Brenda A. Finney

CLEC-2 and Syk in the megakaryocytic/platelet lineage are essential for development.

The C-type lectin receptor CLEC-2 signals through a pathway that is critically dependent on the tyrosine kinase Syk. We show that homozygous loss of either protein results in defects in brain vascular and lymphatic development, lung inflation, and perinatal lethality. Furthermore, we find that conditional deletion of Syk in the hematopoietic lineage, or conditional deletion of CLEC-2 or Syk in the megakaryocyte/platelet lineage, also causes defects in brain vascular and lymphatic development, although the mice are viable. In contrast, conditional deletion of Syk in other hematopoietic lineages had no effect on viability or brain vasculature and lymphatic development. We show that platelets, but not platelet releasate, modulate the migration and intercellular adhesion of lymphatic endothelial cells through a pathway that depends on CLEC-2 and Syk. These studies found that megakaryocyte/platelet expression of CLEC-2 and Syk is required for normal brain vasculature and lymphatic development and that platelet CLEC-2 and Syk directly modulate lymphatic endothelial cell behavior in vitro.

Combined In Vivo Depletion of Glycoprotein VI and C-Type Lectin-Like Receptor 2 Severely Compromises Hemostasis and Abrogates Arterial Thrombosis in Mice

Objective—Platelet inhibition is a major strategy to prevent acute ischemic cardiovascular and cerebrovascular events, which may, however, be associated with an increased bleeding risk. The (hem)immunoreceptor tyrosine activation motif–bearing platelet receptors, glycoprotein VI (GPVI) and C-type lectin-like receptor 2 (CLEC-2), might be promising antithrombotic targets because they can be depleted from circulating platelets by antibody treatment, leading to sustained antithrombotic protection, but only moderately increased bleeding times in mice. Approach and Results—We investigated whether both (hem)immunoreceptor tyrosine activation motif–bearing receptors can be targeted simultaneously and what the in vivo consequences of such a combined therapeutic GPVI/CLEC-2 deficiency are. We demonstrate that isolated targeting of either GPVI or CLEC-2 in vivo does not affect expression or function of the respective other receptor. Moreover, simultaneous treatment with both antibodies resulted in the sustained loss of both GPVI and CLEC-2, while leaving other activation pathways intact. However, GPVI/CLEC-2–depleted mice displayed a dramatic hemostatic defect and profound impairment of arterial thrombus formation. Furthermore, a strongly diminished hemostatic response could also be reproduced in mice genetically lacking GPVI and CLEC-2. Conclusions—These results demonstrate that GPVI and CLEC-2 can be simultaneously downregulated in platelets in vivo and reveal an unexpected functional redundancy of the 2 receptors in hemostasis and thrombosis. These findings may have important implications of the potential use of anti-GPVI and anti–CLEC-2–based agents in the prevention of thrombotic diseases.

CLEC-2 is not required for platelet aggregation at arteriolar shear

The C-type lectin receptor CLEC-2 is expressed primarily on the surface of platelets, where it is present as a dimer, and is found at low level on a subpopulation of other hematopoietic cells, including mouse neutrophils [1–4] Clustering of CLEC-2 by the snake venom toxin rhodocytin, specific antibodies or its endogenous ligand, podoplanin, elicits powerful activation of platelets through a pathway that is similar to that used by the collagen receptor glycoprotein VI (GPVI) [4–6]. The cytosolic tail of CLEC-2 contains a conserved YxxL sequence preceded by three upstream acidic amino acid residues, which together form a novel motif known as a hemITAM. Ligand engagement induces tyrosine phosphorylation of the hemITAM sequence providing docking sites for the tandem-SH2 domains of the tyrosine kinase Syk across a CLEC-2 receptor dimer [3]. Tyrosine phosphorylation of Syk by Src family kinases and through autophosphorylation leads to stimulation of a downstream signaling cascade that culminates in activation of phospholipase C γ2 (PLCγ2) [4,6]. Recently, CLEC-2 has been proposed to play a major role in supporting activation of platelets at arteriolar rates of flow [1]. Injection of a CLEC-2 antibody into mice causes a sustained depletion of the C-type lectin receptor from the platelet surface [1]. The CLEC-2-depleted platelets were unresponsive to rhodocytin but underwent normal aggregation and secretion responses after stimulation of other platelet receptors, including GPVI [1]. In contrast, there was a marked decrease in aggregate formation relative to controls when CLEC-2-depleted blood was flowed at arteriolar rates of shear over collagen (1000 s−1 and 1700 s−1) [1]. Furthermore, antibody treatment significantly increased tail bleeding times and mice were unable to occlude their vessels after ferric chloride injury [1]. These data provide evidence for a critical role for CLEC-2 in supporting platelet aggregation at arteriolar rates of flow. The underlying mechanism is unclear as platelets do not express podoplanin, the only known endogenous ligand of CLEC-2. In the present study, we have investigated the role of CLEC-2 in platelet aggregation and thrombus formation using platelets from a novel mutant mouse model that lacks functional CLEC-2.

