Neuroprotective effect of TNFalpha against the beta-amyloid neurotoxicity mediated by CDK5 kinase

The tumor necrosis factor alpha (TNFalpha) plays a dual role in producing either neurodegeneration or neuroprotection in the central nervous system. Despite that TNFalpha was initially described as a cell death inductor, neuroprotective effects against cell death induced by several neurotoxic insults have been reported. Tau hyperphosphorylation and neuronal death found in Alzheimer disease is mediated by deregulation of the cdk5/p35 complex induced by Abeta treatments. Since TNFalpha affects cdk5 activity, we investigated its possible protective role against the Abeta-induced neurodegeneration, as mediated by cdk5. TNFalpha pretreatments significantly reduced the hippocampal neuronal cell death induced by the effects of Abeta(42) peptide. In addition, this pretreatment reduced the increase in the activity of cdk5 induced by Abeta(42) in primary neurons. Next, we investigated the Alzheimer type phosphorylation of tau protein induced by Abeta(42). We observed that the pretreatment of neurons with TNFalpha reduces tau hyperphosphorylation. Taken together, these results define a novel neuroprotective effect of TNFalpha in preventing neuronal cell death and cdk5-dependent tau hyperphosphorylation. This phenomenon, taken together with other previous findings, suggests that the inflammatory response due to Abeta peptide plays a key role in the development of Alzheimer etiopathogenesis.     

Signaling events in amyloid beta-peptide-induced neuronal death and insulin-like growth factor I protection

Amyloid beta-peptide (Abeta) is implicated as the toxic agent in Alzheimer's disease and is the major component of brain amyloid plaques. In vitro, Abeta causes cell death, but the molecular mechanisms are unclear. We analyzed the early signaling mechanisms involved in Abeta toxicity using the SH-SY5Y neuroblastoma cell line. Abeta caused cell death and induced a 2- to 3-fold activation of JNK. JNK activation and cell death were inhibited by overexpression of a dominant-negative SEK1 (SEK1-AL) construct. Butyrolactone I, a cdk5 inhibitor, had an additional protective effect against Abeta toxicity in these SEK1-AL-expressing cells suggesting that cdk5 and JNK activation independently contributed to this toxicity. Abeta also weakly activated ERK and Akt but had no effect on p38 kinase. Inhibitors of ERK and phosphoinositide 3-kinase (PI3K) pathways did not affect Abeta-induced cell death, suggesting that these pathways were not important in Abeta toxicity. Insulin-like growth factor I protected against Abeta toxicity by strongly activating ERK and Akt and blocking JNK activation in a PI3K-dependent manner. Pertussis toxin also blocked Abeta-induced cell death and JNK activation suggesting that G(i/o) proteins were upstream activators of JNK. The results suggest that activation of the JNK pathway and cdk5 may be initial signaling cascades in Abeta-induced cell death.     

Species-specific and conserved epitopes on mouse and human E-selectin important for leukocyte adhesion

The cdk5 and its activator p35 constitute one of the main tau-phosphorylating systems in neuronal cells. Under normal conditions for neurons, its activity is required for modulating tau involvement in neuronal polarity and in development of the mammalian central nervous system. Recently, we reported that the treatment of rat hippocampal cells in culture with fibrillary beta-amyloid (Abeta) results in deregulation of the protein kinase cdk5. The neurotoxic effects of Abeta fibrils were prevented by inhibition of cdk5 activity by butyrolactone I or by using antisense oligonucleotides that control the expression of this kinase. Here, we show that the Abeta-promoted increase of cdk5 activity is associated with changes in tau phosphorylation patterns and in the intraneuronal distribution of tau. In addition to hippocampal cells, deregulation of cdk5 was observed in other cell types. However, butyrolactone I prevented Abeta-induced cell death only in neuronal cells in which cdk5 activation was sensitive to Abeta fibrils. This lost of cdk5 regulation in hippocampal cells exposed to Abeta fibrils appears to be associated with an increase in the cdk5-p35 complex stability. Complex stabilization was sensitive to phosphorylation of cdk5. However, no changes in cdk5 and p35 mRNAs were observed, suggesting that the main effects on cdk5 occur at the posttranslational level. These studies indicate that cdk5 phosphorylation and the formation of an abnormally active cdk5-p35 complex are directly involved in the molecular paths leading to the neurodegenerative process of rat hippocampal neurons triggered by Abeta fibrils.     

