Phosphorylation of human high molecular weight neurofilament protein (hNF-H) by neuronal cyclin-dependent kinase 5 (cdk5)

Cyclin-dependent Kinase 5 and Neurodegenerative Diseases

Neurodegenerative diseases are a group of diseases characterized by the progressive loss of neurons, including Alzheimer's disease, Parkinson's disease, and Amyotrophic lateral sclerosis. These diseases have a high incidence and mortality rate globally, placing a heavy burden on patients and their families. The pathogenesis of neurodegenerative diseases is complex, and there are no effective treatments at present. Cyclin-dependent kinase 5 is a proline-directed serine/threonine protein kinase that is closely related to the development and function of the nervous system. Under physiological conditions, it is involved in regulating the process of neuronal proliferation, differentiation, migration, and synaptic plasticity. Moreover, there is increasing evidence that cyclin-dependent kinase 5 also plays an important role in the pathogenesis of neurodegenerative diseases. In this review, we address the biological characteristics of cyclin-dependent kinase 5 and its role in neurodegenerative diseases. In particular, this review highlights the underlying mechanistic linkages between cyclin-dependent kinase 5 and mitochondrial dysfunction, oxidative stress and neuroinflammation in the context of neurodegeneration. Finally, we also summarize the currently available cyclin-dependent kinase 5 inhibitors and their prospects for the treatment of neurodegenerative diseases. Taken together, a better understanding of the molecular mechanisms of cyclin-dependent kinase 5 involved in neurodegenerative diseases can lead to the development of new strategies for the prevention and treatment of these devastating diseases.

Searching for novel Cdk5 substrates in brain by comparative phosphoproteomics of wild type and Cdk5-/- mice

Protein phosphorylation is the most common post-translational modification that regulates several pivotal functions in cells. Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase which is mostly active in the nervous system. It regulates several biological processes such as neuronal migration, cytoskeletal dynamics, axonal guidance and synaptic plasticity among others. In search for novel substrates of Cdk5 in the brain we performed quantitative phosphoproteomics analysis, isolating phosphoproteins from whole brain derived from E18.5 Cdk5^+/+ and Cdk5^−/− embryos, using an Immobilized Metal-Ion Affinity Chromatography (IMAC), which specifically binds to phosphorylated proteins. The isolated phosphoproteins were eluted and isotopically labeled for relative and absolute quantitation (iTRAQ) and mass spectrometry identification. We found 40 proteins that showed decreased phosphorylation at Cdk5^−/− brains. In addition, out of these 40 hypophosphorylated proteins we characterized two proteins, :MARCKS (Myristoylated Alanine-Rich protein Kinase C substrate) and Grin1 (G protein regulated inducer of neurite outgrowth 1). MARCKS is known to be phosphorylated by Cdk5 in chick neural cells while Grin1 has not been reported to be phosphorylated by Cdk5. When these proteins were overexpressed in N2A neuroblastoma cell line along with p35, serine phosphorylation in their Cdk5 motifs was found to be increased. In contrast, treatments with roscovitine, the Cdk5 inhibitor, resulted in an opposite effect on serine phosphorylation in N2A cells and primary hippocampal neurons transfected with MARCKS. In summary, the results presented here identify Grin 1 as novel Cdk5 substrate and confirm previously identified MARCKS as a a bona fide Cdk5 substrate.

The cyclin-dependent kinase inhibitor roscovitine inhibits kinase activity, cell proliferation, multicellular development, and Cdk5 nuclear translocation in Dictyostelium discoideum

Roscovitine, a cyclin-dependent kinase (Cdk) inhibitor, inhibited kinase activity and the axenic growth of Dictyostelium discoideum at micromolar concentrations. Growth was almost fully rescued in 50 µM and ≈ 50% rescued in 100 µM roscovitine-treated cultures by the over-expression of Cdk5-GFP. This supports the importance of Cdk5 function during cell proliferation in Dictyostelium and indicates that Cdk5 is a primary target of the drug. Roscovitine did not affect the expression of Cdk5 protein during axenic growth but did inhibit its nuclear translocation. This novel result suggests that the effects of roscovitine could be due in part to altering Cdk5 translocation in other systems as well. Kinase activity was inhibited by roscovitine in assays using AX3 whole cell lysates, but not in assays using lysates from Cdk5-GFP over-expressing cells. At higher concentrations, roscovitine impaired slug and fruiting body formation. Fruiting bodies that did form were small and produced relatively fewer spores many of which were round. However, roscovitine did not affect stalk cell differentiation. Together with previous findings, these data reveal that roscovitine inhibits Cdk5 during growth and as yet undefined Cdks during mid-late development.

Axonal outgrowth and dendritic plasticity in the cortical peri-infarct area after experimental stroke

BACKGROUND AND PURPOSE

Axonal remodeling is critical to brain repair after stroke. The present study investigated axonal outgrowth after stroke and the signaling pathways mediating axonal outgrowth in cortical neurons.

METHODS

Using a rodent model of middle cerebral artery occlusion, we examined high-molecular weight neurofilament (NFH) immunoreactive axons and myelin basic protein-positive oligodendrocytes in the peri-infarct area. In vitro, using cultured cortical neurons in a microfluidic chamber challenged by oxygen-glucose deprivation (OGD), we investigated mechanisms selectively regulating axonal outgrowth after OGD.

RESULTS

NFH(+) axons and MBP(+) oligodendrocytes substantially increased in the peri-infarct area during stroke recovery, concomitantly with an increase in dendrites and spines identified by Golgi-Cox staining. In vitro, cortical neurons subjected to OGD exhibited significant increases in axonal outgrowth and in phosphorylated NFH protein levels, concurrently with downregulation of phosphatase tensin homolog deleted on chromosome 10, activation of Akt, and inactivation of glycogen synthase kinase-3β in regenerated axons. Blockage of phosphoinositide 3-kinase with pharmacological inhibitors suppressed Akt activation and attenuated phosphorylation of glycogen synthase kinase-3β, which resulted in suppression of phosphorylated NFH and axonal outgrowth after OGD; whereas GSK-3 inhibitors augmented axonal regeneration and elevated phosphorylated NFH levels after OGD.

CONCLUSIONS

Stroke induces axonal outgrowth and myelination in rodent ischemic brain during stroke recovery, and the phosphoinositide 3-kinase/Akt/glycogen synthase kinase-3β signaling pathway mediates axonal regeneration of cortical neurons after OGD.

Signaling mechanisms downstream of quinolinic acid targeting the cytoskeleton of rat striatal neurons and astrocytes

The studies of signaling mechanisms involved in the disruption of the cytoskeleton homeostasis were performed in a model of quinolinic acid (QUIN) neurotoxicity in vitro. This investigation focused on the phosphorylation level of intermediate filament (IF) subunits of astrocytes (glial fibrillary acidic protein - GFAP) and neurons (low, medium and high molecular weight neurofilament subunits - NFL, NFM and NFH, respectively). The activity of the phosphorylating system associated with the IFs was investigated in striatal slices of rat exposed to QUIN or treated simultaneously with QUIN plus glutamate receptor antagonists, calcium channel blockers or kinase inhibitors. Results showed that in astrocytes, the action of 100 μM QUIN was mainly due to increased Ca(2+) influx through NMDA and L-type voltage-dependent Ca(2+) channels (L-VDCC). In neuronal cells QUIN acted through metabotropic glutamate receptor (mGluR) activation and influx of Ca(2+) through NMDA receptors and L-VDCC, as well as Ca(2+) release from intracellular stores. These mechanisms then set off a cascade of events including activation of PKA, PKCaMII and PKC, which phosphorylate head domain sites on GFAP and NFL. Also, Cdk5 was activated downstream of mGluR5, phosphorylating the KSP repeats on NFM and NFH. mGluR1 was upstream of phospholipase C (PLC) which, in turn, produced diacylglycerol (DAG) and inositol 3,4,5 triphosphate (IP3). DAG is important to activate PKC and phosphorylate NFL, while IP(3) contributed to Ca(2+) release from internal stores promoting hyperphosphorylation of KSP repeats on the tail domain of NFM and NFH. The present study supports the concept of glutamate and Ca(2+) contribution in excitotoxic neuronal damage provoked by QUIN associated to dysfunction of the cytoskeleton homeostasis and highlights the differential signaling mechanisms elicited in striatal astrocytes and neurons.

