VEGFR2 pY949 signalling regulates adherens junction integrity and metastatic spread

The specific role of VEGFA-induced permeability and vascular leakage in physiology and pathology has remained unclear. Here we show that VEGFA-induced vascular leakage depends on signalling initiated via the VEGFR2 phosphosite Y949, regulating dynamic c-Src and VE-cadherin phosphorylation. Abolished Y949 signalling in the mouse mutant Vegfr2^Y949F/Y949F leads to VEGFA-resistant endothelial adherens junctions and a block in molecular extravasation. Vessels in Vegfr2^Y949F/Y949F mice remain sensitive to inflammatory cytokines, and vascular morphology, blood pressure and flow parameters are normal. Tumour-bearing Vegfr2^Y949F/Y949F mice display reduced vascular leakage and oedema, improved response to chemotherapy and, importantly, reduced metastatic spread. The inflammatory infiltration in the tumour micro-environment is unaffected. Blocking VEGFA-induced disassembly of endothelial junctions, thereby suppressing tumour oedema and metastatic spread, may be preferable to full vascular suppression in the treatment of certain cancer forms.

Signals through VEGF receptor 2 (VEGFR2) increase vascular permeability, promoting cancer progression. Here the authors show that a point mutation in VEGFR2 preventing its auto-phosphorylation leads to reduced metastatic spread and improved response to chemotherapy in tumor-bearing mice, without affecting tumor inflammation.

An Endothelial Gene Signature Score Predicts Poor Outcome in Patients with Endocrine-Treated, Low Genomic Grade Breast Tumors

PURPOSE

The ability of vascular genes to provide treatment predictive information in breast cancer patients remains unclear. As such, we assessed the expression of genes representative of normal endothelial microvasculature (MV) in relation to treatment-specific patient subgroups.

EXPERIMENTAL DESIGN

We used expression data from 993 breast tumors to assess 57 MV genes (summarized to yield an MV score) as well as the genomic grade index (GGI) and PAM50 signatures. MV score was compared with CD31 staining by correlation and gene ontology (GO) analysis, along with clinicopathologic characteristics and PAM50 subtypes. Uni-, multivariate, and/or t-test analyses were performed in all and treatment-specific subgroups, along with a clinical trial cohort of patients with metastatic breast cancer, seven of whom received antiangiogenic therapy.

RESULTS

MV score did not correlate with microvessel density (correlation = 0.096), but displayed enrichment for angiogenic GO terms, and was lower in Luminal B tumors. In endocrine-treated patients, a high MV score was associated with decreased risk of metastasis [HR 0.58; 95% confidence interval (CI), 0.38-0.89], even after adjusting for histologic grade, but not GGI or PAM50. Subgroup analysis showed the prognostic strength of the MV score resided in low genomic grade tumors and MV score was significantly increased in metastatic breast tumors after treatment with sunitinib + docetaxel (P = 0.031).

CONCLUSIONS

MV score identifies two groups of better and worse survival in low-risk endocrine-treated breast cancer patients. We also show normalization of tumor vasculature on a transcriptional level in response to an angiogenic inhibitor in human breast cancer samples. Clin Cancer Res; 22(10); 2417-26. ©2016 AACR.

Functional Characterization of Germline Mutations in PDGFB and PDGFRB in Primary Familial Brain Calcification

Primary Familial Brain Calcification (PFBC), a neurodegenerative disease characterized by progressive pericapillary calcifications, has recently been linked to heterozygous mutations in PDGFB and PDGFRB genes. Here, we functionally analyzed several of these mutations in vitro. All six analyzed PDGFB mutations led to complete loss of PDGF-B function either through abolished protein synthesis or through defective binding and/or stimulation of PDGF-Rβ. The three analyzed PDGFRB mutations had more diverse consequences. Whereas PDGF-Rβ autophosphorylation was almost totally abolished in the PDGFRB L658P mutation, the two sporadic PDGFRB mutations R987W and E1071V caused reductions in protein levels and specific changes in the intensity and kinetics of PLCγ activation, respectively. Since at least some of the PDGFB mutations were predicted to act through haploinsufficiency, we explored the consequences of reduced Pdgfb or Pdgfrb transcript and protein levels in mice. Heterozygous Pdgfb or Pdgfrb knockouts, as well as double Pdgfb^+/-;Pdgfrb^+/- mice did not develop brain calcification, nor did Pdgfrb^{redeye/redeye} mice, which show a 90% reduction of PDGFRβ protein levels. In contrast, Pdgfb^ret/ret mice, which have altered tissue distribution of PDGF-B protein due to loss of a proteoglycan binding motif, developed brain calcifications. We also determined pericyte coverage in calcification-prone and non-calcification-prone brain regions in Pdgfb^ret/ret mice. Surprisingly and contrary to our hypothesis, we found that the calcification-prone brain regions in Pdgfb^ret/ret mice model had a higher pericyte coverage and a more intact blood-brain barrier (BBB) compared to non-calcification-prone brain regions. While our findings provide clear evidence that loss-of-function mutations in PDGFB or PDGFRB cause PFBC, they also demonstrate species differences in the threshold levels of PDGF-B/PDGF-Rβ signaling that protect against small-vessel calcification in the brain. They further implicate region-specific susceptibility factor(s) in PFBC pathogenesis that are distinct from pericyte and BBB deficiency.

Gpr116 Receptor Regulates Distinctive Functions in Pneumocytes and Vascular Endothelium

Despite its known expression in both the vascular endothelium and the lung epithelium, until recently the physiological role of the adhesion receptor Gpr116/ADGRF5 has remained elusive. We generated a new mouse model of constitutive Gpr116 inactivation, with a large genetic deletion encompassing exon 4 to exon 21 of the Gpr116 gene. This model allowed us to confirm recent results defining Gpr116 as necessary regulator of surfactant homeostasis. The loss of Gpr116 provokes an early accumulation of surfactant in the lungs, followed by a massive infiltration of macrophages, and eventually progresses into an emphysema-like pathology. Further analysis of this knockout model revealed cerebral vascular leakage, beginning at around 1.5 months of age. Additionally, endothelial-specific deletion of Gpr116 resulted in a significant increase of the brain vascular leakage. Mice devoid of Gpr116 developed an anatomically normal and largely functional vascular network, surprisingly exhibited an attenuated pathological retinal vascular response in a model of oxygen-induced retinopathy. These data suggest that Gpr116 modulates endothelial properties, a previously unappreciated function despite the pan-vascular expression of this receptor. Our results support the key pulmonary function of Gpr116 and describe a new role in the central nervous system vasculature.

