Lit-Fui Lau

Anesthesia leads to tau hyperphosphorylation through inhibition of phosphatase activity by hypothermia.

Postoperative cognitive dysfunction, confusion, and delirium are common after general anesthesia in the elderly, with symptoms persisting for months or years in some patients. Even middle-aged patients are likely to have postoperative cognitive dysfunction for months after surgery, and Alzheimers disease (AD) patients appear to be particularly at risk of deterioration after anesthesia. Several investigators have thus examined whether general anesthesia is associated with AD, with some studies suggesting that exposure to anesthetics may increase the risk of AD. However, little is known on the biochemical consequences of anesthesia on pathogenic pathways in vivo. Here, we investigated the effect of anesthesia on tau phosphorylation and amyloid precursor protein (APP) metabolism in mouse brain. We found that, regardless of the anesthetic used, anesthesia induced rapid and massive hyperphosphorylation of tau, rapid and prolonged hypothermia, inhibition of Ser/Thr PP2A (protein phosphatase 2A), but no changes in APP metabolism or Aβ (β-amyloid peptide) accumulation. Reestablishing normothermia during anesthesia completely rescued tau phosphorylation to normal levels. Our results indicate that changes in tau phosphorylation were not a result of anesthesia per se, but a consequence of anesthesia-induced hypothermia, which led to inhibition of phosphatase activity and subsequent hyperphosphorylation of tau. These findings call for careful monitoring of core temperature during anesthesia in laboratory animals to avoid artifactual elevation of protein phosphorylation. Furthermore, a thorough examination of the effect of anesthesia-induced hypothermia on the risk and progression of AD is warranted.

Regulation of the NMDA Receptor Complex and Trafficking by Activity-Dependent Phosphorylation of the NR2B Subunit PDZ Ligand

Interactions between NMDA receptors (NMDARs) and the PDZ [postsynaptic density-95 (PSD-95)/Discs large/zona occludens-1] domains of PSD-95/SAP90 (synapse-associated protein with a molecular weight of 90 kDa) family proteins play important roles in the synaptic targeting and signaling of NMDARs. However, little is known about the mechanisms that regulate these PDZ domain-mediated interactions. Here we show that casein kinase II (CK2) phosphorylates the serine residue (Ser1480) within the C-terminal PDZ ligand (IESDV) of the NR2B subunit of NMDAR in vitro and in vivo. Phosphorylation of Ser1480 disrupts the interaction of NR2B with the PDZ domains of PSD-95 and SAP102 and decreases surface NR2B expression in neurons. Interestingly, activity of the NMDAR and Ca2+/calmodulin-dependent protein kinase II regulates CK2 phosphorylation of Ser1480. Furthermore, CK2 colocalizes with NR1 and PSD-95 at synaptic sites. These results indicate that activity-dependent CK2 phosphorylation of the NR2B PDZ ligand regulates the interaction of NMDAR with PSD-95/SAP90 family proteins as well as surface NMDAR expression and may be a critical mechanism for modulating excitatory synaptic function and plasticity.

Concentration-Dependent Modulation of Amyloid-β in Vivo and in Vitro Using the γ-Secretase Inhibitor, LY-450139

LY-450139 is a γ-secretase inhibitor shown to have efficacy in multiple cellular and animal models. Paradoxically, robust elevations of plasma amyloid-β (Aβ) have been reported in dogs and humans after administration of subefficacious doses. The present study sought to further evaluate Aβ responses to LY-450139 in the guinea pig, a nontransgenic model that has an Aβ sequence identical to that of human. Male guinea pigs were treated with LY-450139 (0.2–60 mg/kg), and brain, cerebrospinal fluid, and plasma Aβ levels were characterized at 1, 3, 6, 9, and 14 h postdose. Low doses significantly elevated plasma Aβ levels at early time points, with return to baseline within hours. Higher doses inhibited Aβ levels in all compartments at early time points, but elevated plasma Aβ levels at later time points. To determine whether this phenomenon occurs under steady-state drug exposure, guinea pigs were implanted with subcutaneous minipumps delivering LY-450139 (0.3–30 mg/kg/day) for 5 days. Plasma Aβ was significantly inhibited at 10–30 mg/kg/day, but significantly elevated at 1 mg/kg/day. To further understand the mechanism of Aβ elevation by LY-450139, H4 cells overexpressing the Swedish mutant of amyloid-precursor protein and a mouse embryonic stem cell-derived neuronal cell line were studied. In both cellular models, elevated levels of secreted Aβ were observed at subefficacious concentrations, whereas dose-responsive inhibition was observed at higher concentrations. These results suggest that LY-450139 modulates the γ-secretase complex, eliciting Aβ lowering at high concentrations but Aβ elevation at low concentrations.

