A histochemical and immunohistochemical analysis of the subependymal layer in the normal and Huntington's disease brain

Adult Neurogenesis in the Subventricular Zone of Patients with Huntington's and Parkinson's Diseases and following Long-Term Treatment with Deep Brain Stimulation

OBJECTIVE

Parkinson's and Huntington's diseases are characterized by progressive neuronal loss. Previous studies using human postmortem tissues have shown the impact of neurodegenerative disorders on adult neurogenesis. The extent to which adult neural stem cells are activated in the subventricular zone and whether therapeutic treatments such as deep brain stimulation promote adult neurogenesis remains unclear. The goal of the present study is to assess adult neural stem cells activation and neurogenesis in the subventricular zone of patients with Huntington's and Parkinson's diseases who were treated or not by deep brain stimulation.

METHODS

Postmortem brain samples from Huntington's and Parkinson's disease patients who had received or not long-term deep brain stimulation of the subthalamic nucleus were used.

RESULTS

Our results indicate a significant increase in the thickness of the subventricular zone and in the density of proliferating cells and activated stem cells in the brain of Huntington's disease subjects and Parkinson's disease patients treated with deep brain stimulation. We also observed an increase in the density of immature neurons in the brain of these patients.

INTERPRETATION

Overall, our data indicate that long-term deep brain stimulation of the subthalamic nucleus promotes cell proliferation and neurogenesis in the subventricular zone that are reduced in Parkinson's disease. Taken together, our results also provide a detailed characterization of the cellular composition of the adult human subventricular zone and caudate nucleus in normal condition and in Parkinson's and Huntington's diseases and demonstrate the plasticity of these regions in response to neurodegeneration. ANN NEUROL 2025;97:894-906.

Intersecting impact of CAG repeat and huntingtin knockout in stem cell-derived cortical neurons

Huntington's Disease (HD) is caused by a CAG repeat expansion in the gene encoding Huntingtin (HTT). While normal HTT function appears impacted by the mutation, the specific pathways unique to CAG repeat expansion versus loss of normal function are unclear. To understand the impact of the CAG repeat expansion, we evaluated biological signatures of HTT knockout (HTT KO) versus those that occur from the CAG repeat expansion by applying multi-omics, live cell imaging, survival analysis and a novel feature- based pipeline to study cortical neurons (eCNs) derived from an isogenic human embryonic stem cell series (RUES2). HTT KO and the CAG repeat expansion influence developmental trajectories of eCNs, with opposing effects on the growth. Network analyses of differentially expressed genes and proteins associated with enriched epigenetic motifs identified subnetworks common to CAG repeat expansion and HTT KO that include neuronal differentiation, cell cycle regulation, and mechanisms related to transcriptional repression and may represent gain-of-function mechanisms that cannot be explained by HTT loss of function alone. A combination of dominant and loss-of-function mechanisms are likely involved in the aberrant neurodevelopmental and neurodegenerative features of HD that can help inform therapeutic strategies.

Distinct characteristics of microglia from neurogenic and non-neurogenic regions of the human brain in patients with Mesial Temporal Lobe Epilepsy

The study of microglia isolated from adult human brain tissue provides unique insight into the physiology of these brain immune cells and their role in adult human brain disorders. Reports of microglia in post-mortem adult human brain tissue show regional differences in microglial populations, however, these differences have not been fully explored in living microglia. In this study biopsy tissue was obtained from epileptic patients undergoing surgery and consisted of both cortical areas and neurogenic ventricular and hippocampal (Hp) areas. Microglia were concurrently isolated from both regions and compared by immunochemistry. Our initial observation was that a greater number of microglia resulted from isolation and culture of ventricular/Hp tissue than cortical tissue. This was found to be due to a greater proliferative capacity of microglia from ventricular/Hp regions compared to the cortex. Additionally, ventricular/Hp microglia had a greater proliferative response to the microglial mitogen Macrophage Colony-Stimulating Factor (M-CSF). This enhanced response was found to be associated with higher M-CSF receptor expression and higher expression of proteins involved in M-CSF signalling DAP12 and C/EBPβ. Microglia from the ventricular/Hp region also displayed higher expression of the receptor for Insulin-like Growth Factor-1, a molecule with some functional similarity to M-CSF. Compared to microglia isolated from the cortex, ventricular/Hp microglia showed increased HLA-DP, DQ, DR antigen presentation protein expression and a rounded morphology. These findings show that microglia from adult human brain neurogenic regions are more proliferative than cortical microglia and have a distinct protein expression profile. The data present a case for differential microglial phenotype and function in different regions of the adult human brain and suggest that microglia in adult neurogenic regions are "primed" to an activated state by their unique tissue environment.

