Wnt-associated adult stem cell marker Lgr6 is required for osteogenesis and fracture healing

Despite the remarkable regenerative capacity of skeletal tissues, nonunion of bone and failure of fractures to heal properly presents a significant clinical concern. Stem and progenitor cells are present in bone and become activated following injury; thus, elucidating mechanisms that promote adult stem cell-mediated healing is important. Wnt-associated adult stem marker Lgr6 is implicated in the regeneration of tissues with well-defined stem cell niches in stem cell-reliant organs. Here, we demonstrate that Lgr6 is dynamically expressed in osteoprogenitors in response to fracture injury. We used an Lgr6-null mouse model and found that Lgr6 expression is necessary for maintaining bone volume and efficient postnatal bone regeneration in adult mice. Skeletal progenitors isolated from Lgr6-null mice have reduced colony-forming potential and reduced osteogenic differentiation capacity due to attenuated cWnt signaling. Lgr6-null mice consist of a lower proportion of self-renewing stem cells. In response to fracture injury, Lgr6-null mice have a deficiency in the proliferation of periosteal progenitors and reduced ALP activity. Further, analysis of the bone regeneration phase and remodeling phase of fracture healing in Lgr6-null mice showed impaired endochondral ossification and decreased mineralization. We propose that in contrast to not being required for successful skeletal development, Lgr6-positive cells have a direct role in endochondral bone repair.

Interactions of B-lymphocytes and bone cells in health and disease

Bone remodeling occurs through the interactions of three major cell lineages, osteoblasts, which mediate bone formation, osteocytes, which derive from osteoblasts, sense mechanical force and direct bone turnover, and osteoclasts, which mediate bone resorption. However, multiple additional cell types within the bone marrow, including macrophages, T lymphocytes and B lymphocytes influence the process. The bone marrow microenvironment, which is supported, in part, by bone cells, forms a nurturing network for B lymphopoiesis. In turn, developing B lymphocytes influence bone cells. Bone health during homeostasis depends on the normal interactions of bone cells with other lineages in the bone marrow. In disease state these interactions become pathologic and can cause abnormal function of bone cells and inadequate repair of bone after a fracture. This review summarizes what is known about the development of B lymphocytes and the interactions of B lymphocytes with bone cells in both health and disease.

PDGF inhibits BMP2-induced bone healing

Bone regeneration depends on a pool of bone/cartilage stem/progenitor cells and signaling mechanisms regulating their differentiation. Using in vitro approach, we have shown that PDGF signaling through PDGFRβ inhibits BMP2-induced osteogenesis, and significantly attenuates expression of BMP2 target genes. We evaluated outcomes of treatment with two anabolic agents, PDGF and BMP2 using different bone healing models. Targeted deletion of PDGFRβ in αSMA osteoprogenitors, led to increased callus bone mass, resulting in improved biomechanical properties of fractures. In critical size bone defects BMP2 treatment increased proportion of osteoprogenitors, while the combined treatment of PDGF BB with BMP2 decreased progenitor number at the injury site. BMP2 treatment induced significant bone formation and increased number of osteoblasts, while in contrast combined treatment with PDGF BB decreased osteoblast numbers. This is in vivo study showing that PDGF inhibits BMP2-induced osteogenesis, but inhibiting PDGF signaling early in healing process does not improve BMP2-induced bone healing.

Diabetes impairs periosteal progenitor regenerative potential

Diabetics are at increased risk for fracture, and experience severely impaired skeletal healing characterized by delayed union or nonunion of the bone. The periosteum harbors osteochondral progenitors that can differentiate into chondrocytes and osteoblasts, and this connective tissue layer is required for efficient fracture healing. While bone marrow-derived stromal cells have been studied extensively in the context of diabetic skeletal repair and osteogenesis, the effect of diabetes on the periosteum and its ability to contribute to bone regeneration has not yet been explicitly evaluated. Within this study, we utilized an established murine model of type I diabetes to evaluate periosteal cell differentiation capacity, proliferation, and availability under the effect of a diabetic environment. Periosteal cells from diabetic mice were deficient in osteogenic differentiation ability in vitro, and diabetic mice had reduced periosteal populations of mesenchymal progenitors with a corresponding reduction in proliferation capacity following injury. Additionally, fracture callus mineralization and mature osteoblast activity during periosteum-mediated healing was impaired in diabetic mice compared to controls. We propose that the effect of diabetes on periosteal progenitors and their ability to aid in skeletal repair directly impairs fracture healing.

LGRs in Skeletal Tissues: An Emerging Role for Wnt-Associated Adult Stem Cell Markers in Bone

Leucine‐rich repeat‐containing G protein‐coupled receptors (LGRs) are adult stem cell markers that have been described across various stem cell niches, and expression of LGRs and their corresponding ligands (R‐spondins) has now been reported in multiple bone‐specific cell types. The skeleton harbors elusive somatic stem cell populations that are exceedingly compartment‐specific and under tight regulation from various signaling pathways. Skeletal progenitors give rise to multiple tissues during development and during regenerative processes of bone, requiring postnatal endochondral and intramembranous ossification. The relevance of LGRs and the LGR/R‐spondin ligand interaction in bone and tooth biology is becoming increasingly appreciated. LGRs may define specific stem cell and progenitor populations and their behavior during both development and regeneration, and their role as Wnt‐associated receptors with specific ligands poses these proteins as unique therapeutic targets via potential R‐spondin agonism. This review seeks to outline the current literature on LGRs in the context of bone and its associated tissues, and points to key future directions for studying the functional role of LGRs and ligands in skeletal biology. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Investigating global gene expression changes in a murine model of cherubism

Cherubism is a rare genetic disorder caused primarily by mutations in SH3BP2 resulting in excessive bone resorption and fibrous tissue overgrowth in the lower portions of the face. Bone marrow derived cell cultures derived from a murine model of cherubism display poor osteogenesis and spontaneous osteoclast formation. To develop a deeper understanding for the potential underlying mechanisms contributing to these phenotypes in mice, we compared global gene expression changes in hematopoietic and mesenchymal cell populations between cherubism and wild type mice. In the hematopoietic population, not surprisingly, upregulated genes were significantly enriched for functions related to osteoclastogenesis. However, these upregulated genes were also significantly enriched for functions associated with inflammation including arachidonic acid/prostaglandin signaling, regulators of coagulation and autoinflammation, extracellular matrix remodeling, and chemokine expression. In the mesenchymal population, we observed down regulation of osteoblast and adventitial reticular cell marker genes. Regulators of BMP and Wnt pathway associated genes showed numerous changes in gene expression, likely implicating the down regulation of BMP signaling and possibly the activation of certain Wnt pathways. Analyses of the cherubism derived mesenchymal population also revealed interesting changes in gene expression related to inflammation including the expression of distinct granzymes, chemokines, and sulfotransferases. These studies reveal complex changes in gene expression elicited from a cherubic mutation in Sh3bp2 that are informative to the mechanisms responding to inflammatory stimuli and repressing osteogenesis. The outcomes of this work are likely to have relevance not only to cherubism, but other inflammatory conditions impacting the skeleton.

A PDGFRβ-PI3K signaling axis mediates periosteal cell activation during fracture healing

Insufficient and delayed fracture healing remain significant public health problems with limited therapeutic options. Phosphoinositide 3-kinase (PI3K) signaling, a major pathway involved in regulation of fracture healing, promotes proliferation, migration, and differentiation of osteoprogenitors. We have recently reported that knock-in mice with a global increase in PI3K signaling (gCbl^YF) show enhanced femoral fracture healing characterized by an extraordinary periosteal response to injury. Interestingly, of all growth factor receptors involved in fracture healing, PI3K directly binds only to PDGFR. Given these findings, we hypothesized a PDGFR-PI3K interaction is necessary for mediating robust periosteal cell activation following fracture. In this study, we isolated primary periosteal cells from gCbl^YF mice to analyze cross-talk between the PDGFRβ and PI3K signaling pathways. We found PDGFRβ signaling contributes to robust Akt phosphorylation in periosteal cells in comparison with other growth factor signaling pathways. Additionally, we performed femoral fractures on gCbl^YF mice with a conditional removal of PDGFRβ in mesenchymal progenitors using inducible alpha smooth muscle actin (αSMA) CreER^T2 mice. Our studies showed that depletion of PDGFRβ signaling within these progenitors in the early phase of fracture healing significantly abrogates PI3K-mediated periosteal activation and proliferation three days after fracture. Combined, these results suggest that PDGFRβ signaling through PI3K is necessary for robust periosteal activation in the earliest phases of fracture healing.

