Kinase activity profiling reveals active signal transduction pathways in pediatric acute lymphoblastic leukemia: a new approach for target discovery

Still about 20% of patients with acute lymphoblastic leukemia (ALL) struggle with relapse, despite intensive chemotherapy. We and others have shown that kinase activity profiling is able to give more insights in active signal transduction pathways and point out interesting signaling hubs as well as new potential druggable targets. With this technique the gap between newly designed drugs and ALL may be bridged. The aim of this study was to perform kinome profiling on 20 pediatric ALL samples (14 BCP-ALL and six T-ALL) to identify signaling proteins relevant to ALL. We defined 250 peptides commonly activated in both BCP-ALL and T-ALL representing major signal transduction pathways including MAPK, PI3K/Akt, and regulators of the cell cycle/p53 pathway. For 27 peptides, differentially phosphorylation between BCP-ALL and T-ALL was observed. Among these, ten peptides were more highly phosphorylated in BCP-ALL while 17 peptides showed increased phosphorylation in T-ALL. Furthermore we selected one lead of the list of commonly activated peptides (HGFR_Y1235) in order to test its efficacy as a potential target and provide proof of principle for this approach. In conclusion kinome profiling is an elegant approach to study active signaling and identify interesting potential druggable targets.

Influence of genetic variants in TPMT and COMT associated with cisplatin induced hearing loss in patients with cancer: two new cohorts and a meta-analysis reveal significant heterogeneity between cohorts

Treatment with cisplatin-containing chemotherapy regimens causes hearing loss in 40–60% of cancer patients. It has been suggested that genetic variants in the genes encoding thiopurine S-methyltransferase (TPMT) and catechol O-methyltransferase (COMT) can predict the development of cisplatin-induced ototoxicity and may explain interindividual variability in sensitivity to cisplatin-induced hearing loss. Two recently published studies however, sought to validate these findings and showed inconsistent results. The aim of this study was to evaluate the role of polymorphisms in the TPMT and COMT genes in cisplatin-induced ototoxicity. Therefore we investigated two independent cohorts of 110 Dutch and 38 Spanish patients with osteosarcoma and performed a meta-analysis including all previously published studies resulting in a total population of 664 patients with cancer. With this largest meta-analysis performed to date, we show that the influence of TPMT and COMT on the development of cisplatin-induced hearing loss may be less important than previously suggested.

DNA copy number alterations mark disease progression in paediatric chronic myeloid leukaemia

Early recognition of children with chronic phase chronic myeloid leukaemia (CML-CP) at risk for developing a lymphoid blast crisis (LyBC) is desirable, because therapy options in CML-LyBC are limited. We used Multiplex Ligation-dependent Probe Amplification to determine whether B-cell lymphoid leukaemia-specific copy number alterations (CNAs) (e.g. IKZF1, PAX5, CDKN2A deletions) could be detected in CML-CP and may be used to predict disease progression to LyBC. CNAs were detected in all patients with CML-LyBC, but in none of the 77 patients with CML-CP. Based on this study we conclude that CNAs remain a hallmark of disease progression.

TRPM7 triggers Ca2+ sparks and invadosome formation in neuroblastoma cells

Cell migration depends on the dynamic formation and turnover of cell adhesions and is tightly controlled by actomyosin contractility and local Ca2+ signals. The divalent cation channel TRPM7 (Transient Receptor Potential cation channel, subfamily Melastatin, member 7) has recently received much attention as a regulator of cell adhesion, migration and (localized) Ca2+ signaling. Overexpression and knockdown of TRPM7 affects actomyosin contractility and the formation of cell adhesions such as invadosomes and focal adhesions, but the role of TRPM7-mediated Ca2+ signals herein is currently not understood. Using Total Internal Reflection Fluorescence (TIRF) Ca2+ fluorometry and a novel automated analysis routine we have addressed the role of Ca2+ in the control of invadosome dynamics in N1E-115 mouse neuroblastoma cells. We find that TRPM7 promotes the formation of highly repetitive and localized Ca2+ microdomains or "Ca2+ sparking hotspots" at the ventral plasma membrane. Ca2+ sparking appears strictly dependent on extracellular Ca2+ and is abolished by TRPM7 channel inhibitors such as waixenicin-A. TRPM7 inhibition also induces invadosome dissolution. However, invadosome formation is (functionally and spatially) dissociated from TRPM7-mediated Ca2+ sparks. Rather, our data indicate that TRPM7 affects actomyosin contractility and invadosome formation independent of Ca2+ influx.

