Mesangial cells stimulate differentiation of endothelial cells to form capillary-like networks in a three-dimensional culture system

Crosstalk among podocytes, glomerular endothelial cells and mesangial cells in diabetic kidney disease: an updated review

Diabetic kidney disease (DKD) is a long-term and serious complication of diabetes that affects millions of people worldwide. It is characterized by proteinuria, glomerular damage, and renal fibrosis, leading to end-stage renal disease, and the pathogenesis is complex and involves multiple cellular and molecular mechanisms. Among three kinds of intraglomerular cells including podocytes, glomerular endothelial cells (GECs) and mesangial cells (MCs), the alterations in one cell type can produce changes in the others. The cell-to-cell crosstalk plays a crucial role in maintaining the glomerular filtration barrier (GFB) and homeostasis. In this review, we summarized the recent advances in understanding the pathological changes and interactions of these three types of cells in DKD and then focused on the signaling pathways and factors that mediate the crosstalk, such as angiopoietins, vascular endothelial growth factors, transforming growth factor-β, Krüppel-like factors, retinoic acid receptor response protein 1 and exosomes, etc. Furthermore, we also simply introduce the application of the latest technologies in studying cell interactions within glomerular cells and new promising mediators for cell crosstalk in DKD. In conclusion, this review provides a comprehensive and updated overview of the glomerular crosstalk in DKD and highlights its importance for the development of novel intervention approaches.

[Models of glomerular filtration barrier : New developments]

In this article, we present the latest innovations to generate in vitro models of the glomerular filtration barrier. There is currently a growing interest for such model systems that allow to reduce the use of animal models. Methodologies to improve their physiological relevance have taken advantage of the development of induced pluripotent stem cells and of bioengineering, particularly tissue engineering. Here, we first introduce the methods to overcome the limitations of the currently used glomerular cells based on the use of stem cells. The different approaches to obtain podocytes, the most important cells in the glomerulus, are presented. Finally, we emphasize the importance of the glomerular microenvironment in maintaining the glomerular cell phenotype, which can be achieved by co-culturing different glomerular cells, integration of biomaterials mimicking the extracellular matrix and introduction of flows with microfluidics.

Slit2/Robo1 signaling is involved in angiogenesis of glomerular endothelial cells exposed to a diabetic-like environment

Abnormal angiogenesis plays a pathological role in diabetic nephropathy (DN), contributing to glomerular hypertrophy and microalbuminuria. Slit2/Robo1 signaling participates in angiogenesis in some pathological contexts, but whether it is involved in glomerular abnormal angiogenesis of early DN is unclear. The present study evaluated the effects of Slit2/Robo1 signaling pathway on angiogenesis of human renal glomerular endothelial cells (HRGECs) exposed to a diabetic-like environment or recombinant Slit2-N. To remove the effect of Slit2 derived from mesangial cells, human renal mesangial cells (HRMCs) grown in high glucose (HG) medium (33 mM) were transfected with Slit2 siRNA and then the HG-HRMCs-CM with Slit2 depletion was collected after 48 h. HRGECs were cultured in the HG-HRMCs-CM or recombinant Slit2-N for 0, 6, 12, 24, or 48 h. The mRNA and protein expressions of Slit2/Robo1, PI3K/Akt and HIF-1α/VEGF signaling pathways were detected by quantitative real-time PCR, western blotting, and ELISA, respectively. The CCK-8 cell proliferation assay, flow cytometry and the scratch wound-healing assay were used to assess cell proliferation, cycles, and migration, respectively. Matrigel was used to perform a tubule formation assay. Our results showed that the HG-HRMCs-CM with Slit2 depletion enhanced the activation of Slit2/Robo1, PI3K/Akt, and HIF-1α/VEGF signaling in HRGECs in time-dependent manner (0-24 h post-treatment). In addition, the HG-HRMCs-CM with Slit2 depletion significantly promoted HRGECs proliferation, migration, and tube formation. Pretreatment of HRGECs with Robo1 siRNA suppressed the activation of PI3K/Akt and HIF-1α/VEGF signaling and inhibited angiogenesis, whereas PI3K inhibitor suppressed HIF-1α/VEGF signaling, without influencing Robo1 expression. In the HRGECs treated with Slit2-N, Slit2-N time-dependently enhanced the activation of Robo1/PI3K/Akt/VEGF pathway but not HIF-1α activity, and promoted HRGECs proliferation, migration, and tube formation. The effects induced by Slit2 were also abolished by Robo1 siRNA and PI3K inhibitor. Taken together, our findings indicate that in a diabetic-like environment, in addition to mesangial cells, autocrine activation of Slit2/Robo1 signaling of HRGECs may contribute to angiogenesis of HRGECs through PI3K/Akt/VEGF pathway; therefore, Slit2/Robo1 signaling may be a potent therapeutic target for the treatment of abnormal angiogenesis in early DN and may have broad implications for the treatment of other diseases dependent on pathologic angiogenesis.

