Additional parameters for the morphometry of angiogenesis and lymphangiogenesis in corneal flat mounts

Lipid droplet degradation by autophagy connects mitochondria metabolism to Prox1-driven expression of lymphatic genes and lymphangiogenesis

Autophagy has vasculoprotective roles, but whether and how it regulates lymphatic endothelial cells (LEC) homeostasis and lymphangiogenesis is unknown. Here, we show that genetic deficiency of autophagy in LEC impairs responses to VEGF-C and injury-driven corneal lymphangiogenesis. Autophagy loss in LEC compromises the expression of main effectors of LEC identity, like VEGFR3, affects mitochondrial dynamics and causes an accumulation of lipid droplets (LDs) in vitro and in vivo. When lipophagy is impaired, mitochondrial ATP production, fatty acid oxidation, acetyl-CoA/CoA ratio and expression of lymphangiogenic PROX1 target genes are dwindled. Enforcing mitochondria fusion by silencing dynamin-related-protein 1 (DRP1) in autophagy-deficient LEC fails to restore LDs turnover and lymphatic gene expression, whereas supplementing the fatty acid precursor acetate rescues VEGFR3 levels and signaling, and lymphangiogenesis in LEC-Atg5^-/- mice. Our findings reveal that lipophagy in LEC by supporting FAO, preserves a mitochondrial-PROX1 gene expression circuit that safeguards LEC responsiveness to lymphangiogenic mediators and lymphangiogenesis.

ADAMTS2 and ADAMTS14 substitute ADAMTS3 in adults for proVEGFC activation and lymphatic homeostasis

The capacity of ADAMTS3 to cleave pro-VEGFC into active VEGFC able to bind its receptors and to stimulate lymphangiogenesis has been clearly established during embryonic life. However, this function of ADAMTS3 is unlikely to persist in adulthood because of its restricted expression pattern after birth. Because ADAMTS2 and ADAMTS14 are closely related to ADAMTS3 and are mainly expressed in connective tissues where the lymphatic network extends, we hypothesized that they could substitute for ADAMTS3 during adulthood in mammals allowing proteolytic activation of pro-VEGFC. Here, we demonstrated that ADAMTS2 and ADAMTS14 are able to process pro-VEGFC into active VEGFC as efficiently as ADAMTS3. In vivo, adult mice lacking Adamts2 developed skin lymphedema due to a reduction of the density and diameter of lymphatic vessels, leading to a decrease of lymphatic functionality, while genetic ablation of Adamts14 had no impact. In a model of thermal cauterization of cornea, lymphangiogenesis was significantly reduced in Adamts2- and Adamts14-KO mice and further repressed in Adamts2/Adamts14 double-KO mice. In summary, we have demonstrated that ADAMTS2 and ADAMTS14 are as efficient as ADAMTS3 in activation of pro-VEGFC and are involved in the homeostasis of the lymphatic vasculature in adulthood, both in physiological and pathological processes.

Efficacy of a new automated method for quantification of corneal neovascularisation

BACKGROUND/AIMS

To evaluate the efficacy of a new automated method for quantification of corneal neovascularisation (NV).

METHODS

An in-house software for automated measurement of corneal NV was developed. Anterior segment photographs (ASPs) of 81 consecutive patients with corneal NV were analysed using our newly developed software. Manual measurements were performed by three independent examiners using ImageJ software V.1.48 (National Institute of Health, Bethesda, Maryland, USA). Interobserver reliability of the automated and manual methods, and correlations between the results of both methods were evaluated.

RESULTS

The automated method showed a strong interexaminer reliability (intraclass correlation coefficient (ICC)=0.994), which was slightly better than the manual method (ICC=0.958). A significant correlation was found between the results of both methods (p<0.001 for all three examiners). The time spent for analysis of each ASP was significantly reduced in the automated method compared with the manual method (p<0.001 for all three examiners).

