The human body contains over 10 trillion cells spanning hundreds of morphologically distinct cell types. These cells must work together for our bodies to function correctly. However, it remains a mystery how such an enormous diversity of cells coordinate their behaviors.
Tissue structure - or the composition and physical arrangement of cells, extracellular matrix, and diffusible molecules - helps to coordinate cellular behaviors by organizing the flow of chemical, mechanical, and electrical information between cells. Thus, building tissue structure correctly and maintaining tissue structure over time are prerequisites for engineering functional organs and stopping the progression of diseases like cancer.
We are interested in three general questions about how tissue structure forms and functions:
To answer these questions we take a synthetic approach, building human tissues from the bottom-up. This approach allows us to measure and perturb the molecular and physical properties of individual cells, reconstitute them into living tissue, then observe their interactions to reveal the underlying "rules" guiding their collective behaviors. We focus primarily on the cells and tissues of the human breast, and our work incorporates experimental principles from the chemical, biological, and engineering sciences.
(i) How does tissue structure form through the process of self-organization?
(ii) How does tissue structure help cells to arrive at collective decisions and to organize collective behaviors?
(iii) How does tissue structure breakdown during the progression of diseases like cancer?
Chemically Programmed Assembly of 3D Tissues
Engineering complex human tissues from simpler cellular building blocks remains a challenge, but such tissues have the potential to improve models of disease, facilitate drug screening, and bring the promises of regenerative medicine closer to the clinic. We have developed a chemical approach for synthesizing human tissues that incorporate multiple cell types, arranged with high spatial precision, and assembled from the bottom-up and across three-dimensions. Cells are functionalized with chemical “Velcro” – short DNA oligonucleotides that impart specific adhesive properties between neighboring cells. Hybridization of complementary DNA sequences enables the assembly of multicellular structures with defined cell-cell contacts. We call this approach DNA Programmed Assembly.
Our current efforts aim to extend the synthetic capabilities of DNA Programmed Assembly with the goal of building fully integrated and functional human tissue. For example, we are working to program the assembly of the human mammary gland from purified cellular components in vitro. We are using the assembled structures as models to learn how cell-cell signaling networks regulate the behaviors of cells in vivo, and to reveal how these networks breakdown or are co-opted during the early stages of breast cancer. We are also exploring applications of these tissues as preclinical models for drug screening: a fully integrated model of the human mammary gland would advance the development of new therapies for breast cancer, or provide new opportunities for testing and diagnosis in personalized medicine.
We are also developing next generation approaches for assembling cells with the long-term goal of synthesizing functional tissues and organs for regenerative medicine, drug testing, and diagnostic applications.
Cells are living materials – their properties are not static but rather change dynamically in response to their surroundings. This property of cells as materials gives them the capacity to self-organize into specific three dimensional tissue structures. Indeed, the capacity of cells to self-organize is critical to their normal developmental and their ability to self-repair. A better understanding of how tissues self-organize will improve our ability to synthesize tissues and organs in the lab, and suggest new strategies to slow the breakdown of tissue structure that contributes to the initiation and progression of diseases such as cancer.
We are working to understand the “rules” used by cells to self-organize robustly into specific tissue structures. For example, we are characterizing the interfacial interactions between the two cell types in the human mammary epithelium to reveal the physical rules that guide their self-organization. We are particularly interested in how these rules render self-organization robust to the heterogeneous and dynamic cell behaviors that are characteristic of mammary gland development. We are also investigating how paracrine signals act to regulate these interfacial properties, allowing tissues to undergo shape changes and cell rearrangements during different stages of development and disease.
Collective behaviors and decision making
Individual cells of the same type can behave heterogeneously, even when in the same tissue. These behavioral differences can combine in non-intuitive ways to drive new and emergent behaviors at the level of cell communities. For example, many cellular behaviors related to growth, motility, differentiation and adhesion are controlled by signaling pathways downstream of the protein Ras. Ras integrates signals impinging on cells from their extracellular environment, and is also one of the most frequently mutated proteins in cancer. We recently demonstrated that sustained patterns of heterogeneity (also known as cell-to-cell variability) in signaling downstream of Ras are sufficient to drive several collective behaviors that are not observed when the pathways are activated homogeneously among all the cells of a tissue. These emergent behaviors include dissemination of single hypermotile cells, basal cell extrusions, as well as collective and motile protrusions (see video below). These phenomona are frequently associated with aggressive tumors, where heterogeneity can become exaggerated. We are therefore working to understand how these new behaviors emerge on both the phenotypic (single cell behaviors and material properties) and molecular (signaling pathway activation) levels.
Many cell-surface recptors for extracellular signals can organize into larger, nanoscale structures. How do these nanoscale structures affect the propagation of signals from the outside to the inside of the cell? This has been a difficult question to answer because few tools are available to observe or alter the organization and dynamics of these signaling complexes at the nanoscale. We are developing new tools to image (e.g. monovalent quantum dots) and perturb (e.g. DNA-scaffolded multivalent ligands) the dynamics and organization of these receptors with a focus on the biology of the human Notch and Epidermal Growth Factor Receptors. Our goal is to provide new insight into the spatial organization of these signaling complexes, and how their organization contributes to cell-cell interactions affecting cell-fate decisions and cancer.