Name: in vitro aggregation assays using hyperphosphorylated tau protein
Uploaded: 2015-01-02T13:00:00
Description: Watch this Scientific Journal Video about In Vitro Aggregation Assays Using Hyperphosphorylated Tau Protein at JoVE.com

This work was supported by the National Institute on Aging (AG039768) to MHK. We thank Drs. Thomas Sharkey and Honggao Yan for generously providing the instruments, as well Sean Weise and Yan Wu for technical assistance.

1. Preparation of Reagents
<ol>
	<li>Prepare aggregation buffer (20 mM Tris, pH 7.4, 100 mM NaCl, 1 mM EDTA). Store at RT, stable for months. Supplement 1 mM dithiothreitol (DTT) before use. 
	NOTE: a HEPES-based buffer (10 mM HEPES, pH 7.5, 0.1 mM EDTA, 5 mM DTT) also produces similar results in tau aggregation.</li>
	<li>Prepare thioflavin T or thioflavin S stock solution (3 mM, dissolved in aggregation buffer), and filter by 0.22 &#956;m sterile filter unit. Store at &#8211;20 &#176;C in a tube covered by alumi...

This protocol demonstrates different assay conditions and instruments that detect the phosphorylation-dependent fast tau aggregation kinetics. In the terminal assay, the fluorescence dye ThT is added to a portion of the reaction removed from the master mix at each time point. Amyloid binding-induced fluorescence is then measured 26. In the second, with-dye mode, tau aggregation is carried out in the presence of ThT or ThS, rendering this type of reaction suitable for real-time automatic assessment of the growt...

