Intrinsic and spontaneous neurogenesis in the postnatal slice culture of rat hippocampus

Replenishment of Organotypic Hippocampal Slice Cultures with Neonatal or Adult Microglia

This protocol describes a method to deplete and repopulate organotypic hippocampal slice cultures with ramified microglia. We describe the slice culture preparation from newborn mice, standard culturing of neonatal microglia, and the acute isolation of microglia from adult mouse brain. Furthermore, we outline the technique for the replenishment of microglia-depleted slice cultures with different microglia populations and subsequent morphological analysis. We show that neonatal and adult microglia acquire specific ramified morphologies, which in case of adult microglia are indistinguishable from the in vivo situation. This procedure not only allows the functional investigation of microglia with different degrees of ramification but also enables the construction of chimeric slice cultures with respect to the microglia phenotype. Preparation of slice cultures can be completed in 3.5 h, preparation of mixed-glial cultures in 4 h, isolation of adult microglia can be accomplished in 3.5 h, and replenishment in 30 min.

Anti-aggregant tau mutant promotes neurogenesis

BACKGROUND

The microtubule-associated protein Tau plays a role in neurodegeneration as well as neurogenesis. Previous work has shown that the expression of the pro-aggregant mutant Tau repeat domain causes strong aggregation and pronounced neuronal loss in the hippocampus whereas the anti-aggregant form has no deleterious effects. These two proteins differ mainly in their propensity to form ß structure and hence to aggregate.

METHODS

To elucidate the basis of these contrasting effects, we analyzed organotypic hippocampal slice cultures (OHSCs) from transgenic mice expressing the repeat domain (RD) of Tau with the anti-aggregant mutation (Tau^RDΔKPP) and compared them with slices containing pro-aggregant Tau^RDΔK. Transgene expression in the hippocampus was monitored via a sensitive bioluminescence reporter gene assay (luciferase).

RESULTS

The expression of the anti-aggregant Tau^RDΔKPP leads to a larger volume of the hippocampus at a young age due to enhanced neurogenesis, resulting in an increase in neuronal number. There were no signs of activation of microglia and astrocytes, indicating the absence of an inflammatory reaction. Investigation of signaling pathways showed that Wnt-5a was strongly decreased whereas Wnt3 was increased. A pronounced increase in hippocampal stem cell proliferation (seen by BrdU) was observed as early as P8, in the CA regions where neurogenesis is normally not observed. The increase in neurons persisted up to 16 months of age.

CONCLUSION

The data suggest that the expression of anti-aggregant Tau^RDΔKPP enhances hippocampal neurogenesis mediated by the canonical Wnt signaling pathway, without an inflammatory reaction. This study points to a role of tau in brain development and neurogenesis, in contrast to its detrimental role in neurodegeneration at later age.

Time-lapse imaging reveals highly dynamic structural maturation of postnatally born dentate granule cells in organotypic entorhino-hippocampal slice cultures

Neurogenesis of hippocampal granule cells (GCs) persists throughout mammalian life and is important for learning and memory. How newborn GCs differentiate and mature into an existing circuit during this time period is not yet fully understood. We established a method to visualize postnatally generated GCs in organotypic entorhino-hippocampal slice cultures (OTCs) using retroviral (RV) GFP-labeling and performed time-lapse imaging to study their morphological development in vitro. Using anterograde tracing we could, furthermore, demonstrate that the postnatally generated GCs in OTCs, similar to adult born GCs, grow into an existing entorhino-dentate circuitry. RV-labeled GCs were identified and individual cells were followed for up to four weeks post injection. Postnatally born GCs exhibited highly dynamic structural changes, including dendritic growth spurts but also retraction of dendrites and phases of dendritic stabilization. In contrast, older, presumably prenatally born GCs labeled with an adeno-associated virus (AAV), were far less dynamic. We propose that the high degree of structural flexibility seen in our preparations is necessary for the integration of newborn granule cells into an already existing neuronal circuit of the dentate gyrus in which they have to compete for entorhinal input with cells generated and integrated earlier.

Persistent Gliosis Interferes with Neurogenesis in Organotypic Hippocampal Slice Cultures

Neurogenesis in the adult hippocampus has become an intensively investigated research topic, as it is essential for proper hippocampal function and considered to bear therapeutic potential for the replacement of pathologically lost neurons. On the other hand, neurogenesis itself is frequently affected by CNS insults. To identify processes leading to the disturbance of neurogenesis, we made use of organotypic hippocampal slice cultures (OHSC), which, for unknown reasons, lose their neurogenic potential during cultivation. In the present study, we show by BrdU/Prox1 double-immunostaining that the generation of new granule cells drops by 90% during the first week of cultivation. Monitoring neurogenesis dynamically in OHSC from POMC-eGFP mice, in which immature granule cells are endogenously labeled, revealed a gradual decay of the eGFP signal, reaching 10% of initial values within 7 days of cultivation. Accordingly, reverse transcription quantitative polymerase chain reaction analysis showed the downregulation of the neurogenesis-related genes doublecortin and Hes5, a crucial target of the stem cell-maintaining Notch signaling pathway. In parallel, we demonstrate a strong and long-lasting activation of astrocytes and microglial cells, both, morphologically and on the level of gene expression. Enhancement of astroglial activation by treating OHSC with ciliary neurotrophic factor accelerated the loss of neurogenesis, whereas treatment with indomethacin or an antagonist of the purinergic P2Y12 receptor exhibited potent protective effects on the neurogenic outcome. Therefore, we conclude that OHSC rapidly lose their neurogenic capacity due to persistent inflammatory processes taking place after the slice preparation. As inflammation is also considered to affect neurogenesis in many CNS pathologies, OHSC appear as a useful tool to study this interplay and its molecular basis. Furthermore, we propose that modification of glial activation might bear the therapeutic potential of enabling neurogenesis under neuropathological conditions.

