A nuclear magnetic resonance biomarker for neural progenitor cells: is it all neurogenesis?

Adult Hippocampal Neurogenesis in the Human Brain: Updates, Challenges, and Perspectives

The existence of neurogenesis in the adult human hippocampus has been under considerable debate within the past three decades due to the diverging conclusions originating mostly from immunohistochemistry studies. While some of these reports conclude that hippocampal neurogenesis in humans occurs throughout physiologic aging, others indicate that this phenomenon ends by early childhood. More recently, some groups have adopted next-generation sequencing technologies to characterize with more acuity the extent of this phenomenon in humans. Here, we review the current state of research on adult hippocampal neurogenesis in the human brain with an emphasis on the challenges and limitations of using immunohistochemistry and next-generation sequencing technologies for its study.

Drivers of Chronic Pathology Following Ischemic Stroke: A Descriptive Review

Stroke is the third leading cause of death and long-term disability in the world. Considered largely a disease of aging, its global economic and healthcare burden is expected to rise as more people survive into advanced age. With recent advances in acute stroke management, including the expansion of time windows for treatment with intravenous thrombolysis and mechanical thrombectomy, we are likely to see an increase in survival rates. It is therefore critically important to understand the complete pathophysiology of ischemic stroke, both in the acute and subacute stages and during the chronic phase in the months and years following an ischemic event. One of the most clinically relevant aspects of the chronic sequelae of stroke is its extended negative effect on cognition. Cognitive impairment may be related to the deterioration and dysfunctional reorganization of white matter seen at later timepoints after stroke, as well as ongoing progressive neurodegeneration. The vasculature of the brain also undergoes significant insult and remodeling following stroke, undergoing changes which may further contribute to chronic stroke pathology. While inflammation and the immune response are well established drivers of acute stroke pathology, the chronicity and functional role of innate and adaptive immune responses in the post-ischemic brain and in the peripheral environment remain largely uncharacterized. In this review, we summarize the current literature on post-stroke injury progression, its chronic pathological features, and the putative secondary injury mechanisms underlying the development of cognitive impairment and dementia. We present findings from clinical and experimental studies and discuss the long-term effects of ischemic stroke on both brain anatomy and functional outcome. Identifying mechanisms that occur months to years after injury could lead to treatment strategies in the chronic phase of stroke to help mitigate stroke-associated cognitive decline in patients.

A transient magnetic resonance spectroscopy peri-ischemic peak: a possible radiological biomarker of post-stroke neurogenesis

BACKGROUND AND AIMS

In adult human brain, neurogenesis seems to persist throughout life and ischemic stroke was proved to stimulate this process. Using magnetic resonance spectroscopy (MRS), a 1.28-ppm peak, putative biomarker of neural progenitor cells (NPCs), was identified both in vitro and in vivo, i.e., in normal rat and healthy human brain. The aim of our study was to identify a 1.28-ppm peak in adult human ischemic brain by using 3.0 T multivoxel MRS.

METHODS

We studied 10 patients, six males, and four females, with a mean (± SD) age of 59.3 (± 17.3), at three different time points from ischemic stroke onset (T0: < 5 days; T14: 14 ± 2 days; T30: 30 ± 2 days).

RESULTS

In all patients except one, a 1.28-ppm peak at T14 was detected at the ischemic boundary (all p values < 0.05). MRS performed on six voluntary age-matched healthy subjects did not detect any 1.28-ppm peak.

CONCLUSIONS

The nature of this 1.28-pm peak is uncertain; however, our data support the hypothesis that it might represent a marker of NPCs in post-stroke human brain.

Neurogenesis in aging and age-related neurodegenerative diseases

Adult neurogenesis, the process by which neurons are generated in certain areas of the adult brain, declines in an age-dependent manner and is one potential target for extending cognitive healthspan. Aging is a major risk factor for neurodegenerative diseases and, as lifespans are increasing, these health challenges are becoming more prevalent. An age-associated loss in neural stem cell number and/or activity could cause this decline in brain function, so interventions that reverse aging in stem cells might increase the human cognitive healthspan. In this review, we describe the involvement of adult neurogenesis in neurodegenerative diseases and address the molecular mechanistic aspects of neurogenesis that involve some of the key aggregation-prone proteins in the brain (i.e., tau, Aβ, α-synuclein, …). We summarize the research pertaining to interventions that increase neurogenesis and regulate known targets in aging research, such as mTOR and sirtuins. Lastly, we share our outlook on restoring the levels of neurogenesis to physiological levels in elderly individuals and those with neurodegeneration. We suggest that modulating neurogenesis represents a potential target for interventions that could help in the fight against neurodegeneration and cognitive decline.

Targeting impaired adult hippocampal neurogenesis in ageing by leveraging intrinsic mechanisms regulating Neural Stem Cell activity

Deficits in adult neurogenesis may contribute to the aetiology of many neurodevelopmental, psychiatric and neurodegenerative diseases. Genetic ablation of neurogenesis provides proof of concept that adult neurogenesis is required to sustain complex and dynamic cognitive functions, such as learning and memory, mostly by providing a high degree of plasticity to neuronal circuits. In addition, adult neurogenesis is reactive to external stimuli and the environment making it particularly susceptible to impairment and consequently contributing to comorbidity. In the human brain, the dentate gyrus of the hippocampus is the main active source of neural stem cells that generate granule neurons throughout life. The regulation and preservation of the pool of neural stem cells is central to ensure continuous and healthy adult hippocampal neurogenesis (AHN). Recent advances in genetic and metabolic profiling alongside development of more predictive animal models have contributed to the development of new concepts and the emergence of molecular mechanisms that could pave the way to the implementation of new therapeutic strategies to treat neurological diseases. In this review, we discuss emerging molecular mechanisms underlying AHN that could be embraced in drug discovery to generate novel concepts and targets to treat diseases of ageing including neurodegeneration. To support this, we review cellular and molecular mechanisms that have recently been identified to assess how AHN is sustained throughout life and how AHN is associated with diseases. We also provide an outlook on strategies for developing correlated biomarkers that may accelerate the translation of pre-clinical and clinical data and review clinical trials for which modulation of AHN is part of the therapeutic strategy.

