DOCK2 is Required for Chemokine-Promoted Human T Lymphocyte Adhesion Under Shear Stress Mediated by the Integrin α4β1

Endothelial cell-specific deletion of a microRNA accelerates atherosclerosis

BACKGROUND AND AIMS

Chronic vascular endothelial inflammation predisposes to atherosclerosis; however, the cell-autonomous roles for endothelial-expressing microRNAs (miRNAs) are poorly understood in this process. MiR-181b is expressed in several cellular constituents relevant to lesion formation. The aim of this study is to examine the role of genetic deficiency of the miR-181b locus in endothelial cells during atherogenesis.

METHODS AND RESULTS

Using a proprotein convertase subtilisin/kexin type 9 (PCSK9)-induced atherosclerosis mouse model, we demonstrated that endothelial cell (EC)-specific deletion of miR-181a2b2 significantly promoted atherosclerotic lesion formation, cell adhesion molecule expression, and the influx of lesional macrophages in the vessel wall. Yet, endothelium deletion of miR-181a2b2 did not affect body weight, lipid metabolism, anti-inflammatory Ly6C^low or the pro-inflammatory Ly6C^interm and Ly6C^high fractions in circulating peripheral blood mononuclear cells (PBMCs), and pro-inflammatory or anti-inflammatory mediators in both bone marrow (BM) and PBMCs. Mechanistically, bulk RNA-seq and gene set enrichment analysis of ECs enriched from the aortic arch intima, as well as single cell RNA-seq from atherosclerotic lesions, revealed that endothelial miR-181a2b2 serves as a critical regulatory hub in controlling endothelial inflammation, cell adhesion, cell cycle, and immune response during atherosclerosis.

CONCLUSIONS

Our study establishes that deficiency of a miRNA specifically in the vascular endothelium is sufficient to profoundly impact atherogenesis. Endothelial miR-181a2b2 deficiency regulates multiple key pathways related to endothelial inflammation, cell adhesion, cell cycle, and immune response involved in the development of atherosclerosis.

ICAP-1 loss impairs CD8+ thymocyte development and leads to reduced marginal zone B cells in mice

ICAP‐1 regulates β1‐integrin activation and cell adhesion. Here, we used ICAP‐1‐null mice to study ICAP‐1 potential involvement during immune cell development and function. Integrin α4β1‐dependent adhesion was comparable between ICAP‐1‐null and control thymocytes, but lack of ICAP‐1 caused a defective single‐positive (SP) CD8⁺ cell generation, thus, unveiling an ICAP‐1 involvement in SP thymocyte development. ICAP‐1 bears a nuclear localization signal and we found it displayed a strong nuclear distribution in thymocytes. Interestingly, there was a direct correlation between the lack of ICAP‐1 and reduced levels in SP CD8⁺ thymocytes of Runx3, a transcription factor required for CD8⁺ thymocyte generation. In the spleen, ICAP‐1 was found evenly distributed between cytoplasm and nuclear fractions, and ICAP‐1^–/– spleen T and B cells displayed upregulation of α4β1‐mediated adhesion, indicating that ICAP‐1 negatively controls their attachment. Furthermore, CD3⁺‐ and CD19⁺‐selected spleen cells from ICAP‐1‐null mice showed reduced proliferation in response to T‐ and B‐cell stimuli, respectively. Finally, loss of ICAP‐1 caused a remarkable decrease in marginal zone B‐ cell frequencies and a moderate increase in follicular B cells. Together, these data unravel an ICAP‐1 involvement in the generation of SP CD8⁺ thymocytes and in the control of marginal zone B‐cell numbers.

Dedicator of Cytokinesis 2 (DOCK2) Deficiency Attenuates Lung Injury Associated with Chronic High-Fat and High-Fructose Diet-Induced Obesity

Obesity is a major risk factor for lung disease development. However, little is known about the impact of chronic high-fat and high-fructose (HFHF) diet-induced obesity on lung inflammation and subsequent pulmonary fibrosis. Herein we hypothesized that dedicator of cytokinesis 2 (DOCK2) promotes a proinflammatory phenotype of lung fibroblasts (LFs) to elicit lung injury and fibrosis in chronic HFHF diet-induced obesity. An HFHF diet for 20 weeks induced lung inflammation and profibrotic changes in wild-type C57BL/6 mice. CD68 and monocyte chemoattractant protein-1 (MCP-1) expression were notably increased in the lungs of wild-type mice fed an HFHF diet. An HFHF diet further increased lung DOCK2 expression that co-localized with fibroblast-specific protein 1, suggesting a role of DOCK2 in regulating proinflammatory phenotype of LFs. Importantly, DOCK2 knockout protected mice from lung inflammation and fibrosis induced by a HFHF diet. In primary human LFs, tumor necrosis factor-α (TNF-α) and IL-1β induced DOCK2 expression concurrent with MCP-1, IL-6, and matrix metallopeptidase 2. DOCK2 knockdown suppressed TNF-α-induced expression of these molecules and activation of phosphatidylinositol 3-kinase/AKT and NF-κB signaling pathways, suggesting a mechanism of DOCK2-mediated proinflammatory and profibrotic changes in human LFs. Taken together, these findings reveal a previously unrecognized role of DOCK2 in regulating proinflammatory phenotype of LFs, potentiation of lung inflammation, and pulmonary fibrosis in chronic HFHF diet-caused obesity.

