Signal Regulatory Proteins Research Articles

Crafting figures to describe cellular processes

Crafting figures to describe cellular processes

Antitumor Effects and Mechanisms of Snake Venom: A Systematic Review

Snake venom, as a natural biotoxin, is widely present in nature and can act in coordination with a variety of signal regulatory proteins, playing a significant role in the regulation of tumor cell proliferation, metastasis, invasion, and angiogenesis. Malignant tumors have long attracted the attention of the medical community as the leading cause of death in humans. In this paper, we focus on reviewing the progress of snake venom in the regulation of apoptosis, proliferation, metastasis, invasion, and angiogenesis of tumor cells in malignant tumors in order to clarify the mechanisms by which snake venom suppresses malignant tumors and to provide a reference for the study of malignant tumors.

Characterization of ELA026, a Clinical-Stage Monoclonal Antibody That Rapidly and Potently Depletes Myeloid Cells and T Lymphocytes

BACKGROUND: Hemophagocytic lymphohistiocytosis (HLH) is a rare and life-threatening syndrome of excessive immune activation. The disease is associated with a massive systemic inflammatory response requiring immediate and aggressive treatment. Secondary HLH (sHLH) is triggered by malignancy, autoimmune disease, or infection and believed to be caused by uncontrolled activation of immune cells, particularly monocytes/macrophages and CD8+ T cells. Currently there are no approved therapies for sHLH. Signal regulatory proteins (SIRPs) are cell surface receptors expressed on cells of the myeloid lineage and T cells. ELA026 is a fully human, monoclonal immunoglobulin G1 (IgG1) SIRP-directed antibody that binds to and marks for destruction SIRP-expressing cells. By reducing myeloid-derived antigen presenting cells and interferon gamma-producing CD8+ T cells, ELA026 has the potential to halt the initiation and progression of the inflammatory process in sHLH. A proof-of-concept Phase 1b trial for assessing ELA026 efficacy in sHLH is currently underway (ClinicalTrials.gov Identifier: NCT05416307). OBJECTIVES: To characterize the molecular pharmacology of ELA026 and to evaluate its pharmacokinetic (PK) profile and pharmacodynamic (PD) effects in non-human primates (NHP). RESULTS: ELA026 was identified from a hybridoma library generated from a traditional murine immunization campaign and selected for its binding to SIRPs. ELA026 does not inhibit binding of CD47 to SIRPs and does not interfere with SIRP signaling. In vitro studies with ELA026 have demonstrated that compared to humans, the cynomolgus monkey (Macaca fascicularis) is a pharmacologically-relevant species with comparable cellular SIRP expression, demonstrates similar ELA026 affinity to homologous SIRP isoforms, and has equivalent ELA026-induced antibody-dependent cellular cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP) potency of SIRP+ primary monocytes. ELA026 also induces ADCC of human SIRP+ T lymphocytes. In in vitro human cell systems, ELA026's effective concentration for inducing 90% maximal (EC90) ADCC (Fig. 1) and ADCP was ~0.1 µg/mL. In vivo, intravenous (IV) administration of ELA026 to NHP demonstrated reproducible and dose-dependent PD including a rapid (<24 hrs) and significant reduction of circulating monocyte, granulocyte, and T cell counts (60-80% depending on cell type; monocytes shown in Fig. 2). Duration of PD effect was observed to be dose-dependent, where higher doses lead to increased durability of cellular depletion. A threshold of ~0.1 µg/mL was determined to be the minimum concentration necessary to establish and maintain PD effect, consistent with the effective concentrations for ADCC and ADCP determined in vitro. Reconstitution of all affected cell types to pre-dose levels is observed within days of ELA026 washout (i.e. - dropping below ~0.1 µg/mL circulating plasma concentrations), suggesting that bone marrow hematopoiesis is unaffected by ELA026. Duration of effect was impacted by high levels of anti-drug antibodies in the cynomolgus monkey, though this did not affect interpretation of the ELA026 concentration-effect relationship. CONCLUSION: In vitro pharmacological studies demonstrate that ELA026 binds to SIRP proteins on the surface of primary human monocytes and T cells, inducing potent ADCC and ADCP. Consistent with these in vitro observations, in vivo administration of ELA026 in cynomolgus macaques shows a rapid and potent depletion of SIRP expressing monocytes and lymphocytes with a well-defined PK/PD relationship consistent with in vitro pharmacological results identifying ELA026 mechanisms of action. Reversibility of the PD effect was achieved following washout suggesting that ELA026 treatment is not associated with long-term immunosuppression observed with other similar cell depleting agents. These results support the clinical development of ELA026 for the treatment of myeloid and T lymphocyte driven disorders including sHLH. Disclosures: All authors are employees and stake holders of Electra Therapeutics, Inc. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

