Transient activation of Notch signaling in the injured adult brain

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.

A novel mouse model for the study of endogenous neural stem and progenitor cells after traumatic brain injury

Traumatic brain injury (TBI) is a leading cause of death and disability in the US. Neural stem/progenitor cells (NSPCs) persist in the adult brain and represent a potential cell source for tissue regeneration and wound healing after injury. The Notch signaling pathway is critical for embryonic development and adult brain injury response. However, the specific role of Notch signaling in the injured brain is not well characterized. Our previous study has established a Notch1CR2-GFP reporter mouse line in which the Notch1CR2 enhancer directs GFP expression in NSPCs and their progeny. In this study, we performed closed head injury (CHI) in the Notch1CR2-GFP mice to study the response of injury-activated NSPCs. We show that CHI induces neuroinflammation, cell death, and the expression of typical TBI markers (e.g., ApoE, Il1b, and Tau), validating the animal model. In addition, CHI induces cell proliferation in GFP+ cells expressing NSPC markers, e.g., Notch1 and Nestin. A significant higher percentage of GFP+ astrocytes and GABAergic neurons was observed in the injured brain, with no significant change in oligodendrocyte lineage between the CHI and sham animal groups. Since injury is known to activate astrogliosis, our results suggest that injury-induced GFP+ NSPCs preferentially differentiate into GABAergic neurons. Our study establishes that Notch1CR2-GFP transgenic mouse is a useful tool for the study of NSPC behavior in vivo after TBI. Unveiling the potential of NSPCs response to TBI (e.g., proliferation and differentiation) will identify new therapeutic strategy for the treatment of brain trauma.

The Notch pathway in CNS homeostasis and neurodegeneration

The role of the Notch signaling pathway in neural development has been well established over many years. More recent studies, however, have demonstrated that Notch continues to be expressed and active throughout adulthood in many areas of the central nervous system. Notch signals have been implicated in adult neurogenesis, memory formation, and synaptic plasticity in the adult organism, as well as linked to acute brain trauma and chronic neurodegenerative conditions. NOTCH3 mutations are responsible for the most common form of hereditary stroke, the progressive disorder cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy. Notch has also been associated with several progressive neurodegenerative diseases, including Alzheimer's disease, multiple sclerosis, and amyotrophic lateral sclerosis. Although numerous studies link Notch activity with CNS homeostasis and neurodegenerative diseases, the data thus far are primarily correlative, rather than functional. Nevertheless, the evidence for Notch pathway activity in specific neural cellular contexts is strong, and certainly intriguing, and points to the possibility that the pathway carries therapeutic promise. This article is categorized under: Nervous System Development > Flies Signaling Pathways > Cell Fate Signaling Nervous System Development > Vertebrates: General Principles.

The Absence of TLR4 Prevents Fetal Brain Injury in the Setting of Intrauterine Inflammation

BACKGROUND

Exposure to intrauterine inflammation during pregnancy is linked to brain injury and neurobehavioral disorders in affected children. Innate immunity, specifically Toll-like receptor (TLR) signaling pathways are present throughout the reproductive tract as well as in the placenta, fetal membranes, and fetus. The TLR pathways are mechanistically involved in host responses to foreign pathogens and may lead to brain injury associated with prenatal inflammation.

OBJECTIVE

We aimed to determine whether the activation of the TLR4 signaling pathway, in the mother and fetus, is critical to fetal brain injury in the setting of intrauterine inflammation.

METHODS

A mini-laparotomy was performed on time pregnant C57B6 mice and 2 knockout mouse strains lacking the function of the Tlr4 and Myd88 genes on embryonic day 15. Intrauterine injections of Escherichia coli lipopolysaccharide or saline were administered as described previously. Dams were killed 6 hours postsurgery, and placental, amniotic fluid, and fetal brain tissue were collected. To assess brain injury, quantitative polymerase chain reaction (qPCR) analysis was performed on multiple components of the NOTCH signaling pathway, including Hes genes. Interleukin (IL) IL6, IL1β, and CCL5 expression was assessed using qPCR and enzyme-linked immunosorbent assay.

RESULTS

Using an established mouse model of intrauterine inflammation, we demonstrate that the abrogation of TLR4 signaling eliminates the cytokine response in mother and fetus and prevents brain injury associated with increased expression of transcriptional effectors of the NOTCH signaling pathway, Hes1 and Hes5.

CONCLUSIONS

These data show that the activation of the TLR4 signaling pathway is necessary for the development of fetal brain injury in response to intrauterine inflammation.

