Absence of the transcription factor E2F1 attenuates brain injury and improves behavior after focal ischemia in mice

TSG-6-Mediated Extracellular Matrix Modifications Regulate Hypoxic-Ischemic Brain Injury

Proteoglycans containing link domains modify the extracellular matrix (ECM) to regulate cellular homeostasis and can also sensitize tissues/organs to injury and stress. Hypoxic-ischemic (H-I) injury disrupts cellular homeostasis by activating inflammation and attenuating regeneration and repair pathways. In the brain, the main component of the ECM is the glycosaminoglycan hyaluronic acid (HA), but whether HA modifications of the ECM regulate cellular homeostasis and response to H-I injury is not known. In this report, employing both male and female mice, we demonstrate that link-domain-containing proteoglycan, TNFα-stimulated gene-6 (TSG-6), is active in the brain from birth onward and differentially modifies ECM HA during discrete neurodevelopmental windows. ECM HA modification by TSG-6 enables it to serve as a developmental switch to regulate the activity of the Hippo pathway effector protein, yes-associated protein 1 (YAP1), in the maturing brain and in response to H-I injury. Mice that lack TSG-6 expression display dysregulated expression of YAP1 targets, excitatory amino acid transporter 1 (EAAT1; glutamate-aspartate transporter) and 2 (EAAT2; glutamate transporter-1). Dysregulation of YAP1 activation in TSG-6-/- mice coincides with age- and sex-dependent sensitization of the brain to H-I injury such that 1-week-old neonates display an anti-inflammatory response in contrast to an enhanced proinflammatory injury reaction in 3-month-old adult males but not females. Our findings thus support that a key regulator of age- and sex-dependent H-I injury response in the mouse brain is modulation of the Hippo-YAP1 pathway by TSG-6-dependent ECM modifications.

Insight into the transcription factors regulating Ischemic Stroke and Glioma in Response to Shared Stimuli

Cerebral ischemic stroke and glioma are the two leading causes of patient mortality globally. Despite physiological variations, 1 in 10 people who have an ischemic stroke go on to develop brain cancer, most notably gliomas. In addition, glioma treatments have also been shown to increase the risk of ischemic strokes. Stroke occurs more frequently in cancer patients than in the general population, according to traditional literature. Unbelievably, these events share multiple pathways, but the precise mechanism underlying their co-occurrence remains unknown. Transcription factors (TFs), the main components of gene expression programmes, finally determine the fate of cells and homeostasis. Both ischemic stroke and glioma exhibit aberrant expression of a large number of TFs, which are strongly linked to the pathophysiology and progression of both diseases. The precise genomic binding locations of TFs and how TF binding ultimately relates to transcriptional regulation remain elusive despite a strong interest in understanding how TFs regulate gene expression in both stroke and glioma. As a result, the importance of continuing efforts to understand TF-mediated gene regulation is highlighted in this review, along with some of the primary shared events in stroke and glioma.

Tissue-specific response of the RB-E2F1 complex during mammalian hibernation

Metabolic rate depression during prolonged bouts of torpor is characteristic of mammalian hibernation, reducing energy expenditures over the winter. Cell cycle arrest is observed in quiescent cells during dormancy, partly due to the retinoblastoma (Rb) protein at G₁ /S, given cell division and proliferation are metabolic-costly processes. Rb binds to E2F transcription factors and recruits corepressors (e.g., SUV39H1) to E2F target genes, blocking their transcription and cell cycle passage. Phosphorylation by cyclin-CDK complexes at S780 or S795 abolishes Rb-mediated repression, allowing transition into S phase. The present study compares Rb-E2F1 responses between euthermic and torpid states in five organs (brain, heart, kidney, liver, skeletal muscle) of 13-lined ground squirrels (Ictidomys tridecemlineatus). Immunoblotting assessed the expression of Rb, pRb (S780, S795), E2F1, and SUV39H1. Our findings demonstrate multi-tissue upregulation of Rb and SUV39H1 during torpor, with tissue-specific changes to E2F1 and pRb (S780), suggesting Rb-E2F1 contributes to cell cycle control in hibernation.

E2F1 Expression and Apoptosis Initiation in Crayfish and Rat Peripheral Neurons and Glial Cells after Axonal Injury

Neurotrauma is among the main causes of human disability and mortality. The transcription factor E2F1 is one of the key proteins that determine the fate of cells. The involvement of E2F1 in the regulation of survival and death of peripheral nerve cells after axotomy has not been previously studied. We, for the first time, studied axotomy-induced changes in the expression and localization of E2F1 following axonal injury in rats and crayfish. Immunoblotting and immunofluorescence microscopy were used for the analysis of the expression and intracellular localization of E2F1 and its changes after axotomy. To evaluate whether this transcription factor promotes cell apoptosis, we examined the effect of pharmacological inhibition of E2F activity in axotomized rat models. In this work, axotomy caused increased expression of E2F1 as early as 4 h and even 1 h after axotomy of mechanoreceptor neurons and ganglia of crayfish ventral nerve cord (VNC), as well as rat dorsal root ganglia (DRG). The level of E2F1 expression increased both in the cytoplasm and the nuclei of neurons. Pharmacological inhibition of E2F demonstrated a pronounced neuroprotective activity against axotomized DRGs. E2F1 and downstream targets could be considered promising molecular targets for the development of potential neuroprotective agents.

p53-mediated neurodegeneration in the absence of the nuclear protein Akirin2

Proper gene regulation is critical for both neuronal development and maintenance as the brain matures. We previously demonstrated that Akirin2, an essential nuclear protein that interacts with transcription factors and chromatin remodeling complexes, is required for the embryonic formation of the cerebral cortex. Here we show that Akirin2 plays a mechanistically distinct role in maintaining healthy neurons during cortical maturation. Restricting Akirin2 loss to excitatory cortical neurons resulted in progressive neurodegeneration via necroptosis and severe cortical atrophy with age. Comparing transcriptomes from Akirin2-null postnatal neurons and cortical progenitors revealed that targets of the tumor suppressor p53, a regulator of both proliferation and cell death encoded by Trp53, were consistently upregulated. Reduction of Trp53 rescued neurodegeneration in Akirin2-null neurons. These data: (1) implicate Akirin2 as a critical neuronal maintenance protein, (2) identify p53 pathways as mediators of Akirin2 functions, and (3) suggest Akirin2 dysfunction may be relevant to neurodegenerative diseases.

