Vascular endothelial growth factor mRNA in pericytes is upregulated by phorbol myristate acetate

The pericyte: A critical cell in the pathogenesis of CADASIL

•

CADASIL is the most common hereditary small vessel disease presenting with strokes and subcortical vascular dementia caused by mutations in the NOTCH3 gene.

•

CADASIL is a vasculopathy primarily involving vascular smooth-muscle cells.

•

Arteriolar and capillary pericyte damage or deficiency is a key feature in disease pathogenesis.

•

Pericyte-mediated cerebral venous insufficiency may explain white matter lesions and increased perivascular spaces.

•

Central role of the pericyte offers novel approaches to the treatment of CADASIL.

Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is a hereditary small vessel disease presenting with migraine, mood and cognitive disorders, focal neurological deficits, recurrent ischemic attacks, lacunar infarcts and brain white matter changes. As they age, CADASIL patients invariably develop cognitive impairment and subcortical dementia. CADASIL is caused by missense mutations in the NOTCH3 gene resulting in a profound cerebral vasculopathy affecting primarily arterial vascular smooth muscle cells, which target the microcirculation and perfusion. Based on a thorough review of morphological lesions in arteries, veins, and capillaries in CADASIL, we surmise that arteriolar and capillary pericyte damage or deficiency appears a key feature in the pathogenesis of the disease. This may affect critical pericyte-endothelial interactions causing stroke injury and vasomotor disturbances. Changes in microvascular permeability due to perhaps localized blood-brain barrier alterations and pericyte secretory dysfunction likely contribute to delayed neuronal as well as glial cell death. Moreover, pericyte-mediated cerebral venous insufficiency may explain white matter lesions and the dilatation of Virchow-Robin perivascular spaces typical of CADASIL. The postulated central role of the pericyte offers some novel approaches to the study and treatment of CADASIL and enable elucidation of other forms of cerebral small vessel diseases and subcortical vascular dementia.

Maintenance of vascular integrity by pericytes is essential for normal kidney function

Pericytes are tissue-resident mesenchymal progenitor cells anatomically associated with the vasculature that have been shown to participate in tissue regeneration. Here, we tested the hypothesis that kidney pericytes, derived from FoxD1⁺ mesodermal progenitors during embryogenesis, are necessary for postnatal kidney homeostasis. Diphtheria toxin delivery to FoxD1Cre::RsDTR transgenic mice resulted in selective ablation of >90% of kidney pericytes but not other cell lineages. Abrupt increases in plasma creatinine, blood urea nitrogen, and albuminuria within 96 h indicated acute kidney injury in pericyte-ablated mice. Loss of pericytes led to a rapid accumulation of neutral lipid vacuoles, swollen mitochondria, and apoptosis in tubular epithelial cells. Pericyte ablation led to endothelial cell swelling, reduced expression of vascular homeostasis markers, and peritubular capillary loss. Despite the observed injury, no signs of the acute inflammatory response were observed. Pathway enrichment analysis of genes expressed in kidney pericytes in vivo identified basement membrane proteins, angiogenic factors, and factors regulating vascular tone as major regulators of vascular function. Using novel microphysiological devices, we recapitulated human kidney peritubular capillaries coated with pericytes and showed that pericytes regulate permeability, basement membrane deposition, and microvascular tone. These findings suggest that through the active support of the microvasculature, pericytes are essential to adult kidney homeostasis.

Blood vessel maturation, vascular phenotype and angiogenic potential in malignant melanoma: one step forward for overcoming anti-angiogenic drug resistance?

Angiogenesis is a pivotal process for growth, invasion and spread of the majority of solid tumors including melanoma. Anti-angiogenic agents have not been systematically tested in patients with advanced melanoma. Clinical efficacy of angiogenesis inhibitors targeting endothelial cells has not been as affirmative as initially hoped and improved clinical outcomes have been observed in combination with chemotherapy or additional drugs for many types of human cancer. However, angiogenesis is not only dependent on endothelial cell invasion and proliferation, it also requires pericyte coverage of vascular sprouts for stabilization and maturation of vascular walls. Recent data suggest that pericytes might be able to confer resistance to anti-vascular endothelial growth factor (VEGF) therapy. This review will focus on the significance of the vascular phenotype but also on the impact of pericyte-mediated vessel maturation for the susceptibility to anti-angiogenic therapy, including malignant melanoma, which we identified as crucial factor regarding therapeutic efficacy.