Immunohistochemical evidence for adult human neurogenesis in health and disease

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

Intrinsic Mechanisms Regulating Neuronal Migration in the Postnatal Brain

Neuronal migration is a fundamental brain development process that allows cells to move from their birthplaces to their sites of integration. Although neuronal migration largely ceases during embryonic and early postnatal development, neuroblasts continue to be produced and to migrate to a few regions of the adult brain such as the dentate gyrus and the subventricular zone (SVZ). In the SVZ, a large number of neuroblasts migrate into the olfactory bulb (OB) along the rostral migratory stream (RMS). Neuroblasts migrate in chains in a tightly organized micro-environment composed of astrocytes that ensheath the chains of neuroblasts and regulate their migration; the blood vessels that are used by neuroblasts as a physical scaffold and a source of molecular factors; and axons that modulate neuronal migration. In addition to diverse sets of extrinsic micro-environmental cues, long-distance neuronal migration involves a number of intrinsic mechanisms, including membrane and cytoskeleton remodeling, Ca²⁺ signaling, mitochondria dynamics, energy consumption, and autophagy. All these mechanisms are required to cope with the different micro-environment signals and maintain cellular homeostasis in order to sustain the proper dynamics of migrating neuroblasts and their faithful arrival in the target regions. Neuroblasts in the postnatal brain not only migrate into the OB but may also deviate from their normal path to migrate to a site of injury induced by a stroke or by certain neurodegenerative disorders. In this review, we will focus on the intrinsic mechanisms that regulate long-distance neuroblast migration in the adult brain and on how these pathways may be modulated to control the recruitment of neuroblasts to damaged/diseased brain areas.

Neuroimaging and neuropathology studies of X-linked dystonia parkinsonism

X-linked Dystonia Parkinsonism (XDP) is a recessive, genetically inherited neurodegenerative disorder endemic to Panay Island in the Philippines. Clinical symptoms include the initial appearance of dystonia, followed by parkinsonian traits after 10-15 years. The basal ganglia, particularly the striatum, is an area of focus in XDP neuropathology research, as the striatum shows marked atrophy that correlates with disease progression. Thus, XDP shares features of Parkinson's disease symptomatology, in addition to the genetic predisposition and presence of striatal atrophy resembling Huntington's disease. However, further research is required to reveal the detailed pathology and indicators of disease in the XDP brain. First, there are limited neuropathological studies that have investigated neuronal changes and neuroinflammation in the XDP brain. However, multiple neuroimaging studies on XDP patients provide clues to other affected brain regions. Furthermore, molecular pathological studies have elucidated that the main genetic cause of XDP is in the TAF-1 gene, but how this mutation relates to XDP neuropathology still remains to be fully investigated. Hence, we aim to provide an extensive overview of the current literature describing neuropathological changes within the XDP brain, and discuss future research avenues, which will provide a better understanding of XDP neuropathogenesis.

Subventricular zone lipidomic architecture loss in Huntington's disease

The human subventricular zone (SVZ) has a defined cytological and neurochemical architecture, with four constituent laminae that act in concert to support its neurogenic activity. Lipidomic specialisation has previously been demonstrated in the neurologically normal human SVZ, with enrichment of functionally important lipid classes in each lamina. The SVZ is also responsive to neurodegenerative disorders, where thickening of the niche and enhanced proliferation of resident cells were observed in Huntington's disease (HD) brains. In this study, we hypothesised lipidomic changes in the HD SVZ. Using matrix-assisted laser desorption/ionisation (MALDI) imaging mass spectrometry, this analysis shows differences in the lipidomic architecture in the post-mortem Vonsattel grade III cases. Relative to matched, neurologically normal specimens (N = 4), the lipidomic signature of the HD SVZ (N = 4) was characterized by loss of sulfatides and triglycerides in the myelin layer, with an ectopic and focal accumulation of sphingomyelins and ceramide-1-phosphate observed in this lamina. A striking loss of lipidomic patterning was also observed in the ependymal layer, where the local abundance of phosphatidylinositols was significantly reduced in HD. This comprehensive spatially resolved lipidomic analysis of the human HD SVZ identifies alterations in lipid architecture that may shed light on the mechanisms of SVZ responses to neurodegeneration in HD. Open Science: This manuscript was awarded with the Open Materials Badge. For more information see: https://cos.io/our-services/open-science-badges/.

Differential Fatty Acid-Binding Protein Expression in Persistent Radial Glia in the Human and Sheep Subventricular Zone

Fatty acid-binding proteins (FABPs) are a family of transport proteins that facilitate intracellular transport of fatty acids. Despite abundant expression in the brain, the role that FABPs play in the process of cell proliferation and migration in the subventricular zone (SVZ) remains unclear. Our results provide a detailed characterisation of FABP3, 5, and 7 expression in adult and fetal human and sheep SVZ. High FABP5 expression was specifically observed in the adult human SVZ and co-labelled with polysialylated neural cell adhesion molecule (PSA-NCAM), glial fibrillary acidic protein (GFAP), GFAPδ, and proliferating cell nuclear antigen (PCNA), indicating a role for FABP5 throughout the full maturation process of astrocytes and neuroblasts. Some FABP5+ cells had a radial glial-like appearance and co-labelled with the radial glia markers vimentin (40E-C) and GFAP. In the fetal human brain, FABP5 was expressed by radial glia cells throughout the ventricular zone. In contrast, radial glia-like cells in sheep highly expressed FABP3. Taken together, these differences highlight the species-specific expression profile of FABPs in the SVZ. In this study, we demonstrate the distribution of FABP in the adult human SVZ and fetal ventricular zone and reveal its expression on persistent radial glia that may be involved in adult neurogenesis.

