Dr. Clare M. Waterman is a cell biologist who has made seminal contributions to understanding the role of the cytoskeleton in cell migration. Waterman pioneered the invention and application of novel quantitative and super-resolution light microscopy methods. She utilized these tools to reveal the architectural blueprint and dynamics of protein-based nano-machines that self-assemble in cells to generate, organize, and transmit the forces that drive cell movement, and she defined molecular pathways governing the orchestration of these protein machines in space and time. This unique and innovative body of work constitutes a major advance in knowledge of the molecular and biophysical basis of cell movement. Waterman a Distinguished Investigator, Chief of the Laboratory of Cell and Tissue Morphodynamics, and Director of the Cell Biology and Physiology Center at the National Heart, Lung, and Blood Institute, in the National Institutes of Health in Bethesda MD, USA. Waterman has received numerous awards and honours, including the Sackler International prize in Biophysics, the NIH Director’s Pioneer Award, and the Arthur S. Flemming Award for Public Service. In 2018, she was elected to the National Academy of Sciences. She currently serves on the editorial boards of eLife, Current Biology and Journal of Microscopy.
Actin-adhesion cross-talk and the “molecular clutch" hypothesis
The ability of a cell to adhere to and move relative to its environment is mediated by cell surface proteins called integrins that bind extracellular components and mediate their linkage to the intracellular, force-generating actin cytoskeleton. However, the specific molecules mediating the links between integrins and actin and how they are organized at the cell membrane was not known. Dr. Waterman used Fluorescent Speckle Microscopy and super-resolution light microscopy methods to show in migrating cells that actin polymerizes at the leading edge, undergoes a rearward movement towards the cell center, and engages locally to integrins at the plasma membrane via a trilaminar nanoscale structure consisting of specific adapter proteins. The engagement of actin to the focal adhesion results in slowing of actin motion in the leading edge and transmission of its motion to the adapter proteins, and the protein vinculin is critical for this engagement. Dr. Waterman’s work supports a model that draws on analogy of the focal adhesion adapter proteins acting as a mechanical “clutch” to engage the moving actin with the matrix-bound integrins, and thus transmit intracellular force to friction on the extracellular environment to drive cell movement. Dr. Waterman’s works has led to the acceptance of the molecular clutch hypothesis of cell migration.
Extracellular matrix mechanosensing
Waterman has contributed to the understanding of the mechanisms by which cells sense physical cues such as stiffness or tension in their environment, a process known as cellular mechanosensing. Cellular mechanosensing is critical to embryonic development and is thought to drive cancer metastasis. Dr. Waterman and colleagues developed light microscopy methods for mapping cellular forces on a sub-micron scale while simultaneously performing fluorescent speckle microscopy of protein dynamics. She used these tools to show that generation of forces on the cell’s surroundings is driven by specific patterns of actin and adhesion dynamics within cells. Furthermore, she showed that such dynamics were critical for cells’ ability to sense environmental stiffness, and identified a molecular pathway critical for this mechanosensing. This work provided a biophysical mechanism for the role of actin and adhesion dynamics in cell movement and mechanosensation.
Fluorescent speckle microscopy
In the mid-1990s Dr. Clare Waterman and colleagues developed a microscopy technique called “fluorescent speckle microscopy”, that can be used to measure the dynamics of the subunits making up macromolecular structures in living cells, such as the cytoskeletal elements f-actin and microtubules. Developments in fluorescent protein technology and microscopy in the early 1990s led to the ability of cell biologists to label specific cellular proteins with fluorescent markers and follow their dynamics in living cells by time-lapse high resolution microscopy. However, quantitative analysis of these dynamics was primarily descriptive or very limited in spatial resolution. Dr. Waterman discovered that high-resolution imaging of very low levels of fluorescently labeled tubulin in living cells showed a speckle pattern on microtubules. She demonstrated that the speckles serve as diffraction-limited markers at submicron size and density on the cytoskeleton, whose dynamics over time encode cytoskeletal assembly and motion. Together with computer vision engineers, image analysis software was developed that is capable of extracting high resolution maps of cytoskeletal assembly/disassembly and motion in migrating cells from fluorescent speckle microscopy images. Fluorescent Speckle Microscopy is used by cell biologists as a sensitive assay for determining the effects of drug or genetic perturbations on cytoskeletal dynamics in living cells and animal models.
Microtubule/actin interactions
Both of the cytoskeletal polymers, microtubules and actin, are required for directed cell motility. There was some evidence that microtubules provide spatial and temporal orchestration of f-actin-based protrusive and contractile activities, however there was no direct demonstration of binding interactions between the two polymer systems that would result in interdependent dynamics and regulation. Waterman used multispectral Fluorescent Speckle Microscopy to show that microtubules and f-actin exhibit specific structural interactions in cytoplasmic extracts, migrating cells, and neuronal growth cones that result in interdependent dynamics and organization She dissected the role of the small GTPase signaling protein Rac1 in the co-regulation of actin and microtubules in migrating cells. Since then, the notion of “cytoskeletal crosstalk” has become well-accepted as critical to the processes of polarity development in embryos and cells, neuronal growth cone guidance, and directed cell movement during wound healing.
Research overview
Waterman has made fundamental advances in the understanding of the molecular and biophysical basis of cellular motility. Such events are of critical importance in development, the immune response and wound healing, as well as in metastatic cancer. Dr. Waterman’s past work consists of novel findings related to the development of experimental approaches, the cytoskeletal elements of a cell, including microtubules and actin, and the extracellular matrix.
Although Waterman has never been professionally associated with a University, she has made important contributions to scientific education. Waterman has directly overseen the training of many PhD students and post-doctoral scientists in her own research laboratory. She has also trained hundreds of PhD candidates and post-doctoral scholars through her teaching in the Physiology Course at the Marine Biological Laboratory in Woods Hole, where she served as faculty from 2000-2009, and as its first female director from 2009 – 2014. The Physiology Course is an intensive seven-week laboratory summer course that has run for over 125 years. It is designed to bring together senior PhD candidates and early post-doctoral researchers to work on cutting-edge questions in cell physiology. Work carried out by students and faculty at the course have led to numerous primary research publications and also a Nobel Prize. Waterman has also participated as a faculty member in many courses in advanced quantitative light microscopy, both within the United States as well as in Austria, Portugal, India, and Germany. A large fraction of Waterman’s former trainees now hold faculty positions at academic and research institutions around the world.
