Adult Neurogenesis in Fish

Adult neurogenesis in the Uruguayan teleost species Austrolebias charrua and Gymnotus omarorum

Neurogenesis -the process by which new neurons are generated in the brain- is critical for nervous system development and plasticity. Adult neurogenesis underlies growth, repair, and adaptation to environmental changes. Vertebrates differ in their neurogenic and regenerative capacity, being particularly prominent in teleost as adult neurogenesis occurs throughout the rostral-caudal brain axis. This review examines adult proliferation and neurogenesis in the autochthonous Uruguayan teleost Austrolebias charrua and Gymnotus omarorum. A. charrua are annual fishes that live in temporary freshwater pools that dry up in the summer. The luminosity of the puddles varies greatly, and both vision and olfaction are crucial for the survival of this species. G. omarorum inhabits freshwater lagoons and rivers beneath dense masses of floating plants and have nocturnal habits. They rely on the electrosensory modality to navigate and communicate with conspecifics. These differences in habitats and predominant sensory modalities are reflected in the distinct brain morphotypes of G. omarorum and A. charrua. While G. omarorum is characterized by the hypertrophy of rhombencephalic cerebellum and electrosensory lateral line lobe, A. charrua has a well-developed olfactory bulb, mesencephalic tectum opticum, and torus longitudinalis. Accordingly, these regions have notorious neurogenic activity. Differences in neuroanatomy and distribution of neurogenesis in the brains of both species are discussed considering their life cycle and lifestyle. The comparison of these results with those reported in other teleost and vertebrates contributes to the understanding of the key role of neurogenesis in brain plasticity and evolution.

Life History-Dependent Brain Transcriptomic Signatures in Nothobranchids: Insights into Aging, Neurogenesis, and Life History Evolution

The African turquoise killifish Nothobranchius furzeri is a powerful model organism in aging research. Within the family Nothobranchiidae, a wide range of lifespan is observed in annual, semi-annual, and non-annual life histories. In this study, we examined the brain transcriptomic signatures of adult nothobranchids across life history variations. Our results show that the brain gene expression profiles exhibit strong life history signatures compared to the liver tissue. Semi-annual Fundulopanchax species shows upregulation in cell division and mitosis compared to non-annual Aphyosemion species. We identified genes related to neurogenesis such as DNMT3A, SOX2, and FGF10 that show downregulation in the short-lived annual species compared to other life histories. The Notch signaling pathway is enriched in the non-annual species suggesting the importance of this pathway in longer-lived killifish. Our study demonstrates that other non-model nothobranchids can be used as comparative species to N .  furzeri in the study of aging, neurogenesis, and life history.

Fish microglia: Beyond the resident macrophages of the central nervous system - A review of their morphofunctional characteristics

From classical to modern literature on microglia, the importance of the potential and variability of these immune cells in vertebrates has been pointed out. Recent aspects such as relationships and interactions between microglia and neurons in both normal and injured neural tissues, as well as their nexus with other organs and with the microbiota, or how these cells are modulated during development and adulthood are current topics of major interest. State-of-the-art research methodologies, including microscopy and potent in vivo imaging techniques, genomic and proteomic methods, current culture conditions together with the easy maintenance and manipulation of some fish embryos and adult specimens such as zebrafish (Danio rerio), have emerged and adapted to the phylogenetic position of some fish species. Furthermore, these advancements have facilitated the development of successful protocols aimed at addressing significant hypotheses and unresolved questions regarding vertebrate glia. The present review aims to analyse the available information on fish microglia, mainly the most recent one concerning teleosts, to establish an overview of their structural and immune functional features as a basis for their potentialities, heterogeneity, diversification, involvement, and relationships with neurons under normal and pathological conditions.

Diversity of Intraspecific Patterns of Brain Region Size Covariation in Fish

Traits often do not evolve in isolation or vary independently of other traits. Instead, they can be affected by covariation, both within and across species. However, the importance of within-species trait covariation and, critically, the degree to which it varies between species has yet to be thoroughly studied. Brain morphology is a trait of great ecological and behavioral importance, with regions that are hypothesized to vary in size based on behavioral and cognitive demands. Sizes of brain regions have also been shown to covary with each other across various taxa. Here, we test the degree to which covariation in brain region sizes within species has been conserved across 10 teleost fish species. These 10 species span five orders, allowing us to examine how phylogenetic proximity influences similarities in intraspecific trait covariation. Our results showed a trend that similar patterns of brain region size covariation occur in more closely related species. Interestingly, there were certain brain region pairs that showed similar levels of covariation across all species regardless of phylogenetic distance, such as the telencephalon and optic tectum, while others, such as the olfactory bulb and the hypothalamus, varied more independently. Ultimately, the patterns of brain region covariation shown here suggest that evolutionary mechanisms or constraints can act on specific brain regions independently, and that these constraints can change over evolutionary time.

Concerted monoamine oxidase activity following exposure to di-2-ethylhexyl phthalate is associated with aggressive neurobehavioral response and neurodegeneration in zebrafish brain

Di-2-ethylhexyl phthalate (DEHP) is the most commonly preferred synthetic organic chemical in plastics and its products for making them ductile, flexible and durable. As DEHP is not chemically bound to the macromolecular polymer of plastics, it can be easily leached out to accumulate in food and environment. Our recent report advocated that exposure to DEHP significantly transformed the innate bottom-dwelling and scototaxis behaviour of zebrafish. Our present study aimed to understand the possible role of DEHP exposure pertaining towards the development of aggressive behaviour and its association with amplified monoamine oxidase activity and neurodegeneration in the zebrafish brain. As heightened monoamine oxidase (MAO) is linked with genesis of aggressive behaviour, our observation also coincides with DEHP-persuaded aggressive neurobehavioral transformation in zebrafish. Our preliminary findings also showed that DEHP epitomized as a prime factor in transforming native explorative behaviour and genesis of aggressive behaviour through oxidative stress induction and changes in the neuromorphology in the periventricular grey zone (PGZ) of the zebrafish brain. With the finding demarcating towards heightened chromatin condensation in the PGZ of zebrafish brain, our further observation by immunohistochemistry showed a profound augmentation in apoptotic cell death marker cleaved caspase 3 (CC3) expression following exposure to DEHP. Our further observation by immunoblotting study also demarcated a temporal augmentation in CC3 and tyrosine hydroxylase expression in the zebrafish brain. Therefore, the gross findings of the present study delineate the idea that chronic exposure to DEHP is associated with MAO-instigated aggressive neurobehavioral transformation and neurodegeneration in the zebrafish brain.

The Effects of Rearing Environment on Organization of the Olfactory System and Brain of Juvenile Sockeye Salmon, Oncorhynchus nerka

Pacific salmon (Oncorhynchus spp.) hatch and feed in freshwater habitats, migrate to sea to mature, and return to spawn at natal sites. The final, riverine stages of the return migrations are mediated by chemical properties of the natal stream that they learned as juveniles. Like some other fish, salmon growth is asymptotic; they grow continuously throughout life toward a maximum size. The continued growth of the nervous system may be plastic in response to environmental variables. Due to the ecological, cultural, and economic importance of Pacific salmon, individuals are often reared in hatcheries and released into the wild as juveniles to supplement natural populations. However, hatchery-reared individuals display lower survivorship and may also stray (i.e., spawn in a non-natal stream) at higher rates than their wild counterparts. Hatchery environments may lack stimuli needed to promote normal development of the nervous system, thus leading to behavioral deficits and a higher incidence of straying. This study compared the peripheral olfactory system and brain organization of hatchery-reared and wild-origin sockeye salmon fry (Oncorhynchus nerka). Surface area of the olfactory rosette, diameter of the olfactory nerve, total brain size, and size of major brain regions were measured from histological sections and compared between wild and hatchery-origin individuals. Hatchery-origin fish had significantly larger optic tecta, and marginally insignificant, yet noteworthy trends, existed in the valvula cerebelli (hatchery > wild) and olfactory bulbs (hatchery < wild). We also found a putative difference in olfactory nerve diameter (dmin) (hatchery > wild), but the validity of this finding needs further analyses with higher resolution methods. Overall, these results provide insight into the potential effects of hatchery rearing on nervous system development in salmonids, and may explain behavioral deficits displayed by hatchery-origin individuals post-release.

