The intramembrane protease SPPL2a promotes B cell development and controls endosomal traffic by cleavage of the invariant chain

Selective regulation of aspartyl intramembrane protease activity by calnexin

Signal peptide peptidase-like 2c (SPPL2c) is a testis-specific aspartyl intramembrane protease that contributes to male gamete function both by catalytic and non-proteolytic mechanisms. Here, we provide an unbiased characterisation of the in vivo interactome of SPPL2c identifying the ER chaperone calnexin as novel binding partner of this enzyme. Recruitment of calnexin specifically required the N-glycosylation within the N-terminal protease-associated domain of SPPL2c. Importantly, mutation of the single glycosylation site of SPPL2c or loss of calnexin expression completely prevented SPPL2c-mediated intramembrane proteolysis of all tested substrates. By contrast and despite rather promiscuous binding of calnexin to other SPP/SPPL proteases, expression of the chaperone was exclusively required for SPPL2c-mediated proteolysis. Despite some impact on the stability of SPPL2c most presumably due to assistance in folding of the luminal domain of the protease, calnexin appeared to be recruited rather constitutively to the protease thereby boosting its catalytic activity. In summary, we describe a novel, highly specific mode of intramembrane protease regulation, highlighting the need to systematically approach control mechanisms governing the proteolytic activity of other members of the aspartyl intramembrane protease family.

WISP1 and Macrophage Migration Inhibitory Factor in Respiratory Inflammation: Novel Insights and Therapeutic Potentials for Asthma and COPD

Recent advancements highlight the intricate interplay between the extracellular matrix (ECM) and immune responses, notably in respiratory diseases such as asthma and Chronic Obstructive Pulmonary Disease (COPD). The ECM, a dynamic structural framework within tissues, orches-trates a plethora of cellular processes, including immune cell behavior and tissue repair mecha-nisms. WNT1-inducible-signaling pathway protein 1 (WISP1), a key ECM regulator, controls immune cell behavior, cytokine production, and tissue repair by modulating integrins, PI3K, Akt, β-catenin, and mTOR signaling pathways. WISP1 also induces macrophage migration inhibitory factor (MIF) expression via Src kinases and epidermal growth factor receptor (EGFR) activation. MIF, through its wide range of activities, enhances inflammation and tissue restructuring. Rec-ognized for its versatile roles in regulating the immune system, MIF interacts with multiple immune components, such as the NLRP3 inflammasome, thereby sustaining inflammatory pro-cesses. The WISP1-MIF axis potentially unveils complex molecular mechanisms governing im-mune responses and inflammation. Understanding the intricate roles of WISP1 and MIF in the pathogenesis of chronic respiratory diseases such as asthma and COPD could lead to the identi-fication of novel targets for therapeutic intervention to alleviate disease severity and enhance patient outcomes.

CD74 is a functional MIF receptor on activated CD4+ T cells

Next to its classical role in MHC II-mediated antigen presentation, CD74 was identified as a high-affinity receptor for macrophage migration inhibitory factor (MIF), a pleiotropic cytokine and major determinant of various acute and chronic inflammatory conditions, cardiovascular diseases and cancer. Recent evidence suggests that CD74 is expressed in T cells, but the functional relevance of this observation is poorly understood. Here, we characterized the regulation of CD74 expression and that of the MIF chemokine receptors during activation of human CD4+ T cells and studied links to MIF-induced T-cell migration, function, and COVID-19 disease stage. MIF receptor profiling of resting primary human CD4+ T cells via flow cytometry revealed high surface expression of CXCR4, while CD74, CXCR2 and ACKR3/CXCR7 were not measurably expressed. However, CD4+ T cells constitutively expressed CD74 intracellularly, which upon T-cell activation was significantly upregulated, post-translationally modified by chondroitin sulfate and could be detected on the cell surface, as determined by flow cytometry, Western blot, immunohistochemistry, and re-analysis of available RNA-sequencing and proteomic data sets. Applying 3D-matrix-based live cell-imaging and receptor pathway-specific inhibitors, we determined a causal involvement of CD74 and CXCR4 in MIF-induced CD4+ T-cell migration. Mechanistically, proximity ligation assay visualized CD74/CXCR4 heterocomplexes on activated CD4+ T cells, which were significantly diminished after MIF treatment, pointing towards a MIF-mediated internalization process. Lastly, in a cohort of 30 COVID-19 patients, CD74 surface expression was found to be significantly upregulated on CD4+ and CD8+ T cells in patients with severe compared to patients with only mild disease course. Together, our study characterizes the MIF receptor network in the course of T-cell activation and reveals CD74 as a novel functional MIF receptor and MHC II-independent activation marker of primary human CD4+ T cells.

Genome-Wide Identification of the SPP/SPPL Gene Family and BnaSPPL4 Regulating Male Fertility in Rapeseed (Brassica napus L.)

Signal peptide peptidase (SPP) and its homologs, signal peptide peptidase-like (SPPL) proteases, are members of the GxGD-type aspartyl protease family, which is widespread in plants and animals and is a class of transmembrane proteins with significant biological functions. SPP/SPPLs have been identified; however, the functions of SPP/SPPL in rapeseed (Brassica napus L.) have not been reported. In this study, 26 SPP/SPPLs were identified in rapeseed and categorized into three groups: SPP, SPPL2, and SPPL3. These members mainly contained the Peptidase_A22 and PA domains, which were distributed on 17 out of 19 chromosomes. Evolutionary analyses indicated that BnaSPP/SPPLs evolved with a large number of whole-genome duplication (WGD) events and strong purifying selection. Members are widely expressed and play a key role in the growth and development of rapeseed. The regulation of rapeseed pollen fertility by the BnaSPPL4 gene was further validated through experiments based on bioinformatics analysis, concluding that BnaSPPL4 silencing causes male sterility. Cytological observation showed that male infertility caused by loss of BnaSPPL4 gene function occurs late in the mononucleate stage due to microspore dysplasia.

The role of SPP/SPPL intramembrane proteases in membrane protein homeostasis

Signal peptide peptidase (SPP) and the four SPP-like proteases SPPL2a, SPPL2b, SPPL2c and SPPL3 constitute a family of aspartyl intramembrane proteases with homology to presenilins. The different members reside in distinct cellular localisations within the secretory pathway and the endo-lysosomal system. Despite individual cleavage characteristics, they all cleave single-span transmembrane proteins with a type II orientation exhibiting a cytosolic N-terminus. Though the identification of substrates is not complete, SPP/SPPL-mediated proteolysis appears to be rather selective. Therefore, according to our current understanding cleavage by SPP/SPPL proteases rather seems to serve a regulatory function than being a bulk proteolytic pathway. In the present review, we will summarise our state of knowledge on SPP/SPPL proteases and in particular highlight recently identified substrates and the functional and/or (patho)-physiological implications of these cleavage events. Based on this, we aim to provide an overview of the current open questions in the field. These are connected to the regulation of these proteases at the cellular level but also in context of disease and patho-physiological processes. Furthermore, the interplay with other proteostatic systems capable of degrading membrane proteins is beginning to emerge.

