The levels of cdk5 and p35 proteins and tau phosphorylation are reduced during neuronal apoptosis

Free d-aspartate triggers NMDA receptor-dependent cell death in primary cortical neurons and perturbs JNK activation, Tau phosphorylation, and protein SUMOylation in the cerebral cortex of mice lacking d-aspartate oxidase activity

In mammals, free d-aspartate (D-Asp) is abundant in the embryonic brain, while levels remain very low during adulthood as a result of the postnatal expression and activity of the catabolizing enzyme d-aspartate oxidase (DDO). Previous studies have shown that long-lasting exposure to nonphysiological, higher D-Asp concentrations in Ddo knockout (Ddo^-/-) mice elicits a precocious decay of synaptic plasticity and cognitive functions, along with a dramatic age-dependent expression of active caspase 3, associated with increased cell death in different brain regions, including hippocampus, prefrontal cortex, and substantia nigra pars compacta. Here, we investigate the yet unclear molecular and cellular events associated with the exposure of abnormally high D-Asp concentrations in cortical primary neurons and in the brain of Ddo^-/- mice. For the first time, our in vitro findings document that D-Asp induces in a time-, dose-, and NMDA receptor-dependent manner alterations in JNK and Tau phosphorylation levels, associated with pronounced cell death in primary cortical neurons. Moreover, observations obtained in Ddo^-/- animals confirmed that high in vivo levels of D-Asp altered cortical JNK signaling, Tau phosphorylation and enhanced protein SUMOylation, indicating a robust indirect role of DDO activity in regulating these biochemical NMDA receptor-related processes. Finally, no gross modifications in D-Asp concentrations and DDO mRNA expression were detected in the cortex of patients with Alzheimer's disease when compared to age-matched healthy controls.

Increased phosphorylation of collapsin response mediator protein-2 at Thr514 correlates with β-amyloid burden and synaptic deficits in Lewy body dementias

Collapsin response mediator protein-2 (CRMP2) regulates axonal growth cone extension, and increased CRMP2 phosphorylation may lead to axonal degeneration. Axonal and synaptic pathology is an important feature of Lewy body dementias (LBD), but the state of CRMP2 phosphorylation (pCRMP2) as well as its correlations with markers of neurodegeneration have not been studied in these dementias. Hence, we measured CRMP2 phosphorylation at Thr509, Thr514 and Ser522, as well as markers of β-amyloid (Aβ), tau-phosphorylation, α-synuclein and synaptic function in the postmortem neocortex of a longitudinally assessed cohort of LBD patients characterized by low (Parkinson’s disease dementia, PDD) and high (dementia with Lewy bodies, DLB) burden of Alzheimer type pathology. We found specific increases of pCRMP2 at Thr514 in DLB, but not PDD. The increased CRMP2 phosphorylation correlated with fibrillogenic Aβ as well as with losses of markers for axon regeneration (β-III-tubulin) and synaptic integrity (synaptophysin) in LBD. In contrast, pCRMP2 alterations did not correlate with tau-phosphorylation or α-synuclein, and also appear unrelated to immunoreactivities of putative upstream kinases glycogen synthase kinase 3β and cyclin-dependent kinase 5, as well as to protein phosphatase 2A. In conclusion, increased pCRMP2 may underlie the axonal pathology of DLB, and may be a novel therapeutic target. However, antecedent signaling events as well as the nature of pCRMP2 association with Aβ and other neuropathologic markers require further study.

Specific inhibition of p25/Cdk5 activity by the Cdk5 inhibitory peptide reduces neurodegeneration in vivo

The aberrant hyperactivation of Cyclin-dependent kinase 5 (Cdk5), by the production of its truncated activator p25, results in the formation of hyperphosphorylated tau, neuroinflammation, amyloid deposition, and neuronal death in vitro and in vivo. Mechanistically, this occurs as a result of a neurotoxic insult that invokes the intracellular elevation of calcium to activate calpain, which cleaves the Cdk5 activator p35 into p25. It has been shown previously that the p25 transgenic mouse as a model to investigate the mechanistic implications of p25 production in the brain, which recapitulates deregulated Cdk5-mediated neuropathological changes, such as hyperphosphorylated tau and neuronal death. To date, strategies to inhibit Cdk5 activity have not been successful in targeting selectively aberrant activity without affecting normal Cdk5 activity. Here we show that the selective inhibition of p25/Cdk5 hyperactivation in vivo, through overexpression of the Cdk5 inhibitory peptide (CIP), rescues against the neurodegenerative pathologies caused by p25/Cdk5 hyperactivation without affecting normal neurodevelopment afforded by normal p35/Cdk5 activity. Tau and amyloid pathologies as well as neuroinflammation are significantly reduced in the CIP-p25 tetra transgenic mice, whereas brain atrophy and subsequent cognitive decline are reversed in these mice. The findings reported here represent an important breakthrough in elucidating approaches to selectively inhibit the p25/Cdk5 hyperactivation as a potential therapeutic target to reduce neurodegeneration.

