Dynamic Changes in the Neurogenic Potential in the Ventricular-Subventricular Zone of Common Marmoset during Postnatal Brain Development

Therapeutic modulation of neurogenesis to improve hippocampal plasticity and cognition in aging and Alzheimer's disease

Alzheimer's disease is characterized by progressive memory loss and cognitive decline. The hippocampal formation is the most vulnerable brain area in Alzheimer's disease. Neurons in layer II of the entorhinal cortex and the CA1 region of the hippocampus are lost at early stages of the disease. A unique feature of the hippocampus is the formation of new neurons that incorporate in the dentate gyrus of the hippocampus. New neurons form synapses with neurons in layer II of the entorhinal cortex and with the CA3 region. Immature and new neurons are characterized by high level of plasticity. They play important roles in learning and memory. Hippocampal neurogenesis is impaired early in mouse models of Alzheimer's disease and in human patients. In fact, neurogenesis is compromised in mild cognitive impairment (MCI), suggesting that rescuing neurogenesis may restore hippocampal plasticity and attenuate neuronal vulnerability and memory loss. This review will discuss the current understanding of therapies that target neurogenesis or modulate it, for the treatment of Alzheimer's disease.

Which neurodevelopmental processes continue in humans after birth?

Once we are born, the number and location of nerve cells in most parts of the brain remain unchanged. These types of structural changes are therefore a significant form of flexibility for the neural circuits where they occur. In humans, the postnatal birth of neurons is limited; however, neurons do continue to migrate into some brain regions throughout infancy and even into adolescence. In human infants, multiple migratory pathways deliver interneurons to destinations across the frontal and temporal lobe cortex. Shorter-range migration of excitatory neurons also appears to continue during adolescence, particularly near the amygdala paralaminar nucleus, a region that follows a delayed trajectory of growth from infancy to adulthood. The significance of the timing for when different brain regions recruit new neurons through these methods is unknown; however, both processes of protracted migration and maturation are prominent in humans. Mechanisms like these that reconfigure neuronal circuits are a substrate for critical periods of plasticity and could contribute to distinctive circuit functionality in human brains.

Neuraminidase inhibition promotes the collective migration of neurons and recovery of brain function

In the injured brain, new neurons produced from endogenous neural stem cells form chains and migrate to injured areas and contribute to the regeneration of lost neurons. However, this endogenous regenerative capacity of the brain has not yet been leveraged for the treatment of brain injury. Here, we show that in healthy brain chains of migrating new neurons maintain unexpectedly large non-adherent areas between neighboring cells, allowing for efficient migration. In instances of brain injury, neuraminidase reduces polysialic acid levels, which negatively regulates adhesion, leading to increased cell-cell adhesion and reduced migration efficiency. The administration of zanamivir, a neuraminidase inhibitor used for influenza treatment, promotes neuronal migration toward damaged regions, fosters neuronal regeneration, and facilitates functional recovery. Together, these findings shed light on a new mechanism governing efficient neuronal migration in the adult brain under physiological conditions, pinpoint the disruption of this mechanism during brain injury, and propose a promising therapeutic avenue for brain injury through drug repositioning.

Amphiphilic peptide-tagged N-cadherin forms radial glial-like fibers that enhance neuronal migration in injured brain and promote sensorimotor recovery

The mammalian brain has very limited ability to regenerate lost neurons and recover function after injury. Promoting the migration of young neurons (neuroblasts) derived from endogenous neural stem cells using biomaterials is a new and promising approach to aid recovery of the brain after injury. However, the delivery of sufficient neuroblasts to distant injured sites is a major challenge because of the limited number of scaffold cells that are available to guide neuroblast migration. To address this issue, we have developed an amphiphilic peptide [(RADA)₃-(RADG)] (mRADA)-tagged N-cadherin extracellular domain (Ncad-mRADA), which can remain in mRADA hydrogels and be injected into deep brain tissue to facilitate neuroblast migration. Migrating neuroblasts directly contacted the fiber-like Ncad-mRADA hydrogel and efficiently migrated toward an injured site in the striatum, a deep brain area. Furthermore, application of Ncad-mRADA to neonatal cortical brain injury efficiently promoted neuronal regeneration and functional recovery. These results demonstrate that self-assembling Ncad-mRADA peptides mimic both the function and structure of endogenous scaffold cells and provide a novel strategy for regenerative therapy.

