Reconstitution of neuronal Cdc2-like kinase from bacteria-expressed Cdk5 and an active fragment of the brain-specific activator. Kinase activation in the absence of Cdk5 phosphorylation

Antibody-free time-resolved terbium luminescence assays designed for cyclin-dependent kinase 5 (CDK5)

Novel time-resolved terbium luminescence assays were developed for CDK5 and CDK2 by designing synthetic substrates which incorporate phospho-inducible terbium sensitizing motifs with kinase substrate consensus sequences. Substrates designed for CDK5 showed no phosphorylation by CDK2, opening the possibility for CDK5-specific assay development for selective drug discovery.

The role of cyclin-dependent kinase 5 in neuropathic pain

The chronification of pain can be attributed to changes in membrane receptors and channels underlying neuronal plasticity and signal transduction largely within nociceptive neurons that initiate and maintain pathological pain states. These proteins are subject to dynamic modification by posttranslational modifications, creating a code that controls protein function in time and space. Phosphorylation is an important posttranslational modification that affects ∼30% of proteins in vivo. Increased phosphorylation of various nociceptive ion channels and of their modulators underlies sensitization of different pain states. Cyclin-dependent kinases are proline-directed serine/threonine kinases that impact various biological and cellular systems. Cyclin-dependent kinase 5 (Cdk5), one member of this kinase family, and its activators p35 and p39 are expressed in spinal nerves, dorsal root ganglia, and the dorsal horn of the spinal cord. In neuropathic pain conditions, expression and/or activity of Cdk5 is increased, implicating Cdk5 in nociception. Experimental evidence suggests that Cdk5 is regulated through its own phosphorylation, through increasing p35's interaction with Cdk5, and through cleavage of p35 into p25. This narrative review discusses the molecular mechanisms of Cdk5-mediated regulation of target proteins involved in neuropathic pain. We focus on Cdk5 substrates that have been linked to nociceptive pathways, including channels (eg, transient receptor potential cation channel and voltage-gated calcium channel), proteins involved in neurotransmitter release (eg, synaptophysin and collapsin response mediator protein 2), and receptors (eg, glutamate, purinergic, and opioid). By altering the phosphoregulatory "set point" of proteins involved in pain signaling, Cdk5 thus appears to be an attractive target for treating neuropathic pain conditions.

CDK5: Key Regulator of Apoptosis and Cell Survival

The atypical cyclin-dependent kinase 5 (CDK5) is considered as a neuron-specific kinase that plays important roles in many cellular functions including cell motility and survival. The activation of CDK5 is dependent on interaction with its activator p35, p39, or p25. These activators share a CDK5-binding domain and form a tertiary structure similar to that of cyclins. Upon activation, CDK5/p35 complexes localize primarily in the plasma membrane, cytosol, and perinuclear region. Although other CDKs are activated by cyclins, binding of cyclin D and E showed no effect on CDK5 activation. However, it has been shown that CDK5 can be activated by cyclin I, which results in anti-apoptotic functions due to the increased expression of Bcl-2 family proteins. Treatment with the CDK5 inhibitor roscovitine sensitizes cells to heat-induced apoptosis and its phosphorylation, which results in prevention of the apoptotic protein functions. Here, we highlight the regulatory mechanisms of CDK5 and its roles in cellular processes such as gene regulation, cell survival, and apoptosis.

Preparation of CDK/Cyclin Inhibitor Complexes for Structural Determination

The abundance of biochemical and structural knowledge on the Cyclin-Dependent Kinases (CDKs) has provided a comprehensive but not exhaustive insight into the molecular determinants that govern their function mechanisms. The implementation of structural and functional CDK models towards developing novel anticancer strategies that will specifically target individual or multiple CDKs remains a critical need.More than 250 CDKs crystal structures are available to-date, including truncated or whole, modified or not, active or inactive forms, co-crystallized with the cyclins and/or their respective putative inhibitors, though, to our knowledge, there is no NMR solved structure available to date. We hitherto attempt to provide a useful guide from protein production to crystallization for CDK/Inhibitors complexes based on an overview of the already elucidated CDK structures, constructs and the preferable expression vectors in each case, in order to yield the respective crystals.

Physiological and pathological phosphorylation of tau by Cdk5

Hyperphosphorylation of microtubule-associated protein tau is one of the major pathological events in Alzheimer’s disease (AD) and other related neurodegenerative diseases, including frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). Mutations in the tau gene MAPT are a cause of FTDP-17, and the mutated tau proteins are hyperphosphorylated in patient brains. Thus, it is important to determine the molecular mechanism of hyperphosphorylation of tau to understand the pathology of these diseases collectively called tauopathy. Tau is phosphorylated at many sites via several protein kinases, and a characteristic is phosphorylation at Ser/Thr residues in Ser/Thr-Pro sequences, which are targeted by proline-directed protein kinases such as ERK, GSK3β, and Cdk5. Among these kinases, Cdk5 is particularly interesting because it could be abnormally activated in AD. Cdk5 is a member of the cyclin-dependent kinases (Cdks), but in contrast to the major Cdks, which promote cell cycle progression in proliferating cells, Cdk5 is activated in post-mitotic neurons via the neuron-specific activator p35. Cdk5-p35 plays a critical role in brain development and physiological synaptic activity. In contrast, in disease brains, Cdk5 is thought to be hyperactivated by p25, which is the N-terminal truncated form of p35 and is generated by cleavage with calpain. Several reports have indicated that tau is hyperphosphorylated by Cdk5-p25. However, normal and abnormal phosphorylation of tau by Cdk5 is still not completely understood. In this article, we summarize the physiological and pathological phosphorylation of tau via Cdk5.

New structural insights into phosphorylation-free mechanism for full cyclin-dependent kinase (CDK)-cyclin activity and substrate recognition

Pho85 is a versatile cyclin-dependent kinase (CDK) found in budding yeast that regulates a myriad of eukaryotic cellular functions in concert with 10 cyclins (called Pcls). Unlike cell cycle CDKs that require phosphorylation of a serine/threonine residue by a CDK-activating kinase (CAK) for full activation, Pho85 requires no phosphorylation despite the presence of an equivalent residue. The Pho85-Pcl10 complex is a key regulator of glycogen metabolism by phosphorylating the substrate Gsy2, the predominant, nutritionally regulated form of glycogen synthase. Here we report the crystal structures of Pho85-Pcl10 and its complex with the ATP analog, ATPγS. The structure solidified the mechanism for bypassing CDK phosphorylation to achieve full catalytic activity. An aspartate residue, invariant in all Pcls, acts as a surrogate for the phosphoryl adduct of the phosphorylated, fully activated CDK2, the prototypic cell cycle CDK, complexed with cyclin A. Unlike the canonical recognition motif, SPX(K/R), of phosphorylation sites of substrates of several cell cycle CDKs, the motif in the Gys2 substrate of Pho85-Pcl10 is SPXX. CDK5, an important signal transducer in neural development and the closest known functional homolog of Pho85, does not require phosphorylation either, and we found that in its crystal structure complexed with p25 cyclin a water/hydroxide molecule remarkably plays a similar role to the phosphoryl or aspartate group. Comparison between Pho85-Pcl10, phosphorylated CDK2-cyclin A, and CDK5-p25 complexes reveals the convergent structural characteristics necessary for full kinase activity and the variations in the substrate recognition mechanism.