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.     

Small molecule, non-peptide p75 ligands inhibit Abeta-induced neurodegeneration and synaptic impairment

The p75 neurotrophin receptor (p75(NTR)) is expressed by neurons particularly vulnerable in Alzheimer's disease (AD). We tested the hypothesis that non-peptide, small molecule p75(NTR) ligands found to promote survival signaling might prevent Abeta-induced degeneration and synaptic dysfunction. These ligands inhibited Abeta-induced neuritic dystrophy, death of cultured neurons and Abeta-induced death of pyramidal neurons in hippocampal slice cultures. Moreover, ligands inhibited Abeta-induced activation of molecules involved in AD pathology including calpain/cdk5, GSK3beta and c-Jun, and tau phosphorylation, and prevented Abeta-induced inactivation of AKT and CREB. Finally, a p75(NTR) ligand blocked Abeta-induced hippocampal LTP impairment. These studies support an extensive intersection between p75(NTR) signaling and Abeta pathogenic mechanisms, and introduce a class of specific small molecule ligands with the unique ability to block multiple fundamental AD-related signaling pathways, reverse synaptic impairment and inhibit Abeta-induced neuronal dystrophy and death.     

Role of cdk5 in the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is characterized by two pathological hallmarks, namely, senile plaques and neurofibrillary tangles (NFTs). The former are mainly composed of amyloid-beta peptides (Abeta) while the latter consists mainly of filaments of hyperphosphorylated tau. Cyclin-dependent kinase 5 (cdk5) has been implicated not only in the tangle pathology, but recent data also implicate cdk5 in the generation of Abeta peptides. Since both Abeta peptides and NFTs are believed to play a role in neurodegeneration in AD, this proline-directed serine/threonine protein kinase is likely to contribute to the pathogenesis of AD. In vitro and in vivo animal data demonstrate the ability of cdk5 to induce phosphorylation and aggregation of tau, and NFT deposition and neurodegeneration. Findings from AD brain samples also show an elevated cdk5 activity and conditions that support the activation of cdk5. Evidence for the role of cdk5 in regulating Abeta production is just emerging. The mechanisms for this potentially damaging activity of cdk5 are largely unknown although amyloid precursor protein and presenilin-1 are both cdk5 substrates.     

Inhibition of tau phosphorylating protein kinase cdk5 prevents beta-amyloid-induced neuronal death.

The key target of this study was the tau protein kinase II system (TPK II) involving the catalytic subunit cdk5 and the regulatory component p35. TPK II is one of the tau phosphorylating systems in neuronal cells, thus regulating its functions in the cytoskeletal dynamics and the extension of neuronal processes. This research led to demonstration that the treatment of rat hippocampal cells in culture with fibrillary beta-amyloid (Abeta) results in a significant increase of the cdk5 enzymatic activity. Interestingly, the data also showed that the neurotoxic effect of 1-20 microM Abeta on primary cultures markedly diminished with co-incubation of hippocampal cells with the amyloid fibers plus the cdk5 inhibitor butyrolactone I. This inhibitor protected brain cells against Abeta-induced cell death in a concentration dependent fashion. Moreover, death was also prevented by a cdk5 antisense probe, but not by an oligonucleotide with a random sequence. The cdk5 antisense also reduced neuronal expression of cdk5 compared with the random oligonucleotide. The studies indicate that cdk5 plays a major role in the molecular path leading to the neurodegenerative process triggered by the amyloid fibers in primary cultures of rat hippocampal neurons. These findings are of interest in the context of the pathogenesis of Alzheimer's disease.     

Inhibition of protein phosphatase 2A overrides tau protein kinase I/glycogen synthase kinase 3 beta and cyclin-dependent kinase 5 inhibition and results in tau hyperphosphorylation in the hippocampus of starved mouse.

