Immunostimulatory effects of RACK1 pseudosubstrate in human leukocytes obtained from young and old donors

Systematically characterizing the roles of E3-ligase family members in inflammatory responses with massively parallel Perturb-seq

E3 ligases regulate key processes, but many of their roles remain unknown. Using Perturb-seq, we interrogated the function of 1,130 E3 ligases, partners and substrates in the inflammatory response in primary dendritic cells (DCs). Dozens impacted the balance of DC1, DC2, migratory DC and macrophage states and a gradient of DC maturation. Family members grouped into co-functional modules that were enriched for physical interactions and impacted specific programs through substrate transcription factors. E3s and their adaptors co-regulated the same processes, but partnered with different substrate recognition adaptors to impact distinct aspects of the DC life cycle. Genetic interactions were more prevalent within than between modules, and a deep learning model, comβVAE, predicts the outcome of new combinations by leveraging modularity. The E3 regulatory network was associated with heritable variation and aberrant gene expression in immune cells in human inflammatory diseases. Our study provides a general approach to dissect gene function.

RACK1 plays a critical role in mast cell secretion and Ca2+ mobilization by modulating F-actin dynamics

Although RACK1 is known to act as a signaling hub in immune cells, its presence and role in mast cells (MCs) is undetermined. MC activation via antigen stimulation results in mediator release and is preceded by cytoskeleton reorganization and Ca2+ mobilization. In this study, we found that RACK1 was distributed throughout the MC cytoplasm both in vivo and in vitro. After RACK1 knockdown (KD), MCs were rounded, and the cortical F-actin was fragmented. Following antigen stimulation, in RACK1 KD MCs, there was a reduction in cortical F-actin, an increase in monomeric G-actin and a failure to organize F-actin. RACK1 KD also increased and accelerated degranulation. CD63+ secretory granules were localized in F-actin-free cortical regions in non-stimulated RACK1 KD MCs. Additionally, RACK1 KD increased antigen-stimulated Ca2+ mobilization, but attenuated antigen-stimulated depletion of ER Ca2+ stores and thapsigargin-induced Ca2+ entry. Following MC activation there was also an increase in interaction of RACK1 with Orai1 Ca2+-channels, β-actin and the actin-binding proteins vinculin and MyoVa. These results show that RACK1 is a critical regulator of actin dynamics, affecting mediator secretion and Ca2+ signaling in MCs. This article has an associated First Person interview with the first author of the paper.

The role of HSP27 in RACK1-mediated PKC activation in THP-1 cells

Receptor for Activated C Kinase 1 (RACK1) pseudosubstrate is a commercially available peptide that directly activates protein kinase C-β (PKCβ). We have recently shown that RACK1 pseudosubstrate, alone or in combination with classical immune activators, results in increased cytokine production and CD86 upregulation in primary leukocytes. Furthermore, we demonstrated a role of PKCβ and RACK1 in chemical allergen-induced CD86 expression and IL-8 production in both THP-1 cells and primary human dendritic cells. Aim of this study was to shed light on the mechanisms underlying RACK1 pseudosubstrate-induced immune activation and to compare it to lipopolysaccharide (LPS). The human promyelocytic cell line THP-1 was used throughout the study. RACK1 pseudosubstrate induced rapid (5 min) and dose-related PKCβ activation as assessed by its membrane translocation. Among the proteins phosphorylated, we identified Hsp27. Both RACK1 pseudosubstrate and LPS induce its phosphorylation and release in culture medium. The release of Hsp27 induced by RACK1 pseudosubstrate was also confirmed in peripheral blood mononuclear cells. To evaluate the role of Hsp27 in RACK1 pseudosubstrate or LPS-induced cell activation, we conducted Hsp27 silencing and neutralization experiments. Both strategies confirmed the central role of Hsp27 in RACK1 pseudosubstrate or LPS-induced cell activation, as assessed by IL-8 production and upregulation of CD86.