Pericytes and vessel maturation during tumor angiogenesis and metastasis

Despite promising results in preclinical and clinical studies, the therapeutic efficacy of antiangiogenic therapies has been restricted by a narrow focus on inhibiting the growth of endothelial cells. Other cell types in the tumor stroma are also critical to the progression of cancer, including mural cells. Mural cells are vascular support cells that range in phenotype from pericytes to vascular smooth muscle cells. Although the role of pericytes and pericyte-like cells in the pathophysiology of cancer is still unclear, evidence indicates that aberrations in pericyte-endothelial cell signaling networks could contribute to tumor angiogenesis and metastasis. The purpose of this review is to evaluate critically recent evidence on the role of pericytes in tumor biology and discuss potential therapeutic targets for anticancer intervention.

LPS-induced vascular endothelial growth factor expression in rat lung pericytes

Vascular endothelial growth factor (VEGF) is a potent angiogenic and vascular permeability factor. Recent studies have shown that the VEGF levels increase in several cell types, for example, macrophages and smooth muscle cells after LPS stimulation, suggesting that it is important in the initiation and development of sepsis. In particular, LPS-regulated contractility in lung pericytes may play an important role in mediating pulmonary microvascular fluid hemodynamics during sepsis. This study investigated the production of VEGF by rat lung pericytes in response to LPS. LPS was found to enhance VEGF mRNA expression in a concentration-dependent manner peaking 2 h after stimulation in pericytes. Vascular endothelial growth factor protein levels in conditioned medium and in cell lysate also increased on increasing LPS and peaked after 24 to 48 h. LPS also significantly augmented iNOS expression in lung pericytes within 6 h. However, iNOS mRNA induction occurred later than LPS-induced VEGF mRNA increases. Interestingly, attempted inhibition with nuclear factor-kappaB or tyrosine kinase did not suppress LPS-induced augmented VEGF mRNA expression in lung pericytes, although both inhibitors markedly inhibited LPS-induced iNOS mRNA expression. SB203580, a p38 MAP kinase inhibitor, repressed LPS-induced VEGF mRNA expression. Furthermore, LPS stimulated a rapid and sustained phosphorylation of p38 MAP kinase. These results show that pericytes produce VEGF in response to LPS stimulation, and that this may be partly mediated by the p38 MAP kinase pathway. More research should be done to establish the regulation of capillary hemodynamics and identify mechanisms of their regulation.

Brain pericytes contribute to the induction and up-regulation of blood-brain barrier functions through transforming growth factor-beta production

The blood-brain barrier (BBB) is a highly organized multicellular complex consisting of an endothelium, brain pericytes and astrocytes. The present study was aimed at evaluating the role of brain pericytes in the induction and maintenance of BBB functions and involvement of transforming growth factor-beta (TGF-beta) in the functional properties of pericytes. We used an in vitro BBB model established by coculturing immortalized mouse brain capillary endothelial (MBEC4) cells with a primary culture of rat brain pericytes. The coculture with rat pericytes significantly decreased the permeability to sodium fluorescein and the accumulation of rhodamine 123 in MBEC4 cells, suggesting that brain pericytes induce and up-regulate the BBB functions. Rat brain pericytes expressed TGF-beta1 mRNA. The pericyte-induced enhancement of BBB functions was significantly inhibited when cells were treated with anti-TGF-beta1 antibody (10 microg/ml) or a TGF-beta type I receptor antagonist (SB431542) (10 microM) for 12 h. In MBEC4 monolayers, a 12 h exposure to TGF-beta1 (1 ng/ml) significantly facilitated the BBB functions, this facilitation being blocked by SB431542. These findings suggest that brain pericytes contribute to the up-regulation of BBB functions through continuous TGF-beta production.

Effects of hypoxia on endothelial/pericytic co-culture model of the blood-brain barrier

The blood-brain barrier (BBB) is composed of endothelial cells, pericytes and astrocytic foot processes. Most research for the in vitro BBB is performed endothelial cells with or without astrocytes. Hypoxia damage to the BBB induces vasogenic brain edema. We have generated a new model of the BBB with brain endothelial cells and pericytes and have examined the effects of hypoxia using this model. Brain microvascular endothelial cells and pericytes were isolated from three-week-old male Wister rats using enzyme and mechanical homogenization. Three models (A: only endothelial monolayer, B: endothelial monolayer with pericytes non-contact condition, and C: contact condition) were made by culturing these cells using Transwell co-culture system and were exposed to hypoxic condition. We evaluated barrier function with transendothelial electrical resistance (TEER) and permeability of Evans blue-albumin and sodium fluorescein. The tightest barrier was observed in the endothelial/pericytic contact model. Despite hypoxia-induced disruption of the barrier in endothelial monolayer and non-contact co-culture models, a minimum of dysfunction was seen in the contact co-culture model. Therefore, it is considered that pericytes effect on the endothelia by secreting factors or through a gap junction. In short, pericytes induce endothelial maturation and a tighter barrier function, which supports the function against the hypoxic injury. Intercellular communication might be important to keep the BBB functional and stabilize in hypoxia.

