Molecular Mechanisms of Neurogenic Aging in the Adult Mouse Subventricular Zone

The NDR family of kinases: essential regulators of aging

Aging is defined as a progressive decline of cognitive and physiological functions over lifetime. Since the definition of the nine hallmarks of aging in 2013 by López-Otin, numerous studies have attempted to identify the main regulators and contributors in the aging process. One interesting group of proteins whose participation has been implicated in several aging hallmarks are the nuclear DBF2-related (NDR) family of serine-threonine AGC kinases. They are one of the core components of the Hippo signaling pathway and include NDR1, NDR2, LATS1 and LATS2 in mammals, along with its highly conserved metazoan orthologs; Trc in Drosophila melanogaster, SAX-1 in Caenorhabditis elegans, CBK1, DBF20 in Saccharomyces cerevisiae and orb6 in Saccharomyces pombe. These kinases have been independently linked to the regulation of widely diverse cellular processes disrupted during aging such as the cell cycle progression, transcription, intercellular communication, nutrient homeostasis, autophagy, apoptosis, and stem cell differentiation. However, a comprehensive overview of the state-of-the-art knowledge regarding the post-translational modifications of and by NDR kinases in aging has not been conducted. In this review, we summarize the current understanding of the NDR family of kinases, focusing on their relevance to various aging hallmarks, and emphasize the growing body of evidence that suggests NDR kinases are essential regulators of aging across species.

A Novel Network Pharmacology Strategy Based on the Universal Effectiveness-Common Mechanism of Medical Herbs Uncovers Therapeutic Targets in Traumatic Brain Injury

Purpose

Many herbs can promote neurological recovery following traumatic brain injury (TBI). There must lie a shared mechanism behind the common effectiveness. We aimed to explore the key therapeutic targets for TBI based on the common effectiveness of the medicinal plants.

Material and methods

The TBI-effective herbs were retrieved from the literature as imputes of network pharmacology. Then, the active ingredients in at least two herbs were screened out as common components. The hub targets of all active compounds were identified through Cytohubba. Next, AutoDock vina was used to rank the common compound-hub target interactions by molecular docking. A highly scored compound-target pair was selected for in vivo validation.

Results

We enrolled sixteen TBI-effective medicinal herbs and screened out twenty-one common compounds, such as luteolin. Ten hub targets were recognized according to the topology of the protein-protein interaction network of targets, including epidermal growth factor receptor (EGFR). Molecular docking analysis suggested that luteolin could bind strongly to the active pocket of EGFR. Administration of luteolin or the selective EGFR inhibitor AZD3759 to TBI mice promoted the recovery of body weight and neurological function, reduced astrocyte activation and EGFR expression, decreased chondroitin sulfate proteoglycans deposition, and upregulated GAP43 levels in the cortex. The effects were similar to those when treated with the selective EGFR inhibitor.

Conclusion

The common effectiveness-based, common target screening strategy suggests that inhibition of EGFR can be an effective therapy for TBI. This strategy can be applied to discover core targets and therapeutic compounds in other diseases.

Glucocorticoids rescue cell surface trafficking of R451C Neuroligin3 and enhance synapse formation

Neuroligins are synaptic cell adhesion proteins with a role in synaptic function, implicated in neurodevelopmental disorders. The autism spectrum disorder-associated substitution Arg451Cys (R451C) in NLGN3 promotes a partial misfolding of the extracellular domain of the protein leading to retention in the endoplasmic reticulum (ER) and the induction of the unfolded protein response (UPR). The reduced trafficking of R451C NLGN3 to the cell surface leads to altered synaptic function and social behavior. A screening in HEK-293 cells overexpressing NLGN3 of 2662 compounds (FDA-approved small molecule drug library), led to the identification of several glucocorticoids such as alclometasone dipropionate, desonide, prednisolone sodium phosphate, and dexamethasone (DEX), with the ability to favor the exit of full-length R451C NLGN3 from the ER. DEX improved the stability of R451C NLGN3 and trafficking to the cell surface, reduced the activation of the UPR, and increased the formation of artificial synapses between HEK-293 and hippocampal primary neurons. The effect of DEX was validated on a novel model system represented by neural stem progenitor cells and differentiated neurons derived from the R451C NLGN3 knock-in mouse, expressing the endogenous protein. This work shows a potential rescue strategy for an autism-linked mutation affecting cell surface trafficking of a synaptic protein.