Osthole promotes endogenous neural stem cell proliferation and improved neurological function through Notch signaling pathway in mice acute mechanical brain injury

Mechanical brain injury (MBI) is a common neurotrosis disorder of the central nervous system (CNS), which has a higher mortality and disability. In the case of MBI, neurons death leads to loss of nerve function. To date, there was no satisfactory way to restore neural deficits caused by MBI. Endogenous neural stem cells (NSCs) can proliferate, differentiate and migrate to the lesions after brain injury, to replace and repair the damaged neural cells in the subventricular zone (SVZ), hippocampus and the regions of brain injury. In the present study, we first prepared a mouse model of cortical stab wound brain injury. Using the immunohistochemical and hematoxylin-eosin (H&E) staining method, we demonstrated that osthole (Ost), a natural coumarin derivative, was capable of promoting the proliferation of endogenous NSCs and improving neuronal restoration. Then, using the Morris water maze (MWM) test, we revealed that Ost significantly improved the learning and memory function in the MBI mice, increased the number of neurons in the regions of brain injury, hippocampus DG and CA3 regions. Additionally, we found that Ost up-regulated the expression of self-renewal genes Notch 1 and Hes 1. However, when Notch activity was blocked by the γ-secretase inhibitor DAPT, the expression of Notch 1 and Hes 1 mRNA was down-regulated, augmentation of NICD and Hes 1 protein was ameliorated, the proliferation-inducing effect of Ost was abolished. These results suggested that the effects of Ost were at least in part mediated by activation of Notch signaling pathway. Our findings support that Ost is a potential drug for treating MBI due to its neuronal restoration.

Axon regeneration impediment: the role of paired immunoglobulin-like receptor B

Regenerative capacity is weak after central nervous system injury because of the absence of an enhancing microenvironment and presence of an inhibitory microenvironment for neuronal and axonal repair. In addition to the Nogo receptor (NgR), the paired immunoglobulin-like receptor B (PirB) is a recently discovered coreceptor of Nogo, myelin-associated glycoprotein, and myelin oligodendrocyte glycoprotein. Concurrent blocking of NgR and PirB almost completely eliminates the inhibitory effect of myelin-associated inhibitory molecules on axonal regeneration. PirB participates in a key pathological process of the nervous system, specifically axonal regeneration inhibition. PirB is an inhibitory receptor similar to NgR, but their effects are not identical. This study summarizes the structure, distribution, relationship with common nervous system diseases, and known mechanisms of PirB, and concludes that PirB is also distributed in cells of the immune and hematopoietic systems. Further investigations are needed to determine if immunomodulation and blood cell migration involve inhibition of axonal regeneration.

Notch signaling in response to excitotoxicity induces neurodegeneration via erroneous cell cycle reentry

Neurological disorders such as Alzheimer's disease, stroke and epilepsy are currently marred by the lack of effective treatments to prevent neuronal death. Erroneous cell cycle reentry (CCR) is hypothesized to have a causative role in neurodegeneration. We show that forcing S-phase reentry in cultured hippocampal neurons is sufficient to induce neurodegeneration. We found that kainic-acid treatment in vivo induces erroneous CCR and neuronal death through a Notch-dependent mechanism. Ablating Notch signaling in neurons provides neuroprotection against kainic acid-induced neuronal death. We further show that kainic-acid treatment activates Notch signaling, which increases the bioavailability of CyclinD1 through Akt/GSK3β pathway, leading to aberrant CCR via activation of CyclinD1-Rb-E2F1 axis. In addition, pharmacological blockade of this pathway at critical steps is sufficient to confer resistance to kainic acid-induced neurotoxicity in mice. Taken together, our results demonstrate that excitotoxicity leads to neuronal death in a Notch-dependent manner through erroneous CCR.

Exploring the potential relationship between Notch pathway genes expression and their promoter methylation in mice hippocampal neurogenesis

The Notch pathway is a highly conserved pathway that regulates hippocampal neurogenesis during embryonic development and adulthood. It has become apparent that intracellular epigenetic modification including DNA methylation is deeply involved in fate specification of neural stem cells (NSCs). However, it is still unclear whether the Notch pathway regulates hippocampal neurogenesis by changing the Notch genes' DNA methylation status. Here, we present the evidence from DNA methylation profiling of Notch1, Hes1 and Ngn2 promoters during neurogenesis in the dentate gyrus (DG) of postnatal, adult and traumatic brains. We observed the expression of Notch1, Hes1 and Ngn2 in hippocampal DG with qPCR, Western blot and immunofluorescence staining. In addition, we investigated the methylation status of Notch pathway genes using the bisulfite sequencing PCR (BSP) method. The number of Notch1 or Hes1 (+) and BrdU (+) cells decreased in the subgranular zone (SGZ) of the DG in the hippocampus following TBI. Nevertheless, the number of Ngn2-positive cells in the DG of injured mice was markedly higher than in the DG of non-TBI mice. Accordingly, the DNA methylation level of the three gene promoters changed with their expression in the DG. These findings suggest that the strict spatio-temporal expression of Notch effector genes plays an important role during hippocampal neurogenesis and suggests the possibility that Notch1, Hes1 and Ngn2 were regulated by changing some specific CpG sites of their promoters to further orchestrate neurogenesis in vivo.

