Regulation of T-lymphocyte motility, adhesion and de-adhesion by a cell surface mechanism directed by low density lipoprotein receptor-related protein 1 and endogenous thrombospondin-1

Platelet-derived thrombospondin 1 promotes immune cell liver infiltration and exacerbates diet-induced steatohepatitis

Background & Aims

Recent studies have implicated platelets, particularly α-granules, in the development of non-alcoholic steatohepatitis (NASH). However, the specific mechanisms involved have yet to be determined. Notably, thrombospondin 1 (TSP1) is a major component of the platelet α-granules released during platelet activation. Hence, we aimed to determine the role of platelet-derived TSP1 in NASH.

Methods

Platelet-specific Tsp1 knockout mice (TSP1Δpf4) and their wild-type littermates (TSP1F/F) were used. NASH was induced by feeding the mice with a diet enriched in fat, sucrose, fructose, and cholesterol (AMLN diet). A human liver NASH organoid model was also employed.

Results

Although TSP1 deletion in platelets did not affect diet-induced steatosis, TSP1Δpf4 mice exhibited attenuated NASH and liver fibrosis, accompanied by improvements in plasma glucose and lipid homeostasis. Furthermore, TSP1Δpf4 mice showed reduced intrahepatic platelet accumulation, activation, and chemokine production, correlating with decreased immune cell infiltration into the liver. Consequently, this diminished proinflammatory signaling in the liver, thereby mitigating the progression of NAFLD. Moreover, in vitro data revealed that co-culturing TSP1-deficient platelets in a human liver NASH organoid model attenuated hepatic stellate cell activation and NASH progression. Additionally, TSP1-deficient platelets play a role in regulating brown fat endocrine function, specifically affecting Nrg4 (neuregulin 4) production. Crosstalk between brown fat and the liver may also influence the progression of NAFLD.

Conclusions

These data suggest that platelet α-granule-derived TSP1 is a significant contributor to diet-induced NASH and fibrosis, potentially serving as a new therapeutic target for this severe liver disease.

Impact and implications

Recent studies have implicated platelets, specifically α-granules, in the development of non-alcoholic steatohepatitis, yet the precise mechanisms remain unknown. In this study, through the utilization of a tissue-specific knockout mouse model and human 3D liver organoid, we demonstrated that platelet α-granule-derived TSP1 significantly contributes to diet-induced non-alcoholic steatohepatitis and fibrosis. This contribution is, in part, attributed to the regulation of intrahepatic immune cell infiltration and potential crosstalk between fat and the liver. These findings suggest that platelet-derived TSP1 may represent a novel therapeutic target in non-alcoholic fatty liver disease.

Forsythiaside A suppresses renal fibrosis and partial epithelial-mesenchymal transition by targeting THBS1 through the PI3K/AKT signaling pathway

Renal fibrosis is a key feature of chronic kidney disease (CKD) progression, whereas no proven effective anti-fibrotic treatments. Forsythiaside A (FTA), derived from Forsythia suspense, has been found to possess nephroprotective properties. However, there is limited research on its anti-fibrotic effects, and its mechanism of action remains unknown. This study aimed to investigate the suppressive effects of FTA on renal fibrosis and explore the underlying mechanisms. In vitro, we established a HK2 cell model induced by transforming growth factor β1 (TGF-β1), and in vivo, we used a mice model induced by unilateral ureteral obstruction (UUO). CCK-8 assay, qRT-PCR, Western blotting, immunofluorescence, flow cytometry, histological staining, immunohistochemistry, TUNEL assay, RNA transcriptome sequencing, and molecular docking were performed. The results showed that FTA (40 μM or 80 μM) treatment improved cell viability and suppressed TGF-β1-induced fibrotic changes and partial epithelial-mesenchymal transition (EMT). Furthermore, FTA treatment reversed the activation of the PI3K/AKT signaling pathway, and THBS1 was identified as the target gene. We found that THBS1 knockdown suppressed the activation of the PI3K/AKT signaling pathway and reduced the fibrosis and partial EMT-related protein level. Conversely, THBS1 overexpression activated the PI3K/AKT signaling pathway and exacerbated renal fibrosis and partial EMT. In vivo, mice were administered FTA (30 or 60 mg/kg) for 2 weeks, and the results demonstrated that FTA administration significantly mitigated tubular injury, tubulointerstitial fibrosis, partial EMT, and apoptosis. In conclusion, FTA inhibited renal fibrosis and partial EMT by targeting THBS1 and inhibiting activation of the PI3K/AKT pathway.

