Shayn M. Peirce

Ischemia-reperfusion injury in chronic pressure ulcer formation: a skin model in the rat.

Most animal models of chronic pressure ulcers were designed to study only the role of ischemic injury in wound formation, often using single applications of constant pressure. The purpose of this study was to develop and characterize a reproducible model of cyclic ischemia‐reperfusion injury in the skin of small un‐anesthetized animals using clinically relevant pressures and durations. Ischemia‐reperfusion injury was created in a 9 cm2 region of dorsal skin in male rats by periodically compressing skin under a pressure of 50 mm Hg using an implanted metal plate and an overlying magnet. We varied the total number of ischemia‐reperfusion cycles, examined the effect of varying the frequency and duration of ischemic insult, and compared ischemia‐induced injury to ischemia‐reperfusion‐induced injury with this model. Tissue injury increased with an increasing number of total ischemia‐reperfusion cycles, duration of ischemia, and frequency of ischemia‐reperfusion cycles. This model generates reproducible ischemia‐reperfusion skin injury as characterized by tissue necrosis, wound thickness, leukocyte infiltration, transcutaneous oxygen tension, and wound blood flow. Using this model, the biological markers of ischemia‐reperfusion‐induced wound development can be studied and therapeutic interventions can be evaluated in a cost‐effective manner.

Multicellular simulation predicts microvascular patterning and in silico tissue assembly

Remodeling of microvascular networks in mammals is critical for physiological adaptations and therapeutic revascularization. Cellular behaviors such as proliferation, differentiation, and migration are coordinated in these remodeling events via combinations of biochemical and biomechanical signals. We developed a cellular automata (CA) computational simulation that integrates epigenetic stimuli, molecular signals, and cellular behaviors to predict microvascular network patterning events. Over 50 rules obtained from published experimental data govern independent behaviors (including proliferation, differentiation, and migration) of thousands of interacting cells and diffusible growth factors in their tissue environment. From initial network patterns of in vivo blood vessel networks, the model predicts emergent patterning responses to two stimuli: 1) network‐wide changes in hemodynamic mechanical stresses, and 2) exogenous focal delivery of an angiogenic growth factor. The CA model predicts comparable increases in vascular density (370±29 mm/mm3) 14 days after treatment with exogenous growth factor to that in vivo (480±41 mm/mm3) and approximately a twofold increase in contractile vessel lengths 5–10 days after 10% increase in circumferential wall strain, consistent with in vivo results. The CA simulation was thus able to identify a functional patterning module capable of quantitatively predicting vessel network remodeling in response to two important epigenetic stimuli.

IFATS collection: The role of human adipose-derived stromal cells in inflammatory microvascular remodeling and evidence of a perivascular phenotype.

A growing body of literature suggests that human adipose‐derived stromal cells (hASCs) possess developmental plasticity both in vitro and in vivo, and might represent a viable cell source for therapeutic angiogenesis and tissue engineering. We investigate their phenotypic similarity to perivascular cell types, ability to contribute to in vivo microvascular remodeling, and ability to modulate vascular stability. We evaluated hASC surface expression of vascular and stem/progenitor cell markers in vitro, as well as any effects of platelet‐derived growth factor B chain (PDGF‐BB) and vascular endothelial growth factor 165 on in vitro hASC migration. To ascertain in vivo behavior of hASCs in an angiogenic environment, hASCs were isolated, expanded in culture, labeled with a fluorescent marker, and injected into adult nude rat mesenteries that were stimulated to undergo microvascular remodeling. Ten, 30, and 60 days after injection, tissues from anesthetized animals were harvested and processed with immunohistochemical techniques to determine hASC quantity, positional fate in relation to microvessels, and expression of endothelial and perivascular cell markers. After 60 days, 29% of hASCs exhibited perivascular morphologies compared with 11% of injected human lung fibroblasts. hASCs exhibiting perivascular morphologies also expressed markers characteristic of vascular pericytes: smooth muscle α‐actin (10%) and neuron‐glia antigen 2 (8%). In tissues treated with hASCs, vascular density was significantly increased over age‐matched controls lacking hASCs. This study demonstrates that hASCs express pericyte lineage markers in vivo and in vitro, exhibit increased migration in response to PDGF‐BB in vitro, exhibit perivascular morphology when injected in vivo, and contribute to increases in microvascular density during angiogenesis by migrating toward vessels.

Computational and Mathematical Modeling of Angiogenesis

Over the past two decades, a number of mathematical and computational models have been developed to study different aspects of angiogenesis that span the spatial and temporal scales encompassed by this complex process. For example, models have been built to investigate how growth factors and receptors signal endothelial cell proliferation, how groups of endothelial cells assemble into individual vessels, and how tumors recruit the ingrowth of whole microvascular networks. A prudent question to pose is: “what have we learned from these models?” This review aims to answer this question as it pertains to angiogenesis in the context of normal physiological growth, tumorigenesis, wound healing, tissue engineering, and the design of therapeutic strategies. We also provide a framework for parsing angiogenesis models into categories, according to the type of modeling approach used, the spatial and temporal scales simulated, and the overarching question being posed to the model. Finally, this review introduces some of the simplification strategies and assumptions used in model building, discusses model validation, and makes recommendations for application of modeling approaches to unresolved questions in the field.

