Proliferative status of cells in adult human dentate gyrus

Which neurodevelopmental processes continue in humans after birth?

Once we are born, the number and location of nerve cells in most parts of the brain remain unchanged. These types of structural changes are therefore a significant form of flexibility for the neural circuits where they occur. In humans, the postnatal birth of neurons is limited; however, neurons do continue to migrate into some brain regions throughout infancy and even into adolescence. In human infants, multiple migratory pathways deliver interneurons to destinations across the frontal and temporal lobe cortex. Shorter-range migration of excitatory neurons also appears to continue during adolescence, particularly near the amygdala paralaminar nucleus, a region that follows a delayed trajectory of growth from infancy to adulthood. The significance of the timing for when different brain regions recruit new neurons through these methods is unknown; however, both processes of protracted migration and maturation are prominent in humans. Mechanisms like these that reconfigure neuronal circuits are a substrate for critical periods of plasticity and could contribute to distinctive circuit functionality in human brains.

Extracranial 125I Seed Implantation Allows Non-invasive Stereotactic Radioablation of Hippocampal Adult Neurogenesis in Guinea Pigs

Adult hippocampal neurogenesis (AHN) is important for multiple cognitive functions. We sort to establish a minimal or non-invasive radiation approach to ablate AHN using guinea pigs as an animal model. ¹²⁵I seeds with different radiation dosages (1.0, 0.8, 0.6, 0.3 mCi) were implanted unilaterally between the scalp and skull above the temporal lobe for 30 and 60 days, with the radiation effect on proliferating cells, immature neurons, and mature neurons in the hippocampal formation determined by assessment of immunolabeled (+) cells for Ki67, doublecortin (DCX), and neuron-specific nuclear antigen (NeuN), as well as Nissl stain cells. Spatially, the ablation effect of radiation occurred across the entire rostrocaudal and largely the dorsoventral dimensions of the hippocampus, evidenced by a loss of DCX⁺ cells in the subgranular zone (SGZ) of dentate gyrus (DG) in the ipsilateral relative to contralateral hemispheres in reference to the ¹²⁵I seed implant. Quantitatively, Ki67⁺ and DCX⁺ cells at the SGZ in the dorsal hippocampus were reduced in all dosage groups at the two surviving time points, more significant in the ipsilateral than contralateral sides, relative to sham controls. NeuN⁺ neurons and Nissl-stained cells were reduced in the granule cell layer of DG and the stratum pyramidale of CA1 in the groups with 0.6-mCi radiation for 60 days and 1.0 mCi for 30 and 60 days. Minimal cranial trauma was observed in the groups with 0.3– 1.0-mCi radiation at 60 days. These results suggest that extracranial radiation with ¹²⁵I seed implantation can be used to deplete HAN in a radioactivity-, duration-, and space-controllable manner, with a “non-invasive” stereotactic ablation achievable by using ¹²⁵I seeds with relatively low radioactivity dosages.

Immunohistochemical evidence for adult human neurogenesis in health and disease

Postnatal and adult neurogenesis in the subventricular zone and subgranular zone of animals such as rodents and non-human primates has been observed with many different technical approaches. Since most techniques used in animals cannot be used in humans, the majority of human neurogenesis studies rely on postmortem immunohistochemistry. This technique is difficult in human tissue, due to poor and variable preservation of antigens and samples. Nevertheless, a survey of the literature reveals that most published studies provide evidence for childhood and adult neurogenesis in the human brain stem cell niches. There are some conflicting results even when assessing the same markers and when using the same antibodies. Focusing on immunohistochemical studies on post-mortem human sections, we discuss the relative robustness of the literature on adult neurogenesis. We also discuss the response of the subventricular and subgranular zones to human disease, showing that the two niches can respond differently and that the stage of disease impacts neurogenesis levels. Thus, we highlight strong evidence for adult human neurogenesis, discuss other work that did not find it, describe obstacles in analysis, and offer other approaches to evaluate the neurogenic potential of the subventricular and subgranular zones of Homo sapiens. This article is categorized under: Neurological Diseases > Stem Cells and Development Reproductive System Diseases > Stem Cells and Development.

Reviving through human hippocampal newborn neurons

Recent contradictory data has renewed discussion regarding the existence of adult hippocampal neurogenesis (AHN) in humans, i.e., the continued production of new neurons in the brain after birth. The present review revisits the debate of AHN in humans from a historical point of view in the face of contradictory evidence, analyzing the methods employed to investigate this phenomenon. Thus, to date, of the 57 studies performed in humans that we reviewed, 84% (48) concluded in favor of the presence of newborn neurons in the human adult hippocampus. Besides quality of the tissue (such as postmortem intervals below 26hours as well as tissue conservation and fixation), considerations for assessing and quantify AHN in the human brain require the use of stereology and toxicological analyses of clinical data of the patient.