On the relationship between extant innate immune receptors and the evolutionary origins of jawed vertebrate adaptive immunity.

For over half a century, deciphering the origins of the genomic loci that form the jawed vertebrate adaptive immune response has been a major topic in comparative immunogenetics. Vertebrate adaptive immunity relies on an extensive and highly diverse repertoire of tandem arrays of variable (V), diversity (D), and joining (J) gene segments that recombine to produce different immunoglobulin (Ig) and T cell receptor (TCR) genes. The current consensus is that a recombination-activating gene (RAG)-like transposon invaded an exon of an ancient innate immune VJ-bearing receptor, giving rise to the extant diversity of Ig and TCR loci across jawed vertebrates. However, a model for the evolutionary relationships between extant non-recombining innate immune receptors and the V(D)J receptors of the jawed vertebrate adaptive immune system has only recently begun to come into focus. In this review, we provide an overview of non-recombining VJ genes, including CD8β, CD79b, natural cytotoxicity receptor 3 (NCR3/NKp30), putative remnants of an antigen receptor precursor (PRARPs), and the multigene family of signal-regulatory proteins (SIRPs), that play a wide range of roles in immune function. We then focus in detail on the VJ-containing novel immune-type receptors (NITRs) from ray-finned fishes, as recent work has indicated that these genes are at least 50 million years older than originally thought. We conclude by providing a conceptual model of the evolutionary origins and phylogenetic distribution of known VJ-containing innate immune receptors, highlighting opportunities for future comparative research that are empowered by this emerging evolutionary perspective.

Abstract 5615: Anti-human CD47 antibody Hu5F9 inhibits MC38 tumor growth in hCD47/hSIRPα double knock-in mice

Abstract Introduction: CD47 is broadly expressed on all normal cells or tissues, such as hematopoietic cells, T cells, B cells, monocytes, thymocytes, and platelets as well as other cell lineages, but has reduced expression on Rh erythrocytes. Signal regulatory proteins (SIRP) are a multigene family of immune receptors encoded by a cluster of genes. The most conserved member, SIRPα, binds to CD47 to engage the “don't eat me” signal and avoid macrophage phagocytosis. Notably, CD47 is often highly expressed on various tumors, and is an important immune checkpoint inhibiting macrophage phagocytosis. Therefore, blockade of CD47/SIRPα signalling by an anti-CD47 antibody offers a promising therapeutic approach by unleashing the phagocytosis activity of macrophages against tumor cells. It is well known that the antitumor efficacy of anti-human CD47 antibodies is largely macrophage-dependent in immunocompromised mice (NOD-SCID or NOG®/NSG™) engrafted with xenograft tumors (PDX or cell line derived xenografts). However, the therapeutic effects of anti-CD47 antibodies in immunocompetent mice engrafted with syngeneic tumors have also shown the involvement of cytosolic DNA sensors, dendritic cells, cytotoxic T cells, and type I/II interferons (IFNs). A preclinical animal model with a competent immune system, and humanized CD47/SIRPα is needed to fully recapitulate the mode of action of anti-CD47 therapeutic antibodies. Methods: We developed the human CD47 and SIRPα double knock-in mouse model (CD47/SIRPα HuGEMM™) via CRISPR/Cas9 targeting. We have also engineered mouse syngeneic MC38 cells to express human CD47 (hCD47-MC38 HuCELL™). We engrafted the hCD47-MC38 HuCELL into CD47/SIRPα HuGEMM mice to evaluate the efficacy of the anti-human CD47 antibody Hu5F9. Results: FACS analysis verified that the expression patterns of chimeric h/m CD47 and h/m SIRPα in HuGEMM mice are consistent with the mouse proteins in wild-type C57BL/6 mice. We have also confirmed that an anti-human CD47 antibody (Hu5F9-hIgG4) can bind to human CD47 expressed on hCD47-MC38 HuCELL tumor cells. Finally, we demonstrated robust antitumor effects of an anti-human CD47 antibody by treating the HuGEMM mice bearing hCD47-MC38 tumors with Hu5F9 monotherapy or in combination with anti-PD-1 antibody. Conclusions: Collectively, our hCD47/hSIRPα double knock-in humanized model, along with hCD47-MC38 HuCELL, provides a powerful preclinical platform to evaluate the mode of action, and in vivo efficacy of human-specific anti-CD47 antibodies. It may also serve as a model for in vivo evaluation of human SIRPα antibodies, as well as for proof-of-concept of combination therapies with other immune checkpoint inhibitors or immune modulators. Citation Format: Daniel Xianfei He, Chenpan Nie, Lei Zheng, Ruilin Sun, Annie Xiaoyu An, Henry Q. x. Li, Davy Xuesong Ouyang. Anti-human CD47 antibody Hu5F9 inhibits MC38 tumor growth in hCD47/hSIRPα double knock-in mice [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5615.