Hyperphosphorylated tau is the major component of paired helical filaments in neurofibrillary tangles found in Alzheimer's disease (AD) brain. Starvation of adult mice induces tau hyperphosphorylation at many paired helical filaments sites and with a similar regional selectivity as those in AD, suggesting that a common mechanism may be mobilized. Here we investigated the mechanism of starvation-induced tau hyperphosphorylation in terms of tau kinases and Ser/Thr protein phosphatases (PP), and the results were compared with those reported in AD brain. During starvation, tau hyperphosphorylation at specific epitopes was accompanied by decreases in tau protein kinase I/glycogen synthase kinase 3 beta (TPKI/GSK3 beta), cyclin-dependent kinase 5 (cdk5), and PP2A activities toward tau. These results demonstrate that the activation of TPKI/GSK3 beta and cdk5 is not necessary to obtain hyperphosphorylated tau in vivo, and indicate that inhibition of PP2A is likely the dominant factor in inducing tau hyperphosphorylation in the starved mouse, overriding the inhibition of key tau kinases such as TPKI/GSK3 beta and cdk5. Furthermore, these data give strong support to the hypothesis that PP2A is important for the regulation of tau phosphorylation in the adult brain, and provide in vivo evidence in support of a central role of PP2A in tau hyperphosphorylation in AD.     

Kainate induces AKT, ERK and cdk5/GSK3beta pathway deregulation, phosphorylates tau protein in mouse hippocampus

Acute treatment with kainate 30 mg/kg (KA) produced behavioral alterations and reactive gliosis. However, it did not produce major death of mouse hippocampal neurons, indicating that concentrations were not cytotoxic. KA caused rapid and temporal Erk phosphorylation (at 6h) and Akt dephosphorylation (1-3 days). Concomitantly, the activation of GSK3beta was increased 1-3 days after KA. After 7 days, a reduction in GSK3beta activation was observed. Caspase-3 activity increased, but to a lesser extent than calpain activation (measured by fluorimetry and calpain-cleaved alpha-spectrin). As calpain is involved in cdk5 activation, and cdk5 is related to GSK3beta, the cdk5/p25 pathway was examined. Results showed that the p25/p35 ratio in KA-injected mice for 3 days was 73.6% higher than control levels. However, no changes in cdk5 expression were detected. Both Western blot and immunohistochemistry against p-Tau(Thr(231)) indicated an increase at this phosphorylated site of tau protein. Indeed an increase in p-Tau(Ser(199)) and p-Tau(Ser(396)) was observed by Western blot. Our results demonstrate that tau hyperphosphorylation, induced by KA, is due to an increase in GSK3beta/cdk5 activity in combination with an inactivation of Akt. This indicates that the calpain/cdk5 pathway for tau phosphorylation has a potential role in delayed apoptotic death evoked by excitotoxicity. Moreover, the subsequent activation of caspase and calpain proteases leads to dephosphorylation of tau, thus increasing microtubular destructuration. Taken together, our results provide new insights in the activation of several kinase-pathways implicated in cytoskeletal alterations that are a common feature of neurodegenerative diseases.     

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

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

Cdk5 is a key factor in tau aggregation and tangle formation in vivo

Tau aggregation is a common feature of neurodegenerative diseases such as Alzheimer's disease, and hyperphosphorylation of tau has been implicated as a fundamental pathogenic mechanism in this process. To examine the impact of cdk5 in tau aggregation and tangle formation, we crossed transgenic mice overexpressing the cdk5 activator p25, with transgenic mice overexpressing mutant (P301L) human tau. Tau was hyperphosphorylated at several sites in the double transgenics, and there was a highly significant accumulation of aggregated tau in brainstem and cortex. This was accompanied by increased numbers of silver-stained neurofibrillary tangles (NFTs). Insoluble tau was also associated with active GSK. Thus, cdk5 can initiate a major impact on tau pathology progression that probably involves several kinases. Kinase inhibitors may thus be beneficial therapeutically.     

