Apolipoprotein E decreases tau kinases and phospho-tau levels in primary neurons
© Hoe et al; licensee BioMed Central Ltd. 2006
Received: 21 September 2006
Accepted: 13 December 2006
Published: 13 December 2006
Apolipoprotein E (apoE) receptors act as signaling molecules in neurons, altering phosphorylation of numerous proteins after extracellular ligand binding and affecting neurite outgrowth, synapse formation, and neuronal migration. Since apoE is important in the pathogenesis of Alzheimer's disease (AD), we tested whether apoE treatment of neurons affected molecules important to phosphorylation of tau, such as GSK 3β, P35, and CDK5, and the phosphorylation of tau itself. Treatment of primary neurons with 2 uM apoE (or an apoE-derived peptide) decreased levels of phospho-GSK 3β, P35 and CDK5, and decreased levels of phosphorylated forms of tau. A lower concentration of apoE (100 nM) had no effect on these molecules. The alteration of tau phosphorylation by apoE was blocked by an inhibitor of the low-density lipoprotein receptor family, demonstrating the effects were due to receptor interactions. These results demonstrate that apoE affects several downstream signaling cascades in neurons: decreased tau kinases phosphorylation and inhibition of tau phosphorylation at Thr171 and Ser202/Thr205 epitopes. We conclude that apoE can alter levels of tau kinases and phospho-tau epitopes, potentially affecting tau neuropathological changes seen in AD brains.
Alzheimer's disease (AD) is defined neuropathologically by the presence of two types of protein aggregates: extracellular senile plaques, which are composed of the Aβ peptide, and intraneuronal neurofibrillary tangles (NFT), which are composed of phosphorylated forms of the tau protein [1–3]. Tau is a microtubule-associated protein with multiple phosphorylation sites ; hyperphosphorylation of tau in the AD brain is potentially promoted by several kinases, including GSK 3β, CDK5, and MARK . Much AD-related research focuses on identifying factors that affect these neuropathological lesions and the risk of AD. One genetic factor that has been identified is the APOE genotype . The APOE e4 allele is associated with increased Aβ deposition in brain [7–9]; evidence on whether APOE genotype also affects the accumulation of neurofibrillary tangles is more mixed .
The apoE protein is associated with high-density lipoproteins in the CNS , and is increased after several types of brain damage [12, 13]. ApoE-lipoproteins bind members of the low-density lipoprotein (LDL) receptor family , receptors with complex ligand binding domains that allow interactions with large numbers of ligands. These receptors mediate uptake of apoE-containing lipoproteins, suggesting that apoE receptors could be important in the clearance of lipids after damage . But stimulation of these receptors by ligands also mediates various neuronal signaling mechanisms. Binding of Reelin to LDL receptor family members promotes phosphorylation of the cytoplasmic disabled protein (Dab1) , and induces activation of Src and PKB kinases [17, 18]. These processes are necessary for correct neuronal migration during development. Furthermore, Reelin inhibits phosphorylation of GSK 3β, but does not affect the activity of CDK5 . We have found that apoE binding to these receptors also promotes Dab phosphorylation and stimulates intracellular activation of Src and PKB kinase ; it is unknown whether apoE also affects activation of tau kinases, and this question was the basis for the present study. ApoE induces neurite outgrowth and microtubule stability [21, 22], and several studies have suggested that apoE or apoE fragments can access the cytoplasmic compartment of cells and directly bind to tau  or induce NTF-like inclusions . Because apoE affects intracellular kinases through binding to its receptors, we examined the effects of apoE signaling on the activation of tau kinases and the phosphorylation of tau in vitro, using full-length apoE, or an apoE peptide derived from the receptor-binding region of apoE. Our results in primary neurons show that apoE treatment inhibited tau kinases (e.g., P35, P-GSK3β, and CDK5) and tau phosphorylation. These data suggest that apoE could alter tau phosphorylation and thus potentially affect the accumulation of NFT in the AD brain.
