Biomedical subjects

Marcus Fruttiger

Publications and source records attributed to Marcus Fruttiger.

9 recordsLinked to original sources

PubMed2005

Abnormal maturation of the retinal vasculature in type XVIII collagen/endostatin deficient mice and changes in retinal glial cells due to lack of collagen types XV and XVIII.

Type XVIII collagen is important in the early phase of retinal vascular development and for the regression of the primary vasculature in the vitreous body after birth. We show here that the retina in Col18a1-/- mice becomes densely vascularized by anomalous anastomoses from the persistent hyaloid vasculature by day 10 after birth. In situ hybridizations revealed normal VEGF mRNA expression, but the phenotype of collagen XVIII deficient mice closely resembled that of mice expressing VEGF120 and VEGF188 isoforms only, suggesting that type XVIII collagen may be involved in VEGF function. Type XVIII collagen was found to be indispensable for angiogenesis in the eye, as also oxygen-induced neovascularization was less intense than normal in the Col18a1-/- mice. We observed a marked increase in the amount of retinal astrocytes in the Col18a1-/- mice. Whereas the retinal vessels of wild-type mice are covered by astrocytes and the regressing, thin hyaloid vessels are devoid of astrocytes, the retinal vessels in the Col18a1-/- mice were similarly covered by astrocytes but not the persistent hyaloid vessels in the vitreous body. Interestingly, double null mice lacking type XVIII collagen and its homologue type XV collagen had the persistent hyaloid vessels covered by astrocytes, including the parts located in the vitreous body. We thus hypothesize that type XV collagen is a regulator of glial cell recruitment around vessels and that type XVIII collagen regulates their proliferation.

Animals↗

PubMed2005

Oxygen modifies artery differentiation and network morphogenesis in the retinal vasculature.

The mechanisms that control differentiation of immature blood vessels into either arteries or veins are not well understood. Because oxygen tension in arteries is higher than in veins, oxygen has the potential to be an instructive signal for artery/vein (AV) differentiation. We test this hypothesis by exposing newborn mice to moderate hypoxia (10% atmospheric oxygen) and studying AV differentiation in the developing retinal vasculature. Forming retinal arteries fail to express the artery-specific markers Delta-like 4 (Dll4) and EphrinB2 during hypoxia. However, other aspects of AV differentiation are retained such as high levels of alpha smooth muscle actin in arterial mural cells and vein-specific expression of the msr/apj gene. The capillary network between arteries and veins is denser, and capillaries expressing the venous marker msr/apj are found in territories normally occupied by arterial capillaries. Thus, it appears that high oxygen in arterial blood is required for arterial expression of Dll4 and EphrinB2, which could be involved in cell-cell repulsion pathways that dictate the normal segregation of arteries and veins.

Animals↗

PubMed2005

Stabilization of the retinal vascular network by reciprocal feedback between blood vessels and astrocytes.

Development of the retinal vasculature is controlled by a hierarchy of interactions among retinal neurons, astrocytes and blood vessels. Retinal neurons release platelet-derived growth factor (PDGFA) to stimulate proliferation of astrocytes, which in turn stimulate blood vessel growth by secreting vascular endothelial cell growth factor (VEGF). Presumably, there must be counteractive mechanisms for limiting astrocyte proliferation and VEGF production to prevent runaway angiogenesis. Here, we present evidence that the developing vessels provide feedback signals that trigger astrocyte differentiation--marked by cessation of cell division, upregulation of glial fibrillary acidic protein (GFAP) and downregulation of VEGF. We prevented retinal vessel development by raising newborn mice in a high-oxygen atmosphere, which leads, paradoxically, to retinal hypoxia (confirmed by using the oxygen-sensing reagent EF5). The forced absence of vessels caused prolonged astrocyte proliferation and inhibited astrocyte differentiation in vivo. We could reproduce these effects by culturing retinal astrocytes in a low oxygen atmosphere, raising the possibility that blood-borne oxygen itself might induce astrocyte differentiation and indirectly prevent further elaboration of the vascular network.

Animals↗

PubMed2004

Platelet-derived growth factor regulates oligodendrocyte progenitor numbers in adult CNS and their response following CNS demyelination.

To design therapies for demyelinating diseases such as multiple sclerosis, it will be important to understand the mechanisms that control oligodendrocyte progenitor cell (OPC) numbers in the adult central nervous system (CNS). During development, OPC numbers are limited by the supply of platelet-derived growth factor-A (PDGF-A). Here, we examine the role of PDGF-A in regulating OPC numbers in normal and demyelinated adult CNS using transgenic mice that overexpress PDGF-A in astrocytes under the control of the glial fibrillary acidic protein (GFAP) gene promoter (GFAP-PDGF-A mice). In adult GFAP-PDGF-A mice, there was a marked increase in OPC density, particularly in white matter tracts, indicating that the PDGF-A supply controls OPC numbers in the adult CNS as well as during development. To discover whether increasing PDGF expression increases the number of OPCs following demyelination and whether this enhances the efficiency of remyelination, we induced demyelination in GFAP-PDGF-A transgenic mice by intraspinal injection of lysolecithin or dietary administration of cuprizone. In both demyelinating models, OPC density within lesions was significantly increased compared to wild-type mice. However, morphological analysis of lysolecithin lesions did not reveal any difference in the time course or extent of remyelination between GFAP-PDGF-A and wild-type mice. We conclude that the availability of OPCs is not rate limiting for remyelination of focal demyelinated lesions in the mouse. Nevertheless, our experiments show that it is possible to increase OPC population density in demyelinated areas by artificially increasing the supply of PDGF.

Animals↗

PubMed2004

Periodic Delta-like 4 expression in developing retinal arteries.

