Increased numbers of caveolae in retinal endothelium and pericytes in hypertensive diabetic rats

Rodent Models of Diabetic Retinopathy as a Useful Research Tool to Study Neurovascular Cross-Talk

Diabetes is a group of metabolic diseases leading to dysfunction of various organs, including ocular complications such as diabetic retinopathy (DR). Nowadays, DR treatments involve invasive options and are applied at the sight-threatening stages of DR. It is important to investigate noninvasive or pharmacological methods enabling the disease to be controlled at the early stage or to prevent ocular complications. Animal models are useful in DR laboratory practice, and this review is dedicated to them. The first part describes the characteristics of the most commonly used genetic rodent models in DR research. The second part focuses on the main chemically induced models. The authors pay particular attention to the streptozotocin model. Moreover, this section is enriched with practical aspects and contains the current protocols used in research in the last three years. Both parts include suggestions on which aspect of DR can be tested using a given model and the disadvantages of each model. Although animal models show huge variability, they are still an important and irreplaceable research tool. Note that the choice of a research model should be thoroughly considered and dependent on the aspect of the disease to be analyzed.

Coronary Large Conductance Ca2+-Activated K+ Channel Dysfunction in Diabetes Mellitus

Diabetes mellitus (DM) is an independent risk of macrovascular and microvascular complications, while cardiovascular diseases remain a leading cause of death in both men and women with diabetes. Large conductance Ca2+-activated K+ (BK) channels are abundantly expressed in arteries and are the key ionic determinant of vascular tone and organ perfusion. It is well established that the downregulation of vascular BK channel function with reduced BK channel protein expression and altered intrinsic BK channel biophysical properties is associated with diabetic vasculopathy. Recent efforts also showed that diabetes-associated changes in signaling pathways and transcriptional factors contribute to the downregulation of BK channel expression. This manuscript will review our current understandings on the molecular, physiological, and biophysical mechanisms that underlie coronary BK channelopathy in diabetes mellitus.

Ultrastructural analysis of submacular choriocapillaris and its transport systems in AMD and aged control eyes

The choriocapillaris is the source of nutrients and oxygen for photoreceptors, which consume more oxygen per gram of tissue than any other cell in the body. The purpose of this study was to evaluate and compare the ultrastructure of the choriocapillaris and its transport systems in patients with and without age-related macular degeneration (AMD). Ultrastructural changes were also evaluated in subjects that were homozygous for polymorphisms in high risk CFH alleles (Pure 1) only or homozygous only for high risk ARMS2/HTRA1 (Pure 10) alleles. Tissue samples were obtained from the macular region of forty male (n = 24) and female (n = 16) donor eyes and prepared for ultrastructural studies with transmission electron microscopy (TEM). The average age of the aged donors was 74 ± 7.2 (n = 30) and the young donors 31.7 ± 11.25 (n = 10). There was no significant difference in average ages between the adult groups. TEM images of the capillaries in the choriocapillaris (CC) were taken at 4,000X and 25,000X and used to measure the area of endothelial cell somas, the number of fenestrations, and area of caveolae within the endothelial cells per length of Bruchs membrane (BrMb). The Student t-test and Wilcoxon sum rank test were used to determine significant differences. There was no significant difference between young subjects and aged controls in any of the morphological criteria assessed. There was a significant decrease in the number of fenestrations/mm of BrMb in atrophic areas of GA eyes (p = 0.007) when compared with aged control eyes. A significant increase was found in the caveolae area as a percent of the endothelial cell soma of capillaries from GA subjects as compared with the controls (p = 0.03). Loss of capillary segments in choriocapillaris was also evident, especially in areas of geographic atrophy and CNV. In eyes from patients with sequence variations, the capillary endothelial cells often appeared degenerative and exhibited atypical fenestrations and pericytes covering the blood vessels. Subjects that were homozygous for polymorphisms in high risk CFH alleles only had more fenestrations/mm of BrMb than subjects that were homozygous only for high risk ARMS2/HTRA1 alleles (p = 0.04), while the latter had greater caveolae area/endothelial cell area than the former (p = 0.007). This study demonstrated an attenuation of CC and a significant decline in the two major transport systems in CC endothelial cells in AMD. This may contribute to drusen deposition, nutrient transport, and vision loss in AMD subjects.