Small Rho GTPases and their associated RhoGEFs mutations promote immunological defects in primary immunodeficiencies

Primary immunodeficiencies (PIDs) are associated with deleterious mutations of genes that encode proteins involved in actin cytoskeleton reorganisation. This deficiency affects haematopoietic cells. PID results in the defective function of immune cells, such as impaired chemokine-induced motility, receptor signalling, development and maturation. Some of the genes mutated in PIDs are related to small Ras homologous (Rho) guanosine triphosphatase (GTPase), one of the families of the Ras superfamily. Most of these genes act as molecular switches by cycling between active guanosine triphosphate-bound and inactive guanosine diphosphate-bound forms to control multiple cellular functions. They are best studied for their role in promoting cytoskeleton reorganisation, cell adhesion and motility. Currently, only three small Rho GTPases, namely, Rac2, Cdc42 and RhoH, have been identified in PIDs. However, several other Rho small G proteins might also contribute to the deregulation and phenotype observed in PIDs. Their contribution in PIDs may involve their main regulator, Rho guanine nucleotide exchange factors such as DOCK2 and DOCK8, wherein mutations may result in the impairment of small Rho GTPase activation. Thus, this review outlines the potential contribution of several small Rho GTPases to the promotion of PIDs.

Spontaneous Loss-of-Function Dock2 Mutation Alters Murine Colitis Sensitivity and Is a Confounding Variable in Inflammatory Bowel Disease Research

Background

Dextran sodium sulfate (DSS)-induced colitis is the most commonly used mouse model of inflammatory bowel disease (IBD) due to its acute nature, reproducibility, and phenotypic overlap with human disease. Following an unexpected and sharp decline in DSS-induced colitis susceptibility in our commercially acquired C57Bl/6 wild-type mice, we discovered that a spontaneous loss-of-function mutation in dedicator of cytokinesis 2 (Dock2Hsd) was responsible. Presence of this mutation in research colonies has the capacity to broadly impact preclinical IBD studies.

Methods

DSS-colitis was induced in weight-, age-, and gender-matched C57Bl/6NHsd mice. Daily treatment with vehicle or the glutamate carboxypeptidase II (GCPII) inhibitor, 2-PMPA (100 mg/kg IP), was performed and disease activity index was monitored. At termination, colon GCPII activity was measured.

Results

DSS-treated Dock2Hsd mice developed more severe colitis, had significantly increased colon GCPII activity and were more sensitive to pharmacologic inhibition of GCPII.

Conclusions

The Dock2Hsd mutation is a confounding variable of high relevance to the IBD research community. Dock2Hsd mice were distributed as wild-type C57Bl/6 for multiple years and thus it is unknown how prevalent this mutation is in investigator-maintained colonies of C57Bl/6-derived mice. In our research, presence of the Dock2Hsd mutation caused enhanced GCPII colon activity more closely resembling human disease, providing a useful platform for screening GCPII inhibitors for preclinical efficacy. However, unanticipated presence of Dock2Hsd in genetically modified mice used to study IBD pathobiology can confound conclusions. Thus, care must be taken when interpreting studies performed in mice of C57Bl/6 lineage where Dock2 status is unknown.

Overexpression of dedicator of cytokinesis 2 correlates with good prognosis in colorectal cancer associated with more prominent CD8+ lymphocytes infiltration: a colorectal cancer analysis

Recently, dedicator of cytokinesis 2 (DOCK2) has been reportedly exhibited high mutation prevalence in the Asian colorectal cancer (CRC) cohort. However, the expression pattern of DOCK2 and its clinical significance in CRC were still unknown. To characterize the role of DOCK2, a tissue microarray (TMA) containing 481 archived paraffin-embedded CRC specimens was performed by immunohistochemistry. Among which, 54 primary CRC tissues showed high expression of DOCK2 protein, while others were negative. Moreover, DOCK2 expression was positively associated with invasion depth (P < .001) and tumor size (P = .016). Significantly, Kaplan-Meier survival analysis revealed that patients with higher DOCK2 expression had a longer overall survival time (P = .017). Furthermore, univariate and multivariate Cox regression analysis confirmed that DOCK2 is an independent prognostic marker in CRC (P = .049,; HR, 0.519; 95% CI, 0.270 to 0.997). In addition, we observed a strong correlation between the infiltration of CD8⁺ T lymphocytes and DOCK2 expression (P = .0119). Our findings demonstrated that overexpressed DOCK2 might involve in recruiting CD8⁺ T lymphocytes and serve as a novel prognostic indicator and indicated a potential therapeutic strategy by restoring DOCK2 for CRC.

