There are 5 distinct aspects to this study. (i) Two histological stains for collagen were compared against each other for the first time, namely Herovici's technique and picrosirius-polarization. (ii) Skin samples from embalmed cadaveric tissue from human cadavers were compared against samples taken from surgical patients. (iii) Skin samples were studied from different regions of the body to assess if dermal structure correlates with scarring potential. (iv) Skin samples were sectioned in a plane parallel to the epidermis to gain further insight into dermal structure. (v) A novel basement membrane stain was produced. Type I and type III collagen are important structural constituents of dermis and play a crucial role in wound healing. Only two traditional histological methods are thought to differentiate between them, so avoiding the need for antibodies. These were compared against each other for the first time in order to establish differences in image quality and discrimination between Type I and type III collagen. Neither technique requires antibodies, however picrosirius requires polarisation microscopy. to result in a clearer, consistently reproducible collagen staining pattern than the picrosirius method and more importantly did not require elaborate apparatus to analyze. Additionally other cellular elements were visible. Skin samples for research are often obtained from surgical excision. This clearly limits which tissues are available for comparative study to those areas operated on. Studying samples from embalmed medical school cadavers has the great advantage of studying areas of the body not routinely available from common surgical procedures. It was therefore desirable to assess whether embalmed cadaveric tissues exhibited different properties by virtue of their age and the embalming process compared to fresh surgical specimens, in order to give confidence that studies utilising the former would be equally valid. To test this, 58 skin samples from embalmed medical school cadavers were compared to skin samples from 38 fresh operative specimens. The levels of tissue preservation and processing artefacts were similar in both groups. Embalmed medical school cadavers clearly offer an opportunity to study tissue areas not routinely available during surgery. This is the first time such a comparison has been made. Many things will affect the final appearance of the scar, but the single most important determinant is the body region affected. The most common areas for unfavourable scarring, specifically keloid or hypertrophic scarring have been shown to be the ear, deltoid and sternal areas. To test the hypothesis that there is no difference in histological structure of skin that correlates to body region, comparative histology was undertaken exploring the regional variations of skin characteristics in 58 cadaveric samples. Closely comparable samples were taken from the deltoid (9), abdomen (13), sternum (10), post-auricular (5), earlobe (12) and eyelid (9). Epidermal thickness, epidermal appendage density and collagen fibre orientation were examined and qualitative structural differences were assessed for each region Skin samples were then grouped by both topographical location of the body and scarring potential. Skin samples exhibited qualitative and quantifiable regional variations in the characteristics studied. Epidermal thickness and appendage counts did not correlate with scarring potential. Both however were statistically significantly higher in skin sampled from the head compared to the trunk. Bundles of collagen fibres in the reticular dermis were grouped according to their orientation in relation to the coronal plane; either parallel, oblique or perpendicular. The ratio of oblique to parallel fibres was statistically significantly higher in body areas with poorer scarring prognosis. This corresponds to a more disorganised arrangement of collagen fibres in these areas. Further qualitative understanding of dermal collagen fibres came from perpendicular to conventional histological samples. This new method stained basement membranes purple, cytoplasm was stained greenish-brown and nuclei dark brown. Collagen fibres were either thin and blue or thick and green. This method was compared to PAS staining and although required more preparative steps allows greater identification of other cellular structures.