Positive Controls in Adults and Children Support That Very Few, If Any, New Neurons Are Born in the Adult Human Hippocampus

Adult hippocampal neurogenesis was originally discovered in rodents. Subsequent studies identified the adult neural stem cells and found important links between adult neurogenesis and plasticity, behavior, and disease. However, whether new neurons are produced in the human dentate gyrus (DG) during healthy aging is still debated. We and others readily observe proliferating neural progenitors in the infant hippocampus near immature cells expressing doublecortin (DCX), but the number of such cells decreases in children and few, if any, are present in adults. Recent investigations using dual antigen retrieval find many cells stained by DCX antibodies in adult human DG. This has been interpreted as evidence for high rates of adult neurogenesis, even at older ages. However, most of these DCX-labeled cells have mature morphology. Furthermore, studies in the adult human DG have not found a germinal region containing dividing progenitor cells. In this Dual Perspectives article, we show that dual antigen retrieval is not required for the detection of DCX in multiple human brain regions of infants or adults. We review prior studies and present new data showing that DCX is not uniquely expressed by newly born neurons: DCX is present in adult amygdala, entorhinal and parahippocampal cortex neurons despite being absent in the neighboring DG. Analysis of available RNA-sequencing datasets supports the view that DG neurogenesis is rare or absent in the adult human brain. To resolve the conflicting interpretations in humans, it is necessary to identify and visualize dividing neuronal precursors or develop new methods to evaluate the age of a neuron at the single-cell level.

Intraventricular Medium B Treatment Benefits an Ischemic Stroke Rodent Model via Enhancement of Neurogenesis and Anti-apoptosis

Enhancement of endogenous neurogenesis after ischemic stroke may improve functional recovery. We previously demonstrated that medium B, which is a combination with epidermal growth factor (EGF) and fibronectin, can promote neural stem/progenitor cell (NSPC) proliferation and migration. Here, we showed that medium B promoted proliferation and migration of cultured NSPCs onto various 3-dimentional structures. When rat cortical neurons with oxygen glucose deprivation (OGD) were co-cultured with NSPCs, medium B treatment increased neuronal viability and reduced cell apoptosis. In a rat model with transient middle cerebral artery occlusion (MCAO), post-insult intraventricular medium B treatment enhanced proliferation, migration, and neuronal differentiation of NSPCs and diminished cell apoptosis in the infarct brain. In cultured post-OGD neuronal cells and the infarct brain from MCAO rats, medium B treatment increased protein levels of Bcl-xL, Bcl-2, phospho-Akt, phospho-GSK-3β, and β-catenin and decreased the cleaved caspase-3 level, which may be associated with the effects of anti-apoptosis. Notably, intraventricular medium B treatment increased neuronal density, improved motor function and reduced infarct size in MCAO rats. In summary, medium B treatment results in less neuronal death and better functional outcome in both cellular and rodent models of ischemic stroke, probably via promotion of neurogenesis and reduction of apoptosis.

Recalibrating the Relevance of Adult Neurogenesis

Conflicting reports about whether adult hippocampal neurogenesis occurs in humans raise questions about its significance for human health and the relevance of animal models. Drawing upon published data, I review species' neurogenesis rates across the lifespan and propose that accelerated neurodevelopmental timing is consistent with lower rates of neurogenesis in adult primates and humans. Nonetheless, protracted neurogenesis may produce populations of neurons that retain plastic properties for long intervals, and have distinct functions depending on when in the lifespan they were born. With some conceptual recalibration we may therefore be able to reconcile seemingly disparate findings and continue to ask how adult neurogenesis, as studied in animals, is relevant for human health.

Lateralized hippocampal volume increase following high-frequency left prefrontal repetitive transcranial magnetic stimulation in patients with major depression

AIM

Repetitive transcranial magnetic stimulation (rTMS) has been applied as a treatment for patients with treatment-resistant depression in recent years, and a large body of evidence has demonstrated its therapeutic efficacy through stimulating neuronal plasticity. The aim of this study was to investigate structural alterations in the hippocampus (HIPP) and amygdala (AM) following conventional rTMS in patients with depression.

METHODS

Twenty-eight patients with depression underwent 10 daily 20-Hz left prefrontal rTMS over 2 weeks. The left dorsolateral prefrontal cortex (DLPFC) was identified using magnetic resonance imaging-guided neuronavigation prior to stimulation. Magnetic resonance imaging scans were obtained at baseline and after the completion of rTMS sessions. The therapeutic effects of rTMS were evaluated with the 17-item Hamilton Depression Rating Scale (HAM-D₁₇ ), and the volumes of the HIPP and AM were measured by a manual tracing method.

