Doublecortin (DCX) immunoreactivity in hippocampus of chronic refractory temporal lobe epilepsy patients with hippocampal sclerosis

Long-Term Neuropsychiatric Developmental Defects after Neonatal Organophosphate Exposure: Mitigation by Synthetic Neurosteroids

Children are much more susceptible to the neurotoxic effects of organophosphate (OP) pesticides and nerve agents than adults. OP poisoning in children leads to acute seizures and neuropsychiatric sequela, including the development of long-term disabilities and cognitive impairments. Despite these risks, there are few chronic rodent models that use pediatric OP exposure for studying neurodevelopmental consequences and interventions. Here, we investigated the protective effect of the neurosteroid ganaxolone (GX) on the long-term developmental impact of neonatal exposure to the OP compound, diisopropyl-fluorophosphate (DFP). Pediatric postnatal day-28 rats were acutely exposed to DFP, and at 3 and 10 months after exposure, they were evaluated using a series of cognitive and behavioral tests with or without the postexposure treatment of GX. Analysis of the neuropathology was performed after 10 months. DFP-exposed animals displayed significant long-term deficits in mood, anxiety, depression, and aggressive traits. In spatial and nonspatial cognitive tests, they displayed striking impairments in learning and memory. Analysis of brain sections showed significant loss of neuronal nuclei antigen(+) principal neurons, parvalbumin(+) inhibitory interneurons, and neurogenesis, along with increased astrogliosis, microglial neuroinflammation, and mossy fiber sprouting. These detrimental neuropathological changes are consistent with behavioral dysfunctions. In the neurosteroid GX-treated cohort, behavioral and cognitive deficits were significantly reduced and were associated with strong protection against long-term neuroinflammation and neurodegeneration. In conclusion, this pediatric model replicates the salient features of children exposed to OPs, and the protective outcomes from neurosteroid intervention support the viability of developing this strategy for mitigating the long-term effects of acute OP exposure in children. SIGNIFICANCE STATEMENT: An estimated 3 million organophosphate exposures occur annually worldwide, with children comprising over 30% of all victims. Our understanding of the neurodevelopmental consequences in children exposed to organophosphates is limited. Here, we investigated the long-term impact of neonatal exposure to diisopropyl-fluorophosphate in pediatric rats. Neurosteroid treatment protected against major deficits in behavior and memory and was well correlated with neuropathological changes. Overall, this pediatric model is helpful to screen novel therapies to mitigate long-term developmental deficits of organophosphate exposure.

Methods to study adult hippocampal neurogenesis in humans and across the phylogeny

The hippocampus hosts the continuous addition of new neurons throughout life-a phenomenon named adult hippocampal neurogenesis (AHN). Here we revisit the occurrence of AHN in more than 110 mammalian species, including humans, and discuss the further validation of these data by single-cell RNAseq and other alternative techniques. In this regard, our recent studies have addressed the long-standing controversy in the field, namely whether cells positive for AHN markers are present in the adult human dentate gyrus (DG). Here we review how we developed a tightly controlled methodology, based on the use of high-quality brain samples (characterized by short postmortem delays and ≤24 h of fixation in freshly prepared 4% paraformaldehyde), to address human AHN. We review that the detection of AHN markers in samples fixed for 24 h required mild antigen retrieval and chemical elimination of autofluorescence. However, these steps were not necessary for samples subjected to shorter fixation periods. Moreover, the detection of labile epitopes (such as Nestin) in the human hippocampus required the use of mild detergents. The application of this strictly controlled methodology allowed reconstruction of the entire AHN process, thus revealing the presence of neural stem cells, proliferative progenitors, neuroblasts, and immature neurons at distinct stages of differentiation in the human DG. The data reviewed here demonstrate that methodology is of utmost importance when studying AHN by means of distinct techniques across the phylogenetic scale. In this regard, we summarize the major findings made by our group that emphasize that overlooking fundamental technical principles might have consequences for any given research field.

