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

Cyclin D1 gene ablation confers neuroprotection in traumatic brain injury

Cell cycle activation (CCA) is one of the principal secondary injury mechanisms following brain trauma, and it leads to neuronal cell death, microglial activation, and neurological dysfunction. Cyclin D1 (CD1) is a key modulator of CCA and is upregulated in neurons and microglia following traumatic brain injury (TBI). In this study we subjected CD1-wild-type (CD1(+/+)) and knockout (CD1(-/-)) mice to controlled cortical impact (CCI) injury to evaluate the role of CD1 in post-traumatic neurodegeneration and neuroinflammation. As early as 24 h post-injury, CD1(+/+) mice showed markers of CCA in the injured hemisphere, including increased CD1, E2F1, and proliferating cell nuclear antigen (PCNA), as well as increased Fluoro-Jade B staining, indicating neuronal degeneration. Progressive neuronal loss in the hippocampus was observed through 21 days post-injury in these mice, which correlated with a decline in cognitive function. Microglial activation in the injured hemisphere peaked at 7 days post-injury, with sustained increases at 21 days. In contrast, CD1(-/-) mice showed reduced CCA and neurodegeneration at 24 h, as well as improved cognitive function, attenuated hippocampal neuronal cell loss, decreased lesion volume, and cortical microglial activation at 21 days post-injury. These findings indicate that CD1-dependent CCA plays a significant role in the neuroinflammation, progressive neurodegeneration, and related neurological dysfunction resulting from TBI. Our results further substantiate the proposed role of CCA in post-traumatic secondary injury, and suggest that inhibition of CD1 may be a key therapeutic target for TBI.

Tissue-type plasminogen activator has a neuroprotective effect in the ischemic brain mediated by neuronal TNF-α

Cerebral cortical neurons have a heightened sensitivity to hypoxia and their survival depends on their ability to accommodate to changes in the concentration of oxygen in their environment. Tissue-type plasminogen activator (tPA) is a serine proteinase that activates the zymogen plasminogen into plasmin. Hypoxia induces the release of tPA from cerebral cortical neurons, and it has been proposed that tPA mediates hypoxic and ischemic neuronal death. Here, we show that tPA is devoid of neurotoxic effects and instead is an endogenous neuroprotectant that renders neurons resistant to the effects of lethal hypoxia and ischemia. We present in vitro and in vivo evidence indicating that endogenous tPA and recombinant tPA induce the expression of neuronal tumor necrosis factor-α. This effect, mediated by plasmin and the N-methyl-D-aspartate receptor, leads to increased expression of the cyclin-dependent kinase inhibitor p21 and p21-mediated development of early hypoxic and ischemic tolerance.

Sublethal doses of β-amyloid peptide abrogate DNA-dependent protein kinase activity

Accumulation of DNA damage and deficiency in DNA repair potentially contribute to the progressive neuronal loss in neurodegenerative disorders, including Alzheimer disease (AD). In multicellular eukaryotes, double strand breaks (DSBs), the most lethal form of DNA damage, are mainly repaired by the nonhomologous end joining pathway, which relies on DNA-PK complex activity. Both the presence of DSBs and a decreased end joining activity have been reported in AD brains, but the molecular player causing DNA repair dysfunction is still undetermined. β-Amyloid (Aβ), a potential proximate effector of neurotoxicity in AD, might exert cytotoxic effects by reactive oxygen species generation and oxidative stress induction, which may then cause DNA damage. Here, we show that in PC12 cells sublethal concentrations of aggregated Aβ(25-35) inhibit DNA-PK kinase activity, compromising DSB repair and sensitizing cells to nonlethal oxidative injury. The inhibition of DNA-PK activity is associated with down-regulation of the catalytic subunit DNA-PK (DNA-PKcs) protein levels, caused by oxidative stress and reversed by antioxidant treatment. Moreover, we show that sublethal doses of Aβ(1-42) oligomers enter the nucleus of PC12 cells, accumulate as insoluble oligomeric species, and reduce DNA-PK kinase activity, although in the absence of oxidative stress. Overall, these findings suggest that Aβ mediates inhibition of the DNA-PK-dependent nonhomologous end joining pathway contributing to the accumulation of DSBs that, if not efficiently repaired, may lead to the neuronal loss observed in AD.

Phospho-Rb mediating cell cycle reentry induces early apoptosis following oxygen-glucose deprivation in rat cortical neurons

The aim of this study was to investigate the relationship between cell cycle reentry and apoptosis in cultured cortical neurons following oxygen-glucose deprivation (OGD). We found that the percentage of neurons with BrdU uptake, TUNEL staining, and colocalized BrdU uptake and TUNEL staining was increased relative to control 6, 12 and 24 h after 1 h of OGD. The number of neurons with colocalized BrdU and TUNEL staining was decreased relative to the number of TUNEL-positive neurons at 24 h. The expression of phosphorylated retinoblastoma protein (phospho-Rb) was significantly increased 6, 12 and 24 h after OGD, parallel with the changes in BrdU uptake. Phospho-Rb and TUNEL staining were colocalized in neurons 6 and 12 h after OGD. This colocalization was strikingly decreased 24 h after OGD. Treatment with the cyclin-dependent kinase inhibitor roscovitine (100 μM) decreased the expression of phospho-Rb and reduced neuronal apoptosis in vitro. These results demonstrated that attempted cell cycle reentry with phosphorylation of Rb induce early apoptosis in neurons after OGD and there must be other mechanisms involved in the later stages of neuronal apoptosis besides cell cycle reentry. Phosphoralated Rb may be an important factor which closely associates aberrant cell cycle reentry with the early stages of neuronal apoptosis following ischemia/hypoxia in vitro, and pharmacological interventions for neuroprotection may be useful directed at this keypoint.

Cell cycle reactivation in mature neurons: a link with brain plasticity, neuronal injury and neurodegenerative diseases?