Sevoflurane Offers Neuroprotection in a Cerebral Ischemia/Reperfusion Injury Rat Model Through the E2F1/EZH2/TIMP2 Regulatory Axis

Cerebral ischemia/reperfusion (I/R) injury contributes considerably to the poor prognosis in patients with ischemic stroke. This study is aimed to delineate the molecular mechanistic actions by which sevoflurane protects against cerebral I/R injury. A rat model of cerebral I/R injury was established and pre-treated with sevoflurane, in which hippocampal neuron apoptosis was found to be repressed and the level of E2F transcription factor 1 (E2F1) was observed to be down-regulated. Then, the up-regulated expression of E2F1 was validated in rats with cerebral I/R injury, responsible for stimulated neuron apoptosis. Further, the binding of E2F1 to enhancer of zeste homolog 2 (EZH2) and EZH2 to tissue inhibitor of metalloproteinases-2 (TIMP2) was identified. The stimulative effect of the E2F1/EZH2/TIMP2 regulatory axis on neuron apoptosis was subsequently demonstrated through functional assays. After that, it was substantiated in vivo that sevoflurane suppressed the apoptosis of hippocampal neurons in rats with cerebral I/R injury by down-regulating E2F1 to activate the EZH2/TIMP2 axis. Taken together, our data elucidated that sevoflurane reduced neuron apoptosis through mediating the E2F1/EZH2/TIMP2 regulatory axis, thus protecting rats against cerebral I/R injury.

Role of Non-Coding RNA of Human Platelet in Cardiovascular Disease

Cardiovascular diseases (CVD) are the major cause of death in the world. Numerous genetic studies involving transcriptomic approaches aimed at the detailed understanding of the disease and the development of new therapeutic strategies have been conducted over recent years. There has been an increase in research on platelets, which are implicated in CVD due to their capacity to release regulatory molecules that affect various pathways. Platelets secrete over 500 various kinds of molecules to plasma including large amounts of non-coding (nc) RNA (miRNA, lncRNA or circRNA). These ncRNA correspond to 98% of transcripts that are not translated into proteins as they are important regulators in physiology and disease. Thus, miRNAs can direct protein complexes to mRNAs through base-pairing interactions, thus causing translation blockage or/and transcript degradation. The lncRNAs act via different mechanisms by binding to transcription factors. Finally, circRNAs act as regulators of miRNAs, interfering with their action. Alteration in the repertoire and/or the amount of the platelet-secreted ncRNA can trigger CVD as well as other diseases. NcRNAs can serve as effective biomarkers for the disease or as therapeutic targets due to their disease involvement. In this review, we will focus on the most important ncRNAs that are secreted by platelets (9 miRNA, 9 lncRNA and 5 circRNA), their association with CVD, and the contribution of these ncRNA to CVD risk to better understand the relation between ncRNA of human platelet and CVD.

ZEB2, interacting with MDM2, contributes to the dysfuntion of brain microvascular endothelial cells and brain injury after intracerebral hemorrhage

ZEB2 has been shown to be upregulated in the brain tissues of rats with intracerebral hemorrhage (ICH), but its role in ICH-caused brain injury remains unclear. Here, an ICH rat model was established via intracerebral injection of autologous blood, and the lentivirus-mediated ZEB2 short hairpin RNA (sh-ZEB2) or negative control (scramble) were administered 0.5 hours after ICH. Silencing ZEB2 alleviated ICH-induced neurologic deficits and the increase of BBB permeability, brain water content and ZEB2 expression. Next, OGD (oxygen glucose deprivation) plus hemin was used to treat primary brain microvascular endothelial cells (BMECs) to simulate the ICH condition in vitro. OGD plus hemin upregulated ZEB2 expression and apoptosis, but reduced cell viability, migration, TEER (transendothelial electric resistance) and the expression of vascular-endothelial (VE-) cadherin, occludin and claudin-5, which was reversed by inhibiting ZEB2. Mechanism researches showed that ZEB2 interacted with MDM2 to up-regulate MDM2 protein expression, and then increased E2F1 protein level by suppressing its ubiquitination, which in turn promoted the transcription of ZEB2 to induce its protein expression, so as to enhance the interaction between ZEB2 and MDM2, thereby contributing to OGD plus hemin-induced endothelial dysfunction. Additionally, the joint interference of ZEB2 and MDM2 in vivo had better mitigative effects on ICH-induced brain injury compared with silencing ZEB2 alone. In summary, ZEB2 interacted with MDM2 to promote BMEC dysfunction and brain damage after ICH.