Cbl-PI3K interaction regulates Cathepsin K secretion in osteoclasts

Effective bone resorption by osteoclasts is critical for balanced bone remodeling. We have previously reported that mice harboring a substitution mutation of tyrosine 737 to phenylalanine in the adapter protein Cbl (Cbl^Y737F, YF) have increased bone volume partly due to decreased osteoclast-mediated bone resorption. The Cbl^Y737F mutation abrogates interaction between Cbl and the p85 subunit of PI3K. Here, we studied the mechanism for defective resorptive function of YF mutant osteoclasts. The YF osteoclasts had intact actin cytoskeletons and sealing zones. Expression and localization of proteins needed for acidification of the resorptive lacunae were also comparable between the WT and YF osteoclasts. In contrast, secretion of Cathepsin K, a major protease needed to degrade collagen, was diminished in the conditioned media derived from YF osteoclasts. The targeting of Cathepsin K into LAMP2-positive vesicles was also compromised due to decreased number of LAMP2-positive vesicles in YF osteoclasts. Further, we found that in contrast to WT, conditioned media derived from YF osteoclasts promoted increased numbers of alkaline phosphatase positive colonies, and increased expression of osteogenic markers in WT calvarial cultures. Cumulatively, our results suggest that the Cbl-PI3K interaction regulates Cathepsin K secretion required for proper bone resorption, and secretion of factors which promote osteogenesis.

Protease-Activated Receptor 1 Deletion Causes Enhanced Osteoclastogenesis in Response to Inflammatory Signals through a Notch2-Dependent Mechanism

We found that protease-activated receptor 1 (PAR1) was transiently induced in cultured osteoclast precursor cells. Therefore, we examined the bone phenotype and response to resorptive stimuli of PAR1-deficient (knockout [KO]) mice. Bones and bone marrow-derived cells from PAR1 KO and wild-type (WT) mice were assessed using microcomputed tomography, histomorphometry, in vitro cultures, and RT-PCR. Osteoclastic responses to TNF-α (TNF) challenge in calvaria were analyzed with and without a specific neutralizing Ab to the Notch2-negative regulatory region (N2-NRR Ab). In vivo under homeostatic conditions, there were minimal differences in bone mass or bone cells between PAR1 KO and WT mice. However, PAR1 KO myeloid cells demonstrated enhanced osteoclastogenesis in response to receptor activator of NF-κB ligand (RANKL) or the combination of RANKL and TNF. Strikingly, in vivo osteoclastogenic responses of PAR1 KO mice to TNF were markedly enhanced. We found that N2-NRR Ab reduced TNF-induced osteoclastogenesis in PAR1 KO mice to WT levels without affecting WT responses. Similarly, in vitro N2-NRR Ab reduced RANKL-induced osteoclastogenesis in PAR1 KO cells to WT levels without altering WT responses. We conclude that PAR1 functions to limit Notch2 signaling in responses to RANKL and TNF and moderates osteoclastogenic response to these cytokines. This effect appears, at least in part, to be cell autonomous because enhanced osteoclastogenesis was seen in highly purified PAR1 KO osteoclast precursor cells. It is likely that this pathway is involved in regulating the response of bone to diseases associated with inflammatory signals.

PI3K activation increases SDF-1 production and number of osteoclast precursors, and enhances SDF-1-mediated osteoclast precursor migration

Our previous studies showed that in a mouse model in which PI3K-AKT activation was increased (YF mice), osteoclast numbers and levels of SDF-1, a chemokine, were augmented. The purpose of this study was to delineate the role of PI3K activation in regulating SDF-1 production and examine whether SDF-1 can stimulate differentiation and/or migration of osteoclast precursors. Using flow cytometric analysis, we demonstrated that compared to wild type mice, bone marrow of YF mice had increased numbers of CXCL12 abundant reticular (CAR) cells, that are a major cell type responsible for producing SDF-1. At the molecular level, transcription factor specificity protein 1 (Sp1) induced an increased transcription of SDF-1 that was dependent on PI3K/AKT activation. YF mice also contained an increased number of osteoclast precursors, in which expression of CXCR4, a major receptor for SDF-1, was increased. SDF-1 did not induce differentiation of osteoclast precursors into mature osteoclasts; compared to cells derived from WT mice, cells obtained from YF mice were more responsive to SDF-1. In conclusion, we demonstrate that PI3K activation resulted in increased SDF-1, increased the number of osteoclast precursors, and enhanced osteoclast precursor migration in response to SDF-1.

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PI3K activation regulates the number of CAR cells in mouse bone marrow.

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PI3K activation regulates SDF-1/CXCL12 production by CAR cells in bone marrow.

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PI3K/AKT activation mediates transcription of SDF-1 by regulating transcription factor Sp1.

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SDF-1 enhances migration of osteoclast precursors via CXCR4.

PDGF Modulates BMP2-Induced Osteogenesis in Periosteal Progenitor Cells

BMPs are used in various clinical applications to promote bone formation. The limited success of the BMPs in clinical settings and supraphysiological doses required for their effects prompted us to evaluate the influence of other signaling molecules, specifically platelet‐derived growth factor (PDGF) on BMP2‐induced osteogenesis. Periosteal cells make a major contribution to fracture healing. We detected broad expression of PDGF receptor beta (PDGFRβ) within the intact periosteum and healing callus during fracture repair. In vitro, periosteum‐derived progenitor cells were highly responsive to PDGF as demonstrated by increased proliferation and decreased apoptosis. However, PDGF blocked BMP2‐induced osteogenesis by inhibiting the canonical BMP2/Smad pathway and downstream target gene expression. This effect is mediated via PDGFRβ and involves ERK1/2 MAPK and PI3K/AKT signaling pathways. Therapeutic targeting of the PDGFRβ pathway in periosteum‐mediated bone repair might have profound implications in the treatment of bone disease in the future. © 2018 The Authors JBMR Plus is published by Wiley Periodicals, Inc. on behalf of the American Society for Bone and Mineral Research.

Induction of the Hajdu-Cheney Syndrome Mutation in CD19 B Cells in Mice Alters B-Cell Allocation but Not Skeletal Homeostasis

Mice harboring Notch2 mutations replicating Hajdu-Cheney syndrome (Notch2^tm1.1ECan) have osteopenia and exhibit an increase in splenic marginal zone B cells with a decrease in follicular B cells. Whether the altered B-cell allocation is responsible for the osteopenia of Notch2^tm1.1ECan mutants is unknown. To determine the effect of NOTCH2 activation in B cells on splenic B-cell allocation and skeletal phenotype, a conditional-by-inversion (COIN) Hajdu-Cheney syndrome allele of Notch2 (Notch2^[ΔPEST]COIN) was used. Cre recombination generates a permanent Notch2^ΔPEST allele expressing a transcript for which sequences coding for the proline, glutamic acid, serine, and threonine-rich (PEST) domain are replaced by a stop codon. CD19-Cre drivers were backcrossed into Notch2^{[ΔPEST]COIN/[ΔPEST]COIN} to generate CD19-specific Notch2^{ΔPEST/ΔPEST} mutants and control Notch2^{[ΔPEST]COIN/[ΔPEST]COIN} littermates. There was an increase in marginal zone B cells and a decrease in follicular B cells in the spleen of CD19^Cre/WT;Notch2^{ΔPEST/ΔPEST} mice, recapitulating the splenic phenotype of Notch2^tm1.1ECan mice. The effect was reproduced when the NOTCH1 intracellular domain was induced in CD19-expressing cells (CD19^Cre/WT;Rosa^Notch1/WT mice). However, neither CD19^Cre/WT;Notch2^{ΔPEST/ΔPEST} nor CD19^Cre/WT;Rosa^Notch1/WT mice had a skeletal phenotype. Moreover, splenectomies in Notch2^tm1.1ECan mice did not reverse their osteopenic phenotype. In conclusion, Notch2 activation in CD19-expressing cells determines B-cell allocation in the spleen but has no skeletal consequences.