Although the cell cycle machinery is essentially linked to cellular proliferation, recent findings suggest that neuronal cell death is frequently concurrent with the aberrant expression of cell cycle proteins in post-mitotic neurons. The present work reviews the evidence of cell cycle reentry and expression of cell cycle-associated proteins as a complex response of neurons to insults in the adult brain but also as a mechanism underlying brain plasticity. The basic aspects of cell cycle mechanisms, as well as the evidence showing cell cycle protein expression in the injured brain, are reviewed. The discussion includes recent experimental work attempting to establish a correlation between altered brain plasticity and neuronal death, and an analysis of recent evidence on how neural cell cycle dysregulation is related to neurodegenerative diseases especially the Alzheimer's disease. Understanding the mechanisms that control reexpression of proteins required for cell cycle progression which is involved in brain remodeling, may shed new light into the mechanisms involved in neuronal demise under diverse pathological circumstances. This would provide valuable clues about the possible therapeutic targets, leading to potential treatment of presently challenging neurodegenerative diseases.

Stereotyped fetal brain disorganization is induced by hypoxia and requires lysophosphatidic acid receptor 1 (LPA1) signaling

Fetal hypoxia is a common risk factor that has been associated with a range of CNS disorders including epilepsy, schizophrenia, and autism. Cellular and molecular mechanisms through which hypoxia may damage the developing brain are incompletely understood but are likely to involve disruption of the laminar organization of the cerebral cortex. Lysophosphatidic acid (LPA) is a bioactive lipid capable of cortical influences via one or more of six cognate G protein-coupled receptors, LPA(1-6), several of which are enriched in fetal neural progenitor cells (NPCs). Here we report that fetal hypoxia induces cortical disruption via increased LPA(1) signaling involving stereotyped effects on NPCs: N-cadherin disruption, displacement of mitotic NPCs, and impaired neuronal migration, as assessed both ex vivo and in vivo. Importantly, genetic removal or pharmacological inhibition of LPA(1) prevented the occurrence of these hypoxia-induced phenomena. Hypoxia resulted in overactivation of LPA(1) through selective inhibition of G protein-coupled receptor kinase 2 expression and activation of downstream pathways including G(αi) and Ras-related C3 botulinum toxin substrate 1. These data identify stereotyped and selective hypoxia-induced cerebral cortical disruption requiring LPA(1) signaling, inhibition of which can reduce or prevent disease-associated sequelae, and may take us closer to therapeutic treatment of fetal hypoxia-induced CNS disorders and possibly other forms of hypoxic injury.

Cdk5: multitasking between physiological and pathological conditions

Cyclin-dependent kinase 5 (Cdk5) is a peculiar proline-directed serine/threonine kinase. Unlike the other members of the Cdk family, Cdk5 is not directly involved in cell cycle regulation, being normally associated with neuronal processes such as migration, cortical layering and synaptic plasticity. This kinase is present mainly in post-mitotic neurons and its activity is tightly regulated by the interaction with the specific activators, p35 and p39. Despite its pivotal role in CNS development, Cdk5 dysregulation has been implicated in different pathologies, such as Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD) and, most recently, prion-related encephalopathies (PRE). In these neurodegenerative conditions, Cdk5 overactivation and relocalization occurs upon association with p25, a truncated form of the normal activator p35. This activator switching will cause a shift in the phosphorylative pattern of Cdk5, with an alteration both in targets and activity, ultimately leading to neuronal demise. In AD and PRE, two disorders that share clinical and neuropathological features, Cdk5 dysregulation is a linking event between the major neuropathological markers: amyloid plaques, tau hyperphosphorylation and synaptic and neuronal loss. Moreover, this kinase was shown to be involved in abortive cell cycle re-entry, a feature recently proposed as a possible step in the neuronal apoptosis mechanism of several neurological diseases. This review focuses on the role of Cdk5 in neurons, namely in the regulation of cytoskeletal dynamics, synaptic function and cell survival, both in physiological and in pathological conditions, highlighting the relevance of Cdk5 in the main mechanisms of neurodegeneration in Alzheimer's disease and other brain pathologies.

Cell cycle activation and spinal cord injury

Traumatic spinal cord injury (SCI) evokes a complex cascade of events with initial mechanical damage leading to secondary injury processes that contribute to further tissue loss and functional impairment. Growing evidence suggests that the cell cycle is activated following SCI. Up-regulation of cell cycle proteins after injury appears to contribute not only to apoptotic cell death of postmitotic cells, including neurons and oligodendrocytes, but also to post-traumatic gliosis and microglial activation. Inhibition of key cell cycle regulatory pathways reduces injury-induced cell death, as well as microglial and astroglial proliferation both in vitro and in vivo. Treatment with cell cycle inhibitors in rodent SCI models prevents neuronal cell death and reduces inflammation, as well as the surrounding glial scar, resulting in markedly reduced lesion volumes and improved motor recovery. Here we review the effects of SCI on cell cycle pathways, as well as the therapeutic potential and mechanism of action of cell cycle inhibitors for this disorder.

Structural Bioinformatics Approach of Cyclin-Dependent Kinases 1 and 3 Complexed with Inhibitors

The cyclin-dependent kinases or CDKs participate in the regulation of both the cell progression cycle and the RNA polymerase-II transcription cycle. In several human tumours deregulation of CDK-related mechanisms have been detected, e.g., overexpression of cyclins or deletion of genes encoding for CKIs. Regarding these observations, CDKs came up to be interesting targets for elaboration of novel antitumour drugs. Based on the importance of the CDKs, this research aimed to describe, to characterize and to compare the molecular models of CDK1 and CDK3. Since the structures of human CDK1 and CDK3 are unavailable in the Protein Data Bank -PDB, homology models were created based on the CDK2 as the template, once they share a substantial identity. The structural studies of the CDK1 and CDK3 biding sites were conducted by molecular docking with 15 different CDK inhibitors previously identified to CDK2. This study allowed the understanding of the structure of the complexes between CDK1/ CDK3 with inhibitors. The knowledge of their structural features mainly the biding sites might be useful to discovery and rationalization of drug design process.

The emerging role of epigenetics in stroke: II. RNA regulatory circuitry

Recent scientific advances have demonstrated the existence of extensive RNA-based regulatory networks involved in orchestrating nearly every cellular process in health and various disease states. This previously hidden layer of functional RNAs is derived largely from non-protein-coding DNA sequences that constitute more than 98% of the genome in humans. These non-protein-coding RNAs (ncRNAs) include subclasses that are well known, such as transfer RNAs and ribosomal RNAs, as well as those that have more recently been characterized, such as microRNAs, small nucleolar RNAs, and long ncRNAs. In this review, we examine the role of these novel ncRNAs in the nervous system and highlight emerging evidence that implicates RNA-based networks in the molecular pathogenesis of stroke. We also describe RNA editing, a related epigenetic mechanism that is partly responsible for generating the exquisite degrees of environmental responsiveness and molecular diversity that characterize ncRNAs. In addition, we discuss the development of future therapeutic strategies for locus-specific and genome-wide regulation of genes and functional gene networks through the modulation of RNA transcription, posttranscriptional RNA processing (eg, RNA modifications, quality control, intracellular trafficking, and local and long-distance intercellular transport), and RNA translation. These novel approaches for neural cell- and tissue-selective reprogramming of epigenetic regulatory mechanisms are likely to promote more effective neuroprotective and neural regenerative responses for safeguarding and even restoring central nervous system function.