An assessment of the existence of adult neurogenesis in humans and value of its rodent models for neuropsychiatric diseases

In sub-mammalian vertebrates like fishes, amphibians, and reptiles, new neurons are produced during the entire lifespan. This capacity diminishes considerably in birds and even more in mammals where it persists only in the olfactory system and hippocampal dentate gyrus. Adult neurogenesis declines even more drastically in nonhuman primates and recent evidence shows that this is basically extinct in humans. Why should such seemingly useful capacity diminish during primate evolution? It has been proposed that this occurs because of the need to retain acquired complex knowledge in stable populations of neurons and their synaptic connections during many decades of human life. In this review, we will assess critically the claim of significant adult neurogenesis in humans and show how current evidence strongly indicates that humans lack this trait. In addition, we will discuss the allegation of many rodent studies that adult neurogenesis is involved in psychiatric diseases and that it is a potential mechanism for human neuron replacement and regeneration. We argue that these reports, which usually neglect significant structural and functional species-specific differences, mislead the general population into believing that there might be a cure for a variety of neuropsychiatric diseases as well as stroke and brain trauma by genesis of new neurons and their incorporation into existing synaptic circuitry.

Evidences for Adult Hippocampal Neurogenesis in Humans

The rodent hippocampus generates new neurons throughout life. This process, named adult hippocampal neurogenesis (AHN), is a striking form of neural plasticity that occurs in the brains of numerous mammalian species. Direct evidence of adult neurogenesis in humans has remained elusive, although the occurrence of this phenomenon in the human dentate gyrus has been demonstrated in seminal studies and recent research that have applied distinct approaches to birthdate newly generated neurons and to validate markers of adult-born neurons. Our data point to the persistence of AHN until the 10th decade of human life, as well as to marked impairments in this process in patients with Alzheimer's disease. Moreover, our work demonstrates that the methods used to process and analyze postmortem human brain samples can limit the detection of various markers of AHN to the point of making them undetectable. In this Dual Perspectives article, we highlight the critical methodological aspects that should be strictly controlled in human studies and the robust evidence that supports the occurrence of AHN in humans. We also put forward reasons that may account for current discrepancies on this topic. Finally, the unresolved questions and future challenges awaiting the field are highlighted.

Exosomal 2',3'-CNP from mesenchymal stem cells promotes hippocampus CA1 neurogenesis/neuritogenesis and contributes to rescue of cognition/learning deficiencies of damaged brain

Mesenchymal stem cells (MSCs) have been used in clinical studies to treat neurological diseases and damage. However, implanted MSCs do not achieve their regenerative effects by differentiating into and replacing neural cells. Instead, MSC secretome components mediate the regenerative effects of MSCs. MSC-derived extracellular vesicles (EVs)/exosomes carry cargo responsible for rescuing brain damage. We previously showed that EP₄ antagonist-induced MSC EVs/exosomes have enhanced regenerative potential to rescue hippocampal damage, compared with EVs/exosomes from untreated MSCs. Here we show that EP₄ antagonist-induced MSC EVs/exosomes promote neurosphere formation in vitro and increase neurogenesis and neuritogenesis in damaged hippocampi; basal MSC EVs/exosomes do not contribute to these regenerative effects. 2',3'-Cyclic nucleotide 3'-phosphodiesterase (CNP) levels in EP₄ antagonist-induced MSC EVs/exosomes are 20-fold higher than CNP levels in basal MSC EVs/exosomes. Decreasing elevated exosomal CNP levels in EP₄ antagonist-induced MSC EVs/exosomes reduced the efficacy of these EVs/exosomes in promoting β3-tubulin polymerization and in converting toxic 2',3'-cAMP into neuroprotective adenosine. CNP-depleted EP₄ antagonist-induced MSC EVs/exosomes lost the ability to promote neurogenesis and neuritogenesis in damaged hippocampi. Systemic administration of EV/exosomes from EP₄ -antagonist derived MSC EVs/exosomes repaired cognition, learning, and memory deficiencies in mice caused by hippocampal damage. In contrast, CNP-depleted EP₄ antagonist-induced MSC EVs/exosomes failed to repair this damage. Exosomal CNP contributes to the ability of EP₄ antagonist-elicited MSC EVs/exosomes to promote neurogenesis and neuritogenesis in damaged hippocampi and recovery of cognition, memory, and learning. This experimental approach should be generally applicable to identifying the role of EV/exosomal components in eliciting a variety of biological responses.