Oxidative stress in neurodegeneration

It has been demonstrated that oxidative stress has a ubiquitous role in neurodegenerative diseases. Major source of oxidative stress due to reactive oxygen species (ROS) is related to mitochondria as an endogenous source. Although there is ample evidence from tissues of patients with neurodegenerative disorders of morphological, biochemical, and molecular abnormalities in mitochondria, it is still not very clear whether the oxidative stress itself contributes to the onset of neurodegeneration or it is part of the neurodegenerative process as secondary manifestation. This paper begins with an overview of how oxidative stress occurs, discussing various oxidants and antioxidants, and role of oxidative stress in diseases in general. It highlights the role of oxidative stress in neurodegenerative diseases like Alzheimer's, Parkinson's, and Huntington's diseases and amyotrophic lateral sclerosis. The last part of the paper describes the role of oxidative stress causing deregulation of cyclin-dependent kinase 5 (Cdk5) hyperactivity associated with neurodegeneration.

Quantitative phosphoproteomic analysis of neuronal intermediate filament proteins (NF-M/H) in Alzheimer's disease by iTRAQ

Aberrant hyperphosphorylation of neuronal cytoskeletal proteins is one of the major pathological hallmarks of neurodegenerative disorders such as Alzheimer disease (AD), amyotrophic lateral sclerosis (ALS), and Parkinson's disease (PD). Human NF-M/H display a large number of multiple KSP repeats in the carboxy-terminal tail domain, which are phosphorylation sites of proline-directed serine/threonine (pSer/Thr-Pro, KS/T-P) kinases. The phosphorylation sites of NF-M/H have not been characterized in AD brain. Here, we use quantitative phosphoproteomic methodology, isobaric tag for relative and absolute quantitation (iTRAQ), for the characterization of NF-M/H phosphorylation sites in AD brain. We identified 13 hyperphosphorylated sites of NF-M; 9 Lys-Ser-Pro (KSP) sites; 2 variant motifs, Glu-Ser-Pro (ESP) Ser-736 and Leu-Ser-Pro (LSP) Ser-837; and 2 non-S/T-P motifs, Ser-783 and Ser-788. All the Ser/Thr residues are phosphorylated at significantly greater abundance in AD brain compared with control brain. Ten hyperphosphorylated KSP sites have been identified on the C-terminal tail domain of NF-H, with greater abundance of phosphorylation in AD brain compared with control brain. Our data provide the direct evidence that NF-M/H are hyperphosphorylated in AD compared with control brain and suggest the role of both proline-directed and non-proline-directed protein kinases in AD. This study represents the first comprehensive iTRAQ analyses and quantification of phosphorylation sites of human NF-M and NF-H from AD brain and suggests that aberrant hyperphosphorylation of neuronal intermediate filament proteins is involved in AD.

Declining phosphatases underlie aging-related hyperphosphorylation of neurofilaments

Cytoskeletal protein phosphorylation is frequently altered in neuropathologic states but little is known about changes during normal aging. Here we report that declining protein phosphatase activity, rather than activation of kinases, underlies aging-related neurofilament hyperphosphorylation. Purified PP2A or PP2B dephosphorylated the heavy neurofilament (NFH) subunit or its extensively phorphorylated carboxyl-terminal domain in vitro. In cultured primary hippocampal neurons, inhibiting either phosphatase induced NFH phosphorylation without activating known neurofilament kinases. Neurofilament phosphorylation in the mouse CNS, as reflected by levels of the RT-97 phosphoepitope associated with late axon maturation, more than doubled during the 12-month period after NFH expression plateaued at p21. This was accompanied by declines in levels and activity of PP2A but not PP2B, and no rise in activities of neurofilament kinases (Erk1,2, cdk5 and JNK1,2). Inhibiting PP2A in mice in vivo restored brain RT-97 to levels seen in young mice. Declining PP2A activity, therefore, can account for rising neurofilament phosphorylation in maturing brain, potentially compounding similar changes associated with adult-onset neurodegenerative diseases.

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.

Pin1-dependent prolyl isomerization modulates the stress-induced phosphorylation of high molecular weight neurofilament protein

Aberrant phosphorylation of neuronal cytoskeletal proteins is a key pathological event in neurodegenerative disorders such as Alzheimer disease (AD) and amyotrophic lateral sclerosis, but the underlying mechanisms are still unclear. Previous studies have shown that Pin1, a peptidylprolyl cis/trans-isomerase, may be actively involved in the regulation of Tau hyperphosphorylation in AD. Here, we show that Pin1 modulates oxidative stress-induced NF-H phosphorylation. In an in vitro kinase assay, the addition of Pin1 substantially increased phosphorylation of NF-H KSP repeats by proline-directed kinases, Erk1/2, Cdk5/p35, and JNK3 in a concentration-dependent manner. In vivo, dominant-negative (DN) Pin1 and Pin1 small interfering RNA inhibited epidermal growth factor-induced NF-H phosphorylation. Because oxidative stress plays an important role in the pathogenesis of neurodegenerative diseases, we studied the role of Pin1 in stressed cortical neurons and HEK293 cells. Both hydrogen peroxide (H(2)O(2)) and heat stresses induce phosphorylation of NF-H in transfected HEK293 cells and primary cortical cultures. Knockdown of Pin1 by transfected Pin1 short interference RNA and DN-Pin1 rescues the effect of stress-induced NF-H phosphorylation. The H(2)O(2) and heat shock induced perikaryal phospho-NF-H accumulations, and neuronal apoptosis was rescued by inhibition of Pin1 in cortical neurons. JNK3, a brain-specific JNK isoform, is activated under oxidative and heat stresses, and inhibition of Pin1 by Pin1 short interference RNA and DN-Pin1 inhibits this pathway. These results implicate Pin1 as a possible modulator of stress-induced NF-H phosphorylation as seen in neurodegenerative disorders like AD and amyotrophic lateral sclerosis. Thus, Pin1 may be a potential therapeutic target for these diseases.