CLEC-2 is required for development and maintenance of lymph nodes.

The importance of CLEC-2, a natural ligand/receptor for Gp38/Podoplanin, in the formation of the lymphatic vasculature has recently been demonstrated. As the development and maintenance of lymph nodes (LNs) is dependent on the formation of the lymphatic vasculature and the differentiation of Gp38/Podoplanin(+) stromal cells, we investigated the role of CLEC-2 in lymphoneogenesis and LN homeostasis. Using constitutive Clec1b(-/-) mice, we showed that while CLEC-2 was not necessary for initiation of the LN anlage, it was required at late stages of development. Constitutive deletion of CLEC-2 induced a profound defect in lymphatic endothelial cell proliferation, resulting in lack of LNs at birth. In contrast, conditional deletion of CLEC-2 in the megakaryocyte/platelet lineage in Clec1b(fl/fl)PF4-Cre mice led to the development of blood-filled LNs and fibrosis, in absence of a proliferative defect of the lymphatic endothelial compartment. This phenotype was also observed in chimeric mice reconstituted with Clec1b(fl/fl)PF4-Cre bone marrow, indicating that CLEC-2 expression in platelets was required for LN integrity. We demonstrated that LNs of Clec1b(fl/fl)PF4-Cre mice are able to sustain primary immune responses but show a defect in immune cell recirculation after repeated immunizations, thus suggesting CLEC-2 as target in chronic immune response.

Mice Lacking the ITIM-Containing Receptor G6b-B Exhibit Macrothrombocytopenia and Aberrant Platelet Function

An inhibitory receptor ensures that megakaryocytes produce proper numbers of functional platelets. Controlling Platelet Production Megakaryocytes reside in the bone marrow, where they produce platelets, cell fragments that form clots to prevent blood loss at sites of damage to the vasculature. Platelets and megakaryocytes share many activating receptors on their surface, but unlike platelets, megakaryocytes fail to become activated when exposed to components of the extracellular matrix. Mazharian et al. found that mice deficient in the immunoreceptor tyrosine–based inhibition motif–containing receptor G6b-B had fewer and larger platelets than did their wild-type counterparts. In addition, G6b-B–deficient mice exhibited increased bleeding in response to damage and had activated megakaryocytes, which resulted in the production of defective platelets. Together, these data suggest that G6b-B dampens activating signals in megakaryocytes to enable the generation of the appropriate number of functional platelets. Platelets are highly reactive cell fragments that adhere to exposed extracellular matrix (ECM) and prevent excessive blood loss by forming clots. Paradoxically, megakaryocytes, which produce platelets in the bone marrow, remain relatively refractory to the ECM-rich environment of the bone marrow despite having the same repertoire of receptors as platelets. These include the ITAM (immunoreceptor tyrosine–based activation motif)–containing collagen receptor complex, which consists of glycoprotein VI (GPVI) and the Fc receptor γ-chain, and the ITIM (immunoreceptor tyrosine–based inhibition motif)–containing receptor G6b-B. We showed that mice lacking G6b-B exhibited macrothrombocytopenia (reduced platelet numbers and the presence of enlarged platelets) and a susceptibility to bleeding as a result of aberrant platelet production and function. Platelet numbers were markedly reduced in G6b-B–deficient mice compared to those in wild-type mice because of increased platelet turnover. Furthermore, megakaryocytes in G6b-B–deficient mice showed enhanced metalloproteinase production, which led to increased shedding of cell-surface receptors, including GPVI and GPIbα. In addition, G6b-B–deficient megakaryocytes exhibited reduced integrin-mediated functions and defective formation of proplatelets, the long filamentous projections from which platelets bud off. Together, these findings establish G6b-B as a major inhibitory receptor regulating megakaryocyte activation, function, and platelet production.