A novel role for cathepsin K in periosteal osteoclast precursors during fracture repair

Osteoporosis management is currently centered around bisphosphonates, which inhibit osteoclast (OC) bone resorption but do not affect bone formation. This reduces fracture risk, but fails to restore healthy bone remodeling. Studies in animal models showed that cathepsin K (CatK) inhibition by genetic deletion or chemical inhibitors maintained bone formation while abrogating resorption during bone remodeling and stimulated periosteal bone modeling. Recently, periosteal mononuclear tartrate-resistant acid phosphatase-positive (TRAP⁺ ) osteoclast precursors (OCPs) were shown to augment angiogenesis-coupled osteogenesis. CatK gene deletion increased osteoblast differentiation via enhanced OCP and OC secretion of platelet-derived growth factor (PDGF)-BB and sphingosine 1 phosphate. The effects of periosteum-derived OCPs on bone remodeling are unknown, particularly with regard to fracture repair. We hypothesized that periosteal OCPs derived from CatK-null (Ctsk^-/- ) mice may enhance periosteal bone formation during fracture repair. We found fewer periosteal OCPs in Ctsk^-/- mice under homeostatic conditions; however, after fracture, this population increased in number relative to that seen in wild-type (WT) mice. Enhanced TRAP staining and greater expression of PDGF-BB were observed in fractured Ctsk^-/- femurs relative to WT femurs. This early pattern of augmented PDGF-BB expression in Ctsk^-/- mice may contribute to improved fracture healing by enhancing callus mineralization in Ctsk^-/- mice.

The Hajdu Cheney Mutation Is a Determinant of B-Cell Allocation of the Splenic Marginal Zone

The neurogenic locus notch homolog protein (Notch)-2 receptor is a determinant of B-cell allocation, and gain-of-NOTCH2-function mutations are associated with Hajdu-Cheney syndrome (HCS), a disease presenting with osteoporosis and acro-osteolysis. We generated a mouse model reproducing the HCS mutation (Notch2HCS), and heterozygous global mutant mice displayed gain-of-Notch2 function. In the mutant spleen, the characteristic perifollicular rim marking the marginal zone (MZ), which is the interface between the nonlymphoid red pulp and the lymphoid white pulp, merged with components of the white pulp. As a consequence, the MZ of Notch2HCS mice occupied most of the splenic structure. To explore the mechanisms involved, lymphocyte populations from the bone marrow and spleen were harvested from heterozygous Notch2HCS mice and sex-matched control littermates and analyzed by flow cytometry. Notch2HCS mice had an increase in CD21/35^highCD23^- splenic MZ B cells of approximately fivefold and a proportional decrease in splenic follicular B cells (CD21/35^intCD23⁺) at 1, 2, and 12 months of age. Western blot analysis revealed that Notch2HCS mutant splenocytes had increased phospho-Akt and phospho-Jun N-terminal kinase, and gene expression analysis of splenic CD19⁺ B cells demonstrated induction of Hes1 and Hes5 in Notch2HCS mutants. Anti-Notch2 antibodies decreased MZ B cells in control and Notch2HCS mice. In conclusion, Notch2HCS mutant mice have increased mature B cells in the MZ of the spleen.

Sustained Notch2 signaling in osteoblasts, but not in osteoclasts, is linked to osteopenia in a mouse model of Hajdu-Cheney syndrome

Individuals with Hajdu-Cheney syndrome (HCS) present with osteoporosis, and HCS is associated with NOTCH2 mutations causing deletions of the proline-, glutamic acid-, serine-, and threonine-rich (PEST) domain that are predicted to enhance NOTCH2 stability and cause gain-of-function. Previously, we demonstrated that mice harboring Notch2 mutations analogous to those in HCS (Notch2HCS) are severely osteopenic because of enhanced bone resorption. We attributed this phenotype to osteoclastic sensitization to the receptor activator of nuclear factor-κB ligand and increased osteoblastic tumor necrosis factor superfamily member 11 (Tnfsf11) expression. Here, to determine the individual contributions of osteoclasts and osteoblasts to HCS osteopenia, we created a conditional-by-inversion (Notch2^COIN ) model in which Cre recombination generates a Notch2^ΔPEST allele expressing a Notch2 mutant lacking the PEST domain. Germ line Notch2^COIN inversion phenocopied the Notch2HCS mutant, validating the model. To activate Notch2 in osteoclasts or osteoblasts, Notch2^COIN mice were bred with mice expressing Cre from the Lyz2 or the BGLAP promoter, respectively. These crosses created experimental mice harboring a Notch2^ΔPEST allele in Cre-expressing cells and control littermates expressing a wild-type Notch2 transcript. Notch2^COIN inversion in Lyz2-expressing cells had no skeletal consequences and did not affect the capacity of bone marrow macrophages to form osteoclasts in vitro In contrast, Notch2^COIN inversion in osteoblasts led to generalized osteopenia associated with enhanced bone resorption in the cancellous bone compartment and with suppressed endocortical mineral apposition rate. Accordingly, Notch2 activation in osteoblast-enriched cultures from Notch2^COIN mice induced Tnfsf11 expression. In conclusion, introduction of the HCS mutation in osteoblasts, but not in osteoclasts, causes osteopenia.

Dual growth factor delivery from biofunctionalized allografts: Sequential VEGF and BMP-2 release to stimulate allograft remodeling

Autografts have been shown to stimulate osteogenesis, osteoclastogenesis, and angiogenesis, and subsequent rapid graft incorporation. Large structural allografts, however, suffer from limited new bone formation and remodeling, both of which are directly associated with clinical failure due to non-unions, late graft fractures, and infections, making it a priority to improve large structural allograft healing. We have previously shown the osteogenic ability of a polymer-coated allograft that delivers bone morphogenetic protein-2 both in vitro and in vivo through both burst release and sustained release kinetics. In this study, we have demonstrated largely sequential delivery of bone morphogenetic protein-2 and vascular endothelial growth factor from the same coated allograft. Release data showed that loading both growth factors onto a polymeric coating with two different techniques resulted in short-term (95% release within 2 weeks) and long-term (95% release within 5 weeks) delivery kinetics. We have also demonstrated how released VEGF, traditionally associated with angiogenesis, can also provide a stimulus for allograft remodeling via resorption. Bone marrow derived mononuclear cells were co-cultured with VEGF released from the coated allograft and showed a statistically significant (p < 0.05) and dose dependent increase in the number of tartrate-resistant acid phosphatase-positive multinucleated osteoclasts. Functionality of these osteoclasts was assessed quantitatively and qualitatively by evaluating resorption pit area from both osteo-assay plates and harvested bone. Data indicated a statistically significant higher resorption area from the cells exposed to VEGF released from the allografts over controls (p < 0.05). These results indicate that by using different loading protocols temporal control can be achieved when delivering multiple growth factors from a polymer-coated allograft. Further, released VEGF can also stimulate osteoclastogenesis that may enhance allograft incorporation, and thus mitigate long-term clinical complications. © 2017 Orthopedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1086-1095, 2017.

An Antibody to Notch2 Reverses the Osteopenic Phenotype of Hajdu-Cheney Mutant Male Mice

Notch receptors play a central role in skeletal development and bone remodeling. Hajdu-Cheney syndrome (HCS), a disease characterized by osteoporosis and fractures, is associated with gain-of-NOTCH2 function mutations. To study HCS, we created a mouse model harboring a point 6955C>T mutation in the Notch2 locus upstream of the proline, glutamic acid, serine, and threonine domain, leading to a Q2319X change at the amino acid level. Notch2Q2319X heterozygous mutants exhibited cancellous and cortical bone osteopenia. Microcomputed tomography demonstrated that the cancellous and cortical osteopenic phenotype was reversed by the administration of antibodies generated against the negative regulatory region (NRR) of Notch2, previously shown to neutralize Notch2 activity. Bone histomorphometry revealed that anti-Notch2 NRR antibodies decreased the osteoclast number and eroded surface in cancellous bone of Notch2Q2319X mice. An increase in osteoclasts on the endocortical surface of Notch2Q2319X mice was not observed in the presence of anti-Notch2 NRR antibodies. The anti-Notch2 NRR antibody decreased the induction of Notch target genes and Tnfsf11 messenger RNA levels in bone extracts and osteoblasts from Notch2Q2319X mice. In vitro experiments demonstrated increased osteoclastogenesis in Notch2Q2319X mutants in response to macrophage colony-stimulating factor and receptor activator of nuclear factor-κB ligand, and these effects were suppressed by the anti-Notch2 NRR. In conclusion, Notch2Q2319X mice exhibit cancellous and cortical bone osteopenia that can be corrected by the administration of anti-Notch2 NRR antibodies.