Cardiovascular disease is a leading cause of morbidity worldwide. Many cardiomyopathies and developmental defects arise from misregulation of the cardiac extracellular matrix (ECM), a dynamic network of proteins, growth factors, and signaling molecules that form a protective sheath around organs and tissues. Changes in ECM composition are mediated in part by matrix metalloproteinases (MMPs) and tissue inhibitors of MMPs (TIMPs). ECM dysregulation leads to outcomes such as fibrotic scarring, hypertrophy, and myocardial infarction. Although fundamental to heart formation and function, the regulation of ECM integration and remodeling during growth is poorly understood. To investigate this, I developed a novel adaptation of fluorescence recovery after photobleaching (FRAP), which, for the first time, allows us to assess ECM protein incorporation during growth in live, intact Drosophila larvae. As such, recovery of fluorescently tagged proteins is a proxy for addition or relocation of ECM protein. We focus on Collagen IV (Viking), a conserved protein and major constituent of the basement membrane (BM). Integration and stabilization of Collagen IV in the BM is poorly understood, however is known to be mediated in part by Collagen modifying proteins secreted protein acidic and rich in cysteine (SPARC) and lysyl oxidase (Lox) are known. We established a time course for Vkg-GFP fluorescence accretion in the heart and body wall muscle throughout larval development, under normal conditions and those in which mmp2 or timp is overexpressed. We also observed the effects reducing the activity of SPARC and Lox Vkg dynamics in the early third instar cardiac ECM. In wildtype, we report a strong phasic pattern of Vkg accumulation at second to third instar ecdysis, potentially to support growth of the succeeding instar. Heart-specific overexpression of mmp2 and timp, the inhibitor of mmp2, perturbs net fluorescence recovery as well as estimated turnover of Vkg-GFP. Our results suggests that MMPs are positive regulators of Vkg/Col IV turnover in the ECM, which is in alignment with other recent studies (Davis et al., 2022; Töpfer et al., 2022). Loss of SPARC and Lox appears to affect estimated Vkg turnover in the cardiac ECM, consistent with a role for these proteins in integrating and stabilizing Collagen IV in the BM. These findings have implications in cardiac conditions and in other ECM-related disorders and diseases such as connective tissue disorders, muscular dystrophy, fibrosis, and cancer. / Thesis / Doctor of Science (PhD)

<p>Basement membrane (BM) is the thin, dense, highly cross-linked form of extracellular matrix that underlies all epithelia and endothelia, as well as surrounds muscle, nerve and fat. These sheet-like networks function as physiological barriers to maintain tissue homeostasis. During normal developmental processes and immune surveillance, cells invade through BM to establish tissues and fight infection. Similarly, metastatic cancer cells are thought to co-opt normal programs for BM transmigration as they spread from primary tumors and colonize distant tissues. The difficulty of visualizing cell-BM interactions during invasion in vivo has left the cellular and molecular mechanisms used to breach BM undefined. Specialized F-actin-rich matrix-degrading membrane protrusions, termed invadosomes, have been described in cultured invasive cell lines for more 30 years. Invadosomes are hypothesized to mediate BM penetration during cancer metastasis. Despite promising advances in intravital imaging technologies, however, invadosomes have yet to be observed in cells transmigrating BM in vivo, leaving their physiological relevance unclear. Anchor cell invasion in C. elegans is a simple in vivo model of cell invasion that allows for combined visual and genetic analysis of BM transmigration. In this dissertation I develop high-resolution time-lapse imaging approaches to understand the dynamic interactions that occur at the AC-BM interface during invasion. Through the course of this work we identify an integrin-based mechanism that polarizes the AC towards the BM. We further discover protrusive F-actin-based invadosome structures that mediate BM breach during anchor cell (AC) invasion. We find that in most cases only one or two invadosomes penetrate the BM and then transform into an invasive protrusion that guides the AC through a single BM gap. Using genetics and quantitative single-cell image analysis we characterize several molecular regulators of invadosome formation in vivo. Our findings establish an essential role for invadosomes during BM transmigration in vivo, and support the idea that these structures are a core, conserved element of a normal invasive cellular strategy activated during cancer metastasis.</p> / Dissertation