Structure and function of SPP/SPPL proteases: insights from biochemical evidence and predictive modeling

More than 20 years ago, signal peptide peptidase (SPP) and its homologues, the signal peptide peptidase-like (SPPL) proteases have been identified based on their sequence similarity to presenilins, a related family of intramembrane aspartyl proteases. Other than those for the presenilins, no high-resolution structures for the SPP/SPPL proteases are available. Despite this limitation, over the years bioinformatical and biochemical data have accumulated, which altogether have provided a picture of the overall structure and topology of these proteases, their localization in the cell, the process of substrate recognition, their cleavage mechanism, and their function. Recently, the artificial intelligence-based structure prediction tool AlphaFold has added high-confidence models of the expected fold of SPP/SPPL proteases. In this review, we summarize known structural aspects of the SPP/SPPL family as well as their substrates. Of particular interest are the emerging substrate recognition and catalytic mechanisms that might lead to the prediction and identification of more potential substrates and deeper insight into physiological and pathophysiological roles of proteolysis.

Genome-wide detection of human intronic AG-gain variants located between splicing branchpoints and canonical splice acceptor sites

Human genetic variants that introduce an AG into the intronic region between the branchpoint (BP) and the canonical splice acceptor site (ACC) of protein-coding genes can disrupt pre-mRNA splicing. Using our genome-wide BP database, we delineated the BP-ACC segments of all human introns and found extreme depletion of AG/YAG in the [BP+8, ACC-4] high-risk region. We developed AGAIN as a genome-wide computational approach to systematically and precisely pinpoint intronic AG-gain variants within the BP-ACC regions. AGAIN identified 350 AG-gain variants from the Human Gene Mutation Database, all of which alter splicing and cause disease. Among them, 74% created new acceptor sites, whereas 31% resulted in complete exon skipping. AGAIN also predicts the protein-level products resulting from these two consequences. We performed AGAIN on our exome/genomes database of patients with severe infectious diseases but without known genetic etiology and identified a private homozygous intronic AG-gain variant in the antimycobacterial gene SPPL2A in a patient with mycobacterial disease. AGAIN also predicts a retention of six intronic nucleotides that encode an in-frame stop codon, turning AG-gain into stop-gain. This allele was then confirmed experimentally to lead to loss of function by disrupting splicing. We further showed that AG-gain variants inside the high-risk region led to misspliced products, while those outside the region did not, by two case studies in genes STAT1 and IRF7. We finally evaluated AGAIN on our 14 paired exome-RNAseq samples and found that 82% of AG-gain variants in high-risk regions showed evidence of missplicing. AGAIN is publicly available from https://hgidsoft.rockefeller.edu/AGAIN and https://github.com/casanova-lab/AGAIN.

Dynamic association of the intramembrane proteases SPPL2a/b and their substrates with tetraspanin-enriched microdomains

Signal peptide peptidase-like 2a and b (SPPL2a/b) are aspartyl intramembrane proteases and cleave tail-anchored proteins as well as N-terminal fragments (NTFs) derived from type II-oriented transmembrane proteins. How these proteases recruit substrates and cleavage is regulated, is still incompletely understood. We found that SPPL2a/b localize to detergent-resistant membrane (DRM) domains with the characteristics of tetraspanin-enriched microdomains (TEMs). Based on this, association with several tetraspanins was evaluated. We demonstrate that not only SPPL2a/b but also their substrates tumor necrosis factor (TNF) and CD74 associate with tetraspanins like CD9, CD81, and CD82 and/or TEMs and analyze the stability of these complexes in different detergents. CD9 and CD81 deficiency has protease- and substrate-selective effects on SPPL2a/b function. Our findings suggest that reciprocal interactions with tetraspanins may assist protease-substrate encounters of SPPL2a/b within the membrane. Beyond SPP/SPPL proteases, this supports previous concepts that tetraspanins facilitate membrane-embedded proteolytic processes.

The transmembrane domain of Frey1 harbors a transplantable inhibitory motif for intramembrane proteases

Although aspartic intramembrane-cleaving proteases (I-CLIPs) are crucial switches of multiple signaling pathways and involved in several devastating diseases, little is known about their physiological regulation. We have recently identified Frey regulator of sperm-oocyte fusion 1 (Frey1) as an inhibitory protein of Signal Peptide Peptidase-like 2c (SPPL2c), a member of this protease family. Employing structure modeling along with cell-based inhibition and interaction studies, we identify a short motif within the Frey1 transmembrane domain essential for inhibition of SPPL2c. Intriguingly, this motif can be transplanted to the SPPL2c substrate PLN, thereby transforming it into an inhibitor of this enzyme. It can be adopted for the generation of Notch1-based γ-Secretase inhibitors demonstrating its versatile use among aspartic I-CLIPs. In summary, we describe a mechanism of aspartic I-CLIP inhibition which allows the targeted generation of specific inhibitors of these enzymes and might enable the identification of endogenous negative regulators of these enzymes.

Mendelian Susceptibility to Mycobacterial Disease (MSMD): Clinical, Immunological, and Genetic Features of 22 Patients from 15 Moroccan Kindreds

PURPOSE

The first molecular evidence of a monogenic predisposition to mycobacteria came from the study of Mendelian susceptibility to mycobacterial disease (MSMD). We aimed to study this Mendelian susceptibility to mycobacterial diseases in Moroccan kindreds through clinical, immunological, and genetic analysis.

METHODS

Patients presented with clinical features of MSMD were recruited into this study. We used whole blood samples from patients and age-matched healthy controls. To measure IL-12 and IFN-γ production, samples were activated by BCG plus recombinant human IFN-γ or recombinant human IL-12. Immunological assessments and genetic analysis were also done for patients and their relatives.

RESULTS

Our study involved 22 cases from 15 unrelated Moroccan kindreds. The average age at diagnosis is 4 years. Fourteen patients (64%) were born to consanguineous parents. All patients were vaccinated with the BCG vaccine, and twelve of them (55%) developed locoregional or disseminated BCG infections. The other symptomatic patients had severe tuberculosis and/or recurrent salmonellosis. Genetic mutations were identified on the following genes: IL12RB1 in 8 patients, STAT1 in 7 patients; SPPL2A, IFNGR1, and TYK2 in two patients each; and TBX21 in one patient, with different modes of inheritance. All identified mutations/variants altered production or response to IFN-γ or both.

CONCLUSION

Severe forms of tuberculosis and complications of BCG vaccination may imply a genetic predisposition present in the Moroccan population. In the presence of these infections, systematic genetic studies became necessary. BCG vaccination is contraindicated in MSMD patients and should be delayed in newborn siblings until the exclusion of a genetic predisposition to mycobacteria.