Subcortical neurodegeneration in chorea: Similarities and differences between chorea-acanthocytosis and Huntington's disease

INTRODUCTION

Chorea-acanthocytosis (ChAc) and Huntington's disease (HD) are neurodegenerative conditions that share clinical and neuropathological features, despite their distinct genetic etiologies.

METHODS

In order to compare these neuropathologies, serial gallocyanin-stained brain sections from three subjects with ChAc were analyzed and compared with our previous studies of eight HD cases, in addition to three hemispheres from two male controls.

RESULTS

Astrogliosis was much greater in the ChAc striatum, as compared to that found in HD, with dramatic increase in total striatal glia numbers and the number of glia per striatal neuron. Striatal astrocytes are most likely derived from the striatal subependymal layer in ChAc, which showed massive proliferation. The thalamic centromedian-parafascicular complex is reciprocally connected to the striatum and is more heavily affected in HD than in ChAc.

CONCLUSION

The distinct patterns of selective vulnerability and gliosis observed in HD and ChAc challenge simplistic views on the pathogenesis of these two diseases with rather similar clinical signs. The particular roles played by astroglia in ChAc and in HD clearly need to be elucidated in more detail.

Methamphetamine increases HIV infectivity in neural progenitor cells

HIV-1 infection and methamphetamine (METH) abuse frequently occur simultaneously and may have synergistic pathological effects. Although HIV-positive/active METH users have been shown to have higher HIV viral loads and experience more severe neurological complications than non-users, the direct impact of METH on HIV infection and its link to the development of neurocognitive alternations are still poorly understood. In the present study, we hypothesized that METH impacts HIV infection of neural progenitor cells (NPCs) by a mechanism encompassing NFκB/SP1-mediated HIV LTR activation. Mouse and human NPCs were infected with EcoHIV (modified HIV virus infectious to mice) and HIV, respectively, in the presence or absence of METH (50 or 100 μm). Pretreatment with METH, but not simultaneous exposure, significantly increased HIV production in both mouse and human NPCs. To determine the mechanisms underlying these effects, cells were transfected with different variants of HIV LTR promoters and then exposed to METH. METH treatment induced transcriptional activity of the HIV LTR promotor, an effect that required both NFκB and SP1 signaling. Pretreatment with METH also decreased neuronal differentiation of HIV-infected NPCs in both in vitro and in vivo settings. Importantly, NPC-derived daughter cells appeared to be latently infected with HIV. This study indicates that METH increases HIV infectivity of NPCs, through the NFκB/SP1-dependent activation of the HIV LTR and with the subsequent alterations of NPC neurogenesis. Such events may underlie METH- exacerbated neurocognitive dysfunction in HIV-infected patients.

Concise Review: The Use of Stem Cells for Understanding and Treating Huntington's Disease

Two decades ago, researchers identified that a CAG expansion mutation in the huntingtin (HTT) gene was involved in the pathogenesis of Huntington's disease (HD). However, since the identification of the HTT gene, there has been no advance in the development of therapeutic strategies to prevent or reduce the progression of HD. With the recent advances in stem cell biology and human cell reprogramming technologies, several novel and exciting pathways have emerged allowing researchers to enhance their understanding of the pathogenesis of HD, to identify and screen potential drug targets, and to explore alternative donor cell sources for cell replacement therapy. This review will discuss the role of compensatory neurogenesis in the HD brain, the use of stem cell-based therapies for HD to replace or prevent cell loss, and the recent advance of cell reprogramming to model and/or treat HD. These new technologies, coupled with advances in genome editing herald a promising new era for HD research with the potential to identify a therapeutic strategy to alleviate this debilitating disorder. Stem Cells 2018;36:146-160.

Developmental alterations in Huntington's disease neural cells and pharmacological rescue in cells and mice

Neural cultures derived from Huntington's disease (HD) patient-derived induced pluripotent stem cells were used for 'omics' analyses to identify mechanisms underlying neurodegeneration. RNA-seq analysis identified genes in glutamate and GABA signaling, axonal guidance and calcium influx whose expression was decreased in HD cultures. One-third of gene changes were in pathways regulating neuronal development and maturation. When mapped to stages of mouse striatal development, the profiles aligned with earlier embryonic stages of neuronal differentiation. We observed a strong correlation between HD-related histone marks, gene expression and unique peak profiles associated with dysregulated genes, suggesting a coordinated epigenetic program. Treatment with isoxazole-9, which targets key dysregulated pathways, led to amelioration of expanded polyglutamine repeat-associated phenotypes in neural cells and of cognitive impairment and synaptic pathology in HD model R6/2 mice. These data suggest that mutant huntingtin impairs neurodevelopmental pathways that could disrupt synaptic homeostasis and increase vulnerability to the pathologic consequence of expanded polyglutamine repeats over time.

Molecular Mechanism of Adult Neurogenesis and its Association with Human Brain Diseases

Recent advances in neuroscience challenge the old dogma that neurogenesis occurs only during embryonic development. Mounting evidence suggests that functional neurogenesis occurs throughout adulthood. This review article discusses molecular factors that affect adult neurogenesis, including morphogens, growth factors, neurotransmitters, transcription factors, and epigenetic factors. Furthermore, we summarize and compare current evidence of associations between adult neurogenesis and human brain diseases such as Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, and brain tumors.

