This three-year P&F study will examine the role of Lrp5/6 and Wnt signaling in the determination ofbone density and the feasibility of modulating this function with vitamin supplementation. We have recentlyidentified a missense mutation in a spontaneously arising mouse mutant named Crooked tail (Cd) thatoccurs in the low-density lipoprotein receptor related protein 6 (Lrp6) (Carter et al., submitted). Biochemicaland cell biological data indicate that the Lrp6Cd mutant protein supports Wnt canonical signaling butcannot be antagonized by dikkopf1 (Dkk1), resulting in a net Wnt hyperactivity. Crooked tail (Cd) is a semidominantspontaneous mouse mutation in which heterozygous animals have a crooked tail whilehomozygous mice display a pleiomorphic phenotype that includes early embryonic lethality (25-30% ofCd/Cd embryos), the cranial neural tube defect (NTD) exencephaly (20-25%), or viable but runted pupswith more prominent tail and lumbar skeletal defects in those Cd/Cd mice that close the neural tube(Morgan, 1954;Carter et al., 1999). Preliminary data suggest that mechanical bone strength is increased inLrp6Cd/+ mice. We have demonstrated in a controlled dietary study that folic acid (FA) supplementation ofthis strain begun prior to conception can rescue the early lethality and NTD in a manner that closelyparallels clinical studies (Carter et al., 1999). While our primary focus of the RO1-funded investigations ofthis mouse is on neurulation and brain development in Cd, the skeletal defects are extremely interesting,adding another piece to the puzzle of Wnt signaling contributions to human disorders involving the closefamily member, Lrp5, that affect bone density. Moreover, as neuroscientists, my lab lacks the expertise andessential tools to study the skeletal aspects of Lrp6 function and the Cd mutation.The P&F studies will establish the baseline material properties, cellular composition and strength ofbone in Lrp6Cd+/- (putative hyperactive allele), Lrp6+/- (null allele) and Dkk1 hypomorphic d/d (henceLrp5/6 hyperactive) mice. They will determine whether Dkk1 is a primary antagoinst acting during bonedevelopment and homeostasis-therefore whether Dkk1 is a rational target for therapeutic intervention. TheP&F studies will further determine whether FA can restore bone homeostasis in Lrp6Cd/+ and/or Lrp6+/-mice and if so, whether this is associated with an alteration in absolute numbers or ratio ofosteoblast/osteoclast cells.If funded, this P&F study would provide invaluable resources for gathering essential data andexperience through the Imaging, Biomechanical and Biomaterial Assessment and Analytical MicroscopyCores of the Center that could lead to a successful grant submission through another mechanism.
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