Spatial segmentation and metabolite annotation involved in sperm maturation in the rat epididymis by MALDI imaging mass spectrometry

Spermatozoa acquire their fertilizing capacity during a complex maturation process that occurs in the epididymis. This process involves a substantial molecular remodeling at the surface of the gamete. Epididymis is divided into three regions (the caput, corpus, and cauda) or into 19 intraregional segments based on histology. Most studies carried out on epididymal maturation have been performed on sperm samples or tissue extracts. Here, we used MALDI imaging mass spectrometry in the positive and negative ion modes combined with spatial segmentation and automated metabolite annotation to study the precise localization of metabolites directly in the rat epididymis. The spatial segmentation revealed that the rat epididymis could be divided into several molecular clusters different from the 19 intraregional segments. The discriminative m/z values that contributed the most to each molecular cluster were then annotated and corresponded mainly to phosphatidylcholines, sphingolipids, glycerophosphates, triacylglycerols, plasmalogens, phosphatidylethanolamines, and lysophosphatidylcholines. A substantial remodeling of lipid composition during epididymal maturation was observed. It was characterized in particular by an increase in the number of sphingolipids and plasmalogens and a decrease in the proportion of triacylglycerols annotated from caput to cauda. Ion images reveal that molecules belonging to the same family can have very different localizations along the epididymis. For some of them, annotation was confirmed by on-tissue MS/MS experiments. A 3D model of the epididymis head was reconstructed from 61 sections analyzed with a lateral resolution of 50 μm and can be used to obtain information on the localization of a given analyte in the whole volume of the tissue.