Microscopy, holography team up to reveal neuronal activity

Ashley N. Paddock, ashley.paddock@photonics.com

Digital holographic microscopy (DHM) allows observation of neuronal activity in real time and in three dimensions – with up to 50 times greater resolution than ever before – showing promise in testing new drugs to fight neurodegenerative diseases such as Parkinson’s and Alzheimer’s.

DHM is a noninvasive approach that enables extended observation of neural processes without electrodes or dyes that damage cells. The method yields important information about the shape, dynamics and activity of neurons, and creates 3-D navigable images down to 10-nm precision, according to senior team member Pierre Marquet of École Polytechnique Fédérale de Lausanne (EPFL).

The method consists of pointing a single-wavelength laser beam at neurons, collecting the distorted wave on the other side and comparing it to a reference beam. A computer then numerically reconstructs a 3-D image of the neurons using an algorithm developed by the researchers. The laser beam travels through the transparent cells to obtain important information about their internal composition.

A 3-D representation of a live mouse cortical neuron in culture. Each pixel represents a color-coded quantitative measurement of the phase shift induced by the neuron on the transmitted wavefront. Courtesy of Pierre Marquet, EPFL.

Although DHM normally is used to detect minute defects in materials, the team decided to try using it for neurobiological applications. In the study, the group induced an electric charge in a culture of neurons using glutamate, the main neurotransmitter in the brain. This charge transfer carries water inside the neurons and changes their optical properties in a way that can be detected only by DHM. Thus, the technique accurately visualized the electrical activities of hundreds of neurons simultaneously, in real time, without harming them with electrodes, which can record activity from only a few neurons at a time. The group’s findings appeared in the Aug. 17 issue of The Journal of Neuroscience (doi: 10.1523/jneurosci.0286-11.2011).

“This work has permitted us to obtain an intrinsic optical signature of the activity of many neurons in culture simultaneously, in real time and in a noninvasive manner,” Marquet said. “Practically, the high phase measurement accuracy allows us to rapidly observe the effects of various pharmacological substances mediated by the activation of specific membrane receptors/transponders.

“Within this framework, DHM represents a very promising approach to develop a unique, label-free, high-content screening technique which is highly relevant for the discovery of new drugs.”

This advance in high-content screening has important ramifications for the discovery of new drugs that combat or prevent neurodegenerative diseases, since new molecules can be tested more quickly and in greater numbers. The researchers have found that lab work that previously took 12 hours can now be done in 15 to 30 minutes.

The success of DHM for high-content screening has resulted in a startup company called Lyncée Tec SA.