Common Genetic Variations Associated with the Persistence of Immunity following Childhood Immunization

Vaccines have revolutionized public health, preventing millions of deaths each year, particularly in childhood.Yet, there is considerable variability in the magnitude and persistence of vaccine-induced immunity. Maintenance of specific antibody is essential for continuity of vaccine-induced serological protection. We conducted a genome-wide association study into the persistence of immunity to three childhood vaccines: capsular group C meningococcal (MenC), Haemophilus influenzae type b, and tetanus toxoid (TT) vaccines. We detail associations between variants in a locus containing a family of signal-regulatory proteins and the persistence MenC immunity. We postulate a regulatory role for the lead SNP, with supporting epigenetic and expression quantitative trait loci data. Furthermore, we define associations between SNPs in the human leukocyte antigen (HLA) locus and the persistence of TT-specific immunity. Moreover, we describe four classical HLA alleles, HLA DRB1∗0301, HLA DQB1∗0201, HLA DQB1∗0602, and HLA DRB1∗1501, associated with TT-specific immunity, independent of the lead SNP association.

Network-Based Methods for Identifying Key Active Proteins in the Extracellular Electron Transfer Process in Shewanella oneidensis MR-1.

Shewanella oneidensis MR-1 can transfer electrons from the intracellular environment to the extracellular space of the cells to reduce the extracellular insoluble electron acceptors (Extracellular Electron Transfer, EET). Benefiting from this EET capability, Shewanella has been widely used in different areas, such as energy production, wastewater treatment, and bioremediation. Genome-wide proteomics data was used to determine the active proteins involved in activating the EET process. We identified 1012 proteins with decreased expression and 811 proteins with increased expression when the EET process changed from inactivation to activation. We then networked these proteins to construct the active protein networks, and identified the top 20 key active proteins by network centralization analysis, including metabolism- and energy-related proteins, signal and transcriptional regulatory proteins, translation-related proteins, and the EET-related proteins. We also constructed the integrated protein interaction and transcriptional regulatory networks for the active proteins, then found three exclusive active network motifs involved in activating the EET process—Bi-feedforward Loop, Regulatory Cascade with a Feedback, and Feedback with a Protein–Protein Interaction (PPI)—and identified the active proteins involved in these motifs. Both enrichment analysis and comparative analysis to the whole-genome data implicated the multiheme c-type cytochromes and multiple signal processing proteins involved in the process. Furthermore, the interactions of these motif-guided active proteins and the involved functional modules were discussed. Collectively, by using network-based methods, this work reported a proteome-wide search for the key active proteins that potentially activate the EET process.

