Cerebral ischemia, cell cycle elements and Cdk5

Cellular and Molecular Mechanisms of R/S-Roscovitine and CDKs Related Inhibition under Both Focal and Global Cerebral Ischemia: A Focus on Neurovascular Unit and Immune Cells

Ischemic stroke is the second leading cause of death worldwide. Following ischemic stroke, Neurovascular Unit (NVU) inflammation and peripheral leucocytes infiltration are major contributors to the extension of brain lesions. For a long time restricted to neurons, the 10 past years have shown the emergence of an increasing number of studies focusing on the role of Cyclin-Dependent Kinases (CDKs) on the other cells of NVU, as well as on the leucocytes. The most widely used CDKs inhibitor, (R)-roscovitine, and its (S) isomer both decreased brain lesions in models of global and focal cerebral ischemia. We previously showed that (S)-roscovitine acted, at least, by modulating NVU response to ischemia. Interestingly, roscovitine was shown to decrease leucocytes-mediated inflammation in several inflammatory models. Specific inhibition of roscovitine majors target CDK 1, 2, 5, 7, and 9 showed that these CDKs played key roles in inflammatory processes of NVU cells and leucocytes after brain lesions, including ischemic stroke. The data summarized here support the investigation of roscovitine as a potential therapeutic agent for the treatment of ischemic stroke, and provide an overview of CDK 1, 2, 5, 7, and 9 functions in brain cells and leucocytes during cerebral ischemia.

CCL11 Differentially Affects Post-Stroke Brain Injury and Neuroregeneration in Mice Depending on Age

CCL11 has recently been shown to differentially affect cell survival under various pathological conditions including stroke. Indeed, CCL11 promotes neuroregeneration in neonatal stroke mice. The impact of CCL11 on the adult ischemic brain, however, remains elusive. We therefore studied the effect of ectopic CCL11 on both adolescent (six-week) and adult (six-month) C57BL6 mice exposed to stroke. Intraperitoneal application of CCL11 significantly aggravated acute brain injury in adult mice but not in adolescent mice. Likewise, post-stroke neurological recovery after four weeks was significantly impaired in adult mice whilst CCL11 was present. On the contrary, CCL11 stimulated gliogenesis and neurogenesis in adolescent mice. Flow cytometry analysis of blood and brain samples revealed a modification of inflammation by CCL11 at subacute stages of the disease. In adolescent mice, CCL11 enhances microglial cell, B and T lymphocyte migration towards the brain, whereas only the number of B lymphocytes is increased in the adult brain. Finally, the CCL11 inhibitor SB297006 significantly reversed the aforementioned effects. Our study, for the first time, demonstrates CCL11 to be a key player in mediating secondary cell injury under stroke conditions. Interfering with this pathway, as shown for SB297006, might thus be an interesting approach for future stroke treatment paradigms.

Upregulation of Cdh1 in the trigeminal spinal subnucleus caudalis attenuates trigeminal neuropathic pain via inhibiting GABAergic neuronal apoptosis

Trigeminal neuropathic pain (TNP) remains a tremendous clinical challenge due to its elusive mechanisms. Previous studies showed that peripheral nerve injury facilitated a selective GABAergic neuronal apoptosis in the superficial dorsal horn and contributed to the development and maintenance of neuropathic pain. It has also demonstrated that downregulation of the anaphase-promoting complex/cyclosome(APC/C) and its coactivator Cdh1 contribute to neuronal apoptosis in diverse neurodegenerative diseases. However, whether APC/C-Cdh1 downregulation could induce GABAergic neuronal apoptosis in trigeminal caudalis nucleus (Vc), and then contribute to the development and maintenance of TNP remains unknown. In this study, we aimed to investigate the role of APC/C-Cdh1 in a TNP rat model and its underlying mechanisms. Our results showed that Cdh1 was primarily distributed in superficial laminae of Vc and significantly downregulated in Vc at day 14 post trigeminal nerve injury. Furthermore, trigerminal nerve injury leads to neuronal apoptosis, especially GABAergic interneurons in the superficial of Vc. Upregulating Cdh1 in Vc ameliorated mechanical allodynia and inhibited GABAergic neuronal apoptosis induced by chronic constriction injury of trigeminal infraorbital nerve (CCI-ION).

