Expression of cyclin-dependent kinase 5 and its activator p35 in models of induced apoptotic death in neurons of the substantia nigra in vivo

Unraveling the links between neurodegeneration and Epstein-Barr virus-mediated cell cycle dysregulation

The Epstein-Barr virus is a well-known cell cycle modulator. To establish successful infection in the host, EBV alters the cell cycle at multiple steps via antigens such as EBNAs, LMPs, and certain other EBV-encoded transcripts. Interestingly, several recent studies have indicated the possibility of EBV's neurotrophic potential. However, the effects and outcomes of EBV infection in the CNS are under-explored. Additionally, more and more epidemiological evidence implicates the cell-cycle dysregulation in neurodegeneration. Numerous hypotheses which describe the triggers that force post-mitotic neurons to re-enter the cell cycle are prevalent. Apart from the known genetic and epigenetic factors responsible, several reports have shown the association of microbial infections with neurodegenerative pathology. Although, studies implicating the herpesvirus family members in neurodegeneration exist, the involvement of Epstein-Barr virus (EBV), in particular, is under-evaluated. Interestingly, a few clinical studies have reported patients of AD or PD to be seropositive for EBV. Based on the findings mentioned above, in this review, we propose that EBV infection in neurons could drive it towards neurodegeneration through dysregulation of cell-cycle events and induction of apoptosis.

Cdk5 at crossroads of protein oligomerization in neurodegenerative diseases: facts and hypotheses

Cyclin-dependent kinase 5 (Cdk5) is involved in proper neurodevelopment and brain function and serves as a switch between neuronal survival and death. Overactivation of Cdk5 is associated with many neurodegenerative disorders such as Alzheimer's or Parkinson's diseases. It is believed that in those diseases Cdk5 may be an important link between disease-initiating factors and cell death effectors. A common hallmark of neurodegenerative disorders is incorrect folding of specific proteins, thus leading to their intra- and extracellular accumulation in the nervous system. Abnormal Cdk5 signaling contributes to dysfunction of individual proteins and has a substantial role in either direct or indirect interactions of proteins common to, and critical in, different neurodegenerative diseases. While the roles of Cdk5 in α-synuclein (ASN) - tau or β-amyloid peptide (Aβ) - tau interactions are well documented, its contribution to many other pertinent interactions, such as that of ASN with Aβ, or interactions of the Aβ - ASN - tau triad with prion proteins, did not get beyond plausible hypotheses and remains to be proven. Understanding of the exact position of Cdk5 in the deleterious feed-forward loop critical for development and progression of neurodegenerative diseases may help designing successful therapeutic strategies of several fatal neurodegenerative diseases. Cyclin-dependent kinase 5 (Cdk5) is associated with many neurodegenerative disorders such as Alzheimer's or Parkinson's diseases. It is believed that in those diseases Cdk5 may be an important factor involved in protein misfolding, toxicity and interaction. We suggest that Cdk5 may contribute to the vicious circle of neurotoxic events involved in the pathogenesis of different neurodegenerative diseases.

Cyclin I and p35 determine the subcellular distribution of Cdk5

The atypical cyclin-dependent kinase 5 (Cdk5) serves an array of different functions in cell biology. Among these are axonal guidance, regulation of intercellular contacts, cell differentiation, and prosurvival signaling. The variance of these functions suggests that Cdk5 activation comes to pass in different cellular compartments. The kinase activity, half-life, and substrate specificity of Cdk5 largely depend on specific activators, such as p25, p35, p39, and cyclin I. We hypothesized that the subcellular distribution of Cdk5 activators also determines the localization of the Cdk5 protein and sets the stage for targeted kinase activity within distinct cellular compartments to suit the varying roles of Cdk5. Cdk5 localization was analyzed in murine kidney and brain slices of wild-type and cyclin I- and/or p35-null mice by immunohistochemistry and in cultured mouse podocytes using immunofluorescence labeling, as well as cell fractionation experiments. The predominance of cyclin I mediates the nuclear localization of Cdk5, whereas the predominance of p35 results in a membranous localization of Cdk5. These findings were further substantiated by overexpression of cyclin I and p35 with altered targeting characteristics in human embryonic kidney 293T cells. These studies reveal that the subcellular localization of Cdk5 is determined by its specific activators. This results in the directed Cdk5 kinase activity in specific cellular compartments dependent on the activator present and allows Cdk5 to serve multiple independent roles.