p10, the N-terminal domain of p35, protects against CDK5/p25-induced neurotoxicity

Cyclin-dependent kinase 5(CDK5) in complex with its activator, p35 (protein of 35 kDa), is essential for early neurodevelopment in mammals. However, endogenous cleavage of p35 to p25 is associated with neuron death and neurodegenerative disease. Here we show that a peptide (p10') encoding the N-terminal domain of p35 protects against CDK5/p25-induced toxicity in neurons. p10' also prevented the death of neurons treated with the neurotoxin, 1-methyl-4-phenylpyridinium (MPP(+)), which induces conversion of endogenous p35 to p25, and Parkinson disease (PD)-like symptoms in animals. MPP(+) induces CDK5/p25-dependent phosphorylation of peroxiredoxin 2 (Prx2), resulting in inhibition of its peroxireductase activity and accumulation of reactive oxygen species (ROS). We found that p10' expression inhibited both Prx2 phosphorylation and ROS accumulation in neurons. In addition, p10' inhibited the p25-induced appearance of antigen of the Ki67 antibody (Ki67) and phosphohistone H2AX (γH2AX), classic markers of cell cycle activity and DNA double-strand breakage, respectively, associated with neuron death. Our results suggest that p10 (protein of 10 kDa) is a unique prosurvival domain in p35, essential for normal CDK5/p35 function in neurons. Loss of the p10 domain results in CDK5/p25 toxicity and neurodegeneration in vivo.

Cyclin-dependent kinases in brain development and disease

Cyclin-dependent kinase 5 (Cdk5) is a multifaceted serine/threonine kinase protein with important roles in the nervous system. Two related proteins, p35 and p39, activate Cdk5 upon direct binding. Over the past decade, Cdk5 activity has been demonstrated to regulate many events during brain development, including neuronal migration as well as axon and dendrite development. Recent evidence also suggests a pivotal role for Cdk5 in synaptic plasticity, behavior, and cognition. Dysfunction of Cdk5 has been implicated in a number of neurological disorders and neurodegenerative diseases including Alzheimer's disease, amyotrophic lateral sclerosis, Niemann-Pick type C disease, and ischemia. Hyperactivation of Cdk5 due to the conversion of p35 to p25 by the calcium-dependent protease calpain during neurotoxicity also contributes to the pathological state. This review surveys recent literature surrounding Cdk5 in synaptic plasticity and homeostasis, with particular emphasis on Cdk5 kinase activity under neurodegenerative conditions.

A dinoflagellate CDK5-like cyclin-dependent kinase

BACKGROUND INFORMATION

Mitosis during the dinoflagellate cell cycle is unusual in that the nuclear envelope remains intact and segregation of the permanently condensed chromosomes uses a cytoplasmic mitotic spindle. To examine regulation of the dinoflagellate cell cycle in the context of these unusual nuclear features, it is necessary to isolate and characterize cell cycle regulators such as CDK (cyclin-dependent kinase).

RESULTS

We report the characterization of a CDK from the dinoflagellate Lingulodinium polyedrum. This CDK reacts with an anti-PSTAIRE antibody and was identified by protein microsequencing after partial purification. The protein microsequence shows homology toward the Pho85/CDK5 clade of CDKs. Neither the amount nor the phosphorylation state changed over the course of the cell cycle, in agreement with results reported for CDK5 family members in other systems.

CONCLUSIONS

We conclude we have probably isolated a dinoflagellate CDK5-like protein. The data reported here support the identification of this protein as a CDK5 homologue, and suggest that dinoflagellates may contain several CDK families.

Cyclin-dependent kinases: bridging their structure and function through computations

Cyclin-dependent kinases (CDKs) are one of the most promising target families for drug discovery for several diseases, such as cancer and neurodegenerative disorders. Over the years, structural insights on CDKs have demonstrated high protein plasticity, with several cases where two or more structures of the same protein adopt different conformations. This has generated a great deal of interest in understanding the relationship between CDK structure and function. Here, we highlight how computer simulations have recently contributed in characterizing some key rare and transient events in CDKs, such as the reaction transition state and activation loop movement. Although not yet fully defined, we can now portray the enzymatic mechanism and plasticity of CDKs at high spatial and temporal resolution. These theoretical studies bridge with experiments and highlight structural determinants that could help in designing specific CDK inhibitors.

Twice switched at birth: cell cycle-independent roles of the "neuron-specific" cyclin-dependent kinase 5 (Cdk5) in non-neuronal cells

Cdk5 (cyclin-dependent kinase 5 or initially NCLK for neuronal CDC2-like kinase) was switched twice at its birth nearly twenty years ago: first it was thought to be cyclin-dependent, second it was assumed to be primarily of importance in neuronal cells-both turned out not to be the case. In this review we want to discuss issues of pharmacological inhibition, to highlight the versatile roles, and to summarize the growing evidence for the functional importance of Cdk5 in non-neuronal tissues, such as blood cells, tumor cells, epithelial cells, the vascular endothelium, testis, adipose and endocrine tissues. The organizing principles we follow are apoptosis/cell death, migration/motility, aspects of inflammation, and, finally, secretion/metabolism.

Early growth response 1 (Egr-1) regulates phosphorylation of microtubule-associated protein tau in mammalian brain

In the normal brain, tau protein is phosphorylated at a number of proline- and non-proline directed sites, which reduce tau microtubule binding and thus regulate microtubule dynamics. In Alzheimer disease (AD), tau is abnormally hyperphosphorylated, leading to neurofibrillary tangle formation and microtubule disruption, suggesting a loss of regulatory mechanisms controlling tau phosphorylation. Early growth response 1 (Egr-1) is a transcription factor that is significantly up-regulated in AD brain. The pathological significance of this up-regulation is not known. In this study, we found that lentivirus-mediated overexpression of Egr-1 in rat brain hippocampus and primary neurons in culture activates proline-directed kinase Cdk5, inactivates PP1, promotes tau phosphorylation at both proline-directed Ser(396/404) and non-proline-directed Ser(262) sites, and destabilizes microtubules. Furthermore, in Egr-1(-/-) mouse brain, Cdk5 activity was decreased, PP1 activity was increased, and tau phosphorylation was reduced at both proline-directed and non-proline-directed sites. By using nerve growth factor-exposed PC12 cells, we determined that Egr-1 activates Cdk5 to promote phosphorylation of tau and inactivates PP1 via phosphorylation. When Cdk5 was inhibited, tau phosphorylation at both proline- and non-proline directed sites and PP1 phosphorylation were blocked, indicating that Egr-1 acts through Cdk5. By using an in vitro kinase assay and HEK-293 cells transfected with tau, PP1, and Cdk5, we found that Cdk5 phosphorylates Ser(396/404) directly. In addition, by phosphorylating and inactivating PP1, Cdk5 promotes tau phosphorylation at Ser(262) indirectly. Our results indicate that Egr-1 is an in vivo regulator of tau phosphorylation and suggest that in AD brain increased levels of Egr-1 aberrantly activate an Egr-1/Cdk5/PP1 pathway, leading to accumulation of hyperphosphorylated tau, thus destabilizing the microtubule cytoskeleton.

