Fluid flow-induced soluble vascular endothelial growth factor isoforms regulate actin adaptation in osteoblasts

Expression of the HIF-1α/VEGF pathway is upregulated to protect alveolar bone density reduction in nasal-obstructed rats

BACKGROUND

Hypoxia and mouth breathing are closely related to maxillofacial bone metabolism and are characteristic of obstructive sleep apnea-hypopnea syndrome (OSAHS). Being key factors in the hypoxia response, hypoxia-inducible factor 1α (HIF-1α) and HIF-responsive gene vascular endothelial growth factor (VEGF) are essential for bone remodeling. This study focuses on the role of the HIF-1α/VEGF pathway in alveolar bone metabolism during OSAHS.

MATERIALS AND METHODS

36 three-week-old male Wistar rats were divided into three groups: twelve control rats, twelve bilateral nasal obstructed (BNO) rats, twelve BNO rats treated with intraperitoneal injection of Dimethyloxalylglycine (DMOG). After two weeks, the microstructure and bone mineral density (BMD) of alveolar bone were evaluated using micro-computed tomography (micro-CT). The expressions of HIF-1α and VEGF in the alveolar bone were then assessed via immunohistochemistry staining, quantitative real-time polymerase chain reaction (qRT-PCR) and Western blot. Alkaline phosphatase (ALP) staining and Alizarin red S staining were performed to evaluate osteogenesis of bone marrow-derived mesenchymal stem cells (BMSCs).

RESULTS

Significant reductions in alveolar bone density were noted in BNO rats. Bilateral nasal obstruction increased the expressions of HIF-1α and VEGF in alveolar bone. With upregulation of HIF-1α/VEGF via DMOG, alveolar bone density of BNO rats increased. Furthermore, DMOG promoted the osteogenic differentiation of BMSCs by stabilizing the HIF-1α protein and increasing the expression of VEGF.

CONCLUSION

Bilateral nasal obstruction changes alveolar bone structure and leads to a reduction in alveolar bone density. Moreover, the expression of the HIF-1α/VEGF signaling pathway increases to protect alveolar bone density reduction in BNO rats.

MiRNAs regulate cell communication in osteogenesis-angiogenesis coupling during bone regeneration

Bone regeneration is a complex process that requires not only the participation of multiple cell types, but also signal communication between cells. The two basic processes of osteogenesis and angiogenesis are closely related to bone regeneration and bone homeostasis. H-type vessels are a subtype of bone vessels characterized by high expression of CD31 and EMCN. These vessels play a key role in the regulation of bone regeneration and are important mediators of coupling between osteogenesis and angiogenesis. Molecular regulation between different cell types is important for coordination of osteogenesis and angiogenesis that promotes bone regeneration. MiRNAs are small non-coding RNAs that predominantly regulate gene expression at the post-transcriptional level and are closely related to cell communication. Specifically, miRNAs transduce external stimuli through various cell signaling pathways and cause a series of physiological and pathological effects. They are also deeply involved in the bone repair process. This review focuses on three signaling pathways related to osteogenesis-angiogenesis coupling, as well as the miRNAs involved in these pathways. Elucidation of the molecular mechanisms governing osteogenesis and angiogenesis is of great significance for bone regeneration.

Lowry M, Almasri R, Gorgun C, Eichholz K, Stavenschi E, Hokamp K, Roche F, O’Driscoll L, Hoey D. Mechanically activated mesenchymal-derived bone cells drive vessel formation via an extracellular vesicle mediated mechanism

Blood vessel formation is an important initial step for bone formation during development as well as during remodelling and repair in the adult skeleton. This results in a heavily vascularized tissue where endothelial cells and skeletal cells are constantly in crosstalk to facilitate homeostasis, a process that is mediated by numerous environmental signals, including mechanical loading. Breakdown in this communication can lead to disease and/or poor fracture repair. Therefore, this study aimed to determine the role of mature bone cells in regulating angiogenesis, how this is influenced by a dynamic mechanical environment, and understand the mechanism by which this could occur. Herein, we demonstrate that both osteoblasts and osteocytes coordinate endothelial cell proliferation, migration, and blood vessel formation via a mechanically dependent paracrine mechanism. Moreover, we identified that this process is mediated via the secretion of extracellular vesicles (EVs), as isolated EVs from mechanically stimulated bone cells elicited the same response as seen with the full secretome, while the EV-depleted secretome did not elicit any effect. Despite mechanically activated bone cell-derived EVs (MA-EVs) driving a similar response to VEGF treatment, MA-EVs contain minimal quantities of this angiogenic factor. Lastly, a miRNA screen identified mechanoresponsive miRNAs packaged within MA-EVs which are linked with angiogenesis. Taken together, this study has highlighted an important mechanism in osteogenic-angiogenic coupling in bone and has identified the mechanically activated bone cell-derived EVs as a therapeutic to promote angiogenesis and potentially bone repair.

