Reconstitution of Rab- and SNARE-dependent membrane fusion by synthetic endosomes.

Rab GTPases and SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) are evolutionarily conserved essential components of the eukaryotic intracellular transport system. Although pairing of cognate SNAREs is sufficient to fuse membranes in vitro, a complete reconstitution of the Rab-SNARE machinery has never been achieved. Here we report the reconstitution of the early endosomal canine Rab5 GTPase, its key regulators and effectors together with SNAREs into proteoliposomes using a set of 17 recombinant human proteins. These vesicles behave like minimal 'synthetic' endosomes, fusing with purified early endosomes or with each other in vitro. Membrane fusion measured by content-mixing and morphological assays requires the cooperativity between Rab5 effectors and cognate SNAREs which, together, form a more efficient 'core machinery' than SNAREs alone. In reconstituting a fusion mechanism dependent on both a Rab GTPase and SNAREs, our work shows that the two machineries act coordinately to increase the specificity and efficiency of the membrane tethering and fusion process.

Endothelial transdifferentiation in hepatocellular carcinoma: loss of Stabilin-2 expression in peri-tumourous liver correlates with increased survival.

BACKGROUND & AIMS: Hepatocellular carcinoma (HCC) is a malignant tumour that is characterized by extensive vascular remodelling and responsiveness to treatment with the anti-angiogenic multikinase inhibitor sorafenib. The aim was to study endothelial remodelling in HCC. METHODS: The murine inducible albumin-SV40-large T-antigen model and two tissue microarrays (TMA) with 295 tumourous and 83 peri-tumourous samples of 296 patients with HCC were analysed for expression of liver sinusoidal endothelial cell (LSEC)-specific marker proteins, stabilin-1 and stabilin-2, LYVE-1 and CD32b. RESULTS: LSEC marker proteins were sequentially lost during HCC progression in the murine HCC model being absent from tumour nodules larger than 800 µm in diameter. Similarly, the TMA analysis of human HCCs revealed loss of all four marker proteins in the majority of tumourous tissue samples. Preservation of LYVE-1 expression showed a significant correlation with low grading (G1). In corresponding peri-tumourous liver tissue, loss of all marker proteins was seen in a minor proportion of cases (34%) while the majority of cases retained expression of at least one of the marker proteins. Loss of stabilin-2 expression in peri-tumourous liver tissue of patients with HCC was significantly less likely to occur (38%) than loss of the other marker proteins (63-95%) and it was associated with significantly longer tumour-specific (P = 0.0523) and overall (P = 0.0338) survival. Loss of stabilin-2 may enhance survival in HCC by preventing endothelial-tumour cell adhesive interactions and microvascular invasion. CONCLUSIONS: In summary, endothelial transdifferentiation is a major pathogenic event in HCC development indicating a switch from vessel co-option/intussusceptive angiogenesis to sprouting angiogenesis.

HIF-dependent hematopoietic factors regulate the development of the embryonic vasculature.

Hypoxia inducible factors (HIFs) regulate adaptive responses to changes in oxygen (O(2)) tension during embryogenesis, tissue ischemia, and tumorigenesis. Because HIF-deficient embryos exhibit a number of developmental defects, the precise role of HIF in early vascular morphogenesis has been uncertain. Using para-aortic splanchnopleural (P-Sp) explant cultures, we show that deletion of the HIF-beta subunit (ARNT) results in defective hematopoiesis and the inhibition of both vasculogenesis and angiogenesis. These defects are rescued upon the addition of wild-type Sca-1(+) hematopoietic cells or recombinant VEGF. Arnt(-/-) embryos exhibit reduced levels of VEGF protein and increased numbers of apoptotic hematopoietic cells. These results suggest that HIF coordinates early endothelial cell emergence and vessel development by promoting hematopoietic cell survival and paracrine growth factor production.

Endothelial deletion of hypoxia-inducible factor-2alpha (HIF-2alpha) alters vascular function and tumor angiogenesis.