Bioprinting: uncovering the utility layer-by-layer

Bioprinting is becoming a must have capability in tissue engineering research. Key to the growth of the field is the inherent flexibility, which can be used to answer basic scientific questions that can only be addressed under 3D culture conditions, or organ-on-chip systems that could quickly replace underperforming animal models. Almost certainly the most challenging application of bioprinting will be for bottom-up tissue construction, which faces many of the same challenges as scaffold-based tissue engineering. In this review, the current state-of-the-art approaches to 3D bioprinting are discussed in terms of performance and suitability. This is complemented by an overview of hydrogel-based bioinks, with a special emphasis on composite biomaterial systems.

Leaf-inspired microcontact printing vascular patterns

The vascularization of tissue grafts is critical for maintaining viability of the cells within a transplanted graft. A number of strategies are currently being investigated including very promising microfluidics systems. Here, we explored the potential for generating a vasculature-patterned endothelial cells that could be integrated into distinct layers between sheets of primary cells. Bioinspired from the leaf veins, we generated a reverse mold with a fractal vascular-branching pattern that models the unique spatial arrangement over multiple length scales that precisely mimic branching vasculature. By coating the reverse mold with 50 μg ml^-1 of fibronectin and stamping enabled selective adhesion of the human umbilical vein endothelial cells (HUVECs) to the patterned adhesive matrix, we show that a vascular-branching pattern can be transferred by microcontact printing. Moreover, this pattern can be maintained and transferred to a 3D hydrogel matrix and remains stable for up to 4 d. After 4 d, HUVECs can be observed migrating and sprouting into Matrigel. These printed vascular branching patterns, especially after transfer to 3D hydrogels, provide a viable alternative strategy to the prevascularization of complex tissues.

Activation of endothelial NAD(P)H oxidase accelerates early glomerular injury in diabetic mice

Increased generation of reactive oxygen species (ROS) is a common denominative pathogenic mechanism underlying vascular and renal complications in diabetes mellitus. Endothelial NAD(P)H oxidase is a major source of vascular ROS, and it has an important role in endothelial dysfunction. We hypothesized that activation of endothelial NAD(P)H oxidase initiates and worsens the progression of diabetic nephropathy, particularly in the development of albuminuria. We used transgenic mice with endothelial-targeted overexpression of the catalytic subunit of NAD(P)H oxidase, Nox2 (NOX2TG). NOX2TG mice were crossed with Akita insulin-dependent diabetic (Akita) mice that develop progressive hyperglycemia. We compared the progression of diabetic nephropathy in Akita versus NOX2TG-Akita mice. NOX2TG-Akita mice and Akita mice developed significant albuminuria above the baseline at 6 and 10 weeks of age, respectively. Compared with Akita mice, NOX2TG-Akita mice exhibited higher levels of NAD(P)H oxidase activity in glomeruli, developed glomerular endothelial perturbations, and attenuated expression of glomerular glycocalyx. Moreover, in contrast to Akita mice, the NOX2TG-Akita mice had numerous endothelial microparticles (blebs), as detected by scanning electron microscopy, and increased glomerular permeability. Furthermore, NOX2TG-Akita mice exhibited distinct phenotypic changes in glomerular mesangial cells expressing α-smooth muscle actin, and in podocytes expressing increased levels of desmin, whereas the glomeruli generated increased levels of ROS. In conclusion, activation of endothelial NAD(P)H oxidase in the presence of hyperglycemia initiated and exacerbated diabetic nephropathy characterized by the development of albuminuria. Moreover, ROS generated in the endothelium compounded glomerular dysfunctions by altering the phenotypes of mesangial cells and compromising the integrity of the podocytes.