RESULTS

Statistical analyses revealed a significant volume increase in the left HIPP (+3.4%) after rTMS but no significant volume change in the AM. No correlation was found between the left HIPP volume increase and clinical improvement, as measured by the HAM-D₁₇ .

CONCLUSION

The present study demonstrated that conventional left prefrontal rTMS increases the HIPP volume in the stimulated side, indicating a remote neuroplastic effect through the cingulum bundle.

The dynamics of adult neurogenesis in human hippocampus

The phenomenon of adult neurogenesis is now an accepted occurrence in mammals and also in humans. At least two discrete places house stem cells for generation of neurons in adult brain. These are olfactory system and the hippocampus. In animals, newly generated neurons have been directly or indirectly demonstrated to generate a significant amount of new neurons to have a functional role. However, the data in humans on the extent of this process is still scanty and such as difficult to comprehend its functional role in humans. This paper explores the available data on as extent of adult hippocampal neurogenesis in humans and makes comparison to animal data.

Alterations of neuronal precursor cells in stages of human adult neurogenesis in heroin addicts

BACKGROUND

Adult neurogenesis has been shown to occur throughout life and different brain pathologies were demonstrated to be associated with altered neurogenesis. Here, an impact of heroin addiction on neurogenesis in humans is hypothesised.

METHODS

Post mortem hippocampal specimens of drug addicts with known heroin abuse and a group of non-addictive control subjects were analysed, using antibodies indicating different stages of neurogenesis. The subgranular zone of the dentate gyrus was examined qualitatively and quantitatively.

RESULTS

The data indicate (i) a decreased number of neural precursor cells, (ii) accompanied by low rates of proliferation and (iii) a marked loss of dendritic trees in targeting cells in heroin fatalities. (iv) The age-dependent increase of differentiating cells in the healthy controls was not observed in the addicts. Additionally, double immunofluorescence labelling indicated the precursor nature of Musashi-1 positive cells in the human subgranular zone of the dentate gyrus.

CONCLUSIONS

Present data firstly demonstrate the influence of drug addiction with known heroin abuse on different developmental stages of progenitors in the dentate gyrus. The patterns of antibody staining suggest a distinct inhibition of neurogenesis at the stage of neural precursor cells and revealed morphological changes in targeting cells in cases of heroin addicts as compared to healthy controls. These alterations could be considerable for memory and cognitive deficits as well as addictive behaviour in chronic drug abusers and may give rise to specific pro-neurogenic therapies.

Increased Ki-67 immunoreactivity in the white matter in hemimegalencephaly

Hemimegalencephaly (HMG) is a developmental brain disorder characterized by an enlarged unilateral hemisphere with cortical malformation comprising abnormal hypertrophic cells. To address the proliferative status of HMG, Ki-67 immunoreactivity was investigated in HMG specimens obtained during epilepsy surgery. Nine HMG tissues were stained with a Ki-67 antibody and Ki-67 labeling index in the malformed cortex, and the underlying white matter was measured separately and compared with tissues from focal cortical dysplasias and normal brains from autopsy. In HMG tissues, Ki-67-positive cells were scattered in both the gray and white matter, with a significantly higher Ki-67 labeling index in the white matter compared with gray matter. No dysmorphic neuron or balloon cell was stained for Ki-67. As Ki-67 immunoreactivity overlapped with that of ionized calcium-binding adaptor protein-1, Ki-67-positive cells were identified as microglia. In HMG, microglia were activated and entered into a proliferative status with higher distribution in the white matter, implying an ongoing neuroinflammatory process involving the white matter.

Defining clinico-neuropathological subtypes of mesial temporal lobe epilepsy with hippocampal sclerosis

Hippocampal sclerosis (HS) is the most frequent cause of drug-resistant focal epilepsies (ie, mesial temporal lobe epilepsy with hippocampal sclerosis; mTLE-HS), and presents a broad spectrum of electroclinical, structural and molecular pathology patterns. Many patients become drug resistant during the course of the disease, and surgical treatment was proven helpful to achieve seizure control. Hence, up to 40% of patients suffer from early or late surgical failures. Different patterns of hippocampal cell loss, involvement of other mesial temporal structures, as well as temporal neocortex including focal cortical dysplasia, may contribute to the extent of the epileptogenic network and will be discussed. An international consensus is mandatory to clarify terminology use and to reliably distinguish mTLE-HS subtypes. High-resolution imaging with confirmed histopathologic diagnosis, as well as advanced neurophysiologic and molecular genetic measures, will be a powerful tool in the future to address these issues and help to predict each patient's probability to control their epilepsy in mTLE-HS conditions.