Constitutive Neurogenesis and Neuronal Plasticity in the Adult Cerebellum and Brainstem of Rainbow Trout, Oncorhynchus mykiss

The central nervous system of Pacific salmon retains signs of embryonic structure throughout life and a large number of neuroepithelial neural stem cells (NSCs) in the proliferative areas of the brain, in particular. However, the adult nervous system and neurogenesis studies on rainbow trout, Oncorhynchus mykiss, are limited. Here, we studied the localization of glutamine synthetase (GS), vimentin (Vim), and nestin (Nes), as well as the neurons formed in the postembryonic period, labeled with doublecortin (DC), under conditions of homeostatic growth in adult cerebellum and brainstem of Oncorhynchus mykiss using immunohistochemical methods and Western Immunoblotting. We observed that the distribution of vimentin (Vim), nestin (Nes), and glutamine synthetase (GS), which are found in the aNSPCs of both embryonic types (neuroepithelial cells) and in the adult type (radial glia) in the cerebellum and the brainstem of trout, has certain features. Populations of the adult neural stem/progenitor cells (aNSPCs) expressing GS, Vim, and Nes have different morphologies, localizations, and patterns of cluster formation in the trout cerebellum and brainstem, which indicates the morphological and, obviously, functional heterogeneity of these cells. Immunolabeling of PCNA revealed areas in the cerebellum and brainstem of rainbow trout containing proliferating cells which coincide with areas expressing Vim, Nes, and GS. Double immunolabeling revealed the PCNA/GS PCNA/Vim coexpression patterns in the neuroepithelial-type cells in the PVZ of the brainstem. PCNA/GS coexpression in the RG was detected in the submarginal zone of the brainstem. The results of immunohistochemical study of the DC distribution in the cerebellum and brainstem of trout have showed a high level of expression of this marker in various cell populations. This may indicate: (i) high production of the adult-born neurons in the cerebellum and brainstem of adult trout, (ii) high plasticity of neurons in the cerebellum and brainstem of trout. We assume that the source of new cells in the trout brain, along with PVZ and SMZ, containing proliferating cells, may be local neurogenic niches containing the PCNA-positive and silent (PCNA-negative), but expressing NSC markers, cells. The identification of cells expressing DC, Vim, and Nes in the IX-X cranial nerve nuclei of trout was carried out.

Adult Neurogenesis of Teleost Fish Determines High Neuronal Plasticity and Regeneration

Studying the properties of neural stem progenitor cells (NSPCs) in a fish model will provide new information about the organization of neurogenic niches containing embryonic and adult neural stem cells, reflecting their development, origin cell lines and proliferative dynamics. Currently, the molecular signatures of these populations in homeostasis and repair in the vertebrate forebrain are being intensively studied. Outside the telencephalon, the regenerative plasticity of NSPCs and their biological significance have not yet been practically studied. The impressive capacity of juvenile salmon to regenerate brain suggests that most NSPCs are likely multipotent, as they are capable of replacing virtually all cell lineages lost during injury, including neuroepithelial cells, radial glia, oligodendrocytes, and neurons. However, the unique regenerative profile of individual cell phenotypes in the diverse niches of brain stem cells remains unclear. Various types of neuronal precursors, as previously shown, are contained in sufficient numbers in different parts of the brain in juvenile Pacific salmon. This review article aims to provide an update on NSPCs in the brain of common models of zebrafish and other fish species, including Pacific salmon, and the involvement of these cells in homeostatic brain growth as well as reparative processes during the postraumatic period. Additionally, new data are presented on the participation of astrocytic glia in the functioning of neural circuits and animal behavior. Thus, from a molecular aspect, zebrafish radial glia cells are seen to be similar to mammalian astrocytes, and can therefore also be referred to as astroglia. However, a question exists as to if zebrafish astroglia cells interact functionally with neurons, in a similar way to their mammalian counterparts. Future studies of this fish will complement those on rodents and provide important information about the cellular and physiological processes underlying astroglial function that modulate neural activity and behavior in animals.

Neurosphere culture derived from aged hippocampal dentate gyrus

The neurosphere assay is the gold standard for determining proliferative and differentiation potential of neural progenitor cells (NPCs) in neurogenesis studies 1-3 . While several in vitro assays have been developed to model the process of neurogenesis, they have predominantly used embryonic and early postnatal NPCs derived from the dentate gyrus (DG). A limitation of these approaches is that they do not provide insight into adult-born NPCs, which are modeled to affect hippocampal function and diseases later in life. Here, we show a novel free-floating neurosphere culture system using NPCs isolated from the DG of mature adult and aged mice. The protocol outlines detailed steps on the isolation, propagation, and maintenance of neurospheres from adult and aged (>12 months old) mouse brain and how to differentiate cultured neurospheres into neurons and astrocytes. Culturing adult and aged NPCs provides an important in vitro model to (1) investigate cellular and molecular properties of this unique cell population and (2) expand the understanding of plasticity in the adult and aging brain. This protocol requires ∼2 hours to complete dissection, dissociation and culture plating, while differentiation to neuronal and astrocytic lineages takes 9 days. By focusing on neurospheres obtained from animals at later ages this model facilitates investigation of important biological questions related to development and differentiation of hippocampal neurons generated throughout adult life.

Post-Traumatic Expressions of Aromatase B, Glutamine Synthetase, and Cystathionine-Beta-Synthase in the Cerebellum of Juvenile Chum Salmon, Oncorhynchus keta

In adult fish, neurogenesis occurs in many areas of the brain, including the cerebellum, with the ratio of newly formed cells relative to the total number of brain cells being several orders of magnitude greater than in mammals. Our study aimed to compare the expressions of aromatase B (AroB), glutamine synthetase (GS), and cystathionine-beta-synthase (CBS) in the cerebellum of intact juvenile chum salmon, Oncorhynchus keta. To identify the dynamics that determine the involvement of AroB, GS, and CBS in the cellular mechanisms of regeneration, we performed a comprehensive assessment of the expressions of these molecular markers during a long-term primary traumatic brain injury (TBI) and after a repeated acute TBI to the cerebellum of O. keta juveniles. As a result, in intact juveniles, weak or moderate expressions of AroB, GS, and CBS were detected in four cell types, including cells of the neuroepithelial type, migrating, and differentiated cells (graphic abstract, A). At 90 days post injury, local hypercellular areas were found in the molecular layer containing moderately labeled AroB+, GS+, and CBS+ cells of the neuroepithelial type and larger AroB+, GS+, and CBS+ cells (possibly analogous to the reactive glia of mammals); patterns of cells migration and neovascularization were also observed. A repeated TBI caused the number of AroB+, GS+, and CBS+ cells to further increase; an increased intensity of immunolabeling was recorded from all cell types (graphic abstract, C). Thus, the results of this study provide a better understanding of adult neurogenesis in teleost fishes, which is expected to clarify the issue of the reactivation of adult neurogenesis in mammalian species.

Echinoderm radial glia in adult cell renewal, indeterminate growth, and regeneration

Echinoderms are a phylum of marine deterostomes with a range of interesting biological features. One remarkable ability is their impressive capacity to regenerate most of their adult tissues, including the central nervous system (CNS). The research community has accumulated data that demonstrates that, in spite of the pentaradial adult body plan, echinoderms share deep similarities with their bilateral sister taxa such as hemichordates and chordates. Some of the new data reveal the complexity of the nervous system in echinoderms. In terms of the cellular architecture, one of the traits that is shared between the CNS of echinoderms and chordates is the presence of radial glia. In chordates, these cells act as the main progenitor population in CNS development. In mammals, radial glia are spent in embryogenesis and are no longer present in adults, being replaced with other neural cell types. In non-mammalian chordates, they are still detected in the mature CNS along with other types of glia. In echinoderms, radial glia also persist into the adulthood, but unlike in chordates, it is the only known glial cell type that is present in the fully developed CNS. The echinoderm radial glia is a multifunctional cell type. Radial glia forms the supporting scaffold of the neuroepithelium, exhibits secretory activity, clears up dying or damaged cells by phagocytosis, and, most importantly, acts as a major progenitor cell population. The latter function is critical for the outstanding developmental plasticity of the adult echinoderm CNS, including physiological cell turnover, indeterminate growth, and a remarkable capacity to regenerate major parts following autotomy or traumatic injury. In this review we summarize the current knowledge on the organization and function of the echinoderm radial glia, with a focus on the role of this cell type in adult neurogenesis.

Cellular profiling of a recently-evolved social behavior in cichlid fishes

Social behaviors are diverse in nature, but it is unclear how conserved genes, brain regions, and cell populations generate this diversity. Here we investigate bower-building, a recently-evolved social behavior in cichlid fishes. We use single nucleus RNA-sequencing in 38 individuals to show signatures of recent behavior in specific neuronal populations, and building-associated rebalancing of neuronal proportions in the putative homolog of the hippocampal formation. Using comparative genomics across 27 species, we trace bower-associated genome evolution to a subpopulation of glia lining the dorsal telencephalon. We show evidence that building-associated neural activity and a departure from quiescence in this glial subpopulation together regulate hippocampal-like neuronal rebalancing. Our work links behavior-associated genomic variation to specific brain cell types and their functions, and suggests a social behavior has evolved through changes in glia.