Lanosterol induces mitochondrial uncoupling and protects dopaminergic neurons from cell death in a model for Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder marked by the selective degeneration of dopaminergic neurons in the nigrostriatal pathway. Several lines of evidence indicate that mitochondrial dysfunction contributes to its etiology. Other studies have suggested that alterations in sterol homeostasis correlate with increased risk for PD. Whether these observations are functionally related is, however, unknown. In this study, we used a toxin-induced mouse model of PD and measured levels of nine sterol intermediates. We found that lanosterol is significantly (∼50%) and specifically reduced in the nigrostriatal regions of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated mice, indicative of altered lanosterol metabolism during PD pathogenesis. Remarkably, exogenous addition of lanosterol rescued dopaminergic neurons from 1-methyl-4-phenylpyridinium (MPP+)-induced cell death in culture. Furthermore, we observed a marked redistribution of lanosterol synthase from the endoplasmic reticulum to mitochondria in dopaminergic neurons exposed to MPP+, suggesting that lanosterol might exert its survival effect by regulating mitochondrial function. Consistent with this model, we find that lanosterol induces mild depolarization of mitochondria and promotes autophagy. Collectively, our results highlight a novel sterol-based neuroprotective mechanism with direct relevance to PD.

Intracellular signalling pathways in dopamine cell death and axonal degeneration

The pathways of programmed cell death (PCD) are now understood in extraordinary detail at the molecular level. Although much evidence suggests that they are likely to play a role in Parkinson's disease (PD), the precise nature of that role remains unknown. Two pathways of cell death that are especially well characterized are cyclin-dependent kinase 5-mediated phosphorylation of myocyte enhancer factor 2 and the mitogen-activated protein kinase signalling cascade. Although blockade of these pathways in animals has achieved a truly remarkable degree of neuroprotection of the neuron cell soma, it has not achieved protection of axons. Thus, there is a need to explore beyond the canonical pathways of PCD and investigate mechanisms of axon destruction. We also need to move beyond the narrow classic concept that the mechanisms of PCD are activated exclusively 'downstream', following cellular injury. Studies in the genetics of PD suggest that in some forms of the disease, activation may be an early 'upstream' event. Additionally, recent observations suggest that cell death in some contexts may not be initiated by injury, but instead by a failure of intrinsic cell survival signalling. These new points of view offer new opportunities for molecular targeting.

Commitment of 1-methyl-4-phenylpyrinidinium ion-induced neuronal cell death by proteasome-mediated degradation of p35 cyclin-dependent kinase 5 activator

The dysfunction of proteasomes and mitochondria has been implicated in the pathogenesis of Parkinson disease. However, the mechanism by which this dysfunction causes neuronal cell death is unknown. We studied the role of cyclin-dependent kinase 5 (Cdk5)-p35 in the neuronal cell death induced by 1-methyl-4-phenylpyrinidinium ion (MPP+), which has been used as an in vitro model of Parkinson disease. When cultured neurons were treated with 100 microM MPP+, p35 was degraded by proteasomes at 3 h, much earlier than the neurons underwent cell death at 12-24 h. The degradation of p35 was accompanied by the down-regulation of Cdk5 activity. We looked for the primary target of MPP+ that triggered the proteasome-mediated degradation of p35. MPP+ treatment for 3 h induced the fragmentation of the mitochondria, reduced complex I activity of the respiratory chain without affecting ATP levels, and impaired the mitochondrial import system. The dysfunction of the mitochondrial import system is suggested to up-regulate proteasome activity, leading to the ubiquitin-independent degradation of p35. The overexpression of p35 attenuated MPP+-induced neuronal cell death. In contrast, depletion of p35 with short hairpin RNA not only induced cell death but also sensitized to MPP+ treatment. These results indicate that a brief MPP+ treatment triggers the delayed neuronal cell death by the down-regulation of Cdk5 activity via mitochondrial dysfunction-induced up-regulation of proteasome activity. We propose a role for Cdk5-p35 as a survival factor in countering MPP+-induced neuronal cell death.