Live Imaging of Adult Neural Stem Cells in Rodents

The generation of cells of the neural lineage within the brain is not restricted to early development. New neurons, oligodendrocytes, and astrocytes are produced in the adult brain throughout the entire murine life. However, despite the extensive research performed in the field of adult neurogenesis during the past years, fundamental questions regarding the cell biology of adult neural stem cells (aNSCs) remain to be uncovered. For instance, it is crucial to elucidate whether a single aNSC is capable of differentiating into all three different macroglial cell types in vivo or these distinct progenies constitute entirely separate lineages. Similarly, the cell cycle length, the time and mode of division (symmetric vs. asymmetric) that these cells undergo within their lineage progression are interesting questions under current investigation. In this sense, live imaging constitutes a valuable ally in the search of reliable answers to the previous questions. In spite of the current limitations of technology new approaches are being developed and outstanding amount of knowledge is being piled up providing interesting insights in the behavior of aNSCs. Here, we will review the state of the art of live imaging as well as the alternative models that currently offer new answers to critical questions.

Cochlear nucleus whole mount explants promote the differentiation of neuronal stem cells from the cochlear nucleus in co-culture experiments

The cochlear nucleus is the first brainstem nucleus to receive sensory input from the cochlea. Depriving this nucleus of auditory input leads to cellular and molecular disorganization which may potentially be counteracted by the activation or application of stem cells. Neuronal stem cells (NSCs) have recently been identified in the neonatal cochlear nucleus and a persistent neurogenic niche was demonstrated in this brainstem nucleus until adulthood. The present work investigates whether the neurogenic environment of the cochlear nucleus can promote the survival of engrafted NSCs and whether cochlear nucleus-derived NSCs can differentiate into neurons and glia in brain tissue. Therefore, cochlear nucleus whole-mount explants were co-cultured with NSCs extracted from either the cochlear nucleus or the hippocampus and compared to a second environment using whole-mount explants from the hippocampus. Factors that are known to induce neuronal differentiation were also investigated in these NSC-explant experiments. NSCs derived from the cochlear nucleus engrafted in the brain tissue and differentiated into all cells of the neuronal lineage. Hippocampal NSCs also immigrated in cochlear nucleus explants and differentiated into neurons, astrocytes and oligodendrocytes. Laminin expression was up-regulated in the cochlear nucleus whole-mounts and regulated the in vitro differentiation of NSCs from the cochlear nucleus. These experiments confirm a neurogenic environment in the cochlear nucleus and the capacity of cochlear nucleus-derived NSCs to differentiate into neurons and glia. Consequently, the presented results provide a first step for the possible application of stem cells to repair the disorganization of the cochlear nucleus, which occurs after hearing loss.

Organotypic slice cultures for studies of postnatal neurogenesis

Here we describe a technique for studying hippocampal postnatal neurogenesis in the rodent brain using the organotypic slice culture technique. This method maintains the characteristic topographical morphology of the hippocampus while allowing direct application of pharmacological agents to the developing hippocampal dentate gyrus. Additionally, slice cultures can be maintained for up to 4 weeks and thus, allow one to study the maturation process of newborn granule neurons. Slice cultures allow for efficient pharmacological manipulation of hippocampal slices while excluding complex variables such as uncertainties related to the deep anatomic location of the hippocampus as well as the blood brain barrier. For these reasons, we sought to optimize organotypic slice cultures specifically for postnatal neurogenesis research.

Sex and regional differences in effects of chronic intermittent ethanol exposure on subsequent excitotoxic challenges in hippocampal slice cultures

The organotypic hippocampal slice culture technique was used to study how the effects of repeated ethanol withdrawal might differ between males and females at the cellular level, including potential modulation of subsequent insults. A chronic intermittent ethanol (CIE) exposure paradigm was employed, with 3 days of exposure followed by 24 h withdrawal for 3 cycles. Slices were next exposed to corticosterone (CORT) or pentylenetetrazol (PTZ) for 24 h then imaged for propidium iodide (PI) signal intensities. There were sex-selective responses in the CA1 region and dentate gyrus of the hippocampal slice cultures to treatment with CIE and/or CORT or PTZ. The 50 mM CIE alone generally did not increase the PI signal, but enhanced sensitivity to the toxic effects of CORT (particularly for females) and PTZ (particularly for males). In contrast, 100 mM CIE elicited a toxic response that was greater in females than males, and was exacerbated by exposure to PTZ. These data showed that hippocampal sexual dimorphism influences sensitivity to ethanol and other toxic chemicals even in an immature state. Low-dose CIE may attenuate harm from additional challenges in a hippocampal sex- and region-selective manner. These findings add to the growing evidence of important neurobiological sex differences in responses to chronic ethanol exposure and withdrawal.

