CD93 interacts with the PDZ domain-containing adaptor protein GIPC: implications in the modulation of phagocytosis

Role of CD93 in Health and Disease

CD93 (also known as complement protein 1 q subcomponent receptor C1qR1 or C1qRp), is a transmembrane glycoprotein encoded by a gene located on 20p11.21 and composed of 652 amino acids. CD93 can be present in two forms: soluble (sCD93) and membrane-bound (CD93). CD93 is mainly expressed on endothelial cells, where it plays a key role in promoting angiogenesis both in physiology and disease, such as age-related macular degeneration and tumor angiogenesis. In fact, CD93 is highly expressed in tumor-associated vessels and its presence correlates with a poor prognosis, poor immunotherapy response, immune cell infiltration and high tumor, node and metastasis (TNM) stage in many cancer types. CD93 is also expressed in hematopoietic stem cells, cytotrophoblast cells, platelets and many immune cells, i.e., monocytes, neutrophils, B cells and natural killer (NK) cells. Accordingly, CD93 is involved in modulating important inflammatory-associated diseases including systemic sclerosis and neuroinflammation. Finally, CD93 plays a role in cardiovascular disease development and progression. In this article, we reviewed the current literature regarding the role of CD93 in modulating angiogenesis, inflammation and tumor growth in order to understand where this glycoprotein could be a potential therapeutic target and could modify the outcome of the abovementioned pathologies.

Interactions of the protein tyrosine phosphatase PTPN3 with viral and cellular partners through its PDZ domain: insights into structural determinants and phosphatase activity

The human protein tyrosine phosphatase non-receptor type 3 (PTPN3) is a phosphatase containing a PDZ (PSD-95/Dlg/ZO-1) domain that has been found to play both tumor-suppressive and tumor-promoting roles in various cancers, despite limited knowledge of its cellular partners and signaling functions. Notably, the high-risk genital human papillomavirus (HPV) types 16 and 18 and the hepatitis B virus (HBV) target the PDZ domain of PTPN3 through PDZ-binding motifs (PBMs) in their E6 and HBc proteins respectively. This study focuses on the interactions between the PTPN3 PDZ domain (PTPN3-PDZ) and PBMs of viral and cellular protein partners. We solved the X-ray structures of complexes between PTPN3-PDZ and PBMs of E6 of HPV18 and the tumor necrosis factor-alpha converting enzyme (TACE). We provide new insights into key structural determinants of PBM recognition by PTPN3 by screening the selectivity of PTPN3-PDZ recognition of PBMs, and by comparing the PDZome binding profiles of PTPN3-recognized PBMs and the interactome of PTPN3-PDZ. The PDZ domain of PTPN3 was known to auto-inhibit the protein's phosphatase activity. We discovered that the linker connecting the PDZ and phosphatase domains is involved in this inhibition, and that the binding of PBMs does not impact this catalytic regulation. Overall, the study sheds light on the interactions and structural determinants of PTPN3 with its cellular and viral partners, as well as on the inhibitory role of its PDZ domain on its phosphatase activity.

A proteomic-informed view of the changes induced by loss of cellular adherence: The example of mouse macrophages

Except cells circulating in the bloodstream, most cells in vertebrates are adherent. Studying the repercussions of adherence per se in cell physiology is thus very difficult to carry out, although it plays an important role in cancer biology, e.g. in the metastasis process. In order to study how adherence impacts major cell functions, we used a murine macrophage cell line. Opposite to the monocyte/macrophage system, where adherence is associated with the acquisition of differentiated functions, these cells can be grown in both adherent or suspension conditions without altering their differentiated functions (phagocytosis and inflammation signaling). We used a proteomic approach to cover a large panel of proteins potentially modified by the adherence status. Targeted experiments were carried out to validate the proteomic results, e.g. on metabolic enzymes, mitochondrial and cytoskeletal proteins. The mitochondrial activity was increased in non-adherent cells compared with adherent cells, without differences in glucose consumption. Concerning the cytoskeleton, a rearrangement of the actin organization (filopodia vs sub-cortical network) and of the microtubule network were observed between adherent and non-adherent cells. Taken together, these data show the mechanisms at play for the modification of the cytoskeleton and also modifications of the metabolic activity between adherent and non-adherent cells.

Metoclopramide treatment blocks CD93-signaling-mediated self-renewal of chronic myeloid leukemia stem cells

Self-renewal is a key characteristic of leukemia stem cells (LSCs) responsible for the development and maintenance of leukemia. In this study, we identify CD93 as an important regulator of self-renewal and proliferation of murine and human LSCs, but not hematopoietic stem cells (HSCs). The intracellular domain of CD93 promotes gene transcription via the transcriptional regulator SCY1-like pseudokinase 1 independently of ligation of the extracellular domain. In a drug library screen, we identify the anti-emetic agent metoclopramide as an efficient blocker of CD93 signaling. Metoclopramide treatment reduces murine and human LSCs in vitro and prolongs survival of chronic myeloid leukemia (CML) mice through downregulation of pathways related to stemness and proliferation in LSCs. Overall, these results identify CD93 signaling as an LSC-specific regulator of self-renewal and proliferation and a targetable pathway to eliminate LSCs in CML.