Hypoxia-inducible factor-2alpha (HIF-2alpha) is highly expressed in embryonic vascular endothelial cells (ECs) and activates the expression of target genes whose products modulate vascular function and angiogenesis. In this report, we describe a genetic model designed to test the physiologic consequences of deleting HIF-2alpha in murine endothelial cells. Surprisingly, mice with HIF-2alpha-deficient ECs developed normally but displayed a variety of phenotypes, including increased vessel permeability, aberrant endothelial cell ultrastructure, and pulmonary hypertension. Moreover, these animals exhibited defective tumor angiogenesis associated with increased hypoxic stress and tumor cell apoptosis. Immortalized HIF-2alpha-deficient ECs displayed decreased adhesion to extracellular matrix proteins and expressed reduced levels of transcripts encoding fibronectin, integrins, endothelin B receptor, angiopoietin 2, and delta-like ligand 4 (Dll4). Together, these data identify unique cell-autonomous functions for HIF-2alpha in vascular endothelial cells.

Metastatic Dissemination Mimicked in a Multicellular Transwell Assay.

Tumor progression and metastasis are multistep processes that are critically dependent on the interaction of metastasizing tumor cells with other cells of the local microenvironment. Mimicking the single steps of the metastatic cascade in vitro is therefore challenging when investigating not only tumor cell behavior alone but also cellular crosstalk between different cell populations. In particular, the crosstalk of tumor cells with pericytes and endothelial cells when accessing the bloodstream is of great importance for successful intravasation and metastatic dissemination. To examine metastatic intravasation and analyze the interaction of tumor cells with pericytes, which reside within the basement membrane and endothelial cells, aligning the vessel wall, we have designed a 3D in vitro transwell assay mimicking tumor cell intravasation into a vessel. Modifying the Boyden chamber transwell assay by seeding first an endothelial cell layer onto the transwell membrane and covering it with pericytes before adding the tumor cells allows us to investigate the role of pericytes and endothelial cells on tumor cell intravasation and at the same time to study their crosstalk at the molecular level and how this interaction influences tumor cell behavior. It further allows the manipulation of the system for proof-of-principle experimentation.

EPAS1 Is Required for Spermatogenesis in the Postnatal Mouse Testis.

Spermatogenesis, a process involving the differentiation of spermatogonial stem cells into mature spermatozoa, takes place throughout masculine life. A complex system in the testis, including endocrine signaling, physical interactions between germ and somatic cells, spermatocyte meiosis, and timely release of spermatozoa, controls this cycle. We demonstrate herein that decreased O(2) levels and Epas1 activation are critical components of spermatogenesis. Postnatal Epas1 ablation leads to male infertility, with reduced testis size and weight. While immature spermatogonia and spermatocytes are present in Epas1(Delta/Delta) testes, spermatid and spermatozoan numbers are dramatically reduced. This is not due to germ cell-intrinsic defects. Rather, Epas(Delta/Delta) Sertoli cells exhibit decreased ability to form tight junctions, thereby disrupting the blood-testis barrier necessary for proper spermatogenesis. Reduced numbers of tight junction complexes are due to decreased expression of multiple genes encoding tight junction proteins, including TJP1 (ZO1), TJP2 (ZO2), and occludin. Furthermore, Epas1(Delta/Delta) testes exhibit disrupted basement membranes surrounding the seminiferous tubules, causing the premature release of incompletely differentiated germ cells. We conclude that low O(2) levels in the male gonad regulate germ cell homeostasis in this organ via EPAS1.

Hepatic stellate cells limit hepatocellular carcinoma progression through the orphan receptor endosialin.

Hepatocellular carcinoma (HCC) is among the most common and deadliest cancers worldwide. A major contributor to HCC progression is the cross talk between tumor cells and the surrounding stroma including activated hepatic stellate cells (HSC). Activation of HSC during liver damage leads to upregulation of the orphan receptor endosialin (CD248), which contributes to regulating the balance of liver regeneration and fibrosis. Based on the established role of endosialin in regulating HSC/hepatocyte cross talk, we hypothesized that HSC-expressed endosialin might similarly affect cell proliferation during hepatocarcinogenesis. Indeed, the histological analysis of human HCC samples revealed an inverse correlation between tumor cell proliferation and stromal endosialin expression. Correspondingly, global genetic inactivation of endosialin resulted in accelerated tumor growth in an inducible mouse HCC model. A candidate-based screen of tumor lysates and differential protein arrays of cultured HSC identified several established hepatotropic cytokines, including IGF2, RBP4, DKK1, and CCL5 as being negatively regulated by endosialin. Taken together, the experiments identify endosialin-expressing HSC as a negative regulator of HCC progression.