Brain-derived neurotrophic factor regulates early postnatal developmental cell death of dopamine neurons of the substantia nigra in vivo

Brain-derived neurotrophic factor (BDNF) was the first purified molecule identified to directly support the development of mesencephalic dopamine neurons. However, its physiologic role has remained unknown. Based on patterns of expression, it is unlikely to serve as a target-derived neurotrophic factor, but it may instead act locally in the mesencephalon, either released by afferent projections, or in autocrine fashion. To assess a possible local role, we blocked BDNF signaling in the substantia nigra (SN) of postnatal rats by injection of either neutralizing antibodies or a peptide antagonist. These treatments increased the magnitude of developmental cell death in the SN, indicating that endogenous local BDNF does play a regulatory role. However, we also find that elimination of BDNF in brain throughout postnatal development in BDNF(fl/fl):Nestin-Cre mice has no effect on the adult number of SN dopamine neurons. We postulate that other forms of trophic support may compensate for the elimination of BDNF during early development. Although the number of SN dopamine neurons is unchanged, their organization is disrupted. We conclude that BDNF plays a physiologic role in the postnatal development of SN dopamine neurons.

Zebrafish Rohon-Beard neuron development: cdk5 in the midst

Cyclin-dependent kinase 5 (cdk5) is a proline-directed serine/threonine kinase that is activated mostly by association with its activators, p35 and p39. Initially projected as a neuron-specific kinase, cdk5 is expressed ubiquitously and its kinase activity solely depends on the presence of its activators, which are also found in some non-neuronal tissues. As a multifunctional protein, cdk5 has been linked to axonogenesis, cell migration, exocytosis, neuronal differentiation and apoptosis. Cdk5 plays a critical role in functions other than normal physiology, especially in neurodegeneration. Its contribution to both normal physiological as well as pathological processes is mediated by its specific substrates. Cdk5-null mice are embryonically lethal, therefore making it difficult to study precisely what cdk5 does to the nervous system at early stages of development, be it neuron development or programmed cell death. Zebrafish model system bypasses the impediment, as it is amenable to reverse genetics studies. One of the functions that we have followed for the cdk5 ortholog in zebrafish in vivo is its effect on the Rohon-Beard (RB) neurons. RB neurons are the primary sensory spinal neurons that die during the first two days of zebrafish development eventually to be replaced by the dorsal root ganglia (DRG). Based on ours studies and others', here we discuss possible mechanisms that may be involved in cdk5's role in RB neuron development and survival.

Negative regulation of cyclin-dependent kinase 5 targets by protein kinase C

Cyclin-dependent kinase 5 (Cdk5) is a proline-directed protein serine/threonine kinase essential for brain development and implicated in synaptic plasticity, dopaminergic neurotransmission, drug addiction, and neurodegenerative disorders. Relatively little is known about the molecular mechanisms that regulate the activity of Cdk5 in vivo. In order to determine whether protein kinase C (PKC) regulates Cdk5 activity in the central nervous system, the phosphorylation levels of two Cdk5 substrates were evaluated under conditions of altered PKC activity in vivo. Treatment of acute striatal slices with a PKC-activating phorbol ester caused a time- and dose-dependent decrease in the levels of phospho-Ser6 inhibitor-1, phospho-Ser67 inhibitor-1, and phospho-Thr75 dopamine- and cAMP-regulated phosphoprotein, Mr 32,000 (DARPP-32). This effect was reversed by the PKC inhibitor, Ro-32-0432. Moreover, phospho-Ser6 inhibitor-1, phospho-Ser67 inhibitor-1, and phospho-Thr75 DARPP-32 levels were elevated in brain tissue from mice lacking the gene for PKC-alpha. PKC did not phosphorylate Cdk5 or its cofactor, p25, in vitro. Striatal levels of the Cdk5 cofactor, p35, did not change in response to phorbol ester treatment. Furthermore, Cdk5 immunoprecipitated from striatal slices treated with phorbol ester had unaltered activity toward a control substrate in vitro. These results suggest that PKC exerts its effects on the phosphorylation state of Cdk5 substrates through an indirect mechanism that may involve the regulatory binding partners of Cdk5 other than its neuronal cofactors.

