α(1) adrenergic receptor agonist, phenylephrine, actively contracts early rat rib fracture callus ex vivo

Comparative study of coronary artery bypass graft materials: reduced contraction and ADMA levels in internal mammary artery versus saphenous vein

BACKGROUND

Vasospasm and atherosclerosis due to low endothelial capacity are the most important causes of coronary artery bypass graft failure observed in internal mammary artery (IMA) and saphenous vein (SV). Vasospasm can be mimicked in in vitro studies by inducing vasoconstriction of graft materials. In the present study, we aimed to compare the vascular contraction induced by several spasmogens including prostaglandin E2 (PGE2), prostaglandin F2 alpha (PGF2α), phenylephrine (PE), leukotriene C4 (LTC4), LTD4, potassium chloride (KCl), and arachidonic acid between IMA and SV preparations. Furthermore, endothelial capacity, nitrite and asymmetric dimethylarginine (ADMA) levels were compared between two grafts.

METHODS

By using organ bath, contractile responses induced by different spasmogens were compared between IMA and SV preparations derived from patients underwent coronary artery bypass surgery (n=35). The endothelial capacity was determined by acetylcholine (ACh) -induced relaxation in PE-precontracted vessels. Nitrite and ADMA levels were measured in organ culture supernatant of IMA and SV preparations.

RESULTS

Contractile responses induced by PGE2, PGF2α, PE, LTC4, LTD4, KCl and arachidonic acid were significantly lower in IMA preparations versus SV preparations. ACh-induced relaxation was significantly more prominent in IMA than SV preparations. Nitrite levels were greater and ADMA levels were lower in IMA versus SV preparations.

CONCLUSIONS

IMA has reduced capacity to constrict to several vasoconstrictor agents. Furthermore, IMA has greater endothelial capacity associated with higher nitrite levels and lower ADMA levels. Our results support the greater patency rate observed in IMA versus SV preparations.

A Concomitant Muscle Injury Does Not Worsen Traumatic Brain Injury Outcomes in Mice

Traumatic brain injury (TBI) often involves multitrauma in which concurrent extracranial injury occurs. We previously demonstrated that a long bone fracture exacerbates neuroinflammation and functional outcomes in mice given a TBI. Whether other forms of concomitant peripheral trauma that are common in the TBI setting, such as skeletal muscle injury, have similar effects is unknown. As such, here we developed a novel mouse multitrauma model by combining a closed-skull TBI with a cardiotoxin (CTX)-induced muscle injury to investigate whether muscle injury affects TBI outcomes. Adult male mice were assigned to four groups: sham-TBI + sham-muscle injury (SHAM); sham-TBI + CTX-muscle injury (CTX); TBI + sham-muscle injury (TBI); TBI + CTX-muscle injury (MULTI). Some mice were euthanized at 24 h post-injury to assess neuroinflammation and cerebral edema. The remaining mice underwent behavioral testing after a 30-day recovery period, and were euthanized at 35 days post-injury for post-mortem analysis. At 24 h post-injury, both TBI and MULTI mice had elevated edema, increased expression of GFAP (i.e., a marker for reactive astrocytes), and increased mRNA levels of inflammatory chemokines. There was also an effect of injury on cytokine levels at 35 days post-injury. However, the TBI and MULTI mice did not significantly differ on any of the measures assessed. These initial findings suggest that a concomitant muscle injury does not significantly affect preclinical TBI outcomes. Future studies should investigate the combination of different injury models, additional outcomes, and other post-injury time points.

Suppression of osteoclastogenesis via α2-adrenergic receptors

The sympathetic nervous system is known to regulate osteoclast development. However, the involvement of α2-adrenergic receptors (α2-ARs) in osteoclastogenesis is not well understood. In the present study, their potential role in osteoclastogenesis was investigated. Guanabenz, clonidine and xylazine were used as agonists of α2-ARs, while yohimbine and idazoxan were employed as antagonists. Using RAW264.7 pre-osteoclast and primary bone marrow cells, the mRNA expression of the osteoclast-related genes nuclear factor of activated T-cells, cytoplasmic 1 (NFATc1), tartrate-resistant acid phosphatase (TRAP) and cathepsin K was evaluated following induction with receptor activator of nuclear factor κB ligand (RANKL). TRAP staining was also conducted to assess effects on osteoclastogenesis in mouse bone marrow cells in vitro. Administration of 5–20 µM guanabenz (P<0.01, for RANKL-only treatment), 20 µM clonidine (P<0.05, for RANKL-only treatment) and 20 µM xylazine (P<0.05, for RANKL-only treatment) attenuated RANKL-induced upregulation of NFATc1, TRAP and cathepsin K mRNA. Furthermore, the reductions in these mRNAs by 10 µM guanabenz and 20 µM clonidine in the presence of RANKL were attenuated by 20 µM yohimbine or idazoxan (P<0.05). The administration of 5–20 µM guanabenz (P<0.01, for RANKL-only treatment) and 10–20 µM clonidine (P<0.05, for RANKL-only treatment) also decreased the number of TRAP-positive multi-nucleated osteoclasts. Collectively, the present study demonstrates that α2-ARs may be involved in the regulation of osteoclastogenesis.

