Caspase-2 maintains bone homeostasis by inducing apoptosis of oxidatively-damaged osteoclasts

PKM2 is a Novel Osteoporosis-Associated Protein in Chinese

                       Purpose:Osteoporosis is characterized by low bone mineral density (BMD) and high risk of osteoporotic fracture (OF). Peripheral blood monocytes (PBM) can differentiate into osteoclasts to resorb bone. This study was to identify PBM-expressed proteins significant for osteoporosis in Chinese Han elderly population (>65 years), and focused on two phenotypes of osteoporosis: low BMD and OF.

METHODS

Label-free quantitative proteomics was employed to profile PBM proteome and to identify differentially expressed proteins (DEPs) between OF (N=27) vs. non-fractured (NF, N=24) subjects and between low BMD (N=12) vs. high BMD (N=12) subjects in women. Western blotting (WB) was conducted to validate differential expression, and ELISA to evaluate translational value for secretory protein of interest.

RESULTS

We discovered 59 DEPs with fold change (FC)>1.3 (P<1×10-5), and validated the significant up-regulation of pyruvate kinase isozyme 2 (PKM2) with osteoporosis (P<0.001). PKM2 protein upregulation with OF was replicated with PBM in men (P=0.04). Plasma PKM2 protein level was significantly elevated with OF in an independent sample (N=100, FC=1.68, P=0.01). Pursuant functional assays showed that extracellular PKM2 protein supplement not only promoted monocyte trans-endothelial migration, growth, and osteoclast differentiation (marker gene expression), but also inhibited osteoblast growth, differentiation (ALP gene expression), and activity.

CONCLUSION

The above findings suggest that PKM2 protein is a novel osteoporosis-associated functional protein in Chinese Han elderly population. It may serve as a risk biomarker and drug target for osteoporosis.

Validation of the Intermolecular Disulfide Bond in Caspase-2

Caspases are a family of proteins involved in cell death. Although several caspase members have been well characterized, caspase-2 remains enigmatic. Caspase-2 has been implicated in several phenotypes, but there has been no consensus in the field about its upstream activating signals or its downstream protein targets. In addition, the unique ability of caspase-2 to form a disulfide-bonded dimer has not been studied in depth. Herein, we investigate the disulfide bond in the context of inducible dimerization, showing that disulfide bond formation is dimerization dependent. We also explore and review several stimuli published in the caspase-2 field, test ferroptosis-inducing stimuli, and study in vivo infection models. We hypothesize that the disulfide bond will ultimately prove to be essential for the evolved function of caspase-2. Proving this will require the discovery of cell death phenotypes where caspase-2 is definitively essential.

Making the head: Caspases in life and death

The term apoptosis, as a way of programmed cell death, was coined a half century ago and since its discovery the process has been extensively investigated. The anatomy and physiology of the head are complex and thus apoptosis has mostly been followed in separate structures, tissues or cell types. This review aims to provide a comprehensive overview of recent knowledge concerning apoptosis-related molecules involved in the development of structures of head with a particular focus on caspases, cysteine proteases having a key position in apoptotic pathways. Since many classical apoptosis-related molecules, including caspases, are emerging in several non-apoptotic processes, these were also considered. The largest organ of the head region is the brain and its development has been extensively investigated, including the roles of apoptosis and related molecules. Neurogenesis research also includes sensory organs such as the eye and ear, efferent nervous system and associated muscles and glands. Caspases have been also associated with normal function of the skin and hair follicles. Regarding mineralised tissues within craniofacial morphogenesis, apoptosis in bones has been of interest along with palate fusion and tooth development. Finally, the role of apoptosis and caspases in angiogenesis, necessary for any tissue/organ development and maintenance/homeostasis, are discussed. Additionally, this review points to abnormalities of development resulting from improper expression/activation of apoptosis-related molecules.