TRPM7 is required for breast tumor cell metastasis

TRPM7 encodes a Ca2+-permeable nonselective cation channel with kinase activity. TRPM7 has been implicated in control of cell adhesion and migration, but whether TRPM7 activity contributes to cancer progression has not been established. Here we report that high levels of TRPM7 expression independently predict poor outcome in breast cancer patients and that it is functionally required for metastasis formation in a mouse xenograft model of human breast cancer. Mechanistic investigation revealed that TRPM7 regulated myosin II-based cellular tension, thereby modifying focal adhesion number, cell-cell adhesion and polarized cell movement. Our findings therefore suggest that TRPM7 is part of a mechanosensory complex adopted by cancer cells to drive metastasis formation.

Mechanoregulation of cytoskeletal dynamics by TRP channels

The ability of cells to respond to mechanical stimulation is crucial to a variety of biological processes, including cell migration, axonal outgrowth, perception of pain, cardiovascular responses and kidney physiology. The translation of mechanical cues into cellular responses, a process known as mechanotransduction, typically takes place in specialized multiprotein structures such as cilia, cell-cell or cell-matrix adhesions. Within these structures, mechanical forces such as shear stress and membrane stretch activate mechanosensitive proteins, which set off a series of events that lead to altered cell behavior. Members of the transient receptor potential (TRP) family of cation channels are emerging as important players in mechanotransductory pathways. Localized within mechanosensory structures, they are activated by mechanical stimuli and trigger fast as well as sustained cytoskeletal responses. In this review, we will provide an overview of how TRP channels affect cytoskeletal dynamics in various mechano-regulated processes.

The origin and nature of tightly clustered BTG1 deletions in precursor B-cell acute lymphoblastic leukemia support a model of multiclonal evolution

Recurrent submicroscopic deletions in genes affecting key cellular pathways are a hallmark of pediatric acute lymphoblastic leukemia (ALL). To gain more insight into the mechanism underlying these deletions, we have studied the occurrence and nature of abnormalities in one of these genes, the B-cell translocation gene 1 (BTG1), in a large cohort of pediatric ALL cases. BTG1 was found to be exclusively affected by genomic deletions, which were detected in 65 out of 722 B-cell precursor ALL (BCP-ALL) patient samples (9%), but not in 109 T-ALL cases. Eight different deletion sizes were identified, which all clustered at the telomeric site in a hotspot region within the second (and last) exon of the BTG1 gene, resulting in the expression of truncated BTG1 read-through transcripts. The presence of V(D)J recombination signal sequences at both sites of virtually all deletions strongly suggests illegitimate RAG1/RAG2-mediated recombination as the responsible mechanism. Moreover, high levels of histone H3 lysine 4 trimethylation (H3K4me3), which is known to tether the RAG enzyme complex to DNA, were found within the BTG1 gene body in BCP-ALL cells, but not T-ALL cells. BTG1 deletions were rarely found in hyperdiploid BCP-ALLs, but were predominant in other cytogenetic subgroups, including the ETV6-RUNX1 and BCR-ABL1 positive BCP-ALL subgroups. Through sensitive PCR-based screening, we identified multiple additional BTG1 deletions at the subclonal level in BCP-ALL, with equal cytogenetic distribution which, in some cases, grew out into the major clone at relapse. Taken together, our results indicate that BTG1 deletions may act as “drivers” of leukemogenesis in specific BCP-ALL subgroups, in which they can arise independently in multiple subclones at sites that are prone to aberrant RAG1/RAG2-mediated recombination events. These findings provide further evidence for a complex and multiclonal evolution of ALL.