Cdk5 protein inhibition and Aβ42 increase BACE1 protein level in primary neurons by a post-transcriptional mechanism: implications of CDK5 as a therapeutic target for Alzheimer disease

The β-secretase enzyme BACE1 initiates production of the amyloid-β (Aβ) peptide that comprises plaques in Alzheimer disease (AD) brain. BACE1 levels are increased in AD, potentially accelerating Aβ generation, but the mechanisms of BACE1 elevation are not fully understood. Cdk5/p25 has been implicated in neurodegeneration and BACE1 regulation, suggesting therapeutic Cdk5 inhibition for AD. In addition, caspase 3 has been implicated in BACE1 elevation. Here, we show that the Cdk5 level and p25:p35 ratio were elevated and correlated with BACE1 level in brains of AD patients and 5XFAD transgenic mice. Mouse primary cortical neurons treated with Aβ42 oligomers had increased BACE1 level and p25:p35 ratio. Surprisingly, the Aβ42-induced BACE1 elevation was not blocked by Cdk5 inhibitors CP68130 and roscovitine, and instead the BACE1 level was increased greater than with Aβ42 treatment alone. Moreover, Cdk5 inhibitors alone elevated BACE1 in a time- and dose-dependent manner that coincided with increased caspase 3 cleavage and decreased Cdk5 level. Caspase 3 inhibitor benzyloxycarbonyl-VAD failed to prevent the Aβ42-induced BACE1 increase. Further experiments suggested that the Aβ42-induced BACE1 elevation was the result of a post-transcriptional mechanism. We conclude that Aβ42 may increase the BACE1 level independently of either Cdk5 or caspase 3 and that Cdk5 inhibition for AD may cause BACE1 elevation, a potentially negative therapeutic outcome.

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.

An unusual member of the Cdk family: Cdk5

The proline-directed serine threonine kinase, Cdk5, is an unusual molecule that belongs to the well-known large family of proteins, cyclin-dependent kinases (Cdks). While it has significant homology with the mammalian Cdk2 and yeast cdc2, unlike the other Cdks, it has little role to play in cell cycle regulation and is activated by non-cyclin proteins, p35 and p39. It phosphorylates a spectrum of proteins, most of them associated with cell morphology and motility. A majority of known substrates of Cdk5 are cytoskeletal elements, signalling molecules or regulatory proteins. It also appears to be an important player in cell-cell communication. Highly conserved, Cdk5 is most abundant in the nervous system and is of special interest to neuroscientists as it appears to be indispensable for normal neural development and function. In normal cells, transcription and activity of Cdk5 is tightly regulated. Present essentially in post-mitotic neurons, its normal activity is obligatory for migration and differentiation of neurons in developing brain. Deregulation of Cdk5 has been implicated in Alzheimer's disease, amyotrophic lateral sclerosis, Parkinson's disease, Huntington's disease and acute neuronal injury. Regulators of Cdk5 activity are considered as potential therapeutic molecules for degenerative diseases. This review focuses on the role of Cdk5 in neural cells as regulator of cytoskeletal elements, axonal guidance, membrane transport, synaptogenesis and cell survival in normal and pathological conditions.

