The proteoglycan (heparan sulfate proteoglycan) binding domain of APRIL serves as a platform for ligand multimerization andcross‐linking

Abstract

A proliferation-inducing ligand (APRIL) (also known as TALL-2 and TRDL-1) is a member of the tumor necrosis factor (TNF) superfamily that has tumorigenic properties but is also important for the induction of humoral immune responses. APRIL binds two TNF receptors: transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI) and B-cell maturation antigen (BCMA) as well as heparan sulfate proteoglycans (HSPGs). The aim of this study was to clarify the role of the HSPG interaction in canonical APRIL signaling, because it has been proposed to act as a docking site and also to play a role in direct signaling. In this study, we generated point mutants of soluble APRIL that lack either the capacity to bind HSPGs or TACI and BCMA and then tested the function of these mutants in mouse B-cell assays. In contrast to previous reports, we found that APRIL alone is sufficient to costimulate B-cell proliferation and drive IgA production and does not require artificial antibody cross-linking. We found no evidence that APRIL requires signaling through HSPGs but, notably, were able to show that binding of APRIL to HSPGs is crucial for mediating natural APRIL cross-linking to allow for optimal activation of murine B cells.

Intrarenal production of B-cell survival factors in human lupus nephritis

“…42,43 Mesangial cell-derived APRIL may promote survival and proliferation in an autocrine or paracrine, e.g., proteoglycan-receptor-mediated, manner leading to increased glomerular cellularity. 6,44,45 On the other hand, the expression of BLyS and BAFF-R in inflammatory cells of the interstitium in lupus nephritis, as described by us, may help sustain autocrine B 31 and T lymphocytemediated inflammation in the tubulointerstitial compartment of kidneys with lupus nephritis. In the glomerular compartment, few T cells, even fewer B cells and no dendritic cells are typically present.…”

The Design and Characterization of Receptor-selective APRIL Variants

“…A pcDNA3.1 construct containing FLAG-tagged wild-type soluble APRIL (amino acids 105-250, numbering according to UniprotKB/ Swiss-Prot O75888) was used as the PCR template and was described previously (14). Plasmid DNA of clones was isolated (BIOKÉ, Leiden, The Netherlands), and the presence of the mutation(s) was verified by DNA sequencing.…”

“…1E). R231A, previously shown by us to be a mutation that leads to loss of both TACI and BCMA binding, but not binding to heparan sulfate proteoglycans, was included as a negative control (14,32). Binding to human BCMA-Fc was retained by all variants predicted to selectively bind BCMA (Fig.…”

“…This is a validated approach that has been used previously (14,32). Protein expression was quantified by Western blot using an anti-FLAG tag antibody ( Fig.…”

“…In addition, APRIL binds to heparan sulfate proteoglycans, which appear to play a predominantly structural role by enabling APRIL cross-linking (13,14), although a distinct signaling role in different contexts cannot be eliminated. In addition to APRIL, TACI can also bind to heparan sulfate proteoglycans, which is suggested to lead to its activation (15).…”