Membrane Diffusion Occurs by Continuous-Time Random Walk Sustained by Vesicular Trafficking

Diffusion in cellular membranes is regulated by processes that occur over a range of spatial and temporal scales. These processes include membrane fluidity, interprotein and interlipid interactions, interactions with membrane microdomains, interactions with the underlying cytoskeleton, and cellular processes that result in net membrane movement. The complex, non-Brownian diffusion that results from these processes has been difficult to characterize, and moreover, the impact of factors such as membrane recycling on membrane diffusion remains largely unexplored. We have used a careful statistical analysis of single-particle tracking data of the single-pass plasma membrane protein CD93 to show that the diffusion of this protein is well described by a continuous-time random walk in parallel with an aging process mediated by membrane corrals. The overall result is an evolution in the diffusion of CD93: proteins initially diffuse freely on the cell surface but over time become increasingly trapped within diffusion-limiting membrane corrals. Stable populations of freely diffusing and corralled CD93 are maintained by an endocytic/exocytic process in which corralled CD93 is selectively endocytosed, whereas freely diffusing CD93 is replenished by exocytosis of newly synthesized and recycled CD93. This trafficking not only maintained CD93 diffusivity but also maintained the heterogeneous distribution of CD93 in the plasma membrane. These results provide insight into the nature of the biological and biophysical processes that can lead to significantly non-Brownian diffusion of membrane proteins and demonstrate that ongoing membrane recycling is critical to maintaining steady-state diffusion and distribution of proteins in the plasma membrane.

C-type lectin domain group 14 proteins in vascular biology, cancer and inflammation

The C‐type lectin domain (CTLD) group 14 family of transmembrane glycoproteins consist of thrombomodulin, CD93, CLEC14A and CD248 (endosialin or tumour endothelial marker‐1). These cell surface proteins exhibit similar ectodomain architecture and yet mediate a diverse range of cellular functions, including but not restricted to angiogenesis, inflammation and cell adhesion. Thrombomodulin, CD93 and CLEC14A can be expressed by endothelial cells, whereas CD248 is expressed by vasculature associated pericytes, activated fibroblasts and tumour cells among other cell types. In this article, we review the current literature of these family members including their expression profiles, interacting partners, as well as established and speculated functions. We focus primarily on their roles in the vasculature and inflammation as well as their contributions to tumour immunology. The CTLD group 14 family shares several characteristic features including their ability to be proteolytically cleaved and engagement of some shared extracellular matrix ligands. Each family member has strong links to tumour development and in particular CD93, CLEC14A and CD248 have been proposed as attractive candidate targets for cancer therapy.

Soluble CD93 is an apoptotic cell opsonin recognized by αx β2

Efferocytosis is essential for homeostasis and prevention of the inflammatory and autoimmune diseases resulting from apoptotic cell lysis. CD93 is a transmembrane glycoprotein previously implicated in efferocytosis, with mutations in CD93 predisposing patients to efferocytosis-associated diseases. CD93 is a cell surface protein, which is proteolytically shed under inflammatory conditions, but it is unknown how CD93 mediates efferocytosis or whether its efferocytic activity is mediated by the soluble or membrane-bound form. Herein, using cell lines and human monocytes and macrophages, we demonstrate that soluble CD93 (sCD93) potently opsonizes apoptotic cells but not a broad range of microorganisms, whereas membrane-bound CD93 has no phagocytic, efferocytic, or tethering activity. Using mass spectrometry, we identified α_x β₂ as the receptor that recognizes sCD93, and via deletion mutagenesis determined that sCD93 binds to apoptotic cells via its C-type lectin-like domain and to α_x β₂ by its EGF-like repeats. The bridging of apoptotic cells to α_x β₂ markedly enhanced efferocytosis by macrophages and was abrogated by α_x β₂ knockdown. Combined, these data elucidate the mechanism by which CD93 regulates efferocytosis and identifies a previously unreported opsonin-receptor system utilized by phagocytes for the efferocytic clearance of apoptotic cells.

Complement Protein C1q Enhances Macrophage Foam Cell Survival and Efferocytosis

In the atherosclerotic lesion, macrophages ingest high levels of damaged modified low-density lipoproteins (LDLs), generating macrophage foam cells. Foam cells undergo apoptosis and, if not efficiently cleared by efferocytosis, can undergo secondary necrosis, leading to plaque instability and rupture. As a component of the innate immune complement cascade, C1q recognizes and opsonizes modified forms of LDL, such as oxidized or acetylated LDL, and promotes ingestion by macrophages in vitro. C1q was shown to be protective in an atherosclerosis model in vivo. Therefore, this study aimed to investigate whether ingestion of modified LDL in the presence of C1q alters macrophage foam cell survival or function. In an unbiased transcriptome analysis, C1q was shown to modulate expression of clusters of genes involved in cell death and apoptosis pathways in human monocyte-derived macrophages ingesting modified LDL; this was validated by quantitative PCR in human and murine macrophages. C1q downregulated levels and activity of active caspase-3 and PARP-1 in human and mouse macrophages during ingestion of modified LDL. This led to a measurable increase in survival and decrease in cell death, as measured by alamarBlue and propidium iodide assays, respectively. C1q opsonization also increased phagocytosis and efferocytosis in macrophage foam cells. These data suggest that C1q promotes macrophage survival during ingestion of excess cholesterol, as well as improves foam cell efferocytic function. This may be important in slowing disease progression and provides insight into the protective role of C1q in early atherosclerosis.

Short-Lived Cages Restrict Protein Diffusion in the Plasma Membrane

The plasma membrane is a heterogeneous environment characterized by anomalous diffusion and the presence of microdomains that are molecularly distinct from the bulk membrane. Using single particle tracking of the C-type lectin CD93, we have identified for the first time the transient trapping of transmembrane proteins in cage-like microdomains which restrict protein diffusion. These cages are stabilized by actin-dependent confinement regions, but are separate structures with sizes and lifespans uncorrelated to those of the underlying actin corral. These membrane cages require cholesterol for their strength and stability, with cholesterol depletion decreasing both. Despite this, cages are much larger in size and are longer lived than lipid rafts, suggesting instead that cholesterol-dependent effects on membrane fluidity or molecular packing play a role in cage formation. This diffusional compartment in the plasma membrane has characteristics of both a diffusional barrier and a membrane microdomain, with a size and lifespan intermediate between short-lived microdomains such as lipid rafts and long-lasting diffusional barriers created by the actin cytoskeleton.

