WNK1 kinase balances T cell adhesion versus migration in vivo.

Adhesion and migration of T cells are controlled by chemokines and by adhesion molecules, especially integrins, and have critical roles in the normal physiological function of T lymphocytes. Using an RNA-mediated interference screen, we identified the WNK1 kinase as a regulator of both integrin-mediated adhesion and T cell migration. We found that WNK1 is a negative regulator of integrin-mediated adhesion, whereas it acts as a positive regulator of migration via the kinases OXSR1 and STK39 and the ion co-transporter SLC12A2. WNK1-deficient T cells home less efficiently to lymphoid organs and migrate more slowly through them. Our results reveal that a pathway previously known only to regulate salt homeostasis in the kidney functions to balance T cell adhesion and migration.

LFA-1 in T cell priming, differentiation, and effector functions.

The integrin LFA-1 is crucial for T cell entry into mammalian lymph nodes and tissues, and for promoting interactions with antigen-presenting cells (APCs). However, it is increasingly evident that LFA-1 has additional key roles beyond the mere support of adhesion between T cells, the endothelium, and/or APCs. These include roles in homotypic T cell-T cell (T-T) communication, the induction of intracellular complement activity underlying Th1 effector cell polarization, and the support of long-lasting T cell memory. Here, we briefly summarize current knowledge of LFA-1 biology, discuss novel cytoskeletal regulators of LFA-1 functions, and review new aspects of LFA-1 mechanobiology that are relevant to its function in immunological synapses and in specific pathologies arising from LFA-1 dysregulation.

Extracellular matrix rigidity modulates physical properties of SCS macrophage-B cell immune synapses.

Subcapsular sinus (SCS) macrophages (SSMs) play a key role in immune defence by forming immunological barriers that control the transport of antigens from lymph into lymph node follicles. SSMs participate in antibody responses by presenting antigens directly to naive B cells and by supplying antigens to follicular dendritic cells to propagate germinal centre reactions. Despite the prominent roles that SSMs play during immune responses, little is known about their cell biology because they are technically challenging to isolate and study in vitro. Here, we used multi-colour fluorescence microscopy to identify lymph node-derived SSMs in culture. We focused on the role of SSMs as antigen-presenting cells, and found that their actin cytoskeleton regulates the spatial organisation and mobility of multivalent antigens (immune complexes, ICs) displayed on the cell surface. Moreover, we determined that SSMs are mechanosensitive cells that respond to changes in extracellular matrix (ECM) rigidity by altering the architecture of the actin cytoskeleton, leading to changes in cell morphology, membrane topography, and immune complex mobility. Changes to ECM rigidity also modulate actin remodelling by both SSMs and B cells when they form an immune synapse. This alters synapse duration but not IC internalisation nor NF-κB activation in the B cell. Taken together, our data reveal that the mechanical microenvironment may influence B cell responses by modulating physical characteristics of antigen presentation by SSMs.

Mechanism and function of Vav1 localisation in TCR signalling.

The antigen-specific binding of T cells to antigen presenting cells results in recruitment of signalling proteins to microclusters at the cell-cell interface known as the immunological synapse (IS). The Vav1 guanine nucleotide exchange factor plays a critical role in T cell antigen receptor (TCR) signalling, leading to the activation of multiple pathways. We now show that it is recruited to microclusters and to the IS in primary CD4(+) and CD8(+) T cells. Furthermore, we show that this recruitment depends on the SH2 and C-terminal SH3 (SH3(B)) domains of Vav1, and on phosphotyrosines 112 and 128 of the SLP76 adaptor protein. Biophysical measurements show that Vav1 binds directly to these residues on SLP76 and that efficient binding depends on the SH2 and SH3(B) domains of Vav1. Finally, we show that the same two domains are critical for the phosphorylation of Vav1 and its signalling function in TCR-induced calcium flux. We propose that Vav1 is recruited to the IS by binding to SLP76 and that this interaction is critical for the transduction of signals leading to calcium flux.

B cell-intrinsic requirement for WNK1 kinase in antibody responses in mice.