Emerging functions of thrombospondin-1 in immunity

Thrombospondin-1 is a secreted matricellular glycoprotein that modulates cell behavior by interacting with components of the extracellular matrix and with several cell surface receptors. Its presence in the extracellular matrix is induced by injuries that cause thrombospondin-1 release from platelets and conditions including hyperglycemia, ischemia, and aging that stimulate its expression by many cell types. Conversely, rapid receptor-mediated clearance of thrombospondin-1 from the extracellular space limits its sustained presence in the extracellular space and maintains sub-nanomolar physiological concentrations in blood plasma. Roles for thrombospondin-1 signaling, mediated by specific cellular receptors or by activation of latent TGFβ, have been defined in T and B lymphocytes, natural killer cells, macrophages, neutrophils, and dendritic cells. In addition to regulating physiological nitric oxide signaling and responses of cells to stress, studies in mice lacking thrombospondin-1 or its receptors have revealed important roles for thrombospondin-1 in regulating immune responses in infectious and autoimmune diseases and antitumor immunity.

Role of HIV-1 Tat Protein Interactions with Host Receptors in HIV Infection and Pathogenesis

Each time the virus starts a new round of expression/replication, even under effective antiretroviral therapy (ART), the transactivator of viral transcription Tat is one of the first HIV-1 protein to be produced, as it is strictly required for HIV replication and spreading. At this stage, most of the Tat protein exits infected cells, accumulates in the extracellular matrix and exerts profound effects on both the virus and neighbor cells, mostly of the innate and adaptive immune systems. Through these effects, extracellular Tat contributes to the acquisition of infection, spreading and progression to AIDS in untreated patients, or to non-AIDS co-morbidities in ART-treated individuals, who experience inflammation and immune activation despite virus suppression. Here, we review the role of extracellular Tat in both the virus life cycle and on cells of the innate and adaptive immune system, and we provide epidemiological and experimental evidence of the importance of targeting Tat to block residual HIV expression and replication. Finally, we briefly review vaccine studies showing that a therapeutic Tat vaccine intensifies ART, while its inclusion in a preventative vaccine may blunt escape from neutralizing antibodies and block early events in HIV acquisition.

Controlled extracellular proteolysis of thrombospondins

Limited proteolysis of thrombospondins is a powerful mechanism to ensure dynamic tuning of their activities in the extracellular space. Thrombospondins are multifunctional matricellular proteins composed of multiple domains, each with a specific pattern of interactions with cell receptors, matrix components and soluble factors (growth factors, cytokines and proteases), thus with different effects on cell behavior and responses to changes in the microenvironment. Therefore, the proteolytic degradation of thrombospondins has multiple functional consequences, reflecting the local release of active fragments and isolated domains, exposure or disruption of active sequences, altered protein location, and changes in the composition and function of TSP-based pericellular interaction networks. In this review current data from the literature and databases is employed to provide an overview of cleavage of mammalian thrombospondins by different proteases. The roles of the fragments generated in specific pathological settings, with particular focus on cancer and the tumor microenvironment, are discussed.

Morphology of Biomaterials Affect O-Glycosylation of HUVECs

Biomaterials have been widely used as substitutes for diseased tissue in surgery and have gained great success and attention. At present, the biocompatibility of biomaterials such as PET woven fabrics is often evaluated both in vitro and in vivo. However, the current experimental methods cannot reveal the relationship between material surfaces and cell adhesion, and few research works have focused on the mechanisms of how the surface morphology of biomaterials affects cell adhesion and proliferation. Thus, it is meaningful to find out how the altered surfaces could affect cell adhesion and growth. In this study, we employed Ar low-temperature plasma treatment technology to create nano-grooves on the warp yarn of PET woven fabrics and seeded human umbellar vein endothelial cells (HUVEC) on these fabrics. We then assessed the O-glycan and N-glycan profiles of the cells grown on different structures of the polyester woven fabrics. The result showed that the surface morphology of polyester woven fabrics could affect the O-glycan profile but not the N-glycan profile of cultured HUVEC. Taken together, the study describes the effects of the surface morphology of biomaterial on the biosynthesis of cellular glycans and may provide new insights into the design and manufacture of biomaterials used as blood vessels based on the expression profiles of O-glycans on cultured cells.