Tissue-Specific Ferritin- and GFP-Based Genetic Vectors Visualize Neurons by MRI in the Intact and Post-Ischemic Rat Brain

(1) Background: Neurogenesis is considered to be a potential brain repair mechanism and is enhanced in stroke. It is difficult to reconstruct the neurogenesis process only from the histological sections taken from different animals at different stages of brain damage and restoration. Study of neurogenesis would greatly benefit from development of tissue-specific visualization probes. (2) Purpose: The study aimed to explore if overexpression of ferritin, a nontoxic iron-binding protein, under a doublecortin promoter can be used for non-invasive visualization of neurogenesis using magnetic resonance imaging (MRI). (3) Methods: Ferritin heavy chain (FerrH) was expressed in the adeno-associated viral backbone (AAV) under the doublecortin promoter (pDCX), specific for young neurons, in the viral construct AAV-pDCX-FerrH. Expression of the enhanced green fluorescent protein (eGFP) was used as an expression control (AAV-pDCX-eGFP). The viral vectors or phosphate-buffered saline (PBS) were injected intracerebrally into 18 adult male Sprague–Dawley rats. Three days before injection, rats underwent transient middle-cerebral-artery occlusion or sham operation. Animals were subjected to In vivo MRI study before surgery and on days 7, 14, 21, and 28 days after injection using a Bruker BioSpec 11.7 T scanner. Brain sections obtained on day 28 after injection were immunostained for ferritin, young (DCX) and mature (NeuN) neurons, and activated microglia/macrophages (CD68). Additionally, RT-PCR was performed to confirm ferritin expression. (4) Results: T2* images in post-ischemic brains of animals injected with AAV-pDCX-FerrH showed two distinct zones of MRI signal hypointensity in the ipsilesioned hemisphere starting from 14 days after viral injection—in the ischemic lesion and near the lateral ventricle and subventricular zone (SVZ). In sham-operated animals, only one zone of hypointensity near the lateral ventricle and SVZ was revealed. Immunochemistry showed that ferritin-expressing cells in ischemic lesions were macrophages (88.1%), while ferritin-expressing cells near the lateral ventricle in animals both after ischemia and sham operation were mostly mature (55.7% and 61.8%, respectively) and young (30.6% and 7.1%, respectively) neurons. RT-PCR confirmed upregulated expression of ferritin in the caudoputamen and corpus callosum. Surprisingly, in animals injected with AAV-pDCX-eGFP we similarly observed two zones of hypointensity on T2* images. Cellular studies also showed the presence of mature (81.5%) and young neurons (6.1%) near the lateral ventricle in both postischemic and sham-operated animals, while macrophages in ischemic lesions were ferritin-positive (98.2%). (5) Conclusion: Ferritin overexpression induced by injection of AAV-pDCX-FerrH was detected by MRI using T2*-weighted images, which was confirmed by immunochemistry showing ferritin in young and mature neurons. Expression of eGFP also caused a comparable reduced MR signal intensity in T2*-weighted images. Additional studies are needed to investigate the potential and tissue-specific features of the use of eGFP and ferritin expression in MRI studies.

Engineered Perineural Vascular Plexus for Modeling Developmental Toxicity

There is a vital need to develop in vitro models of the developing human brain to recapitulate the biological effects that toxic compounds have on the brain. To model perineural vascular plexus (PNVP) in vitro, which is a key stage in embryonic development, human embryonic stem cells (hESC)-derived endothelial cells (ECs), neural progenitor cells, and microglia (MG) with primary pericytes (PCs) in synthetic hydrogels in a custom-designed microfluidics device are cocultured. The formation of a vascular plexus that includes networks of ECs (CD31+, VE-cadherin+), MG (IBA1+), and PCs (PDGFRβ+), and an overlying neuronal layer that includes differentiated neuronal cells (βIII Tubulin+, GFAP+) and radial glia (Nestin+, Notch2NL+), are characterized. Increased brain-derived neurotrophic factor secretion and differential metabolite secretion by the vascular plexus and the neuronal cells over time are consistent with PNVP functionality. Multiple concentrations of developmental toxicants (teratogens, microglial disruptor, and vascular network disruptors) significantly reduce the migration of ECs and MG toward the neuronal layer, inhibit formation of the vascular network, and decrease vascular endothelial growth factor A (VEGFA) secretion. By quantifying 3D cell migration, metabolic activity, vascular network disruption, and cytotoxicity, the PNVP model may be a useful tool to make physiologically relevant predictions of developmental toxicity.

The Appearance of the Warburg Effect in the Developing Avian Eye Characterized In Ovo: How Neurogenesis Can Remodel Neuroenergetics

Purpose

The avian eye is an established model for exploring mechanisms that coordinate morphogenesis and metabolism during embryonic development. Less is known, however, about trafficking of bioenergetic and metabolic signaling molecules that are involved in retinal neurogenesis.

Methods

Here we tested whether the known 3-day delayed neurogenesis occurring in the pigeon compared with the chick was associated with a deferred reshaping of eye metabolism in vivo. Developmental metabolic remodeling was explored using 1H-magnetic resonance spectroscopy of the whole eye and vitreous body, in ovo, in parallel with biochemical and molecular analyses of retinal, vitreous, and lens extracts from bird embryos.

Results

Cross-species comparisons enabled us to show that a major glycolytic switch in the retina is related to neurogenesis rather than to eye growth. We further show that the temporal emergence of an interlocking regulatory cascade controlling retinal oxidative phosphorylation and glycolysis results in the exchange of lactate and citrate between the retina and vitreous.