Polydopamine functionalized VEGF gene-activated 3D printed scaffolds for bone regeneration

Bone is a highly vascularized organ and the formation of new blood vessels is essential to regenerate large critical bone defects.

Asperosaponin VI Injection Enhances Orthodontic Tooth Movement in Rats

Background

This study was performed to investigate the effect of local injection of asperosaponin VI (ASA VI) on the orthodontic tooth movement in rats.

Material/Methods

A total of 64 healthy female Sprague-Dawley rats were selected and divided into 2 groups randomly: the ASA VI group and the control group. For the ASA VI group, 10 mg/kg ASA VI solution was injected into buccal submucoperiosteal of bilaterally first maxillary molars, and the same volume of normal saline was given to the control group. The orthodontic force was applied to the maxillary first molars. All rats were sacrificed on days 3, 7, or 14. Tooth movement effects on the periodontium were analyzed through hematoxylin and eosin (H&E) staining, tartrate-resistant acid phosphatase (TRAP) staining and immunohistochemistry analysis. Tooth movement measurements and alveolar bone volumetric changes were analyzed using a micro-computed tomography (CT) scan. Molecular changes were evaluated by quantitative real-time polymerase chain reaction (qRT-PCR) and western blot.

Results

The ASA VI group presented with a significant increase of tooth movement, osteoclast number, and the expression of osteoclast differentiation factor (ODF) compared with the control group. ASA VI also induced a significant decrease in bone volume and density and an increase in trabecular spacing and RANKL (receptor activator of nuclear factor kappa-B ligand) expression at the compression side. Furthermore, ASA VI stimulated bone formation on the tension side by enhancing OCN (osteocalcin) expression and RUNX2 (runt-related transcription factor 2) expression, increasing bone volume and density and decreasing in trabecular spacing.

Conclusions

Injection of ASA VI may accelerate tooth movement via increasing the activity of osteoclasts, stimulating bone resorption at the compression side. Furthermore, ASA VI has a positive effect on bone formation at the tension side.

Applied mechanical loading to mouse hindlimb acutely increases skeletal perfusion and chronically enhanced vascular porosity

Blood supply is essential for osteogenesis, yet its relationship to load-related increases in bone mass is poorly defined. Herein, we aim to investigate the link between load-induced osteogenesis and the blood supply (bone perfusion and vascular porosity) using an established osteogenic noninvasive model of axial loading. Accordingly, 12 N mechanical loads were applied to the right tibiae of six male C57BL6 mice at 10-12 wk of age, 3 times/wk for 2 wk. Skeletal perfusion was measured acutely (postloading) and chronically in loaded and contralateral, nonloaded hindlimbs by laser-Doppler imaging. Vascular and lacunar porosity of the cortical bone and tibia load-related changes in trabecular and cortical bone was measured by nanoCT and micro-CT, respectively. We found that the mean skeletal perfusion (loaded: nonloaded limb ratio) increased by 56% immediately following the first loading episode (vs. baseline, P < 0.01), and a similar increase was observed after all loading episodes, demonstrating that these acute responses were conserved for 2 wk of loading. Loading failed, however, to engender any significant chronic changes in mean perfusion between the beginning and the end of the experiment. In contrast, 2 wk of loading engendered an increased vascular canal number in the tibial cortical compartment (midshaft) and, as expected, also increased trabecular and cortical bone volumes and modified tibial architecture in the loaded limb. Our results indicate that each episode of loading both generates acute enhancement in skeletal blood perfusion and also stimulates chronic vascular architectural changes in the bone cortices, which coincide with load-induced increases in bone mass.NEW & NOTEWORTHY This study investigated modifications to the blood supply (bone perfusion and intracortical vascular canals) in mechanoadaptive responses in C57BL6 mice. Each episode of mechanical loading acutely increases skeletal perfusion. Two weeks of mechanical loading increased bone mass and cortical vascular canal number, while there was no chronic increase in hindlimb perfusion. Our findings suggest that the blood supply may participate in the processes that govern load-induced bone formation.

