Cyclin-dependent kinase 5 is associated with apoptotic cell death during development and tissue remodeling

Developmental mechanisms in the pathogenesis of neurodegenerative diseases

Cellular genes that are mutated in neurodegenerative diseases code for proteins that are expressed throughout neural development. Genetic analysis suggests that these genes are essential for a broad range of normal neurodevelopmental processes. The proteins they code for interact with numerous other cellular proteins that are components of signaling pathways involved in patterning of the neural tube and in regional specification of neuronal subtypes. Further, pathogenetic mutations of these genes can cause progressive, sublethal alterations in the cellular homeostasis of evolving regional neuronal subpopulations, culminating in late-onset cell death. Therefore, as a consequence of the disease mutations, targeted cell populations may retain molecular traces of abnormal interactions with disease-associated proteins by exhibiting changes in a spectrum of normal cellular functions and enhanced vulnerability to a host of environmental stressors. These observations suggest that the normal functions of these disease-associated proteins are to ensure the fidelity and integration of developmental events associated with the progressive elaboration of neuronal subtypes as well as the maintenance of mature neuronal populations during adult life. The ability to identify alterations within vulnerable neuronal precursors present in pre-symptomatic individuals prior to the onset of irrevocable cellular injury may help foster the development of effective therapeutic interventions using evolving pharmacologic, gene and stem cell technologies.

Tau protein is hyperphosphorylated in a site-specific manner in apoptotic neuronal PC12 cells

Alterations in the status of microtubules contribute to the cytoskeletal rearrangements that occur during apoptosis. The microtubule-associated protein tau regulates microtubule dynamics and thus is likely to play an important role in the cytoskeletal changes that occur in apoptotic cells. Previously, we demonstrated that the phosphorylation of tau at the Tau-1 epitope was increased during neuronal PC12 cell apoptosis, and further that the microtubule binding of tau from apoptotic cells was significantly impaired because of altered phosphorylation. The fact that the microtubule-binding capacity of tau from apoptotic cells was reduced to approximately 30% of control values indicated that sites in addition to those within the Tau-1 epitope were hyperphosphorylated during apoptosis. In this study using a combination of immunological and biochemical approaches, numerous sites were found to be hyperphosphorylated on tau isolated from apoptotic cells. Further, during apoptosis, the activities of cell division control protein kinase (cdc2) and cyclin-dependent kinase 5 (cdk5) were selectively and significantly increased. The association of these two protein kinases with tau was also increased during apoptosis. These findings are intriguing because many of the sites found to be hyperphosphorylated on tau during apoptosis are also hyperphosphorylated on tau from Alzheimer's disease brain. Likewise, there are data indicating that in Alzheimer's disease the activities of cdc2 and cdk5 are also increased.

Association of cyclin-dependent kinase 5 and its activator p35 with apoptotic cell death

We have investigated the role of cyclin-dependent kinases in cell death and found that the expression of cyclin-dependent kinase 5 (Cdk5) is associated with apoptotic cell death in both adult and embryonic tissues. By double labeling immunohistochemistry and confocal microscopy, we specifically associated the expression of Cdk5 to dying cells. The association of Cdks with cell death is unique to Cdk5 as this association is not found with the other Cdks (Cdk 1-8) and cell death. The differential increase in Cdk5 expression is at the level of protein only, and no differences can be detected at the level of mRNA Using both limbs of mutant mice detective in the pattern of interdigital cell death and limbs with increased interdigital cell death by retinoic acid treatment, we confirmed the specificity of Cdk5 protein expression in dying cells. To investigate the regulation of Cdk5 during cell death, we examined the expression of a regulatory protein of Cdk5, p35, and found p35 to be expressed in the dying cells as well. Similar to Cdk5, there is also no specific differential expression of the p35 mRNA in dying cells. Our results suggest a role for Cdk5 and p35 proteins in cell death. This protein complex may function in the rearrangement of the cytoskeleton during apoptosis.

