Impact of hippocampal neuronal ablation on neurogenesis and cognition in the aged brain

Neurogenesis and glial impairments in congenital hydrocephalus: insights from a BioGlue-induced fetal lamb model

BACKGROUND

Congenital hydrocephalus (HCP) is a prevalent condition, that leads to fetal cerebral ventricle dilation and increased intracranial pressure. It is associated with significant neurological impairments, partly due to the disruption of neurogenesis and gliogenesis. This study aims to investigate alterations in the proliferation and differentiation of neural progenitor cells (NPCs) in a fetal lamb model of obstructive HCP induced by intracisternal BioGlue injection, to identify the potential optimal intervention time for prenatal surgery.

METHODS

This study involved 22 fetal lambs, divided into control (n = 10) and HCP (n = 12) groups with hydrocephalus induced at approximately 85-90 gestational days. Histological and molecular techniques, including hematoxylin and eosin staining, triple immunofluorescence, Western blot analysis, and RT-qPCR, were utilized to assess changes in NPCs, astrocytes, and oligodendrocytes across three different gestational stages (E105, E125, and E140). The analysis of data was done by using multiple (unpaired) two-sample t-test and was represented as mean and standard deviation.

RESULTS

HCP led to significant disruptions in the ventricular zone (VZ), with the translocation of NPCs into the intraventricular CSF and formation of periventricular heterotopias. This study revealed an initial surge in the expression of NPC markers (Pax6 and Sox2), which decreased as HCP progressed. Astroglia reaction intensified, as indicated by increased expression of GFAP, vimentin, and aquaporin 4, particularly at later stages of pregnancy (p < 0.001, p < 0.001 and p < 0.001, control and HCP E140, respectively). Myelin formation was also adversely affected, with reduced expression of oligodendrocyte markers (Olig2 and Sox10, p = 0.01 and p = 0.009, control and HCP E140, respectively) and myelin proteins (MOBP, MOG and MBP, p = 0.02, p = 0.049 and p = 0.02 control and HCP E140, respectively).

CONCLUSIONS

This study contributed to clarify the profound impact of congenital HCP on neurogenesis and gliogenesis in an experimental fetal lamb model. The VZ disruption and altered expression of key neurogenic and glial markers suggested a significant pathological process underlying neurodevelopmental abnormalities. The findings suggested a potential window for prenatal surgical intervention between E105 and E125 in the sheep model, offering new avenues for prenatal therapeutic approaches and improving surgical outcomes in affected fetuses and neonates.

Environmental enrichment in middle age rats improves spatial and object memory discrimination deficits

Changes in memory performance are one of the main symptoms of normal aging. The storage of similar experiences as different memories (ie. behavioral pattern separation), becomes less efficient as aging progresses. Studies have focused on hippocampus dependent spatial memories and their role in the aging related deficits in behavioral pattern separation (BPS) by targeting high similarity interference conditions. However, parahippocampal cortices such as the perirhinal cortex are also particularly vulnerable to aging. Middle age is thought to be the stage where mild mnemonic deficits begin to emerge. Therefore, a better understanding of the timing of the spatial and object domain memory impairment could shed light over how plasticity changes in the parahipocampal-hippocampal system affects mnemonic function in early aging. In the present work, we compared the performance of young and middle-aged rats in both spatial (spontaneous location recognition) and non-spatial (spontaneous object recognition) behavioral pattern separation tasks to understand the comparative progression of these deficits from early stages of aging. Moreover, we explored the impact of environmental enrichment (EE) as an intervention with important translational value. Although a bulk of studies have examined the contribution of EE for preventing age related memory decline in diverse cognitive domains, there is limited knowledge of how this intervention could specifically impact on BPS function in middle-aged animals. Here we evaluate the effects of EE as modulator of BPS, and its ability to revert the deficits caused by normal aging at early stages. We reveal a domain-dependent impairment in behavioral pattern separation in middle-aged rats, with spatial memories affected independently of the similarity of the experiences and object memories only affected when the stimuli are similar, an effect that could be linked to the higher interference seen in this group. Moreover, we found that EE significantly enhanced behavioral performance in middle-aged rats in the spatial and object domain, and this improvement is specific of the high similarity load condition. In conclusion, these results suggest that memory is differentially affected by aging in the object and spatial domains, but that BPS function is responsive to an EE intervention in a multidomain manner.

