Raman-Based Approach Images Biological Processes in Action

A fast, novel imaging technique could improve molecular-scale understanding of the biological processes involved in the early stages of disease. The technique uses polarization-resolved coherent Raman scattering (polar-CRS) to reveal a sample’s chemical makeup as well as the orientation of molecules that comprise it. The technique takes only seconds, making it possible to view the progression of disease in a living animal model at the molecular level. With further development, this technique could be used to detect early signs of neurodegenerative diseases in people.

Researchers from Institut Fresnel, CNRS, Aix Marseille Université demonstrated fast-polar-CRS imaging based on combined electro-optic polarization and acousto-optic amplitude modulations, applicable to both stimulated Raman scattering and coherent anti-Stokes Raman scattering imaging.

A new technique that provides both chemical composition and molecular orientation information at subsecond timescales could reveal new information about what occurs on the molecular level as diseases such as Alzheimer's and multiple sclerosis progress. Courtesy of Sophie Brasselet, Institut Fresnel, CNRS, Aix Marseille Université.

To obtain molecular orientation information from the coherent Raman signal, the researchers used a Pockels cell to quickly modulate the laser’s polarization.

“We took the concept of intensity modulation used for stimulated Raman scattering and transposed it to polarization modulation using an off-the-shelf device,” said researcher Sophie Brasselet. “The signal detection for our technique is very similar to what is done with stimulated Raman scattering, except that instead of detecting only the intensity of the light, we detect polarization information that tells us if molecules are highly oriented or totally disorganized.”

By modulating the laser polarization very fast, the researchers could take measurements pixel by pixel, in real time. This enabled them to acquire orientation information fast enough to capture highly dynamic biological processes on a molecular level.

The researchers demonstrated subsecond timescale imaging of lipid order packing and local lipid membrane deformations in artificial lipid multilayers and in red blood cell ghosts, illustrating the technique’s high degree of sensitivity down to a single lipid bilayer membrane.

The packed layers of lipids that were used in the study are similar to those found in the myelin sheath that covers axons to help electrical impulses move quickly and efficiently. As diseases such as Alzheimer's and multiple sclerosis progress, these lipids start to disorganize and the lipid layers lose their adhesion. This ultimately causes the myelin sheath to detach from the axon and leads to malfunctioning neural signals.

“We designed a technique able to image molecular organization in cells and tissues that can ultimately let us see the early stage of this detachment and how lipids are organized within this myelin sheath,” said Brasselet. “This could help us understand the progression of diseases by identifying the stage at which lipids start disorganizing, for example, and what molecular changes are occurring during this time. This could allow new targeted drug treatments that work differently than those used now.

“Even though we only demonstrated the technique with model membranes and single cells, this technique is translatable to biological tissue,” said Brasselet. “It will show us how molecules behave — information that is not available from the micron-scale morphological images taken with traditional microscopy techniques.”

Brasselet said that the technique could be used in the near future to better understand progression in diseases that involve a breakdown of the myelin sheath. For example, it could be used to image neurons in living mice by combining the Raman scattering technique with existing methods in which tiny windows are implanted in the brains and spinal cords of laboratory animals.

“Ultimately, we would like to develop coherent Raman imaging so that it could be used in the body to detect diseases in their early stages,” said Brasselet. “To do this, the technique would have to be adapted to work with endoscopes or other tools in development that allow light-based imaging inside the body.”

The research was published in Optica, a publication of The Optical Society (doi: 10.1364/OPTICA.4.000795).