Loss of Cbl-PI3K interaction modulates the periosteal response to fracture by enhancing osteogenic commitment and differentiation

The periosteum contains multipotent skeletal progenitors that contribute to bone repair. The signaling pathways regulating the response of periosteal cells to fracture are largely unknown. Phosphatidylinositol-3 Kinase (PI3K), a prominent lipid kinase, is a major signaling protein downstream of several factors that regulate osteoblast differentiation. Cbl is an E3 ubiquitin ligase and a major adaptor protein that binds to the p85 regulatory subunit and modulates PI3K activity. Substitution of tyrosine 737 to phenylalanine (Y737F) in Cbl abolishes the interaction between Cbl and p85 subunit without affecting the Cbl's ubiquitin ligase function. Here, we investigated the role of PI3K signaling during the very early stages of fracture healing using Osterix^RFP reporter mice. We found that the absence of PI3K regulation by Cbl resulted in robust periosteal thickening, with increased proliferation of periosteal cells. While the multipotent properties of periosteal progenitors to differentiate into chondrocytes and adipocytes did not change, osteogenic differentiation in the absence of Cbl-PI3K interaction was highly augmented. The increased stability and nuclear localization of Osterix observed in periosteal cells lacking Cbl-PI3K interaction may explain this enhanced osteogenic differentiation since the expression of Osterix transcriptional target genes including osteocalcin and BSP are increased in YF cells. Overall, our findings highlight a hitherto unexplored and novel role for Cbl and PI3K in modulating the osteogenic response of periosteal cells during the early stages of fracture repair.

The Multifaceted Osteoclast; Far and Beyond Bone Resorption

The accepted function of the bone resorbing cell, osteoclast, has been linked to bone remodeling and pathological osteolysis. Emerging evidence points to novel functions of osteoclasts in controlling bone formation and angiogenesis. Thus, while the concept of a "clastokine" with the potential to regulate osteogenesis during remodeling did not come as a surprise, new evidence provided unique insight into the mechanisms underlying osteoclastic control of bone formation. The question still remains as to whether osteoclast precursors or a unique trap positive mononuclear cell, can govern any aspect of bone formation. The novel paradigm eloquently proposed by leaders in the field brings together the concept of clastokines and osteoclast precursor-mediated bone formation, potentially though enhanced angiogenesis. These fascinating advances in osteoclast biology have motivated this short review, in which we discuss these new roles of osteoclasts. J. Cell. Biochem. 117: 1753-1756, 2016. © 2016 Wiley Periodicals, Inc.

Role of Cbl-PI3K Interaction during Skeletal Remodeling in a Murine Model of Bone Repair

Mice in which Cbl is unable to bind PI3K (YF mice) display increased bone volume due to enhanced bone formation and repressed bone resorption during normal bone homeostasis. We investigated the effects of disrupted Cbl-PI3K interaction on fracture healing to determine whether this interaction has an effect on bone repair. Mid-diaphyseal femoral fractures induced in wild type (WT) and YF mice were temporally evaluated via micro-computed tomography scans, biomechanical testing, histological and histomorphometric analyses. Imaging analyses revealed no change in soft callus formation, increased bony callus formation, and delayed callus remodeling in YF mice compared to WT mice. Histomorphometric analyses showed significantly increased osteoblast surface per bone surface and osteoclast numbers in the calluses of YF fractured mice, as well as increased incorporation of dynamic bone labels. Furthermore, using laser capture micro-dissection of the fracture callus we found that cells lacking Cbl-PI3K interaction have higher expression of Osterix, TRAP, and Cathepsin K. We also found increased expression of genes involved in propagating PI3K signaling in cells isolated from the YF fracture callus, suggesting that the lack of Cbl-PI3K interaction perhaps results in enhanced PI3K signaling, leading to increased bone formation, but delayed remodeling in the healing femora.

Altered hematopoietic stem cell and osteoclast precursor frequency in cathepsin K null mice

Cathepsin K (CatK) is a lysosomal cysteine protease necessary for bone resorption by osteoclasts (OCs), which originate from myeloid hematopoietic precursors. CatK-deficient (CatK(-/-) ) mice show osteopetrosis due to defective resorption by OCs, which are increased in number in these mice. We investigated whether genetic ablation of CatK altered the number of hematopoietic stem cells (HSCs) and OC precursor cells (OCPs) using two mouse models: CatK(-/-) mice and a knock-in mouse model in which the CatK gene (ctsk) is replaced by cre recombinase. We found that CatK deletion in mice significantly increased the number of HSCs in the spleen and decreased their number in bone marrow. In contrast, the number of early OCPs was unchanged in the bone marrow. However, the number of committed CD11b(+) OCPs was increased in the bone marrow of CatK(-/-) compared to wild-type (WT) mice. In addition, the percentage but not the number of OCPs was decreased in the spleen of CatK(-/-) mice relative to WT. To understand whether increased commitment to OC lineage in CatK(-/-) mice is influenced by the bone marrow microenvironment, CatK(Cre/+) or CatK(Cre/Cre) red fluorescently labeled OCPs were injected into WT mice, which were also subjected to a mid-diaphyseal femoral fracture. The number of OCs derived from the intravenously injected CatK(Cre/Cre) OCPs was lower in the fracture callus compared to mice injected with CatK(+/Cre) OCPs. Hence, in addition to its other effects, the absence of CatK in OCP limits their ability to engraft in a repairing fracture callus compared to WT OCP.

Loss of Cbl-PI3K interaction enhances osteoclast survival due to p21-Ras mediated PI3K activation independent of Cbl-b

Cbl family proteins, Cbl and Cbl-b, are E3 ubiquitin ligases and adaptor proteins, which play important roles in bone-resorbing osteoclasts. Loss of Cbl in mice decreases osteoclast migration, resulting in delayed bone development where as absence of Cbl-b decreases bone volume due to hyper-resorptive osteoclasts. A major structural difference between Cbl and Cbl-b is tyrosine 737 (in YEAM motif) only on Cbl, which upon phosphorylation interacts with the p85 subunit of phosphatidylinositol-3 Kinase (PI3K). In contrast to Cbl(-/-) and Cbl-b(-/-) , mice lacking Cbl-PI3K interaction due to a Y737F (tyrosine to phenylalanine, YF) mutation showed enhanced osteoclast survival, but defective bone resorption. To investigate whether Cbl-PI3K interaction contributes to distinct roles of Cbl and Cbl-b in osteoclasts, mice bearing CblY737F mutation in the Cbl-b(-/-) background (YF/YF;Cbl-b(-/-) ) were generated. The differentiation and survival were augmented similarly in YF/YF and YF/YF;Cbl-b(-/-) osteoclasts, associated with enhanced PI3K signaling suggesting an exclusive role of Cbl-PI3K interaction, independent of Cbl-b. In addition to PI3K, the small GTPase Ras also regulates osteoclast survival. In the absence of Cbl-PI3K interaction, increased Ras GTPase activity and Ras-PI3K binding were observed and inhibition of Ras activation attenuated PI3K mediated osteoclast survival. In contrast to differentiation and survival, increased osteoclast activity observed in Cbl-b(-/-) mice persisted even after introduction of the resorption-defective YF mutation in YF/YF;Cbl-b(-/-) mice. Hence, Cbl and Cbl-b play mutually exclusive roles in osteoclasts. Whereas Cbl-PI3K interaction regulates differentiation and survival, bone resorption is predominantly regulated by Cbl-b in osteoclasts.