Bone Cells Metabolic Changes Induced by Ageing

Bone is a living organ that exhibits active metabolic processes, presenting constant bone formation and resorption. The bone cells that maintain local homeostasis are osteoblasts, osteoclasts, osteocytes and bone marrow stem cells, their progenitor cells. Osteoblasts are the main cells that govern bone formation, osteoclasts are involved in bone resorption, and osteocytes, the most abundant bone cells, also participate in bone remodeling. All these cells have active metabolic activities, are interconnected and influence each other, having both autocrine and paracrine effects. Ageing is associated with multiple and complex bone metabolic changes, some of which are currently incompletely elucidated. Ageing causes important functional changes in bone metabolism, influencing all resident cells, including the mineralization process of the extracellular matrix. With advancing age, a decrease in bone mass, the appearance of specific changes in the local microarchitecture, a reduction in mineralized components and in load-bearing capacity, as well as the appearance of an abnormal response to different humoral molecules have been observed. The present review points out the most important data regarding the formation, activation, functioning, and interconnection of these bone cells, as well as data on the metabolic changes that occur due to ageing.

Astaxanthin Induces Apoptosis in Human Osteosarcoma MG-63 Cells

We explored the mechanism of human osteosarcoma MG-63 cell apoptosis induced by asta-xanthin. The MTT assay was used to detect the effect of astaxanthin on cell viability. Morphological changes associated with apoptosis were observed after DAPI staining. Early and late stages of apoptosis were detected by flow cytometry with annexin V-FITC/PI staining. Activation of caspases-8, -9 and -3 was detected by enzyme activity in vitro. Changes in the mitochondrial membrane potential were detected by MitoCapture staining. Western blot was used to detect the cleavage of PARP, which is a caspase-3 substrate, the release of cytochrome c and Smac into the cytosol, the translocation of pro-apoptotic proteins Bax and Bak, and the expression of mitochondrial pathway-related proteins. The translocation of Bax was also detected by immunofluorescence assay. Astaxanthin significantly inhibited the viability of human osteosarcoma MG-63 cells with an IC50 value of 12.36 μg/ml. The DAPI-stained cells showed characteristic apoptotic morphological changes - cell shrinkage, cell membrane blebbing, nuclear condensation, and apoptotic body formation. Cytochrome c and Smac were released from mitochondria to the cytosol. Pro-apoptotic proteins Bax and Bak were rapidly translocated to mitochondria after six hours of astaxanthin action. Caspases-9 and -3 were activated and PARP was cleaved. The expression of anti-apoptotic proteins Bcl-2, Bcl-xL and XIAP was significantly decreased. Astaxanthin induced human osteosarcoma MG-63 cell apoptosis through the mitochondria-mediated endogenous apoptosis pathway.

Ginsenoside Rg1 Reduces Oxidative Stress Via Nrf2 Activation to Regulate Age-Related Mesenchymal Stem Cells Fate Switch Between Osteoblasts and Adipocytes

Background

An important feature of aging cells is the gradual loss of physiological integrity. As aging progresses, MSCs change preferring to differentiate toward adipocytes rather than osteoblasts. Oxidative stress accumulation is an important factor in age-related bone loss. Many experiments have demonstrated the good therapeutic effect of Ginsenoside (Rg1) on oxidative stress injury. In this study, we investigated the effect of Rg1 on the osteogenic-adipogenic differentiation balance of bone marrow mesenchymal stem cells (BMMSC).

Objective

To analyze the potential application value of Rg1 in the treatment of senile osteoporosis.

Methods

BMMSCs were isolated from healthy donors of different ages and identified based on isotype and by multi-differentiation induction. Rg1 was used to treat BMMSCs, The differentiation propensity was analyzed by induction of differentiation assay. Antioxidant capacity of BMMSCs as measured by oxidative stress product assay Related mechanism studies were confirmed by quantitative real-time reverse transcription-polymerase chain reaction (qRT-PCR), immunofluorescence, western blotting, and inhibitor treatment. Moreover, Observation of the effects of Rg1 on aging BMMSCs under in vivo conditions by treatment of aged mice with Rg1 injections.