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.

Activation of cyclin-dependent kinase 5 by calpains contributes to human immunodeficiency virus-induced neurotoxicity

Although the specific mechanism of neuronal damage in human immunodeficiency virus (HIV) -associated dementia is not known, a prominent role for NMDA receptor (NMDAR)-induced excitotoxicity has been demonstrated in neurons exposed to HIV-infected/activated macrophages. We hypothesized NMDAR-mediated activation of the calcium-dependent protease, calpain, would contribute to cell death by induction of cyclin-dependent kinase 5 (CDK5) activity. Using an in vitro model of HIV neurotoxicity, in which primary rat cortical cultures are exposed to supernatants from primary human HIV-infected macrophages, we have observed increased calpain-dependent cleavage of the CDK5 regulatory subunit, p35, to the constitutively active isoform, p25. Formation of p25 is dependent upon NMDAR activation and calpain activity and is coincident with increased CDK5 activity in this model. Further, inhibition of CDK5 by roscovitine provided neuroprotection in our in vitro model. Consistent with our observations in vitro, we have observed a significant increase in calpain activity and p25 levels in midfrontal cortex of patients infected with HIV, particularly those with HIV-associated cognitive impairment. Taken together, our data suggest calpain activation of CDK5, a pathway activated in HIV-infected individuals, can mediate neuronal damage and death in a model of HIV-induced neurotoxicity.

Cyclin-dependent kinase 5 influences Rohon-Beard neuron survival in zebrafish

Cyclin-dependent kinase 5 (cdk5), a member of the cyclin-dependent kinase family, is expressed predominantly in post-mitotic cell populations. Unlike the other cdks, cdk5 is abundant and most active in differentiated neurons. Here, we describe the function of a cdk5 ortholog in zebrafish. Cdk5 catalytic activity is meager but present in early stages of development. However, at 24 h post-fertilization (hpf), the activity is remarkably higher and continues to be high through 48 and 72 hpf. Knocking down cdk5 by micro-injection of a specific siRNA resulted in decreased cdk5 protein level accompanied by reduced kinase activity. In the cdk5 siRNA-injected embryos, the number of primary sensory Rohon-Beard (RB) neurons was significantly reduced and there were more apoptotic cells in the brain. These phenotypes were rescued by co-injection of cdk5 mRNA. Within the first two days of development, RB neurons undergo apoptosis in zebrafish. To examine whether cdk5 has a role in RB neuron survival, cdk5 mRNA was injected into the one- to two-cell embryos. In these embryos, RB neuron apoptosis was inhibited compared with the uninjected control embryos. These results suggest that in zebrafish, cdk5 influences RB neuron survival and potentially regulates early neuronal development.

