A mathematical model to serve as a clinical tool for assessing obstructive sleep apnea severity

Obstructive sleep apnea (OSA) is a sleep disorder caused by periodic airway obstructions and has been associated with numerous health consequences, which are thought to result from tissue hypoxia. However, challenges in the direct measurement of tissue-level oxygenation make it difficult to analyze the hypoxia exposure pattern in patients. Furthermore, current clinical practice relies on the apnea-hypopnea index (AHI) and pulse oximetry to assess OSA severity, both of which have limitations. To overcome this, we developed a clinically deployable mathematical model, which outputs tissue-level oxygenation. The model incorporates spatial pulmonary oxygen uptake, considers dissolved oxygen, and can use time-dependent patient inputs. It was applied to explore a series of breathing patterns that are clinically differentiated. Supporting previous studies, the result of this analysis indicated that the AHI is an unreliable indicator of hypoxia burden. As a proof of principle, polysomnography data from two patients was analyzed with this model. The model showed greater sensitivity to breathing in comparison with pulse oximetry and provided systemic venous oxygenation, which is absent from clinical measurements. In addition, the dissolved oxygen output was used to calculate hypoxia burden scores for each patient and compared to the clinical assessment, highlighting the importance of event length and cumulative impact of obstructions. Furthermore, an intra-patient statistical analysis was used to underscore the significance of closely occurring obstructive events and to highlight the utility of the model for quantitative data processing. Looking ahead, our model can be used with polysomnography data to predict hypoxic burden on the tissues and help guide patient treatment decisions

Modeling of cardiac muscle thin films: Pre-stretch, passive and active behavior

Recent progress in tissue engineering has made it possible to build contractile bio-hybrid materials that undergo conformational changes by growing a layer of cardiac muscle on elastic polymeric membranes. Further development of such muscular thin films for building actuators and powering devices requires exploring several design parameters, which include the alignment of the cardiac myocytes and the thickness/Young's modulus of elastomeric film. To more efficiently explore these design parameters, we propose a 3-D phenomenological constitutive model, which accounts for both the passive deformation including pre-stretch and the active behavior of the cardiomyocytes. The proposed 3-D constitutive model is implemented within a finite element framework, and can be used to improve the current design of bio-hybrid thin films and help developing bio-hybrid constructs capable of complex conformational changes.Engineering and Applied Science

Quality Metrics for Stem Cell-Derived Cardiac Myocytes

Summary Advances in stem cell manufacturing methods have made it possible to produce stem cell-derived cardiac myocytes at industrial scales for in vitro muscle physiology research purposes. Although FDA-mandated quality assurance metrics address safety issues in the manufacture of stem cell-based products, no standardized guidelines currently exist for the evaluation of stem cell-derived myocyte functionality. As a result, it is unclear whether the various stem cell-derived myocyte cell lines on the market perform similarly, or whether any of them accurately recapitulate the characteristics of native cardiac myocytes. We propose a multiparametric quality assessment rubric in which genetic, structural, electrophysiological, and contractile measurements are coupled with comparison against values for these measurements that are representative of the ventricular myocyte phenotype. We demonstrated this procedure using commercially available, mass-produced murine embryonic stem cell- and induced pluripotent stem cell-derived myocytes compared with a neonatal mouse ventricular myocyte target phenotype in coupled in vitro assays

A tissue-engineered jellyfish with biomimetic propulsion

Reverse engineering of biological form and function requires hierarchical design over several orders of space and time. Recent advances in the mechanistic understanding of biosynthetic compound materials, computer-aided design approaches in molecular synthetic biology and traditional soft robotics, and increasing aptitude in generating structural and chemical microenvironments that promote cellular self-organization have enhanced the ability to recapitulate such hierarchical architecture in engineered biological systems. Here we combined these capabilities in a systematic design strategy to reverse engineer a muscular pump. We report the construction of a freely swimming jellyfish from chemically dissociated rat tissue and silicone polymer as a proof of concept. The constructs, termed 'medusoids', were designed with computer simulations and experiments to match key determinants of jellyfish propulsion and feeding performance by quantitatively mimicking structural design, stroke kinematics and animal-fluid interactions. The combination of the engineering design algorithm with quantitative benchmarks of physiological performance suggests that our strategy is broadly applicable to reverse engineering of muscular organs or simple life forms that pump to survive

