Monitoring of lipid storage in <i>Caenorhabditis elegans</i> using coherent anti-Stokes Raman scattering (CARS) microscopy

<jats:p> Better understanding of the fundamental mechanisms behind metabolic diseases requires methods to monitor lipid stores on single-cell level <jats:italic>in vivo</jats:italic> . We have used <jats:italic>Caenorhabditis elegans</jats:italic> as a model organism to demonstrate the limitations of fluorescence microscopy for imaging of lipids compared with coherent anti-Stokes Raman scattering (CARS) microscopy, the latter allowing chemically specific and label-free imaging in living organisms. CARS microscopy was used to quantitatively monitor the impact of genetic variations in metabolic pathways on lipid storage in 60 specimens of <jats:italic>C. elegans</jats:italic> . We found that the feeding-defective mutant <jats:italic>pha-3</jats:italic> contained a lipid volume fraction one-third of that found in control worms. In contrast, mutants ( <jats:italic>daf-2</jats:italic> , <jats:italic>daf-4</jats:italic> dauer) with deficiencies in the insulin and transforming growth factors (IGF and TGF-β) signaling pathways had lipid volume fractions that were 1.4 and 2 times larger than controls, respectively. This was observed as an accumulation of small-sized lipid droplets in the hypodermal cells, hosting as much as 40% of the total lipid volume in contrast to the 9% for the wild-type larvae. Spectral CARS microscopy measurements indicated that this is accompanied by a shift in the ordering of the lipids from gel to liquid phase. We conclude that the degree of hypodermal lipid storage and the lipid phase can be used as a marker of lipid metabolism shift. This study shows that CARS microscopy has the potential to become a sensitive and important tool for studies of lipid storage mechanisms, improving our understanding of phenomena underlying metabolic disorders. </jats:p>