How to grow bone to treat osteoporosis and mend fractures

Granular PEG hydrogels mediate osteoporotic MSC clustering via N-cadherin influencing the pro-resorptive bias of their secretory profile

Postmenopausal osteoporosis results from a pro-resorptive bone environment, which decreases bone mineral density causing increased fracture risk. Bone marrow derived mesenchymal stem/stromal cells (MSCs) secrete factors involved in bone homeostasis, but osteoporosis mediated changes to their secretions remain understudied. Herein, we examined the secretome of MSCs isolated from ovariectomized rats (OVX rMSCs), a model of post-menopausal osteoporosis, as a function of cell-cell interactions. Specifically, we controlled clustering of OVX and SHAM rMSCs by assembling them in granular hydrogels synthesized from poly(ethylene glycol) microgels with average diameters of ∼10, 100, and 200 µm. We directed both the sizes of rMSC clusters (single cells to ∼30 cells/cluster) and the percentages of cells within clusters (∼20-90%) by controlling the scaffold pore dimensions. Large clusters of OVX rMSCs had a pro-resorptive secretory profile, with increased concentrations of Activin A, CXCL1, CX3CL1, MCP-1, TIMP-1, and TNF-ɑ, compared to SHAM rMSCs. As this pro-resorptive bias was only observed in large cell clusters, we characterized the expression of several cadherins, mediators of cell-cell contacts. N-cadherin expression was elevated (∼4-fold) in OVX relative to SHAM rMSCs, in both cell clusters and single cells. Finally, TIMP-1 and MCP-1 secretion was only decreased in large cell clusters of OVX rMSCs when N-cadherin interactions were blocked, highlighting the dependence of OVX rMSC secretion of pro-resorptive cytokines on N-cadherin mediated cell-cell contacts. Further elucidation of the N-cadherin mediated osteoporotic MSC secretome may have implications for developing therapies for postmenopausal osteoporosis. STATEMENT OF SIGNIFICANCE: Postmenopausal osteoporosis is a prevalent bone disorder that affects tens of millions of women worldwide. This disease is characterized by severe bone loss resulting from a pro-resorptive bone marrow environment, where the rates of bone resorption outpace the rates of bone deposition. The paracrine factors secreted by bone marrow MSCs can influence cell types responsible for bone homeostasis, but the osteoporosis-mediated changes to MSC secretory properties remains understudied. In this study, we used PEG-based porous granular scaffolds to study the influence of cell clustering on the secretory properties of osteoporotic MSCs. We observed increased secretion of several pro-resorptive factors by osteoporotic MSCs in large clusters. Further, we explored the dependence of this altered secretion profile on N-cadherin mediated cell-cell contacts.

Effects of intermittent administration of parathyroid hormone on bone augmentation in rat calvarium

OBJECTIVES

This study examined the effects of parathyroid hormone (PTH) on bone augmentation beyond the skeletal envelope within a plastic cap in rat calvaria.

MATERIALS AND METHODS

The calvaria of 30 rats were exposed, and 2 plastic caps were placed on each. Each of the 10 rats was treated with 35 or 105 μg/kg (PTH-35, PTH-105) PTH 3 times per week. The control group was injected with sterile saline 3 times per week. Micro-computed tomography (CT) imaging was performed every 2 weeks for 12 weeks. Micro-CT and histological sections were used to determine the amount of bone augmentation within the plastic caps. Bone volume (BV) was calculated using BV-measuring software.

RESULTS

The histomorphometric and histological analyses showed that the amount of bone augmentation was increased significantly in the PTH groups compared with the controls at 12 weeks. The PTH-105 group showed significantly more bone augmentation and osteoblasts compared with the PTH-35 group.

CONCLUSIONS

These results indicate that the higher the dose of intermittent PTH administered, the greater the amount of bone formation beyond the skeletal envelop in the rat calvarium.

Parathyroid hormone and leptin--new peptides, expanding clinical prospects

There are three injectable and one oral bone-building (i.e., bone anabolic) parathyroid hormone (PTH) peptides. One of the four, Lilly's injectable teriparatide (Forteo), is currently being used, and the other three are in clinical trials. They are being used or assessed only for treating postmenopausal osteoporosis. However, their potential clinical targets now extend far beyond osteoporosis. They can accelerate the mending of even severe non-union fractures; they will probably be used to strengthen the anchorage of pros-theses to bone; they have been shown to treat psoriasis that has resisted other treatments; they can increase the size of haematopoietic stem cell proliferation and accelerate the endogenous repopulation or repopulation by donor transplants of bone marrow depleted by chemotherapeutic drugs; and they may prevent vascular ossification. Leptin, a member of the cytokine superfamily has a PTH-like osteogenic activity and may even partly mediate PTH action. But leptin has two drawbacks that cloud its therapeutic future. First, apart from directly stimulating osteoblastic cells, it targets cells in the hypothalamic ventromedial nuclei and through them it reduces oestrogenic activity by promoting osteoblast-suppressing adrenergic activity. Second, it stimulates vascular and heart valve ossification, which leads to such events as heart failure and diabetic limb amputations.

Taming psoriatic keratinocytes?PTHs' uses go up another notch

The native parathyroid hormone (PTH) and several of its N-terminal adenylyl cyclase-activating fragments and their analogs have become the star stimulators of bone growth for treating osteoporosis, accelerating fracture healing, and strengthening the anchorage of prosthetic bone implants and one of them (Lilly's Forteo--recombinant hPTH-(1-34) has recently arrived in the clinic. But something entirely different has been lurking in the background-the ability of the adenylyl cyclase stimulating hPTH-(1-34) to calm hyperproliferating keratinocytes and reduce psoriatic lesions. By contrast PTH-(7-34) which cannot stimulate adenylyl cyclase actually stimulates keratinocyte proliferation. Normal keratinocytes make PTHrP after they lift off the basal lamina and have stopped cycling. But they have an unconventional PTH/PTHrP receptor which is not coupled to adenylyl cyclase. Psoriatic keratinocytes do not make PTHrP and have only a broken-down, proliferation-limiting terminal differentiation-driving Notch-Notch ligand mechanism. Putting these and other facts together produces a possible picture of an exogenously applied adenylyl cyclase-activating PTH pinch hitting for the missing PTHrP and restoring normal keratinocyte proliferative activity epidermal structure by stimulating dermal fibroblasts which do have the conventional adenylyl cyclase-linked PTHR1 and in response directly or indirectly restore the overlying basal keratinocytes' Notch-Notch ligand terminal differentiation-driving mechanism and consequently a normal epidermal structure.