Soluble CD93 Is Involved in Metabolic Dysregulation but Does Not Influence Carotid Intima-Media Thickness

Type 2 diabetes and cardiovascular disease are complex disorders involving metabolic and inflammatory mechanisms. Here we investigated whether sCD93, a group XIV c-type lectin of the endosialin family, plays a role in metabolic dysregulation or carotid intima-media thickness (IMT). Although no association was observed between sCD93 and IMT, sCD93 levels were significantly lower in subjects with type 2 diabetes (n = 901, mean ± SD 156.6 ± 40.0 ng/mL) compared with subjects without diabetes (n = 2,470, 164.1 ± 44.8 ng/mL, P < 0.0001). Genetic variants associated with diabetes risk (DIAGRAM Consortium) did not influence sCD93 levels (individually or combined in a single nucleotide polymorphism score). In a prospective cohort, lower sCD93 levels preceded the development of diabetes. Consistent with this, a cd93-deficient mouse model (in addition to apoe deficiency) demonstrated no difference in atherosclerotic lesion development compared with apoe(-/-) cd93-sufficient littermates. However, cd93-deficient mice showed impaired glucose clearance and insulin sensitivity (compared with littermate controls) after eating a high-fat diet. The expression of cd93 was observed in pancreatic islets, and leaky vessels were apparent in cd93-deficient pancreases. We further demonstrated that stress-induced release of sCD93 is impaired by hyperglycemia. Therefore, we propose CD93 as an important component in glucometabolic regulation.

CD93 and GIPC expression and localization during central nervous system inflammation

CD93 and GAIP-interacting protein, C termius (GIPC) have been shown to interactively alter phagocytic processes of immune cells. CD93 and GIPC expression and localization during central nervous system inflammation have not yet been reported. In this study, we established a rat model of brain inflammation by lipopolysaccharide injection to the lateral ventricle. In the brain of rats with inflammation, western blots showed increased CD93 expression that decreased over time. GIPC expression was unaltered. Immunohistochemistry demonstrated extensive distribution of CD93 expression mainly in cell membranes in the cerebral cortex. After lipopolysaccharide stimulation, CD93 expression increased and then reduced, with distinct staining in the cytoplasm and nucleus. Double immunofluorescence staining in cerebral cortex of normal rats showed that CD93 and GIPC widely expressed in resting microglia and neurons. CD93 was mainly expressed in microglial and neuronal cell membranes, while GIPC was expressed in both cell membrane and cytoplasm. In the cerebral cortex at 9 hours after model establishment, CD93-immunoreactive signal diminished in microglial membrane, with cytoplasmic translocation and aggregation detected. GIPC localization was unaltered in neurons and microglia. These results are the first to demonstrate CD93 participation in pathophysiological processes of central nervous system inflammation.

Regulation of CD93 Cell Surface Expression by Protein Kinase C Isoenzymes

Human CD93, also known as complement protein 1, q subcomponent, receptor (C1qRp), is selectively expressed by cells with a myeloid lineage, endothelial cells, platelets, and microglia and was originally reported to be involved in the complement protein 1, q subcomponent (C1q)-mediated enhancement of phagocytosis. The intracellular molecular events responsible for the regulation of its expression on the cell surface, however, have not been determined. In this study, the effect of protein kinases in the regulation of CD93 expression on the cell surface of a human monocyte-like cell line (U937), a human NK-like cell line (KHYG-1), and a human umbilical vein endothelial cell line (HUV-EC-C) was investigated using four types of protein kinase inhibitors, the classical protein kinase C (cPKC) inhibitor Go6976, the novel PKC (nPKC) inhibitor Rottlerin, the protein kinase A (PKA) inhibitor H-89 and the protein tyrosine kinase (PTK) inhibitor herbimycin A at their optimum concentrations for 24 hr. CD93 expression was analyzed using flow cytometry and glutaraldehyde-fixed cellular enzyme-linked immunoassay (EIA) techniques utilizing a CD93 monoclonal antibody (mAb), mNI-11, that was originally established in our laboratory as a CD93 detection probe. The nPKC inhibitor Rottlerin strongly down-regulated CD93 expression on the U937 cells in a dose-dependent manner, whereas the other inhibitors had little or no effect. CD93 expression was down-regulated by Go6976, but not by Rottlerin, in the KHYG-1 cells and by both Rottlerin and Go6976 in the HUV-EC-C cells. The PKC stimulator, phorbol myristate acetate (PMA), strongly up-regulated CD93 expression on the cell surface of all three cell-lines and induced interleukin-8 (IL-8) production by the U937 cells and interferon-gamma (IFN-gamma) production by the KHYG-1 cells. In addition, both Go6976 and Rottlerin inhibited the up-regulation of CD93 expression induced by PMA and IL-8 or IFN-gamma production in the respective cell-lines. Whereas recombinant tumor necrosis factor-alpha (rTNF-alpha) slightly up-regulated CD93 expression on the U937 cells, recombinant interleukin-1beta (rIL-1beta), recombinant interleukin-2 (rIL-2), recombinant interferon-gamma (rIFN-gamma) and lipopolysaccharide (LPS) had no effect. Taken together, these findings indicate that the regulation of CD93 expression on these cells involves the PKC isoenzymes.