Increased oligodendrogenesis and myelination in the subventricular zone of aged mice and gray mouse lemurs

The adult rodent subventricular zone (SVZ) generates neural stem cells (NSCs) throughout life that migrate to the olfactory bulbs (OBs) and differentiate into olfactory interneurons. Few SVZ NSCs generate oligodendrocyte precursor cells (OPCs). We investigated how neurogliogenesis is regulated during aging in mice and in a non-human primate (NHP) model, the gray mouse lemur. In both species, neuronal commitment decreased with age, while OPC generation and myelin content unexpectedly increased. In the OBs, more tyrosine hydroxylase interneurons in old mice, but fewer in lemurs, marked a surprising interspecies difference that could relate to our observation of a continuous ventricle in lemurs. In the corpus callosum, aging promoted maturation of OPCs into mature oligodendrocytes in mice but blocked it in lemurs. The present study highlights similarities and dissimilarities between rodents and NHPs, revealing that NHPs are a more relevant model than mice to study the evolution of biomarkers of aging.

Consistency and Variation in Doublecortin and Ki67 Antigen Detection in the Brain Tissue of Different Mammals, including Humans

Recently, a population of "immature" neurons generated prenatally, retaining immaturity for long periods and finally integrating in adult circuits has been described in the cerebral cortex. Moreover, comparative studies revealed differences in occurrence/rate of different forms of neurogenic plasticity across mammals, the "immature" neurons prevailing in gyrencephalic species. To extend experimentation from laboratory mice to large-brained mammals, including humans, it is important to detect cell markers of neurogenic plasticity in brain tissues obtained from different procedures (e.g., post-mortem/intraoperative specimens vs. intracardiac perfusion). This variability overlaps with species-specific differences in antigen distribution or antibody species specificity, making it difficult for proper comparison. In this work, we detect the presence of doublecortin and Ki67 antigen, markers for neuronal immaturity and cell division, in six mammals characterized by widely different brain size. We tested seven commercial antibodies in four selected brain regions known to host immature neurons (paleocortex, neocortex) and newly born neurons (hippocampus, subventricular zone). In selected human brains, we confirmed the specificity of DCX antibody by performing co-staining with fluorescent probe for DCX mRNA. Our results indicate that, in spite of various types of fixations, most differences were due to the use of different antibodies and the existence of real interspecies variation.

Postnatal and Adult Neurogenesis in Mammals, Including Marsupials

In mammals, neurogenesis occurs during both embryonic and postnatal development. In eutherians, most brain structures develop embryonically; conversely, in marsupials, a number of brain structures develop after birth. The exception is the generation of granule cells in the dentate gyrus, olfactory bulb, and cerebellum of eutherian species. The formation of these structures starts during embryogenesis and continues postnatally. In both eutherians and marsupials, neurogenesis continues in the subventricular zone of the lateral ventricle (SVZ) and the dentate gyrus of the hippocampal formation throughout life. The majority of proliferated cells from the SVZ migrate to the olfactory bulb, whereas, in the dentate gyrus, cells reside within this structure after division and differentiation into neurons. A key aim of this review is to evaluate advances in understanding developmental neurogenesis that occurs postnatally in both marsupials and eutherians, with a particular emphasis on the generation of granule cells during the formation of the olfactory bulb, dentate gyrus, and cerebellum. We debate the significance of immature neurons in the piriform cortex of young mammals. We also synthesize the knowledge of adult neurogenesis in the olfactory bulb and the dentate gyrus of marsupials by considering whether adult-born neurons are essential for the functioning of a given area.

Self-tuition as an essential design feature of the brain

We are curious by nature, particularly when young. Evolution has endowed our brain with an inbuilt obligation to educate itself. In this perspectives article, we posit that self-tuition is an evolved principle of vertebrate brain design that is reflected in its basic architecture and critical for its normal development. Self-tuition involves coordination between functionally distinct components of the brain, with one set of areas motivating exploration that leads to the experiences that train another set. We review key hypothalamic and telencephalic structures involved in this interplay, including their anatomical connections and placement within the segmental architecture of conserved forebrain circuits. We discuss the nature of educative behaviours motivated by the hypothalamus, innate stimulus biases, the relationship to survival in early life, and mechanisms by which telencephalic areas gradually accumulate knowledge. We argue that this aspect of brain function is of paramount importance for systems neuroscience, as it confers neural specialization and allows animals to attain far more sophisticated behaviours than would be possible through genetic mechanisms alone. Self-tuition is of particular importance in humans and other primates, whose large brains and complex social cognition rely critically on experience-based learning during a protracted childhood period.

