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ID: 185 (Conflict of Interest: K)

Defizienz der intestinalen alkalischen Phosphatase führt zu signifikanten chronischen Veränderungen des Knochens

F.Kühn1, F.Adiliaghdam2, R.Vasan2, S.Hamarneh2, E.Liu2, Y.Liu2, F.Ko2, M.Wein2, M.Bouxsein2, M.Demay2, R.Hodin2
1Universitätsklinikum der LMU München, München
2Massachusetts General Hospital, Boston

Einleitung

The gut is becoming increasingly recognized as the source of various systemic diseases, and recently it has been linked to bone metabolism and formation via the so-called gut-bone axis. The microbiome and gut-derived mediators such as lipopolysaccharides (LPS) and flagellin are thought to impact bone metabolism and administration of beneficial bacteria (probiotics) such as lactobacillus has been shown to result in higher bone mineralization and greater bone strength. The gut brush border enzyme intestinal alkaline phosphatase (IAP) plays an important role in controlling calcium absorption, inhibiting LPS and other inflammatory mediators responsible for endotoxemia, and, also appears to preserve the normal of gut microbiota. Interestingly, IAP-deficient mice (AKP3-/-) also display a significant decrease in Lactobacillus, the genus shown to be beneficial to bone. Based on this background, we sought to determine the impact of IAP-deficiency on bone formation in mice.

 

Material und Methoden

Transcript levels of IAP in mouse bone were measured using qPCR. Femurs of young (3 months) and old (21 months) IAP-KO mice and their WT counterparts were analyzed by micro-CT scan and histopathology. Serum levels of alkaline phosphatase (AP) as a marker of active bone formation, and, serum calcium and phosphorus levels were measured in IAP-KO and WT mice. In-vitro target cell response upon exposure to blood serum from IAP-KO and WT mice was measured using primary bone marrow macrophages.

Ergebnisse

IAP was not expressed in bone of WT mice. Transcript levels were similar to IAP-KO counterparts (IAP-WT vs. IAP-KO, 56.9 ± 17.7 vs. 41.4 ± 21.3 Relative Expression, p=0.4) and vanishingly low when compared to duodenum expression levels (bone vs. duodenum, 56.9 ± 17.7 vs. 25,430.3 ± 10,884.5 Relative Expression, p=0.01). Histological examination of 3-month old IAP-KO and WT did not show any differences, whereas older IAP-deficient mice showed a very distinct bone phenotype in histology (image 1) and CT-scan (image 2). Younger KO mice did not display any abnormal levels in blood chemistry analysis but older IAP-KO animals showed an isolated increase in AP serum levels, significantly higher than in control animals (IAP-WT vs. IAP-KO, 80 ± 27.4 U/I vs. 453 ± 107.5 U/I, p =0.004). Normal GGT levels ruled out an underlying hepatic pathology causing the AP increase. Finally, to determine to the extent that systemic serum may contribute to inflammation in peripheral organs such as the bone marrow we tested the inflammatory response of target cells exposed to systemic blood serum from IAP-KO and WT mice. Serum from IAP-KO mice induced a significantly higher inflammatory target cell response than serum derived from WT counterparts (p<0.01).

Schlussfolgerung

Through its multiple functions, IAP seems to play a crucial role in connecting the gut to the bone. IAP-deficiency leads to chronic changes in bone formation, probably through dysbiosis and dissemination of pro-inflammatory mediators.