Treatment with an interleukin-1 receptor antagonist mitigates neuroinflammation and brain damage after polytrauma

Traumatic brain injury (TBI) and long bone fracture are common in polytrauma. This injury combination in mice results in elevated levels of the pro-inflammatory cytokine interleukin-1β (IL-1β) and exacerbated neuropathology when compared to isolated-TBI. Here we examined the effect of treatment with an IL-1 receptor antagonist (IL-1ra) in mice given a TBI and a concomitant tibial fracture (i.e., polytrauma). Adult male C57BL/6 mice were given sham-injuries or polytrauma and treated with saline-vehicle or IL-1ra (100mg/kg). Treatments were subcutaneously injected at 1, 6, and 24h, and then once daily for one week post-injury. 7-8 mice/group were euthanized at 48h post-injury. 12-16 mice/group underwent behavioral testing at 12weeks post-injury and MRI at 14weeks post-injury before being euthanized at 16weeks post-injury. At 48h post-injury, markers for activated microglia and astrocytes, as well as neutrophils and edema, were decreased in polytrauma mice treated with IL-1ra compared to polytrauma mice treated with vehicle. At 14weeks post-injury, MRI analysis demonstrated that IL-1ra treatment after polytrauma reduced volumetric loss in the injured cortex and mitigated track-weighted MRI markers for axonal injury. As IL-1ra (Anakinra) is approved for human use, it may represent a promising therapy in polytrauma cases involving TBI and fracture.

Experimental Traumatic Brain Injury Induces Bone Loss in Rats

Few studies have investigated the influence of traumatic brain injury (TBI) on bone homeostasis; however, pathophysiological mechanisms involved in TBI have potential to be detrimental to bone. The current study assessed the effect of experimental TBI in rats on the quantity and quality of two different weight-bearing bones, the femur and humerus. Rats were randomly assigned into either sham or lateral fluid percussion injury (FPI) groups. Open-field testing to assess locomotion was conducted at 1, 4, and 12 weeks post-injury, with the rats killed at 1 and 12 weeks post-injury. Bones were analyzed using peripheral quantitative computed tomography (pQCT), histomorphometric analysis, and three-point bending. pQCT analysis revealed that at 1 and 12 weeks post-injury, the distal metaphyseal region of femora from FPI rats had reduced cortical content (10% decrease at 1 week, 8% decrease at 12 weeks; p < 0.01) and cortical thickness (10% decrease at 1 week, 11% decrease at 12 weeks p < 0.001). There was also a 23% reduction in trabecular bone volume ratio at 1 week post-injury and a 27% reduction at 12 weeks post-injury in FPI rats compared to sham (p < 0.001). There were no differences in bone quantity and mechanical properties of the femoral midshaft between sham and TBI animals. There were no differences in locomotor outcomes, which suggested that post-TBI changes in bone were not attributed to immobility. Taken together, these findings indicate that this rat model of TBI was detrimental to bone and suggests a link between TBI and altered bone remodeling.

Loss of Gi G-Protein-Coupled Receptor Signaling in Osteoblasts Accelerates Bone Fracture Healing

G-protein-coupled receptors (GPCRs) are key regulators of skeletal homeostasis and are likely important in fracture healing. Because GPCRs can activate multiple signaling pathways simultaneously, we used targeted disruption of G(i) -GPCR or activation of G(s) -GPCR pathways to test how each pathway functions in the skeleton. We previously demonstrated that blockade of G(i) signaling by pertussis toxin (PTX) transgene expression in maturing osteoblastic cells enhanced cortical and trabecular bone formation and prevented age-related bone loss in female mice. In addition, activation of G(s) signaling by expressing the G(s) -coupled engineered receptor Rs1 in maturing osteoblastic cells induced massive trabecular bone formation but cortical bone loss. Here, we test our hypothesis that the G(i) and G(s) pathways also have distinct functions in fracture repair. We applied closed, nonstabilized tibial fractures to mice in which endogenous G(i) signaling was inhibited by PTX, or to mice with activated G(s) signaling mediated by Rs1. Blockade of endogenous G(i) resulted in a smaller callus but increased bone formation in both young and old mice. PTX treatment decreased expression of Dkk1 and increased Lef1 mRNAs during fracture healing, suggesting a role for endogenous G(i) signaling in maintaining Dkk1 expression and suppressing Wnt signaling. In contrast, adult mice with activated Gs signaling showed a slight increase in the initial callus size with increased callus bone formation. These results show that G(i) blockade and G(s) activation of the same osteoblastic lineage cell can induce different biological responses during fracture healing. Our findings also show that manipulating the GPCR/cAMP signaling pathway by selective timing of G(s) and G(i) -GPCR activation may be important for optimizing fracture repair.

