Preface/Front Matter

The neurodegenerative disorders parkinson’s disease (PD), Huntington’s disease (HD), Alzheimer’s disease (AD), and human immunodeficiency virus (HIV)-associated cognitive impairment (HACI) all present with a gradual loss of relatively well-defined neuronal populations. Under all of these conditions, progression is slow. In some cases, the neuropathology is relatively restricted, leaving significant parts of the nervous system unaffected. They have therefore become interesting targets for restorative therapies. One of the most exciting ideas for repair is the concept that one might be able to harness the adult brain’s endogenous capacity for cell renewal. Thus, it might be possible to direct newborn cells in the adult brain to migrate to the regions affected by the disease and there differentiate into the specific types of neurons that succumb due to the disease (Jordan et al. 2006). This concept is based on the realization that the adult mammalian brain also has the capacity to generate new neurons. The interaction between neurogenesis in the adult brain and neurodegenerative disease can also be viewed from another angle. It is conceivable that failure of a normal reparative process, i.e., adult neurogenesis, contributes to the development of the disease. Taken to its extreme, this idea has even led to the hypothesis that the symptoms in some neurodegenerative diseases may partly be the consequence of reduced adult neurogenesis, resulting in a failed replacement of dying neurons (Armstrong and Barker 2001). The objectives of this chapter are to describe neurogenesis in the adult brain and to determine to what extent it is...

Over most of the past century, it was thought that the adult brain was completely incapable of generating new neurons. However, in the last decade, the development of new techniques has resulted in an explosion of new research showing that (i) neurogenesis, the birth of new neurons, is not restricted to embryonic development, but normally also occurs in two limited regions of the adult mammalian brain (the olfactory bulb and the dentate gyrus of the hippocampus); (ii) that there are significant numbers of multipotent neural precursors in many parts of the adult mammalian brain; and (iii) that it is possible to induce neurogenesis even in regions of the adult mammalian brain, like the neocortex, where it does not normally occur, via manipulation of endogenous multipotent precursors in situ. In the neocortex, recruitment of small numbers of new neurons can be induced in a region-specific, layer-specific, and neuronal type-specific manner, and newly recruited neurons can form long-distance connections to appropriate targets. This suggests that elucidation of the relevant molecular controls over adult neurogenesis from endogenous neural precursors/stem cells may allow the development of neuronal replacement therapies for neurodegenerative disease and other central nervous system injuries that may not require transplantation of exogenous cells.

Compared to the adult mammalian brain, the brain of the adult zebrafish Danio rerio exhibits a very high proliferative and regenerative potential. The adult mammalian brain in contrast has a very limited neurogenic capacity mainly restricted to two zones, the subventricular zone of the lateral telencephalic ventricles and the subgranular zone of the dentate gyrus of the hippocampus. In contrast, the zebrafish brain harbours 16 proliferation zones distributed all over the brain. The zebrafish has thus become a model for the study of adult neurogenesis and regeneration of nervous tissue. I characterized the expression of the two transcription factors p53 and p73 in the adult zebrafish brain. Both p53 and p73 were shown to play crucial roles in mammalian adult neurogenesis: p53 suppresses the self-renewal of adult neural stem cells and is involved in apoptotic death of neurons following damage. p73 is relevant for the survival of neurons, self-renewal and maintenance of neural stem cells as well as differentiation of precursor cells. It was thus of interest whether these genes have similar roles in the adult zebrafish brain. I established a detailed map of the expression pattern of p53 and p73 mRNA and p53 protein in the adult zebrafish brain. p53 and p73 mRNA expression overlaps in many regions including neurogenic zones. The p53 protein is expressed in most of these regions indicating that the mRNA expression reflects the protein expression. The p53 protein is expressed in mature neurons, Type I cells (non-dividing radial glial cells) and Type IIIa and Type IIIb cells (neuroblasts) in the adult zebrafish telencephalon. In cells of the oligodendrocyte lineage and in Type II cells (dividing radial glial cells) an expression of the p53 protein is not detectable. After stab injury of the adult zebrafish telencephalon both p53 and p73 genes are up-regulated. p53 is up-regulated in Type I cells. In contrast to the uninjured brain, p53 is expressed in cells of the oligodendrocyte lineage following injury. Furthermore, target genes of p53 are up-regulated and apoptosis is induced after stab injury. These results suggest a role for p53 in constitutive and regenerative neurogenesis. However, tp53M214K mutant zebrafish do not show any phenotype. The structurally related p73 is expressed in a very similar pattern as p53 in the uninjured and injured zebrafish brain. Therefore, redundancy between p53 and p73 may occlude the manifestation of a phenotype in the p53 mutant. Taken together, the analysis of expression of both p53 and p73 in the adult zebrafish brain suggests a role of these genes during constitutive and regenerative neurogenesis. The future elucidation of the precise function of the two genes in these processes requires, however, double mutant analysis.