GABA mediates the network activity-dependent facilitation of axonal outgrowth from the newborn granule cells in the early postnatal rat hippocampus

Neural network activity regulates the development of hippocampal newborn granule cells (GCs). Excitatory GABAergic input is known to be a key player in this regulation. Although calcium signaling is thought to be a downstream mediator of GABA, GABA-induced calcium signaling in newborn GCs is not well understood. We investigated Ca(2+) signaling and its regulatory role in axon and dendrite outgrowth in newborn GCs identified in the organotypic slice culture of early postnatal rat hippocampus. Here, we report that hippocampal network activity can induce calcium transients (CaTs) in newborn GCs during the first post-mitotic week via GABAergic inputs. The GABA-induced CaTs were mediated mainly by L-type Ca(2+) channels. Furthermore, we found that inhibiting any step in the signaling pathway, network activity → GABA → L-type Ca(2+) channels, selectively suppressed the axonal outgrowth and pruning of newborn GCs, but not dendritic outgrowth. The GABA(A) receptor blocker bicuculline significantly suppressed axonal outgrowth, despite increasing network activity, thus indicating an essential role of GABAergic inputs. Therefore, we conclude that network activity-dependent GABAergic inputs open L-type Ca(2+) channels and promote axonal outgrowth in newborn GC during the first post-mitotic week.

Enhanced neurogenesis in organotypic cultures of rat hippocampus after transient subfield-selective excitotoxic insult induced by domoic acid

New neurons are continuously generated in the hippocampus and may play an important role in many physiological and pathological conditions. Here we present evidence of cell proliferation and neurogenesis after a selective and transient excitotoxic injury to the hippocampal cornu ammonis 1 (CA1) area induced by low concentrations of domoic acid (DOM) in rat organotypic hippocampal slice cultures (OHSC). DOM is an excitatory amino acid analog to kainic acid that acts through glutamate receptors to elicit a rapid and potent excitotoxic response. Exposure of slice cultures to varying concentrations of DOM for 24 h induced dose-dependent neuronal toxicity that was independent of activation of classic apoptotic markers. Treatment with 2 μM DOM for 24 h caused a selective yet transient neurotoxic injury in the CA1 subfield of the hippocampus that appeared recovered after 7 days of incubation in a DOM-free medium and showed significant microgliosis but no sign of astrogliosis. The DOM insult (2 μM, 24 h) resulted in a significant upregulation of cell proliferation, as assessed by 5-bromo-2-deoxyuridine (BrdU) incorporation, and a concurrent increase of the neuronal precursor cell marker doublecortin (DCX) within the subgranular zone of the dentate gyrus and area CA1. Neurogenesis occurred primarily during the first week after termination of the DOM exposure. Our study shows that exposure of OHSC to concentrations of DOM below those required to induce permanent neurotoxicity can induce proliferation and differentiation of neural progenitor cells that may contribute to recovery from mild injury and to develop abnormal circuits relevant to disease.

Time-lapse imaging reveals symmetric neurogenic cell division of GFAP-expressing progenitors for expansion of postnatal dentate granule neurons

Granule cells in the hippocampus, a region critical for memory and learning, are generated mainly during the early postnatal period but neurogenesis continues in adulthood. Postnatal neuronal production is carried out by primary progenitors that express glial fibrillary acidic protein (GFAP) and they are assumed to function as stem cells. A central question regarding postnatal dentate neurogenesis is how astrocyte-like progenitors produce neurons. To reveal cell division patterns and the process of neuronal differentiation of astrocyte-like neural progenitors, we performed time-lapse imaging in cultured hippocampal slices from early postnatal transgenic mice with mouse GFAP promoter-controlled enhanced green fluorescent protein (mGFAP-eGFP Tg mice) in combination with a retrovirus carrying a red fluorescent protein gene. Our results showed that the majority of GFAP-eGFP+ progenitor cells that express GFAP, Sox2 and nestin divided symmetrically to produce pairs of GFAP+ cells (45%) or pairs of neuron-committed cells (45%), whereas a minority divided asymmetrically to generate GFAP+ cells and neuron-committed cells (10%). The present results suggest that a substantial number of GFAP-expressing progenitors functions as transient amplifying progenitors, at least in an early postnatal dentate gyrus, although a small population appears to be stem cell-like progenitors. From the present data, we discuss possible cell division patterns of adult GFAP+ progenitors.

Optically controlled contraction of photosensitive skeletal muscle cells

As the skeletal muscle cell is an efficient force transducer, it has been incorporated in bio-microdevices using electrical field stimulation for generating contractile patterns. To improve both the spatial and temporal resolutions, we made photosensitive skeletal muscle cells from murine C2C12 myoblasts, which express channelrhodopsin-2 (ChR2), one of archaea-type rhodopsins derived from green algae Chlamydomonas reinhardtii. The cloned ChR2-expressing C2C12 myoblasts were made and fused with untransfected C2C12 to form multinucleated myotubes. The maturation of myotubes was facilitated by electrical field stimulation. Blue LED light pulse depolarized the membrane potential of a ChR2-expressing myotube and eventually evoked an action potential. It also induced a twitch-like contraction in a concurrent manner. A contraction pattern was thus made with a given pattern of LED pulses. This technique would have many applications in the bioengineering field, such as wireless drive of muscle-powered actuators/microdevices.

Electric field-guided neuron migration: a novel approach in neurogenesis

Effective directional neuron migration is crucial in development of the central nervous system and for neurogenesis. Endogenous electrical signals are present in many developing systems and crucial cellular behaviors such as neuronal cell division, cell migration, and cell differentiation are all under the influence of such endogenous electrical cues. Preclinical in vivo studies have used electric fields (EFs) to attempt to enhance regrowth of damaged spinal cord axons with some success. Recent evidence shows that small EFs not only guide axonal growth, but also direct the earlier events of neuronal migration and neuronal cell division. This raises the possibility that applied or endogenous EFs, perhaps in combination, may direct transplanted neural stem cells, or regenerating neurons, to the desired site after brain injury or neuron degeneration. The high complexity of both structure and function of the nervous system, however, poses significant challenges to techniques for applying EFs to promote neurogenesis. The evolution of functional biomaterials and nanotechnology may provide promising solutions for the application of EFs in guiding neuron migration and neurogenesis within the central nervous system.