Neural stem cells promote neuroplasticity: a promising therapeutic strategy for the treatment of Alzheimer's disease

Recent studies have demonstrated that neuroplasticity, such as synaptic plasticity and neurogenesis, exists throughout the normal lifespan but declines with age and is significantly impaired in individuals with Alzheimer's disease. Hence, promoting neuroplasticity may represent an effective strategy with which Alzheimer's disease can be alleviated. Due to their significant ability to self-renew, differentiate, and migrate, neural stem cells play an essential role in reversing synaptic and neuronal damage, reducing the pathology of Alzheimer's disease, including amyloid-β, tau protein, and neuroinflammation, and secreting neurotrophic factors and growth factors that are related to plasticity. These events can promote synaptic plasticity and neurogenesis to repair the microenvironment of the mammalian brain. Consequently, neural stem cells are considered to represent a potential regenerative therapy with which to improve Alzheimer's disease and other neurodegenerative diseases. In this review, we discuss how neural stem cells regulate neuroplasticity and optimize their effects to enhance their potential for treating Alzheimer's disease in the clinic.

Predicting progression to Alzheimer's disease with human hippocampal progenitors exposed to serum

Adult hippocampal neurogenesis is important for learning and memory and is altered early in Alzheimer's disease. As hippocampal neurogenesis is modulated by the circulatory systemic environment, evaluating a proxy of how hippocampal neurogenesis is affected by the systemic milieu could serve as an early biomarker for Alzheimer's disease progression. Here, we used an in vitro assay to model the impact of systemic environment on hippocampal neurogenesis. A human hippocampal progenitor cell line was treated with longitudinal serum samples from individuals with mild cognitive impairment, who either progressed to Alzheimer's disease or remained cognitively stable. Mild cognitive impairment to Alzheimer's disease progression was characterized most prominently with decreased proliferation, increased cell death and increased neurogenesis. A subset of 'baseline' cellular readouts together with education level were able to predict Alzheimer's disease progression. The assay could provide a powerful platform for early prognosis, monitoring disease progression and further mechanistic studies.

Bibliometric analysis: Research trends of acupuncture treatment to cognitive impairment in recent 15 years

Objectives

Acupuncture therapy has been used for cognitive impairment-related diseases, however, there are still few studies on the overall trend of acupuncture therapy on cognitive impairment based on bibliometric analysis. The purpose of this study was to explore the research trend of the impact of acupuncture on cognitive impairment in the past 15 years, analyze the research trends and hotspots, and provide new ideas and theoretical basis for future research directions.

Methods

From the Web of Science Core Collection (WoSCC), the relevant literature on the treatment of cognitive impairment with acupuncture from 2007 to 2022 was retrieved. Then, based on the CiteSpace and VOSviewer software of the Java platform, the cooperation between countries and institutions in this field, the co-citation of journals and documents, and the cooperation between authors and authors, etc. were analyzed. In addition, the co-occurrence and burst analysis of keywords are also carried out, and a visual knowledge map is drawn.

Results

As of August 08, 2022, a total of 394 records related to the treatment of cognitive impairment with acupuncture were identified. The analysis results show: The number and rate of annual publications have steadily increased, with some fluctuations from year to year. The countries that contribute the most to this field are China and the USA. Among them, Beijing University of Chinese Medicine and Capital Medical University are tied for first place in terms of the number of published papers. Tao Jing is the most prolific author and the number one cited author.

Conclusions

The number of publications on acupuncture for cognitive impairment is expected to increase rapidly in future research, suggesting a bright future for the field. Future research hotspots will focus on pain, injury, protocol, diagnosis, guidelines, etc. It is also necessary to strengthen cross-regional and cross-country cooperation among various academic groups.

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.

Reproduction-Associated Hormones and Adult Hippocampal Neurogenesis

The levels of reproduction-associated hormones in females, such as estrogen, progesterone, prolactin, and oxytocin, change dramatically during pregnancy and postpartum. Reproduction-associated hormones can affect adult hippocampal neurogenesis (AHN), thereby regulating mothers' behavior after delivery. In this review, we first briefly introduce the overall functional significance of AHN and the methods commonly used to explore this front. Then, we attempt to reconcile the changes of reproduction-associated hormones during pregnancy. We further update the findings on how reproduction-related hormones influence adult hippocampal neurogenesis. This review is aimed at emphasizing a potential role of AHN in reproduction-related brain plasticity and its neurobiological relevance to motherhood behavior.

Co-Ultra PEALut Enhances Endogenous Repair Response Following Moderate Traumatic Brain Injury

This study aimed to assess the neuro-regenerative properties of co-ultramicronized PEALut (Glialia^®), composed of palmitoylethanolamide (PEA) and the flavonoid luteolin (Lut), in an in vivo model of traumatic brain injury (TBI) and patients affected by moderate TBI. An increase in neurogenesis was seen in the mice at 72 h and 7 d after TBI. The co-ultra PEALut treatment helped the neuronal reconstitution process to restore the basal level of both novel and mature neurons; moreover, it induced a significant upregulation of the neurotrophic factors, which ultimately led to progress in terms of memory recall during behavioral testing. Moreover, our preliminary findings in a clinical trial suggested that Glialia^® treatment facilitated neural recovery on working memory. Thus, co-ultra PEALut (Glialia^®) could represent a valuable therapeutic agent for intensifying the endogenous repair response in order to better treat TBI.

