The Intramembrane Proteases Signal Peptide Peptidase-Like 2a and 2b Have Distinct Functions In Vivo

Abstract

We reported recently that the presenilin homologue signal peptide peptidase-like 2a (SPPL2a) is essential for B cell development by cleaving the N-terminal fragment (NTF) of the invariant chain (li, CD74). Based on this, we suggested that pharmacological modulation of SPPL2a may represent a novel approach to deplete B cells in autoimmune disorders. With regard to reported overlapping substrate spectra of SPPL2a and its close homologue, SPPL2b, we investigated the role of SPPL2b in CD74 NTF proteolysis and its impact on B and dendritic cell homeostasis. In heterologous expression experiments, SPPL2b was found to cleave CD74 NTF with an efficiency simliar to that of SPPL2a. For in vivo analysis, SPPL2b single-deficient and SPPL2a/SPPL2b double-deficient mice were generated and examined for CD74 NTF turnover/accumulation, B cell maturation and functionality, and dendritic cell homeostasis. We demonstrate that in vivo SPPL2b does not exhibit a physiologically relevant contribution to CD74 proteolysis in B and dendritic cells. Furthermore, we reveal that both proteases exhibit divergent subcellular localizations in B cells and different expression profiles in murine tissues. These findings suggest distinct functions of SPPL2a and SPPL2b and, based on a high abundance of SPPL2b in brain, a physiological role of this protease in the central nervous system.

Processing of CD74 by the Intramembrane Protease SPPL2a Is Critical for B Cell Receptor Signaling in Transitional B Cells

“…9B). Because SPPL2a deficiency has no impact on the abundance of CD74 full-length protein (13,14), this signal increase most likely reflects the accumulation of the CD74 NTF. The Journal of Immunology…”

“…We recently found that clearance of the final membrane-bound N-terminal fragment (NTF) of CD74 requires activity of the GxGD-type intramembrane protease signal peptide peptidase-like (SPPL)2a (13), but not of its closely related homolog SPPL2b (14). This unique role of SPPL2a is also conserved in human B cells (15).…”

“…SPPL2a is part of the SPP/SPPL protease family with specificity for transmembrane proteins in type II orientation (16). Within this family, SPPL2a is unique with its localization in lysosomes/late endosomes (14,17). Additional substrates of SPPL2a include TNF-a, Fas ligand, Bri2, and TMEM106B (16,18).…”

Regulated Intramembrane Proteolysis of the Frontotemporal Lobar Degeneration Risk Factor, TMEM106B, by Signal Peptide Peptidase-like 2a (SPPL2a)

“…To date, five known substrates have been identified: TNF␣, Fas Ligand, British dementia protein-2/ITMB2 (Bri2), Transferrin Receptor 1, and CD74 (33)(34)(35)(36)(37)(38)(39)(40)(41). In some cases, SPPL2a and SPPL2b appear to share a common substrate such as TNF␣ and Bri2.…”

A Cell‐Based Assay Reveals Nuclear Translocation of Intracellular Domains Released by SPPL Proteases

“…Though this phenotype is clearly driven by the CD74 NTF as documented by its alleviation in Sppl2a −/− Cd74 −/− mice (12), the fate and putative downstream function of the CD74 intracellular domain (ICD) that is released by SPPL2a is insufficiently resolved. In contrast to SPPL2a, SPPL2b was reported to reside primarily at the plasma membrane (5,6). Although the substrate spectra of SPPL2a and SPPL2b were found to overlap significantly in cell-based systems, SPPL2b was described to be dispensable for CD74 proteolysis under endogenous conditions, indicating that both proteases exhibit distinct functions in vivo (5).…”