Changes in Cyclin D1, cdk4, and Their Associated Molecules in Ischemic Pyramidal Neurons in Gerbil Hippocampus after Transient Ischemia and Neuroprotective Effects of Ischemic Preconditioning by Keeping the Molecules in the Ischemic Neurons

Simple Summary

Cyclin D1 and cyclin-dependent kinase 4 (cdk4) is implicated in neuronal death induced by various pathological conditions. Ischemic preconditioning (IPC) confers neuroprotective effect, but underlying mechanisms have been poorly addressed. In this study, IPC protected pyramidal neurons (cells) in gerbil hippocampus after transient ischemia. Additionally, IPC controlled expressions of cyclin D1, cdk4, phosphorylated retinoblastoma (p-Rb), and E2 promoter binding factor 1 (E2F1). In particular, the expression of p16INK4a was not different by IPC. These findings indicate that cyclin D1/cdk4-related signals may play important roles in events in neurons related to damage/death following ischemic insults. Especially, the preservation of p16INK4a by IPC may be crucial in attenuating neuronal death/damage or protecting neurons after brain ischemic insults.

Abstract

Inadequate activation of cell cycle proteins including cyclin D1 and cdk4 is involved in neuronal cell death induced by diverse pathological stresses, including transient global brain ischemia. The neuroprotective effect of ischemic preconditioning is well-established, but the underlying mechanism is still unknown. In this study, we examined changes in cyclin D1, cdk4, and related molecules in cells or neurons located in Cornu Ammonis 1 (CA1) of gerbil hippocampus after transient ischemia for 5 min (ischemia and reperfusion) and investigated the effects of IPC on these molecules after ischemia. Four groups were used in this study as follows: sham group, ischemia group, IPC plus (+) sham group, and IPC+ischemia group. IPC was developed by inducing 2-min ischemia at 24 h before 5-min ischemia (real ischemia). Most pyramidal cells located in CA1 of the ischemia group died five days after ischemia. CA1 pyramidal cells in the IPC+ischemia group were protected. In the ischemia group, the expressions of cyclin D1, cdk4, phosphorylated retinoblastoma (p-Rb), and E2F1 (a transcription factor regulated by p-Rb) were significantly altered in the pyramidal cells with time after ischemia; in the IPC+ischemia group, they were controlled at the level shown in the sham group. In particular, the expression of p16^INK4a (an endogenous cdk inhibitor) in the ischemia group was reversely altered in the pyramidal cells; in the IPC+TI group, the expression of p16^INK4a was not different from that shown in the sham group. Our current results indicate that cyclin D1/cdk4-related signals may have important roles in events in neurons related to damage/death following ischemia and reperfusion. In particular, the preservation of p16^INK4a by IPC may be crucial in attenuating neuronal death/damage or protecting neurons after brain ischemic insults.

The Role of Post-Translational Acetylation and Deacetylation of Signaling Proteins and Transcription Factors after Cerebral Ischemia: Facts and Hypotheses

Histone deacetylase (HDAC) and histone acetyltransferase (HAT) regulate transcription and the most important functions of cells by acetylating/deacetylating histones and non-histone proteins. These proteins are involved in cell survival and death, replication, DNA repair, the cell cycle, and cell responses to stress and aging. HDAC/HAT balance in cells affects gene expression and cell signaling. There are very few studies on the effects of stroke on non-histone protein acetylation/deacetylation in brain cells. HDAC inhibitors have been shown to be effective in protecting the brain from ischemic damage. However, the role of different HDAC isoforms in the survival and death of brain cells after stroke is still controversial. HAT/HDAC activity depends on the acetylation site and the acetylation/deacetylation of the main proteins (c-Myc, E2F1, p53, ERK1/2, Akt) considered in this review, that are involved in the regulation of cell fate decisions. Our review aims to analyze the possible role of the acetylation/deacetylation of transcription factors and signaling proteins involved in the regulation of survival and death in cerebral ischemia.

Distinct peripheral blood monocyte and neutrophil transcriptional programs following intracerebral hemorrhage and different etiologies of ischemic stroke

Understanding cell-specific transcriptome responses following intracerebral hemorrhage (ICH) and ischemic stroke (IS) will improve knowledge of the immune response to brain injury. Transcriptomic profiles of 141 samples from 48 subjects with ICH, different IS etiologies, and vascular risk factor controls were characterized using RNA-seq in isolated neutrophils, monocytes and whole blood. In both IS and ICH, monocyte genes were down-regulated, whereas neutrophil gene expression changes were generally up-regulated. The monocyte down-regulated response to ICH included innate, adaptive immune, dendritic, NK cell and atherosclerosis signaling. Neutrophil responses to ICH included tRNA charging, mitochondrial dysfunction, and ER stress pathways. Common monocyte and neutrophil responses to ICH included interferon signaling, neuroinflammation, death receptor signaling, and NFAT pathways. Suppressed monocyte responses to IS included interferon and dendritic cell maturation signaling, phagosome formation, and IL-15 signaling. Activated neutrophil responses to IS included oxidative phosphorylation, mTOR, BMP, growth factor signaling, and calpain proteases-mediated blood-brain barrier (BBB) dysfunction. Common monocyte and neutrophil responses to IS included JAK1, JAK3, STAT3, and thrombopoietin signaling. Cell-type and cause-specific approaches will assist the search for future IS and ICH biomarkers and treatments.