Folic acid prevents habituation memory impairment and oxidative stress in an aging model induced by D-galactose

The present study aimed to evaluate the effect of folic acid treatment in an animal model of aging induced by D-galactose (D-gal). For this propose, adult male Wistar rats received D-gal intraperitoneally (100 mg/kg) and/or folic acid orally (5 mg/kg, 10 mg/kg or 50 mg/kg) for 8 weeks. D-gal caused habituation memory impairment, and folic acid (10 mg/kg and 50 mg/kg) reversed this effect. However, folic acid 50 mg/kg per se caused habituation memory impairment. D-gal increased the lipid peroxidation and oxidative damage to proteins in the prefrontal cortex and hippocampus from rats. Folic acid (5 mg/kg, 10 mg/kg, or 50 mg/kg) partially reversed the oxidative damage to lipids in the hippocampus, but not in the prefrontal cortex, and reversed protein oxidative damage in the prefrontal cortex and hippocampus. D-gal induced synaptophysin and BCL-2 decrease in the hippocampus and phosphorylated tau increase in the prefrontal cortex. Folic acid was able to reverse these D-gal-related alterations in the protein content. The present study shows folic acid supplementation as an alternative during the aging to prevent cognitive impairment and brain alterations that can cause neurodegenerative diseases. However, additional studies are necessary to elucidate the effect of folic acid in aging.

Aggravated brain injury after neonatal hypoxic ischemia in microglia-depleted mice

Background

Neuroinflammation plays an important role in neonatal hypoxic-ischemic encephalopathy (HIE). Although microglia are largely responsible for injury-induced inflammatory response, they play beneficial roles in both normal and disease states. However, the effects of microglial depletion on neonatal HIE remain unclear.

Methods

Tamoxifen was administered to Cx3cr1^CreER/+Rosa26^DTA/+ (microglia-depleted model) and Cx3cr1^CreER/+Rosa26^DTA/− (control) mice at P8 and P9 to assess the effect of microglial depletion. The density of microglia was quantified using Iba-1 staining. Moreover, the proportion of resident microglia after the HI insult was analyzed using flow cytometric analysis. At P10, the HI insult was conducted using the Rice-Vannucci procedure at P10. The infarct size and apoptotic cells were analyzed at P13. Cytokine analyses were performed using quantitative polymerase chain reaction and enzyme-linked immunosorbent assay (ELISA) at P13.

Results

At P10, tamoxifen administration induced > 99% microglial depletion in DTA⁺ mice. Following HI insult, there was persisted microglial depletion over 97% at P13. Compared to male DTA⁻ mice, male DTA⁺ mice exhibited significantly larger infarct volumes; however, there were no significant differences among females. Moreover, compared to male DTA⁻ mice, male DTA⁺ mice had a significantly higher density of TUNEL⁺ cells in the caudoputamen, cerebral cortex, and thalamus. Moreover, compared to female DTA⁻ mice, female DTA⁺ mice showed a significantly greater number of TUNEL⁺ cells in the hippocampus and thalamus. Compared to DTA⁻ mice, ELISA revealed significantly lower IL-10 and TGF-β levels in both male and female DTA⁺ mice under both normal conditions and after HI (more pronounced).

Conclusion

We established a microglial depletion model that aggravated neuronal damage and apoptosis after the HI insult, which was predominantly observed in males.

Neuroregeneration: Regulation in Neurodegenerative Diseases and Aging

It had been commonly believed for a long time, that once established, degeneration of the central nervous system (CNS) is irreparable, and that adult person merely cannot restore dead or injured neurons. The existence of stem cells (SCs) in the mature brain, an organ with minimal regenerative ability, had been ignored for many years. Currently accepted that specific structures of the adult brain contain neural SCs (NSCs) that can self-renew and generate terminally differentiated brain cells, including neurons and glia. However, their contribution to the regulation of brain activity and brain regeneration in natural aging and pathology is still a subject of ongoing studies. Since the 1970s, when Fuad Lechin suggested the existence of repair mechanisms in the brain, new exhilarating data from scientists around the world have expanded our knowledge on the mechanisms implicated in the generation of various cell phenotypes supporting the brain, regulation of brain activity by these newly generated cells, and participation of SCs in brain homeostasis and regeneration. The prospects of the SC research are truthfully infinite and hitherto challenging to forecast. Once researchers resolve the issues regarding SC expansion and maintenance, the implementation of the SC-based platform could help to treat tissues and organs impaired or damaged in many devastating human diseases. Over the past 10 years, the number of studies on SCs has increased exponentially, and we have already become witnesses of crucial discoveries in SC biology. Comprehension of the mechanisms of neurogenesis regulation is essential for the development of new therapeutic approaches for currently incurable neurodegenerative diseases and neuroblastomas. In this review, we present the latest achievements in this fast-moving field and discuss essential aspects of NSC biology, including SC regulation by hormones, neurotransmitters, and transcription factors, along with the achievements of genetic and chemical reprogramming for the safe use of SCs in vitro and in vivo.