New cells added to the preoptic area during sex change in the common clownfish Amphiprion ocellaris

Sex differences in cell number in the preoptic area of the hypothalamus (POA) are documented across all major vertebrate lineages and contribute to differential regulation of the hypothalamic-pituitary-gonad axis and reproductive behavior between the sexes. Sex-changing fishes provide a unique opportunity to study mechanisms underlying sexual differentiation of the POA. In anemonefish (clownfish), which change sex from male to female, females have approximately twice the number of medium-sized cells in the anterior POA compared to males. This sex difference transitions from male-like to female-like during sex change. However, it is not known how this sex difference in POA cell number is established. This study tests the hypothesis that new cell addition plays a role. We initiated adult male-to-female sex change in 30 anemonefish (Amphiprion ocellaris) and administered BrdU to label new cells added to the POA at regular intervals throughout sex change. Sex-changing fish added more new cells to the anterior POA than non-changing fish, supporting the hypothesis. The observed effects could be accounted for by differences in POA volume, but they are also consistent with a steady trickle of new cells being gradually accumulated in the anterior POA before vitellogenic oocytes develop in the gonads. These results provide insight into the unique characteristics of protandrous sex change in anemonefish relative to other modes of sex change, and support the potential for future research in sex-changing fishes to provide a richer understanding of the mechanisms for sexual differentiation of the brain.

Prdm16 and Vcam1 regulate the postnatal disappearance of embryonic radial glia and the ending of cortical neurogenesis

Embryonic neural stem cells (NSCs, i.e., radial glia) in the ventricular-subventricular zone (V-SVZ) generate the majority of neurons and glia in the forebrain. Postnatally, embryonic radial glia disappear and a subpopulation of radial glia transition into adult NSCs. As this transition occurs, widespread neurogenesis in brain regions such as the cerebral cortex ends. The mechanisms that regulate the postnatal disappearance of radial glia and the ending of embryonic neurogenesis remain poorly understood. Here, we show that PR domain-containing 16 (Prdm16) promotes the disappearance of radial glia and the ending of neurogenesis in the cerebral cortex. Genetic deletion of Prdm16 from NSCs leads to the persistence of radial glia in the adult V-SVZ and prolonged postnatal cortical neurogenesis. Mechanistically, Prdm16 induces the postnatal reduction in Vascular Cell Adhesion Molecule 1 (Vcam1). The postnatal disappearance of radial glia and the ending of cortical neurogenesis occur normally in Prdm16-Vcam1 double conditional knockout mice. These observations reveal novel molecular regulators of the postnatal disappearance of radial glia and the ending of embryonic neurogenesis, filling a key knowledge gap in NSC biology.

Localization and Characterization of Major Neurogenic Niches in the Brain of the Lesser-Spotted Dogfish Scyliorhinus canicula

Adult neurogenesis is defined as the ability of specialized cells in the postnatal brain to produce new functional neurons and to integrate them into the already-established neuronal network. This phenomenon is common in all vertebrates and has been found to be extremely relevant for numerous processes, such as long-term memory, learning, and anxiety responses, and it has been also found to be involved in neurodegenerative and psychiatric disorders. Adult neurogenesis has been studied extensively in many vertebrate models, from fish to human, and observed also in the more basal cartilaginous fish, such as the lesser-spotted dogfish, Scyliorhinus canicula, but a detailed description of neurogenic niches in this animal is, to date, limited to the telencephalic areas. With this article, we aim to extend the characterization of the neurogenic niches of S. canicula in other main areas of the brain: we analyzed via double immunofluorescence sections of telencephalon, optic tectum, and cerebellum with markers of proliferation (PCNA) and mitosis (pH3) in conjunction with glial cell (S100β) and stem cell (Msi1) markers, to identify the actively proliferating cells inside the neurogenic niches. We also labeled adult postmitotic neurons (NeuN) to exclude double labeling with actively proliferating cells (PCNA). Lastly, we observed the presence of the autofluorescent aging marker, lipofuscin, contained inside lysosomes in neurogenic areas.

Allometric model of brain morphology of Hemiculter leucisculus and its variation along climatic gradients

Prior studies on Hemiculter leucisculus, which is a widespread native fish in China, mainly focused on its growth, feeding habits, and individual fecundity, but few have investigated the brain. In this research, we explored the developmental patterns of the Hemiculter leucisculus brain and found the brain showed allometry through sample time points and three age groups. At the same time, we found that the brain varied along climatic gradients. The volumes of the olfactory bulbs, telencephalic lobes, optic tectum, corpus cerebelli, and total brain in the south were larger than those in the north, while the volume of the hypothalamus in the north was larger than in the south. This study provides a view for the in-depth study of the acclimatized mechanism of the teleost brain, lays a foundation for the further study of evolutionary ecology, and provides a reference for the phenotypic plasticity of the teleost brain.

Uncovering the spectrum of adult zebrafish neural stem cell cycle regulators

Adult neural stem and progenitor cells (aNSPCs) persist lifelong in teleost models in diverse stem cell niches of the brain and spinal cord. Fish maintain developmental stem cell populations throughout life, including both neuro-epithelial cells (NECs) and radial-glial cells (RGCs). Within stem cell domains of the brain, RGCs persist in a cycling or quiescent state, whereas NECs continuously divide. Heterogeneous populations of RGCs also sit adjacent the central canal of the spinal cord, showing infrequent proliferative activity under homeostasis. With the rise of the zebrafish (Danio rerio) model to study adult neurogenesis and neuroregeneration in the central nervous system (CNS), it has become evident that aNSPC proliferation is regulated by a wealth of stimuli that may be coupled with biological function. Growing evidence suggests that aNSPCs are sensitive to environmental cues, social interactions, nutrient availability, and neurotrauma for example, and that distinct stem and progenitor cell populations alter their cell cycle activity accordingly. Such stimuli appear to act as triggers to either turn on normally dormant aNSPCs or modulate constitutive rates of niche-specific cell cycle behaviour. Defining the various forms of stimuli that influence RGC and NEC proliferation, and identifying the molecular regulators responsible, will strengthen our understanding of the connection between aNSPC activity and their biological significance. In this review, we aim to bring together the current state of knowledge on aNSPCs from studies investigating the zebrafish CNS, while highlighting emerging cell cycle regulators and outstanding questions that will help to advance this fascinating field of stem cell biology.

The costs and benefits of larger brains in fishes

The astonishing diversity of brain sizes observed across the animal kingdom is typically explained in the context of trade-offs: the benefits of a larger brain, such as enhanced cognitive ability, are balanced against potential costs, such as increased energetic demands. Several hypotheses have been formulated in this framework, placing different emphasis on ecological, behavioural, or physiological aspects of trade-offs in brain size evolution. Within this body of work, there exists considerable taxonomic bias towards studies of birds and mammals, leaving some uncertainty about the generality of the respective arguments. Here, we test three of the most prominent such hypotheses, the 'expensive tissue', 'social brain' and 'cognitive buffer' hypotheses, in a large dataset of fishes, derived from a publicly available resource (FishBase). In accordance with predictions from the 'expensive tissue' and the 'social brain' hypothesis, larger brains co-occur with reduced fecundity and increased sociality in at least some Classes of fish. Contrary to expectations, however, lifespan is reduced in large-brained fishes, and there is a tendency for species that perform parental care to have smaller brains. As such, it appears that some potential costs (reduced fecundity) and benefits (increased sociality) of large brains are near universal to vertebrates, whereas others have more lineage-specific effects. We discuss our findings in the context of fundamental differences between the classically studied birds and mammals and the fishes we analyse here, namely divergent patterns of growth, parenting and neurogenesis. As such, our work highlights the need for a taxonomically diverse approach to any fundamental question in evolutionary biology.