Myosin VI and Associated Proteins Are Expressed in Human Macrophages but Do Not Play a Role in Foam Cell Formation in THP-1 Cells

Myosin VI (Myo6) functions in endocytosis in conjunction with binding partners including adaptor protein (AP)-2, disabled 2 (Dab2), and GAIP interacting protein C terminus 1 (GIPC1). This study aimed to investigate the expression and function of Myo6 in macrophages and its possible role in the endocytosis of lipoproteins during the induction of foam cell formation. Expression of Myo6, AP-2 (α2 subunit), and Dab2 in THP-1 macrophages and primary human monocyte-derived macrophages was demonstrated at the mRNA and protein level, but GIPC1 was only detected at the mRNA level. Immunofluorescence showed that Myo6 was distributed similarly to F-actin in both macrophage types. AP-2α2 was found to have a similar subcellular distribution to Myo6 and Dab2 in THP-1 cells. Myo6 was located within membrane ruffles and protrusions of the plasma membrane. These results suggest that in macrophages Myo6 is required for several functions including cell adhesion, cell progression, and macropinocytosis. Low-density lipoprotein (LDL) and oxidised LDL (oxLDL) decreased Myo6 and GIPC1 mRNA expression in THP-1 cells, but uptake of the fluorescence-labelled lipoproteins was unaffected by knockdown of the expression of Myo6 or associated proteins with siRNA. Our findings, therefore, do not support the idea that Myo6 plays a major role in foam cell formation.

Functional proteomics, human genetics and cancer biology of GIPC family members

GIPC1, GIPC2 and GIPC3 consist of GIPC homology 1 (GH1) domain, PDZ domain and GH2 domain. The regions around the GH1 and GH2 domains of GIPC1 are involved in dimerization and interaction with myosin VI (MYO6), respectively. The PDZ domain of GIPC1 is involved in interactions with transmembrane proteins [IGF1R, NTRK1, ADRB1, DRD2, TGFβR3 (transforming growth factorβ receptor type III), SDC4, SEMA4C, LRP1, NRP1, GLUT1, integrin α5 and VANGL2], cytosolic signaling regulators (APPL1 and RGS19) and viral proteins (HBc and HPV-18 E6). GIPC1 is an adaptor protein with dimerizing ability that loads PDZ ligands as cargoes for MYO6-dependent endosomal trafficking. GIPC1 is required for cell-surface expression of IGF1R and TGFβR3. GIPC1 is also required for integrin recycling during cell migration, angiogenesis and cytokinesis. On early endosomes, GIPC1 assembles receptor tyrosine kinases (RTKs) and APPL1 for activation of PI3K-AKT signaling, and G protein-coupled receptors (GPCRs) and RGS19 for attenuation of inhibitory Gα signaling. GIPC1 upregulation in breast, ovarian and pancreatic cancers promotes tumor proliferation and invasion, whereas GIPC1 downregulation in cervical cancer with human papillomavirus type 18 infection leads to resistance to cytostatic transforming growth factorβ signaling. GIPC2 is downregulated in acute lymphocytic leukemia owing to epigenetic silencing, while Gipc2 is upregulated in estrogen-induced mammary tumors. Somatic mutations of GIPC2 occur in malignant melanoma, and colorectal and ovarian cancers. Germ-line mutations of the GIPC3 or MYO6 gene cause nonsyndromic hearing loss. As GIPC proteins are involved in trafficking, signaling and recycling of RTKs, GPCRs, integrins and other transmembrane proteins, dysregulation of GIPCs results in human pathologies, such as cancer and hereditary deafness.

Molecular characterization and ligand binding specificity of the PDZ domain-containing protein GIPC3 from Schistosoma japonicum

Background

Schistosomiasis is a serious global health problem that afflicts more than 230 million people in 77 countries. Long-term mass treatments with the only available drug, praziquantel, have caused growing concerns about drug resistance. PSD-95/Dlg/ZO-1 (PDZ) domain-containing proteins are recognized as potential targets for the next generation of drug development. However, the PDZ domain-containing protein family in parasites has largely been unexplored.

Methods

We present the molecular characteristics of a PDZ domain-containing protein, GIPC3, from Schistosoma japonicum (SjGIPC3) according to bioinformatics analysis and experimental approaches. The ligand binding specificity of the PDZ domain of SjGIPC3 was confirmed by screening an arbitrary peptide library in yeast two-hybrid (Y2H) assays. The native ligand candidates were predicted by Tailfit software based on the C-terminal binding specificity, and further validated by Y2H assays.

Results

SjGIPC3 is a single PDZ domain-containing protein comprised of 328 amino acid residues. Structural prediction revealed that a conserved PDZ domain was presented in the middle region of the protein. Phylogenetic analysis revealed that SjGIPC3 and other trematode orthologues clustered into a well-defined cluster but were distinguishable from those of other phyla. Transcriptional analysis by quantitative RT-PCR revealed that the SjGIPC3 gene was relatively highly expressed in the stages within the host, especially in male adult worms. By using Y2H assays to screen an arbitrary peptide library, we confirmed the C-terminal binding specificity of the SjGIPC3-PDZ domain, which could be deduced as a consensus sequence, -[SDEC]-[STIL]-[HSNQDE]-[VIL]*. Furthermore, six proteins were predicted to be native ligand candidates of SjGIPC3 based on the C-terminal binding properties and other biological information; four of these were confirmed to be potential ligands using the Y2H system.

Conclusions

In this study, we first characterized a PDZ domain-containing protein GIPC3 in S. japonicum. The SjGIPC3-PDZ domain is able to bind both type I and II ligand C-terminal motifs. The identification of native ligand will help reveal the potential biological function of SjGIPC3. These data will facilitate the identification of novel drug targets against S. japonicum infections.