Serum Amyloid A Proteins Induce Pathogenic Th17 Cells and Promote Inflammatory Disease

Lymphoid cells that produce interleukin (IL)-17 cytokines protect barrier tissues from pathogenic microbes but are also prominent effectors of inflammation and autoimmune disease. T helper 17 (Th17) cells, defined by RORγt-dependent production of IL-17A and IL-17F, exert homeostatic functions in the gut upon microbiota-directed differentiation from naive CD4⁺ T cells. In the non-pathogenic setting, their cytokine production is regulated by serum amyloid A proteins (SAA1 and SAA2) secreted by adjacent intestinal epithelial cells. However, Th17 cell behaviors vary markedly according to their environment. Here, we show that SAAs additionally direct a pathogenic pro-inflammatory Th17 cell differentiation program, acting directly on T cells in collaboration with STAT3-activating cytokines. Using loss- and gain-of-function mouse models, we show that SAA1, SAA2, and SAA3 have distinct systemic and local functions in promoting Th17-mediated inflammatory diseases. These studies suggest that T cell signaling pathways modulated by the SAAs may be attractive targets for anti-inflammatory therapies.

Born this way: Hippocampal neurogenesis across the lifespan

The capability of the mammalian brain to generate new neurons through the lifespan has gained much attention for the promise of new therapeutic possibilities especially for the aging brain. One of the brain regions that maintains a neurogenesis-permissive environment is the dentate gyrus of the hippocampus. Here, new neurons are generated from a pool of multipotent neural progenitor cells to become fully functional neurons that are integrated into the brain circuitry. A growing body of evidence points to the fact that neurogenesis in the adult hippocampus is necessary for certain memory processes, and in mood regulation, while alterations in hippocampal neurogenesis have been associated with a myriad of neurological and psychiatric disorders. More recently, evidence has come to light that new neurons may differ in their vulnerability to environmental and disease-related influences depending on the time during the life course at which they are exposed. Thus, it has been the topic of intense research in recent years. In this review, we will discuss the complex process and associated functional relevance of hippocampal neurogenesis during the embryonic/postnatal period and in adulthood. We consider the implications of hippocampal neurogenesis during the developmentally critical periods of adolescence and older age. We will further consider the literature surrounding hippocampal neurogenesis and its functional role during these critical periods with a view to providing insight into the potential of harnessing neurogenesis for health and therapeutic benefit.

Propofol Regulates Neural Stem Cell Proliferation and Differentiation via Calmodulin-Dependent Protein Kinase II/AMPK/ATF5 Signaling Axis

BACKGROUND

Propofol can cause degeneration of developing brain cells and subsequent long-term learning or memory impairment. However, at the early stage of embryonic development, the molecular mechanism of propofol-induced inhibition in neural stem cells (NSCs) neurogenesis is still unclear. The aim of this study was to determine the role of propofol in NSCs neurogenesis and, more importantly, to explore the underlying mechanism.

METHODS

First, a single intraperitoneal injection of propofol was performed in pregnant mice, and 6 hours after administration of propofol, the hippocampus RNA and the protein of the embryos' brains was extracted to analyze the expression of neuron-specific markers. Second, the primary NSCs were isolated from the hippocampus of mouse embryonic brain and then treated with propofol for cell viability, immunostaining, and transwell assays; more importantly, we performed RNA sequencing (RNA-seq) and q-reverse transcription polymerase chain reaction assays to identify genes regulated by propofol; the Western blot, small interfering RNA (SiRNA), and luciferase reporter assays were used to study the effects of propofol on calmodulin-dependent protein kinase (CaMk) II/5' adenosine monophosphate-activated protein kinase (AMPK)/activating transcription factor 5 (ATF5) signaling pathway.

RESULTS

Our results indicated that propofol treatment could inhibit the proliferation, migration, and differentiation of NSCs. The results of RNA-seq assays showed that propofol treatment resulted in downregulation of a group of Ca-dependent genes. The following mechanism studies showed that propofol regulates the proliferation, differentiation, and migration of NSCs through the CaMkII/phosphorylation of serine at amino acid position 485 (pS485)/AMPK/ATF5 signaling pathway.