Conclusions

Our results point to the vitreous as a reservoir and buffer of energy metabolites that provides trophic support to oxidative neurons, such as retinal ganglion cells, in early development. Through its control of key glycolytic regulatory enzymes, citrate, exchanged between extracellular and intracellular compartments between the retina and vitreous, is a key metabolite in the initiation of a glycolytic switch.

Human Hippocampal Neurogenesis Persists throughout Aging

Adult hippocampal neurogenesis declines in aging rodents and primates. Aging humans are thought to exhibit waning neurogenesis and exercise-induced angiogenesis, with a resulting volumetric decrease in the neurogenic hippocampal dentate gyrus (DG) region, although concurrent changes in these parameters are not well studied. Here we assessed whole autopsy hippocampi from healthy human individuals ranging from 14 to 79 years of age. We found similar numbers of intermediate neural progenitors and thousands of immature neurons in the DG, comparable numbers of glia and mature granule neurons, and equivalent DG volume across ages. Nevertheless, older individuals have less angiogenesis and neuroplasticity and a smaller quiescent progenitor pool in anterior-mid DG, with no changes in posterior DG. Thus, healthy older subjects without cognitive impairment, neuropsychiatric disease, or treatment display preserved neurogenesis. It is possible that ongoing hippocampal neurogenesis sustains human-specific cognitive function throughout life and that declines may be linked to compromised cognitive-emotional resilience.

NMR Metabolomics for Stem Cell type discrimination

Cell metabolism is a key determinant factor for the pluripotency and fate commitment of Stem Cells (SCs) during development, ageing, pathological onset and progression. We derived and cultured selected subpopulations of rodent fetal, postnatal, adult Neural SCs (NSCs) and postnatal glial progenitors, Olfactory Ensheathing Cells (OECs), respectively from the subventricular zone (SVZ) and the olfactory bulb (OB). Cell lysates were analyzed by proton Nuclear Magnetic Resonance (¹H-NMR) spectroscopy leading to metabolites identification and quantitation. Subsequent multivariate analysis of NMR data by Principal Component Analysis (PCA), and Partial Least Square Discriminant Analysis (PLS-DA) allowed data reduction and cluster analysis. This strategy ensures the definition of specific features in the metabolic content of phenotypically similar SCs sharing a common developmental origin. The metabolic fingerprints for selective metabolites or for the whole spectra demonstrated enhanced peculiarities among cell types. The key result of our work is a neat divergence between OECs and the remaining NSC cells. We also show that statistically significant differences for selective metabolites characterizes NSCs of different ages. Finally, the retrived metabolome in cell cultures correlates to the physiological SC features, thus allowing an integrated bioengineering approach for biologic fingerprints able to dissect the (neural) SC molecular specificities.

The Role of Lipid Metabolism for Neural Stem Cell Regulation

Neural stem/progenitor cells (NSPCs) give rise to billions of cells during development and are critical for proper brain formation. The finding that NSPCs persist throughout adulthood has challenged the view that the brain has poor regenerative abilities and raised hope for stem cell-based regenerative therapies. For decades there has been a strong movement towards understanding the requirements of NSPCs and their regulation, resulting in the discovery of many transcription factors and signaling pathways that can influence NSPC behavior and neurogenesis. However, the role of metabolism for NSPC regulation has only gained attention recently. Lipid metabolism in particular has been shown to influence proliferation and neurogenesis, offering exciting new possible mechanisms of NSPC regulation, as lipids are not only the building blocks of membranes, but can also act as alternative energy sources and signaling entities. Here I review the recent literature examining the role of lipid metabolism for NSPC regulation and neurogenesis.

Aerobic fitness associates with mnemonic discrimination as a mediator of physical activity effects: evidence for memory flexibility in young adults

A physically active lifestyle has beneficial effects on hippocampal memory function. A potential mechanism for this effect is exercise-enhanced hippocampal plasticity, particularly in the dentate gyrus (DG). Within hippocampal memory formation, the DG plays a crucial role in pattern separation, which is the ability to discriminate among similar experiences. Computational models propose a theoretical hypothesis that enhanced DG-mediated pattern separation leads to “memory flexibility”–a selective improvement in the ability to overcome moderate levels of mnemonic interference. Thus, in the current cross-sectional study of healthy young adults, we tested the working hypothesis that aerobic fitness, as a physiological indicator of endurance capacity associated with physical activity, is strongly associated with mnemonic discrimination at moderate interference levels. When divided the sample (n = 75) based on a median split of aerobic fitness, the higher fitness group had better discrimination performance for moderate interference levels compared to the lower fitness group, namely, exhibited memory flexibility. Moreover, aerobic fitness levels were positively associated with discrimination performance for moderate interference levels, as a mediator of physical activity effects. This evidence suggests that aerobic fitness levels are associated with hippocampal DG-related memory, which is consistent with literature showing positive effect of physical exercise on hippocampal memory.

Distinguishing tendon and ligament fibroblasts based on 1H nuclear magnetic resonance spectroscopy

Tendon and ligament (T/L) have been known to be obviously different from each other in tissue level. However, due to the overlapping gene markers, distinction in cellular level has not been clearly verified yet. Recently, the use of nuclear magnetic resonance (NMR) spectroscopy has shown the potential to detect biological markers in cellular level. Therefore, in this study we applied a non-invasive technique based on NMR spectroscopy to establish biomarkers to distinguish between T/L fibroblasts. In addition the cellular morphologies and gene expression patterns were also investigated for comparison through optical microscopy and real-time polymerase chain reaction (PCR). No difference was observed from morphology and real-time PCR results, either as expected. However, we found clear differences in their metabolomic spectra using 1H NMR spectroscopy. The calculated integral values of fatty acids (with chemical shifts at ~0.9, 1.26, 1.59, 2.05, 2.25, and 2.81 ppm), lactate (~1.33 ppm), and leucine (~2.72 ppm) were significantly different between the two types of fibroblasts. To be specific tendon group exhibited higher level of the metabolite than ligament group. In conclusion, in-cell metabolomic evaluation by NMR technique used in this study is believed to provide a promising tool in distinguishing cell types, especially T/L cells, which cannot be classified by conventional biological assays.