Serine protease autotransporters from Shigella flexneri and pathogenic Escherichia coli target a broad range of leukocyte glycoproteins

The serine protease autotransporters of Enterobacteriaceae (SPATEs) are secreted by pathogenic Gram-negative bacteria through the autotransporter pathway. We previously classified SPATE proteins into two classes: cytotoxic (class 1) and noncytotoxic (class 2). Here, we show that Pic, a class 2 SPATE protein produced by Shigella flexneri 2a, uropathogenic and enteroaggregative Escherichia coli strains, targets a broad range of human leukocyte adhesion proteins. Substrate specificity was restricted to glycoproteins rich in O-linked glycans, including CD43, CD44, CD45, CD93, CD162 (PSGL-1; P-selectin glycoprotein ligand 1), and the surface-attached chemokine fractalkine, all implicated in leukocyte trafficking, migration, and inflammation. N-terminal sequencing of proteolytic products revealed Pic (protease involved in colonization) cleavage sites to occur before Thr or Ser residues. The purified carbohydrate sLewis-X implied in inflammation and malignancy inhibited cleavage of PSGL-1 by Pic. Exposure of human leukocytes to purified Pic resulted in polymorphonuclear cell activation, but impaired chemotaxis and transmigration; Pic-treated T cells underwent programmed cell death. We also show that the Pic-related protease Tsh/Hbp, implicated in extraintestinal infections, exhibited a spectrum of substrates similar to those cleaved by Pic. In the guinea pig keratoconjunctivitis model, a Shigella pic mutant induced greater inflammation than its parent strain. We suggest that the class-2 SPATEs represent unique immune-modulating bacterial virulence factors.

CD248 and its cytoplasmic domain: a therapeutic target for arthritis

OBJECTIVE

CD248 is a transmembrane glycoprotein expressed on the surface of activated perivascular and fibroblast-like cells. This study was undertaken to explore the function of CD248 and its cytoplasmic domain in arthritis.

METHODS

Synovial tissue biopsy samples from healthy controls, from patients with psoriatic arthritis (PsA), and from patients with rheumatoid arthritis (RA) were stained for CD248. Transgenic mice that were CD248-deficient (CD248-knockout [CD248(KO/KO) ]) or mice with CD248 lacking the cytoplasmic domain (CD248(CyD/CyD) ) were generated. Collagen antibody-induced arthritis (CAIA) was induced in these mice and in corresponding wild-type (WT) mice as controls. Clinical signs and histologic features of arthritis were evaluated. Cytokine levels were determined by enzyme-linked immunosorbent assay, and the number of infiltrating inflammatory cells was quantified by immunohistochemistry. In vitro studies were performed with fibroblasts from CD248-transgenic mouse embryos to explain the observed effects on inflammation.

RESULTS

Immunostaining of synovium from patients with PsA and patients with RA and that from mice after the induction of CAIA revealed strong CD248 expression in perivascular and fibroblast-like stromal cells. CD248(KO/KO) and CD248(CyD/CyD) mice had less severe arthritis, with lower plasma levels of proinflammatory cytokines, as compared with WT controls. Moreover, the joints of these mice had less synovial hyperplasia, reduced accumulation of inflammatory cells, and less articular cartilage and bone damage. Tumor necrosis factor α-induced monocyte adhesion to CD248(CyD/CyD) fibroblasts was impaired. CD248(CyD/CyD) fibroblasts exhibited reduced expression of hypoxia-inducible factor 1α, placental growth factor, vascular endothelial growth factor, and matrix metalloproteinase 9 activity in response to transforming growth factor β.

CONCLUSION

CD248 contributes to synovial hyperplasia and leukocyte accumulation in inflammatory arthritis, the effects of which are mediated partly via its cytoplasmic domain. CD248 is therefore a potential new target in the treatment of arthritis.

Interactions between GIPC-APPL and GIPC-TRP1 regulate melanosomal protein trafficking and melanogenesis in human melanocytes

By virtue of the presence of multiple protein-protein interaction and signaling domains, PDZ proteins play important roles in assembling protein complexes that participate in diverse cell biological processes. GIPC is a versatile PDZ protein that binds a variety of target proteins in different cell types. In previous studies we showed that, in epidermal melanocytes, GIPC interacts with newly synthesized melanosomal protein TRP1 in the Golgi region and proposed that this interaction may facilitate intracellular trafficking of TRP1. However, since GIPC contains a single PDZ domain and no other known protein interaction motifs, it is not known how GIPC-TRP1 interaction affects melanosome biogenesis and/or melanin pigmentation. Here, we show that in human primary melanocytes GIPC interacts with AKT-binding protein APPL (adaptor protein containing pleckstrin homology, leucine zipper and phosphotyrosine binding domains), which readily co-precipitates with newly synthesized TRP1. Knockdown of either GIPC or APPL inhibits melanogenesis by decreasing tyrosinase protein levels and enzyme activity. In melanocytes, APPL exists in a complex with GIPC and phospho-AKT. Inhibition of AKT phosphorylation using a PI3-kinase inhibitor abolishes this interaction and results in retardation TRP1 in the Golgi. These data suggest that interactions between TRP1-GIPC and GIPC-APPL-AKT provide a potential link between melanogenesis and PI3 kinase signaling.

dGIPC is required for the locomotive activity and longevity in Drosophila

To identify genes that function in the adult neural system, we screened pools of P element-mediated mutants and tested locomotor activity of homozygous flies. Of 1014 P element-mutagenized lines, 638 were homozygous viable. These lines were tested for climbing ability and lifespan. We isolated dGIPC, a Drosophila homolog of GIPC, that produced a 50% premature loss of locomotor activity and a 30% reduction in life span. We found that dGIPC is expressed in the central brain of adult flies, especially in glia and dopaminergic (DA) neurons. Inhibition of dGIPC expression in DA neurons significantly affected climbing ability and survival. In vertebrates, interactions between GIPC with dopamine receptors have been reported. Our findings, together with those obtained from vertebrate models, suggest that DrosophiladGIPC acts in the adult central nervous system and may be required to regulate the trafficking of dopamine receptors needed for proper functioning of dopaminergic neurons.

