Comparison between thaw-mounting and use of conductive tape for sample preparation in ToF-SIMS imaging of lipids in Drosophila microRNA-14 model

Abstract

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) imaging elucidates molecular distributions in tissue sections, providing useful information about the metabolic pathways linked to diseases. However, delocalization of the analytes and inadequate tissue adherence during sample preparation are among some of the unfortunate phenomena associated with this technique due to their role in the reduction of the quality, reliability, and spatial resolution of the ToF-SIMS images. For these reasons, ToF-SIMS imaging requires a more rigorous sample preparation method in order to preserve the natural state of the tissues. The traditional thaw-mounting method is particularly vulnerable to altered distributions of the analytes due to thermal effects, as well as to tissue shrinkage. In the present study, the authors made comparisons of different tissue mounting methods, including the thaw-mounting method. The authors used conductive tape as the tissue-mounting material on the substrate because it does not require heat from the finger for the tissue section to adhere to the substrate and can reduce charge accumulation during data acquisition. With the conductive-tape sampling method, they were able to acquire reproducible tissue sections and high-quality images without redistribution of the molecules. Also, the authors were successful in preserving the natural states and chemical distributions of the different components of fat metabolites such as diacylglycerol and fatty acids by using the tape-supported sampling in microRNA-14 (miR-14) deleted Drosophila models. The method highlighted here shows an improvement in the accuracy of mass spectrometric imaging of tissue samples.

Biological tissue sample preparation for time-of-flight secondary ion mass spectrometry (ToF–SIMS) imaging

“…According to the authors, this thermal conductivity difference prevents cholesterol migration during freeze-drying when the tissue is mounted with adhesive tape, so there is no difference in the ion signal intensity between the measurement temperatures as shown in Fig. 2 b and e. ToF–SIMS imaging results by Uyen et al [ 84 ] also support this explanation. Positive ion mass spectral images of a Drosophila head section prepared by the adhesive tape-supported and freeze-dried methods were observed to be very similar to the images obtained from frozen hydrate analysis as shown in Fig.…”

“…Analyte relocation has also been observed in samples other than rat brain tissue. Uyen et al [ 84 ] performed ToF–SIMS imaging of Drosophila head sections that were prepared by the two mounting methods and freeze-dried using the same method. In order to prevent charge accumulation during analysis, a conductive double-sided carbon tape was used for mounting.…”

“…MC stands for maximal counts (Reproduced from ref. [ 84 ] with permission, © John Wiley & Sons, Ltd)

Fig. 4 ToF–SIMS images of corn seed tissue prepared by the adhesive tape-supported and thaw-mounted methods.

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“…Ion signal intensity is the most critical factor in ToF–SIMS image quality and is dependent on the ionization efficiency of the analytes. To increase ionization efficiency, additional aiding compounds are applied to the tissue sample [ 89 , 90 ], or GCIB [ 11 , 12 , 50 – 52 , 82 – 84 ] may be used to enhance the survival yield of the molecular ions with high molecular weights or fragile biological polymers.…”

Mass spectrometry imaging: An emerging technology for the analysis of metabolites in insects

“…Lipids play a fundamental role in a number of biological functions, such as maintenance of cell membrane structure, energy storage, intracellular and extracellular signaling, insect pheromones, chemical communication, and social behavior (Beenakkers, van der Horst, & van Marrewijk, 1981;Gilbert & Chino, 1974). D. melanogaster, as a biological model insect, has been widely used in many perspectives of MSI, such as three-dimensional spatial distribution of neutral lipids on the surface of adults (Kaftan et al, 2014), lipid distribution in the body (Niehoff et al, 2014), lipid structure in brain (Le et al, 2018;Phan, Munem, Ewing, & Fletcher, 2017), lipids of the wing (Marty et al, 2017;Vrkoslav, Muck, Cvačka, & Svatoš, 2010), and phospholipid distribution of the malpighian tubules (E. Yang, Gamberi, & Chaurand, 2019). In addition to D. melanogaster, MSI has also been used to detect and localize phospholipid, phosphatidylcholine, or triacylglyceride composition and distribution in the mosquito (Anopheles stephensi or Aedes aegypti; Castellanos et al, 2019;Khalil, Römpp, Pretzel, Becker, & Spengler, 2015), and phospholipids in locusts (Scistocerca gregaria; Olsen et al, 2015).…”