Dock5, a promising new target against osteolytic bone diseases? – BONEDISEASE

Osteoclasts, the unique body cells able to resorb bone, are an important element of the homeostatic mechanisms maintaining skeletal integrity. This involves resorption of bone by osteoclasts followed by its replacement by osteoblasts. In diverse physiopathological situations: menopause, rheumatoid arthritis, bone metastases, lack of physical activity, disability and genetic disorders, bone resorption exceeds its formation due to excessive activity of osteoclasts. It causes skeletal fragility, higher risk of fractures, disabilities and pain. In particular due to general population aging, the frequency of diseases associated with an osteoporotic skeleton is increasing, which has important health and socioeconomic consequences. The direct costs of osteoporotic fractures were over 30 billion € for 2000 in Europe, expected to increase to over 75 in 2050 based on demographical changes. The high prevalence and considerable functional and socioeconomic impact of osteolytic diseases raise dreadful challenges in the near future and motivate the active ongoing research to improve their treatment. In this context, we identified Dock5 as a new regulator essential for bone resorption. By activating the GTPase Rac downstream of integrin avß3, Dock5 controls the formation of the sealing zone, the osteoclast-specific actin-based adhesion structure essential for bone resorption. Genetic suppression of Dock5 hinders osteoclast activity in vitro and in vivo without affecting their differentiation. Predominantly expressed in osteoclasts, Dock5 is absent in osteoblasts, the bone forming cells. We further identified chemical inhibitors of Dock5 activity that inhibit bone resorption in vitro.

Based on these results, our hypothesis is that Dock5 could be a novel target to develop antiosteolytic treatments. Our project involves Team 1, specialized in osteoclast biology and Rho GTPase signaling and Team 2, expert for structural and biochemical analyses of GTPase exchange factors and the development of inhibitors.
Our first aim is to validate Dock5 as a therapeutic target in the context of pathological inscrease of bone loss. To test if the inhibition of Dock5 can have beneficial effects, we will use the classical model of ovariectomy to induce bone loss in the mouse. We will use Dock5-/- animals or chemical inhibitors of Dock5 we identified. A reduction in bone resorption upon Dock5 inhibition in this model would validate the suppression of Dock5 activity as a potential therapeutic approach against osteoporosis.
Our second aim is to identify the patners of Dock5 in osteoclasts to unravel novel regulatory mechanisms downstream of integrin avß3 and driving actin reorganization to form the sealing zone. We will develop SILAC quantitative proteomic in mature osteocasts to identify Dock5 partners. They will be tested for their role in integrin avß3 signaling and Rac activation in osteoclasts and the regulation of bone resorption. We aim at the identification of novel signaling complexes that act as integrin downstream effectors to regulate actin organization in osteoclasts.
Our third aim is to characterize the structural and biochemical determinants of Dock5 activity and inhibition. We will establish biochemical assays to reconstitute the exchange activity of Dock5 towards Rac on artificial membranes. This will provide key insight into the exchange mechanism of the novel family of Dock exchange factors and allow the analysis of the effects of Dock5 inhibitors. We will set out to solve the crystal structure of Dock5 and its complex with Rac, which would be the first representative of this sub-class of Dock exchange factors. The objective is to understand the mechanism of Rac activation by Dock5 and its inhibition.

Overall, our project will establish the basis to screen, characterize and validate novel inhibitors of osteoclast activity in vitro, in culture and in vivo, an essential starting point for the development of novel antiosteoporotic strategies.