Memory in receptor–ligand-mediated cell adhesion

Abstract

Single-molecule biomechanical measurements, such as the force to unfold a protein domain or the lifetime of a receptor-ligand bond, are inherently stochastic, thereby requiring a large number of data for statistical analysis. Sequentially repeated tests are generally used to obtain a data ensemble, implicitly assuming that the test sequence consists of independent and identically distributed (i.i.d.) random variables, i.e., a Bernoulli sequence. We tested this assumption by using data from the micropipette adhesion frequency assay that generates sequences of two random outcomes: adhesion and no adhesion. Analysis of distributions of consecutive adhesion events revealed violation of the i.i.d. assumption, depending on the receptor-ligand systems studied. These include Markov sequences with positive (T cell receptor interacting with antigen peptide bound to a major histocompatibility complex) or negative (homotypic interaction between C-cadherins) feedbacks, where adhesion probability in the next test was increased or decreased, respectively, by adhesion in the immediate past test. These molecular interactions mediate cell adhesion and cell signaling. The ability to ''remember'' the previous adhesion event may represent a mechanism by which the cell regulates adhesion and signaling.adhesion frequency assay ͉ Markov sequence ͉ single-molecule mechanics ͉ Bernoulli sequence B iomechanical studies of protein, DNA, and RNA at the level of single molecules provide insights that complement information obtained from conventional measurements on ensembles of large numbers of molecules (1). These experiments employ ultrasensitive force techniques, for example, atomic force microscopy (2) and the biomembrane force probe technique (3), to mechanically characterize a single molecule that physically links the force sensor to a sample surface. Fig. 1 [and supporting information (SI) Movie 1] illustrate a simple experiment: the micropipette adhesion frequency assay (4). A human red blood cell (RBC) pressurized by micropipette aspiration is used as an adhesion sensor to test interactions between ligands coated on the RBC membrane and receptors expressed on a second cell (Fig. 1 A). The receptor-expressing cell is put into contact with the RBC for a given duration (Fig. 1B) and then retracted. If adhesion results, retraction will stretch the RBC (Fig. 1C), otherwise the RBC will smoothly return to its initial shape (Fig. 1D). When adhesion does occur, additional quantities can be measured by using the RBC picoforce transducer or any other ultrasensitive force technique, including rupture force (3), adhesion lifetime (2), molecular elasticity (5), protein unfolding (6), and protein refolding (7).Single-molecule biomechanical measurements are inherently stochastic because molecular events (e.g., unfolding of a protein domain or unbinding of a receptor-ligand bond) are determined not only by the weak, noncovalent interactions (within a single molecule or between two interacting molecules) but also by thermal excitations from the envi...

Increased Sensitivity of Antigen-Experienced T Cells through the Enrichment of Oligomeric T Cell Receptor Complexes

“…This suggests cooperativity in binding between TCRs, which could be achieved by their organization in TCR oligomers in protein islands. Micropipette adhesion frequency assays used to measure TCR-MHCp interactions showed not only a similar increase in TCR-MHCp on rate (Huang et al, 2010), but also showed that interaction events clustered in time, meaning that the probability of interaction was higher when an interaction had occurred in the immediately previous cycle of this assay (Zarnitsyna et al, 2007). One possible explanation for this effect could be that such clusters of interaction represent sequential interaction of the individual TCRs complexes within a TCR oligomer with the single MHCp ligand on the opposing membrane.…”

Regulation of Catch Bonds by Rate of Force Application

“…However, the concept of kinetic rate memory has been used by different authors to describe different phenomena, which are also different from the one reported here. Previously, we have reported a form of memory in receptor-ligand interaction where the likelihood of bond formation in the next contact is affected by the outcome of the previous contact (44). Lu et al (45) showed that enzymatic turnover of cholesterol oxidase was dependent on previous turnovers, a non-Markovian molecular memory attributed to a slow fluctuation of protein conformation.…”

Dynamic coupling of ALCAM to the actin cortex strengthens cell adhesion to CD6

“…S4A). This demonstrates that when delay times are used between sequential adhesion cycles, the cell adhesion shows no memory of previous adhesion events (Zarnitsyna et al, 2007;Friedrichs et al, 2010;Dao et al, 2013). Therefore, repetitive cell adhesion measurements were treated as uncorrelated events.…”