Recombinant human apoE2, E3 and E4 were purchased from Oxford Biomedical Research. The apoE peptide (EP; sequence LRKLRKRLLLRKLRKRLL) was synthesized by Johns Hopkins University of Medicine (Biosynthesis and sequencing facility, Baltimore, MD). This peptide, containing a tandem repeat of the receptor binding domain of apoE, has the same signaling properties as full length apoE . Poly-D-lysine (P-7280) and phosphatase inhibitor cocktails (P-2850 and P-5726) were purchased from Sigma (St Louis, MO). CytoTox-ONE™ homogeneous Membrane Integrity Assay (G7891) was purchased from Promega (Madison, WI).
Antibodies against phospho-GSK-3β (pY216, diluted 1:1000) and total GSK-3β were from BD Transduction Laboratories. Mouse monoclonal antibodies against p35 (diluted 1:1000), CDK5 (1:1000) and β-actin (1:1000) were purchased from Santa Cruz Biotechnology and BD Pharmingen. To examine the phosphorylation of tau, the following antibodies were used: mouse monoclonal antibody 5E2 (Upstate Biotechnology, diluted 1:1000), which recognizes a phosphorylation-independent tau epitope and is used to measure total tau; anti Tau-1 (Chemicon International, diluted 1:1000), which recognizes an unphosphorylated tau epitope; pS396 (Biosource International, diluted 1:1000), AT8 (Innogenetics, diluted 1:1000) and AT270 (Innogenetics, diluted 1:1000), which recognize phosphorylated tau epitopes.
Primary mouse embryonic cortical neuron cultures were prepared from embryonic day 16 Swiss-Webster mice as previously described . Briefly, mouse E16 embryos brain cortices were chopped and trypsinized for 10 min at 37°C. After trypsinization, 0.4 μg/ml trypsin inhibitor, 0.025% DNase and 12 mM MgSO4 were added and mixed until the tissue was thoroughly homogenized. Cells were then transferred to Neurobasal medium containing B27 serum supplement (Invitrogen), 1 mM glutamine, gentamycin and cytosine-β-D-arabinofuranoside (Ara-C, 5 μg/ml) (Sigma). Neurons were seeded on 50 μg/ml poly-D- lysine coated 12 well tissue culture plates at a density of 2 × 10 6 per well. Toxicity was monitored by LDH release using the CytoTox-ONE™ Assay. To measure the effects of apoE on tau proteins, primary neuronal cells were treated with buffer or apoE peptide for 2 or 12 hours. For extraction of cells in RIPA buffer, neurons were washed with ice-cold PBS containing 10 mM NaF, ice-cold RIPA buffer (10 mM Tris-HCl, pH 7.2, 150 mM NaCl, 1% Triton X-100, 1% deoxycholate, 5 mM EDTA, protease inhibitor and phosphatase inhibitor cocktails) was added, then cells were centrifuged at 10,000 rpm for 5 min and the supernatant was collected. For extraction of cells in SDS buffer, the pellet after centrifugation was incubated with SDS sample buffer (62.5 mM Tris-HCl, pH 6.8, 2% SDS, 25% glycerol, 0.01% Bromophenol Blue)(Bio-Rad) including DNase for 30 min at 37°C. Additionally, cells were extracted directly with SDS sample buffer including DNase and incubated for 30 min at 37°C. Phosphorylation of tau and total tau levels were then measured in the SDS buffer extract, RIPA buffer extract, or in the SDS buffer extract following RIPA buffer extraction.
Proteins from cell extracts were separated under denatured and reduced conditions by Tris-glycine polyacrylamide gel electrophoresis. Separated proteins were transferred onto PVDF membrane at 170 mA for 2 hours and blocked with 5% nonfat dry milk or 4% BSA. The blots were incubated with antibodies at room temperature for 1 hour. Horseradish peroxidase-conjugated secondary antibody was visualized by ECL detection system and exposed to film. Films were scanned using a ScanMaker 9800XL densitometer (Microtek), and relative levels of bands on a film were compared using Jandel SigmaStat software.
Experiments were repeated a minimum of four times unless otherwise noted. All data was analyzed using ANOVA with Graphpad Prism 4 software, using Tukey's Multiple Comparison test for post-hoc analyses with significance determined as P < 0.05. Descriptive statistics were calculated with StatView 4.1 and displayed as an expressed mean ± S.E.M.