Cyclic strain induces dualmode endothelial-mesenchymal transformation of the cardiac valve

Endothelial-mesenchymal transformation (EMT) is a critical event for the embryonic morphogenesis of cardiac valves. Inducers of EMT during valvulogenesis include VEGF, TGF-β1, and wnt/β-catenin (where wnt refers to the wingless-type mammary tumor virus integration site family of proteins), that are regulated in a spatiotemporal manner. EMT has also been observed in diseased, strain-overloaded valve leaflets, suggesting a regulatory role for mechanical strain. Although the preponderance of studies have focused on the role of soluble mitogens, we asked if the valve tissue microenvironment contributed to EMT. To recapitulate these microenvironments in a controlled, in vitro environment, we engineered 2D valve endothelium from sheep valve endothelial cells, using microcontact printing to mimic the regions of isotropy and anisotropy of the leaflet, and applied cyclic mechanical strain in an attempt to induce EMT. We measured EMT in response to both low (10%) and high strain (20%), where low-strain EMT occurred via increased TGF-β1 signaling and high strain via increased wnt/β-catenin signaling, suggesting dual strain-dependent routes to distinguish EMT in healthy versus diseased valve tissue. The effect was also directionally dependent, where cyclic strain applied orthogonal to axis of the engineered valve endothelium alignment resulted in severe disruption of cell microarchitecture and greater EMT. Once transformed, these tissues exhibited increased contractility in the presence of endothelin-1 and larger basal mechanical tone in a unique assay developed to measure the contractile tone of the engineered valve tissues. This finding is important, because it implies that the functional properties of the valve are sensitive to EMT. Our results suggest that cyclic mechanical strain regulates EMT in a strain magnitude and directionally dependent manner. tight junctions | cytokines | activated myofibroblast C ardiac valves are sophisticated structures that function in a complex mechanical environment, opening and closing more than 3 billion times during the average human lifetime (1). Initially considered passive flaps of tissue, it is now acknowledged that valves contain a highly heterogeneous population of endothelial (VEC) and interstitial (VIC) cells. The VICs exist as synthetic, myofibroblast, or smooth muscle-like phenotypes (2, 3) and alter their tone in response to vasoactive mediators (4-7). The VECs line the surface of the valve leaflet and are unique in their ability to undergo endothelial-mesenchymal transformation (EMT), a process that is crucial for valvulogenesis (8, 9). Recent clinical evidence of EMT has been observed in pathologies such as ischemic cardiomyopathy and concomitant mitral regurgitation and is correlated with increased leaflet mechanical strains (10, 11). These pathological strains can be oriented obliquely to cell and tissue orientation (12, 13), suggesting the possible interaction between mechanical forces and tissue architecture in regulating EMT. Prior work has focused on the regulation of EMT via soluble factors. Modulation of VEGF and increases in wnt/β-catenin and TGF-β1 expression, among other factors, direct EMT during valvulogenesis (8, 14) and in the mature valve (15, 16). Additionally, mechanical forces are known to modulate valve remodeling and disease progression (17, 18). However, the influence of mechanical forces and its synergy with tissue architecture in influencing cardiac valve EMT is unknown. During embryonic development, valve morphogenesis has been correlated with an increase in fluid shear stresses, mechanical strains, and altered geometry of the developing heart (19-22). These observations potentially suggests interaction between mechanical forces and the factors that regulate EMT. Additionally, it is also unknown if EMT results in a functional change of the VEC to a contractile myofibroblast-like VIC. We hypothesized that cyclic strain may potentiate valve EMT in a manner dependent on cell orientation and the direction of applied strain. We developed an in vitro model that combines cyclic stretching of engineered valve endothelium reconstituted from primary sheep VECs for biochemical and expression studies. In addition, we present a functional assay for EMT using valve thin films (vTFs), a biohybrid construct of the engineered valve endothelium on an elastomer thin film that is deformed during tissue contraction. We report strain-dependent dual-mode EMT, with TGF-β1 signaling triggering EMT under low strain (10%) and wnt/β-catenin signaling under high strain (20%). We also report strain-dependent increased contractility of transformed VEC tissues when treated with endothelin-1, suggesting transformation of the normally noncontractile VEC to a contractile VIC-like cell

