How do genes and the environment shape the body and organs of developing embryos?
Answering this question remains a central goal of modern biology. Unlocking the mysteries of development and organogenesis are critical to the effort to advance human health and improve the human condition. Since the discovery of DNA and elucidation of the genetic code, biologists have been focused on the genes, proteins, and molecular pathways that are involved in morphogenesis and the closely related topics of regeneration and cancer. These efforts are now turning to understand how physical principles of mechanics, diffusion, and transport are coupled to genetic processes. These efforts are leading to a new paradigm in biology and medicine that recognizes the key roles played by physical principles in maintaining both healthy homeostasis and regeneration, and the roles of those same principles in pathological cases of morphogenesis that drive cancer and cardiovascular disease.
The overarching research focus of the lab is to understand the role of physical principles in development and disease and to turn that understanding into a technology that can be used to engineer new tissues. We have directed our efforts to understand how genetic factors guide the physical mechanical processes that shape organisms and their organs; how failure or co-option of these processes contribute to birth defects and the spread of cancer; and how engineers might use genetic factors to gain direct control and command over these processes and to fabricate novel tissues for regenerative medicine.
Building the Frog Embryo: molecular-mechanical principles of embryonic morphogenesis.
Our group has pioneered innovative methodologies and tools to measure and modulate the mechanical properties of small, rare tissues in the embryo, and assess the forces they are capable of generating. The ability to manipulate these tissues allows us to test the role of environmental cues in guiding development, e.g. how cells and tissues respond to specific mechanical constraints. We uniquely combine these bioengineering tools with state-of-the-art live cell imaging and molecular genetic manipulation.
Amphibian embryos, specifically those of the aquatic frog Xenopus laevis, offer unique access to cells and tissues and allow us to manipulate the genetic “programs” running in the embryo and directly assess the physical consequences at the cell-, tissue-, and embryo-scale. In a series papers since the lab was founded in 2006 we have tested and identified several important molecular pathways, including those that regulate cytoskeletal dynamics and the extracellular matrix, and their role in shaping the vertebrate animal. We have identified novel biological processes that depend explicitly on the physical mechanical events in the embryo — enabling the embryo to adapt and maintain robust programs of development.
Mechanical control of cellular phenotype and establishment of organ primordia and regeneration: Heart and Skin
Our most complex organs are formed much as a new building is constructed. Like the complex schedule of contractors at a building site, multiple sources of cells are scheduled to arrive and contribute at precise times to add specific components. As an organ develops, naive cells arrive, join with pre-existing tissues, and differentiate to elaborate the function of the organ. We know little about the factors that bring cells into forming organs or the changing conditions in organ primordia during their assembly.
We have recently discovered that cells in the developing embryo respond to changing conditions at the site of organ primordia. Two very different types of cells, one adjacent to the embryonic skin and one migrating to lay the foundation of the embryonic heart, sense the mechanical properties of their environment and use that information to change their behavior and their architecture. The cell under the embryo’s skin appears to recognize changes in its environment and can transdifferentiate to a goblet cell type to regenerate the skin. Heart progenitor cells typically migrate half-way around the embryo as a loose assembly of cells. As they approach their target they sense the changing mechanical conditions and change their behavior and architecture. While they appear to sense similar changes to their environment, heart progenitor cells do not change their type but rather begin a complex program of tissue assembly. These cells assemble new laminar structures leading to formation of the pericardial sac, a fluid filled environment that supports the beating heart. The transition of cardiac progenitors is critical to the physiological function of the embryonic and adult heart. By contrast, we know little about the regenerative biology of goblet cells. However, their remarkable ability of goblet cells to regenerate based on cues from their mechanical environment is of biomedical importance to those seeking either to restore their function or to limit their dysregulation in the respiratory and gastrointestinal tracts
Endothelial Cell Responses to Shear Stress and BMP-ligands: integrating flow driven shear stress and signaling during cardiovascular development and disease:
Cardiovascular health depends on both the initial establishment of the cardiovascular system during development and adaptive remodeling of blood vessels during growth and adult life. Clinicians and basic science researchers have long recognized the role of hemodynamics in both vessel growth and remodeling as well as its role in emergent cases of cardiovascular disease such as atherosclerosis, aortic aneurysms, and arteriovenous malformations.