Live Imaging of Adult Neural Stem Cells in Rodents

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

In Vivo Targeted Magnetic Resonance Imaging of Endogenous Neural Stem Cells in the Adult Rodent Brain

Neural stem cells in the adult mammalian brain have a significant level of neurogenesis plasticity. In vivo monitoring of adult endogenous NSCs would be of great benefit to the understanding of the neurogenesis plasticity under normal and pathological conditions. Here we show the feasibility of in vivo targeted MR imaging of endogenous NSCs in adult mouse brain by intraventricular delivery of monoclonal anti-CD15 antibody conjugated superparamagnetic iron oxide nanoparticles. After intraventricular administration of these nanoparticles, the subpopulation of NSCs in the anterior subventricular zone and the beginning of the rostral migratory stream could be in situ labeled and were in vivo visualized with 7.0-T MR imaging during a period from 1 day to 7 days after the injection. Histology confirmed that the injected targeted nanoparticles were specifically bound to CD15 positive cells and their surrounding extracellular matrix. Our results suggest that in vivo targeted MR imaging of endogenous neural stem cells in adult rodent brain could be achieved by using anti-CD15-SPIONs as the molecular probe; and this targeting imaging strategy has the advantage of a rapid in vivo monitoring of the subpopulation of endogenous NSCs in adult brains.

Adult Neurogenesis in Humans

Adult neurogenesis appears very well conserved among mammals. It was, however, not until recently that quantitative data on the extent of this process became available in humans, largely because of methodological challenges to study this process in man. There is substantial hippocampal neurogenesis in adult humans, but humans appear unique among mammals in that there is no detectable olfactory bulb neurogenesis but continuous addition of new neurons in the striatum.

In vivo and ex vivo magnetic resonance spectroscopy of the infarct and the subventricular zone in experimental stroke

Ex vivo high-resolution magic-angle spinning (HRMAS) provides metabolic information with higher sensitivity and spectral resolution than in vivo magnetic resonance spectroscopy (MRS). Therefore, we used both techniques to better characterize the metabolic pattern of the infarct and the neural progenitor cells (NPCs) in the ipsilateral subventricular zone (SVZi). Ischemic stroke rats were divided into three groups: G0 (non-stroke controls, n = 6), G1 (day 1 after stroke, n = 6), and G7 (days 6 to 8 after stroke, n = 12). All the rats underwent MRS. Three rats per group were analyzed by HRMAS. The remaining rats were used for immunohistochemical studies. In the infarct, both techniques detected significant metabolic changes. The most relevant change was in mobile lipids (2.80 ppm) in the G7 group (a 5.53- and a 3.95-fold increase by MRS and HRMAS, respectively). In the SVZi, MRS did not detect any significant metabolic change. However, HRMAS detected a 2.70-fold increase in lactate and a 0.68-fold decrease in N-acetylaspartate in the G1 group. None of the metabolites correlated with the 1.37-fold increase in NPCs detected by immunohistochemistry in the G7 group. In conclusion, HRMAS improves the metabolic characterization of the brain in experimental ischemic stroke. However, none of the metabolites qualifies as a surrogate biomarker of NPCs.

(1) H NMR spectroscopy of glioblastoma stem-like cells identifies alpha-aminoadipate as a marker of tumor aggressiveness

Patients suffering from glioblastoma multiforme (GBM) face a poor prognosis with median survival of about 14 months. High recurrence rate and failure of conventional treatments are attributed to the presence of GBM cells with stem-like properties (GSCs). Metabolite profiles of 42 GSC lines established from the tumor tissue of adult GBM patients were screened with (1) H NMR spectroscopy and compared with human neural progenitor cells from human adult olfactory bulb (OB-NPCs) and from the developing human brain (HNPCs). A first subset (n=12) of GSCs exhibited a dramatic accumulation of the metabolite α-aminoadipate (αAAD), product of the oxidation of α-aminoadipic semialdehyde catalyzed by the ALDH7A1 aldehyde dehydrogenase (ALDH) family in lysine catabolism. αAAD was low/not detectable in a second GSC subset (n=13) with the same neural metabolic profile as well as in a third GSC subset (n=17) characterized by intense lipid signals. Likewise, αAAD was not detected in the spectra of OB-NPCs or HNPCs. Inhibition of mitochondrial ATP synthase by oligomycin treatment revealed that the lysine degradative pathway leading to αAAD formation proceeds through saccharopine, as usually observed in developing brain. Survival curves indicated that high αAAD levels in GSCs significantly correlated with poor patient survival, similarly to prostate and non-small-cell-lung cancers, where activity of ALDH7A1 correlates with tumor aggressiveness.

In vivo imaging of endogenous neural stem cells in the adult brain

The discovery of endogenous neural stem cells (eNSCs) in the adult mammalian brain with their ability to self-renew and differentiate into functional neurons, astrocytes and oligodendrocytes has raised the hope for novel therapies of neurological diseases. Experimentally, those eNSCs can be mobilized in vivo, enhancing regeneration and accelerating functional recovery after, e.g., focal cerebral ischemia, thus constituting a most promising approach in stem cell research. In order to translate those current experimental approaches into a clinical setting in the future, non-invasive imaging methods are required to monitor eNSC activation in a longitudinal and intra-individual manner. As yet, imaging protocols to assess eNSC mobilization non-invasively in the live brain remain scarce, but considerable progress has been made in this field in recent years. This review summarizes and discusses the current imaging modalities suitable to monitor eNSCs in individual experimental animals over time, including optical imaging, magnetic resonance tomography and-spectroscopy, as well as positron emission tomography (PET). Special emphasis is put on the potential of each imaging method for a possible clinical translation, and on the specificity of the signal obtained. PET-imaging with the radiotracer 3’-deoxy-3’-[¹⁸F]fluoro-L-thymidine in particular constitutes a modality with excellent potential for clinical translation but low specificity; however, concomitant imaging of neuroinflammation is feasible and increases its specificity. The non-invasive imaging strategies presented here allow for the exploitation of novel treatment strategies based upon the regenerative potential of eNSCs, and will help to facilitate a translation into the clinical setting.

