Skeletal development, bone remodeling, and hematopoiesis

Protein tyrosine phosphatases in osteoclast differentiation, adhesion, and bone resorption

Osteoclasts are large cells derived from the monocyte-macrophage hematopoietic cell lineage. Their primary function is to degrade bone in various physiological contexts. Osteoclasts adhere to bone via podosomes, specialized adhesion structures whose structure and subcellular organization are affected by mechanical contact of the cell with bone matrix. Ample evidence indicates that reversible tyrosine phosphorylation of podosomal proteins plays a major role in determining the organization and dynamics of podosomes. Although roles of several tyrosine kinases are known in detail in this respect, little is known concerning the roles of protein tyrosine phosphatases (PTPs) in regulating osteoclast adhesion. Here we summarize available information concerning the known and hypothesized roles of the best-researched PTPs in osteoclasts - PTPRO, PTP epsilon, SHP-1, and PTP-PEST. Of these, PTPRO, PTP epsilon, and PTP-PEST appear to support osteoclast activity while SHP-1 inhibits it. Additional studies are required to provide full molecular details of the roles of these PTPs in regulating osteoclast adhesion, and to uncover additional PTPs that participate in this process.

Flexible and dynamic organization of bone marrow stromal compartment

The bone marrow mesenchymal compartment contains putative stem/progenitors of skeletal tissue components such as bone, cartilage, haematopoiesis-supporting stroma and adipocytes. Previously appreciated as vital to the support of haematopoiesis, these cells have also been recently recognized as having significant immunomodulatory properties with implications for allogeneic haematopoietic cell transplantation. Despite having been studied for more than three decades and currently being used in different clinical settings, their biology remains elusive. The aim of this review is to critically analyse the field of mesenchymal stem/progenitor cell biology, in respect of their relationship with other mesenchymal cell-types. Several issues concerning lineage commitment and inter-conversion potential between different mesenchymal cell-types are reviewed.

Distinct activities of stromal cells involved in the organogenesis of lymph nodes and Peyer's patches

It is now well established that the interaction between "inducer" cells of hemopoietic origin and "organizer" cells of mesenchymal lineage is involved in the organogenesis of lymph node (LN) and Peyer's patch (PP). Organizer cells are defined by the expression of VCAM-1 and ICAM-1 and the production of homeostatic chemokines. However, several studies suggested the presence of a diversity among these cells from different lymphoid tissues. Thus, we attempted to define the difference of organizer cells of LN and PP in terms of gene expression profile. Microarray analyses of organizer cells revealed that these cells isolated from embryonic mesenteric LN expressed higher levels of genes that are related to inflammation, tissue remodeling, and development of mesenchymal lineage compared with those from PP. Several transcription factors related to epithelial-mesenchymal interactions were also up-regulated in organizer cells from LN. These results indicate that organizer cells in LN and PP are indeed distinct and suggest that the organizer cells in LN are at a more activated stage than those in PP.

Noncanonical Wnt-4 signaling enhances bone regeneration of mesenchymal stem cells in craniofacial defects through activation of p38 MAPK

Mesenchymal stem cells (MSCs) are multipotent cells that can be differentiated into osteoblasts and provide an excellent cell source for bone regeneration and repair. Recently, the canonical Wnt/beta-catenin signaling pathway has been found to play a critical role in skeletal development and osteogenesis, implying that Wnts can be utilized to improve de novo bone formation mediated by MSCs. However, it is unknown whether noncanonical Wnt signaling regulates osteogenic differentiation. Here, we find that Wnt-4 enhanced in vitro osteogenic differentiation of MSCs isolated from human adult craniofacial tissues and promoted bone formation in vivo. Whereas Wnt-4 did not stabilize beta-catenin, it activated p38 MAPK in a novel noncanonical signaling pathway. The activation of p38 was dependent on Axin and was required for the enhancement of MSC differentiation by Wnt-4. Moreover, using two different models of craniofacial bone injury, we found that MSCs genetically engineered to express Wnt-4 enhanced osteogenesis and improved the repair of craniofacial defects in vivo. Taken together, our results reveal that noncanonical Wnt signaling could also play a role in osteogenic differentiation. Wnt-4 may have a potential use in improving bone regeneration and repair of craniofacial defects.