Peroxisome proliferator-activated receptor gamma promotes neuroprotection by modulating cyclin D1 expression after focal cerebral ischemia

Peroxisome proliferator-activated receptor gamma (PPARgamma) has been shown to protect against stroke and improve neurological outcome after cerebral ischemia. This study investigated whether activation of cerebral PPARgamma improves recovery from focal cerebral ischemia by reducing expression of cyclin D1, which is associated with programmed neuron death. Focal cerebral ischemia was induced by 90 min of middle cerebral artery occlusion (MCAO), followed by reperfusion. Intracerebroventricular (i.c.v.) infusion of the PPARgamma agonist ciglitazone, beginning 5 days before and continuing through 1 day after MCAO, reduced infarct size and cyclin D1 expression in the peri-infarct cortical region. Furthermore, primary cortical neurons treated with ciglitazone showed suppressed expression of cyclin D1 in response to hypoxia-reoxygenation. This protective effect was reversed after cotreatment with the selective PPAR-gamma antagonist GW 9662 (2-chloro-5-nitrobenzanilide), clearly demonstrating the involvement of a PPARgamma-dependent mechanism. Our data provide evidence that activation of neuronal PPARgamma makes a substantial contribution to neuroprotection by preventing cyclin D1 up-regulation in vitro and in vivo.

The mitochondrial inner membrane GTPase, optic atrophy 1 (Opa1), restores mitochondrial morphology and promotes neuronal survival following excitotoxicity

Mitochondrial dynamics have been extensively studied in the context of classical cell death models involving Bax-mediated cytochrome c release. Excitotoxic neuronal loss is a non-classical death signaling pathway that occurs following overactivation of glutamate receptors independent of Bax activation. Presently, the role of mitochondrial dynamics in the regulation of excitotoxicity remains largely unknown. Here, we report that NMDA-induced excitotoxicity results in defects in mitochondrial morphology as evident by the presence of excessive fragmented mitochondria, cessation of mitochondrial fusion, and cristae dilation. Up-regulation of the mitochondrial inner membrane GTPase, Opa1, is able to restore mitochondrial morphology and protect neurons against excitotoxic injury. Opa1 functions downstream of the calcium-dependent protease, calpain. Inhibition of calpain activity by calpastatin, an endogenous calpain inhibitor, significantly rescued mitochondrial defects and maintained neuronal survival. Opa1 was required for calpastatin-mediated neuroprotection because the enhanced survival found following NMDA-induced toxicity was significantly reduced upon loss of Opa1. Our results define a mechanism whereby breakdown of the mitochondrial network mediated through loss of Opa1 function contributes to neuronal death following excitotoxic neuronal injury. These studies suggest Opa1 as a potential therapeutic target to promote neuronal survival following acute brain damage and neurodegenerative diseases.

Essential role of cytoplasmic cdk5 and Prx2 in multiple ischemic injury models, in vivo

Recent evidence suggests that abnormal activation of cyclin-dependent kinase 5 (cdk5) is a critical prodeath signal in stroke. However, the mechanism(s) by which cdk5 promotes death is unclear. Complicating the role of cdk5 are the observations that cdk5 can exist in multiple cellular regions and possess both prosurvival and prodeath characteristics. In particular, the critical role of cytoplasmic or nuclear cdk5 in neuronal jury, in vivo, is unclear. Therefore, we determined where cdk5 was activated in models of ischemia and how manipulation of cdk5 in differing compartments may affect neuronal death. Here, we show a critical function for cytoplasmic cdk5 in both focal and global models of stroke, in vivo. Cdk5 is activated in the cytoplasm and expression of DNcdk5 localized to the cytoplasm is protective. Importantly, we also demonstrate the antioxidant enzyme Prx2 (peroxiredoxin 2) as a critical cytoplasmic target of cdk5. In contrast, the role of cdk5 in the nucleus is context-dependent. Following focal ischemia, nuclear cdk5 is activated and functionally relevant while there is no evidence for such activation following global ischemia. Importantly, myocyte enhancer factor 2D (MEF2D), a previously described nuclear target of cdk5 in vitro, is also phosphorylated by cdk5 following focal ischemia. In addition, MEF2D expression in this paradigm ameliorates death. Together, our results address the critical issue of cdk5 activity compartmentalization, as well as define critical substrates for both cytoplasmic and nuclear cdk5 activity in adult models of stroke.

Pim-1 kinase as activator of the cell cycle pathway in neuronal death induced by DNA damage

DNA damage is a critical component of neuronal death underlying neurodegenerative diseases and injury. Neuronal death evoked by DNA damage is characterized by inappropriate activation of multiple cell cycle components. However, the mechanism regulating this activation is not fully understood. We demonstrated previously that the cell division cycle (Cdc) 25A phosphatase mediates the activation of cyclin-dependent kinases and neuronal death evoked by the DNA damaging agent camptothecin. We also showed that Cdc25A activation is blocked by constitutive checkpoint kinase 1 activity under basal conditions in neurons. Presently, we report that an additional factor is central to regulation of Cdc25A phosphatase in neuronal death. In a gene array screen, we first identified Pim-1 as a potential factor up-regulated following DNA damage. We confirmed the up-regulation of Pim-1 transcript, protein and kinase activity following DNA damage. This induction of Pim-1 is regulated by the nuclear factor kappa beta (NF-kappaB) pathway as Pim-1 expression and activity are significantly blocked by siRNA-mediated knockdown of NF-kappaB or NF-kappaB pharmacological inhibitors. Importantly, Pim-1 activity is critical for neuronal death in this paradigm and its deficiency blocks camptothecin-mediated neuronal death. It does so by activating Cdc25A with consequent activation of cyclin D1-associated kinases. Taken together, our results demonstrate that Pim-1 kinase plays a central role in DNA damage-evoked neuronal death by regulating aberrant neuronal cell cycle activation.