Cdk5-p39 is a labile complex with the similar substrate specificity to Cdk5-p35

Cyclin-dependent kinase 5 (Cdk5) is a proline-directed Ser/Thr kinase that plays important roles in various neuronal activities, including neuronal migration, synaptic activity, and neuronal cell death. Cdk5 is activated by association with a neuron-specific activator, p35 or its isoform p39, but little is known about the kinase activity of Cdk5--p39. In fact, kinase-active Cdk5--p39 was not prepared from rat brain extracts nor from HEK293 cells expressing Cdk5 and p39 by immunoprecipitation in the presence of non-ionic detergent, under conditions with which active Cdk5--p35 could be isolated. p39 dissociated from Cdk5 in the presence of detergent, indicating that p39 has a lower binding affinity for Cdk5 than p35. We developed a method for purifying kinase-active Cdk5--p39 from Sf9 cells infected with baculovirus encoding Cdk5 and p39. The purified Cdk5--p39 complex showed similar substrate specificity to that of Cdk5--p35, but with opposite sensitivity to detergent. Cdk5--p39 was inactivated by Triton X-100, whereas Cdk5--p35 was activated. The N-terminal deletion from p35 and p39, the amino acid sequences of which are different, did not change the stability or substrate specificity of either Cdk5 complex. The different stability between Cdk5--p35 and Cdk5--p39 suggests their distinct roles under different regulation mechanisms in neurons.

Suppression of Calpain-dependent Cleavage of the CDK5 Activator p35 to p25 by Site-specific Phosphorylation

Cdk5 is a proline-directed Ser/Thr protein kinase predominantly expressed in postmitotic neurons together with its activator, p35. N-terminal truncation of p35 to p25 by calpain results in deregulation of Cdk5 and contributes to neuronal cell death associated with several neurodegenerative diseases. Previously we reported that p35 occurred as a phosphoprotein, phospho-p35 levels changed with neuronal maturation, and that phosphorylation of p35 affected its vulnerability to calpain cleavage. Here, we identify the p35 residues Ser(8) and Thr(138) as the major sites of phosphorylation by Cdk5. Mutagenesis of these sites to unphosphorylatable Ala increased susceptibility to calpain in cultured cells and neurons while changing them to phosphomimetic glutamate-attenuated cleavage. Furthermore, phosphorylation state-specific antibodies to these sites revealed that Thr(138) was dephosphorylated in adult rat, although both Ser(8) and Thr(138) were phosphorylated in prenatal brains. In cultured neurons, inhibition of protein phosphatases converted phosho-Ser(8) p35 to dual phospho-Ser(8)/Thr(138) p35 and conferred resistance to calpain cleavage. These results suggest phosphorylation of Thr(138) predominantly defines the susceptibility of p35 to calpain-dependent cleavage and that dephosphorylation of this site is a critical determinant of Cdk5-p25-induced cell death associated with neurodegeneration.

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.

Cholesterol, statins and tau

Many of the known risk factors for Alzheimer's disease (AD) are associated with cholesterol metabolism. Interestingly, it seems as if higher doses of statins, i.e. inhibitors of the cholesterol biosynthesis by blocking formation of mevalonate, might lower the progression of AD. The mechanisms, however, by which statins or cholesterol levels exert their influence are unknown. A hereditary cholesterol-storage disorder, Niemann Pick C, shows Alzheimer-like tau-pathology in youth or adolescence but with no amyloid plaques. This gives rise to the possibility that disturbances in cholesterol metabolism induce changes in tau without interposition of Abeta-protein aggregates. Experimental data suggest that manipulation of cholesterol levels may lead to changes in tau phosphorylation. These changes vary depending on how cholesterol metabolism is manipulated. Effects seem to be either mild and transient, or drastic and related to neurodegeneration, or independent of the mevalonate pathway.

Differential roles of nuclear and cytoplasmic cyclin-dependent kinase 5 in apoptotic and excitotoxic neuronal death

Cyclin-dependent kinase 5 (cdk5) is a member of the cyclin-dependent kinase family whose activity is localized mainly to postmitotic neurons attributable to the selective expression of its activating partners p35 and p39. Deregulation of cdk5, as a result of calpain cleavage of p35 to a smaller p25 form, has been suggested to be a central component of neuronal death underlying numerous neurodegenerative diseases. However, the relevance of cdk5 in apoptotic death that relies on the mitochondrial pathway is unknown. Furthermore, evidence that cdk5 can also promote neuronal survival has necessitated a more complex understanding of cdk5 in the control of neuronal fate. Here we explore each of these issues using apoptotic and excitotoxic death models. We find that apoptotic death induced by the DNA-damaging agent camptothecin is associated with early transcription-mediated loss of p35 and with late production of p25 that is dependent on Bax, Apaf1, and caspases. In contrast, during excitotoxic death induced by glutamate, neurons rapidly produce p25 independent of the mitochondrial pathway. Analysis of the localization of p35 and p25 revealed that p35 is mainly cytoplasmic, whereas p25 accumulates selectively in the nucleus. By targeting a dominant-negative cdk5 to either the cytoplasm or nucleus, we show that cdk5 has a death-promoting activity within the nucleus and that this activity is required in excitotoxic death but not apoptotic death. Moreover, we also find that cdk5 contributes to pro-survival signaling selectively within the cytoplasm, and manipulation of this signal can modify death induced by both excitotoxicity and DNA damage.

