Increase in the expression of the neuronal cyclin-dependent protein kinase cdk-5 during differentiation of N2A neuroblastoma cells

hnRNP-Q1 represses nascent axon growth in cortical neurons by inhibiting Gap-43 mRNA translation

A novel posttranscriptional mechanism for regulating the neuronal protein GAP-43 is reported. The mRNA-binding protein hnRNP-Q1 represses Gap-43 mRNA translation by a mechanism involving a 5′ untranslated region G-quadruplex structure, which affects GAP-43 function, as demonstrated by a GAP-43–dependent increase in neurite length and number with hnRNP-Q1 knockdown.

Posttranscriptional regulation of gene expression by mRNA-binding proteins is critical for neuronal development and function. hnRNP-Q1 is an mRNA-binding protein that regulates mRNA processing events, including translational repression. hnRNP-Q1 is highly expressed in brain tissue, suggesting a function in regulating genes critical for neuronal development. In this study, we have identified Growth-associated protein 43 (Gap-43) mRNA as a novel target of hnRNP-Q1 and have demonstrated that hnRNP-Q1 represses Gap-43 mRNA translation and consequently GAP-43 function. GAP-43 is a neuronal protein that regulates actin dynamics in growth cones and facilitates axonal growth. Previous studies have identified factors that regulate Gap-43 mRNA stability and localization, but it remains unclear whether Gap-43 mRNA translation is also regulated. Our results reveal that hnRNP-Q1 knockdown increased nascent axon length, total neurite length, and neurite number in mouse embryonic cortical neurons and enhanced Neuro2a cell process extension; these phenotypes were rescued by GAP-43 knockdown. Additionally, we have identified a G-quadruplex structure in the 5′ untranslated region of Gap-43 mRNA that directly interacts with hnRNP-Q1 as a means to inhibit Gap-43 mRNA translation. Therefore hnRNP-Q1–mediated repression of Gap-43 mRNA translation provides an additional mechanism for regulating GAP-43 expression and function and may be critical for neuronal development.

Cyclin-dependent kinase 5 activator p35 over-expression and amyloid beta synergism increase apoptosis in cultured neuronal cells

Alzheimer disease (AD) is a neurodegenerative disorder characterized by neuronal loss, dementia and pain. Two main protein aggregates, extracellular (senile plaques, SP) and intracellular (neurofibrillary tangles, NFT), are associated with AD. NFT are mainly composed of hyperphosphorylated microtubule-associated protein tau. Nowadays several protein kinases have been implicated in the phosphorylation of tau, including glycogen synthase kinase 3 beta (GSK3beta), MAP kinase, protein kinase A and cyclin-dependent kinase 5 (Cdk5). A deregulation in the activity of Cdk5 has been postulated to participate in the abnormal tau hyperphosphorylation in AD. Activation of Cdk5 occurs after its association with p35, a neuron-specific activator, predominantly in the nervous system. Therefore, in this study we used the tetracycline transactivator system to increase p35/GFP in neuronal cells, treated with amyloid beta 1-42 (Abeta(1-42)) peptide. These cells showed an increase of terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) and cleaved caspase-3 staining, indicating increased apoptosis of neuronal cells. This effect could be reversed by the addition of tetracycline in the culture medium, suggesting synergistic effects of p35 over-expression and Abeta treatment in the apoptosis of neuronal cells. These results represent a linkage between amyloidogenic and cdk5 pathways leading to apoptosis of neuronal cells.

cdk5 modulates beta- and delta-catenin/Pin1 interactions in neuronal cells

The cdk5/p35 complex has been implicated in a variety of functions related to brain development, including axonal outgrown and neuronal migration. In this study, by co-immunoprecipitation and pull-down experiments, we have shown that the cdk5/p35 complex associates with and phosphorylates the neuronal delta-catenin. Immunocytochemical studies of delta-catenin and the cdk5-activator p35 in primary cortical neurons indicated that these proteins co-localize in the cell body of neuronal cells. In addition, cdk5 co-localized with beta-catenin in the cell-cell contacts and plasma membrane of undifferentiated and differentiated N2A cells. In this context, we identified Ser(191) and Ser(246) on beta-catenin structure as specific phosphorylation sites for cdk5/p35 complex. Moreover, Pin1, a peptidyl-prolyl isomerase (PPIase) directly bound to both, beta- and delta-catenin, once they have been phosphorylated by the cdk5/p35 complex. Studies indicate that the cdk5/p35 protein kinase system is directly involved in the regulatory mechanisms of neuronal beta- and delta-catenin.