Depression and Temporal Lobe Epilepsy: Expression Pattern of Calbindin Immunoreactivity in Hippocampal Dentate Gyrus of Patients Who Underwent Epilepsy Surgery with and without Comorbid Depression

Purpose

Changes in calbindin (CB) expression have been reported in patients with temporal lobe epilepsy (TLE) with controversial implications on hippocampal functions. The aim of this study was to determine the CB immunoreactivity in hippocampal dentate gyrus of patients who underwent epilepsy surgery for drug-resistant TLE with and without comorbid depression and/or memory deficits.

Methods

Selected hippocampal samples from patients with TLE who underwent epilepsy surgery were included. Clinical and complementary assessment: EEG, video-EEG, MRI, psychiatric assessment (structured clinical interview, DSM-IV), and memory assessment (Rey auditory verbal learning test, RAVLT; Rey-Osterrieth complex figure test, RCFT), were determined before surgery. Hippocampal sections were processed using immunoperoxidase with the anti-calbindin antibody. The semiquantitative analysis of CB immunoreactivity was determined in dentate gyrus by computerized image analysis (ImageJ).

Results

Hippocampal sections of patients with TLE and HS (n = 24) and postmortem controls (n = 5) were included. A significant reduction of CB+ cells was found in patients with TLE (p < 0.05, Student's t-test). Among TLE cases (n = 24), depression (n = 12) and memory deficit (n = 17) were determined. Depression was associated with a higher % of cells with the CB dendritic expression (CB-sprouted cells) (F(1, 20) = 11.81, p = 0.003, hp² = 0.37), a higher CB+ area (μm²) (F(1, 20) = 5.33, p = 0.032, hp² = 0.21), and a higher optical density (F(1, 20) = 15.09, p = 0.001, hp² = 0.43) (two-way ANOVA). The GAF scale (general assessment of functioning) of DSM-IV inversely correlated with the % of CB-sprouted cells (r = −0.52, p = 0.008) and with the CB+ area (r = −0.46, p = 0.022).

Conclusions

In this exploratory study, comorbid depression was associated with a differential pattern of CB cell loss in dentate gyrus combined with a higher CB sprouting. These changes may indicate granular cell dysmaturation associated to the epileptic hyperexcitability phenomena. Further investigations should be carried out to confirm these preliminary findings.

Sustained somatostatin gene expression reverses kindling-induced increases in the number of dividing Type-1 neural stem cells in the hippocampi of behaviorally responsive rats

Neurogenesis persists throughout life in the hippocampi of all mammals, including humans. In the healthy hippocampus, relatively quiescent Type-1 neural stem cells (NSCs) can give rise to more proliferative Type-2a neural progenitor cells (NPCs), which generate neuronal-committed Type-2b NPCs that mature into Type-3 neuroblasts. Many Type-3 neuroblasts survive and mature into functionally integrated granule neurons over several weeks. In kindling models of epilepsy, neurogenesis is drastically upregulated and many new neurons form aberrant connections that could support epileptogenesis and/or seizures. We have shown that sustained vector-mediated hippocampal somatostatin (SST) expression can both block epileptogenesis and reverse seizure susceptibility in fully kindled rats. Here we test whether adeno-associated virus (AAV) vector-mediated sustained SST expression modulates hippocampal neurogenesis and microglial activation in fully kindled rats. We found significantly more dividing Type-1 NSCs and a corresponding increased number of surviving new neurons in the hippocampi of kindled versus sham-kindled rats. Increased numbers of activated microglia were found in the granule cell layer and hilus of kindled rats at both time points. After intrahippocampal injection with either eGFP or SST-eGFP vector, we found similar numbers of dividing Type-1 NSCs and -2 NPCs and surviving BrdU⁺ neurons and glia in the hippocampi of kindled rats. Upon observed variability in responses to SST-eGFP (2/4 rats exhibited Grade 0 seizures in the test session), we conducted an additional experiment. We found significantly fewer dividing Type-1 NSCs in the hippocampi of SST-eGFP vector-treated responder rats (5/13 rats) relative to SST-eGFP vector-treated non-responders and eGFP vector-treated controls that exhibited high-grade seizures on the test session. The number of activated microglia was upregulated in the GCL and hilus of kindled rats, regardless of vector treatment. These data support the hypothesis that sustained SST expression exerts antiepileptic effects potentially through normalization of neurogenesis and suggests that abnormally high proliferating Type-1 NSC numbers may be a cellular mechanism of epilepsy.