Imaging of cellular aging in human retinal blood vessels

To date two main aging vascular lesions have been reported in elderly human retinas: acellular capillaries and microaneurysms. However, their exact mechanism of formation remains unclear. Using high resolution microscopy techniques we revise cellular alterations observed in aged human retinal vessels, such as lipofuscin accumulation, caveolae malfunction, blood basement membrane disruption and enhanced apoptosis that could trigger the development of these aging vascular lesions. Moreover, we have generated a set of original images comparing retinal vasculature between middle and old aged healthy humans to show in a comprehensive manner the main structural and ultrastructural alterations occurred during age in retinal blood vessels.

Patients with long bone fracture have altered Caveolin-1 expression in their peripheral blood mononuclear cells

INTRODUCTION

Fracture triggers a cascade of systemic and local responses including inflammatory mediator signaling, chemotaxis, osteogenic cell recruitment, differentiation and proliferation at the fracture site. Early signaling between immune cells and repair cells in fracture repair is not well understood. Caveolin-1, a 21-24 kDa membrane protein plays key roles in transmembrane signaling. This study was to investigate the expression of caveolin-1 in human peripheral blood mononuclear cells (PBMNCs) following long bone fracture.

METHODS

PBMNCs were obtained from healthy volunteers or fracture patients at three time points following fracture by density-gradient-centrifugation procedure. Caveolin-1 gene expression and protein characterization was examined by semi-quantitative RT-PCR, immunocytochemistry and Western blot analysis.

RESULTS

Caveolin-1 mRNA and protein was expressed at low levels in the PBMNCs of non-fracture samples. In contrast, caveolin-1 expression was greatly increased in the PBMNCs of fracture patients 9-12 days and reduced at 16-21 days following long bone fracture.

CONCLUSION

The identification of caveolin-1 in PBMNCs and osteoblasts makes this cellular domain a new focus for further investigation. We speculate that caveolin-1 expression in PBMNCs and osteoblasts play an important role in signal transduction during the early stages of fracture healing and may be an indicator for normal or abnormal fracture repair.

Inhibition of cytokine signaling in human retinal endothelial cells through modification of caveolae/lipid rafts by docosahexaenoic acid

PURPOSE

Docosahexaenoic acid (DHA(22:6,n3)) is the principal n3 polyunsaturated fatty acid (PUFA) in the retina. The authors previously demonstrated that DHA(22:6,n3) inhibited cytokine-induced adhesion molecule expression in primary human retinal vascular endothelial (hRVE) cells, the target tissue affected by diabetic retinopathy. Despite the importance of vascular inflammation in diabetic retinopathy, the mechanisms underlying anti-inflammatory effects of DHA(22:6,n3) in vascular endothelial cells are not understood. In this study the authors address the hypothesis that DHA(22:6,n3) acts through modifying lipid composition of caveolae/lipid rafts, thereby changing the outcome of important signaling events in these specialized plasma membrane microdomains.

METHODS

hRVE cells were cultured in the presence or absence of DHA(22:6,n3). Isolated caveolae/lipid raft-enriched detergent-resistant membrane domains were prepared using sucrose gradient ultracentrifugation. Fatty acid composition and cholesterol content of caveolae/lipid rafts before and after treatment were measured by HPLC. The expression of Src family kinases was assayed by Western blotting and immunohistochemistry.

RESULTS

Disruption of the caveolae/lipid raft structure with a cholesterol-depleting agent, methyl-cyclodextrin (MCD), diminished cytokine-induced signaling in hRVE cells. Growth of hRVE cells in media enriched in DHA(22:6,n3) resulted in significant incorporation of DHA(22:6,n3) into the major phospholipids of caveolae/lipid rafts, causing an increase in the unsaturation index in the membrane microdomain. DHA(22:6,n3) enrichment in the caveolae/raft was accompanied by a 70% depletion of cholesterol from caveolae/lipid rafts and displacement of the SFK, Fyn, and c-Yes from caveolae/lipid rafts. Adding water-soluble cholesterol to DHA(22:6,n3)-treated cells replenished cholesterol in caveolae/lipid rafts and reversed the effect of DHA(22:6,n3) on cytokine-induced signaling.

CONCLUSIONS

Incorporation of DHA(22:6,n3) into fatty acyl chains of phospholipids in caveolae/lipid rafts, followed by cholesterol depletion and displacement of important signaling molecules, provides a potential mechanism for anti-inflammatory effect of DHA(22:6,n3) in hRVE cells.