Role of adult hippocampal neurogenesis in cognition in physiology and disease: pharmacological targets and biomarkers

Adult hippocampal neurogenesis is a remarkable form of brain structural plasticity by which new functional neurons are generated from adult neural stem cells/precursors. Although the precise role of this process remains elusive, adult hippocampal neurogenesis is important for learning and memory and it is affected in disease conditions associated with cognitive impairment, depression, and anxiety. Immature neurons in the adult brain exhibit an enhanced structural and synaptic plasticity during their maturation representing a unique population of neurons to mediate specific hippocampal function. Compelling preclinical evidence suggests that hippocampal neurogenesis is modulated by a broad range of physiological stimuli which are relevant in cognitive and emotional states. Moreover, multiple pharmacological interventions targeting cognition modulate adult hippocampal neurogenesis. In addition, recent genetic approaches have shown that promoting neurogenesis can positively modulate cognition associated with both physiology and disease. Thus the discovery of signaling pathways that enhance adult neurogenesis may lead to therapeutic strategies for improving memory loss due to aging or disease. This chapter endeavors to review the literature in the field, with particular focus on (1) the role of hippocampal neurogenesis in cognition in physiology and disease; (2) extrinsic and intrinsic signals that modulate hippocampal neurogenesis with a focus on pharmacological targets; and (3) efforts toward novel strategies pharmacologically targeting neurogenesis and identification of biomarkers of human neurogenesis.

The hippocampus and exercise: histological correlates of MR-detected volume changes

Growing evidence indicates that physical exercise increases hippocampal volume. This has consistently been shown in mice and men using magnetic resonance imaging. On the other hand, histological studies have reported profound alterations on a cellular level including increased adult hippocampal neurogenesis after exercise. A combined investigation of both phenomena has not been documented so far although a causal role of adult neurogenesis for increased hippocampal volume has been suggested before. We investigated 20 voluntary wheel running and 20 sedentary mice after a period of 2 month voluntary wheel running. Half of each group received focalized hippocampal irradiation to inhibit neurogenesis prior to wheel running. Structural MRI and histological investigations concerning newborn neurons (DCX), glial cells (GFAP), microglia, proliferating and pyknotic cells, neuronal activation, as well as blood vessel density and arborisation were performed. In a regression model, neurogenesis was the marker best explaining hippocampal gray matter volume. Individual analyses showed a positive correlation of gray matter volume with DCX-positive newborn neurons in the subgroups, too. GFAP-positive cells significantly interacted with gray matter volume with a positive correlation in sham-irradiated mice and no correlation in irradiated mice. Although neurogenesis appears to be an important marker of higher hippocampal gray matter volume, a monocausal relationship was not indicated, requesting further investigations.

A ventral view on antidepressant action: roles for adult hippocampal neurogenesis along the dorsoventral axis

Adult hippocampal neurogenesis is implicated in antidepressant action, stress responses, and cognitive functioning. The hippocampus is functionally segregated along its longitudinal axis into dorsal (dHi) and ventral (vHi) regions in rodents, and analogous posterior and anterior regions in primates, whereby the vHi preferentially regulates stress and anxiety, while the dHi preferentially regulates spatial learning and memory. Given the role of neurogenesis in functions preferentially regulated by the dHi or vHi, it is plausible that neurogenesis is preferentially regulated in either the dHi or vHi depending upon the stimulus. We appraise here the literature on the effects of stress and antidepressants on neurogenesis along the hippocampal longitudinal axis and explore whether preferential regulation of neurogenesis in the vHi/anterior hippocampus contributes to stress resilience and antidepressant action.

Hippocampal neurogenesis and antidepressive therapy: shocking relations

Speculations on the involvement of hippocampal neurogenesis, a form of neuronal plasticity, in the aetiology of depression and the mode of action of antidepressive therapies, started to arise more than a decade ago. But still, conclusive evidence that adult neurogenesis contributes to antidepressive effects of pharmacological and physical therapies has not been generated yet. This review revisits recent findings on the close relation between the mode(s) of action of electroconvulsive therapy (ECT), a powerful intervention used as second-line treatment of major depression disorders, and the neurogenic response to ECT. Following application of electroconvulsive shocks, intricate interactions between neurogenesis, angiogenesis, and microglia activation, the hypothalamic-pituitary-adrenal axis and the secretion of neurotrophic factors have been documented. Furthermore, considering the fact that neurogenesis strongly diminishes along aging, we investigated the response to electroconvulsive shocks in young as well as in aged cohorts of mice.

TGF-beta signalling in the adult neurogenic niche promotes stem cell quiescence as well as generation of new neurons

Members of the transforming growth factor (TGF)-β family govern a wide range of mechanisms in brain development and in the adult, in particular neuronal/glial differentiation and survival, but also cell cycle regulation and neural stem cell maintenance. This clearly created some discrepancies in the field with some studies favouring neuronal differentiation/survival of progenitors and others favouring cell cycle exit and neural stem cell quiescence/maintenance. Here, we provide a unifying hypothesis claiming that through its regulation of neural progenitor cell (NPC) proliferation, TGF-β signalling might be responsible for (i) maintaining stem cells in a quiescent stage, and (ii) promoting survival of newly generated neurons and their functional differentiation. Therefore, we performed a detailed histological analysis of TGF-β1 signalling in the hippocampal neural stem cell niche of a transgenic mouse that was previously generated to express TGF-β1 under a tetracycline regulatable Ca-Calmodulin kinase promoter. We also analysed NPC proliferation, quiescence, neuronal survival and differentiation in relation to elevated levels of TGF-β1 in vitro and in vivo conditions. Finally, we performed a gene expression profiling to identify the targets of TGF-β1 signalling in adult NPCs. The results demonstrate that TGF-β1 promotes stem cell quiescence on one side, but also neuronal survival on the other side. Thus, considering the elevated levels of TGF-β1 in ageing and neurodegenerative diseases, TGF-β1 signalling presents a molecular target for future interventions in such conditions.