Knockdown of Tmem234 in zebrafish results in proteinuria

Podocytes are highly specialized epithelial cells located at the outer aspects of the glomerular capillary tuft and critical components of the kidney filtration barrier. To maintain their unique features, podocytes express a number of proteins that are only sparsely found elsewhere in the body. In this study, we have identified four (Tmem234, Znf185, Lrrc49, and Slfn5) new highly podocyte-enriched proteins. The proteins are strongly expressed by podocytes, while other parts of the kidney show only weak or no expression. Tmem234, Slfn5, and Lrrc49 are located in foot processes, whereas Znf185 is found in both foot and major processes. Expressional studies in developing kidneys show that these proteins are first expressed at the capillary stage glomerulus, the same stage when the formation of major and foot processes begins. We identified zebrafish orthologs for Tmem234 and Znf185 genes and knocked down their expression using morpholino technology. Studies in zebrafish larvae indicate that Tmem234 is essential for the organization and functional integrity of the pronephric glomerulus filtration barrier, as inactivation of Tmem234 expression results in foot process effacement and proteinuria. In summary, we have identified four novel highly podocyte-enriched proteins and show that one of them, Tmem234, is essential for the normal filtration barrier in the zebrafish pronephric glomerulus. Identification of new molecular components of the kidney filtration barrier opens up possibilities to study their role in glomerulus biology and diseases.

PDGF, pericytes and the pathogenesis of idiopathic basal ganglia calcification (IBGC)

Platelet-derived growth factors (PDGFs) are important mitogens for various types of mesenchymal cells, and as such, they exert critical functions during organogenesis in mammalian embryonic and early postnatal development. Increased or ectopic PDGF activity may also cause or contribute to diseases such as cancer and tissue fibrosis. Until recently, no loss-of-function (LOF) mutations in PDGF or PDGF receptor genes were reported as causally linked to a human disease. This changed in 2013 when reports appeared on presumed LOF mutations in the genes encoding PDGF-B and its receptor PDGF receptor-beta (PDGF-Rβ) in familial idiopathic basal ganglia calcification (IBGC), a brain disease characterized by anatomically localized calcifications in or near the blood microvessels. Here, we review PDGF-B and PDGF-Rβ biology with special reference to their functions in brain-blood vessel development, pericyte recruitment and the regulation of the blood-brain barrier. We also discuss various scenarios for IBGC pathogenesis suggested by observations in patients and genetically engineered animal models of the disease.

Increased flux of the plant sterols campesterol and sitosterol across a disrupted blood brain barrier

The intact blood-brain barrier in mammalians prevents exchange of cholesterol loaden particles between periphery and brain and thus nearly all cholesterol in this organ originates from de novo synthesis. Dietary cholesterol homologues from plants, campesterol and sitosterol, are known to get enriched to some extent in the mammalian brain. We recently showed that Pdgfb(ret)(/)(ret) mice, with a pericyte deficiency and a leaking blood-brain barrier phenotype, have significantly higher levels of plant sterols in the brain compared to their heterozygous Pdgfb(ret)(/)(+) controls keeping the integrity of the blood-brain barrier (BBB). In order to further study the protective functionality of the BBB we synthesized a mixture of [(2)H6]campesterol/sitosterol and fed it for 10-40days to genetically different types of animals. There was a significant enrichment of both deuterium stable isotope labeled plant sterols in the brain of both strains of mice, however, with a lower enrichment in the controls. As expected, the percentage and absolute enrichment was higher for [(2)H6]campesterol than for the more lipophilic [(2)H6]sitosterol. The results confirm that a leaking BBB causes increased flux of plant sterols into the brain. The significant flux of the labeled plant sterols into the brain of the control mice illustrates that the presence of an alkyl group in the 24-position of the steroid side chain markedly increases the ability of cholesterol to pass an intact BBB. We discuss the possibility that there is a specific transport mechanism involved in the flux of alkylated cholesterol species across the BBB.

TGF-β1-induced EMT promotes targeted migration of breast cancer cells through the lymphatic system by the activation of CCR7/CCL21-mediated chemotaxis

Tumor cells frequently disseminate through the lymphatic system during metastatic spread of breast cancer and many other types of cancer. Yet it is not clear how tumor cells make their way into the lymphatic system and how they choose between lymphatic and blood vessels for migration. Here we report that mammary tumor cells undergoing epithelial–mesenchymal transition (EMT) in response to transforming growth factor-β (TGF-β1) become activated for targeted migration through the lymphatic system, similar to dendritic cells (DCs) during inflammation. EMT cells preferentially migrated toward lymphatic vessels compared with blood vessels, both in vivo and in 3D cultures. A mechanism of this targeted migration was traced to the capacity of TGF-β1 to promote CCR7/CCL21-mediated crosstalk between tumor cells and lymphatic endothelial cells. On one hand, TGF-β1 promoted CCR7 expression in EMT cells through p38 MAP kinase-mediated activation of the JunB transcription factor. Blockade of CCR7, or treatment with a p38 MAP kinase inhibitor, reduced lymphatic dissemination of EMT cells in syngeneic mice. On the other hand, TGF-β1 promoted CCL21 expression in lymphatic endothelial cells. CCL21 acted in a paracrine fashion to mediate chemotactic migration of EMT cells toward lymphatic endothelial cells. The results identify TGF-β1-induced EMT as a mechanism, which activates tumor cells for targeted, DC-like migration through the lymphatic system. Furthermore, it suggests that p38 MAP kinase inhibition may be a useful strategy to inhibit EMT and lymphogenic spread of tumor cells.

Schip1 is a novel podocyte foot process protein that mediates actin cytoskeleton rearrangements and forms a complex with Nherf2 and ezrin

Background

Podocyte foot process effacement accompanied by actin cytoskeleton rearrangements is a cardinal feature of many progressive human proteinuric diseases.

Results

By microarray profiling of mouse glomerulus, SCHIP1 emerged as one of the most highly enriched transcripts. We detected Schip1 protein in the kidney glomerulus, specifically in podocytes foot processes. Functionally, Schip1 inactivation in zebrafish by morpholino knock-down results in foot process disorganization and podocyte loss leading to proteinuria. In cultured podocytes Schip1 localizes to cortical actin-rich regions of lamellipodia, where it forms a complex with Nherf2 and ezrin, proteins known to participate in actin remodeling stimulated by PDGFβ signaling. Mechanistically, overexpression of Schip1 in vitro causes accumulation of cortical F-actin with dissolution of transversal stress fibers and promotes cell migration in response to PDGF-BB stimulation. Upon actin disassembly by latrunculin A treatment, Schip1 remains associated with the residual F-actin-containing structures, suggesting a functional connection with actin cytoskeleton possibly via its interaction partners. A similar assay with cytochalasin D points to stabilization of cortical actin cytoskeleton in Schip1 overexpressing cells by attenuation of actin depolymerisation.

