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One more step in nanoscopy

Lattice light sheet microscope

Every biologist dreams to dive into the heart of cells to observe in detail the mechanisms that take place inside them, and to do so by disturbing as little as possible. By combining their expertise, three groups of researchers including one of the Institut Curie have just created a kind of “nanocage” to enlighten the world of the infinitely small.

Specialists call it the QD. These are the Quantum Dots, nanostructures which because of their size of a few nanometers – a few billionths of a meter – have specific properties. One of them is the fluorescence (1). “But until now marking QD limited their use. Adding multiple ligands per QD in order to allow them to bind to the desired location entailed changes in the properties of these ligands and therefore biases”, says Ludger Johannes, head of the Endocytic Trafficking and Intracellular Delivery (CNRS/Inserm/Institut Curie). A major restriction and difficulty to overcome since it took the skills of two other research teams – Team Yamuna Krishnan (formerly the National Centre for Biological Sciences (NCBS) in Bangalore, India, now at the University of Chicago the USA) and Benedict Dubertret (ESPCI, Paris), in addition to that of the Institut Curie, to construct a new class of cellular imaging agents which consists of:

  • A biomolecular label or ligand, of which we want to study the intracellular properties;
  • a DNA polyhedron, a 3D structure that encapsulates the QD, and on which by nanometer design is grafted one copy of the ligand mentioned above;
  • a QD in charge of emitting light and thus of locating the complex.

In reality, the driving force of the development of these compounds is the postdoc Dhiraj Bhatia. He began this work during his thesis in the team of Yamuna Krishnan at the National Center for Biological Sciences and then continued in the team of Ludger Johannes, Inserm research director at the Institut Curie. “The major advance of these new nanostructures lies in setting a single ligand to the DNA hull, which circumvents the limitations observed with previous systems,” explains the young researcher.

Now the researchers plan to use these nanostructures to purify molecules within cells, simply by replacing the QD by a ferromagnetic element. “The hope is to enter the era of nanobiochemistry and access trace amounts of product”, explains Ludger Johannes, who develops this project with Prof. Satyajit Mayor of the NCBS in Bangalore.

The Institut Curie, always at the cutting edge of cell imaging

The other outcome of these nanostructures obviously concerns imaging. By fixing these agents to cellular components, it is possible to follow their dynamics over long time frames. “But the phototoxicity of current microscopes causes a fairly rapid destruction of the imaged cells, continues the researcher. This limit will become ancient history thanks to the new microscope, unique in Europe, installed in early September at the institute. The “lattice light sheet,” this is its name, makes it possible to illuminate extremely thin layers of the cell.

“Thanks to this combination, unique in the world to this day, the light sheet and our nanostrcutures, we will tackle new challenges in fundamental cell biology and biomedical research on cancer” concludes Ludger Johannes.

(1) Like other molecules, QD emit a wavelength of light in specific response to excitation by a different wavelength, but the light they emit depends on their size. Biologists therefore use in recent years this specificity to observe the cells.

Quantum dot-loaded monofunctionalized DNA Icosahedra for single particle tracking of endocytic pathways
Nature Nanotechnology (2016)

Dhiraj Bhatia, Senthil Arumugam , Michel Nasilowski, Himanshu Joshi, Christian Wunder, Valerie Chambon, Ved Prakash, Chloé Grazon, Brice Nadal, Prabal K. Maiti, Ludger Johannes, Benoit Dubertret, Yamuna Krishnan