Location, Location, Location

In 1981, then a senior scientist at LBNL, Bissell challenged scientific dogma about how much cell culture studies can reveal about whole organisms, asserting that changes in gene expression and function in culture differ from patterns within tissues and organisms, and therefore, that microenvironment regulates cell function. The following year, she proposed that the insoluble extracellular matrix (ECM) outside of cells is in direct communication with the cell nucleus through both physical and chemical signaling, dubbing it the “dynamic reciprocity” model. Reaction to the proposal was lukewarm at best, and Bissell and her students and postdocs have continued to chip away at the naysayers’ objections for the last 35 years, providing a steady stream of evidence for direct communication between cells and the ECM using the mammary gland as a model system.

“I realized that when you put cells in culture, everything changed—the function, their shape. Clearly there was something that is missing in culture that is present in vivo.”

For Bissell, having an opinion and voicing it is nothing new. She grew up in Iran, in close quarters with a large, highly educated multigenerational extended family (her father was the oldest of 10 children). Discussions and debates were the norm. “They were all intellectuals, empathetic and extremely passionate about literature, issues, and politics, and I was immersed in all of that,” she says. From an early age, Bissell was curious, argumentative, and loved the sciences, which came easily to her. By 1979, at the tail end of the Iranian Revolution, Bissell had already built a life and a science career in the U.S., and although her parents and most of her extended family were back in Iran, she didn’t see herself returning to her country of birth. Neither did her mother: “‘Mina is not coming back, not even to visit,’ I remember my mom saying, because she knew that I hadn’t learned not to speak my mind and would upset someone. I was also very sensitive and had been raised to have a deep sense of justice, that people should not be mistreated. Because I was so outspoken, she knew I would argue and likely end up in jail.” Instead, Bissell focused on her scientific pursuits: providing evidence for the idea that the seemingly inert ECM actually has important functions, guiding the biology of cells and tissues that surround it.

Here, Bissell discusses her mother’s reaction to Bissell’s becoming pregnant while in graduate school, her active choice not to research trendy topics, and her conviction that students need to question scientific dogma.

BRIGHT BISSELL

A drive to excel. Schoolwork came easily to Bissell. She received a medal from the Shah of Iran for being the top high school senior in the country and, in her last year of high school, she took an exam that earned her one of five scholarships from the Iranian government to partially cover the cost of attending university abroad. She applied to schools in the U.S., despite her “rudimentary English,” and was accepted to Bryn Mawr College. “I left the county at barely 18 and all on my own,” Bissell says. At Bryn Mawr, Bissell excelled in math and chemistry courses but anything involving English was a challenge. “I took a literature course with Ann Berthoff, one of my most favorite teachers, who is 93 now, and I struggled so much with the books she assigned, especially Faulkner. Still, I wish every teacher taught the way she did. She had so much passion; her drive and work ethic came out in every class. She was a huge influence on me.”

Against the odds. After two years at Bryn Mawr, Bissell transferred to Radcliffe College, following her fiancé, who was a graduate student in political science at Harvard University. In 1963, she graduated with a chemistry major, got married, and, because her husband was still working towards his PhD, entered the bacterial genetics graduate program at Harvard Medical School. “I thought graduate school was better than medical school,” says Bissell. “But I don’t know why I chose microbiology.”

Sitting in a lecture during her first year, she answered a technical microscopy question that made the teaching assistant, a member of Luigi Gorini’s lab, take note. The postdoc told her to come and meet Gorini, who was in his late 60s, and he decided to recruit her as a graduate student. A few months later, Bissell was visibly pregnant with her first child, and Gorini assumed she would be quitting graduate school. “‘What would your mother say?’ he asked me,” Bissell recalls. “What my mother said, from Iran, was ‘You’re not quitting, are you?’ Now how many mothers at that time would say that? But I came from a family and country where education is valued and expected; I was pregnant and going to school and my family saw nothing wrong with that.”  

Against the grain. In Gorini’s lab, Bissell chose to probe the mechanism by which bacterial cells synthesize and excrete a proteinase. While the main focus of the lab, and the department, was studying bacterial resistance to antibiotics, Bissell’s strategy was to avoid the pressure associated with this topic. “Even within the same department at Harvard, there was so much competition and friction that I wanted to work on something that wouldn’t push me into that crowd,” she says. The enzyme translation project was something that two prior postdocs in Gorini’s lab had attempted, but they had failed to generate any publishable results. Bissell persevered for four years—but she could not confirm the model Gorini and his two former postdocs had developed for the mechanism by which this enzyme controlled its own synthesis from outside the bacteria.

Bissell was fascinated with the puzzle of how some proteins in a given bacterium get secreted, while most never exit the cell membrane or exert their functions only intracellularly. “I thought maybe the proteins that get out are synthesized in a different compartment and that a floppy version of the protein comes out of the membrane unfolded and then gets stabilized at the extracellular surface. Thanks to my chemistry background, I thought it might be calcium that acts as the stabilizer. But when I showed Gorini some of my results and explained the new model, he said, ‘Mina, what do you think these proteins are, spaghetti? You will never make it in science.’” With the help of another professor in the department, Bissell devised a way to label the protein and measure its enzymatic activity, showing that only in the presence of calcium was the enzyme active. She proposed a new model of co-translational secretion to describe the process.