Differentiation-dependent expression of hypothetical proteins in the neuroblastoma cell line N1E-115

Several protein cascades, including signaling, cytoskeletal, chaperones, metabolic, and antioxidant proteins, have been shown to be involved in the process of neuronal differentiation (ND) of neuroblastoma cell lines. No systematic approach to detect hitherto unknown and unnamed proteins or structures that have been predicted upon nucleic acid sequences in ND has been published so far. We therefore decided to screen hypothetical protein (HP) expression by protein profiling. Two-dimensional gel electrophoresis with subsequent matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF/TOF) identification was used for expression analysis of undifferentiated and dimethylsulfoxide-induced neuronally differentiated N1E-115 cells. We unambiguously identified six HPs: Q8C520, Q99LF4, Q9CXS1, Q9DAF8, Q91WT0, and Q8C5G2. A prefoldin domain in Q91WT0, a t-SNARE domain in Q9CXS1, and a bromodomain were observed in Q8C5G2. For the three remaining proteins, no putative function using Pfam, BLOCKS, PROSITE, PRINTS, InterPro, Superfamily, CoPS, and ExPASy could be assigned. While two proteins were present in both cell lines, Q9CXS1 was switched off (i.e., undetectably low) in differentiated cells only, and Q9DAF8, Q91WT0, and Q8C5G2 were switched on in differentiated cells exclusively. Herein, using a proteomic approach suitable for screening and identification of HP, we present HP structures that have been only predicted so far based upon nucleic acid sequences. The four differentially regulated HPs may play a putative role in the process of ND.

Cyclin-dependent kinase 5 activators p35 and p39 facilitate formation of functional synapses

Cyclin-dependent kinase 5 (Cdk5) has emerged as a key coordinator of cell signaling in neurite outgrowth. Cdk5 needs to associate with one of the regulatory proteins p35 or p39 to be an active enzyme. To investigate if Cdk5 plays a role in the establishment of functional synapses, we have characterized the expression of Cdk5, p35, and p39 in the neuroblastoma-glioma cell line NG108-15, and recorded postsynaptic activity in myotubes in response to presynaptic overexpression of Cdk5, p35, and p39. Endogenous Cdk5 and p35 protein levels increased with cellular differentiation and preferentially distributed to soluble pools, whereas the level of p39 protein remained low and primarily was present in membrane and cytoskeletal fractions. Transient transfection of a dominant-negative mutant of Cdk5 in NG108-15 cells and subsequent culturing on differentiating muscle cells resulted in a significant reduction in synaptic activity, as measured by postsynaptic miniature endplate potentials (mEPPs). Overexpression of either Cdk5/p35 or Cdk5/p39 resulted in a substantial increase in synaptic structures that displayed postsynaptic activities, as well as mEPP frequency. These findings demonstrate that Cdk5, p35, and p39 are endogenously expressed in NG108-15 cells, exhibit distinct subcellular localizations, and that both Cdk5/p35 and Cdk5/p39 are central in formation of functional synapses.