SOS1, ARHGEF1, and DOCK2 rho-GEFs Mediate JAK-Dependent LFA-1 Activation by Chemokines

JAK-dependent activation of the rho module of integrin affinity triggering mediates chemokine-induced leukocyte adhesion. However, the signaling events linking JAKs to rho small GTPase activation by chemokines is still incompletely described. In this study, we show that son of sevenless 1 (SOS1), rho guanine nucleotide exchange factor (GEF)1 (ARHGEF1), and dedicator of cytokinesis (DOCK)2 GEFs mediate CXCL12-induced LFA-1 activation in human primary T lymphocytes. Downregulated expression of SOS1, ARHGEF1, and DOCK2 impairs LFA-1-mediated rapid T lymphocyte adhesion as well as underflow arrest on ICAM-1 induced by CXCL12. Moreover, LFA-1 affinity triggering by CXCL12 is impaired by SOS1, ARHGEF1, and DOCK2 downregulation. Notably, the three GEFs are all critically involved in chemokine-induced RhoA and Rac1 activation, thus suggesting the occurrence of a SOS1 specificity shift in the context of chemokine signaling. Accordingly, SOS1, ARHGEF1, and DOCK2 are tyrosine phosphorylated upon chemokine signaling with timing coherent with rapid LFA-1 affinity activation. Importantly, chemokine-induced tyrosine phosphorylation of these GEFs is fully mediated by JAK protein tyrosine kinases. Unexpectedly, and differently from VAV1, tyrosine phosphorylation of SOS1, ARHGEF1, and DOCK2 is completely inhibited by pertussis toxin pretreatment, thus suggesting different routes of rho-GEF triggering upon CXCR4 engagement. Taken together, these findings reveal a deeper level of complexity in the rho-signaling module, with at least four different rho-GEFs cooperating in the regulation of chemokine-induced integrin activation, possibly suggesting the emergence of stochastic concurrency in signaling mechanisms controlling leukocyte trafficking.

DOCK2 interacts with FLT3 and modulates the survival of FLT3-expressing leukemia cells

The FMS-like tyrosine kinase-3 (FLT3) gene is the most commonly mutated gene in acute myeloid leukemia (AML), and patients carrying internal tandem duplication (ITD) mutations have a poor prognosis. Long-term inhibition of FLT3 activity in these patients has been elusive. To provide a more complete understanding of FLT3 biology, a mass spectroscopy-based screen was performed to search for FLT3-interacting proteins. The screen identified dedicator of cytokinesis 2 (DOCK2), which is a guanine nucleotide exchange factor for Rho GTPases, and its expression is limited to hematolymphoid cells. We show that DOCK2 is expressed in leukemia cell lines and primary AML samples, and DOCK2 co-immunoprecipitates with wild-type FLT3 and FLT3/ITD. Knock-down (KD) of DOCK2 by shRNA selectively reduced cell proliferation and colony formation in leukemia cell lines with increased FLT3 activity, and greatly sensitized these cells to cytarabine treatment, alone and in combination with FLT3 tyrosine kinase inhibitors. DOCK2 KD in a FLT3/ITD-positive leukemia cell line also significantly prolonged survival in a mouse xenograft model. These findings suggest that DOCK2 is a potential therapeutic target for novel AML treatments, as this protein regulates the survival of leukemia cells with elevated FLT3 activity and sensitizes FLT3/ITD leukemic cells to conventional anti-leukemic agents.

In vivo adhesion of malignant B cells to bone marrow microvasculature is regulated by α4β1 cytoplasmic-binding proteins

Multiple myeloma (MM) and chronic lymphocytic leukemia (CLL) cells must attach to the bone marrow (BM) microvasculature before lodging in the BM microenvironment. Using intravital microscopy (IVM) of the BM calvariae we demonstrate that the α4β1 integrin is required for MM and CLL cell firm arrest onto the BM microvasculature, while endothelial P-selectin and E-selectin mediate cell rolling. Talin, kindlin-3 and ICAP-1 are β1-integrin-binding partners that regulate β1-mediated cell adhesion. We show that talin and kindlin-3 cooperatively stimulate high affinity and strength of α4β1-dependent MM and CLL cell attachment, whereas ICAP-1 negatively regulates this adhesion. A functional connection between talin/kindlin-3 and Rac1 was found to be required for MM cell attachment mediated by α4β1. Importantly, IVM analyses with talin- and kindlin-3-silenced MM cells indicate that these proteins are needed for cell arrest on the BM microvasculature. Instead, MM cell arrest is repressed by ICAP-1. Moreover, MM cells silenced for talin and kindlin-3, and cultured on α4β1 ligands showed higher susceptibility to bortezomib-mediated cell apoptosis. Our results highlight the requirement of α4β1 and selectins for the in vivo attachment of MM and CLL cells to the BM microvasculature, and indicate that talin, kindlin-3 and ICAP-1 differentially control physiological adhesion by regulating α4β1 activity.

