Effects of endotoxin on lung pericytes in vitro

The Role of Pericytes in Lipopolysaccharide-Induced Murine Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome (ARDS) is a heterogeneous clinical syndrome that is most commonly triggered by infection-related inflammation. Lung pericytes can respond to infection and act as immune and proangiogenic cells; moreover, these cells can differentiate into myofibroblasts in nonresolving ARDS and contribute to the development of pulmonary fibrosis. Here, we aimed to characterize the role of lung cells, which present characteristics of pericytes, such as peri-endothelial location and expression of a panel of specific markers. To study their role in ARDS, we used a murine model of lipopolysaccharide (LPS)-induced resolving ARDS. We confirmed the development of ARDS after LPS instillation, which was resolved 14 days after onset. Using immunofluorescence and flow cytometry, we observed early expansion of neural-glial antigen 2+ β-type platelet-derived growth factor receptor+ pericytes in murine lungs with loss of CD31+ β-type platelet-derived growth factor receptor+ endothelial cells. These changes were accompanied by specific changes in lung structure and loss of vascular integrity. On day 14 after ARDS onset, the composition of pericytes and endothelial cells returned to baseline values. LPS-induced ARDS activated NOTCH signaling in lung pericytes, the inhibition of which during LPS stimulation reduced the expression of its downstream target genes, pericyte markers, and angiogenic factors. Together, lung pericytes in response to inflammatory injury activate NOTCH signaling that supports their maintenance and in turn can contribute to recovery of the microvascular endothelium.

Lipopolysaccharide decreases cochlear blood flow dose dependently in a guinea pig animal model via TNF signaling

OBJECTIVE

To evaluate the effect of Lipopolysaccharide (LPS), a bacterial endotoxin on cochlear microcirculation, and its mode of action.

METHODS

Twenty-five Dunkin-Hartley guinea pigs were divided into five groups of five animals each. After surgical preparation, cochlear microcirculation was quantified by in vivo fluorescence microscopy. Placebo or LPS (1 mg, 10 µg, and 100 ng) was applied topically, and microcirculation was measured before and twice after application. A fifth group was pretreated with etanercept, a tumor necrosis factor (TNF) antagonist, and afterward the lowest LPS concentrations that yielded significant results (10 µg) were applied.

RESULTS

In the groups that had been treated with 1 mg and 10 µg LPS, a significant drop in cochlear microcirculation was observed after 30 (.791 ± .089 Arbitrary Units (AU), compared to baseline, and .888 ± .071AU) and 60 (.756 ± .101 AU and .817 ± .124 AU, respectively) minutes. The groups that had been treated with 100 ng LPS and that had been pretreated with etanercept showed no significant change in cochlear blood flow compared to placebo.

CONCLUSION

Lipopolysaccharide shows a dose-dependent effect on cochlear microcirculation; this effect can already be observed after 30 min. Pretreatment with etanercept can abrogate this effect, indicating that TNF mediates the effect of LPS on cochlear microcirculation.

Controversial Role of Toll-like Receptor 4 in Adult Stem Cells

Adult or somatic stem cells are tissue-resident cells with the ability to proliferate, exhibit self-maintenance as well as to generate new cells with the principal phenotypes of the tissue in response to injury or disease. Due to their easy accessibility and their potential use in regenerative medicine, adult stem cells raise the hope for future personalisable therapies. After infection or during injury, they are exposed to broad range of pathogen or damage-associated molecules leading to changes in their proliferation, migration and differentiation. The sensing of such damage and infection signals is mostly achieved by Toll-Like Receptors (TLRs) with Toll-like receptor 4 being responsible for recognition of bacterial lipopolysaccharides (LPS) and endogenous danger-associated molecular patterns (DAMPs). In this review, we examine the current state of knowledge on the TLR4-mediated signalling in different adult stem cell populations. Specifically, we elaborate on the role of TLR4 and its ligands on proliferation, differentiation and migration of mesenchymal stem cells, hematopoietic stem cells as well as neural stem cells. Finally, we discuss conceptual and technical pitfalls in investigation of TLR4 signalling in stem cells.

