Reproducibility and validity of a novel invasive method of assessing peripheral microvascular vasomotor function.

In healthy arteries, blood flow is regulated by microvascular tone assessed by changes in blood flow volume and vascular resistance to endothelium-dependent and -independent vasodilators. We developed a novel method of using intravascular ultrasound (IVUS) and a Doppler flow wire to measure changes in blood flow volume and vascular resistance of the profunda arterial bed. We assessed the variability over 6 months in measuring microvascular endothelium-dependent dilation to acetylcholine and endothelium-independent dilation to adenosine in 20 subjects who were part of a larger study of Gulf War Illness without obstructive peripheral artery disease. Vasomotor function was assessed by Infusions of control (dextrose), acetylcholine (10-6M), adenosine (50µg), and nitroglycerin (25µg/ml). 400 IVUS and 240 flow velocity images were measured a mean 6 (SD = 2) months apart blind to measurement and infusion stage. The mean (SD) baseline profunda flow was 227 (172) ml/min and vascular resistance 4.6 x 104 (2.4 x 104) dynes-s/cm5. The intraclass correlation coefficients for 6-month variability for vascular function were excellent (range 0.827-0.995). Bland-Altman analyses showed mean differences of less than 2% for microvascular endothelium-dependent function (flow volume and resistance) and less than 1% for macrovascular endothelium-dependent function with acceptable limits of agreement. In 49 subjects assessing concurrent validity of the technique against atherosclerosis risk factors, we observed greater impairment in microvascular endothelium-dependent function per year of age (flow volume = -1.4% (p = 0.018), vascular resistance = 1.5% (p = 0.015)) and current smoking (flow volume = -36.7% (p = .006), vascular resistance = 50.0% (p<0.001)). This novel method of assessing microvascular vasomotor function had acceptable measurement reproducibility and validity.

Long-Term Outcomes in Patients With Diabetes Mellitus Related to Prolonging Clopidogrel More Than 12 Months After Coronary Stenting.

BACKGROUND: Recent large clinical trials show lower rates of late cardiovascular events by extending clopidogrel >12 months after percutaneous coronary revascularization (PCI). However, concerns of increased bleeding have elicited support for limiting prolonged treatment to high-risk patients. OBJECTIVES: The aim of this analysis was to determine the effect of prolonging clopidogrel therapy >12 months versus ≤12 months after PCI on very late outcomes in patients with diabetes mellitus (DM). METHODS: Using the Veterans Health Administration, 28,849 patients undergoing PCI between 2002 and 2006 were categorized into 3 groups: 1) 16,332 without DM; 2) 9,905 with DM treated with oral medications or diet; and 3) 2,612 with DM treated with insulin. Clinical outcomes, stratified by stent type, ≤4 years after PCI were determined from the Veterans Health Administration and Medicare databases and risk was assessed by multivariable and propensity score analyses using a landmark analysis starting 1 year after the index PCI. The primary endpoint of the study was the risk of all-cause death or myocardial infarction (MI). RESULTS: In patients with DM treated with insulin who received drug-eluting stents (DES), prolonged clopidogrel treatment was associated with a decreased risk of death (hazard ratio [HR]: 0.59; 95% confidence interval [CI]: 0.42 to 0.82) and death or MI (HR: 0.67; 95% CI: 0.49 to 0.92). Similarly, in patients with noninsulin-treated DM receiving DES, prolonged clopidogrel treatment was associated with less death (HR: 0.61; 95% CI: 0.48 to 0.77) and death or MI (HR: 0.61; 95% CI: 0.5 to 0.75). Prolonged clopidogrel treatment was not associated with a lower risk in patients without DM or in any group receiving bare-metal stents. CONCLUSIONS: Extending the duration of clopidogrel treatment >12 months may decrease very late death or MI only in patients with DM receiving first-generation DES. Future studies should address this question in patients receiving second-generation DES.

Outcomes with prolonged clopidogrel therapy after coronary stenting in patients with chronic kidney disease.

