Jesper Hjortnaes

Hydrogel bioprinted microchannel networks for vascularization of tissue engineering constructs

Vascularization remains a critical challenge in tissue engineering. The development of vascular networks within densely populated and metabolically functional tissues facilitate transport of nutrients and removal of waste products, thus preserving cellular viability over a long period of time. Despite tremendous progress in fabricating complex tissue constructs in the past few years, approaches for controlled vascularization within hydrogel based engineered tissue constructs have remained limited. Here, we report a three dimensional (3D) micromolding technique utilizing bioprinted agarose template fibers to fabricate microchannel networks with various architectural features within photocrosslinkable hydrogel constructs. Using the proposed approach, we were able to successfully embed functional and perfusable microchannels inside methacrylated gelatin (GelMA), star poly(ethylene glycol-co-lactide) acrylate (SPELA), poly(ethylene glycol) dimethacrylate (PEGDMA) and poly(ethylene glycol) diacrylate (PEGDA) hydrogels at different concentrations. In particular, GelMA hydrogels were used as a model to demonstrate the functionality of the fabricated vascular networks in improving mass transport, cellular viability and differentiation within the cell-laden tissue constructs. In addition, successful formation of endothelial monolayers within the fabricated channels was confirmed. Overall, our proposed strategy represents an effective technique for vascularization of hydrogel constructs with useful applications in tissue engineering and organs on a chip.

Arterial and aortic valve calcification inversely correlates with osteoporotic bone remodelling: a role for inflammation.

Aims Westernized countries face a growing burden of cardiovascular calcification and osteoporosis. Despite its vast clinical significance, the precise nature of this reciprocal relationship remains obscure. We hypothesize that cardiovascular calcification progresses with inflammation and inversely correlates with bone tissue mineral density (TMD). Methods and results Arterial, valvular, and bone metabolism were visualized using near-infrared fluorescence (NIRF) molecular imaging agents, targeting macrophages and osteogenesis. We detected significant arterial and aortic valve calcification in apoE−/− mice with or without chronic renal disease (CRD, 30 weeks old; n = 28), correlating with the severity of atherosclerosis. We demonstrated decreases in osteogenic activity in the femurs of apoE−/− mice when compared with WT mice, which was further reduced with CRD. Three-dimensional micro-computed tomography imaging of the cortical and cancellous regions of femurs quantified structural remodelling and reductions in TMD in apoE−/− and CRD apoE−/− mice. We established significant correlations between arterial and valvular calcification and loss of TMD (R2 = 0.67 and 0.71, respectively). Finally, we performed macrophage-targeted molecular imaging to explore a link between inflammation and osteoporosis in vivo. Although macrophage burden, visualized as uptake of NIRF-conjugated iron nanoparticles, was directly related to the degree of arterial and valvular inflammation and calcification, the same method inversely correlated inflammation with TMD (R2 = 0.73; 0.83; 0.75, respectively). Conclusion This study provides direct in vivo evidence that in arteries and aortic valves, macrophage burden and calcification associate with each other, whereas inflammation inversely correlates with bone mineralization. Thus, understanding inflammatory signalling mechanisms may offer insight into selective abrogation of divergent calcific phenomena.