Contribution of hypoxia to Alzheimer's disease: is HIF-1alpha a mediator of neurodegeneration?

The mammalian brain is extremely sensitive to alterations in cellular homeostasis as a result of environmental or physiological insults. In particular, hypoxic/ischemic challenges (i.e. reduced oxygen and/or glucose delivery) cause severe and detrimental alterations in brain function and can trigger neuronal cell death within minutes. Unfortunately, as we age, oxygen delivery to cells and tissues is impaired, thereby increasing the susceptibility of neurons to damage. Thus, hypoxic (neuronal) adaptation is significantly compromised during aging. Many neurological diseases, such as stroke, Alzheimer's disease (AD), Parkinson's disease and diabetes, are characterized by hypoxia, a state that is believed to only exacerbate disease progression. However, the contribution of hypoxia and hypoxia-mediated pathways to neurodegeneration remains unclear. This review discusses current evidence on the contribution of oxygen deprivation to AD, with an emphasis on hypoxia inducible transcription factor-1 (HIF-1)-mediated pathways and the association of AD with the cytoskeleton regulator cyclin-dependent kinase 5.

Dynamic changes of cell cycle elements in the ischemic brain after bone marrow stromal cells transplantation in rats

Transplantation of bone marrow stromal cells (BMSCs) improves animal neurological functional recovery after stroke. But the mechanism remains unclear. As cell cycle machinery plays an important role in stroke, we investigated the dynamic changes of cell cycle elements in a rat model of middle cerebral artery occlusion. We found the cell cycle markers, cdk4 along with its activator cyclin D1, and proliferating cell nuclear antigen (PCNA), increased after brain ischemia-reperfusion. Phosphorylation of the retinoblastoma protein (pRb, on ser-795), the cyclin D/cdk4 complex mutual target, was upregulated accordingly. However, intravenously administrated BMSCs facilitated cyclin D1, cdk4, and PCNA decrease in the ischemic cortex. Meanwhile, phospho-pRb (ser-795) was completely inhibited. On the contrary, endogenous cdk inhibitor p27 reduced before but enhanced after BMSCs treatment. These findings suggested BMSCs might modulate cell cycle progression in injured brain via downregulation of the cyclin D1/cdk4/pRb pathway as well as upregulation of p27 level. These results provide another way by which BMSCs may contribute to the recovery from stroke.

c-Abl tyrosine kinase modulates tau pathology and Cdk5 phosphorylation in AD transgenic mice

The c-Abl tyrosine kinase is an important link in signal transduction pathways that promote cytoskeletal rearrangement and apoptotic signalling. We have previously shown that amyloid-β-peptide (Aβ) activates c-Abl. Herein we show that c-Abl participates in Aβ-induced tau phosphorylation through Cdk5 activation. We found that intraperitoneal administration of STI571, a specific inhibitor for c-Abl kinase, decreased tau phosphorylation in the APPswe/PSEN1ΔE9 transgenic mouse brain. In addition, when neurons were treated with Aβ we observed: (i) an increase in active c-Abl and tau phosphorylation, (ii) the prevention of tau phosphorylation by STI571 and (iii) the inhibition of c-Abl expression by shRNA, as well as the expression of a c-Abl kinase death mutant, decreased AT8 and PHF1 signals. Furthermore, the increase of c-Abl was associated with Tyr15 phosphorylation of Cdk5 and its association with c-Abl. Brains from APPswe/PSEN1ΔE9 mice showed higher levels of c-Abl and phospho-Cdk5 than wild-type mice. Moreover, STI571 treatment decreased the phospho-Cdk5 levels. Together, the evidence suggests that activation of c-Abl by Aβ promotes tau phosphorylation through Tyr15 phosphorylation-mediated Cdk5 activation.

A sequel to the tale of p25/Cdk5 in neurodegeneration

p25/Cdk5 dysregulation may contribute to neurodegeneration. In this issue of Neuron, Kim et al. show that cdk5 inactivates HDAC-1, leading to cell cycle deregulation and DNA damage accumulation. This study provides further insights into the function of p25/Cdk5 in neurons and points to HDAC-1 as a target for therapeutic interventions.

Multifunctional drug treatment in neurotrauma

Although the concepts of secondary injury and neuroprotection after neurotrauma are experimentally well supported, clinical trials of neuroprotective agents in traumatic brain injury or spinal cord injury have been disappointing. Most strategies to date have used drugs directed toward a single pathophysiological mechanism that contributes to early necrotic cell death. Given these failures, recent research has increasingly focused on multifunctional (i.e., multipotential, pluripotential) agents that target multiple injury mechanisms, particularly those that occur later after the insult. Here we review two such approaches that show particular promise in experimental neurotrauma: cell cycle inhibitors and small cyclized peptides. Both show extended therapeutic windows for treatment and appear to share at least one important target.

SOX7, down-regulated in colorectal cancer, induces apoptosis and inhibits proliferation of colorectal cancer cells

The sex-determining region Y-box 7 (Sox7) is a member of high mobility group (HMG) transcription factor family, essential for embryonic development and endoderm differentiation. Deregulation of Wnt signaling pathway is a hallmark of colorectal cancer. Our results showed that the expression level of SOX7 was frequently down-regulated in human colorectal cancer cell lines and in primary colorectal tumor tissues, and the SOX7 silencing was partially due to the aberrant DNA methylation of the gene. Restoration of SOX7 induced colorectal cancer cell apoptosis, inhibited cell proliferation and colony formation. In addition, SOX7 efficiently suppressed beta-catenin-mediated transcriptional activity.

The expression of cyclins in neurons of rats after focal cerebral ischemia

The change of the expression of Cyclins in neurons of rats after focal cerebral ischemia was investigated. Ischemia was induced by temporary middle cerebral artery occlusion (MCAO). The experimental rats induced by MCAO were sacrificed on 7th and 14th day after reperfusion. The brain was taken out at 7th and 14th day after injury, and the expression of Cyclin D1, E, A and B1 in neurons of cerebral cortex or hippocampal CA1 region was detected by immunofluorescence and confocal microscope. The results showed that after MCAO, in the ipsilateral CA1 subfield of hippocampus the expression of Cyclin D1, E, A and B1 in neurons was significantly gradually up-regulated at 7th and 14th day after reperfusion (P<0.05) as compared with that in control group. In the ipsilateral cerebral cortex the expression of Cyclin D1 and B1 in neurons was notably gradually down-regulated at 7th and 14th day, and that of Cyclin E and A was significantly up-regulated at 14th day after reperfusion as compared with that in control group (all P<0.05). It was concluded that there was a differential sensitivity among neurons from different brain regions to ischemic injury. But all of them re-enter into cell cycle after MCAO.