Tissue-engineered kidney disease models

Renal disease represents a major health problem that often results in end-stage renal failure necessitating dialysis and eventually transplantation. Historically these diseases have been studied with patient observation and screening, animal models, and two-dimensional cell culture. In this review, we focus on recent advances in tissue engineered kidney disease models that have the capacity to compensate for the limitations of traditional modalities. The cells and materials utilized to develop these models are discussed and tissue engineered models of polycystic kidney disease, drug-induced nephrotoxicity, and the glomerulus are examined in detail. The application of these models has the potential to direct future disease treatments and preclinical drug development.

Optimizing the fabrication processes for manufacturing a hybrid hierarchical polyurethane–cell/hydrogel construct

It is essential to control the overall composition and internal architecture for complex organ manufacturing. In this study, several subprocesses were optimized to produce hybrid hierarchical polyurethane–cell/hydrogel constructs with an intrinsic network of grid and branched channels using a double-nozzle low-temperature deposition rapid prototyping system. The formation quality was mainly determined by the polymer concentration and composition. However, the cell viability was mainly determined by the formation time. Cell sensitivities to the inner nozzle diameter and extrusion flux were not significantly different within the given parameter ranges. The integrity of the two material systems can be varied by the formation routes and layer thickness. Under the optimal fabrication parameters, such as formation time within 20 min and gelatin:alginate:fibrinogen ratio of 2:1:1, a high cell survival rate of 80% was attained. The design and fabrication strategies used to create such a complex heterogeneous objects directly from a computer-aided design model represent a promising route for robotic hybrid hierarchical construct implementations, which would allow easy expansion of the subprocessing capabilities and scale up manufacturing capabilities.

Ets-1 transcription is required in tissue factor driven microvessel formation and stabilization

Tissue factor (TF) has well-recognized roles as initiator of blood coagulation as well as an intracellular signaling receptor. TF signaling regulates gene transcription and protein translation. Recently, we have shown that TF-induced mature neovessel formation is ultimately driven by CCL2 expression. However, the signaling process induced by TF to promote microvessel formation remains to be determined. This study was designed with the objective to investigate the mechanisms involved in TF-induced neovessel formation. Here, we have identified that Ets-1 expression is a downstream effector of TF signaling. TF-siRNA induced a highly significant reduction in Ets-1 expression levels and in Ets-1/DNA binding while inducing abrogation of microvessel formation. Activation of Ets-1 rescued the effect of TF inhibition and restored microvessel formation confirming the critical role of Ets-1 in TF-induced angiogenesis. VE-cadherin expression, a key regulator of endothelial intercellular junctions, and an Ets-1 target molecule was dependent of TF-inhibition. We show that TF signals through ERK1/2 to activate Ets-1 and induce CCL2 gene expression by binding to its promoter region. We conclude that endothelial cell TF signals through ERK1/2 and Ets-1 to trigger microvessel formation.

Tissue Factor Regulates Microvessel Formation and Stabilization by Induction of Chemokine (C-C motif) Ligand 2 Expression

Objective—

Tissue factor (TF) triggers arterial thrombosis. TF is also able to initiate cellular signaling mechanisms leading to angiogenesis. Because high cardiovascular risk atherosclerotic plaques show significant angiogenesis, our objective was to investigate whether TF is able to trigger and stabilize atherosclerotic plaque neovessel formation.