Conclusions

Schip1 is a novel glomerular protein predominantly expressed in podocytes, necessary for the zebrafish pronephros development and function. Schip1 associates with the cortical actin cytoskeleton network and modulates its dynamics in response to PDGF signaling via interaction with the Nherf2/ezrin complex. Its implication in proteinuric diseases remains to be further investigated.

Brain structure. Cell types in the mouse cortex and hippocampus revealed by single-cell RNA-seq

The mammalian cerebral cortex supports cognitive functions such as sensorimotor integration, memory, and social behaviors. Normal brain function relies on a diverse set of differentiated cell types, including neurons, glia, and vasculature. Here, we have used large-scale single-cell RNA sequencing (RNA-seq) to classify cells in the mouse somatosensory cortex and hippocampal CA1 region. We found 47 molecularly distinct subclasses, comprising all known major cell types in the cortex. We identified numerous marker genes, which allowed alignment with known cell types, morphology, and location. We found a layer I interneuron expressing Pax6 and a distinct postmitotic oligodendrocyte subclass marked by Itpr2. Across the diversity of cortical cell types, transcription factors formed a complex, layered regulatory code, suggesting a mechanism for the maintenance of adult cell type identity.

Notch3 is necessary for blood vessel integrity in the central nervous system

OBJECTIVE

Vascular smooth muscle cells (VSMC) are important for contraction, blood flow distribution, and regulation of blood vessel diameter, but to what extent they contribute to the integrity of blood vessels and blood-brain barrier function is less well understood. In this report, we explored the impact of the loss of VSMC in the Notch3(-/-) mouse on blood vessel integrity in the central nervous system.

APPROACH AND RESULTS

Notch3(-/-) mice showed focal disruptions of the blood-brain barrier demonstrated by extravasation of tracers accompanied by fibrin deposition in the retinal vasculature. This blood-brain barrier leakage was accompanied by a regionalized and patchy loss of VSMC, with VSMC gaps predominantly in arterial resistance vessels of larger caliber. The loss of VSMC appeared to be caused by progressive degeneration of VSMC resulting in a gradual loss of VSMC marker expression and a progressive acquisition of an aberrant VSMC phenotype closer to the gaps, followed by enhanced apoptosis and cellular disintegration in the gaps. Arterial VSMC were the only mural cell type that was morphologically affected, despite Notch3 also being expressed in pericytes. Transcriptome analysis of isolated brain microvessels revealed gene expression changes in Notch3(-/-) mice consistent with loss of arterial VSMC and presumably secondary transcriptional changes were observed in endothelial genes, which may explain the compromised vascular integrity.

CONCLUSIONS

We demonstrate that Notch3 is important for survival of VSMC, and reveal a critical role for Notch3 and VSMC in blood vessel integrity and blood-brain barrier function in the mammalian vasculature.

Excessive vascular sprouting underlies cerebral hemorrhage in mice lacking αVβ8-TGFβ signaling in the brain

Vascular development of the central nervous system and blood-brain barrier (BBB) induction are closely linked processes. The role of factors that promote endothelial sprouting and vascular leak, such as vascular endothelial growth factor A, are well described, but the factors that suppress angiogenic sprouting and their impact on the BBB are poorly understood. Here, we show that integrin αVβ8 activates angiosuppressive TGFβ gradients in the brain, which inhibit endothelial cell sprouting. Loss of αVβ8 in the brain or downstream TGFβ1-TGFBR2-ALK5-Smad3 signaling in endothelial cells increases vascular sprouting, branching and proliferation, leading to vascular dysplasia and hemorrhage. Importantly, BBB function in Itgb8 mutants is intact during early stages of vascular dysgenesis before hemorrhage. By contrast, Pdgfb(ret/ret) mice, which exhibit severe BBB disruption and vascular leak due to pericyte deficiency, have comparatively normal vascular morphogenesis and do not exhibit brain hemorrhage. Our data therefore suggest that abnormal vascular sprouting and patterning, not BBB dysfunction, underlie developmental cerebral hemorrhage.

Characterization of platelet-derived growth factor-A expression in mouse tissues using a lacZ knock-in approach

Expression of the platelet-derived growth factor A-chain gene (Pdgfa) occurs widely in the developing mouse, where it is mainly localized to various epithelial and neuronal structures. Until now, in situ mRNA hybridization (ISH) has been the only reliable method to identify Pdgfa expression in tissue sections or whole mount preparations. Validated protocols for in situ detection of PDGF-A protein by immunohistochemistry is lacking. In particular, this has hampered understanding of Pdgfa expression pattern in adult tissues, where ISH is technically challenging. Here, we report a gene targeted mouse Pdgfa allele, Pdgfa^ex4COIN, which is a combined conditional knockout and reporter allele. Cre-mediated inversion of the COIN cassette inactivates Pdgfa coding while simultaneously activating a beta-galactosidase (lacZ) reporter under endogenous Pdgfa transcription control. The generated Pdgfa^{ex4COIN-INV-lacZ} allele can next be used to identify cells carrying a Pdgfa null allele, as well as to map endogenous Pdgfa expression. We evaluated the Pdgfa^{ex4COIN-INV-lacZ} allele as a reporter for endogenous Pdgfa expression patterns in mouse embryos and adults. We conclude that the expression pattern of Pdgfa^{ex4COIN-INV-lacZ} recapitulates known expression patterns of Pdgfa. We also report on novel embryonic and adult Pdgfa expression patterns in the mouse and discuss their implications for Pdgfa physiology.

Effects of a disrupted blood-brain barrier on cholesterol homeostasis in the brain

The presence of the blood-brain barrier (BBB) is critical for cholesterol metabolism in the brain, preventing uptake of lipoprotein-bound cholesterol from the circulation. The metabolic consequences of a leaking BBB for cholesterol metabolism have not been studied previously. Here we used a pericyte-deficient mouse model, Pdgfb^ret/ret, shown to have increased permeability of the BBB to a range of low-molecular mass and high-molecular mass tracers. There was a significant accumulation of plant sterols in the brains of the Pdgfb^ret/ret mice. By dietary treatment with 0.3% deuterium-labeled cholesterol, we could demonstrate a significant flux of cholesterol from the circulation into the brains of the mutant mice roughly corresponding to about half of the measured turnover of cholesterol in the brain. We expected the cholesterol flux into the brain to cause a down-regulation of cholesterol synthesis. Instead, cholesterol synthesis was increased by about 60%. The levels of 24(S)-hydroxycholesterol (24S-OHC) were significantly reduced in the brains of the pericyte-deficient mice but increased in the circulation. After treatment with 1% cholesterol in diet, the difference in cholesterol synthesis between mutants and controls disappeared. The findings are consistent with increased leakage of 24S-OHC from the brain into the circulation in the pericyte-deficient mice. This oxysterol is an efficient suppressor of cholesterol synthesis, and the results are consistent with a regulatory role of 24S-OHC in the brain. To our knowledge, this is the first demonstration that a defective BBB may lead to increased flux of a lipophilic compound out from the brain. The relevance of the findings for the human situation is discussed.