Bone Marrow Macrophages Induce Inflammation by Efferocytosis of Apoptotic Prostate Cancer Cells via HIF-1α Stabilization

The clearance of apoptotic cancer cells by macrophages, known as efferocytosis, fuels the bone-metastatic growth of prostate cancer cells via pro-inflammatory and immunosuppressive processes. However, the exact molecular mechanisms remain unclear. In this study, single-cell transcriptomics of bone marrow (BM) macrophages undergoing efferocytosis of apoptotic prostate cancer cells revealed a significant enrichment in their cellular response to hypoxia. Here, we show that BM macrophage efferocytosis increased hypoxia inducible factor-1alpha (HIF-1α) and STAT3 phosphorylation (p-STAT3 at Tyr705) under normoxic conditions, while inhibitors of p-STAT3 reduced HIF-1α. Efferocytosis promoted HIF-1α stabilization, reduced its ubiquitination, and induced HIF-1α and p-STAT3 nuclear translocation. HIF-1α stabilization in efferocytic BM macrophages resulted in enhanced expression of pro-inflammatory cytokine MIF, whereas BM macrophages with inactive HIF-1α reduced MIF expression upon efferocytosis. Stabilization of HIF-1α using the HIF-prolyl-hydroxylase inhibitor, Roxadustat, enhanced MIF expression in BM macrophages. Furthermore, BM macrophages treated with recombinant MIF protein activated NF-κB (p65) signaling and increased the expression of pro-inflammatory cytokines. Altogether, these findings suggest that the clearance of apoptotic cancer cells by BM macrophages triggers p-STAT3/HIF-1α/MIF signaling to promote further inflammation in the bone tumor microenvironment where a significant number of apoptotic cancer cells are present.

The intramembrane proteases SPPL2a and SPPL2b regulate the homeostasis of selected SNARE proteins

Signal peptide peptidase (SPP) and SPP-like (SPPL) aspartyl intramembrane proteases are known to contribute to sequential processing of type II-oriented membrane proteins referred to as regulated intramembrane proteolysis. The ER-resident family members SPP and SPPL2c were shown to also cleave tail-anchored proteins, including selected SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins facilitating membrane fusion events. Here, we analysed whether the related SPPL2a and SPPL2b proteases, which localise to the endocytic or late secretory pathway, are also able to process SNARE proteins. Therefore, we screened 18 SNARE proteins for cleavage by SPPL2a and SPPL2b based on cellular co-expression assays, of which the proteins VAMP1, VAMP2, VAMP3 and VAMP4 were processed by SPPL2a/b demonstrating the capability of these two proteases to proteolyse tail-anchored proteins. Cleavage of the four SNARE proteins was scrutinised at the endogenous level upon SPPL2a/b inhibition in different cell lines as well as by analysing VAMP1-4 levels in tissues and primary cells of SPPL2a/b double-deficient (dKO) mice. Loss of SPPL2a/b activity resulted in an accumulation of VAMP1-4 in a cell type- and tissue-dependent manner, identifying these proteins as SPPL2a/b substrates validated in vivo. Therefore, we propose that SPPL2a/b control cellular levels of VAMP1-4 by initiating the degradation of these proteins, which might impact cellular trafficking.

Phagosomal signalling of the C-type lectin receptor Dectin-1 is terminated by intramembrane proteolysis

Sensing of pathogens by pattern recognition receptors (PRR) is critical to initiate protective host defence reactions. However, activation of the immune system has to be carefully titrated to avoid tissue damage necessitating mechanisms to control and terminate PRR signalling. Dectin-1 is a PRR for fungal β-glucans on immune cells that is rapidly internalised after ligand-binding. Here, we demonstrate that pathogen recognition by the Dectin-1a isoform results in the formation of a stable receptor fragment devoid of the ligand binding domain. This fragment persists in phagosomal membranes and contributes to signal transduction which is terminated by the intramembrane proteases Signal Peptide Peptidase-like (SPPL) 2a and 2b. Consequently, immune cells lacking SPPL2b demonstrate increased anti-fungal ROS production, killing capacity and cytokine responses. The identified mechanism allows to uncouple the PRR signalling response from delivery of the pathogen to degradative compartments and identifies intramembrane proteases as part of a regulatory circuit to control anti-fungal immune responses.

Dectin-1 is a critical component of the innate sensing repertoire which is involved in pattern based recognition of fungal pathogens. Here the authors show that intramembrane proteolysis is involved in the regulation of the antifungal host response by termination of the phagosomal signalling of Dectin-1.

Lysosomes and lysosome‐related organelles in immune responses

The catabolic, degradative capacity of the endo‐lysosome system is put to good use in mammalian immune responses as is their recently established status as signaling platforms. From the ‘creative destruction’ of antigenic and ‘self’ material for antigen presentation to T cells to the re‐purposing of lysosomes as toxic exocytosable lysosome‐related organelles (granules) in leukocytes such as CD8 T cells and eosinophils, endo‐lysosomes are key players in host defense. Signaled responses to some pathogen products initiate in endo‐lysosomes and these organelles are emerging as important in distinct ways in the unique immunobiology of dendritic cells. Potential self‐inflicted toxicity from lysosomal and granule proteases is countered by expression of serpin and cystatin family members.

Signal peptide peptidase-like 2 proteases: Regulatory switches or proteasome of the membrane?

Signal peptide peptidase-like 2 (SPPL) proteases constitute a subfamily of SPP/SPPL intramembrane proteases which are homologues of the presenilins, the catalytic core of the γ-secretase complex. The three SPPL2 proteases SPPL2a, SPPL2b and SPPL2c proteolyse single-span, type II-oriented transmembrane proteins and/or tail-anchored proteins within their hydrophobic transmembrane segments. We review recent progress in defining substrate spectra and in vivo functions of these proteases. Characterisation of the respective knockout mice has implicated SPPL2 proteases in immune cell differentiation and function, prevention of atherosclerotic plaque development and spermatogenesis. Mechanisms how substrates are selected by these enzymes are still incompletely understood. We will discuss current views on how selective SPPL2-mediated cleavage is or whether these proteases may exhibit a generalised role in the turnover of membrane proteins. This has been suggested previously for the mechanistically related γ-secretase for which the term "proteasome of the membrane" has been coined based on its broad substrate spectrum. With regard to individual substrates, potential signalling functions of the resulting cytosolic cleavage fragments remain a controversial aspect. However, it has been clearly shown that SPPL2 proteases can influence cellular signalling and membrane trafficking by controlling levels of their membrane-bound substrate proteins which highlights these enzymes as regulatory switches. Based on this, regulatory mechanisms controlling activity of SPPL2 proteases would need to be postulated, which are just beginning to emerge. These different questions, which are relevant for other families of intramembrane proteases in a similar way, will be critically discussed based on the current state of knowledge.

The transmembrane domain and luminal C-terminal region independently support invariant chain trimerization and assembly with MHCII into nonamers

Background

Invariant chain (CD74, Ii) is a multifunctional protein expressed in antigen presenting cells. It assists the ER exit of various cargos and serves as a receptor for the macrophage migration inhibitory factor. The newly translated Ii chains trimerize, a structural feature that is not readily understood in the context of its MHCII chaperoning function. Two segments of Ii, the luminal C-terminal region (TRIM) and the transmembrane domain (TM), have been shown to participate in the trimerization process but their relative importance and impact on the assembly with MHCII molecules remains debated. Here, we addressed the requirement of these domains in the trimerization of human Ii as well as in the oligomerization with MHCII molecules. We used site-directed mutagenesis to generate series of Ii and DR mutants. These were transiently transfected in HEK293T cells to test their cell surface expression and analyse their interactions by co-immunoprecipitations.