Down-regulation of cyclin-dependent kinase 5 attenuates p53-dependent apoptosis of hippocampal CA1 pyramidal neurons following transient cerebral ischemia

Abnormal activation of cyclin-dependent kinase 5 (Cdk5) is associated with pathophysiological conditions. Ischemic preconditioning (IPC) can provide neuroprotective effects against subsequent lethal ischemic insult. The objective of this study was to determine how Cdk5 and related molecules could affect neuroprotection in the hippocampus of gerbils after with IPC [a 2-min transient cerebral ischemia (TCI)] followed by 5-min subsequent TCI. Hippocampal CA1 pyramidal neurons were dead at 5 days post-TCI. However, treatment with roscovitine (a potent inhibitor of Cdk5) and IPC protected CA1 pyramidal neurons from TCI. Expression levels of Cdk5, p25, phospho (p)-Rb and p-p53 were increased in nuclei of CA1 pyramidal neurons at 1 and 2 days after TCI. However, these expressions were attenuated by roscovitine treatment and IPC. In particular, Cdk5, p-Rb and p-p53 immunoreactivities in their nuclei were decreased. Furthermore, TUNEL-positive CA1 pyramidal neurons were found at 5 days after TCI with increased expression levels of Bax, PUMA, and activated caspase-3. These TUNEL-positive cells and increased molecules were decreased by roscovitine treatment and IPC. Thus, roscovitine treatment and IPC could protect CA1 pyramidal neurons from TCI through down-regulating Cdk5, p25, and p-p53 in their nuclei. These findings indicate that down-regulating Cdk5 might be a key strategy to attenuate p53-dependent apoptosis of CA1 pyramidal neurons following TCI.

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.

Sustained (S)-roscovitine delivery promotes neuroprotection associated with functional recovery and decrease in brain edema in a randomized blind focal cerebral ischemia study

Stroke is a devastating disorder that significantly contributes to death, disability and healthcare costs. In ischemic stroke, the only current acute therapy is recanalization, but the narrow therapeutic window less than 6 h limits its application. The current challenge is to prevent late cell death, with concomitant therapy targeting the ischemic cascade to widen the therapeutic window. Among potential neuroprotective drugs, cyclin-dependent kinase inhibitors such as (S)-roscovitine are of particular relevance. We previously showed that (S)-roscovitine crossed the blood-brain barrier and was neuroprotective in a dose-dependent manner in two models of middle cerebral artery occlusion (MCAo). According to the Stroke Therapy Academic Industry Roundtable guidelines, the pharmacokinetics of (S)-roscovitine and the optimal mode of delivery and therapeutic dose in rats were investigated. Combination of intravenous (IV) and continuous sub-cutaneous (SC) infusion led to early and sustained delivery of (S)-roscovitine. Furthermore, in a randomized blind study on a transient MCAo rat model, we showed that this mode of delivery reduced both infarct and edema volume and was beneficial to neurological outcome. Within the framework of preclinical studies for stroke therapy development, we here provide data to improve translation of pre-clinical studies into successful clinical human trials.

Changes in the expression level of MAPK pathway components induced by monosodium glutamate-administration produce neuronal death in the hippocampus from neonatal rats