Glutamatergic stimulation triggers rapid Krüpple-like factor 4 expression in neurons and the overexpression of KLF4 sensitizes neurons to NMDA-induced caspase-3 activity

We report the first demonstration that Krüpple-like factor 4 (KLF4) mRNA is dramatically and rapidly upregulated by NMDA application in primary cortical neuron cultures. We also report that NMDA induced significant and transient upregulation of KLF4 protein expression, in both cortical neuron cultures and native brain slices. The increase of KLF4 mRNA and protein expression in response to NMDA was time-dependent, and required NMDA receptor-mediated Ca(2+) influx. In addition, AMPA exposure caused a time-dependent increase in KLF4 mRNA expression, which was also Ca(2+)-dependent and involved activation of AMPA/kainate receptors and L-type voltage-sensitive calcium channels. To assess the downstream signaling pathways and functions of KLF4 activation, we used lentiviral vectors to induce ectopic overexpression of KLF4 in cultured neurons. KLF4 overexpression induced the activation of caspase-3 after a normally subtoxic dose of NMDA (10 microM). KLF4 overexpression also increased both protein and mRNA levels of the cell cycle protein cyclin D1, but reduced p21(Waf1/Cip1) protein levels. After the NMDA treatment, cyclin D1 levels increased after a short delay (4 h), but fell back to control levels after 20 h. The effects of NMDA and KLF4 overexpression on cyclin D1 induction were additive. We conclude that glutamatergic stimulation can trigger rapid elevation of KLF4 mRNA and protein levels, and that the overexpression of KLF4 can regulate neuronal cell cycle proteins and sensitize neurons to NMDA-induced caspase-3 activity.

Roscovitine reduces neuronal loss, glial activation, and neurologic deficits after brain trauma

Traumatic brain injury (TBI) causes both direct and delayed tissue damage. The latter is associated with secondary biochemical changes such as cell cycle activation, which leads to neuronal death, inflammation, and glial scarring. Flavopiridol--a cyclin-dependent kinase (CDK) inhibitor that is neither specific nor selective--is neuroprotective. To examine the role of more specific CDK inhibitors as potential neuroprotective agents, we studied the effects of roscovitine in TBI. Central administration of roscovitine 30 mins after injury resulted in significantly decreased lesion volume, as well as improved motor and cognitive recovery. Roscovitine attenuated neuronal death and inhibited activation of cell cycle pathways in neurons after TBI, as indicated by attenuated cyclin G1 accumulation and phosphorylation of retinoblastoma protein. Treatment also decreased microglial activation after TBI, as reflected by reductions in ED1, galectin-3, p22(PHOX), and Iba-1 levels, and attenuated astrogliosis, as shown by decreased accumulation of glial fibrillary acidic protein. In primary cortical microglia and neuronal cultures, roscovitine and other selective CDK inhibitors attenuated neuronal cell death, as well as decreasing microglial activation and microglial-dependent neurotoxicity. These data support a multifactorial neuroprotective effect of cell cycle inhibition after TBI--likely related to inhibition of neuronal apoptosis, microglial-induced inflammation, and gliosis--and suggest that multiple CDKs are potentially involved in this process.

Cyclin-dependent kinase 2 signaling regulates myocardial ischemia/reperfusion injury

Ischemia/reperfusion (I/R) injury to the heart is accompanied by the upregulation and posttranslational modification of a number of proteins normally involved in regulating cell cycle progression. Two such proteins, cyclin-dependent kinase-2 (Cdk2) and its downstream target, the retinoblastoma gene product (Rb), also play a critical role in the control of apoptosis. Myocardial ischemia activates Cdk2, resulting in the phosphorylation and inactivation of Rb. Blocking Cdk2 activity reduces apoptosis in cultured cardiac myocytes. Genetic or pharmacological inhibition of Cdk2 activity in vivo during I/R injury led to a 36% reduction in infarct size (IFS), when compared to control mice, associated with a reduction in apoptotic myocytes. To confirm that Rb was the critical target in Cdk2-mediated I/R injury, we determined the consequences of I/R injury in cardiac-specific Rb-deficient mice (CRb(L/L)). IFS was increased 140% in CRb(L/L) mice compared to CRb+/+ controls. TUNEL positive nuclei and caspase-3 activity were augmented by 92% and 36%, respectively, following injury in the CRb(L/L) mice demonstrating that loss of Rb in the heart significantly exacerbates I/R injury. These data suggest that Cdk2 signaling pathways are critical regulators of cardiac I/R injury in vivo and support a cardioprotective role for Rb.

Calpain-mediated signaling mechanisms in neuronal injury and neurodegeneration

Calpain is a ubiquitous calcium-sensitive protease that is essential for normal physiologic neuronal function. However, alterations in calcium homeostasis lead to persistent, pathologic activation of calpain in a number of neurodegenerative diseases. Pathologic activation of calpain results in the cleavage of a number of neuronal substrates that negatively affect neuronal structure and function, leading to inhibition of essential neuronal survival mechanisms. In this review, we examine the mechanistic underpinnings of calcium dysregulation resulting in calpain activation in the acute neurodegenerative diseases such as cerebral ischemia and in the chronic neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Huntington's disease, multiple sclerosis, prion-related encephalopathy, and amylotrophic lateral sclerosis. The premise of this paper is that analysis of the signaling and transcriptional consequences of calpain-mediated cleavage of its various substrates for any neurodegenerative disease can be extrapolated to all of the neurodegenerative diseases vulnerable to calcium dysregulation.