The Notch signaling inhibitor DAPT down-regulates cdk5 activity and modulates the distribution of neuronal cytoskeletal proteins

Notch signaling is critical for the development of the nervous system. Cyclin-dependent kinase 5 (cdk5) is a neuronal kinase involved in neuronal development and phosphorylates a number of neuronal cytoskeletal proteins. To determine the relationship between Notch and cdk5 signaling, we tested the effects of the Notch inhibitor, N-[N-(3,5-difluorophenacetyl)-1-alanyl]-S-phenylglycine t-butyl ester (DAPT) on cdk5 expression, activity and cytoskeletal protein distribution in the rat cortical neurons in primary cultures. Neurons treated with 10 microM DAPT showed attenuated cdk5 activity in spite of an up-regulation of cdk5 protein level, consistent with a phenomenon reported in the cdk5 transgenic mice. Immunoblot and immunofluorescence analyses showed an increased level of cdk5, but not p35. Phospho-tau and phospho-neurofilament showed a shift from axons to cell bodies in DAPT-treated cells. DAPT-induced attenuation of cdk5 activity was restored by over-expression of p35 indicating that it interacted with cdk5 and up-regulated nascent cdk5 activity. p35 over-expression also rescued DAPT-induced translocation of phospho-tau and phospho-neurofilament. Immunoprecipitation followed by immunoblotting demonstrated that DAPT does not disrupt cdk5 and p35 interaction. Moreover, DAPT up-regulated neurogenin that is negatively regulated by Notch, and down-regulated Hes1, a downstream target of Notch, suggesting that Notch signaling in the cortical neurons was disrupted. Semi-quantitative and quantitative RT-PCR analyses confirmed that DAPT up-regulated cdk5 expression at the transcriptional level. These results establish a link between Notch signaling and cdk5 expression regulating neuronal cytoskeletal protein dynamics.

Neurofilament tail phosphorylation: identity of the RT-97 phosphoepitope and regulation in neurons by cross-talk among proline-directed kinases

As axons myelinate, establish a stable neurofilament network, and expand in caliber, neurofilament proteins are extensively phosphorylated along their C-terminal tails, which is recognized by the monoclonal antibody, RT-97. Here, we demonstrate in vivo that RT-97 immunoreactivity (IR) is generated by phosphorylation at KSPXK or KSPXXXK motifs and requires flanking lysines at specific positions. extracellular signal regulated kinase 1,2 (ERK1,2) and pERK1,2 levels increase in parallel with phosphorylation at the RT-97 epitope during early postnatal brain development. Purified ERK1,2 generated RT-97 on both KSP motifs on recombinant NF-H tail domain proteins, while cdk5 phosphorylated only KSPXK motifs. RT-97 epitope generation in primary hippocampal neurons was regulated by extensive cross-talk among ERK1,2, c-Jun N-terminal kinase 1,2 (JNK1,2) and cdk5. Inhibition of both ERK1,2 and JNK1,2 completely blocked RT-97 generation. Cdk5 influenced RT-97 generation indirectly by modulating JNK activation. In mice, cdk5 gene deletion did not significantly alter RT-97 IR or ERK1,2 and JNK activation. In mice lacking the cdk5 activator P35, the partial suppression of cdk5 activity increased RT-97 IR by activating ERK1,2. Thus, cdk5 influences RT-97 epitope generation partly by modulating ERKs and JNKs, which are the two principal kinases regulating neurofilament phosphorylation. The regulation of a single target by multiple protein kinases underscores the importance of monitoring other relevant kinases when the activity of a particular one is blocked.

Hyperphosphorylation of tau and neurofilaments and activation of CDK5 and ERK1/2 in PTEN-deficient cerebella

Inherited mutations to the tumor suppressor PTEN sporadically lead to cerebellar gangliocytoma characterized by migration defects. This has been modeled by CNS-specific PTEN ablation in mice, but the underlying mechanism cannot be explained by the known role of PTEN in Akt/PKB inactivation. Here we show that the loss of PTEN in mouse cerebellar neurons causes neurodegeneration by hyperphosphorylation of tau and neurofilaments, and activation of Cdk5 and pERK1/2, suggesting that dysregulation of the PTEN/pAkt pathway can mediate neurodegeneration.

Cyclin-dependent kinase 5 activity controls cell motility and metastatic potential of prostate cancer cells

We show here that cyclin-dependent kinase 5 (CDK5), a known regulator of migration in neuronal development, plays an important role in prostate cancer motility and metastasis. P35, an activator of CDK5 that is indicative of its activity, is expressed in a panel of human and rat prostate cancer cell lines, and is also expressed in 87.5% of the human metastatic prostate cancers we examined. Blocking of CDK5 activity with a dominant-negative CDK5 construct, small interfering RNA, or roscovitine resulted in changes in the microtubule cytoskeleton, loss of cellular polarity, and loss of motility. Expression of a dominant-negative CDK5 in the highly metastatic Dunning AT6.3 prostate cancer cell line also greatly impaired invasive capacity. CDK5 activity was important for spontaneous metastasis in vivo; xenografts of AT6.3 cells expressing dominant-negative CDK5 had less than one-fourth the number of lung metastases exhibited by AT6.3 cells expressing the empty vector. These results show that CDK5 activity controls cell motility and metastatic potential in prostate cancer.

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.

Cyclin-dependent kinase 5 activity regulates pain signaling

Several molecules and cellular pathways have been implicated in nociceptive signaling, but their precise molecular mechanisms have not been clearly defined. Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase implicated in the development and disease of the mammalian nervous system. The precise role of this kinase in sensory pathways has not been well characterized. Here we report a molecular role for Cdk5 in nociception. We identified the expression of Cdk5 and its activator p35 in nociceptive neurons, which is modulated during a peripheral inflammatory response. Increased calpain activity in sensory neurons after inflammation resulted in the cleavage of p35 to p25, which forms a more stable complex with Cdk5 and, consequently, leads to elevation of Cdk5 activity. p35 knockout mice (p35(-/-)), which exhibit significantly decreased Cdk5 activity, showed delayed responses to painful thermal stimulation compared with WT controls. In contrast, mice overexpressing p35, which exhibit elevated levels of Cdk5 activity, were more sensitive to painful thermal stimuli than were controls. In conclusion, our data demonstrate a role for Cdk5/p35 activity in primary afferent nociceptive signaling, suggesting that Cdk5/p35 may be a target for the development of analgesic drugs.

Cdk5 inhibits anterograde axonal transport of neurofilaments but not that of tau by inhibition of mitogen-activated protein kinase activity

Cyclin-dependent kinase 5 (cdk5) inhibits neurofilament (NF) anterograde axonal transport while p42/44 mitogen-activated protein kinase (MAPk) promotes it. Since cdk5 is known to inhibit MAP kinase activity, we examined whether or not cdk5 inhibits anterograde NF transport via inhibition of MAPk activity. To accomplish this, we manipulated the activity of these kinases in differentiated NB2a/d1 cells, and monitored anterograde axonal transport of green fluorescent protein-conjugated-NF-M (GFP-M) and cyan fluorescent protein-conjugated (CFP)-tau. The cdk5 inhibitor roscovitine increased anterograde axonal transport of GFP-M and CFP-tau; transfection with cdk5/p25 inhibited transport of both. Inhibition of MAPk activity by PD98059 or expression of dominant-negative MAPk inhibited anterograde GFP-M transport, while expression of constitutively active MAPk enhanced it; these treatments did not affect CFP-tau transport. PD98059 prevented roscovitine-mediated enhancement of GFP-M transport, but did not prevent enhancement of CFP-tau transport. Co-transfection with constitutively activated MAPk prevented the inhibition of GFP-M transport that normally accompanied transfection with cdk5/p25, but did not prevent inhibition of tau transport by cdk5/p25. Finally, the extent of inhibition of GFP-M axonal transport by PD98059 was not additive to that derived from transfection with cdk5/p35, and the increase in NF transport that accompanies roscovitine treatment was not additive to that derived from transfection with constitutively activated MAPk, suggesting that the influence of these kinases on NF transport was within the same, rather than distinct, pathways. These findings suggest that axonal transport of tau and NFs is under the control of distinct kinase cascades, and that cdk5 inhibits NF transport at least in part by inhibiting MAPk.

