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

item LI, XIANGKAI - Lanzhou University
item YANG, JIAN - Children'S Nutrition Research Center (CNRC)
item MORRIS, JAY - University Of Texas Health Science Center
item HESTER, ASHLEY - Children'S Nutrition Research Center (CNRC)
item Nakata, Paul
item HIRSCHI, KENDAL - Children'S Nutrition Research Center (CNRC)

Submitted to: Journal of Bioequivalence and Bioavailability
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/21/2012
Publication Date: 12/27/2012
Citation: Li, X., Yang, J., Morris, J., Hester, A., Nakata, P.A., Hirschi, K.D. 2012. Genetically modified Medicago truncatula lacking calcium oxalate has increased calcium bioavailability and partially rescues vitamin D receptor knockout mice phenotypes. Journal of Bioequivalence and Bioavailability. 5:47-52.

Interpretive Summary: Oxalate is an antinutrient that locks calcium up in the form of a calcium oxalate crystal. Calcium in foods found in this crystalline form cannot easily be absorbed. Other factors such as the presence of vitamin D also affect our ability to absorb calcium from foods. It has been shown that mice lacking a functional Vitamin D Receptor (VDR) absorb less calcium from foods than wild-type mice. This decrease in calcium absorption results in visible symptoms of calcium deficiencies which include reduced weight gain and bone density. In this report we assess the ability modified plants with increased calcium bioavailability to alleviate the calcium deficiency symptoms observed in VDR mice. The study was performed by feeding VDR mice either a diet of plant material containing crystals of calcium oxalate (wild-type) or plant material that was engineered to have these crystals removed (modified). In order to monitor calcium absorption and utilization both the wild-type and modified plants were grown in the presence of radiolabeled calcium. The ability of the mice to absorb and utilize the calcium from the two diets was determined by measuring the amount of radiolabeled calcium incorporated into the bones of the mice. We found that the leaves from the plant lacking crystals allowed for greater calcium absorption and utilization by the mice compared to the plant containing the crystals. Our study extends our earlier investigations by showing that the plant material without crystals not only had higher calcium bioavailability but could alleviate, in part, the calcium deficiency symptoms exhibited by the VDR mice. Such a finding shows the potential benefits of improving calcium bioavailability in plant foods through genetic manipulation of its oxalate content.

Technical Abstract: How the distribution and sequestered form of plant macro/micro-nutrients influence their bioavailability, and ultimately impact human health, is poorly understood. The legume Medicago truncatula has a portion of its tissue calcium sequestered in the form of the calcium oxalate crystal, which reduces its nutritional value in terms of calcium bioavailability. The calcium oxalate deficient 5 (cod5) mutant has a total calcium (Ca) content similar to wild type (WT) plants, but sequesters less of its tissue calcium in the form of the calcium oxalate crystal. Previous short-term mice feeding studies suggest that this difference is responsible for the improved Ca bioavailability of the cod5 plants compared to WT plants. We performed long-term feeding studies with Vitamin D Receptor Knockout (VDR-KO) mice and the nutritional improved cod5 line to assess the impact of increased Ca bioavailability on VDR-KO calcium deficiency phenotypes. To assess the ability of an improved bioavailable Ca plant food (cod5) to rescue the calcium 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. 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). 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 calcium 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 calcium related disorders.