TSP1 promotes fibroblast proliferation and extracellular matrix deposition via the IL6/JAK2/STAT3 signalling pathway in keloids

Keloids are benign fibroproliferative diseases with abnormally proliferated bulges beyond the edge of the skin lesions, and they are characterized by uncontrolled fibroblast proliferation and excessive extracellular matrix deposition in the dermis. However, the definite mechanisms that increase fibroblast proliferation and collagen deposition in keloids remain unclear. Thrombospondin 1 (TSP1) has been suggested to play an important role in wound healing and fibrotic disorders, but its role in keloids is unknown. In this study, we aimed to clarify the specific role of TSP1 in keloids and explore the potential mechanism. Our results demonstrated that TSP1 was highly expressed in keloid lesions compared to normal skin. Knockdown of TSP1 in keloid fibroblasts decreased cell proliferation and collagen I deposition. Exogenous TSP1 treatment increased cell proliferation and collagen I deposition in normal fibroblasts. We further investigated the underlying mechanism and found that TSP1 promoted fibroblast proliferation and extracellular matrix deposition by upregulating the IL6/JAK2/STAT3 pathway. Moreover, we verified that TSP1 expression was positively correlated with IL6/STAT3 signalling activity in keloids. Taken together, our findings indicate that TSP1 promotes keloid development via the IL6/JAK2/STAT3 signalling pathway and blocking TSP1 may represent a potential strategy for keloid therapy.

ECM stiffness-tuned exosomes drive breast cancer motility through thrombospondin-1

Breast cancer progression features ECM stiffening due to excess deposition and crosslinking of collagen, which dramatically influence tumor behaviour and fate. The mechanisms by which extracellular matrix (ECM) stiffening drives breast cancer invasion is an area of active research. Here we demonstrate the role of exosomes in ECM stiffness triggered breast cancer invasiveness. Using stiffness tuneable hydrogel ECM scaffolds, we show that stiff ECMs promote exosome secretion in a YAP/TAZ pathway-dependent manner. Interestingly, blocking exosome synthesis and secretion by GW4869 abrogated stiffness regulated motility and contractility in breast cancer cells. Reciprocally, exogenous addition of ECM stiffness-tuned exosomes orchestrated a series of changes in cell morphology, adhesion, protrusion dynamics resulting in fostered cell motility and invasion. Proteomic analysis of exosomal lysates followed by overrepresentation analysis and interactome studies revealed enrichment of cell adhesion and cell migration proteins in exosomes from stiff ECM cultures compared to that of soft ones. Quantitative proteomics of exosomes combined with genomic analysis of human breast tumor tissues (TCGA database) identified thrombospondin-1 (THBS1) as a prospective regulator of stiffness-dependent cancer invasion. Knockdown studies confirmed that the pro-invasive effects of stiffness-tuned exosomes are fuelled by exosomal THBS1. We further demonstrated that exosomal THBS1 mediates these stiffness-induced effects by engaging matrix metalloproteinase and focal adhesion kinase. Our studies establish the pivotal role of exosomal communication in ECM stiffness dependent cell migration with exosomal THBS1 as a master regulator of cancer invasion, which can be further exploited as a potential theranostic for improved breast cancer management.

Exosomes derived from LPS-stimulated human thymic mesenchymal stromal cells enhance inflammation via thrombospondin-1

Inflammatory response mediated by immune cells is either directly or indirectly regulated by mesenchymal stromal cells (MSCs). Accumulating evidence suggests that thrombospondin-1 (TSP-1) is highly expressed in response to inflammation. In this work, we isolated and identified human thymic mesenchymal stromal cells (tMSCs) and detected the expression of TSP-1. We found that tMSCs expressed TSP-1 and Poly (I:C) or LPS treatment promoted the expression of TSP-1. Further, we isolated and identified exosomes originating from tMSCs (MEXs). Notably, exosomes derived from LPS-pretreated tMSCs (MEXs^LPS) promoted the polarization of macrophages to M1-like phenotype and IL-6, TNF-α secretion as well as the pro-inflammatory differentiation of CD4⁺T cells into Th17 cells. Upon silencing the expression of TSP-1 in tMSCs, the pro-inflammatory effects of MEXs^LPS were suppressed. Therefore, these findings uncovered TSP-1 as the principal factor in MEXs^LPS pro-inflammatory regulation.

