Celebrating the creative scientific life of Ken Jacobson

Our friend and colleague Kenneth Jacobson passed away on February 7, 2022. With Ken’s passing, we lost both a leading light in biophysics and a force for collegiality and understanding. Ken is perhaps best known for his development of fluorescence recovery after photobleaching (FRAP, which he used for his email address and license plate!) but also left groundbreaking contributions in membrane biophysics, cytoskeletal dynamics, cell motility, and the use of computational modeling to understand cellular events. Among other honors, Ken was the Kenan Distinguished Professor at the University of North Carolina at Chapel Hill, a fellow of the American Association for the Advancement of Science, and winner of the Gregorio Weber Award for contributions to biophysics. Attesting to his compassion and broad vision, later in life, Ken devoted himself to developing solar-powered vaccine coolers to serve remote areas. More than his scientific contributions, though, Ken will also be fondly remembered by his colleagues and former trainees for his warm guidance and selfless support. Simply put, he was an amazing scientist, mentor, and man. We are delighted that Biophysical Journal has agreed to produce this special issue in his honor and that we can offer this brief summary of his career.

Quantitative microscopy on living cells

Throughout his scientific career, Ken Jacobson pioneered the use of the light microscope to follow the dynamics of living cells and the cellular molecules that control their biology. Although the methods and tools that he developed were always driven by the biological questions he was asking, many of the approaches he pioneered have been much more widely adopted and applied to broader areas of research.

First and perhaps foremost among these methods is FRAP. As any cell or membrane biophysicist knows, in FRAP, a strong laser pulse bleaches a small patch of fluorescence, and the rate of recovery is measured to determine the diffusion and/or binding kinetics of the fluorescently labeled molecule. Ken’s first FRAP papers, in which he coined the acronym, were in 1976 (1,2). He was motivated by a deep interest in the dynamic organization of cell membranes, as further described elsewhere in this article. Coincidentally, and as is often the case for great scientific advances, two other laboratories published the same idea that year (3,4). But, not surprisingly, considering that Ken was involved, the competition was apparently very friendly. As recounted in a recent reminiscence published in this journal (5), Ken traveled from his laboratory in Buffalo, NY, to Ithaca with dishes of cultured cells to help the physicists at Cornell actually do some biology with this exciting technique, resulting in a joint paper (6). Initially, FRAP experiments were difficult and required complex homebuilt microscopy rigs coupled to expensive light detectors and lasers. However, with the invention and commercialization of confocal microscopes, FRAP became a standard tool for cell biologists. In this special issue, a perspective by Anne Kenworthy discusses the past and future of analysis with FRAP (7), and in a new study by Cowan and Loew (8), a series of models are presented to simulate various complex experimental FRAP protocols in cell membranes, cytoplasm, and biomolecular condensates.

In addition to FRAP, Ken’s interest in membrane biophysics motivated another important methodological innovation, single-particle tracking, to follow the motion of individual gold particles anchored to single-membrane lipid or protein molecules (9,10); although others had also worked on this technique, Ken developed the analysis of particle tracks with Michael Saxton to establish the ability of the technique to distinguish modes of motion such as directed, confined, tethered, normal diffusion, and anomalous diffusion (11). Because the method was able to localize particle positions with nanometer precision, it arguably was the precursor to today’s super-resolution single-molecule imaging methods.

As discussed in more detail below, Ken focused much of his later science on trying to understand cell motility, and this spurred him to develop several methods that were, much as with FRAP, subsequently adopted and expanded. He published a seminal paper for defining the kinematics of migrating cells on the basis of frame-by-frame analysis of microscope videos (12), which set off a surge in the use of fish keratocytes as model systems for cell migration. He also developed a quantitative method to assess the forces across, within, and around migrating cells. Dubbed “traction force microscopy,” it involved precisely measuring the displacement of 1-μm latex beads embedded in an elastic silicone substrate upon which fish keratocytes were rapidly crawling (13,14); a paper by Mollica et al. in this issue uses a variant of traction force microscopy, “black dots analysis,” to analyze traction forces in platelets on varying adhesive substrates (15).

To determine how cell migration is regulated, Ken developed methods for sensing and controlling the molecules involved. He came up with the idea that actin binding proteins could be photoactivated to locally sequester G-actin and control the direction of lamellipodial extension (16). He also developed the use of EGFP labeling to implement “chromophore-assisted laser inactivation” to locally ablate the labeled molecule with brief laser excitation that required significantly more power than FRAP (17,18). Importantly, his laboratory subsequently used a combination of chromophore-assisted laser inactivation and FRAP of the actin capping protein, dissected with the help of computational modeling, to show that capping protein dissociation from the ends of actin filaments is much faster in cells than had been measured in vitro (19). Papers from the Hahn and Sharp laboratories in this issue (20,21) develop and use fluorescent sensors to spatially localize cell motility regulators and their conformational changes.

