Michael Pazianas

A ryanodine receptor-like molecule expressed in the osteoclast plasma membrane functions in extracellular Ca2+ sensing.

Ryanodine receptors (RyRs) reside in microsomal membranes where they gate Ca2+ release in response to changes in the cytosolic Ca2+ concentration. In the osteoclast, a divalent cation sensor, the Ca2+ receptor (CaR), located within the cells plasma membrane, monitors changes in the extracellular Ca2+ concentration. Here we show that a RyR-like molecule is a functional component of this receptor. We have demonstrated that [3H] ryanodine specifically binds to freshly isolated rat osteoclasts. The binding was displaced by ryanodine itself, the CaR agonist Ni2+ and the RyR antagonist ruthenium red. The latter also inhibited cytosolic Ca2+ elevations induced by Ni2+. In contrast, the responses to Ni2+ were strongly potentiated by an antiserum Ab129 raised to an epitope located within the channel-forming domain of the type II RyR. The antiserum also stained the surface of intact, unfixed, trypan blue-negative osteoclasts. Serial confocal sections and immunogold scanning electron microscopy confirmed a plasma membrane localization of this staining. Antiserum Ab34 directed to a putatively intracellular RyR epitope expectedly did not stain live osteoclasts nor did it potentiate CaR activation. It did, however, stain fixed, permeabilized cells in a distinctive cytoplasmic pattern. We conclude that an RyR-like molecule resides within the osteoclast plasma membrane and plays in important role in extracellular Ca2+ sensing.

Bone Mineral Density in Patients with Human Immunodeficiency Virus Infection

Abstract. The Object of this study was to determine whether HIV infection is associated with decreased bone mineral density (BMD). BMD was measured by dual-energy X-ray absorptiometry at total body, lumbar spine, and hip in 45 men with HIV infection and compared with sex, age, and weight-matched controls. Repeat scans were performed after a mean interval of 15 months in 21 patients to determine whether there were detectable losses of BMD. Compared with controls, the HIV patients had marginally lower BMD at the lumbar spine (P= 0.04) but there was no significant difference in total body or hip BMD. None of the patients had reduced BMD to levels associated with a diagnosis of osteoporosis. On longitudinal follow-up, a small decrease in total body BMD (−1.6%; P= 0.02) was observed but there was no significant change in spine and hip BMD. In spite of the many features of HIV infection that might be expected to cause a reduction in BMD such as cytokine activation, decreased physical activity, small bowel disease, hypogonadism, and direct infection of osteogenic cells by HIV, we found only minimal differences in BMD between HIV patients and controls. Furthermore, the HIV patients studied did not appear to show excessive loss in bone mineral over time.

Longitudinal changes in body composition measured with a variety of methods in patients with AIDS

We test the hypothesis that human immunodeficiency virus (HIV)-related weight loss is accompanied by inappropriately large losses of fat-free mass (FFM). Our secondary aims were to examine whether FFM increases during weight gain and to compare several techniques for measuring FFM change. FFM was measured at intervals averaging 5 months in 21 AIDS patients by means of skinfold thickness (SF), dual-energy x-ray absorptiometry (DEXA), total body water (TBW), and bioelectrical impedance using the equation of the manufacturer of the equipment (BIA(EZComp)) and a published prediction equation (BIA(Segal)). The FFM content of weight loss was similar for SF (57%), DEXA (60%), TBW (55%) and BIA(EZComp) (65%), but the result from BIA(Segal) (78%) was higher. The results were close to predicted starvation values apart from the results with BIA(Segal), which were significantly higher than predicted values. Weight gain was also composed of a large proportion of FFM. There were large intermethod differences in measurements of absolute FFM, but for measuring changes in FFM, the bias between SF, DEXA, and TBW was minimal. The results of BIA vary with the prediction equation used. In this group of patients with the acquired immune deficiency syndrome (AIDS), weight loss was composed of a large proportion of FFM, but in general this is compatible with undernutrition as the underlying cause and does not support the hypothesis of excessive FFM catabolism in HIV disease. SF, DEXA, TBW, and BIA(Segal) show reasonable agreement for measuring body composition changes. This information should be considered in the design of future intervention studies for HIV-related wasting.

Osteonecrosis of the Jaw and the Role of Macrophages

Nitrogen-containing bisphosphonates have been associated with the development of osteonecrosis of the jaws (ONJ), but the lack of reliable epidemiological data and appropriate animal models has restricted our understanding of ONJ pathophysiology and limited its management. The best available information is from histopathologic findings, which implicate bone necrosis and infection, although it is not clear which is primary. However, there are data suggesting that macrophages could well be the central factor in allowing the infection to develop first, followed by local necrosis, which could also account for the development of ONJ in patients treated with denosumab, a human monoclonal antibody to the receptor activator of nuclear factor-κB ligand. This review examines the evidence that macrophages could play a prominent role in development of ONJ and the proposal that it may be more appropriate to view ONJ as a drug and not only a bisphosphonate-related complication.

Safety of bisphosphonates

Bisphosphonates are one of the most well studied groups of medications and they are bone specific. This tissue specificity is a rare property for a drug introduced into clinical practice as long as 40 years ago. Over the years, the therapeutic boundaries of bisphosphonates were explored and their safety profile has withstood the challenges of the harsh clinical reality and widespread use. Certainly, the esophageal or gastric irritation caused by the oral preparations is an established adverse effect, the risk of which can be reduced by the recommended routine of taking the medication. From the other reported associations with adverse events, osteonecrosis of the jaw (ONJ) and subtrochanteric fractures have attracted most of the attention mainly because their pathophysiology remains unclear. However, overall, only a very small proportion of patients treated with bisphosphonates, especially with the oral formulations, experience adverse events and the overall benefits have consistently outweighed their potential risks. Furthermore, bisphosphonates improve the quality of life in patients with metastatic bone cancer and delay the development of adverse skeletal effects.