Valvular interstitial cells suppress calcification of valvular endothelial cells

BACKGROUND Calcific aortic valve disease (CAVD) is the most common heart valve disease in the Western world. We previously proposed that valvular endothelial cells (VECs) replenish injured adult valve leaflets via endothelial-to-mesenchymal transformation (EndMT); however, whether EndMT contributes to valvular calcification is unknown. We hypothesized that aortic VECs undergo osteogenic differentiation via an EndMT process that can be inhibited by valvular interstitial cells (VICs). APPROACH AND RESULTS VEC clones underwent TGF-β1-mediated EndMT, shown by significantly increased mRNA expression of the EndMT markers α-SMA (5.3 ± 1.2), MMP-2 (13.5 ± 0.6) and Slug (12 ± 2.1) (p < 0.05), (compared to unstimulated controls). To study the effects of VIC on VEC EndMT, clonal populations of VICs were derived from the same valve leaflets, placed in co-culture with VECs, and grown in control/TGF-β1 supplemented media. In the presence of VICs, EndMT was inhibited, shown by decreased mRNA expression of α-SMA (0.1 ± 0.5), MMP-2 (0.1 ± 0.1), and Slug (0.2 ± 0.2) (p < 0.05). When cultured in osteogenic media, VECs demonstrated osteogenic changes confirmed by increase in mRNA expression of osteocalcin (8.6 ± 1.3), osteopontin (3.7 ± 0.3), and Runx2 (5.5 ± 1.5). The VIC presence inhibited VEC osteogenesis, demonstrated by decreased expression of osteocalcin (0.4 ± 0.1) and osteopontin (0.2 ± 0.1) (p < 0.05). Time course analysis suggested that EndMT precedes osteogenesis, shown by an initial increase of α-SMA and MMP-2 (day 7), followed by an increase of osteopontin and osteocalcin (day 14). CONCLUSIONS The data indicate that EndMT may precede VEC osteogenesis. This study shows that VICs inhibit VEC EndMT and osteogenesis, indicating the importance of VEC-VIC interactions in valve homeostasis.

Directing Valvular Interstitial Cell Myofibroblast‐Like Differentiation in a Hybrid Hydrogel Platform

Three dimensional (3D) hydrogel platforms are powerful tools, providing controllable, physiologically relevant microenvironments that could aid in understanding how various environmental factors direct valvular interstitial cell (VIC) phenotype. Continuous activation of VICs and their transformation from quiescent fibroblast to activated myofibroblast phenotype is considered to be an initiating event in the onset of valve disease. However, the relative contribution VIC phenotypes is poorly understood since most 2D culture systems lead to spontaneous VIC myofibroblastic activation. Here, a hydrogel platform composed of photocrosslinkable versions of native valvular extracellular matrix components-methacrylated hyaluronic acid (HAMA) and methacrylated gelatin (GelMA)-is proposed as a 3D culture system to study VIC phenotypic changes. These results show that VIC myofibroblast-like differentiation occurs spontaneously in mechanically soft GelMA hydrogels. Conversely, differentiation of VICs encapsulated in HAMA-GelMA hybrid hydrogels, does not occur spontaneously and requires exogenous delivery of TGFβ1, indicating that hybrid hydrogels can be used to study cytokine-dependent transition of VICs. This study demonstrates that a hybrid hydrogel platform can be used to maintain a quiescent VIC phenotype and study the effect of environmental cues on VIC activation, which will aid in understanding pathobiology of valvular disease.

Visualizing novel concepts of cardiovascular calcification

Cardiovascular calcification is currently viewed as an active disease process similar to embryonic bone formation. Cardiovascular calcification mainly affects the aortic valve and arteries and is associated with increased mortality risk. Aortic valve and arterial calcification share similar risk factors, including age, gender, diabetes, chronic renal disease, and smoking. However, the exact cellular and molecular mechanism of cardiovascular calcification is unknown. Late-stage cardiovascular calcification can be visualized with conventional imaging modalities such as echocardiography and computed tomography. However, these modalities are limited in their ability to detect the development of early calcification and the progression of calcification until advanced tissue mineralization is apparent. Due to the subsequent late diagnosis of cardiovascular calcification, treatment is usually comprised of invasive interventions such as surgery. The need to understand the process of calcification is therefore warranted and requires new imaging modalities which are able to visualize early cardiovascular calcification. This review focuses on the use of new imaging techniques to visualize novel concepts of cardiovascular calcification.