The Cytoarchitecture of the Tectal-Related Pallium of Squirrelfish, Holocentrus sp

The squirrelfish, which live in visually complex coral reefs, have very large eyes and a special dual-system “day and night vision” retina. They also have atypical expansions of brain areas involved in processing visual information. The midbrain tectum sends information via diencephalic relay to two enlarged dorsal telencephalic regions. The latter include a superficial dorsal/lateral “cortex-like area” of small to medium-sized cells [area dorsalis telencephali, pars lateralis-dorsal region (dorsal segment); Dld1] which projects to an underlying dorsocentral region of relatively large cells (the area dorsalis telencephali, pars centralis-dorsal region; Dcd) which in turn reconnects with the tectum. Additionally, the cerebellum is also involved in this pathway. The hypertrophied pallial regions, termed the tectal-related pallium (TRP), most likely exert major influences on a variety of visually-related sensorimotor systems. This research aimed at better establishing the cellular structures and possible connections within the TRP. Nissl and rapid Golgi staining, biotinylated dextran amine tracing and cell-filling, and electron microscopy were used in this study. For gross observation of the pallial areas and plotting of the study sites, a mini-atlas of transverse and horizontal sections was constructed. This research better documented the known cellular elements of the TRP and defined two novel cell types. Species differences in the TRP may be related to possible differences in behavior and ecology.

Expression of Doublecortin, Glial Fibrillar Acidic Protein, and Vimentin in the Intact Subpallium and after Traumatic Injury to the Pallium in Juvenile Salmon, Oncorhynchus masou

Fetalization associated with a delay in development and the preservation of the features of the embryonic structure of the brain dominates the ontogeny of salmonids. The aim of the present study was to comparatively analyze the distribution of the glial-type aNSC markers such as vimentin and glial fibrillar acidic protein (GFAP) and the migratory neuronal precursors such as doublecortin in the telencephalon subpallium of juvenile masu salmon, Oncorhynchus masou, in normal conditions and at 1 week after an injury to the dorsal pallium. Immunohistochemical labeling of vimentin, GFAP, and doublecortin in the pallium of intact juvenile masu salmon revealed single cells with similar morphologies corresponding to a persistent pool of neuronal and/or glial progenitors. The study of the posttraumatic process showed the presence of intensely GFAP-labeled cells of the neuroepithelial type that form reactive neurogenic zones in all areas of the subpallial zone of juvenile masu salmon. A comparative analysis of the distribution of radial glia in the dorsal, ventral, and lateral zones of the subpallium showed a maximum concentration of cells in the dorsal part of subpallium (VD) and a minimum concentration in the lateral part of subpallium VL. An essential feature of posttraumatic immunolabeling in the masu salmon subpallium is the GFAP distribution patterns that are granular intracellular in the apical periventricular zone (PVZ) and fibrillar extracellular in the subventricular (SVZ) and parenchymal zones (PZ). In contrast to those in intact animals, most of the GFAP+ granules and constitutive neurogenic niches in injured fish were localized in the basal part of the PVZ. With the traumatic injury to the subpallium, the number of Vim+ cells in the lateral and ventral regions significantly increased. At 1 week post-injury, the total immunolabeling of vimentin cells in the PVZ was replaced by the granular pattern of Vim immunodistribution spreading from the PVZ to the SVZ and deeper parenchymal layers of the brain in all areas of the subpallium. A significant increase in the number of DC+ cells was observed also in all areas of the subpallium. The number of cells increased both in the PVZ and in the SVZ, as well as in the deeper PZ. Thus, at 1 week after the injury to the dorsal pallium, the number of DC, Vim, and GFAP expressing cells of the neuroepithelial type in the subpallium of juvenile masu salmon increased, and additionally GFAP+ radial glia appeared in VD, which was absent from intact animals.

Molecular Markers of Adult Neurogenesis in the Telencephalon and Tectum of Rainbow Trout, Oncorhynchus mykiss

In the brain of teleost fish, radial glial cells are the major type of astroglial cells. To answer the question as to how radial glia structures adapt to the continuous growth of the brain, which is characteristic of salmonids, it is necessary to study various types of cells (neuronal precursors, astroglial cells, and cells in a state of neuronal differentiation) in the major integrative centers of the salmon brain (telencephalon and tectum opticum), using rainbow trout, Oncorhynchus mykiss, as a model. A study of the distribution of several molecular markers in the telencephalon and tectum with the identification of neural stem/progenitor cells, neuroblasts, and radial glia was carried out on juvenile (three-year-old) O. mykiss. The presence of all of these cell types provides specific conditions for the adult neurogenesis processes in the trout telencephalon and tectum. The distribution of glutamine synthetase, a molecular marker of neural stem cells, in the trout telencephalon revealed a large population of radial glia (RG) corresponding to adult-type neural stem cells (NSCs). RG dominated the pallial region of the telencephalon, while, in the subpallial region, RG was found in the lateral and ventral zones. In the optic tectum, RG fibers were widespread and localized both in the marginal layer and in the periventricular gray layer. Doublecortin (DC) immunolabeling revealed a large population of neuroblasts formed in the postembryonic period, which is indicative of intense adult neurogenesis in the trout brain. The pallial and subpallial regions of the telencephalon contained numerous DC+ cells and their clusters. In the tectum, DC+ cells were found not only in the stratum griseum periventriculare (SGP) and longitudinal torus (TL) containing proliferating cells, but also in the layers containing differentiated neurons: the central gray layer, the periventricular gray and white layers, and the superficial white layer. A study of the localization patterns of vimentin and nestin in the trout telencephalon and tectum showed the presence of neuroepithelial neural stem cells (eNSCs) and ependymoglial cells in the periventricular matrix zones of the brain. The presence of vimentin and nestin in the functionally heterogeneous cell types of adult trout indicates new functional properties of these proteins and their heterogeneous involvement in intracellular motility and adult neurogenesis. Investigation into the later stages of neuronal development in various regions of the fish brain can substantially elucidate the major mechanisms of adult neurogenesis, but it can also contribute to understanding the patterns of formation of certain brain regions and the involvement of RG in the construction of the definite brain structure.

Zebrafish Blunt-Force TBI Induces Heterogenous Injury Pathologies That Mimic Human TBI and Responds with Sonic Hedgehog-Dependent Cell Proliferation across the Neuroaxis

Blunt-force traumatic brain injury (TBI) affects an increasing number of people worldwide as the range of injury severity and heterogeneity of injury pathologies have been recognized. Most current damage models utilize non-regenerative organisms, less common TBI mechanisms (penetrating, chemical, blast), and are limited in scalability of injury severity. We describe a scalable blunt-force TBI model that exhibits a wide range of human clinical pathologies and allows for the study of both injury pathology/progression and mechanisms of regenerative recovery. We modified the Marmarou weight drop model for adult zebrafish, which delivers a scalable injury spanning mild, moderate, and severe phenotypes. Following injury, zebrafish display a wide range of severity-dependent, injury-induced pathologies, including seizures, blood–brain barrier disruption, neuroinflammation, edema, vascular injury, decreased recovery rate, neuronal cell death, sensorimotor difficulties, and cognitive deficits. Injury-induced pathologies rapidly dissipate 4–7 days post-injury as robust cell proliferation is observed across the neuroaxis. In the cerebellum, proliferating nestin:GFP-positive cells originated from the cerebellar crest by 60 h post-injury, which then infiltrated into the granule cell layer and differentiated into neurons. Shh pathway genes increased in expression shortly following injury. Injection of the Shh agonist purmorphamine in undamaged fish induced a significant proliferative response, while the proliferative response was inhibited in injured fish treated with cyclopamine, a Shh antagonist. Collectively, these data demonstrate that a scalable blunt-force TBI to adult zebrafish results in many pathologies similar to human TBI, followed by recovery, and neuronal regeneration in a Shh-dependent manner.

Social stimuli increase activity of adult-born cells in the telencephalon of zebrafish, Danio rerio

Fish have particularly high levels of adult neurogenesis, and this high neurogenic capacity may contribute to behavioural plasticity. While it is known that adult-born cells can differentiate into neurons and incorporate into neural circuits, it is unclear whether they are responsive to external stimuli and thereby capable of contributing to behavioural change. We tested whether cells born in the telencephalon of adult zebrafish are activated by social stimuli. We marked cell birth with BrdU and, 40 d later, exposed fish to brief (15 min) visual social stimuli and assayed cellular activity through immunolocalization of phospho-S6-ribosomal protein (pS6). BrdU+/pS6+ colabeled cells were found in six brain regions, and, in four regions (D, Dl, Dm and POA), the number of colabelled cells and fraction of BrdU+ cells that labeled pS6+ increased during social stimulation. These results are consistent with the hypothesis that adult-born neurons play a role in regulating social behaviour.