Neuroprotective effects of crocin against traumatic brain injury in mice: Involvement of notch signaling pathway

This study investigated the protective effects and mechanisms of crocin, an extract of saffron, on brain damage after traumatic brain injury (TBI) in mice. C57BL/6 mice were subjected to controlled cortical impact (CCI)-induced TBI. Pretreatment with crocin (20mg/kg) had protective effects against TBI, demonstrated by improved neurological severity score (NSS) and brain edema, decreased microglial activation and release of several pro-inflammatory cytokines, and decreased cell apoptosis. TBI activated Notch signaling, as shown by upregulated levels of Notch intracellular domain (NICD) and Hes1 mRNA, and pretreatment with crocin further increased Notch activation. However, pretreatment with DAPT (100mg/kg), a gamma-secretase inhibitor, significantly suppressed crocin-induced activation of Notch signaling and attenuated the ability of crocin to protect mice against TBI-induced inflammation and apoptosis. Therefore, these results suggest that crocin has neuroprotective effects against TBI in mice, and these effects are at least partially dependent on activation of Notch signaling.

Electroacupuncture pretreatment induces tolerance against focal cerebral ischemia through activation of canonical Notch pathway

Background

Electroacupuncture (EA) pretreatment can induce the tolerance against focal cerebral ischemia. However, the underlying mechanisms have not been fully understood. Emerging evidences suggest that canonical Notch signaling may be involved in ischemic brain injury. In the present study, we tested the hypothesis that EA pretreatment-induced tolerance against focal cerebral ischemia is mediated by Notch signaling.

Results

EA pretreatment significantly enhanced Notch1, Notch4 and Jag1 gene transcriptions in the striatum, except Notch1 intracellular domain level, which could be increased evidently by ischemia. After ischemia and reperfusion, Hes1 mRNA and Notch1 intracellular domain level in ischemic striatum in EA pretreatment group were increased and reached the peak at 2 h and 24 h, respectively, which were both earlier than the peak achieved in control group. Intraventricular injection with the γ-secretase inhibitor MW167 attenuated the neuroprotective effect of EA pretreatment.

Conclusions

EA pretreatment induces the tolerance against focal cerebral ischemia through activation of canonical Notch pathway.

Controlling the stem cell compartment and regeneration in vivo: the role of pluripotency pathways

Since the realization that embryonic stem cells are maintained in a pluripotent state through the interplay of a number of key signal transduction pathways, it is becoming increasingly clear that stemness and pluripotency are defined by the complex molecular convergence of these pathways. Perhaps this has most clearly been demonstrated by the capacity to induce pluripotency in differentiated cell types, so termed iPS cells. We are therefore building an understanding of how cells may be maintained in a pluripotent state, and how we may manipulate cells to drive them between committed and pluripotent compartments. However, it is less clear how cells normally pass in and out of the stem cell compartment under normal and diseased physiological states in vivo, and indeed, how important these pathways are in these settings. It is also clear that there is a potential "dark side" to manipulating the stem cell compartment, as deregulation of somatic stem cells is being increasingly implicated in carcinogenesis and the generation of "cancer stem cells." This review explores these relationships, with a particular focus on the role played by key molecular regulators of stemness in tissue repair, and the possibility that a better understanding of this control may open the door to novel repair strategies in vivo. The successful development of such strategies has the potential to replace or augment intervention-based strategies (cell replacement therapies), although it is clear they must be developed with a full understanding of how such approaches might also influence tumorigenesis.

ADAM-17/tumor necrosis factor-α-converting enzyme inhibits neurogenesis and promotes gliogenesis from neural stem cells

