Mass spectrometry imaging (MSI) of metals in mouse spinal cord by laser ablation ICP-MS

Abstract

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been developed as a powerful MS imaging (MSI) tool for the direct investigation of element distributions in biological tissues. Here, this technique was adapted for the analysis of native mouse spinal cord cryosections of 3.1 mm × 1.7 mm by implementing a new conventional ablation system (NWR-213) and improving the spatial resolution from 120 μm to 65 μm in routine mode. Element images of the spinal cord are provided for the first time and the metalloarchitecture was established using a multimodal atlas approach. Furthermore, the spatial distribution of Rb was mapped for the first time in biological tissue. Metal concentrations were quantified using matrix-matched laboratory standards and normalization of the respective ion intensities to the average ¹³C ion intensity of standards and samples as a surrogate of slice thickness. The “butterfly” shape of the central spinal grey matter was visualized in positive contrast by the distributions of Fe, Mn, Cu and Zn and in negative contrast by C and P. Mg, Na, K, S and Rb showed a more homogenous distribution. The concentrations averaged throughout grey matter and white matter were 8 and 4 μg g⁻¹ of Fe, 3 and 2 μg g⁻¹ of Cu, 8 and 5 μg g⁻¹ of Zn, 0.4 and 0.2 μg g⁻¹ of Mn. The carbon concentration in white matter exceeded that of grey matter by a factor of 1.44. Zn and Cu at 9 and 4 μg g⁻¹, respectively, were particularly enriched in the laminae I and II, in line with the high synaptic and cellular density there. Surprisingly Zn but not Cu was enriched in the central channel. Rb occurred at 0.3 μg g⁻¹ with a distribution pattern congruent to that of K. The coefficients of variation were 6%, 5%, 8% and 10% for Fe, Cu, Zn and Mn, respectively, throughout three different animals measured on different days. These MSI analyses of healthy wild type spinal cords demonstrate the suitability of the established techniques for investigating diseased or transgenic states in future imaging studies.