OBJECTIVES: Patients with chronic kidney disease (CKD) are at high risk of death or myocardial infarction (MI) after percutaneous coronary interventions (PCI). We assessed whether prolonged dual antiplatelet therapy beyond the recommended 12 months may prevent adverse outcomes in patients with CKD receiving drug-eluting stents (DES) or bare-metal stents (BMS). METHODS: We studied all Veterans receiving PCI with BMS or first-generation DES in the Veterans Affairs (VA) Healthcare System between 2002 and 2006, classified by CKD (estimated glomerular filtration rate <60 mL/min) or normal renal function. We used landmark analyses from 12 months after PCI with Cox proportional hazards multivariable and propensity-adjusted models to assess the effect of prolonged clopidogrel (more than 12  months) versus 12 months or less after PCI on clinical outcomes from 1 year to 4 years after PCI. RESULTS: Of 23 042 eligible subjects receiving PCI, 4880 (21%) had CKD. Compared with normal renal function, patients with CKD had higher risks of death or MI 1-4 years after DES (21% vs 12%, HR=1.75; 95% CI 1.51 to 2.04) or BMS (28% vs 15%, HR=2.10; 95% CI 1.90 to 2.32). In patients with CKD receiving DES, clopidogrel use of more than 12 months after PCI was associated with lower risks of death or MI (18% vs 24%, HR=0.74; 95% CI 0.58 to 0.95), and death (15% vs 23%, HR=0.61; 95% CI 0.47 to 0.80), but had no effect on repeat revascularisation 1-4 years after PCI. CONCLUSIONS: In patients with CKD, prolonging clopidogrel beyond 12 months after PCI may decrease the risk of death or MI only in patients receiving first-generation DES. These results support a patient-tailored approach to prolonging clopidogrel after PCI.

CXCR4 antagonism attenuates load-induced periosteal bone formation in mice.

Mechanical loading is a key anabolic regulator of bone mass. Stromal cell-derived factor-1 (SDF-1) is a stem cell homing factor that is important in hematopoiesis, angiogenesis, and fracture healing, though its involvement in skeletal mechanoadaptation is virtually unknown. The objective of this study was to characterize skeletal expression patterns of SDF-1 and CXCR4, the receptor for SDF-1, and to determine the role of SDF-1 signaling in load-induced periosteal bone formation. Sixteen-week-old C57BL/6 mice were treated with PBS or AMD3100, an antagonist against CXCR4, and exposed to in vivo ulnar loading (2.8 N peak-to-peak, 2 Hz, 120 cycles). SDF-1 was expressed in cortical and trabecular osteocytes and marrow cells, and CXCR4 was primarily expressed in marrow cells. SDF-1 and CXCR4 expression was enhanced in response to mechanical stimulation. The CXCR4 receptor antagonist AMD3100 significantly attenuated load-induced bone formation and led to smaller adaptive changes in cortical geometric properties as determined by histomorphometric analysis. Our data suggest that SDF-1/CXCR4 signaling plays a critical role in skeletal mechanoadaptation, and may represent a unique therapeutic target for prevention and treatment of age-related and disuse bone loss.

Mechanosensing by the primary cilium: deletion of Kif3A reduces bone formation due to loading.

Primary cilia, solitary microtubule-based structures that grow from the centriole and extend into the extracellular space, have increasingly been implicated as sensors of a variety of biochemical and biophysical signals. Mutations in primary cilium-related genes have been linked to a number of rare developmental disorders as well as dysregulation of cell proliferation. We propose that primary cilia are also important in mechanically regulated bone formation in adults and that their malfunction could play a role in complex multi-factorial bone diseases, such as osteoporosis. In this study, we generated mice with an osteoblast- and osteocyte-specific knockout of Kif3a, a subunit of the kinesin II intraflagellar transport (IFT) protein; IFT is required for primary cilia formation, maintenance, and function. These Colα1(I) 2.3-Cre;Kif3a(fl/fl) mice exhibited no obvious morphological skeletal abnormalities. Skeletally mature Colα1(I) 2.3-Cre;Kif3a(fl/fl) and control mice were exposed to 3 consecutive days of cyclic axial ulna loading, which resulted in a significant increase in bone formation in both the conditional knockouts and controls. However, Colα1(I) 2.3-Cre;Kif3a(fl/fl) mice did exhibit decreased formation of new bone in response to mechanical ulnar loading compared to control mice. These results suggest that primary cilia act as cellular mechanosensors in bone and that their function may be critical for the regulation of bone physiology due to mechanical loading in adults.

Osteocyte mechanobiology and pericellular mechanics.

An impressive range of tissues and cells are regulated by mechanical loading, and this regulation is central to disease processes such as osteoporosis, atherosclerosis, and osteoarthritis. However, other than a small number of specialized excitable cells involved in hearing and touch, cellular mechanosensing mechanisms are generally quite poorly understood. A lack of mechanistic understanding of these processes is one of the primary foci of the nascent field of mechanobiology, which, as a consequence, enjoys enormous potential to make critical new insights into both physiological function and etiology of disease. In this review we outline the process in bone by tracing mechanical effects from the organ level to the cellular and molecular levels and by integrating the biological response from molecule to organ. A case is made that a fundamental roadblock to advances in mechanobiology is the dearth of information in the area of pericellular mechanics.