Differential Arterial/Venous Expression of NG2 Proteoglycan in Perivascular Cells Along Microvessels: Identifying a Venule-Specific Phenotype

Objective: Similar to other vascular pericyte markers, including smooth muscle (SM) α‐actin, desmin, and PDGF‐β‐receptor, NG2 proteoglycan is not pericyte specific. Therefore, the use of NG2 as a pericyte marker, especially in cell lineage studies, in comparison to other nonspecific pericyte markers requires an understanding of how its expression varies spatially within a microvascular network. The objective of this study was to characterize NG2 expression along vessels within rat microvascular networks and compare this to SM α‐actin expression.

Vascular Assembly in Natural and Engineered Tissues

Abstract: With the advent of molecular embryology and exploitation of genetic models systems, many genes necessary for normal blood vessel formation during early development have been identified. These genes include soluble effectors and their receptors, as well as components of cell‐cell junctions and mediators of cell‐matrix interactions. In vitro model systems (2‐D and 3‐D) to study paracrine and autocrine interactions of vascular cells and their progenitors have also been created. These systems are being combined to study the behavior of genetically altered cells to dissect and define the cellular role(s) of specific genes and gene families in directing the migration, proliferation, and differentiation needed for blood vessel assembly. It is clear that a complex spatial and temporal interplay of signals, including both genetic and environmental, modulates the assembly process. The development of real‐time imaging and image analysis will enable us to gain further insights into this process. Collaborative efforts among vascular biologists, biomedical engineers, mathematicians, and physicists will allow us to bridge the gap between understanding vessel assembly in vivo and assembling vessels ex vivo.

Multi-cell Agent-based Simulation of the Microvasculature to Study the Dynamics of Circulating Inflammatory Cell Trafficking

Leukocyte trafficking through the microcirculation and into tissues is central in angiogenesis, inflammation, and the immune response. Although the literature is rich with mechanistic detail describing molecular mediators of these processes, integration of signaling events and cell behaviors within a unified spatial and temporal framework at the multi-cell tissue-level is needed to achieve a fuller understanding. We have developed a novel computational framework that combines agent-based modeling (ABM) with a network flow analysis to study monocyte homing. A microvascular network architecture derived from mouse muscle was incorporated into the ABM. Each individual cell was represented by an individual agent in the simulation. The network flow model calculates hemodynamic parameters (blood flow rates, fluid shear stress, and hydrostatic pressures) throughout the simulated microvascular network. These are incorporated into the ABM to affect monocyte transit through the network and chemokine/cytokine concentrations. In turn, simulated monocytes respond to their local mechanical and biochemical environments and make behavioral decisions based on a rule set derived from independent literature. Simulated cell behaviors give rise to emergent leukocyte rolling, adhesion, and extravasation. Molecular knockout simulations were performed to validate the model, and predictions of monocyte adhesion, rolling, and extravasation show good agreement with the independently published corresponding mouse studies.

Microvascular Remodeling: A Complex Continuum Spanning Angiogenesis to Arteriogenesis

Angiogenesis, the arterialization of capillaries, and arteriogenesis are specific manifestations of the complex continuum of blood vessel‐remodeling processes that are produced by environmental stimuli. Together, they determine the integrative control of vascular assembly and pattern formation. Vascular assembly and pattern formation are critical elements of therapeutic vascular collateralization of progressively ischemic organs and in the tissue engineering or organogenesis of various tissue substitutes. An integrative systems approach is useful to measure the dynamics of vascular assembly in vivo across time scales from the embryo to the adult, and spanning spatial scales from cells to whole networks, to understand the complex interplay of multiple interacting cells and signal molecules. This requires in vivo observations, multiscale computer simulations, and tools for the genetic regulation of cell interactions. The new view of vascular remodeling as a continuum that can be manipulated in various tissues and in different size blood vessels, using appropriately coordinated multisignal stimuli, should open new therapeutic avenues.

Multiscale Computational Models of Complex Biological Systems

Integration of data across spatial, temporal, and functional scales is a primary focus of biomedical engineering efforts. The advent of powerful computing platforms, coupled with quantitative data from high-throughput experimental methodologies, has allowed multiscale modeling to expand as a means to more comprehensively investigate biological phenomena in experimentally relevant ways. This review aims to highlight recently published multiscale models of biological systems, using their successes to propose the best practices for future model development. We demonstrate that coupling continuous and discrete systems best captures biological information across spatial scales by selecting modeling techniques that are suited to the task. Further, we suggest how to leverage these multiscale models to gain insight into biological systems using quantitative biomedical engineering methods to analyze data in nonintuitive ways. These topics are discussed with a focus on the future of the field, current challenges encountered, and opportunities yet to be realized.

Characterizing emergent properties of immunological systems with multi- cellular rule-based computational modeling

The immune system is comprised of numerous components that interact with one another to give rise to phenotypic behaviors that are sometimes unexpected. Agent-based modeling (ABM) and cellular automata (CA) belong to a class of discrete mathematical approaches in which autonomous entities detect local information and act over time according to logical rules. The power of this approach lies in the emergence of behavior that arises from interactions between agents, which would otherwise be impossible to know a priori. Recent work exploring the immune system with ABM and CA has revealed novel insights into immunological processes. Here, we summarize these applications to immunology and, particularly, how ABM can help formulate hypotheses that might drive further experimental investigations of disease mechanisms.