Methods and Results—

In this study, we showed, by real-time confocal microscopy in 3-dimensional basement membrane cocultures, that TF in human microvascular endothelial cells (HMEC-1) and in human vascular smooth muscle cells (HVSMCs) plays an important role in the formation of capillary-like networks. TF silencing in endothelial cells and smooth muscle cells inhibits the formation of tube-like structures with stable phenotype. Using an in vivo model, we observed that TF inhibition in either HMEC-1 or HVSMCs reduced their shared ability to form new capillaries. The phenotypic changes induced by TF silencing were linked to reduced chemokine (C-C motif) ligand 2 (CCL2) expression in endothelial cells. Wound healing and chemotactic assays demonstrated that TF-induced release of CCL2 stimulated HVSMC migration to HMEC-1.

Conclusion—

Endogenous TF regulates CCL2 production in endothelial cells. Secreted CCL2 mediates the angiogenic effect of TF by recruiting smooth muscle cells toward endothelial cells and facilitates the maturation of newly formed microvessels.

An in vitro model of the glomerular capillary wall using electrospun collagen nanofibres in a bioartificial composite basement membrane

The filtering unit of the kidney, the glomerulus, contains capillaries whose walls function as a biological sieve, the glomerular filtration barrier. This comprises layers of two specialised cells, glomerular endothelial cells (GEnC) and podocytes, separated by a basement membrane. Glomerular filtration barrier function, and dysfunction in disease, remains incompletely understood, partly due to difficulties in studying the relevant cell types in vitro. We have addressed this by generation of unique conditionally immortalised human GEnC and podocytes. However, because the glomerular filtration barrier functions as a whole, it is necessary to develop three dimensional co-culture models to maximise the benefit of the availability of these cells. Here we have developed the first two tri-layer models of the glomerular capillary wall. The first is based on tissue culture inserts and provides evidence of cell-cell interaction via soluble mediators. In the second model the synthetic support of the tissue culture insert is replaced with a novel composite bioartificial membrane. This consists of a nanofibre membrane containing collagen I, electrospun directly onto a micro-photoelectroformed fine nickel supporting mesh. GEnC and podocytes grew in monolayers on either side of the insert support or the novel membrane to form a tri-layer model recapitulating the human glomerular capillary in vitro. These models will advance the study of both the physiology of normal glomerular filtration and of its disruption in glomerular disease.

Gene targeting and expression analysis of mouse Tem1/endosialin using a lacZ reporter

TEM1 (endosialin) expression is increased in the stroma and tumor vasculature of several common human cancers. The exact physiological role of TEM1 is still unknown since Tem1-deficient mice are viable and show only a lower rate of abdominal site-specific tumor invasion in tumor transplantation experiments. Previous studies have reported Tem1 expression in mouse embryos and adults, but did not determine the timing or location of the earliest expression, and did not examine all organ systems. Using the highly sensitive Bluo-Gal staining method for detecting temporal and spatial Tem1-lacZ activity in lacZ knock-in (+/lacZ) mice, we found that Tem1 gene expression was initially detectable in the dorsal aortic wall, the heart, the umbilical vessels, the first branchial arch, and the cephalic mesenchyme at E9.5. From E10.5 to E14.5, Tem1 gene expression was additionally seen mainly in the genital tubercle, the mesonephros, the whisker follicles, the mesenchymal tissues around the eye, and the lung. Remarkably, the kidney expressed abundant Tem1-lacZ starting from E16.5. Postnatally, Tem1 expression decreased in most organs but elevated expression persisted in the renal glomerulus and the uterus, where the expression pattern varied at different estrous cycle stages. Co-localization studies indicated that most vimentin-positive cells co-expressed Tem1-lacZ, while a large portion of CD31- or desmin-positive cells were also positive for Tem1-lacZ. Taken together, our observations suggest that Tem1 is expressed throughout embryonic and adult development in several types of mesenchymal cells closely related to blood vessels.

