Two different PDGF beta-receptor cohorts in human pericytes mediate distinct biological endpoints

Glomerulopathic Light Chain-Mesangial Cell Interactions: Sortilin-Related Receptor (SORL1) and Signaling

INTRODUCTION

Deciphering the intricacies of the interactions of glomerulopathic Ig light chains with mesangial cells is key to delineate signaling events responsible for the mesangial pathologic alterations that ensue.

METHODS

Human mesangial cells, caveolin 1 (CAV1), wild type (WT) ,and knockout (KO), were incubated with glomerulopathic light chains purified from the urine of patients with light chain-associated (AL) amyloidosis or light chain deposition disease. Associated signaling events induced by surface interactions of glomerulopathic light chains with caveolins and other membrane proteins, as well as the effect of epigallocatechin-3-gallate (EGCG) on the capacity of mesangial cells to intracellularly process AL light chains were investigated using a variety of techniques, including chemical crosslinking with mass spectroscopy, immunofluorescence, and ultrastructural immunolabeling.

RESULTS

Crosslinking experiments provide evidence suggesting that sortilin-related receptor (SORL1), a transmembrane sorting receptor that regulates cellular trafficking of proteins, is a component of the receptor on mesangial cells for glomerulopathic light chains. Colocalization of glomerulopathic light chains with SORL1 in caveolae and also in lysosomes when light chain internalization occurred, was documented using double immunofluorescence and immunogold labeling ultrastructural techniques. It was found that EGCG directly blocks c-Fos cytoplasmic to nuclei signal translocation after interactions of AL light chains with mesangial cells, resulting in a decrease in amyloid formation.

CONCLUSION

Our findings document for the first time a role for SORL1 linked to glomerular pathology and signaling events that take place when certain monoclonal light chains interact with mesangial cells. This finding may lead to novel therapies for treating renal injury caused by glomerulopathic light chains.

Mural cell dysfunction leads to altered cerebrovascular tau uptake following repetitive head trauma

A pathological characteristic of repetitive traumatic brain injury (TBI) is the deposition of hyperphosphorylated and aggregated tau species in the brain and increased levels of extracellular monomeric tau are believed to play a role in the pathogenesis of neurodegenerative tauopathies. The pathways by which extracellular tau is eliminated from the brain, however, remains elusive. The purpose of this study was to examine tau uptake by cerebrovascular cells and the effect of TBI on these processes. We found monomeric tau interacts with brain vascular mural cells (pericytes and smooth muscle cells) to a greater extent than other cerebrovascular cells, indicating mural cells may contribute to the elimination of extracellular tau, as previously described for other solutes such as beta-amyloid. Consistent with other neurodegenerative disorders, we observed a progressive decline in cerebrovascular mural cell markers up to 12 months post-injury in a mouse model of repetitive mild TBI (r-mTBI) and human TBI brain specimens, when compared to control. These changes appear to reflect mural cell degeneration and not cellular loss as no difference in the mural cell population was observed between r-mTBI and r-sham animals as determined through flow cytometry. Moreover, freshly isolated r-mTBI cerebrovessels showed reduced tau uptake at 6 and 12 months post-injury compared to r-sham animals, which may be the result of diminished cerebrovascular endocytosis, as caveolin-1 levels were significantly decreased in mouse r-mTBI and human TBI cerebrovessels compared to their respective controls. Further emphasizing the interaction between mural cells and tau, similar reductions in mural cell markers, tau uptake, and caveolin-1 were observed in cerebrovessels from transgenic mural cell-depleted animals. In conclusion, our studies indicate repeated injuries to the brain causes chronic mural cell degeneration, reducing the caveolar-mediated uptake of tau by these cells. Alterations in tau uptake by vascular mural cells may contribute to tau deposition in the brain following head trauma and could represent a novel therapeutic target for TBI or other neurodegenerative disorders.

Glucose withdrawal induces Endothelin 1 release with significant angiogenic effect from first trimester (FTM), but not term human umbilical cord perivascular cells (HUCPVC)

BACKGROUND

Perivascular cells (PVC) and their "progeny," mesenchymal stromal cells (MSC), have high therapeutic potential for ischemic diseases. While hypoxia can increase their angiogenic properties, the other aspect of ischemic conditions-glucose shortage-is deleterious for MSC and limits their therapeutic applicability. Regenerative cells in developing vascular tissues, however, can adapt to varying glucose environment and react in a tissue-protective manner. Placental development and fetal insulin production generate different glucose fluxes in early and late extraembryonic tissues. We hypothesized that FTM HUCPVC, which are isolated from a developing vascular tissue with varying glucose availability react to low-glucose conditions in a pro-angiogenic manner in vitro.