Podoplanin-expressing inflammatory macrophages activate murine platelets via CLEC-2

Podoplanin, a small transmembrane glycoprotein, is expressed at high levels on lymphatic endothelial cells, kidney podocytes and lung type I alveolar cells, but absent on blood endothelial cells. It has also been described on splenic and peritoneal macrophages, but the functional significance of this is unknown [1]. Podoplanin is an endogenous ligand for CLEC-2, a C-type lectin expressed on platelets and at a lower level on subsets of other hematopoietic cells [2–4]. The interaction of CLEC-2 on platelets with podoplanin on lymphatic endothelial cells is essential for the separation of lymphatic and blood vessels [5,6]. In the present study, we further investigated the expression of podoplanin on macrophages and explored the ability of macrophage-expressed podoplanin to function as a CLEC-2 ligand. We probed various macrophage populations with a podoplanin-specific antibody and FcCLEC-2 (a probe for CLEC-2 ligands [7]). We were unable to detect specific binding of either the podoplanin antibody or FcCLEC-2 on RAW264.7 macrophages, bone marrow-derived macrophages (BMDMs) and tissue-resident macrophages, including alveolar and resident peritoneal macrophages, indicating the absence of podoplanin (Fig. 1A). Conversely, podoplanin was detected on thioglycollate-elicited ‘inflammatory’ peritoneal macrophages and lipopolysaccharide (LPS)-treated RAW264.7 cells using the same two ligands (Fig. 1A). Podoplanin expression was also markedly up-regulated on BMDMs following stimulation with LPS and weakly up-regulated in response to the TLR2/1 and TLR2/6 agonists, Pam3CSK4 and Pam2CSK4, respectively (Fig. 1B). There was also a slight, but significant, up-regulation induced by the inflammatory cytokine tumor necrosis factor (TNF), but not by several other cytokines tested (Fig. 1B). These results demonstrate that podoplanin is expressed on inflammatory but not tissue-resident macrophages, and is up-regulated in response to TLR stimulation and some inflammatory cytokines. The present results extend a previous report of podoplanin expression on macrophages [1], by revealing that expression is limited to inflammatory subsets. Fig. 1 Podoplanin expression on macrophages. (A) Anti-podoplanin and FcCLEC-2 staining of various macrophage populations as indicated. Resident and thioglycollate-elicited peritoneal macrophages and resident alveolar macrophages were isolated from Balb/c mice ... To investigate whether macrophage-expressed podoplanin could induce CLEC-2 activation we used a reporter system based on BWZ.36 cells expressing a chimeric CLEC-2/CD3ζ receptor linked to β-galactosidase expression and IL-2 secretion [8]. Incubation of these cells with podoplanin-expressing macrophages induced IL-2 and β-galactosidase, whereas podoplanin-negative macrophages had no effect (Figs 1C and D). We also investigated whether podoplanin-expressing macrophages could directly induce platelet activation (determined by analysing aggregation and ATP secretion). Untreated podoplanin-negative RAW264.7 macrophages had no effect on platelet aggregation or ATP secretion, whereas the subsequent addition of thrombin induced powerful activation. In contrast, LPS-activated RAW264.7 macrophages stimulated platelet aggregation and ATP secretion after a delay of several minutes (Fig. 1E). A delay in activation is reminiscent of platelet activation by the CLEC-2 ligand rhodocytin [2]. The subsequent lack of an effect of addition of thrombin confirmed that these platelets had undergone full activation. Aggregation and secretion induced by LPS-stimulated RAW264.7 cells was blocked by the αIIbβ3 inhibitor, lotrafiban, confirming integrin-dependent platelet activation, and by the Src kinase inhibitor, PP1, consistent with CLEC-2-mediated platelet activation (Fig. 1F). In line with this, activation was blocked in CLEC-2-deficient mouse platelets [9], whereas thrombin was still able to induce full activation of these cells (Fig. 1G). Another mechanism by which platelets and macrophages interact is via binding of P-selectin on activated platelets to PSGL-1 on macrophages. We investigated whether blocking this interaction would modulate platelet activation triggered by podoplanin-expressing macrophages; however, the inclusion of a blocking antibody against PSGL-1 did not have any effect on platelet aggregation or secretion (data not shown). These results demonstrate that inflammatory macrophages expressing podoplanin can directly activate platelets via CLEC-2. This could represent a novel mechanism for extravascular platelet activation during clotting, wound healing and vascular inflammatory processes such as atherosclerosis. Expression of podoplanin promotes migration of a variety of cell types, including tumor, MDCK and lymphatic endothelial cells [10–12]. The presence of podoplanin on macrophages could therefore underlie a mechanism through which platelets regulate migration of inflammatory macrophages as well as other functions.