Excessive Glutamate (Glu) release may trigger excitotoxic cellular death by the activation of intracellular signaling pathways that transduce extracellular signals to the cell nucleus, which determines the onset of a death program. One such signaling pathway is the mitogen-activated protein kinases (MAPK), which is involved in both survival and cell death. Experimental evidences from the use of specific inhibitors supports the participation of some MAPK pathway components in the excitotoxicity mechanism, but the complete process of this activation, which terminates in cell damage and death, is not clearly understood. The present work, we investigated the changes in the expression level of some MAPK-pathway components in hippocampal excitotoxic cell death in the neonatal rats using an experimental model of subcutaneous monosodium glutamate (MSG) administration on postnatal days (PD) 1, 3, 5 and 7. Data were collected at different ages through PD 14. Cell viability was evaluated using fluorescein diacetate mixed with propidium iodide (FDA-PI), and the Nissl-staining technique was used to evaluate histological damage. Transcriptional changes were also investigated in 98 components of the MAPK pathway that are associated with cell damage. These results are an evidence of that repetitive use of MSG, in neonatal rats, induces cell damage-associated transcriptional changes of MAPK components, that might reflect a differential stage of both biochemical and molecular brain maturation. This work also suggests that some of the proteins evaluated such as phosphorylated retinoblastoma (pRb) protein, which was up-regulated, could regulate the response to excitotoxic through modulation of the process of re-entry into the cell cycle in the hippocampus of rats treated with MSG.

TFP5 peptide, derived from CDK5-activating cofactor p35, provides neuroprotection in early-stage of adult ischemic stroke

Cyclin-dependent kinase 5 (CDK5) is a multifaceted protein shown to play important roles in the central nervous system. Abundant evidence indicates that CDK5 hyperactivities associated with neuronal apoptosis and death following ischemic stroke. CDK5 activity increases when its cofactor p35 cleaves into p25 during ischemia. Theoretically, inhibition of CDK5/p25 activity or reduction of p25 would be neuroprotective. TFP5, a modified 24-aa peptide (Lys254-Ala277) derived from p35, was found to effectively inhibit CDK5 hyperactivity and improve the outcomes of Alzheimer's disease and Parkinson's disease in vivo. Here, we showed that intraperitoneal injection of TFP5 significantly decreased the size of ischemia in early-stage of adult ischemic stroke rats. Relative to controls, rats treated with TFP5 displayed reduced excitotoxicity, neuroinflammation, apoptosis, astrocytes damage, and blood-brain barrier disruption. Our findings suggested that TFP5 might serve as a potential therapeutic candidate for acute adult ischemic stroke.

Neuroprotection Induced by Transplanted CDK5 Knockdown Astrocytes in Global Cerebral Ischemic Rats

Cerebral ischemia is a cerebrovascular episode that generates a high incidence of death and physical and mental disabilities worldwide. Excitotoxicity, release of free radicals, and exacerbated immune response cause serious complications in motor and cognitive areas during both short and long time frames post-ischemia. CDK5 is a kinase that is widely involved in the functions of neurons and astrocytes, and its over-activation is implicated in neurodegenerative processes. In this study, we evaluated the brain parenchymal response to the transplantation of CDK5-knockdown astrocytes into the somatosensory cortex after ischemia in rats. Male Wistar rats were subjected to the two-vessel occlusion (2VO) model of global cerebral ischemia and immediately transplanted with shCDK5miR- or shSCRmiR-transduced astrocytes or with untransduced astrocytes (Control). Our findings showed that animals transplanted with shCDK5miR astrocytes recovered motor and neurological performance better than with those transplanted with WT or shSCRmiR astrocytes. Cell transplantation produced an overall prevention of neuronal loss, and CDK5-knockdown astrocytes significantly increased the immunoreactivity (IR) of endogenous GFAP in branches surrounding blood vessels, accompanied by the upregulation of PECAM-1 IR in the walls of vessels in the motor and somatosensory regions and by an increase in Ki67 IR in the subventricular zone (SVZ), partially associated with the production of BDNF. Together, our data suggest that transplantation of shCDK5miR astrocytes protects the neurovascular unit in ischemic rats, allowing the motor and neurological function recovery.

