cerebral cortex and its connections in man, four brains of human fetuses at 25, 26, 30, and 32 weeks postovulation were used to investigate the following: 1) the radial distribution of callosal neurons in the cingulate cortex at the immediate postmigratory period; 2) the existence of callosally projecting neurons in the cortical subplate; and 3) the dendritic morphology of developing callosal neurons. The carbocyanine dye (1,1′‐dioctadecyl‐3,3,3′,3′‐tetramethylindocarbocyanine perchlorate) (DiI) was used as a fluorescent postmortem tracer for the identification and morphological description of callosal neurons, 4–6 months after the insertion of DiI crystals at the callosal midplane. Sixty‐one completely labeled neurons were selected for microscopical analysis, drawn by use of a camera lucida and photographed.

Cell migration in the postnatal subventricular zone

New neurons are constantly added to the olfactory bulb of rodents from birth to adulthood. This accretion is not only dependent on sustained neurogenesis, but also on the migration of neuroblasts and immature neurons from the cortical and striatal subventricular zone (SVZ) to the olfactory bulb. Migration along this long tangential pathway, known as the rostral migratory stream (RMS), is in many ways opposite to the classical radial migration of immature neurons: it is faster, spans a longer distance, does not require radial glial guidance, and is not limited to postmitotic neurons. In recent years many molecules have been found to be expressed specifically in this pathway and to directly affect this migration. Soluble factors with inhibitory, attractive and inductive roles in migration have been described, as well as molecules mediating cell-to-cell and cell-substrate interactions. However, it is still unclear how the various molecules and cells interact to account for the special migratory behavior in the RMS. Here we will propose some candidate mechanisms for roles in initiating and stopping SVZ/RMS migration.

Adult neural stem cells: plastic or restricted neuronal fates?

During embryonic development, the telencephalon is specified along its axis through morphogenetic gradients, leading to the positional-dependent generation of multiple neuronal types. After embryogenesis, however, the fate of neuronal progenitors becomes more restricted, and they generate only a subset of neurons. Here, we review studies of postnatal and adult neurogenesis, challenging the notion that fixed genetic programs restrict neuronal fate. We hypothesize that the adult brain maintains plastic neural stem cells that are capable of responding to changes in environmental cues and generating diverse neuronal types. Thus, the limited diversity of neurons generated under normal conditions must be actively maintained by the adult milieu.

The 9-O-acetyl GD3 gangliosides are expressed by migrating chains of subventricular zone neurons in vitro

Neurons from the anterior subventricular zone (SVZ) of the cerebral cortex migrate tangentially to become interneurons in the olfactory bulb during development and in adult rodents. This migration was defined as neuronophilic, independent of a radial glial substrate. The cortical SVZ and the rostral migratory stream to the olfactory bulb were shown to be rich in 9-O-acetyl GD3 gangliosides (9-O-acGD3), which have been previously shown to be implicated in gliophilic migration in the rodent cerebral cortex and cerebellum. In the present study, we performed SVZ explant cultures using rats during their first postnatal week to analyze the expression of these gangliosides in chain migration of neuronal precursors. We characterized migrating chains of these neuroblasts through morphological analysis and immunocytochemistry for the neural cell adhesion molecule. By using the Jones monoclonal antibody which binds specifically to 9-O-acGD3 we showed that migrating chains from the SVZ explants express 9-O-acGD3 which is distributed in a punctate manner in individual cells. 9-O-acGD3 is also present in migrating chains that form in the absence of radial glia, typical of the neuronophilic chain migration of the SVZ. Our data indicate that 9-O-acetylated gangliosides may participate in neuronophilic as well as gliophilic migration.

Putrescine as an important source of GABA in the postnatal rat subventricular zone

The subventricular zone (SVZ) is a neurogenic region that continually gives rise to olfactory bulb (OB) GABAergic interneurons in mammals. The newly generated neuroblasts already express GABA while migrating to this structure along the rostral migratory stream (RMS). Here, we investigate in early postnatal rat if SVZ/RMS cells undertake the same synthetic pathway by which GABA is produced in differentiated neurons, i.e. the decarboxylation of glutamate by the glutamic acid decarboxylase (GAD), or, if an alternative pathway, the conversion of putrescine into GABA, also contributes to GABA synthesis. We show here that GAD immunoreactivity is not significantly detectable within the SVZ/RMS. However, strong immunolabeling is found within the OB. Nevertheless, low GAD enzymatic activity (as compared with OB) is detected in the SVZ/RMS. SVZ/RMS explants convert approximately 30% of all captured radiolabeled putrescine into GABA in vitro, showing that this pathway is important for GABA synthesis in the SVZ. We also show that SVZ/RMS, OB and choroid plexus explants are able to synthesize putrescine, as analyzed by ornithine decarboxylase (ODC) activity, providing neuroblasts with different sources of putrescine for GABA production. During early stages of neuroblast differentiation, in which neurotransmitter choice may still be undefined, an alternative pathway for GABA synthesis guarantees the production of GABA, necessary for neuroblast proliferation and migration in the SVZ/RMS.