Inhibition of neuronal apoptosis by the cyclin‐dependent kinase inhibitor GW8510: Identification of 3′ substituted indolones as a scaffold for the development of neuroprotective drugs

Increasing evidence suggests that neuronal apoptosis is triggered by the inappropriate activation of cyclin-dependent kinases leading to an abortive re-entry of neurons into the cell cycle. Pharmacological inhibitors of cell-cycle progression may therefore have value in the treatment of neurodegenerative diseases in humans. GW8510 is a 3' substituted indolone that was developed recently as an inhibitor of cyclin-dependent kinase 2 (CDK2). We found that GW8510 inhibits the death of cerebellar granule neurons caused by switching them from high potassium (HK) medium to low potassium (LK) medium. Although GW8510 inhibits CDK2 and other CDKs when tested in in vitro biochemical assays, when used on cultured neurons it only inhibits CDK5, a cytoplasmic CDK that is not associated with cell-cycle progression. Treatment of cultured HEK293T cells with GW8510 does not inhibit cell-cycle progression, consistent with its inability to inhibit mitotic CDKs in intact cells. Neuroprotection by GW8510 is independent of Akt and MEK-ERK signaling. Furthermore, GW8510 does not block the LK-induced activation of Gsk3beta and, while inhibiting c-jun phosphorylation, does not inhibit the increase in c-jun expression observed in apoptotic neurons. We also examined the effectiveness of other 3' substituted indolone compounds to protect against neuronal apoptosis. We found that like GW8510, the VEGF Receptor 2 Kinase Inhibitors [3-(1H-pyrrol-2-ylmethylene)-1,3-dihydroindol-2-one], {(Z)-3-[2,4-Dimethyl-3-(ethoxycarbonyl)pyrrol-5-yl)methylidenyl]indol-2-one} and [(Z)-5-Bromo-3-(4,5,6,6-tetrahydro-1H-indol-2-ylmethylene)-1,3-dihydroindol-2-one], the Src family kinase inhibitor SU6656 and a commercially available inactive structural analog of an RNA-dependent protein kinase inhibitor 5-Chloro-3-(3,5-dichloro-4-hydroxybenzylidene)-1,3-dihydro-indol-2-one, are all neuroprotective when tested on LK-treated neurons. Along with our recent identification of the c-Raf inhibitor GW5074 (also a 3' substituted indolone) as a neuroprotective compound, our findings identify the 3' substituted indolone as a core structure for the designing of neuroprotective drugs that may be used to treat neurodegenerative diseases in humans.

Pathogenic factors in vascular dementia and Alzheimer's disease. Multiple actions of heparin that probably are beneficial

The following areas are discussed in this review: atherogenesis; cerebrovascular factors; hypoperfusion; beta-amyloid production; beta-amyloid fibril formation; beta-sheets; metal cations; reactive oxygen species/free radicals; chronic inflammatory factors; endogenous plasma heparin; lipoprotein lipase; polyamines; protein kinase C; casein kinases; phospholipase A2; serine proteases; myeloperoxidase; cyclooxygenase 2; cysteine proteases; caspases; proprotein convertases; aspartic proteases; cyclin proteinases; thrombin; tau hyperphosphorylation; advanced glycosylation end products; activator protein 1; calcium; apolipoprotein E epsilon4; histamine; blood-brain barrier; glutamate; transglutaminase; insulin-like growth factor 1.

Effects of 6-hydroxydopamine on primary cultures of substantia nigra: specific damage to dopamine neurons and the impact of glial cell line-derived neurotrophic factor

6-Hydroxydopamine (6-OHDA)-induced loss of dopamine (DA) neurons has served to produce an animal model of DA neuron loss in Parkinson's disease. We report here the use of 6-OHDA to produce an in vitro model of this phenomena using dissociated cultures prepared from neonatal rat mesencephalon. Cultures were exposed to 6-OHDA (40-100 microm, 15 min) in an antioxidant medium, and DA and GABA neurons evaluated by immunocytochemistry. 6-OHDA induced morphological and biochemical signs of cell death in DA neurons within 3 h, followed by loss of tyrosine hydroxylase immunoreactive neurons within 2 days. In substantia nigra (SN) cultures, DA neurons were much more affected by 6-OHDA than were GABA neurons. In contrast, DA neurons from the ventral tegmental area were only lost at higher, non-specific concentrations of 6-OHDA. The effects of 6-OHDA on nigral DA neurons were blocked by inhibitors of high affinity DA transport and by z-DEVD-fmk (150 microm), a caspase inhibitor. Glial cell line-derived neurotrophic factor (GDNF) treatment reduced TUNEL labeling 3 h after 6-OHDA exposure, but did not prevent loss of DA neurons at 48 h. Thus, 6-OHDA can selectively destroy DA neurons in post-natal cultures of SN, acting at least in part by initiating caspase-dependent apoptosis, and this effect can be attenuated early but not late by GDNF.