Apoptosis regulation in the penumbra after ischemic stroke: expression of pro- and antiapoptotic proteins

Ischemic stroke is the leading cause of human disability and mortality in the world. The main problem in stroke therapy is the search of efficient neuroprotector capable to rescue neurons in the potentially salvageable transition zone (penumbra), which is expanding after brain damage. The data on molecular mechanisms of penumbra formation and expression of diverse signaling proteins in the penumbra during first 24 h after ischemic stroke are discussed. Two basic features of cell death regulation in the ischemic penumbra were observed: (1) both apoptotic and anti-apoptotic proteins are simultaneously over-expressed in the penumbra, so that the fate of individual cells is determined by the balance between these opposite tendencies. (2) Similtaneous and concerted up-regulation in the ischemic penumbra of proteins that execute apoptosis (caspases 3, 6, 7; Bcl-10, SMAC/DIABLO, AIF, PSR), signaling proteins that regulate different apoptosis pathways (p38, JNK, DYRK1A, neurotrophin receptor p75); transcription factors that control expression of various apoptosis regulation proteins (E2F1, p53, c-Myc, GADD153); and proteins, which are normally involved in diverse cellular functions, but stimulate apoptosis in specific situations (NMDAR2a, Par4, GAD65/67, caspase 11). Hence, diverse apoptosis initiation and regulation pathways are induced simultaneously in penumbra from very different initial positions. Similarly, various anti-apoptotic proteins (Bcl-x, p21/WAF-1, MDM2, p63, PKBα, ERK1, RAF1, ERK5, MAKAPK2, protein phosphatases 1α and MKP-1, estrogen and EGF receptors, calmodulin, CaMKII, CaMKIV) are upregulated. These data provide an integral view of neurodegeneration and neuroprotection in penumbra. Some discussed proteins may serve as potential targets for anti-stroke therapy.

Transcription Factor E2F1 Aggravates Neurological Injury in Ischemic Stroke via microRNA-122-Targeted Sprouty2

BACKGROUND

It has been documented that microRNAs (miRs) assume a pivotal role in the development of ischemic stroke (IS). However, it remains poorly identified about the role of miR-122 in IS. Herein, this study was intended to explore the mechanism of E2F1-orchestrated miR-122 in IS.

PATIENTS AND METHODS

E2F1, miR-122, and SPRY2 expression in serum from patients with IS and oxygen-glucose deprivation (OGD)-treated N2a cells was detected by RT-qPCR. After gain- and loss-of-function approaches in OGD-induced N2a cells, GAFP staining, flow cytometry, and Western blot analysis were adopted to assess neuronal viability, cell cycle and apoptosis, and expression of apoptosis- and autophagy-related proteins, respectively. Meanwhile, mice with IS were induced, in which E2F1, miR-122, and SPRY2 were overexpressed, followed by evaluation of neurological deficit and cerebral infarction area. The MAPK pathway activity in tissues of mice and cells was determined.

RESULTS

miR-122 was down-regulated, and E2F1 and SPRY2 were up-regulated in IS patients and OGD-induced N2a cells. E2F1 inhibited miR-122 transcription, while miR-122 targeted SPRY2. Overexpression (OE) of miR-122 or down-regulation of E2F1 or SPRY2 increased viability, but decreased apoptosis, cell cycle arrest, and autophagy in OGD-induced N2a cells. In IS mice, the neurological deficit score and cerebral infarction area were elevated, which was aggravated by up-regulating E2F1 or SPRY2 but attenuated by overexpressing miR-122. E2F1/miR-122/SPRY2 axis mediated the MAPK pathway in vivo and in vitro.

CONCLUSION

Collectively, E2F1 reduced miR-122 transcription to up-regulate SPRY2, which inactivated MAPK pathway and promoted neurological deficit in IS.

PTEN Inhibition Protects Against Experimental Intracerebral Hemorrhage-Induced Brain Injury Through PTEN/E2F1/β-Catenin Pathway

Intracerebral hemorrhage (ICH) is a subtype of stroke with highest mortality and morbidity. We have previously demonstrated that dipotassium bisperoxo (picolinato) oxovanadate (V), (bpV[pic]) inhibits phosphatase and tensin homolog (PTEN) and activates extracellular signal-regulated kinase (ERK)1/2. In this study, we examined the effect of bpV[pic] in the rat ICH model in vivo and the hemin-induced injury model in rat cortical cultures. The rat model of ICH was created by injecting autologous blood into the striatum, and bpV[pic] was intraperitoneally injected. The effects of bpV[pic] were evaluated by neurological tests, Fluoro-Jade C (FJC) staining, and Nissl staining. We demonstrate that bpV[pic] attenuates ICH-induced brain injury in vivo and hemin-induced neuron injury in vitro. The expression of E2F1 was increased, but β-catenin expression was decreased after ICH, and the altered expressions of E2F1 and β-catenin after ICH were blocked by bpV[pic] treatment. Our results further show that bpV[pic] increases β-catenin expression through downregulating E2F1 in cortical neurons and prevents hemin-induced neuronal damage through E2F1 downregulation and subsequent upregulation of β-catenin. By testing the effect of PTEN-siRNA, PTEN cDNA, or combined use of ERK1/2 inhibitor and bpV[pic] in cultured cortical neurons after hemin-induced injury, we provide evidence suggesting that PTEN inhibition by bpV[pic] confers neuroprotection through E2F1 and β-catenin pathway, but the neuroprotective role of ERK1/2 activation by bpV[pic] cannot be excluded.

The pro-death role of Cited2 in stroke is regulated by E2F1/4 transcription factors

We previously reported that the cell cycle-related cyclin-dependent kinase 4-retinoblastoma (RB) transcriptional corepressor pathway is essential for stroke-induced cell death both in vitro and in vivo However, how this signaling pathway induces cell death is unclear. Previously, we found that the cyclin-dependent kinase 4 pathway activates the pro-apoptotic transcriptional co-regulator Cited2 in vitro after DNA damage. In the present study, we report that Cited2 protein expression is also dramatically increased following stroke/ischemic insult. Critically, utilizing conditional knockout mice, we show that Cited2 is required for neuronal cell death, both in culture and in mice after ischemic insult. Importantly, determining the mechanism by which Cited2 levels are regulated, we found that E2F transcription factor (E2F) family members participate in Cited2 regulation. First, E2F1 expression induced Cited2 transcription, and E2F1 deficiency reduced Cited2 expression. Moreover, determining the potential E2F-binding regions on the Cited2 gene regulatory sequence by ChIP analysis, we provide evidence that E2F1/4 proteins bind to this DNA region. A luciferase reporter assay to probe the functional outcomes of this interaction revealed that E2F1 activates and E2F4 inhibits Cited2 transcription. Moreover, we identified the functional binding motif for E2F1 in the Cited2 gene promoter by demonstrating that mutation of this site dramatically reduces E2F1-mediated Cited2 transcription. Finally, E2F1 and E2F4 regulated Cited2 expression in neurons after stroke-related insults. Taken together, these results indicate that the E2F-Cited2 regulatory pathway is critically involved in stroke injury.