Investigating the composition and dynamics of membrane domains

Over the course of five decades, Ken was engaged in understanding the lateral organization and particularly the dynamic mobility between components of the cell’s plasma membrane. Ken was intrigued by debate regarding whether noncaveolar glycosphingolipid-enriched domains exist on the plasma membrane of cells in vivo, which intensified after the suggestion by Kai Simons and Elina Ikonen that “lipid rafts” could function as mobile platforms that supported apical sorting in polarized epithelial cells (22). Ken maintained an integrative perspective that included not only the rapid dynamics and exchange of lipid components, but also their interaction with membrane proteins, often occurring at the nanoscale (23,24,25). Ken realized that the questions regarding the nanoscale organization and rapid dynamics of the plasma membrane were ideal candidates to be addressed with the concurrent breakthroughs occurring in the development of new optical imaging techniques.

Ken turned his attention to studying plasma membrane domains that were known to be critical for viral budding and assembly, notably first to HIV-1 and later to Dengue virus. He found these viruses, which could be studied by using noninfectious virus-like particles of ∼100–150 nm in diameter, to be ideal candidates for imaging techniques that required nanoscale dynamic reorganization of lipid and protein components and could be observed close to the imaging coverslip. In the past few decades, members of Ken’s laboratory applied complementary biophysical imaging techniques to determine the composition, structure, and dynamics of plasma membrane domains relevant to viral budding and binding (26,27,28). Ken believed strongly that to fully interrogate these critical structures, in vivo dynamic studies should be complemented with high-resolution structural composition interrogation, leading to studies that often combined the contributions from multiple technique developers and biological viral assembly experts. These large-scale collaborations, combining not only techniques like two-color super-resolution “Blink” microscopy and line-scan fluorescence correlation spectroscopy, but expertise from highly diverse scientific backgrounds, are representative of Ken’s deep respect for subject matter experts and incredible capacity for creativity, collaboration, and integrating information. The continuation of this work is represented in part by the contributions from Morales et al. (29), Anaya et al. (30), and Dasgupta et al. (31) in this special issue.

Understanding the mechanics of the cytoskeleton and cell migration

Almost a decade after establishing his laboratory, Ken expanded his research interests to include the contributions the cytoskeleton makes toward maintaining cell shape and moving it through a physical environment. This included studies into the forces that cells exert onto their substrates (14,32), modeling the principles of cell locomotion (12), using sophisticated microscopy techniques to reveal the importance of spatiotemporal control of individual cytoskeletal components in cell migration (16,17,33), deciphering the signaling mechanisms that control focal adhesion dynamics (34,35,36), and using oscillatory behavior that occurs during cell spreading to perform systems-level investigations into the biochemical and biomechanical mechanisms that drive changes in cell shape (37). Ken was a member of the NIH-funded Cell Migration Consortium, an interdisciplinary and multiinstitutional effort to develop tools, reagents, and shared information to allow huge leaps forward in our understanding of cell movement (38). Apart from the work performed in his laboratory, Ken would also travel to Germany in the summers to collaborate with Manfred Radmacher, where they performed atomic force microscopy experiments. This work, which continued into his sixties and had Ken doing some of the actual experiments himself, provided some of the first direct physical measurements of furrow stiffening (39) and lamellipodia extension (40,41).

This special issue contains many articles that continue the legacy of Ken Jacobson in understanding the biophysical principles of the cytoskeleton and cell migration. This includes studies revealing novel mechanisms whereby Rho-dependent cell signaling controls cell movement (20,42), manuscripts that deepen our understanding of how membrane protrusions form and behave (43,44), an analysis of the actin patterns and traction forces made by platelets (15), and papers that depict new mechanisms through which actomyosin structures form (45,46) and control cell shape (47). There are also two exciting perspectives from Chen et al. and Mogilner and Savinov that explore the biophysical mechanisms that underlie cell motility (48,49).

A generous mentor, collaborator, and colleague

During the online memorial services for Ken, when many people had a chance to say just a few words about him, it became abundantly clear that no one there had an inkling of the impact he had had on so many individuals and their careers. There was an outpouring of stories, quotes, and moving attestations to his generosity. One of the most important of Ken’s contributions was the example he set in interactions with students, postdocs, and colleagues. Dr. Jacobson did not see science as a competition but rather as a great endeavor we were privileged to work on together. He was extraordinarily generous—asking the people in his laboratory to speak when he was invited to present at major conferences and putting young professors in the corresponding author position on joint papers and reviews. Long after people left his laboratory, they came to him for help finding jobs or with other career issues, and he worked hard to find the right connections for them.

Due to his extensive collaborations, Ken met and mentored quite a few people who were not part of his group. They came to him because of his approach to science and the example that he set. Even when he asked very probing questions in research meetings or at presentations, he was encouraging. Those of us who knew him were impressed by his calm, supportive demeanor and his expert use of humor and stories of famous scientists. Ken had lunch with his group almost every day, which they genuinely enjoyed. He was both approachable and commanded great respect. One of us, who as a student regularly attended those lunches and is now a principal investigator, has a sign on the back of his office door that says “Have you been a Ken Jacobson today?”

Editor: Vasanthi Jayaraman.