GIPC1 interacts with MyoGEF and promotes MDA-MB-231 breast cancer cell invasion

GIPC1/synectin, a single PDZ domain-containing protein, binds to numerous proteins and is involved in multiple biological processes, including cell migration. We reported previously that MyoGEF, a guanine nucleotide exchange factor, plays a role in regulating breast cancer cell polarization and invasion. Here, we identify GIPC1 as an interacting partner of MyoGEF. Both in vitro and in vivo binding assays show that the GIPC1 PDZ domain binds to the PDZ-binding motif at the C terminus of MyoGEF. Immunofluorescence analysis shows that GIPC1 and MyoGEF colocalize to the cell leading edge. Depletion of GIPC1 by RNAi in MDA-MB-231 cells causes cells to shift from a polarized to a rounded morphology. Matrigel invasion assays show that RNAi-mediated depletion of GIPC1 dramatically decreases MDA-MB-231 cell invasion. Notably, an anti-MyoGEF peptide antibody, whose epitope is located at the C terminus of MyoGEF, interferes with GIPC1-MyoGEF complex formation. Treatment of MDA-MB-231 cells with the anti-MyoGEF peptide antibody disrupts cell polarization and invasion. Thus, our results suggest that GIPC1-MyoGEF complex formation plays an important role in regulating MDA-MB-231 breast cancer cell polarization and invasion.

Flow cytometric identification of CD93 expression on naive T lymphocytes (CD4(+)CD45RA (+) cells) in human neonatal umbilical cord blood

Human CD93 has a molecular weight of about 100 kDa and is selectively expressed by myeloid cell lineages in peripheral blood (PB) mononuclear cells. Although CD93 was initially identified as a receptor for complement component 1, subcomponent q phagocytosis (C1qRp) involved in the C1q-mediated enhancement of the phagocytosis of various antigens, several recent studies have reported that CD93 is not a receptor for the C1q-mediated enhancement of phagocytosis. The expression patterns of CD93 have been previously investigated in PB mononuclear cells (lymphocytes, monocytes, and granulocytes) from adult PB and neonatal umbilical cord blood (UCB), and the expression of CD93 was not found on lymphocytes from either normal adult PB or neonatal UCB. However, the detection of CD93 expression in neonatal UCB using CD93 monoclonal antibodies (mAbs) that recognize different antigenic epitopes remains poorly understood. In this study, we examined the expression of CD93 on lymphocytes, monocytes, and granulocytes from neonatal UCB using four different types of CD93 mAb detection probes, mNI-11, R139, R3, and X-2, using flow cytometric and western blot analyses. We found that CD93, as defined using all four mAbs, was expressed on monocytes and granulocytes in PB mononuclear cells from adult PB and neonatal UCB. On the other hand, we observed for the first time that the expression of CD93 on lymphocytes in neonatal UCB can only be detected using the mNI-11 mAb, established in our laboratory, and not with commercially available CD93 mAbs (R139, R3, and X-2). However, CD93 expression on lymphocytes from normal adults was not detected using any of the four CD93 mAbs. Two-color flow cytometric analyses showed that the CD93 recognized by mNI-11 mAb was expressed on CD3(+) T lymphocytes (mainly CD4(+) helper T lymphocytes), but not on CD19(+) B lymphocytes or on CD8(+) suppressor/cytotoxic T lymphocytes from neonatal UCB. In addition, CD93 was expressed on CD45RA(+) (naive antigen) lymphocytes from neonatal UCB, but not on CD45RO(+) (memory antigen) lymphocytes from neonatal UCB or on CD45RA(+) and CD45RO(+) lymphocytes from normal adult PB. Three-color flow cytometric analysis showed that CD93 was co-expressed on naive T lymphocytes (CD4(+)CD45RA(+)) from neonatal UCB. In a western blot analysis, the CD93 mAb (mNI-11) immunoprecipitated at a molecular weight of 98 kDa, identified as a CD93 molecule, in the CD4(+)CD45RA(+) cells from neonatal UCB but not from adult PB, similar to the results in the human monocyte-like cell line U937 (human CD93-positive cells). Taken together, these results provide the first direct evidence of a novel/naive cell population (CD4(+)CD45RA(+)CD93(+)) in neonatal UCB that may have an important role in cell biology, transplantation, and immature/mature immune responses.

CD93/AA4.1: a novel regulator of inflammation in murine focal cerebral ischemia

The stem-cell marker CD93 (AA4.1/C1qRp) has been described as a potential complement C1q-receptor. Its exact molecular function, however, remains unknown. By using global expression profiling we showed that CD93-mRNA is highly induced after transient focal cerebral ischemia. CD93 protein is upregulated in endothelial cells, but also in selected macrophages and microglia. To elucidate the potential functional role of CD93 in postischemic brain damage, we used mice with a targeted deletion of the CD93 gene. After 30 min of occlusion of the middle cerebral artery and 3 d of reperfusion these mice displayed increased leukocyte infiltration into the brain, increased edema, and significantly larger infarct volumes (60.8 +/- 52.2 versus 23.9 +/- 16.6 mm(3)) when compared with wild-type (WT) mice. When the MCA was occluded for 60 min, after 2 d of reperfusion the CD93 knockout mice still showed more leukocytes in the brain, but the infarct volumes were not different from those seen in WT animals. To further explore CD93-dependent signaling pathways, we determined global transcription profiles and compared CD93-deficient and WT mice at various time points after induction of focal cerebral ischemia. We found a highly significant upregulation of the chemokine CCL21/Exodus-2 in untreated and treated CD93-deficient mice at all time points. Induction of CCL21 mRNA and protein was confirmed by PCR and immunohistochemistry. CCL21, which was formerly shown to be released by damaged neurons and to activate microglia, contributes to neurodegeneration. Thus, we speculate that CD93-neuroprotection is mediated via suppression of the neuroinflammatory response through downregulation of CCL21.