When good Cdk5 turns bad

The cyclin-dependent kinase-5 (Cdk5) is critical to normal mammalian development and has been implicated in synaptic plasticity, learning, and memory in the adult brain. But Cdk-5 activity has also been linked to neurodegenerative diseases. Could a single protein have opposing effects? A new study shows that production of a neuronal protein capable of regulating Cdk-5 activity can turn Cdk-5 from "good" to "bad." The findings may have implications for the development and treatment of conditions like Alzheimer's disease.

Very low levels of methylmercury induce cell death of cultured rat cerebellar neurons via calpain activation

Methylmercury, an environmental neurotoxicant, induces the apoptotic death of cerebellar granule cells in vitro at a low concentration. To further understand the mechanism of cell death, we used a rat cerebellar granule cell culture system to investigate whether the calpain/cyclin-dependent kinase 5 (cdk5)/p35 cascade, an important cascade for neuronal apoptosis, is involved in the methylmercury-induced death. A noteworthy finding was that the cerebellar granular cell death was increased at a very low concentration of methylmercury, 30 nM, which is lower than that previously reported. The high sensitivity to methylmercury indicates that this culture system is useful for studying methylmercury toxicity at very low concentrations. Using this system, we here found that the methylmercury-induced death was inhibited by the calpain inhibitor II. Furthermore, it was shown that, in methylmercury-exposed cells, alpha-fodrin and tau, calpain substrates, were cleaved to the fragments that disappeared by treatment with the calpain inhibitor II. We next assayed and showed that the intracellular Ca(2+) concentration in cerebellar granule cells increased after methylmercury exposure in a time- and dose-dependent manner, significantly even at 30 nM. These results indicated that a very low concentration of methylmercury causes the intracellular Ca(2+) concentration to increase, activates calpain in the cells, and then induces cell death. We further found that the p35 protein was also processed to p25 that forms the cdk5-p25 complex, a hyperactive kinase for tau. However, an immunoblot using the anti-phosphorylated tau antibody showed that there was no increase of phosphorylated tau in methylmercury-exposed cells. These results suggested that methylmercury-induced cell death via calpain activation should not involve the stimulation of tau phosphorylation activity.

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.

Both N-methyl-d-aspartate (NMDA) and non-NMDA receptors mediate glutamate-induced cleavage of the cyclin-dependent kinase 5 (cdk5) activator p35 in cultured rat hippocampal neurons

Cyclin-dependent kinase 5 (cdk5) regulates crucial neurobiological events, and deregulation of cdk5 has been implicated in several neurodegenerative disorders. The deregulation is suggested to occur due to cleavage of the cdk5 activator protein p35 to a smaller p25 fragment by the calcium-activated protease calpain. Here we have elucidated the role of different calcium-permeable ionotropic glutamate receptors in the induction of p35 cleavage in cultured rat hippocampal neurons. The glutamate receptor agonists glutamic acid, N-methyl-D-aspartate (NMDA), kainic acid, and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) were all able to induce p35 cleavage, in a manner depending on extracellular calcium. The effect of glutamate was mediated by NMDA receptors, as it was prevented by the NMDA antagonist MK-801. Cyclothiazide (CTZ), an inhibitor of AMPA receptor desensitization, enhanced glutamate-induced p35 cleavage. In immature 6-day-old cultures the non-NMDA agonist kainic acid provoked p35 cleavage, whereas glutamate and NMDA were ineffective. The data suggest that both NMDA and non-NMDA receptors are able to induce p35 cleavage. Different factors, such as maturation state of neurons or desensitization properties of non-NMDA receptors, may determine which receptor predominantly mediates the effect of glutamate on p35 cleavage.