Organization of radial glia reveals growth pattern in the telencephalon of a percomorph fish Astatotilapia burtoni

In the brain of teleost fish, radial glial cells are the main astroglial cell type. To understand how radial glia structures are adapting to continuous growth of the brain, we studied the astroglial cells in the telencephalon of the cichlid fish Astatotilapia burtoni in small fry to large specimens. These animals grow to a standard length of 10-12 cm in this fish species, corresponding to a more than 100-fold increase in brain volume. Focusing on the telencephalon where glial cells are arranged radially in the everted (dorsal) pallium, immunocytochemistry for glial markers revealed an aberrant pattern of radial glial fibers in the central division of the dorsal pallium (DC, i.e., DC4 and DC5). The main glial processes curved around these nuclei, especially in the posterior part of the telencephalon. This was verified in tissue-cleared brains stained for glial markers. We further analyzed the growth of radial glia by immunocytochemically applied stem cell (proliferating cell nuclear antigen [PCNA], Sox2) and differentiation marker (doublecortin) and found that these markers were expressed at the ventricular surface consistent with a stacking growth pattern. In addition, we detected doublecortin and Sox2 positive cells in deeper nuclei of DC areas. Our data suggest that radial glial cells give rise to migrating cells providing new neurons and glia to deeper pallial regions. This results in expansion of the central pallial areas and displacement of existing radial glial. In summary, we show that radial glial cells can adapt to morphological growth processes in the adult fish brain and contribute to this growth.

Loss of Active Neurogenesis in the Adult Shark Retina

Neurogenesis is the process by which progenitor cells generate new neurons. As development progresses neurogenesis becomes restricted to discrete neurogenic niches, where it persists during postnatal life. The retina of teleost fishes is thought to proliferate and produce new cells throughout life. Whether this capacity may be an ancestral characteristic of gnathostome vertebrates is completely unknown. Cartilaginous fishes occupy a key phylogenetic position to infer ancestral states fixed prior to the gnathostome radiation. Previous work from our group revealed that the juvenile retina of the catshark Scyliorhinus canicula, a cartilaginous fish, shows active proliferation and neurogenesis. Here, we compared the morphology and proliferative status of the retina in catshark juveniles and adults. Histological and immunohistochemical analyses revealed an important reduction in the size of the peripheral retina (where progenitor cells are mainly located), a decrease in the thickness of the inner nuclear layer (INL), an increase in the thickness of the inner plexiform layer and a decrease in the cell density in the INL and in the ganglion cell layer in adults. Contrary to what has been reported in teleost fish, mitotic activity in the catshark retina was virtually absent after sexual maturation. Based on these results, we carried out RNA-Sequencing (RNA-Seq) analyses comparing the retinal transcriptome of juveniles and adults, which revealed a statistically significant decrease in the expression of many genes involved in cell proliferation and neurogenesis in adult catsharks. Our RNA-Seq data provides an excellent resource to identify new signaling pathways controlling neurogenesis in the vertebrate retina.

Role of cyclooxygenases and prostaglandins in adult brain neurogenesis

The telencephalon of adult mammals shows constitutive neurogenesis, and disease or traumatic injuries alter the rate of neurogenesis in the adult brain. Understanding the molecular signals that control adult brain neurogenesis is of crucial importance for the development of therapies to promote regeneration in the injured or diseased brain. Here, I reviewed our current knowledge on the role of cyclooxygenases and prostaglandins in controlling adult brain neurogenesis. Current data indicate that cyclooxygenase-2 derived prostaglandin E2 acting through EP receptors promotes neurogenesis in adult neurogenic niches of the telencephalon and that manipulations of this signalling pathway could be used to promote neurogenesis under pathological conditions. In this review article, I also propose new research directions to increase our knowledge on the role of this signalling pathway in neurogenesis.

Hydrogen Sulfide Modulates Adult and Reparative Neurogenesis in the Cerebellum of Juvenile Masu Salmon, Oncorhynchus masou

Fish are a convenient model for the study of reparative and post-traumatic processes of central nervous system (CNS) recovery, because the formation of new cells in their CNS continues throughout life. After a traumatic injury to the cerebellum of juvenile masu salmon, Oncorhynchus masou, the cell composition of the neurogenic zones containing neural stem cells (NSCs)/neural progenitor cells (NPCs) in the acute period (two days post-injury) changes. The presence of neuroepithelial (NE) and radial glial (RG) neuronal precursors located in the dorsal, lateral, and basal zones of the cerebellar body was shown by the immunohistochemical (IHC) labeling of glutamine synthetase (GS). Progenitors of both types are sources of neurons in the cerebellum of juvenile O. masou during constitutive growth, thus, playing an important role in CNS homeostasis and neuronal plasticity during ontogenesis. Precursors with the RG phenotype were found in the same regions of the molecular layer as part of heterogeneous constitutive neurogenic niches. The presence of neuroepithelial and radial glia GS+ cells indicates a certain proportion of embryonic and adult progenitors and, obviously, different contributions of these cells to constitutive and reparative neurogenesis in the acute post-traumatic period. Expression of nestin and vimentin was revealed in neuroepithelial cerebellar progenitors of juvenile O. masou. Patterns of granular expression of these markers were found in neurogenic niches and adjacent areas, which probably indicates the neurotrophic and proneurogenic effects of vimentin and nestin in constitutive and post-traumatic neurogenesis and a high level of constructive metabolism. No expression of vimentin and nestin was detected in the cerebellar RG of juvenile O. masou. Thus, the molecular markers of NSCs/NPCs in the cerebellum of juvenile O. masou are as follows: vimentin, nestin, and glutamine synthetase label NE cells in intact animals and in the post-traumatic period, while GS expression is present in the RG of intact animals and decreases in the acute post-traumatic period. A study of distribution of cystathionine β-synthase (CBS) in the cerebellum of intact young O. masou showed the expression of the marker mainly in type 1 cells, corresponding to NSCs/NCPs for other molecular markers. In the post-traumatic period, the number of CBS+ cells sharply increased, which indicates the involvement of H₂S in the post-traumatic response. Induction of CBS in type 3 cells indicates the involvement of H₂S in the metabolism of extracellular glutamate in the cerebellum, a decrease in the production of reactive oxygen species, and also arrest of the oxidative stress development, a weakening of the toxic effects of glutamate, and a reduction in excitotoxicity. The obtained results allow us to consider H₂S as a biologically active substance, the numerous known effects of which can be supplemented by participation in the processes of constitutive neurogenesis and neuronal regeneration.

Adult neuronal regeneration in the telencephalon of the killifish Aphaniops hormuzensis

The potential of central nervous system regeneration was evaluated for the first time in the injured brain of the old world killifish Aphaniops hormuzensis. The histomorphological organization in the regeneration procedure was evaluated using the hematoxylin and eosin (H&E) staining and the bromodeoxyuridine (BrdU) immunohistochemistry technique. The histological tissue sections were sampled daily for 10 days. Based on the H&E staining, a large gliosis reaction was detected along with vacuolization and telencephalon deformation on 1-day post-lesion (dpl). The vacuolated zone declined fast and the telencephalon hemisphere recovered on 3 dpl. The symptoms of injured telencephalon nervous tissue were resolved within 7 dpl in both genders. In the BrdU test of the control group, BrdU-labeled cells were observed in the ventricular zone (VZ), pallium (Pa), and lateral pallium (LPa). On 1 dpl, the BrdU⁺ cells accumulated in the VZ, Pa, and LPa (located near the injury area). From 3 dpl onwards, the BrdU⁺ cells were reduced in the telencephalic VZ, Pa, and LPa. Based on the BrdU⁺ results, the adult brain in A. hormuzensis possesses a remarkable capacity for neuronal regeneration. By taking into account the high neural regeneration potency of A. hormuzensis and its relatively short lifespan, it could be concluded that besides the currently known models, the members of aphaniid fishes could probably be valuable animals to study the regeneration phenomenon in the vertebrates.

Common and Distinct Features of Adult Neurogenesis and Regeneration in the Telencephalon of Zebrafish and Mammals

In contrast to mammals, the adult zebrafish brain shows neurogenic activity in a multitude of niches present in almost all brain subdivisions. Irrespectively, constitutive neurogenesis in the adult zebrafish and mouse telencephalon share many similarities at the cellular and molecular level. However, upon injury during tissue repair, the situation is entirely different. In zebrafish, inflammation caused by traumatic brain injury or by induced neurodegeneration initiates specific and distinct neurogenic programs that, in combination with signaling pathways implicated in constitutive neurogenesis, quickly, and efficiently overcome the loss of neurons. In the mouse brain, injury-induced inflammation promotes gliosis leading to glial scar formation and inhibition of regeneration. A better understanding of the regenerative mechanisms occurring in the zebrafish brain could help to develop new therapies to combat the debilitating consequences of brain injury, stroke, and neurodegeneration. The aim of this review is to compare the properties of neural progenitors and the signaling pathways, which control adult neurogenesis and regeneration in the zebrafish and mammalian telencephalon.

