Conversion of p35 to p25 deregulates Cdk5 activity and promotes neurodegeneration

Nuclear inclusions in oculopharyngeal muscular dystrophy consist of poly(A) binding protein 2 aggregates which sequester poly(A) RNA

Oculopharyngeal muscular dystrophy (OPMD) is an adult-onset disease characterized by progressive eyelid drooping, swallowing difficulties and proximal limb weakness. The autosomal dominant form of the disease is caused by short (GCG)(8-13) expansions in the PABP2 gene. This gene encodes the poly(A) binding protein 2 (PABP2), an abundant nuclear protein that binds with high affinity to nascent poly(A) tails, stimulating their extension and controlling their length. In this work we report that PABP2 is detected in filamentous nuclear inclusions, which are the pathological hallmark of OPMD. Using both immunoelectron microscopy and fluorescence confocal microscopy, the OPMD-specific nuclear inclusions appeared decorated by anti-PABP2 antibodies. In addition, the inclusions were labeled with antibodies directed against ubiquitin and the subunits of the proteasome and contained a form of PABP2 that was more resistant to salt extraction than the protein dispersed in the nucleoplasm. This suggests that the polyalanine expansions in PABP2 induce a misfolding and aggregation of the protein into insoluble inclusions, similarly to events in neurodegenerative diseases caused by CAG/polyglutamine expansions. No significant differences were observed in the steady-state poly(A) tail length in OPMD and normal myoblasts. However, the nuclear inclusions were shown to sequester poly(A) RNA. This raises the possibility that in OPMD the polyalanine expansions in the PABP2 protein may interfere with the cellular traffic of poly(A) RNA.

Neuron-specific phosphorylation of Alzheimer's beta-amyloid precursor protein by cyclin-dependent kinase 5

The mature form of Alzheimer's beta-amyloid precursor protein (APP) is phosphorylated specifically at Thr(668) in neurons. In mature neurons, phosphorylated APP is detected in neurites, with dephosphorylated APP being found mostly in the cell body. In vitro, active cyclin-dependent kinase 5 (Cdk5) phosphorylated the cytoplasmic domain of APP at Thr(668). Treatment of mature neurons with an antisense oligonucleotide to Cdk5 suppressed Cdk5 expression and significantly diminished the level of phosphorylated APP. The expression of APP was unaffected in antisense-treated neurons. These results indicate that in neurons APP is phosphorylated by Cdk5, and that this may play a role in its localization.

Integrin alpha(1) beta(1)-mediated activation of cyclin-dependent kinase 5 activity is involved in neurite outgrowth and human neurofilament protein H Lys-Ser-Pro tail domain phosphorylation

Cellular adhesion to the extracellular matrix is mediated by a diverse class of alpha/beta heterodimeric receptors known as integrins, which transduce signals to activate multiple intracellular signal transduction pathways within the cells. The signaling pathway linking integrins to mediate neuronal process outgrowth is not well understood. Here, we have provided evidence that intracellular signaling by the alpha(1)beta(1) integrin-induced activation of cyclin-dependent kinase 5 (cdk5) is involved in neurite outgrowth and human neurofilament protein H (hNF-H) Lys-Ser-Pro (KSP) tail domain phosphorylation in differentiated human SH-SY5Y cells. The integrin alpha(1) and beta(1) monoclonal antibodies and BL-1, a specific cdk5 inhibitor, inhibited these effects. We also demonstrated that cdk5 activity and hNF-H KSP tail domain phosphorylation were increased in cdk5/p35 and hNF-H tail domain co-transfected HEK293 cells grown on laminin. This increased hNF-H tail domain phosphorylation was triggered by cdk5 activation. Taken together, these results indicated that cdk5 may play an important role in promoting neurite outgrowth and hNF-H tail KSP domain phosphorylation through the integrin alpha(1)beta(1) signaling pathway.

Cyclin-dependent kinases as a therapeutic target for stroke

Cyclin-dependent kinases (CDKs) are commonly known to regulate cell proliferation. However, previous reports suggest that in cultured postmitotic neurons, activation of CDKs is a signal for death rather than cell division. We determined whether CDK activation occurs in mature adult neurons during focal stroke in vivo and whether this signal was required for neuronal death after reperfusion injury. Cdk4/cyclin D1 levels and phosphorylation of its substrate retinoblastoma protein (pRb) increase after stroke. Deregulated levels of E2F1, a transcription factor regulated by pRb, are also observed. Administration of a CDK inhibitor blocks pRb phosphorylation and the increase in E2F1 levels and dramatically reduces neuronal death by 80%. These results indicate that CDKs are an important therapeutic target for the treatment of reperfusion injury after ischemia.