Effect of amyloid peptides on serum withdrawal-induced cell differentiation and cell viability

Abnormal deposition of amyloid-beta(A beta) peptides and formation of neuritic plaques are recognized as pathological processes in Alzheimer's disease (AD) brain. By using amyloid precursor protein (APP) transfected cells, this study aims to investigate the effect of overproduction of A beta on cell differentiation and cell viability. It was shown that after serum withdrawal, untransfected cell (N2a/Wt) and vector transfected cells (N2a/vector) extended long and branched cell processes, whereas no neurites was induced in wild type APP (N2a/APP695) and Swedish mutant APP (N2a/APPswe) transfected N2a cells. After differentiation by serum withdrawal, the localization of APP/A beta and neurofilament was extended to neurites, whereas those of APP-transfected cells were still restricted within the cell body. Levels of both APP and A beta were significantly higher in N2a/APP695 and N2a/APPswe than in N2a/Wt, as determined by Western blot and Sandwich ELISA, respectively. To further investigate the effect of A beta on the inhibition of cell differentiation, we added exogenously the similar level or about 10-times of the A beta level produced by N2a/APP695 and N2a/APPswe to the culture medium and co-cultured with N2a/Wt for 12 h, and we found that the inhibition of serum withdrawal-induced differentiation observed in N2a/APP695 and N2a/APPswe could not be reproduced by exogenous administration of A beta into N2a/Wt. We also observed that neither endogenous production nor exogenous addition of A beta 1-40 or A beta 1- 42, even to hundreds fold of the physiological concentration, affected obviously the cell viability. These results suggest that the overproduction of A beta could not arrest cell differentiation induced by serum deprivation and that, at least to a certain degree and in a limited time period, is not toxic to cell viability.

Effects of melatonin on wortmannin-induced tau hyperphosphorylation

AIM

To explore the underlying mechanism of tau hyperphosphorylation in an Alzheimeros-affected brain and the possible arresting strategies.

METHODS

MTT(3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl-tetrazolium bromide), crystal violet assay, phase-contrast, dead end colorimetric apoptosis detection system (TUNEL) and electron microscopy were used to detect cell viability, morphology and apoptosis. Western blot, 32P-labeling and the detection of malondialdehyde level and superoxide dismutase activity were used respectively for the phosphorylation level of tau, the activity of glycogen synthase kinase (GSK-3), and oxidative stress measurement.

RESULTS

Exposure of the cells to wortmannin resulted in an obvious lipid peroxidation, reduction of cell viability, cell process retraction, and plasma vacuolation, but with no obvious cell apoptosis. We also found that preincubation of the cells with melatonin or vitamin E attenuated differentially wortmannin-induced oxidative stress as well as GSK-3 overactivation and tau hyperphosphorylation.

CONCLUSION

Wortmannin is an effective tool for reproducing Alzheimer-like tau hyperphosphorylation cell model and melatonin/vitamin E can effectively protect the cells from wortmannin-induced impairments.

Dishevelled promotes neurite outgrowth in neuronal differentiating neuroblastoma 2A cells, via a DIX-domain dependent pathway

Dishevelled (Dvl) is a cytoplasmic protein involved in the Wnt-Frizzled signaling cascade, which has also been shown to interact with the cytoskeleton in part through inhibition of glycogen synthase kinase 3beta (GSK3beta). Using mouse neuroblastoma 2A (N2A) cells as a model system, we have found that overexpression of Dvl promotes the outgrowth of neurite-like processes, and leads to the induction of a striking, bipolar morphologic phenotype during neuronal differentiation. In contrast, suppression of Dvl expression using isoform-specific siRNAs led to an inhibition of neurite outgrowth in these cells. In order to further elucidate the mechanism(s) responsible for this effect, we overexpressed several mutant forms of Dvl in the N2A cells, including deletions in each of the three major functional subdomains of the protein (DeltaDIX, DeltaPDZ, DeltaDEP) and point mutations in the two well-defined interaction motifs within the DIX domain (the actin-binding and vesicle-association elements; K58A and K68A/E69A, respectively). These experiments revealed that the DIX domain (and its vesicle-binding subregion) was essential for Dvl's effect on neurite extension and morphogenesis in N2A cells. In contrast, direct overexpression of a degradation-resistant form of beta-catenin (S37A), or a dominant negative GSK3beta mutant (K85R), had no effect on neurite outgrowth or morphology in neuronally differentiating N2A cells; exposure of cells to a pharmacologic inhibitor of GSK3beta (lithium) also had no effect. Taken together, these results suggest that Dvl induces cytoskeletal and morphologic rearrangements in neuronal differentiating N2A cells through a mechanism that cannot be attributed exclusively to modulation of GSK3beta or beta-catenin activity, but which does depend upon a DIX-domain/vesicle-association-dependent signaling pathway.

