Molecular control of mechanical forces driving buckling morphogenesis of the small intestine

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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.