Effects of cyclin-dependent kinase-5 activity on apoptosis and tau phosphorylation in immortalized mouse brain cortical cells

Cyclin-dependent kinase-5 (CDK5), a unique CDK family member, is active primarily in the central nervous system (CNS). Previous studies suggest that CDK5 is proapoptotic and contributes to tau hyperphosphorylation and neurodegeneration in Alzheimer's disease. The objective of this study was to examine CDK5 effects on apoptotic progression and tau phosphorylation. Immortalized embryonic mouse brain cortical cells were used to establish a stable cell line that overexpressed wild-type human tau. In these studies, thapsigargin, which induces endoplasmic reticulum stress and can cause accumulation of misfolded proteins, was used to induce apoptosis. Caspase-3 activity and poly-(ADP-ribose)-polymerase (PARP) cleavage, as measures of apoptosis, were significantly increased 24 and 48 hr after thapsigargin treatment, and these events were unaffected by tau expression. Although transient coexpression of CDK5 and its activator, p25, increased CDK5 activity greater than tenfold, increases in caspase-3 activity in response to thapsigargin treatment were unaffected by the presence of CDK5/p25. Tau phosphorylation at the PHF-1 epitope, but not the Tau-1 epitope, was increased significantly in CDK5/p25-transfected cells compared to cells transfected with dominant negative CDK5 (DNCDK5). The PHF-1 epitope remained phosphorylated until 48 hr after thapsigargin treatment in the CDK5/p25-transfected cells. Over the course of apoptosis in this model, phosphorylation of the Tau-1 epitope was unaffected in cells transfected with DNCDK5, vector, or CDK5/p25. In summary, these results demonstrate that CDK5 does not have a significant impact on tau phosphorylation and thapsigargin-induced apoptosis in this neuronal cell model.

Cdk5: mediator of neuronal death and survival

Cdk5 (cyclin-dependent kinase 5) is a serine/threonine kinase implicated to play pivotal roles in neuronal development. Recently, its potential involvement as a regulator of neuronal death and survival has attracted considerable interests. Importantly, increasing evidence has linked Cdk5 to the etiopathology of neurodegenerative diseases such as Alzheimer's disease and amyotrophic lateral sclerosis. Here we summarize the recent findings on Cdk5 not only as an important participant in neuronal death, but also a key player in neuronal survival. Elucidating the mechanisms of regulation of Cdk5 and its downstream signaling might prove to be crucial in the therapeutic treatment of neurodegenerative diseases.

Cyclin-dependent kinase-5 in neurodegeneration

Cyclin-dependent kinase-5 (CDK5) is predominantly active in the nervous system and it is well established that CDK5 is essential in neuronal development. In addition to its recognized role in development, there is increasing evidence that CDK5 may be involved in the pathogenesis of several neurodegenerative disorders. Although studies have shown that CDK5 can modulate cell death and survival, controversy still exists as to the exact role CDK5 may play in neurodegenerative processes. This review will highlight recent data on the possible roles of CDK5 in neurodegeneration.

Cyclin-dependent kinase 5 is a mediator of dopaminergic neuron loss in a mouse model of Parkinson's disease

Recent evidence indicates that cyclin-dependent kinases (CDKs, cdks) may be inappropriately activated in several neurodegenerative conditions. Here, we report that cdk5 expression and activity are elevated after administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a toxin that damages the nigrostriatal dopaminergic pathway. Supporting the pathogenic significance of the cdk5 alterations are the findings that the general cdk inhibitor, flavopiridol, or expression of dominant-negative cdk5, and to a lesser extent dominant-negative cdk2, attenuates the loss of dopaminergic neurons caused by MPTP. In addition, CDK inhibition strategies attenuate MPTP-induced hypolocomotion and markers of striatal function independent of striatal dopamine. We propose that cdk5 is a key regulator in the degeneration of dopaminergic neurons in Parkinson's disease.