The mammalian brain is a complex organ composed of trillions of neurons connected with each other in a highly stereotyped yet modifiable manner. Most neurons are born during embryonic development and persist throughout life in the adult brain circuit, in contrast to many other adult tissues, including most from epithelial origins that usually harbor stem cells to maintain homeostatic cellular turnover (Weissman et al. 2001; Li and Xie 2005). The relative stability of neural circuits at the cellular level, especially in higher processing centers of the brain such as the cerebral cortex, was thought to be essential to maintain the ongoing information processing, and any loss or addition to the circuitry component could undermine the cognitive process as a whole (Rakic 1985). Therefore, the discovery of adult neurogenesis—that new neurons are indeed generated in specific regions of adult brains and undergo developmental maturation to become functionally integrated into local neural circuits (Fig. 1a)—came as a surprise (Altman and Das 1965; van Praag et al. 2002). During adult neurogenesis, neural stem cells (NSCs) generate functional neurons through coordinated steps, including cell-fate specification, migration, axonal and dendritic growth, and finally synaptic integration into the adult brain (Fig. 1d). Since the pioneering studies of Altman in the early 1960s (Altman 1962), the process of adult neurogenesis has been unambiguously established in all mammals examined, including humans (Eriksson et al. 1998; Gage 2000; Lie et al. 2004; Abrous et al. 2005; Ming and Song 2005; Lledo et al. 2006; Merkle and Alvarez-Buylla...

New ideas pass through a series of stages from initial rejection to skepticism, to reluctant acceptance (without true belief in its importance), to a final casual acknowledgment of the obvious. It is fair to say that the acceptance of the idea that new neurons are generated in the adult brain of all mammals has been a slow process, and along the way, the idea has been met with skepticism and resistance. It is still not yet casually accepted as obvious. Rather, adult neurogenesis remains in the stage of reluctant acceptance, without a clear understanding of its importance, but the search for its function is in full gear. Joseph Altman’s original observations in the 1960s were met with significant reservation, as were attempted confirmations by a handful of investigators in the next 20 years. Somehow, Fernando Nottebohm and Steve Goldman’s observation of neurogenesis in the brains of adult canaries was received more positively but—because it took place in birds—was not considered as much of a threat to the prevailing belief (often even termed “dogma”) that there are no new neurons in the adult mammalian brain. Why this resistance to the capacity of the adult brain to generate new neurons? It was well accepted that other systems, like blood, liver, and skin, could generate new cells, so why not the brain? The most straightforward explanation is that the brain is not just any organ. At a philosophical and metaphysical level, the brain is thought to be the place where the...