1H NMR detects different metabolic profiles in glioblastoma stem-like cells

The metabolic profiles of glioblastoma stem-like cells (GSCs) growing in neurospheres were examined by (1)H NMR spectroscopy. Spectra of two GSC lines, labelled 1 and 83, from tumours close to the subventricular zone of the temporal lobe were studied in detail and compared with those of neural stem/progenitor cells from the adult olfactory bulb (OB-NPCs) and of the T98G glioblastoma cell line. In both GSCs, signals from myoinositol (Myo-I), UDP-hexosamines (UDP-Hex) and glycine indicated an astrocyte/glioma metabolism. For line 1, the presence of signals from N-acetyl aspartate, GABA and creatine pointed to a neuronal fingerprint. These metabolites were almost absent from line 83 spectra, whereas lipid signals, absent from normal neural lineages, were intense in line 83 spectra and remained low in those of line 1, irrespective of apoptotic fate. Spectra of OB-NPC cells displayed strong similarities with those from line 1, with low lipid signals and clearly detectable neuronal signals. In contrast, the spectral profile of line 83 was more similar to that of T98G, displaying high lipids and nearly complete absence of the neuronal markers. A mixed neural-astrocyte metabolic phenotype with a strong neuronal fingerprint was therefore found in line 1, while an astrocytic/glioma-like metabolism prevailed in line 83. We found a signal assigned to the amide proton of N-acetyl galactosamine in GSC lines and in OB-NPC spectra, whereas it was absent from those of T98G cells. This signal may be related to a stem-cell-specific protein glycosylation pattern and is therefore suggested as a marker of cell multipotency. Other GSC lines from patients with different clinical outcomes were then examined. Unsupervised analysis of spectral data from 13 lines yielded two clusters, with six lines resembling spectral features of line 1 and seven resembling those of line 83, suggesting that distinct metabolic phenotypes may be present in GSC lines.

Neurogenesis and neuronal regeneration in status epilepticus

Neurogenesis in the adult central nervous system has been well documented in several mammals including humans. By now, a plethora of data has been generated with the aim of understanding the molecular and cellular events governing neurogenesis. This growing comprehension will provide the basis for modulation of neurogenesis for therapeutic purposes, in particular in neurodegenerative diseases. Herein, we review the current knowledge on neurogenesis, in particular in the frame of epilepsy, since seizures have massive effects on neurogenesis. Conversely, some studies have suggested that aberrant neurogenesis might contribute to the development or manifestation of epilepsy and, moreover, chronic inhibition of neurogenesis in epilepsy might contribute to comorbidities of epilepsy such as cognitive deficits. Therefore, a better understanding of neurogenesis in the context of epilepsy is still required for future therapeutic purposes.

In vivo imaging of adult human hippocampal neurogenesis: progress, pitfalls and promise

New neurons are produced within the hippocampus of the mammalian brain throughout life. Evidence from animal studies has suggested that the function of these adult-born neurons is linked to cognition and emotion. Until we are able to detect and measure levels of adult neurogenesis in living human brains-a formidable challenge for now-we cannot establish its functional importance in human health, disease and new treatment development. Current non-invasive neuroimaging modalities can provide live snapshots of the brain's structure, chemistry, activity and connectivity. This review explores whether existing macroscopic imaging methods can be used to understand the microscopic dynamics of adult hippocampal neurogenesis in living individuals. We discuss recent studies that have found correlations between neuroimaging measures of human hippocampal biology and levels of pro- or anti-neurogenic stimuli, weigh whether these correlations reflect changes in adult neurogenesis, detail the conceptual and technical limitations of these studies and elaborate on what will be needed to validate in vivo neuroimaging measures of adult neurogenesis for future investigations.

NMR spectroscopy of macrophages loaded with native, oxidized or enzymatically degraded lipoproteins

Oxidized and enzymatically modified low-density lipoproteins (oxLDL and eLDL) play a key role in early stages of atherogenesis. Their uptake by recruited macrophages leads to endolysosomal phospholipidosis or foam cell formation, respectively, each of which is preceded by highly differential lipid restructuring processes. We applied ¹H-NMR spectroscopy (NMRS) to elucidate these structural rearrangements both in consequence of lipoprotein modifications and following phagocytosis. Being specifically sensitive to the mobile lipid subset, NMRS of oxLDL and eLDL revealed a partial and total immobilization of lipids, respectively. NMRS of intact macrophages showed a sixfold increase in mobile lipids in case of loading with eLDL but no significant changes for oxLDL or native LDL. This finding reflected the disparate lipid storage in lipid droplets and in multilamellar endolysosomal clusters when loaded with either eLDL or oxLDL, respectively. Moreover, a significant shift of the degree of saturation towards mainly polyunsaturated fatty acid chains was found for the mobile lipid pool in eLDL-loaded macrophages. Additional analyses of lipid extracts by NMRS and mass spectrometry (MS) reflected these changes in lipid content and in fatty acid composition only partially. In summary, in-cell NMRS represents a unique lipidomics tool to investigate structural changes within the mobile lipid pool following atherogenic triggers that can be not detected by the analysis of lipid extracts by MS or NMRS.

