Comment to the Paper of Michael J. Saxton: "A Biological Interpretation of Transient Anomalous Subdiffusion. I. Qualitative Model"

TL;DR

This paper critiques Saxton's interpretation of transient subdiffusion in cellular molecule tracking data, suggesting that the observed MSD behavior can be explained by confined diffusion with a superimposed slower diffusion, rather than true anomalous diffusion.

Contribution

It offers an alternative explanation for MSD patterns in single molecule tracking, emphasizing confined diffusion and superimposed diffusion over anomalous diffusion models.

Findings

MSD data can be fitted by confined diffusion with superimposed slower diffusion.

Transition in MSD plots may be misinterpreted as anomalous diffusion.

Confined diffusion models can explain transient subdiffusive regimes.

Abstract

In a recent paper, Michael J. Saxton proposes to interpret as anomalous diffusion the occurrence of apparent transient sub-diffusive regimes in mean-squared displacements (MSD) plots, calculated from experimental trajectories of molecules diffusing in living cells, acquired by Single Particle (or Molecule) Tracking techniques (SPT or SMT). In this Comment, without questioning the existence of sub-diffusive behaviors, which certainly play a key role in numbers of mechanisms in living systems, we point out that the data used by J.M. Saxton can as well be fitted by a simple law, resulting from confined diffusion at short times, with a slower free diffusion superimposed at larger times. When visualizing MSD plots, the transition from short-term diffusion confined in domains of size L, to slower, longer-term free diffusion, can be confused with anomalous diffusion over several orders of magnitude of time.