Effects of natural flavones and flavonols on the kinase activity of Cdk5

A number of natural and synthetic flavonoids have been assessed previously with regard to their effects on the activity of cyclin-dependent kinases (Cdk1 and -2) related to the inhibition of cell cycle progression. On the other hand, the Cdk5/p35 system is of major importance in neuronal migration phenomena and brain development, and its deregulation is implicated in neurodegenerative diseases, particularly Alzheimer's. Here we show that some natural flavonoids inhibit the activity of the Cdk5/p35 system in the micromolar range, while others are practically inactive. Ring B-unsubstituted and highly methoxylated flavones were inactive or gave irreproducible results, and 6-methoxyapigenin and 6-methoxyluteolin were the most potent Cdk5 complex inhibitors within this series, while the common flavonols kaempferol and quercetin showed intermediate behavior. The reported crystal structure of the Cdk5 complex with its activator p25 was used for docking studies, which also led to the identification of the two 6-methoxyflavones, kaempferol and quercetin, as well as the untested 6-methoxy derivatives of kaempferol and quercetin and the corresponding 6-hydroxy analogues as compounds exhibiting a good fit to the active site of the enzyme.

Amyloid-β-induced toxicity of primary neurons is dependent upon differentiation-associated increases in tau and cyclin-dependent kinase 5 expression

It has previously been reported that amyloid-beta (Abeta) peptide is neurotrophic to undifferentiated but neurotoxic to differentiated primary neurons. The underlying reasons for this differential effect is not understood. Recently, the toxicity of Abeta to neurons was shown to be dependent upon the activation of cyclin-dependent kinase 5 (Cdk5), thought to promote tau phosphorylation that leads to cytoskeletal disruption, morphological degeneration and apoptosis. Here we report that Cdk5, tau, and phosphorylated-tau (P-tau) are expressed at very low levels in undifferentiated primary neurons, but that the expression of Cdk5 and tau and the phosphorylation of tau increase markedly between 4 and 8 days of differentiation in vitro. Tau expression decreased after this time, as did the level of P-tau, to low levels by 17 days. Abeta induced tau phosphorylation of neurons only after >or= 4 days of differentiation, a time that coincides with the onset of Abeta toxicity. Blocking tau expression (and therefore tau phosphorylation) with an antisense oligonucleotide completely blocked Abeta toxicity of differentiated primary neurons, thereby confirming that tau was essential for mediating Abeta toxicity. Our results demonstrate that differentiation-associated changes in tau and Cdk-5 modulate the toxicity of Abeta and explain the opposite responses of differentiated and undifferentiated neurons to Abeta. Our results predict that only cells containing appreciable levels of tau are susceptible to Abeta-induced toxicity and may explain why Abeta is more toxic to neurons compared with other cell types.

Regulation of p27 in the process of neuroblastoma N2A differentiation

Neuronal differentiation implies morphological and biochemical changes to generate a specialized neuron. N2A neuroblastoma cells can be promoted to undergo differentiation associated to neurites outgrowth, a process linked to the arrest of cell division. Using N2A cells as a model, we investigated the detailed molecular aspects on the involvement of p27 in dibutyryl cAMP-induced neuronal differentiation. In the undifferentiated N2A phenotype, an unusually high level of accumulated p27 protein mass was evidenced. Data suggest that in proliferating cells, p27 could be sequestered by direct interaction with cyclin D1, thus preventing its inhibitory action on cell cycle Cdks. Studies also indicate that p27 is functionally active and that its loss of action on Cdks in proliferating cells is due to its strong association with cyclin D1. Therefore, when cell differentiation is triggered, the action of p27 on Cdks seems to depend on both p27 and cyclin D1 degradation during the early steps of differentiation followed by late events of re-synthesis of active p27. In this context, an overexpression of p27 after N2A transfection with a mouse p27 clone induces the outgrowth of neurites associated with a decrease in cyclin D1 expression. On the other hand, treatment of N2A undifferentiated cells with c-myc antisense oligonucleotides led to a decrease in p27 and cyclin D1 levels, similar events as those in early stages of cell differentiation. Studies suggest that blockage in c-myc expression triggers early events in neuronal differentiation. These studies are of the utmost importance to elucidate regulatory mechanisms of molecules that play a critical role in the transition from a proliferating phenotype to differentiated cells.