Vascularization--the conduit to viable engineered tissues

Long-term viability of thick three-dimensional engineered tissue constructs is a major challenge. Addressing it requires development of vessel-like network that will allow the survival of the construct in vitro and its integration in vivo owing to improved vascularization after implantation. Resulting from work of various research groups, several approaches were developed aiming engineered tissue vascularization: (1) embodiment of angiogenesis growth factors in the polymeric scaffolds for prolonged release, (2) coculture of endothelial cells with target tissue cells and angiogenesis signaling cells, (3) use of microfabrication methods for creating designed channels for allowing nutrients to flow and/or for directing endothelial cells attachment, and (4) decellularization of organs and blood vessels for creating extracellular matrix. A synergistic effect is expected by combining several of these approaches as already demonstrated in some of the latest studies. Current paper reviews the progress in each approach and recent achievements toward vascularization of engineered tissues.

An optimized three-dimensional in vitro model for the analysis of angiogenesis

Angiogenesis is the formation of new blood vessels from the existing vasculature. It is a multistage process in which activated endothelial cells (EC) degrade basement membrane, sprout from the parent vessel, migrate, proliferate, align, undergo tube formation, and eventually branch and anastomose with adjacent vessels. Here we describe a three-dimensional in vitro assay that reproduces each of these steps. Human umbilical vein endothelial cells (HUVEC) are cultured on microcarrier beads, which are then embedded in a fibrin gel. Fibroblasts cultured on top of the gel provide factors that synergize with bFGF and VEGF to promote optimal sprouting and tube formation. Sprouts appear around day 2, lumen formation begins at day 4, and at day 10 an extensive anastomosing network of capillary-like tubes is established. The EC express a similar complement of genes as angiogenic EC in vivo and undergo identical morphologic changes during tube formation. This model, therefore, recapitulates in vivo angiogenesis in several critical aspects and provides a system that is easy to manipulate genetically, can be visualized in real time, and allows for easy purification of angiogenic EC for downstream analysis.

Differentiation of multipotent adult progenitor cells into functional endothelial and smooth muscle cells

Stem cells are not only a promising in vivo tool for the treatment of diseases characterized by irreversible tissue damage, but can also be exploited as in vitro systems to study the conditions required to generate molecularly and functionally defined cell types. Constructing functional arteries with luminal arterial endothelial cells stabilized by a medial layer of smooth muscle cells is one of the challenges of regenerative medicine. This unit describes the conditions for generating endothelial and smooth muscle cells from multipotent adult progenitor cells (MAPCs). It elaborates on the importance of certain parameters, e.g., quality control of the stem cell population used, serum lot variations, seeding density, use of appropriate cytokines, critical to obtaining high differentiation efficiencies. It further focuses on the molecular and functional characterization of the obtained cell types.

Properties of the Glomerular Barrier and Mechanisms of Proteinuria

This review focuses on the intricate properties of the glomerular barrier. Other reviews have focused on podocyte biology, mesangial cells, and the glomerular basement membrane (GBM). However, since all components of the glomerular membrane are important for its function, proteinuria will occur regardless of which layer is affected by disease. We review the properties of endothelial cells and their surface layer, the GBM, and podocytes, discuss various methods of studying glomerular permeability, and analyze data concerning the restriction of solutes by size, charge, and shape. We also review the physical principles of transport across biological or artificial membranes and various theoretical models used to predict the fluxes of solutes and water. The glomerular barrier is highly size and charge selective, in qualitative agreement with the classical studies performed 30 years ago. The small amounts of albumin filtered will be reabsorbed by the megalin-cubulin complex and degraded by the proximal tubular cells. At present, there is no unequivocal evidence for reuptake of intact albumin from urine. The cellular components are the key players in restricting solute transport, while the GBM is responsible for most of the resistance to water flow across the glomerular barrier.

