Unpacking the mechanical correlates of Brillouin microscopy in living cells

 In All Presenters, Overby Darryl R

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 [1]. 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 [2]. 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 [3].

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 [4]. The relationship between  and  followed the biphasic model for  proposed by Wu et al. [5]. 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: [1] Scarcelli et al., Nat. Methods 12:1132, 2015. [2] Green & Meuth Cell 3:127, 1974. [3] Antonacci et al., J Roy Soc Interface 12:20150843, 2015. [4] Guo et al. Proc Natl Acad Sci USA 114:E8618, 2017. [5] Wu et al. Nat Methods 15, 561, 2018.

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