Transcriptional profiling of adult neural stem-like cells from the human brain

There is a great potential for the development of new cell replacement strategies based on adult human neural stem-like cells. However, little is known about the hierarchy of cells and the unique molecular properties of stem- and progenitor cells of the nervous system. Stem cells from the adult human brain can be propagated and expanded in vitro as free floating neurospheres that are capable of self-renewal and differentiation into all three cell types of the central nervous system. Here we report the first global gene expression study of adult human neural stem-like cells originating from five human subventricular zone biopsies (mean age 42, range 33–60). Compared to adult human brain tissue, we identified 1,189 genes that were significantly up- and down-regulated in adult human neural stem-like cells (1% false discovery rate). We found that adult human neural stem-like cells express stem cell markers and have reduced levels of markers that are typical of the mature cells in the nervous system. We report that the genes being highly expressed in adult human neural stem-like cells are associated with developmental processes and the extracellular region of the cell. The calcium signaling pathway and neuroactive ligand-receptor interactions are enriched among the most differentially regulated genes between adult human neural stem-like cells and adult human brain tissue. We confirmed the expression of 10 of the most up-regulated genes in adult human neural stem-like cells in an additional sample set that included adult human neural stem-like cells (n = 6), foetal human neural stem cells (n = 1) and human brain tissues (n = 12). The NGFR, SLITRK6 and KCNS3 receptors were further investigated by immunofluorescence and shown to be heterogeneously expressed in spheres. These receptors could potentially serve as new markers for the identification and characterisation of neural stem- and progenitor cells or as targets for manipulation of cellular fate.

Physical exercise-induced adult neurogenesis: a good strategy to prevent cognitive decline in neurodegenerative diseases?

Cumulative evidence has indicated that there is an important role for adult hippocampal neurogenesis in cognitive function. With the increasing prevalence of cognitive decline associated with neurodegenerative diseases among the ageing population, physical exercise, a potent enhancer of adult hippocampal neurogenesis, has emerged as a potential preventative strategy/treatment to reduce cognitive decline. Here we review the functional role of adult hippocampal neurogenesis in learning and memory, and how this form of structural plasticity is altered in neurodegenerative diseases known to involve cognitive impairment. We further discuss how physical exercise may contribute to cognitive improvement in the ageing brain by preserving adult neurogenesis, and review the recent approaches for measuring changes in neurogenesis in the live human brain.

Interactions of HIV and drugs of abuse: the importance of glia, neural progenitors, and host genetic factors

Considerable insight has been gained into the comorbid, interactive effects of HIV and drug abuse in the brain using experimental models. This review, which considers opiates, methamphetamine, and cocaine, emphasizes the importance of host genetics and glial plasticity in driving the pathogenic neuron remodeling underlying neuro-acquired immunodeficiency syndrome and drug abuse comorbidity. Clinical findings are less concordant than experimental work, and the response of individuals to HIV and to drug abuse can vary tremendously. Host-genetic variability is important in determining viral tropism, neuropathogenesis, drug responses, and addictive behavior. However, genetic differences alone cannot account for individual variability in the brain "connectome." Environment and experience are critical determinants in the evolution of synaptic circuitry throughout life. Neurons and glia both exercise control over determinants of synaptic plasticity that are disrupted by HIV and drug abuse. Perivascular macrophages, microglia, and to a lesser extent astroglia can harbor the infection. Uninfected bystanders, especially astroglia, propagate and amplify inflammatory signals. Drug abuse by itself derails neuronal and glial function, and the outcome of chronic exposure is maladaptive plasticity. The negative consequences of coexposure to HIV and drug abuse are determined by numerous factors including genetics, sex, age, and multidrug exposure. Glia and some neurons are generated throughout life, and their progenitors appear to be targets of HIV and opiates/psychostimulants. The chronic nature of HIV and drug abuse appears to result in sustained alterations in the maturation and fate of neural progenitors, which may affect the balance of glial populations within multiple brain regions.

GABA(A) receptor characterization and subunit localization in the human sub-ventricular zone

It is now well established that the human brain continuously produces new stem cells until well into old age. One of these stem-cell rich areas in the human brain is the sub-ventricular zone (SVZ). The human SVZ is organized in four distinctive layers containing type A, B and C cells. To date, no studies have investigated the distribution of inhibitory neurotransmitters such as γ-aminobutyric acid (GABA) and their respective receptors on the different cell types in the human SVZ. GABA(A) receptors (GABA(A)R) are ubiquitously expressed, inhibitory heteropentameric chloride ion channels comprised of a variety of subunits that are targeted by many prescribed drugs. In this study we present detailed immunohistochemical data on the regional and cellular localization of α₁, α₂, α3, β₂,₃ and γ₂ subunits of GABA(A)R in the human SVZ. The results from our double and triple labeling studies demonstrate that the cell types and subunit composition throughout the SVZ is heterogeneous; the thickness of the SVZ and GABA(A)R α₂ and γ₂ expression is increased especially in the vicinity of large SVZ blood vessels. GABA(A)R γ₂ is the most specific to the SVZ and present on various cells that express, either glial fibrillary acidic protein (GFAPδ) or polysialic acid-neural cell adhesion molecule (PSA-NCAM) separately, or together in a respective ratio of 7:6:2. Proliferating (type C) cells in the SVZ express GAD65/67, GFAPδ and GABA(A)R β₂,₃ receptor subunits. Within the SVZ the majority of cells have an unexpected nuclear GABA(A)R β₂,₃ expression that is inversely proportional to that of PCNA (proliferating cell nuclear antigen marker), which is a very different pattern of expression compared with underlying caudate nucleus cells. Taken together our results provide a detailed description of the chemo-architecture of the adult human SVZ demonstrating the importance of GABA and GABA(A) receptors on the various cell types in the SVZ.