Src/Syk/IRAK1-targeted anti-inflammatory action of Torreya nucifera butanol fraction in lipopolysaccharide-activated RAW264.7 cells

Ethnopharmacological relevanceSeed of Torreya nucifera (L.) Siebold & Zucc is used to treat several diseases in Asia. Reports document that T. nucifera has anti-cancer, anti-inflammatory, anti-oxidative activities. In spite of numerous findings on its pharmacological effects, the understanding of the molecular inhibitory mechanisms of the plant remains to be studied. Therefore, we aimed to explore in vitro anti-inflammatory mechanisms of ethyl acetate fraction (Tn-EE-BF) prepared from the seed of T. nucifera in LPS-stimulated macrophage inflammatory responses. Materials and methodsFor this purpose, we measured nitric oxide (NO) and prostaglandin E2 (PGE2) in LPS-stimulated macrophages. Additionally, using RT-PCR, luciferase reporter gene assay, immunoblotting analysis, and kinase assay, the levels of inflammatory genes, transcription factors, and inflammatory signal-regulatory proteins were investigated. Finally, the constituent of Tn-EE-BF was identified using HPLC. ResultsTn-EE-BF inhibits NO and PGE2 production and also blocks mRNA levels of inducible NO synthase (iNOS), tumor necrosis factor (TNF)-α, and cyclooxygenase (COX)-2 in a dose dependent manner. Tn-EE-BF reduces nuclear levels of the transcriptional factors NF-κB (p65) and AP-1 (c-Jun and FRA-1). Surprisingly, we found that Tn-EE-BF inhibits phosphorylation levels of Src and Syk in the NF-κB pathway, as well as, IRAK1 at the protein level, part of the AP-1 pathway. By kinase assay, we confirmed that Src, Syk, and IRAK1 are suppressed directly. HPLC analysis indicates that arctigenin, amentoflavone, and quercetin may be active components with anti-inflammatory activities. ConclusionTn-EE-BF exhibits anti-inflammatory activities by direct inhibition of Src/Syk/NF-κB and IRAK1/AP-1.

Activating Mutations Affecting the Dbl Homology Domain of SOS2 Cause Noonan Syndrome.

The RASopathies constitute a family of autosomal-dominant disorders whose major features include facial dysmorphism, cardiac defects, reduced postnatal growth, variable cognitive deficits, ectodermal and skeletal anomalies, and susceptibility to certain malignancies. Noonan syndrome (NS), the commonest RASopathy, is genetically heterogeneous and caused by functional dysregulation of signal transducers and regulatory proteins with roles in the RAS/extracellular signal-regulated kinase (ERK) signal transduction pathway. Mutations in known disease genes account for approximately 80% of affected individuals. Here, we report that missense mutations altering Son of Sevenless, Drosophila, homolog 2 (SOS2), which encodes a RAS guanine nucleotide exchange factor, occur in a small percentage of subjects with NS. Four missense mutations were identified in five unrelated sporadic cases and families transmitting NS. Disease-causing mutations affected three conserved residues located in the Dbl homology (DH) domain, of which two are directly involved in the intramolecular binding network maintaining SOS2 in its autoinhibited conformation. All mutations were found to promote enhanced signaling from RAS to ERK. Similar to NS-causing SOS1 mutations, the phenotype associated with SOS2 defects is characterized by normal development and growth, as well as marked ectodermal involvement. Unlike SOS1 mutations, however, those in SOS2 are restricted to the DH domain.

Regulatory Role of Endothelial CD47 during Leukocyte Transmigration

The vascular endothelium plays a critical role in inflammation and leukocyte recruitment by expression of adhesion molecules and leukocyte chemoattractants. Adhesion molecules bind leukocytes and transmit signals to the endothelial cells (EC) required for transendothelial migration (TEM). An important unanswered question is how ECs integrate the signals arising from leukocyte engagement of multiple adhesion molecules. CD47, also known as Integrin Associated Protein, is a broadly expressed glycoprotein and binds thrombospondin‐1 and leukocyte expressed Signal Regulatory Proteins (SIRPα and SIRPg). Besides these “in trans” interactions, CD47 also interacts “in cis” (in the same plasma membrane) with multiple integrins. We previously reported that endothelial CD47 plays a role in TEM and transmits intracellular signals upon engagement. Interestingly, CD47 signals phenocopy those occurring after engagement of ICAM‐1. We hypothesized CD47 interacts with ICAM‐1 to form a protein complex in EC plasma membrane and that this complex binds leukocytes and transmits intracellular signals during leukocyte TEM. Here we report CD47 associates with ICAM‐1 in EC by Fluorescence Lifetime Imaging Microscopy of Förster Resonance Energy Transfer (FLIM‐FRET). Furthermore, ICAM‐1 crosslinking‐induced signaling through pyk2 and src kinases was impaired in CD47‐/‐ mouse heart ECs (MHEC) as compared to WT MHEC. This was also demonstrated in HUVEC silenced for CD47. Understanding how CD47 interacts with different molecules will clarify its role in leukocyte recruitment and EC dependent mechanisms that regulate TEM.Supported by grants from National Research Fund of Luxembourg (FNR, Award # 4800397) and NIH HL‐36028.