HMG-CoA reductase inhibitors induce apoptosis in pericytes

Pericytes, which surround endothelial cells in precapillary arterioles, capillaries, and postcapillary venules, are important for the development, maturation, and maintenance of the vascular system. Pericytes are also pluripotent cells that can differentiate into a variety of mesenchymal cells including smooth muscle cells and osteoblasts. Possibly because of their vasculature regulating activities and ability to differentiate in situ, pericytes are implicated in several diseases with vascular complications, including diabetic retinopathy, as well as Reynaud's Syndrome, central nervous system dementias, and vascular calcification among others. Statin drugs, which block the conversion of HMG-CoA to mevalonate in the cholesterol synthesis pathway, are known to have apoptotic and growth inhibitory effects on cells in vitro and complex pleiotropic effects on cells and tissues in vivo. Recently, evidence has emerged that statin drug use in human patients results in a significant 20% reduction in cancer incidence. It is not known whether these results are due to direct statin action on normal tissue, growth inhibitory/pro-apoptotic effects on tumor cells, and/or effects on angiogenesis. Because of the role of pericytes in angiogenesis and the effects of statins on cancer incidence, we tested the direct effects of statins on pericytes. Specifically, we demonstrate that 3 statins, simvastatin, lovastatin, and mevastatin induce dose-dependent apoptosis in the TR-PCT1 pericyte cell line, that simvastatin (empirically shown to be the most potent of the 3 statins) induces similar levels of apoptosis in freshly isolated pericytes, and that simvastatin-induced apoptosis in pericytes is cholesterol, caspase-3, and caspase-7 mediated.

Caveolin-1 expression is critical for vascular endothelial growth factor-induced ischemic hindlimb collateralization and nitric oxide-mediated angiogenesis

Nitric oxide (NO) is a powerful angiogenic mediator acting downstream of vascular endothelial growth factor (VEGF). Both the endothelial NO synthase (eNOS) and the VEGFR-2 receptor colocalize in caveolae. Because the structural protein of these signaling platforms, caveolin, also represses eNOS activity, changes in its abundance are likely to influence the angiogenic process in various ways. In this study, we used mice deficient for the caveolin-1 gene (Cav-/-) to examine the impact of caveolae suppression in a model of adaptive angiogenesis obtained after femoral artery resection. Evaluation of the ischemic tissue perfusion and histochemical analyses revealed that contrary to Cav+/+ mice, Cav-/- mice failed to recover a functional vasculature and actually lost part of the ligated limbs, thereby recapitulating the effects of the NOS inhibitor L-NAME administered to operated Cav+/+ mice. We also isolated endothelial cells (ECs) from Cav-/- aorta and showed that on VEGF stimulation, NO production and endothelial tube formation were dramatically abrogated when compared with Cav+/+ ECs. The Ser1177 eNOS phosphorylation and Thr495 dephosphorylation but also the ERK phosphorylation were similarly altered in VEGF-treated Cav-/- ECs. Interestingly, caveolin transfection in Cav-/- ECs redirected the VEGFR-2 in caveolar membranes and restored the VEGF-induced ERK and eNOS activation. However, when high levels of recombinant caveolin were reached, VEGF exposure failed to activate ERK and eNOS. These results emphasize the critical role of caveolae in ensuring the coupling between VEGFR-2 stimulation and downstream mediators of angiogenesis. This study also provides new insights to understand the paradoxical roles of caveolin (eg, repressing basal enzyme activity but facilitating activation on agonist stimulation) in cardiovascular pathophysiology.

Structure and function of endothelial caveolae

Caveolae are spherical invaginations of the plasma membrane and associated vesicles that are found at high surface densities in most cells, endothelia included. Their structural framework has been shown to consist of oligomerized caveolin molecules interacting with cholesterol and sphingolipids. Caveolae have been involved in many cellular functions such as endocytosis, signal transduction, mechano-transduction, potocytosis, and cholesterol trafficking. Some confusion still persists in the field with respect to the relationship between caveolae and the lipid rafts, which have been involved in many of the above functions. In addition to all these, endothelial caveolae have been involved in capillary permeability by their participation in the process of transcytosis. This short review will focus on their structure and components, methods used to determine these components, and the role of caveolae in the transendothelial exchanges between blood plasma and the interstitial fluid.