Comparing effects of CDK inhibition and E2F1/2 ablation on neuronal cell death pathways in vitro and after traumatic brain injury

Traumatic brain injury (TBI) activates multiple neuronal cell death mechanisms, leading to post-traumatic neuronal loss and neurological deficits. TBI-induced cell cycle activation (CCA) in post-mitotic neurons causes regulated cell death involving cyclin-dependent kinase (CDK) activation and initiation of an E2F transcription factor-mediated pro-apoptotic program. Here we examine the mechanisms of CCA-dependent neuronal apoptosis in primary neurons in vitro and in mice exposed to controlled cortical impact (CCI). In contrast to our prior work demonstrating robust neuroprotective effects by CDK inhibitors after TBI, examination of neuronal apoptotic mechanisms in E2F1^−/−/E2F2^−/− or E2F2^−/− transgenic mice following CCI suggests that E2F1 and/or E2F2 likely play only a modest role in neuronal cell loss after brain trauma. To elucidate more critical CCA molecular pathways involved in post-traumatic neuronal cell death, we investigated the neuroprotective effects and mechanisms of the potent CDK inhibitor CR8 in a DNA damage model of cell death in primary cortical neurons. CR8 treatment significantly reduced caspase activation and cleavage of caspase substrates, attenuating neuronal cell death. CR8 neuroprotective effects appeared to reflect inhibition of multiple pathways converging on the mitochondrion, including injury-induced elevation of pro-apoptotic Bcl-2 homology region 3 (BH3)-only proteins Puma and Noxa, thereby attenuating mitochondrial permeabilization and release of cytochrome c and AIF, with reduction of both caspase-dependent and -independent apoptosis. CR8 administration also limited injury-induced deficits in mitochondrial respiration. These neuroprotective effects may be explained by CR8-mediated inhibition of key upstream injury responses, including attenuation of c-Jun phosphorylation/activation as well as inhibition of p53 transactivation of BH3-only targets.

Cdc25A Is a Critical Mediator of Ischemic Neuronal Death In Vitro and In Vivo

Dysregulation of cell cycle machinery is implicated in a number of neuronal death contexts, including stroke. Increasing evidence suggests that cyclin-dependent kinases (Cdks) are inappropriately activated in mature neurons under ischemic stress conditions. We previously demonstrated a functional role for the cyclin D1/Cdk4/pRb (retinoblastoma tumor suppressor protein) pathway in delayed neuronal death induced by ischemia. However, the molecular signals leading to cyclin D/Cdk4/pRb activation following ischemic insult are presently not clear. Here, we investigate the cell division cycle 25 (Cdc25) dual-specificity phosphatases as potential upstream regulators of ischemic neuronal death and Cdk4 activation. We show that a pharmacologic inhibitor of Cdc25 family members (A, B, and C) protects mouse primary neurons from hypoxia-induced delayed death. The major contributor to the death process appears to be Cdc25A. shRNA-mediated knockdown of Cdc25A protects neurons in a delayed model of hypoxia-induced death in vitro Similar results were observed in vivo following global ischemia in the rat. In contrast, neurons singly or doubly deficient for Cdc25B/C were not significantly protective. We show that Cdc25A activity, but not level, is upregulated in vitro following hypoxia and global ischemic insult in vivo Finally, we show that shRNA targeting Cdc25A blocks Ser795 pRb phosphorylation. Overall, our results indicate a role for Cdc25A in delayed neuronal death mediated by ischemia.SIGNIFICANCE STATEMENT A major challenge in stroke is finding an effective neuroprotective strategy to treat cerebral ischemic injury. Cdc25 family member A (Cdc25A) is a phosphatase normally activated during cell division in proliferating cells. We found that Cdc25A is activated in neurons undergoing ischemic stress mediated by hypoxia in vitro and global cerebral ischemia in rats in vivo We show that pharmacologic or genetic inhibition of Cdc25A activity protects neurons from delayed death in vitro and in vivo Downregulation of Cdc25A led to reduction in retinoblastoma tumor suppressor protein (pRb) phosphorylation. An increase in pRb phosphorylation has been previously linked to ischemic neuronal death. Our results identify Cdc25A as a potential target for neuroprotectant strategy for the treatment of delayed ischemic neuronal death.