Regulation and role of cyclin-dependent kinase activity in neuronal survival and death

Cyclin-dependent kinase (Cdk)5 is a proline-directed Ser/Thr protein kinase that functions mainly in neurons and is activated by binding to a regulatory subunit, p35 or p39. Kinase activity is mainly determined by the amount of p35 available, which is controlled by a balance between synthesis and degradation. Kinase activity is also regulated by Cdk5 phosphorylation, but the activity of phosphorylated Cdk5 is in contrast to that of cycling Cdks. Cdk5 is a versatile protein kinase that regulates multiple neuronal activities including neuronal migration and synaptic signaling. Further, Cdk5 plays a role in both survival and death of neurons. Long-term inactivation of Cdk5 triggers cell death, and the survival activity of Cdk5 is apparent when neurons suffer from stress. In contrast, hyper-activation of Cdk5 by p25 promotes cell death, probably by reactivating cell-cycle machinery in the nucleus. The pro-death activity is suppressed by membrane association of Cdk5 via myristoylation of p35. Appropriate activity, localization, and regulation of Cdk5 may be critical for long-term survival of neurons, which is more than 80 years in the case of humans.

Membrane association facilitates degradation and cleavage of the cyclin-dependent kinase 5 activators p35 and p39

Cyclin-dependent kinase 5 (Cdk5) is activated by binding to its activators, p35 and p39. The level of Cdk5 activity is determined by the amount of p35 and p39, which is regulated not only by transcription but also via proteasomal degradation. Alternatively, calpain-induced cleavage of p35 to p25 can induce aberrant Cdk5 activation. As the regulation of p35 and p39 proteolysis is not well understood, we have studied here the mechanisms governing their degradation and cleavage. We find that p35 and p39 undergo proteasomal degradation in neurons, with p39 showing a slower degradation rate than p35. Degradation of the activators is dependent on their respective N-terminal p10 region, as indicated by experiments in which cognate p10 regions were swapped between p35 and p39. The effect of the p10 region on degradation and cleavage could be assigned to its membrane binding properties, mediated predominantly by myristoylation. Together, these results indicate that both proteasomal degradation and calpain cleavage of p35 and p39 are stimulated by membrane association, which is in turn mediated via myristoylation of their p10 regions. However, p35 and p39 show differences in degradation and cleavage rates, which may in fact underlie the distinct physiological and pathological functions of these two Cdk5 activators.

Overexpression of Dyrk1A contributes to neurofibrillary degeneration in Down syndrome

Adults with Down syndrome (DS) develop Alzheimer neurofibrillary degeneration in the brain, but the underlying molecular mechanism is unknown. Here, we report that the presence of an extra copy of the dual-specificity tyrosine-phosphorylated and regulated kinase 1A (Dyrk1A) gene due to trisomy 21 resulted in overexpression of Dyrk1A and elevated kinase activity in DS brain. Dyrk1A phosphorylated tau at several sites, and these sites were hyperphosphorylated in adult DS brains. Phosphorylation of tau by Dyrk1A primed its further phosphorylation by glycogen synthase kinase-3beta (GSK-3beta). Dyrk1A-induced tau phosphorylation inhibited tau's biological activity and promoted its self-aggregation. In Ts65Dn mouse brain, an extra copy of the Dyrk1A gene caused increased expression and activity of Dyrk1A and resulted in increased tau phosphorylation. These findings strongly suggest a novel mechanism by which the overexpression of Dyrk1A in DS brain causes neurofibrillary degeneration via hyperphosphorylating tau.

Regulation of Kalirin by Cdk5

Kalirin, one of the few Rho guanine nucleotide exchange factors (GEFs) that contains spectrin-like repeats, plays a critical role in axon extension and maintenance of dendritic spines. PC12 cells were used to determine whether Cdk5, a critical participant in both processes, regulates the action of Kalirin. Expression of Kalirin-7 in nondifferentiated PC12 cells caused GEF-activity-dependent extension of broad cytoplasmic protrusions; coexpression of dominant-negative Cdk5 largely eliminated this response. The spectrin-like repeat region of Kalirin plays an essential role in this response, which is not mimicked by the GEF domain alone. Thr1590, which follows the first GEF domain of Kalirin, is the only Cdk5 phosphorylation site in Kalirin-7. Although mutant Kalirin-7 with Ala1590 retains GEF activity, it is unable to cause extension of protrusions. Kalirin-7 with an Asp1590 mutation has slightly increased GEF activity and dominant-negative Cdk5 fails to block its ability to cause extension of protrusions. Phosphorylation of Thr1590 causes a slight increase in GEF activity and Kalirin-7 solubility. Dendritic spines formed by cortical neurons in response to the expression of Kalirin-7 with Ala1590 differ in shape from those formed in response to wild-type Kalirin-7 or Kalirin-7 containing Asp1590. The presence of Thr1590 in each major Kalirin isoform would allow Cdk5 to regulate Kalirin function throughout development.

Regulation of membrane association and kinase activity of Cdk5-p35 by phosphorylation of p35

Although protein kinase Cdk5-p35 is important in many aspects of the development and function of the central nervous system, relatively little is known about its regulation. In the present study, we examined the relationship between the association of this kinase with membranes and its activity in perinatal and adult rat brains. Cdk5-p35 in perinatal brain exhibited higher activity than that found in adult tissue. Gel filtration chromatography revealed that a portion of Cdk5-p35 from fetal brain occurred as a soluble complex, whereas Cdk5-p35 in adult brain occurred predominantly as a membrane-bound complex. Furthermore, soluble Cdk5-p35 in perinatal brain displayed elevated kinase activity, whereas membrane-bound Cdk5-p35 was highly active only in the presence of detergent. This more active soluble form of Cdk5-p35 correlated to a form in which p35 was phosphorylated, whereas the less active membrane-bound form of Cdk5 correlated to the dephosphorylated form of p35, as evidenced by a downward shift in electrophoretic mobility. Cdk5 activity and transition from soluble to membrane-associated compartments could be modulated by conditions that affected the phosphorylation or dephosphorylation of p35. For example, dephosphorylation of p35 in brain extracts was suppressed by selective inhibition of protein phosphatase-1. Together, these results suggest that the kinase activity of Cdk5-p35 is regulated through its association with membranes, which in turn is under the control of Cdk5-dependent phosphorylation and protein phosphatase-1-dependent dephosphorylation of p35.

A cell-based screening method for specifically detecting kinase activity

No universal approach has been reported for specific monitoring of the catalytic activity of a wide range of kinases in cells. The present study describes an original platform for detecting the autonomous activity of serine/threonine kinases in cells through the introduction of expression vectors encoding modified substrate kinase fusion proteins. The surrogate substrate used consists of the p53 tumor suppressor protein fused with individual kinase domains (Chk1, DYRK3, and Cdk5) at its carboxy-terminal through four tandem Gly-Gly-Gly-Gly-Ser repeats. After transfection into cells, phosphorylation of the p53 moiety could be specifically induced by the catalytic activity of kinases contained in the fusion protein. Moreover, p53 phosphorylation was significantly blocked when a kinase-inactive mutant was used as the fusion partner instead of the active kinase. Using this system, the cell-based evaluation of several Cdk5 inhibitors was demonstrated. Thus, this approach provides a novel platform for the specific, cell-based screening of inhibitors of a wide prospective range of protein kinases and is of tremendous potential for drug discovery efforts.

