Porous Graphitic Carbon-Based Imprint Mass Spectrometry Imaging with an Ambient Liquid Extraction Technique for Enhancing Coverage of Glycerolipids and Sphingolipids in Brain Tissue

Abstract

Localization of lipidomes and tracking their spatial changes by mass spectrometry imaging (MSI) is critical for the mechanism studies on living process, disease, and therapeutic treatment. However, due to the strong ion suppression in complex biotissue, the imaging coverage for lipids with low polarity or low abundances, such as glycerolipids and sphingolipids, is usually limited. To address this issue, we utilized a porous graphitic carbon (PGC) material to imprint brain tissue sections for selective enrichment of neutral lipids with polar phospholipids removed. Then, the tissue imprint was scanned for desorption by the ambient liquid extraction MSI system. It was found that on the PGC surface, hydrophobic interaction dominates in protic solvents, and polar interaction dominates in aprotic solvents. Accordingly, methanol was selected as the spray solvent for tissue imprinting, and 75% acetonitrile–methanol was selected as the desorption solvent for the ambient liquid extraction MSI system. The results showed that glycerides had high recoveries after the imprinting–desorption process (recovery ∼ 70%) with phospholipids eliminated (recovery < 7%). To increase the transferring efficiencies of lipids from tissue onto PGC, electrospray was used for solvent application during imprinting, and the signals of diglycerides (DGs) in the imprint MSI of brain tissue increased by 2–3 times as compared to those via air spray. Finally, the new imprint MSI approach was applied to the imaging of the rat cerebellum and was compared with direct tissue MSI. The results showed that with imprint MSI, the coverage of DGs, sphingomyelins (SMs), and ceramides was enhanced by 4–5-fold (32 vs 6, 4 vs 1, and 5 vs 0). The ion images showed that with imprint MSI, higher signal intensities and clearer spatial distribution of DGs and SMs were obtained without interference from phosphatidylcholine signals compared with tissue MSI. This new method provides a complementary approach for traditional MSI to address the issues in imaging poorly ionizable or low-abundance lipids.

Lewis Acidic Metal–Organic Framework Assisted Ambient Liquid Extraction Mass Spectrometry Imaging for Enhancing the Coverage of Poorly Ionizable Lipids in Brain Tissue

“…When the thickness increased to 10 μm, the time for completely removing phospholipid slightly increased, and the S4, and the imaging signal−time curves and the path map of the imaging referred to the published work of our group. 18 After the background peaks were removed from the blank glass slide and the bare Zr 6 OTf-BTB sheet (Figure S5), the peaks detected by the two methods in mouse brain imaging were identified by searching their MS or MS/MS spectra in the public databases. The results are shown in Tables S2 and S3 and Figure S6.…”

“…The sample sheet was placed on a triaxial platform, and the single probe was fixed on the triaxial platform with its tip perpendicular to the sample surface. The fabrication of the single probe refers to our previous work . A syringe pump was used to deliver 1% FA-MeOH as extraction solvent into the probe at a flow rate of 5 μL·min –1 , whereas a vacuum pump was used to aspirate the solvent into the probe, forming a stable liquid junction between the probe tip and sample surface.…”

“…The fabrication of the single probe refers to our previous work. 18 A syringe pump was used to deliver 1% FA-MeOH as extraction solvent into the probe at a flow rate of 5 μL•min −1 , whereas a vacuum pump was used to aspirate the solvent into the probe, forming a stable liquid junction between the probe tip and sample surface. The distance between the single probe and the sample was 10 μm.…”

“…Our group recently developed a novel imprint MSI approach by combining porous graphitic carbon imprinting with ambient liquid extraction for selectively enriching glycerolipids and removing phospholipids. 18 The imaging coverage to glycer- olipids was enhanced by 5 times. However, in those approaches, multistep sample preparation processes including imprinting, solvent spraying, and tissue rinsing were inevitable, which would result in low recoveries due to transferring processes and incomplete tissue rinsing.…”

“…However, novel nanomaterials with high surface area and highly selective surface interaction were seldom used in imprint MSI. Our group recently developed a novel imprint MSI approach by combining porous graphitic carbon imprinting with ambient liquid extraction for selectively enriching glycerolipids and removing phospholipids . The imaging coverage to glycerolipids was enhanced by 5 times.…”