Neurogenesis and progenitor cells in the adult human brain: a comparison between hippocampal and subventricular progenitor proliferation

For more than a decade, we have known that the human brain harbors progenitor cells capable of becoming mature neurons in the adult human brain. Since the original landmark article by Eriksson et al. in 1998 (Nat Med 4:1313-1317), there have been many studies investigating the effect that depression, epilepsy, Alzheimer's disease, Huntington's disease, and Parkinson's disease have on the germinal zones in the adult human brain. Of particular interest is the demonstration that there are far fewer progenitor cells in the hippocampal subgranular zone (SGZ) compared with the subventricular zone (SVZ) in the human brain. Furthermore, the quantity of progenitor cell proliferation in human neurodegenerative diseases differs from that of animal models of neurodegenerative diseases; there is minimal progenitor proliferation in the SGZ and extensive proliferation in the SVZ in the human. In this review, we will present the data from a range of human and rodent studies from which we can compare the amount of proliferation of cells in the SVZ and SGZ in different neurodegenerative diseases.

Regulation of neural stem cell in the human SVZ by trophic and morphogenic factors

The subventricular zone (SVZ), lining the lateral ventricular system, is the largest germinal region in mammals. In there, neural stem cells express markers related to astoglial lineage that give rise to new neurons and oligodendrocytes in vivo. In the adult human brain, in vitro evidence has also shown that astrocytic cells isolated from the SVZ can generate new neurons and oligodendrocytes. These proliferative cells are strongly controlled by a number of signals and molecules that modulate, activate or repress the cell division, renewal, proliferation and fate of neural stem cells. In this review, we summarize the cellular composition of the adult human SVZ (hSVZ) and discuss the increasing evidence showing that some trophic modulators strongly control the function of neural stem cells in the SVZ.

The role of mitochondria in brain aging and the effects of melatonin

Melatonin is an endogenous indoleamine present in different tissues, cellular compartments and organelles including mitochondria. When melatonin is administered orally, it is readily available to the brain where it counteracts different processes that occur during aging and age-related neurodegenerative disorders. These aging processes include oxidative stress and oxidative damage, chronic and acute inflammation, mitochondrial dysfunction and loss of neural regeneration. This review summarizes age related changes in the brain and the importance of oxidative/nitrosative stress and mitochondrial dysfunction in brain aging. The data and mechanisms of action of melatonin in relation to aging of the brain are reviewed as well.

Neurotransmitters couple brain activity to subventricular zone neurogenesis

Adult neurogenesis occurs in two privileged microenvironments, the hippocampal subgranular zone of the dentate gyrus and the subventricular zone (SVZ) along the lateral ventricle. This review focuses on accumulating evidence suggesting that the activity of specific brain regions or bodily states influences SVZ cell proliferation and neurogenesis. Neuromodulators such as dopamine and serotonin have been shown to have long-range effects through neuronal projections into the SVZ. Local γ-aminobutyric acid and glutamate signaling have demonstrated effects on SVZ proliferation and neurogenesis, but an extra-niche source of these neurotransmitters remains to be explored and options will be discussed. There is also accumulating evidence that diseases and bodily states such as Alzheimer's disease, seizures, sleep and pregnancy influence SVZ cell proliferation. With such complex behavior and environmentally-driven factors that control subregion-specific activity, it will become necessary to account for overlapping roles of multiple neurotransmitter systems on neurogenesis when developing cell therapies or drug treatments.

Neurogenesis in humans

Since 1944 increasing evidence has been emerging that the adult human brain harbours progenitor cells with the potential to produce neuroblasts. However, it was not until 1998 that this fact was confirmed in the adult human brain. With the purpose of human neurogenesis being hotly debated, many research groups have focussed on the effect of neurodegenerative diseases in the brain to determine the strength of the endogenous regenerative response. Although most of the human studies have focussed on the hippocampus, there is a groundswell of evidence that there is greater plasticity in the subventricular zone and in the ventriculo-olfactory neurogenic system. In this review, we present the evidence for increased or decreased plasticity and neurogenesis in different diseases and with different drug treatments in the adult human brain. Whilst there is a paucity of studies on human neurogenesis, there are sufficient to draw some conclusions about the potential of plasticity in the human brain.

Strategies for regenerating striatal neurons in the adult brain by using endogenous neural stem cells

Currently, there is no effective treatment for the marked neuronal loss caused by neurodegenerative diseases, such as Huntington's disease (HD) or ischemic stroke. However, recent studies have shown that new neurons are continuously generated by endogenous neural stem cells in the subventricular zone (SVZ) of the adult mammalian brain, including the human brain. Because some of these new neurons migrate to the injured striatum and differentiate into mature neurons, such new neurons may be able to replace degenerated neurons and improve or repair neurological deficits. To establish a neuroregenerative therapy using this endogenous system, endogenous regulatory mechanisms that can be co-opted for efficient regenerative interventions must be understood, along with any potential drawbacks. Here, we review current knowledge on the generation of new neurons in the adult brain and discuss their potential for use in replacing striatal neurons lost to neurodegenerative diseases, including HD, and to ischemic stroke.

