Since 1999, Yoshiyuki Takizawa has been working on the Extreme Universe Space Observatory (EUSO), an international project to develop a super wide-field telescope capable of observing large volumes of Earth’satmosphere in order to detect the arrival of high-energy cosmic particles. Along with other international astrophysicists, Takizawa has been developing a photon detector that will be a critical part of the new 2.5-meter EUSO telescope onboard the Kibo Japanese experiment module of the International Space Station. The module is to be launched by 2015. The detector will consist of six thousand 1-inch-square photomultiplier tubes, and will allow an area of about 400 kilometers in diameter of Earth’s atmosphere to be imaged in each shot.
The technologies for photon detection and the associated readout algorithm are so innovative that Takizawa, a research scientist at the Computational Astrophysics Laboratory of the RIKEN Advanced Science Institute (ASI) in Wako, decided to apply his expertise to observing a completely opposite object: molecules of nanometer size.
From space to single molecules
The original idea came from Takizawa’s supervisor, Toshikazu Ebisuzaki. Having conversations with various researchers, Ebisuzaki learned that biologists are frustrated with their inability to observe many important biological phenomena.
In collaboration with three biologists at RIKEN’s Wako campus—Yasushi Sako, Kiminori Ushida and Etsuko Muto—Takizawa obtained a research grant from the RIKEN Strategic Research Programs for Research and Development, or the ‘President’s Fund’ as it is known. The two-year interdisciplinary project, commenced in October 2008, aims to develop an ultra-sensitive camera based on Takizawa’s detector. “Unlike space science, a research target is on hand in biology. It is appealing to share the joy with biologists to uncover new mechanisms of living things,” Takizawa says.
The project comprises a dozen researchers from four laboratories. The new camera will be able to image molecules of 1–100 nanometers in size in just one microsecond, making it 10–100 times faster than equivalent detectors. Although there are existing technologies for looking at single molecules, none has so far delivered these capabilities.
The three biologists collaborating with Takizawa study different subjects but share a fundamental objective to observe molecular behaviors with microsecond resolution. “We have individually chosen research topics best suited to test the detector’s performance,” says Ushida, a senior scientist at the Supermolecular Science Laboratory of the RIKEN ASI.
Three years ago, Ushida developed a novel method of fluorescence correlation spectroscopy to directly observe anomalous diffusion in hyaluronan solution, which plays an important role in controlling the transport of molecules in many biological media such as extracellular matrices. In the past, such phenomena were difficult to detect, forcing researchers to depend on hypotheses based on the simple rule that diffusion coefficients are constant. “I’d like to handle the diffusion of a single molecule in a precise manner using the best optical spectroscope, which will be possible with the new detector,” says Ushida.
Sako, a chief scientist at the Cellular Informatics Laboratory of the RIKEN ASI, is using single-molecule fluorescence microscopy to observe the orientation of membrane proteins, such as hormone receptors, through the movements of molecules and the changes of their structures and shapes. Researchers have many assumptions for the binding process between a hormone receptor and a specific hormone in membranes, but no one has directly seen such a reaction, Sako says.
Muto, team leader of the Laboratory of Molecular Biophysics at the RIKEN Brain Science Institute, and her colleagues are looking at the relationship between motor proteins and microtubules using dark-field microscopy. Microtubules exist within neurons and play an important role in the transport of intracellular substances. Malfunction of the transportation system is known to trigger certain diseases. “Higher temporal resolution could enable us to discover new types of fluctuations at smaller timescales,” says Itsushi Minoura, a postdoctoral fellow in Muto’s laboratory.
A prototype is now being used with a microscope in Sako’s laboratory (Fig.2). When they want to test the device, Kenji Okamoto, a researcher in Sako’s lab, sets up the device and gives meticulous instructions on its use. Another researcher in Sako’s lab, Kayo Hibino, is busy making novel fluorescent probes and particle probes that she will use to label sample cells so researchers can maximize the quality of images.
The participants say the project wouldn’t have started without the ‘ubiquitous circuit board’ developed by Yasushi Watanabe, a research scientist in the Radiation Laboratory of the RIKEN Nishina Center for Accelerator-Based Science. The board is mounted with a DAP/DNA chip that has the special ability to reconfigure its signal processing logic circuit in one clock cycle—just six nanoseconds. Equivalent system chips, such as field-programmable gate arrays, require up to a few seconds to rewrite configuration programs. Watanabe’s circuit board is innovative in that it is capable of storing numerous programs and reading out optimal programs according to the type of signal detected. The device, also developed with the support of the President’s Fund, was completed in 2007.
Takizawa and his colleague Yoshiya Kawasaki are developing a key technology: a G-APD photon detection system (Fig. 3). The system consists of a Geiger-mode avalanche photodiode (G-APD), an application-specific integrated circuit (ASIC) and Watanabe’s ubiquitous circuit board. The ASIC is another type of system chip that was originally developed jointly by the Ebisuzaki laboratory and the Institute of Space and Astronautical Science (ISAS) of the Japan Aerospace Exploration Agency. Takizawa and Kawasaki have upgraded the chip design with ISAS collaborators and adjusted it for use in the current project. They are now pursuing “the most delicate part of the detector system development,” as Takizawa says, to connect their new circuit to the ubiquitous circuit board in order to relay ultra-fast signals without electrical noise.
The first prototype will likely be completed by the end of 2009, but the participants won’t be satisfied to leave the development there. “The most important thing in this project is not simply to make the prototype camera work, but what we will do next,” says Sako. Takizawa adds, “We’d like to keep improving our technologies so as many people as possible can benefit from our work.”