Vertebrate brain regeneration - a community effort of fate-restricted precursor cell types

The process of regeneration describes the full restoration of tissue after destruction from injury or disease. Most mammals show very limited ability for regeneration of adult organs, while vertebrate models of regeneration such as fish and salamanders, allow to study regeneration mechanism of the brain, heart, limbs, retina, and other organs in adults. The regenerative abilities of teleost fish are well documented, but the cellular sources for regeneration, the specificity of source cells for restored cell types, as well as the extent and fidelity of cell replacement are only beginning to be revealed for many regeneration paradigms. Here, we highlight recent analyses of adult neurogenesis and regeneration after injury in teleost fish that address these issues, and we discuss how such analyses can help to evaluate the role of different cells in tissues in the regeneration process.

Phylogenetic variation in cortical layer II immature neuron reservoir of mammals

The adult mammalian brain is mainly composed of mature neurons. A limited amount of stem cell-driven neurogenesis persists in postnatal life and is reduced in large-brained species. Another source of immature neurons in adult brains is cortical layer II. These cortical immature neurons (cINs) retain developmentally undifferentiated states in adulthood, though they are generated before birth. Here, the occurrence, distribution and cellular features of cINs were systematically studied in 12 diverse mammalian species spanning from small-lissencephalic to large-gyrencephalic brains. In spite of well-preserved morphological and molecular features, the distribution of cINs was highly heterogeneous, particularly in neocortex. While virtually absent in rodents, they are present in the entire neocortex of many other species and their linear density in cortical layer II generally increased with brain size. These findings suggest an evolutionary developmental mechanism for plasticity that varies among mammalian species, granting a reservoir of young cells for the cerebral cortex.

To acquire new skills or recover after injuries, the mammalian brain relies on plasticity, the ability for the brain to change its architecture and its connections during the lifetime of an animal.

Creating new nerve cells is one way to achieve plasticity, but this process is rarer in humans than it is in mammals with smaller brains. In particular, it is absent in the human cortex: this region is enlarged in species with large brains, where it carries out complex tasks such as learning and memory. Producing new cells in the cortex would threaten the stability of the structures that retain long-term memories.

Another route to plasticity is to reshape the connections between existing, mature nerve cells. This process takes place in the human brain during childhood and adolescence, as some connections are strengthened and others pruned away.

An alternative mechanism relies on keeping some nerve cells in an immature, ‘adolescent’ state. When needed, these nerve cells emerge from their state of arrested development and ‘grow up’, connecting with the appropriate brain circuits. This mechanism does not involve producing new nerve cells, and so it would be suitable to maintain plasticity in the cortex. Consistent with this idea, in mice some dormant nerve cells are present in a small, primitive part of the cortex.

La Rosa et al. therefore wanted to determine if the location and number of immature cells in the cortex differed between mammals, and if so, whether these differences depended on brain size. The study spanned 12 mammal species, from small-brained species like mice to larger-brained animals including sheep and non-human primates.

Microscopy imaging was used to identify immature nerve cells in brain samples, which revealed that the cortex in larger-brained species contained more adolescent cells than its mouse counterpart. The difference was greatest in a region called the neocortex, which has evolved most recently. This area is most pronounced in primates – especially humans – where it carries out high-level cognitive tasks.

These results identify immature nerve cells as a potential mechanism for plasticity in the cortex. La Rosa et al. hope that the work will inspire searches for similar reservoirs of young cells in humans, which could perhaps lead to new treatments for brain disorders like dementia.

High-resolution mapping of injury-site dependent functional recovery in a single axon in zebrafish

In non-mammalian vertebrates, some neurons can regenerate after spinal cord injury. One of these, the giant Mauthner (M-) neuron shows a uniquely direct link to a robust survival-critical escape behavior but appears to regenerate poorly. Here we use two-photon microscopy in parallel with behavioral assays in zebrafish to show that the M-axon can regenerate very rapidly and that the recovery of functionality lags by just days. However, we also find that the site of the injury is critical: While regeneration is poor both close and far from the soma, rapid regeneration and recovery of function occurs for injuries between 10% and 50% of total axon length. Our findings show that rapid regeneration and the recovery of function can be studied at remarkable temporal resolution after targeted injury of one single M-axon and that the decision between poor and rapid regeneration can be studied in this one axon.

Alexander Hecker et al. study the regeneration potential of the axon of the giant Mauthner (M) neuron in zebrafish. Using two-photon microscopy and behavioral assays, they show that the M-axon can recover rapidly days after injury. They also characterize the optimal injury site that enables rapid regeneration and functional recovery.

Toxicity of mercury: Molecular evidence

Minamata disease in Japan and the large-scale poisoning by methylmercury (MeHg) in Iraq caused wide public concerns about the risk emanating from mercury for human health. Nowadays, it is widely known that all forms of mercury induce toxic effects in mammals, and increasing evidence supports the concern that environmentally relevant levels of MeHg could impact normal biological functions in wildlife. The information of mechanism involved in mercurial toxicity is growing but knowledge gaps still exist between the adverse effects and mechanisms of action, especially at the molecular level. A body of data obtained from experimental studies on mechanisms of mercurial toxicity in vivo and in vitro points to that disruption of the antioxidant system may play an important role in the mercurial toxic effects. Moreover, the accumulating evidence indicates that signaling transduction, protein or/and enzyme activity, and gene regulation are involving in mediating toxic and adaptive response to mercury exposure. We conducted here a comprehensive review of mercurial toxic effects on wildlife and human, in particular synthesized key findings of molecular pathways involved in mercurial toxicity from the cells to human. We discuss the molecular evidence related mercurial toxicity to the adverse effects, with particular emphasis on the gene regulation. The further studies relying on Omic analysis connected to adverse effects and modes of action of mercury will aid in the evaluation and validation of causative relationship between health outcomes and gene expression.

Characterization of neurogenic niches in the telencephalon of juvenile and adult sharks

Neurogenesis is a multistep process by which progenitor cells become terminally differentiated neurons. Adult neurogenesis has gathered increasing interest with the aim of developing new cell-based treatments for neurodegenerative diseases in humans. Active sites of adult neurogenesis exist from fish to mammals, although in the adult mammalian brain the number and extension of neurogenic areas is considerably reduced in comparison to non-mammalian vertebrates and they become mostly reduced to the telencephalon. Much of our understanding in this field is based in studies on mammals and zebrafish, a modern bony fish. The use of the cartilaginous fish Scyliorhinus canicula (representative of basal gnathostomes) as a model expands the comparative framework to a species that shows highly neurogenic activity in the adult brain. In this work, we studied the proliferation pattern in the telencephalon of juvenile and adult specimens of S. canicula using antibodies against the proliferation marker proliferating cell nuclear antigen (PCNA). We have characterized proliferating niches using stem cell markers (Sex determining region Y-box 2), glial markers (glial fibrillary acidic protein, brain lipid binding protein and glutamine synthase), intermediate progenitor cell markers (Dlx2 and Tbr2) and markers for migrating neuroblasts (Doublecortin). Based in the expression pattern of these markers, we demonstrate the existence of different cell subtypes within the PCNA immunoreactive zones including non-glial stem cells, glial progenitors, intermediate progenitor-like cells and migratory neuroblasts, which were widely distributed in the ventricular zone of the pallium, suggesting that the main progenitor types that constitute the neurogenic niche in mammals are already present in cartilaginous fishes.

Brain Structural Plasticity: From Adult Neurogenesis to Immature Neurons

Brain structural plasticity is an extraordinary tool that allows the mature brain to adapt to environmental changes, to learn, to repair itself after lesions or disease, and to slow aging. A long history of neuroscience research led to fascinating discoveries of different types of plasticity, involving changes in the genetically determined structure of nervous tissue, up to the ultimate dream of neuronal replacement: a stem cell-driven “adult neurogenesis” (AN). Yet, this road does not seem a straight one, since mutable dogmas, conflicting results and conflicting interpretations continue to warm the field. As a result, after more than 10,000 papers published on AN, we still do not know its time course, rate or features with respect to other kinds of structural plasticity in our brain. The solution does not appear to be behind the next curve, as differences among mammals reveal a very complex landscape that cannot be easily understood from rodents models alone. By considering evolutionary aspects, some pitfalls in the interpretation of cell markers, and a novel population of undifferentiated cells that are not newly generated [immature neurons (INs)], we address some conflicting results and controversies in order to find the right road forward. We suggest that considering plasticity in a comparative framework might help assemble the evolutionary, anatomical and functional pieces of a very complex biological process with extraordinary translational potential.

