Brigitte Burt-Pichat

Histomorphometric and μCT Analysis of Bone Biopsies From Postmenopausal Osteoporotic Women Treated With Strontium Ranelate

Strontium ranelate is a new anti‐osteoporotic treatment. On bone biopsies collected from humans receiving long‐term treatment over 5 yr, it has been shown that strontium ranelate has good bone safety and better results than placebo on 3D microarchitecture. Hence, these effects may explain the decreased fracture rate.

Bone sialoprotein plays a functional role in bone formation and osteoclastogenesis.

Bone sialoprotein (BSP) and osteopontin (OPN) are both highly expressed in bone, but their functional specificities are unknown. OPN knockout (−/−) mice do not lose bone in a model of hindlimb disuse (tail suspension), showing the importance of OPN in bone remodeling. We report that BSP−/− mice are viable and breed normally, but their weight and size are lower than wild-type (WT) mice. Bone is undermineralized in fetuses and young adults, but not in older (≥12 mo) BSP−/− mice. At 4 mo, BSP−/− mice display thinner cortical bones than WT, but greater trabecular bone volume with very low bone formation rate, which indicates reduced resorption, as confirmed by lower osteoclast surfaces. Although the frequency of total colonies and committed osteoblast colonies is the same, fewer mineralized colonies expressing decreased levels of osteoblast markers form in BSP−/− versus WT bone marrow stromal cultures. BSP−/− hematopoietic progenitors form fewer osteoclasts, but their resorptive activity on dentin is normal. Tail-suspended BSP−/− mice lose bone in hindlimbs, as expected. In conclusion, BSP deficiency impairs bone growth and mineralization, concomitant with dramatically reduced bone formation. It does not, however, prevent the bone loss resulting from loss of mechanical stimulation, a phenotype that is clearly different from OPN−/− mice.

Microarchitecture Influences Microdamage Accumulation in Human Vertebral Trabecular Bone

It has been suggested that accumulation of microdamage with age contributes to skeletal fragility. However, data on the age‐related increase in microdamage and the association between microdamage and trabecular microarchitecture in human vertebral cancellous bone are limited. We quantified microdamage in cancellous bone from human lumbar (L2) vertebral bodies obtained from 23 donors 54–93 yr of age (8 men and 15 women). Damage was measured using histologic techniques of sequential labeling with chelating agents and was related to 3D microarchitecture, as assessed by high‐resolution μCT. There were no significant differences between sexes, although women tended to have a higher microcrack density (Cr.Dn) than men. Cr.Dn increased exponentially with age (r = 0.65, p < 0.001) and was correlated with bone volume fraction (BV/TV; r = −0.55; p < 0.01), trabecular number (Tb.N; r = −0.56 p = 0.008), structure model index (SMI; r = 0.59; p = 0.005), and trabecular separation (Tb.Sp; r = 0.59; p < 0.009). All architecture parameters were strongly correlated with each other and with BV/TV. Stepwise regression showed that SMI was the best predictor of microdamage, explaining 35% of the variance in Cr.Dn and 20% of the variance in diffuse damage accumulation. In addition, microcrack length was significantly greater in the highest versus lowest tertiles of SMI. In conclusion, in human vertebral cancellous bone, microdamage increases with age and is associated with low BV/TV and a rod‐like trabecular architecture.

Effects of preexisting microdamage, collagen cross-links, degree of mineralization, age, and architecture on compressive mechanical properties of elderly human vertebral trabecular bone.

Previous studies have shown that the mechanical properties of trabecular bone are determined by bone volume fraction (BV/TV) and microarchitecture. The purpose of this study was to explore other possible determinants of the mechanical properties of vertebral trabecular bone, namely collagen cross‐link content, microdamage, and mineralization. Trabecular bone cores were collected from human L2 vertebrae (n = 49) from recently deceased donors 54–95 years of age (21 men and 27 women). Two trabecular cores were obtained from each vertebra, one for preexisting microdamage and mineralization measurements, and one for BV/TV and quasi‐static compression tests. Collagen cross‐link content (PYD, DPD, and PEN) was measured on surrounding trabecular bone. Advancing age was associated with impaired mechanical properties, and with increased microdamage, even after adjustment by BV/TV. BV/TV was the strongest determinant of elastic modulus and ultimate strength (r2 = 0.44 and 0.55, respectively). Microdamage, mineralization parameters, and collagen cross‐link content were not associated with mechanical properties. These data indicate that the compressive strength of human vertebral trabecular bone is primarily determined by the amount of trabecular bone, and notably unaffected by normal variation in other factors, such as cross‐link profile, microdamage and mineralization.

Molecular histology of mitochondrial and nuclear transcripts in the muscle of patients harbouring a single mitochondrial DNA deletion

Abstract The distribution of transcripts of mitochondrial and nuclear genes involved in oxidative phosphorylation and of the mitochondrial creatine kinase nuclear gene was examined, using in situ hybridisation, in the skeletal muscle of 11 patients harbouring a heteroplasmic mitochondrial DNA (mtDNA) single deletion. Levels of mRNAs transcribed from genes located within the deletions were not decreased, suggesting that the remaining wild-type mtDNA was still transcribed. Those muscle fibres with characteristic abnormal mitochondrial proliferation always showed overexpression of mRNAs and rRNAs transcribed from mitochondrial genes located outside the deletions. Interestingly, they also showed overexpression of the nuclear-encoded ATP synthase β subunit mRNA, but not of mitochondrial creatine kinase mRNA. These observations lead to three proposals: (1) overexpression of mitochondrial transcripts within fibres harbouring mitochondrial proliferation, together with the apparently normal expression of the remaining wild-type mtDNA, is not related to decreased mitochondrial translation; (2) it is more probably related to an up-regulation mechanism which co-ordinates both mitochondrial and nuclear expression; and (3) this mechanism is restricted to transcripts directly involved in oxidative phosphorylation and to fibres with mitochondrial accumulation.

Bone sialoprotein plays a functional role in bone formation and osteoclastogenesis

Malaval et al. 2008. J. Exp. Med. doi:10.1084/jem.20071294 [OpenUrl][1][Abstract/FREE Full Text][2] [1]: {openurl}?query=rft_id%253Dinfo%253Adoi%252F10.1084%252Fjem.20071294%26rft_id%253Dinfo%253Apmid%252F18458111%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%