METHODS

Xeno-free (Human Platelet Lysate 2.5%) expanded FTM (n = 3) and term (n = 3) HUCPVC lines were cultured in low (2 mM) and regular (4 mM) glucose conditions. After 72 h, the expression (Next Generation Sequencing) and secretion (Proteome Profiler) of angiogenic factors and the functional angiogenic effect (rat aortic ring assay and Matrigel™ plug) of the conditioned media were quantified and statistically compared between all cultures.

RESULTS

Low-glucose conditions had a significant post-transcriptional inductive effect on FTM HUCPVC angiogenic factor secretion, resulting in significantly higher VEGFc and Endothelin 1 release in 3 days compared to term counterparts. Conditioned media from low-glucose FTM HUCPVC cultures had a significantly higher endothelial network enhancing effect compared to all other experimental groups both in vitro aortic ring assay and in subcutan Matrigel™ plugs. Endothelin 1 depletion of the low-glucose FTM HUCPVC conditioned media significantly diminished its angiogenic effect CONCLUSIONS: FTM HUCPVC isolated from an early extraembryonic tissue show significant pro-angiogenic paracrine reaction in low-glucose conditions at least in part through the excess release of Endothelin 1. This can be a substantial advantage in cell therapy applications for ischemic injuries.

Vitamin D Analogues Tacalcitol and Calcipotriol Inhibit Proliferation and Migration of T98G Human Glioblastoma Cells

The active form of vitamin D (1α,25-dihydroxyvitamin D) acts as a steroid hormone and binds to the vitamin D receptor. This receptor is expressed in most cell types including cells in the central nervous system (CNS). Vitamin D has several functions in the body including effects on brain development, neuroprotection and immunological regulation. It has been shown that vitamin D has antiproliferative activities in different cancer cell lines. Tacalcitol and calcipotriol are synthetic analogues of 1α,25-dihydroxyvitamin D with reduced effect on calcium metabolism. The aim of this study was to analyse the effects of tacalcitol and calcipotriol on cell viability, proliferation and migration in the human glioblastoma cell line T98G. Glioblastoma is the most lethal type of primary tumours in the CNS. Both analogues decreased cell viability and/or growth, dose-dependently, in concentrations between 1 nM and 10 μM. Manual counting indicated suppressive effects by the vitamin D analogues on proliferation. Treatment with tacalcitol strongly suppressed thymidine incorporation, indicating that the vitamin D analogues mainly inhibit proliferation. Also, effects on cell migration were measured with wound-healing assay. Both calcipotriol and tacalcitol reduced the migration rate of T98G cells compared to vehicle-treated cells. However, they had no effect on caspase-3 and -7 activities, suggesting that their mechanism of action does not involve induction of apoptosis. The current results indicate that the vitamin D analogues tacalcitol and calcipotriol strongly reduce proliferation and migration of human glioblastoma T98G cells, suggesting a potential role for this type of compounds in treatment of brain cancer.

Pharmacological activation of PPARγ inhibits hypoxia-induced proliferation through a caveolin-1-targeted and -dependent mechanism in PASMCs

Previously, we and others have demonstrated that activation of peroxisome proliferator-activated receptor γ (PPARγ) by specific pharmacological agonists inhibits the pathogenesis of chronic hypoxia-induced pulmonary hypertension (CHPH) by suppressing the proliferation and migration in distal pulmonary arterial smooth muscle cells (PASMCs). Moreover, these beneficial effects of PPARγ are mediated by targeting the intracellular calcium homeostasis and store-operated calcium channel (SOCC) proteins, including the main caveolae component caveolin-1. However, other than the caveolin-1 targeted mechanism, in this study, we further uncovered a caveolin-1 dependent mechanism within the activation of PPARγ by the specific agonist GW1929. First, effective knockdown of caveolin-1 by small-interfering RNA (siRNA) markedly abolished the upregulation of GW1929 on PPARγ expression at both mRNA and protein levels; Then, in HEK293T, which has previously been reported with low endogenous caveolin-1 expression, exogenous expression of caveolin-1 significantly enhanced the upregulation of GW1929 on PPARγ expression compared with nontransfection control. In addition, inhibition of PPARγ by either siRNA or pharmacological inhibitor T0070907 led to increased phosphorylation of cellular mitogen-activated protein kinases ERK1/2 and p38. In parallel, GW1929 dramatically decreased the expression of the proliferative regulators (cyclin D1 and PCNA), whereas it increased the apoptotic factors (p21, p53, and mdm2) in hypoxic PASMCs. Furthermore, these effects of GW1929 could be partially reversed by recovery of the drug treatment. In combination, PPARγ activation by GW1929 reversibly drove the cell toward an antiproliferative and proapoptotic phenotype in a caveolin-1-dependent and -targeted mechanism.