Tuberous sclerosis complex (TSC) is a relatively rare genetic disease characterized by the formation of benign tumors or hamartomas in multiple organs. The tumors are noninvasive and rarely transform to metastatic lesions. TSC is an autosomal dominant disorder that results from mutations in the TSC1 or TSC2 genes. Neurologically, individuals with TSC have severe complications, including refractory seizures, autism, mental retardation, learning difficulties, and changes in behavior. Tubers in the cerebral cortex, subependymal nodules (SENs) along the lateral walls of the lateral ventricles, and subependymal giant cell (GC) astrocytomas are characteristic brain lesions in patients with TSC. Astrocytic-like cells immunopositive for both glial and neuronal markers, dysplastic neurons (DNs), and GCs immunopositive for nestin and vimentin, as well as for proliferation markers such as proliferating nuclear cell antigen (PCNA) and Ki-67, are histological hallmarks of the disease. DNs and GCs retain their ability to re-enter the cell cycle and are immunopositive for markers of neural progenitor and stem cells. Neurogenesis occurs in the adult brain of mammals, particularly in the hippocampus and subventricular zone (SVZ). In the SVZ, newly generated neuronal cells migrate along the ventricle and a SVZ origin for brain tumors in the adult brain have been reported. These brain tumors express markers of neural progenitor and stem cells. The study of analogies and differences between SENs in TSC, neurogenesis in the SVZ, and tumors in the adult brain would reveal clues on the development and origin of SENs and brain tumors. Keywords: Epilepsy, Cancer, Drug, Disease, Neural stem cells, Rapamycin, Therapy, Tumor DOI:10.5055/jndr.2013.0012

Secreted Frizzled-Related Protein 3 Regulates Activity-Dependent Adult Hippocampal Neurogenesis

When discussing a brain function such as memory, one should relate it to brain plasticity. One definition of plasticity is an alternative way of performing the same function. Anecdotal evidence suggests that the human brain can perform amazing memory feats in unexpected, alternative ways. For example, the established ability of savants (individuals with partial brain damage) to memorize events, sequences of numbers, letters, or musical notes, and to perform arithmetical calculations, suggests that compensatory rewiring of brain circuits after injury can affect learning. Which particular form of brain plasticity could be responsible for such astounding learning abilities as those seen in Kim Peek (“Rain Man”) and Daniel Tammet (“Brainman”), two individuals diagnosed as autistic savants (www.savantsyndrome.com)? In this chapter, we describe a radical form of plasticity, adult neurogenesis, in hippocampal formation (HF). The discovery of adult neurogenesis (production of new neurons in adult brain) has radically changed our ideas of how the brain can adapt to physiological and environmental challenges. The process of neuronal production is highly regulated and is involved in hippocampal functions under physiological conditions. In some cases, neurogenesis can respond to hippocampus-related pathologies such as epilepsy, ischemia, mood disorders, and addiction. Understanding neurogenesis, along with other forms of brain plasticity, may help us to understand normal memory and perhaps the enhanced memory such as that seen in individuals with the Savant Syndrome (Treffert and Christensen 2005). LESIONS OF THE NEUROGENIC REGION The HF is part of an integrated network involved in learning and memory (Eichenbaum 2000, 2001;...

