. Cdk5 is a key factor in tau aggregation and tangle formation in vivo. Neuron. 2003 May 22;38(4):555-65. PubMed.

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  1. The combination of mutant tau-P301L with the CDK5-activating cofactor p25 in brain of double-transgenic mice is proven here to increase phosphorylation of tau and its aggregation into filaments. This outcome is not totally unexpected, and confirms the fact that CDK5 was proposed and identified as tau-kinase II—and GSK3β as tau-kinase I (Ishiguro et al., 1992). This was confirmed by many studies since then, at least in cell culture. Nevertheless, in brain in vivo, the situation was and is more complex, since even overexpression of CDK5 and p35 with human tau-4R in brain of triple-transgenic mice was not sufficient to increase tau-phosphorylation appreciably (Van den Haute et al., 2001) as opposed to GSK3β (Spittaels et al., 2000).

    Given the phenotype of the parental transgenic mouse strains, and of other strains as published, comparing them to the current presented double-transgenic strain brings up some interesting questions.

    First, the criteria used here to define increased CDK5 activity as pathological are biochemical and histological in nature. No mention is made of neurotoxicity or neuronal loss, defects in LTP or cognition, deviations in behavior or in any other aspect that would indicate that more aggregated tau is bad for neuronal health (although certainly refraining from claiming that it is in any way good or healthy).

    Second, both parental mouse strains suffer motoric and health problems at an early age: Whole-body exertion tremors in p25 mice at age four to nine weeks (Ahlijanian et al., 2000) and absence of escape extension at age 6.5 months in hemizygous tau-P301L mice (Lewis et al., 2000). These phenotypic aspects appear to be missing (completely?) in the double-transgenic mice analyzed here, despite their advanced age of 12 months. Could that mean that moderately increased CDK5 activity has the same effect as moderately increased GSK3β activity, namely rescue of the motoric problems (Spittaels et al., 2000)?

    Third, the relative levels of kinases and their site of action, both in terms of subcellular site and in brain regional site, will be determining their actual activity and specificity. For CDK5 and GSK3β to induce or promote "tau pathology" as defined by intraneuronal tau aggregates might indeed require "complexes" of some sort, as tentatively identified and proposed by the authors. And I also agree with them when they state that we need to implement in the models the actual—and still elusive—link of increased tau phosphorylation to amyloid (or vice versa, depending on your school of thought …) (Terwel et al., 2002).

    View all comments by Fred Van Leuven
  2. This paper is very relevant to our work and that of others. It begins to bring together the roles of tau dysregulation and aberrant phosphorylation in a mammalian model. It further demonstrates the role for tau hyperphosphorylation in accelerating NFT pathology.

    Although in the discussion it is stated that the other mouse models and fly models are at odds, or inconclusive, they actually are quite in line with these current findings. The one fly model that explores the role of phosphorylation (Jackson et al., 2002) shows that both tau dysregulation and dysregulation of kinase activity (in this case GSK3β, but I suspect CDK5 would be similar) are needed to form NFTs, and that altered kinase activity alone, or wild-type tau overexpression alone, are insufficient. This current paper shows that hyperactive kinase accelerates NFT formation, just as it does in the fly.

    Quite importantly, this demonstrates nicely a role for CDK5 in addition to the previously demonstrated role for GSK3β in NFT formation. It also confirms in vivo that CDK5 activation leads to GSK3β activation, by a mechanism that has yet to be uncovered. From a therapeutic standpoint, this work supports continued development of treatments focused on kinase inhibition. However, given the multifaceted roles of these kinases, the potential toxicity of GSK3β or CDK5 inhibition remains an issue.

    View all comments by Daniel Geschwind
  3. Tau aggregation is a central issue for understanding tauopathies, including AD. Crossbreeding CDK5-activator p25 transgenics with P301L transgenics resulted in hyperphosphorylation of human tau and induced tau aggregation in neurons. The correlation between hyperphosphorylation and aggregation of tau is not simple. In utero, tau is highly phosphorylated, but not aggregated. The different phosphorylation sites between fetal tau and PHF tau have been reported and may provide answers regarding which kinases are essential for formation of tau aggregates. Regarding this point, this paper did not satisfy the criteria for phosphorylation sites of PHF-tau, because Serine 202 and 404 of tau are phosphorylated in fetal and PHF tau, although they are phosphorylation sites of CDK5. The phosphorylation of Ser422, Ser262, and AT100 epitopes are specific to PHF tau, but the activation of CDK5 alone cannot explain the phosphorylation of these sites, even in synergistic activation with GSK3. For this reason, it is thought that tau phosphorylation by CDK5 in these mice might not itself induce tau aggregation; this may occur through other cascades. Nevertheless, CDK5 inhibitor administration into tau Tg mice may clarify the involvement of CDK5 on tau aggregation.