Physiological and proteome analysis suggest critical roles for the photosynthetic system for high water-use efficiency under drought stress in Malus

Water use efficiency is an important indicator for plant adaptation and resistance to drought conditions. We previously found that under moderate drought stress, the water use efficiency of cv. ‘Qinguan’ apple (Malus domestica Borkh.) (tolerant to drought) was enhanced, while that of cv. ‘Naganofuji No. 2’ was not enhanced. In this research, we also found that instantaneous water-use efficiency of cv. ‘Qinguan’ was higher than that of cv. ‘Naganofuji No. 2’, mainly because of its higher net photosynthesis rate. To dissect the potential mechanisms underlying this phenomenon, we performed a comparative iTRAQ-based proteomics analysis with leaves of drought-treated cv. ‘Qinguan’ and ‘Naganofuji No. 2’. We identified 4078 proteins, of which 594 were differentially abundant between drought and well-watered leaves. The majority of increased proteins were predicted to be involved in photosynthetic pathway in drought treated cv. ‘Qinguan’ leaves, indicating that regulation of photosynthesis plays an important role for higher water use efficiency under drought stress. Enzyme activity assays were performed to validate the proteomics data. Our results suggested that the main regulatory mechanisms for high water use efficiency of cv. ‘Qinguan’ under moderate drought stress included the maintaining of Calvin cycle function by increasing key enzymes, stabilization of photosynthetic electron transfer and keeping reactive oxygen species at normal level by regulation of photosynthetic electron transfer chain, photorespiration and reactive oxygen species scavenging capability, thus prevented photoinhibition, reduced reactive oxygen species production and enhanced net photosynthesis rate. In addition, the response of signal regulatory proteins and abiotic stress-responsive proteins to drought also helped plants to cope with such stress.

β-Caryophyllene attenuates dextran sulfate sodium-induced colitis in mice via modulation of gene expression associated mainly with colon inflammation

We examined the modulatory activity of β-caryophyllene (CA) and gene expression in colitic colon tissues in a dextran sulfate sodium (DSS)-induced colitis model. Experimental colitis was induced by exposing male BALB/c mice to 5% DSS in drinking water for 7 days. CA (30 or 300mg/kg) was administered orally once a day together with DSS. CA administration attenuated the increases in the disease activity index, colon weight/length ratio, inflammation score, and myeloperoxidase activity in DSS-treated mice. Microarray analysis showed that CA administration regulated the expression in colon tissue of inflammation-related genes including those for cytokines and chemokines (Ccl2, Ccl7, Ccl11, Ifitm3, IL-1β, IL-28, Tnfrsf1b, Tnfrsf12a); acute-phase proteins (S100a8, Saa3, Hp); adhesion molecules (Cd14, Cd55, Cd68, Mmp3, Mmp10, Sema6b, Sema7a, Anax13); and signal regulatory proteins induced by DSS. CA significantly suppressed NF-κB activity, which mediates the expression of a different set of genes. These results suggest that CA attenuates DSS-induced colitis, possibly by modulating the expression of genes associated mainly with colon inflammation through inhibition of DSS-induced NF-κB activity.