Osteocyte primary cilium and its role in bone mechanotransduction.

Bone is a dynamic tissue that adapts to its local loading environment. Mechanotransduction, the process by which cells convert mechanical forces into biochemical signals, is important for maintaining bone health and homeostasis. It is less clear, however, what the cellular mechanosensor(s) are that sense and initiate these signaling cascades. Primary cilia are solitary rigid structures that extend from the cell body into the extracellular space and as a consequence are prime candidates for mechanosensing in bone. Primary cilia have been shown to be critical in development and have been implicated in mechanosensing in other tissue types, including liver and kidney. In this review we discuss the potential for primary cilia to play an important role in bone mechanotransduction and possible avenues for future study.

Primary cilium-dependent mechanosensing is mediated by adenylyl cyclase 6 and cyclic AMP in bone cells.

Primary cilia are chemosensing and mechanosensing organelles that regulate remarkably diverse processes in a variety of cells. We previously showed that primary cilia play a role in mediating mechanosensing in bone cells through an unknown mechanism that does not involve extracellular Ca(2+)-dependent intracellular Ca(2+) release, which has been implicated in all other cells that transduce mechanical signals via the cilium. Here, we identify a molecular mechanism linking primary cilia and bone cell mechanotransduction that involves adenylyl cyclase 6 (AC6) and cAMP. Intracellular cAMP was quantified in MLO-Y4 cells exposed to dynamic flow, and AC6 and primary cilia were inhibited using RNA interference. When exposed to flow, cells rapidly (<2 min) and transiently decreased cAMP production in a primary cilium-dependent manner. RT-PCR revealed differential expression of the membrane-bound isoforms of adenylyl cyclase, while immunostaining revealed one, AC6, preferentially localized to the cilium. Further studies showed that decreases in cAMP in response to flow were dependent on AC6 and Gd(3+)-sensitive channels but not intracellular Ca(2+) release and that this response mediated flow-induced COX-2 gene expression. The signaling events identified provide important details of a novel early mechanosensing mechanism in bone and advances our understanding of how signal transduction occurs at the primary cilium.

Osteocytes as mechanosensors in the inhibition of bone resorption due to mechanical loading.

Bone has the ability to adjust its structure to meet its mechanical environment. The prevailing view of bone mechanobiology is that osteocytes are responsible for detecting and responding to mechanical loading and initiating the bone adaptation process. However, how osteocytes signal effector cells and initiate bone turnover is not well understood. Recent in vitro studies have shown that osteocytes support osteoclast formation and activation when co-cultured with osteoclast precursors. In this study, we examined the osteocytes' role in the mechanical regulation of osteoclast formation and activation. We demonstrated here that (1) mechanical stimulation of MLO-Y4 osteocyte-like cells decreases their osteoclastogenic-support potential when co-cultured with RAW264.7 monocyte osteoclast precursors; (2) soluble factors released by these mechanically stimulated MLO-Y4 cells inhibit osteoclastogenesis induced by ST2 bone marrow stromal cells or MLO-Y4 cells; and (3) soluble RANKL and OPG were released by MLO-Y4 cells, and the expressions of both were found to be mechanically regulated. Our data suggest that mechanical loading decreases the osteocyte's potential to induce osteoclast formation by direct cell-cell contact. However, it is not clear that osteocytes in vivo are able to form contacts with osteoclast precursors. Our data also demonstrate that mechanically stimulated osteocytes release soluble factors that can inhibit osteoclastogenesis induced by other supporting cells including bone marrow stromal cells. In summary, we conclude that osteocytes may function as mechanotransducers by regulating local osteoclastogenesis via soluble signals.

Age specificity of inbreeding load in Drosophila melanogaster and implications for the evolution of late-life mortality plateaus.

Current evolutionary theories explain the origin of aging as a byproduct of the decline in the force of natural selection with age. These theories seem inconsistent with the well-documented occurrence of late-life mortality plateaus, since under traditional evolutionary models mortality rates should increase monotonically after sexual maturity. However, the equilibrium frequencies of deleterious alleles affecting late life are lower than predicted under traditional models, and thus evolutionary models can accommodate mortality plateaus if deleterious alleles are allowed to have effects spanning a range of neighboring age classes. Here we test the degree of age specificity of segregating alleles affecting fitness in Drosophila melanogaster. We assessed age specificity by measuring the homozygous fitness effects of segregating alleles across the adult life span and calculated genetic correlations of these effects across age classes. For both males and females, we found that allelic effects are age specific with effects extending over 1-2 weeks across all age classes, consistent with modified mutation-accumulation theory. These results indicate that a modified mutation-accumulation theory can both explain the origin of senescence and predict late-life mortality plateaus.