Aromatase deficiency in transplanted bone marrow cells improves vertebral trabecular bone quantity...

Aromatase deficiency in transplanted bone marrow cells improves vertebral trabecular bone quantity, connectivity, and mineralization and decreases cortical porosity in murine bone marrow transplant recipients

AUTHORS

Katie Rubitschung, Amber Sherwood, Rasesh Kapadia, Yin Xi, Asghar Hajibeigi, Katya B. Rubinow, Joseph E. Zerwekh, Orhan K. Öz

ABSTRACT

Estradiol is an important regulator of bone accumulation and maintenance. Circulating estrogens are primarily produced by the gonads. Aromatase, the enzyme responsible for the conversion of androgens to estrogen, is expressed by bone marrow cells (BMCs) of both hematopoietic and nonhematopoietic origin. While the significance of gonad-derived estradiol to bone health has been investigated, there is limited understanding regarding the relative contribution of BMC derived estrogens to bone metabolism. To elucidate the role of BMC derived estrogens in male bone, irradiated wild-type C57BL/6J mice received bone marrow cells transplanted from either WT (WT(WT)) or aromatase-deficient (WT(ArKO)) mice. MicroCT was acquired on lumbar vertebra to assess bone quantity and quality. WT(ArKO) animals had greater trabecular bone volume (BV/TV p = 0.002), with a higher trabecular number (p = 0.008), connectivity density (p = 0.017), and bone mineral content (p = 0.004). In cortical bone, WT(ArKO) animals exhibited smaller cortical pores and lower cortical porosity (p = 0.02). Static histomorphometry revealed fewer osteoclasts per bone surface (Oc.S/BS%), osteoclasts on the erosion surface (ES(Oc+)/BS, p = 0.04) and low number of osteoclasts per bone perimeter (N.Oc/B.Pm, p = 0.01) in WT(ArKO). Osteoblast-associated parameters in WT(ArKO) were lower but not statistically different from WT(WT). Dynamic histomorphometry suggested similar bone formation indices’ patterns with lower mean values in mineral apposition rate, label separation, and BFR/BS in WT(ArKO) animals. Ex vivo bone cell differentiation assays demonstrated relative decreased osteoblast differentiation and ability to form mineralized nodules. This study demonstrates a role of local 17β-estradiol production by BMCs for regulating the quantity and quality of bone in male mice. Underlying in vivo cellular and molecular mechanisms require further study.

Loss of Notch signaling in skeletal stem cells enhances bone formation with aging

AUTHORS

Lindsey H. Remark, Kevin Leclerc, Malissa Ramsukh, Ziyan Lin, Sooyeon Lee, Backialakshmi Dharmalingam, Lauren Gillinov, Vasudev V. Nayak, Paulo El Parente, Margaux Sambon, Pablo J. Atria, Mohamed A. E. Ali, Lukasz Witek, Alesha B. Castillo, Christopher Y, Park, Ralf H. Adams, Aristotelis Tsirigos, Sophie M. Morgani & Philipp Leucht

ABSTRACT

Skeletal stem and progenitor cells (SSPCs) perform bone maintenance and repair. With age, they produce fewer osteoblasts and more adipocytes leading to a loss of skeletal integrity. The molecular mechanisms that underlie this detrimental transformation are largely unknown. Single-cell RNA sequencing revealed that Notch signaling becomes elevated in SSPCs during aging. To examine the role of increased Notch activity, we deleted Nicastrin, an essential Notch pathway component, in SSPCs in vivo. Middle-aged conditional knockout mice displayed elevated SSPC osteo-lineage gene expression, increased trabecular bone mass, reduced bone marrow adiposity, and enhanced bone repair. Thus, Notch regulates SSPC cell fate decisions, and moderating Notch signaling ameliorates the skeletal aging phenotype, increasing bone mass even beyond that of young mice. Finally, we identified the transcription factor Ebf3 as a downstream mediator of Notch signaling in SSPCs that is dysregulated with aging, highlighting it as a promising therapeutic target to rejuvenate the aged skeleton.

Multi-organ phenotypes in mice lacking latent TGFβ binding protein 2 (LTBP2)

AUTHORS

Nicholas K. Bodmer, Russell H. Knutsen, Robyn A. Roth, Ryan M. Castile, Michael D. Brodt, Carrie M. Gierasch, Thomas J. Broekelmann, Mark A. Gibson, Jeffrey A. Haspel, Spencer P. Lake, Jeffrey R. Koenitzer, Steven L. Brody, Matthew J. Silva, Robert P. Mecham, David M. Ornitz

ABSTRACT

Background

Latent TGFβ binding protein-2 (LTBP2) is a fibrillin 1 binding component of the microfibril. LTBP2 is the only LTBP protein that does not bind any isoforms of TGFβ, although it may interfere with the function of other LTBPs or interact with other signaling pathways.

Results

Here, we investigate mice lacking Ltbp2 (Ltbp2−/−) and identify multiple phenotypes that impact bodyweight and fat mass, and affect bone and skin development. The alterations in skin and bone development are particularly noteworthy since the strength of these tissues is differentially affected by loss of Ltbp2. Interestingly, some tissues that express high levels of Ltbp2, such as the aorta and lung, do not have a developmental or homeostatic phenotype.

Conclusions

Analysis of these mice show that LTBP2 has complex effects on development through direct effects on the extracellular matrix (ECM) or on signaling pathways that are known to regulate the ECM.

