Biomedical subjects

Alexandra Abramsson

Publications and source records attributed to Alexandra Abramsson.

12 recordsLinked to original sources

PubMed2006

Nonradioactive in situ hybridization on frozen sections and whole mounts.

Nonradioactive in situ hybridization offers a unique opportunity to study gene expression on samples with preserved histological information. This method makes it possible to locate not only where in a tissue a particular gene is expressed, but in many cases also in which specific cell type it is active. Here, we describe our current protocols for in situ hybridization on frozen sections or whole mounts of mouse embryos. The protocols included describe synthesis of a digoxigenin-labeled probe, tissue handling, hybridization of the probe to the mRNA expressed in the sample and signal detection.

Animals↗

PubMed2005

Endothelial/pericyte interactions.

Interactions between endothelial cells and mural cells (pericytes and vascular smooth muscle cells) in the blood vessel wall have recently come into focus as central processes in the regulation of vascular formation, stabilization, remodeling, and function. Failure of the interactions between the 2 cell types, as seen in numerous genetic mouse models, results in severe and often lethal cardiovascular defects. Abnormal interactions between the 2 cell types are also implicated in a number of human pathological conditions, including tumor angiogenesis, diabetic microangiopathy, ectopic tissue calcification, and stroke and dementia syndrome CADASIL. In the present review, we summarize current knowledge concerning the identity, characteristics, diversity, ontogeny, and plasticity of pericytes. We focus on the advancement in recent years of the understanding of intercellular communication between endothelial and mural cells with a focus on transforming growth factor beta, angiopoietins, platelet-derived growth factor, spingosine-1-phosphate, and Notch ligands and their respective receptors. We finally highlight recent important data contributing to the understanding of the role of pericytes in tumor angiogenesis, diabetic retinopathy, and hereditary lymphedema.

Angiopoietin-1↗

PubMed2004

Platelet-derived growth factor production by B16 melanoma cells leads to increased pericyte abundance in tumors and an associated increase in tumor growth rate.

Platelet-derived growth factor (PDGF) receptor signaling participates in different processes in solid tumors, including autocrine stimulation of tumor cell growth, recruitment of tumor stroma fibroblasts, and stimulation of tumor angiogenesis. In the present study, the B16 mouse melanoma tumor model was used to investigate the functional consequences of paracrine PDGF stimulation of host-derived cells. Production of PDGF-BB or PDGF-DD by tumor cells was associated with an increased tumor growth rate. Characterization of tumors revealed an increase in pericyte abundance in tumors derived from B16 cells producing PDGF-BB or PDGF-DD. The increased tumor growth rate associated with PDGF-DD production was not seen in mice expressing an attenuated PDGF beta-receptor and was thus dependent on host PDGF beta-receptor signaling. The increased pericyte abundance was not associated with an increased tumor vessel density. However, tumor cell apoptosis, but not proliferation, was reduced in tumors displaying PDGF-induced increased pericyte coverage. Our findings thus demonstrate that paracrine PDGF production stimulates pericyte recruitment to tumor vessels and suggest that pericyte abundance influences tumor cell apoptosis and tumor growth.

Animals↗

PubMed2004

Neuropilin-1 is required for endothelial tip cell guidance in the developing central nervous system.

Recent evidence indicates that sprouting angiogenesis in the central nervous system (CNS) is a guided process similar to the guidance of axons and insect tracheal tubes. Specialized tip cells of vessel sprouts navigate in response to local depots or gradients of vascular endothelial growth factor (VEGF-A). Neuropilin-1 (Nrp-1) is a transmembrane receptor with a repulsive function in axon guidance. Nrp-1 also binds the VEGF-A splice isoform VEGF165, stimulates angiogenesis, and is necessary for vascular development in the mouse. However, the morphogenetic events controlled by Nrp-1 in angiogenesis have not been defined. Here, we analyzed endothelial tip cell guidance in the CNS of Nrp-1-deficient mice. We focused our attention on the developing hindbrain, which is normally vascularized in a stereotyped manner. Initially, angiogenic sprouts extend along radial glia from the pial surface toward the ventricles, but in the subventricular zone (SVZ), they leave the radial path, turn laterally, and fuse to form a capillary plexus. Radial sprout elongation correlated with tip cell filopodia extensions along nestin-positive radial glial processes, but in the SVZ, the tip cell filopodia also extended perpendicular to the glial tracks and made contact with filopodia of the neighboring sprouts. In Nrp-1-deficient mice, the tip cell filopodia remained associated with the radial glia in the SVZ, which correlated with a failure of sprout turning and elongation across this region. As a result, the sprouts remained blind-ended forming glomeruloid tufts in the SVZ. These observations suggest that Nrp-1 plays an important role in allowing the endothelial tip cell filopodia to switch substrate and protrude in a new direction at a specific location in the developing brain.