Hepatic stellate cell-expressed endosialin balances fibrogenesis and hepatocyte proliferation during liver damage.

Liver fibrosis is a reversible wound-healing response to injury reflecting the critical balance between liver repair and scar formation. Chronic damage leads to progressive substitution of liver parenchyma by scar tissue and ultimately results in liver cirrhosis. Stromal cells (hepatic stellate cells [HSC] and endothelial cells) have been proposed to control the balance between liver fibrosis and regeneration. Here, we show that endosialin, a C-type lectin, expressed in the liver exclusively by HSC and portal fibroblasts, is upregulated in liver fibrosis in mouse and man. Chronic chemically induced liver damage resulted in reduced fibrosis and enhanced hepatocyte proliferation in endosialin-deficient (EN(KO)) mice. Correspondingly, acute-liver-damage-induced hepatocyte proliferation (partial hepatectomy) was increased in EN(KO) mice. A candidate-based screen of known regulators of hepatocyte proliferation identified insulin-like growth factor 2 (IGF2) as selectively endosialin-dependent hepatocyte mitogen. Collectively, the study establishes a critical role of HSC in the reciprocal regulation of fibrogenesis vs. hepatocyte proliferation and identifies endosialin as a therapeutic target in non-neoplastic settings.

Endothelial cell-derived angiopoietin-2 controls liver regeneration as a spatiotemporal rheostat.

Liver regeneration requires spatially and temporally precisely coordinated proliferation of the two major hepatic cell populations, hepatocytes and liver sinusoidal endothelial cells (LSECs), to reconstitute liver structure and function. The underlying mechanisms of this complex molecular cross-talk remain elusive. Here, we show that the expression of Angiopoietin-2 (Ang2) in LSECs is dynamically regulated after partial hepatectomy. During the early inductive phase of liver regeneration, Ang2 down-regulation leads to reduced LSEC transforming growth factor-ß1 production, enabling hepatocyte proliferation by releasing an angiocrine proliferative brake. During the later angiogenic phase of liver regeneration, recovery of endothelial Ang2 expression enables regenerative angiogenesis by controlling LSEC vascular endothelial growth factor receptor 2 expression. The data establish LSECs as a dynamic rheostat of liver regeneration, spatiotemporally orchestrating hepatocyte and LSEC proliferation through angiocrine- and autocrine-acting Ang2, respectively.

An inducible hepatocellular carcinoma model for preclinical evaluation of antiangiogenic therapy in adult mice.

The limited availability of experimental tumor models that faithfully mimic the progression of human tumors and their response to therapy remains a major bottleneck to the clinical translation and application of novel therapeutic principles. To address this challenge in hepatocellular carcinoma (HCC), one of the deadliest and most common cancers in the world, we developed and validated an inducible model of hepatocarcinogenesis in adult mice. Tumorigenesis was triggered by intravenous adenoviral delivery of Cre recombinase in transgenic mice expressing the hepatocyte-specific albumin promoter, a loxP-flanked stop cassette, and the SV40 large T-antigen (iAST). Cre recombinase-mediated excision of the stop cassette led to a transient viral hepatitis and resulted in multinodular tumorigenesis within 5 to 8 weeks. Tumor nodules with histologic characteristics of human HCC established a functional vasculature by cooption, remodeling, and angiogenic expansion of the preexisting sinusoidal liver vasculature with increasing signs of vascular immaturity during tumor progression. Treatment of mice with sorafenib rapidly resulted in the induction of vascular regression, inhibition of tumor growth, and enhanced overall survival. Vascular regression was characterized by loss of endothelial cells leaving behind avascular type IV collagen-positive empty sleeves with remaining pericytes. Sorafenib treatment led to transcriptional changes of Igf1, Id1, and cMet over time, which may reflect the emergence of potential escape mechanisms. Taken together, our results established the iAST model of inducible hepatocarcinogenesis as a robust and versatile preclinical model to study HCC progression and validate novel therapies.

