Direct detection of cell membrane slope fluctuations upon adding Latrunculin B using optical tweezers and single probe particle

The cell membrane has fluctuations due to thermal and athermal sources. That causes the membrane to flicker. Conventionally, only the normal (perpendicular to the membrane) fluctuations are studied and then used to ascertain the membrane properties like the bending rigidity. It is here that we introduce a different concept, namely the slope fluctuations of the cell membrane which can be modelled as a gradient of the normal fluctuations. This can be studied using a new technique where a birefringent particle placed on the membrane turns in the out of plane sense, called the pitch sense. We introduce the pitch detection technique in optical tweezers relying upon asymmetric scattering from a birefringent particle under crossed polarizers. We then go on to use this pitch detection technique to ascertain the power spectral density of membrane slope fluctuations and find it to be (frequency)-1 while the normal fluctuations yields (frequency)-5/3. We also explore a different regime where the cell is applied with the drug Latrunculin-B which inhibits actin polymerization and find the effect on membrane fluctuations. We find that even as the normal fluctuations now become (frequency)-4/3, the slope fluctuations spectrum still remains (frequency)-1, with exactly the same coefficient as the case when the drug was not applied. Thus, this presents a convenient opportunity to study the membrane parameters like bending rigidity as a function of time after applying the drug. This would be the first time the membrane bending rigidity could be studied as a function of time upon the application of Lat-B without reverting to AFM.