Interferon-γ blocks signalling through PDGFRβ in human brain pericytes

BACKGROUND

Neuroinflammation and blood-brain barrier (BBB) disruption are common features of many brain disorders, including Alzheimer's disease, epilepsy, and motor neuron disease. Inflammation is thought to be a driver of BBB breakdown, but the underlying mechanisms for this are unclear. Brain pericytes are critical cells for maintaining the BBB and are immunologically active. We sought to test the hypothesis that inflammation regulates the BBB by altering pericyte biology.

METHODS

We exposed primary adult human brain pericytes to chronic interferon-gamma (IFNγ) for 4 days and measured associated functional aspects of pericyte biology. Specifically, we examined the influence of inflammation on platelet-derived growth factor receptor-beta (PDGFRβ) expression and signalling, as well as pericyte proliferation and migration by qRT-PCR, immunocytochemistry, flow cytometry, and western blotting.

RESULTS

Chronic IFNγ treatment had marked effects on pericyte biology most notably through the PDGFRβ, by enhancing agonist (PDGF-BB)-induced receptor phosphorylation, internalization, and subsequent degradation. Functionally, chronic IFNγ prevented PDGF-BB-mediated pericyte proliferation and migration.

CONCLUSIONS

Because PDGFRβ is critical for pericyte function and its removal leads to BBB leakage, our results pinpoint a mechanism linking chronic brain inflammation to BBB dysfunction.

Ephrin-B2 controls PDGFRβ internalization and signaling

Ephrin-B2 is essential for supporting mural cells; namely, pericytes and vascular smooth muscle cells (VSMCs). Nakayama et al. find that ephrin-B2 controls platelet-derived growth factor receptor β (PDGFRβ) distribution in the VSMC plasma membrane, endocytosis, and signaling. VSMCs lacking ephrin-B2 exhibited a redistribution of PDGFRβ from caveolin-positive to clathrin-associated membrane fractions and enhanced PDGF-B-induced PDGFRβ internalization. Mice lacking ephrin-B2 in vascular smooth muscle developed vessel wall defects and aortic aneurysms. These results suggest that ephrin-B2 is an important regulator of PDGFRβ endocytosis in mural cells.

B-class ephrins, ligands for EphB receptor tyrosine kinases, are critical regulators of growth and patterning processes in many organs and species. In the endothelium of the developing vasculature, ephrin-B2 controls endothelial sprouting and proliferation, which has been linked to vascular endothelial growth factor (VEGF) receptor endocytosis and signaling. Ephrin-B2 also has essential roles in supporting mural cells (namely, pericytes and vascular smooth muscle cells [VSMCs]), but the underlying mechanism is not understood. Here, we show that ephrin-B2 controls platelet-derived growth factor receptor β (PDGFRβ) distribution in the VSMC plasma membrane, endocytosis, and signaling in a fashion that is highly distinct from its role in the endothelium. Absence of ephrin-B2 in cultured VSMCs led to the redistribution of PDGFRβ from caveolin-positive to clathrin-associated membrane fractions, enhanced PDGF-B-induced PDGFRβ internalization, and augmented downstream mitogen-activated protein (MAP) kinase and c-Jun N-terminal kinase (JNK) activation but impaired Tiam1–Rac1 signaling and proliferation. Accordingly, mutant mice lacking ephrin-B2 expression in vascular smooth muscle developed vessel wall defects and aortic aneurysms, which were associated with impaired Tiam1 expression and excessive activation of MAP kinase and JNK. Our results establish that ephrin-B2 is an important regulator of PDGFRβ endocytosis and thereby acts as a molecular switch controlling the downstream signaling activity of this receptor in mural cells.