Podoplanin and CLEC-2 drive cerebrovascular patterning and integrity during development

Mice with a constitutive or platelet-specific deletion of the C-type-lectin-like receptor (CLEC-2) exhibit hemorrhaging in the brain at mid-gestation. We sought to investigate the basis of this defect, hypothesizing that it is mediated by the loss of CLEC-2 activation by its endogenous ligand, podoplanin, which is expressed on the developing neural tube. To induce deletion of podoplanin at the 2-cell stage, we generated a podoplanin(fl/fl) mouse crossed to a PGK-Cre mouse. Using 3-dimensional light-sheet microscopy, we observed cerebral vessels were tortuous and aberrantly patterned at embryonic (E) day 10.5 in podoplanin- and CLEC-2-deficient mice, preceding the formation of large hemorrhages throughout the fore-, mid-, and hindbrain by E11.5. Immunofluorescence and electron microscopy revealed defective pericyte recruitment and misconnections between the endothelium of developing blood vessels and surrounding pericytes and neuro-epithelial cells. Nestin-Cre-driven deletion of podoplanin on neural progenitors also caused widespread cerebral hemorrhaging. Hemorrhaging was also seen in the ventricles of embryos deficient in the platelet integrin subunit glycoprotein IIb or in embryos in which platelet α-granule and dense granule secretion is abolished. We propose a novel role for podoplanin on the neuro-epithelium, which interacts with CLEC-2 on platelets, mediating platelet adhesion, aggregation, and secretion to guide the maturation and integrity of the developing vasculature and prevent hemorrhage.

Regulation of mouse lung development by the extracellular calcium-sensing receptor, CaR

Postnatal lung function is critically dependent upon optimal embryonic lung development. As the free ionized plasma calcium concentration ([Ca2+]o) of the fetus is higher than that of the adult, the process of lung development occurs in a hypercalcaemic environment. In the adult, [Ca2+]o is monitored by the G‐protein coupled, extracellular calcium‐sensing receptor (CaR), but neither its ontogeny nor its potential role in lung development are known. Here, we demonstrate that CaR is expressed in the mouse lung epithelium, and that its expression is developmentally regulated, with a peak of expression at embryonic day 12.5 (E12.5) and a subsequent decrease by E18, after which the receptor is absent. Experiments carried out using the lung explant culture model in vitro show that lung branching morphogenesis is sensitive to [Ca2+]o, being maximal at physiological adult [Ca2+]o (i.e. 1.0–1.3 mm) and lowest at the higher, fetal (i.e. 1.7 mm) [Ca2+]o. Administration of the specific CaR positive allosteric modulator, the calcimimetic R‐568, mimics the suppressive effects of high [Ca2+]o on branching morphogenesis while both phospholipase C and PI3 kinase inhibition reverse these effects. CaR activation suppresses cell proliferation while it enhances intracellular calcium signalling, lung distension and fluid secretion. Conditions which are restrictive either to branching or to secretion can be rescued by manipulating [Ca2+]o in the culture medium. In conclusion, fetal Ca2+o, acting through a developmentally regulated CaR, is an important extrinsic factor that modulates the intrinsic lung developmental programme. Our observations support a novel role for the CaR in preventing hyperplastic lung disease in utero.