This article is part of the theme issue ‘Systems neuroscience through the lens of evolutionary theory’.

Neural Stem Cells in Adult Mammals are not Astrocytes

At the turn of the 21st century studies of the cells that resided in the adult mammalian subventricular zone (SVZ) characterized the neural stem cells (NSCs) as a subtype of astrocyte. Over the ensuing years, numerous studies have further characterized the properties of these NSCs and compared them to parenchymal astrocytes. Here we have evaluated the evidence collected to date to establish whether classifying the NSCs as astrocytes is appropriate and useful. We also performed a meta-analysis with 4 previously published datasets that used cell sorting and unbiased single-cell RNAseq to highlight the distinct gene expression profiles of adult murine NSCs and niche astrocytes. On the basis of our understanding of the properties and functions of astrocytes versus the properties and functions of NSCs, and from our comparative transcriptomic analyses we conclude that classifying the adult mammalian NSC as an astrocyte is potentially misleading. From our vantage point, it is more appropriate to refer to the cells in the adult mammalian SVZ that retain the capacity to produce new neurons and macroglia as NSCs without attaching the term "astrocyte-like."

Phosphorylation of GAP-43 T172 is a molecular marker of growing axons in a wide range of mammals including primates

GAP-43 is a vertebrate neuron-specific protein and that is strongly related to axon growth and regeneration; thus, this protein has been utilized as a classical molecular marker of these events and growth cones. Although GAP-43 was biochemically characterized more than a quarter century ago, how this protein is related to these events is still not clear. Recently, we identified many phosphorylation sites in the growth cone membrane proteins of rodent brains. Two phosphorylation sites of GAP-43, S96 and T172, were found within the top 10 hit sites among all proteins. S96 has already been characterized (Kawasaki et al., 2018), and here, phosphorylation of T172 was characterized. In vitro (cultured neurons) and in vivo, an antibody specific to phosphorylated T172 (pT172 antibody) specifically recognized cultured growth cones and growing axons in developing mouse neurons, respectively. Immunoblotting showed that pT172 antigens were more rapidly downregulated throughout development than those of pS96 antibody. From the primary structure, this phosphorylation site was predicted to be conserved in a wide range of animals including primates. In the developing marmoset brainstem and in differentiated neurons derived from human induced pluripotent stem cells, immunoreactivity with pT172 antibody revealed patterns similar to those in mice. pT172 antibody also labeled regenerating axons following sciatic nerve injury. Taken together, the T172 residue is widely conserved in a wide range of mammals including primates, and pT172 is a new candidate molecular marker for growing axons.

Positive Controls in Adults and Children Support That Very Few, If Any, New Neurons Are Born in the Adult Human Hippocampus

Adult hippocampal neurogenesis was originally discovered in rodents. Subsequent studies identified the adult neural stem cells and found important links between adult neurogenesis and plasticity, behavior, and disease. However, whether new neurons are produced in the human dentate gyrus (DG) during healthy aging is still debated. We and others readily observe proliferating neural progenitors in the infant hippocampus near immature cells expressing doublecortin (DCX), but the number of such cells decreases in children and few, if any, are present in adults. Recent investigations using dual antigen retrieval find many cells stained by DCX antibodies in adult human DG. This has been interpreted as evidence for high rates of adult neurogenesis, even at older ages. However, most of these DCX-labeled cells have mature morphology. Furthermore, studies in the adult human DG have not found a germinal region containing dividing progenitor cells. In this Dual Perspectives article, we show that dual antigen retrieval is not required for the detection of DCX in multiple human brain regions of infants or adults. We review prior studies and present new data showing that DCX is not uniquely expressed by newly born neurons: DCX is present in adult amygdala, entorhinal and parahippocampal cortex neurons despite being absent in the neighboring DG. Analysis of available RNA-sequencing datasets supports the view that DG neurogenesis is rare or absent in the adult human brain. To resolve the conflicting interpretations in humans, it is necessary to identify and visualize dividing neuronal precursors or develop new methods to evaluate the age of a neuron at the single-cell level.