Stem cell metabolic and spectroscopic profiling

Stem cells offer great potential for regenerative medicine because they regenerate damaged tissue by cell replacement and/or by stimulating endogenous repair mechanisms. Although stem cells are defined by their functional properties, such as the potential to proliferate, to self-renew, and to differentiate into specific cell types, their identification based on the expression of specific markers remains vague. Here, profiles of stem cell metabolism might highlight stem cell function more than the expression of single genes/markers. Thus, systematic approaches including spectroscopy might yield insight into stem cell function, identity, and stemness. We review the findings gained by means of metabolic and spectroscopic profiling methodologies, for example, nuclear magnetic resonance spectroscopy (NMRS), mass spectrometry (MS), and Raman spectroscopy (RS), with a focus on neural stem cells and neurogenesis.

New neurons for 'survival of the fittest'

Adult neurogenesis is often considered an archaic trait that has undergone a 'phylogenetic reduction' from amphibian ancestors to humans. However, adult neurogenesis in the hippocampal dentate gyrus might actually be a late-evolved trait. In non-mammals, adult hippocampal neurogenesis is not restricted to the equivalents of the dentate gyrus, which also show different connectivity and functionality compared to their mammalian counterpart. Moving actively in a changing world and dealing with novelty and complexity regulate adult neurogenesis. New neurons might thus provide the cognitive adaptability to conquer ecological niches rich with challenging stimuli.

Neurodegenerative disease and adult neurogenesis

The generation and cell death of newly generated cells have critical roles in brain development and maintenance in the embryonic and adult brain. Alterations in these processes are also seen in neurodegenerative diseases. Genes that are key players in neurodegenerative diseases (α-synuclein, presenilin-1, tau, huntingtin) are also physiologically involved in modulating brain plasticity. Interestingly, in some neurodegenerative diseases, the specific alterations in neurogenic areas such as the dentate gyrus and subventricular zone/olfactory bulb system parallel the early or premotor symptoms that are seen in the early stages of these diseases, such as depression, anxiety or olfactory dysfunction. We will review the modulation of neurogenesis in animal models and human brains of Parkinson's disease, Huntington's disease and Alzheimer's disease.

In vivo imaging of adult neurogenesis

As our understanding of adult neurogenesis increases, hopes rise that neurological disorders and neuronal losses might be addressed one day by neural stem cell-based regenerative therapies. However, evaluating the efficacy and safety of any neurogenesis-based intervention requires a means to monitor neurogenesis in vivo and, so far, no such imaging techniques are available for human studies. Nevertheless, using imaging techniques presently available to clinicians, i.e. magnetic resonance imaging, positron emission tomography and optical imaging, significant progress has been made in this direction over the last decade. This review describes the current state-of-the-art for each imaging technique, and shows that detection of neurogenesis could theoretically be achieved using current imaging devices. Indeed, in vivo imaging of neurogenesis has already been achieved in mice using transgenic model systems. However, the imaging of human neurogenesis still requires the development of methods to reliably target the neural stem cells and the neuronal precursors in vivo.

In vivo monitoring of adult neurogenesis in health and disease

Adult neurogenesis, i.e., the generation of new neurons in the adult brain, presents an enormous potential for regenerative therapies of the central nervous system. While 5-bromo-2'-deoxyuridine labeling and subsequent histology or immunohistochemistry for cell-type-specific markers is still the gold standard in studies of neurogenesis, novel techniques, and tools for in vivo imaging of neurogenesis have been recently developed and successfully applied. Here, we review the latest progress on these developments, in particular in the area of magnetic resonance imaging (MRI) and optical imaging. In vivo in situ labeling of neural progenitor cells (NPCs) with micron-sized iron oxide particles enables longitudinal visualization of endogenous progenitor cell migration by MRI. The possibility of genetic labeling for cellular MRI was demonstrated by using the iron storage protein ferritin as the MR reporter-gene. However, reliable and consistent results using ferritin imaging for monitoring endogenous progenitor cell migration have not yet been reported. In contrast, genetic labeling of NPCs with a fluorescent or bioluminescent reporter has led to the development of some powerful tools for in vivo imaging of neurogenesis. Here, two strategies, i.e., viral labeling of stem/progenitor cells and transgenic approaches, have been used. In addition, the use of specific promoters for neuronal progenitor cells such as doublecortin increases the neurogenesis-specificity of the labeling. Naturally, the ultimate challenge will be to develop neurogenesis imaging methods applicable in humans. Therefore, we certainly need to consider other modalities such as positron emission tomography and proton magnetic resonance spectroscopy ((1)H-MRS), which have already been implemented for both animals and humans. Further improvements of sensitivity and neurogenesis-specificity are nevertheless required for all imaging techniques currently available.

Proton MR spectroscopy of neural stem cells: does the proton-NMR peak at 1.28 ppm function as a biomarker for cell type or state?

Recently, a peak at 1.28 ppm in proton magnetic resonance spectroscopy ((1)H-MRS) of neural stem cells (NSCs) was introduced as a noninterventional biomarker for neurogenesis in vivo. This would be an urgently needed requisite for translational studies in humans regarding the beneficial role of adult neurogenesis for the structural and functional integrity of the brain. However, many concerns have risen about the validity of the proposed signal as a specific marker for NSCs. The peak has also been related to cell-type-independent phenomena such as apoptosis or necrosis. Thus, we compared the 1.28-ppm peak in various immature stem cell populations, including embryonic stem cells, mouse embryonic fibroblasts, embryonic stem cell- and induced pluripotent stem cell-derived NSCs, ex vivo isolated embryonic NSCs, as well as mature and tumor cell types from different germ layers. To correlate the integral peak intensity with cell death, we induced both apoptosis with camptothecin and necrosis with sodium azide. A peak at 1.28 ppm was found in most cell types, and in most, but not all, NSCH cultures, demonstrating no specificity for NSCs. The intensities of the 1.28-ppm resonance significantly correlated with the rate of apoptosis, but not with the rate of necrosis, cell cycle phase distribution, cell size, or type. Multiple regression analysis displayed a significant predictive value of the peak intensity for apoptosis only. In this context, its specificity for apoptosis as a major selection process during neurogenesis may suggest this resonance as an indirect marker for neurogenesis in vivo.

