Capillary endothelial cells secrete a heparin-binding mitogen for pericytes

Pericytes regulate VEGF-induced endothelial sprouting through VEGFR1

Pericytes adhere to the abluminal surface of endothelial tubules and are required for the formation of stable vascular networks. Defective endothelial cell-pericyte interactions are frequently observed in diseases characterized by compromised vascular integrity such as diabetic retinopathy. Many functional properties of pericytes and their exact role in the regulation of angiogenic blood vessel growth remain elusive. Here we show that pericytes promote endothelial sprouting in the postnatal retinal vasculature. Using genetic and pharmacological approaches, we show that the expression of vascular endothelial growth factor receptor 1 (VEGFR1) by pericytes spatially restricts VEGF signalling. Angiogenic defects caused by pericyte depletion are phenocopied by intraocular injection of VEGF-A or pericyte-specific inactivation of the murine gene encoding VEGFR1. Our findings establish that pericytes promote endothelial sprouting, which results in the loss of side branches and the enlargement of vessels when pericyte function is impaired or lost.

Isolation and functional characterization of pericytes derived from hamster skeletal muscle

AIM

At the interface of tissue and capillaries, pericytes (PC) may generate electrical signals to be conducted along the skeletal muscle vascular network, but they are functionally not well characterized. We aimed to isolate and cultivate muscle PC allowing to analyse functional properties considered important for signal generation and conduction.

METHODS

Pericytes were enzymatically isolated from hamster thigh muscles and further selected during a 16-30 days' cultivation period. PC markers were studied by fluorescence activated cell scanning (FACS) and immunocytochemistry. Electrical properties of the cultured PC were investigated by patch clamp technique as well as the membrane potential sensitive dye DiBAC(4) (3).

RESULTS

The cultured cells showed typical PC morphology and were positive for NG2, alpha smooth muscle actin, PDGFR-β and the gap junction protein Cx43. Expressions of at least one single or combinations of several markers were found in 80-90% of subpopulations. A subset of the patched cells expressed channel activities consistent with a Kv1.5 channel. In vivo presence of the channels was confirmed in sections of hamster thigh muscles. Interleukin-8, a myokine known to be released from exercising muscle, increased the expression but not the activity of this channel. Pharmacologic stimulation of the channel activity by flufenamic acid induced hyperpolarization of PC alone but not of endothelial cells [human umbilical vein endothelial cells (HUVEC)] alone. However, hyperpolarization was observed in HUVEC adjacent to PC when kept in co-culture.

CONCLUSION

We established a culture method for PC from skeletal muscle. A first functional characterization revealed properties which potentially enable these cells to generate hyperpolarizing signals and to communicate them to endothelial cells.

Heparin-binding EGF-like growth factor protects pericytes from injury

BACKGROUND

We have previously shown that heparin-binding EGF-like growth factor (HB-EGF) promotes angiogenesis and preserves mesenteric microvascular blood flow in several models of intestinal injury. The current study was designed to evaluate the effect of HB-EGF on pericytes, since these cells function to regulate capillary blood flow and new capillary growth.

MATERIALS AND METHODS

C3H/10T1/2 mouse mesenchymal cells were differentiated into pericyte-like cells in vitro using transforming growth factor-β1 (TGF-β1). In addition, primary pericyte cultures were established from rat brain. The effect of HB-EGF on pericyte proliferation was assessed. In addition, cells were stressed by exposure to anoxia, and apoptosis determined. In vivo, we examined the effect of HB-EGF on pericytes in a model of intestinal I/R injury based on superior mesenteric artery occlusion (SMAO) in mice.

RESULTS

Differentiated C3H/10T1/2 cells (pericyte-like cells) demonstrated morphologic characteristics of pericytes, and expressed pericyte specific markers. Addition of HB-EGF led to significant cell proliferation in differentiated pericyte-like cells, even under conditions of anoxic stress. Addition of the EGF receptor inhibitor AG 1478 led to complete inhibition of the proliferative effects of HB-EGF on pericyte-like cells. In addition, HB-EGF protected pericyte-like cells from anoxia-induced apoptosis. In addition, HB-EGF promoted cell proliferation in primary pericyte cultures. In vivo, administration of HB-EGF to mice subjected to intestinal I/R injury led to protection of pericytes from injury.

CONCLUSIONS

These results suggest that HB-EGF may function as a microcirculatory blood flow regulator, at least in part, via its effects on pericytes.

The role of vasculature and blood circulation in zebrafish swimbladder development

BACKGROUND

Recently we have performed a detailed analysis of early development of zebrafish swimbladder, a homologous organ of tetrapod lung; however, the events of swimbladder development are still poorly characterized. Many studies have implicated the role of vascular system in development of many organs in vertebrates. As the swimbladder is lined with an intricate network of blood capillaries, it is of interest to investigate the role of the vascular system during early development of swimbladder.