Renal medullary effects of transient prehypertensive treatment in young spontaneously hypertensive rats

AIM

Transient angiotensin II receptor blockade (ARB) leads to prolonged blood pressure (BP) lowering, but the underlying mechanism remains uncertain. Long-term BP control is regulated by the medullary microcirculation with the pericyte as contractile cell. We hypothesize that the prolonged BP effect is caused by increased medullary blood flow (MBF) associated with structural alterations based on reduced medullary pericyte number.

METHODS

Four-week-old spontaneously hypertensive rats (SHR) were treated for 4 weeks with losartan (SHR-Los: 20 mg kg(-1) day(-1)), hydralazine (SHR-Hyd: 15 mg kg(-1) day(-1)), losartan and pan-caspase inhibitor zVAD (SHR-Los + 1 mg kg(-1) day(-1) zVAD), losartan and glycogen synthase kinase-3beta (GSK) inhibitor valproate (SHR-Los + 10 mg kg(-1) day(-1) Val) or placebo. BP, MBF and pericyte number were determined under and after treatment (8 and 12 weeks). Apoptotic pericytes were determined with alpha-actin and TUNEL double staining. Sodium concentration was determined in renal medulla and urine.

RESULTS

Antihypertensive treatment equipotently reduced BP at 8 weeks of age. After drug withdrawal (12 weeks of age) BP reduction was restricted to SHR-Los (SHR-Los: 153 +/- 5, SHR-Hyd: 177 +/- 2, SHR: 184 +/- 3 mmHg). Simultaneously, MBF was increased and pericyte number reduced, while medullary and urinary sodium concentration increased. Transient ARB in combination with zVAD or valproate resulted in more medullary pericytes and higher BP (SHR-Los/zVAD: 164 +/- 7; SHR-Los/Val: 168 +/- 6 mmHg) compared with transient ARB alone.

CONCLUSION

After drug withdrawal, transient ARB leads to increased MBF and is associated with a reduction in medullary pericytes. This may be associated with pericyte apoptosis as anti-apoptosis during transient ARB increases pericyte number and BP.

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.

Endotoxemia and endotoxin tolerance in patients with ARDS

BACKGROUND

The significance of endotoxemia in man is controversial, induces cytokine release and stimulates the immune system. Exaggerated cytokine release of mononuclear cells was observed in acute lung injury/acute respiratory distress syndrome (ALI/ARDS). However, repetitive administration of endotoxin can cause tolerance.

OBJECTIVE

To investigate endotoxemia, plasma TNFalpha, IL-1beta, IL-6, the liberation capacity of those cytokines from mononuclear cells after LPS challenge (Delta values), and plasma antibodies to endotoxins and alpha-hemolysin of Staphylococcus aureus in ALI/ARDS.

DESIGN

A prospective clinical study was conducted.

SETTING

The study was carried out at the University Hospital Ulm, Ulm, Germany.

SUBJECTS

The respondents were 23 patients with ALI/ARDS.

INTERVENTIONS

ALI/ARDS was defined according to the American-European Consensus Conference on ARDS. Blood was collected periodically. Parameters were measured by LAL or ELISA.

RESULTS

ARDS (P(a)O(2)/F(i)O(2) < 200) revealed higher endotoxemia (0.22-0.46 [0.06-1.15] EU/mL vs 0.05-0.14 [0.02-0.63] EU/mL) than ALI (P(a)O(2)/F(i)O(2) > 200) but lower DeltaIL-6 (124-209 [10-1214] pg/mL vs 298-746 [5-1797] pg/mL), DeltaTNFalpha (50-100 [6-660] pg/mL vs 143-243 [12-2795] pg/mL), and DeltaIL-1 (2-3 [0-26] pg/mL vs 2-14 [0-99] pg/mL). Endotoxemia correlated negative with P(a)O(2)/F(i)O(2) (r, -0.44 to -0.50). All patients presented antibodies to lipopolysaccharides and alpha-hemolysin, but the level did not correlate with P(a)O(2)/F(i)O(2).

