A Fluorescent Intravital Imaging Approach to Study Load-Induced Calcium Signaling Dynamics in Mouse Osteocytes

Bone tissue is exquisitely sensitive to differences in mechanical load magnitude. Osteocytes, dendritic cells that form a syncytium throughout the bone, are responsible for the mechanosensory function of bone tissue. Studies employing histology, mathematical modeling, cell culture, and ex vivo bone organ cultures have greatly advanced the understanding of osteocyte mechanobiology. However, the fundamental question of how osteocytes respond to and encode mechanical information at the molecular level in vivo is not well understood. Intracellular calcium concentration fluctuations in osteocytes offer a useful target for learning more about acute bone mechanotransduction mechanisms. Here, we report a method for studying osteocyte mechanobiology in vivo, combining a mouse strain with a fluorescently genetically encoded calcium indicator expressed in osteocytes with an in vivo loading and imaging system to directly detect osteocyte calcium levels during loading. This is achieved with a three-point bending device that can deliver well-defined mechanical loads to the third metatarsal of living mice while simultaneously monitoring fluorescently indicated calcium responses of osteocytes using two-photon microscopy. This technique allows for direct in vivo observation of osteocyte calcium signaling events in response to whole bone loading and is useful in the endeavor to reveal mechanisms in osteocyte mechanobiology.

Estrogen depletion on In vivo osteocyte calcium signaling responses to mechanical loading

Microstructural adaptation of bone in response to mechanical stimuli is diminished with estrogen deprivation. Here we tested in vivo whether ovariectomy (OVX) alters the acute response of osteocytes, the principal mechanosensory cells of bone, to mechanical loading in mice. We also used super resolution microscopy (Structured Illumination microscopy or SIM) in conjunction with immunohistochemistry to assess changes in the number and organization of "osteocyte mechanosomes" - complexes of Panx1 channels, P2X7 receptors and CaV3 voltage-gated Ca²⁺ channels clustered around α_vβ₃ integrin foci on osteocyte processes. Third metatarsals bones of mice expressing an osteocyte-targeted genetically encoded Ca²⁺ indicator (DMP1-GCaMP3) were cyclically loaded in vivo to strains from 250 to 3000 με and osteocyte intracellular Ca²⁺ signaling responses were assessed in mid-diaphyses using multiphoton microscopy. The number of Ca²⁺ signaling osteocytes in control mice increase monotonically with applied strain magnitude for the physiological range of strains. The relationship between the number of Ca²⁺ signaling osteocytes and loading was unchanged at 2 days post-OVX. However, it was altered markedly at 28 days post-OVX. At loads up to 1000 με, there was a dramatic reduction in number of responding (i.e. Ca²⁺ signaling) osteocytes; however, at higher strains the numbers of Ca²⁺ signaling osteocytes were similar to control mice. OVX significantly altered the abundance, make-up and organization of osteocyte mechanosome complexes on dendritic processes. Numbers of α_vβ₃ foci also staining with either Panx 1, P2X7R or CaV3 declined by nearly half after OVX, pointing to a loss of osteocyte mechanosomes on the dendritic processes with estrogen depletion. At the same time, the areas of the remaining foci that stained for α_vβ₃ and channel proteins increased significantly, a redistribution of mechanosome components suggesting a potential compensatory response. These results demonstrate that the deleterious effects of estrogen depletion on skeletal mechanical adaptation appear at the level of mechanosensation; osteocytes lose the ability to sense small (physiological) mechanical stimuli. This decline may result at least partly from changes in the structure and organization of osteocyte mechanosomes, which contribute to the distinctive sensitivity of osteocytes (particularly their dendritic processes) to mechanical stimulation.