Microtubule association of the neuronal p35 activator of Cdk5

Cdk5 and its neuronal activator p35 play an important role in neuronal migration and proper development of the brain cortex. We show that p35 binds directly to alpha/beta-tubulin and microtubules. Microtubule polymers but not the alpha/beta-tubulin heterodimer block p35 interaction with Cdk5 and therefore inhibit Cdk5-p35 activity. p25, a neurotoxin-induced and truncated form of p35, does not have tubulin and microtubule binding activities, and Cdk5-p25 is inert to the inhibitory effect of microtubules. p35 displays strong activity in promoting microtubule assembly and inducing formation of microtubule bundles. Furthermore, microtubules stabilized by p35 are resistant to cold-induced disassembly. In cultured cortical neurons, a significant proportion of p35 localizes to microtubules. When microtubules were isolated from rat brain extracts, p35 co-assembled with microtubules, including cold-stable microtubules. Together, these findings suggest that p35 is a microtubule-associated protein that modulates microtubule dynamics. Also, microtubules play an important role in the control of Cdk5 activation.

Inhibition of Melatonin Biosynthesis Induces Neurofilament Hyperphosphorylation with Activation of Cyclin-dependent Kinase 5

Decreased level of melatonin and hyperphosphorylation of neurofilament proteins have been reported in Alzheimer's disease (AD). However, the direct evidence linking melatonin and neurofilament phosphorylation is still lacking. Here, we investigated the effect of inhibiting melatonin biosynthesis on phosphorylation of neurofilament proteins and the involvement of cyclin-dependent kinase 5 (cdk-5) in rats. We observed that injection of haloperidol, a specific inhibitor of 5-hydroxyindole-O-methyltransferase, resulted in significantly decreased level of serum melatonin with a concomitantly increased phosphorylation of neurofilament proteins and activation of cdk-5 in rats. Exogenous supplementation of melatonin partially arrested the hyperphosphorylation of neurofilament and the activation of cdk-5. These results suggest that inhibition of melatonin biosynthesis may activate cdk-5 and thus induces Alzheimer-like hyperphosphorylation of neurofilament proteins.

Regulation of s6 kinase 1 activation by phosphorylation at ser-411

Ribosomal S6 kinase 1 (S6K1), as a key regulator of mRNA translation, plays an important role in cell cycle progression through the G(1) phase of proliferating cells and in the synaptic plasticity of terminally differentiated neurons. Activation of S6K1 involves the phosphorylation of its multiple Ser/Thr residues, including the proline-directed sites (Ser-411, Ser-418, Thr-421, and Ser-424) in the autoinhibitory domain near the C terminus. Phosphorylation at Thr-389 is also a crucial event in S6K1 activation. Here, we report that S6K1 phosphorylation at Ser-411 is required for the rapamycin-sensitive phosphorylation of Thr-389 and the subsequent activation of S6K1. Mutation of Ser-411 to Ala ablated insulin-induced Thr-389 phosphorylation and S6K1 activation, whereas mutation mimicking Ser-411 phosphorylation did not show any effect. Furthermore, phosphomimetic mutation of Thr-389 overcame the inhibitory effect of the mutation S411A. Thus, Ser-411 phosphorylation regulates S6K1 activation via the control of Thr-389 phosphorylation. In nervous system neurons, Cdk5-p35 kinase associates with S6K1 via the direct interaction between p35 and S6K1 and catalyzes S6K1 phosphorylation specifically at Ser-411. Inhibition of the Cdk5 activity or suppression of Cdk5 expression blocked S6K1 phosphorylation at Ser-411 and Thr-389, resulting in S6K1 inactivation. Similar results were obtained by treating asynchronous populations of proliferating cells with the CDK inhibitor compound roscovitine. Altogether, our findings suggest a novel mechanism by which the CDK-mediated phosphorylation regulates the activation of S6K1.

Identification of Nuclear Import Mechanisms for the Neuronal Cdk5 Activator

The activation of Cdk5 by p35 plays a pivotal role in a multitude of nervous system activities ranging from neuronal differentiation to degeneration. A fraction of Cdk5 and p35 localizes in the nucleus where Cdk5-p35 exerts its functions via protein phosphorylation, and p35 displays a dynamic localization between the cytoplasm and the nucleus. Here, we examined the nuclear import properties of p35. In nuclear import assays, p35 was actively transported into the nuclei of digitonin-permeabilized HeLa cells and cortical neurons by cytoplasmic carrier-mediated mechanisms. Importin-beta, importin-5, and importin-7 were identified to import p35 into the nuclei via a direct interaction with it. An N-terminal region of p35 was defined to interact with the above importins, serving as a nuclear localization signal. Finally, we show that the nuclear localization of p35 does not require the association of Cdk5. Furthermore, Cdk5 and importin-beta/5/7 are mutually exclusive in binding to p35. These results suggest that p35 employs pathways distinct from that used by Cdk5 for transport to the nucleus.

PKA modulates GSK-3beta- and cdk5-catalyzed phosphorylation of tau in site- and kinase-specific manners

Phosphorylation of tau protein is regulated by several kinases, especially glycogen synthase kinase 3beta (GSK-3beta), cyclin-dependent protein kinase 5 (cdk5) and cAMP-dependent protein kinase (PKA). Phosphorylation of tau by PKA primes it for phosphorylation by GSK-3beta, but the site-specific modulation of GSK-3beta-catalyzed tau phosphorylation by the prephosphorylation has not been well investigated. Here, we found that prephosphorylation by PKA promotes GSK-3beta-catalyzed tau phosphorylation at Thr181, Ser199, Ser202, Thr205, Thr217, Thr231, Ser396 and Ser422, but inhibits its phosphorylation at Thr212 and Ser404. In contrast, the prephosphorylation had no significant effect on its subsequent phosphorylation by cdk5 at Thr181, Ser199, Thr205, Thr231 and Ser422; inhibited it at Ser202, Thr212, Thr217 and Ser404; and slightly promoted it at Ser396. These studies reveal the nature of the inter-regulation of tau phosphorylation by the three major tau kinases.

Regulation of phosphorylation of tau by cyclin-dependent kinase 5 and glycogen synthase kinase-3 at substrate level

Microtubule associated protein tau, which is expressed in six alternatively spliced molecular isoforms in human brain, is abnormally hyperphosphorylated in Alzheimer disease and related tauopathies. Here, we show (i) that GSK-3alpha and neither GSK-3beta nor cdk5 can phosphorylate tau at Ser262 and phosphorylation at Ser235 by cdk5 primes phosphorylation at Thr231 by GSK-3alpha/beta; (ii) that tau isoforms with two N-terminal inserts (tau4L, tau3L) are phosphorylated by cdk5 plus GSK-3 at Thr231 markedly more than isoforms lacking these inserts (tau4, tau3); and (iii) that Thr231 is phosphorylated approximately 50% more in free tau than in microtubule-bound tau, and the phosphorylation at this site results in the dissociation of tau from microtubules. These findings suggest that the phosphorylation of tau at Thr231 and Ser262 by cdk5 plus GSK-3, which inhibits its normal biological activity, is regulated both by its amino terminal inserts and its physical state.

