Throwing new light on cellular networks

Biologists have made remarkable progress in understanding life at the cellular and molecular levels over the past 20 years, but they have been hobbled in moving forward by their inability to easily study how the multitude of cells in tissues and organs form complex signaling networks, as happens in the nervous and immune systems. To date, experiments aimed at exploring these cellular networks have mostly relied on limiting, invasive techniques, such as implanting electrodes in just a few cells in animals’ brains or observing how collections of isolated immune-system cells behave in a dish.

“It is clear that the next big frontier is to understand not only the individual players in the orchestra,” says Gero Miesenböck, M.D., associate professor of cell biology, “but the orchestra itself—that is, how multicellular life functions.”

Now, with a 3-year, $2 million grant from the W.M. Keck Foundation in hand, Miesenböck and Ira Mellman, Ph.D., chair and Sterling Professor of Cell Biology, are collaborating on a research program that will study cellular networks in the nervous and immune systems in a wholly new way. The research will extend groundbreaking techniques pioneered by Miesenböck that combine tools to precisely conduct the orchestras of cellular networks with the ability to record the physiological “music” these networks produce.

Over the past several years, Miesenböck has devised ways to genetically engineer specific sets of cells so that “remote controls”—flashes of light, zaps of heat, or floods of molecules—will simultaneously activate them. To determine the downstream effects of such activation on other cells in a network, he has invented molecular sensors that can be inserted into cells and emit light when those cells are activated.

In one remote-control experiment so widely reported that Jay Leno incorporated it into two comedy routines on The Tonight Show, Miesenböck introduced “optical actuators,” proteins engineered to be sensitive to light, into nerve cells that govern an escape behavior in the fruit fly Drosophila. A flash of bright light was sufficient to activate the cell network, which caused the flies to execute their typical escape response.

When Miesenböck couples this actuator technique with his light-emitting sensors, the propagation of light-activated nerve impulses throughout a cell network can be observed over time, as downstream cells erupt in blooms of green fluorescence. “We can essentially watch information processing,” says Miesenböck.

The nervous and immune systems are quite different in organization and function. In the brain, information is conducted among nerve-cell networks that are hard-wired in place, whereas immune-system networks rely on the physical movement of various cells to specialized information transfer sites, such as lymph nodes. But Mellman and Miesenböck say that the two systems share the fundamental property of “coordinated reaction of diverse cell types to seemingly limitless numbers of inputs”—pathogens in the case of the immune system and sensory information in the case of the nervous system.

These inputs, whether a complex visual scene or a bacterial infection, broadly activate cellular networks in our bodies to prepare us for an appropriate behavioral or immune response. In the new Keck-funded research, a beam of light (or a temperature change or some other noninvasive stimulus) will be the carefully controlled stand-in for such real-world events. Miesenböck will investigate neuronal circuits to gain insight into how the brain controls behavior, while Mellman, also professor of immunobiology, will explore how immune-system networks distinguish the body’s own cells from foreign invaders such as bacteria and viruses.

The researchers will genetically insert suites of actuators and sensors into specific classes of neurons or immune cells in mice, and they will then excise tissues containing many thousands of these cells—a chunk of the cerebral cortex, say, or a whole lymph node—for experiments in which light or another stimulus will simultaneously activate many inputs to a cellular network.

The network’s coordinated response to the stimulus, reflected in illumination patterns made by the downstream sensors, will be recorded in real time by custom-designed microscopes. Finally, this rich pool of stimulus and response data will be analyzed and interpreted using powerful computers and specialized software.

Mellman says that Miesenböck’s technology holds the potential to forever change the way that scientists study cells as systems.

“The primary goal,” Mellman explains, “is to establish the scientific and intellectual base of an essential new direction for cell biology in particular and biomedical science in general: learning how to apply reductionist approaches to understand the behavior of cells in complex tissue contexts. Unless we can do this, we will never really understand biology or have the tools to uncover the basis for complex diseases.”

Mellman, who has received numerous accolades and honors over more than two decades of research, says his new partnership with Miesenböck is a high point. “This is possibly the first time in a very long scientific career when I feel as if I am involved in something truly profound,” he says. “It is remarkably exciting.”