Generation and Characterization of Immortalized Mouse Cortical Astrocytes From Wildtype and Connexin43 Knockout Mice

We transduced mouse cortical astrocytes cultured from four litters of embryonic wildtype (WT) and connexin43 (Cx43) null mouse pups with lentiviral vector encoding hTERT and measured expression of astrocyte-specific markers up to passage 10 (p10). The immortalized cell lines thus generated (designated IWCA and IKOCA, respectively) expressed biomarkers consistent with those of neonatal astrocytes, including Cx43 from wildtype but not from Cx43-null mice, lack of Cx30, and presence of Cx26. AQP4, the water channel that is found in high abundance in astrocyte end-feet, was expressed at moderately high levels in early passages, and its mRNA and protein declined to low but still detectable levels by p10. The mRNA levels of the astrocyte biomarkers aldehyde dehydrogenase 1L1 (ALDH1L1), glutamine synthetase (GS) and glial fibrillary acidic protein (GFAP) remained relatively constant during successive passages. GS protein expression was maintained while GFAP declined with cell passaging but was still detectable at p10. Both mRNA and protein levels of glutamate transporter 1 (GLT-1) declined with passage number. Immunostaining at corresponding times was consistent with the data from Western blots and provided evidence that these proteins were expressed at appropriate intracellular locations. Consistent with our goal of generating immortalized cell lines in which Cx43 was either functionally expressed or absent, IWCA cells were found to be well coupled with respect to intercellular dye transfer and similar to primary astrocyte cultures in terms of time course of junction formation, electrical coupling strength and voltage sensitivity. Moreover, barrier function was enhanced in co-culture of the IWCA cell line with bEnd.3 microvascular endothelial cells. In addition, immunostaining revealed oblate endogenous Cx43 gap junction plaques in IWCA that were similar in appearance to those plaques obtained following transfection of IKOCA cells with fluorescent protein tagged Cx43. Re-expression of Cx43 in IKOCA cells allows experimental manipulation of connexins and live imaging of interactions between connexins and other proteins. We conclude that properties of these cell lines resemble those of primary cultured astrocytes, and they may provide useful tools in functional studies by facilitating genetic and pharmacological manipulations in the context of an astrocyte-appropriate cellular environment.

CITED2 mediates the cross-talk between mechanical loading and IL-4 to promote chondroprotection

Osteoarthritis (OA) pathogenesis is mediated largely through the actions of proteolytic enzymes such as matrix metalloproteinase (MMP) 13. The transcriptional regulator CITED2, which suppresses the expression of MMP13 in chondrocytes, is induced by interleukin (IL)-4 in T cells and macrophages, and by moderate mechanical loading in chondrocytes. We tested the hypothesis that CITED2 mediates cross-talk between IL-4 signaling and mechanical loading-induced pathways that result in chondroprotection, at least in part, by downregulating MMP13. IL-4 induced CITED2 gene expression in human chondrocytes in a dose- and time-dependent manner through JAK/STAT signaling. Mechanical loading combined with IL-4 resulted in additive effects on inducing CITED2 expression and downregulating of MMP13 in human chondrocytes in vitro. In vivo, IL-4 gene knockout (KO) mice exhibited reduced basal levels of CITED2 expression in chondrocytes. While moderate treadmill running induced CITED2 expression and reduced MMP13 expression in wild-type mice, these effects were blunted (for CITED2) or abolished (for MMP13) in chondrocytes of IL-4 gene KO mice. Moreover, intra-articular injections of mouse recombinant IL-4 combined with regular cage activity mitigated post-traumatic OA to a greater degree compared to immobilized mice treated with IL-4 alone. These data suggest that using moderate loading to enhance IL-4 may be a potential therapeutic strategy for chondroprotection in OA.

Role of pannexin 1 channels in load-induced skeletal response

The pannexin 1 (Panx1) channel is a mechanosensitive channel that interacts with P2X7 receptors (P2X7R) to form a functional complex that has been shown in vitro to play an essential role in osteocyte mechanosignaling. While the participation of P2X7R in skeletal responses to mechanical loading has been demonstrated, the role of Panx1 and its interplay with P2X7R still remain to be determined. In this study, we use a global Panx1^-/- mouse model and in vivo mechanical loading to demonstrate that Panx1 channels play an essential role in load-induced skeletal responses. We found that absence of Panx1 not only disrupts the P2X7R-Panx1 signaling complex, but also alters load-induced regulation of P2X7R expression. Moreover, lack of Panx1 completely abolished load-induced periosteal bone formation. Load-induced regulation of β-catenin and sclerostin expression was dysregulated in Panx1^-/- , compared to wild-type, bone. This finding suggests that Panx1 deficiency disrupts Wnt/β-catenin signaling by lowering β-catenin while favoring inhibition of bone formation by increasing load-induced sclerostin expression. This study demonstrates the existence of a Panx1-dependent mechanosensitive mechanism that not only modulates ATP signaling but also coordinates Wnt/β-catenin signaling that is essential for proper skeletal response to mechanical loading.

