Clare Waterman

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.[2] 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,[3] and engages locally to integrins at the plasma membrane via a trilaminar nanoscale structure consisting of specific adapter proteins.[4] 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,[5][6] and the protein vinculin is critical for this engagement.[7][8] 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,[9][10] 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.

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.[11] 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.[12][13] Furthermore, she showed that such dynamics were critical for cells’ ability to sense environmental stiffness, and identified a molecular pathway critical for this mechanosensing.[14] This work provided a biophysical mechanism for the role of actin and adhesion dynamics in cell movement and mechanosensation.

In the mid-1990s Dr. Clare Waterman and colleagues developed a microscopy technique called “fluorescent speckle microscopy” (FSM),[15] 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.[16] 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.[17] 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.[18][19] 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.[20][21][22][23][24]

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[25][26][27] She dissected the role of the small GTPase signaling protein Rac1 in the co-regulation of actin and microtubules in migrating cells.[28][29][30][31][32][33] 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.