Background Pancreatic ductal adenocarcinoma (PDAC) patients have the poorest 5-year survival rates of all cancer forms. It is difficult to diagnose at early disease stages, tumour relapse after surgery is common, and current chemotherapies are ineffective. Carbohydrate antigen 19-9 (Ca 19-9), the only clinically implemented PDAC biomarker, is insufficient for diagnostic and screening purposes. PDAC tumours are characterised by a voluminous stroma that is rich in extracellular matrix (ECM) molecules such as collagens, hyaluronan (HA) and matricellular proteins. These stromal components have been suggested to promote PDAC cell migration, proliferation, evasion of apoptosis and chemotherapy resistance. Those events are mediated via interactions with adhesion receptors, such as integrins and CD44 receptors expressed on cancer cell surfaces. Micro-RNAs (miRNA) post-transcriptionally regulate gene expression in health and disease. At the time of PDAC diagnosis, miRNA levels are altered both in plasma and tumour tissue. Before PDAC diagnosis, tissue miRNA levels are altered in precursor lesions, raising the possibility that plasma miRNAs might aid in early detection. In this thesis, it is hypothesised that stromal components and miRNAs can serve as tissue or blood based biomarkers in PDAC. The aims are: (1) to characterise the expression of stromal components and their receptors in normal and cancerous tissue; (2) to find potential stroma-associated tissue and blood-based biomarkers for diagnosis and prognosis estimates; (3) to determine the cellular effects of type IV collagen (Col IV) in PDAC; (4) to determine if plasma miRNAs that are altered in manifest PDAC can be used to diagnose PDAC earlier. Methods The expression patterns of Col IV, Col IV-binding integrin subunits (α1, α2, β1), Endostatin, Osteopontin (OPN) and Tenascin C (TNC) were analysed in frozen PDAC and normal pancreatic tissue. A tissue microarray (TMA) was constructed using formalin-fixed, paraffin-embedded primary tumours and lymph node metastases. The TMA was used to study the expression levels and associations with survival of the standard CD44 receptor (CD44s), its variant isoform 6 (CD44v6), HA, OPN and Col IV. Circulating levels of HA, Col IV, Endostatin, OPN and TNC were measured in PDAC patients and healthy individuals, and compared with conventional tumour markers (Ca 19-9, CEA, Ca 125 and TPS). The functional roles of Col IV were studied in PDAC cell lines by: (1) growth on different matrices (2) blocking Col IV binding integrin subunits, (3) blocking the Col IV domains 7s, CB3 and NC1, and (4) by down regulation of PDAC cell synthesis of Col IV using siRNA transfection. Plasma miRNAs alterations were screened for in samples from patients with manifest disease, using real-time quantitative PCR (RT-qPCR). To find early miRNA alterations, levels of those miRNAs that were altered at diagnosis were measured in prediagnostic plasma samples. Results High tissue expression of both the standard CD44 receptor (CD44s) and its variant isoform CD44v6 as well as low expression of stromal OPN were associated with poor survival. In addition, high CD44s and low OPN predicted poor survival independent of established prognostic factors. Circulating Col IV, Endostatin, OPN, TNC and HA were increased in preoperative samples from PDAC patients. Preoperatively, higher levels of serum-HA and plasma-Endostatin were associated with shorter survival. Postoperatively, higher levels of Col IV, Endostatin and OPN were associated with shorter survival. On the contrary, only one of the conventional tumour markers was associated with survival (Ca 125). Col IV stimulated PDAC cell proliferation and migration and inhibited apoptosis in vitro, dependent on the collagenous domain (CB3) of Col IV and the Col IV binding integrin subunit β1. Reduced endogenous Col IV synthesis inhibited these effects, suggesting that PDAC cells synthesise Col IV to stimulate tumour-promoting events via a newly discovered autocrine loop. 15 miRNAs were altered in early stage PDAC patients and the combination of these markers outperformed Ca 19-9 in discriminating patients from healthy individuals. However, none of the miRNAs were altered in prediagnostic samples, suggesting that plasma miRNA alterations appear late in the disease course. Conclusions Up regulated stromal components in PDAC tumours are detectable in blood samples and are potential diagnostic and prognostic biomarkers in PDAC. High circulating levels of Col IV, Endostatin, OPN and HA predict poor survival, as well as high expression of CD44s and CD44v6 and low expression of OPN in tumour tissue. PDAC cells synthesise Col IV, which forms BM-like structures close to cancer cells and promote tumour progression in vitro via an autocrine loop. Several plasma-miRNAs are altered in PDAC, but are not useful for early discovery.