Simulation of early calcific aortic valve disease in a 3D platform: A role for myofibroblast differentiation

PURPOSE Calcific aortic valve disease (CAVD) is the most prevalent valve disease in the Western world. Recent difficulty in translating experimental results on statins to beneficial clinical effects warrants the need for understanding the role of valvular interstitial cells (VICs) in CAVD. In two-dimensional culture conditions, VICs undergo spontaneous activation similar to pathological differentiation, which intrinsically limits the use of in vitro models to study CAVD. Here, we hypothesized that a three-dimensional (3D) culture system based on naturally derived extracellular matrix polymers, mimicking the microenvironment of native valve tissue, could serve as a physiologically relevant platform to study the osteogenic differentiation of VICs. PRINCIPAL RESULTS Aortic VICs loaded into 3D hydrogel constructs maintained a quiescent phenotype, similar to healthy human valves. In contrast, osteogenic environment induced an initial myofibroblast differentiation (hallmarked by increased alpha smooth muscle actin [α-SMA] expression), followed by an osteoblastic differentiation, characterized by elevated Runx2 expression, and subsequent calcific nodule formation recapitulating CAVD conditions. Silencing of α-SMA under osteogenic conditions diminished VIC osteoblast-like differentiation and calcification, indicating that a VIC myofibroblast-like phenotype may precede osteogenic differentiation in CAVD. MAJOR CONCLUSIONS Using a 3D hydrogel model, we simulated events that occur during early CAVD in vivo and provided a platform to investigate mechanisms of CAVD. Differentiation of valvular interstitial cells to myofibroblasts was a key mechanistic step in the process of early mineralization. This novel approach can provide important insight into valve pathobiology and serve as a promising tool for drug screening.

Mortality after cardiac surgery in patients with liver cirrhosis classified by the Child-Pugh score

Liver cirrhosis is a known risk factor for postoperative mortality in patients undergoing cardiac surgery. Clinical assessment of liver cirrhosis using the widely accepted Child-Pugh (CP) score is thus vital for evaluation of surgical options and perioperative care. However, detailed mortality rates as a consequence of liver cirrhosis are unclear. This review aimed to stratify the risk of short-term (<30 days) and overall (up to 10 years) mortality after cardiac surgery in patients with liver cirrhosis, classified by the CP score. Thus, PubMed, Embase, CINAHL and the Cochrane Library were systematically reviewed by two independent investigators for studies published up to February 2014, in which mortality in cirrhotic patients, classified by the CP classification, undergoing cardiac surgery was evaluated postoperatively. A total of 993 articles were identified. After critical appraisal of 21 articles, 19 were selected for final analysis. Weighted short-term mortality of cirrhotic patients undergoing cardiac surgery was 19.3% [95% confidence interval (CI): 16.4-22.5%]. Across the different CP groups, short-term mortality appeared to be 9.0% (95% CI: 6.6-12.2%), 37.7% (95% CI: 30.8-44.3%) and 52.0% (95% CI: 33.5-70.0%) in Groups A, B and C, respectively. Weighted overall mortality within 1 year was 42.0% (95% CI: 36.0-48.3%) in all cirrhotic patients. Subdivided in groups, overall mortality within that 1 year was 27.2% (95% CI: 20.9-34.7%), 66.2% (95% CI: 54.3-76.3%) and 78.9% (95% CI: 56.1-92.1%) in Groups A, B and C, respectively. In conclusion, short-term mortality is considerably increased in patients with liver cirrhosis CP class B and C. Overall mortality is significantly high in all classes of liver cirrhosis.

Modeling the Human Scarred Heart In Vitro: Toward New Tissue Engineered Models

Cardiac remodeling is critical for effective tissue healing, however, excessive production and deposition of extracellular matrix components contribute to scarring and failing of the heart. Despite the fact that novel therapies have emerged, there are still no lifelong solutions for this problem. An urgent need exists to improve the understanding of adverse cardiac remodeling in order to develop new therapeutic interventions that will prevent, reverse, or regenerate the fibrotic changes in the failing heart. With recent advances in both disease biology and cardiac tissue engineering, the translation of fundamental laboratory research toward the treatment of chronic heart failure patients becomes a more realistic option. Here, the current understanding of cardiac fibrosis and the great potential of tissue engineering are presented. Approaches using hydrogel-based tissue engineered heart constructs are discussed to contemplate key challenges for modeling tissue engineered cardiac fibrosis and to provide a future outlook for preclinical and clinical applications.