Neural precursor cells (NPCs) are activated in central nervous system injury. However, despite being multipotential, their progeny differentiates into astrocytes rather than neurons in situ. We have investigated the role of epidermal growth factor receptor (EGFR) in the generation of non-neurogenic conditions. Cultured mouse subventricular zone NPCs exposed to differentiating conditions for 4 days generated approximately 50% astrocytes and 30% neuroblasts. Inhibition of EGFR with 4-(3-chloroanilino)-6,7-dimethoxyquinazoline significantly increased the number of neuroblasts and decreased that of astrocytes. The same effects were observed upon treatment with the metalloprotease inhibitor galardin, N-[(2R)-2-(hydroxamidocarbonylmethyl)-4-methylpentanoyl]-L-tryptophan methylamide (GM 6001), which prevented endogenous transforming growth factor-α (TGF-α) release. These results suggested that metalloprotease-dependent EGFR-ligand shedding maintained EGFR activation and favored gliogenesis over neurogenesis. Using a disintegrin and metalloprotease 17 (ADAM-17) small interference RNAs transfection of NPCs, ADAM-17 was identified as the metalloprotease involved in cell differentiation in these cultures. In vivo experiments revealed a significant upregulation of ADAM-17 mRNA and de novo expression of ADAM-17 protein in areas of cortical injury in adult mice. Local NPCs, identified by nestin staining, expressed high levels of ADAM-17, as well as TGF-α and EGFR, the three molecules necessary to prevent neurogenesis and promote glial differentiation in vitro. Chronic local infusions of GM6001 resulted in a notable increase in the number of neuroblasts around the lesion. These results indicate that, in vivo, the activation of a metalloprotease, most probably ADAM-17, initiates EGFR-ligand shedding and EGFR activation in an autocrine manner, preventing the generation of new neurons from NPCs. Inhibition of ADAM-17, the limiting step in this sequence, may contribute to the generation of neurogenic niches in areas of brain damage.

Neuronal regeneration in a zebrafish model of adult brain injury

Neural stem cells in the subventricular zone (SVZ) of the adult mammalian forebrain are a potential source of neurons for neural tissue repair after brain insults such as ischemic stroke and traumatic brain injury (TBI). Recent studies show that neurogenesis in the ventricular zone (VZ) of the adult zebrafish telencephalon has features in common with neurogenesis in the adult mammalian SVZ. Here, we established a zebrafish model to study injury-induced neurogenesis in the adult brain. We show that the adult zebrafish brain possesses a remarkable capacity for neuronal regeneration. Telencephalon injury prompted the proliferation of neuronal precursor cells (NPCs) in the VZ of the injured hemisphere, compared with in the contralateral hemisphere. The distribution of NPCs, viewed by BrdU labeling and ngn1-promoter-driven GFP, suggested that they migrated laterally and reached the injury site via the subpallium and pallium. The number of NPCs reaching the injury site significantly decreased when the fish were treated with an inhibitor of γ-secretase, a component of the Notch signaling pathway, suggesting that injury-induced neurogenesis mechanisms are at least partly conserved between fish and mammals. The injury-induced NPCs differentiated into mature neurons in the regions surrounding the injury site within a week after the injury. Most of these cells expressed T-box brain protein (Tbr1), suggesting they had adopted the normal neuronal fate in this region. These results suggest that the telencephalic VZ contributes to neural tissue recovery following telencephalic injury in the adult zebrafish, and that the adult zebrafish is a useful model for regenerative medicine.

Notch signaling proteins HES-1 and Hey-1 bind to insulin degrading enzyme (IDE) proximal promoter and repress its transcription and activity: implications for cellular Aβ metabolism

Cerebral amyloid β (Aβ) accumulation is pathogenically associated with sporadic Alzheimer's disease (SAD). BACE-1 is involved in Aβ generation while insulin-degrading enzyme (IDE) partakes in Aβ proteolytic clearance. Vulnerable regions in AD brains show increased BACE-1 protein levels and enzymatic activity while the opposite occurs with IDE. Another common feature in SAD brains is Notch1 overexpression. Here we demonstrate an increase in mRNA levels of Hey-1, a Notch target gene, and a decrease of IDE transcripts in the hippocampus of SAD brains as compared to controls. Transient transfection of Notch intracellular domain (NICD) in N2aSW cells, mouse neuroblastoma cells (N2a) stably expressing human amyloid precursor protein (APP) Swedish mutation, reduce IDE mRNA levels, promoting extracellular Aβ accumulation. Also, NICD, HES-1 and Hey-1 overexpression result in decreased IDE proximal promoter activity. This effect was mediated by 2 functional sites located at -379/-372 and -310-303 from the first translation start site in the -575/-19 (556 bp) fragment of IDE proximal promoter. By site-directed mutagenesis of the IDE promoter region we reverted the inhibitory effect mediated by NICD transfection suggesting that these sites are indeed responsible for the Notch-mediated inhibition of the IDE gene expression. Intracranial injection of the Notch ligand JAG-1 in Tg2576 mice, expressing the Swedish mutation in human APP, induced overexpression of HES-1 and Hey-1 and reduction of IDE mRNA levels, respectively. Our results support our theory that a Notch-dependent IDE transcriptional modulation may impact on Aβ metabolism providing a functional link between Notch signaling and the amyloidogenic pathway in SAD.