Lim domain binding 2: a key driver of transendothelial migration of leukocytes and atherosclerosis

OBJECTIVE

Using a multi-tissue, genome-wide gene expression approach, we recently identified a gene module linked to the extent of human atherosclerosis. This atherosclerosis module was enriched with inherited risk for coronary and carotid artery disease (CAD) and overlapped with genes in the transendothelial migration of leukocyte (TEML) pathway. Among the atherosclerosis module genes, the transcription cofactor Lim domain binding 2 (LDB2) was the most connected in a CAD vascular wall regulatory gene network. Here, we used human genomics and atherosclerosis-prone mice to evaluate the possible role of LDB2 in TEML and atherosclerosis.

APPROACH AND RESULTS

mRNA profiles generated from blood macrophages in patients with CAD were used to infer transcription factor regulatory gene networks; Ldlr(-/-)Apob(100/100) mice were used to study the effects of Ldb2 deficiency on TEML activity and atherogenesis. LDB2 was the most connected gene in a transcription factor regulatory network inferred from TEML and atherosclerosis module genes in CAD macrophages. In Ldlr(-/-)Apob(100/100) mice, loss of Ldb2 increased atherosclerotic lesion size ≈2-fold and decreased plaque stability. The exacerbated atherosclerosis was caused by increased TEML activity, as demonstrated in air-pouch and retinal vasculature models in vivo, by ex vivo perfusion of primary leukocytes, and by leukocyte migration in vitro. In THP1 cells, migration was increased by overexpression and decreased by small interfering RNA inhibition of LDB2. A functional LDB2 variant (rs10939673) was associated with the risk and extent of CAD across several cohorts.

CONCLUSIONS

As a key driver of the TEML pathway in CAD macrophages, LDB2 is a novel candidate to target CAD by inhibiting the overall activity of TEML.

Lmx1b and FoxC combinatorially regulate podocin expression in podocytes

Podocin is a key protein of the kidney podocyte slit diaphragm protein complex, an important part of the glomerular filtration barrier. Mutations in the human podocin gene NPHS2 cause familial or sporadic forms of renal disease owing to the disruption of filtration barrier integrity. The exclusive expression of NPHS2 in podocytes reflects its unique function and raises interesting questions about its transcriptional regulation. Here, we further define a 2.5-kb zebrafish nphs2 promoter fragment previously described and identify a 49-bp podocyte-specific transcriptional enhancer using Tol2-mediated G0 transgenesis in zebrafish. Within this enhancer, we identified a cis-acting element composed of two adjacent DNA-binding sites (FLAT-E and forkhead) bound by transcription factors Lmx1b and FoxC. In zebrafish, double knockdown of Lmx1b and FoxC orthologs using morpholino doses that caused no or minimal phenotypic changes upon individual knockdown completely disrupted podocyte development in 40% of injected embryos. Co-overexpression of the two genes potently induced endogenous nphs2 expression in zebrafish podocytes. We found that the NPHS2 promoter also contains a cis-acting Lmx1b-FoxC motif that binds LMX1B and FoxC2. Furthermore, a genome-wide search identified several genes that carry the Lmx1b-FoxC motif in their promoter regions. Among these candidates, motif-driven podocyte enhancer activity of CCNC and MEIS2 was functionally analyzed in vivo. Our results show that podocyte expression of some genes is combinatorially regulated by two transcription factors interacting synergistically with a common enhancer. This finding provides insights into transcriptional mechanisms required for normal and pathologic podocyte functions.

Wtip- and gadd45a-interacting protein dendrin is not crucial for the development or maintenance of the glomerular filtration barrier

Glomerular podocyte cells are critical for the function of the renal ultrafiltration barrier. Especially, the highly specialized cell–cell junction of podocytes, the slit diaphragm, has a central role in the filtration barrier. This is highlighted by the fact that mutations in molecular components of the slit diaphragm, including nephrin and Cd2-associated protein (Cd2ap), result in proteinuric diseases in man. Dendrin is a poorly characterized cytosolic component of the slit diaphragm in where it interacts with nephrin and Cd2ap. Dendrin is highly specific for the podocyte slit diaphragm, suggesting that it has a dedicated role in the glomerular filtration barrier. In this study, we have generated a dendrin knockout mouse line and explored the molecular interactions of dendrin. Dendrin-deficient mice were viable, fertile, and had a normal life span. Morphologically, the glomerulogenesis proceeded normally and adult dendrin-deficient mice showed normal glomerular histology. No significant proteinuria was observed. Following glomerular injury, lack of dendrin did not affect the severity of the damage or the recovery process. Yeast two-hybrid screen and co-immunoprecipitation experiments showed that dendrin binds to Wt1-interacting protein (Wtip) and growth arrest and DNA-damage-inducible 45 alpha (Gadd45a). Wtip and Gadd45a mediate gene transcription in the nucleus, suggesting that dendrin may have similar functions in podocytes. In line with this, we observed the relocation of dendrin to nucleus in adriamycin nephropathy model. Our results indicate that dendrin is dispensable for the function of the normal glomerular filtration barrier and that dendrin interacts with Wtip and Gadd45a.

Development of renal renin-expressing cells does not involve PDGF-B-PDGFR-β signaling

Apart from their endocrine functions renin-expressing cells play an important functional role as mural cells of the developing preglomerular arteriolar vessel tree in the kidney. The recruitment of renin-expressing cells from the mesenchyme to the vessel wall is not well understood. Assuming that it may follow more general lines of pericyte recruitment to endothelial tubes we have now investigated the relevance of the platelet-derived growth factor (PDGF)-B-PDGFR-β signaling pathway in this context. We studied renin expression in kidneys lacking PDGFR-β in these cells and in kidneys with reduced endothelial PDGF-B expression. We found that expression of renin in the kidneys under normal and stimulated conditions was not different from wild-type kidneys. As expected, PDGFR-β immunoreactivity was found in mesangial, adventitial and tubulo-interstitial cells but not in renin-expressing cells. These findings suggest that the PDGF-B-PDGFR-β signaling pathway is not essential for the recruitment of renin-expressing cells to preglomerular vessel walls in the kidney.

Correction: The importance of microglia in the development of the vasculature in the central nervous system

CORRECTION

After the publication of this work 1 it was brought to our attention that citations in the article were not correspondingly numbered in the reference list. To avoid confusion, the article is republished here in its entirety, with the citations referenced correctly.The Publisher and authors apologize to the readers for the inconvenience caused.