Le mélanome cutané est le plus agressif des cancers de la peau. Sa dangerosité provient de sa grande capacité à former des métastases. A l’heure actuelle, la prévention et la détection précoce de la lésion primitive restent les seuls moyens d’aboutir à une guérison. En effet, ce cancer est particulièrement reconnu pour sa résistance aux thérapies actuelles sous sa forme métastatique. Le mélanome se développe à partir des mélanocytes de la peau, qui après des évènements oncogéniques, prolifèrent de manière anarchique dans l’épiderme. Par la suite, certaines cellules de la tumeur acquièrent un phénotype invasif qui leur permet de franchir la jonction dermo-épidermique et envahissent le derme, pour y proliférer et le coloniser en profondeur. Une fois dans le derme, les cellules de mélanome peuvent disséminer à distance par voie lymphatique et sanguine pour former des métastases dans les organes cibles. La dégradation de la jonction dermo-épidermique conduisant à l’invasion du derme est la première étape qui mène à la formation de métastase. Dans l’équipe on s’intéresse à l’étude des mécanismes de l’invasion dermique. Grâce au travail fourni, nous avons pu identifier un nouvel acteur moléculaire intervenant dans l’invasion précoce du mélanome cutané : La Tétraspanine 8. Le but de ce travail de thèse a donc été de définir le rôle et le mode d’action de cette protéine. Nous avons réussi à établir l’implication de la Tétraspanine 8 dans la perte d’adhérence des cellules de mélanome aux protéines de la matrice extracellulaire, en association avec une inhibition de l’activation de l’intégrine β1 et avec une diminution de la phosphorylation de la kinase ERK. Nous avons également obtenu des résultats impliquant la Tspan8 dans l’invasion du derme par les cellules de mélanome, en association avec augmentation de la stabilité de β-caténine. Enfin, les tests de tumorigénicité, réalisés dans les souris Nude, permettent d’établir que l’expression de Tspan8 procure aux cellules de mélanome un pouvoir tumorigène. À long terme, ce travail vise à faire émerger de nouveaux marqueurs de pronostic mais aussi de nouvelles cibles thérapeutiques visant à bloquer la progression du mélanome avant l’apparition des métastases / Cutaneous melanoma is the most aggressive skin cancer since its great ability to form metastasis. Successful treatment depends on its early detection, as the metastatic form is resistant to current therapies. Melanoma cells, developing from the melanocytes of the skin, proliferate first in the epidermis. Subsequently, some tumor cells acquire an invasive phenotype, degrade the dermal-epidermal junction and invade the dermis where they proliferate and deeply colonize the tissue. Melanoma cells can then enter the circulatory system and disseminate to form metastases in the target organs. Degradation of the basement membrane and invasion of the dermis are therefore the early events of melanoma tumor invasion and the first step leading to the formation of metastases. In the team, we are interested in the study of the poorly understood mechanisms of dermal invasion. We have identified a novel molecular mediator of the early cutaneous melanoma invasion : the Tetraspanin 8. The aim of this thesis was therefore to define the role and the mode of action of this protein. We have established the involvement of Tetraspanin 8 in the loss of melanoma cells anchorage to matrix proteins and also in the degradation of dermalepidermal junction components. We have then obteined data that indicate the involvement of ERK/MAPK and PI3K/AKT signaling pathways, in the invasive phenotype, downstream of the Tetraspanin 8. Although much is yet to be learned ragarding the clinical relevace of Tetraspanin 8 function, we postulate that it could be a promising new therapeutic target in anti-invasive therapies for cutaneous melanoma

INTRODUÇÃO: Embora seja reconhecida a existência do espessamento da membrana basal e da fibrose da concha nasal na rinite alérgica, não há estudos descritivos do comportamento da mucosa nasal nos pacientes com rinite idiopática. O propósito desse estudo é descrever possíveis alterações na membrana basal e na lâmina própria da concha nasal inferior em pacientes com rinite alérgica ou idiopática, além do estudo quantitativo das fibras colágenas nesta localização. MÉTODOS: Analisou-se na concha nasal inferior obtida através de turbinectomia bilateral em 28 pacientes - 14 com rinite alérgica e 14 com rinite idiopática - a área ocupada pelo colágeno, a espessura da membrana basal e o diâmetro das fibrilas de colágeno através do uso de microscopia óptica (coloração Hematoxilina-eosina e Picrossírius-hematoxilina), microscopia eletrônica e imunoistoquímica para laminina e colágeno IV. RESULTADOS: 1) pacientes com rinite alérgica apresentaram significantemente maior área da concha nasal ocupada por colágeno do que o grupo com rinite idiopática. 2) a membrana basal de pacientes com rinite alérgica foi significantemente mais espessa. 3) a lâmina reticular da membrana basal dos pacientes com rinite alérgica apresentaram fibrilas de colágeno com menor diâmetro que os pacientes com rinite idiopática. 