NSF/MCB-BSF:Elucidating the role of ERM proteins in cytoskeletal orientation in a contractile tissue

Northeastern University, Boston MA

Investigators

Abstract

Animal bodies are full of biological tubing: blood vessels, lung airways, intestines, the reproductive system, and ducts and glands of various sorts. These tubes are composed of cells that squeeze and relax to move contents through the tubes in the correct direction, at the correct rate, and in a coordinated manner. The squeezing is driven by a molecular machine called the acto-myosin cytoskeleton, which is comprised of tiny motors that pull on a girdle-like mesh of fibers to contract the cells. The "girdle" must be lined up properly and coordinated between cells for tubes to work properly. However, little is known about how this "girdle" knows how to squeeze the right amount at the right time or how its fiber alignment changes in response to changing mechanical conditions. To better understand how this process works, the Cram and Zaidel-Bar labs study the reproductive system of a small nematode worm called C. elegans. This worm can produce its body weight in babies every day, so many eggs have to be squeezed through the reproductive system in a coordinated and robust manner. This system was chosen because the worm is transparent, which makes it easy to see the response of the "girdle" in real time in a living animal, and the system uses the same cellular components as do other animals. This project will determine, molecularly, how the worm's acto-myosin "girdle" responds to the stretch of eggs entering and exiting the system, and how it contracts just the right amount to push the eggs through in the right direction and without mangling them. Because similar genes regulate acto-myosin contraction in many animals, the results should be broadly applicable up to and including primates. Broader impacts of this project include developing materials to help undergraduates learn how to be scientists, and involving high school teachers in research experiences and helping them design projects that they can take back to their classrooms. Actin networks in contractile cells, such as the smooth muscle and endothelial cells of the vasculature, are critical for cell contractility, motility, and tissue function. But, how do cells organize their actin cytoskeletons in response to changing mechanical conditions? And how is cytoskeletal alignment coordinated between cells to produce a cohesive tissue-level response? To address these questions, the Cram and Zaidel-Bar labs have developed a new in vivo model system: the C. elegans spermatheca, a stretch-responsive and contractile tissue in the nematode reproductive system. The Cram lab has discovered that during the first ovulation, myosin becomes activated and pulls a network of loose actin fibers into aligned and oriented stress-fiber like acto-myosin bundles. In a screen through all actin binding proteins in C. elegans, the ezrin-radixin-moesin (ERM) proteins Merlin (NFM-1) and Ezrin (ERM-1) were identified as key tissue-level regulators of actin fiber orientation. ERM proteins can bind plasma membrane, actin, and transmembrane proteins, placing them ideally to regulate cell responses to stretch. Using live imaging, biochemical, genetic, and optogenetic approaches, this collaborative team will elucidate the role ERM proteins play in actin organization within and between cells of the spermatheca. Aim 1 will determine the dynamics of the ERM proteins during spermathecal stretch and contraction and discover the mechanism by which these proteins regulate the apical and basal actin networks and tissue-level organization of the actin cytoskeleton. Aim 2 will determine how ERM proteins both regulate and are regulated by the small GTPase Rho, including analysis of a novel regulator C45G9.7/Tip1, to promote cytoskeletal organization and orientation. This project will reveal that Merlin and Ezrin, important regulators of the actin cytoskeleton, act not only within individual cells, but in tissue-level coordination of actin cytoskeletal alignment. Novel roles for ERM proteins in cell response to physiological levels of strain will be revealed, including, how the cytoskeleton adapts its orientation and alignment for optimal tissue contractility. The Broader Impacts of this project include 1) developing a structured mentoring approach for undergraduate research and 2) involving science teachers in independent research with the goal of enhancing K-12 STEM education. This collaborative US/Israel project is supported by the US National Science Foundation and the Israeli Binational Science Foundation. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.