Single cell sequencing of radial glia progeny reveals the diversity of newborn neurons in the adult zebrafish brain

Zebrafish display widespread and pronounced adult neurogenesis, which is fundamental for their regeneration capability after central nervous system injury. However, the cellular identity and the biological properties of adult newborn neurons are elusive for most brain areas. Here, we have used short-term lineage tracing of radial glia progeny to prospectively isolate newborn neurons from the her4.1⁺ radial glia lineage in the homeostatic adult forebrain. Transcriptome analysis of radial glia, newborn neurons and mature neurons using single cell sequencing identified distinct transcriptional profiles, including novel markers for each population. Specifically, we detected two separate newborn neuron types, which showed diversity of cell fate commitment and location. Further analyses showed that these cell types are homologous to neurogenic cells in the mammalian brain, identified neurogenic commitment in proliferating radial glia and indicated that glutamatergic projection neurons are generated in the adult zebrafish telencephalon. Thus, we prospectively isolated adult newborn neurons from the adult zebrafish forebrain, identified markers for newborn and mature neurons in the adult brain, and revealed intrinsic heterogeneity among adult newborn neurons and their homology with mammalian adult neurogenic cell types.

Predation drives the evolution of brain cell proliferation and brain allometry in male Trinidadian killifish, Rivulus hartii

The external environment influences brain cell proliferation, and this might contribute to brain plasticity underlying adaptive behavioural changes. Additionally, internal genetic factors influence the brain cell proliferation rate. However, to date, researchers have not examined the importance of environmental versus genetic factors in causing natural variation in brain cell proliferation. Here, we examine brain cell proliferation and brain growth trajectories in free-living populations of Trinidadian killifish, Rivulus hartii, exposed to contrasting predation environments. Compared to populations without predators, populations in high predation (HP) environments exhibited higher rates of brain cell proliferation and a steeper brain growth trajectory (relative to body size). To test whether these differences in the wild persist in a common garden environment, we reared first-generation fish originating from both predation environments in uniform laboratory conditions. Just as in the wild, brain cell proliferation and brain growth in the common garden were greater in HP populations than in no predation populations. The differences in cell proliferation observed across the brain in both the field and common garden studies indicate that the differences are probably genetically based and are mediated by evolutionary shifts in overall brain growth and life-history traits.

A conceptual framework for understanding sexual differentiation of the teleost brain

Vertebrate brains are sexually differentiated, giving rise to differences in various physiological and behavioral phenotypes between the sexes. In developing mammals and birds, the neural substrate underlying sex-dependent physiology and behavior undergoes an irreversible process of sexual differentiation due to the effects of perinatal gonadal steroids and sex chromosome complement. The differentiated neural substrate is then activated in the adult by the sex-specific steroid milieu to facilitate the expression of sex-typical phenotypes. However, this well-established concept does not hold for teleost fish, whose sexual phenotypes (behavioral or otherwise) are highly labile throughout life and can be reversed even in adulthood. Indeed, the available evidence suggests that, in teleosts, neither gonadal steroids early in development nor the sex chromosome complement contribute much to brain sexual differentiation; instead, steroids in adulthood serve to both differentiate the neural substrate and activate it to elicit sex-typical phenotypes in a transient and reversible manner. Evidence further suggests that marked sexual dimorphisms and adult steroid-dependent lability in the neural expression of sex steroid receptors constitute the primary molecular basis for sexual differentiation and lability of the teleost brain. The consequent sexually dimorphic but reversible steroid sensitivity in response to the adult steroid milieu may enable the teleost brain to maintain lifelong sexual lability and to undergo phenotypic sex reversal.

Neuropeptide B mediates female sexual receptivity in medaka fish, acting in a female-specific but reversible manner

Male and female animals display innate sex-specific mating behaviors. In teleost fish, altering the adult sex steroid milieu can effectively reverse sex-typical mating behaviors, suggesting remarkable sexual lability of their brains as adults. In the teleost medaka, neuropeptide B (NPB) is expressed female-specifically in the brain nuclei implicated in mating behavior. Here, we demonstrate that NPB is a direct mediator of estrogen action on female mating behavior, acting in a female-specific but reversible manner. Analysis of regulatory mechanisms revealed that the female-specific expression of NPB is dependent on direct transcriptional activation by estrogen via an estrogen-responsive element and is reversed in response to changes in the adult sex steroid milieu. Behavioral studies of NPB knockouts revealed that female-specific NBP mediates female receptivity to male courtship. The female-specific NPB signaling identified herein is presumably a critical element of the neural circuitry underlying sexual dimorphism and lability of mating behaviors in teleosts.

Newly Generated and Non-Newly Generated "Immature" Neurons in the Mammalian Brain: A Possible Reservoir of Young Cells to Prevent Brain Aging and Disease?

Brain plasticity is important for translational purposes since most neurological disorders and brain aging problems remain substantially incurable. In the mammalian nervous system, neurons are mostly not renewed throughout life and cannot be replaced. In humans, the increasing life expectancy explains the increase in brain health problems, also producing heavy social and economic burden. An exception to the “static” brain is represented by stem cell niches leading to the production of new neurons. Such adult neurogenesis is dramatically reduced from fish to mammals, and in large-brained mammals with respect to rodents. Some examples of neurogenesis occurring outside the neurogenic niches have been reported, yet these new neurons actually do not integrate in the mature nervous tissue. Non-newly generated, “immature” neurons (nng-INs) are also present: Prenatally generated cells continuing to express molecules of immaturity (mostly shared with the newly born neurons). Of interest, nng-INs seem to show an inverse phylogenetic trend across mammals, being abundant in higher-order brain regions not served by neurogenesis and providing structural plasticity in rather stable areas. Both newly generated and nng-INs represent a potential reservoir of young cells (a “brain reserve”) that might be exploited for preventing the damage of aging and/or delay the onset/reduce the impact of neurological disorders.

Distribution and density of mixed-input ON bipolar cells of the goldfish (Carassius auratus) during growth

Neurons are continuously produced at different rates and locations in the teleost retina. Goldfish rods are homogeneously distributed and maintain a stable density throughout growth, whereas little is known about their postsynaptic partners. We examined the distribution and density of mixed-input ON bipolar cells (ON mBCs) in 57 goldfish of various sizes by immunolabeling their retinas with an antibody against PKCα and counting PKCα-positive neurons in wholemounts. Cell densities were correlated with morphometric data for the same animals, and the spatial resolution of the ON mBC mosaic was calculated in each case. The distribution of ON mBCs is homogeneous throughout growth. For a 10-fold change in body size (i.e., from 20 to 200 mm), the total number of ON mBCs increases 2.8 times, while retinal area expands around 10 times. As a consequence, the density of ON mBCs in large fish falls to ∼1/3 of that of small animals, and intercellular spacing doubles. The eye and the lens become around three times larger from small to large fish. This causes the retinal magnification factor (and thereby the image projected onto retina) to augment by the same amount. Because the retinal magnification factor rises more than the intercellular spacing in the same animals, the spatial resolution of the ON mBC mosaic improves from 0.8 to 1.4 cycles/degree as the body size increases from 20 to 200 mm. As ON mBCs are mostly rod-driven, our results suggest that the scotopic acuity of the goldfish may improve as the animal grows.

Sexually Dimorphic Neuropeptide B Neurons in Medaka Exhibit Activated Cellular Phenotypes Dependent on Estrogen

Brain and behavior of teleosts are highly sexually plastic throughout life, yet the underlying neural mechanisms are largely unknown. On examining brain morphology in the teleost medaka (Oryzias latipes), we identified distinctively large neurons in the magnocellular preoptic nucleus that occurred much more abundantly in females than in males. Examination of sex-reversed medaka showed that the sexually dimorphic abundance of these neurons is dependent on gonadal phenotype, but independent of sex chromosome complement. Most of these neurons in females, but none in males, produced neuropeptide B (Npb), whose expression is known to be estrogen-dependent and associated with female sexual receptivity. In phenotypic analysis, the female-specific Npb neurons had a large euchromatic nucleus with an abundant cytoplasm containing plentiful rough endoplasmic reticulum, exhibited increased overall transcriptional activity, and typically displayed a spontaneous regular firing pattern. These phenotypes, which are probably indicative of cellular activation, were attenuated by ovariectomy and restored by estrogen replacement. Furthermore, the population of Npb-expressing neurons emerged in adult males treated with estrogen, not through frequently occurring neurogenesis in the adult teleost brain, but through the activation of preexisting, quiescent male counterpart neurons. Collectively, our results demonstrate that the morphological, transcriptional, and electrophysiological phenotypes of sexually dimorphic preoptic Npb neurons are highly dependent on estrogen and can be switched between female and male patterns. These properties of the preoptic Npb neurons presumably underpin the neural mechanism for sexual differentiation and plasticity of brain and behavior in teleosts.