Loss of Cbl-PI3K interaction in mice prevents significant bone loss following ovariectomy

Cbl and Cbl-b are E3 ubiquitin ligases and adaptor proteins, which perform regulatory roles in bone remodeling. Cbl-/- mice have delayed bone development due to decreased osteoclast migration. Cbl-b-/- mice are osteopenic due to increased bone resorbing activity of osteoclasts. Unique to Cbl, but not present in Cbl-b, is tyrosine 737 in the YEAM motif, which upon phosphorylation provides a binding site for the regulatory p85 subunit of PI3K. Substitution of tyrosine 737 with phenylalanine (Y737F, CblYF/YF mice) prevents Y737 phosphorylation and abrogates the Cbl-PI3K interaction. We have previously reported that CblYF/YF mice had increased bone volume due to defective bone resorption and increased bone formation. Here we show that the lumbar vertebra from CblYF/YF mice did not have significant bone loss following ovariectomy. Our data also suggests that abrogation of Cbl-PI3K interaction in mice results in the loss of coupling between bone resorption and formation, since ovariectomized CblYF/YF mice did not show significant changes in serum levels of c-terminal telopeptide (CTX), whereas the serum levels of pro-collagen type-1 amino-terminal pro-peptide (P1NP) were decreased. In contrast, following ovariectomy, Cbl-/- and Cbl-b-/- mice showed significant bone loss in the tibiae and L2 vertebrae, concomitant with increased serum CTX and P1NP levels. These data indicate that while lack of Cbl or Cbl-b distinctly affects bone remodeling, only the loss of Cbl-PI3K interaction protects mice from significant bone loss following ovariectomy.

c-Cbl inhibition improves cardiac function and survival in response to myocardial ischemia

BACKGROUND

The proto-oncogene Casitas b-lineage lymphoma (c-Cbl) is an adaptor protein with an intrinsic E3 ubiquitin ligase activity that targets receptor and nonreceptor tyrosine kinases, resulting in their ubiquitination and downregulation. However, the function of c-Cbl in the control of cardiac function is currently unknown. In this study, we examined the role of c-Cbl in myocyte death and cardiac function after myocardial ischemia.

METHODS AND RESULTS

We show increased c-Cbl expression in human ischemic and dilated cardiomyopathy hearts and in response to pathological stress stimuli in mice. c-Cbl-deficient mice demonstrated a more robust functional recovery after myocardial ischemia/reperfusion injury and significantly reduced myocyte apoptosis and improved cardiac function. Ubiquitination and downregulation of key survival c-Cbl targets, epidermal growth factor receptors and focal adhesion kinase, were significantly reduced in c-Cbl knockout mice. Inhibition of c-Cbl expression or its ubiquitin ligase activity in cardiac myocytes offered protection against H2O2 stress. Interestingly, c-Cbl deletion reduced the risk of death and increased cardiac functional recovery after chronic myocardial ischemia. This beneficial effect of c-Cbl deletion was associated with enhanced neoangiogenesis and increased expression of vascular endothelial growth factor-a and vascular endothelial growth factor receptor type 2 in the infarcted region.

CONCLUSIONS

c-Cbl activation promotes myocyte apoptosis, inhibits angiogenesis, and causes adverse cardiac remodeling after myocardial infarction. These findings point to c-Cbl as a potential therapeutic target for the maintenance of cardiac function and remodeling after myocardial ischemia.

Abstract 214: Cbl Inhibition Improves Cardiac Function and Survival in Response to Myocardial Ischemia

Introduction and Hypothesis: Proteasome degradation of ubiquitin-targeted proteins plays an important role in the development of cardiomyopathy and heart failure. The mechanisms that control proteasome function and protein ubiquitination and turnover remain poorly defined. The proto-oncogene Casitas b-lineage lymphoma (Cbl) is an adaptor protein with an intrinsic E3 ubiquitin ligase activity that targets tyrosine kinases, resulting in their ubiquitination and down-regulation. Although Cbl is a crucial signaling molecule in many cells, the role of Cbl in the heart is unknown.

Methods and Results: We show increased Cbl expression in human failing hearts and in response to pathological stress stimuli in mice. Cbl deficient mice demonstrated a more robust functional recovery after ischemia reperfusion injury, as well as significantly reduced myocyte apoptosis and epidermal growth factor receptor and focal adhesion (FA) protein ubiquitination and downregulation. Cbl Knockdown in-vitro protected cardiomyocytes against H2O2-induced apoptosis. Cbl deficient mice also show greater functional recovery after myocardial ischemia. This cardioprotective effect of Cbl deletion can be attributed, at least in part, to enhanced neoangiogenesis in the infarcted region via upregulation of vascular endothelial growth factor-a.

Conclusions: Cbl activation regulates myocyte apoptosis and angiogenesis and contributes to adverse cardiac remodeling post-MI. These findings point to Cbl as a potential therapeutic target for the maintenance of cardiac function and remodeling post-MI.

cSrc is necessary for epididymal development and is incorporated into sperm during epididymal transit

Changes that occur to mammalian sperm upon epididymal transit and maturation render these cells capable of moving progressively and capacitating. Signaling events leading to mammalian sperm capacitation depend on the modulation of proteins by phosphorylation and dephosphorylation cascades. Recent experiments have demonstrated that the Src family of kinases plays an important role in the regulation of these events. However, sperm from cSrc null mice display normal tyrosine phosphorylation associated with capacitation. We report here that, despite normal phosphorylation, sperm from cSrc null mice display a severe reduction in forward motility, and are unable to fertilize in vitro. Histological analysis of seminiferous tubules in the testes, caput and corpus epididymis do not reveal obvious defects. However, the cauda epididymis is significantly smaller, and expression of key transport proteins in the epithelial cells lining this region is reduced in cSrc null mice compared to wild type littermates. Although previously, we and others have shown the presence of cSrc in mature sperm from cauda epididymis, a closer evaluation indicates that this tyrosine kinase is not present in sperm from the caput epididymis, suggesting that this protein is acquired by sperm later during epididymal maturation. Consistent with this observation, cSrc is enriched in vesicles released by the epididymal epithelium known as epididymosomes. Altogether, these observations indicate that cSrc is essential for cauda epididymal development and suggest an essential role of this kinase in epididymal sperm maturation involving cSrc extracellular trafficking.

c-Cbl ubiquitin ligase regulates focal adhesion protein turnover and myofibril degeneration induced by neutrophil protease cathepsin G

The neutrophil-derived serine protease, cathepsin G (Cat.G), has been shown to induce myocyte detachment and apoptosis by anoikis through down-regulation of focal adhesion (FA) signaling. However, the mechanisms that control FA protein stability and turnover in myocytes are not well understood. Here, we have shown that the Casitas b-lineage lymphoma (c-Cbl), adaptor protein with an intrinsic E3 ubiquitin ligase activity, is involved in FA and myofibrillar protein stability and turnover in myocytes. Cat.G treatment induced c-Cbl activation and its interaction with FA proteins. Deletion of c-Cbl using c-Cbl knock-out derived myocytes or inhibition of c-Cbl ligase activity significantly reduced FA protein degradation, myofibrillar degeneration, and myocyte apoptosis induced by Cat.G. We also found that inhibition of the proteasome activity, but not the lysosome or the calpain activity, markedly attenuated FA and myofibrillar protein degradation induced by Cat.G. Interestingly, c-Cbl activation induced by Cat.G was mediated through epidermal growth factor receptor (EGFR) transactivation as inhibition of EGFR kinase activity markedly attenuated c-Cbl phosphorylation and FA protein degradation induced by Cat.G. These findings support a model in which neutrophil protease Cat.G promotes c-Cbl interaction with FA proteins, resulting in enhanced c-Cbl-mediated FA protein ubiquitination and degradation, myofibril degradation, and subsequent down-regulation of myocyte survival signaling.

Abrogation of Cbl-PI3K interaction increases bone formation and osteoblast proliferation

Cbl is an adaptor protein and E3 ligase that plays both positive and negative roles in several signaling pathways that affect various cellular functions. Tyrosine 737 is unique to Cbl and phosphorylated by Src family kinases. Phosphorylated CblY737 creates a binding site for the p85 regulatory subunit of phosphatidylinositol 3 kinase (PI3K) that also plays an important role in the regulation of bone homeostasis. To investigate the role of Cbl-PI3K interaction in bone homeostasis, we examined knock-in mice in which the PI3K binding site on Cbl was ablated due to the substitution of tyrosine 737 to phenylalanine (Cbl(YF/YF), YF mice). We previously reported that bone volume in these mice is increased due to decreased osteoclast function (Adapala et al., J Biol Chem 285:36745-36758, 19). Here, we report that YF mice also have increased bone formation and osteoblast numbers. In ex vivo cultures bone marrow-derived YF osteoblasts showed increased Col1A expression and their proliferation was also significantly augmented. Moreover, proliferation of MC3T3-E1 cells was increased after treatment with conditioned medium generated by culturing YF bone marrow stromal cells. Expression of stromal derived factor-1 (SDF-1) was increased in YF bone marrow stromal cells compared to wild type. Increased immunostaining of SDF-1 and CXCR4 was observed in YF bone marrow stromal cells compared to wild type. Treatment of YF condition medium with neutralizing anti-SDF-1 and anti-CXCR4 antibodies attenuated MC3T3-E1 cell proliferation. Cumulatively, these results show that abrogation of Cbl-PI3K interaction perturbs bone homeostasis, affecting both osteoclast function and osteoblast proliferation.