Results

Our results showed that the TRIM domain is not essential for Ii trimerization nor for intracellular trafficking with MHCII molecules. We also gathered evidence that in the absence of TM, TRIM allows the formation of multi-subunit complexes with HLA-DR. Similarly, in the absence of TRIM, Ii can assemble into high-order structures with MHCII molecules.

Conclusions

Altogether, our data show that trimerization of Ii through either TM or TRIM sustains nonameric complex formation with MHCII molecules.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12865-021-00444-6.

An updated review on Mendelian susceptibility to mycobacterial diseases - a silver jubilee celebration of its first genetic diagnosis

INTRODUCTION

Mendelian susceptibility to mycobacterial diseases (MSMD), a group of at least 18 different genetic disorders, encompasses a specific class of inborn errors of immunity that result in predilection to infection with mycobacteria including the weakly virulent strains. Primarily, these consist of defects in the IFN-γ-IL-12/23 circuit that is crucial for immunity against intracellular microorganisms. Although the first genetic etiology of MSMD was discovered in 1996, molecular diagnosis of MSMD in resource-constrained settings may remain far-fetched. Recently, original studies have emerged from developing countries, including India, wherein the genetic diagnosis was confirmed within the country itself. A lag of about 25 years, hence, seems to exist.

AREAS COVERED

Herein, we review the clinical, laboratory, and mutational profile of the genetic defects responsible for causing MSMD. We intend to enhance the recognition of these disorders in settings endemic for tuberculosis and bridge the gap between the developed and developing countries in the field of MSMD research and therapeutics.

EXPERT OPINION

Research in the field of MSMD in developing countries, including India, can uncover novel genetic etiologies, as the population exceeds 1.3 billion, a huge burden of tuberculosis (across all clinical spectrums) exists, and BCG vaccination is given universally at birth.

Integration of Alzheimer's disease genetics and myeloid genomics identifies disease risk regulatory elements and genes

Genome-wide association studies (GWAS) have identified more than 40 loci associated with Alzheimer's disease (AD), but the causal variants, regulatory elements, genes and pathways remain largely unknown, impeding a mechanistic understanding of AD pathogenesis. Previously, we showed that AD risk alleles are enriched in myeloid-specific epigenomic annotations. Here, we show that they are specifically enriched in active enhancers of monocytes, macrophages and microglia. We integrated AD GWAS with myeloid epigenomic and transcriptomic datasets using analytical approaches to link myeloid enhancer activity to target gene expression regulation and AD risk modification. We identify AD risk enhancers and nominate candidate causal genes among their likely targets (including AP4E1, AP4M1, APBB3, BIN1, MS4A4A, MS4A6A, PILRA, RABEP1, SPI1, TP53INP1, and ZYX) in twenty loci. Fine-mapping of these enhancers nominates candidate functional variants that likely modify AD risk by regulating gene expression in myeloid cells. In the MS4A locus we identified a single candidate functional variant and validated it in human induced pluripotent stem cell (hiPSC)-derived microglia and brain. Taken together, this study integrates AD GWAS with multiple myeloid genomic datasets to investigate the mechanisms of AD risk alleles and nominates candidate functional variants, regulatory elements and genes that likely modulate disease susceptibility.

Deficiency of the Intramembrane Protease SPPL2a Alters Antimycobacterial Cytokine Responses of Dendritic Cells

Signal peptide peptidase-like 2a (SPPL2a) is an aspartyl intramembrane protease essential for degradation of the invariant chain CD74. In humans, absence of SPPL2a leads to Mendelian susceptibility to mycobacterial disease, which is attributed to a loss of the dendritic cell (DC) subset conventional DC2. In this study, we confirm depletion of conventional DC2 in lymphatic tissues of SPPL2a^-/- mice and demonstrate dependence on CD74 using SPPL2a^-/- CD74^-/- mice. Upon contact with mycobacteria, SPPL2a^-/- bone marrow-derived DCs show enhanced secretion of IL-1β, whereas production of IL-10 and IFN-β is reduced. These effects correlated with modulated responses upon selective stimulation of the pattern recognition receptors TLR4 and Dectin-1. In SPPL2a^-/- bone marrow-derived DCs, Dectin-1 is redistributed to endosomal compartments. Thus, SPPL2a deficiency alters pattern recognition receptor pathways in a CD74-dependent way, shifting the balance from anti- to proinflammatory cytokines in antimycobacterial responses. We propose that in addition to the DC reduction, this altered DC functionality contributes to Mendelian susceptibility to mycobacterial disease upon SPPL2a deficiency.

Signaling Functions of Intramembrane Aspartyl-Proteases

Intramembrane proteolysis is more than a mechanism to "clean" the membranes from proteins no longer needed. By non-reversibly modifying transmembrane proteins, intramembrane cleaving proteases hold key roles in multiple signaling pathways and often distinguish physiological from pathological conditions. Signal peptide peptidase (SPP) and signal peptide peptidase-like proteases (SPPLs) recently have been associated with multiple functions in the field of signal transduction. SPP/SPPLs together with presenilins (PSs) are the only two families of intramembrane cleaving aspartyl proteases known in mammals. PS1 or PS2 comprise the catalytic center of the γ-secretase complex, which is well-studied in the context of Alzheimer's disease. The mammalian SPP/SPPL family of intramembrane cleaving proteases consists of five members: SPP and its homologous proteins SPPL2a, SPPL2b, SPPL2c, and SPPL3. Although these proteases were discovered due to their homology to PSs, it became evident in the past two decades that no physiological functions are shared between these two families. Based on studies in cell culture models various substrates of SPP/SPPL proteases have been identified in the past years and recently-developed mouse lines lacking individual members of this protease family, will help to further clarify the physiological functions of these proteases. In this review we concentrate on signaling roles of mammalian intramembrane cleaving aspartyl proteases. In particular, we will highlight the signaling roles of PS via its substrates NOTCH, VEGF, and others, mainly focusing on its involvement in vasculature. Delineating also signaling pathways that are affected and/or controlled by SPP/SPPL proteases. From SPP's participation in tumor progression and survival, to SPPL3's regulation of protein glycosylation and SPPL2c's control over cellular calcium stores, various crossovers between proteolytic activity of intramembrane proteases and cell signaling will be described.

How Lysosomes Sense, Integrate, and Cope with Stress

Lysosomes are in the center of the cellular control of catabolic and anabolic processes. These membrane-surrounded acidic organelles contain around 70 hydrolases, 200 membrane proteins, and numerous accessory proteins associated with the cytosolic surface of lysosomes. Accessory and transmembrane proteins assemble in signaling complexes that sense and integrate multiple signals and transmit the information to the nucleus. This communication allows cells to respond to changes in multiple environmental conditions, including nutrient levels, pathogens, energy availability, and lysosomal damage, with the goal of restoring cellular homeostasis. This review summarizes our current understanding of the major molecular players and known pathways that are involved in control of metabolic and stress responses that either originate from lysosomes or regulate lysosomal functions.