Isolation and characterization of neural stem cells from the neonatal rat cochlear nucleus

Neural stem cells have been identified in multiple parts of the postnatal mammalian brain, as well as in the inner ear. No investigation of potential neural stem cells in the cochlear nucleus has yet been performed. The aim of this study was to investigate potential neural stem cells from the cochlear nucleus by neurosphere assay and in histological sections to prove their capacity for self-renewal and for differentiation into progenitor cells and cells of the neuronal lineage. For this purpose, cells of the cochlear nucleus of postnatal day 6 rats were isolated and cultured for generation of primary neurospheres. Spheres were dissociated and cells analyzed for capacity for mitosis and differentiation. Cell division was detected by cell-counting assay and BrdU incorporation. Differentiated neural progenitor cells showed distinct labeling for Nestin and for Atoh1. Positive staining of ß-III Tubulin, glial fibrillary acid protein (GFAP) and myelin basic protein (MBP) showed differentiation into neurons, astrocytes and oligodendrocytes. Furthermore, Nestin- and BrdU-labeled cells could also be detected in histological sections. In conclusion, the isolated cells from the cochlear nucleus presented all the features of neural stem cells: cell division, presence of progenitor cells and differentiation into different cells of the neuronal lineage. The existence of neural stem cells may add to the understanding of developmental features in the cochlear nucleus.

Erythropoietin promotes hippocampal neurogenesis in in vitro models of neonatal stroke

The hippocampus is often injured in neonatal stroke. We have investigated the effect of erythropoietin (EPO) on oxygen-glucose deprived hippocampal slices and hypoxic progenitor cells. EPO improved survival of the organotypic hippocampal slices with significantly less cell death in the dentate gyrus and an increased number of proliferating cells 4-5 days after insult. Significantly fewer markers of neurogenesis were seen after the insult but when EPO was added to the culture medium, neurogenesis was sustained. When hippocampal progenitor cultures were stimulated into differentiation, more cells chose a neuronal cell fate when treated with EPO. These findings support the hypothesis that EPO not only prevents ischemia induced cell death but promotes neuronal cell fate commitment in in vitro models of neonatal stroke.

Combination therapies for traumatic brain injury: prospective considerations

Traumatic brain injury (TBI) initiates a cascade of numerous pathophysiological events that evolve over time.Despite the complexity of TBI, research aimed at therapy development has almost exclusively focused on single therapies, all of which have failed in multicenter clinical trials. Therefore, in February 2008 the National Institute of Neurological Disorders and Stroke, with support from the National Institute of Child Health and Development, the National Heart, Lung, and Blood Institute, and the Department of Veterans Affairs, convened a workshop to discuss the opportunities and challenges of testing combination therapies for TBI. Workshop participants included clinicians and scientists from a variety of disciplines, institutions, and agencies. The objectives of the workshop were to: (1) identify the most promising combinations of therapies for TBI; (2) identify challenges of testing combination therapies in clinical and pre-clinical studies; and (3) propose research methodologies and study designs to overcome these challenges. Several promising combination therapies were discussed, but no one combination was identified as being the most promising. Rather, the general recommendation was to combine agents with complementary targets and effects (e.g., mechanisms and time-points), rather than focusing on a single target with multiple agents. In addition, it was recommended that clinical management guidelines be carefully considered when designing pre-clinical studies for therapeutic development.To overcome the challenges of testing combination therapies it was recommended that statisticians and the U.S. Food and Drug Administration be included in early discussions of experimental design. Furthermore, it was agreed that an efficient and validated screening platform for candidate therapeutics, sensitive and clinically relevant biomarkers and outcome measures, and standardization and data sharing across centers would greatly facilitate the development of successful combination therapies for TBI. Overall there was great enthusiasm for working collaboratively to act on these recommendations.

Primary target cells of herpes simplex virus type 1 in the hippocampus

Herpes simplex virus type 1 (HSV-1) causes fatal and sporadic encephalitis in human. The encephalitis-survivors frequently suffer from symptoms of memory deficits. It remains unclear how HSV-1 induces tissue damages in memory formation-associated brain tissues such as the hippocampus. In this study, we examined HSV-1 infection in the hippocampus using a rat HSV-1 infection model. We found profound pathological changes in the hippocampus and large numbers of HSV-1 antigen-positive cells in the dentate gyrus (DG) subfield of HSV-1-infected rats. To understand the precise mechanism of HSV-1-induced tissue damages in the hippocampus, we employed rat organotypic hippocampal slice cultures (OHC) as an in vitro HSV-1 infection model. In OHC, HSV-1 infection predominated in neuronal cells and the infected neuronal cells were severely damaged. Longitudinal analysis indicated that granule cells in DG subfield were extremely vulnerable to HSV-1 infection among neuronal cells in the hippocampus. Since DG granule cells play a crucial role in memory formation, disruption of these cells may be a primary step leading to memory deficits.