ApoE inhibits tau kinases in primary neurons
ApoE decreases tau phosphorylation in primary neurons
Surprisingly, we found that the levels of total tau were also significantly altered by the treatments with apoE peptide in this short (2 hour) period. Immunoblots with the antibody 5E2 (which recognizes tau irrespective of its phosphorylation state) demonstrated that levels of tau in cell extracts decreased by 69% (Figure 3A). We were concerned that all tau species were efficiently extracted in the RIPA buffer used, given that tau is microtubule-associated and thus might remain associated with cell debris after extraction. Therefore, we repeated the experiments, extracting the cells in RIPA buffer, and then extracting the insoluble fractions in SDS buffer (after treatment with DNase to decrease the viscosity of the extracts). Under these conditions, we did not observe significant decrease of total tau after EP treatment (Fig 3F), suggesting that apoE affected the proportion of total tau solubilized by RIPA, and apoE did not decrease the total levels of tau. As a further test of this hypothesis, we also extracted cells directly into the stronger buffer containing SDS. We found that the total level of tau observed was not decreased, but apparently increased after apoE peptide treatment (Figure 3G). We also found that the level of unphosphorylated tau (Tau-1) was still increased and the level of phospho-tau (AT8) was still decreased (Figure 3G) in the SDS extraction buffer. Thus, the apoE peptide treatment caused both a change in the phosphorylation state of tau, as well as a change in the distribution of the tau protein to alter its extraction in different buffers.
We tested whether similar changes occurred after treatment of cells with each of the full-length apoE isoforms. Each of the three apoE isoforms caused a decrease in the levels of phosphorylated tau (Fig. 3B, C, D). No significant changes in β-actin were observed. Interestingly, quantification of these blots revealed that apoE4 had the smallest effect of the three isoforms on tau phosphorylation state: it slightly increased levels of unphosphorylated tau (21%) and decreased phospho-tau proteins (35%).
Reduction in tau phosphorylation depends on apoE-receptor interaction
In order to test whether the observed changes to tau phosphorylation in primary neurons were mediated by members of LDL receptor family or were some direct effect of apoE, we pre-treated cells with RAP, an inhibitor of these receptors. RAP causes a prolonged redistribution of apoE receptors from dendritic to somatic compartments, potentially preventing signaling by treatments with subsequent ligands . RAP treatment alone did not affect levels of phospho-tau epitopes (Fig. 5A, B). Treatment of neurons with the apoE peptide caused the significant decrease in phospho-taus (Thr 171, Ser202/Thr205), but these decreases were inhibited by pretreatment of cells with RAP (Fig. 5A, B, first row). The Src kinase inhibitor PP2 and the ERK kinase inhibitor PD98059 did not block the decrease in tau phosphorylation by apoE peptide (Fig. 5A, second and third row). No significant changes in β-actin were observed. These data suggest that decreases in phosphorylation of tau required binding of the apoE peptide to LDL receptor family members, but did not involve ERK or Src kinase cascades.
We have investigated how apoE signaling affects tau kinase activity and tau phosphorylation in primary neurons. In primary neurons, treatment of cells with 2 uM apoE (but not 100 nM) inhibited phosphorylation of GSK 3β, and decreased levels of P35 and CDK5 (Fig. 1). ApoE also decreased the levels of phosphorylated tau and increased the levels of unphosphorylated tau (Fig. 3). These treatments may also have altered the distribution of tau in the neurons, since increased levels of tau (but decreased levels of phosphorylated tau) were recovered in easily solubilized cell fractions (Fig. 3). These effects were mediated by members of the LDL receptor family, since their effects were blocked by the inhibitor RAP (Fig. 5).
Although apoE receptors were initially thought to function to endocytose ligands, several other functions of ligand binding have now been described. ApoE receptors transduce signals important for neuronal migration during development . Ligand interactions with apoE receptors result in transient changes in Src, ERK and JNK phosphorylation [20, 28]. ApoER2 interactions with NMDA receptors are modulated by ligand binding . In glia, apoE receptors mediate anti-inflammatory effects of apoE . Finally, in several cell types, ligand binding to apoE receptors increases the surface cleavage of the receptors . Ligands for this family of receptors differ somewhat in whether they are freely able to be endocytosed (such as apoE), or whether they are attached to other cells or to the extracellular matrix (such as reelin). It will be interesting to determine whether there are differences in the signaling effects of ligands depending on whether their metabolism depends on endocytosis by apoE receptors.