Animals↗

PubMed2004

Endothelium-specific ablation of PDGFB leads to pericyte loss and glomerular, cardiac and placental abnormalities.

Platelet-derived growth factor-B (PDGFB) is necessary for normal cardiovascular development, but the relative importance of different cellular sources of PDGFB has not been established. Using Cre-lox techniques, we show here that genetic ablation of Pdgfb in endothelial cells leads to impaired recruitment of pericytes to blood vessels. The endothelium-restricted Pdgfb knockout mutants also developed organ defects including cardiac, placental and renal abnormalities. These defects were similar to those observed in Pdgfb null mice. However, in marked contrast to the embryonic lethality of Pdgfb null mutants, the endothelium-specific mutants survived into adulthood with persistent pathological changes, including brain microhemorrhages, focal astrogliosis, and kidney glomerulus abnormalities. This spectrum of pathological changes is reminiscent of diabetic microangiopathy, suggesting that the endothelium-restricted Pdgfb knockouts may serve as models for some of the pathogenic events of vascular complications to diabetes.

Animals↗

PubMed2003

Endothelial PDGF-B retention is required for proper investment of pericytes in the microvessel wall.

Several platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF) family members display C-terminal protein motifs that confer retention of the secreted factors within the pericellular space. To address the role of PDGF-B retention in vivo, we deleted the retention motif by gene targeting in mice. This resulted in defective investment of pericytes in the microvessel wall and delayed formation of the renal glomerulus mesangium. Long-term effects of lack of PDGF-B retention included severe retinal deterioration, glomerulosclerosis, and proteinuria. We conclude that retention of PDGF-B in microvessels is essential for proper recruitment and organization of pericytes and for renal and retinal function in adult mice.

Alleles↗

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

Transcription profiling of platelet-derived growth factor-B-deficient mouse embryos identifies RGS5 as a novel marker for pericytes and vascular smooth muscle cells.

All blood capillaries consist of endothelial tubes surrounded by mural cells referred to as pericytes. The origin, recruitment, and function of the pericytes is poorly understood, but the importance of these cells is underscored by the severe cardiovascular defects in mice genetically devoid of factors regulating pericyte recruitment to embryonic vessels, and by the association between pericyte loss and microangiopathy in diabetes mellitus. A general problem in the study of pericytes is the shortage of markers for these cells. To identify new markers for pericytes, we have taken advantage of the platelet-derived growth factor (PDGF)-B knockout mouse model, in which developing blood vessels in the central nervous system are almost completely devoid of pericytes. Using cDNA microarrays, we analyzed the gene expression in PDGF-B null embryos in comparison with corresponding wild-type embryos and searched for down-regulated genes. The most down-regulated gene present on our microarray was RGS5, a member of the RGS family of GTPase-activating proteins for G proteins. In situ hybridization identified RGS5 expression in brain pericytes, and in pericytes and vascular smooth muscle cells in certain other, but not all, locations. Absence of RGS5 expression in PDGF-B and PDGFR beta-null embryos correlated with pericyte loss in these mice. Residual RGS5 expression in rare pericytes suggested that RGS5 is a pericyte marker expressed independently of PDGF-B/R beta signaling. With RGS5 as a proof-of-principle, our data demonstrate the usefulness of microarray analysis of mouse models for abnormal pericyte development in the identification of new pericyte-specific markers.

Animals↗

PubMed2003

Neuron-specific ablation of PDGF-B is compatible with normal central nervous system development and astroglial response to injury.