A simple rocker-induced mechanical stimulus upregulates mineralization by human osteoprogenitor cells in fibrous scaffolds

Biodegradable electrospun polycaprolactone scaffolds can be used to support bone-forming cells and could fill a thin bony defect, such as in cleft palate. Oscillatory fluid flow has been shown to stimulate bone production in human progenitor cells in monolayer culture. The aim of this study was to examine whether bone matrix production by primary human mesenchymal stem cells from bone marrow or jaw periosteal tissue could be stimulated using oscillatory fluid flow supplied by a standard see-saw rocker. This was investigated for cells in two-dimensional culture and within electrospun polycaprolactone scaffolds. From day 4 of culture onwards, samples were rocked at 45 cycles/min for 1 h/day, 5 days/week (rocking group). Cell viability, calcium deposition, collagen production, alkaline phosphatase activity and vascular endothelial growth factor secretion were evaluated to assess the ability of the cells to undergo bone differentiation and induce vascularisation. Both cell types produced more mineralized tissue when subjected to rocking and supplemented with dexamethasone. Mesenchymal progenitors and primary human mesenchymal stem cells from bone marrow in three-dimensional scaffolds upregulated mineral deposition after rocking culture as assessed by micro-computed tomography and alizarin red staining. Interestingly, vascular endothelial growth factor secretion, which has previously been shown to be mechanically sensitive, was not altered by rocking in this system and was inhibited by dexamethasone. Rocker culture may be a cost effective, simple pretreatment for bone tissue engineering for small defects such as cleft palate.

Enhancement of bone regeneration with the accordion technique via HIF-1α/VEGF activation in a rat distraction osteogenesis model

Axial micromotion of bone fragments promotes callus formation and bone healing during the process of distraction osteogenesis (DO). This study investigated the effects of the combined axial compression and distraction (accordion) technique on bone regeneration in rat DO model. Male Sprague-Dawley rats (n = 62) underwent right tibial transverse osteotomy and were randomly divided into four groups after lengthening: control (no manipulation) and three experimental groups assigned on the basis of the period of accordion manoeuvres in the consolidation phase (Groups 1, 2, and 3 with accordion technique applied at Weeks 1, 3, and 5, respectively). Animals were terminated at 1 week after each accordion phase (i.e., Weeks 2, 4, and 6). Callus formation was monitored by X-ray radiography; new bone quality was evaluated by microcomputed tomography, histological analysis, and mechanical testing. Serum levels of hypoxia-inducible factor (HIF)-1α and vascular endothelial growth factor (VEGF) were measured. Callus formation after accordion manoeuvre at Week 3 (Group 2) increased significantly over time of consolidation. The microcomputed tomography and mechanical analysis revealed Group 2 had more newly formed bone and superior mechanical properties in contrast to the other groups at termination. Histomorphological and immunohistochemical analyses confirmed a greater degree of osteogenesis and angiogenesis corresponding to increased serum levels of HIF-1α and VEGF in Group 2. The accordion technique was effective in promoting bone consolidation via activation of HIF-1α/VEGF during DO. The accordion technique may be used in the middle phase of bone consolidation to promote bone formation in patients undergoing DO treatment.

The roles of vascular endothelial growth factor in bone repair and regeneration

Vascular endothelial growth factor-A (VEGF) is one of the most important growth factors for regulation of vascular development and angiogenesis. Since bone is a highly vascularized organ and angiogenesis plays an important role in osteogenesis, VEGF also influences skeletal development and postnatal bone repair. Compromised bone repair and regeneration in many patients can be attributed to impaired blood supply; thus, modulation of VEGF levels in bones represents a potential strategy for treating compromised bone repair and improving bone regeneration. This review (i) summarizes the roles of VEGF at different stages of bone repair, including the phases of inflammation, endochondral ossification, intramembranous ossification during callus formation and bone remodeling; (ii) discusses different mechanisms underlying the effects of VEGF on osteoblast function, including paracrine, autocrine and intracrine signaling during bone repair; (iii) summarizes the role of VEGF in the bone regenerative procedure, distraction osteogenesis; and (iv) reviews evidence for the effects of VEGF in the context of repair and regeneration techniques involving the use of scaffolds, skeletal stem cells and growth factors.