Doublecortin-expressing cell types in temporal lobe epilepsy

Doublecortin (DCX) is widely regarded as a marker of immature and migrating neurons during development. While DCX expression persists in adults, particularly in the temporal lobe and neurogenic regions, it is unknown how seizures influence its expression. The aim of the present study was to explore the distribution and characteristics of DCX-expressing cells in surgical and postmortem samples from 40 adult and paediatric patients, with epilepsy and with or without hippocampal sclerosis (HS), compared to post mortem controls. The hippocampus (pes and body), parahippocampal gyrus, amygdala, temporal pole and temporal cortex were examined with DCX immunohistochemistry using four commercially-available DCX antibodies, labelled cells were quantified in different regions of interest as well as their co-expression with cell type specific markers (CD68, Iba1, GFAP, GFAP∂, nestin, SOX2, CD34, OLIG2, PDGFRβ, NeuN) and cell cycle marker (MCM2). Histological findings were compared with clinical data, as well as gene expression data obtained from the temporal cortex of 83 temporal lobe epilepsy cases with HS. DCX immunohistochemistry identified immature (Nestin⁻/NeuN⁻) neurons in layer II of the temporal neocortex in patients with and without epilepsy. Their number declined significantly with age but was not associated with the presence of hippocampal sclerosis, seizure semiology or memory dysfunction. DCX⁺ cells were prominent in the paralaminar nuclei and periamygdalar cortex and these declined with age but were not significantly associated with epilepsy history. DCX expressing cells with ramified processes were prominent in all regions, particularly in the hippocampal subgranular zone, where significantly increased numbers were observed in epilepsy samples compared to controls. DCX ramified cells co-expressed Iba1, CD68 and PDGFRβ, and less frequently MCM2, OLIG2 and SOX2, but no co-localization was observed with CD34, nestin or GFAP/GFAP ∂. Gene expression data from neocortical samples in patients with TLE and HS supported ongoing DCX expression in adults. We conclude that DCX identifies a range of morphological cell types in temporal lobe epilepsy, including immature populations, glial and microglial cell types. Their clinical relevance and biological function requires further study but we show some evidence for alteration with age and in epilepsy.

Effects of anterior thalamic nuclei deep brain stimulation on neurogenesis in epileptic and healthy rats

BACKGROUND

The efficacy of anterior thalamic nuclei (ANT) deep brain stimulation (DBS) in mitigating epileptic seizures has been established. Though the neuroprotection of ANT-DBS has been illustrated, the seizure mitigating mechanism of ANT-DBS has not been thoroughly elucidated. In particular, the effect of ANT-DBS on neurogenesis has not been reported previously.

METHOD

Thirty-two male Sprague Dawley rats were randomly assigned to the following groups: sham-DBS-healthy (HL) (n=8), DBS-HL (n=8), sham-DBS-epilepsy (EP) (n=8) and DBS-EP (n=8). Normal saline and kainic acid were injected, respectively, into the former and later two groups, and seizures were monitored. One month later, rats received electrode implantation. Stimulation was exerted in the DBS group but not in the sham-DBS group. Next, all rats were sacrificed, and the ipsilateral hippocampus was dissected and prepared for quantitative real time PCR (qPCR) and western blot analysis in order to measure neuronal nuclear (NeuN), brain-derived neurotrophic factor (BDNF), doublecortin (DCX) and Ki-67 expressions.

RESULTS

A 44.4% seizure frequency reduction was obtained after ANT-DBS, and no seizures was observed in healthy rats. NeuN, BDNF, Ki-67 and DCX expression levels were significantly decreased in the epileptic rats compared to healthy rats (P<0.01 or P<0.05). Obvious increases in NeuN, Ki-67 and DCX expressions were observed in epileptic and healthy rats receiving stimulation compared to rats receiving no stimulation (all Ps<0.01). However, BDNF expression was not affected by ANT-DBS (all Ps>0.05).