Effects of cyclin-dependent kinase-5 activity on apoptosis and tau phosphorylation in immortalized mouse brain cortical cells

Cyclin-dependent kinase-5 (CDK5), a unique CDK family member, is active primarily in the central nervous system (CNS). Previous studies suggest that CDK5 is proapoptotic and contributes to tau hyperphosphorylation and neurodegeneration in Alzheimer's disease. The objective of this study was to examine CDK5 effects on apoptotic progression and tau phosphorylation. Immortalized embryonic mouse brain cortical cells were used to establish a stable cell line that overexpressed wild-type human tau. In these studies, thapsigargin, which induces endoplasmic reticulum stress and can cause accumulation of misfolded proteins, was used to induce apoptosis. Caspase-3 activity and poly-(ADP-ribose)-polymerase (PARP) cleavage, as measures of apoptosis, were significantly increased 24 and 48 hr after thapsigargin treatment, and these events were unaffected by tau expression. Although transient coexpression of CDK5 and its activator, p25, increased CDK5 activity greater than tenfold, increases in caspase-3 activity in response to thapsigargin treatment were unaffected by the presence of CDK5/p25. Tau phosphorylation at the PHF-1 epitope, but not the Tau-1 epitope, was increased significantly in CDK5/p25-transfected cells compared to cells transfected with dominant negative CDK5 (DNCDK5). The PHF-1 epitope remained phosphorylated until 48 hr after thapsigargin treatment in the CDK5/p25-transfected cells. Over the course of apoptosis in this model, phosphorylation of the Tau-1 epitope was unaffected in cells transfected with DNCDK5, vector, or CDK5/p25. In summary, these results demonstrate that CDK5 does not have a significant impact on tau phosphorylation and thapsigargin-induced apoptosis in this neuronal cell model.

Cdk5: mediator of neuronal death and survival

Cdk5 (cyclin-dependent kinase 5) is a serine/threonine kinase implicated to play pivotal roles in neuronal development. Recently, its potential involvement as a regulator of neuronal death and survival has attracted considerable interests. Importantly, increasing evidence has linked Cdk5 to the etiopathology of neurodegenerative diseases such as Alzheimer's disease and amyotrophic lateral sclerosis. Here we summarize the recent findings on Cdk5 not only as an important participant in neuronal death, but also a key player in neuronal survival. Elucidating the mechanisms of regulation of Cdk5 and its downstream signaling might prove to be crucial in the therapeutic treatment of neurodegenerative diseases.

In vitro cultured neurons for molecular studies correlating apoptosis with events related to Alzheimer disease

This short review analyses the possible molecular events linking a general program of death such as apoptosis to highly specific intracellular pathways involving the function and degradation of two proteins--tau and amyloid precursor protein--which in their aggregated state constitute the hallmark of Alzheimer disease. By surveying the recent studies carried out in 'in vitro' neuronal cultures--with special emphasis to cerebellar granule neurons--the apparent correlation between onset of apoptosis, tau cleavage with formation of potential toxic fragments, and activation of an amyloidogenic route are discussed. Within this framework, proteasomes seem to play a crucial role upstream of the proteolytic cascade involving calpain(s) and caspase(s) by contributing to tau and amyloid precursor protein-altered breakdown and consequent tendency to aggregation of their degradation fragments. Thus, apoptotic death due to altered supply of anti apoptotic agents, neurotrophic factors, deafferentiation or other causes, may constitute a major trigger of the onset of Alzheimer disease.

Cyclin-dependent kinase-5 in neurodegeneration

Cyclin-dependent kinase-5 (CDK5) is predominantly active in the nervous system and it is well established that CDK5 is essential in neuronal development. In addition to its recognized role in development, there is increasing evidence that CDK5 may be involved in the pathogenesis of several neurodegenerative disorders. Although studies have shown that CDK5 can modulate cell death and survival, controversy still exists as to the exact role CDK5 may play in neurodegenerative processes. This review will highlight recent data on the possible roles of CDK5 in neurodegeneration.

Cyclosporin A enhances colchicine-induced apoptosis in rat cerebellar granule neurons

1. Cyclosporin A (CsA, 1-50 microM), an immunosuppressive drug with known neurotoxic effects, did not decrease the viability of primary cultures of rat cerebellar granule neurons (CGN) or induce apoptotic features. However, CsA specifically enhanced the cytotoxicity and apoptosis induced by colchicine (1 microM). 2. Flavopiridol, an inhibitor of cyclin-dependent kinases (CDKs), prevented the neurotoxic effects of colchicine plus CsA. At 0.1-5 microM, it also showed antiapoptotic effects, as revealed by propidium iodide staining, flow cytometry and counting of cell nuclei. 3. Roscovitine (25-50 microM), a selective cdk1, 2 and 5 inhibitor, showed an antiapoptotic effect against colchicine- and colchicine plus CsA-induced apoptosis. 4. CsA increased the expression of cdk5 and cdk5/p25 mediated by colchicine, a CDK involved in neuronal apoptosis. After treatment of CGN with colchicine plus CsA, the changes in the p25/p35 ratio pointed to cdk5 activation. 5. Immunohistochemical results showed a nuclear localization of cdk5 after neurotoxic treatment, which was prevented by cdk inhibitors. Thus, we propose a new mechanism of modulation of CsA neurotoxicity mediated by cdk5.