The emerging roles of circular RNAs in vessel co-option and vasculogenic mimicry: clinical insights for anti-angiogenic therapy in cancers

Unexpected resistance to anti-angiogenic treatment prompted the investigation of non-angiogenic tumor processes. Vessel co-option (VC) and vasculogenic mimicry (VM) are recognized as primary non-angiogenic mechanisms. In VC, cancer cells utilize pre-existing blood vessels for support, whereas in VM, cancer cells channel and provide blood flow to rapidly growing tumors. Both processes have been implicated in the development of tumor and resistance to anti-angiogenic drugs in many tumor types. The morphology, but rare molecular alterations have been investigated in VC and VM. There is a pressing need to better understand the underlying cellular and molecular mechanisms. Here, we review the emerging circular RNA (circRNA)-mediated regulation of non-angiogenic processes, VC and VM.

Functions of Thrombospondin-1 in the Tumor Microenvironment

The identification of thrombospondin-1 as an angiogenesis inhibitor in 1990 prompted interest in its role in cancer biology and potential as a therapeutic target. Decreased thrombospondin-1 mRNA and protein expression are associated with progression in several cancers, while expression by nonmalignant cells in the tumor microenvironment and circulating levels in cancer patients can be elevated. THBS1 is not a tumor suppressor gene, but the regulation of its expression in malignant cells by oncogenes and tumor suppressor genes mediates some of their effects on carcinogenesis, tumor progression, and metastasis. In addition to regulating angiogenesis and perfusion of the tumor vasculature, thrombospondin-1 limits antitumor immunity by CD47-dependent regulation of innate and adaptive immune cells. Conversely, thrombospondin-1 is a component of particles released by immune cells that mediate tumor cell killing. Thrombospondin-1 differentially regulates the sensitivity of malignant and nonmalignant cells to genotoxic stress caused by radiotherapy and chemotherapy. The diverse activities of thrombospondin-1 to regulate autophagy, senescence, stem cell maintenance, extracellular vesicle function, and metabolic responses to ischemic and genotoxic stress are mediated by several cell surface receptors and by regulating the functions of several secreted proteins. This review highlights progress in understanding thrombospondin-1 functions in cancer and the challenges that remain in harnessing its therapeutic potential.

The circular RNA aplacirc_13267 upregulates duck granulosa cell apoptosis by the apla-miR-1-13/THBS1 signaling pathway

Follicle development is a key factor that determines the reproductive performance of poultry. The existing evidence suggests that circular RNAs (circRNAs) play an important role in a variety of biological processes, especially in posttranscriptional regulation, but the regulatory mechanism of circRNAs in duck follicle development has rarely been reported. To better explore the molecular mechanism of follicle development in ducks, we sequenced and analyzed the follicular circRNAs; 4,204 circRNAs were predicted in the duck follicles. Fourteen circRNAs were differentially expressed between the white follicles and yellow follicles. The results of our studies showed that aplacirc_013267 promoted cell apoptosis in duck GCs. Moreover, a bioinformatics prediction analysis demonstrated that aplacirc_013267 was involved in a circRNA-miRNA-mRNA coexpression network and was observed to sponge two follicle-related miRNAs by a luciferase activity assay. Furthermore, we found that overexpression of aplacirc_013267 significantly increased thrombospondin-1 (THBS1) expression and downregulated granulosa cell apoptosis. The mechanistic study showed that aplacirc_013267 directly binds to and inhibits apla-mir-1-13; then, aplacirc_013267 increases the expression of THBS1 and upregulates granulosa cell apoptosis. Taken together, our findings demonstrate that circRNAs have potential effects in duck ovarian follicles and that circRNAs may represent a new avenue to understand follicular development.

Impact of Novel Monoclonal Antibody Therapeutics on Blood Bank Pretransfusion Testing

Novel monoclonal antibody therapies are increasing in number and clinical significance as their role in oncologic formularies expands. Anti-CD38 and anti-CD47/SIRPα agents commonly interfere with pretransfusion compatibility testing. Anti-CD38 interference is mitigated by dithiothreitol, which disrupts CD38 antigen on reagent red cells; however, this modification limits rule-out of all clinically significant antibodies. Several anti-CD47 agents are in clinical trials and demonstrate wide variability in pretransfusion testing interference. Modifications to pretransfusion testing can limit interference by anti-CD47 agents. Rapid dissemination of knowledge of these monoclonal antibody agents to the broader transfusion medicine community is paramount for continued patient transfusion safety.