Osteoblast-derived paracrine factors regulate angiogenesis in response to mechanical stimulation

Angiogenesis is a process by which new blood vessels emerge from existing vessels through endothelial cell sprouting, migration, proliferation, and tubule formation. Angiogenesis during skeletal growth, homeostasis and repair is a complex and incompletely understood process. As the skeleton adapts to mechanical loading, we hypothesized that mechanical stimulation regulates "osteo-angio" crosstalk in the context of angiogenesis. We showed that conditioned media (CM) from osteoblasts exposed to fluid shear stress enhanced endothelial cell proliferation and migration, but not tubule formation, relative to CM from static cultures. Endothelial cell sprouting was studied using a dual-channel collagen gel-based microfluidic device that mimics vessel geometry. Static CM enhanced endothelial cell sprouting frequency, whereas loaded CM significantly enhanced both frequency and length. Both sprouting frequency and length were significantly enhanced in response to factors released from osteoblasts exposed to fluid shear stress in an adjacent channel. Osteoblasts released angiogenic factors, of which osteopontin, PDGF-AA, IGBP-2, MCP-1, and Pentraxin-3 were upregulated in response to mechanical loading. These data suggest that in vivo mechanical forces regulate angiogenesis in bone by modulating "osteo-angio" crosstalk through release of paracrine factors, which we term "osteokines".

P2X7R-Panx1 Complex Impairs Bone Mechanosignaling under High Glucose Levels Associated with Type-1 Diabetes

Type 1 diabetes (T1D) causes a range of skeletal problems, including reduced bone density and increased risk for bone fractures. However, mechanisms underlying skeletal complications in diabetes are still not well understood. We hypothesize that high glucose levels in T1D alters expression and function of purinergic receptors (P2Rs) and pannexin 1 (Panx1) channels, and thereby impairs ATP signaling that is essential for proper bone response to mechanical loading and maintenance of skeletal integrity. We first established a key role for P2X7 receptor-Panx1 in osteocyte mechanosignaling by showing that these proteins are co-expressed to provide a major pathway for flow-induced ATP release. To simulate in vitro the glucose levels to which bone cells are exposed in healthy vs. diabetic bones, we cultured osteoblast and osteocyte cell lines for 10 days in medium containing 5.5 or 25 mM glucose. High glucose effects on expression and function of P2Rs and Panx1 channels were determined by Western Blot analysis, quantification of Ca²⁺ responses to P2R agonists and oscillatory fluid shear stress (± 10 dyne/cm²), and measurement of flow-induced ATP release. Diabetic C57BL/6J-Ins2^Akita mice were used to evaluate in vivo effects of high glucose on P2R and Panx1. Western blotting indicated altered P2X7R, P2Y₂R and P2Y₄R expression in high glucose exposed bone cells, and in diabetic bone tissue. Moreover, high glucose blunted normal P2R- and flow-induced Ca²⁺ signaling and ATP release from osteocytes. These findings indicate that T1D impairs load-induced ATP signaling in osteocytes and affects osteoblast function, which are essential for maintaining bone health.

EGFL7 is expressed in bone microenvironment and promotes angiogenesis via ERK, STAT3, and integrin signaling cascades

