Factors Influencing the Oxygen Consumption Rate of Aortic Valve Interstitial Cells: Application to Tissue Engineering

Relative Effects of Fluid Oscillations and Nutrient Transport in the In Vitro Growth of Valvular Tissues

Engineered valvular tissues are cultured dynamically, and involve specimen movement. We previously demonstrated that oscillatory shear stresses (OSS) under combined steady flow and specimen cyclic flexure (flex-flow) promote tissue formation. However, localized efficiency of specimen mass transport is also important in the context of cell viability within the growing tissues. Here, we investigated the delivery of two essential species for cell survival, glucose and oxygen, to 3-dimensional (3D) engineered valvular tissues. We applied a convective-diffusive model to characterize glucose and oxygen mass transport with and without valve-like specimen flexural movement. We found the mass transport effects for glucose and oxygen to be negligible for scaffold porosities typically present during in vitro experiments and non-essential unless the porosity was unusually low (<40%). For more typical scaffold porosities (75%) however, we found negligible variation in the specimen mass fraction of glucose and oxygen in both non-moving and moving constructs (p > 0.05). Based on this result, we conducted an experiment using bone marrow stem cell (BMSC)-seeded scaffolds under Pulsatile flow-alone states to permit OSS without any specimen movement. BMSC-seeded specimen collagen from the pulsatile flow and flex-flow environments were subsequently found to be comparable (p > 0.05) and exhibited some gene expression similarities. We conclude that a critical magnitude of fluid-induced, OSS created by either pulsatile flow or flex-flow conditions, particularly when the oscillations are physiologically-relevant, is the direct, principal stimulus that promotes engineered valvular tissues and its phenotype, whereas mass transport benefits derived from specimen movement are minimal.

System for measuring oxygen consumption rates of mammalian cells in static culture under hypoxic conditions

Estimating the oxygen consumption rates (OCRs) of mammalian cells in hypoxic environments is essential for designing and developing a three-dimensional (3-D) cell culture system. However, OCR measurements under hypoxic conditions are infrequently reported in the literature. Here, we developed a system for measuring OCRs at low oxygen levels. The system injects nitrogen gas into the environment and measures the oxygen concentration by an optical oxygen microsensor that consumes no oxygen. The developed system was applied to HepG2 cells in static culture. Specifically, we measured the spatial profiles of the local dissolved oxygen concentration in the medium, then estimated the OCRs of the cells. The OCRs, and also the pericellular oxygen concentrations, decreased nonlinearly as the oxygen partial pressure in the environment decreased from 19% to 1%. The OCRs also depended on the culture period and the matrix used for coating the dish surface. Using this system, we can precisely estimate the OCRs of various cell types under environments that mimic 3-D culture conditions, contributing crucial data for an efficient 3-D culture system design.

In vitro models of aortic valve calcification: solidifying a system

Calcific aortic valve disease (CAVD) affects 25% of people over 65, and the late-stage stenotic state can only be treated with total valve replacement, requiring 85,000 surgeries annually in the US alone (University of Maryland Medical Center, 2013, http://umm.edu/programs/services/heart-center-programs/cardiothoracic-surgery/valve-surgery/facts). As CAVD is an age-related disease, many of the affected patients are unable to undergo the open-chest surgery that is its only current cure. This challenge motivates the elucidation of the mechanisms involved in calcification, with the eventual goal of alternative preventative and therapeutic strategies. There is no sufficient animal model of CAVD, so we turn to potential in vitro models. In general, in vitro models have the advantages of shortened experiment time and better control over multiple variables compared to in vivo models. As with all models, the hypothesis being tested dictates the most important characteristics of the in vivo physiology to recapitulate. Here, we collate the relevant pieces of designing and evaluating aortic valve calcification so that investigators can more effectively draw significant conclusions from their results.

Metabolic regulation of collagen gel contraction by porcine aortic valvular interstitial cells

Despite a high incidence of calcific aortic valve disease in metabolic syndrome, there is little information about the fundamental metabolism of heart valves. Cell metabolism is a first responder to chemical and mechanical stimuli, but it is unknown how such signals employed in valve tissue engineering impact valvular interstitial cell (VIC) biology and valvular disease pathogenesis. In this study porcine aortic VICs were seeded into three-dimensional collagen gels and analysed for gel contraction, lactate production and glucose consumption in response to manipulation of metabolic substrates, including glucose, galactose, pyruvate and glutamine. Cell viability was also assessed in two-dimensional culture. We found that gel contraction was sensitive to metabolic manipulation, particularly in nutrient-depleted medium. Contraction was optimal at an intermediate glucose concentration (2 g l(-1)) with less contraction with excess (4.5 g l(-1)) or reduced glucose (1 g l(-1)). Substitution with galactose delayed contraction and decreased lactate production. In low sugar concentrations, pyruvate depletion reduced contraction. Glutamine depletion reduced cell metabolism and viability. Our results suggest that nutrient depletion and manipulation of metabolic substrates impacts the viability, metabolism and contractile behaviour of VICs. Particularly, hyperglycaemic conditions can reduce VIC interaction with and remodelling of the extracellular matrix. These results begin to link VIC metabolism and macroscopic behaviour such as cell-matrix interaction.

Design strategies of biodegradable scaffolds for tissue regeneration

There are numerous available biodegradable materials that can be used as scaffolds in regenerative medicine. Currently, there is a huge emphasis on the designing phase of the scaffolds. Materials can be designed to have different properties in order to match the specific application. Modifying scaffolds enhances their bioactivity and improves the regeneration capacity. Modifications of the scaffolds can be later characterized using several tissue engineering tools. In addition to the material, cell source is an important component of the regeneration process. Modified materials must be able to support survival and growth of different cell types. Together, cells and modified biomaterials contribute to the remodeling of the engineered tissue, which affects its performance. This review focuses on the recent advancements in the designs of the scaffolds including the physical and chemical modifications. The last part of this review also discusses designing processes that involve viability of cells.

Heart valve tissue engineering: quo vadis?

Surgical replacement of diseased heart valves by mechanical and tissue valve substitutes is now commonplace and generally enhances survival and quality of life. However, a fundamental problem inherent to the use of existing mechanical and biological prostheses in the pediatric population is their failure to grow, repair, and remodel. A tissue engineered heart valve could, in principle, accommodate these requirements, especially somatic growth. This review provides a brief overview of the field of heart valve tissue engineering, with emphasis on recent studies and evolving concepts, especially those that establish design criteria and key hurdles that must be surmounted before clinical implementation.

Preparation of collagen modified photopolymers: a new type of biodegradable gel for cell growth

In this study a new branched methacrylated poly(propylene glycol-co-lactic acid) (PPG-PLA-IEM) and methacrylated cellulose acetate butyrate resin (CAB-IEM) were synthesized. Hydrogels with various amounts of PPG-PLA-IEM and CAB-IEM (25, 50 and 75 wt% IEM modified) were prepared by photopolymerization. Collagen tethered PEG-monoacrylate (PEGMA-collagen) was prepared and introduced as a bioactive moiety to modify the hydrogel in order to enhance cell affinity. In vitro attachment and growth of 3T3 mouse fibroblasts and human umbilical vein endothelial cells (HUVEC) on the hydrogels with and without collagen were also investigated. It was observed that, the collagen improves the cell adhesion onto the hydrogel surface. With the increasing amount of collagen, cell viability increased by 28% for ECV304 (P < 0.05) and 30% for 3T3 (P < 0.05).