Recent studies have alluded to the existence of a complex clonal cellular architecture in acute lymphoblastic leukemia (ALL), where multiple subclones contribute to leukemogenesis. Here, we show that in pediatric B-cell precursor ALL (BCP-ALL) monoallelic deletions in the tumor suppressor BTG1 locus, which were found to occur in 9% of the patients studied, result in truncations of the gene rather than in complete allelic losses. Using both genetic and epigenetic approaches, we show that these deletions most likely originate from illegitimate RAG recombination. Sensitive backtracking using deletion-spanning PCRs revealed that these BTG1 deletions occur in specific BCP-ALL subtypes, with frequencies higher than previously anticipated, often in one minor subclone or in multiple independent subclones within individual patients. Subclones that carry a BTG1 deletion at diagnosis can evolve into the major clone at relapse. These findings link a mechanism of tumor suppressor gene deletion to the multiclonal evolution of ALL.

Geometry sensing by dendritic cells dictates spatial organization and PGE(2)-induced dissolution of podosomes

Assembly and disassembly of adhesion structures such as focal adhesions (FAs) and podosomes regulate cell adhesion and differentiation. On antigen-presenting dendritic cells (DCs), acquisition of a migratory and immunostimulatory phenotype depends on podosome dissolution by prostaglandin E₂ (PGE₂). Whereas the effects of physico-chemical and topographical cues have been extensively studied on FAs, little is known about how podosomes respond to these signals. Here, we show that, unlike for FAs, podosome formation is not controlled by substrate physico-chemical properties. We demonstrate that cell adhesion is the only prerequisite for podosome formation and that substrate availability dictates podosome density. Interestingly, we show that DCs sense 3-dimensional (3-D) geometry by aligning podosomes along the edges of 3-D micropatterned surfaces. Finally, whereas on a 2-dimensional (2-D) surface PGE₂ causes a rapid increase in activated RhoA levels leading to fast podosome dissolution, 3-D geometric cues prevent PGE₂-mediated RhoA activation resulting in impaired podosome dissolution even after prolonged stimulation. Our findings indicate that 2-D and 3-D geometric cues control the spatial organization of podosomes. More importantly, our studies demonstrate the importance of substrate dimensionality in regulating podosome dissolution and suggest that substrate dimensionality plays an important role in controlling DC activation, a key process in initiating immune responses.

Vascular smooth muscle cell differentiation to an osteogenic phenotype involves TRPM7 modulation by magnesium

Arterial calcification, common in vascular diseases, involves vascular smooth muscle cell (VSMC) transformation to an osteoblast phenotype. Clinical studies suggest that magnesium may prevent this, but mechanisms are unclear. We assessed whether increasing magnesium levels reduce VSMC calcification and differentiation and questioned the role of the Mg(2+) transporter, transient receptor potential melastatin (TRPM)7 cation channels in this process. Rat VSMCs were exposed to calcification medium in the absence and presence of magnesium (2.0 to 3.0 mmol/L) or 2-aminoethoxy-diphenylborate (2-APB) (TRPM7 inhibitor). VSMCs from mice with genetically low (MgL) or high-normal (MgH) [Mg(2+)](i) were also studied. Calcification was assessed by von Kossa staining. Expression of osteocalcin, osteopontin, bone morphogenetic protein (BMP)-2, BMP-4, BMP-7, and matrix Gla protein and activity of TRPM7 (cytosol:membrane translocation) were determined by immunoblotting. Calcification medium induced osteogenic differentiation, reduced matrix Gla protein content, and increased expression of the sodium-dependent cotransporter Pit-1. Magnesium prevented calcification and decreased osteocalcin expression and BMP-2 activity and increased expression of calcification inhibitors, osteopontin and matrix Gla protein. TRPM 7 activation was decreased by calcification medium, an effect reversed by magnesium. 2-APB recapitulated the VSMC osteoblastic phenotype in VSMCs. Osteocalcin was increased by calcification medium in VSMCs and intact vessels from MgL but not MgH, whereas osteopontin was increased in MgH, but not in MgL mice. Magnesium negatively regulates vascular calcification and osteogenic differentiation through increased/restored TRPM7 activity and increased expression of anticalcification proteins, including osteopontin, BMP-7, and matrix Gla protein. New molecular insights are provided whereby magnesium could protect against VSMC calcification.

