THE FOUR PHASES OF RICHARD LOSICK'S MICROBIAL CAREER
June 16th, 2008
A lot has changed since microbiologist Richard Losick came to Harvard University as a Junior Fellow in 1968. He is now a renowned Harvard College Professor, and biology has become a much more interdisciplinary field, which affects both Losick's teaching and research. What has remained constant is Losick's quest for an ever deeper understanding of microorganisms, which he and others have shown are much more complex entities than anyone imagined four decades ago.
In Losick's hands, bacteria, particularly spore-forming bacteria, are microcosms of much larger events. “I wanted to know under what circumstance bacteria form spores, but I also saw spore formation as a simple model for development and how cells differentiate and go through morphogenesis,” he explains.
Usually when bacteria divide, they form two equal daughter cells, each with the same shape and behavior. In sporulation, though, a bacterium decides - often when enduring environmental stress - to divide into two unequal cells with diverging destinies. The “mother” cell engulfs the smaller cell, as if nurturing it in a womb. This tiny “prespore” later becomes a hard, durable spore while, duty done, the mother self-destructs. The spore can survive years of duress before, phoenix-like, it reproduces more bacteria to repopulate a more amenable environment.
For many years, Losick worked on two types of bacteria, Bacillus subtilis, a harmless relative of the bacterium that causes anthrax, and Streptomyces coelicolor, a shape-shifting bacterium that grows aerial hyhae on which spores form. Recently, though, he has concentrated on B. subtilis, which has accompanied him through four phases of research.
Phase 1: Which Genes Become Active, and When?
After receiving a bachelor's degree in chemistry from Princeton University, Losick earned a PhD in biochemistry from MIT before coming to Harvard and the precursor to the Molecular and Cellular Biology (MCB) Department. As a Junior Fellow, Losick never did a “proper” post-doc, but Jack Strominger gave him lab space and acted as his surrogate post-doctoral advisor. “I was going to work on membranes, but Jim Watson was down the hall creating lots of excitement about gene transcription,” he recalls. “I became interested in how different genes are turned on and off during sporulation.”
This work remained “crude” until technology for cloning genes became available in the late 1970s. Cloning changed Losick's life, he claims, because he could then isolate specific genes involved in sporulation and differentiation.
Scientists understood that the cell operates transcription machinery involving RNA polymerase, an enzyme that moves up and down the DNA to selectively activate genes. Regulatory proteins called sigma factors had just emerged as the part of that machinery that tells the enzyme which genes to activate. People thought that bacteria had just one sigma factor that expressed all the genes. But Losick and his wife, Jan Pero, who now runs a Cambridge biotech firm, showed that bacteria, and the viruses that live in them, encode alternate sigma factors that direct the RNA polymerase to express different categories of genes. This work, published in a 1975 Nature paper, was the first of several image changes Losick's work brought to bacteria.
Over the years, Losick has identified five alternate sigma factors dedicated to sporulation. He showed that when a cell makes a spore, the genetic programs of the mother and prespore cells are regulated by their own alternate sigma factors.
Phase 2: What Are Cells Signaling Each Other, and How?
During the 1980s Losick made the surprising discovery that the two adjacent cells talk to each other, and they are linked to each other by an intercellular communication called signal transduction.
In time, he worked out a “crisscross regulation” involving the five sigma factors. The original cell tells sigma factor F in the smaller cell to turn on certain genes that will send it down the path to becoming a spore. The small cell calls on sigma E in the large cell to activate genes that cause it to swallow the small cell. This swallowing activates sigma G in what is now the engulfed prespore, which in turn tells sigma K in the mother cell to switch on. The action of sigma K in the mother cell ultimately directs it to release the now mature spore.
In other words, the two cells, though different, are codependent. Each relies on the other for the signal to initiate its own genetic program. He summarized this crisscross regulation in a review article in Nature in 1992.
Phase 3: Where Do Proteins Go, and Why?
By that time, Losick had observed that, during sporulation, different proteins ended up at different locations in the cell. He could visualize these dynamic movements because his lab had developed techniques of introducing florescent labels that made proteins easy to see within the cell.
“Until then, the traditional view of bacteria was that they were amorphous vessels with enzymes floating around,” he recalls. “Several of us in the field realized that just as in higher cells, proteins have addresses and go different places, and where they are and the dynamics of their movements matter because they drive gene control, cell differentiation, and morphogenesis.”
At the time, not many researchers bought into this new image of bacterial cell biology, but it is now a large part of microbiology.
Phase 4: How Do Bacteria Build Communities?
Likewise, scientists now realize that bacteria in the wild do not look or behave the same as they do in “boring, smooth, flat colonies” seen in laboratories. Bacillus subtilis has inhabited such monotonous lab cultures since Ferdinand Cohn discovered the strain in the 1870s (during the same time that Robert Koch showed that germs directly cause disease using the related B. anthracis). But Cohn's drawings show that his original isolates of B. subtilis formed multicellular assemblies.
“We hadn't seen this communal aspect in the lab because we'd inadvertently domesticated the bacterium. It's fun to go back to the wild strains and discover the biology that got lost in the laboratory,” Losick comments, who is working with biofilm expert Roberto Kolter at Harvard Medical School. Wild strains of B. subtilis live in dynamic communities housing cells in different states that form complex architectures with aerial structures. Sporulation preferentially takes place at the tips of these structures. “So now we see spore formation not as an individualistic behavior, as we had traditionally viewed it, but rather as a community affair.”
Losick is finding that the same intimate crisscross regulation dedicated to making a spore also takes place within the biofilm community. He expects that learning more about this intercellular cross talk will yield insights relevant to higher development.
The Macro Community
Just as interrelationships are proving essential at the microbial level, they are also critical at the macro level of interdisciplinary research and teaching at MCB and beyond. About five years ago, Losick participated in a microbial collaborative that connects disciplines from biology, chemistry, and genetics to physics, geology, and planetary science. He also stands at the forefront of reforming undergraduate science education into a more interdisciplinary, hands-on affair. This dedication has twice earned him the prestigious appointment of Howard Hughes Medical Institute (HHMI) Professor.
With the first HHMI grant of $1 million in 2002, he established the experimental inquiry course MCB100, which is now a self-sustaining part of the curriculum and is being replicated in other courses and disciplines at Harvard; Biological Sciences 52, with interactive teaching and Web-based scientific animations; and the FEEDS (Freshman from Economically or Educationally Disadvantaged Backgrounds in Science) program. The renewed $500,000 grant in 2006 is mainly dedicated to enhancing FEEDS.
Among the many honors Losick has received, he is a member of the National Academy of Sciences (NAS); a fellow of the American Academy of Arts and Sciences, the American Academy of Microbiology, and the American Philosophical Society; and a former visiting scholar of the Phi Beta Kappa Society. In 2007, he received the NAS's Selman A. Waksman Award.
Still, Losick keeps undergraduate teaching “near and dear” to his heart, and thinks it has positively influenced his research. In teaching at an introductory level, he stays focused on the important big questions and broader themes in biology, in the same way that he finds the macrocosm of cell differentiation and morphogenesis in the miniature spore-forming system.
“I've been working on the same general problem since 1968, on the same floor in the same building in the same university,” Losick muses. “Some say I'm in the deepest rut ever,” but using the same biological system to answer new sets of problems involved in larger pathways is what attracted him to microbiology from the beginning.