Comparison of thermal and athermal dynamics of the cell membrane slope fluctuations in the presence and absence of Latrunculin-B.

Conventionally, only the normal cell membrane fluctuations have been studied and used to ascertain membrane properties like the bending rigidity. A new concept, the membrane local slope fluctuations was introduced recently[Vaippully, et al. Soft Matter 16, 7606 (2020)], which can be modelled as a gradient of the normal fluctuations. It has been found that the Power Spectral Density (PSD) of slope fluctuations behave as (frequency)$^{-1}$ while the normal fluctuations yields (frequency)$^{-5/3}$ even on the apical cell membrane in the high frequency region. In this manuscript, we explore a different situation 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 show a power law (frequency)$^{-5/3}$ as is the case for a free membrane, the slope fluctuations PSD remains (frequency)$^{-1}$, with exactly the same coefficient as the case when the drug was not applied. Moreover, while sometimes, when the normal fluctuations at high frequency yield a power law of (frequency)$^{-4/3}$, the pitch PSD still yields (frequency)$^{-1}$. Thus, this presents a convenient opportunity to study membrane parameters like bending rigidity as a function of time after application of the drug, while the membrane softens. We also investigate the active athermal fluctuations of the membrane appearing in the PSD at low frequencies and find active timescales of slower than 1 sec.