CD47 contributes to neutrophil recruitment (146.2)

Leukocyte recruitment plays a key role in an inflammatory response. CD47 associates “in cis” with β1, β2 and β3 integrins. CD47 also interacts “in trans” with Signal Regulatory Proteins (SIRP‐α and ‐γ) and thrombospondin (TSP). CD47 plays an important but incompletely understood role in the innate and adaptive immune responses. Neutrophil recruitment in CD47KO mice is reduced in models of peritonitis, lung injury and dermal air pouch inflammation, but the exact contribution(s) of CD47 to neutrophil (PMN) extravasation is unclear. Here we report that in the TNF‐α‐stimulated cremaster muscle microcirculation, PMN rolling velocity is elevated compared to WT (17.8±1.5μm/s vs. 14.3±1.0μm/s, respectively; p&lt;0.05), whereas there was no difference between WT and CD47KO mice in CXCL1‐induced arrest. Also, PMN transendothelial migration (TEM) is reduced by 30% in CD47KO mice compared to WT mice (p&lt;0.05). In vitro flow chamber assays demonstrate reduced adhesion (30% reduction vs. WT; p&lt;0.05) and TEM (60% reduction vs. WT; p&lt;0.05) of CD47KO PMN compared to WT PMN across WT TNF‐activated murine endothelium. We have previously reported that CD47 physically interacts with β2 integrins and regulates integrin affinity on T cells (Azcutia et al., Mol Biol Cell 2013). These previous observations and our current results suggest that CD47 plays a role in PMN recruitment through the regulation of β2 integrin adhesive function.Grant Funding Source: Supported by NIH HL‐36028 and AHA fellowship 11POST7730055.

CD47 update: a multifaceted actor in the tumour microenvironment of potential therapeutic interest

CD47 is a ubiquitous 50 kDa five-spanning membrane receptor that belongs to the immunoglobulin superfamily. This receptor, also known as integrin-associated protein, mediates cell-to-cell communication by ligation to transmembrane signal-regulatory proteins SIRPα and SIRPγ and interacts with integrins. CD47 is also implicated in cell-extracellular matrix interactions via ligation with thrombospondins. Furthermore, CD47 is involved in many and diverse cellular processes, including apoptosis, proliferation, adhesion and migration. It also plays a key role in many immune and cardiovascular responses. Thus, this multifaceted receptor might be a central actor in the tumour microenvironment. Solid tumours are composed of not only cancer cells that actively proliferate but also other cell types including immune cells and fibroblasts that make up the tumour microenvironment. Tumour cell proliferation is strongly sustained by continuous sprouting of new vessels, which also represents a gate for metastasis. Moreover, infiltration of inflammatory cells is observed in most neoplasms. Much evidence has accumulated indicating that infiltrating leukocytes promote cancer progression. Given its ubiquitous expression on all the different cell types that compose the tumour microenvironment, targeting CD47 could represent an original therapeutic strategy in the field of oncology. We present a current overview of the biological effects associated with CD47 on cancer cells and stromal cells.

Dynamics of heat shock protein 90 C-terminal dimerization is an important part of its conformational cycle

The molecular chaperone heat shock protein 90 (Hsp90) is an important and abundant protein in eukaryotic cells, essential for the activation of a large set of signal transduction and regulatory proteins. During the functional cycle, the Hsp90 dimer performs large conformational rearrangements. The transient N-terminal dimerization of Hsp90 has been extensively investigated, under the assumption that the C-terminal interface is stably dimerized. Using a fluorescence-based single molecule assay and Hsp90 dimers caged in lipid vesicles, we were able to separately observe and kinetically analyze N- and C-terminal dimerizations. Surprisingly, the C-terminal dimer opens and closes with fast kinetics. The occupancy of the unexpected C-terminal open conformation can be modulated by nucleotides bound to the N-terminal domain and by N-terminal deletion mutations, clearly showing a communication between the two terminal domains. Moreover our findings suggest that the C- and N-terminal dimerizations are anticorrelated. This changes our view on the conformational cycle of Hsp90 and shows the interaction of two dimerization domains.

Genome-wide comparison of FGFRL1 with structurally related surface receptors

FGFRL1 (FGFR5) is a member of the fibroblast growth factor receptor family, which lacks the intracellular tyrosine kinase domain required for signal transduction by trans-phosphorylation. Since it still contains a cytoplasmic domain of 100 amino acid residues with a peculiar histidine-rich element, it might be involved in an alternative pathway of downstream signaling. To get a clue about a possible mechanism, we compared the overall structure of FGFRL1 with all proteins from the UniProt databank. We found that the human genome encodes 42 structurally related proteins with a signal peptide, three Ig-like domains and a transmembrane domain. These proteins can be grouped in seven families, fibroblast growth factor receptors (FGFRs), Fc receptor-like proteins, IL-1 receptor-like proteins, killer cell Ig-like receptors (KIRs), nectin-like proteins, sialic acid binding lectins (SIGLECs) and signal regulatory proteins (SIRPs). The 7 families utilize four different strategies for signaling, namely a protein tyrosine kinase domain, a TIR (Toll/IL-1 receptor) domain, ITIM/ITAM motifs as well as carboxy-terminal peptides that interact with the PDZ domain of an adaptor protein. It remains to be determined whether FGFRL1 might also utilize one of these strategies for signaling.