Different Mechanisms of CDK5 and CDK2 Activation as Revealed by CDK5/p25 and CDK2/Cyclin A Dynamics

A detailed analysis is presented of the dynamics of human CDK5 in complexes with the protein activator p25 and the purine-like inhibitor roscovitine. These and other findings related to the activation of CDK5 are critically reviewed from a molecular perspective. In addition, the results obtained on the behavior of CDK5 are compared with data on CDK2 to assess the differences and similarities between the two kinases in terms of (i) roscovitine binding, (ii) regulatory subunit association, (iii) conformational changes in the T-loop following CDK/regulatory subunit complex formation, and (iv) specificity in CDK/regulatory subunit recognition. An energy decomposition analysis, used for these purposes, revealed why the binding of p25 alone is sufficient to stabilize the extended active T-loop conformation of CDK5, whereas the equivalent conformational change in CDK2 requires both the binding of cyclin A and phosphorylation of the Thr(160) residue. The interaction energy of the CDK5 T-loop with p25 is about 26 kcal.mol(-1) greater than that of the CDK2 T-loop with cyclin A. The binding pattern between CDK5 and p25 was compared with that of CDK2/cyclin A to find specific regions involved in CDK/regulatory subunit recognition. The analyses performed revealed that the alphaNT-helix of cyclin A interacts with the alpha6-alpha7 loop and the alpha7 helix of CDK2, but these regions do not interact in the CDK5/p25 complex. Further differences between the CDK5/p25 and CDK2/cyclin A systems studied are discussed with respect to their specific functionality.

Contributions of protein phosphatases PP1, PP2A, PP2B and PP5 to the regulation of tau phosphorylation

Abnormal hyperphosphorylation of tau is believed to lead to neurofibrillary degeneration in Alzheimer's disease (AD) and other tauopathies. Recent studies have shown that protein phosphatases (PPs) PP1, PP2A, PP2B and PP5 dephosphorylate tau in vitro, but the exact role of each of these phosphatases in the regulation of site-specific phosphorylation of tau in the human brain was unknown. Hence, we investigated the contributions of these PPs to the regulation of tau phosphorylation quantitatively. We found that these four phosphatases all dephosphorylated tau at Ser199, Ser202, Thr205, Thr212, Ser214, Ser235, Ser262, Ser396, Ser404 and Ser409, but with different efficiencies toward different sites. The K(m) values of tau dephosphorylation catalysed by PP1, PP2A and PP5 were 8-12 microm, similar to the intraneuronal tau concentration of human brain, whereas the K(m) of PP2B was fivefold higher. PP2A, PP1, PP5 and PP2B accounted for approximately 71%, approximately 11%, approximately 10% and approximately 7%, respectively, of the total tau phosphatase activity of human brain. The total phosphatase activity and the activities of PP2A and PP5 toward tau were significantly decreased, whereas that of PP2B was increased in AD brain. PP2A activity negatively correlated to the level of tau phosphorylation at the most phosphorylation sites in human brains. Our findings indicate that PP2A is the major tau phosphatase that regulates its phosphorylation at multiple sites in human brain. The abnormal hyperphosphorylation of tau is partially due to a downregulation of PP2A activity in AD brain.

Activation of latent cyclin-dependent kinase 5 (Cdk5)-p35 complexes by membrane dissociation

Cyclin-dependent kinase 5 (Cdk5) is a Ser/Thr kinase of increasingly recognized importance in a large number of fields, ranging from neuronal migration to synaptic plasticity and neurodegeneration. However, little is known about its mechanism of activation beyond its requirement for binding to p35 or p39. We have examined membrane interactions as one method of regulating the Cdk5-p35 complex. The kinase activity of Cdk5-p35 is low when it is bound to membranes. The Cdk5-p35 found in rat brain extract associates with membranes in two ways. Approximately 75% of complexes associate with membranes via ionic interactions only, and the remaining 25% associate with membranes via ionic interactions together with lipidic interactions. Solubilization with detergent or high-salt solution activates Cdk5-p35 several fold, and this activation is reversible. Therefore, membrane interactions represent a novel mechanism for the regulation of Cdk5-p35 kinase activity.

Cdk5 and Trio modulate endocrine cell exocytosis

Hormone secretion by pituitary cells is decreased by roscovitine, an inhibitor of cyclin-dependent kinase 5 (Cdk5). Roscovitine treatment reorganizes cortical actin and ultrastructural analysis demonstrates that roscovitine limits the ability of secretory granules to approach the plasma membrane or one another. Trio, a multifunctional RhoGEF expressed in pituitary cells, interacts with peptidylglycine α-amidating monooxygenase, a secretory granule membrane protein known to affect the actin cytoskeleton. Roscovitine inhibits the ability of Trio to activate Rac, and peptides corresponding to the Cdk5 consensus sites in Trio are phosphorylated by Cdk5. Together, these data suggest that control of the cortical actin cytoskeleton, long known to modulate hormone exocytosis and subsequent endocytosis, involves Cdk5-mediated activation of Trio.

Protein kinase CK2 is an inhibitor of the neuronal Cdk5 kinase

The complex of Cdk5 and its neuronal activator p35 is a proline-directed Ser/Thr kinase that plays an important role in various neuronal functions. Deregulation of the Cdk5 enzymatic activity was found to associate with a number of neurodegenerative diseases. To search for regulatory factors of Cdk5-p35 in the brain, we developed biochemical affinity isolation using a recombinant protein comprising the N-terminal 149 amino acids of p35. The catalytic alpha-subunit of protein kinase CK2 (formerly known as casein kinase 2) was identified by mass spectrometry from the isolation. The association of CK2 with p35 and Cdk5 was demonstrated, and the CK2-binding sites were delineated in p35. Furthermore, CK2 displayed strong inhibition toward the Cdk5 activation by p35. The Cdk5 inhibition is dissociated from the kinase function of CK2 because the kinase-dead mutant of CK2 displayed the similar Cdk5 inhibitory activity as the wild-type enzyme. Further characterization showed that CK2 blocks the complex formation of Cdk5 and p35. Together, these findings suggest that CK2 acts as an inhibitor of Cdk5 in the brain.

Alpha-chimaerin exists in a functional complex with the Cdk5 kinase in brain

Cyclin-dependent kinase 5 (Cdk5) in association with its neuronal activators p35 and p39 shows a complex involvement in the control of neurocytoskeletal dynamics. Here we show that alpha-chimaerin, a GTPase-activating protein specific for Rac and Cdc42, is a p35-binding protein. The interaction domains of p35 and alpha-chimaerin were delineated. In transfected HeLa cells, p35 and alpha-chimaerin displayed an overlapping distribution pattern and they could be co-immunoprecipitated from the cell lysate. As alpha-chimaerin has a regulatory function in actin repolymerization, these results suggested that the regulation of neurocytoskeleton dynamics by Cdk5 is mediated at least in part via alpha-chimaerin.