Physiological functions of SPP/SPPL intramembrane proteases

Intramembrane proteolysis describes the cleavage of substrate proteins within their hydrophobic transmembrane segments. Several families of intramembrane proteases have been identified including the aspartyl proteases Signal peptide peptidase (SPP) and its homologues, the SPP-like (SPPL) proteases SPPL2a, SPPL2b, SPPL2c and SPPL3. As presenilin homologues, they employ a similar catalytic mechanism as the well-studied γ-secretase. However, SPP/SPPL proteases cleave transmembrane proteins with a type II topology. The characterisation of SPP/SPPL-deficient mouse models has highlighted a still growing spectrum of biological functions and also promoted the substrate discovery of these proteases. In this review, we will summarise the current hypotheses how phenotypes of these mouse models are linked to the molecular function of the enzymes. At the cellular level, SPP/SPPL-mediated cleavage events rather provide specific regulatory switches than unspecific bulk proteolysis. By this means, a plethora of different cell biological pathways is influenced including signal transduction, membrane trafficking and protein glycosylation.

Role of MIF Cytokine/CD74 Receptor Pathway in Protecting Against Injury and Promoting Repair

Wound healing after an injury is essential for life. An in-depth understanding of the healing process is necessary to ultimately improve the currently limited treatment options for patients suffering as a result of damage to various organs and tissues. Injuries, even the most minor, trigger an inflammatory response that protects the host and activates repair pathways. In recent years, substantial progress has been made in delineating the mechanisms by which inflammatory cytokines and their receptors facilitate tissue repair and regeneration. This mini review focuses on emerging literature on the role of the cytokine macrophage migration inhibitory factor (MIF) and its cell membrane receptor CD74, in protecting against injury and promoting healing in different parts of the body.

Impaired proteolysis by SPPL2a causes CD74 fragment accumulation that can be recognized by anti-CD74 autoantibodies in human ankylosing spondylitis

Ankylosing spondylitis (AS) is associated with autoantibody production to class II MHC‐associated invariant chain peptide, CD74/CLIP. In this study, we considered that anti‐CD74/CLIP autoantibodies present in sera from AS might recognize CD74 degradation products that accumulate upon deficiency of the enzyme signal peptide peptidase‐like 2A (SPPL2a). We analyzed monocytes from healthy controls (n = 42), psoriatic arthritis (n = 25), rheumatoid arthritis (n = 16), and AS patients (n = 15) for SPPL2a enzyme activity and complemented the experiments using SPPL2a‐sufficient and ‐deficient THP‐1 cells. We found defects in SPPL2a function and CD74 processing in a subset of AS patients, which culminated in CD74 and HLA class II display at the cell surface. These findings were verified in SPPL2a‐deficient THP‐1 cells, which showed expedited expression of MHC class II, total CD74 and CD74 N‐terminal degradation products at the plasma membrane upon receipt of an inflammatory trigger. Furthermore, we observed that IgG anti‐CD74/CLIP autoantibodies recognize CD74 N‐terminal degradation products that accumulate upon SPPL2a defect. In conclusion, reduced activity of SPPL2a protease in monocytes from AS predisposes to endosomal accumulation of CD74 and CD74 N‐terminal fragments, which, upon IFN‐γ‐exposure, is deposited at the plasma membrane and can be recognized by anti‐CD74/CLIP autoantibodies.

Heme binding of transmembrane signaling proteins undergoing regulated intramembrane proteolysis

Transmembrane signaling proteins play a crucial role in the transduction of information across cell membranes. One function of regulated intramembrane proteolysis (RIP) is the release of signaling factors from transmembrane proteins. To study the role of transmembrane domains (TMDs) in modulating structure and activity of released signaling factors, we purified heterologously expressed human transmembrane proteins and their proteolytic processing products from Escherichia coli. Here we show that CD74 and TNFα are heme binding proteins. Heme coordination depends on both a cysteine residue proximal to the membrane and on the oligomerization of the TMD. Furthermore, we show that the various processing products have different modes of heme coordination. We suggest that RIP changes the mode of heme binding of these proteins and generates heme binding peptides with yet unexplored functions. The identification of a RIP modulated cofactor binding of transmembrane signaling proteins sheds new light on the regulation of cell signaling pathways.

Kupke et al. study regulated intramembrane proteolysis (RIP) using recombinant transmembrane proteins CD74 and TNFα and find they are heme binding proteins that change their mode of heme binding after proteolytic processing. These data suggest that RIP of type II transmembrane proteins can generate intracellular heme sensor peptides.

Mendelian susceptibility to mycobacterial disease: recent discoveries

Mendelian susceptibility to mycobacterial disease (MSMD) is caused by inborn errors of IFN-γ immunity. Affected patients are highly and selectively susceptible to weakly virulent mycobacteria, such as environmental mycobacteria and Bacillus Calmette-Guérin vaccines. Since 1996, disease-causing mutations have been reported in 15 genes, with allelic heterogeneity leading to 30 genetic disorders. Here, we briefly review the progress made in molecular, cellular, immunological, and clinical studies of MSMD since the last review published in 2018. Highlights include the discoveries of new genetic etiologies of MSMD: autosomal recessive (AR) complete deficiencies of (1) SPPL2a, (2) IL-12Rβ2, and (3) IL-23R, and (4) homozygosity for TYK2 P1104A, resulting in selective impairment of responses to IL-23. The penetrance of SPPL2a deficiency for MSMD is high, probably complete, whereas that of IL-12Rβ2 and IL-23R deficiencies, and TYK2 P1104A homozygosity, is incomplete, and probably low. SPPL2a deficiency has added weight to the notion that human cDC2 and Th1* cells are important for antimycobacterial immunity. Studies of IL-12Rβ2 and IL-23R deficiencies, and of homozygosity for P1104A TYK2, have shown that both IL-12 and IL-23 are required for optimal levels of IFN-γ. These recent findings illustrate how forward genetic studies of MSMD are continuing to shed light on the mechanisms of protective immunity to mycobacteria in humans.

Transcriptomic Analysis Reveals Involvement of the Macrophage Migration Inhibitory Factor Gene Network in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a progressive hereditary muscular disease with X-linked recessive inheritance, that leads patients to premature death. The loss of dystrophin determines membrane instability, causing cell damage and inflammatory response. Macrophage migration inhibitory factor (MIF) is a cytokine that exerts pleiotropic properties and is implicated in the pathogenesis of a variety of diseases. Recently, converging data from independent studies have pointed to a possible role of MIF in dystrophic muscle disorders, including DMD. In the present study, we have investigated the modulation of MIF and MIF-related genes in degenerative muscle disorders, by making use of publicly available whole-genome expression datasets. We show here a significant enrichment of MIF and related genes in muscle samples from DMD patients, as well as from patients suffering from Becker’s disease and limb-girdle muscular dystrophy type 2B. On the other hand, transcriptomic analysis of in vitro differentiated myotubes from healthy controls and DMD patients revealed no significant alteration in the expression levels of MIF-related genes. Finally, by analyzing DMD samples as a time series, we show that the modulation of the genes belonging to the MIF network is an early event in the DMD muscle and does not change with the increasing age of the patients, Overall, our analysis suggests that MIF may play a role in vivo during muscle degeneration, likely promoting inflammation and local microenvironment reaction.

