The role of syndecan-3 in bone metabolism

The skeleton is an important organ in the body of all vertebrates including man. It gives the body its shape, protects internal organs from injury, allows for movement, stores the bone marrow, where the blood cells are produced and provides minerals (especially calcium and phosphate) for the function of all cells in the organism. In order that all these tasks are accomplished bones have to be strong throughout life. Thus, bone tissue has the ability to repair and adapt to changes in its environment, for example expand and toughen in areas of greater mechanical load.
This ability is due to the function of specialised bone cells: firstly osteoclasts, which resorb (or dissolve) damaged or unwanted bone and secondly osteoblasts, which come along and form (or build) new bone to fill the gaps. This process normally starts with osteoclasts, which seem to guide osteoblasts as to where to deposit new bone (known as the bone remodelling cycle). In the overall control of this process, osteocytes, the most abundant bone cells (and which sense mechanical loading) seem to play a prominent role. Unfortunately, with ageing bone remodelling deteriorates which leads to bone loss, weakening of bone and eventually bone fragility (osteoporosis).
Interestingly, a separate process of bone formation, independent of prior bone resorption (modelling-based formation, in contrast to the above described re-modelling) has been observed decades ago, but so far understudied. This modelling-based bone formation occurs during the development and growth of bones. During adulthood it is thought to play an important role in the adaptation of bone to mechanical loading, such that osteoblasts make bone where it is necessary. The question arises, however, how do osteoblasts know where to start making new bone without the guidance from osteoclasts?
There are a number of communication pathways that cells use for passing information between one another, known as signalling pathways. One such pathway is Notch-signalling, which in bone promotes modelling-based bone formation. Muscle scientists have found that Notch-signalling needs an additional receptor (on/off switch) called Syndecan-3. Interestingly, we have made an observation that mice engineered to lack Syndecan-3 (Syndecan-3 knock-out mice) develop low bone mass at 3 months of age and that this worsens at 6 months of age. We do currently not know whether this is due to a defect in bone growth, low adult peak bone mass, or premature age-related bone loss, however it suggests that Syndecan-3 is important not only for normal functioning of muscle cells, but also of bone cells. On the basis of the known Syndecan-3 and Notch interaction in muscle cells we think that a similar mechanism may be operating in osteoblasts, which express Syndecan-3. Syndecan-3, responds to a number of other signalling molecules important in bone formation, such as HB-GAM and FGF2, which are produced by bone cells. In this project we plan to study these interactions in detail.
The overall aim of this proposal is to understand how Syndycan-3 regulates bone development and function, and the underlying molecular mechanisms. To achieve this we will study the architecture of bone and bone turnover in Syndycan-3 KO mice at a range of ages from early postnatal, to 12-month old mice, by a number of specialised imaging techniques (including uCT) and by measuring markers of bone turnover. We will investigate the role of Syndycan-3 in mechanical loading-induced bone formation, a process that involves bone modelling. We will use osteoblast cultures to study Syndycan-3 involvement in osteoblast differentiation and function, as well as signalling of important osteoblast regulators: HB-GAM, FGF2 and Notch. In summary, these studies will increase our understanding of modelling based bone formation, and may identify Sdc3 mediated pathways as targets to help devise drugs promoting bone formation.

In ageing the balance between bone formation and resorption is skewed towards bone resorption, resulting in a decrease in bone mass and strength leading to fragility fractures and deformities. With increasing life-expectancy and an ageing population, there is a clear need to understand the mechanisms underlying age-related bone diseases. Syndecan-3 (Sdc3) is a transmembrane heparan sulfate proteoglycan receptor involved in the development of the central nervous and the muscular systems, and previous research suggests that Sdc3 may regulate signalling pathways important in bone homeostasis. In pilot studies we have found that adult mice lacking the Sdc3 gene (Sdc3 KO) are characterised by a 40% decrease in trabecular bone volume. However, we do currently not know whether this is due to a defect of bone metabolism during bone growth, low adult peak bone mass, or premature age-related bone loss. The aim of this proposal is to understand how Sdc3 regulates bone development and homeostasis. We will study bone architecture and metabolism in Sdc3 KO mice from early postnatal, to 12-month old osteopenic mice, by microCT, dynamic bone histomorphometry and measuring bone turnover markers. We will investigate the role of Sdc3 in mechanical loading induced bone formation by performing in vivo loading studies of the tibia. To determine whether the bone defect is due to a direct effect on bone formation we will a Sdc3 KO mouse with rescue of expression in the osteoblast lineage. We will study Sdc3 involvement in osteoblast differentiation and bone matrix deposition and mineralisation, as well as signalling of important osteoblast regulators: HB-GAM, FGF2 and Notch in tissue culture. These studies will increase our understanding of bone metabolism, and may identify Sdc3 mediated pathways as targets for novel bone anabolic strategies.

Age related osteoporosis poses an increasing burden on the health services in an ageing population such as the UK's. However, the mechanisms underlying age-related bone loss are incompletely understood. The role of Sdc3 in maintenance of bone homeostasis and age-related bone loss may indicate new pathways for targeting, especially age-related, bone loss, and the knowledge generated by the experiments described here may, over a longer time period, allow development of new therapeutic, in particular anabolic, interventions. Bisphosphonate treatment is effective in reducing bone resorption, but compliance is poor and prolonged use of these drugs is associated with atypical bone fractures, possibly due to suppression of bone renewal through the remodeling process. Duration of treatment is therefore limited to 5-10 years.
There is currently only one anabolic treatment available, teriparatide, which use is limited to 2 years because of concerns about the risk of osteosarcomas. Another anabolic, anti-sclerostin antibody, romosozumab, is still in phase 3 trials, whereas a cathepsin-K inhibitor, odanacatib (which preserves bone remodelling, increases modelling and reduces fracture risk) has recently been withdrawn due to increased risk of cardiovascular events. Thus, long term treatment of age-related osteoporosis is therefore still limited and sub-optimal and there is a need for new anabolic agents. Especially if we can show that Sdc3-mediated signals can stimulate bone modelling (bone formation independent of bone resorption), treatment based on this pathway may be of great benefit to patients, including patients previously treated with bisphosphonates, as these drugs exert long term inhibition of bone resorption and thereby suppress remodeling based bone formation. Moreover, Sdc3 mediated signaling is an attractive pathway to explore in the broader context of musculoskeletal disorders because of the role Sdc3 plays in muscle (especially regeneration) and (yet unsolved, but obvious from the Sdc3 KO) in bone. Potentially one could envisage targeting this pathway in both age related bone loss and sarcopenia in future studies.
The development of alternative effective treatment strategies specifically targeted to age-related osteoporosis and suitable for long term treatment would therefore be of great benefit to both the health services and individual patients. A reduction in fracture rates in the elderly would reduce hospitalisation and disability. This could potentially lead to increased independent living in older age, and increased mobility leading to improved cardiovascular health. Reduced costs of care for elderly patients immobilised due to age-related bone fractures, would benefit the UK economy as a whole.