Patterns of gene dysregulation in the frontal cortex of patients with HIV encephalitis

The neurodegenerative process in HIV encephalitis (HIVE) is associated with extensive damage to the dendritic and synaptic structure that often leads to cognitive impairment. Several mechanisms might be at play, including release of neurotoxins, oxidative stress and decreased activity of neurotrophic factors. Furthermore, HIV-mediated dysregulation of genes involved in neuronal maintenance might play an important role. For this purpose, cRNA was prepared from the brains of 17 AIDS patients for analysis with the Affymetrix Human U95Av2 GeneChip and analyzed with the GeneSpring Expression Analysis Software. Out of 12,625 genes analyzed, 74 were downregulated and 59 were upregulated compared to controls. Initial alternative analysis of RNA was performed by ribonuclease protection assay (RPA). In cases with HIVE, downregulated genes included neuronal molecules involved in synaptic plasticity and transmission (ion channels, synaptogyrin, synapsin II), cell cycle (p35, p39, CDC-L2, CDC42, PAK1) and signaling molecules (PI3K, Ras-Raf-MEK1), transcription factors and cytoskeletal components (MAP-1B, MAP-2, tubulin, adducin-2). Upregulated genes included those involved in neuroimmune (IgG, MHC, beta2microglobulin) and anti-viral responses (interferon-inducible molecules), transcription (STAT1, OLIG2, Pax-6) and signaling modulation (MEK3, EphB1) of the cytoskeleton (myosin, aduccin-3, radixin, dystrobrevin). Taken together, this study suggests that HIV proteins released from infected macrophages might not only induce a neuroinflammatory response, but also may promote neurodegeneration by interfering with neuronal transcription of genes involved in regulating signaling and cytoskeletal molecules important in maintaining synapto-dendritic functioning and integrity.

p35/cyclin-dependent kinase 5 phosphorylation of ras guanine nucleotide releasing factor 2 (RasGRF2) mediates Rac-dependent Extracellular Signal-regulated kinase 1/2 activity, altering RasGRF2 and microtubule-associated protein 1b distribution in neurons

Cyclin-dependent kinase 5 (Cdk5) is a proline-directed kinase the activity of which is dependent on association with its neuron-specific activators, p35 and p39. Cdk5 activity is critical for the proper formation of cortical structures and lamination during development. In the adult nervous system, Cdk5 function is implicated in cellular adhesion, dopamine signaling, neurotransmitter release, and synaptic activity. In addition, Cdk5 is also involved in "cross-talk" with other signal transduction pathways. To further examine its involvement in cross-talk with other pathways, we identified proteins that interacted with p35 using the yeast two-hybrid system. We report here that p35 associates with Ras guanine nucleotide releasing factor 2 (RasGRF2) in coimmunoprecipitation and colocalization studies using transfected cell lines as well as primary cortical neurons. Additionally, Cdk5 phosphorylates RasGRF2 both in vitro and in vivo, leading to a decrease in Rac-guanidine exchange factor activity and a subsequent reduction in extracellular signal-regulated kinase 1/2 activity. We show that p35/Cdk5 phosphorylates RasGRF2 on serine737, which leads to an accumulation of RasGRF2 in the neuronal cell bodies coinciding with an accumulation of microtubule-associated protein 1b. The membrane association of p35 and subsequent localization of Cdk5 activity toward RasGRF2 and Rac provide insights into important cellular signaling processes that occur at the membrane, resulting in downstream effects on signal transduction cascades.

Long-term regulation of signalling components of adenylyl cyclase and mitogen-activated protein kinase in the pre-frontal cortex of human opiate addicts

Opiate addiction involves the development of chronic adaptive changes in micro -opioid receptors and associated pathways (e.g. cAMP signalling) which lead to neuronal plasticity in the brain. This study assessed the status of cAMP and mitogen-activated protein kinase (MAPK) pathways in brains (pre-frontal cortex) of chronic opiate addicts. In these subjects (n = 24), the immunodensities of adenylyl cyclase-I, PKA Calpha, total and phosphorylated CREB were not different from those in sex-, age- and PMD-matched controls. Moreover, the ratio pCREB/tCREB was similar in opiate addicts (0.74) and controls (0.76), further indicating that opiate addiction in humans is not associated with an upregulation of several key components of cAMP signalling in the pre-frontal cortex. In contrast, the components of MAPK cascade (Ras/c-Raf-1/MEK/ERK) were decreased in the same brains. Notably, pronounced downregulations of phosphorylated MEK (85%) and ERK1/2 (pERK1: 81%; pERK2: 80%) were quantitated in brains of opiate addicts. Chronic morphine treatment in rats (10-100 mg/kg for 5 days) was also associated with decreases of pERK1/2 (59-68%) in the cortex. In SH-SY5Y cells, morphine also stimulated the activity of pERK1/2 (2.5-fold) and the MEK inhibitor PD98059 blocked this effect (90%). The abnormalities of MAPK signalling might have important consequences in the long term development of various forms of neural plasticity associated with opiate addiction in humans.

Cyclin-dependent kinase 5 increases perikaryal neurofilament phosphorylation and inhibits neurofilament axonal transport in response to oxidative stress

Cyclin-dependent kinase 5 (cdk5) phosphorylates the high molecular weight neurofilament (NF) protein. Overexpression of cdk5 inhibits NF axonal transport and induces perikaryal accumulation of disordered phospho-NF cables. Experimental and clinical motor neuron disease is characterized by oxidative stress, increased cdk5 activity, and accumulation of phospho-NFs within perikarya or proximal axons. Because oxidative stress increases cdk5 activity in experimental motor neuron disease, we examined whether oxidative stress induced cdk5-mediated NF phosphorylation. Treatment of cultured neuronal cells with hydrogen peroxide inhibited axonal transport of green fluorescent protein-tagged NF subunits and induced perikaryal accumulation of NF phosphoepitopes normally confined to axons. These effects were prevented by treatment with the cdk5 inhibitor roscovitine or transfection with a construct expressing the endogenous cdk5 inhibitor peptide. These findings indicate that oxidative stress can compromise NF dynamics via hyperactivation of cdk5 and suggest that antioxidants may alleviate multiple aspects of neuropathology in motor neuron disease.

Neuronal cyclin-dependent kinase 5: role in nervous system function and its specific inhibition by the Cdk5 inhibitory peptide

Cyclin-dependent kinase 5 (Cdk5) is a member of the cyclin-dependent kinase family that is involved in the regulation of the cell cycle. As their name suggests, the Cdks require association with activator proteins called cyclins for their activity. Cdk5, however, is unique to this family of proline-directed serine/threonine kinases on two accounts. Firstly, Cdk5 has not been found to function in the cell cycle and, although expressed in a number of tissues, its activity is restricted to the nervous system. Secondly, unlike the other members of the Cdk family, Cdk5 is not activated by association with a cyclin, although it can bind them. Instead, Cdk5 is activated by the activator proteins p35 and p39 that are structurally distinct from cyclins and have, for the most part, a neuronal-specific expression pattern. In the past decade of research on Cdk5, it is now established that Cdk5 activity is critical for the proper formation and function of the brain. Moreover, its role as a central kinase, phosphorylating its substrates in its 'cross-talk' control of other kinase and signal transduction pathways, has also been determined. In addition to the normal physiological role of Cdk5, the kinase has been implicated in certain neurodegenerative disorders. For example, Cdk5 associates with the proteolytic, more active p25 fragment that is derived through the cleavage of p35. In turn, the p25/Cdk5 complex aberrantly phosphorylates its substrates tau and neurofilaments, which has been implicated in the pathogenesis of these disorders. Here, we attempt to review the past decade of research on Cdk5 from our laboratory and others, on the roles of Cdk5 in nervous system function. Additionally, our research has recently uncovered a possible therapeutic avenue of research, focusing on inhibition of aberrant Cdk5 hyperactivity which may well be used to treat the symptoms of a number of neurodegenerative diseases. The elucidation of a specific inhibitor of p25/Cdk5, termed CIP, also inhibits p25/Cdk5-mediated tau phosphorylation. This may well provide us with avenues of research focusing on the inhibition of pathologically damaging p25/Cdk5 species.