Macrophage response to apoptotic cells varies with the apoptotic trigger and is not altered by a deficiency in LRP expression

Rapid engulfment of apoptotic cells in the absence of inflammation is required for maintenance of normal tissue homeostasis. The low-density lipoprotein receptor-related protein-1 (LRP/CD91) is a receptor mediating interactions between macrophages and apoptotic cells, but recent reports have challenged the requirement of this surface protein in this process. To explore the role of LRP in the recognition of apoptotic cells, target cells were generated with two distinct inducers of apoptotic cell death, etoposide and actinomycin-D. Jurkat T cells rendered apoptotic with etoposide exposed phosphatidylserine (PtdSer) and triggered engulfment by murine bone marrow-derived macrophages (BMDM), however they failed to suppress lipopolysaccharide-driven inflammatory cytokine secretion or, correspondingly, NF kappaB-dependent or TNFalpha promoter-driven transcriptional activity in transfected RAW264.7 macrophages. In contrast, induction of apoptosis in either Jurkat cells or HeLa epithelial cells with actinomycin-D resulted in diminution of proinflammatory signaling from RAW264.7 cells and BMDM. Treatment of actinomycin-treated Jurkat cells with Q-VD-OPh, an irreversible inhibitor of caspase activity, blocked apoptosis, as assessed by the inhibition of PtdSer exposure; however, the cells maintained anti-inflammatory activity. Anti-inflammatory signaling mediated by actinomycin-treated cells was not affected by a macrophage-specific deletion in LRP. Moreover, the presence of LRP on macrophages did not alter the efficiency of engulfment of apoptotic cells in vitro or in vivo. These data demonstrate that the method of induction of apoptosis of target cells influences subsequent macrophage responsiveness, and that LRP is not required for engulfment of apoptotic cells regardless of the method of induction.

A role for connexin43 in macrophage phagocytosis and host survival after bacterial peritoneal infection

The pathways that lead to the internalization of pathogens via phagocytosis remain incompletely understood. We now demonstrate a previously unrecognized role for the gap junction protein connexin43 (Cx43) in the regulation of phagocytosis by macrophages and in the host response to bacterial infection of the peritoneal cavity. Primary and cultured macrophages were found to express Cx43, which localized to the phagosome upon the internalization of IgG-opsonized particles. The inhibition of Cx43 using small interfering RNA or by obtaining macrophages from Cx43 heterozygous or knockout mice resulted in significantly impaired phagocytosis, while transfection of Cx43 into Fc-receptor expressing HeLa cells, which do not express endogenous Cx43, conferred the ability of these cells to undergo phagocytosis. Infection of macrophages with adenoviruses expressing wild-type Cx43 restored phagocytic ability in macrophages from Cx43 heterozygous or deficient mice, while infection with viruses that expressed mutant Cx43 had no effect. In understanding the mechanisms involved, Cx43 was required for RhoA-dependent actin cup formation under adherent particles, and transfection with constitutively active RhoA restored a phagocytic phenotype after Cx43 inactivation. Remarkably, mortality was significantly increased in a mouse model of bacterial peritonitis after Cx43 inhibition and in Cx43 heterozygous mice compared with untreated and wild-type counterparts. These findings reveal a novel role for Cx43 in the regulation of phagocytosis and rearrangement of the F-actin cytoskeleton, and they implicate Cx43 in the regulation of the host response to microbial infection.

The Double-Edged Flower: Roles of Complement Protein C1q in Neurodegenerative Diseases

A role for the complement cascade in AD neuropathology was hypothesized over a decade ago, and the results of a significant number of in vitro studies are consistent with the involvement of this pathway in AD pathogenesis (reviewed in). Since C1q is colocalized with thioflavine-positive plaques and the C5b-9 complement membrane attack complex is detected in AD brain at autopsy, it is reasonable to hypothesize that complement activation has a role in the manifestation of AD either by its lytic capacity or as a trigger of glial infiltration and initiation of potentially damaging inflammation. The observed diminished glial activation and reduced loss of neuronal integrity in a murine model overexpressing mutant human APP but lacking the ability to activate the classical complement cascade provide the first direct evidence for a detrimental role of C1q, and presumably activation of the classical complement pathway in an animal model of AD. Research is now focused on generating mouse models that more closely mimic the human disease, so that the role of complement activation and inflammation on the behavioral/learning and memory dysfunction that occurs in this disease can be assessed. In addition, candidate therapies such as targeted inhibition of complement activation will need to be tested in these animal models as a step toward treatment of humans with the disease. However, it is important that the potential for a protective effect of C1q early on in disease progression should not be overlooked. Rather, strategies that enhance or mimic the protective effects of C1q as well as strategies that inhibit the detrimental processes should be fully investigated.

Kermit 2/XGIPC, an IGF1 receptor interacting protein, is required for IGF signaling in Xenopus eye development

GIPC is a PDZ-domain-containing protein identified in vertebrate and invertebrate organisms through its interaction with a variety of binding partners including many membrane proteins. Despite the multiple reports identifying GIPC, its endogenous function and the physiological significance of these interactions are much less studied. We have previously identified the Xenopus GIPC homolog kermit as a frizzled 3 interacting protein that is required for frizzled 3 induction of neural crest in ectodermal explants. We identified a second Xenopus GIPC homolog, named kermit 2 (also recently described as an IGF receptor interacting protein and named XGIPC). Despite its high amino acid similarity with kermit, kermit 2/XGIPC has a distinct function in Xenopus embryos. Loss-of-function analysis indicates that kermit 2/XGIPC is specifically required for Xenopus eye development. Kermit 2/XGIPC functions downstream of IGF in eye formation and is required for maintaining IGF-induced AKT activation. A constitutively active PI3 kinase partially rescues the Kermit 2/XGIPC loss-of-function phenotype. Our results provide the first in vivo loss of function analysis of GIPC in embryonic development and also indicate that kermit 2/XGIPC is a novel component of the IGF pathway, potentially functioning through modulation of the IGF1 receptor.