Specific inhibition of cyclin-dependent kinases and cell proliferation by harmine

As key regulators of the cell proliferation cycle, cyclin-dependent kinases (CDKs) are attractive targets for the development of anti-tumor drugs. In the present study, harmine was identified from a collection of herbal compounds to be a specific inhibitor of Cdk1/cyclin B, Cdk2/cyclin A, and Cdk5/p25 with IC50 values at low micromoles. It displayed little effect on other serine/threonine and tyrosine kinases tested. The CDK inhibition by harmine is competitive with ATP-Mg2+, suggesting that it binds to the ATP-Mg2+-binding pocket of CDKs. In cytotoxicity assays, harmine exhibited a strong inhibitory effect on the growth and proliferation of carcinoma cells whereas it had no significant effect on quiescent fibroblasts. Further, harmine was found to block DNA replication in the carcinoma cells. Taken together, harmine is a selective inhibitor of CDKs and cell proliferation.

Apoptosis-associated tyrosine kinase is a Cdk5 activator p35 binding protein

A 3(')-terminal fragment of a splice variant of KIAA0641, a human homologue of apoptosis-associated tyrosine kinase (AATYK), was screened from human brain cDNA libraries by a yeast two-hybrid system using a Cdk5 activator p35 as a bait. The cloned cDNA encoded 477 amino acids, composed of internal 458 amino acids of KIAA0641 and 19 amino acids unique to this variant after splicing, then referred to this clone as hAATYKs-p35BP (human AATYK short isoform-p35 binding polypeptide). Using GST-fusion protein, hAATYKs-p35BP was shown to bind to Cdk5/p35 in a rat brain extract. hAATYKs made by fusing the kinase domain of KIAA0641 to the N-terminus of hAATYKs-p35BP was used for binding to Cdk5/p35 in HEK293 cells. Both hAATYKs and KIAA0641 bound to and were phosphorylated by Cdk5/p35. These results suggest that both isoforms of hAATYK are novel Cdk5/p35-binding and substrate proteins.

Cyclin-dependent kinase 5 (CDK5) and neuronal cell death

Many neurological disorders like Parkinson's and Alzheimer's disease, amyotrophic lateral sclerosis (ALS) or stroke have in common a definite loss of CNS neurons due to apoptotic or necrotic neuronal cell death. Previous studies suggested that proapoptotic stimuli may trigger an abortive and, therefore, eventually fatal cell cycle reentry in postmitotic neurons. Neuroprotective effects of small molecule inhibitors of cyclin-dependent kinases (CDKs), which are key regulators of cell cycle progression, support the cell cycle theory of neuronal apoptosis. However, growing evidence suggests that deregulated CDK5, which is not involved in cell cycle control, rather than cell cycle relevant members of the CDK family, promotes neuronal cell death. Here we summarize the current knowledge about the involvement of CDK5 in neuronal cell death and discuss possible up- or downstream partners of CDK5. Moreover, we discuss potential therapeutic options that might arise from the identification of CDK5 as an important upstream element of neuronal cell death cascades.

Involvement of aberrant glycosylation in phosphorylation of tau by cdk5 and GSK-3beta

Microtubule-associated protein tau is abnormally hyperphosphorylated, glycosylated, and aggregated in affected neurons in the brains of individuals with Alzheimer's disease (AD). We recently found that the glycosylation might precede hyperphosphorylation of tau in AD. In this study, we investigated the effect of glycosylation on phosphorylation of tau catalyzed by cyclin-dependent kinase 5 (cdk5) and glycogen synthase kinase-3beta (GSK-3beta). The phosphorylation of the longest isoform of recombinant human brain tau, tau(441), at various sites was detected by Western blots and by radioimmuno-dot-blot assay with phosphorylation-dependent and site-specific tau antibodies. We found that cdk5 phosphorylated tau(441) at Thr-181, Ser-199, Ser-202, Thr-205, Thr-212, Ser-214, Thr-217, Thr-231, Ser-235, Ser-396, and Ser-404, but not at Ser-262, Ser-400, Thr-403, Ser-409, Ser-413, or Ser-422. GSK-3beta phosphorylated all the cdk5-catalyzed sites above except Ser-235. Deglycosylation by glycosidases depressed the subsequent phosphorylation of AD-tau (i) with cdk5 at Thr-181, Ser-199, Ser-202, Thr-205, and Ser-404, but not at Thr-212; and (ii) with GSK-3beta at Thr-181, Ser-202, Thr-205, Ser-217, and Ser-404, but not at Ser-199, Thr-212, Thr-231, or Ser-396. These data suggest that aberrant glycosylation of tau in AD might be involved in neurofibrillary degeneration by promoting abnormal hyperphosphorylation by cdk5 and GSK-3beta.

The cyclin-dependent kinase 5 activators p35 and p39 interact with the alpha-subunit of Ca2+/calmodulin-dependent protein kinase II and alpha-actinin-1 in a calcium-dependent manner

Cyclin-dependent kinase 5 (Cdk5) is a critical regulator of neuronal migration in the developing CNS, and recent studies have revealed a role for Cdk5 in synaptogenesis and regulation of synaptic transmission. Deregulation of Cdk5 has been linked to the pathology of neurodegenerative diseases such as Alzheimer's disease. Activation of Cdk5 requires its association with a regulatory subunit, and two Cdk5 activators, p35 and p39, have been identified. To gain further insight into the functions of Cdk5, we identified proteins that interact with p39 in a yeast two-hybrid screen. In this study we report that alpha-actinin-1 and the alpha-subunit of Ca2+/calmodulin-dependent protein kinase II (CaMKIIalpha), two proteins localized at the postsynaptic density, interact with Cdk5 via their association with p35 and p39. CaMKIIalpha and alpha-actinin-1 bind to distinct regions of p35 and p39 and also can interact with each other. The association of CaMKIIalpha and alpha-actinin-1 to the Cdk5 activators, as well as to each other, is stimulated by calcium. Further, the activation of glutamate receptors increases the association of p35 and p39 with CaMKIIalpha, and the inhibition of CaMKII activation diminishes this effect. The glutamate-mediated increase in association of p35 and CaMKIIalpha is mediated in large part by NMDA receptors, suggesting that cross talk between the Cdk5 and CaMKII signal transduction pathways may be a component of the complex molecular mechanisms contributing to synaptic plasticity, memory, and learning.

Cell death during development

There are many ways to measure apoptosis and other forms of programmed cell death in development. Once nonmammalian embryos have passed the midblastula transition, or much earlier in mammalian embryos, apoptosis is similar to that seen in adult organisms, and is used to sculpt the animal, fuse bilateral tissues, and establish the structure of the nervous system and the immune system. Embryos present unique problems in that, in naturally occurring cell deaths, few cells are involved and they are frequently in very restricted regions. Thus, identification of apoptotic or other dying cells is more effectively achieved by microscopy-based techniques than by electrophoretic or cell-sorting techniques. Since embryos have many mitotic cells and are frequently more difficult to fix than adult tissues, it is best to confirm interpretations by the use of two or more independent techniques. Although natural embryonic deaths are frequently programmed and require protein synthesis, activation of a cell death pathway is often post-translational and assays for transcriptional or translational changes-as opposed to changes in aggregation of death-related molecules or proteolytic activation of enzymes-is likely to be uninformative. Also, embryos can frequently exploit partially redundant pathways, such that the phenotype of a knockout or upregulated death-related gene is often rather modest, even though the adult may develop response or regulation problems. For these reasons, the study of cell death in embryos is fascinating but researchers should be cautious in their analyses.