NF-M is an essential target for the myelin-directed "outside-in" signaling cascade that mediates radial axonal growth

Neurofilaments are essential for acquisition of normal axonal calibers. Several lines of evidence have suggested that neurofilament-dependent structuring of axoplasm arises through an “outside-in” signaling cascade originating from myelinating cells. Implicated as targets in this cascade are the highly phosphorylated KSP domains of neurofilament subunits NF-H and NF-M. These are nearly stoichiometrically phosphorylated in myelinated internodes where radial axonal growth takes place, but not in the smaller, unmyelinated nodes. Gene replacement has now been used to produce mice expressing normal levels of the three neurofilament subunits, but which are deleted in the known phosphorylation sites within either NF-M or within both NF-M and NF-H. This has revealed that the tail domain of NF-M, with seven KSP motifs, is an essential target for the myelination-dependent outside-in signaling cascade that determines axonal caliber and conduction velocity of motor axons.

Cdk5 regulates the organization of Nestin and its association with p35

The intermediate filament protein nestin is characterized by its specific expression during the development of neuronal and myogenic tissues. We identify nestin as a novel in vivo target for cdk5 and p35 kinase, a critical signaling determinant in development. Two cdk5-specific phosphorylation sites on nestin, Thr-1495 and Thr-316, were established, the latter of which was used as a marker for cdk5-specific phosphorylation in vivo. Ectopic expression of cdk5 and p35 in central nervous system progenitor cells and in myogenic precursor cells induced elevated phosphorylation and reorganization of nestin. The kinetics of nestin expression corresponded to elevated expression and activation of cdk5 during differentiation of myoblast cell cultures and during regeneration of skeletal muscle. In the myoblasts, a disassembly-linked phosphorylation of Thr-316 indicated active phosphorylation of nestin by cdk5. Moreover, cdk5 occurred in physical association with nestin. Inhibition of cdk5 activity-either by transfection with dominant-negative cdk5 or by using a specific cdk5 inhibitor-blocked myoblast differentiation and phosphorylation of nestin at Thr-316, and this inhibition markedly disturbed the organization of nestin. Interestingly, the interaction between p35, the cdk5 activator, and nestin appeared to be regulated by cdk5. In differentiating myoblasts, p35 was not complexed with nestin phosphorylated at Thr-316, and inhibition of cdk5 activity during differentiation induced a marked association of p35 with nestin. These results demonstrate that there is a continuous turnover of cdk5 and p35 activity on a scaffold formed by nestin. This association is likely to affect the organization and operation of both cdk5 and nestin during development.

Downregulation of neuronal cdk5/p35 in opioid addicts and opiate-treated rats: relation to neurofilament phosphorylation

Neuronal cyclin-dependent kinase-5 (Cdk5) and its neuron-specific activator p35 play a major role in regulating the cytoskeleton dynamics. Since opioid addiction was associated with hyperphosphorylation of neurofilament (NF) in postmortem human brains, this study was undertaken to assess the status of the cdk5/p35 complex and its relation with NF-H phosphorylation in brains of chronic opioid abusers. Decreased immunodensities of cdk5 (18%) and p35 (26-44%) were found in the prefrontal cortex of opioid addicts compared with matched controls. In the same brains, the densities of p25 (a truncated neurotoxic form of p35), phosphatase PP2Ac and mu-calpain were found unaltered. Acute treatment of rats with morphine (30 mg/kg, 2 h) increased the density of cdk5 (35%), but not that of p35, in the cerebral cortex. In contrast, chronic morphine (10-100 mg/kg for 5 days) induced marked decreases in cdk5 (40%) and p35 (47%) in rat brain. In brains of opioid addicts, the density of phosphorylated NF-H was increased (43%) as well as the ratio of phosphorylated to nonphosphorylated NF-H forms (two-fold). In these brains, phosphorylated NF-H significantly correlated with p35 (r=0.58) but not with cdk5 (r=0.03). The results suggest that opiate addiction is associated with downregulation of cdk5/p35 levels in the brain. This downregulation and the aberrant hyperphosphorylation of NF-H proteins might have important consequences in the development of neural plasticity associated with opiate addiction in humans.

Cdk5 regulates cell-matrix and cell-cell adhesion in lens epithelial cells

Cdk5 is a member of the cyclin-dependent kinase family, which is expressed predominantly in terminally differentiated neurons. Lower levels of Cdk5 are also found in a wide variety of cell types, including the lens. Although Cdk5 has been shown to play an important role in neuronal migration and neurite outgrowth, its function in non-neuronal cells is not known. Therefore, this study was undertaken to explore the role of Cdk5 in the lens. Results showed that, within the adult mouse lens, Cdk5 was localized to the cytoplasm,especially along the lateral membranes of differentiating primary fiber cells,which suggests a role in cell-cell adhesion. Staining at the tips of elongating fiber cells was also particularly strong, suggesting a role in cell-matrix adhesion. To examine the possible role of Cdk5 in lens epithelial cell adhesion, we stably transfected N/N1003A rabbit lens epithelial cells with cDNAs for Cdk5 or a dominant-negative mutation, Cdk5-T33. Attachment to a fibronectin matrix, as measured with substrate-coated cell adhesion strips,was increased by Cdk5 overexpression, while an equivalent overexpression of Cdk5-T33 had no effect. Cdk5 also increased the rate of cell attachment and spreading as measured by electric cell-substrate impedance sensing (ECIS). In addition, Cdk5 overexpression decreased cell-cell adhesion as measured by a cell aggregation assay. These findings suggest that Cdk5 plays a role in regulating both cell-matrix and cell-cell interactions in the lens.

GC box-binding transcription factors control the neuronal specific transcription of the cyclin-dependent kinase 5 regulator p35

Cyclin-dependent kinase 5 (cdk5)/p35 kinase activity is highest in post-mitotic neurons of the central nervous system and is critical for development and function of the brain. The neuronal specific activity of the cdk5/p35 kinase is achieved through the regulated expression of p35 mRNA. We have identified a small 200-bp fragment of the p35 promoter that is sufficient for high levels of neuronal specific expression. Mutational analysis of this TATA-less promoter has identified a 17-bp GC-rich element, present twice, that is both required for promoter activity and sufficient for neuronal specific transcription. A GC box within the 17-bp element is critical for both promoter activity and protein-DNA complex formation. The related transcription factors Sp1, Sp3, and Sp4 constitute most of the GC box DNA binding activity in neurons. We have found that both the relative contribution of the Sp family proteins to GC box binding and the transcriptional activity of these proteins is regulated during neuronal differentiation. Thus, our data show that the GC box-binding Sp proteins contribute to the regulation of p35 expression in neurons, suggesting changes in the Sp transcription factors level and activity may contribute to cell type-specific expression of many genes in the central nervous system.