The neuroprotective effects of phytoestrogens on amyloid beta protein-induced toxicity are mediated by abrogating the activation of caspase cascade in rat cortical neurons

Amyloid beta protein (Abeta) elicits a toxic effect on neurons in vitro and in vivo. In present study we attempt to elucidate the mechanism by which Abeta confers its neurotoxicity. The neuroprotective effects of phytoestrogens on Abeta-mediated toxicity were also investigated. Cortical neurons treated with 5 microm Abeta-(25-35) for 40 h decreased the cell viability by 45.5 +/- 4.6% concomitant with the appearance of apoptotic morphology. 50 microm kaempferol and apigenin decreased the Abeta-induced cell death by 81.5 +/- 9.4% and 49.2 +/- 9.9%, respectively. Abeta increased the activity of caspase 3 by 10.6-fold and to a lesser extent for caspase 2, 8, and 9. The Abeta-induced activation of caspase 3 and release of cytochrome c showed a biphasic pattern. Apigenin abrogated Abeta-induced cytochrome c release, and the activation of caspase cascade. Kaempferol showed a similar effect but to a less extent. Kaempferol was also capable of eliminating Abeta-induced accumulation of reactive oxygen species. These two events accounted for the remarkable effect of kaempferol on neuroprotection. Quercetin and probucol did not affect the Abeta-mediated neurotoxicity. However, they potentiated the protective effect of apigenin. Therefore, these results demonstrate that Abeta elicited activation of caspase cascades and reactive oxygen species accumulation, thereby causing neuronal death. The blockade of caspase activation conferred the major neuroprotective effect of phytoestrogens. The antioxidative activity of phytoestrogens also modulated their neuroprotective effects on Abeta-mediated toxicity.     

Alzheimer neurofibrillary degeneration: significance, etiopathogenesis, therapeutics and prevention

Alzheimer disease (AD) is multi-factorial and heterogeneous. Independent of the aetiology, this disease is characterized clinically by chronic and progressive dementia and histopathologically by neurofibrillary degeneration of abnormally hyperphosphorylated tau seen as intraneuronal neurofibrillary tangles, neuropil threads and dystrophic neurites, and by neuritic (senile) plaques of beta-amyloid. The neurofibrillary degeneration is apparently required for the clinical expression of AD, and in related tauopathies it leads to dementia in the absence of amyloid plaques. While normal tau promotes assembly and stabilizes microtubules, the abnormally hyperphosphorylated tau sequesters normal tau, MAP1 and MAP2, and disrupts microtubules. The abnormal hyperphosphorylation of tau also promotes its self-assembly into tangles of paired helical and or straight filaments. Tau is phosphorylated by a number of protein kinases. Glycogen synthase kinase-3 (GSK-3) and cyclin dependent protein kinase 5 (cdk5) are among the kinases most implicated in the abnormal hyperphosphorylation of tau. Among the phosphatases which regulate the phosphorylation of tau, protein phosphatase-2A (PP-2A), the activity of which is down-regulated in AD brain, is by far the major enzyme. The inhibition of abnormal hyperphosphorylation of tau is one of the most promising therapeutic targets for the development of disease modifying drugs. A great advantage of inhibiting neurofibrillary degeneration is that it can be monitored by evaluating the levels of total tau and tau phosphorylated at various known abnormally hyperphosphorylated sites in the cerebrospinal fluid of patients, obtained by lumbar puncture. There are at least five subgroups of AD, each is probably caused by a different etiopathogenic mechanism. The AD subgroup identification of patients can help increase the success of clinical trials and the development of specific and potent disease modifying drugs.     