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.

1H-nuclear magnetic resonance spectroscopy of glioblastoma cancer stem cells

The noninvasive tracking of glioblastoma cancer stem cells (CSCs) in vivo constitutes a prerequisite for the development of CSC-specific therapies. Therefore, as a pilot study to identify CSC biomarkers for clinical magnetic resonance spectroscopy, 10 CSC lines were investigated using high-resolution (1)H-nuclear magnetic resonance ((1)H-NMR) spectroscopy at 600 and 800 MHz (14.4 and 18.8 T) under reproducible in vitro conditions. The spectra were analyzed using principal component analysis (PCA), and spectral regions of high variability were evaluated regarding correlations to stem cell-related properties (clonogenic index and CD133 positivity) and cell death. PCA revealed that duplicates of CSC lines clustered together suggesting a characteristic (1)H-NMR pattern of each CSC line. PCA enabled discrimination between samples with high and low clonogenicity, that is, clustering according to one of the hallmarks of stemness in samples with high viability. High/moderate correlations to clonogenicity and CD133 were found in spectral regions with high variability. In contrast, the mobile lipid signal at 1.28 ppm correlated to cell death, but not to stemness, as published previously for neural progenitor cells. In conclusion, our exploratory study demonstrates the correlation of specific resonances within (1)H-NMR spectra with stem cell properties and advocates the use of the 1.28 ppm resonance as biomarker for cell death also in CSCs.

Cell-based therapy for stroke

Current treatments for stroke, such as the use of thrombolytic agents, are often limited by a narrow therapeutic time window. However, the regeneration of the brain after damage is still active days even weeks after stroke occurs, which might provide a second window for treatment. Cell-based therapy can be categorized into two strategies. One is transplantation of exogenous cells into the injured brain to replace the lost cells or support the remaining cells. The other strategy is to enhance the proliferation, differentiation, migration of endogenous stem or progenitor cells. Recent development in adult stem cell research and advancement in the induction of pluripotent stem cells from somatic adult cells provide a tremendous opportunity for transplantation therapy. Understanding the mechanisms and regulations involved in the endogenous neurogenesis will also help develop novel therapeutic interventions to promote neurogenesis and functional recovery in stroke. This review describes up-to-date progresses in cell-based therapy for the treatment of stroke.

13C-labelling studies indicate compartmentalized synthesis of triacylglycerols in C6 rat glioma cells

NMR-visible mobile lipid (ML) signals have been detected in (1)H-NMR spectra of tissues in vivo, ex vivo and in vitro, and have been shown to change in apparent intensity in association with pathology (necrosis in brain tumours) and normal processes (cell differentiation, cell growth arrest and apoptosis). Although it is widely accepted that ML signals originate mainly from fatty-acyl chains in triacylglycerols (TAG) contained in cytosolic lipid droplets (LD), the dynamics of TAG in LD is not yet fully understood. In order to better understand the synthesis of cellular TAG and its relationship to ML dynamics we carried out a set of labelling experiments with C6 rat glioma cells in culture. TAG and phospholipid metabolism was monitored by incubating C6 cells with [1-(13)C]-glucose at two time points during cell growth curve -24 h incubation starting at log-phase; 48 h incubation starting at saturation density- and by acquiring the 2D-HMQC NMR spectra of the respective total lipid extracts. The resulting TAG, diacylglycerol (DAG) and phospholipid labelling patterns can only be explained if TAG synthesis takes place in two different subcellular compartments. One compartment would be the endoplasmic reticulum, which is known to be involved in TAG metabolism, while the other compartment could be the plasma membrane and/or the LD. This possible role of LD is further supported by the recent description of diacylglycerolacyltranferase-activity associated with LD. Accordingly, we postulate the existence of a carbon-shuttling mechanism between plasma membrane phospholipids and endoplasmic reticulum by way of LD content. The results we have obtained with C6 cells may also apply to other cellular systems and should be taken into account when interpreting ML dynamics detected by NMR in vivo.

Transcranial electro-hyperthermia combined with alkylating chemotherapy in patients with relapsed high-grade gliomas: phase I clinical results

Non-invasive loco-regional electro-hyperthermia (EHT) plus alkylating chemotherapy is occasionally used as salvage treatment in the relapse of patients with high-grade gliomas. Experimental data and retrospective studies suggest potential effects. However, no prospective clinical results are available. We performed a single-center prospective non-controlled single-arm Phase I trial. Main inclusion criteria were recurrent high-grade glioma WHO Grade III or IV, age 18-70, and Karnofsky performance score > or = 70. Primary endpoints were dose-limiting toxicities (DLT) and maximum tolerated dose (MTD) with the combined regimen. Groups of 3 or 4 patients were treated 2-5 times a week in a dose-escalation scheme with EHT. Alkylating chemotherapy (ACNU, nimustin) was administered at a dose of 90 mg/m(2) on day 1 of 42 days for up to six cycles or until tumor progression (PD) or DLT occurred. Fifteen patients with high-grade gliomas were included. Relevant toxicities were local pain and increased focal neurological signs or intracranial pressure. No DLT occurred. In some patients, the administration of mannitol during EHT or long-term use of corticosteroids was necessary to resolve symptoms. Although some patients showed responses in their primarily treated sites, the pattern of response was not well defined. EHT plus alkylating chemotherapy is tolerable in patients with relapse of high-grade gliomas. Episodes of intracranial pressure were, at least, possibly attributed to EHT but did not cause DLTs. A Phase II trial targeting treatment effects is warranted on the basis of the results raised in this trial.