The imaging FCS diffusion law in the presence of multiple diffusive modes

Area of Science:

• Cellular Biology
• Biophysics
• Membrane Dynamics

Background:

• The plasma membrane's organization and dynamics are crucial for cellular functions like sensing and metabolism.
• Heterogeneity within the plasma membrane, including lipid/protein domains and cytoskeleton meshworks, hinders biomolecule diffusion.
• These structures are often below the optical diffraction limit, making direct observation difficult, necessitating indirect dynamic measurements.

Purpose of the Study:

• To address the interpretation challenges of the y-intercept (τ₀) in the Fluorescence Correlation Spectroscopy (FCS) diffusion law for complex diffusion modes.
• To investigate the absolute value of τ₀ when molecules exhibit combined confined and hindered diffusion or when multiple molecules show different diffusion behaviors.
• To examine how τ₀ changes upon drug-induced disruption of membrane domains or cytoskeleton, altering probe diffusion modes.

Main Methods:

• Utilized the Fluorescence Correlation Spectroscopy (FCS) diffusion law, plotting diffusion time (τd) versus observation area.
• Employed a combination of experimental studies and computational simulations to analyze diffusion behaviors.
• Investigated scenarios with complex diffusion modes, including combined domain confinement and cytoskeleton hindrance, and dual-molecule diffusion.

Main Results:

• Demonstrated that τ₀ is not influenced equally by different diffusion modes in cellular environments.
• Showed that the relative change in τ₀, rather than its absolute value, provides significant information about the diffusion mode.
• Observed alterations in τ₀ upon drug treatments that disrupted membrane domains or cytoskeleton, correlating with changes in diffusion.

Conclusions:

• The interpretation of the FCS diffusion law's y-intercept (τ₀) requires careful consideration in complex diffusion scenarios.
• Relative changes in τ₀ offer a more robust indicator of diffusion modes and membrane structural influences than absolute τ₀ values.
• This study provides a refined understanding for interpreting membrane dynamics using FCS, particularly when dealing with heterogeneous environments and drug perturbations.