Ablation of the transcription factors E2F1-2 limits neuroinflammation and associated neurological deficits after contusive spinal cord injury

Traumatic spinal cord injury (SCI) induces cell cycle activation (CCA) that contributes to secondary injury and related functional impairments such as motor deficits and hyperpathia. E2F1 and E2F2 are members of the activator sub-family of E2F transcription factors that play an important role in proliferating cells and in cell cycle-related neuronal death, but no comprehensive study have been performed in SCI to determine the relative importance of these factors. Here we examined the temporal distribution and cell-type specificity of E2F1 and E2F2 expression following mouse SCI, as well as the effects of genetic deletion of E2F1-2 on neuronal cell death, neuroinflammation and associated neurological dysfunction. SCI significantly increased E2F1 and E2F2 expression in active caspase-3(+) neurons/oligodendrocytes as well as in activated microglia/astrocytes. Injury-induced up-regulation of cell cycle-related genes and protein was significantly reduced by intrathecal injection of high specificity E2F decoy oligodeoxynucleotides against the E2F-binding site or in E2F1-2 null mice. Combined E2F1+2 siRNA treatment show greater neuroprotection in vivo than E2F1 or E2F2 single siRNA treatment. Knockout of both E2F1 and E2F2 genes (E2Fdko) significantly reduced neuronal death, neuroinflammation, and tissue damage, as well as limiting motor dysfunction and hyperpathia after SCI. Both CCA reduction and functional improvement in E2Fdko mice were greater than those in E2F2ko model. These studies demonstrate that SCI-induced activation of E2F1-2 mediates CCA, contributing to gliopathy and neuronal/tissue loss associated with motor impairments and post-traumatic hyperesthesia. Thus, E2F1-2 provide a therapeutic target for decreasing secondary tissue damage and promoting recovery of function after SCI.

Regulation of ischemic neuronal death by E2F4-p130 protein complexes

Inappropriate activation of cell cycle proteins, in particular cyclin D/Cdk4, is implicated in neuronal death induced by various pathologic stresses, including DNA damage and ischemia. Key targets of Cdk4 in proliferating cells include members of the E2F transcription factors, which mediate the expression of cell cycle proteins as well as death-inducing genes. However, the presence of multiple E2F family members complicates our understanding of their role in death. We focused on whether E2F4, an E2F member believed to exhibit crucial control over the maintenance of a differentiated state of neurons, may be critical in ischemic neuronal death. We observed that, in contrast to E2F1 and E2F3, which sensitize to death, E2F4 plays a crucial protective role in neuronal death evoked by DNA damage, hypoxia, and global ischemic insult both in vitro and in vivo. E2F4 occupies promoter regions of proapoptotic factors, such as B-Myb, under basal conditions. Following stress exposure, E2F4-p130 complexes are lost rapidly along with the presence of E2F4 at E2F-containing B-Myb promoter sites. In contrast, the presence of E2F1 at B-Myb sites increases with stress. Furthermore, B-Myb and C-Myb expression increases with ischemic insult. Taken together, we propose a model by which E2F4 plays a protective role in neurons from ischemic insult by forming repressive complexes that prevent prodeath factors such as Myb from being expressed.

Neuropilin 1 directly interacts with Fer kinase to mediate semaphorin 3A-induced death of cortical neurons

Neuropilins (NRPs) are receptors for the major chemorepulsive axonal guidance cue semaphorins (Sema). The interaction of Sema3A/NRP1 during development leads to the collapse of growth cones. Here we show that Sema3A also induces death of cultured cortical neurons through NRP1. A specific NRP1 inhibitory peptide ameliorated Sema3A-evoked cortical axonal retraction and neuronal death. Moreover, Sema3A was also involved in cerebral ischemia-induced neuronal death. Expression levels of Sema3A and NRP1, but not NRP2, were significantly increased early during brain reperfusion following transient focal cerebral ischemia. NRP1 inhibitory peptide delivered to the ischemic brain was potently neuroprotective and prevented the loss of motor functions in mice. The integrity of the injected NRP1 inhibitory peptide into the brain remained unchanged, and the intact peptide permeated the ischemic hemisphere of the brain as determined using MALDI-MS-based imaging. Mechanistically, NRP1-mediated axonal collapse and neuronal death is through direct and selective interaction with the cytoplasmic tyrosine kinase Fer. Fer RNA interference effectively attenuated Sema3A-induced neurite retraction and neuronal death in cortical neurons. More importantly, down-regulation of Fer expression using Fer-specific RNA interference attenuated cerebral ischemia-induced brain damage. Together, these studies revealed a previously unknown function of NRP1 in signaling Sema3A-evoked neuronal death through Fer in cortical neurons.

Dynamic Analysis of the Blood-Brain Barrier Disruption in Experimental Stroke Using Time Domain in Vivo Fluorescence Imaging

The blood-brain barrier (BBB) disruption following cerebral ischemia can be exploited to deliver imaging agents and therapeutics into the brain. The aim of this study was (a) to establish novel in vivo optical imaging methods for longitudinal assessment of the BBB disruption and (b) to assess size selectivity and temporal patterns of the BBB disruption after a transient focal ischemia. The BBB permeability was assessed using in vivo time domain near-infrared optical imaging after contrast enhancement with either free Cy5.5 (1 kDa) or Cy5.5 conjugated with bovine serum albumin (BSA) (67 kDa) in mice subjected to either 60- or 20-minute transient middle cerebral artery occlusion (MCAO) and various times of reperfusion (up to 14 days). In vivo imaging observations were corroborated by ex vivo brain imaging and microscopic analyses of fluorescent tracer extravasation. The in vivo optical contrast enhancement with Cy5.5 was spatially larger than that observed with BSA-Cy5.5. Longitudinal studies after a transient 20-minute MCAO suggested a bilateral BBB disruption, more pronounced in the ipsilateral hemisphere, peaking at day 7 and resolving at day 14 after ischemia. The area differential between the BBB disruption for small and large molecules could potentially be useful as a surrogate imaging marker for assessing perinfarct tissues to which neuroprotective therapies of appropriate sizes could be delivered.