Thyroid hormone regulation of adult neural stem cell fate: A comparative analysis between rodents and primates

Thyroid hormone (TH) signaling, a highly conserved pathway across vertebrates, is crucial for brain development and function throughout life. In the adult mammalian brain, including that of humans, multipotent neural stem cells (NSCs) proliferate and generate neuronal and glial progenitors. The role of TH has been intensively investigated in the two main neurogenic niches of the adult mouse brain, the subventricular and the subgranular zone. A key finding is that T3, the biologically active form of THs, promotes NSC commitment toward a neuronal fate. In this review, we first discuss the roles of THs in the regulation of adult rodent neurogenesis, as well as how it relates to functional behavior, notably olfaction and cognition. Most research uncovering these roles of TH in adult neurogenesis was conducted in rodents, whose genetic background, brain structure and rate of neurogenesis are considerably different from that of humans. To bridge the phylogenetic gap, we also explore the similarities and divergences of TH-dependent adult neurogenesis in non-human primate models. Lastly, we examine how photoperiodic length changes TH homeostasis, and how that might affect adult neurogenesis in seasonal species to increase fitness. Several aspects by which TH acts on adult NSCs seem to be conserved among mammals, while we only start to uncover the molecular pathways, as well as how other in- and extrinsic factors are intertwined. A multispecies approach delivering more insights in the matter will pave the way for novel NSC-based therapies to combat neurological disorders.

Postnatal neuronal migration in health and disease

Postnatal neuronal migration modulates neuronal circuit formation and function throughout life and is conserved among species. Pathological conditions activate the generation of neuroblasts in the ventricular-subventricular zone (V-SVZ) and promote their migration towards a lesion. However, the neuroblasts generally terminate their migration before reaching the lesion site unless their intrinsic capacity is modified or the environment is improved. It is important to understand which factors impede neuronal migration for functional recovery of the brain. We highlight similarities and differences in the mechanisms of neuroblast migration under physiological and pathological conditions to provide novel insights into endogenous neuronal regeneration.

Effects of interferon-alpha on hippocampal neurogenesis and behavior in common marmosets

In many mammalian species, the production of new neurons in the hippocampal dentate gyrus continues throughout life. Previous studies using rodents suggest that adult-born neurons are involved in memory and cognition tasks and mood regulation. Interferon-alpha (IFNα), a proinflammatory cytokine used for the treatment of chronic viral hepatitis and malignancies, frequently causes depressive symptoms in patients and animals, including non-human primates. We have previously demonstrated that chronic IFNα treatment decreases hippocampal neurogenesis in mice. Here, we investigated the effects of four-week human pegylated IFNα treatment on hippocampal neurogenesis and behavior in common marmosets. Continuous monitoring of voluntary activity levels using an actigraphy device suggested that adaptive ability is impaired in IFNα-treated animals. Analyses of BrdU-labeled cells expressing a marker for immature or mature neurons revealed a significant reduction in the number of new neurons in the hippocampus of IFNα-treated animals. These data indicate that chronic human IFNα treatment causes behavioral changes and a decrease in hippocampal neurogenesis in common marmosets.

Neurogenesis and neuronal migration in the postnatal ventricular-subventricular zone: Similarities and dissimilarities between rodents and primates

The ventricular-subventricular zone (V-SVZ) is located in the walls of the lateral ventricles and produces new neurons in the postnatal brain of mammals, including humans. Immature new neurons called "neuroblasts" generated by neural stem cells in the V-SVZ migrate toward their final destinations and contribute to brain development and plasticity. In this review, we describe recent progress in understanding the similarities and dissimilarities in postnatal neurogenesis and neuronal migration between rodents and primates. In rodents, most new V-SVZ-derived neurons migrate along the rostral migratory stream towards the olfactory bulb, where they differentiate into interneurons. In contrast, in humans, the extensive migration of new neurons towards the neocortex continues for several months after birth and might be involved in the development of the expanded neocortex. The mode of migration and the fate of neuroblasts seem to change depending on their environment, destination, and roles in the brain. A better understanding of these similarities and differences between rodents and primates will help translate important findings from animal models and may contribute to the development of clinical strategies for brain repair.