Cleavage of the cyclin-dependent kinase 5 activator p35 to p25 does not induce tau hyperphosphorylation

Hyperphosphorylated tau protein is the primary component of neurofibrillary tangles observed in several neurodegenerative disorders. It has been hypothesized that in certain pathological conditions, the calcium activated protease, calpain, would cleave the cyclin-dependent kinase 5 (cdk5) activator p35 to a p25 fragment, which would lead to augmented cdk5 activity, and cdk5-mediated tau hyperphosphorylation. To test this hypothesis, we induced calpain-mediated p35 cleavage in rat hippocampal neuronal cultures and studied the relationship between p25 production, cdk5 activity, and tau phosphorylation. In glutamate-treated cells p35 was cleaved to p25 and this was associated with elevated cdk5 activity. However, tau phosphorylation was concomitantly decreased at multiple sites. The calpain inhibitor MDL28170 prevented the cleavage of p35 but had no effect on tau phosphorylation, suggesting that calpain-mediated processes, i.e., the cleavage of p35 to p25 and cdk5 activation, do not contribute to tau phosphorylation in these conditions. Treatment of the neuronal cultures with N-methyl-D-aspartic acid or with calcium ionophores resulted in an outcome highly similar to that of glutamate. We conclude that, in neuronal cells, the cleavage of p35 to p25 is associated with increased activity of cdk5 but not with tau hyperphosphorylation.

Influence of phosphorylation of p35, an activator of cyclin-dependent kinase 5 (cdk5), on the proteolysis of p35

Cyclin-dependent kinase 5 (cdk5) is involved in the development of the nervous system and neuronal process outgrowth, and it regulates several intracellular processes including cytoskeletal dynamics. Dysregulation of cdk5 has been implicated in many disorders of the nervous system. The activity of the kinase is regulated by binding of cdk5 activators (p35, p39, p67). We examined the phosphorylation of p35, and the role of phosphorylation in regulating the proteolysis of the p35 protein. By detecting changes in electrophoretic mobility, we observed that a significant proportion of p35 is phosphorylated in rat brain tissue. In cultured neurons, the phosphorylation was prevented by roscovitine, an inhibitor of cdk5 and some other cdks. The phosphatase inhibitor okadaic acid induced p35 degradation in neuronal cultures which was sensitive to the proteasome inhibitor lactacystin. These latter results agree with some previous studies showing that phosphorylation regulates proteasomal degradation of p35. Treatment of brain homogenate with okadaic acid in the presence of ATP led to accumulation of p35 phosphorylated also by a kinase that was not inhibited by roscovitine. This implies that the effect of okadaic acid on p35 degradation could also be contributed by a non-cdk kinase. The calpain protease has been shown to cleave p35. Our results suggest that this process may also be modulated by p35 phosphorylation under some conditions. We conclude that p35 phosphorylation influences the proteasome-mediated degradation of p35 and calpain-mediated cleavage of p35 to p25.

Calpain activates caspase-3 during UV-induced neuronal death but only calpain is necessary for death

While caspases have been strongly implicated in delayed neuronal death in a variety of experimental paradigms, other proteases such as calpain can also contribute to neuronal death. To evaluate the relative roles of caspase and calpain, we used a model system wherein UV treatment induced moderate or severe delayed cortical neuronal death, as quantified by propidium iodide and calcein AM. UV treatment led to increases in both caspase and calpain activation. Calpain inhibitor III (MDL-28170) reduced caspase activation, suggesting that caspase activation was mediated by calpain. Calpain contributed to neuronal death, as indicated by strong neuroprotection provided by calpain inhibitor III, calpeptin, or Ca2+-free medium. In contrast, caspase inhibitors were not neuroprotective. These results suggest that UV neurotoxicity is mediated by a loss of Ca2+ homeostasis which leads to a calpain-dependent, caspase-independent cell death. That calpain, but not caspase, may mediate death in instances involving the activation of both proteases may have relevance to other neuronal death models.