CONCLUSIONS

Our newly developed automated method for quantification of corneal NV was more reproducible and time-saving compared with the manual method. Our method can be useful for diagnosis and monitoring diseases causing corneal NV.

uPARAP/Endo180 receptor is a gatekeeper of VEGFR-2/VEGFR-3 heterodimerisation during pathological lymphangiogenesis

The development of new lymphatic vessels occurs in many cancerous and inflammatory diseases through the binding of VEGF-C to its receptors, VEGFR-2 and VEGFR-3. The regulation of VEGFR-2/VEGFR-3 heterodimerisation and its downstream signaling in lymphatic endothelial cells (LECs) remain poorly understood. Here, we identify the endocytic receptor, uPARAP, as a partner of VEGFR-2 and VEGFR-3 that regulates their heterodimerisation. Genetic ablation of uPARAP leads to hyperbranched lymphatic vasculatures in pathological conditions without affecting concomitant angiogenesis. In vitro, uPARAP controls LEC migration in response to VEGF-C but not VEGF-A or VEGF-CCys156Ser. uPARAP restricts VEGFR-2/VEGFR-3 heterodimerisation and subsequent VEGFR-2-mediated phosphorylation and inactivation of Crk-II adaptor. uPARAP promotes VEGFR-3 signaling through the Crk-II/JNK/paxillin/Rac1 pathway. Pharmacological Rac1 inhibition in uPARAP knockout mice restores the wild-type phenotype. In summary, our study identifies a molecular regulator of lymphangiogenesis, and uncovers novel molecular features of VEGFR-2/VEGFR-3 crosstalk and downstream signaling during VEGF-C-driven LEC sprouting in pathological conditions.

Serine Synthesis via PHGDH Is Essential for Heme Production in Endothelial Cells

The role of phosphoglycerate dehydrogenase (PHGDH), a key enzyme of the serine synthesis pathway (SSP), in endothelial cells (ECs) remains poorly characterized. We report that mouse neonates with EC-specific PHGDH deficiency suffer lethal vascular defects within days of gene inactivation, due to reduced EC proliferation and survival. In addition to nucleotide synthesis impairment, PHGDH knockdown (PHGDH^KD) caused oxidative stress, due not only to decreased glutathione and NADPH synthesis but also to mitochondrial dysfunction. Electron transport chain (ETC) enzyme activities were compromised upon PHGDH^KD because of insufficient heme production due to cellular serine depletion, not observed in other cell types. As a result of heme depletion, elevated reactive oxygen species levels caused EC demise. Supplementation of hemin in PHGDH^KD ECs restored ETC function and rescued the apoptosis and angiogenesis defects. These data argue that ECs die upon PHGDH inhibition, even without external serine deprivation, illustrating an unusual importance of serine synthesis for ECs.

Targeting VEGFR-3/-2 signaling pathways with AD0157: a potential strategy against tumor-associated lymphangiogenesis and lymphatic metastases

Background

Lymphatic metastasis is one of the leading causes of death in patients with different types of cancer and is the main prognostic factor for the disease survival. The formation of new lymphatic vessels (lymphangiogenesis) in primary tumors facilitates tumor cell dissemination to regional lymph nodes and correlates with distant metastases. Lymphangiogenesis has thus emerged as a suitable therapeutic target to block metastases, but no anti-lymphangiogenic compounds have been approved for clinical use to date. Therefore, new or improved therapies blocking lymphatic metastases are urgently required.

Methods

We established murine breast tumors to assess the effect of AD0157 on tumor growth, lymphangiogenesis, and lymphatic dissemination. Then, a battery of in vivo (mouse corneal neovascularization and ear sponges), ex vivo (mouse lymphatic rings and rat mesentery explants), and in vitro (proliferation, tubulogenesis, wound-healing, Boyden chambers, and spheroids) assays was used to give insight into the lymphangiogenic steps affected by AD0157. Finally, we investigated the molecular pathways controlled by this drug.

Results

AD0157 was found to inhibit the growth of human breast cancer xenografts in mice, to strongly reduce tumor-associated lymphangiogenesis and to block metastatic dissemination to both lymph nodes and distant organs. The high anti-lymphangiogenic potency of AD0157 was further supported by its inhibitory activity at low micromolar range in two in vivo pathological models and in two ex vivo assays. In addition, AD0157 inhibited lymphatic endothelial cell proliferation, migration and invasion, cellular sprouting, and tube formation. Mechanistically, this compound induced apoptosis in lymphatic endothelial cells and decreased VEGFR-3/-2, ERK1/2, and Akt phosphorylations.