Angiogenesis plays a pivotal role in bone formation, remodeling, and fracture healing. The regulation of angiogenesis in the bone microenvironment is highly complex and orchestrated by intercellular communication between bone cells and endothelial cells. Here, we report that EGF-like domain 7 (EGFL7), a member of the epidermal growth factor (EGF) repeat protein superfamily is expressed in both the osteoclast and osteoblast lineages, and promotes endothelial cell activities. Addition of exogenous recombinant EGFL7 potentiates SVEC (simian virus 40-transformed mouse microvascular endothelial cell line) cell migration and tube-like structure formation in vitro. Moreover, recombinant EGFL7 promotes angiogenesis featuring web-like structures in ex vivo fetal mouse metatarsal angiogenesis assay. We show that recombinant EGFL7 induces phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2), signal transducer and activator of transcription 3 (STAT3), and focal adhesion kinase (FAK) in SVEC cells. Inhibition of ERK1/2 and STAT3 signaling impairs EGFL7-induced endothelial cell migration, and angiogenesis in fetal mouse metatarsal explants. Bioinformatic analyses indicate that EGFL7 contains a conserved RGD/QGD motif and EGFL7-induced endothelial cell migration is significantly reduced in the presence of RGD peptides. Moreover, EGFL7 gene expression is significantly upregulated during growth plate injury repair. Together, these results demonstrate that EGFL7 expressed by bone cells regulates endothelial cell activities through integrin-mediated signaling. This study highlights the important role that EGFL7, like EGFL6, expressed in bone microenvironment plays in the regulation of angiogenesis in bone.

Distinct VEGF functions during bone development and homeostasis

Vascular endothelial growth factor-A (VEGF) is a key regulator of physiological hemangiogenesis during development, postnatal growth, and homeostasis. It is well known that VEGF is required for effective coupling of angiogenesis to endochondral and membranous bone formation during skeletal development. However, less well known are the roles of VEGF in regulating the differentiation and/or functions of skeletal cells such as chondrocytes, osteoblasts, and osteoclasts. In this review, we discuss some of these functions. During early skeletal development, VEGF is important for the survival of chondrocytes in the hypoxic regions of the cartilage models of future bones, the vascularization of developing bones and proliferation and differentiation of osteoblastic cells. Postnatally, osteoblast-derived VEGF is critical for maintaining bone homeostasis by stimulating the differentiation of mesenchymal stem cells to osteoblasts and repressing their differentiation to adipocytes. Recent data indicate that these effects of VEGF on osteogenic/adipogenic stem cell fates are based on an intracellular (intracrine) mechanism. In contrast, osteoblast-derived VEGF is also known to stimulate the differentiation of monocytes to osteoclasts by a paracrine mechanism. Mice with VEGF-deficient osteoblastic lineage cells exhibit age-dependent loss of bone mass and an increase in bone marrow fat. These changes are similar to the changes associated with osteoporosis in humans. Thus, a better understanding of the intracellular mechanisms by which VEGF regulates osteoblastic and adipogenic differentiation may lead to the identification of new targets for therapies to prevent osteoporotic bone loss.

Prostaglandin E2 modulates F-actin stress fiber in FSS-stimulated MC3T3-E1 cells in a PKA-dependent manner

The effect of prostaglandin E2 (PGE2) on bone mass has been well-established in vivo. Previous studies have showed that PGE2 increases differentiation, proliferation, and regulates cell morphology through F-actin stress fiber in statically cultured osteoblasts. However, the effect of PGE2 on osteoblasts in the presence of fluid shear stress (FSS), which could better uncover the anabolic effect of PGE2 in vivo, has yet to be examined. Here, we hypothesized that PGE2 modulates F-actin stress fiber in FSS-stimulated MC3T3-E1 osteoblastic cells through protein kinase A (PKA) pathway. Furthermore, this PGE2-induced F-actin remodeling was associated with the recovery of cellular mechanosensitivity. Our data showed that treatment with 10 nM dmPGE2 for 15 min significantly suppressed the F-actin stress fiber intensity in FSS-stimulated cells in a PKA-dependent manner. In addition, dmPGE2 treatment enhanced the cells' calcium peak magnitude and the percentage of responding cells in the second FSS stimulation, though these effects were abolished and attenuated by co-treatment with phalloidin. Our results demonstrated that 10 nM dmPGE2 was able to accelerate the 'reset' process of F-actin stress fiber to its pre-stimulated level partially through PKA pathway, and thus promoted the recovery of cellular mechanosensitivity. Our finding provided a novel cellular mechanism by which PGE2 increased bone formation as shown in vivo, suggesting that PGE2 could be a potential target for treatments of bone formation-related diseases.