Cross-talk between two cysteine protease families. Activation of caspase-12 by calpain in apoptosis

Calpains and caspases are two cysteine protease families that play important roles in regulating pathological cell death. Here, we report that m-calpain may be responsible for cleaving procaspase-12, a caspase localized in the ER, to generate active caspase-12. In addition, calpain may be responsible for cleaving the loop region in Bcl-xL and, therefore, turning an antiapoptotic molecule into a proapoptotic molecule. We propose that disturbance to intracellular calcium storage as a result of ischemic injury or amyloid beta peptide cytotoxicity may induce apoptosis through calpain- mediated caspase-12 activation and Bcl-xL inactivation. These data suggest a novel apoptotic pathway involving calcium-mediated calpain activation and cross-talks between calpain and caspase families.

14-3-3zeta is an effector of tau protein phosphorylation

Neurofibrillary tangles associated with Alzheimer's disease are composed mainly of paired helical filaments that are formed by the aggregation of abnormally phosphorylated microtubule-associated protein tau. 14-3-3, a highly conserved protein family that exists as seven isoforms and regulates diverse cellular processes is present in neurofibrillary tangles (Layfield, R., Fergusson, J., Aitken, A., Lowe, J., Landon, M., Mayer, R. J. (1996) Neurosci. Lett. 209, 57-60). The role of 14-3-3 in Alzheimer's disease pathogenesis is not known. In this study, we found that the 14-3-3zeta isoform is associated with tau in brain extract and profoundly stimulates cAMP-dependent protein kinase catalyzed in vitro phosphorylation on Ser(262)/Ser(356) located within the microtubule-binding region of tau. 14-3-3zeta binds to both phosphorylated and nonphosphorylated tau, and the binding site is located within the microtubule-binding region of tau. From brain extract, 14-3-3zeta co-purifies with microtubules, and tubulin blocks 14-3-3zeta-tau binding. Among four 14-3-3 isoforms tested, beta and zeta but not gamma and epsilon associate with tau. Our data suggest that 14-3-3zeta is a tau protein effector and may be involved in the abnormal tau phosphorylation occurring during Alzheimer's disease ontogeny.

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.

Processing of cdk5 activator p35 to its truncated form (p25) by calpain in acutely injured neuronal cells

Recently, it was shown that conversion of cdk5 activator protein p35 to a C-terminal fragment p25 promotes a deregulation of cdk5 activity, which may contribute to neurodegeneration in Alzheimer's disease. In this study, we present evidence that calpain is a protease involved in the conversion of p35 to p25. To activate calpain, rat cerebellar granule neurons were treated with maitotoxin (MTX). A C-terminus-directed anti-p35 antibody detected that p35 conversion to p25 paralleled the formation of calpain-generated alpha-spectrin (alpha-fodrin) breakdown products (SBDP's) in a maitotoxin-dose-dependent manner. Two calpain inhibitors (MDl28170 and SJA6017) reduced p35 processing but were unchanged when exposed to the caspase inhibitor carbobenzoxy-Asp-CH(2)OC(=O)-2, 6-dichlorobenzene or the proteasome inhibitors (lactacystin and Z-Ile-Glu(OtBu)Ala-Leu-CHO). p35 protein was also degraded to p25 when rat brain lysate was subjected to in vitro digestion with purified mu- and m-calpains. Additionally, in a rat temporary middle cerebral artery occlusion model, p35 processing to p25 again paralleled SBDP formation in the ischemic core. Lastly, in malonate-injured rat brains, the ipsilateral side showed a striking correlation of SBDP formation with p35 to p25 conversion and tau phosphorylation (at Ser202 and Thr205) increase. These data suggest that calpain is a major neuronal protease capable of converting p35 to p25 and might play a pathological role of activating cdk5 and its phosphorylation of tau in Alzheimer's disease.

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.

Tau aggregation into fibrillar polymers: taupathies

Different neurological disorders, known as taupathies have been recently described. In these disorders it has been suggested that modifications in the microtubule-associated protein tau could cause neural degeneration in specific regions. Although these regions are different in the different taupathies, some common features appear to occur in all of them: abnormal hyperphosphorylation of tau and aberrant tau aggregation. These two features are commented upon in this review.

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.

Neurotoxicity induces cleavage of p35 to p25 by calpain

Cyclin-dependent kinase 5 (cdk5) and its neuron-specific activator p35 are required for neurite outgrowth and cortical lamination. Proteolytic cleavage of p35 produces p25, which accumulates in the brains of patients with Alzheimer's disease. Conversion of p35 to p25 causes prolonged activation and mislocalization of cdk5. Consequently, the p25/cdk5 kinase hyperphosphorylates tau, disrupts the cytoskeleton and promotes the death (apoptosis) of primary neurons. Here we describe the mechanism of conversion of p35 to p25. In cultured primary cortical neurons, excitotoxins, hypoxic stress and calcium influx induce the production of p25. In fresh brain lysates, addition of calcium can stimulate cleavage of p35 to p25. Specific inhibitors of calpain, a calcium-dependent cysteine protease, effectively inhibit the calcium-induced cleavage of p35. In vitro, calpain directly cleaves p35 to release a fragment with relative molecular mass 25,000. The sequence of the calpain cleavage product corresponds precisely to that of p25. Application of the amyloid beta-peptide A beta(1-42) induces the conversion of p35 to p25 in primary cortical neurons. Furthermore, inhibition of cdk5 or calpain activity reduces cell death in A beta-treated cortical neurons. These observations indicate that cleavage of p35 to p25 by calpain may be involved in the pathogenesis of Alzheimer's disease.

