Volume 3, Issue 24 December 2006/January 2007
The 3D Language of Cells
Jay Groves is revealing how the information embedded in cell membrane patterns and other complex molecular environments affect cell behavior and organization. (courtesy A. Demond)
Beneath your skin, out of your sight, your cells are locked in heated conversation. Like teens glued to a telephone, they have much to say to one another. Who are you? What have you sensed? What do you need? Do you belong?
Though blind, deaf, and mute, cells face no communication quandary. They send out signaling molecules, tiny versions of a castaway's message in a bottle. They stud themselves with receptors, to snag other signaling molecules floating by. Each also plasters itself with molecular identity tags proclaiming citizenship in the body.
Now Jay Groves, a UC Berkeley professor of chemistry, is deciphering yet another mode of cellular discourse: spatial patterning. An expert in molecular self-organization, Groves' current research focuses on immune system components called T cells. Like customs agents, T cells use receptors on their surfaces to examine the identification molecules of every cell they encounter. When a T cell encounters another cell carrying foreign ID, two of its membrane proteins bond to other proteins on the alien cell. Then an astonishing thing occurs: those bonded proteins arrange themselves into a distinctive bull's-eye pattern. The T cell then releases a flood of intracellular calcium to sound the invasion alarm.
Groves has invented a half-live, half silicon cell junction by floating cell membrane molecules atop a silicon chip. Tiny walls (black) built atop the chip (white) restrict the movement of cell membrane molecules (red and yellow) floating in a sea of lipid molecules (blue). (Dr. Raghu Parthasarathy, former Groves postdoc)
Groves suspected the pattern conveyed additional information to the cell. But to find out for sure, he had to manipulate the physical locations of the membrane proteins themselves. "The goal was to see how forcing these clusters into different patterns might change how the T cells react," Groves says.
So Groves built a totally new experimental platform—a half-living, half artificial cell. He patterned bars of chromium in various patterns atop silicon wafers using the same techniques used to make computer chips. Fusing lipid molecules and cell membrane proteins to the surface of the chip results in what Groves calls a hybrid live cell-supported membrane junction.
T cells placed on top of the hybrid membrane junction can't tell the difference between the hybrid cell and a natural cell. "The T cell never sees the metal or the semiconductor. The only thing it interacts with is the protein," Groves says. "But with these structures we've snuck in underneath, we control the way the proteins move."
The patterned chips radically altered the T cells' behavior. Instead of forming one large, central bull's-eye pattern, the proteins clustered into many miniature bull's-eyes, each constrained within its own chromium-walled room. "It's like putting little barricades in there; the proteins can't get over them, so they stop the receptor clusters from moving around. The cell tries to drag them around, but they get stuck or redirected."
T cells arrange their receptors into a distinctive bull's-eye pattern (A) when they bind to foreign cells. Groves uses patterned silicon chips to disrupt this pattern (B-D) and determine its importance in cell signaling. (courtesy Jay Groves)
Disrupting the pattern clearly impacted the T cells' behavior. Those unable to pull their receptors into a central area released a much stronger alarm signal than normal. "The cell is using the bull's-eye pattern to control its sensitivity. It's moving its receptors out of a region of very high sensitivity to turn down its signal," Groves says.
This mechanism helps the body modulate its immune response. "If you have an extensive viral infection, the T cell in that case will initiate a response immediately because it will detect a large number of viral protein antigens. If it sees a smaller signal, it doesn't react that way, because you don't want to initiate a huge immune response to a small thing. That would take a huge amount of energy and you'd constantly feel sick."
Now Groves has developed a means to alter the reactive properties of molecules anywhere in a cell. The method takes advantage of the special properties of a compound known as NVOC. NVOC readily attaches to other molecules with a bond that dissolves in 320 nm UV light.
In the November issue of the Journal of the American Chemical Society, Groves reports using NVOC to cage the identity tag of a particular foreign protein. He then added the protein to one of his hybrid cell membrane junctions. T cells crawling atop the membrane never paused. But when Groves hit the membrane with UV light, the T cells stopped to form bull's-eye patterns. The beam of light had successfully uncaged the foreign protein sequences, allowing them to be recognized by the T cells.
Groves is now moving beyond manipulating surface proteins. "We can pick a spot inside of a cell, and uncage a protein or drug molecule only at that point," Groves says. By focusing an intense beam of longer wavelength light in a particular spot, he can uncage only proteins at the focal point, leaving all other cell molecules unaffected.
Groves' goal is to understand the forces of molecular organization that bring a cell to life. "It's like examining the sound wave of a symphony. It's fantastically complicated, but if you look at a larger scale, there are some patterns—a periodic beat there, and repeated phrases. And you realize there might be levels higher up in the complexity that might be simple, that organize the entire piece. There's something big out there that's regulating the chemistry of life, and that's where we're headed."