Identification of a neuronal Cdk5 activator-binding protein as Cdk5 inhibitor

Neuronal Cdc2-like kinase (Nclk) plays an important role in a variety of cellular processes, including neuronal cell differentiation, apoptosis, neuron migration, and formation of neuromuscular junction. The active kinase consists of a catalytic subunit, Cdk5, and an essential regulatory subunit, neuronal Cdk5 activator (p35(nck5a) or p25(nck5a)), which is expressed primarily in neurons of central nervous tissue. In our previous study using the yeast two-hybrid screening method, three novel p35(nck5a)-associated proteins were isolated. Here we show that one of these proteins, called C42, specifically inhibits the activation of Cdk5 by Nck5a. Co-immunoprecipitation data suggested that C42 and p35(nck5a) could form a complex within cultured mammalian cells. Deletion analysis has mapped the inhibitory domain of C42 to a region of 135 amino acids, which is conserved in Pho81, a yeast protein that inhibits the yeast cyclin-dependent protein kinase Pho85. The Pho85.Pho80 kinase complex has been shown to be the yeast functional homologue of the mammalian Cdk5/p35(nck5a) kinase.

Beta-carbolines as specific inhibitors of cyclin-dependent kinases

Harmine (3), 7-fluoro-1-methyl beta-carboline (35) and 1-(5-methyl-imidazol-4-yl) beta-carboline (41) were potent and specific inhibitors of cyclin-dependent kinases. The degree of aromaticity of the tricyclic ring and the positioning of substituents are important for inhibitory activity. While most beta-carbolines inhibited CDK2 and CDK5 to the same extent, selective inhibition against CDK2 was observed in 1-(2-chlorophenyl)- (12), 1-(2-fluorophenyl)- (15), and 1-(2-chloro-5-nitrophenyl)- (28) beta-carbolines.

The protein SET binds the neuronal Cdk5 activator p35nck5a and modulates Cdk5/p35nck5a activity

The neuronal Cdk5 kinase is composed of the catalytic subunit Cdk5 and the activator protein p35(nck5a) or its isoform, p39(nck5ai). To identify novel p35(nck5a)- and p39(nck5ai)-binding proteins, fragments of p35(nck5a) and p39(nck5ai) were utilized in affinity isolation of binding proteins from rat brain homogenates, and the isolated proteins were identified using mass spectrometry. With this approach, the nuclear protein SET was shown to interact with the N-terminal regions of p35(nck5a) and p39(nck5ai). Our detailed characterization showed that the SET protein formed a complex with Cdk5/p35(nck5a) through its binding to p35(nck5a). The p35(nck5a)-interacting region was mapped to a predicted alpha-helix in SET. When cotransfected into COS-7 cells, SET and p35(nck5a) displayed overlapping intracellular distribution in the nucleus. The nuclear co-localization was corroborated by immunostaining data of endogenous SET and Cdk5/p35(nck5a) from cultured cortical neurons. Finally, we demonstrated that the activity of Cdk5/p35(nck5a), but not that of Cdk5/p25(nck5a), was enhanced upon binding to the SET protein. The tail region of SET, which is rich in acidic residues, is required for the stimulatory effect on Cdk5/p35(nck5a).

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 decade of CDK5

Since it was identified a decade ago, cyclin-dependent kinase 5 (CDK5) has emerged as a crucial regulator of neuronal migration in the developing central nervous system. CDK5 phosphorylates a diverse list of substrates, implicating it in the regulation of a range of cellular processes - from adhesion and motility, to synaptic plasticity and drug addiction. Recent evidence indicates that deregulation of this kinase is involved in the pathology of neurodegenerative diseases.

Structure and regulation of the CDK5-p25(nck5a) complex

CDK5 plays an indispensable role in the central nervous system, and its deregulation is involved in neurodegeneration. We report the crystal structure of a complex between CDK5 and p25, a fragment of the p35 activator. Despite its partial structural similarity with the cyclins, p25 displays an unprecedented mechanism for the regulation of a cyclin-dependent kinase. p25 tethers the unphosphorylated T loop of CDK5 in the active conformation. Residue Ser159, equivalent to Thr160 on CDK2, contributes to the specificity of the CDK5-p35 interaction. Its substitution with threonine prevents p35 binding, while the presence of alanine affects neither binding nor kinase activity. Finally, we provide evidence that the CDK5-p25 complex employs a distinct mechanism from the phospho-CDK2-cyclin A complex to establish substrate specificity.

Structural Insights into Cdk5 activation by a neuronal Cdk5 activator

Although Cdk5 shows high sequence identity to Cdk1 and Cdk2, it can be fully activated by its neuronal activators p35/p25(nck5a) and p39(nck5ai) in a phosphorylation-independent manner. To understand structural basis of the Cdk5/p25(nck5a) activation, the complex is modelled to assume either an obstructed or an opened conformation based on X-ray structures of the unphosphorylated or the phosphorylated Cdk2/cyclin A complex, respectively. Comparison and analysis of the two models, along with mutagenesis studies of p25(nck5a), suggest that the opened form represents more closely the structure of active Cdk5/p25(nck5a). The results provide a rationale basis for understanding the phosphorylation-independent activation of Cdk5/p25(nck5a).

Phosphorylation by cyclin-dependent protein kinase 5 of the regulatory subunit of retinal cGMP phosphodiesterase. II. Its role in the turnoff of phosphodiesterase in vivo

Retinal cGMP phosphodiesterase (PDE) is regulated by Pgamma, the regulatory subunit of PDE, and GTP/Talpha, the GTP-bound alpha subunit of transducin. In the accompanying paper (Matsuura, I., Bondarenko, V. A., Maeda, T., Kachi, S., Yamazaki, M., Usukura, J., Hayashi, F., and Yamazaki, A. (2000) J. Biol. Chem. 275, 32950-32957), we have shown that all known Pgammas contain a specific phosphorylation motif for cyclin-dependent protein kinase 5 (Cdk5) and that the unknown kinase is Cdk5 complexed with its activator. Here, using frog rod photoreceptor outer segments (ROS) isolated by a new method, we show that Cdk5 is involved in light-dependent Pgamma phosphorylation in vivo. Under dark conditions only negligible amounts of Pgamma were phosphorylated. However, under illumination that bleached less than 0.3% of the rhodopsin, approximately 4% of the total Pgamma was phosphorylated in less than 10 s. Pgamma dephosphorylation occurred in less than 1 s after the light was turned off. Analysis of the phosphorylated amino acid, inhibition of Pgamma phosphorylation by Cdk inhibitors in vivo and in vitro, and two-dimensional peptide map analysis of Pgamma phosphorylated in vivo and in vitro indicate that Cdk5 phosphorylates a Pgamma threonine in the same manner in vivo and in vitro. These observations, together with immunological data showing the presence of Cdk5 in ROS, suggest that Cdk5 is involved in light-dependent Pgamma phosphorylation in ROS and that the phosphorylation is significant and reversible. In an homogenate of frog ROS, PDE activated by light/guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) was inhibited by Pgamma alone, but not by Pgamma complexed with GDP/Talpha or GTPgammaS/Talpha. Under these conditions, Pgamma phosphorylated by Cdk5 inhibited the light/GTPgammaS-activated PDE even in the presence of GTPgammaS/Talpha. These observations suggest that phosphorylated Pgamma interacts with and inhibits light/GTPgammaS-activated PDE, but does not interact with GTPgammaS/Talpha in the homogenate. Together, our results strongly suggest that after activation of PDE by light/GTP, Pgamma is phosphorylated by Cdk5 and the phosphorylated Pgamma inhibits GTP/Talpha-activated PDE, even in the presence of GTP/Talpha in ROS.