The cyclin-dependent kinase Cdk5 controls multiple aspects of axon patterning in vivo

Cyclin-dependent kinase 5 (Cdk5) is one of a subfamily of Cdks involved in the control of cell differentiation and morphology rather than cell division. Specifically, Cdk5 and its activating subunit, p35, have been implicated in growth cone motility during axon extension. Both Cdk5 and p35 are expressed in post-mitotic neurons and are localized to growth cones [1] [2] [3] [4]. The Cdk5-p35 complex interacts with the Rac GTPase, a protein required for growth cone motility [5]. Studies using cultured neurons have suggested that Cdk5 activity controls the efficiency of neurite extension [3] [4]. Mutant mice lacking p35 exhibit subtle axon-guidance defects [6], but these mice have severe defects in neuronal migration [6] [7] [8], making it difficult to define precisely the role of the Cdk5-p35 complex in vivo. Here, we examined Cdk5 function in axon patterning in the Drosophila embryo. Although our data support the idea that Cdk5-p35 is involved in axonogenesis, they do not support the view that Cdk5 simply promotes growth cone motility. Instead, we found that disrupting Cdk5 function caused widespread errors in axon patterning.

Cortical development: Cdk5 gets into sticky situations

Cyclin-dependent kinase 5 (Cdk5) is much more than its name implies; it plays a role in neuronal migration, neurite outgrowth and degeneration. Recent evidence suggests that Cdk5 regulates neuronal adhesion and cytoskeletal dynamics.

Regulation of N-cadherin-mediated adhesion by the p35-Cdk5 kinase

BACKGROUND

The p35-Cdk5 kinase has been implicated in a variety of functions in the central nervous system (CNS), including axon outgrowth, axon guidance, fasciculation, and neuronal migration during cortical development. In p35(-/-) mice, embryonic cortical neurons are unable to migrate past their predecessors, leading to an inversion of cortical layers in the adult cortex.

RESULTS

In order to identify molecules important for p35-Cdk5-dependent function in the cortex, we screened for p35-interacting proteins using the two-hybrid system. In this study, we report the identification of a novel interaction between p35 and the versatile cell adhesion signaling molecule beta-catenin. The p35 and beta-catenin proteins interacted in vitro and colocalized in transfected COS cells. In addition, the p35-Cdk5 kinase was associated with a beta-catenin-N-cadherin complex in the cortex. In N-cadherin-mediated aggregation assays, inhibition of Cdk5 kinase activity using the Cdk5 inhibitor roscovitine led to the formation of larger aggregates of embryonic cortical neurons. This finding was recapitulated in p35(-/-) cortical neurons, which aggregated to a greater degree than wild-type neurons. In addition, introduction of active p35-Cdk5 kinase into COS cells led to a decreased beta-catenin-N-cadherin interaction and loss of cell adhesion.

CONCLUSIONS

The association between p35-Cdk5 and an N-cadherin adhesion complex in cortical neurons and the modulation of N-cadherin-mediated aggregation by p35-Cdk5 suggests that the p35-Cdk5 kinase is involved in the regulation of N-cadherin-mediated adhesion in cortical neurons.

Cyclin-dependent kinases inhibitors as potential anticancer, antineurodegenerative, antiviral and antiparasitic agents

Cyclin-dependent kinases (CDKs) play a key role in the cell division cycle, in neuronal functions, in transcription and in apoptosis. Intensive screening with these kinases as targets has lead to the identification of highly selective and potent small - molecule inhibitors. Co-crystallization with CDK2 shows that these flat heterocyclic hydrophobic compounds bind through two or three hydrogen bonds with the side chains of two amino acids located in the ATP-binding pocket of the kinase. These inhibitors are anti-proliferative; they arrest cells in G1 and in G2/M phase. Furthermore they facilitate or even trigger apoptosis in proliferating cells while they protect neuronal cells and thymocytes from apoptosis. The potential use of these inhibitors is being extensively evaluated for cancer chemotherapy and also in other therapeutic areas: neurology (Alzheimer's disease), cardiovascular (restenosis, angiogenesis), nephrology (glomerulonephritis), parasitology (Plasmodium, Trypanosoma, Toxoplasma, etc.) and virology (cytomegalovirus, HIV, herpes virus). Copyright 2000 Harcourt Publishers Ltd.