Dbx2, an Aging-Related Homeobox Gene, Inhibits the Proliferation of Adult Neural Progenitors

In the adult mouse brain, the subventricular zone (SVZ) underlying the lateral ventricles harbours a population of quiescent neural stem cells, which can be activated (aNSCs) to initiate proliferation and generate a neurogenic lineage consisting of transit amplifying progenitors (TAPs), neuroblasts (NBs) and newborn neurons. This process is markedly reduced during aging. Recent studies suggest that the aged SVZ niche decreases the pool of proliferating neural/stem progenitor cells (NSPCs), and hence adult neurogenesis, by causing transcriptomic changes that promote NSC quiescence. The transcription factors that mediate these changes, however, remain unclear. We previously found that the homeobox gene Dbx2 is upregulated in NSPCs of the aged mouse SVZ and can inhibit the growth of NSPC cultures. Here, we further investigate its role as a candidate transcriptional regulator of neurogenic decline. We show that Dbx2 expression is downregulated by Epidermal Growth Factor receptor signaling, which promotes NSPC proliferation and decreases in the aged SVZ. By means of transgenic NSPC lines overexpressing Dbx2, we also show that this gene inhibits NSPC proliferation by hindering the G2/M transition. Furthermore, we exploit RNA sequencing of transgenic NSPCs to elucidate the transcriptomic networks modulated by Dbx2. Among the top hits, we report the downregulation of the molecular pathways implicated in cell cycle progression. Accordingly, we find that Dbx2 function is negatively correlated with the transcriptional signatures of proliferative NSPCs (aNSCs, TAPs and early NBs). These results point to Dbx2 as a transcription factor relaying the anti-neurogenic input of the aged niche to the NSPC transcriptome.

Adult hippocampal neurogenesis and social behavioural deficits in the R451C Neuroligin3 mouse model of autism are reverted by the antidepressant fluoxetine

Neuron generation persists throughout life in the hippocampus but is altered in animal models of neurological and neuropsychiatric diseases, suggesting that disease-associated decline in cognitive and emotional hippocampal-dependent behaviours might be functionally linked with dysregulation of postnatal neurogenesis. Depletion of the adult neural stem/progenitor cell (NSPCs) pool and neurogenic decline have been recently described in mice expressing synaptic susceptibility genes associated with autism spectrum disorder (ASDs). To gain further insight into mechanisms regulating neurogenesis in mice carrying mutations in synaptic genes related to monogenic ASDs, we used the R451C Neuroligin3 knock-in (Nlgn3 KI) mouse, which is characterized by structural brain abnormalities, deficits in synaptic functions and reduced sociability. We show that the number of adult-born neurons, but not the size of the NSPC pool, was reduced in the ventral dentate gyrus in knock-in mice. Notably, this neurogenic decline was rescued by daily injecting mice with 10 mg/Kg of the antidepressant fluoxetine for 20 consecutive days. Sustained treatment also improved KI mice's sociability and increased the number of c-Fos active adult-born neurons, compared with vehicle-injected KI mice. Our study uncovers neurogenesis-mediated alterations in the brain of R451C KI mouse, showing that the R451C Nlgn3 mutation leads to lasting, albeit pharmacologically reversible, changes in the brain, affecting neuron formation in the adult hippocampus. Our results suggest that fluoxetine can ameliorate social behaviour in KI mice, at least in part, by rescuing adult hippocampal neurogenesis, which may be relevant for the pharmacological treatment of ASDs.