Positive and negative regulation by SLP-76/ADAP and Pyk2 of chemokine-stimulated T-lymphocyte adhesion mediated by integrin α4β1

Stimulation by chemokines of integrin α4β1-dependent T-lymphocyte adhesion is a crucial step for lymphocyte trafficking. The adaptor Vav1 is required for chemokine-activated T-cell adhesion mediated by α4β1. Conceivably, proteins associating with Vav1 could potentially modulate this adhesion. Correlating with activation by the chemokine CXCL12 of T-lymphocyte attachment to α4β1 ligands, a transient stimulation in the association of Vav1 with SLP-76, Pyk2, and ADAP was observed. Using T-cells depleted for SLP-76, ADAP, or Pyk2, or expressing Pyk2 kinase-inactive forms, we show that SLP-76 and ADAP stimulate chemokine-activated, α4β1-mediated adhesion, whereas Pyk2 opposes T-cell attachment. While CXCL12-promoted generation of high-affinity α4β1 is independent of SLP-76, ADAP, and Pyk2, the strength of α4β1-VCAM-1 interaction and cell spreading on VCAM-1 are targets of regulation by these three proteins. GTPase assays, expression of activated or dominant-negative Rac1, or combined ADAP and Pyk2 silencing indicated that Rac1 activation by CXCL12 is a common mediator response in SLP-76-, ADAP-, and Pyk2-regulated cell adhesion involving α4β1. Our data strongly suggest that chemokine-stimulated associations between Vav1, SLP-76, and ADAP facilitate Rac1 activation and α4β1-mediated adhesion, whereas Pyk2 opposes this adhesion by limiting Rac1 activation.

Leukocyte migration into inflamed tissues

Leukocyte migration through activated venular walls is a fundamental immune response that is prerequisite to the entry of effector cells such as neutrophils, monocytes, and effector T cells to sites of infection, injury, and stress within the interstitium. Stimulation of leukocytes is instrumental in this process with enhanced temporally controlled leukocyte adhesiveness and shape-changes promoting leukocyte attachment to the inner wall of blood vessels under hydrodynamic forces. This initiates polarized motility of leukocytes within and through venular walls and transient barrier disruption facilitated sequentially by stimulated vascular cells, i.e., endothelial cells and their associated pericytes. Perivascular cells such as macrophages and mast cells that act as tissue inflammatory sentinels can also directly and indirectly regulate the exit of leukocytes from the vascular lumen. In this review, we discuss current knowledge and open questions regarding the mechanisms involved in the interactions of different effector leukocytes with peripheral vessels in extralymphoid organs.

Dock-family exchange factors in cell migration and disease

Dock family proteins are evolutionary conserved exchange factors for the Rho GTPases Rac and Cdc42. There are 11 Dock proteins in mammals, named Dock1 (or Dock180) to Dock11 that play different cellular functions. In particular, Dock proteins regulate actin cytoskeleton, cell adhesion and migration. Not surprisingly, members of the Dock family have been involved in various pathologies, including cancer and defects in the central nervous and immune systems. This review proposes an update of the recent findings regarding the function of Dock proteins, focusing on their role in the control of cell migration and invasion and the consequences in human diseases.

Sphingosine-1-phosphate activates chemokine-promoted myeloma cell adhesion and migration involving α4β1 integrin function

Myeloma cell adhesion dependent on α4β1 integrin is crucial for the progression of multiple myeloma (MM). The α4β1-dependent myeloma cell adhesion is up-regulated by the chemokine CXCL12, and pharmacological blockade of the CXCL12 receptor CXCR4 leads to defective myeloma cell homing to bone marrow (BM). Sphingosine-1-phosphate (S1P) regulates immune cell trafficking upon binding to G-protein-coupled receptors. Here we show that myeloma cells express S1P1, a receptor for S1P. We found that S1P up-regulated the α4β1-mediated myeloma cell adhesion and transendothelial migration stimulated by CXCL12. S1P promoted generation of high-affinity α4β1 that efficiently bound the α4β1 ligand VCAM-1, a finding that was associated with S1P-triggered increase in talin-β1 integrin association. Furthermore, S1P cooperated with CXCL12 for enhancement of α4β1-dependent adhesion strengthening and spreading. CXCL12 and S1P activated the DOCK2-Rac1 pathway, which was required for stimulation of myeloma cell adhesion involving α4β1. Moreover, in vivo analyses indicated that S1P contributes to optimizing the interactions of MM cells with the BM microvasculture and for their lodging inside the bone marrow. The regulation of α4β1-dependent adhesion and migration of myeloma cells by CXCL12-S1P combined activities might have important consequences for myeloma disease progression.

Endothelial endoglin is involved in inflammation: role in leukocyte adhesion and transmigration

Human endoglin is an RGD-containing transmembrane glycoprotein identified in vascular endothelial cells. Although endoglin is essential for angiogenesis and its expression is up-regulated in inflammation and at sites of leukocyte extravasation, its role in leukocyte trafficking is unknown. This function was tested in endoglin heterozygous mice (Eng(+/-)) and their wild-type siblings Eng(+/+) treated with carrageenan or LPS as inflammatory agents. Both stimuli showed that inflammation-induced leukocyte transendothelial migration to peritoneum or lungs was significantly lower in Eng(+/-) than in Eng(+/+) mice. Leukocyte transmigration through cell monolayers of endoglin transfectants was clearly enhanced in the presence of endoglin. Coating transwells with the RGD-containing extracellular domain of endoglin, enhanced leukocyte transmigration, and this increased motility was inhibited by soluble endoglin. Leukocytes stimulated with CXCL12, a chemokine involved in inflammation, strongly adhered to endoglin-coated plates and to endoglin-expressing endothelial cells. This endoglin-dependent adhesion was abolished by soluble endoglin, RGD peptides, the anti-integrin α5β1 inhibitory antibody LIA1/2 and the chemokine receptor inhibitor AMD3100. These results demonstrate for the first time that endothelial endoglin interacts with leukocyte integrin α5β1 via its RGD motif, and this adhesion process is stimulated by the inflammatory chemokine CXCL12, suggesting a regulatory role for endoglin in transendothelial leukocyte trafficking.