Adult human neurogenesis: from microscopy to magnetic resonance imaging

Neural stem cells reside in well-defined areas of the adult human brain and are capable of generating new neurons throughout the life span. In rodents, it is well established that the new born neurons are involved in olfaction as well as in certain forms of memory and learning. In humans, the functional relevance of adult human neurogenesis is being investigated, in particular its implication in the etiopathology of a variety of brain disorders. Adult neurogenesis in the human brain was discovered by utilizing methodologies directly imported from the rodent research, such as immunohistological detection of proliferation and cell-type specific biomarkers in postmortem or biopsy tissue. However, in the vast majority of cases, these methods do not support longitudinal studies; thus, the capacity of the putative stem cells to form new neurons under different disease conditions cannot be tested. More recently, new technologies have been specifically developed for the detection and quantification of neural stem cells in the living human brain. These technologies rely on the use of magnetic resonance imaging, available in hospitals worldwide. Although they require further validation in rodents and primates, these new methods hold the potential to test the contribution of adult human neurogenesis to brain function in both health and disease. This review reports on the current knowledge on adult human neurogenesis. We first review the different methods available to assess human neurogenesis, both ex vivo and in vivo and then appraise the changes of adult neurogenesis in human diseases.

Grape seed extract enhances neurogenesis in the hippocampal dentate gyrus in C57BL/6 mice

The effects of grape seed extract (GSE), a major source of phenolic compounds, were examined on cell proliferation, neuroblast differentiation and integration into granule cells in the hippocampal dentate gyrus (DG) of middle-aged (12 month-old) mice using Ki67, doublecortin (DCX) immunohistochemistry and 5'-bromo-2-deoxyguanosine (BrdU)/calbindin D-28k (CB) double immunofluorescence study, respectively. GSE (25, 50 and 100 mg/kg) was administered orally for 28 days, and the animals were treated with 50 mg/kg BrdU intraperitoneally on the day of first GSE treatment. In the vehicle-treated group, Ki67 and DCX immunoreactivity was detected in the subgranular zone of the DG (SZDG). GSE treatment dose-dependently increased the number of Ki67 and DCX immunoreactive cells, particularly the number of DCX immunoreactive neuroblasts with well-developed (tertiary) dendrites. GSE also dose-dependently increased DCX protein levels. In addition, GSE treatment increased significantly the number of BrdU/CB double labeled granule cells. These results suggest that GSE significantly increases cell proliferation, neuroblast differentiation and integration into granule cells in the DG, and the consumption of GSE enhances the plasticity of hippocampus in middle-aged mice.

NG2+/Olig2+ cells are the major cycle-related cell population of the adult human normal brain

A persistent cycling cell population in the normal adult human brain is well established. Neural stem cells or neural progenitors have been identified in the subventricular zone and the dentate gyrus subgranular layer (SGL), two areas of persistent neurogenesis. Cycling cells in other human normal brain areas, however, remains to be established. Here, we determined the distribution and identity of these cells in the cortex, the white matter and the hippocampal formation of adult patients with and without chronic temporal lobe epilepsy using immunohistochemistry for the cell cycle markers Ki-67 (Mib-1) and minichromosome maintenance protein 2. Rare proliferative neuronal precursors expressing the neuronal antigen neuronal nuclei were restricted to the SGL. In contrast, the oligodendrocyte progenitor cell markers Olig2 and the surface antigen NG2 were expressed by the vast majority of cycling cells scattered throughout the cortex and white matter of both control and epileptic patients. Most of these cycling cells were in early G1 phase, and were significantly more numerous in epileptic than in non-epileptic patients. These results provide evidence for a persistent gliogenesis in the human cortex and white matter that is enhanced in an epileptic environment.

