Level crossing statistics in a biologically motivated model of a long dynamic protrusion: passage times, random and extreme excursions

TL;DR

This paper presents a stochastic model of flagellum length fluctuations, deriving analytical expressions for passage times and extreme lengths, and demonstrates how mutations affect these statistics independently of steady-state length.

Contribution

It introduces a novel analytical framework for level-crossing statistics in a biologically motivated flagellum model, linking mutation effects to length fluctuation dynamics.

Findings

Derived analytical expressions for passage and sojourn times.

Identified parameter regimes mimicking wildtype and mutants.

Showed mutation effects on fluctuation statistics independent of steady-state length.

Abstract

Long cell protrusions, which are effectively one-dimensional, are highly dynamic subcellular structures. Length of many such protrusions keep fluctuating about the mean value even in the the steady state. We develop here a stochastic model motivated by length fluctuations of a type of appendage of an eukaryotic cell called flagellum (also called cilium). Exploiting the techniques developed for the calculation of level-crossing statistics of random excursions of stochastic process, we have derived analytical expressions of passage times for hitting various thresholds, sojourn times of random excursions beyond the threshold and the extreme lengths attained during the lifetime of these model flagella. We identify different parameter regimes of this model flagellum that mimic those of the wildtype and mutants of a well known flagellated cell. By analysing our model in these different parameter regimes, we demonstrate how mutation can alter the level-crossing statistics even when the steady state length remains unaffected by the same mutation. Comparison of the theoretically predicted level crossing statistics, in addition to mean and variance of the length, in the steady state with the corresponding experimental data can be used in near future as stringent tests for the validity of the models of flagellar length control. The experimental data required for this purpose, though never reported till now, can be collected, in principle, using a method developed very recently for flagellar length fluctuations.