Human immunodeficiency virus type-1 vulnerates nascent neuronal cells

Macrophages or microglial cells are the major target cells for HIV-1 infection in the brain. The infected cells release neurotoxic factors that may cause severe neuronal cell damage, especially in the basal ganglia and hippocampus. In this study, we used rat OHC to examine the region-specific neuronal cell damage caused by HIV-1-infected macrophages. When OHC was cocultured with HIV-1-infected MDM, we found that neuronal cells at the GCL of the DG were preferentially killed via apoptosis, and that projection of MF from GCL to PCL of the CA3 region was severely disturbed. We marked precursor cells around the DG region by using an EGFP-expressing retrovirus vector and found that these cells lost the ability to differentiate into neurons when exposed to HIV-1-infected MDM. In the DG, new neurons are normally incorporated into GCL or PCL, while in the presence of HIV-1-infected MDM, mature neurons failed to be incorporated into those layers. These data indicate that the neurotoxic factor(s) released from HIV-1-infected macrophages impede(s) neuronal cell repair in brain tissue. This suggests that DG is the region of the hippocampus most vulnerable to neuronal damage caused by HIV-1 infection, and that its selective vulnerability is most likely due to the highly active neurogenesis that takes place in this region.

Class III beta-tubulin is constitutively coexpressed with glial fibrillary acidic protein and nestin in midgestational human fetal astrocytes: implications for phenotypic identity

Class III beta-tubulin isotype (betaIII-tubulin) is widely regarded as a neuronal marker in developmental neurobiology and stem cell research. To test the specificity of this marker protein, we determined its expression and distribution in primary cultures of glial fibrillary acidic protein (GFAP)-expressing astrocytes isolated from the cerebral hemispheres of 2 human fetuses at 18 to 20 weeks of gestation. Cells were maintained as monolayer cultures for 1 to 21 days without differentiation induction. By immunofluorescence microscopy, coexpression of betaIII-tubulin and GFAP was detected in cells at all time points but in spatially distinct patterns. The numbers of GFAP+ cells gradually decreased from Days 1 to 21 in vitro, whereas betaIII-tubulin immunoreactivity was present in 100% of cells at all time points. beta-III-tubulin mRNA and protein expression were demonstrated in cultured cells by reverse-transcriptase-polymerase chain reaction and immunoblotting, respectively. Glial fibrillary acidic protein+/beta-III-tubulin-positive cells coexpressed nestin and vimentin but lacked neurofilament proteins, CD133, and glutamate-aspartate transporter. Weak cytoplasmic staining was detected with antibodies against microtubule-associated protein 2 isoforms. Confocal microscopy, performed on autopsy brain samples of human fetuses at 16 to 20 gestational weeks, revealed widespread colocalization of GFAP and betaIII-tubulin in cells of the ventricular/subventricular zones and the cortical plate. Our results indicate that in the midgestational human brain, betaIII-tubulin is not neuron specific because it is constitutively expressed in GFAP+/nestin+ presumptive fetal astrocytes.

Human immunodeficiency virus type 1 Vpr inhibits axonal outgrowth through induction of mitochondrial dysfunction

Human immunodeficiency virus type 1 (HIV-1)-infected macrophages damage mature neurons in the brain, although their effect on neuronal development has not been clarified. In this study, we show that HIV-1-infected macrophages produce factors that impair the development of neuronal precursor cells and that soluble viral protein R (Vpr) is one of the factors that has the ability to suppress axonal growth. Cell biological analysis revealed that extracellularly administered recombinant Vpr (rVpr) clearly accumulated in mitochondria where a Vpr-binding protein adenine nucleotide translocator localizes and also decreased the mitochondrial membrane potential, which led to ATP synthesis. The depletion of ATP synthesis reduced the transportation of mitochondria within neurites. This mitochondrial dysfunction inhibited axonal growth even when the frequency of apoptosis was not significant. We also found that point mutations of arginine (R) residues to alanine (A) residues at positions 73, 77, and 80 rendered rVpr incapable of causing mitochondrial membrane depolarization and axonal growth inhibition. Moreover, the Vpr-induced inhibition was suppressed after treatment with a ubiquinone analogue (ubiquinone-10). Our results suggest that soluble Vpr is a major viral factor that causes a disturbance in neuronal development through the induction of mitochondrial dysfunction. Since ubiquinone-10 protects the neuronal plasticity in vitro, it may be a therapeutic agent that can offer defense against HIV-1-associated neurological disease.

Postnatal neurogenesis in hippocampal slice cultures: early in vitro labeling of neural precursor cells leads to efficient neuronal production

Neurogenesis continues throughout life in the hippocampus. To study postnatal neurogenesis in vitro, hippocampal slices from rats on postnatal day 5 (P5) were cultured on a porous membrane for 14 or 21 days. In the initial experiments, precursor cells were labeled with bromodeoxyuridine (BrdU) after 7 days in culture because hippocampal slices are generally used in experiments after 1-2 weeks in culture. Fourteen days after labeling, however, only about 10% of BrdU-labeled cells expressed neuronal markers, although in living rats, about 80% of cells labeled with BrdU on P5 had become neurons by P19. Next, rats were injected with BrdU 30 min before culture, after which hippocampal slices were cultured for 14 days to examine the capacity of in vivo-labeled neural precursors to differentiate into neurons in vitro. In this case, more than two-thirds of BrdU-labeled cells expressed neuronal markers, such as Hu, NeuN, and PSA-NCAM. Furthermore, precursor cells underwent early in vitro labeling by incubation with BrdU or a modified retrovirus vector carrying EGFP for 30 min from the beginning of the culture. This procedure resulted in a similar high rate of neuronal differentiation and normal development into granule cells. In addition, time-lapse imaging with retrovirus-EGFP revealed migration of neural precursors from the hilus to the granule cell layer. These results indicate that in vivo- and early in vitro-labeled cultures are readily available ex vivo models for studying postnatal neurogenesis and suggest that the capacity of neural precursors to differentiate into neurons is reduced during the culture period.