Neurogenesis in the human hippocampus and its relevance to temporal lobe epilepsies

Ample evidence points to the dentate gyrus as anatomical region for persistent neurogenesis in the adult mammalian brain. This has been confirmed in a variety of animal models under physiological as well as pathophysiological conditions. Notwithstanding, similar experiments are difficult to perform in humans. Postmortem studies demonstrated persisting neurogenesis in the elderly human brain. In addition, neural precursor cells can be isolated from surgical specimens obtained from patients with intractable temporal lobe epilepsy (TLE) and propagated or differentiated into neuronal and glial lineages. It remains a controversial issue, whether epileptic seizures have an effect on or even increase hippocampal neurogenesis in humans. Recent data support the notion that seizures induce neurogenesis in young patients, whereas the capacity of neuronal recruitment and proliferation decreases with age. Animal models of TLE further indicate that these newly generated neurons integrate into epileptogenic networks and contribute to increased seizure susceptibility. However, pathomorphological disturbances within the epileptic hippocampus, such as granule cell dispersion (GCD), may not directly result from compromised neurogenesis. Still, the majority of adult TLE patients present with significant dentate granule cell loss at an end stage of the disease, which relates to severe memory and learning disabilities. In conclusion, surgical specimens obtained from TLE patients represent an important tool to study mechanisms of stem cell recruitment, proliferation and differentiation in the human brain. In addition, increasing availability of surgical specimens opens new avenues to systematically explore disease pathomechanisms in chronic epilepsies.

Changes in adult neurogenesis in neurodegenerative diseases: cause or consequence?

This review addresses the role of adult hippocampal neurogenesis and stem cells in some of the most common neurodegenerative disorders and their related animal models. We discuss recent literature in relation to Alzheimer's disease and dementia, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, alcoholism, ischemia, epilepsy and major depression.

What causes the hippocampal volume decrease in depression? Are neurogenesis, glial changes and apoptosis implicated?

Even though in vivo imaging studies document significant reductions of hippocampal volume in depressed patients, the exact underlying cellular mechanisms are unclear. Since stressful life events are associated with an increased risk of developing depression, preclinical studies in which animals are exposed to chronic stress have been used to understand the hippocampal shrinkage in depressed patients. Based on morphometrical studies in these models, parameters like dendritic retraction, suppressed adult neurogenesis and neuronal death, all due to elevated levels of glucocorticoids, have been suggested as major causative factors in hippocampal shrinkage. However, histopathological studies examining hippocampi of depressed individuals have so far failed to confirm either a massive neuronal loss or a suppression of dentate neurogenesis, an event that is notably very rare in adult or elderly humans. In fact, many of the structural changes and the volume reduction appear to be reversible. Clearly, more histopathological studies are needed; especially ones that (a) employ stereological quantification, (b) focus on specific cellular elements and populations, and (c) are performed in nonmedicated depressed patients. We conclude that mainly other factors, like alterations in the somatodendritic, axonal, and synaptic components and putative glial changes are most likely to explain the hippocampal shrinkage in depression, while shifts in fluid balance or changes in the extracellular space cannot be excluded either.

Changes in neurogenesis in dementia and Alzheimer mouse models: are they functionally relevant?

Alzheimer's disease and related dementias are devastating disorders that lead to the progressive decline of cognitive functions. Characteristic features are severe brain atrophy, paralleled by accumulation of beta amyloid and neurofibrillary tangles. With the discovery of neurogenesis in the adult brain, the hopes have risen that these neurodegenerative conditions could be overcome, or at least ameliorated, by the generation of new neurons. The location of the adult neurogenic zones in the hippocampus and the lateral ventricle wall, close to corpus callosum and neocortex, indicates strategic positions for potential repair processes. However, we also need to consider that the generation of new neurons is possibly involved in cognitive functions and could, therefore, be influenced by disease pathology. Moreover, aberrant neurogenic mechanisms could even be a part of the pathological events of neurodegenerative diseases. It is the scope of this review to summarize and analyze the recent data from neurogenesis research with respect to Alzheimer's disease and its animal models.

Evidence of increased cell proliferation in the hippocampus in children with Ammon's horn sclerosis

Previous studies of Ammon's horn sclerosis (AHS) suggest that AHS is both the result of and the cause of seizures, and support the idea that seizures cause alterations in cell numbers and location. To test the hypothesis that epilepsy induces neurogenesis/gliogenesis, hippocampal cell proliferation was assessed in AHS. Twelve and four resected hippocampi in patients with AHS and with tumor-related epilepsy (TRE), respectively, and 11 autopsy controls were immunostained for Ki-67. Total number of Ki-67-positive cells (KiPC) in each hippocampal area was counted. Selected cases were further studied with double immunohistochemical labeling. KiPC were observed in all three groups. Total numbers of KiPC were significantly higher in AHS cases than in controls, but were not significantly different between TRE cases and controls. Significant differences were observed in the dentate gyrus, the cornu ammonis (CA)-4 region, and the fissura hippocampi between the AHS and control groups. In double immunolabeling, nestin was positive in some KiPC. The existence of neurogenesis/gliogenesis was shown in the hippocampi of pediatric patients with AHS. Increased numbers of progenitor cells in the hippocampi with AHS appear not to be due to seizures per se, but to be more associated with the specific cause of epilepsy.