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.

Proneural transcription factors Dlx2 and Pax6 are altered in adult SVZ neural precursor cells following striatal cell loss

Compensatory replacement of neurons by endogenous subventricular zone (SVZ)-derived neural precursor cells has been demonstrated in the adult brain following striatal cell loss. Such cell replacement is associated with increased SVZ cell proliferation and neuroblast expansion in the rostral migratory stream (RMS). SVZ-derived neural precursor cells co-express multiple transcription factors involved in lineage restriction and cell fate determination. We propose that compensatory neurogenesis in response to striatal cell loss will alter the temporal expression of transcription factors in discrete populations of SVZ-derived neural precursor cells. We therefore examined the expression of Mash1, Dlx2, Pax6 and Olig2 in SVZ-derived neural precursor cell populations across a range of times following quinolinic acid (QA) induced striatal cell death. We have identified a heterogeneous population of SVZ-derived neural precursor cells that respond independently to striatal cell loss. In both the anterior SVZ (aSVZ) and RMS we observed an increase in a sub-population of Dlx2+ transit amplifying precursor (TAP) cells and neuroblasts following QA lesioning when compared to controls. Subsequently, the number of Pax6+ TAPs and neuroblasts in the QA lesioned aSVZ and RMS was also increased. Olig2 expression was not however altered in response to QA-induced cell loss. Our results suggest Dlx2 and Pax6 may play a prominent role in directing neural precursor cell proliferation and neuroblast generation following striatal cell loss. Selective alteration of specific transcription factors in the SVZ and during migration through the RMS in response to cell loss may predetermine the subsequent generation of specific neuronal subclasses for endogenous replacement.

Genetics and neuropathology of Huntington's disease

Huntington's disease (HD) is an autosomal dominant progressive neurodegenerative disorder that prominently affects the basal ganglia, leading to affective, cognitive, behavioral and motor decline. The basis of HD is a CAG repeat expansion to >35 CAG in a gene that codes for a ubiquitous protein known as huntingtin, resulting in an expanded N-terminal polyglutamine tract. The size of the expansion is correlated with disease severity, with increasing CAG accelerating the age of onset. A variety of possibilities have been proposed as to the mechanism by which the mutation causes preferential injury to the basal ganglia. The present chapter provides a basic overview of the genetics and pathology of HD.

Human multipotent stromal cells (MSCs) increase neurogenesis and decrease atrophy of the striatum in a transgenic mouse model for Huntington's disease

Background

Implantation of human multipotent stromal cells from bone marrow (hMSCs) into the dentate gyrus of the hippocampus of mice was previously shown to stimulate proliferation, migration and neural differentiation of endogenous neural stem cells. We hypothesized that hMSCs would be beneficial in a mouse model of Huntington disease (HD) due to these neurogenic effects.

Results

We implanted hMSCs into the striatum of transgenic mice (N171-82Q) that are a model for HD. The implanted hMSCs rapidly disappeared over 3 to 15 days. However, they increased proliferation and neural differentiation of endogenous neural stem cells for up to 30 days. They also increased neurotrophic signaling and decreased atrophy of the striatum in 3-month old HD mice implanted with hMSCs one month earlier.

Conclusions

The results therefore suggested that neural implantation of hMSCs may be of benefit in HD but a number of parameters of dose, treatment schedule, and route of administration need to be optimized.

Transplantation of embryonic and adult neural stem cells in the granuloprival cerebellum of the weaver mutant mouse

Numerous studies have explored the potential of different stem and progenitor cells to replace at-risk neuronal populations in a variety of neurodegenerative disease models. This study presents data from a side-by-side approach of engrafting two different stem/progenitor cell populations within the postnatal cerebellum of the weaver neurological mutant mouse--cerebellar-derived multipotent astrocytic stem cells and embryonic stem cell-derived neural precursors--for comparative analysis. We show here that both donor populations survive, migrate, and appear to initiate differentiation into neurons within the granuloprival host environment. Neither of these disparate stem/progenitor cell populations adopted significant region-specific identities, despite earlier studies that suggested the potential of these cells to respond to in vivo cues when placed in a permissive/instructive milieu. However, data presented here suggest that molecular and cellular deficits present within weaver homozygous or heterozygous brains may promote a slightly more positive donor cell response toward acquisition of a neuronal phenotype. Hence, it is likely that a fine balance exists between a compromised host environment that is amenable to cell replacement and that of a degenerating cellular milieu where it is perhaps too deleterious to support extensive neuronal differentiation and functional cellular integration. These findings join a growing list of studies that show successful cell replacement depends largely on the interplay between the potentiality of the donor cells and the specific pathological conditions of the recipient environment, and that emergent therapies for neurological disorders involving the use of neural stem cells still require refinement.