Cell cycle activation in striatal neurons from Huntington's disease patients and rats treated with 3-nitropropionic acid

This study was undertaken to investigate the potential role of cell cycle re-entry in an experimental model of Huntington's disease and in human brain samples. We found that after treatment of rats with the mitochondrial neurotoxin 3-nitropropionic acid, the expression of cell cycle markers of G1 phase measured by immunohistochemistry was induced in the striatal brain region. Furthermore, we detected an increase in the nuclear and also cytoplasmatic E2F-1 expression, suggesting that this protein could activate the apoptotic cascade in rat brain. Western blot analysis of post-mortem brain samples from patients also showed an increase in the expression of E2F-1 and cyclin D1 in comparison with control samples. These results indicate that cell cycle re-entry is activated in Huntington's disease and may contribute to the neurodegenerative process.

Cerebral ischemia, cell cycle elements and Cdk5

Stroke is a devastating disorder that significantly contributes to death, disability, and healthcare costs. New therapeutic strategies have been recently focusing on the development of neuroprotective agents that could halt the underlying mechanisms of neuronal death leading to brain damage. Accumulating evidence implicates proteins that are normally involved in the regulation of the cell cycle to neuronal death following ischemic insult, suggesting that these proteins could be suitable targets for stroke therapy. In this brief review, we present in vitro and in vivo arguments linking cell cycle molecules, i.e., cyclins, mitotic cyclin-dependent kinases (Cdks), as well as non-mitotic Cdk5, to ischemic neuronal death. We also report the evaluation of the potential of Cdk inhibitors as neuroprotective strategy for ischemic injury.

Implication of the transcription factor E2F-1 in the modulation of neuronal apoptosis

Neurodegenerative diseases as Alzheimer's disease, Parkinson's disease and other neurological disorders remain major problem worldwide since is currently no effective treatment. Thus, studying the mechanisms involved in neuronal apoptotic pathways is imperative if drugs that might stop or delay these disease processes are to be synthesized. In recent years it has become evident that mitochondria are key component of the neuronal apoptotic route. In addition to mitochondria, other intracellular components have been implicated in this process. Thus, DNA damage and re-entry into the cell cycle may constitute a common pathway in apoptosis in neurological diseases. The implication of cell cycle in neurodegenerative disorders is supported by data on the brain of patients who showed an increase in cell cycle protein expression. Indeed, studies performed in neuronal cell preparations indicate that re-entry into the cell cycle and, more specifically, an increase in the expression of E2F-1 transcription role of DNA damage/repair as a potential mechanism in cell cycle re-entry. In this context, ataxia telangiectasia mutated protein could be the enzyme responsible for neuronal apoptosis activation. Furthermore, the potential routes involved in E2F-1 induced apoptosis, p53-dependent and p53-independent, are similarly reviewed. Under this hypothesis, multiple pathways have been suggested, including the route of caspases. Finally, given the increasing experimental data on the neuroprotective and antiapoptotic effects of cyclin dependent kinase CDK inhibitory drugs, including flavopiridol, their application for the treatment of neurological disorders is proposed.

Cell cycle machinery and stroke

Stroke results from a transient or permanent reduction in blood flow to the brain. The mechanisms involving neuronal death following ischemic insult are complex and not fully understood. One signal which may control ischemic neuronal death is the inappropriate activation of cell cycle regulators including cyclins, cyclin dependent kinases (CDKs) and endogenous cyclin dependent kinase inhibitors (CDKIs). In dividing cells, activation of cell cycle machinery induces cell proliferation. In the context of terminally differentiated-neurons, however, aberrant activation of these elements triggers neuronal death. Indeed, there are several lines of correlative and functional evidence supporting this "cell cycle/neuronal death hypothesis". The objective of this review is to summarize the findings implicating cell cycle machinery in ischemic neuronal death from in vitro and in vivo studies. Importantly, determining and blocking the signaling pathway(s) by which these molecules act to mediate ischemic neuronal death, in conjunction with other targets may provide a viable therapeutic strategy for stroke damage.

Neuropilin-1 is a direct target of the transcription factor E2F1 during cerebral ischemia-induced neuronal death in vivo

The nuclear transcription factor E2F1 plays an important role in modulating neuronal death in response to excitotoxicity and cerebral ischemia. Here, by comparing gene expression in brain cortices from E2F1(+/+) and E2F1(-/-) mice using a custom high-density DNA microarray, we identified a group of putative E2F1 target genes that might be responsible for ischemia-induced E2F1-dependent neuronal death. Neuropilin 1 (NRP-1), a receptor for semaphorin 3A-mediated axon growth cone collapse and retraction, was confirmed to be a direct target of E2F1 based on (i) the fact that the NRP-1 promoter sequence contains an E2F1 binding site, (ii) reactivation of NRP-1 expression in E2F1(-/-) neurons when the E2F1 gene was replaced, (iii) activation of the NRP-1 promoter by E2F1 in a luciferase reporter assay, (iv) electrophoretic mobility gel shift analysis confirmation of the presence of an E2F binding sequence in the NRP-1 promoter, and (v) the fact that a chromatin immunoprecipitation assay showed that E2F1 binds directly to the endogenous NRP-1 promoter. Interestingly, the temporal induction in cerebral ischemia-induced E2F1 binding to the NRP-1 promoter correlated with the temporal-induction profile of NRP-1 mRNA, confirming that E2F1 positively regulates NRP-1 during cerebral ischemia. Functional analysis also showed that NRP-1 receptor expression was extremely low in E2F1(-/-) neurons, which led to the diminished response to semaphorin 3A-induced axonal shortening and neuronal death. An NRP-1 selective peptide inhibitor provided neuroprotection against oxygen-glucose deprivation. Taken together, these findings support a model in which E2F1 targets NRP-1 to modulate axonal damage and neuronal death in response to cerebral ischemia.