The alpha-kinase family: an exceptional branch on the protein kinase tree

The alpha-kinase family represents a class of atypical protein kinases that display little sequence similarity to conventional protein kinases. Early studies on myosin heavy chain kinases in Dictyostelium discoideum revealed their unusual propensity to phosphorylate serine and threonine residues in the context of an alpha-helix. Although recent studies show that some members of this family can also phosphorylate residues in non-helical regions, the name alpha-kinase has remained. During evolution, the alpha-kinase domains combined with many different functional subdomains such as von Willebrand factor-like motifs (vWKa) and even cation channels (TRPM6 and TRPM7). As a result, these kinases are implicated in a large variety of cellular processes such as protein translation, Mg(2+) homeostasis, intracellular transport, cell migration, adhesion, and proliferation. Here, we review the current state of knowledge on different members of this kinase family and discuss the potential use of alpha-kinases as drug targets in diseases such as cancer.

TLR4-mediated podosome loss discriminates gram-negative from gram-positive bacteria in their capacity to induce dendritic cell migration and maturation

Chronic infections are caused by microorganisms that display effective immune evasion mechanisms. Dendritic cell (DC)-dependent T cell-mediated adaptive immunity is one of the mechanisms that have evolved to prevent the occurrence of chronic bacterial infections. In turn, bacterial pathogens have developed strategies to evade immune recognition. In this study, we show that gram-negative and gram-positive bacteria differ in their ability to activate DCs and that gram-negative bacteria are far more effective inducers of DC maturation. Moreover, we observed that only gram-negative bacteria can induce loss of adhesive podosome structures in DCs, a response necessary for the induction of effective DC migration. We demonstrate that the ability of gram-negative bacteria to trigger podosome turnover and induce DC migration reflects their capacity to selectively activate TLR4. Examining mice defective in TLR4 signaling, we show that this DC maturation and migration are mainly Toll/IL-1 receptor domain-containing adaptor-inducing IFNbeta-dependent. Furthermore, we show that these processes depend on the production of PGs by these DCs, suggesting a direct link between TLR4-mediated signaling and arachidonic metabolism. These findings demonstrate that gram-positive and gram-negative bacteria profoundly differ in their capacity to activate DCs. We propose that this inability of gram-positive bacteria to induce DC maturation and migration is part of the armamentarium necessary for avoiding the induction of an effective cellular immune response and may explain the frequent involvement of these pathogens in chronic infections.

Diffusion-driven device for a high-resolution dose-response profiling of combination chemotherapy

Combination therapies have proven vital in the fight against HIV and cancer. However, the identification and optimization of such combination therapies is largely experience driven and an activity of clinicians rather than of systematic screening efforts. Here we present a diffusion device, compatible with the format of a 12-well microtiter plate, to create and test all possible mixtures of two substances with only two pipetting steps. Applications to the testing of different drug combinations and the parallel screening of different leukemia cell lines as well as primary patient cells are presented. The diffusion device yields qualitatively and quantitatively comparable results to an MTT viability assay conducted in a standard 96-well format albeit with a tremendous reduction of processing steps. In addition, a fluorescence-based annexin V binding assay of cell death was implemented. Next to the reduction of processing steps, the diffusion device constitutes a considerable assay miniaturization that overcomes the problems typically associated with miniaturization as a consequence of small sample volumes. Given its ease of handling, the device will greatly advance the development and optimization of combination drugs and the identification of optimum drug combinations in personalized medicine.

Regulation of MYCN expression in human neuroblastoma cells

Background

Amplification of the MYCN gene in neuroblastoma (NB) is associated with a poor prognosis. However, MYCN-amplification does not automatically result in higher expression of MYCN in children with NB. We hypothesized that the discrepancy between MYCN gene expression and prognosis in these children might be explained by the expression of either MYCN-opposite strand (MYCNOS) or the shortened MYCN-isoform (ΔMYCN) that was recently identified in fetal tissues. Both MYCNOS and ΔMYCN are potential inhibitors of MYCN either at the mRNA or at the protein level.

Methods

Expression of MYCN, MYCNOS and ΔMYCN was measured in human NB tissues of different stages. Transcript levels were quantified using a real-time reverse transcriptase polymerase chain reaction assay (QPCR). In addition, relative expression of these three transcripts was compared to the number of MYCN copies, which was determined by genomic real-time PCR (gQPCR).