ABSTRACT

The body's vascular system is thought to have developed in order to supply oxygen and nutrients to cells beyond the reach of simple diffusion. Hence, relative hypoxia in the growing central nervous system (CNS) is a major driving force for the ingression and refinement of the complex vascular bed that serves it. However, even before the establishment of this CNS vascular system, CNS-specific macrophages (microglia) migrate into the brain. Recent studies in mice point to the fundamental importance of microglia in shaping CNS vasculature during development, and re-shaping these vessels during pathological insults. In this review, we discuss the origin of CNS microglia and their localization within the brain based on data obtained in mice. We then review evidence supporting a functional role of these microglia in developmental angiogenesis. Although pathologic processes such as CNS ischemia may subvert the developmental functions of microglia/macrophages with significant effects on brain neo-angiogenesis, we have left this topic to other recent reviews 23.

The importance of microglia in the development of the vasculature in the central nervous system

The body’s vascular system is thought to have developed in order to supply oxygen and nutrients to cells beyond the reach of simple diffusion. Hence, relative hypoxia in the growing central nervous system (CNS) is a major driving force for the ingression and refinement of the complex vascular bed that serves it. However, even before the establishment of this CNS vascular system, CNS-specific macrophages (microglia) migrate into the brain. Recent studies in mice point to the fundamental importance of microglia in shaping CNS vasculature during development, and re-shaping these vessels during pathological insults. In this review, we discuss the origin of CNS microglia and their localization within the brain based on data obtained in mice. We then review evidence supporting a functional role of these microglia in developmental angiogenesis. Although pathologic processes such as CNS ischemia may subvert the developmental functions of microglia/macrophages with significant effects on brain neo-angiogenesis, we have left this topic to other recent reviews (Nat Rev Immunol 9:259–270, 2009 and Trends Mol Med 17:743–752, 2011).

The sphingosine-1-phosphate receptor S1PR1 restricts sprouting angiogenesis by regulating the interplay between VE-cadherin and VEGFR2

Angiogenesis, the process by which new blood vessels arise from preexisting ones, is critical for embryonic development and is an integral part of many disease processes. Recent studies have provided detailed information on how angiogenic sprouts initiate, elongate, and branch, but less is known about how these processes cease. Here, we show that S1PR1, a receptor for the blood-borne bioactive lipid sphingosine-1-phosphate (S1P), is critical for inhibition of angiogenesis and acquisition of vascular stability. Loss of S1PR1 leads to increased endothelial cell sprouting and the formation of ectopic vessel branches. Conversely, S1PR1 signaling inhibits angiogenic sprouting and enhances cell-to-cell adhesion. This correlates with inhibition of vascular endothelial growth factor-A (VEGF-A)-induced signaling and stabilization of vascular endothelial (VE)-cadherin localization at endothelial junctions. Our data suggest that S1PR1 signaling acts as a vascular-intrinsic stabilization mechanism, protecting developing blood vessels against aberrant angiogenic responses.

Neuronal proteins are novel components of podocyte major processes and their expression in glomerular crescents supports their role in crescent formation

The podocyte has a central role in the glomerular filtration barrier typified by a sophisticated morphology of highly organized primary (major) and secondary (foot) processes. The molecular makeup of foot processes is well characterized, but that of major processes is poorly known. Previously, we profiled the glomerular transcriptome through large-scale sequencing and microarray profiling. Unexpectedly, the survey found expression of three neuronal proteins (Huntingtin interacting protein 1 (Hip1), neurofascin (Nfasc), and olfactomedin-like 2a (Olfml2a)), all enriched in the glomerulus. These proteins were expressed exclusively by podocytes, wherein they localized to major processes as verified by RT-PCR, western blotting, immunofluorescence, and immunoelectron microscopy. During podocyte development, these proteins colocalized with vimentin, confirming their association with major processes. Using immunohistochemistry, we found coexpression of Hip1 and Olfml2a along with the recognized podocyte markers synaptopodin and Pdlim2 in glomerular crescents of human kidneys, indicating the presence of podocytes in these lesions. Thus, three neuronal proteins are highly expressed in podocyte major process. Using these new markers we found that podocytes contribute to the formation of glomerular crescents.

Plekhh2, a novel podocyte protein downregulated in human focal segmental glomerulosclerosis, is involved in matrix adhesion and actin dynamics

Pleckstrin homology domain-containing, family H (with MyTH4 domain), member 2 (Plekhh2) is a 1491-residue intracellular protein highly enriched in renal glomerular podocytes for which no function has been ascribed. Analysis of renal biopsies from patients with focal segmental glomerulosclerosis revealed a significant reduction in total podocyte Plekhh2 expression compared to controls. Sequence analysis indicated a putative α-helical coiled-coil segment as the only recognizable domain within the N-terminal half of the polypeptide, while the C-terminal half contains two PH, a MyTH4, and a FERM domain. We identified a phosphatidylinositol-3-phosphate consensus-binding site in the PH1 domain required for Plekhh2 localization to peripheral regions of cell lamellipodia. The N-terminal half of Plekkh2 is not necessary for lamellipodial targeting but mediates self-association. Yeast two-hybrid screening showed that Plekhh2 directly interacts through its FERM domain with the focal adhesion protein Hic-5 and actin. Plekhh2 and Hic-5 coprecipitated and colocalized at the soles of podocyte foot processes in situ and Hic-5 partially relocated from focal adhesions to lamellipodia in Plekhh2-expressing podocytes. In addition, Plekhh2 stabilizes the cortical actin cytoskeleton by attenuating actin depolymerization. Our findings suggest a structural and functional role for Plekhh2 in the podocyte foot processes.

Apolipoprotein E controls cerebrovascular integrity via cyclophilin A

Human apolipoprotein E has three isoforms: APOE2, APOE3 and APOE4. APOE4 is a major genetic risk factor for Alzheimer's disease and is associated with Down's syndrome dementia and poor neurological outcome after traumatic brain injury and haemorrhage. Neurovascular dysfunction is present in normal APOE4 carriers and individuals with APOE4-associated disorders. In mice, lack of Apoe leads to blood-brain barrier (BBB) breakdown, whereas APOE4 increases BBB susceptibility to injury. How APOE genotype affects brain microcirculation remains elusive. Using different APOE transgenic mice, including mice with ablation and/or inhibition of cyclophilin A (CypA), here we show that expression of APOE4 and lack of murine Apoe, but not APOE2 and APOE3, leads to BBB breakdown by activating a proinflammatory CypA-nuclear factor-κB-matrix-metalloproteinase-9 pathway in pericytes. This, in turn, leads to neuronal uptake of multiple blood-derived neurotoxic proteins, and microvascular and cerebral blood flow reductions. We show that the vascular defects in Apoe-deficient and APOE4-expressing mice precede neuronal dysfunction and can initiate neurodegenerative changes. Astrocyte-secreted APOE3, but not APOE4, suppressed the CypA-nuclear factor-κB-matrix-metalloproteinase-9 pathway in pericytes through a lipoprotein receptor. Our data suggest that CypA is a key target for treating APOE4-mediated neurovascular injury and the resulting neuronal dysfunction and degeneration.