    View all comments by Akihiko Takashima
  4. An interesting observation emanating from the studies of Noble et al. is that the double-transgenic mice overexpressing p25 and mutant tau show an increase in the active form of GSK3β while total GSK3β levels remain unaltered. Although the GSK3β kinase activity was not directly determined in these studies, this observation raises a question: What is the link between elevated p25 levels (which activate CDK5) and the active form of GSK3β? To date, there is no evidence that GSK3β can be directly phosphorylated and activated by CDK5. Nevertheless, these findings suggest that hyperphosphorylation of tau observed in the p25/T double-transgenic animals is likely caused by both kinases. Earlier studies had shown that phosphorylation of tau by CDK5 makes tau a "better" substrate for GSK3β. Thus, these observations suggest two ways by which sustained activation of CDK5 causes hyperphosphorylation of tau: by direct phosphorylation and by activation of GSK3β, which will further phosphorylate tau. Hyperphosphorylated tau is released from microtubules and forms aggregates and tangles leading to neuronal pathology.

    There is, however, a wrinkle to this conclusion. An earlier study by Spittaels et al. showed that constitutive overexpression of human GSK3β in tau-transgenic mice actually reduced the axonal pathology. On the other hand, Lucas et al. observed that conditional expression of Xenopus GSK3β in mice promoted tau hyperphosphorylation and neurodegeneration. Although these differences could be explained by differences in the experimental designs (constitutive vs. conditional expression of transgene, species and strain differences, etc.), these observations exhibit the complexities of interpreting data from transgene studies.

    Finally, the "AβPpists" might wonder what connection AβPP or Aβ might have to these observations. One possibility suggested by the observations of Town et al. is that Aβ stimulates p25 production in a calpain-dependent fashion. Increased levels of p25 will activate CDK5 (and indirectly GSK3β). Considering the "cross-talk" between various kinases, such a simple scenario probably represents oversimplification. Also, direct evidence supporting this chain of "cause-and-effect" remains lacking. Future studies focused on understanding the molecular mechanism by which p25 (and, therefore, activated CDK5) stimulates GSK3β will likely bring us a step closer to understanding tangle formation.

    View all comments by Sanjay Pimplikar
  5. Neurofibrillary tangles consist of tau protein that has forsaken its role as a stabilizer of microtubules to polymerize into abnormal fibrils within neurons. Preclinical and clinical evidence leaves little doubt that abnormal tau polymerization is injurious to neurons. A prominent characteristic of tangles is a high degree of site-specific phosphorylation that is thought to contribute to the dysfunction and polymerization of tau, as well as to the stability of tau filaments. Several kinases are implicated in tau hyperphosphorylation in brain, and one that has garnered attention in Alzheimer's pathogenesis is cyclin-dependent kinase-5 (CDK5). There is evidence in AD that CDK5 is overactivated by an excess of a protein fragment called p25; the resulting increase in phosphorylation is hypothesized to facilitate tau polymerization and tangle formation.

    Transgenic mice are ideal for testing such hypotheses in vivo. Mice overexpressing p25 develop axonopathy and movement dysfunction, but the neurons show no evidence of neurofibrillary tangles (Ahlijanian et al., 2000; Bian et al., 2002), indicating that normal mouse tau is refractory to p25/CDK5-induced tangle formation. Noble and colleagues took the interesting step of crossing p25-transgenics (Ahlijanian et al., 2000) with mice expressing mutant human tau (4R0N, P301L mutation; Lewis et al., 2000). Tau in these dual-transgenic mice is hyperphosphorylated at sites linked to CDK5, and accumulates intraneuronally in several brain areas. The finding that phosphorylated sites are detectable on soluble tau (in addition to the insoluble aggregates) suggests that phosphorylation precedes, and thus may facilitate, atypical polymerization. These findings support the notion that activation of CDK5 (and, as the authors note, probably other kinases) contributes to the hyperphosphorylation and consequent autophilicity of tau in vivo. Surprisingly, the age of onset of dystonia seen in P301L mice was not noticeably advanced by the presence of the p25 transgene (nor is there yet evidence for frank neuronal loss). Perhaps older mice will show motoric (and, with luck, mnemonic) impairments.

    The CDKs regulate a number of important cellular functions such as cell division, differentiation, and apoptosis. In fact, CDK inhibitors are being considered as therapies for cancer, atherosclerosis, and viral infections, among other disorders (Knockaert et al., 2002). Hence, for the treatment of chronic neurodegenerative disorders, highly selective and demonstrably safe inhibitors of CDK5 will be needed. When such agents are found, they will find fertile ground for preclinical efficacy testing in p25/P301L-dual transgenic mice. Ultimately, as with all new, disease-modifying approaches, only clinical trials will prove the benefit of CDK5 inhibition for the treatment of AD and other tauopathies.

    View all comments by Lary Walker

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  1. Aiding and Abetting, Hyperactive CDK5 Gives Mouse Tangles