In collaboration with Dr. Beth Roman, a zebrafish developmental geneticist at the University of Pittsburgh we are working on the mechanistic origin of Hereditary Hemorrhagic Telangiectasia (HHT). HHT is a genetically-linked autosomal dominant cardiovascular disease affecting 1 in 5000 people. Clinical manifestations of HHT are highly variable but it is thought that disease progression and severity are the result of a complex interaction between endothelial cell signaling and the exposure of endothelial cells to blood-flow driven shear stress.
Our goal is to investigate this interaction and to understand its impact on cell migration in a quantitative and predictive fashion. With such predictions, clinicians may be able to restore proper cell migratory behaviors to endothelial cells in HHT patients. To carry out this study we have developed microfluidic devices that can be integrated with high throughput imaging tools allowing us to visualize endothelial cells under varying flow conditions while simultaneously varying concentrations of cell signaling factors, including ligands BMP9 and BMP10. With precise spatial and temporal control of cell signaling and shear stresses we are building statistical models correlating cell behaviors with behaviors. With such models we will be able to build more predictive, computational models that invoke both biological programs of signal transduction and cell motility to predict the outcome of new experiments where different branches of cell signaling and shear stress sensing are altered. Such predictive models will help connect specific genetic mutations, there are hundreds associated with HHT, to functional consequences and suggest specific targets for clinical therapeutics.
Integrating Mechanics of Morphogenesis and Evolution of the Posterior Lobe.
As we consider the origins of human birth defects we are simultaneously struck by the role played by evolution and humbled by deep gaps in our understanding of how evolution shapes the processes of morphogenesis. Variation in morphogenesis and the catastrophic events of a birth defect are closely coupled as variations may be harmless indicators of a robust system or may alternatively trigger a series of changes in development that impact growth and physiological function. Since the modern synthesis of the field of evolution, the major questions of the field have been to connect changes in DNA with geologic-rates of variation and selection. A new discipline is emerging, one seeking to answer fundamental questions concerning the mechanisms by which DNA changes induce variation and how large changes in body structures and organs arise.
We are contributing to this new discipline in collaboration with Dr. Mark Rebeiz at the University of Pittsburgh. Mark works on evolution of the posterior lobe in Drosophila, e.g. fruit fly, species. Mark has worked on genetic mechanisms that control these structures and several years ago we started to ask “how are genetics translated into physical structures?” Drosophila species offer both a powerful, well established genetic model in Drosophila melanogaster as well as a well characterized set of closely related Drosophila species with dramatically different lobe structures. We are currently working to combine both computational and experimental approaches to explore the biophysics of lobe formation and the physico-evolutionary origins of their differences.
Application of “morphogenetic” design principles to tissue engineering.
The goal of tissue engineering is improve human health by providing clinicians with biocompatible materials that can be used in replacement and regenerative medicine applications. We are working toward this goal by first comparing and contrasting strategies of tissue assembly used by embryos to those used by engineers. Tissue engineers try to mimic the end-product of development by shaping scaffold geometry in such a way that cells embedded within these scaffolds form a tissue, for instance aligning to synthetic fibers, and assembling native extracellular matrix to form the desired tissue-like structure. Although self-assembly is a dominant process that guides tissue assembly both within the embryo and within artificial tissue constructs, we know little about these critical processes. Our goals in this project are to use tissue and cellular engineering principles to recreate the processes that produce embryonic tissues and organ primordia. We are proposing to leverage engineering principles to drive large scale self-assembly through reiterated cycles of patterning and morphogenesis – relying more on the initial conditions and intrinsic programs of development found in embryos.
Normal embryogenesis is made possible by the release of long and short range biochemical signaling molecules that serve to generate cell patterns across the embryo, for instance inducing brain and muscle in different regions. For engineers to use similar methods they will need both spatial and temporal control of signaling ligands. In one of our early studies we used microfluidics to deliver diffusible ligands to a living tissue. Synthetic green fluorescent protein receptor proteins were expressed in the tissue allowing us minute-by-minute updates on cell responses to the ligand. Our efforts to control delivery and measure cell-by-cell responses in an embryonic tissue illustrate how such control can be achieved. Ongoing work on this project involves microfabricated structures to control how cells migrate and colonize a territory as well as microfluidic devices to deliver etchants to mimic the removal of tissues by cell death. These demonstration projects are carried out with the aim of illustrating how a particular developmental process can be co-opted by engineers to enable a specific cellular outcome.