Low Density Receptor-Related Protein 1 Interactions With the Extracellular Matrix: More Than Meets the Eye

The extracellular matrix (ECM) is a biological substrate composed of collagens, proteoglycans and glycoproteins that ensures proper cell migration and adhesion and keeps the cell architecture intact. The regulation of the ECM composition is a vital process strictly controlled by, among others, proteases, growth factors and adhesion receptors. As it appears, ECM remodeling is also essential for proper neuronal and glial development and the establishment of adequate synaptic signaling. Hence, disturbances in ECM functioning are often present in neurodegenerative diseases like Alzheimer’s disease. Moreover, mutations in ECM molecules are found in some forms of epilepsy and malfunctioning of ECM-related genes and pathways can be seen in, for example, cancer or ischemic injury. Low density lipoprotein receptor-related protein 1 (Lrp1) is a member of the low density lipoprotein receptor family. Lrp1 is involved not only in ligand uptake, receptor mediated endocytosis and lipoprotein transport—functions shared by low density lipoprotein receptor family members—but also regulates cell surface protease activity, controls cellular entry and binding of toxins and viruses, protects against atherosclerosis and acts on many cell signaling pathways. Given the plethora of functions, it is not surprising that Lrp1 also impacts the ECM and is involved in its remodeling. This review focuses on the role of Lrp1 and some of its major ligands on ECM function. Specifically, interactions with two Lrp1 ligands, integrins and tissue plasminogen activator are described in more detail.

Multiscale simulation of the interaction of calreticulin-thrombospondin-1 complex with a model membrane microdomain

Cell surface calreticulin (CRT) binding to thrombospondin-1 (TSP1), regulates cell adhesion, migration, anoikis resistance, and collagen production. Due to the essential role of membrane microdomains in CRT-mediated focal adhesion disassembly, we previously studied the effect of raft-like bilayers on TSP1-CRT interactions with all-atom molecular dynamics (AAMD) simulations. However, the simulated systems of protein on the surface of the bilayer(s) in the explicit solvent are too large for long timescale AAMD simulations due to computational expense. In this study, we adopted a multiscale modeling approach of combining AAMD, coarse-grained molecule dynamics (CGMD), and reversed AAMD (REV AAMD) simulations to investigate the interactions of single CRT or of the TSP1-CRT complex with a membrane microdomain at microsecond timescale. Results showed that CRT conformational stabilization by binding of TSP1 in AAMD simulation was undetectable in CGMD simulation, but it was recovered in REV AAMD simulation. Similarly, interactions of the CRT N-domain and TSP1 with the membrane microdomain were lost in CGMD simulations but they were re-gained in the REV AAMD simulations. There was the higher coordination of the CRT P-domain in the TSP1-CRT complex with the lipid components of membrane microdomain compared to that of single CRT, which could directly affect the conformation of CRT and further mediate CRT recruitment of LDL receptor-related protein for signaling events. This study provides structural and molecular insights into TSP1-CRT interactions in a membrane microdomain environment and demonstrates the feasibility of using multiscale simulations to investigate the interactions between protein and membrane microdomains at a long timescale.

T-cell regulation through a basic suppressive mechanism targeting low-density lipoprotein receptor-related protein 1

Cell adhesion is generally considered to depend on positive regulation through ligation of integrins and cytokine receptors. However, here we show that T-cell adhesion, and notably also T-cell receptor (TCR) -induced activation, are subject to constant suppression through shedding of low-density lipoprotein receptor-related protein 1 (LRP1). The broad-spectrum metalloprotease inhibitor GM6001 abrogated shedding, so inducing prominent cell surface expression of LRP1 while enhancing TCR-induced activation and adhesion to β₁ and β₂ integrin ligands, hence arresting the cells. Integrin ligands also inhibited shedding but the effect was less potent than that of GM6001. Unlike GM6001, integrin ligands also induced cell surface expression of full-length thrombospondin-1 (TSP170) and TSP130, which associated with LRP1, and TSP110, which did not associate with LRP1. Cell surface expression of LRP1 and TSP130 were induced exclusively in adhering cells, expression of TSP110 preferentially in non-adhering cells and expression of TSP170 correlated with T-cell motility. The pro-adhesive chemokine CXCL12 also inhibited LRP1 shedding and induced surface expression of TSP170 and TSP130 while inhibiting TSP110. Exogenous TSP-1 and ligation of CD28 inhibited shedding although less effectively than GM6001, and the inhibition through CD28 was independent of TSP-1. Small interfering RNA silencing experiments confirmed involvement of LRP1 and TSP-1 in integrin-dependent adhesion and TCR-induced activation. Hence, the poor LRP1 expression in T cells depends on shedding. Integrin ligands and CXCL12 antagonize shedding through a TSP-1-dependent pathway and ligation of CD28 antagonizes shedding independent of TSP-1. The disappearance of LRP1 from the cell surface may provide basic immunosuppression at the T-cell level.

