Retractile processes in T lymphocyte orientation on a stimulatory substrate: morphology and dynamics

Microtubule appendages mediating T-cell motility and polarity

Polarization of the centrosome and the Golgi apparatus in the T cell (TC) toward the antigen-presenting cell (APC) is essential for the specificity of the immune response on the cellular level. Previously we reported the existence of thin, long processes on the TC surface, which emanated predominantly from the area next to the Golgi apparatus. They appeared to be involved in the orientation of the TC during the initial phases of its attachment, which preceded the formation of the immunological synapse mediated by lamellipodia. Here we improve the visualization of the long, thin protrusions in the cultured TC and demonstrate using cytoskeleton inhibitors and immunofluorescence that microtubules form their cytoskeletal basis. The protrusions are seen prior to the attachment and the development of the broad lamellipodia (within a few minutes). We propose the term "tubulopodia" for this distinct type of cell appendage. Using an established experimental model that replaces the APC surface with a biomimetic substrate coated with antibodies against the TC receptor (TCR), we demonstrate that abrogation of the lamellipodium-mediated synapse formation does not impede the orientation of the TC Golgi apparatus and the centrosome to the contact area. Video microscopy reveals the spreading of the tubulopodia on the TCR-binding substrate, which results in the area of their emanation, and consequently the Golgi apparatus and the centrosome, being closely apposed (polarized) to the TCR-binding surface. Treatment with paclitaxel made the tubulopodia rigid, preventing their attachment to the TCR-binding surface and the reorientation of the cell body with the intracellular structures. We speculate that the motility and polarity of the TC in vivo may be mediated on a large scale by differential adhesion through the long, flexible tubulopodia.

Systems biomechanics of centrosome positioning: A conserved complexity

Positioning of centrosomes within cells determines the directionality of cell division, as well as directionality of cellular activities in the interphase. This brief review focuses on similarities (and differences) of centrosome positioning during early divisions in the Caenorhabditis embryo and during the interaction of T lymphocytes with other cells in the course of immune response. In the study of the two phenomena, a synergy of experimentation and numerical mechanical analysis has recently been achieved. The picture that emerges from these studies is one in which simple physical forces under the constraints of the basic cell structure lead to complex, "life-like" mechanical behavior. This behavior includes instability of equilibria, irreversibility of structural transitions and multidimensional, multiperiodic oscillations. This new picture of cell mechanics may form an interesting paradigm for future research.

Symmetry, stability, and reversibility properties of idealized confined microtubule cytoskeletons

Many cell cytoskeletons include an aster of microtubules, with the centrosome serving as the focal point. The position of the centrosome within the cell is important in such directional activities as wound closure and interactions of immune cells. Here we analyzed the centrosome positioning as it is dictated by microtubule elasticity alone in a mechanical model of an intrinsically fully symmetric microtubule aster. We demonstrate that the symmetry and the central position of the centrosome are unstable. The equilibrium deviation of the centrosome from the center is approximately proportional to the difference of the microtubule length and cell radius. The proportionality coefficient is 1 in flat cells and 2 in three-dimensional cells. The loss of symmetry is irreversible, and in general, the equilibrium form of the aster exhibits memory of past perturbations. The equilibrium position of the centrosome as a function of the microtubule length exhibits hysteresis, and the history of the length variation is reflected in the aster form. These properties of the simple aster of elastic microtubules must be taken into account in the analysis of more comprehensive theoretical models, and in the design and interpretation of experiments addressing the complex process of cytoskeleton morphogenesis.

An experimental and computational study of effects of microtubule stabilization on T-cell polarity

T-killer cells eliminate infected and cancerous cells with precision by positioning their centrosome near the interface (immunological synapse) with the target cell. The mechanism of centrosome positioning has remained controversial, in particular the role of microtubule dynamics in it. We re-examined the issue in the experimental model of Jurkat cells presented with a T cell receptor-binding artificial substrate, which permits controlled stimulation and reproducible measurements. Neither 1-µM taxol nor 100-nM nocodazole inhibited the centrosome positioning at the “synapse” with the biomimetic substrate. At the same time, in micromolar taxol but not in nanomolar nocodazole the centrosome adopted a distinct peripheral rather than the normally central position within the synapse. This effect was reproduced in a computational energy-minimization model that assumed no microtubule dynamics, but only a taxol-induced increase in the length of the microtubules. Together, the experimental and computational results indicate that microtubule dynamics are not essential for the centrosome positioning, but that the fit of the microtubule array in the deformed body of the conjugated T cell is a major factor. The possibility of modulating the T-cell centrosome position with well-studied drugs and of predicting their effects in silico appears attractive for designing anti-cancer and antiviral therapies.