Comparison of gene expression during in vivo and in vitro postnatal retina development

Retina explants are widely used as a model of neural development. To define the molecular basis of differences between the development of retina in vivo and in vitro during the early postnatal period, we carried out a series of microarray comparisons using mouse retinas. About 75% of 8,880 expressed genes from retina explants kept the same expression volume and pattern as the retina in vivo. Fewer than 6% of the total gene population was changed at two consecutive time points, and only about 1% genes showed more than a threefold change at any time point studied. Functional Gene Ontology (GO) mapping for both changed and unchanged genes showed similar distribution patterns, except that more genes were changed in the GO clusters of response to stimuli and carbohydrate metabolism. Three distinct expression patterns of genes preferentially expressed in rod photoreceptors were observed in the retina explants. Some genes showed a lag in increased expression, some showed no change, and some continued to have a reduced level of expression. An early downregulation of cyclin D1 in the explanted retina might explain the reduction in numbers of precursors in explanted retina and suggests that external factors are required for maintenance of cyclin D1. The global view of gene profiles presented in this study will help define the molecular changes in retina explants over time and will provide criteria to define future changes that improve this model system.

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (doi:10.1007/s12177-008-9009-z) contains supplementary material, which is available to authorized users.

CITED2 signals through peroxisome proliferator-activated receptor-gamma to regulate death of cortical neurons after DNA damage

DNA damage is an important initiator of neuronal apoptosis and activates signaling events not yet fully defined. Using the camptothecin-induced DNA damage model in neurons, we previously showed that cyclin D1-associated cell cycle cyclin-dependent kinases (Cdks) (Cdk4/6) and p53 activation are two major events leading to activation of the mitochondrial apoptotic pathway. With gene array analyses, we detected upregulation of Cited2, a CBP (cAMP response element-binding protein-binding protein)/p300 interacting transactivator, in response to DNA damage. This upregulation was confirmed by reverse transcription-PCR and Western blot. CITED2 was functionally important because CITED2 overexpression promotes death, whereas CITED2 deficiency protects. Cited2 upregulation is upstream of the mitochondrial death pathway (BAX, Apaf1, or cytochrome c release) and appears to be independent of p53. However, inhibition of the Cdk4 blocked Cited2 induction. The Cited2 prodeath mechanism does not involve Bmi-1 or p53. Instead, Cited2 activates peroxisome proliferator-activated receptor-gamma (PPARgamma), an activity that we demonstrate is critical for DNA damage-induced death. These results define a novel neuronal prodeath pathway in which Cdk4-mediated regulation of Cited2 activates PPARgamma and consequently caspase.

Delayed administration of a potent cyclin dependent kinase and glycogen synthase kinase 3 beta inhibitor produces long-term neuroprotection in a hypoxia-ischemia model of brain injury

We compared the neuroprotective efficacy of a potent and CNS-penetrant cyclin dependent kinase (CDK) and glycogen synthase kinase 3 beta (GSK3beta) inhibitor (Compound 1) in juvenile (postnatal day 21; P21) and adult C57Bl/6 mice (postnatal day 60; P60) using a model of hypoxic-ischemic brain injury (HI). Neuronal cell counts and density measures from brain sections stained with Cresyl Violet revealed that exposure of P21 mice to 60 min of HI resulted in extensive damage to the ipsilateral cornu ammonis 1 (CA1) region of the hippocampus (40% cell loss) and striatum (30% cell loss) 7 days later. Exposure of P60 mice to 40 min of HI produced a similar pattern of cell loss. Intraperitoneal administration of Compound 1 (3 mg/kg) 1, 5 and 9 h after 60 min of HI did not reduce brain injury in P21 mice relative to vehicle controls. By contrast, in P60 mice, this treatment significantly decreased cell loss in the ipsilateral hippocampus (10% cell loss) and striatum (15% loss) relative to vehicle controls. Terminal uridine deoxynucleotidyl transferase (TUNNEL) positive cell counts and infarct volume were also substantially reduced in P60 mice treated with Compound 1. A motor coordination test performed twice weekly until 5 weeks post-HI confirmed that Compound 1 produced long lasting functional recovery. Our results indicate that Compound 1 produced long lasting neuroprotective effects in adult but not juvenile mice suggesting that inhibition of the CDKs and GSK3beta plays a distinct neuroprotective role in the juvenile and adult brain.

Meriolins (3-(pyrimidin-4-yl)-7-azaindoles): synthesis, kinase inhibitory activity, cellular effects, and structure of a CDK2/cyclin A/meriolin complex

We report the synthesis and biological characterization of 3-(pyrimidin-4-yl)-7-azaindoles (meriolins), a chemical hybrid between the natural products meridianins and variolins, derived from marine organisms. Meriolins display potent inhibitory activities toward cyclin-dependent kinases (CDKs) and, to a lesser extent, other kinases (GSK-3, DYRK1A). The crystal structures of 1e (meriolin 5) and variolin B (Bettayeb, K.; Tirado, O. M.; Marionneau-Lambert, S.; Ferandin, Y.; Lozach, O.; Morris, J.; Mateo-Lozano, S.; Drückes, P.; Schächtele, C.; Kubbutat, M.; Liger, F.; Marquet, B.; Joseph, B.; Echalier, A.; Endicott, J.; Notario, V.; Meijer, L. Cancer Res. 2007, 67, 8325-8334) in complex with CDK2/cyclin A reveal that the two inhibitors are orientated in very different ways inside the ATP-binding pocket of the kinase. A structure-activity relationship provides further insight into the molecular mechanism of action of this family of kinase inhibitors. Meriolins are also potent antiproliferative and proapoptotic agents in cells cultured either as monolayers or in spheroids. Proapoptotic efficacy of meriolins correlates best with their CDK2 and CDK9 inhibitory activity. Meriolins thus constitute a promising class of pharmacological agents to be further evaluated against the numerous human diseases that imply abnormal regulation of CDKs including cancers, neurodegenerative disorders, and polycystic kidney disease.

Rat focal cerebral ischemia induced astrocyte proliferation and delayed neuronal death are attenuated by cyclin-dependent kinase inhibition

Astroglial proliferation and delayed neuronal death are two common pathological processes in the ischemic brain. However, it is not clear if astrogliosis causes delayed neuronal death. In this study, we addressed this potential linkage by examining the relationship between attenuated astrocyte proliferation, induced by cyclin-dependent kinase (CDK) inhibition, and delayed neuronal death in rat ischemic hippocampus. Our results show that following middle cerebral artery occlusion (MCAO), astrocyte hypertrophy and proliferation were closely associated with delayed neuronal death. Importantly, administration of olomoucine, a selective CDK inhibitor, not only suppressed astroglial proliferation and glial scar formation, but also decreased neuronal cell death in the ischemic boundary zone and hippocampal CA1 region at days 1 and 30 after MCAO. These results indicate that reactive astrogliosis and delayed neuronal death, at least in rat hippocampus, are sequential pathological events following MCAO. Therefore, suppressing astroglial cell cycle progression in acute focal cerebral ischemia may be beneficial to neuronal survival. Our study also implies that cell cycle regulation should be considered as a promising future therapeutic intervention in treating those neurological diseases characterized by an excessive astrocyte proliferation.

