Investigation of Free
Fatty Acid Associated Recombinant Membrane Receptor Protein
Expression in HEK293 Cells Using Raman Spectroscopy, Calcium Imaging,
and Atomic Force Microscopy
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Abstract
G-protein-coupled receptor 120 (GPR120) is a previously
orphaned
G-protein-coupled receptor that apparently functions as a sensor for
dietary fat in the gustatory and digestive systems. In this study,
a cDNA sequence encoding a doxycycline (Dox)-inducible mature peptide
of GPR120 was inserted into an expression vector and transfected in
HEK293 cells. We measured Raman spectra of single HEK293 cells as
well as GPR120-expressing HEK293–GPR120 cells at a 48 h period
following the additions of Dox at several concentrations. We found
that the spectral intensity of HEK293–GPR120 cells is dependent
upon the dose of Dox, which correlates with the accumulation of GPR120
protein in the cells. However, the amount of the fatty acid activated
changes in intracellular calcium (Ca<sup>2+</sup>) as measured by
ratiometric calcium imaging was not correlated with Dox concentration.
Principal components analysis (PCA) of Raman spectra reveals that
the spectra from different treatments of HEK293–GPR120 cells
form distinct, completely separated clusters with the receiver operating
characteristic (ROC) area of 1, while those spectra for the HEK293
cells form small overlap clusters with the ROC area of 0.836. It was
also found that expression of GPR120 altered the physiochemical and
biomechanical properties of the parental cell membrane surface, which
was quantitated by atomic force microscopy (AFM). These findings demonstrate
that the combination of Raman spectroscopy, calcium imaging, and AFM
may provide new tools in noninvasive and quantitative monitoring of
membrane receptor expression induced alterations in the biophysical
and signaling properties of single living cells