These results complement studies of mouse knock-out models. Brains of APOE knock-out mice have increased levels of tau phosphorylation compared to wild-type controls [32, 33]. Similarly, brains of Reeler mice, ApoER2-/-/VLDLr-/- mice, and Dab-1 knock-out mice have hyperphosphorylated tau . Thus, chronic interruption of the apoE signaling pathway leads to increased tau phosphorylation in brain. Acute treatment of neurons with reelin caused decreased GSK-3β activation , and we report that treatment with apoE also caused decreased GSK-3β activation as well as reduced tau phosphorylation. Thus the acute effects of apoE and reelin are similar to each other, and consistent with the effects of chronic absence of apoE and reelin in vivo.
The fact that apoE affects the phosphorylation of the microtubule-associated protein tau is supportive of a function of apoE in promoting neurite outgrowth [25, 35, 36]. ApoE levels are induced after several types of brain damage [12, 13], presumably as a mechanism for clearance of damaged membranes . However, this apoE could also act as a signal for neurons to stabilize new neurites as a mechanism for regeneration after damage . ApoE could alter the stability of the microtubules by altering tau and phospho-tau levels, allowing permanent changes to the neuronal cytoskeleton that need to accompany neurite outgrowth.
In the AD brain, the APOE genotype affects levels of Aβ deposition [7, 8], and apoE is a component of most plaques [39, 40]. In fact, because of its strong affinity for Aβ, apoE has been referred to as a pathological chaperone . The presence of the apoE receptor binding domain on amyloid deposits  suggests that apoE on plaques could provide chronic signals via apoE receptors to neurons in the vicinity of plaques. These signals could include increased phosphorylation of substrates of kinases such as ERK and Src  or decreased phosphorylation of substrates of kinases such as JNK , GSK 3β and CDK5 (Fig 1). Since kinases in both groups have been linked to increased tau phosphorylation (e.g. ERK  and GSK-3β ), apoE presence on plaques could contribute both to the formation and the inhibition of phospho-tau-positive dystrophic neurites found around neuritic plaques. We previously have found different strengths of the signaling effects for the apoE isoforms. ApoE2 had the most effect on ERK activation and JNK inhibition  and apoE4 the least; a similar pattern was observed for the different apoE isoforms on receptor cleavage . In this study, apoE2 decreased levels of phospho-GSK-3β and CDK5 more than apoE4 (Figure 2). If the acute effects of apoE on neurons applied to more chronic exposures, apoE2 on plaques would be expected to inhibit phospho-tau levels the most, and apoE4 would be expected to have the least effect. Several studies have indicated that higher levels of phospho-tau in AD brain are associated with apoE4 than with apoE3 or apoE2 , correlating with the in vitro effects of these isoforms on GSK-3β and CDK5 inhibition (and not with their effects on ERK activation).
Animal models have also implicated apoE in disruption of neuronal cytoskeleton. Neuronal expression of apoE4 is associated with prominent axonopathy  and increased tau phosphorylation [42, 45]. These processes may involve intraneuronal proteolysis of apoE . Expression of APOE is glial under most conditions, but studies of the APOE promoter demonstrate that under some kainic acid treatment condition , APOE may be expressed in neurons. Under some conditions, apoE may be present within the neuronal cytoplasm , where more direct effects on tau and microtubules are possible. ApoE directly inhibits GSK-3β phosphorylation of tau  and apoE alters the pattern of tau phosphorylation by GSK-3β . These effects of apoE would not depend on interactions with apoE receptors, as we observed here (Figure 5).
In this study, we report that apoE can decrease levels of tau kinases and phospho-tau proteins via extracellular interactions with apoE receptors. These effects of apoE could be important for the strong association of the APOE gene with risk of AD.
This study was supported by NIH AG14473.
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