Low-intensity pulsed ultrasound (LIPUS) induces RANKL, MCP-1, and MIP-1beta expression in osteoblasts through the angiotensin II type 1 receptor

Constant mechanical stress is essential for the maintenance of bone mass and strength, which is achieved through the cooperative functions of osteoblasts and osteoclasts. However, it has not been fully elucidated how these cell types mediate mechanical signals. Low-intensity pulsed ultrasound (LIPUS) therapy is a recently developed method for application of mechanical stress, and is used clinically to promote bone fracture healing. In the present study, we applied LIPUS to osteoblasts at different stages of maturation and analyzed their chemokine and cytokine expression. In comparison with their immature counterparts, mature osteoblasts expressed significantly higher levels of mRNAs for the receptor activator of nuclear factor kappa B ligand (RANKL), monocyte chemoattractant protein (MCP)-1, and macrophage-inflammatory protein (MIP)-1beta after a few hours of LIPUS treatment. Intriguingly, protein and mRNA expression of angiotensin II type 1 receptor (AT1), a known mechanoreceptor in cardiomyocytes, was detected in osteoblasts, and the level of expression increased significantly during cell maturation. Furthermore, LIPUS-induced extracellular signal-regulated kinase (ERK) phosphorylation and RANKL/chemokine expression was abrogated by a specific AT1 inhibitor. Thus, AT1 may play one of the essential roles in bone metabolism as a mechanoreceptor of osteoblasts.

Regulation of hematopoietic niches by sympathetic innervation

Once hematopoiesis is established in the bone marrow, a continuous egress of hematopoietic stem cells (HSCs) to the periphery occurs at a low frequency. It has been proposed that this phenomenon is part of a regenerative homeostatic mechanism that ensures the maintenance of hematopoiesis through the life of the individual. The administration of certain cytotoxic drugs or cytokines can enhance the mobilization of hematopoietic progenitors to the periphery. During the past 15 years, granulocyte-colony stimulating factor (G-CSF) has been used as a standard cytokine for mobilization protocols in experimental models and in humans. Despite extensive efforts by multiple groups, a definitive mechanism explaining its role in mobilization has not been provided. In a recent paper, Katayama et al., through a series of clever associations supported by well-defined experimental systems, proposed that signals through the sympathetic nervous system modify the activity of the hematopoietic niche, acting as regulators of the mobilization of hematopoietic progenitors. This surprising finding adds a new level of complexity to the cellular milieu responsible for generation and maintenance of the hematopoietic niche.

Osteoclasts, rheumatoid arthritis, and osteoimmunology

PURPOSE OF REVIEW

Osteoclasts are terminally differentiated cells of the monocyte/macrophage lineage that resorb bone matrix. Bone destruction in rheumatoid arthritis is mainly attributable to the abnormal activation of osteoclasts, and studies on activation of osteoclasts by the immune system have led to the new research field called osteoimmunology. This interdisciplinary field is very important to biologic research and to the treatment of diseases associated with the bone and immune systems.

RECENT FINDINGS

The T-cell-mediated regulation of osteoclast differentiation is dependent on cytokines and membrane-bound factors expressed by T cells. The cross-talk between receptor activator of nuclear factor-kappaB ligand and interferon-gamma has been shown to be crucial for the regulation of osteoclast formation in arthritic joints. Recent studies indicate that an increasing number of immunomodulatory factors are associated with the regulation of bone metabolism: nuclear factor of activated T cells c1 has been shown to be the key transcription factor for osteoclastogenesis, the activation of which requires calcium signaling induced by the immunoglobulin-like receptors.

SUMMARY

New findings in osteoimmunology will be instrumental in the development of strategies for research into the treatment of various diseases afflicting the skeletal and immune systems.

Concise Review: Embryonic Stem Cells: A New Tool to Study Osteoblast and Osteoclast Differentiation

Bone remodeling involves synthesis of organic matrix by osteoblasts and bone resorption by osteoclasts. A tight collaboration between these two cell types is essential to maintain a physiological bone homeostasis. Thus, osteoblasts control bone-resorbing activities and are also involved in osteoclast differentiation. Any disturbance between these effectors leads to the development of skeletal abnormalities and/or bone diseases. In this context, the determination of key genes involved in bone cell differentiation is a new challenge to treat any skeletal disorders. Different models are used to study the differentiation process of these cells, but all of them use pre-engaged progenitor cells, allowing us to study only the latest stages of the differentiation. Embryonic stem (ES) cells come from the inner mass of the blastocyst prior its implantation to the uterine wall. Because of their capacity to differentiate into all germ layers, and so into all tissues of the body, ES cells represent the best model by which to study earliest stages of bone cell differentiation. Osteoblasts are generated by two methods, one including the generation of embryoid body, the other not. Mineralizing cells are obtained after 2 weeks of culture and express all the specific osteoblastic markers (alkaline phosphatase, type I collagen, osteocalcin, and others). Osteoclasts are generated from a single-cell suspension of ES cells seeded on a feeder monolayer, and bone-resorbing cells expressing osteoclastic markers such as tartrate-resistant alkaline phosphatase or receptor activator of nuclear factor kappaB are obtained within 11 days. The aim of this review is to present recent discoveries and advances in the differentiation of both osteoblasts and osteoclasts from ES cells.