Plasticity of 5‐HT1Areceptor‐mediated signaling during early postnatal brain development

The presence of serotonin 1A receptor (5-HT(1A)-R) in the hippocampus, amygdala, and most regions of the frontal cortex is essential between postnatal day-5-21 (P5-21) for the expression of normal anxiety levels in adult mice. Thus, the 5-HT(1A)-R plays a crucial role in this time window of brain development. We show that the 5-HT(1A)-R-mediated stimulation of extracellular signal-regulated kinases 1 and 2 (Erk1/2) in the hippocampus undergoes a transition between P6 and P15. At P6, a protein kinase C (PKC) isozyme is required for the 5-HT(1A)-R -->Erk1/2 cascade, which causes increased cell division in the dentate gyrus. By contrast, at P15, PKC alpha participates downstream of Erk1/2 to augment synaptic transmission through the Schaffer Collateral pathway but does not cause increased cell division. Our data demonstrate that the 5-HT(1A)-R -->Erk1/2 cascade uses PKC isozymes differentially, first boosting the cell division to form new hippocampal neurons at P6 and then undergoing a plastic change in mechanism to strengthen synaptic connections in the hippocampus at P15.

Neurotropism of herpes simplex virus type 1 in brain organ cultures

The mechanism of herpes simplex virus type 1 (HSV-1) penetration into the brain and its predilection to infect certain neuronal regions is unknown. In order to study HSV-1 neurotropism, an ex vivo system of mice organotypic brain slices was established and the tissue was infected with HSV-1 vectors. Neonate tissues showed restricted infection confined to leptomeningeal, periventricular and cortical brain regions. The hippocampus was the primary parenchymatous structure that was also infected. Infection was localized to early progenitor and ependymal cells. Increasing viral inoculum increased the intensity and enlarged the infected territory, but the distinctive pattern of infection was maintained and differed from that observed with adenovirus and Vaccinia virus. Neonate brain tissues were much more permissive for HSV-1 infection than adult mouse brain tissues. Taken together, these results indicate a complex interaction of HSV-1 with different brain-cell types and provide a useful vehicle to elucidate the mechanisms of viral neurotropism.

Potential Value of Changes in Cell Markers in Organotypic Hippocampal Cultures Associated With Chronic EtOH Exposure and Withdrawal: Comparison with NMDA-Induced Changes

BACKGROUND

Previous studies have shown that withdrawal from ethanol (EtOH) exposure induces neuronal damage, as indicated by propidium iodide (PI) uptake, in organotypic hippocampal slice cultures. This is prevented by MK801, suggesting that damage is "excitotoxic," resulting from activation of N-methyl-d-aspartate (NMDA) receptors by endogenous glutamate. To avoid reliance on a single indicator, and to enable assessment of recovery from the EtOH withdrawal (EWD) insult, we assessed changes in cell markers for neurons and glia, as well as cell division, following either EWD or NMDA challenge (as a positive control).

METHODS

Organotypic cultures from postnatal day (PND) 8 rats were cultured for 5 days before exposure to EtOH (mean concentration approximately 65 mM) for 10 days before EWD. Cultures of the same "days in vitro" age (DIV16) were exposed to NMDA (200 microM) for 1 hour. Neuronal injury was visualized using PI and indices of neurons, glia, or cell division were measured at intervals up to 10 days following the neurotoxic insults. Each time point and measurement used separate slice cultures, and these were treated as separate experiments with paired controls. Regional neuronal content was assessed by neuronal nuclear protein (NeuN) and calbindin D28k (Calb), glial content by glial fibrillary acidic protein (GFAP), and cell division by bromodeoxyuridine (BrdU) incorporation, all measured immunohistochemically.

RESULTS

Chronic exposure to EtOH was associated with a dramatic reduction in BrdU incorporation in all regions of cultures. Propidium iodide fluorescence in the CA1 region was elevated significantly after EWD and more so after NMDA challenge. Reduced immunoreactivity (IR) of NeuN and Calb suggested that loss of neurons resulted from the EWD insult. Bromodeoxyuridine incorporation was initially depressed even further by EWD, but had returned to control levels after 3 days. In contrast, following NMDA insult, BrdU incorporation was significantly and persistently elevated above control levels after 3 days. Glial fibrillary acidic protein was reduced immediately after both EWD and NMDA challenge. Several days after EWD, expression of neuronal and glial markers, although variable, had generally returned to control levels. In contrast, NeuN IR remained significantly reduced after NMDA challenge.

CONCLUSIONS

In general, the use of additional markers supports data obtained with PI uptake alone and suggests that neurons (and glia) are lost from the culture following EWD or NMDA challenge. These cell markers recover several days after EWD, but it is unclear whether functional recovery accompanies these changes. If the dramatic effect of EtOH exposure and EWD on BrdU incorporation reflects reduced neuro- and gliogenesis, it is likely that this adversely affects long-term recovery from EWD. Finally, some markers showed significant and consistent changes after EWD, whereas others did not. This information may facilitate the use of this model in evaluation of potential medications that protect against and/or promote recovery from neurotoxicity.