Trophic factor induction of human umbilical cord blood cells in vitro and in vivo

The mononuclear fraction of human umbilical cord blood (HUCBmnf) is a mixed cell population that multiple research groups have shown contains cells that can express neural proteins. In these studies, we have examined the ability of the HUCBmnf to express neural antigens after in vitro exposure to defined media supplemented with a cocktail of growth and neurotrophic factors. It is our hypothesis that by treating the HUCBmnf with these developmentally-relevant factors, we can expand the population, enhance the expression of neural antigens and increase cell survival upon transplantation. Prior to growth factor treatment in culture, expression of stem cell antigens is greater in the non-adherent HUCBmnf cells compared to the adherent cells (p < 0.05). Furthermore, treatment of the non-adherent cells with growth factors, increases BrdU incorporation, especially after 14 days in vitro (DIV). In HUCBmnf-embryonic mouse striata co-culture, a small number of growth factor treated HUCBmnf cells were able to integrate into the growing neural network and express immature (nestin and TuJ1) and mature (GFAP and MAP2) neural markers. Treated HUCBmnf cells implanted in the subventricular zone predominantly expressed GFAP although some grafted HUCBmnf cells were MAP2 positive. While short-term treatment of HUCBmnf cells with growth and neurotrophic factors enhanced proliferative capacity in vitro and survival of the cells in vivo, the treatment regimen employed was not enough to ensure long-term survival of HUCBmnf-derived neurons necessary for cell replacement therapies for neurodegenerative diseases.

Phenotypic heterogeneity of the endothelium: II. Representative vascular beds

Endothelial cells, which form the inner cellular lining of blood vessels and lymphatics, display remarkable heterogeneity in structure and function. This is the second of a 2-part review on the phenotypic heterogeneity of blood vessel endothelial cells. The first part discusses the scope, the underlying mechanisms, and the diagnostic and therapeutic implications of phenotypic heterogeneity. Here, these principles are applied to an understanding of organ-specific phenotypes in representative vascular beds including arteries and veins, heart, lung, liver, and kidney. The goal is to underscore the importance of site-specific properties of the endothelium in mediating homeostasis and focal vascular pathology, while at the same time emphasizing the value of approaching the endothelium as an integrated system.

IL-1beta induces VEGF, independently of PGE2 induction, mainly through the PI3-K/mTOR pathway in renal mesangial cells

Vascular endothelial growth factor (VEGF) could play a relevant role in angiogenesis associated with chronic allograft nephropathy. Interleukin-1beta (IL-1beta) has a key role in inflammatory response. It induces prostaglandin (PG) E2, which is involved in VEGF release by some normal and tumor cells. In the present work, we studied the effect of IL-1beta on VEGF release by rat mesangial cells, the transduction signal, and whether or not PGE2 is involved in this effect. IL-1beta induced a time-dependent formation of VEGF (analyzed by enzyme-linked immunosorbent assay) and PGE2 (analyzed by enzyme immunoassay). The latter correlated with microsomal-PGE-synthase (mPGES)-1 expression rather than with cyclooxygenase (COX)-2 in terms of protein, determined by Western blotting. No effect of IL-1beta on COX-1, cytosolic PGES, or mPGES-2 expression was observed. Indomethacin exerted a nonsignificant effect on IL-1beta-induced VEGF, and exogenously added PGE2 exhibited a nonsignificant stimulatory effect on VEGF formation. SB 203580, a p38 mitogen-activated protein kinase inhibitor, weakly inhibited the induction of VEGF by IL-1beta in a concentration-dependent manner, whereas LY 294002, a phosphoinoside 3-kinase (PI3-K) inhibitor, and rapamycin, a mammalian target of rapamycin (mTOR) inhibitor, strongly inhibited both IL-1beta- and tumor necrosis factor-alpha-induced VEGF formation in a concentration-dependent manner. Rapamycin also decreased glomerular VEGF levels in the anti-Thy1.1 model of experimental glomerulonephritis. In conclusion, the PI3-K-mTOR pathway seems to be essential in cytokine-induced release of VEGF in mesangial cells.