Results

Rg1 treatment rescued age-induced switch of BMMSCs differentiation fate in vitro. In elderly people, Rg1 markedly increased osteogenic differentiation of BMMSCs by decreasing oxidative stress, while inhibiting adipogenic differentiation. However, this effect was abolished in BMMSCs by an Nrf2-inhibitor. Notably, aging mice showed a reduction in adipocyte distribution in the bone marrow and a decrease in oxidative stress products after a 3-month period of Rg1 treatment.

Conclusion

We have uncovered a novel function for Rg1 that involves attenuating bone loss via Nrf2 antioxidant signaling, which in turn may potentially be utilized as a therapeutic agent for improving osteogenic differentiation in aging BMMSCs.

Recent Advances in Osteoclast Biological Behavior

With the progress of the aging population, bone-related diseases such as osteoporosis and osteoarthritis have become urgent problems. Recent studies have demonstrated the importance of osteoclasts in bone homeostasis, implying these will be an important mediator in the treatment of bone-related diseases. Up to now, several reviews have been performed on part of osteoclast biological behaviors such as differentiation, function, or apoptosis. However, few reviews have shown the complete osteoclast biology and research advances in recent years. Therefore, in this review, we focus on the origin, differentiation, apoptosis, behavior changes and coupling signals with osteoblasts, providing a simple but comprehensive overview of osteoclasts for subsequent studies.

SIRT1, a promising regulator of bone homeostasis

Sirtuin 1 (SIRT1), a nicotinamide adenine dinucleotide-dependent deacetylase, epigenetically regulates various cell metabolisms, including inflammation, tumorigenesis, and bone metabolism. Many clinical studies have found the potential of SIRT1 in predicting and treating bone-related disorders, such as osteoporosis and osteonecrosis, suggesting that SIRT1 might be a regulator of bone homeostasis. In order to identify the mechanisms that underlie the pivotal role of SIRT1 in bone homeostasis, many studies revealed that SIRT1 could maintain the balance between bone formation and absorption via regulating the ratio of osteoblasts to osteoclasts. SIRT1 controls the differentiation of mesenchymal stem cells (MSCs) and bone marrow-derived macrophages, increasing osteogenesis and reducing osteoclastogenesis. Besides, SIRT1 can enhance bone-forming cells' viability, including MSCs and osteoblasts under adverse conditions by resisting senescence, suppressing apoptosis, and promoting autophagy in favor of osteogenesis. Furthermore, the effect on bone vasculature homeostasis enables SIRT1 to become a valuable strategy for ischemic osteonecrosis and senile osteoporosis. The review systemically discusses SIRT1 pathways and the critical role in bone homeostasis and assesses whether SIRT1 is a potential target for manipulation and therapy, to lay a solid foundation for further researches in the future.

Various pathways of zoledronic acid against osteoclasts and bone cancer metastasis: a brief review

Zoledronic acid (ZA) is one of the most important and effective class of anti-resorptive drug available among bisphosphonate (BP), which could effectively reduce the risk of skeletal-related events, and lead to a treatment paradigm for patients with skeletal involvement from advanced cancers. However, the exact molecular mechanisms of its anticancer effects have only recently been identified. In this review, we elaborate the detail mechanisms of ZA through inhibiting osteoclasts and cancer cells, which include the inhibition of differentiation of osteoclasts via suppressing receptor activator of nuclear factor κB ligand (RANKL)/receptor activator of nuclear factor κB (RANK) pathway, non-canonical Wnt/Ca2+/calmodulin dependent protein kinase II (CaMKII) pathway, and preventing of macrophage differentiation into osteoclasts, in addition, induction of apoptosis of osteoclasts through inhibiting farnesyl pyrophosphate synthase (FPPS)-mediated mevalonate pathway, and activation of reactive oxygen species (ROS)-induced pathway. Furthermore, ZA also inhibits cancer cells proliferation, viability, motility, invasion and angiogenesis; induces cancer cell apoptosis; reverts chemoresistance and stimulates immune response; and acts in synergy with other anti-cancer drugs. In addition, some new ways for delivering ZA against cancer is introduced. We hope this review will provide more information in support of future studies of ZA in the treatment of cancers and bone cancer metastasis.