CONCLUSIONS

The results from study demonstrated that propofol inhibits the proliferation, differentiation, and migration of NSCs, and these effects are partially mediated by CaMkII/pS485/AMPK/ATF5 signaling pathway.

Electroacupuncture ameliorate learning and memory by improving N-acetylaspartate and glutamate metabolism in APP/PS1 mice

Objective

To explore the precise mechanism of electroacupuncture (EA) to delay cognitive impairment in Alzheimer disease.

Methods

N-Acetylaspartate (NAA), glutamate (Glu) and myoinositol (mI) metabolism were measured by magnetic resonance spectroscopy, learning and memory of APP/PS1 mouse was evaluated by the Morris water maze test and the step-down avoidance test, neuron survival number and neuronal structure in the hippocampus were observed by Nissl staining, and BDNF and phosphorylated TrkB detected by Western blot.

Results

EA at DU20 acupuncture significantly improve learning and memory in behavioral tests, up-regulate NAA, Glu and mI metabolism, increase the surviving neurons in hippocampus, and promote the expression of BDNF and TrkB in the APP/PS1 transgenic mice.

Conclusion

These findings suggested that EA is a potential therapeutic for ameliorate cognitive dysfunction, and it might be due to EA could improve NAA and Glu metabolism by upregulation of BDNF in APP/PS1 mice.

Comparative approaches to understanding thyroid hormone regulation of neurogenesis

Thyroid hormone (TH) signalling, an evolutionary conserved pathway, is crucial for brain function and cognition throughout life, from early development to ageing. In humans, TH deficiency during pregnancy alters offspring brain development, increasing the risk of cognitive disorders. How TH regulates neurogenesis and subsequent behaviour and cognitive functions remains a major research challenge. Cellular and molecular mechanisms underlying TH signalling on proliferation, survival, determination, migration, differentiation and maturation have been studied in mammalian animal models for over a century. However, recent data show that THs also influence embryonic and adult neurogenesis throughout vertebrates (from mammals to teleosts). These latest observations raise the question of how TH availability is controlled during neurogenesis and particularly in specific neural stem cell populations. This review deals with the role of TH in regulating neurogenesis in the developing and the adult brain across different vertebrate species. Such evo-devo approaches can shed new light on (i) the evolution of the nervous system and (ii) the evolutionary control of neurogenesis by TH across animal phyla. We also discuss the role of thyroid disruptors on brain development in an evolutionary context.

Long-term Fate Mapping to Assess the Impact of Postnatal Isoflurane Exposure on Hippocampal Progenitor Cell Productivity

BACKGROUND

Exposure to isoflurane increases apoptosis among postnatally generated hippocampal dentate granule cells. These neurons play important roles in cognition and behavior, so their permanent loss could explain deficits after surgical procedures.

METHODS

To determine whether developmental anesthesia exposure leads to persistent deficits in granule cell numbers, a genetic fate-mapping approach to label a cohort of postnatally generated granule cells in Gli1-CreER::GFP bitransgenic mice was utilized. Green fluorescent protein (GFP) expression was induced on postnatal day 7 (P7) to fate map progenitor cells, and mice were exposed to 6 h of 1.5% isoflurane or room air 2 weeks later (P21). Brain structure was assessed immediately after anesthesia exposure (n = 7 controls and 8 anesthesia-treated mice) or after a 60-day recovery (n = 8 controls and 8 anesthesia-treated mice). A final group of C57BL/6 mice was exposed to isoflurane at P21 and examined using neurogenesis and cell death markers after a 14-day recovery (n = 10 controls and 16 anesthesia-treated mice).

RESULTS

Isoflurane significantly increased apoptosis immediately after exposure, leading to cell death among 11% of GFP-labeled cells. Sixty days after isoflurane exposure, the number of GFP-expressing granule cells in treated animals was indistinguishable from control animals. Rates of neurogenesis were equivalent among groups at both 2 weeks and 2 months after treatment.

CONCLUSIONS

These findings suggest that the dentate gyrus can restore normal neuron numbers after a single, developmental exposure to isoflurane. The authors' results do not preclude the possibility that the affected population may exhibit more subtle structural or functional deficits. Nonetheless, the dentate appears to exhibit greater resiliency relative to nonneurogenic brain regions, which exhibit permanent neuron loss after isoflurane exposure.