Absent or low rate of adult neurogenesis in the hippocampus of bats (Chiroptera)

Bats are the only flying mammals and have well developed navigation abilities for 3D-space. Even bats with comparatively small home ranges cover much larger territories than rodents, and long-distance migration by some species is unique among small mammals. Adult proliferation of neurons, i.e., adult neurogenesis, in the dentate gyrus of rodents is thought to play an important role in spatial memory and learning, as indicated by lesion studies and recordings of neurons active during spatial behavior. Assuming a role of adult neurogenesis in hippocampal function, one might expect high levels of adult neurogenesis in bats, particularly among fruit- and nectar-eating bats in need of excellent spatial working memory. The dentate gyrus of 12 tropical bat species was examined immunohistochemically, using multiple antibodies against proteins specific for proliferating cells (Ki-67, MCM2), and migrating and differentiating neurons (Doublecortin, NeuroD). Our data show a complete lack of hippocampal neurogenesis in nine of the species (Glossophaga soricina, Carollia perspicillata, Phyllostomus discolor, Nycteris macrotis, Nycteris thebaica, Hipposideros cyclops, Neoromicia rendalli, Pipistrellus guineensis, and Scotophilus leucogaster), while it was present at low levels in three species (Chaerephon pumila, Mops condylurus and Hipposideros caffer). Although not all antigens were recognized in all species, proliferation activity in the subventricular zone and rostral migratory stream was found in all species, confirming the appropriateness of our methods for detecting neurogenesis. The small variation of adult hippocampal neurogenesis within our sample of bats showed no indication of a correlation with phylogenetic relationship, foraging strategy, type of hunting habitat or diet. Our data indicate that the widely accepted notion of adult neurogenesis supporting spatial abilities needs to be considered carefully. Given their astonishing longevity, certain bat species may be useful subjects to compare adult neurogenesis with other long-living species, such as monkeys and humans, showing low rates of adult hippocampal neurogenesis.

Proliferation and apoptosis of hippocampal granule cells require local oestrogen synthesis

Ovarian oestrogens have been demonstrated to influence neurogenesis in the dentate gyrus. As considerable amounts of oestrogens are synthesized in hippocampal neurones, we focused on the role of hippocampus-derived estradiol on proliferation and apoptosis of granule cells in vitro. We used hippocampal dispersion cultures, which allowed for cultivation of the cells under steroid- and serum-free conditions and monitoring of oestrogen synthesis. To address the influence of hippocampus-derived estradiol on neurogenesis, we inhibited oestrogen synthesis by treatment of hippocampal cell cultures with letrozole, a specific aromatase inhibitor. Alternatively, we used siRNA against steroidogenic acute regulatory protein (StAR). The number of proliferative cells decreased whereas the number of apoptotic cells increased dose-dependently, in response to reduced estradiol release into the medium after treatment with letrozole. This also held true for siRNA against StAR transfected cell cultures. Application of estradiol to the medium had no effect on proliferation and apoptosis whereas the anti-proliferative and pro-apoptotic effects of StAR knock-down and letrozole treatment were restored by treatment of the cultures with estradiol. Our findings suggest that neurogenesis and apoptosis in the hippocampus require a defined range of estradiol concentrations that is physiologically provided by hippocampal cells but not by gonads.

Increased number of neural progenitors in human temporal lobe epilepsy

An increased neurogenesis is reported in animal models of mesial temporal lobe epilepsy (MTLE) but the fate of newborn cells is unknown. Here, we attempted to demonstrate neurogenesis in adult epileptic tissue obtained after hippocampectomy. MTLE hippocampi showed increased expression of division markers and of Musashi-1, a marker of neural progenitors, compared to control hippocampi. Large quantities of Musashi-1+ cells were obvious in the subgranular layer and the subventricular zone, both known neurogenic areas, and in the fissura hippocampi. Musashi-1 was expressed by small cells that were mainly vimentin+ or nestin+, rarely Dcx+ or PSA-NCAM+ and negative for markers of mature neurons or astrocytes. Some of them are present in the granular layer, the hilus, and CA1 area resembling the ectopic positions described in rodents. These findings demonstrate that neural progenitors proliferate in chronic epilepsy and suggest that the fissura hippocampi behaves like another neurogenic area.