Uncovering the PIDDosome and caspase-2 as regulators of organogenesis and cellular differentiation

The PIDDosome is a multiprotein complex that drives activation of caspase-2, an endopeptidase originally implicated in apoptosis. Yet, unlike other caspases involved in cell death and inflammation, caspase-2 seems to exert additional versatile functions unrelated to cell death. These emerging roles range from control of transcription factor activity to ploidy surveillance. Thus, caspase-2 and the PIDDosome act as a critical regulatory unit controlling cellular differentiation processes during organogenesis and regeneration. These newly established functions of the PIDDosome and its downstream effector render its components attractive targets for drug-development aiming to prevent fatty liver diseases, neurodegenerative disorders or osteoporosis.

Positive and negative regulators of osteoclast apoptosis

Survival and apoptosis are of major importance in the osteoclast life cycle. As osteoclasts have short lifespan, any alteration that prolongs their viability may cause enhanced osteoclast activity. Hence, the regulation of OC apoptosis has been recognized as a critical factor in bone remodeling. An imbalance in bone remodeling due to increased osteoclast activity leads to most adult bone diseases such as osteoporosis, rheumatoid arthritis and multiple myeloma. Therefore, manipulating osteoclast death would be a viable therapeutic approach in ameliorating bone diseases, with accelerated resorption. Over the last few decades we have witnessed the unraveling of many of the intracellular mechanisms responsible for osteoclast apoptosis. Thus, an understanding of the underlying mechanisms by which osteoclasts undergo programmed cell death and the regulators that modulate that activity will undoubtedly provide an insight into the development of pharmacological agents to treat such pathological bone diseases.

The triptolide-induced apoptosis of osteoclast precursor by degradation of cIAP2 and treatment of rheumatoid arthritis of TNF-transgenic mice

This study aims to discuss the effect of triptolide (TPL) on rheumatoid arthritis (RA) and the mechanism related to osteoclast precursor (OCP) and osteoclast (OC). TNF-transgenic RA mice were treated with different doses of TPL by gavage. After the administration was finished, the curative effects were evaluated and compared, and the OCP apoptosis rates, the OC number, and the OC differentiation ability in vitro were detected. Finally, splenocytes of wild-type mice were cultured in vitro and induced to differentiate into OCP, and the cell apoptosis rate, cIAP2, and apoptotic effectors expression level were detected after cIAP2 overexpression and TPL administration. After TPL administration, the RA symptoms in the TPL groups were all better, the apoptosis rate of OCP was higher, and the amount of OC in vitro were lower than that in the control group (all P < 0.05), and all of the changes in the high-dose group were more obvious than the low-dose group. In splenocytes cells cultured in vitro, cIAP2 overexpression could decrease the apoptosis rate of OCPs and increase the OC number, and TPL treatment could down-regulate the cIAP2 and promote OCP apoptosis and OC reduction. In conclusion, TPL could induce OCP apoptosis and inhibit OC formation to effectively treat RA by mediating cIAP2 degradation.

Aging of the Bone

Besides mechanical and protective function, bone serves as a keeper for marrow cells and an organ for regulation of calcium ion homeostasis. During aging, significant amounts of the bone are lost due to the loss of this delicate balance toward increased bone resorption coupled with decreased formation, which leads to net bone loss of the aging people. Osteoblasts, osteoclasts, and osteocytes are defined by their respective functions of bone formation and bone resorption. So, during bone aging, how the bone and bone cells will change are key issues for understanding osteoporosis. In this chapter, we focus on the changes of these factors during aging of the bone.