Evidence for the generation of young neurons out of precursor cells in the adult brain, i.e. adult neurogenesis, exists for at least two brain regions. New nerve cells are generated in the subventricular zone of the olfactory bulb and in the subgranular zone of hippocampal dentate gyrus. Young neurons of the subgranular zone migrate along the rostral migratory stream to the olfactory bulb, where they functionally integrate and contribute to the discrimination of odors. In the hippocampus the function of newly formed granule cells is still a matter of debate, yet it is thought that adult neurogenesis functionally contributes to hippocampal functions.
\nOver the last twenty years of extensive research it became clear that adult hippocampal neurogenesis (AHN) in laboratory rodents can be up-and down regulated by different internal and external stimuli. Physical exercise in a running wheel being among the factors that have been most investigated. Since voluntary exercise not only increases adult neurogenesis in the hippocampus but also beneficially affects learning and memory in laboratory mice and rats, a widespread assumption holds a direct relationship between AHN and cognitive brain health also in higher order species, including humans. However, translating findings in laboratory rodents to the human condition faces difficulties. Enormous differences in basal rates of adult neurogenesis have been reported between mammalian species. The low level of AHN in primates and the complete lack of adult neurogenesis in bat species indicate species-specific differences in adult neurogenesis not only on a regulatory but also on a functional level. For a better understanding of species-specific differences in the regulation of AHN, we investigated basal rates of adult neurogenesis in laboratory mice and closely related wild mouse strains as well as the reaction of AHN to motivationally different running conditions. Testing different wild- and laboratory mice in the same environment allowed the identification of species-specific differences as well as possible domestication effects.
\nBasal rates of adult hippocampal neurogenesis in equally-aged and genetically identical laboratory C57BL/6 mice show individual differences possibly reflecting epigenetic factors. However, the initial level of adult neurogenesis does not influence the response to wheel-exercise. Voluntary physical exercise in laboratory mice always increases AHN but this positive effect cannot be additively stimulated by enhanced running and is even lost as soon as the mice are forced to run. Rewarding the mice for their performance leads to an increase in wheel activity
\nbut does not translate into a corresponding additive increase in adult neurogenesis. Likewise, a more naturalistic situation, in which laboratory mice must run to obtain their daily food does not lead to an increase in cell proliferation and entails only a small increase in the number of young neurons, far below the one in voluntary running mice.
\nWild wood mice (Apodemus sylvaticus) and wild-derived western house mice (Mus musculus domesticus), both close relatives of the common laboratory mouse strains, were tested in the same running situations as laboratory C57BL/6 mice. Besides species-differences in basal neurogenesis rate, we find adult neurogenesis in wild mice remaining relatively constant in response to external influences. None of the factors that normally affect AHN in laboratory animals, such as stress, environmental changes or physical exercise, have an effect on adult neurogenesis in these animals. In wood mice, neither voluntary wheel running nor stress or an impoverished cage environment affect the number of newly generated neurons. House mice also show a stable adult neurogenesis, which shows no significant change after voluntary running or running for food.
\nAdult neurogenesis in the dentate gyrus is thus regulated differently in laboratory and wild mice. However, even in laboratory mice it is not as plastic as initially suggested: laboratory mice, which are tested in a more naturalistic and complex running situation, show rather weak plasticity of AHN, resembling wild mice. Hence, it seems that the regulatory difference in adult neurogenesis between laboratory- and wild mice is, that laboratory animals react to a single stimulus in absence of other inputs. We believe that the constant exposure to different stimuli potentially affecting AHN has led to a natural selection that stabilizes adult neurogenesis in the wild. In contrast, during domestication - including inbreeding - much of the homeostatic capacity in regulating adult neurogenesis might have been lost.
\nTaken together, our data imply that genetic (species-specific differences as well as within-species variation) play an important role in determining basal rates of adult neurogenesis, while motivational-contextual factors modulate the response of AHN to physical exercise, albeit chiefly in domesticated laboratory strains . As such differences appear already between phylogenetically closely related species, extrapolating findings in laboratory mice to distantly related taxonomic groups, such as humans, obviously requires much caution.

New neurons are generated throughout life. This discovery has challenged firmly held concepts about the structural plasticity and regenerative capacity of the mammalian brain. In this special issue of the EJN, leaders in the field summarize and review recent advances aiming to understand the molecular mechanisms underlying adult neurogenesis and the impact of new neurons on brain function in health and disease. Below we discuss pivotal yet unsolved aspects of adult neurogenesis as well as potential future directions in the field.

Adult hippocampal neurogenesis in Alzheimer's disease: A roadmap to clinical relevance.

Adult neurogenesis adds an entirely new level of plasticity to the brain and raises hope to use stem cell therapy to repair damaged nervous tissue. To understand the role of neurogenesis in the adult brain and to harness its potential it is of utmost importance to understand the regulation of the stem cell niches. Our group previously showed that the endozepine DBI is expressed in neuronal progenitors in the SVZ and that it reduces GABA signalling in these cells. Via this mechanism, DBI promotes the proliferation of fast dividing progenitors which leads to a strong increase in neurogenesis. Here I investigated the presence of DBI in other neurogenic niches and its role in regulating postnatal and adult neurogenesis. I found that DBI is strongly expressed in the SGZ and in the walls of the 3rd ventricle both postnatally and in adult mice. Furthermore, I showed that DBI is present in RG cells during embryonic development. I found that DBI is expressed not only in all mouse postnatal and adult neurogenic niches but also across species in the SGZ of the Rhesus monkey and in humans. High expression levels of DBI were detected in all stem cells and in the early population of amplifying progenitors, suggesting that this protein could be considered as an indicator for stemness in the nervous tissue. Focusing on the SGZ, I showed that DBI negatively modulates the activity of the GABAA receptor in stem cells, thereby increasing their proliferation, self-renewal and astrocyte production. In summary, DBI together with GABA regulate the balance between preserving the stem cell pool and neuronal production. External factors such as environmental enrichment and physical exercise strongly enhance neurogenesis. I found in this study that DBI is essential for the pro-proliferative and pro-neurogenic effects of enriched environment and exercise. Therefore, DBI and GABA regulate SGZ neurogenesis in a close partnership enabling multiple levels of control which makes the niche dynamic and capable of reacting promptly to changes in the environment.