Potential role for a specialized β3 integrin-based structure on osteocyte processes in bone mechanosensation

Osteocyte processes are an order of magnitude more sensitive to mechanical loading than their cell bodies. The mechanisms underlying this remarkable mechanosensitivity are not clear, but may be related to the infrequent αV β3 integrin sites where the osteocyte cell processes attach to canalicular walls. These sites develop dramatically elevated strains during load-induced fluid flow in the lacunar-canalicular system and were recently shown to be primary sites for osteocyte-like MLO-Y4 cell mechanotransduction. These αV β3 integrin sites lack typical integrin transduction mechanisms. Rather, stimulation at these sites alters Ca2+ signaling, ATP release and membrane potential. In the current studies, we tested the hypothesis that in authentic osteocytes in situ, key membrane proteins implicated in osteocyte mechanotransduction are preferentially localized at or near to β3 integrin-foci. We analyzed these spatial relationships in mouse bone osteocytes using immunohistochemistry combined with Structured Illumination Super Resolution Microscopy, a method that permits structural resolution at near electron microscopy levels in tissue sections. We discovered that the purinergic channel pannexin1, the ATP-gated purinergic receptor P2 × 7R and the low voltage transiently opened T-type calcium channel CaV3.2-1 all reside in close proximity to β3 integrin attachment foci on osteocyte processes, suggesting a specialized mechanotransduction complex at these sites. We further confirmed this observation on isolated osteocytes in culture using STochasitc Optical Resonance Microscopy. These findings identify a possible structural basis for the unique mechanosensation and transduction capabilities of the osteocyte process. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:642-652, 2018.

A new perspective on mechanisms governing skeletal complications in type 1 diabetes

This review focuses on bone mechanobiology in type 1 diabetes (T1D), an area of research on diabetes-associated skeletal complications that is still in its infancy. We first provide a brief overview of the deleterious effects of diabetes on the skeleton and of the knowledge gained from studies with rodent models of T1D. Second, we discuss two specific hallmarks of T1D, low insulin and high glucose, and address the extent to which they affect skeletal health. Third, we highlight the mechanosensitive nature of bone tissue and the importance of mechanical loading for bone health. We also summarize recent advances in bone mechanobiology that implicate osteocytes as the mechanosensors and major regulatory cells in the bone. Finally, we discuss recent evidence indicating that the diabetic bone is "deaf" to mechanical loading and that osteocytes are central players in mechanisms that lead to bone loss in T1D.

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.

Pannexin-1 and P2X7-Receptor Are Required for Apoptotic Osteocytes in Fatigued Bone to Trigger RANKL Production in Neighboring Bystander Osteocytes

Osteocyte apoptosis is required to induce intracortical bone remodeling after microdamage in animal models, but how apoptotic osteocytes signal neighboring "bystander" cells to initiate the remodeling process is unknown. Apoptosis has been shown to open pannexin-1 (Panx1) channels to release adenosine diphosphate (ATP) as a "find-me" signal for phagocytic cells. To address whether apoptotic osteocytes use this signaling mechanism, we adapted the rat ulnar fatigue-loading model to reproducibly introduce microdamage into mouse cortical bone and measured subsequent changes in osteocyte apoptosis, receptor activator of NF-κB ligand (RANKL) expression and osteoclastic bone resorption in wild-type (WT; C57Bl/6) mice and in mice genetically deficient in Panx1 (Panx1KO). Mouse ulnar loading produced linear microcracks comparable in number and location to the rat model. WT mice showed increased osteocyte apoptosis and RANKL expression at microdamage sites at 3 days after loading and increased intracortical remodeling and endocortical tunneling at day 14. With fatigue, Panx1KO mice exhibited levels of microdamage and osteocyte apoptosis identical to WT mice. However, they did not upregulate RANKL in bystander osteocytes or initiate resorption. Panx1 interacts with P2X7 R in ATP release; thus, we examined P2X7 R-deficient mice and WT mice treated with P2X7 R antagonist Brilliant Blue G (BBG) to test the possible role of ATP as a find-me signal. P2X7 RKO mice failed to upregulate RANKL in osteocytes or induce resorption despite normally elevated osteocyte apoptosis after fatigue loading. Similarly, treatment of fatigued C57Bl/6 mice with BBG mimicked behavior of both Panx1KO and P2X7 RKO mice; BBG had no effect on osteocyte apoptosis in fatigued bone but completely prevented increases in bystander osteocyte RANKL expression and attenuated activation of resorption by more than 50%. These results indicate that activation of Panx1 and P2X7 R are required for apoptotic osteocytes in fatigued bone to trigger RANKL production in neighboring bystander osteocytes and implicate ATP as an essential signal mediating this process.