Adult neurogenesis and its alteration under pathological conditions

Even in the adult brain, neural stem cells in the dentate gyrus and subventricular zone continue to produce neuronal precursors, which migrate and differentiate into functional mature neurons. This physiological neurogenesis is thought to be involved in neuronal plasticity. Moreover, recent studies indicate that adult neurogenesis can change in response to various brain insults, including psychiatric diseases, stroke, and neurodegenerative disorders. Although increased neurogenesis in these pathological conditions could contribute to the restoration and regeneration of the damaged brain, an inadequate and/or excessive supply of new neurons, or suppressed neurogenesis, could contribute to their pathophysiology. To develop successful regenerative treatments for the injured brain, we need to understand more precisely and comprehensively the mechanisms regulating adult neurogenesis under both physiological and pathological conditions.

Expanded CAG repeats in the murine Huntington's disease gene increases neuronal differentiation of embryonic and neural stem cells

Huntington's disease is an uncommon autosomal dominant neurodegenerative disorder caused by expanded polyglutamine repeats. Increased neurogenesis was demonstrated recently in Huntington's disease post-mortem samples. In this manuscript, neuronally differentiated embryonic stem cells with expanded CAG repeats in the murine Huntington's disease homologue and neural progenitors isolated from the subventricular zone of an accurate mouse Huntington's disease were examined for increased neurogenesis. Embryonic stem cells with expanded CAG repeats in the murine Huntington's disease homologue were demonstrated to undergo facilitated differentiation first into neural progenitors, then into more mature neurons. Neural progenitor cells isolated from the subventricular zone of a Huntington's disease knock-in animal displayed increased production of neural progenitors and increased neurogenesis. These findings suggested that neuronally differentiating embryonic stem cells with expanded CAG repeats is a reasonable system to identify factors responsible for increased neurogenesis in Huntington's disease. Expression profiling analysis comparing neuronally differentiating embryonic stem cells with expanded CAG repeats to neuronally differentiating embryonic stem cells without expanded CAG repeats identified transcripts involved in development and transcriptional regulation as factors possibly mediating increased neurogenesis in response to expanded CAG repeats.

Cell therapy in Huntington disease

Huntington disease (HD), caused by polyglutamate expansions in the huntingtin protein, is a progressive neurodegenerative disease resulting in cognitive and motor impairments and death. Neuronal dysfunction and degeneration contribute to progressive physiological, motor, cognitive, and emotional disturbances characteristic of HD. A major impetus for research into the treatment of HD has centered on cell therapy strategies to protect vulnerable neuronal cell populations or to replace dysfunctional or dying cells. The work underlying 3 approaches to HD cell therapy includes the potential for self-repair through the manipulation of endogenous stem cells and/or neurogenesis, the use of fetal or stem cell transplantation as a cell replacement strategy, and the administration of neurotrophic factors to protect susceptible neuronal populations. These approaches have shown some promising results in animal models of HD. Although striatal transplantation of fetal-derived cells has undergone clinical assessment since the 1990s, many cell therapy strategies have yet to be applied in the clinic environment. A more thorough understanding of the pathophysiologies underlying HD as well as the response of both endogenous and exogenous cells to the degenerating brain will inform their merit as potential therapeutic agents and enhance the framework by which the success of such strategies are determined.

Huntington disease models and human neuropathology: similarities and differences

Huntington disease (HD) occurs only in humans. Thus, its natural pathogenesis takes place exclusively within the human brains expressing the causative, mutated protein huntingtin (mhtt). The techniques applicable to postmortem human HD brains are inadequate for investigating the cellular pathogenesis. The creation of genetically engineered animals represents a critical moment in neuroscience. Monitoring the actions of either normal, or abnormal proteins at subcellular levels, and at different time points is now possible thanks to these models. They are the necessary substitutes to investigate the wild type (whtt), or mhtt. The postmortem neuropathologic phenotype of the human HD is well documented. Its pattern and spectrum are highly predictable. From this point of view, the existent models do not exhibit the phenotypic constellation of changes seen in the human HD brains. On one hand, this deficit reflects the limitations of the methods of evaluation used in a clinical setting. On the other hand, it highlights the limitations of the animals. The validity of the models probably should be measured by their capacity of reproducing the cellular dysfunctions of HD rather than the phenotype of the postmortem human brains. Although not perfect, these models are essential for modeling the human disease in cells, which is not feasible with postmortem human HD brains. Nonetheless, their relevance to the patient population remains to be determined. Ultimately needed are means preventing the disease to occur, the discovery of which probably depends on these models.

Progenitor cells and adult neurogenesis in neurodegenerative diseases and injuries of the basal ganglia

1. The subventricular zone (SVZ) of the forebrain that overlies the caudate nucleus is one of the principal brain regions in which neurogenesis occurs in the human brain, throughout life. 2. In response to the degeneration that occurs in the caudate nucleus in Huntington's disease, or in the caudate nucleus or cortex in stroke models, the SVZ increases the production of progenitor cells that migrate towards the site of the damage where they can differentiate into mature neurons and glial cells. The SVZ contains three main cell types and these are progenitor cells, glial cells and migratory neuroblasts; glial cells are the most common cell type and, in response to Huntington's disease, most of the SVZ cell proliferation is glial, but the number of precursor and neuroblasts is also increased. 3. The SVZ is enriched in neuroactive compounds, such as neuropeptide Y and gamma-aminobutyric acid receptor subunits gamma2, which stimulate ongoing neurogenesis. Interestingly, these stimulating cues are upregulated in the SVZ in response to Huntington's disease. Thus, the SVZ comprises heterogeneous cell types that are maintained in an environment that is permissive to neurogenesis and gliogenesis, and responds to neurodegenerative changes in adjacent brain regions by increasing progenitor cell proliferation and neurogenesis in an attempt to replace the cells that die as a result of neurodegeneration.