Extracellular engagement of ADAM12 induces clusters of invadopodia with localized ectodomain shedding activity

Invadopodia are dynamic actin structures at the cell surface that degrade extracellular matrix and act as sites of signal transduction. The biogenesis of invadopodia, including the mechanisms regulating their formation, composition, and turnover is not entirely understood. Here, we demonstrate that antibody ligation of ADAM12, a transmembrane disintegrin and metalloprotease, resulted in the rapid accumulation of invadopodia with extracellular matrix-degrading capacity in epithelial cells expressing the αvβ3 integrin and active c-Src kinase. The induction of invadopodia clusters required an intact c-Src interaction site in the ADAM12 cytoplasmic domain, but was independent of the catalytic activity of ADAM12. Caveolin-1 and transmembrane protease MMP14/MT1-MMP were both present in the ADAM12-induced clusters of invadopodia, and cholesterol depletion prevented their formation, suggesting that lipid-raft microdomains are involved in the process. Importantly, our data demonstrate that ADAM12-mediated ectodomain shedding of epidermal growth factor receptor ligands can occur within these invadopodia. Such localized growth factor signalling offers an interesting novel biological concept highly relevant to the properties of carcinoma cells, which often show upregulated ADAM12 and β3 integrin expression, together with high levels of c-Src kinase activity.

The loss of Cbl-phosphatidylinositol 3-kinase interaction perturbs RANKL-mediated signaling, inhibiting bone resorption and promoting osteoclast survival

Cbl is an adaptor protein and an E3 ligase that plays both positive and negative roles in several signaling pathways that affect various cellular functions. Tyrosine 737 is unique to Cbl and is phosphorylated by Syk and Src family kinases. Phosphorylated Cbl Tyr(737) creates a binding site for the p85 regulatory subunit of PI3K, which also plays an important role in the regulation of bone resorption by osteoclasts. To investigate the role of Cbl-PI3K interaction in bone homeostasis, we examined the knock-in mice (Cbl(YF/YF)) in which the PI3K binding site in Cbl is ablated due to the mutation in the regulatory tyrosine. We report that in Cbl(YF/YF) mice, despite increased numbers of osteoclasts, bone volume is increased due to defective osteoclast function. Additionally, in ex vivo cultures, mature Cbl(YF/YF) osteoclasts showed an increased ability to survive in the presence of RANKL due to delayed onset of apoptosis. RANKL-mediated signaling is perturbed in Cbl(YF/YF) osteoclasts, and most interestingly, AKT phosphorylation is up-regulated, suggesting that the lack of PI3K sequestration by Cbl results in increased survival and decreased bone resorption. Cumulatively, these in vivo and in vitro results show that, on one hand, binding of Cbl to PI3K negatively regulates osteoclast differentiation, survival, and signaling events (e.g. AKT phosphorylation), whereas on the other hand it positively influences osteoclast function.

Src is a major signaling component for CTGF induction by TGF-beta1 in osteoblasts

Connective tissue growth factor (CTGF/CCN2) is induced by transforming growth factor beta1 (TGF-beta1) where it acts as a downstream mediator of TGF-beta1 induced matrix production in osteoblasts. We have shown the requirement of Src, Erk, and Smad signaling for CTGF induction by TGF-beta1 in osteoblasts; however, the potential interaction among these signaling pathways remains undetermined. In this study we demonstrate that TGF-beta1 activates Src kinase in ROS17/2.8 cells and that treatment with the Src family kinase inhibitor PP2 prevents Src activation and CTGF induction by TGF-beta1. Additionally, inhibiting Src activation prevented Erk activation, Smads 2 and 3 activation and nuclear translocation by TGF-beta1, demonstrating that Src is an essential upstream signaling partner of both Erk and Smads in osteoblasts. MAPKs such as Erk can modulate the Smad pathway directly by mediating the phosphorylation of Smads or indirectly through activation/inactivation of required nuclear co-activators that mediate Smad DNA binding. When we treated cells with the Erk inhibitor, PD98059, it inhibited TGF-beta1-induced CTGF protein expression but had no effect on Src activation, Smad activation or Smad nuclear translocation. However PD98059 impaired transcriptional complex formation on the Smad binding element (SBE) of the CTGF promoter, demonstrating that Erk activation was required for SBE transactivation. These data demonstrate that Src is an essential upstream signaling transducer of Erk and Smad signaling with respect to TGF-beta1 in osteoblasts and that Smads and Erk function independently but are both essential for forming a transcriptionally active complex on the CTGF promoter in osteoblasts.

Cbl-phosphatidylinositol 3 kinase interaction differentially regulates macrophage colony-stimulating factor-mediated osteoclast survival and cytoskeletal reorganization

The Cbl protein is a key player in macrophage colony-stimulating factor (M-CSF)-induced signaling. To examine the role of Cbl in M-CSF-mediated cellular events, we used Cbl(YF/YF) knockin mice in which the regulatory tyrosine 737, which when phosphorylated binds to the p85 subunit of phosphatidylinositol 3 kinase (PI3K), is substituted to phenylalanine. In ex vivo cultures, M-CSF and receptor activator of nuclear factor-kappaB ligand-mediated differentiation of bone marrow precursors from Cbl(YF/YF) mice generated increased number of osteoclasts; however, osteoclast numbers in Cbl(YF/YF) cultures were unchanged with increasing doses of M-CSF. We found that Cbl(YF/YF) osteoclasts have enhanced intrinsic ability to survive, and this response was further augmented upon exposure to M-CSF. Treatment of osteoclasts with M-CSF-induced actin reorganization and lamellipodia formation in wild-type osteoclasts; however, in Cbl(YF/YF) osteoclasts lamellipodia formation was compromised. Collectively, these results indicate that abrogation of the Cbl-PI3K interaction, although not affecting M-CSF-induced proliferation and differentiation of precursors, is required for regulation of survival and actin cytoskeletal reorganization of mature osteoclasts.

Cbl-b is a novel physiologic regulator of glycoprotein VI-dependent platelet activation

Cbl-b, a member of the Cbl family of E3 ubiquitin ligases, plays an important role in the activation of lymphocytes. However, its function in platelets remains unknown. We show that Cbl-b is expressed in human platelets along with c-Cbl, but in contrast to c-Cbl, it is not tyrosine-phosphorylated upon glycoprotein VI (GPVI) stimulation. Cbl-b, unlike c-Cbl, is not required for Syk ubiquitylation downstream of GPVI activation. Phospholipase Cgamma2 (PLCgamma2) and Bruton's tyrosine kinase (BTK) are constituently associated with Cbl-b. Cbl-b-deficient (Cbl-b(-/-)) platelets display an inhibition in the concentration-response curve for GPVI-specific agonist-induced aggregation, secretion, and Ca(2+) mobilization. A parallel inhibition is found for activation of PLCgamma2 and BTK. However, Syk activation is not affected by the absence of Cbl-b, indicating that Cbl-b acts downstream of Syk but upstream of BTK and PLCgamma2. When Cbl-b(-/-) mice were tested in the ferric chloride thrombosis model, occlusion time was increased and clot stability was reduced compared with wild type controls. These data indicate that Cbl-b plays a positive modulatory role in GPVI-dependent platelet signaling, which translates to an important regulatory role in hemostasis and thrombosis in vivo.

