Quantitative Mass Spectrometry Imaging of Prostaglandins as Silver Ion Adducts with Nanospray Desorption Electrospray Ionization

Prostaglandins (PG) are an important class of lipid biomolecules that are essential in many biological processes, including inflammation and successful pregnancy. Despite a high bioactivity, physiological concentrations are typically low, which makes direct mass spectrometric analysis of endogenous PG species challenging. Consequently, there have not been any studies investigating PG localization to specific morphological regions in tissue sections using mass spectrometry imaging (MSI) techniques. Herein, we show that silver ions, added to the solvent used for nanospray desorption electrospray ionization (nano-DESI) MSI, enhances the ionization of PGs and enables nano-DESI MSI of several species in uterine tissue from day 4 pregnant mice. It was found that detection of [PG + Ag]⁺ ions increased the sensitivity by ∼30 times, when compared to [PG – H]⁻ ions. Further, the addition of isotopically labeled internal standards enabled generation of quantitative ion images for the detected PG species. Increased sensitivity and quantitative MSI enabled the first proof-of-principle results detailing PG localization in mouse uterus tissue sections. These results show that PG species primarily localized to cellular regions of the luminal epithelium and glandular epithelium in uterine tissue. Further, this study provides a unique scaffold for future studies investigating the PG distribution within biological tissue samples

High-Speed Tandem Mass Spectrometric in Situ Imaging by Nanospray Desorption Electrospray Ionization Mass Spectrometry

Nanospray desorption electrospray ionization (nano-DESI) combined with tandem mass spectrometry (MS/MS), high-resolution mass analysis of the fragment ions (<i>m</i>/Δ<i>m</i> = 17 500 at <i>m</i>/<i>z</i> 200), and rapid spectral acquisition enabled simultaneous imaging and identification of a large number of metabolites and lipids from 92 selected <i>m</i>/<i>z</i> windows (±1 Da) with a spatial resolution of better than 150 μm. Mouse uterine sections of implantation sites on day 6 of pregnancy were analyzed in the ambient environment without any sample pretreatment. MS/MS imaging was performed by scanning the sample under the nano-DESI probe at 10 μm/s, while higher-energy collision-induced dissociation (HCD) spectra were acquired for a targeted inclusion list of 92 <i>m</i>/<i>z</i> values at a rate of ∼6.3 spectra/s. Molecular ions and their corresponding fragments, separated by high-resolution mass analysis, were assigned on the basis of accurate mass measurement. Using this approach, we were able to identify and image both abundant and low-abundance isobaric and isomeric species within each <i>m</i>/<i>z</i> window. MS/MS analysis enabled efficient separation and identification of isomeric and isobaric phospholipids that are difficult to separate in full-scan mode. Furthermore, we identified several metabolites associated with early pregnancy and obtained the first 2D images of these molecules

CB1 expression in the Fallopian tube.

<p>CB1 mRNA expression was lower in the follicular compared to the luteal phase. In Fallopian tube from women with ectopic pregnancy, CB1 mRNA expression was also low (p<0.05).</p

Immunohistochemistry for CB1 protein expression in Fallopian tube.

<p>CB1 protein was expressed in the smooth muscle of the wall (A and B), the smooth muscle of the endothelial vessels (A and C) and in the cytoplasm of the luminal epithelium of the Fallopian tube (A, B and D). There was no evidence of nuclear or stromal expression. EPI = epithelium, STR = stroma, SM = smooth muscle, BV = blood vessel. Scale bar = 50 microns.</p

Allelic frequencies of rs1049353 (1359G/A) and rs806368 (3′UTR) single nucleotide polymorphisms (SNPs) of the <i>CNR1</i> gene in women with ectopic and intra-uterine (surgical termination of pregnancies and miscarriage groups combined) pregnancies.

<p>While rs1049353 GG homozygotes constituted 55 versus 29% of ectopic and intra-uterine cohorts, respectively, differences in genotype distributions were not statistically significant (Likelihood Ratio and Pearson analyses had p = 0.18 and 0.15, respectively). The rs806368 SNP was not informative as all ectopic and 13/14 intra-uterine subject DNAs were TT homozygotes.</p

Demographics for Fallopian tube and endometrial biopsies from women undergoing surgery for benign gynecological conditions.

<p>TAH = total abdominal hysterectomy.</p><p>STAH = sub-total hysterectomy.</p><p>LAVH = laparoscopically-assisted vaginal hysterectomy.</p><p>HMB = heavy menstrual bleeding.</p

Genomic organization of <i>CNR1</i> gene and SNPs.

<p>The <i>CNR1</i> gene is a single exon gene located on chromosome 6q14–15 that is represented by a rectangle box (the orientation of the transcription of the gene is indicated by the arrow). The SNPs genotyped are represented by the vertical lines and their position in base pairs according to their chromosomal location. The names of the <i>CNR1</i> SNPs according to the NCBI database are indicated by the prefix ‘rs’.</p