A Wnt/Shh signaling loop controls intervertebral disc growth and differentiation

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DESCRIPTION (provided by applicant): This project aims to identify the molecular mechanisms of postnatal growth and differentiation of the intervertebral disc (IVD), and how these mechanisms are down-regulated, leading to age-related changes in the IVD. Each IVD consists of a central semi-liquid nucleus pulposus (NP), surrounded by a multi-layered annulus fibrosus (AF), and connected to the bodies of the adjacent vertebral bodies by cartilagenous end plates (EP). Together, these components form a strong joint that resists both tension and compression forces between vertebrae. Degenerative disc disease (DDD) is a major cause of lower back pain, and other neurological symptoms leading to a decreased quality of life. DDD is extremely common, affecting as many as one in seven people. The treatment costs are high, and often include surgical intervention. However, treatment is essentially palliative, since it treats the effects of disc degeneration rather than the causes. The long-term goal of this project is to identify potential biological approaches to therapy, using the same pathways by which the IVD normally grows and differentiates. We have developed the mouse lumbar IVD as a model, and preliminary data shows that it can be used to identify the signaling pathways in the IVD that control postnatal growth and differentiation. We have found that during postnatal growth, the NP acts as a signaling center that controls both growth and differentiation. NP cells express SHH, and blockade of Shh signaling both in vitro and in vivo shows that it is essential for cell proliferation and differentiation of the IVD. NP cells also express several Wnt ligands, which are required to maintain Shh signaling in the IVD. Both these signaling pathways, and the downstream targets of Shh signaling, are down-regulated by the end of the growth period. However, Shh signaling, and the expression of differentiation markers, can be re-activated after the growth period by signals present in fetal bovine serum, or by Wnt agonists, showing that down-regulation of growth and differentiation is not irreversible, and offers potential future targets for therapy. Aim 1 of the project tests the hypothesis that the duration of the postnatal growth period is determined by active Wnt signaling. Aim 2 tests the hypothesis that a feedback loop between Wnt and Shh signaling is established by Wnt inhibitors expressed downstream of Shh signaling. Aim 3 tests the hypothesis that circulating signals control the synchronous growth of all the IVDs by acting on Wnt/Shh signaling loop in the NP. The data from this project will provide the necessary information to explore further the nature of aging of the IVD, the basis of possible biological therapies for disc injury or degeneration, and novel scientific insights into te way growth and differentiation of the IVDs are controlled during postnatal growth.