Aging Modulates the Ability of Quiescent Radial Glia-Like Stem Cells in the Hippocampal Dentate Gyrus to be Recruited into Division by Pro-neurogenic Stimuli

A delicate balance between quiescence and division of the radial glia-like stem cells (RGLs) ensures continuation of adult hippocampal neurogenesis (AHN) over the lifespan. Transient or persistent perturbations of this balance due to a brain pathology, drug administration, or therapy can lead to unfavorable long-term outcomes such as premature depletion of the RGLs, decreased AHN, and cognitive deficit. Memantine, a drug used for alleviating the symptoms of Alzheimer's disease, and electroconvulsive seizure (ECS), a procedure used for treating drug-resistant major depression or bipolar disorder, are known strong AHN inducers; they were earlier demonstrated to increase numbers of dividing RGLs. Here, we demonstrated that 1-month stimulation of quiescent RGLs by either memantine or ECS leads to premature exhaustion of their pool and altered AHN at later stages of life and that aging of the brain modulates the ability of the quiescent RGLs to be recruited into the cell cycle by these AHN inducers. Our findings support the aging-related divergence of functional features of quiescent RGLs and have a number of implications for the practical assessment of drugs and treatments with respect to their action on quiescent RGLs at different stages of life in animal preclinical studies.

The effects of peripheral hormone responses to exercise on adult hippocampal neurogenesis

Over the last decade, a considerable amount of new data have revealed the beneficial effects of exercise on hippocampal neurogenesis and the maintenance or improvement of cognitive function. Investigations with animal models, as well as human studies, have yielded novel understanding of the mechanisms through which endocrine signaling can stimulate neurogenesis, as well as the effects of exercise on acute and/or chronic levels of these circulating hormones. Considering the effects of aging on the decline of specific endocrine factors that affect brain health, insights in this area of research are particularly important. In this review, we discuss how different forms of exercise influence the peripheral production of specific endocrine factors, with particular emphasis on brain-derived neurotrophic factor, growth hormone, insulin-like growth factor-1, ghrelin, estrogen, testosterone, irisin, vascular endothelial growth factor, erythropoietin, and cortisol. We also describe mechanisms through which these endocrine responses to exercise induce cellular changes that increase hippocampal neurogenesis and improve cognitive function.

BDNF promotes mouse follicular development and reverses ovarian aging by promoting cell proliferation

BACKGROUND

Brain-derived neurotrophic factor (BDNF) plays an important role in ovarian function including follicle development and oocyte maturation, and embryonic development. However, whether BDNF treatment can reimpose ovarian aging and impaired fertility remains elusive. In this study, we investigated the reproductive outcomes of BDNF treatment and potential mechanisms in aged mice.

METHOD

"Aged" mice (35-37 weeks old, n = 68) were treated with recombinant human BDNF protein (rhBDNF, 1 µg/200 µL) through daily intraperitoneal (IP) injection for 10 days with/without ovulation induction. Reproductive age mice (8-10 weeks old, n = 28) were treated with ANA 12 (a selective BDNF receptor, TrkB antagonist) through daily IP injection for 5 days with/without ovulation induction. Ovarian function was assessed by ovarian weight, number of follicles, and sex hormone productions. Following induction of ovulation, the number of total oocytes or abnormal oocytes, and blastocyst formation were assessed. Reproductive functions of the mice were evaluated, including pregnancy rate, mating duration for conception, implantation sites, litter size, and weight of offspring. Finally, the molecular mechanism of the effects of BDNF on ovarian cell functions in mice were examined by Western blot and immunofluorescence.

RESULTS

rhBDNF treatment increased the ovarian weight, number of follicles, number and quality of oocytes including increased blastocysts formation, blood estrogen levels, and pregnancy rate in 35-37-week-old mice. Conversely, BDNF receptor antagonist, ANA 12, treatment decreased the ovarian volume and number of antral follicles and increased the proportion of abnormal oocytes in 8-10-week-old mice. We further demonstrated that BDNF treatment promoted ovarian cell proliferation as well as activation of TrkB and cyclinD1-creb signalling.