Structure analysis between the SWAP-70 RHO-GEF and the newly described PLD2-GEF

Small GTPases like Rac2 are crucial regulators of many cell functions central to life itself. Our laboratory has recently found that phospholipase D2 (PLD2) can act as a guanine nucleotide exchange factor (GEF) for Rac2. PLD2 has a Pleckstrin Homology (PH) domain but does not bear a Dbl homology (DH) or DOCK homology region (DHR) domain. It has, however, a Phox (PX) domain upstream of its PH domain. To better understand the novel finding of PLD2 as an enhancer of GDP/GTP exchange, we modeled the N-terminal portion of PLD2 (as the crystal structure of this protein has not as of yet been resolved), and studied the correlation with two known GEFs, SWAP-70 and the Leukemic Associated RhoGEF (LARG). Structural similarities between PLD2's PH and SWAP-70s or LARG's PH domain are very extensive, while similarities between PLD2's PX and SWAP-70s or LARG's DH domains are less evident. This indicates that PLD functions as a GEF utilizing its PH domain and part of its PX domain and possibly other regions. All this makes PLD unique, and an entirely new class of GEF. By bearing two enzymatic activities (break down of PC and GDP/GTP exchange), it is realistic to assume that PLD is an important signaling node for several intracellular pathways. Future experiments will ascertain how the newly described PLD2's GEF is regulated in the context of cell activation.

Chemokines and the signaling modules regulating integrin affinity

Integrin-mediated adhesion is a general concept referring to a series of adhesive phenomena including tethering–rolling, affinity, valency, and binding stabilization altogether controlling cell avidity (adhesiveness) for the substrate. Arrest chemokines modulate each aspect of integrin activation, although integrin affinity regulation has been recognized as the prominent event in rapid leukocyte arrest induced by chemokines. A variety of inside-out and outside-in signaling mechanisms have been related to the process of integrin-mediated adhesion in different cellular models, but only few of them have been clearly contextualized to rapid integrin affinity modulation by arrest chemokines in primary leukocytes. Complex signaling processes triggered by arrest chemokines and controlling leukocyte integrin activation have been described for ras-related rap and for rho-related small GTPases. We summarize the role of rap and rho small GTPases in the regulation of rapid integrin affinity in primary leukocytes and provide a modular view of these pro-adhesive signaling events. A potential, albeit still speculative, mechanism of rho-mediated regulation of cytoskeletal proteins controlling the last step of integrin activation is also discussed. We also discuss data suggesting a functional integration between the rho- and rap-modules of integrin activation. Finally we examine the universality of signaling mechanisms regulating integrin triggering by arrest chemokines.

Rho GTPases: masters of T lymphocyte migration and activation

Rho GTPases are key signal transducer elements activated in T cells by both chemokine and antigen receptors. These two signalling pathways control the two main functions of T lymphocytes: motility and activation. Rho GTPases are thus crucial for the development of an adequate immune response. In this review, we mostly focus on the roles of RhoA, Rac1 and Cdc42 in T cells. We show their importance in phenomena such as adhesion, morphological polarization, migration and antigen recognition.

RGS10 restricts upregulation by chemokines of T cell adhesion mediated by α4β1 and αLβ2 integrins

Chemokines rapidly and transiently upregulate α4β1 and αLβ2 integrin-mediated adhesion during T lymphocyte extravasation by activating Gα-dependent inside-out signaling. To limit and terminate Gα-mediated signaling, cells can use several mechanisms, including the action of regulator of G protein signaling (RGS) proteins, which accelerate the GTPase activity of Gα subunits. Using human T cells silenced for or overexpressing RGS10, we show in this article that RGS10 functions as an inhibitor of Gα(i)-dependent, chemokine-upregulated T cell adhesion mediated by α4β1 and αLβ2. Shear stress-dependent detachment and cell spreading analyses revealed that RGS10 action mainly targets the adhesion strengthening and spreading phases of α4β1-mediated cell attachment. Associated with these observations, chemokine-stimulated Vav1-Rac1 activation was longer sustained and of higher intensity in RGS10-silenced T cells, or inhibited in cells overexpressing RGS10. Of importance, expression of constitutively activated Rac1 forms in cells overexpressing RGS10 led to the rescue of CXCL12-stimulated adhesion to VCAM-1 to levels similar to those in control transfectants. Instead, adhesion under flow conditions, soluble binding experiment, flow cytometry, and biochemical analyses revealed that the earlier chemokine-triggered integrin activation step was mostly independent of RGS10 actions. The data strongly suggest that RGS10 opposes activation by chemokines of the Vav1-Rac1 pathway in T cells, leading to repression of adhesion strengthening mediated by α4β1. In addition to control chemokine-upregulated T cell attachment, RGS10 also limited adhesion-independent cell chemotaxis and activation of cdc42. These results identify RGS10 as a key molecule that contributes to the termination of Gα-dependent signaling during chemokine-activated α4β1- and αLβ2-dependent T cell adhesion.