Cyclin-dependent kinase 5 (cdk5) activation requires interaction with three domains of p35

Cyclin-dependent kinase 5 (cdk5), in contrast to other members of the cyclin-dependent kinase family, is not activated by cyclins but instead is activated by complexing with neuron-specific activator molecules (p35, p39, and p67). The most effective activator of cdk5 both in vitro and in vivo is p35. We have taken a kinetic approach to study the interaction between p35, its various truncated forms, and cdk5 to understand better the mechanism of its activation. The cdk5 complexes formed with the truncated forms p25 and p21 produced similar maximum active kinase, whereas the cdk5 complexed with full-length p35 and a further truncated form spanning amino acid residues from 138 to 291, with approximate molecular weight of 16 kDa (p16), produced slightly less (80%) activation than p25. P16 was the smallest fragment of p35 that produced activation equal to or greater than that of full-length p35. By examination of further truncations of p16, we found that a small number of residues, 11 and 4 at the N- and C-termini, respectively, of p16, are essential for cdk5 activation. Further truncation, removing both essential N- and C-terminal domains, produces a peptide with markedly higher affinity for cdk5 compared with the peptides that retain either of these domains. Using these inactive truncated peptides as inhibitors, we examined the kinetics of activation. From these studies we conclude that activation involves at least three cdk5-interacting domains, one located at each end of p16 and at least one located in a central domain. The cdk5 activation process is slow: The second-order rate constant for p16 is about 1.2 microM(-1) hr(-1). On the basis of kinetic data, we suggest that cdk5 exists in two conformations. The inactive kinase conformation predominates in the absence of the activator. Activation occurs in two stages: a rapid and reversible interaction of cdk5 with its activator, which involves only one or two binding domains, followed by a slow stabilization of the active conformation as interaction with all three domains is achieved.

A scintillation proximity assay for studying inhibitors of human tau protein kinase II/cdk5 using a 96-well format

Dysregulation of the brain-specific tau protein kinase II (TPK II)/cdk5 is reported to play an important role in the pathogenesis of Alzheimer's disease. We report here a quantitative scintillation proximity assay (SPA), which is suitable for determining TPK II/cdk5 activity and its inhibition. It depends upon the phosphorylation of a synthetic histone-based peptide substrate (PKTPKKAKKL), which has been biotinylated at its C-terminus. When this biotinylated peptide is incubated with [gamma-33P] ATP and TPK II/cdk5 under defined assay conditions, product formation is linear with respect to time and enzyme concentration. The production of [33P] phosphorylated peptide is inhibited in the presence of a known TPK II/cdk5 inhibitor but is unaffected in the presence of 1% DMSO. A signal-to-noise ratio of 16:1 was obtained in a 60-min assay with an intra-assay variability of <10% in the 96-well microtiter format. The TPK II/cdk5 SPA is very robust, sensitive and simple to perform.

p35 and p39 are essential for cyclin-dependent kinase 5 function during neurodevelopment

Cyclin-dependent kinase 5 (Cdk5) plays a pivotal role in brain development and neuronal migration. Cdk5 is abundant in postmitotic, terminally differentiated neurons. The ability of Cdk5 to phosphorylate substrates is dependent on activation by its neuronal-specific activators p35 and p39. There exist striking differences in the phenotypic severity of Cdk5-deficient mice and p35-deficient mice. Cdk5-null mutants show a more severe disruption of lamination in the cerebral cortex, hippocampus, and cerebellum. In addition, Cdk5-null mice display perinatal lethality, whereas p35-null mice are viable. These discrepancies have been attributed to the function of other Cdk5 activators, such as p39. To understand the roles of p39 and p35, we created p39-null mice and p35/p39 compound-mutant mice. Interestingly, p39-null mice show no obvious detectable abnormalities, whereas p35(-/-)p39(-/-) double-null mutants are perinatal lethal. We show here that the p35(-/-)p39(-/-) mutants exhibit phenotypes identical to those of the Cdk5-null mutant mice. Other compound-mutant mice with intermediate phenotypes allow us to determine the distinct and redundant functions between p35 and p39. Our data strongly suggest that p35 and p39 are essential for Cdk5 activity during the development of the nervous system. Thus, p35 and p39 are likely to be the principal, if not the only, activators of Cdk5.

Cyclin-dependent kinase 5 promotes insulin exocytosis

Cyclin-dependent kinase 5 (Cdk5) is widely expressed although kinase activity has been described preferentially in neuronal systems. Cdk5 has an impact on actin polymerization during neuronal migration and neurite outgrowth and deregulation of the kinase has been implicated in the promotion of neurodegeneration. Recently it was shown that Cdk5 modulates dopamine signaling in neurons by regulating DARPP-32 function. In addition, Cdk5 phosphorylates munc-18 and synapsin I, two essential components of the exocytotic machinery. We have shown by reverse transcriptase-polymerase chain reaction, immunocytochemistry, and Western blotting that Cdk5 is present in the insulin-secreting pancreatic beta-cell. Subcellular fractionation of isolated beta-cells revealed a glucose-induced translocation of membrane-bound Cdk5 protein to lower density fractions. Inhibition of Cdk5 with roscovitine reduced insulin secretion with approximately 35% compared with control after glucose stimulation and with approximately 65% after depolarization with glucose and KCl. Capacitance measurements performed on single beta-cells that expressed a dominant-negative Cdk5 mutant showed impaired exocytosis. The effect on exocytosis by Cdk5 appeared to be independent of changes in free cytoplasmic Ca(2+) concentration. Taken together these results show that Cdk5 is present in beta-cells and acts as a positive regulator of insulin exocytosis.

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

Cyclin-dependent kinase 5 is predominantly expressed in postmitotic neurons and plays a role in neurite elongation during development. It has also been postulated to play a role in apoptosis in a variety of cells, including neurons, but little is known about the generality and functional significance of cdk5 expression in neuronal apoptosis in living brain. We have therefore examined its expression and that of its known activators, p35, p39 and p67, in models of induced apoptosis in neurons of the substantia nigra. We find that cdk5 is expressed in apoptotic profiles following intrastriatal injection of 6-hydroxydopamine and axotomy. It is expressed exclusively in profiles which are in late morphologic stages of apoptosis. In these late stages, derivation of the profiles from neurons, and localization of expression to the nucleus, can be demonstrated by co-labeling with a neuron-specific nuclear marker, NeuN. In another model of induced apoptotic death in nigra, produced by developmental striatal lesion, kinase activity increases in parallel with cell death. While mRNAs for all three cdk5 activators are expressed in nigra during development, only p35 protein is expressed in apoptotic profiles. We conclude that cdk5/p35 expression is a general feature of apoptotic neuron death in substantia nigra neurons in vivo.