Prevention and treatment of osteoporosis in inflammatory bowel disease

&NA; The following are guidelines for evaluation and consideration for treatment of patients with inflammatory bone disease (IBD) after bone mineral density (BMD) measurements. The Crohns & Colitis Foundation of America (CCFA) has indicated that its recommendations are intended to serve as reference points for clinical decision‐making, not as rigid standards, limits, or rules. They should not be interpreted as quality standards.

Long-term treatment with bisphosphonates and their safety in postmenopausal osteoporosis

Bisphosphonates are the leading drugs for the treatment of osteoporosis. In randomized controlled trials (RCTs), alendronate, risedronate, and zoledronate have shown to reduce the risk of vertebral, nonvertebral, and hip fractures, whereas RCTs with ibandronate show antifracture efficacy at vertebral sites. Bisphosphonates are generally well tolerated and safe. Nevertheless, adverse events have been noted, and it is important to consider the strength of the evidence for causal relationships. Effects on the gastrointestinal tract and kidney function are well recognized, as are transient acute-phase reactions. Atrial fibrillation was first identified as a potential adverse event in a zoledronate trial, but subsequent trials and analyses failed to substantiate an association with bisphosphonates. Case reports have suggested a relationship between oral bisphosphonates and esophageal cancer, but this has not been demonstrated in epidemiologic studies. A possible association between bisphosphonate use and osteonecrosis of the jaw (ONJ) has also been suggested. However, the risk of ONJ in patients with osteoporosis appears to be very low, with no evidence from prospective RCTs of a causal association. There are reports of occasional occurrence of subtrochanteric or diaphyseal fractures in osteoporotic patients, but an association with bisphosphonate therapy is not substantiated by epidemiologic studies or prospective RCTs.

Calcium absorption and bone mineral density in celiacs after long term treatment with gluten-free diet and adequate calcium intake

Calcium malabsorption, hypocalcemia and skeletal demineralization are well-recognized features of untreated celiac disease. This study investigates calcium absorption and bone mineral density (BMD) after a prolonged, over 4 years, treatment with a gluten-free diet. Twenty-four adult females with treated celiac disease and twenty age- and sex-matched control subjects were studied. Mean body mass index (MBI), energy intake, serum calcium, and serum 25(OH)D concentrations in treated celiacs did not differ from controls. However, while both dietary calcium and protein intake were significantly higher in celiacs (P<0.012), fractional calcium absorption was lower (mean percentage±SD; treated 39.8±12 versus controls 52.3±10, P<0.001). Thus, after adjusting for calcium intake, the estimated amount of calcium absorbed daily was similar in both groups. Whole body, spine and trochanter BMD were significantly lower in treated celiac patients compared with controls (P<0.05). There were significant inverse correlations between: serum parathyroid hormone (PTH) and femoral neck or total body BMD (P<0.01), PTH and duration of gluten-free diet (P=0.05), and fractional calcium absorption and alkaline phosphatase (P=0.022). Increased calcium intake could potentially compensate for the reduced fractional calcium absorption in treated adult celiac patients, but may not normalize the BMD. In addition, the inverse correlation between PTH and time following treatment is suggestive of a continuing long-term benefit of gluten withdrawal on bone metabolism in celiac patients.

Role of the endothelial cell in osteoclast control: New perspectives

The osteoclast is of central importance in the process of bone remodeling. Its function is regulated by hormones and locally produced factors. Endothelial cells occur in close proximity to the osteoclast. Some endothelial cell-derived products, including endothelins, nitric oxide, and reactive oxygen species, have been recently implicated as modulators of osteoclast function. Endothelins inhibit bone resorption and osteoclast margin ruffling (quiescence or Q effect) at concentrations similar to those effective for their primary vasoconstrictive action. Contrary to expectations, however, it has been shown that endothelin action on the osteoclast is not mediated through an elevation of cytosolic Ca2+. Nitric oxide (NO) produces marked cell retraction (retraction or R effect), but its detailed mode of action is unknown. However, it is clear that the effects of this autocoid are not due to enhanced cyclic guanosine monophosphate (cGMP) production, a transduction system commonly used by NO. Finally, the reactive oxygen species H2O2 has been shown recently to enhance osteoclastic activity. Thus, the reported effects of the endothelial cell-derived products on the osteoclast are generally consistent with a regulatory role for endothelial cells in osteoclast control and suggest the existence of unique activation pathways, well worth exploring further. Unravelling the responsible mechanisms may also help understand the pathophysiology of a range of bone and joint diseases. For example, in rheumatoid arthritis, there is increased H2O2 production from activated neutrophils, and bone resorption is a major pathophysiological feature.

Calcific Aortic Stenosis: A Complication of Chronic Uraemia

The incidence of aortic stenosis was studied in 174 consecutive patients aged less than 55 years at initiation of maintenance haemodialysis. Severe, calcific aortic stenosis developed in 6 patients a mean 9.7 years after starting haemodialysis (p = 0.0004 compared to population incidence). In 5 patients stenosis was due to severe premature calcification of a tricuspid aortic valve. Necropsy proven aortic valve calcification (with or without stenosis) was associated with an increased duration of haemodialysis, age greater than 35 years at starting dialysis and mitral annular calcification. There is an increased incidence of aortic stenosis in patients with chronic renal failure due to premature valvular calcification, which may be related to abnormal calcium and phosphate metabolism in uraemia.