Chemokine triggered integrin activation and actin remodeling events guiding lymphocyte migration across vascular barriers

Chemokine signals activate leukocyte integrins and actin remodeling machineries critical for leukocyte adhesion and motility across vascular barriers. The arrest of leukocytes at target blood vessel sites depends on rapid conformational activation of their α4 and β2 integrins by the binding of endothelial-displayed chemokines to leukocyte Gi-protein coupled receptors (GPCRs). A universal regulator of this event is the integrin-actin adaptor, talin1. Chemokine-stimulated GPCRs can transmit within fractions of seconds signals via multiple Rho GTPases, which locally raise plasma membrane levels of the talin activating phosphatidyl inositol, PtdIns(4,5)P2 (PIP2). Additional pools of GPCR stimulated Rac-1 and Rap-1 GTPases together with GPCR stimulated PLC and PI3K family members regulate the turnover of focal contacts of leukocyte integrins, induce the collapse of leukocyte microvilli, and promote polarized leukocyte crawling in search of exit cues. Concomitantly, other leukocyte GTPases trigger invasive protrusions into and between endothelial cells in search of basolateral chemokine exit cues. We will review here major findings and open questions related to these sequential guiding activities of endothelial presented chemokines, focusing mainly on lymphocyte-endothelial interactions as a paradigm for other leukocytes.

Integrin activation in the immune system

Modulation of leukocyte adhesiveness is critical to leukocyte function during the immune response. A central paradigm in this phenomenon is represented by integrin activation, which is controlled by inside-out signal transduction mechanisms triggered by selectins, chemoattractants and TcR-bound Ag and facilitated by mechanochemical forces. Integrins are heterodimeric adhesive receptors differently expressed on all leukocyte subtypes. At least two distinct modalities of integrin activation are known, namely conformational changes, leading to increased affinity, and lateral mobility leading to increased valency, both enhancing cell avidity (adhesiveness). Several signal transduction events have been correlated to integrin activation in leukocytes. The complexity of intracellular signaling networks leading to leukocyte integrin activation is likely functional to generate robustness and fine tuning of integrin activation allowing integration of qualitative and quantitative variations of extracellular signals leading to leukocyte-, agonist- and integrin-specific control of adhesion. In this context, the recent modular abstraction proposed for the functional architecture of biological networks may provide a powerful paradigm to understand regulation and specificity of signaling events. Accordingly, pro-adhesive intracellular signaling networks may be organized in regulatory signalosomes, or modules, corresponding to discrete clusters of interacting signaling proteins, with some devoted to context-dependent regulation of specificity and dynamics of integrin activation. The principles and technologies of systems biology, and more specifically of network theory, may help to address this complexity and unveil the inner logic governing leukocyte recruitment during the immune response.

African signatures of recent positive selection in human FOXI1

Background

The human FOXI1 gene codes for a transcription factor involved in the physiology of the inner ear, testis, and kidney. Using three interspecies comparisons, it has been suggested that this may be a gene under human-specific selection. We sought to confirm this finding by using an extended set of orthologous sequences. Additionally, we explored for signals of natural selection within humans by sequencing the gene in 20 Europeans, 20 East Asians and 20 Yorubas and by analysing SNP variation in a 2 Mb region centered on FOXI1 in 39 worldwide human populations from the HGDP-CEPH diversity panel.

Results

The genome sequences recently available from other primate and non-primate species showed that FOXI1 divergence patterns are compatible with neutral evolution. Sequence-based neutrality tests were not significant in Europeans, East Asians or Yorubas. However, the Long Range Haplotype (LRH) test, as well as the iHS and XP-Rsb statistics revealed significantly extended tracks of homozygosity around FOXI1 in Africa, suggesting a recent episode of positive selection acting on this gene. A functionally relevant SNP, as well as several SNPs either on the putatively selected core haplotypes or with significant iHS or XP-Rsb values, displayed allele frequencies strongly correlated with the absolute geographical latitude of the populations sampled.

Conclusions

We present evidence for recent positive selection in the FOXI1 gene region in Africa. Climate might be related to this recent adaptive event in humans. Of the multiple functions of FOXI1, its role in kidney-mediated water-electrolyte homeostasis is the most obvious candidate for explaining a climate-related adaptation.

