Technical Advance: Introducing a novel metric, directionality time, to quantify human neutrophil chemotaxis as a function of matrix composition and stiffness

Abstract

A direct consequence of cellular movement and navigation, migration incorporates elements of speed, direction, and persistence of motion. Current techniques to parameterize the trajectory of a chemotaxing cell most commonly pair migration speed with some measure of persistence by calculating MSD, RMS speed, TAD, and/or CI. We address inherent limitations in TAD and CI for comparative analysis by introducing two new analytical tools to quantify persistence: directionality index and directionality time. With the use of these tools, we show that the mechanical properties of the underlying substrate contribute significantly to the regulation of human neutrophil chemotaxis toward fMLP on Fgn-, Col-, and Fn-coated gels of varying elasticity. The β₁-integrin ligand Col demonstrated mechanosensitive speed. In contrast, β₂-integrin ligand Fgn supported mechanosensitive persistence. Fn, recognized by β₁ and β₂ integrins, mechanoregulated speed and persistence. Blocking β₂ integrins of cells migrating on Fn identified an underlying β₂-integrin-directed modulation of persistence. These data demonstrate that individual components of the neutrophil chemotactic response show integrin dependence and are finely tunable with different ligand, mechanotactic, and chemotactic cues, underscoring the need for sensitive analytical methods.

Describing Directional Cell Migration with a Characteristic Directionality Time

“…Put in terms of log-log MSD slope, β ( τ ) transitions towards 2 as τ → ∞ for a directionally biased random walk. The idea of using log-log MSD slope to measure the transition time was recently proposed in a preceding article about neutrophil chemotaxis [ 17 ]. We suggested an empirical fit function for β ( t ) to quantify this time interval that we called directionality time.…”

“…The TASD of the i th migration path is given by where the overline denotes an average over time t , leaving TASD a function of the time interval, τ . Squared displacement that is first time averaged and then ensemble averaged, , is hereby referred to as MSD as this is how it is often defined in other studies [ 14 , 17 , 26 ].…”

“…Differential interference contrast (DIC) image sequences capturing directionally migrating Polymorphonuclear Human Neutrophils were obtained from Ref. [ 17 ] along with cell centroid positions, r i ( t n ), where i is the migration path index and t n is the time measured in multiples of the sampling interval, t n = n Δ t ( n = 1, 2, …). Centroid measurement error was estimated as follows.…”

“…Mean squared displacement (MSD) is a common metric for measuring migration speed and distance traveled because it is easily interpretable and readily derived from mathematical models of motion. Numerous studies that characterize directional migration use MSD in conjunction with at least one other metric for quantifying path persistence or tortuosity [ 11 – 17 ]. Three examples of such metrics used are: the distribution of turning angles between discrete measurements of centroid displacements (turning angle distribution, TAD); tortuosity (also known as straightness index [ 18 , 19 ], chemotactic index [ 15 , 20 ], or directionality ratio [ 21 ]) defined as the end-to-end distance traveled divided by the total migration path length; and tangent-tangent correlation, which describes the correlation in migration path orientation over a specified length or time interval.…”

Cardiac extracellular matrix–fibrin hybrid scaffolds with tunable properties for cardiovascular tissue engineering

“…However, the cardiac ECM is comprised of a complex and specific mixture of proteins, and this composition changes throughout heart development and maturation [14, 27, 43, 44]. In addition, the mechanical properties of tissues can play a critical role in directing cell response [13, 14, 45, 46], and the interplay between ECM composition and stiffness has only recently begun to be appreciated [34, 47-49]. Therefore, we sought to develop a cardiac ECM-based scaffold that could be tuned to mimic both the complex composition and stiffness of the myocardium during development and maturation.…”