Parkin is a component of an SCF-like ubiquitin ligase complex and protects postmitotic neurons from kainate excitotoxicity

Mutations in parkin, which encodes a RING domain protein associated with ubiquitin ligase activity, lead to autosomal recessive Parkinson's disease characterized by midbrain dopamine neuron loss. Here we show that parkin functions in a multiprotein ubiquitin ligase complex that includes the F-box/WD repeat protein hSel-10 and Cullin-1. HSel-10 serves to target the parkin ubiquitin ligase activity to cyclin E, an hSel-10-interacting protein previously implicated in the regulation of neuronal apoptosis. Consistent with the notion that cyclin E is a substrate of the parkin ubiquitin ligase complex, parkin deficiency potentiates the accumulation of cyclin E in cultured postmitotic neurons exposed to the glutamatergic excitotoxin kainate and promotes their apoptosis. Furthermore, parkin overexpression attenuates the accumulation of cyclin E in toxin-treated primary neurons, including midbrain dopamine neurons, and protects them from apoptosis.

Ectopic expression of cell cycle markers in models of induced programmed cell death in dopamine neurons of the rat substantia nigra pars compacta

There is increasing evidence that proteins normally involved in the cell cycle can regulate neuronal programmed cell death (PCD). However, it remains unknown whether cell cycle markers are expressed in normal, postmitotic, postmigratory neurons undergoing PCD in vivo. We have previously shown that natural cell death occurs postnatally in dopamine neurons of the substantia nigra pars compacta (SNpc). PCD can be induced postnatally in these neurons either by intrastriatal injection of the neurotoxin 6-hydroxydopamine (6-OHDA) or by medial forebrain bundle (MFB) axotomy. At the time of induction of death in these models, these neurons are long postmitotic and postmigratory. We have studied three cell cycle markers in these models: 5-bromo-2'-deoxyuridine (BrdU) incorporation (a marker of S phase), cdc2 protein expression (a marker of G2 phase), and expression of MPM2 (a marker of M phase), an epitope phosphorylated by cdc2. We report here that postmitotic dopaminergic neurons undergoing PCD in the SNpc following 6-OHDA and axotomy lesions incorporate BrdU and overexpress cdc2, but do not express MPM2. This is the first in vivo evidence that postmitotic dopamine neurons of the SNpc undergoing apoptosis express markers for S phase and G2 phase. These results raise the possibility that cell cycle regulatory proteins may play a role in the demise of dopaminergic neurons in Parkinson's disease, in which PCD has been postulated to play a role.

Aspirin provides cyclin-dependent kinase 5-dependent protection against subsequent hypoxia/reoxygenation damage in culture

Aspirin [acetylsalicylic acid (ASA)] is an anti-inflammatory drug that protects against cellular injury by inhibiting cyclooxygenases (COX), inducible nitric oxide synthase (iNOS) and p44/42 mitogen-activated protein kinase (p44/42 MAPK), or by preventing translocation of nuclear factor kappaB (NF-kappaB). We studied the effect of ASA pre-treatment on neuronal survival after hypoxia/reoxygenation damage in rat spinal cord (SC) cultures. In this injury model, COX, iNOS and NF-kappaB played no role in the early neuronal death. A 20-h treatment with 3 mm ASA prior to hypoxia/reoxygenation blocked the hypoxia/reoxygenation-induced lactate dehydrogenase (LDH) release from neurons. This neuroprotection was associated with increased phosphorylation of neurofilaments, which are substrates of p44/42 MAPK and cyclin-dependent kinase 5 (Cdk5). PD90859, a p44/42 MAPK inhibitor, had no effect on ASA-induced tolerance, but olomoucine and roscovitine, Cdk5 inhibitors, reduced ASA neuroprotection. Hypoxia/reoxygenation alone reduced both the protein amount and activity of Cdk5, and this reduction was inhibited by pre-treatment with ASA. Moreover, the protein amount of a neuronal Cdk5 activator, p35, recovered after reoxygenation only in ASA-treated samples. The prevention of the loss in Cdk5 activity during reoxygenation was crucial for ASA-induced protection, because co-administration of Cdk5 inhibitors at the onset ofreoxygenation abolished the protection. In conclusion, pre-treatment with ASA induces tolerance against hypoxia/reoxygenation damage in spinal cord cultures by restoring Cdk5 and p35 protein expression.