Migration and adhesion play critical roles in B cells, regulating recirculation between lymphoid organs, migration within lymphoid tissue, and interaction with CD4+ T cells. However, there is limited knowledge of how B cells integrate chemokine receptor and integrin signaling with B cell activation to generate efficient humoral responses. Here, we show that the WNK1 kinase, a regulator of migration and adhesion, is essential in B cells for T-dependent and -independent antibody responses. We demonstrate that WNK1 transduces signals from the BCR, CXCR5, and CD40, and using intravital imaging, we show that WNK1 regulates migration of naive and activated B cells, and their interactions with T cells. Unexpectedly, we show that WNK1 is required for BCR- and CD40-induced proliferation, acting through the OXSR1 and STK39 kinases, and for efficient B cell-T cell collaboration in vivo. Thus, WNK1 is critical for humoral immune responses, by regulating B cell migration, adhesion, and T cell-dependent activation.

T cell migration requires ion and water influx to regulate actin polymerization.

Migration of T cells is essential for their ability to mount immune responses. Chemokine-induced T cell migration requires WNK1, a kinase that regulates ion influx into the cell. However, it is not known why ion entry is necessary for T cell movement. Here we show that signaling from the chemokine receptor CCR7 leads to activation of WNK1 and its downstream pathway at the leading edge of migrating CD4+ T cells, resulting in ion influx and water entry by osmosis. We propose that WNK1-induced water entry is required to swell the membrane at the leading edge, generating space into which actin filaments can polymerize, thereby facilitating forward movement of the cell. Given the broad expression of WNK1 pathway proteins, our study suggests that ion and water influx are likely to be essential for migration in many cell types, including leukocytes and metastatic tumor cells.

Chloride sensing by WNK1 regulates NLRP3 inflammasome activation and pyroptosis.

The NLRP3 inflammasome mediates the production of proinflammatory cytokines and initiates inflammatory cell death. Although NLRP3 is essential for innate immunity, aberrant NLRP3 inflammasome activation contributes to a wide variety of inflammatory diseases. Understanding the pathways that control NLRP3 activation will help develop strategies to treat these diseases. Here we identify WNK1 as a negative regulator of the NLRP3 inflammasome. Macrophages deficient in WNK1 protein or kinase activity have increased NLRP3 activation and pyroptosis compared with control macrophages. Mice with conditional knockout of WNK1 in macrophages have increased IL-1ß production in response to NLRP3 stimulation compared with control mice. Mechanistically, WNK1 tempers NLRP3 activation by balancing intracellular Cl- and K+ concentrations during NLRP3 activation. Collectively, this work shows that the WNK1 pathway has a critical function in suppressing NLRP3 activation and suggests that pharmacological inhibition of this pathway to treat hypertension might have negative clinical implications.

Nox2-deficient Tregs improve heart transplant outcomes via their increased graft recruitment and enhanced potency.

Nox2 is a ROS-generating enzyme, deficiency of which increases suppression by Tregs in vitro and in an in vivo model of cardiac remodeling. As Tregs have emerged as a candidate therapy in autoimmunity and transplantation, we hypothesized that Nox2 deficiency in Tregs in recipient mice may improve outcomes in a heart transplant model. We generated a potentially novel B6129 mouse model with Treg-targeted Nox2 deletion (Nox2fl/flFoxP3Cre+ mice) and transplanted with hearts from CB6F1 donors. As compared with those of littermate controls, Nox2fl/flFoxP3Cre+ mice had lower plasma levels of alloantibodies and troponin-I, reduced levels of IFN-γ in heart allograft homogenates, and diminished cardiomyocyte necrosis and allograft fibrosis. Single-cell analyses of allografts revealed higher absolute numbers of Tregs and lower CD8+ T cell infiltration in Nox2-deficient recipients compared with Nox2-replete mice. Mechanistically, in addition to a greater suppression of CD8+CD25- T effector cell proliferation and IFN-γ production, Nox2-deficient Tregs expressed higher levels of CCR4 and CCR8, driving cell migration to allografts; this was associated with increased expression of miR-214-3p. These data indicate that Nox2 deletion in Tregs enhances their suppressive ability and migration to heart allografts. Therefore, Nox2 inhibition in Tregs may be a useful approach to improve their therapeutic efficacy.

Nance-Horan Syndrome-like 1 protein negatively regulates Scar/WAVE-Arp2/3 activity and inhibits lamellipodia stability and cell migration.