Alzheimer&#8217;s disease (AD) is one of a large collection of neurodegenerative disorders known as tauopathies. The quintessential pathology underlying tauopathy is the neurofibrillary tangles, NFTs, in neurons, astrocytes and microglia 1-4. The NFT density correlates with cognitive impairment 3,5 and neuron loss 6. NFT contains primarily hyperphosphorylated tau protein (referred to as &#8220;p-tau&#8221; henceforth) that forms straight or paired helical filaments (PHF) 7,8. Tau is a microtubule associated protein thought to facilitate axonal transport that is essential for neuronal signaling and trafficking 9,10. Each tau molecule contains 2 to 3 phosphates in normal brain, but the phosphoryl content increases by several folds in tauopathy patients 11. Multiple kinases are likely to contribute to tau hyperphosphorylation including GSK3&#946; (glycogen synthase kinase 3&#946;) and CDK5 (cyclin-dependent kinase 5) 12,13, but the direct trigger for the pathological phosphorylation remains elusive 14. Abnormal phosphorylation in or near the microtubule-binding motifs dissociates tau from the microtubule 15, and causes tau mis-localization to the somatodendritic compartment, where p-tau oligomerizes into straight or paired helical filaments that can eventually polymerize into NFT inclusions. The close tie between tau hyperphosphorylation, NFT formation, and neurodegeneration led to a prevalent hypothesis that p-tau tangles elicit apoptotic and other cytotoxic responses, and thus is the underlying cause for tauopathy neurodegeneration 16,17. Drug screens and early clinical tests based on this premise have been launched 18. However, this hypothesis faces challenges 19,20. For example, SantaCruz et al. showed that cognitive functions of transgenic mice can be improved by suppressing the expression of a mutant human tau, even though NFTs continued to form from existing tau molecules 21. In a Drosophila model, NFT was shown to sequester the toxic cytosolic tau to protect the underlying neuron cells 22,23. Clearly, the pathogenesis role of NFT, if any, will greatly impact the direction of tauopathy therapeutics development.
In high concentrations, recombinant or normal brain tau protein spontaneously but slowly polymerizes into a PHF-like structure in vitro, as indicated by the binding of several &#946;-sheet preferred fluorescent dyes, electron microscopy, and light scattering spectroscopy 24-27. Adding heparin or arachidonic acid, an abundant fatty acid in human brain, drastically accelerates PHF formation in tau isoform- and inducer concentration-dependent manners 28-32. Intriguingly, hyperphosphorylated tau purified from AD brains or prepared by exhaustive in vitro phosphorylation reactions aggregates faster and more efficiently 26,33-35. These results are in excellent agreement with the pathological roles of p-tau. An in vitro system based on the aggregation of p-tau may thus serve as a powerful tool for AD drug screening.
Given the close association between tau aggregation and the progressive neurodegeneration of AD, as well as the recent failure in drug development targeting the A&#946; plaque, another key histological marker of AD 36-38, the interest in discovering drugs that control tau aggregation is rising. Indeed, several groups have already begun drug screens at different throughput, using in vitro tau aggregation reactions as the primary assay. A number of chemicals were found to exhibit inhibitory or reversal activities on tau aggregation in vitro 39-42. However, all current tau aggregation regulator screens use unmodified tau that misses the key pathological mark of phosphorylation, raising a concern for the specificity and efficacy of using these compounds in AD treatment.
One of the major hurdles of developing aggregation assays for biochemical characterization and AD drug screening is the production of sufficient quantities of the pathophysiologically relevant hyperphosphorylated tau protein. Using the Zippers-Assisted Catalysis system in which the 1N4R isoform of tau and the GSK-3&#946; kinase are co-expressed in E. coli as leucine zipper fusion proteins, we have overcome this challenge (Sui et al., submitted; see Figure 1 for the final products of tau and p-tau; also see 43 for preliminary mass spectrometry characterization of p-tau). From a panel of nine antibodies specific for different phosphorylation sites of tau, positive signals were seen in eight positions (data not shown). Below, we describe protocols and instrumentations that can differentiate the aggregation kinetic differences between unmodified tau and p-tau species. These assays were modified from published protocols that measured the increase of fluorescence of thioflavin T (ThT) or thioflavin S (ThS) upon amyloid (tau aggregates) binding 26. In the first &#8220;terminal&#8221;, no-dye approach, aggregation reactions are assembled and incubated in the absence of the amyloid dye. At different time points, an aliquot of each reaction is removed and mixed with equal volume of the ThT-containing buffer to stop aggregation and allow ThT to bind tau aggregates. Fluorescence is measured by an IAP FluoroMax-2 fluorometer. In the second &#8220;with-dye&#8221; continual monitoring assay, ThT or ThS is included in the aggregation reactions. Fluorescence can be measured continuously throughout the entire experiment manually or using a multi-plate reader. In addition, we describe an assay that uses a near-physiological concentration of tau and p-tau for aggregation in the continual measurement mode. The effect of phosphorylation remains readily detectable. Below, we will describe step-by-step operation procedures, and show representative results of these assays. Discussion of some of the pros and cons of each approach, as well as potential drug screening applications will follow.
At a high concentration, tau aggregates into amyloid-like structures spontaneously. However, in the laboratory, tau fibrillization is typically accelerated by such inducers as heparin (average molecular weight, 6,000 g/mol) and arachidonic acid. Examples shown herein include 30 &#181;M heparin. The formation of tau amyloid aggregates is monitored by the fluorescence resulting from amyloid binding by thioflavin T (ThT) or thioflavin S (ThS). Upon binding to tau aggregates, ThT exhibits a red shift in fluorescence (excitation: 450 nm; peak emission: 485 nm). ThS, on the other hand, has weak emission at 510 nm (excitation at 450 nm) before amyloid binding, but this fluorescence increases significantly in the presence of an amyloid protein such as the aggregated tau 44. Both dyes work well in detecting tau and p-tau aggregation. Because of the strong and relatively broad emission peak of ThT (see Figure 2), there is only 30% reduction in the fluorescence unit at 510 nm. For convenience, we use the same combination of excitation/emission wavelengths (i.e., 450 nm/510 nm) to monitor tau aggregation when using either dye.
Tau aggregation can be done in the presence or absence of the dye, depending on the purpose of the assay and the availability of tau protein. Both modes of reactions are shown below. In addition, we demonstrate the operation of two different instruments &#8211; a single-sample fluorometer (ISA-SPEX FluoroMax-2) and a multi-plate reader (SpectraMax M2). Readers should be able to adapt these protocols to suit their specific needs and instrument availability.