Getting to know the cast - cellular interactions and signaling at the neurovascular unit

The neurovascular unit (NVU), consisting of endothelial cells, basement membrane, pericytes, astrocytes and microglial cells, couples local neuronal function to local cerebral blood flow and regulates transport of blood-borne molecules across the blood-brain barrier (BBB). The building blocks and the phenotype of the NVU are well-established but the intercellular signaling between the different components remains elusive. A better understanding of the cellular interactions and signaling within the NVU is critical for the development of efficient therapeutics for the treatment of a variety of brain diseases, such as brain cancer, stroke, neuroinflammation and neurodegeneration. This review gives an overview about the current in vivo knowledge of the NVU and the communication between its different cellular constituents. We also discuss the usefulness of various model organisms for studies of the brain vasculature.

Platelet-derived growth factor C deficiency in C57BL/6 mice leads to abnormal cerebral vascularization, loss of neuroependymal integrity, and ventricular abnormalities

Platelet-derived growth factors (PDGFs) and their tyrosine kinase receptors (PDGFRs) are known to play important roles during development of the lungs, central nervous system (CNS), and skeleton and in several diseases. PDGF-C is a ligand for the tyrosine kinase receptor PDGFRα. Mutations in the gene encoding PDGF-C have been linked to clefts of the lip and/or palate in humans, and ablation of PDGF-C in 129/Sv background mice results in death during the perinatal period. In this study, we report that ablation of PDGF-C in C57BL/6 mice results in a milder phenotype than in 129/Sv mice, and we present a phenotypic characterization of PDGF-C deficiency in the adult murine CNS. Multiple congenital defects were observed in the CNS of PDGF-C-null C57BL/6 mice, including cerebral vascular abnormalities with abnormal vascular smooth muscle cell coverage. In vivo imaging of mice deficient in PDGF-C also revealed cerebral ventricular abnormalities, such as asymmetry of the lateral ventricles and hypoplasia of the septum, reminiscent of cavum septum pellucidum in humans. We further noted that PDGF-C-deficient mice displayed a distorted ependymal lining of the lateral ventricles, and we found evidence of misplaced neurons in the ventricular lining. We conclude that PDGF-C plays a critical role in the development of normal cerebral ventricles and neuroependymal integrity as well as in normal cerebral vascularization.

Pericytes: developmental, physiological, and pathological perspectives, problems, and promises

Pericytes, the mural cells of blood microvessels, have recently come into focus as regulators of vascular morphogenesis and function during development, cardiovascular homeostasis, and disease. Pericytes are implicated in the development of diabetic retinopathy and tissue fibrosis, and they are potential stromal targets for cancer therapy. Some pericytes are probably mesenchymal stem or progenitor cells, which give rise to adipocytes, cartilage, bone, and muscle. However, there is still confusion about the identity, ontogeny, and progeny of pericytes. Here, we review the history of these investigations, indicate emerging concepts, and point out problems and promise in the field of pericyte biology.

Glcci1 deficiency leads to proteinuria

Unbiased transcriptome profiling and functional genomics approaches identified glucocorticoid-induced transcript 1 (GLCCI1) as being a transcript highly specific for the glomerulus, but its role in glomerular development and disease is unknown. Here, we report that mouse glomeruli express far greater amounts of Glcci1 protein compared with the rest of the kidney. RT-PCR and Western blotting demonstrated that mouse glomerular Glcci1 is approximately 60 kD and localizes to the cytoplasm of podocytes in mature glomeruli. In the fetal kidney, intense Glcci1 expression occurs at the capillary-loop stage of glomerular development. Using gene knockdown in zebrafish with morpholinos, morphants lacking Glcci1 function had collapsed glomeruli with foot-process effacement. Permeability studies of the glomerular filtration barrier in these zebrafish morphants demonstrated a disruption of the selective glomerular permeability filter. Taken together, these data suggest that Glcci1 promotes the normal development and maintenance of podocyte structure and function.

Loss of PDGF-B activity increases hepatic vascular permeability and enhances insulin sensitivity

Hepatic vasculature is not thought to pose a permeability barrier for diffusion of macromolecules from the bloodstream to hepatocytes. In contrast, in extrahepatic tissues, the microvasculature is critically important for insulin action, because transport of insulin across the endothelial cell layer is rate limiting for insulin-stimulated glucose disposal. However, very little is known concerning the role in this process of pericytes, the mural cells lining the basolateral membrane of endothelial cells. PDGF-B is a growth factor involved in the recruitment and function of pericytes. We studied insulin action in mice expressing PDGF-B lacking the proteoglycan binding domain, producing a protein with a partial loss of function (PDGF-B(ret/ret)). Insulin action was assessed through measurements of insulin signaling and insulin and glucose tolerance tests. PDGF-B deficiency enhanced hepatic vascular transendothelial transport. One outcome of this change was an increase in hepatic insulin signaling. This correlated with enhanced whole body glucose homeostasis and increased insulin clearance from the circulation during an insulin tolerance test. In obese mice, PDGF-B deficiency was associated with an 80% reduction in fasting insulin and drastically reduced insulin secretion. These mice did not have significantly higher glucose levels, reflecting a dramatic increase in insulin action. Our findings show that, despite already having a high permeability, hepatic transendothelial transport can be further enhanced. To the best of our knowledge, this is the first study to connect PDGF-B-induced changes in hepatic sinusoidal transport to changes in insulin action, demonstrating a link between PDGF-B signaling and insulin sensitivity.

Podocin-green fluorescence protein allows visualization and functional analysis of podocytes

Podocytes do not remain fully differentiated when cultured, and they are difficult to image in vivo, making the study of podocyte biology challenging. Zebrafish embryos are transparent and develop a single, midline, pronephric glomerulus accessible for imaging and systematic functional analysis. Here, we describe a transgenic zebrafish line that expresses green fluorescence protein (GFP) from the zebrafish podocin promoter. The line recapitulates the endogenous pronephric podocin expression pattern, showing GFP expression exclusively in podocytes starting 2 days postfertilization. Using the podocyte GFP signal as a guide for dissection, we examined the pronephric glomerulus by scanning electron microscopy; the surface ultrastructure exhibited fine, interdigitating podocyte foot processes surrounding glomerular capillaries. To determine whether the GFP signal could serve as a direct readout of developmental abnormalities or injury to the glomerulus, we knocked down the podocyte-associated protein crb2b; this led to a loss of GFP signal. Thus, podocin-GFP zebrafish provide a model for ultrastructural studies and in vivo visualization and functional analysis of glomerular podocytes. This model should also be useful for high-throughput genetic or chemical analysis of glomerular development and function.