4) não houve diferenças significativas entre os grupos na distribuição de laminina e de colágeno IV. CONCLUSÕES: Alterações na mucosa nasal ocorrem na rinite alérgica, sendo caracterizadas pelo aumento da espessura da membrana basal e por fibrose. Na rinite idiopática, observou-se uma mucosa com aspecto estrutural semelhante aos pacientes normais / INTRODUCTION: Despite our knowledge about nasal conchae fibrosis and basement membrane thickening in allergic rhinitis, there are no descriptive studies on nasal mucosa behavior in patients with idiopathic rhinitis. The aim of our study was to describe possible changes in the basement membrane and lamina propria of the inferior concha in patients with idiopathic or allergic rhinitis, in addition to a quantitative study of collagen fibers in this site. METHODS: The inferior nasal concha obtained from 28 patients submitted to bilateral turbinectomy was examined - 14 with allergic rhinitis and 14 with idiopathic rhinitis; analyzing the collagen area, the basement membrane thickness and the collagen fibrils diameter using optical microscopy (Hematoxylin-eosin and Picrosirius-hematoxylin staining), electron microscopy and immunohistochemistry for laminin and collagen IV. RESULTS: 1) patients with allergic rhinitis had a significantly larger area of the nasal concha occupied by collagen than the group with idiopathic rhinitis. 2) the basement membrane of patients with allergic rhinitis was significantly thicker. 3) the reticular lamina of the basement membrane of patients with allergic rhinitis had collagen fibrils with diameters which were smaller than those from patients with idiopathic rhinitis. 4) there were no significant differences between the groups concerning the distribution of laminin and collagen IV. CONCLUSIONS: Alterations to the nasal mucosa that happen in allergic rhinitis are characterized by basement membrane thickening and fibrosis. In idiopathic rhinitis the patients\' mucosae were structurally similar to those from normal patients

<p>Basement membranes are thin extracellular sheets that surround muscle, fat and peripheral nerve cells and underlay epithelial and endothelial cells. Laminins are one of the main protein families of these matrices. Integrins and dystroglycan are receptors for laminins, connecting cells to basement membranes. Each laminin consists of three different chains, (α, β, γ). Laminin-1 (α1β1γ1) was the first laminin to be found and is the most frequently studied. Despite this, it was unclear where its α1 chain was expressed. A restricted distribution of the α1 chain in the adult epithelial basement membranes was demonstrated in the present study. In contrast, dystroglycan was found to have a much broader distribution. Dystroglycan is an important receptor for α2-laminins in muscle, but binds also α1-laminins. The more ubiquitous α5-laminins were also shown to bind dystroglycan, but with distinctly lower affinity than α1- and α2- laminins. </p><p>The biological roles of different laminin isoforms have been investigated. Differences were found in the capacity of various tested laminins to promote epithelial cell adhesion. The α5-laminins were potent adhesive substrates, a property shown to be dependent on α3 and α6 integrins. Each receptor alone could promote efficient epithelial cell adhesion to α5-laminins. Yet, only α6 integrin and in particular the α6A cytoplasmic splice variant could be linked to laminin-mediated activation of a mitogen-activated protein kinase (MAP kinase) pathway. Attachment to either α1- or α5-laminins activated extracellular-signal regulated kinase (ERK) in cells expressing the integrin α6A variant, but not in cells expressing α6B. A new role for dystroglycan as a suppressor of this activation was demonstrated. Dystroglycan antibodies, or recombinant fragments with high affinity for dystroglycan, decreased ERK activation induced by integrin α6 antibodies. Integrin αvβ3 was identified as a novel co-receptor for α5-laminin trimers. Cell attachment to α5-laminins was found to facilitate growth factor induced cell proliferation. This proliferation could be blocked by antibodies against integrin αvβ3 or by an inhibitor of the MEK/ERK pathway. Therefore, integrin αvβ3 binding to α5-laminins could be of biological significance.</p>

<p>Blood vessels transport blood with essential nutrients and oxygen to the cells in our body. In a healthy adult, formation of new vessels (angiogenesis) occurs only in case of tissue repair and growth. Physiological angiogenesis requires precise regulation of multiple signaling components, a process which is deregulated in a number of pathological conditions, such as cancer. This thesis is focused on the role of laminins, heparan sulfate proteoglycans (HSPGs) and vascular endothelial growth factor (VEGF)-A in regulation of vascular development and angiogenesis. As a model, we have used embryonic stem cells that differentiate to form blood vessels in a manner faithfully recapitulating the <i>in vivo</i> processes. </p><p>We show that the basement membrane (BM) protein laminin-111 promotes maturation of endothelial cells in the presence of fibroblast growth factor-2, a known endothelial cell mitogen. However, embryonic stem cells are able to differentiate into endothelial cells also in the absence of laminin deposition in the vascular BM. Sprouting angiogenesis, induced by VEGF-A, is also not strictly dependent on laminin deposition. On the other hand, in the absence of laminins, vessels are enlarged. These data suggest an important role for laminins in regulation of the vessel diameter.