Results

Both ΔMYCN and MYCNOS are expressed in all NBs examined. In NBs with MYCN-amplification, these transcripts are significantly higher expressed. The ratio of MYCN:ΔMYCN expression was identical in all tested NBs. This indicates that ΔMYCN and MYCN are co-regulated, which suggests that ΔMYCN is not a regulator of MYCN in NB. However, the ratio of MYCNOS:MYCN expression is directly correlated with NB disease stage (p = 0.007). In the more advanced NB stages and NBs with MYCN-amplification, relatively more MYCNOS is present as compared to MYCN. Expression of the antisense gene MYCNOS might be relevant to the progression of NB, potentially by directly inhibiting MYCN transcription by transcriptional interference at the DNA level.

Conclusion

The MYCNOS:MYCN-ratio in NBs is significantly correlated with both MYCN-amplification and NB-stage. Our data indicate that in NB, MYCN expression levels might be influenced by MYCNOS but not by ΔMYCN.

Creatine kinase-mediated ATP supply fuels actin-based events in phagocytosis

Phagocytosis requires locally coordinated cytoskeletal rearrangements driven by actin polymerization and myosin motor activity. How this actomyosin dynamics is dependent upon systems that provide access to ATP at phagosome microdomains has not been determined. We analyzed the role of brain-type creatine kinase (CK-B), an enzyme involved in high-energy phosphoryl transfer. We demonstrate that endogenous CK-B in macrophages is mobilized from the cytosolic pool and coaccumulates with F-actin at nascent phagosomes. Live cell imaging with XFP-tagged CK-B and β-actin revealed the transient and specific nature of this partitioning process. Overexpression of a catalytic dead CK-B or CK-specific cyclocreatine inhibition caused a significant reduction of actin accumulation in the phagocytic cup area, and reduced complement receptor–mediated, but not Fc-γR–mediated, ingestion capacity of macrophages. Finally, we found that inhibition of CK-B affected phagocytosis already at the stage of particle adhesion, most likely via effects on actin polymerization behavior. We propose that CK-B activity in macrophages contributes to complement-induced F-actin assembly events in early phagocytosis by providing local ATP supply.

To do work, cells need energy in the form of ATP. High and sudden energy demand is seen during cell-shape change, a process in which ATP fuels the cytoskeletal machinery that drives cell-morphology alteration. How a cell organizes high-energy surges without disrupting global ATP homeostasis remains an important research question. One view proposes that ATP is heterogeneously distributed, but the cytoskeletal proteins actin and myosin receive regional and preferential access to ATP. Yet this model raises another question: how is ATP funneled to these proteins from distant sources? To address some of these questions, we studied the highly localized molecular events controlling actin dynamics around phagocytic activity of macrophages. We demonstrate that actin and creatine kinase-B (CK-B), a long-known enzyme involved in ATP supply, are simultaneously recruited into the sites of action during the early phases of particle ingestion. Local availability of CK-activity and local generation of ATP promotes on-site actin remodeling and particle capture efficiency, and thus supports successful initiation of the first phases of phagocytosis. Interestingly, this coupling between local CK-activity and actin regulation is only relevant for complement-mediated phagocytosis (used by immune cells to target specific particles for ingestion). We predict that our findings may also shed light on how shape dynamics is energized in other cell types.

A tight connection exists between local recruitment of creatine kinase-B (CK-B) and actin remodeling activity in phagocytic cups of macrophages. Complement-mediated phagocytosis is stimulated in the presence of enzymatic-active CK-B, indicating that local ATP supply stimulates actin-driven particle uptake events.

Interplay between TRP channels and the cytoskeleton in health and disease

Transient receptor potential (TRP) channels are a family of cation channels that play a key role in ion homeostasis and cell volume regulation. In addition, TRP channels are considered universal integrators of sensory information required for taste, vision, hearing, touch, temperature, and the detection of mechanical force. Seminal investigations exploring the molecular mechanisms of phototransduction in Drosophila have demonstrated that TRP channels operate within macromolecular complexes closely associated with the cytoskeleton. More recent evidence shows that mammalian TRP channels similarly connect to the cytoskeleton to affect cytoskeletal organization and cell adhesion via ion-transport-dependent and -independent mechanisms. In this review, we discuss new insights into the interplay between TRP channels and the cytoskeleton and provide recent examples of such interactions in different physiological systems.