BISSELL BUBBLES

Questioning the dogma. Outside the lab, Bissell went through a divorce, became a single mother, and, in her final year of graduate school, married Montgomery (Monty) Bissell, a medical school student who was doing research in her department. “I had no opinion yet about what I wanted to study. I opened The New York Times one day and saw an editorial by Harry Rubin, a biology professor in Berkeley, who had published a paper suggesting that the reason chicken embryo fibroblasts become malignant is that they secrete a particular protease. I quickly decided that I wanted to do a postdoc to isolate and study this protease,” says Bissell. In 1970, the family moved to California, where Bissell began an American Cancer Society Fellowship in Rubin’s lab at the University of California, Berkeley (UCB). Once she began her research, Bissell realized that that particular protease was an artifact created by cell lysis. They published the results, but now she needed something else to work on.

Revealing literature gaps. At UCB, Bissell became interested in cell culture techniques and how viral transformation changes metabolism, working with virus-transformed chicken cells to study how glucose metabolism differs from that of normal cells in culture. She began to read the literature on the Warburg effect—the observation that cancer cells produce energy by aerobic glycolysis and lactic acid fermentation rather than through the typical ATP-producing oxidative phosphorylation cycle. In typical fashion, Bissell quickly became critical of what she read: “The literature was a huge mess. No one was asking what relevance results of experiments from a monolayer of fibroblasts grown in 5 percent carbon dioxide and 20 percent oxygen had to what happens in the body. And no one seemed to be measuring both the input and the output of glucose metabolism.” Bissell felt that those who criticized Warburg were performing poorly controlled experiments and decided to do a complete reanalysis of the Warburg effect on transformed chicken cells. She showed that with the same amount of glucose input, the level of lactate produced by transformed cells was always higher compared to nontransformed cells, independent of culture cell density.

Two years later, in 1974, now in her own laboratory at the LBNL, Bissell, along with James Bassham and colleagues, designed a steady-state machine to measure the kinetics of metabolism and other processes in cultured cells by keeping them in precise growing conditions, including constant temperature and pH, and in isolation from the outside environment. Using the device, Bissell’s lab again confirmed that transformed cells rely more heavily on aerobic glycolysis for energy, but that the switch to this energy pathway did not result from the impairment of the hydrogen-transfer pathway. The results, says Bissell, went against the other half of Warburg’s hypothesis: that the reason for the increased glycolysis is impaired hydrogen transfer.

Everything is in flux. Bissell’s lab at the LBNL continued to study the metabolism of virus-transformed cells in culture. Some of their results from the late 1970s, including the role of cell shape in sugar transport and the potential of microtubules to influence cell growth, would later help shape Bissell’s controversial 1982 proposal that the ECM directly communicates with cells, influencing their behavior and morphology. (See “May the Force Be With You,” The Scientist, February 2016.) “I didn’t know much about the ECM, but I had three postdocs, Richard Schwartz, Glenn Hall, and Joanne Emerman, who had worked with and thought about the components of the ECM. We observed that cells grown on a collagen gel more resembled the look of cells in vivo. It also occurred to me that in vivo, cells have a polarity that they don’t have in culture. And I realized that when you put cells in culture, everything changed—the function, their shape. Clearly there was something that is missing in culture that is present in vivo,” says Bissell.

She scoured the literature, the vast majority of it descriptive, for information about the biochemical components and structure of the molecules that made up the ECM, which at the time, was thought to be inert. In 1981, Bissell first proposed that gene expression, and therefore cell function, changes depending on context and that the cell’s microenvironment influences these changes. That thinking led her to propose, in 1982, that the microenvironmental influence is the ECM, which both chemically and physically interacts with cells. According to Bissell’s ‘dynamic reciprocity’ model, signals from the ECM traveled through transmembrane receptors to a cell’s interior and nucleus, altering its gene expression. “I began to think that the ECM played a role in tissue and organ specificity, because the cells all had the same genetic material, but I realized that there is no constitutive gene expression, that the context changes and so do the cells.”

BOLD BISSELL

Evidence builds. To provide evidence for the model, Bissell’s lab developed 3-D culture techniques, allowing differentiation and creation of at least partial tissue architecture of the mammary gland in culture. “If the cellular and tissue architecture is so important, I thought we should be able to take a malignant cell and change its structure and make it normal and also vice versa,” says Bissell. By the early 1980s, integrins—proteins that physically attach the ECM to the cell cytoskeleton—had been discovered. Valerie Weaver, a postdoc in Bissell’s lab, showed that blocking integrins with an inhibitory antibody could revert the malignant phenotype of human breast cancer cells in 3-D culture. Then, in collaboration with Zena Werb of the University of California, San Francisco, the labs showed that proteins called matrix metalloproteinases (MMPs), when upregulated, promote tumor formation, providing evidence that the ECM can encourage malignant transformation and proliferation. Six years later, the two labs revealed that signaling from the MMPs resulted in genomic instability in cells that led to malignancy.

Still at it. Bissell’s lab is still buzzing with excitement, continuing to bolster the validity of her dynamic reciprocity model. “When I would give talks and say that laminin [a large extracellular protein that is a major component of the basement membrane] is as important as p53, people would laugh. We have been working on the story of what laminin does for the last eight years, and it is almost complete,” says Bissell. “It probably will be considered one of my most important studies.”

Paving a way. “I tell the people I train, don’t listen to what the literature says, do your experiments and understand why you found something different—as long as you can reproduce the data and the process. I tell them to trust themselves.” 

Greatest Hits

• Over a period of 40 years, was instrumental in developing the field of tumor microenvironments
• Developed the concepts that phenotype is dominant over genotype, that context matters, and that cellular and tissue architecture relays messages to cells
• Used a “steady-state machine” she helped develop to show that the level of sugar in culture media determined whether chicken cells remained normal or displayed malignant metabolic patterns
• In her model of dynamic reciprocity, proposed that the extracellular matrix directly signals to the nucleus and chromatin biochemically and mechanically to regulate gene expression  
• Developed three-dimensional culture techniques using basement membrane gels to study organ specificity in mammary organoids