Activation of Cdk5/p25 and tau phosphorylation following chronic brain hypoperfusion in rats involves microRNA-195 down-regulation

Chronic brain hypoperfusion (CBH) is a common clinical feature of Alzheimer's disease and vascular dementia, but the underlying molecular mechanism is unclear. Our previous study reported that the down-regulation of microRNA-195 (miR-195) promotes amyloidogenesis via regulation of amyloid precursor protein and β-site amyloid precursor protein cleaving enzyme 1 (BACE1) expression at the post-transcriptional level in CBH rats with bilateral common carotid artery occlusion (2VO). CBH owing to unilateral common carotid artery occlusion (UCCAO) increases tau phosphorylation levels at multiple phosphorylation sites in the brain, but the molecular mechanism is poorly understood. The purpose of this study was to investigate whether miR-195 could both deregulate amyloid metabolism and indirectly deregulate tau phosphorylation in CBH. We observed that 2VO leads to tau hyperphosphorylation at Ser202/Thr205, Ser262, Thr231, and Ser422 and to the conversion from cyclin-dependent kinase 5 (Cdk5)/p35 to Cdk5/p25 in rat hippocampi. Endogenous miR-195 was knocked down using over-expression of its antisense molecule (pre-AMO-miR-195) via a lentivirus (lenti-pre-AMO-miR-195); this knockdown increased the tau phosphorylation at Ser202/Thr205, Ser262, Thr231, Ser422, and the Cdk5/p25 activation, but over-expression of miR-195 using lenti-pre-miR-195 decreased the tau phosphorylation and Cdk5/p25 activation. Further in vitro studies demonstrated that miR-195 over-expression prevented tau hyperphosphorylation and Cdk5/p35 activity, which were increased by miR-195 inhibition. A dual luciferase reporter assay showed that miR-195 bound to the Cdk5r1 gene, which encodes p35 protein, in the 3'UTR and inhibited p35 expression. We concluded that tau hyperphosphorylation involves the down-regulation of miR-195, which is mediated by Cdk5/p25 activation in 2VO rats. Our findings demonstrated that down-regulation of miR-195 led to increased vulnerability via the regulation of multiple targets. Schematic diagram of miR-195 mediated Aβ aggregation and tau hyperphosphorylation in chronic brain hypoperfusion (CBH). First, CBH results in the elevation of nuclear factor-κB (NF-κB), which binds with the promoter sequences of miR-195 and negatively regulates the expression of miR-195. Second, down-regulated miR-195 induces up-regulation of APP and BACE1 and leads to an increase in Aβ levels. Third, some of the elevated Aβ then enter the intracellular space and activate calpain, which promotes the conversion of Cdk5/p35 to Cdk5/p25 and catalyzes the degradation of IκB; IκB is an inhibitor of NF-κB, which activates NF-κB. Cdk5/p25 directly phosphorylates Tau. Fourth, down-regulated miR-195 induces an up-regulation of p35, which provides the active substrates of p25. Our findings demonstrated that the down-regulation of miR-195 plays a key role in the increased vulnerability to dementia via the regulation of multiple targets following CBH.

PFKFB3-mediated glycolysis is involved in reactive astrocyte proliferation after oxygen-glucose deprivation/reperfusion and is regulated by Cdh1

Reactive astrocyte proliferation is involved in many central degenerative diseases. The enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase isoform 3 (PFKFB3), an allosteric activator of 6-phosphofructo-1-kinase (PFK1), controls glycolytic flux. Furthermore, APC/C-Cdh1 plays a crucial role in brain metabolism by regulating PFKFB3 expression. Previous studies have defined the roles of PFKFB3-mediated glycolysis in pathological angiogenesis, cell autophagy, and amyloid plaque deposition in proliferating cells. However, the role of PFKFB3 in reactive astrocyte proliferation after cerebral ischemia is unknown. In this study, we cultured rat primary cortical astrocytes and established an oxygen-glucose deprivation/reperfusion (OGD/R) model to mimic cerebral ischemia in vivo. Astrocyte proliferation was measured by western blotting for proliferating cell nuclear antigen (PCNA) and by EdU incorporation. We found that OGD/R up-regulated PFKFB3 and PFK1 expression, which was accompanied by reactive astrocyte proliferation. Knockdown of PFKFB3 by siRNA transfection significantly inhibited reactive astrocyte proliferation and lactate release, an indicator of glycolysis. We found that PFKFB3 and PFK1 expression were down-regulated and lactate release was decreased when OGD/R-induced astrocyte proliferation was inhibited by a Cdh1-expressing lentivirus. Thus, reactive astrocyte proliferation can be effectively suppressed by down-regulation of PFKFB3 through control of glycolytic flux, which is downstream of APC/C-Cdh1.