Cell proliferation and granule cell dispersion in human hippocampal sclerosis

Granule cell dispersion (GCD) is observed in approximately 40% of cases of hippocampal sclerosis (HS) in patients with epilepsy. Studies in animal models suggest that GCD may be a consequence of enhanced proliferation of granule cell precursors as a result of seizures. We quantified the number of cells in cycle in subfields of the hippocampus with immunohistochemistry for Mcm2 in 14 HS cases with or without severe GCD compared to 6 epilepsy patients without classical HS or GCD as well as 5 postmortem controls. Higher numbers of Mcm2-positive cells were seen in the region of the granule cell layer in patients with severe GCD, and immunolabeling with Geminin and Ki-67 confirmed a proportion were progressing through cycle. Double labeling with Mcm2 and GFAP confirmed the majority of these cycling cells were GFAP-negative and occasional cells stained colocalized with stem cell marker nestin. These findings support the view that GCD may be a phenomenon related to increased progenitor cell proliferation in patients with hippocampal damage and chronic epilepsy.

Ki67 and doublecortin positive cells in the human prefrontal cortices of normal aging and vascular dementia

Immunohistochemical localizing of the proliferation of Ki67 nuclei and doublecortin positive cells were performed in the prefrontal cortex of normal aged and vascular dementia (multiple infarct dementia) patients. Positive Ki67 nuclei and doublecortin positive cells were observed in both groups, with slightly higher density in the prefrontal cortex of vascular dementia. When the Ki67 sites were superimposed with the neuronal specific enolase localizations, only about 5% of the cells was doubly labeled, indicating few proliferating cells were neurons. This percentage did not vary between specimens of normal aging and those of vascular dementia.

Apoptosis and neurogenesis after transient hypoxia in the developing rat brain

Perinatal brain damage following a hypoxic-ischemic episode has been considered for a long time as an irreversible phenomenon. However, recent studies have shown that various insults may induce de novo neurogenesis in the adult rodent brain. The present study tested the hypothesis that acute hypoxia may trigger neurogenesis in the developing brain. In vitro, the influence of transient hypoxia was analyzed on the outcome of embryonic rat neurons in culture. In vivo, the temporal profile of brain damage was monitored at the level of the CA1 layer of the hippocampus after the exposure to hypoxia of 1-day-old rats. The extent of cell loss and regeneration was evaluated after staining with DAPI. The characterization of newly generated cells was performed in the subventricular zone at 20 days postexposure by immunohistochemistry. Following hypoxia for 6 hours, neuronal viability in the culture dishes was reduced by 36% at 96 hours, with a significant number of cell nuclei showing apoptosis features. In contrast, a 3-hour hypoxia apparently did not damage cultured neurons whose number increased by 14%. The Bax/Bcl-2 ratio tended to increase after 6-hour hypoxia and to decrease after 3-hour hypoxia. In vivo, hypoxia induced cell damage in the CA1 subfield of the hippocampus, where the total number of cells was reduced by 27% at days 6-7 postreoxygenation, with histopathological hallmarks of apoptosis. This cell deficit was followed by a gradual recovery observable from day 20, suggesting a repair mechanism. Brain incorporation of BrdU in the subventricular zone revealed an accumulation of proliferating cells expressing the neuronal marker NeuroD. The present data demonstrate that a posthypoxic neurogenesis does occur during development and may account for brain protection.

Ammon's horn sclerosis: a maldevelopmental disorder associated with temporal lobe epilepsy

Ammon's horn sclerosis (AHS) is the major neuropathological substrate in patients with temporal lobe epilepsy (TLE). Histopathological hallmarks include segmental loss of pyramidal neurons, granule cell dispersion and reactive gliosis. Pathogenetic mechanisms underlying this distinct hippocampal pathology have not yet been identified and it remains to be resolved whether AHS represents the cause or the consequence of chronic seizure activity and pharmacoresistant TLE. Whereas the clinical history indicates an early onset in most patients, ie, occurrence of febrile seizures at a young age, surgical treatment is usually carried out at an end stage of the disease. It has, therefore, been difficult to analyse the sequential development of hippocampal pathology in TLE patients. Recent molecular neuropathological studies focusing on developmental aspects of hippocampal organization revealed 2 intriguing findings in AHS specimens: i) The persistence of Cajal-Retzius cells in AHS patients points towards an early insult and an altered Reelin signaling pathway and ii) increased neurogenesis in and abnormal architectural organization of the dentate granule cell layer can be observed in young patients with early hippocampal seizure onset. These findings would be compatible with a model that involves a neurodevelopmental component in the formation of AHS. Its association with a lowered seizure threshold and an increased susceptibility for segmental cell loss in the hippocampus during the long course of the disease may constitute additional elements in a pathogenic cascade.