Calcium influx pathways in rat CNS pericytes

In central nervous system (CNS), pericytes have been proposed to play a role in broad functional activities including blood-brain barrier, microcirculation, and macrophage activity. However, contractile responses and Ca2+ signaling in CNS pericytes have not been elucidated. The aim of the present study is to investigate contractility and Ca2+ influx pathway in CNS pericytes. CNS pericytes were cultured from rat brain. Contraction of the pericytes in response to various stimuli was evaluated by the change in surface area measured by a light microscope with a digital camera. Reverse transcription and polymerase chain reaction (RT-PCR) was performed to examine the expression of mRNA of alpha-smooth muscle actin. Intracellular Ca2+ was measured using fura-2 fluorescence spectroscopy. A23187 (Ca2+ ionophore), high external K+ (4 x 10(-2) mol/l), endothelin-1, and serotonin induced contraction of CNS pericytes. RT-PCR analysis revealed the expression of alpha-smooth muscle actin in CNS pericytes. Cytosolic Ca2+ ([Ca2+]i) increased after application of high concentration of external K+, tetraethylammonium, and charybdotoxin, which was inhibited by nicardipine and removal of external Ca2+. Angiotensin-II, serotonin, acetylcholine, ATP, and endothelin-1 caused biphasic response in [Ca2+]i. In response to these agents, [Ca2+]i rapidly increased and then decayed to a relatively constant Ca2+ plateau. The Ca2+ plateau was partially inhibited by nicardipine and completely abolished by omission of external Ca2+. After intracellular Ca2+ store was depleted by the removal of external Ca2+ and addition of thapsigargin, reapplication of external Ca2+ evoked increases in [Ca2+]i. These results indicate that CNS pericytes express mRNA of alpha-smooth muscle actin and possess contractile ability. In CNS pericytes, resting membrane potential is regulated by large conductance Ca2+-activated K+ channels and Ca2+ enters into the cells via L-type voltage-dependent Ca2+ channels, agonist-activated Ca2+ permeable channels, and capacitative Ca2+ entry pathways.

Hypertonic saline impedes tumor cell–endothelial cell interaction by reducing adhesion molecule and laminin expression

BACKGROUND

Hypertonic saline infusion dampens inflammatory responses and suppresses neutrophil-endothelial interaction by reducing adhesion molecule expression. This study tested the hypothesis that hypertonic saline attenuates tumor cell adhesion to the endothelium through a similar mechanism.

METHODS

Human colon cancer cells (LS174T) were transfected with green fluorescent protein and exposed to lipopolysaccharide, tumor necrosis factor-alpha, and interleukin-6 under hypertonic and isotonic conditions for 1 and 4 hours. Confluent human umbilical vein endothelial cells were similarly exposed. Cellular apoptosis and expression of adhesion molecules and laminin were measured by flow cytometry. Tumor cell adhesion to endothelium and laminin was assessed with fluorescence microscopy. Data are represented as mean +/- standard error of mean, and an ANOVA test was performed to gauge statistical significance, with P <.05 considered significant.

RESULTS

Hypertonic exposure significantly reduced tumor cell adhesion despite the presence of the perioperative cell stressors (42 +/- 2.9 vs 172.5 +/- 12.4, P <.05), attenuated tumor cell beta-1 integrin (14.43 vs 23.84, P <.05), and endothelial cell laminin expression (22.78 +/- 2.2 vs 33.74 +/- 2.4, P <.05), but did not significantly alter cell viability.

CONCLUSION

Hypertonic saline significantly attenuates tumor cell adhesion to endothelium by inhibiting adhesion molecule and laminin expression. This may halt the metastatic behavior of tumor cells shed at surgery.