A two-way communication between microglial cells and angiogenic sprouts regulates angiogenesis in aortic ring cultures

BACKGROUND

Myeloid cells have been associated with physiological and pathological angiogenesis, but their exact functions in these processes remain poorly defined. Monocyte-derived tissue macrophages of the CNS, or microglial cells, invade the mammalian retina before it becomes vascularized. Recent studies correlate the presence of microglia in the developing CNS with vascular network formation, but it is not clear whether the effect is directly caused by microglia and their contact with the endothelium.

METHODOLOGY/PRINCIPAL FINDINGS

We combined in vivo studies of the developing mouse retina with in vitro studies using the aortic ring model to address the role of microglia in developmental angiogenesis. Our in vivo analyses are consistent with previous findings that microglia are present at sites of endothelial tip-cell anastomosis, and genetic ablation of microglia caused a sparser vascular network associated with reduced number of filopodia-bearing sprouts. Addition of microglia in the aortic ring model was sufficient to stimulate vessel sprouting. The effect was independent of physical contact between microglia and endothelial cells, and could be partly mimicked using microglial cell-conditioned medium. Addition of VEGF-A promoted angiogenic sprouts of different morphology in comparison with the microglial cells, and inhibition of VEGF-A did not affect the microglia-induced angiogenic response, arguing that the proangiogenic factor(s) released by microglia is distinct from VEGF-A. Finally, microglia exhibited oriented migration towards the vessels in the aortic ring cultures.

CONCLUSIONS/SIGNIFICANCE

Microglia stimulate vessel sprouting in the aortic ring cultures via a soluble microglial-derived product(s), rather than direct contact with endothelial cells. The observed migration of microglia towards the growing sprouts suggests that their position near endothelial tip-cells could result from attractive cues secreted by the vessels. Our data reveals a two-way communication between microglia and vessels that depends on soluble factors and should extend the understanding of how microglia promote vascular network formation.

Differential Endothelial Transcriptomics Identifies Semaphorin 3G as a Vascular Class 3 Semaphorin

Objective—

To characterize the role of a vascular-expressed class 3 semaphorin (semaphorin 3G [Sema3G]).

Methods and Results—

Semaphorins have been identified as axon guidance molecules. Yet, they have more recently also been characterized as attractive and repulsive regulators of angiogenesis. Through a transcriptomic screen, we identified Sema3G as a molecule of angiogenic endothelial cells. Sema3G-deficient mice are viable and exhibit no overt vascular phenotype. Yet, LacZ expression in the Sema3G locus revealed intense arterial vascular staining in the angiogenic vasculature, starting at E9.5, which was detectable throughout adolescence and downregulated in adult vasculature. Sema3G is expressed as a full-length 100-kDa secreted molecule that is processed by furin proteases to yield 95- and a 65-kDa Sema domain–containing subunits. Full-length Sema3G binds to NP2, whereas processed Sema3G binds to NP1 and NP2. Expression profiling and cellular experiments identified autocrine effects of Sema3G on endothelial cells and paracrine effects on smooth muscle cells.

Conclusion—

Although the mouse knockout phenotype suggests compensatory mechanisms, the experiments identify Sema3G as a primarily endothelial cell–expressed class 3 semaphorin that controls endothelial and smooth muscle cell functions in autocrine and paracrine manners, respectively.

Glomerular podocytes express type 1 adenylate cyclase: inactivation results in susceptibility to proteinuria

BACKGROUND/AIMS

The organization of actin cytoskeleton in podocyte foot processes plays a critical role in the maintenance of the glomerular filtration barrier. The cAMP pathway is an important regulator of the actin network assembly in cells. However, the role of the cAMP pathway in podocytes is not well understood. Type 1 adenylate cyclase (Adcy1), previously thought to be specific for neuronal tissue, is a member of the family of enzymes that catalyses the formation of cAMP. In this study, we characterized the expression and role of Adcy1 in the kidney.

METHODS

Expression of Adcy1 was studied by RT-PCR, Northern blotting and in situ hybridization. The role of Adcy1 in podocytes was investigated by analyzing Adcy1 knockout mice (Adcy1-/-).

RESULTS AND CONCLUSION

Adcy1 is expressed in the kidney specifically by podocytes. In the kidney, Adcy1 does not have a critical role in normal physiological functioning as kidney histology and function are normal in Adcy1-/- mice. However, albumin overload resulted in severe albuminuria in Adcy1-/- mice, whereas wild-type control mice showed only mild albumin leakage to urine. In conclusion, we have identified Adcy1 as a novel podocyte signaling protein that seems to have a role in compensatory physiological processes in the glomerulus.

Cardiac malformations in Pdgfralpha mutant embryos are associated with increased expression of WT1 and Nkx2.5 in the second heart field

Platelet-derived growth factor receptor alpha (Pdgfralpha) identifies cardiac progenitor cells in the posterior part of the second heart field. We aim to elucidate the role of Pdgfralpha in this region. Hearts of Pdgfralpha-deficient mouse embryos (E9.5-E14.5) showed cardiac malformations consisting of atrial and sinus venosus myocardium hypoplasia, including venous valves and sinoatrial node. In vivo staining for Nkx2.5 showed increased myocardial expression in Pdgfralpha mutants, confirmed by Western blot analysis. Due to hypoplasia of the primary atrial septum, mesenchymal cap, and dorsal mesenchymal protrusion, the atrioventricular septal complex failed to fuse. Impaired epicardial development and severe blebbing coincided with diminished migration of epicardium-derived cells and myocardial thinning, which could be linked to increased WT1 and altered alpha4-integrin expression. Our data provide novel insight for a possible role for Pdgfralpha in transduction pathways that lead to repression of Nkx2.5 and WT1 during development of posterior heart field-derived cardiac structures.

Expression of novel podocyte-associated proteins sult1b1 and ankrd25

BACKGROUND/AIMS

Podocytes have a unique function in the renal ultrafiltration that is achieved by expressing proteins that are highly specific to podocytes. In this study, we identified two novel podocyte-associated proteins.

METHODS

The expression of sult1b1 and ankrd25 in mouse tissues was studied by RT-PCR. The protein expression was studied by generating polyclonal antibodies that were used in Western blotting and immunohistochemistry.

RESULTS

By RT-PCR we detected sult1b1 expression only in glomerular, liver and brain tissues. By immunohistochemistry, sult1b1 was detected in the kidney exclusively in the Golgi apparatus of the podocyte. No expression outside the glomerulus was observed in the kidney. The ankrd25 transcript was detected in most mouse tissues analyzed by RT-PCR. In the kidney, however, immunohistochemistry showed that this protein was expressed only by podocyte, mesangial, and smooth muscle cells. In podocytes, ankrd25 was localized to foot processes.