Dihydromyricetin Protects against Bone Loss in Ovariectomized Mice by Suppressing Osteoclast Activity

Dihydromyricetin (DMY), the main flavonoid component of Ampelopsis grossedentata, possesses pharmacological activities useful for treatment of diseases associated with inflammation and oxidative damage. Because osteoclasts are often involved in chronic low-grade systemic inflammation and oxidative damage, we hypothesized that DMY may be an effective treatment for osteoclast-related diseases. The effects of DMY on osteoclast formation and activity were examined in vitro. Female C57BL/6 mice were ovariectomized to mimic menopause-induced bone loss and treated with DMY, and femur samples were subjected to bone structure and histological analysis, serum biochemical indicators were also measured. DMY suppressed the activation of nuclear factor-κB, c-Fos and mitogen-activated protein kinase, and prevented production of reactive oxygen species. DMY decreased expression of osteoclast-specific genes, including Trap, Mmp-9, Cathepsin K, C-Fos, Nfatc1, and Rank. In addition, DMY prevented bone loss and decreased serum levels of tumor necrosis factor-α, interleukin-1β, and interleukin-6, and with a decrease in the ratio between receptor activator of nuclear factor-κB (RANK) ligand (RANKL) and osteoprotegerin (OPG) in vivo. These findings demonstrate that DMY attenuates bone loss and inhibits osteoclast formation and activity through modulation of multiple pathways both upstream and downstream of RANKL signaling. DMY may thus be a useful option for treatment of osteoclast-related diseases such as rheumatoid arthritis and osteoporosis.

Mitochondrial Oxidative Stress Reduces the Immunopotency of Mesenchymal Stromal Cells in Adults With Coronary Artery Disease

RATIONALE

Mesenchymal stromal cells (MSCs) are promising therapeutic strategies for coronary artery disease; however, donor-related variability in cell quality is a main cause of discrepancies in preclinical studies. In vitro, MSCs from individuals with coronary artery disease have reduced ability to suppress activated T-cells. The mechanisms underlying the altered immunomodulatory capacity of MSCs in the context of atherosclerosis remain elusive.

OBJECTIVE

The aim of this study was to assess the role of mitochondrial dysfunction in the impaired immunomodulatory properties of MSCs from patients with atherosclerosis.

METHODS AND RESULTS

Adipose tissue-derived MSCs were isolated from atherosclerotic (n=38) and nonatherosclerotic (n=42) donors. MSCs:CD4⁺T-cell suppression was assessed in allogeneic coculture systems. Compared with nonatherosclerotic-MSCs, atherosclerotic-MSCs displayed higher levels of both intracellular (P=0.006) and mitochondrial (P=0.03) reactive oxygen species reflecting altered mitochondrial function. The increased mitochondrial reactive oxygen species levels of atherosclerotic-MSCs promoted a phenotypic switch characterized by enhanced glycolysis and an altered cytokine secretion (interleukin-6 P<0.0001, interleukin-8/C-X-C motif chemokine ligand 8 P=0.04, and monocyte chemoattractant protein-1/chemokine ligand 2 P=0.01). Furthermore, treatment of atherosclerotic-MSCs with the reactive oxygen species scavenger N-acetyl-l-cysteine reduced the levels of interleukin-6, interleukin-8/C-X-C motif chemokine ligand 8, and monocyte chemoattractant protein-1/chemokine ligand 2 in the MSC secretome and improved MSCs immunosuppressive capacity (P=0.03).

CONCLUSIONS

An impaired mitochondrial function of atherosclerotic-MSCs underlies their altered secretome and reduced immunopotency. Interventions aimed at restoring the mitochondrial function of atherosclerotic-MSCs improve their in vitro immunosuppressive ability and may translate into enhanced therapeutic efficiency.

