Matrix pre-coated targets for high throughput MALDI imaging of proteins

Abstract

We have developed matrix pre-coated targets for imaging proteins in thin tissue sections by matrix-assisted laser desorption/ionization mass spectrometry. Gold covered microscope slides were coated with sinapinic acid (SA) in batches in advance and were shown to be stable for over 6 months when kept in the dark. The sample preparation protocol using these SA pre-coated targets involves treatment with diisopropylethylamine (DIEA)-H2 O vapor, transforming the matrix layer to a viscous ionic liquid. This SA-DIEA ionic liquid layer extracts proteins and other analytes from tissue sections that are thaw mounted to this target. DIEA is removed by the immersion of the target into diluted acetic acid, allowing SA to co-crystallize with extracted analytes directly on the target. Ion images (3-70 kDa) of sections of mouse brain and rat kidney at spatial resolution down to 10 µm were obtained. Use of pre-coated slides greatly reduces sample preparation time for matrix-assisted laser desorption/ionization imaging while providing high throughput, low cost and high spatial resolution images.

Keywords: MALDI IMS; imaging; pre-coated; proteins; target.

Figure 1

Sample preparation with a sinapinic acid pre-coated slide (Yellow: SA; green: SA-DIEA-H₂O)

Figure 2

(A) 3 μm thick chicken liver sections placed on slides with different amounts of SA coating; (B) different thicknesses of rat brain sections imprinted on SA pre-coated slides with a coating of 2.7 mg/cm². Average spectra were taken from 100 locations on the tissue.

Figure 3

Average spectra from a rat kidney from the images in Figure 4. (A) 3–30 kDa; (B) 30–70 kDa. These were acquired from two serial sections.

Figure 4

Representative m/z ion images from 3–30 and 30–70 kDa from two serial sections of a rat kidney (scale bar: 2 mm). M/z values are noted below the images.

Figure 5

Ion images at 10 μm spatial resolution from a mouse brain section. M/z values are noted below the images. Scale bars: 200 μm

Figure 6

Reproducibility and stability of SA pre-coated targets. Four serial sections of a rat brain were imaged at 150 μm spatial resolution using SA pre-coated slides stored for 1, 2, 30 and 180 days. M/z values are noted above each column.