Neurotrophin Signaling and Stem Cells-Implications for Neurodegenerative Diseases and Stem Cell Therapy

Neurotrophins (NTs) are members of a neuronal growth factor protein family whose action is mediated by the tropomyosin receptor kinase (TRK) receptor family receptors and the p75 NT receptor (p75NTR), a member of the tumor necrosis factor (TNF) receptor family. Although NTs were first discovered in neurons, recent studies have suggested that NTs and their receptors are expressed in various types of stem cells mediating pivotal signaling events in stem cell biology. The concept of stem cell therapy has already attracted much attention as a potential strategy for the treatment of neurodegenerative diseases (NDs). Strikingly, NTs, proNTs, and their receptors are gaining interest as key regulators of stem cells differentiation, survival, self-renewal, plasticity, and migration. In this review, we elaborate the recent progress in understanding of NTs and their action on various stem cells. First, we provide current knowledge of NTs, proNTs, and their receptor isoforms and signaling pathways. Subsequently, we describe recent advances in the understanding of NT activities in various stem cells and their role in NDs, particularly Alzheimer's disease (AD) and Parkinson's disease (PD). Finally, we compile the implications of NTs and stem cells from a clinical perspective and discuss the challenges with regard to transplantation therapy for treatment of AD and PD.

Structure-function integrity of the adult hippocampus depends on the transcription factor Bcl11b/Ctip2

The dentate gyrus is one of the only two brain regions where adult neurogenesis occurs. Throughout life, cells of the neuronal stem cell niche undergo proliferation, differentiation and integration into the hippocampal neural circuitry. Ongoing adult neurogenesis is a prerequisite for the maintenance of adult hippocampal functionality. Bcl11b, a zinc finger transcription factor, is expressed by postmitotic granule cells in the developing as well as adult dentate gyrus. We previously showed a critical role of Bcl11b for hippocampal development. Whether Bcl11b is also required for adult hippocampal functions has not been investigated. Using a tetracycline‐dependent inducible mouse model under the control of the forebrain‐specific CaMKIIα promoter, we show here that the adult expression of Bcl11b is essential for survival, differentiation and functional integration of adult‐born granule cell neurons. In addition, Bcl11b is required for survival of pre‐existing mature neurons. Consequently, loss of Bcl11b expression selectively in the adult hippocampus results in impaired spatial working memory. Together, our data uncover for the first time a specific role of Bcl11b in adult hippocampal neurogenesis and function.

Testosterone depletion increases the susceptibility of brain tissue to oxidative damage in a restraint stress mouse model

Among sex hormones, estrogen is particularly well known to act as neuroprotective agent. Unlike estrogen, testosterone has not been well investigated in regard to its effects on the brain, especially under psychological stress. To investigate the role of testosterone in oxidative brain injuries under psychological stress, we adapted an orchiectomy and restraint stress model. BALB/c mice were subjected to either an orchiectomy or sham operation. After allowing 15 days for recovery, mice were re-divided into four groups according to exposure of restraint stress: sham, sham plus stress, orchiectomy, and orchiectomy plus stress. Serum testosterone was undetectable in orchiectomized groups and restraint-induced stress significantly reduced testosterone levels in sham plus stress group. The serum levels of corticosterone and adrenaline were notably elevated by restraint stress, and these elevated hormones were markedly augmented by orchiectomy. Two oxidative stressors and biomarkers for lipid and protein peroxidation were significantly increased in the cerebral cortex and hippocampus by restraint stress, while the reverse pattern was observed in antioxidant enzymes. These results were supported by histopathological findings, with 4-hydroxynonenal staining for oxidative injury and Fluoro-Jade B staining showing the degenerating neurons. The aforementioned patterns of oxidative injury were accelerated by orchiectomy. These findings strongly suggest the conclusion that testosterone exerts a protective effect against oxidative brain damage, especially under stressed conditions. Unlike estrogen, the effects of testosterone on the brain have not been thoroughly investigated. In order to investigate the role of testosterone in oxidative brain injuries under psychological stress, we adapted an orchiectomy and restraint stress model. Orchiectomy markedly augmented the restraint stress-induced elevation of serum corticosterone and adrenaline levels as well as oxidative alterations in brain tissues, especially in the hippocampus. These findings are the first evidence that testosterone depletion makes the brain prone to oxidative injury.

Stimulation of neuronal cells by culture supernatant of T lymphocytes triggered by anti-CD3 mAb followed by propagation in the presence of interleukin-2

Performance status (PS) frequently improves occurs in cancer patients who have been infused with their own lymphokine-activated killer T cells (LAK-T). In the present study, a culture supernatant of LAK-T (LAK-T sup) administered to 8-week-old rats caused neurogenesis as evidenced by increased 5-ethynyl-2'-deoxyuridine staining of brain tissues. Intravenous injection of granulocyte-macrophage colony stimulating factor (GM-CSF), a major cytokine in LAK-T sup, had a similar effect. Furthermore, LAK-T sup induced Ca(++) increase in rat hippocampal brain slices that was detected in neuronal cells by emission of Fluo-8 NW at 520 nm. The same effect was observed with an rGM-CSF solution. GM-CSF may activate neuronal cells by stimulating the glial cells that surround and attach to them. If so, GM-CSF and LAK-T sup may improve the motor neurons of patients with amyotrophic lateral sclerosis. The neurogenerative effect of GM-CSF in LAK-T sup may also help improve brain function in aged adults including those with dementia such as Alzheimer's disease.