<table border="0" cellpadding="0" cellspacing="0" width="449">
	<tbody>
		<tr height="64">
			<td height="64" style="height:64px;width:116px;">Name of the reagent</td>
			<td style="width:128px;">Company</td>
			<td style="width:103px;">Catalogue number</td>
			<td style="width:103px;">Comments</td>
		</tr>
		<tr height="43">
			<td height="43" style="height:43px;width:116px;">Trizma&#160;base</td>
			<td style="width:128px;">Sigma</td>
			<td style="width:103px;">T1503</td>
			<td style="width:103px;"></td>
		</tr>
		<tr height="85">
			<td height="85" style="height:85px;width:116px;">NaCl</td>
			<td style="width:128px;">Macron Fine Chemicals</td>
			<td style="width:103px;">MAL-7581-06</td>
			<td style="width:103px;"></td>
		</tr>
		<tr height="106">
			<td height="106" style="height:106px;width:116px;">Ethylenediaminetetraacetic Acid (EDTA)</td>
			<td style="width:128px;">Invitrogen</td>
			<td style="width:103px;">15576-028</td>
			<td style="width:103px;"></td>
		</tr>
		<tr height="43">
			<td height="43" style="height:43px;width:116px;">Thioflavin T</td>
			<td style="width:128px;">Sigma</td>
			<td style="width:103px;">T3516</td>
			<td style="width:103px;">Stored in dark</td>
		</tr>
		<tr height="43">
			<td height="43" style="height:43px;width:116px;">Thioflavin S</td>
			<td style="width:128px;">Sigma</td>
			<td style="width:103px;">T1892</td>
			<td style="width:103px;">Stored in dark</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:116px;">heparin</td>
			<td style="width:128px;">Sigma</td>
			<td style="width:103px;">H3393</td>
			<td style="width:103px;"></td>
		</tr>
		<tr height="85">
			<td height="85" style="height:85px;width:116px;">DL-Dithiothreitol (DTT)</td>
			<td style="width:128px;">Sigma</td>
			<td style="width:103px;">D9779</td>
			<td style="width:103px;">Stored at 4&#176;C</td>
		</tr>
		<tr height="43">
			<td height="43" style="height:43px;width:116px;">96-well plate</td>
			<td style="width:128px;">Corning</td>
			<td style="width:103px;">3917</td>
			<td style="width:103px;"></td>
		</tr>
		<tr height="64">
			<td height="64" style="height:64px;width:116px;">ISA SPEX FluoroMax-2</td>
			<td style="width:128px;">Horiba</td>
			<td style="width:103px;"></td>
			<td style="width:103px;"></td>
		</tr>
		<tr height="148">
			<td height="148" style="height:148px;width:116px;">SpectraMax M2 Multi-Mode Microlate Reader</td>
			<td style="width:128px;">Molecular Devices</td>
			<td style="width:103px;"></td>
			<td style="width:103px;"></td>
		</tr>
		<tr height="106">
			<td height="106" style="height:106px;width:116px;">Mouse Anti-Tau Monoclonal Antibody</td>
			<td style="width:128px;">R&#38;D Systems</td>
			<td style="width:103px;">MAB3494</td>
			<td style="width:103px;">Stored at &#8211;80&#176;</td>
		</tr>
	</tbody>
</table>

in vitro aggregation assays using hyperphosphorylated tau protein

Alzheimer's disease is one of a large group of neurodegenerative disorders known as tauopathies that are manifested by the neuronal deposits of hyperphosphorylated tau protein in the form of neurofibrillary tangles (NFTs). The density of NFT correlates well with cognitive impairment and other neurodegenerative symptoms, thus prompting the endeavor of developing tau aggregation-based therapeutics. Thus far, however, tau aggregation assays use recombinant or synthetic tau that is devoid of the pathology-related phosphorylation marks. Here we describe two assays using recombinant, hyperphosphorylated tau as the subject. These assays can be scaled up for high-throughput screens for compounds that can modulate the kinetics or stability of hyperphosphorylated tau aggregates. Novel therapeutics for Alzheimer's disease and other tauopathies can potentially be discovered using hyperphosphorylated tau isoforms.

Using recombinant tau and p-tau (Figure 1), we established two different protocols to compare the kinetics of aggregation of tau and p-tau, taking advantage of the strong fluorescence emission of ThT and ThS upon binding to amyloidal protein aggregates, including tau and p-tau (Figure 2). With or without the fluorescent dye in the aggregation reaction, we observed consistent enhancement of tau aggregation by hyperphosphorylation (Figures 3&#8211;5). This stimulation is i...

Watch this Scientific Journal Video about In Vitro Aggregation Assays Using Hyperphosphorylated Tau Protein at JoVE.com

In Vitro Aggregation Assays Using Hyperphosphorylated Tau Protein

Unmodified and hyperphosphorylated tau proteins were used in two in vitro aggregation assays to reveal the hyperphosphorylation-dependent fast aggregation kinetics. These assays pave the way for future screens for compounds that can modulate the propensity of hyperphosphorylated tau to form fibrils that underlie the progression of Alzheimer&#8217;s disease.

In Vitro Aggregation Assays Using Hyperphosphorylated Tau Protein

Alzheimer's disease is one of a large group of neurodegenerative disorders known as tauopathies that are manifested by the neuronal deposits of ...

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