Noradrenaline stimulates cell proliferation by suppressing potassium channels via G(i/o) -protein-coupled α(1B) -adrenoceptors in human osteoblasts

BACKGROUND AND PURPOSE

Recent studies demonstrated that the sympathetic nervous system regulates bone metabolism via β(2) -adrenoceptors. Although α-adrenoceptors are also expressed in osteogenic cells, their functions in bone metabolism have been less studied. We previously demonstrated that noradrenaline suppressed potassium currents via α(1B) -adrenoceptors in the human osteoblast SaM-1 cell line. The aim of this study was to investigate the signal transduction pathway and the physiological role of noradrenaline in human osteoblasts in more detail.

EXPERIMENTAL APPROACH

To investigate signal transduction through α(1B) -adrenoceptors, we used whole-cell patch clamp recording and Ca fluorescence imaging. Potassium channels regulate membrane potential and cell proliferation activity in non-excitable cells, so we evaluated cell proliferation activity by BrdU incorporation and WST assay.

KEY RESULTS

In SaM-1 cells, bath-applied noradrenaline elevated intracellular Ca(2+) concentration and this effect was abolished by both chloroethylclonidine, an α(1B) -adrenoceptor antagonist, and U73122, a PLC inhibitor. However, the inhibitory effect of noradrenaline on whole-cell current was unaffected by U73122. In contrast, in cells pretreated with either Pertussis toxin, a G(i/o) -protein-coupled receptor inhibitor, or gallein, a Gβγ-protein inhibitor, the inhibitory effect of noradrenaline on whole-cell current was significantly suppressed. Noradrenaline-induced enhancement of cell proliferation was inhibited by CsCl, a non-selective potassium channel blocker, gallein and H89, a PKA inhibitor, but not by U73122.

CONCLUSIONS AND IMPLICATIONS

Noradrenaline facilitated cell proliferation by regulation of potassium currents in human osteoblasts via G(i/o) -protein-coupled α(1B) -adrenoceptors, not via coupling to Gq-proteins.

Transient expression of myofibroblast-like cells in rat rib fracture callus

BACKGROUND AND PURPOSE

We have previously shown that early fracture callus of rat rib has viscoelastic and contractile properties resembling those of smooth muscle. The cells responsible for this contractility have been hypothesized to be myofibroblast-like in nature. In soft-tissue healing, force generated by contraction of myofibroblasts promotes healing. Accordingly, we tried to identify myofibroblast-like cells in early fibrous callus.

ANIMALS AND METHODS

Calluses from rat rib fractures were removed 7, 14, and 21 days after fracture and unfractured ribs acted as controls. All tissues were analyzed using qPCR and immunohistochemistry. We analyzed expression of smooth muscle- and myofibroblast-associated genes and proteins including alpha smooth muscle actin (αSMA), non-muscle myosin, fibronectin extra domain A variant (EDA-fibronectin), OB-cadherin, connexin-43, basic calponin (h1CaP), and h-caldesmon.

RESULTS

In calluses at 7 days post-fracture, there were statistically significant increases in expression of αSMA mRNA (2.5 fold), h1CaP mRNA (2.1 fold), EDA-fibronectin mRNA (14 fold), and connexin-43 mRNA (1.8 fold) compared to unfractured ribs, and by 21 days post-fracture mRNA expression in calluses had decreased to levels approaching those in unfractured rib. Immunohistochemistry of 7 day fibrous callus localized calponin, EDA-fibronectin and co-immunolabeling of OB-cadherin and αSMA (thus confirming a myofibroblastic phenotype) within various cell populations.

INTERPRETATION

This study provides further evidence that early rat rib callus is not only smooth muscle-like in nature but also contains a notable population of cells that have a distinct myofibroblastic phenotype. The presence of these cells indicates that in vivo contraction of early callus is a mechanism that may occur in fractures so as to facilitate healing, as it does in soft tissue wound repair.