Differential patterns of apoptosis in resolving and nonresolving bacterial pneumonia

Infection with either Streptococcus sanguis or Streptococcus pneumoniae type 25 causes acute pneumonitis in rats. Pneumonia caused by S. sanguis resolves over the course of 8 d, whereas pneumonia caused by S. pneumoniae type 25 progresses to fibrosis. To examine the role of apoptosis in these models, we performed assays with the terminal deoxynucleotidyltransferase-uridine nucleotide end-labeling technique on tissue sections from rat lungs at various times, and quantified the results with image analysis. Apoptosis was a feature of both the acute and resolving stages of pneumonia. The pattern and extent of apoptosis were similar in both models during the acute stage, and the number of apoptotic nuclei increased in both models through 4 d after infection. Although there were differences in the cellular pattern of apoptosis after 2 d and 4 d of infection, the extent of apoptosis was the same in both models. After 8 d, major differences were observed. In the resolving model, apoptosis was limited primarily to an abscess in the base of the lung. In the nonresolving model, apoptosis was persistent. We also found that cyclin-dependent kinase-5 expression is upregulated during apoptosis induced by bacterial infection. These data indicate that the location and timing of apoptosis may determine whether pneumonia resolves or progresses to fibrosis.

Cyclin-dependent kinase 5 (Cdk5) associated with Lewy bodies in diffuse Lewy body disease

We investigated the cyclin-dependent kinase (Cdk) 5 distribution pattern in diffuse Lewy body disease brains using immunohistochemistry. Cdk5 immunoreactivity was detected in both brainstem-type Lewy bodies (LBs) and cortical LBs. The number of Cdk5-positive LBs was less than that of ubiquitin- or alpha-synuclein-positive LBs, and more than that of phosphorylated neurofilament-positive LBs. Immunoelectron microscopy revealed Cdk5-immunolabeled granulo-filamentous components in LBs and LB-related neurites. These data suggest that Cdk5 may be associated with LB formation.

Unscheduled re-entry into the cell cycle induced by NGF precedes cell death in nascent retinal neurones

During their early postmitotic life, a proportion of the nascent retinal ganglion cells (RGCs) are induced to die as a result of the interaction of nerve growth factor (NGF) with the neurotrophin receptor p75. To analyse the mechanisms by which NGF promotes apoptosis, an in vitro culture system consisting of dissociated E5 retinal cells was established. In this system, NGF-induced apoptosis was only observed in the presence of insulin and neurotrophin-3, conditions that favour the birth of RGCs and other neurones expressing the glycoprotein G4. The pro-apoptotic effect of NGF on the G4-positive neurones was evident after 10 hours in vitro and was preceded by a significant upregulation of cyclin B2, but not cyclin D1, and the presence of mitotic nuclei in these cells. Brain-derived neurotrophic factor prevented both the increase of cyclin B2 expression in the G4-positive neurones and the NGF-induced cell death. Finally, pharmacologically blocking cell-cycle progression using the cyclin-dependent kinase inhibitor roscovitine prevented NGF-induced cell death in a dose-dependent manner. These results strongly suggest that the apoptotic signalling initiated by NGF requires a driving stimulus manifested by the neuronal birth and is preceded by the unscheduled re-entry of postmitotic neurones into the cell cycle.

Bone morphogenetic proteins regulate interdigital cell death in the avian embryo

The embryonic limb bud provides an excellent model for analyzing the mechanisms that regulate programmed cell death during development. At the time of digit formation in the developing autopod, the undifferentiated distal mesodermal cells may undergo or chondrogenic differentiation or apoptosis depending whether they are incorporated into the future digital rays or into the interdigital spaces. Both chondrogenesis or apoptosis are induced by local BMPS. However, whereas the chondrogenic-promoting activity of BMPs appears to be regulated through the BMPR-1b receptor, the mechanism by which the BMPs execute the death program remains unknown. The BMP proapoptotic activity requires the expression of members of the msx family of closely related homeobox-containing genes and is finally mediated by caspase activation, but the nature of the caspase(s) directly responsible for the cell death is also unknown. Finally, other growth factors present in the developing autopod at the stages of digit formation such as members of the FGF and TGF beta families modulate the ability of BMPs to induce cell death or chondrogenesis.