Mitotic reorganization of the intermediate filament protein nestin involves phosphorylation by cdc2 kinase

The intermediate filament protein nestin is expressed during early stages of development in the central nervous system and in muscle tissues. Nestin expression is associated with morphologically dynamic cells, such as dividing and migrating cells. However, little is known about regulation of nestin during these cellular processes. We have characterized the phosphorylation-based regulation of nestin during different stages of the cell cycle in a neuronal progenitor cell line, ST15A. Confocal microscopy of nestin organization and (32)P in vivo labeling studies show that the mitotic reorganization of nestin is accompanied by elevated phosphorylation of nestin. The phosphorylation-induced alterations in nestin organization during mitosis in ST15A cells are associated with partial disassembly of nestin filaments. Comparative in vitro and in vivo phosphorylation studies identified cdc2 as the primary mitotic kinase and Thr(316) as a cdc2-specific phosphorylation site on nestin. We generated a phosphospecific nestin antibody recognizing the phosphorylated form of this site. By using this antibody we observed that nestin shows constitutive phosphorylation at Thr(316), which is increased during mitosis. This study shows that nestin is reorganized during mitosis and that cdc2-mediated phosphorylation is an important regulator of nestin organization and dynamics during mitosis.

Cyclin-dependent protein kinase 5 (Cdk5) and the regulation of neurofilament metabolism

Cyclin-dependent kinase 5 (Cdk5), a complex of Cdk5 and its activator p35 (Cdk5/p35), phosphorylates diverse substrates which have multifunctional roles in the nervous system. During development, it participates in neuronal differentiation, migration, axon outgrowth and synaptogenesis. Cdk5, acting together with other kinases, phosphorylates numerous KSPXK consensus motifs in diverse cytoskeletal protein target molecules, including neurofilaments, and microtubule associated proteins, tau and MAPs. Phosphorylation regulates the dynamic interactions of cytoskeletal proteins with one another during all aspects of neurogenesis and axon radial growth. In this review we shall focus on Cdk5 and its regulation as it modulates neurofilament metabolism in axon outgrowth, cytoskeletal stabilization and radial growth. We suggest that Cdk5/p35 forms compartmentalized macromolecular complexes of cytoskeletal substrates, other neuronal kinases, phosphatases and activators ('phosphorylation machines') which facilitate the dynamic molecular interactions that underlie these processes.

Neuronal cyclin-dependent kinase 5 activity is critical for survival

Cyclin-dependent kinase 5 (Cdk5) null mice exhibit a unique phenotype characterized by perinatal mortality, disrupted cerebral cortical layering attributable to abnormal neuronal migration, lack of cerebellar foliation, and chromatolytic changes of neurons in the brainstem and the spinal cord. Because Cdk5 is expressed in both neurons and astrocytes, it has been unclear whether this phenotype is primarily attributable to defects in neurons or in astrocytes. Herein we report reconstitution of Cdk5 expression in neurons in Cdk5 null mice and its effect on the null phenotype. Unlike the Cdk5 null mice, the reconstituted Cdk5 null mice that express the Cdk5 transgene under the p35 promoter (TgKO mice) were viable and fertile. Because Cdk5 expression is mainly limited to neurons in these mice and rescues the defects in the nervous system of the Cdk5 null phenotype, it clearly demonstrates that Cdk5 activity is necessary for normal development and survival of p35-expressing neurons.

Cdk5 mediates changes in morphology and promotes apoptosis of astrocytoma cells in response to heat shock

The cyclin-dependent kinase member, Cdk5, is expressed in a variety of cell types, but neuron-specific expression of its activator, p35, is thought to limit its activity to neurons. Here we demonstrate that both Cdk5 and p35 are expressed in the human astrocytoma cell line, U373. Cdk5 and p35 are present in the detergent-insoluble cytoskeletal fraction of this cell line and Cdk5 localizes to filopodia and vinculin-rich regions of cell-matrix contact in lamellopodia. When exposed to a 46(o)C heat shock, U373 cells change shape, lose cell-matrix contacts and show increased levels of apoptosis. To test whether Cdk5 activation might play a role in these events, U373 cells were stably transfected with histidine-tagged or green fluorescent protein-tagged constructs of Cdk5 or a dominant negative mutation, Cdk5T33. Under normal growth conditions, growth characteristics of the stably transfected lines were indistinguishable from untransfected U373 cells and Cdk5 localization was not changed. However, when subjected to heat shock, cells stably transfected with Cdk5-T33 remained flattened, showed little loss of cell-matrix adhesion, and exhibited significantly lower levels of apoptosis. In contrast, cells that overexpressed wild-type Cdk5 showed morphological changes similar to those seen in untransfected U373 cells in response to heat shock and had significantly higher levels of apoptosis. Heat-shocked cells showed changes in p35 mobility and stability of the Cdk5/p35 complex consistent with endogenous Cdk5 activity. Together these findings suggest that endogenous Cdk5 activity may play a key role in regulating morphology, attachment, and apoptosis in U373 cells, and raise the possibility that Cdk5 may be a general regulator of cytoskeletal organization and cell adhesion in both neuronal and non-neuronal cells.

Developmental changes in the localization of activated C-JUN N-terminal kinase (JNK/SAPK) in the chick spinal cord

To examine the role of c-Jun N-terminal kinase (JNK/SAPK) in the developing nervous system of vertebrates, the localization of an active form of JNK, phosphorylated JNK (p-JNK), was studied in the lumbosacral spinal cord of the chick embryo. We also examined the localization of phosphorylated neurofilaments (NFs, potential targets of p-JNK) and cyclin-dependent kinase 5 (Cdk5), which is known to phosphorylate cytoskeletal proteins, including NFs, and compared their expression with that of p-JNK. Additionally, the localization of phosphorylated forms of c-Jun and ATF-2 was compared with that of p-JNK. On embryonic day 3 (E3), the expression of p-JNK was observed in regions containing early-projecting axons. Axons in these regions also expressed phosphorylated NFs. Subsequently, on E5 and E8, the expression of both p-JNK and phosphorylated NFs increased concomitantly in the axonal tracts in the spinal white matter. Thus, white matter expressed both p-JNK and phosphorylated NFs, whereas there was only weak expression of Cdk5. By E13, the spinal cord expression pattern of p-JNK and phosphorylated NFs had changed compared to earlier ages. Although phosphorylated NFs were still expressed in the white matter, the expression of p-JNK was decreased in axons in the white matter, whereas strong p-JNK expression appeared in cell nuclei in the gray matter. In summary, the present study revealed that the localization of p-JNK in the spinal cord changes dramatically from axons to cell nuclei during development, suggesting multiple roles of p-JNK, depending on the developmental age.

Neuronal migration disorders: from genetic diseases to developmental mechanisms

Neurons that constitute the cerebral cortex must migrate hundreds of cell-body distances from their place of birth, and through several anatomical boundaries, to reach their final position within the correct cortical layer. Human neurological conditions associated with abnormal neuronal migration, together with spontaneous and engineered mouse mutants, define at least four distinct steps in cortical neuronal migration. Many of the genes that control neuronal migration have strong genetic or biochemical links to the cytoskeleton, suggesting that the field of neuronal migration might be closing in on the underlying cytoskeletal events.

The expression of Cdk5, p35, p39, and Cdk5 kinase activity in developing, adult, and aged rat brains

The expression of cyclin-dependent kinase 5 (Cdk5) and its regulatory subunits, p35 and p39, was investigated in rat brain from embryonic day 12 (E12) to postnatal 18 months (18M). The Cdk5 protein levels increased from E12 to postnatal day 7 (P7) and remained at this level until 18M. The Cdk5 kinase activity and the levels of both p35 mRNA and protein were low at E12, became prominent at E18-P14 but then decreased in the adult and aged rat brains of 3M to 18M. In comparison, the expression pattern of p39 appeared to have an inverse relationship to that of Cdk5 and p35. In regional distribution studies, p35 protein levels and Cdk5 kinase activity were significantly higher in the cerebral cortex and hippocampus, but lower in the cerebellum and striatum. These results suggested that Cdk5, p35 and p39 might have region-specific and developmental stage-specific functions in rat brain.