Cyclin-dependent kinase 5 is required for associative learning

Transient stressful experiences may persistently facilitate associative and nonassociative learning, possibly through alterations of gene expression. Here we identify, by subtractive hybridization, differential expression of the Cdk5 gene in response to stress. The Cdk5 protein is selectively induced in the fibers of septohippocampal cholinergic neurons but not in other regions of prominent Cdk5 production. This upregulation is accompanied by increased Cdk5 kinase activity, which is blocked completely by the Cdk5 inhibitor butyrolactone I. Microinjection of butyrolactone I into the lateral septum and hippocampus prevents the acquisition of conditioned context-dependent fear as well as its stress-induced facilitation. By demonstrating that a transient increase of Cdk5 activity within the septohippocampal system is required for associative learning, an important novel role of Cdk5 has been identified.

Distinct nuclear and cytoplasmic localization of caspase cleavage products in two models of induced apoptotic death in dopamine neurons of the substantia nigra

An emerging theme in programmed cell death (PCD) of neurons is that the mechanisms involved depend on the cellular context and the death-inducing stimulus. One particular class of neurons for which it is important to identify the mechanisms of PCD are the dopamine neurons of the substantia nigra, the neurons which degenerate in Parkinson's disease. PCD has been shown to occur in these neurons during normal development and to be induced in neurotoxin models of parkinsonism. Conventional histologic stains and TUNEL labeling have not revealed morphologic differences in the apoptosis observed in these neurons in any context. We now show that in two models of induced PCD in postmitotic dopamine neurons, one induced by early striatal target injury and another induced by the neurotoxin 6-hydroxydopamine (6OHDA), there are differences in the cellular localization and type of caspase cleavage products. Using two antibodies to caspase cleavage products (fractin and AB127), we show that in the target lesion model immunostaining is localized to the nucleus, whereas in the 6OHDA model intense cytoplasmic as well as nuclear staining is observed. Another antibody, AB246, to a caspase cleavage product of spectrin, immunostains apoptotic profiles only in the 6OHDA model. These findings suggest that the cellular compartment and therefore the role of the caspases may differ in apoptosis induced in pathologic settings, such as that due to neurotoxins, from that observed in models of natural or induced natural cell death. It will be important to recognize these differences in the consideration of caspase inhibitors in the treatment of degenerative neurologic disease.

Antioxidant compounds protect dopamine neurons from death due to oxidative stress in vitro

Using tissue culture models of oxidative stress caused by serum deprivation or MPTP/MPP+ toxicity, the present study establishes that the antioxidants epigallocatechin gallate, lazaroids U74389G and U83836E, reservatrol, MnTBAP, MCI 186, trolox, and melatonin protect 68-100% of dopamine (DA) neurons from cell death. In contrast, the nitric oxide inhibitor LY83583, the caspase inhibitors Z-VAD-FMK, Ac-DQMD-CHO and Z-DEVD-FMK, and the CDK-5 inhibitor, roscovotine were not neuroprotective, although death was often delayed by 1 day in vitro. We conclude that antioxidants are more effective at preventing cell death in vitro than are inhibitors at later stages in the death cascade.