Acute respiratory distress syndrome in the septic surgical patient

The acute respiratory distress syndrome (ARDS) is a process of acute inflammatory lung injury that affects a diverse array of surgical and medical patients. The syndrome is mediated by a complex and interacting system of chemical mediators produced by several types of pulmonary cells. Regardless of the predisposing causes, activation of the nuclear factor kappa B seems to be, at the molecular level, a signature event of ARDS, leading to the rapid activation of intracellular signaling pathways, which coordinate the induction of multiple genes encoding inflammatory mediators. There are at least two compelling reasons for promoting an understanding of these interactions and their molecular mediators and second messengers: new therapies intended to modulate these factors continue to be developed, and the levels of some of these molecules, most notably cytokines, may serve as early indicators of the onset of ARDS.

Attenuation of pancreatitis-induced pulmonary injury by aerosolized hypertonic saline

BACKGROUND

The immunomodulatory effects of hypertonic saline (HTS) provide potential strategies to attenuate inappropriate inflammatory reactions. This study tested the hypothesis that administration of intratracheal aerosolized HTS modulates the development of lung injury in pancreatitis.

METHODS

Pancreatitis was induced in 24 male Sprague-Dawley rats by intraperitoneal injection of 20% L-arginine (500 mg/100 g body weight). At 24 and 48 h, intratracheal aerosolized HTS (7.5% NaCl, 0.5 mL) was administered to 8 rats, while a further 8 received 0.5 mL of aerosolized normal saline (NS). At 72 hours, pulmonary neutrophil infiltration (myeloperoxidase activity) and endothelial permeability (bronchoalveolar lavage and wet:dry weight ratios) were assessed. In addition, histological assessment of representative lung tissue was performed by a blinded assessor. In a separate experiment, polymorphonucleocytes (PMN) were isolated from human donors, and exposed to increments of HTS. Neutrophil transmigration across an endothelial cell layer, VEGF release, and apoptosis at 1, 6, 12, 18, and 24 h were assessed.

RESULTS

Histopathological lung injury scores were significantly reduced in the HTS group (4.78 +/- 1.43 vs. 8.64 +/- 0.86); p < 0.001). Pulmonary neutrophil sequestration (1.40 +/- 0.2) and increased endothelial permeability (6.77 +/- 1.14) were evident in the animals resuscitated with normal saline when compared with HTS (0.70 +/- 0.1 and 3.57 +/- 1.32), respectively; p < 0.04). HTS significantly reduced PMN transmigration (by 97.1, p = 0.002, and induced PMN apoptosis (p < 0.03). HTS did not impact significantly upon neutrophil VEGF release (p > 0.05).

CONCLUSIONS

Intratracheal aerosolized HTS attenuates the neutrophil-mediated pulmonary insult subsequent to pancreatitis. This may represent a novel therapeutic strategy.

Induction of a potential paracrine angiogenic loop between human THP-1 macrophages and human microvascular endothelial cells during Bartonella henselae infection

Bartonella henselae is responsible for various disease syndromes that loosely correlate with the immune status of the host. In the immunocompromised individual, B. henselae-induced angiogenesis, or bacillary angiomatosis, is characterized by vascular proliferative lesions similar to those in Kaposi's sarcoma. We hypothesize that B. henselae-mediated interaction with immune cells, namely, macrophages, induces potential angiogenic growth factors and cytokines which contribute in a paracrine manner to the proliferation of endothelial cells. Vascular endothelial growth factor (VEGF), a direct inducer of angiogenesis, and interleukin-1beta (IL-1beta), a potentiator of VEGF, were detected within 12 and 6 h, respectively, in supernatants from phorbol 12-myristate 13-acetate-differentiated human THP-1 macrophages exposed to live B. henselae. Pretreatment of macrophages with cytochalasin D, a phagocytosis inhibitor, yielded comparable results, suggesting that bacterium-cell attachment is sufficient for VEGF and IL-1beta induction. IL-8, an angiogenic cytokine with chemotactic properties, was induced in human microvascular endothelial cells (HMEC-1) within 6 h of infection, whereas no IL-8 induction was observed in infected THP-1 cells. In addition, conditioned medium from infected macrophages induced the proliferation of HMEC-1, thus demonstrating angiogenic potential. These data suggest that Bartonella modulation of host or target cell cytokines and growth factors, rather than a direct role of the bacterium as an endothelial cell mitogen, is the predominant mechanism responsible for angiogenesis. B. henselae induction of VEGF, IL-1beta, and IL-8 outlines a broader potential paracrine angiogenic loop whereby macrophages play the predominant role as the effector cell and endothelial cells are the final target cell, resulting in their proliferation.