PGE2-mediated podosome loss in dendritic cells is dependent on actomyosin contraction downstream of the RhoA-Rho-kinase axis

Podosomes are dynamic adhesion structures found in dendritic cells (DCs) and other cells of the myeloid lineage. We previously showed that prostaglandin E2 (PGE2), an important proinflammatory mediator produced during DC maturation, induces podosome disassembly within minutes after stimulation. Here, we demonstrate that this response is mediated by cAMP elevation, occurs downstream of Rho kinase and is dependent on myosin II. Whereas PGE2 stimulation leads to activation of the small GTPase RhoA, decreased levels of Rac1-GTP and Cdc42-GTP are observed. These results show that PGE2 stimulation leads to activation of the RhoA-Rho-kinase axis to promote actomyosin-based contraction and subsequent podosome dissolution. Because podosome disassembly is accompanied by de novo formation of focal adhesions, we propose that the disassembly/formation of these two different adhesion structures is oppositely regulated by actomyosin contractility and relative activities of RhoA, Rac1 and Cdc42.

Distinct kinetic and mechanical properties govern ALCAM-mediated interactions as shown by single-molecule force spectroscopy

The activated leukocyte cell adhesion molecule (ALCAM) mediates dynamic homotypic and heterotypic cellular interactions. Whereas homotypic ALCAM-ALCAM interactions have been implicated in the development and maintenance of tissue architecture and tumor progression, heterotypic ALCAM-CD6 interactions act to initiate and stabilize T-cell–dendritic-cell interactions affecting T-cell activation. The ability to resist the forces acting on the individual bonds during these highly dynamic cellular contacts is thought to be crucial for the (patho)physiology of ALCAM-mediated cell adhesion. Here, we used atomic force microscopy to characterize the relationship between affinity, avidity and the stability of ALCAM-mediated interactions under external loading, at the single-molecule level. Disruption of the actin cytoskeleton resulted in enhanced ALCAM binding avidity, without affecting the tensile strength of the individual bonds. Force spectroscopy revealed that the ALCAM-CD6 bond displayed a significantly higher tensile strength, a smaller reactive compliance and an up to 100-fold lower dissociation rate in the physiological force window in comparison to the homotypic interaction. These results indicate that homotypic and heterotypic ALCAM-mediated adhesion are governed by significantly distinct kinetic and mechanical properties, providing novel insight into the role of ALCAM during highly dynamic cellular interactions.

Myosin II and mechanotransduction: a balancing act

Adherent cells respond to mechanical properties of the surrounding extracellular matrix. Mechanical forces, sensed at specialized cell-matrix adhesion sites, promote actomyosin-based contraction within the cell. By manipulating matrix rigidity and adhesion strength, new roles for actomyosin contractility in the regulation of basic cellular functions, including cell proliferation, migration and stem cell differentiation, have recently been discovered. These investigations demonstrate that a balance of forces between cell adhesion on the outside and myosin II-based contractility on the inside of the cell controls many aspects of cell behavior. Disturbing this balance contributes to the pathogenesis of various human diseases. Therefore, elaborate signaling networks have evolved that modulate myosin II activity to maintain tensional homeostasis. These include signaling pathways that regulate myosin light chain phosphorylation as well as myosin II heavy chain interactions.

Activation of TRPM7 Channels by Phospholipase C-coupled Receptor Agonists

TRPM7 is a ubiquitously expressed nonspecific cation channel that has been implicated in cellular Mg(2+) homeostasis. We have recently shown that moderate overexpression of TRPM7 in neuroblastoma N1E-115 cells elevates cytosolic Ca(2+) levels and enhances cell-matrix adhesion. Furthermore, activation of TRPM7 by phospholipase C (PLC)-coupled receptor agonists caused a further increase in intracellular Ca(2+) levels and augmented cell adhesion and spreading in a Ca(2+)-dependent manner (1). Regulation of the TRPM7 channel is not well understood, although it has been reported that PIP(2) hydrolysis closes the channel. Here we have examined the regulation of TRPM7 by PLC-coupled receptor agonists such as bradykinin, lysophosphatidic acid, and thrombin. Using FRET assays for second messengers, we have shown that the TRPM7-dependent Ca(2+) increase closely correlates with activation of PLC. Under non-invasive "perforated patch clamp" conditions, we have found similar activation of TRPM7 by PLC-coupled receptor agonists. Although we could confirm that, under whole-cell conditions, the TRPM7 currents were significantly inhibited following PLC activation, this PLC-dependent inhibition was only observed when [Mg(2+)](i) was reduced below physiological levels. Thus, under physiological ionic conditions, TRPM7 currents were activated rather than inhibited by PLC-activating receptor agonists.

