ARPC5 Isoforms and Their Regulation by Calcium-Calmodulin-N-WASP Drive Distinct Arp2/3-dependent Actin Remodeling Events in CD4 T Cells

Abstract

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.

ArpC5 isoforms regulate Arp2/3 complex–dependent protrusion through differential Ena/VASP positioning

“…Our results show that the ArpC5 isoform is clearly associated with more prolific actin assemblies in lamellipodia of Rat2 fibroblasts and B16-F1 melanoma cells. These observations are in line with recently published work revealing that ArpC5 has the same effect in peripheral actin rings of CD4 T cells ( 61 ). This supports the notion that ArpC5/5L isoform–specific activities on cellular actin networks are generalizable over different cell types and species.…”

Arp2/3 complex-dependent actin regulation protects the survival of tissue-resident mast cells

“…The reported defects, resulting in combined immunodeficiencies with severe inflammation and early-onset autoimmunity, inflammation, and mortality, highlight the clinical relevance of Arp2/3 complex dysfunction (64)(65)(66). Notably, isoform diversity within the Arp2/3 complex may determine cellspecific phenotypes (67,68), prompting further exploration of individual ARPC isoforms for MC biology.…”

CK-666 and CK-869 differentially inhibit Arp2/3 iso-complexes

“…Structural analysis reveals that the higher branching efficiency of ArpC5L complexes is mediated by its disordered N-terminus (von Loeffelholz et al , 2020). Not surprisingly given their different properties, Arp2/3 iso-complexes have different cellular functions (Fassler et al , 2023; Molinie et al , 2019; Roman et al , 2017; Sadhu et al , 2023). The importance of Arp2/3 iso-complexes in cell and tissue homeostasis is also underscored by the observation that loss of function mutations in human ArpC1B lead to severe inflammation and immunodeficiency (Brigida et al , 2018; Kahr et al , 2017; Kuijpers et al , 2017; Randzavola et al , 2019; Somech et al , 2017; Volpi et al , 2019).…”