Phenotypical differences in connective tissue cells emerging from microvascular pericytes in response to overexpression of PDGF-B and TGF-β1 in normal skin in vivo

Fibrosis is a deleterious consequence of chronic inflammation in a number of human pathologies ultimately leading to organ dysfunction and failure. Two growth factors that are important in blood vessel physiology and tissue fibrosis, platelet-derived growth factor (PDGF)-B and transforming growth factor (TGF)-β1, were investigated. Adenoviral vectors were used to induce transient overexpression of these growth factors in mouse skin. Changes in tissue structure and protein and mRNA expressions were investigated. Both PDGF-B and TGF-β1 could initiate but neither could sustain angiogenesis. Instead, vascular regression was observed. Overexpression of both TGF-β1 and PDGF-B led to a marked macrophage influx and an expansion of the connective tissue cell population. Over time, this effect was sustained in mice treated with TGF-β1, whereas it was partially reversible in mice treated with PDGF-B. On the basis of structure and expression of phenotypical markers, the emerging connective tissue cell population may originate from microvascular pericytes. TGF-β1 induced expansion of connective tissue cells with a myofibroblast phenotype, whereas PDGF-B induced a fibroblast phenotype negative for α-smooth muscle actin. TGF-β1 and PDGF-B overexpressions mediated distinct effects on mRNA transcript levels of fibrillar procollagens, their modifying enzymes, small leucin-rich repeat proteoglycans, and matricellular proteins affecting both the composition and the quantity of the extracellular matrix. This study offers new insight into the effects of PDGF-B and TGF-β1 on the vasculature and connective tissue in vivo.

Soluble FLT1 binds lipid microdomains in podocytes to control cell morphology and glomerular barrier function

Vascular endothelial growth factor and its receptors, FLK1/KDR and FLT1, are key regulators of angiogenesis. Unlike FLK1/KDR, the role of FLT1 has remained elusive. FLT1 is produced as soluble (sFLT1) and full-length isoforms. Here, we show that pericytes from multiple tissues produce sFLT1. To define the biologic role of sFLT1, we chose the glomerular microvasculature as a model system. Deletion of Flt1 from specialized glomerular pericytes, known as podocytes, causes reorganization of their cytoskeleton with massive proteinuria and kidney failure, characteristic features of nephrotic syndrome in humans. The kinase-deficient allele of Flt1 rescues this phenotype, demonstrating dispensability of the full-length isoform. Using cell imaging, proteomics, and lipidomics, we show that sFLT1 binds to the glycosphingolipid GM3 in lipid rafts on the surface of podocytes, promoting adhesion and rapid actin reorganization. sFLT1 also regulates pericyte function in vessels outside of the kidney. Our findings demonstrate an autocrine function for sFLT1 to control pericyte behavior.

Expression of VEGF receptors on endothelial cells in mouse skeletal muscle

VEGFR surface localization plays a critical role in converting extracellular VEGF signaling towards angiogenic outcomes, and the quantitative characterization of these parameters is critical for advancing computational models; however the levels of these receptors on blood vessels is currently unknown. Therefore our aim is to quantitatively determine the VEGFR localization on endothelial cells from mouse hindlimb skeletal muscles. We contextualize this VEGFR quantification through comparison to VEGFR-levels on cells in vitro. Using quantitative fluorescence we measure and compare the levels of VEGFR1 and VEGFR2 on endothelial cells isolated from C57BL/6 and BALB/c gastrocnemius and tibialis anterior hindlimb muscles. Fluorescence measurements are calibrated using beads with known numbers of phycoerythrin molecules. The data show a 2-fold higher VEGFR1 surface localization relative to VEGFR2 with 2,000-3,700 VEGFR1/endothelial cell and 1,300-2,000 VEGFR2/endothelial cell. We determine that endothelial cells from the highly glycolytic muscle, tibialis anterior, contain 30% higher number of VEGFR1 surface receptors than gastrocnemius; BALB/c mice display ~17% higher number of VEGFR1 than C57BL/6. When we compare these results to mouse fibroblasts in vitro, we observe high levels of VEGFR1 (35,800/cell) and very low levels of VEGFR2 (700/cell), while in human endothelial cells in vitro, we observe that the balance of VEGFRs is inverted, with higher levels VEGFR2 (5,800/cell) and lower levels of VEGFR1 (1,800/cell). Our studies also reveal significant cell-to-cell heterogeneity in receptor expression, and the quantification of these dissimilarities ex vivo for the first time provides insight into the balance of anti-angiogenic or modulatory (VEGFR1) and pro-angiogenic (VEGFR2) signaling.