Delayed combinatorial treatment with flavopiridol and minocycline provides longer term protection for neuronal soma but not dendrites following global ischemia

We previously reported that delayed administration of the general cyclin-dependent kinase inhibitor flavopiridol following global ischemia provided transient neuroprotection and improved behavioral performance. However, it failed to provide longer term protection. In the present study, we investigate the ability of delayed flavopiridol in combination with delayed minocycline, another neuroprotectant to provide sustained protection following global ischemia. We report that a delayed combinatorial treatment of flavopiridol and minocycline provides synergistic protection both 2 and 10 weeks following ischemia. However, protected neurons in the hippocampal CA1 are synaptically impaired as assessed by electrophysio logical field potential recordings. This is likely because of the presence of degenerated processes in the CA1 even with combinatorial therapy. This indicates that while we have addressed one important pre-clinical parameter by dramatically improving long-term neuronal survival with delayed combinatorial therapy, the issue of synaptic preservation of protected neurons still exists. These results also highlight the important observation that protection does not always lead to proper function.

An investigation of the expression of G1-phase cell cycle proteins in focal cortical dysplasia type IIB

Balloon cells (BCs) are the pathologic hallmark of focal cortical dysplasia type IIB, a common cause of pharmacoresistent epilepsy. Expression of markers of cell immaturity and of the proliferation marker minichromosome maintenance protein 2 (mcm2) have been previously shown in BCs, suggesting that these cells might represent a pool of less-differentiated cells licensed for replication. An alternative explanation is that these cells are the remnants of early cortical plate cells that have failed to differentiate or to be eliminated during development and are arrested in the cell cycle, a hypothesis that this study aims to explore. Using immunohistochemical methods and semiquantitative analysis in 19 cases of focal cortical dysplasia (ages 1-81 years), we studied the expression of cell cycle proteins important either in regulating progression through the G1 phase or inducing cell arrest and promoting premature senescence. Only a small fraction of BCs expressed geminin, suggesting that few BCs enter the S phase or complete the cell cycle. Variable expression of nonphosphorylated retinoblastoma protein (Rb), cdk4, and p53 was noted in BCs. Cyclin E, D1, cdk2, phosphorylated Rb (795 and 807/811), and checkpoint 2 expression levels were low in BCs. These findings suggest early rather than late G1 arrest. Cell senescence could be induced by an undefined cerebral insult during development or alternatively represent a physiologic replicative senescence. These findings also suggest that dysregulation of cell cycle pathways may occur in focal cortical dysplasia, which opens further areas for exploration as potential new treatment avenues.

The Parkinson's disease gene DJ-1 is also a key regulator of stroke-induced damage

Recent evidence has indicated that common mechanisms play roles among multiple neurological diseases. However, the specifics of these pathways are not completely understood. Stroke is caused by the interruption of blood flow to the brain, and cumulative evidence supports the critical role of oxidative stress in the ensuing neuronal death process. DJ-1 (PARK7) has been identified as the gene linked to early-onset familial Parkinson's disease. Currently, our work also shows that DJ-1 is central to death in both in Vitro and in Vivo models of stroke. Loss of DJ-1 increases the sensitivity to excitotoxicity and ischemia, whereas expression of DJ-1 can reverse this sensitivity and indeed provide further protection. Importantly, DJ-1 expression decreases markers of oxidative stress after stroke insult in Vivo, suggesting that DJ-1 protects through alleviation of oxidative stress. Consistent with this finding, we demonstrate the essential role of the oxidation-sensitive cysteine-106 residue in the neuroprotective activity of DJ-1 after stroke. Our work provides an important example of how a gene seemingly specific for one disease, in this case Parkinson's disease, also appears to be central in other neuropathological conditions such as stroke. It also highlights the important commonalities among differing neuropathologies.

Role of Cdk5-mediated phosphorylation of Prx2 in MPTP toxicity and Parkinson's disease

We reported previously that calpain-mediated Cdk5 activation is critical for mitochondrial toxin-induced dopaminergic death. Here, we report a target that mediates this loss. Prx2, an antioxidant enzyme, binds Cdk5/p35. Prx2 is phosphorylated at T89 in neurons treated with MPP+ and/or MPTP in animals in a calpain/Cdk5/p35-dependent manner. This phosphorylation reduces Prx2 peroxidase activity. Consistent with this, p35-/- neurons show reduced oxidative stress upon MPP+ treatment. Expression of Prx2 and Prx2T89A, but not the phosphorylation mimic Prx2T89E, protects cultured and adult neurons following mitochondrial insult. Finally, downregulation of Prx2 increases oxidative stress and sensitivity to MPP+. We propose a mechanistic model by which mitochondrial toxin leads to calpain-mediated Cdk5 activation, reduced Prx2 activity, and decreased capacity to eliminate ROS. Importantly, increased Prx2 phosphorylation also occurs in nigral neurons from postmortem tissue from Parkinson's disease patients when compared to control, suggesting the relevance of this pathway in the human condition.

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.

The nuclear localization of SET mediated by impalpha3/impbeta attenuates its cytosolic toxicity in neurons

SET is a multi-functional protein in proliferating cells. Some of the proposed functions of SET suggest an important nuclear role. However, the nuclear import pathway of SET is also unknown and the function of SET in neurons is unclear. Presently, using cortical neurons, we report that the nuclear import of SET is mediated by an impalpha/impbeta-dependent pathway. Nuclear localization signal, (168)KRSSQTQNKASRKR(181), in SET interacts with impalpha3, which recruits impbeta to form a ternary complex, resulting in efficient transportation of SET into nucleus. By in vitro nuclear import assay based on digitonin-permeabilized neurons, we further demonstrated that the nuclear import of SET relies on Ran GTPase. We provide evidence that this nuclear localization of SET is important in neuronal survival. Under basal conditions, SET is predominately nuclear. However, upon death induced by genotoxic stress, endogenous SET decreases in the nucleus and increases in the cytoplasm. Consistent with a toxic role of SET in the cytoplasm, targeted expression of SET to the cytoplasm exacerbates death compared to wild type SET expression which is protective following DNA damage. Taken together, our results indicate that SET is imported into the nucleus through its association with impalpha3/impbeta, and that localization of SET is important in regulation of neuronal death.

