Imaging of Corticosteroids in Mouse Kidney Using Mass Spectrometry.

Aldosterone regulates renal sodium reabsorption which is important for blood pressure homeostasis. Glucocorticoids may also contribute to this process although this is less well understood. Analysis of steroid hormones and the metabolites in the plasma and urine gives insights into corticosteroid biology but steroid concentrations at a cellular level are largely unknown, and the kidney remains a "black box". Mass spectrometry imaging (MSI) permits localisation of steroids in histological zones based on regional markers. This approach has been previously applied to localise steroids in brain and testes, and here is applied to kidney. Our aim was to map and quantify glucocorticoids and aldosterone in different histological zones (cortex, medulla) of kidneys from mice receiving different dietary salt, to provide new information relevant to the physiological actions of these hormones. Cryosections kidney from male C57BL6 mouse (age 12 weeks, n=6: dietary salt low 0.03% vs normal 0.3% vs high 3%) were subject to MSI analysis following Girard T reagent derivatisation and α-cyano-4-hydroxycinnamic acid matrix application. Matrix assisted laser desorption/ionisation (MALDI) was used as a sampling method, coupled to Fourier Transform Ion cyclotron mass spectrometry. Ions with m/z 458.3010 (Δppm=0.65), 460.3166 (Δppm=0.65), and 474.2957 (Δppm=1.05) were detected, using MALDI, in renal sections for derivatives of 11-dehydrocorticosterone, corticosterone and aldosterone respectively. Untargeted evaluation of ions was conducted to find regional markers that would allow definition of kidney histological zones. Heat maps indicated that corticosterone intensity was higher in the inner cortex than the rest of the kidney. In contrast 11-dehydrocorticosterone, a metabolite produced by the enzymic activity of 11β-hydroxysteroid dehydrogenase type II, was detected at highest levels in the medulla. The aldosterone signal was equally strong in medulla and outer cortex. Steroid localisation after treatment with different salt intakes indicated that the intensity of the corticosterone signal increases in the outer cortex in kidneys from mice receiving low salt diet, while 11-dehydrocorticosterone was not affected. The intensity of the aldosterone signal within the kidney was not changed by different salt intakes. Our approach shows that it is feasible to image corticosteroids at 75μm spatial resolution within the kidney. This technology provides new insights into the physiological control of sodium transport by steroids and opens doors to understanding changes in disorders of blood pressure.