RESULTS

To investigate the role of endothelial cells (ECs) and blood circulation during development of the swimbladder, phenotypes of swimbladder were analysed at three different stages (approximately 2, 3 and 5 dpf [day postfertilization]) in cloche (clo) mutant and Tnnt2 morphants, in the background of transgenic lines Et(krt4:EGFP)sq33-2 and Et(krt4:EGFP)sqet3 which express EGFP in the swimbladder epithelium and outer mesothelium respectively. Analyses of the three tissue layers of the swimbladder were performed using molecular markers hb9, fgf10a, acta2, and anxa5 to distinguish epithelium, mesenchyme, and outer mesothelium. We showed that the budding stage was independent of ECs and blood flow, while early epithelial growth, mesenchymal organization and its differentiation into smooth muscle, as well as outer mesothelial organization, were dependent on ECs. Blood circulation contributed to later stage of epithelial growth, smooth muscle differentiation, and organization of the outer mesothelium. Inflation of the swimbladder was also affected as a result of absence of ECs and blood flow.

CONCLUSION

Our data demonstrated that the vascular system, though not essential in swimbladder budding, plays an important role in the development of the swimbladder starting from the early growth stage, including mesenchyme organization and smooth muscle differentiation, and outer mesothelial organization, which in turn may be essential for the function of the swimbladder as reflected in its eventual inflation.

The effect of moderate hypothermia in acute ischemic stroke on pericyte migration: an ultrastructural study

Pericytes are essential components of the blood-brain barrier together with endothelial cells and astrocytes. Any disturbance of brain perfusion may result in blood-brain barrier dysfunction due to pericyte migration from the microvascular wall. The neuroprotective influence of hypothermia on ischemic brain injury has been clearly shown in models of both global and focal ischemia. Leakage of plasma proteins contributes to the extension of neuronal injury and hypothermia has a neuroprotective influence during the ischemic insult. This line of thinking impelled us to investigate the possible role of the pericytes in the occurrence of hypothermic protection during cerebral ischemia. In this study, we examined at the ultrastructural level the effect of moderate hypothermia on microvascular pericyte responses using a rat model of permanent middle cerebral artery occlusion. Twenty rats were divided into four groups. Middle cerebral artery occlusion was performed in all rats except the control group (first group), which was used to determine the pericyte morphology under normal conditions. In the second group, pericyte response to irreversible ischemia under normothermic conditions was examined at the end of the first hour. In the third group, pericyte response to hypoxia was examined under normothermic conditions three hours after ischemia. In the fourth group, temporalis muscle temperature was maintained at 27-29 degrees C for 1h after middle cerebral artery occlusion and pericyte response was then examined at the ultrastructural level. In ischemic normothermic conditions at the end of the first hour (Group 2), a separation was observed between pericytes and the basement membrane and this was interpreted as pericyte migration from the microvascular wall. In ischemic normothermic conditions at the end of the third hour (Group 3), basement membrane disorganization and increased space between the basement membranes were seen in addition to the differentiation of second group. In ischemic hypothermic conditions at the end of the first hour (Group 4), pericyte separation or migration from basement membrane were not seen and the blood-brain barrier remained firm. These findings were interpreted by the authors as a possible relationship between pericyte behavior and neural protection during hypothermia. We suggest that hypothermia may delay the pericyte response but not necessarily attenuate it, and should be associated with hypothermic protection.

Microarray analysis of in vitro pericyte differentiation reveals an angiogenic program of gene expression

The vasculature consists of endothelial cells (ECs) lined by pericyte/vascular smooth muscle cells (vSMCs). Pericyte/vSMCs provide support to the mature vasculature but are also essential for normal blood vessel development. To determine how pericyte-EC communication influences vascular development, we used the well-established in vitro model of TGFbeta-stimulated differentiation of 10T1/2 cells into pericyte/vSMCs. Microarray analysis was performed to identify genes that were differentially expressed by induced vs. uninduced 10T1/2 cells. We discovered that these cells show an angiogenic program of gene expression, with up-regulation of several genes previously implicated in angiogenesis, including VEGF, IL-6, VEGF-C, HB-EGF, CTGF, tenascin C, integrin alpha5, and Eph receptor A2. Up-regulation of some genes was validated by Western blots and immunocytochemistry. We also examined the functional significance of these gene expression changes. VEGF and IL-6 alone and in combination were important in 10T1/2 cell differentiation. Furthermore, we used a coculture system of 10T1/2 and human umbilical vein ECs (HUVECs), resulting in the formation of cordlike structures by the HUVECs. This cordlike structure formation was disrupted when neutralizing antibodies to VEGF or IL-6 were added to the coculture system. The results of these studies show that factors produced by pericytes may be responsible for recruiting ECs and promoting angiogenesis. Therefore, a further understanding of the genes involved in pericyte differentiation could provide a novel approach for developing anti-angiogenic therapies.

Early pericyte response to brain hypoxia in cats: an ultrastructural study

Hypoxic condition in the brain result in microvascular dysfunction. Pericytes are one of the blood-brain barrier constituents with the endothelial cells and astrocytes. Pericytes of blood-brain barrier are the first cells to react to hypoxia of brain. We showed, at the ultrastructural level, microvascular pericyte responses to the brain hypoxia in early stage of hypoxia in cats. In first 2 h of hypoxia, pericytes start to migrate and one of every three pericytes migrates from original location. In the first stage of migration spikes occur at the abluminal surface of pericytes. At the same time basal lamina thickens and endothelial cells remain the same.