Conclusions

These findings demonstrate the suitability of AD0157 to suppress tumor-associated lymphangiogenesis. Beyond discovering a new potent anti-lymphangiogenic drug that is worth considering in future clinical settings, our study supports the interest of designing anti-lymphangiogenic therapies to avoid distant metastatic processes.

Electronic supplementary material

The online version of this article (doi:10.1186/s13045-017-0484-1) contains supplementary material, which is available to authorized users.

The role of fatty acid β-oxidation in lymphangiogenesis

Lymphatic vessels are lined by lymphatic endothelial cells (LECs), and are critical for health. However, the role of metabolism in lymphatic development has not yet been elucidated. Here we report that in transgenic mouse models, LEC-specific loss of CPT1A, a rate-controlling enzyme in fatty acid β-oxidation, impairs lymphatic development. LECs use fatty acid β-oxidation to proliferate and for epigenetic regulation of lymphatic marker expression during LEC differentiation. Mechanistically, the transcription factor PROX1 upregulates CPT1A expression, which increases acetyl coenzyme A production dependent on fatty acid β-oxidation. Acetyl coenzyme A is used by the histone acetyltransferase p300 to acetylate histones at lymphangiogenic genes. PROX1-p300 interaction facilitates preferential histone acetylation at PROX1-target genes. Through this metabolism-dependent mechanism, PROX1 mediates epigenetic changes that promote lymphangiogenesis. Notably, blockade of CPT1 enzymes inhibits injury-induced lymphangiogenesis, and replenishing acetyl coenzyme A by supplementing acetate rescues this process in vivo.

Quantitative Assessment of Mouse Mammary Gland Morphology Using Automated Digital Image Processing and TEB Detection

The assessment of rodent mammary gland morphology is largely used to study the molecular mechanisms driving breast development and to analyze the impact of various endocrine disruptors with putative pathological implications. In this work, we propose a methodology relying on fully automated digital image analysis methods including image processing and quantification of the whole ductal tree and of the terminal end buds as well. It allows to accurately and objectively measure both growth parameters and fine morphological glandular structures. Mammary gland elongation was characterized by 2 parameters: the length and the epithelial area of the ductal tree. Ductal tree fine structures were characterized by: 1) branch end-point density, 2) branching density, and 3) branch length distribution. The proposed methodology was compared with quantification methods classically used in the literature. This procedure can be transposed to several software and thus largely used by scientists studying rodent mammary gland morphology.

Antilymphangiogenic therapy to promote transplant survival and to reduce cancer metastasis: what can we learn from the eye?

The lymphatic vasculature is - amongst other tasks - essentially involved in inflammation, (auto)immunity, graft rejection and cancer metastasis. The eye is mainly devoid of lymphatic vessels except for its adnexa, the conjunctiva and the limbus. However, several pathologic conditions can result in the secondary ingrowth of lymphatic vessels into physiologically alymphatic parts of the eye such as the cornea or the inner eye. Therefore, the cornea has served as an excellent in vivo model system to study lymphangiogenesis, and findings from such studies have substantially contributed to the understanding of central principles of lymphangiogenesis also with relevance outside the eye. Grafting experiments at the cornea have been extensively used to analyze the role of lymphangiogenesis in transplant immunology. In this regard, we recently demonstrated the crucial role of lymphatic vessels in mediating corneal allograft rejection and could show that antilymphangiogenic therapy increases graft survival. In the field of cancer research, we recently detected tumor-associated lymphangiogenesis in the most common malignant tumors of the eye, such as conjunctival carcinoma and melanoma, and ciliochoroidal melanoma with extraocular extension. These neolymphatics correlate with an increased risk of local recurrence, metastasis and tumor related death, and may offer potential therapeutic targets for the treatment of these tumors. This review will focus on corneal and tumor-associated ocular lymphangiogenesis. First, we will describe common experimentally used corneal lymphangiogenesis models and will recapitulate recent findings regarding the involvement of lymphatic vessels in corneal diseases and transplant immunology. The second part of this article will summarize findings about the participation of tumor-associated lymphangiogenesis in ocular malignancies and their implications for the development of future therapeutic strategies.