Conditional labeling of newborn granule cells to visualize their integration into established circuits in hippocampal slice cultures

The dentate gyrus continues to produce new granule cells throughout life. Understanding the mechanisms underlying their integration into the pre-existing hippocampal circuitry is of crucial importance. In the present study, we developed an approach allowing visual tracking of newborn granule cells in hippocampal organotypic slices. By crossing neurogenin 2 (Ngn2-CreER) with Cre-reporter mice expressing YFP or GFP reporter genes, it was possible to observe living cells after treating slice cultures with 4-hydroxytamoxifen to induce Cre recombinase activation. Colocalization of GFP with the mitotic marker BrdU demonstrated that the GFP-expressing granule cells were born in vitro. They mature and integrate normally into the hippocampal circuitry, as shown using morphological and electrophysiological techniques. This ex vivo approach therefore offers a highly accessible model to study the effects of long-term treatments on maturation and integration of newborn granule cells.

Anti-inflammatory treatment in oxygen-glucose-deprived hippocampal slice cultures is neuroprotective and associated with reduced cell proliferation and intact neurogenesis

Increased neurogenesis in response to brain injury is considered a mechanism of regeneration after neuronal loss. Using organotypic hippocampal cultures (OHC), we investigated the interplay between neuronal damage (propidium iodide uptake), microglia activation (OX-42 immunohistochemistry), cell proliferation (bromodeoxyuridine incorporation), and neurogenesis (double labeling of bromodeoxyuridine with doublecortin or beta-III tubulin) after oxygen-glucose deprivation (OGD). We observed that microglia activation and upregulation of pro-inflammatory cytokines mRNA preceded neuronal loss and was followed by increased cell proliferation. Neurogenesis was inhibited 3 days after OGD in both neurogenic zones of the slice, the dentate gyrus and the posterior periventricle (pPV). After 6 days, neurogenesis was restored and significantly increased in the pPV. Indomethacin or minocycline reduced the OGD-induced damage, proliferation, and increase of microglia. Both agents did not interfere with OGD-induced pPV neurogenesis. Our study shows for the first time that neuroprotection against OGD-induced damage in OHC by anti-inflammatory treatment is associated with intact neurogenesis.

Functional maturation of adult-generated granule cells

The excitability and connectivity of adult-generated granule cells dictate to what extent newborn neurons participate in the hippocampal network. These functional parameters evolve as newborn cells mature and interact with the existing circuit. The progression of granule cell maturation during neonatal development appears to be reiterated in the adult, but with some caveats. New approaches to identify and track newborn neurons are revealing the timing of this process, as well as its sensitivity to activity-dependent regulation.

Adult neurogenesis and functional plasticity in neuronal circuits

The adult brain is a plastic place. To ensure that the mature nervous system's control of behaviour is flexible in the face of a varying environment, morphological and physiological changes are possible at many levels, including that of the entire cell. In two areas of the adult brain - the olfactory bulb and the dentate gyrus - new neurons are generated throughout life and form an integral part of the normal functional circuitry. This process is not fixed, but highly modulated, revealing a plastic mechanism by which the brain's performance can be optimized for a given environment. The functional benefits of this whole-cell plasticity, however, remain a matter for debate.

AMPA receptor-induced intracellular calcium response in the paraventricular nucleus is modulated by nitric oxide: calcium imaging in a hypothalamic organotypic cell culture model

An organotypic cell culture (OCC) model of the rat hypothalamic paraventricular nucleus (PVN) was established to monitor intracellular calcium levels ([Ca(2+)](i)) of magnocellular neurons in response to glutamate and nitric oxide (NO). The histoarchitectural organization of these cultures was characterized either by immunohistochemical labeling of vasopressin, neuronal nitric oxide synthase (nNOS) and the neuronal marker NeuN or by the enzyme histochemical NADPH-diaphorase staining. A distinct NeuN positive cell population in 14-days old OCC's was confirmed as being the PVN by its vasopressin- and nNOS-immunostained neurons as well as by its NADPH-diaphorase labeling. Life cell imaging was performed using the [Ca(2+)](i) sensor Fluo-4 to measure [Ca(2+)](i) transients in response to bath applications of glutamate, high potassium (60 mM), and ATP. The glutamate-induced [Ca(2+)](i) response was mimicked by AMPA but not NMDA in the PVN. NMDA, however, elicited a [Ca(2+)](i) transient in a different area of the OCC that corresponds to the suprachiasmatic nucleus indicating the potential effectiveness of the stimulus. The AMPA-receptor blocker NBQX abolished the glutamate-induced response in the PVN. An inhibition of endogenous NO production by the NOS inhibitor L-NAME decreased the amplitude of AMPA- and glutamate-induced [Ca(2+)](i) rises. Taken together, these data suggest that AMPA mediates the glutamate-induced [Ca(2+)](i) rises within the PVN, where endogenous NO is able to modulate such glutamate signaling in OCC.

Examination of granule layer cell count, cell density, and single-pulse brdu incorporation in rat organotypic hippocampal slice cultures with respect to culture medium, septotemporal position, and time in vitro

Adult neurogenesis in the dentate gyrus is assuming an increasingly important role in supporting hippocampal-dependent learning and the modulation of mood and anxiety. Moreover, injury to the developing postnatal dentate gyrus has profound effects on neurogenesis and hippocampal learning throughout life. Organotypic hippocampal slice cultures represent an attractive model for studying neurogenesis both in the early postnatal and adult hippocampus, as they retain much of their anatomical and functional circuitry in vitro. Ongoing neurogenesis has been recently demonstrated in organotypic hippocampal slice cultures. However, cell proliferation, one of the critical components of neurogenesis, has yet to be characterized in this culture system. We examined single-pulse S-phase bromo-deoxyuridine (BrdU) labeling in the dentate granule layer with respect to the septotemporal position of origin of the slice culture, the medium the cultures were grown in, and the time the cultures were maintained in vitro up to 14 days, when they are believed to have matured to a near adult state. Using single 10-microm sections through a culture as our reference volume, we report significant effects of septotemporal position on the number of granule layer cells and the number of cells in S-phase, as estimated by short-survival (2 hours) BrdU studies. We report a declining rate of BrdU incorporation, evidence of significant structural changes within the granule cell layer, and differences in cell death between culture media over the first 14 days in vitro. We report caution with the use of BrdU cell density and changes in cell number to indirectly estimate proliferation.