Endogenous Neural Stem Cell Mediated Oligodendrogenesis in the Adult Mammalian Brain

Oligodendrogenesis is essential for replacing worn-out oligodendrocytes, promoting myelin plasticity, and for myelin repair following a demyelinating injury in the adult mammalian brain. Neural stem cells are an important source of oligodendrocytes in the adult brain; however, there are considerable differences in oligodendrogenesis from neural stem cells residing in different areas of the adult brain. Amongst the distinct niches containing neural stem cells, the subventricular zone lining the lateral ventricles and the subgranular zone in the dentate gyrus of the hippocampus are considered the principle areas of adult neurogenesis. In addition to these areas, radial glia-like cells, which are the precursors of neural stem cells, are found in the lining of the third ventricle, where they are called tanycytes, and in the cerebellum, where they are called Bergmann glia. In this review, we will describe the contribution and regulation of each of these niches in adult oligodendrogenesis.

Assessing the Role of Ependymal and Vascular Cells as Sources of Extracellular Cues Regulating the Mouse Ventricular-Subventricular Zone Neurogenic Niche

Neurogenesis persists in selected regions of the adult mouse brain; among them, the ventricular-subventricular zone (V-SVZ) of the lateral ventricles represents a major experimental paradigm due to its conspicuous neurogenic output. Postnatal V-SVZ neurogenesis is maintained by a resident population of neural stem cells (NSCs). Although V-SVZ NSCs are largely quiescent, they can be activated to enter the cell cycle, self-renew and generate progeny that gives rise to olfactory bulb interneurons. These adult-born neurons integrate into existing circuits to modify cognitive functions in response to external stimuli, but cells shed by V-SVZ NSCs can also reach injured brain regions, suggesting a latent regenerative potential. The V-SVZ is endowed with a specialized microenvironment, which is essential to maintain the proliferative and neurogenic potential of NSCs, and to preserve the NSC pool from exhaustion by finely tuning their quiescent and active states. Intercellular communication is paramount to the stem cell niche properties of the V-SVZ, and several extracellular signals acting in the niche milieu have been identified. An important part of these signals comes from non-neural cell types, such as local vascular cells, ependymal and glial cells. Understanding the crosstalk between NSCs and other niche components may aid therapeutic approaches for neuropathological conditions, since neurodevelopmental disorders, age-related cognitive decline and neurodegenerative diseases have been associated with dysfunctional neurogenic niches. Here, we review recent advances in the study of the complex interactions between V-SVZ NSCs and their cellular niche. We focus on the extracellular cues produced by ependymal and vascular cells that regulate NSC behavior in the mouse postnatal V-SVZ, and discuss the potential implication of these molecular signals in pathological conditions.

Repurposed anti-cancer epidermal growth factor receptor inhibitors: mechanisms of neuroprotective effects in Alzheimer's disease

Numerous molecular mechanisms are being examined in an attempt to discover disease-modifying drugs to slow down the underlying neurodegeneration in Alzheimer’s disease. Recent studies have shown the beneficial effects of epidermal growth factor receptor inhibitors on the enhancement of behavioral and pathological sequelae in Alzheimer’s disease. Despite the promising effects of epidermal growth factor receptor inhibitors in Alzheimer’s disease, there is no irrefutable neuroprotective evidence in well-established animal models using epidermal growth factor receptor inhibitors due to many un-explored downstream signaling pathways. This caused controversy about the potential involvement of epidermal growth factor receptor inhibitors in any prospective clinical trial. In this review, the mystery beyond the under-investigation of epidermal growth factor receptor in Alzheimer’s disease will be discussed. Furthermore, their molecular mechanisms in neurodegeneration will be explained. Also, we will shed light on SARS-COVID-19 induced neurological manifestations mediated by epidermal growth factor modulation. Finally, we will discuss future perspectives and under-examined epidermal growth factor receptor downstream signaling pathways that warrant more exploration. We conclude that epidermal growth factor receptor inhibitors are novel effective therapeutic approaches that require further research in attempts to be repositioned in the delay of Alzheimer’s disease progression.