Expression of cyclin-dependent kinase 5 mRNA and protein in the human brain following acute ischemic stroke

Neuronal cell death after brain ischemia may be regulated by activation of cyclin-dependent kinase 5 (Cdk5). In this study, expression of Cdk5 and its activator p35/p25 was examined in human post-mortem stroke tissue and in human cerebral cortical fetal neurons and human brain microvascular endothelial cells exposed to oxygen-glucose deficiency and reperfusion. The majority of patients demonstrated increased expression of Cdk5 and p-Cdk5 in stroke-affected tissue, with about a third showing increased p35 and p25 cleaved fragment as determined by Western blotting. An increase in Cdk5-, p-Cdk5- and p35-positive neurons and microvessels occurred in stroke-affected regions of patients. Staining of neurons became irregular and clumped in the cytoplasm, and nuclear translocation occurred, with colocalization of p35 and Cdk5. Association of Cdk5 with nuclear damage was demonstrated by coexpression of nuclear Cdk5 in TUNEL-positive neurons and microvessels in peri-infarcted regions. In vitro studies showed up-regulation and/or nuclear translocation of Cdk5, p-Cdk5 and p35 in neurons and endothelial cells subjected to oxygen-glucose deficiency, and strong staining was associated with propidium iodide positive nuclei, an indicator of cellular damage. These results provide new evidence for a role of Cdk5 in the events associated with response to ischemic injury in humans.

Novel rat Alzheimer's disease models based on AAV-mediated gene transfer to selectively increase hippocampal Abeta levels

Background

Alzheimer's disease (AD) is characterized by a decline in cognitive function and accumulation of amyloid-β peptide (Aβ) in extracellular plaques. Mutations in amyloid precursor protein (APP) and presenilins alter APP metabolism resulting in accumulation of Aβ42, a peptide essential for the formation of amyloid deposits and proposed to initiate the cascade leading to AD. However, the role of Aβ40, the more prevalent Aβ peptide secreted by cells and a major component of cerebral Aβ deposits, is less clear. In this study, virally-mediated gene transfer was used to selectively increase hippocampal levels of human Aβ42 and Aβ40 in adult Wistar rats, allowing examination of the contribution of each to the cognitive deficits and pathology seen in AD.

Results

Adeno-associated viral (AAV) vectors encoding BRI-Aβ cDNAs were generated resulting in high-level hippocampal expression and secretion of the specific encoded Aβ peptide. As a comparison the effect of AAV-mediated overexpression of APPsw was also examined. Animals were tested for development of learning and memory deficits (open field, Morris water maze, passive avoidance, novel object recognition) three months after infusion of AAV. A range of impairments was found, with the most pronounced deficits observed in animals co-injected with both AAV-BRI-Aβ40 and AAV-BRI-Aβ42. Brain tissue was analyzed by ELISA and immunohistochemistry to quantify levels of detergent soluble and insoluble Aβ peptides. BRI-Aβ42 and the combination of BRI-Aβ40+42 overexpression resulted in elevated levels of detergent-insoluble Aβ. No significant increase in detergent-insoluble Aβ was seen in the rats expressing APPsw or BRI-Aβ40. No pathological features were noted in any rats, except the AAV-BRI-Aβ42 rats which showed focal, amorphous, Thioflavin-negative Aβ42 deposits.

Conclusion

The results show that AAV-mediated gene transfer is a valuable tool to model aspects of AD pathology in vivo, and demonstrate that whilst expression of Aβ42 alone is sufficient to initiate Aβ deposition, both Aβ40 and Aβ42 may contribute to cognitive deficits.

Broad spectrum neuroprotection profile of phosphodiesterase inhibitors as related to modulation of cell-cycle elements and caspase-3 activation

Cellular injury can involve the aberrant stimulation of cell cycle proteins in part through activation of phosphodiesterases (PDEs) and downstream expression of cell-cycle components such as cyclin D1. In mature non-proliferating cells activation of the cell cycle can lead to the induction of programmed cell death. In the present study, we investigated the in vitro neuroprotective efficacy and mechanism of action of vinpocetine (PDE1 inhibitor), trequinsin (PDE3 inhibitor), and rolipram (PDE4 inhibitor) in four mechanistically-distinct models of injury to primary rat cortical neurons as related to cell cycle regulation and apoptosis. Cellular injury was induced by hypoxia/hypoglycemia, veratridine (10 microM), staurosporine (1 microM), or glutamate (100 microM), resulting in average neuronal cell death rates of 43-48% as determined by MTT assay. Treatment with each PDE inhibitor (PDEI) resulted in a similar concentration-dependent neuroprotection profile with maximal effective concentrations of 5-10 microM (55-77% neuroprotection) in all four neurotoxicity models. Direct cytotoxicity due to PDE inhibition alone was not observed at concentrations below 100 microM. Further studies indicated that PDEIs can suppress the excitotoxic upregulation of cyclin D1 similar to the effects of flavopiridol, a cyclin-dependent kinase inhibitor, including suppression of pro-apoptotic caspase-3 activity. Overall, these data indicate that PDEIs are broad-spectrum neuroprotective agents acting through modulation of cell cycle elements and may offer a novel mode of therapy against acute injury to the brain.

Role of Cell Cycle Proteins in CNS Injury

Following trauma or ischemia to the central nervous system (CNS), there is a marked increase in the expression of cell cycle-related proteins. This up-regulation is associated with apoptosis of post-mitotic cells, including neurons and oligodendrocytes, both in vitro and in vivo. Cell cycle activation also induces proliferation of astrocytes and microglia, contributing to the glial scar and microglial activation with release of inflammatory factors. Treatment with cell cycle inhibitors in CNS injury models inhibits glial scar formation and neuronal cell death, resulting in substantially decreased lesion volumes and improved behavioral recovery. Here we critically review the role of cell cycle pathways in the pathophysiology of experimental stroke, traumatic brain injury and spinal cord injury, and discuss the potential of cell cycle inhibitors as neuroprotective agents.

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.