Regulation of PTHrP expression by cyclic mechanical strain in postnatal growth plate chondrocytes

Mechanical loading has been widely considered to be a crucial regulatory factor for growth plate development, but the exact mechanisms of this regulation are still not completely understood. In the growth plate, parathyroid hormone-related protein (PTHrP) regulates chondrocyte differentiation and longitudinal growth. Cyclic mechanical strain has been demonstrated to influence growth plate chondrocyte differentiation and metabolism, whereas the relationship between cyclic mechanical strain and PTHrP expression is not clear. The objective of this study was to investigate whether short-term cyclic tensile strain regulates PTHrP expression in postnatal growth plate chondrocytes in vitro and to explore whether the organization of cytoskeletal F-actin microfilaments is involved in this process. To this end, we obtained growth plate chondrocytes from 2-week-old Sprague-Dawley rats and sorted prehypertrophic and hypertrophic chondrocytes using immunomagnetic beads coated with anti-CD200 antibody. The sorted chondrocytes were subjected to cyclic tensile strain of varying magnitude and duration at a frequency of 0.5 Hz. We found that cyclic strain regulates PTHrP expression in a magnitude- and time-dependent manner. Incubation of chondrocytes with cytochalasin D, an actin microfilament-disrupting reagent, blocked the induction of PTHrP expression in response to strain. The results suggest that short-term cyclic tensile strain induces PTHrP expression in postnatal growth plate prehypertrophic and hypertrophic chondrocytes and that PTHrP expression by these chondrocytes may subsequently affect growth plate development. The results also support the idea that the organization of cytoskeletal F-actin microfilaments plays an important role in mechanotransduction.

Matrix-dependent adhesion mediates network responses to physiological stimulation of the osteocyte cell process

Osteocytes are bone cells that form cellular networks that sense mechanical loads distributed throughout the bone tissue. Interstitial fluid flow in the lacunar canalicular system produces focal strains at localized attachment sites around the osteocyte cell process. These regions of periodic attachment between the osteocyte cell membrane and its canalicular wall are sites where pN-level fluid-flow induced forces are generated in vivo. In this study, we show that focally applied forces of this magnitude using a newly developed Stokesian fluid stimulus probe initiate rapid and transient intercellular electrical signals in vitro. Our experiments demonstrate both direct gap junction coupling and extracellular purinergic P2 receptor signaling between MLO-Y4 cells in a connected bone cell network. Intercellular signaling was initiated by pN-level forces applied at integrin attachment sites along both appositional and distal unapposed cell processes, but not initiated at their cell bodies with equivalent forces. Electrical coupling was evident in 58% of all cell pairs tested with appositional connections; coupling strength increased with the increasing number of junctional connections. Apyrase, a nucleotide-degrading enzyme, suppressed and abolished force-induced effector responses, indicating a contribution from ATP released by the stimulated cell. This work extends the understanding of how osteocytes modulate their microenvironment in response to mechanical signals and highlights mechanisms of intercellular relay of mechanoresponsive signals in the bone network.

VEGF and bone cell signalling: an essential vessel for communication?

Vascular endothelial growth factor (VEGF) is an endothelial cell survival factor and is required for effective coupling of angiogenesis and osteogenesis. Although central to bone homeostasis, repair and the pathobiology that affect these processes, the precise mechanisms coupling endothelial cell function within bone formation and remodelling remain unclarified. This review will (i) focus on the potential directionality of VEGF signalling in adult bone by identifying the predominant source of VEGF within the bone microenvironment, (ii) will summarize current VEGF receptor expression studies by bone cells and (iii) will provide evidence for a role for VEGF signalling during postnatal repair and osteoporosis. A means of understanding the directionality of VEGF signalling in adult bone would allow us to most effectively target angiogenic pathways in diseases characterized by changes in bone remodelling rates and enhance bone repair when compromised.

Plexin structures are coming: opportunities for multilevel investigations of semaphorin guidance receptors, their cell signaling mechanisms, and functions