Cdk5 and munc-18/p67 co-localization in early stage neurofibrillary tangles-bearing neurons in Alzheimer type dementia brains

Hyperphosphorylation of tau protein occurs during the formation of paired helical filament (PHF) in the brain with Alzheimer's disease. As previously reported, cyclin-dependent kinase (cdk) 5 can phosphorylate tau at the site of abnormally phosphorylated in PHF. To characterize the relationship between cdk5 and PHF-tau, we investigated the localization of cdk5 and its regulator, p67 (munc 18), in the hippocampus and temporal lobes from 12 Alzheimer type dementia (ATD) patients and 5 controls using immunohistochemical procedures. The specificity of antibodies was confirmed with Western blot analysis. Anti-cdk5 antibody diffusely stained the perikarya of some tau2-positive or neurofibrillary tangle (NFT)-bearing neurons in ATD brains, while cdk5-positive staining was scarcely found in control brains. Anti-p67 antibody also showed stronger immunoreactivity of pyramidal neurons in ATD brains than in control brains. Double immunostaining with anti-cdk5 and anti-p67 antibodies revealed co-localization of both molecules in some pyramidal neurons. These findings suggest that cdk5 is activated by p67 at the early stage of NFT formation and accelerates NFT formation. In cdk5-positive and p67-negative neurons, cdk5 may be activated by other regulator molecules such as p35. In addition, cdk5-positive reactive astrocytes were found close to cdk5-positive NFT-bearing neurons m ATD brains but not in control brains, suggesting a correlation between NFT and reactive astrocytes.

Insulin-like growth factor-1 affects expressions of cyclin-dependent kinase 5 and its activator p35 in reperfused rat brain

Insulin-like growth factor-1 (IGF-1) was applied topically on the brain surface of reperfused rat brain after 60 min of transient middle cerebral artery (MCA) occlusion. In contrast to the cases treated with vehicle, the infarct volume was greatly reduced at 24 h of reperfusion by the treatment with IGF-1. Immunohistochemical analysis in the MCA territory showed that the increase of cyclin-dependent kinase 5 (cdk5) was greatly reduced, and that the decrease of the critical regulatory subunit of cdk5, p35, was preserved with treatment of IGF-1. The present results suggest that IGF-1 has a significant effect on ameliorating brain injury after transient focal brain ischemia with affecting the expressions of cdk5 and its activator p35.

Conversion of p35 to p25 deregulates Cdk5 activity and promotes neurodegeneration

Cyclin-dependent kinase 5 (Cdk5) is required for proper development of the mammalian central nervous system. To be activated, Cdk5 has to associate with its regulatory subunit, p35. We have found that p25, a truncated form of p35, accumulates in neurons in the brains of patients with Alzheimer's disease. This accumulation correlates with an increase in Cdk5 kinase activity. Unlike p35, p25 is not readily degraded, and binding of p25 to Cdk5 constitutively activates Cdk5, changes its cellular location and alters its substrate specificity. In vivo the p25/Cdk5 complex hyperphosphorylates tau, which reduces tau's ability to associate with microtubules. Moreover, expression of the p25/Cdk5 complex in cultured primary neurons induces cytoskeletal disruption, morphological degeneration and apoptosis. These findings indicate that cleavage of p35, followed by accumulation of p25, may be involved in the pathogenesis of cytoskeletal abnormalities and neuronal death in neurodegenerative diseases.

Changes in cyclin dependent kinase expression and activity accompanying lens fiber cell differentiation

Previous studies from this laboratory have shown that differentiating lens fiber cells contain two active cyclin dependent kinases (Cdks), Cdk1 and Cdk5. The present study was undertaken to explore the expression and regulation of six additional members of the Cdk family (Cdk2, Cdk3, Cdk4, Cdk6, Cdk7 and Cdk8) during lens differentiation. Differentiating lens fiber cells were separated from lens epithelial cells by microdissection of developing rat lenses [embryonic day 16 (E16) to postnatal day 8 (P8)] and Cdk expression was assessed by RT-PCR and immunoblotting. Two Cdks (Cdk3 and Cdk6) were not expressed in lens fiber cells or epithelial cells during this developmental period. In the lens epithelium, we detected proteins and mRNAs corresponding to all other Cdks examined (Cdk2, Cdk4, Cdk7, Cdk8) throughout this developmental period. Epithelial cells showed significant Cdk2 activity, which decreased with developmental age, but no significant activity was detected for Cdk4, Cdk7, or Cdk8. Fiber cells contained all four Cdk proteins and the corresponding Cdk mRNAs except for Cdk2 mRNA. None of the Cdks examined showed significant kinase activity in fiber cells. Immunoprecipitates of Cdk2 and Cdk4 from fiber cells contained p57(kip2), supporting the view that this Cdk inhibitor blocks the activity of these Cdks in lens fibers. In contrast, p57(kip2)did not co-immunoprecipitate with Cdk5 from lens fibers. These findings suggest that the differential affinity of p57(kip2)for members of the Cdk family may provide a mechanism for specific regulation of individual Cdks during fiber cell differentiation.