Is epilepsy a progressive disorder? Prospects for new therapeutic approaches in temporal-lobe epilepsy

During the past decade, it has become apparent that neural circuits undergo activity-dependent reorganisation. In pathological disorders with recurring episodes of excessive neural activity, such as temporal-lobe epilepsy, brain circuits can undergo continual remodelling. For clinical practice, seizure-induced remodelling implies that after a diagnosis of epilepsy, recurring seizures can cause continuing neural reorganisation and potentially contribute to progressive severity of the epilepsy and to cognitive and behavioural consequences. The alterations induced by seizures include neuronal death and birth, axonal and dendritic sprouting, gliosis, molecular reorganisation of membrane and extracellular-matrix proteins, and intermediates involved in cellular homoeostasis. These changes are influenced by genetic background and seizure type, thus identification of genetic risk factors should be a priority. Therapeutic modification of seizure-induced molecular and cellular responses offers new opportunities for intervention beyond seizure suppression.

Cytoarchitectural abnormalities in hippocampal sclerosis

Hippocampal sclerosis (HS) is the most common pathological substrate for temporal lobe epilepsy with a characteristic pattern of loss of principle neurons primarily in CA1 and hilar subfields. Other cytoarchitectural abnormalities have been identified in human HS specimens, including dispersion of dentate granule cells and cytoskeletal abnormalities in residual hilar cells. The incidence of these features, their relationship to the severity of HS and potential indication of underlying hippocampal maldevelopment is unverified. In a series of 183 hippocampectomies we identified classical HS (grades 3 and 4) in 90% of specimens, granule cell disorganization or severe dispersion in 40% of cases with a bilaminar pattern in 10%, and cytoskeletal abnormalities in hilar cells in 55% of cases. The severity of granule cell disorganization correlated closely with the degree of hippocampal neuronal loss but not with the age at first seizure or a history of a precipitating event for epilepsy such as prolonged febrile seizures. These findings suggest that granule cell disorganization is closely linked with the progression of HS rather than a hallmark of impaired hippocampal maturation. Furthermore, stereological quantitation of granule cells showed evidence of cell loss but greater numbers in regions of maximal dispersion, which may indicate enhanced neurogenesis of these cells. Quantitation of reelin-and calretinin-positive Cajal-Retzius cells in the dentate gyrus molecular layer in 26 cases showed no correlation between the number of these cells and the severity of granule cell dispersion, but increased numbers of these cells were present in HS with respect to control groups. Although a role for Cajal-Retzius cells is therefore not implicated in the mechanism of granule cell disorganization, their excess number may be indicative of underlying hippocampal maldevelopment in HS.

Immunohistochemical localization of GDNF in the human hippocampal formation from prenatal life to adulthood

In this study, we examined the immunohistochemical occurrence and distribution of glial cell line-derived neurotrophic factor (GDNF) in autoptic specimens of normal human hippocampus at different ages, from 22 weeks of gestation (w.g.) to adult life. Two different anti-GDNF polyclonal antibodies were used. Western blot analysis on homogenates of human and rat brain and recombinant human GDNF resulted in differential detection of monomeric and dimeric forms of the proteins. The ABC immunohistochemical technique revealed that in the Ammon's horn, numerous positive cell bodies occurred in the pyramidal layer, the majority of them being present in the proximal CA1 and in CA2. Sparse positive neurons could be observed in the stratum oriens and moleculare. In the fascia dentata many granule cells showed a light punctate staining, whereas more heavily labelled neurons occurred in the polymorphic layer and, occasionally, in the molecular layer. The distribution pattern of GDNF-like immunoreactivity appeared consistently similar throughout life stages from 29 w.g. to adult age. However, intensity of labelling and frequency of neuronal cell bodies was highest in the neonate and decreased in adulthood. The present data provide a comprehensive map of the localization of GDNF-like immunoreactive neurons in the human archicortex at developmental ages and in the mature tissue and represent a first step towards the identification of hippocampal neurons which express the protein and/or are responsive to it. They further suggest that GDNF may play a role in the development of intrahippocampal circuitry and in neuronal function and maintenance throughout life.

Spatial learning affects immature granule cell survival in adult rat dentate gyrus

Neurogenesis occurs throughout life in mammalian dentate gyrus. The effect of learning on newborn cell survival was studied in rat. Rats were trained on a hippocampus-dependent spatial learning task by using Morris water maze. Neurogenesis was evaluated by 5-bromo-2'deoxyuridine administered before learning. Several newborn cells expressed the immature neuron marker TOAD-64. The main findings were as follows: (1) the survival of newborn cells was enhanced by learning at early stage of differentiation; (2) the newborn cells saved by learning were mainly located in the rostral part of external blade of granule cell layer and (3) there was a correlation between the actual individual learning and newborn cell survival.