Remembrances of Steve Ellmann, Still Present

In Memoriam: Steven Ellman

Self-Organization of Muscle Cell Structure and Function

The organization of muscle is the product of functional adaptation over several length scales spanning from the sarcomere to the muscle bundle. One possible strategy for solving this multiscale coupling problem is to physically constrain the muscle cells in microenvironments that potentiate the organization of their intracellular space. We hypothesized that boundary conditions in the extracellular space potentiate the organization of cytoskeletal scaffolds for directed sarcomeregenesis. We developed a quantitative model of how the cytoskeleton of neonatal rat ventricular myocytes organizes with respect to geometric cues in the extracellular matrix. Numerical results and in vitro assays to control myocyte shape indicated that distinct cytoskeletal architectures arise from two temporally-ordered, organizational processes: the interaction between actin fibers, premyofibrils and focal adhesions, as well as cooperative alignment and parallel bundling of nascent myofibrils. Our results suggest that a hierarchy of mechanisms regulate the self-organization of the contractile cytoskeleton and that a positive feedback loop is responsible for initiating the break in symmetry, potentiated by extracellular boundary conditions, is required to polarize the contractile cytoskeleton

Monoculture of Leafcutter Ant Gardens

Background -- Leafcutter ants depend on the cultivation of symbiotic Attamyces fungi for food, which are thought to be grown by the ants in single-strain, clonal monoculture throughout the hundreds to thousands of gardens within a leafcutter nest. Monoculture eliminates cultivar-cultivar competition that would select for competitive fungal traits that are detrimental to the ants, whereas polyculture of several fungi could increase nutritional diversity and disease resistance of genetically variable gardens. Methodology/Principal Findings -- Using three experimental approaches, we assessed cultivar diversity within nests of Atta leafcutter ants, which are most likely among all fungus-growing ants to cultivate distinct cultivar genotypes per nest because of the nests' enormous sizes (up to 5000 gardens) and extended lifespans (10–20 years). In Atta texana and in A. cephalotes, we resampled nests over a 5-year period to test for persistence of resident cultivar genotypes within each nest, and we tested for genetic differences between fungi from different nest sectors accessed through excavation. In A. texana, we also determined the number of Attamyces cells carried as a starter inoculum by a dispersing queens (minimally several thousand Attamyces cells), and we tested for genetic differences between Attamyces carried by sister queens dispersing from the same nest. Except for mutational variation arising during clonal Attamyces propagation, DNA fingerprinting revealed no evidence for fungal polyculture and no genotype turnover during the 5-year surveys. Conclusions/Significance -- Atta leafcutter ants can achieve stable, fungal monoculture over many years. Mutational variation emerging within an Attamyces monoculture could provide genetic diversity for symbiont choice (gardening biases of the ants favoring specific mutational variants), an analog of artificial selection.The research was supported by National Science Foundation awards DEB-0920138, DEB-0639879, and DEB-0110073 to UGM; DEB-0949689 to T.R. Schultz, N. Mehdiabadi, and UGM; and a Fellowship (02/05) from the Conselho Nacional de Desenvolvimento Científico e Tecnológico to AR. The funding agencies had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.Biological Sciences, School o

A Bioinspired Computational Model of Cardiac Mechanics: Pathology and Development

In this work we study the function and development of the myocardium by creating models that have been stripped down to essentials. The model for the adult myocardium is based on the double helical band formation of the heart muscle fibers, observed in both histological studies and advanced DTMRI images. The muscle fibers in the embryonic myocardium are modeled as a helical band wound around a tubular chamber. We model the myocardium as an elastic body, utilizing the finite element method for the computations. We show that when the spiral band architecture is combined with spatial wave excitations the structure is twisted, thus driving the development of the embryonic heart into an adult heart. The double helical band model of the adult heart allows us to gain insight into the long standing paradox between the modest, by only 15 %, ability of muscle fibers to contract, and the large left ventricular volume ejection fraction of 60 %. We show that the double helical band structure is the essential factor behind such efficiency. Additionally, when the double helical band model is excited following the path of the Purkinje nerve network, physiological twist behavior is reproduced. As an additional validation, we show that when the stripped down double helical band is placed inside a sack of soft collagen-like tissue it is capable of producing physiologically high pressures. We further develop the model to understand the different factors behind the loss of efficiency in heart with a common pathology such as dilated cardiomyopathy. Using the stripped down model we are able to show that the change to fiber angle is the much more important factor to heart function than the change in gross geometry. This finding has the potential to greatly impact the strategy used in certain surgical procedures.</p