RGS17/RGSZ2 and the RZ/A family of regulators of G-protein signaling

Regulators of G-protein signaling (RGS proteins) comprise over 20 different proteins that have been classified into subfamilies on the basis of structural homology. The RZ/A family includes RGSZ2/RGS17 (the most recently discovered member of this family), GAIP/RGS19, RGSZ1/RGS20, and the RGSZ1 variant Ret-RGS. The RGS proteins are GTPase activating proteins (GAPs) that turn off G-proteins and thus negatively regulate the signaling of G-protein coupled receptors (GPCRs). In addition, some RZ/A family RGS proteins are able to modify signaling through interactions with adapter proteins (such as GIPC and GIPN). The RZ/A proteins have a simple structure that includes a conserved amino-terminal cysteine string motif, RGS box and short carboxyl-terminal, which confer GAP activity (RGS box) and the ability to undergo covalent modification and interact with other proteins (amino-terminal). This review focuses on RGS17 and its RZ/A sibling proteins and discusses the similarities and differences among these proteins in terms of their palmitoylation, phosphorylation, intracellular localization and interactions with GPCRs and adapter proteins. The specificity of these RGS protein for different Galpha proteins and receptors, and the consequences for signaling are discussed. The tissue and brain distribution, and the evolving understanding of the roles of this family of RGS proteins in receptor signaling and brain function are highlighted.

CD93 is rapidly shed from the surface of human myeloid cells and the soluble form is detected in human plasma

CD93 is a highly glycosylated transmembrane protein expressed on monocytes, neutrophils, endothelial cells, and stem cells. Antibodies directed at CD93 modulate phagocytosis, and CD93-deficient mice are defective in the clearance of apoptotic cells from the inflamed peritoneum. In this study we observe that CD93, expressed on human monocytes and neutrophils, is susceptible to phorbol dibutyrate-induced protein ectodomain shedding in a time- and dose-dependent manner. The soluble fragment found in culture supernatant retains the N-terminal carbohydrate recognition domain and the epidermal growth factor repeats after ectodomain cleavage. Importantly, a soluble form of the CD93 ectodomain was detected in human plasma, demonstrating that shedding is a physiologically relevant process. Inhibition of metalloproteinases with 1,10-phenanthroline inhibited shedding, but shedding was independent of TNF-alpha-converting enzyme (a disintegrin and metalloproteinase 17). Phorbol dibutyrate-induced CD93 shedding on monocytes was accompanied by decreased surface expression, whereas neutrophils displayed an increase in surface expression, suggesting that CD93 shed from the neutrophil surface was rapidly replaced by CD93 from intracellular stores. Cross-linking CD93 on human monocytes with immobilized anti-CD93 mAbs triggered shedding, as demonstrated by a decrease in cell-associated, full-length CD93 concomitant with an increase in CD93 intracellular domain-containing cleavage products. In addition, the inflammatory mediators, TNF-alpha and LPS, stimulated ectodomain cleavage of CD93 from monocytes. These data demonstrate that CD93 is susceptible to ectodomain shedding, identify multiple stimuli that trigger shedding, and identify both a soluble form of CD93 in human plasma and intracellular domain containing cleavage products within cells that may contribute to the physiologic role of CD93.

GLUT1CBP(TIP2/GIPC1) interactions with GLUT1 and myosin VI: evidence supporting an adapter function for GLUT1CBP

We identified a novel interaction between myosin VI and the GLUT1 transporter binding protein GLUT1CBP(GIPC1) and first proposed that as an adapter molecule it might function to couple vesicle-bound proteins to myosin VI movement. This study refines the model by identifying two myosin VI binding domains in the GIPC1 C terminus, assigning respective oligomerization and myosin VI binding functions to separate N- and C-terminal domains, and defining a central region in the myosin VI tail that binds GIPC1. Data further supporting the model demonstrate that 1) myosin VI and GIPC1 interactions do not require a mediating protein; 2) the myosin VI binding domain in GIPC1 is necessary for intracellular interactions of GIPC1 with myosin VI and recruitment of overexpressed myosin VI to membrane structures, but not for the association of GIPC1 with such structures; 3) GIPC1/myosin VI complexes coordinately move within cellular extensions of the cell in an actin-dependent and microtubule-independent manner; and 4) blocking either GIPC1 interactions with myosin VI or GLUT1 interactions with GIPC1 disrupts normal GLUT1 trafficking in polarized epithelial cells, leading to a reduction in the level of GLUT1 in the plasma membrane and concomitant accumulation in internal membrane structures.

Modulated interaction of the ERM protein, moesin, with CD93

CD93 is a cell-surface glycoprotein that has been shown to influence defence collagen-enhanced Fc-receptor or CR1-mediated phagocytosis of suboptimally opsonized targets in vitro, and CD93-deficient mice are defective in the clearance of apoptotic cells in vivo. To investigate the mechanism of CD93 modulation of phagocytic activity, GST fusion proteins containing the 47 amino acid intracellular domain (GST-Cyto), or various mutants of the intracellular domain of CD93, were constructed and used to identify intracellular CD93-binding molecules. The intracellular protein moesin, well characterized for its role in linking transmembrane proteins to the cytoskeleton and in cytoskeletal remodelling, bound to GST-Cyto when either cell lysates or recombinant moesin were used as a source of interacting molecules. An association of moesin with CD93 within intact cells was confirmed by co-capping moesin with CD93 in human monocytes. The moesin-binding site on CD93 mapped to the first four positively charged amino acids in the juxtamembrane region of the CD93 cytoplasmic tail. Interestingly, deletion of the last 11 amino acids from the C terminus of CD93 (GST-Cyto-C11) dramatically increased moesin binding to the cytoplasmic tail of CD93 in the cell lysate assay, but not when the binding of purified recombinant moesin was assessed. Furthermore, moesin binding to CD93 was enhanced by the addition of phosphatidylinositol 4,5-bisphosphate (PIP(2)). Taken together, these data suggest that the interaction of moesin with the CD93 cytoplasmic domain is modulated by binding of other intracellular molecules to the C11 region and implies that a PIP(2) signalling pathway is involved in CD93 function.