Phosphorylation by cyclin-dependent protein kinase 5 of the regulatory subunit of retinal cGMP phosphodiesterase. I. Identification of the kinase and its role in the turnoff of phosphodiesterase in vitro

Cyclic GMP phosphodiesterase (PDE) is an essential component in retinal phototransduction. PDE is regulated by Pgamma, the regulatory subunit of PDE, and GTP/Talpha, the GTP-bound alpha subunit of transducin. In previous studies (Tsuboi, S., Matsumoto, H. , Jackson, K. W., Tsujimoto, K., Williamas, T., and Yamazaki, A. (1994) J. Biol. Chem. 269, 15016-15023; Tsuboi, S., Matsumoto, H., and Yamazaki, A. (1994) J. Biol. Chem. 269, 15024-15029), we showed that Pgamma is phosphorylated by a previously unknown kinase (Pgamma kinase) in a GTP-dependent manner in photoreceptor outer segment membranes. We also showed that phosphorylated Pgamma loses its ability to interact with GTP/Talpha, but gains a 10-15 times higher ability to inhibit GTP/Talpha-activated PDE than that of nonphosphorylated Pgamma. Thus, we propose that the Pgamma phosphorylation is probably involved in the recovery phase of phototransduction through shut off of GTP/Talpha-activated PDE. Here we demonstrate that all known Pgammas preserve a consensus motif for cyclin-dependent protein kinase 5 (Cdk5), a protein kinase believed to be involved in neuronal cell development, and that Pgamma kinase is Cdk5 complexed with p35, a neuronal Cdk5 activator. Mutational analysis of Pgamma indicates that all known Pgammas contain a P-X-T-P-R sequence and that this sequence is required for the Pgamma phosphorylation by Pgamma kinase. In three different column chromatographies of a cytosolic fraction of frog photoreceptor outer segments, the Pgamma kinase activity exactly coelutes with Cdk5 and p35. The Pgamma kinase activity ( approximately 85%) is also immunoprecipitated by a Cdk5-specific antibody, and the immunoprecipitate phosphorylates Pgamma. Finally, recombinant Cdk5/p35, which were expressed using clones from a bovine retina cDNA library, phosphorylates Pgamma in frog outer segment membranes in a GTP-dependent manner. These observations suggest that Cdk5 is probably involved in the recovery phase of phototransduction through phosphorylation of Pgamma complexed with GTP/Talpha in mature vertebrate retinal photoreceptors.

Tau phosphorylation at serine 396 and serine 404 by human recombinant tau protein kinase II inhibits tau's ability to promote microtubule assembly

In Alzheimer's disease, hyperphosphorylated tau is an integral part of the neurofibrillary tangles that form within neuronal cell bodies and fails to promote microtubule assembly. Dysregulation of the brain-specific tau protein kinase II is reported to play an important role in the pathogenesis of Alzheimer's disease (Patrick, G. N., Zukerberg, L., Nikolic, M., De La Monte, S., Dikkes, P., and Tsai, L.-H. (1999) Nature 402, 615-622). We report here that in vitro phosphorylation of human tau by human recombinant tau protein kinase II severely inhibits the ability of tau to promote microtubule assembly as monitored by tubulin polymerization. The ultrastructure of tau-mediated polymerized tubulin was visualized by electron microscopy and compared with phosphorylated tau. Consistent with the observed slower kinetics of tubulin polymerization, phosphorylated tau is compromised in its ability to generate microtubules. Moreover, we show that phosphorylation of microtubule-associated tau results in tau's dissociation from the microtubules and tubulin depolymerization. Mutational studies with human tau indicate that phosphorylation by tau protein kinase II at serine 396 and serine 404 is primarily responsible for the functional loss of tau-mediated tubulin polymerization. These in vitro results suggest a possible role for tau protein kinase II-mediated tau phosphorylation in initiating the destabilization of microtubules.

Calpain-dependent proteolytic cleavage of the p35 cyclin-dependent kinase 5 activator to p25

Cyclin-dependent kinase 5 (CDK5) is a unique CDK, the activity of which can be detected in postmitotic neurons. To date, CDK5 purified from mammalian brains has always been associated with a truncated form of the 35-kDa major brain specific activator (p35, also known as nck5a) of CDK5, known as p25. In this study, we report that p35 can be cleaved to p25 both in vitro and in vivo by calpain. In a rat brain extract, p35 was cleaved to p25 by incubation with Ca(2+). This cleavage was inhibited by a calpain inhibitor peptide derived from calpastatin and was ablated by separating the p35.CDK5 from calpain by centrifugation. The p35 recovered in the pellet after centrifugation could then be cleaved to p25 by purified calpain. Cleavage of p35 was also induced in primary cultured neurons by treatment with a Ca(2+) ionophore and Ca(2+) and inhibited by calpain inhibitor I. The cleavage changed the solubility of the CDK5 active complex from the particulate fraction to the soluble fraction but did not affect the histone H1 kinase activity. Increased cleavage was detected in cultured neurons undergoing cell death, suggesting a role of the cleavage in neuronal cell death.

Interaction of neuronal Cdc2-like protein kinase with microtubule-associated protein tau

Neuronal Cdc2-like protein kinase (NCLK), a approximately 58-kDa heterodimer, was isolated from neuronal microtubules (Ishiguro, K., Takamatsu, M., Tomizawa, K., Omori, A., Takahashi, M., Arioka, M., Uchida, T. and Imahori, K. (1992) J. Biol. Chem. 267, 10897-10901). The biochemical nature of NCLK-microtubule association is not known. In this study we found that NCLK is released from microtubules upon microtubule disassembly as a 450-kDa species. The 450-kDa species is an NCLK.tau complex, and NCLK-bound tau is in a nonphosphorylated state. Tau phosphorylation causes NCLK.tau complex dissociation, and phosphorylated tau does not bind to NCLK. In vitro, the Cdk5 subunit of NCLK binds to the microtubule-binding region of tau and NCLK associates with microtubules only in the presence of tau. Our data indicate that in brain extract NCLK is complexed with tau in a tau phosphorylation-dependent manner and that tau anchors NCLK to microtubules. Recently NCLK has been suggested to be aberrantly activated and to hyperphosphorylate tau in Alzheimer's disease brain (Patrick, G. N., Zukerberg, L., Nikolic, M., de la Monte, S., Dikkes, P, and Tsai, L.-H. (1999) Nature 402, 615-622). Our findings may explain why in Alzheimer's disease NCLK specifically hyperphosphorylates tau, although this kinase has a number of protein substrates in the brain.