CONCLUSION

We demonstrated that ten consecutive days of daily IP injection of rhBDNF rescued ovarian function in aged mice. Our results further indicate that TrkB and cyclin D1-creb signaling may underlie the BDNF function in ovaries. Targeting BDNF-TrkB signaling is a potential novel therapeutic strategy to reverse ovarian aging.

Long noncoding RNA H19 mediates neural stem/progenitor cells proliferation, differentiation and apoptosis through the p53 signaling pathway after ischemic stroke

Long non-coding RNA (LncRNA) H19 plays an important role on the biological functions of endogenous neural stem/progenitor cells (NSPCs). Our study aimed to explore the functions of H19 in NSPCs induced by oxygen-glucose deprivation/reperfusion (OGD/R) in vitro and the underlying mechanisms. In this study, our results showed that knockdown of H19 significantly inhibited NSPCs proliferation. Additionally, the apoptosis of NSPCs after ODG/R injury was notably promoted by H19 knockdown. Cell cycle arrest was induced in NSPCs at G0/G1 phase after OGD/R, while knockdown of H19 decreased the percentage of cells at G2/S phase. The results of immunofluorescence analysis revealed that H19 knockdown reduced the staining intensity of Ki-67 and DCX. Furthermore, H19 knockdown enhanced the expression of p53, Bax and Cleaved Caspase-3, while Bcl-2 expression was decreased. Silencing of H19 suppressed the NSPCs proliferation, cell cycle progression and differentiation, whereas cell apoptosis was promoted. Upregulation of H19 abolished OGD/R-induced NSPCs apoptosis, while cell proliferation and differentiation were promoted. Furthermore, the effects of overexpressed H19 on NSPCs proliferation, differentiation and apoptosis were abrogated by the upregulation of p53. In summary, overexpressed H19 resulted in the inactivation of p53, which promoted NSPCs proliferation, differentiation, and inhibited cell apoptosis. These findings suggested that H19 could promote cell proliferation and differentiation after OGD/R through suppressing the p53 signaling.

Fading memories in aging and neurodegeneration: Is p75 neurotrophin receptor a culprit?

Aging and age-related neurodegenerative diseases have become one of the major concerns in modern times as cognitive abilities tend to decline when we get older. It is well known that the main cause of this age-related cognitive deficit is due to aberrant changes in cellular, molecular circuitry and signaling pathways underlying synaptic plasticity and neuronal connections. The p75 neurotrophin receptor (p75^NTR) is one of the important mediators regulating the fate of the neurons in the nervous system. Its importance in neuronal apoptosis is well documented. However, the mechanisms involving the regulation of p75^NTR in synaptic plasticity and cognitive function remain obscure, although cognitive impairment has been associated with a higher expression of p75^NTR in neurons. In this review, we discuss the current understanding of how neurons are influenced by p75^NTR function to maintain normal neuronal synaptic strength and connectivity, particularly to support learning and memory in the hippocampus. We then discuss the age-associated alterations in neurophysiological mechanisms of synaptic plasticity and cognitive function. Furthermore, we also describe current evidence that has begun to elucidate how p75^NTR regulates synaptic changes in aging and age-related neurodegenerative diseases, focusing on the hippocampus. Elucidating the role that p75^NTR signaling plays in regulating synaptic plasticity will contribute to a better understanding of cognitive processes and pathological conditions. This will in turn provide novel approaches to improve therapies for the treatment of neurological diseases in which p75^NTR dysfunction has been demonstrated.

Stress-Related Dysfunction of Adult Hippocampal Neurogenesis-An Attempt for Understanding Resilience?

Newborn neurons in the adult hippocampus are regulated by many intrinsic and extrinsic cues. It is well accepted that elevated glucocorticoid levels lead to downregulation of adult neurogenesis, which this review discusses as one reason why psychiatric diseases, such as major depression, develop after long-term stress exposure. In reverse, adult neurogenesis has been suggested to protect against stress-induced major depression, and hence, could serve as a resilience mechanism. In this review, we will summarize current knowledge about the functional relation of adult neurogenesis and stress in health and disease. A special focus will lie on the mechanisms underlying the cascades of events from prolonged high glucocorticoid concentrations to reduced numbers of newborn neurons. In addition to neurotransmitter and neurotrophic factor dysregulation, these mechanisms include immunomodulatory pathways, as well as microbiota changes influencing the gut-brain axis. Finally, we discuss recent findings delineating the role of adult neurogenesis in stress resilience.