Loss of hepatic chaperone-mediated autophagy accelerates proteostasis failure in aging

Chaperone-mediated autophagy (CMA), a cellular process that contributes to protein quality control through targeting of a subset of cytosolic proteins to lysosomes for degradation, undergoes a functional decline with age. We have used a mouse model with liver-specific defective CMA to identify changes in proteostasis attributable to reduced CMA activity in this organ with age. We have found that other proteolytic systems compensate for CMA loss in young mice which helps to preserve proteostasis. However, these compensatory responses are not sufficient for protection against proteotoxicity induced by stress (oxidative stress, lipid challenges) or associated with aging. Livers from old mice with CMA blockage exhibit altered protein homeostasis, enhanced susceptibility to oxidative stress and hepatic dysfunction manifested by a diminished ability to metabolize drugs, and a worsening of the metabolic dysregulation identified in young mice. Our study reveals that while the regulatory function of CMA cannot be compensated for in young organisms, its contribution to protein homeostasis can be handled by other proteolytic systems. However, the decline in the compensatory ability identified with age explains the more severe consequences of CMA impairment in older organisms and the contribution of CMA malfunction to the gradual decline in proteostasis and stress resistance observed during aging.

Age-related carbonylation of fibrocartilage structural proteins drives tissue degenerative modification

Aging-related oxidative stress has been linked to degenerative modifications in different organs and tissues. Using redox proteomic analysis and illustrative tandem mass spectrometry mapping, we demonstrate oxidative posttranslational modifications in structural proteins of intervertebral discs (IVDs) isolated from aging mice. Increased protein carbonylation was associated with protein fragmentation and aggregation. Complementing these findings, a significant loss of elasticity and increased stiffness was measured in fibrocartilage from aging mice. Studies using circular dichroism and intrinsic tryptophan fluorescence revealed a significant loss of secondary and tertiary structures of purified collagens following oxidation. Collagen unfolding and oxidation promoted both nonenzymatic and enzymatic degradation. Importantly, induction of oxidative modification in healthy fibrocartilage recapitulated the biochemical and biophysical modifications observed in the aging IVD. Together, these results suggest that protein carbonylation, glycation, and lipoxidation could be early events in promoting IVD degenerative changes.

Gap junctions and Bystander Effects: Good Samaritans and executioners

The "Bystander" and "Good Samaritan" effects involve the transfer of toxic or beneficial compounds from one cell to a generally adjacent other through gap junction channels and through extracellular routes. The variety of injuries in which bystander cell killing or protection occurs has greatly expanded in the last decade to include infectious agents and therapeutic compounds, radiation injury, chaperones in cell therapy and apoptosis in development. This has been accompanied by the appreciation that both gap junction mediated and paracrine routes are used for the signaling of the "kiss of life" and the "kiss of death" and that manipulations of these pathways and the molecules that use them may find therapeutic utility in treatment of a variety of pathological conditions.

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.