A critical role for prostaglandin E2 in podosome dissolution and induction of high-speed migration during dendritic cell maturation

Dendritic cells (DCs) are professional APCs of the immune system that play a key role in regulating T cell-based immunity. The capacity of DCs to activate T cells depends on their maturation state as well as their ability to migrate to the T cell areas of draining lymph nodes. In this study, we investigated the effects of DC maturation stimuli on the actin cytoskeleton and beta(1) integrin-dependent adhesion and migration. Podosomes, specialized adhesion structures found in immature monocyte-derived DCs as well as myeloid DCs, rapidly dissolve in response to maturation stimuli such as TNF-alpha and PGE(2), whereas the TLR agonist LPS induces podosome dissolution only after a long lag time. We demonstrate that LPS-mediated podosome disassembly as well as the onset of high-speed DC migration are dependent on the production of PGs by the DCs. Moreover, both of these processes are inhibited by Ab-induced activation of beta(1) integrins. Together, these results show that maturation-induced podosome dissolution and loss of alpha(5)beta(1) integrin activity allow human DCs to undergo the transition from an adhesive to a highly migratory phenotype.

Organization of the integrin LFA-1 in nanoclusters regulates its activity

The beta2-integrin LFA-1 facilitates extravasation of monocytes (MOs) into the underlying tissues, where MOs can differentiate into dendritic cells (DCs). Although DCs express LFA-1, unlike MOs, they cannot bind to ICAM-1. We hypothesized that an altered integrin organization on the DC plasma membrane might cause this effect and investigated the relationship between membrane organization and function of LFA-1 on MOs and DCs. High-resolution mapping of LFA-1 surface distribution revealed that on MOs LFA-1 function is associated with a distribution in well-defined nanoclusters (100-150-nm diameter). Interestingly, a fraction of these nanoclusters contains primed LFA-1 molecules expressing the specific activation-dependent L16-epitope. Live imaging of MO-T-cell conjugates showed that only these primed nanoclusters are dynamically recruited to the cellular interface forming micrometer-sized assemblies engaged in ligand binding and linked to talin. We conclude that besides affinity regulation, LFA-1 function is controlled by at least three different avidity patterns: random distributed inactive molecules, well-defined ligand-independent proactive nanoclusters, and ligand-triggered micrometer-sized macroclusters.

Long-term engagement of CD6 and ALCAM is essential for T-cell proliferation induced by dendritic cells

Interactions between T cells and antigen-presenting cells (APCs) are the first step in the induction of an adaptive immune response. Here, we show that CD6 and its ligand activated leukocyte cell adhesion molecule (ALCAM) are actively recruited to the antigen-induced dendritic cell (DC)–T-cell contact zone. Moreover, ALCAM-blocking antibodies interfere with DC–T-cell conjugate formation, demonstrating that CD6-ALCAM binding is essential for stable T-cell–APC contact. We now demonstrate that besides their role in establishing initial contacts, CD6-ALCAM interactions are also required during the proliferative phase of the T-cell response; the presence of CD6-blocking antibodies or recombinant ALCAM-Fc proteins results in a strong and sustained inhibition of T-cell proliferation. Furthermore, simultaneous crosslinking of CD6 and CD3 induces enhanced proliferation and transcriptional activity to a similar level as observed after CD3 and CD28 co-crosslinking, demonstrating that CD6 is an important costimulatory molecule. The stability of ALCAM-CD6 binding, which contrasts with transient homotypic ALCAM-ALCAM interactions, further supports the long-lasting effects observed on T-cell proliferation. Taken together, we demonstrate that CD6 and ALCAM form a key adhesive receptor-ligand pair that is not only involved in early DC-T-cell binding but also in sustaining DC-induced T-cell proliferation long after the initial contact has been established.