Selective Ablation of BDNF from Microglia Reveals Novel Roles in Self-Renewal and Hippocampal Neurogenesis

Microglia, the resident immune cells of the CNS, have emerged as key regulators of neural precursor cell activity in the adult brain. However, the microglia-derived factors that mediate these effects remain largely unknown. In the present study, we investigated a role for microglial brain-derived neurotrophic factor (BDNF), a neurotrophic factor with well known effects on neuronal survival and plasticity. Surprisingly, we found that selective genetic ablation of BDNF from microglia increased the production of newborn neurons under both physiological and inflammatory conditions (e.g., LPS-induced infection and traumatic brain injury). Genetic ablation of BDNF from microglia otherwise also interfered with self-renewal/proliferation, reducing their overall density. In conclusion, we identify microglial BDNF as an important factor regulating microglia population dynamics and states, which in turn influences neurogenesis under both homeostatic and pathologic conditions.SIGNIFICANCE STATEMENT (1) Microglial BDNF contributes to self-renewal and density of microglia in the brain. (2) Selective ablation of BDNF in microglia stimulates neural precursor proliferation. (3) Loss of microglial BDNF augments working memory following traumatic brain injury. (4) Benefits of repopulating microglia on brain injury are not mediated via microglial BDNF.

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.

Intervention of Brain-Derived Neurotrophic Factor and Other Neurotrophins in Adult Neurogenesis

Cell survival during adult neurogenesis and the modulation of each step, namely, proliferation, lineage differentiation, migration, maturation, and functional integration of the newborn cells into the existing circuitry, is regulated by intrinsic and extrinsic factors. Transduction of extracellular niche signals triggers the activation of intracellular mechanisms that regulate adult neurogenesis by affecting gene expression. While the intrinsic factors include transcription factors and epigenetic regulators, the extrinsic factors are molecular signals that are present in the neurogenic niche microenvironment. These include morphogens, growth factors, neurotransmitters, and signaling molecules secreted as soluble factors or associated to the extracellular matrix. Among these molecular mechanisms are neurotrophins and neurotrophin receptors which have been implicated in the regulation of adult neurogenesis at different levels, with brain-derived neurotrophic factor (BDNF) being the most studied neurotrophin. In this chapter, we review the current knowledge about the role of neurotrophins in the regulation of adult neurogenesis in both the subventricular zone (SVZ) and the hippocampal subgranular zone (SGZ).

Fluoxetine May Enhance VEGF, BDNF and Cognition in Patients with Vascular Cognitive Impairment No Dementia: An Open-Label Randomized Clinical Study

PURPOSE

Selective serotonin reuptake inhibitors (SSRIs) enhance angiogenesis and neurogenesis. Brain-derived neurotrophic factor (BDNF) and vascular endothelial growth factor (VEGF) play an important role in neurogenesis and angiogenesis. However, the effect of SSRIs on cognition and serum BDNF and VEGF in patients with vascular cognitive impairment no dementia (VCIND) is largely unknown.

PATIENTS AND METHODS

It was an open label study. Fifty VCIND patients were randomly allocated to receive fluoxetine (20 mg/d; n = 25) or no fluoxetine (control group; n = 25) for 12 weeks. VCIND patients received fluoxetine 20 mg/d and secondary prevention of stroke for 12 weeks in the fluoxetine group, whereas the control group received only secondary prevention of stroke for 12 weeks. The primary outcome and secondary outcome were of assessment of Alzheimer's Disease Assessment Scale cognitive subscale (ADAS-cog) score, Ten Point Clock drawing test score (TPC), Verbal Fluency Test (VFT), Trail Making Test form a (TMTa), Trail Making Test form b (TMTb) and Digit Span Test score at baseline and week 12 in the both groups. And serum concentration of BDNF and VEGF was also tested at baseline and week 12 in both groups.

RESULTS

After 12 weeks, TPC scores increased more significantly in the fluoxetine group than in the control group, while TMTa score and TMTb score were decreased more significantly in the fluoxetine group than in the control group. We also found that the serum concentration of BDNF and VEGF in the fluoxetine group increased more significantly than in the control group. However, we found no significant differences in mean change from baseline between fluoxetine and control group in ADAS-Cog score, Digit Span Test score and VFT score.