Matrix metalloproteinase-2 governs lymphatic vessel formation as an interstitial collagenase

Lymphatic dysfunctions are associated with several human diseases, including lymphedema and metastatic spread of cancer. Although it is well recognized that lymphatic capillaries attach directly to interstitial matrix mainly composed of fibrillar type I collagen, the interactions occurring between lymphatics and their surrounding matrix have been overlooked. In this study, we demonstrate how matrix metalloproteinase (MMP)-2 drives lymphatic morphogenesis through Mmp2-gene ablation in mice, mmp2 knockdown in zebrafish and in 3D-culture systems, and through MMP2 inhibition. In all models used in vivo (3 murine models and thoracic duct development in zebrafish) and in vitro (lymphatic ring and spheroid assays), MMP2 blockage or down-regulation leads to reduced lymphangiogenesis or altered vessel branching. Our data show that lymphatic endothelial cell (LEC) migration through collagen fibers is affected by physical matrix constraints (matrix composition, density, and cross-linking). Transmission electron microscopy and confocal reflection microscopy using DQ-collagen highlight the contribution of MMP2 to mesenchymal-like migration of LECs associated with collagen fiber remodeling. Our findings provide new mechanistic insight into how LECs negotiate an interstitial type I collagen barrier and reveal an unexpected MMP2-driven collagenolytic pathway for lymphatic vessel formation and morphogenesis.

The proteolytic activity of MT4-MMP is required for its pro-angiogenic and pro-metastatic promoting effects

Membrane-type 4 matrix metalloprotease (MT4-MMP) expression in breast adenocarcinoma stimulates tumor growth and metastatic spreading to the lung. However, whether these pro-tumorigenic and pro-metastatic effects of MT4-MMP are related to a proteolytic action is not yet known. Through site directed mutagenesis MT4-MMP has been inactivated in cancer cells through Glutamic acid 249 substitution by Alanine in the active site. Active MT4-MMP triggered an angiogenic switch at day 7 after tumor implantation and drastically accelerated subcutaneous tumor growth as well as lung colonization in recombination activating gene-1-deficient mice. All these effects were abrogated upon MT4-MMP inactivation. In sharp contrast to most MMPs being primarily of stromal origin, we provide evidence that tumor-derived MT4-MMP, but not host-derived MT4-MMP contributes to angiogenesis. A genetic approach using MT4-MMP-deficient mice revealed that the status of MT4-MMP produced by host cells did not affect the angiogenic response. Despite of this tumor intrinsic feature, to exert its tumor promoting effect, MT4-MMP requires a permissive microenvironment. Indeed, tumor-derived MT4-MMP failed to circumvent the lack of an host angio-promoting factor such as plasminogen activator inhibitor-1. Overall, our study demonstrates the key contribution of MT4-MMP catalytic activity in the tumor compartment, at the interface with host cells. It identifies MT4-MMP as a key intrinsic tumor cell determinant that contributes to the elaboration of a permissive microenvironment for metastatic dissemination.

Digging deeper into lymphatic vessel formation in vitro and in vivo

BACKGROUND

Abnormal lymphatic vessel formation (lymphangiogenesis) is associated with different pathologies such as cancer, lymphedema, psoriasis and graft rejection. Lymphatic vasculature displays distinctive features than blood vasculature, and mechanisms underlying the formation of new lymphatic vessels during physiological and pathological processes are still poorly documented. Most studies on lymphatic vessel formation are focused on organism development rather than lymphangiogenic events occurring in adults. We have here studied lymphatic vessel formation in two in vivo models of pathological lymphangiogenesis (corneal assay and lymphangioma). These data have been confronted to those generated in the recently set up in vitro model of lymphatic ring assay. Ultrastructural analyses through Transmission Electron Microscopy (TEM) were performed to investigate tube morphogenesis, an important differentiating process observed during endothelial cell organization into capillary structures.