Involvement of cyclin D1-cdk5 overexpression and MCM3 cleavage in bax-associated spontaneous apoptosis and differentiation in an A253 human head and neck carcinoma xenograft model

Time-dependent ladder-type DNA fragmentation and morphological alterations consistent with apoptosis were observed among A253 human head and neck squamous cell carcinoma (HNSCC) cells in nude mice from 15 to 18 days after transplantation, without any drug treatment. No evidence of ladder-type DNA fragmentation was detected in A253 cells in vitro or in normal nude mouse tissues (skin and muscle). Our aim was to explore molecular factors associated with such spontaneous apoptosis. Bcl-2 protein expression decreased, while bax protein expression increased from day 9 after transplantation. Moreover, altered expression of bcl-2 and bax was accompanied by the increased proteolytic cleavage of poly(ADP-ribose) polymerase (PARP). Time-dependent dephosphorylation of Rb, followed by proteolytic cleavage, was also observed from day 9 after transplantation. The data indicate that the caspase-3 activation and cleavage of Rb protein may represent important steps in the regulation pathway of bax-mediated spontaneous apoptosis. Interestingly, the time-dependent activation of spontaneous apoptosis was almost simultaneous with the induction of differentiation and increased expression of several differentiation-associated regulatory proteins. An increased expression of cyclin D1 and cyclin-dependent kinase-5 (cdk5) was observed from day 9 after transplantation, whereas only slight alteration of cdk4 expression was found. The time-dependent activation of cyclin D1 and cdk5 preceded both the induction of ladder-type DNA fragmentation and increased keratin pearl formation. Furthermore, MCM3 was cleaved early in spontaneous apoptosis and differentiation. Our observations suggest the involvement of cyclin D1-cdk5 overexpression and MCM3 cleavage in bax-mediated spontaneous apoptosis and differentiation in A253 xenografts. P53 and WAF1 proteins were not expressed in the xenografts, indicating that the changes in the regulatory proteins during apoptosis and differentiation were not p53 or WAF1 dependent.

Interferon gamma induces the expression of p21waf-1 and arrests macrophage cell cycle, preventing induction of apoptosis

Incubation of bone marrow macrophages with lipopolysaccharide (LPS) or interferon gamma (IFN gamma) blocks macrophage proliferation. LPS treatment or M-CSF withdrawal arrests the cell cycle at early G1 and induces apoptosis. Treatment of macrophages with IFN gamma stops the cell cycle later, at the G1/S boundary, induces p21Waf1, and does not induce apoptosis. Moreover, pretreatment of macrophages with IFN gamma protects from apoptosis induced by several stimuli. Inhibition of p21Waf1 with antisense oligonucleotides or using KO mice shows that the induction of p21Waf1 by IFN gamma mediates this protection. Thus, IFN gamma makes macrophages unresponsive to apoptotic stimuli by inducing p21Waf1 and arresting the cell cycle at the G1/S boundary. Therefore, the cells of the innate immune system could only survive while they were functionally active.

Cyclin dependent kinase-5 (CDK-5) phosphorylates neurofilament heavy (NF-H) chain to generate epitopes for antibodies that label neurofilament accumulations in amyotrophic lateral sclerosis (ALS) and is present in affected motor neurones in ALS

1. Accumulations of phosphorylated neurofilaments are seen in affected motor neurones in amyotrophic lateral sclerosis (ALS). 2. The authors demonstrate that cyclin dependent kinase-5 (cdk-5) will induce cellular phosphorylation of neurofilament heavy chain side-arms to generate epitopes for several antibodies that label these neurofilament accumulations. 3. By creating recombinant neurofilament fragments of NF-H side-arm domains, the authors also map the epitopes for several of these antibodies. 4. Finally, the authors demonstrate that cdk-5 is also present in affected motor neurones in ALS. These studies implicate cdk-5 in the pathogenesis of ALS.

