Evolution of SWATH® Acquisition provides large gains in quantified proteins
Overview of key methodology improvements on TripleTOF® Systems
Christie Hunter1, Nick Morrice2, Joerg Dojahn3
1SCIEX, USA, 2SCIEX, UK, 3SCIEX, Germany
The data independent acquisition strategy termed SWATH Acquisition was first described1 and commercialized in 2012. The acquisition strategy used was to step 25 Da Q1 windows across the peptide mass range, collecting high resolution MS/MS data in each step. Then the resulting data was interrogated using a targeted data extraction strategy guided by a small ion library.
Since the first description of the technique, there has been a lot of progress made by the SWATH Acquisition user community in better understanding the best way to collect and process data for large scale protein quantitation experiments. Innovations in data acquisition, improvements in instrument hardware, advances in library generation and large increases in scale of studies have all resulted in large improvements, resulting in the workflow used today.
Here the improvements made due to the various method improvements and quantified and summarized. Holding much of the parameters constant (LC, instrument, sample load) and running the various acquisition methods back to back, a good measure of the impact of the techniques can be obtained (Figure 1). Comparing to the method used in 2012 (34 fixed window (fx) x 25Da with a small library), significant gains (>100% protein and 300% peptides quantified) are observed when comparing to results obtained by using a 100 variable window (vw) approach with a large ion library.
Key advances in SWATH® Acquisition
- Variable Q1 Window SWATH Acquisition2
- Specificity of detection and quantitation can be significantly improved through using more, smaller Q1 isolation windows
- By optimizing window width based on m/z density of precursors provides further specificity gains while allowing for broad mass range coverage
- Instrumentation3 – the ability to collect high resolution MS/MS (~30 000 resolution) at high acquisition rates with good dynamic range is key for DIA
- Library generation4,5 – 2 dimensional fractionation for generating deeper ion libraries is important to maximize the quantitative information extracted from SWATH Acquisition data
- Industrialization of workflow through using microflow chromatography provides high robustness and throughput for performing larger scale studies6
Chromatography: Separation of a trypsin digest of human cell lysate was performed on a NanoLC™ 425 System (SCIEX) operating in trap elute mode at microflow rates. A 0.3x150 cm ChromXP™ column (SCIEX) was used with a short gradient (4-32% acetonitrile in 43 min in 0.1% formic acid) at 5 µL/min (total run time 57min6). Total protein digest injected on column was 5 µg.
Mass spectrometry: The MS analysis was performed on multiple TripleTOF® 6600 and 5600 systems (SCIEX), each equipped with Turbo V™ Sources and 25 μm I.D. hybrid electrodes (SCIEX). Variable window SWATH Acquisition methods were built using Analyst® TF Software 1.7. Four different SWATH Acquisition methods were built with different window strategies for comparison (Table 1).
Data processing: Replicate injections of each acquisition condition were processed using SWATH® 2.0 in PeakView® Software 2.2 using 3 different libraries (Table 2). Shared peptides were specifically excluded from quantitation. Results analysis was performed in Excel using the SWATH Acquisition Replicates template7. All protein and peptide numbers reported were determined at <1%FDR and <20% CV across the 5 SWATH acquisition replicates collected.
More, smaller windows significantly improve specificity
One of the most significant improvements to the SWATH Acquisition strategy is the use of variable sized Q1 isolation windows. By optimizing the size of the window based on the precursor m/z density2, much smaller windows can be used in dense m/z windows, significantly improving the specificity of the data for those targeted peptides. In addition, larger windows can be used across the higher m/z region where far fewer peptide precursors are observed to ensure that proteome coverage is maintained. To test this, we set up a series of experiments on multiple TripleTOF systems where we used increasingly smaller Q1 windows and compared the data to the original 25 Da window strategy (Table 1). After data extraction, we compared the XICs for the same peptide across the different methods to observe the impact on data specificity (Figure 2). There were many cases where significantly improved XIC specificity was observed in the 100 variable window method over the original 25 Da window method.
Improved peptide quantitation with large number of windows
As expected, this had a large impact on the number of peptides and proteins that could be quantified reproducibly from the replicate datafiles. Comparing just the data using a single large library (PHL, Figure 3, solid bars), there is a ~70-100% gain in the number of peptides quantified between the 25 Da window method (34 windows) and the 60 variable window method. Even more improvements are observed in quantitation (~110-150% peptide increase) when comparing to the 100 variable window strategy. These significant gains are mainly due to the improved S/N obtained on the quantitative XICs on so many of the peptides (Figure 2).
Interestingly, if smaller libraries were used like the 1D library, much smaller gains were seen across the different method (Figure 3, dotted bars). Only 40-50% more peptides were observed in the 60 VW method and ~70-80% more peptides in the 100 VW method over the 25 Da method. To look at this in more detail, a plot of library size vs method was done (Figure 4) and this highlights that when using 60 or 100 VW window SWATH Acquisition methods, it is recommended that larger libraries are also used to fully leverage the amount of quality quantitative data obtained.
The gains were reasonably consistent between the TripleTOF 6600 and 5600 Systems, confirming that the 100 variable window strategy is the optimal strategy for analyzing complex proteomes using SWATH Acquisition.
SWATH Acquisition in comparison to other DIA strategies
Finally, an additional acquisition method was tested, a method strategy used on other accurate mass systems where high resolution MS/MS acquisition rates are not as high as TripleTOF systems. Here a typical method with 20 fixed windows of 20 Da width is used to interrogate a limited mass range centered around the peptide m/z distribution (500-900m/z). This reduced mass range had to be used, in order to keep the cycle time short to acquire enough data points across the peak for proper quantitation. The concept is that enough peptides are found in the more narrow range to represent most proteins. However in direct comparison, this method was still found to significantly underperform relative to using the 60 or 100 variable window strategy.
The SWATH Acquisition data independent acquisition strategy has been rapidly adopted by researchers needing robust, comprehensive protein quantitation data for their biological and biomarker research.
Significant gains in data quality have been achieved over the last 5 years since launch of the workflow, through innovations in hardware, acquisition methods and library developments. Comparing to the original implementation of the SWATH Acquisition method (34fx x 25Da with a small 1D library), significant gains (>100% protein and 300% peptides quantified) are observed when comparing to results obtained by using a 100 variable window approach with a large ion library.