RESULTS

In both in vivo models (lymphangiogenic corneal assay and lymphangioma), migrating lymphatic endothelial cells extended long processes exploring the neighboring environment and organized into cord-like structures. Signs of intense extracellular matrix remodeling were observed extracellularly and inside cytoplasmic vacuoles. The formation of intercellular spaces between endothelial cells led to tube formation. Proliferating lymphatic endothelial cells were detected both at the tips of sprouting capillaries and inside extending sprouts. The different steps of lymphangiogenesis observed in vivo are fully recapitulated in vitro, in the lymphatic ring assay and include: (1) endothelial cell alignment in cord like structure, (2) intracellular vacuole formation and (3) matrix degradation.

CONCLUSIONS

In this study, we are providing evidence for lymphatic vessel formation through tunneling relying on extensive matrix remodeling, migration and alignment of sprouting endothelial cells into tubular structures. In addition, our data emphasize the suitability of the lymphatic ring assay to unravel mechanisms underlying lymphangiogenesis.

Does plasminogen activator inhibitor-1 drive lymphangiogenesis?

The purpose of this study is to explore the function of plasminogen activator inhibitor-1 (PAI-1) during pathological lymphangiogenesis. PAI-1, the main physiological inhibitor of plasminogen activators is involved in pathological angiogenesis at least by controlling extracellular proteolysis and by regulating endothelial cell survival and migration. Protease system's role in lymphangiogenesis is unknown yet. Thus, based on its important pro-angiogenic effect, we hypothesized that PAI-1 may regulate lymphangiogenesis associated at least with metastatic dissemination of cancer cells. To address this issue, we studied the impact of PAI-1 deficiency in various murine models of tumoral lymphangiogenesis. Wild-type PAI-1 proficient mice were used as controls. We provide for the first time evidence that PAI-1 is dispensable for tumoral lymphangiogenesis associated with breast cancers either induced by mammary carcinoma cell injection or spontaneously appearing in transgenic mice expressing the polyomavirus middle T antigen (PymT) under the control of a mouse mammary tumor virus long-terminal repeat promoter (MMTV-LTR). We also investigated inflammation-related lymphatic vessel recruitment by using two inflammatory models. PAI-1 deficiency did neither affect the development of lymphangioma nor burn-induced corneal lymphangiogenesis. These novel data suggest that vascular remodelling associated with lymphangiogenesis and angiogenesis involve different molecular determinants. PAI-1 does not appear as a potential therapeutic target to counteract pathological lymphangiogenesis.

Effects of topical and subconjunctival bevacizumab in high-risk corneal transplant survival

PURPOSE

To investigate whether corneal graft survival could be improved by topical or subconjunctival bevacizumab in a murine model of vascularized high-risk corneal transplantation.

METHODS

Before corneal transplantation, intrastromal sutures were placed for 2 weeks in the corneas of BALB/c mice, inducing intense angiogenesis. Allogeneic corneal transplantation was performed using C57BL/6 donor mice. Topical bevacizumab (2.5%) was delivered 3 times a day for 3 weeks in one treatment group, and 0.02 mL (0.5 mg) bevacizumab was injected subconjunctivally at days 0, 4, 8, and 15 after transplantation in the other treatment group. The control group received no treatment. Grafts were examined twice a week for 8 weeks by slit-lamp microscopy and were photographed once a week by slit-lamp digital camera and scored for opacity. For assessment of corneal neovascularization (NV), a quantitative method was used to measure three primary metrics including neovascular area, vessel caliber, and neovessel invasion area.

RESULTS

Both topical and subconjunctival bevacizumab treatment reduced neovascular area and vessel caliber; however, the regression of corneal NV was more profound when treated subconjunctivally. The mean percentage reduction of neovascular area was 55% (P < 0.05) by week 8 in the subconjunctival treatment group and 33% (P = 0.15) in the topical group. Only subconjunctival bevacizumab treatment resulted in significant regression of neovessel invasion area (P < 0.05). All corneal transplants in both the control and the topical groups were rejected by 4 weeks after transplantation. However, in the subconjunctival treatment group, 33% of corneal grafts survived (P < 0.01).

CONCLUSIONS

Subconjunctival bevacizumab may offer an adjunctive measure to conventional therapies in preventing graft rejection in high-risk corneal transplantation.