Aging entails distinct requirements for Rb at maintaining adult neurogenesis

Cell cycle proteins play essential roles in regulating embryonic and adult neurogenesis in the mammalian brain. A key example is the Retinoblastoma protein (Rb) whose loss disrupts the whole neurogenic program during brain development, but only results in increased progenitor proliferation in the adult subventricular zone (SVZ) and compromised long-term neuronal survival in the adult olfactory bulb (OB). Whether this holds true of neurogenesis in the aged brain remains unknown. In this study, we find no evidence of irregular proliferation or early commitment defects in the mid-aged (12-month-old) and old-aged (20-month-old) SVZ following tamoxifen-inducible Rb knockout (Rb iKO) in mice. However, we highlight a striking defect in early maturation of Rb-deficient migrating neuroblasts along the rostral migratory stream (RMS), followed by massive decline in neuronal generation inside the aged OB. In the absence of Rb, we also show evidence of incomplete cell cycle re-entry (CCE) along with DNA damage in the young OB, while we find a similar trend towards CCE but no clear signs of DNA damage or neurodegenerative signatures (pTau or Synuclein accumulation) in the aged OB. However, such phenotype could be masked by the severe maturation defect reported above in addition to the natural decline in adult neurogenesis with age. Overall, we show that Rb is required to prevent CCE and DNA damage in adult-born OB neurons, hence maintain neuronal survival. Moreover, while loss of Rb alone is insufficient to trigger seeding of neurotoxic species, this study reveals age-dependent non-monotonic dynamics in regulating neurogenesis by Rb.

Principles of brain aging: Status and challenges of modeling human molecular changes in mice

Due to the extension of human life expectancy, the prevalence of cognitive impairment is rising in the older portion of society. Developing new strategies to delay or attenuate cognitive decline is vital. For this purpose, it is imperative to understand the cellular and molecular events at the basis of brain aging. While several organs are directly accessible to molecular analysis through biopsies, the brain constitutes a notable exception. Most of the molecular studies are performed on postmortem tissues, where cell death and tissue damage have already occurred. Hence, the study of the molecular aspects of cognitive decline largely relies on animal models and in particular on small mammals such as mice. What have we learned from these models? Do these animals recapitulate the changes observed in humans? What should we expect from future mouse studies? In this review we answer these questions by summarizing the state of the research that has addressed cognitive decline in mice from several perspectives, including genetic manipulation and omics strategies. We conclude that, while extremely valuable, mouse models have limitations that can be addressed by the optimal design of future studies and by ensuring that results are cross-validated in the human context.

Differential hippocampal protein expression between normal mice and mice with the perioperative neurocognitive disorder: a proteomic analysis

OBJECTIVES

To compare differential expression protein in hippocampal tissues from mice of perioperative neurocognitive disorder (PND) and normal control mice and to explore the possible mechanism of PND.

METHODS

Mice were randomly divided into a PND group (n = 9) and a control group (n = 9).The mice in the PND group were treated with open tibial fracture with intramedullary fixation under isoflurane anesthesia, while the mice in the control group received pure oxygen without surgery. The cognitive functions of the two groups were examined using Morris water maze experiment, Open field test and Fear conditioning test. The protein expression of the hippocampus of mice was analyzed by high-performance liquid chromatography-mass spectrometry (HPLC-MS/MS). Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses were performed to explore the principal functions of dysregulated proteins.

RESULTS

A total of 21 proteins were differentially expressed between PND and control mice on days 1, 3, and 7 after the operation. These proteins were involved in many pathological processes, such as neuroinflammatory responses, mitochondrial oxidative stress, impaired synaptic plasticity, and neuronal cell apoptosis. Also, the dysregulated proteins were involved in MAPK, AMPK, and ErbB signaling pathways.

CONCLUSION

The occurrence of PND could be attributed to multiple mechanisms.

Dysfunction of the proteoglycan Tsukushi causes hydrocephalus through altered neurogenesis in the subventricular zone in mice