Nitric oxide protects osteoblasts from oxidative stress-induced apoptotic insults via a mitochondria-dependent mechanism

Nitric oxide (NO) contributes to the regulation of osteoblast activities. In this study, we evaluated the protective effects of NO pretreatment on oxidative stress-induced osteoblast apoptosis and its possible mechanism using neonatal rat calvarial osteoblasts as the experimental model. Exposure of osteoblasts to sodium nitroprusside (SNP) at a low concentration of 0.3 mM significantly increased cellular NO levels without affecting cell viability. However, when the concentration reached a high concentration of 2 mM, SNP increased the levels of intracellular reactive oxygen species and induced osteoblast injuries. Thus, administration of 0.3 and 2 mM SNP in osteoblasts were respectively used as sources of NO and oxidative stress. Pretreatment with NO for 24 h significantly ameliorated the oxidative stress-caused morphological alterations and decreases in alkaline phosphatase activity, and reduced cell death. Oxidative stress induced osteoblast death via an apoptotic mechanism, but NO pretreatment protected osteoblasts against the toxic effects. The mitochondrial membrane potential was significantly reduced following exposure to the oxidative stress. However, pretreatment with NO significantly lowered the suppressive effects. Oxidative stress increased cellular Bax protein production and cytochrome c release from mitochondria. Pretreatment with NO significantly decreased oxidative stress-caused augmentation of Bax and cytochrome c protein levels. In parallel with cytochrome c release, oxidative stress induced caspase-3 activation and DNA fragmentation. Pretreatment with NO significantly reduced the oxidative stress-enhanced caspase-3 activation and DNA damage. Results of this study show that NO pretreatment can protect osteoblasts from oxidative stress-induced apoptotic insults. The protective action involves a mitochondria-dependent mechanism.

Nothing but skin and bone

Skin and bone - what comes to mind at hearing this phrase? While certainly a metaphor for disease, it also defines two very different tissues, one a flexible and contiguous outer covering, the other a morphologically diverse hard tissue distributed at over 200 sites in the body. As the accompanying series of Reviews highlights, these tissues are indeed diverse, but there are also surprising similarities. Skin is the interface between the internal organs and the environment, and as such plays a crucial role in the body's defense mechanism. The skin and its many appendages are responsible for functions as diverse as epidermal barrier and defense, immune surveillance, UV protection, thermoregulation, sweating, lubrication, pigmentation, the sensations of pain and touch, and, importantly, the protection of various stem cell niches in the skin. Bone serves a number of purposes: it provides protection for vital organs, a lever for locomotion, a reservoir for calcium, and the site of adult hematopoiesis. The tissue is composed of osteoblasts, osteoclasts, and their individual precursors plus a complex mixture of mesenchymal, myeloid, and lymphoid cells in the marrow space. Finally, the endothelial microenvironment provides nutrition and is a conduit for the influx and emigration of cells that impact bone biology in several important ways. This Review series guides the reader through these various facets of 2 diverse, yet interdependent, tissues.

Microvascular adaptation to growth in rat humeral head

The aim of this work is to investigate the growth of the vasculature in the rat humeral head cartilage after the initial development of the secondary ossification centre until the adult organization. Rats aging from 5 weeks to 12 months were used. Histological observations on humeral heads were implemented with morphometrical analysis. Subsequently, vascular corrosion cast, that permits a three-dimensional observation of the vasculature, were prepared and observed by scanning electron microscopy. In young animals the epiphysis contains thin bone trabeculae and most of the epiphysis is occupied by bone marrow spaces. With age, the bone trabeculae progressively enlarge up to double their thickness. The percentage of bone tissue increases from 33.6 to 58.6% of the entire epiphysis, while the bone marrow spaces tend to increase very little in their mean dimension. Vascular corrosion casts show that the epiphyseal microcirculation is well distinguished from that of the diaphysis, and arises from the vessels present in the capsule and the periosteal networks. In young animals the only capillaries are bone marrow sinusoids and few subchondral capillaries. In adult animals small vessels run between the clusters of sinusoids forming the trabecular circulation. Capillary sprouts from sinusoids are always observed both in the young and adult animals. Thus, in adult rats different proper microcirculatory districts can be distinguished in the epiphysis: (a) the sinusoidal network, that supplies the hematopoiesis of the bone marrow and the adjacent osteogenic tissue; (b) the bone tissue microcirculation, limited to small vessels that supply the metabolism and the remodelling of the bone tissue. The reported microvascular organization and its adaptation to the epiphyseal growth represent the morphological basis for understanding the reciprocal interaction among the different tissues in developing and adult rat epiphysis.