PROJECT SUMMARY/ABSTRACT
The broad goal of this work is to understand how molecular cues orchestrate and interact with the physical
forces to drive vertebrate morphogenesis. Specifically, we focus on looping of the small intestine, a process
essential for packing of the lengthy intestine within the abdomen, that when defective leads to devastating
congenital disorders. Loops arise due to buckling of the intestinal tube as it elongates against the constraint of
its attached mesentery. The resulting loop wavelength and curvature can be predicted from a handful of
experimentally measured physical properties, comprising tissue geometry, growth rate, and stiffness. Buckling
has emerged as a core mechanism of shaping various tissues and organs in the embryo. However, the elegant
simplicity of buckling mechanics often betrays the biological complexity that engenders and constrains this
physical process. Indeed, an understanding of buckling morphogenesis that integrates physics with the
underlying molecular cues and dynamic cell behaviors is lacking in most contexts. We recently identified BMP
signaling as a key pathway controlling gut looping. With this pathway in hand, the present application exploits a
well-developed understanding of the associated mechanics to study the molecular and cell biological control of
buckling morphogenesis, as well as how forces generated during development feed back to modulate these
controls. We begin by asking how BMP-dependent acto-myosin activity in the mesentery contributes to tissue
mechanics through manipulation of extracellular matrix organization (Aim 1), focusing on the ability of this
tissue to accommodate large strains (>100%) before stiffening; this behavior, known as constitutive
nonlinearity, is a critical determinant of looping morphology, but its biological basis and morphological function
are often overlooked in development. Next, we build upon the striking observation that BMP establishes
differential growth by restricting mesentery elongation in a proliferation-independent manner (Aim 2), testing
the hypothesis that BMP regulates cell size to set up differential growth, driving buckling. Therefore, Aims 1
and 2 focus on BMP-dependent mechanisms of elastic energy storage within the mesentery. This energy
storage must be precisely balanced with energy dissipation to generate stereotyped looping. To address this,
we examine the control of proliferative growth of the mesentery (Aim 3), focusing on the Hippo signaling
pathway and how forces generated by differential growth may feedback on proliferation. These cross-
disciplinary studies combine retroviral gene misexpression, analyses of cell behavior, force and stiffness
measurements, tensile bioreactor studies, and mathematical modeling. The long term vision is to establish
mechano-molecular rules or design principles of embryogenesis, enabling a true engineering approach to
regenerative medicine, wherein stiffness, stress, and strain can be biologically programmed alongside cell type
specification to instruct the assembly of functional three dimensional tissues and organs.