The lateral ventricle (LV) is flanked by the subventricular zone (SVZ), a neural stem cell (NSC) niche rich in extrinsic growth factors regulating NSC maintenance, proliferation, and neuronal differentiation. Dysregulation of the SVZ niche causes LV expansion, a condition known as hydrocephalus; however, the underlying pathological mechanisms are unclear. We show that deficiency of the proteoglycan Tsukushi (TSK) in ependymal cells at the LV surface and in the cerebrospinal fluid results in hydrocephalus with neurodevelopmental disorder-like symptoms in mice. These symptoms are accompanied by altered differentiation and survival of the NSC lineage, disrupted ependymal structure, and dysregulated Wnt signaling. Multiple TSK variants found in patients with hydrocephalus exhibit reduced physiological activity in mice in vivo and in vitro. Administration of wild-type TSK protein or Wnt antagonists, but not of hydrocephalus-related TSK variants, in the LV of TSK knockout mice prevented hydrocephalus and preserved SVZ neurogenesis. These observations suggest that TSK plays a crucial role as a niche molecule modulating the fate of SVZ NSCs and point to TSK as a candidate for the diagnosis and therapy of hydrocephalus.

Oxidative stress sensing and response in neural stem cell fate

Neural stem/progenitor cells (NSPCs) contribute to the physiological cellular turnover of the adult brain and make up its regenerative potential. It is thus essential to understand how different factors influence their proliferation and differentiation to gain better insight into potential therapeutic targets in neurodegenerative diseases and traumatic brain injuries. Recent evidences indicate the roles of redox stress sensing and coping mechanisms in mediating the balance between NSPC self-renewal and differentiation. Such mechanisms involve direct cysteine modification, signaling and metabolic reprogramming, epigenetic alterations and transcription changes leading to adaptive responses like autophagy. Here, we discuss emerging findings on the involvement of redox sensors and effectors and their mechanisms in influencing changes in cellular redox potential and NSPC fate.

Manipulation of EGFR-Induced Signaling for the Recruitment of Quiescent Neural Stem Cells in the Adult Mouse Forebrain

The ventricular-subventricular zone (V-SVZ) is the principal neurogenic niche in the adult mammalian forebrain. Neural stem/progenitor cell (NSPC) activity within the V-SVZ is controlled by numerous of extrinsic factors, whose downstream effects on NSPC proliferation, survival and differentiation are transduced via a limited number of intracellular signaling pathways. Here, we investigated the relationship between age-related changes in NSPC output and activity of signaling pathways downstream of the epidermal growth factor receptor (EGFR), a major regulator of NSPC activity. Biochemical experiments indicated that age-related decline of NSPC activity in vivo is accompanied by selective deficits amongst various EGFR-induced signal pathways within the V-SVZ niche. Pharmacological loss-of-function signaling experiments with cultured NSPCs revealed both overlap and selectivity in the biological functions modulated by the EGFR-induced PI3K/AKT, MEK/ERK and mTOR signaling modules. Specifically, while all three modules promoted EGFR-mediated NSPC proliferation, only mTOR contributed to NSPC survival and only MEK/ERK repressed NSPC differentiation. Using a gain-of-function in vivo genetic approach, we electroporated a constitutively active EGFR construct into a subpopulation of quiescent, EGFR-negative neural stem cells (qNSCs); this ectopic activation of EGFR signaling enabled qNSCs to divide in 3-month-old early adult mice, but not in mice at middle-age or carrying familial Alzheimer disease mutations. Thus, (i) individual EGFR-induced signaling pathways have dissociable effects on NSPC proliferation, survival, and differentiation, (ii) activation of EGFR signaling is sufficient to stimulate qNSC cell cycle entry during early adulthood, and (iii) the proliferative effects of EGFR-induced signaling are dominantly overridden by anti-proliferative signals associated with aging and Alzheimer's disease.

Wnt/β-Catenin Signaling Promotes Differentiation of Ischemia-Activated Adult Neural Stem/Progenitor Cells to Neuronal Precursors

