Dynamic Microtubule Arrays in Leukocytes and Their Role in Cell Migration and Immune Synapse Formation

Abstract

The organization of microtubule arrays in immune cells is critically important for a properly operating immune system. Leukocytes are white blood cells of hematopoietic origin, which exert effector functions of innate and adaptive immune responses. During these processes the microtubule cytoskeleton plays a crucial role for establishing cell polarization and directed migration, targeted secretion of vesicles for T cell activation and cellular cytotoxicity as well as the maintenance of cell integrity. Considering this large spectrum of distinct effector functions, leukocytes require flexible microtubule arrays, which timely and spatially reorganize allowing the cells to accommodate their specific tasks. In contrast to other specialized cell types, which typically nucleate microtubule filaments from non-centrosomal microtubule organizing centers (MTOCs), leukocytes mainly utilize centrosomes for sites of microtubule nucleation. Yet, MTOC localization as well as microtubule organization and dynamics are highly plastic in leukocytes thus allowing the cells to adapt to different environmental constraints. Here we summarize our current knowledge on microtubule organization and dynamics during immune processes and how these microtubule arrays affect immune cell effector functions. We particularly highlight emerging concepts of microtubule involvement during maintenance of cell shape and physical coherence.

Keywords: cell coherence; cell migration; immune synapse; leukocytes; microtubules.

FIGURE 1

Differential organization of microtubule arrays in distinct cell types. Terminal differentiation into effector cells such as neurons is associated with a reorganization of centrosomal microtubule filaments into non-centrosomal arrays (Muroyama and Lechler, 2017). In highly specialized leukocytes, the centrosome acts as single MTOC, from which a radial array of microtubule filaments project to the cell periphery. In unpolarized leukocytes, the centrosome is located centrally close to the nucleus (left panel). During polarization the MTOC reorients toward the uropod leading to higher densities of microtubule filaments at the cell’s back (right panel). By contrast, fibroblasts exhibit an anterior MTOC localization relative to the nucleus. MTOC positioning is highly plastic in neutrophils and differs depending on the complexity of the environment (Yoo et al., 2013).

FIGURE 2

Microtubule function during leukocyte migration in environments of different complexity. Microtubule depletion leads to uncoordinated actomyosin activation, yet with different consequences on cell shape and coherence depending on the geometry of the cell’s surrounding. Oscillating myosin activation across the cortex yields in uncoordinated and unstable polarization in 2D while in linear channels (1D) leukocytes maintain their polarized shape. In complex 3D environments, microtubules mediate the coordination of multiple competing protrusions (Renkawitz et al., 2019; Kopf et al., 2020). Disruption of microtubule integrity impairs protrusion-retraction dynamics of competing extensions resulting in loss of cell shape and induction of cell fragmentation.

FIGURE 3

Mechanisms of MTOC translocation during immune synapse formation. Antigen recognition on the surface of an antigen presenting cell by the T cell receptor leads to the formation of a tight contact zone between two adjacent cells. Microtubule filaments inside the T cell project toward the contact area and subsequently the MTOC gets recruited to the immune synapse. Two major models have been proposed how MTOC relocalization is achieved, both of which depend on the presence of the microtubule-associated motor protein dynein at the plasma membrane. (A) In the “cortical sliding model” dynein is held in place at the cell cortex and simultaneously walks toward the minus-end of the microtubule filaments. This leads to cortical sliding of microtubules and pulling of the MTOC toward the contact site (Martin-Cofreces et al., 2008). (B) The “capture-shrinkage model” suggests that the force, which is required for MTOC translocation is generated by the dynamic instability of microtubule filaments and transmitted via dynein (Yi et al., 2013). Here dynein couples microtubule depolymerization to the cell cortex to move the MTOC toward the synaptic membrane. Recently a revised model for MTOC translocation to the immune synapse was proposed, in which centrally localized dynein first pulls the centrosome straight toward the immune synapse. Afterward, dynein congresses at the peripheral SMAC, which leads to opposing forces acting laterally on the centrosome (Maskalenko et al., 2020). Such lateral pulling forces lead to oscillating movements of the centrosome at the immune synapse.

FIGURE 4

Microtubule-actin crosstalk at the B cell synapse. Upper panel: in resting B cells, Arp2/3-mediated actin nucleation at the centrosome tethers the MTOC to the nucleus. Upon B cell receptor activation, Arp2/3 and F-actin are recruited to the immune synapse thus facilitating centrosome-nucleus separation and repositioning of the MTOC to the cell-cell contact side. Lower panel: proteasomal activity is required for centrosomal actin depletion and subsequent centrosome relocalization to the immune synapse (Obino et al., 2016; Ibañez-Vega et al., 2019).

FIGURE 5

Stabilization of centrosomal microtubules support lysosome fusion at the synaptic membrane by releasing Exo70 and GEF-H1. Concomitantly to the repositioning of the centrosome, lysosomes are recruited to the immune synapse and become locally released, allowing acidification of the B cell synapse and secretion of lysosomal proteases that promote the extraction of immobilized surface antigen. In resting B cells, GEF-H1 is bound to microtubule filaments and kept in its inactive state, while Exo70 is mainly associated to the centrosome. BCR engagement triggers microtubule acetylation, which results in the release of Exo70 and GEF-H1 and subsequent recruitment to the immune synapse. There, GEF-H1 promotes the assembly of the exocyst complex to enable local lysosome fusion at the synaptic membrane (Yuseff et al., 2011; Sáez et al., 2019).

FIGURE 6

Microtubules and lytic granule convergence. Cytotoxic cells assemble lytic granules around the MTOC, which improves the efficiency of targeted killing (upper panel). Impaired lytic granule convergence results in non-directed degranulation and bystander killing of healthy cells (Hsu et al., 2016). The role of centrosome polarization for efficient cytotoxic T lymphocytes and natural killer cell-mediated killing is still unclear. Under certain experimental conditions, MTOC relocalization toward the immune synapse is dispensable for polarized granule secretion and cytotoxicity (Bertrand et al., 2013; Tamzalit et al., 2020). Under these conditions plus-end directed transport along microtubule filaments might deliver lytic granules to the synapse.