Targeting TGF-β signaling, oxidative stress, and cellular senescence rescues osteoporosis in gerodermia osteodysplastica

GORAB is a key regulator of Golgi vesicle transport and protein glycanation. Loss of GORAB function in gerodermia osteodysplastica (GO) causes shortening of glycosaminoglycan chains, leading to extracellular matrix disorganization that results in wrinkled skin, osteoporosis and elevated TGF-beta signaling. In this study, we investigated the role of TGF-beta-signaling, oxidative stress, and resulting cellular senescence in the osteoporosis phenotype of GO. Treatment of GorabPrx1 conditional knockouts with the TGF-beta neutralizing antibody 1D11 rescued the trabecular bone loss, indicating that TGF-beta overactivation causes osteoporosis in GO. Using an inducible knockout system, we demonstrated that TGF-beta dysregulation was not a cell-intrinsic effect of GORAB inactivation, but a consequence of a disorganized extracellular matrix. Enhanced TGF-beta signaling caused elevated Nox4 expression in GorabPrx1 mutants and in GO patients' fibroblasts, resulting in overproduction of mitochondrial superoxide. The resulting oxidative stress was detected in GORAB null cells and also in wildtype bystander cells. The same effect was observed in zebrafish after TALEN-mediated gorab inactivation, indicating that the pathway is evolutionarily conserved. Treating Gorab(Prx1) mutants with the antioxidant N-acetylcysteine ameliorated the osteoporosis phenotype. TGF-beta induced oxidative stress coincided with accumulation of DNA damage and elevated expression of senescence markers. Inactivation of Cdkn2a in the Gorab(Prx1 )rescued the osteoporosis phenotype. Reduced colony formation and altered subpopulations of bone marrow stromal cells were normalized upon inactivation of Cdkn2a, thus further demonstrating the relevance of cellular senescence in the pathogenesis. Our results shed light on the causative role of a TGF-beta-Nox4-senescence axis and therapeutic strategies for GO.