Impaired proteoglycan glycosylation, elevated TGF-β signaling, and abnormal osteoblast differentiation as the basis for bone fragility in a mouse model for gerodermia osteodysplastica.

Wing Lee Chan,Kazuyuki Sugahara,Michael Amling,Petra Knaus,Uwe Kornak,Antonio Rossi,Shuji Mizumoto,Mateusz Kolanczyk,Jan M Pestka,Björn Busse,Stefan Mundlos,Chiara Paganini,Magdalena Steiner,Haikuo Zhang,Johannes Egerer,Martin Lowe,Layal Abo Khayal,Thorsten Schinke,D Chan ,Ingrid Haußer ,Bettina M Willie ,Claus‐Eric Ott ,Tomasz M Witkos

doi:10.1371/journal.pgen.1007242

Abstract

Gerodermia osteodysplastica (GO) is characterized by skin laxity and early-onset osteoporosis. GORAB, the responsible disease gene, encodes a small Golgi protein of poorly characterized function. To circumvent neonatal lethality of the GorabNull full knockout, Gorab was conditionally inactivated in mesenchymal progenitor cells (Prx1-cre), pre-osteoblasts (Runx2-cre), and late osteoblasts/osteocytes (Dmp1-cre), respectively. While in all three lines a reduction in trabecular bone density was evident, only GorabPrx1 and GorabRunx2 mutants showed dramatically thinned, porous cortical bone and spontaneous fractures. Collagen fibrils in the skin of GorabNull mutants and in bone of GorabPrx1 mutants were disorganized, which was also seen in a bone biopsy from a GO patient. Measurement of glycosaminoglycan contents revealed a reduction of dermatan sulfate levels in skin and cartilage from GorabNull mutants. In bone from GorabPrx1 mutants total glycosaminoglycan levels and the relative percentage of dermatan sulfate were both strongly diminished. Accordingly, the proteoglycans biglycan and decorin showed reduced glycanation. Also in cultured GORAB-deficient fibroblasts reduced decorin glycanation was evident. The Golgi compartment of these cells showed an accumulation of decorin, but reduced signals for dermatan sulfate. Moreover, we found elevated activation of TGF-β in GorabPrx1 bone tissue leading to enhanced downstream signalling, which was reproduced in GORAB-deficient fibroblasts. Our data suggest that the loss of Gorab primarily perturbs pre-osteoblasts. GO may be regarded as a congenital disorder of glycosylation affecting proteoglycan synthesis due to delayed transport and impaired posttranslational modification in the Golgi compartment.

Highlights

Bone mass is highly heritable and largely determined by bone growth during childhood and adolescence leading to the so-called peak bone mass, and the rate of subsequent bone loss at older ages [1]
Using several mouse models and patient-derived primary cells we elucidate that loss of Gorab elicits a defect in proteoglycan glycanation, which is associated with collagen disorganization in dermis and bone
If these changes occur in mesenchymal stem cells or early osteoblasts they impair osteoblast differentiation resulting in cortical thinning and spontaneous fractures

Summary

Introduction

Bone mass is highly heritable and largely determined by bone growth during childhood and adolescence leading to the so-called peak bone mass, and the rate of subsequent bone loss at older ages [1]. GORAB, the gene product defective in GO, is a coiled-coil containing peripheral membrane protein that is recruited to the Golgi compartment via a specific, GTP-dependent interaction with the small GTPases ARF5 and RAB6 [5]. Due to this fact, GORAB has been suggested to belong to the group of golgins, small GTPase effector proteins involved in different steps of Golgi-related transport processes. The physiological role or GORAB in development and homeostasis of the skeleton and of connective tissues is not well understood

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Conclusion