Engineered 3D Cardiac Fibrotic Tissue to Study Fibrotic Remodeling

Activation of cardiac fibroblasts into myofibroblasts is considered to play an essential role in cardiac remodeling and fibrosis. A limiting factor in studying this process is the spontaneous activation of cardiac fibroblasts when cultured on two-dimensional (2D) culture plates. In this study, a simplified three-dimensional (3D) hydrogel platform of contractile cardiac tissue, stimulated by transforming growth factor-β1 (TGF-β1), is presented to recapitulate a fibrogenic microenvironment. It is hypothesized that the quiescent state of cardiac fibroblasts can be maintained by mimicking the mechanical stiffness of native heart tissue. To test this hypothesis, a 3D cell culture model consisting of cardiomyocytes and cardiac fibroblasts encapsulated within a mechanically engineered gelatin methacryloyl hydrogel, is developed. The study shows that cardiac fibroblasts maintain their quiescent phenotype in mechanically tuned hydrogels. Additionally, treatment with a beta-adrenergic agonist increases beating frequency, demonstrating physiologic-like behavior of the heart constructs. Subsequently, quiescent cardiac fibroblasts within the constructs are activated by the exogenous addition of TGF-β1. The expression of fibrotic protein markers (and the functional changes in mechanical stiffness) in the fibrotic-like tissues are analyzed to validate the model. Overall, this 3D engineered culture model of contractile cardiac tissue enables controlled activation of cardiac fibroblasts, demonstrating the usability of this platform to study fibrotic remodeling.

Comparative Histopathological Analysis of Mitral Valves in Barlow Disease and Fibroelastic Deficiency

Whether Barlow disease (BD) and fibroelastic deficiency (FED), the main causes of mitral valve prolapse (MVP), should be considered 2 distinct diseases remains unknown. Mitral valves from patients who required surgery for severe mitral regurgitation due to degenerative nonsyndromic MVP were analyzed. Intraoperative diagnosis of BD or FED was based on leaflet redundancy and thickness, number of segments involved, and annular dimension. The removed medial scallop of the posterior leaflet and attached chordae were used for histopathological and immunohistological assessment. Histologically, compared to normal controls (n = 3), BD (n = 14), and FED (n = 9) leaflets demonstrated an altered architecture and increased thickness. Leaflet thickness was greater and chordae thickness lower in BD than FED (P < 0.0001). In BD, increased thickness was owing to spongiosa expansion (proteoglycan accumulation) and intimal thickening on fibrosa and atrialis; in FED, local thickening was predominant on the fibrosa side, with accumulation of proteoglycan-like material around the chordae. Collagen accumulation was observed in FED leaflets and chords and decreased in BD. Fragmented elastin fibers were present in BD and FED; elastin decreased in BD but increased in FED leaflets and around chordae. Activated myofibroblasts accumulate in both diseased leaflets and chords, but more abundantly in FED chordae (P < 0.0001), independently of age, suggesting a role of these cells in chordal rupture. There were more CD34-positive cells in BD leaflets and in FED chordae (P < 0.01). In BD leaflets (but not chordae) proliferative Ki67-positive cells were more abundant (P < 0.01) and matrix metalloproteinase 2 levels were increased (P < 0.01) indicating tissue remodeling. Upregulation of transforming growth factor beta and pERK signaling pathways was evident in both diseases but more prominent in FED leaflets (continued on next page)(P < 0.001), with pERK upregulation in FED chordae (P < 0.0001). Most cellular and signaling markers were negligible in control valves. Quantitative immunohistopathological analyses demonstrated distinct changes between BD and FED valves: predominant matrix degradation in BD and increased profibrotic signaling pathways in FED, indicating that BD and FED are 2 different entities. These results may pave the way for genetic studies of MVP and development of preventive drug therapies.