Altered cytoskeletal organization characterized lethal but not surviving Brtl^+/−mice: insight on phenotypic variability in osteogenesis imperfecta

Abstract

Osteogenesis imperfecta (OI) is a heritable bone disease with dominant and recessive transmission. It is characterized by a wide spectrum of clinical outcomes ranging from very mild to lethal in the perinatal period. The intra- and inter-familiar OI phenotypic variability in the presence of an identical molecular defect is still puzzling to the research field. We used the OI murine model Brtl(+/-) to investigate the molecular basis of OI phenotypic variability. Brtl(+/-) resembles classical dominant OI and shows either a moderately severe or a lethal outcome associated with the same Gly349Cys substitution in the α1 chain of type I collagen. A systems biology approach was used. We took advantage of proteomic pathway analysis to functionally link proteins differentially expressed in bone and skin of Brtl(+/-) mice with different outcomes to define possible phenotype modulators. The skin/bone and bone/skin hybrid networks highlighted three focal proteins: vimentin, stathmin and cofilin-1, belonging to or involved in cytoskeletal organization. Abnormal cytoskeleton was indeed demonstrated by immunohistochemistry to occur only in tissues from Brtl(+/-) lethal mice. The aberrant cytoskeleton affected osteoblast proliferation, collagen deposition, integrin and TGF-β signaling with impairment of bone structural properties. Finally, aberrant cytoskeletal assembly was detected in fibroblasts obtained from lethal, but not from non-lethal, OI patients carrying an identical glycine substitution. Our data demonstrated that compromised cytoskeletal assembly impaired both cell signaling and cellular trafficking in mutant lethal mice, altering bone properties. These results point to the cytoskeleton as a phenotypic modulator and potential novel target for OI treatment.

The Osteocyte Transcriptome Is Extensively Dysregulated in Mouse Models of Osteogenesis Imperfecta

“…The results of our analysis confirm that in OI there is a significant dysregulation of transcripts involved in dendrite formation, ECM organization, collagen fibril organization, integrin‐mediated signaling pathway, tissue development, among others. This implies a profound alteration of cellular‐matrix‐tissue interactions that impact cellular function, morphology, and cytoskeletal organization and is consistent with recent evidence of cytoskeletal dysregulation in OI . Of the genes implicated in bone formation, transcripts involved in the WNT signaling pathway were differentially expressed in both our OI models.…”

“…This implies a profound alteration of cellularmatrix-tissue interactions that impact cellular function, morphology, and cytoskeletal organization and is consistent with recent evidence of cytoskeletal dysregulation in OI. (56,57) Of the genes implicated in bone formation, transcripts involved in the WNT signaling pathway were differentially expressed in both our OI models. These included at least 3 WNT ligands but also inhibitors of WNT signaling, suggesting a cellular attempt to increase osteoblast differentiation and function, perhaps triggered by the osteocyte sensing a defective matrix.…”

Cytoskeleton and nuclear lamina affection in recessive osteogenesis imperfecta: A functional proteomics perspective

“…Previously, we demonstrated consistent similarities in cellular pathway affection between bone and skin specimens in the Brtl −/+ mouse, model for dominant OI [19] , [28] . Such alterations were also detected in primary fibroblasts of dominant OI patients [29] .…”

Genetic causes and mechanisms of Osteogenesis Imperfecta

“…In vitro , binding of recombinant decorin to type I collagen of Crtap −/− mice is reduced compared with wildtype collagen, which raises the possibility that the abnormal collagen in OI impairs the ability of decorin, and potentially other SLRPs, to modulate TGF-β signaling [135]. Abnormalities in TGF-β signaling were also observed in Brtl +/− mice, which exhibit phenotypic variability with either a moderately severe or lethal phenotype [140]. Bones from lethal Brtl +/− mice showed increased expression of TGF-β, but no change in the phosphorylation of Smad2/3, whereas bone from non-lethal Brtl +/− mice demonstrated no increased TGF-β expression, but increased Smad2/3 activation.…”

“…Furthermore, lethal Brtl +/− mice showed abnormal cytoskeletal structure and function, which affected intracellular trafficking and integrin-mediated signaling. Because integrins have been shown to activate TGF-β in the ECM, it is possible that defects in integrin function may be a contributing factor to abnormal TGF-β signaling in OI [140,141]. Together, these findings indicate roles for dysregulated matrix-cell signaling in the pathogenesis of OI.…”