Members of the PDGF family have multiple roles during embryogenesis and in a variety of pathological situations in the adult. One of the major sites of PDGF-B expression in adult mammals are postmitotic CNS neurons. Combined with reported neurotrophic and neuroprotective effects of exogenously administered PDGFs, this has led to the speculation that PDGF-B may have a role in CNS development, in maintenance, or in response to CNS injury. To test these hypotheses, we developed mice in which PDGF-B was ablated genetically in postmitotic neurons at sites where PDGF-B is normally expressed. We found that these mice develop to adulthood without apparent defects. We demonstrate PDGF-B expression in the postnatal mouse hippocampus and forebrain cortex. We show that neuron-specific knockout of PDGF-B does not influence the astroglial and angiogenic responses to injury in the hippocampus or forebrain cortex. We conclude that the role of neuron-derived PDGF-B remains obscure. A role for neuron-derived PDGF-B, if existing, might be redundant with other CNS growth factors. Alternatively, other and more specific analyses of CNS functions in the normal and injured states will be required to demonstrate such a role.

Animals↗

PubMed2003

Endothelial and nonendothelial sources of PDGF-B regulate pericyte recruitment and influence vascular pattern formation in tumors.

Tumor-infiltrating blood vessels deviate morphologically and biochemically from normal vessels, raising the prospect of selective pharmacological targeting. Current antiangiogenic approaches focus mainly on endothelial cells, but recent data imply that targeting pericytes may provide additional benefits. Further development of these concepts will require deeper insight into mechanisms of pericyte recruitment and function in tumors. Here, we applied genetic tools to decipher the function of PDGF-B and PDGF-Rbeta in pericyte recruitment in a mouse fibrosarcoma model. In tumors transplanted into PDGF-B retention motif-deficient (pdgf-b(ret/ret)) mice, pericytes were fewer and were partially detached from the vessel wall, coinciding with increased tumor vessel diameter and hemorrhaging. Transgenic PDGF-B expression in tumor cells was able to increase the pericyte density in both WT and pdgf-b(ret/ret) mice but failed to correct the pericyte detachment in pdgf-b(ret/ret) mice. Coinjection of exogenous pericytes and tumor cells showed that pericytes require PDGF-Rbeta for recruitment to tumor vessels, whereas endothelial PDGF-B retention is indispensable for proper integration of pericytes in the vessel wall. Our data support the notion that pericytes serve an important function in tumor vessels and highlight PDGF-B and PDGF-Rbeta as promising molecular targets for therapeutic intervention.

Animals↗

PubMed2002

Analysis of mural cell recruitment to tumor vessels.

BACKGROUND: Tumor blood vessels are both structurally and functionally abnormal compared with normal vessels. A limited support of mural cells may contribute to these abnormalities. Here, we characterized mural cell recruitment in 2 mouse tumor models and addressed the question of why tumor vessels fail to recruit a proper coat of mural cells. METHODS AND RESULTS: We studied mural cell recruitment to the vasculature of 2 transplantable mouse tumor models, T241 fibrosarcoma and KRIB osteosarcoma. We found that both tumors formed a vessel network with heterogeneous and highly abnormal organization of mural cells. Transplantation of tumors to mice expressing lacZ in mural cells demonstrated that these cells were host-derived. Although tumor vessel endothelium expressed PDGF-B, an embryonic mitogen for mural cells, only very few PDGFRbeta-positive cells were found to be associated with the developing tumor vasculature, suggesting a limited pool of recruitable mural cells. We tested whether exogenous mural cells could be recruited to tumor vessels by injecting mixtures of T241 tumor cells and embryonic mesenchymal cells isolated from mice expressing lacZ in mural cells. In the tumors that arose, lacZ-positive cells were efficiently recruited to the tumor vessels. CONCLUSIONS: T241 and KRIB tumors show a similar highly abnormal organization of vessel-associated mural cells. T241 tumor vessels seem highly capable of recruiting exogenously added mural cells. The sparse mural cell coat of tumor vessels may result from a limited pool of mural cells available for recruitment.

Actins↗

PubMed2002

Expression analysis of PDGF-C in adult and developing mouse tissues.

There are four members of the platelet-derived growth factor (PDGF) family; PDGF-A, PDGF-B, PDGF-C and PDGF-D. Their biological effects are mediated via two tyrosine kinase receptors, PDGFR-alpha and PDGFR-beta, and PDGF-mediated signaling is critical for development of many organ systems. Analysis in adult tissues showed that PDGF-C was mainly expressed in kidney, testis, liver, heart and brain. During development, PDGF-C expression was widespread and dynamic, and found in somites and their derivatives, in kidney, lung, brain, and in several other tissues, particularly at sites of developing epidermal openings. PDGF-C may therefore have unique functions during tissue development and maintenance.

Animals↗