Differentiation-related changes in the cell cycle traverse

This review examines recent developments relating to the interface between cell proliferation and differentiation. It is suggested that the mechanism responsible for this transition is more akin to a "dimmer" than to a "switch," that it is more useful to refer to early and late stages of differentiation rather than to "terminal" differentiation, and examples of the reversibility of differentiation are provided. An outline of the established paradigm of cell cycle regulation is followed by summaries of recent studies that suggest that this paradigm is overly simplified and should be interpreted in the context of different cell types. The role of inhibitors of cyclin-dependent kinases in differentiation is discussed, but the data are still inconclusive. An increasing interest in the changes in G2/M transition during differentiation is illustrated by examples of polyploidization during differentiation, such as megakaryocyte maturation. Although the retinoblastoma protein is currently maintaining its prominent role in control of proliferation and differentiation, it is anticipated that equally important regulators will be discovered and provide an explanation at the molecular level for the gradual transition from proliferation to differentiation.

A link between cell cycle and cell death: Bax and Bcl-2 modulate Cdk2 activation during thymocyte apoptosis

Resting thymocytes undergoing apoptosis in response to specific stimuli degrade the cdk inhibitor p27(Kip1) and upregulate Cdk2 kinase activity. Inhibition of Cdk2 kinase activity efficiently blocks cell death via certain apoptosis pathways whereas overexpression of Cdk2 accelerates such cell death, suggesting its involvement in the signal transduction pathways activated by certain apoptotic stimuli. We found that Cdk2 activation during thymocyte apoptosis can be regulated by p53, Bax and Bcl-2. The highly elevated Cdk2 kinase activity in the apoptosing thymocytes is not associated with its canonical cyclins, cyclin E and cyclin A, and requires de novo synthesis of proteins for activation to take place. We therefore propose Cdk2 activation to be a crucial event in distinct pathways of apoptosis and the point at which the cell cycle and cell death pathways interact.

Apoptosis in Degenerative Diseases of the Basal Ganglia

Degenerative disorders of the basal ganglia are characterized by disturbances of motor control. Prototypic examples are Parkinson's disease, which is caused by degeneration of dopamine neurons of the substantia nigra, and Huntington's disease, which is caused by degeneration of neurons of the striatum. In recent years, it has been postulated that some of these disorders may be caused by programmed cell death or apoptosis, a genetically regulated form of cell death. There is clear evidence that apoptosis occurs in neurons of the basal ganglia during normal development, that it can be regulated, and that it can be induced in some animal models of these disorders. Although there is some suggestive direct evidence that apoptosis may occur in the human brain in these disorders, the evidence to date is partial and not yet compelling. Nevertheless, programmed cell death is an important new hypothesis for the pathogenesis of these disorders and warrants vigorous further investigation, particularly with molecular markers in addition to classic morphological markers. The concept of programmed cell death is relevant not only to the pathogenesis of these diseases but also to therapeutic issues, such as transplantation approaches.

The identification and characterization of expression of Pftaire-1, a novel Cdk family member, suggest its function in the mouse testis and nervous system

We have isolated a murine cDNA encoding for a novel putative Cdk-related protein kinase, which has been named Pftaire-1, by screening a testis cDNA library for new serine/threonine kinases. Pftaire-1 showed 50% and 49% amino acid identity with Cdk5 and Pctaire-3, respectively, and contains the eleven subdomains characteristic of the protein kinases. By northern blot analysis we detected two transcripts of approximately 5.5 and 4.9 kb in size. These transcripts were expressed at low level in all murine tissues tested, except in the brain, testis and embryo, where high expression was detected. Cellular localization of the mRNAs by in situ hybridization analysis shows that Pftaire-1 is expressed in late pachytene spermatocytes in the testis and in post mitotic neuronal cells both in the brain and the embryo, suggesting a role of Pftaire-1 both in the process of meiosis as well as neuron differentiation and/or function.