Brain levels of CDK5 activator p25 are not increased in Alzheimer's or other neurodegenerative diseases with neurofibrillary tangles

Elevated levels of p25 and constitutive activation of CDK5 have been observed in AD brains. This has led to the hypothesis that increased p25 levels could promote neurofibrillary tangles (NFT) through CDK5-mediated hyperphosphorylation of tau, the principal component of NFTs. We examined p25 immunoreactivity in brains from sporadic and familial AD cases, as well as other neurologic diseases that exhibit NFT, such as Down's syndrome (DS), Pick's disease (Pick), corticobasal degeneration (CBD), progressive supranuclear palsy (PSP), frontotemporal dementia (FTD). Neither the p25 immunoreactivity nor the p25/p35 ratio was elevated in the AD brains or in the other tauopathies (n = 34) compared with controls (n = 11). Although Abeta peptides have been suggested to activate calpain-mediated cleavage of p35 to p25 in cultured neurons, p25 levels in brains of TgCRND8 mice, which express high levels of brain Abeta peptides, were similar to those of non-Tg littermates. Our data suggest that high Abeta levels in brain do not activate p35 proteolysis, and p25 is unlikely to be a causative agent for NFT formation in AD or other tauopathies.     

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-3β (GSK-3β). The phosphorylation of the longest isoform of recombinant human brain tau, tau₄₄₁, 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 tau₄₄₁ 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-3β 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-3β 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-3β.     

22R-Hydroxycholesterol protects neuronal cells from beta-amyloid-induced cytotoxicity by binding to beta-amyloid peptide

22R-hydroxycholesterol, a steroid intermediate in the pathway of pregnenolone formation from cholesterol, was found at lower levels in Alzheimer's disease (AD) hippocampus and frontal cortex tissue specimens compared to age-matched controls. beta-Amyloid (Abeta) peptide has been shown to be neurotoxic and its presence in brain has been linked to AD pathology. 22R-hydroxycholesterol was found to protect, in a dose-dependent manner, against Abeta-induced rat sympathetic nerve pheochromocytoma (PC12) and differentiated human Ntera2/D1 teratocarcinoma (NT2N) neuron cell death. Other steroids tested were either inactive or acted on rodent neurons only. The effect of 22R-hydroxycholesterol was found to be stereospecific because its enantiomer 22S-hydroxycholesterol failed to protect the neurons from Abeta-induced cell death. Moreover, the effect of 22R-hydroxycholesterol was specific for Abeta-induced cell death because it did not protect against glutamate-induced neurotoxicity. The neuroprotective effect of 22R-hydroxycholesterol was seen when using Abeta1-42 but not the Abeta25-35 peptide. To investigate the mechanism of action of 22R-hydroxycholesterol we examined the direct binding of this steroid to Abeta using a novel cholesterol-protein binding blot assay. Using this method the direct specific binding, under native conditions, of 22R-hydroxycholesterol to Abeta1-42 and Abeta17-40, but not Abeta25-35, was observed. These data suggest that 22R-hydroxycholesterol binds to Abeta and the formed 22R-hydroxycholesterol/Abeta complex is not toxic to rodent and human neurons. We propose that 22R-hydroxycholesterol offers a new means of neuroprotection against Abeta toxicity by inactivating the peptide.     

c-Jun contributes to amyloid beta-induced neuronal apoptosis but is not necessary for amyloid beta-induced c-jun induction

The role of gene expression in neuronal apoptosis may be cell- and apoptotic stimulus-specific. Previously, we and others showed that amyloid beta (Abeta)-induced neuronal apoptosis is accompanied by c-jun induction. Moreover, c-Jun contributes to neuronal death in several apoptosis paradigms involving survival factor withdrawal. To evaluate the role of c-Jun in Abeta toxicity, we compared Abeta-induced apoptosis in neurons from murine fetal littermates that were deficient or wild-type with respect to c-Jun. We report that neurons deficient for c-jun are relatively resistant to Abeta toxicity, suggesting that c-Jun contributes to apoptosis in this model. When changes in gene expression were quantified in neurons treated in parallel, we found that Abeta treatment surprisingly led to an apparent activation of the c-jun promoter in both the c-jun-deficient and wild-type neurons, suggesting that c-Jun is not necessary for activation of the c-jun promoter. Indeed, several genes induced by Abeta in wild-type neurons were also induced in c-jun-deficient neurons, including c-fos, fosB, ngfi-B, and ikappaB. In summary, these results indicate that c-Jun contributes to Abeta-induced neuronal death but that c-Jun is not necessary for c-jun induction.     