CONCLUSIONS

Identification of these two novel glomerulus-associated proteins opens up possibilities to investigate their role in the renal filter physiology and diseases. We speculate that sult1b1 may be involved in the sulfonylation of podocyte protein podocalyxin, whereas ankrd25 may contribute to controlling actin dynamics in podocyte foot processes.

The absence of pericytes does not increase the sensitivity of tumor vasculature to vascular endothelial growth factor-A blockade

Recent progress with therapies targeting endothelial cells has drawn attention also to the pericytes as potential target cells for antiangiogenic therapy. Published data suggest that pericytes might confer resistance to vascular endothelial growth factor (VEGF) withdrawal in tumors. This hypothesis has been supported by experiments using tumors with reversible transgenic expression of VEGF-A as well as by individual pharmacologically targeting VEGF and platelet-derived growth factor receptor signaling in endothelial cells and pericytes using receptor tyrosine kinase (RTK) inhibitors with different specificities. However, the RTK inhibitors applied thus far are not entirely specific to the mentioned pathways, and therefore, the effects putatively attributed to pericyte targeting might reflect other antitumor effects. Here, we have reinvestigated the putative benefits of doubly targeting endothelial cells and pericytes in the treatment of experimental tumors. For this purpose, we used two highly specific tools, the pericyte-deficient pdgfb(ret/ret) mouse and the recently developed specific anti-VEGF-A antibody G6-31, which neutralizes both murine and human VEGF-A. We generated B16, Lewis lung carcinoma, and T241 subcutaneous tumors in both pdgfb(ret/ret) and control mice and treated these mice with G6-31. Our results fail to show any improved effect of VEGF inhibition, as measured by tumor growth or decrease in vascular density, in pericyte-deficient tumors compared with controls. Our observations suggest that additional targeting of pericytes does not increase the antitumor effect already generated by anti-VEGF drugs.

Antibodies blocking adhesion and matrix binding domains of laminin-332 inhibit tumor growth and metastasis in vivo

Laminin-332 (LN-332), which is essential for epithelial cell adhesion and migration, is up-regulated in most invasive carcinomas. Association between LN-332 and carcinoma cell integrins and stroma collagen is thought to be important for tumor growth and metastasis. Here, we show that function blocking LN-332 antibodies interfering with cellular adhesion and migration in vitro evoke apoptotic pathways. The antibodies also target epithelial tumors in vivo. Antibodies against the cell binding domain of the alpha3 chain of LN-332 inhibited tumor growth by up to 68%, and antibodies against the matrix binding domains of the beta3 and gamma2 chains significantly decreased lung metastases. The LN-332 antibodies appear to induce tumor cell anoikis and subsequent programmed cell death and reduce migration by interfering with tumor cell matrix interactions.

A reverse genetic screen in the zebrafish identifies crb2b as a regulator of the glomerular filtration barrier

The glomerular filtration barrier is necessary for the selective passage of low molecular weight waste products and the retention of blood plasma proteins. Damage to the filter results in proteinuria. The filtration barrier is the major pathogenic site in almost all glomerular diseases and its study is therefore of clinical significance. We have taken advantage of the zebrafish pronephros as a system for studying glomerular filtration. In order to identify new regulators of filtration barrier assembly, we have performed a reverse genetic screen in the zebrafish testing a group of genes which are enriched in their expression within the mammalian glomerulus. In this novel screen, we have coupled gene knockdown using morpholinos with a physiological glomerular dye filtration assay to test for selective glomerular permeability in living zebrafish larvae. Screening 20 genes resulted in the identification of ralgps1, rapgef2, rabgef1, and crb2b. The crumbs (crb) genes encode a family of evolutionarily conserved proteins important for apical-basal polarity within epithelia. The crb2b gene is expressed in zebrafish podocytes. Electron microscopic analysis of crb2b morphants reveals a gross disorganization of podocyte foot process architecture and loss of slit diaphragms while overall polarity is maintained. Nephrin, a major component of the slit diaphragm, is apically mis-localized in podocytes from crb2b morphants suggesting that crb2b is required for the proper protein trafficking of Nephrin. This report is the first to show a role for crb function in podocyte differentiation. Furthermore, these results suggest a novel link between epithelial polarization and the maintenance of a functional filtration barrier.

Dysfunctional microvasculature as a consequence of shb gene inactivation causes impaired tumor growth

Shb (Src homology 2 protein B) is an adapter protein downstream of the vascular endothelial growth factor receptor receptor-2 (VEGFR-2). Previous experiments have suggested a role for Shb in endothelial cell function. Recently, the Shb gene was inactivated and Shb null mice were obtained on a mixed genetic background, but not on C57Bl6 mice. The present study was performed to address endothelial function in the Shb knockout mouse and its relevance for tumor angiogenesis. Tumor growth was retarded in Shb mutant mice, and this correlated with decreased angiogenesis both in tumors and in Matrigel plugs. Shb null mice display an abnormal endothelial ultrastructure in liver sinusoids and heart capillaries with cytoplasmic extensions projecting toward the lumen. Shb null heart VE-cadherin staining was less distinct than that of control heart, exhibiting in the former case a wavy and punctuate pattern. Experiments on isolated endothelial cells suggest that these changes could partly reflect cytoskeletal abnormalities. Vascular permeability was increased in Shb null mice in heart, kidney, and skin, whereas VEGF-stimulated vascular permeability was reduced in Shb null mice. It is concluded that Shb plays an important role in maintaining a functional vasculature in adult mice, and that interference with Shb signaling may provide novel means to regulate tumor angiogenesis.

Paracrine signaling by platelet-derived growth factor-CC promotes tumor growth by recruitment of cancer-associated fibroblasts

Cancer results from the concerted performance of malignant cells and stromal cells. Cell types populating the microenvironment are enlisted by the tumor to secrete a host of growth-promoting cues, thus upholding tumor initiation and progression. Platelet-derived growth factors (PDGF) support the formation of a prominent tumor stromal compartment by as of yet unidentified molecular effectors. Whereas PDGF-CC induces fibroblast reactivity and fibrosis in a range of tissues, little is known about the function of PDGF-CC in shaping the tumor-stroma interplay. Herein, we present evidence for a paracrine signaling network involving PDGF-CC and PDGF receptor-alpha in malignant melanoma. Expression of PDGFC in a mouse model accelerated tumor growth through recruitment and activation of different subsets of cancer-associated fibroblasts. In seeking the molecular identity of the supporting factors provided by cancer-associated fibroblasts, we made use of antibody arrays and an in vivo coinjection model to identify osteopontin as the effector of the augmented tumor growth induced by PDGF-CC. In conclusion, we establish paracrine signaling by PDGF-CC as a potential drug target to reduce stromal support in malignant melanoma.