Neurogenesis in the adult hippocampus: history, regulation, and prospective roles

BACKGROUND

The hippocampus is one of the sites in the mammalian brain that is capable of continuously generating controversy. Adult neurogenesis is a remarkable process, and yet an intensely debatable topic in contemporary neuroscience due to its distinctiveness and conceivable impact on neural activity. The belief that neurogenesis continues through adulthood has provoked remarkable efforts to describe how newborn neurons differentiate and incorporate into the adult brain. It has also encouraged studies that investigate the consequences of inadequate neurogenesis in neuropsychiatric and neurodegenerative diseases and explore the potential role of neural progenitor cells in brain repair. The adult nervous system is not static; it is subjected to morphological and physiological alterations at various levels. This plastic mechanism guarantees that the behavioral regulation of the adult nervous system is adaptable in response to varying environmental stimuli. Three regions of the adult brain, the olfactory bulb, the hypothalamus, and the hippocampal dentate gyrus, contain new-born neurons that exhibit an essential role in the natural functional circuitry of the adult brain. Purpose/Aim: This article explores current advancements in adult hippocampal neurogenesis by presenting its history and evolution and studying its association with neural plasticity. The article also discusses the prospective roles of adult hippocampal neurogenesis and describes the intracellular, extracellular, pathological, and environmental factors involved in its regulation. Abbreviations AHN Adult hippocampal neurogenesis AKT Protein kinase B BMP Bone Morphogenic Protein BrdU Bromodeoxyuridine CNS Central nervous system DG Dentate gyrus DISC1 Disrupted-in-schizophrenia 1 FGF-2 Fibroblast Growth Factor 2 GABA Gamma-aminobutyric acid Mbd1 Methyl-CpG-binding domain protein 1 Mecp2 Methyl-CpG-binding protein 2 mTOR Mammalian target of rapamycin NSCs Neural stem cells OB Olfactory bulb; P21: cyclin-dependent kinase inhibitor 1 RBPj Recombination Signal Binding protein for Immunoglobulin Kappa J Region RMS Rostral migratory Stream SGZ Subgranular zone Shh Sonic hedgehog SOX2 SRY (sex determining region Y)-box 2 SVZ Subventricular zone Wnt3 Wingless-type mouse mammary tumor virus.

Epigenetic Regulation of Organ Regeneration in Zebrafish

The zebrafish is broadly used for investigating de novo organ regeneration, because of its strong regenerative potential. Over the past two decades of intense study, significant advances have been made in identifying both the regenerative cell sources and molecular signaling pathways in a variety of organs in adult zebrafish. Epigenetic regulation has gradually moved into the center-stage of this research area, aided by comprehensive work demonstrating that DNA methylation, histone modifications, chromatin remodeling complexes, and microRNAs are essential for organ regeneration. Here, we present a brief review of how these epigenetic components are induced upon injury, and how they are involved in sophisticated organ regeneration. In addition, we highlight several prospective research directions and their potential implications for regenerative medicine.

Brain Plasticity in Mammals: An Example for the Role of Comparative Medicine in the Neurosciences

Comparative medicine deals with similarities and differences between veterinary and human medicine. All mammals share most basic cellular and molecular mechanisms, thus justifying murine animal models in a translational perspective; yet “mice are not men,” thus some biases can emerge when complex biological processes are concerned. Brain plasticity is a cutting-edge, expanding topic in the field of Neurosciences with important translational implications, yet, with remarkable differences among mammals, as emerging from comparative studies. In particular, adult neurogenesis (the genesis of new neurons from brain stem cell niches) is a life-long process in laboratory rodents but a vestigial, mostly postnatal remnant in humans and dolphins. Another form of “whole cell” plasticity consisting of a population of “immature” neurons which are generated prenatally but continue to express markers of immaturity during adulthood has gained interest more recently, as a reservoir of young neurons in the adult brain. The distribution of the immature neurons also seems quite heterogeneous among different animal species, being confined within the paleocortex in rodents while extending into neocortex in other mammals. A recent study carried out in sheep, definitely showed that gyrencephalic, large-sized brains do host higher amounts of immature neurons, also involving subcortical, white, and gray matter regions. Hence, “whole cell” plasticity such as adult neurogenesis and immature neurons are biological processes which, as a whole, cannot be studied exclusively in laboratory rodents, but require investigation in comparative medicine, involving large-sized, long-living mammals, in order to gain insights for translational purposes.

Enhanced brain expression of genes related to cell proliferation and neural differentiation is associated with cortisol receptor expression in fishes

Stress enhances or inhibits neurogenesis in mammals and some fish species. The link between the two processes is still unclear. Most studies have been performed in very specific stressful or altered environments. Despite the known inter-individual divergence in coping abilities within populations, the relationship between the stress axis and neurogenesis has never been addressed in unstressed individuals. Here we correlate brain expression of the pcna (proliferating cell nuclear antigen) and neurod1 (neurogenic differentiation factor 1) genes, two markers of neurogenesis, with transcripts of cortisol receptors in three fish species living in very distinct environments. Within the three species, individuals with the highest expression of neurogenesis genes were also those that expressed the high levels of cortisol receptors. Based on these correlations and the hypothesis that mRNA levels are proxies of protein levels, we hypothesize that within unstressed animals, individuals sensitive to cortisol perceive a similar environment to be more stimulating, leading to increased neurogenesis. Although it is difficult to determine whether it is sensitivity to cortisol that affects neurogenesis capacities or the opposite, the proposed pathway is a potentially fruitful avenue that warrants further mechanistic experiments.

Magnitude Assessment of Adult Neurogenesis in the Octopus vulgaris Brain Using a Flow Cytometry-Based Technique

Adult neurogenesis is widespread among metazoans, it occurs in animals with a network nervous system, as cnidarians, and in animals with a complex and centralized brain, such as mammals, non-mammalian vertebrates, ecdysozoans, and a lophotrochozoan, Octopus vulgaris. Nevertheless, there are important differences among taxa, especially in the number of the regions involved and in cell proliferation rate during the life-cycle. The comparative evaluation of adult neurogenesis among different brain regions is an arduous task to achieve with only stereological techniques. However, in Octopus vulgaris we recently confirmed the presence of active proliferation in the learning-memory centers, multisensory integration centers, and the motor centers of the adult brain. Here, using a flow cytometry technique, we provide a method to quantify the active proliferation in octopus nervous system using a BrdU in vitro administration without exposing the animals to stress or painful injections usually used. This method is in line with the current animal welfare regulations regarding cephalopods, and the flow cytometry-based technique enabled us to measure adult neurogenesis more quickly and reliably than histological techniques, with the additional advantage of processing multiple samples in parallel. Flow cytometry is thus an appropriate technique for measuring and comparing adult neurogenesis in animals that are in a different physiological and/or environmental contexts. A BrdU immunoreactivity distribution, to define the neurogenic areas, and the effective penetration in vitro of the BrdU is also provided.

Cypermethrin, chlorpyrifos, deltamethrin, and imidacloprid exposure up-regulates the mRNA and protein levels of bdnf and c-fos in the brain of adult zebrafish (Danio rerio)

The aim of the present study is to investigate the toxicity effects of frequently used pesticides, involving cypermethrin, deltamethrin, chlorpyrifos and imidacloprid, on the expression of bdnf and c-fos genes in zebrafish brain tissues. Therefore, brain tissues exposed to intoxication was primarily analyzed by indirect immunofluorescence assay. Afterwards, the mRNA transcription levels of BNDF and c-fos genes and the protein levels were measured by qRT-PCR and Western blotting, respectively. The data of the immunofluorescence assay revealed intensive immunopositivity for bdnf and c-fos genes in the tissues exposed to pesticide intoxication in comparison to the control group (p<0.05). Moreover, the transcription levels of BNDF and c-fos genes, and protein levels were elevated following the intoxication (p<0.05, p<0.01, and p<0.001, respectively). These results showed that the exposure to the acute cypermethrin, deltamethrin, chlorpyrifos and imidacloprid intoxication disrupted the normal neuronal activity, resulting in neurotoxic effect, also DNA-binding Increasing c-fos activation, an oncoprotein from the family of the Nuclear Proteins, is also true of the knowledge that these chemicals are oncogenic in zebrafish brain tissues. Thus, the use of these pesticides poses a potential neuronal and oncogenic risk to the non-target organisms.