CONCLUSION

Fluoxetine may enhance cognition in certain cognitive domains and serum concentration of BDNF and VEGF in patients with VCIND.

Early-Life Multiple Sevoflurane Exposures Alleviate Long-term Anxiety-Like Behaviors in Mice via the proBDNF/ERK Pathway

Early-life multiple anesthetics exposure causes neurotoxicity and hence cognitive dysfunction on developing brain. However, the effects of early-life multiple sevoflurane exposures on emotional changes, especially upon stress, are far beyond understood. In young male C57BL6/J mice, the present study showed that 3% sevoflurane inhalation for 2 h in three consecutive days did not influence anxiety-like behaviors as measured by open field test, light dark transition, and elevated plus maze test. In addition, foot shocks stress induced both the short- and long-term anxiety-like behaviors. However, triple sevoflurane exposures ameliorated the long-term anxiety-like behaviors induced by the foot shocks. In parallel, foot shocks stress upregulated the expression of phosphorylated extracellular signal-regulated kinase (p-ERK) and brain-derived neurotrophic factor precursor (proBDNF) in the anterior cingulate cortex (ACC), which were significantly inhibited by triple sevoflurane exposures. Immunofluorescence further indicated that the increased p-ERK was mainly expressed in the proBDNF-positive staining cells. Intra-ACC injection of recombinant proBDNF protein upregulated the p-ERK expression and blocked the anxiolytic effect of sevoflurane exposure on long-term anxiety-like behaviors. Therefore, our study demonstrated that multiple sevoflurane exposures alleviate long-term anxiety-like behaviors upon acute stress in young mice by inhibiting proBDNF-ERK signaling in the ACC.

Saikosaponin-d impedes hippocampal neurogenesis and causes cognitive deficits by inhibiting the survival of neural stem/progenitor cells via neurotrophin receptor signaling in mice

Neural stem/progenitor cells (NPCs) are multipotent stem cells in the central nervous system. Damage to NPCs has been demonstrated to cause adverse effects on neurogenesis and to contribute to neurological diseases. Our previous research suggested that saikosaponin-d (SSd), a cytostatic drug belonging to the bioactive triterpenoid saponins, exhibited neurotoxicity by inhibiting hippocampal neurogenesis, but the underlying mechanism remained elusive. This study was performed to clarify the role of SSd in cognitive function and the mechanism by which SSd induced damage to hippocampal neurogenesis and NPCs. Our results indicated that SSd caused hippocampus-dependent cognitive deficits and inhibited hippocampal neurogenesis by reducing the numbers of newborn neurons in mice. RNA sequencing analysis revealed that SSd-induced neurotoxicity in the hippocampus involved neurotrophin receptor-interacting MAGE (NRAGE)/neurotrophin receptor interacting factor (NRIF)/p75NTR -associated cell death executor (NADE) cell signaling activated by the p75 neurotrophin receptor (p75NTR ). Mechanistic studies showed that a short hairpin RNA targeting p75NTR intracellular domain reversed SSd-increased NRAGE/NRIF/NADE signaling and the c-Jun N-terminal kinase/caspase apoptotic pathway, subsequently contributing to the survival of NPCs, as well as cell proliferation and differentiation. The addition of recombinant brain-derived neurotrophic factor (BDNF) ameliorated the SSd-induced inhibition of BDNF/Tyrosine kinase receptor B (TrkB) neurotrophic signaling, but did not affect SSd-activated pro-BDNF/p75NTR signaling. Moreover, the SSd-induced elevation of cytosolic Ca2+ concentration was responsible for damage to NPCs. The extracellular Ca2+ chelator ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA), rather than the intracellular Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester) (BAPTA/AM), attenuated SSd-induced cytosolic Ca2+ dysfunction and SSd-disordered TrkB/p75NTR signaling. Overall, this study demonstrated a new mechanism for the neurotoxic effect of SSd, which has emerging implications for pharmacological research of SSd and provides a better understanding of neurotoxicity induced by cytostatic drugs.