Most migrating cells extrude their front by the force of actin polymerization. Polymerization requires an initial nucleation step, which is mediated by factors establishing either parallel filaments in the case of filopodia or branched filaments that form the branched lamellipodial network. Branches are considered essential for regular cell motility and are initiated by the Arp2/3 complex, which in turn is activated by nucleation-promoting factors of the WASP and WAVE families. Here we employed rapid amoeboid crawling leukocytes and found that deletion of the WAVE complex eliminated actin branching and thus lamellipodia formation. The cells were left with parallel filaments at the leading edge, which translated, depending on the differentiation status of the cell, into a unipolar pointed cell shape or cells with multiple filopodia. Remarkably, unipolar cells migrated with increased speed and enormous directional persistence, while they were unable to turn towards chemotactic gradients. Cells with multiple filopodia retained chemotactic activity but their migration was progressively impaired with increasing geometrical complexity of the extracellular environment. These findings establish that diversified leading edge protrusions serve as explorative structures while they slow down actual locomotion.

Control of integrin-mediated adhesion and migration by chemokines plays a critical role in B cell development, differentiation, and function; however, the underlying signaling mechanisms are poorly defined. Here we show that the chemokine SDF-1 induced activation of Bruton's tyrosine kinase (Btk) and that integrin-mediated adhesion and migration in response to SDF-1 or CXCL13, as well as in vivo homing to lymphoid organs, was impaired in Btk-deficient (pre-)B cells. Furthermore, SDF-1 induced tyrosine phosphorylation of Phospholipase Cgamma2 (PLCgamma2), which, unlike activation of the migration regulatory GTPases Rac or Rap1, was mediated by Btk. PLCgamma2-deficient B cells also exhibited impaired SDF-1-controlled migration. These results reveal that Btk and PLCgamma2 mediate chemokine-controlled migration, thereby providing insights into the control of B cell homeostasis, trafficking, and function, as well as into the pathogenesis of the immunodeficiency disease X-linked agammaglobulinemia (XLA).

PurposeThis review evaluates the application of bone morphogenetic proteins (BMPs) in delayed bone repair, aiming at a broad audience from clinicians to scientists. Next to an overview of the role of the different BMPs, their antagonists and their current applications, special attention is focused on new scientific developments improving the effects of BMP-based therapy for bone repair.MethodsPublication searches in PubMed and Embase revealed 850 relevant articles on the criteria ‘BMP’ AND ‘bone repair’ (as of May 2011). The abstracts were carefully reviewed and papers were selected according to the content.ResultsThe resulting publications showed that BMP-2 and BMP-7 are clearly the most extensively evaluated BMPs, in general with positive results on bone healing, comparable to the use of unspecific preparations such as autologous bone grafts or platelet-rich plasma.ConclusionsAlthough the efficacy of BMPs as stimulators of bone repair has been demonstrated in model systems and clinical studies, the use of BMPs to enhance fracture healing in the clinical setting is still controversial. Issues such as when, where and how much of which BMP is the most effective and profitable to use still have to be elucidated. But optimisation of the BMP products used in combination with cheaper production methods will inevitably stimulate the clinical use of BMPs for bone fracture healing in the near future.

The orchestration of systemic immune responses is critically dependent on coordinated lymphocyte migration and recirculation. This "homing" guides lymphocytes to the microenvironments that control their differentiation and survival, disperses the immunologic repertoire, and targets effector lymphocytes to sites of antigenic insult. Lymphocyte homing is a multistep process that requires chemotaxis and cell adhesion coupled with strategies to overcome physical barriers. At the molecular level, it is regulated by adhesion molecules and chemokines, and facilitated by intrinsic molecular programs that allow "ameboid" shape change, allowing highly effective lymphocyte traffic between different tissue compartments. In case of malignant transformation, however, the fact that lymphocytes are "licensed to move" forms a serious threat to the organism, because it permits rapid tumor dissemination irrespective of the conventional anatomic boundaries limiting early spread in most types of cancer. Thus, unlike the metastatic spread of other cancers, lymphoma dissemination generally is not a reflection of tumor progression but of conserved physiological behavior. The dissemination patterns often reflect basic rules of lymphocyte homing, explaining the strikingly tissue-specific dissemination of, for example, mucosal lymphomas, cutaneous lymphomas, and multiple myeloma. Understanding the molecular mechanisms underlying this behavior may provide novel targets for treatment of lymphoma patients. (Blood.

Fibrodysplasia ossificans progressiva (FOP) is a rare disabling disease characterized by heterotopic ossification for which there is currently no treatment available. FOP has been linked recently to a heterozygous R206H mutation in the bone morphogenetic protein (BMP) type I receptor activin receptor-like kinase 2 (ALK2). Expression of the mutant ALK2-R206H receptor (FOP-ALK2) results in increased phosphorylation of the downstream Smad1 effector proteins and elevated basal BMP-dependent transcriptional reporter activity, indicating that FOP-ALK2 is constitutively active. FOP-ALK2-induced transcriptional activity could be blocked by overexpressing either of the inhibitory Smads, Smad6 or -7, or by treatment with the pharmacological BMP type I receptor inhibitor dorsomorphin. However, in contrast to wild-type ALK2, FOP-ALK2 is not inhibited by the negative regulator FKBP12. Mesenchymal cells expressing the FOP-ALK2 receptor are more sensitive to undergoing BMP-induced osteoblast differentiation and mineralization. In vivo bone formation was assessed by loading human mesenchymal stem cells (hMSCs) expressing the ALK2-R206H receptor onto calcium phosphate scaffolds and implantation in nude mice. Compared with control cells FOP-ALK2-expressing cells induced increased bone formation. Taken together, the R206H mutation in ALK2 confers constitutive activity to the mutant receptor, sensitizes mesenchymal cells to BMP-induced osteoblast differentiation, and stimulates new bone formation. We have generated an animal model that can be used as a stepping stone for preclinical studies aimed at inhibiting the heterotopic ossification characteristic of FOP.