Activity Dependency and Aging in the Regulation of Adult Neurogenesis

Age and activity might be considered the two antagonistic key regulators of adult neurogenesis. Adult neurogenesis decreases with age but remains present, albeit at a very low level, even in the oldest individuals. Activity, be it physical or cognitive, increases adult neurogenesis and thereby seems to counteract age effects. It is, thus, proposed that activity-dependent regulation of adult neurogenesis might contribute to some sort of "neural reserve," the brain's ability to compensate functional loss associated with aging or neurodegeneration. Activity can have nonspecific and specific effects on adult neurogenesis. Mechanistically, nonspecific stimuli that largely affect precursor cell stages might be related by the local microenvironment, whereas more specific, survival-promoting effects take place at later stages of neuronal development and require the synaptic integration of the new cell and its particular synaptic plasticity.

Downregulation of survivin regulates adult hippocampal neurogenesis and apoptosis, and inhibits spatial learning and memory following traumatic brain injury

Survivin, a unique member of the inhibitor of the apoptosis protein (IAP) family, has been suggested to play a crucial role in promoting the cell cycle and mediates mitosis during embryonic development. However, the role of survivin following traumatic brain injury (TBI) in adult neurogenesis and apoptosis in the mouse dentate gyrus (DG) remains only partially understood. We adopted adenovirus-mediated RNA interference (RNAi) as a means of suppressing the expression of survivin and observed its effects on adult regeneration and neurological function in mice after brain injury. The mice were subjected to TBI, and the ipsilateral hippocampus was then examined using reverse transcription polymerase chain reaction (RT-PCR) and Western blotting analyses. Brain slices were stained for 5'-bromo-2'-deoxyuridine (BrdU) and doublecortin (DCX). Our data showed that survivin knockdown inhibits the proliferation and differentiation of neural precursor cells (NPCs) in the DG of the hippocampus soon after TBI. Furthermore, downregulation of survivin results in a significant increase in programmed cell death in the DG, as assessed using terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) and 4',6-diamidino-2-phenylindole (DAPI) double staining. The Morris water maze (MWM) test was adopted to evaluate neurological function, which confirmed that knockdown of survivin worsened the memory capacity that was already compromised following TBI. Survivin in adult mice brains after TBI can be successfully down-regulated by RNAi, which inhibited adult hippocampal neurogenesis, promoted apoptotic cell death, and resulted in a negative role in the recovery of dysfunction following injury.

Bisphenol-A Mediated Inhibition of Hippocampal Neurogenesis Attenuated by Curcumin via Canonical Wnt Pathway

Bisphenol A (BPA) is an environmental xenoestrogenic endocrine disruptor, utilized for production of consumer products, and exerts adverse effects on the developing nervous system. Recently, we found that BPA impairs the finely tuned dynamic processes of neurogenesis (generation of new neurons) in the hippocampus of the developing rat brain. Curcumin is a natural polyphenolic compound, which provides neuroprotection against various environmental neurotoxicants and in the cellular and animal models of neurodegenerative disorders. Here, we have assessed the neuroprotective efficacy of curcumin against BPA-mediated reduced neurogenesis and the underlying cellular and molecular mechanism(s). Both in vitro and in vivo studies showed that curcumin protects against BPA-induced hippocampal neurotoxicity. Curcumin protects against BPA-mediated reduced neural stem cells (NSC) proliferation and neuronal differentiation and enhanced neurodegeneration. Curcumin also enhances the expression/levels of neurogenic and the Wnt pathway genes/proteins, which were reduced due to BPA exposure in the hippocampus. Curcumin-mediated neuroprotection against BPA-induced neurotoxicity involved activation of the Wnt/β-catenin signaling pathway, which was confirmed by the use of Wnt specific activators (LiCl and GSK-3β siRNA) and inhibitor (Dkk-1). BPA-mediated increased β-catenin phosphorylation, decreased GSK-3β levels, and β-catenin nuclear translocation were significantly reversed by curcumin, leading to enhanced neurogenesis. Curcumin-induced protective effects on neurogenesis were blocked by Dkk-1 in NSC culture treated with BPA. Curcumin-mediated enhanced neurogenesis was correlated well with improved learning and memory in BPA-treated rats. Overall, our results conclude that curcumin provides neuroprotection against BPA-mediated impaired neurogenesis via activation of the Wnt/β-catenin signaling pathway.