Organotypic Hippocampal Slice Cultures: A Model System to Study Basic Cellular and Molecular Mechanisms of Neuronal Cell Death, Neuroprotection, and Synaptic Plasticity

The hippocampus has become one of the most extensively studied areas of the mammalian brain, and its proper function is of utmost importance, particularly for learning and memory. The hippocampus is the most susceptible brain region for damage, and its impaired function has been documented in many human brain diseases, e.g. hypoxia, ischemia, and epilepsy regardless of the age of the affected patients. In addition to experimental in vivo models of these disorders, the investigation of basic anatomical, physiological, and molecular aspects requires an adequate experimental in vitro model, which should meet the requirements for well-preserved representation of various cell types, and functional information processing properties in the hippocampus. In this review, the characteristics of organotypic hippocampal slice cultures (OHCs) together with the main differences between the in vivo and in vitro preparations are first briefly outlined. Thereafter, the use of OHCs in studies focusing on neuron cell death and synaptic plasticity is discussed.

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

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

Identification and characterization of two neurogenic zones in interface organotypic hippocampal slice cultures

Neurogenesis plays a role in many physiological (memory formation) and pathological (stroke, depression) processes. However the mechanisms of postnatal stem cell proliferation and neurogenesis are still poorly understood. We characterized early neurogenesis in vitro in rat organotypic hippocampal slice cultures. Proliferation was assessed by bromodeoxyuridine incorporation, neurogenesis by bromodeoxyuridine-double labeling with doublecortin or beta-III tubulin. We showed for the first time that in addition to the dentate gyrus organotypic hippocampal slice cultures include a second neurogenic zone: the posterior periventricle, which is a part of the lateral ventricle wall. This structure lining the stratum oriens contained Nestin+ precursors. We could identify morphological and functional differences between dentate gyrus and posterior periventricle precursor populations. Our data demonstrate that basic fibroblast growth factor treatment induced a fast but short-lasting neurogenic response in the dentate gyrus while the posterior periventricle showed a more pronounced and long lasting neurogenic effect of basic fibroblast growth factor. Thus two neurogenic zones with different neurogenic properties were identified in organotypic hippocampal slice cultures.

GABAergic Excitation Promotes Neuronal Differentiation in Adult Hippocampal Progenitor Cells

Hippocampal activity influences neurogenesis in the adult dentate gyrus; however, little is known about the involvement of the hippocampal circuitry in this process. In the subgranular zone of the adult dentate gyrus, neurogenesis involves a series of differentiation steps from radial glia-like stem/progenitor (type-1) cells, to transiently amplifying neuronal progenitor (type-2) cells, to postmitotic neurons. In this study, we conducted GFP-targeted recordings of progenitor cells in fresh hippocampal slices from nestin-GFP mice and found that neuronal progenitor (type-2) cells receive active direct neural inputs from the hippocampal circuitry. This input was GABAergic but not glutamatergic. The GABAergic inputs depolarized type-2 cells because of their elevated [Cl(-)](i). This excitation initiated an increase of [Ca(2+)](i) and the expression of NeuroD. A BrdU-pulse labeling study with GABA(A)-R agonists demonstrated the promotion of neuronal differentiation via this GABAergic excitation. Thus, it appears that GABAergic inputs to hippocampal progenitor cells promote activity-dependent neuronal differentiation.

Spontaneous recurrent network activity in organotypic rat hippocampal slices

Organotypic hippocampal slices were prepared from postnatal day 4 rats and maintained in culture for >6 weeks. Cultured slices exhibited from 12 days in vitro spontaneous events which closely resembled giant depolarizing potentials (GDPs) recorded in neonatal hippocampal slices. GDP-like events occurred over the entire hippocampus with a delay of 30-60 ms between two adjacent regions as demonstrated by pair recordings from CA3-CA3, CA3-CA1 and interneurone-CA3 pyramidal cells. As in acute slices, spontaneous recurrent events were generated by the interplay of GABA and glutamate acting on AMPA receptors as they were reversibly blocked by bicuculline and 6,7-dinitroquinoxaline-2,3-dione but not by dl-2-amino-5-phosphonopentaoic acid. The equilibrium potentials for GABA measured in whole cell and gramicidin-perforated patch from interconnected interneurones-CA3 pyramidal cells were -70 and -56 mV, respectively. The resting membrane potential estimated from the reversal of N-methyl-D-aspartate-induced single-channel currents in cell-attach experiments was -75 mV. In spite of its depolarizing action, in the majority of cases GABA was still inhibitory as it blocked the firing of principal cells. The increased level of glutamatergic connectivity certainly contributed to network synchronization and to the development of interictal discharges after prolonged exposure to bicuculline. In spite of its inhibitory action, in a minority of cells GABA was still depolarizing and excitatory as it was able to bring principal cells to fire, suggesting that a certain degree of immaturity is still present in cultured slices. This was in line with the transient bicuculline-induced block of GDPs and with the isoguvacine-induced increase of GDP frequency.