HIV-1 Nef inhibits ruffles, induces filopodia, and modulates migration of infected lymphocytes

The HIV-1 Nef protein is a pathogenic factor modulating the behavior of infected cells. Nef induces actin cytoskeleton changes and impairs cell migration toward chemokines. We further characterized the morphology, cytoskeleton dynamics, and motility of HIV-1-infected lymphocytes. By using scanning electron microscopy, confocal immunofluorescence microscopy, and ImageStream technology, which combines flow cytometry and automated imaging, we report that HIV-1 induces a characteristic remodeling of the actin cytoskeleton. In infected lymphocytes, ruffle formation is inhibited, whereas long, thin filopodium-like protrusions are induced. Cells infected with HIV with nef deleted display a normal phenotype, and Nef expression alone, in the absence of other viral proteins, induces morphological changes. We also used an innovative imaging system to immobilize and visualize living individual cells in suspension. When combined with confocal "axial tomography," this technique greatly enhances three-dimensional optical resolution. With this technique, we confirmed the induction of long filopodium-like structures in unfixed Nef-expressing lymphocytes. The cytoskeleton reorganization induced by Nef is associated with an important impairment of cell movements. The adhesion and spreading of infected cells to fibronectin, their spontaneous motility, and their migration toward chemokines (CXCL12, CCL3, and CCL19) were all significantly decreased. Therefore, Nef induces complex effects on the lymphocyte actin cytoskeleton and cellular morphology, which likely impacts the capacity of infected cells to circulate and to encounter and communicate with bystander cells.

Chemokine-induced Zap70 kinase-mediated dissociation of the Vav1-talin complex activates alpha4beta1 integrin for T cell adhesion

Lymphocyte integrins mediate cell arrest on endothelium during immune surveillance after activation by chemokine-stimulated inside-out signals. Here we show that a Vav1-talin complex in T cells is a key target for chemokine-triggered inside-out signaling leading to integrin alpha4beta1 activation. Thus, Vav1 dissociation from talin was required to generate high-affinity alpha4beta1 conformations. Assembly of the Vav1-talin complex required PtdIns(4,5)P(2), which was provided by talin-bound phosphatidylinositol phosphate kinase Igamma. Chemokine-promoted Vav1 dissociation from talin followed an initial increase in talin binding to alpha4beta1. This process was dependent on ZAP-70, which binds to and phosphorylates Vav1 in the complex, leading to further alpha4beta1 activation and cell adhesion strengthening. Moreover, Vav1-talin dissociation was needed for Rac1 activation, thus indicating that alpha4beta1 and Rac1 activation can be coupled by chemokine-stimulated ZAP-70 function. Our data suggest that Vav1 might function as a repressive adaptor of talin that must dissociate from alpha4beta1-talin complexes for efficient integrin activation.

Mechanisms of chemokine and antigen-dependent T-lymphocyte navigation

T-lymphocyte trafficking is targeted to specific organs by selective molecular interactions depending on their differentiation and functional properties. Specific chemokine receptors have been associated with organ-specific trafficking of memory and effector T-cells, as well as the recirculation of naïve T-cells to secondary lymphoid organs. In addition to the acquisition of tissue-selective integrins and chemokine receptors, an additional level of specificity for T-cell trafficking into the tissue is provided by specific recognition of antigen displayed by the endothelium involving the TCRs (T-cell antigen receptors) and co-stimulatory receptors. Activation of PI3K (phosphoinositide 3-kinase) is a robust signalling event shared by most chemokine receptors as well as the TCR and co-stimulatory receptors, contributing to several aspects of T-lymphocyte homing as well as actin reorganization and other components of the general migratory machinery. Accordingly, inhibition of PI3K has been considered seriously as a potential therapeutic strategy by which to combat various T-lymphocyte-dependent pathologies, including autoimmune and inflammatory diseases, as well as to prevent transplant rejection. However, there is substantial evidence for PI3K-independent mechanisms that facilitate T-lymphocyte migration. In this regard, several other signalling-pathway components, including small GTPases, PLC (phospholipase C) and PKC (protein kinase C) isoforms, have also been implicated in T-lymphocyte migration in response to chemokine stimulation. The present review will therefore examine the PI3K-dependent and -independent signal-transduction pathways involved in T-cell migration during distinct modes of T-cell trafficking in response to either chemokines or the TCR and co-stimulatory molecules.

Reduced notch activity is associated with an impaired marginal zone B cell development and function in Sly1 mutant mice

MZ B cells represent a distinct lineage of naive B lymphocytes, apart from FO B cells and peritoneal B1 cells, and mediate humoral immune responses against blood-borne type 2 T-independent antigens. Regulation of MZ B cell development involves the Notch receptor signaling, the intensity of B cell receptor signals, and cell compartmentalization by adhesion and chemokine receptors. Our previous work showed that gene-targeted mice expressing a truncated form of the putative signaling adapter protein SLy1 exhibit reduced numbers of a splenic B cell population enriched in MZ B cells. Here, we demonstrate that Sly1(d/d) mice exhibit a partial, but selective, block in the transition from pre-MZ to mature MZ B cells. Development of both T1 and T2 precursor subsets and FO B cells was normal in Sly1(d/d) mice. Consistent with the loss of MZ B cells, the production of antigen-specific IgM antibodies following immunization with pneumococcal polysaccharides was severely impaired in Sly1(d/d) mice. Importantly, expression of the Notch signaling mediator RBP-J and the Notch target genes Hes-1 and Hes-5 was markedly reduced in MZ but not FO B cells of Sly1(d/d) mice. In contrast, B cell receptor signaling, expression and function of LFA-1 and alpha4-integrins, and expression of chemokine receptors appeared intact in Sly1(d/d) cells. Collectively, these results provide strong evidence that SLy1 is important for the generation and function of MZ B cells and suggest a novel link between SLy1 and the activity of the Notch pathway in the development of MZ B cells.