Modulating endogenous regenerative processes may represent a suitable treatment for central nervous system (CNS) injuries, such as stroke or trauma. Neural stem/progenitor cells (NS/PCs), which naturally reside in the subventricular zone (SVZ) of the adult brain, proliferate and differentiate to other cell types, and therefore may compensate the negative consequences of ischemic injury. The fate of NS/PCs in the developing brain is largely influenced by Wingless/Integrated (Wnt) signaling; however, its role in the differentiation of adult NS/PCs under ischemic conditions is still enigmatic. In our previous study, we identified the Wnt/β-catenin signaling pathway as a factor promoting neurogenesis at the expense of gliogenesis in neonatal mice. In this study, we used adult transgenic mice in order to assess the impact of the canonical Wnt pathway modulation (inhibition or hyper-activation) on NS/PCs derived from the SVZ, and combined it with the middle cerebral artery occlusion (MCAO) to disclose the effect of focal cerebral ischemia (FCI). Based on the electrophysiological properties of cultured cells, we first identified three cell types that represented in vitro differentiated NS/PCs – astrocytes, neuron-like cells, and precursor cells. Following FCI, we detected fewer neuron-like cells after Wnt signaling inhibition. Furthermore, the immunohistochemical analysis revealed an overall higher expression of cell-type-specific proteins after FCI, indicating increased proliferation and differentiation rates of NS/PCs in the SVZ. Remarkably, Wnt signaling hyper-activation increased the abundance of proliferating and neuron-like cells, while Wnt pathway inhibition had the opposite effect. Finally, the expression profiling at the single cell level revealed an increased proportion of neural stem cells and neuroblasts after FCI. These observations indicate that Wnt signaling enhances NS/PCs-based regeneration in the adult mouse brain following FCI, and supports neuronal differentiation in the SVZ.

Neurotrophins Time Point Intervention after Traumatic Brain Injury: From Zebrafish to Human

Traumatic brain injury (TBI) remains the leading cause of long-term disability, which annually involves millions of individuals. Several studies on mammals reported that neurotrophins could play a significant role in both protection and recovery of function following neurodegenerative diseases such as stroke and TBI. This protective role of neurotrophins after an event of TBI has also been reported in the zebrafish model. Nevertheless, reparative mechanisms in mammalian brain are limited, and newly formed neurons do not survive for a long time. In contrast, the brain of adult fish has high regenerative properties after brain injury. The evident differences in regenerative properties between mammalian and fish brain have been ascribed to remarkable different adult neurogenesis processes. However, it is not clear if the specific role and time point contribution of each neurotrophin and receptor after TBI is conserved during vertebrate evolution. Therefore, in this review, I reported the specific role and time point of intervention for each neurotrophic factor and receptor after an event of TBI in zebrafish and mammals.

Syndecan-1 Stimulates Adult Neurogenesis in the Mouse Ventricular-Subventricular Zone after Injury

The production of neurons from neural stem cells (NSCs) persists throughout life in the mouse ventricular-subventricular zone (V-SVZ). We have previously reported that NSCs from adult V-SVZ are contained in cell populations expressing the carbohydrate SSEA-1/LeX, which exhibit either characteristics of quiescent NSCs (qNSCs) or of actively dividing NSCs (aNSCs) based on the absence or the presence of EGF-receptor, respectively. Using the fluorescence ubiquitination cell cycle indicator-Cdt1 transgenic mice to mark cells in G₀/G₁ phase of the cell cycle, we uncovered a subpopulation of qNSCs which were primed to enter the cell cycle in vitro. Besides, we found that treatment with Syndecan-1, a heparan sulfate proteoglycan involved in NSC proliferation, hastened the division of qNSCs and increased proliferation of aNSCs shortening their G₁ phase in vitro. Furthermore, administration of Syndecan-1 ameliorated the recovery of neurogenic populations in the V-SVZ after radiation-induced injury providing potential cure for neurogenesis decline during brain aging or after injury.

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A subpopulation of quiescent NSCs are primed to enter cell cycle

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The content of primed quiescent NSCs decreases rapidly with age

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Syndecan-1 favors cell cycle progression of NSCs in vitro and in vivo

Tissue Engineering and Biomaterial Strategies to Elicit Endogenous Neuronal Replacement in the Brain

