Genetically modified Medicago truncatula lacking calcium oxalate has increased calcium bioavailability and partially rescues vitamin D receptor knockout mice phenotypes

Abstract

Background: How the distribution and sequestered form of plant macro/micro-nutrients influence their bioavailability, and ultimately impact human health, are poorly understood. The legume Medicago truncatula has a portion of its tissue calcium (Ca) sequestered in the form of the Ca oxalate (CaOx) crystal which reduces its nutritional value in terms of Ca bioavailability. The calcium oxalate deficient 5 (cod5) mutant has a total Ca content similar to wild-type (WT) plants, but sequesters less of its tissue Ca in the form of the CaOx crystal. Previous shortterm mice feeding studies suggest that this difference is responsible for the improved Ca bioavailability of the cod5 plants compared to WT plants. Objectives: To perform long term feeding studies with Vitamin D Receptor Knockout (VDR-KO) mice and the nutritionally improved cod5 line to assess the impact of increased Ca bioavailability on VDR-KO Ca deficiency phenotypes. Methods: To assess the ability of diets containing cod5 plant material as the sole Ca source to rescue the Ca deficiency phenotypes of the VDR-KO mice we conducted both short term and long term experiments. Specifically, Ca absorption and utilization were measured short term (24-hour) in the hind limb bones and duodenum tissue of VDR-KO mice that were fed either an intrinsically 45Ca labeled cod5 or WT Medicago diet. Long term (20-day) bodyweight gain and change in Bone Mineral Density (BMD) were also measured over a 20 day period in VDR-KO mice fed either a cod5 or WT Medicago diet. Results: In the 24-hour feeding study, 45Ca incorporation was found to be 46.3% (male) or 53.9% (female) higher in hind limb bones (P<0.01); and 32.5% (male) or 38.5% (female) higher in duodenums (P<0.01) in VDR-KO mice fed cod5 than those fed WT plants. In the 20-day feeding study, the VDR-KO mice (male) fed cod5 gained 38.1% more bodyweight than those fed WT plants (P=0.06). The increase of BMD after 20 days in the VDR-KO mice (male) fed cod5 diets was 22.5% higher than those fed WT diets (P=0.17). Conclusions: Our study confirms and extends an earlier study by showing that cod5 Medicago not only had higher Ca bioavailability but it can also rescue, in part, the VDR-KO Ca deficiency phenotypes. Thus, the removal of CaOx from a plant-based diet appears to be a viable long-term dietary option to boost bioavailable Ca levels and help combat Ca related disorders. © 2013 Li X, et al.