Discovery of Cellular Roles of Intramembrane Proteases

Intramembrane proteases (IMPs) are localized within lipid bilayers of membranes-either the cell membrane or membranes of various organelles. Cleavage of substrates often results in release from the membrane, leading to a downstream biological effect. This mechanism allows different signaling events to happen through intramembrane proteolysis. Over the years, various mechanistically distinct families of IMPs have been discovered, but the research progress has generally been slower than for soluble proteases due to the challenges associated with membrane proteins. In this review we summarize how each mechanistic family of IMPs was discovered, which chemical tools are available for the study of IMPs, and which techniques have been developed for the discovery of IMP substrates. Finally, we discuss the various roles in cellular physiology of some of these IMPs.

Knockdown of CD-74 in the Proliferative and Apoptotic Activity of Breast Cancer Cells

BACKGROUND

The cluster of differentiation (CD) 74 is known for its immunological functions and its elevated level was reported in various cancer cells.

AIM

The aim of the present study was to investigate the expression and potential roles of CD74 in the proliferative and apoptotic activity of breast cancer.

METHODS

Expression of CD74, macrophage migration inhibitory factor (MIF) and CD44 was assayed in CAMA-1 and MDA-MB-231 cell lines using flow cytometry. CD74 was knocked down using CD74 siRNA-transfection in CAMA-1, and MDA-MB-231 cells and proliferation and apoptosis were determined in the transfected breast cancer cells.

RESULTS

The data showed that CD74, MIF and CD44 were expressed in breast cancer cell lines and were associated with cell proliferation and apoptosis. Correlation analysis revealed that CD74 was positively correlated and colocalised with MIF on the cell-surface of CAMA-1 and MDA-MB-231. The knockdown of CD74 significantly reduced CAMA-1 and MDA-MB-231 cell proliferation and increased the level of apoptotic cells.

CONCLUSION

We concluded that the interactions of CD74 with MIF and CD74 with CD44 could be a potential tumour marker for breast cancer cells. Moreover, the level of co-expression of MIF and CD74 or CD44 could be a surrogate marker for the efficacy of anti-angiogenic drugs, particularly in breast cancer tumours. In short, the study revealed the potential roles of CD74 in the proliferation and apoptosis of breast cancer which may serve as a potential therapeutic target for breast cancer.

Transcription analysis of the response of the porcine adrenal cortex to a single subclinical dose of lipopolysaccharide from Salmonella Enteritidis

Lipopolysaccharide (LPS) is a bacterial endotoxin which can participate in the induction of inflammatory responses. LPS may also play a significant role in some neurodegenerative, oncological and metabolic disorders. The aim of the current study was to determine the effect of a subclinical low single dose of LPS from Salmonella Enteritidis administrated in vivo on the transcriptome of porcine adrenal cortex cells, especially gene expression levels, long non-coding RNA (lncRNA) profiles, alternative splicing events and RNA editing sites using RNA-seq technology. The subclinical dose of LPS changed the expression of 354 genes, 27 lncRNA loci and other unclassified RNAs. An analysis of alternative splicing events revealed 104 genes with differentially expressed splice junction sites, and the single nucleotide variant calling approach supported the identification of 376 canonical RNA editing candidates and 7249 allele-specific expression variants. The obtained results suggest that the RIG-I-like receptor signaling pathway, may play a more important role than the Toll-like signaling pathway after the administration of a subclinical dose of LPS. Single subclinical dose of LPS can affect the expression profiles of genes coding peptide hormones, steroidogenic enzymes and transcriptional factors, and modulate the endocrine functions of the gland.

HIV protease inhibitors block parasite signal peptide peptidases and prevent growth of Babesia microti parasites in erythrocytes

Malaria and babesiosis are bloodborne protozoan infections for which the emergence of drug-resistant strains poses a threat. Our previous phage display cDNA screens established the essentiality of Plasmodium falciparum signal peptide peptidase (SPP) in asexual development at the blood stage of malaria infection. Given the structural similarities between SPP inhibitors and HIV protease inhibitors, we screened ten HIV protease inhibitors and selected Lopinavir and Atazanavir for their ability to inhibit PfSPP activity. Using a transcription-based assay, we observed that Lopinavir inhibits both parasite-and host-derived SPP activities whereas Atazanavir inhibited only parasite derived SPP activity. Consistent with their inhibitory effect on Plasmodium growth, both Lopinavir and Atazanavir strongly inhibited intraerythrocytic Babesia microti growth ex vivo. Moreover, Lopinavir prevented the steep rise in Babesia microti parasitemia typically observed in rag1-deficient mice. Our data provide first evidence that inhibition of parasite-derived SPPs by HIV protease inhibitors offers a promising therapeutic avenue for the treatment of severe babesiosis and infections caused by other Apicomplexa parasites.

Macrophage migration inhibitory factor (MIF) as a therapeutic target for rheumatoid arthritis and systemic lupus erythematosus

Introduction. Macrophage migration inhibitory factor (MIF) is a pleiotropic inflammatory cytokine with upstream regulatory roles in innate and adaptive immunity and is implicated in the pathogenesis of autoimmune diseases including rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). Several classes of MIF inhibitors such as small molecule inhibitors and peptide inhibitors are in clinical development. Areas covered. The role of MIF in the pathogenesis of RA and SLE is examined; the authors review the structure, physiology and signaling characteristics of MIF and the related cytokine D-DT/MIF-2. The preclinical and clinical trial data for MIF inhibitors are also reviewed; information was retrieved from PubMed and ClinicalTrials.gov using the keywords MIF, D-DT/MIF-2, CD74, CD44, CXCR2, CXCR4, Jab-1, rheumatoid arthritis, systemic lupus erythematosus, MIF inhibitor, small molecule, anti-MIF, anti-CD74, and peptide inhibitor. Expert opinion. Studies in mice and in humans demonstrate the therapeutic potential of MIF inhibition for RA and SLE. MIF- directed approaches could be particularly efficacious in patients with high expression MIF genetic polymorphisms. In patients with RA and SLE and high expression MIF alleles, targeted MIF inhibition could be a precision medicine approach to treatment. Anti-MIF pharmacotherapies could also be steroid-sparing in patients with chronic glucocorticoid dependence or refractory autoimmune disease.

Intramembrane proteolysis at a glance: from signalling to protein degradation

Over the last two decades, a group of unusual proteases, so-called intramembrane proteases, have become increasingly recognized for their unique ability to cleave peptide bonds within cellular membranes. They are found in all kingdoms of life and fulfil versatile functions ranging from protein maturation, to activation of signalling molecules, to protein degradation. In this Cell Science at a Glance article and the accompanying poster, we focus on intramembrane proteases in mammalian cells. By comparing intramembrane proteases in different cellular organelles, we set out to review their functions within the context of the roles of individual cellular compartments. Additionally, we exemplify their mode of action in relation to known substrates by distinguishing cleavage events that promote degradation of substrate from those that release active domains from the membrane bilayer.