Plexin transmembrane receptors and their semaphorin ligands, as well as their co-receptors (Neuropilin, Integrin, VEGFR2, ErbB2, and Met kinase) are emerging as key regulatory proteins in a wide variety of developmental, regenerative, but also pathological processes. The diverse arenas of plexin function are surveyed, including roles in the nervous, cardiovascular, bone and skeletal, and immune systems. Such different settings require considerable specificity among the plexin and semaphorin family members which in turn are accompanied by a variety of cell signaling networks. Underlying the latter are the mechanistic details of the interactions and catalytic events at the molecular level. Very recently, dramatic progress has been made in solving the structures of plexins and of their complexes with associated proteins. This molecular level information is now suggesting detailed mechanisms for the function of both the extracellular as well as the intracellular plexin regions. Specifically, several groups have solved structures for extracellular domains for plexin-A2, -B1, and -C1, many in complex with semaphorin ligands. On the intracellular side, the role of small Rho GTPases has been of particular interest. These directly associate with plexin and stimulate a GTPase activating (GAP) function in the plexin catalytic domain to downregulate Ras GTPases. Structures for the Rho GTPase binding domains have been presented for several plexins, some with Rnd1 bound. The entire intracellular domain structure of plexin-A1, -A3, and -B1 have also been solved alone and in complex with Rac1. However, key aspects of the interplay between GTPases and plexins remain far from clear. The structural information is helping the plexin field to focus on key questions at the protein structural, cellular, as well as organism level that collaboratoria of investigations are likely to answer.

Connexin43 and pannexin1 channels in osteoblasts: who is the "hemichannel"?

Osteoblasts sense and respond to mechanical stimuli in a process involving influx and release of large ions and signaling molecules. Unapposed gap junction hemichannels formed of connexin43 (Cx43) have been proposed as a major route for such exchange, in particular for release of ATP and prostaglandin E₂ (PGE₂) in osteocytes. However, we have found that Cx43-null osteoblasts have unaltered, mechanically induced PGE₂ release and ATP-induced YoPro dye uptake. In contrast, PGE₂ release in response to fluid shear stress is abolished in P2X₇ receptor (P2X₇R)-null osteoblasts, and ATP-induced dye uptake is attenuated following treatment of wild-type cells with a P2X₇R or Pannexin1 (Panx1) channel blocker. These data indicate that Panx1 channels, in concert with P2X₇R, likely form a molecular complex that performs the hemichannel function in osteoblast mechanosignaling.

An Integrative Review of Mechanotransduction in Endothelial, Epithelial (Renal) and Dendritic Cells (Osteocytes)

In this review we will examine from a biomechanical and ultrastructural viewpoint how the cytoskeletal specialization of three basic cell types, endothelial cells (ECs), epithelial cells (renal tubule) and dendritic cells (osteocytes), enables the mechano-sensing of fluid flow in both their native in vivo environment and in culture, and the downstream signaling that is initiated at the molecular level in response to fluid flow. These cellular responses will be discussed in terms of basic mysteries and paradoxes encountered by each cell type. In ECs fluid shear stress (FSS) is nearly entirely attenuated by the endothelial glycocalyx that covers their apical membrane and yet FSS is communicated to both intracellular and junctional molecular components in activating a wide variety of signaling pathways. The same is true in proximal tubule (PT) cells where a dense brush border of microvilli covers the apical surface and the flow at the apical membrane is negligible. A four decade old unexplained mystery is the ability of PT epithelia to reliably reabsorb 60% of the flow entering the tubule regardless of the glomerular filtration rate. In the cortical collecting duct (CCD) the flow rates are so low that a special sensing apparatus, a primary cilia is needed to detect very small variations in tubular flow. In bone it has been a century old mystery as to how osteocytes embedded in a stiff mineralized tissue are able to sense miniscule whole tissue strains that are far smaller than the cellular level strains required to activate osteocytes in vitro.

Transcriptional control mechanisms associated with the nucleotide receptor P2X7, a critical regulator of immunologic, osteogenic, and neurologic functions

The nucleotide receptor P2X(7) is an attractive therapeutic target and potential biomarker for multiple inflammatory and neurologic disorders, and it is expressed in several immune, osteogenic, and neurologic cell types. Aside from its role in the nervous system, it is activated by ATP released at sites of tissue damage, inflammation, and infection. Ligand binding to P2X(7) stimulates many cell responses, including calcium fluxes, MAPK activation, inflammatory mediator release, and apoptosis. Much work has centered on P2X(7) action in cell death and mediator processing (e.g., pro-interleukin-1 cleavage by the inflammasome), but the contribution of P2X(7) to transcriptional regulation is less well defined. This review will focus on the growing evidence for the importance of nucleotide-mediated gene expression, highlight several animal models, human genetic, and clinical studies that support P2X(7) as a therapeutic target, and discuss the latest developments in anti-P2X(7) clinical trials.