Computational study of T cell repolarization during target elimination

T Cells are one of the most important players of the immune system. They are responsible for the elimination of the pathogen-infected or tumorigenic cells (target cells). When a target cell is recognized, the T Cell establishes a contact zone called the immunological synapse (IS). Subsequently, the cytoskeleton rotates and the MTOC relocates to the IS. The cytoskeleton rotation is correlated with a movement of organelles attached to microtubules (MT). The MTOC repositioning results from an interplay between MTs and dyneins in the IS pulling MTs via two mechanisms: cortical sliding and capture-shrinkage. Since many aspects of the process remain unknown, we designed a theoretical model for the molecular-motor-driven motion of the MT cytoskeleton in the cell with one or two IS. The model offers explanations of several experimental results including the biphasic nature of the MTOC movement. We also compared the two mechanisms in different cell configurations and found that the T Cell performs one of the most important immune reactions with stunning efficiency by the advantageous placement of dyneins and by employing two mechanisms acting in synergy. We also analyzed Ca2+ diffusion in the T Cell following the MTOC repositioning. We provided the evidence that mitochondria relocate towards the IS with the MTOC and their placement together with their ability of absorption and redistribution significantly increase the Ca2+ concentration.T Zellen sind einer der wichtigsten Spieler des Immunsystems. Sie sind verantwortlich für die Beseitigung von infizierten-oder tumorösen Zellen (Zielzellen). Wenn eine Zielzelle erkannt ist, schafft die T-Zelle eine Immunologische Synapse (IS) genannte Kontaktzone. Dann rotiert das Zytoskelett und das MTOC zieht zur IS. Die Rotation ist mit einer Bewegung von an Mikrotubuli (MT) angehefteten Organellen korreliert. Die MOTC Umpositionierung ergibt sich aus dem Zusammenspiel zwischen MT und Dyneinen in der IS wobei MTs über zwei Mechanismen gezogen werden: ”cortical slidingünd ”captureshrinkage”. Da viele Aspekte des Prozesses unbekannt bleiben entwarfen wir ein theoretisches Modell für die durch molekulare Dyneinen Bewegung des MT Zytoskeletts in der Zelle mit einer oder zwei IS. Das Modell bietet Erklärungen mehrerer experimenteller Ergebnisse einschließlich der biphasischen Natur der MTOC Bewebung. Ebenso verglichen wir die beiden Mechanismen unter verschiedenen Konfigurationen und fanden, dass die T-Zelle eine der wichtigsten Immunreaktionen durch nutzbar Anordnung von Dyneinen und Einsatzes zweier in Synergie arbeitenden Mechanismen mit erstaunlicher Effizienz durchführt. Wir analysierten auch folgenden Ca2+ Diffusion in der T-Zelle. Wir liefern den Nachweis, dass Mitochondrien mit das MTOC zu der IS ziehen und ihre Plazierung, zusammen mit der Fähigkeit der Absorption und Umverteilung, die global Ca2+ Konzentration signifikant steiger

Cytoskeleton rotation relocates mitochondria to the immunological synapse and increases calcium signals

Ca2+ microdomains and spatially resolved Ca2+ signals are highly relevant for cell function. In T cells, local Ca2+ signaling at the immunological synapse (IS) is required for downstream effector functions. We present experimental evidence that the relocation of the MTOC towards the IS during polarization drags mitochondria along with the microtubule network. From time-lapse fluorescence microscopy we conclude that mitochondria rotate together with the cytoskeleton towards the IS. We hypothesize that this movement of mitochondria towards the IS together with their functionality of absorption and spatial redistribution of Ca2+ is sufficient to significantly increase the cytosolic Ca2+ concentration. To test this hypothesis we developed a whole cell model for Ca2+ homoeostasis involving specific geometries for mitochondria and use the model to calculate the spatial distribution of Ca2+ concentrations within the cell body as a function of the rotation angle and the distance from the IS. We find that an inhomogeneous distribution of PMCA pumps on the cell membrane, in particular an accumulation of PMCA at the IS, increases the global Ca2+ concentration and decreases the local Ca2+ concentration at the IS with decreasing distance of the MTOC from the IS. Unexpectedly, a change of CRAC/Orai activity is not required to explain the observed Ca2+ changes. We conclude that rotation-driven relocation of the MTOC towards the IS together with an accumulation of PMCA pumps at the IS are sufficient to control the observed Ca2+ dynamics in T-cells during polarization