Role of CDK5/cyclin complexes in ischemia-induced death and survival of renal tubular cells

Ischemia reperfusion processes induce damage in renal tubules and compromise the viability of kidney transplants. Understanding the molecular events responsible for tubule damage and recovery would help to develop new strategies for organ preservation. CDK5 has been traditionally considered a neuronal kinase with dual roles in cell death and survival. Here, we demonstrate that CDK5 and their regulators p35/p25 and cyclin I are also expressed in renal tubular cells. We show that treatment with CDK inhibitors promotes the formation of pro-survival CDK5/cyclin I complexes and enhances cell survival upon an ischemia reperfusion pro-apoptotic insult. These findings support the benefit of treating with CDK inhibitors for renal preservation, assisting renal tubule protection.

Cdh1 inhibits reactive astrocyte proliferation after oxygen-glucose deprivation and reperfusion

Anaphase-promoting complex (APC) and its co-activator Cdh1 are required for cell cycle regulation in proliferating cells. Recent studies have defined diverse functions of APC-Cdh1 in nervous system development and injury. Our previous studies have demonstrated the activity of APC-Cdh1 is down-regulated in hippocampus after global cerebral ischemia. But the detailed mechanisms of APC-Cdh1 in ischemic nervous injury are unclear. It is known that astrocyte proliferation is an important pathophysiological process following cerebral ischemia. However, the role of APC-Cdh1 in reactive astrocyte proliferation is not determined yet. In the present study, we cultured primary cerebral astrocytes and set up in vitro oxygen-glucose deprivation and reperfusion model. Our results showed that the expression of Cdh1 was decreased while Skp2 (the downstream substrate of APC-Cdh1) was increased in astrocytes after 1h oxygen-glucose deprivation and reperfusion. The down-regulation of APC-Cdh1 was coupled with reactive astrocyte proliferation. By constructing Cdh1 expressing lentivirus system, we also found exogenous Cdh1 can down-regulate Skp2 and inhibit reactive astrocyte proliferation induced by oxygen-glucose deprivation and reperfusion. Moreover, Western blot showed that other downstream proteins of APC-Cdh1, PFK-1 and SnoN, were decreased in the inhibition of reactive astrocyte proliferation with Cdh1 expressing lentivirus treatment. These results suggest that Cdh1 plays an important role in the regulation of reactive astrocyte proliferation induced by oxygen-glucose deprivation and reperfusion.

Inhibition of calpain-regulated p35/cdk5 plays a central role in sildenafil-induced protection against chemical hypoxia produced by malonate

Phosphodiesterase 5 (PDE5) inhibitors have recently been reported to exert beneficial effects against ischemia-reperfusion injury in several organs but their neuroprotective effects in brain stroke models are scarce. The present study was undertaken to assess the effects of sildenafil against cell death caused by intrastriatal injection of malonate, an inhibitor of succinate dehydrogenase; which produces both energy depletion and lesions similar to those seen in cerebral ischemia. Our data demonstrate that sildenafil (1.5mg/kg by mouth (p.o.)), given 30min before malonate (1.5μmol/2μL), significantly decreased the lesion volume caused by this toxin. This protective effect can be probably related to the inhibition of excitotoxic pathways. Thus, malonate induced the activation of the calcium-dependent protease, calpain and the cyclin-dependent kinase 5, cdk5; which resulted in the hyperphosphorylation of tau and the cleavage of the protective transcription factor, myocyte enhancer factor 2, MEF2. All these effects were also significantly reduced by sildenafil pre-treatment, suggesting that sildenafil protects against malonate-induced cell death through the regulation of the calpain/p25/cdk5 signaling pathway. Similar findings were obtained using inhibitors of calpain or cdk5, further supporting our contention. Sildenafil also increased MEF2 phosphorylation and Bcl-2/Bax and Bcl-xL/Bax ratios, effects that might as well contribute to prevent cell death. Finally, sildenafil neuroprotection was extended not only to rat hippocampal slices subjected to oxygen and glucose deprivation when added at the time of reoxygenation, but also, in vivo when administered after malonate injection. Thus, the therapeutic window for sildenafil against malonate-induced hypoxia was set at 3h.