CONCLUSIONS

ALI/ARDS is associated with endotoxemia. The more severe the disease, the more intense is endotoxemia but the lower is the capacity of mononuclear cells to release cytokines (tolerance). Antibodies against Gram-positive and Gram-negative bacteria are detectable in the plasma but without relation to P(a)O(2)/F(i)O(2).

Isolated Anxa5+/Sca-1+ perivascular cells from mouse meningeal vasculature retain their perivascular phenotype in vitro and in vivo

Pericytes are closely associated with endothelial cells, contribute to vascular stability and represent a potential source of mesenchymal progenitor cells. Using the specifically expressed annexin A5-LacZ fusion gene (Anxa5-LacZ), it became possible to isolate perivascular cells (PVC) from mouse tissues. These cells proliferate and can be cultured without undergoing senescence for multiple passages. PVC display phenotypic characteristics of pericytes, as they express pericyte-specific markers (NG2-proteoglycan, desmin, alphaSMA, PDGFR-beta). They also express stem cell marker Sca-1, whereas endothelial (PECAM), hematopoietic (CD45) or myeloid (F4/80, CD11b) lineage markers are not detectable. These characteristics are in common with the pericyte-like cell line 10T1/2. PVC also display a phagocytoic activity higher than 10T1/2 cells. During coculture with endothelial cells both cell types stimulate angiogenic processes indicated by an increased expression of PECAM in endothelial cells and specific deposition of basement membrane proteins. PVC show a significantly increased induction of endothelial specific PECAM expression compared to 10T1/2 cells. Accordingly, in vivo grafts of PVC aggregates onto chorioallantoic membranes of quail embryos recruit endothelial cells, get highly vascularized and deposit basement membrane components. These data demonstrate that isolated Anxa5-LacZ(+) PVC from mouse meninges retain their capacity for differentiation to pericyte-like cells and contribute to angiogenic processes.

Pericytes and their role in microvasculature homeostasis

BACKGROUND

The microvascular pericyte was first described in 1873, though it is a cell that has largely been ignored in the clinical literature. Pericytes are multifunctional, polymorphic, perivascular cells that lie within, and contribute to the production of the microvessel basil lamina.

MATERIALS

The pericyte is the second cell that comprises the capillary wall, and is in a prime location to be involved with microvascular permeability. The exact sequence of events in Acute Respiratory Distress Syndrome (ARDS) is unknown, though increased permeability (pulmonary edema) is the primary physiological abnormality seen in the early stages. Pericytes are crucial in the development of capillary leak and pulmonary edema seen in ARDS. Pericytes regulate permeability through contractility and apoptosis.

RESULTS

Changes in pericyte contractility alter the physical capillary barrier by opening the endothelial junctional space, and are reversible. Pericyte apoptosis leads to a compromise of the barrier effect of the capillary wall, and is a more permanent change.

CONCLUSIONS

The purpose of this paper is to review publications of pericyte physiological and pathophysiologic interactions in regards to contractility, apoptosis, and permeability.

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.

Heparin-like molecules inhibit pulmonary vascular pericyte proliferation in vitro

Proliferation of vascular pericytes (PCs), smooth muscle-like cells found in the distal microvasculature, contributes to vascular remodeling in pulmonary hypertension. The factors controlling lung PC quiescence in normal states are poorly understood. We demonstrate that exogenous heparin and heparan sulfate proteoglycans inhibit rat lung PC proliferation in vitro as does pulmonary vascular subendothelial matrix, particularly its heparan sulfate component. Heparin inhibits the intracellular alkalinization essential to proliferation, and we show that inhibition of alkalinization by 5-(N, N-dimethyl)amiloride also reduces PC proliferation. As shown by DNA staining and fluorescence-activated cell sorting analysis, heparin does not induce apoptosis in PCs. However, heparin maintains lung PCs in the G(0)/G(1) growth phase. Heparin induces production of p21, a potent inhibitor of cyclin-dependent kinases, thereby potentially identifying a fundamental mechanism by which heparin inhibits proliferation in smooth muscle-like cells. These studies establish additional similarities between lung PCs and smooth muscle cells and provide further understanding of growth control in the lung microvasculature. They also further support the rationale that heparin-like molecules might be therapeutically beneficial in pulmonary hypertension.