Lung injury in acute pancreatitis: mechanisms, prevention, and therapy

Lung injury is the most pertinent manifestation of extra-abdominal organ dysfunction in pancreatitis. The propensity of this retroperitoneal inflammatory condition to engender a diffuse and life-threatening lung injury is significant. Approximately one third of patients will develop acute lung injury and acute respiratory distress syndrome, which account for 60% of all deaths within the first week. The variability in the clinical course of pancreatitis renders it a vexing entity and makes demonstration of the efficacy of any specific intervention difficult. The distinct pathologic entity of pancreatitis-associated lung injury is reviewed with a focus on etiology and potential therapeutic maneuvers.

Abnormalities in pericytes on blood vessels and endothelial sprouts in tumors

Endothelial cells of tumor vessels have well-documented alterations, but it is less clear whether pericytes on these vessels are abnormal or even absent. Here we report that alpha-smooth muscle actin (alpha-SMA) and desmin-immunoreactive pericytes were present on >97% of blood vessels viewed by confocal microscopy in 100-microm-thick sections of three different spontaneous or implanted tumors in mice. However, the cells had multiple abnormalities. Unlike pericytes on capillaries in normal pancreatic islets, which had desmin but not alpha-SMA immunoreactivity, pericytes on capillary-size vessels in insulinomas in RIP-Tag2 transgenic mice expressed both desmin and alpha-SMA. Furthermore, pericytes in RIP-Tag2 tumors, as well as those in MCa-IV breast carcinomas and Lewis lung carcinomas, had an abnormally loose association with endothelial cells and extended cytoplasmic processes deep into the tumor tissue. alpha-SMA-positive pericytes also covered 73% of endothelial sprouts in RIP-Tag2 tumors and 92% of sprouts in the other tumors. Indeed, pericyte sleeves were significantly longer than the CD31-immunoreactive endothelial cell sprouts themselves in all three types of tumors. All three tumors also contained alpha-SMA-positive myofibroblasts that resembled pericytes but were not associated with blood vessels. We conclude that pericytes are present on most tumor vessels but have multiple abnormalities, including altered expression of marker proteins. In contrast to some previous studies, the almost ubiquitous presence of pericytes on tumor vessels found in the present study may be attributed to our use of both desmin and alpha-SMA as markers and 100-microm-thick tissue sections. The association of pericytes with endothelial sprouts raises the possibility of an involvement in sprout growth or retraction in tumors.

Angiotensin II stimulates migration of retinal microvascular pericytes: involvement of TGF-beta and PDGF-BB

We studied the promigratory effect of angiotensin II (ANG II) on cultured bovine retinal microvascular pericytes. ANG II stimulated migration of pericytes by 86% at 10(-8) M, but this effect was lost at 10(-4) M. Migratory responses were inhibited by the ANG II type 1 (AT(1)) receptor antagonist losartan but not by PD-123319, an AT(2) antagonist. Addition of PD-123319 to the 10(-4) M ANG II dose restored migratory responses. The promigratory effect of ANG II (10(-7) M) was reduced by 59% in absence of gradient. Although ANG II augmented the latent matrix metalloproteinase-2 (MMP-2) activity of the pericyte by 35%, it also doubled tissue inhibitors of MMPs. ANG II-induced migration was not altered by a broad-spectrum MMP inhibitor (GM6001); it was inhibited by ~50% by antibodies against transforming growth factor (TGF)-beta(1/2/3) and was abolished by antibodies against platelet-derived growth factor (PDGF)-BB. We conclude that ANG II induces chemotactic responses on retinal microvascular pericytes acting through the AT(1) receptor. This effect is opposed by the AT(2) receptor. ANG II-induced chemotaxis is mediated by PDGF-BB and involves TGF-beta, but it is independent of MMP activity. It is also independent of vascular endothelial growth factor (VEGF) because VEGF did not stimulate pericyte migration. ANG II can contribute to the regulation of retinal neovascularization by stimulating pericyte migration.

Protein kinase C signalling pathway is involved in the regulation of vascular endothelial growth factor expression in human bladder transitional carcinoma cells

Vascular endothelial growth factor (VEGF) is a potent angiogenic factor associated with the growth and metastasis of various cancers and plays a prominent role in vesical angiogenesis regulation. In this study, we investigated the effect of the phorbol 12-myristate 13-acetate (PMA) on the expression of VEGF in human bladder transitional carcinoma cells (RT4). RT4 cells expressed three VEGF isoforms (VEGF(189), VEGF(165), VEGF(121)). PMA increased VEGF mRNA expression time-dependently with a peak at 4 h. PMA increased the half-life of VEGF mRNA. The amount of VEGF protein in conditioned media was increased by PMA in a dose-dependent manner with a maximal effect at 10(-7) M. Staurosporine and calphostin C (PKC inhibitors) decreased PMA-induced VEGF mRNA expression as opposed to protein kinase A or cyclic nucleotide-dependent protein kinase inhibitors. Thus, in RT4 cells, VEGF expression is up-regulated by PMA via the PKC signalling pathway and according to a posttranscriptional mechanism.