Gene profiling identifies commonalities in neuronal pathways in excitotoxicity: evidence favouring cell cycle re-activation in concert with oxidative stress

Excitotoxicity, induced by the aberrant rise in cytosolic Ca(2+) level, is a major neuropathological process in numerous neurodegenerative disorders. It is triggered when extracellular glutamate (Glu) concentration reaches neuropathological levels resulting in dysregulation and hyper-activation of ionotropic glutamate receptor subtype (iGluRs). Even though all three members of the iGluRs, namely N-methyl-d-aspartate (NMDAR), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPAR) and kainate (KAR) receptors are implicated in excitotoxicity, their individual contributions to downstream signaling transduction have not been explored. In this study, we report a comprehensive description of the recruitment of cellular processes in neurons upon iGluR activation during excitotoxicity through temporal (5h, 15h, and 24h) global gene profiling of AMPA, KA, NMDA, and Glu excitotoxic models. DNA microarray analyses of mouse primary cortical neurons treated with these four pharmacological agonists are further validated via real-time PCR. Bi-model analyses against Glu model demonstrate that NMDARs and KARs play a more pivotal role in Glu-mediated excitotoxicity, with a higher degree of global gene profiling overlaps, as compared to that of AMPARs. Comparison of global transcriptomic profiles reveals aberrant calcium ion binding and homeostasis, organellar (lysosomal and endoplasmic reticulum) stress, oxidative stress, cell cycle re-entry and activation of cell death processes as the main pathways that are significantly modulated across all excitotoxicity models. Singular profile analyses demonstrate substantial transcriptional regulation of numerous cell cycle proteins. For the first time, we show that iGluR activation forms the basis of cell cycle re-activation, and together with oxidative stress fulfill the "two-hit" hypothesis that accelerates neurodegeneration.

Effects of roscovitine, a cell cyclin [correction of cycling]-dependent kinase inhibitor, on intraocular pressure of rabbit and retinal ganglion cell damage

Glaucoma is characterized by increased intraocular pressure (IOP) and the death of retinal ganglion cells. Previously, we reported that roscovitine, a cell cyclin-dependent kinase (CDK) inhibitor, strongly induced relaxation of porcine trabecular meshwork cells, implicating an interaction with lowered IOP. In addition, the activity of CDKs is known to increase in response to high IOP, which is linked to retinal ganglion cell damage. However, the effects of roscovitine on IOP and retinal damage have not been investigated. Roscovitine has racemic isomers that differ in their inhibition of CDKs. Therefore, we investigated the effects of both the R-isomer and the S-isomer on the IOP of rabbits and on the death of cultured retinal ganglion cells. In the in vivo rabbit experiment, instillation of both isomers significantly lowered the IOP. In the in vitro cell experiment, the R-isomer amplified the effects of tunicamycin, an endoplasmic reticulum stress inducer, and increased oxygen-glucose deprivation-induced cell death, whereas S-isomer significantly inhibited this cell death. Therefore, both isomers of roscovitine can lower the IOP, but from the perspective of neuroprotective effects, the S-isomer was superior to the R-isomer. The S-isomer of roscovitine may be useful as an agent for lowering IOP and its neuroprotective effects.