A novel, extraneuronal role for cyclin-dependent protein kinase 5 (CDK5): modulation of cAMP-induced apoptosis in rat leukemia cells

A number of cyclin-dependent protein kinase (CDK) inhibitors were tested for the ability to protect IPC-81 rat leukemic cells against cAMP-induced apoptosis. A near perfect proportionality was observed between inhibitor potency to protect against cAMP-induced apoptosis and to antagonize CDK5, and to a lesser extent, CDK2 and CDK1. Enforced expression of dominant negative CDK5 (but not CDK1-dn or CDK2-dn) protected against death, indicating that CDK5 activity was necessary for cAMP-induced apoptosis. The CDK inhibitors failed to protect the cells against daunorubicine-, staurosporine-, or okadaic acid-induced apoptosis. The inhibition of CDK5 prevented the cleavage of pro-caspase-3 in cAMP-treated cells. The cells could be saved closer to the moment of their onset of death by inhibitors of caspases than by inhibitors of CDK5. This suggested that the action of CDK5 was upstream of caspase activation. The cAMP treatment resulted in a moderate increase of the level of CDK5 mRNA and protein in IPC-81 wild-type cells. Such cAMP induction of CDK5 was not observed in cells expressing the inducible cAMP early repressor. The cAMP-induced increase of CDK5 contributed to apoptosis since cells overexpressing CDK5-wt were more sensitive for cAMP-induced death. These results demonstrate the first example of a proapoptotic CDK action upstream of caspase activation and of an extra-neuronal effect of CDK5.

The molecular bases of Alzheimer's disease and other neurodegenerative disorders

Alzheimer's disease, the cause of one of the most common types of dementia, is a brain disorder affecting the elderly and is characterized by the formation of two main protein aggregates: senile plaques and neurofibrillary tangles, which are involved in the process leading to progressive neuronal degeneration and death. Neurodegeneration in Alzheimer's disease is a pathologic condition of cells rather than an accelerated way of aging. The senile plaques are generated by a deposition in the human brain of fibrils of the beta-amyloid peptide (Abeta), a fragment derived from the proteolytic processing of the amyloid precursor protein (APP). Tau protein is the major component of paired helical filaments (PHFs), which form a compact filamentous network described as neurofibrillary tangles (NFTs). Experiments with hippocampal cells in culture have indicated a relationship between fibrillary amyloid and the cascade of molecular signals that trigger tau hyperphosphorylations. Two main protein kinases have been shown to be involved in anomalous tau phosphorylations: the cyclin-dependent kinase Cdk5 and glycogen synthase kinase GSK3beta. Cdk5 plays a critical role in brain development and is associated with neurogenesis as revealed by studies in brain cells in culture and neuroblastoma cells. Deregulation of this protein kinase as induced by extracellular amyloid loading results in tau hyperphosphorylations, thus triggering a sequence of molecular events that lead to neuronal degeneration. Inhibitors of Cdk5 and GSK3beta and antisense oligonucleotides exert protection against neuronal death. On the other hand, there is cumulative evidence from studies in cultured brain cells and on brains that oxidative stress constitutes a main factor in the modification of normal signaling pathways in neuronal cells, leading to biochemical and structural abnormalities and neurodegeneration as related to the pathogenesis of Alzheimer's disease. This review is focused on the main protein aggregates responsible for neuronal death in both sporadic and familial forms of Alzheimer's disease, as well as on the alterations in the normal signaling pathways of functional neurons directly involved in neurodegeneration. The analysis is extended to the action of neuroprotective factors including selective inhibitors of tau phosphorylating protein kinases, estrogens, and antioxidants among other molecules that apparently prevent neuronal degeneration.