</p><p>We also show that HSPGs serve as coreceptors for VEGF-A to regulate vascular development. The mode of presentation of HSPGs, <i>in</i> <i>cis</i> (on the endothelial cell) or <i>in</i> <i>trans</i> (on an adjacent cell such as pericytes), is critical in regulation of VEGF receptor-2 activation and stimulation of vascular development. Binding of VEGF-A to HSPGs <i>in</i> <i>trans</i> leads to accumulation of activated VEGFR-2 in endothelial cells and to prolonged signaling. This demonstrates a potential role for HSPGs in regulation of receptor trafficking and signaling kinetics, with possible implications also for other HS-binding ligand/receptor systems.</p>

Basement membranes are thin extracellular sheets that surround muscle, fat and peripheral nerve cells and underlay epithelial and endothelial cells. Laminins are one of the main protein families of these matrices. Integrins and dystroglycan are receptors for laminins, connecting cells to basement membranes. Each laminin consists of three different chains, (α, β, γ). Laminin-1 (α1β1γ1) was the first laminin to be found and is the most frequently studied. Despite this, it was unclear where its α1 chain was expressed. A restricted distribution of the α1 chain in the adult epithelial basement membranes was demonstrated in the present study. In contrast, dystroglycan was found to have a much broader distribution. Dystroglycan is an important receptor for α2-laminins in muscle, but binds also α1-laminins. The more ubiquitous α5-laminins were also shown to bind dystroglycan, but with distinctly lower affinity than α1- and α2- laminins. The biological roles of different laminin isoforms have been investigated. Differences were found in the capacity of various tested laminins to promote epithelial cell adhesion. The α5-laminins were potent adhesive substrates, a property shown to be dependent on α3 and α6 integrins. Each receptor alone could promote efficient epithelial cell adhesion to α5-laminins. Yet, only α6 integrin and in particular the α6A cytoplasmic splice variant could be linked to laminin-mediated activation of a mitogen-activated protein kinase (MAP kinase) pathway. Attachment to either α1- or α5-laminins activated extracellular-signal regulated kinase (ERK) in cells expressing the integrin α6A variant, but not in cells expressing α6B. A new role for dystroglycan as a suppressor of this activation was demonstrated. Dystroglycan antibodies, or recombinant fragments with high affinity for dystroglycan, decreased ERK activation induced by integrin α6 antibodies. Integrin αvβ3 was identified as a novel co-receptor for α5-laminin trimers. Cell attachment to α5-laminins was found to facilitate growth factor induced cell proliferation. This proliferation could be blocked by antibodies against integrin αvβ3 or by an inhibitor of the MEK/ERK pathway. Therefore, integrin αvβ3 binding to α5-laminins could be of biological significance.

Blood vessels transport blood with essential nutrients and oxygen to the cells in our body. In a healthy adult, formation of new vessels (angiogenesis) occurs only in case of tissue repair and growth. Physiological angiogenesis requires precise regulation of multiple signaling components, a process which is deregulated in a number of pathological conditions, such as cancer. This thesis is focused on the role of laminins, heparan sulfate proteoglycans (HSPGs) and vascular endothelial growth factor (VEGF)-A in regulation of vascular development and angiogenesis. As a model, we have used embryonic stem cells that differentiate to form blood vessels in a manner faithfully recapitulating the in vivo processes. We show that the basement membrane (BM) protein laminin-111 promotes maturation of endothelial cells in the presence of fibroblast growth factor-2, a known endothelial cell mitogen. However, embryonic stem cells are able to differentiate into endothelial cells also in the absence of laminin deposition in the vascular BM. Sprouting angiogenesis, induced by VEGF-A, is also not strictly dependent on laminin deposition. On the other hand, in the absence of laminins, vessels are enlarged. These data suggest an important role for laminins in regulation of the vessel diameter. We also show that HSPGs serve as coreceptors for VEGF-A to regulate vascular development. The mode of presentation of HSPGs, in cis (on the endothelial cell) or in trans (on an adjacent cell such as pericytes), is critical in regulation of VEGF receptor-2 activation and stimulation of vascular development. Binding of VEGF-A to HSPGs in trans leads to accumulation of activated VEGFR-2 in endothelial cells and to prolonged signaling. This demonstrates a potential role for HSPGs in regulation of receptor trafficking and signaling kinetics, with possible implications also for other HS-binding ligand/receptor systems.