Inhibition of Ser/Thr phosphatases induces capacitation-associated signaling in the presence of Src kinase inhibitors

Signaling events leading to mammalian sperm capacitation rely on activation/deactivation of proteins by phosphorylation. This cascade includes soluble adenylyl cyclase, an atypical bicarbonate-stimulated adenylyl cyclase, and is mediated by protein kinase A and the subsequent stimulation of protein tyrosine phosphorylation. Recently, it has been proposed that the capacitation-associated increase in tyrosine phosphorylation is governed by Src tyrosine kinase activity. This conclusion was based mostly on the observation that Src is present in sperm and that the Src kinase family inhibitor SU6656 blocked the capacitation-associated increase in tyrosine phosphorylation. Results in the present manuscript confirmed these observations and provided evidence that these inhibitors were also able to inhibit protein kinase A phosphorylation, sperm motility, and in vitro fertilization. However, the block of capacitation-associated parameters was overcome when sperm were incubated in the presence of Ser/Thr phosphatase inhibitors such as okadaic acid and calyculin-A at concentrations reported to affect only PP2A. Altogether, these data indicate that Src is not directly involved in the observed increase in tyrosine phosphorylation. More importantly, this work presents strong evidence that capacitation is regulated by two parallel pathways. One of them requiring activation of protein kinase A and the second one involving inactivation of Ser/Thr phosphatases.

ADAM12 localizes with c-Src to actin-rich structures at the cell periphery and regulates Src kinase activity

ADAM12 is an active metalloprotease playing an important role in tumour progression. Human ADAM12 exists in two splice variants: a long transmembrane form, ADAM12-L, and a secreted form, ADAM12-S. The subcellular localization of ADAM12-L is tightly regulated and involves intracellular interaction partners and signalling proteins. We demonstrate here a c-Src-dependent redistribution of ADAM12-L from perinuclear areas to actin-rich Src-positive structures at the cell periphery, and identified two separate c-Src binding sites in the cytoplasmic tail of ADAM12-L that interact with the SH3 domain of c-Src with different binding affinities. The association between ADAM12-L and c-Src is transient, but greatly stabilized when the c-Src kinase activity is disrupted. In agreement with this observation, kinase-active forms of c-Src induce ADAM12-L tyrosine phosphorylation. Interestingly, ADAM12-L was also found to enhance Src kinase activity in response to external signals, such as integrin engagement. Thus, we suggest that activated c-Src binds, phosphorylates, and redistributes ADAM12-L to specific sites at the cell periphery, which may in turn promote signalling mechanisms regulating cellular processes with importance in cancer.

FGFR2-Cbl interaction in lipid rafts triggers attenuation of PI3K/Akt signaling and osteoblast survival

Fibroblast growth factor receptor (FGFR) signaling plays an important role in skeletogenesis. The molecular mechanisms triggered by activated FGFR in bone forming cells are however not fully understood. In this study, we identify a role for phosphatidylinositol 3-kinase (PI3K) signaling in cell apoptosis induced by FGFR2 activation in osteoblasts. We show that FGFR2 activation leads to decrease PI3K protein levels, resulting in attenuation of PI3K signaling in human osteoblasts. Biochemical and molecular analyses revealed that the attenuated PI3K signaling induced by FGFR2 activation is due to increased Cbl-PI3K molecular interaction mediated by the Cbl Y731 residue, which results in increased PI3K ubiquitination and proteasome degradation. Biochemical and immunocytochemical analyses showed that FGFR2 and Cbl interact in raft micro-domains at the plasma membrane. FGFR2 activation increases FGFR2 and Cbl recruitment in micro-domains, resulting in increased molecular interactions. Consistently, functional analyses showed that the attenuation of PI3K/Akt signaling triggered by FGFR2 activation results in increased osteoblast apoptosis. These results identify a functional molecular mechanism by which activated FGFR2 recruits Cbl in raft micro-domains to trigger PI3K ubiquitination and proteasome degradation, and reveal a novel role for PI3K/Akt attenuation in the control of osteoblast survival by FGFR2 signaling.

Molecular requirements for induction of CTGF expression by TGF-beta1 in primary osteoblasts

Connective tissue growth factor (CTGF/CCN2) is a cysteine rich, extracellular matrix protein that acts as an anabolic growth factor to regulate osteoblast differentiation and function. In osteoblasts, CTGF is induced by TGF-beta1 where it acts as a downstream mediator of TGF-beta1 induced matrix production. The molecular mechanisms that control CTGF induction by TGF-beta1 in osteoblasts are not known. To assess the role of individual Smads in mediating the induction of CTGF by TGF-beta1, we used specific Smad siRNAs to block Smad expression. These studies demonstrated that Smads 3 and 4, but not Smad 2, are required for TGF-beta1 induced CTGF promoter activity and expression in osteoblasts. Since the activation of MAPKs (Erk, Jnk and p38) by TGF-beta1 is cell type specific, we were interested in determining the role of individual MAPKs in TGF-beta1 induction of CTGF promoter activity and expression. Using dominant negative (DN) mutants for Erk, Jnk and p38, we demonstrated that the expression of DN-Erk caused a significant inhibition of TGF-beta1 induced CTGF promoter activity. In contrast, the expression of DN-p38 or DN-Jnk failed to inhibit activation of CTGF promoter activity. To confirm the vital role of Erk, we used the Erk inhibitor (PD98059) to block its activation, demonstrating that it prevented TGF-beta1 activation of the CTGF promoter and up-regulation of CTGF expression in osteoblasts. Since Src can also act as a downstream signaling effector for TGF-beta in some cell types, we determined its role in TGF-beta1 induction of CTGF in osteoblasts. Treatment of osteoblasts with a Src family kinase inhibitor, PP2, or the expression of two independent kinase-dead Src mutant constructs caused significant inhibition of TGF-beta1 induced CTGF promoter activity and expression. Additionally, blocking Src activation prevented Erk activation by TGF-beta1 demonstrating a role for Src as an upstream mediator of Erk in regulating CTGF expression in osteoblasts. To investigate the involvement of the TGF-beta1 response element (TRE) and the SMAD binding element (SBE) in CTGF induction, we cloned the rat CTGF proximal promoter (-787 to +1) containing the TRE and SBE motifs into a pGL3-Luciferase reporter construct. Using a combination of CTGF promoter deletion constructs and site-directed mutants, we demonstrated the unique requirement of both the TRE and SBE for CTGF induction by TGF-beta1 in osteoblasts. Electro-mobility shift assays using specific probes containing the TRE, SBE or both showed TGF-beta1 inducible complexes that can be ablated by mutation of the respective motif, confirming their requirement for TGF-beta1 induced CTGF promoter activity. In conclusion, these studies demonstrate that CTGF induction by TGF-beta1 in osteoblasts involves Smads 3 and 4, the Erk and Src signaling pathways, and requires both the TRE and SBE motifs in the CTGF proximal promoter.

Novel role of protein kinase C-delta Tyr 311 phosphorylation in vascular smooth muscle cell hypertrophy by angiotensin II

We have shown previously that activation of protein kinase C-delta (PKC delta) is required for angiotensin II (Ang II)-induced migration of vascular smooth muscle cells (VSMCs). Here, we have hypothesized that PKC delta phosphorylation at Tyr(311) plays a critical role in VSMC hypertrophy induced by Ang II. Immunoblotting was used to monitor PKC delta phosphorylation at Tyr(311), and cell size and protein measurements were used to detect hypertrophy in VSMCs. PKC delta was rapidly (0.5 to 10.0 minutes) phosphorylated at Tyr(311) by Ang II. This phosphorylation was markedly blocked by an Src family kinase inhibitor and dominant-negative Src but not by an epidermal growth factor receptor kinase inhibitor. Ang II-induced Akt phosphorylation and hypertrophic responses were significantly enhanced in VSMCs expressing PKC delta wild-type compared with VSMCs expressing control vector, whereas the enhancements were markedly diminished in VSMCs expressing a PKC delta Y311F mutant. Also, these responses were significantly inhibited in VSMCs expressing kinase-inactive PKC delta K376A compared with VSMCs expressing control vector. From these data, we conclude that not only PKC delta kinase activation but also the Src-dependent Tyr(311) phosphorylation contributes to Akt activation and subsequent VSMC hypertrophy induced by Ang II, thus signifying a novel molecular mechanism for enhancement of cardiovascular diseases induced by Ang II.