Cell migration is important for development and its aberrant regulation contributes to many diseases. The Scar/WAVE complex is essential for Arp2/3 mediated lamellipodia formation during mesenchymal cell migration and several coinciding signals activate it. However, so far, no direct negative regulators are known. Here we identify Nance-Horan Syndrome-like 1 protein (NHSL1) as a direct binding partner of the Scar/WAVE complex, which co-localise at protruding lamellipodia. This interaction is mediated by the Abi SH3 domain and two binding sites in NHSL1. Furthermore, active Rac binds to NHSL1 at two regions that mediate leading edge targeting of NHSL1. Surprisingly, NHSL1 inhibits cell migration through its interaction with the Scar/WAVE complex. Mechanistically, NHSL1 may reduce cell migration efficiency by impeding Arp2/3 activity, as measured in cells using a Arp2/3 FRET-FLIM biosensor, resulting in reduced F-actin density of lamellipodia, and consequently impairing the stability of lamellipodia protrusions.

Critical role of WNK1 in MYC-dependent early mouse thymocyte development.

WNK1, a kinase that controls kidney salt homeostasis, also regulates adhesion and migration in CD4+ T cells. Wnk1 is highly expressed in thymocytes, and since migration is important for thymocyte maturation, we investigated a role for WNK1 in mouse thymocyte development. We find that WNK1 is required for the transition of double negative (DN) thymocytes through the ß-selection checkpoint and subsequent proliferation and differentiation into double positive (DP) thymocytes. Furthermore, we show that WNK1 negatively regulates LFA1-mediated adhesion and positively regulates CXCL12-induced migration in DN thymocytes. Despite this, migration defects of WNK1-deficient thymocytes do not account for the developmental arrest. Instead, we show that in DN thymocytes WNK1 transduces pre-TCR signals via OXSR1 and STK39 kinases, and the SLC12A2 ion co-transporter that are required for post-transcriptional upregulation of MYC and subsequent proliferation and differentiation into DP thymocytes. Thus, a pathway regulating ion homeostasis is a critical regulator of thymocyte development.

The signal peptide of the rat corticotropin-releasing factor receptor 1 promotes receptor expression but is not essential for establishing a functional receptor.

Approximately 5-10% of the GPCRs (G-protein-coupled receptors) contain N-terminal signal peptides that are cleaved off during receptor insertion into the ER (endoplasmic reticulum) membrane by the signal peptidases of the ER. The reason as to why only a subset of GPCRs requires these additional signal peptides is not known. We have recently shown that the signal peptide of the human ET(B)-R (endothelin B receptor) does not influence receptor expression but is necessary for the translocation of the receptor's N-tail across the ER membrane and thus for the establishment of a functional receptor [Köchl, Alken, Rutz, Krause, Oksche, Rosenthal and Schülein (2002) J. Biol. Chem. 277, 16131-16138]. In the present study, we show that the signal peptide of the rat CRF-R1 (corticotropin-releasing factor receptor 1) has a different function: a mutant of the CRF-R1 lacking the signal peptide was functional and displayed wild-type properties with respect to ligand binding and activation of adenylate cyclase. However, immunoblot analysis and confocal laser scanning microscopy revealed that the mutant receptor was expressed at 10-fold lower levels than the wild-type receptor. Northern-blot and in vitro transcription translation analyses precluded the possibility that the reduced receptor expression is due to decreased transcription or translation levels. Thus the signal peptide of the CRF-R1 promotes an early step of receptor biogenesis, such as targeting of the nascent chain to the ER membrane and/or the gating of the protein-conducting translocon of the ER membrane.

In vitro reconstitution of fusion between immature autophagosomes and endosomes.

Autophagy is a highly conserved degradative pathway whereby a double membrane engulfs cytoplasmic constituents to form an autophagic vacuole or autophagosome. An essential requirement for efficient autophagy is the acquisition of an adequate degradative capacity by the autophagosomes. To acquire this capacity the immature autophagic vacuoles (AVis) obtain lysosomal hydrolases by fusion with endosomes. The current models suggest that at least two types of endosomes, early and late, fuse with AVis to form mature, degradative AVds. This fusion and maturation requires proteins also involved in endosome maturation such as Rab7. However, it is not known if there are molecular requirements unique to AVi-endosome fusion. To identify and investigate the molecular requirements of this fusion we developed a cell-free fusion assay based on content mixing, which occurs after fusion of isolated AVis and different endosomal fractions. Our assay shows that isolated AVis can fuse to a similar extent in vitro with both early and late endosomes. Furthermore, fusion between autophagosomes and endosomes requires cytosolic and endosomal proteins, but does not show a nucleotide-dependence, and is partially N-ethylmaleimide sensitive. We also demonstrate that the lipidated form of the autophagosomal protein LC3 is dispensable for this fusion event.