Human neuroblasts migrate to the olfactory bulb via a lateral ventricular extension

The rostral migratory stream (RMS) is the main pathway by which newly born subventricular zone cells reach the olfactory bulb (OB) in rodents. However, the RMS in the adult human brain has been elusive. We demonstrate the presence of a human RMS, which is unexpectedly organized around a lateral ventricular extension reaching the OB, and illustrate the neuroblasts in it. The RMS ensheathing the lateral olfactory ventricular extension, as seen by magnetic resonance imaging, cell-specific markers, and electron microscopy, contains progenitor cells with migratory characteristics and cells that incorporate 5-bromo-2'-deoxyuridine and become mature neurons in the OB.

A progressive and cell non-autonomous increase in striatal neural stem cells in the Huntington's disease R6/2 mouse

Neural stem and progenitor cells are located in the subependyma of the adult forebrain. An increase in adult subependymal cell proliferation is reported after various kinds of brain injury. We demonstrate an expansion of neural precursor cells in the postnatal subependyma in a murine genetic disease model of Huntington's disease (HD), the R6/2 mouse. We used the in vitro neurosphere assay as an index of the number of neural stem cells in vivo and to assess proliferation kinetics in vitro and in vivo bromodeoxyuridine labeling to assess the progenitor cell population and their fates. Disease progression in this model leads to an increase in the numbers of neural stem cells in the adult striatal subependyma. This increase is produced cell non-autonomously by events in the R6/2 brains as the mice become increasingly symptomatic. Once the neural stem cell increase is induced in vivo, it is maintained during in vitro passaging of neural stem cells, but the neural stem cell increase is not reproduced during in vitro passaging of neural stem cells from presymptomatic R6/2 mice. In addition, we show that some of the R6/2 neural progenitor cells show a change from their normal migration destiny toward the olfactory bulb. Instead, some of these cells migrate into the striatum, one of the main affected areas in HD. Our findings demonstrate that HD damage recruits precursor cells in two ways: expansion of neural stem cells and altered migration of progenitor cells.

Neurogenesis in diseases of the central nervous system

Neurogenesis is altered in ageing, and diseases of the central nervous system (CNS) such as neurodegenerative disorders. We discuss the process of neurogenesis, its relevance for disorders of the CNS, the dynamic nature of neurogenesis, how and why it may be abnormal in ageing, and disease, and possibilities to ameliorate abnormal neurogenesis in disease.

Neurogenesis in the R6/1 transgenic mouse model of Huntington's disease: effects of environmental enrichment

Previous work has demonstrated that the transgenic R6/1 mouse model of Huntington's disease has decreased proliferation of neural precursor cells (NPCs) in the dentate gyrus of the hippocampus. This study therefore examined the survival and differentiation of NPCs in presymptomatic and symptomatic R6/1 mice and the effects of environmental enrichment on these variables. Here it is demonstrated that the survival of bromodeoxyuridine-positive (BrdU+) NPCs in the dentate gyrus is decreased in the transgenic mice. In addition, the number of doublecortin-positive (DCX+) cells is greatly reduced in these mice, as is the total number of new mature neurons, while the proportion of BrdU+ cells differentiating into mature neurons was not significantly different between genotypes. Furthermore, the DCX+ cells in the R6/1 mice had smaller and irregular-shaped somas, shorter neurites, and migrated a shorter distance into the granular cell layer compared with wild-type mice. Older symptomatic mice housed in an enriched environment had an increased number of BrdU+ and DCX+ cells as well as longer neurites and increased migration of DCX+ cells. There was no significant difference between genotypes or environments in the number of BrdU+ cells in the subventricular zone. These results suggest that decreased neurogenesis might be responsible, in part, for the hippocampal deficits observed in these mice and that environmental enrichment produces morphological changes in newborn granule neurons in both wild-type and R6/1 mice, which could underlie some of the beneficial effects of enrichment.

Immunohistochemical staining of post-mortem adult human brain sections

One of the challenges for modern neuroscience is to understand the basis of coordinated neuronal function and networking in the human brain. Some of these questions can be addressed using low- and high-resolution imaging techniques on post-mortem human brain tissue. We have established a versatile protocol for fixation of post-mortem adult human brain tissue, storage of the tissue in a human brain bank, and immunohistochemical analysis in order to understand human brain functions in normal controls and in neuropathological conditions. The brains are fixed by perfusion through the internal carotid and basilar arteries to enhance the penetration of fixative throughout the brain, then blocked, postfixed, cryoprotected, snap-frozen and stored at -80 degrees C. Sections are processed for immunohistochemical single- or double-label staining and conventional-, electron- or confocal laser scanning-microscopy analysis. The results gained using this tissue and protocol are vital for determining the localization of neurochemicals throughout the human brain and to document the changes that occur in neurological diseases.