Light-based measurement of cell elasticity

A label-free optical microscopy technique based on Brillouin light scattering that is capable of measuring intrinsic longitudinal modulus at sub-cellular resolution has been developed. Cell mechanical properties are involved in many cell functions such as migration and differentiation, and can influence system level behavior such as tissue morphogenesis. However, current measurements of the biomechanical properties of cells generally require physical contact and thus are limited to cells cultured on 2D flat substrates. We obtained elasticity maps of cells in 2D and 3D microenvironments, which capture a wide variety of phenomena involving cellular mechanical changes through cytoskeletal modulation and cell volume regulation.

HFSP Young Investigator Grant holder Giuliano Scarcelli and colleagues
authored on Thu, 22 October 2015

The cellular microenvironment critically regulates cellular function by providing a complex mixture of biochemical and biophysical stimuli. Among the components of the cell-microenvironment interaction, the role of biomechanical factors is recognized to be crucial. In recent years, tremendous effort has been placed on developing tools for mechanical stimulation and mechanical probing of single cells.  One area which required improvement is the non-invasive measurement of intracellular elasticity. Current technology for cell and extracellular matrix elasticity measurements is limited to point-sample analysis or requires contact. These are important limitations since cells are heterogeneous, alter their properties upon mechanical perturbation, and need to be studied in 3D microenvironments.

To overcome these limitations, we developed Brillouin cell microscopy, an all-optical technique to measure intracellular and extracellular mechanical properties without contact at high 3D resolution. Brillouin scattering is the result of the interaction of light with spontaneous acoustic phonons within material. Thus, by measuring the frequency shift of the scattered light, Brillouin microscopy probes the local pressure waves due to thermodynamic fluctuations within the intracellular environments. From this information, the longitudinal modulus of material in the gigahertz frequency range can be determined at high spatial resolution. In this study, we have developed and validated Brillouin microscopy against the gold-standard atomic force microscopy. Moreover, we have verified that Brillouin microscopy is sensitive to both changes in liquid-solid volume fraction of the cytoplasm and to the important cytoskeletal modifications, such as actin polymerization and branching, that regulate cell mechanical properties.

The development of a Brilluoin microscope is one component of a broader project funded by HFSP in 2013 that integrates expertise in optics, biophysics and neurobiology to investigate the role of mechanics in CNS development.  The development of the central nervous system (CNS) is one of the most spectacular processes in biology. Key aspects include the formation of neuronal axons, their subsequent growth and guidance through thick layers of nerve tissue, and the folding of the brain. All these processes involve motion and are driven by forces. However, while our understanding of the biochemical and molecular control of these processes is increasing rapidly, the contribution of the dynamic interplay between cellular forces and tissue elasticity remains poorly understood – mostly due to a lack of suitable measurement techniques.

To address this need in the interdisciplinary project, a novel photonic toolbox is developed for in situ, label-free and non-contact measurements of cellular forces and elasticity.  Force sensing (developed in the lab of Dr. Malte Gather at the University of St. Andrews, UK) is based on spatially mapping nanoscale deformations of an ultra-flexible planar optical microcavity in response to local stress. Elasticity measurements (developed in the lab of Dr. Scarcelli at the University of Maryland, USA) is based on the high-resolution Brillouin microscopy described here. Force and elasticity measurements are combined to illuminate how forces exerted by neurons contribute to axon formation and neuronal guidance (led by the lab of Dr. Kristian Franze at the University of Cambridge UK).

Reference

Noncontact three-dimensional mapping of intracellular hydromechanical properties by Brillouin microscopy. G Scarcelli, WJ Polacheck, HT Nia, K Patel, AJ Grodzinsky, RD Kamm, SH Yun. Nature Methods (2015) doi:10.1038/nmeth.3616.

Link to Nature Methods article

Pubmed link