Microtubules facilitate autophagosome formation and fusion of autophagosomes with endosomes.

Nutrient deprivation of eukaryotic cells provokes a variety of stress responses, including autophagy. Autophagy is carried out by autophagosomes which sequester cytosolic components and organelles for degradation after fusion with protease-containing endosomes. To determine the role of microtubules in autophagy, we used nocodazole and vinblastine to disrupt microtubules and independently measured formation and fusion of autophagsosomes in primary rat hepatocytes. By measuring the translocation of GFP-LC3, an autophagosomal marker, to autophagosomes and the lipidation of GFP-LC3, we quantified the rate and magnitude of autophagosome formation. Starvation increased both the rate of autophagosome formation over the basal level and the total number of autophagosomes per cell. Maximal autophagosome formation required an intact microtubule network. Fusion of autophagosomes with endosomes, assayed by acquisition of protease-inhibitor sensitivity as well as overlap with LysoTracker Red-positive endosomes, required intact microtubules. Live-cell imaging demonstrated that autophagosomes were motile structures, and their movement also required microtubules. Interestingly, vinblastine stimulated autophagosome formation more than twofold before any discernable change in the microtubule network was observed. Stimulation of autophagosome formation by vinblastine was independent of nutrients and mTOR activity but was inhibited by depletion of the Autophagy proteins Atg5 and Atg6, known to be required for autophagy.

Starvation and ULK1-dependent cycling of mammalian Atg9 between the TGN and endosomes.

Autophagy, fundamentally a lysosomal degradation pathway, functions in cells during normal growth and certain pathological conditions, including starvation, to maintain homeostasis. Autophagosomes are formed through a mechanism that is not well understood, despite the identification of many genes required for autophagy. We have studied the mammalian homologue of Atg9p, a multi-spanning transmembrane protein essential in yeast for autophagy, to gain a better understanding of the function of this ubiquitious protein. We show that both the N- and C-termini of mammalian Atg9 (mAtg9) are cytosolic, and predict that mAtg9 spans the membrane six times. We find that mAtg9 is located in the trans-Golgi network and late endosomes and colocalizes with TGN46, the cation-independent mannose-6-phosphate receptor, Rab7 and Rab9. Amino acid starvation or rapamycin treatment, which upregulates autophagy, causes a redistribution of mAtg9 from the TGN to peripheral, endosomal membranes, which are positive for the autophagosomal marker GFP-LC3. siRNA-mediated depletion of the putative mammalian homologue of Atg1p, ULK1, inhibits this starvation-induced redistribution. The redistribution of mAtg9 also requires PI 3-kinase activity, and is reversed after restoration of amino acids. We speculate that starvation-induced autophagy, which requires mAtg9, may rely on an alteration of the steady-state trafficking of mAtg9, in a Atg1-dependent manner.

The signal peptide of the G protein-coupled human endothelin B receptor is necessary for translocation of the N-terminal tail across the endoplasmic reticulum membrane.

The initial step of the intracellular transport of G protein-coupled receptors, their insertion into the membrane of the endoplasmic reticulum, follows one of two different pathways. Whereas one group uses the first transmembrane domain of the mature receptor as an uncleaved signal anchor sequence for this process, a second group possesses additional cleavable signal peptides. The reason this second subset requires the additional signal peptide is not known. Here we have assessed the functional significance of the signal peptide of the endothelin B (ET(B)) receptor in transiently transfected COS.M6 cells. A green fluorescent protein-tagged ET(B) receptor mutant lacking the signal peptide was nonfunctional and retained in the endoplasmic reticulum, suggesting that it has a folding defect. To determine the defect in more detail, ET(B) receptor fragments containing the N-terminal tail, first transmembrane domain, and first cytoplasmic loop were constructed. We assessed N tail translocation across the endoplasmic reticulum membrane in the presence and absence of a signal peptide and show that the signal peptide is necessary for N tail translocation. We postulate that signal peptides are necessary for those G protein-coupled receptors for which post-translational translocation of the N terminus is impaired or blocked by the presence of stably folded domains.