Generation of Glutamatergic Neurons from Postnatal and Adult Subventricular Zone with Pyramidal-Like Morphology

The mammalian subventricular zone (SVZ) contains progenitors derived from cerebral cortex radial glia cells, which give rise to glutamatergic pyramidal neurons during embryogenesis. However, during postnatal life, SVZ generates neurons that migrate and differentiate into olfactory bulb γ-aminobutyric acid (GABA)ergic interneurons. In this work, we tested if SVZ cells are able to produce glutamatergic neurons if confronted with the embryonic cortical ventricular zone environment. Different from typical SVZ chain migration, cells from P9-P11 SVZ explants migrate into embryonic cortical slices individually, many of which radially oriented. An average of 82.5% of green fluorescent protein-positive cells were immunolabeled for neuronal marker class III β-tubulin. Invading cells differentiate into multiple morphologies, including a pyramidal-like morphotype. A subset of these cells are GABAergic; however, about 28% of SVZ-derived cells are immunoreactive for glutamate. Adult SVZ explants also give rise to glutamatergic neurons in these conditions. Taken together, our results indicate that SVZ can be a source of glutamatergic cortical neurons when submitted to proper environmental cues.

Does cell lineage in the developing cerebral cortex contribute to its columnar organization

Since the pioneer work of Lorente de Nó, Ramón y Cajal, Brodmann, Mountcastle, Hubel and Wiesel and others, the cerebral cortex has been seen as a jigsaw of anatomic and functional modules involved in the processing of different sets of information. In fact, a columnar distribution of neurons displaying similar functional properties throughout the cerebral cortex has been observed by many researchers. Although it has been suggested that much of the anatomical substrate for such organization would be already specified at early developmental stages, before activity-dependent mechanisms could take place, it is still unclear whether gene expression in the ventricular zone (VZ) could play a role in the development of discrete functional units, such as minicolumns or columns. Cell lineage experiments using replication-incompetent retroviral vectors have shown that the progeny of a single neuroepithelial/radial glial cell in the dorsal telencephalon is organized into discrete radial clusters of sibling excitatory neurons, which have a higher propensity for developing chemical synapses with each other rather than with neighboring non-siblings. Here, we will discuss the possibility that the cell lineage of single neuroepithelial/radial glia cells could contribute for the columnar organization of the neocortex by generating radial columns of sibling, interconnected neurons. Borrowing some concepts from the studies on cell–cell recognition and transcription factor networks, we will also touch upon the potential molecular mechanisms involved in the establishment of sibling-neuron circuits.

Role of the 9-O-acetyl GD3 in subventricular zone neuroblast migration.

In the mammalian central nervous system the subventricular zone (SVZ) is one of the few neurogenic regions that persist postnatally. Neuroblasts generated in the SVZ migrate from this region tangentially towards the olfactory bulbs via the rostral migratory stream (RMS) and give rise to interneurons. In previous studies, an important role in radial migration of cerebellar granule neurons has been attributed to the 9-O-acetylated GD3 ganglioside. Previous data demonstrated the expression of 9-O-acetyl GD3 in the rostral migratory stream in vivo as well as in chains of neuroblasts that migrate from SVZ explants in vitro. Herein, using the Jones monoclonal antibody (Jones mAb), we combined SVZ explant migration measurements and time-lapse videomicroscopy of migrating neuroblasts to show that SVZ neuroblast migration is inhibited by the antibody that recognizes 9-O-acetyl GD3 but not by A2B5, an antibody that recognizes c-series gangliosides. In addition, inhibition of ganglioside synthesis results in reduction of migratory halos around SVZ explants. Coherently, we show that most migratory neuroblasts which express the embryonic form of NCAM co-express 9acGD3. Also, we observe that some of the ganglioside positive neuroblasts also express nestin consistent with their maintained proliferative capacity. These results strongly support that the 9-O-acetyl GD3 has a pivotal role in neuroblast migration from SVZ, being fundamental for cell-cell and cell-substrate interactions in this region.