Interplay between Cyclin-Dependent Kinase 5 and Glycogen Synthase Kinase 3β Mediated by Neuregulin Signaling Leads to Differential Effects on Tau Phosphorylation and Amyloid Precursor Protein Processing

Cyclin-dependent kinase 5 (cdk5) and glycogen synthase kinase 3β (GSK3β) have been implicated in pathogenic processes associated with Alzheimers disease because both kinases regulate tau hyperphosphorylation and enhance amyloid precursor protein (APP) processing leading to an increase in amyloid β (Aβ) production. Here we show that young p25 overexpressing mice have enhanced cdk5 activity but reduced GSK3β activity attributable to phosphorylation at the inhibitory GSK3β–serine 9 (GSK3β–S9) site. Phosphorylation at this site was mediated by enhanced activity of the neuregulin receptor complex, ErbB, and activation of the downstream phosphatidylinositol 3 kinase/Akt pathway. Young p25 mice had elevated Aβ peptide levels, but phospho-tau levels were decreased overall. Thus, cdk5 appears to play a dominant role in the regulation of amyloidogenic APP processing, whereas GSK3β plays a dominant role in overall tau phosphorylation. In older mice, GSK3β inhibitory phosphorylation at S9 was reduced relative to young mice. Aβ peptide levels were still elevated but phospho-tau levels were either unchanged or showed a trend to increase, suggesting that GSK3β activity increases with aging. Inhibition of cdk5 by a specific inhibitor reduced cdk5 activity in p25 mice, leading to reduced Aβ production in both young and old mice. However, in young mice, cdk5 inhibition reversed GSK3β inhibition, leading to an increase in overall tau phosphorylation. When cdk5 inhibitor was administered to very old, nontransgenic mice, inhibition of cdk5 reduced Aβ levels, and phospho-tau levels showed a trend to increase. Thus, cdk5 inhibitors may not be effective in targeting tau phosphorylation in the elderly.

Tau Protein Phosphorylation as a Therapeutic Target in Alzheimers Disease

Neurofibrillary tangles (NFTs) are a distinguishing neuropathological feature found in postmortem brains of Alzheimer s disease (AD) and tauopathy patients. The density of these lesions correlates with severity of AD and their distribution follows a characteristic pattern of expansion as the disease progresses. The principle components of NFTs are highly phosphorylated forms of the microtubule-associated protein, tau. Tau phosphorylation is believed to initiate or facilitate dissociation from microtubules leading to microtubule destabilization, decay of cellular transport properties, and cell death. This review summarizes recent data and prevailing views on the roles of protein kinases and phosphatases in the regulation of tau phosphorylation in vitro and in vivo, taking into account data from human neurodegenerative diseases and from transgenic rodent models. Small molecule inhibitors of tau phosphorylation that serve as important research tools and possibly the basis of potential new therapeutics, are also described. Key challenges in developing effective therapeutic agents include identification of the relevant kinase(s) responsible for aberrant tau phosphorylation in AD, synthesis of inhibitors selectively targeting those kinases and establishment of appropriate animal models.

Cdk5 as a drug target for the treatment of Alzheimer's disease.

Cyclin-dependent kinase-5 (cdk5) is suggested to play a role in tau phosphorylation and contribute to the pathogenesis of Alzheimer’s disease (AD). One of its activators, p25, is dramatically increased in AD brains where p25 and cdk5 are colocalized with neurofibrillary tangles. Several animal models have shown a correlation of p25/cdk5 activities with tau phosphorylation. Overexpression of p25/cdk5 in nueronal cultures not only leads to tau phosphorylation but also cytoskeletal abnormalities and neurodegeneration. Therefore, cdk5 kinase inhibitors are potential therapeutic agents for the treatment of AD. Availability of potent, selective, brain permeable cdk5 inhibitors and relevant animal models in which their efficacy can be treated will be critical in the development of these inhibitors.

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-β peptides (Aβ) 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 Aβ peptides. Since both Aβ 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 Aβ 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.

Therapeutic Approaches for the Treatment of Alzheimer's Disease: An Overview

Alzheimers disease (AD) is a neurodegenerative disease that robs the minds of our elderly population. Approximately one in every eight adults over the age of 65 and nearly half of those over 85 are afflicted with this disease. Aging and other risk factors (e.g. cardiovascular diseases, obesity and diabetes) in developed societies will impose an ever increasing socioeconomic threat in the future. Current medicines for AD patients are mainly symptomatic treatments and a huge unmet medical need exists to slow, stop or reverse the progression of this disease. A great deal of research has been dedicated to understanding the pathogenesis of AD from which come many ideas for intervening in its progression. They can be grossly categorized into those targeting the amyloid pathology, tau pathology, microgliosis (neuroinflammation) and functional deficits. Some of these ideas have been fast-tracked to clinical trials due to the availability of medicines with proven clinical efficacies for other diseases while others represent novel chemical entities. Our continued commitment in searching for efficacious treatments together with a healthier lifestyle will be important in fighting against the growing threat of this deteriorating disease.

Animal models of tau phosphorylation and tauopathy – what have they taught us?

Abstract Alzheimers disease (AD) is characterized by the presence of senile plaques and neurofibrillary tangles (NFTs) in brain. NFTs are mainly composed of aggregates of hyperphosphorylated tau proteins. Various animal models have been generated that recapitulate different aspects of tau pathologies, including tau hyperphosphorylation, somatodendritic accumulation of tau in the pretangle stage, and deposition of NFTs. Amyloid peptides appear to potentiate but not initiate NFT deposition in transgenic mice. Tau phosphorylation and aging in various animal models also promote NFT deposition. In addition to tau pathologies, axonal degeneration, neuronal cell death, and/or behavioral deficits are apparent in a number of animal models. While mechanisms leading to neurodegeneration are believed to involve functional disruption of microtubules by either overexpression or hyperphosphorylation of tau, the formation of NFTs may confer additional toxicities. Although the present animal models of tau phosphorylation and tauopathy do not fully recapitulate AD pathologies, they provide significant insights into mechanisms leading to tau pathology and associated neurodegeneration, and may serve as in vivo models for testing potential drug candidates for the treatment of this debilitating disease.