Discovery of Orally Active Hydroxyethylamine Based SPPL2a Inhibitors

SPPL2a (Signal Peptide Peptidase Like 2a) is an intramembrane aspartyl protease engaged in the function of B-cells and dendritic cells. Despite being an attractive target for modulation of the immune system, selective SPPL2a inhibitors are barely described in the literature. Recently, we have disclosed a selective, small molecular weight agent SPL-707 which confirmed that pharmacological inhibition of SPPL2a leads to the accumulation of its substrate CD74/p8 and as a consequence to a reduction in the number of B-cells as well as myeloid dendritic cells in mice. In this paper we describe the discovery of novel hydroxyethylamine based SPPL2a inhibitors. Starting from a rather lipophilic screening hit, several iterative optimization cycles allowed for its transformation into a highly potent and selective compound 15 (SPL-410) which inhibited in vivo CD74/p8 fragment processing in mice at 10 mg/kg oral dose.

Measurement of CD74 N-terminal Fragment Accumulation in Cells Treated with SPPL2a Inhibitor

The recent discovery of human signal peptide peptidase-like 2a (SPPL2a) deficiency in humans revealed the toxicity associated with the accumulation of one of its substrates, CD74 N-terminal fragment (CD74-NTF), for certain type of dendritic cells (cDC2). We developed a two-step protocol for monitoring the accumulation of this molecule in different subsets of PBMCs and immortalized B cells, in which SPPL2a is chemically inhibited and CD74-NTF levels are then assessed by flow cytometry or western blotting. The chemical inhibition of SPPL2a has been described elsewhere, but this is the first time that this inhibition has been reported as a protocol.

Inhibitors of signal peptide peptidase and subtilisin/kexin-isozyme 1 inhibit Ebola virus glycoprotein-driven cell entry by interfering with activity and cellular localization of endosomal cathepsins

Emerging viruses such as severe fever and thrombocytopenia syndrome virus (SFTSV) and Ebola virus (EBOV) are responsible for significant morbidity and mortality. Host cell proteases that process the glycoproteins of these viruses are potential targets for antiviral intervention. The aspartyl protease signal peptide peptidase (SPP) has recently been shown to be required for processing of the glycoprotein precursor, Gn/Gc, of Bunyamwera virus and for viral infectivity. Here, we investigated whether SPP is also required for infectivity of particles bearing SFTSV-Gn/Gc. Entry driven by the EBOV glycoprotein (GP) and the Lassa virus glycoprotein (LASV-GPC) depends on the cysteine proteases cathepsin B and L (CatB/CatL) and the serine protease subtilisin/kexin-isozyme 1 (SKI-1), respectively, and was examined in parallel for control purposes. We found that inhibition of SPP and SKI-1 did not interfere with SFTSV Gn + Gc-driven entry but, unexpectedly, blocked entry mediated by EBOV-GP. The inhibition occurred at the stage of proteolytic activation and the SPP inhibitor was found to block CatL/CatB activity. In contrast, the SKI-1 inhibitor did not interfere with CatB/CatL activity but disrupted CatB localization in endo/lysosomes, the site of EBOV-GP processing. These results underline the potential of protease inhibitors for antiviral therapy but also show that previously characterized compounds might exert broader specificity than initially appreciated and might block viral entry via diverse mechanisms.

Mendelian susceptibility to mycobacterial disease: 2014-2018 update

Mendelian susceptibility to mycobacterial disease (MSMD) is caused by inborn errors of IFN-γ immunity. Since 1996, disease-causing mutations have been found in 11 genes, which, through allelic heterogeneity, underlie 21 different genetic disorders. We briefly review here progress in the study of molecular, cellular and clinical aspects of MSMD since the last comprehensive review published in 2014. Highlights include the discoveries of (1) a new genetic etiology, autosomal recessive signal peptide peptidase-like 2 A deficiency, (2) TYK2-deficient patients with a clinical phenotype of MSMD, (3) an allelic form of partial recessive IFN-γR2 deficiency, and (4) two forms of syndromic MSMD: RORγ/RORγT and JAK1 deficiencies. These recent findings illustrate how genetic and immunological studies of MSMD can shed a unique light onto the mechanisms of protective immunity to mycobacteria in humans.

Signal peptide peptidase-like 2c impairs vesicular transport and cleaves SNARE proteins

Members of the GxGD-type intramembrane aspartyl proteases have emerged as key players not only in fundamental cellular processes such as B-cell development or protein glycosylation, but also in development of pathologies, such as Alzheimer's disease or hepatitis virus infections. However, one member of this protease family, signal peptide peptidase-like 2c (SPPL2c), remains orphan and its capability of proteolysis as well as its physiological function is still enigmatic. Here, we demonstrate that SPPL2c is catalytically active and identify a variety of SPPL2c candidate substrates using proteomics. The majority of the SPPL2c candidate substrates cluster to the biological process of vesicular trafficking. Analysis of selected SNARE proteins reveals proteolytic processing by SPPL2c that impairs vesicular transport and causes retention of cargo proteins in the endoplasmic reticulum. As a consequence, the integrity of subcellular compartments, in particular the Golgi, is disturbed. Together with a strikingly high physiological SPPL2c expression in testis, our data suggest involvement of SPPL2c in acrosome formation during spermatogenesis.

The intramembrane protease SPPL2c promotes male germ cell development by cleaving phospholamban

Signal peptide peptidase (SPP) and the four homologous SPP-like (SPPL) proteases constitute a family of intramembrane aspartyl proteases with selectivity for type II-oriented transmembrane segments. Here, we analyse the physiological function of the orphan protease SPPL2c, previously considered to represent a non-expressed pseudogene. We demonstrate proteolytic activity of SPPL2c towards selected tail-anchored proteins. Despite shared ER localisation, SPPL2c and SPP exhibit distinct, though partially overlapping substrate spectra and inhibitory profiles, and are organised in different high molecular weight complexes. Interestingly, SPPL2c is specifically expressed in murine and human testis where it is primarily localised in spermatids. In mice, SPPL2c deficiency leads to a partial loss of elongated spermatids and reduced motility of mature spermatozoa, but preserved fertility. However, matings of male and female SPPL2c-/- mice exhibit reduced litter sizes. Using proteomics we identify the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA2)-regulating protein phospholamban (PLN) as a physiological SPPL2c substrate. Accumulation of PLN correlates with a decrease in intracellular Ca2+ levels in elongated spermatids that likely contribute to the compromised male germ cell differentiation and function of SPPL2c-/- mice.

Atherogenic LOX-1 signaling is controlled by SPPL2-mediated intramembrane proteolysis

The intramembrane proteases SPPL2a/b control pro-atherogenic signaling of membrane-bound proteolytic fragments derived from the oxLDL receptor LOX-1. In mice deficient for these proteases, plaque development and fibrosis is enhanced. This highlights SPPL2a/b as crucial players of a novel athero-protective mechanism, which is conserved in humans.