Human dermal pericytes express 3G5 ganglioside--a new approach for microvessel histology in the skin

BACKGROUND

Pericytes cover the abluminal surface of microvessels and play an important role in capillary regulation and pathology. Studies on pericytes have been hindered by the lack of specific markers with which to facilitate microscopic identification of this cell type. Expression of the cell surface 3G5 ganglioside antigen has been reported in cultured retinal and cardiac pericytes. The objective of this study was to determine the usefulness of monoclonal antibody 3G5 as a pericyte marker in human skin.

METHODS

Cryosections of 21 skin biopsies were examined by direct fluorescence technique with anti-3G5, anti-von Willebrand factor, anti-alpha-smooth muscle actin or DNA fluorochrome.

RESULTS

In human dermis, 3G5 expression is limited to pericytes discriminating this cell type from all other cells including smooth muscle cells, myofibroblasts and myoepithelial cells. We found a pericyte: endothelial cell ratio of 1:12.4 (+/-7.1), and a difference of alpha-smooth muscle actin expression between the subpapillary plexus and the microvessels of the Stratum reticulare.

CONCLUSIONS

3G5 mAB is an excellent and so far the only reported tool for identification of dermal pericytes by fluorescent light microscopy. Moreover, this is the first report of the application of 3G5 technique to the microvasculature in tissue sections at the light microscopic level.

Isolation and in vitro characterization of human dermal microvascular pericytes

Pericytes cover the abluminal surface of capillaries and venules and are thought to play an important role in microvascular regulation and pathology. The purpose of this study was to isolate and characterize human dermal microvascular pericytes (HDMPC), a minor cell type in the skin but a relatively easily obtainable human source of tissue. We developed and compared two procedures that differed in the preselection method. Isolation of dermal microvessel fragments from neonatal foreskins by trypsin digestion was followed by mechanical release of subepidermal tissue, collagenase treatment, and sieving through 100- and 30-microm meshes. After subcultivation, pericytes were preselected either by isolation of outgrowing capillary fragments or by 3G5-coupled magnetic beads. Pericytes were selected finally by cultivation of single cells in endothelial cell-conditioned media. Cultured HDMPC were seen to be large and well spread with irregular edges and prominent stress fibers. They lack contact inhibition, are positive for 3G5 antigen, alpha-smooth muscle actin, and vimentin, and are negative for the endothelial cell marker CD31, diI-acetylated low-density lipoprotein uptake, cytokeratin 5, 6, and 18, and S100 protein. Using both preselection methods, we could establish purified cell cultures of HDMPC. The results of these studies represent the first report of HDMPC isolation.

Cellular mechanisms of CNS pericytes

Three major functional roles have been ascribed to pericytes associated with central nervous system microvasculature-contractility, regulation o f endothelial cell activity, and macrophage activity. A host of different cell factors and signalling agents appear to be involved with these cellular functions, some effecting the pericyte and others produced by this cell. These include neuromodulators, vasoactive peptides, metabolic factors, growth factors and cytokines. The specific compounds and their actions are collectively viewed in an effort to provide an overall picture of the regulation of pericyte functional activity. This small vascular cell is emerging as a significant entity in several physiological processes through the functions of above; these processes include control of blood flow, regulation of vascular development and immune responses. Defining the regulatory agents and their mechanisms is key to understanding the role that pericytes play in these processes. Because these cells have begun to receive increasing attention in neurobiological studies, an overview of signalling properties should be timely and beneficial.

Role of the CNS microvascular pericyte in the blood-brain barrier

Pericytes are a very important cellular constituent of the blood-brain barrier. They play a regulatory role in brain angiogenesis, endothelial cell tight junction formation, blood-brain barrier differentiation, as well as contribute to the microvascular vasodynamic capacity and structural stability. Central nervous system pericytes express macrophage functions and are actively involved in the neuroimmune network operating at the blood-brain barrier. They exhibit unique functional characteristics critical for the pathogenesis of a number of cerebrovascular, neurodegenerative, and neuroimmune diseases.