Secondary allergic T cell responses are regulated by dendritic cell-derived thrombospondin-1 in the setting of allergic eye disease

Allergic eye disease, as in most forms of atopy, ranges in severity among individuals from immediate hypersensitivity to a severe and debilitating chronic disease. Dendritic cells play a key role in stimulating pathogenic T cells in allergen re-exposure, or secondary responses. However, molecular cues by dendritic cells underpinning allergic T cell response levels and the impact that this control has on consequent severity of allergic disease are poorly understood. Here, we show that a deficiency in thrombospondin-1, a matricellular protein known to affect immune function, has subsequent effects on downstream T cell responses during allergy, as revealed in an established mouse model of allergic eye disease. More specifically, we demonstrate that a thrombospondin-1 deficiency specific to dendritic cells leads to heightened secondary T cell responses and consequent clinical disease. Interestingly, whereas thrombospondin-1-deficient dendritic cells augmented activity of allergen-primed T cells, this increase was not recapitulated with naïve T cells in vitro. The role of dendritic cell-derived thrombospondin-1 in regulating secondary allergic T cell responses was confirmed in vivo, as local transfer of thrombospondin-1-sufficient dendritic cells to the ocular mucosa of thrombospondin-1 null hosts prevented the development of augmented secondary T cell responses and heightened allergic eye disease clinical responses. Finally, we demonstrate that topical instillation of thrombospondin-1-derived peptide reduces T cell activity and clinical progression of allergic eye disease. Taken together, this study reveals an important modulatory role of dendritic cell-derived thrombospondin-1 on secondary allergic T cell responses and suggests the possible dysregulation of dendritic cell-derived thrombospondin-1 expression as a factor in allergic eye disease severity.

Antigen-induced regulation of T-cell motility, interaction with antigen-presenting cells and activation through endogenous thrombospondin-1 and its receptors

Antigen recognition reduces T-cell motility, and induces prolonged contact with antigen-presenting cells and activation through mechanisms that remain unclear. Here we show that the T-cell receptor (TCR) and CD28 regulate T-cell motility, contact with antigen-presenting cells and activation through endogenous thrombospondin-1 (TSP-1) and its receptors low-density lipoprotein receptor-related protein 1 (LRP1), calreticulin and CD47. Antigen stimulation induced a prominent up-regulation of TSP-1 expression, and transiently increased and subsequently decreased LRP1 expression whereas calreticulin was unaffected. This antigen-induced TSP-1/LRP1 response down-regulated a motogenic mechanism directed by LRP1-mediated processing of TSP-1 in cis within the same plasma membrane while promoting contact with antigen-presenting cells and activation through cis interaction of the C-terminal domain of TSP-1 with CD47 in response to N-terminal TSP-1 triggering by calreticulin. The antigen-induced TSP-1/LRP1 response maintained a reduced but significant motility level in activated cells. Blocking CD28 co-stimulation abrogated LRP1 and TSP-1 expression and motility. TCR/CD3 ligation alone enhanced TSP-1 expression whereas CD28 ligation alone enhanced LRP1 expression. Silencing of TSP-1 inhibited T-cell conjugation to antigen-presenting cells and T helper type 1 (Th1) and Th2 cytokine responses. The Th1 response enhanced motility and increased TSP-1 expression through interleukin-2, whereas the Th2 response weakened motility and reduced LRP1 expression through interleukin-4. Ligation of the TCR and CD28 therefore elicits a TSP-1/LRP1 response that stimulates prolonged contact with antigen-presenting cells and, although down-regulating motility, maintains a significant motility level to allow serial contacts and activation. Th1 and Th2 cytokine responses differentially regulate T-cell expression of TSP-1 and LRP1 and motility.