The role of pericytes in controlling angiogenesis in vivo

In order to evaluate the interaction between endothelial cells and the perivascular pericytes during physiological angiogenesis, stereological analysis of fine structure was performed on samples of rat skeletal muscle where capillary growth was induced to a similar extent by three different interventions (indirect electrical stimulation, vasodilatation by alpha 1-blockade, stretch due to synergist extirpation). There was a significant reduction in the relative area of contact between pericytes and the capillary abluminal surface with stimulation, and withdrawal of pericyte processes coincided with an increase in anatomical capillary supply. These data indicate that pericytes may play an anti-angiogenic role in vivo in normal adult tissue similar to that proposed for in vitro models of angiogenesis, with their retraction during increased muscle activity possibly releasing endothelial cells from their contact inhibition. However, following long-term peripheral vasodilatation expansion of the capillary bed was accompanied by a co-ordinated increase in pericytes, such that coverage of capillaries was similar to that in control muscles. In addition, growth of capillaries following prolonged stretch resulted in a slightly greater increase in the pericyte population, suggesting they may be permissive for endothelial cell migration. Thus, the role of pericytes in controlling physiological angiogenesis is dependent on the nature of the initial stimulus, suggesting that in vitro data have to be interpreted with caution when discussing the mechanism of capillary growth in vivo.

Diversity within pericytes

1. Pericytes are cells of microvessels (arterioles, capillaries and venules) that wrap around endothelial cells. They are most abundant on venules and are common on capillaries. 2. The pericyte population is highly variable between different tissues and organs, probably in a manner reflecting postarteriolar hydrostatic pressures. Pericytes are more abundant in the distal legs and feet, again suggesting a hydrostatic pressure-driven mechanical role for pericytes as protectors of microvessel wall integrity. 3. Pericyte alteration or degeneration is linked directly with microangiopathy in diabetes, scleroderma, hypertension, dementias and, possibly, inappropriate calcification of blood vessels. 4. Pericytes are functionally codependent on endothelial cells. Each cell type influences each others' mitotic rate and probably phenotypic expression. 5. Pericytes are not randomly located around microvessels. Instead, they are located adjacent to or over endothelial cell junctions of venules and especially over gaps between endothelial cells during inflammation. Pericytes are emerging as essential components of the microvessel wall, with metabolic, signalling and mechanical roles to support the endothelial cell.

In vivo formation of complex microvessels lined by human endothelial cells in an immunodeficient mouse

We have identified conditions for forming cultured human umbilical vein endothelial cells (HUVEC) into tubes within a three-dimensional gel that on implantation into immunoincompetent mice undergo remodeling into complex microvessels lined by human endothelium. HUVEC suspended in mixed collagen/fibronectin gels organize into cords with early lumena by 24 h and then apoptose. Twenty-hour constructs, s.c. implanted in immunodeficient mice, display HUVEC-lined thin-walled microvessels within the gel 31 days after implantation. Retroviral-mediated overexpression of a caspase-resistant Bcl-2 protein delays HUVEC apoptosis in vitro for over 7 days. Bcl-2-transduced HUVEC produce an increased density of HUVEC-lined perfused microvessels in vivo compared with untransduced or control-transduced HUVEC. Remarkably, Bcl-2- but not control-transduced HUVEC recruit an ingrowth of perivascular smooth-muscle alpha-actin-expressing mouse cells at 31 days, which organize by 60 days into HUVEC-lined multilayered structures resembling true microvessels. This system provides an in vivo model for dissecting mechanisms of microvascular remodeling by using genetically modified endothelium. Incorporation of such human endothelial-lined microvessels into engineered synthetic skin may improve graft viability, especially in recipients with impaired angiogenesis.

Regulation of a blood-brain barrier-specific enzyme expressed by cerebral pericytes (pericytic aminopeptidase N/pAPN) under cell culture conditions

In this study we show that the aminopeptidase N of cerebral pericytes (pAPN) associated with the blood-brain barrier (BBB) is downregulated in pericytic cell cultures. This observation is in accordance with previous data describing comparable in vitro effects for BBB-specific enzymes of endothelial or pericytic origin, such as gamma-glutamyl transpeptidase or alkaline phosphatase. By polymerase chain reaction and in situ hybridization we were able to determine that the down-regulation of pAPN occurs at the posttranscriptional level. The mRNA of pAPN was found to be constitutively expressed even when the protein is no longer detectable. Culturing the pericytes in an endothelial cell-conditioned medium allowed pAPN to be reexpressed. However, the reexpression effect depended largely on the culturing conditions of the pericytes. Although purified pericytes deprived of endothelial cells did not reveal a reexpression effect, pericytes that were kept in contact with endothelial cells were able to acquire a pAPN-positive phenotype, indicating that endothelial cells constitute an essential requirement for the in vitro reexpression of pAPN. Astrocytes, however, were insufficient in exerting any reexpression effect.