Perivascular human endometrial mesenchymal stem cells express pathways relevant to self-renewal, lineage specification, and functional phenotype

Human endometrium regenerates on a cyclic basis from candidate stem/progenitors whose genetic programs are yet to be determined. A subpopulation of endometrial stromal cells, displaying key properties of mesenchymal stem cells (MSCs), has been characterized. The endometrial MSC (eMSC) is likely the precursor of the endometrial stromal fibroblast. The goal of this study was to determine the transcriptome and signaling pathways in the eMSC to understand its functional phenotype. Endometrial stromal cells from oocyte donors (n = 20) and patients undergoing benign gynecologic surgery (n = 7) were fluorescence-activated cell sorted into MCAM (CD146)(+)/PDGFRB(+) (eMSC), MCAM (CD146)(-)/PDGFRB(+) (fibroblast), and MCAM (CD146)(+)/PDGFRB(-) (endothelial) populations. The eMSC population contained clonogenic cells with a mesenchymal phenotype differentiating into adipocytes when cultured in adipogenic medium. Gene expression profiling using Affymetrix Human Gene 1.0 ST arrays revealed 762 and 1518 significantly differentially expressed genes in eMSCs vs. stromal fibroblasts and eMSCs vs. endothelial cells, respectively. By principal component and hierarchical clustering analyses, eMSCs clustered with fibroblasts and distinctly from endothelial cells. Endometrial MSCs expressed pericyte markers and were localized by immunofluorescence to the perivascular space of endometrial small vessels. Endometrial MSCs also expressed genes involved in angiogenesis/vasculogenesis, steroid hormone/hypoxia responses, inflammation, immunomodulation, cell communication, and proteolysis/inhibition, and exhibited increased Notch, TGFB, IGF, Hedgehog, and G-protein-coupled receptor signaling pathways, characteristic of adult tissue MSC self-renewal and multipotency. Overall, the data support the eMSC as a clonogenic, multipotent pericyte that displays pathways of self-renewal and lineage specification, the potential to respond to conditions during endometrial desquamation and regeneration, and a genetic program predictive of its differentiated lineage, the stromal fibroblast.

Brain pericytes: emerging concepts and functional roles in brain homeostasis

Brain pericytes are an important constituent of neurovascular unit. They encircle endothelial cells and contribute to the maturation and stabilization of the capillaries in the brain. Recent studies have revealed that brain pericytes play pivotal roles in a variety of brain functions, such as regulation of capillary flow, angiogenesis, blood brain barrier, immune responses, and hemostasis. In addition, brain pericytes are pluripotent and can differentiate into different lineages similar to mesenchymal stem cells. The brain pericytes are revisited as a key player to maintain brain function and repair brain damage.

Metronomic administration of the drug GMX1777, a cellular NAD synthesis inhibitor, results in neuroblastoma regression and vessel maturation without inducing drug resistance

High-risk neuroblastoma is a rapidly growing tumor with a survival rate below 50%. A new treatment strategy is to administer chemotherapeutic drugs metronomically, i.e., at lower doses and frequent intervals. The aim of the study was to investigate the effects of GMX1777, a chemotherapeutic drug affecting cellular energy metabolism, in a mouse model for high-risk neuroblastoma. Female SCID mice were injected s.c. with MYCN-amplified human neuroblastoma cells and randomized to either treatment with GMX1777 or vehicle. In some animals, treatment was discontinued allowing tumor relapse. Treatment response was evaluated using the pediatric preclinical testing program (PPTP). Immunohistochemistry and qRT-PCR was performed on tumor cryosections to investigate the microscopic and molecular changes in tumors in response to GMX1777. Despite an increase in vessel density, tumor regression and a high group response score according to PPTP criteria was induced by GMX1777 without inducing drug resistance. Treatment resulted in inhibition of tumor cell proliferation, vessel maturation, reduced hypoxia, increased infiltration of MHC class II negative macrophages and expansion of the nonvascular stromal compartment. Decreased stromal VEGF-A and PDGF-B mRNA in response to treatment together with the structural data suggest a "deactivation" or "silencing" of the tumor stroma as a paracrine entity. In conclusion, GMX1777 was highly efficient against high-risk neuroblastoma xenografts through modulation of both the tumor cell and stromal compartment.