Regulation of APC/C-Cdh1 and its function in neuronal survival

Neurons are post-mitotic cells that undergo an active downregulation of cell cycle-related proteins to survive. The activity of the anaphase-promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase that regulates cell cycle progression in proliferating cells, plays a relevant role in post-mitotic neurons. Recent advances in the study of the regulation of APC/C have documented that the APC/C-activating cofactor, Cdh1, is essential for the function(s) of APC/C in neuronal survival. Here, we review the normal regulation of APC/C activity in proliferating cells and neurons. We conclude that in neurons the APC/C-Cdh1 complex actively downregulates the stability of the cell cycle protein cyclin B1 and the glycolytic enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase-3. Keeping these proteins destabilized is critical both for preventing the aberrant reentry of post-mitotic neurons into the cell cycle and for maintaining their reduced antioxidant status. Further understanding of the pathophysiological regulation of these proteins by APC/C-Cdh1 in neurons will be important for the search for novel therapeutic targets against neurodegeneration.

Cyclin-dependent kinase inhibition with roscovitine: neuroprotection in acute ischemic stroke

Stroke is the third most common cause of mortality and the leading cause of disability in industrialized country. According to population based-studies, ischemic stroke accounts for 67-80% of all strokes. Thrombolysis is used during the acute phase in only 2-5% of ischemic patients. Clinical trials of candidate neuroprotective agents have failed to identify viable therapies for ischemic stroke in humans. There is therefore a great need for new therapeutic strategies, considering that not all brain cells die immediately after ischemic stroke.

Delayed treatment with systemic (S)-roscovitine provides neuroprotection and inhibits in vivo CDK5 activity increase in animal stroke models

Background

Although quite challenging, neuroprotective therapies in ischemic stroke remain an interesting strategy to counter mechanisms of ischemic injury and reduce brain tissue damage. Among potential neuroprotective drug, cyclin-dependent kinases (CDK) inhibitors represent interesting therapeutic candidates. Increasing evidence indisputably links cell cycle CDKs and CDK5 to the pathogenesis of stroke. Although recent studies have demonstrated promising neuroprotective efficacies of pharmacological CDK inhibitors in related animal models, none of them were however clinically relevant to human treatment.

Methodology/Principal Findings

In the present study, we report that systemic delivery of (S)-roscovitine, a well known inhibitor of mitotic CDKs and CDK5, was neuroprotective in a dose-dependent manner in two models of focal ischemia, as recommended by STAIR guidelines. We show that (S)-roscovitine was able to cross the blood brain barrier. (S)-roscovitine significant in vivo positive effect remained when the compound was systemically administered 2 hrs after the insult. Moreover, we validate one of (S)-roscovitine in vivo target after ischemia. Cerebral increase of CDK5/p25 activity was observed 3 hrs after the insult and prevented by systemic (S)-roscovitine administration. Our results show therefore that roscovitine protects in vivo neurons possibly through CDK5 dependent mechanisms.

Conclusions/Significance

Altogether, our data bring new evidences for the further development of pharmacological CDK inhibitors in stroke therapy.

An unusual member of the Cdk family: Cdk5

The proline-directed serine threonine kinase, Cdk5, is an unusual molecule that belongs to the well-known large family of proteins, cyclin-dependent kinases (Cdks). While it has significant homology with the mammalian Cdk2 and yeast cdc2, unlike the other Cdks, it has little role to play in cell cycle regulation and is activated by non-cyclin proteins, p35 and p39. It phosphorylates a spectrum of proteins, most of them associated with cell morphology and motility. A majority of known substrates of Cdk5 are cytoskeletal elements, signalling molecules or regulatory proteins. It also appears to be an important player in cell-cell communication. Highly conserved, Cdk5 is most abundant in the nervous system and is of special interest to neuroscientists as it appears to be indispensable for normal neural development and function. In normal cells, transcription and activity of Cdk5 is tightly regulated. Present essentially in post-mitotic neurons, its normal activity is obligatory for migration and differentiation of neurons in developing brain. Deregulation of Cdk5 has been implicated in Alzheimer's disease, amyotrophic lateral sclerosis, Parkinson's disease, Huntington's disease and acute neuronal injury. Regulators of Cdk5 activity are considered as potential therapeutic molecules for degenerative diseases. This review focuses on the role of Cdk5 in neural cells as regulator of cytoskeletal elements, axonal guidance, membrane transport, synaptogenesis and cell survival in normal and pathological conditions.