Regulation of neurofilament axonal transport by phosphorylation in optic axons in situ

Axonal transport of neurofilament (NFs) is considered to be regulated by phosphorylation. While existing evidence for this hypothesis is compelling, supportive studies have been largely restricted to correlative evidence and/or experimental systems involving mutants. We tested this hypothesis in retinal ganglion cells of normal mice in situ by comparing subunit transport with regional phosphorylation state coupled with inhibition of phosphatases. NF subunits were radiolabeled by intravitreal injection of 35S-methionine. NF axonal transport was monitored by following the location of the peak of radiolabeled subunits immunoprecipitated from 9x1.1 mm segments of optic axons. An abrupt decline transport rate was observed between days 1 and 6, which corresponded to translocation of the peak of radiolabeled subunits from axonal segment 2 into segment 3. Notably, this is far downstream from the only caliber increase of optic axons at 150 mu from the retina. Immunoblot analysis demonstrated a unique threefold increase between segments 2 and 3 in levels of a "late-appearing" C-terminal NF-H phospho-epitope (RT97). Intravitreal injection of the phosphatase inhibitor okadaic acid increased RT97 immunoreactivity within retinas and proximal axons, and markedly decreased NF transport rate out of retinas and proximal axons. These findings provide in situ experimental evidence for regulation of NF transport by site-specific phosphorylation.

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.

Regulation of cyclin-dependent kinase 5 catalytic activity by phosphorylation

Cyclin-dependent kinase 5 (cdk5) is found in an active form only in neuronal cells. Activation by virtue of association with the cyclin-like neuronal proteins p35 (or its truncated form p25) and p39 is the only mechanism currently shown to regulate cdk5 catalytic activity. In addition to cyclin binding, other members of the cdk family require for maximal activation phosphorylation of a Ser/Thr residue (Thr(160) in the case of cdk-2) that is conserved in all cdks except cdk8. This site is phosphorylated by cdk-activating kinases, which, however, do not phosphorylate cdk5. To examine the possible existence of a phosphorylation-dependent regulatory mechanism in the case of cdk5, we have metabolically labeled PC12 cells with (32)P(i) and shown that the endogenous cdk5 is phosphorylated. Bacterially expressed cdk5 also can be phosphorylated by PC12 cell lysates. Phosphorylation of cdk5 by a PC12 cell lysate results in a significant increase in cdk5/p25 catalytic activity. Ser(159) in cdk5 is homologous to the regulatory Thr(160) in cdk2. A Ser(159)-to-Ala (S159A) cdk5 mutant did not show similar activation, which suggests that cdk5 is also regulated by phosphorylation at this site. Like other members of the cdk family, cdk5 catalytic activity is influenced by both p25 binding and phosphorylation. We show that the cdk5-activating kinase (cdk5AK) is distinct from the cdk-activating kinase (cyclin H/cdk7) that was reported previously to neither phosphorylate cdk5 nor affect its activity. We also show that casein kinase I, but not casein kinase II, can phosphorylate and activate cdk5 in vitro.

CDK-5-mediated neurofilament phosphorylation in SHSY5Y human neuroblastoma cells

Cyclin-dependent kinase-5 (CDK-5) has been shown to play important roles in neuronal development and neurogenesis. In vitro studies indicate a role of CDK-5 in phosphorylation of neurofilaments (NFs). In this study, we have chosen the human neuroblastoma cell line SHSY5Y as a model system to study the in vivo phosphorylation of NF proteins by CDK-5. Upon differentiation of SHSY5Y cells with retinoic acid, we found that the phosphorylation of high molecular mass (NF-H) and medium molecular mass (NF-M) NFs increased, whereas the CDK-5 protein level and kinase activity were unaffected. The role of CDK-5 in the phosphorylation of cytoskeletal proteins was studied by using antisense oligonucleotides (ONs) to inhibit the expression of the CDK-5 gene. We found that inhibition of CDK-5 levels by antisense ON treatment resulted in a decrease in phosphorylation of NF-H that correlated with a decline in neurite outgrowth. These results demonstrate that CDK-5 is a major proline-directed kinase phosphorylating the human NF-H tail domain.

Dendritic changes in Alzheimer's disease and factors that may underlie these changes

It seems likely that the Alzheimer disease (AD)-related dendritic changes addressed in this article are induced by two principally different processes. One process is linked to the plastic response associated with deafferentation, that is, long-lasting transneuronally induced regressive changes in dendritic geometry and structure. The other process is associated with severe alterations of the dendritic- and perikaryal cytoskeleton as seen in neurons with the neurofibrillary pathology of AD, that is, the formation of paired helical filaments formed by hyperphosphorylated microtubule-associated protein tau. As the development of dendritic and cytoskeletal abnormalities are at least mediated by alterations in signal transduction, this article also reviews changes in signal pathways in AD. We also discuss transgenic approaches developed to model and understand cytoskeletal abnormalities.

Mitogen-activated protein kinases (Erk1,2) phosphorylate Lys-Ser-Pro (KSP) repeats in neurofilament proteins NF-H and NF-M

Mammalian neurofilament proteins, particularly midsized (NF-M) and heavy (NF-H) molecular weight neurofilament proteins, are highly phosphorylated in axons. Neurofilament function depends on the state of phosphorylation of the numerous serine/threonine residues in these proteins. Most phosphorylation occurs in the lys-ser-pro (KSP) repeats in the C-terminal tail domains of NF-H and NF-M. In our previous study, cyclin-dependent kinase 5 (cdk5) was shown to phosphorylate specifically the KSPXK repeats in rat NF-H. Because 80% of the repeats are of the KSPXXXK type, it was of interest to determine which kinase phosphorylates these motifs. Using a synthetic KSPXXXK peptide to screen for a specific kinase, we fractionated rat brain extracts by column chromatography and identified extracellular signal-regulated kinase (Erk2) activated by an upstream activator, the mitogen-activated protein kinase kinase MAPKK (MEK), by Western blot analysis, sequence identification, and inhibition by a specific MEK inhibitor (PD 98059). The fraction containing Erk2, as well as bacterially expressed Erk1 and Erk2, phosphorylated all types of KSP motifs in peptides (KSPXK, KSPXXK, KSPXXXK, and KSPXXXXK) derived from NF-M and NF-H. They also phosphorylated an expressed 24 KSPXXXK repeat NF-H polypeptide, an expressed NF-H as well as dephosphorylated native rat NF-H, and NF-M proteins with accompanying decreases in their respective electrophoretic mobilities. A comparative kinetic study of Erk2 and cdk5 phosphorylation of KSPXK and KSPXXXK peptides revealed that, in contrast to cdk5, which phosphorylated only the KSPXK peptide, Erk2 could phosphorylate both. The preferred substrate for Erk2 was KSPXXXK peptide. The MEK inhibitor PD 98059 also inhibited phosphorylation of NF-H, NF-M, and microtubule-associated protein (MAP) in primary rat hippocampal cells and caused a decrease in neurite outgrowth, suggesting that Erk1,2 may play an important role in neurite growth and branching. These data suggest that neuronal Erk1 and Erk2 are capable of phosphorylating serine residues in diverse KSP repeat motifs in NF-M and NF-H.