CONCLUSIONS

(1) ANT-DBS reduces neuronal loss during the chronic stage of epilepsy. (2) Neurogenesis is elevated by ANT-DBS in both epileptic and healthy rats, and this elevation may not be regulated via a BDNF pathway.

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.

Hyperactive mTOR signals in the proopiomelanocortin-expressing hippocampal neurons cause age-dependent epilepsy and premature death in mice

Epilepsy is a frequent comorbidity in patients with focal cortical dysplasia (FCD). Recent studies utilizing massive sequencing data identified subsets of genes that are associated with epilepsy and FCD. AKT and mTOR-related signals have been recently implicated in the pathogenic processes of epilepsy and FCD. To clarify the functional roles of the AKT-mTOR pathway in the hippocampal neurons, we generated conditional knockout mice harboring the deletion of Pten (Pten-cKO) in Proopiomelanocortin-expressing neurons. The Pten-cKO mice developed normally until 8 weeks of age, then presented generalized seizures at 8–10 weeks of age. Video-monitored electroencephalograms detected paroxysmal discharges emerging from the cerebral cortex and hippocampus. These mice showed progressive hypertrophy of the dentate gyrus (DG) with increased expressions of excitatory synaptic markers (Psd95, Shank3 and Homer). In contrast, the expression of inhibitory neurons (Gad67) was decreased at 6–8 weeks of age. Immunofluorescence studies revealed the abnormal sprouting of mossy fibers in the DG of the Pten-cKO mice prior to the onset of seizures. The treatment of these mice with an mTOR inhibitor rapamycin successfully prevented the development of seizures and reversed these molecular phenotypes. These data indicate that the mTOR pathway regulates hippocampal excitability in the postnatal brain.

Interictal psychosis following temporal lobe surgery: dentate gyrus pathology

BACKGROUND

De novo interictal psychosis, albeit uncommon, can develop in patients following temporal lobe surgery for epilepsy. Pathological alterations of the dentate gyrus, including cytoarchitectural changes, immaturity and axonal reorganization that occur in epilepsy, may also underpin co-morbid psychiatric disorders. Our aim was to study candidate pathways that may be associated with the development of interictal psychosis post-operatively in patients with hippocampal sclerosis (HS).

METHOD

A total of 11 patients with HS who developed interictal psychosis (HS-P) post-operatively were compared with a matched surgical HS group without psychosis (HS-NP). Resected tissues were investigated for the extent of granule cell dispersion, mossy fibre sprouting and calbindin expression in the granule cells. We quantified doublecortin, mini-chromosome maintenance protein 2 (MCM2) and reelin-expressing neuronal populations in the dentate gyrus as well as the distribution of cannabinoid type 1 receptor (CBR1).

RESULTS

The patterns of neuronal loss and gliosis were similar in both groups. HS-P patients demonstrated less mossy fibre sprouting and granule cell dispersion (p < 0.01) and more frequent reduction in calbindin expression in granule cells. There were no group differences in the densities of immature MCM2, doublecortin and reelin-positive cells. CBR1 labelling was significantly lower in Cornu ammonis area CA4 relative to other subfields (p < 0.01); although reduced staining in all hippocampal regions was noted in HS-P compared with HS-NP patients, the differences were not statistically significant.

CONCLUSIONS

The alterations in dentate gyrus pathology found in HS-P patients could indicate underlying differences in the cellular response to seizures. These mechanisms may predispose to the development of psychosis in epilepsy and warrant further investigation.

Phytoestrogen α-zearalanol improves memory impairment and hippocampal neurogenesis in ovariectomized mice