Reactive oxygen species are required for zoledronic acid-induced apoptosis in osteoclast precursors and mature osteoclast-like cells

Inhibiting osteoclasts and osteoclast precursors to reduce bone resorption is an important strategy to treat osteoclast-related diseases, such as osteoporosis, inflammatory bone loss, and malignant bone metastasis. However, the mechanism by which apoptosis is induced in the osteoclasts and their precursors are not completely understood. Here, we used nitrogen-containing bisphosphonate zoledronic acid (ZA) to induce cell apoptosis in human and murine osteoclast precursors and mature osteoclast-like cells. Caspase-3-mediated cell apoptosis occurred following the ZA (100 μM) treatment. Reactive oxygen species (ROS) were also generated in a time-dependent manner. Following knock-down of the p47^phox expression, which is required for ROS activation, or co-treatment with the ROS inhibitor, N-acetyl-L-cysteine, ZA-induced apoptosis was significantly suppressed in both osteoclast precursors and mature osteoclast-like cells. The ROS-activated mitogen-activated protein kinases pathways did not trigger cell apoptosis. However, a ROS-regulated Mcl-1 decrease simultaneously with glycogen synthase kinase (GSK)-3β promoted cell apoptosis. These findings show that ZA induces apoptosis in osteoclast precursors and mature osteoclast-like cells by triggering ROS- and GSK-3β-mediated Mcl-1 down-regulation.

Is Diabetic Skeletal Fragility Associated with Microvascular Complications in Bone?

PURPOSE OF REVIEW

The objective of this literature review is to determine whether there are indications that microvascular complications occur in diabetic bone. Evidence definitively linking diabetic skeletal fragility with microvascular complications in bone remains elusive.

RECENT FINDINGS

Circumstantial evidence, some recent and some lost to time, suggests that atherosclerotic vascular diseases such as peripheral arterial disease cause poor blood perfusion of bone and subsequent hypoxia and contribute to low bone density and high cortical porosity, patterns similar to some recently observed in diabetic subjects. Evidence also exists to suggest that potentially anti-angiogenic conditions, such as impaired vascular endothelial growth factor (VEGF) signaling, predominate in diabetic bone. Microvascular complications may contribute, in part, to diabetic skeletal fragility but data supporting this interpretation are primarily circumstantial at this time. This review highlights gaps in our knowledge and hopefully spurs further discussions and research on this topic.

Redox Status in Patients with Femoral Neck Fractures

The femur transfers the body weight from the pelvic bone to the shinbone. Femur fractures are a significant cause of morbidity and mortality among the group of locomotor apparatus injuries, especially in the elderly population. Considering that oxidative stress occurs as a result of increased production of free radicals that damage cell function and cause numerous pathological conditions and diseases, the aim of this study was to investigate oxidative stress parameters in older patients with femoral neck fractures. This clinical study included 70 patients, of which 35 had femoral neck fractures (26 males and 9 females), while the other half of the patients formed the matched control group. Markers of oxidative stress (NO2 −, TBARS, H2O2 and O2 -) and anti-oxidative enzymes (SOD, CAT, and GSH) were measured. Results showed that the levels of O2 - increased, while levels of NO2 -, H2O2 and all the antioxidative enzymes decreased in patients with femoral neck fractures. These findings indicate that fractures cause oxidative stress, probably because of the reduced activity of osteoblasts and the increased activity of osteoclasts.

Dihydroartemisinin attenuates lipopolysaccharide-induced osteoclastogenesis and bone loss via the mitochondria-dependent apoptosis pathway

Dihydroartemisinin (DHA) is a widely used antimalarial drug isolated from the plant Artemisia annua. Recent studies suggested that DHA has antitumor effects utilizing its reactive oxygen species (ROS) yielding mechanism. Here, we reported that DHA is inhibitory on lipopolysaccharide (LPS)-induced osteoclast (OC) differentiation, fusion and bone-resorption activity in vitro. Intracellular ROS detection revealed that DHA could remarkably increase ROS accumulation during LPS-induced osteoclastogenesis. Moreover, cell apoptosis was also increased by DHA treatment. We found that DHA-activated caspase-3 increased Bax/Bcl-2 ratio during LPS-induced osteoclastogenesis. Meanwhile, the translocation of apoptotic inducing factor (AIF) and the release of cytochrome c from the mitochondria into the cytosol were observed, indicating that ROS-mediated mitochondrial dysfunction is crucial in DHA-induced apoptosis during LPS-induced osteoclastogenesis. In vivo study showed that DHA treatment decreased OC number, prevents bone loss, rescues bone microarchitecture and restores bone strength in LPS-induced bone-loss mouse model. Together, our findings indicate that DHA is protective against LPS-induced bone loss through apoptosis induction of osteoclasts via ROS accumulation and the mitochondria-dependent apoptosis pathway. Therefore, DHA may be considered as a new therapeutic candidate for treating inflammatory bone loss.