During vascular development, Notch signalling plays important roles in cell-cell communication and cell fate decisions. We studied expression of Notch 1-4 and its ligand Delta-like 4 (Dll4) in the developing retinal vasculature. Dll4 mRNA is strongly expressed in endothelial cells at the very tips of growing vessels ('tip cells') and also in arteries, where it is expressed in a segmented 'tiger's tail' pattern. This implies that developing retinal arteries contain different types of endothelial cells, Dll4-positive and Dll4-negative. The Dll4-positive stripes do not correspond to any obvious morphological property of the vascular network but correlate to some extent with the distribution of platelet derived growth factor B (PDGF-B) mRNA. However, PDGF-B expression is neither as artery-specific nor as clearly segmented as Dll4. Possible target cells for Dll4 signalling are retinal astrocytes (Notch1 positive), arterial pericytes (Notch3 positive) or arterial endothelial cells themselves (Notch4 positive). However, there is no clear reciprocity of Notch and Dll4 expression that allows identification of the interacting cells. Nevertheless, Dll4 stripes are a novel property of immature arteries, the origin and function of which remain to be explained.

Animals↗

PubMed2003

VEGF guides angiogenic sprouting utilizing endothelial tip cell filopodia.

Vascular endothelial growth factor (VEGF-A) is a major regulator of blood vessel formation and function. It controls several processes in endothelial cells, such as proliferation, survival, and migration, but it is not known how these are coordinately regulated to result in more complex morphogenetic events, such as tubular sprouting, fusion, and network formation. We show here that VEGF-A controls angiogenic sprouting in the early postnatal retina by guiding filopodial extension from specialized endothelial cells situated at the tips of the vascular sprouts. The tip cells respond to VEGF-A only by guided migration; the proliferative response to VEGF-A occurs in the sprout stalks. These two cellular responses are both mediated by agonistic activity of VEGF-A on VEGF receptor 2. Whereas tip cell migration depends on a gradient of VEGF-A, proliferation is regulated by its concentration. Thus, vessel patterning during retinal angiogenesis depends on the balance between two different qualities of the extracellular VEGF-A distribution, which regulate distinct cellular responses in defined populations of endothelial cells.

Animals↗

PubMed2003

Role of arteries in oxygen induced vaso-obliteration.

In mice the retinal vasculature develops in the first postnatal week by spreading from the optic nerve head towards the retinal periphery. During this growth period, exposure to hyperoxia causes vaso-obliteration of capillaries in the retinal center but not in peripheral regions. High oxygen levels lead to downregulation of vascular endothelial growth factor (VEGF), an important survival factor for vascular endothelial cells, which could explain the vaso-obliteration caused by hyperoxia. However, it is not clear why only capillaries in the center of the retina are affected. We therefore investigated how capillary obliteration correlates with VEGF mRNA distribution by in situ hybridization in retinal whole mount preparations. In mouse pups reared under normoxic conditions VEGF mRNA was detectable across the entire vascular network but was virtually absent in the immediate vicinity of arteries. This was true along developing retinal arteries but also around the optic nerve head through which the entire arterial blood supply for the retinal and hyaloid vasculature passes. In these areas capillaries were absent, resulting in so-called capillary free zones. Exposure to hyperoxia caused an expansion of areas with low VEGF mRNA which correlated with capillary obliteration in these regions. Combined capillary obliteration around the optic nerve head and along retinal arteries lead to a large capillary free zone in the center of the retina. Thus, our observations suggest that hyperoxia affects the retinal vasculature by reducing VEGF mRNA levels near arteries and causing a widening of capillary free zones.

Animals↗

PubMed2002

Arteriolar and venular patterning in retinas of mice selectively expressing VEGF isoforms.

The murine VEGF gene is alternatively transcribed to yield the VEGF(120), VEGF(164), and VEGF(188) isoforms, which differ in their potential to bind to heparan sulfate and neuropilin-1 and to stimulate endothelial growth. Here, their role in retinal vascular development was studied in mice selectively expressing single isoforms. VEGF(164/164) mice were normal, healthy, and had normal retinal angiogenesis. In contrast, VEGF(120/120) mice exhibited severe defects in vascular outgrowth and patterning, whereas VEGF(188/188) mice displayed normal venular outgrowth but impaired arterial development. It is noteworthy that neuropilin-1, a receptor for VEGF(164), was predominantly expressed in retinal arterioles. These findings reveal distinct roles of the various VEGF isoforms in vascular patterning and arterial development in the retina.

Animals↗

PubMed2002

Development of the mouse retinal vasculature: angiogenesis versus vasculogenesis.

PURPOSE: The inner vasculature of the retina develops as a spreading network, which is preceded by spindle-shaped cells. These cells are alleged to be vascular precursor cells (angioblasts). This study was designed to test whether such angioblasts exist in neonatal mouse retina. METHODS: In situ hybridization and immunohistochemistry on mouse retinal wholemount preparations were used to visualize specific vascular cell types. RESULTS: In situ hybridization with an RNA probe against vascular endothelial growth factor receptor (VEGFR)-2 (a marker for endothelial cells and angioblasts) labeled the vascular network but failed to label the spindle-shaped cells in front of it. A probe against VEGFR1, a marker for endothelial cells only, revealed the same staining pattern. Pericytes, visualized with a probe against platelet-derived growth receptor (PDGFR)-beta, were spread over the entire vessel network, but not beyond it. However, in situ hybridization with a probe against PDGFRalpha (a marker for retinal astrocytes) labeled spindle-shaped cells preceding the vessel network. CONCLUSIONS: These observations imply that in the mouse retina the spindle-shaped cells preceding the forming vasculature are immature retinal astrocytes and not vascular precursor cells and that the primary vascular network in the retina develops by angiogenesis (budding from existing vessels) and not vasculogenesis (assembly of dispersed angioblasts).

Animals↗