CD4 T cell activation induces nuclear and cytoplasmic actin polymerization via the Arp2/3 complex to activate cytokine expression and strengthen T cell receptor (TCR) signaling. Actin polymerization dynamics and filament morphology differ between nucleus and cytoplasm. However, it is unclear how the Arp2/3 complex mediates distinct nuclear and cytoplasmic actin polymerization in response to a common stimulus. In humans, the ARP3, ARPC1, and ARPC5 subunits of the Arp2/3 complex exist as two different isoforms, resulting in complexes with different properties. Here, we show that the Arp2/3 subunit isoforms ARPC5 and ARPC5L play a central role in coordinating distinct actin polymerization events in CD4 T cells. While ARPC5L is heterogeneously expressed in individual CD4 T cells, it specifically drives nuclear actin polymerization upon T cell activation. In contrast, ARPC5 is evenly expressed in CD4 T cell populations and is required for cytoplasmic actin dynamics. Interestingly, nuclear actin polymerization triggered by a different stimulus, DNA replication stress, specifically requires ARPC5 but not ARPC5L. TCR signaling but not DNA replication stress induces nuclear actin polymerization via nuclear calcium-calmodulin signaling and N-WASP. Diversity in the molecular properties and individual expression patterns of ARPC5 subunit isoforms thus tailors Arp2/3-mediated actin polymerization to different physiological stimuli.
Nuclear actin polymerization is observed in an increasing number of biological processes including DNA replication stress and T cell receptor (TCR) signaling in CD4 T cells. TCR activation induces the formation of F-actin in the cytoplasm and the nucleus to strengthen contacts to antigen presenting cells and drive a gene expression program to shape humoral immune responses, respectively. Interestingly, these two actin remodeling events are phenotypically different and appear to be mechanistically uncoupled from each other but both involve actin polymerization by the Arp2/3 complex. The Arp2/3-complex consists of 7 subunits where ARP3, ARPC1 and ARPC5 exist as two different isoforms in humans that can assemble in complexes with different properties. Here we examined whether specific Arp2/3 subunit isoforms are responsible for distinct actin remodeling events in CD4 T cells. Transient silencing or knock out of individual subunit isoforms demonstrates that in response to TCR signaling, the ARPC5L isoform is involved in nuclear actin polymerization, while cytoplasmic actin dynamics selectively relies on ARPC5. In contrast, nuclear actin polymerization triggered by DNA replication stress in CD4 T cells required ARPC5 and was independent of ARPC5L. Moreover, nuclear Ca2+ transients, which are essential for TCR-induced nuclear actin polymerization, were dispensable for nuclear actin filament formation during DNA replication stress. Our results reveal that the selective involvement of ARPC5 isoforms governs the activity of Arp2/3 complex in distinct actin polymerization events and imply nuclear Ca2+ transients as selective trigger for ARPC5L-dependent nuclear actin polymerization.
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