Thrombospondin-1 (TSP1) contributes to the development of vascular inflammation by regulating monocytic cell motility in mouse models of abdominal aortic aneurysm

RATIONALE

Histological examination of abdominal aortic aneurysm (AAA) tissues demonstrates extracellular matrix destruction and infiltration of inflammatory cells. Previous work with mouse models of AAA has shown that anti-inflammatory strategies can effectively attenuate aneurysm formation. Thrombospondin-1 is a matricellular protein involved in the maintenance of vascular structure and homeostasis through the regulation of biological functions, such as cell proliferation, apoptosis, and adhesion. Expression levels of thrombospondin-1 correlate with vascular disease conditions.

OBJECTIVE

To use thrombospondin-1-deficient (Thbs1(-/-)) mice to test the hypothesis that thrombospondin-1 contributes to pathogenesis of AAAs.

METHODS AND RESULTS

Mouse experimental AAA was induced through perivascular treatment with calcium phosphate, intraluminal perfusion with porcine elastase, or systemic administration of angiotensin II. Induction of AAA increased thrombospondin-1 expression in aortas of C57BL/6 or apoE-/- mice. Compared with Thbs1(+/+) mice, Thbs1(-/-) mice developed significantly smaller aortic expansion when subjected to AAA inductions, which was associated with diminished infiltration of macrophages. Thbs1(-/-) monocytic cells had reduced adhesion and migratory capacity in vitro compared with wild-type counterparts. Adoptive transfer of Thbs1(+/+) monocytic cells or bone marrow reconstitution rescued aneurysm development in Thbs1(-/-) mice.

CONCLUSIONS

Thrombospondin-1 expression plays a significant role in regulation of migration and adhesion of mononuclear cells, contributing to vascular inflammation during AAA development.

Molecular insight into the effect of lipid bilayer environments on thrombospondin-1 and calreticulin interactions

Thrombospondin-1 (TSP1) binding to cell surface calreticulin (CRT) stimulates the association of CRT with low-density lipoprotein (LDL) receptor-related protein (LRP1) to signal focal adhesion disassembly and engagement of cellular activities. A recent study demonstrated that membrane rafts are necessary for TSP1-mediated focal adhesion disassembly, but the molecular role of membrane rafts in mediating TSP1-CRT-LRP1 signaling is unknown. In this study, we investigated the effect of lipid bilayer environments on TSP1 and CRT interactions via atomically detailed molecular dynamics simulations. Results showed that the microscopic structural properties of lipid molecules and mesoscopic mechanical properties and electrostatic potential of the bilayer were significantly different between a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer and a raftlike lipid bilayer [a POPC/cholesterol (CHOL) raftlike lipid bilayer or a POPC/CHOL/sphingomyelin (SM) raftlike lipid bilayer], and the difference was enhanced by SM lipids in a raftlike lipid bilayer. These bilayer property differences affect the interactions of CRT with the bilayer, further influencing CRT conformation and TSP1-CRT interactions. A raftlike lipid bilayer stabilized CRT conformation as compared to a POPC bilayer environment. TSP1 binding to CRT resulted in a conformation for the CRT N-domain more "open" than that of the CRT P-domain in a raftlike lipid bilayer environment, which could facilitate binding of CRT to LRP1 to engage downstream signaling. The open conformational changes of CRT by binding to TSP1 in a raftlike lipid bilayer were enhanced by SM lipids in a lipid bilayer. The direct interactions of both the N- and P-domains of CRT with the bilayer contribute to the more open conformation of CRT in the TSP1-CRT complex on a raftlike lipid bilayer as compared to that on a POPC bilayer. The interactions of CRT or the TSP1-CRT complex with the lipid bilayer also caused CHOL molecules and/or lipids to be more coordinated and to aggregate into patchlike regions in the raftlike lipid bilayers. The lipid and CHOL molecule coordination and aggregation could in turn affect the interactions of CRT with the membrane raft, thereby altering TSP1-CRT interactions and CRT conformational changes that potentially regulate its interactions with LRP1. This study provides molecular insights into the role of lipid bilayer environments in TSP1-CRT interactions and in the CRT conformational changes that are predicted to facilitate binding of CRT to LRP1 to engage downstream signaling events.