Defective microtubule-dependent podosome organization in osteoclasts leads to increased bone density in Pyk2(-/-) mice

The protein tyrosine kinase Pyk2 is highly expressed in osteoclasts, where it is primarily localized in podosomes. Deletion of Pyk2 in mice leads to mild osteopetrosis due to impairment in osteoclast function. Pyk2-null osteoclasts were unable to transform podosome clusters into a podosome belt at the cell periphery; instead of a sealing zone only small actin rings were formed, resulting in impaired bone resorption. Furthermore, in Pyk2-null osteoclasts, Rho activity was enhanced while microtubule acetylation and stability were significantly reduced. Rescue experiments by ectopic expression of wild-type or a variety of Pyk2 mutants in osteoclasts from Pyk2^−/− mice have shown that the FAT domain of Pyk2 is essential for podosome belt and sealing zone formation as well as for bone resorption. These experiments underscore an important role of Pyk2 in microtubule-dependent podosome organization, bone resorption, and other osteoclast functions.

The tyrosine kinase activity of c-Src regulates actin dynamics and organization of podosomes in osteoclasts

Podosomes are dynamic actin-rich structures composed of a dense F-actin core surrounded by a cloud of more diffuse F-actin. Src performs one or more unique functions in osteoclasts (OCLs), and podosome belts and bone resorption are impaired in the absence of Src. Using Src(-/-) OCLs, we investigated the specific functions of Src in the organization and dynamics of podosomes. We found that podosome number and the podosome-associated actin cloud were decreased in Src(-/-) OCLs. Videomicroscopy and fluorescence recovery after photobleaching analysis revealed that the life span of Src(-/-) podosomes was increased fourfold and that the rate of actin flux in the core was decreased by 40%. Thus, Src regulates the formation, structure, life span, and rate of actin polymerization in podosomes and in the actin cloud. Rescue of Src(-/-) OCLs with Src mutants showed that both the kinase activity and either the SH2 or the SH3 binding domain are required for Src to restore normal podosome organization and dynamics. Moreover, inhibition of Src family kinase activities in Src(-/-) OCLs by Src inhibitors or by expressing dominant-negative Src(K295M) induced the formation of abnormal podosomes. Thus, Src is an essential regulator of podosome structure, dynamics and organization.

Identification and functional characterization of an Src homology domain 3 domain-binding site on Cbl

Cbl is an adaptor protein and ubiquitin ligase that binds and is phosphorylated by the nonreceptor tyrosine kinase Src. We previously showed that the primary interaction between Src and Cbl is mediated by the Src homology domain 3 (SH3) of Src binding to proline-rich sequences of Cbl. The peptide Cbl RDLPPPPPPDRP(540-551), which corresponds to residues 540-551 of Cbl, inhibited the binding of a GST-Src SH3 fusion protein to Cbl, whereas RDLAPPAPPPDR(540-551) did not, suggesting that Src binds to this site on Cbl in a class I orientation. Mutating prolines 543-548 reduced Src binding to the Cbl 479-636 fragment significantly more than mutating the prolines in the PPVPPR(494-499) motif, which was previously reported to bind Src SH3. Mutating Cbl prolines 543-548 to alanines substantially reduced Src binding to Cbl, Src-induced phosphorylation of Cbl, and the inhibition of Src kinase activity by Cbl. Expressing the mutated Cbl in osteoclasts induced a moderate reduction in bone-resorbing activity and increased amounts of Src protein. In contrast, disabling the tyrosine kinase-binding domain of full-length Cbl by mutating glycine 306 to glutamic acid, and thereby preventing the previously described binding of the tyrosine kinase-binding domain to the Src phosphotyrosine 416, had no effect on Cbl phosphorylation, the inhibition of Src activity by full-length Cbl, or bone resorption. These data indicate that the Cbl RDLPPPP(540-546) sequence is a functionally important binding site for Src.

The role of c-Src kinase in the regulation of osteoclast function

The targeted disruption of c-Src impairs osteoclast bone resorbing activity, causing osteopetrosis. Although it has been reported that restoring only the c-Src adaptor function at least partly rescues the skeletal phenotypes, the importance of c-Src kinase activity remains controversial. We here highlight the contributions of the Src adaptor and kinase activities in cytoskeletal organization and osteoclast function using adenovirus vectors containing various mutants of Src or Pyk2. In addition, we describe the importance of c-Src in mitochondria, where it phosphorylates cytochrome c oxidase (Cox). Src-induced Cox activity is also required for bone resorbing activity of osteoclasts that require high levels of ATP. Thus, c-Src kinase activity not only on the plasma membrane but also within mitochondria is essential for the regulation of osteoclastic bone resorption.

The role(s) of Src kinase and Cbl proteins in the regulation of osteoclast differentiation and function

The osteoclast resorbs mineralized bone during bone development, homeostasis, and repair. The deletion of the gene encoding the nonreceptor tyrosine kinase c-Src produces an osteopetrotic skeletal phenotype that is the consequence of the inability of the mature osteoclast to efficiently resorb bone. Src-/- osteoclasts exhibit reduced motility and abnormal organization of the apical secretory domain (the ruffled border) and attachment-related cytoskeletal elements that are necessary for bone resorption. A key function of Src in osteoclasts is to promote the rapid assembly and disassembly of the podosomes, the specialized integrin-based attachment structures of osteoclasts and other highly motile cells. Once recruited to the activated integrins, especially alphavbeta3), by the adhesion tyrosine kinase Pyk2, Src binds and phosphorylates Cbl and Cbl-b, homologous multisite adapter proteins with ubiquitin ligase activity. The Cbl proteins in turn recruit and activate additional signaling effectors, including phosphatidylinositol 3-kinase and dynamin, which play key roles in the development of cell polarity and the regulation of cell attachment and motility. In addition, Src and the Cbl proteins contribute to signaling cascades that are activated by several important receptors, including receptor activator of nuclear factor kappaB and the macrophage colony-stimulating factor receptor, and also downregulate the signaling from many of these receptors.

Cholesterol and steroid synthesizing smooth endoplasmic reticulum of adrenocortical cells contains high levels of proteins associated with the translocation channel

Steroid-secreting cells are characterized by abundant smooth endoplasmic reticulum whose membranes contain many enzymes involved in sterol and steroid synthesis. Yet they have relatively little morphologically identifiable rough endoplasmic reticulum, presumably required for synthesis and maintenance of the smooth membranes. In this study, we demonstrate that adrenal smooth microsomal subfractions enriched in smooth endoplasmic reticulum membranes contain high levels of translocation apparatus and oligosaccharyltransferase complex proteins, previously thought confined to rough endoplasmic reticulum. We further demonstrate that these smooth microsomal subfractions are capable of effecting cotranslational translocation, signal peptide cleavage, and N-glycosylation of newly synthesized polypeptides. This shifts the paradigm for distinction between smooth and rough endoplasmic reticulum. Confocal microscopy revealed the proteins to be distributed throughout the abundant tubular endoplasmic reticulum in these cells, which is predominantly smooth surfaced. We hypothesize that the broadly distributed translocon and oligosaccharyltransferase proteins participate in local synthesis and/or quality control of membrane proteins involved in cholesterol and steroid metabolism in a sterol-dependent and hormonally regulated manner.

Dynamin forms a Src kinase-sensitive complex with Cbl and regulates podosomes and osteoclast activity

Podosomes are highly dynamic actin-containing adhesion structures found in osteoclasts, macrophages, and Rous sarcoma virus (RSV)-transformed fibroblasts. After integrin engagement, Pyk2 recruits Src and the adaptor protein Cbl, forming a molecular signaling complex that is critical for cell migration, and deletion of any molecule in this complex disrupts podosome ring formation and/or decreases osteoclast migration. Dynamin, a GTPase essential for endocytosis, is also involved in actin cytoskeleton remodeling and is localized to podosomes where it has a role in actin turnover. We found that dynamin colocalizes with Cbl in the actin-rich podosome belt of osteoclasts and that dynamin forms a complex with Cbl in osteoclasts and when overexpressed in 293VnR or SYF cells. The association of dynamin with Cbl in osteoclasts was decreased by Src tyrosine kinase activity and we found that destabilization of the dynamin-Cbl complex involves the recruitment of Src through the proline-rich domain of Cbl. Overexpression of dynamin increased osteoclast bone resorbing activity and migration, whereas overexpression of dynK44A decreased osteoclast resorption and migration. These studies suggest that dynamin, Cbl, and Src coordinately participate in signaling complexes that are important in the assembly and remodeling of the actin cytoskeleton, leading to changes in osteoclast adhesion, migration, and resorption.