Endothelial HIF-2α regulates murine pathological angiogenesis and revascularization processes.

Localized tissue hypoxia is a consequence of vascular compromise or rapid cellular proliferation and is a potent inducer of compensatory angiogenesis. The oxygen-responsive transcriptional regulator hypoxia-inducible factor 2α (HIF-2α) is highly expressed in vascular ECs and, along with HIF-1α, activates expression of target genes whose products modulate vascular functions and angiogenesis. However, the mechanisms by which HIF-2α regulates EC function and tissue perfusion under physiological and pathological conditions are poorly understood. Using mice in which Hif2a was specifically deleted in ECs, we demonstrate here that HIF-2α expression is required for angiogenic responses during hindlimb ischemia and for the growth of autochthonous skin tumors. EC-specific Hif2a deletion resulted in increased vessel formation in both models; however, these vessels failed to undergo proper arteriogenesis, resulting in poor perfusion. Analysis of cultured HIF-2α-deficient ECs revealed cell-autonomous increases in migration, invasion, and morphogenetic activity, which correlated with HIF-2α-dependent expression of specific angiogenic factors, including delta-like ligand 4 (Dll4), a Notch ligand, and angiopoietin 2. By stimulating Dll4 signaling in cultured ECs or restoring Dll4 expression in ischemic muscle tissue, we rescued most of the HIF-2α-dependent EC phenotypes in vitro and in vivo, emphasizing the critical role of Dll4/Notch signaling as a downstream target of HIF-2α in ECs. These results indicate that HIF-1α and HIF-2α fulfill complementary, but largely nonoverlapping, essential functions in pathophysiological angiogenesis.

Modulation of receptor recycling and degradation by the endosomal kinesin KIF16B.

Different classes of endosomes exhibit a characteristic intracellular steady-state distribution governed by interactions with the cytoskeleton. We found a kinesin-3, KIF16B, that transports early endosomes to the plus end of microtubules in a process regulated by the small GTPase Rab5 and its effector, the phosphatidylinositol-3-OH kinase hVPS34. In vivo, KIF16B overexpression relocated early endosomes to the cell periphery and inhibited transport to the degradative pathway. Conversely, expression of dominant-negative mutants or ablation of KIF16B by RNAi caused the clustering of early endosomes to the perinuclear region, delayed receptor recycling to the plasma membrane, and accelerated degradation. These results suggest that KIF16B, by regulating the plus end motility of early endosomes, modulates the intracellular localization of early endosomes and the balance between receptor recycling and degradation. We propose that this mechanism could have important implications for signaling.

Hypoxia affects mesoderm and enhances hemangioblast specification during early development.

Hypoxia Inducible Factor (HIF), consisting of HIF1alpha and ARNT (HIF1beta) subunits, activates multiple genes in response to oxygen (O(2)) deprivation. Arnt(-/-) mice exhibit substantial defects in blood cell and vessel development. We demonstrate that hypoxia accelerates the expression of Brachyury (a mesoderm-specific transcription factor), BMP4 (a mesoderm-promoting growth factor) and FLK1 (a marker of hemangioblasts, the bipotential progenitor of endothelial and hematopoietic cells) in differentiating ES cell cultures. Significantly, proliferation of embryonic hemangioblasts (BL-CFCs) is regulated by hypoxia, as Arnt(+/+) ES cells generate increased numbers of FLK1(+) cells, and BL-CFCs with accelerated kinetics in response to low O(2). This response is HIF-dependent as Arnt(-/-) ES cells produce fewer FLK1(+) cells and BL-CFCs, under both normoxic and hypoxic conditions. Interestingly, this defect is rescued when Arnt(-/-) ES cells are co-cultured with Arnt(+/+) ES cells. Vegf(+/-)or Vegf(-/-) ES cells generate proper numbers of FLK1(+) cells but fewer BL-CFCs, suggesting that additional factors regulated by HIF (other than VEGF) are involved in these early events. Thus, hypoxic responses are important for the establishment of various progenitor cells, including early mesoderm and its differentiation into hemangioblasts. Together these data suggest that ineffective responses to hypoxia in Arnt(-/-) embryos abrogate proper cardiovascular development during early embryogenesis, including the pathways controlling hemangioblast differentiation.