Bone Abstracts

Bone tissue homeostasis requires the coordinated activity of osteoblasts, the bone forming cells, of osteoclasts, the bone resorbing cells, and of osteocytes, generally referred as the bone mechano-sensors. In this contest, osteoblasts are the mesenchymal cells secreting the extracellular matrix components on which hydroxyapatite crystals are then deposited. The most abundant protein of this organic matrix is type I collagen, a heterotrimeric secretory protein, synthesized as procollagen precursor in the endoplasmic reticulum (ER) where it undergoes a series of post translational modification events necessary for proper folding, secretion, extracellular processing and self-assembly in collagen fibers. Mild ER stress in osteoblasts is a physiological event during osteoblasts differentiation, whereas constitutively prolonged and severe ER stress negatively affects osteoblasts homeostasis, causing the activation of unfolded protein response, leading to apoptosis, and ultimately impairing the bone properties. Osteogenesis imperfecta (OI) is a collagen-related heritable disorder affecting several connective tissues, but mainly characterized by skeletal deformity and bone fragility. Together with the dominant forms caused by collagen type I mutations and representing over 85% of OI cases, recessive and X-linked OI have been also described, characterized by defects in proteins involved in collagen type I folding, post-translational modifications, intracellular trafficking, extracellular processing or in proteins important for osteoblasts maturation. Traditionally, the OI bone phenotype was attributed only to the reduced amount or to the presence of structurally abnormal collagen type I in the extracellular matrix. In the last decade it became clear, from our research as well as from data from other groups, that the accumulation of abnormal collagen in the endoplasmic reticulum is responsible for ER stress and indeed is modulating the OI bone outcome. In vitro and in vivo studies using OI models support this conclusion and will be presented and discussed.

Disclosure: Funding from Telethon [grant n. GGP13098] and the European Community, FP7, ‘Sybil’ project [grant n. 602300].

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