Characterization of hTERT-immortalized osteoblast cell lines generated from wild-type and connexin43-null mouse calvaria

The gap junction protein connexin43 (Cx43) has been proposed to play key roles in bone differentiation and mineralization, but underlying cellular mechanisms are not totally understood. To further explore roles of Cx43 in these processes, we immortalized calvarial osteoblasts from wild-type and Cx43-null mice using human telomerase reverse transcriptase (hTERT). Osteoblastic (MOB) cell lines were generated from three individual wild-type and three individual Cx43-null mouse calvaria. Average population doubling times of the cell lines were higher than of the primary osteoblasts but did not greatly differ with regard to genotype. Modest to high level of Cx45 expression was detected in MOBs of both genotypes. Most of the cell lines expressed osteoblastic markers [Type I collagen, osteopontin, osteocalcin, parathyroid hormone/parathyroid hormone-related peptide receptor (PTH/PTHrP), periostin (OSF-2), osterix (Osx), runt-related transcription factor 2 (Runx2), alkaline phosphatase (ALP)], and mineralization was comparable to that of primary osteoblasts. Two MOB cell lines from each genotype with most robust maintenance of osteoblast lineage markers were analyzed in greater detail, revealing that the Cx43-null cell lines showed a significant delay in early differentiation (up to 9 days in culture). Matrix mineralization was markedly delayed in one of the Cx43-null lines and slightly delayed in the other. These findings comparing new and very stable wild-type and Cx43-null osteoblastic cell lines define a role for Cx43 in early differentiation and mineralization stages of osteoblasts and further support the concept that Cx43 plays important role in the cellular processes associated with skeleton function.

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

Although load-induced mechanical signals play a key role in bone formation and maintenance of bone mass and structure, the cellular mechanisms involved in the translation of these signals are still not well understood. Recent identification of a novel flow-induced mechanosignaling pathway involving VEGF in osteoblasts and the known VEGF regulation of actin reorganization in various cell types has led us to hypothesize that fluid shear stress-induced Vegf up-regulation underlies the actin cytoskeleton adaptation observed in osteoblasts during mechanotransduction. Our results show that MC3T3-E1 cells secrete significant VEGF in response to 5 h of pulsatile fluid shear stress (PFSS; 5 dynes/cm(2) at 1 Hz), whereas expression of VEGF receptors (VEGFR-1, VEGFR-2, or NRP1) is unaffected. These receptors, in particular VEGFR-2, participate in PFSS-induced VEGF release. Exposure to flow-conditioned medium or exogenous VEGF significantly induces stress fiber formation in osteoblasts that is comparable with PFSS-induced stress fiber formation, whereas VEGF knockdown abrogates this response to PFSS, thereby providing evidence that flow-induced VEGF release plays a role in actin polymerization. Using neutralizing antibodies against the receptors and VEGF isoforms, we found that soluble VEGFs, in particular VEGF(164), play a crucial role in transient stress fiber formation during osteoblast mechanotransduction, most likely through VEGFR-2 and NRP1. Based on these data we conclude that flow-induced VEGF release from osteoblasts regulates osteoblast actin adaptation during mechanotransduction and that VEGF paracrine signaling may provide potent cross-talk among bone cells and endothelial cells that is essential for fracture healing, bone remodeling, and osteogenesis.

Aquaporin-4 water channels in enteric neurons

Aquaporin-4 is a water channel predominantly found in astrocytes in the central nervous system and is believed to play a critical role in the formation and maintenance of the blood-brain barrier and in water secretion from the brain. As enteric glial cells were found to share several similarities with astrocytes, we hypothesized that enteric glia might also contain aquaporin-4. We used immunohistochemistry to identify aquaporin-4 in the myenteric and submucosal plexuses of the mouse and the rat colon. We found that subpopulations of neurons in both enteric plexuses were positively labeled for human aquaporin-4. Double staining of the enteric ganglia with antibodies to the neuronal marker neurofilament-heavy chain 100 and to aquaporin-4 showed that a minority of myenteric neurons were aquaporin-4 positive (about 12% in the mouse and 13% in the rat). In contrast, in the submucosal plexus significant numbers of neurons were positive for aquaporin-4 (about 79% in both the mouse and the rat). Double labeling for aquaporin-4 and for the glial marker glial fibrillary acidic protein verified that glial cells were not immunoreactive to aquaporin-4. We further confirmed our findings with additional aquaporin-4 antibodies and Western blot analysis. We found that, in addition to expressing aquaporin-4, the myenteric plexus and, to a greater extent, the submucosal plexus both expressed aquaporin-1. We conclude that neurons rather than glial cells contain aquaporin-4 in the colonic enteric plexuses. It is known that submucosal neurons control transport processes in the intestinal mucosa, and the high percentage of aquaporin-4-positive submucosal neurons suggests that aquaporin-4 contributes to this function.