Estrogen is known to provide robust protection of memory in postmenopausal women, but the fact that estrogen may increase the incidence of uterine and breast tumors has undoubtedly limited the clinical use of estrogen. In the present study, the effect of α-zearalanol (α-ZAL), a plant-derived phytoestrogen with low side-effect on uterine and breast, on memory has been evaluated in ovariectomized (OVX) mice when using 17β-estradiol (17β-E2) as an estrogen positive control. Our findings demonstrated that OVX resulted in impaired spatial learning and memory and reduced numbers of newborn neurons in the dentate gyrus of the hippocampus, while 17β-E2 or α-ZAL treatment significantly improved memory performance and restored hippocampal neurogenesis. We also found the reduction of brain derived neurotrophic factor (BDNF) and TrkB expression in OVX mice, which were ameliorated by 17β-E2 or α-ZAL supplementation. These results indicated that α-ZAL may improve memory impairments induced by OVX and modulate the expression of BDNF-TrkB benefit to neurogenesis which may be involved in the memory protection from α-ZAL, in a manner similar to that of 17β-E2. The present findings suggested that α-ZAL may be a plausible substitute of 17β-E2 in improving memory in postmenopausal women.

Adult human brain neural progenitor cells (NPCs) and fibroblast-like cells have similar properties in vitro but only NPCs differentiate into neurons

The ability to culture neural progenitor cells from the adult human brain has provided an exciting opportunity to develop and test potential therapies on adult human brain cells. To achieve a reliable and reproducible adult human neural progenitor cell (AhNPC) culture system for this purpose, this study fully characterized the cellular composition of the AhNPC cultures, as well as the possible changes to this in vitro system over prolonged culture periods. We isolated cells from the neurogenic subventricular zone/hippocampus (SVZ/HP) of the adult human brain and found a heterogeneous culture population comprised of several types of post-mitotic brain cells (neurons, astrocytes, and microglia), and more importantly, two distinct mitotic cell populations; the AhNPCs, and the fibroblast-like cells (FbCs). These two populations can easily be mistaken for a single population of AhNPCs, as they both proliferate under AhNPC culture conditions, form spheres and express neural progenitor cell and early neuronal markers, all of which are characteristics of AhNPCs in vitro. However, despite these similarities under proliferating conditions, under neuronal differentiation conditions, only the AhNPCs differentiated into functional neurons and glia. Furthermore, AhNPCs showed limited proliferative capacity that resulted in their depletion from culture by 5–6 passages, while the FbCs, which appear to be from a neurovascular origin, displayed a greater proliferative capacity and dominated the long-term cultures. This gradual change in cellular composition resulted in a progressive decline in neurogenic potential without the apparent loss of self-renewal in our cultures. These results demonstrate that while AhNPCs and FbCs behave similarly under proliferative conditions, they are two different cell populations. This information is vital for the interpretation and reproducibility of AhNPC experiments and suggests an ideal time frame for conducting AhNPC-based experiments.

The interesting interplay between interneurons and adult hippocampal neurogenesis

Adult neurogenesis is a unique form of plasticity found in the hippocampus, a brain region key to learning and memory formation. While many external stimuli are known to modulate the generation of new neurons in the hippocampus, little is known about the local circuitry mechanisms that regulate the process of adult neurogenesis. The neurogenic niche in the hippocampus is highly complex and consists of a heterogeneous population of cells including interneurons. Because interneurons are already highly integrated into the hippocampal circuitry, they are in a prime position to influence the proliferation, survival, and maturation of adult-generated cells in the dentate gyrus. Here, we review the current state of our understanding on the interplay between interneurons and adult hippocampal neurogenesis. We focus on activity- and signaling-dependent mechanisms, as well as research on human diseases that could provide better insight into how interneurons in general might add to our comprehension of the regulation and function of adult hippocampal neurogenesis.

The beneficial effects of physical activity on impaired adult neurogenesis and cognitive performance

Neurogenesis occurs in two neurogenic zones in the adult brain: new neurons are born at the subventricular zone of the lateral ventricles and then migrate to the olfactory bulb, and at the subgranular zone to integrate the granular cell layer of the dentate gyrus. The hippocampus is involved in learning and memory and the generation of new hippocampal neurons has been suggested to be a new form of plasticity implicated in these processes. In the last decades, diverse intrinsic and epigenetic factors have been identified to influence adult neurogenesis but the underlying mechanisms remain unclear. In a recent study, Lafenetre et al. (2010) showed the beneficial influence of physical voluntary activity on adult neurogenesis and cognitive performance in a transgenic mouse, the synRas mouse via brain-derived neurotrophic factor. Here we review how hippocampal neurogenesis can be regulated by environmental factors and the possible role of the newly generated cells in learning and memory.