Neurogenesis in the postnatal mammalian brain is known to occur in the dentate gyrus of the hippocampus and the subventricular zone. These neurogenic niches serve as endogenous sources of neural precursor cells that could potentially replace neurons that have been lost or damaged throughout the brain. As an example, manipulation of the subventricular zone to augment neurogenesis has become a popular strategy for attempting to replace neurons that have been lost due to acute brain injury or neurodegenerative disease. In this review article, we describe current experimental strategies to enhance the regenerative potential of endogenous neural precursor cell sources by enhancing cell proliferation in neurogenic regions and/or redirecting migration, including pharmacological, biomaterial, and tissue engineering strategies. In particular, we discuss a novel replacement strategy based on exogenously biofabricated "living scaffolds" that could enhance and redirect endogenous neuroblast migration from the subventricular zone to specified regions throughout the brain. This approach utilizes the first implantable, biomimetic tissue-engineered rostral migratory stream, thereby leveraging the brain's natural mechanism for sustained neuronal replacement by replicating the structure and function of the native rostral migratory stream. Across all these strategies, we discuss several challenges that need to be overcome to successfully harness endogenous neural precursor cells to promote nervous system repair and functional restoration. With further development, the diverse and innovative tissue engineering and biomaterial strategies explored in this review have the potential to facilitate functional neuronal replacement to mitigate neurological and psychiatric symptoms caused by injury, developmental disorders, or neurodegenerative disease.

X-ray irradiated cultures of mouse cortical neural stem/progenitor cells recover cell viability and proliferation with dose-dependent kinetics

Exposure of the developing or adult brain to ionizing radiation (IR) can cause cognitive impairment and/or brain cancer, by targeting neural stem/progenitor cells (NSPCs). IR effects on NSPCs include transient cell cycle arrest, permanent cell cycle exit/differentiation, or cell death, depending on the experimental conditions. In vivo studies suggest that brain age influences NSPC response to IR, but whether this is due to intrinsic NSPC changes or to niche environment modifications remains unclear. Here, we describe the dose-dependent, time-dependent effects of X-ray IR in NSPC cultures derived from the mouse foetal cerebral cortex. We show that, although cortical NSPCs are resistant to low/moderate IR doses, high level IR exposure causes cell death, accumulation of DNA double-strand breaks, activation of p53-related molecular pathways and cell cycle alterations. Irradiated NSPC cultures transiently upregulate differentiation markers, but recover control levels of proliferation, viability and gene expression in the second week post-irradiation. These results are consistent with previously described in vivo effects of IR in the developing mouse cortex, and distinct from those observed in adult NSPC niches or in vitro adult NSPC cultures, suggesting that intrinsic differences in NSPCs of different origins might determine, at least in part, their response to IR.

Granulins Regulate Aging Kinetics in the Adult Zebrafish Telencephalon

Granulins (GRN) are secreted factors that promote neuronal survival and regulate inflammation in various pathological conditions. However, their roles in physiological conditions in the brain remain poorly understood. To address this knowledge gap, we analysed the telencephalon in Grn-deficient zebrafish and identified morphological and transcriptional changes in microglial cells, indicative of a pro-inflammatory phenotype in the absence of any insult. Unexpectedly, activated mutant microglia shared part of their transcriptional signature with aged human microglia. Furthermore, transcriptome profiles of the entire telencephali isolated from young Grn-deficient animals showed remarkable similarities with the profiles of the telencephali isolated from aged wildtype animals. Additionally, 50% of differentially regulated genes during aging were regulated in the telencephalon of young Grn-deficient animals compared to their wildtype littermates. Importantly, the telencephalon transcriptome in young Grn-deficent animals changed only mildly with aging, further suggesting premature aging of Grn-deficient brain. Indeed, Grn loss led to decreased neurogenesis and oligodendrogenesis, and to shortening of telomeres at young ages, to an extent comparable to that observed during aging. Altogether, our data demonstrate a role of Grn in regulating aging kinetics in the zebrafish telencephalon, thus providing a valuable tool for the development of new therapeutic approaches to treat age-associated pathologies.