Cyclin-dependent kinase-5 is associated with lipofuscin in motor neurones in amyotrophic lateral sclerosis

We have studied the distribution of cyclin dependent kinase-5 (cdk-5) within spinal cord in sporadic and two superoxide dismutase type 1 (SOD1) familial cases of amyotrophic lateral sclerosis (ALS). Although most neurofilament accumulations in ALS motor neurones did not appear to contain high levels of cdk-5, intense cdk-5 immunoreactivity was observed in perikarya of degenerating neurones in many ALS cases. Here, cdk-5 co-localised with lipofuscin. Co-localisation of cdk-5 with lipofuscin was also observed in some aged non-affected controls although this labelling was less intense than the ALS cases. The biogenesis of lipofuscin is believed to be linked to oxidative stress and oxidative stress and free radical damage have been suggested to be part of the pathogenic process of ALS, possibly involving apoptotic mechanisms. cdk-5 has recently been associated with apoptosis. These observations suggest a role for cdk-5 in the pathogenesis of ALS.

Changes in the activity of cdk2 and cdk5 accompany differentiation of rat primary oligodendrocytes

Oligodendrocytes, the myelinating cells of the central nervous system, are terminally differentiated cells that originate through asynchronous waves of proliferation and differentiation of precursors present at birth. Withdrawal from cell cycle and onset of differentiation are tightly linked and depend on an intrinsic program modulated by the action of growth factors. p27 plays a central and obligatory role in the initiation of oligodendrocyte differentiation and cessation of proliferation. In this paper, we have characterized the role of modulation of cdk2 and cdk5 kinase activity during the process of oligodendrocyte precursor differentiation. As rat primary oligodendrocytes differentiate in culture there is a fall in cdk2 activity and a rise in cdk5 activity as well as an increase in the cdk inhibitor, p27 protein. The decline in cdk2 activity is not accompanied by a drop in cdk2 protein level, suggesting that it results from inhibition of cdk2 activation rather than decreased protein expression. Taken together, these data suggest that oligodendrocytes may withdraw from the cell cycle at G1-S transition through inactivation of cdk2 activity, possibly initiated by increasing amount of p27, and that cdk5 may have a role until now unrecognized in the differentiation of oligodendrocytes.

Transient activation of cyclin B/Cdc2 during terminal differentiation of lens fiber cells

Previous work has shown that postmitotic, differentiating fiber cells of the embryonic chicken lens express cyclin B and Cdc2. The present study explores the possible physiological role of these proteins in lens differentiation by examining the developmental regulation of cyclin B/Cdc2 expression and activity in lens fiber cells of embryonic and newborn rats. Cyclin B mRNA and protein were detected not only in the lens epithelium, which contains proliferating cells, but also in postmitotic, differentiating fiber cells. In contrast, cyclin A mRNA and protein were detected only in epithelial cells. Immunoprecipitation with cyclin B antibody coprecipitated Cdc2 from both epithelial and fiber cell extracts. Immunoprecipitates of cyclin B from both epithelial cells and fiber cells showed H1 kinase activity when assayed in vitro, but the developmental pattern of cyclin B-associated kinase activity in these two lens fractions was markedly different. In the epithelium, H1 kinase activity decreased gradually with developmental age in parallel with the decrease in epithelial cell proliferation, whereas, in the fiber cells, kinase activity peaked sharply at embryonic day 18 (E18) and E19. Microscopic examination of rat lenses indicated that peak cyclin B/Cdc2 activity was correlated with changes in chromatin structure and nuclear envelope breakdown in the terminally differentiating primary lens fiber cells. These findings suggest that cyclin B/Cdc2 activity may play an active role in nuclear changes leading to primary fiber cell denucleation.

Increased expression of cyclin-dependent kinase 5 in induced apoptotic neuron death in rat substantia nigra

We reported previously that striatal excitotoxic lesion with quinolinic acid of rat pups during the first 2 weeks of postnatal life results in loss of dopaminergic neurons of the substantia nigra (SN) due to induced apoptosis. Here we demonstrate by immunohistochemistry that, following such a lesion, high levels of cyclin-dependent kinase 5 (cdk5) protein are present exclusively in apoptotic cells over and above basal levels of diffuse axonal staining. Furthermore, localization of high levels of cdk5 is associated also with normal developmental programmed cell death in the SN and other regions of the central nervous system, including the cerebral cortex. These findings suggest a novel role for cdk5 during neuron apoptosis and may provide insight into mechanisms of loss of dopaminergic neurons in Parkinson's disease.