The lectin-like oxidized LDL receptor 1 (LOX-1) is a key player in the development of atherosclerosis. LOX-1 promotes endothelial activation and dysfunction by mediating uptake of oxidized LDL and inducing pro-atherogenic signaling. However, little is known about modulators of LOX-1–mediated responses. Here, we show that the function of LOX-1 is controlled proteolytically. Ectodomain shedding by the metalloprotease ADAM10 and lysosomal degradation generate membrane-bound N-terminal fragments (NTFs), which we identified as novel substrates of the intramembrane proteases signal peptide peptidase–like 2a and b (SPPL2a/b). SPPL2a/b control cellular LOX-1 NTF levels which, following self-association via their transmembrane domain, can activate MAP kinases in a ligand-independent manner. This leads to an up-regulation of several pro-atherogenic and pro-fibrotic targets including ICAM-1 and the connective tissue growth factor CTGF. Consequently, SPPL2a/b-deficient mice, which accumulate LOX-1 NTFs, develop larger and more advanced atherosclerotic plaques than controls. This identifies intramembrane proteolysis by SPPL2a/b as a novel atheroprotective mechanism via negative regulation of LOX-1 signaling.

Disruption of an antimycobacterial circuit between dendritic and helper T cells in human SPPL2a deficiency

Human inborn errors of IFN-γ immunity underlie mycobacterial diseases. We describe patients with Mycobacterium bovis (BCG) disease who are homozygous for loss-of-function mutations of SPPL2A. This gene encodes a transmembrane protease that degrades the N-terminal fragment (NTF) of CD74 (HLA invariant chain) in antigen-presenting cells. The CD74 NTF therefore accumulates in the HLA class II+ myeloid and lymphoid cells of SPPL2a-deficient patients. This toxic fragment selectively depletes IL-12- and IL-23-producing CD1c+ conventional dendritic cells (cDC2s) and their circulating progenitors. Moreover, SPPL2a-deficient memory TH1* cells selectively fail to produce IFN-γ when stimulated with mycobacterial antigens in vitro. Finally, Sppl2a-/- mice lack cDC2s, have CD4+ T cells that produce small amounts of IFN-γ after BCG infection, and are highly susceptible to infection with BCG or Mycobacterium tuberculosis. These findings suggest that inherited SPPL2a deficiency in humans underlies mycobacterial disease by decreasing the numbers of cDC2s and impairing IFN-γ production by mycobacterium-specific memory TH1* cells.

Old vaccines, new diseases: when BCG meets SPPL2a

In the current issue of Nature Immunology , Casanova and colleagues demonstrate that humans (and mouse models) with autosomal-recessive SPPL2a deficiency have a severe defect in conventional dendritic cell 2 survival and production of IL-12 and IL-23, and diminished IFN-γ secretion by mycobacterium-specific memory T cells, thus resulting in increased susceptibility to mycobacterial diseases.

Proteolytic ectodomain shedding of membrane proteins in mammals-hardware, concepts, and recent developments

Proteolytic removal of membrane protein ectodomains (ectodomain shedding) is a post-translational modification that controls levels and function of hundreds of membrane proteins. The contributing proteases, referred to as sheddases, act as important molecular switches in processes ranging from signaling to cell adhesion. When deregulated, ectodomain shedding is linked to pathologies such as inflammation and Alzheimer's disease. While proteases of the "a disintegrin and metalloprotease" (ADAM) and "beta-site APP cleaving enzyme" (BACE) families are widely considered as sheddases, in recent years a much broader range of proteases, including intramembrane and soluble proteases, were shown to catalyze similar cleavage reactions. This review demonstrates that shedding is a fundamental process in cell biology and discusses the current understanding of sheddases and their substrates, molecular mechanisms and cellular localizations, as well as physiological functions of protein ectodomain shedding. Moreover, we provide an operational definition of shedding and highlight recent conceptual advances in the field. While new developments in proteomics facilitate substrate discovery, we expect that shedding is not a rare exception, but rather the rule for many membrane proteins, and that many more interesting shedding functions await discovery.

Transcriptomic profiles of post-smolt Atlantic salmon challenged with Piscirickettsia salmonis reveal a strategy to evade the adaptive immune response and modify cell-autonomous immunity

Piscirickettsiosis is the main bacterial disease affecting the Chilean salmon farming industry and is responsible for high economic losses. The development of effective strategies to control piscirickettsiosis has been limited in part by insufficient knowledge of the host response. The aim of this study was to use RNA sequencing to describe the transcriptional profiles of the responses of post-smolt Atlantic salmon infected with LF-89-like or EM-90-like Piscirickettsia salmonis. Enrichment and pathway analyses of the differentially expressed genes revealed several central signatures following infection, including positive regulation of DC-SIGN and TLR5 signalling, which converged at the NF-κB level to modulate the pro-inflammatory cytokine response, particularly in the PS-EM-90-infected fish. P. salmonis induced an IFN-inducible response (e.g., IRF-1 and GBP-1) but inhibited the humoral and cell-mediated immune responses. P. salmonis induced significant cytoskeletal reorganization but decreased lysosomal protease activity and caused the degradation of proteins associated with cellular stress. Infection with these isolates also delayed protein transport, antigen processing, vesicle trafficking and autophagy. Both P. salmonis isolates promoted cell survival and proliferation and inhibited apoptosis. Both groups of Trojan fish used similar pathways to modulate the immune response at 5 dpi, but the transcriptomic profiles in the head kidneys of the cohabitant fish infected with PS-LF-89 and PS-MS-90 were relatively different at day 35 post-infection of the Trojan fish, probably due to the different degree of pathogenicity of each isolate. Our study showed the most important biological mechanisms used by P. salmonis, regardless of the isolate, to evade the immune response, maintain the viability of host cells and increase intracellular replication and persistence at the infection site. These results improve the understanding of the mechanisms by which P. salmonis interacts with its host and may serve as a basis for the development of effective strategies for the control of piscirickettsiosis.

Functional characterization of the lysosomal membrane protein TMEM192 in mice

The Transmembrane protein 192 (TMEM192) is a lysosomal/late endosomal protein initially discovered by organellar proteomics. TMEM192 exhibits four transmembrane segments with cytosolic N- and C-termini and forms homodimers. Devoid of significant homologies, the molecular function of TMEM192 is currently unknown. Upon TMEM192 knockdown in hepatoma cells, a dysregulation of autophagy and increased apoptosis were reported. Here, we aimed to define the physiological role of TMEM192 by analysing consequences of TMEM192 ablation in mice. Therefore, we compared the biochemical properties of murine TMEM192 to those of the human orthologue. We reveal lysosomal residence of murine TMEM192 and demonstrate ubiquitous tissue expression. In brain, TMEM192 expression was pronounced in the hippocampus but also present in the cortex and cerebellum, as analysed based on a lacZ reporter allele. Murine TMEM192 undergoes proteolytic processing in a tissue-specific manner. Thereby, a 17 kDa fragment is generated which was detected in most murine tissues except liver. TMEM192 processing occurs after lysosomal targeting by pH-dependent lysosomal proteases. TMEM192-/- murine embryonic fibroblasts (MEFs) exhibited a regular morphology of endo-/lysosomes and were capable of performing autophagy and lysosomal exocytosis. Histopathological, ultrastructural and biochemical analyses of all major tissues of TMEM192-/- mice demonstrated normal lysosomal functions without apparent lysosomal storage. Furthermore, the abundance of the major immune cells was comparable in TMEM192-/- and wild type mice. Based on this, we conclude that under basal conditions in vivo the loss of TMEM192 can be efficiently compensated by alternative pathways. Further studies will be required to decipher its molecular function.