Redox stress and the contributions of BH3-only proteins to infarction

Ischemia followed by reperfusion is the primary cause of tissue injury and infarction during heart attack and stroke. The initiating stimulus is believed to involve reactive oxygen species that are produced during reperfusion when electron transport resumes in the mitochondria after suppression by ischemia. Programmed death has been shown to be a significant component of infarction, and evidence indicates that multiple pathways are initiated during both ischemia and reperfusion phases. Major infarction is preceded by severe ischemia that includes hypoxia, intracellular acidosis, glucose depletion, loss of ATP, and elevation of cytoplasmic calcium. The superimposition of a reactive oxygen surge on the latter condition provides the impetus for maximal damage. Compelling evidence implicates mitochondria not only as the source of initiating ROS but also as the focal sensors that translate the redox stress signal into a cellular-death response. Pivotal to this response are the BH3-only proteins that are activated by death signals and regulate mitochondrial communication with executioner proteins in the cytoplasm. The BH3-only proteins do this by controlling the activity of pores and channels in the outer mitochondrial membrane. To date at least six BH3-only proteins have been shown to contribute to ischemia-reperfusion death pathways in heart and/or brain; these include Bnip3, PUMA, Bid, Bad, HGTD-P, and Noxa. Here we review the evidence for these cell-death pathways and discuss their relevance to ischemic disease and infarction.

Calpain-cleaved collapsin response mediator protein-3 induces neuronal death after glutamate toxicity and cerebral ischemia

Collapsin response mediator proteins (CRMPs) mediate growth cone collapse during development, but their roles in adult brains are not clear. Here we report the findings that the full-length CRMP-3 (p63) is a direct target of calpain that cleaves CRMP-3 at the N terminus (+76 amino acid). Interestingly, activated calpain in response to excitotoxicity in vitro and cerebral ischemia in vivo also cleaved CRMP-3, and the cleavage product of CRMP-3 (p54) underwent nuclear translocation during neuronal death. The expression of p54 was colocalized with the terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling-positive nuclei in glutamate-treated cerebellar granule neurons (CGNs) and in ischemic neurons located in the infarct core after focal cerebral ischemia, suggesting that p54 might be involved in neuronal death. Overexpression studies showed that p54, but not p63, caused death of human embryonic kidney cells and CGNs, whereas knock-down CRMP-3 expression by selective small interfering RNA protected neurons against glutamate toxicity. Collectively, these results reveal a novel role of CRMP-3 in that calpain cleavage of CRMP-3 and the subsequent nuclear translocation of the truncated CRMP-3 evokes neuronal death in response to excitotoxicity and cerebral ischemia. Our findings also establish a novel route of how calpain signals neuron death.

Chlortetracycline and Demeclocycline Inhibit Calpains and Protect Mouse Neurons against Glutamate Toxicity and Cerebral Ischemia

Minocycline is a potent neuroprotective tetracycline in animal models of cerebral ischemia. We examined the protective properties of chlortetracycline (CTC) and demeclocycline (DMC) and showed that these two tetracyclines were also potent neuroprotective against glutamate-induced neuronal death in vitro and cerebral ischemia in vivo. However, CTC and DMC appeared to confer neuroprotection through a unique mechanism compared with minocycline. Rather than inhibiting microglial activation and caspase, CTC and DMC suppressed calpain activities. In addition, CTC and DMC only weakly antagonized N-methyl-D-aspartate (NMDA) receptor activities causing 16 and 14%, respectively, inhibition of NMDA-induced whole cell currents and partially blocked NMDA-induced Ca2+ influx, commonly regarded as the major trigger of neuronal death. In vitro and in vivo experiments demonstrated that the two compounds selectively inhibited the activities of calpain I and II activated following glutamate treatment and cerebral ischemia. In contrast, minocycline did not significantly inhibit calpain activity. Taken together, these results suggested that CTC and DMC provide neuroprotection through suppression of a rise in intracellular Ca2+ and inhibition of calpains.

Multiple cyclin-dependent kinases signals are critical mediators of ischemia/hypoxic neuronal death in vitro and in vivo

The mechanisms involving neuronal death after ischemic/hypoxic insult are complex, involving both rapid (excitotoxic) and delayed (apoptotic-like) processes. Recent evidence suggests that cell cycle regulators such as cyclin-dependent kinases are abnormally activated in neuropathological conditions, including stroke. However, the function of this activation is unclear. Here, we provide evidence that inhibition of the cell cycle regulator, Cdk4, and its activator, cyclinD1, plays critical roles in the delayed death component of ischemic/hypoxic stress by regulating the tumor suppressor retinoblastoma protein. In contrast, the excitotoxic component of ischemia/hypoxia is predominately regulated by Cdk5 and its activator p35, components of a cyclin-dependent kinase complex associated with neuronal development. Hence, our data both characterize the functional significance of the cell cycle Cdk4 and neuronal Cdk5 signals as well as define the pathways and circumstances by which they act to control ischemic/hypoxic damage.

Inhibition of adenine nucleotide translocator pore function and protection against apoptosis in vivo by an HIV protease inhibitor

Inhibitors of HIV protease have been shown to have antiapoptotic effects in vitro, yet whether these effects are seen in vivo remains controversial. In this study, we have evaluated the impact of the HIV protease inhibitor (PI) nelfinavir, boosted with ritonavir, in models of nonviral disease associated with excessive apoptosis. In mice with Fas-induced fatal hepatitis, Staphylococcal enterotoxin B-induced shock, and middle cerebral artery occlusion-induced stroke, we demonstrate that PIs significantly reduce apoptosis and improve histology, function, and/or behavioral recovery in each of these models. Further, we demonstrate that both in vitro and in vivo, PIs block apoptosis through the preservation of mitochondrial integrity and that in vitro PIs act to prevent pore function of the adenine nucleotide translocator (ANT) subunit of the mitochondrial permeability transition pore complex.