Fluid shear stress upregulates vascular endothelial growth factor gene expression in osteoblasts

Fluid-induced shear stress is widely recognized as an important biophysical signal in cell-cell mechanotransduction. To identify cellular signaling pathways that are regulated by fluid shear stress, we applied the unbiased approach of transcriptional profiling. Our cDNA array analysis detected that 1,165 of the 6,288 sampled unigenes were significantly affected by pulsatile fluid flow. GenMapp 2.1 analysis revealed pathways of genes regulated by shear stress: angiogenesis, blood vessel morphogenesis, regulation of endothelial cell proliferation, and prostaglandin biosynthesis. Individual genes significantly up-/downregulated by shear stress included vascular endothelial growth factor A (Vegf a), cysteine-rich protein 61 (Cyr61), platelet-derived growth factor-alpha (Pdgf a), connective tissue growth factor (Ctgf), Neuropilin 1 (Nrp1), angiotensin II receptor, type 1 a (Agtr1 a) and fibroblast growth factor 1 (Fgf1). Based on these findings, we hypothesize that fluid shear stress-regulated Vegf most likely stimulates MC3T3-E1 cells through autocrine/paracrine release and may provide a powerful recruitment signal for osteoclasts, endothelial cells, and/or stem cells during bone remodeling.

The role of the glycocalyx in reorganization of the actin cytoskeleton under fluid shear stress: a "bumper-car" model

We propose a conceptual model for the cytoskeletal organization of endothelial cells (ECs) based on a major dichotomy in structure and function at basal and apical aspects of the cells. Intracellular distributions of filamentous actin (F-actin), vinculin, paxillin, ZO-1, and Cx43 were analyzed from confocal micrographs of rat fat-pad ECs after 5 h of shear stress. With intact glycocalyx, there was severe disruption of the dense peripheral actin bands (DPABs) and migration of vinculin to cell borders under a uniform shear stress (10.5 dyne/cm2; 1 dyne = 10 microN). This behavior was augmented in corner flow regions of the flow chamber where high shear stress gradients were present. In striking contrast, no such reorganization was observed if the glycocalyx was compromised. These results are explained in terms of a "bumper-car" model, in which the actin cortical web and DPAB are only loosely connected to basal attachment sites, allowing for two distinct cellular signaling pathways in response to fluid shear stress, one transmitted by glycocalyx core proteins as a torque that acts on the actin cortical web (ACW) and DPAB, and the other emanating from focal adhesions and stress fibers at the basal and apical membranes of the cell.

Fluid shear stress remodels expression and function of junctional proteins in cultured bone cells

We tested the hypothesis that fluid shear stress (tau) modifies the expression, function, and distribution of junctional proteins [connexin (Cx)43, Cx45, and zona occludens (ZO)-1] in cultured bone cells. Cell lines with osteoblastic (MC3T3-E1 cells) and osteocytic (MLO-Y4 cells) phenotypes were exposed to tau-values of 5 or 20 dyn/cm(2) for 1-3 h. Immunostaining indicated that at 5 dyn/cm(2), the distribution of Cx43, Cx45, and ZO-1 was moderately disrupted at cell membranes; at 20 dyn/cm(2), disruption was more severe. Intercellular coupling was significantly decreased at both shear stress levels. Western blots showed the downregulation of membrane-bound Cx43 and ZO-1 and the upregulation of cytosolic Cx43 and Cx45 at different levels of shear stress. Similarly, Northern blots revealed that expression of Cx43, Cx45, and ZO-1 was selectively up- and downregulated in response to different shear stress levels. These results indicate that in cultured bone cells, fluid shear stress disrupts junctional communication, rearranges junctional proteins, and determines de novo synthesis of specific connexins to an extent that depends on the magnitude of the shear stress. Such disconnection from the bone cell network may provide part of the signal whereby the disconnected cells or the remaining network initiate focal bone remodeling.