Genome-wide comparison of FGFRL1 with structurally related surface receptors

FGFRL1 (FGFR5) is a member of the fibroblast growth factor receptor family, which lacks the intracellular tyrosine kinase domain required for signal transduction by trans-phosphorylation. Since it still contains a cytoplasmic domain of 100 amino acid residues with a peculiar histidine-rich element, it might be involved in an alternative pathway of downstream signaling. To get a clue about a possible mechanism, we compared the overall structure of FGFRL1 with all proteins from the UniProt databank. We found that the human genome encodes 42 structurally related proteins with a signal peptide, three Ig-like domains and a transmembrane domain. These proteins can be grouped in seven families, fibroblast growth factor receptors (FGFRs), Fc receptor-like proteins, IL-1 receptor-like proteins, killer cell Ig-like receptors (KIRs), nectin-like proteins, sialic acid binding lectins (SIGLECs) and signal regulatory proteins (SIRPs). The 7 families utilize four different strategies for signaling, namely a protein tyrosine kinase domain, a TIR (Toll/IL-1 receptor) domain, ITIM/ITAM motifs as well as carboxy-terminal peptides that interact with the PDZ domain of an adaptor protein. It remains to be determined whether FGFRL1 might also utilize one of these strategies for signaling.

Signal Regulatory Proteins (SIRPS) Are Secreted Presynaptic Organizing Molecules

Formation of chemical synapses requires exchange of organizing signals between the synaptic partners. Using synaptic vesicle aggregation in cultured neurons as a marker of presynaptic differentiation, we purified candidate presynaptic organizers from mouse brain. A major bioactive species was the extracellular domain of signal regulatory protein alpha (SIRP-alpha), a transmembrane immunoglobulin superfamily member concentrated at synapses. The extracellular domain of SIRP-alpha is cleaved and shed in a developmentally regulated manner. The presynaptic organizing activity of SIRP-alpha is mediated in part by CD47. SIRP-alpha homologues, SIRP-beta and -gamma also have synaptic vesicle clustering activity. The effects of SIRP-alpha are distinct from those of another presynaptic organizer, FGF22: the two proteins induced vesicle clusters of different sizes, differed in their ability to promote neurite branching, and acted through different receptors and signaling pathways. SIRP family proteins may act together with other organizing molecules to pattern synapses.

Paired Receptor Specificity Explained by Structures of Signal Regulatory Proteins Alone and Complexed with CD47

CD47 is a widely distributed cell-surface protein that acts a marker of self through interactions of myeloid and neural cells. We describe the high-resolution X-ray crystallographic structures of the immunoglobulin superfamily domain of CD47 alone and in complex with the N-terminal ligand-binding domain of signal regulatory protein alpha (SIRPalpha). The unusual and convoluted interacting face of CD47, comprising the N terminus and loops at the end of the domain, intercalates with the corresponding regions in SIRPalpha. We have also determined structures of the N-terminal domains of SIRPbeta, SIRPbeta(2), and SIRPgamma; proteins that are closely related to SIRPalpha but bind CD47 with negligible or reduced affinity. These results explain the specificity of CD47 for the SIRP family of paired receptors in atomic detail. Analysis of SIRPalpha polymorphisms suggests that these, as well as the activating SIRPs, may have evolved to counteract pathogen binding to the inhibitory SIRPalpha receptor.

Phagocytic Signaling: You Can Touch, but You Can't Eat

The ability of phagocytes to discriminate between viable/healthy and apoptotic/foreign/abnormal cells is of fundamental importance; a recent study provides new molecular insights into the function of CD47-SIRP alpha signaling in this discrimination.