Transcriptional regulation of adult neural stem/progenitor cells: tales from the subventricular zone

In rodents, well characterized neurogenic niches of the adult brain, such as the subventricular zone of the lateral ventricles and the subgranular zone of the hippocampus, support the maintenance of neural/stem progenitor cells (NSPCs) and the production of new neurons throughout the lifespan. The adult neurogenic process is dependent on the intrinsic gene expression signatures of NSPCs that make them competent for self-renewal and neuronal differentiation. At the same time, it is receptive to regulation by various extracellular signals that allow the modulation of neuronal production and integration into brain circuitries by various physiological stimuli. A drawback of this plasticity is the sensitivity of adult neurogenesis to alterations of the niche environment that can occur due to aging, injury or disease. At the core of the molecular mechanisms regulating neurogenesis, several transcription factors have been identified that maintain NSPC identity and mediate NSPC response to extrinsic cues. Here, we focus on REST, Egr1 and Dbx2 and their roles in adult neurogenesis, especially in the subventricular zone. We review recent work from our and other laboratories implicating these transcription factors in the control of NSPC proliferation and differentiation and in the response of NSPCs to extrinsic influences from the niche. We also discuss how their altered regulation may affect the neurogenic process in the aged and in the diseased brain. Finally, we highlight key open questions that need to be addressed to foster our understanding of the transcriptional mechanisms controlling adult neurogenesis.

Omega-3 Docosahexaenoic Acid Is a Mediator of Fate-Decision of Adult Neural Stem Cells

The mammalian brain is enriched with lipids that serve as energy catalyzers or secondary messengers of essential signaling pathways. Docosahexaenoic acid (DHA) is an omega-3 fatty acid synthesized de novo at low levels in humans, an endogenous supply from its precursors, and is mainly incorporated from nutrition, an exogeneous supply. Decreased levels of DHA have been reported in the brains of patients with neurodegenerative diseases. Preventing this decrease or supplementing the brain with DHA has been considered as a therapy for the DHA brain deficiency that could be linked with neuronal death or neurodegeneration. The mammalian brain has, however, a mechanism of compensation for loss of neurons in the brain: neurogenesis, the birth of neurons from neural stem cells. In adulthood, neurogenesis is still present, although at a slower rate and with low efficiency, where most of the newly born neurons die. Neural stem/progenitor cells (NSPCs) have been shown to require lipids for proper metabolism for proliferation maintenance and neurogenesis induction. Recent studies have focused on the effects of these essential lipids on the neurobiology of NSPCs. This review aimed to introduce the possible use of DHA to impact NSPC fate-decision as a therapy for neurodegenerative diseases.

Neuroprotective Potential of GDF11: Myth or Reality?

In the brain, aging is accompanied by cellular and functional deficiencies that promote vulnerability to neurodegenerative disorders. In blood plasma from young and old animals, various factors such as growth differentiation factor 11 (GDF11), whose levels are elevated in young animals, have been identified. The blood concentrations of these factors appear to be inversely correlated with the age-related decline of neurogenesis. The identification of GDF11 as a "rejuvenating factor" opens up perspectives for the treatment of neurodegenerative diseases. As a pro-neurogenic and pro-angiogenic agent, GDF11 may constitute a basis for novel therapeutic strategies.

Molecular Signatures of the Aging Brain: Finding the Links Between Genes and Phenotypes

Aging is associated with cognitive decline and increased vulnerability to neurodegenerative diseases. The progressive extension of the average human lifespan is bound to lead to a corresponding increase in the fraction of cognitively impaired elderly individuals among the human population, with an enormous societal and economic burden. At the cellular and tissue levels, cognitive decline is linked to a reduction in specific neuronal subpopulations, a widespread decrease in synaptic plasticity and an increase in neuroinflammation due to an enhanced activation of astrocytes and microglia, but the molecular mechanisms underlying these functional changes during normal aging and in neuropathological conditions remain poorly understood. In this review, we summarize very recent and outstanding progress in elucidating the molecular changes associated with cognitive decline through the genome-wide profiling of aging brain cells at different molecular levels (genomic, epigenomic, transcriptomic, proteomic). We discuss how the correlation of different molecular and phenotypic traits driven by mathematical and computational analyses of large datasets has led to the prediction of key molecular nodes of neurodegenerative pathways, and provide a few examples of candidate regulators of cognitive decline identified with these approaches. Furthermore, we highlight the dysregulation of the synaptic transcriptome in neuronal cells and of the inflammatory transcriptome in glial cells as some of the key events during normal and neuropathological human brain aging.