Regulation of T-cell migration and effector functions: insights from in vivo imaging studies

Studies of the immune system are providing us with ever more detailed information on the cellular and molecular mechanisms that underlie our evolutionarily conserved ability to fend off infectious pathogens. Progress has probably been fastest at two levels: the various basic biological functions of isolated cells on one side and the significance of individual molecules or cells to the organism as a whole on the other. In both cases, direct phenomenological observation has been an invaluable methodological approach. Where we know least is the middle ground, i.e. how immune functions are integrated through the dynamic interplay of immune cell subsets within the organism. Most of our knowledge in this area has been obtained through inference from static snapshots of dynamic processes, such as histological sections, or from surrogate cell co-culture models. The latter are employed under the assumption that an in vivo equivalent exists for each type of cellular contact artificially enforced in absence of anatomical compartmentalization. In this review, we summarize recent insights on migration and effector functions of T cells, focusing on observations gained from their dynamic microscopic visualization in physiological tissue environments.

Specific recognition of Rac2 and Cdc42 by DOCK2 and DOCK9 guanine nucleotide exchange factors

Recognition of cognate Rho GTPases by guanine-nucleotide exchange factors (GEF) is fundamental to Rho GTPase signaling specificity. Two main GEF families use either the Dbl homology (DH) or the DOCK homology region 2 (DHR-2) catalytic domain. How DHR-2-containing GEFs distinguish between the GTPases Rac and Cdc42 is not known. To determine how these GEFs specifically recognize the two Rho GTPases, we studied the amino acid sequences in Rac2 and Cdc42 that are crucial for activation by DOCK2, a Rac-specific GEF, and DOCK9, a distantly related Cdc42-specific GEF. Two elements in the N-terminal regions of Rac2 and Cdc42 were found to be essential for specific interactions with DOCK2 and DOCK9. One element consists of divergent amino acid residues in the switch 1 regions of the GTPases. Significantly, these residues were also found to be important for GTPase recognition by Rac-specific DOCK180, DOCK3, and DOCK4 GEFs. These findings were unexpected because the same residues were shown previously to interact with GTPase effectors rather than GEFs. The other element comprises divergent residues in the beta3 strand that are known to mediate specific recognition by DH domain containing GEFs. Remarkably, Rac2-to-Cdc42 substitutions of four of these residues were sufficient for Rac2 to be specifically activated by DOCK9. Thus, DOCK2 and DOCK9 specifically recognize Rac2 and Cdc42 through their switch 1 as well as beta2-beta3 regions and the mode of recognition via switch 1 appears to be conserved among diverse Rac-specific DHR-2 GEFs.

Dock2 participates in bone marrow lympho-hematopoiesis

Dock2 has been shown to be indispensable for chemotaxis of mature lymphocytes as a critical Rac activator. However, the functional expression of Dock2 in immature hematopoietic cells is unclear. In this study, we demonstrate that Dock2 is broadly expressed in bone marrow (BM) hematopoietic compartment, including hematopoietic stem/progenitor cell (HSC/HPC) fraction. Response of Dock2-/- HPCs to CXCL12 in chemotaxis and actin polymerization in vitro was impaired, although alpha4 integrin activation by CXCL12 was not altered. Myelosuppressive stress on HSCs in vivo, such as consecutive 5-FU administration and serial bone marrow transplantation, did not show hematopoietic defect in Dock2-/- mice. Long-term engraftment of transplanted Dock2-/- BM cells was severely impaired in competitive reconstitution. However, this was not intrinsic to HSCs but originated from the defective competition of Dock2-/- lymphoid precursors. These results suggest that Dock2 plays a significant role in BM lymphopoiesis, but is dispensable for HSC engraftment and self-renewal.

Posterolateral approach for open reduction and internal fixation of trimalleolar ankle fractures

Cell-cell and cell-matrix interactions are of critical importance in immunobiology. Leukocytes make extensive use of a specialized repertoire of receptors to mediate such processes. Among these receptors, integrins are known to be of crucial importance. This review deals with the central role of integrins and their counterreceptors during the establishment of leukocyte-endothelium contacts, interstitial migration, and final encounter with antigen-presenting cells to develop an appropriate immune response. Particularly, we have addressed the molecular events occurring during these sequential processes, leading to the dynamic subcellular redistribution of adhesion receptors and the reorganization of the actin cytoskeleton, which is reflected in changes in cytoarchitecture, including leukocyte polarization, endothelial docking structure formation, or immune synapse organization. The roles of signaling and structural actin cytoskeleton-associated proteins and organized membrane microdomains in the regulation of receptor adhesiveness are also discussed.