Inhibiting WNT secretion reduces high bone mass caused by Sost loss-of-function or gain-of-function mutations in Lrp5

AUTHORS

Cassandra R. Diegel, Ina Kramer, Charles Moes, Gabrielle E. Foxa, Mitchell J. McDonald, Zachary B. Madaj, Sabine Guth, Jun Liu, Jennifer L. Harris, Michaela Kneissel & Bart O. Williams

ABSTRACT

Proper regulation of Wnt signaling is critical for normal bone development and homeostasis. Mutations in several Wnt signaling components, which increase the activity of the pathway in the skeleton, cause high bone mass in human subjects and mouse models. Increased bone mass is often accompanied by severe headaches from increased intracranial pressure, which can lead to fatality and loss of vision or hearing due to the entrapment of cranial nerves. In addition, progressive forehead bossing and mandibular overgrowth occur in almost all subjects. Treatments that would provide symptomatic relief in these subjects are limited. Porcupine-mediated palmitoylation is necessary for Wnt secretion and binding to the frizzled receptor. Chemical inhibition of porcupine is a highly selective method of Wnt signaling inhibition. We treated three different mouse models of high bone mass caused by aberrant Wnt signaling, including homozygosity for loss-of-function in Sost, which models sclerosteosis, and two strains of mice carrying different point mutations in Lrp5 (equivalent to human G171V and A214V), at 3 months of age with porcupine inhibitors for 5–6 weeks. Treatment significantly reduced both trabecular and cortical bone mass in all three models. This demonstrates that porcupine inhibition is potentially therapeutic for symptomatic relief in subjects who suffer from these disorders and further establishes that the continued production of Wnts is necessary for sustaining high bone mass in these models.

Buds of new bone formation within the femoral head of hip fracture patients coincide with zones of low osteocyte sclerostin

AUTHORS

Hiroshige Sano, Tristan Whitmarsh, Linda Skingle, Taketoshi Shimakura, Noriaki Yamamoto, Juliet E. Compston, Hideaki E. Takahashi, Kenneth E. S. Poole

ABSTRACT

Romosozumab treatment reduces the rate of hip fractures and increases hip bone density, increasing bone formation by inhibiting sclerostin protein. We studied the normal pattern of bone formation and osteocyte expression in the human proximal femur because it is relevant to both anti-sclerostin treatment effects and fracture. Having visualized and quantified buds of new bone formation in trabeculae, we hypothesized that they would coincide with areas of a) higher mechanical stress and b) low sclerostin expression by osteocytes. In patients with hip fracture, we visualised each bud of active modeling-based formation, (Forming Minimodeling Structure, FMiS) in trabecular cores taken from different parts of the femoral head. Trabecular bone structure was also measured with high resolution imaging.

More buds of new bone formation (by volume) were present in the higher stress supero-medial zone (FMiS density, N.FMiS/T.Ar) than lower stress supero-lateral (p < 0.05), and inferomedial (p < 0.001) regions. There were fewer sclerostin expressing osteocytes close to, or within FMiS. FMiS density correlated with greater amount, thickness, number and connectivity of trabeculae (bone volume BV/TV, r = 0.65, p < 0.0001; bone surface BS/TV, r = 0.47, p < 0.01; trabecular thickness Tb.Th, r = 0.55, p < 0.001; trabecular number Tb.N, r = 0.47, p < 0.01; and connectivity density Conn.D, r = 0.40, p < 0.05) and lower trabecular separation (Tb.Sp, r = −0.56, p < 0.001).

These results demonstrate modeling-based bone formation in femoral trabeculae from patients with hip fracture as a potential therapeutic target to enhance bone structure.

Engineering 3D-Printed Strontium-Titanium Scaffold-Integrated Highly Bioactive Serum Exosomes for Critical Bone Defects by Osteogenesis and Angiogenesis

AUTHORS

Hao Liu, Ranli Gu, Wei Li, Lijun Zeng, Yuan Zhu, Boon Chin Heng, Yunsong Liu, and Yongsheng Zhou

ABSTRACT

Currently, healing of large bone defects faces significant challenges such as a bulk of bone regeneration and revascularization on the bone defect region. Here, a “cell-free scaffold engineering” strategy that integrates strontium (Sr) and highly bioactive serum exosomes (sEXOs) inside a three-dimensional (3D)-printed titanium (Ti) scaffold (Sc) is first developed. The constructed SrTi Sc can serve as a sophisticated biomaterial platform for maintaining bone morphological characteristics of the radius during the period of critical bone defect (CBD) repair and further accelerating bone formation and fibroblastic suppression via the controlled release of Sr from the superficial layer of the scaffold. Moreover, compared with sEXO from healthy donors, the sEXO extracted from the serum of the femoral fracture rabbit model at the stage of fracture healing, named BF EXO, is robustly capable of facilitating osteogenesis and angiogenesis. In addition, the underlying therapeutic mechanism is elucidated, whereby altering miRNAs shuttled by BF EXO enables osteogenesis and angiogenesis. Further, the in vivo study revealed that the SrTi Sc + BF EXO composite dramatically accelerated bone repair via osteoconduction, osteoinduction, and revascularization in radial CBD of rabbits. This study broadens the source and biomedical potential of specifically functionalized exosomes and provides a comprehensive clinically feasible strategy for therapeutics on large bone defects.