Genetically augmenting tau levels does not modulate the onset or progression of Abeta pathology in transgenic mice

The two hallmark pathologies of Alzheimer's disease (AD) are amyloid plaques, composed of the small amyloid-beta (Abeta) peptide, and neurofibrillary tangles, comprised aggregates of the microtubule binding protein, tau. The molecular linkage between these two lesions, however, remains unknown. Based on human and mouse studies, it is clear that the development of Abeta pathology can trigger tau pathology, either directly or indirectly. However, it remains to be established if the interaction between Abeta and tau is bidirectional and whether the modulation of tau will influence Abeta pathology. To address this question, we used the 3xTg-AD mouse model, which is characterized by the age-dependent buildup of both plaques and tangles. Here we show that genetically augmenting tau levels and hyperphosphorylation in the 3xTg-AD mice has no effect on the onset and progression of Abeta pathology. These data suggest that the link between Abeta and tau is predominantly if not exclusively unidirectional, which is consistent with the Abeta cascade hypothesis and may explain why tauopathy-only disorders are devoid of any Abeta pathology.     

Tauroursodeoxycholic acid prevents amyloid-beta peptide-induced neuronal death via a phosphatidylinositol 3-kinase-dependent signaling pathway

Tauroursodeoxycholic acid (TUDCA), an endogenous bile acid, modulates cell death by interrupting classic pathways of apoptosis. Amyloid-beta (Abeta) peptide has been implicated in the pathogenesis of Alzheimer's disease, where a significant loss of neuronal cells is thought to occur by apoptosis. In this study, we explored the cell death pathway and signaling mechanisms involved in Abeta-induced toxicity and further investigated the anti-apoptotic effect(s) of TUDCA. Our data show significant induction of apoptosis in isolated cortical neurons incubated with Abeta peptide. Apoptosis was associated with translocation of pro-apoptotic Bax to the mitochondria, followed by cytochrome c release, caspase activation, and DNA and nuclear fragmentation. In addition, there was almost immediate but weak activation of the serine/threonine protein kinase Akt. Inhibition of the phosphatidylinositide 3 prime-OH kinase (PI3K) pathway with wortmannin did not markedly affect Abeta-induced cell death, suggesting that this signaling pathway is not crucial for Abeta-mediated toxicity. Notably, co-incubation with TUDCA significantly modulated each of the Abeta-induced apoptotic events. Moreover, wortmannin decreased TUDCA protection against Abeta-induced apoptosis, reduced Akt phosphorylation, and increased Bax translocation to mitochondria. Together, these findings indicate that Abeta-induced apoptosis of cortical neurons proceeds through a Bax mitochondrial pathway. Further, the PI3K signaling cascade plays a role in regulating the anti-apoptotic effects of TUDCA.     

A Cdk5 inhibitory peptide reduces tau hyperphosphorylation and apoptosis in neurons

The extracellular aggregation of amyloid beta (Abeta) peptides and the intracellular hyperphosphorylation of tau at specific epitopes are pathological hallmarks of neurodegenerative diseases such as Alzheimer's disease (AD). Cdk5 phosphorylates tau at AD-specific phospho-epitopes when it associates with p25. p25 is a truncated activator, which is produced from the physiological Cdk5 activator p35 upon exposure to Abeta peptides. We show that neuronal infections with Cdk5 inhibitory peptide (CIP) selectively inhibit p25/Cdk5 activity and suppress the aberrant tau phosphorylation in cortical neurons. Furthermore, Abeta(1-42)-induced apoptosis of these cortical neurons was also reduced by coinfection with CIP. Of particular importance is our finding that CIP did not inhibit endogenous or transfected p35/Cdk5 activity, nor did it inhibit the other cyclin-dependent kinases such as Cdc2, Cdk2, Cdk4 and Cdk6. These results, therefore, provide a strategy to address, and possibly ameliorate, the pathology of neurodegenerative diseases that may be a consequence of aberrant p25 activation of Cdk5, without affecting 'normal' Cdk5 activity.