A long-standing problem in the field of adult neurogenesis has been the need to identify newborn neurons and their precursors within a much larger population of preexisting mature neurons and glia. If these nascent cells could be identified, it would be possible to visualize and enumerate such cells in vivo, to access them for electrophysiological and molecular studies, to identify their connections in the neuronal networks, and to alter their activity and function. Several strategies have been developed to solve this problem of finding the proverbial needle in a haystack. Methods such as labeling with thymidine analogs, phenotypic analysis based on the expression of developmental markers, and retro- and lentiviral labeling have each had an important role in advancing our understanding of the proliferation and maturation of newborn neurons in the adult brain. As with all methods, these techniques have advantages and limits that demarcate their appropriate application. In this review, we focus on genetic approaches to studying adult mammalian neurogenesis, describing reporter lines of transgenic mice and summarizing recent advances that employ these emerging technologies. The general strategy of these genetic approaches is to drive the expression of “live” markers such as green fluorescent protein (GFP) in a defined population of neurons, neuronal progenitors, or stem cells. Cytoplasmic expression of fluorescent proteins (FPs) allows the full morphology of labeled cells to be visualized, whereas nuclear expression of such proteins facilitates cell enumeration. FP expression also allows labeled cells to be identified and accessed in live animals and in acute...

The ongoing production of neurons in selected areas of the adult mammalian brain is tantalizing and has become an active area of research for many investigators. It is exciting to consider the functional importance of adding new neurons to mature circuits, as well as the intricate biological processes regulating their production (Meltzer et al. 2005; Ming and Song 2005; Lledo et al. 2006). Many investigators are also fascinated by the potential of repairing the injured nervous system with adult-generated neurons, those either produced in specialized adult neurogenic niches or induced in regions where little (if any) neurogenesis normally persists, such as the spinal cord or neocortex. Whether the inspiration is systems neuroscience, basic biology, or biomedical applications, the advancement of the field of neurogenesis depends on understanding the underlying molecular mechanisms regulating this process. When considering this central issue, most investigators have posited that molecular pathways important for development must have similar roles in the adult (Deisseroth et al. 2004; Meltzer et al. 2005; Ming and Song 2005). It is important to realize, however, that the number of studies demonstrating a required and specific role for any developmental regulators in adult neurogenesis is quite small. Adult neurogenesis is contingent on the functioning of the neurogenic niche, which must be produced during development, maintained during postnatal life, and regulated during adulthood. This presents a significant barrier for interpreting most genetic manipulations, as it is virtually impossible to distinguish adult requirements from developmental insults in most studies examining these pathways. To establish...

Adult neurogenesis in birds offers unique opportunities to study basic questions addressing the birth, migration and differentiation of neurons. Neurons in adult canaries originate from discrete proliferative regions on the walls of the lateral ventricles. They migrate away from their site of birth, initially at high rates, along the processes of radical cells. The rates of dispersal diminish as the young neurons invade regions devoid of radial fibers, probably under the guidance of other cues. The discrete sites of birth in the ventricular zone generate neurons that end up differentiating throughout the telencephalon. New neurons may become interneurons or projection neurons; the latter connect two song control nuclei between neostriatum and archistriatum. Radial cells, that in mammals disappear as neurogenesis comes to an end, persist in the adult avian brain. The presence of radial cells may be key to adult neurogenesis. Not only do they serve as guides for initial dispersal, they also divide and may be the progenitors of new neurons.