Loss of Intestinal Alkaline Phosphatase Leads to Distinct Chronic Changes in Bone Phenotype.

BACKGROUND: The gut is becoming increasingly recognized as the source of various systemic diseases, and recently, it has been linked to bone metabolism via the so-called gut-bone axis. The microbiome and gut-derived mediators are thought to impact upon bone metabolism, and administration of probiotics has been shown to have beneficial effects in bone. The gut brush border enzyme intestinal alkaline phosphatase (IAP) plays an important role in controlling calcium absorption, inhibiting lipopolysaccharides, and other inflammatory mediators responsible for endotoxemia and appears to preserve the normal gut microbiota. Interestingly, IAP-deficient mice (AKP3-/-) also display a significant decrease in fecal Lactobacillus, the genus shown to be beneficial to bone. MATERIALS AND METHODS: IAP mRNA levels in mouse bone were measured using quantitative real-time polymerase chain reaction. Femurs of IAP-knockout (KO) and wild-type (WT) mice were analyzed by microcomputed tomography and histopathology. Serum levels of alkaline phosphatase, calcium, and phosphorus were measured. Target cell response upon exposure to serum from IAP-KO and WT mice was quantified using primary bone marrow macrophages. RESULTS: IAP was not significantly expressed in bones of WT or KO animals. IAP (alkaline phosphatase 3) expression in bone was vanishingly low compared to the duodenum (bone versus duodenum, 56.9 ± 17.7 versus 25,430.3 ± 10,884.5 relative expression, P = 0.01). Bone histology of younger IAP-KO and WT animals was indistinguishable, whereas older IAP-deficient mice showed a distinctly altered phenotype on histology and computed tomography scan. Younger KO mice did not display any abnormal levels in blood chemistry. Older IAP-KO animals showed an isolated increase in serum alkaline phosphatase levels reflecting an environment of active bone formation (IAP-WT versus IAP-KO, 80 ± 27.4 U/I versus 453 ± 107.5 U/I, P = 0.004). There was no significant difference in serum calcium or phosphorus levels between KO and WT mice. Serum from IAP-KO mice induced a significantly higher inflammatory target cell response. CONCLUSIONS: 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, most likely through dysbiosis and systemic dissemination of proinflammatory mediators.

An Inverse Agonist Ligand of the PTH Receptor Partially Rescues Skeletal Defects in a Mouse Model of Jansen's Metaphyseal Chondrodysplasia.

Jansen's metaphyseal chondrodysplasia (JMC) is a rare disease of bone and mineral ion physiology that is caused by activating mutations in PTHR1. Ligand-independent signaling by the mutant receptors in cells of bone and kidney results in abnormal skeletal growth, excessive bone turnover, and chronic hypercalcemia and hyperphosphaturia. Clinical features further include short stature, limb deformities, nephrocalcinosis, and progressive losses in kidney function. There is no effective treatment option available for JMC. In previous cell-based assays, we found that certain N-terminally truncated PTH and PTHrP antagonist peptides function as inverse agonists and thus can reduce the high rates of basal cAMP signaling exhibited by the mutant PTHR1s of JMC in vitro. Here we explored whether one such inverse agonist ligand, [Leu11 ,dTrp12 ,Trp23 ,Tyr36 ]-PTHrP(7-36)NH2 (IA), can be effective in vivo and thus ameliorate the skeletal abnormalities that occur in transgenic mice expressing the PTHR1-H223R allele of JMC in osteoblastic cells via the collagen-1α1 promoter (C1HR mice). We observed that after 2 weeks of twice-daily injection and relative to vehicle controls, the IA analog resulted in significant improvements in key skeletal parameters that characterize the C1HR mice, because it reduced the excess trabecular bone mass, bone marrow fibrosis, and levels of bone turnover markers in blood and urine. The overall findings provide proof-of-concept support for the notion that inverse agonist ligands targeted to the mutant PTHR1 variants of JMC can have efficacy in vivo. Further studies of such PTHR1 ligand analogs could help open paths toward the first treatment option for this debilitating skeletal disorder. © 2019 American Society for Bone and Mineral Research.