Fetal Calcium Regulates Branching Morphogenesis in the Developing Human and Mouse Lung: Involvement of Voltage-Gated Calcium Channels

Airway branching morphogenesis in utero is essential for optimal postnatal lung function. In the fetus, branching morphogenesis occurs during the pseudoglandular stage (weeks 9–17 of human gestation, embryonic days (E)11.5–16.5 in mouse) in a hypercalcaemic environment (∼1.7 in the fetus vs. ∼1.1–1.3 mM for an adult). Previously we have shown that fetal hypercalcemia exerts an inhibitory brake on branching morphogenesis via the calcium-sensing receptor. In addition, earlier studies have shown that nifedipine, a selective blocker of L-type voltage-gated Ca2+ channels (VGCC), inhibits fetal lung growth, suggesting a role for VGCC in lung development. The aim of this work was to investigate the expression of VGCC in the pseudoglandular human and mouse lung, and their role in branching morphogenesis. Expression of L-type (CaV1.2 and CaV1.3), P/Q type (CaV2.1), N-type (CaV2.2), R-type (CaV2.3), and T-type (CaV3.2 and CaV3.3) VGCC was investigated in paraffin sections from week 9 human fetal lungs and E12.5 mouse embryos. Here we show, for the first time, that Cav1.2 and Cav1.3 are expressed in both the smooth muscle and epithelium of the developing human and mouse lung. Additionally, Cav2.3 was expressed in the lung epithelium of both species. Incubating E12.5 mouse lung rudiments in the presence of nifedipine doubled the amount of branching, an effect which was partly mimicked by the Cav2.3 inhibitor, SNX-482. Direct measurements of changes in epithelial cell membrane potential, using the voltage-sensitive fluorescent dye DiSBAC2(3), demonstrated that cyclic depolarisations occur within the developing epithelium and coincide with rhythmic occlusions of the lumen, driven by the naturally occurring airway peristalsis. We conclude that VGCC are expressed and functional in the fetal human and mouse lung, where they play a role in branching morphogenesis. Furthermore, rhythmic epithelial depolarisations evoked by airway peristalsis would allow for branching to match growth and distension within the developing lung.

The extracellular calcium-sensing receptor regulates human fetal lung development via CFTR

Optimal fetal lung growth requires anion-driven fluid secretion into the lumen of the developing organ. The fetus is hypercalcemic compared to the mother and here we show that in the developing human lung this hypercalcaemia acts on the extracellular calcium-sensing receptor, CaSR, to promote fluid-driven lung expansion through activation of the cystic fibrosis transmembrane conductance regulator, CFTR. Several chloride channels including TMEM16, bestrophin, CFTR, CLCN2 and CLCA1, are also expressed in the developing human fetal lung at gestational stages when CaSR expression is maximal. Measurements of Cl−-driven fluid secretion in organ explant cultures show that pharmacological CaSR activation by calcimimetics stimulates lung fluid secretion through CFTR, an effect which in humans, but not mice, was also mimicked by fetal hypercalcemic conditions, demonstrating that the physiological relevance of such a mechanism appears to be species-specific. Calcimimetics promote CFTR opening by activating adenylate cyclase and we show that Ca2+-stimulated type I adenylate cyclase is expressed in the developing human lung. Together, these observations suggest that physiological fetal hypercalcemia, acting on the CaSR, promotes human fetal lung development via cAMP-dependent opening of CFTR. Disturbances in this process would be expected to permanently impact lung structure and might predispose to certain postnatal respiratory diseases.