Unpacking the mechanical correlates of Brillouin microscopy in living cells
PRESENTING AUTHOR’S FULL NAME: Darryl R Overby
INSTITUTION: Imperial College London, Department of Bioengineering
ADDITIONAL AUTHORS NAMES, AS TO BE PUBLISHED: Pei-Jung Wu, Carl Paterson, Peter Török
BACKGROUND/PURPOSE: Brillouin microscopy provides a non-invasive optical method to map the mechanical properties of cells and tissues at confocal resolution. The technique relies on measuring a frequency shift in scattered light that is proportional to the speed of acoustic waves within a material. Acoustic wave speed is related to the longitudinal elastic modulus , which is a measure of compressibility. However, recent studies have shown that in cells correlates with Young’s modulus , which is a measure of deformability . In soft tissue, is many orders of magnitude larger than and it is not obvious how changes in of the order of kPa could result in changes in equivalent to changes of the order of GPa in . The purpose of this study was to determine why and appear correlated in cells, and to determine whether this correlation is related to water content that may affect both and .
METHOD: Experiments were performed using 3T3-L1 preadipocytes or adipocytes that were differentiated according to established protocols . Water content was varied by osmotic perturbation (200-800 mOsm). and were measured by atomic force microscopy (JPK). was calculated as , where is the smallest achievable cell volume under hyperosmotic conditions. was measured using a custom-built Brillouin confocal microscope .
RESULTS: Increasing osmolarity caused a decrease in and an increase in . Increasing osmolarity also increased in the cytoplasm, but was unchanged in lipid droplets. In preadipocytes, the relationship between and followed the ideal gas model proposed by Guo et al . The relationship between and followed the biphasic model for proposed by Wu et al. . Together, these two models were able to fully explain the observed relationship between and over a wide range of (15 – 78%; Fig. 1).
CONCLUSION: Brillouin microscopy does not directly measure in cells. Instead, Brillouin microscopy is sensitive to , which tends to correlate with due to a mutual dependence of and on .
SUPPORT: NIH grants EY022359 and EY019696.
Figure 1: Brillouin microscopy of a 3T3-L1 adipocyte in cell culture. (a) Brightfield image revealing two lipid droplets (LDs), surrounded by a thin cytoplasm (C) and nucleus (N). (b) A confocal image of the Brillouin frequency shift of the same cell in (a). (c) Changing osmolarity increases both and in preadipocytes, but the relationship between and can be explained by existing models (red) [4,5].
REFERENCES:  Scarcelli et al., Nat. Methods 12:1132, 2015.  Green & Meuth Cell 3:127, 1974.  Antonacci et al., J Roy Soc Interface 12:20150843, 2015.  Guo et al. Proc Natl Acad Sci USA 114:E8618, 2017.  Wu et al. Nat Methods 15, 561, 2018.