Sclerosteosis and Van Buchem disease are rare, high-bone-mass disorders that have been linked to deficiency in the SOST gene, encoding sclerostin. Sclerostin belongs to the DAN family of glycoproteins, of which multiple family members have been shown to antagonize bone morphogenetic protein (BMP) and/or Wnt activity. Sclerostin is specifically expressed by osteocytes and inhibits BMP-induced osteoblast differentiation and ectopic bone formation. Sclerostin binds only weakly to BMPs and does not inhibit direct BMP-induced responses. Instead, sclerostin antagonizes canonical Wnt signaling by binding to Wnt coreceptors, low-density lipoprotein receptor-related protein 5 and 6. Several lipoprotein receptor-related protein-5 mutants that cause the high-bone-mass trait are defective in sclerostin binding. Thus, high bone mass in sclerosteosis and Van Buchem disease may result from increased Wnt signaling due to the absence of or insensitivity to sclerostin.

The evolution of multiple myeloma (MM) depends on complex signals from the bone marrow (BM) microenvironment, supporting the proliferation and survival of malignant plasma cells. An interesting candidate signal is hepatocyte growth factor/ scatter factor (HGF), since its receptor Met is expressed on MM cells, while HGF is produced by BM stromal cells and by some MM cell lines, enabling para-or autocrine interaction. To explore this hypothesis, we studied the biological effects of HGF stimulation on MM cell lines and on primary MMs. We observed that Met is expressed by the majority of MM cell lines and by approximately half of the primary plasma cell neoplasms tested. Stimulation of MM cells with HGF led to the activation of the RAS/mitogen-activated protein kinase and phosphatidylinositol 3-kinase/protein kinase B (PI3K/PKB) pathways, signaling routes that have been implicated in the regulation of cell proliferation and survival. Indeed, functional studies demonstrated that HGF has strong proliferative and antiapoptotic effects on both MM cell lines and primary MM cells. Furthermore, by applying specific signal-transduction inhibitors, we demonstrated that MEK is required for HGF-induced proliferation, whereas activation of PI3K is required for both HGF-induced proliferation and for rescue of MM cells from apoptosis. Taken together, our data indicate that HGF is a potent myeloma growth and survival factor and suggest that the HGF/Met pathway is a potential therapeutic target in MM.

Sclerostin is expressed by osteocytes and has catabolic effects on bone. It has been shown to antagonize bone morphogenetic protein (BMP) and/or Wnt activity, although at present the underlying mechanisms are unclear. Consistent with previous findings, Sclerostin opposed direct Wnt3a-induced but not direct BMP7-induced responses when both ligand and antagonist were provided exogenously to cells. However, we found that when both proteins are expressed in the same cell, sclerostin can antagonize BMP signaling directly by inhibiting BMP7 secretion. Sclerostin interacts with both the BMP7 mature domain and pro-domain, leading to intracellular retention and proteasomal degradation of BMP7. Analysis of sclerostin knock-out mice revealed an inhibitory action of sclerostin on Wnt signaling in both osteoblasts and osteocytes in cortical and cancellous bones. BMP7 signaling was predominantly inhibited by sclerostin in osteocytes of the calcaneus and the cortical bone of the tibia. Our results suggest that sclerostin exerts its potent bone catabolic effects by antagonizing Wnt signaling in a paracrine and autocrine manner and antagonizing BMP signaling selectively in the osteocytes that synthesize simultaneously both sclerostin and BMP7 proteins.Sclerosteosis (OMIM accession number 269500) and van Buchem disease (OMIM accession number 239100) are closely related, rare high bone mass disorders that have been linked to a deficiency in expression of sclerostin, encoded by the SOST gene (1-8). Sclerostin is an osteocyte-derived negative regulator of bone formation belonging to the DAN family of secreted glycoproteins. Members of the DAN family were shown to have the capacity to inhibit BMP and/or Wnt activity (9 -13). Because sclerostin binds, albeit weakly, to mature bone morphogenetic proteins (BMPs), 4 it initially was presumed to be a BMP antagonist; however, currently sclerostin is believed to mediate its inhibitory effect on bone formation by directly blocking the Wnt signaling pathway (9, 14, 15).Canonical Wnt signaling has been described to play a crucial role in several bone mass disorders. Wnt proteins transduce their signals via seven-transmembrane-spanning receptors of the frizzled family and lipoprotein receptor-related protein-5/6 (LRP5/6), thereby controlling the stability of cytoplasmic ␤-catenin. In the absence of Wnt ligands, ␤-catenin forms a complex with APC (adenomatous polyposis coli), axin, GSK3 (glycogen synthase kinase 3), and CK1 (casein kinase I). This complex facilitates phosphorylation and subsequent proteasomal degradation of ␤-catenin. In the presence of Wnt ligands, this complex dissociates, and ␤-catenin accumulates and translocates into the nucleus, where it forms complexes with TCF/Lef1 transcription factors and initiates transcription of target genes (16). The importance of Wnt signaling in bone formation is illustrated by the low bone mass osteoporosis-pseudoglioma syndrome or high bone mass phenotype caused by missense loss or gain of function mutations in LRP5,. Sclerostin was found to ac...