Hippocampal adaptive response following extensive neuronal loss in an inducible transgenic mouse model

Neuronal loss is a common component of a variety of neurodegenerative disorders (including Alzheimer's, Parkinson's, and Huntington's disease) and brain traumas (stroke, epilepsy, and traumatic brain injury). One brain region that commonly exhibits neuronal loss in several neurodegenerative disorders is the hippocampus, an area of the brain critical for the formation and retrieval of memories. Long-lasting and sometimes unrecoverable deficits caused by neuronal loss present a unique challenge for clinicians and for researchers who attempt to model these traumas in animals. Can these deficits be recovered, and if so, is the brain capable of regeneration following neuronal loss? To address this significant question, we utilized the innovative CaM/Tet-DT(A) mouse model that selectively induces neuronal ablation. We found that we are able to inflict a consistent and significant lesion to the hippocampus, resulting in hippocampally-dependent behavioral deficits and a long-lasting upregulation in neurogenesis, suggesting that this process might be a critical part of hippocampal recovery. In addition, we provide novel evidence of angiogenic and vasculature changes following hippocampal neuronal loss in CaM/Tet-DTA mice. We posit that angiogenesis may be an important factor that promotes neurogenic upregulation following hippocampal neuronal loss, and both factors, angiogenesis and neurogenesis, can contribute to the adaptive response of the brain for behavioral recovery.

Changes in neural stem cells in the subventricular zone in a rat model of communicating hydrocephalus

Communicating hydrocephalus is a common type of hydrocephalus. At present, the prevalent treatment is to perform a ventriculo-peritoneal shunt, which, for reasons that are not clear, is sometimes ineffective. The subventricular zone (SVZ) of the lateral ventricles has been established as the primary site of adult neurogenesis. Following cerebral ischemia or brain injury, neural stem cells (NSCs) increase in the SVZ and can both differentiate into neurons and glial cells and respond to the injury. Neural stem cells, enabled by a complex repertoire of factors that precisely regulate the activation, proliferation, differentiation and integration of newborn cells, continuously generate new neurons. However, only a few systematic studies of the role of NSCs in hydrocephalus have been reported. In a rat model of communicating hydrocephalus, we recently showed that hydrocephalus caused the ventricular system to expand over time. We found that the number of NSCs in the SVZ peaked rapidly after hydrocephalus was established and decreased gradually over time until the cells disappeared. NSCs may be involved in the pathophysiology changes and repair process of hydrocephalus.

7,8-Dihydroxyflavone, a small molecule TrkB agonist, improves spatial memory and increases thin spine density in a mouse model of Alzheimer disease-like neuronal loss

Augmenting BDNF/TrkB signaling has been demonstrated to be a promising strategy for reversing cognitive deficits in preclinical models of Alzheimer disease (AD). Although these studies highlight the potential of targeting BDNF/TrkB signaling, this strategy has not yet been tested in a model that develops the disease features that are most closely associated with cognitive decline in AD: severe synaptic and neuronal loss. In the present study, we investigated the impact of 7,8-dihydroxyflavone (DHF), a TrkB agonist, in CaM/Tet-DT_A mice, an inducible model of severe neuronal loss in the hippocampus and cortex. Systemic 7,8-DHF treatment significantly improved spatial memory in lesioned mice, as measured by water maze. Analysis of GFP-labeled neurons in CaM/Tet-DT_A mice revealed that 7,8-DHF induced a significant and selective increase in the density of thin spines in CA1 of lesioned mice, without affecting mushroom or stubby spines. These findings suggest chronic upregulation of TrkB signaling with 7,8-DHF may be an effective and practical strategy for improving function in AD, even after substantial neuronal loss has occurred.

Thyroid hormone signaling and adult neurogenesis in mammals

The vital roles of thyroid hormone in multiple aspects of perinatal brain development have been known for over a century. In the last decades, the molecular mechanisms underlying effects of thyroid hormone on proliferation, differentiation, migration, synaptogenesis, and myelination in the developing nervous system have been gradually dissected. However, recent data reveal that thyroid signaling influences neuronal development throughout life, from early embryogenesis to the neurogenesis in the adult brain. This review deals with the latter phase and analyses current knowledge on the role of T3, the active form of thyroid hormone, and its receptors in regulating neural stem cell function in the hippocampus and the subventricular zone, the two principal sites harboring neurogenesis in the adult mammalian brain. In particular, we discuss the critical roles of T3 and TRα1 in commitment to a neuronal phenotype, a process that entails the repression of a number of genes notably that encoding the pluripotency factor, Sox2. Furthermore, the question of the relevance of thyroid hormone control of adult neurogenesis is considered in the context of brain aging, cognitive decline, and neurodegenerative disease.