Analysis of the molecular and physiological influence of Wnt1 on the skeleton

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The Wnt signaling pathway is emerging as one of the most important regulatory mechanisms controlling bone remodeling. Especially the ligand Wnt1 turned out to play a major role in this context as demonstrated by several studies including our own previous investigations. Mutations of WNT1 in patients cause low bone mass pathologies and deterioration of bone matrix quality that were recapitulated in our respective murine disease models. Furthermore, conditional Wnt1 loss-of-function mouse lines displayed similar phenotypes with increased skeletal fragility. Most importantly, we were able to demonstrate the tremendous osteoanabolic potential of Wnt1 in an inducible mouse model (Wnt1-Tg).Despite these advances, several important questions regarding the influence of Wnt1 on the skeleton remain or have even been raised by the previous investigations: 1) A molecular influence of Wnt1 on cultured osteoblast progenitor cells was only observed in complex with Sfrp1. Recent data indicates that Sfrp1, generally considered to be a Wnt-signaling inhibitor, is required to solubilize Wnt1. Therefore we plan to investigate the necessity of Sfrp1 for the osteoanabolic effect of Wnt1 by cell culture experiments with recombinant proteins and cross breeding Wnt1-Tg with Sfrp1-deficient mice. 2) The main receptor for Wnt1 in the skeleton remains elusive. Importantly, our previous investigations indicate Fzd4 as a potential candidate. We plan to prove or refute this hypothesis by extensive cell culture experiments and cross breeding of Wnt1-Tg with Fzd4-deficient mice. These experiments are set up to be extended to other receptor candidates if necessary. 3) Although Wnt1 has been demonstrated to influence bone material quality in addition to bone mass, the underlying molecular mechanism has not been established. Results from our previous studies demonstrate that Osteomodulin is a downstream target of Wnt1 and that Osteomodulin may be a major effector facilitating the impact of Wnt1 signaling on bone quality. Published in vitro data from other groups show that Osteomodulin can regulate collagen fiber thickness, but no in vivo analyses have been performed to date. Therefore we plan to thoroughly investigate the skeletal phenotype of Osteomodulin-deficient mice and elucidate the connection between Wnt1 and Osteomodulin by cross breeding experiments with Wnt1-Tg mice. Overall, the investigations planned in this grant proposal have a high potential to generate novel and also clinically relevant insights into the molecular mechanisms explaining the influence of Wnt1 on the skeleton. While an understanding of the downstream effectors of Wnt1 may improve treatment of patients suffering from Wnt1-related skeletal pathologies, the identification of the receptor and crucial supportive molecules of the signal transduction pathway may highlight novel targets that can potentially be modulated to treat common skeletal disorders such as osteoporosis.