Poly(ADP-ribose) in the bone: from oxidative stress signal to structural element

Contrary to common perception bone is a dynamic organ flexibly adapting to changes in mechanical loading by shifting the delicate balance between bone formation and bone resorption carried out by osteoblasts and osteoclasts, respectively. In the past decades numerous studies demonstrating production of reactive oxygen or nitrogen intermediates, effects of different antioxidants, and involvement of prototypical redox control mechanisms (Nrf2-Keap1, Steap4, FoxO, PAMM, caspase-2) have proven the central role of redox regulation in the bone. Poly(ADP-ribosyl)ation (PARylation), a NAD-dependent protein modification carried out by poly(ADP-ribose) polymerase (PARP) enzymes recently emerged as a new regulatory mechanism fine-tuning osteoblast differentiation and mineralization. Interestingly PARylation does not simply serve as a signaling mechanism during osteoblast differentiation but also couples it to osteoblast death. Even more strikingly, the poly(ADP-ribose) polymer likely released from succumbed cells at the terminal stage of differentiation is incorporated into the bone matrix representing the first structural role of this versatile biopolymer. Moreover, this new paradigm explains why and how osteodifferentiation and death of cells entering this pathway are closely coupled to each other. Here we review the role of reactive oxygen and nitrogen intermediates as well as PARylation in osteoblast and osteoclast differentiation, function, and cell death.

Variability in reference point microindentation and recommendations for testing cortical bone: maximum load, sample orientation, mode of use, sample preparation and measurement spacing

Reference Point Indentation (RPI) is a novel microindentation tool that has emerging clinical potential for the assessment of fracture risk as well as use as a laboratory tool for straight-forward mechanical characterisation of bone. Despite increasing use of the tool, little research is available to advise the set-up of testing protocols or optimisation of testing parameters. Here we consider five such parameters: maximum load, sample orientation, mode of use, sample preparation and measurement spacing, to investigate how they affect the Indentation Distance Increase (IDI), the most published measurement parameter associated with the RPI device. The RPI tool was applied to bovine bone; indenting in the proximal midshaft of five femora and human bone; indenting five femoral heads and five femoral neck samples. Based on the findings of these studies we recommend the following as the best practice. (1) Repeat measurements should be utilised to reduce the coefficient of variation (e.g. 8-15 repeats to achieve a 5-10% error, however the 3-5 measurements used here gives a 15-20% error). (2) IDI is dependent on maximum load (r=0.45 on the periosteal surface and r=0.94 on the machined surface, p<0.05), mode of use (i.e. comparing the device held freehand compared to fixed in its stand, p=0.04) and surface preparation (p=0.004) so these should be kept consistent throughout testing. Though sample orientation appears to have minimal influence on IDI (p>0.05), care should also be taken in combining measurements from different orientations. (3) The coefficient of variation is higher (p=0.04) when holding the device freehand, so it should ideally be kept supported in its stand. (4) Removing the periosteum (p=0.04) and machining the surface of the bone (p=0.08) reduces the coefficient of variation, so should be performed where practical. (5) There is a hyperbolic relationship between thickness and IDI (p<0.001) with a sample thickness 10 fold greater than the maximum indentation depth recommended, to ensure a representative measurement. (6) Measurement spacing does not appear to influence the IDI (p>0.05), so it can be as low as 500 µm. By following these recommendations, RPI users can minimise the potential confounding effects associated with the variables investigated here and reduce the coefficient of variation, hence achieving more consistent testing. This optimisation of the technique enhances both the clinical and laboratory potential of the tool.